Georges Hadziioannou, Paul F. van Hutten (Eels.)

Senticonducting Poly01ers Chemistry, Physics and Engineering

Editors: Prof. G. Hadziioannou Departmc'l.t of Polymer Chemistry and Ma.termls Science Centre Univcrsi.ty of Groningen Nijenborgh 4 NL-9747 AG Groningen The Netherlands

Or. P. F. van Huucn Oepartmen1 of Polymer Chen1istry and Materials Science Centre University of Groningen Nijenborgh 4 NL-9747 AG G roningen 'The Ne.therlam.ls

1111:. book wa.~ carefully prut.luccU. Ncvcrthclc:..". wthor., , ooitor.. 111 mind tllu l>tatc-menls, data. illus.tr.tlioru;, proccdur.tl delails or other item.~ may ina.Jvcncntly be inaccur.ue.

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Foreword

The science and technology of conducting polymers are inherently interdisciplin-ary; they fall at the intersection of three established disciplines: chemistry, physics and engineering; hence the name for this volume. These macromolecular materials are synthesized by the methods of organic chemistry. Their electronic structure and electronic properties faiJ within the domain of condensed matter physics. Effi-cient processing of conjugated polymer rnatcri

VIII Foreword

considered as "dirty" and poorly chamcterized materiuls. Therefore, the recent de-monstration of high-brightness polymer emissive displays with opemting lifetimes of 10000-20000 hours was a particularly important step; it is now clear that semiconducting polymers c:m be used to fabricate consumer product-; that meet commercial specifications.

As a result of the remarlwble p1Vgress in the clzemiscry, physics uml c:11gi11eer-ing (device physics) of semiconductillg and metallic polymers, we are now witnes-sing the beguwing of a revolution in "Plastic Electronics".

The field of semiconducting and metallic polymers remains vital; again and again the science anc technology have moved into new directions. Specific exam-ples of recent advances (within the I 990s) of special importance include the fol-lowing: Metallic polymers which are stable, soluble and processible, and therefore suit-

able for industrial applications; e The science and technology of high-efficiency light emission from polymer

light-emitting diodes and polymer light-emitting electrochemical cells; Ultrafast photoinduced electron transfer in semiconducting polymers mixed

with controlled amounts of acceptors; this phenomenon has opened the wuy to a variety of applications including high-sensitivity plastic photodiodes, and efti-

cien~ plastic solm cells; Semiconducting polymers as mate&ials for solid-state lasers. This book, "Semiconducting Polymers - Chemistry, Physics and Engineering", edited by Georges Hadziiounnou and Paul van Hutten of the University of Gro-ningen (The Netherl~mds) summarizes progress in areas of current activity within the field. The various chapters, all contributed by leading researchers, provide a summary and review of the field that will be useful and important to anyone seek-ing a strong buckground in the basic interdisciplinary science and an up-IO-d4tte "snapshot" of the current status of research. Emphasis is on the basic physics and chemistry of conjugated polymers as electronic and opto-electronic materials

X

2

2.1 2.2 2.21 2.2.2 2.2.3 2.3 2.4 2.4.1 2.4.2 2.4.3

2.5

3

3.1 3.2 3.3 3.4 3..5

4

4. 1 4.2 4.3 4.4 4.4.1 4.4.2 4.4.3

4.5 4.6

Conlents

Oligo- a nd Poly(phenylene)s 37 Ullrich Scherf and Klaus Mullen Introduction 37 Polymers 37 Oxidative Condensation of Benzene Derivatives 38 Transition Metal-Mediated Couplings 39 Other Routes to Poly(p-phenylene)s 48 Oligomers 50 Dendtitic and Hyperbranched Poly(phenylene)s 55 Hyperbranched Poly(phenylene) Derivatives 55 Oligo(phenylene)s Composed of Orthogonally Arranged Arms 56 Dendritic Poly(phenylene)s and Giant Polyaromatic Hydrocarbons (PAHs) 57 Conclusion 60 References 61

Disorder and Solitons in Trans-Polyacetylene 63 Jasper Knoester and Maxim Mostovuy Introduction 63 The Peierls Instability and Solitons 65 Disorder: The Auctualing Gap Model 7 1 Disorder-Induced Kinks 76 Concluding Remarks 82 Acknowledgements 83 References 84

Gas Phase to Solid State Evolution of the Electronic and Optical Properties of Conjugated Chains: A Theoretical Investigation 87 Jerome Comi/, Donhelli A. dos Santos, David Beljumw, Zhigang Shuai, and Jetm-Luc Bredas Introduction 87 Theoretical Methodology 89 Wavefunction AnInterface Formmion Between Aluminum ;.md PPV 130 Interrace Formation Bclwccn C.tlcium and PPV 132 Polythiophene 134 lmerface Fornmtion Between Aluminum

XII

7

7.1 7.2 7.2. 1 7.2.2 7.2.3 7.2.4 7.3 7.3.1 7.3.2 7.3.3 7.3.4 7.4 7.4.1 7.4.2 7.4.3 7.5 7.5.1 7.6

8

8.1 8.2 8.3 8.4 8.4. 1 8.4.2 8.5 8.5. 1 8.5.2 8.5.3 8.6 8.7 8.8

Con/ellis

Spectroscopy of Photoexcitation$ in Conjugated Polymers 189 Paul A. Lane, Sergey V. Frolov. and Zev V. Vardeny Introduction 189 Experimental Techniques 190 CW Photomodulation Spectroscopy I 90 Optically Detected Magnetic Resonance 193 Transient Photomodulation Spectroscopy 196 Non-Linear Optical Spectroscopy 199 Poly(para-phenylene vinylene) 202 Absorption and Photoluminescence 202 Transient Photomodulation 204 Non-Linear Spectroscopy (TPA and EA) 208 CW Photomodulation 21 1 Polythiophcnc 2 14 Linear and Non-LinC

XIV

11.3 11.3.1

11.3.2 11.4 11.4.1 11.4.2 11.5 11.5.1 11.5.2 11.5.3 11.6

12

12.1 12.2 12.2.1 12.2.2 12.3 12.3.1 12.3.2 12.4 12.4.1 12.4.2 12.4.3 12.4.3.1 12.4.4 12.4.5

13

13.1 13.2 13.2.1 13.2.2 13.2.3 13.2.4

13.2.5

Contents

Device Electronic Structure 339 Internal Photoemission Measurements of Schouky Energy Barriers 340 Buill-In Potentials in Device Structures 342 Single-Layer Devices 345 Single-Carrier Suuctures 346 Two-Carrier Structures 352 Multi-Layer Devices 355 Blocking Layers 356 Transport Layers 359 Two-Carrier Multi-Layer Devices 361 Conclusion 362 References 363

Charge Transport in Random Organic Semiconductors 365 Heinz Biissler Introduction 365 Charge Carrier Injection 367 Concepts 367 Comparison with Experiment 373 Space Charge Limited (SCL) Currents 379 The Concept 379 Experimental Results 381 Charge Carrier Transport 384 Concepts 384 Transport in the Presence of Extrinsic Traps 390 Charge Carrier Transport in Conjugated Polymers 398 Time-of-Flight Studies 398 Transient Absorption of Radical Cations 403 Some Remarks Concerning the Nature of Charge-Carrying Moieties 406 Acknowledgements 407 References 408

The Chemistry, Physics and Engineering of Organic Light-Emitiing Diodes 411 John Campbell Scott and George G. Ma/liaras Introduction 411 Materials 413 Conjugated Polymers 413 Small Molecules 416 Molecularly Doped Polymers and Polymer Blends 419 Self-Assembled Layers, Langmuir-Biodgeu Layers, and Liquid Crystals 420 Electrodes and Interface Modification 421

13.2.5.1 13.2.5.2 13.2.5.3 13.2.5.4 13.2.5.5 13.3 13.4 13.4. 1 13.4.2 13.4.3 13.4.4 13.4.5 13.4.6 13.5 13.5.1 13.5.2 13.5.2.1 13.5.2.2 13.5.2.3 13.5.3 13.5.4 13.5.5 13.6 13.7 13.7.1 13.7.2 13.7.3 13.7.4 13.7.5

14

14.1 14.2 14.2.1 14.2.1.1 14.2.1.1.1 14.2. 1. 1.2 14.2.1.1.3 14.2. 1.2 14.2.2 14.2.2. 1 14.2.2.1.1

Anodes 421 Cathodes 422 Electrode Modification 423 B

XVI Contents

14.2.2.1.2 14.2.2.2 14.2.2.3 14.2.2.3.1 14.2.2.3.2 14.2.2.3.3 14.2.2.3.4 14.3 14.3. 1 14.3.2 14.3.2.1 14.3.2.2

. 14.3.2.2.1 14.3.2.2.2 14.3.3 14.3.4 14.4 14.4.1 14.4. 1.1 14.4. 1.2 14.4.1.3 14.4.1.4 14.4.2 14.5 14.5.1 14.5.2 14.5.2.1 14.5.2.2 14.5.2.3 14.5.3 14.6 14.6.1 14.6.1.1 14.6.1.2 14.6.2 14.6.2. 1 14.6.2.2 14.7

15

15. 1 15.2 15.2. 1 15.2.1.1

Current-Voltage Characteristic 473 Metal-Semiconductor FET (MESFE1) 475 Thin-Film Transistor (TFI') 476 Accumulation Mode, Linear Regime 477 Depletion Mooe 478 Accumulation Mode, Saturation Regime 480 Mobility Threshold 481 Charge Transport in Organic Materials 481 Localized Versus Dclocalizcd States 481 Hopping 483 Hopping Rate 483 Polarons 484 Polarons in Conjugated Polymers 484 Transport Mechanism of Polarons 484 Multiple Tmpping and Release 486 Field-Dependent Mobility 487 Fabrication Techniques 488 Deposition of the Semiconductor 488 Elcctropolymeriw tion 489 Solution-Processed Deposition 489 Vacuum Evaporation 489 Langmuir-Blodgett 490 All-Organic Devices 490 Materials 491 Oligothiophenes 492 Other Sm:~ll Molecules 495 Phthalocyanines 495 Pentaccne 496 n-Typc Semiconductors 498 Polymers 499 Models 500 Temper.llure and Gate Bias Dependence 50 I Trap-Limited Tr.msport 501 Polaron and Hopping Models 504 Current-Voltage Characteristics 507 Short-Channel Effects 507 On-Off Current R:.1tio 508 Concluding Remarks 510 References 5 11

