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MSU I. An Affirmative Mai/E“ OppommIIy Infiltwon
Wanna-o.

 

SYNTHESIS AND CHARACTERIZATION OF NOVEL
CONJUGATED POLYMERS BASED ON THIOPHENE
BUILDING BLOCKS

By

Chenggang Wang

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements

for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry

1995

ABSTRACT

SYNTHESIS AND CHARACTERIZATION OF NOVEL,
CONJUGATED POLYMERS BASED ON THIOPHENE
BUILDING BLOCKS

By
Chenggang Wang

Our investigation of conductive polymers has focused on the
thiophene-based polymers/copolymers. 3',4'-Dibutyl-2,2':5',2"-terthio-
phene was designed and used as a building block for the synthesis of new
conjugated polymers with regular structure. Poly(3',4'-dibutyl-2,2':5',2"-
terthi0phene) [poly(DBTT)] and its pyrrole analog, i.e. poly[3,4-dibutyl-
2,5-bis(2-thienyl)-pyrrole] [poly(DBTP)] were prepared by chemical
oxidative polymerization of the corresponding monomers. Both
poly(DBTI‘) and poly(DBTP) are ordered, soluble, conjugated polymers
with a predominance of thiophene ring cud-couplings in the polymer
backbone. They also bear strong similarity in their physicochemical
properties, except that poly(DBTP) is not stable in the presence of oxygen
as compared with poly(DBTI‘). The doped polymers (with FeCl4’ or 13')
show p-type metallic behavior with room temperature electrical
conductivities of ~ 0.1-5 S/cm.

To extend the use of DBTT as a desirable building block, 2,5"-
diformyl-3',4'-dibutyl-2,2':5',2"-terthiophene (DFDBT), a dialdehyde

functionalized DBTT, has been utilized to synthesize various copolymers.

Exploiting of the Wittig reaction affords four tailor-made copolymers with
"tunable" properties while maintaining the enhanced solubility imparted by
the DBTT building block. Poly( 3',4'-dibutyl-oc—tertl'iiophene-vinylene-1,4—
phenylene-vinylene) (PBTPV—p), poly(3',4'—dibutyl-0t-tcrthiophene-
vinylene-1,3-phenylene-vinylene) (PBTPV-m), poly(3',4'-dibutyl-0L-
terthiophene-vinylene-methylene-vinylene) (PBTMV), poly(3'.4'-dibutyl-
OL-terthiophene-vinylene—ethylene-vinylene) (PBTEV) show a increase of
optical bandgap from 2.05 to 2.47 eV, as a result of the decrease of
conjugation in the polymer backbone. Polycondensation of DFDBT with
1,4-phenylenediamine gives another new conjugated copolymer, poly(3',4'-
dibutyl-oc-terthiophene-azomethine-1,4-pheny1ene-azomethine) (PB TPI),
which is formally isoelectronic with PBTPV—p. The incorporation of imine
nitrogens on the PBTPI backbone gives rise a significant change in its
properties such as reversible protonation and Lewis acid complexation, as
compared to PBTPV-p.

Additional chemical modification of the thiOphene ring leads to two
new derivatives, i.e. 3,4-ethylenedithiathiophene (EDTT) and 3,4-
vinylenedithiathiophene (VDTT). EDTT can be both chemically and
electrochemically polymerized to give a new soluble conductive
poly(EDTT), while VDTT cannot be polymerized under similar
conditions. Instead, a stable radical cation salt was obtained by the
electrolysis of VDTT in 0.1 M (Bu4N)ClO4 acetonitn'le solution.

In this dissertation, the detailed synthesis, ‘characterization and

prOperties of the above materials will be discussed.

To My Lovely Wife, Xurong (Echo) and
To My Wonderful Parents, Yi Fang and Xiu Zhi Wang

ACKNOWLEDGMENT

The research in this dissertation would not have been successful
without the assistance and support of many sincere persons. First and
foremost, I would like to thank my advisor Professor Mercouri G.
Kanatzidis for his encouragement, wise guidance and support. I would also
like to thank Professor Kim Dunbar for her commitment as a second
reader, Professor Eugene LeGoff for his help with organic synthesis, and

Professor Jeffrey Ledford for serving in my graduate committee.

I would like to thank Professor H. Eick and Professor C. K. Chang
for their help and fruitful discussion. I thank Dr. M. Benz and Dr. S. Shieh
for the synthesis of 3',4'-dibutyl-2,2':5',2"-terthiophene and its dialdehyde
derivative, Professor C. R. Kannewurf‘s group at Northwestern University
for charge transport measurements, Dr. J.-H. Liao, Dr. J.-H. Chou and Dr.
X. Zhang for their help with X-ray single crystal structure analysis. Many
thanks go to all members in the Kanatzidis' group for their friendship and
kindness.

I would like to acknowledge the Center for Fundamental Materials
Research, the Department of Chemistry, Michigan State university, and the
National Science Foundation for financial support during my graduate

studies.

Finally, I would like to express my deepest gratitude to my parents,
my sister, my wife and parents-in-law for their love, support and

understanding.

TABLE OF CONTENTS

Page
LIST OF TABLES .................................................................. xiv
LIST OF FIGURES ................................................................. xvi
LIST OF SCHEMES ................................................................ xxviii
ABBREVIATIONS .................................................................. xxix
CHAPTER 1
1.1. General Introduction ................................................................ 2
1.2. Description of the Starting Materials Used in This Work ............. 28
List of References .......................................................................... 35
CHAPTER 2

Preparation, Spectroscopic and Charge Transport Properties of A New
Soluble Polythiophene Derivative: Poly(3',4'-dibutyl-2,2':5',2"-

terthiophene) ................................................................................ 48
ABSTRACT .................................................................................. 49
2.1. Introduction ......................................................................... 50
2.2. Experimental Section ............................................................ 52
2.2.1. Materials ......................................................................... 52
2.2.2. Synthesis of 2,5-Dibromo-3,4-dibutyl-terthiophene .............. 52

2.2.3. Synthesis of 3',4'-Dibutyl-2,2':5',2"-terthiophene, (DBTT)... 53

vi

2.2.4. Preparation of Neutral Poly(3',4'_dibutyl-2,2':5',2"-
terthiophene), poly(DBTT) ............................................... 54
2.2.5. Isolation of Fraction I of Poly(DBTT) ............................... 54
2.2.6. Isolation of Fraction II of Poly(DBTT) .............................. 55
2.2.7. Synthesis of D0ped Poly(DBTT) ........................................ 55
2.2.8. Physicochemical Measurements .......................................... 56
2.3. Results and Discussion ........................................................... 58
2.3.1. Polymer Synthesis ............................................................ 58
2.3.2. Chromatographic Molecular Weigh Studies ........................ 62
2.3.3. Infrared Spectroscopy ...................................................... 64
2.3.4. Electronic Spectroscopy, UV-vis/Near-IR ......................... 67
2.3.5. Photoluminescence Spectroscopy ....................................... 72
2.3.6. NMR Spectroscopy ........................................................... 74
2.3.7. Thermal Analysis ............................................................. 78
2.3.8. Electron Spin Resonance Spectroscopy ............................... 79
2.3.9. Magnetic Susceptibility Measurements ................................ 83
2.3.10. Charge Transport Measurements ........................................ 84
2.4. Conclusion ........................................................................... 93
List of References .......................................................................... 94
CHAPTER 3

Synthesis and Characterization of A New Hybrid Conjugated

Polymer: Poly[3,4-dibutyl-2,5-bis(2-thienyl)pyrrole] ........................ 100
ABSTRACT ................................................................................. 101
3.1. Introduction ............................................................. A ........... 102

vii

3.2. Experimental Section ............................................................ 105

3.2.1. Materials ......................................................................... 105
3.2.2. Synthesis of 3,4-dibutyl-2,5-bis(2-thienyl)-pyrrole .............. 105
3.2.3. Synthesis of Neutral
Poly[3,4—dibutyl-2,5-bis(2-thienyl)-pyrrole] ....................... 106
3.2.4. Isolation of the Chloroform-Soluble, Fraction 1 .................. 107
3.2.5. Isolation of the THF-Soluble, Fraction II ............................ 107
3.2.6. Preparation of Doped Poly(DBTP) .................................... 107
3.2.7. Physicochemical Measurements ......................................... 108
3.3. Results and Discussion .......................................................... 109
3.3.1. Oxidative Polymerization ................................................. 109
3.3.2. Chromatographic Molecular Weight Studies ....................... 110
3.3.3. Infrared Spectroscopy ...................................................... 113
3.3.4. NMR Spectroscopy .......................................................... 115
3.3.5. Electronic Spectroscopy, UV-Visible-NIR .......................... 117
3.3.6. Photoluminescence Spectrosc0py ....................................... 123
3.3.7. Thermal Analysis ........................................................... .. 123
3.3.8. Electron Spin Resonance Spectroscopy ............................... 127
3.3.9. Charge Transport Properties ............................................. 127
3.4. Conclusion ........................................................................... 131
List of References ......................................................................... 132
CHAPTER 4
Synthesis and Characterization of Conjugated and Non-Conjugated
Copolymers Containing 3',4'-Dibutyl-0t-Terthiophene Moiety via
Wittig Reactions ............ 136

viii

ABSTRACT .................................................................................. 137

4.1. Introduction ......................................................................... 138
4.2. Experimental Section ............................................................ 139
4.2.1. Materials ......................................................................... 139
4.2.2. Synthesis of 2,5"-Diformyl-3',4'-dibutyl-2,2':5',2"-
terthiophene, (DFDBT) .................................................... 140
4.2.3. Synthesis of Di-Wittig Salts ............................................... 141
4.2.3.1. Synthesis of p—Xylenebis(triphenylphosphonium
bromide), (I) .............................................................. 141
4.2.3.2. Synthesis of m-Xylenebis(triphenylphosphonium
bromide), (11) ............................................................. 142
4.2.3.3. Synthesis of 1,3-Bis(tn'phenylphosphonium)propane
Dibromide, (III) .......................................................... 142
4.2.3.4. Synthesis of 1,4-Bis(triphenylphosphonium)butane
Dibromide, (IV) .......................................................... 142
4.2.4. Synthesis of Copolymers via Wittig Reactions ..................... 143
4.2.4.1. Synthesis of Poly(3',4'-dibutyl-oc-terthiophene-
vinylene- 1,4-phenylene-vinylene), (PB TPV-p) ............... 143
4.2.4.2. Synthesis of Poly(3',4'-dibutyl-a-terthiophene-
vinylene- 1 ,3-pheny1ene-vinylene), (PBTPV—m) .............. 144
4.2.4.3. Synthesis of Poly(3',4'-dibuty1-(at-terthiophene-
vinylene-methylene-vinylene), (PBTMV) ....................... 145
4.2.4.4. Synthesis of Poly(3',4'~dibutyl—a-terthiophene-
vinylene-ethylene-vinylene), (PBTEV) .......................... 146
4.2.5. Synthesis of Iodine-Doped Copolymers .............................. 146
4.2.6. Physicochemical Methods ................................................. 147
4.3. Results and Discussmn ............ 149

ix

4.3.1. Polymer Synthesis ............................................................ 149 '
4.3.2. Chromatographic Molecular Weight Studies ....................... 152
4.3.3. Infrared Spectroscopy ...................................................... 154
4.3.4. NMR Spectroscopy .......................................................... 156
4.3.5. Electronic Spectroscopy, UV-Visible-NIR .......................... 160
4.3.6. Photoluminescence Spectroscopy ....................................... 165
4.3.7. Thermal Analysis ............................................................. 170
4.3.8. Electron Spin Resonance Spectroscopy ............................... 175
4.3.9. Polymer Electrochemistry ................................................ 178
4.3.10. Charge Transport Properties ............................................. 181
4.4. Conclusion ........................................................................... 183
List of References .......................................................................... 184
CHAPTER 5

Synthesis and Characterization of A New Conjugated Aromatic

Poly(azomethine) Derivative Containing

3',4'-Dibutyl-0t-Terthiophene Linkages ........................................... 188
ABSTRACT ................................................................................. 189
5.1. Introduction ........................................................................ 190
5.2. Experimental Section ............................................................ 195
5.2.1. Materials ......... p ................................................................ 195
5.2.2. Synthesis of ‘2,5"-Diformy1-3',4'-dibutyl-2,2':5',2"-
terthiophene, (DFDBT) .................................................... 196
5.2.3. Synthesis of Poly(3',4'-dibutyl-(at-terthiophene-

azomethine-p-phenylene-azomethine) (PBTPI)....... ............. 196

X

5.2.4. Synthesis of Protonated PBTPI .......................................... 197

5.2.5. Synthesis of Iodine-Doped PBTPI ...................................... 197
5.2.6. Physicochemical Methods .................................................. 197
5.3. Results and Discussion ........................................................... 199
5.3.1. Polymer Synthesis and Characterization .............................. 199
5.3.2. NMR Spectroscopy .......................................................... 206
5.3.3. Infrared Spectroscopy ...................................................... 209
5.3.4. Electronic Absorption Spectra of the Neutral PBTPI and
Complexes ...................................................................... 214
5.3.5. Photoluminescence Spectrosc0py ...................................... 223
5.3.6. Thermal Analysis ............................................................ 223
5.3.7. Electron Spin Resonance (ESR) Spectroscopy ..................... 226
5.3.8. Electrochemistry ............................................................. 230
5.3.9. Electrical Conductivity .................................................... 233
5.4. Conclusion .......................................................................... 233
List of References ......................................................................... 235
CHAPTER 6

