optical and fluorescent properties of thiophene … · 2005. 2. 11. · optical and fluorescent...

OPTICAL AND FLUORESCENT PROPERTIES OF

THIOPHENE-BASED CONJUGATED POLYMERS

Cheng Yang

M.Sc., University of Victoria, 1994

THESIS SUBMITTED IN PARTIAL FULFILLMENT OF

THE REQUIREMENTS FOR THE OEGREE OF

DOCTOR OF PHILOSOPHY

in the Department

of

Chemistry

O Cheng Yang 2000

SIMON FRASER UNIVERSITY

June 2000

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The first part of the thesis deals with thermochromism and band-gap

tuning of poly(3-alkylthiop henes) (P3ATs). Conflicting results on the

themtochromism of P3ATs are found in the literature: some researchers have

observed a continuous blue shift of the absorption maximum upon heating P3AT

films, suggesting a multiphase morphology; others have documented a dear

isosbestic point, indicating a huo-phase morphology. To address this issue, a

series of P3ATs (A = hexyî, octyl, dodecyl, and hexadecyl) with different head-to-

tail (HT) regio-regularities have been synthesked and investigated. It is shown in

this work that the therrnochromic properties of P3ATs are controlled by the head-

to-taP dyad content and the alkyl side chain length. P3Afs with moderate HT

dyad content give rise to a clear isosbestic point, while polymers with high HT

dyad content and short alkyl side chains exhibit no isosbestic point with

increasing temperature. This is due to a morphological effect. A

phenomenological rnodel for predicüng the existence or absence of an isosbestic

point is proposed and verified based on experimental results.

Pdy(3-(6-acryloyloxy)hexyithiophene) (93AHT) films undergo an

irreversible themmftromic change with increasing temperature. The absorption

maximum blue shifts from 489 nm to 435 nm upon heating. The band-gap

changes from 1.85 eV before heating to 2.24 eV after heating. Accordingly, the

emission maximum blue shifts from 642 nm to 594 nm, upon heating. This is due

to the thermal crosslinking of the acryloyloxy functionality at elevated

temperatures, which "locks in" the twisted conformation of the polymer chain.

This work demonstrates that the band-gap of functionalized P3ATs can be easily

tuned by a post-synthetic step.

The second part of the thesis deals with synthesis and characterization of

novel thiophene based polymers with high luminescent efficiency. A series of

regiospecific 1,4di(hexylthienyl)benzenes (DHTBs), 2,5di(hexylthienyl)furans

(DHTFs) and corresponding polymers have been synthesized, and their

fluorescence properties studied. The large Stokes shifts obsenred for both

trimers and polymers are attributed to the skeletal rearrangement upon

excitation. Fluorescence quantum yields (ais) of DHTBs and DHTFs are found

to be substantially higher than the conesponding ones of terthiophenes. The Qr's

of the polymers in THF solution range from 25% to 54%; The Of's of P44QHTB

and P33DHTB in solid state are 20% and 18%, respectively, orders of magnitude

higher than ordinary thiophene-based conjugated polymers. The large difference

in Qf is attributed to heavy atom and steric effects. The high luminescence

efficiencies of OHf6 polymers make them good candidates as emissive

materials for LEDs. The electtocfiemical properties of these polymers are also

investigated. The band-gaps and the work functions of the polyrners are

estimated from optical and electrochernical data.

To Fenglin, Harvey, and Charlie

Acknowledgments

I would like to thank my senior supervisor Dr. Steven Holdcroft. This work

would never be fulfilled without his guidance, patience, and encouragement. I

would tike to thank Drs. Paul Percival and Andrew Bennet for their guidance.

I would like to thank Mr. Jianfei Yu, Mr. Frank Orfino, Ms. Sara Villanureva

Diez, and Mr. Michael Abley. I really enjoyed collaborating with them.

I would also like to thank al1 the members of the research group for

making the lab such a pleasant work place. I will definitely miss those wonderful

birthday and holiday season parties.

I would like to thank Mr. George Vamvounis for proofreading this

manuscript.

Last, but not the least, I would like to thank Dr. Holdcroft, Simon fraser

University, NSERC, and Petro-Canada for scholarships and financial assistance.

Table of Contents

.. ...................................................................................................... Approval. ,...II

.m. ............................................................................................................ Abstract III

........................................................................................................ Dedication .v

Acknowledgments ............................................................................................ .vi

List of Schemes ................................................................................................xi m.

List of Tables ....................................................................................................xl~

..- ............................................................................................... List of Figures XIII

... List of Abbreviations ......................................................................................... XVIM

1 General Introduction.. ........................................................................ 1

... 1.1 Electrochemical Pdymerization of Thiophene and 3-Alkylthiophenes 3

1.2 Chernical Preparation of PT and P3ATs ................................................ 7

1.2.1 Chernical Preparation of PT ..................................................... 7

1.2.2 Chernical Preparation of P3ATs ............................................... 9

1.2.2.1 Grignard Coupling Method ..................................... 9

1.2.2.2 Chernical Oxidation or lron Chloride Method ......... 1 1

1.2.2.3 Curtis' Dernercuratian Polymerization .................... 13

1.2.3 Synthesis of Regioregular P3ATs ............................................. 14

1 .2.3.1 The McCullough Method ........................................ 14

................................................. 1.2.3.2 The Rieke Method 16

......... 1.2.3.3 Other Methods for Preparation of R-P3ATs. -18

........................................... 1.3 Characterization of P3ATs and Derivatives 20

.............. 1.3.1 UV-visi ble Spectroscopie Characterization of P3ATs .20

vii

....................... 1.3.2 NMR Spectroscopie Characteriration of P3ATs 22

1.3.3 Molecular Weight Measurements of P3ATs ............................. 26

..................................................... 1.3.4 Thermal Analysis of P3ATs 27

1.3.5 X-Ray Diffraction Studies of P3ATs .......................................... 28

................................................................ Thermochrcrmism of P3ATs 32

Photoluminescence of Oligothiophenes and P3ATs .............................. 34

Research Objectives ........................................................................... 40

Thermochromism of Regioregutar and Non-Regioregular

Poly(3-alkyhhiophenes): A Phenomenological Model ...................... 43

................................................................................................... Results 43

2.1 . 1 Preparation of Samples ............................................................ 43

......... 2.1.2 Temperature Dependence of UV-vis Absorption Spectra 46

................................ 2.1.3 Oifferential Scanning Calorimetry (DSC) -58

............................................................ 2.1.4 X-Ray Diffraction (XRD) 63

.............................................................................................. Discussion 73

.............................................................................................. Summary û4

.......................................................................................... Experimental 85

................................................................................. 2.4.1 Materials 85

2.4.2 Preparation of 3-Akylthiophenes and 2-Bromo-3-

......................................................................... alkyithiophenes 85

..................................... 2.4.3 Preparation of Poly(3-alkylthiophenes) 87

2.4.4 Measurements .......................................................................... 09

Synthesis and B a n d a p Tuning of Poly(3-(6-acryloyloxy)-

................................ hexylthiophene) (P3AHT) 9

Introduction ............................................................................................ 89

Results and Discussion .......................................................................... 90

3.2.1 Synthesis of P3AHT ................................................................. 90

3.2.2 Optical and Fluorescent Properties of P3AHT .......................... 92

................................................................................................ Sumrnary 98

Experimental .......................................................................................... 98

Synthesis of Di(24hienyl)furans. Di(24hienyl)benzenes

............................................................. and Conesponding Polymers 105

.......................................................................................... Introduction 105

.......................................................................... Results and Discussion 109

Synthesis and Characterization of 2.5.Di( 24hienyl)furan ......... 109

Synthesis and Characterization of 2.5.Di(2.(hexylthienyl) ).

....................................................................................... furans 113

Synthesis and Characterization of 1.4.Di( 2.

(hexylthieny1))-benzenes and 2.5.Di(2.(3.hexylthienyl) ).

................................................................................. thiophene 121

.......................................................................... Polymerization 123

................................................................................................ Summary 126

.......................................................................................... Experimental 127

................................................................................... 4.4.1 Materials 127

.......................................................................... 4.4.2 Measurements 128

..................................................... 4.4.3 Synthesis of Dithienylfurans 129

4.4.4 Pre~aration of 1 3-Di(2-(hexvlthienvlNbenzenes

.................................... and 2.5~0i(2.(3.hexylthienyl))thiophene 139

......................................................................... 4.4.5 Polymerization f42

Optical. Fluorescent, and Electrochemical Properties of

Novel Thiophene-Based Heteroaromatic Trimers and

.................................................................... Corresponding Polymers 145

Optical and Fluorescent Properties of Thiophene-Based

......................................................................... Heteroaromatic Trimers 145

Optical and Fluorescent Properties of Regiospecific Thiophene-

Based Conjugated Polyme rs .................................................................. 152

........................... Cyclic Voltammetric Study of Heteroaromatic Trimers 160

............................................... Cyclic Voltamrnetric Study of Polymers 167

................................................................................................ Summafy 174

Experimental ........................................................................................ 175

......................................................................................... Conclusions 177

...................................................................................................... References 180

List of Schemes

Scheme 1.1

Scheme 1.2

Scheme 1.3

Scheme 1.4

Scheme 1 . 5

Scheme 1.6

Scheme 1.7

Scheme 1.8

Scheme 1.9

Scheme 1.1 O

Scheme 1.1 1

Scheme 1.1 2

Scheme 1.1 3

Scheme 1.14

Scheme 1.1 5

Structure and abbreviations of representative

conjugated polymers .............................................................. 1

Mechanistic scheme for electrochemical polymerization

of thiophene via anodic oxidation ........................................... 4

Chernical oxidation and Grignard coupling methods

for polymerization of thiophene .............................................. 8

The Grignard coupling rnetfiod for P3AT preparation ............ 8

Possible diad couplings of P3ATs .......................................... 10 Possible triad couplings of P3ATs ......................................... 10

The iron chloriâe method for preparation of P3AT ................. 11

A radical mechanism for the iron chloride method ................. 12 Curtis' demerwration polyrnerization method ..................... 1 3

The McCullough method for synthesis of R-P3ATs ............... 15

The Rieke method for synthesis of R-P3ATs ......................... 17

The Grignard metathesis method for synthesis of R0P3ATs .. 19

The Suzuki couplhg method for synthesis of R-P3ATs ......... 19 1 H NMR chernical shiits in various triad linkages of

P3ATs in solutions ...................................................... 23 ............................................................................................. An

interdigitated model for grstal structure of P3ATs .................... 31 Scheme 3.1 Synthesis of poly(3-(6-acryloyloxy)hexylthiophene) ............... 93 Scheme 4.1 Synthesis of 2.5di( 24hienyl)furan (DTF) ............................... 109 Scheme 4.2 Preparation of 2-fomylthiophene ........................................... 110

.............. Scheme 4.3 Synthesis of 2.5di( 2.(hexylthienyl))furans (DHTFs) 112

..................... Scheme 4.4 Preparation of 2-formyld(or 4)-hexylthiophene 114

Scheme 4.5 Preparation of 2-bromo-4-hexylthiophene ............................. 115

Scheme 4.6 Mechanistic scheme of the Stetter reaction mediated by

................................... thiazolium salt under basic conditions 119

.................................. Scheme 4.7 Preparation of DHTBs and 3,3 '-DHTT l22

........... Scheme 4.8 Polymerization of DHT Bs, 4,4'-DHTF, and 3,3'-DHTT 123

........... Scheme 4.9 Preparation of P33DHTF via the McCullough method 125

List of Tables

Table 1.1

Table 1.2

Table 1.3

Table 1.4

Table 1.5

Table 2.1

Table 2.2

Table 2.3

Table 2.4

UV-vis absorption characteristic of P3HTs ............................. 20

13c NMR chernical shifts in different triads of P3ATs ............ 25

Thermal properties of P3ATs from DSC analysis ................. 27

Fluorescence properties of oligothiophenes in solution ........ 35

Fluorescence properties of P3HT in solution and in

solid state .............................................................................. 36

HT content. molecular weights and molecular weight

distributions of P3ATs .......................................................... 46

Thermal properties of P3ATs with various HT

regioregularity and side chain length obtained

................................................................ from DSC analysis 59

....................... XRD data for various poly(3-alkylthiophenes) 68

Representative results of thermochromic behavior of

.......................................................... P3AT films in literature 74

2di

Table 5.1

Table 5.2

Table 5.3

Table 5.4

Table 5.5

Absorption and fluorescence characteristics of thiophene

bas& heteroaromatic trimers in hexanes at room

temperature .......................................................................... 146

Absorption and fluorescence characteristics of the

polyrners in THF solution ..................................................... 154

Absorption and fluorescence characteristics of the

polymers in solid state .......................................................... 155

Oxidation potentials of regiochemically controlled

thiophene-based heteroaromatic trimers in acetonitrile

solution ................................................................................. 167

Energy levels of regiochemically controlled thiophene-

based conjugated polymers .................................................. 173

List of Figures

Figure 1 .S

Figure 2.1

Figure 2.2

Figure 2.3

Figure 2.4

Plot of d-spacing (parameter a) versus the number of

carbon atoms in the side chains of P3ATs, reproduced

from literature data ................................................................ 30

Structures and abbreviations of P3ATs prepared and

used in this work ................................................................ 45

Temperature dependence of the UV-visible absorption

spectra of a PDHBT film under nitrogen ............................... 47


spectra of a P3HT55 film under nibogen .............................. 48


xiii

Figure 2.5

Figure 2.6

Figure 2.7

Figure 2.8

Figure 2.9

Figure 2.10

Figure 2.1 t

Figure 2.12

Figure 2.1 3

Figure 2.i4

Figure 2.15

spectra of a P3HT80 film under nitrogen .............................. 49


spectra of a P30T80 film under nitrogen .............................. 50


spectra of a P3DDT70 film under nitmgen ............................ 51


spectra of a P3HDT80 film under nitrogen ............ , .... . ........ 52


spectra of a P3HT100 film under nitrogen .................... ........ 53


spectra of a P30T100 film under nitrogen ............................ 54


spectra of a P3DDT100 film under nitrogen .......................... 55


spectra of a P3HDTlOO film under nitrogen .......................... 56

Temperature dependence of absorption maxima for

P3ATs: (top) regioirregular P3ATs; (bottom)

regioregular P3ATs . . .. . . . . .. ... . .... .. . .. ... .... .. . .. .. .. . . . . . . . . . . . . . . . . . .. . . . . . 57

DSC thermograms of P3ATs: (top) P3HT80 and P3HT100;

(bottom) P30T 80 and P3OTlOO .......................................... 60

DSC thermograms of P3ATs: (top) P3HT80 and P3HT100;

(bottom) P30T 80 and P30T100 .........................+................ 61

Temperature dependence of XRD spectra of P3HT80

and P3HT100 films cast from solution

(baselines of the wrves are offset for clarity) ................ .... ... 64

xiv

Figure 2.16

Figure 2.17

Figure 2.18

Figure 2.19

Figure 2.20

Figure 2.21

Figure 2.22

Figure 2.23

Figure 2.24

Figure 3.1

Figure 3.2

Temperature dependence of XRD spectra of a P30T100

film (baselines of the curves are offset for clarity) ................ 65

Temperature dependence of XRD spectra of a P3DDT100

film (baselines of the curves are offset for clarity) ................ 65

Temperature dependence of XRD spectra of a P3HDT100

film (baselines of the cwes are offset for clarity) ................ 66

Temperature dependence of: (a) the lattice spacing and

(b) the crystallite size for P3HT8O and P3HT1 O0 ............... .. .67

Plot of d-spacing vs alkyl side chain length of R-P3ATs

n = number of carôon atoms in the side chain .................... .. 69

Temperature dependence of: (a) the lattice spacing and

(b) the crystallite size for P3HDT80 and P3HDT100 ............. 72

Schematic representation of: (a) crystalline, (b) quasi-

ordered, and (c) disordered phases of P3ATs

(viewing atong the thiophene chain) .............................. ... . 7 8

Therrno-morphologicai transitions for R-P3ATs: (a) direct

transition from the crystalline to the disordered phase;

(b) gradua1 increase in the lattice spacing. Parallel lines

represent x-stacked polymer chains .................................... 82

Therrno-morphological transition for regio-irregular P3ATs

depicting the interconversion between quasisrdered and

disordered phases. Parallel lines represent x-stacked

polymer chains ................................................................... 83

UV-visible spectra of P3AHT in solution and in solid state ... 94


XV

Figure 3.3

Figure 3.4

Figure 3.5

Figure 4.1

Figure 4.2

Figure 5.1

Figure 5.2

Figure 5.3

Figure 5.4

Figure 5.5

Figure 5.6

spectra of P3AHT film (first heating cycle) ............................ 95


spectra of P3AHT film (second heating cycle) ...................... 97

Fluorescence emission spectrum of P3AHT in chlorofom ... 98

Fluorescence emission spectra d P3AHT film: (a)

uncmsslinked; (b) crosslinked at 200°C ................................ 99

Heteroaromatic trimers synthesized and studied in

this work ................................................................................ 107

Structure and abbreviations of polymers synthesized and

studied in Chapters 4 and 5 ................................................. 108

Nomalized fluorescence excitation and emission spectra

d a-3T and 33-DHfT .......................................................... 147

Nomalized fluorescence exception and emission spectra

........................................ of DTÇ, 3,3'-DHTF and 4,4'-DHTF 147

Normalired fluorescence excitation and emission spectra

............................................... of 3,3'-OHTB and 4,4'-DHTB 148

Energy diagram illustrating configuration rearrangernent

of thiophene-based trimers and polymers upon photo-

excitation. FC and eq standard for Franck-Condon

....... and equilibrium states, respectively. X = S, O, HC=CH. 149

Nomalized UV-vis absorption spectra of regiochernically-

........ wntmlled thiophene-based polyrners in THF solution.. 152

Nomalized UV-vis absorption spectra of regiochemically-

.............. controlled thiophenebased polymers in solid state 153

Figure 5.7

Figure 5.8

Figure 5.9

Figure 5.10

Figure 5.1 1

Figure 5.12

Figure 5.13

Figure 5.14

Figure 5.15

Figure 5.16

Figure 5.17

Figure 5.18

Normalized fluorescence emission spectra of

regiochemically-controlled thiophene-based

polymers in THF solution ...................................................... 156

Normalized fluorescence emission spectra of

reg iochemicall y-controlled th iophene-based

polyrners in solid state .......................................................... 158

Cyclic voltammogram of a 5 mM a-terthiophene (a-3T)

solution in 0.1 M LiClOdacetonitrile ...................................... 160

Cyclic voltammogram of a 5 mM 3,3'-DHTT solution

.................................................... in 0.1 M LiCIO~/acetonitnle 162

Cyclic voltammogram of a 5 mM DTF solution

in 0.1 M LiCIOdacetonitrilel25% (vlv) H20 ............................ 163

Cyclic voltammogram of 3,3'-DHTF solution

in O. 1 M LiCIOdacetonitrile ................................................... 164

Cyclic voltammogram of 4,4'-DHTF solution

.................................................... in 0.1 M LiCIOdacetonitrile 165

Cydic voltammogram of 3,3'-DHTB solution

................................................... in 0.1 M LiClOdacetonitrile 165

Cyclic voltammogram of 4,4'-DHTB solution

in 0.1 M LiCIOdacetonitrile .................................................. 166

Cyclic voltammogram of a spin-cast film of P33DHTT on

platinum electrode in 0.5 M LiClOdacetonitrile ..................... 168

Cyclic voltammogram of a spin-cast film of P33DHTF on


Cyclic voltammogram of a spin-cast film of P44DHTF on

xvü

platinum electde in 0.5 M LiClOdacetonitrile ..................... 170

Figure 5.19 Cyclic voltammogram of a spin-cast film of P33DHTB on


Figure 5.20 Cyclic voltammogram of a spin-cast film of P44DHTB on


List of Abbrevistions

3,3'-DHTB

3,3'-DHTF

3,3'-DHTT

4,4'-DHTB

4,4'-DHTF

abs.

BT

cv

DHTB

DHTF

DMF

~ P P P

DTB

DTF

EL

1,4di(2-(3-hexy1thienyt))benzene

2,5di(2-(3-hexylthienyl))furan

2,Sdi(2-(3-hexylthien yl))tttiophene

1,44i(2-(4-hexyîthienyi))benzene

2,Sdi(2-(4-hexyithieny1))furan

absorption

bithiophene

cyclic voltammetry, cyclic voltammogram

1,4di(2-(hexylthieny1))benzene

2,5di(2-(hexyit hienyl))furan

N, Ndimethylformamide

(dipheny1phosphino)propane

1,4di(24hienyI)benzene

2,5di(24hienyl)furan

electroluminescence

em.

eq*

FC

HH

HT

IC

ISC

LDA

LED

Mn

MW

00

P33DHTB

P33DHTF

P33DHlT

P3AT

P3BT

P3DDT

emission

equilibrium

Franck-Condon

head-to-head

heat-to-tail

intemal conversion

intersystem crossing

lithium diiospropyiamine

light emitting diode, light emitting devices

nurnber average molecular weight

weight average molecular weight

optical density

poly(i,4-di(2-(3-hexyithienyl))benzene)

poly(2,S-di(2-(3-hexylthienyt))furan)

poly(2,5-di(2-(3-hexylthienyl))thiophene)

poly(3-alkylthiophene)

poly(3-butylthiophene)

paly(3-dodecylthiophene)

P3DOTl00 ploy(34odecylthbphene) with 100% HT diad content

P3DDTïO ploy(3-dodecylthiophene) with 70% H f diad content

P3DST poly(3-docosylthiophene)

P3DT poly(3decylthiophene)

P3HDT

P3HDT1 O0

P3HDT80

P3HT

P3HT1 O0

P3HT55

P3HT80

P3MT

P30T

P3OTlOO

P30T80

P440HT6

P44DHTF

PA

PPP

PANi

PDABT

PDHBT

PDHTB

PDHTF

PF

PL

poly(3-hexadecylthiophene)

ploy(3-hexadecylthiophene) with 100% HT diad content

ploy(3-hexadecylthiophene) with 80% HT diad content

ploy(3-hexylthiophene)

ploy(3-hexylthiophene) with 100% HT diad content

ploy(3-hexylthiophene) with 55% H l diad content

ploy(3-hexylthiophene) with 80% H l diad content

poly(3-methylthiophene)

poly(3octylt hiop hene)

ploy(3-octylthiophene) with 100% HT diad content

ploy(3~thiophene) with 80% HT diad content

poly(l,4di(2-(4-hexy1thienyl))benzene)

poly(2,5di(2-(4-hexyl thieny1))furan)

polyacety lene

poly(ppheny1ene)

polyaniline

paly(3.3'dialkyl-2,2'-bithiophene)

poly(3,3'dihexyl-2,2'-bithiophene)

poly(l,4-di(2-(hexylthienyl)) benzene)

poiy(2,5-di(2-(hexylthienyl))furan)

polyfuran

photoluminescence

PPF

PPS

PPT

PPV

TFT

T g

THF

TMEDA

VPO

poly(pheny1ene-CO-furan)

poly(phenylene sulfde)

poly(pheny1ene-CO-thiophene)

poly(phenylene vinyiene)

PO~Y PY Kole

polythiophene

regioregular poly(3-al kylthiop hene)

thin film transistor

glass transition temperature

tetrahydrofuran

melting temperature of main chain ordering

tetramethylethylenediamine

melting temperature of side chah ordering

tail-to-tail

vapor pressure osometry

heat of fusion for main chah crystatlinity

heat of fusion for side chain crystaltinity

fluorescence quantum yield

intersystem crossing quantum yield

a-terthiophene

absorption maximum

hm, (em.) emission maximum

Chapter 1

General Introduction

Conjugated polymers are a class of polymers possessing an extended n-

conjugated system, i.e., double and single bonds alternating along the polyrner

backbone. In the literature, conjugated polymers are often refened to as

"conducting or conductive polymersn; due to their ability of conducting electricity

when partidly oxidized or reduced.1-14 Although polyaniline (PANi, see Scheme

PPV PPS 1 R

PPP

PANi

Scheme 1.1 Stnicture and abbreviations of

representative conjugated polyrners

1.1) was Rrst prepared in 1862.15 it was polyacetylene (PA) which actually

launched this new area of research. The first PA film was prepared by Shirakawa

and coworkers in 1974. using a Ziegler-Natta-type ~atal~st.16 The silvery-like

pristine PA fiim is virtually an insulator with electric wnductivity varying fmrn los

S cm-' for cis-PA to loJ S cm-' for trans-PA. Partial oxidation (doping) of either

PA isomer with 12, FeCI3, AsF5, or other electron-accepting species renders the

polymer with metallic properties, including an increase in conductivity of 10

orders of magnitude.17f18 It has been demonstrated that a stretched PA film

doped with AsFs shows an initial conductivity of, 10' S crne'.19 In the wake of

this pioneering work, world wide research efforts have resulted in the discovery

of a whole class of conjugated polymers with unique electric, electronic, and

electrooptical properties. Listed in Scheme 1.1 are some tepresentalie

rnernbers of this family, i.e., PA, poly@-phenylene) (PPP), poly(pheny1ene

vinylene) (PPV) and derivatives, polythiophene (PT), poly(3-alkythiophene)

(P3AT) and derivatives, poly(pheny1ene sulfide) (PPS), polypyrrole (PPy},

polyfuran (PF). and polyaniline PAN^).^.^^ These materials have practical

andlor potential applications in light emitting diodes ( L E D S ) ? ~ , ~ ~ thin film

transistors ( T F T S ) ~ ~ rechargeable batteries. electrochromic devices.23

nonlinear optical materials, photolithography.24-26 micmsensors~7 and

antistatic coatings.1.11,12

Since the mid 198OFs, P3ATs and various derivatives have attracted much

research attention owing to their ease in preparation, ready solubility in common

organic solvents and good eledronic pmperties.g-ll Many P3ATs and other

substitutsd PT denvatives have been synthesized, characterized, and their

properties investigated.

The objectives of mis work are to develop novel thiophene based

plyrners with tunable band gap, high luminescent efficiency, and other unique

properties. Therefore, the synthesis, characterization, and properties of PT and

P3ATs pertaining to this research will be addressed briefly in the following

sections.

1.1 Electrochemical Polymerization of Thiophene and 3-Alkythiophenes

Tourillon and Garnier first applied the anodic oxidation rnethod to prepare

PT and pdy(3-meth ylthiophene) ( ~ 3 ~ ~ ) . 2 8 W hen thiophene or 3-

methylthiop hene is electmchemically oxidized in acetonitriie using

tetrabutylamrnonium fluomborate as suppotting electrolyte, a thin polymer film is

formed on the platinum electrode. The as-prepared fdms are highly conductive

because the polymers are doped with the anions of the supporting electrolyte. IR

and NMR analysis indicated that thienylene moieties are mainly coupled by

aa'-linkages.28-30 However, a nonnegligible amount of a$'- coupling has been

deteded by IR and XPS analysis.3~

Scheme 1.2 Mechanistic scheme for electrochemical

polymerization of thiophene via anodic oxidation

It is generally accepted that anodic electrochemical polymerization of

thiophene and its denvatives proceed via a radical cationic mechanism as shown

in Scheme 1.2.617.9.31 In the first step of polyrnerkation. an electron is removed

from the monomer to fom the corresponding radical cation. Since the electron 4

transfer reaction is much faster than the diffusion of monomer from the bulk

solution, a high concentration of radical cation is generated and maintained near

the surface of electrode. In the second step, two radical cations undergo a

coupling reaction to fom a dihydro dimeric dication as shown in reaction (a).

Alternatively, The radical cation can react with a neutral monomer through

reaction (b) to yield a dimeric radical cation, which then loses an electron to

afford the same dication. The dication tends to split off two a-protons and

rearomatizes to yield a neutral dimer. The driving force for mis step is the

reammatization. The dimers, possessing a lower oxidizing potential than the

monomers, are more easily oxidized to the corresponding radical cations, which

then further couple with either a monomeric radical cation or a neutral monomer.

This procedure repeats until the polymer becomes insoluble and precipiîates on

the electrode surface.

Many efforts to optimize the electropolymerization of thiophene and its

derivatives have been made since Touriilon and Garnier's first repc~rt.69799~32 In

general, PTs with high electroactivity are obtained in rigorously deoxygenated,

anhydrous aprotic solvents with high dielectric constant and low nucleophilicity,

such as acetonitrile, nitrobenzene, benzonitrile, and propylene carbonate. Other

experimental variables such as monomer concentration, reaction temperature,

applied electrical conditions, supporting electrolytes, electrode materials and

configuration also exert significant effects on the structure and electrochemical

properties of PT films.

Sato and c ~ w o r k e r s 3 3 ~ ~ documented the electrochemical preparation of

poly(3dodecylthiophene) (P3DDT), the first wnjugated polymer that is soluble in

wmmon organic solvents such as chloroform and toluene. Hotta and other

research groups then conducted systematic studies on the electrochernical

preparation of poly(5alkylthiophenes) (~3~~s).617,9,35.36 Introduction of a

flexible alkyl side chain into the PT backbone significantly irnproves the solubility

of the polymers. P3ATs with butyl or longer alkyl groups are soluble in common

organic solvents, while their electroactivity remains in similar to that of the parent

PT. These findings, together with the discovery of various chernical methods for

preparation of P3AT and derivatives (vide infra), have attracted increasing

interest in these polymers.

Comparative studies have show that the polymerization potential

decreases from thiophene, bithiophene (BT), to terthiophene (u-3~).37138

However, the resulting polymer showed a decrease in conjugation length, and

hence conductivities, from PT to polyterthiophene (P3T). This result has been

attributed to the irregular intern'ng couplings in P3T. An increase in conjugation

length of starting materials results in a decrease in the relative reactivity of a-

positions over P-positions. fherefom, the content of the deleterious u$'-

couplings increases frorn PT to P3T. Tefthiophenes with alkyl or phenyl

substituents on the central thienylene ring were synthesized and their

electrochemical polymerization have b e n compared with the parent 3i.39.40

Electropolyrnerization of thiophene based heteroaromatic trimers, such as 2,s-

6

di(24hienyl)furan (DTF), 2,5di(Zœthienyi)pyrroIe (DTP), 1,4di(2-thieny1)benzene

(DTB), have been pursued and mmpared to a-3T by several research groups

(vide infra) . 4 1 4

1.2 Chemical Preparation of PT and P3ATs

Electropolymerization of thiophene and derivatives provides a convenient

and rapid method of preparing PT and derivatives. It is especially useful in

preparing freestanding films of insoluble PTs. One of the shortcomings of

electropolymerization is its limitation in large scale preparation. Therefore,

intense efforts in developing methds for chemical preparation of PTs and P3ATs

have been warranted since the early 1980's.

1.2.1 Chemical Preparation of PT

The first chemical preparations of PT involve a metal catalyzed Grignard

coupling of 2-halo-5magnesiohalothiophene~~~48~49 via an extended Kumada

coupling reactionso This method was developed independently by two research

groups (Scheme 1.3 (b)). Treatrnent of 2,5dihalothiophene with Mg in

anhydrous THF results in the formation of 2-halo-5-magnesiohalothiophene,

followed by cross-coupling in the presence of a metal or metal complex, e.g. Ni

or Ni(dppp)C12 (dppp = 1,3diphenylphosphinopropane, affords PT.

(a) Chemical Oxidation

1

(b) Grignard Coupling (X = Br, 1)

Scheme 1.3 Chemical oxidation and Grignard

wupiing rnethods for polymerization of thiophene

An alternative way for the chernical preparation of PT is shown in Scheme

1.3 (a).51 Yashino and m-workers reported that thiophene can be chemically

oxidized by iron (III) chloride to afford PT. This polymerization is believed to

proceed via a radical or a radical cation mechanism (vide infra).

