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Fundamentals and Technical Applications
With Contributions of Stefan WeigelMichael P Schluumlsener and Jens A Andresen
W Schnabel
Polymers and Light
W Schnabel
Polymers and Light
Each generation has its unique needs and aspirations When Charles Wiley firstopened his small printing shop in lower Manhattan in 1807 it was a generationof boundless potential searching for an identity And we were there helping todefine a new American literary tradition Over half a century later in the midstof the Second Industrial Revolution it was a generation focused on buildingthe future Once again we were there supplying the critical scientific technicaland engineering knowledge that helped frame the world Throughout the 20thCentury and into the new millennium nations began to reach out beyond theirown borders and a new international community was born Wiley was there ex-panding its operations around the world to enable a global exchange of ideasopinions and know-how
For 200 years Wiley has been an integral part of each generationrsquos journeyenabling the flow of information and understanding necessary to meet theirneeds and fulfill their aspirations Today bold new technologies are changingthe way we live and learn Wiley will be there providing you the must-haveknowledge you need to imagine new worlds new possibilities and new oppor-tunities
Generations come and go but you can always count on Wiley to provide youthe knowledge you need when and where you need it
William J Pesce Peter Booth WileyPresident and Chief Executive Officer Chairman of the Board
1807ndash2007 Knowledge for Generations
Fundamentals and Technical Applications
With Contributions of Stefan WeigelMichael P Schluumlsener and Jens A Andresen
W Schnabel
Polymers and Light
The Author
Prof Dr W SchnabelDivison of Solar Energy ResearchHahn-Meitner-InstitutGlienicker Str 10014109 BerlinGermany
Library of Congress Card No applied for
British Library Cataloguing-in-Publication DataA catalogue record for this book is availablefrom the British Library
Bibliographic information published bythe Deutsche NationalbibliothekThe Deutsche Nationalbibliothek lists this publica-tion in the Deutsche Nationalbibliografie detailedbibliographic data are available in the Internet athttpdnbd-nbde
copy 2007 WILEY-VCH Verlag GmbH amp Co KGaAWeinheim
All rights reserved (including those of translationinto other languages) No part of this book maybe reproduced in any form ndash by photoprintingmicrofilm or any other means ndash nor transmittedor translated into a machine language withoutwritten permission from the publishersRegistered names trademarks etc used in thisbook even when not specifically marked as suchare not to be considered unprotected by law
Composition K+V Fotosatz GmbH BeerfeldenPrinting betz-druck GmbH DarmstadtBookbinding Litges amp Dopf GmbH HeppenheimCover Adam Design WeinheimWiley Bicentennial Logo Richard J Pacifico
Printed in the Federal Republic of GermanyPrinted on acid-free paper
ISBN 978-3-527-31866-7
All books published by Wiley-VCH are carefullyproduced Nevertheless authors editors andpublisher do not warrant the information containedin these books including this book to be free oferrors Readers are advised to keep in mind thatstatements data illustrations procedural details orother items may inadvertently be inaccurate
Preface XIII
Introduction 1
Part I Light-induced physical processes in polymers
1 Absorption of light and subsequent photophysical processes 511 Principal aspects 512 The molecular orbital model 713 The Jablonski diagram 1014 Absorption in non-conjugated polymers 1015 Absorption in conjugated polymers 1216 Deactivation of electronically excited states 13161 Intramolecular deactivation 13162 Intermolecular deactivation 14163 Energy migration and photon harvesting 16164 Deactivation by chemical reactions 2117 Absorption and emission of polarized light 22171 Absorption 22172 Absorption by chiral molecules 23173 Emission 2618 Applications 30181 Absorption spectroscopy 301811 UVVis spectroscopy 301812 Circular dichroism spectroscopy 321813 IR spectroscopy 35182 Luminescence 37183 Time-resolved spectroscopy 381831 General aspects 381832 Experimental techniques 391833 Applications of time-resolved techniques 4118331 Optical absorption 41
V
Contents
18332 Luminescence 44References 45
2 Photoconductivity 4921 Introductory remarks 4922 Photogeneration of charge carriers 50221 General aspects 50222 The exciton model 52223 Chemical nature of charge carriers 54224 Kinetics of charge carrier generation 55225 Quantum yield of charge carrier generation 5723 Transport of charge carriers 6024 Mechanism of charge carrier transport in amorphous poly-
mers 6425 Doping 6626 Photoconductive polymers produced by thermal or high-energy
radiation treatment 6927 Photoconductive polymers produced by plasma polymerization or
glow discharge 70References 70
3 Electro-optic and nonlinear optical phenomena 7331 Introductory remarks 7332 Fundamentals 74321 Electric field dependence of polarization and dipole moment 74322 Electric field dependence of the index of refraction 7833 Characterization techniques 79331 Second-order phenomena 793311 Determination of the hyperpolarizability 793312 Determination of the susceptibility (2) 81332 Third-order phenomena 823321 Third harmonic generation 833322 Self-focusingdefocusing 843323 Two-photon absorption (TPA) 853324 Degenerate four-wave mixing (DFWM) and optical phase
conjugation 8634 Nonlinear optical materials 87341 General aspects 87342 Second-order NLO materials 893421 Guest-host systems and NLO polymers 893422 Orientation techniques 92343 Third-order NLO materials 9335 Applications of NLO polymers 96351 Applications relating to telecommunications 96352 Applications relating to optical data storage 99
ContentsVI
353 Additional applications 100References 101
4 Photorefractivity 10341 The photorefractive effect 10342 Photorefractive formulations 10543 Orientational photorefractivity 10744 Characterization of PR materials 10845 Applications 110
References 112
5 Photochromism 11351 Introductory remarks 11352 Conformational changes in linear polymers 115521 Solutions 115522 Membranes 12253 Photocontrol of enzymatic activity 12354 Photoinduced anisotropy (PIA) 12355 Photoalignment of liquid-crystal systems 12656 Photomechanical effects 130561 Bulk materials 130562 Monolayers 13357 Light-induced activation of second-order NLO properties 13458 Applicationss 136581 Plastic photochromic eyewear 136582 Data storage 137
References 139
6 Technical developments related to photophysical processesin polymers 143
61 Electrophotography ndash Xerography 14362 Polymeric light sources 146621 Light-emitting diodes 1476211 General aspects 1476212 Mechanism 1506213 Polarized light from OLEDs 1546214 White-light OLEDs 155622 Lasers 1566221 General aspects 1566222 Lasing mechanism 1586223 Optical resonator structures 1596224 Prospects for electrically pumped polymer lasers 16263 Polymers in photovoltaic devices 16264 Polymer optical waveguides 167641 General aspects 167
Contents VII
642 Optical fibers 1686421 Polymer versus silica fibers 1686422 Compositions of polymer optical fibers (POFs) 1696423 Step-index and graded-index polymer optical fibers 170643 Polymer planar waveguides 170644 Polymer claddings 170
References 171
Part II Light-induced chemical processes in polymers
7 Photoreactions in synthetic polymers 17771 Introductory remarks 177711 Amplification effects 178712 Multiplicity of photoproducts 178713 Impurity chromophores 180714 Photoreactions of carbonyl groups 18272 Cross-linking 183721 Cross-linking by cycloaddition of C=C bonds 184722 Cross-linking by polymerization of reactive moieties
in pendant groups 186723 Cross-linking by photogenerated reactive species 188724 Cross-linking by cleavage of phenolic OH groups 19273 Simultaneous cross-linking and main-chain cleavage
of linear polymers 19374 Photodegradation of selected polymers 196741 Poly(vinyl chloride) 196742 Polysilanes 19875 Oxidation 19976 Singlet oxygen reactions 20277 Rearrangements 202
References 205
8 Photoreactions in biopolymers 20781 Introductory remarks 20782 Direct light effects 2118 21 Photoreactions in deoxyribonucleic acids (DNA) 2118211 Dimeric photoproducts 2128212 Other DNA photoproducts 214822 Photoreactions in proteins 2148221 Chemical alterations by UV light 2158222 Formation of stress proteins 2168223 Effects of visible light ndash photoreceptor action 2178224 Repair of lesions with the aid of DNA photolyases 219823 Photoreactions in cellulose 221824 Photoreactions in lignins and wood 221
ContentsVIII
83 Photosensitized reactions 222References 228
9 Technical developments related to photochemical processesin polymers 231
91 Polymers in photolithography 231911 Introductory remarks 231912 Lithographic processes 2319121 Projection optical lithography 2339122 Maskless lithography 235913 Resists 2369131 Classical polymeric resists ndash positive and negative resist
systems 2369132 Chemical amplification resists 2399133 Resists for ArF (193 nm) lithography 2429134 Resists for F2 (157 nm) lithography 245914 The importance of photolithography for macro- micro-
and nanofabrication 24692 Laser ablation of polymers 248921 General aspects 2489211 Introductory remarks 2489212 Phenomenological aspects 2489213 Molecular mechanism 250922 Dopant-enhanced ablation 250923 Polymers designed for laser ablation 251924 Film deposition and synthesis of organic compounds
by laser ablation 252925 Laser desorption mass spectrometry and matrix-assisted laser
desorptionionization (MALDI) 254926 Generation of periodic nanostructures in polymer surfaces 256927 Laser plasma thrusters 25693 Stabilization of commercial polymers 257931 Introductory remarks 257932 UV absorbers 2589321 Phenolic and non-phenolic UV absorbers 2589322 Mechanistic aspects 259933 Energy quenchers 260934 Chain terminators (radical scavengers) 262935 Hydroperoxide decomposers 265936 Stabilizer packages and synergism 266937 Sacrificial consumption and depletion of stabilizers 267
References 268
Contents IX
Part III Light-induced synthesis of polymers
10 Photopolymerization 275101 Introduction 275102 Photoinitiation of free radical polymerizations 2761021 General remarks 2761022 Generation of reactive free radicals 27610221 Unimolecular fragmentation of type I photoinitiators 27610222 Bimolecular reactions of type II photoinitiators 27910223 Macromolecular photoinitiators 27910224 Photoinitiators for visible light 281102241 Metal-based initiators 282102242 Dyeco-initiator systems 284102243 Quinones and 12-diketones 28510225 Inorganic photoinitiators 287103 Photoinitiation of ionic polymerizations 2881031 Cationic polymerization 28810311 General remarks 28810312 Generation of reactive cations 290103121 Direct photolysis of the initiator 290103122 Sensitized photolysis of the initiator 291103123 Free-radical-mediated generation of cations 2921031231 Oxidation of radicals 2921031232 Addition-fragmentation reactions 2941032 Anionic polymerization 29510321 General remarks 29510322 Generation of reactive species 295103221 Photo-release of reactive anions 295103222 Photo-production of reactive organic bases 296104 Topochemical polymerizations 2981041 General remarks 2981042 Topochemical photopolymerization of diacetylenes 2991043 Topochemical photopolymerization of dialkenes 301
References 302
11 Technical developments related to photopolymerization 305111 General remarks 305112 Curing of coatings sealants and structural adhesives 3071121 Free radical curing 30711211 Solvent-free formulations 30711212 Waterborn formulations 3091122 Cationic curing 3091123 Dual curing 310113 Curing of dental preventive and restorative systems 312114 Stereolithography ndash microfabrication 313
ContentsX
115 Printing plates 3161151 Introductory remarks 3161152 Structure of polymer letterpress plates 3171153 Composition of the photosensitive layer 3171154 Generation of the relief structure 317116 Curing of printing inks 318117 Holography 3191171 Principal aspects 3191172 Mechanism of hologram formation 3211173 Multicolor holographic recording 3211174 Holographic materials 3221175 Holographic applications 323118 Light-induced synthesis of block and graft copolymers 3241181 Principal aspects 3241182 Surface modification by photografting 328
References 329
Part IV Miscellaneous technical developments
12 Polymers in optical memories 337121 General aspects 337122 Current optical data storage systems 3381221 Compact disk (CD) and digital versatile disk (DVD) 3381222 Blue-ray disks 340123 Future optical data storage systems 3411231 General aspects 3411232 Volume holography 34212321 Storage mechanism 34212322 Storage materials 3431233 Photo-induced surface relief storing 345
References 345
13 Polymeric photosensors 347131 General aspects 347132 Polymers as active chemical sensors 3491321 Conjugated polymers 34913211 Turn-off fluorescence detection 35013212 Turn-on fluorescence detection 35013213 ssDNA base sequence detection 35213214 Sensors for metal ions 35213215 Image sensors 3531322 Optical fiber sensors 3531323 Displacement sensors 354133 Polymers as transducer supports 355
References 356
Contents XI
14 Polymeric photocatalysts 359141 General aspects 359142 Polymers as active photocatalysts 3591421 Conjugated polymers 3591422 Linear polymers bearing pendant aromatic groups 361143 Polymers as supports for inorganic photocatalysts 362
References 364
Subject Index 365
ContentsXII
Light can do a lot of quite different things to polymers and light is employedin various quite different technical applications related to polymers that have be-come beneficial to humans and are influencing the daily lives of many peopleThese applications include photocopying machines computer chips compactdisks polymer optical fiber systems in local area networks and printing platesThere are many other very useful practical applications Since these are com-monly dealt with separately in monographs or review articles the idea arose tocomprehend and combine in a single book all important developments relatedto polymers and light that concern industrially employed practical applicationsor show potential for future applications Actually I first contemplated writing abook dealing with both physical and chemical aspects related to the interactionof light with polymers and to the synthesis of polymers with the aid of lightwhile I was lecturing on certain topics of this field at the Technical Universityin Berlin and at Rika Daigaku (Science University) in Tokyo However I onlystarted to immerse myself in this extensive project when I retired from activeservice some time ago Upon retrieving and studying the salient literature I be-came fascinated by the broadness of the field The results of this project are pre-sented here for the first time In referring to the different topics I have tried todeal with the fundamentals only to the extent necessary for an understandingof described effects In attempting to be as concise as possible descriptions oftechnical processes and tools have had to be restricted to a minimum in orderto keep the extent of the book within reasonable limits To somewhat compen-sate for this flaw a rather comprehensive list of literature references also cover-ing technical aspects is presented at the end of each chapter
Writing a monograph implies that the author can both concentrate on thesubject in a quiet office and rely on the cooperation of an effectively functioninglibrary Both were provided by the Hahn-Meitner-Institute HMI and I am verygrateful to the management of this institute especially to Prof Dr M SteinerScientific Director Chief Executive for giving me the opportunity to work onthis book after my transfer to emeritus status Special thanks are due to ProfDr H Tributsch head of the Solar Energy Research Division of HMI for appreciat-ing my intention to write this book and for providing a quiet room The HMIlibrary under the direction of Dr E Kupfer and his successor Dr W Fritsch has sub-
XIII
Preface
stantially contributed to the preparation and completion of the manuscript bydelivering necessary resources and executing many retrievals The latter yieldedmost of the literature citations upon which this book is based In this context Iwish to express my special gratitude to senior librarian Mr M Wiencken whohas performed an excellent job Other people who proved very helpful in thisproject are Mr D Gaszligen who has kept the computer running and Mrs PKampfenkel who has scanned various figures
The personnel of the publisher Wiley-VCH worked carefully and rapidly onthe editing of the manuscript after its completion in the summer of 2006 Thisis gratefully acknowledged
Last but not least credit has to be given to the efforts of the authorrsquos familyMy wife Hildegard has accompanied the progress of the project with encourag-ing sympathy and moral support and my two sons Dr Ronald Schnabel andDr Rainer Florian Schnabel have given substantial advice The latter has criti-cally read all chapters of the manuscript
Berlin November 2006 Wolfram Schnabel
PrefaceXIV
The technological developments of the last decades have been essentially deter-mined by trends to invent new materials and to establish new technical meth-ods These trends encompass the synthesis of novel polymeric materials andthe employment of light in industrial processes To an increasing extent techni-cal processes based on the interaction of light with polymers have become im-portant for various applications To mention a few examples polymers are usedas nonlinear optical materials as core materials for optical wave guides and asphotoresists in the production of computer chips Polymers serve as photo-switches and optical memories and are employed in photocopying machinesand in solar cells for the generation of energy Moreover certain polymeric ma-terials can be utilized for the generation of light
On the other hand light serves also as a tool for the synthesis of polymersie for the initiation of the polymerization of small molecules a method whichis applied in technical processes involving the curing of coatings and adhesivesand even by the dentist to cure tooth inlays
Obviously the field related to the topic polymers and light is a very broad oneA principle of order derived from the distinction of photophysical from photo-chemical processes may help to steer us through this wide field Hence photo-physical and photochemical processes are addressed in separate parts of thisbook (Part I and Part II) where both fundamentals and related practical applica-tions are dealt with Regarding pure photophysical processes that are not com-bined with chemical alterations of the polymers (Part I) separate chapters aredevoted to fundamentals concerning the interaction of light with polymersphotoconductivity electro-optic and nonlinear phenomena photorefractivity andphotochromism (Chapters 1ndash5 respectively) Important technical applicationsrelated to photophysical processes in polymers are dealt with in Chapter 6These applications include xerography light-emitting diodes (LEDs) lasers solarcells optical wave guides and optical fibers
In Part II fundamentals of light-induced chemical processes are discussed bymaking a distinction between synthetic organic polymers (Chapter 7) and biopo-lymers (Chapter 8) Also in Part II important technical applications related tophotochemical processes in polymers are dealt with separately in Chapter 9Here important practical applications such as photolithography which is a nec-
1
Introduction
essary tool for the production of computer chips and laser ablation are coveredMoreover one section of Chapter 9 is devoted to the stabilization of commercialpolymers a very important subject regarding the long-time stability of plasticmaterials
The light-induced synthesis of polymers is the topic of Part III While the var-ious modes of photoinitiation of polymerization processes are discussed inChapter 10 related technical applications are treated in Chapter 11 The latterinclude curing of coatings and dental systems printing plates (used to printnewspapers) holography (important for data storage) and the synthesis ofblock-and-graft copolymers
Finally Part IV reviews miscellaneous technical developments that do not fitneatly into the scheme of the preceding parts These concern in particular theapplication of polymers in the field of optical memories treated in Chapter 12which refers also to currently important data storage systems (compact disksdigital versatile disks and blue-ray disks) Moreover the application potential ofpolymers in the fields of photosensors and photocatalysts is outlined in Chap-ters 13 and 14 respectively
Introduction2
Part ILight-induced physical processes in polymers
To open the way into the wide-ranging fields covered in this book some ele-mentary facts essential for an understanding of the material covered are out-lined at the beginning Since books [1ndash6] are available that comprehensivelytreat the principles of the interaction of light with matter the aim here is topresent the salient points in a very concise manner Nevertheless in citing typi-cal cases close adherence to the actual subject of the book has been sought byreferring to polymers wherever possible
11Principal aspects
Photons are absorbed by matter on a time scale of about 10ndash15 s During thisvery short time the electronic structure of the absorbing molecule is alteredwhereas the positions of the atomic nuclei in the molecule vibrating on a timescale of 10ndash12 s are not changed There are two prerequisites for the absorptionof a photon of energy h by a molecule (1) the molecule must contain a chro-mophoric group with excitable energy states corresponding to the photon en-ergy according to Eq (1-1)
h En E0 1-1
En and E0 denote the energies of the excited and the ground state respectivelyTypical chromophoric groups are listed in Table 11
(2) The transition between the two energy states must cause a change in thecharge distribution in the molecule ie a change in the dipole moment Interms of quantum mechanics absorption of a photon is possible (allowed) ifthe transition moment M has a non-zero value Since M is a vector composedof three components parallel to the three coordinates [Eq (1-2)] at least onecomponent must have a non-zero value
M Mx My Mz 1-2
5
1Absorption of light and subsequent photophysical processes
The higher the value of M the more efficient is the absorption As described byEq (1-3) M is composed of three integrals
M
vvdv
edpede
ssds 1-3
where v e and s are the vibronic electronic and electron-spin wave func-tions of the absorbing molecule respectively The asterisk denotes ldquoexcitedstaterdquo dp is the electronic dipole moment operator dv de and ds refer tothe three respective coordinates d= dxmiddotdymiddotdz
The three integrals in Eq (1-3) are the basis of the so-called selection rules whichdetermine whether a transition is allowed or forbidden v
vd2 is the Franck-Condon factor and
ssds applies to the spin properties of the excited and the
ground states If any of the three integrals in Eq (1-3) is zero the correspondingtransition is forbidden ie a final probability could only result from a second-orderapproximation This applies eg to the forbidden transitions between levels of thesinglet and the triplet system The magnitude of the Franck-Condon factor deter-mines the probability of transitions with respect to molecular geometry The rulestates that the transition probability is highest if the geometries of the ground andexcited states are equal A more detailed treatment of these aspects is beyond thescope of this book and the reader is referred to relevant monographs [2ndash4]
The probability of the occurrence of an electronic transition is given by the(dimensionless) oscillator strength f which is proportional to the square of thetransition moment [Eq (1-4)]
1 Absorption of light and subsequent photophysical processes6
Table 11 Typical chromophoric groups [4]
Chromophore Typical compound max
(nm) a)max
(L molndash1 cmndash1) b)Mode of electrontransition
Ethene 193 104
Ethyne 173 6103
Acetone 187271
103
15 n
Azomethane 347 5 n
t-Nitrosobutane 300665
10020
n
Amyl nitrite 219357
219357
n
a) Wavelength of maximum optical absorptionb) Decadic molar extinction coefficient (log I0I = cd)
f 875 102EM2 1-4
Here E is equal to EnndashE0 (given in eV) A large value of f corresponds to astrong absorption band and a short lifetime of the excited state The maximumvalue is f = 1
Experimentally the absorption of light is recorded as a function of the wave-length or the wave number =ndash1 by measuring the change in the intensityof a light beam passing through a sample of unit path length (1 cm) For ahomogeneous isotropic medium containing an absorbing compound at concen-tration c (mol Lndash1) the light absorption is described by Eq (1-5) the Lambert-Beer law
A lg10I0I cd 1-5
where A is the absorbance (extinction optical density) and I0 and I denote thelight intensity before and after absorption Equivalent denotations for I0 and Iare incident and transmitted radiant flux respectively (L molndash1 cmndash1) is thedecadic molar extinction coefficient at a given wavelength The Lambert-Beerlaw does not hold at high light intensities as experienced eg with lasers Theoscillator strength f is related to the measured integrated extinction coefficientd by Eq (1-6) where and have to be given in units of L molndash1 cmndash1 and
cmndash1 respectively
f 23 103c2mNe2F
d 432 109 F
d 1-6
Here c is the velocity of light m and e are the mass and charge of an electronrespectively and N is Avogadrorsquos number The factor F which reflects solvent ef-fects and depends on the refractive index of the absorbing medium is close tounity max the extinction coefficient at the maximum of an absorption band isa measure of the intensity (magnitude) of the band and an indicator of the al-lowedness of the corresponding electronic transition
12The molecular orbital model
Changes in the electronic structure of a molecule can be visualized with the aidof the molecular orbital (MO) model [3 4] Molecular orbitals are thought to beformed by the linear combination of the valence shell orbitals of the atomslinked together in the molecule The combination of two single orbitals of twoadjacent atoms results in two molecular orbitals one of lower and the other ofhigher energy than before combination The low-energy orbital denoted as thebonding orbital is occupied by a pair of electrons of antiparallel spin The high-energy molecular orbital is called an antibonding orbital It is unoccupied in the
12 The molecular orbital model 7
ground state but may be occupied by an electron upon electronic excitation ofthe molecule
There are different kinds of molecular orbitals bonding and orbitals non-bonding n orbitals and antibonding and orbitals and orbitals arecompletely symmetrical about the internuclear axis whereas and orbitalsare antisymmetric about a plane including the internuclear axis n orbitalswhich are located on heteroatoms such as oxygen nitrogen or phosphorus arenonbonding and are of almost the same energy as in the case of the isolatedatom A pair of electrons occupying an n orbital is regarded as a lone pair onthe atom in question
The simple MO model is based on several assumptions For instance and orbitals are assumed not to interact Moreover molecules are described by lo-calized orbitals each covering two nuclei only Delocalized orbitals involvingmore than two nuclei are thought to exist only in the case of -bonding in con-jugated systems
When a molecule in its ground state absorbs a photon an electron occupyinga or n orbital is promoted to a higher-energy or orbital In principlethe following transitions are possible n and n As
1 Absorption of light and subsequent photophysical processes8
Fig 11 Molecular orbitals (not to scale) and electronictransitions induced by the absorption of a photon
can be seen in Fig 11 the orbital energy increases in the series n
According to the differences in the orbital energies the electron transitionsindicated in Fig 11 correspond to light absorption in different wavelength re-gions This is illustrated in Table 12
It follows that under conveniently practicable conditions (gt 200 nm) photonabsorption initiates transitions of n or electrons rather than those of elec-trons
Commonly molecular orbitals are classified as occupied (doubly) singly occu-pied and unoccupied The acronyms HOMO and LUMO denote the frontier orbi-tals ie the Highest Occupied and the Lowest Unoccupied Molecular Orbitalrespectively SOMO stands for Singly Occupied Molecular Orbital (see Fig 12)
12 The molecular orbital model 9
Table 12 The correspondence of electron transition and optical absorption
Electron transition Absorption region(nm)
Extinction coefficient(L molndash1 cmndash1)
100ndash200 103
n 150ndash250 102ndash103
(Isolated -bonds)(Conjugated -bonds)
180ndash250220ndashIR
102ndash104
n (Isolated groups)(Conjugated segments)
220ndash320250ndashIR
1ndash400
Fig 12 Classification of molecular orbitals with respect to electron occupancy
13The Jablonski diagram
Photon-induced excitations of molecules also include vibrations of nuclei Thisfact can be visualized with the aid of the Jablonski diagram (see Fig 13)
The diagram shows the various energy states of a molecule and further indi-cates the transitions related to the formation and deactivation of excited statesHere photon absorption leads to electron transitions from the ground state S0
to the excited states S1 S2 etc Electron release occurs when the photon energyexceeds the ionization energy EI This is not the case within the wavelengthrange of UV and visible light ie = 200ndash800 nm (h= 62ndash16 eV)
14Absorption in non-conjugated polymers
Figure 14 shows absorption spectra of the typical unconjugated linear polymerspresented in Chart 11
Due to the fact that electronic excitations also involve vibronic and rotationalsublevels (the latter are not shown in Fig 13) the absorption spectra of mole-cules consist of bands rather than single lines It is notable that the maxima ofthe absorption spectra shown in Fig 14 are located in the UV region They re-flect spin-state-conserving electronic transitions ie transitions in the singletmanifold upon photon absorption molecules in the singlet ground state S0 are
1 Absorption of light and subsequent photophysical processes10
Fig 13 Jablonski-type diagram Abbreviations and acronymsAbs absorption Fl fluorescence Phos phosphorescenceIC internal conversion ISC intersystem crossing
converted into molecules in an excited singlet state Sn At long wavelengths(low photon energies) photon absorption generates S1 states At shorter wave-lengths S2 and higher states are excited In the case of polymers containing car-bonyl groups the absorption bands located at long wavelengths correspond ton transitions with low extinction coefficients ie low values of the transi-tion moment At shorter wavelengths transitions with larger transitionmoments are excited In this connection the readerrsquos attention is directed to Ta-ble 12 which indicates the relative orders of magnitude of the extinction coeffi-cients of the different electron transitions
14 Absorption in non-conjugated polymers 11
Chart 11 Chemical structures of poly(vinyl acetate) PVAcpoly(methyl methacrylate) PMMA polystyrene PSt poly-(methyl vinyl ketone) PMVK poly(phenyl vinyl ketone) PPVK
Fig 14 Absorption spectra of non-conjugated polymersAdapted from Schnabel [7] with permissionfrom Carl Hanser
15Absorption in conjugated polymers
In recent years various aromatic polymers with conjugated double bonds so-called conjugated polymers have been synthesized and thoroughly investigatedwith regard to applications in the fields of electroluminescence (organic light-emitting diodes) and photovoltaics (energy conversion of sunlight) Figure 15presents typical absorption spectra of conjugated polymers (see Chart 12)
The maxima of the absorption spectra of conjugated polymers are located inthe visible wavelength region
Certain phenomena observed with conjugated polymers cannot be rational-ized in terms of the model described in Section 11 This concerns above allthe generation of charge carriers with the aid of UV and visible light and theconduction of photogenerated charge carriers A rationale for these phenomenais provided by the exciton model which was originally developed for inorganicsemiconductors and dielectrics [9ndash11] According to this model the absorption
1 Absorption of light and subsequent photophysical processes12
Fig 15 Absorption spectra of conjugated polymers Adaptedfrom Shim et al [8] with permission from Springer
Chart 12 Chemical structures of poly(14-phenylene vinylene) PPV and three PPV derivatives
of a photon by a conjugated polymer promotes an electron from the groundstate to an upper electronically excited state which takes on the quality of a qua-si-particle resembling a hydrogen-like system and can be considered as an elec-tronhole pair The electron and hole are bound together ie they cannot moveindependently of one another in the medium Significantly however excitonsare considered to be able to diffuse and under certain circumstances to dissoci-ate into free charge carriers This aspect is also treated in Section 222
16Deactivation of electronically excited states
161Intramolecular deactivation
In condensed media vibrational relaxation (internal conversion) is usually so fastthat molecules excited to vibronically excited states S1v S2v etc relax to the lowestexcited singlet state S1 before they can undergo other processes Further intramo-lecular deactivation processes of S1 states (see the Jablonski diagram in Fig 13)may be radiative or non-radiative There is one radiative deactivation path result-ing in photon emission termed fluorescence and two non-radiative processes com-peting with fluorescence internal conversion (IC) to the ground state and intersys-tem crossing (ISC) to the triplet manifold The latter process involves a change inelectron spin ie a molecule excited to the singlet state having solely pairs of elec-trons with antiparallel spins is converted into a molecule in an excited triplet statepossessing one pair of electrons with parallel spins Triplet states are commonlyformed via this route The direct formation of triplet states from the ground statethrough photon uptake is strongly spin-forbidden In other words S0T1 transi-tions are very unlikely ie the respective extinction coefficients are very low Inanalogy T1S0 transitions are also spin-forbidden which implies that the life-time of triplet states is quite long and significantly exceeds that of S1 states Tripletstates can deactivate radiatively The emission of photons from triplet states istermed phosphorescence Both luminescence processes fluorescence and phosphor-escence cover a variety of transitions to the various vibronic levels of the S0 state(see Fig 16) and therefore yield emission spectra with several bands instead of asingle line as would be expected for the sole occurrence of 0-0 transitions Fig-ure 17 presents as a typical example the emission spectrum of poly(25-diocty-loxy-p-phenylene vinylene) DOO-PPV (see Chart 12) [12]
Since fluorescence is emitted from the non-vibronically excited S1 state (seeFig 16) and absorption involves higher ie vibronically excited S1 states themaximum of the fluorescence spectrum is shifted to lower energy (higher wave-lengths) relative to the absorption maximum (Stokes shift) The maximum ofthe phosphorescence spectrum is located at even higher wavelengths since phos-phorescence originates from the non-vibronically excited T1 state which is of low-er energy than the corresponding S1 state (see Fig 13) The emission spectrum
16 Deactivation of electronically excited states 13
presented in Fig 17 features three bands at 215 eV (577 nm) 198 eV (626 nm)and 18 eV (689 nm) which may be attributed to the zero-phonon (0-0) the one-phonon (1-0) and the two-phonon (2-0) transitions respectively
162Intermolecular deactivation
Energy transfer from electronically excited molecules to ground-state molecules ofdifferent chemical composition represents a highly important intermolecular de-activation path In general terms energy transfer occurs according to Eq (1-7)from a donor to an acceptor the latter frequently being referred to as a quencher
1 Absorption of light and subsequent photophysical processes14
Fig 16 Schematic depiction of transitions occurring duringabsorption fluorescence and phosphorescence
Fig 17 Emission spectrum (full curve) and part of theabsorption spectrum (dotted curve) of DOO-PPV Adaptedfrom Lane et al [12] with permission from Wiley-VCH
D A D A 1-7
This process is energetically favorable in the case of exothermicity ie if the ex-citation energy of D exceeds that of A E (D) gt E (A) A typical case concernsthe stabilization of polymeric plastics If an electronically excited macromoleculeP transfers its excitation energy to an additive A according to Eq (1-8) hydro-gen abstraction [Eq (1-9)] is inhibited and the macromolecule remains intact
P A P A 1-8
P RH PH R 1-9
There are two major mechanisms by which energy transfer can occur (1) Thedipole-dipole (coulombic) mechanism also denoted as the Foumlrster mechanismoperating through mutual repulsion of the electrons in the two molecules It ischaracterized by relatively large interaction distances ranging up to a molecularseparation of 5 nm (2) The exchange mechanism also denoted as the Dextermechanism according to which a transient complex is formed on close approachof the partner molecules
The dependence of the rate constant kET of intermolecular energy-transferprocesses on the distance R is given by Eqs (1-10) and (1-11) [13]
Long-range interaction kET k0DR0R6 1-10
Short-range interaction kET k0D expR 1-11
Here kD0 is the unimolecular decay rate constant of the excited donor and R0 is
the critical distance between D and A at which the probabilities of sponta-neous deactivation and of energy transfer are equal Typical R0 values are listedin Table 13 which also includes values for self-transfer [14] The latter processis of relevance for down-chain energy transfer (energy migration) which is re-ferred to below
In principle energy-transfer processes from both singlet and triplet exciteddonors to ground-state acceptors are possible [see Eqs (1-12) and (1-13) respec-tively]
16 Deactivation of electronically excited states 15
Table 13 Typical R0 values (in Aring) for aromatic chromophores [14]
Naphthalene Phenanthrene Pyrene Anthracene
Naphthalene 735 1316 2897 2316Phenanthrene 877 1443 2172Pyrene 1003 2130Anthracene 2181
DS1 AS0 DS0 AS1 1-12
DT1 AS0 DS0 AT1 1-13
Commonly singlet energy transfer takes place by the dipole-dipole mechanismwhereas triplet energy transfer occurs by the exchange mechanism since the di-pole-dipole mechanism is spin-forbidden in this case
If electronically excited chemically identical species are generated at a highconcentration for example at high absorbed dose rates or during the simulta-neous excitation of various chromophores attached to the same polymer chainannihilation processes according to Eq (1-14) can become important
M M M M 1-14
M denotes a highly excited species that can emit a photon differing in energyto that emitted by M or can undergo ionization or bond breakage Annihila-tion is a self-reaction of excited species that may be singlets or triplets
163Energy migration and photon harvesting
A polymer-specific mode of energy transfer concerns energy migration in linearhomopolymers ie in macromolecules composed of identical repeating unitsSince all of the repeating units contain identical chromophores excitation en-ergy can travel down the chain provided that the geometrical conditions are ap-propriate (large R0 for self-transfer) and the lifetime of the excited state exc islonger than the energy-hopping time h ie exc gt h There are various path-ways that may ensue following the absorption of a photon by a certain chromo-phoric group Figure 18 shows besides the energy migration process energytransfer to an external acceptor molecule and light emission
Actually monomer emission needs to be distinguished from excimer emissionThe latter process originates from a transient complex formed eg in the caseof aromatic compounds by the interaction of an excited molecule with a non-ex-cited chemically identical molecule leading to an excited dimer denoted as anexcimer (see Scheme 11) In linear macromolecules bearing pendant aromaticgroups this process corresponds to the interaction between neighboring repeat-ing units as demonstrated in Scheme 11
Excimers can usually be detected by a shift of the fluorescence emission maxi-mum to a wavelength longer than in the case of monomer emission
After down-chain energy migration in linear polymers had been evidenced bytriplet-triplet annihilation and enhanced phosphorescence quenching [15ndash17]the idea arose to guide electronic excitation energy along the chain to definedsites where it might serve to initiate chemical or physical processes Obviouslysuch a mechanism is relevant to photon harvesting processes employed by naturein photosynthetic systems operating on the following principle which is also re-
1 Absorption of light and subsequent photophysical processes16
ferred to as the antenna effect [18] a large number of chromophores collectphotons and guide the absorbed energy to one reaction center As regards syn-thetic polymers early studies on photon harvesting were devoted to linear poly-mers composed overwhelmingly of repeating units bearing the same donorchromophore (naphthalene) and to a very small extent the acceptor chromo-phore (anthracene) acting as an energy trap [15 19] Relevant work concerninglinear polymers has been thoroughly reviewed by Webber [13] Very interestingrecent studies concerning multiporphyrin systems of various nonlinear struc-tures have been reviewed by Choi et al [20] and are considered below In thecase of the linear polymers mentioned above practically all photons are ab-sorbed by naphthalene moieties upon exposure to light in the wavelength range290ndash320 nm As illustrated in Scheme 12 excitation energy taken up by anaphthalene chromophore migrates down the chain and eventually reaches ananthracene trap
This process is evidenced by the anthracene fluorescence which is quite dis-tinct from that of naphthalene The quantum yield of anthracene sensitization
16 Deactivation of electronically excited states 17
Fig 18 Pathways of excitation energy in a linear macromolecule
Scheme 11 Excimer formation (a) general description (b) in polystyrene
13S ie the number of sensitized acceptors per directly excited donor can beobtained from Eq (1-15)
13S 1 13IDID0 1-15
Here I(D)0 and I(D) are the donor fluorescence intensities in the absence andin the presence of the acceptor respectively 13S values varying between 01 and07 have been found by examining in aqueous or organic solvents a variety ofpolymers having naphthalene and anthracene groups attached to the main
1 Absorption of light and subsequent photophysical processes18
Scheme 12 Mechanism of photonharvesting Illustration of thetransport of excitation energy byself-transfer through donor moieties(naphthalene) to an acceptor trap(anthracene)
Chart 13 Chemical structures of repeating units bearingnaphthalene and anthracene groups contained in copolymersemployed in photon-harvesting studies [13]
16 Deactivation of electronically excited states 19
Cha
rt1
4C
hem
ical
stru
ctur
eof
ade
ndri
tic21
-por
phyr
inar
ray
cons
istin
gof
20Z
npo
rphy
rin
units
atta
ched
toa
Zn-
free
porp
hyrin
foca
lco
re[2
122
]
chain in different modes (see Chart 13) The largest 13S values were found incases in which excimer formation was unlikely [13]
Obviously excimer formation represents a serious obstacle to energy migra-tion since the excimer site itself functions as a trap and after excitation ismostly deactivated by emission of a photon rather than by energy transfer to aneighboring donor moiety (exc lth) Moreover any effect on coil density exertedby the choice of temperature or solvent can dramatically effect the efficiency ofenergy trapping
1 Absorption of light and subsequent photophysical processes20
Chart 15 Chemical structure of a dendritic multiporphyrinarray consisting of four wedges of a Zn porphyrin heptameranchored to a Zn-free porphyrin focal core [22]
The light-harvesting multiporphyrin arrays synthesized in recent years seemto mimic natural photosynthetic systems much more closely than the linearpolymers of the early studies As outlined in the review by Choi [20] strategiesfor the synthesis of multiporphyrin arrays of various architectures have been de-veloped These comprise besides ring- star- and windmill-shaped structuresalso dendritic arrays With the aim of a high photon-harvesting efficiency com-bined with vectorial energy transfer over a long distance to a designated pointdendritic light-harvesting antennae have proved to be most promising A typicalexample is the system shown in Chart 14 It consists of a total of 21 porphyrinunits ie 20 PZn Zn-complexing porphyrin moieties which are connected viadiarylethyne linkers to one centrally located Pfree unit ie a non-complexing por-phyrin moiety The quantum yield for the energy transfer PZnPfree is13ET = 092 [21]
The structure of another large dendritic system is depicted in Chart 15 Itconsists of four heptameric Zn-porphyrin segments acting as energy donorsThey are anchored to a central Pfree moiety acting as the acceptor [22] Photonabsorption by the PZn moieties at = 589 nm or 637 nm results in very effec-tive PZnPfree energy transfer (13ET = 071 kET = 104109 sndash1) as indicated by astrongly increased light emission from the Pfree moieties
164Deactivation by chemical reactions
Triplet excited molecules formed in condensed media are liable to undergo bi-molecular chemical reactions since their long lifetimes permit a large numberof encounters between the reaction partners The hydrogen abstraction reactionEq (1-16) of triplet excited carbonyl groups is a typical example
C O RH C OH R
1-16
Singlet excited molecules are usually relatively short-lived and therefore are notvery likely to undergo bimolecular reactions In many cases however chemicalbond cleavage competes with physical monomolecular deactivation paths Forexample singlet excited carbonyl groups contained in a polyethylene chain canundergo the Norrish type I reaction resulting in a free radical couple [seeEq (1-17)]
CH2 CH2 CH2 C CH2 CH2 CH2 CH2 CH2 C CH2 CH2
1-17
O O
More details of chemical deactivation processes are provided in Chapter 7
16 Deactivation of electronically excited states 21
17Absorption and emission of polarized light
171Absorption
The absorption of linearly polarized light is characterized by the fact that onlythose chromophores with a component of the absorption transition moment lo-cated in the same direction as the electric (polarization) vector of the incidentlight can be excited No light will be absorbed if the direction of the transitionmoment is perpendicular to the electric vector of the incident light This di-chroic behavior is exhibited by anisotropic organic materials in the solid statesuch as single crystals of certain substances in which the transition moments ofall molecules are fixed in a parallel orientation In the case of linear polymersit is possible to generate some degree of optical anisotropy in highly viscous orrigid samples by aligning the macromolecules in a specific direction Variousmethods have been employed to achieve orientation such as mechanical align-ment Langmuir-Blodgett (LB) film deposition liquid-crystalline self-organiza-tion and alignment on specific substrates As a typical example Fig 19 showsabsorption spectra recorded from an LB film placed on the surface of a fused si-lica substrate and consisting of 100 monolayers of DPOPP (see Chart 16) [23]
Electron microscopy revealed that the LB film had a liquid-crystalline-likestructure This means that many polymer chains were oriented parallel to thesubstrate plane and exhibited a preferential orientation of their backbones alongthe dipping direction Absorption spectra recorded with the incident light polar-ized either parallel or perpendicular to the dipping direction show a maximumat 330 nm (376 eV) in both cases but A|| and A the absorbances parallel and
1 Absorption of light and subsequent photophysical processes22
Fig 19 Absorption spectra of an LBfilm consisting of 100 monolayers ofDPOPP recorded with linearlypolarized incident light (|| and parallel and perpendicular to thedipping direction respectively)Adapted from Cimrova et al [23] withpermission from Wiley-VCH
perpendicular to the dipping direction respectively differ by a factor of aboutfive the in-plane order parameter S= (A|| ndash A)(A|| + A) being 067
It might be noted that in principle it is possible to create anisotropy upon ir-radiating an ensemble of randomly oriented photochromic chromophores withlinearly polarized light since photons are only absorbed by chromophores withtransition moments parallel to the electric vector of the incident light This ap-plies eg to thin films of poly(vinyl cinnamate) (see Chart 17) and its deriva-tives Exposure to linearly polarized light induces the preferential orientation ofliquid-crystal molecules in contact with the film surface [24] The photoalign-ment is likely to be caused by the trans-cis isomerization of the cinnamoylgroups a separate process to cross-linking through [2+2] addition which is amajor photoreaction of this polymer
The creation of anisotropy is treated in some detail in Section 44 which dealswith the trans-cis isomerization of azobenzene compounds
172Absorption by chiral molecules
A chiral molecule is one that is not superimposable on its mirror image It con-tains one or more elements of asymmetry which can be for example carbonatoms bearing four different substituent groups In principle chiral moleculescan exist in either of two mirror-image forms which are not identical and arecalled enantiomers Chiral molecules have the property of rotating the plane of po-larization of traversing linearly polarized monochromatic light a phenomenoncalled optical activity Linearly polarized light can be viewed as the result of thesuperposition of opposite circularly polarized light waves of equal amplitudeand phase The two circularly polarized components traverse a medium contain-ing chiral molecules with different velocities Thereby the wave remains plane-po-
17 Absorption and emission of polarized light 23
Chart 16 Chemical structure of poly(25-di-isopentyloxy-p-phenylene) DPOPP
Chart 17 Chemical structure of poly(vinyl cinnamate)
larized but its plane of polarization is rotated through a certain angle the opticalrotation OR In other words optical activity stems from the fact that nr and nl therefractive indices for the two circularly polarized components of linearly polarizedlight are different a phenomenon referred to as circular birefringence
Optically active compounds are commonly characterized by their specific rota-tion [] measured in solution [see Eq (1-18)]
13 100cd deg cm3 dm1 g1 1-18
where c is the concentration in units of g100 cm3 and d is the path length of thelight in dm [] depends on the wavelength of the light and the temperatureActually [] is proportional to the difference in the refractive indices nr and nl[] nrndashnl Since nr and nl have different dependences on [] also dependson A plot of [] vs yields the optical rotary dispersion (ORD) curve of the sub-stance In many cases ORD curves exhibit at wavelengths of light absorption asine-wave form which is referred to as the Cotton effect (see Fig 110) [25] The in-version point of the S-shaped curve (c) in Fig 110 corresponds to max the wave-length of the absorption maximum at which nr is equal to nl
In addition to their optical activity chiral molecules are characterized by theproperty of absorbing the two components of incident linearly polarized lightie left- and right-circularly polarized light to different extents This phenome-non called circular dichroism CD can be quantified by the difference in molarextinction coefficients l r CD is characterized by the fact that a linearlypolarized light wave passing through an optically active medium is transformedinto an elliptically polarized light wave With the aid of commercially availableinstruments the actual absorbance A of each circularly polarized light compo-
