Photoluminescence Studies of Silsesquioxanes

(SiO1.5)nRn and Some Selected Organosilicon

Compounds

Jun Cai

Institut für Chemie Anorganischer Materialien

Technischen Universität München

Institut für Chemie Anorganischer Materialien

der Technischen Universität München

Photoluminescence Studies of Silsesquioxanes (SiO1.5)nRn

and Some Selected Organosilicon Compounds

Jun Cai

Vollständiger Abdruck der von der Fakultät für Chemie der Technischen Universität München

zur Erlangung des akademischen Grades eines

Doktors der Naturwissenschaften

genehmigten Dissertation.

Vorsitzender: Univ. – Prof. Dr. W. Domcke

Prüfer der Dissertation: 1. Univ. – Prof. Dr. Dr. h. c. St. Veprek

2. Univ. – Prof. Dr. H. Schmidbaur

Die Dissertation wurde am 06.06.2002 bei der Technischen Universität München eingereicht

und durch die Fakultät für Chemie am 01.07.2002 angenommen.

Die vorliegende Arbeit entstand in der Zeit von März 1998 bis Juli 2002 unter

Anleitung von Herrn Professor Dr. Dr. h. c. Stan Veprek am Institut für Chemie

Anorganischer Materialien der Technischen Universität München

I would like to express my deep gratitude to my academic supervisors

Prof. Dr. Dr. h. c. Stan Veprek

For the advice and the continuous support of my work.

To Yan-Xia

With my deep love

and gratitude

“The search for truth is in one way hard and in another easy. For it is evident that no

one can master it fully nor miss it wholly. But, each adds a little to our knowledge of

nature and from all the facts assembled, there arises a certain grandeur.”

———— Aristotle

Symbol Quantity Value and unit

ε Dielectric constant

λ Wavelength nm c Velocity of light in vacuum 2.99792×108m/s h Planck’s constant 6.62618×10-34Js M Multiplicity

Molar concentration mol/l

PL Photoluminescence S Spin quantum number

Singlet state +1/2, -1/2

T Cage structure formula (SiO1.5)n Triplet state

UV Ultraviolet light 120 – 400 nm VIS Visible light 400 – 700 nm R Ligand A Absorbance I Intensity of light φF Quantum yield of fluorescence φP Quantum yield of phosphorescence

Contents

Abstract

1 Introduction……………………………………………………….…..….….1

1.1 Silsesquioxanes..……….……………………………………….….……..1

1.2 Fundamentals of photoluminescence…………………………….…….…6

1.3 Purpose of this work……………………………………………….…….12

2 Experimental……………………………….……………………….………14

2.1 Experimental procedure…………………………………………………..14

2.1.1 Sample…………………..…………………………………….…..14

2.1.2 The cuvette…………….…….……………………………….……16

2.2 Apparatus………………………………………………………………..17

2.2.1 Absorption spectrometer…………………..….…….……………..17

2.2.2 Fluorescence spectrometer…..…………………….….…………...20

2.2.2.1 Emission and excitation spectrum………………………….20

2.2.2.2 Laser induced fluorescence………………….……………...23

2.2.3 HPLC purification………….………………………………….…..23

3 Results and discussion………………………………………………………25

3.1 Influence of molecular structure………………………………………….25

3.1.1 The influence of cage size……………………………………….25

3.1.1.1 UV-absorption spectra………………………………………25

3.1.1.2 Emission and excitation spectra…………………………….29

3.1.1.2.1 0.001 M solution ………………….………………...29

3.1.1.2.2 0.01 M solution …………………….……………….41

3.1.1.3 Discussion and summary………………………………….....49

3.1.2 The influence of ligand length……………………………………54

3.1.2.1 UV-absorption spectra……………………………………….55

3.1.2.2 Emission and excitation spectra……………………….……..61

3.1.1.2.3 0.001 M solution …………………..………………..61

3.1.1.2.4 0.01 M solution …………………..…………………69

3.1.2.3 Discussion and summary…………………………………….77

3.2 Photoluminescence from selected branched polysilanes…………………..85

3.2.1 Tetrakis(trimethylsilyl)silane [(Me3Si)4Si]…….……….…………85

3.2.2 [Me3SiMe2Si)3Si]2 and its radicals during photolysis…………….88

4 Conclusion…………………………………………………………………….94

5 Appendix Influence of Solvent……………………………………………..96

a) Hexane solvent……………………………………………………………98

b) Pentane solvent………………………………………...…………………100

c) THF (tetrahydrofuran) solvent……………………………………………100

d) Conclusion………………………………………………………………..104

6 References……….……………………………………………………………105

7 Acknowledgement……………………………………………………………111

i

Abstract

Silsesquioxanes (SiO1.5)nRn form an important class of organosilicon compounds

among silicon-based materials which attain increasing interest in various fields of

materials science and industrial application. Unlike currently common

organosilicon compounds with chain or “ladder” structure, in which the

photoluminescence (PL) is associated with the σ* - σ transition, with their

specific spherical cage structure, silsesquioxanes show much higher

photochemical stability and their PL does not show any photo-degradation under

similar conditions. These materials can be used in a wide range of applications

from catalysts or catalyst support, opto-electronic and sensor devices, low ε

dielectrics thin films to selective permeability membranes and so on.

Systematic studies of the PL property of silsesquioxanes, such as the influence of

cage size, ligand length, concentration of solution and excitation energy, will be

of great help in understanding the PL mechanism.

In this work, the PL spectra of MnTn with cage size of 6, 8, 10 and 12 and with

trimethylsiloxy [(CH3)3SiO] groups as ligands, octa-silsesquioxane (SiO1.5)8R8

with mono alkyl ligands with 1 to 10 carbon atoms and hydrogen ligands in

n-hexane solution have been systematically investigated. Besides, several types

ii

of solvent have been used in this research, and the influence of the solvent was

studied and summarized in the Appendix.

Due to the fact that under UV light, many polysilanes may undergo photolysis

and form radicals, some researchers raise the question as to what extent these

radicals may contribute to the observed PL. To answer this question, we selected

some branched organosilicon compounds, such as tetrakis(trimethylsilyl)silane

[(Me3Si)4Si], and the star-dimer [(Me3SiMe2Si)3Si]2, to check the possibility of

PL from radicals.

1

1. Introduction

1.1 Silsesquioxanes

Silsesquioxanes, also called functionalized polyhedral siloxanes, form an

important class of organosilicon compounds among silicon-based materials [1].

Their general formula is (SiO1.5)nRn (n = even number), denoted as :

SiO1.5

R

This class of materials has a three-dimensional polyhedral siloxane-containing

cage with alternating Si and O atoms in the backbone [2, 3]. The mainframe of

these compounds, i.e., (SiO1.5)n cage, is usually denoted as Tn unit [4, 5, 6]. For

example, T8 structure consists of a cage of 8 silicon atoms, which are connected

with each other by oxygen atoms, whereas the T12 structure has a cage with 12

silicon atoms and 18 oxygen atoms. Because of their spherical mainframe

structure, the term “spherosiloxanes” is sometimes used to describe such

molecules [7, 36]. The ligands in these materials are referring to as R, which

can be elements such as hydrogen (R = H) or halogen (e.g. R = Cl, Br), or

organic groups, typically either of alkyl, aryl, alkoxy groups or

organometallofunctionalized groups [3 - 7]. The ligands are attached to the

cage via Si atoms on the main frame. Figure 1.1 shows structure models of

Hexa-R6T6, Octa-R8T8, Deca-R10T10 and Dodeca-R12T12 Silsesquioxanes with

D3h, Oh, D5h and D2d symmetry, respectively [1, 8]. These four kinds of

2

silsesquioxanes are studied in this work and the results will be discussed in

chapter 3.

(a) D3h-R6T6 (b) Oh-R8T8

(c) D5h-R10T10 (d) D2d-R12T12

Fig. 1.1 Structure models for the (a) Hexa-, (b) Octa-, (c) Deca- (d) Dodeca - Silsesquioxanes [1]

Since the first report of synthesizing silsesquioxanes in 1940s [9, 68], they have

attained increasing attention in various fields of material science and industrial

application [10 - 34]. Their wide use ranges from the preparation of

high-performance optical fibers [12 - 14] to applications in cosmetics [14].

They are possible sources for new organosilicon polymers [15] and as

3

precursors to organolithic macromolecular materials (OMM’s) [16, 17] or

porous hybrid inorganic-organic materials. They are considered as “building

blocks” or structuralizing agents for the sol-gel preparation of ceramic or

polymeric materials showing specific features reminiscent of the cubic

precursors [18]; In the field of supramolecular chemistry, novel

octasilasesquioxane based 3-D cores for dendritic supermolecules have also

been described [19].

Metal organic derivatives of silsesquioxanes are involved in the generation of

catalysts such as siliceous matrix-encapsulated cobalt carbide nanoparticles (R

= Co(CO)4) [20] or in the design of new electrochemical devices (R =

ferrocenyl) [21]. Their attraction stems from the appealing combination of

unusual structural features (inorganic moiety) with potentially useful chemical

functionalities (organic or organo-metallically fuctionalized ligands). They can

also be fruitfully viewed as readily available model compounds for studying

specific aspects of zeolites [22 - 30]. Silsesquioxanes fulfill the structural

requirements for this purpose as they exhibit polyhedral Si-O skeletons, which

fit closely the characteristic building elements of zeolites; especially, these

compounds with ligands of organic groups or organometallofunctionalized

groups increases the number of suitable molecules with which the different

kinds of zeolites and silica-supported transition-metal complexes [31 - 33] or

catalysts [34] can be modeled. All these studies will be of great help in

4

understanding the role of zeolites in heterogeneous catalysis, and in developing

catalysts or supporting materials for catalysts based on silsesquioxanes.

Another distinguished property of silsesquioxanes is their low dielectric

constant [2, 3, 10]. In addition, they are rather stable molecules, especially of

high photochemical stability. These advantages make them to be one of the

promising materials for making opto-electronic devices [2,3,10] and for use in

the microelectronic industry, especially for the multi-layer structure in the

nowadays’ ultra large-scale integrated circuit (ULSI) [2,11,35].

Because of the promising prospects of their application in various fields as

already mentioned above, extensive investigations have been carried out for

synthesizing silsesquioxanes with specific structures [1, 36, 37]. However, the

synthesis of silsesquioxanes is found to be a rather complex, multistep process.

The first report of synthesizing silsesquioxanes can be traced back to 1940s [9,

68]. Details about the synthesis of some of the silsesquioxanes such as

octahydrosilsesquioxane and homo-substituted silsesquioxane can be found in

Refs. 8, 38 and 39. In brief, they are obtained by the hydrolytic condensation of

monomers such as trichloro- or trialkoxysilanes or the condensation of

trihydroxysilanes [36, 37] to form a cross-linked gel and then be air-dried [10],

the overall reaction can be described by the following formula:

5

( )3 1.5 n

solventR SiX R SiOcatalyst

− → −

Here the subscript number n are even numbers such as 6, 8, 10, 12 etc. It is

found that different substituent R require different condensation conditions [8,

36]. It is reported that some other different structures with different molar-mass

can be frequently found in such a process, like linear, “ladder”, cagelike

fragment structure or the mixture of these different structures [37]. Some

researches also indicated that catalyst plays an important role in such

molar-mass distribution phenomena [8, 37, 40].

Various techniques have been employed to characterize the structure and

properties of silsesquioxanes. X-ray photoelectron spectroscopy (XPS or ESCA)

[2], size exclusion chromatography (SEC) [37, 41], matrix-assisted ultraviolet

laser desorption/ionization time-of-fight mass spectrometry (UV-MALDI-TOF

MS) [37] techniques are conventionally used in determining and controlling the

structures during the synthesis of silsesquioxanes [37].

Photoluminescence (PL) spectroscopy is also one of the powerful tools for

providing molecular level structure information of organosilicon materials [5,

40]. Figure 1.2 gives one example for characterizing the structure of

organosilicon compounds by this method. Like many other organosilicon

6

compounds, silsesquioxanes show an efficient PL [4, 5]. However, in the chain,

“ladder” and other organosilicon compounds, the PL is associated with the σ -

σ* transition, which always results in photo degradation. Silsesquioxanes do

not show any photo degradation under similar conditions. This makes them

interesting for possible optoelectronic application. For this reason, we

investigated in this work the mechanism of the PL from a variety of

silsesquioxanes. Since this is the main technique used in this study, details of

the theory on PL will be given in the following section.

Fig. 1.2 Absorption and PL spectra of organosilicon compounds with the structure of (a) chain, (b) “ladder”, and (c) cubic cage [40]

1.2 Fundamentals of photoluminescence

Photoluminescence spectroscopy is one of the most effective methods for the

7

characterization of organic compounds [42 - 46]. Its extremely fast response

time and high sensitivity, coupled with its ease of use, have made it an

economical technique in industry [42, 43] and scientific research [44 - 46].

Luminescence is a process by which a molecule in an electronic excited state

returns to the ground state by emission of a photon [45]. When molecules are

excited by interaction with photons, the form of luminescence is called

photoluminescence (PL) [42]. If the release of the absorbed radiation energy is

immediately (actually it always takes some time, usually about 10-7 – 10-9 sec),

the process is called fluorescence, whereas a delayed release of this energy

from a triplet state or a state excited in some energy transfer process, it is

named phosphorescence. Because most of work in this thesis deals with

fluorescence, we shall use the term photoluminescence or PL as a synonymous

to fluorescence.

Besides of the photoluminescence, there also exist other forms of luminescence,

such as chemiluminescence in which the excitation energy is from chemical

reaction, electrochemiluminescence where the excitation is due to electron

transfer in solution [44] and bioluminescence from living organisms.

In the fluorescence process, the emitted photons usually have a lower energy

(larger wavelength) than the absorbed ones. It is called Stokes-Transitions

8

(bands). This is the normally observed case in solution and for room

temperature solids, which we will discuss in detail in the following chapters.

When the emitted light has a higher energy (shorter wavelength) than the

incoming one, e.g. in diluted gases at a high temperature, this phenomenon is

called anti-Stokes-bands (radiation) [42].

The energy of absorbed photons can be expressed by the following equation:

hcE hν

λ= = [1.1]

Where h is the Planck’s constant, c the speed of light, ν is the frequency of light,

and λ is its wavelength.

Figure 1.3 shows a simplified schematic energy level diagram of a diatomic

molecule illustrating the principle of photoluminescence, in which part A is the

potential energy-level diagram and part B is the Jablonski energy-level diagram.

In the potential curve diagram A of Fig. 1.3, the horizontal lines between each

main electronic state are the various vibrational levels of the molecule. The

process (a) is an absorption transition, which occurs from a lower electronic

state to some vibrational levels of an upper electronic state. This process is

governed by the Frank-Condon principle [45, 46]. During an electronic

transition between different states, the internuclear separation remains constant.

According to this principle, the absorption line in Fig 1.3A is presented as a

vertical line, the time scale of this process is around 10-15 sec [45, 46]. (b)

9

illustrates a fluorescence process, in which the molecule emits a photon by a

transition from the lowest vibrational level of that excited electronic state to the

ground electronic state. The reason for the loss of vibrational energy in

electronically excited states is thermal decay (collisions with other molecules),

because this process is faster than spontaneous emission of the radiation [47]

(fluorescence occurs in around 10-7 - 10-9 s [44 - 46]). Transition (c) shows a

phosphorescence process, which usually occurs at the time scale of 10 - 10-3 s

[44 - 46]. Its efficiency depends on the rate coefficient of intersystem crossing

from an excited singlet state to a triplet state of similar energy. When highly

excited electronic states are involved allowing triplet – triplet transitions, the

resulting luminescence can be fast. For example, potassium molecule K2

undergoes an intersystem transition 1 32 2u g+Σ → Π and then emits a photon

after transition from 32 gΠ to 31 uΣ , which takes about 20 ns.

The difference of fluorescence and phosphorescence lies in the multiplicities of

the two electronic states (singlet and triplet) within which the transition occurs.

The fluorescence involves a transition between two electronic states with the

same multiplicity, whereas the phosphorescence is with different one. The term

multiplicity (M) is related to the spin by equation:

M = 2S + 1 [1.2]

S is the total spin quantum number. For each electron, spin can be directed

either up (↑, +1/2) or down (↓,-1/2). When all electrons are paired, the overall

10

spin quantum number of the molecule is zero, this is called a singlet electronic

state, while when the spin of a single electron is reversed, the molecule will

have two unpaired electrons, the overall spin quantum number equal to one and

M = 2 × (½ + ½ ) + 1 = 3, this is a triplet state.