Scheme 1.4 The Grignard coupling method for P3AT preparation

1.2.2 Chernical Preparation of P3ATs

1.2.2.1 Grignard Coupling Method

The aforementioned methds for chemical preparation of PTs were then

extended to prepare P3ATs. Elsenbaumer and co-workers reported the first

chemical preparation of soluble P3ATs via the extended Kumada coupling

reaction as shown in Scheme 1.4.52953 When a THF solution of 2,5-diiodo-3-

alkylthiophene (4) is treated with one equivalent of magnesium, a mixture of 2-

iodo-5-magnesioiodo-3-alkylthiophene and 5-iodo-2-rnagnesioiodo-3-

alkylthiophene is formed. After addition of a catalytic amount of nickel catalyst,

the Grignard species cross-couple with each other to afford the corresponding

P3AT. The molecular weights of Elsenbaumer's P3ATs are relatively low,

ranging from 3000 to 8000. A later report by Chen and Tsai has shown that high

molecular weights can be achieved by optimizing reaction conditions.54 Only

a,al-couplings are presented in P3ATs prepared by this method, owing to the

nature of the reaction. However, coupling of Ssubstituted thiophenes via the 2-

and 5-positions leads to three different regiochemical dimeric units, Le., head-to-

tail (Hf), head-to-head (HH), and tail-to-tail (TT) diads as shown in Scheme

1.5.55 These diads lead to four possible triad units, Le., head-to-tail-to-head-to-

tail (Hf-HT), head-to-tail-to-head-to-head (HT-HH), tail-to-tail-to-head-to-tail (TT-

HT), and tail-to-tail-to-head-to-head (TT-HH) triads as shown in Scheme 1.6. The

electrical and optical properties of P3ATs are largely controlled by the

regioregularity of the polyrners (vide infra). Holdcroft and co-workers have show

9

that the regioregularity, Le., HT diad content, of P3ATs prepared by the Grignard

coupling method can be tuned by varying the polymerization conditions and the

reaction times.55156

Head-to-Tail (HT)

Scheme 1.5

* R Rk R

Head-to-Head (HH) Tail-to-Tail (TT)

Possible diad cou plings of P3ATs

oRQ \ 1 dR (-J+Ro R R

HT-HT HT-HH

TT-HT TT-HH

Scheme 1.6 Possible triad couplings of P3ATs

1.2.2.2 Chemical Oxidation or lron Chloride Method

Scheme 1.7 The iron chloride method for preparation of P3AT

Sugimoto and coworkers have shown that 3-alkylthiophenes can be ready

polymerized by transition metal halides, such as FeCI3, MoC~~, and RuCl3 as

shown in Scheme 1.7.57 This method is generally referred to as the iron chloride

method and is the most widely used method for preparation of P3ATs. In a

typical experiment, a solution of 1 equivafent of 3-alkylthiophene is added into a

suspension of 4 equivalents of FeCI3 in CHCI3 or other appropriate solvents. The

reaction mixture is stirred at room temperature under nitrogen for 2 hours or

longer, and then poured into methanol to quench the polyrnerization. The

resulting black precipitates are P3ATs doped with Fe& which are subsequently

reduced by aqueous amrnonia. P3ATs with alkyî chains longer than four carbon

atoms prepared by the iron chloride method are fully soluble in cornrnon organic

solvents, such as CHC13, THF, and bluene.57-60 The iron chloride method

affords P3ATs with good yield (- 70%) and retatively high molecular weight with

Mn up b 300,000.58 It has b e n show that thienylene moieties are primarily

coupled at %a'-positions. Elongated polyrnerization may result in some irregular

[email protected] HT diad content of the polymers varies with reaction conditions.

In general, oxidation of 3-alkylthiophene by FeCI3 in CHCI3 solution gives rise to

P3ATs with - 80% HT diad content.6143 One of the major shortwmings of the

iron chloride method is that the resulting P3ATs contain non-negligible amounts

of a residual iron impurity, which may significantly affect the photochernical and

optoeledronic pmperties.48,49 Holdcroft and CO-workers have demonstrated that

the concentration of residual iron impurity varies with the purification

procedure.62 The iron content of a P3HT sample was found to be 0.15 mol%,

even after extensive purification.@

Scheme 1.8 A radical mechanism for the iron chloride method

The imn chloride pdymerization is a heterogeneous reaction, The active

sites have been found to be iron (III) ions on the crystal surface. Soluble iron

chloride is ineft.64.65 A plausible mechanism for this polyrnerization is the radical

cation pathway as illustrated by Scheme 1.2 for elecîropolyrnerization of

thiophene derivatives. However, Niemi and co-workers have argued that the

polyrnerization might pmceed through a free radical mechanism as illustrated by

Scheme 1.8.65 In the free radical mechanism, the Crst step involves a

heterogeneous electron transfer reaction between monomer and crystal iron (III)

ion. The monomeric radical cation produced loses a proton to afford thé

corresponding radical, which reacts with another monomer to form a dimeric

radical. This process proceeds until a long chain polymer is formed and

precipitates out of the solution.

1.2.2.3 Curtis' Demercuration Polymerization

Scherne 1.9 Curtis' demercuration polymerkation method

Curtis and coworkerç66~67 have developed a new preparation method for

P3ATs based on the Pd-catalyzed, reductive coupling reaction of 23-

13

bis(chloromercurio)-3-alkylthiophenes as shown in Scheme 1.9. The Curtis

method ensures only %a'-coupling between thienylene moieties. The most

appealing feature of this method is its tolerance to electrophilic groups, such as

carbonyls, esters, and nitriles. This method has also been extended ta prepare

poly(3-al kylthienyl ketones).67168

1.2.3 Synthesis of Regioregular P3ATs

Electrochemical and conventional chemical polymerization methods as

described previously give rise to P3ATs with Ca. 60 - 80% HT diad content. The

HH and T i diad defects present in the polymers strongly affect their electronic

and electrooptical properties by disnipting the conjugation and preventing the

polymer chains from close packing in the solid state. Methodologies for

preparation of regioregular HT coupled P3ATs (R-P3ATs) have been widely

pursued. Two different methods based on organometallic coupling reactions, as

illustrated in Schemes 1.10 and 1.11, were reported independently by two

* 69-72 and ~ieke's.73-76 in the early 1990's. research groups, Le., McCullough s

1.2.3.1 The McCullough Method

A one-pot, rnulti-step procedure for preparation of R-P3ATs (Scheme

1 . I O ) was reported by McCullough and Lowe in 1992.69 This method is based on

the extended Kumada cross-coupling reaction of 2-bromo-5-(magnesiobromo)3-

alkylthiophene (9).50977 In the first step of this approach, 2-bromo-3-

14

alkyithiophene (7) with very high purity (free of 2-bromo-4alkyl-içomer) is

selectively lithiated at the Scposition by lithium diisopropyiamine (LDA) at

cryogenic temperatures to afford the cotresponding 2-bromo-3-alkyl-5-

lithiothiophene (8). Organotithium intermediate 8, which is stable at cryogenic

temperatures, is then converted to Grignard reagent 9 by reacting with

recrystallized MgBr2Et20. In situ treatment of 9 with a catalytic amount of

Ni(dppp)Cla gives rise to the desired R-P3AT. The nickel mediated cross-coupling

of Grignard reagents is believed to proceed thmugh a mechanism cbmprised of

the following steps: (1) oxidative-addition of an aryl halide to the metal catalyst.

(2) transrnetalation between the catalyst cornplex formed in step one and a

reactive Grignard reagent to fom a diorganometallic complex, and (3) reductive-

elimination of the coupled dimeric produd and regeneration of the catalyst.78a

The ratedeterrnining step is believed to be the transmetalation reaction.

N ~ ( ~ P P P ) C ~ _ BrMg Br -5 OC to r.t.

ovemight

Scheme 1.10 The McCullough method for synthesis of R-P3ATs

The whole procedure usually takes from 18 to 36 hours with moderate to

good yields (40 - 70%). R-P3ATs prepared by this approach contain 95% - 98%

HT-HT couplings. The number averaged molecular weight (Mn) of the

McCullough's R-P3ATs ranges from 1OK to 40K with polydispersity index (PDI)

ranging from 1.4 to 2.0.

This approach is suitable for making R-P3ATs in gram scale (10 mmol

scale of starting materials). Homopolymers and copolymers of 3-alkylthiophenes

with different length of alkyl chains (alkyl = methyl, butyl, hexyl, octyl, decyl,

dodecyl, etc.), 3-alkoxylthiophenes, and other 3-substituted-thiophenes with

functional groups inert to organometallic reagents have been prepared by the

McCullough approach.24,25,6g-72.75181-84 Major drawbacks for this method

include the requirements for highly purified starting materials and ctyogenic

reaction ternperatures.

1.2.3.2 The Rieke Method

An alternative method for preparation of R-P3ATs was reported at about

Me same time by Rieke and coworkers73~6 as shown in Scheme Id l . At

cryogenic ternperatures, one equivalent of highly reactive Rieke Zinc undergoes

a regioselective oxidative addition to 2,5-dibrorno-9alkylthiophene (IO) to afford

2-bromo-5-bromozincio-3-alkylthiophene ( l ia) as major product and 5-bromo-2-

bromozincio-3-alkylthiophene (11 b) as the minor product. This reaction is almost

quantitative. No 2,s-bisbromozincio-3-alkylthiophene is fomd. The Rieke Zinc is

prepared by reacting Zn& with lithium in the presence of a catalytic amount of

naphthalene and it is used right away. In situ treatment of 11 with an

organometallic catalyst affords the corresponding P3AT. Interestingly, it has been

found that the regioregularity of the P3AT proâuœd is controlled by the nature of

the catalyst: Ni(dppp)C12 gives rise to a regiospecific R-P3AT. while Pd(PPh&

Regiorandom P3AT

R-P3AT

Scheme 1.11 The Rieke method for synthesis of R-P3ATs

yields a totally regiorandom polymer. In general, catalysts with smaller rnetal

cations and larger ligands favor the formation of P3ATs with higher HT

regioregularity, while catalysts with larger metal cations and smaller ligands favor

the fonnation regiorandom P3ATs. It is, therefore, proposed that the degree of

HT regioregularity is wntrolled by steric congestion of the rate determining

transmetalation step.

P3ATs with different alkyl side chains and poly(3-al kyithiothiophenes) with

different alkylthio chains have been prepared by this rneth0d.~3-~6,85 It has

been reported that the Rieke method affords P3ATs in high to very high yields.

lsolated yields range from 70 to 80% after extensive Soxhlet extraction with

hexanes. Number average molecular weights of P3ATs prepared by this method

range between 25K to 35K with PD1 ranging from 1.1 - 1 S. R-P3ATs prepared by

Rieke's method are comparable with those prepared by McCullough's method.

No spectroscopie difference have been identified between Rieke's and

McCullough's R-P3ATs. Major drawbacks for the Rieke's method include the

utilization of highly reactive Rieke Zinc and cryogenic reaction temperatures.

1.2.3.3 Other Methods for Preparation of R-P3ATs

More recently, McCullough and wworkerseb reported a wnvenient way of

preparing R-P3ATs based on a Grignard metathesis reaction as shown in

Scheme 1.12. When 2,s-dibromo-3-alkyithiophene (1 0) is treated with one

equivalent of methylmagnesium bromide or other Grignard reagents in refluxing

THF, a Grignard metathesis reaction takes place to afford a 80:20 mixture of

regio-isomers of 12 (80% of 2-bromo-5-bromomagenesio-3-alkyithiophene and

Br (0 Br - Br MgBr S S S

10 12 R-P3AT

(i) R9MgBr/THFlreflux, 1 hr; (ii) Ni(dppp)Cl2Ireflux, 2 hr

Scheme 1.12 The Grignard methathesis method for synthesis of R-P3ATs

20% of 5-bromo-2-bromomagnesio-3-alkylthiophene). /n situ beatment of 12

with a catalytic amount of Ni(dppp)Clz gives rise to the corresponding R-P3AT in

60-70% yield. This method provides a quick and easy way for preparing R-

P3ATs in large scale.

(i) LDCVTHFI-~~C~~O min; (ii) B(OM~)~I-~O~C to r.t.; (iii) H', H20; (iv) 2,2-dimethyl-1 ,3-propanediollNa2S0,/Et20;

(v) Pd ( O A C ) ~ I K ~ C O ~ K H F I E ~ O H I H ~ O ~ ~ ~ ~ ~ ~ ~ - ~ 6 hr

Scheme 1.13 The Suzuki coupling method for synttiesis of R-P3ATs

Guillerez and ~idan87 have devekped a method of preparing R-P3ATs

based on the Suzuki coupling reaction as shown in Scheme 1.13. Due to the

compatibility of the stable intemediate 14 with a variety of functional groups, it is

hoped that this method may be utilized to prepare functionalized P3ATs.

1.3 Characterization of P3ATs and Derivatives

Molecular and morphological characterization of P3ATs and derivatives

has been intensively studied. A brief aceount on the UV-vis, NMR, GPC, thermal,

and X-ray diffraction characterization of P3ATs will be given in this section.

1.3.1 UV-visible Spectroscopie: Characterization of P3ATs

P3ATs possess intensive and bmad absorption bands in the UV-visible

region, indicating an extensive wonjugatian in the thienylene backbone. The

La, depends on the HT regioregularity of the polymer chain and the aggregation

state of the polymer. Listed in Table 1.1 are the &s in both solution and sdid

state for a series of P3HT samples with difFerent Hf diad contents.

Poly(3,3'dihexyl-2,2'-bithiophene) (PDHBT) is a regioregular polyrner

containing 50% each of HH and Il diads. Severe steric interactions between the

two a-methylene groups, dualrepulsive interactions between the methylenes and

the lone pair in the sp2 orbital of the suifur atom forces the adjacent thienylene

moieties to twist with resped to each other.55988 The twisted conformation

severely disnipts the K-conjugation along the thienylene skeleton. Therefore, a

20

very blue shifted A,,,, at 389 nrn is obsenred for PDHBT in chlorofom solution.88

On the other hand, normal P3HTs possess various percentages of HT diad units.

There is only a single methylene-lone pair interaction in the HT diad. Hence, a

more planar conformation is allowed. Therefote, a higher conjugation is actiieved

and a red shifted & is ohe~ed.11.55.~0,73188 The higher the H f diad

content, the higher the conjugation, hence the longer the Amax. NO significant

difference in solution Lx is obsenied for P3ATs possessing different alkyl side

chain length . & of Hf regiorandom poly(3-butylthiophene) (P3BT), P3HT,

poly(3-octylthiophene} (P30T) samples in chlomfom are al1 found to be at 428

nm. Solution Iq, for the HT regioregular P3ATs with alkyl side chains ranging

from butyl to tetradecyl are found b be amund 460 nm?3

Table 1.1 UV-vis absorption characteristics of P3HTs

HT diad content (%)

A red shift of hm,

except for P3DHBT. The

is observed when going frorn solution to solid state,

higher the Hl content, the larger the &, between

hmax

(nm)

088

389

389

O

solution

film

mm)

5073

428

438

10

6055

420

444

24

70%

434

488

54

8055

440

505

65

>9870,73

442,456

556

1 O0

solution and the solid state. This observation is attnbuted to an increase in

coplanarity of the thienylene backbone in the solid state, due to the possible z-z

stacking in the solid state.55 Spedra of non-regioregular P3AT films are broad

and structureless. The hm, red shifts with increasing HT content. R-P3AT films

possess the longest &. Vibronic structures are also observed for R-P3AT

films.11.70.73 The band edge of RP3ATs is found to be 1.7-1.8 eV, which is

significantly lower than the 2.1 eV observed for non-regioregular

~3~~~.11,69,70.73,89 Interestingly, the h.u of R-P3AT films are found to

increase with length of the alkyl side chain, indicating a higher order in polymers

with longer side chain.70.73

1.3.2 NMR Spectroscopie Characteriztition of P3ATs

The ready solubility of P3ATs and other poly(3-substituted thiophenes in

common organic solvents renders them characterizable by proton ('H) and

carbon ('k) NMR spect ro~cop~.~~~61 170173g90-94 As discussed in Setction 1.2,

P3ATs synthesized by electrochemical, chemical oxidation, and Grignard

coupling methods are al1 non-regioregular. Therefore, al1 three possible diad

units, Le., HT, TT, and HH, and al1 four possible triad units, Le., HT-HT, HT-TT,

HT-HH, and TT-HH, are present in these polymers. 'H NMR analysis of non-

regioregular P3ATs reveals two resonance peaks for the a-methylene protons at

2.8 and 2.6 ppm.55.61-90-94 These signals are assigned to the HT and HH diad

units, respectively. Only a single peak centered at 2.8 ppm is detected for R-

P3ATs prepared by the McCullough and Rieke rnethods.70173 indicating an

almost 100% HT diad content of the polymers. The ratio of H l to HH diad peaks

are found to be around 4 ta 1 for P3ATs prepared by the iron chloride method,

indicating approxirnately 80% HT diad content.7~l0~11 HT diad contents of

electrochemically pnpared P3ATs are found to depend on the polymerization

conditions. A totally regiorandom P3HT sample prepared by Rieke and

m r k e r s gives nse to two a-methylene peaks with eqwl intensity.73

HT-HT, S 6.98 HH-TT, 6 7+05

HT-HH, 6 7.03 TT-Hf, S 7.00

Scheme 1 A4 'H NMR chernical shifts in various triad

linkages of P3ATs in CDC13 solutions

In the aromatic region, four tesonance peaks centered at 6.98, 7.00, 7.02,

and 7.05 ppm are presented in spectra of non-regioregular P3ATç, owing to the

presence of four different triad units.55170173.90-94 Relative intensity of the

peaks varies with HT regioregularity of the polymers. Four peaks of equal

intensity are obsenred for regiorandom P3ATs. The most intensive peak appears

23

at 6.98 ppm for P3ATs prepared by electrochernical and iron chloride methods,

while only the 6.98 ppm peak is detected for R-P3ATs. Therefore, the 6.98 ppm

resonance is unambiguously assigned to the 4-proton on the center thienylene

moiety of the HT-HT triads. Peaks at 67.00, 7.02, and 7.05 were assigned to the

4-proton on the central thienyiene moiety of HT-HH, TH-HT, and TT-HH,

respectively by Sato and ~orii,92,93 as well as Stein and coworkers.~o~~1

Mao and ~oldcmft,55 however, have proposed a different assignment for

the HT-HH and TT-HT moieties based on a qualitative analysis of the

conformationdependent ring curent effects on the aromatic proton chemical

shifts of P3HT. The HT configuration facilitates coplanarity between adjacent

thienylene moieties, which enhances the electron delocalization among the

triads. The enhanced delocaliration when associated with the electron-donating

inductive effect of the alkyl group on the adjacent thienylene ring, results in a

resonance at higher field for the 4-proton on the central ring of the HT-HT triads.

The TT configuration also favors coplanarity between adjacent rings and electron

delocalization. The inductive effect of the alkyl group is, however, diminished

owing to the increased distance between the alkyî group and the 4-proton on the

central ring. Severe steric interactions between a-methylenes and the lone pair

of the sulfur atom forces adjacent rings out of coplanarity in the HH configuration.

The twisted confotmation significantly reduces the electron delocalization and

diminishes the inductive effect of the alkyl group on adjacent rings. Therefore, the

4-proton of the central ring on the triads containing an HH configuration

experience less deshielding and appear at lower field than those of triads

containing HT and TT configurations. On the basis of this argument, resonance

peaks at 6.98, 7.00, 7.02, and 7.05 ppm are assigned to the HT-Hf, Tl-HT, HT-

HH, and TT-HH, respectively, as shown in Scheme 1.14. These assignrnents

were then confirmed by an elegant piece of work by Barbarella and coworkers.95

Table 1.2 "C NMR chernical shift (ppm) for different triads in P3ATs

Only four sharp resonance peaks at 130.5, 139.9, 128.6, and 133.7 ppm,

assignable to C2, C3, Cd. and Cs of the Hi-HT linkage of the thienyl rings,

respectively, are obsewed in the aromatic region of 13c NMR spectra for R-

P ~ A T s . ~ O , ~ ~ A whole set of 16 peaks ranging fmm 125 ppm to 144 ppm are

present in 13c NMR spectra of non-regioregular and regiorandom P3ATs. A

c2

c3

c4

Cs

Hl-Hl

130.5

139.9

128.6

133.7

n'-Hl

128.3

142.8

126.6

135.7

Hl-HH

129.7

140.3

127.3

134.7

TT-HH

127.2

143.4

125.1

136.8

detailed assignment of these peaks has b e n given by Rieke and wworkers73

as shown in Table 1.2.

1.3.3 Molecular Weight Measurements of P3ATs

Although molecular weights of P3ATs have been measured and reported

by vapor phase osometry (vPo).~~. 'H NMR integration.55 vismmetryl light

scattering.97 and other methads. gel pemeation chromatography (GPC) remains

the most popular method. Almost al1 of the GPC molecular weights are quoted

against polystyrene standards.10~11.61.73~96 Heffner and Pearson have

reported that weight average molecular weights (MW) measured by GPC in THF

solution using a polystyrene calibration cunre are in good agreement with that

measured by light scattering. This result appears to contradict the findings by the

same authors that the rigidity of P3HT is about 2-3 times that of p0l~styrene.97

True molecular weight values for an unknown polymer sample may be

obtained by GPC using the universal calibration cunre, if the Mark-Houwink

constants of the polymer are known. ~oldcrdtM has reported that Mark-Houwink

constants of P3HT can be estirnated from intrinsic viscosity and GPC data.

Number averaged molecular weights (Mn) of P3HT determined with the universal

calibration curves agree well with the absolute Mn determined by absolute

methods. Molecular weights estimated using unmodified polystyrene calibration

curves are almost two times the absolute values.

1.3.4 Thermal Analysis of P3ATs

P3ATs are thermally and environmsntally stable. TGA analysis shows that

P30T samples are stable up to at least 300°C under a nitrogen atmosphere. In

oxygen, P30T prepared by the Grignard coupling methodolgy starts to

decornpose at 250°C, while samples prepared by the iron chloride method

decompose rapidly at 230°C, due to the presence of Fe and CI impurity.98

Table 1.3 Thermal pmperties of P3Afs fmm DSC analysis89

a Ts: melting temperature of side chain crystallinity.

AHs: heat of fusion for side chah crystallinity, measured in kJlmol of

repeating unit.

AH,: heat of fusion for main chain crystallinity, measured in kJ/mol of

repeating unit.

nlo: not observeâ up to 300°C

mm

(kJlrn~l)~

TB ("Cl m. (k~lrnol)~

P3BT

P30T

P3DDT

L ("cla

59

-7

-1 9

Tm ("Cl

56

n/od

1 50

116 6.1

3.6

3.6

Differential scanning calorimetry (DSC) analysis of P3ATs has been

punued by various research groups.89.g*-108 Listed in Tabk 1.3 are

representative DSC data reported by Chen and ~ i 8 9 for a series of P3ATs

prepared by the iron chloride method. It is evident that both Tg and Tm of P3ATs

decrease with increasing side chain length, indicating that the flexible alkyl side

chain serves as an interna1 plasticizer. The rather low heat of fusion implies that

the degree of ordering is very low for P3ATs prepared by the iron chloride

method. The themial properties are also found to be molecular weight dependent

by these and other authon.89.107 A broad peak centered at 56°C for P3DDT is

assigned to the melting of side chain crystallinity. Similar observation has k e n

made for P3ATs possessing Cs or longer alkyl side chains by other

researchers.100-103t107 Side chain crystallization has been reported as a

general phenomenon for comblike polymers containing alkyl side chains longer

than octyl group. It seems that the rigid backbone exerts strong influence on the

side chain ~~stallization.109-111 lt has been reported that regioregularity of

P3ATs affects their thermal behavior.107 However, a systematic comparative

DSC study of the influence of HT regioregularity on morphological structure has

not been reported in the literature.

1.3.5 X-Ray Diffraction Studies of P3ATs

Extensive experimental investigations and theoretical simulations of the

crystalline structure of P3ATs by using X-ray diffraction have been made since

28

late 1980'~.55-~1*-135 P3ATs are known b be only partially crystalline

consisting of an ordered and a disordered phase. The "crystallinityn of stretched

P3AT films prepared by the iron chloride method has been found to be only 10%

or less.135 Most XRD studies have been perfomed on stretched films or fibers,

due to their low degree of ordering. XRD analysis reveals a broad amorphous

halo centsred at 20 = 20 - 25" for P3ATs. In the ordered phase, XRD analysis

reveals that the thienyiene backbone adopts an anti-planar conformation that

leads to straight chains with an orthorhombic unit ce11.55,112-136 On top of the

amorphous halo, a peak at 20 = 24" with moderate to weak intensity is also

observed for P3ATs with a side chain length up to dodecyl. The peak shifts

slightly to lower angles for P3ATs with side chain longer than dodecyl group. This

peak with d-spacing of - 3.8 A has been attnbuted to the m-facial stacking of

thiophene main chain, ie., the 6-axis of the cell. û-Spacing for the c-axis is found

to be 7.8 A, indicating a fully extended anti-planar conformation of the thiophene

skeleton. A strong diffraction peak is observed at low angle. The d-spacing of

this low angle diffraction increases with alkyl side chain length as shown in

Figure 1 .l. Second and third order peaks of this low angle diffraction are also

observed for P3ATs with longer side chains. These peaks correspond to space

between tvm neighboring coplanar thiophene main chains on the same plane,

which are filled with the alkyl side chains. The existence of second and third

order diffraction peaks suggests that the alkyl side chains are packed in a higher

degree of ordering. This is consistent with the existence of side chain ordering of

29

P3ATs with side chains longer than octyi group, as revealed by DSC

Two major models for the crystalline structure of P3ATs have been

proposed based on XRD investigations. Winokur and coworkers112-115 have

interpreted their result in ternis of their model of an altemating inverse wmb

structure, in which the all-tram alkyl side chains are strongly tilted away from the

direction of the wplanar thiophene main chain.

Figure 1.1 Plot of d-spacing (parameter a) versus the number of carbon atoms

in the side chain of P3ATs, reproduced fmm literature data1281132

Scheme 1.15 An interdigitated mode1 for crystal structure of P3ATs

The dependence of d-spacing along the a-axis against the alkyl side chah

length is shown in Figure 1.1. The increment of d-spacing per CH2 unit

decreases with increasing alkyi chain length, starting from the decyl group. This

phenomenon can not be explained by the inverse comb model. An interdigitated

model is thus proposed to account for this observation as shown in Scheme

1 . 15 . 74,102,111-1 13,116,121 ,123,124 In this model, the thiophene main chains,

with the thienylene moieties linked in a planar anti-conformation, co-facially stack

3 1

to form a lamellar structure. In each layer of the larnellar structure, the alkyl side

chains, adopting rnainly all-tmns conformation, stretch between the fully

extended thiophene main chains. Partial interdigitation between the alkyl side

chains occurs for P3ATs with octyl or longer side chains.

Both inverse comb and interdigitation models assume an all-tram

confonnation for the side chains. Recent investigations, however, indicate that

thenna! agitation at room temperature may generate some gauche conformers in

the side chains.1341135

H f regioregularity of P3ATs are found ta strongly affect their

morphological structure by XRD. Poly(3,3'dialkyl-2,2'-bithiophenes) (PDABTs)

are virtually amarphous in their solid state.119.120 R-P3ATs are found to adopt a

more planar structure and possess higher crystallinity than the corresponding

non-regioregular ~ 3 ~ ~ 1 . 1 2 3 It is reported that self-assembiy occurs in R-

~ 3 ~ ~ s . 1 1 Consistent with previously reported DSC results,89.138 P3ATs with

higher molecutar weight are found to have a higher aystallinity.124

1.4 Themochromism of P3ATs

An interesting property of P3ATs is their themochmmism.~38-146 The

solution thermochromism of P3ATs was first reported by Hotta and co-

workerç.139 The solid-state themochromism of P3ATs was first reported by

Inganas' group.14*They have found that the absorption maximum of the film of

an electrochemically prepared P3HT sample blue shifts from 51 5 nm (2.41eV) at

room temperature to 41 9 nm (2.96 eV) at 190°C. Accordingly, the color of the film

changes from red-violet at room temperature to yellow at elevated temperatures.

This color change is almost fully reversible in an inert atmosphere or

vacuum. 41-143 Similar observations have subsequently been reported by other

gro~ps.~0.89.99,1 444 53 The phenomenon has been attnbuted to the twisting of

the lhiophene chain with a wbsequent decrease in the conjugation length.153 It

is generally accepted that, at low temperature, adjacent thienylene units of the

P3AT main chain adopt an antiplanar conformation with the alkyl side chains

stretching out in the same plane in an all-transplanar conformation. This planar

conformation favors a longer conjugation length and a red-shifted absorption

maximum. At elevated temperatures, the trans-planar conformation of the side

chains is less stable and a tram to gauche conversion occurs with an increase in

disarder of the side chains as the result of thermal agitation. This conversion

forces the thienylene rings along the main chain to twist with respect ta each

other, resulting in a coiled chain with shorter conjugation lengths and a

corresponding blue shift of the absorption maximum.

Detailed thermochromic analysis of P3ATs has y ielded conflicting results.

lnganis1141-143 and ~eegeh146 groups have obsewed a continuous blue shift

upon heating the P3AT samples and proposed that the polymer possesses a

multiphase morphology which changes continuously upon heating as a result of a

change in the chain conformation. In contrast, others daim that a clear

33

thermochromic isosbestic point is obsewed which implies that the polymer

possesses a two-phase morphology.99, 449 45,1 53 More recently , Leclerc and

CO-workers have proposed that the existence of an isosbestic point is due to the

formation of delocalized confornational defects ("twistonsu) which occur upon

heating147-151 as a resuk of canfonnational changes related to individual

chains.152

As discussed in previous sections, coupling of the 3-alkylthiophenes

results in three different regiochemical diads: the "head-to-tail" diad (HT), i.e.2,5'-

coupling, the "head-to-head" diad (HH), Le. 2,2'-coupling, and 'Yail-to-tail" diad

(TT), Le. 5,5'-coupling. The HT diad content of P3ATs can be tuned by employing

appropriate synthetic rnethod~.57,~0t73 To date, there have been few attempts

to integrate the regioregularity of P3ATs into models for thermochromism. This is

surprising given that many properties of P3ATs are attenuated by changing their

regioregularity. The most ment attempt to correlate regioregularity with

thermochromism was fmm Lech 's gr0up,~~7-~51 who showed that the

thermochromic conformational transition of P3ATs is strongly dependent on the

substitution pattern. However, there does not yet exist a model which integrates

al1 the thermochromic data reported to date.

1.5 Photoluminescence of Oligothiophenes and P3ATs

Thiophene itself is non-fluorescent, while oligothiophenes (T,) and

oligo(alkylthiophenes) with two or more thienyl rnoieties are al1 fluorescent.lw

34

159 Bithiophene shows a very weak fluorescence emission band centered at MO

nm. The fluorescence quantum yield of bithiophene is only about 1%. The

emission maxima, fluorescence quantum yields, and fluorescence of T,, have

been found to increase with increasing number of thienylene moieties from n = 2

to n = 5, as illustrated by Table 1.4. These parameters are found to increase

much slower when n > 5 and reach a plateau at n = I O - 12.158~160 161

Structured emission bands and large Stokes shifts are observed for T, with n * 2,

indicating the relaxed Si state possessing a more planar and more rigid

configuration (vide supra). 154-159 The fluorescent behavior of

oligo(alkylthiophenes) is affected by the substitution pattern of the alkyl side

chains.1559158

Table 1.4 Fluorescence properties of oligothiophenes in solution1541155, 161

7,

T2

1 3

T4

Ts

T6

1 7

La (em)

mm)

360

408

446

480

508

522

@f (a)

1

8

20

28,154 36161

42,154 32161

34

q (ns)

0.24

0.52

0.88

0.83

0.85

+ (%)

93

90

7 1

63

60

Stokes Shift

mm)

59

53

56

66

76

81

The radiative (kR) and nonradiative (kN) rate constants of oligothiophenes

(1, ) have been calculated from their fluorescence data. No significant alteration

of kR with chain length is obsetved. However, k~ is found to decrease

monotonically with increasing chain 1en~th.~s.1=,159 Major nonradiative decay

pathways of oligothiophenes include intersystem crossing from SI to Tr and

intemal conversion (IC). 80th ISC and IC are found to decrease with increasing

chain length.161 1% of oligothiophenes has been reported to decrease

from 93% of a-3T to 60% of T7.161

Table 1 .S Fluorescence pmperties of P3HT in solution and solid state.163

HT diad

Based on the results of the photophysi~l studies on oligothiophenes, one

would expect that P3ATs show strong fluorescence in solution. This has been

confirmed by Magnani and coworkers. They have show that ais of extensively

purified P3ATs in gmd solvents are in the range of 40%162, compared to that of

content

(%)

50 v

60

Solution Solid State (Thin Film)

( m . )

(nm)

608

608,643

Stokes

Win (nm)

1 54

1 52

( m . )

(nm)

567,600

572,600

*(%)

9

12

a (%)

0.8

0.3

SWes

Shift (nm)

188

176

9 to 14% for P3ATs reported previously.~63 This discrepancy illustrates the

importance of fluorescence quenching by impurity in P3ATs.