Poly(3,4-ethy1enedithiathi0phene). A New Soluble Conductive

Polythiophene Derivative ............................................................... 241
ABSTRACT ................................................................................. 242 _
6.1. Introduction ........................................................................ 243
6.2. Experimental Section ........................................................... 245
6.2.1. Materials ........................................................................ 245
6.2.2. Synthesis of Thieno[3,4-d]-1,3-dithiole-2-thione ............ 246

xi

6.2.3. Synthesis of 3,4-Ethylenedithiathiophene (EDTT) .............. 247
6.2.4. Chemical Polymerization of 3,4-Ethy1enedithiathiophene.... 247
6.2.5. Electrochemical Polymerization of
3,4-Ethylenedithiathiophene ............................................. 248
6.2.6. Physicochemical Measurements ........................................ 249
6.3. Results and Discussion ......................................................... 251
6.3.1. Monomer and Polymer Syntheses ..................................... 251
6.3.2. Polymer Electrochemistry ............................................... 254
6.3.3. Gel Permeation Chromatography (GPC) Molecular
Weight Analysis .............................................................. 258
6.3.4. Infrared Spectroscopy ..................................................... 261
6.3.5. UV-vis/NIR Spectroscopy ................................................ 266
6.3.6. Photoluminescence Spectroscopy ...................................... 269
6.3.7. Thermal Analysis ............................................................ 272
6.3.8. Electron Spin Resonance Spectroscopy .............................. 272
6.3.9. Charge Transport Properties ............................................ 279
6.4. Concluding Remarks ............................................................ 285
List of References ......................................................................... 286
CHAPTER 7
3,4-Vinylenedithiathiophene Perchlorate. A New Conductive
Charge Transfer Compound ........................................................... 291
ABSTRACT ................................................................................. 292
7.1. Introduction ........................................................................ 293
7.2. Experimental Section ........... 295

xii

7.2.1. Materials ........................................................................ 295
7.2.2. Synthesis of T hieno[3,4-d]-1,3-dithiole-2-thione ................. 296
7.2.3. Synthesis of 3,4-Vinylenedithiathiophene, (VDTT) ............. 296
7.2.4. Preparation of 3,4-Vinylenedithiathiophene Perchlorate,
(VDTT-C104) ................................................................. 297
7.2.5. Physicochemical Methods ................................................. 297
7.2.6. Crystallographic Studies ................................................... 298
7.3. Results and Discussion .......................................................... 302
7.3.1. Synthesis and Electrochemistry ......................................... 302
7.3.2. Spectroscopic Characterization ......................................... 304
7.3.3. Description of the Structure of the VDTT-C104 Salt ........... 310
7.3.4. Electron Spin Resonance (ESR) Spectroscopy .................... 314
7.3.5. Electrical Conductivity .................................................... 314
7.4. Concluding Remarks ............................................................ 316
List of References ......................................................................... 317

xiii

1.1.

2.1.

2.2.

3.1.

4.1.

4.2.

4.3.

4.4.

4.5.

4.6.

5.1.
5.2.

LIST OF TABLES

Page
The Conductivities of Some Prototypical
Conductive Polymers ........................................................... 4
Comparison of Infrared Band Positions (cm'l) and Their
Assignments for Poly(DBTT) and
Various Poly(3-a1kylthiophenes) .......................................... 66
ESR Data for Three Different Samples ................................. 83

Comparison of Infrared Band Positions (cm-1) and Their
Assignments for Poly(DBTP) and Some Related Polymers ..... 115
Molecular Weight Data for Four Copolymers Determined

by the GPC Method ............................................................ 152
Comparison of Infrared Band Positions (cm-1) and

Their Assignments for C0polymers and Related Polymers ...... 154

Electronic Absorption Data of Four Copolymers in

Solution and as Solid State Thin Films .................................. 162
Photoluminescence Emission Maxima in Solution and Solid

State of Four Copolymers at 23 0C ...................................... 167
Summary of Thermal Analysis Data for Four

Copolymers and PDBTTa ................................................... 170
ESR Data for Four Copolymers at 23 0C ............................... 175
X-ray Diffraction Data for PBTPI at 23 0C .......................... 202

Comparison of Infrared Band Positions (cm-1) and Their

Assignments for PBTPI and Related Polymers ...................... 213

xiv

5.3.

6.1.

6.2.

7.1.

7.2.

7.3.

Electronic Absorption Maxima, Absorption Edge, and

301"

Solid-State Bandgap of Conjugated Poly(azomethines) ........... 220
Comparison of Infrared Band Positions (cm’l) and Their
Assignments for Some Monomeric and Polymeric

Polythiophene Derivativesa ................................................ 263
ESR Data for Various Poly(EDTT) Samples ....................... 278
Summary of Crystallographic Data and Structure Analysis

for the VDTT-C104 Salt .................................................... 300
Fractional Atomic Coordinates and B eq Values for the
Non-Hydrogen Atoms in the VDTT-C104 Salt with

Estimated Standard Deviations in Parentheses .......................
Distances (A) and Angles (deg) in the VDTT-C104 salt

with Estimated Standard Deviations in Parentheses ................ 311

XV

1.1.

1.2.

1.3.

1.4.

1.5.
1.6.

1.7.
1.8.
1.9.

1.10.

1.11.
1.12.

1.13.

LIST OF FIGURES

Page
Electrical conductivities of materials ................................... 3
(a) Chemical-structure diagram of cis-(CH)X and
polythiOphene. (b) Two inequivalent structures for the
thiophene heterocycle in polythiophene ................................ 5

Proposed mechanism of polymerization of thiophene

(adopted from ref 18) ........................................................ 9
The band structure of polythiophene upon oxidative

doping: (a) neutral state, (b) less than 1% doping,

(c) few present doping and ((1) heavy doping ....................... 1.1
Three modes of coupling in the polythiophene backbone ....... 13
Structures of polythiophenes with different

regiochemistry of substitution arising from different

modes of coupling of substituted thiophene monomers .......... 14
General Wittig reaction ..................................................... 18
Bis-Wittig reaction ............................................................ 19
Chemical structures of PPP, PEMB, PPV and PPI ............... 22
Chemical structures of previously investigated

poly(azomethines). (reprinted from ref. 65 b) ..................... 24
General Schiff' reaction. (ref 68) ........................................ 25

X-ray single crystal structure of 3',4'-dibutyl-2,2':5',2"-
terthiophene (see reference 78) .......................................... 30
Schematic representation of the synthesis of

thieno[3,4-d]-1,3-dithiole-2-thione ........................ ' .............. 33

xvi

1.14.
2.1.

2.4.

2.5.

2.6.

2.7.

2.8.

Precursor route to new thiophene derivatives ...................... 34
X-ray powder diffraction profiles for annealed and
unannealed samples of (A) fraction I and (B) fraction II ..... 61

Typical GPC trace of fraction II of poly(DBTT) in

THF solution at room temperature ..................................... 63
FT-IR transmission spectra(KBr pellets) of (A) DBTT
and (B) neutral poly(DBTT) ............................................. 65

FT-IR transmission spectra(KBr pellets) of the

doped polymer:.(A) neutral poly(DBTT) dOped

with FeCl3 in CH3N02 solution; (B) neutral

poly(DBTT) doped with 12 in CH3N02 solution ................. 68
Solution UV-Vis absorption spectra of two soluble-

fractions of poly(DBTT) in chloroform solution

at room temperature: (A) fraction 1 (Max:446 um);

(B) fraction II (kmax=499 nm) ........................................... 69
UV-Vis-NIR absorption spectra of neutral and

doped poly(DBTT) films at room temperature:

(A) fraction 1; (B) fraction II ............................................ 70
Photoluminescence emission spectra of two soluble

fractions in chloroform solution at room temperature:

(A) fraction I (Max:557 um);

(B) fraction II (kmax=620 nm) ........................................... 73
500 MHz 1H NMR spectra of the two-soluble fractions

of poly(DBTT) at room temperature (in CDC13).

(A) fraction 1; (B) fraction II. The peak integration

associated with peaks f and e gives 1:1 ratio ........................ 75

xvii

2.9.

2.10.

2.11.

2.12.

2.13.

2.14.

2.15.

2.16.

2.17.

2.18.

13C NMR spectra of fraction I of poly(DBTT) at

room temperature (in CDC13) .......................................... 77
TGA thermograms of neutral bulk poly(DBTT)

(A) under nitrogen; (B) under oxygen ............................... 80
DSC thermogram of neutral bulk poly(DBTT)

(under nitrogen) .............................................................. 81
ESR spectra of (A) neutral bulk poly(DBTT) and

(B) polymer doped with iodine (33% mol) at room
temperature ..................................................................... 82
Four-probe variable-temperature electrical conductivity

of poly(DBTT) doped with ferric chloride:

(A) fraction 1; (B) fraction II ............................................ 86
Four-probe variable-temperature electrical conductivity

of bulk poly(DBTT) doped with iodine:

(A) poly(DBTT)(13-)o.33; (B) poly(DBTT)(I3-)0,12 ................ 87
Four probe variable—temperature electrical conductivity

of poly(DBTT), doped with ferric chloride, in a logo vs.

T'“2 format. (A) fraction 1; (B) fraction II ......................... 88
Four-probe variable-temperature electrical conductivity

of poly(DBTT), doped with ferric chloride, in a logo vs.

T-”4 format. (A) fraction I (B) fraction II .......................... 89
Variable-temperature thermoelectric power data for
poly(DBTT) doped with ferric chloride: (A) fraction 1;

(B) fraction II ................................................................. 91
Variable-temperature thermoelectric power data for

bulk poly(DBTT) doped with iodine:

(A) poly(DBTT)(I3-)o,33; (B) poly(DBTT)(I3-)0.1249 ............. 92

xviii

3.1.

3.2.

3.3.

3.4.

3.5.

3.6.

3.7.

3.8.

3.9.

3.10.

3.11.

X—ray powder diffraction profile of neutral bulk
poly(DBTP) .................................................................... 1 11
Typical GPC trace of the two soluble-fractions of

poly(DBTP) at room temperature ..................................... 112
FT-IR transmission spectra (KBr pellets) of (A)

monomer DBTP, (B) neutral bulk poly(DBTP) .................. 114
1H NMR spectrum of fraction I of poly(DBTP) in

CDC13 at room temperature .............................................. 116

Solution UV-vis absorption spectra of fraction 1 of

poly(DBTP) in CHC13 at room temperature ....................... 118
Solution UV-vis absorption spectra of fraction II of
poly(DBTP) in THF at room temperature .......................... 119
UV-vis—NIR absorption spectra of the CHCl3-solution cast

thin films of fraction I of poly(DBTP) in

(A) "neutral form" (B) iodine doped form ......................... 121
UV-vis-N IR absorption spectra of the THF-solution cast

thin films of fraction II of poly(DBTP) in

(A) "neutral form" (B) iodine doped form ......................... 122
Photoluminescence spectrum of the two soluble—fractions

of poly(DBTP) at room temperature. (A) fraction I in

CHC13 (B) fraction II in THF ............................................ 124
TGA thermograms of neutral bulk poly(DBTP)
(A) under nitrogen flow and (B) under oxygen flow ........... 125

DSC thermograms of neutral bulk poly(DBTP) under
nitrogen flow (A) first heating cycle (B) second
heating cycle ................................................................... 126

xix

3.12.

3.13.

3.14.

4.1.

4.2.

4.3.

4.4.

4.5.

4.6.

4.7.

ESR spectra of neutral bulk poly(DBTP) at room

temperature ..................................................................... 1 28
Four-probe variable-temperature electrical conductivity

of bulk poly(DBTP) doped with ferric chloride .................. 129
Variable-temperature thermoelectric power data for

bulk poly(DBTP) doped with ferric chloride ...................... 130
X-ray powder diffraction patterns of the copolymers at

23 0C (A) PBTPV-p, (B) PBTPV-m, (C) PBTMV,

and (D) PBTEV ............................................................... 151
Typical GPC traces of the copolymers in THF solution at

room temperature (A) PBTPV-p, (B) PBTPV-m,

(C) PBTMV, and (D) PBTEV ........................................... 153
FT -IR transmission spectra (KBr pellets) of the dialdehyde
DFDBT and copolymers (A) DFDBT, (B) PBTPV-p,

and (C) PBTPV-m ........................................................... 155
FT-IR transmission spectra (KBr pellets) of copolymers
(A) PBTMV and (B) PBTEV ............................................ 157

300 MHz 1H NMR spectra of the copolymers
(A) PBTPV-p and (B) PBTPV-m at room temperature

(in CDC13) ...................................................................... 158
300 MHz 1H N MR spectra of the copolymers (A) PBTMV
and (B) PBTEV at room temperature (in CDC13) ................ 159

Solution UV-vis absorption spectra of copolymers in THF
solution at room temperature (A) PBTPV-p,
(B) PBTPV-m, (C) PBTMV, and (D) PBTEV .................... 161

XX

4.8.

4.9.

4.10.

4.11.

4.12.

4.13.

4.14.

4.15.

4.16.

4.17.

UV-vis-NIR absorption spectra of solution—cast films of
the copolymers (A) PBTPV—p and (B) PBTPV-m at

neutral and iodine doped states .........................................

UV-Vis-NIR absorption spectra of solution-cast films of
the copolymers (A) PBTMV and (B) PBTEV at neutral,

iodine and FeCl3 doped states ...........................................

Photoluminescence emission spectra of the copolymers in

THF solution at room temperature (a) PBTPV-p,

(b) PBTPV-m, (c) PBTMV, and (d) PBTEV ......................

Photoluminescence emission spectra of the c0polymers in

the solid state at room temperature (a) PBTPV-p,

(b) PBTPV-m, (c) PBTMV, and (d) PBTEV ......................

TGA thermograms of the copolymers under nitrogen (A)

PBTPV-p, (B) PBTPV-m, (C) PBTMV, and (D) PBTEV...

TGA thermograms of the copolymers under oxygen (A)

PBTPV-p, (B) PBTPV-m, (C) PBTMV, and (D) PBTEV...

DSC thermograms of the'copolymers under nitrogen (A)

PBTPV-p, (B) PBTPV-m, (C) PBTMV, and (D) PBTEV....

ESR spectra of the copolymers at room temperature

(A) PBTPV-p and (B) PBTPV-m ......................................

ESR spectra of the copolymers at room temperature

(A) PBTMV and (B) PBTEV ...........................................

Typical cyclic voltammograms of cast films of the
copolymers (A) PBTPV-p and (B) PBTPV-m on Pt
electrode in CH3CN/0.1 M (Bu4N)ClO4 at room

temperature. Scan rate 20 mV/s ........................................

xxi

. 163

.164

168

169

172

173

174

176

. 177

. 1'79

4.18.