A B

Fig. 1.3 Schematic energy level diagram for a diatomic molecule, A [46] is the potential energy-level diagram and B [42] is the Jablonski energy-level diagram. In which S is singlet and T is triplet, the subscript numbers indicate individual states, (a) is an absorption process, (b) is a fluorescence process, and (c) is a phosphorescence process

In the example of a simple Jablonski energy-level diagram (B), there are shown

three distinct singlet electronic states (S0, S1, S2) and two triplet states (T1 and

T2). The ground electronic state for molecules with an even number of

electrons is often a singlet state because in the lowest energy state, all spins are

11

paired [44 - 46]. At ambient temperature and under equilibrium conditions,

nearly all of molecules exist in the ground state (S0). With the absorption of

visible (VIS, 400-700nm or 3.09-1.77 eV) or ultraviolet (UV, 120-400 nm, or

10.31-3.09 eV; 120-250 nm is usually called “far ultraviolet” and 250-400 nm

is called “near ultraviolet”) light, molecules may be promoted to excited

electronic states, such as to S1 or S2, while the transitions between S0 and T1 or

T2 are forbidden by the quantum mechanical selection rules [44 - 47]. The

excess energy possessed by molecules in higher singlet states, like S2 may

undergo an internal conversion process to the S1 state. By return to S0, the

excess energy in S1 may be dissipated in two ways, either by fluorescence or by

a non radiative internal conversion processes. The later one is inefficient as

compared to the first way because of the relatively wide energy gap between S1

and S0 [44-46]. Some molecules in the S1 state will find a way known as

non-radiative intersystem crossing to undergo a transition to a lower energy T1

state. The condition for it is that the energy gap between S1 and T1 is small due

to the change in electronic spin in such system [44 - 46]. Electrons in T1 state

will return to S0 by either radiative or non-radiative internal conversion. The

rates of these two transitions are usually small. Therefore, the lifetime of state

T1 to decay to S0 is many orders of magnitude longer than for the singlet –

singlet transitions. This makes phosphorescence a much slower process than

fluorescence.

12

Figure 1.4 summarizes the approximate lifetimes of all different processes

depicted in Fig. 1.3 [44 - 46]. Various techniques of PL spectroscopy, which are

used in this study will be discussed in detail in the experimental section

(Chapter 2).

0

1

0

1

-1510 sec (absorption)

11 1410 10 sec (internal conversion)

7 910 10 sec +h (fluorescence)

10

n

n

S S

S S

S

S

ν

→− −−→

− −−→−

−

0

01

8sec (internal conversion)5 710 10 sec (internal conversion)

310 10 sec +h (phosphorescence)310 10 sec

nT

S

ST

ν

→ − −− →

−−→−

−− → 0 (internal conversion)S

Fig. 1.4 Time scale of different processes happening in PL

1.3 Purpose of this work

It is well known that some linear polysilanes and porous silicon are widely used

materials for the study of photoluminescence. However, PL in those

compounds is due to the σ - σ* excitations which can result in photo

degradation or even decomposition [48]. With specific spherical cage structure,

silsesquioxanes show much higher photochemical stability, as compared to

polysilanes. Thus silsesquioxanes may be alternative materials with promising

13

prospect for applications in the field of opto-electronic devices, especially for

light-emitting, silicon compatible, structures with reduced dimensions [2, 3, 10].

Systematic PL studies of silsesquioxanes, such as the effect of cage size, ligand

length, concentration of solution and excitation energy on the PL behavior, will

be of great help in understanding of the PL process. This will not only help us

to understand why silsesquioxanes are more photochemically stable under

irradiation than linear polysilanes, but will also consequently provide

guidelines for selecting appropriate materials for such applications.

In this work, the influence of solvent on the absorption and PL is investigated

and discussed in the Appendix. The PL spectra of MnTn with cage size of 6, 8,

10 and 12 and with ligand as trimethylsiloxy [(CH3)3SiO] group are studied and

discussed in detail in Section 3.1.1. The PL spectra of octa-silsesquioxanes

(SiO1.5)8R8 with mono alkyl - substituted and hydrido - substituted ligands will

be studied in Section 3.1.2. It is well known that branched polysilanes show a

strong PL with a high quantum yield [48, 53]. Because many of such

polysilanes may undergo photolysis with the formation of radicals, some

researchers raise the question as to what extent these radicals may contribute to

the observed PL. To answer this question, in Section 3.2, we selected some

branched organosilicon compounds, such as tetrakis(trimethylsilyl)silane

[(Me3Si)4Si], and star-dimer [(Me3SiMe2Si)3Si]2, to check the possibility of PL

from radicals.

14

2 Experimental instruments and techniques

2.1 Experimental procedure

2.1.1 Sample

The UV absorption, UV light induced fluorescence and laser induced

fluorescence (LIF, with He-Cd laser (3.8 eV)) of mono substituted

silsesquioxanes (SiO1.5)nRn in solution have been studied. The influence of

molecular structure, including cage size n (n = 6, 8, 10, 12) and the length of

the saturated aliphatic ligand R (CmH2m+1, m = 0, 1, … 10, except m = 5) with n

= 8 was extensively studied in this work. Several types of solvents have been

used in these photoluminescence (PL) experiments, such as n-hexane, c-hexane,

THF (tetrahydrofuran, C4H8O) and pentane in order to check the influences of

the effect of the solvent. The results are summarized in the Appendix. From

these studies, n-hexane is the best solvent among all of them. Table 2.1

summarizes all measured silsesquioxane samples with their used abbreviation,

chemical formulas, molecular masses, dimensions, and solubility in n-hexane,

which is the main solvent used in this work.

Apart from the silsesquioxanes, the PL behavior of tetrakis(trimethylsilyl)silane

[(Me3Si)4Si] and star dimer [(Me3SiMe2Si)3Si]2 have also been investigated in

solution. During the photo-excitation of these materials, new species are often

formed by photolysis which may be responsible for the observed PL. These

15

species are reactive short living and therefore difficult to study separately from

the parent molecules and final products [48, 53]. In order to check the possible

contribution of these species to the observed PL, we selected some of the above

organosilicon compounds for this study.

Table 2.1 The chemical formulas, molecular masses, dimensions, and solubility in

n-hexane of the silsesquioxanes measured in this thesis [5]

Silsesquioxane

Chemical formula Short Name

Molecular Mass [g/mol]

Molecular Dimension

[Å]

Solubility in n-hexane, THF

and pentane

H8Si8O12 H8T8 424.74 8.3 < 0.01 M

(CH3)8Si8O12 Me8T8 536.95 9.1 <0.001 M

(C2H5)8Si8O12 Et8T8 649.17 11.6

(C3H7)8Si8O12 Prop8T8 761.38 14.1

(C4H9)8Si8O12 Bu8T8 873.60 16.6

(C5H11)8Si8O12 Pent8T8 985 18.7

(C6H13)8Si8O12 Hex8T8 1098.03 21.6

(C7H15)8Si8O12 Hept8T8 1210.24 24.1

(C8H17)8Si8O12 Oct8T8 1322.46 26.6

(C9H19)8Si8O12 Non8T8 1434.67 29.1

(C10H21)8Si8O12 Dec8T8 1546.89 31.6

[(CH3)3SiO]6Si6O9 M6T6 847.66 15.9

[(CH3)3SiO]8Si8O12 M8T8 1130.24 16.5

[(CH3)3SiO]10Si10O15 M10T10 1412.8 17.3

[(CH3)3SiO]12Si12O18 M12T12 1695.36 18

> 0.1 M

All silsesquioxane samples were synthesized by the method described in

Chapter 1 by Prof. H. C. Marsmann and his co-workers at the Institute of

Chemistry and Chemical Technique of the University of Paderborn, Germany.

The other compounds, such as [(Me3Si)4Si] and [(Me3SiMe2Si)3Si]2 were

16

synthesized in the Technion, Israel Institute of Technology, at Haifa, Israel.

A series of concentrations of solutions have been investigated from saturated to

a very low concentration for above samples. Most experiments were done with

the concentrations of 0.01M and 0.001M. In 0.01 M solution, the mean distance

between two dissolved molecules is 6.2 nm, while for 0.001 M solution, it is

13.2 nm [4].

All samples were sealed and stored in a clean environment. In order to further

exclude the possible artifacts of PL due to impurities and reaction with air, high

pressure liquid phase chromatography (HPLC) was used in order to purify the

samples prior to the measurements. The principle of HPLC will be described

below. In this work, no differences in PL spectrum before and after HPLC were

found. In addition, silsesquioxanes do not react with air.

2.1.2 The Cuvette

The cuvette used in this work for PL, absorption and LIF spectra is a Hellma

Halb-Mikro 108F Spectrosil Quartz Glass Cuvette. The empty cuvette has a

transmission of about 90% from 200 to 2500 nm. Its shape and dimension are

described in Fig. 2.1b. After each use, the cuvette is cleaned first with pentane,

immersed in 10% deconex for about 6 – 8 hours, then washed with deionized

17

water and dried in air.

2.2 Apparatus

2.2.1 Absorption spectrometer

The absorption measurements were done using a Perkin Elmer® Lambda 9

UV/VIS/NIR spectrometer, which can measure absorption spectra in the range

of 171-3200 nm (7.2 to 0.39 eV). The light source is a combination of

deuterium and tungsten lamps for UV measurements and a tungsten lamp for

visible and NIR measurements. The selection of the wavelength from these

continuous light sources is made by two grating monochromators. Figure 2.1a

represents a schematic diagram of the construction of the spectrometer.

A parallel beam of light with intensity of I0 coming through a cuvette (Fig. 2.1b)

can be divided into four parts according to the different physical process [51].

Part 1 is the reflection at each interface, denoted as IR, part 2 is the scattering

within the solution (IS), part 3 is the absorbed (IA) and part 4 the transmitted (IT)

light. Thus:

0 R S A TI I I I I= + + + [2.1]

18

(a)

(b)

Fig. 2.1 (a) Schematic diagram of light path in UV-absorption spectrometer, (b) Arrangement of the silica cuvette and the pinhole dimension used in experiments.

19

In the experiment, a pinhole with around 2.8 mm diameter is put in front of the

cuvette (Fig. 2.1b). As the width of the cuvette is about 4 mm, there are only

two interfaces left to be considered for the light reflections. One is on the light

incoming side and the other is on the light outcoming side. On the air/glass

interface, IR losses are approximately 4% at each of these two interfaces, while

on the solution/glass interface, this loss is related to the concentration of

solution and the relative molecular mass of the solute, and also to the intensity

of incoming light. The relationship is expressed as [5]:

( )0R rI K I c M g θ= ⋅ ⋅ ⋅ ⋅ [2.2]

in which ( ) 21 cosg θ θ= + for unpolarized light, θ is the incident angle, in our

case, equal to 900, so the ( )g θ equals 1. K is a constant, c is the concentration

and Mr is the relative molecular mass of the solute. In the experimental

arrangement, the IR losses are compensated by subtraction of the absorption

background of the solvent and the empty cuvette. The amount of the light

scattering in homogeneous solution is small and can be ignored [51]. Thus in

the absorption experiment, only absorption and transmission will be taken into

account.

The linear relationship between absorbance and concentration of the absorbing

species is expressed by the Beer - Lambert law, which in general can be written

as:

clA ⋅⋅= ε [2.3]

20

where A is the absorbance, which equals to ( )To IIlog , ε is the

wavelength-dependent molar extinction coefficient with units of M-1cm-1, l is

the absorption path length (1 cm in our case, as shown in Fig. 2.1b), c is the

molar concentration of the measured sample.

In the following chapters, the absorption experimental conditions will be set as

follow: scan range: 175 - 450 nm, interval 0.8 nm, 1 time cycle, spectral

resolution of 1 nm, pinhole = 2 mm, speed = 15 nm/min, smooth = 1, light

sources are a deuterium and a tungsten lamp.

2.2.2 Fluorescence spectrometer

The fluorescence spectra of silsesquioxanes were measured using a Perkin

Elmer® LS 50B Luminescence Spectrometer. The arrangement of its optical

system is shown in Fig. 2.2. The excitation source is a xenon lamp, with an

excitation monochromator ranging from 200 to 800 nm and an emission

monochromator ranging from 200 to 900 nm.

2.2.2.1 Emission and excitation spectrum

The fluorescence spectra include the emission spectrum and the excitation

spectrum. The emission spectrum measures the relative intensity of emitted

21

light at a given range of wavelengths, which can be used to characterize the

electronic transitions, intermolecular interactions and dynamical properties on

the molecular scale [4, 42, 46, 51]. The excitation spectrum measures the

relative efficiency of various wavelengths to excite the emission. It gives

additional information about the structure of electronically excited states, about

the minimal energy gap between ground and excited states, and it also shows

the influence of the energy transfer processes.

Fig 2.2 Schematic diagram of the optical system arrangement of Perkin Elmer® LS 50B

An important quantity which describes the fluorescence behavior of a molecule

is the quantum yield or quantum efficiency. It is expressed as:

F

number of fluorescence quanta emittednumber of quanta absorbed

φ = [2.4]

22

A higher value of quantum yield means a higher intensity of emission, whereas

a lower value of quantum yield means that more absorbed energy is lost by

non-radiative (e.g. collisional) deactivation [42]. For many cases, the quantum

yield is dependent on the excitation energy and the environmental temperature

[42, 51, 52].

Unlike absorbance measurement, which is based on the ratio of Io/IT so that the

instrument-related influence will be compensated, the fluorescence spectra are

less reproducible because they are not measured relative to a blank. They can

be affected by two factors [42]: one is how the intensity of the light source

varies with wavelength, and the other is how the response of the detector varies

with wavelength. In our system, the spectral response is automatically

corrected by the software which comes with the spectrometer.

In this study, all emission and excitation spectra were measured under the

following experimental conditions: scan speed = 20 nm/min, scan 1 time,

emission slit and excitation slit = 7 nm, no filter used. Emission spectra given

in Chapter 3 are all normalized to the spectral distribution of the excitation light

source.

23

2.2.2 Laser induced fluorescence

The PL is excited by a He-Cd laser with a photon energy of 3.8 eV which can

only excite the PL bands below this value. In this work, we used this excitation

as a supplement in order to compare our results with those of Ossadnik et al. [5,

54]. The fluorescence spectra measured with the Perkin Elmer® LS 50B

Luminescence Spectrometer cover the full spectral range of the PL from

silsesquioxanes because this instrument allows us to use much higher excitation

energy. (The instrument was not available to Ossadnik et al. in course of their

work.)

2.2.3 HPLC purification

In order to rule out any impurity effect on the PL spectra, HPLC (High Pressure

Liquid Chromatograph) pretreatment has been used to purify the compounds

prior to the measurements. Figure 2.4 shows the schematic diagram of the

HPLC system. The reverse phase column is made of frontosil nitriles, which

means that the more polar compounds will be eluted first. The sample dissolved

in MeOH is injected and passes through the HPLC pump together with the

mixture of MeOH and NH4OH (or NH4OAc, the relative concentration of the

mixture depends on the experimental efficiency) at a speed of 3 ml/min. The

outcoming liquid is examined by a UV detector. According to the spectra of the

detector, the purified sample can be collected separately.

24

Fig. 2.4 The schematic diagram of the HPLC system

25

3 Results and discussion

3.1 Influence of molecular structure

Molecular structure properties of silsesquioxanes, including their cage size,

ligands type, bonding states (lengths, angles), symmetry and total energy will

be studied in this chapter. Energy difference between HOMOs (Highest

Occupied Molecular Orbital) and LUMOs (Lowest Unoccupied Molecular

Orbital) causes various differences in the characteristics in the PL and UV

absorption spectra, as already discussed in the introduction. Systematic studies

of the effects of these parameters on the PL spectra will help us to understand

the optical properties of these materials and facilitate their application in the

field of optoelectronics. In the following section, PL spectra of silsesquioxanes

with different molecular structures will be discussed.

3.1.1 The influence of cage size

3.1.1.1 UV-absorption spectra

The molecular structure models of Hexa-, Octa-, Deca-, and Dodeca-

Silsesquioxanes are presented in Fig. 1.1. They have the symmetry of T6(D3h),

T8(Oh), T10(D5h), and T12(D2d), respectively. The ligand used here is

trimethylsiloxy [(CH3)3SiO]. The chemical formula, molecular mass,

26

dimension and solubility of these series compounds have been given in Table

2.1.

Figure 3.1 shows the UV-absorption spectra of M6T6, M8T8, M10T10 and M12T12

in n-hexane solution with concentration of 0.001 M (diagram a) and 0.01 M

(diagram b, except of M12T12), respectively. In Fig. 3.1a, the absorption of

M6T6 and M8T8 is barely discernable in the energy range from 3.0 to 5.5 eV.