1 Absorption of light and subsequent photophysical processes24
Fig 110 Schematic depiction of opticalrotary dispersion (ORD) curves for positiveand negative rotation (a) and (b) respec-tively for wavelength regions without
absorption The S-shaped curve (c) is typicalof the Cotton effect reflecting lightabsorption Adapted from Perkampus [25]with permission from Wiley-VCH
nent is measured yielding the difference Al Ar The latter is related to the el-lipticity given either in degrees (deg) or radians (rad) according to Eqs (1-19) and (1-20) respectively
2303Al Ar1804 deg 1-19
23034Al Ar rad 1-20
Commonly for the sake of comparison the molar ellipticity [] = 100 cd inunits of deg cm2 dmolndash1 is recorded where c is the concentration in mol Lndash1
and d is the optical path length If in the case of polymers such as proteinsthe molar concentration is related to the molar mass of the residue ie to therepeating (base) unit the mean residue weight ellipticity []MRW is obtained
In recent years circular dichroism spectroscopy has been widely applied ininvestigations concerning the molecular structure of chiral polymers It is apowerful tool for revealing the secondary structures of biological macromole-cules for instance of polypeptides proteins and nucleic acids in solution An
17 Absorption and emission of polarized light 25
Fig 111 Circular dichroism spectra of poly(L-lysine) in its-helical -sheet and random coil conformations Adaptedfrom Greenfield et al [26] with permission from the AmericanChemical Society
important feature is the possibility of monitoring conformational alterations ofoptically active macromolecules by CD measurements Typical data are pre-sented in Fig 111 which shows CD spectra of poly(L-lysine) in three differentconformations [26] Poly(L-lysine) adopts three different conformations depend-ing on the pH and temperature random coil at pH 70 -helix at pH 108 and-sheet at pH 111 (after heating to 52 C and cooling to room temperature oncemore) These conformational transitions are due to changes in the long-rangeorder of the amide chromophores For detailed information on circular dichro-ism of chiral polymers the reader is referred to relevant publications [27ndash30]
173Emission
Provided that the transition moment does not change direction during the lifetimeof an excited state fluorescent light is polarized parallel to the incident light Forlinearly polarized incident light this implies that the direction of the electric vec-tor of both the incident and the emitted light is the same Therefore in the case oforiented polymers fluorescence can only be generated with linearly polarized lightif the components of the absorption transition moments of the chromophores arealigned parallel to the electric vector of the incident light If the alignment of themacromolecules is not perfect the emitted light is not perfectly polarized This iscommonly characterized by the degree of polarization P defined by Eq (1-21)
P I II I
1-21
Here I|| and I are the intensities of the fluorescence polarized parallel and per-pendicular to the electric vector of the incident light Usually set-ups with thegeometry shown in Fig 112 are employed for fluorescence measurements The
1 Absorption of light and subsequent photophysical processes26
Fig 112 Geometry of experimentalset-ups employed in fluorescencedepolarization measurements
sample is excited with light incident along the x-axis and the fluorescence ismonitored along the y-axis M denotes the transition dipole moment
As a typical example Fig 113 shows fluorescence spectra recorded from anLB film of DPOPP (for the absorption spectra see Fig 19) The exciting lightwas polarized parallel to the dipping direction
In accordance with the conclusion derived from the absorption spectra theemission spectra also reveal the partially ordered structure of the film As in thecase of absorption I|| and I the fluorescence intensities parallel and perpendic-ular to the dipping direction respectively differ appreciably in this case by afactor of three to four Much higher dichroic ratios have been found with otheroriented systems eg with highly aligned films consisting of blends of poly-ethylene with 1 wt MEH-PPV (see Chart 18) [31 32] The films fabricated bytensile drawing over a hot pin at 110ndash120 C proved to be highly anisotropic (di-chroic ratio gt 60) with the preferred direction parallel to the draw axis
In principle oriented polymeric systems capable of generating linearly polar-ized light have the potential to be used as backlights for conventional liquid-crystal displays (LCDs) a subject reviewed by Grell and Bradley [33] In thisconnection systems generating circularly polarized (CP) light also became at-tractive CP light can be utilized for backlighting LCDs either directly with theaid of appropriate systems or after transformation into linearly polarized lightwith the aid of a suitable 4 plate [33] CP light has been generated for exam-ple with a highly ordered polythiophene bearing chiral pendant groups
17 Absorption and emission of polarized light 27
Chart 18 Chemical structure of poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene] MEH-PPV
Fig 113 Fluorescence spectra of a DPOPP filmprepared by the LB technique I|| and I fluores-cence intensities parallel and perpendicular tothe dipping direction Exciting lightexc = 320 nm polarized parallel to the dippingdirection Adapted from Cimrova et al [23] withpermission from Wiley-VCH
poly34-di[(S)-2-methylbutoxy]thiophene (see Chart 112) [34] In this casehowever the dissymmetry factor ge was low ge is defined as 2(IL ndashIR)(IL + IR)and |ge| is equal to two for pure single-handed circularly polarized light IL andIR denote the left- and right-handed emissions respectively Circularly polarizedlight is produced quite efficiently when a conventional luminophore is em-bedded within a chiral nematic matrix consisting of a mixture of compounds Aand B (see Chart 19) [35] When this system was exposed to unpolarized lightof = 370 nm the dissymmetry factor ge approached ndash2 in the 400ndash420 nmwavelength range
Another aspect also considered in Subsection 18332 concerns fundamentaltime-resolved fluorescence studies Here the emphasis is placed on fluores-cence depolarization measurements which are very helpful in following rota-tional and segmental motions and for studying the flexibility of macromole-cules If the polymer under investigation does not contain intrinsically fluores-cent probes (eg certain amino acid moieties in proteins) then the macromole-cules have to be labeled with fluorescent markers Information concerning therate of rotation or segmental motion then becomes available provided that theemission rate is on a similar time scale Only when this condition is met canthe rate of depolarization be measured If the emission rate is much fasterthere is no depolarization whereas if it is much slower the depolarization willbe total
Commonly the emission anisotropy r(t) is determined as a function of timer(t) is defined by Eq (1-22)
rt It ItIt 2It 1-22
By irradiating a sample with a short pulse of linearly polarized light and separa-tely recording I|| and I as a function of time t after the pulse the sum S(t) =I|| + 2I and the difference D(t) = I|| ndash I may be obtained The application of anappropriate correlation function to r(t) = D(t)S(t) yields the relaxation time In
1 Absorption of light and subsequent photophysical processes28
Chart 19 Chemical structures of compounds A and Bforming a chiral nematic matrix and of an oligomericluminophore
general the time dependence of r(t) is rather complex ie the decay of r(t) doesnot follow a single exponential decay function Theories have been developed toanalyze the experimentally observed decay functions However it is beyond thescope of this book to deal with the relevant theoretical work which has beenthoroughly reviewed elsewhere as part of the overall subject of fluorescence de-polarization [36 37] In simple cases r(t) decays according to a single exponen-tial decay law Provided that this applies to the rotational motion of macromole-cules the rotational relaxation time r can be evaluated by assuming sphericallyshaped macromolecules For a rotating spherical body r(t) is expressed byEq (1-23)
rt 25exp6Drt 1-23
The rotational diffusion constant Dr is given by Eq (1-24) the Einstein law
Dr 1r kTV 1-24
Here V is the volume of the sphere and is the viscosity of the solventAs can be seen in Table 14 the r values of proteins such as bovine serum al-
bumin and trypsin in aqueous solution lie in the ns range and become largerwith increasing molar mass The proteins were labeled with fluorescent markerssuch as 1-dimethylamino-5-sulfonyl-naphthalene groups (see Chart 110) [38]
Segmental motions and molecular flexibility have been studied for variouspolymers such as polystyrene and the Y-shaped immunoglobulins IgA and IgGRelaxation times in the range of 10ndash100 ns were found In these studies the
17 Absorption and emission of polarized light 29
Table 14 Rotational correlation times r of proteins inaqueous solution at 25 C determined by time-resolvedfluorescence depolarization measurements [37]
Protein Molar mass (g molndash1) r (ns)
Apomyoglobin 17000 83Trypsin 25000 129Chymotrypsin 25000 151-Lactoglobulin 36000 203Apoperoxidase 40000 252Serum albumin 66000 417
Chart 110 Chemical structure of the 1-dimethylamino-5-sulfonyl-naphthalene group
polymers were labeled with small amounts of appropriate fluorescent markerssuch as anthracene in the case of PSt [39]
Again it is a prerequisite for such measurements that the fluorescence decaysat a rate similar to that of the motion under investigation Measurable rotationalrelaxation times are in the range 1 ns to 1 s corresponding to the rotation ofspecies with molar masses up to 106 g molndash1 in aqueous solution
18Applications
181Absorption spectroscopy
1811 UVVis spectroscopyThere are numerous applications reliant upon the ultraviolet and visible (UVVis) wavelength range For example absorption spectroscopy is applied to ana-lyze and identify polymers and copolymers containing chromophores that ab-sorb in this wavelength range such as aromatic or carbonyl groups In this con-text the investigation of photochemical reactions for instance of reactions oc-curring in degradation processes is noteworthy Moreover absorption measure-ments allow the monitoring of alterations in the tertiary structure ofmacromolecular systems for instance in the case of the denaturation of bio-macromolecules especially proteins and nucleic acids Figure 114 demonstratesthe increase in the optical absorption observed upon heating an aqueous solu-
1 Absorption of light and subsequent photophysical processes30
Fig 114 Thermal denaturation of lysozyme in aqueoussolution Differential absorption vs temperature [lysozyme]10 g Lndash1 pH 145 [KCl] 02 m Adapted from Nicolai et al[40] with permission from John Wiley amp Sons Inc
tion of lysozyme a globular protein that acts as an enzyme in the cleavage ofcertain polysaccharides [40] The absorption change reflects the unfolding of thepolypeptide chains due to the destruction of intramolecular interactions such ashydrogen bonds (see Scheme 13)
The thermal denaturation of other superstructures such as those of collagenand deoxyribonucleic acid (DNA) may also be monitored by following the in-crease in the optical absorption Collagen is the most abundant protein in con-nective tissues and constitutes a major part of the matrix of bones In its nativestate it adopts a three-stranded helical structure Dissociation of the threechains at temperatures above 40 C is accompanied by an increase in optical ab-sorption DNA the carrier of genetic information and an essential constituentof the nuclei of biological cells contains the bases adenine guanine cytosineand thymine and hence absorbs UV light The intensity of its absorption spec-trum (max = 260 nm) is reduced by about 30 when single strands combine toform the double-stranded helix Conversely the optical absorption increasesupon denaturation [41] This is illustrated in Fig 115
Generally changes in optical absorption related to molecular alterations notinvolving chemical bond breakage are denoted by the terms hypochromicity (alsohypochromy) and hyperchromicity (also hyperchromy) depending on whether theoptical absorption decreases or increases respectively As regards nucleic acidsin solution hypochromicity applies to a decrease in optical absorbance whensingle-stranded nucleic acids combine to form double-stranded helices The hy-pochromic effect is not restricted to nucleic acids proteins and other polymersbut has also been observed with aggregates of dyes and clusters of aromaticcompounds In interpreting this effect it has been assumed that the electronclouds of chromophores brought into close proximity are strongly interactingThe resulting alteration in the electron density causes changes in the absorptionspectrum The hypochromicity phenomenon and relevant theories are discussedin detail in a recent monograph [42]
18 Applications 31
N H O C N H O C
Scheme 13 Destruction of hydrogen bonds
Fig 115 Thermal denaturation of DNA (E coli)Relative absorbance at 260 nm vs temperature atvarious concentrations of KCl (given in the graphin units of mol Lndash1) Adapted from Marmur et al[41] with permission from Elsevier
1812 Circular dichroism spectroscopyCircular dichroism (CD) spectroscopy is a form of absorption spectroscopy basedon measuring the difference in the absorbances of right- and left-circularly polar-ized light by a substance (see Section 172) Regarding polypeptides proteins andnucleic acids it is a powerful tool for analyzing secondary and tertiary structuresand for monitoring conformational changes In the case of proteins it allows thediscrimination of different structural types such as -helix parallel and antiparal-lel -pleated sheets and -turns and moreover allows estimation of the relativecontents of these structures Details are given in review articles [43ndash45]
Since appropriate instruments have become commercially available CD spec-troscopy has developed into a routine method for the characterization of thechirality of newly synthesized polymers As a typical example the rather highchiro-optical activity of the ladder-type poly(p-phenylene) of the structure shownin Chart 111 was revealed CD spectroscopically molar ellipticity [] = 22106 rad cm2 molndash1 (at max = 461 nm) corresponding to an anisotropy factor ofg == 0003 [46]
The following three examples serve to demonstrate the general importance ofCD spectroscopy (1) Consider first the case of optically active polythiophene de-rivatives They belong to the class of polymers of which the optical activity isbased on the enantioselective induction of main-chain chirality by the presenceof enantiomerically pure side groups In the case of PDMBT (Chart 112) CDspectroscopy permits the detection of a pronounced thermochromic effectWhen dichloromethane solutions that do not exhibit chiro-optical activity relatedto the transition at = 438 nm at 20 C are cooled to ndash30 C the onset ofabsorption is significantly red-shifted Moreover a CD spectrum exhibiting astrong bisignate Cotton effect (see Fig 116) is recorded The chiro-optical activ-ity which is observed for n-decanol solutions even at room temperature (g = = 002) is ascribed to highly ordered packing of the polythiophene chains inchiral aggregates [34]
(2) In the case of thin films of PMBET (see Chart 113) another optically ac-tive polythiophene derivative CD spectroscopy reveals stereomutation of themain chain As can be seen in Fig 117 a CD spectrum that is the mirror im-
1 Absorption of light and subsequent photophysical processes32
Chart 111 Chemical structure of a ladder-typepoly(p-phenylene)
Chart 112 Chemical structure of poly34-di[(S)-2-methylbutoxy]thiophene PDMBT
age of the original spectrum is recorded when PMBET is rapidly cooled fromthe disordered melt to the crystalline state Apparently by rapid cooling of themelt a metastable chiral associated form of the polymer that exhibits the mir-ror-image main-chain chirality is frozen-in [47]
(3) A final example demonstrating the usefulness of CD spectroscopy con-cerns the detection of light-induced switching of the helical sense in polyisocya-nates bearing chiral pendant groups [48] Polyisocyanates (see Chart 114) existas stiff helices comprising equal populations of dynamically interconvertingright- and left-handed helical segments The relative population of these seg-ments is extraordinarily sensitive to chiral perturbations This is demonstratedby the CD spectra shown in Fig 118 They were recorded from polyisocyanatePICS (see Chart 114) that had been irradiated with circularly polarized light(CPL) of opposite handedness Initially the pendant groups consist of a racemicmixture of the two enantiomers and a CD spectrum is not observed Absorption
18 Applications 33
Fig 116 Normalized absorptionspectrum (dashed line)and CD spectrum (solid line) ofPDMBT recorded in dichloro-methane solution at -30 C Dottedline first derivative of theabsorption spectrum Adapted fromLangeveld-Voss et al [34] withpermission the American ChemicalSociety
Chart 113 Chemical structure of poly(3-2-[(S)-2-methylbutoxy]ethylthiophene) PMBET
Scheme 14 Isomerization of the pendant groups of PICS
of light induces isomerization at the C-C double bond (see Scheme 14) Thusirradiation with circularly polarized light which is absorbed by the two enantio-mers to different extents results in an optically active partially resolved mixtureand the CD spectra shown in Fig 118 are observed Remarkably an enantio-meric excess of just a few percent ie close to the racemic state converts thepolymer into one having a disproportionate excess of one helical sense In otherwords chiral amplification takes place since the minor enantiomeric grouptakes on the helical sense of the major enantiomeric group
Interestingly the helical sense of the polymer may be reversibly switched byalternating irradiation with (+)- or (ndash)-CPL or returned to the racemic state byirradiation with unpolarized light
1 Absorption of light and subsequent photophysical processes34
Fig 117 CD spectra of PMBET recorded at room temperaturefrom thin films spin-coated onto glass plates after fast (a)and slow (b) cooling from 200 C to 20 C Adapted from Bou-man et al [47] with permission from Wiley-VCH
Chart 114 Chemical structure of polyisocyanates General structure left PICS right
1813 IR spectroscopyInfrared (IR) spectroscopy has become a very powerful chemical-analytical toolin the analysis and identification of polymers It also plays a prominent role intests related to chemical alterations generated by extrinsic forces and serves forexample in the monitoring of polymer degradation The wavelength regime ofimportance ranges from about 25 to 50 m (4000 to 200 cmndash1) This corre-sponds to the energies required to excite vibrations of atoms in molecules Pre-cisely speaking the full spectrum of infrared radiation covers the wavelengthrange from 075 to 103 m ie besides the aforementioned mid-IR region thereis the near-IR region (075 to 25 m) and the far-IR region (50 to 103 m)
IR light is absorbed when the oscillating dipole moment corresponding to amolecular vibration interacts with the oscillating vector of the IR beam The ab-sorption spectra recorded with the aid of IR spectrometers consist of bands at-tributable to different kinds of vibrations of atom groups in a molecule espe-cially valence and deformation (bending) vibrations as can be seen in Fig 119
Figure 120 presents a typical example of the application of IR spectroscopyHere the UV radiation-induced chemical modification of a polyester containingin-chain cinnamoyl groups (see Chart 115) is illustrated [49]
As can be seen in Fig 120 the FTIR spectrum of the unirradiated polymerfeatures absorption bands at 1630 1725 and 1761 cmndash1 which may be assigned
18 Applications 35
Fig 118 CD spectra of polyisocyanate PICS irradiated withcircularly polarized light (CPL) of opposite handedness atgt 305 nm The spectra were recorded in dichloromethanetetrahydrofuran (1 1) solution Adapted from Li et al [48]with permission from the American Chemical Society
Fig 119 Notation of group vibrations
to the stretching vibrations of vinylene double bonds and conjugated and non-conjugated carbonyl bonds respectively Upon irradiation the intensities of thevinylene and the conjugated carbonyl bands decrease whereas the band due tothe non-conjugated carbonyl groups intensifies with increasing absorbed doseThis behavior may be explained in terms of simultaneously occurring trans-cisisomerizations and [2+ 2] cycloadditions (dimerizations) The band at 1630 cmndash1
decreases since the extinction coefficient of cis C=C bonds is lower than that oftrans C=C bonds The growth in the intensity of the band at 1761 cmndash1 indicatesthe occurrence of dimerizations
Modern commercial IR spectrometers operating with the aid of a Michelsoninterferometer produce interferograms which upon mathematical decoding bymeans of the Fourier transformation deliver absorption spectra commonly re-ferred to as Fourier-transform infrared (FTIR) spectra [50] Comprehensive col-lections of IR spectra of polymers monomers and additives are available [51]Moreover the readerrsquos attention is directed to several books [52ndash58]
1 Absorption of light and subsequent photophysical processes36
Chart 115 Chemical structure of the polyester referred to in Fig 120
Fig 120 FTIR spectra of a Cn-polyester recordedbefore and after irradiation with UV light (260ndash380 nm) to different absorbed doses Adaptedfrom Chae et al [49] with permission fromElsevier
182Luminescence
Many problems in the physics and chemistry of polymers have been investi-gated by means of fluorescence techniques Within the scope of this book it ismerely possible to point out the high versatility of these techniques rather thanto discuss the innumerable publications Among the features of luminescencethat account for the variety of its applications is the fact that emission spectracan be recorded at extremely low chromophore concentrations Thus a polymermay be labeled with such a small amount of luminophore that the labeling doesnot perturb the properties of the system As regards linear polymers in solutionit is possible to derive information on the conformational state and the behaviorof the macromolecules This concerns such topics as the interpenetration ofpolymer chains the microheterogeneity of polymer solutions conformationaltransitions of polymer chains and the structures of polymer associates Relevantwork has been reviewed by Morawetz [59] Here only one typical example is de-scribed which concerns the kinetics of HCl transfer from aromatic amino moi-eties to much more basic aliphatic amino groups attached to discrete macromol-ecules in this case poly(methyl methacrylate)s (see Scheme 15)
18 Applications 37
Scheme 15 HCl transfer from aromatic to aliphatic amino groups
The release of HCl from the aminostyrene groups increases the fluorescenceintensity since protonation prevents light emission Thus the rate of HCl trans-fer between the different macromolecules can be measured in a stopped-flowexperiment It was found that the rate constant of the reaction decreased withincreasing chain length of the interacting polymers [60] This result may be in-terpreted in terms of the excluded volume effect flexible polymer chains ingood solvent media strongly resist mutual interpenetration a phenomenon thatbecomes more pronounced with increasing chain length
Another quite different kind of luminescence application pertains to the gen-eration of polarized light with the aid of aligned systems Here the concept ofpolarizing excitonic energy transfer EET comes to prominence Thus in appro-priate systems randomly oriented sensitizer molecules harvest the incomingunpolarized light by isotropic absorption and subsequently transfer the energyto a uniaxially oriented polymer The latter emits light with a high degree of lin-ear polarization According to this concept all incident light can be funnelledinto the same polarization The incorporation of the polarizing EET process intocolored liquid-crystal displays (LCDs) would imply that dichroic polarizers areno longer required for the generation of polarized backlights in conventionalLCDs A system functioning in this way consists of a ternary blend of high mo-lar mass (4106 g molndash1) polyethylene 2 wt of a derivative of PPE and 2 wtof the sensitizer DMC (see Chart 116) [61] Blend films prepared by solution-casting from xylene are uniaxially drawn at 120 C to a draw ratio of about 80
183Time-resolved spectroscopy
1831 General aspectsWith the advent of powerful lasers capable of generating short light pulses a newera of research commenced [62ndash64] Notably the new light sources permit themeasurement of lifetimes of excited states and the detection of short-lived inter-mediates such as free radicals and ions The concomitant development of sophis-ticated detection methods has also brought about continuous progress during the
1 Absorption of light and subsequent photophysical processes38
Chart 116 Chemical structures of a poly(25-dialkoxy-p-phenylene ethynylene) PPE and 7-diethylamino-4-methyl-coumarin DMC
last decades in the fields of polymer physics and chemistry [9 65ndash68] While re-searchers were initially fascinated by studying processes on the microsecond(1 s= 10ndash6 s) and nanosecond (1 ns= 10ndash9 s) time scale more recent researchhas concentrated on the picosecond (1 ps= 10ndash12 s) and femtosecond (1 fs= 10ndash
15 s) time region In this way a wealth of information has become available thatallows the identification of extremely short-lived intermediates and elucidatesthe mechanisms of many photophysical and photochemical processes The aimhere is not to review work on the technical development of pulsed lasers andon the invention of highly sensitive detection methods In a more general way in-formation is given on the wide-ranging potential of time-resolved measurementsand their benefits in the fields of polymer photophysics and photochemistry
Time-resolved measurements were initiated both by physicists who wereprincipally interested in photophysical processes that left the chemical struc-tures of the molecules intact and by chemists who were mainly interested inthe chemical alterations of the irradiated molecules but also in the associatedphotophysical steps The parallel development of these two lines of research isreflected in the terminology For example the term flash photolysis as used bychemists applies to time-resolved measurements of physical property changescaused by chemical processes induced by the absorption of a light flash (pulse)Flash photolysis serves to identify short-lived intermediates generated by bondbreakage such as free radicals and radical ions Moreover it allows the determi-nation of rate constants of reactions of intermediates Therefore this method isappropriate for elucidating reaction mechanisms
1832 Experimental techniquesFor pico- and femtosecond studies time-resolved measurements require power-ful pulsed laser systems operated in conjunction with effective detection tech-niques Relevant commercially available laser systems are based on Ti sapphireoscillators tunable between 720 and 930 nm (optimum laser power around800 nm) For nanosecond work Nd3+ YAG (neodymium-doped yttrium-alumi-num-garnet) (1064 nm) and ruby (6943 nm) laser systems are commonly em-ployed For many applications light pulses of lower wavelength are producedwith the aid of appropriate nonlinear crystals through second third or fourthharmonic generation For example short pulses of = 532 355 and 266 nm aregenerated in this way by means of Nd3+ YAG systems Moreover systems based
18 Applications 39
Fig 121 Schematic depiction of a set-upfor time-resolved optical absorptionmeasurements
on mode-locked dye lasers have occasionally been employed for ultrafast mea-surements in the fs and ps time domain [12]
Principally the pump and probe technique depicted in Fig 121 is applied intime-resolved transient absorption experiments A pump beam directed ontothe sample generates excited species or reactive intermediates such as free radi-cals The formation and decay of these species can be monitored with the aid ofan analyzing (probe) light beam that passes through the sample perpendicularto the direction of the pump beam In principle a set-up of this kind is alsosuitable for recording luminescence if it is operated without the probe beam
1 Absorption of light and subsequent photophysical processes40
Fig 122 Schematic depiction of a set-up for time-resolvedoptical absorption measurements in the femtosecond timedomain SHG second harmonic generation crystal PDphotodiode OMA optical multichannel analyzer Adaptedfrom Lanzani et al [68] with permission from Wiley-VCH
A typical set-up employed for time-resolved measurements in the femtose-cond time domain is presented in Fig 122 [68] Here a Ti sapphire system op-erated in conjunction with a LiB3O5 crystal functioning as a frequency doublerprovides the pump pulse (= 390 nm repetition rate 1 kHz) The pulse intensity(excitation density) can be varied between 03 and 12 mJ cmndash2 For the genera-tion of the analyzing white light a fraction of the pump pulse is split off andfocused through a thin sapphire plate The resulting supercontinuum which ex-tends from 450 to 1100 nm is passed through the sample prior to hitting thedetector Through mechanical operation of the delay line transient absorptionspectra are recorded at various times after the pump pulse by averaging over100 to 1000 laser pulses
Modern detection systems are based on the charge-coupled device (CCD) tech-nique which is not indicated in the schematic of Fig 122
Prior to the advent of powerful lasers high-speed flash techniques were em-ployed as light sources in time-resolved studies Research was focused mainlyon luminescence studies aimed at determining fluorescence and phosphores-cence lifetimes In this connection the development and successful applicationof sophisticated methods such as the single-photon time-correlation methodand high-speed photography methods (streak camera) are worthy of note De-tailed technical information on these topics is available in a book by Rabek [69]The physical principles of lifetime determinations have been described by Birks[70]
1833 Applications of time-resolved techniques
18331 Optical absorptionOptical absorption measurements are much more difficult to perform thanemission measurements This applies for instance to the detection of specieshaving a low extinction coefficient at the relevant wavelengths The surroundingmolecules should be transparent which is important in the case of solutionsMoreover it has to be taken into account that invariably one has to measure anabsorbance difference and not an absolute quantity as in the case of lumines-cence In principle molecules that have been promoted to an excited state ofsufficiently long lifetime can absorb photons Provided that the absorption coef-ficients are large enough the absorption spectrum can permit identification ofthe excited state and from its decay the lifetime of the excited state is obtainedIn the relevant literature this kind of absorption is frequently denoted by theacronyms PIA or PA referring to photoinduced absorption In many cases ex-cited triplet states are relatively long-lived and can easily be detected by light ab-sorption measurements As a typical result Fig 123 shows the T-T absorptionspectrum ie the spectrum of excited triplet states of the polymer PPVK (seeChart 117) generated by irradiation in benzene solution at room temperaturewith a 15 ns pulse of 347 nm light The triplet lifetime amounts to several mi-croseconds in this case [71]
18 Applications 41
Commonly excited singlet states have very short lifetimes and can only be de-tected by means of femtosecond absorption spectroscopy A typical case is illus-trated in Fig 124 which shows the differential transmission spectrum ofMEH-DSB (see Chart 118)
The differential transmission is defined as TT = (TndashT0)T0 where T and T0
are the transmissions in the presence and the absence of the pump beam re-spectively It may be recalled that T = (II0) = endashd where I0 and I denote the lightintensities before and after the sample and d are the absorption coefficientand the sample thickness respectively The absorbance A is equal to d In thesmall signal limit commonly 10ndash5 to 10ndash3 ie (TT) 1 TT is proportionalto the change in the absorption coefficient (TT)ndashd Negative valuesof TT correspond to photoinduced absorption (PIA) Thus in Fig 124 theband between 600 and 1100 nm with a peak at about 900 nm reflects the ab-sorption of singlet intrachain excitons [72] Positive values of TT correspondto bleaching or stimulated emission SE Thus in Fig 124 the band between450 and 500 nm is assigned to bleaching due to depopulation of ground-stateelectrons and the band at around 535 nm coinciding with the photolumines-cence (PL) spectrum is ascribed to SE [72] The spectral features shown by thesolid line in Fig 124 are similar to those reported for many poly(arylene viny-lene)s The phenomenon of stimulated emission is dealt with in more detail inSection 622 Also typical of poly(arylene vinylene)s Fig 125 presents differen-tial transmission kinetic traces recorded at 800 nm at varying pulse intensitiesfor a thin film of poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylene vinylene]MEH-PPV The absorption decays on the ps time scale and the decay dynamicsdepends on the excitation density The higher the pulse intensity the faster is
1 Absorption of light and subsequent photophysical processes42
Fig 123 Triplet-triplet absorption spectrumof poly(phenyl vinyl ketone) in benzenesolution at room temperature Recorded atthe end of a 15 ns pulse of 347 nm light
Chart 117 Chemical structure of poly(phenyl vinyl ketone)
18 Applications 43
Chart 118 Chemical structure of a phenylene vinylene oligomer
Fig 124 Femtosecond spectroscopy atexc = 400 nm pulse length 150 fs pulseenergy 1 mJ pulse repetition rate 1 kHzDifferential transmission spectrum of a thinfilm of MEH-DSB (solid line) recorded at theend of the pulse Also shown ground-state
absorption coefficient (dashed line) andphotoluminescence spectrum PL (dottedline) Adapted from Maniloff et al [72] withpermission from the American PhysicalSociety
Fig 125 Femtosecond spectroscopyDifferential transmission traces recorded atrec = 850 nm from thin films of poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene] MEH-PPV irradiated as indicatedin the legend of Fig 124 at varying photon
fluences from upper to lower curves101013 311014 and 931014 cmndash2respectively Adapted from Maniloff et al[72] with permission from the AmericanPhysical Society
the decay Since the decay dynamics of the PIA band at around 800 nm and ofthe SE band at 535 nm are correlated it is concluded that both bands arise fromthe same species namely intrachain excitons The intensity-dependent decay dy-namics may be interpreted in terms of exciton-exciton annihilation a processinvolving interaction of nearby excitons and resulting in non-radiative relaxationto the ground state [72]
18332 LuminescenceDuring the past decades time-resolved fluorescence measurements have helpedto address many problems in the polymer field A typical example concerns thedetermination of the rate of rotational and segmental motions of macromole-cules in solutions as dealt with in Section 173 Moreover time-resolved fluores-cence measurements permit the investigation of energy migration and excimerformation in linear polymers Down-chain energy migration in a linear polymerbearing overwhelmingly naphthalene plus a few anthracene pendant groupswas evidenced by a decrease in the naphthalene fluorescence and a concomitantincrease in anthracene fluorescence [17] Similarly the decay of the monomeremission was found to be correlated with the build-up of the excimer fluores-cence in the case of polystyrene in dilute solution in dichloromethane [73] Thisis illustrated in Fig 126
The remainder of this section focuses on the phenomenon of spectral or gainnarrowing which has been discovered in more recent fluorescence studies Ascan be seen in Fig 127 the shape of the spectrum of light emitted fromBuEH-PPV (see Chart 119) changes drastically when the intensity of the excit-ing light pulse is increased beyond a threshold value The broad emission spec-trum extending over a wavelength range of about 200 nm recorded at low inci-dent light intensity is transformed into a narrow band with 10 nm at highlight intensity [74]
The phenomenon of spectral narrowing is attributed to a cooperative effect inlight emission the so-called amplified spontaneous emission effect which involvesthe coherent coupling of a large number of emitting sites in a polymer matrix
1 Absorption of light and subsequent photophysical processes44
Fig 126 Fluorescence spectra ofpolystyrene in oxygen-free CH2Cl2solution (1 g Lndash1) I Monomeremission recorded at the end of a10 ns flash (exc = 257 nm) IIExcimer emission recorded 45 nsafter the flash Adapted fromBeavan et al [73] with permissionfrom John Wiley amp Sons Inc
Spectral narrowing has been observed for thin polymer films (200ndash300 nmthick) on planar glass substrates The films act as wave guides since the refrac-tive index of the polymer is larger than that of the surrounding air or the glasssubstrate Immediately after absorption of a light pulse some photons are spon-taneously emitted from certain excited sites These photons are coupled into theguided-wave mode and stimulate radiative deactivation processes of other ex-cited sites upon propagation through the film a process denoted as amplifiedspontaneous emission The phenomenon of spectral narrowing is explained bythe fact that the emission of photons with the highest net gain coefficient is fa-vored [75]
References 45
Fig 127 Spectral narrowing in the caseof BuEH-PPV Emission spectrarecorded at different excitation pulseenergies Pulse duration 10 nsexc = 532 nm Film thickness 210 nm[74] Adapted from Lemmer et al [75]with permission from Wiley-VCH
Chart 119 Chemical structure of poly[2-butyl-5-(2-ethylhexyl)-14-phenylene vinylene] BuEH-PPV
References
1 J D Coyle Introduction to Organic Photo-chemistry Wiley Chichester (1986)
2 HH Jaffe M Orchin Theory and Appli-cations of Ultraviolet Spectroscopy WileyNew York (1962)
3 G M Barrow Introduction to MolecularSpectroscopy McGraw-Hill KogakushaTokyo (1962)
4 HG O Becker (ed) Einfuumlhrung in diePhotochemie Thieme Stuttgart (1983)
5 J Kopecky Organic Photochemistry A Vi-sual Approach VCH Weinheim (1992)
6 M Pope C E Swenberg Electronic Pro-cesses in Organic Crystals and Polymers2nd Edition Oxford University PressNew York (1999)
7 W Schnabel Polymer Degradation Princi-ples and Practical Applications HanserMuumlnchen (1981)
8 H-K Shim J-I Jin Light-Emitting Char-acteristics of Conjugated Polymers in K-SLee (ed) Polymers for Photonics Applica-tions I Springer Berlin Adv Polym Sci158 (2002) 193
1 Absorption of light and subsequent photophysical processes46
9 NS Sariciftci (ed) Primary Photoexcita-tions in Conjugated Polymers MolecularExciton versus Semiconductor Band ModelWorld Scientific Singapore (1997)
10 J Cornil D A dos Santos D BeljonneZ Shuai J-L Bredas Gas Phase to SolidState Evolution of the Electronic and Opti-cal Properties of Conjugated Chains ATheoretical Investigation in G Hadziioan-nou PF van Hutten (eds) Semicon-ducting Polymers Wiley-VCH Weinheim(2000) p 235
11 K Pichler D Halliday DC BradleyPL Burn R H Friend A B Holmes JPhys Cond Matter 5 (1993) 7155
12 PA Lane SV Frolov Z V VardenySpectroscopy of Photoexcitations in Conju-gated Polymers in G Hadziioannou PFvan Hutten (eds) Semiconducting Poly-mers Wiley-VCH Weinheim (2000)p 189
13 SE Webber Chem Rev 90 (1990) 146914 IB Berlman Energy Transfer Parameters
of Aromatic Compounds Academic PressNew York (1973)
15 R F Cozzens R B Fox J Chem Phys50 (1969) 1532
16 C David M Lempereur G GeuskensEur Polym J 8 (1972) 417
17 J W Longworths MD Battista Photo-chem Photobiol 11 (1970) 207
18 J E Guillet Polymer Photophysics andPhotochemistry Cambridge UniversityPress Cambridge UK (1985)
19 J S Aspler CE Hoyle J E GuilletMacromolecules 11 (1978) 925
20 MS Choi T Yamazaki I Yamazaki TAida Angew Chem Int Ed 43 (2004)150
21 MR Benites ET Johnson S WeghornL Yu PD Rao J R Diers S I Yang CKirmaier D J Bocian D Holten J SLindsey J Mater Chem 12 (2002) 65
22 MS Choi T Aida T Yamazaki I Ya-mazaki T Aida Angew Chem Int Ed40 (2001) 3194
23 V Cimrova M Remmers D Neher GWegner Adv Mater 8 (1996) 146
24 K Ichimura Y Akita H Akiyama KKudo Y Hayashi Macromolecules 30(1997) 903
25 H-H Perkampus Encyclopedia of Spec-troscopy VCH Weinheim (1995)
26 NJ Greenfield G D Fasman ComputedCircular Dichroism Spectra for the Evalua-tion of Protein Conformation Biochemis-try 8 (1969) 4108
27 A Rodger B Norden Circular Dichroismand Linear Dichroism Oxford UniversityPress Oxford (1997)
28 G D Fasman (ed) Circular Dichroismand the Conformational Analysis of Biomo-lecules Plenum Press New York (1996)
29 K Nakanishi N Berova R W Woody(eds) Circular Dichroism Principles andApplications VCH Publishers Weinheim(1994)
30 R W Woody Circular Dichroism of Pep-tides in E Gross J Meienhofer (eds)The Peptides Analysis Synthesis BiologyAcademic Press New York (1985) Vol 7p 14
31 TW Hagler K Pakbaz J Moulton FWudl P Smith A J Heeger PolymCommun 32 (1991) 339
32 TW Hagler K Pakbaz K F Voss A JHeeger Polym Commun Phys Rev B44 (1991) 8652
33 M Grell DD C Bradley Adv Mater 11(1999) 895
34 BMW Langeveld-Voss RA J JanssenMPT Christiaans SC J MeskersHP JM Dekkers E W Meijer J AmChem Soc 118 (1996) 4908
35 SH Chen D Katsis A W Schmid J CMastrangelo T Tsutsui N T BlantonNature 397 (1999) 506
36 EA Anufrieva Yu Ya Gotlib Investiga-tion of Polymers in Solution by PolarizedLuminescence Adv Polym Sci 40Springer Berlin (1981) p 1
37 K P Ghiggino A Roberts D PhillipsTime-Resolved Fluorescence Techniques inPolymer and Biopolymer Studies AdvPolym Sci 40 Springer Berlin (1981)p 69
38 P Wahl CR Acad Sci 263 (1966)1525
39 See literature cited in [37]40 DF Nicolai GB Benedek Biopolymers
15 (1976) 242141 J Marmur P Doty J Mol Biol 5 (1962)
10942 NL Veksin Photonics of Biopolymers
Springer Berlin (2002)
References 47
43 R W Woody Circular Dichroism Meth-ods Enzymol 246 (1995) 34
44 W C Johnson Jr Methods Enzymol 210(1992) 426
45 W C Johnson Jr Proteins 7 (1990) 20546 R Fiesel J Huber U Scherf Angew
Chem 108 (1996) 223347 MM Bouman E W Meijer Adv Mater
7 (1995) 38548 J Li G B Schuster K-S Cheon MM
Green J V Selinger J Am Chem Soc122 (2000) 2603
49 B Chae SW Lee M Ree S B Kim Vi-brational Spectrosc 29 (2002) 69
50 W Kloumlpffer Introduction to Polymer Spec-troscopy Springer Berlin (1984)
51 DO Hummel Atlas of Polymer and Plas-tics Analysis 3rd Edition Wiley-VCHWeinheim (2005)
52 A Elliott Infrared Spectra and Structureof Organic Long-Chain Polymers ArnoldLondon (1969)
53 M Claybourn Infrared Reflectance Spec-troscopy of Polymers Analysis of Films Sur-faces and Interfaces Adhesion SocietyBlacksburg VA (1998)
54 R A Meyers (ed) Encyclopedia of Analyt-ical Chemistry Application Theory and In-strumentation Wiley Chichester (2000)
55 J M Chalmers P R Griffiths (eds)Handbook of Vibrational Spectroscopy Wi-ley Chichester (2002)
56 HW Siesler Y Ozaki S Kawata HMHeise Near-Infrared Spectroscopy Wiley-VCH Weinheim (2002)
57 J Workman Jr Handbook of OrganicCompounds NIR IR Raman and UV-VisSpectra Featuring Polymers and Surfac-tants Academic Press San Diego (2000)
58 HM Mantsch D Chapman InfraredSpectroscopy of Biomolecules Wiley NewYork (1996)
59 H Morawetz J Polym Sci Part APolym Chem 37 (1999) 1725
60 Y Wang H Morawetz Macromolecules23 (1990) 1753
61 A Montali C Bastiaansen P Smith CWeder Nature 392 (1998) 261
62 R R Alfano Semiconductors Probed byUltrafast Laser Spectroscopy AcademicPress New York (1984)
63 J L Martin A Mignus G A MourouA H Zewail (eds) Ultrafast Phenomena
Springer Series in Chemical PhysicsVol 55 Springer Berlin (1992)
64 G Porter Flash Photolysis into the Femto-second ndash A Race against Time in J ManzL Woumlste (eds) Femtosecond ChemistryWiley-VCH Weinheim (1995)
65 FC DeSchryver S De Feyter GSchweitzer (eds) Femtochemistry Wiley-VCH Weinheim (2001)
66 DW McBranch MB Sinclair UltrafastPhoto-Induced Absorption in Nondegener-ate Ground State Conjugated PolymersSignatures of Excited States in [9] p 587
67 J-Y Bigot T Barisien Excited-State Dy-namics of Conjugated Polymers and Oligo-mers in FC DeSchryver S De FeyterG Schweitzer (eds) Femtochemistry Wi-ley-VCH Weinheim (2001)
68 G Lanzani S De Silvestre G CerulloS Stagira M Nisoli W Graupner GLeising U Scherf K Muumlllen Photophys-ics of Methyl-Substituted Poly(para-Phenyl-ene)-Type Ladder Polymers in G Hadziio-annou PF van Hutten (eds) Semicon-ducting Polymers Wiley-VCH Weinheim(2000) p 235
69 J F Rabek Experimental Methods inPhotochemistry and Photophysics WileyChichester (1982)
70 J B Birks Photophysics of Aromatic Mole-cules Wiley-Interscience London (1970)p 94
71 W Schnabel J Kiwi Photodegradationin HHG Jellinek (ed) Aspects of Deg-radation and Stabilization of PolymersElsevier Scientific Publ Amsterdam(1978) p 195
72 ES Maniloff V I Klimov DWMcBranch Phys Rev B 56 (1997) 1876
73 SW Beavan JS Hargreaves D Phil-lips Photoluminescence in PolymerScience Adv Photochem 11 (1978) 207
74 F Hide MA Diaz-Garcia B J SchartzMR Anderson P Qining A J HeegerScience 273 (1996) 1833
75 U Lemmer A Haugeneder C Kallin-ger J Feldmann Lasing in ConjugatedPolymers in G Hadziioannou PF vanHutten (eds) Semiconducting PolymersWiley-VCH Weinheim (2000) p 309
21Introductory remarks
A photoconductive solid material is characterized by the fact that an electric cur-rent flows through it under the influence of an external electric field when it ab-sorbs UV or visible light There are two essential requirements for photoconduc-tivity (1) the absorbed photons must induce the formation of charge carriersand (2) the charge carriers must be mobile ie they must be able to move inde-pendently under the influence of an external electric field Photoconductivitywas first detected in inorganic materials for example in crystals of alkali metalhalides containing color centers (trapped electrons in anion vacancies) or in ma-terials possessing atomic disorder such as amorphous silicon or selenium Asregards organic materials dye crystals and more recently also various polymer-ic systems have been found to exhibit photoconductivity Two groups of photo-conducting polymeric systems may be distinguished (a) solid solutions of activecompounds of low molar mass in inert polymeric matrices also denoted as mo-lecularly doped polymers and (b) polymers possessing active centers in themain chain or in pendant groups Examples belonging to group (a) are polycar-bonate and polystyrene molecularly doped with derivatives of triphenylaminehydrazone pyrazoline or certain dyes (see Table 21) Molecularly doped poly-mers are widely used as transport layers in the photoreceptor assemblies ofphotocopying machines
Typical examples of photoconductive polymers (group (b)) are listed in Ta-ble 22 Concerning the field of conducting polymers including photoconduct-ing polymers the reader is referred to various books and reviews [1ndash21]
49
2Photoconductivity
22Photogeneration of charge carriers
221General aspects
Regarding inorganic semiconductors the photogeneration of charge carriers hasbeen explained in terms of the so-called band model according to which thenuclei of atoms are situated at fixed sites in a lattice [22] Since the charges ofthe nuclei are largely compensated by their inner-shell electrons an averageconstant potential is attributed to the outer-shell electrons denoted as valenceelectrons The energy levels of the valence electrons differ only slightly and aretherefore considered as being located in the so-called valence band (seeFig 21)
At T = 0 the absolute zero temperature all valence electrons reside in the va-lence band at higher temperatures some electrons are promoted to the so-called conduction band The probability of an electron being in a quantum stateof energy E is given by Eq (2-1)
2 Photoconductivity50
Table 21 Typical dyes applied as dopants in photoconducting polymeric systems
Chemical structure Denotation
Perylene dye
Azo dye
Quinone dye
Squaraine dye
M CdZnTiO etc Phthalocyanine dye
f E EF exp13E EF1 exp13E EF
2-1
Here f(E ndash EF) is the Fermi distribution function is equal to (kT)ndash1 where kis the Boltzmann constant T is the absolute temperature and EF is the Fermienergy
The Fermi level of inorganic semiconductors lies between the valence bandand the conduction band in contrast to metals for which the Fermi level lieswithin the valence band According to this model the phenomenon of dark con-ductivity is feasible Photoconductivity implies that upon irradiation electrons
22 Photogeneration of charge carriers 51
Table 22 Chemical structures of typical photoconducting polymers
Chemical structure Acronym Denotation
PVC Poly(N-vinyl carbazole)
PAC trans-Polyacetylene
PT Polythiophene
PFO Poly(dialkyl fluorene)
PPV Poly(p-phenylene vinylene)
PPP Poly(p-phenylene)
m-LPPP Methyl-substituted ladder-typepoly(p-phenylene)
R1 and R2 alkyl or aryl groups Polysilylene
PANI Polyaniline
are promoted from the valence band to the conduction band Thus the totalelectrical conductivity is composed of two terms representing the dark con-ductivity d and the photoconductivity p
d p 2-2
Band-to-band transitions of electrons require photon energies exceeding the en-ergy of the band gap Since the energy states of the conduction band are not lo-calized ie not attributable to specific atomic nuclei electrons transferred to theconduction band lose their local binding and become mobile Regarding poly-meric systems this aspect is at variance with recent experimental and theoreti-cal work which overwhelmingly led to the conclusion that in such systems lo-calized states are involved both in the photogeneration of charges and in thecarrier transport and that the theoretical model developed for inorganic semi-conductors is not applicable for polymeric systems At present the generationof charge carriers is explained in terms of the exciton concept and a generally ac-cepted carrier transport mechanism presumes charge hopping among discretesites as will be described in the following subsections
222The exciton model
The exciton model is based on the fact that in organic photoconductors thelight-induced transition of an electron to an excited state causes a pronouncedpolarization of the chromophoric group Because of the relatively high stabilityof this state it is considered to be an entity of special nature This entity calledan exciton is an excited state of quasi-particle character located above the va-lence band It resembles a hydrogen-like system with a certain binding energy
2 Photoconductivity52
Fig 21 Energy levels of a semiconductor Also shown energylevel of an exciton state as generated upon photonabsorption
which can besides other non-radiative or radiative deactivation routes also giverise to the formation of a geminate electronhole pair Under certain conditionsthe latter can dissociate and thus give rise to the generation of free ie indepen-dent charge carriers
exciton 13he h e 2-3