Regioregularity, which dictates the effective conjugation length, of P3ATs

plays an important roîe in their luminescence. Xu and Holdcroft reported that

emission maxima of P3HT samples possessing lower Hf diad content are

significantly blue shifted, owing to the double repulsive interactions between or-

methylene and the lone pair of the sulfur atom as illustrated by the data given in

Tabie 1.5.163 Similar observations have been made by Hadziioannou's and

Durochets groups.157.158.164 They have dernonstrated that fluorescence

bands of P3ATs can be tuned by controlling the effective conjugation length,

through either stereochemically induced conjugation breaks,158,164 or

copolymerization with chemically distinct blocks.157

Ws of P3HT samples in solution are also found to increase with

increasing HT diad mtent.163 ai of a conjugated system is detemined by the

relative rate constants of radiative and nonradiative decay processes. Longer

effective conjugation length leads to larger dipole and oscillator strengths, larger

fluorescence rate constant, and hence, higher quantum yields. Furthemore, one

of the nonradiative decay channels for conjugated armatic systems is torsional

vibrational relaxation. The torsional vibration is enhanceci by twisted inter-annular

conformation.165 Therefore, a lower af for P3ATs with higher HH diad content is

expected.

As aforementioned, even extensively purified R-P3ATs with almost 100%

HT diad content still show fluorescence quantum yields much lower than unity,

indicating the existence of nonradiative decay channels other than torsional

vibration. Extrapolation of 9isc of oligothiophenes to n w predicts a Oise of 0.4

- 0.5 for PTs, suggesting that intersystem crossing h m SI or higher singlet

states to Tl might be a major nonradiative decay paaiway for P3ATs. Xu and

Holdcroft have observed a long lived (z = 15 ps) photoluminescence centered at

826 nrn for thin films of P3HT at cryogenic temperatures.166-168 This band has

been attributed to a radiative, spin forbidden Tl to So transition, i-e.,

phosphorescence. This is the first report of phosphorescence for canjugated

polymers. ISC and the dynamics of the triplet state of P3ATs were then explored

by Heegets and Itok groups.1607169 These authors have show that ISC

competes effectively with fluorescence. This has been attributed to the existence

of a relatively heavy sulfur atom in the thienylene moieties, which facilitates tSC

through spinilrbit wupling. Interestingly, the ISC rate is not affected by the HT

regioregularity of the polymer.

Structured emission bands with large Stokes shifis have been obsewed

for both oligothiophenes and P3ATs as shown by the data listed in Tables 4 and

5. This has been attributed to the nuclear geometry change of the thienylene

backbone upon optical excitation.154.163 In gmnd state, the thienylene

moieties mainly adopt the aromatic form with an inter-annular bond order of -1.

Upon excitation to the Frank-Condon SI state, the nuclear geometry of the

38

thienylene moieties rearranges rapidly to the more stable quinoid forrn. The

quinoid form, or the equilibrium excited state, possesses an intrinsically higher

degree of inter-annular coplanarity, and hence a longer effective conjugation

length.

The Qf's of P3ATs in the solid state and in paor solvents are 1-2 orders of

magnitude lower than that in good solvents.160.162.163,168.169 ln mtrast to

solution, P3ATs with lower HT content show higher Of's in the solid state. ar of

fitms of poly(4.4'didecylbithiophene) is found to be one order of magnitude

higher than that of P3DDT fihs.170 9 1 of P3HT (50% HT) films is about 4 times

higher Man that of P3HT (80% HT) films (Table 5).163 No further decrease in ü+

is obserwd from P3HT (80% HT) b P3HT (100% HT) films.162 8;s of P3ATs

increase with increasing temperature in the solid state. This increase is

enhanced with increasing alkyl chain length.171 On the other hand, af's of P3AT

films decrease markedly with increasing pressure.172 The above observations

have been explained on a molecular level in terms of intermolecular quenctiing.

As revealed by XRD studies, in the solid state P3ATs CO-facially stack on top of

each other to form a lamella structure. The z-K stacking favors the formation of a

non-emissive excimer.163 The higher the HT regioregularity . the doser the

stacking, the more efficient fomiation of excimer, and hence the lower the

fluorescence quantum yields. lncreasing pressure and decreasing temperature

force P3ATs to pack more closely, therefore, reduce their fluorescence quantum

yields.

1.6 Research Objectives

Therrnochromism of P3ATs is a well-known phenomenon and a large

number of research articles on this phenomenon have been published. However,

conflicting results are presented in the literature on the thermochromism of

P3ATs. Some groups have observed a continuous blue shift of the absorption

maximum upon heating the P3AT film, suggesting a multiphase morphology,

white others have documented a clear isosbestic point, indicating a two phase

morphology (vide supra). The apparent contradiction might be related to HT

regioregularity and side chain ordering. However, there have been few attempts

to integrate the HT regioregularity and side chain ordering into models for

thermochromism. To address this issue, a systematic investigation of the

influence of HT regioregularity and side chain ordering on thermochromism and

morphological structures of P3ATs has been conducted. Thus, a series of

P3ATs (A = hexyl, octyl, dodecyl, and hexadecyl) with different HT

regioregularities were synthesized and their thennochromic behavior, thermal,

and morphological properties studied. A phenomenological model for predicting

the existence or absence of an isosbestic point is proposed and verified based

on the results of this work and from literature.

One of the most promising applications of conjugated polyrners is as

emissive materials in light emitting devices (LEDs). Two major challenges in this

application are the emission color, Le., the band-gap, tuning, and the

development of conjugated polymeric materials possessing high luminescent

efficiency in the solid state.

Currently, the band-gap tuning can be achieved by either stereochemically

induced conjugation breaksl158.Ia or copolymerization with chemically distinct

blocks.157 These methods. however, involve lengthy syntheses and result in low

yields. In this work, it is demonstrated that the band-gap of acrylated P3AT films

can be tuned by a post-synthetic step, by taking advantage of their

thermochromic properties. Thus poly(3-(6-acryloytoxy)hexylthiophene) (P3AHT)

have been synthesized and crosslinked at different temperatures. It is anticipated

that P3AHT films without crosslinking or cmsslinked at low temperature afford a

more ordered morphology, a long wavelength of absorption and emission and a

low luminescence yield. Altematively, films crosslinked at high temperature give

rise to an amorphous morphology. a shorter wavelength absorption and emission

and a higher luminescence yield.

Despite their g d solubility in common solvents, their excellent

environmental stability, and their good optical and electronic properties, the

application of P3ATs in polyrnenc LEDs has been limited, mainly due to their low

luminescence efficiency in the solid state. As discussed in Section 1.5, the

relatively low intrinsic fluorescence efficiency of P3ATs might be attributed to the

existence of a sulfur atom in the thienylene moiety. The relatively heavy sulfur

atom favors intersystem crossing, and hence reduces the fluorescence

efficiency. On the other hand, the very low efficiency in the solid state can be

attributed to the cofacial K-x stacking which promotes the formation of a non-

emissive excimer. It is, therefore, anticipated that replacing a fraction of the

thienylene moieties with groups possessing lighter atoms, e.g., phenylene and

furylene, should significantly enhance their intrinsic luminescence, while retaining

the desirable versatility of P3ATs. The solid state luminescence might be

enhanced by introducing steric constraints in a regiochemically controlled

rnanner to reduce molecular aggregation. Therefore, a series of regiochemically-

controlled 1,4-di(2-(hexylthieny1))benzenes (DHTBs), 2,5di(2-

(hexylthieny1))furans (DHTF), and 2,5-di(2-(3-hexylthieny1))thiophene (3,3'-DHTT)

and their corresponding polymers have been designed and synthesized. A

systematic study on the fluorescence efficiency of the trimers and the polymers

has been conducted. It is anticipated that this work can shine some ligtit on the

molecular design of highly luminescent conjugated polymers. It is also

anticipated that this work would result in a new class of thiophene based

conjugated polymers suitable for applications as emissive materials of LEDs. The

electrochemical properties of the trimers and the polymers are also studied.

Chapter 2

Themiochromism of Regioregular and Non-Regioregular

Poly(3-alkylthiophenes): A Phenomenological Model

2.1. Results

2.1.1. Prepamfion of Sampks

3-Alkylthiophenes (alkyl = hexyl, octyl, dodecyl, and hexadecyl) were

prepared by Grignard wupling of n-magnesiobromoalkanes with 3-

bromothiophene in the presence of Ni(dppp)C12 catalyst, following the procedure

outlined by Zimmeh The resulting 3-hexylthiophene. 3-

octylthiophene, and 3dodecylthiophene were purified by vacuum distillation. 3-

Hexadecylthiophene was purified by recrystailization h m 95% ethanol. The 3-

alkylthiophenes were characterized by 'H NMR spectroswpy and the

spectfoscopic data were found to be consistent with literature vâliies.

2-Bromo-3-alkylthiophenes (alkyl = hexyl, octyl, ddecyl, and hexadecyl)

were prepared by the selective bmrnination of the 2-position of the thienyl ring of

the conesponding 3-alkylthiophenes using bmmine in acetic acid174 or NBS in

polar salvents.175 The product mixture mntains 2-brorno-3-alkylthiophene as the

major product, some unreacted 3-alkylthiophenes, and some 2,5dibromo-3-

alkylthiophenes. Vacuum distillation (alkyl = hexyl, octyi, dodecyl), or

recrystailization fram 95% ethanol (alkyl = hexadecyl) of the product mixture

afforded pure 2-bromo-3-alkylthiophenes. The final products were characterized

by 'H NMR spectroscopy and the spectroscopic data were found to be consistent

with reported literature values.

Poly(3-alkylthiophenes) (P3ATs) containing 70 - 80% HT diad content

were prepared by chemical oxidation of 3-alkylthiophene in chloroform using

~ecl3.57 The polymers were formed in the oxidized fotm and were reduced by a

solution of triethylamine in methanol. The polymers were then further purified by

Soxhlet extraction using methanol, hexanes and methanol, consecutively.

Structure and diad content of the polymen were determined by 'H NMR

spectroscopic analysis.

Regioregular poly(3-alkylthiophenes) containing -100% HT diad content

were synthesized using the McCullough rnethod.69-72,81182 As discussed in the

previous chapter, this method involves an one-pot multi-step reaction. Reaction

of 2-bromo-3-alkylthiophene with LDA selectively lithiates the 5-position of the

thienyl ring to afford the corresponding 2-bromo-Slithiothiophene, which is then

converted to 2-bromo-5-magnesiobromothiophene by reaction with magnesium

bromide etherate. Cross coupling of the 2-bromo-5-magnesiobromothiophene by

the Kumada methods50,77 affords the desired polyrner with a reasonable yield.

No HH diad signal couM be detected by 'H NMR spectroscopy for P3ATs

synthesized by this method.

Poly(3-hexylthiophene) (P3HT)

P3HTIOO (>98% HT diad content)

P3HT80 (79% HT diad content)

P3HT55 (55% HT diad content)

Poly(3dodecylthiophene (P3DDT)

P3DDT100 (>98% HT diad content)

P3DDT70 (70% HT content)

Poly(3-octyîthiophene (P30T)

P30T100 (~98% HT diad content)

P30T80 (82% HT content)

Poly(3-hexadecylthiophene (P3HDT)

P3HDTlOO (>98% HT diad content)

P3HDT80 (84% HT content)

Poly(3,3'dihexyl-22-bithiophene) (PDHBT)

Figure 2.1 Structures and abbreviations of

P3ATs prepared and used in this work

The structure of the polymers prepared for this study and their

abbreviations are given in Figunr 2.1. The HT diad content, molecular weight

and molecular weight distribution are sumrnarized in Table 2.1.

Table 2.1 HT content, molecular weights

and molecular weight distributions of P3ATs

Sample

P3HT80

P3HT1 O0

P30T80

P3OTlOO

P3DT70

P3DT1 O0

P3HDT80

P3HDT100

2.1.2 Temperature Dependence of UV-Vls Absorption Spectra

UV-Vis absorption spectra of P3ATs with different regioregularity were

recorded from room temperature to above their melting temperature. The

temperature dependence of the electronic absorption spectnim of a PDHBT film

Wavelength (nm)

Figure 2.2 Temperature dependence of the UV-visible absorption

spectra of a PDHBT film under nitrogen

is shown in Figure 2.2. POHBT is a regioregular polymer with 50% each of HH

and lT diads content. No H f diads are present in PDHBT. Strong intra-chain

alkyî-alkyl and alkyl- sulfur atom repulsion force adjacent thienylene moieties to

twist with respect to each other. The twisted conformation significantly impairs

the conjugation along the thienylene skeleton, hence, an absorption band at

lower wavelengths (vide supra).55.88 A cast film of PDHBT shows a of 390

nm at m m temperature which only undergoes a small blue shiit with increasing

temperature. A reasonable explanation for this observation is that some localized

fluctuations in its torsional angles may exist in the already twisted confornation

47

of PDHBT. This fluctuation leads to a further reduction in its effective conjugation

length.147

Wavelength (nm)


spectra of a P3HT55 film under nitrogen

Introduction of H f diads into P3AT molecules facilitates a more planar

conformation, hence a red-shifted absorption band is observed. hm, of a P3HT55

film is found at 435 nm, 45 nm red-shifted from that of a PDHBT film. The

intensity of this band decreases with increasing temperature. A new band

centered at 400 nm emerges, and a clear isosbestic point is observed at about

430 nm.

Re lat iv e Int 0îï sit Y (a- u.)

Wavelength (nm)


spectra of a P3HT80 film under nitrogen

The ha, of P3HT films at room temperature red shifts with increasing HT

diad content. The temperature dependence of the electronic absorption spectnim

of a P3HT80 film is shown in Figure 2.4. At room temperature an absorption

maximum at 502 nm is obsenred. Two shoulders at - 550 nm and 600 nm are

also been seen. The intensities of the shoulders are apptoximately 70% and 40%

of the Lax peak, respectively. Upon heating, the intensiîy of this band decreases

and a new band emerges at - 420 nm. The 550 nm shoulder disappears at

1 30°C, Mi le the 600 nm shoulder can still be seen at 175°C. Above 200°C, only

the 420 nm band can be obsenred and, at - 450 nm, a clear isosbestic point is

observed. A two-phase morphology of the sample may be inferred fmm the

presence of an isosbestic point.

300 400 500 600 700

Wavelength (nm)


spectra of a P30T80 film under nitrogen

A film of P30T80 gives rise to an absorption maximum at 510 nm at m m

temperature (Figure 2.5). Two shoulders at - 560 nm (-70% of peak intensity)

and 610 nm (-30% intensity) are also okrved. As for the previous sample, a

Wavelength (nm)


spectra of a P3DDTïO film under nitrogen

new band centered at 420 nm appears with increasing temperature at the

expense of the 510 nm band. At temperatures higher than 150°C, only the high

energy band can be observed. Again, a clear isosbestic point is observed. The

thermochromic behavior of P3HT80 and P30T80 is essentially the same, except

that the transition between the two absorbing phases occurs at a lower

temperature for P30T80. Similar thetmochromic behavior is observed for

samples of P3DDTïO and P3HDT80, although the temperature required to

convert fmm the lower energy absorbing phase to the higher energy absorbing

phase is even lower (Figure 2.6 and Figure 2.7).

Wavelength (nm)


spectra of a P3HDTSO film under nitrogen

R-P3ATs show a different temperature dependence of the electronic

absorption spectra. Shown in Figure 2.8 and Figure 2.9 are the absorption

spectra of P3HTIOO and P30T100 films, respectively. At room temperature, the

P3HTIOO film gives rise to an absorption maximum at 520 nm, with two

shoulders at 550 nm and 600 nm, respectively. The intensities of the shoulders

are about 95% and 60% of that of the peak. The red shifted L,,, together with

existence of high intensity lower energy shoulders, implies that P3HTIOO

passesses a higher degree of ordering than its HT irregular analogues. The 550

nm shoulder disappears at - 160°C; however, the 600 nm shoulder can still be

observed at 210°C and disappears at 220°C. The absorption maximum at this

temperature occurs at 450 nm. The absorption maximum blue shifts

continuously with increasing temperature; and no isosbestic point is observed.

This observation implies a continuous decrease in conjugation length or a rnulti-

phase morphology of the sample.

300 400 500 600 700

Wavelength (nm)


spectra of a P3HTIOO film under nitrogen

400 500 600 700

Wavelength (nm)


spectra of a P3OTlOO film under nitrogen

Similar to P3HT 100 films, P30T100 films show an absorption maximum at

520 nm, together with two shoulders at - 550 nm (90% of peak intensity) and 600

nm (60% of peak intensity) (Figure 2.9). The absorption maximum of a P30T100

film continuously blue shifts fram 520 nm at room temperature to a single band

centered at 425 nm at 200°C. A mntinuous blue shift of the absorption band

upon heating can be observed up to a temperature of 170°C. Above this

temperature, a broad isosbestic point could be seen to emerge.

400 500 600 700

Wavelength (nm)


spectra of a P3ODT film under nitrogen

Shown in Figure 2.10 and Figure 2.11 are the temperature dependence

of electronic absorption spectra of P3DDT 100 and P3HDT100 films. For a film of

P3DDTlO0, an absorption maximum at 520 nm, together with two shoulders at

558 nm (90% of peak intensity) and 606 nm (50% of peak intensity), are

observed at room temperature. The absorbance of these bands decreases,

while a new band centered at 430 nm appears, and increases in intensity with

increasing temperature. A broad isosbestic point at - 460 nm is observed.

Similarly, heating a film of P3HDT100 results in a new band at 430 nm at the

55

expense of the tong wavelength absorption band centered at 528 nm. A

thermochromic isosbestic point is observed at 475 nm.

400 500 600 700

Wavelength (nm)


spectra of a P3HDT film under narogen

A convenient way to illustrate how the tegiochemistry and side chain

length affect the temperature over which thennochromic transitions occur is ta

plot the wavelength of absorption maximum, L, against the temperature

(Figure 2.12). By inspection, al1 sampies exhibited similar behavior, except tbat

P30T100, P3DDTIO0, and P3HDT100 show a sharper transition. The

temperature at the inflection point of the cuwe is taken to be indicative of the

Temperature ( O C )

Figuré 2.12 Temperature dependence of absorption maxima for P3ATs: (top)

regioirregular P3ATs; (bottom) regioregular P3ATs

temperature required to interconvert the two absorbing phases. For P3HT80,

P30T80, P3DTï0, and P3HDT00 this occurs at Ca. 180, 140, 80, and 75°C-

respectively (Figure 2.12 (top)). For regioregular samples, P3HTlOO shows a

continuous blue shift, Mile P30T100, P3DTlO0, and P3HDT100, show steep

transitions at Ca. 195, 150, and 140°C, respectively (Figure 2.12 (bottom)).

2.1.3. DIffemntial Scanning Calon'metty (WC)

DSC thennograms of the heating scan of P3ATs are shown in Figures

2.13 and 2.14; and the thermal properties of P3ATs revealed by DSC are

summarized in Table 2.2.

DSC therrnograms of P3HT80, P3HTlO0, P30T80, and P30T100

samples are shown in Figure 2.13. A firstsrder transition is not observed for

P3HT80, indicating that the sample is virtually amorphous. In contrast, an

endothermic transition is oh rved at 220°C. The enthalpy change associated

with this transition is found to be 3.4 kJImol per repeating unit for P3HTlOO. This

first-order transition is interpreted as being caused by the melting of crystallites.

The relatively small enthalpy change suggests that the sample is only semi-

crystalline.

For P30T80, a very small endothemic transition at 164°C is obsenred (

AH c 0.3 kJ1mol per repeating unit). This indicates that P30T80 film is also

formally amorphous. P30T100, however, gives rise to a first-order endothermic

transition at 175°C (AH = 2.3 kJlmole of repeating unit), indicating its semi-

crystalline nature.

a melting temperature of side chain ordering;

"elting temperature of main chain ordering;

heat of fusion for side chah ordering, measured in kJ1mol per repeating

Table 2.2 Thermal properties of P3ATs with various HT regioregularity and

side chain length obtained from DSC analyses

unit;

unit;

P3AT

P3HT80

P3HT1 O0

P30T80

P30T100

P3DDïïO

P3DDT100

P3HDT80

P3HDT100

heat of fusion for main chain ordering, measured in kJlmol per repeating

not observed.

Ts ("CI'

nloe

nlo

nlo

nlo

75

69

67,91

93

Tm mb nlo

220

1 64

175

nlo

147

126

145

AHs(kJlm~l)C

7.2

3.8

5.1

6.0

A H ~ ( ~ J / ~ O I ) ~

3.4

0.3

2.3

nlo

3.9

1 .O

4.0

1 O0 150 200

Temperature (OC)

I 1 I

1 O0 150 200

Temperature ( O C )

Figure 2.13 DSC thennograms of P3ATs: (top) P3HT80 and P3HT100;

(bottom) P30T80 and P3OTlOO

Figure 2.14 DSC thermograms of P3Als: (top) P3DDl7O and P3DDT100;

(bottom) P3HDT80 and P3HDT100

The DSC thermograms of heating scans for P3DDl70, P3DDTIO0,

P3HDT80, and P3HDTlOO samples are shown in Figure 2.14. P3DDTïO gives

rise to a first order endothennic transition peak at 75OC (AH = 7.2 kJ1mol of

repeating unit). This observation is consistent with previous DSC studies and is

attributed to the disruption of ordered side chains.891100-103,107 However, the

absence of a melting transition associated with crystallinity of the main chain

indicates that the polymer is not crystalline. For P3DDT100, first order transitions

are observed at 6g°C and 147OC, respectively. The enthalpy changes for the two

peaks are 3.8 and 3.9 kJlmol per repeating unit, respectively. The first transition

is due to the melting of ordered side chains while the second transition is

attributed to the melting of main chain crystallites. The existence of separate

melting transitions of ordered side chains and main chains is often observed for

comblike polyrners possessing alkyl side chains with eight or more carbon

atomç.89,100-1 O3,lO7,lO9,llOI 152

For P3HDT80, a very broad band with two peaks centered at 67°C and

91°C with an enthalpy change of 5.1 kJlmol per repeating unit is observed,

inferring the existence of two types of side chain crystallinity. Also, a small first

order transition at 126°C with an enthalpy change of 1 .O kJlmol per repeating unit

is observed whicb appears to be due to the melting of a small number of

crystallites. For P3HDT100, a broad peak associated with the melting of side

chain aggregates at 93°C with an enthalpy change of 6.0 kJ/mol per repeating

unit is observed. A peak associated with the rnelting of the main chain

62

crystallites at 145°C with an enthalpy change of 4.0 kJ/mol per repeating unit

indicates that P3HDT100 is semiçrystalline.

In summary, DSC analysis reveals that non-regioregular P3ATs are

fomally amorphous in their solid state, while R-P3ATs are semicrystalline. Side

chain ordering is present in both regioregular and non-regioregular P3ATs

possessing longer alkyl side chains.

2.1.4. X-Ray Dlfhcfion (XRD)

X-ray diffraction spedra were obtained for al1 samples. All the data sets

were similar in that regioregular samples exhibit sharper peaks than the non-

regioregular counterparts. A sh'i to lower angles of the diffracted peak positions

is obsenred for samples containing longer side chains. Due to their similarity,

only the temperature dependences of the diffraction spectra for P3HT80 and

P3HTlOO films are shown here (Figure 2.15). At raom temperature, diffraction

peaks corresponding to the (lm), (200), and (300) planes were observed for

both samples.55,1 14,115.1 17-1 1% 126.1 27,130.132.1 33, 176 The diffraction

intensity of P3HT100 peaks was found to be much higher than the corresponding

ones of P3HT80. In addition, P3HT80 exhibits an amorphous peak at 28 = 21"

due to an amorphous phase. Similar obsetvations were made for P30Ts,

P3DDTs, and P3HDTs. The temperature dependence of XRD spectra for

P30T100, P3DDTlOO. and P3HDTlOO are shown in Figures 2.16, 2.17 and

2.18, respectively. It is, therefore, evident that R-P3ATs possess a higher degree

10 20 30

2 - Theta Angle (Degree)

- . f i 250 O C ~ I C - - - C C C d - - - . I - - - - ---- -ri

280 O C - 1 1 I

- 10 20 30

2 - Theta Angle (degree)

Figure 2.15 Temperature dependence of XRD spectra of

P3HT80 and P3HT100 films cast from chlorofom solution (baselines of the

curves are offset for clarity)

2 6 10 14 16 22 26 30 2-Thdi Angh (dagm)

Figure 2.16 Temperature dependence of XRD spectra of aP30T100 film

(baselines of the cuwes are offset for clarity)

Figure 2.17

2 6 10 14 18 22 26 30 34

2 -n i ta Anglo (âagm)

Temperature dependence of XRD spectra of a P3DDTlOO film

(baselines of the curves are offset for clarity)

2 6 10 14 18 22 26 30 34 2-Thdi Angk (drgna)

Figure 2.18 Temperature dependence of XRD spectra of a P3HDT100 film

(baselines of the curves are offset for clarity)

of ordering than their non-regioregular ~ounter~arts.123 The peak intensity of

P3HT80 decreased with increasing temperature; and vanished at 2 1 0°C. While

the peak intensity of P3HT100 remains virtually unchanged until 230°C; and

vanished at ca. 250°C. Similar observation was made for other P3ATs, with the

peak vanishing temperature decreasing with alkyl side chain length. The

extraordinarily wide melting range observed for non-regioregular P3ATs implies

that the diffraction peaks might arise from some quashrdered aggregates, while

the relatively narrow melting range for R-P3ATs indicates the existence of real

crystallites (vide infra).

+ P3HT80: cl-value T+ P3HT100: d-value

- 7 P * 1 + PSHTBO: Crystallite Si;,

+ P3HT100: Crystallite Size

_+ f O 1 00 200

Temperature ( O C )

Figure 2.19 Temperature dependence of: (a) the lattice spacing and

(b) the crystallite sire for P3HT80 and P3HTIOO

The angular positions of the peaks were used to detemine the interlayer

spacing accotding to the standard diffraction condition.ln In Figure 2.19 the d-

spacings are plotted as a function of temperature for both regioregular and non-

regioregular P3HT samples. A similar trend was obsewed, namely, an initial

increase in spacing upon heating followed by a decrease at high temperature

where the polymer begins to melt.

Table 2.3. Room temperature XRD data for various poly(3-alkylthiophenes)

a annealed by heating to 185 O C .

a-axis.

calculated acmrding to the method given by reference. 89

Crystallite Size (nm) Experimental

P3HT100

P30T100

P3DT100

P3HDT100

Table 2.3 tabulates the rwm temperature values of the (100) peak

Calculated

positions for the 100% HT regioregular polyrners and their corresponding d-

spacings. The d-spacing values calculated h m the anti-planar, non-

interdigitated model are also listed in Table 2.3. For P3HT100 and P30T100,

the calculated d-spacing values are very close to the expenmental values. For

P3DDf 100 and P3HDT100, however, the experimental values are substantially

smaller than the calculated ones. This obsenration is manifested more deariy in

Figure 2.20, where the d-spacing is plotted against number of carbon atoms in

the alkyl side chain. The experimental d-spacing deviates downward from the

(100)

5.25

4.20

3.15

2.50a

d-value (A)b

16.8

21 .O

28.0

35.3a

d-value (A)'

17.4

21.7

30.4

39.4

13.3

22.9

120.1

-

theoretical line. The longer the alkyi side chain, the larger the deviation. This is

consistent with previous reports and supports the interdigitated

mo~e~.74,10Z,I~1-I13,116,~21,1Z3,124

I

O d-spacing (cal.) - d-spacing (exp.)

t - ! 1 -

-

-

Figure 2.20. Plot of d-spacing vs alkyl side chain length of R- P3ATs; n = number of carbon atorns in the side chain.

The angular dispersion of the diffraction peaks (FWHM) yields information

about the dimensions of the crystalline region responsible for scattering the X-

rays. The crystallite size, as it pertains to the c-axis, and for crystallite sires

smaller than 100 nm, can be calculated using the Schener formula178 shown in

Equation 2.1.

KA t = - B cos e (eq. 2.1)

where t is the crystallite size (nm), K is a constant (0.9), h is the wavelength of

the X-rays (Cu Ka, 0.15418 nm), 8 is the angular dispersion of the peak

(radians), and 8 is the peak's position in degrees. It is recognized that the

calculated crystallites are onedimensional values and do not truly reflect the

absolute sires of grstallites. Thus, they are only used to examine trends in

changes in lattice dimension. The temperature dependence of the crystallite size

for P3HT80 and P3HT1 O0 is shown in Figure 2.19. Upon heating, the crystallite

size of P3HT80, calculated from the diffracted peaks, remained constant while for

P3HT1 O0 it increased from 13 to 24 nm. The sharp decrease in crystallite size of

P3HT100 at high temperature is due to the melting of the main chain at 220°C,

consistent with the DSC results.

For the P30T100 sample (not shown), a small amorphous background

was noted in the XRD spectra. Fitted XRD spectra, at room temperature, also

indicate the presence of a broad peak at 20 = 23.9" (d-value of 3.73 A) which

corresponds to the (010) reflection and may be attributed to the interlayer

separation due to the ir - stacking of the thienyl units. The crystalline diffraction

peak associated with the (100) plane in P30T100 was shifted to lower 28 values

wmpared to P3HT100 as show in Table 2.3. The shift is not surprising since it

indicates that the inter-planar distance is larger for the octyl than for the hexyl

polymer as a wnsequence of the longer alkyl side chain. Upon heating, the

lattice spacing for P30T100 is wnsistently larger than P3HTlO0, as expected,

due to the longer alkyl chain. The spacing increased from 21 A to 23 A upon

heating from room temperature to 160°C.

10 30 50 70 90 110 130 150 170 Temperature ( O C )

Figure 2.21. Temperature dependence of: (a) the lattice spacing and

(b) the crystallite size for P3HDT80 and P3HDT100

The plot of d-spacing, (100) plane, and the corresponding crystallite sire

for P3DDTlOO as a function of temperature (not shown) indicates that both

parameters increase as a function of temperature, up to the onset of melting at

147"C, whereupon the diffraction peaks disappeared. The d-spacing increases

from 28 A to - 31.5 A and the crystallite size increases from 13 to 32 nm upon

heating from room temperature to 140°C. Thus, for this polymer, even though

melting of the side chains was obsewed by DSC (Figure 2.14), the crystallite

size still increased. This observation implies that the re-ordering of the main

chain is not accompanied by a decrease in the crystallite size but that the

crystallites' boundaries keep expanding.

Figura 2.21 shows the plots of the d-spacings and the crystallite size as a

function of temperature for samples of P3HDT80 and P3HDTIOO. A large

decrease in both of these parameters occurred for P3HDT80 at ca. 70°CC, which

coincides with the side chain rnelting transition as show by DSC rneasurements.