When energy is higher than 5.5 eV, there is only a very weak absorption which

increases slightly with source energy to the experimental cut off for both M6T6

and M8T8 while M10T10 shows much stronger absorption in this range. The

absorption structure can be attributed to two weak absorption bands and one

strong band (see Fig. 3.1a). However, for M10T10 and the group of compounds

M6T6 and M8T8, the absorption bands are located in different regions. For

M10T10, band I shows a weak structure located between 4.31 and 4.95 eV. Band

II is much stronger than band I (about 8 times) and it appears as a peak from

4.95 to 5.72 eV; Above 5.72 eV band III appears which peaks at E > 6 eV. For

M6T6 and M8T8, all the bands shift to higher energy range, a very weak

structure from 4.4 to 5.2 eV is attributed to band I. Band II is located between

5.2 to 5.9 eV with a weak peak at about 5.5 eV. Band III appears at energy

higher than 5.9 eV. One can clearly see from Fig.3.1a that the absorption

intensity increases in the order of M6T6 ≈ M8T8<M10T10. The integral intensity

of the absorption from 4.0 to 6.0 eV for all four molecules is shown in Fig.3.2a.

27

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

0.0

0.5

1.0

1.5

2.0

IM

10T

10

M6T

6

M8T

8

M10

T10

M12

T12

0.001 M

IIIM

10T

10

IIM

10T

10

Abs

orba

nce

ln(I o/I

T)

Energy (eV)

(a)

400 350 300 250 200Wavelength (nm)

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

0.0

0.5

1.0

1.5

2.0

IIIIII

M6T

6

M8T

8

M10

T10

Abs

orba

nce

ln(I

o/IT)

Energy (eV)

(b)

400 300 200

X 1/3

0.01 M

Wavelength (nm)

Fig. 3.1 Absorption spectra of M6T6, M8T8, M10T10 and M12T12 with concentration of (a) 0.001 M and (b) 0.01 M in n-hexane solution. The absorbance of M10T10 in diagram b is divided by 3 intentionally to make it comparable with other compounds in the same concentration and those with 0.001 M concentration.

28

The absorption of this cage series in 0.01 M solution is shown in Fig. 3.1b,

where the absorption of M10T10 is also the strongest among all. For these

molecules, the absorption shows three different bands in the absorption spectra.

Two weak absorption bands are located at 3.96 - 4.8 eV (denoted as band I) and

4.8 - 5.73 eV (band II), and a strong absorption band (band III) appears from

about 5.73 eV up to the experimental cut off of our spectrometer (6.1 eV, 200

nm). The absorbance increases in the order of M6T6 ≈ M8T8 < M10T10.

In Fig. 3.2, one can clearly see that with the increase of concentration from

0.001 M to 0.01 M, the integral absorbance of M10T10 shows a strong increase,

whereas for M6T6 and M8T8, at an appropriate scale, it also shows a small

increase with the increase of concentration.

6 8 10 120

1

2

3

4

5

6

Inte

gral

Abs

orpt

ion

Inte

nsity

(a.

u.)

Cage Size

(a)

0.001 M

6 8 10 120

1

2

3

4

5

6

Inte

gral

Abs

orpt

ion

Inte

nsity

(a.

u.)

Cage Size

(b)

0.01 M

Fig. 3.2. Dependence of integral absorbances on cage size from M6T6, M8T8 to M10T10 in n-hexane solutions, (a) 0.001 M (b) 0.01 M.

In summary, for both 0.001 and 0.01 M n-hexane solutions, the absorbance

increases with increasing cage size. For the same cage size, the absorption

29

increases with the increase of the concentration. M10T10 shows the strongest

absorption among all compounds in both concentration solutions (Fig. 3.1b,

3.2b). Fluorescence behavior of these cages series will be discussed in the

following section. In order to correlate the absorption spectra with the

fluorescence spectra of these four MnTn molecules, we summarize the

approximate absorption band location of these cage series in 0.01 M and 0.001

M n-hexane solutions in Table 3.1.

Table 3.1 Absorption bands’ location for M6T6, M8T8 and M10T10 with 0.01 M and 0.001 M concentration in n-hexane solution

0.01 M concentration 0.001 M concentration Band

Cage I II III I II III M6T6 M8T8

4.4 – 5.2 eV 5.2 – 5.8 eV 5.8 eV – cut off

M10T10 3.96-4.8 eV 4.8-5.73 eV

5.73eV – cut off

4.31–4.95 eV 4.95-5.7 eV 5.7 eV – cut off

3.1.1.2 Emission and excitation spectra

3.1.1.2.1 0.001 M solution

Figure 3.1 shows the fluorescence spectra of M6T6, M8T8, M10T10 and M12T12 in

0.001 M n-hexane solution. The excitation energies used are 6.10, 5.89, 5.63,

5.38, 4.95, 4.58, 4.27, and 4.06 eV, which correspond to wavelengths of 203,

210, 220, 230, 250, 270, and 290 nm, respectively. The arrows indicate the

Raman peak from the solvent, which originates from the C-H vibration [4]. The

Raman shift is 0.38 eV.

30

At excitation energy of 6.1 eV, M8T8, M10T10 and M12T12 show one broad PL

band with a maximum at 4.2 eV. With an increase of the cage size, the PL band

becomes more symmetric. M6T6 has one broad PL with maxima position at

4.14 eV and one small shoulder at 3.7 eV.

At excitation energy between 5.89 – 5.38 eV, for all cage sizes, the PL spectra

consist of two mixed broad bands with maxima located at around 3.7 and 4.2

eV (M6T6 is at 4.14 eV), respectively. With increase of excitation energy, the

band intensity with maxima position around 4.2 eV increases while that around

3.7 eV decreases. In general, for all cage sizes of these compounds, the PL

band at 4.2 eV (4.14 eV for M6T6) has the maximum intensity under excitation

energy of 6.1 eV, while the band at 3.7 eV has the maximum intensity under

excitation energy of 5.38 eV.

At excitation energy below 5.38 eV, e.g. between 4.95 – 4.06 eV, the PL band at

3.7 eV nearly vanished in all spectra except of M10T10 and M6T6, where a very

weak band around 4 eV can still be seen. All the spectra show a weak low

energy tail of the PL band of 4.2 eV.

31

2.0 2.5 3.0 3.5 4.0 4.5 5.0

M6T

6

4.14 eV

3.70 eV

Eex

= 6.10 eV

Emission Energy (eV)(a)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV

Eex

= 4.58 eV

E

ex = 4.27 eV

2.0 2.5 3.0 3.5 4.0 4.5 5.0

M8T

8

4.20 eV

3.70 eV

Eex

= 6.10 eV

Emission Energy (eV)(b)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 4.95 eV

Eex

= 4.58 eV

Eex

= 4.27 eV

Eex

= 4.06 eV

2.0 2.5 3.0 3.5 4.0 4.5 5.0

M10

T10

4.20 eV

3.70 eV

Eex

= 6.10 eV

Emission Energy (eV)(c)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 4.95 eV

Eex

= 4.58 eV

Eex

= 4.27 eV

Eex

= 4.06 eV

2.0 2.5 3.0 3.5 4.0 4.5 5.0

M12

T12

4.20 eV

3.70 eV

Eex

= 6.10 eV

Emission Energy (eV)(d)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 4.95 eV

Eex

= 4.58 eV E

ex = 4.27 eV

Eex

= 4.06 eV

Fig. 3.3 Fluorescence spectra of 0.001 M concentration (a) M6T6, (b) M8T8, (c) M10T10, and (d) M12T12 in n-hexane solution for the excitation wavelengths at 203, 210, 220, 230, 250, 270, 290, and 305 nm, the corresponding excitation energies are presented inside the diagram.

32

In order to obtain a better comparison of the PL for the different compounds,

such as M6T6, M8T8, M10T10 and M12T12, we show the PL spectra of these

compounds under the same excitation energy in Fig. 3.4. The intensity scales

are the same in each diagram, and the excitation energies are (a) 6.1 eV, (b)

5.89 eV, (c) 5.63 eV, (d) 5.38 eV, (e) 4.95 eV and (f) 4.58 eV, respectively.

As it has been mentioned in the introduction, after absorbing light, excited

molecules can undergo inter- or intra-molecular conversion to release their

excitation energies. Classifying the excitation energy according to its position

in the absorption spectrum will help us to make these emission spectra in Fig.

3.4 more understandable. The excitation energy at 6.1 eV is located in

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

4.14 eV Eex

= 6.10 eV

M6T

6

M8T

8

M10

T10

M12

T12

4.20 eV

Emission Energy (eV)(a)

3.68 eV

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

Eex

= 5.89 eV

M6T

6

M8T

8

M10

T10

M12

T12

4.20 eV

Emission Energy (eV)(b)

3.68 eV

33

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

Eex

= 5.63 eV

M6T

6

M8T

8

M10

T10

M12

T12

4.20 eV

Emission Energy (eV)(c)

3.68 eV

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

Eex

= 5.38 eV

M6T

6

M8T

8

M10

T10

M12

T12

4.17 eV

Emission Energy (eV)(d)

3.68 eV

2.0 2.5 3.0 3.5 4.0 4.5

3.68 eV

Eex

= 4.95 eV

M6T

6

M8T

8

M10

T10

M12

T12

4.00 eV

Emission Energy (eV)(e)

3.50 eV

2.0 2.5 3.0 3.5 4.0 4.5

3.68 eV

Eex

= 4.58 eV

M6T

6

M8T

8

M10

T10

M12

T12

4.00 eV

Emission Energy (eV)(f)

3.50 eV

Fig. 3.4 The PL spectra of 0.001 M M6T6 , M8T8 M10T10 and M12T12 in n-hexane at excitation energies of (a) 6.1 eV, (b) 5.89 eV, (c) 5.63 eV, (d) 5.38 eV, (e) 4.95 eV and (f) 4.58 eV. The arrows in (d) and (f) indicate the Raman peak from the solvent.

34

absorption band III for all four molecules, as can be seen clearly in Fig. 3.5,

which is an enlargement of Fig. 3.1a (the absorbances of M10T10 are divided by

10 to fit in the scale). Returning to Fig. 3.4a, we can find that at an excitation

energy of 6.1 eV, the emission intensities around 4.2 eV follow the sequence of

M10T10 > M12T12 > M8T8 ≈ M6T6. This correlates well with the absorption

spectra. As shown in Fig. 3.1a, the absorption in this energy range also

decreases in the same order as the PL spectra, e.g. M10T10 > M8T8 ≈ M6T6.

4.50 4.75 5.00 5.25 5.50 5.75 6.00 6.25 6.50

0.01

0.1

x 1/10

4.58 eV

4.95 eV

5.38 eV

5.63 eV

5.89 eV

M6T

6

M8T

8

M10

T10

6.10 eV

Abs

orba

nce

In (

I 0/IT)

Energy (eV)

275 250 225 200Wavelength (nm)

Fig. 3.5 UV-absorption spectra of M6T6, M8T8, M10T10 and M12T12 in n-hexane solution with concentration of 0.001 M, in which vertical dashed lines indicate the position of excitation energy values used in Fig. 3.4. The absorbances of M10T10 and M12T12 are divided by 10.

Under an excitation energy of 6.1 eV, the emission spectra of M6T6 and M8T8

have one broad shoulder around 3.68 eV, which is the position where the

35

second emission band appears at lower excitation energy for all four

compounds, see Fig. 3.4 a and d. M10T10 also shows a weak tail at 3.68 eV, and

it is located in a similar absorption region as that of M6T6 and M8T8.

In Fig. 3.4b, with excitation energy of 5.89 eV, the emission spectra for all four

cages become broader compared to those under 6.1 eV in Fig. 3.4a. All these

spectra have an emission band at 3.68 eV. With reference to Fig. 3.5, we can

see that all three cages are excited within the region of band III. Comparing the

PL spectra of these three compounds, one can see that within this absorbing

region, the emission spectrum shows a broad band between 4.2 and 3.68 eV.

The emission tails of M10T10 and M12T12 are growing in intensity at 3.68 eV.

For all samples, the emission intensities of the band centered at 4.2 eV

decreases with decreasing excitation energy from 6.1 to 5.89 eV. Figure 3.6

summarizes the dependences discussed in this subsection.

With excitation energy further decreasing to 5.63 eV, from Fig. 3.5 and Fig.

3.1b one can see that the excitation is located in the absorption band II. The

intensity of the emission band with maximum at 3.68 eV increases for all cage

size molecules compared to that under higher excitation energy. For the band

with maximum at 4.2 eV, the situation is inversed (Fig. 3.6a) though with

different slope (Fig. 3.6b).

36

5.4 5.5 5.6 5.7 5.8 5.9 6.0 6.1

0

2

4

6

8

10

12

14

16

18 M6T

6

M8T

8

M10

T12

M12

T12

Inte

gral

em

issi

on in

tens

ity a

t 4.2

eV

Excitation energy (eV)(a)

230 225 220 215 210 205Wavelength (nm)

6 8 10 12

0

2

4

6

8

10

12

14

16

18

Inte

gral

em

issi

on in

tens

ity a

t 4.2

eV

Cage Size (n)(b)

Eex

= 6.10 eV E

ex = 5.89 eV

Eex

= 5.63 eV E

ex = 5.38 eV

Fig. 3.6 Dependence of the integral emission intensities with maxima of 4.2 eV on (a) excitation energy for different cage sizes and (b) cage sizes with different excitation energy, for M6T6, M8T8, M10T10 and M12T12.

37

At an excitation energy of 5.38 eV, the emission spectra show two bands with

maxima at 3.68 and 4.17 eV for M10T10 and M12T12 (Fig. 3.4d). This excitation

energy value is located in the absorption band II for cage 6 and 8, while for

cage 10, it is in the left edge of band II. Thus we can conclude that the emission

band at 3.68 eV is associated with absorption band II. Similar phenomena have

also been found and published in our previous work on silsesquioxanes with

different alkyl ligands at cage size of 8 in pentane solution [4] where the

emission band is at 3.7 eV. The integral emission intensities of the 3.68 eV

band can be found in Fig. 3.7 for cages 6, 8, 10 and 12. Due to the difficulty of

the peak de-convolution, only emission spectra with excitation energies at 5.63

and 5.38 eV are shown in this diagram.

With excitation energy reduced to 4.95 and 4.58 eV, one can find that the

emission band has a maximum at 4 eV, while the emission at 3.68 eV begins to

decrease and vanishes (Fig. 3.3, 3.4). Such a value of excitation energy is

located around the absorption band I for all these four cage size molecules.

This confirms that the emission band at 3.68 eV originates from the region of

absorption band II. The emission band around 4 eV undergoes a red-shift from

4.2 to 4.17 and to 4 eV with the excitation energy decreasing from ≥ 5.63 to

5.38 and to ≤ 4.95 eV. It appears in all absorption regions.

38

6 8 10 12

1

2

3

4

5

Inte

gral

em

issi

on in

tens

ity a

t 3.6

8 eV

Cage Size (n)(a)

Eex

= 5.63 eV E

ex = 5.38 eV

5.25 5.30 5.35 5.40 5.45 5.50 5.55 5.60 5.65 5.70

0

1

2

3

4

5

M6T

6 M

10T

10

M8T

8 M

12T

12

Inte

gral

em

issi

on in

tens

ity a

t 3.7

eV

Excitation energy (eV)(b)

235 230 225 220Wavelength (nm)

Fig. 3.7 Dependence of the integral emission intensities with maxima of 3.68 eV on (a) cage sizes with different excitation energy and (b) excitation energy for different cage sizes, for M6T6, M8T8, M10T10 and M12T12.

39

Figure 3.8 shows the excitation spectra of M6T6, M8T8, M10T10 and M12T12 at

the position of emission bands of 4.2 eV and 3.68 eV in 0.001 M in n-hexane

solution. In our previous work [4], we have shown that at an excitation energy

higher than 5.8 eV solvent molecules (n-hexane) can also be excited. Therefore,

there exists a probability for collision energy transfer between excited solvent

and silsesquioxane states. At excitation energies larger than 5.7 eV, a strong

re-absorption causes these selective absorption “peaks”.

4.5 5.0 5.5 6.0

6.00 eV

5.44 eV

5.57 eV

Eem

= 4.20 eV

M6T

6

M8T

8

M10

T10

M12

T12

5.70 eV

Excitation Energy (eV)(a)

Raman Peaksfrom Solvent

4.0 4.5 5.0 5.5 6.0

Raman Peaksfrom Solvent 6.10 eV

5.40 eV

Eem

= 3.68eV

M6T

6

M8T

8

M10

T10

M12

T12

Excitation Energy (eV)(b)

Fig. 3.8 Excitation spectra of M6T6, M8T8, M10T10 and M12T12 at emission position of (a) 4.2 eV and (b) 3.68 eV with concentration of 0.001 M in n-hexane solution.