Conjugated Polymer Based Plastic Solar Cells 515 Christoph J. Brabec and N. Serdar Sariciftci Introduction 5 I 5 Conjugated Polymers as Photoexcited Donors 516 Optical Properties 518 Linear Optic:.1l Propert1es 518

15.2. 1.2

15.2. 1.3 15.2.2 15.2.2.1 15.3 15.3. 1 15.3.2 15.3.3 15.3.4 15.4 15.4. 1 15.5 15.5. 1 15.5.2

15.6 15.6.1 15.6.2

15.7

16

16.1 16.1.1 16. 1.2

16.1.3 16.2 16.2. 1 16.2.1. 1 16.2. 1.2 16.2.1 .3 16.2.1.3. 1 16.2. 1.3.2 16.2.1.3.3 16.2. 1.3.4 16.2.2 16.2.2. 1 16.2.2.1.1

Cumems XVII

Photoinduced Absorption 521 Quenching of the Intersystem Crossing to the Triplet State 521 Photoinduced IRAV Studies 522 lime-Resolved Photoinduced Studies 524 Sensitiwtion of Photoconductivity 525 Magnetic Properties 526 Light-Induced Electron Spin Resonance (LESR) 526 Pure Conjugated Polymer Photovoltaic Devices 528 Definitions 528 Basic Transport Properties 528 MetaUConjugated Polymer Contacts 53 1 Spectml Response 532 -Conjugated Polymer Bih1yer Devices 536 Conjugated Polymer/C(~l Heterojunction PhOtodiodes 539 Conjugated Polymer Bulk Heterojum:tion Dio

XVIII Contents

16.2.2. 1.2 16.2.2.2 16.2.2.3 16.3 16.3. 1 16.3.1.1 16.3.1.2 16.3.1.2.1 16.3.1.2.2 16.3.1.2.3 16.3.1.3 16.3.1.3. 1 16.3.1.3.2 16.3. 1.3.3 16.3.1.3.4 16.3.1.3.5 16.3.1.3.6 16.3.1.4 16.3.2 16.3.3 16.3.3.1 16.3.3.2 16.3.3.2. 1

16.3.3.2.2 16.3.3.2.3 16.3.3.2.4 16.3.3.3 16.3.3.3.1 16.3.3.3.2 16.4 16.4.1 16.4.2 16.4.2.1 16.4.2.2 16.4.2.3 16.4.3 16.4.3. 1 16.4.3.2 16.5

Index 615

Five-Ring Chromophores 572 Substitution Effects in OPV5s 573 About the Geometry of the Excited State 576 OPVs in the Condensed State 577 Single Crystals 577 Introduction 577 Crystal Structures of Five-Ring OPVs 579 Ooct-OPV5 579 Ooct-OPV5-CN' 580 Ooct-OPV5-CN" 581 Crystal Structures of Three-Ring OPVs 582 Ooct-OPV3 582 Ome-OPV3 582 Ooct-OPV3-CN' 583 Ooct-OPV3-CN" 583 Oct-OPV3 584 Oct-OPV3-CN" 584 Optical Properties of Single Cryst~tls 585 Them1al Properties: Liquid-Crystalline Phases 586 Thin Films 588 Introduction 588 Thin-Film Structure 589 Ooct-OPV5 589 Optical Microscopy 589 X-Ray Diffraction (XRD) 589 Atomic Force Microscopy (AFM) 590 Ooct-OPV5-CN' 592 Ooct-OPV5-CN" 592 Ooct-OPV3 593 Optical Properties 593 Five-Ring OPVs 593 Ooct-OPV3 597 Light Emission Applications of OPVs 598 Introduction 598 Light-Emitting Diodes 599 Single-Layer Devices 599 Influence of Morphology on Device Performance 602

Double~Layer Devices 603 Stimulated Emission 604 Single Crystals 604 Vacuum-Deposited Films 605 Summary and Outlook 608 Acknowledgements 6 10 References 6 10

List of Contributors

H. Biissler Institute of Physical, Nuclear ;md Macromolcculm Chemistry Philipps-Univers ity of M~trburg Hans-Meerwein-Stral3e D-35032 Marburg Germany

D. Bdjonne Service de Chimie des Matcriaux Nouveaux Centre de Recherche en Electroni4ue et Photonique Moleculaires Univcrsite de Mons-Ho.tinaut Place du Po.trc, 20 B-7000 Mons Belgium

F. Biscarini lstituto di Spenroscopht Molecolme Consiglio Nazionale delle Riccrche Via P. Gobeui, I 0 I 1-40129 Bologna Italy

C.J. Brabec Christian Doppler Laboratory for Plastic Solar Cells Physical Chemistry Johannes Kepler University Linz AltenbergerstraBe 69 A-4040 Linz Austria

J.-L. Brcdas Service de Chimic ties Materi

XX w t of Comributors

S. De Silvestri Istituto Nazionale per La Fisica della Materia Dipartimento di Fisica Politecnico di Milano Piazza Leonardo

XX:ll List of Contributors

W. R. Salaneck Department of Physics. IFM Linkoping University S-581 83 Linkoping Sweden

N. S. Sariciftci Christian Doppler Laboratory for Plastic Solar Cells Physical Chemistry Johannes Kepler University Linz Altenbergerstra8e 69 A-4040 Linz Austria

U. Scherf Max-Pianck-lnstitut fur Polymerfon;chung Ackermannweg 10 D-55128 Mainz Germany

J.C. Scott IBM Research Division Almaden Research Center 650 Harry Road San Jose CA 95120-6099 USA

Z. Shuai Service de Chimie des Materiaux Nouveaux Centre de Recherche en Electroniquc et Photonique Molcculaires Universite de Mons-Hainaut Place du Pare, 20 B-7000 Mons Belgium

D.L. Smith Electronics Research Group Los Alamos National Laboratory Mail Stop 0429 Los Alamos NM 87545 USA

S. Stagira Istituto Nazionale per Ia Fisica della Materia Dipartimento di Fisica Politecnico di Milano Piazza Leonardo da Vinci, 32 I-20133 Milano Italy

C. Taliani Istituto di Speuroscopia Molecolare Consiglio Nazionale delle Ricerche Via P. Gobetti. 101 1-40 129 Bologna Italy

S. Ta.,ch Instttut fur Fcstkorpcrphy:-.ik Tcchnisdu; Univcrsitat Gr.tz Petcrga:-.se 16 A-8010 Graz Austria

P. F. van Huuen Department of Polymer Chemistry and Materials Science Centre University of Groningen Nijenborgh 4 NL-9747 AG Groningen The Netherland-;

Z. V. Vardeny Department or Physics University of Utah Salt Lake -City UT 84112 USA

j II List of Abbreviations

c

AFM AOM ASE BDAD BEH-PPV BuEH-PPV CB CEO CP CPG CTE cw DASMB OAT DBR DFB DH6T DHPPV DJA 000-PPY DOS DOS DOYS DPOP-PPY DSC DT DTA EA ECC ED EDC EL ESCA ESR FET

dielectric constant Atomic Force Microscopy acousto-optic modulator amplified spontaneous emission bis-( 4' -diphcnylaminostyryl)-2,5-dimethox ybenzene poly(2,5-bis(2'-ethylhexyloxy)-para-phenylene vinylene) poly(2-butyl-5-(2' -ethylhexyl)-1 .4-phenylene vinylenc) conduction band coupled electronic oscillator conducting polymer charge photogenemtion charge-transfer excitons Continuous Wave diphenylaminostyi)'Ibenzcne di-para-anisyl-para-tolylamine distributed Bragg reflector distributed feedback dihexyl-substituted 6T poly(2.5-diheptyl-para-phenylene vinylene) doping-induced absorption poly(2,5-dioctyloxy-para-phenylene vinylene) distribution of states density-of-states density of valence states poly( I ,4-phenylene-1 ,2-diphenoxyphenylvinylene) DiiTercntial scanning calorimeuy difle rcntial transmission di-para-tolyl-pam-anisylaminc clcctroabsorption cxtcmal color conversion electron ditrraction energy distribution curve Elcctroluminescencc Electron Spectroscopy for Chemical Application electron spin resonance licld-cffect transistor

XXIV List of Abbreviations

FGM FN FTO FWHM GP GPC HOMO H-T HV ICC IGFET INOO IPCE !RAY ISC ITO LB LCD LEC LED LESR LPPP LUMO MBE MEH-DSB MEH-PPV MeLPPP MIM MIMIC MIS MISFET m-LPPP MNDO MOSFET MS MSA MSM MTR NBS NTCDA ODMR OFET OGM OLEO OMA OMBD

Fluctuating Gap Model Fowler-Nordheim fluorine-doped tin dioxide full width at half maximum geminate pair Gel Permeation Chromatography highest occupied molecular orbital Herzberg-Teller high vacuum internal color conversion insulated gate FET Intermediate Neglect of Differential Overlap incident photon to converted electron infrared active vibrational modes intersystem crossing indium-tin oxide Langmuir-Blodgett liquid crystal display light-emitting electrochemical cell light-emitting diode Light-Induced Electron Spin Resonance laddered poly(para-phenylene) lowest unoccupied molecular orbital molecular beam epitaxy 2 -methox y-5-(2' -ethyl hex y loxy )-1 ,4-bis( 4-styry lstyry I )benzene Poly(2-methoxy-5-(2' -ethylhexyloxy)-1 ,4-phenylene vinylene methyl-substituted poly(para-phenylene)-type ladder polymer a metaVinsulator/metal micromolding in capillary metal-insulator-semiconductor metal-insulator-semiconductor FET methyl-substituted poly(pam-phenylene)-type ladder polymer Modified Neglect of Diatomic Overlap silicon metal-oxide-semiconductor FET metal-semiconductor tris(p-methoxystilbene)amine mctaVsemiconductor/mctal multiple trapping and thermal release N-bromosuccinimide naphthalene tetracarboxylic dianhydride optically detected magnetic resonance organic field-effect tmnsistor oriented gas model organic light-emitting diodes optical multichannel analyzer organic molecular beam deposition