This suggests that intercalation of the side chains occurs at this temperature with

a consequent decrease in the size of the crystallites and the inter-lamella

spacing. Upon a further increase in the temperature, the d-value increased from

38 to 40 A and the crystallite size increased from 10 to 20 nm. The dramatic

decrease in crystallite dimensions at 70°C was not obsewed for the P3HDTIOO

sample, which may indicate that the side chains in this polymer are already in a

substantially intercalated geometry at m m temperature. This conclusion is

supported by the observation that alaiough the lattice constant for P3HDT100 is

already in the same range as for P3HDT80, Le. 36 - 42 A, the crystallite regions

do not expand upon further heating.

2.2. Discussion

The nature of the thermochromic behavior of P3ATs has led to an

apparent confiict in the literature. Yoshino's group99,144.145 reports a clear

therrnochromic isosbestic point, while Inganas' and Heeger's groups claim a

continuous blue shift with increasing temperature.141-143,146 In the late

1980's, at the time these reports were published, the scientific community paid

little attention to the effect of regioregularity on the properties of P3ATs; hence,

neither group reported the HT regularity of their samples. Inganas' samples were

prepared by nickel-catalyzed dehalogenating polycondenzation of 3-alkyl-23-

diiodothiophene, while Yoshino's samples were prepared by chemical oxidation

of 3-alkylthiophenes with Fe&. Gallaui et al. have since shown that the former

method yields P3ATs with greater than 90% HT diad content, while the latter

method yields P3ATs with - 80% HT diad c0ntent.a A careful study of Meir

results shows that Inganas 41,141-143 used PJHT films which exhibited a room

temperature absorption maximum at 516 nm (2.41 eV); indicating a > 90% H f

diad content. In contrast, samples used by ~oshinogg~144~145 exhibited an

absorption maximum at - 500 nm, indicating a HT diad content of approximately

80%. Thus the controversy in the literature appears to be related to the

difference in the HT regularity.

Table 2.4 Representative results of therrnochromic

behavior of P3AT films in literature

P3BTa

P30T

P3DT

P3DDT

Polyrnerization Reference

Method

FeCI3

FeC13

FeC13

a poly (Sbutylthiophene).

No

No

No

Yes

89,153

89

89,99,138,144,

145,148,153

FeCI3

FeCI3

Electrochemical

Grignard

Coupling

>90

>90

>90

- 1ûOd

138

99,144,145

146

141-143

Grignard

Coupling

Grignard

Coupling

Grignard

Coupling

Rieke Zinc

141

141

141

1 52

poly (3-docosylthiophene).

estimated from UV-Vis spectra.

measured by 'H NMR.

Themochromic behavior of P3AT films as reported in the literature, are

shown in Table 2.4. In this table, we classiiy the HT content of samptes as Ca.

80% or > 90% using reported UV-vis absorption data in cases where the HT

content was not reported. The results show that P3ATs with - 80% HT diad

content yield a thermochromic isosbestic point, while P3AT films possessing >

90% HT diad content yield a continuous blue shift. The only anomaly tabulated

is that a thennochromic isosbestic point was reported for P3DDT with 100% HT

diad content.

Our results confimi that P3AT samples of different HT diad content give

rise to different themochromic behavior. Specifically, P3HT80, P30T80,

P3DDTi0, P3HDT80, P3DDTIO0, and P3HDTIOO exhibit a themochromic

isosbestic point. P3HT100 yields only a continuous blue shift while P30T100

yields a continuous blue shift up to a temperature of 170°C and shows evidence

of an isosbestic point at higher temperatures. Thus, the thermochromic behavior

of P3ATs is controlled by the regioregularity and the side chain tength. As a

consequence, some assertions may be made: samples with moderate HT

content exhibit a thermochromic isosbestic point irrespective of the alkyl side

chain length. HT regioregular samples with short side chains (octyi and shorter)

yield a continuous blue shift of the electronic absorption band upon heating, while

75

H f regioregutar samples with longer side chains (dodecyl and longer) exhibit an

isosbestic point. The discussion that follows addresses the origin of these

differences.

We note that it is well known that P3ATs adopt different conformations

depending on the degree of freedom of rotation about the inter-annular bond. In

this regard, three cases can be considered:

(a) In one extrerne case, rotation is completely restricted and the

polymer adopts a rigid-rod configuration in which adjacent thienylenes are CO-

planar. Stacking of polymer chains in this configuration leads to highly crystalline

regions in whicti the alkyl side chains adopt an al1 Crans-planar

mnfomation89~9Qv~41-153v17g as depicted in Figure 2.22a. This conformation

facilitates a relatively long conjugation length, hence an absorption band at

longer wavelengths,

(b) In the other extrerne case, inter-annular bond rotation can ocwr

with little impediment, as in the case of the polymer chain in solution or in a

polymer melt. In this instance, P3AT molecules assume a wiled conformation in

which the thiophene rings are twisted with respect to each other, and the alkyl

side chains adopt a gauchconfomation. The twisted conformation possesses

a coiled thiophene chain with relaüvely shorter conjugation length, and it absorbs

at a relatively shorter wa~elength.89,99.~~~-~531~~9 In the solid state,

aggregation of P3AT molecules with the twisted conformation gives rise to a

"disoderetf or "amorphwsn phase (Figure 2.22~).

(c) A third state exists which lies in between the rigid coplanar and

flexible coi1 conformations. In this state, the inter-annular bond experiences

some degree of freedom which allows a rocking vibration or partial twisting of

adjacent thienylenes, but there is insufficient freedom for adjacent thienyîenes ta

fully rotate and adopt a coiled conformation. Such a stage may occur if there are

weak steric interactions due to head-to-head linkages or if the polymer

possesses suffident themal energy. This is the "quasi-ordered phasen

described by ~erbi's179 and ~oshino'sl38 grwps in which the chains stack in a

manner similar to the crystalline polymer but the mt interactions are weaker and

the interchain distances larger. This situation is depicted in Figure 2.22b. In the

following discussion, we propose a model which describes how the interplay and

dynamic nature of these morphologies determines the thermochromic behavior of

P3ATs.

It would appear reasonable to assume that heating a semi-crystalline

sample of poly(3-alkylthiophene) leads to an expansion of the crystal lattice due

to an increase in the vibrational energy content of the polymer. In fact, for

P3HT100 we observe an increase in the inter-lamella spacing from 16.8 to 19.1A

(14% increase) and an increase in the relative crystallite site from 13.3 to 18.3

nm (38% increase) as the temperature is incteased from 30 to 230°C (see

Figure 2.19). Since the Iattice expands with increasing temperature so too will

the average degree of twist along a polymer chain. As a consequence the

average degree of conjugation will decrease until the copianarity between

(a) Crysbline Phase

(b) Quasi.Ordered Phase

(c) Disorderd Phase

Figure 2.22 Schematic representation of (a) crystalline, (b) quasiordered, and

(c) disordered phases of P3ATs (viewing along the thiophene chain).

adjacent thienylenes is totally disnipted. Essentially, this process can be viewed

as convertirtg the crystalline phase into the disordered phase via a quasiordered

intermediate phase. Therefore, a continuously thermochromic blue-shift woutd

be observed for this multiphase rnorphology, as in fact it is, for P3HT100 and

P3OTlOO.

The thermochromic behavior of P3DDTlOO and P3HDT100 are different,

but it should be noted that both XRD and DSC analyses confimi the fact that the

rnelting temperature of the polymer decreases as the length of the side chain is

increased. For example, DSC analyses yield melting transition peaks for

P3HTIO0, P30T100, P3DDT100, and P3tiDTlûO at ca. 220, 175, 147, and

145"C, respectively. Thus, the temperature range over which P3HTlOO is

observed to exhibi a continuous blue shift is not sufficiently high to induce its

melting. P30T100 appears to undergo a continuously themochrornic blue shift

in the temperature region 25 - 170%. Above this temperature, a region in which

the polyrner melts, an isosbestic point can be obsewed. In the case of

P3DDTlOO and P3HDT100, the polymers melt at much lower temperature and

the dramatic change in the absorption spectra (Figure 2.12) coincides with the

melting temperature. Thermochmmic, DSC, and XRD data indicate that upon

heating P30DT100 and P3HDTiOO convert directly ftom the crystalline state to

the polyrner melt. Since these phases have difietent wavelengths of

maximum absorption, an isosbestic point is observed upon heating. It is also

noted that above the melting temperature, iCm, is 430 nrn k10 nm for al1

polyrners, except PDHBT, which indicates a common degree of conjugation and

a common conformation.

Given that P3HT100 and P30T100 show a continuous blue shift with

increasing temperature, it might be reasonable to assume that a longer alkyl side

chain derivatives, e.g., P3DDT100 and P3HDT100, might atso show a

continuous blue shift up until they undergo melting. However, in contrast to

P3HT100 and P30T100, plots of Lx against temperature for P3DDTlOO and

P3HDT100 show that & is constant until the melting temperature is reached

(Figure 2.12). Furthemore, XRD data indicate both the crystallite sire and

lattice spacing of P3HDT100 remain relatively constant over this temperature

range, at Ca. 15 f 1 nm and 38.5 & 0.5 A, respectively. As indicated by this and

other x-ray diffraction results, there iç a significant interdigitation of the alkyl side

chains in the crystalline phase for P3ATs possessing long side chains. 89.138

This interdigitation provides extra stabilization of the fully planar conformation

and gives rise to the crystalline structure due to the so-called "zipper effedU.l38

Therefore, upon heating at temperatures below the melting range, only a modest

blue shift of the absorption band is observed. When the melting range is

reached, the entropy-favored Crans-gauche transformation resutts in an

instantaneous twisting of the skeletal chain; thus, the crystalline phase converts

directly to the disordered phase and the quasisrdered phase is by-passed. This

two phase process yields an isosbestic point. Figure 2.23 depicl the two types

of thermo-morphological changes discussed above:

(a) A direct transition from the crystalline to the disordered phase upon

reaching the melting temperature, as observed, for P3DDT100 and P3HDT100;

(b) A gradua1 increase in the lattice spacing which enables a

continuous dismption of K-conjugation (observed for P3HTlOO and P30T100).

As indicated previously, P30T100 appears to pass between the crystalline and

quasi-ordered state before finally undergoing a melting transition at higher

temperature.

In contrast to regioregular P3ATs, samples with lower HT content, e.g.,

P3AT80s, contain a significant proportion of sterically hindered HH couplings.

Extensive crystallization of these polymers is prevented due to a twisting of

adjacent thienylene units. Samples containing a moderate percentage of HT

linkages, however, can still possess considerable coplanarity which enables

polymer chains to pack in a quasi-ordered state. There is some evidence of

partial crystalline character in some of these regioirregular samples but the

extent of crystallinity compared to the regioregular P3ATs is negligible when one

compares DSC and XRD data. Thus, it is reasonable to postulate that

regioirregular P3ATs are formally amorphous but wntain quasmrdered regions.

The quasiadered and disordered phases have been reported to CO-exist and

equilibrate with each other but at low temperatures the quasi-ordered phase

dominates.153.179 It has been reported that the relative concentrations of quasi-

ordered and disordered phases at room temperature are 55% and 45%

respectively for a sample with ca. 80% HT diad content.17g The phase

Disordered Phase

Ordered Phase

Quasi- Ordered Phase

Figure 2.23 Themo-morphological transitions for R-P3ATs: (a) direct transition

from the crystalline to the disordered phase; (b) gradua1 increase in the lattice

spacings. Parallel lines represent rr-stacked ~olymer chains

boundary between the quasi-ordered and disordered phases can be envisaged

to be somewhat indistinct.

haterfacial Boundary

Figure 2.24 Thermo-morphological transition for regio-irregular P3ATs depicting

the interconversion between quasi-ordered and disordered phases. Parallel lines

represent x-stacked polymer chains

In our model, the percentage of disordered phase increases with

increasing temperature at the expense of the quashrdered phase. When the

temperature is sufficientty high, almost al1 of the quashrdered phase has been

converted to the disordered phase. The continuous interconversion of the two

absorbing phases without an intermediate during thermal cycling is responsible

for the observed isosbestic point. The interconversion of the two phases is

from the change of crystallite sire detemined by XRD. In contrast to the 38%

increase in crystallite size for P3HT100 as the temperature is increased from 30

to 230°C (see Figure 2.19), the crystallite size associated with P3HT80 remains

constant. Since the lattice spacing of P3HT80 was found to increase by 9% over

the same temperature range (see Figure 2.19), the number of chains per

crystallite must be decreasing with increasing temperature.

2.3. Summary

The thennochromic properties of P3ATs are controlled by the head-to-tail

diad content and the alkyl side chain length of the sample. Samples with

moderate Hl diad content gave rise to a clear isosbestic point, while samples

with high H f diad content and short alkyl side chains exhibit no isosbestic point

with increasing temperature. This is due to a morphological effect. P3ATs with

moderate HT diad content are fomally amorphous with some short range

ordered structure dispersed in the disordered bulk. The coexistence and

interconversion of the two phases is believed to be responsible for the observed

isosbestic point. P3ATs with high HT diad content and short alkyl side chains are

semi-crystalline. The crystalline, quasi-ordered, and disordered phases

equilibrate with each other in the thin film. The isosbestic point is destroyed by

this multiphase equilibrium. P3ATs with high HT diad content and long alkyl side

chains are also semi-crystalline. These polymers melt at much lower temperature

and crystalline phase is converted directly into disordered phases; therefore, a

broad isosbestic point is observed.

2.4. Experimental

2.4.1. Meterials

All reagents were purchased from Sigma-Aldrich and were used as

received, unless otherwise specified. Solvents were purchased from BDH.

Diethyl ether and tetrahydrofuran (THF) for Grignard wupling reactions were

dried over sodium. Diisopropylamine was dned over calcium hydride. THF for

GPC analysis was Fisher Scientific HPLC grade reagent. Chloroform for

spectroscopie analysis was BDH spectrograde reagent.

2.4.2 Pmparation of 3-AlkyIthiophenes and 2-Brome3-al~lthiophenes

3-Alkylthiophenes (alkyl = hexyl, octyl , dodecyl, and hexadecyl). In a

typical procedure, to a suspension of 0.10 mole of magnesium in 20 mL of

anhydrous (sodium dried) diethyi ether was added dropwise 0.10 mole of n-

alkylbromide in 20 mL of anhydmus ether. After complete disappearance of

magnesium 0.080 mole of 3-bromathiophene and 60 mg of Ni(dppp)Q were

added. The reaction is slightly exothermic and a red brown coloration was

observed. After stimng and heating for 15 hours, the reaction mixture was

poured into a mixture of crushed ice and 2 M HCI and extracted from ether. The

combined ether layer was then dried over MgS04. After removal of the solvent

under reduced pressure, the residue was vacuum distilled (alkyl = hexyl, octyl,

and dodecyl), or recrystallized from ethanol (alkyl = hexadecyl) to afford the

desired 3-alkylthiophene in 60-70% yields.

3-Hexylthiophene: 'H NMR (100 MHz, CDCl3, ppm): 0.89 (3H. t), 1.2-1.8

(8H, m), 2.65 (2H, t), 6.9 - 7.3 (3H, m).

3-Octylthiophene: 'H NMR (100 MHzl CDCl3, ppm): 0.90 (3H. t), 1.2-1.8

(12H, m), 2.66 (2H, t), 6.9 - 7.3 (3H, m).

3-Dodecylthiophene: 'H NMR (100 MHz. CDClj ppm): 0.91 (3H. 1). 1.2-1.8

(20H, m), 2.64 (2H, t), 6.9 - 7.3 (3H, m).

3-Hexadecylthiophene: 'H NMR (100 MHz, CDC13, ppm): 0.90 (3H. t), 1.2-

1.7 (28H, m), 2.64 (2H, t), 6.9 - 7.3 (3H, m).

2-Brorno-3-alkylthiophenes (alkyl = hexyl, octyl, dodecyl, and

hexadecyl). In a typical reaction, to a stirring 50:50 (vlv) solution of chloroform-

acetic acid (60 mL) in a round bottom flask was added 20 mmol of 3-

alkylthiophene and 3.65 g of NBS. Reaction occurs spontaneously at room

temperature. After the NBS was dissolved, the reaction mixture was allowed to

stir at room temperature for another hour. The reaction mixture was then diluted

with 60 mL of water. The chloroform layer was separated and the aqueous layer

was extracted with chloroform (75 ml x 5). The combineci chloroform extracts

were then washed with 6 N NaOH, water, and dried over magnesium sulfate

consecutively. After removal of solvent under reduced pressure, the residue was

vacuum distilled (alkyl = hexyl, octyl, and dodecyl), or recrystaltized from ethanol

(alkyl = hexadecyl) in 70 -75% yields.

2-Bromo-3-hexylthiophene: 1 H NMR (t O0 MHz, CDC131 ppm): 0.89 (3H, t),

1.2-1.6 (8 Hl m), 2.59 (2H, t), 6.9 (IH, dl J = 5.5 Hz), 7.2 (AH, d, J = 5.5 Hz).

2-Brorn~3-octylthiophene: 1H NMR (100 MHz, CDCI3, ppm): 0.90 (3H, t),

1.2-1.6 (12 Hl m), 2.61 (2H, t), 6.9 (IH, dl J = 5.4 Hz), 7.2 (lt l , dl J = 5.4 Hz).

2-Bromo-3-dodecylthiophene: 1 H NMR (100 MHz, CDCb, ppm): 0.90 (3H,

0, i.2-1.7(20 Hl m). 2.56 (2H. 1). 6.8 (lti, d, J = 5.5 HZ). 7.2 (AH, d, J = 5.5 Hz).

2-Bromo-3-hexadecylthiophene: 1 H NMR (1 00 MHz, CDCI3, ppm): 0.89

(3H, t ) , 1.2-1.8 (28 Hl m), 2.58 (2H, t), 6.9 (IH, d, J = 5.5 Hz), 7.2 (IH, dl J = 5.5

Hz).

2.4.3 Preparafion of Poly(39lkyithiophenes)

Chemical Oxidation Method. P3ATs (A = tiexyl, octyl, dodecyl, and

hexadecyl) with Iow to moderate H f regkregularity were prepared by oxidative

coupling of the conesponding alkyithiophene using Fe&. 57 'H NMR

spectroscopy was used to determine the Hf diad content of the samples and the

HT diad contents were found to be 70 - 84%.55~6~,92-95 In a typical

experiment, poly(3-alkylthiophene) was synthesized by the following procedure.

To a stirred solution of 40 mmol of FeCI3 in 250 mL of CHCI3 purged with

nitmgen, was added I O mmol of Salkylthiophene. The mixture was stirred for 2

hours at room temperature piior to the addition of methanol whereupon a black

precipitate was obtained. The precipitate was consecutively filtered, washed with

methanol, 28% ammonia solution, and acidic methanol. A reddish solid was

obtained (50 - 60% yield) which was subsequently dried under reduced pressure.

The McCullough Method. In a typical experiment, HT regioregular P3AT

was synthesized by the following procedure. 69-72.81982.149 lnto a dry round-

bottom flask was placed 15 mm01 of dry diisopropylamine and 75 mL of freshly

distilled THF. To the mixture was added 15 mmol of n-butyllithium in hexane at

room temperature. The mixture was then cooled to - 40°C and stirred for 40

minutes. The reaction mixture containing LDA was cooled to - 78°C and 15

mmol of 2-bromo-3-alkylthiophene were added. After being stirred for 40

minutes at - 40°C, the mixture was cooled to - 60°C and 15 mmol of MgBr2.EtzO

were added. After k i n g stirred at - 60°C for 20 minutes, the reaction mixture

was allowed to warm slowly to - SOC, whereupon the MgBr2*Et20 had reacted.

Then 0.5 mol % of Ni(dppp)zClz was added and the mixture was allowed to wann

to room temperature overnight (- 18 h). The reaction was quenched by MeOH,

and solvents were removed under reduced pressure. The red residue was

subjected to Soxhlet extractions using MeOH, H20, MeOH, and hexane solvents

consecutively, in order to remove oligomers and impurities. The polymer was

then dissolved in CHCI3 using a Soxhlet extractor. Removal of solvent afforded a

30 - 40% yield of desired polymer.

PDHBT, P3HT55, P3DDT7O were prepared by Dr. Jimmy Lowe and other

previous members of Dr. Holdcroft's research group.

2.4.1 Meusumments.

'H NMR spectra were recorded on either a Bruker WP100 or a Bruker

AMX4OO instrument. Molecular weights and motecular weight distributions of

P3ATs. calibrated against poly(3-hexylthiophene) standards.96 were

characterized by gel permeation chromatography (GPC) (Waters Model 510)

using p-styragel columns at 25°C. Polymers were eluted with tetrahydrofuran at

a flow rate of 1 mumin. and detected using a UV-visible spectrophotometer

(Waters Model486) at 480 nm.

Thin films of P3ATs for UV-Vis absorption studies were cast on glass

substrates from chlorofom solution at room temperature. Optical absorption

spectral measurements were camed out using either a Hewlett-Packard Model

HP8452A diode array spectrophotometer or a Cary 3E UV-visible

spectrophotometer, equipped with a home-made temperature control cell (k 2°C).

The temperature range was varied room temperature to above the P3AT's

melting temperature. P3DTlOO and P3HDTlOO samples were annealed prior to

temperature dependence measurements. ALI measurements were perfomed

under a nitrogen atmosphere.

Differential scanning calorimetry (DSC) measurements were perforrned on

a Du Pont Model 2100 Thermal Analyst equipped with a rnodel 910 DSC unit.

The sample, typically 8 mg, was pressed in a sealed aluminum pan, and the

measurements were carried out using a heating rate of 10°Clmin. in ambient

atmosphere. The transition temperatures were reproducible to i 2"C, and the

enthalpies of the transitions were calculated by integrating the area under the

endothetmic peaks. These are reported in units of kilojoules per mole of

repeating unit.

Samples for X-ray diffraction studies were prepared by casting films on a

copper substrat8 from chloroform solution at room temperature. The X-ray

diffractions were obtained bennieen r o m temperature and above the sample's

melting point using a Siemens 05000 diffractometer with a Cu X-ray tube. The

samples were mounted horizontally in a Bragg-Brentano geometry and the data

were collected in theta-theta mode h m 2" to 35" in 0.1" intervals with a 3.6

sec./point dwell time and 1°Clmin. heating rate; thus a typical scan took

approximately 20 minutes. The peaks in each spectrum were fitted with Voigt

lineshapes using a fitting routine integrated with the operating system of the

diffractometer.

Chapter 3

Synthesis and Band-Gap Tuning of

Poly(b(6-acryloylox yhexyl)thiophene) (P3AHT)

3.1 Introduction

Devetopment of solid-state light emitting devices (LEDs) has drawn much

attention in the last decade. It is hoped that color computer screens based on

flat LED boards will eventually replace large and expensive cathode ray

tubes.180 Both inorganic and organic semiconductor based LEDs have been

developed, however, low luminescent efficiencies, difficulty in large-area

fabrication, andlor poor reliability prevent them from large-scale

app~ications.20~2~ .180

Recent advances in the field of electrotuminescence of conjugated

pdymers provide a bright future for ~ ~ ~ s . 2 0 ~ 2 1 Conjugated polymers offer a

number of advantages over conventional inorganidorganic materials. The

processibility promises a significant advantage in large-area fabrication and the

flexibility promises the fabrication of displays with unusual non-standard shapes.

Also, the inherently high radiative decay efficiency promises a greater potential

for polymeric ratkr than inorganic ~ ~ ~ s . 2 ~ 1 2 ~

It is essential to control the emission color for full-color displays; thus,

much effort has been devoted in developing wnjugated polymers with tunable

ernission.2O,21.181 Altering the n-nt band gap, Le., the conjugation length, will

change their emission; thus the color perceived. Currently, the band-gap tuning

can be achieved through either stereochemically induced conjugation

breaks.158.164 or copolymerization with chemically distinct blocks.157 These

methods, however, involve lengthy syntheses and result in low yields. ln this

work, it is demonstrated that, by taking advantage of their thermochmmic

properties, the band-gap of acrylated P3AT films can be tuned by a post-

synthetic step.

3.2 Results and Discussion

3.2.1 Synthesis of P3AHT

P3AHT was prepared according the route outlined in Scheme 3.1. 345-

Hexeny1)thiophene (16) was prepared by the Grignard coupling of 6-

magnesiobromo-1-hexene with Sbromothiophene in the presence of 1,3-

bis(diphenylphosphino)propane]nickel (II) chloride (Ni(dppp)C12) as

catalyst.182,1*3 Vacuum distillation of the cnide product afiorded pure 16 in

good yield (66%). The proton NMR spectroscopie data are consistent with

Merature results.64

3-(6-Hydroxyhexy1)thiophene (17) was obtained by hydroboration of 16

following a procedure reported by ~ane.l84 The readion is almost quantitative

and about 5% of 3-(5-hydroxyhexy1)thiophene was found in the crude product

mixture. Chrornatographic purification on silica gel afforded pure 17 tee of the 5'-

isomer in high yield (99%).

Scheme 3.1 Synthesis of poly(3-(6-acryloyloxyhexyl)thiophene) ( P3AHT)

Esterification of 3-(6-hydroxyhexy1)thiophene (17) with acryloyl chloride

was carried out under a very mild condition. After dropwise addition of acryloyl

chloride into a solution of 17 in methylene chloride on an ice bath, the system

was stirred at room temperature ovemight. The solution was continuously purged

with a stream of nitrogen to remove HCI produced during the reaction process.

Chrornatographic separation of the product mixture afforded 52% of the desired

product 3-(6-acryloyloxyhexyl)thiophene (18) and 25% of recovered starting

material 17.

Polyrnerization of 18 with iron chloride in chlorofonn gave rise to 10% of

soluble poly(3-(6-acryloyloxy hexy1)thiophene) (P3AHT). The resulting P3AHT

contains - 80% of HT diad linkage as revealed by NMR analysis.

!

Q solution

31 O 41 O 51 0 61 O Wavelength (nm)

Figure 3.1 UV-visible spectra of P3AHT in solution and in solid state

3.2.2 Optical and Fluorescent Properties of P3AHT

P3AHT possesses a strong, broad, and structureless absorption band

centered at 436 nm in chlorofom (Figure 3.1). The absorption band is very

similar to that observed for P3ATs prepared by chemical oxidation method,

implying that placement of an acryioyloxy moiety at the end of the alkyl side

chain exerts no signifiant effect on the conformation of the thienylene backbone

in solution.

1.0 ,

4 00 500 600 700 Wavelength (nm)


spectra of P3AHT film (first heating cycle)

A red-shift in &, is obsewed when going from solution to solid state. The

solid state absorption band, centered at 489 nm, possesses a low energy

shoulder at - 600 nm. Extrapolation of the low energy edge of the solid state

absorption spectrum yields a band-gap of 1.85 eV. The absorption band of

P3AHT is slightly blue-shifted with respect to P3ATs prepared by the same

method, indicating that the acryloyloxy moiety impairs the packing of the polymer

chains to some extent.

Shown in Figure 3.2 is the temperature dependence of electronic

absorption spectra of a P3AHT film. Upon heating, P3AHT undergoes a

thermochromic change. lntensity of the 489 nrn band, together with the 600 nm

shoulder, decreases with increasing temperature. A new band centered at 428

nm emerges with increasing temperature. At 200°C, only the 428 nm band is

observed. The blue-shift in Lx corresponds to a color change from red to

yellow. No isosbestic point was observed. This color change, however, is

irreversible. When the sample was cooled back to room temperature, the

absorption maximum stays at 435 nm and the color of the film remains yellow.

Heating up the same sample for a second cycle resulted in only a slight blue shift

(Figure 3.3).

The irreversible color change and the absence of an isosbestic point are

due to the thermal crosslinking of the acryiate functionality in the side chain. The

crosslinking is facilitated by the iron impurity in the polymer. At elevated

temperatures, P3AHT chains adopt a coiled conformation in which the adjacent

thienylenes are twisted with respect to each other. Crosslinking of the acrylate

functionality at these temperatures "locks in" the twisted conformation, hence the

polymer possesses a higher band-gap. The band-gap of the crosslinked sample

is estimated to be 2.24 eV, obtained from extrapolation of the low energy edge of

the absorption band. This band-gap, which is 0.39 eV higher than that of the

uncrosslinked sarnple, does not change upon further thermal treatment due to

the existence of the chemical crosslinks. It thus shows that the band-gap of

P3AHT film can be tuned between 1.85 eV and 2.24 eV, by crosslinking the film

at appropriate temperatures. These results indicate that, by changing the

crosslinking temperature, a series of P3AHT netwrks with various bandqaps

can be created.

.*..... 100 oc ---- 150 OC

-.-..- 180 o c

0.0 I I I

400 500 600 700 Wavelength (nm)


spectra of P3AHT film (second heating cycle)

The steady state fluorescence emission spectrum of P3AHT in chloroform

solution was obtained at ambient temperature and was shown in Figure 3.4. The

solution was purged with argon prior to measurement. A structured ernission

spectrum with a ka, at 572 nm and a shoulder at 603 nm is observed. The

fluorescence quantum yield (af) of P3AHT solution was obtained using quinine

bisulfate (af = 0.546 in 1.0 N H2SO4) as secondary standard and calculated

according to Equation 3.1. 185

d$ = ( I ~ / I ~ ) ( A ~ I A ~ ) ( ~ ~ / ~ ~ ) ~ ~ (eq. 3.1)

Where, @? and O{ denote the fluorescence quantum yields of the sample

and the standard, respectively; 1' and Ir denote the optical density of the sample

and the standard at excitation wavelength, respectively; AS and Ar denote the

area under the fluorescence emission curve of the sample and the reference,

respectively; nS and nr denote the refractive indices of the solvent used with the

sample and the standard, respectively.

. -- - - - - - - - - - - - --

400 500 600 700 800

Wavehngth (nm)

Figure 3.4 Fluorescence emission spectrum of P3AHT in chloroform

@t of P3AHT in chloroform is found to be 20%. We have recently obtained

a @f of 36% for an extensively purified P3HT sample with - 80% HT diad content,

186 and a value of 41% was reported for an extensively purified regioregular

P3HT sample.162 The relatively low af of P3AHT may be attributed to the

existence of carbonyl groups and iron impuriües in the sample.1879188

Wavelength (nrn) Figure 3.5 Fluorescence emission spectra of P3AHT films: (a)

uncrosslinkeâ ; (b) crosslinked at 200°C

Shown in Figure 3.5 are fluorescence spectra of P3AHT films. The

uncrosslinked film shows a very broad and structureless emission band with a

A,,,, of 642 nm. The film crosslinked at 200°C ais0 possesses a broad and

structureless emission band. f he A- is, however, blue-shifted by 58 nm to 594

nm. This shift indicates that the emission spectra of P3AHT films may also be

tuned by 'Iocking in" the conformation at an appropriate crosslinking temperature.

The photoluminescence efficiency of themally wosslinked films appears be

orders of magnitude higher than the uncrosslinked films.

3.3. Summary

A synthesis of poly(3-(6-acryloyloxyhexyl)thiophene) (P3AHT) is reported.

P3AHT possesses strong solution and solid state absorption bands; and is

fluorescent in both solution and solid state. P3AHT films undergo an irreversible

thermochromic change Ath inaeasing temperature. The absorption maximum

blue shifts from 489 nm to 435 nm upon heating. The band-gap changes from

1.85 eV before heating to 2.24 eV after heating. Accordingly, the emission

maximum blue shifts from 642 nm to 594 nm, upon heating. This is due to the

thermal crosslinking of the acryloyloxy functionality at elevated temperatures,

which "locks inn the twisted conformation of the polymer chain. This work

dernonstrates that the band-gap of functionalked P3ATs can be easily tuned by

a post-synthetic crosslinking step.

3.4. Expetimental

Diethyl ether and THF were dried over sodium under a nitrogen

atmosphere. Methylene chloride was dried over calcium hydride. Acryloyl

100

chloride was distilled under nitrogen prior to use. Other materials were

commercially available reagents and used as received. Flash chromatograph

was performed on silica gel 60 (E. Mer& No. 9385,230-400 mesh) as described

by StiII and CO-workers.189 GPC, UV-visible absorption. and NMR analyses were

perfonned as described in Chapter 2.