4.19.

5.1.
5.2.

5.3.

5.4.

5.5.

5.6.

5.7.

5.8.

5.9.

A typical cyclic voltammogram of a cast film of the

copolymer PBTMV on ITO electrode in CH3CN/0.1 M
(Bu4N)ClO4 at room temperature. Scan rate 20 mV/s .......... 180
(A) Four-probe variable-temperature electrical

conductivity of PBTPV-p doped with iodine.

(B) Variable-temperature thermoelectric power data for
PBTPV-p doped with iodine ............................................. 182
X—ray powder diffraction profile of pristine PBTPI ............ 201
Typical GPC trace of the THF-soluble fraction of

PBTPI at room temperature ............................................. 205
1H NMR spectrum of PBTPI in CD3NOz/A1Cl3

and its assignment ............................................................ 207
13C NMR spectrum of PBTPI in CD3NOz/A1Cl3 ................ 208
FT-IR transmission spectra (KBr pellets) of

(A) dialdehyde DFDBT, (B) pristine PBTPI and

(C) prontonated (with HCl) PBTPI .................................... 210
Solution UV-vis absorption spectra of PBTPI in THF

(solid line, kmax = 457 nm) and

NMP (dash line, hmax = 464 nm) ...................................... 215
Solution UV-vis absorption spectra of PBTPI in (A)
H2804 and (B) CH3N02 containing ca 2 wt% A1C13 ............ 216

UV-vis-NIR absorption spectra of the THF-solution
cast thin films of PBTPI in (A) neutral form

(B) iodine doped form ..................................................... 218
Optical absorption spectra of pristine and protonated
(with HCl) bulk PBTPI .................................................... 221

xxii

5.10. (A) UV-vis-NIR absorption spectra of THF-solution
cast thin films of PBTPI treated with HCl solution at

various pH values. (B) A plot of absorbance at 1.8 eV

vs. pH values ................................................................... 222
5.11. Photoluminescence spectrum of PBTPI in THF solution
at room temperature ........................................................ 224

5.12. Photoluminescence spectra of bulk PBTPI solid at
(A) 300 K and (B) 77 K ................................................... 225
5.13. TGA thermograms of pristine bulk PBTPI (A) under

nitrogen flow and (B) under oxygen flow .......................... 227
5.14. TGA thermogram of protonated (with HCl) bulk
PBTPI under nitrogen flow .............................................. 228

5.15. ESR spectra of (A) pristine PBTPI and (B) protonated

PBTPI at room temperature. Experimental setting:

frequency, 9.425 GHz; power, 1 mW; gain, 2 x 104;

center field, 3360 G; scan range, 100 G; time constant,

0.3 sec; scan time, 16 min ................................................ 229
5.16. Typical cyclic voltammogram of a solution-cast PBTPI

film on a Pt electrode in CH3CN/0.1 M (Bu4N)ClO4.

Scan rate, 20 mV/s .......................................................... 231
5.17. Typical cyclic voltammogram of a solution-cast PBTPI

film protonated with HCl on a Pt electrode in

CH3CN/0.1 M (BU4N)CIO4. Scan rate, 20 mV/s ................. 232

xxiii

6.1.

6.2.

6.3.

6.4.

Cyclic voltammogram of 3,4-ethylenedithiathiophene

(ED'I'I‘) monomer (5 mM) in CH3CN/0.1M (Bu4N)ClO4.

on a Pt disk electrode. Scan rate, 100 mV/s. (A) First

scan showing nucleation loop on the Pt electrode. (B)

Fourth to eighth scan showing polymer deposition on

the Pt electrode ............................................................... 253
(A) Typical cyclic voltammogram of a directly

electrogenerated poly(EDTT)-E film on Pt electrode

in CH3CN/0.1M (Bu4N)ClO4. Scan rate, 20 mV/s. (B)

Plot of peak current density vs. the scan rate for 3

directly electrogenerated poly(EDTT)-E film on Pt

electrode in CH3CN/0.1M (Bu4N)ClO4 ............................. 256
(A) Typical cyclic voltammogram of a cast

poly(EDTT)-C film on Pt electrode in CH3CN/0.1M
(Bu4N)ClO4. Scan rate, 20 mV/s. (B) Plot of peak current
density vs. the scan rate for a cast poly(EDTT)-C film on

Pt electrode in CH3CN/0.1M (Bu4N)ClO4 ......................... 257
GPC traces of neutral poly(3,4-ethylenedithiathiophene)

in NMP solution at room temperature. (A) poly(EDTT)-C,
without LiCl; (B) poly(EDTT)-C, with addition of LiCl: (C)
poly(EDTT)-E, without LiCl; (D) poly(EDTT)-E, with
addition of LiCl. The Li+ ions are thought to serve as

Lewis acid centers probably complexing the sulfur atoms

on the polymer chains, interfering with chain-to-chain
interactions and helps dissolve the large agglomerated

particles to single chains .................................................. 260

xxiv

6.5.

6.6.

6.7.

6.8.

6.9.

6.10.

6.11.

6.12.

6.13.

6.14.

FT -IR transmission spectra (KBr pellets) of (A) monomer
3,4-ethylenedithiathiophene (EDTT); (B) neutral

poly(EDTT)-C; (C) neutral poly(EDTT)-E .......................

PT-IR transmission spectra (KBr pellets) of (A)
poly(EDTT)-C doped with FeCl4'; (B) poly(EDTT)-C;
doped with 13" (C) poly(EDTT)-E doped with ClO4‘ ........
UV-Visible absorption spectra of poly(EDTT)-C and
poly(EDTT)-E in NMP solution at room temperature .......
UV—Visible-NIR absorption spectra of (A) neutral and
doped poly(EDTT)—C films (B) neutral and doped

poly(EDTT)-E films at room temperature ........................

(A) Fluorescence emission spectra of poly(EDTT)-C
and poly(EDTT)-E in dilute NMP solution at room

temperature Over—"400 nm) (B) excitation spectra of

poly(EDTT)-C and poly(EDTT)-E in dilute NMP

262

265

267

268

solution at room temperature ............................................ 270

Fluorescence emission spectra of poly(EDTT)-C in the

solid state at room temperature (Xex=350 nm) ................... 271

TGA thermograms of neutral poly(EDTT)-C

(A) under nitrogen and (B) under oxygen .......................... 273

DSC thermogram of neutral poly(EDTT)-C_

(under nitrogen). (* due to the baseline of empty A1 pan)...

ESR spectra of neutral poly(EDTT)-C (A) at 23 0C and

(B) ~157 0C ....................................................................

ESR spectra of (A) doped poly(EDTT)-C (13') and

(B) doped poly(EDTT)-E (ClO4') at 23 0C ......................

XXV

274

276

277

6.15.

6.16.

6.17.

7.1.

7.2.

7.3.

7.4.

Four-probe variable-temperature electrical conductivity
of (A) poly(EDTT)-C doped with FeC14" and (B)
poly(EDTT)-E doped with ClO4'. The temperature

range is 5300 K ............................................................

Four probe variable-temperature electrical conductivity
of (A) poly(EDTT)-C , doped with FeCl4- and (B)
poly(EDTT)-E, doped with C104: : (TOP) in a logo

vs. T“2 format. (BOTTOM) in a logo vs. T”4 format.

The temperature range is 5-300 K ...................................

Variable temperature thermopower data for several
pressed pellet samples of doped poly(EDTT).
Regardless of chemical or electrochemical origin of the

sample, both positive and negative thermopower values

have been observed ........................................................

Typical cyclic voltammogram of
3,4-vinylenedithiathiophene (VDTT) (5 mM) in
CH3CN/0.1 M (Bu4N)ClO4 on a Pt-disk electrode.

Scan rate, 100 mV/s .......................................................

Typical cyclic voltammogram of
3,4-vinylenedithiathiophene (VDTT) (5 mM) in
CH3CN/0.1 M (Bu4N)ClO4 on an ITO electrode.

Scan rate, 100 mV/s .......................................................

FT-IR transmission spectra (KBr pellets) of (A) donor
VDTT and (B) the VDTT-C104 salt.

(* peaks are due to ClO4') ..............................................
Optical absorption spectrum of the VDTT-C104 salt .........

xxvi

. 281

. 283

284

. 303

.. 305

.. 306
.. 307

7.5.

7.6.

7.7.

7.8.

X-ray powder diffraction pattern of the VDTT-C104 salt
at room temperature. Note: the broad peaks are due
to the background of a supportive plastic tape ..................... 309

Molecular geometry and atom labeling scheme of the
cation and anion in the VDTT-C104 salt ............................. 312

ORTEP packing diagram of the VDTT-C104 salt

looking down the c-axis .................................................... 313
ESR spectrum of the VDTT-C104 salt at room temperature. 315

xxvii

2.1.
2.2.

4.1

4.2.

5.1.
5.2.

5.3.

6.1.

7.1.
7.2.

LIST OF SCHEMES

Page
Fractionation of poly(DBTT) ............................................ 60
Possible conformations of poly(DBTT) backbone ................ 78
Synthesis of copolymers via the Wittig reaction .................. 150
Pr0posed structural change of PBTMV upon
oxidation with FeCl3 ........................................................ 166
Chemical structures of PPV and PPI .................................. 191
Chemical structures of previously investigated
poly(azomethines). (reprinted from ref. 19 b) .................... 193
Proposed structures of PBTPI: (A) protonated with HCl
and (B) complexed with A1C13 .......................................... 212
Synthetic route of 3,4-ethylenedithiathiophene .................... 251
Some known hybrid n-electron donor systems .................... 294
Synthetic route to 3,4-vinylenedithiathiophene .................... 302

xxviii

ABBREVIATIONS

8H8 673/116]
\/s\/ \/N\/
H

3',4'-dibutyl-2,2':5',2"-terthiophene 3,4-dibutyl-2,5-bis(2-thienyl)pyrrole
(DBTT) (DBTP)

OHC (S, ’S‘ ‘5, CH0

2,5'-diformyl-3',4'-dibutyl-2,2':5',2"-terthiophene
(DFDBT)

-(DB'IT— CH= cu—Q—cu: CHa- PBTPV-p
Il
DB'IT— CH= Chi-Q
CH: CH 11 PBTPV-m

_(.DB'I'I‘— CH: CH— CHz—‘CH: CH?‘ PBTMV
II

—(-DB'I'I‘— CH= C1—l-(CH2)2—CH= CH-)-n PBTEV

+DB'1'T— CH=N—©—N= CH-)-;1 PBTPI

r—\ /—\
S S S S
H EDTT fl VDTT
S S

3,4-ethylenedithiathiophene 3,4-vinylenedithiathiophene

xxix

CHAPTER 1

INTRODUCTION

1.1. General Introduction

1.1.1. General Review of Organic Conducting Polymers

The first organic conducting polymer (CP) was discovered by
Heeger and MacDiarmid in 1977 who found that polyacetylene (CH)x could
undergo a 12 order of magnitude increase of electronic conductivity upon
oxidative doping.l Since then a number of CPS have been synthesized and
characterized such as polyphenylene, polypyrrole, polyfuran,
polythiophene and polyaniline.2 Figure 1.1 shows electrical conductivities
of various materials. Table 1.1. lists some archetype conductive polymers
and the highest conductivity achieved to date. The essential structural
characteristic of CPS is their conjugated 1t system extending over a large
number of repeat monomer units. Partial oxidation or reduction
(commonly referred to as doping) of the polymer chain leads to a
delocalization of positive (hole) or negative (electron) charge over the
polymer framework. It is this delocalized charge that is responsible for
electronic conductivity in conducting polymers.

Poly(heterocycles) (2) can be viewed as an spzpz carbon chain in
which the structure, analogous to that of CiS-(CH)X (1), is stabilized by the
heteroatom (X: N, O, S). These CPS differ from (CH)x by (1) their
nondegenerate ground state related to the nonenergetic equivalence of their
two limiting mesomeric forms, aromatic and quinoid (Figure 1.2.), (2)
their higher environmental stability, and (3) their structural versatility
which allows the modulation of their electronic, electrochemical properties

and processabilities by manipulation of the monomer structure.

Conductivity (ohmocm) ' 1

p-

 

I'l—

Copper

Silver }
Gold I.
Bismut

Mercury

Metals—w

 

lntrin sic
Semi- —

conductors Silicon F1
Graphite

Germanium—

Boron

- Soda-lime
glass

Fused cum
Polyvinyl chloride—
White phosphorous—
Insulators Alumina —

Mica. diamond -4

 

"Yb“ -—
Polyethylene —

Sulfur. Tellon -—

 

I'l'l'l'lTl'l'l'l'l‘l

 

I
I- r—10

.4— 10'

d

10°

10'

10'

10

10

10

10

10

c.
O
I

l
I
r
O

.10

42

-14

d.

d.

TTF-TCNO
Polyacetylene "
Polypyrrole
Polythiophene
Polyaniline

Conductive
panic
Crystals

Doped
__ Conductive
Polymers

 

Approximate electrical conductivities of materials. Actual observed
omducuvhieedependmdopingkvehaedhnpundes.mebcuooicomducuvida
are for room temperature in units ofohm" an" (or siemens (8) per centimeter). ‘Ihe
classification on the left of metals, sunioooduetor, or iris-nator is only approximate.
[Adapted with permission from D. 0. Conan and F. M. Wiysnl. Oren. Eng. NM.
July 2!. I986. p. 29. Copyright I986 American Chemical Society.)

Figure 1.1. Electrical conductivities of materials

4

Table 1.1. The Conductivities of Some Prototypical Conductive Polymers

 

Polymer Chemical Structure Conductivity (S/cm)

 

 

Polyacetylene (CH)x 105, highly oriented

Polythiophene ~1000, highly oriented

Polypyrrole ~500-7500, highly oriented

Polyphenylvinylene M ~ 10000, highly oriented

Polyaniline 57 ~200, highly oriented

 

my
3 n
W)
N n
H
Poly-p-phenylene ~1000, highly oriented
Q \
11
ff N
H n

 

fl—U—fl— -©—<ir®

(a) S S
(1) (2)

(b) S S
aromatic form quinoid form

Figure 1.2. (a) Chemical-structure diagram of CiS-(CH)X and
polythiophene. (b) Two inequivalent structures for the thiophene

heterocycle in polythiophene.