It is generally accepted that the dissociation of electronhole pairs is induced orat least strongly assisted by an external electric field Whether electronholepair dissociation generally also occurs intrinsically ie in the absence of an ex-ternal electric field has not yet been fully established In certain cases such asin m-LPPP [23] or in PPV [24] this process has been evidenced However inthese and similar cases electronhole pair dissociation is likely to be due to thepresence of impurities such as molecular oxygen andor structural defects inthe macromolecular system such as conformational kinks or chain twists thatfunction as dissociation sites The existence of these sites and the capability ofexcitons to approach them are presumably prerequisites for dissociation In thisconnection it is notable that excitons are conjectured to diffuse over certain dis-tances It has been suggested that charge generation ie the formation of freecharge carriers occurs preferentially at specially structured sites on the surfaceof the sample
In view of the highly variable nature of photoconducting materials differenttypes of exciton states have been postulated For instance an exciton state witha radius of the order of 100 Aring a so-called Wannier exciton is assumed to beformed in amorphous silicon in which the wave function spreads over the elec-tronic orbitals of many Si atoms In contrast in conjugated polymers such aspoly(phenyl vinylene) or polysilanes (see Table 22) the formation of less ex-tended so-called Frenkel excitons with radii of the order of 10 Aring is assumed Inthis case the polymer system is considered to be an ensemble of short molecu-lar segments that are characterized by localized wave functions and discrete en-ergy levels and an exciton generated by the absorption of a photon exists withinthe intra-chain delocalization length For systems permitting the formation ofcharge-transfer (CT) states the existence of charge-transfer or quasi-Wannier ex-citons having radii exceeding those of Frenkel excitons is postulated This ap-plies for example to poly(methyl phenyl silylene) [25] In this case the absorp-tion of photons in main-chain segments generates Frenkel excitons which areconverted to CT excitons through intramolecular interaction with pendant phe-nyl groups (see Scheme 21)
Moreover CT excitons are thought to be formed by intermolecular interactionin certain polymeric systems containing small molecules A typical example ispoly(N-vinyl carbazole) doped with trinitrofluorenone (TNF) a system whichplayed a major role in early photoconductive studies on polymeric systems (seeChart 21)
As regards the nature of the so-called dissociation sites referred to above it maybe noted that generally any kind of disorder-induced kink may play an activating
22 Photogeneration of charge carriers 53
role in the dissociation of electronhole pairs In the case of trans-polyacetylenewhich has been examined quite extensively so-called neutral solitons (seeChart 22) resulting from incomplete cis-trans isomerization are postulated to func-tion as dissociation sites Neutral solitons are characterized by a free spin and aretherefore detectable by electron-spin resonance (ESR) measurements [26]
223Chemical nature of charge carriers
In the earlier literature charge carriers generated in polymers are frequently de-noted as polarons and bipolarons and it is assumed that these charged speciesare formed instantaneously upon optical excitation [27] The fundamental andoften quite controversial debate on the nature of the primary photoexcitations
2 Photoconductivity54
Scheme 21 Generation of charge-transfer excitons in poly(methyl phenyl silylene) [25]
Chart 21 Chemical structures of poly(N -vinyl carbazole) and trinitrofluorenone
Chart 22 Chemical structures of solitons formed in trans-polyacetylene
in -conjugated polymers has attracted much attention in the scientific commu-nity and has resulted in a series of articles being compiled in a book edited bySariciftci [9] This book is wholeheartedly recommended for further readingThe currently accepted notion that optical absorption generates primarily neu-tral excitations (excitons) rather than charged species was adopted in Sec-tion 222 The earlier model is based to some extent on the assignment of tran-sient optical absorption bands at around 06 and 16 eV recorded with PPV-typepolymers to bipolarons However this assignment was contradicted by unam-biguous experimental evidence for an attribution of these transient absorptionbands to singly-charged ions [28] The definition of the term polaron which cansometimes be rather elusive in older work has been subject to alterations andmany authors now denote the products of the dissociation of electronhole pairsas negative and positive polarons However by doing so the difficulty of pre-cisely describing the chemical nature of the charge carriers is merely circum-vented As a matter of fact the release of an electron should lead to a radicalcation and the capture of an electron to a radical anion Actually relatively littlework has hitherto been dedicated to clarifying the nature of photogeneratedcharge carriers Time-resolved spectroscopy has helped to evidence the existenceof radical cations acting as charge carriers in certain polymeric systems In thiscase radical cations were generated by hole injection from an indium tin oxide(ITO) electrode by applying an external electric field to polysulfone systems con-taining tris(stilbene) amine derivatives [29] Moreover the formation of radicalcations in poly(methyl phenyl silylene) with 13CC110ndash3 was evidenced bymeans of transient optical absorption measurements (absorption bands ataround 375 and 460 nm formed upon irradiation with 20 ns laser pulses= 347 nm) [25] In the case of m-LPPP irradiated with 380 nm laser pulses atransient optical absorption band at around 691 nm (191 eV) attributed to posi-tive polarons was detected (see below) [23] Obviously quite different charge car-riers will be produced depending on the chemical nature of the polymer For ex-ample in the case of trans-polyacetylene the dissociation of electronhole pairsat neutral solitons is considered to give rise to positively and negatively chargedsolitons (see Chart 22) [30]
224Kinetics of charge carrier generation
The research concerning the mechanism and kinetics of the photogeneration ofcharge carriers has focused on conjugated polymers since these are of great im-portance for applications in light-emitting diodes and organic photovoltaic cells(see Sections 621 and 63) Typical work performed with m-LPPP (see Table 22)revealed that charge carriers are generated within a few hundred femtosecondsin a very small yield in the absence of an external electric field [23] The poly-mer was irradiated with 180 fs pulses of 380 nm light at 77 K Transmission dif-ference spectra plotted as TT exhibited besides the emission and absorptionbands of excitons an absorption band at 19 eV (650 nm) attributable to individ-
22 Photogeneration of charge carriers 55
ual positive polarons (holes) This band was formed within the duration of thepulse When an external electric field was applied the yield of charge carrierswas significantly increased As can be seen from the kinetic traces shown inFig 22 the formation of the polaron absorption corresponds to the decay of theexciton emission thus demonstrating that excitons dissociate into charge car-riers
Upon applying a field modulation technique it was possible to record directlyfield-induced changes in the TT spectra Therefore the kinetic traces inFig 22 reflect the time dependence of the field-induced differential transmis-sion (TT)FM which is the difference between TT recorded in the presenceand absence of the electric field (TT)FM = (TT)F ndash (TT)F = 0
2 Photoconductivity56
Fig 22 Dissociation of excitons into chargecarriers in m-LPPP under the influence of anexternal electric field (13 V) Kinetic traceson different time scales demonstratingchanges in the field-induced differentialtransmission (TT)FM at 191 eV (hole
absorption) and 253 eV (exciton emission)following irradiation of a 100 nm thickpolymer film at 77 K with 180 fs pulses of380 nm light Trace (a) also shows the pulseprofile (dashed line) Adapted from Lanzaniet al [23] with permission from Wiley-VCH
225Quantum yield of charge carrier generation
It has been pointed out above that the deactivation of excitons may result in theformation of geminate electronhole pairs that can eventually form free chargecarriers This process proceeds with strong competition from charge recombina-tion and can be affected by an external electric field According to the Onsagertheory [31] the probability Pr of recombination can be estimated with the aid ofEq (2-4)
Pr exp rc
r
exp eFr
2kT131 cos
2-4
Here e is the elementary charge F is the electric field strength k is the Boltz-mann constant T is the temperature and is the angle between the vectorconnecting the charges and the direction of the electric field
The Onsager theory considers two potentials determining the fate of an elec-tronhole pair the Coulomb potential e2r (= dielectric constant) and the ther-mal energy kT Pairs having a radial distance r larger than rc will escape recom-bination At the critical radial distance rc the thermal energy is equal to theCoulomb potential [see Eq (2-5)]
kT e2
rc2-5
According to Eq (2-4) the recombination probability decreases with increasingfield strength ie the escape probability Pe = 1ndash Pr increases Therefore thequantum yield for charge carrier generation 13cc should increase with increasingfield strength Figure 23 shows a double logarithmic plot of the dependence of13cc on the electric field strength measured at T = 295 K for three polysilylenes[32]
The quantum yield increases dramatically by about three orders of magnitudein the cases of the polysilylenes PBMSi and PMPSi having aromatic substitu-ents whereas the fully aliphatic polysilane PDHeSi is quite ineffective in chargecarrier production presumably because CT excitons cannot be formed in thiscase Interestingly 13cc is markedly higher for the biphenyl-substituted polysi-lane than for the phenyl-substituted one which might be due to a larger initialelectronhole distance in the former case The curves in Fig 23 were obtainedwith the aid of Eq (2-6) [33] which is based on calculations by Mozumder [34]
13cc 13cc0
4r2f rFTgrdr 2-6
Here 13cc0 denotes the primary quantum yield f(r F T) is the dissociationprobability of pairs at radial distance r and g(r) is the initial spatial distribution
22 Photogeneration of charge carriers 57
of electronhole pairs Satisfactory data fits were obtained by applying a Gaus-sian distribution function for electronhole pair distances [see Eq (2-7)]
gr 323 exp r2
2
2-7
Here is a material parameterRegarding the curves in Fig 23 data fitting was performed with 13cc0 = 085
and = 16 nm in the case of PBMSi and 13cc0 = 045 and = 13 nm in the caseof PMPSi These data are in accordance with the assumption that 13cc0 in-creases with increasing initial electronhole radial distance r0 since statistically is a measure of r013cc values are most accurately determined by the xerographic (electrophoto-
graphic) discharge method which is based on the determination of the light-in-duced change in the surface potential U= QC generated by a corona processQ and C denote the surface charge density and the capacitance per unit area re-spectively U is recorded at a given sampling frequency and the dischargequantum yield is obtained with the aid of Eq (2-8)
13cc 1efI
Q
t
tt0
CefI
Ut
tt0
0
edfI
Ut
tt0
2-8
with the following denotations dielectric constant (dimensionless) vacuumdielectric constant 0 = 88510ndash14 A s Vndash1 cmndash1 elementary charge e= 1602210ndash19 A s sample thickness d [cm] light intensity I [photons cmndash2 sndash1] surfacepotential U [V] and fraction of absorbed light f Figure 24 shows a schematicdepiction of a typical experimental set-up which includes a rotating metal disk
2 Photoconductivity58
Fig 23 Quantum yield for charge carriergeneration as a function of the electric fieldstrength determined at 295 K for three poly-silylenes poly(biphenyl methyl silylene)
PBMSi poly(methyl phenyl silylene) PMPSiand poly(dihexyl silylene) PDHeSi Adaptedfrom Eckhardt [32] with permission from theauthor
carrying the sample Upon rotation (600ndash2400 rpm) the sample passes a contin-uous light beam and a condenser plate for determination of the change in thesurface potential
A typical result obtained upon irradiation of poly(methyl phenyl silylene) atexc = 337 nm is shown in Fig 25 [32]
22 Photogeneration of charge carriers 59
Fig 24 Schematic illustration of a set-up used to determine13cc by means of the xerographic discharge method Adaptedfrom Eckhardt [32] with permission from the author
Fig 25 Light-induced decrease in the surface potentialrecorded for poly(methyl phenyl silylene) at exc = 337 nmt0 = onset of irradiation Adapted from Eckhardt [32] withpermission from the author
23Transport of charge carriers
The transport of charge carriers through a solid is characterized by the drift mo-bility which is defined as the hole or electron velocity per unit electric fieldstrength frequently given in units of cm2 Vndash1 sndash1 can be obtained with theaid of Eq (2-9) by measuring the transit time tr which is the time required forcharge carriers to pass a sample of thickness d when an external electric field ofstrength F is applied
dtrF
2-9
Commonly the so-called time-of-flight (TOF) method is applied to determine Figure 26 shows a schematic depiction of a typical set-up
A sandwich-type sample consisting of a semi-transparent ITO electrode apolymer layer and a metal (usually aluminum) electrode (see Fig 27a) is irra-diated with a short laser flash through the ITO electrode During the light flashwhich is totally absorbed by a very thin sheet at the surface of the polymer layercharge carriers are generated and start to drift towards the metal electrode un-der the influence of an external electric field The photocurrent is recorded as afunction of time after the flash Notably the transport of both sorts of chargecarriers cannot be recorded simultaneously In the case of a negatively polarizedmetal electrode hole migration can be observed while electron migration canbe followed with a positively polarized metal electrode For mobility measure-ments in thin samples or materials inappropriate for photochemical charge car-
2 Photoconductivity60
Fig 26 Schematic illustration of a typical time-of-flight (TOF)set-up used for the determination of the mobility
rier generation (low absorption coefficient low quantum yield 13cc) a sandwich-type arrangement consisting of goldsiliconpolymergold layers (see Fig 27b)is used [35] Here after passing through the lower gold layer the light is totallyabsorbed by the silicon substrate thus generating charges that are injected intothe polymer layer
Usually only one sort of charge carrier is capable of migrating through thepolymer film In the cases of carbon-catenated -conjugated and silicon-cate-nated -conjugated polymers the photoconductivity is due to hole conductionOn the other hand electrical conductivity due to electron conduction has beenobserved with low molar mass compounds such as tris(8-oxyquinolato)alumi-num Alq3 dispersed in polymethacrylates bearing special pendant groups (seeChart 23 and also Table 63 in Section 6212)
Figure 28 shows a typical result obtained for conjugated polymers [36] Herecharge carriers are generated in a poly(methyl phenyl silylene) sample by a15 ns flash of 347 nm light The photocurrent is formed during the flash and afraction decays very rapidly until a plateau is reached In the subsequent phasethe current decreases slowly The initial phase after the flash is characterized bythe rapid formation of charge carriers and the rapid recombination of a fractionof them The plateau corresponds to the migration of the holes which drift atdifferent velocities through the sample and the end of the plateau correspondsto the time at which the fastest holes arrive at the metal electrode
23 Transport of charge carriers 61
Fig 27 Sandwich-type assembliesapplied in time-of-flight determinations ofcharge carrier mobility (a) carriergeneration in the polymer layer (b) carriergeneration in the silicon substrate
Chart 23 Chemical structure of tris(8-oxyquinolato)-aluminum Alq3
From Table 23 which lists typical values it can be seen that the hole mo-bility in conjugated polymers is lower than that in organic crystals and amor-phous silicon but much larger than that in undoped poly(N-vinyl carbazole)Therefore conjugated polymers have potential for applications in conductingopto-electronic and photonic devices In principle this also applies to liquid-crystal systems that can exhibit enhanced molecular order due to their self-orga-nizing ability as has been pointed out in a progress report [42]
The fact that liquid crystallinity enhances carrier transport as compared tonon-ordered systems was convincingly demonstrated in the case of poly(99-dioctylfluorene) A relatively high hole mobility of 910ndash3 cm2 Vndash1 sndash1 was ob-tained when the polymer was examined as a uniformly aligned nematic glassThis value is significantly larger than the = 410ndash4 cm2 Vndash1 sndash1 measured foran isotropic film of the same polymer [43] Although significant progress hasbeen made in developing materials with improved charge carrier mobilities itseems that future applications will require materials possessing much furtherimproved transport properties Apparently interchain interactions and morpho-logical complexities strongly control charge carrier transport in bulk polymericsystems Taking this into account recent work on hole transport has led to quitehigh mobility values For example high mobilities were measured for very thinfilms (70ndash100 nm) of poly(3-hexylthiophene) P3HT having a regioregularity of96 [40] (Regioregularity denotes the percentage of stereoregular head-to-tail at-
2 Photoconductivity62
Table 23 Hole mobilities at T= 295 K and F105 V cmndash1
Polymer (cm2 Vndash1 sndash1) References
Crystals of low molar mass organic compounds 10ndash1ndash100 [5 28]Amorphous silicon 10ndash1 [5]m-LPPP 10ndash3 [37]Poly(99-dioctylfluorene) 10ndash4 [38]Poly(methyl phenyl silylene) 10ndash4 [32]Poly(p-phenylene vinylene) 10ndash5 [39]Polythiophene 10ndash5 [40]Poly(N-vinyl carbazole) 10ndash7ndash10ndash6 [41]
Fig 28 Time-of-flight experiment performedwith poly(methyl phenyl silylene) Photocurrenttrace recorded with a positively biased ITOelectrode at F= 25107 V mndash1 d = 2 mexc = 347 nm flash duration 20 ns Adaptedfrom Eckhardt et al [36] with permission fromTaylor amp Francis Ltd
tachments of thiophene rings bearing hexyl groups in the 3-position) The filmsconsisted of large amounts of microcrystalline domains embedded in an amor-phous matrix During film processing the macromolecules arranged by self-orga-nization into a lamellar structure composed of two-dimensional conjugated sheetsFor a lamellae orientation parallel to the substrate hole mobility values as high as01 cm2 Vndash1 sndash1 were found In this context work with isolated linear polymerchains (molecular wires) is also noteworthy [44] It revealed that the hole transportmobility along isolated polymer chains can exceed 01 cm2 Vndash1 sndash1 as can be seen inTable 24 Here values were obtained from a pulse radiolysis study on dilute poly-mer solutions Holes were generated by charge transfer from benzene radical ca-tions to the polymer By means of a time-resolved microwave conductivity methodit was shown that the conductivity of the solution increased significantly after theholes were produced indicating that the mobility of holes in the polymer chainsis considerably higher than the mobility of the initially formed benzene radicalcations
Interestingly electron transport has been observed with a diene compound ofthe structure shown in Chart 24
23 Transport of charge carriers 63
Table 24 Hole mobility in linear polymers in dilute solution in benzene [44]
Chemical structure Acronym (cm2 Vndash1 sndash1)
DEH-PF 074
MEH-PPV 043
m-LPPP 016
P3HT 002
PAPS6 023
For this compound which forms a smectic C phase at room temperature anelectron mobility of 1510ndash5 cm2 Vndash1 sndash1 was reported By virtue of its reactivegroups this diene compound can be photopolymerized to form a polymeric net-work [45]
24Mechanism of charge carrier transport in amorphous polymers
At present a hopping mechanism is generally accepted for the transport ofcharge carriers through amorphous polymeric media under the influence of anexternal electric field [23 46] After separation of electronhole pairs the inde-pendent charge carriers are temporarily trapped at certain sites The latter havethe quality of potential wells formed by single molecules or segments of poly-mer chains Assisted by an external electric field the carriers are removed fromthese sites by thermal activation and move until recaptured by other sites Withregard to this model Gill has formulated an empirical relationship [Eq (2-10)]for the dependence of the mobility on electric field strength and temperature[47]
FT 0 exp Ea0 F12
kTeff
2-10
Here Ea0 is the average activation energy = (e30)12 is the Poole-Frenkelfactor and Teff is an effective temperature where Teff
ndash1 = Tndash1 ndashT0ndash1 T0 is the tem-
perature at which Arrhenius plots of with varying F intersect and 0 =(T = T0)
More recently a relationship for the dependence of on F and T was derivedby Baumlssler [21 28] on the basis of the so-called disorder concept The latter takesinto account that carrier hopping in amorphous polymers is determined by theenergy state of the transport sites and by the geometrical localization of thesites The values of the energy states of the sites vary within a certain distribu-tion the so-called density of states (DOS) distribution which is referred to as di-agonal disorder The width of this distribution is characterized by a parameter Regarding the geometrical localization of the sites it is taken into accountthat they are randomly distributed within the three-dimensional system whichis referred to as off-diagonal disorder The width of this distribution is character-ized by the geometrical disorder parameter The two distributions can be il-
2 Photoconductivity64
Chart 24 Chemical structure of a diene compound amenable to electron transport [45]
lustrated as follows Diagonal disorder transport sites are traps of varyingdepths off-diagonal disorder the trajectories of carriers do not follow lines par-allel to the field direction but show significant deviations therefrom especiallyat low electric field strengths as is demonstrated in Fig 29
In conclusion charge transport in amorphous polymers occurs by way of car-rier hopping within a positionally random and energetically disordered systemof localized states [48] The dependence of the carrier mobility on diagonal andoff-diagonal disorder is taken into account by Eq (2-11)
FT 0 exp 42
9
exp C2 2F12
132-11
Here kT with being the width of the Gaussian distribution of energystates C is an empirical constant and 0 is a material constant
According to Eq (2-11) ln is proportional to F12 and 1T2 Regarding thefield strength dependence of typical results obtained with poly(methyl phenylsilylene) are presented in Fig 210 [32]
Note that the square-root dependence does not hold for the entire field regimewhich is in accordance with findings for other polymers [28] Note also that Eq (2-11) predicts that the field dependence changes sign if gtkT and that the phe-nomenologically defined Gill temperature T0 is related to the disorder parameter of the system T0 =k For example T0 is equal to 387 K for = 3 and = 01 eV[28] The applicability of the model described above was scrutinized by Baumlssler [28]and is still being examined as indicated by recent publications [49ndash51] It has beenpointed out for instance that in the case of m-LPPP the dependence of on elec-tric field strength and temperature resembles that of molecular crystals exceptthat is two orders of magnitude lower a behavior at variance with the presentversion of the disorder model Attempts to modify the disorder model have tosome extent been focussed on the interaction of charge carriers with the surround-ing matrix ie on the so-called polaronic effect The latter implies that a localized
24 Mechanism of charge carrier transport in amorphous polymers 65
Fig 29 Schematic depiction of a carrier trajectory in apolymeric matrix reflecting the geometrical (off-diagonal) disorder The electric field acts along the DndashAdirection jump rate Adapted from Baumlssler et al [21]with permission from Wiley-VCH
carrier is strongly coupled either to local polarization or to vibrations andor rota-tions of the molecule at which it resides Since the coupling is induced by thecharge carrier itself the process is referred to as self-trapping and gives rise tothe denotation of charge carriers as polarons When a polaron moves it carriesalong the associated structural deformation As regards the hopping model po-laronic effects can be taken into account by considering that the activation energyfor the mobility in a random hopping system is composed of two components apolaronic component Ea
(p) and a disorder component Ea(d) [see Eq (2-12)]
Ea Epa Ed
a 2-12
Therefore the dependence of the charge carrier mobility on electric fieldstrength and temperature can be described by Eq (2-13)
FT 0 exp Ep
2kT 42
9
exp C 2 2
F12
132-13
Here Ep denotes the polaron binding energy
25Doping
It is possible to make inert polymers photoconductive and to improve the photo-conduction performance of conducting polymers by doping ie by the additionof appropriate low molar mass substances to the polymers Relevant work hasbeen reviewed by Mylnikov [3] Early studies with inert polymers such as poly-
2 Photoconductivity66
Fig 210 Electric field dependence ofthe mobility of holes in poly-(methyl phenyl silylene) at varioustemperatures (1) 295 K (2) 312 K(3) 325 K (4) 355 K (5) 385 KAdapted from Eckhardt [32] withpermission from the author
carbonate polystyrene and poly(vinyl chloride) revealed that the hole mobili-ty and 13cc the quantum yield of charge carrier generation were increasedwhen electron-donating compounds such as those presented in Chart 25 wereincorporated as dopants Actually large amounts of dopants have to be appliedto accomplish significant variations in 13cc and
Figure 211 depicts the increase in 13cc with increasing triphenylamine con-tent in commercial bisphenol A polycarbonate (see Chart 26) [52] and Fig 212shows a plot of log vs 1T It can be seen that the hole mobility may be variedover several orders of magnitude by changing the TPA concentration [53] Hereirradiations were performed at wavelengths of exc = 300 and 337 nm respec-
25 Doping 67
Chart 25 Chemical structures of electron-donatingcompounds triphenylamine (TPA) isopropylcarbazole (IPC)and phenylcarbazole (PhC)
Fig 211 Doping of an inert polymer bisphenol Apolycarbonate with triphenylamine (TPA) The quantum yieldof charge carrier formation 13cc as a function of the TPAcontent exc = 300 nm Adapted from Borsenberger et al [52]with permission from the American Institute of Physics
tively at which the polycarbonate is transparent and the light is absorbed solelyby TPA
As regards photoconducting polymers typical work has been carried out withpoly(N-vinylcarbazole) PVK and polysilylenes The first commercial photocon-ductor was based on a 1 1 charge-transfer (CT) complex between PVK and trini-trofluorenone (TNF) [11] Similar photoconductor properties were found with a1 1 CT complex of TNF with poly[bis(2-naphthoxy)phosphazene] (see Chart 27)which is an insulator if dopant-free [54]
Results obtained with poly(methyl phenyl silylene) are presented in Table 25which demonstrate that at low concentration (3 mol) electron-accepting do-pants having zero dipole moment are capable of increasing both and 13cc Theincrease in 13cc is more pronounced the higher the value of the electron affinity
2 Photoconductivity68
Chart 26 Chemical structure of bisphenol Apolycarbonate poly(oxycarbonyloxy-14-pheny-lene-isopropylidene-14-phenylene)
Fig 212 Doping of an inert polymerbisphenol A polycarbonate with triphenyl-amine (TPA) Temperature dependence ofthe hole mobility Plot of log vs 1T forvarious TPA contents denoted as weight
fraction x exc = 337 nm F= 7105 V cmndash1 denotes the activation energy Adaptedfrom Pfister [53] with permission from theAmerican Physical Society
EA Polar dopants also cause an increase in the quantum yield but the holemobility is concomitantly decreased [55]
Fullerene C60 is quite an effective dopant It is an excellent electron acceptorcapable of accepting up to six electrons Photoinduced electron transfer fromconducting polymers such as poly(3-octylthiophene) P3OT and poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylene vinylene] MEH-PPV to fullerene C60 occurs ona timescale of less than 1 ps A C60 content of a few percent is sufficient to en-hance 13cc in the ps time domain by more than an order of magnitude [56]
26Photoconductive polymers produced by thermal or high-energy radiation treatment
Certain polymers become photoconductive upon exposure to heat or high-en-ergy radiation an aspect that has been reviewed by Mylnikov [3] For examplepolyacrylonitrile (maximum sensitivity at = 420 nm) or polypyrrole (maximumsensitivity at = 500ndash600 nm) exhibit photoconductivity after heat treatmentwhich is thought to be due to the formation of conjugated double bonds High-
26 Photoconductive polymers produced by thermal or high-energy radiation treatment 69
Chart 27 Chemical structure of poly[bis(2-naphthox-y)phosphazene] P2NP
Table 25 The photoconduction performance of poly(methylphenyl silylene) containing electron-acceptor-type dopants[55]
Additive(3 mol)
EA a)
(eV)Dipole moment(Debye)
b)
(cm2 Vndash1 sndash1)cc
c)
None 22810ndash4 1910ndash2 d)
o-DNB g) 00 60 50210ndash5 2310ndash2 d)
m-DNB 03 38 14210ndash4 2310ndash2 d)
p-DNB 07 00 31010ndash4 3410ndash2 d)
Tetracene 10 00 30610ndash4 9610ndash2 e)
Chloranil 13 00 41210ndash4 12510ndash2 e)
TCNQ f) 17 00 57110ndash4 10010ndash2 e)
a) Electron affinityb) Hole mobilityc) Quantum yield of charge carrier formationd) exc =355 nme) exc =339 nmf) TCNQ tetracyanoquinoneg) DNB dinitrobenzene
energy electron irradiation on the other hand renders polyethylene photocon-ductive with maximum sensitivity in the near-infrared region This phenome-non was postulated as being due to radiation-generated donor- and acceptor-typetraps
27Photoconductive polymers produced by plasma polymerization or glow discharge
Various polymeric materials prepared by plasma polymerization or glow dis-charge become conductive when exposed to UV light This applies for exampleto a polymer obtained by plasma polymerization of styrene The polymer wasexamined as a thin sheet coated with gold layers on both sides [57] Also thinpolymer layers deposited by glow discharge of tetramethylsilane tetramethylger-manium or tetramethyltin on conducting substrates were found to be photocon-ductive in the wavelength region 200ndash350 nm [58]
2 Photoconductivity70
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2 D Mort N Pfister (eds) Electronic Prop-erties of Polymers Wiley-InterscienceNew York (1982)
3 V Mylnikov Photoconducting PolymersAdv Polym Sci 115 (1994) 1
4 D Haarer Photoconductive Polymers AComparison with Inorganic Materials AdvSolid State Phys 30 (1990) 157
5 D Haarer Angew Makromol Chem183 (1990) 197
6 TA Skotheim (ed) Handbook of Con-ducting Polymers Marcel Dekker NewYork (1986)
7 TA Skotheim R L Elsenbaumer J RReynolds (eds) Handbook of ConductingPolymers 2nd Edition Marcel DekkerNew York (1997)
8 G Zerbi Organic Materials for PhotonicsElsevier Science Amsterdam (1993)
9 NS Sariciftci (ed) Primary Photoexcita-tions in Conjugated Polymers MolecularExciton versus Semiconductor Band ModelWorld Scientific Singapore (1997)
10 K Y Law Chem Rev 93 (1993) 44911 PM Borsenberger D S Weiss Organic
Photoreceptors for Xerography Marcel Dek-ker New York (1998)
12 PM Borsenberger D S Weiss OrganicPhotoreceptors for Imaging Systems MarcelDekker New York (1993)
13 NV Joshi Photoconductivity MarcelDekker New York (1990)
14 HS Nalwa (ed) Handbook of OrganicConductive Molecules and Polymers Vol 3Wiley New York (1997)
15 HS Nalwa (ed) Handbook of AdvancedElectronic and Photonic Materials and De-vices Academic Press San Diego (2001)
16 G Hadziioannou P F van Hutten(eds) Semiconducting Polymers Wiley-VCH Weinheim (2000)
17 M Pope C E Swenberg Electronic Pro-cesses in Organic Crystals and Polymers2nd ed University Press Oxford (1999)
18 D Fichou (ed) Handbook of Oligo- andPolythiophenes Wiley-VCH Weinheim(1998)
19 A Pron P Rannou Processible Conjugat-ed Polymers From Organic Semiconductorsto Organic Metals and SuperconductorsProg Polym Sci 27 (2002) 135
20 H Kies Conjugated Conducting PolymersSpringer Berlin (1992)
21 H Baumlssler Phys Stat Sol B 175 (1993)15
References 71
22 G von Buumlnau T Wolff PhotochemieGrundlagen Methoden AnwendungenVCH Weinheim (1987)
23 G Lanzani S de Sylvestre G CerulloS Stagira M Nisoli W Graupner GLeising U Scherf K Muumlllen Photo-physics of Methyl-Substituted Poly(para-phenylene)-Type Ladder Polymers in [16]p 235
24 K Pichler D Halliday DC BradleyPL Burn R H Friend A B Holmes JPhys Cond Matter 5 (1993) 7155
25 S Nespurek V Herden W Schnabel AEckhardt Czechoslovak J Phys 48(1998) 477
26 J Knoester M Mostovoy Disorder andSolitons in trans-Polyacetylene in [16]p 63
27 R H Friend DDC Bradley P DTownsend J Phys D Appl Phys 20(1987) 1367
28 H Baumlssler Charge Transport in RandomOrganic Semiconductors in [16] p 365
29 M Redecker H Baumlssler HH HoumlrholdJ Phys Chem 101 (1997) 7398
30 M Loumlgdlund W R Salaneck ElectronicStructure of Surfaces and Interfaces in Con-jugated Polymers in [16] p 115
31 L Onsager Phys Rev 54 (1938) 55432 A Eckhardt PhD Thesis Technical
University Berlin (1995)33 V Cimrova I Kminek S Nespurek W
Schnabel Synth Metals 64 (1994) 27134 A Mozumder J Chem Phys 60 (1974)
430035 B J Chen C S Lee S T Lee P Webb
YC Chan W Gambling H Tian WHZhu Jpn Appl Phys 39 (2000) 1190
36 A Eckhardt V Herden S Nespurek WSchnabel Phil Mag B 71 (1995) 239
37 D Hertel U Scherf H Baumlssler AdvMat 10 (1998) 1119
38 M Redecker DD C Bradley M Inbase-karan EP Woo Appl Phys Lett 73(1998) 1565
39 E Lebedev T Dittrich V Petrova-KochS Karg W Bruumltting Appl Phys Lett 71(1997) 2686
40 H Sirringhaus P J Brown R HFriend MM Nielsen K Bechgaard
BMW Langeveld-Voss A I SpieringR A J Janssen E W Meijer D M deLeeuw Nature 401 (1999) 685
41 E Muumlller-Horsche D Haarer H ScherPhys Rev B 35 (1987) 1273
42 M OrsquoNeill S M Kelly Adv Mater 15(2003) 1135
43 M Redecker DD C Bradley M Inbase-karan EP Woo Appl Phys Lett 74(1998) 1400
44 FC Grozema LDA Siebbeles JMWarman S Seki S Tagawa U ScherfAdv Mater 14 (2002) 228
45 P Vlachos S M Kelly B Mansoor MOrsquoNeill Chem Commun (2002) 874
46 M Abkowitz H Baumlssler M Stolka PhilMag B 63 (1991) 201
47 W D Gill J Appl Phys 43 (1972) 503348 V I Arkhipov P Heremans EV Eme-
lianova G J Andriaenssens H BaumlsslerAppl Phys Lett 82 (2003) 3245
49 S Nespurek Macromol Symp 104(1996) 285
50 V I Arkhipov J Reynaert Y D Jin PHeremans EV Emelianova G J An-driaenssens H Baumlssler Synth Met 138(2003) 209
51 V I Arkhipov P Heremans EV Eme-lianova G J Andriaenssens H BaumlsslerChem Phys 288 (2003) 51
52 P Borsenberger G Contois A Ateya JAppl Phys 50 (1979) 914
53 G Pfister Phys Rev B 16 (1977) 367654 PG Di Marco G M Gleria S Lora
Thin Solid Films 135 (1986) 15755 A Eckhardt V Herden W Schnabel
Photoconductivity in Polysilylenes Dopingwith Electron Acceptors in N Auner JWeis (eds) Organosilicon Chemistry IIIWiley-VCH Weinheim (1997) p 617
56 B Kraabel CH Lee D McBranch DMoses NS Sariciftci A J HeegerChem Phys Lett 213 (1993) 389
57 S Morita M Shen J Polym Sci PhysEd 15 (1977) 981
58 N Inagaki M Mitsuuchi Polym SciLett Ed 22 (1978) 301
31Introductory remarks
Electro-optic (EO) phenomena are related to the interaction of an electric fieldwith an optical process The classical electro-optic effects the Pockels and theKerr effect discovered in 1893 and 1875 with quartz and carbon disulfide re-spectively refer to the induction of birefringence in certain materials under theinfluence of an external electric field Application of an electric field to the sam-ple causes a change in the refractive index In the case of the Pockels effect nis linearly proportional to E the strength of the applied electric field [see Eq (3-1)] Hence it is also called the linear electro-optic effect In contrast n is pro-portional to E2 in the case of the Kerr effect [see Eq (3-2)]
Linear electro-optical effect Pockels effect n rE 3-1
Quadratic electro-optical effect Kerr effect n qE2 3-2
r (m Vndash1) and q (m Vndash2) are the Pockels and the Kerr constants respectively Eis the electric field strength (V mndash1) and (m) is the wavelength of the light
Pockels cells containing an appropriate crystal such as potassium dihydrogenphosphate and Kerr cells containing an appropriate liquid eg nitrobenzeneare used as light shutters (in conjunction with polarizers) and intensity modula-tors of linearly polarized laser light beams Actually the technical importance ofEO effects is increasing because of various applications in optical communica-tion devices particularly concerning EO modulators that are used in fiber-opticcommunication links In the search for novel EO materials organic compoundsand particularly polymeric systems have also been explored While polymers arecheap and easily processable many of them are inferior to inorganic crystals be-cause of their low thermal stabilities Therefore the application potential ofpolymeric systems is limited Nevertheless a large volume of research has beendevoted to the use of polymers in photonic devices based on EO effects Somehighlights regarding the achievements in this field are reported in this chapter
It should be emphasized that the Kerr effect refers to a quadratic ie a non-linear dependence of the refractive index on the strength of the externally ap-
73
3Electro-optic and nonlinear optical phenomena
plied electric field In this respect the Kerr effect is the first nonlinear opticalphenomenon that has gained both fundamental and practical importance Theinterest in nonlinear phenomena flourished after the construction of the firstruby laser in 1960 by TH Maiman [1] and the observation of second harmonicgeneration (SHG) ie frequency doubling of laser light in 1961 [2] Since thenthe field of nonlinear optics has developed very rapidly as demonstrated by aplethora of articles and books To a large extent these also cover research on or-ganic materials including polymers [3ndash14]
32Fundamentals
321Electric field dependence of polarization and dipole moment
Electric field-induced changes in refractive index can be explained with the aidof the following model under the influence of the electric field the charge dis-tribution in the molecules is perturbed and the molecules are polarized The di-pole moment pi induced by an electric field along the molecular axis can be ex-pressed by an expansion [see Eq (3-3)] [15]
pi 0
j
ijEj
jk
ijkEjEk
jkl
ijklEjEkEl 3-3
Here 0 denotes the permanent dipole moment The coefficients are tensorstermed as linear polarizability ij and first and second molecular hyperpolariz-abilities ijk and ijkl respectively The indices refer to the tensor elements ex-pressed in the frame of the molecule using Cartesian coordinates Ej Ek and El
denote the applied electric field strength components Commonly the responsetime ranges from picoseconds to femtoseconds Therefore if an alternating elec-tric field with a frequency of less than 1012 Hz is applied the direction of thepolarization alternates with the oscillations of the applied field
The polarization induced at the molecular level can cause a polarization inthe bulk of the sample and lead to macroscopically detectable property changesfor instance in the refractive index The macroscopic polarization PI induced bythe electric field can be expressed by the expansion given by Eq (3-4)
PI P0
J
1IJ EJ
JK
2IJKEJEK
JKL
3IJKLEJEKEL 3-4
Here P0 is the permanent polarization and (2) and (3) denote the second- andthird-order nonlinear optical three-dimensional susceptibility tensors The in-dices attached to the tensors refer to the tensor elements and the indices as-sociated with the E values refer to the components of the electric field strengthhere expressed in the laboratory frame
3 Electro-optic and nonlinear optical phenomena74
In the case of weak applied fields the higher terms in Eq (3-4) can be ne-glected and if the sample is not permanently polarized Eq (3-4) reduces toEq (3-5)
Plinear 1E 3-5
If the medium is isotropic (1) is a scalar ie the relationship between E andPlinear is independent of the direction of the field vector E and the polarizationis parallel to E Many polymers possess amorphous structures and their opticalproperties are isotropic However electro-optic polymeric systems containing po-lar moieties can be made anisotropic by orienting these moieties for exampleby electric field-induced or optical alignment In this case the polarization isnot necessarily parallel to the direction of E and its component in one directionis related to the field components in all three directions
PX 11EX 12EY 13EZ
PY 21EX 22EY 23EZ PI
J
IJEJ 3-6
PZ 31EX 32EY 33EZ
Note that the indices X Y and Z expressed in upper-case letters represent thecoordinates of the macroscopic laboratory frame As indicated in Fig 31 lower-case letters are used to denote the coordinates of the molecular frame
The susceptibility of an anisotropic medium is represented by a tensor Ten-sors are composed of 3a+1 elements where a is the number of interacting vec-tors and a+1 denotes the rank With a = 1 (1) is a second-rank tensor with32 = 9 elements which can be expressed by the matrix given in Eq (3-7)
1 11 12 1321 22 2331 32 33
3-7
Polarization can be induced in matter not only by an externally applied electricfield but also by the electric field of a passing light beam This kind of interac-tion does not lead to a loss of intensity of the beam in contrast to absorptionwhich reduces the intensity The overall situation taking into account both
32 Fundamentals 75
Fig 31 The macroscopic laboratory frame (X Y Z) and themolecular frame (x y z) Adapted from Kippelen et al [15] withpermission from Springer
kinds of interaction ie polarization and absorption can be described on the ba-sis of complex and frequency-dependent entities consisting of a real and animaginary part This concerns the dielectric constant the optical susceptibilityand the refractive index For example the complex refractive index n [seeEqs (3-8) and (3-9)] is given by the sum of the real part n and the imaginarypart ik the latter corresponding to light absorption [15]
n n ik 3-8
2kc
3-9
Here (cmndash1) is the linear absorption coefficient (sndash1) is the frequency ofthe optical field and c (cm sndash1) is the speed of light
When a high-intensity laser beam impinges on material its electromagneticfield induces electrical polarization that gives rise to a variety of nonlinear opti-cal properties because in this case the higher terms in Eq (3-4) are not negligi-ble The determination of the coefficients (2) and (3) that serve to characterizethe nonlinear properties is complicated by the fact that they are composed ofmany elements With a being equal to two and three (2) and (3) are composedof 3a+1 = 27 and 81 elements respectively Fortunately these tensors possesssymmetry properties that can be invoked to reduce the number of independentelements for instance when the optical frequencies involved in the nonlinearinteraction are far away from resonance (absorption) [15]
In the case of second harmonic generation for example the second-order sus-ceptibility tensor elements are symmetrical in their last two indices Therefore thenumber of independent tensor elements is reduced from 27 to 18 Moreover thetensor elements
2IJK can be expressed in contracted form
2IJ The index I takes
the value 1 2 or 3 corresponding to the three Cartesian coordinates and the indexL varies from 1 to 6 The values of L refer to the six different combinations of theindices J and K according to the following convention [15]
L 1 2 3 4 5 6
JK 11 22 33 23 or 32 13 or 31 12 or 21
Therefore (2) can be expressed by the matrix given by Eq (3-10)
2 211
212
213
214
215
216
221
222
223
224
225
226
231
232
233
234
235
236
3-10
For poled polymers that belong to the mm symmetry group some of the ten-sor elements vanish and the (2) tensor reduces to Eq (3-11) [15]
3 Electro-optic and nonlinear optical phenomena76
2 0 0 0 0
215 0
0 0 0 215 0 0
231
232
233 0 0 0
3-11
When Kleinman symmetry 2ijk
2ikj
2jkl
2jik
2kij
2kji
is valid [16]
215 is equal to
231 Therefore only two independent tensor elements namely
231 and
233 remain Methods that are commonly applied to determine macro-
scopic susceptibilities are based on geometrical arrangements permitting theusage of these simplifications Regarding the relationship between the macro-scopic susceptibilities and the molecular hyperpolarizabilities equations havebeen derived for the practically very important case of rigid polar moieties con-taining polymeric systems that have been or are subject to an alignment process[15] It is beyond the scope of this book to treat this subject in detail A typical re-sult concerning the relation of (2) to is given by Eqs (3-12) and (3-13) [17] Inthis case it was assumed that the macroscopic susceptibility of a given volumeis the sum of all corresponding molecular contributions in this volume and thateach molecular component is mapped onto the corresponding macroscopic vector
2ZZZ NFzzz cos3
3-12
2XXZ
2YYZ
2XZY
2YZY
2ZXX
2ZYY 1
2NFzzz cos sin2
3-13
Here N is the number of hyperpolarizable groups per unit volume (numberdensity) F is a factor correcting for local field effects and is the angle be-tween the permanent dipole 0 of the molecule (z direction) and the directionof the poling field (Z direction) The brackets indicate an averaging over all mo-lecular orientations weighted by an orientational distribution function
The importance of the hyperpolarizability and susceptibility values relates tothe fact that provided these values are sufficiently large a material exposed to ahigh-intensity laser beam exhibits nonlinear optical (NLO) properties Remark-ably the optical properties of the material are altered by the light itselfalthough neither physical nor chemical alterations remain after the light isswitched off The quality of nonlinear optical effects is crucially determined bysymmetry parameters With respect to the electric field dependence of the vectorP given by Eq (3-4) second- and third-order NLO processes may be discrimi-nated depending on whether (2) or (3) determines the process The discrimi-nation between second- and third-order effects stems from the fact that second-order NLO processes are forbidden in centrosymmetric materials a restrictionthat does not hold for third-order NLO processes In the case of centrosym-metric materials (2) is equal to zero and the nonlinear dependence of the vec-tor P is solely determined by (3) Consequently third-order NLO processes canoccur with all materials whereas second-order optical nonlinearity requiresnon-centrosymmetric materials
32 Fundamentals 77
The significances of the susceptibilities (1) (2) and (3) are related to specificphenomena (1) relates to optical refraction and absorption Common effects re-lated to (2) are frequency doubling (second harmonic generation SHG) andthe linear electro-optic effect (Pockels effect) Typical effects connected with (3)
are frequency tripling (third harmonic generation THG) sum and differencefrequency mixing two-photon absorption and degenerate four-wave mixing
322Electric field dependence of the index of refraction
Regarding light frequencies in the non-resonant regime electro-optic (EO) activityrelates to the control of the index of refraction of a material by application of anexternal electric field Either DC or AC (ranging from 1 Hz to more than100 GHz) voltages are applied The index of refraction n corresponds to the speedof light c in the material (n = c0c with c0 being the speed of light in vacuo) There-fore the electro-optic activity relates to a voltage-controlled phase shift of the lightThe change in the refractive index of a non-centrosymmetric material in a modu-lating electric field E can be represented by the expansion given by Eq (3-14) [18]
nIJ 12
n3IJrIJKEK 1
2n3
IJpIJKKE2K 3-14
Provided that higher terms are negligible Eq (3-14) reduces to Eq (3-15) whichrelates to the Pockels effect
nIJ 12
n3IJrIJKEK 3-15
The susceptibility tensor 2IJK is related to the Pockels tensor rIJK [Eq (3-16)] [19]
2IJK 1
2n4
I rIJK 3-16
2IJK is invariant under permutation of the first two indices Therefore a con-
densed notation resulting in only two indices L and K can be used The firstindex L represents the combination IJ and may have the value 1= XX 2 = YY3 = ZZ 4 = YZ 5 = ZX or 6 = XY= YX and the second index K may have the val-ue 1 = X 2= Y or 3 = Z [17]
Technical applications based on the Pockels effect require systems that are non-centrosymmetric on a macroscopic level This relates particularly to polymeric sys-tems containing physically admixed or chemically incorporated components withpermanent dipoles In such cases macroscopic second-order nonlinearity can beaccomplished by poling ie by aligning the permanent dipole moments of thecomponents with the aid of an external electric field that is applied at tempera-tures in the vicinity of the polymerrsquos glass transition temperature Tg The orderthus obtained is frozen-in by cooling to a low temperature TTg The refractive
3 Electro-optic and nonlinear optical phenomena78
index of the uniformly poled polymer is uniaxial with a long axis ne in the polingdirection (direction 3) and a short axis n0 perpendicular to the poling direction (di-rections 1 and 2) If a modulating electric field is applied in the poling directionthe two Pockels coefficients r33 and r31 can be discriminated They are described byEqs (3-17) and (3-18) in relation to the susceptibilities
2333 and
2311 and are re-
lated to the hyperpolarizability through Eqs (3-12) and (3-13)
n 12
n3er33Emod
2333Emod
ne NF cos3 Emod
ne3-17
n 12
n30r13Emod
2113Emod
n0
NF12
cos sin2
Emod
n03-18
Here N is the number density of hyperpolarizable groups is the angle be-tween the permanent dipole 0 of the molecule (z direction) and the directionof the poling field (Z direction) and F is a local field factor Commonlycos3 is larger than 05 cos sin2
Therefore the most efficient EO mod-
ulation is achieved if r33 is used rather than r13 [17]
33Characterization techniques
331Second-order phenomena
3311 Determination of the hyperpolarizability
Commonly two methods are employed to determine the hyperpolarizability (1)electric field-induced second harmonic generation EFISH and (2) hyper-Raleighscattering HRS HRS is applicable to both nonpolar and polar molecules as wellas ions but EFISH applies only to polar non-ionic molecules While in the EFISHmethod only the component of parallel to the dipole moment is measured HRSyields several of the tensor components In the case of EFISH one measures I2the intensity of light at frequency 2 emitted from a solution of the sample that issubmitted to an external electric field E0 and simultaneously irradiated with laserlight of frequency Provided that the external electric field is applied along the Z-axis in the laboratory frame and the laser light is polarized along the same axis themacroscopic polarization P(2) induced in the solution by the electric field of theincident laser wave E is given by Eq (3-19)
PZ2 3ZZZZE0EZEZ 3-19
Here 3ZZZZ is the macroscopic third-order susceptibility which is related to thefirst and second molecular hyperpolarizabilities and by Eq (3-20) [20]
33 Characterization techniques 79
3ZZZZ NF2F2
F0 z
5kT
3-20
N is the number density of chromophoric groups F2 F and F0 are local fieldfactors at frequencies 2 and zero is the ground-state dipole momentand z is the vectorial component of along the ground-state dipole momenttaken to be oriented along the z-axis in the molecular frameworkz zxx zyy zzz In the case of -conjugated chromophores is negligi-bly small in comparison with z5kT Therefore according to Eq (3-20) theproduct z is directly available from
3ZZZZ obtained by measuring the intensity
I2 of the second harmonic generated by sample solutions I2 is proportionalto
3ZZZZ [see Eq (3-21)]
I2 3ZZZZI2
E20 3-21
Commonly the evaluation of the susceptibility 3ZZZZ is related to a reference