Steady state fluorescence measurement was performed on a SLM 4800C

spectrofluorometer at ambient temperature. Solutions (OD = 0.05 - 0.10) in four-

sided suprasil cuvettes were deoxygenated by purging with argon for 10 min.

prior to the measurement. Fluorescence quantum yields (5 10% error) were

measured by using quinine bisulfate (Qt = 0.546 in 1 .O N HzSOs) as secondary

standard. Fluorescence quantum yields were calculated according to Equation

3.1 .le5

Films for solid state fluorescence measurement were spin cast from

chloroform solution on quartz substrates.

3-(S=Hexenyl)thiophene (16) To a suspension of 1.73 g (71 mmol) of

magnesium in 100 mL of anhydrous ether was added dropwise 11.7 g (71 mmol)

of 6-bromo-1-hexene. After wmplete disappearance of magnesium, 115 mg of

Ni(dppp)C12 (0.3 mol %) and 11.70 g (71 mmol) of 3-bromothiophene were

added. The reaction is exothemic and a red brown coloration was obsewed.

After stirring and refluxing ovemight, the reaction mixture was poured into a

mixture of crushed ice and 2N HCI and extracted with ether. The combined ether

101

layers were washed with saturated NaHC03, saturated Nt-14Cl consecutively.

After drying over MgS04, ether was removed under reduced pressure to afford

an oily crude product. Simple distillation afforded 7.78 g (66%) pure product.

NMR (100 MHz, CDCI3, ppm): 1.3 -1.8 (4H, m), 2.2 (2H, dt), 2.7 (2H, t), 4.9 - 5.1

(2H, m), 5.7 -6.1 (IH, m), 7.0 (2H, m), 7.3 (1H, m).

3=(tHydroxyhexyl)thiophene (17) A dry 3-neck flask equipped with

a pressure-equalizing dropping funnel and a refluxing condenser was flushed

with dry nitrogen and maintained under a positive nitrogen pressure. The flask

was then charged with 4.33 g (26 mmol) of 3-(5-hexeny1)thiophene and 50 rnL of

dry THF and cooled to - 10°C with an ice-bath. Hydroboration was achieved by

the dropwise addition of 5.0 mL (10 mmol) of borane dimethyl sulfide (BMS).

Following the addition of the hydride, the cooling bath was removed and the

solution was stirred for 3 hr at room temperature. Water (10 mL) was then added

followed by 24 mL of 1 .O M NaOH. After cooling to O - 5°C in an ice-water bath,

hydrogen peroxide (1.7 mL, 30 mmol) was added dropwise at such a rate that

the reaction mixture warmed to 25 - 35°C. lmmediately following the addition of

the peroxide (1 hr), the cooling bath was removed and the reaction mixture was

heated at reflux for 1 hr. The reaction mixture was then poured into 200 mL of

ice water. The ether layer was separated and the aqueous layer was extracted

with ether (75 mL x 5). The ether extracts were then combined and washed with

saturated sodium bicarbonate solution, saturated NaCl solution, dried over

MgS04, consecutively. Removal of solvent afforded - 6 g of crude product,

Mich was purified by column chromatograph (etherhexane 21, rf = 0.31) to

afford 4.08 g (89%) of pure product. 'H NMR (100 MHz, CDCI3,ppm): 1.47 - 1.69

(9H, including the -OH, m), 2.68 (2H, t, J = 7.2 Hz), 3.66 (2H, t, J = 5.7 Hz), 6.94

- 7.31 (3H, m).

5(6Acryloyloxyhexyl)thiophene (1 8) To an ovendried 2 neck

round bottom flask under positive nitrogen pressure, 1.84 g of 3-(6-

hydroxyhexyl)thiophene (10 mmol) and 50 mL of chloroform were added. To the

above system on an ice bath, a solution of acryloyl chloride (2.4 mL, 30 mmol) in

10 mL CHCI3 was added by dropping funnel. The solution was stirred and

wntinuously purged with nitrogen ovemight at room temperature. The reaction

was then quenched by ice-water. The CH2C12 layer was separated and the water

layer extracted by CH$& (4 x 75 mL). The organic layers were then mmbined

and washed by sodium carbonate solution, saturated ammonium chloride,

saturated NaCI, and water consecutively, and then dned over MgS04. The

solvent was removed under reduced pressure. GC analysis suggests that 67% of

starting material converted to the desired product (rf = 0.45 in ether:hexane, 1:4)

and 31% remained unreacted (rf = 0.05 in etherhexane, 1 : ) The product

mixture was subject to column chromatography (silica gel) to afford 1.24 g (52%

yield) of pure product (etherhexane; 1:4), and 0.45 g of the starting material

(ether). 'H NMR (100 MHz. CDC13. ppm): 1.08-1.98 (8H. m). 2.67 (2H. t), 4.18

(2H, t), 5.60-6.71 (3H, m), 8.80-7.40 (3H, m). MS (CI, Mlz): 239 (M+l).

Poly(3-(6acryloyloxyhexyi)thiophene) (P3AHT) To a stirred solution

of 4.40 g of (27 mmol) FeC13 in CHCt3 (200 mL) purged with nitrogen was added

1.42 g (6 mmol) of 3-(6-acryioyloxyhexyl)thiophene. The reaction mixture was

stirred for 2 hours at room temperature, and then was added to methanol. The

black precipitate was washed by methanol and dissolved in chlorofom. The

polymer was then precipitated, washed with methanol, and re-dissolved in

chlorofotm. This step was repeated several tirnes to afford 0.14 g (10% yield) of

pure P3AHT. 'H NMR (400 MHz, CDC13, ppm): 1.2-1.8 (8H, m), 2.6-2.8 (2H, m),

4.2 (2H, b), 5.6-6.4 (3H, m), 6.9-7.2 (2H, m). Molecular weight (GPC, P3HT

standard): MW = 20,000, Mn = 7,000, MwlMn = 2.86.

Chapter 4

Synthesis of Di(Mhienyl)furans, Di(?-thienyl)benzenes

and Corresponding Polyrners

4.1. Introduction

Poly(3-alkylthiophenes) (P3ATs) are a promising class of polyrner for

electroluminescence (EL) due to their ease of preparation, versatility and tunable

band gap.9110 but their solid state luminescent efiiciency is too low.163.164.190

This low efficiency is attributed to intemal conversion of excitation through

molecular aggregates and the existence of sulfur in the thienyl moiety which

promotes intersystem crossing via spin-orbit wupling, i.e. the heavy atom

effect.l60~191~192 Replacing a fraction of the thienylene moieties with gmups

possessing lighter atoms, e.g., phenylene and furylene, or introducing steric

wnstraints to reduce molecular aggregation should significantly enhance solid

state luminescence, while retaining the desirable versatility of P3ATs. It has been

reported that the fluorescent quantum efiiciency for a solution of 2,5-di(2-

thienyl)furan (DTF) (af = 50%) is significantly higher than that for the

corresponding a-terthiophene (a-3T. * = 8%).193 More recently, Yu and co-

workers192 demonstrated that EL efficiency in a simple single layer device using

poly(phenyîene-co-furan) (PPF) was 0.1%; much larger than the corresponding

polyrner, poly(pheny1ene-co-thiophene) (PPT) (0.03%). The lower EL efficiency

I OS

for PPT was attributed to the heavy atom effect. Chan and CO-workersl94

synthesized poly(lI44i(2-(3-alkylthienyl)benzenes) (PDATBs) with different alkyl

side chains. The PDATBs were only partially soluble in common organic

solvents. The soluble fractions of the polymers were found to be stronger

emitters than analogous P3ATs. Reynolds and coworkers reported syntheses

and characterization of a series of poly(di-2-thienyl-2,5-

dialkyl(alkoxy)phenylenes).43 Yoshino and mworkers found these polymers to

be highly luminescent but no quantitative data was repwted.195

There are numerous quantitative reports on EL of conjugated polymers

but relatively few on solid state photoluminescence (PL). The latter, however,

are necessary in order to understand the role of polymer structure and

morphology on EL efficiency since the former is complicated by

polymerlelectrode, and other interfacial processes. In this Chapter, the design,

syntheses, and characterization of a series of regiospecific 1,4di(2-

thieny1)benzenes (DTBs), 2.5-di(24hienyi)furans (DTFs), and 2,5di(2-

(3hexylthienyl)thiophene (3,3'-DHTT) (Figure 4.1), and their corresponding

polymers (Figure 4.2) are reported. A systematic investigation of PL quantum

yields (solution and solid state) and relevant properties of these regiochemically-

controlled oligomers and polyrners are addressed in Chapter S.

19 (3,3'-OHTB): R = 3-hexyl, R'- = 3-hexyl 20 (4,4'-DHT B): R = 4-hexyIl R' = 4'-hexyl

21 (DTF) 22 (a-3T)

25 (3,3'-DHTT): R = hexyl

Figure 4.1 Heteroaromatic trimer- synthesued and studied in this work

Figure 4.2 Stnicture and abbreviation of polyrners

synthesized and studied in Chapters 4 and 5.

4.2 Results and Discussion

4.2.1 Synthesis and Characterization of 2,bDi(24hienyl)furan (DTF)

(ii)

S

(iii)

(i) CH3COCIISnCI4; (ii) (H2CO)nlMe2NHCIIC2H50H; (iii) NaCN; (iv) HCI (ga~) / (CH~C0)~0

Scheme 4.1 Synthesis of 2.5-di(24hienyi)furan (DTF)

DTF is a known compound and several synthetic approaches to DTF have

been reported. In this work. a modified procedure based on ~uo's.196.197

~akker's,198 and ~agan'slgg approaches as shown by Scheme 4.1 was 109

utilized. Treatment of thiophene with acetyl chloride in the presence of one

equivalent of tin (IV) chloride, a Lewis acid, under anhydrous conditions gave rise

to 2-acetyîthiophene (26) in high yields (81% isolated yield of this work,

compared to 7883% in literature).200 Similady, forrnylation of thiophene was

achieved by reaction of thiophene with one equivalent of N-methyCN-

phenylformamide in the presence of one equivalent of freshly distilled oxychloride

under anhydrous conditions as show by Scheme 42.201 Vacuum distillation of

the oily crude product afforded pure 2-formylthiophene (27) in 70% isolated yield.

Scheme 4.2 Preparation of 2-formylthiophene

The hydrochloric salt of Mannich base 28 was prepared by refluxing

paraformaldehyde, dimethylamine hydrochloride and concentrated HCI with 26 in

ethanol solution over night.441202 The resulting sait of 28 predpitated out of

solution upon cooling with 83% yield and was further purifieci by recrystallization

from 95% ethanol. Free Mannich base 28 was then obtained by neutralization

with aqueous ammonia and was used immediately.

1,4-Di(2-thieny1)-l,4-butanedione (29) was prepared following the Stetter

pmcedure.441197~202-207 In the presence of NaCN or a thiazolium salt as

catalyst, aldehyde 27 underwent Michael-type addition smaothly to afford the

Mannich base 28 in dry DMF at room temperature to afford the desired diketone

29 in high yield (90%). 'H NMR analysis showed a singlet resonance peak at

3.43 ppm, which is assignable to the methylene groups. Three aromatic peaks at

7.18, 7.77, and 7.85 are assigned to the thienyi moieties.

Ring closure of 1,4diketone 29 to afford the desired 2,5di(24hienyl)furan

(21) wuld be achieved by treatment with a catalytic amount of concentrated

hydrochloric in acetic anhydride at 85 - gOC44196.197 The reaction is.

however, not clean and yields Vary fmm batch to batch. In this work, the ring

closure was accomplished by purging an acetic anhydride solution of 1,4-

diketone 29 with hydrogen chloride gas at r o m temperature for 1 to 1.5

hours.199 This reaction is very dean. Only the starting material 29 and the

desired product 21 were found in the product mixture. The conversion of the

starting material c m be increased by simply extending the reaction time. A

singlet resonance peak in the 'H NMR spectnim at 6.54 ppm was assigned to

the furylene moiety, and the three aromatic peaks obsenred at 7.05, 7.23, and

7.31 were assigned to 4-. 5 and 3-protons of the thienyl groups, r e ~ ~ e c t i v e l ~ . 2 ~ ~

34 or 35 - (iii)

(i) CH3COCIISnCI4; (ii) (H2CO)nlMe2NHCIIC2H50H; (iii) thiazolium salt; (iv) HCI (ga~) l (CH~C0)~0

Scheme 4.3 Synthesis of 2,5di(2-(hexyithienyI)furans (DHTFs)

4.2.2 Synthesis and Characterization of 2,5-Di(2-hexyithienyl)furans

(DHTFs)

The syntheses of 23-di(2-(3-hexylthienyl))furan (3,3'-DHTF, 23) and 2,5-

di(2-(4-hexylthieny1))furan (4,4'-DHTF, 24) have not been reported in the

literature. In this work, syntheses of DHTFs 23 and 24 were accomplished by a

procedure similar to the synthesis of DTF (21) as outlined in Scheme 4.3.

It is well known that an electrophilic reaction of mono-substituted

thiophenes is affected by electronic effects, i.e., relative directing power of the

sulfur atom and the substituent, and steric effects, Le., steric hindrance of the

substituents. For 3-alkylthiophenes electrophilic substitution mainly takes place at

the 2- and 5-positions.209 Electronic effeds favor the formation of 2-substituted

products. The percentage of 2-substituted products, however, decreases with

increasing steric hindrance of the 3-alkyl group. In this work, acetyiation of 3-

hexylthiophene was accomplished by reacting 3-hexylthiophene with one

equivalent of acetyl chloride in the presence of one equivalent of tin (IV) chloride

under anhydrous conditions, following a procedure similar to that reported by

Johnson and May for acetylation of thi0~hene.200 The conversion of the starting

material was found to be quantitative by GC analysis. The oily crude prociuct was

found to be a mixture of 2-acetyl-3-hexylthiophene (30) and 2-acetyl-4-

hexylthiophene (31) in 2:l ratio. Fortunately, acetylthiophenes 30 and 31 were

readily separable by flash column chromatograph using a 1:10 mixture of diethyl

ether and hexanes as eluent. 'H NMR resonance peaks of the methyl groups of

the acetyl moiety of 30 and 31 were found to be singlets at 2.57 and 2.54 pprn,

respectively. Aromatic protons of 30 were found at 7.02 and 7.43 ppm,

respectively. The 2,3-disubstituted structure of thiophene was characteristic by

the observed coupling constant of 5.0 Hz, while aromatic proton peaks of 31

were found at 7.23 and 7.52 pprn, respetiively. The 2,4disubstituted pattern was

evident by the observeci coupling constant of 1.4 Hz208

Scheme 4.4 Preparation of 2-fomiyt-$or (4)-hexyithiophene

3-Hexyithiophene could also be formylated by N-rnethyl-N-

pheny Iformamide in the presence of phosphorus oxychloride. Similar to

acetylation, a mixture of 2-fomyt-Shexylthiophene (34) and 2-formyl-4-

hexylthiophene (35) was formed. The ratio of 34 to 35 was also found to be 2 to

1. The two isomers, however, are not readily separable by conventional methods.

In this work, compounds 34 and 35 were prepared following a method reported

by Gronowitz and wworkers as shown by Scheme 44.2109211 2-Bromo-3-

hexylthiophene (7) was prepared by the method described in Chapter 2. 2-

Bromo-4-hexyithiophene (38) was prepared via the procedure shown in Scheme

4.5.2121213 3-Hexylthiophene was readily lithiated by n-butyllithium under

anhydrous conditions. Quenching studies revealed that the product mixture

41

TMEDA: Me2NCH2CH2NMe2

Schem 4.5 Preparation of 2-bromo-4-hexylthiophene

contained about 86Oh of 5-lithium derivative, i.e., 2-lithio-4-hexylthiophene (41),

and 14% of 2-lithium derivative, i.e., 2-lithio-3-hexylthiophene. In the presence of

N,N,N,N-tetramethylethylenediamine (TMEDA), selectivity of the 5-position was

enhanced and 92 - 95% of 41 was fonned as revealed by quenching studies.

In situ treatment of 41 with carbon tetrabromide under cryogenic temperature

gave rise to the desired product (38). One mole of carbon tetrabromide can

brominate more than one mole of thienyl llhium 41. Thus, only 0.67 mole

equivalent of carbon tetrabromide was added dropwise to the thienyl lithium at - 78°C. After stirring at -78°C ovemight, the reaction was quenched by ice-water

and worked up by conventional methods. Vacuum distillation of the crude product

afforded 38 in 59% yield with > 95% pure (by GC). The product may contain a

small amount of carbon tetrabmmide. No significant amount of 2-bromo-3-

hexylthiophene was detected by NMR analysis, suggesting that the 2-lithio-3-

115

hexylthiophene does not react with carbon tetrabromide or only reacts very

slowly. The low reactivity of carbon tetrabromide towards 2-lithio-3-

hexylthiophene might be attributed to the steric effect, i.e., the bulky carbon

tetrabromide can not approach the already congested 2-position.

Treatrnent of the isomeric 2-bromo-hexylthiophenes 7 and 38 with

magnesium afforded the corresponding Grignard reagents 39 and 40,

respectively. In situ quenching of 39 and 40 with anhydrous NIN-

dimethylfomamide gave rise in good yields to the corresponding 2-formyl-3-

hexylthiophene (34) and 2-formyl-4-hexylthiophene (35), respectively. Proton

NMR spectroscopic analysis of 34 showed a doublet at a chemical shift of 10.07

ppm, wtiich is assignable to the aldehyde proton. In the aromatic region, a

doublet at 7.04 ppm, assignable to the 4-proton, and a doublet of doublets at

7.67 ppm, assignable to the 5-proton, were observed. The 2,3-disu bstituted

pattern was confirmed by the observed coupling constant or 5.0 Hz between the

4-, and 5-protons. The coupling constant between the aldehyde proton and 5-

proton was found to be 1.0 Hz. For 35, the aldehyde proton was observed at

9.87 ppm as a doublet with a coupling constant of 1.2 Hz. The 3-proton was

found at 7.60 ppm as a doublet with a coupling constant of 1.5 Hz, indicating a

2,4-disubstituted pattern of the thiophene ring. The 5-proton was observed at

7.37 ppm as a multiplet due to multi-coupling between it and the 3-proton, the

aldehyde proton, and the a-methylene group.208

Similar to acetylthiophene 26, the isomeric 2-acetyl-(hexyl)thiophenes 30

and 31 also undergo a Mannich reaction to afford the corresponding salts of

Mannich base 32 and 33, respectively, when refluxing with parafonaldehyde,

dimethylamine hydrochlonde and a catalytic amount of concentrated HCI in

ethanol solution. For 2-acetyl-3-hexyithiophene 30 the equilibrium was found to

favor the reactants side, presumably due to the steric hindrance of the 3-hexyl

group. Therefore, parafomaldehyde and dimethylamine hydrochloride was used

in excess to ensure a higher conversion of 30. When 4 equivalents of

paraformaldehyde and dimethylamine hydrochloride were used, > 95% of 30 was

converted to 32. The large excess of reactants is, however, not desirable due to

the difficulty in work up. Thus, in this work, 2 equivalents of parafonaldehyde

and dimethylamine hydrochloride were used and the reaction mixture was

refluxed ovemight. Under these conditions the conversion of 30 was found to be

around '10%. The unreacted 30 (29%) was recovered by quenching the reaction

with a mixture of 2M HCI and crushed ice, and subsequently extracting with

diethyl ether. The aqueous solution was then basified with aqueous ammonia

and extracted with diethyl ether to isolated crude Mannich base 32, which

following acidification was purified by recrystallization from ethyl acetate to afford

the hydrochloric salt fonn of 32 in 64% yield. Rebasification of the salt with

aqueous ammonia yielded the pure Mannich base 32, which was used right

away. The Mannich base 33 was prepared following the same procedure.

Proton NMR spectroscopic analysis of 32 revealed a singlet of 6

hydrogens at 2.27 ppm, assignable to the N-methyl groups, a triplet of 2

hydrogens at 2.60 ppm, assignable to the a-methylene, a multiplet of 4

hydrogens between 2.95 and 3.05 ppm, assignable to the methylenes adjacent

to carbonyl and dimethylamino groups. The aromatic peaks were found at

chemical shifts of 6.99 and 7.39 ppm, respectively. For the Mannich base 33, the

resonances for the N-methyi groups, the a-methylene, methylene adjacent to

dimethylamino moiety and the methylene adjacent to carbonyl were obsenred at

2.28, 2.60, 2.74, and 3.04 ppm, respectively. Two coupled aromatic peaks were

observed at 7.23 and 7.55 ppm with a coupling constant of 1.4 Hz.

Attempts to prepare 14-butadione 36 using cyanide as catalyst were

unsuccessful. Therefore, 1,4-butadiones 36 and 37 were prepared by the

thiazolium salt mediated Stetter reaction under basic conditions. The thiazolium

sait used in this work is 3-ethyl-5-(2-hydroxyethy1)-4-methylthiazolium bromide.

The reaction proceeds via an elimination-addition mechanism as shown by

Schrme 4.6.203-207 In the presence of a base, e.g., triethylamine, 3-ethyl-5(2-

hydroxyethyi)Q-methylthiazolium bromide is transfomed to a ylide-type

intermediate 42, which reacts with the aldehyde 34 (35) to afford the

corresponding carbanion 43 (44). Carbanion 43 (44) then reacts with the a$-

unsaturated ketone 45 (46), formed in situ from its precursor Mannich base 32

(33), to afford adduct 47 (48). Elimination of ylide 42 h m 47 (48) gives nse to

the desired 1 &butadione 36 (37). The reaction was carried out in a DMF

Scheme 4.6 Mechanistic scheme of the Stetter reaction

mediated by thiazolium Salt under basic conditions

solution at 80 - 90°C for - 10 hours. Some unident'ied oily by-products was

present in the product mixture. Separation of the by-products from the desired

1,4diketone 36 (37) were very diffiwlt. Recrystallization from hexanes afforded

pure 36 (37) in low yield (24% for 36 and 26% for 37).

FTlR spectroscopie analysis revealed a strong absorption at 1654 cm",

characteristic stretching of a conjugated carbonyl group, for both 36 and 37.

Characteristic NMR resonance peaks of 36 were found at chemical shifts of 3.00,

3.31, 7.00, and 7.40 ppm, and these are assignable to the a-methylene, the

methylene adjacent to the carbonyl, the 4- and the 5-protons on the thienyl

moiety, respectively. The coupling constant between 4- and 5-protons was found

to be 5.0 Hz. For 37, NMR peaks corresponding to a-methylene, methylene

adjacent to carbonyi, 5-, and Sprotons were found at 2.61, 3.35, 7.24, and 7.64

ppm, respectively. The coupling constant was 1.4 Hz, indicating a 3,5-

disubstituted pattern.

Treatment of 1,4diketone 36 in acetyl anhydride with hydrogen chloride

gas at room temperature gave rise to 1,4-di(2-(3-hexylthieny1)furan (3,3'-DHTF,

23) in high yield (82% isolated yield). Accordingly, treatment of 1,4diketone 37

with HCI gas afforded the corresponding 1,4di(2-(4-hexylthienyl)furan (4,4'-

DHTF, 24) with 68% isolated yield. NMR analysis of 3,s-DHTF (23) revealed a

singlet at 6.49 ppm, which is assigned to the furylene moiety. Resonance peaks

of a-methylene, 4- and 5-protons of thienyl moieties were found at chemical

shifts of 2.81, 6.91 and 7.16 ppm, respectively. The 4,s-coupling constant was

founâ to be 5.0 Hz. For the 4,4-DHTF (24), resonance peaks of the kirylene and

the a-methylene were found at 6.52 and 2.64 ppm, respectively. The 3- and 5-

protons of the thienyl moieties were detected at 7.17 and 6.85 ppm, respectively,

with a coupling constant of 1 .O Hz.

4.2.3 Synthesis and Characterization of 1 ,dDi(2-(hexylthienyi)) bmenes

and 2,5-Di(P(3-hexy1thienyl))thiaphena

Preparation of 1,4-di(2-(3-hexylthienyl))benzene (3,3'-DHTB, 19) and 1,4-

di(2-(4-hexy1thienyl))benzene (4,4'-DHTB, 20 were shown in Scheme

4.7.439194v214.215 Dropwise addition of a mixture of 1 equivalent of 1,4-

dibromobenzene and catalytic amount of Ni(dppp)C12 in anhydrous ether into a

refluxing solution of 2 equivalent of magnesiobromide 39 or 46 gave rise to the

corresponding dithienylbenzene 19 or 20, respectively, in gmd yields. The crude

products were purified by column chromatograph using etherhexanes as eluent.

Proton NMR analysis of 3,3'-DHTB (19) revealed a singlet of 4 protons at 7.46

pprn, which is assigned to the phenylene moiety. Remance peaks due to the 4-

and the 5-protons of the thienyl moieties were detected at 7.00 and 7.24 ppm,

respectively, with a coupling constant of 5.2 Hz. Resonance of the a-methylene

was found at 2.69 ppm as a triplet. Accordingly, resonance peaks of the

phenytene moieîy of 4,4'-DHTB (20) was obsewed at 7.56. Two doublets with a

coupling constant of t .2 Hz at 6.87 and 7.17 ppm were assigned to the 5- and

the 3-protons of the thienylene moieties. The triplet at 2.61 ppm was assgned to

the a-methylene group.

Schem 4.7 Preparation of OHTBs and 3,3-DHTT

Similady, 2,5di(2-(3-hexylthienyl))thiophene (3,3'-DHTT, 25) was

prepared by reacting rnagnesiobromide 39 with 2,5aibromothiophene, in the

presence of catalytic arnount of Ni(dppp)Cl*. NMR resonance of the thienylene

moiety of 25 was obsewed as a singlet at 7.05 ppm. Doublets with a coupling

constant of 5.2 Hz at 6.94 and 7.14 pprn were assigned to the 4- and the 5-

protons of the thienyl rnoieties, respectively. A triplet at 2.78 was due to the a-

methylene group.

24: R = hexyl P44DHTF: R = hexyi

25: R = hexyl P33DHTT: R = hexyl

Scheme 4.8 Polyrnerization of DHTBs, 4,4'-DHTF, and 3,3-DHTT

As shown by Scheme 4.8, chemical oxidation of 3,3'-OHTB, 4,4'-DHTB,

4,4'-DHTF and 3,3'-DHll by 4 equivalents of iron (III) chloride yielded the

corresponding polymers, poly(l14di(2-(3-hexylthieny1))benzene) (P33DHTB),

poly(1,4-di(2-(4-hexylthieny1))benzene) (PUDHTB), poly(2,Mi(2-(4-

hexylthieny1))furan) (P44DHTF), and poly(2,5-di(2-(3-hexylthieny1))thiophene)

(P33DHlT). re~~ec t i ve l~ .57 , *~~ The solubility of the polymers was found to

depend on polymerization conditions. Polymerization of the trimers in chlorofom

probably allows the formation of polymers with very high rnolecular weight.

Therefore, these polymers are only sparingly soluble in common organic

sd~ents.~94.214 Polymeriraüon in carbon tetrachloride, however, gave rise to

polymers fully soluble in chloroform, THF, toluene, and other common organic

solvents.214 Thus, in this work, P33DHT6, PUDHTB, PUDHTF, and P33I)HTT

were al1 prepared in carbon tetrabromide. The conversion of the trimers was

found to be nearly complete in 2 hours at room temperature. No significant

increase in molecular weight was found with extended polymerization time.

Chemical oxidation of 3,3'-DHTF in carbon tetrabromide gave flse to a

mixture of polymer and low molecular weight oligomers. The polymer and

oligomers are not separable by conventional methods. Thus P33DHTF was

prepared following the McCullough method as shown by Schem 4.9.11.69

7218~~83 2 - ( 2 - ( 5 - b r o m ~ 3 - h e x y ( t h i e n y l ) ) - 5 - ( 2 - ( 3 - h e ~ n (49. 5'-Br-

3,3'-DHTF) was prepared by reacting 3,3'-DHTF (23) with 1 equivalent NBS in

DMF in high yield.218~232 No significant amount of dibromide was fomed. 5'8r-

1 24

3,3'-OHTF was then treated with LDA, magnesium bromide etherate and

Ni(dppp)Cl2, consecutively under cryogenic temperature to give rise to P33DHTF

in reasonable yield (31 %).

(1) LDA, 40 '~ ; (2) MgBr2(OE$). - 60 '~ to -40'~. then -Soc; (3) Ni(dppp)C12, -5 C to rat.

Scheme 4.9 Preparation of P33DHTF via the McCullough method

As-prepared polyrners were precipitated by methanol, reduced by a

mixture of Methylamine and methanol, and extensively purified by Soxhlet

extraction and characterized by NMR, UV-vis, and infrared spectroscopie

analyses. Resonance peaks due to a-methyiene, thienylene, and phenylene

moieties for P33DHTB were found at 2.70, 7.10, and 7.51 ppm, respectively.

Corresponding peaks for P44DHTB were at 2.59. 7.25, and 7.62 ppm,

respectively. Peaks due to a-rnethyiene, furylene, and thienyiene moieties for

125

P33DHTF were at 2.76, 6.50, and 7.00 ppm, respectively. Corresponding peaks

for P44DttTF were at 2.55, 6.55, and 7.18 ppm, respectively. Peaks due to a-

methylene and thienylene moieties for P33DHTT were at 2.76, 7.00, and 7.09

ppm, respectively. NMR analyses indicated that only a,a'-coupling of the trimeric

units was presented in the pdymers. FTlR analysis showed a peak at 1654 an"

for both P33DHTF and P44DHTF. This peak was assigned to the carbonyl

stretching conjugated with a thienyl ring by comparing with FTlR spectra of

diketones 36 and 37. The observation of the carbonyl peak indicates that partial

ring opening of the furylene moiety occurs during the polymerization or work up

process. GPC analysis against a poly(3-hexyithiophene) calibration curve

indicated that number averaged molecular weights of the polymers range from

4000 to 9000 (10 - 25 trimeric unit@

4.3 Summary

The synthesis and characterization of 3,3'-DHTF (23), 4,4'-DHTF (24),

3,3'-DHTB (19), 4,4'-DHTB (20), and 3,3'-DHTT (25) are reported in this Chapter.

3-3'-DHTF (23) and 4,4'-DHTF (24) were synthesized following a multi-step

procedure employing the Stetter reaction and acid catalyzed ring closure of 1,4-

diketones as the key steps. The others were prepared by the nickel complex

mediated Grignard coupling reaction.

P33DHT6, P44DHT6, P44DHTF, and P33DHTT were prepared by

chernical oxidation of the corresponding heteroaromatic trimers with iron (III)

126

chloride in carbon tetrabromide in very high yield. While P33DHTF was prepared

following the McCullough method. The polymers are fully soluble in common

organic solvents such as chloroform, THF, and toluene. The molecular weights of

the polyrners range between 4000 to 9000 (10 to 20 trimeric units).

4.4 Experimental

4.4.1 Materials

Acetyl chloride (Mallinckrodt), acetic anhydride (Caledon), triethylamine

(Anachemia), and phosphorus oxychloride (Anachemia) were distilled before

use. Magnesium (Fisher) was treated with dilute HCI, rinsed with acetone and

ether, and dried under vacuum. THF (Caledon) and diethyl ether (Caledon) were

dried over sodium (Aldrich). N,N,N8,N'-tetramethylethylenediamine (TMEDA)

(ICN Biomedicals), diisopropylamine (Sigma), benzene (Caledon), and CHCI3

(Mallinckrodt) were dried over CaH2 (Sigma). DMF (BDH) was dried over 4A

molecular sieve (Sigma). 3-ethyl-5-(2-hydroxyethy1)-4-methylthiaiiu brornide

(Aldrich) was recrystallized from amixture of isopropyl alwhol and diethyl ether

(BDH). Other materials are wmmercially available reagents and used as

received.