OPY P3HT P30T PA PADMR PAH PAni PB PBD Pc PD PDA PDOT PTCDA PEOOT-PSS PEDT PEOPT PES PF PHP PIA PLQY PLDMR PM PMMA PP3YE

ppp pppy ppy PS PSD PT PTFE PTV PVK r.m.s. RS SCI SCLC SE SF Si-PPV

STM TAA

oligo(phenylcnc vinylene) poly(3-hexylthiophene) poly(3-octylthiophene) photoinduced absorption PA-Jetected magnetic resonance polyaromatic hydrocarbon polyaniline photobleaching

Lisr of Abbni(llious XXV

2-(4-biphenyl)-5-( 4-tert-butylphenyl)-1 ,3,4-oxadiaz.ole Phthalocyanine photodiode personal digital assistant poly(dodecyloxy-terthienyl) perylene tetracarboxylic dianhydride poly(3,4-ethylencdioxythiophene)-poly(styrenesultonate) poly(ethylcnedioxythiophcne) poly(3-( 4' -(I " ,4", 7" -trioxaoctyl)-phenyl)thiophene) photoelectron spectroscopy Poole-Frenkel para-hexaphcnyl photoinduced absorption photoluminescence quantum yield PL-detected magnetic resonance photomodulation spectroscopy polymethylmethacrylate copolymer containing phenylene, vinylene and non-conju-gated ethylidene units poly(para-phenylene) poly(phenylphcnylene vinylcne) poly(pam-phenylenc vinylene) polystyrene power spectral density polythiophene polytetmt1uoroethylene poly(2,5-thienylcnevinylene) polyvinylcarbazole root mean S(JU

XXVl List of Abbreviations

TCNQ IDAE TE TEM TFf

TOF TPA TIA UHV UPS uv VB VEH XPS XRD

tetracyanoquinodimethane tetrakisdimethylaminoethylene transverse electric modes transmission electron microscopy thin-film transistor transverse magnetic modes time of flight two-photon absorption tri-para-tolylamine ultra-high vacuum Ultraviolet Photoelectron Spectroscopy (UPS) ultraviolet valence band Valence Effective Hamiltonian X-ray photoelectron spectroscopy X-Ray Diffraction

1 Poly(arylene vinylene)s - Synthesis and Applications in Semiconductor Devices

Michael M. Murray and Andrew B. Holmes

1.1 Introduction

Polymers have traditionally been considered as insulating materials by chemist~ and physicists alike. Indeed a conventional application of polymers is the safe iso-lation of metallic conductors. The serendipitous discovery of highly conducting polyacetylene, however, marked the birth of a new tidd Ll ]. The study of thi~ new class of comtx>Unds is often termed .. molecular electronics" and the area ha~ proven to be highly interdisciplinary in nature. Chemists, physicists and theorist' alike arc continually exploring new materials and devising novel technologies. Early studies focused on improving the conductivity of organic poly1ners upon doping, but it has been generally accepted that most real devices are likely to ex-ploit the intrinsic semiconductive prope11ies of these materials.

Poly(arylene vinylene)s form an important class of conducting polymers. Two representative examples of this class of materials will be discussed in some detail here. There are poly( I ,4-phenylene vinylene) (PPY) I, poly(l ,4-thienylene viny-lene) (PTV) 2 and their derivatives. The polymers are conceptually similar~ PTV may be considered

2 1 Poly(arylt!lle vinyle11e)s - Symhesis alld Applicutio11s u1 Semiconductor Devices

1~2 Poly(1,4-phenylene vinylene) and its Derivatives

Eectroluminescence was first demonstrated for conjugated polymers (e.g. PPV 1) at Cambridge in 1990 12]. PPV is the cheapest and simplest poly(arylene viny lene), comprising of alternating benzene and vinylene units. T he material is highly fluores-cent and is bright yellow in color. Its emission maxima are in the yellow-green re-gion of the visible spectrum and two distinct emission peaks are readily idemified: at 551 nm (2.25 eV) and at 520 nm (2.4 eV). The synthesis of PPV was first described in the 1960s but the final material obtained was insoluble, infusible, and difficult to process. The high crystallinity of conjugated polymers is be lieved to arise from strong interchain n- n stacking interactions. Solution processing is a highly desir-able chamctcristic as it allows the matclial to be coated cheaply and incorporated into EL devices; PPV fonns excellent transparent films when cast from solution or upon spincoming. Synthetic routes have been developed to allow thcmtal conver-sion to PPV from a variety o f soluble precursors. Wessling and Zimmennan intro-duced (3, 4] the sulfonium precursor route to PPV. '01e route has subscyucntly been adopted and modified by other research groups 15-7 ]. It has also been used to good effect in the synthesis of several de1ivativcs of PPV.

The chemistry behind the synthesis of parent PPV is relatively straightforward and is outlined in Scheme l-2. A sultide such as tetrahydrothiophcne is reacted

a

3

6

~ n 1

Scheme 1-2. Synthesis of PPV 1: a) tetrahydrothiophene. MeOH, 65 "C; b) NaOH, MeOHI H20 or Bu4NOH, MeOH, O"C; c) neutralization (HCI); d) dialysis (water); e) MeOH, soc; f) 220"C, HCI(g)/Ar, 22 h; g) 220- 300 "C, vacuum, 12 h.

1.2 Poly( 1,4 -pheuyleue viuyle11e) tmd its Derivatives 3

with a,a'-dichloro-p-xylene 3 to yield a bis-sulfonium salt 4. Dimethyl sulfide has also been used but dimethylsulfoniurn groups were found to under

4 1 Poly(arylene vinylene)s- Synthesis and Applications in Semic01uluctor Devices

3

~CI Cl~

8

c ~ X=CI, Br n

10

-o-"" Ph#CH2 _" CH~Ph3 c1e c,e

e ~ n 11

Scheme 1-3. Alternative synthetic routes to PPV 1: a) 500-700 C, 0.01 rnbar; b) 580"C, 10 Pa; c) 800-000''C, O.Q1 rnbar. then 60"C, 0.1 rnbar: d) 200 c. vacuum; e) CH3CN, 5.5 v.

cyclophane 8 [20]. In each case the intermediate a-halo precursor polymer 10 is thermally converted to PPV at 200C under vacuum (Scheme 1-3). These meth-ods yield PPV with low (0.002%) device efficiencies [ 19, 21 j.

A xylylene-bis-phosphonium salt 11 gave films of PPV 1 upon clectropolymer-ization. The absorption and emission spectra of the resultant material were blue-shifted with respect to PPV produced by other routes, suggesting that the electro-polymerized material has a shorter effective conjugation length, possibly because of incomplete elimination of phosphonium groups [22).

A potential drawback of all the routes discussed thus far is that there is linle control over polydispersity and molecular weight of the resultant polymer. Ring-opening metathesis polymerization (ROMP) is a living polymerization method and, in theory, affords materials with low polydispersities and predictable molecu-lar weights. This methodology has been applied to the synthesis of polyacetylene by Feast [23), and has recently been exploited in the synthesis of PPV. Bicyclic monomer 12 [24] and cyclophane 13 [25) afford well-defined precursor polymers which may be converted into PPV 1 by thermal elimination as described in Scheme 1-4.

1.2.1 The Basic Polymer LED Device Architecture

The simplest polymer-based EL device consists of a single layer of semiconduct-ing fluorescent polymer, e.g., PPV, sandwiched between two electrodes, one of . which has to be u-ansparent (Fig. 1-1). When a voltage or bias is applied to the material, charged carriers (electrons and holes) arc injected into the emissive layer and these ca1Tiers arc mobile under the influence of the high (> 105 V cm- 1) dec-

@OSiM~IBu a -

12

1.2 Poly( 1,4-phenylelle vi11yle11e) aiiCI its Derivatives 5

b

n n

Scheme 1-4. ROMP routes to PPV 1: a) [Mo(=NAr)(:::CHCMe2Ph)(OCMe2(CF3))2}; b) Bu4NF; c) HCI(g). 190 c: d) 108"C; e) 280 C.

8

(f) Fluorescern

~ t polymer film ~--~~~--~~~

tl.

Figure 1-1. S.:hema1ic dr

6

a)

I Poly(arylene vinylene)s - Synthesis ami Apfllicaticms in Scmiccmduuor Dt!vicc.1

LUMO C=:J c=:=:J hY , ---~ hY' C - --!.. - -+-HOMO~ c=:=:J

AaOocal anaoo

Singlet excited slate

Sdaglel exoted state

hv'

Figure 1-2. a) Irr.H.Iiatiun of a nuun:M:cnt polymer "".:ate~ an ckctruu frum IIOMO In LUMO. In a typical conjug:aloo pt>lymcr, two new energy ~l.ilc' an: geueratcJ upon rclaxatllm wllhiu the original HOMO-LUMO enc11;y gap anc.l arc each lillcJ with om: cle.:trun ur llJlJl'l'itc ~ign (singlet cx.:itcc.l slate). TIIC cxcitc..-J polylllcr may 1hcn relax to the gruunc.l >IOIIe with emi"mn of light at a longer wave-lcnglh than lhat absorbcc.l (phololumiucsccncc ). b) In a (l\llymer LED. delymcr. lhc r~ulling ch:ugcs migraJc from polymer chain to polymer ch:tin under the in llu-

e~-e of the applicc.l chxtric Iield. When a mc.lical anion ant.! a r.ac.lical cation a:ombtnc on a ~inglc conju-gated ~egmcnt, ~inglet anc.l triplet excitet! slates arc formcc.l, of whicll the singleL~ can emil lighl.

visible to the viewer. Some of the photons emitted are reflected by the polymer itself and the external efficiency, therefore, is a factor of 2n2 smaller than the in-ternal efficiency, where 11 is the refractive index of the polymer (typically '' = 1.4 ). Typical external quantum efficiencies for this class of materials is 0.1- 2% [26).