Among these numerous CPS, polythiophenes (PTS) have rapidly
become of the subject of considerable interest because of their high
electrical conductivity, thermal and chemical stability, amenability to
chemical modification, reversible redox properties, and other interesting
physical properties.3 Polythiophenes and their derivatives have
demonstrated potential multiple important technological applications
ranging from antistatics packaging materials,4 chemical sensors,5
rechargeable battery electrodes,6 field-effect transistors,7 electrochromic
devices,8 electroluminescent devices9 and nonlinear optical devices.10

Polythiophenes are essentially prepared by two main routes, i.e. the
chemical and electrochemical syntheses.

In chemical syntheses several methods have been reported. Grignard

coupling of 2,5—dihalothiophene in presence of transition metal complexes

6

has been extensively employed for the synthesis of poly(thiophenes).11
Polycondensation of 2,5-dibromothiophene using 2,2'-bipyridinenickel
dichloride or nickel acetylacetonate as catalyst was also reported.12
Poly(thiophenes) can be synthesized by oxidative coupling of bis-lithiated
thiophene derivatives.13 The use of ferric chloride as a coupling agent was
first reported by Yoshino et al.14. in 1984. In their work a ferric chloride
solution in anhydrous chloroform or dioxane was poured onto a flat
substrate and the liquid layer was exposed to gaseous thiophene; the
polymer was obtained as a film on the surface of the nonconducting

substrate. More recently, poly(3-alky1thiophenes) have been obtained by
the coupling of the alkylthiophenes through metal halides such as FeCl3,

MoCl5, RuCl3, and A1C13 in dilute solutions (commonly using CHC13,
CH2C12 and CH3CN) and a ratio of metal halide to monomer of 3 or 4:1.15

Because of the excess of the oxidative agent and the lower oxidation
potential of the polymer than that of the monomer, the polymer is obtained
in the oxidized, conducting form, which can be undoped chemically by
"compensation" with reducing agents such as ammonia or hydrazine. The
polymers obtained have a well-defined poly(3-alkyl-2,5-thienylene)
structure. The yields are on the order of 60%.

The electropolymerization of bithiophene was first reported in 1981
by Diaz,16 followed by the electropolymerization of thiophene by Garnier
and Tourillon in 1982.17 Electrochemical syntheses involves the
electrolysis of a solution of a monomer and an electrolyte salt in an organic
solvent. One-compartment cells are normally used, the most commonly
used solvents are rigorously anhydrous aprotic solvents of high dielectric
constant and low nucleophilicity such as acetonitrile, benzonitrile,

nitrobenzene, and propylene carbonate. Lithium or tetraalkylammonium

7

salts of strong acids such as perchlorate (ClO4'), hexafluorophosphate

(PF6-), tetrafluorobonate (BF4') and hexafluoroarsenate (ASF6') are

commonly used as electrolyte salts. Platinum and indium-tin oxide (ITO)
coated glass are generally chosen to be the anode material, while platinum
wire or ITO glass as the counter electrode, and saturated calomel electrode
(SCE) as a reference electrode. The polymerization is carried out in a
moisture- and air-free environment, and the electrolytic solution is
degassed prior to polymerization.

The polymerization consists of the oxidative coupling of the
monomers with deposition of the polymer as a film on the anode. The
polymer is obtained in the oxidized form with the counterion coming from
the supporting electrolyte. The polymerization continues even after the
electrode has been totally coated with the film. By controlling the charge
passed to the cell, one may obtain conducting films of variable thickness.
The polymer can be undoped electrochemically, by reversal of the polarity
of the cell after synthesis, or chemically by "compensation" with reducing
agents such as ammonia or hydrazine.

The mechanism for the electropolymerization of pyrrole was
extensively studied by Genies et a1.18 and can be extended to the other
heterocycles. The general mechanism for the electrochemical process is
depicted in Figure 1.3. The first electrochemical step (E) consists in the
oxidation of the monomer to its radical cation. Since the electron-transfer
reaction is much faster than the diffusion of the monomer from the bulk
solution, it follows that a high concentration of radicals is continuously
maintained near the electrode surface. The second step involves the
coupling of two radicals to produce a dihydro dimer dication which leads

to a dimer after loss of two protons and rearomatization. This

8

rearomatization constitutes the driving force of the chemical step (C). Due
to the applied potential, the dimer, which is more easily oxidized than the
monomer, occurs in its radical form and undergoes a further coupling with
a monomeric radical. Electropolymerization proceeds then through
successive electrochemical and chemical steps according to a general
E(CE)n scheme, until the oligomer or polymer insoluble in the electrolytic
medium and precipitates onto the anode surface.

Compared to other chemical syntheses of conducting
poly(heterocycles), the anodic electropolymerization of the monomer is
very convenient and has several distinct advantages such as absence of
catalyst, direct grafting of the doped conducting polymer onto the electrode
surface (which is of particular interest for electrochemical applications),
easy control of the film thickness by the deposition charge, and possibility
to perform in-situ characterization of the growing process or of the
polymer by electrochemical and/or spectroscopic techniques. One major
drawback of the electropolymerization is its low yield [as low as 3% for

poly(3-hexylthiophene)] reaction. 19

. -—>1©1+
215314» @4534 CW
00, , WHM] [C1,
O—O—O W

Figure 1.3. Proposed mechanism of polymerization of thiophene
(adopted from ref 18)

 

10

In the neutral state, these conjugated polymers are either insulators
or semiconductors. Upon doping they become electrically conducting and
display nearly metal-like behavior. At the same time, the color and
physicochemical properties also change. This doping-undoping process is
reversible.

The identification of the charged Species responsible for the
conduction mechanism in CPS has attracted much attention. In
polyacetylenes, which has a degenerate ground state, solitons (cations) have
been Shown to be the dominant charge Species.20 In contrast,
polyheterocycles such as polythiophene, poly(pyrrole) and poly(furan)
have a nondegenerate ground state and the two limiting mesomeric
structures, i.e. aromatic and quinoidal are not energetically equivalent, the
quinoidal form having a Slightly lower energy.21 In these polymers, the
removal of an electron from the conjugated 71: system provokes a local
distortion of the chain and the appearance of two states in the gap
corresponding to a polaron (radical cation) with spin 1/2. Theoretical
calculations have Shown that adjacent polarons are unstable and lead to the
formation of spinless doubly charged defects (e.g. bipolarons) which has
been proposed as the dominant charge storage species.22 The evolution of
the band structure of polythiophene on doping21 is shown in Figure 1.4. In
the neutral state, the band gap of polythiophene iS equal to 2.2 eV which is
a semiconductor. At a low doping level (< 1%), the formation of polarons
is evident. As the doping level increases to a few percent, polarons start to
bind in pairs to form spinless bipolaron states. At higher doping level (up
to 30%), the bipolaron states overlap to form two new bands inside the
band gap. Consequently, the band gap decreases to as small as 0.14 eV and
the polymer shows high electrical conductivity.

11

It is worth noting that, although the bipolaron theory has received a
large general agreement, the nature of the charged species responsible for
the conduction in polythiophenes is still a matter of debate. Recently,
diamagnetic interchain dicationic n-dimers have been proposed as
alternatives to intrachain bipolarons (dications) responsible for the

conduction mechanism in CPS.23‘25

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

__.J _ _ _
I _ _ 0.71 eV *—
2.2 eV 4 1k [:1
i . . 0.61 eV 1:: .
rmmm Illlllllll Illllllllll mnnm
mrmm Illlllllll uruum mumu
rmmm "mum mumu mrmm
IIIIIIIIII mrmm urnmn r umum
Illlllllll mrrum Illlllllllt mmrm
(a) neutral (b) < 1% doping (0) 5% doping (d) heavy doping

Figure 1.4. The band structure of polythiophene upon oxidative doping:
(a) neutral state, (b) less than 1% doping, (0) few percent doping and ((1)
heavy doping.

12

Due to their rigid n-conjugated backbone and strong interchain
interactions, all unsubstituted organic conducting polymers are insoluble
and infusible. This constituted a major obstacle in their study and in the
development of practical applications.

Among the various possible strategies for modification of
polythiophenes, the polymerization of monomers modified by the covalent
grafting of different Side groups represents the most straightforward
method to increase their processability (by way of solution or melt
processing) and achieve electronic tunability (in terms of conducting and
optical properties).

It has been known that incorporation of long alkyl chains on polymer
backbone will increase their processability. Processable poly(3-
alkylthiophenes) have been extensively studied.3 It was expected that
substitution of long chains in the polymer backbone would seriously affect
its conductivity by affecting the planarity of the backbone and interfering
with the electron transport between polymer chains. Indeed, the
conductivities of the long-chain substituted poly(thiophenes) were lower
than those of the parent polymer, but the loss in conductivity was not
dramatic and it was well compensated by the acquired solubility.

The structural effects of substitution (inductive, mesomeric and
steric) drastically affect the properties of the resulting polymers. The
substitution of thiophene by electron-donating groups produces a decrease
of the oxidation potential, while the oxidation potential of the monomer
increase as thiophene substituted by electron-withdrawing groups.26 The
steric interaction between substitutent grafted on adjacent monomers results
in a decrease of the overlap between the it orbitals on consecutive rings and

hence in a shortening of the effective mean conjugation length. This

13

distortion of the conjugated backbone from planarity ultimately cause a
drop of the maximum conductivity upon doping. Besides the electronic and
steric effects, the stereoregularity in the poly(thiophenes) also plays an
important role in governing the macromolecular and macroscopic
properties of the polymers.

The stereoregularity in the poly(thiophenes) is determined by the
regiochemistry of coupling and the regiochemistry of substitution.
Coupling of the five-member heterocyclic rings can occur through the or-
or B-position. In the thiophene ring the reactivity of the or-position is much
higher than that of the B-position so that coupling of thiophene units to
form the polymer is expected to happen essentially through the or,0t'-
coupling. However, 0L,B'- or [HT-linkages also occur, (Figure 1.5.) which
cause defects in the backbone, shorten the conjugation length, reduced the

electrical conductivity”,28
S / \
\ / S \ / / \ /
S / \
S

0t,0t'—coupling 0t,[3'-coupling {MT-coupling
Figure 1.5. Three modes of coupling in the polythiophene backbone.

The presence of substitutents at one or two B-positions of the starting
monomers Should increase the stereoregularity of the polymer by
decreasing the chance of 0t,B- or [Hi-coupling and possibly improve the

processability of the resulting polymer.

14

In general, substituted poly(thiophenes) can be classified according to
the type of substitution patterns. There are three common types of
substitution and these are (a) poly(3-substituted thiophenes), (b) poly(3,4-
disubstituted thiophenes), (c) fused ring systems.

Monosubstitution on the thiophene ring removes the original
symmetry of the thiophene repeat unit. This structural asymmetry leads to
two possible structures via head-to-head or head-to-tail coupling (Figure
1.6). Even through head-to-tail coupling is the preferred mode, the
polymer obtained from 3-monosubstituted monomers contains head-to-head
coupling to an extent of about 10-20%, as determined by 1H NMR
spectroscopy.29 Recently, McCullough et al. reported the regioselectivity
synthesis of ~100% head-to-tail polyalkylthiophenes which exhibit
Significantly higher conductivities.3O

   

head-to-head coupling head-to-tail coupling

Figure 1.6. Structures of polythiophenes with different regiochemistry of
substitution arising from different modes of coupling of substituted

thiophene monomers.

Disubstitution on both B-positions will virtually eliminate the
possibility of 0t,B- or [3,[3-mislinkages. This approach has been drastically

limited by the large steric hindrance of the two substitutents3l which forces

15

the polymer backbone out of planarity and results in the loss of conjugation
and reduced electrical conductivity. However, cyclization between 3,4-
positions of thiophene considerably reduces the steric hindrance.32

Fused-ring systems are of particular interest to obtain a low energy
band gap and possibly intrinsically conducting polymers.33 Wudl et al.34
reported that poly(isothianaphthene) had a small energy gap (1 eV) because
of an increase in the quinoid contribution to the electronic structure caused
by the fusion of the benzene to the thiophene rings. Recently, several other
fused-ring systems also Show a Significant decrease of band gap of
compared to the polymers.35

Another strategy used to obtain a polymer with fewer 0t,B- or 13,13-
mislinkages is to use dimers or longer oligomers as starting materials. A
number of soluble thiophene oligomers containing from three to Sixteen
thiophene units have been prepared and their properties investigated as
precursors or as subunits of the idealized regularly or-linked
polythiophene.3‘5“39 The longest oligothiophene to date is the 64 A-long
oligomer made up with Sixteen thiophene units.39 These oligomers can also
be considered components of "molecular wires" and serve as model
compounds for the study of charge transport in electrically conducting
polymers.

Use of the oligomers decreases the number of mislinkages through
the B-position in the final polymer because some met—linkages have been
built-in in the oligomers. However, higher oligomers (n>3) are rarely used
for the synthesis of polymers because theoretical calculations (on pyrrole)
Show that as the number of units increases the difference in the reactivity
between the or- and B-positions of the radical cation, generated in the first

step of the polymerization, decreases and gives rise to stable oligomers and

16

not to polymers. Thus, well-defined alkyl substituted bithiophenes and
terthiophenes are commonly used as starting materials for polymer
synthesis. 3

Recently, there is increasing interest in the design and synthesis of
novel hybrid polymers (or copolymers) as second generation materials
with novel electrical and optical properties.40 Depending on the synthetic
methods, the copolymers may be obtained as random or block
copolymers.41

Because of the structural Similarity of thiophene and pyrrole, hybrid
polymers containing varying proportions of them have been examined. The
direct oxidation of mixtures of thiophene or bithiophene or terthiophene
and pyrrole produces a random copolymer with a highly irregular
structure.42 The electrochemical polymerization of biheterocyclic
monomer 2-(2-thienyl)-pyrrole (l) and some of its N-alkyl derivatives,43
has been predicted to form random copolymers with highly irregular
structure due to the uncertainty of the linkages of the monomer. However,
the electrochemical and chemical polymerization of triheterocyclic
monomer 2,5-bis(2-thienyl)-pyrrole (2) and some of its N-alkyl derivatives
give relatively ordered block-copolymers.44 Some other higher
thienylpyrrole oligomers (from 4 to 7 heterocyclic units) have been
synthesized.45 However, the electrochemical oxidative polymerization of
these higher thienylpyrrole oligomers has not resulted in the formation of
polymer.45 Thus, among the various monomer substrates, the
triheterocyclic monomer system seems to be the best candidate expected to
form polymers of greater linearity and symmetry due to the symmetrical
nature of the monomer. However, a recent electrochemical study of a

variety of substitution products of 2 has provided strong kinetic evidence

17

for considerable branching in the oxidative polymerization of 2 and its N-

methyl derivatives.