3-Hexylthiophene and 2-bromo-3-hexylthiophene were prepared

according to literature procedure and the spectroswpic data were consistent with

literature.175

'H NMR spectra were taken on either a Bruker WP100 or a Bruker

AMX4OO instrument. Chemical shifts were reported in ppm reference to TMS. IR

spectra were recorded on a Bomem 8-155 FT-IR spectrophotometer. Mass

spectra were perforrned on a Hewlett-Packard 59858 GClMS equipped with a

DB-1 capiilary column operating at 70 eV for electron impact (El) ionization.

Molecular weight and molecular weight distribution, calibrated against

pdy(3-hexyithiophene) standards.96 were characterized by gel penneation

chromatography (GPC) (Waters Model 510) using a pstyragel column at 25°C.

Polyrners were eluted with tetrahydrofuran at a flow rate of 1 mumin. and

detected using a UV-vis spectrophotometer (Waters Model486) at 480 nm.

4.4.3 Synthesis of Dithienylfurans

2-Acetylthiophene (26). lnto a 200 mL 3-necked round bottomed flask

equipped with a thennometer, a dropping funnel and a magnetic stirrer were

charged with 10 g (0.12 mol) of thiophene, 9.33 g (0.12 mol) of acetyl chloride,

and - 120 mL of CaH2-dried benzene under positive nitrogen pressure. The

solution was cooled to O°C, and 30.96 g (13.9 mL, 0.12 mol) of tin (IV) chloride

was added dropwise, with efficient stimng. The reaction assumed a purpie color

when the first few drops of tin (IV) chloride were added, and soon after, a purple

solid precipitated. After al1 the tin (IV) chloride had been added, the cooling bath

was removed and the mixture was stirred for one hour at ambient temperatures.

The addition product is hydrolyzed by the slow addition of a mixture of 54 mL of

water and 6 mL of concentrated HCI. The yellow benzene layer was separated.

The aqueous layer was extracted with benzene. The organic layers were

wmbined, washed with water, and dried over MgSO4. Removal of solvent gave

rise to an oily product, which after vacuum distillation afforded 12.19 g (81%

yieM) of pure product. 'H NMR (100 MHz, CDCl3, ppm): 2.65 (3H. s), 7.15 (IH,

m), 7.60 (2H, m); MS (El, mlz): 126 (hi+).

2-Acetyl-3-hexylthiophene (30) and 2-acetyl4hexylthiop hene (31 ).

lnto a 100 mL 3-necked round bottomed flask equipped with a thennometer, a

dropping funnel and a magnetic stirrer were charged with 3.36 g (20 mmol) of 3-

hexylthiophene, 1.57 g (1,4 mL, 20 mmol) of acetyl chloride, and - 20 mL of

CaHdried benzene under positive nitmgen pressure. The solution was cooled

to O°C, and 5.21 g (2.4 mL, 20 mmol) of tin (IV) chloride was added dropwise,

with efficient stirring. After al1 the ün (IV) chloride was added, the cooling bath

was removed and the mixture was stirred for one hour at ambient temperatures.

The addition product was hydrolyzed by the slow addition of a mixture of 9 mL of

water and 1 mL of wncentrated HCI. The yellow benzene layer was separated.

The aqueous layer was extracted with benzene. The organic layers were

combined, washed with water, and dned over MgSO4. GC analysis showed that

the starting material was completely converteci. Removal of solvent gave an oily

crude product, which is a mixture of 30 (62% by GC) and 31 (31% by GC). The

mixture was then purified using column chromatography (etherhexane 1:10) to

afford 2.05 g (49% yield, r,= 0.37) of 30 and 1.02 g (24% yield, rf = 0.21) of 31.

'H NMR (100 MHz, CDCI3. ppm): 30: 0.91 (3H. t, J = 6.1 Hz). 1.20 - 1.90 (8H, m),

2.57(3H,s), 3.03(2H.t, J=7.4Hz), 7.02(1H,dI J =5.0Hz), 7.43(1HI d , 3 = 5 . 0

Hz); 31: 0.87 (3H, t. J = 7.01 Hz), 1.20 - 1 .?O (8H, m), 2.54 (3H, s), 2.61 (2H, t, J

= 7.7 Hz), 7.23 (IH, d, J = 1.4 Hz), 7.52 (IH, d, J = 1.4 Hz). MS (El, rnlz): 210

(M+) for both isomers.

2-Fonnyithiophene (27). A 100 mL 3-necked round bottom flask

equipped with a thermometer, dropping funnel and a magnetic stirrer was

purged with dry nitrogen. lnto the flask were placed 16.07 g (0.12 mol) of N-

methyl formanilide and 18.22 g (0.12 mol) of freshly distilled phosphorus

oxychloride. The mixture was allowed to stand for 30 min. Then 10.00 g (0.12

mol) of thiophene was added dropwise, with efficient stimng, at such a rate that

the temperature was maintained at 25 - 35". The reaction mixture was allowed

to stand at rwm temperature for ovemight. The resulting dark, viscous solution

was poured into a vigorously stirred mixture of crushed ice and water (- 250 mL).

The aqueous mixture was then extracted with ether. The ether extracts were

then combined and washed with 2 M HCI to remove unreacted N-methyl

formanilide. The aqueous washings were then extracted with ether, and the

ether extracts were combined with the original ether extracts. The combined

ether extracts were then washed with sahirated sodium bicarbonate, water, and

dned over anhydrous magnesiurn sulfate. Removal of solvent gave an oily cnide

proâuct, which after vacuum distillation afforded 9.44 g (70% yield) of 27. 'H

NMR (100 MHz, CDCI3, ppm): 7.25 (IH, m), 7.85 (2H, m), 10.0 (AH, s); MS (El,

mlz): 1 12 (M+), 1 1 1 (base peak, M-1).

2-Fonnyl-3-hexylthiophene (34). To a suspension of 0.48 g (20 mmol)

of magnesium in 40 mL of anhydrous diethyl ether (sodium dried) was added

dropwise 2.47 g (10 mmol) of 2-bromo-3-hexylthiophene in 10 mL of anhydrous

ether. The reaction mixture was refluxed for 3 hours under nitrogen atmosphere.

The resulting Grignard reagent was then pressed with nitrogen into a stirred

solution of 1.83 g (25 mmol) of DMF in 50 mL of sodium dried ether. The

reaction mixture was refluxed for 2 hours and quenched by a mixture of crushed

ice and 2 M HCI (- 200 mL). The ether layer was separated and the aqueous

layer was extracted with ether. The organic extracts were then combined and

washed with saturated sodium bicarbonate and water, dned over magnesiurn

sulfate. Removal of solvent afforded an oily crude product. The crude product is

chromatographed (silica gel, 1:10 ether:hexane, rt = 0.31) to afford 1 A4 g (73%

yield) of pure 34. 'H NMR (100 MHz, CDC13, ppm): 0.91 (3H, t, J = 6.4 Hz), 1.20

- 1.90 (8H, m), 3.00 (2H, 1, J = 7.3 Hz), 7.04 (2H, d, J = 5.0 Hz), 7.67 (ZH, dd, Jt

= 5.0 Hz, J2 = 1.0 HZ), 10.07 (IH, dl J = 1.0 HZ); MS (El, mlz): 196 (M+).

2-Bromo-4-hexylthiophene (38). A mixture of 50 mL (0.125 mol) of 2.5

M n-butyllithium and 18.8 mL (14.50 g, 0.12 mol) of TMEDA (dried over Calcium

Hydride) was slowly added into a solution of 20.00 g (0.12 mol) of 3-

hexylthiophene in 90 mL ether at m m temperature at such a rate that the

solution was gently refiuxed. The reaction mixture was stirred at room

temperature for 30 min. and then coded down to -78°C. A solution of 26.01 g

(0.078 mol) of carbon tetrabromide in 50 mL of ether was added dropwise. The

reaction mixture was stirred overnight at -78°C and then poured into a mixture of

crushed ice-water (200 mL). The ether layer was separated and aqueous layer

extracted with ether. The organic extracts were then combined and washed with

2M HCI, sodium bicarbonate solution, water, and dned over magnesium sulfate.

Removal of solvent afforded a black oily cnide product, which was purified by

vacuum distillation to afford 17.09 g (59% yield) of the desired product. GC

analysis showed that the final product was 95% pure. 'H NMR (100 MHz, CDC13,

ppm): 0.91 (3H, t, J = 5.7 Hz), 1.2 - 1.8 (8H, m), 2.58 (2H, t, J = 7.3 Hz), 6.84

(IH, m), 6.91 (IH, dl J = 1.5 Hz).

2-Fonnyl4hexylthiophene (35). To a suspension of 1.09 g (45 mmol)

of magnesium in 40 mL of anhydrous diettiyî ether (sodium dried) was added

dropwise 7.41 g (30 mmol) of 2-bromo4-hexyithiophene in 10 mL of anhydrous

ether. The reaction mixture was refluxed for 3 hours under nitrogen atrnosphere.

The resulting Grignard reagent was then pressed with nitrogen into a stirred

solution of 4.39 g (60 mmol) of DMF in 40 mC of sodium dried ether. The

reaction mixture was refluxed for 2 hours and quenched by a mixture of crushed


layer extracted W.& eether. The organic extracts were then combined and

washed with saturated sodium bicarbonate and), dried over rnagnesium sulfate.

Removal of solvent afforded an oily crude product. The crude product was

chromatographed (silica gel, 1:10 etherhexane, rf = 0.31) to afford 4.00 g (74%

yield) of pure 35. GC analysis showed the final product was 95% pure. 'H NMR

(400 MHz, CDCI3, ppm): 0.88 (3H, t, J = 6.9 Hz), 1.3 (6H, m), 1.62 (2H, p, J = 8.0

Hz), 2.64 (2H, f, J = 7.7 Hz), ?.37(2H, rn) 7.60 (2H, d, J = 1.4 Hz), 9.87 (IH, d, J

= 1.4 Hz).

3-(N,N-0imethylamino)-1-(2-thieny1)propanone (28). A mixture of 21.6

g (0.17 mol) of 2-acetyithiophene, 6.17 g of parafomialdehyde, 76.76 g of

dimethylamine hydrochloride, and 1 mL of concentrated HCI in 20 mL of 95%

ettianol were refluxed ovemight under nitrogen. The resulting Mannich base

hydrochloride (31 g, 8296 yield) was precipitated out of the solution upon cooling.

The cmde product was then purified by recrystallization h m 95% ethanol and

basified with ammonia to afford the free Mannich base 28.

3-(N,N-Dimethyiamino)Ii-(2=(3-hexylthienyl))ptopanone (32). A

mixture of 5.25 g (25 mmol) of 2-acyî-3-hexyîthiophene, 1.50 g (50 rnmol) of

parafomaldehyde, 4.08 g (50 mmot) of dimethylamine hydrochloride and 0.70

mL of concentrated hydrochloric acid in 20 mL of 95% ethyl alcohol was heated

under reflux ovemight. The reaction mixture was cooled and poured into - 150

mL of a mixture of 2 M HCI and cnistied ice. The aqueous mixture was extracted

with ether. The organic extracts were combined and washed. Removal of

solvent recovered the unreacted starting materials (1.5 g). The aqueous phase

was then basified with NH40H and extracted with ether. The extracts were

combined and washed with water, dned over MgS04. Removal of solvent

afforded free Mannich base 32. The crude Mannich base was then acidified

again and purified by recrystallization from ethyi acetate to afford 4.83 g (64%

yield) of the Mannich base hydrochloride. Re-basification with NHaOH aiforded

3.84 g (90% yield) of pure free Mannich base 32. 'H NMR (400 MHz, CDCI3,

ppm): 0.87(3H, t, J = 7.0 Hz) 1.2 - 1.4 (6H, m), 1.59 (2H, p, 3 = 7.8 Hz), 2.27 (6H,

s), 2.74 (2H, t, J = 7.3 Hz), 2.95 - 3.05 (4H, m), 6.99 (IH, d, J = 5.0 Hz), 7.39

(IH, d, J = 5.0 Hz).

3-(N,N-Dimethylamino~1-(2-(+hexylthienyl))propanone (33). A

mixture of 8.00 g (38 mmol) of 2-a~etyl4hexylthiophene~ 2.28 (76 mmol) of

parafomaldehyde, 6-20 g (76 mmol) of dimethyiamine hydrochloride and 0.20

mL of concentrated hydrochloric acid in 30 mL of 95% ethyl alcohol was heated

under reflux overnight. The reaction mixture was worked up to afford 4.01 g

(40% yield) of the desired free Mannich base 33. 'H NMR (400 MHz, CDC13,

ppm): 0.88 (3H, t, J = 7.0 Hz), 1.2 - 1.4 (6H, m), 1.60 (2H, m), 2.28 (6H, s), 2.60

(2H, t, J = 7.7 Hz), 2.74 (2H, t, J = 7.4 Hz), 3.04 (2H, t, J = 7.4 Hz), 7.23 (IH, m),

7.55 (IH, d, J = 1.4 Hz).

1,4-Di(2-thienyl)-1,4=butanedione (29). lnto a suspension of 0.27 g (5.5

mmol) of NaCN in 4 mL of dry DMF was added a solution of 1.55 g (14 mmol) of

2-formylthiophene in 3 mL of dry DMF over a period over 15 min under a nitrogen

atmosphere. After the mixture had been stirred for 15 min, 2.03 g (1 1 mmol) of

freshly made free 3-(N,Ndimethylamino)-1-(2-thieny1)propanone in 5 mL of dry

DMF was added over a period of 1 hour. The solution was allowed to stand at

room temperature ovemight. Water was added and the product was extracted

with chlorofotm. The extracts were combined, washed with water and dried over

magnesium sulfate. Evaporation of solvent afforded 2.80 g of crude product.

The cnide product was recrystallized from ethanol to afford 2.05 g (82% yield) of

pure 29. 'H NMR (100 MHz, CD&, ppm): 3.43 (4H, s), 7.18 (2H, t, J = 4.2 Hz),

7.77(2H, d, J =3.9 Hz),7.85(2H, d, J =3.9Hz).

1,4-Di(2-(3-hexylthienyl))-l,4butanedione (36). lnto a 3-neck round

bottom flask, equipped with a magnetic stir, a condenser with nitrogen adapter,

and a dropping funnel, were placed 2.67 g (10 mmol) of Mannich base 32,0.50 g

(2 mmol) of 3-ethyl-5-(2-hydroxyethyl)-4-methylthiazolium bromide, and 10 mL of

molecular sieve dried DMF. The solution was heated to 80-90°C and 1.1 mL (8

mmol) of triethylamine was added. Then 2.7 g (15 mmol) of 2-forrnyl-3-

hexylthiophene was added dropwise over a period of 1.5 hours. The reaction

mixture was stirred at 80-90°C for 8 hours. The solvent was then distilled and

the residue was poured into a mixture of crushed ice and water (- 150 mL). The

aqueous solution was acidiied with 2M HCI and extracted with ether. The

organic extracts were combined and washed with saturateci sodium bicarbonate

solution, water, and dried over magnesium suifate. Evaporation of solvent

afforded - 2.5 g of crude product, which was recrystallized from hexanes to

afford 0.94 g (24% yield) of pure 36. 'H NMR (400 MHz, CDCI3, ppm): 0.87 ( 6 4

t, J = 6.6 Hz), 1.2 - 1.4 (ISH, m), 1.6 (4H, pl J = 5.8 Hz), 3.00 (4H, t, J = 7.9 Hz),

3.31 (4H, s), 7.00 (2H, d, J = 5.OHr). 7.40 (2t1, dl J = 5.0 Hz). FTlR (KBr pellet):

1654 cm" (C=O stretching).

1,4-Di(2-(4-hexyithieny1))-1 ,&butanedione (37). lnto a 3-neck round

bottom flask, equipped with a magnetic stir, a condenser with nitrogen, and a

dropping funnel, were placed 4.01 g (15 mmol) of 3,3-dimethylamino-1-(2-(4-

hexylthieny1)-1-propanone (33), 0.60 g (2.4 mmol) of 3-ethyl-5-(2-hydroxyethy1)-

4-methylthiazolium bromide, and 15 mL of molecular sieve dried DMF. The

solution was heated to 80-90°C and 1.2 mL (9 mmol) of triethylamine was added.

Then 2.7 g (15 mmol) of 2-fomyi-4-hexylthiophene was added dropwise over a

period of 1.5 hours. The reaction mixture was stirred at 80-90°C for 5 hours.

The reaction mixture was worked up and the resulting cade product

recrystallized from hexanes to afford 1.56 g (26%) of pure 37. 'H NMR (400MHz,

CDCI3, ppm): 0.89 (6H t, J = 6.7 Hz), 1.31 (12H, m), 1.62 (4H, m), 2.61 (4H, t, J = 136

7.7 Hz), 3.35 (4H, s), 7.24 (2H, d, J = 1.4 Hz), 7.64 (2H, d, J = 1.4 Hz). FTlR (KBr

pellet): 1654 cm-' (C=O stretching).

2,SDi(2-thienyl)furan (DTF) (21). A stirred solution of 0.22 g (1 mmol)

of 1,4di(2-thieny1)-1,4-butanedione (29), 10 mL (10 mmol) of acetic anhydride

was treated with HCI gas at m temperature. After stimng for 1.5 hrs, the

solution was quenched by a mixture of crushed ice and water. The aqueous

mixture was extracted with CH2CI2. The wmbined organic extracts were washed

with water dried over MgS04. Removal of solvent in vacuo afforded an oily crude

product. The cnide product was column chromatographed (silica gel, ethyl

acetate-hexanes (1 :15), rf = 0.40) to give 0.12 g (60% yield) of pure 21. 'H NMR

(400MHz, CDCI3, ppm): 6.54 (2H, s), 7.05 (2H, dd, 3% = 3.66 Hz, Jds = 5.03 Hz),

7.23 (2H, dd, J35 = 1 .O7 HZ, J45 = 5.03 HZ), 7.31 (2H, dd, J35 = 1 .O7 HZ, J3 = 3.66

Hz). MS (El, mlz): 232 (M+).

2,SDi(2-(3-hexylthienyl))furan (23). A stirred solution of 0.609 (1.5

mmol) of 1,4di(2-(3-hexy)thienyî)-l,4-butanediine (36), 25 mL (25 mmol) of

acetic anhydride was treated with HCI gas at room temperature. After stimng for

1.5 hours, the solution was quenched by a mixture of cnished ice and water. The

aqueous mixture was extracted with CH2CI2. The combined organic extracts

were washed with water, dried over MgSOd. Removal of solvent in vacuo

afforded an oily cnide product. The crude product was purified by column

chromatography (silica gel. hexanes ) to give 0.47 g (82% yield) of pure 23. 'H

NMR (400MHz, CDC13, ppm): 0.88 (6H, t, J = 7.0 Hz), 1.25 -1.45 (16H, m), 2.81

(4H, t, J = 7.9 Hz), 6.49 (2H, s), 6.91 (2H, dl J = 5.1 Hz), 7.16 (SHI dl J = 5.1 Hz).

MS (El, d z ) : 400 (M+).

2,5-Di(2-(4-hexyithienyl))furan (24). A stirred solution of 1.54 g (3.9

mmol) of 1,4di(2-(4-hexy)thienyl)-l,4-butanedione (37), 30 mL (30 mmol) of

acetic anhydride was treated with HCI gas at room temperature. After stirring for

1.5 hour, the solution was quenched by a mixture of crushed ice and water. The

aqueous mixture was extracted with CH2CI2. The combined organic extracts

were washed with water, dried over MgSOs. Removal of solvent in vacuo

afforded an oily crude product. The crude product was purified using column

chromatography (silica gel. hexanes) to give 1.0 g (68% yield) of pure 24. 'H

NMR (IOOMHz, CDCI3, ppm): 0.93 (6H, t, J = 6.0 Hz), 1.2 - 1.9 (16H, m), 2.64

(4H, t, J = 7.3 Hz), 6.52 (2H, s), 6.85 (2H, m), 7.17 (2H, d, J = 1.0 Hz); MS (El,

mlz): 400 (hi+).

4.4.4 Preparation of 1,4-Di(2-(hexylthienyI))ben%enes and 2,5=Di(2-(3-

hexyîthieny1))thiophene

2,SDi(2-(3-hexyithieny1)thiophene (25). To a suspension of 0.49 g (20

mmol) of Magnesium in 30 mL of anhydrous diethyl ether (sodium dried) was

added dropwise 5.0 g (20 mmol) of 2-bromo-3-hexylthiophene in 15 mL of

anhydrous ether. The reaction mixture was refluxed for 2 hours under nitrogen

atmosphere. A suspension made up of 2.49 g (10 mmol) of 2,s

dibromothiophene, 60 mg (0.1 mmol) of Ni(dppp)Clp, and 15 mL of anhydrous

ether was added dropwise to the Grignard over a period of 1.2 hours. The

reaction mixture was refluxed overnight and quenched by a mixture of crushed


layer was extracted with ether. The organic extracts were then combineci and

washed with saturated sodium bicarbonate and water, dried over magnesium

sulfate. Removal of solvent afforded an oily crude product. The c ~ d e p d u c t

was purified using column chromatography (silica gel, ether:hexane) to afford

2.34 g (56% yield) of 25. 'H NMR (400MHz. CDCIî. ppm): 0.88 (6H. t, J = 6.8

Hz), 1.25 - 1.40 (12H, m), 1.65 (4H, pl J = 7.6 Hz), 2.78 (4H, t, J = 7.6 Hz), 6.94

(2H, dl J = 5.2 Hz), 7.05 (4H, s), 7.14 (2H, dl J = 5.2 Hz). MS (El, mlz): 416 (M+).

1,4Di(2-(3-hexylthieny1))benzene (19). To a suspension of 0.60 g (25


added dropwise 6.0 g (24 mmol) of 2-bromo-3-hexylthiophene (7) in 15 mL of

anhydrous ether. The reaction mixture was refluxed for 2 hours under nitrogen

atmosphere. A suspension made up of 2.78 g (12 mmol) of 1,4-

dibmmobenzene, 64 mg (0.12 mmol) of Ni(dppp)C12, and 15 mL of anhydrous

ether was added dropwise to the Grignard over a period of 1 hour. The reaction

mixture was refluxed ovemight and quenched by a mixture of crushed ice and 2

M HCI (- 200 mL). The ether layer was separated and the aqueous layer

extracted with ether. The organic extracts were then combined and washed with

saturated sodium bicarbonate and water, dried over magnesium sulfate.

Removal of solvent afforded an oily crude product. The crude product was

purified using column chromatography (silica gel, ether:hexane) to afford 3.16 g

(66% yieM) of 19. 'H NMR (400MHz. CDCl3. ppm): 0.86 (6H, 1, J = 6.9 Hz). 1.20

- 1.35 (12H, m), t.63 (4H, p, J = 7.9 Hz), 2.69 (4H, t, J = 7.9 Hz), 7.00 (2H, dl J =

5.2 Hz), 7.24 (2H, dl J = 5.2 Hz), 7.46 (4H, s). MS (CI, rnlz): 41 1 (M + 1).

1,4Di(2-(4hexyithienyl))benzene (20). To a suspension of 0.50 g (20


added dropwise 5.00 g (20 mmol) of 2-bromo4hexylthiophene (38) in 15 mL of

anhydrous ether. After refluxing for 2 hours under nitrogen atmosphere, a

suspension made up of 2.36 g (10 mmol) of 1,4-dibromobenzene, 60 mg (0.55

mol %) of Ni(dppp)Clz, and 15 mL of anhydrous ether was added dropwise to the

Grignard over a period of 1 hour. The reaction mixture was refluxed ovemight,

quenched by a mixture of crashed ice and 2 M HCI, and worked up by

conventional method. The crude product was purified using column

chromatography (silica gel, ether:hexane) to afford 2.45 g (60% yield) of 20. 'H

NMR (400MHz. CDC13, ppm): 0.89 (6H, t, J = 6.9 Hz), 1.25 - 1.40 (12H, m), 1.63

(4H,p, J =7.7 Hz), 2.61 (4H, t, J =7.7Hz),6.87(2HI dl J = 1.2Hz), 7.17 (2H, d,

J = 1.2 Hz), 7.56 (4H. s). MS (CI. mlz): 411 (M + 1).

2-(2-(S-bromo13-hexyIthienyl))-Q(2-(3-hexyl-thienyl))hirsn (49). Into a

two neck round bottom flask were charged 2.00 g (5 mmol) of 3,3-DHTF and 15

mL of molecular sieve dried DMF. The solution was then coolad down to - 0°C

with an ice bath and, in darkness, a solution of 0.90 g (5 mmol) of N8S in 5 mL of

DMF was added dropwise under nitrogen. After stifflng at m m temperature for

24 hours, the reaction mixture was poured into a mixture of cmshed ice and

water (- 200 mL), extracted by ether (3 x 75 mL), washed with sodium

carbonate, water, dried over magnesium sulfate. Removal of solvent afforded

2.25 g of crude product, which was then purified by column chromatography to

afford 2.05 g (85%) of the desired product. 1H NMR (400 MHz, CDCI3, ppm):

0.88 (6H, m), 1.25-1.45 (12H, m), 1.65 (4H, m), 2.74 (2H, t, J = 7.4 Hz), 2.79 (ZH,

t, J 7.8 Hz), 6.45 (2H, dd), 6.86 (1H, s), 6.91 (lH, d), 7.16 (1H, d).

4.4.5 Polymerization

Poly(l,Mi(2-(3-hexylthienyl)benrene) (P33DHTB) To a stirred solution

of 1.00 g (2.4 mmol) of 3,3'-DHTB in CC14 (40 mL) purged with nitrogen was

added 1.62 g (10 mmol) FeCI3. The reaction mixture was stirred for 2 hours at

room temperature and was quenched by methanol. The black precipitate was

filtered and reduced by a mixture of methanol-triethylamine. The resulting

polyrner was further purified by Sexhlet extraction with methanol. A yellow solid

(0.91gI 91% yield) was obtained and dried under reduced pressure. 'H NMR

(400 MHz, CDCI3, ppm): 0.89 (6H, b). 1.3 - 1.4 (12H, m), 1.69 (4H. b), 2.70 (4H,

b), 7.10 (2H, b), 7-51 (2H, b). Aromatic protons due ta the end moieties were

detected at 7.00, 7.09, and 7.49. Molecular weight (GPC, P3HT standard): MW =

35,000, Mn = 7,700, MwlMn = 4.55.

Paly(1,4-di(2-(4-hexylthienyî)benzene) (P44DHTB) 0.62 g (1.5 mmol) of

4,4'-DHTB in CC4 (20 mL) readed with 1.00 g (6.2 mmol) FeCI3 to afford 0.57 g

(92%) of P44DHTB. 'H NMR (400 MHz, CDC13, ppm): 0.88 (6H, b), 1.3 - 1.4

(12H, m), 1.63 (4H, b), 2.58 (4H, b), 7.25 (2H, b), 7.62 (2H, b). Aromatic protons

due 10 the end moiety were detected at 6.88, 7.23, and 7.60. Molecular weight

(WC, P3HT standard): MW = 20,000, Mn = 6,400, MwlMn = 3.07.

Poly(2,5=di(2-(4-hexyIthienyl)furan) (P440HTF) 0.60 g (1.5 mmol) of

4,4'-DHTB in CC14 (20 mL) reacted with 1.00 g (6.2 mmol) FeC13 to afford 0.55 g

(92%) of P44DHTB. 'H NMR (400 MHz, CDC13, ppm): 0.89 (64 b), 1.3 - 1.4

(12H, m), 1.60 (4H, b), 2.55 (4H, b), 6.55 (2H, b), 7.18 (2H, b). Molecular weight

(GPC, P3HT standard): MW = lg,OOû, Mn = 5,900, MwlMn = 3.1 9.

Poly(2,5.di(2-(3-hexylthienyl)thiophene) (P33DHTT) 1 .O0 g (2.4 mrnol)

of 3,s-OHll in CC14 (40 mL) reacted 1.62 g (10 mmol) FeCI3 to afford 0.94 g of

P33DHTT (94%). 'H NMR (400 MHz, CDC13, ppm): 0.91 (6H, b), 1.3 - 1.4 (12H,

m), 1.60 (4H, b), 2.76 (4H, b), 7.00 (2H, b), 7.09 (2H, b). Aromatic protons due to

the end rnoiety were detected at 5.95 and 7.20. FTlR (KBr pellet): 1654 (weak,

carbonyl stretching). Molecular weight (GPC, P3HT standard): MW = 8,800, Mn =

3,100, MwlMn = 2.82.

Poly(2,S-âi(2-(3=hexylthienyl)furan) (P33DHTF) Into a dry round-bottom

fiask was placed dry diisopropylamine (0.35 mL , 2.5 mmol) and freshly distilled,

dry THF (- 20 mL). To the mixture was added 1 .O mL of 2.5 M n-butyllithium (2.5

mmol) at room temperature. The mixture was m l e d to - 40 OC and stirred for

40 min. The reaction mixture containing LDA was then cooled to - 78 OC, and

2-bromo-3,3'-DHTF (1.20 g, 2.5 mmol) was added. The mixture was stirred for

40 min at -400C. The mixture was then m l e d to -600C, MgBr2-Et20 (0.65 g, 2.5

mmol) was added, and the reaction was stirred at -60% for 20 min. The reaction

was then allowed to slowiy wann to -5W, whereupon al1 of the MgBr2-Et20 had

reacted. At -5% catalytic amount of Ni(dppp)&I2 (41 mg) was added. The

mixture was allowed to warm to room temperature overnight (- 18 h). The

reaction was then quenched by MeOH and the solvents were removed under

reduced pressure. The red residue was then subjected to Soxhlet extractions

with MeOH, H20, MeOH and hexane consecutively, to remove oligomers and

impurities. The polymer was then dissolved in CHCI3 using a Soxhlet extractor,

the CHCI3 was removed and the residue was dried under reduced pressure. 0.32

g (31%) of pure P33DHTF was isolated. 1H NMR (400 MHz, CDC13, ppm): 0.90

(6H, b), 1.3-1.5 (12H, b), 1-72 (4H, b), 2.76 (4H, b), 6.50 (2H, b), 7.00 (2H, b).

Aromatic peaks due to the end thienyl rnoiety were detected at 6.88, 6.92, and

143

7.25, respectively. FTlR (KBr pellet): 1654 (weak, carbonyl stretching). Molecular

weight (GPC, P3HT standard): Molecular weight (GPC, P3HT standard): MW =

8,000, Mn = 3,800, MwlMn = 2.10.

Chapter 5

Optical, Fluorescent, and Electrochemical Properties of Novel

Thiophene-Based Heteroaromatic Trimers and Componding

Polymers

5.1 Optical and Fluorescent Properties of Thiophene-Based

Heteroaromatic Trimers

UV-vis absorption spectra were recorded at ambient temperature.

Absorption and fluorescence characteristics of regiospecific thiophene-based

heteroaromatic trimers in hexanes are summarized in Table 1. The absorption

spectra of the trimers are broad and featureless. A low energy shoulder is

observed for absorption bands of 2,5dithienylfuran (DTF) and 2,5di(2-

(hexylthieny1))furans (DHTFs). Lx ranged from 300 nm for 1,4di(2-(3-

hexylthieny1))benezene (3,3'-DHTB) to 352 nm for a-3T. Placement of the hexyl

groups at the 3,s- positions of the thienyî moiety does not appear to significantly

reduce the effective conjugation between adjacent rings for dithienylfuran- and

dithienylthiophene-based timers, but it does for the phenylene analogues. As

expected the introduction of the phenylene unit appears to reduce the effective

wnjugation of the molecule by increasing the torsional angle.157.163

Steady state fluorescence spectra of the trimers were taken at ambient

temperature. Solutions (OD = 0.05 - 0.10) in four-sided suprasil cuvettes were

purged with a stream of argon prior to the measurement. Fluorescent excitation

and emission spectra of the trimers are shown in Figures 5.1 through 5.3.