For M6T6, the emission band at about 4.2 eV can be excited with energy from

5.25 eV to the cut off of the emission spectrometer (6.1 eV). According to Fig.

3.8a, with increase of the excitation energy, the PL intensity increases, in

40

agreement with Fig. 3.3a. In Fig. 3.8b, the emission band at 3.68 eV can be

most strongly excited with an excitation energy of 5.4 eV, which agrees with

Fig. 3.7b.

In Fig. 3.8a, the excitation spectrum of M8T8 shows similarity to that of M6T6.

To excite the emission band at 4.2 eV, the excitation energy should be in the

range from 5.25 eV to the cut off of the emission spectrometer. The differences

between these two compounds are that, for M8T8, the most efficient excitation

energy to excite the 4.2 eV band is at 5.44 eV. Referring to Fig. 3.3b, with the

excitation energy of 5.38 eV, the emission spectrum shows the clearest contour

of such band, which agrees very well with this excitation spectrum.

Both M10T10 and M12T12 have the re-absorption edge at 5.7 eV. The emission

band at 4.2 eV is excited with an excitation energy ranging from 5.2 eV to the

cut-off of spectrometer. Comparing with M6T6 and M8T8, these two compounds

have much higher re-absorption at E > 5.7 eV. In all of the selective absorption

diagram of M8T8, M10T10 and M12T12, a shoulder at left side of maxima with

value around 5.3-5.4 eV is found, in which at 5.38 eV the emission spectrum

also shows an emission band at 3.68 eV.

Figure 3.8b shows the excitation spectra for the emission band at 3.68 eV.

There is one maximum at 5.4 eV for all four cages. At higher excitation energy,

41

due to the strong re-absorption, there are fewer chances to excite such band.

For an excitation energy range from 5.0 to about 5.7 eV, this band can be

excited. Figure 3.3 agrees well with this conclusion.

3.1.1.2.2 0.01 M solution

Figure 3.9 shows the emission spectra and band positions for different

excitation energy for each cage in 0.01 M n-hexane solution. Figure 3.10 shows

the emission spectra and position for different cage size at excitation energies

of 6.1, 5.89, 5.63, 5.38, 5.16 and 4.95 eV. These two diagrams can be compared

with Fig. 3.3 and 3.4, which were measured in 0.001 M solutions.

In Fig. 3.9 and 3.10, the emission spectra of silsesquioxanes with all four cage

sizes have two emission bands. These two emission bands are overlapping, and

the maxima of these two bands are located at the same position for all samples

regardless of their cage size. The maximum of emission band I appears at 4.17

eV, and the maximum of emission band II is located around 3.66 eV.

In Fig. 3.9, at excitation energies lower than 5.63 eV, the intensity of the

emission band I becomes very low, and it is obscured by the weak tail of

emission band II. The positions of these two bands are about 0.02 eV

42

2.0 2.5 3.0 3.5 4.0 4.5 5.0

M6T

6

3.52 eV

3.33 eV

4.17 eV

3.66 eV

Eex

= 6.10 eV

Emission Energy (eV)(a)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV

Eex

= 4.58 eV

E

ex = 4.27 eV

2.0 2.5 3.0 3.5 4.0 4.5 5.0

M8T

8

3.52 eV

3.33 eV

4.17 eV

3.66 eV

Eex

= 6.10 eV

Emission Energy (eV)(b)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV

Eex

= 4.58 eV

Eex

= 4.27 eV

2.0 2.5 3.0 3.5 4.0 4.5 5.0

M10

T10

3.52 eV

3.33 eV 4.17 eV

3.66 eV

Eex

= 6.10 eV

Emission Energy (eV)(c)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV

Eex

= 4.58 eV

Eex

= 4.27 eV

2.0 2.5 3.0 3.5 4.0 4.5 5.0

M12

T12

3.52 eV

3.33 eV

4.17 eV

3.66 eV

Eex

= 6.10 eV

Emission Energy (eV)(d)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV

Eex

= 4.58 eV

Eex

= 4.27 eV

Fig. 3.9 Fluorescence spectra of 0.01 M silsesquioxane with different cage sizes in n-hexane solution for the excitation wavelengths of 203, 210, 220, 230, 240, 250, 270, and 290 nm. The corresponding excitation energies are shown in the diagram.

43

red-shifted compared to those in the case of 0.001 M solutions (Fig. 3.3, 3.4).

The emission band II has two shoulders at 3.33 and 3.52 eV, respectively. With

a decrease of excitation energy, the emission intensity at these two shoulders

position increases. At excitation energies in the range of 4.95 to 5.16 eV, the

maximum position of emission band II shifts to 3.52 eV, and two bands at 3.33

and 3.66 eV superimpose to this center peak at 3.52 eV. Within this excitation

energy range, the emission intensity at the lower energy shoulder position (3.33

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

Eex

= 6.10 eV

4.17 eV

M6T

6

M8T

8

M10

T10

M12

T12

Emission Energy (eV)(a)

3.66 eV

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

Eex

= 5.89 eV

M6T

6

M8T

8

M10

T10

M12

T124.17 eV

Emission Energy (eV)(b)

3.66 eV

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

Eex

= 5.63 eV

M6T

6

M8T

8

M10

T10

M12

T12

4.17 eV

Emission Energy (eV)(c)

3.66 eV

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

Eex

= 5.38 eV

M6T

6

M8T

8

M10

T10

M12

T12

4.13 eV

Emission Energy (eV)(d)

3.66 eV

44

2.0 2.5 3.0 3.5 4.0 4.5 5.0

4.06 eV3.33 eV

3.66 eVE

ex = 5.16 eV

M6T

6

M8T

8

M10

T10

M12

T12

Emission Energy (eV)(e)

3.52 eV

2.0 2.5 3.0 3.5 4.0 4.5 5.0

3.66 eV

3.33 eV

Eex

= 4.95 eV

M6T

6

M10

T10

M12

T12

4.06 eV

Emission Energy (eV)(f)

M8T

8

3.52 eV

2.0 2.5 3.0 3.5 4.0 4.5

3.84 eV

3.52 eV

Eex

= 4.58 eV

M6T

6

M8T

8

M10

T10

M12

T12

4.06 eV

Emission Energy (eV)(g)

3.33 eV

3.66 eV

2.0 2.5 3.0 3.5 4.0

3.52 eVEex

= 4.27 eV

M6T

6

M8T

8

M10

T10

M12

T12

Emission Energy (eV)(h)

3.33 eV

3.66 eV

Fig. 3.10 Dependence of emission intensity on cage size of M6T6, M8T8, M10T10 and M12T12 (0.01 M) at excitation energy of (a) 6.10 eV, (b) 5.89 eV, (c) 5.63 eV, (d) 3.68 eV, (e) 5.16 eV, (f) 4.95 eV, (g) 4.58 eV and (h) 4.27 eV.

eV) increases with the decrease of excitation energy, whereas at the same time,

the higher energy one at 3.66 eV decreases in intensity. When the excitation

energy is in the range of 4.58 – 4.27 eV, the maximum position of this emission

band II stays at a value between 3.52 and 3.66 eV, and still one shoulder on the

45

lower emission energy side with a value of 3.33 eV is seen.

Figure 3.11 presents the dependence the integral emission intensities of band I

on (a) the excitation energy for each of these silsesquioxanes with different

cage sizes and (b) on the cage size of the molecule at different excitation

energies. One can clearly see that this emission band I has the highest

intensities at the values of excitation energy from 5.89 to 5.63 eV. M10T10 has

the strongest re-absorption rate (see more details in Fig. 3.15) because its

absorbance is the strongest between 5 and 6.5 eV.

Figure 3.12 presents the dependence the integral emission intensities of band II

(a) on the excitation energy for each of these silsesquioxanes with different

cage sizes and (b) on the cage size of these molecules at different excitation

energies. For all silsesquioxanes, the emission intensities of band II increases

with decrease of the excitation energy from 6.1 eV. Among all these molecules,

M10T10 has the lowest emission intensity of band II for all excitation energies

(Fig. 3.12a).

Figure 3.13 compares the integral intensities of emission bands I and II at

different excitation energies. We find that the ratio of the emission intensities

from these two bands is nearly constant at excitation energy between 5.63 and

6.10 eV. At an excitation energy of 5.38 eV, the PL is dominated by emission

46

from band II.

5.4 5.6 5.8 6.00

5

10

15

20

25 M6T

6

M8T

8

M10

T10

M12

T12

Inte

grat

ed B

and

Inte

nsity

at 4

.17

eV (

a. u

.)

Excitation energy (eV)(a)

230 225 220 215 210 205Wavelength (nm)

6 8 10 120

2

4

6

8

10

12

14

16

18

20

Inte

grat

ed e

mis

sion

inte

nsity

at 4

.17

eV

Cage Size (n)(b)

Eex

= 6.10 eV E

ex = 5.89 eV

Eex

= 5.63 eV E

ex = 5.38 eV

Fig. 3.11 Dependence of the integral emission intensities with maxima of 4.17 eV on (a) excitation energy for different cage sizes and (b) cage sizes with different excitation energy, for M6T6, M8T8, M10T10 and M12T12

47

5.4 5.6 5.8 6.00

10

20

30

40

50

60

70 M6T

6

M8T

8

M10

T10

M12

T12

Inte

grat

ed b

and

Inte

nsity

at 3

.66

eV (

a. u

.)

Excitation energy (eV)(a)

230 225 220 215 210 205wavelength (nm)

6 8 10 120

10

20

30

40

50

60

70

80

Inte

gral

em

issi

on in

tens

ity a

t 3.6

6 eV

Cage Size (n)(b)

Eex

= 6.10 eV E

ex = 5.89 eV

Eex

= 5.63 eV E

ex = 5.38 eV

Fig. 3.12 Dependence of the integral emission intensities with maxima of 3.66 eV on (a) excitation energy for different cage sizes and (b) cage sizes with different excitation energy, for M6T6, M8T8, M10T10 and M12T12

48

5.4 5.6 5.8 6.00

5

10

15

20

( )

( )

3.66

4. 17

Band II eVIPI

Band I eV

JR

J=

M6T

6

M8T

8

M10

T10

M12

T12

Rat

io o

f int

egra

ted

band

inte

nsity

(IP

I)be

twee

n em

issi

on b

and

II an

d I

Emission energy (eV)

230 225 220 215 210 205wavelength (nm)

Fig. 3.13 Dependence of the ratio of integral intensities between emission band II and I on excitation energy for M6T6, M8T8, M10T10 and M12T12

In order to verify the value of the excitation energy for exciting these two

emission bands, Fig. 3.14 shows their excitation spectra. It is seen that for all

samples with different cage sizes, emission band I can be excited with an

excitation energy higher than 5.23 eV, while an excitation energy higher than

4.8 eV is enough to excite band II.

The maximum position for the selective absorption shifts from 5.7 eV (M12T12)

to 5.75 eV (M6T6) in the case of exciting the emission band I at 4.17 eV. For the

excitation of the emission band II at 3.66 eV, this maximum position remains

constant regardless of the change in cage sizes. Based on the above

49

4.5 5.0 5.5 6.0

5.75 eV

5.23 eV

5.70 eV

Eem

= 4.17eV

M6T

6

M8T

8

M10

T10

M12

T12

Excitation Energy (eV)(a)

Raman Peaksfrom the solvent

4.5 5.0 5.5 6.0

4.8 eV

5.48 eVEem

= 3.66 eV

M6T

6

M8T

8

M10

T10

M12

T12

Excitation Energy (eV)(b)

Fig. 3.14 Excitation spectra of M6T6, M8T8, M10T10 and M12T12 at emission position of (a) 4.17 eV and (b) 3.66 eV with concentration of 0.01 M in n-hexane solution.

discussion and the results obtained from 0.001 M solutions, one can conclude

that the emission band I originates from the molecule itself, while band II is

related to the interaction with other molecules. As for these series of

silsesquioxanes, the difference is only the cage size, in which the ligand is kept

the same (trimethylsiloxy).

3.1.1.2.3 Discussion and summary

In both 0.001 and 0.01 M n-hexane solutions, the PL spectra of silsesquioxanes

MnTn (n = 6, 8, 10, 12) show two overlapping emission bands (I and II). The

maxima positions of these two bands are slightly red-shifted in the 0.01 M

50

n-hexane solution in comparison with the one in 0.001 M solution. In the 0.01

M solution, the maximum of band I is at 4.17 eV, whereas that of band II varies

between 3.52 and 3.66 eV, depending on the energy of the excitation light. In

solution with concentration of 0.001M, the maxima position in emission

spectra of these two bands are located at 4.2 and 3.68 eV for band I and band II,

respectively.

The integral intensities of emission band I are comparable for both

concentrations (Fig. 3.5, 3.11), which means that the PL intensity of band I is

independent of concentration. In contrast, the integral intensity of emission

band II varies in a wide range (Fig. 3.7, 3.12). Figure 3.15 shows one example

of this difference at an excitation energy value of 5.38 eV, together with the

integrated intensity ratio of band II to band I at concentrations of 0.001 and

0.01 M. It shows clearly that the PL intensity of band II is strongly dependent

on the concentration.

Table 3.2 summarizes the excitation energies for both band I and II. One can

see that the excitation energies for these two bands are comparable in both

solutions.

51

6 8 10 120

10

20

30

40

50

60

70

Eex

= 5.38 eV

Inte

grat

ed in

tens

ity o

f em

issi

on b

and

II

Cage Size (Tn)

(a)

0.001 M 0.01 M

6 8 10 120

5

10

15

20

em i ss io n b an d IIIPI

e m i ss io n b an d I

JR

J=

Eex

= 5.38 V

Rat

io o

f int

egra

ted

peak

inte

nsity

(IP

I)be

twee

n em

issi

on b

and

II an

d I

Cage Size (Tn)

(b)

0.001 M 0.01 M

Fig. 3.15 Dependence of (a) integral intensities of emission band II and (b) the ratio of integral intensities between emission band II and I on molecules’ cage size at excitation energy of 5.38 eV with concentration of 0.01 and 0.001 M for M6T6, M8T8, M10T10 and M12T12

52

Table 3.2 The value of excitation energy for exciting emission band I and II in n-hexane solution with concentration of 0.001 and 0.01 M

0.001 M concentration 0.01 M concentration Band

Cage I II I II

M6T6 5.7 eV- cut off 5.13 eV- cut off

M8T8 4.9 eV- cut off

M10T10

M12T12

5.2 eV- cut off 4.23 eV- cut off

5.23 eV- cut off 4.8 eV- cut off

In solution, molecular vibration and rotation modes can be excited to add some

extra energy to the ground electronic state, for example, the Si-O-Si stretching

mode (ω ≈ 1150 cm-1, ∼ 0.144 eV), O-Si-O “ring opening” vibration mode

(ω ≈ 420 cm-1, ∼ 0.052 eV) and C-H stretching mode (ω ≈ 3050 cm-1, ∼ 0.381

eV) [4]. In trimethylsiloxy silsesquioxane, such additional modes can explain

the following points:

1. The red-shift of the position of emission band I from 4.2 to 4 eV in 0.001

M solution and from 4.17 to 4.06 eV in 0.01 M solution with the decrease

of the excitation energy.

2. The shoulder structure in emission band II at positions of 3.33 – 3.52 –

3.66 eV in 0.01 M solution within the whole range of excitation energy. In

0.001 M solution, the maximum of this band II has a width of about 0.14

eV, see Fig. 3.3& 3.4.

The origin of the two PL bands can be explained by analogy to our work on the

53

PL from silsesquioxanes R8(SiO1.5)8 with alkyl ligands in pentane solution [3].

Accordingly, the emission band I with a maximum at about 4.17 to 4.2 eV is

due to a charge transfer from the Si-O-cage to the ligands. Because the HOMO

are predominantly the non-bonding states, no photo-degradation was observed.

The second emission band II at the lower energy is most probably due to an

inter-molecular interaction, which results in dimers or exiplexes (“excited

complex” formation). This interpretation is in agreement with the theoretical

calculations of Calzaferri et al. [24] and Xiang et al. [6].

54

3.1.2 The influence of ligand length

In this section, the PL spectra of octa-silsesquioxanes (SiO1.5)8R8 with alkyl and

hydride ligands were studied. The LIF spectra of such molecules have been

studied by Ossadnik et al. [5, 54] and the PL spectra of these molecules in

pentane solution and in gel state have been studied in our previous work [4]. In

Ossadnik’s work, the observed intensity of the emission excited by He-Cd laser

at 3.8 eV gradually decreased from 3.8 to 2.0 eV. This LIF was attributed to

charge transfer transitions, where in the electronically excited state, some

negative charge density was pushed from the Si-O cage to the alkyl ligands.