1.2.2 Substituted Poly(phenylene vinylene)s

Conjugated polymers in general have.! a propensity to aggregate or s tack as a con-sequence of their extensive n-delocalization. This is a limitation of PPV itself as it requires thermal processing during device fabrication. LEOs fabricated from PPV derivatives that arc soluble in the conjugatc.:d fonn were first reported by Ohnishi et ul. at Sumimoto [27] and by Hceger and Bmun at Santa Barbam [28, 29]. The solubility of the materials results from the presence of long alkyl chains which af-ford some conformational mobility to the polymer chains. As a consequence, the materials tend to have lower glass transition temperatures than PPV itself. Further-more, the sulfonium precursor polymer 6 is soluble in methanolic solvenL-;, whereas almost all soluble PPV derivatives are proces~ible in solvl!nts that do not cause swelling of PPV or its sulfunium precursor 6. Multilayer or sandwich type

1.2 Poly( / ,4 -plrenylene vinylene) ancl its Derivatives 7

structures comprising of different polymers (e.g. PPV and substituted PPV) may be fabricated, and these exploit this difference in solubilities [30, 3 1 ].

Poly[2,5-diatkoxy- 1,4-phenylene) vinylene}s with long solubilizing alkoxy chains dissolve in conventional organic solvents such as chloroform, toluene, or tctmhydrofuran l27, 28, 32-36]. Their emission .and absorption spectm are red-shifted relative to PPV itself, and the polymers' fluorescence and e lectrolumines-cence quantum yields are greater than parent PPV. This benefit may be a conse-quence of the long alkyl l:hains isolating the polymer chains from each other.

The sulfonium precursor route may also be applied to alkoxy-substituted PPVs, but a dehydrohalogenation--condensation polymerization route, pioneered by Gilch, h; favored 137}. The polymerization again proceeds via a quinomethide in-tcnncdiatt:, but the synthesis of the conjugated polymer requires only two steps and proceeds often in improved yields. Till! synthesis of the much-studied polyl2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylenej, MEH-PPV IS is outlined in Scheme 1-5 133. 35). The.! solubility of MEH-PPV is believed to be enhanCI!d by the branch!.!d nature of its side-chain.

14

oy ~ MeO n

15

Scheme 15. Preparation of MEH-PPV 15: a) 2-ethylhexyt bromide, KOH, EtOH, reflux, 16 h ; b) HCHO, cone. HCI, dioxane. 20' C, 18 h, reflux, 4 h; c) KOtBu, THF, 20"C, 24 h.

4-Methoxyphenol is alkylated and then chloromethylated to yield the bis-chlor-omethyl derivative 14 [28, 32- 351. Polymerization and elimination to fully conju-gated matl!rial is promoted by the addition of a 1 0-fold excess of a base such a.-; potassium terr-butoxide in tctrahydrofuran. The concentration of base and mono-mer must be carefully controlled to prevent cross-linking and gelation. TI1e resul-tant MEH-PPV is bright red-orange.! in color. A single layer elcctroluminescence device (ITO/MEH-PPV/Ca) achieves illl eiTiciency of 1.0% [28, 38, 39]. The qu~tntum etliciency may be improved four-fold by the incorpomtion of a thin hole-blocking layer of poly(mcthyl methacrylate) deposited by Langmuir-Blodgcu techniques and by using aluminum as a cathode.

Other poly(2,5-dialkoxy- l ,4-phenylcne vinylenc)s have been prepared in a sim-ilar fashion [34, 35, 40, 41 ). Ahl!mativcly, a soluble a-halo precursor polymer 17 may be obtained by using less than one.! equivalent of base (Scheme 1-6). This may then be conve11ed into fu lly conjugated material 16 by them1al trcatml!nt. This halo-precursor route may be preferred if the fully conjugated mate1ial has limited solubility ur if incomplete conversion is desired.

8 1 Poly(arylene vinylene)s - Synthesis and Applications in Semiconductor Devices

OR OR

H,C-P-CH3 a Br0i2-P-CH~r

-

X X

R = Ct;H1s. CeHe X= OR, Br, Cl, F3 1 b

OR OR

~ c ~ X n X n 16 17

Scheme 1-6. The Halo-precursor route to substituted PPVs: a) NBS, CC!.t. hv; b) KOtBu, THF; c) 160-220"C, vacuum, 4 h.

a

c

20

(Cti:!lnH n 23

Scheme 1-7. Preparation of DP-PPV derivative 23: a) H-cs C-{CH2)n-H; b) UAIH4; c) SOCI2; d) KOtBu, 2.2.

1.2 Poly( /,4-phenylene vinyle11e) UIU[ its Derivatives 9

Control of gelation and polydispcn;ity has recently been improved by the use of a benzylic halide 22 as a terminator (Scheme 1-7) [42]. The halide serves as a non-polymerizable acidic additive, and high molecular weight PPV derivatives 23 with relatively small polydispersities can be attained. A hexyl-substituted deriva-tive 23a, DP6-PPV, shows a remarkably high photoluminescence quantum effi-ciency of 65% in the solid state. Its emission maximum (490 nm) is in the blue-green region of the spectrum. The synthetic route is versatile and utilises the Diets-Alder reaction as a novel way of appending alkyl groups onto the benzene ring.

The effects of the length and degree of bnmching of the alkoxy side chain on EL etliciency have been examined by a number of research groups (Scheme 1-8) [27]. Efficiencies pass through a maximum with increasing size of side chain be-fore falling off tor very large substituents such as cholestanoxy, e.g. 26 [40, 43). Il may be envisaged that such lar

10 1 Poly(arylene vinylene)s - Symhesi.1. w1d Applications ill Semiconductor Dcvias

Qrdered dialkoxy PPY derivative has been prepared by Yoshino e1 u/. r49l. ~1}'{2.5-nonoyloxy-1,4-phenylene vinylene) 27a fonns a nematic liquid-crystal-line phase upon 1nelting. TI1e material retains its order upon cooling to room tem-perature, and its band gap (2.08 eY) is measurably smaller than in an unoriented sample. Oriented clectroluminescence may be achieved by rubbing a thin film of the material to induce molecular orientation [50].

1.2.2.1 Poly(anthrylene vinylene)s The scope of Wessling route has been extended by Mullen and co-workers to de-velop a soluble precursor route to poly(anthrylene vinylene)s (PAYs) [51). It was anticipated that the energy differences between the quinoid and aromatic reso-nance structures would be diminished in PAY relative to PPY itself. An optical band gap of 2.12 eY was detem1incd for 1,4-PAY 29, some 0.3 eY lower than the value observed in PPY. Interestingly, the 9, I 0-bis-sulfonium salt does not poly-merize, possibly due to steric cftccts (Scheme 1-9).

~~B-4 l

12 1 Poly(wylt!l1e vinylene)s - Synthesis and Applications in Semiconductor Devices

la,b,c,d

OMe

34

1'

~ MeO n 35

Scheme 1-11. Synthesis of partially conjugated PPV copolymers: a) B~NOH, MeOH; b) MeOH, 50C; c) 300C, vacuum, 12 h; d) 220C, HCI(g)/Ar, 22 h.

lithographically patterned copolymers [58, 59). High EL efficiencies have also been demonstrated in alkoxy- and alkyl-substituted poly(phenylene vinylene)s with interrupted conjugation. These systems may also be synthesized by the base-induced dehydrohalogenation method [61 ].

The efficiency of PPV may also be mised by introducing disorder into the polymer chains. The crystallinity of PPV may be lowered by employing a modi-tied Wessling method utilizing a xanthate leaving group 163]. PPV produced by this method is believed to contain a mixture of cis- and trans-alkene units. The efficiency of the material is 0.22% when employed in a single layer device with

1.3 Refining the Properties of PPV - Multilayer Devitc:s

R R

o~ ~o. ~y~ 36

Roo R Y=NC6Hs.O

9 0f'~

16 1 Poly( urylene vinylene)s - Synthesis and Applications in Semiconductor Device.s

43

Scheme 1-14. Examples of a oxadiazole- and a quinoxaline-containing electron-transporting polymer.

A hyperbmnched polymer 42 compnsmg oxadiazole subunits has been synthe-sized, but defect formation in such a structure appears to limit its use as a hole-blocking material [74].

Other electron-deficient heterocyclic systems have also been invcstig~ued as electron-transporting materials. In particular, devices employing poly(phenyl qui-noxaline) 43 as an ECHB layer have shown improvements in device efliciency when used in conjunction with an emissive PPV layer [75).

1.3.2 Electron-Deficient Polymers - Luminescent Transport Layers

It would be preferable to incorporate both fluorescent and electron transport prop-erties in the same material so as to dispense entirely with the need for elcclron-tmnsport layers in LEOs. Raising the aflinity of the polymer facilitates the use of metal electrodes other than calcium, thus avoiding the need to encapsulate the cathode. It has bt:en shown computationally [76] that the presence of a cyano sub-stituent on the aromatic ring or on the vinylenc portion of PPV lowers both the HOMO and LUMO of the material. 1l1e barrier for electron injection in the mate-rial is lowered considerably as a result. However, the We.-;sling route is incompati-ble with strongly electron-withdrawing substitucnts, and an alternative synthetic route to this class of matCJials must be employed. 1l1e Knocvcnagcl c."Onden~1tion

1.3 Refining the Properties of PPV - Multilayer Devices

45 46

47

Scheme 1-15. Synlhesis ol CN-PPV 41: a) NaOAc; b) KOH, EtOH; c) pyridinium chlorochrc mate; d) NaCN; e) KOtBu or Bu4 NOH, tBuOH, ll-IF, SO "C.

177.1 h~s bee1~ utilized in the synthesis of CN-PPV 47, a dialkoxy-substituted PP\ dcnvat1ve wnh a c~ano substituent on the vinylene position. TI1e reaction in vol~es .the co~densauon of a tcrephthaldehyde 45 and a benzene-1,4-diacetonitrilt denva~1vc 46 Ill the ~rescnce of a base such as tetrabutylammonium hydroxide o1 potasstum te~t-butoxtde in a solvent mixture of tetrahydrofuran and tert-butano (Scheme l.-1.)) [78 ). Both monomers arc readily available from the dichloride 44 The matcr~al- may be puril}ed by ~epeated precipitation. CN-PPV is a highly fluo-rescent, bn!hant red matenal and Its band gap [590 nm (2.1 eV)) is very similar rc other 2,~-dalkoxy PPY derivatives. The molecular weight of the polymer is mod-~st . albell accept~ble lor step-growth polymerizations of this nature. The polymc1 ts l~eely _soluble 111 solv~n~s such as chloroform, by virtue-of its long alkoxy side chams. 1 He e~cctron-dehcent nature of the material is exemplified by cyclic vol-tammetry studtes. These demonstrate that the cyano substituent reduces the onsc1 of reduction by 0.6 V relative to dialkoxy PPV (74].