3 / \ S / \ S
\ / If \ / [i] \ /
R R
l 2
(R = H, CH3) (R: H, CH3, C2115, C7H152 C18H37)

1.1.2. Wittig Reaction in Copolymer Synthesis

Since the discovery of p-phenylenevinylene-based electroluminescent
(EL) devices in 1990,46 tremendous effort has been focused on the design
and synthesis of new soluble electro-active polymers to gain control of
color and efficiency of light emission.47 Among a variety of approaches,
copolymerization of p-phenylenevinylene with other moieties are of
considerable interest. Recently, two papers have demonstrated the use of
appropriate Wittig reactions to synthesize PPV derivatives in order to vary
the color of emitted light.48

The Wittig reaction,49 which was discovered by Wittig and Geissler
in 1953, involves the reaction of a large variety of different
triphenylalkylidenephosphoranes 1 with carbonyl compounds 2 to give
alkenes 3. (Figure 1.7.)

18

 

>C=P(R)3 0=c : , /C=C
2 R5 '(R)3P=0 R2 R5
1 2 3a
+
R1 \ / R5
/C=C\
R2 R4
3 b

Figure 1.7. General Wittig reaction.

The Wittig reaction has become one of the favorites among the
numerous methods of olefin synthesis. The Wittig reaction has several
advantages: As a rule, the new C=C double bond appears exclusively at the
Site of the former CO function. The starting material, aldehydes or ketones
and phosphonium salts derived from aryl or alkyl halide and
triphenylphosphine, are readily available. The geometry of the double bond
(E/Z) may be influenced to a considerable extent by the choice of the
reaction conditions. The main drawback of the Wittig reaction is its
susceptibility to steric hindrance. Whereas aldehydes normally give olefins
in high yields, ketones often react less satisfactorily. The Wittig reaction

usually fails in the synthesis of a tetrasubstituted olefin.

19

/CH=O (C6H5)3P=CH \ 2 / —\
Y1 + Y > Y1 Y2

\CH=O (C6H5)3P=CH/ ‘ 2 (C6H5)3P=O \_/

 

4 5 6a

01'
+YL C=C—Y1)—
11

6b

Figure 1.8. Bis-Wittig reaction.

The double intermolecular Wittig reactions50 (termed bis-Wittig
reactions) between one mol of a dicarbonyl compound 4 and one mole of a
biS-alkylidenetriphenylphosphorane 5 (bis-ylide) leads to the unsaturated
(conjugated or non-conjugated) cyclic compounds (6a) or linear oligomers,
or even polymers (6b). The cyclic compounds are usually formed under
highly dilute conditions in low yields, while the linear condensation
products prefer to form under concentrated conditions. There are two
general methods of performing the bis-Wittig reaction. In the first of these,
a bis-ylide 5 is generated by the addition of a suitable base (usually
organolithium compounds in ether, hexane, or benzene; sodamide in liquid
ammonia; alcoholates in alcohol and/or dimethylformamide; dimethyl
sulfoxylate in dimethylsufoxide) to the bis-phosphonium dihalide and then
allowed to react with a dicarbonyl compound 4. In many cases bis-ylides 5
are too unstable to be prepared in this manner, and then the second method

of performing the bis-Wittig reaction is preferred. In this method, the bis-

20

ylide is generated in-situ by adding base (usually alkali alkoxide) to a
solution containing both the bis-phosphonium salt and the dicarbonyl
compound in equimolar amounts. The alternative method suffers from the
disadvantage that many dialdehydes are unstable in the basic medium.

The resulting products could, in principle, contain new double bonds
in the cis and/or trans configuration. Experimentally, the stereochemistry
is found to depend on the nature of the ylide, the carbonyl component, the
solvent, and the presence or absence of salts.

Poly(2,5—thieny1ene-vinylene) was prepared by the Wittig reaction
about 25 years ago.51 It can be viewed as the intermediate structure
between (CH)x and polythiophenes. Poly(alkylthiophenes) also Show
interesting photoluminescence properties and have been studied in EL
devices.9 The Wittig reaction is a logical choice to combine thiophene units
and phenylenevinylene units with C=C bonds to form new block
copolymers. Once one chooses an appropriate dialdehyde, the reaction with
a variety of ylides may result in the electronic tunability in the resulting

polymers.

1.1.3. Organic Polymers Containing Nitrogen on Backbone

Among conjugated polymers, the extended 7: systems consisting of
alternating of C=C and C-C bonds are dominated. For example, many
prototype conjugated polymers,2 such as polyacetylene, poly(p—phenylene),
poly(p-phenylene-vinylene), are conjugated through all-sp2 carbons in
polymer backbone. Because the -CH=N- group is isoelectronic with the
-CH=CH- group, the incorporation of nitrogen atoms into the conjugated
system is an interesting approach to form materials with similar electronic

and optical properties.

21

Prototype polyaniline is a well-known example, which is constructed
by the alternating p-phenylene ring and nitrogen atom in the polymer
backbone.52 Polyaniline shows unique features such as the degree of
oxidation depends on the fraction two types nitrogen atoms, i.e., imine
(=N-, sp2) and amine (>N-, Sp3), and non-oxidizing doping by
protonation.53

Polyazines, [-N=C(R)-C(R)=N-]x (R = H, alkyl), have been
intensively investigated recently.54'56 The unsubstituted polyazine, [-
N=CH-CH=N-]x, is formally isoelectronic with the prototype and simplest
conjugated polymer, i.e., polyacetylene, [-CH=CH-CH=CH-]x. Unlike
polyacetylene, the presence of the nitrogen in polyazines stabilizes them in
air; i.e., they are not oxidized by oxygen.54 Polyazines are synthesized via
acid-catalyzed condensation reactions between an 0t,|3-dihydrazone and an
0t,B-dicarbonyl. Polyazines can be doped with iodine to give air-stable
electrically conducting materials with room temperature conductivity as
high as 1.3 S/cm.54

Conjugated poly(azomethines), polyimines or poly(Schiff bases) are
another interesting class of conjugated polymers containing nitrogen atom
in polymer backbone. The first poly(azomethines) were prepared by
Adams and co-workers from terephthalaldehyde and benzidine and
dianisidine in 1923.57 The basic aromatic conjugated poly(azomethine) is
poly( 1 ,4-phenylenemethylidynenitrilo- 1 ,4-phenylene-nitrilomethylidyne)58
(PPI), which is isoelectronic with poly(p-phenylene-vinylene) (PPV) and
intermediate between PPV and poly(emeraldine base) (PEMB) in
molecular structure, as Shown in Figure 1.9. Unlike PPV, the imine
nitrogen of the PPI backbone introduces novel features and chemical

flexibility.53'55

22

“’1’
Ow-O—O—=©=— PEMB
_Q_CH=H._Q_.H=HC_
_Q_CH= N_Q_~= HC_ pp.

Figure 1.9. Chemical structures of PPP, PEMB, PPV and PPI.

Recently, conjugated poly(azomethines) have been of growing
interest and extensively investigated for their synthesis,58’59 high thermal
stability,58,59 good mechanical strength and fiber-forming properties59
thermotropic liquid crystallinity,59 lyotropic liquid crystallinity in sulfuric
or methanesulfonic acid,59,60 third-order nonlinear optical properties,61
and electrical properties.62 It has been reported that the electrical
conductivity of conjugated polyazomethines may be increased by about
eight orders of magnitude (up to the level of semiconductors) when they
are doped by iodine.62 However, a major obstacle to characterizing and
developing the conjugated aromatic poly(azomethines) has been their
intractability and insolubility. Several methods have been reported to
improve the processability of conjugated poly(azomethines) by
modification and selection of polymer structure, for example,
unsymmetrical63 or symmetrical64 substitutions of main-chain aromatic

benzene ring units with flexible alkyl or alkoxy Side chains lead to

23

processable and dopable poly(azomethines). A recent approach based on the
reversible Lewis acid-base complexation has also been very successfully
applied to the processing of non-substituted and substituted
poly(azomethines).65 Figure 1.10 shows the chemical structures of some
previously investigated conjugated aromatic poly(azomethines) with
various backbone and side—group substitution structures.63'65 These known
structures of the poly(azomethines) include p-phenylene, p-biphenylene,
1,5-naphthalene, and vinylene, ether linkages in the polymer backbone, and
alkyl, alkoxy, and hydroxyl side-group substitutions.63'65 The solid state
electronic absorption spectra of these poly(azomethines) reveals that band—
gaps are in the range of 2.03—2.83 eV.

To our knowledge, oligothiophene-linked conjugated aromatic
poly(azomethines) have not been reported. However, it is notable that the
replacement of a phenylene linkage by a thiophene linkage in the
polyquinolines or polyanthrazoline backbone has been reported recently,66
and the products show a significant reduction of optical band-gap by 0.3-
0.5 eV. We expect that by substitution of benzene ring with 3',4'-dibutyl-
2,2':5',2"-terthiophene building block in conjugated poly(azomethine)
systems will not only improve the solubility but also reduce further the

band gap of the resulting polymers.

24

{0:949_N7531%

1. R1=R2=H (PPI) 10. R=H (PSPI)
2, R1=CH3, R2=H (PMPI) 11. R=OCH3 (PSMOPI)
3. R1=H, R2=OCH3 (PMOPI)

4. Rle, R2=OH (PHOPI)

5. R1=OCH3, R2=H (MO-PPI)

6. R1=R2=OCH3 (P3MOPI)

7. R1=OCH3, R2=OH (MO-PHOPI) 6 : N R
\
\S $—\\
R N9-

12. R=H (1,5-PNI)

—€©— N\\ Q 13. R=OCH3 (1,5-PMONI)
@‘N¥Q—\\Nfi
N9—

 

8. PPI/PMPI

@—o-©— N\\ 0 \N9_

m_ N\ : \N>— 14. PBEPI

9. PBPI

Figure 1.10. Chemical structures of previously investigated

POIY(azomethines). (reprinted from ref. 65 b)

25
Poly(azomethines) usually can be prepared by Schiff

polycondensation between diamines and dialdehydes in either solution or
solid melt state.59 A general Schiff reaction,67 discovered by Hugo Schiff,
a German chemist in 1864, is shown in Figure 1.11. In such reaction,
aldehydes and ketones react with primary amines, compounds of the type
RNH2 or ArNH2 to form a nucleophilic addition species known as a
carbinolamine. Once formed, the carbinolamine undergoes dehydration to
yield the product of the reaction, an N-alkyl or N-aryl-substituted imine:
(Figure 1.11)

 

 

H : Rn
I? .. addition ? elimination¥ I'll '
RCR' + R"NH —‘-——" RICR' ‘ RCR + H20
PIN’R"
Aldehyde Primary Carbinolamine N-substituted Water
or Ketone amine imine

Figure 1.11. General Schiff reaction. (ref 68)

By using dialdehyde and diamine, stepwise polycondensation will
occur to give poly(azomethines). In solution method, the reaction is
initiated in solution in water-free solvent, such as dimethylacetamide,
ethanol, or benzene. No catalysts are necessary but removal of water
expedites polymerization. The poly(azomethines) may be obtained in high
yield with low to intermediate molecular weight. High molecular weights
are attained by conducting the reaction at elevated temperature in a solid or

a molten state.

26

1.1.4. Investigations of Organic Conducting Polymers in

Our laboratory

During the past several years, our group has intensively investigated
the intercalation chemistry of conducting/non-conducting polymers in
various layered materials such as FeOCl,69 V205 xerogel,70 uranyl
phosphate,71 M003,72 and M08273 The resulting nanocomposite materials
show interesting physical and chemical properties.

To expand our study on conductive polymers, we are interested in
the synthesis and characterization of novel thiophene-based polymers or
copolymers. Without the limiting structural restriction imparted by the
layered galleries, we expect to obtain new processable and highly ordered
polymers/copolymers by the means of judicious choice of well-defined
monomers and specific chemical synthetic routes.

We have found that 3',4'-dibutyl-2,2':5',2"-terthiophene (DBTT) is
an excellent starting material for the synthesis of soluble, structurally-
ordered, and conjugated polythiophene.74 In Chapter 2, we present the
synthesis, chemical and molecular weight, X-ray diffraction
characterization, as well as spectroscopic, magnetic and charge transport
properties of this new polythiophene derivative, i.e. poly(3',4'-dibutyl-
2,2':5',2"-terthiophene).

As mentioned above, hybrid polymers containing varying
proportions of thiophene and pyrrole are particularly of considerable
interest due to their structural similarity.42'45 We have found that 3,4-
dibutyl-2,5-bis(2-thienyl)-pyrrole (DBTP) is a good candidate for the
synthesis of soluble and structurally regular hybrid polymer containing

thiophene and pyrrole. (Chapter 3)

27

With the goal of achieving both processability and electronic
tunability in the resulting polymers, we have synthesized several interesting
copolymers by the Wittig reaction and Schiff reaction using a dialdehyde-
functionalized building block, i.e. 2,5'-diformyl-3',4'-dibutyl-2,2':5',2"-
terthiophene (DFDBT).75 (Chapter 4 & 5)

To search for new polythiophene derivatives with possible low
energy band-gap, we have synthesized two new fused ring thiophene
monomers through a novel precursor method.76 Although the resulting
polymer and the charge-transfer salt do not show a significant decrease of
the band gap, both show interesting chemical and physical properties.
(Chapter 6 & 7)

28
1.2. Description of the Starting Materials Used in This Work

1.2.1. Oligomers for the Syntheses of Polymers

a) 3',4'-Dibutyl-2,2':5',2"-Terthiophene (DBTT)
3',4'—Dibutyl-2,2':5',2"-terthiophene (DBTT) was synthesized by
Michael E. Benz in Professor Eugene LeGoff's group. The details of the

synthesis are described in references 74 and 77.