The role and energy position of the oxygen lone pair state was not clearly

defined. In our work [4], two intensive emission bands at about 3.7 and 4.2 eV

were observed in pentane solution of various concentrations, in the solid state

and in the gel state. Intensive absorption bands at about 6 eV were also found

in these states. From this work, the emission band I (4.2 eV) was explained by

charge transfer transition from the Si-O cage to the alkyl ligands, and emission

band II (3.7 eV) is presumed to originate from a dimer or an exciplex (“excited

complex”) due to intermolecular interaction. In the following parts, UV

absorption, PL-emission and PL-excitation spectra will be investigated in

n-hexane solution. The concentrations of the solution used are of 0.001 M and

0.01 M, which makes it comparable with the work on the cage size influence

(Section 3.1.1) and also with the work in pentane solution [4]. The length of the

55

alkyl ligands ranges from 1 to 10 carbon atoms in the chains as described in

Table 2.1.

3.1.2.1 UV-absorption spectra

Figure 3.16 (a) and (b) show the dependence of absorbance on the length of

alkyl-ligand with concentration of 0.001 M and 0.01 M in octa-silsesquioxane

n-hexane solution. The absorbance scale for both Fig.3.16a and 3.16b are the

same. Me8T8 has a lower solubility than 0.001 M (see Table 2.1), therefore it is

not included in the diagram. Absorption was measured under the same

condition as described in Fig. 3.1. For 0.001 M concentration solutions, the

error of weighing is about 30%.

In 0.001 M concentration n-hexane solution (Fig. 3.16a), except Et8T8, Prop8T8

and Bu8T8, all show three absorption bands. Referring to Fig. 3.1, these bands

can be classified as I (≤ 4.72 eV, with proper scale), II (4.72 - 5.39 eV) and III

(5.39 eV to the cut off of spectrometer). In band III, the absorbance shows

saturation when the energy of incoming light exceeds 5.61 eV. With the

decrease of the length of the alkyl ligand, the absorption bands I and II become

very weak and can not be identified from the diagram, while the lower energy

side of band III shows a blue shift compared to that in Dec8T8. The absorptions

in band III decrease with the decrease of ligand length, which can be connected

56

4.0 4.5 5.0 5.5 6.0

IIIIIIEt

8T

8, 2C

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec8T

8, 10C 0.

07

Abs

orba

nce

Scal

e

Energy (eV)(a)

4.0 4.5 5.0 5.5 6.0

IIIIIIEt

8T

8, 2C

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec8T

8, 10C 0.

07A

bsor

banc

eSc

ale

Energy (eV)(b)

Fig. 3.16 Dependence of absorbances on the lengths of the alkyl ligands with concentration of (a) 0.001 M and (b) 0.01 M in octa-silsesquioxane n-hexane solution. Me8T8 has a lower solubility than 0.001 M and, therefore, is not included here.

with variations of the symmetry of the silsesquioxane [4]. H8T8 and Me8T8 are

nearly spherical top molecules in which all three moments of inertia are equal.

With the increase of the ligand length, there are more than two carbon atoms in

the ligand, these will take different positions in the frame defined by the cage

and the ligand, which will break the symmetry of the molecule. These

molecules do not follow strictly the selection rules for rotational quantum

numbers like symmetric or spherical molecules [4, 67]. For short ligand

molecules, like methyl, ethyl, propyl and butyl octa-silsesquioxanes, the

absorbances become very weak, but the three absorption bands can be

identified by enlarging the diagram. The exact energy positions of these bands

57

are given in Table. 3.3.

In the 0.01 M n-hexane solution (Fig. 3.16b), for compounds with all ligand

lengths, the absorbance shows three bands and these bands are located in the

same energy ranges as in Fig. 3.16a. The absorbances are stronger than that in

Fig. 3.16a as expected for the higher concentration. For comparison, Fig. 3.17

shows the absorbances of Hept8T8 for concentration 0f 0.001 M (solid line) and

0.01 M (dashed line). With the increase of the concentration by a factor of 10,

the integral absorbance increases about 2.3 times. At an appropriate scale, both

show typical three absorption bands.

3.5 4.0 4.5 5.0 5.5 6.00.00

0.05

0.10

0.15

0.20

0.25 0.001 M 0.01 M

Abs

orba

nce

ln(I O

/I T)

Energy (eV)

350 300 250Wavelength (nm)

Fig. 3.17 Dependence of absorbances on molar concentration of Hept8T8 in 0.001 M (solid line), and 0.01 M n-hexane solution (dashed line)

In our previous work [4] we found that in pentane solution, the integral

absorption intensities of these molecules have different dependence on the

58

1 2 3 4 5 6 7 8 9 10 110.00

0.05

0.10 0.001 M in n-Hexane

Et8T

8 Bu8T

8

Hex8T

8

Oct8T

8

Dec8T

8Non

8T

8

Hept8T

8

Prop8T

8

Inte

gral

Abs

orpt

ion

Inte

nsity

(a.

u.)

Number of Carbon Atoms in the Ligands(a)

1 2 3 4 5 6 7 8 9 10 11

0.00

0.05

0.10

0.15

0.200.01 M in n-hexane

Et8T

8

Bu8T

8Hex

8T

8

Oct8T

8

Dec8T

8

Non8T

8

Hept8T

8

Prop8T

8

Inte

gral

Abs

orpt

ion

Inte

nsity

(a.

u.)

Number of Carbon Atoms in the Ligands(b)

0 1 2 3 4 5 6 7 8 9 10

0.00

0.25

0.50

0.750.005 M in pentane

H8T

8Et

8T

8Bu

8T

8

Hex8T

8

Oct8T

8

Dec8T

8

Non8T

8

Hept8T

8

Prop8T

8

Me8T

8Inte

gral

Abs

orpt

ion

Inte

nsity

(a.u

.)

Number of Carbon Atoms in the Ligands(c)

Fig. 3.18 Dependence of integral absorbances on length of alkyl ligands in octasilsesquioxanes in (a) 0.001 M n-hexane solution (with 30% weighing error), (b) 0.01 M n-hexane solution, (c) 0.005 M pentane solution. ( Notice that Me8T8 has a lower solubility than 0.001)

59

number of carbon atoms in the ligands for even or odd numbers. Molecules

with odd numbers of carbon atoms show stronger absorption. Figure 3.18

shows the dependence of the integral absorption intensity on the ligand length

in 0.001 M and 0.01 M n-hexane solution. In both concentration solutions, the

integral absorption of molecules with ligand of odd numbers of carbon atoms

show stronger absorption than those with ligands of even number carbon atoms.

There is about 30% of weighing error for 0.001 M concentration solutions, as

shown in Fig. 3.18a. Similar dependence was also found in n-pentane solution

(Fig. 3.18c) [4].

In general, for both concentrations in n-hexane and also in pentane, the integral

absorption intensity increases with the increase of ligand length. The

differences of the absorption behavior, which relate to even-odd numbers of

carbon atoms of the ligand, are connected with the variation of symmetries

through the number of carbon atoms. Even and odd number of carbon atoms in

the ligands give different selection rules, which contribute to the variation of

the absorption [46].

Figure 3.19 shows the effect of isomeric structures on the absorbance. One

notices that the absorbance of iso-Bu8T8 is much stronger than that of the

n-Bu8T8. With increase of concentration from 0.001 M to 0.01 M, the

absorption bands show a blue-shift, from which one can conclude that the

60

carbon atoms in branched chain contribute to the absorption.

3.0 3.5 4.0 4.5 5.0 5.5 6.00.00

0.25

0.50

0.75

1.00

1.25

1.50

¡Á3

¡Á3

0.001M Bu8T

8,4C

iso-Bu8T

8,4C

Abs

orba

nce

ln(I O

/I T)

Energy (eV)(a)

400 375 350 325 300 275 250 225Wavelength (nm)

3.5 4.0 4.5 5.0 5.5 6.00.00

0.25

0.50

0.75

1.00

1.25

1.50 0.01M Bu8T

8,4C

iso-Bu8T

8,4C

Abs

orba

nce

ln(I O

/I T)

Energy (eV)(b)

375 350 325 300 275 250 225Wavelength (nm)

Fig. 3.19 Dependence of absorbances on Bu8T8 with and without isomeric ligand structure in (a) 0.001 M n-hexane solution, (b) 0.01 M n-hexane solution.

61

The absorbance of iso-Bu8T8 saturates at energy higher than 5.5 eV. In 0.001 M

concentration solutions, the integral absorption intensity of iso-Bu8T8 is about 4

times stronger than that of n-Bu8T8. In 0.01 M concentration solution, this

factor is about 8. These results are similar to that of Ch. Ossadnik [5] in 0.01 M

THF solutions where the absorption of iso-Bu8T8 was about 3 times stronger

than that of n-Bu8T8.

3.1.2.2 Emission and excitation spectra

3.1.2.2.1 0.001 M Solution

Figure 3.20 shows the fluorescence spectra of octa-silsesquioxanes with

hydrogen and alkyl ligands in 0.001 M n-hexane solution. Because Me8T8 has a

lower solubility than 0.001 M in n-hexane, it is not included. The excitation

energies were 4.27, 4.58, 4.95, 5.16, 5.38, 5.63, 5.89, and 6.10 eV, which

correspond to wavelengths of 290, 270, 250, 240, 230, 220, 210, and 203 nm.

These values of excitation energy are comparable with those in section 3.1.1.2.

At an excitation energy ranging from 5.38 to 6.10 eV, the fluorescence spectra

show two emission bands, similar to those found in section 3.1.1.2.

For hydrogen and all alkyl ligands of octasilsesquioxanes, the maximum of

emission band II is at 3.68 eV in agreement with the band II of 0.001 M

62

3.0 3.3 3.6 3.9 4.2 4.5 4.8

H8T

8

4.17 eV3.68 eV Eex

= 6.10 eV

Emission Energy (eV)(a)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV E

ex = 4.58 eV

Eex

= 4.27 eV

3.0 3.3 3.6 3.9 4.2 4.5 4.8

Et8T

8

4.09 eV

3.68 eV

Eex

= 6.10 eV

Emission Energy (eV)

(b)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV

E

ex = 4.58 eV

Eex

= 4.27 eV

3.0 3.3 3.6 3.9 4.2 4.5 4.8

Prop8T

8

4.12 eV

3.68 eV

Eex

= 6.10 eV

Emission Energy (eV)(c)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV

Eex

= 4.58 eV

Eex

= 4.27 eV

3.0 3.3 3.6 3.9 4.2 4.5 4.8

Bu8T

8

4.12 eV3.68 eV

Eex

= 6.10 eV

Emission Energy (eV)(d)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV

Eex

= 4.58 eV

Eex

= 4.27 eV

63

3.0 3.3 3.6 3.9 4.2 4.5 4.8

iso-Bu8T

8

4.16 eV

3.68 eV Eex

= 6.10 eV

Emission Energy (eV)(e)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV E

ex = 4.58 eV

Eex

= 4.27 eV

3.0 3.3 3.6 3.9 4.2 4.5 4.8

Hex8T

8

4.17 eV

3.68 eV

Eex

= 6.10 eV

Emission Energy (eV)(f)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV

Eex

= 4.58 eV

Eex

= 4.27 eV

3.0 3.3 3.6 3.9 4.2 4.5 4.8

Hept8T

8

4.19 eV

3.68 eV

Eex

= 6.10 eV

Emission Energy (eV)(g)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV E

ex = 4.58 eV

Eex

= 4.27 eV

3.0 3.3 3.6 3.9 4.2 4.5 4.8

Oct8T

8

4.13 eV

3.68 eV

Eex

= 6.10 eV

Emission Energy (eV)(h)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV

Eex

= 4.58 eV

Eex

= 4.27 eV

64

3.0 3.3 3.6 3.9 4.2 4.5 4.8

4.29 eV

Non8T

8

4.12 eV

3.68 eV

Eex

= 6.10 eV

Emission Energy (eV)(i)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV E

ex = 4.58 eV

Eex

= 4.27 eV

3.0 3.3 3.6 3.9 4.2 4.5 4.8

Dec8T

8

4.26 eV3.68 eV

Eex

= 6.10 eV

Emission Energy (eV)(j)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV

Eex

= 4.58 eV

Eex

= 4.27 eV

Fig. 3.20 Fluorescence spectra of 0.001 M hydrogen and alkyl ligand octa-silsesquioxanes with different ligand lengths in n-hexane solution for excitation energies indicated in the diagram. The arrows indicate the Raman peak from the solvent.

cage-dependence-silsesquioxane (Sect. 3.1.1.2) and also with our results in

pentane solution [4]. This is a strong support for the explanation that it

originates from intermolecular interaction. This intermolecular interaction is

independent of the alkyl solvent (at least for pentane and n-hexane), and of the

molecular structure (variation in cage and ligand size). For the emission band I,

the maximum positions are within the range of 4.09 - 4.3 eV, and for the

sample of Bu8T8, iso-Bu8T8, Oct8T8, and Dec8T8, there exist broader peaks.

Figure 3.21 summarizes the change of position of the maximum of the emission

65

bands with the increase of alkyl ligand lengths, from two to ten carbon atoms at

excitation energy used as in Fig. 3.20. At an excitation energy of 6.10 eV (Fig.

3.21a), the maximum position of emission band I is blue-shifted from 4.09 eV

(Et8T8) to 4.27 eV (Dec8T8) with the increase of ligand length. There is no

emission band II at such excitation energy, except that there exist broad

shoulders for Prop8T8, Bu8T8, Oct8T8, and Dec8T8 which stride over this

position. In Fig. 3.21b with excitation energy of 5.89 eV, this blue shift of

emission band I can also be observed. With the excitation energy decreasing to

5.63eV, emission band II around 3.68 eV appears for samples with all ligand

lengths. Its maximum position occurs at the same energy of 3.68 eV regardless

of ligand length. At excitation energy less than 5.38 eV, emission band I

decreases sharply and vanish at excitation energy below 5.16 eV.

At an excitation energy of 4.95 eV, a new emission peak appears at the position

of bands I & II, in which for the samples of Bu8T8, Hex8T8, and Hept8T8, there

still remains a double peak structure. When exciting with a low energy, such as

4.58, 4.27 and 4.06 eV, one can only find a red tail of the emission band I.

The two band structure can be seen most clearly at an excitation energy of 5.63

eV (see Fig. 3.21). Because both bands are fairly symmetric, their contribution

to the total PL intensity can be obtained from numerical fitting of the measured

spectra by two Gaussian functions. Figure 3.22 shows the dependence of the

ratio of the integral intensity of band II to band I on the number of carbon

66

3.0 3.3 3.6 3.9 4.2 4.5 4.8

4.27 eVEex

= 6.10 eV

4.09 eV

3.68 eV Et8T

8, 2C

Emission Energy (eV)(a)

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec

8T

8, 10C

3.0 3.3 3.6 3.9 4.2 4.5 4.8

4.30 eVE

ex= 5.89 eV

4.00 eV

Et8T

8, 2C

Emission Energy (eV)(b)

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec8T

8, 10C

3.0 3.3 3.6 3.9 4.2 4.5 4.8

4.27 eV

Eex

= 5.63 eV

4.00 eV

3.68 eV Et8T

8, 2C

Emission Energy (eV)(c)

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec8T

8, 10C

3.0 3.3 3.6 3.9 4.2 4.5 4.8

4.25 eV

Eex

= 5.38 eV

4.00 eV

3.68 eV

Et8T

8, 2C

Emission Energy (eV)(d)

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec8T

8, 10C

67

3.0 3.3 3.6 3.9 4.2 4.5 4.8

4.15 eV

Eex

= 5.16 eV

4.00 eV

3.68 eVEt8T

8, 2C

Emission Energy (eV)(e)

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec8T

8, 10C

3.0 3.3 3.6 3.9 4.2 4.5 4.8

3.80 eV

Eex

= 4.95 eV

4.15 eV3.50 eV

Et8T

8, 2C Emission Energy (eV)

(f)

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec8T

8, 10C

3.0 3.3 3.6 3.9 4.2 4.5 4.8

Eex

= 4.58 eV

Et8T

8, 2C

Emission Energy (eV)(g)

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec8T

8, 10C

3.0 3.3 3.6 3.9 4.2 4.5 4.8

Eex

= 4.27 eVEt8T

8, 2C

Emission Energy (eV)(i)

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec8T

8, 10C

68

3.0 3.3 3.6 3.9 4.2 4.5 4.8

Eex

= 4.06 eV

Et8T

8, 2C

Emission Energy (eV)(i)

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec8T

8, 10C

3 4 5 6 7 8 9 100.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Bu8T

8

Hex8T

8

Oct8T

8

Prop8T

8

Hept8T

8

Non8T

8

Dec8T

8

Inte

nsity

rat

io o

f the

Ban

d II

vs I

Number of Carbon Atoms in the Ligands

Fig. 3.22 Dependence of the integral ratio of emission band II vs. I on the number of carbon atoms in the ligand at an excitation energy of 5.63 eV

atoms in the ligands. The results show that the intensity ratio decreases with

Fig. 3.21 Dependence of emission intensity on ligand lengths in 0.001 M solution at excitation energy of (a) 6.1 eV, (b) 5.89 eV, (c) 5.63 eV, (d) 3.68 eV, (e) 5.16 eV, (f) 4.95 eV, (g) 4.58 eV, (h) 4.27 eV. The arrows indicate the Raman peak from the solvent

69

increasing number of C-atoms in the ligand except Dec8T8. For ligand with odd

number of carbon atoms, they has a higher slope than those with even number

of carbon atoms. The reason for that of Dec8T8 is still unclear.