In contrao;t with .L"O_njugated polymers, such as PPV, devices employing CN-PPV 47 _as the enusstve layer can achieve respectable intemal e rticiencies (ca. ~.2%) wllh both cal~ium and aluminum electrodes. EL elliciency may be further unproved by cmploymg a hole-transporting layer such as PPV in COilJunction with

14 1 Poly(ary/eJ1e vi11ylene)s- Sy111hesis and ApplicaJions in Semiccmductur Devices

1.3.1 Multilayer Devices: The Incorporation of Charge-Transporting Layers

To maximize light output. it would be ideal to create roughly equal amounts of both charge carriers, i.e., electrons and holes. Most devices described thus far have been single-layer devices where the semiconducting polymer is sandwiched between the cathode and anode. It has already been mentioned that PPV has a higher barrier for electron injection than for hole injection, and holes are preferen-tially injected in a single-layer device. It is common. therefore, to use additional layers of materials to enhance electron injection. These layers arc termed electron-conducting/hole-blocking (ECHB) layers, and the configuration of a typical de-vice incorpor.uing such a layer is depicted in Figure 1-3. To work efficiently, the electron affinity of the ECHB layer must be higher than that of PPV. In such a case charge carrier recombination takes place away from the polymer metal inter-face, which is known to act as a quenching site. EL efficiency may thus be con-siderably improved relative to single-layer devices.

The earliest ECHB materials studied were the oxadiazolcs. These are electron-deficient materials and 2-( 4-biphcnylyl)-5-( 4-te rt-but ylphcnyl)- 1 ,3,4-oxadiazole (PBD) 39 was the first member of this class of compounds to be used success-fully in a sublimed film EL device [66, 671. The material is available in high pur-ity, and may be sublimed or spin-coated as a dispersion in poly(mcthyl methacry-late) (PMMA) (68]. It may also be blended to good effect with electroluminescent polymers [47, 69, 70], although it is preferred to disperse PBD in an insulating polymer such as PMMA to overcome potential problems with phase separation. An increase in device thickness is llil inevitable consequence of this approach, and this manifests itself as a rise in driving voltages in an ITO/PPV/PBD-PMMA/ Ca device. This disadvantage, however, is outweighed by a rise in EL efticiency from 0. 1% (for an .ITO/PPV/Ca device) to 0.8% (for ITO/PPVfPBD-PMMA/Ca).

8

hv

Figure l-3. In lhi~ improved bilayer dcvi

18 1 Pvly(uryle11e viuyltmc:).~ - Syutlw.~is am/ Applic

20 1 Poly(aryleue viuylene)s - Synthesis and Applicmions in Semiconductor Devices

55

Bu

Kht n

56 57 58

Scheme 1-18. Pyridine-based analogs of PPV.

a

-

Cl...._ _ f N\ __ ~CI

59

b -

Q.. ;=N, ::.b-~

cr S+ 60

/ Xfr. d

-trOt d ~ ---n ~ !J n 62 55 61

l d / M{!Ot ~ !J n

63

Scheme 1-19. Wessling route to PPyV 49: a) NCS, CC4; b) tetrahydrothiophene, MeOH, sooc; c) aq. NaOH, MeOH; d) heat; e) 0.98 eq. KOtBu, THF; f) heat, MeOH.

length of the head-to-head isomer is found to be the longest. Its emission spec-trum is maximal at 605 nm, whereas the head-to-tail and random isomers emit at 584 nm and 575 nm, respectively 194].

Poly(pyridine vinylene) 55 may also be prepared by the Wessling route (Scheme 1-19) (95]. Fully conjugated material may be obtained by thennal elimi-nation of the sulfonium salt 61 or the halide 62. The method, however, does not

I .-J Full Color Displ been the slowest in development, both in traditional inorganil materials as well a-; in polymers. There are a number of different approaches to suitable polymeric analog. It has already been discussed in Section 1-2 how th1 HOMO-LUMO gap of PPV may be raised by incomplete conversion to the full) conjugated polymer. Furthermore, the band gap may be increased by appendin~ sterically demanding side groups onto the polymer main chain. The useful opto electronic properties of phenyl-substituted PPV derivatives has already been dis cussed (sec Section 1-2.2) 1421. The following section will enunciate a number o other approaches to matc1ials that emit in the blue region of the spectrum.

1.4.1 Isolated Chromophores - Towards Blue Emission

An alternative pproach to tailoring the band gap of a polymeric material involve~ the use of polymer blends. 1l1e blend of a fluorescent dye in an inert polymc matrix yields a polymeric EL material with useful luminescent charactetistics. h practice, however, the fluorescent yield in such a blend is very low as a consc quence of the low mole fraction of the luminescent species. Device brightnes~ may be enhanced considerably by the use of an clectroactive polymer, e.g., poly-(vinyl carbazole) 64. TI1e improvement in performance manifests itself even a1 very low concentrations of fluorescent species. Alternatively, the active chromo-phore may be incorporated into the polymer backbone ito;elf and the light-emittinf species can be separated by varying lengths of inert .. spacer'' units. This has beer demonstrated in the preparation of copolymers such as 65. These are prepared h) conducting a Willig condensation of a bis-ylidc in the presence of varyin~ amounts of saturated dialdehydes (Scheme 1-20) 197].

More precise control over the emission color may be achieved by employin~ equimolar quantities of a his-phosphonium salt, e.g., 67 and a dialdehyde contain-ing a flexible unit such as 66. This approach has been exploited by a number ol researchers 19X- 1021. The emission wavelength of these materials is in the blue tc hlue-grcen region of the spectrum (470-495 nrn).

22 I Pol)(urylene vulyle11e)s - Symhesis and Applications i11 Semiconductor Devices

OMe OMe

n 0- (CH2}8 -0~ MeO m

64 65

Scheme 1-20. Poly(vinyl carbazole) and an example of a polymer with an isolated chromo-phore.

OMe MeO

OHC-Q-0- (CH2}e -0-o-CHO OMe MeO

66

+

67

I MeO

OMe 68

Scheme 1-21. Synthesis of a blue-emitting copolymer with isolated chtomophore by Wittig reaction: a) base.

1.4.2 Comb Polymers with Chromophores on the Side-Chain

The luminescent moiety may also be attached to the polymer backbone as a side-chain, yielding a comb polymer. In this instance, the main polymer chain is often non-conjugated, and classical approaches to high molecular weight polymers may be employed. Distyrylbenzenc has proved to be a u:scful lumophore in this con-text. It has been attached to polymethacrylates such as 69 bCaJing electron-trans-porting diaryloxadiazole units [74, 103, 104]. A bilayer LED employing PPV as a hole-transporting layer and 69 as an emitting layer showed emission in the blue region (457 nan) of the spectrum with an efficiency of 0.037%. A tail into the yel-low-green region of tl1e spectrum was observed, and this was attributed to emis-sion from PPV itself.

0~MeO 0 'I

MeO

70

/ .5 Chiral PPV - Pulurized Emission

OMe

OMe

Scheme 1-22. Examples of electroluminescent polymers with side-ch,;n""' ho ... "'oromop res.

The precise control of ROMP methodolo has bee . co-workers in the 1 , gy n cxploated by Schrock and distyrylbe~zcne siJ:Jl'~~n;;;tal~nOSolf aW nloerlx>~lle~le n~onomer functionalized with a

. l n ca caum 1s used as a .. thod . nal dev1ce efficiency of 0 J% . b . cd . . . ca e, an lnter-(475 nm). .. IS o serv and the peak emission is in the blue

l.S ChiraJ PPV - Polarized Emission

Meijer and co-workers I 106j have recently re 011 d . . I I . nescence (CPEL) . . h . P c cncu ar Y polanzed elcctrolumi-

. usmg a c lral 7H.:CmJugatcd PPV deriva( 71 S .. , such a system the absorebcd and emiued lioht hav' lrtl ave . ( ~hem~ 1-23). In and rioht (R) circularly ..,. 1 "' . d . o . c 1 c.:rcnt antcnsatiCS lor left (L) o , . > auzc.: components To unpmvc >I b T al and racemic monomers had to be c .I. . h' S Sl u a _aty pn>pcalles, char-f 1 0Pl> ymcn;rNU. mall ch1ral aggrc . t. . orml-'"l Ill these matcriah. as a consequence of the rciospll- .. b t . ga c.:s arc

o ~~ ~ su s llutaon paucna

24 I Poly(arylene vinylene)s- Sy111hesis and Applicutions in Semicunductur Devices

R=CH3 71

Scheme 1-23. Chirooptical PPV derivatives.

of the enantiomcrically pure side groups on the chain. The conjugated polymer obtained maintains its intrinsic chirality in both solution and solid state, and circu-larly polarized emission requires no extemal macroscopic ordering of the chromo-phores. Funhem1orc, it was found that the cin:ular polarization in absorption, ~"h" was larger in an aggregated solution than in a solid lilm, sugges_ting fut~h~r orderin_g of polymer chains in an appropriate solvent. The dissymmetnc hght-cmttll~g layer ~s a consequence of the polymer's constitution, the solvent and the proccss.ng condi-tions employed.

1.6 Poly(thienylene vinylene)s-A Stable Class of Low Band-Gap Materials

Polyacetylene is considered to be the prototypical low band~gap pol~n~er, but its potential uses in device applications have been hampered by ~s senstllv_ty to both oxygen and moisture in its pristine and doped states. Pol_y(thtenylene vmylene) 2_ has been extensively studied because it shares many ot the useful aunbutes ol polyacetylene but shows considcr.1bly improved enviromn~mal stabil~ty. T_he low band gap of PTV and its derivatives lends itself to potenttal apphcallons n both its pristine and highly conductive doped state. Furthermore, the vinylcnc spacers between thiophene uniL'i allow substitution on the thiophene ring without disrupt-ing the conjugation along the polymer backbone. . .

Polymers such as PTV have potential applications as the acllve semtconductor layer in thin-film transistors (TFTs).