S/\S
\/s\/

DBTT

DBTT is extremely soluble in chloroform, dichloromethane, and
ether. It forms pale green single crystals with a melting point of 36.0-36.5
0C. The solid state single crystal X-ray structure of DBTT has been
determined by Dr. Ju-Hsiou Laio in Professor Kanatzidis' group73 (Figure
1.12.). The thiophene rings take anti conformation with respect to the
position of the S atoms. The torsion angle between the adjacent rings is
about 6-90. The two n-butyl groups on the center ring are expected to
enhance the solubility of the corresponding polymer in common organic
solvents. On the other hand, the reduced number of side chains will
minimize extensive steric effects and give the corresponding polymer a
more coplanar backbone. In each repeating unit, more space has been
provided for the extension of the two butyl groups. In addition, the lower
density of the side n-butyl groups per thiophene ring compared to other

poly(3-alkylthiophenes), in principle, could lead to a better supramolecular

29

association of polymer chains thereby facilitating charge carrier hopping.
As mentioned above, the use of oligomer (here is a trimmer) as a starting
material would decrease the 0t,[3- or B,B-mislinkages during the
polymerization. Because the two terminal (at-positions of DBTT are
geometrically equivalent, the problem of head-to-head vs. head-to-tail
coupling which is commonly seen in the polymerization of 3-
alkylthiophenes is also eliminated. The relatively low oxidation potential of
DBTT (0.86 V vs. SCE) allows both chemical and electrochemical
polymerization under mild conditions. This avoids overoxidation and
potential side reactions of the product. Thus, we chose the novel monomer
DBTT to synthesize a soluble, well-ordered, and conjugated
polythiophene. (Chapter 2)

30

 

 

Figure 1.12. X-ray single crystal structure of 3',4'~dibutyl-2,2':5',2"-

terthiophene (see reference 78).

a.“

"At
. .—

‘¥\1'

t
a.

o I
l .
u

- \.
1
"b.

d.-
N‘\

 

l‘ g
."

I-

_s

_ w-

. s

31
b) 3,4-Dibutyl-2,5-Bis(2-thienyl)-Pyrrole (DBTP)
3,4—Dibutyl-2,5-bis(2-thienyl)-pyrrole (DBTP) was synthesized by
Seaver Shieh in Professor Eugene LeGoff's group. The details of the

synthesis are described in reference 79.

S /\s
\/ N\/

H
DBTP
DBTP is soluble in common organic solvents such as chloroform,
acetonitrile. It is expected to have a similar structure as its all sulfur
analog. It is also expected to have the same advantages as described above
for the synthesis of new hybrid polymer, i.e. poly(3,4-dibutyl-2,5-bis(2-
thienyl)-pyrrole. However, the electronic difference between the pyrrole
and thiophene rings causes significant chemical and physical changes in the

properties of the resulting polymers. (Chapter 3)

1.2.2. A Functionlized Oligomer for the Syntheses of

Copolymers

The successful synthesis of the new soluble and highly conjugated
poly(DBTT) justified our idea of using the monomer DBTT as a building
block to achieve better conductive polythiophenes. To broaden the
applications of this useful building block, a dialdehyde derivative of 3',4'-
dibutyl-2,2':5',2"-terthiophene was synthesized by Seaver Shieh in
Professor Eugene LeGoff‘s group. The synthetic details are described in

reference 79.

32

OHC \S/ /\ \8/ CH0
DFDBT

2,5'-Diformyl-3',4'-dibutyl-2,2':5',2"-terthiophene (DFDBT) is
extremely soluble in tetrahydrofuran, chloroform, dichloromethane, and
other common organic solvents. It forms gold-orange single crystals with a
melting point of 90-91 0C. This compound not only has the structural
advantages described for its parent compound (see above), but also has the
dialdehyde functional groups which give it the ability to be used in many
kinds of reactions involving these functional groups. Among these possible
reactions, two specific reactions are particularly of interest to us, i.e. the
Wittig reaction and the Schiff base reaction. Under certain reaction

conditions, both produce condensation polymers.

1.2.3. A Precursor for the Syntheses of New Monomers

and Polymers

The precursor method has been widely used in the syntheses of
polymers. A prime example is the use of soluble precursor poly(p-xylene-
a-dimethylsulfonium) salts to obtain high molecular weight
poly(phenylene-vinylene) (PPV).80

During our investigations on new polythiophene derivatives, we
discovered that the known compound thieno[3,4—d]-1,3-dithiole-2-thione
(TDT)81 could be used as a good precursor to synthesize new thiophene

derivative monomers.76

33

The precursor thieno[3,4-d]-1,3-dithiole-2-thione was prepared from

3,4—dibromothiophene as shown in Figure 1.13.

Br , SLi
\ 1.n-BuL1(1eq.) 3. n-BuLi(1 eq.) \
> >
2. s 1 .
/ Br ( 6C1 ) 4 S (1 eq~) / SLi
csz, NaOH (aq) \ \
>

reflux, N2 / /

 

 

C=S

 

thieno[3,4-d]-1,3—dithiole-2-thione

Figure 1.13. Schematic representation of the synthesis of thieno[3,4-d]-
1 ,3-dithiole-2-thione.

Excess potassium reacts with the thione in methanol or ethanol to
form the dipotassium salt intermediate via the GS bond cleavage. The
dipotassium salt of the thiophene-dithiolate intermediate reacts with
dibromoethane or dibromoethylene to give new monomer 3,4-
ethylenedithiathiophene (EDTT) and 3,4-vinylenedithiathiophene (VDTT)
in good yield, respectively. The synthetic route is outlined in Figure 1.14.
The dipotassium salt of the thiophene-dithiolate intermediate also reacts
with metal halide (such as NiC12 and CuClz) to form several transition
metal complexes with the new multisulfur ligand, 3,4-thiophene-dithiolate
C4H4S32' (tdsz'), as shown in Figure 1.14. However, the synthesis and
characterization of these metal complexes will not be included in this

dissertation. They will be published separately as a research paper.82

34

Chemical and S

BrCH ZCH 231' Electrochemical
> ——>

 

S
y‘\ RT' 1 D Polymerization
MeOK/MeOH [<:ESK 3.4-Ethylenedithiathiophene Poly(EDTT)
_—>
U EtOK/EtOH SK l

Thieno[3 ,-4-d] 1,3- Intermediate BrCH= CHBr/ Elwwcrystallization/
dithiole-Z-thione \CIO4
RT, 1 D Bu4NCIo,/CH,CN

3,4-Vinylenedithiathiophene Charge-transfer Salt
(VDT'I)

NiC12,CuC12 @@ 2-
.—___>

/

S

M =Ni, Cu

11

 

 

Figure 1.14. Precursor route to new thiophene derivatives.

In Chapter 6, we describe the synthesis and characterization of a new
soluble conductive polythiophene derivative, i.e. poly(3,4-
ethylenedithiathiophene). The properties of poly(EDTT) are compared
with some known related polythiophenes.

In Chapter 7, we present the synthesis and characterization of a new
charge transfer complex, 3,4-vinylenedithiathiophene perchlorate (VDTT-
C104) by the electrolysis of the monomer 3,4-vinylenedithiathiophene in
0.1 M (Bu4N)ClO4 acetonitrile solution.

(1)

(2)

(3)

(4)

(5)

(6)

(7)

35
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Chiang, C. K.; Park, Y. W.; Hegger, A. J.; Shirakawa, H.; Louis, E.
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36

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(a) Yamamoto, T.; Sanechika, K.; Yamamoto, A. J. Polym.
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37

Yoshino, K.; Hayashi, S.; Sugimoto, R. Jpn. J. Appl. Phys. 1984, Q,
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(82) Wang,C; Liao, J .-H.; Chou, J.-H.; Kanatzidis, M. G. manuscript in

preparation.

CHAPTER 2

Preparation, Spectroscopic, and Charge-Transport
Properties of A New Soluble Polythiophene
Derivative:
Poly(3',4'-dibutyl-2,2':5',2"-terthiophene)

48

49
ABSTRACT

A new polythiophene derivative has been synthesized by chemical
oxidative polymerization of 3',4'-dibutyl-2,2':5'2"-terthiophene (DBTT),
a monomer designed to yield an ordered, soluble, conjugated polymer.
The new polymer, poly(DBTT), contains two soluble fractions of differing
molecular weights, both of which were characterized by X-ray-diffraction,
IR, N MR, UVNis/NIR, photoluminescence and ESR spectroscopies, as well
as magnetic susceptibility and charge transport measurements. The
molecular weights of both fractions were determined by gel permeation
chromatography. The high molecular weight fraction (Mw~9.1x103) has
one of the longest n-conjugation lengths known for poly(alkyl-thiophenes)
and high conductivity. The low molecular weight fraction (Mw~4.3x103)
has a shorter conjugation length and two orders of magnitude lower
conductivity (~10-2 S/cm) at room temperature. Thermal gravimetric
analysis studies show that the polymer is stable in nitrogen up to 3800C.
Variable temperature charge transport data (conductivity and
thermopower) for both doped polymer fractions indicate p-type metallic
behavior. These results are compared to previously characterized

polythiophenes.

50

2.1. Introduction

In the field of conjugated polymers, polythiophenes occupy a
prominent position because of their high electrical conductivity, thermal
and chemical stability, amenability to chemical modification, reversible
redox properties, and other interesting physical properties.‘»2 Chemical
modifications on the polythiophene backbone yield processable
polythiophenes.3 Numerous applications have been demonstrated using
these materials. Examples include rechargeable battery electrodes4, field-
effect transistors5, electroluminescent devices6 and nonlinear optical
devices.7 Polythiophenes and their derivatives are prepared by either
chemical or electrochemical methods.

Recently, the study of polythiophenes derived from well-defined
oligomers has attracted increased attention.8-10 The main goal in these
studies is to exercise regio- and stereochemical control in the polymer so
that undesirable thiophene ring linkages such as 01, B- and [3, B— are
reduced.“ In poly(3-alkylthiophenes)8 (P3AT), the added chains can result
in lower electrical conductivity than the unsubstituted polymer, due to
steric hindrance which disrupts the coplanarity of the thiophene rings.
Another cause of structural irregularity originates from the fact that in 3-
alkyl-thiophene, the 2- and 5- positions are geometrically inequivalent,
making possible two types of 01,01-couplings. Although head-to-tail
couplings are favored, 10-20% of coupling defects involve head-to-head
couplings”. Several studies on di-substituted polythiophenes (e.g.
poly(4',4'-dialkyl-2,2'-bithiophene)13a and poly(3,3'-dialky1-2,2'-
bithiophene)13b), in which the head-to-head couplings are predominant,

showed significantly reduced conjugation lengths compared to the

51

unsubstituted material.l3 Steric effects such as intrachain sulfur-alkyl
repulsions become surprisingly dominant, forcing the thiophene rings out
of coplanarity and thus reducing n-conjugation. Recently, significant
progress has been reported in the regiospecific synthesis of ~100% head-to-
tail polyalkylthiophenes that exhibit significantly higher conductivities.l4
With the goal of obtaining a soluble conjugated polythiophene,
without the irregular couplings mentioned above, we synthesized and

polymerized the novel monomer 3',4'-dibutyl-01-terthiophene (DBTT) :

S/\S
\/s\/

DBTT

DBTT has a known structure (a detailed X-ray diffraction single
crystal structure determination has been carried out in our laboratory).15
The two butyl groups on the center ring are expected to enhance the
solubility of the corresponding polymer in common organic solvents.
However, because the two opposite oc-positions of DB'IT are geometrically
equivalent, the problem of head-to-head vs. head-to-tail coupling which is
commonly seen in the polymerization of 3-alkylthiophenes is eliminated. A
preliminary account of this work has appeared.16

Polymerization of 3,4-dialkyl-thiophene yields a polymer which also
avoids the head-to-head vs. head-to-tail coupling problem but steric
hindrance among the large number of alkyl groups does not help in
achieving planarity.17a We also here must mention that the polymerization
of an isomeric terthiophene derivative 3,3-dihexyl-terthiophene and several
monoalkylated analogs of DBTT have been briefly described in the
literature.17 DBTT, on the other hand, is expected to produce a polymer

which, because of the reduced number of side chains, minimizes extensive

52

steric effects and thus exhibit a more coplanar backbone. In each repeating
unit, more space has been provided for the extension of the two butyl
groups. In addition, the lower density of the side n-butyl groups per
thiophene ring compared to other poly(3-alkylthiophenes), in principle,
could lead to a better supramolecular association of polymer chains thereby
facilitating charge carrier hopping. Also, the relatively low oxidation
potential of DBTT (0.86 V vs. SCE) allows both chemical and
electrochemical18 polymerization under mild conditions. This avoids
overoxidation and potential side reactions of the product. In this chapter,
we present the synthesis, chemical and molecular weight characterization,
as well as X-ray diffraction, spectroscopic, magnetic and charge transport

properties of this new polythiophene derivative.

2.2. Experimental Section

2.2.1. Materials

FeCl3 (anhydrous), CHCl3, CH3CN, CH3NOz, MeOH, 12 were
purchased from commercial sources and used as received. 2-
bromothiophene was purchased from Aldrich Chemical Co. Pd(dppf)C12
was prepared according to ref. 19a (dppf= 1,3-bis(diphenylphosphino)-
ferrocene). The synthesis of 3',4'-Dibutyl-2,2':5'2"-terthiophene was
carried out by Dr. M. Benz in Professor LeGoff's group.