3.1.2.2.2 0.01 M Solution

Figure 3.23 shows the fluorescence spectra of the octa-silsesquioxane samples

with hydrogen and alkyl ligand in 0.01 M n-hexane solution (H8T8 has a lower

molar concentration than 0.01 M). Because the saturated concentration of

Me8T8 is lower than 0.001 M, it is not included. The excitation energies used in

these series are the same as in section 3.1.2.2.1 (4.27, 4.58, 4.95, 5.16, 5.38,

5.63, 5.89, and 6.10 eV, corresponding to wavelength 290, 270, 250, 240, 230,

220, 210, and 203 nm). At excitation energies ranging from 4.58 to 6.10 eV, the

fluorescence spectra show two emission bands. Comparing with section

3.1.2.2.1 (0.001 M concentration), the lowest value of excitation energy for the

emission band II is red shifted from 5.38 to 4.58 eV. Considering that this

emission band II is supposed to originate from interactions between molecules,

this red shift of the excitation energy value strongly supports this hypothesis.

For the hydrogen ligands, the maximum position of emission band II is at 3.68

eV, while for all alkyl ligand lengths of octasilsesquioxanes, this position

changes to 3.65 eV. This value is 0.03 eV red-shifted from the case of 0.001 M,

70

3.0 3.3 3.6 3.9 4.2 4.5 4.8

H8T

8

4.18 eV3.68 eV Eex

= 6.10 eV

Emission Energy (eV)(a)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV

Eex

= 4.58 eV

Eex

= 4.27 eV

3.0 3.3 3.6 3.9 4.2 4.5 4.8

Et8T

8

4.04 eV

3.65 eV Eex

= 6.10 eV

Emission Energy (eV)(b)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV E

ex = 4.58 eV

Eex

= 4.27 eV

3.0 3.3 3.6 3.9 4.2 4.5 4.8

Prop8T

8

4.20 eV

3.65 eV

Eex

= 6.10 eV

Emission Energy (eV)(c)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV

Eex

= 4.58 eV

Eex

= 4.27 eV

3.0 3.3 3.6 3.9 4.2 4.5 4.8

Bu8T

8

4.15 eV

3.65 eV

Eex

= 6.10 eV

Emission Energy (eV)(d)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV E

ex = 4.58 eV

Eex

= 4.27 eV

71

3.0 3.3 3.6 3.9 4.2 4.5 4.8

iso-Bu8T

8

4.17 eV

3.65 eV

Eex

= 6.10 eV

Emission Energy (eV)(e)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV E

ex = 4.58 eV

Eex

= 4.27 eV

3.0 3.3 3.6 3.9 4.2 4.5 4.8

Hex8T

8

4.20 eV

3.65 eV

Eex

= 6.10 eV

Emission Energy (eV)(f)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV

Eex

= 4.58 eV

Eex

= 4.27 eV

3.0 3.3 3.6 3.9 4.2 4.5 4.8

Hept8T

8

4.20 eV

3.65 eV

Eex

= 6.10 eV

Emission Energy (eV)(g)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV E

ex = 4.58 eV

Eex

= 4.27 eV

3.0 3.3 3.6 3.9 4.2 4.5 4.8

Oct8T

8

4.04 eV

3.65 eV

Eex

= 6.10 eV

Emission Energy (eV)(h)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV

Eex

= 4.58 eV

Eex

= 4.27 eV

72

3.0 3.3 3.6 3.9 4.2 4.5 4.8

Non8T

8

4.27 eV

3.65 eV

Eex

= 6.10 eV

Emission Energy (eV)(j)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV

Eex

= 4.58 eV

Eex

= 4.27 eV

3.0 3.3 3.6 3.9 4.2 4.5 4.8

Dec8T

8

4.04 eV

3.65 eV

Eex

= 6.10 eV

Emission Energy (eV)(k)

Eex

= 5.89 eV

Eex

= 5.63 eV

Eex

= 5.38 eV

Eex

= 5.16 eV

Eex

= 4.95 eV E

ex = 4.58 eV

Eex

= 4.27 eV

Fig. 3.23 PL spectra of 0.01 M hydrogen and alkyl ligand octa-silsesquioxanes with different ligand lengths in n-hexane solution for the excitation wavelengths of 203, 210, 220, 230, 240, 250, 270, and 290 nm. The corresponding excitation energies are shown in the diagram.

which is at 3.68 eV. The intensity of this band is much higher than that in 0.001

M solution, whereas for the maximum of emission band I, for hydrogen ligand

is at 4.18 eV which is comparable with that in 0.001M solution. For alkyl

ligands, the maxima are within the range of 4.02 - 4.27 eV, which is also about

0.03 eV red shifted compared to the 0.001 M case at 4.09 – 4.3 eV.

In order to show the influence of the ligand length on the position of the

maximum of the emission band at a given excitation energy, Figure 3.24

summarizes the PL spectra of samples with ligand lengths from ethyl to decyl

73

3.0 3.3 3.6 3.9 4.2 4.5 4.8

4.26 eVEex

= 6.10 eV

4.12 eVEt8T

8, 2C

Emission Energy (eV)(a)

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec8T

8, 10C

3.0 3.3 3.6 3.9 4.2 4.5 4.8

3.83 eV

4.26 eVEex

= 5.89 eV

4.02 eVEt

8T

8, 2C

Emission Energy (eV)(b)

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec8T

8, 10C

3.0 3.3 3.6 3.9 4.2 4.5 4.8

4.07 eV

3.80 eV

4.26 eV

Eex

= 5.63 eV

3.65 eV

Et8T

8, 2C

Emission Energy (eV)(c)

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec8T

8, 10C

3.0 3.3 3.6 3.9 4.2 4.5 4.8

4.03 eV

3.78 eV

4.26 eV

Eex

= 5.38 eV

3.65 eVEt

8T

8, 2C

Emission Energy (eV)(d)

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec8T

8, 10C

74

3.0 3.3 3.6 3.9 4.2 4.5 4.8

3.51 eV

4.02 eV

3.74 eV

4.17 eV

Eex

= 5.16 eV

3.59 eV

Et8T

8, 2C

Emission Energy (eV)(e)

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec

8T

8, 10C

3.0 3.3 3.6 3.9 4.2 4.5 4.8

3.82 eV3.35 eV

4.03 eV

Eex

= 4.95 eV

3.50 eV

Et8T

8, 2C

Emission Energy (eV)(f)

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec8T

8, 10C

3.0 3.3 3.6 3.9 4.2 4.5 4.8

4.02 eV

3.80 eV

4.23 eV

Eex

= 4.58 eV

3.65 eV

Et8T

8, 2C

Emission Energy (eV)(g)

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec8T

8, 10C

3.0 3.3 3.6 3.9 4.2 4.5 4.8

3.68 eV

3.90 eV

Eex

= 4.27 eV

3.53 eV

Et8T

8, 2C

Emission Energy (eV)(h)

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec8T

8, 10C

75

3.0 3.3 3.6 3.9 4.2 4.5 4.8

3.52 eV

Eex

= 4.06 eV

3.34 eV

Et8T

8, 2C

Emission Energy (eV)(i)

Prop8T

8, 3C

Bu8T

8, 4C

Hex8T

8, 6C

Hept8T

8, 7C

Oct8T

8, 8C

Non8T

8, 9C

Dec8T

8, 10C

except pentyl. Hydro-silsesquioxanes are not included because their solubility

is lower than 0.01 M.

For excitation energy of 6.1 eV, emission spectra show a broad peak with

maxima located at band I, and a red tail across the position of band II. For

excitation energy of 5.89 eV, emission band I still dominates while the intensity

of emission band II begins to increase. With the excitation energy decreasing to

5.63 eV, emission band II becomes prominent in the spectra. Below the

excitation energy of 5.38 eV, emission band I appears as a weak tail of the band

II. At excitation energies of 4.95 to 4.58 eV, emission band II becomes very

weak.

Fig. 3.24 Dependence of emission intensity on ligand lengths for 0.01 M concentration at excitation energy of (a) 6.1 eV, (b) 5.89 eV, (c) 5.63 eV, (d) 3.68 eV, (e) 5.16 eV, (f) 4.95 eV, (g) 4.58 eV, (h) 4.27 eV. The arrows indicate the Raman peak from the solvent

76

At all excitation energies, the maximum position of emission band II is

centered at 3.65 eV, while for emission band I, the maximum position shows a

blue shift with increasing ligand length. For example, at an excitation energy of

5.89 eV, this position changes from 4.02 (Et8T8) to 4.27 eV (Dec8T8).

In 0.01M n-hexane solution, with an excitation energy of 5.63 eV, the integral

ratio of emission band II vs. band I also shows a dependence on the even or odd

numbers of carbon atoms of the ligand, as can be seen in Fig. 3.25. The reason

for choosing this excitation energy is the same as in the case of 0.001M

solution: The emission spectra have the most prominent two bands structure.

Comparing to Fig. 3.22 (0.001M n-hexane solution), the relative intensity of

the band II to band I is much higher. For example, in 0.001 M solutions, the

maximum value of this ratio is 1.4 for Hept8T8, while in 0.01 M solution, the

maximum value is around 4.6 for Et8T8. Unlike the 0.001 M case, where for

even numbers of carbon atoms in the ligand the ratio of integral intensity of

band II to band I is nearly constant of about 0.8, whereas for odd numbers of

carbon atoms, it increases from 0.45 to 1.4, in 0.01 M n-hexane solution, this

ratio decreases for all molecules with the increase of ligand length except of

Dec8T8 .

Because in solution of a higher concentrations there is a higher chance for

aggregation or interaction between molecules, the increase of relative

77

intensities of emission band II compared with band I is a logical consequence.

From Fig. 3.25 it can be concluded that, in 0.01 M solution (similar with that in

0.001M), short ligand molecules have more possibilities to form intermolecular

excimers.

2 3 4 5 6 7 8 9 10

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0Et

8T

8

Bu8T

8

Hex8T

8

Oct8T

8

Prop8T

8

Hept8T

8Non

8T

8

Dec8T

8

Inte

nsity

rat

io o

f the

Ban

d II

vs I

Carbon Atom Number in the Ligands

Fig. 3.25 Dependence of the integral ratio of emission band II vs. I on the number of carbon atoms of the ligand at an excitation energy of 5.63 eV

3.1.2.3 Discussion and summary

In both 0.001 M and 0.01 M n-hexane solutions the emission spectra of

octa-silsesquioxanes (SiO1.5)8R8 with alkyl and hydride ligands contain two

main emission bands. For molecules with the hydrogen ligands, the maximum

of emission band II appears at the same position for both concentrations, while

78

for the alkyl-substituted samples, this position shows a 0.3 eV red shift in 0.01

M (3.65 eV) as compared to that in 0.001 M solutions (3.68 eV).

The position of the maximum of emission band I can be influenced by two

factors:

1. The concentration of solution;

2. The length of the ligand.

At a given excitation energy, this maximum moves to higher energy with the

increase of the concentration. For example, for Hex8T8 at excitation energy of

6.1 eV the maximum position is at 4.17 eV in 0.001M solution while it is

shifted to 4.2 eV in 0.01 M solution. The value of this blue shift with the

increase of concentration is related to the ligand length, as is shown in Fig. 3.26.

Oct8T8 shows a somewhat different behavior, the reason for it is still not clear.

The position of the maximum displays a blue shift with the increase of ligand

length at the same excitation energy and in the same concentration (see Fig.

3.21a and 3. 24a).

The origin of these two emission bands can be rationalized on the basis of the

available theoretical calculations of the electronic structure of H8Si8O12 and

substituted silsesquioxanes [2, 5, 24]. Calzafferri et al. calculated the energies

of X8Si8O12 with X = H, Cl, and CH3 [24]. For X = H, the highest occupied

79

molecular orbital (HOMO) was found to be of pure oxygen lone pair character

with A2g symmetry, whereas the lowest unoccupied molecular orbital (LUMO)

is a 4A1 state, in which contributions from Si, O and H are mixed. The

photoelectron spectra reproduced the nature of the occupied molecular orbitals

in agreement with the calculations. The first sharp peak at –10.7 eV was

identified as an oxygen lone pair ground state of octahydrosilsesquioxane and

the second intensive maximum as a superposition of the 5T1u, 3T2u, 2Eu, 2T1g,

4T2g, 2T2u, 2Eg, 4T1u and 2A2u states. These states are mostly due to the

interactions of oxygen and silicon p orbitals. The binding energy of 10.7 eV in

the photoelectron spectrum is the first ionization energy of H8T8.

2 3 4 5 6 7 8 9 104.00

4.05

4.10

4.15

4.20

4.25

4.30E

ex=6.10 eV

0.01 M 0.001 M

Et8T

8

Bu8T

8

Hex8T

8

Oct8T

8

Prop8T

8

Hept8T

8

Non8T

8

Dec8T

8

Pea

k po

sitio

n of

em

issi

on b

and

I

Carbon Atom Number in the Ligands

Fig. 3.26 Dependence of the maximum position of the emission band I on the number of carbon atoms of the ligand at excitation energy of 6.1 eV in 0.01 M and 0.001 M n-hexane solution

Silicon-oxygen polymers (siloxanes and silsesquioxanes) with CH3 ligands

80

have similar structures of the occupied molecular orbitals [2]. The oxygen lone

pair states dominate the HOMO with some small amount of Si- and negligible

C- orbitals. For these compounds, the lowest photoelectron energies are

between 4 and 6 eV.

Xiang et al. calculated the electronic structures of occupied and unoccupied

molecular orbitals for different hydrogen substituted silsesquioxanes with cage

sizes from 6 to 16 Si atoms [6]. HOMO and LUMO of H8T8O12, calculated by

non-local density approximation, consists of a 100% A2g state of oxygen for

HOMO and a 30% Si, 54% O and 16% H mixture in the A1g LUMO state.

Since both states are of gerade symmetry, the direct A2g – A1g transition is

dipole forbidden. The next excited state is 3Eu.

There are several states from which the absorption is allowed. Allowed

transitions with increasing absorption energy are 5T1u – 4A1g, 4T1u – 4A1g, 2T1g

–3Eu and 2Eg – 2Eu. If the lowest observed absorption band A at 4.4 eV

corresponds to the lowest energy transition from 5T1u to 4A1g, then the other

three transitions are at 5.3, 5.8 and 6.2 eV, respectively, according to the

energies of the states involved in those transitions [24] presented in Fig. 3.27.

This is in good agreement with the measured positions of absorption bands II

and III (Fig. 3.16).

81

-1

0

1

2

3

4

5

6 E

nerg

y (e

V)

6.2

eV

5.8

eV C

5.3

eV B

4T1u

2Eg

2T2u

4T2g

2T1g

2Eu

3T2u

5T1u

A2g

3Eu

4A1g

-1

0

1

2

3

4

5

6

Eex

p=4.

4 eV

A

2A2u

Ene

rgy

(eV

)

-2

-1

0

0

1

2Densities of states

H8T

8

Fig. 3.27 Model for absorption and emission of band I using calculated potential energies [24] and densities of states [6]

The corresponding densities of states calculated by Xiang et al. [6] are shown

82

on the right-hand side of this diagram. Supposing the intensity of the transitions

to be a multiplication of densities of states, then the calculated intensity ratio of

transitions at 4.4, 5.3, 5.8 and 6.2 eV is 1:9:20:60. Such an estimated intensity

ratio is in a good agreement with the measured absorption intensities of these

three absorption bands (Fig. 3.1 and 3.16).

Based on this model, the emission band I at about 4.2 eV originates from the

4A1g – 5T1u transition and, due to the emission to the higher vibration levels in

the ground state (Stokes), its maximum position is red-shifted from the 4.4 eV

of the maximum of the absorption band.

For silsesquioxanes with long ligands, the gerade – ungerade symmetry

vanishes because the probability that each ligand atom is within the exact

equilibrium position is low. Therefore, additional electronic transitions can be

allowed which results in a higher density of allowed states and consequently a

higher intensity of absorption and emission bands.