standard A detailed description of both experimental techniques and data evalu-ation is given in the article by Singer et al [20]
In contrast to the EFISH method the hyperpolarizability can be measureddirectly by means of the HRS method developed by Clays and Persoons [21 22]This method involves measuring the intensity of the incoherently scattered fre-quency-doubled light from isotropic solutions As shown in Fig 32 an infraredlaser beam is focused on the center of a cell containing a solution of the NLO-active compound
3 Electro-optic and nonlinear optical phenomena80
Fig 32 Schematic depiction of a set-up for measuringsecond-order hyperpolarizability by means of the hyper-Rayleigh scattering method
The intensity of the scattered light I2 is proportional to the square of theintensity of the incident light I as given by Eq (3-22)
I2 g N1 2IJKsolvent
N2 2
IJKsolute
I2 3-22
Here g is a set-up dependent factor N1 and N2 are the number densities of solventand solute molecules respectively and 2
IJK
is the mean value of the square of
hyperpolarizability tensor components in the laboratory framework [23] It mustbe noted that the HRS process is extremely inefficient Typically the number ofscattered photons is 10ndash14 times the number of incident photons [20] In principlea low output intensity would be expected for an isotropic solution where the fieldsemitted from the individual NLO molecules interfere destructively That a measur-able amount of incoherently scattered harmonic light can be generated may be ra-tionalized by assuming that fluctuations in orientation can produce regions ofalignment [22] The rather low intensity of the scattered light requires the applica-tion of powerful lasers such as an Nd-YAG system producing 1064 nm lightpulses in conjunction with a sampling technique involving more than 100 pulses
3312 Determination of the susceptibility (2)
Several techniques have been developed for determining the second-order suscep-tibility (2) [24] Of practical importance are methods that may be employed foraligned polymeric systems containing polar moieties [4 8] Methods makinguse of the Pockels or linear electro-optic (EO) effect are based on the measurementof the variation in the refractive index of thin polymer films induced by an externalelectric field In this way values of the electro-optic coefficients r33 and r13 are ob-tained which are related to the corresponding (2) values through Eq (316)
A quite direct method for measuring (2) is based on second harmonic gen-eration SHG Figure 33 depicts a typical set-up used to determine the SHGcoefficients d31 and d33 defined as d =(2)2 by way of SHG measurements
A polarized laser beam of frequency passes through the polymer sampleand an IR-blocking filter The SHG signal is selected by means of an interfer-ence filter operating at the frequency 2 and is measured using a photomulti-plier tube connected to a boxcar integrator The intensity I2 is proportional tothe square of the SHG coefficient d and to the square of the intensity of thefundamental laser beam [see Eqs (3-23) and (3-24)] [8]
I2 Kd2I2 3-23
K 512t4T2t2
0p2 sin2 An2
n22
3-24
Here A is the area of the laser beam is the incident angle t0 t and T2
are transmission factors p is a projection factor () is an angular factor re-
33 Characterization techniques 81
lated to the sample thickness the fundamental wavelength and the refractionangles and n and n2 are the refractive indices of the sample at and 2The coefficient d of the polymer is obtained by comparing the I2 value withthat measured for a standard reference sample commonly Y-cut quartz withd11 = 049 pm Vndash1 at = 1064 m
332Third-order phenomena
Several measuring techniques giving evidence of third-order nonlinear behaviorare listed in Table 31 [26 27]
It is difficult to compare the third-order susceptibilities of systems examinedusing different measuring techniques Since they are based on fundamentallydifferent origins they do not yield identical (3) values Different nonlinearmechanisms contribute in a specific manner to (3) and values measured forthe same material by different techniques may differ by several orders of mag-nitude This applies for instance to the case of the combined resonant andnon-resonant interaction of light with matter A full expression of (3) reflectsnon-resonant and resonant contributions [see Eq (3-25)]
3 3nonresonant
3resonant 3-25
Resonance occurs at wavelengths around that of the absorption band Moreoverthe strong frequency (wavelength) dependence of (3) and the influence of repe-tition frequency and pulse duration of the laser on (3) have to be taken into ac-count It is beyond the scope of this book to describe the various measuring
3 Electro-optic and nonlinear optical phenomena82
Fig 33 Schematic depiction of a set-up for measuring secondharmonic generation (SHG) BS beam splitter PDphotodiode PMT photomultiplier tube Adapted fromJerphagnon et al [25] with permission from the AmericanInstitute of Physics
techniques However some of the most widely used methods are briefly consid-ered below with the additional aim of providing some insight into the fascinat-ing field of third-order nonlinear effects
3321 Third harmonic generationThe term third harmonic generation THG refers to the generation of a lightbeam that consists of photons having three times the energy of the photons ofthe input beam THG can be easily detected and is therefore widely employedin the third-order nonlinear characterization of newly developed materials [28]THG is a four-photon process in which three incident photons with angularfrequency create a photon with frequency 3 The off-resonant THG processcan be represented by a transition between virtual excited states as shown bythe dashed lines in Fig 34
In the case of THG the third-order susceptibility corresponds to a nonlinearpolarization component which oscillates at the third harmonic frequency of theincident laser beam Regarding the simplified case of an isotropic solution onlythe element
3XXXX3 of the third-order susceptibility tensor creates
a polarization at 3 which is parallel to the incident electrical field E as-sumed to be parallel to the X-axis [see Eq (3-26)]
P3 143XXXX3E3
3-26
For THG measurements pulsed laser systems operating at infrared wavelengths(typically 1064 1850 1907 or 2100 nm) are used Most commonly 3XXXX is ob-tained by relating the third-harmonic signal of the sample to that measured si-
33 Characterization techniques 83
Table 31 Measuring techniques for third-order susceptibilities
Method Acronym Denotation of process
Third harmonic generation THG (3) (3)Z-scan (3) (ndash ndash)Two-photon absorption TPA (3) (ndash ndash)Degenerate four-wave mixing DFWM (3) (ndash ndash)Electric field-induced second harmonic generation EFISH (3) (ndash2 0)Optical Kerr gate OKG (3) (ndashndash)
Fig 34 Energy level diagram illustrating third harmonic generationArrows denote photon energies horizontal solid lines represent energystates of the medium and dashed lines represent virtual excited states
multaneously with a fused silica plate serving as a reference The incident beamis focused on the sample in a vacuum chamber and a water filter removes thefundamental frequency from the output beam which is further attenuated sothat it lies within the linear range of the photomultiplier
3322 Self-focusingdefocusingThin polymer sheets allowing unhindered passage of a low-intensity light beamof a given non-resonant wavelength can act as lenses if a high-intensity beam ispassed through them This is a consequence of the intensity dependence of therefractive index n [see Eq (3-27)]
n n0 n2I 3-27
Here n0 denotes the linear refractive index (at low intensity I) and n2 is thenonlinear refractive index which can be measured by means of a Z-scan experi-ment [29 30] A typical set-up is schematically depicted in Fig 35 a
The incoming beam is split into two equal parts one part is guided to the de-tector D1 while the other is passed through the sample and an aperture priorto reaching the detector D2 Provided that the sample is nonlinearly active thephenomena outlined below will be observed if the sample is moved through thefocused laser beam along the optical axis Thus the transmission through theaperture is reduced if the sample is moved to the left of the original focus z0 be-cause the beam is defocused On the other hand if the sample is placed to the
3 Electro-optic and nonlinear optical phenomena84
Fig 35 (a) Schematic depiction of the Z-scan experimentBS beam splitter (b) Typical Z-scan curves for n2 gt 0 andn2 lt 0 Adapted from Gubler et al [30] with permission fromSpringer
right of z0 the beam is focused on the aperture and the transmission through it isincreased This applies in the case of n2 gt 0 The opposite behavior is observed ifn2 lt 0 Both cases are shown schematically in Fig 35b in which the signal ratioD2D1 is plotted against the distance z The nonlinear refractive index n2 can beobtained from the z-scan in the following way Tpv the difference in the transmit-tance between peak and valley is proportional to the phase distortion 130 accord-ing to the empirical relationship Tpv = k 130 where k is a constant determined bythe lay-out of the apparatus With 130 = (2)n2I0L one obtains Eq (3-28) [29]
n2 Tpv
2kI0L3-28
Here I0 and L denote the light intensity and the thickness of the sample re-spectively The third-order susceptibility (3) can then be obtained by usingEq (3-29) [26]
n2 122
cn03 3-29
This applies when esu units are used for both n2 and (3) It is interesting tonote that the set-up shown in Fig 35 a can also be used to determine the two-photon absorption coefficient 2 In this case the Z-scan experiment is per-formed without the aperture
3323 Two-photon absorption (TPA)The simultaneous absorption of two photons of equal energy can occur if a la-ser beam (ps or fs pulses) is focused within a material [31 32] The process de-picted schematically in Fig 36 is related to the excitation of a molecule to anenergy level h1 = 2 h2 by the simultaneous absorption of two photons of en-ergy h2 (=2)
Two-photon absorption is possible provided that both photons are spatiallyand temporally coincident It occurs with a probability proportional to thesquare of the light intensity
TPA can be measured by the transmission method or by the Z-scan techniqueMoreover two-photon fluorescence can serve to measure TPA absorption cross-sections provided that a fluorescent excited state is reached by TPA In nonlinear
33 Characterization techniques 85
Fig 36 Energy level diagram depictingsingle-photon and two-photonabsorptions
transmission experiments the transmission of the sample Tr is measured as afunction of the input intensity I0 At high incident intensities TPA is proportionalto I2
0 and there is a linear relationship between 1Tr and I0 [see Eq (3-30)]
1Tr
I0
I 1 2I0L 3-30
Here L is the sample thickness and 2 is the absorption coefficient for the puretwo-photon absorption process
3324 Degenerate four-wave mixing (DFWM) and optical phase conjugationDegenerate four-wave mixing (DFWM) is frequently employed to measure (3)
values and response times of polymeric systems The DFWM technique is basedon the interaction between three spatially distinguishable light beams of equalfrequency The interaction results in the generation of a fourth beam of thesame frequency Figure 37 shows the commonly used backward-wave geome-try with three incident beams spatially overlapping in the sample
The pump beams 1 and 3 are counterpropagating The signal beam 4 isemitted in the direction opposite to the probe beam 2 Its intensity depends on(3) and on the intensities of beams 1 2 and 3 according to Eq (3-31) [27]
I4 2
4c2n2 32L2I1I3I2 3-31
Here c n and L denote the velocity of light in vacuo the refractive index of thesample and the pathlength in the medium respectively Equation (3-31) holdsin the case of there being no linear or nonlinear light absorption The retrace-ment of the probe beam is characteristic of the phenomenon of optical phaseconjugation OPC [33] This refers to the property of materials to act as mirrorsand to reflect an incident light beam exactly in phase with its former phase Un-like a conventional mirror whereby rays are redirected according to the ordinarylaw of reflection a phase conjugate mirror also called a phase conjugator retro-reflects all incoming rays back to their origin Figure 38 illustrates the differ-ence between a conventional and a phase conjugate mirror
At a conventional mirror only the wave vector component normal to the surfacechanges sign while the tangential components remain unchanged The propaga-tion direction of the reflected ray depends on the angle between the surface normal
3 Electro-optic and nonlinear optical phenomena86
Fig 37 Degenerating four-wave mixing withcounterpropagating pump beams 1 and 3BS beam stopper
and the incident ray A phase conjugate mirror on the other hand changes the signof the complex wave vector so that the reflected ray is antiparallel to the incidentray Phase conjugation by degenerate four-wave mixing may result in reflectivitiesR = I4I2 exceeding 100 For example using picosecond pulses R = 25 has beenfound for poly(methyl methacrylate) doped with 510ndash4 mol Lndash1 rhodamine 6G[34] For detailed information concerning the DFWM technique and additionaltechniques not dealt with here the reader is referred to the literature [26 27]
34Nonlinear optical materials
341General aspects
Second-order NLO materials Originally second-order nonlinear optics was devel-oped with the aid of inorganic crystals such as lithium niobate LiNbO3 and po-tassium dihydrogen phosphate KH2PO4 (KDP) The nonlinear optical behaviorof these crystals is due to light-induced displacement of the ions in the latticeCertain organic substances having a non-centrosymmetric structure and con-taining delocalized -electrons behave similarly They undergo very fast light-in-duced intramolecular perturbations of their charge distributions In otherwords irradiation with light at non-resonant wavelengths causes an almost in-stantaneous shift in the -electron density over the molecule which accountsfor the large and fast polarization 2-Methyl-4-nitroaniline MNA and 4-di-methylamino-4-nitrostilbene DANS are typical organic compounds exhibitingsecond-order NLO activity (see Chart 31)
These compounds are so-called charge-transfer molecules having the generalstructure shown in Chart 32
Here an electron-donating and an electron-accepting moiety are connected byan extended -electron system In such compounds the electron displacementoccurs on a subpicosecond time scale and can be much more pronounced thanin inorganic crystals Polymeric organic systems are of practical importance
34 Nonlinear optical materials 87
Fig 38 The reflection of a ray of light off an ordinary mirrorand off a phase conjugate mirror
They consist either of polymers containing admixed AD compounds or of poly-mers with AD moieties chemically incorporated into the main chain or in pen-dant groups As pointed out above for an organic material to undergo a signifi-cant change in its dipole moment upon exposure to an intense light beam itneeds to have a non-centrosymmetric molecular structure This requirementalso pertains to the macroscopic level In other words both a large hyperpolariz-ability of the molecular constituents and a large macroscopic susceptibility (2)
are required Macroscopic non-centrosymmetry can be attained by aligning theassemblies so that the individual tensor components of add constructively
Third-order NLO materials Unlike for second-order NLO activities there areno molecular symmetry restrictions for the third-order nonlinear response ofmaterials In principle all materials including air are capable of exhibitingthird-order NLO activity Generally for most centrosymmetric compounds thehyperpolarizability is very small This does not apply however for organic -conjugated compounds It is the almost instantaneous shift in -electron densityover the whole molecule or extended parts of it that occurs upon irradiationwhich accounts for the large susceptibilities (3) of conjugated compounds Asregards the field of macromolecules -conjugated polymers such as polyacetyl-enes or polydiacetylenes (see Chart 33) exhibit pronounced third-order NLO ac-tivities (3) values of non-conjugated polymers such as poly(methyl methacry-late) are several orders of magnitude lower than those of conjugated polymers
3 Electro-optic and nonlinear optical phenomena88
Chart 31 Chemical structures of 2-methyl-4-nitroanilineMNA and 4-dimethylamino-4-nitrostilbene DANS
ACCEPTOR mdashndash[-conjugated system]mdashndash DONORChart 32 General structure of charge-transfer molecules (AD molecules)
Chart 33 Polymers exhibiting third-order NLO activitiesR R1 and R2 denote aliphatic or aromatic groups
Interestingly -conjugated polymers such as polysilanes (see Chart 33) also ex-hibit remarkably large third-order susceptibilities (3)
342Second-order NLO materials
3421 Guest-host systems and NLO polymersFundamentally there are two categories of second-order NLO polymeric sys-tems commonly also referred to as electro-optically active polymeric systems [435] (1) guest-host systems consisting of rigid solutions of small AD com-pounds in polymeric matrices and (2) systems consisting of polymers withAD moieties incorporated into either the main chain or side groups [36] Inthe latter case the rigidity of the polymeric matrix can be improved by chemicalcrosslinking General structures of such polymers are depicted in Fig 39
In this context research concerning non-centrosymmetric structures with su-pramolecular helical organization is interesting In the case of thermally stable(up to 400 C) polyesters containing -conjugated donor-acceptor segments (seeChart 34) the hyperpolarizability values turned out to be much larger thanthose of the respective monomeric chromophores
At a chiral unit content of 50 the second harmonic generation (SHG) effi-ciency of the polymer (at = 532 nm) is 48 times that of the monomer and isequal to 20710ndash30 esu This enhancement may be rationalized in terms of thedirectional orientation of dipole segments in the polymer as a consequence ofthe chiral organization of the polymer chains [37]
Typical low molar mass AD compounds and polymers containing AD moi-eties are listed in Table 32 [38] and Table 33 [39 40] In this context it is no-ticeable that electro-optically active compounds have been tabulated [7]
34 Nonlinear optical materials 89
Fig 39 Schematic depiction of the structures of polymeric matrices containing AD moieties
3 Electro-optic and nonlinear optical phenomena90
Chart 34 Chemical structures of an electro-optically activepolyester and a chemically related monomer
Table 32 Characteristics of electro-optically activechromophores determined in chloroform solutionAdapted from Swalen and Moylan [38]
Denotation Structure maxa)
(nm) b)
(Debye)0
c)
(10ndash30 esu)
I 438 67 813
II 494 80 952
III 602 71 259
IV 698 104 359
V 680 83 479
a) Wavelength of maximumb) Dipole momentc) Off-resonance hyperpolarizability
At present various compounds are commercially available [41] Typical exam-ples are given in Table 34
Second-order NLO polymers have potential for technical applications (see Sec-tion 35 below) for example in electro-optic modulation and switching or fre-quency doubling A large body of compounds has hitherto been explored andat present relevant research is mainly focused on optimizing secondary proper-ties such as thermal stability adhesion thermal expansion etc
34 Nonlinear optical materials 91
Table 33 Characteristics of electro-optically active poled polymer filmsAdapted from Bertram et al [39] and Lipscomb et al [40]
Chemical structure Acronym faca) TPol
b)
(C) c)
(m)r33
d)
(pm Vndash1)
Ber-1 100 155 42
3RDCYXY 15 mol 140 13 30
GT-P3 62 wt 180 1541 12
ROI-4 17 mol 215 13 16
a) Fraction of active compoundb) Poling temperaturec) Wavelengthd) Component of the Pockels coefficient tensor directed parallel to the applied elec-
tric field
3422 Orientation techniquesPractical applications demand optimum alignment of the AD moieties in thesample in a non-centrosymmetric fashion To this end the most commonapproach involves electric field-induced alignment of glassy ie amorphouspolymer films a process commonly referred to as poling Thereby a net orienta-tion of the molecular dipole moments along a polar axis of the macroscopicsample is attained Poling is carried out at a temperature close to the glass tran-sition temperature of the polymer matrix at which the molecules are relativelymobile Electric field-induced alignment can be achieved either by sandwichingthe polymer samples between electrodes which is referred to as electrode pol-ing or by corona poling Figure 310 shows a schematic diagram of a coronapoling set-up with wire-to-plane configuration
A corona discharge is induced upon application of an electric potential of sev-eral kV across the electrodes Ionized molecules from the air are forced by theelectric field to move to the surface of the sample The deposited ions induceimage charges on the earthed electrode Thereby a static electric field of about
3 Electro-optic and nonlinear optical phenomena92
Table 34 Commercially available NLO polymers [41]
Denotation Chemical Structure
Poly[4-(22-dicyanovinyl)-N-bis(hydroxyethyl)aniline-alt-(isophoronediisocyanate)]urethane
Poly4-(22-dicyanovinyl)-N-bis(hydroxyethyl)aniline-alt-[44-methylenebis(phenyl)isocyanate]urethane
Poly[4-(22-dicyanovinyl)-N-bis(hydroxyethyl)aniline-alt-p-phenylenediacrylate]
Poly[1-methoxy-4-(0-disperse red 1)-25-bis(2-methoxyethyl)-benzene]
Poly[1-methoxy-4-(0-disperse red 1)-25-phenylenevinylene]
106 V cmndash1 is generated across the sample which induces alignment of theNLO moieties with respect to the direction of the electric field Poled samplesare represented by Cv symmetry Alternative alignment methods are based onthe Langmuir-Blodgett (LB) and self-assembly techniques both of which are dif-ficult to perform
In the case of polymer systems containing photochromic chromophores egazo groups alignment can be achieved upon exposure to light instead of a staticelectric field This method is referred to as optical poling (see also Section 55)With such systems optimum results have been obtained by applying a com-bined electro-optical poling method As can be seen in Table 33 Pockels coeffi-cients exceeding 10 pm Vndash1 have been measured for appropriate polymers poledby the combined electro-optical method More detailed information concerningthe various alignment techniques can be obtained from review articles [4 8 4344]
343Third-order NLO materials
Table 35 presents a selection of (3) values of various conjugated polymers de-termined by THG measurements while Table 36 shows (3) values of somefull-ladder and semi-ladder polymers determined by means of the DFWM tech-nique
It must be noted that the (3) values reported in the literature vary over broadranges Therefore the values listed here reflect only the general behavior of sev-eral classes of compounds It can be seen in Table 35 that trans-polyacetylenes(PAs) and polydiacetylenes (PDAs) exhibit the largest third-order NLO suscept-ibilities The (3) value of cis-PA (not shown) is more than an order of magni-tude smaller than that of trans-PA Derivatives of poly-p-phenylene poly(phenyl-ene vinylene) and polythiophene also exhibit NLO activity but to a much lesserextent than PAs and PDAs As pointed out above polysilanes also possess quitelarge (3) values This is explained by the -conjugation of the silicon chainwhich implies a pronounced delocalization of -electrons A very large (3) value
34 Nonlinear optical materials 93
Fig 310 Schematic diagramshowing a corona poling set-upwith wire-to-plane configurationThe tungsten wire is placedabove and parallel to the sampleAdapted from Eich et al [42] withpermission from the OpticalSociety of America
3 Electro-optic and nonlinear optical phenomena94
Table 35 Third-order susceptibilities (3) obtained by third harmonic generationmeasurements Adapted from Kajzar [28] and Nalwa [45]
Polymer Acronym c) (3)(esu) a) (nm) Remarks
trans-PA 5610ndash9 1907 Isotropic film
trans-PA 2710ndash8 1907 Oriented film
PDA-C4UC4 2910ndash10 1907 Oriented film
PDA-CH 1010ndash10 1907
PPV 1410ndash10 1450 Isotropic film
PBT 2910ndash11 1907 Spun film
PTV 3210ndash11 1850
PTT 210ndash11 1907 Isotropic film
PDES 3010ndash9 b) 620H
34 Nonlinear optical materials 95
Table 35 (continued)
Polymer Acronym c) (3)(esu) a) (nm) Remarks
PDHS 110ndash11 1064
PVT 310ndash14 1907
a) Fundamental wavelengthb) Determined by the DFWM methodc) Abbreviations trans-PA trans-polyacetylene PDA-C4UC4 poly[57-dodecadiyne-
112-diol-bis(n-butoxycarbonyl methylurethane)] PDA-CH poly[16-di-(N-carba-zoyl)-24-hexadiyne] PPV poly(p-phenylene vinylene) PBT poly(3-butylthio-phene) PTV poly(25-thienylene vinylene) PTT poly(thieno-32-bithiophene)PDES poly(diethynylsilane) PDHS poly(di-n-hexylsilane) PVT poly(vinyl-toluene)
Table 36 Third-order susceptibilities (3) obtained by theDFWM method Adapted from Wijekoon et al [46]
Polymer Acronym (3) (esu) a) (nm)
PBT 1010ndash10 602
PBO 1010ndash10 602
LARC-TPI 2010ndash12 602
BBL 1510ndash11 1064
BBB 5510ndash12 1064
a) Fundamental wavelength
(310ndash9 esu) has been found for poly(diethynylsilane) PDES In this case a re-sponse time of 135 fs was measured [47] Compared with those of conjugatedpolymers the (3) values of non-conjugated polymers are very low For example(3) values of 40 and 3410ndash14 esu have been measured for poly(methyl meth-acrylate) and poly(vinyltoluene) respectively As regards the polymers listed inTable 36 it is notable that some of them for instance BBL and BBB are solu-ble and film-forming in spite of their quasi-two-dimensional structures Forpractical applications materials with large (3) values low optical losses andultrafast response times tresp are desired Ideal targets set for device applicationsare (3) 10ndash7 esu 102 cmndash1 and tresp1 ps Therefore appropriate materialsshould possess a figure of merit (3) of 10ndash9 esu cm Although most polymer-ic materials exhibit much lower (3) values various promising devices havebeen proposed and fabricated [45] For detailed information concerning third-or-der NLO properties of polymers and other compounds the reader is referred tothe literature [28 45 46]
35Applications of NLO polymers
The application potential of the effects dealt with in this chapter covers a broadfield extending from specific electro-optical devices to the all-optical computerFor many applications polymeric materials have proven appropriate and equiva-lent to inorganic materials This section is focused on two aspects the electro-optical (EO) or Pockels effect and two-photon absorption which have beenexploited extensively Technical developments relating to polymeric modulatorsoperating on the basis of the Pockels effect have reached the stage of commer-cialization [5]
351Applications relating to telecommunications
With the advent of optical fibers in telecommunications in the late 1970s practi-cal applications for nonlinear optical devices operating on the basis of the EOeffect became a serious goal Besides inorganic materials which were used ex-clusively in the early days more recently polymeric electro-optic materials havealso found use in a variety of device configurations They can function as tun-able Bragg wavelength filters ultra-high bandwidth signal modulators for tele-communications fast modulators for optical 3D sensing electrical-to-optical sig-nal transducers switches at nodes in optical networks and controllers of thephase of radiofrequency optical signals etc [5] Typical configurations theMach-Zehnder (MZ) interferometer and the birefringent modulator are depictedschematically in Fig 311
In the case of the MZ interferometer (Fig 311 a) application of an electricfield to one arm results in a phase retardation relative to the signal traversing
3 Electro-optic and nonlinear optical phenomena96
the second arm and in destructive interference at the output The phase retarda-tion of light traversing the material of optical path length L under the in-fluence of an electric field E is proportional to n the change in the index of re-fraction [see Eq (3-32)]
2nL
n3ErL
3-32
As a consequence of the voltage-controlled destructive interference the appliedelectrical signal is transduced onto the optical beam as an amplitude modula-tion The birefringent modulator depicted in Fig 311 b functions as an electri-cal-to-optical signal transducer Here both TM and TE optical modes traverse
35 Applications of NLO polymers 97
Fig 311 Electro-optic device configurations (a) Mach-Zehnder interferometer (b) birefringent modulator TM andTE denote transverse magnetic and transverse electricpolarization respectively
the EO material The application of an electric field produces a voltage-depen-dent birefringence which is turned into amplitude modulation with the aid of apolarizer positioned at the output of the device
The drive voltage VD required to achieve full-wave modulation is inverselyproportional to the EO coefficient of the material Since drive voltages of the or-der of 1 V or less are required for lossless communication links materials withlarge EO coefficients are desirable VD depends on the device configuration Forexample VD for the birefringent modulator exceeds that for the MZ-type modu-lator by a factor of 15 [5] It should be noted that the change in the refractiveindex (n = 05 n3rE) is rather small For example if n3 = 5 r = 510ndash12 m Vndash1and E = 106 V mndash1 n is equal to 12510ndash5
Very successful efforts in employing polymeric materials as modulators havebeen made with the guesthost systems shown in Table 37 The guest com-pounds are characterized by the cyanofuran moiety A thermally rather stablehost matrix denoted as APC is a copolymer poly[bisphenol A carbonate-co-44-(335-trimethylcyclohexylidene)diphenol] The systems shown in Table 37 areemployed in commercially available modulators the relevant industrial compa-nies are cited in Daltonrsquos review article [5] These polymeric systems are
3 Electro-optic and nonlinear optical phenomena98
Table 37 Characteristics of electro-optically activechromophores in a PMMA matrix Adapted from Dalton [5]
Denotation Chemical Structure a) (Debye) r b) (pm Vndash1)
FTC 1219 50
CLD 1342 70
GLD 1388 105
a) Dipole moment obtained by quantum mechanicalcalculation
b) Pockels coefficient at a number density of about151020 molecules cmndash3 measured at =13 m
superior to lithium niobate with respect to various important properties as canbe seen in Table 38
Pockels coefficients measured at the technologically important wavelengths13 and 155 m are higher than in the case of lithium niobate Moreover thedifference in the dielectric constants is important = 28 (LiNbO3) and = 25ndash4(EO polymer) The lower value corresponds to a decreased device power con-sumption and an enhanced speed of operation
352Applications relating to optical data storage
Potential applications of polymeric materials with large (3) values concernphotonic devices in various fields such as optical fiber communication opticalcomputing imaging dynamic holography optical switching and optical datastorage Two-photon absorption a third-order nonlinear effect (see Section3323) has gained importance for optical data storage [48] Two-photon absorp-tion is possible provided that both photons are spatially and temporally coinci-dent As this requirement has to be fulfilled optical sectioning can be accom-plished ie absorption events can be directed to selected layers In other wordsinformation can be recorded in previously defined layers of a film and therebythree-dimensional bit optical data storage within the volume of a recording me-dium is possible Photochemical free radical polymerization (see Section 102)can be employed to achieve optical data storage at a density as high as04 Tb cmndash3 with a bit spacing of 1 m and a layer spacing of 3 m [49 50] Forthis technique a recording medium consisting of a monomer solution contain-ing a photoinitiator is typically used Since the initiation is restricted to two-photon absorption the polymerization is confined to the region of the focusspot To prevent distortion of the recorded planes through shrinkage or flow ge-lation of the system by UV pre-irradiation is carried out Polymerization at therecorded bit changes the refractive index The pattern of recorded bits can thus
35 Applications of NLO polymers 99
Table 38 Comparison of lithium niobate and polymeric EOmaterials Adapted from Dalton [5]
Property LiNbO3 EO Polymer
Pockels coefficient r (pm Vndash1) at = 13 m 31 gt 70Dielectric constant 28 25ndash4Refractive index n 22 16ndash17Figure of merit (n3r) 12 gt 100Optical loss (dB cmndash1) at = 13 m 02 02ndash11Maximum optical power (mW) 250 250Bandwidth length product a) f L (GHz cm) 10 gt 100
a) f Bandwidth in a device of Mach-Zehnder configurationL Interaction length of light with the modulating electricfield
be read by producing a phaseintensity map by means of differential interfer-ence contrast microscopy [51]
353Additional applications
Additional potential applications based on other nonlinear phenomena such assecond harmonic generation (frequency doubling of laser light) phase conjuga-tion and optical bistability may be envisaged Phase conjugation (see Sec-tion 3324) allows the distortionless transmission of images because upon re-tracement the beam reflected from a phase conjugator corrects every distortionof the probe beam Optical bistability is the basis for the transphasor the opticaltransistor a device switching light with light without the aid of an electrical cur-rent This can be achieved by focusing two laser beams a strong constant beamand a weak variable probe beam onto the front face of a Fabry-Perot interferom-eter containing a substance having a nonlinear refractive index Since the latterdepends on the light intensity constructive interference sets in at a certain in-tensity of the probe beam and the transmittance increases to a high level asshown in Fig 312 The term bistability refers to the existence of two quasi-stable levels
Another potential application relates to optical limiters ie materials that canbe used for the protection of eyes and sensors from intense light pulses andgenerally for devices that are required to have a high transmittance at low in-tensities and a low transmittance at high intensities [52 53] Appropriate sub-stances contain chromophores that exhibit nonlinear light absorption termedreverse saturable absorption Such chromophores become more strongly absorb-ing as the incident light intensity is increased The nonlinear response may beexhibited when chromophores absorb weakly in the ground state and stronglyin the excited state Optical limiting may also be due to two-photon (or moregenerally multi-photon) absorption (see Section 3323)
3 Electro-optic and nonlinear optical phenomena100
Fig 312 The transmittance behaviorof a transphasor (optical transistor)Plot of the transmitted intensity as afunction of the incident intensity
References 101
References
1 H-H Perkampus Encyclopedia of Spec-troscopy VCH Weinheim (1995)
2 PA Franken LE Hill CW Peters GWeinreich Phys Rev Lett 7 (1961) 118
3 SK Yesodha CKS Pillai N TsutsumiStable Polymeric Materials for NonlinearOptics A Review Based on AzobenzeneSystems Prog Polym Sci 29 (2004) 45
4 F Kajzar K-S Lee AK-Y Jen Polymer-ic Materials and their Orientation Tech-niques for Second-Order Nonlinear OpticsAdv Polym Sci 161 (2003) 1
5 L Dalton Nonlinear Optical PolymericMaterials From Chromophore Design toCommercial Applications Adv Polym Sci158 (2002) 1
6 Z Sekkat W Knoll (eds) PhotoreactiveOrganic Thin Films Academic PressAmsterdam (2002)
7 MG Kuzyk CW Dirk (eds) Character-ization Techniques and Tabulations for Or-ganic Nonlinear Optical Materials MarcelDekker New York (1998)
8 J I Chen S Marturunkakul L Li S TTripathy Second-Order Nonlinear OpticalMaterials in TA Skotheim R L Elsen-baumer J R Reynolds (eds) Handbookof Conducting Polymers 2nd Edition Mar-cel Dekker New York (1998) p 727
9 S Bauer-Gogonea R Gerhard-Multhaupt Nonlinear Optical Electrets inR Gerhard-Multhaupt (ed) Electrets 3rd
Edition Vol 2 Laplacian Press MorganHill CA (1999) p 260
10 HS Nalwa S Miyata (eds) NonlinearOptics of Organic Molecules and PolymersCRC Press Boca Raton FL USA (1997)
11 DM Burland R D Miller C A WalshSecond-Order Nonlinearity in Poled Poly-mer Systems Chem Rev 94 (1994) 31
12 NP Prasad D J Williams Introductionto Nonlinear Optical Effects in Moleculesand Polymers Wiley New York (1991)
13 BS Wherrett in C Flytzanis J L Ou-dar (eds) Nonlinear Optics Materials andDevices Springer Berlin (1986)
14 M Canva G I Stegeman QuadraticParametric Interactions in Organic Wave-guides Adv Polym Sci 158 (2002) 87
15 B Kippelen N Peyghambarian Photore-fractive Polymers and their Applications
Springer Berlin Adv Polym Sci 161(2003) 87
16 DA Kleinmann Phys Rev 126 (1962)1977
17 G R Moumlhlmann C P J M van derVorst R A Huijts CT J WreesmannProc SPIE 971 (1988) 252
18 E Cavicchi J Kumar S Tripathy Non-linear Optical Spectroscopy of Polymers inH Baumlssler (ed) Optical Techniques toCharacterize Polymer Systems ElsevierAmsterdam (1989) p 325
19 CP J M van der Vorst D J PickenElectric Field Poling of Nonlinear OpticalSide Chain Polymers in VP Shibaev(ed) Polymers as Electrooptical and Photo-optical Active Media Springer Berlin(1996)
20 K D Singer SF Hubbard A SchoberLM Hayden K Johnson Second Har-monic Generation in [7] p 311
21 K Clays A Persoons Phys Rev Lett 66(1991) 2980 Rev Sci Instrum 63 (1992)3285
22 K Clays A Persoons L De Mayer Mod-ern Linear Optics Part 3 Adv ChemPhys Wiley New York (1993)
23 J A Delaire E Ishov K NakataniPhotoassisted Poling and Photoswitching ofNLO Properties of Spiropyrans and otherPhotochromic Molecules in Polymers andCrystals in Z Sekkat W Knoll (eds)Photoreactive Organic Thin Films Aca-demic Press Amsterdam (2002)
24 T Watanabe HS Nalwa S MiyataMeasurement Techniques for Refractive In-dex and Second-Order Optical Nonlineari-ties Chapter 3 in [10]
25 J Jerphagnon SK Kurtz J Appl Phys41 (1970) 1667
26 HS Nalwa Measurement Techniques forThird-Order Optical Nonlinearities Chap-ter 10 in [10]
27 J L Bredas C Adant P Tackx A Per-soons Third-Order Optical Response inOrganic Materials Theoretical and Experi-mental Aspects Chem Rev 94 (1994)243
28 F Kajzar Third Harmonic Generation in[7]
3 Electro-optic and nonlinear optical phenomena102
29 EW Van Stryland M Sheik-Bahae Z-Scan Chapter 8 in [7]
30 U Gubler C Bosshard Molecular Designfor Third-Order Optics Adv Polym Sci158 (2002) 125
31 T-C Lin S-J Chung K-S Kim XWang G S He J Swiatkiewicz HEPudavar P N Prasal Organics and Poly-mers with High Two-Photon Activities andtheir Applications Springer Berlin AdvPolym Sci 161 (2003) 157
32 S Kershaw Two-Photon AbsorptionChapter 7 in [7]
33 M Gower D Proch (eds) Optical PhaseConjugation Springer Berlin (1994)
34 K Abe M Amano T Omatsu OpticsExpress 12 (2004) 1243
35 HS Nalwa T Watanabe S Miyata Or-ganic Materials for Second-Order NonlinearOptics Chapter 4 in [10]
36 N Pereda J Extebarria CL Focia JOrtega C Artal MR Ros J C SeranoJ Appl Phys 87 (2000) 217
37 B Philip K Sreekumar J Polym SciPart A Polym Chem 40 (2002) 2868
38 J D Swalen CR Moylan Linear OpticalProperties Chapter 4 in [7]
39 R P Bertram E Soergel H Blank NBenter K Buse R Hagen SG Kostro-mine J Appl Phys 94 (2003) 6208
40 G F Lipscomb J I Thackara R LytelElectro-Optic Effect in [7]
41 Aldrich ChemFiles 4 (2004) 442 M Eich H Looser D Yoon R Twieg
G Bjorklund J Baumert J Opt SocAm B 6 (1989) 1590
43 F Kajzar J M Nunzi Molecular Orienta-tion Techniques in F Kajzar R Reinisch(eds) Beam Shaping Control with Non-
linear Optics Plenum Press New York(1998) p 101
44 S Bauer Appl Phys Rev 80 (1996)5531
45 HS Nalwa Organic Materials for Third-Order Nonlinear Optics Chapter 11 in[10]
46 W MK P Wijekoon PN Prasad Non-linear Optical Properties of Polymers inJ E Mark (ed) Physical Properties ofPolymers Handbook AIP Press Wood-bury NY (1995) Chapter 38
47 K S Wong S G Han ZV Vardeny JShinar Y Pang I Maghsoodi T J Bar-ton S Grigoras B Parbhoo Appl PhysLett 58 (1991) 1695
48 P Boffi D Piccinin MC Ubaldi (eds)Infrared Holography for Optical Communi-cations Techniques Materials and DevicesTopics in Applied Physics 86 SpringerBerlin (2003)
49 BH Cumpton S P Ananthavel S Bar-low D Dyer J E Ehrlich LL ErskineA A Heikal SM Kuebler IY S LeeD McCord-Maughon J Qin H RoumlckelM Rumi XL Wu S R Marder JWPerry Nature 398 (1999) 51
50 HB Sun S Matsuo H Misawa ApplPhys Lett 74 (1999) 786
51 D Day M Gu A Smallridge Review ofOptical Data Storage in [48] p 1
52 J W Perry Organic and Metal-ContainingReverse Saturable Absorbers for OpticalLimiters Chapter 13 in [10]
53 EW Van Stryland D J Hagan T XiaA A Said Application of Nonlinear Opticsto Passive Optical Limiting Chapter 14 in[10]
41The photorefractive effect
The photorefractive (PR) effect refers to the spatial modulation of the index ofrefraction in an electro-optically active material that is non-uniformly irradiatedNotably the refractive index of an electro-optically active material is electric fielddependent The PR effect is based on the light-induced generation and subse-quent migration of charge carriers and therefore is strongly connected to thephenomena of photogeneration and conduction of charge carriers in polymericsystems dealt with in Chapter 2 The PR effect was first observed in inorganicmaterials such as LiNbO3 BaTiO3 InP Fe and GaAs [1ndash9] and later also in or-ganic materials Work related to polymers has been reviewed [10ndash12] Materialsexhibiting the PR effect should be capable of forming charge carriers ie pairsof positively and negatively charged ions in a sufficiently high quantum yieldupon exposure to light and these charge carriers should migrate with a suffi-ciently high mobility A prerequisite for the occurrence of the PR effect is sepa-ration of the charges which is commonly accomplished if only one type ofcharge carrier is mobile and the material contains traps where the migratingcarriers are captured A non-uniform irradiation of polymeric materials can beaccomplished by placing foils in the interference region of two coherent lightwaves In this way a fringe pattern of brighter and darker regions ie ofstrongly and weakly or not at all irradiated regions is produced Notably thecharge separation due to the exclusive migration of charge carriers of the samesign from the irradiated to the non-irradiated regions results in the build-up ofa space-charge field ie of an internal electric field between the irradiated andunirradiated regions which allows the linear electro-optic effect (Pockels effectsee Section 31) to become operative In other words the formation of thespace-charge field gives rise to a change in the refractive index and in this waya refractive index fringe pattern is generated The magnitude of the refractiveindex modulation n frequently also referred to as the dynamic range dependson the space-charge field strength ESC according to Eq (4-1)
n n3reESC
24-1
103
4Photorefractivity
Here re is the electro-optic (or Pockels) coefficient for a given geometry and nis the refractive index
Commonly holes are the mobile charge carriers in photorefractive polymersSince the migration of holes by diffusion is a rather slow process a drift is en-forced by the application of an external electric field The latter not only pro-motes hole migration but also provides essential assistance during the photo-
4 Photorefractivity104
Fig 41 The photorefractive effect One-dimensional illustration of the chargegeneration by non-uniform irradiation of apolymer film and the subsequent generationof a refractive index grating through
transport and trapping of the mobile holesAdapted from Valley and Klein [13] andMoerner and Silence [12] with permissionfrom the American Chemical Society
Fig 42 Schematic depiction of the experimental geometryemployed for writing a refractive index grating in a PRpolymer Adapted from Moerner and Silence [12] withpermission from the American Chemical Society
generation process (see Section 22) Significantly there is a phase shift betweenthe irradiation pattern and the refractive index pattern as can be seen inFig 41 which illustrates the mechanism of grating formation
A schematic depiction of the formation of a grating in a polymer film locatedin an external electric field is shown in Fig 42
The grating is written by beams 1 and 2 which enter the film at angles of in-cidence 1 and 2 with respect to the sample normal The grating is written at awave vector KG at an angle with respect to the external electric field E0 Thespatial periodicity G of the grating is given by Eq (4-2)
G 0
2n sin131 22 4-2
Here n is the refractive index and 0 is the wavelength of the light in vacuo
42Photorefractive formulations
An organic photorefractive system has to contain different functional groupsproviding for the generation transport and trapping of charge carriers More-over a plasticizing function is required for certain formulations Apart from thelatter these requirements may in principle be met by fully functionalized poly-mers ie by polymers containing in their main chain and side chains the var-ious requisite functional groups However since this approach is rather difficultto implement research activities have concentrated mostly on the so-calledhostguest approach which is based on formulations consisting of a host poly-mer and various low molar mass guest compounds Typical polymers and lowmolar mass compounds used for formulations exhibiting a photorefractive effectare shown in Chart 41 and Chart 42 respectively
The system PMMA-PNA DEHTNF is a typical photorefractive formulationwith PMMA-PNA acting as the host polymer and DEH (30 wt) and TNF(01 wt) as charge-transporting agent and charge-generating sensitizer respec-tively In order to ensure bulk transport of the photogenerated holes by the hop-ping mechanism the concentration of the transporting agent has to be ratherhigh Typical examples of fully functionalized polymers are also presented inChart 41 (polymers VI [14] and VII [15]) In the case of polymer VI photoexcita-tion of the chromophores MHB+Brndash at = 647 nm induces electron transfer fromthe aromatic amino groups (Am) according to reaction (b) in Scheme 41 In thisway trapped electrons MHBBrndash and mobile radical cations Am+ are formedThe hole transport according to reaction (c) is a multiple successive electron-hop-ping process from neutral Am groups to neighboring radical cations
Polymer VII belongs to a group of conjugated polymers containing porphyrinor phthalocyanine complexes synthesized by Lu et al [16] Here the polymerbackbone consists of phenylene vinylene moieties which facilitate hole trans-
42 Photorefractive formulations 105
port through intramolecular migration and interchain hopping Charge carriersare formed as a result of the selective absorption of near-infrared light (eg He-Ne laser light = 6328 nm) by the porphyrin or phthalocyanine complexes andtrapping might occur at the side groups
4 Photorefractivity106
Chart 41 Polymers employed in photorefractive formulations
43Orientational photorefractivity
During the development of new photorefractive materials the employment ofchromophoric compounds with a permanent dipole moment turned out to leadto unexpectedly high n values provided that the glass transition temperatureof the formulation was close to ambient temperature such that the chromo-phores were mobile and could become oriented under the influence of an elec-tric field a process referred to as poling Poling-induced orientation of the chro-mophoric molecules leads to macroscopic electro-optical properties and espe-cially to birefringence Notably the total effective electric field in a photorefrac-tivity experiment results from a superposition of the internal space-charge fieldand the externally applied electric field Consequently the spatial refractive in-dex modulation is controlled not only by the space-charge field but also by astrong contribution from the orientational birefringence a fact referred to bythe term orientational photorefractivity Notably in this case the refractive indexchange has a quadratic dependence on the total electric field which is a super-position of the internal space-charge field and the externally applied field andto a rough approximation the dependence of the dynamic range n on the fieldstrength E is given by Eq (4-3)
n pE2 pV2
d2 4-3
43 Orientational photorefractivity 107
Chart 42 Low molar mass compounds employed in photorefractive formulations
MHBBr h MHBBr aMHBBr Am MHBBr Am b
Am Am Am Am etc cScheme 41 Generation and transport of charge carriers in polymer VI
Here p is a material parameter V is the applied voltage and d is the samplethickness
DMNPAA and DHADC-MPN (see Chart 42) are typical optically anisotropiccompounds with permanent dipole moments which can be oriented in an elec-tric field at room temperature in formulations plasticized with ECZ and there-fore have low Tg values Typical values reported in the literature arep = 86 cm2 Vndash2 for the system DMNPAA PVK ECZ TNF and p = 333 cm2 Vndash2 forthe system DHADC-MPNPVK ECZTNFDM [10]
44Characterization of PR materials
Commonly the PR properties of materials are characterized and tested by two-beam coupling and four-wave mixing experiments Two-beam coupling (2BC) re-fers to the energy exchange between the two interfering laser beams employedto write the grating During the formation of the grating the two writing beamsdiffract from the forming grating ie each writing beam is partially diffractedin the direction of the other beam by the forming grating In a 2BC experimentthe change in the transmitted intensity of either of the write beams is recordedas the other write beam is switched on and the grating is formed This can beseen in Fig 43 which shows beam intensity as a function of time as recordedin two experiments in which the intensities of the two writing beams (beforethe sample) were kept equal [14]
4 Photorefractivity108
Fig 43 Two-beam coupling experimentsyielding evidence for the occurrence of thePR effect in a film consisting of polymer VI(MHB+Brndash) The intensity of beam 1 wasmonitored as beam 2 was switched on att= 0 and switched off at t= 90 s and the
intensity of beam 2 was monitored as beam1 was switched on at t= 0 and switched offat t = 90 s = 647 nm E = 26 V mndash1 andd= 194 m I0 (1)= I0 (2)= 78 mW cmndash2Adapted from Vannikov et al [14] withpermission from Elsevier
In the first experiment in which beam 2 was switched on and off and beam1 was monitored the intensity of the latter decreased Conversely when beam 1was switched on and off and beam 2 was monitored the intensity of the latterincreased The occurrence of such asymmetric energy transfer unambiguouslyconfirms the PR nature of the optical encoding and allows a distinction to bemade between a grating based on the PR effect and other types of gratings
From plots of the type shown in Fig 43 the beam coupling ratio 0 as de-fined by Eq (4-4) can be determined
0 ILsat
IL04-4
Here I(L)sat and I(L)0 denote the intensity at saturation and at time t = 0 respec-tively of the writing beam under consideration measured after passage throughthe sample The beam coupling gain coefficient is given by Eq (4-5)
1L13ln0 ln 1 0 4-5
Here is the ratio of the intensities of the two beams before the sample and Lis the optical path length given by Eq (4-6)
L dcos
4-6
Here d is the sample thickness and is the angle of incidence of the beamwith respect to the sample normal
The total refraction index modulation n is given by Eq (4-7)
n
44-7
Typical results obtained with polymer VI at I1(0) = 720 mW cmndash2 = 22E = 8 V mndash1 = 647 nm and d = 74 m are = 313 cmndash1 n= 1610ndash3= sin2(L2) = 21 and = 4 s (grating build-up or response time) [14]
The four-wave mixing technique serves to measure the diffraction efficiency
during the writing process as a function of time and as a function of thestrength of the external electric field Figure 44 shows a schematic representa-tion of a typical set-up employed in four-wave mixing experiments