2.2.2. Synthesis of 2,5-Dibromo-3,4,-dibutylthiophene
To 18.7 g (95.4 mmol) 3,4-dibutylthiophene19b was added to 66 g
(210 mmol) tetramethyl ammonium tribromide (TMAT) in 100 mL of 1:1

acetic/dichloromethane. The reaction was monitored by TLC and was

‘ O
A‘mn‘h)
; “if IVL

-
. l
hl‘jfflll
All1¥l P J

'11:.“
1111,1111:

‘ I
111.: 11

Jun?
gland;

' JJ
._-
(V:

NMR
(DC:

..,._,.-;

mun-)1

«1A,
11 \1

.1.” ,

;1.
alum

53

complete in 25 min. The mixture was diluted to 200 mL with CHzClz and
filtered to remove Me4NBr. The filtrate was washed with water (3x50
mL) and then with saturated NaHCO3 solution until all acetic acid odor was
gone It was dried with MgSO4 and the solvent was removed under reduced
pressure. Purification of the crude product by flash chromatography gave
31.1 g (92%) of 2,5-dibromo-3,4,-dibutylthiophene as a clear liquid. 1H-
NMR (CDC13) 5 2.5 (t, 4H), 1.1-1.3 (m, 8H), 0.95 (t, 6H); l3C-NMR
(CDC13) 5 141.4, 107.8, 31.7, 28.7, 22.6, 13.9; EI-MS m/z (relative
intensity) 356 (m+2, 9.6), 354 (m+, 18.9), 352 (9.1), 269 (49.6), 233
(31.8), 191 (92.8), 189 (base)

2.2.3. Synthesis of 3',4'-Dibutyl-2,2':5'2"-terthiophene,

(DBTT)

A dry 100 mL flask containing 2.0 g Mg was placed in an ice bath
and dry ether was added to cover. To it was added dropwise 6.1 mL 2-
bromothiophene (63 mmol) in 20 mL ether. After addition was complete,
100 mL dry ether was added and the solution was refluxed for one hour. A
dry 500 mL flask containing 5.4 g 2,5-dibromo-3,4,-dibutylthiophene, 100
mg Pd(dppf)C1219a (0.4 mol %) and 300 mL dry ether was placed in a dry
ice/acetone bath. The Grignard solution was transferred to this flask by
double ended needle. The Grignard solution must be added with care to
ensure that the vigorous stirring continues. The cold bath was removed
and the mixture was stirred for 72 h at room temperature. It was then
quenched and washed with water. The organic layer was dried with MgSO4
and the solvent removed under reduced pressure. The product was purified
by flash chromatography using hexane as eluent. Yield was 4.58 g (83%)
of the pale yellow green DBTT (m.p.=36-36.5 0C). lH-NMR (CDC13) 5

54

7.35 (d, 2H), 7.19 (d, 2H), 7.10 (dd, 2H), 2.77 (t, 4H), 1.61 (quintet, 4H),
1.50 (sextet, 4H), 1.00 (t, 6H); 13C-NMR (CDC13) 5 139.7, 135.9, 127.0,
124.9, 124.4, 123.4, 32.3, 27.1, 22.3, 13.1; EI-MS m/z (relative intensity)
360 (m+, 3.63), 317 (6.4), 303 (2.56), 275 (32.4), 166 (18.8), 127 (22.74).
UV (acetonitrile) Max:331 nm (8 15,000).

2.2.4. Preparation of Neutral Poly(3',4'-dibutyl-2,2':5'2"-

terthiophene), poly(DBTT)

To a stirred solution of 1.00 g (2.78 mmol) DBTT dissolved in 40
mL of chloroform was added dropwise a mixture of 1.80 g (11.12 mmol)
anhydrous ferric chloride in 150 mL of chloroform. A deep blue-black
precipitate appeared quickly. The resultant dark mixture was stirred at
room temperature for ~48 h, then poured into 1000 mL of methanol. The
resulting precipitate was filtered from the reaction mixture and
exhaustively extracted with methanol in a Soxhlet extractor for 24 h in
order to remove the residual oxidant and oligomers. This was followed by
extraction with acetone and drying in vacuo at 50 0C for 24 h. There was
obtained 0.87 g (87% yield) of red powder of poly(DBTT). Elemental
analyses by EDS showed that Fe and Cl impurity was less 0.5%.

2.2.5. Isolation of Fraction I of Poly(DBTT)

A sample of 0.42 g poly(DBTT) was exhaustively extracted with ten
250 mL portions of CHC13 at 23 0C until the filtrate was colorless. The
yellow orange extracts were combined and concentrated in vacuo to
approximately 25 mL. To this solution 250 mL of methanol was added to
precipitate a red powder, fraction 1. The product was subsequently washed

with methanol and dried in vacuo at 60 0C for 12 h to give 0.084 g of

55

product (20% yield). lH-NMR (CDC13): 5 (ppm) 7.30, 7.13, 7.05, 2.73,
1.53, 1.46, 0.97; 13C-NMR (CDC13): 5 (Wm) 140.50, 136.96, 136.26,
135.41, 129.98, 129.78, 127.33, 126.58, 126.07, 125.40, 123.94, 32.86.
28.00, 23.03, 13.81.

2.2.6. Isolation of Fraction II of Poly(DBTT)

The insoluble material obtained from the above procedure (ca. 0.33
g) was heated in a sealed thick wall pyrex tube with 300 mL CHC13 at 80
0C for 10 h. Subsequently the tube was opened and the insoluble material
was removed by filtration. The red CHC13 extract was evaporated to
dryness in vacuo to afford 0.24 g of a red product, fraction II. Yield 57%.
lH-NMR (CDC13): 5 (ppm) 7.20, 7.12, 7.05, 2.73, 1.53-1.46, 0.97.

2.2.7. Synthesis of Doped Poly(DBTT)

(A) Doping with iodine in nitromethane. To a stirred solution of
excess iodine in 50 mL of nitromethane, 0.05 g red powder of poly(DBTT)
was added. The red powder turned black immediately. After stirring for 9
h, the black solid was collected, washed several times with nitromethane
and vacuum dried. The yield was quantitative.

(B) Doping with ferric chloride in nitromethane. To a stirred
solution of 0.1M ferric chloride in 50 mL of nitromethane at room
temperature, was added 0.03 g poly(DBTT). A black product formed
immediately. After stirring for 12 h, the black solid was collected from the
reaction mixture, washed several times with nitromethane and vacuum

dried.

56

2.2.8. Physicochemical Measurements

Elemental analyses (S, Fe, Cl, 1) were performed on a JEOL JSM-
35C scanning electron microscope (SEM) equipped with a Tracor Northern
energy dispersive spectroscopy (EDS) detector. Infrared spectra were
recorded on pressed KBr pellets on a N icolet 740 FTIR spectrometer. UV-
visible-NIR absorption spectra were obtained from a Shimadzu UV-
3101PC double beam, double-monochromator spectrophotometer.

X-ray powder diffraction patterns were collected at room
temperature on a Rigaku powder diffractometer using Cu(K01) radiation
generated by a rotating anode operating at 45 kV and 100 mA. The data
were collected at a rate of l deg/min. FeOCl was used as a standard for
estimating the instrumental line broadening in our Scherrer calculations.20

Nuclear magnetic resonance spectra (1H and 13C) were obtained
using a computer controlled Varian VXR-NMR(500 MHz) spectrometer.
The chemical shifts are reported in parts per million (d, ppm) using the
residual solvent resonance peak as reference (CHC13, 5 7.24 ppm for 1H
and 77.00 ppm for 13C).

Photoluminescence spectra were measured at room temperature on a
Perkin Elmer LS-5 fluorescence spectrophotometer. Thermogravimetric
analysis (TGA) and Differential scanning calorimetry (DSC) were
performed on Shimadzu TGA-50 and DSC-50 under nitrogen or oxygen at
5 0C/min or 30C/min heating rate. Electron spin resonance (ESR)
measurements were conducted on a Bruker ER200 ESR spectrometer.
Variable temperature magnetic susceptibility measurements were carried
out using a Quantum Design Inc. SQUID Magnetic Property Measurement

System (MPMS). The magnetization of each sample was examined and was

57

found to vary linearly as a function of applied field (from 500 to 5000
Gauss). Diamagnetic corrections were made for both the container used
(plastic bag) and the contributions of the atoms in the molecule using the
Pascal constants.21

Molecular weight measurements of the soluble fractions of
poly(DBTT) were carried out using gel-permeation chromatography
(GPC) using a Shimadzu LC-lOAS high pressure liquid chromatograph
(HPLC) equipped with a PL—GEL 511 (MIXED-B) column of length 300
mm. Chromatographic grade tetrahydrofuran (THF) was used as an eluent.
Calibration was made with a series of polystyrene standards (Mw in the
range of 3250 to 500800).

Charge-Transport Measurements. DC electrical conductivity
and thermopower measurements were made on pressed polymer pellets.
Conductivity measurements were performed in the usual four-probe
geometry with 60- and 25-um diameter gold wires used for the current and
voltage electrodes, respectively. Measurements of the sample cross-
sectional area and voltage probe separation were made with a calibrated
binocular microscope. Conductivity data were obtained with the computer-
automated system described elsewhere.22 Thermoelectric power (TP)
measurements were made by using a slow ac technique22 with 60-um gold
wires used to support and conduct heat to the sample, as well as to measure
the voltage across the sample resulting from the applied temperature
gradient. Samples were suspended between the quartz block heaters by 60-
ttm gold wires thermally grounded to the blocks with GE 7031 varnish.
The magnitude of the applied temperature gradient was generally 1.0 K.
Smaller temperature gradients gave essentially the same results but with

somewhat lower sensitivity.

58

In both measurements, the gold electrodes were held in place on the
sample with a conductive gold paste. Mounted samples were placed under
vacuum (10'3 Torr) and heated to 320 K for 2-4 h to cure the gold
contacts. For a variable-temperature run, data (conductivity or
thermopower) were acquired during sample warming. The average
temperature drift rate during an experiment was kept below 0.3 K/min.
Several variable-temperature runs were carried out for each sample to
ensure reproducibility and stability. At a given temperature,

reproducibility was within i5%.

2.3. Results and Discussion

2.3.1. Polymer Synthesis
Poly(DBTT) was synthesized by direct chemical oxidative
polymerization of DBTT using ferric chloride as the oxidant in

chloroform, according to Eq. 1.

 

y+
s /\ s 4FCI CHC‘3 : S /\ S -
\/ S \/+63 2d,RT \/ S \/n(F¢CLl)y
Eq.l

The as-made black conductive polymer is doped with FeCl4' (y=0.4).
Undoping can be accomplished by Soxhlet extraction with methanol and
acetone, yielding a red product.

The neutral poly(DBTT) is partly soluble in chloroform, at room
temperature, giving a yellow-orange solution from which fraction I can

be obtained. The room temperature insoluble residue of poly(DBTT) is

59

more soluble in chloroform at an elevated temperature (e.g. 80 0C) giving
fraction 11. Finally, a small fraction is insoluble in all organic solvents,
fraction III. In contrast, due to the high content of n-alkyl groups, P3AT
are completely soluble. These fractions must differ from one another in
molecular weight. The fractionation process is shown in Scheme 2.1. Free-
standing films can be easily cast from solution by slow solvent evaporation.
The films can be doped with either iodine vapor in a closed chamber or
ferric chloride in nitromethane.

The expectation that the DBTT monomer would yield an ordered
product was confirmed by X-ray scattering which shows that both soluble
fractions are polycrystalline, when undoped. Annealing the samples at 180
0C for 15 h followed by cooling to 50 0C at 26 deg/h increased the
crystallinity significantly, as shown in the powder patterns of Figure 2.1.
The most notable feature is the intense low angle reflection at 17.06 A (in
both fractions) and the very broad high angle reflection at 4.40 A.
Annealing not only increased the intensity of the first low angle reflection
but also shifted it to higher 20 angle, indicating a slight contraction of the
repeating length to 16.43 A and 16.74 A for fraction I and II respectively.
Annealing also revealed clearly the presence of the second and third order
reflections at 8.15 and 5.42 A, and 8.36 and 5.51 A for fractions 1 and 11
respectively. These peaks are attributed to considerable coherent ordering
of the polymer chains due to their planar configuration. These reflections
arise from the packing of poly(DBTT) chains which is dominated by the
length and density of the butyl side chains. It should be noted here that
these X-ray diffraction patterns bear strong similarities to those of P3AT
and thus are likely due to the similar overall fundamental packing of alkyl-
substituted thiophenes, including poly(DB'I'I‘).23 The presence of rotational

60

y+
As prepared: \S / /S\ \S / ](FeC14')y

 

U d . Soxhlet extraction
‘1 0ng with MeOH and
Acetone
__ S / \ 8 Red Product
\ / S \ (mixture)
11

Disslove in
CHC13 at room temp

 

Soluble Fraction I
Yellow Xmax=446 nm

   
    

Evaporation
Solvent

Cast Film (Orange)
Max:488 nm
Fraction I

 

 
   
   

l Doped by iodine vapor

brown film

1

Insoluble fraction

 

 

   
 

    
  

Red
Disslove in CHCl3
at 80°C
Soluble Fraction II Insoluble
Red Solution (11) Red Solid
km ax =499 nm Fraction IH

Evaporation
Solvent

 

Cast Film (Red)
Xmax=522 nm

 
 
 

 

  

Fraction H
Doped by iodine vapor

brown film

Scheme 2.1. Fractionation of poly(DBTT).

61

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(A)
61‘
1: Annealed
E W
:1 we: _
a m.
S
E
Un-anncalcd
' I
5. 10. 15. 20. 25. 30. 35. 40. 45. 50. 55. 50.
26(Deg)
(B)

8
E ic_,5/ -v////\\\N‘\_\‘~_ Auunuued
:5 .
:n'
a fi.
3 .
g

// Un-annealcd

'I'I'r'r'r'r'l'r'r'I't'lfi'r'r'r'l'r'r'r'r'l'r I'I'l'l'I'I'U'I'I'I'I'I'I'I'I'I'I'I'I'I'I'I'I‘I‘I'I'I'I'I'I'I'I'I‘

5. to. 15. 20: 25. 30. 35. 40. 45. so. '55. so.
29(ch)

Figure 2.1. X-ray powder diffraction profiles for annealed and un-

annealed samples of (A) fraction 1 and (B) fraction II.