According to the calculations of Xiang et al. [6] for H8T8, the HOMO negative

and positive charge density regions are symmetrically located around the cage

atoms, while for the LUMO some negative charge is also located in the ligand

(H atom). Thus HOMO – LUMO excitation can be described as charge transfer

transition from the Si – O cage (HOMO) to the H ligand (LUMO), where a

83

small negative charge moves from the oxygen atom to the position of H. For

alkyl ligands, this charge should transfer to longer distances from the cage,

depending on the length of ligand. The calculated gap between HOMO and

LUMO of H8T8 is about 6 eV, which is 1.8 eV higher than the experimentally

observed energy of 4.2 eV (Fig. 3.24a). This decrease of the gap energy is due

to the contribution of the negative Coulomb integral, which is estimated to be

about 2 eV for such a molecule [56].

This model cannot explain the origin of the emission band II. Similar spectra

with an additional emission peak at somewhat lower emission energies were

reported for several kinds of silane, siloxane, and carbon polymers, mainly

substituted with aryl groups [57 - 60]. For these molecules, the low energy peak

has been attributed to inter-molecular excimer or exciplex emission. The

excited silsesquioxanes can interact with ground sate molecules and form

excimers. As silsesquioxanes are rather rigid molecules, intra-molecular

excimers seem to be rather unlikely. Nevertheless, the formation of

intermolecular excimers may occur. Due to the charge transfer in the excited

state, these molecules show some negatively charged ligands, which may

interact with another molecule in the ground sate. The typical energy potential

curves for the excimer is shown in Fig. 3.28, with which, one can more

reasonably deduce the reason why band II can be only excited within a small

energy range around 5.45 eV. Excitation with lower energies cannot interact

84

with any states of the dimer whereas excitation with higher energies results in

the dissociation of the excited dimmer.

H8T

8

5.4

eV B

4T1u

2Eg 2T

2u

4T2g

2T1g

2Eu

3T2u

5T1u

A2g

3Eu

4A1g

-1

0

1

2

3

4

5

6

2A2u

Ene

rgy

(eV

)

(H8T

8)*+H

8T

8

H8T

8+H

8T

8

(H8T

8)2

3.7

eV (

band

II)

Fig. 3.28 Model for absorption and emission of band II with the formation of an excimer state

85

3.2 Photoluminescence from selected branched polysilanes

As outlined in the forgoing section, the PL from silsesquioxane involves the

excitation from the non-bonding lone-pair states into the LUMOs of the ligands.

Therefore we did not observed any noticeable photo-degradation comparable to

that of e.g. linear polysilanes in which the PL is due to σ - σ* excitations. It is

well known that branched polysilanes show a strong PL with a high quantum

yield [48]. Because many of such polysilanes may undergo photolysis with the

formation of radicals, several researchers raise the question to what extent these

radicals may contribute to the observed PL.

In order to answer this question we studied several branched polysilanes, which

were synthesized and investigated in terms of the radical formation upon their

photolysis by Professor Y. Apeloig and his coworker.

3.2.1 Tetrakis(trimethylsilyl)silane [(Me3Si)4Si]

Veprek et al [53] reported a fairly strong PL from tetrakis(trimethylsilyl)silane,

the molecular structure of which is given below:

Me3SiSi

SiMe3

SiMe3

SiMe3

86

In solid state the maximum of the PL at energy of about 3 eV was found. In

order to exclude possible artifacts due to impurities and also to be able to

measure the absorption, the present studies were done in n-hexane solution.

Figure 3.29 shows the absorption spectrum of this molecule.

3.25 3.50 3.75 4.00 4.25 4.50 4.75 5.00

0.0

0.1

0.2

0.3

0.4

0.5

III

0.001M 0.004M

Abs

orba

nce

ln(I O

/I T)

Energy (eV)

350 300 250Wavelength (nm)

Fig. 3.29 UV-absorption spectra of tetrakis(trimethylsilyl)silane, in n-hexane for concentration of 0.001 M and 0.004 M. The absorption of solvent was subtracted from that of solution in order to obtain the true absorption spectra of the compound.

The concentrations of tetrakis(trimethylsilyl)silane used in this study are 0.004

M and 0.001 M. There are two absorption bands, band I is located between 4.2

and 4.8 eV and band II appears at E > 4.8 eV. At 0.004 M, the absorption

increases. Above the energy of 4.86 eV, there is a sharp increase of absorption.

Because of a relatively low PL intensity of this compound, we used the

87

concentration of 0.004 M. The PL spectra measured at several excitation

energies are shown in Fig. 3.30.

2.5 3.0 3.5 4.0 4.5 5.0 5.5

0

5

10

15

20

4.24 eV4.35 eV

Eex

= 6.10 eV E

ex = 5.63 eV

Eex

= 4.95 eV E

ex = 4.58 eV

Eex

= 4.27 eV E

ex = 4.06 eV

PL

Inte

nsity

(ar

b. u

nits

)

Emission energy (eV)

450 400 350 300 250wavelength (nm)

0

5

10

15

20

Fig. 3.30 Emission spectrum of Tetrakis(trimethylsilyl)silane in 0.04M concentration of n-hexane solution at excitation energies at 4.06, 4.27, 4.58, 5.63 and 6.1 eV, which correspond to the wavelengths of 305, 290, 270, 250, 220 and 203 nm.

With excitation energies of 4.95 and 5.63 eV (within the absorption band II, see

Fig. 3.29), strong emission peaks with two maxima around of 4.24 and 4.35 eV

were found. For the excitation energy of 6.1 eV, which corresponds to the

saturation region of absorption, there is actually no PL seen because all

incoming excitation light is absorbed within the short path through solution.

When exciting these materials with the excitation energy within the absorption

band I (e.g. 4.58, 4.27 and 4.06 eV) only very weak emission bands can be seen.

88

We attribute the PL from tetrakis(trimethylsilyl)silane to a HOMO-LUMO

transition. Intermolecular interactions or a possible contribution of radicals are

very weak and they cannot contribute to the observed fluorescence.

3.2.1 [(Me3SiMe2Si)3Si]2 and its radicals during photolysis

The molecular structure of this compound (briefly called “star-dimer”) is as

below:

Me3SiMe2SiSi

SiMe2SiMe3

Si

SiMe2SiMe3

Me3SiMe2Si

SiMe2SiMe3

SiMe2SiMe3

During the UV photolysis (1) in solution, it forms radicals (2) which undergo

subsequently reactions leading to products (P), see Scheme 1.

( ) ( ) ( )dissociationrecombination1 2 P←→ →

Scheme 1

Polysilanes absorb UV light strongly, show electric conductivity and nonlinear

89

optical properties [48, 53, 61 – 63, 71]. They have been studied extensively so

far. Figure 3.31 shows the time evolution of absorption during the UV light

photolysis with a high-pressure Hg lamp (250W, strongest emission at 254 nm).

It was measured by my colleague Dr. D. Azinovic. The figure shows two bands

peaking at 4.8 eV (255 nm) and 5.25 eV (234 nm) in the absorption spectrum

of the un-irradiated star-dimer (open circles). During the irradiation, the

intensities of both bands decrease rapidly. In the first 1.5 hours a red tail of

absorbance at 4.1 eV (300 nm) can be observed, which increases in intensity in

the first 1.5 hours and thereafter its intensity decreases, see Fig. 3.31b.

The absorption at 4.8 eV (255 nm) is typical for the parent species 1, and

absorption at 4.1 eV (300 nm) is from the star radicals 2 formed in such

photolysis process, as shown by Apeloig et al. [64].

Figure 3.32 shows the time evolution of the PL spectra of [(Me3SiMe2Si)3Si]2 in

n-hexane solution under continuous irradiation by a He-Cd laser (325.4 nm). The

spectra show two emission bands in the region between 2.1 eV (600 nm) and 3.35 eV

(370 nm), with maxima positions at 2.83 eV (435 nm, blue band) and 3.1 eV (390 nm,

violet band).

90

210 240 270 300 3300

1

2

3

4

0

1

2

3

4

not irradiated

Irradiation time 4 min 48 s 17 min 47 min 1 h 17 min 1 h 27 min 1 h 47 min 2 h 7 min 3 h 17 min 5 h 22 min 7 h 10 min 13h 19 min 20 h 29 min 31 h 34 min

ln(I 0/I

T)

λ (nm)(a)

6 5.5 5 4.5 4energy (eV)

280 290 300 3100.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 not irradiated

Irradiation time 4 min 48 s 17 min 47 min 1 h 17 min 1 h 27 min 1 h 47 min 2 h 7 min 3 h 17 min 5 h 22 min 7 h 10 min 13h 19 min 20 h 29 min 31 h 34 min

ln(I 0/I

T)

λ (nm)(b)

Fig. 3.31 a) Absorption spectra of the star-dimer [(Me3SiMe2Si)3Si]2 in n-hexane solution. Evolution of absorption during the UV light photolysis. b) Change in absorption in the spectral region 275-320 nm.

91

400 450 500 5500

2000

4000

6000

8000

350 400 450 500 550 6000

2000

4000

6000

not irradiated

Irradiation time

50 min 2 h 20 min 21 h 12 min

Inte

nsity

3.4 3.2 3 2.8 2.6 2.4 2.2

a)

Energy (eV)

b)

Wavelength (nm)

Irradiation time

21 h 12 min 24 h 17 min 46 h 47 min 3 days after

irrad.

Inte

nsity

Fig. 3.32 Time evolution of the emission spectrum of [(Me3SiMe2Si)3Si]2 in n-hexane solution under continuous irradiation by a He-Cd laser (325.4 nm).

Figure 3.33 shows the time dependence behavior of the integral PL intensity

and absorbance under various irradiation conditions, as:

92

1) 35 hours of continuous He-Cd laser irradiation with photon flux of 4

W/cm2 (circles), without Hg lamp irradiation.

2) Integral HeCd laser induced PL intensity during 35 hours of continuous

Hg lamp (photon flux 40 mW/cm2) irradiation (squares).

3) Integral PL intensity during 35 hours, when the Hg lamp is turned off after

36 min (open triangles).

4) Absorbance at 300 nm under continuous Hg lamp irradiation (open

circles)

5) Absorbance at 300 nm, when Hg lamp irradiation is turned off after 40

min (open squares)

From the previous investigations [64, 70] it is well known that the radical half

lifetime is about 6 min. They react with other species in the solution forming

the products P, which also show efficient PL. This photolysis reaction is

completed after about 4 hours of a continuous of irradiation, see Fig. 3.33 open

circles. The products also show an efficient PL, which degrades under

irradiation.

These results show that there is no evidence for any noticeable contribution of

radicals to the observed PL during the whole time scale of experiment.

Although the initial increase of the PL intensity somewhat correlates with the

increase of the concentration of the radicals, the maximum of the PL is reached

93

after 4-5 hours when the radicals 2 have almost vanished (see data in Fig. 3.33

under continuous irradiation). Furthermore, after 30 minutes when the intense

Hg lamp is switched off, the concentration of radicals 2 decreases but the PL

intensity increases.

absorbance at 300 nm (lamp off t>40 min)

0 5 10 15 20 25 30 350

10

20

30

40

50

60

70

80

90

100

110

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0.11

lamp off

lamp off

abs

orba

nce

Inte

gral

inte

nsity

time (hours)

without Hg irradiation (only laser) irradiated all the time (Hg+laser) Hg lamp switched off after 30 min absorbance at 300 nm

Fig. 3.33 Chemical kinetics study of 1→2+2→P in absorption and PL for different irradiation conditions.

94

4. Conclusions

In this work, UV-absorption, photoluminescence and excitation spectra of

silsesquioxanes (SiO1.5)nRn and selected branched polysilanes, such as

tetrakis(trimethylsilyl)silane [(Me3Si)4Si], and the star-dimer [(Me3SiMe2Si)3Si]2

have been studied in order to identify the PL mechanism and the possible

contribution of the radicals formed during the photolysis. The influence of the

solvent on the PL spectra has also been investigated.

In the case of silsesquioxanes (SiO1.5)nRn, the influence of molecular structure,

including cage size n (n = 6, 8, 10, 12) and the length of the saturated aliphatic

ligand R (CmH2m+1, m = 0, 1,…10, except m = 5) for n = 8 was systematically

studied in 0.01 M and 0.001 M n-hexane solution. The PL emission spectra

show two bands: emission band I and excimer band II. Band I is due to a

charge transfer transition from the non-bonding oxygen orbitals of the cage to

the LUMO of the ligands. Band II originates from intermolecular exciplex

transitions. The intensity of band I increases with increasing length of the

ligands. The interpretation of the spectra is supported by published theoretical

calculations of the electronic structure of substituted silsesquioxanes.

The emission spectrum of tetrakis(trimethylsilyl)silane has only one emission

band with a maximum in the range of 4.24 to 4.35 eV at excitation energies of

95

4.58 and 5.63 eV. The photoluminescence of this molecule is due to

HOMO-LUMO transition. Intermolecular interactions are too weak to

contribute to the fluorescence.

In the case of branched polysilanes, such as the star-dimer [(Me3SiMe2Si)3Si]2,

the observed PL is of the same origin, i.e., of a HOMO-LUMO transition, but

involves transitions between bonding and anti-bonding orbitals. This results in

a relatively fast photolysis during which the radicals 2 i.e. 3 2 3(Me SiMe Si) Si •

are formed as intermediates with lifetime of several minutes. The hypothesis of

some scientists that the observed PL is due to radicals was disproved in the

present work. It was shown that the observed PL from the “star dimer” 1

illuminated by the exciting UV light is due to this species alone. Under

conditions of the intensive irradiation with a high power mercury lamp, when a

relatively fast photolysis of 1 takes place on the time scale of hours, additional

contributions to the increasing PL intensity are due to the products of the

reactions of the radicals 2 (Time scale of 4 –5 hrs under the conditions of the

experiment). Under an irradiation longer than 5 hours, these products are also

photolysed and the PL intensity decreases. The appendix summarizes briefly

our search for the most suitable solvent to be used for such investigations.

96

Appendix

The influence the of solvent

It is well known and documented in the literature, that the solvent may strongly

influence the absorption and PL spectra of solute molecules. Schaaf et al. [65]

have observed a hypsochromic shift in the absorption bands when changing the

solvent from cyclohexane (c-hexane) to THF (tetrahydrofuran) in the solutions

of Hg(SiMe3-xClx)2. Lee et al. [66] have shown that the solvent-solute

interactions have a significant effect on the luminescence spectra of the film

cast from solution, and suggested that the solvent effect should be considered

seriously in such a manufacturing process. In our previous work, we [4] also

used methanol as a polar solvent, mixing with pentane solution, to produce a

silsesquioxane gel and found differences of the luminescence properties with

respect to those of a pure pentane solution.

By changing the type of solvent, the luminescence spectra of silsesquioxanes

can also show some differences. For example, when changing the solvent from

n-hexane to THF, the three absorption bands increase in intensity and show a

small blue shift. Figure 1 shows one example for Hept8T8 at concentration of

0.001 M. The oxygen atom of THF can interact with H2O from air or H atom of

the ligands and this interaction may influence the spectrum.

97

3.5 4.0 4.5 5.0 5.50.0

0.1

0.2

0.3

0.4

0.5

Hept8T

8 in

n-hexane THF

Abs

orba

nce

ln(I O

/I T)

Energy (eV)

350 300 250

IIa

IIIa

IIIII

Wavelength (nm)

Fig. 1 UV-absorption of Hept8T8 in n-hexane and THF for concentration of 0.001 M. The absorption of solvents was subtracted from that of solution in order to obtain the true absorption spectra of Hept8T8.

2 3 4 5 6 7 8 9 10

0.06

0.08

0.10

0.12

0.14 in n-hexane in THF

Et8T

8

Bu8T

8

Hex8T

8

Oct8T

8

Dec8T

8

Non8T

8

Hept8T

8

Prop8T

8

Inte

gral

Abs

orpt

ion

Inte

nsity

(a.

u.)

Number of Carbon Atoms in the Ligands

Fig. 2 Dependence of integral absorption intensity on the number of carbon atoms in the silsesquioxane ligand with solvents of n-hexane and THF for concentration of 0.001 M. The absorption of solvent was subtracted from that of solution in order to obtain the true absorption spectra of Hept8T8.

98

Figure 2 shows the difference of integral absorption intensity of a series of

silsesquioxanes with a variety of ligands in 0.001 M n-hexane and THF

solution. The absorption of the solvent was subtracted from that of solution in

order to obtain the true absorption spectra of the silsesquioxanes. The

absorptions in THF solution are much stronger than those in n-hexane solution,

except for that of Bu8T8.