Table 5.1. Absorption and fluorescence characteristics of thiophene based

heteroaromatic trimers in hexanes at room temperature

-- -

The excitation spectra of the trimers are found to resemble their

absorption spectra. Unlike the absorption bands, the emission spectra are

stnictured, possessing two resolved peaks and a low energy shoulder.

Stnictured emission has been observed for other thiophene-based oiigomers and

is generally attributed to vibronic cwpling.154.157.1~~161 ,216 The diiference

Trimer

DTF

a-3T

3,3'-DHTF

4,4'-DHTF

3,s-DHTB

4,4'-DHTB

F

3,3'-DHTT

StokesShift

(nm)

af(%) h (abs.)

(nm)

350

352

340

355

300

330

340

h (m.)

(nm)

400

423

407

407

381

386

426

50

7 1

67

52

8 1

56

82

57 + 6

8.1 f 0.8

36 + 4

5 6 I 6

19+2

5 3 f 6

4.6 f 0.5

3T (ex)

Wavelength (nm)

Figure 5.1 Normalized fluorescence excitation

and emission spectra of a-3T and 3,3'-DHTT

A DTF (ex) O 3,s-DHTF (ex)

4,4'-DHTF (ex)

a 3,s-DHTF (em) 4,4'-DHTF (em)

250 300 350 400 450 500 Wavelength (nm)

Figure 5.2 Nomalized fluorescence excitation

and emission spectra of DTF, 3,3'-DHTF, and 4,4'-DHTF

33-DHTB (ex) 4,4'-DHTB (ex) 3.3'-DHTB (em) 4,4'-DHTB (em)

! 4,4'-DHTB (ex)

n 3 . .E/. :.. 3.3'-DHTB (em) t

4 . .: 4,4'-DHTB (em) . . a . œ C .. y ;. . . .

f : * ?

I . . a b .

i . . . : . l b : L

l 0 .

: . . 1

! .

250 300 350 400 450 500

Wavslength (nm)

Figure 5.6 Nomalized fluorescence excitation

and emission spectra of 3,3'-DHTB and 4,4'-DHTB

between the structureless absorption and sttuctured emission bands implies that

the trimers adopt a more rigid and a more planar configuration in their equilibrium

SI state. In the ground state, the molecules mainly adopt the aromatic

configuration with an inter-annular bond order of - 1. In the equilibrium SI state,

the molecules reanange to the energetically favored quinoid configuration with

an inter-annular bond order of 2. 15491571161,163

Figure 5.4 Energy diagram illustrating confguration

rearrangement of thiophene-based trimers and polymers

upon photoexcitation. FC and eq stand for Franck-

Condon and equilibrium states, respectively. X = S, O, HC=CH

Stokes shifts of the trimers range from 50 nm for DTF to 82 nm for 2,5-

di(2-(3-hexylthieny1))thiophene (33-DHTT). The large Stokes shifts may also be

attributed to the configuration rearrangement upon excitation as illustrated by

Figun 5.4154,157,161g163 In the ground state. the aromatic configuration is

more stable than the corresponding quinoid configuration. Therefore, the

molecules mainly adopt the aromatic form. Upon excitation, the energetically

disfavored aromatic configuration, SI (FC), rapidly rearranges to the more stable

quinoid configuration, Si (eq). Emission from S1 (eq) gives rise to the

energetically disfavored quinoid form So (FC), which then relaxes back to the 149

aromatic So configuration. As illustrated in Figure 5.4, the energy gap beîween

SI (eq) and So (FC) is significantly smaller than that between So (eq) and SI (FC).

Therefore, a large Stokes shifî is observed.

Fluorescence quantum yields (5 10% error) of trimers in hexane solution

were measured by using 2-aminopyridine (af = 60% in 0.1 N H Z S Q ) ~ ~ ~ and

calculateci acwrding to Equation 3.1 (see page 96).185 Fluorescent quantum

yields and Stokes shifts of the trimers are summarized in Table 5.1.

It is well known that fluorescence eniciencies of thiophene-based

oligomers are relatively low. The measured value of 8.1% for @f of a-3T is

consistent with previous reports.154~155.217m The dominant non-radiative

decay channel for oligothiophenes and P3ATs has been reported to be singlet-

triplet intersystem crossing (1~~)160.161.191.217219.220 Becker and co-

workers have reported that ISC quantum yields (aisc) of oligothiophenes

decrease with increasing chain length and level off at - 60% when n > 7.161 The

photophysics of a-3T and alkyl substituted a-3Ts has been extensively studied

by several research gmups.161.217.218 Although Qisc of a-3T ranged between

20% to 75% in early reports,lg3 reœnt publicatiions confirmed that *x: of a3T

and a series of a-3Ts are > 90%.161217.218 The high ISC efficiency has been

unambiguously attributed to the existence of relatively heavy sulfur atoms in the

thienylene rings. The sulfur a t m enhances intra-annular intersystem crossing

(ISC) due to spin-orbit coupling through the participation of its d, orbit in the

triplet ~tate.~60.~61~191,217-220 An additional non-radiative decay pathway is

intemal conversion. The bulky sulfur atom engenders the thienylene ring with a

lower rigidity, and hence this favors intemal conversion through skeletal

relaxation.

Fluorescence yields of DTF, 2,s-di(2-(4-hexylthieny1))furan (4.4'-DHTF)

and 1,4-di(2-(4-hexylthienyl))benzene (4,4'-DHTB) are found to be 7 times

greater than a-3T. Steric arguments cannot explain this difference since 4,4'-

DHTB is a relatively twisted molecule whereas 4,4'-DHTF is not. The data are

consistent with a reduction in the number of sulfur atoms, i.e. a heavy atom

effect.

The addition of hexyl groups at the 3,3'-positions in the a-terthiophene (a-

3T) and DTF has only a little effect on the effective conjugation tength of the

molecules. DTF and 4,4'-DHTF possess very similar values of Of, indicating that

the hexyl groups do not interfere with the excited state in this configuration. The

steric hindrance of 3,3'-alkyl groups, however, impairs the formation of the planar

quinoid configuration in the S1 state. Therefore, Ois of 33-DHTT and 3,3'-DHTF

are significantly lower than their parent and 4,4'dialkyi analogues. In the case of

DHTBs, severe steric hindrance of the 3,3'-hexyl groups forces the trimer to

adopt a significantly more twisted conformation in both ground and excited state

than its 4.4'-analog. Therefore, ar of 4,4'-DHTB is found to be 3 times higher than

that of 3,3'-DHTB.

5.2. Optical and Fluonscent Properties of Regiospecific

Thiophene-Based Conjugated Palymers

UV-visible absorption spectra of the polymers in THF solution and as films

are shown in Figure 5.5 and Figure 5.6. Absorption and fluorescence

characteristics of the polymers in solution are summarized in Table 2.

300 400 500 600 700 Wavelingth (nm)

Figure 5.5 Nomalized UV-vis absorption spectra of

regiochemically-controlled thiophene-based polymers in THF solution

Absorption bands in THF solution are broad and structureless. This is

again attributed to a ground state with more contribution frorn the aromatic

resonance structure. & values range from 383 nm for P44DHTB to 470 nm for

P33DHTF. For PUDHTB, severe steric interaction between the 4,4'-hexyl

300 400 500 600 700

Wawlength (nm)

Figure 5.6 Nomlized UV-vis absorption spectra of

regiochemically-contmlled thiophene-based polymers in solid state

groups, dual interactions between the hexyl gmup and the lone pair in the sp2

orbit of the sulfur atom due to the HH orientation of the pendant groups between

the trimeric units, and severe steric hindrance between thienylene and phenylene

moieties force the adjacent rings to with each other. The twisted

conformation severely disnipts sr-conjugation along the polymer chain. Therefore,

a very blue-shifted absorption band is observed for P ~ ~ D H T B . S S . ~ ~ The

values for P33DHTB and P33DHTF are ted-shifted with respect to their 4,4'-

153

counterparts. This is due to the absence of the HH orientation of pedant groups

between the trimeric uni& in the 3,3'dialkyl polymers. The observed Lax0 f 470

nm of P33DHTF in THF is about 20 nm red-shifted wmpared to that of

regioregular poly(3-hexylthiophene) and P33DHTT. This is presumably due to

the existence of more rigid furytene moieties in the backbone, which enables the

polymer to adopt a more exîended and planar conformation in solution.

No red-shift in hmax is obse~ed for PUDHTB, and only a moderate red-

shift is observed for P33DHTB and PUDHTF, when going from solution to solid

state. On the other hand, A- of P33DHTF and P33DHTT are significantly red-

shifted when going from solution to solid state. The band-gap of the polymers,

obtained from extrapolation of the low energy edge of solid state UV-visible

154

Table 5.2. Absorption and fluorescence characteristics

of the pdyrners in THF solution

Polyrner

P33DHTB

P44DHTB

P33DHTF

P44DHTF

P33DHTT

Lu (abs.)

nm

395

383

470

398

450

Stokes Shift

(nm)

95

1 O1

97

121

112

(m.)

nm

490

484

567

519

562

@t (Oh)

5455

25k3

4 2 4

28+3

3724

absorption spectra, range from 2.61 eV for P44DHTB to 1.94 eV for P33DHff

(fable 5.3). This observation is again attributed to the difference in stetic

hindrance between the polymers.

Steady state fluorescence spectra of polymers in THF solution (O0 = 0.05

- 0.10) were recorded at ambient temperature. The solution was purged with

argon prior to measurement. Fluorescence quantum yields (S 10% error) were

obtained using quinine bisuîfate (ar = 54.6% in 1.0 N H2S04) as secondary

1 able 5.3. Absorption and fluorescence characteristics

of the polymers in solid state

standard and calculated according to Equation 3.1.185 Solid state fluorescence

measurements were performed on spin cast films (00 = 0.15 - 0.50) under on

oxygen-free nitrogen atmosphere. Quantum yields (S 30% error) were

2

U+ (%)

1 8 I 6 ' -

20f 7

0.5 + 0.2

0.8 + 0.3

1.6 +_ 0.6

A,,,, (em.)

nm

490

484

567

519

562

Band-Gap

(eV)

2.56

2.61

1.97

2.45

1.94

Polymer

P33DHTB

P44DNTB

P33DHTF

P44DHTF

P330Hil

Stokes Shift

(nm)

95

1 O1

97

121

112

( S . )

nm

410

385

520

410

530

determined relative to 9,lOdiphenylanthracene (< loJ M) in PMMA glas (?+ =

83%).221 Absorption and emission charaderbocs of polymeis in solid state are

summarized in Table 5.3. Fluorescence spectra of the polyrners in solution and

in solid state are shown in Figure 5.7 and Figure 5.8, respectively.

400 500 600 700 800

Wavelength (nm)

Figure 5.7 Normalized fluorescence emission spectra of

regiochemically-controlled thiophene-based polyrners in THF solution

The polyrners are found to be highly fluorescent in THF solutions.

Emission maxima range from 385 nm for P44DHTB to 567 nm for P33DHTF,

indicating a difference of effective conjugation length in the relaxed excited state.

The emission spectra are found to be broad with a low energy shoulder. Very

large Stokes shifts, ranging from 95 nm for P33DHTB to 121 nm for PUDHTF,

are observed for the polymers. This is again attnbuted to relatively large changes

in nuclear geometry during electronic excitation.

The strong greenish-blue fluorescence from P33DHTB solutions can be

seen by naked eye.195 Its 4+ is found to be 54%; one of the highest fluorescent

efficiencies for a solution of thiophene-based polymers reported to date. Ng and

co-workers recently reported the @;s of poly(3-butyl-2,5-thienytene-afi-t,4-

phenylene) to be 48% in solution and 17% in solid state, respectively. These

values are consistent with our results.222 for P44DHTB solutions is haif that of

P33DHTB, due to the severe steric interaction between adjoining trimeric units,

Le. a head-to-head coupling, and the resulting decrease in rigidity of the polymer

in solution.

Based on the fluorescence studies of the trimers, an even higher value of

4+ for DHTF-based polyrners would have been expected because of increased

conjugation resulting from polymerization. As discussed in Chapter 4, both

P33DHTF and P44DHTF contain non-negligible amounts of carbonyl defects. It

is well known that carbonyl defects efficiently quench luminescence of

conjugated polymers.18~1188 for P33DHll' in solutions s 37%, which is

consistent with that for an extensively purified regioregular P3HT ~arn~le.162

400 500 600 700 800

Wavelength (nm)

Figure 5.8 Normalized fluorescence emission spectra of

regiochemically-controlled thiophene-based potymers in solid state

The solid state fluorescence emission spectra of P33DHTB, P44DHTB,

and P44DHTF are broad and virtually structureless. The emission spectra of

P33DHTF and P33DHTT are, however, well resolved with two peaks. Once

again, very large Stokes shifts associated with geometry rearrangement in the

excited state are observed. P44DHTB and P33DHTBs show emission bands at

500 and 511 nm, respectively. Of values of 20% and 18% for P44DHTB and

P33DHTB films are orders of magnitude higher than P3ATs. Steric hindrance

between thienylene and phenylene moieties prevents the polyrners from

achieving long range order in the solid state, as evidenced by the absence of

DSC and X-ray diffraction peaks associated with semicrystalline conjugated

158

polymers. In addition, only a modest difference in hax between solution to solid

state is evident, impiying that the DHTB polymer chains cannot adopt a planar

conformation in the solid state. In combination with the reduced heavy atom

effect, the twisted confoimation and amorphous morphology prevents

radiationless relaxation channel through x-stacking and enables the polyrners to

maintain a high 9f in the solid state. The high luminescent efficiencies of the

DHTB polymers make them very good candidates as emissive materials for

LEDs.

P44DHTF showed a weak and broad emission band at 542 nm in the solid

state. Structured emission bands at 625 and 649 nm were obsewed for

P33DHTF and P33DHTT in solid state, respectively. It is known that fluorescence

quantum yields for thiophene polymers are usually 1-2 orders of magnitude lower

in the solid state than that in solution, due to n-stacking.163 The recorded fi+

value of 1.6% for P33DHTT is consistent with that reported for P3ATs. Similar to

P33DHTT, the very low fluorescence efficiencies found for the DHTF polymers

appear to be attributed to wtacking. This assertion is made because DHTF

segments are relatively planar; even the 33'-DHTF trimeric unit, with its two alkyl

chains directed toward the central ring, can adopt a high degree of coplanarity as

indicated by its spectral properties. Despite these observations. if the

deleterious aggregation of luminescent centers can be reduced, and the

existence of carôonyl defects eliminated, dithienylfuran-based polymers have the

potential to be highly luminescent; perhaps more luminescent than

dithienylbenzene-based polymers.

7

-1 -0.5 O 0.5 1 1.5 2 2.5 E -WWM

Figure 5.9 Cyclic voltammogram of a 5 mM a-terthiophene

(a-3T) solution in 0.1 M LiC104 lacetonitrile

5.3 Cyclic Vdtammatric Study of Heteroarornatic ri mers Cyclic voltammogram of a 5 mM solution a-3T in acetonitrile containing

0.1 M LiC10.t as supporting electrolyte was performed h m 4.80 V vs AgIAgCi to

2.80 V using a clean 1 cm2 platinum electrode at ambient temperature, under an

oxygen-free nitrogen atrnosphere. A typical voltammogram is depicted in Figure

5.9. On the positive scan, three consecutive oxidation peaks, with anodic

potentials of Eol = 1 .O0 V, ECQ = 1.21 V, and Eg3 = 1.74 V, respectively, are

observed. At a potential of - 0.9 V, a dark blue polymer film is fomed on the

electrode. The palyrner film is unifomi and adherent to the electrode until the

pobntial is tiigher than 1.2 V. At higher potential, a bladc, non-uniform, and low-

adherent film is obsewed. Therefore, the O1 peak can be assigned to the

oxidation-poiymerization process of a-3T. The 02 and 03 peaks correspond to

the oxidative degradation of the polymer. On the negative scan, two reduction

peaks, with cathodic potentials of ER1 = 0.74 V and ER2 = -0.36 V, respectively

are observed. Our obsenration is different from that made by Carrasco and ca-

workers, who reported four oxidation and 3 reduction peaks for a-3T in

aceton itrile solution .47

A cyclic voltammogram of 5 mM 33-DHTT solution in 0.1 M LiC104

lacetonitrile is shown in Figure 5.10. Three consecutive oxidation peaks, with

anodic potentials of €01 = 0.98 V, EO2 = 1.13 V, and b3 = 1.66 VI respectively,

are observed. At a potential of - 0.85 V, a blue polymer film sbrts to grow on the

Pt electrode. A uniforni, adherent dark blue film is formed on the electrode M e n

the potential swept up to 0.95 V. At higher potentials (> 1.1 V), a black, non-

uniforni, and tow adherent film is obsenred. Similar to a-3T, the 01 peak is

assigned to the oxidation process of 3,3'-DHlT. The 0 2 and 03 peaks are

assigned to the oxidative degradation process of electrochemically fomed

P330HTT. On the negative scan, two consecutive reduction pea ks, with cathodic

potentials of ER, = 0.74 V and ERZ = -0.37V1 respectively, are observed. The

cathodic peaks are associated with the reduction of pdoped polymer film formed

on the positive scan.

-1 -0.5 O 0.5 1 1.5 2 2.5 3

E vs AgIAgCI (V)

Figure 5-10 Cyclic voltammogram of a 5 mM 3,3'-OHTT

solution in 0.1 M LiCLOs lacetonitrile

Comparing the voltammograms of a-3T and 3,3'-DHTT (Figures 5.9 and

5.10), one can conclude that placement of hexyl groups on the 3,3'-positions

exerts no signifiant impact on the electrochemical behavior of terthiophenes.

A cyclic voltammogram of 5 mM DTF solution in 0.1 M LiCIOs lacetonitrilel

25% (vlv) H20 is shown in Figure 5.11. On the positive scan, three consecutive

oxidation peaks, with anodic potentials at Eol = 0.93 V, Eo2 = 1.29 V, and EOJ =

1.39 V, respectively, are observed. At a potential of - 0.80 VI a dark blue

polyrner film starts to gmw on the platinum electrode. The film is unifom and

adherent up to 1.2 V. At higher potential, a non-uniforni and low adherent

polyrner film is obsewed. Apparently, 01 is associated with the oxidation

polymerization process of DTF. 02 and 03 are associated with oxidative

degradation of the polymer. On the negaüve sweep, two consecutive reduction

peaks, with cathodic potentials of ER, = 0.25 V and Ew = -0.79 VI respectively,

are obsewed. Similar results were repofted by Brillas and CO-workers. 46

Figure 5.11 Cyclic voltammogram of a 5 mM Dl F solution

in 0.1 M LiC104 laœtoniüile 125% (vlv) H20

Cyclic voltammograms of 3,3'-DHTF (Figure 5.12) and 4,4'-DHTF (Figure

5.13) in 0.1 M LiC104 lacetonitrile are very similar. Three oxidation and two

reduction peaks are observed for both compounds. Dark blue, uniform, and

adherent polymer films are formed at lower potentials. At higher potentials, an

oxidation degradation process of the polymer takes place. On the negative scan,

two reduction peaks, with cathodic potentials at ER, = 0.52 V and Ew = 4.25 V,

respectively, are observed.

-2 1 -1 -0.5 0 0.5 1 1.5 2 2.5 3

E vr AgiAgCl (V)

Figure 5.12 Cyclic voltammogram of 3,3'-DHTF in 0.1 M LiC104/acetonltrile

Comparing Figures 5.11, 5.12, and 5.13, one may notice that introducing

hexyl groups at the 3 3 - and 4,4'-positions of DTF exerts tiile effects on the

electrochemical behavior.

4 -1 -0.5 O 0.5 1 1.5 2 2.5 3

E - WWI M Figure 5.13 Cyclic voltammogram of 4,4'-DHTF in 0.1 M LiClOs/cetoniltrile

-1 -0.5 O 0.5 1 1.5 2 2.5 3

E vs AglAg Cl (V)

Figure 5.14 Cyclic voltammogram of 3,3-DHTB in 0.1 M LiCIOs Iacetonitrile

Cyclic voltamrnograms of 3,3'-DHTB and 4,4'-DHTB in 0.1 M

LiClOdacetonitrile are shown in Figures 5.14 and 5.15, respectively. The positive

scans are similar, except that the third oxidation peak is not observed for 4,4-

DHTB. Once again, unifonn and adherent polymer films are observed at lower

potentials. The films b e r n e noniiniforni and non-adherent at higher potentials.

No reduction peak is observed on the negative scan, because the polymer film

dissolves into the solution during the reduction process. However, three

reduction peaks are observed for 4,4'-DHTB. Placement of hexyl groups at the

4,4'-positions does not appear to exert significant effects on the electrochemical

behaviors of DHTBs.

Figure 5.15 Cydic voltarnmogram of 4,4'-DHTB in 0.1 M LiClOdacetonitrile

Table 5.4 Oxidation potentials of regiochemically controlled thiophene-based

heteroaromatic trimers in acetonitrile solution

Trimer Oxidation Potential (V vs AgIAgC1) 1

Oxidation potentials vs AgIAgCI reference electrode of the trimers are

a-31

3,3'-DHlT

DTF

3,3'-DHTF

4'4'-DHT F

tabulated in Table 5.4. It is ciearly shown that alkyl substitution pattern plays little

role in their oxidative polymerization process. The potential values indicate that

O1

1 .O0

0.98

0.92

0.92

0.91

the electrochemical polymerization process takes place in the sequence of DTFs,

3Ts, and DTBs.

5.4 Cyclic Voltammettic Study of Polymers

02

1.21

1.13

1.37

1.48

1.67

Electrochemical polymerization of unsubstituted heteroaromatic trimers

0 3

1.74

1.66

2.20

2.10

and alectrochemical behavior of the electrochemically prepared poiymers have

been reported by several research gnwps.41 142144-47 Electrochemical behavior

167

of some dithienylphenylene-based polymers has been dowmented by Reynolds

Figure 5.16 Cyclic voltammogram of a spin-cast film of P33DHTT

on platinum electrode in 0.5 M LiC104 Iacetonitrile

A film of chernically prepared polymer was spincast on to a flame cleaned

platinum working electrode. Cyclic voltammograms of cast polymer films were

obtained in 0.5 M LiC104 acetonitrile solution using a scan rate of 5 mVls at

ambient temperature. A typical voltamrnogram of P33DHTT film is shown in

Figure 5.16. On the positive scan, three wnsecutive oxidation peaks, with

anodic potentials of E o ~ = 0.46 V, EOS = 0.51 V, and Egj = 0.56 V, respectively,

are obsewed. On the negative sweep, three reduction peaks, with cathodic

potentials of ER, = 0.53 V, ER^ = 0.41 V, and ER3 = 0.35 V, respectively, are

observed. A dear electrochmmic change can be observed. The color changes

from purple to da& blue upon oxidation; and changes back to purple on the

negative scan.

-500 -] l

-200 -100 O 100 200 300 400 500 600

E n WAeCl (mW

Figure 5.17 Cyclic voltammogram of a spin-cast film of P33DHTF

on platinum electrode in 0.5 M LiClO4 Jacetonitrile

A cyclic voltammogram of a spin-cast film of P33DHTF film in 0.5 M LiC104

acetonitrile solution is show in Figure 5.17. One oxidation peak at the potential

of 0.49 V is obsenred during the positive scan. The correspanding reduction peak

is obsenred at 0.39 V during the negative scan. An electrochromic change from

purple in the reduced fom to dark blue in the oxidized fom is obsenred. The

electrochemical behavior of the chemically prepared P33DHTF film is found to be

very similar to the electrochemically polymerized one (CV not shown).

Figure 5.18 Cyclic voltammogram of a spin-cast film of P44DHTF

on platinum electroâe in 0.5 M LiC104 lacetonitrile

In the cyclic voltammogram of a cast film of PUDHTF, an oxidation peak

at 0.72 V and a reduction peak at 0.62 V are observed. The color of the film

changes to blue (oxidized form) on positive scan and reverses back to yellow

(reduced fom) during negative sweep. The electrochemical behavior of this film

is very similar to the electrochernically synthesized one.

4 I

200 400 600 800 IO00 1200 1400

E AglAscl (W Figure 5.19 Cyclic voltammogram of a spin-cast film of P33DHTB

on platinum electrode in 0.5 M LiClOdacetonitrile

Cyclic voltamrnograms of cast films of P33DHTB and P44DHTB in 0.5 M

LiC104 acetonitfile solution are shown in Figures 5.19 and 5.20. For P33DHT0,

an oxidation peak at 1.1 V and the corresponding reâuction peak at 0.79 V are

obsewed. For PUDHTB, the oxidation peak and the corresponding reduction

peak are observed at 1.17 V and 0.77 V, respectively. Electrochromic change

form yellow to dark blue is observed for both films. The electrochemical behavior

of the films is very similar to their electrochemically prepared counterparts.

-4 -600 -1 00 400 900 1400

E vs Ag/AgCl (mV)

Figure 5.20 Cyclic voltammogram of a spin-cast film of P44DHTB

on platinum electrode in 0.5 M LiClOdacetonitrile

The work function of undoped conjugated polymer is correlated to the

onset electrode potential. De Leeuw and CO-workers has proposed that the

HOMO energy level can be calculated by equation 5.1.224.225

ELUm can then be calculated from EHoMo and the band gap (AE) values obtained

from UV-vis absorption spectra (Equation 5.2).

ELUMO = EHOMO + AE (eq. 5.2)

The E H ~ ~ ~ , and ELoMo of the polymers are thus calculateci and listed in

Table 5.5, together with their Eo, EmW values. The energy levels of HOMO and

LUMO provide guidelines in seleeting the electrode materials when consûucting

a LED based on these polymers.

- - -- -- - - - - - - - - - -- -

On the oxidatin scan, both onset and peak potentials of PDHTBs are

significantly higher than that of the corresponding PDHTFs and P33DHTT. The

oxidation potential of the 4,4'dihexyi polymers are also found to be higher than

that of their 3,3-dihexyl analogous. These observations may be attributed to the

decrease in effective conjugation length of the 4,4'dihexyl polymers resulted

from the HH orientation of the pendant hexyl group between the trimeric units.

173

Table 5.5 Energy levels of regioehemiwlly-controlled thiophene-based

conjugated polyrners

Polymer

P33DHTT

P33DHTF

P44DHTF

P33DHTB

P44DHTB

Eo (V)

0.46

0.49

0.72

1.1

1.17

E-t (V)

0.39

0.40

0.64

0.90

0.95

EHOM (V)

-4.75

-4.76

-5.00

-5.26

-5.31

ELUMO (V)

-2.81

-2.79

-2.55

-2.70

-2.70

5.5 Summary

Optical and fluorescence properties of novel regiochemically-controlled

thiophene-based heteroaromatic trimers and their polymers are studied.

Fluorescence quantum yields (Qf) of the trimers range from 5.1% of 3-3'-DHff to

57% of DTF. Emission maxima of polymer solutions range from 484 nm for

P44DHTB to 567 nm for P33DHTF. The solid state emission maxima range from

500 nm for P44DHTB to 649 nm for P330HTT. U+'s of the polymers in THF

solution range from 25% to 54%. The large Stokes shifts observed for both

trimers and polymers are attributed to the skeletal rearrangement upon

excitation. The fluorescence quantum yields of P44DHTB and P33DHTB in solid

state are 20% and 18%, respectively, orders of magnitude higher than ordinary

thiophene-based conjugated polymers. The large difference in Qf is attributed to

heavy atom and steric effects. The high luminescence eficiencies of DHTB

polymers make them good candidates as emissive materials for LEDs.

Cyclic voltammetric study indicates that placement of alkyl groups at the

3,3'-and 4-4'-positions exerts little effect on the electrochemical behavior of the

heteroaromatic trimers. DHTBs are found to be more difficult to oxidize than the

DTFs and 3Ts. Placement of hexyl groups on the 4-4'-positions, however, plays a

significant role in the electrochemical behavior of the polymers. The 4,4'dihexyl

polymers are oxidized at higher potentials than their 3-3'-analogues. PDHTBs are

also found to be more difficult to oxidize than the others. This is attributed to the

1 74

stenc effect which significantly reduces the conjugation length of PDHTBs and

the 4,4'-polymers.

HOMO and LUMO energy levels of the polyrners are estimated based on

electrochemical and optical data.

5.6 Experimental

Hexanes, THF, and chloroform for fluorescence measurement were

spectrograde reagents h m Caledon. Acetonitrile and acetone for

electrochemical measurement were HPLC grade reagents from Caledon. Lithium

perchlorate was dried in an oven at 80°C prior to use. All other chemicals are

commerciaHy available reagents and were used as received.

UV-visible absorption spectra were recorded on a Cary 3E

spectrophotometer at ambient environment. Steady state fluorescence spectra

were taken on a PT1 ~ u a n t u m ~ a s t e r ~ ~ Model QM-1 Fluorescence System at

ambient temperature. Solutions (OD = 0.05 - 0.10) in four-sided suprasil were

deoxygenated by purging with argon for 10 min. prior ta the measurement.

Fluorescence quantum yields (5 10% error) were measured by using 2-

aminopyridine (@t =60% in 0.1 N H2S04) and quinine bisulfate (41 = 54.6% in 1 .O

N H2S04) as secondary standards for trimers and polyrners, respectively.l*5

Solid state fluorescent measurements were performed on spin cast films (00 =

0.15 - 0.50) under oxygen-free nitrogen atmosphere. Quantum yields (I 30%

enor) were determined relative to 9,1O-diphenylanthracene (c 10~) M) in PMMA

175

glass (@v = 83%).221 Fluorescence quantum yields were calculateci according to

equation 3.1.185

Cyclic voltammetric studies were carried out on a cornputer driven PAR

286 (EG8G) potentiostat. The experiments were perfomed in a one-

compartment three electrode cell under inert atmosphere generated by a flow of

oxygen-free nitrogen The working and counter electrodes are two platinum

sheets of 1 and 4 cm4 surface area. respectively. The working elecbode was

rinsed and flame cleaned prior to every scan. An AglAgCl electrode saturated

with NaCl aqueous solution was used as reference electrode. All potentials are

reported reference to this electrode. Cyclic voltammograms of the trimers were

obtained using 0.1 M LiCIOs as supporting electrolyte and at a scan rate of 50

mVls. Polymer films for electrochemical studies were spin-cast fmrn chloroform

solution on to the platinum working electrode. Cyclic voltammograrns of the films

were perforrned using 0.5 M LiC104 as supporting electrolyte and at a scan rate

of 5 mV1s.

Chapter 6

Conclusions

One of the most promising applications of conjugated polymers is as the

emissive layer for LED-based flat panel displays. This work addressed two of the

major challenges in this application for thiophene-based conjugated polymers:

the emission color tuning and the development of thiophene-based materials with

high luminescence efficiency in the solid state. The thermochromism of P3ATs

was also addressed in this research.

Thermochromism of P3ATs is a well-known phenornenon and a large

number of research articles have b e n published. However, conflicting results

have been reported. To address this issue, a series of P3ATs (A = hexyl, octyl,

dodecyl, and hexadecyl) with different head-to-tail (HT) regio-regulanties were

synthesized. Their thennochromic behavior and morphological properties have

been investigated. It was show in this work that the thermochromic properties of

P3ATs are controlled by the head-to-tail diad content and the alkyi side chain

length of the sample. P3ATs with moderate HT diad content give rise to a clear

isosbestic point, while polyrners with high HT diad content and short alkyl side

chains exhibit no isosbestic point with increasing temperature. This is due to a

morphological effect. P3ATs with moderate HT diad content are found to be

formally amorphous with some short range ordered (quasi-ordered) structure

dispersed in the disorder bulk. The coexistence and interconversion of the twa

177

phases is believed to be responsible for the observed isosbestic point. P3ATs

with high HT diad content and short alkyl side chains are found to be semi-

crystalline. The crystalline, quasi-ordered, and disordered phases equilibrate

with each other in the thin film. The isosbestic point is destroyed by this

multiphase equilibrium. P3ATs with high HT diad content and long alkyl side

chains are also semi-crystalline. These polyrners melt at much lower temperature

and crystalline phase is converted directly into disordered phases. Therefore, a

broad isosbestic point was obsenred. A phenomenological mode1 for predicting

the existence or absence of an isosbestic point was proposed and verified based

on experimental results.