1.6 Poly(thieuyleue vinylene)s - A Swble Class of Low Bcmd-Cup Mmerials 2:

1.6.1 Organic Field Efl'ect Transistors (FETs)

Organic materials have recently shown promise as the active layer in organic based thin lilm tr,msistors (TFTs). Such devices have potential applications in th switching element in flat panel displays and smart cards. Organic materials coul have substantial cost advantages over their silicon counterparts if they can be de posited from solution, as this enables the easy fabtication of large-area, tlexibl displays. The geometry of a typical thin-film transistor device is depicted i Figure 1-4. The current passing between two electrodes, the source and drain, i controlled by applying a voltage to a third electrode, the gate. The semiconduc1 ing layer carries the current and the perfonnance of the device is very muc dependent on the mobility, Jt, of the semiconducting material. Another figure c merit is the ON/OFF r.1tio of the device; ideally the semiconducting layer shoul have a low conductivity but a high field-eflcct mobility. Crystalline silicon has mobility of 10-' cm2 v- s- , whereas organics can only attain mobilities of th order 1- 10 cm2 v- s- but these values are sufficient for the intended applicdc. !he gale. In 'ueh a cunfJ!_!Uraliuu 1he device IS capable uf .:urrcnl amlllilkaliun. i.e .. sm.o changes in til

26 1 Poly(wylene vinylene)s- Synthesis cmd Applications in Semiconductor Devices

1.6.2 Synthesis

In common with PPV 1, however, parent PTV is insoluble and infusible. The mate-rial was first synthesized in powder fonn by Kossmehl using a Wittig polycondensa-tion methodology [ 116). The first example of PTV synthesized by a soluble precur-sor route was published by Elsenbaumer and co-workers [ 117). Once again, the Wessling method was exploited (Scheme 1-24): 2,5-(Bis(tetrahydrothiophenonium, methyl)thiophene chloride 73 was treated with one equivalent of water at O"C and rapidly fonned a thick polyelectrolyte. The material may be cast from aqueous solution and converted into PTV by heating to l50 C. The material exhibited a con-ductivity of 60S em- when doped with 12 The watersoluble precursor polymer, however, has a tendency to gel or precipitate during dialysis and storage. Further-more, the soluble precursor is prone to elimination even at room temperature. 1l1e route was modified somewhat by Murase [ 118) and Saito [ 119 J by treating the water-soluble precursor with methanol. As a conscquenc.:e, the sulfonium groups arc replaced by mcthoxy substituents rendering the pn.:cursor polymer 75 soluble in organic solvents. Thermal treatment at 200- 250 '"C affords a shiny material with a large absorption centered arounu 2.3 ~:V (540 mu) and an optical banu gap of 1.8 eV. A considerable amount of effort has been devoted lo optimising this protocol[l20j. lt has been found that the reaction is best carried out in water, anu that low temperatures are benelicial for high monomer conversion and polymer processibility. The optical anisotropy of PTV was examined by orienting thin films of thickness 0. 1 f.un [ 121 ). A red-shift in the absorption peaks on the ma-terial was observed upon stretching indicating more highly-ordered polymer films.

A drawback of the Wessling methodology, however, is that trace amounts of acid catalysts are required to completely eliminate the methoxy groups. A conse-

a~c a O~sO cr cr

72 73

~ 1 b n OMe

75 'Z_

ld ~ ~ n crs~ ~ 0 S n 74

2

Scheme 1-24. Wessling route to PTV 2: a) MeOH, tetrahydrothiophene; b) NaOH, H20; c) heat

/ .6 Puly(tlzieuyleue vinylene)s- A Swble Cluss of Law Bwzd-Cup Materials 27 .

quence is that this synthetic route is not particularly useful for semiconductor de-vice applications. A route employing a xanthate precur.;or has been recently de-scribed by the group at Lucent Technologies.

1.6.3 Aldol Route

Parent poly(thienylene vinylene) has also been synthesized by an ~dol precur~or route [ 122). ln this method, 5-methyl-2-thiophenecarbaldehyde 76 ts treated With a base and the monomer polymerizes yielding a precursor 77 which is soluble in water. 1l1em1al treatment in an acidic solution at 80"C yields the fully conjugated material. Alternatively, the solid polymer may be heated to 280oC to effect elimi-nation of water. Fully conjugated matt:rial exhibits low conductivity (10-41 S em- ') in its pristine stale.

H cJ[)-CHo 3 s

76

a

1 b

~n 2

Scheme 1-25. Aldol route to PTV: a) KOtBu, DMF; b) heat.

1.6.4 Ring-Substituted PTV Derivatives

There have been a number of different synthetic 'approaches to substituted PTV derivatives proposed in the last decade. Almost all focus on the aromatic ring as the site for substitution. Some effort has been made to apply the tmditional base-catalyzed dehydrohalogenation route to PTV and its substituted analogs. 1l1e methodology, however, is not as successful for PTV as it is for PPV and its den-vatives because of the grca1 tendency for the poly(u-chloro thiophene) precursor spontaneously to eliminate at room tempcr.tture. Swager and co-workers at-tempted this route to synthesize a PTV derivative substituted with a crown ether with potential applications as a sensory material (Scheme 1-26) [123). The synthe-sis employs a Fager condensation [ 1241 in its initial step to yield diol 78. Treat-ment with a ditosylate yields a crown ethcr-funetionalized thiophene diester 79. This may be elabor.1ll ... "

28 1 Poly(uryle11e vi11ylelle}s - Symhesis am/ Applicutiolls in SemicvtuluL'Ior Devices

a

78

d

-

Scheme 1-26. Attempted synthesis of a crown ether-functionalized PTV derivative 76: ~~~O(CH2CH20)30Ts, K2C03, NMP; b) liAJH4, THF; c) SOCI2, pyridine, THF; d) KOtBu,

showed solvatochromic behavior, but could not be redissolved when solvent was removed, and the material could only be chamcterizcd by IR spectroscopy.

The first substituted PTV derivatives poly(3-methoxy-2,5-thienylcne vinylene) ~d poly(3-ethoxy-2,5-thienylene vinylene) 84 were synthesized by Elsenbaumer 10 the late 1980s [ 125 ]. The method employed was a step-growth condensation

u~ing a nickel-mediated cross-coupling reaction between a Grignard reagent and a dtchlorocthene (Scheme 1-27). The materials obtained were deep blue solids, but the majority of the sample was insoluble in common organic solvents. Endgroup

OR OR ~ OR d a,b b.c ~ s " S n RO

82 83 84 A = Me, Et

BuO 06u

~ S n 85

Scheme 1-27_ Synthesis of alkoxy PTV derivatives and dibutoxy PTV 79: a) Bull, MgBr2Et20; b) CICH::oCHCI (0.5 equiv.), NiCI2(dppp)2; c) CICH::oCHCI (1 equiv.), NiCI2(dppph.

1.6 Pol_r(tlliellylene Pinylene)s - A Stuble Cluss of ww Band-Cup Materials 2'

analysis of the insoluble proportion suggested that the number average molecula weight M,, was over 9 100 g/mol. The soluble pl)rtion, however, showed in teres I ing optoelectronic properties. It has an absorption maximum of 600 nm and it band gap of 1.5 eV was significantJy lower than that of PTV itself. In additio1 the polymer could be cast into thin fil ms. The electron-donating groups arc lx licvcd to reduce the ionisation potential of the polymer considcmbly compared t unsubstitulcd fYTV. The material could be readily doped by FeC13 and pressed JX lets of poly(3-cthoxy-2.5-thienylene vinylene) treated with FeCI3 in nitromethan solution exhibited conductivities of 1.8 S cm- 1 [126].

The same synthetic route was employed by Blohm and co-workers [1 27] in th synthesis of poly(3,4-dibutoxy-2,5-thienylene vinylene) 85. The alkoxy chair help to solubilise the polymer in conventional organic solvents, and this facilitat' a rca.;onablc estimation of its molecular weight. GPC analysis of the polymer su: gests M,= 12400, concsponding to a degree of polymetization of 49. The efll!t tive conjugation length of this material is an improvement over poly(3-ethox: 2,5-thienylcne vinylene): the absorption maximum of poly(3,4-dibutoxy-2,5-th enylcne vinylcne) 85 is red-shifted to 607 nm with a shoulder at 670 nm an furthcnnorc, the color l)f the material could be reversibly changed from blue nearly transparent in a doping-dedoping cycle. The measured conductivity of tl doped material. however, was of the order I S em 1

The methodoll)gy has been further extended to hexyl-substituted P1V detivativo by Shirakawa et ul. [ 12g]. A regioregular polymer consisting of alternating vinylet and hexyl-substituted thiophene units 88 wa.o; prepared by a nickel-mediated eros coupling (Scheme 1-27). The degree of polymerization was moder~tte (M, == 360 and the ciTectivc conjugation length of the material .:t ..... ~ = 470 nm) was consi

30 1 Poly(arylene vinylene)s - Synthesis and Applications in Semiconductor Devices

C4 H110 OCHe

OHC)i_CHO s 80

b

Scheme 1-29. McMurry coupling route to dibutoxy PTV 79: a) BuU, TMEDA, DMF; b) T1CI4 , Zn.

R R

0 s a R R

.--0-. s ll1

b A R ,f-1 A ' ~s~

ll2

Scheme 1-30. StUie route to alkyl-substituted PTV derivatives: a) 12 , HN03; b) 8u3SnCH=CHSnBu3 , Pd(PPh3)(0Ac).

by the reaction of 1,2-bistributylstannyl ethylene with a diiodothiophene 91 (Scheme l -30).

An attractive route to dialkoxy PTV derivatives has recently been reponed by Elsenbaumer and co-workers 1131, 132). The method employs the thenual elimi-nation of a sulfinyl group from

32 I Poly(arylene vi11ylene)s - Symhesis wul Applicutions in Semico11ductor Devices

semiconducting polymers into real devices. The technology employing poly-(arylene vinylene)s as the active layer in LEOs has advanced considembly over lhe past Len years, and lhc impact of controlled synthesis on device properties can-not be underestimuted. lL is perhups fair to say lhat PPV derivatives have had a greater impact to date in the field of molecular electronics, but research effons arc continuing to devise new synthetic approaches to PTV and its derivatives. As a consequence, it is anticipated lhat novel applications of such low band gap materi-als in solid-state devices will be pursued with increased vigour in the next de-cade.