Notably a reading beam is used in addition to the two writing beams Com-monly the reading beam is of the same wavelength as the two writing beamsbut of a much lower intensity and it is counterpropagating one of the writingbeams is defined according to Eq (4-8) as the ratio of the intensities of thediffracted beam Id and of the incoming reading beam I0
44 Characterization of PR materials 109
Id
I04-8
Usually the electric field is applied to the sample by sandwiching the polymerbetween two transparent electrodes such as ITO (indium tin oxide)-coated glassslides The diffraction efficiency can be obtained from Kogelnikrsquos coupled-wave theory for thick holograms with the aid of Eq (4-9) [17]
sin2 fgdn
4-9
Here fg is a geometrical factor dependent on the polarization of the beams andthe experimental geometry and is the wavelength of the light of the readingbeam
45Applications
Photorefractive polymeric systems can be used to record in real-time and witha high storage density optically encoded information with low-power lasers suchas semiconductor diode lasers They are appropriate for recording hologramsThe storage of a large number of holograms at a single spot in the storage me-dium (multiplexing see Section 123) is possible Therefore there is a significantapplication potential Actually applications concerning dynamic holographic in-terferometry holographic storage and real-time processing have been demon-strated and future technical applications seem likely [18ndash22] With respect tocommercial applications it is noteworthy that the PR effect is reversible ie
4 Photorefractivity110
Fig 44 Schematic depiction of a set-upfor a four-wave mixing experiment asemployed to measure diffraction efficiencyas a function of the strength of an externalelectric field Reading beam counterpropa-gating with writing beam (1) Diffractedbeam counterpropagating with writingbeam (2) Adapted from Kippelen et al[11] with permission from the InternationalSociety for Optical Engineering
previously recorded holograms can be erased by irradiation with a spatially uni-form light beam Moreover holograms can be overwritten
There is a long list of technical requirements for holographic materials suchas optical quality near-IR sensitivity large refractive index modulation short re-sponse time self-processing inertness and long shelf-life non-destructive read-out and low cost Successful technical applications depend on the availability ofmaterials that fulfil all or most of these requirements Interesting proposalshave been made to overcome still existing technical problems such as that con-cerning destructive readout To retrieve information from holograms with goodfidelity the reading and writing beams have to be of the same wavelengthHowever since the material is photosensitive at the relevant wavelength thereadout process partially erases the stored information According to Kippelenet al this problem can be overcome with the aid of a photorefractive systemcontaining a substituted diphenylacetylene (compound VII in Chart 42) that issensitive to two-photon absorption [23] In a system of the composition FTCNPVKBBPECZ (25 55 10 10 wt) charge carriers are generated exclusively bytwo-photon processes and holographic recording is achieved with high-intensitywriting beams (= 650 nm 025 mW each) For readout a low-intensity beam(= 650 nm 025 W) which does not affect the photorefractive system is suffi-cient
The requirements of high near-IR sensitivity and short response time arelargely fulfilled by applying a pre-irradiation method denoted as time-gated holo-graphic imaging [24] Pre-irradiation provides for charge carriers before the writ-ing starts and thus affords a significant reduction in response time Accordingto Mechner et al [24] pre-irradiation at = 633 nm prior to holographic record-ing at = 830 nm improved the response time by a factor of 40 (30 ms) in in-vestigations with a formulation containing TPD-PPV (polymer VIII in Chart 42)(see Table 41)
Note that holograms can also be generated in polymeric media by other meth-ods for instance by photopolymerization of appropriate monomers contained inspecial formulations (see Section 117)
45 Applications 111
Table 41 Composition of a photorefractive material suitablefor holographic recording by means of time-gated holographicimaging [24]
Components Content (wt) Function
Polymer VIII (TPD-PPV) 56 Conductive host matrix1 1 Mixture of 25-dimethyl-(4-p-nitrophenyl-azo)-anisole and 3-methoxy-(4-p-nitrophenylazo)-anisole
30 Electro-optical material
Diphenyl phthalate 13 Plasticizer[66]-Phenyl-C61-butyric acid methyl ester 1 Sensitizer
4 Photorefractivity112
References
1 FS Chen J Appl Phys 38 (1967) 34182 P Guumlnter Holography Coherent Light
Amplification and Optical Phase Conjuga-tion with Photorefractive Materials PhysRep 93 (1982) 199
3 T J Hall R Jaura LM Conners PDFoote The Photorefractive Effect ndash A Re-view Prog Quant Electron 10 (1985)77
4 J Feinberg Photorefractive Nonlinear Op-tics Phys Today 41 (1988) 46
5 P Guumlnter J-P Huignard PhotorefractiveMaterials and Their Applications I and IIin Topics in Applied Physics 61Springer Berlin (1988)
6 MP Petrov SL Stepanov AV Kho-menko Photorefractive Crystals in Coher-ent Optical Systems Springer Berlin(1991)
7 M Gower D Proch (eds) Optical PhaseConjugation Springer Berlin (1994)
8 P Yeh Introduction to PhotorefractiveNonlinear Optics Wiley New York (1993)
9 DD Nolte (ed) Photorefractive Effectsand Materials Kluwer Academic PublBoston (1995)
10 B Kippelen Overview of PhotorefractivePolymers for Holographic Data Storage inJ Coufal D Psaltis G T Sincerbox(eds) Holographic Data StorageSpringer Berlin Series in OpticalSciences 76 (2000) 159
11 B Kippelen N Peyghambarian CurrentStatus and Future of Photorefractive Poly-mers for Photonic Applications Crit RevOpt Sci Technol CR 68 (1997) 343
12 W E Moerner SM Silence PolymericPhotorefractive Materials Chem Rev 94(1994) 127
13 G C Valley M B Klein Opt Eng 22(1983) 704
14 A V Vannikov AD Grishina L Ya Per-eshivko T V Krivenko VV SavelyevL I Kostenko R W Rychwalski JPhotochem Photobiol A Chem 150(2002) 187
15 L Lu J Polym Sci Part A PolymChem 39 (2001) 2557
16 LQ Wang M Wang L Lu Adv Mater12 (2000) 974
17 H Kogelnik Bell Syst Tech J 48 (1969)2909
18 R Bittner K Meerholz G Steckman DPsaltis Appl Phys Lett 81 (2002) 211
19 C Poga PM Lundquist V Lee R MShelby R J Twieg DM Burland ApplPhys Lett 69 (1996) 1047
20 PM Lundquist R Wortmann C Gelet-neky R J Twieg M Jurich VY LeeCR Moylan D M Burland Science 274(1996) 1182
21 BL Volodin Sandalphon K MeerholzB Kippelen N Kukhtarev N Peygham-barian Opt Eng 34 (1995) 2213
22 BL Volodin B Kippelen K MeerholzB Jaridi N Peyghambarian Nature 383(1996) 58
23 B Kippelen P-A Blanche A Schuumllz-gen C Fuentes-Hernandez G Ramos-Ortiz J F Wang N PeyghambarianSR Marder A Leclercq D BeljonneJ-L Bredas Adv Funct Mater 12 (2002)615
24 E Mechner F Gallego-Gomez H Till-mann H-H Houmlrhold J C HummelenK Meerholz Nature 418 (2002) 959
51Introductory remarks
There are substances that are transformed from form A into form B having adifferent absorption spectrum upon the absorption of light of wavelength 1
and that return to the initial state A either thermally or by the absorption oflight of wavelength 2 (see Scheme 51)
Substances capable of undergoing color changes in this way are denoted asphotochromic and the corresponding phenomenon is termed photochromism Ascan be seen from Table 51 in which typical photochromic systems are pre-sented photochromism can be based on various chemical processes
trans-cis (EZ) Isomerization occurs in azobenzene compounds (example (a))and also in the cases of azines stilbenes and certain biological receptors in liv-ing systems Pericyclic reactions (electrocyclizations) occur in the cases of spiro-pyrans and spirooxazines (examples (b) and (c)) and also with diarylethenes (ex-ample (d)) and fulgides (example (e)) Heterolytic bond cleavage resulting inionic dissociation occurs in the case of triphenylmethanes (example (f)) Con-cise information on organic photochromism including details of the variousfamilies of photochromic compounds and the chemical processes involved inphotochromic transformations is given in an IUPAC Technical Report [1]Moreover this subject has been dealt with in various review articles and booksthat emphasize its importance and potential for applications in the fields of mo-lecular switches and information storage [2ndash9] With respect to the present bookvarious publications focusing on polymers have to be pointed out [10ndash21]
The transformations presented in Table 51 are always accompanied bychanges in physical properties Besides the color changes there are alsochanges in dipole moment and in the geometrical structure at the molecularlevel Regarding bulk properties there are changes in the refractive indexwhich give rise to photo-induced birefringence and dichroism
113
5Photochromism
Scheme 51 Photochromic transformation of molecules
5 Photochromism114
Table 51 Typical photochromic processes
trans-cis Isomerization(a) Azobenzene
Pericyclic reactions(b) Spiropyrans
(c) Spirooxazines
(d) Diarylethenes
(e) Fulgides and fulgimides(X = O) (X = NR)
Heterolytic bond cleavage(f) Triarylmethanes
With respect to polymeric systems containing photochromic groups specialaspects have to be addressed For instance in linear macromolecules not onlythe chromophoric moieties but also neighboring units of the polymer chain orsurrounding molecules may be affected upon the absorption of photons by thechromophoric groups Conformational changes in linear polymers in solutioninduced in this way may lead to a change in viscosity or even to phase separa-tion For instance in liquid-crystalline polymeric systems phase transitions canbe generated In the case of rigid polymer matrices photomechanical effects areinduced ie photoisomerization causes shrinkage or expansion Interestinglystable relief surface gratings can be generated in polymer foils containingphotochromic moieties Notably the photostimulated conformational change inpolymers may result in an enormous amplification effect ie the absorption ofa single photon affects not only one moiety but also several neighboring onesor even the whole macromolecule
Potential applications of photochromic transformations relate to the reversiblecontrol of the properties of appropriate materials In this connection polymersoffer the advantage of easy fabrication and therefore a plethora of studies hasbeen devoted to polymers containing photochromic groups or to polymers withadmixed photochromic compounds Apparently among the various photochrom-ic polymeric systems dealt with in the literature those containing azobenzenegroups [19 20] have attracted the main interest although it seems that othersparticularly those containing diarylethenes [5] and furyl fulgides [6] deserve spe-cial attention because of their excellent performance Light-induced colorationdiscoloration cycles could be repeated more than 104 times with certain diaryl-ethenes thus proving their extraordinary resistance to fatigue [5] Thermal irre-versibility and fatigue resistance are prerequisites for applications related to datastorage and switching of photonic devices [21] which are considered in Chap-ter 12 of this book
52Conformational changes in linear polymers
521Solutions
Photochromic transformations may induce conformational changes in linearmacromolecules containing appropriate chromophoric groups Commonly thetransformation of these groups is accompanied by a change in polarity Thischange is most pronounced if the transformation generates electrically chargedgroups eg in the cases of triphenylmethane or spiropyran groups Howeverazobenzene groups also undergo a drastic change in polarity The change in thegeometry of the azobenzene group from the planar (trans or E-form) to the non-planar (cis or Z-form) leads to a decrease in the distance between the para car-bon atoms of the benzene rings from 99 to 55 Aring and to an increase in the di-
52 Conformational changes in linear polymers 115
pole moment from 05 to 55 D Regarding linear polymers containing pendantphotochromic groups the change in polarity affects not only the intermolecularinteraction between the chromophore and surrounding solvent molecules butalso the intramolecular interaction between pendant groups As a consequencerandom coil macromolecules undergo conformational alterations leading to ex-pansion or shrinkage For example a copolymer with pendant azobenzenegroups consisting of styrene and 4ndash6 mol 4-(methylacryloylamino)azobenzeneMAB (see Chart 51) precipitates in dilute cyclohexane solution at temperaturesabove the critical miscibility temperature upon irradiation with UV light Thisphenomenon is explained in terms of cis-azobenzene groups having in contrastto trans-azobenzene groups the capability of interacting rather strongly with sty-rene moieties Therefore immediately after trans-cis isomerization cis-azoben-zene groups interact preferentially with neighboring styrene moieties thuscausing a contraction of the coil Interactions of the cis-azobenzene groups withstyrene moieties of other macromolecules result in aggregation a process thatultimately leads to precipitation [22 23] This is illustrated schematically inFig 51
In solution coil expansion and contraction is readily reflected by changes inviscosity and in the intensity of scattered light As can be seen in Fig 52 theoptical absorption at 620 nm and the reduced viscosity specc increase simulta-neously when a poly(NN-dimethylacrylamide) sample containing 91 mol pen-dant triphenylmethane leucohydroxide groups is irradiated in dilute methanolsolution with UV light (gt 270 nm) In the dark the reduced viscosity returnsto the initial value The development of a green color in conjunction with theincrease in the viscosity indicates the formation of triphenylmethyl cations Ob-viously the polymer coils become expanded due to electrostatic repulsion of io-nized pendant groups formed according to Scheme 52 [24]
In the case of an azobenzene-modified poly(arylether ketone amide) (seeChart 52) a pronounced volume contraction due to photo-induced trans-cis iso-merization of the azobenzene groups was evidenced by means of size-exclusionchromatography (SEC) [25] When irradiated in dilute NN-diethylacetamide so-lution this polymer underwent a reduction in its hydrodynamic radius by a fac-tor of 27 corresponding to a contraction of the hydrodynamic volume by a fac-tor of about 20 This pronounced shrinkage effect is believed to be due to alarge number of conformationally restricted backbone segments because othermore flexible polyamides and polyurea polymers exhibit much weaker contrac-tion effects
5 Photochromism116
Chart 51 Chemical structures of co-monomer moieties styrene (left) and4-(methylacryloylamino)azobenzene(right)
The dynamics of conformational changes can be measured by following thechange in the light-scattering intensity Relevant studies relate to a polyamidecontaining in-chain azobenzene groups (see Chart 53) that was brought intothe compact form through trans-cis isomerization by continuous UV irradiationin NN-dimethylacetamide solution and subsequently exposed to a 20 ns flash of532 nm light On recording the changes in the optical absorption and in thelight-scattering intensity both at = 514 nm as a function of time it turned outthat the cis-trans isomerization was completed within the 20 ns flash and thatthe polymer chains unfolded on the ms time scale Obviously after isomeriza-tion the polymer chains maintain the initial compact conformation and thestrain energy built-up in this way causes coil expansion [26] The whole processis shown schematically in Scheme 53
The possibility of photo-inducing geometrical alteration in polymers in solu-tion has attracted special interest with regard to various polypeptides (seeChart 54)
Besides unordered random coil structures polypeptides are capable of assum-ing stable geometrically ordered structures namely -helix and -structures Asshown in Fig 53 these structures can be conveniently discriminated by record-ing circular dichroism (CD) spectra [14]
52 Conformational changes in linear polymers 117
Fig 51 Coil contraction and precipitation of polystyrenebearing pendant azobenzene groups
5 Photochromism118
Fig 52 Coil expansion of poly(NN-dimethy-lacrylamide) containing pendant triphenyl-methane leucohydroxide (91 mol) inmethanol upon exposure to UV light
(gt 270 nm) (a) Optical absorption at= 620 nm (b) reduced viscosity specc(spec = (solutionsolvent)ndash1) Adapted fromIrie [11] with permission from Springer
Scheme 52 Photogeneration of triphenylmethyl cations inpoly(NN-dimethylacrylamide) containing pendanttriphenylmethane leucohydroxide groups
Chart 52 Chemical structure of an azobenzene-modified poly(arylether ketone amide)
Light-induced transformations from one structure to another have been stud-ied with many modified polypeptides [13 14] bearing pendant photochromicgroups such as azobenzene or spiropyran groups Typical examples are themodified poly(L-glutamic acids) PGA-1 and PGA-2 presented in Chart 55
The spiropyran-modified poly(L-glutamic acid) PGA-2 undergoes a coilhelixtransition upon exposure to visible light in hexafluoro-2-propanol solution Inthe dark the polypeptide containing 30ndash80 mol chromophore units in theopen charged form adopts a random coil conformation Irradiation causes iso-merization in the side chains as indicated by complete bleaching of the coloredsolution (see Scheme 54) The formation of the colorless and uncharged spiro-pyran form induces spiralization of the polypeptide chain The coilhelix tran-sition can be followed with the aid of CD spectra as shown in Fig 54
52 Conformational changes in linear polymers 119
Chart 53 Chemical structure of a polyamide containing in-chain azobenzene groups
Scheme 53 Conformational change of a polyamidecontaining in-chain azobenzene groups due to cis-transisomerization
Chart 54 Chemical structures of poly(L-lysine) and poly(L-glutamic acid)
The coilhelix transition proceeds rapidly within seconds whereas the backreaction requires several hours for full conversion Notably in this case thephotochromic behavior of the spiropyran groups is opposite to that observed inother solvents (see example (b) in Table 5-1) The reverse photochromism is dueto the high polarity of hexafluoro-2-propanol which stabilizes the charged mero-cyanine form better than the uncharged spiropyran form
5 Photochromism120
Chart 55 Chemical structures of modified poly(L-glutamic acids)
Fig 53 Standard circular dichroism (CD)spectra of common polypeptide structures(1) -helix (2) -structure and (3) randomcoil Adapted from Pieroni et al [14] withpermission from Elsevier
52 Conformational changes in linear polymers 121
Scheme 54 Isomerization of the spiropyran-modified poly(L-glutamic acid) PGA-2
Fig 54 Coilhelix transition of poly(glutamic acid) PGA-2containing 80 mol spiropyran units in the side chains CDspectra recorded in hexafluoro-2-propanol solution in the dark(1) and after exposure to sunlight (2) Adapted from Pieroniet al [14] with permission from Elsevier
522Membranes
As an extension of the work described in the previous section one goal was thedevelopment of artificial membranes the physical properties of which such aspermeability electrical conductivity and membrane potential could be con-trolled in response to light Typically in the case of membranes consisting ofpoly(L-glutamic acid) bearing azo groups in the side chains the water contentincreases upon light exposure Concomitantly the dissociation of acid groups isaccelerated and augmented and the potential across the membrane and thecross-membrane conductance are enhanced [15] Typical results are presented inFig 55
Moreover a low molar mass spiropyran compound entrapped in a membraneconsisting of plasticized poly(vinyl chloride) rendered the latter photoresponsiveA membrane potential change of more than 100 mV was induced by irradiationwith light [27] For further details and additional references the reader is re-ferred to the relevant reviews [11 28]
5 Photochromism122
Fig 55 Photoresponsive behavior of membranes of anazo-modified poly(L-glutamic acid) containing 12ndash14 molazobenzene groups at 60 C (a) Membrane potential(b) conductance and (c) absorbance at 350 nm Adaptedfrom Kinoshita [15] with permission from Elsevier
53Photocontrol of enzymatic activity
Photochromic groups covalently attached to enzymes are in certain cases cap-able of affecting the tertiary protein structure upon light-induced isomerizationAs a consequence the biocatalytic activity of the enzymes can be switched onand off [29] For example the catalytic activity of papain is inhibited when 4-carboxy-trans-azobenzene groups covalently linked to the lysine moieties of theenzyme undergo trans-cis isomerization (see Scheme 55) At a loading of fiveunits per enzyme molecule 80 of the catalytic activity is retained
The inactivity of enzyme molecules bearing cis-azobenzene groups is ex-plained by their incapability of binding to the reaction substrate Similarly thebinding of -d-manopyranose to concanavalin A is photocontrollable providedthat the enzyme is modified by the attachment of thiophenefulgide or nitro-spiropyran However the general applicability of this method has to be subjectto scrutiny because the photoswitching behavior is quite sensitive to the level ofloading Low loadings may result in a low switching efficiency and high load-ings often deactivate the biomaterials in both isomeric forms
54Photoinduced anisotropy (PIA)
Exposure of polymer films bearing azobenzene groups to linearly polarized laserlight induces optical dichroism and birefringence This is due to the fact thatduring exposure a major fraction of the chromophores becomes oriented per-pendicular to the polarization direction of the light Photons of linearly polar-ized light are preferentially absorbed by molecules with a transition momentparallel to the polarization plane of the light The absorbed photons inducetrans-cis isomerizations in conjunction with rotational diffusion The relaxationof the cis molecules results in trans molecules with a new orientation distribu-tion ie the fraction of trans molecules with a transition moment parallel to thepolarization plane of the incident light becomes smaller Continuous repetitionof this cycle steadily reduces this fraction and makes the system more transpar-ent to the incident light as the trans molecules can no longer be excited
54 Photoinduced anisotropy (PIA) 123
Scheme 55 Photoisomerization of azobenzene groupscovalently linked to the lysine moieties of papain
To sum up during the irradiation azobenzene groups with transition mo-ments that are not initially perpendicular to the polarization direction of the la-ser light undergo a series of trans-cis-trans isomerization cycles accompanied bya change in orientation until they finally line up in directions approximatelyperpendicular to the polarization direction of the laser light (see Fig 56)
In this way an orientation distribution with an excess of azobenzene groupsoriented in the direction perpendicular to the polarization plane of the laserlight is attained The resulting birefringence can be detected with the aid of an-other laser beam that is not absorbed by the photochromic compound Notablythe anisotropy can be erased if the sample is irradiated with circularly polarizedlaser light or is heated to a temperature in excess of the glass transition tem-perature This behavior is demonstrated for a typical case in Fig 57 Here itcan be seen that the birefringence (monitored at 633 nm) of a 400ndash500 nm thickfilm of pMNAP polymer (see Chart 56) is built up upon irradiation with a lin-early polarized laser beam (= 488 nm) [30] The birefringence relaxes down to acertain level when the writing beam is turned off and is completely eliminatedupon turning on a circularly polarized light beam (= 488 nm)
Photo-induced anisotropy (PIA) is quantitatively described by Eqs (5-1) and(5-2) by n in terms of the induced birefringence and by the parameter S interms of light absorption behavior
n n n 5-1
5 Photochromism124
Fig 56 Schematic illustration ofthe generation of anisotropy uponirradiation of a film containingphotochromic entities with linearlypolarized light
Fig 57 Generation of birefringence uponirradiation of pMNAP polymer with linearlypolarized light (= 488 nm) A light turned onB light turned off C circularly polarized lightturned on Adapted from Meng et al [30] withpermission from John Wiley amp Sons Inc
S A AA 2A 5-2
Here A|| and A and n|| and n denote the absorbances and the refractive in-dices at orientations parallel and perpendicular to the polarization plane of theexciting probe light respectively
In recent years optical dichroism and birefringence based on photo-inducedtrans-cis-trans isomerization of azobenzene groups has been observed with pre-oriented liquid-crystalline polymers [31-35] at temperatures above the glass tran-sition temperature and also with various amorphous polymers at temperatureswell below the glass transition temperature In the case of a polyimide (seeChart 57) a quasi-permanent orientation can be induced [36ndash38] Here the azo-benzene groups are rather rigidly attached to the backbone and photoisomeriza-tion occurs at room temperature ie 325 C below the glass transition tempera-ture Tg = 350 C This behavior is in accordance with the fact that the isomeriza-tion quantum yields of azobenzene compounds are very similar in solution andin polymer matrices 13(trans cis)01 and 13(cis trans) 05
54 Photoinduced anisotropy (PIA) 125
Chart 56 Chemical structure ofpMNAP polymer used for the photo-generation of birefringence(see Fig 57)
Chart 57 Chemical structure of a polyimide bearing pendant azobenzene groups
Because of the importance of the PIA phenomenon for applications in opticaldata storage systems a large variety of homopolymers and copolymers has beenstudied and the reader is referred to the literature cited in a relevant review arti-cle [39] In this connection it is also worthwhile to cite work performed with cy-clic siloxane oligomers bearing pendant photochromic groups Compounds ofthis family possessing relatively high glass transition temperatures and capableof forming cholesteric liquid-crystalline phases have been examined as potentialoptical recording materials [40]
55Photoalignment of liquid-crystal systems
It has been shown in Section 54 that linearly polarized laser light induces achange in the orientation of azobenzene groups contained in polymers Interest-ingly this change in orientation can be greatly amplified if the azobenzenegroups are contained in liquid-crystalline polymers This phenomenon whichhas been the subject of extensive investigations [16 41ndash44] is described here insome detail for the case of a methacrylate-based copolymer consisting mainly ofnon-photosensitive mesogenic side groups and a small fraction of azobenzene-containing side groups (see Chart 58) [45]
Initially this copolymer is an isotropic (polydomain) liquid-crystalline polymerwith a glass transition temperature of Tg = 45 C and a clearing temperature(transition from nematic to isotropic phase) of TN-I = 112 C Irradiation with lin-early polarized light at = 366 nm (28 mW cmndash2) and T = 106 C ie just belowTN-I induces anisotropy By repetitive trans-cis-trans isomerization the opticalaxis of the azobenzene groups becomes aligned perpendicular to the electricvector of the incident light In this way a cooperative motion of the neighboringphotoinactive mesogenic groups is triggered Thus the entire assembly of me-sogenic side groups becomes aligned in one direction and forms a monodomain
5 Photochromism126
Chart 58 Chemical structures of the components of a liquid-crystalline copolymer exhibiting amplified photoalignment(see Fig 58)
nematic phase This was evidenced by measuring the transmittance of an irra-diated (exc = 633 nm) copolymer film placed between a pair of crossed polarizersat various rotation angles As can be seen in Fig 58 the transmittance hasmaxima at 45 135 225 and 315 and minima at 0 90 180 and 270
Materials such as the LC copolymer considered here possess an applicationpotential for image storage This is demonstrated in Fig 59 which shows (a)the transmittance response of the copolymer during alternating irradiation withpolarized and unpolarized light and (b) a one-year-old stored image which wasgenerated by irradiation of a copolymer film through a standard photo mask[45]
The field of liquid-crystalline polymers is still growing and a significant num-ber of the relevant papers deal with subjects related to photochemical andphotophysical problems as has been documented in several reviews [46ndash48]The progress in research is demonstrated here by referring to an interesting de-velopment concerning the photochromic amplification effect based on the sur-face-assisted alignment of liquid-crystalline compounds in cells possessing so-called command surfaces [16 41ndash43] The latter consist of silica glass plates orpolymer films bearing attached photochromic groups at an area density of aboutone unit per nm2 The light-induced isomerization of the photochromic moi-eties triggers reversible alignment alterations of the low molar mass liquid-crys-talline compounds contained in the cell Chemical structures of appropriatecompounds forming nematic crystalline phases are shown in Chart 59
It should be noted that the intermolecular interaction between surface azo-benzene units and liquid-crystal molecules is strongly determined by theirchemical nature an aspect that has been thoroughly investigated [43] but is notelaborated here It is estimated that the amplification involves up to 104 liquid-
55 Photoalignment of liquid-crystal systems 127
Fig 58 Alignment of liquid-crystalcopolymer MACB-CNB6 upon30 min of exposure to polarized lightat = 366 nm (28 mW cmndash2) at106 C (a) Transmittance of probelight (633 nm) through a 2 m thickcopolymer film placed betweencrossed polarizers as a function ofthe rotation angle (b) Experimentalset-up Adapted from Wu et al [45]with permission from Elsevier
crystalline molecules per elementary isomerization process The response timeof the cells is determined by relax the relaxation time of the nematic phase Val-ues of relax typically range from 50 to 300 ms [43] and so are several orders ofmagnitude longer than isomerization times which are of the order of picose-conds Figure 510 schematically depicts for the case of azobenzene chromo-phores as the active entities at the surface how irradiation with unpolarizedlight induces an alignment change from the homeotropic to the planar homoge-neous state
Notably this kind of alignment change can also be accomplished by applyingan electric field On the other hand alignment changes between planar homo-
5 Photochromism128
Fig 59 (a) Transmittance responseof copolymer MACB-CNB6 duringirradiation with polarized light (A toB) and unpolarized light (C to D) at106 C (b) One-year-old imagestored in the liquid-crystalcopolymer The film was coveredwith a photo mask during irradiationwith polarized light at = 366 nm(28 mW cmndash2) and 106 C Adaptedfrom Wu et al [45] with permissionfrom Elsevier
Chart 59 Compounds forming nematic liquid-crystallinephases appropriate for photoalignment [43]
geneous states not realizable with the aid of an electric field can be achievedby employing linearly polarized light An alignment change induced by an azi-muthal in-plane reorientation of the photochromic groups is depicted schemati-cally in Fig 511
It has been reported that cells fabricated with azobenzene-modified surfacesand operating on the basis of alternate irradiation with UV and visible light be-come inactive after about 2000 cycles which is thought to be due to side reac-tions occurring with a quantum yield of about 10ndash4 [43]
55 Photoalignment of liquid-crystal systems 129
Fig 510 Light-induced surface-assisted alignment change ina liquid-crystal cell Schematic depiction of the out-of-planechange from the homeotropic state to the planar homoge-neous state upon exposure to unpolarized UV light Adaptedfrom Ichimura [43] with permission from Springer
Fig 511 Light-induced surface-assisted alignment change ina liquid-crystal cell Schematic depiction of the in-planechange between homogeneous planar states under theinfluence of linearly polarized light Adapted from Ichimura[43] with permission from Springer
56Photomechanical effects
561Bulk materials
The idea of transforming light into mechanical energy has fascinated many re-searchers In the early studies reviewed by Irie [11] contractionexpansion be-havior in conjunction with isomerization of photochromic entities either ad-mixed to or chemically incorporated into polymer films was found Howeverthe dimensional changes were only marginal amounting to 1 or less and onscrutiny turned out in many cases to be due to the local increase in tempera-ture arising from non-radiative transitions rather than to isomerization of thechromophores
Large real effects on the other hand were observed with hydrogels A typicalresult is presented in Fig 512 which shows how a polyacrylamide gel contain-ing 19 mol triphenylmethane leucocyanide swells upon irradiation with UVlight at 25 C [49] The swelling is correlated to a 18-fold increase in the relativeweight
It can also be seen in Fig 512 that in the dark the gel slowly attains the ini-tial weight More recently rigid films (501005 mm) of polyurethanendashacrylateblock copolymers containing nitrospiropyrans and nitro-bis-spiropyrans havebeen irradiated with 325 nm light at 20 C in 5 min lightdark cycles [50] Thefilms expanded during irradiation and shrank in the dark with a response timeof a few seconds in each case The highest photomechanical responses were ob-served at a high acrylate content (72) which rendered the system least elastic
The possibility of converting light into mechanical energy has been impres-sively demonstrated with cross-linked liquid-crystalline polymeric systems con-taining azobenzene groups that were prepared by polymerizing previouslyaligned mixtures of acrylate 1-AC and diacrylate 2-AC (see Chart 510) [51]
Figure 513 shows how a film prepared from an 8020 mol mixture of 1-ACand 2-AC bends upwards towards the incident light (= 360 nm) It becomes flat
5 Photochromism130
Fig 512 Photomechanical effects UV-light-stimulated dilatation of a polyacrylamide gelcontaining pendant triphenylmethane leucocyanidegroups (19 mol) at 25 C Adapted from Irieet al [49] with permission from the AmericanChemical Society
again upon irradiation at = 450 nm These processes are completed within90 s The anisotropic bending phenomenon caused by trans-cis isomerizationmay be explained in terms of a volume contraction The latter is limited to athin surface layer of the 10 m thick film in which the incident light is totallyabsorbed Since the film mobility requires segment relaxation the bending phe-nomenon can be observed with rigid films at T gt Tg in this case at T = 90 C orat room temperature with films swollen in a good solvent such as toluene
The phenomenon of light-induced dimensional alterations in polymer films hasbeen exploited for the generation of regular surface structures in azobenzene-con-taining polymers The technique employed is based on the fact that azobenzenegroups undergo reorientation due to repeated trans-cis-trans isomerization upon
56 Photomechanical effects 131
Chart 510 Monomers used to prepare cross-linked polymericsystems exhibiting photomechanical effects
Fig 513 Photomechanical effects Schematicillustration of UV-light-induced bending of across-linked liquid-crystalline polymer filmcontaining azobenzene groups Light isabsorbed at the upper surface layer of the filmand causes anisotropic contraction Adaptedfrom Ikeda et al [51] with permission fromWiley-VCH
irradiation with polarized light (see Section 54) and that the target film is inhomo-geneously irradiated The reorientation results in a driving force that initiatesmass transport from irradiated to unirradiated areas The experimental set-uporiginally used to generate large surface gratings is shown in Fig 514 a [52 53]
The gratings are optically inscribed onto the films with a single beam of anargon ion laser (488 nm irradiation power between 1 and 100 mW) split by amirror and reflected coincidently onto the film surface which is fixed perpen-dicular to the mirror The diffraction efficiency is monitored with the aid of aHe-Ne laser beam (1 mW = 633 nm) Changing the incident angle of the writ-ing beam allows the intensity profile spacing on the sample and thereby thegrating spacing to be changed Under such conditions irradiation of the poly-mer films for a few seconds at an intensity between 5 and 200 mW cmndash2 pro-duces reversible volume birefringence gratings with low diffraction efficiency If
5 Photochromism132
Fig 514 Photomechanical effectsGeneration of surface relief gratings inpoly(4-(2-acryloyloxy)ethylamino-4-nitroazo-benzene) by light-induced mass transport(a) Experimental set-up (b) Sinusoidal
surface relief profiles examined with the aidof an atomic force microscope Adaptedfrom Rochon et al [53] with permission fromthe American Physical Society
the film is exposed for a longer period (up to a few minutes) an irreversibleprocess creates an overlapping and highly efficient surface grating Thus thereis an initial rapid growth corresponding to the production of the reversible vol-ume birefringence grating and a slower process which irreversibly creates sur-face gratings observable by atomic force microscopy (AFM) with efficiencies ofup to 50 Figure 514 b shows a typical grating generated in this case at thesurface of a film of a polymer having the structure depicted in Chart 511
Surface gratings have been generated in various azobenzene-modified poly-mers epoxy polymers polyacrylates polyesters conjugated polymers poly(4-phenylazophenol) and cellulose [54ndash56]
562Monolayers
Monolayers of a polypeptide consisting of two -helical poly(L-glutamate)slinked by an azobenzene moiety (see Chart 512) become bent in the main
56 Photomechanical effects 133
Chart 511 Chemical structure of poly(4-(2-acry-loyloxy)ethylamino-4-nitroazobenzene)
Chart 512 Chemical structure of a poly(L-glutamate) with in-chain azobenzene groups
Chart 513 Chemical structure of a hairy-rod-type poly(gluta-mate) bearing pendant azobenzene groups
chain to an angle of about 140 upon light-induced trans-cis isomerization As aresult the area of the monolayer shrinks [57]
Photomechanical effects in monolayers have also been observed in othercases for example with so-called hairy-rod type poly(glutamate)s (see Chart 513)[58]
57Light-induced activation of second-order NLO properties
Apart from the aforementioned property alterations photochromicity is fre-quently also connected with changes in nonlinear optical (NLO) properties Thisis due to the fact that the two molecular species in a photochromic couple com-monly exhibit different molecular NLO properties Relevant studies have beenperformed with thin polymer films For example if spiropyran is transformedto merocyanine the first hyperpolarizability increases considerably The sec-ond harmonic generation (SHG) increases by a factor of ten when a previouslyelectric field-poled PMMA film doped with a spiropyran (see Chart 514) is irra-diated at = 355 nm [59] Subsequent irradiation at = 514 nm at which mero-cyanine absorbs strongly induces the reverse reaction resulting in a drop of theSHG signal to almost zero Figure 515 shows how the SHG signal changes inresponse to alternating irradiation with UV and visible light
Clearly the SHG signal decreases with increasing number of cycles indicat-ing that in the absence of an external electric field the chromophores becomeincreasingly disorientated ie the NLO activity of the system is deactivated Ana-logous behavior has been observed with a PMMA film doped with a furyl ful-gide (see Chart 515) In this case the ring-opening and -closure reactions needless free volume Therefore the matrix is less disturbed and the SHG signal de-creases more slowly with increasing number of cycles
Interestingly the disorientation-induced distortion of the matrix can beavoided if the photoswitching is performed under an external electric field Thiswas demonstrated in the case of the polyimide of the structure shown inChart 516 [60]
Here the SHG signal decays under irradiation due to trans-cis isomerizationand recovers almost completely in the dark after the light is switched off Theinfluence of the external electric field is thought to allow a compensation of thephoto-induced distortion through photo-assisted poling
5 Photochromism134
Chart 514 Chemical structure of 6-nitro-133-trimethylspiro[2H-1-benzopyran-22-indoline] [59]
57 Light-induced activation of second-order NLO properties 135
Chart 515 Chemical structure of furyl fulgide FF-1
Fig 515 Light-induced generation ofsecond-order NLO properties in an electricfield-poled PMMA film doped with 25 wtof a spiropyran (see Chart 514) Alternatingirradiation at = 355 nm and =514 nm
Upper part Second harmonic generation(SHG) Lower part Optical absorption of themerocyanine isomer at =532 nm Adaptedfrom Atassi et al [59] with permission fromthe American Chemical Society
Chart 516 Chemical structure of a polyimide with pendant azobenzene groups
58Applications
581Plastic photochromic eyewear
Besides classical inorganic glasses there are certain optical plastics that are em-ployed in the transparency and eyewear industry For instance thermoset resinsbased on allyl diglycol carbonate poly(methyl methacrylate) derivatives and bis-phenol A polycarbonates have been used to produce commercial plastic non-photochromic and photochromic lenses As far as has been disclosed by themanufacturers indolinospironaphthoxazines INSO and pyridobenzoxazines
5 Photochromism136
Fig 516 UV activation and thermal bleach profiles at 10 C20 C and 30 C of a commercial photochromic lens based onindolinospironaphthoxazine Adapted from Crano et al [61]with permission from Springer
Chart 517 Chemical structures of compounds that render plastic lenses photochromic
QISO (see Chart 517) have received much attention among the compoundscapable of rendering plastic lenses photochromic
The photochromic compounds are incorporated at a concentration of 01ndash03either by admixing or by chemical bonding In the latter case modified compoundswith appended polymerizable functionalities are employed Photochromic lensesoperate on the basis of UV activation and thermal bleaching as shown in Fig 516
As with most photochromic lenses the performance of plastic photochromiclenses is temperature-dependent In addition to variable light attenuation photo-chromic lenses offer protection against UV light Photochromic plastics coated ontoclassical glass lenses provide abrasionscratch resistance and highly functional anti-reflectivity For further details the reader is referred to a review article [61]
582Data storage
The availability of two states associated with the common photochromic processis a promising basis for erasable optical data storage systems as outlined in areview article by Irie [62] Besides sufficiently high quantum yields and rapid re-sponses for both the forward and the reverse reaction important requirementsfor device application include a high storage capacity a long archival lifetimeand good intrinsic fatigue characteristics and cyclability ie the number oftimes the interconversion can be made without significant performance lossObviously a development of the recorded image should not be necessary
Photochromic compound families that have been considered for employmentin data storage systems include for example fulgides and diarylethenes Com-pounds that have been examined for instance are the furyl fulgide FF-1 (seeChart 515) [63] and the diarylethene shown in Scheme 56 When dispersed ina polystyrene film the latter system exhibited a strong fatigue resistance in atest using a low-power readout laser (633 nm 20 nW) The initial optical densityof 05 remained unchanged during more than 105 readout cycles [5 64]
In this connection the importance of fatigue resistance should be pointedout If form A of a chromophoric couple AB undergoes a side reaction with aquantum yield 13side = 0001 and B converts to A without loss 63 of the initialmolecules of A will be decomposed after 1000 cycles Thus 13side should be lessthan 00001 if the system is expected to endure more than 104 cycles [65]
58 Applications 137
Scheme 56 Photoisomerization of 3-(1-octyl-2-methyl-3-indolyl)-4-(235-trimethyl-1-thienyl)maleic anhydride
The search for materials appropriate for data storage has also been extendedto liquid-crystalline copolymers containing photochromic moieties and inten-sive studies have been focused on copolymers containing pendant azobenzenegroups because of the possibility of generating anisotropy Indeed alignment al-terations induced in such copolymers by exposure to linearly polarized light canbe permanently frozen-in and stored Since long durability is a prime require-ment for information storage materials with a high glass transition tempera-ture (higher than 100 C) seemed to be most appropriate [66] However in thecase of a liquid-crystalline polyester (P6a12 see Chart 518) containing azoben-zene side groups holographically recorded gratings endured at room tempera-ture over a period of several years and up to 104 write-record-erase cycles couldbe accomplished [67 68] Notably erasure is achieved by heating this polyesterto approximately 80 C This temperature is much higher than the glass transi-tion temperature of about 30 C and corresponds to the clearing temperature atwhich the liquid-crystalline domains form the mesophase melt
Similarly good long-term optical storage properties at room temperature havebeen reported for a liquid-crystalline copolymer composed of the moietiesshown in Chart 519 with phase transitions at 487 C (Tg) 832 C (SC) and969 C (SA) [69]
5 Photochromism138
Chart 518 Chemical structure of a polyester with pendant azobenzene groups
Chart 519 Chemical structures of the constituents of acopolymer with good optical storage properties
Large induced birefringences [see Eq (5-1)] up to n = 036 at 780 nm are ob-tained with liquid-crystalline copolymers containing the methyl methacrylate co-monomer presented in Chart 520 [70 71]
Since such copolymers possess besides a high storage capacity a high storagecyclability and moreover withstand temperatures up to 120 C they are utilizedby Bayer Material Science for high-tech storage systems The holography-relatedapplication potential of these materials includes forgery-proof storage systemsID cards for access control to high security areas etc [72]
Regarding the heat resistance of potential storage materials work on oligo-peptides (see Chart 521) is also noteworthy Holograms written in DNO films(write = 488 nm read = 633 nm) remained stable at room temperature for up toone year and were not erased upon exposure to 80 C for one month [73]
References 139
Chart 520 Chemical structure of a base unit of copolymersused for forgery-proof storage systems
Chart 521 Chemical structure of oligopeptides with good optical storage properties
References
1 H Bouas-Laurent H Duumlrr OrganicPhotochromism Pure Appl Chem 73(2001) 639
2 J C Crano R J Guglielmetti (eds) Or-ganic Photochromic and ThermochromicCompounds Vol 1 Photochromic FamiliesPlenum Press New York (1999)
3 G H Brown (ed) Photochromism Tech-niques in Chemistry III Wiley-Inter-science New York (1971)
4 H Duumlrr H Bouas-Laurent (eds) Photo-chromism Molecules and Systems ElsevierAmsterdam (1990)
5 M Irie Chem Rev 100 (2000) 16856 Y Yokoyama Chem Rev 100 (2000)
17177 G Berkovic V Krongauz V Weiss
Chem Rev 100 (2000) 17418 S Kawata Y Kawata Chem Rev 100
(2000) 17779 N Tamai H Miyasaka Chem Rev 100
(2000) 187510 CB McArdle (ed) Applied Photochromic
Polymer Systems Blackie Glasgow(1992)
11 M Irie Adv Polym Sci 94 (1990) 27
5 Photochromism140
12 O Nuyken C Scherer A Baindl A RBrenner U Dahn R Gaumlrtner S Kiser-Roumlhrich R Kollefrath P Matusche BVoit Prog Polym Sci 22 (1997) 93
13 F Ciardelli O Pieroni PhotoswitchablePolypeptides in [21]
14 O Pieroni A Fissi G Popova ProgPolym Sci 23 (1998) 81
15 T Kinoshita Prog Polym Sci 20 (1995)527
16 K Ichimura Chem Rev 100 (2000)1847
17 N Hampp Chem Rev 100 (2000) 175518 J A Delaire K Nakatani Chem Rev
100 (2000) 181719 S Xie A Natansohn P Rochon Chem
Mater 5 (1993) 40320 G S Kumar G Neckers Chem Rev 89
(1989) 191521 BL Feringa (ed) Molecular Switches
Wiley-VCH Weinheim (2001)22 M Irie H Tanaka Macromolecules 16
(1983) 21023 M Irie W Schnabel Light-Induced Con-
formational Changes in Macromolecules inSolution as Detected by Flash Photolysis inConjunction with Light Scattering Measure-ments in B Sedlacek (ed) Physical Op-tics of Dynamic Phenomena and Processesin Macromolecular Systems de GruyterBerlin (1985) p 287
24 M Irie M Hosoda Makromol ChemRapid Commun 6 (1985) 533
25 MS Beattie C Jackson G D JaycoxPolymer 39 (1998) 2597
26 M Irie W Schnabel Macromolecules 14(1983) 1246
27 J Anzai T Osa Tetrahedron 50 (1994)4039
28 O Pieroni F Ciardelli Trends in PolymSci 3 (1995) 282
29 I Willner Acc Chem Res 30 (1997)347
30 X Meng A Natansohn P Rochon JPolym Sci Polym Phys 34 (1996)1461
31 M Eich J H Wendorff B Reck HRingsdorf Makromol Chem RapidCommun 8 (1987) 59
32 M Eich J H Wendorff MakromolChem Rapid Commun 8 (1987) 467
33 NCR Holme L Nikolova PS Rama-nujam S Hvilsted Appl Phys Lett 70(1997) 1518
34 H Ringsdorf C Urban W Knoll MSawodny Makromol Chem 193 (1992)1235
35 FT Niesel J Rubner J Springer Mak-romol Chem Chem Phys 196 (1995)4103
36 Z Seccat P Pretre A Knoesen WVolksen VY Lee RD Miller J WoodW Knoll J Opt Soc Am B 15 (1998)401
37 Z Seccat J Wood W Knoll W VolksenR D Miller A Knoesen J Opt SocAm B 14 (1997) 829
38 Z Seccat J Wood EF Aust W KnollW Volksen R D Miller J Opt SocAm B 13 (1996) 1713
39 J A Delaire K Nakatani Chem Rev100 (2000) 1817
40 FH Kreuzer Ch Braumluchle A Miller APetri Cyclic Liquid-Crystalline Siloxanes asOptical Recording Materials in [48]
41 K Ichimura Y Suzuki T Hosoki KAoki Langmuir 4 (1988) 1214
42 T Ikeda S Horiuchi DB Karanjit SKrihara S Tazuke Macromolecules 23(1990) 36 and 42
43 K Ichimura Photoregulation of Liquid-Crystal Alignment by Photochromic Mole-cules and Polymeric Thin Films in [48]
44 (a) V P Shibaev S G Kostromin S AIvanov Comb-Shaped Polymers with Meso-genic Side Groups as Electro- and Photoop-tical Active Media in [48] (b) VP Shi-baev A Bobrovsky N Boiko ProgPolym Sci 28 (2003) 729
45 Y Wu A Kanazawa T Shiono T IkedaQ Zhang Polymer 40 (1999) 4787
46 D Creed Photochemistry and Photophysicsof Liquid-Crystalline Polymers in V Rama-murthy K S Schanze (eds) Molecularand Supramolecular Organic and Inorgan-ic Photochemistry Vol 2 Marcel DekkerNew York (1998)
47 CB McArdle (ed) Side-Chain Liquid-Crystal Polymers Blackie Glasgow (1989)
48 V P Shibaev (ed) Polymers as Electroopti-cal and Photooptical Active MediaSpringer Berlin (1996)
49 M Irie D Kungwatchakun Macromole-cules 19 (1986) 2476
50 EA Gonzalez-de los Santos J Lozano-Gonzalez A F Johnson J Appl PolymSci 71 (1999) 267
References 141
51 T Ikeda M Nakano Y Yu O TsutsumiA Kanazawa Adv Mater 15 (2003) 201
52 DY Kim S K Tripathy L Li J KumarAppl Phys Lett 66 (1995) 1166
53 P Rochon E Batalla A NatansohnAppl Phys Lett 66 (1995) 136
54 T Fukuda K Sumaru T Kimura HMatsuda J Photochem Photobiol AChem 145 (2002) 35
55 S Yang L Li A L Cholly J KumarSK Tripathy J Macromol Sci PureAppl Chem A 38 (2001) 1345
56 NK Viswanathan S BalasubramanianJ Kumar SK Tripathy J MacromolSci Pure Appl Chem A 38 (2001)1445
57 M Higuchi N Minoura T KinoshitaColloid Polym Sci 273 (1995) 1022
58 H Menzel Macromol Chem Phys 195(1994) 3747
59 Y Atassi J A Delaire K Nakatani JPhys Chem 99 (1995) 16320
60 Z Sekkat P Pretre A Knoesen WVolksen VY Lee RD Miller J WoodW Knoll J Opt Soc Am B 15 (1998)401
61 J C Crano WS Kwak CN WelchSpiroxazines and Their Use in Photo-chromic Lenses in [10]
62 M Irie High-Density Optical Memory andUltrafine Photofabrication Springer Se-ries in Optical Sciences 84 (2002) 137
63 J Whittall Fulgides and Fulgimides ndash aPromising Class of Photochromes for Appli-cation in [10]