Band-gap tuning of conjugated polyrners usually involves lengthy

synthesis. It is demonstrated in this work that the conformation, and hence band-

gap, of poly(3-(6-acryloyloxy)hexylthiophene) (P3AHT) films can be pemanently

affected by a post-synthetic crosslinking step, by taking advantage of their

therrnochromic property. P3AHT shows strong solution and solid state absorption

bands, and is fluorescent in both solution and solid state. P3AHT films undergo

an irreversible thermochromic change with increasing temperature. The

absorption maximum blue shifts from 489 nm to 435 nm upon heating. The band-

gap changes from 1.85 eV before heating to 2.24 eV after heating. Accordingly,

the emission maximum blue shifts from 642 nm to 594 nm, upon heating. This is

due to the thermal crosslinking of the acryloyloxy functionality at elevated

temperatures, which 'locks in" the twisted conformation of the polymer chain.

The application of P3ATs in polymeric LEDs has been limited owing to

their low luminescence efkiency in the solid state. It is anticipated that the solid

state fluorescence efficiency of P3ATs might be enhanced by replaciement of a

fraction of thienylene moieties with phenylene and furylene groups and by

introducing steric constraints in a regiochemically-controlled manner. Therefore,

a series of regiospecific 2,54(hexylthienyl)furans (DHTFs), 1,4-

di(hexylthieny1)benzenes (DHTBs) and corresponding polyrners were

synthesized, and their fluorescence properties investigated. The large Stokes

shifts observed for both trimers and polymers is attributed to the skeletgl

rearrangement upon excitation. Fluorescence quantum yields (0;s) of DHTBs

and DHTFs are found to be substantially higher than the corresponding ones of

terthiophenes. Emission maxima of polymer solutions range from 484 nm for

P44DHTB to 567 nm for P33DHTF. The solid state emission maxima range from

500 nm for P44DHTB to 649 nm for P33DHTT. 0;s of the polymers in THF

solution range h m 25% to 54%. The fluorescence quantum yieMs of P44DHTB

and P33DHTB in solid state are found to be 20% and 18%, respectively, orders

of magnitude higher than ordinary thiophene-based wnjugated polymers. The

large difference in 0f is attributed to heavy atom and steric effects. The high

luminescence efficiencies of DHTB polymers make them good candidates as

emissive materials for LEDs. The electrochemical properties of these polymers

were also investigated. The band-gaps and the work functions of the polymers

were estimated from optical and electrochemical data.

References

Skotheim, T. A. Handbook of Conducting Polymers; Skotheim, T. A., Ed.;

Marcel Dekker: New York, 1986.

Kiess, H. G. Conjugated Conducting Polymers; Kiess, H. G., Ed.;

Springer-Verlag: Berlin, New York, 1992.

Scorati, B. Application of Electroactive Polymers; Scora ti, B., Ed . ;

Chapman & Hall: London, 1994.

Salaneck, W. R.; LundestrOm, 1.; Ranby, 6. Conjugated Polymers and

Related Materials; Oxford University Press: New York, 1993.

Shirota, Y. Functional Monomers and Polymers; 2nd ed.; Marcel Dekker:

New York, 1997, Chapter4.

Nalwa, H. S. Handbook of Organic Molecules and Polymers; John Wiley 8

Sons: Chichester, 1997; Vol. 1-4.

Schopf, G.; Kobmehl, G. Adv. Polym. Sci. 1996, 129, 1 .

Diaz, A. F.; Nguyen, M. T.; Letclerc, M. Physical Electmchemistry:

Princip/es, Methods, and Applications; Marcel Dekker: New York, 1995;

Chapter i 2.

Roncali, J. Chem. Rev. 1992,92,9711.

Roncali, J. Chem. Rev. 1997, 97, 173.

1 1) McCullough, R. D. Adv. Mater. 1998, 10, 93.

Novak, P.; Müller, K.; Santhanam, K. S. V.; Haas, O. Chem. Rev. 1997,

97,207.

Heinze, J. Top. Cun. Chem. 1990, 752, 1.

Evans, G. P. Adv. Electrochem. Sci, Eng. 1990, 1, 1.

Letheby, H. J. Chem. Soc. 1862, 75,161.

Ito, T.; Shirakawa, H.; Ikeada, S. J. Polym. Sci., Polym. Chem. Ed. 1974,

12, l l .

Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A.

J. J. Chem. Soc., Chem. Commun. 1977,578.

Etemad, S.; Heeger, A. J.; MacDiarmid, A. G. Ann Rev. Phys. Chem.

1982, 33,443.

Naannann, H.; Theophilou, N. Synth. Met. 1987, 22, 1.

Burroughes, J. H.; Bradley, O. D. C.; Brown, A. R.; Marks, R. N.; Mackay,

K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990, 347,539.

Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri, N.;

Heeger, A. J. Nature 1992,357,477.

Horowitz, G. Adv. Mater. 1998, 10,365.

Kumar, A.; Welsh, D, M.; Morvant, M. C.; Piroux, F.; Abboud, K. A.;

Reynolds, J. R. Chem. Mater. 1998, 10,896.

Abdou, M. S. A.; Xie, 2.; A., L.; Holdcroft, S. Synth. Met. 1992, 52, 159.

Yu, 3.; Abley, M.; Yang, C.; Holdcroft, S. Chern. Commun. 1998, 1503.

Holdcroft, S.; Yu, J.; Abley, M.; Yang, C. Polym. Prepnnts 1998, 39,85.

Imisides, M. D.; John, R.; Wallace, G. G. Chemtech 1996,26, 19.

Tourillon, G.; Gamier, F. J. Electroanal. Chem. 1982, f35, 173.

Hotta, S.; Hosaka, T.; Shimotsuma, W. J. Chem. Phys. 19û4, 80,954.

Tourillon, G.; Garnier, F. J. Phys. Chem. 1983, 87,2289.

Hoier, S. N.; Park, S. M. J. Electmchem. Soc. 1993, 140,2454.

Roncali, J.; Yassar, A.; Garnier, F. Synth. Met. 1989, 28, C275.

Sato, M.; Tanaka, S.; Kaeriyama, K. J. Chem. Soc., Chem. Commun.

1986,873.

Sato, M.; Tanaka, S.; Kaeriyama, K. Synth. Met. 1987, f8, 229.

Hotta, S. Synth. Met. 1987, 22,103.

Wang, S.; Takahashi, H.; Yoshino, K.; Tanaka, K.; Yamaber, T. Jpn. J.

Appl. Phys. Part 1 1990,29,772.

Rasch, B.; Vielstich, W. J. J. Electmanal. Chem 1994, 370, 109.

Roncali, J.; Lemaire, M.; Gamau, R.; Garnier, F. Synth. Met. 1987, 18,

139.

Roncali, 3.; Gorgues, A.; Jubault, M. Chem. Mater. 1993, 5, 1546.

Visy, C.; Lukkari, J.; Kankare, J. Macromolecules 1994, 27, 3322.

Ferraris, J. P.; Skiles, G. D. Polymer 1987, 28, 179.

Ferraris, J. P.; Hanlan, T. R. Polymer 1989, 30, 1319.

Reynolds, J. R.; Ruiz, J. P.; Child, A. D.; Nayak, K.; Marynick, D. S.

Macromolecules 1991 , 24,678.

Joshi, M. V.; Hemler, C.; Cava, M. P.; Cain, J. L.; Bakker, M. G.; McKinley,

A. J.; Metzger, R. M. J. Chem. Soc. Perkin Tram. 2 1993,1084.

Brillas, E.; Anton, G.; Otero, T. F.; Carrasco, J. J. Electmanal. Chem.

1998,445,125.

Brillas, E.; Carrasco, 3.; Figueras, A.; Urpi, F.; Otero, T. F. J. Electroanal.

Chem. 1995,392,55.

Carrasco, J.; Otero, T. F.; Brillas, E.; Montilla, M. J. Electroanal. Chem.

1996,418,115.

Yamamoto, T.; Sanechika, K.; Yamamoto, A. J. Polym. Sci., Polym. Le#.

Ed. 1980, 18,9.

Lin, J. W. P.; Duedek, L. P. J. Polym. Sci., Polym. Chem. Ed. 1980, 18,

2869.

Tamao, K.; Sumitani, K.; Kumada, M. J. Am. Chem. Soc. 1972,94,4376.

Yashino, K.; Hayashi, S.; Sugimoto, R. Jpn. J. Appl. Phys. 1984, 23, L899.

Jen, K.-Y.; Miller, G. G.; Elsenbaumer, R. L. J. Chem. Soc., Chem.

Commun. 1986,1346.

Elsenbaumer, R. L.; Jen, K.-Y.; Oboodi, R. Synth. Met. 1986, 15,169.

Chen, S. A.; Tsai, C. C. Macromolecules 1993, 26,2234.

Mao, H.; Xu, B.; Holdcroft, S. Macromolecules 1993, 26,1163.

Mao, H.; Holdcroft, S. Macmmolecules 1992, 25,554.

Sugimoto, R.; Takeda, S.; Gu, H. B.; Yoshino, K. Chem. Express. 1986, f ,

Pomerantz, M.; Tseng, J. J.; Zhu, H.; Sproull, S. J.; Reynolds, J. R.; Uik,

R.; Amott, H. J. Synth. Met. 1991, 4143,825.

Holdcroft, S. Macromolecules 1991, 24.

Laakso, J.; Jiiwinen, H.; Skagerberg, B. Synth. Met. 1993, 5557, 1204.

Leclerc, M.; Diaz, F. M.; Wegner, G. Makromol. Chem. 1989, 190,3105.

Abdou, M. S. A.; Lu, X.; Xie, 2. W.; Orfino, F.; Deen, M. J.; Holdcroft, S.

Chem. Mater. 1995, 7,631.

Abdou, M. S. A.; Hodcroft, S. Macromolecules 1993, 26,2954.

Arroyo-Villan, M. 1.; Diaz-Quijada, G. A.; Abdou, M. S. A.; Holdcroft, S.

Macromolecules 1995, 28,975.

Niemi, V. M.; Knuuttila, P.; Osterholm, J.-E.; Korvola, J. Polymer 1992, 33,

1559.

Curtis, M. O.; McClaim, M. D. Chem. Mater. 1996,8,936.

McClain, M. D.; Whitttingto, D. A.; Mitchell, D. J. J. Am. Chem. Soc. 1995,

1 1 7,3887.

Curtis, M. D.; McClain, M. O. Chem. Mater. 1996, 8,945.

McCullough, R. D.; Lowe, R. D. J. Chem. Soc., Chem. Commun. 1992,

70.

McCullough, R. D.; Lowe, R. D.; Jayaraman, M.; Anderson, D. L. J. OQ.

Chem. 1993,58,904.

McCullough, R. D.; Lowe, R. D.; Jayaraman, M.; Ewbank, P. C.;

Anderson, D. L. Synth. Met. 1993,55-57,1198.

McCullough, R. D.; Tristram-Nagle, S.; Williams, S. P.; Lowe, R. D.;

Jayaraman, M. J. Am. Chem. Soc. 1993, 1 15,4910.

Chen, T A ; Wu, X.; Rieke, R. D. J. Am. Chem. Soc. 1995, 11 7,233.

Chen, T.-A.; O'Brien, R. A.; Rieke, R. D. Macromolecules 1993, 26,3462.

Chen, T.-A.; Rieke, R. O. 3. Am. Chem. Soc. 1992, ll4,lOO87.

Chen, T.-A.; Rieke, R. D. Synrh. Met. 1993, 60, 175.

Tamao, K.; Kodama, S.; Nakajima, J.; Kumada, M. Tetrahedron 1982, 38,

3347.

Bumagin, N. A.; Beleskaya, 1. P. Russ. Chem. Rev. 1990, 59,1174.

Yamamoto, T.; Wakabayashi, S.; Osakada, K. J. Organomet. Chem.

1992,428,223.

Yamamoto, A.; Yamamoto, T.; Ozawa, F. Pure Appl. Chem. 1985, 57,

1779.

McCullough, R. O.; Williams, S. P.; Tristam-Nagle, S.; Jayamaraman, M.;

Ewbank, P. C.; Miller, L. Synth. Met. 1995, 69,279.

McCullough, R. O.; William, S. P. J. Am. Chem. Soc. 1993, 7 15,11608.

McCullough, R. D.; Ewbank, P. C.; Loewe, R. C. J. Am. Chem. Soc. 1997,

119,633.

Bjorholm, T.; Greve, D. R.; Reitzel, N.; Hassenkam, T.; Kiaer, K.; Howes,

P. B.; Larsen, N. B.; Bogeiund, J.; Jayaraman, M.; Ewbank, P. C.;

McCullough, R. D. J. Am. Chem. Soc. 1998, 120,633.

Wu, X.; Chen, T.-A.; Rieke, R. D. Macromolecules 1996, 29,7671.

Loewe, R. S.; Khersonsky, S. M.; McCullough, R. D. Adv. Mater. 1999, 17,

250.

Guillerez, S.; Bidan, G. Synth. Met. 1998, 93,123.

Souto Maior, R. M.; Hinkelman, K.; Eckert, H.; Wudl, F. Macromolecules

1990,23,1268.

Chen, S.-A.; Ni, J.-M. Macromolecules 1992, 25,6081.

Stein, P. C.; Botta, C.; Bolognesi, A.; Catellani, M. Synth. Met. 1995, 69,

305.

Stein, P. C.; Bolognesi, A.; Catellani, M.; Destri, S.; Zetta, L. Synth. Met.

1991,41-43,559.

Sato, M.; Moni, H. Macromolecules 1991, 24, 1 196.

Sato, M.; Shimizu, T.; Yamauchi, A. Synth. Met. 1991, 41-43, 551.

Gallani, M. C.; Castellani, L.; Zerbi, G.; Souani, P. Synth. Met. 1991, 41-

43,495.

Barbarella, G.; Bongini, A.; Zambianchi, M. Macromolecules 1994, 27,

3039.

Holdcroft, S. J. Polym. Sci., Polym. Phys. Ed. 1991, 29, 1585.

Heffner, G. W.; Pearson, D. S. Macromolecules 1991,24,6259.

Ostertiolm, J.-E.; Laakso, J.; Nyholm, P.; Isotalo, H.; Stubb, H.; Inganas,

O.; Salaneck, W. R. Synth. Met. 1989,28, C435.

Tashiro, K.; Ono, K.; Minagawa, Y.; Kobayashi, M.; Kawai, T.; Yoshino, K.

J. Polym. Sci., Polym. Phys. Ed. 1991, 1991, 1223.

100) Bolognesi, A.; Ponio, W.; Provasoli, F.; Gquerra, T. Makromol. Chem.

1993,194,817.

101) Bolognesi, A.; Porzio, W.; Zhou, G.; Ezquerra, T. Eur. Polym. J. 1996, 32,

1097.

102) Ho, K.-S.; Bartus, J.; Levon, K.; Mao, J.; Zheng, W.-Y. Synth. Met. 1993,

55-57,384.

103) Hsu, W.-P.; Levon, K.; Ho, K.-S.; Myerson, A. S.; Kwei, T. K.

Macromolecules 1993, 26, 131 8.

104) Park, K.; Levon, K. Macromolecules 1997,30,3175.

105) Zhao, Y.; Yuan, G.; Roche, P.; Leclerc, M. Polymer1995,36, 2211.

106) Zhao, Y.; Keroack, D.; Yuan, G.; A., M.; Hanna, R.; Leclerc, M. Macromol.

Chem. Phys. 1997, 198,1035.

107) Meille, S. V.; Romita, V.; Caronna, T.; Lovinger, A. J.; Catellani, M.;

Belobrazeckaja, L. Macromolecules 1997, 30,7898.

108) Shimomura, M.; Kaga, M.; Nakayama, N.; Miyauchi, S. Synth. Met. 1995,

69,313,

109) Kricheldorff, J. H.; Domschke, A. Macmmolecules 1996,29, 1337.

1 10) Lee, J. L.; Pearce, E. M.; Kwei, T. K. Macromolecules 1997,30,6877.

1 1 1) Aubery, O.; Barnatt, A. J. Polym. Sci., Polym. Phys. Ed. 1968, 6,241.

112) Winokur, W. J.; Wamsley, P.; Moulton, J.; Smith, P.; Heeger, A. J.

Macromolecules 1991,24,3812.

113) Prosa, T. J.; Winokur, W. J.; Moukon, J.; Smith, P.; Heeger, A. J.


114) Prosa, T. J.; Winokur, W. J.; Moulton, J.; Smith, P.; Heeger, A. J. Synth.

Met. 1993, 5557,370.

115) Prosa, T. J.; Winokur, W. J.; McCullough, R. D. Macmmolecules 1996, 29,

3654.

116) Ihn, K. J.; Moulton, J.; Smith, P. J. Polym. Sci. Polym. Phys. 1993, 31,

735.

117) Mardalen, J.; Samuelsen, E.; Gautun, O. R.; Carlsen, P. H. Synth. Met.

1992, 48,363.

118) Fell, H. J.; Samuelsen, E. J.; Bakken, E.; Carlsen, P. H. J. Synth. Met.

1995, 72,193.

119) Mardalen, J.; Fell, H. J.; Samuelsen, E.; Bakken, E.; Carlsen, P. H. J.;

Andersson, M. R. Macromol. Chem. Phys. 1995, 196,553.

120) Fell, H. J.; Samuelsen, E.; Mardalen, J.; Bakken, E.; Carlsen, P. H. J.

Synth. Met. 1995, 69,301.

121 ) Lunzy, W .; Pron, A. Synth. Met. 1995,69,337.

122) Lunzy, W.; Pron, A. Synth. W. 1996, 79,37.

123) Lunzy, W.; Trznadel, M.; Pron, A. Synth. Met. 1996, 81,71.

124) Kaniowski, T.; Lunzy, W.; Niziol, S.; Sanetra, J.; Trznadel, M. Synth. Met.

1998, 92,7.

125) Fell, H. J.; Samuelsen, E.; Mardalen, J.; Andersson, M. R. Synth. Met. 1

995,69,283.

126) Kawai, T.; Nakazono, M.; Sugimoto, R.; Yoshino, K. Synth. Met. 1993, 55-

57,353.

127) Mardalen, J.; Samuelsen, E.; Gautun, O. R.; Carisen, P. H. Sdid State

Commun. 1991, 77,337.

128) Kawai, T.; Nakazono, M.; Sugimoto, R.; Yoshino, K. J. Phys. Soc. Jpn.

1992,61,3400.

129) Nakazono, M.; Kawai, T.; Yoshino, K. Chem. Mater. 1994,6,864.

130) Tashiro, K.; Kobayashi, M.; Morita, S.; Kawai, T.; Yoshino, K. Synth. Met.

1995, 69,397.

131) Kawai, T.; Nakazono, M.; Yoshino, K. Synth. Met. 1995, 69, 379.

132) Tashiro, K.; Kobayashi, M.; Kawai, T.; Yoshino, K. Polymer 1997, 38,

2867.

133) Bolognesi, A.; Catellani, M.; Destri, S.; Porzio, W. Makromol. Chem.,

Rapid. Commun. 1991, 12,Q.

134) Corish, J.; Morton-Blake, D. A.; Lantoine J. Chem. Soc., Faraday Trans.

1996, 92,671.

135) Chen, S.-A.; Lee, S.-J. Polymer l995,36, 1719.

136) Yamamoto, T.; Komarudin, D.; Ami, M. L., B.-L.; Suganuma, H.; Asakawa,

N.; Inoue, Y.; Kubota, K.; Sasaki, S.; Fukuda, T.; Matsuda, H. J. Am.

Chem. Soc. 1998, 120,2047.

MArdalen, 3.; Samuelsen, E. J.; Gautun, O. R.; Carlsen, P. H. Synth. Met.

1992, 48,363.

fashiro, K.; Minagawa, Y.; Kobayashi, M.; Morita, S.; Kawai, T.; Yoshino,

K. Synth. Met. 1993,5557,321.

Rughooputh, S. D. D. V.; Hotta, S.; Heeger, A. J.; F., W . J. Poly. Sci.

Polym. Phys. Ed. 1987,25, 1 O7 1.

Bao, Z.; Chan, W.; Yu, L. Chem. Mater. 1993, 5, 2.

Inganas, 0.; Gustafsson, G.; Salaneck, W. R. Synth. Met. 1989, 28, C3n.

Inganas, 0.; Salaneck, W. R.; Osterholm, J.-E.; Laakso, J. Synth. Met.

1988, 22,395.

Salaneck, W. R.; Inganas, 0.; Nilsson, J.-O.; C)sterholm, J.-E.; Themas,

B.; Brédas, J.-L. Synth Met. 28 1989, C451.

Yoshino, K.; Park, D. H.; Onoda, M.; Sugimoto, R. Solid State Commun

1988,67,1119.

Yoshino, K.; Park, D. H.; Onoda, M.; Sugimoto, R. Jpn. J. Appl. Phys

1988,27, L1612.

Winokur, M. J.; Spiecel, O.; Kim, Y.; Hotta, S.; Heeger, A. J. Synth. Met.

1989, 28, C419.

Roux, C.; Bergeron, J.-Y.; Leclerc, M. Makmmol. Chem. 1993, 194, 869.

Roux, C.; Faid, K.; Leclerc, M. Makmmol. Chem., Rapid Commun. 1993,

14,461.

149) Roux, C.; Leclerc, M. Chem. Mater. 1994, 6,620.

150) Roux, C.; Leclerc, M. Macmmolecules 1992, 25,2141.

151) Leclerc, M.; Roux, C.; Bergeron, J.-Y. Synth. Met. 1993, 55-57,287.

152) Faid, K.; Frechette, M.; Ranger, M.; Mazerolle, L.; Levesque, 1.; Leclerc,

M.; Chen, T.-A.; Rieke, R. D. Chem. Mater. 1995, 7.

153) Iwasaki, K.; Fujimoto, H.; Matsuraki, S. Synth. Met. 1994, 63, 101.

154) Chosrovian, H.; Rentsch, S.; Grebner, D.; Dahm, D. U.; Brickner, E.

Synth. Met. 1993, 60,23.

155) Herrema, J. K.; van Hutten, P. F.; Gill, R. E.; Wildeman, J.; Wieringa, R.

H.; Hadziioannou, G. Macmmolecules 1995, 28,8102.

156) Kanemitsu, Y.; Suzuki, K.; Masumoto, Y.; Tomiuchi, Y.; Shiraishi, Y.;

Kuroda, M. Phys. Rev. 6 1994,50,2301.

157) Belletete, M.; Mazerolle, L.; Desrosier, N.; Leclerc, M.; Durocher, G.


158) van Hutten, P. F.; Gill, R. E.; Herrema, J. K.; Hadziioannou, G. J. Phys.

Chem. 1995,99,3218.

159) Kanemitsu, Y.; Suzuki, K.; Tomiuchi, Y.; Shiraishi, Y.; Kuroda, M.; Nabeta,

O. Synth. Met. 1995, 71,2209.

160) Kraabel, B.; Moses, D.; Heeger, A. J. J. Chem. Phys. 1995, 103,5102.

161) Becker, R. S.; de Melo, J. S.; Macanita, A. L.; Elisei, F. Pure Appl. Chem.

1995, 67,9.

162) Magnani, L.; Rumbles, G.; Samuel, 1. D. W.; Murray, K.; Moratti, S. C.;

Holmes, A. B.; Friend, R. H. Synth. Met. 1997, 84,899.

163) Xu, B.; Holdcroft, S. Macmmolecules 1993, 26,4457.

164) Gill, R. E.; Malliaras, G. G.; Wildeman, J.; Hadziioannou, G. Adv. Mater.

1994, 6, 132.

165) Beriman, 1. B. Fluorescence Spectra of Aromatic Molecules; 2nd ed.;

Academic Press: New York, 1971.

166) Xu, B.; Holduoft, S. J. Am. Chem. Soc. 1993, 1 15,8447.

167) Xu, B.; Holdcroft, S. Thin Solid Films 1994, 242,74.

168) Xu, B.; Holdcroft, S. Adv. Mater. 1994, 6, 325.

169) Xu, B.; Lowe, J.; Holdcroft, S. Thin Solid Films 1994, 243, 638.

170) Louarn, G.; Kruszka, J.; Lefrant, S.; Zagorska, M.; Kulszewicz-Bayer, 1.;

Pron, A. Synth. Met. 1993, 61,233.

171) Yoshino, K.; Manda, Y.; Sawada, K.; Onoda, M.; Sugimoto, R. Solid State

Commun. 1989,69,143.

172) Hess, B. C.; Kanner, G. S.; Vardeny, Z. Phys. Rev. 8 1993,47,1407.

173) Van Pharrn, C.; Mark, H. B., Jr.; Zimmer, H. Synth. Comm. 1986, 16,689.

174) Consiglio, G.; Gronowitz, S.; Homfeldt, A.-B.; Maltesson, B.; Noto, R.;

Spinelli, D. Chem. Scfipta 1977, 11, 175.

175) Mitchell, R. H.; Lai, 1.-H.; Williams, R. V. J. Org. Chem. 1979,44,4733.

176) Luzny, W.; Pron, A. Synth. Met. 1995, 69,337.

177) Cullity, B. C. Elements of X-Ray Diffraction; 2nd ed.; Addison-Wesley:

Reading, MA, 1978.

178) Bodor, G. Structural Investigation of Polymers; Ellis Horwood Limited:

Chichester, West Sussex, England, 1991.

179) Zerbi, G.; Castellani, L.; Chierichetti, C.; Gallani, C. Chem. Phys. Lett.

1990, 172,143.

180) Grern, G.; Leditzky, G.; Ullrich, B.; Leising, G. Adv. Mater. 1992,4, 36.

181) Holmes, A. B.; Bradley, D. D. C.; Brown, A. R. Synth. Met. 1993, 55-57,

4031.

182) Pham, C. V.; Mark, H. B.; Zimmer, H. Synth. Commun. 1986,689.

183) Tamao, K.; Sumitani, K.; Kiso, Y.; Zembayashi, M.; Fujioka, A.; Kodama,

S.; Nakajima, 1.; Minato, A.; Kumada, M. Bull. Chem. Soc. Jpn. 1976, 49.

184) Lane, F. C. J. Org. Chem. 1974,39.

1 85) Eaton, D. F. Pure Appl. Chem. 1988, 60, 1 1 O?.

1 86) Yang, C.; Holdcroft, S. unpublished result.

187) Yan, M.; Rothberg, L. J.; Padadimitrakopoulos, F.; Galvin, M. E.; Miller, T.

M. Phys. Rev. Lett. 1994, 73,744.

188) Rothberg, L. J.; Yan, M.; Padadimitrakopoulos, F.; Galvin, M. E.; Kwock,

E. W.; Miller, T. M. Synth. Met. 1996, 80,41.

189) Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978, 43,2923.

190) Greenham, N. C.; Samuel, 1. D. W.; Hayes, G. R.; Phillips, R. T.;

Kessener, Y. A. R. R.; Moratti, S. C.; Holmes, A. B.; Friend, R. H. Chem.

Phys. Lett. 1995, 241,89.

191) Kodaira, T.; Watanabe, A.; Fujitsuka, M.; Ito, O. Pdymer 1997, 39,3793.

Saadeh, H.; Goodçon, T. 1.; Yu, L. Macmmolecules 1997, 30,4608.

Reyftmann, J. P.; Kagan, J.; Santus, R.; Morliere, P. Photochem.

Photobiol. 1985, 4 1, 1 .

Ng,S.C.;Xu, J. M.;S.H.O.,C. Synth. Met. 1998, 92,33.

Tada, K.; Onoda, M.; Yoshino, K. J. Phys. D: Appl. Phys. 1997,30,2063.

Chen, L.-H.; Wang, C.-Y.; Luo, T.-M. H. Heterocycles 1994, 38, 1093.

Luo, T. M. H.; LeGoff, E. J. Chinese Chem. Soc. 1992, 39,325.

Joshi, M. V.; Hemler, C.; Cava, M. P.; Cain, J. L.; Bakker, M. G.; McKinley,

A. J.; Metzger, R. M. J. Chem. Soc. Perkin Trans 2 1983,1082.

Kagan, J.; Arora, K. Heterocycles 1983,20, 1941.

Johnson, J. R.; May, G. E. Org. Synth. Coll. Vol. 11 1943,8.

Weston, A. W.; Micheals, R. J., Jr. O g . Synth. Coll. Vol. 11, 1963, 91 5.

Wynberg, H.; Metselaar, J. Synth. Commun. 1984, 14, 1.

Stetter, H.; Kulmann, H. Angew. Chem. Int. Ed. Eng. 1974, 13,539.

Stetter, H. Angew. Chem. Int. Ed. Eng. 1976, 15,639.

Stetter, H.; Kulmann, H. Chem. Ber. 1977,110,1971.

Stetter, H.; Bender, H.-J. Angew. Chem. Int. Ed. Eng. 1978, 17, 131.

Stetter, H.; Kulmann, H. ; Oganic Reactions; John Wiley 8 Sons: New

York, 1991 ; Vol. 40, pp 407-496.

Gronowitz, S.; Homfeldt, A.-B. Thiophene and Its Derivatives, Part IV;

Gtonowitz, S. (Ed.); John Wiley & Sons: New York, 1991, pp Chapter

1.

209) Clementi, S.; Linda, P.; Vergoni, M. Tetrahedron 1971, 27,4667.

210) Gronowitz, S.; Moses, P.; Homfeldt, A.-B.; Hakansson, R. Arkiv Kemi

1961, f7, 165.

21 1 ) Campaigne, E.; Archer, W. L. J. Am. Chem. Soc. 1953, 75,989.

21 2) Consiglio, G.; Spielli, D.; Gronowitz, S.; Hornfeldt, A.-S.; Maltesson, B.;

Noto, R. J. Chem Soc., Perkin trans. 2 1982.

21 3) Gronowitz, S.; Cederlund, B.; Homfeldt, A.-B. Chern. Scripta 1974, 5,217.

214) Andreani, F.; Salatelli, E.; Lanzi, M. Polymer 1996, 37,661.

215) Delabouglise, O.; Hmyene, M.; Horwortz, 2.; Yasar, A.; Garnier, F. Adv.

Mater. l992,4, 1 07.

216) Botta, C.; Luzzati, S.; Tubino, R.; Bofghesi, A. Phys. Rev. 8 11992, 46,

1 3008.

217) Rossi, R.; Ciofalo, M.; Carpita, A.; Ponterini, G. J. Photochem. Photobiol.

A: Chem. 1993,70, 59.

218) Scaiano, J. C.; Redmond, R. W.; Mehta, B.; Amason, J. T. Photochem.

Photobiol. 1990, 52,655.

219) Janssen, R. A.; Smibwitz, L.; SarÎciftci, N. S.; Moses, D. J. Chem. Phys.

1894, 101,1787.

220) Kodaira, f .; Watanabe, A.; Ito, O,; Watanabe, M.; Saito, H.; Koishi, M. J.

Phys. Chem. 1996, IMl, 15309.

221) Guilbault, G. G. Practieal Fluomscence; 2nd ed.; Guilbault, G. G., Ed.;

Marcel Dekker: New York, 1990.

222) Ng, S.-C.; Xu, J.-M.; Chan, H. S. O.; Fujii, A.; Yoshino, K. J. Mater. Chem.

1999, 9,381.

223) Ruiz, J. P.; Dhana, J. R.; Reynolds, J. R. Macmmolecules 1992, 25,849.

224) Pei, J.; Yu, W.-L.; Huang, W.; Heeger, A. 5. Mscromolecules 2000, 33,

2462.

225) de Leeuw, D. M.; Simenon, M. M. J.; Brown, A.R.; Einerhand, R. E. F.

Synth. Met. 1997, 87, 53.

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