Acknowledgemenls We lhank the Engineering and Physical Sciences Research Council (UK), the Commission of the European Community (Marie Curie Fellowship to MMM), and Cambridge Display Technology for financial support. We acknowledge the collaboration in Cambridge of our colleagues Prof. R. H. Friend, Dr. S.C. Moratti, Dr. N.C. Greenham, and Dr. F. Cacialli and we thank Dr. A. Kraft and Dr. A. C. Grimsdale for their interest in this work.

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Rejere11ces 33

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huiz.:n. M. M. F. v. Knippcnb

34 I Puly(wylene vinylene)s - Syntilesis ami Applicutian.~ in Semicmuluc l()r Devices

62. A. KrJO, P. L Bum, A. B. Holmes. D. D. C. Bradley, A. R. Brown. R. H. Friend, R. W. Gymcr, Symh. Met. 1993, 55, 936.

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2 Oligo- and Poly(phenylene)s

Ulhich Scherf and Klaus Mullen

2.1 Introduction

The structural regularity and order in polymeric or oligomeric molecules play a critical role in determining the physical properties of such electronically active compounds. This creates an exciting challenge for the synthetic chemists; with the emphasis on the physical function of such materials, their properties have to be selectively engineered through synthesis and supramolecular assembly. A fruitful design strategy is aimed at controlling both the microscopic and 1he solid-slate macroscopic structure. On the microscopic level this concerns a homogeneous chemical structure (conliguration, tacticity etc.) and, on the macroscopic level, the solid stale morphology (disordered, amorphous arrangemem of the molecules or a controlled supramolecular assembly).

The synthesis-driven approach towards material science can be applied to cre-ate oligomcrs and polymers with optimized properties, e.g. maximized carrier mo-bilities and electrical conductivities or high photo- and electroluminescencc quan-tum yields. It becomes obvious, however, that the abili ty to synthesize structur-ally defined n-architectures is the key to these high performance matetials.

Besides the classical' search for linear, one-dimensional electronically active materials, synthetic approaches are now also focussed on the generation and char-acterization of two- and three-dimensional structures, especially shape-persistent molecules with a well-defined size and geometry on a nanometer-scale. It is there-fore timely and adequate to extend concepts of materials synthesis and processing to meet the needs defined by nanochemistry' since the latter is now emerging as a subdiscipline of material sciences.

2.2 Polymers

A large part of organic and macromolecular chemistry starts with the chcmkal functionalization or benzene, and benzene units serve as building blocks lor im-portant polymers. Naturally, benzene-based aromatic materials also represent au impottant sulx:hL'iS of n-conjugatcd an:hih.:dures. Despite some synthetic Jiflicul-ties related to the gem:ration of structurally well-dctined oligo- and poly(phenyl-

38 2 Oligo- and Poly(phenylene)s

ene)s. both academic and industrial researchers have had constant interest in phe-nylene-type materials.

Poly(p-phcoylcne)s, PPPs. constitute the prototype of rigid-rod polymers and are currently being intensively investigated [I]. The key role of PPPs follows from their conceptually simple and appealing molecular structure, from their chemical stability, and from their superior physical properties [2]. In tum, this i~ the result of important advances made in aromatic chemistry over the last few years. The following section gives an overview of the most common methods to generate poly(p-phenylene)s via different synthetic approaches.

ppp 1

Scheme 2-1.

Unsubstituted poly(p-phenylene) PPP 1 as a parent system of a whole class of polymers is an insoluble and intractable mate rial, available by a variety of syn-thetic methods [3, 4}. The lack of solubility and fusibility hinders both unequivo-cal characterization and the processing of PPP l . Moreover. the inuactabili ty of unsubstituted PPP materials has thwarted any serious commercial development of the polymer.

2.2.1 Oxidative Condensation of Benzene Derivatives

The first allempts to generate poly(p-phenylene) (l) were undertaken in the 1960s. Kovacic et al. [3) reported that the oxidative treatment of benzene with copper(ll) chloride in the presence of strong Lewis acids (aluminum trichloride) gives rise to a coupling of the aromatic rings. During the condensation reaction radical cations are formed as reactive intem1cdiates. The maximum degrees of po-lymerization are ca. 1~12. The benzene subunits are preferentially connected in the 1.4-position, however, crosslinking and oxidative coupling to polycyclic aro-matic hydrocarbon building blocks occur as side reactions. Adapting the initial procedures of Kovacic et a/. other 1,4-substitutcd benzene derivatives were coupled to poly(p-phenylene)s.

Katsuya er a/. [5 1 published the oxidative coupling (agent: copper( II) chloride/ a luminum chloride) of electron-rich benzene derivatives such as 2,5-dimethoxy-benzene to poly(2,5-dimethoxy- 1,4-phenylene) (2). The resulting polymer is only soluble in concentrated sulfuric acid, and is fusible at 32o

40 2 Oligo- und Poly(l'lumylem! )s

plus aryl halide or -tosylate) and the nickei(O)-catalyzed or media~ed coupling ac-cording to Yamamoto [9J (aryl halide or -tosylate plus aryl hahde or -tosylate) have been most successfully employed.

Typically, such reactions were designed for the synthesis of low molecular weight organic compounds and then, after having proven their synthetic potential, applied to repetitive processes in the genemtion of macromolecules. This protocol was shown to greatly improve the chemistry of conjugated polymers. Failure Ill avoid side reactions such as dehalogenation or deboronation of the fu nctionalized monomers often creates, however. an unbalanced stoichiometry and limits the at-tainable molecular weights.

Kaeriyama et al. [ 101 reported on the Ni(O)-catalyzed coupling of I ,4-dibromo-2-methoxycarbonylbenzene to poly(2-methoxycarbonyl-1,4-phenylene) (4) as a soluble, processable precursor for parent PPP 1. The aromatic polyester-type PPP precursor 4 was then saponified to carboxylated PPP 5 and the~ally d~~rbox~latcd to 1 with CuO catalysts. However, due to the harsh reaction cond1t1tms 111 the final step, the reaction cannot be carried out satisfact01ily in the solid state (film).

The strategy of Kaeriyama represents a so-called precursor route and_ was de-veloped to overcome the characteristic shortcomin~s (insolu.bili t~. lac~ ol proce~sabili ty) of previous PPP syntheses. The condensatton rcact1on 1s camed out w1th solubilized monomers, leading to a soluble polymeric intennediate. In the linal reaction step this intermediate is then converted, preferentially in the solid state allowing the formation of homogeneous PPP films or layers, into PPP (or other poly(arylene)s).

~CXMe fd:1 Ni(O) ott r - -!J n ~

~ cue -tot -!J n PPP 1

Scheme2-4.

As has been outlined above, a second, very fruitfu l synthetic principle for obtain-ing structumlly homogeneous, processable PPP de.rivatives involves _the pr~paration of soluble PPPs via introduction of solubilizing s1de groups. The p10neenng work here was canicd out at the end of the eighties by Schluter, Wegner, et a/. [II , 12], who for the lirst time prepared soluble poly(2,5-dialkyl-1,4-phenylene)s 6.

'

~ Scheme 2-5-

R: ..alkyl, ..alkoxy

2.2 Polymer.~ 41

SchlUter et al. [ Ill were the first to describe the coupling of aromatic com-pounds containing aryl magnesium halide and aryl halide functions catalyzed by Ni(O) compounds. Herein, the authors attached solubilizing side-chains at the 2-and 5- positions or the benzene rings and in on.lcr to make soluble products. They yicldt::d oligo(para-phenylene)s with maximum degrees of polymerization of 8-10. The products arc chamctcrizcd by an exclusive 1.4-linking of the benzene rings of the main-chain. However, the molecular weights were quite low.

Several authors developed the method further of Ni(O)-mediated couplings to generate several I~PI~ derivatives (9, 13, 141. They described homocouplings of various 1,4-dihalobenzene derivatives by means of nickci(II)chloride/triphenylpho-sphine/zinc or the nil:kei(O)kyclooctadienc complex.

Ni(O)-mcdi;~ted homocouplings of 2-substituted 1,4-phcnylcnebis(trillate)s have been reported by Pcrcec et a/. [1 5] to provide substituted poly(p-phenylene)s 7 containing alkyl, aryl or ester substitucnts in the 2- and 3-positions of the 1.4-phenylenc skeleton. This method of preparation appears to be broad in scope. especially due to the case of preparation of the bis(trillatc) monomers starting from the con-csponding hydroquinone derivatives.

Nl(O)

R: ..alkyl, 1)henyl, -COOMe Scheme 2-6.

By means of a repetitive Suzuki aryl-aryl cross-coupling method, developed by Schliiter, Wegner and co-workers, the synthesis of solubilized PPP's 6 with a dra-matically increased molecular weight (number average up to 100 1,4-phenylcne units) was possible (12j. 2,5-Dialkyl-substituted PPPs 6 were intensively studied as prototypes of so-called 'hairy-rod' macromolccu!es, composed of a linear, rigid PPI' main-chain and tlexible, 'hairy' alkyl side-chains. The individual, shape-per-sistent macromolecules can be imaged by transition electron microscopy within monolayers r 16]. Poly(2,5-di-n-dodecyl-1 ,4-phcnylene) 6 (R: -C I:!H:!:;) revealed a sandwich-type structure with layers or aliphutie side-chains perpendicular and layers or the PIP main-chain parallel to the substrate surface. Poly(2.5-di-n-uodc-

42 2 Oligo- and Poly(phenylene)s

cyl-1,4-phenylene) of Mw 73000-94000 show a single anismropic liquid crystal-line mesophasc in the molten state and macromolecules with Mw 44000-73000 gave coexisting isotropidanisotropic phases [ I 7 j.

Pd(O) --

R: -alkyl, -alkoxy

Scheme2-7.

In addition to alkyl-substituted derivatives, soluble PPPs 6 are also known to-day containing alkoxy groups as well as ionic side groups (carboxy ancJ sulfonic acid functions) [l8j. Schliilcr cr a/. ~ttently described the gencmtion of soluble PPPs decorated with densely packed stcrically demanding dendmns on the fomm-tion of cylindrically shaped dendrimcrs, so-called cylinder dendrimers 119j.

Scherf er at. [20} reported the synthesis of the PI)P derivative Sa, which is composed of chiml eyclophane subunits, by means of a Suzuki-type aryl-aryl cross-coupling of the corresponding diboronic acid and dibromo monomers. The monomers with the cyclic -0-C 11,H20-0- loops can be resolved into the pure enan-tiomers by preparative high pressure liquid chromatography on chiral stationary phases. The pure enantiomers were used to generate the corresponding stereoregu-lar iso- and syndiotactic PPP derivatives 8b and 8c. Hereby, the isotactic deriva-tive 8b is of special interest due to its main-chain chiral character [20].