64 T Tsujioka F Tatezono T Harada KKuroki M Irie Jpn J Appl Phys 33(1994) 5788
65 M Irie K Uchida Bull Chem SocJpn 71 (1998) 985
66 R Natansohn P Rochon C Barret AHay Chem Mater 7 (1995) 1612
67 NCR Holme S Hvilsted PS Rama-nujam Appl Optics 35 (1996) 4622
68 NCR Holme S Hvilsted PS Rama-nujam Opt Lett 21 (1996) 1902
69 Y Tian J Xie C Wang Y Zhao H FeiPolymer 40 (1999) 3835
70 BL Lachut SA Maier HA AtwaterMJ A de Dood A Polman R HagenS Kostromine Adv Mater 16 (2004)1746
71 R P Bertram N Benter D Apitz ESoergel K Buse R Hagen SG Kostro-mine Phys Rev E 70 (2004) 041802-1
72 Forgery-Proof Information Storage Genu-ine Security Bayer Scientific MagazineResearch 16 (2004)
73 R H Berg S Hvilsted P S Ramanu-jam Nature 383 (1996) 506
61Electrophotography ndash Xerography
According to Schaffertrsquos definition [1] electrophotography concerns the formationof images by the combined interaction of light and electricity and xerography is aform of electrophotography that involves the development of electrostatic chargepatterns created on the surfaces of photoconducting insulators The term xerogra-phy originates from the Greek words xeros (dry) and graphein (to write) which to-gether mean dry writing The xerographic process invented by Carlson in 1938 [2] isthe basis for copying documents with the aid of copying machines The impor-tance of xerography in our daily lives is unquestionable in view of the ubiquitousemployment of copying machines At present virtually all copiers use xerographyWith the advent of semiconductor lasers and light-emitting diodes xerography isalso widely applied in desktop printing [3ndash8] The principle of the xerographic pro-cess is outlined briefly in the following and depicted schematically in Fig 61
The essential part of a copying machine is the photoreceptor which nowadaysconsists mostly of organic material In order to make a copy of a document thephotoreceptor surface is first positively or negatively corona charged and subse-quently exposed to the light reflected from the document The resulting patternof exposed and unexposed areas at the photoreceptor corresponds to areas wherethe corona charges were neutralized or remained unaltered respectively Electro-statically charged toner particles brought into contact with the exposed photorecep-tor adhere exclusively to those areas that still carry charges To complete the copy-ing process the toner particles are transferred to a sheet of paper which is pressedonto the photoreceptor and then fixed (fused) by a thermal (infrared) treatment
Modern copying machines employ dual-layer photoreceptors (see Fig 62) Inthis way charge generation and charge transport are separated The charge genera-tion layer (CGL 05ndash50 m) is optimized for the spectral response and the quan-tum yield of charge carrier formation and the charge transport layer (CTL 15ndash30 m) is optimized for the drift mobility of the charge carriers and for wear re-sistance
Dual-layer systems have the advantages of high sensitivity long process life-time and a reduction in the hysteresis of latent image formation The transportlayer requires the displacement of either electrons or holes Since most trans-
143
6Technical developments related to photophysical processesin polymers
port layers are formulated to transport holes dual-layer receptors are usuallynegatively charged
Numerous compounds have been tested and applied commercially as charge-generation and charge-transport materials as can best be seen from the bookby Borsenberger and Weiss [4]
6 Technical developments related to photophysical processes in polymers144
Fig 61 Schematic depiction of the xerographic process for apositively corona-charged single-layer photoreceptor
Fig 62 Schematic depiction of the light-induced dischargeprocess for a negatively corona-charged dual-layer photo-receptor CGL and CTL denote the charge generation layerand the charge transport layer respectively
The first all-organic photoreceptor was a single-layer device consisting of a1 1 molar mixture an electron-donor polymer poly(N-vinyl carbazole) and anelectron acceptor TNF (see Chart 21) A very effective dual-layer system desig-nated by the acronym TiO(F4-Pc) TTA contains a dispersion of tetrafluorotita-nylphthalocyanine in poly(vinyl butyral) in the charge-generation layer and amixture of tris(p-tolylamine) and polycarbonate in the charge-transport layerHighly sensitive charge-generation systems appropriate for visible and also fornear-infrared light were obtained upon doping polymers with pigment particlesof dyes In this case the CG layers consist of a light-sensitive crystalline phasedispersed in the polymeric matrix Besides phthalocyanines pigments employedcomprise azo compounds squaraines and polycyclic aromatic compounds (thechemical structures of which are shown in Table 21) Improved sensitivitieshave sometimes been achieved with pigment mixtures As a typical exampleFig 63 presents results obtained with a dual-layer system [8 9] Here the CGlayer consisted of a dispersion of the triphenylamine triazo pigment AZO-3 (seeChart 61) in poly(vinyl butyral) in a 4 10 weight ratio while the CT layer con-sisted of a mixture of bisphenol A polycarbonate and the triarylamine derivativeMAPS (see Chart 61) in a 10 9 weight ratio
Note that the value of the quantum yield of charge carrier formation is veryhigh about 045 at F= 3105 V cmndash1 and remains practically constant over theinvestigated wavelength range from 470 to 790 nm Interestingly the quantumyield found for the single-layer system was about one order of magnitude lowerThe very high quantum yield is interpreted in terms of exciton dissociation atthe interface between the two layers and injection of practically all of the holesinto the charge-transport layer
61 Electrophotography ndash Xerography 145
Fig 63 Charge generation in a dual-layerphotoreceptor system The quantum yield ofcharge generation as a function of the wave-length of the incident light at
F = 3105 V cmndash1 () and F= 08105 V cmndash1
() See text for system characterizationAdapted from Williams [8] with permissionfrom John Wiley amp Sons Inc
Regarding the charge-transport layers materials for hole and electron trans-port have to be discriminated A large number of hole-transport materials con-tain arylamine moieties Moreover polysilylenes are well-suited for hole trans-port A key requirement for dual-layer systems is a high efficiency of charge in-jection from the generation layer into the transport layer Moreover it is impor-tant that the charge transport is not impeded by trapping and that the transittime is short compared to the time between exposure and development Formost applications a hole mobility between 10ndash6 and 10ndash5 cm2 Vndash1 sndash1 is suffi-cient
The requirements for electron-transport materials cannot be fulfilled easilyFor instance an appropriate compound should be weakly polar and have a lowreduction potential ie a high electron affinity Actually the electron affinityshould be higher than that of molecular oxygen which is always present Forthis reason and because of some additional difficulties electron-transport layershave not yet been used in commercial applications [4]
62Polymeric light sources
One of the most fascinating developments in recent times concerns the genera-tion of light with the aid of polymers This development is characterized by twoinventions which are described in the following subsections the polymericlight-emitting diode and the polymer laser
6 Technical developments related to photophysical processes in polymers146
Chart 61 Chemical structures of the triphenylamine triazopigment AZO-3 and the triarylamine derivative MAPS
621Light-emitting diodes
6211 General aspectsPolymeric light-emitting diodes operate on the basis of electroluminescence ieluminescence generated by the application of high electric fields to thin poly-mer layers Devices based on the electroluminescence of organic materials com-monly denoted as organic light-emitting diodes OLEDs are used for examplefor mini-displays in wrist watches and chip cards for flexible screens and foremitting wall paper In contrast to liquid-crystal displays (LCDs) OLED displayscan be seen from all viewing angles OLED devices can be extremely thin flex-ible and of low weight Moreover production costs and energy consumptionare low Consequently the potential for making large-area multicolor displaysfrom easily processable polymers has initiated a large number of research pro-
62 Polymeric light sources 147
Table 61 Poly(p-phenylene vinylene)s used in light-emitting diodes [11 12 20]
Polymer Acronym EL Maximum (nm)
PPV 540
PMPPV 560
MEH-PPV 590
PMCYH-PV 590
PDFPV 600
PPFPV 520
jects in the area of polymer light-emitting diodes as has been documented byseveral reviews [10ndash23]
The phenomenon of polymer-based electroluminescence was first demon-strated in the case of poly(p-phenylene vinylene) PPV ( energy gap25 eV) [24] and was later also observed with many PPV derivatives and otherfully -conjugated polymers Typical representatives are shown in Tables 61 and62 Table 61 relates to PPV and some of its derivatives whereas Table 62 listsother classes of polymers that have been employed in LED work
6 Technical developments related to photophysical processes in polymers148
Table 62 Polymers employed in light-emitting diodes [10a]
Polymer class Structure of typical polymer Characteristics
Polythiophenesp-Type (hole-transporting) polymers Alkylgroups provide for solubility in organic sol-vents Emission tunable from UV to IRthrough varying the substituent
Poly-p-phenylenesp-Type polymers of rather high thermal sta-bility mostly used in the form of polymerscontaining oligo-p-phenylene sequencesEmit light in the blue wavelength range
Polyfluorenesp-Type polymers of improved thermal andphotostability (relative to PPV) Emit lightprimarily in the blue wavelength range
R typically hexyl octyl ethylhexyl
Cyano polymersPolymers eg PPV derivatives containingelectron-withdrawing cyano groups The lat-ter provide for electron transport thus com-plementing the hole-transport property
Pyridine-containingpolymers
Highly luminescent polymers soluble in or-ganic solvents High electron affinity affordsimproved electron transportQuaternization of nitrogen allows manipula-tion of the emission wavelength
Oxadiazole-containingpolymers
Oxadiazole groups provide for efficient elec-tron transport Insertion of these groupsinto p-type polymers facilitates bipolar car-rier transport
In this connection the reader is referred to a rather comprehensive reviewdealing with the various classes of polymers tested for LED application [10 a]and to a list of appropriate commercially available materials [25]
As can be seen from Fig 64 a an OLED consists in the simplest case of apolymer film placed between two electrodes one of them being light-transpar-ent such as indium tin oxide (ITO) and the other being a metal of low workfunction eg barium calcium or aluminum
Holes and electrons are injected from the ITO electrode (anode) and the me-tal electrode (cathode) respectively The energy level diagram under forwardbias is shown in Fig 65 More sophisticated OLEDs possess multilayer struc-tures as shown in Fig 64 b
62 Polymeric light sources 149
Fig 64 (a) Structure of a single-layer polymer LED(b) Structure of a multilayer polymer LED
Fig 65 Energy level diagram of a single-layer polymer LEDunder forward bias The z-direction is parallel to the currentdirection and hence perpendicular to the layer Adapted fromGraupner [13] with permission from the Center forPhotochemical Sciences Bowling Green
As can be seen from the typical luminancendashvoltage characteristic presented inFig 66 light generation requires a minimum voltage the turn-on voltage atwhich light emission commences
The luminance increases drastically on further increasing the voltage immedi-ately beyond the onset and later approaches saturation The curve in Fig 66 refersto a 240 1 blend of the polymers denoted as MEH-PPV and PCzDBT20 (seeChart 62) [26] In this case red light with a maximum at about 680 nm is emittedHere the turn-on voltage is quite low (lt 2 V) and the external quantum yield israther high 13ext = 0038 13ext represents the number of photons penetrating thedevice surface to the outside generated per injected electron The availability ofhighly efficient OLEDs emitting light of the primary colors ndash red green and bluendash is important for the realization of full color display applications
6212 MechanismThe injection of charges from the electrodes into the bulk organic material isdetermined by various parameters Since holes are injected into the highest oc-cupied molecular orbital (HOMO) and electrons into the lowest unoccupied mo-lecular orbital (LUMO) matching of energy levels is required This is demon-
6 Technical developments related to photophysical processes in polymers150
Fig 66 Luminancendashvoltage characteristic for the polymerblend PCzDBT20MEH-PPV (1240) Adapted from Niu et al[26] with permission from Wiley-VCH
Chart 62 Polymers contained in the blend referred to in Fig 66
strated for a two-layer OLED of the structure shown in Chart 63 by the energylevel diagram presented in Fig 67 [12]
This diagram illustrates the equivalence of the valence band with the ioniza-tion potential (IP) and the HOMO as well as that of the conduction band withthe electron affinity (EA) and the LUMO Notably electron and hole injectionare controlled by the energy barrier between the contact and the organic materi-al In the absence of surface states and a depletion region due to impurity dop-ing the energy barriers are given by Eqs (6-1) and (6-2)
Eh IP 13anode for holes 6-1
Eel 13cathode EA for electrons 6-2
Here 13anode and 13cathode denote the work functions of the contact materialsDepending on the magnitude of E the current flow through an OLED can beeither space-charge limited (SCL) ie transport-limited or injection-limited Pre-requisites for SCL are that the injection barrier is rather low and that one of thecontacts supplies more charge carries per unit time than can be transportedthrough the organic material layer Commonly injection-limited conduction isdescribed by Fowler-Nordheim (FN) tunneling into the transport band or by Ri-chardson-Schottky (RS) thermionic emission [27 28] The FN model ignores im-age-charge effects and assumes tunneling of electrons from the contact throughthe barrier into a continuum of states The RS model assumes that electronscapable of ejection from the contact have acquired sufficiently high thermal en-ergies to cross the potential maximum resulting from the superposition of theexternal and the image-charge potentials These models were developed forband-type materials However it turned out that they are inadequate for describ-
62 Polymeric light sources 151
ITO anodehole-transporting layer (HTL)emitting layer (EML)metal cathode
Chart 63 Structure of a two-layer OLED
Fig 67 Energy level diagram fora two-layer polymer LEDshowing the ITO anode thehole-transporting layer HTL theemitting and electron-transporting layer EML and themetal cathode EV denotes thevacuum potential
ing the currentndashvoltage dependence measured for disordered organic materials[29] In organic materials the charge carriers are not very mobile because theyare localized and the transport involves localized discrete hopping steps withina distribution of energy states For charge carrier injection of electrons from ametal contact into such organic hopping systems a Monte Carlo simulationyielded excellent agreement with the experimentally observed dependence of theinjection current on electric field strength and temperature [30 31] It is basedon the concept of temperature and field-assisted injection from the Fermi levelof an electrode into the manifold of hopping states Under the influence of theapplied electric field the injected oppositely charged carriers migrate throughthe system towards the electrodes and a portion of them eventually combine toform excited electron-hole singlet states so-called singlet excitons The latter un-dergo radiative decay to only a small extent that is to say electroluminescencequantum yields in terms of emitted photons per injected electron are relativelylow and amount to only a few per cent even in the best cases Competing pro-cesses are operative such as singlet-triplet crossing singlet-exciton quenchingetc Figure 68 shows typical photoluminescence and electroluminescence spec-tra recorded for PPV and two PPV derivatives
6 Technical developments related to photophysical processes in polymers152
Fig 68 Photoluminescence (a)and electroluminescence spec-tra (b) of PPV PMCYH-PV andPPFPV Adapted from Shim etal [11] with permission fromSpringer
Obviously in these cases the maxima of both types of emission spectra arealmost the same indicating that the emission originates from the same speciesIn both cases the peak position is red-shifted when strongly electron-donatinggroups are attached to the conjugated backbone of the polymer Therefore it ispossible to tune the color of the electroluminescent emission by varying thechemical nature of the substituent A blue color can be obtained by wideningthe gap through shortening the conjugation length and lowering the elec-tron density in the conjugated backbone In the case of PPFPV the emissionmaximum lies in the greenish-blue region Here the strong electron-withdraw-ing influence of the perfluorobiphenyl group lowers the electron density in the
62 Polymeric light sources 153
Table 63 Hole and electron transport materials employed in polymer LEDs [10a]
Chemical structure Acronym
Hole transport materials
TPD
PPV
PVK
PMPS
Electron transport materials
PBD
Alq3
PMA-PBD
polymer chain and thus causes a shift of the maximum from 540 nm (PPV) toabout 520 nm
Notably the major steps in the electroluminescence mechanism are injectiontransport and recombination of charge carriers Good carrier transport and effi-cient recombination in the same material are antagonists because the combina-tion probability is low if the charge carriers swiftly migrate to the electrodeswithout interaction with their oppositely charged counterparts A solution tothis dilemma was found with devices consisting of several layers In manycases a layer allowing swift hole transport and blocking of the passage of elec-trons has been combined with a layer permitting only electron transport andserving as an emitting layer Table 63 presents typical hole and electron trans-port materials [10 a]
6213 Polarized light from OLEDsProvided that the macromolecules in a thin film employed as an emitting layerin a LED device are well oriented the emitted light is largely polarized [31] Re-garding conjugated polymers this phenomenon has attracted broad interest be-cause low-cost techniques for chain alignment in such polymers are availablePolarized electroluminescence is useful for certain applications for instance forthe background illumination of liquid-crystal displays (LCDs) [20 32] The firstLED device emitting polarized light was realized with the stretch-oriented poly-thiophene PTOPT (see Chart 64) [33]
The methods commonly used for chain alignment in polymer films havebeen reviewed [34] They comprise the Langmuir-Blodgett technique rubbing ofthe film surface mechanical stretching of the film and orientation on pre-aligned substrates As an example electroluminescence spectra of the orientedsubstituted poly(p-phenylene) presented in Chart 65 are shown in Fig 69 a [35]
The device prepared by the Langmuir-Blodgett (LB) technique had the struc-ture shown in Chart 66
6 Technical developments related to photophysical processes in polymers154
Chart 64 Chemical structure of poly[3-(4-octylphenyl)-22-bithiophene] PTOPT
Chart 65 Chemical structure of an orientedsubstituted poly(p-phenylene) [35]
As demonstrated schematically in Fig 69 b the rigid rod-like macromoleculesare oriented parallel to the substrate plane and their backbones exhibit a prefer-ential orientation along the dipping direction employed during LB processing
From the emission spectra recorded with the polarization of the light paralleland perpendicular to the dipping direction the polarization ratio can be esti-mated to be somewhat greater than three
6214 White-light OLEDsIn many cases OLED devices have been developed that contain polymers ashole-transport media and low molar mass organic or inorganic compounds asemitting materials This pertains for instance to certain white-light-emittingLEDs two of them being exemplified here The first case refers to a device con-taining CdSe nanoparticles in the emitting layer These particles are embeddedin a polymer namely PPV A device having the multilayer structure shown inChart 67 produces almost white light under a forward bias of 35ndash50 V [36]
The second case refers to a device containing a platinum compound such asFPt-1 or FPt-2 in the emitting layer (see Chart 68)
A device having the multilayer structure shown in Chart 69 emits white lightwith 13ext = 19 at a brightness of 100 cd mndash2 (J = 2 mA cmndash2) The white lightresults from the simultaneous monomer (blue) and excimer (green to red)emission of the Pt compound [37]
62 Polymeric light sources 155
Fig 69 (a) Electroluminescence spectra ofthe oriented substituted poly(p-phenylene)SPPP The emission spectra were recordedwith the polarization direction parallel andperpendicular to the dipping direction
employed during preparation by the LBtechnique (b) Schematic depiction of rigidrod-like macromolecules oriented parallel tothe substrate plane Adapted from Cimrovaet al [35] with permission from Wiley-VCH
ITO anode100 monolayers SPPPAl cathodeChart 66 Device used for recording the electroluminescence spectra depicted in Fig 69a
622Lasers
6221 General aspectsThe term laser is an acronym (light amplification by stimulated emission of ra-diation) that denotes a technical device operating on the basis of the stimulatedemission of light A laser emits monochromatic spatially coherent and stronglypolarized light The essential parts of a laser device are an active material and aresonator ie an optical feedback (see Fig 610)
In classical laser systems such as Ti sapphire-based systems or semiconduc-tor laser diodes the active materials are inorganic compounds In recent yearssuitable organic active materials have been introduced [38ndash41] These organicmaterials may be divided into two classes hostguest systems consisting of ahost material doped with organic dye molecules and systems consisting of con-jugated polymers Typical dyes used in hostguest systems are rhodamines cou-marins and pyrromethenes and these are dissolved in polymeric hosts such aspoly(methyl methacrylate) or methacrylate-containing copolymers In some
6 Technical developments related to photophysical processes in polymers156
ITO anodePEI(CdSe-PPV)Al cathodeChart 67 Device used to produce almost white light PEIpoly(ethylene imine) ndash(CH2ndashCH2ndashNH)nndash
Chart 68 Chemical structures of Pt-containing compounds used to produce white light
ITO anodePEDOTPSS(FPt2-CBP)BCPLiFAl cathodeChart 69 Device used to produce white light PEDOTpoly(34-ethylenedioxythiophene) PSS poly(styrene sulfonicacid) CBP 44-di(N-carbazolyl)-biphenyl (see Chart 610)BCP bathocuproine (29-dimethyl-47-diphenyl-1-10-phenan-throline)
Fig 610 Schematic illustration of an opticallypumped laser device Adapted fromKranzelbinder et al [38] with permission fromthe Institute of Physics Publishing Bristol UK
62 Polymeric light sources 157
Chart 610 Chemical structures of 44-di(N-carbazolyl)-biphenyl CBP and 2-(4-biphenyl)-5-(4-tert-butylphenyl)-134-oxadiazole PBD
Table 64 Conjugated polymers used as laser materials
Polymer a) Chemical structure Resonator Excitationconditions
Ithresholdb)
(J cmndash2)Ref
DOO-PPV Microring = 532 nm= 100 ps
01 [43]
BEH-PPV Microring = 555 nm= 100 fs 25 [44]
BuEH-PPV Microcavity = 435 nm= 10 ns
45 [45]
m-LPPPFlexibledistributedfeedback
= 400 nm= 150 ps
37 [46]
PDOPT Microcavity = 530 nm= 90 fs
012 [47]
a) Acronyms used in this column DOO-PPV poly(25-dioctyloxy-p-phenylene viny-lene) BEH-PPV poly[25-di-(2-ethylhexyloxy)-p-phenylene vinylene] BuEH-PPVpoly[2-butyl-5-(2-ethylhexyl)-p-phenylene vinylene] m-LPPP ladder-type poly(p-phenylene) bearing methyl groups PDOPT poly[3-(25-dioctylphenyl)thiophene]
b) Threshold pulse intensity for lasing
cases low molar mass materials have been employed as host materials such asCBP or PBD (see Chart 610)
In systems of the type PBDpoly(p-phenylene vinylene) derivative the hostmaterial PBD absorbs the pump light and transfers the excitation energy tothe polymer here the emitting guest [42] Appropriate conjugated polymers cit-ed in the literature are presented in Table 64
It seems that m-LPPP a ladder-type poly(p-phenylene) is one of the mostpromising materials for laser application It is soluble in nonpolar organic sol-vents thus enabling the facile preparation of thin layers on substrates that maypossess structured uneven surfaces
6222 Lasing mechanismAt present polymer lasers are operated by optical pumping ie through the ab-sorption of light by the active material A four-level energy scheme similar tothat used for organic laser dyes serves to explain the lasing mechanism in thecase of conjugated polymers As can be seen in Fig 611 the absorption of aphoton corresponds to a transition from the lowest vibronic level of the groundstate S0 to a higher-lying vibronic level of the singlet state S1
Rapid (non-radiative) internal conversion leads to the lowest vibronic excita-tion level of the S1 manifold Subsequent transition from this level to one of thevibronic excitation levels of the S0 manifold is radiative and corresponds toeither spontaneous or stimulated emission SE In terms of a simple modelstimulated emission is generated through the interaction of the excited mole-cules with other photons of equal energy This process can only become impor-tant with respect to other competitive processes such as spontaneous emissionwhen the concentration of excited states is very high ie when the populationof the upper state exceeds that of the lower state a situation denoted by theterm population inversion In other words the Boltzmann equilibrium of statesmust be disturbed Notably the lasing transition relates to energy levels that arenot directly involved in the optical pumping process The laser potential of anactive material is characterized by Eq (6-3)
6 Technical developments related to photophysical processes in polymers158
Fig 611 Energy scheme illustrating stimulated emission in conjugated polymers
Iout Iin expNexcL 6-3
Here Iin and Iout denote the intensities of the incoming and outgoing beam re-spectively is the cross-section for stimulated emission Nexc is the concentra-tion of excited S1 states and L is the path length of the light in the sample Theterm gnet = Nexc represents the net gain coefficient of the material
As pointed out above the transition from spontaneous to stimulated emissionrequires population inversion In other words SE becomes significant whenNexc exceeds a critical value Nexc(crit) which characterizes the lasing thresholdExperimentalists frequently denote the threshold in terms of the energy ormore exactly the intensity Ithreshold of the excitation light pulse Figure 612shows a schematic depiction of the dependence of the laser output on the inten-sity of the excitation light pulse
Typical Ithreshold values are given in Table 64 In films of conjugated poly-mers Nexc(crit) is about 1018 cmndash3 if a resonator is not operative Significantlythe employment of appropriate feedback structures lowers the threshold by sev-eral orders of magnitude
6223 Optical resonator structuresAs has been pointed out above a laser basically consists of an active materialand a resonator The latter enables the build-up of certain resonant modes andessentially determines the lasing characteristics In most conventional devicesthe optical feedback is provided by an external cavity with two end mirrorsforming the resonator With the advent of polymers as active materials variousnew feedback structures were invented Initially a microcavity resonator deviceof the type shown schematically in Fig 613 a was employed [48]
This device consisted of a PPV layer placed between a highly reflective distrib-uted Bragg reflector DBR and a vacuum-deposited silver layer functioning as thesecond mirror The emission characteristics at different intensities of the pumpinglight are shown in Fig 613 b At low intensity the emission consisted of three dif-ferent modes whereas at high intensity it was concentrated into the mode of thehighest gain Moreover the directionality of the emitted light was enhanced by in-creasing the intensity of the exciting light Both effects were taken as evidence for
62 Polymeric light sources 159
Fig 612 Schematic depiction of thedependence of the intensity of the lightemitted from a laser device on theintensity of the exciting light
the occurrence of lasing During the ensuing development resonators in theshape of microspheres microrings and flat microdisks were designed As an ex-ample Fig 614a shows a schematic depiction of a cylindrical microring laser de-vice with an outer diameter of D= 11 m and a lateral length of about 100 m con-sisting of a thin DOO-PPV film coated onto an optical fiber
When the device was excited with 532 nm light pulses (= 100 ps) at an intensitybelow the lasing threshold (100 pJpulse) the spectrum shown in Fig 614 b ex-tending over about 100 nm was emitted Dramatic changes occurred when the in-tensity of the excitation light pulse exceeded the lasing threshold the emissionspectrum collapsed into several dominant microcavity modes [43]
Another device the flexible distributed Bragg reflector laser with an activelayer structure supporting second-order feedback makes full use of the advanta-geous properties of polymers namely flexibility large-area fabrication and low-cost processing [41 42] As can be seen in Fig 615 the device consists of aone-dimensionally periodically structured flexible substrate coated with an m-LPPP layer which acts as a planar wave guide The substrate possesses a peri-odic height modulation with a period of = 300 nm
The surface of the polymer layer exhibits a height modulation with the same per-iod but a smaller amplitude (lt 10 nm) It should be pointed out that the polymerlayer in the device considered here functions as a distributed Bragg reflector and the
6 Technical developments related to photophysical processes in polymers160
Fig 613 The microcavity a vertical cavitylasing device (a) Schematic depiction of thedevice consisting of a distributed Braggreflector a PPV layer and a silver layer(b) Spectra emitted at two different pump
laser energies Eexc = 005 Jpulse (dashedline) and Eexc = 11 Jpulse (solid line)Pulse duration 200ndash300 ps Adapted fromTessler et al [48] with permission fromMcMillan Publishers Ltd
resonant modes for laser oscillation in this strongly frequency-selective feedbackdevice correspond to the wavelength satisfying the Bragg condition [see Eq (6-4)]
m 2n 6-4Here m is the order of diffraction n is the refractive index and is the gratingperiod (height modulation period) Optical feedback is accomplished by way ofthe second-order diffraction mode (m= 2) which is fed into the counter-propa-gating wave The first-order light (m= 1) is coupled out from the waveguide andpropagates perpendicular to the film Provided that the energy of the excitinglight pulses (pulse duration 150 fs 400 nm spot size diameter 200 m) ex-ceeds the threshold value Ethreshold = 15 nJ highly polarized laser light(= 488 nm) is emitted perpendicular to the film plane An improvement overthis method of mode selection was achieved with the aid of two-dimensionallynano-patterned substrates [49] The device depicted schematically in Fig 616emits a monomode beam perpendicular to its surface
62 Polymeric light sources 161
Fig 614 Microring laser device (a)and spectra emitted at excitation lightintensities below (b) and above (c) thethreshold intensity Active materialDOO-PPV coated onto an optical fiberAdapted from Frolov et al [43] withpermission from the American Instituteof Physics
Fig 615 Schematic illustration of a one-dimensionally patterned flexible distributed Braggreflector laser device Active layer 400 nm m-LPPPSubstrate 125 m thick poly(ethylene terephthalate)film covered with acrylic coating Adapted fromKallinger et al [46] with permission from Wiley-VCH
Compared to the one-dimensionally structured device the lasing threshold is30 lower and the divergence of the emission is drastically reduced In accor-dance with the 2D laser operation the emitted light is not polarized in this case
6224 Prospects for electrically pumped polymer lasersAt present an electrically driven polymer laser has yet to be realized [39] Never-theless low-cost polymer laser diodes could be an attractive alternative to thewidely used inorganic laser diodes In principle an electrically pumped polymerlaser could be realized with the aid of an appropriate feedback structure pro-vided that the excitation density Nexc(crit) ie the concentration of excitons ex-ceeded the lasing threshold (see Section 6222) From research concerning opti-cally pumped polymer lasers it is known that Nexc(crit) is about 1018 cmndash3 Thisvalue corresponds to a critical current density of 105 to 106 A cmndash2 [50] How-ever the highest current densities hitherto obtained are about 103 A cmndash2 ieseveral orders of magnitude below the required value Therefore besides thesearch for appropriate device structures and appropriate highly conducting ma-terials strategies aiming at an electrically pumped polymer laser are also con-cerned with achieving much higher exciton concentrations An approach in thisdirection may lie in the application of sharp-edge shaped electrodes with the po-tential of generating locally very high electric fields enabling the formation oflocally very high charge carrier concentrations through field-induced emission
63Polymers in photovoltaic devices
Photovoltaic (PV) cells generate electric power when irradiated with sunlight orartificial light Classical PV cells based on inorganic semiconducting materials
6 Technical developments related to photophysical processes in polymers162
Fig 616 Schematic illustration of a flexiblepolymer laser device consisting of anm-LPPP layer spin-coated onto a two-dimensionally structured flexible poly(ethy-lene terephthalate) substrate The laser light
is emitted perpendicular to the substrateAdapted from Riechel et al [49] withpermission from the American Institute ofPhysics
such as silicon GaAs CdTe or CuInSe2 consist of layers doped with smallamounts of additives that provide n-type (electron) or p-type (hole) conductivity[51ndash59] A ldquobuilt-inrdquo electric field exists across the junction between the two layerswhich sweeps electrons from the n to the p side and holes from the p to the n sideFigure 617 shows the essential features of a (sandwich-structured) p-n homojunc-tion silicon solar cell
The absorption of photons having energies greater than the band gap energypromotes electrons from the valence to the conduction band thus generatinghole-electron pairs The latter rapidly dissociate into free carriers that move in-dependently of each other As these approach the junction they come under theinfluence of the internal electric field which actually prevents recombinationAt present most of the industrially produced photovoltaic cells consist of mono-crystalline or polycrystalline and to some extent of amorphous silicon (a-Si) Dif-ferent types of junctions may be distinguished homojunctions are p-n junctionsformed by adjacent p- and n-doped regions in the same semiconductor of bandgap Ug whereas heterojunctions are formed between two chemically differentsemiconductors with different band gaps Moreover there are p-i-n junctionswhich are formed by interposing an intrinsic undoped layer between p and nlayers of the same semiconductor
Certain organic materials also possess semiconductor properties and can beemployed in PV cells a fact that has recently been attracting growing interestsince the advent of novel polymeric materials [22 60ndash66] Table 65 lists sometypical polymers used in solar cells
Criteria commonly used to characterize PV cells comprise Jsc the short-circuitcurrent density Voc the open-circuit voltage 13cc the quantum efficiency for
63 Polymers in photovoltaic devices 163
Fig 617 Schematic depiction of a p-n homojunctioncrystalline silicon solar cell Typical dimensions of commercialwafers 10 cm10 cm03 mm Adapted from Archer [67]with permission from the World Scientific PublishingCompany
6 Technical developments related to photophysical processes in polymers164
Table 65 Chemical structures of semiconducting polymersused in organic solar cell devices [60ndash66]
Chemical structure Acronym Denotation
MDMO-PPV Poly[2-methoxy-5-(37-dimethyl-octyloxy)-14-phenylene vinylene]
MEH-PPV Poly[2-methoxy-5-(2-ethyl-hexyl-oxy)-14-p henylene vinylene]
MEH-CN-PPV Poly[2-methoxy-5-(2-ethyl-hexyl-oxy)-14-phenylene (1-cyano)vinyl-ene]
CN-PPV Poly[25-di-n-hexyloxy-14-phenyl-ene (1-cyano)vinylene]
P3HT Poly(3-hexylthiophene)
POPT Poly[3-(4-octylphenyl)thiophene]
PEOPT Poly3-[4-(147-trioxaoctyl)-phenyl]thiophene
PEDOT Poly(34-ethylenedioxy thiophene)
PDTI Thiophene-isothianaphthenecopolymer
PTPTB Benzothiadiazole-pyrrolecopolymer
charge carrier generation ie the number of electrons formed per absorbedphoton ffill the fill factor and mp the maximum power conversion efficiencyffill and mp are defined by Eqs (6-5) and (6-6) respectively [67]
ffill impVmpIscVoc 6-5
mp impVmpDr ffilliscVocDr 6-6
Here imp and Vmp denote the current and the voltage at maximum power andDr (W cmndash2) is the incident solar irradiance
Compared with inorganic PV cells organic PV cells resemble the heterojunc-tion type apart from the fact that organic materials do not support the forma-tion of a space-charge region at the junction Figure 618 shows a schematic de-piction of a cell simply formed by the superposition of two layers of semicon-ducting organic materials with different electron affinities and ionization poten-tials One layer functions as the electron donor (p-type conductor) and the otheras the electron acceptor (n-type conductor) In this case the absorption of aphoton is confined to a molecule or to a region of a polymer chain where anexcited state is created This localized excited state is frequently termed an exci-ton (see Section 222) It refers to an electron-hole pair in semiconductor termi-nology Charge separation at the interphase requires that the difference in ener-gies of the hole states and the electron states exceeds the binding energy of theelectron-hole pairs This amounts to about 100 meV and is much larger thanthe input energy required for charge separation in inorganic semiconductorsThe efficiency of charge separation is critically determined by the exciton diffu-sion range since after its generation the exciton must reach the junction in or-der to dissociate into two free charge carriers Actually the exciton diffusionrange is at most a few nanometers and therefore a portion of the excitons gen-erated in the bulk of the layer do not dissociate In the course of efforts to over-come this flaw of flat-junction organic solar cells new architectures consistingof phase-separated polymer blends were devised [68ndash70] Figure 619 shows thestructure of such a system and the charge transfer from an exciton at a donoracceptor heterojunction These blend systems consist of interpenetrating bicon-tinuous networks of donor and acceptor phases with domain sizes of 5ndash50 nmand provide donoracceptor heterojunctions distributed throughout the layerthickness In this case the mean distance that the excitons have to travel toreach the interface is within the diffusion range and therefore efficiencies for
63 Polymers in photovoltaic devices 165
Fig 618 Schematic depictionof a flat-heterojunction organicsolar cell
the conversion of incident photons to electric current of over 50 have beenachieved Such systems can be formed for example from blends of donor andacceptor polymers such as MEH-PPV and CN-PPV [68 69] or from compositesof conducting polymers with buckminsterfullerenes such as MEH-PPV+ C60 orpoly(3-hexylthiophene) (P3HT) + C60 [70ndash74] In the latter cases the preparationof appropriate composites is facilitated by using fullerene derivatives with im-proved solubility such as PCBM the structure of which is presented inChart 611 [65 75]
In typical experiments thin (100 nm) films of polymer blends were depositedby spin coating from a solution of the two polymers Alternatively two thin filmsof a hole-accepting and an electron-accepting polymer that had been deposited onITO or metal substrates were laminated together in a controlled annealing pro-
6 Technical developments related to photophysical processes in polymers166
Fig 619 Schematic diagram depicting charge transfer froman exciton at a donoracceptor heterojunction in a compositeof two conducting polymers
Chart 611 Chemical structure of 1-(3-methoxycarbonyl)-propyl-1-phenyl-[66]C61 PCBM
cess In the latter case a 20ndash30 nm deep interpenetration between the two layerswas revealed by atomic force microscopy [76] Performance characteristics of someof these organic PV cells and those of silicon cells are shown in Table 66
Obviously the performance of organic cells having bicontinuous networkstructures with quantum efficiencies of about 50 and power conversion effi-ciencies of about 5 remains far inferior to that of silicon cells but is highlyimproved as compared to that of flat-junction organic cells which have bothquantum efficiencies and power conversion efficiencies of less than 01
In conclusion for various reasons certain organic materials and especiallypolymers are attractive for use in photovoltaics There is the prospect of inex-pensive production of large-area solar cells at ambient temperature since high-throughput manufacture using simple procedures such as spin-casting or spraydeposition and reel-to-reel handling is feasible It is possible to produce verythin flexible devices which may be integrated into appliances or building mate-rials Moreover it seems that new markets will become accessible with the aidof polymer-based photovoltaic elements This concerns daily life consumergoods such as toys chip cards intelligent textiles and electronic equipmentwith low energy consumption
64Polymer optical waveguides
641General aspects
With the advent of semiconductor lasers a new technique of information trans-mission based on optical fibers was developed [77] Instead of propagating dataelectronically by the transport of electrons through coaxial copper cables the
64 Polymer optical waveguides 167
Table 66 Performance characteristics of solar cells
Material system Jsca)
(mA cmndash2)Voc
b)
(V)ffill
c) mpd)
()cc
e) Ref
P3HTPCMB (1 08) 95 063 068 51 [70a]P3HTPCMB (1 1) 106 061 067 44 [70c]MDMO-PPVPCBM 525 082 061 25 050 (470 nm) [70d]POPTMEH-CN-PPV ca 1 ca 1 032 19 029 [76]Amorphous silicon 194 089 074 127 090 [61]Monocrystalline silicon 424 071 083 247 gt090 [61]
a) Short-circuit current densityb) Open-circuit voltagec) Fill factord) Maximum power conversion efficiencye) Quantum efficiency for charge carrier generation
new technique permits optical data transfer by laser light pulses guided throughbranching optical networks operated with the aid of optical fibers Optical fibersconsist of a highly transparent core and a surrounding cladding of refractive in-dices ncore and ncladding respectively Provided that ncore gt ncladding light enteringthe fiber at an angle ltmax is totally reflected at the cladding boundary and isthus transmitted through the fiber
At present copper conductors are still used in short-distance data communi-cation However they can no longer cope with the high bandwidth demands ofmodern communication systems Therefore copper wiring systems are going tobe replaced by high-bandwidth fiber-optic systems The size and weight of opti-cal fiber cables are significantly lower than those of coaxial copper wire cablesin which the single wires must be carefully isolated to prevent electromagneticinterference
642Optical fibers
6421 Polymer versus silica fibersInitially the new fiber-optic technique was based solely on inorganic glass fibersbut in recent years polymeric optical fibers have also become attractive and appearto be in great demand for the transmission and the processing of optical commu-nications compatible with the Internet [78ndash84] As compared with silica fiberspolymer fibers have a larger caliber are cheaper to prepare and easier to processHowever because of their greater light attenuation and their lower frequencybandwidth for signal transmission polymer fibers can only be employed in infor-mation networks over distances of several hundred meters Typical properties ofpolymer and inorganic glass optical fibers are compared in Table 67
Silica fibers are still unsurpassed as regards attenuation and bandwidth buttheir diameter has to be kept rather small to provide for the required cable flex-ibility Consequently skillful hands and high precision tools are required to con-nect silica fibers in a time-consuming process Polymer fibers have a much low-
6 Technical developments related to photophysical processes in polymers168
Table 67 Typical properties of step-index optical fibers [85]
Property PMMA a) Polycarbonate Silica glass
Attenuation coefficient (dB kmndash1) b) 125 at 650 nm 1000 at 650 nm 02 at 1300 nmTransmission capacity Ctrans (MHz km) c) lt 10 lt 10 102 to 103
Numerical aperture 03 to 05 04 to 06 010 to 025Fiber diameter (mm) 025 to 10 025 to 10 910ndash3 to 12510ndash1
Maximum operating temperature (C) 85 85 ca 150
a) Poly(methyl methacrylate)b) = (10L) log (P0PL) P0 and PL input and output power L fiber lengthc) Ctrans product of bandwidth Wband and fiber length L Ctrans = WbandL
Wband044 L tndash1 t t2out t2
in12 tout and tin width (FWHM) of output andinput pulses
er modulus than inorganic glass fibers and can therefore be of a much largerdiameter without compromising their flexibility Since their numerical apertureis larger the acceptance angle ie the light gathering capacity is larger com-pared to that of glass fibers Due to the large core diameter and the high nu-merical aperture the installation of polymer optical fibers is facilitated and in-stallation costs are much lower than for silica glass fiber networks Hence poly-mer optical fibers are suitable for short-distance data communication systemsthat require a large number of connections [85] Generally polymer optical fibersystems are applicable in local area networks (LANs) fiber-to-the-home systemsfiber-optic sensors industrial environments automotive applications eg me-dia-oriented system transport (MOST) devices etc Actually data transmissionrates increase in parallel with the number of devices connected to a system andtransmission rates of 400 Mbit sndash1 or more are envisaged With already existingand commercially available polymer optical fibers of a sufficiently large band-width these requirements can be fulfilled Another interesting field of applica-tion relates to lighting and illumination In this context end or point-sourcelighting and side- or line-lighting devices are to be discriminated The formerare used for motorway signaling and the latter for night illumination of build-ings to give typical examples [85]
The introduction of polymer optical fibers may have an impact on the devel-opment of next-generation light sources for optical communication To date theemission wavelength of semiconductor lasers has been adapted to the absorp-tion characteristics of silica fibers Since polymer optical fibers may be used indifferent wavelength regions a change in an important boundary condition forlight source engineering is anticipated
6422 Compositions of polymer optical fibers (POFs)Polymer optical fibers have been prepared from various amorphous polymerssuch as polycarbonate poly(methyl methacrylate) polystyrene and diglycol dial-lylcarbonate resin [79 80] In these cases the light attenuation of the respectiveoptical fibers is due to absorption by higher harmonics of CndashH vibrations Sub-stitution of hydrogen by deuterium fluorine or chlorine results in a shift of theabsorption due to overtone vibrations to higher wavelengths and reduces the at-tenuation at key communication wavelengths as is apparent from Table 68
64 Polymer optical waveguides 169
Table 68 Light attenuation (approximate values) caused byabsorption due to overtone vibrations at key communicationwavelengths in units of dB kmndash1 [79]
(nm) CndashCl CndashF CndashD CndashH
840 lt10ndash8 10ndash4 101 104
1310 10ndash5 100 103 105
1550 10ndash3 101 105 106
Actually commercial polymeric optical fibers made from a perfluorinatedpolymer (see Chart 612) exhibit an attenuation of 15 dB kmndash1 at = 1300 nmSingle-channel systems can be operated at a transmission rate of 25 Gbit sndash1
over a distance of 550 m at = 840 or 1310 nm [79 86] Besides the intrinsic fac-tors for optical propagation loss mentioned above namely absorption and Ray-leigh light scattering there are extrinsic factors such as dust interface asymme-try between core and cladding variation in core diameter etc that may also af-fect the light transmission
6423 Step-index and graded-index polymer optical fibersTable 67 presents the properties of large-core step-index polymer optical fibers SI-POFs They are characterized by a single refractive index which extends overthe entire core and changes abruptly at the corecladding interface SI-POFspossess a low bandwidth due to extensive pulse broadening An increased band-width is achieved with graded-index polymer optical fibers GI-POFs which pos-sess a refractive index profile over the core Refractive index profiles can be ob-tained by special techniques eg by polymerizing a mixture of two monomersdiffering in size and refractive index in rotating tubes or by photochemical par-tial bleaching of a dopant contained in a polymer [79]
643Polymer planar waveguides
Planar ie rectangular waveguide components are applied in many photonicdevices They can be easily manufactured at low cost Typical applications relateto computer backplanes combining electrical and optical cables [87] thermo-op-tical switches [88] optical splitters of multichannel high-density planar light-wave circuits [89] and polyimide-based electro-optical (EO) modulators [90]
644Polymer claddings
Polymers also play a role in the case of specialized optical equipment wherethe different parts are connected by silica fibers This applies for example toinstruments used for spectroscopic process analysis ie for real-time control ofchemical processes [91] To prevent physical damage the fibers are coated withpoly(vinyl chloride) or acrylate-based polymers Fibers coated with polyimidewithstand temperatures up to 350 C
6 Technical developments related to photophysical processes in polymers170
Chart 612 Chemical structure of a perfluorinatedpolymer used to make optical fibers
References 171
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12 T Bernius M Inbasekaran J OrsquoBrienW-S Wu Progress with Light-EmittingPolymers Adv Mater 12 (2000) 1737
13 W Graupner Science and Technology ofOrganic Light-Emitting Diodes The Spec-trum 15 (2002) 20
14 B Ruhstaller SA Carter S Barth HRiel W Riess J C Scott J Appl Phys89 (2001) 4575
15 DY Kim HN Cho CY Kim BlueLight Emitting Polymers Prog PolymSci 25 (2000) 1089
16 A Greiner Design and Synthesis of Poly-mers for Light-Emitting Diodes PolymAdv Technol 9 (1998) 371
17 J R Sheats YL Chang DB RoitmanA Socking Chemical Aspects of PolymericElectroluminescent Devices Acc ChemRes 32 (1999) 193
18 L J Rothberg A J Lovinger Status andProspects for Organic ElectroluminescenceJ Mater Res 11 (1996) 3174
19 A Kraft A Grimsdale A B HolmesElectroluminescent Conjugated Polymers ndashSeeing Polymers in a New Light AngewChem Int Ed 37 (1998) 402
20 R H Friend RW Gymer A B HolmesJ H Burroughes R N Marks C TalianiDD C Bradley DA dos Santos JLBredas M Loumlgdlund W R SalaneckElectroluminescence in Conjugated Poly-mers Nature 397 (1999) 121
