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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
All rights resemed. This work may not be reproduced in whde or in part, by photocopy
or other rneans, wiüiout permission of the author.
uisitbns and Acquisitions et B iographii Services aenRces bibliographiques 3
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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
Temperature dependence of the UV-visible absorption
spectra of a P3HT55 film under nibogen .............................. 48
Temperature dependence of the UV-visible absorption
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
Temperature dependence of the UV-visible absorption
spectra of a P30T80 film under nitrogen .............................. 50
Temperature dependence of the UV-visible absorption
spectra of a P3DDT70 film under nitmgen ............................ 51
Temperature dependence of the UV-visible absorption
spectra of a P3HDT80 film under nitrogen ............ , .... . ........ 52
Temperature dependence of the UV-visible absorption
spectra of a P3HT100 film under nitrogen .................... ........ 53
Temperature dependence of the UV-visible absorption
spectra of a P30T100 film under nitrogen ............................ 54
Temperature dependence of the UV-visible absorption
spectra of a P3DDT100 film under nitrogen .......................... 55
Temperature dependence of the UV-visible absorption
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
Temperature dependence of the UV-visible absorption
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
Temperature dependence of the UV-visible absorption
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
platinum electrode in 0.5 M LiClOdacetonitrile ..................... 169
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
platinum electrode in 0.5 M LiClOdacetonitrile ..................... 171
Figure 5.20 Cyclic voltammogram of a spin-cast film of P44DHTB on
platinum electrode in 0.5 M LiClOdacetonitrile ..................... 172
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
a@-couplings.61 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)
Figure 2.3 Temperature dependence of the UV-visible absorption
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)
Figure 2.4 Temperature dependence of the UV-visible absorption
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)
Figure 2.5 Temperature dependence of the UV-visible absorption
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)
Figure 2.6 Temperature dependence of the UV-visible absorption
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)
Figure 2.7 Temperature dependence of the UV-visible absorption
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)
Figure 2.8 Temperature dependence of the UV-visible absorption
spectra of a P3HTIOO film under nitrogen
400 500 600 700
Wavelength (nm)
Figure 2.9 Temperature dependence of the UV-visible absorption
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)
Figure 2.10 Temperature dependence of the UV-visible absorption
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)
Figure 2.11 Temperature dependence of the UV-visible absorption
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)
Figure 3.2 Temperature dependence of the UV-visible absorption
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)
Figure 3.3 Temperature dependence of the UV-visible absorption
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
ice and 2 M HCI (- 200 mL). The ether layer was separated and the aqueous
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
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 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
mmol) of Magnesium in 40 mL of anhydrous diethyl ether (sodium dried) was
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
mmol) of Magnesium in 40 mL of anhydrous diethyl ether (sodium dried) was
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.
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