Unsubstituted PPP 1 possesses a 23 twist between adjacent phenylene units [2 I ). Since the n-overlap operates as a function of the cosine of the twist angle, even at 23 there is a fair amount of conjugative interaction remaining. If one places substituents along the PPP backbone (e.g. at the 2- and 5-positions as in 6, 7 and 8), the solubility is drJmatically enhanced, as discussed above, however. the n-overlap is reduced. Twist angles of 60-80'' are reported for alkyl substitu-ents in 2,5-positions [22j. Thus, for poly(2,5-dialkyl-1,4-phenylcne)s. only negli-gible optical absorption can be detected in the wavelength region above 300 nm, which is characteristic for delocalized n-n* tmnsilions.

The results described thus far sketch the synthetic demands for being able to prepare processable. structurally defined PPPs, in which the n-conjugation re-mains fully intact or is even increased as compared to that of the parent PPP 1 system. The key step in the realization of this principle is the prepamtion of a PPP in which the aromatic subunits can be obtained in a planar or only slightly twisted conformation in spite of the introduction of substituents.

One of the first examples was the above mentioned work from Yoshino er a/. PI conceming the synthesis of poly(9,9-dialkylfluorene)s via oxidative coupling of fluorene derivatives. Poly(9,9-dialkylfluorene) derivatives have been also synthc-si:i".cd via Ni- and Pd-catalyzcd aryl-aryl homo- and cross-coupling reactions of sui-

Atactic PPP-Oerlvative Sa

lsotacllc PPP-Oerivative 8b

Syndiotactic PPP-Oerivative ~ Scheme 2-a.

2.2 Polymers 43

tably substituted monomers (2,7-dibromolluorene and fluorene-2.7-diboronic acid derivatives. respectively) 123, 241. These reactions allow the synthesis or structu-rully defined products possessing high molecular weights of up to 100000. Some derivatives, e.g. the 9,9-dioctyl-substituted polylluorcnc form well-defined thermo-II'Opic LC states and can be aligned on tubbed substmtes. These layers show a high degree of orientation, both in absorption and photoluminescence {25].

The next allempt at these reactions were carried out by Yamamoto et ul. 1261. They coupled 2,7-dibromo-9, I 0-dihydrophenanthrcne to give an ethano-bridged pol y(p phcn y lene)

44 2 Oligo- a11d Poly(phe11ylene)s

Consequently, by combining the synthetic procedure of Yamamoto (26] with the introduction of more extended solubilizing substituents, advantageous results were expected. Accordingly, alkyl-substituted dihydrophenanthrenes or tetr-clhydro-pyrenes offered themselves as starting monomers for the preparation of soluble 'step-ladder' PPPs of this type. 2,7-Dibrom

46 2 Oligo- a11d Poly(pltenylene)s

characterized by unusual electronic and optical properties; the absorption maxi-mum undergoes a marked bathochromic shift as a consequence of planarization of the chromophore, to a Amax value of ca. 440 nm (see Fig. 2-1), depending some-what on the substituents -R and -R'. In addition, the longest wavelength n- rc* ab-sorption band possesses an unusually sharp absorption edge as an indication for the fully planarized, geometrically fixed ladder structure.

The photo luminescence (in solution) of LPPP 12 is very intense and blue (AnlaX emissio n: ca. 460 nm). The Stokes shift between absorption and emission is extremely small (ca. 150 cm-1); a consequence of the geometric tixation of the c hromophore in the ladder structure. The PL quantum yields are hig h in compari-son to those of many other conjugated polymers; in solution values between 60 and 90% have been measured, in the solid state up to 40% [29). In comparison, PPJ> 1, synthesized by the lCI-precursor route, shows a PL quantum yield of only 4% [30). Thus, the next step was to investigate the suitability of this new type o f material for application as an active component in organic materials-based LEOs. Init ially, this led to the surprising result that although etlil:ient LEOs can be pre-pared with LPJ>P. the color of the emission in the :-.olid state (lilm) is 111.:verthelcss yellow (PL and EL). In addition to primary emission of the LPPP 12 chromo-phore in the blue region, the PL and EL spcctm exhibit an additional

48 2 Oligo- and Pvly(phenylelle)s

pie fact that they are readily available in sufficient quantities - even in laborato-ries without highly developed synthetic know-how. However, one must be aware that each electrochemical formation of a polypyrrole or a precursor route towards PPV provide individual samples whose performance in devices depends upon the chemical conditions of materials synthesis. Thus. while one would not argue against a 'practical' synthesis, two important aspects must be obeyed; (I ) a selec-tive structure-property relationship necessitates a scrupulous definition of the mo-lecular structure and of possible impurities, and (2) adequate synthetic solutions for physical problems can require a higher level of sophistication and still remain valuable, even from a practical point of view.

2.2.3 Other Routes to Poly(p-phenylene)s

Besides the oxidative and transition-metal-catalyzed condensation reactions discussed above, several other syntheses were developed to generate PPP and PPP derivatives.

Marvel et a/. described [4 1] the polymerization of 5,6-dibromocyclohexa-1,3-? iene (16) to poly(5,6-dibromo- 1,4-cyclohcx-2-ene) 17 followed by a thermally tnduced, solid state elimination of HBr on the formation of tPP l. The products, however, display some indications for several types of structuml defects (incom-plete cycliUttion, crosslinking).

- -

16 11 1 Scheme 2-13.

Later on, Ballard e1 al. [42, 43) developed an improved precursor route starting from 5,6-diacctoxycyclohexa-1,3-diene (18), the so-called ICI route. The soluble precursor polymer 19 is finally aromatized themmHy into PPP 1 via elimination of two molecules of acetic acid per structuml unit. Unfortunately, the polymeriza-tion of the monomer does not proceed as a uniform 1,4-polymerization; in addi-tion to the regular 1,4-linkages ca. 10% of 1,2-linkages arc also formed as result of a 1,2-polymerization of the monomer.

Q-- I MoCOOOOC,...t - 1ot. MeCOO OCOMe n 1 .1j 1

Scheme 2-14.

2.2 Polymers 49

Grubbs [441 and MacDiannid [45) et a/. described in 1992 and 1994, respec-tively, an improved precursor route to high molecular weight, structurally regular PPP 1 starting from the cyclohexa- 1,3-diene derivative 18 and leading to a stereo-regular precursor polymer 20 via transition metal-catalyzed polymerization. The final step of the reaction sequence is the thermal. acid-catalyzed elimination of acetic acid to convert 20 into PPP 1. The authors obtained free-standing PPP films of defined structure, however these tilms still contained large amounts or the acidic reagent polyphosphoric acid. Nevertheless, in this work a reliable value for the longwave absorption maximum Amax of PPP 1 could be obtained, at about 336 nm. This value is of utmost importance in the interpretation of the optical properties of substi tuted PPPs.

Q , .. ~ .... ., ~ MeCOO OCOMe l MeCJ----

50 2 Oligo- and Poly(phenylene)s

Recently, Tour et al. [47) described attempts to use the Bergman cyclicll produce PPP derivatives starting from substituted endiynes, e.g. pvi.: . 1.4-phenylene) 22 starting from 1-phenyl-hex-3-en-1.5-diyne or the structurally re-lated poly(2-phenyl-1,4-naphthalene) 23 starting from 1-phenylethynyl-2-ethynyl-benzene.

\J- -All the above oligomers are characterized hy the presence of solubilizing alkyl groups, resulting in an increased solubility. However, the electronic properties of the 1r-system arc disturbed by the mutual distortion or the phenylenc units in-duced by the substituents. Compared to the parent PPP-system with its 23"' twist between adjacent building blocks, the substituted derivatives display distortion an-gles of 60-XO"", which minimizes the conjugative interaction within the conjugated backbone. One possibility to overcome this drawback is the substitution only at the terminal rings as pcrfonncd by Liillkc eta/. 1511. They generated oligo(phenyl-enc)s 27 with tat butyl substituents at the tenninal 3- and 5-positions using a

52 2 Oligo- and Pvly(phenylenc)s

Scheme 2-20.

Grignard coupling us the key step. However, longer oligo(phenylene)s 27 are not available fo llowing this approach, since the compounds become also insoluble when reaching chain lengths of more than 7 aromatic building blocks.

Scheme 2-21.

As described for the corresponding polymers, a powerful strategy to achieve soluble oligomers with maximum conjugative interaction is to incorporate the PPP backbone into a step-ladder (or ladder, see Section 2.2) fr.unework in combi-nation with the attachment of solubilizing side groups onto the bridging function-alities. Following this, it was possible to generate short-chain tetrahydropyrene oligomers via separation of polydispersc mixtures into their (monodispersc) indi-vidual components 28 with the aid of preparative gel permeation chromatogmphy [52j.

28 (n: o -8)

n R: -alkyl

Scheme 2-22.

2.3 0/igomer.s 53

With such a series of o ligomer.; 28 available, the convergence of optical proper-ties with increasing chain length can be monitored and the conjugation length in the corresponding polymer YfHP 11 can be determined as comprising of about 10 monomer building blocks (i.e., 20 aromatic rings) [53j.

The above approaches to synthesize oligomer.; (i.e. stepwise synthesis, chroma-togmphic resolution of oligodispersc mixtures) have more recently been comple-mented by repetitive modular stmtcgies. Such concepts involve the repetition of directed protcction/couplingldcprotection sequences in a convergent process to minimize the number of reaction steps necessary to genemte the extended oligo-mers. Such a stmtegy was developed e.g. for linear poly(para-phenyleneethynyl-ene)s PPEs by Tour eta/. 154j. Using a similar approach, Schluter et a/. [55] dc-:.cribed the synthesis of monodisperse oligo(phcnylene) rods 29 with up to 16 phcnylcne rings and with well-defined functional end groups. The synthesis is based on a convei"Jent (exponential) growth using the Suzuki reaction as the cou-

TM~ R

/

+ ~ l Pd(O)

1

Zl!. n: 2,4,6,8

R: -n-hexyl

Scheme 2-23.

54 2 Oligo- und Poly(phenylene)s