21 A Bolognesi C Botta D FacchinettiM Jandke K Kreger P Strohriegl ARelini R Rolandi S Blumstengel Polar-ized Electroluminescence in Double-LayerLight-Emitting Diodes with PerpendicularlyOriented Polymers Adv Mater 13 (2001)1072
22 M Schwoerer HC Wolf Elektrolumines-zenz und Photovoltaik Chapter 11 in MSchwoerer HC Wolf Organische Mole-kulare Festkoumlrper Wiley-VCH Weinheim(2005)
23 S Miyata HS Nalwa (eds) OrganicElectroluminescent Materials and DevicesGordon amp Breach Amsterdam (1997)
24 J H Burroughes DD C Bradley ARBrown R N Marks K Mackay R HFriend PL Burns A B Holmes Nature347 (1990) 539
25 OLED Cross Reference by Material Func-tion HW Sands Corp httpwwwhwsandscomproductslistsoledcross_reference_material_function_oledhtm
26 Y-H Niu J Huang Y Cao Adv Mater15 (2003) 807
27 J Kalinowski Electronic Processes in Or-ganic Electroluminescence in S MiyataHS Nalwa (eds) Organic Electrolumi-nescent Materials and Devices Gordon ampBreach Amsterdam (1997) p 1
28 H Baumlssler Polym Adv Technol 9(1998) 402
29 S Barth U Wolf H Baumlssler P MuumlllerH Riel H Vestweber PF Seidler WRieszlig Phys Rev B 60 (1999) 8791
30 (a) U Wolf V I Arkhipov H BaumlsslerPhys Rev B 59 (1999) 7507 (b) V I Ar-
6 Technical developments related to photophysical processes in polymers172
khipov U Wolf H Baumlssler Phys Rev B59 (1999) 7514
31 DD C Bradley RH Friend H Linden-berger S Roth Polymer 27 (1986) 1709
32 M Grell DD C Bradley M Inbasekar-an E R Woo Adv Mater 9 (1997) 798
33 P Dyreklev M Berggren O InganaumlsMR Andersson O Wennerstroumlm THjertberg Adv Mater 7 (1995) 43
34 M Grell DD C Bradley Adv Mater 11(1999) 895
35 V Cimrova M Remmers D Neher GWegner Adv Mater 8 (1996) 146
36 M Gao B Richter S Kirstein Adv Ma-ter 9 (1997) 802
37 BW D Andrade J Brooks V Adamo-vich ME Thompson SR Forrest AdvMater 14 (2002) 1032
38 G Kranzelbinder G Leising OrganicSolid-State Lasers Rep Prog Phys 63(2000) 729
39 IDF Samuel G A Turnbull PolymerLasers Recent Advances Materials Today7 (2004) 28
40 U Lemmer A Haugeneder C Kallin-ger J Feldmann Lasing in ConjugatedPolymers in G Hadziioannou P vanHutton (eds) Semiconducting PolymersChemistry Physics and Engineering Wiley-VCH Weinheim (2000) p 309
41 U Lemmer C Kallinger J FeldmannPhys Blaumltter 56 (2000) 25
42 Z Bao Y M Chen R B Cai L Yu Mac-romolecules 26 (1993) 5228
43 SV Frolov A Fujii D Chinn ZV Var-deny K Yoshino R V Gregory ApplPhys Lett 72 (1998) 2811
44 Y Kawabe Ch Spielberg A SchuumllzgenMF Nabor B Kippelen EA Mash PAllemand M Kuwata-Gonokami K Ta-keda N Peyghambarian Appl PhysLett 72 (1998) 141
45 MD McGehee R Gupta S VeenstraEK Miller MA Diaz-Garcia A J Hee-ger Phys Rev B 58 (1998) 7035
46 C Kallinger M Hilmer A HaugenederM Perner W Spirkl U Lemmer JFeldmann U Scherf K Muumlllen AGombert V Wittwer Adv Mater 10(1998) 920
47 T Granlund M Theander M BerggrenM Andersson A Ruzeckas V Sund-
strom G Bjork M Granstrom O Inga-nas Chem Phys Lett 288 (1998) 879
48 N Tessler G J Denton R H FriendNature 382 (1996) 695
49 S Riechel C Kallinger U Lemmer JFeldmann A Gombert V Wittwer UScherf Appl Phys Lett 77 (2000) 2310
50 F Hide B J Schwartz MA Diaz-Gar-cia A J Heeger Chem Phys Lett 256(1996) 424
51 MD Archer R Hill (eds) CleanElectricity from Photovoltaics ImperialCollege Press London (2001)
52 R Messenger G Ventre PhotovoltaicSystems Engineering CRC Press Boca Ra-ton FL USA (1999)
53 J Perlin From Space to Earth The Storyof Solar Electricity Aatec PublicationsAnn Arbor MI USA (1999)
54 R H Bube Photovoltaic Materials Imper-ial College Press London (1998)
55 H-J Lewerenz H Jungblut Photovol-taik Springer Berlin (1995)
56 MA Green Silicon Solar Cells AdvancedPrinciples and Practice Centre for Photo-voltaic Devices and Systems Universityof New South Wales Sydney (1995)
57 SR Wenham MA Green ME WattApplied Photovoltaics Centre for Photo-voltaic Devices and Systems Universityof New South Wales Sydney (1995)
58 LD Partain (ed) Solar Cells and TheirApplications Wiley-Interscience NewYork (1995)
59 T Markvart (ed) Solar Electricity WileyChichester (1994)
60 (a) N S Sariciftci Plastic Photovoltaic De-vices Materials Today 7 (2004) 36 (b)C J Brabec V Dyakonov J Parisi NSSariciftci (eds) Organic PhotovoltaicsConcept and Realization Springer Berlin(2003)
61 J Nelson (a) Organic and Plastic SolarCells Chapter IIe-2 in T Markvart LCatantildeer (eds) Practical Handbook ofPhotovoltaics Fundamentals and Applica-tions Elsevier Oxford (2003) (b) Materi-als Today 5 (2002) 20
62 J JM Halls R H Friend Organic Photo-voltaic Devices in Ref [51] p 377
63 J-F Nierengarten G Hadziioannou NArmaroli Materials Today 4 (2001) 16
References 173
64 (a) C J Brabec Organic PhotovoltaicsTechnology and Markets Solar Energy Ma-ter Solar Cells 83 (2004) 273 (b) C JBrabec N S Sariciftci J Keppler Mate-rials Today 3 (2000) 5
65 A Dhanabalan J K J van Duren PA vanHal JL J van Dongen R A J JannssenAdv Funct Mater 11 (2001) 255
66 SE Shaheen D Vangeneugden R Kie-booms D Vanderzande T Fromherz FPadinger C J Brabec N S SariciftciSynth Met 121 (2001) 1583
67 MD Archer The Past and Present inRef [51] p 1
68 J JM Halls CA Walsh N C Green-ham EA Marseglia RH Friend S CMoratti A B Holmes Efficient Photo-diodes from Interpenetrating Networks Na-ture 376 (1995) 498
69 G Yu J Gao J C Hummelen F WudlA J Heeger Science 270 (1995) 1789
70 (a) H Hoppe NS Sariciftci Morphologyof PolymerFullerene Bulk HeterojunctionSolar Cells J Mater Chem 16 (2006) 45(b) M Al-Ibrahim H-K Roth U Zho-khavets G Gobsch S Sensfuss SolarEnergy Mater Solar Cells 85 (2005) 13(c) G Li V Shrotriya J Huang Y YadT Moriarty K Emery Y Yang NatureMater 4 (2005) 864 (d) SE ShaheenC J Brabec NS Sariciftci F PadingerT Fromherz J C Hummelen ApplPhys Lett 78 (2001) 841
71 I Riedel M Pientka V DyakonovCharge Carrier Photogeneration and Trans-port in Polymer-Fullerene Bulk-Heterojunc-tion Solar Cells Chapter 15 in W Bruumlt-ting (ed) Physics of Organic Semiconduc-tors Wiley-VCH Weinheim (2005)
72 N Armaroli E Barigefletti P CeroniJ-E Eckert J-F Nicoud J-F Nierengar-ten Chem Commun (2000) 599
73 J-E Eckert J J Nicoud J-F Nierengar-ten S-G Liu L Echegoyen F Barigel-letti N Armaroli L Ouali V KrasnikovG Hadziioannou J Am Chem Soc122 (2000) 7467
74 J-F Nierengarten J-E Eckert J J Ni-coud L Ouali V Krasnikov G Had-ziioannou Chem Commun (1999) 617
75 CJ Brabec V Dyakonov PhotoinducedCharge Transfer in Bulk HeterojunctionComposites in Ref [60b]
76 M Granstroumlm K Petritsch A C AriasA Lux MR Andersson RH FriendNature 395 (1998) 257
77 H Zanger Fiber Optics Communicationand Other Applications McMillan NewYork (1991)
78 HS Nalwa Polymer Optical FibersAmerican Scientific Publishers Steven-son Ranch CA USA (2004)
79 W Daum J Krauser P E Zamzow OZiemann POF ndash Polymer Optical Fibersfor Data Communication Springer Berlin(2002)
80 K Horie H Ushiki FM Winnik Mo-lecular Photonics Fundamentals and Prac-tical Aspects Kodansha-Wiley-VCHWeinheim (2000)
81 A Weinert Plastic Optical Fibers Princi-ples Components Installation MCD Ver-lag Erlangen (1999)
82 J Hecht City of Light The Story of FiberOptics Oxford University Press NewYork (1999)
83 T Kaino Polymers for Light Wave and In-tegrated Optics LA Hornak (ed) Dek-ker New York (1992)
84 M Kitazawa POF Data Book MCRTechno Research Tokyo (1993)
85 MA de Graaf Transmissive and EmissivePolymer Waveguides for Communicationand Illumination University Press Facili-ties Eindhoven The Netherlands (2002)
86 G-D Khoe H van den Boom I T Mon-roy High Capacity Transmission SystemsChapter 6 in [78]
87 J Moisel J Guttman H-P Huber OKrumpholz M Rode R BogenbergerK-P Kuhn Opt Eng 39 (2000) 673
88 N Keil HH Yao C Zawadski KLoumlsch K Satzke W Wischmann J VWirth J Schneider J Bauer M BauerElectron Lett 37 (2001) 89
89 J T Kim CG Choi J Micromech Mi-croeng 15 (2005) 1140
90 S Ermer Applications of Polyimides toPhotonic Devices in K Horie T Yamashi-ta (eds) Photosensitive Polyimides Funda-mentals and Applications TechnomicLancaster PA USA (1995)
91 J Andrews P Dallin Spectroscopy Eu-rope 15 (2003) 23
Part IILight-induced chemical processes in polymers
71Introductory remarks
According to the Grotthus-Draper law chemical changes can only be producedin a system by absorbed radiation It has been pointed out in Chapter 1 thatlight absorption involves electronic transitions As regards organic moleculessuch transitions occur with a high probability if some of the constituent atomsare arranged in special bonding positions Such arrangements are termed chro-mophoric groups (Chapter 1 Table 11) They become resonant at certain light fre-quencies Resonance gives rise to absorption bands in the absorption spectrum(Chapter 1 Figs 14 and 15) The chemical activity of a chromophoric groupmay originate from two features (a) The bonding strength between adjacentatoms is strongly reduced when an electron is promoted to a higher levelTherefore a chemical bond can be cleaved if the atoms separate upon vibrationThis type of monomolecular bond cleavage is a very rapid process (ca 10-12 s)that cannot be prevented by any means after the absorption of a photon (b)The electronic excitation leads to a relatively stable state The lifetime of the ex-cited state is so long (occasionally approaching the ms range) that in the con-densed phase chromophoric groups have many encounters with the surround-ing molecules thus enabling bimolecular chemical interactions Thereby theoriginal chemical bond is relinquished and a new bond is formed This type ofbond cleavage can be prevented by energy quenching (see Chapter 1) ie throughenergy transfer from the excited chromophore to an additive functioning as anenergy acceptor The bond scission processes mentioned above are energeticallyfeasible since the photon energies associated with radiation of wavelengthsranging from 250 nm (496 eV) to 400 nm (31 eV) correspond to the bond dis-sociation energies of common covalent bonds ie about 35 eV for CndashH CndashCand CndashO bonds (in aliphatic compounds) Although these considerations applyto both small and large molecules there are certain aspects pertaining to poly-mers that merit special attention and these are dealt with in this chapter Thesubsequent sections are related overwhelmingly to phenomena associated withapplication aspects Cross-linking and main-chain scission for example playkey roles in lithographic applications and photo-oxidation reactions are ofprominent importance for the behavior of polymers in outdoor applications
177
7Photoreactions in synthetic polymers
It should be emphasized that a plethora of research papers and patents havebeen devoted to the field of photoreactions in synthetic polymers However onlya few important results are highlighted in this chapter For more detailed infor-mation the reader is referred to relevant books and reviews [1ndash28]
711Amplification effects
Photochemical reactions in polymers may result in amplification effects as be-comes obvious if we consider the example of the photochemical coupling of twomolecules In a system of linear chain macromolecules consisting of a largenumber of base units the formation of a given small number of cross-linksmay lead to an enormous property change This is so because each cross-linkconnects two chains with many base units which are all then affected Conse-quently the polymer may become insoluble in solvents if on average each mac-romolecule only contains one cross-link site On the other hand a propertychange is hardly detectable if the same number of cross-links is generated in asystem consisting of small molecules because in this case each cross-link in-volves only two small molecules and leaves the other molecules unaffected
712Multiplicity of photoproducts
The deactivation of identical electronically excited chromophores can result inthe cleavage of different chemical bonds This common phenomenon is demon-strated for two polymers polystyrene and poly(methyl methacrylate) inSchemes 71 and 72 Note that the bond cleavage probabilities are not equalie the quantum yields for the individual processes may differ by orders ofmagnitudes
As indicated in Schemes 71 and 72 several different free radicals are gener-ated upon exposure to light These radicals undergo various reactions eg hy-drogen abstraction reactions thereby generating new free radicals and couplingreactions In this way a variety of products is eventually formed as is demon-strated in Scheme 73 for the case of polystyrene
Notably this scheme does not cover all of the initially formed free radicals(see before Scheme 71) Therefore the number of photoproducts formed inthe case of polystyrene exceeds that shown in Scheme 73
Obviously photochemical methods based on the direct absorption of light bythe polymer can hardly be envisaged for chemical modifications of commercialpolymers Most practical applications especially those devoted to photolithogra-phy concern light-induced changes in the solubility of polymers as a conse-quence of intermolecular cross-linking or main-chain scission In these casesonly reactions causing changes in the average molar mass are important be-cause other photoreactions and the resulting products are ineffective with re-spect to the desired property change
7 Photoreactions in synthetic polymers178
71 Introductory remarks 179
Scheme 71 Primary reactions in the photolysis of polystyrene [9]
Scheme 72 Primary reactions in the photolysis of poly(methyl methacrylate) [14]
713Impurity chromophores
Commonly commercial polymers contain impurities originating from the poly-merization or from processing These impurities although mostly present intrace amounts only play an undesired role because they are capable of absorb-ing the near-UV portion (290ndash400 nm) of the solar radiation reaching the earthand therefore jeopardize or curtail the stability of the polymers in outdoor ap-plications hastening degradation According to the structures of their repeatingunits some of the practically important linear polymers such as polyethylenepolypropylene and poly(vinyl chloride) should be transparent to light ofgt 250 nm However commercial polymer formulations contain impurity chro-mophores (see Table 71) which absorb UV light Consequently these formula-tions are subject to severe degradation in the absence of stabilizers
Some of the chromophores shown in Table 71 are chemically incorporatedinto the polymers such as carbonyl groups or carbon-carbon double bondswhereas others are adventitiously dispersed such as polynuclear aromatic com-pounds and metal salts The latter are almost invariably present in many poly-mers Oxygen-polymer charge-transfer complexes have been postulated as addi-tional UV light-absorbing species Apart from the latter the impurity chromo-phores listed in Table 71 function as free radical generators as illustrated inScheme 74 Hydroperoxide groups the most common and important of chro-mophores yield highly reactive hydroxyl radicals Carbonyl groups can give riseto the formation of various kinds of free radicals as outlined in Section 714Moreover they may act as donors in energy-transfer processes which also ap-
7 Photoreactions in synthetic polymers180
Scheme 73 Reactions of a benzyl-type macroradical formedin the photolysis of polystyrene [9]
plies for polynuclear aromatic compounds Metal salts produce free radicals byelectron-transfer processes In the case of poly(vinyl chloride) allyl-type chlorineatoms are split off
Most of the radicals generated by photoreactions of impurity chromophorescan abstract hydrogen atoms from the surrounding polymer This applies espe-cially to hydroxyl and chlorine radicals
Dioxygen-polymer charge-transfer complexes are assumed to form hydroper-oxide groups [Eq (7-1)]
71 Introductory remarks 181
Table 71 Impurity chromophores commonly contained incommercial polyalkenes or poly(vinyl chloride)s
Structure of chromophore Denotation
Hydroperoxide group
Carbonyl group
-Unsaturated carbonyl group
Double bonds
Conjugated double bonds
Polynuclear aromatics (eg naphthalene an-thracene rubrene)
Metal ions
Charge-transfer complex
13RH O2CTh 13RH O
2 CT R OOH ROOH 7-1
714Photoreactions of carbonyl groups
The detrimental environmental degradation of unstabilized commercial poly-meric products consisting of polyethylene polypropylene poly(vinyl chloride)etc is frequently due to very small amounts of ketonic carbonyl groups Elec-tronically excited ketone groups can undergo different processes in particularthe so-called Norrish type I and Norrish type II reactions as illustrated inScheme 75 for the case of a copolymer of ethylene and carbon monoxide
7 Photoreactions in synthetic polymers182
Scheme 74 Generation of free radicals by photoreactions ofimpurity chromophores and ensuing hydrogen abstractionfrom the polymer
According to the Norrish type I reaction a carbon-carbon bond in a position to the carbonyl group is cleaved The resulting ketyl radical is very likely to re-lease carbon monoxide [Eq (7-2)]
R C R COO
7-2
The Norrish type II process refers to a CndashC bond cleavage initiated by the ab-straction of a hydrogen in a -position with respect to the carbonyl group
Note that Norrish-type reactions are not only of importance in relation to var-ious polymers containing ketonic impurities but they also play a dominant rolein the photolysis of all polymers containing carbonyl groups as constituent moi-eties such as polyacrylates polymethacrylates poly(vinyl acetate) polyestersand polyamides
72Cross-linking
The formation of intermolecular cross-links ie covalent bonds between differ-ent polymer chains causes an increase in the average molar mass and even-tually combines all of the macromolecules into a three-dimensional insolublenetwork Cross-linking can be accomplished in various ways Several methodsrely on reactions of electronically excited pendant groups on the polymerchains others on reactions of various kinds of reactive species in the groundstate that are photogenerated in polymeric systems Typical of the former reac-tion type are [2+2] cycloadditions that occur in the case of linear polymers bear-
72 Cross-linking 183
Scheme 75 Light-induced main-chain cleavage of poly-ethylene containing traces of carbonyl groups
ing pendant C=C bonds typical examples of the latter process are reactions ofnitrenes generated in polymeric systems containing azide groups [17]
Photo-cross-linking of thick polymer films is a difficult task because thepenetration depth is limited to thin layers if the light is strongly absorbed Ahigh absorptivity on the other hand is required for effective photo-cross-link-ing Therefore only the photo-cross-linking of thin films (1 m) is of practicalimportance This process has found widespread application in photolithography(see Section 91) The following subsections are largely devoted to systems thathave been employed for photolithographic applications although some systemsof as yet purely academic interest are also discussed
721Cross-linking by cycloaddition of C=C bonds
The reaction of an excited alkene molecule in its S1 or T1 state with an alkenemolecule in its ground state produces a cyclobutane derivative [Eq (7-3)]
7-3
7 Photoreactions in synthetic polymers184
Scheme 76 Light-induced cross-linking and trans cis isomerization of poly(vinyl cinnamate)
In this reaction which occurs in competition with isomerization two bondsare lost with the formation of two new bonds Since two electrons of eachalkene molecule are involved the reaction is called [2+2] or simply [2+2] cy-cloaddition As discovered by Minsk [29] linear polymers containing C=C bondsin pendant groups also undergo light-induced [2+2] cycloaddition reactions Thisleads to the formation of intermolecular cross-links as demonstrated here forthe classical case of poly(vinyl cinnamate) Exposure of the polymer to UV light(exp = 365 nm) results both in [2+2] cycloaddition and trans cis isomerization(Scheme 76)
Besides cinnamate compounds various other compounds containing C=Cbonds also undergo light-induced cycloaddition reactions (see Chart 71)
Scheme 77 shows as a typical example the photo-cross-linking of a co-poly-peptide [30]
72 Cross-linking 185
Chart 71 Structures of moieties suitable for the cross-linkingof linear polymers through cycloaddition
Scheme 77 Photo-cross-linking of a co-polypeptide consistingof L-ornithine and -7-coumaryloxyacetyl-L-ornithine residues[30]
722Cross-linking by polymerization of reactive moieties in pendant groups
Photo-cross-linking of linear polymers can be achieved by light-induced poly-merization of reactive moieties in pendant groups located on different macro-molecules a process analogous to the polymerization of low molar mass com-pounds which is treated in Chapter 10 Provided that the pendant groups arecapable of approaching to within the reaction distance and their concentrationis high enough they undergo chain reactions which can propagate by way ofvarious mechanisms that are started with the aid of appropriate photoinitiatorsFrom the technical point of view free radical polymerizations of unsaturatedcarbon-carbon bonds are most important In principle cationic polymerizationsinvolving the ring opening of epoxides and glycidyl ethers (see Chart 72) arealso suitable
Although in contrast to free radical polymerizations cationic polymerizationsare unaffected by O2 their importance is somewhat limited by the scarcity ofappropriate macromolecules and suitable photoinitiators [3] However this doesnot apply to the photopolymerization of low molar mass epoxides (see Sec-tion 103) In this context applications of photo-cross-linked epoxides in variousfields such as stereolithography volume holography and surface coating arenotable [16]
A typical example involving the polymerization of unsaturated pendantgroups relates to the fixation of surface relief gratings that are optically in-scribed with the aid of a 488 nm laser beam (see Section 561) onto a film of acopolymer bearing pendant azobenzene groups (chemical structure shown inChart 73)
The generation of the relief gratings involves trans cis isomerization of thependant azobenzene groups and the subsequent fixation is achieved by cross-linking with UV light at 80 C ie by polymerization of the acrylic groups withthe aid of a photoinitiator (see Chart 74)
7 Photoreactions in synthetic polymers186
Chart 72 Structures of moieties suitable for cross-linking by photopolymerization
This process results in an improved thermal stability of the gratings [31] An-other example relates to the photo-cross-linking of a copolymer of the structureshown in Chart 75 [32]
Here the alkynyl side groups are polymerized to form a three-dimensionalnetwork when the copolymer is exposed to UV light (320ndash390 nm) in the pres-ence of 5 mol tungsten hexacarbonyl W(CO)6 (see also Subsection 102241)The polymerization is presumed to be initiated by the formation of a 2-alkynetungsten pentacarbonyl complex 2-RCCRW(CO)5
72 Cross-linking 187
Chart 73 Co-monomers (1 1 molar ratio) contained in apolymer used to generate surface relief gratings
Chart 74 Chemical structure of 4-(methylthio)-2-morpholino-propiophenone used as a photoinitiator in the cross-linking ofthe copolymer of Chart 73
Chart 75 Chemical structure of a copolymer consisting ofpropargyl acrylate (345 left) and methyl methacrylate(655 right)
723Cross-linking by photogenerated reactive species
This mode of photo-cross-linking has attracted attention for applications in re-sist technology since it became apparent that the photodecomposition of organ-ic azides in polymeric systems leads to insolubility Azide groups can be chemi-cally attached to polymer chains as demonstrated here by two examples
Alternatively bisazides ie low molar mass compounds containing two azidegroups can be added to the polymer Several commercially used bisazides arepresented in Table 72 Many linear polymers can be photo-cross-linked with theaid of bisazides [17] Of note in this context is poly(cis-isoprene) which containssome cyclized structures (Chart 77) It has been frequently applied as a resistmaterial in photolithography applications
A water-soluble bisazide (see Chart 78) is applicable for the photo-cross-link-ing of water-processable polymeric systems containing polyacrylamide or poly(vi-nyl pyrrolidone)
7 Photoreactions in synthetic polymers188
Chart 76 Base units of polymers bear-ing pendant azide groups
Table 72 Bisazides of practical importance for the photo-cross-linking of linear polymers [17]
Denotation Chemical structure
26-Bis(4-azidobenzal)-4-methylcyclohexane
44-Diazidostilbene
44-Diazidobenzophenone
44-Diazidobenzalacetone
When an azide group decomposes after absorption of a photon an electricallyneutral very reactive intermediate called a nitrene is formed Immediately afterdecomposition the latter is in an electronically excited singlet state which candecay to the ground state the triplet nitrene [see Eqs (7-4) and (7-5)]
RN3 h 1RN N2 7-41RN 3RN 7-5
Both nitrene species are very reactive since the nitrogen possesses only six va-lence electrons Singlet nitrene can insert into CndashH bonds of the polymer andin the case of unsaturated polymers can add to C=C bonds both in single-stepprocesses (Scheme 78)
As shown in Scheme 79 triplet nitrene can abstract a hydrogen atom fromneighboring macromolecules thus forming an amino radical and a carbonmacroradical (reaction (a)) The two radicals have correlated spins and can
72 Cross-linking 189
Chart 77 Cyclized structure in poly(cis-isoprene)
Chart 78 Chemical structure of a water-soluble bisazide
Scheme 78 Reactions of singlet nitrene with saturated and unsaturated polymers
therefore only couple after spin inversion (reaction (b)) The amino radical mayalso abstract a hydrogen atom from a different site to produce a primary amine(reaction (c)) Cross-links are formed by coupling reactions namely by the com-bination of macroradicals (reaction (d)) and if bisazides are employed after theconversion of both azide groups according to reaction (e) [17]
Free radical mechanisms also serve to explain the photo-cross-linking of var-ious polymers such as that of polyethylene accomplished with the aid of light-absorbing additives such as benzophenone quinone benzoin acetophenone ortheir derivatives When electronically excited by light absorption these additiveseither directly abstract hydrogen from the polymer or decompose into free radi-cals capable of abstracting hydrogen as shown in Schemes 710 and 711
Macroradicals P can form cross-links by combination reactions according toEq (7-6)
P P PP 7-6
7 Photoreactions in synthetic polymers190
Scheme 79 Cross-linking of polymers through the reaction of triplet nitrene
The occurrence of these reactions is restricted to the amorphous phase Thereforethe photo-cross-linking process has to be performed at temperatures exceeding thecrystalline melting point in the case of highly crystalline polymers such as poly-ethylene The cross-linking efficiency can be strongly enhanced by the additionof small amounts of multifunctional compounds such as triallyl cyanurate TAC(see Chart 79) or by the incorporation of special diene moieties into copolymerssuch as ethylene propylene diene copolymers (EPDM elastomers) [33]
72 Cross-linking 191
Scheme 710 Generation of macroradicals by the reaction ofelectronically excited benzophenone and anthraquinone with apolymer PH
Scheme 711 Generation of free radicals by -cleavage inelectronically excited acetophenone and benzoin derivativesand subsequent formation of macroradicals P by hydrogenabstraction from macromolecules PH
The reaction mechanism in this case is shown in Scheme 712 It is based onthe fact that allyl-type hydrogens are readily abstracted by reactive radicals suchas ketyl species Side-chain macroradicals generated in this way combine toform intermolecular cross-links
724Cross-linking by cleavage of phenolic OH groups
Typical of this type of photo-cross-linking is the case of poly(4-hydroxystyrene)(see Chart 710) [34]
The deactivation of excited singlet phenolic groups proceeds by two mainroutes cleavage of the OndashH bonds and intersystem crossing to the triplet stateas shown in Scheme 713
7 Photoreactions in synthetic polymers192
Chart 79 Chemical structure of triallyl cyanurate
Scheme 712 Generation of pendant macroradicals acting asprecursors for the cross-linking of an EPDM elastomercontaining ethylidene norbornene moieties (other co-monomer moieties are not shown) Initiatorhydroxycyclohexyl phenyl ketone [33]
The phenoxyl radicals can couple to form cross-links (Scheme 714)If dioxygen is present additional phenoxyl radicals are formed by reaction ac-
cording to Eq (7-7) ie by the reaction of triplet excited phenolic groups with O2
7-7
Therefore the cross-linking quantum yield is significantly increased if the irra-diation is performed in the presence of dioxygen
73Simultaneous cross-linking and main-chain cleavage of linear polymers
As has been pointed out in Section 712 polymers commonly undergo differentkinds of bond ruptures simultaneously upon exposure to light ie bond cleav-age processes occur both in side chains and in the main chain of linear poly-mers Bond rupture in side chains results in the formation of lateral macroradi-
73 Simultaneous cross-linking and main-chain cleavage of linear polymers 193
Scheme 713 Primary steps in the photolysis of poly(4-hydroxystyrene)
Chart 710 Chemical structure of poly(4-hydroxystyrene)
cals which can give rise to the release of low molar mass compounds and canalso form inter- and intramolecular cross-links Therefore it is often the casethat main-chain scission and cross-linking occur simultaneously These pro-cesses cause changes in the molar mass distribution and in the average molarmass of the polymer which has been treated theoretically [35ndash37] The depen-dence of the weight-average molar mass Mw (g molndash1) of linear polymers under-going simultaneous main-chain cleavage and cross-linking on the absorbed doseDabs (photons gndash1) is given by Eq (7-8)
1MwD
1Mw0
13S2
213X
Dabs
NA7-8
where 13(S) and 13(X) denote the quantum yields for main-chain cleavage andcross-linking respectively and NA is Avogadrorsquos number Equation (7-8) holdsfor the case that the initial molar mass distribution is of the most probable typeand that main-chain ruptures and cross-links are randomly distributed alongthe polymer chains Cross-linking predominates if 13(S) lt 413(X) In this casethe reciprocal average molar mass decreases ie Mw increases with increasingabsorbed dose On the other hand main-chain cleavage predominates if13(S) gt 413(X) In this case the reciprocal average molar mass increases ie Mw
decreases with increasing absorbed dose In this context it should be noted thatpredominant main-chain cleavage causes a deterioration of important mechani-cal properties that are related to the molar mass of the polymer Several linearpolymers are characterized with respect to the predominance of cross-linking ormain-chain cleavage in Table 73
Interestingly polyacrylonitrile poly(methyl acrylate) and polystyrene behavedifferently in the rigid state and in dilute solution This may be explained interms of lateral macroradicals being generated upon the release of side groupsin a primary step The combination of these radicals competes with decomposi-tion through main-chain rupture In dilute solution where radical encountersare much less probable than in the rigid state main-chain rupture predomi-
7 Photoreactions in synthetic polymers194
Scheme 714 Coupling of phenoxyl radicals
nates This mechanism is illustrated for the case of polyacrylonitrile inScheme 715
When linear polymers undergo predominantly cross-linking a three-dimen-sional insoluble network is formed The absorbed dose at which the insolublenetwork begins to form is the gel dose Dgel It corresponds to an average of onecross-link per weight-average molecule [35] and a simple equation may be de-rived from Eq (7-8) for the relationship between Dgel and 13(X)
Dgel NA
13XMw07-9
Equation (7-9) holds in the absence of main-chain scission ie at 13(S) = 0 Inthis case the reciprocal molar mass approaches infinity at the gel dose ie1MwDgel 0
Quantum yields of photoproducts of selected polymers are presented in Ta-ble 74 It can be seen that both 13(S) and 13(X) are low (lt 01) The quantum
73 Simultaneous cross-linking and main-chain cleavage of linear polymers 195
Scheme 715 Main-chain cleavage and cross-linking of polyacrylonitrile
Table 73 Predominant effects upon UV irradiation of polymers in the absence of oxygen [27]
Polymer Rigid state Dilute solution
Poly(methyl methacrylate) degradation degradationPoly(-methyl styrene) degradation degradationPoly(phenyl vinyl ketone) degradation degradationPolyacrylonitrile crosslinking degradationPoly(methyl acrylate) crosslinking degradationPolystyrene crosslinking degradation
yields of volatile products resulting from side-group degradation are also quitelow for most polymers apart from poly(methyl methacrylate)
74Photodegradation of selected polymers
It is not intended to present a comprehensive treatise on the photoreactions inpolymers in this book Actually many polymers exhibit analogous behaviorHowever this certainly does not apply to poly(vinyl chloride) or polysilanes andtherefore these two types of polymers are discussed to some extent in the fol-lowing subsections
741Poly(vinyl chloride)
Poly(vinyl chloride) PVC is one of the most widely used polymers CommercialPVC products commonly contain plasticizers (up to 40) such as phthalates ormellitates If exposed to UV or solar radiation for prolonged periods PVC productssuffer from a deterioration of their mechanical and electrical properties and areeventually discolored [11 19 21] Unsaturated moieties are believed to be the mostimportant initiator species with carbonyl groups as the next most important Thelatter can undergo Norrish-type reactions (see Section 714) Moreover excited car-bonyl groups can transfer energy to unsaturated moieties or abstract hydrogens Inaddition hydroperoxide and peroxide groups formed during autoxidation of thepolymer (see Section 75) can contribute to the initiation process [11]
7 Photoreactions in synthetic polymers196
Table 74 Photoproduct quantum yields of polymers in the rigid state deter-mined at room temperature in vacuo [27]
Polymer SX (S)102 (X)102 (nm) Volatile products
(102 )
Poly--methylstyrene 01ndash06 2537 -methylstyreneH2 (17102)
Poly(methyl methacrylate) 12ndash39 2537 CH3OH (48)HCOOCH3 (14) COH2 CO2
Poly(phenyl vinyl ketone) 60 313Poly(vinyl acetate) 14 66 47 2537 CH3COOH (10) CO2
(065) CO (069) CH4
(038)Poly(ethylene terephthalate) 27 016 006 313Poly(methyl acrylate) 10 019 019 2537 HCHO (2) CH3OH
(02) HCOOCH3 (08)Poly(p-methylstyrene) 052 2537 H2 (6) CH4 (004)
The discoloration is due to a dehydrochlorination process resulting in the for-mation of long conjugated polyene sequences in the polymer chain [Eq (7-10)]Polyenes can give rise to photo-cross-linking reactions
7-10
It is generally accepted that the elimination of HCl occurs by way of a free radi-cal chain reaction As shown in the lower part of Scheme 716 chlorine atomsfunction as propagating species Likely initiation mechanisms involving some ofthe impurity chromophores listed in Table 71 are presented in the upper partof Scheme 716
The solar light-induced dehydrochlorination of PVC plasticized with phtha-lates has been reported to be sensitized by the plasticizer [38 39] In markedcontrast more recent work has revealed a weak protective effect of phthalateswith respect to CndashCl bond cleavage and polyene formation Phthalates are likelyto quench electronically excited states of impurity chromophores [40]
74 Photodegradation of selected polymers 197
Scheme 716 Mechanism of the light-induced dehydrochlorination of poly(vinyl chloride)
742Polysilanes
Polysilanes (alternative denotations polysilylenes poly-catena-silicons) of thegeneral structure shown in Chart 711 exhibit an absorption band in a relativelylong-wavelength region ie between 300 and 400 nm reflecting the -conjuga-tion of electrons in the silicon chain
In addition to their other interesting properties polysilanes are photoconduc-tive [41] (see Chapter 2) and therefore are attractive with regard to practical ap-plications [42 43] However to the detriment of their technical applicabilitypolysilanes show a pronounced trend to suffer photodegradation Light absorp-tion induces main-chain scission and extrusion of silylene as depicted inScheme 717
The lifetime of the excited state giving rise to main-chain cleavage is shorterthan 100 ps [44]
7 Photoreactions in synthetic polymers198
Scheme 717 Main-chain degradation of polysilanes
Chart 711 Chemical structure of a base unit of polysilane
75Oxidation
Oxidation processes are initiated when polymers absorb visible or UV light inthe presence of air [7 12 24-26] In most cases these processes occur as chainreactions initiated by the light-induced generation of free radicals Since someof the reaction products are chromophoric groups capable of initiating new ki-netic chains themselves the oxidation becomes auto-accelerated during expo-sure As a consequence of autoxidation important mechanical properties ofpolymeric materials may suffer a sudden breakdown during continuous expo-sure to light This is demonstrated in Fig 71 which shows how the impactstrength of an ABS polymer drops drastically after a certain exposure time [45]
The schematic representation in Fig 72 shows how at first the oxygen uptakeincreases exponentially with increasing irradiation time ie the reaction rate isaccelerated After prolonged irradiation the autoacceleration is followed by anautoretardation stage due to a depletion in the O2 concentration in the interiorof the sample or to reaction products interfering with the propagation process
The behavior depicted in Fig 72 is observed with many polymers upon expo-sure to sunlight including with commercial polyalkenes such as polyethyleneand polypropylene In the latter cases impurity chromophores act as initiatorsof the autoxidation process (see Scheme 74 in Section 713) Important elemen-tary reactions determining the autoxidation process are described in the follow-ing Free radicals RX
formed during the initiation phase abstract hydrogenatoms from macromolecules PH thus forming macroradicals P [Eq (7-11)]
75 Oxidation 199
Fig 71 Photodegradation of an acrylonitrilebutadienestyrene (ABS) copolymer at 30 C Plot of the impact strengthvs the simulated natural exposure time (xenon-arc radiation055 W mndash2 at 340 nm) Adapted from Davis et al [45] withpermission from Elsevier
RX PH RXH P 7-11
The ensuing chain reaction which is propagated by the macroradicals produceshydroperoxide groups (see Scheme 718)
Hydroperoxide groups can be photolytically cleaved provided that the wave-length of the incident light is lower than about 300 nm [Eq (7-12)]
POOHh PO OH 7-12
The radicals generated in this way can initiate additional chain reactions (chainbranching) by abstracting hydrogens from neighboring macromolecules for in-stance by reaction according to Eq (7-13)
OH PH H2O P 7-13
The kinetic chains are terminated by radical coupling reactions (seeScheme 719)
The combination of peroxyl radicals (reaction (a) in Scheme 719) is assumedto proceed via a tetroxide P-O4-P a short-lived intermediate Various reaction
7 Photoreactions in synthetic polymers200
Fig 72 Autoxidation of polymers Schematic represen-tation of the oxygen uptake as a function of timeAdapted from Schnabel [24] with permission from CarlHanser
Scheme 718 Propagation of the chain reaction in the autoxidation process
P O2 POO
POO PH POOH P
POO POO a Products
POO P b POOP
P P c P P
Scheme 719 Termination reactions in the autoxidation process
paths that may be envisaged in the case of secondary peroxyl radicals are shownin Scheme 720 [46] Reaction (a) in Scheme 720 refers to the so-called Russelmechanism The extent to which each individual reaction occurs depends onthe chemical nature of the polymer as well as on other parameters particularlythe temperature The oxyl radicals formed by reaction (b) can abstract hydrogenin inter- andor intramolecular reactions Alternatively they can decompose withthe formation of carbonyl groups (see Scheme 721)
In conclusion the salient features of the light-induced oxidation of polymersare the formation of hydroperoxide peroxide and carbonyl groups the latter inthe form of both aldehyde and keto groups Moreover certain reactions such asreaction (d) in Scheme 720 and reaction (b) in Scheme 721 result in main-chain cleavage as far as the oxidation of linear macromolecules is concernedMain-chain cleavage leads to a deterioration in certain important mechanicalproperties Therefore the photo-oxidation of polymers is deleterious and shouldbe avoided in commercial polymers Appropriate stabilization measures are dis-cussed in Section 93
75 Oxidation 201
Scheme 720 Decay processes of secondary peroxyl radicals [46]
Scheme 721 Reactions of oxyl radicals
76Singlet oxygen reactions
The ground state of molecular oxygen (3O2) is a triplet state with two unpairedelectrons In addition to the reactions outlined in Section 75 3O2 can undergoenergy-transfer reactions with many compounds such as dyes and polynucleararomatics provided that the difference in the energy levels exceeds 94 kJ molndash1In these reactions the first excited state of molecular oxygen ie singlet oxygen(1O2
) is formed as is illustrated by the reaction of triplet excited carbonylgroups present in a polymer with 3O2 according to Eq (7-14)
7-14
1O2 is unreactive towards saturated hydrocarbons but reacts with unsaturated
substances with a rate constant of 103 to 104 L molndash1 sndash1 [47] This reaction re-sults in the insertion of hydroperoxide groups [Eq (7-15)]
7-15
In conclusion singlet oxygen plays a role in the photo-oxidative degradation ofpolymers containing olefinic unsaturations Polymers that do not contain thesegroups eg poly(vinyl chloride) poly(methyl methacrylate) polystyrene etc areunreactive [24]
77Rearrangements
Certain organic molecules are modified by a rearrangement of some of theirconstituent groups upon light absorption Typical processes that have gainedimportance in the polymer field are the photo-Fries rearrangement of aromaticesters amides and urethanes (see Scheme 722) and the o-nitrobenzyl ester re-arrangement (see Scheme 723) In the latter case nitronic acid forms as a long-lived intermediate Its decay in polymeric matrices is non-exponential (kineticmatrix effect) up to temperatures exceeding the glass transition temperaturerange [49]
7 Photoreactions in synthetic polymers202
Regarding linear polymers rearrangements can involve the main chain as inthe case of a polycarbonate (see Scheme 724) or pendant groups as in the caseof poly(4-acetoxy styrene) which is converted into poly(3-acetyl-4-hydroxy sty-
77 Rearrangements 203
Scheme 722 Photo-Fries rearrangement of a carbonate
Scheme 723 Mechanism of the o-nitrobenzyl ester photo-rearrangement [48 49]
rene) (see Scheme 725) or with polymers bearing o-nitrobenzyl ester pendantgroups (see Scheme 726)
Photo-rearrangements in polymers are important because they can lead topronounced property changes For example polymers containing o-nitrobenzylpendant groups become soluble in aqueous solution since benzyl ester groupsare converted into carboxyl groups Therefore such polymers are applicable aspositive-tone photoresists in lithographic processes [50 51] (see Section 91)
7 Photoreactions in synthetic polymers204
Scheme 724 Photo-rearrangement of a polycarbonate
Scheme 725 Photo-rearrangement of poly(4-acetoxy styrene)
Scheme 726 Photo-rearrangement of polymers bearing o-nitrobenzyl pendant groups
References 205
References
1 (a) J C Salamone (ed) Polymeric Materi-als Encyclopedia CRC Press Boca RatonFL USA (1996) (b) Abridgement of (a)J C Salamone (ed) Concise PolymericMaterials Encyclopedia CRC Press BocaRaton FL USA (1999)
2 G Scott Polymers and the EnvironmentRoyal Society of Chemistry Cambridge(1999)
3 H-J Timpe Polymer Photochemistry andPhoto-Crosslinking in R Arshady (ed)Desk Reference of Functional PolymersSynthesis and Applications AmericanChemical Society Washington DC(1997) p 273
4 S I Hong S Y Joo D W Kang Photo-sensitive Polymers in R Arshady (ed)Desk Reference of Functional PolymersSynthesis and Applications AmericanChemical Society Washington DC(1997) p 293
5 B Raringnby B Qu W Shi Photocrosslink-ing (Overview) in [1(a)] Vol 7 p 5155
6 J Paczkowski Photocrosslinkable Photopo-lymers (Effect of Cinnamate Group Struc-ture) in [1(a)] Vol 7 p 5142
7 J F Rabek Photodegradation of PolymersPhysical Characteristics and ApplicationsSpringer Berlin (1996)
8 R L Clough NC Billingham K T Gil-len (eds) Polymer Durability Stabiliza-tion and Lifetime Prediction AmericanChemical Society Washington DC Ad-vances in Chemistry Series 249 (1996)
9 W Schnabel I Reetz Polystyrene and De-rivatives Photolysis in [1(a)] Vol 9p 6786
10 V V Krongauz AD Trifunac Processesin Photoreactive Polymers Chapman ampHall New York (1995)
11 A L Andrady Ultraviolet Radiation andPolymers in J E Mark Physical Propertiesof Polymers Handbook AIP Press Wood-bury NY (1995) Chapter 40
12 G Scott (ed) Atmospheric Oxidation andAntioxidants Elsevier Amsterdam(1993)
13 NS Allen M Edge Fundamentals ofPolymer Degradation and StabilisationElsevier Applied Science London (1992)
14 Z Osawa Photoinduced Degradation ofPolymers in S H Hamid MB AminA G Maadhah (eds) Handbook of Poly-mer Degradation Dekker New York(1992)
15 H Boumlttcher J Bendig MA Fox GHopf H-J Timpe Technical Applicationsof Photochemistry Deutscher Verlag fuumlrGrundstoffindustrie Leipzig (1991)
16 V Strehmel Epoxies Structures Photoin-duced Cross-Linking Network Propertiesand Applications in HS Nalwa (ed)Handbook of Photochemistry and Photo-biology American Scientific PublishersStevenson Ranch CA USA (2003) Vol2 p 2
17 A Reiser Photoreactive Polymers TheScience and Technology of Resists WileyNew York (1989)
18 J Guillet Polymer Photophysics andPhotochemistry Cambridge UniversityPress Cambridge (1985)
19 C Decker Photodegradation of PVC inED Owen (ed) Degradation and Stabili-zation of PVC Elsevier Applied ScienceLondon (1984) p 81
20 S Tazuke Photocrosslinking of Polymersin NS Allen (ed) Developments in Poly-mer Photochemistry ndash 3 Applied ScienceLondon (1982) Chapter 2 p 53
21 ED Owen Photodegradation and Stabili-zation of PVC in NS Allen (ed) Devel-opments in Polymer Photochemistry ndash 3Applied Science London (1982) Chapter5 p 165
22 Z Ozawa Photodegradation and Stabili-zation of Polyurethanes in NS Allen(ed) Developments in Polymer Photochem-istry ndash 3 Applied Science London(1982) Chapter 6 p 209
23 W Schnabel Laser Flash Photolysis ofPolymers in N S Allen (ed) Develop-ments in Polymer Photochemistry ndash 3 Ap-plied Science London (1982) Chapter 7p 237
24 W Schnabel Polymer Degradation Princi-ples and Practical Applications HanserMuumlnchen (1981) Chapter 4
25 R Arnaud J Lemaire PhotocatalyticOxidation of Polypropylenes and Polyunde-canoamides in N S Allen (ed) Develop-
7 Photoreactions in synthetic polymers206
ments in Polymer Photochemistry ndash 2 Ap-plied Science London (1981) Chapter 4p 135
26 A Garton D J Carlsson DM WilesPhoto-oxidation Mechanisms in Commer-cial Polyolefins in NS Allen (ed) Devel-opments in Polymer Photochemistry ndash 1Applied Science London (1980) Chapter4 p 93
27 W Schnabel J Kiwi Photodegradationin HHG Jellinek (ed) Aspects of Deg-radation and Stabilization of PolymersElsevier Amsterdam (1979)
28 J F McKellar NS Allen Photochemistryof Man-Made Polymers Applied ScienceLondon (1979)
29 LM Minsk J G Smith W P Van Deu-sen J W Wright J Appl Polym Sci 11(1959) 302
30 K Ohkawa K Shoumura M YamadaA Nishida H Shirai H YamamotoMacromol Biosci 1 (2001) 149
31 H Takase A Natansohn P RochonPolymer 44 (2003) 7345
32 C Badaru ZY Wang Macromolecules36 (2000) 6959
33 B Raringnby Photoinitiated Modifications ofSynthetic Polymers Photocrosslinking andSurface Photografting in NS Allen MEdge I R Bellobono E Selli (eds) Cur-rent Trends in Polymer PhotochemistryHorwood New York (1995) Chapter 2p 23
34 K Nakabayashi R Schwalm W Schna-bel Angew Makromol Chem 195(1992) 191
35 A Charlesby Atomic Radiation and Poly-mers Pergamon Press Oxford (1960)Chapter 10
36 O Saito Statistical Theory of Crosslinkingin M Dole (ed) The Radiation Chemistryof Macromolecules Academic Press NewYork (1972) Chapter 11
37 CL Moad D J Windzor Prog PolymSci 23 (1998) 759
38 IS Biggin DL Gerrard G E Wil-liams J Vinyl Technol 4 (1982) 150
39 DL Gerrard HJ Bowley KP J Wil-liams IS Biggin J Vinyl Technol 8(1986) 43
40 A I Balabanovich S Denizligil WSchnabel J Vinyl Add Technol 3 (1997)42
41 R G Kepler J M Zeigler LA HarrahSR Kurtz Phys Rev B 35 (1987) 2818
42 R D Miller J Michl Chem Rev 89(1989) 1359
43 R D Miller Radiation Sensitivity of Solu-ble Polysilane Derivatives in J M ZeiglerFW G Fearon (eds) Silicon-Based Poly-mer Science A Comprehensive ResourceAmerican Chemical Society WashingtonDC (1990) Advances in Chemistry Se-ries 224 Chapter 24
44 Y Ohsako CM Phillips J M ZeiglerR M Hochstrasser J Phys Chem 93(1989) 4408
45 P Davis BE Tiganis L S Burn PolymDegrad Stab 84 (2004) 233
46 C von Sonntag The Chemical Basis ofRadiation Biology Taylor amp Francis Lon-don (1987) Chapter 4
47 H Bortolus S Dellonte G Beggiato WCorio Eur Polym J 13 (1977) 185
48 K H Wong H Schupp W SchnabelMacromolecules 22 (1989) 2176
49 G Feldmann A Winsauer J Pfleger WSchnabel Macromolecules 27 (1994)4393
50 H Barzynski D Saumlnger MakromolChem 93 (1981) 131
51 E Reichmanis R Gooden CW Wil-kins H Schonehorn J Polym SciPolym Chem Ed 21 (1983) 1075
81Introductory remarks
Biopolymers play a key role in many light-triggered biological processes such as inphotomorphological processes in plants and in the photomovements of bacteriaMoreover biopolymers participate in energy transduction processes related tothe conversion of solar energy into chemical energy (photosynthesis) and to theconversion of chemical energy into light (bioluminescence) Apart from these ben-eficial effects light can also have a harmful effect on polymers and cause chemicaldamage resulting in a deactivation of their biological activity While the deleteriousaction is commonly restricted to UVB and UVC light ( 200ndash320 nm) ie to photonshaving energies high enough to cleave chemical bonds the regulatory action relatesto light of longer wavelengths ie UVA ( 320ndash400 nm) and visible light In thelatter case effective biopolymers contain chromophoric groups capable of absorbinglight in the 400ndash800 nm wavelength region This chapter which deals with bothmodes of action of light is organized according to the important biopolymer familiesof nucleic acids proteins lignins and polysaccharides (see Chart 81) However itshould be kept in mind that very often members of these families exist in closeproximity in biological objects and are sometimes even linked by chemical bonds
For relevant literature concerning the broad field of light-induced effects inbiopolymers and biological objects the reader is directed to several reviews andbooks [1ndash17]
The polymers presented in Chart 81 absorb UV light to quite different ex-tents Nucleic acids absorb more strongly than proteins This can be seen inFig 81 which shows absorption spectra of aqueous solutions of DNA and bo-vine serum albumin recorded at equal concentrations In contrast to the ratherstrongly absorbing nucleotide residues in DNA only a few of the amino acid re-sidues in proteins absorb light measurably in the UV region This pertainsmainly to the aromatic amino acids phenylalanine tyrosine and tryptophan (seeChart 82)
Lignins a major component of wood (15ndash30 wt) are phenolic polymersbased on three structural units the content of which depends on the type ofwood trans-p-coumaryl alcohol (I) trans-coniferyl alcohol (II) and trans-sinapylalcohol (III) (see Chart 83)
207
8Photoreactions in biopolymers
The optical absorption spectra of lignins extend into the visible wavelength re-gion and exhibit peaks at about 205 and 280 nm and shoulders at 230 and340 nm [18] Polysaccharides such as cellulose and amylose essentially do notabsorb light at gt 200 nm Very weak absorption bands observable in somecases in the region between 250 and 300 nm are due to intrinsic impuritiessuch as acetal groups or carboxyl groups replacing hydroxyl groups [17 19]