3.2.1.1 Hexane solvent

Figure 3 shows the absorption spectra of pure n-hexane and c-hexane solvent.

Obviously, c-hexane which shows a strong absorption band in the range of 4.5.

– 5.2 eV, and becomes saturated at an energy above 5.6 eV is not suitable for

spectroscopic studies (This results is agree with the standard spectrum of

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9 n-Hexane c-Hexane

Abs

orba

nce

ln(I O

/I T)

Energy (eV)

450 400 350 300 250Wavelength (nm)

Fig. 3 UV-absorption of n-hexane and c-hexane

99

3.0 3.5 4.0 4.5 5.00

10

20

30

40

50 Eex

= 5.63 eV E

ex = 4.95 eV

Eex

= 4.58 eV E

ex = 4.27 eV

Eex

= 4.06 eV

PL

Inte

nsity

(ar

b. u

nits

)

Emission energy (eV)(a) n-hexane

400 350 300 250

Wavelength (nm)

3.0 3.5 4.0 4.5 5.00

10

20

30

40

50

294 nm4.21 eV

291 nm4.25 eV

284 nm4.36 eV

Eex

= 5.63 eV E

ex = 4.95 eV

Eex

= 4.58 eV E

ex = 4.27 eV

Eex

= 4.06 eV

PL

Inte

nsity

(ar

b. u

nits

)

Emission energy (eV)(b) c-hexane

400 350 300 250

Wavelength (nm)

Fig. 4 Emission spectra of fresh n-hexane and c-hexane at excitation energies of 4.06, 4.27, 4.58, 4.95, and 5.63 eV (which corresponds to wavelengths of 305, 290, 270, 250, and 220 nm, respectively).

100

c-hexane [69]). Figure 4 shows the PL spectra of the two solvents (left diagram

is n-hexane, and right diagram is c-hexane) at excitation energies of 4.06, 4.27,

4.58, 4.95 and 5.63 eV (which corresponds to wavelengths of 305, 290, 270,

250, and 220 nm). These spectra were measured with fresh solvent, which

means immediately after opening the storage bottle. No emission from

n-hexane is seen in Fig. 4a, whereas a strong PL in the range of 3.3 – 4.65 eV is

found for c-hexane as shown in Fig. 4b. The PL from silsesquioxanes and other

organosilanes appears in the same range. For this reason, c-hexane is not a

suitable solvent also for the emission spectrum measurements, while n-hexane

is very appropriate for this purpose.

3.2.1.2 Pentane solvent

N-pentane is also a good solvent for emission spectral measurements. It has

similar absorption and emission spectra like n-hexane. However, because it

evaporates much faster and causes stronger erosion effects on the sealing

washer which is used on the top of the cuvette, it was ruled out in this work.

3.2.1.3 THF (Tetrahydrofuran) solvent

Figure 5 shows the absorption spectrum of tetrahydrofuran (THF) solvent. It is

similar to that of n-hexane solvent, but the values of the absorption are larger.

This solvent is unstable in the air. Figure 3.5 compares the emission spectra of

fresh THF solvent (Fig. 6a) and after its exposure to air and after its storage in

101

Argon for 36 hr (Fig. 6b). At an excitation energy of 4.42 eV, the emission

3.5 4.0 4.5 5.0 5.5 6.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Abs

orba

nce

ln(I O

/I T)

Energy (eV)

350 300 250Wavelength (nm)

Fig. 5 UV-absorption spectrum of THF solvent.

spectra do not show much difference between fresh sample and after storage.

But at an excitation energy of 4.58 eV, which corresponds to 270 nm, a strong

emission band with a peak at 4.09 eV appears.

Figure 7 shows the dependence of the emission band at 4.09 eV on the storage

time at such an excitation energy. Figure 8 shows that the integral emission

intensity increases with increase of storage time. For this reason, THF was also

ruled out as a solvent for this study.

102

3.00 3.25 3.50 3.75 4.00 4.25

0.0

0.5

1.0

1.5

2.0 Eex

= 4.58 eV E

ex = 4.42 eV

PL

Inte

nsity

(arb

. uni

ts)

Emission energy (eV)(a) fresh solvent

400 375 350 325 300Wavelength (nm)

3.00 3.25 3.50 3.75 4.00 4.25

0.0

0.5

1.0

1.5

2.0

303 nm4.09 eV

Eex

= 4.58 eV E

ex = 4.42 eV

PL

Inte

nsity

(ar

b. u

nits

)

Emission energy (eV)(b) after 36h storaging in Argon

400 375 350 325 300Wavelength (nm)

Fig. 6 Emission spectra of fresh THF solvent (a) and after storage in pure Argon for 36 hr (b) at excitation energies of 4.42 and 4.58 eV, which correspond to wavelengths of 295 and 270 nm, respectively. The arrows indicate the Raman band

103

3.0 3.5 4.0 4.50

2

4

6

303 nm4.09 eV

Eex

= 4.58 eV

Fresh Sample After 1 day After 2 days After 6 days After 7 days After 15 days

PL

Inte

nsity

(ar

b. u

nits

)

Emission energy (eV)

425 400 375 350 325 300 275Wavelength (nm)

Fig. 7 Dependence of emission intensity on the storage time at an excitation energy of 4.58 eV (270 nm). The arrow indicates the Raman band

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 150.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Eex

= 4.58 eV

Inte

gral

Inte

nsity

of t

he e

mis

sion

Pea

ks

Days after initial using

Fig. 8 Dependence of integral emission intensity of THF on the storage time at an excitation energy of 4.58 eV (270 nm).

104

3.2.1.4 Conclusions

By comparing the absorption and emission spectra of c-hexane, n-hexane,

n-pentane and THF solvent, c-hexane shows extra absorption bands and very

strong emission bands, which are within the range of that of silsesquioxanes,

n-hexane, n-pentane and THF show similar absorption and emission behaviors.

However, n-pentane evaporates too fast and exerts a strong erosion effect on

the sealing materials of the cuvette. THF shows a time-dependent emission

band at an excitation energy of 4.58 eV. Only n-hexane has no such

disadvantages. For this reason, n-hexane solvent is the most suitable solvent for

the study of PL properties of silsesquioxanes in solution.

105

References

1. E. Rikowski and H. C. Marsmann, Polyhedron 16, 3357(1997)

2. G. Ferenczy, A. Toth, I. Bertoti, and Sandor Suhai, J. Phys.: Condens. Matter.

9, 4781(1997)

3. A. Provatas, M. Luft, J. C. Mu, A. H. White, J G. Matisons, and B. W.

Skelton, J. Organometallic Chem. 565, 159(1998)

4. D. Azinovic, J. Cai, C. Eggs, H. Konig, C. Marsmann, and S. Veprek, J.

Luminescence 97, 40(2002)

5. C. Ossadnik, Ph.D. thesis, Technical University Munich, 1999.

6. K. H. Xiang, R. Pandey, U. C. Pernisz, and C. Freeman, J. Phys. Chem.

B102, 8704(1998)

7. P. A. Agaskar, Inorg. Chem. 30, 2707(1991)

8. U. Dittmar, Ph.D. thesis, University of Paderborn, 1993.

9. G. Calzaferri, C. Marcolli, R. Imhof and K. W. Toernroos, J. Chem. Soc.

Dalton Trans. 3313(1996)

10. C. X. Zhang, F. Bavonneau, C. Bonhomme, R. M. Laine, C. L. Soles, H. A.

Hristov and A. F. Yee, J. Am. Chem. Soc. 120, 8380(1998)

11. W. L. Wu and H. C. Liou, Thin Solid Films 312, 73(1998)

12. R. H. Baney, M. Itoh, A. Sakakibara and T. Suzuki, Chem. Rev. 95,

1409(1995)

106

13. G. Calzaferri, Proc. Taylor-Made Silicon-Oxygen Compounds, Mol. Mater.,

Bielefeld, Sept. 3-5, 149(1996)

14. H. Buergy, G. Calzaferri, D. Herren and A. Zhdanov, Chimia 45, 3(1991)

15. D. Hoebbel, I. Pitsch, and D.Heidemann, Z. Anorg. Allg. Chem. 592,

207(1991)

16. P. A. Agaskar, J. Am. Chem. Soc. 111, 6858(1989)

17. P. A. Agaskar, Inorg. Chem. 29, 1603(1990)

18. V. W. Day, W. G. Klemperer, V. V. Mainz and D. M. Millar, J. Am. Chem.

Soc. 107, 8262(1985)

19. A. R. Bassindale and T. E. Gentle, J. Mater. Chem. 3, 1319(1993)

20. P. G. Harrison, and R. Kannengiesser, J. Chem. Soc., Chem. Commun.

2065(1995)

21. M. Mora´n, C. M. Casado, I. Cuadrado and J. Losada, Organometallics 12,

4327(1993)

22. J. B. Nicholas, A. J. Hopfinger, F. R. Trouw and L. E. Iton, J. Am. Chem.

Soc. 113, 4792(1991)

23. B. Beagley and J. O. Titiloye, Struct. Chem. 3, 429(1992)

24. G. Calzaferri and R. Hoffmann, J. Chem. Soc., Dalton Trans. 917, 1991.

25. M. Baertsch, P. Bornhauser, G. Calzaferri and R. Imhof, J. Phys. Chem. 98,

2817(1994)

26. J. A. W. Harkless, D. K. Stillinger and F. H. Stillinger, J. Phys. Chem. 100,

1098(1996)

107

27. V. A. Ermoshin, K. S. Smirnov and D. Bougeard, Chem. Phys. 202,

53(1996)

28. A. J. M. Man and J. Sauer, J. Phys. Chem. 100, 5025(1996)

29. A. M. Bieniok and H. B. Buergi, J. Phys. Chem. 98, 10735(1994)

30. F. J. Feher, D. A. Newman and J. F. Walzer, J. Am. Chem. Soc. 111,

1741(1989)

31. R. Murugavel, V. Chandrasekhar and H. W Roesky, Acc. Chem. Res. 29,

183(1996)

32. A. Voigt, R. Murugavel, E. Parisini and H. W. Roesky, Angew. Chem., Int.

Ed. Engl. 35, 748(1996)

33. M. Montero, A. Voigt, M. Teichert, and H. W. Roesky, Angew. Chem., Int.

Ed. Engl. 34, 2504(1995)

34. F. J. Feher, T. A. Budzichowski, K. Rahimian and J. W. Ziller, J. Am. Chem.

Soc. 114, 3859(1992)

35. C. X. Zhang and R. M. Laine, J. Organometallic Chem. 521, 199(1996)

36. S. Luecke and K. Stoppek-Langner, Appl. Surface Sci. 144 - 145,

713(1999)

37. P. Eisenberg, R. E. Balsells, Y. Ishikawa, J. C. Lucas, A. N. Mauri, H.

Nonami, C. C. Riccardi and R. J. Williams, Macromolecules 33,

1940(2000)

38. C. F. Frye and T. Collins, J. Am. Chem. Soc. 92, 5586(1997)

108

39. G. Calzaferri, Tailor-made Silicon Oxygen Compounds: From Molecules to

Materials, Vieweg, Braunschweig, 1996.

40. Y. Kanemitsu, Phys. Reports 263, 1(1995)

41. C. Marcolli, P. Laine, R. Buehler and G. Calzaferri, J. Phys. Chem. B101,

1171(1997)

42. G. G. Guilbault, Practical Fluorescence, Marcel Dekker Inc., New York,

1973.

43. B. Rangarajan, L. S. Coons, and A. B. Scranton, Biomaterials 17, 649(1996)

44. R. J. Hurtubise, Analytica Chimica Acta 351, 1(1997)

45. S. G. Schulman, Fluorescence and Phosphorescence Spectroscopy:

Physicochemical Principles and Practice, Pergamon Press, Oxford, 1975.

46. R. S. Becker, Theory and Interpretation of Fluorescence and

Phosphorescence, Wiley Interscience, New York, 1969.

47. B. H. Bransden and C. J. Joachain, Physics of Atoms and Molecules,

Longman Scientific & Technical, New York, 1983.

48. R. D. Miller and J. Michl, Chem. Rev. 89, 1359(1989)

49. C. M. Michael, Y. G. Lazarou and P. Papagiannakopoulos, Chem. Phys. Lett.

194, 415( 1992)

50. P. Papagiannakopoulos and Y.G. Lazarou, Int. J. of Chemical Kinetics 26,

857(1994)

109

51. C. A. Parker, Photoluminescence of solution with application to

photochemistry and analytical chemistry, Elsevier Publishing Com.,

Amsterdam-London-New York, 1968.

52. R. F. Chen, Anal. Biochem. 19, 374(1967)

53. S. Veprek, in: Organosilicon Chemistry, eds. N. Auner and J. Weis, VCH

Weinheim, 821, 1996

54. Ch. Ossadnik, S. Veprek, C. Marsmann, and E. Rikowski, Monatshefte f.

Chemie 55, 130(1999)

55. G. Herzberg, Molecular Spectra and Molecular Structure III, Van Nostrand

Reinhold Company, 1980

56. J. C. Slater, Quantum Theory of Molecules and Solids, Vol. 1, Electronic

Structure of Molecules, McGraw – Hill Book Company, Inc. New York,

1963

57. K. Nagai, K. Utsunomiya, N. Takamiya, N. Nemoto, M. Kaneko, J. Polym.

Sci. B, Polym. Phys. 34, 917(1991)

58. K. Hamanishi, H. Shizuka, J. chem. Soc. Faraday Trans. 89, 3007(1993)

59. G. Calzaferri, R. Imhof and K W. Toernroos, J. Chem. Soc. Dalton Trans.

3123(1994)

60. H. Shizuka, H. Obuchi, M. Ishikawa and M. Kumada, J. Chem. Soc.

Faraday Trans. 80, 383 (1984)

61. Y. Kanemitsu, Physics Reports 263, 1(1995)

62. L. Brus, J. Phys. Chem. 98, 3575(1994)

110

63. M. Kira, in: The chemistry of organic silicon compounds, Vol. 2, Edited by

Z. Rappoport and Y. Apeloig, John Wiley & Sons Ltd, 1311, 1998

64. Y. Apeloig, D. Bravo-Zhivotovski, M. Yuzefovich, M. Bendikov and A.I.

Shames, Appl. Magn. Reson. 18, 426(2000)

65. T. F. Schaaf, A. K. Hovland, W. H. Ilsley, and J. P. Oliver, J. Organometallic

Chem. 197, 169(1980)

66. S. Lee, J. Y. Lee and H. Lee, Synthetic Metals 101, 248(1999)

67. W. D. Cheng, K. H. Xiang, R. Pandey and U. C. Pernisz, J. Phys. Chem.

B104, 6737(2000)

68. D. W. Scott, J. Am. Chem. Soc. 68, 356(1946)

69. H. H. Perkampus, UV-VIS Atlas of Organic Compounds, Part 2 Spectra,

D1/1-M19, 2nd edition, VCH Verlagsgesellschaft, Weinheim, 1992

70. H. Kunkely, A. Vogler, Chem. Phys. Lett. 308, 169(1999)

111

Acknowledgements

I would like to thank Prof. Dr. Dr. h. c. Stan Veprek and his wife, Dr. Maritza

Veprek-Heijman, heartfully for their support of my work here and their great

patience in helping me finishing this work. Prof. Veprek is really a great

professor in the fields not only in science, but also in care and love to his

students. Without his instruction, I will face more difficulties and more

complicated situation on the way to the sacred place of science.

Very special thanks to Dr. D. Azinovic for her helpful discussion on my Ph.

D work. Thanks to Ms. U. Madan-Singh for preparing the chemical sample,

to Mr. G. Stohwasser for checking and repairing the electronics, to Mr. A.

Englert for helping in the designs and construction of the apparatus. I would

also like to thank Dr. C. Eggs, Dr. F. Glatz, Dr. C. Ossanik, Dr. Tibor Bolom

and Dr. Xiaochun Wu for their help to make me go through formalities and in

arranging my personal life here.

Thanks to Prof. Li Shizhi for his support and encouragements during my work

here in TU muchen. Thanks to all my colleagues, Dr. Jianli He, Dr. Jian Xu, Mr.

Hans Männling, Mr. Jan Procházka, Ms. Pavla Karvankova, Mr. Hiroshi

Kobayashi, Dr. S. Mukerjee for their cooperation and help. They together built

a comfortable atmosphere in our institute for working.

112

I would also like to thank my wife, Dr. Chen Yanxia for her support and

encouragment. Whenever I face difficulties, she makes me feel life is so

beautiful in this world. At the same time, I want to express my thanks to my

parents and my family members for their love to me.

I wish all the persons I mentioned here and anyone who gave help during my

stay in this institute a fruitful life. THANK YOU ALL!