Special biopolymers containing covalently bound chromophoric groups absorbvisible light and act as photoreceptors They play a regulatory role in important
8 Photoreactions in biopolymers208
Chart 81 Biopolymer structures depicting(a) different nucleotides contained in humandeoxyribonucleic acid DNA (b) part of aprotein chain consisting of various aminoacid residues with R being H (glycine) CH3
(alanine) (CH2)4NH2 (lysine) CH2SH(cysteine) etc (c) the base unit of thecellulose chain representing the class ofpolysaccharides and (d) part of a lignin withtypical structural elements
biological processes Typical photoreceptors are proteins belonging to the carote-noid (rhodopsin) phytochrome and cryptochrome families In this context thechlorophyllic protein complexes are also of note They function as light-harvestingantenna pigments and auxiliary cofactors in the photosynthetic process and are
81 Introductory remarks 209
Fig 81 Optical absorption spectra of aqueous solutions of anucleic acid (calf thymus DNA) and a protein (bovine serumalbumin) both recorded at a concentration of 19710ndash2 g Lndash1Adapted from Harm [12] with permission from CambridgeUniversity Press
Chart 82 Chemical structures of aromatic amino acids
8 Photoreactions in biopolymers210
Chart 83 Substituted phenyl propanols that constitute the structural units of lignins
Table 81 Photoactive chromophores (pigments) of photoreceptor proteins [9 20ndash25]
Typical chromophore Photoreceptor class Typical functions
Carotenoids(a) Photoantennas in the photo-synthetic system of plants (b) Cat-alytic pigments in animal andbacterial rhodopsins
11-cis Retinal
Flavins(a) Photoantennas in enzymes(b) Cofactors for photolyaseblue-light photoreceptors
Flavin
Phytochromes
(a) Photoreceptors exerting mor-phogenic control in plants(b) Accessory antennas in thelight-harvesting complexes ofphotosynthetic systems
Phytochromobilin
PterinsPhotoantennas in the majority ofphotolyasecryptochrome blue-light photoreceptors
510-Methenyltetrahydrofolate(MTHF)
Xanthopsins YellowProteins
Sensory blue light receptorswater-soluble controlling the lifeof bacteria in saline lakes
4-Hydroxycinnamate
therefore of profound biological importance The chemical structures of typicalchromophoric groups contained in these proteins are presented in Table 81
In conclusion proteins play a range of roles in relation to the exposure of bio-logical objects to light of different wavelengths UV light acts harmfully since itcauses chemical changes leading to the deactivation of specifically acting pro-teins such as enzymes However light-induced chemical changes might alsotrigger the synthesis of special proteins As regards irradiation with visible lightit is most important that certain proteins serve as light-harvesting agents inphotosynthesis and as photoreceptors and photosensors in photomorphogenicprocesses in plants The various aspects are referred to briefly in the followingsections
82Direct light effects
8 21Photoreactions in deoxyribonucleic acids (DNA)
The energy-rich UV light portion of the terrestrial solar spectrum ( 280ndash400 nm) is harmful to most organisms and can even cause skin cancer in hu-mans (basal and squamous cell carcinoma melanoma) This is mainly due to
82 Direct light effects 211
Table 81 (continued)
Typical chromophore Photoreceptor class Typical functions
NaphthodianthronesBlepharismins
Photosensors in ciliated protozo-ans exhibiting step-up photopho-bic and negative phototacticresponses
Stentorin
Chlorophylls Photoantennas in the light-harvest-ing complexes and electron donorsin the reaction center of the photo-synthetic system
Chlorophyll a
light-induced chemical modifications in DNA bases commonly termed UV-in-duced DNA lesions The absorption of light converts the bases into their excitedsinglet or triplet states from which chemical reactions can ensue The resultingbase modifications are accompanied by a change in the base-pairing propertieswhich in turn causes mutations [26ndash29] There are a number of feasible photo-lesions based on the cleavage of chemical bonds with the concurrent generationof free radicals Besides these dimeric photoproducts may be formed in greatabundance through a molecular rather than a free radical mechanism Notablypyrimidine bases are essentially involved in the generation of lesions of biologi-cal importance although both purine and pyrimidine residues are rather strongabsorbers in the far-UV region Actually the quantum yield of photodecomposi-tion differs significantly It amounts to about 10ndash4 for purines ie one or two or-ders of magnitude lower than that for pyrimidines [12]
8211 Dimeric photoproductsThe pyrimidine bases thymine (T) and cytosine (C) form dimers at sites withadjacent pyrimidine moieties so-called dipyrimidine sites in the DNA chainwhich have been well characterized with respect to chemical structure and mu-tagenic potential The dimerization presented in Scheme 81 is a [2+2] cy-cloaddition (see Section 73) involving the two C(5)=C(6) double bonds leadingto cyclobutane structures denoted by the symbol T lt gt T or generally Pyr lt gt Pyr
The dimerization can in principle lead to three isomers cis-syn trans-syn Iand trans-syn II but due to the constraints imposed by the DNA double strandthe cis-syn dimer shown in Scheme 81 is the major photoproduct [27]
Another type of dimeric lesions are pyrimidinendashpyrimidone (Pyr[6-4]Pyr) di-mers formed by a Paterno-Buumlchi-type reaction at dipyrimidine sites between theC(5)=C(6) double bond of the first pyrimidine and the C(4)=O carbonyl groupof the second base This kind of dimerization is demonstrated in Scheme 82for the case of adjacent thymine moieties
8 Photoreactions in biopolymers212
Scheme 81 Dimerization of adjacent thymine moieties in DNA by [2+2] cycloaddition
Analogous photoproducts may form between any types of adjacent pyrimi-dines T-T T-C C-T and C-C except that the (6-4) photoproduct does not format C-T sites Adeninendashthymine heterodimers (see Chart 84) have also been de-tected [29 30]
The UV-induced generation of cyclobutane dimers is greatly dependent ondouble-helix conformational factors In dormant spores of various bacillus spe-cies for example a group of small acid-soluble proteins specifically bind toDNA thereby enforcing a particular conformation that is unfavorable for theformation of harmful cyclobutane-type lesions As a consequence these dor-mant spores are much more resistant to UV radiation than the correspondinggrowing cells in which DNA strands reassume conformations favorable for theformation of cyclobutane-type lesions [31]
Notably photodimers of the cyclobutane type are cleaved by irradiation withfar-UV light (240 nm) with a quantum yield of almost unity by way of the so-called [2+2] cycloreversion reaction In living cells dimer lesions can be repairedby the nucleotide excision repair pathway which is based on the excision of asmall piece of DNA around the lesion Lesions not removed from the genomelead to cell death or mutagenesis
82 Direct light effects 213
Chart 84 Structure of an adeninendashthyminephotodimer [29]
Scheme 82 Dimerization of adjacent thymine moieties inDNA by a Paterno-Buumlchi-type reaction
8212 Other DNA photoproductsAdditional photoproducts commonly generated via free radical mechanismshave been identified These include single-strand breaks cross-links betweenthe strands of the same double helix and between different DNA strands andadjacent protein molecules and the so-called photohydrates (see Chart 85)
822Photoreactions in proteins
Gross changes in proteins due to UV irradiation include disturbance of the naturalconformation aggregation and chain cleavage all of which lead to denaturationThe structural proteins keratin (wool) collagen elastin and fibroin (silk) undergolosses in mechanical strength and elasticity (wool tenders) and sometimes colorchanges (yellowing) These changes are due to chemical alterations
In order to assess possible photochemical events one has to take into accountthat proteins are heterogeneously composed linear polymers (see Chart 81)The amino acid residues are connected by amide (peptide) bonds ndashCOndashNHndashNature uses 20 amino acids to synthesize a great variety of proteins which arecharacterized by amino acid sequence size and three-dimensional structureMany proteins are intramolecularly cross-linked by disulfide links (RndashSndashSndashR)ie they consist of several covalently connected chains Alternatively two ormore protein chains can be linked by non-covalent forces Proteins consisting ofthe 20 natural amino acids absorb light at lt 320 nm The low-wavelength por-tion of the terrestrial solar spectrum extending to about 290 nm is mainly ab-sorbed by the aromatic amino acids (see Chart 82) Therefore the sunlight-in-duced photochemistry of proteins essentially relates to these moieties Atlt 290 nm light is also absorbed by the other amino acid residues whichgreatly increases the variety of possible bond ruptures In view of these facts itis clear that the photochemistry of proteins is extremely complex and thereforeonly certain aspects have been thoroughly investigated to date
8 Photoreactions in biopolymers214
Chart 85 Photohydrates of cytosine (a) and of thymine (b) [30]
(a) (b)
8221 Chemical alterations by UV lightTryptophan (Trp) tyrosine (Tyr) cystine (Cys) and phenylalanine (Phe) moietiesplay a determinant role regarding UV light-induced chemical alterations inmany proteins After the absorption of light by these moieties in most casesmainly by Trp and Tyr they undergo photoionization and participate in energy-and electron-transfer processes This not only holds for structural proteins suchas keratin and fibroin [11] but also for enzymes in aqueous media such as lyso-zyme trypsin papain ribonuclease A and insulin [7] The photoionization ofTrp andor Tyr residues is the major initial photochemical event which resultsin inactivation in the case of enzymes A typical mechanism pertaining to Trpresidues (see Scheme 83) commences with the absorption of a photon and thesubsequent release of an electron In aqueous media the latter is rapidly sol-vated By the release of a proton the tryptophan cation radical Trp+ is con-verted to the tryptophan radical Trp
In many proteins such as -lactalbumin which consists of 123 amino acidmoieties the electron released from a Trp moiety is attached by way of an intra-molecular process to a disulfide group of a cystine bridge in a position adjacentto the indole ring of the Trp moiety [32]
As shown in Scheme 84 the resulting disulfide anion radical dissociates intoa thiolate ion RndashSndash and a thiyl radical RndashS Proton transfer from the tryptophancation radical to the thiolate ion leads to the tryptophan radical Trp and thethiol RSH The final stage of the process is governed by radical coupling whichmay result in sulfenylation of the Trp moiety yielding TrpndashSndashR or in inter-molecular cross-linking ie in the formation of enzyme dimers or trimers
Disulfide bridges can also be ruptured by reaction with the triplet excited moi-eties 3Trp or 3Tyr the formation of which accompanies the electron release
82 Direct light effects 215
Scheme 83 Photolysis of proteins Reactions involving tryptophan moieties [7]
In this process the triplet species undergo an electron transfer with cystinemoieties thus forming the disulfide radical anion (see Scheme 85)
Intermediates occurring in these mechanisms have been identified by ESRmeasurements and by flash photolysis studies using optical absorption detec-tion For example ESR measurements on wool keratins revealed the formationof sulfur-centered radicals of the structure RCH2S which in this case are as-sumed to result from a reaction of electronically excited tyrosine moieties withcystine residues [11] In many proteins cross-links are formed In the case ofkeratin and collagen the cross-links are of the tryptophan-histidine and dityro-sine types [11] Cross-links formed by the combination of RndashS or RndashSndashS radi-cals both intermolecularly and intramolecularly with incorrect sites are consid-ered to be an important source of photoaggregation effects [8] ESR measure-ments have also yielded evidence of CndashH and CndashN bond ruptures [8]
8222 Formation of stress proteinsUV light induces the formation (expression) of so-called stress proteins in mam-malian skin cells [34] Stress proteins (shock proteins) are also generated byother stress factors such as hyperthermia and comprise a heterogeneous groupof proteins with molar masses ranging from 104 to 11105 g molndash1 They func-tion as molecular chaperones by transiently binding to unfolded proteins aftersynthesis as well as to denatured proteins in stressed cells thus promoting theirrefolding and correct assembly In this way they protect proteins from misfold-ing and irreversible denaturation The molecular mechanism of the formationof stress proteins has not yet been elucidated although it is supposed that theirformation is triggered by oxidative damage
8 Photoreactions in biopolymers216
Scheme 84 Rupture of cystine bridges by the attachment ofelectrons stemming from the photoionization of tryptophan[32 33]
Scheme 85 Reaction of tryptophan triplets with cystine moieties
8223 Effects of visible light ndash photoreceptor actionPhotoreceptors ie proteins containing chromophores absorbing visible light (seeTable 81) play a key role in many light-triggered biological processes For instancein plants they regulate and participate in energy transduction processes during theconversion of solar energy into chemical energy (photosynthesis) and trigger andsupport photomorphological processes Moreover photoreceptors are responsiblefor the photomovements of certain bacteria and regulate the circadian rhythm ofhigher animals Circadian (circa= round about and dies= day) rhythms are oscilla-tions in the biochemical physiological and behavioral functions of organisms witha periodicity of approximately 24 hours Detailed information on this fascinatingfield is available from the cited literature [6 9 20 22 35ndash44] Upon light absorp-tion the chromophores of photoreceptors undergo molecular transformations thatresult in the formation of signaling states in the protein The regulatory action re-lates to UVA ( 320ndash400 nm) and visible light ( 400ndash800 nm) In most proteinac-eous photoreceptor systems such as cytochromes and phytochromes the chromo-phores are covalently linked to the protein [35] On the other hand chlorophyll moi-eties are specifically associated with intrinsic proteins of the photosynthetic mem-brane thus forming chlorophyll-protein (non-covalent) complexes
Depending on their chemical nature chromophores undergo different modesof light-induced molecular transformation As can be seen in Table 82 thetransformation modes include trans-cis isomerization charge transfer and en-ergy transfer
The chromophores act as photosensing-phototransducing devices because theyare not isolated but rather are embedded in and interacting with a molecular apo-protein framework The latter senses the light-induced molecular modifications inthe chromophores and in turn gives rise to the signaling state The intimate in-teraction between chromophore and protein determines the physiological andspectroscopic properties of the photoreceptors In recent years photobiological re-search has been largely focused on photoreceptors and has revealed some very in-teresting results This is illustrated here for the typical case of the family of phy-tochromes which are present in plants and certain bacteria [20 37ndash39] Certainphytochromes exert morphogenic control functions in higher and lower plants al-gae and mosses relating to for example blooming the opening of hooks ofshoots or the germination of seeds Other phytochromes function as accessorylight-harvesting antennae in conjunction with the photosynthetic systems of cer-tain algae Plant phytochromes consist of polypeptide chains of about 1100 amino
82 Direct light effects 217
Table 82 Transformation modes of chromophores in photoreceptors
Transformation mode Chromophores
trans-cis Isomerization Retinals 4-hydroxy-cinnamate bilinsCharge transfer Flavins stentorins blepharisminsEnergy transfer Pterins flavins
acid moieties (molar mass 12ndash13105 g molndash1) and a single open-chain tetrapyr-role chromophore of the bilin family (see Table 81 and Scheme 86) which iscovalently bound via an S-cysteine linkage to the apoprotein The polypeptidechain is composed of two domains the globular N (amino) terminal domain bear-ing the chromophore and the regulatory C (carboxyl) terminal domain [39] Thetwo domains are connected by a flexible protease-sensitive hinge region contain-ing the Q (Quail) box Active phytochrome entities are dimers ie they consistof two polypeptide strands (see Fig 82)
8 Photoreactions in biopolymers218
Scheme 86 Mechanism of the PrPfr photocycle for phytochromobilin Adapted from [20]
Fig 82 Schematic illustration of the interdo-main signal transmission in a dimeric oatphytochrome Q Quail box PAS Per-Arnt-Sim motif Q and PAS constitute the regula-tory core region HD Histidine kinase-related domain PKS1 Phytochrome kinase
substrate 1 NDPK2 Nucleosidediphosphate kinase 2 PIF3 Phytochromeinteracting factor 3 Adapted from Bhoo etal [39] with permission from RoutledgeTay-lor amp Francis Group LLC
The photomorphogenic control functions are triggered by trans cis and cistrans double-bond isomerizations of the chromophore induced by red (r) and far-red (fr) light respectively The PrPfr photocycle is illustrated in Scheme 86
The Pr to Pfr isomerization induces a transformation from random to -helicalconformation in part of the N-terminal domain and thus triggers a series ofconformational changes in other structural peptide motifs especially in the C-terminal domain (see Fig 82) Here certain regulatory sites become exposedand thus capable of interacting with signal transducer proteins such as PIF3(phytochrome interacting factor 3) NDPK2 (nucleoside diphosphate kinase 2)etc In this way the enzymatic activity of these proteins is significantly in-creased Moreover the Q-box in the hinge region becomes uncovered thus per-mitting the phosphorylation of the serine moiety in position 598 of the chainThe phosphorylation at Ser-598 exerts an accelerating effect on the associationof PIF3 and NDPK2 and the phosphorylation of PKS1 (phytochrome kinasesubstrate 1) The latter is a protein that is complexed to the Pr state of the phy-tochrome and is released from the photoactivated Pfr state after phosphorylationto give downstream signals through a kinase cascade [39] Recall that a kinaseis an enzyme that catalyzes the phosphorylation of a substrate here a proteinIn conclusion the light-induced isomerization of carbon-carbon double bondsin the chromophore causes a series of conformational changes within the twodomains of the phytochrome These changes trigger the association of signaltransducer proteins with the phytochrome and allow phosphorylation and phos-phate transfer at various sites These are key steps initializing the downstreamof processes that eventually result in transcriptional regulation
8224 Repair of lesions with the aid of DNA photolyasesThe repair of dimer lesions induced with the aid of light of relatively long wave-length that is not absorbed by the dimer sites ( 300ndash400 nm) is based on photo-receptor action as dealt with in Section 8223 above It occurs if DNA photolyasesie structure-specific (not sequence-specific) enzymes are present in the systemduring the irradiation [6] Photolyases are proteins of 450-550 amino acids contain-ing two non-covalently bound chromophore cofactors (see Chart 86)
One of the cofactors is always flavin adenine dinucleotide FAD and the sec-ond one is either methenyltetrahydrofolate MTHF or 8-hydroxy-78-dides-methyl-5-deazariboflavin 8-HDF
The repair of lesions by photolyases is the basis of the so-called photoreactiva-tion of organisms A striking example is the resurrection of UV-killed Escheri-chia coli by subsequent exposure to a millisecond light flash which is demon-strated by the results shown in Fig 83
The reaction mechanism can be summarized as follows In a dark reactionthe enzyme binds to DNA and flips out the pyrimidine dimer from the doublehelix into the active cavity After the photochemical repair the reaction partnersare moved out of the cavity As shown in Scheme 87 MTHF (or alternatively 8-HDF) is converted into an excited state MTHF upon absorption of a photon
82 Direct light effects 219
8 Photoreactions in biopolymers220
Chart 86 Cofactors of photolyases
Fig 83 Photoreactivation of UV-killed E coli cells Lower linecells irradiated with UV light and plated on a growth mediumUpper line UV-irradiated cells exposed to a 1 ms light flashbefore plating Adapted from Sancar [6] with permission fromthe American Chemical Society
Excited reduced flavin (FADH) formed by energy transfer from MTHFtransfers an electron to Pyr lt gtPyr the pyrimidine dimer In a subsequent con-certed reaction the latter is split into two pyrimidines and an electron is trans-ferred to the nascently formed FADH
823Photoreactions in cellulose
It was pointed out in Section 81 that polysaccharides do not absorb light atgt 200 nm Therefore photochemical alterations caused by light of longer wave-lengths are due to the action of impurity chromophores This also holds for cel-lulose which is a major component of plants Some plants such as jute flaxhemp and cotton contain up to 90 cellulose Neat cellulose forms gaseousproducts (CO CO2 and H2) upon exposure to UV light (= 2537 nm) ESRstudies have revealed the generation of H radicals and various carbon-centeredfree radicals The degree of crystallinity of the cellulose fibrils is reduced [17] IfO2 is present during the irradiation carbonyl carboxyl and peroxide groups areformed even at gt 340 nm Main-chain scission occurs and the brightness is re-duced [45] This is because irradiation at lt 360 nm leads to homolysis of thepreviously formed hydroperoxide groups (see Scheme 88)
The OH radicals resulting from this process are very reactive ie they ab-stract hydrogens from neighboring molecules and thus initiate further decom-position processes For detailed information concerning the photochemistry ofcellulose the reader is referred to the relevant literature [17 46]
824Photoreactions in lignins and wood
Wood contains 15ndash30 lignin an aromatic UV- and visible-light-absorbing poly-mer with a very complex structure (see Chart 81) and photochemical alterationsof wood are essentially determined by reactions initiated by bond breakages in the
82 Direct light effects 221
Scheme 87 Reaction mechanism of the repair of pyrimidinedimer lesions in DNA with the aid of photolyases
RO OHh RO OH
Scheme 88 Generation of hydroxyl radicals during the photolysis of hydroperoxide groups
lignin component Due to a lack of systematic investigations little is known aboutthe complex mechanism of the photoreactions in lignins Scheme 89 illustratesbond-breakage processes suggested in the literature [16 47]
The formation of phenoxyl radicals has been revealed by ESR measurementsPhenoxyl radicals can be transformed into quinoid structures (see Scheme 810)which are thought to be responsible for the yellowing of the surfaces of woodproducts
Because of the capability of lignins to absorb near-UV and visible light evenindoor yellowing and darkening of wood surfaces due to slow photooxidationprocesses is unavoidable More detailed information concerning the photochem-istry of lignins and wood is available in relevant review articles [16 47]
83Photosensitized reactions
Various applications are based on the indirect action of light on polymers con-tained in biological objects Many biopolymers do not absorb visible light andabsorb UV light only to a limited extent Therefore sensitizers are used to ac-complish light-induced chemical alterations Sensitizers which are in an elec-tronically excited state after light absorption either react directly with substratepolymers or decompose into fragments capable of reacting with the polymers
8 Photoreactions in biopolymers222
Scheme 89 Photoreactions of lignins
Scheme 810 Formation of quinoid structures in lignins
Sensitizers can be employed for agricultural purposes as herbicides and insecti-cides or for medical purposes as antibacterial and antiviral agents Moreoversensitizer-based methods serve as tools for the analysis of the interaction facesof polymer complexes and the sequence-selective photocleavage of double-stranded DNA The ways in which photosensitized reactions are utilized are il-lustrated by the following typical examples The first case relates to the photo-chemotherapy of cancer cells in superficial solid tumors [48] The so-calledphotodynamic therapy PDT is based on the selective incorporation of a photosen-sitizer into tumor cells followed by exposure to light (commonly at = 600 nm)Cytotoxic products namely singlet oxygen 1O2
and superoxide radical anionsO
2 are generated upon irradiation and these are postulated to start a cascadeof biochemical processes that inactivate neoplastic cells The precise mechanismhas not yet been elucidated [49] However it has been established that chemicalalterations of the cytoskeleton trigger a sequence of reactions eventually causingcell apoptosis The cytoskeleton consists of a complex array of highly dynamicprotein structures that organize the cytoplasma of the cell The basic proteinac-eous constituents having molar masses ranging from 4104 to 7104 g molndash1are microtubules and globular or linear microfilaments (actins and keratins re-spectively) The cytoskeleton structure disorganizes and reorganizes continu-ously depending on the shape and state of division of the cells as well as onsignals received from the environment Assembly and disassembly of the cyto-skeletal elements are severely disturbed or inhibited by light-induced damageChart 87 presents the chemical structures of several PDT sensitizers Relevantresearch work has been reviewed [50]
The second example relates to photochemical cross-linking as a tool for study-ing metastable protein-nucleic acid and protein-protein assemblies [51ndash54] Pro-tein-protein and protein-nucleotide interactions are maintained by a multitudeof weak non-covalent interaction forces From an analytical perspective it isuseful to stabilize such complexes by trapping the interaction partners bymeans of a cross-linking technique so as to generate covalent bonds betweenthem The process of protein assembly can be time-resolved in a snapshot man-ner if the cross-linking period is significantly shorter than the lifetimes of inter-mediate stages reached during the complexing of two or more protein mole-cules ie during dimerization or oligomerization respectively The method dis-cussed here denoted by the acronym PICUP (photo-induced cross-linking ofunmodified proteins) in the case of the oligomerization of unmodified proteinsinvolves exposing the assemblies to a short high-power laser pulse therebygenerating a number of cross-links that is sufficient to stabilize the interactionpartners The aim of the subsequent analysis is then to define binding sites byidentifying the composition of the cross-linked domains of the partners Massspectrometry has been successfully applied for this purpose and it appears thatthe desired information can be obtained more quickly and with greater sensitiv-ity in this way than by NMR or X-ray crystallography [53] The information ob-tained can be used as a basis for three-dimensional molecular modeling of pro-tein oligonucleotide interfaces Commonly the cross-linking reaction is per-
83 Photosensitized reactions 223
formed with the aid of sensitizers that absorb light at wavelengths exceeding300 nm since photo-cross-linking by direct irradiation of the complexes withfar-UV light suffers from serious disadvantages such as low cross-linking yieldstrand breakage and oxidation
In studies of the dynamics of protein oligomerization in the context of inves-tigations exploring amyloidoses ie diseases including Alzheimerrsquos disease ruthe-nium(II) complexes are used [52 55] To this end tris(22-bipyridyl)dichloro-ruthenium(II) Ru(II)bpy3Cl2 (see Chart 88) and ammonium persulfate(NH4)2S2O8 are homogeneously dispersed in an aqueous protein solution
8 Photoreactions in biopolymers224
Chart 87 Sensitizers employed in the photochemotherapy ofcancer cells TPP meso-tetraphenylporphine TMPyP meso-tetra(4-N-methylpyridyl)porphine MB methylene blueTB toluidine blue ZnPc zinc(II) phthalocyanine TPPotetraphenylporphyrene
83 Photosensitized reactions 225
Chart 88 Structure of tris(22-bipyridyl)dichloro ruthenium(II) Ru(II)bpy3Cl2
Scheme 811 Photoreaction of Ru(II)bpy32+ complexes with persulfate ions [53]
Table 83 Nucleobases bearing photosensitizer groupscommonly used for nucleic acidprotein cross-linking studies[51 53]
Structure of nucleobase Denotation max
(nm)operation
(nm)
4-Thiouridine 330 gt 300
Azido-substitutednucleobases
280 gt 300
IodouridineIodocytidine
290300 gt300
Bromouridine 275 gt 300
Upon photoexcitation Ru(III) complexes and sulfate radicals are produced(see Scheme 811) Both resultant species Ru(III)bpy3
3+ and SO4ndash are potent
one-electron oxidants and can generate protein radicals by hydrogen abstractionfrom protein molecules The combination of the protein radicals leads to cross-links
If nucleic acidprotein complexes are to be explored photosensitive groupsare synthesized and incorporated into the nucleic acids Typical sensitizer-bear-ing nucleobases are shown in Table 83
A typical cross-linking reaction is presented in Scheme 812A third example concerns the sequence-selective photocleavage of double-
stranded DNA [14 56ndash58] The advantage of using photoreagents for this pur-pose is that they are inert in the dark and react only under irradiation with lightof an appropriate wavelength that is not absorbed by neat DNA Strand cleavagecan be accomplished by attack of either sugar or nucleobase moieties In the lat-ter case cleavage of DNA usually requires alkaline treatment after irradiation
8 Photoreactions in biopolymers226
Scheme 812 Cross-linking of a nucleic acid with a protein bythe reaction of a 5-iodouracil group with a tryptophan sidegroup
Scheme 813 Cleavage of a DNA strand following theabstraction of a hydrogen atom from a sugar moiety by anelectronically excited photoreagent X
On the other hand attack at a sugar moiety can lead to direct cleavage of theDNA strand In this case a common mechanism is based on hydrogen abstrac-tion (see Scheme 813) The resulting sugar radicals can decompose by a varietyof pathways to yield low molar mass products and DNA fragments
83 Photosensitized reactions 227
Scheme 814 Intra-chain hydrogen abstraction from the sugarmoiety in poly(uridylic acid) involving an uracil radical formedby addition of an OH radical
Chart 89 Structures of typical photochemical nucleases usedfor sequence-specific cleavage of DNA strands L LinkerR sequence-specific DNA-binding compound [56]
Although mechanistic details which are discussed in the relevant literature[14 59 60] cannot be dealt with here the following aspect should at least bepointed out an attack at the nucleobase might induce chemical alterations inthe sugar moiety that eventually result in strand breakage This applies for ex-ample to the intramolecular hydrogen abstraction suggested in the case ofpoly(uridylic acid) (see Scheme 814) [59]
The hydrogen abstraction process is in principle unselective since abstract-able hydrogens are present in all sugar moieties Strand ruptures originatingfrom attacks at the nucleobases are also intrinsically unselective However se-quence selectivity can be accomplished if the photoreagent binds to one or afew sequences of the DNA strand The focus of relevant research is on synthe-sizing conjugates composed of a photosensitizer group and a sequence-specificDNA-binding compound also denoted as photochemical nucleases [56] Appropri-ate photoactive groups (listed eg in [14]) include complexes of transition metalions such as Ru(II) Rh(III) and Co(II) polycyclic aromatic compounds such asanthraquinone and naphthalene diimide porphyrins and related compounds(chlorins sapphyrins) phthalocyanines and fullerenes (see Chart 89)
8 Photoreactions in biopolymers228
References
1 W M Horspool F Lenci (eds) CRCHandbook of Organic Photochemistry andPhotobiology 2nd Edition Boca RatonFlorida (2004)
2 W M Horspool P-S Song (eds) CRCHandbook of Organic Photochemistry andPhotobiology 1st Edition Boca RatonFlorida (1995)
3 H Morrison (ed) Bioorganic Photochem-istry Wiley New York (1990)
4 A R Young LO Bjorn J Moan WNultsch (eds) Environmental UV Photo-biology Plenum Press New York (1993)
5 HS Nalwa (ed) Handbook of Photo-chemistry and Photobiology American Sci-entific Publ Stevenson Ranch Califor-nia (2003)
6 A Sancar Structure and Function of DNAPhotolyase and Cryptochrome Blue-LightPhotoreceptors Chem Rev 103 (2003)2203
7 L I Grossweiner Photochemistry of Pro-teins A Review Curr Eye Res 3 (1984)137
8 K M Schaich Free Radical Initiation inProteins and Amino Acids by Ionizing andUltraviolet Radiation and Lipid Oxidationndash Part II Ultraviolet Radiation and Photo-
lysis CRC Crit Rev Food Sci Nutr 13(1980) 131
9 A Sancar Cryptochrome The SecondPhotoactive Pigment in the Eye and its Rolein Circadian Photoreception Ann RevBiochem 69 (2000) 31
10 NL Veksin Photonics of BiopolymersSpringer Berlin Heidelberg (2002)
11 G J Smith New Trends in Photobiology(Invited Review) Photodegradation ofKeratin and other Structural Proteins JPhotochem Photobiol B Biol 27 (1995)187
12 W Harm Biological Effects of UltravioletRadiation Cambridge University PressCambridge (1980)
13 CH Nicholls Photodegradation andPhotoyellowing of Wool in N S Allen(ed) Developments in Polymer Photochem-istry ndash 1 Appl Science Publ London(1980) Chapter 5 p 125
14 B Armitage Photocleavage of NucleicAcids Chem Rev 98 (1998) 1171
15 J Barber (ed) The Light Reactions Else-vier Amsterdam (1987)
16 DN S Hon N Shiraishi (eds) Woodand Cellulosic Chemistry 2nd EditionDekker New York (2001)
References 229
17 P J Baugh Photodegradation and Photo-oxidation of Cellulose in NS Allen (ed)Developments in Polymer Photochemistry ndash2 Appl Science Publ London (1981)Chapter 5 p 165
18 A Sakakibara Y Sano Chemistry of Lig-nin Chapter 4 in [16]
19 A Bos J Appl Polym Sci 16 (1972)2567
20 K Schaffner W Gaumlrtner Open-Chain Tet-rapyrroles in Light Sensor Proteins Phyto-chromes The Spectrum 12 (1999) 1
21 G EO Borgstahl D E Williams E DGetzoff Biochemistry 34 (1995) 6278
22 J Hendriks K J Hellingwerf PhotoactiveYellow Protein the Prototype XanthopsinChapter 123 in [1]
23 Y Muto T Matsuoka A Kida Y OkanoY Kirino FEBS Lett 508 (2001) 423
24 R Dai T Yamazaki I Yamazaki P SSong Biochim Biophys Acta 1231(1995) 58
25 Y Shichida T Yoshizawa PhotochemicalAspects of Rhodopsin Chapter 125 in [1]
26 MG Friedel DNA Damage and RepairPhotochemistry Chapter 141 in [1]
27 SY Wang (ed) Photochemistry andPhotobiology of Nucleic Acids AcademicPress New York (1976)
28 F Cadet P Vigny The Photochemistry ofNucleic Acids Vol 1 Chapter 1 in [3]
29 DL Mitchell D Karentz The Inductionand Repair of DNA Photodamage in theEnvironment p 345 in [4]
30 DL Mitchell DNA Damage and RepairChapter 140 in [1]
31 P Setlow Environ Mol Mutagen 38(2001) 97
32 A Vanhooren B Devreese K Vanhee JVan Beeumen I Hanssens Biochem 41(2002) 11035
33 DV Bent E Hayon J Am Chem Soc97 (1975) 2612
34 F Trautinger Stress Proteins in the Photo-biology of Mammalian Cells Vol 4 Chap-ter 5 in [5]
35 J Breton E Naberdryk Pigment and Pro-tein Organization in Reaction Center andAntenna Complexes Chapter 4 in [15]
36 H Zuber The Structure of Light-Harvest-ing Pigment Protein Complexes Chapter 5in [15]
37 K Schaffner SE Braslavski SE Holz-warth Protein Environment Photophysicsand Photochemistry of Prosthetic BiliproteinChromophores in H-J Schneider HDuumlrr (eds) Frontiers in SupramolecularOrganic Chemistry and PhotochemistryVCH Weinheim (1991) p 421
38 SE Braslavski W Gaumlrtner K SchaffnerPhytochrome Photoconversion Plant Celland Environment 6 (1997) 700
39 SH Bhoo P S Song Phytochrome Mo-lecular Properties Chapter 129 in [1]
40 G Checcuci A Sgarbossa F LenciPhotomovements of Microorganisms An In-troduction Chapter 120 in [1]
41 SC Tu Bacterial Bioluminescence Bio-chemistry Chapter 136 in [1]
42 V Tozzini V Pellegrini F BeltramGreen Fluorescent Proteins and Their Ap-plications to Cell Biology and BioelectronicsChapter 139 in [1]
43 NK Packham J Barber Structural andFunctional Comparison of Anoxygenic andOxygenic Organisms Chapter 1 in [15]
44 M Salomon Higher Plant PhototropinsPhotoreceptors not only for Phototropismin A Batschauer (ed) Photoreceptors andLight Signalling Comprehensive Seriesin Photochemistry and PhotobiologyVol 3 Royal Soc Chem Cambridge(2003) p 272
45 J Malesic J Kolar M Strlic D KocarD Fromageot J Lemaire O HaillandPolym Degrad Stab 89 (2005) 64
46 DN S Hon Weathering and Photochem-istry of Wood Chapter 11 in [16]
47 B George E Suttie A Merlin X De-glise Photodegradation and Photostabilisa-tion of Wood ndash the State of the Art PolymDegrad Stab 88 (2005) 268
48 T J Dougherty J G Levy Clinical Appli-cations of Photodynamic Therapy Chapter147 in [2]
49 BW Henderson S O Gollnick Mechan-istic Principles of Photodynamic TherapyChapter 145 in [2]
50 A Villanueva R Vidania J C StockertM Canete A Juarranz Photodynamic Ef-fects on Cultured Tumor Cells CytoskeletonAlterations and Cell Death MechanismsVol 4 Chapter 3 in [5]
51 K Meisenheimer T Koch Crit Rev Bio-chem Mol Biol 32 (1997) 101
8 Photoreactions in biopolymers230
52 G Bitan DB Teplow Acc Chem Res37 (2004) 357
53 H Steen ON Hensen Analysis of Pro-tein-Nucleic Acid Interaction by Photo-chemical Crosslinking Mass SpectromRev (2002) 163
54 B Bartholomew RT Tinker G A Kas-savetis EP Geiduschek Meth Enzy-mol 262 (1995) 476
55 DA Fancy I Kodadek Proc Natl AcadSci USA 96 (1999) 6020
56 A S Boutorine PB Arimondo Se-quence-Specific Cleavage of Double-Stranded DNA in MA Zenkova (ed)Artificial Nucleases Nucleic Acids andMolecular Biology Vol 13 Springer Ber-lin (2004) p 243
57 T Da Ros G Spalluto A S BoutorineR V Bensasson M Prato DNA-Photo-cleavage Agents Curr Pharm Design 7(2001) 1781
58 IE Kochevar DA Dunn Photosensi-tized Reactions of DNA Cleavage and Ad-dition Vol 1 Chapter 1 p 299 in [3]
59 C von Sonntag The Chemical Basis ofRadiation Biology Taylor amp Francis Lon-don (1987) Chapter 9
60 W K Pogozelski DT Tullius OxidativeStrand Scission of Nucleic Acids RoutesInitiated by Hydrogen Abstraction from theSugar Moiety Chem Rev 98 (1998) 1089
91Polymers in photolithography
911Introductory remarks
In modern-day technical terminology lithography denotes a technology used topattern the surfaces of solid substrates Lithography as invented by Alois Sene-felder in 1798 is a printing technique used by artists who draw (Greek gra-phein) directly onto a stone (Greek lithos) surface with greasy ink which adheresto the dry stone and attracts printing ink while the background absorbs waterand repels the printing ink The patterning of surfaces with the aid of light iscalled photolithography It serves to generate macrostructures in the millimeterrange and is applied for example in the fabrication of printed circuit boardsand printing plates In its currently most important version lithography heredenoted as microlithography refers to the generation of microstructures on topof semiconductor (mostly silicon) wafers Photomicrolithography has served asthe essential tool in the information and electronic revolution It is still unavoid-able in the mass production of computer chips containing fine-line featuresnow in the sub-75 nm range thus permitting an information density exceeding109 integrated circuits (IC) per cm2 This miniaturization technique is renderedpossible by polymers although they are not contained in the final productsStimulated by the demand for further progress in the miniaturization of de-vices outlined by the SIA International Roadmap [1] a large body of researchand development still focuses on the improvement of the classical microlitho-graphic techniques and the development of novel ones [2ndash4]
912Lithographic processes
The lithographic process that is widely used to generate microstructures espe-cially in the context of the fabrication of microdevices is shown schematicallyin Fig 91 It is based on the interaction of electromagnetic or particle radiationwith matter Since direct irradiation of the substrate (eg silicon wafers) does
231
9Technical developments related to photochemical processesin polymers
not result in the generation of microstructures of the required quality the tech-nically utilized processes are performed with wafers coated with a thin layer ofa radiation-sensitive material The required fine-line structures are generatedwithin this thin layer essentially in two steps irradiation through a stencil (herecalled the mask) and subsequent (commonly liquid) development The radiation-sensitive material is called the resist (material) because it has to be resistant toetching agents ie chemicals capable of reacting with the substrate Etching iscarried out after development ie after the removal of either the irradiated orthe unirradiated resist All of these steps are illustrated in Fig 91 which relatesto photolithography Most of the resists that have been employed to date arepolymer-based ie they consist wholly or partly of an amorphous polymer
As regards the manufacture of microdevices photolithography is the key tech-nology On the other hand charged particle beam lithography using electron orion beams (eg H+ He2+ Ar+) serves to fabricate photomasks In this case acomputer-stored pattern is directly converted into the resist layer by addressingthe writing particle beam
In applying the process depicted in Fig 91 the mask may either be placed di-rectly onto the wafer (contact printing) or may be positioned a short distance infront of the wafer (proximity printing) In either case the minimum feature sizeamounts to a couple of micrometers and thus does not satisfy todayrsquos industrialdemands However fine-line features down to the sub-micrometer range can beobtained with projection techniques as described in the next subsection
9 Technical developments related to photochemical processes in polymers232
Fig 91 Schematic illustration ofthe lithographic process
9121 Projection optical lithographyProjection optical lithography has been the mainstream technology in the semi-conductor industry for the last two decades [2] Figure 92 shows a schematic de-piction of an optical projection system consisting of a laser light source a maska projection lens and a resist-coated wafer The projection of the pattern of themask onto the resist layer provides a demagnification ratio of up to 4
Regarding a periodic fine structure assembly consisting of lines and spacesthe minimum line resolution of the pattern in terms of the minimum achiev-able feature size LWmin can be estimated with the aid of Eq (9-1)
LWmin k1
NA9-1
Actually LWmin is equal to p2 Here p denotes the pitch ie the distance madeup of a pair of lines and spaces is the wavelength of the exposure light and k1 isa system factor that depends on various parameters such as resist response pat-tern geometry in the mask etc NA is the numerical aperture given by Eq (9-2)
NA n sin 9-2
Here n is the refractive index and is the acceptance angle of the lens (seeFig 92) According to Eq (9-1) a decrease in LWmin can be accomplished by de-
91 Polymers in photolithography 233
Fig 92 Schematic illustration of an optical pro-jection system
creasing k1 or or by increasing NA In the past all three approaches havebeen implemented in following industryrsquos roadmap for the miniaturization ofelectronic devices [1] For instance a significant enhancement in resolution wasachieved by using excimer lasers operating at short wavelengths 248 nm (KrF)193 nm (ArF) and 157 nm (F2) as can be seen from Table 91 Sub-100 nm fea-tures can be generated with the aid of ArF and F2 lasers and sub-50 nm fea-tures with extreme ultraviolet (EUV) sources The numerical aperture may be in-creased with the aid of lenses with increased acceptance angle Most recently aquite radical approach to enhanced resolution has been introduced althoughnot yet applied in manufacturing namely liquid immersion lithography [5ndash7]This new technology is based on an increase in the refractive index n by repla-cing the ambient gas (air nitrogen) with a transparent liquid Using water withn= 14366 at = 193 nm and T = 215 C the numerical aperture NA is increasedby 44 at a given sin [2] The revolutionary development in miniaturizationbecomes evident if one considers that the storage capacity of dynamic randomaccess memory (DRAM) devices has been increased from less than 1 Megabit(1 Mb= 106 bit) to several Gigabit (1 Gb= 109 bit) This increase in storage capaci-ty has been accomplished by lowering LWmin from gt 1 m to less than 007 m
A different approach whereby the resolution may be improved by 50ndash100is based on the use of phase-shifting transmission masks The latter containopaque regions as conventional masks do but some of the apertures are cov-ered with a transparent phase-shifting material which reverses the phase of thelight passing through them The interaction of phase-shifted with non-phase-
9 Technical developments related to photochemical processes in polymers234
Table 91 Correlation of radiation wavelength and minimumfeature size in dynamic random access memory (DRAM)devices
LWmin (m) Light source Wavelength (nm)
08 Hg discharge lamps 436 (g-line) 365 (i-line)05 Hg discharge lamps 436 365 250035 KrF excimer lasers 248025 KrF excimer lasers
ArF excimer lasers248193
018 ArF excimer lasers 1930090 F2 excimer lasers
ArF excimer lasersa)157193
0065 F2 excimer lasersArF excimer lasersa)
157193
0045 EUV sourcesb) 135 c)
a) Using hard resolution enhancement technology (RET)including the immersion technique and phase-shift masktechnology
b) Laser- and discharge-produced plasmas [8] and compactelectron-driven extreme ultraviolet (EUV) sources [9]
c) Si L-shell emission
shifted light brings about destructive interference at the resist plane This re-sults in sharply defined contrast lines because the resist is only sensitive to theintensity of the light and not to its sign [10]
9122 Maskless lithographyThe tools used for projection optical lithography as described in the previoussection include very expensive parts for instance the mask and the heavy (over1000 kg) reduction lens The projection of the image of the mask onto the sili-con wafer requires such a heavy reduction lens Moreover the design and fabri-cation of the features of the mask are associated with high costs and long de-lays The cost of the masks producing one chip can exceed $2 million Innova-tions that have stemmed from these difficulties concern the development ofmaskless optical techniques Actually non-optical techniques such as electron-beam and ion-beam lithography have existed for many years They are em-ployed in photo-mask production but are inappropriate for the large-scale pro-duction of chips Novel techniques relating to optical projection are based onprotocols differing from that described above in Section 9121 Zone-plate arraylithography ZPAL seems to play a prominent role among the novel techniques[3] In ZPAL an array of diffractive lenses focuses an array of spots onto thesurface of a photoresist-coated substrate This is accomplished by passing lightfrom a continuous-wave laser through a spatial filter and a collimating lens tocreate a clean uniform light beam The latter is incident on a spatial light mod-ulator which replaces the mask Under digital control it splits the beam intoindividually controllable beamlets Subsequently the beamlets are passedthrough a telescope such that each is normally incident upon one zone plate inthe array By simple diffraction the zone plate consisting of circular concentriczones focuses the light on a spot of the resist layer The zones in the platecause a phase shift of the transmitted light The radii of the zones are chosensuch that there is constructive interference at the focus Lines and spaces with adensity of 150 nm have been patterned with a ZPAL system operated at400 nm Sub-100 nm linewidths are expected to be realized with systems operat-ing at lower wavelengths At present continuous-wave lasers emitting at= 198 nm are commercially available [3]
Imprinting lithography is another maskless technique capable of generatingsub-100 nm patterns It is essentially a nanomolding process in which a trans-parent patterned template is pressed into a low-viscosity monomer layer dis-pensed onto the surface of a wafer Thereby the relief structure of the templateis filled After photopolymerization of the monomer with the aid of UV light(see Chapter 10) the template is separated leaving a solid polymer replica ofthe template on the surface of the wafer With the aid of subsequent etchingprocesses the pattern is fixed on the waferrsquos surface [4]
91 Polymers in photolithography 235
913Resists
A resist material suitable for computer chip fabrication has to fulfil various re-quirements the most important of which are the following The material mustbe suited for spin casting from solution into a thin and uniform film that ad-heres to various substrates such as metals semiconductors and insulators Itmust possess high radiation sensitivity and high resolution capability The as-pect ratio of radiation-generated fine-line features (height-to-width ratio of lines)is desired to be high but is limited by the risk of pattern collapse Moreoverthe resist material must withstand extremely harsh environments for examplehigh temperature strong acids and plasmas
On the aforementioned roadmap of progressive miniaturization major advancesin resolution have been achieved through the use of light of shorter wavelengthsNew resist materials with low absorptivities (optical density less than 04) at thesewavelengths had to be found because near-uniform exposure throughout the resistlayer needs to be maintained For example Novolak resists which function well at365 nm are too opaque at 248 nm and protected p-hydroxystyrene-based polymersthat operate well at 248 nm are too opaque at 193 nm at which acrylate- and cy-cloalkene-based polymers are used At 157 nm only transparent fluorocarbon-based polymers containing CndashF bonds appear to operate satisfactorily
Liquid development which is commonly applied in lithographic processes isbased on the radiation-induced alteration of the solubility of the irradiated resistareas (see Fig 91) Solubility is decreased by intermolecular cross-linking (nega-tive mode) or increased by main-chain degradation of the polymer (positivemode) Moreover radiation-induced chemical alterations of functional groupson the polymers can lead to a solubility change Very importantly radiation-in-duced conversion of additives controlling the solubility behavior of the polymercan also bring about the desired effect For example an additive that normallyfunctions as a dissolution inhibitor may accelerate the dissolution after exposureto light In the following subsections typical resist systems are presented With-in the frame of this book the aim is not to provide an exhaustive treatment ofthis subject More information can be obtained from relevant review articles [1-25] In this context one should note that details of the compositions of resistsystems and of the chemical nature of components are commonly withheld bythe manufacturers
9131 Classical polymeric resists ndash positive and negative resist systemsThe earliest photoresists used in integrated circuit manufacture consisted ofpolymers that were rendered insoluble by photo-cross-linking and thus operatedin the negative tone mode For instance partially cyclized poly(cis-isoprene) con-taining a bisazide as additive served for a long time as the ldquoworkhorserdquo resistmaterial in photolithography applications [15] This system has already been de-scribed in Section 723 Subsequently Novolak-based positively functioning sys-
9 Technical developments related to photochemical processes in polymers236