62

defects along the backbone, resulting from 180 deg rotations of the
thiophene rings, particularly among those with no alkyl substitution, is
entirely possible. However, the quality of the X-ray data is not sufficient to
detect them. The broad peak at 3.8 A observed in P3AT is attributed to an
interlayer spacing arising from a lamellar packing of the polymer chains. It
corresponds to the 4.4 A feature observed in the diffraction pattern of
poly(DBTT). The average coherence length of the unannealed fraction I
and II was calculated from the Scherrer formula20 to be 69.5 A and 41 A,
while in the annealed samples it increased to 144 A and 109 A respectively.

The chemical structure and properties of the polymer were further
examined with vibrational, optical, N MR and EPR spectroscopies.
Photoluminescence spectroscopy was also used to explore the excited state
properties of poly(DBTT). The molecular weights of the soluble fractions
were determined by the GPC technique.

2.3.2. Chromatographic Molecular Weight Studies

Molecular weights of the two soluble fractions of poly(DBTT) were
determined by gel permeation chromatography in THF. Figure 2.2 shows a
typical chromatogram representing the molecular weight distribution from
fraction II. The molecular weights were obtained from the common
method of a retention time calibration curve using a series of polystyrene
standards with a UV-Vis detector set at 450 nm. The weight-average
molecular weight (Mw) of fraction I is ~4.3x103 and number-average
molecular weight (Mn) is ~2.6x103 giving a polydispersity index (PD) of
1.65. The Mn corresponds to an average of 21 rings per chain. This is in
good agreement with the value obtained for this fraction by end-group

analysis using NMR spectroscopy, (see below). For fraction 11, MW is

63

 

 

INTENSITY (ARB. UNITS)

 

 

 

 

T

I .
3 5 4 0 4 5 SIC 5 5 6 O
RETENTION TIME (MIN)

q‘I--———I
O

3

Figure 2.2. Typical GPC trace of fraction II of poly(DBTT) in THF

solution at room temperature.

64
~9.1x103 and Mn is ~5.7x103 with PD of 1.60. The Mn corresponds to 48

rings per chain. The rather small value of polydispersity indicates narrow
molecular weight distribution in both soluble fractions of the poly(DBTT).
We note that the measured molecular weights are an order of magnitude
lower than those reported for poly(3-hexylthiophene) (P3HT) and similar
polythiophenes.24 However, a superior polydispersity index is achieved in
poly(DBTT). P3HT has Mw ~1.4xlO5 and Mn ~3.2x104, corresponding to
a polydispersity index of 4.4.13b The lower MW of poly(DBTT) is in
agreement with what was found for polythiophene synthesized from
terthiophene. This is attributed to the increased stability of the radical
cations formed from terthiophene, which slows the polymerization rate and
leads to low molecular weight polymers.25’17b The GPC studies on the two
soluble fractions of poly(DBTT) place a lower limit on the molecular
weight of the completely insoluble fraction III which is expected to be
much higher than 104.

2.3.3. Infrared Spectroscopy

Figure 2.3 shows the infrared spectra of the starting oligomer DBTT
and the neutral poly(DBTT). The principal IR absorption bands observed
in poly(DBTT) and their assignments, together with the corresponding
results for polythiophene and P3AT, are listed in Table 2.1. Both fraction I
and fraction II have very similar infra-red spectra with peaks at essentially
the same energies but with small differences in intensity.

The strong, sharp absorption band at 788 cm-1 (C-H out-of-plane
vibration) is characteristic of an amt-coupled alkyl-substituted
polythiophene ring, suggesting a linear polymer chain structure}.24 The

single broad peak at 3062 cm'1 is due to CB-H stretching modes. The Ca-H

65

I. (A) MIN
:W +

. (B) g
I W /7“ “4

5000 3590 also zivo 2560 1§50 1§§O 1130 #520 510
unVENUHaEH

 

7. TFIHNSH I TTHNCE

 

 

Figure 2.3. FT-IR transmission spectra(KBr pellets) of (A) DBTT and
(B) neutral poly(DBTT).

66

Table 2.1. Comparison of Infrared Band Positions (cm'l) and Their
Assignments for Poly(DBTT) and Various Poly(3-alkylthiophenes)

 

 

arom aliph. methyl arom C-H
sample CB'H Stl' C-H str ring str def out-of-plane
Poly(DBTT) 3062 2951 2925 2856 1492 1456 1377 788
PTha 3063 1491 1453 1441 1377 788
P3BTa 3055 2955 2928 2858 1512 1458 1439 1377 829
P3HTa 3055 2959 2930 2858 1512 1458 1439 1377 825

 

a) see ref. 24: PTH-polythiophene, P3BT-poly(3-butylthiophene), P3HT-
poly(3-hexylthiophene)

stretching mode, observed in the spectrum of DBTT at 3103 cm'1 , is
absent. This provides further support for the predominance of 0t,0I-
couplings in the polymer backbone.24 It is noteworthy that in poly(DBTT)
only two bands are observed at 1456 and 1492 cm’l, which are assigned to
the C=C symmetric and antisymmetric stretching modes respectively. In
the case of P3ATs, three bands are present in the same region (1520-1440
cm'l).24 The decrease in the number of active modes in the C=C region is
probably due to the existence of an inversion center in the poly(DBTT).
Similar features have been reported for poly(4,4'-dialkyl-2,2'-
bithiophene)13a and poly(3,3'—dihexyl-2,2'-bithiophene)13b, which also
contain an inversion center in their ideal structures.

Based on a previous analysis of the vibrational spectra of
polythiophenes and oligomers, the intensity ratio of the symmetric stretch

at 1456 cm-1 to the asymmetric stretch at 1492 cm-1 (Isym/Iasym) is indicative

67

of the degree of conjugation in the polymer backbone.26 Extended
backbone conjugation results in small ratios. Thus, it would be interesting
to compare this ratio in the two soluble fractions of poly(DBTT). The IR
spectra of these fractions show clearly that Isym/Iasym is greater in fraction
1, consistent with a smaller conjugation length, as expected.

The IR spectra of the doped polymer are virtually independent of
dopant and are shown in Figure 2.4. As in other polythiophenes, doping
causes a profound change in the IR spectra due to dramatic changes in the

electronic structure of poly(DBTT).

2.3.4. Electronic Spectroscopy, UV-Visible-NIR

The optical absorption spectra of fraction I and fraction II, in
chloroform, are shown in Figure 2.5. In both fractions of poly(DBTT), the
energy of 1t—1t* transition occurs at lower energy than that observed for
P3HT (maximum absorption at 439 nm in chloroform13b). The
corresponding solution-cast films show absorption maxima at 488 nm for
fraction I and 522 nm for fraction II at room temperature, see Figure 2.6.
The higher Kmax and lower solubility of fraction II together suggest it has
a considerably longer effective conjugated chain length and larger
molecular weight. This conclusion is consistent with the molecular weight
data obtained by GPC.

A red shift of the Tt—Tt* absorption band, in going from solution to
the solid state, is characteristic of all soluble polythiophenes”. This can be
attributed to conformational changes, which decrease the degree of
conjugation in the polymer backbone in solution, as compared to the

condensed state27v23. This decrease in conjugation results from the

'/. THHNSH I TTRNCE

- L

 

68

(A)

(B)

 

$000

3390 also aim 2360 1050 ISIIO 1130 1120 510
uavsuuneEn

Figure 2.4. FT-IR transmission spectra(KBr pellets) of the doped
polymer: (A) neutral poly(DBTT) doped with FeCl3 in CH3N 02 solution;
(B) neutral poly(DBTT) doped with 12 in CH3N 02 solution. A

69

 

 

 

 

 

 

 

 

 

 

I J l 1
fr?
1':
2
D
:0
I
5
LL!
0
Z
<
m
(I
8 ( A)
m
<
I I T I
300 400 500 600 700 ' 800
WAVELENGTH (nm) -
l l l 1
(’5
t:
2
D
as
(I
S.
LU
0
Z
5
II B
o ( )
m .
CD
<
I I I r
300 400 500 600 700 800
WAVELENGTH (nm)

Figure 2.5. Solution UV-Vis absorption spectra of two soluble-fractions
of poly(DBTT) in chloroform solution at room temperature: (A) fraction I

(Max:446 nm); (B) fraction II (Max:499 nm).

7O

 

 

 

 

 

 

 

 

 

 

I l l l
63‘
1: neutral
2
D
:0
a:
:5.
LL!
'0
_Z
<
co
0:
O
U)
:0
<
I I T I
0 1 2 3 4 ' S
ENERGY (eV)
1 J l I
E natural
2
D
:0
0:
s
LU
O
2
<
CD
a:
O
(I)
03
<
I I I 7
O 1 2 3 4 5

ENERGY (eV)

Figure 2.6. UV-Vis-NIR absorption spectra of neutral and doped
poly(DBTT) films at room temperature: (A) fraction 1; (B) fraction II.

71

deviation of adjacent thiophene rings from coplanarity. Of course, a very
small contribution to the red shift would come from the alkyl substitutents
on the backbone. It has been proposed that, in solution, polythiophenes
adopt a coil-like conformation resulting in relatively small conjugation
lengths.13bv243l In the solid state a rod-like structure is expected providing a
more extensive delocalization. It is notable that the shifts of the absorption
maximum for the two poly(DBTT) fractions are smaller (42 and 23 nm,
respectively) than in the case of P3HT (69 nm)13b. The higher kmax in
solution, coupled with the smaller blue shift in going from the solid to the
solution phase, indicate a considerably smaller coil-like contribution to the
structure of poly(DBTT) and thus less extensive deviation from coplanarity
of the thiophene rings. As a result, we conclude that the average
conjugated chain length in poly(DBTT) is longer than that in the typical
P3AT. The conjugation length approaches that of the 100% head-to-tail
P3AT reported recently.14 This is consistent with the fact that two-thirds
of the heterocyclic rings are not alkylated and thus experience minimal
steric repulsion. This justifies our initial expectations in choosing DBTT as
the monomer.

At first glance, it would seem that the longer n—conjugation length in
poly(DBTT) compared to P3AT, suggested by the electronic spectra, are in
contrast with the molecular weight studies, which suggest correspondingly
shorter overall chain lengths. The reason probably lies in the considerably
more planar organization of poly(DBTT) chains resulting from reduced
steric repulsion as explained above. Therefore, poly(DBTT) is the first
polythiophene which exhibits longer effective conjugation lengths than

P3AT while possessing smaller MW (i.e. shorter polymer chain lengths).

72

From the solid state electronic spectra the band-gaps of the
semiconducting poly(DBTT) fractions were determined by extrapolating
the linear portions of (ozhv)2 vs. E plots to (ahv)2—>0. Interestingly,
these direct band-gaps are virtually the same for fraction I and II and very
similar to P3AT, suggesting that inter-chain n-Tt'“ solid state interactions
must contribute significantly to the band-gap.

Upon doping with iodine, the solution cast films turn dark brown.
The UV-Vis-NIR absorption spectra of films for the neutral and doped
state are compared in Figure 2.6. For a film of fraction 1, the 1t—1t*
absorption band at 2.64 eV loses intensity upon oxidation and shifts to
slightly higher energy, while two new subgap absorption bands appear at
0.66 eV and 1.55 eV. For fraction II, the 1t—1t* absorption band also
weakens, while two new subgap absorptions appear at 0.49 eV and 1.47 eV.
The two midgap absorptions which develop in both cases correspond to
transitions-associated with the two localized bipolaron energy levels that
appear within the band gap upon doping“,8 These results are consistent
with charge storage predominantly in bipolaronslc and follow the same
trends as P3AT. Further support for the conclusion that fraction II
possesses a longer average conjugation length, than fraction 1, derives from

the lower energies associated with the midgap transitions of the former.

2.3.5. Photoluminescence Spectroscopy

Figure 2.7 shows the photoluminescence spectra of the two soluble
fractions of poly(DBTT) in chloroform at 23 0C using excitation
wavelength of 400 nm. Photoexcitation of these polymers results in broad
band luminescence (halfwidth~0.20 eV) with a peak maximum at 2.23 eV
(557 nm) for fraction I and a similar band (halfwidth~0.15 eV) with peak

73

 

(A)

INTENSITY (ARB. UNITS)

 

 

 

 

T . l T I I 0
420 460 500 $40 580 620 660 700

 

 

 

 

WAVELENGTH (nm)
(73‘
I:
2
D
In'
(I
S.
E
0)
E
I- (B)
Z
"""'""I“ -7 I I I u
420 460 500 540 580 620 660 700
WAVELENGTH (nm)

Figure 2.7. Photoluminescence emission spectra of two soluble fractions
in chloroform solution at room temperature: (A) fraction I (Max:557 nm);

(B) fraction II (Max:620 nm).

74
maximum at 2.00 eV (620 nm) for fraction II. The peak at 2.00 eV

coincides with the onset of optical absorption of this fraction indicating that
it is a localized excitation.29 The shift of the emission peak maximum to
lower energies in fraction II parallels a similar shift of the absorption
maximum seen in the electronic absorption spectra of Figure 2.5. This is
consistent with a longer effective conjugation length in fraction II. The
energy and profile of the luminescence bands are insensitive to the
excitation frequency in the range of 350-500 nm for both soluble-fractions
of poly(DBTT). By comparison, P3HT shows a peak at ~2.17 eV (571
nm) and halfwidth of 0.3 eV.30

In the solid state, both fractions emit light at lower energies (at 596
nm for fraction I and at 602, 627 and 656 nm for fraction II). This is in
good agreement with the decrease in band gap observed in the absorption
spectra of the solids. By comparison solid films of P3HT emit at 596 nm.
The photoluminescent property of the soluble fractions of poly(DBTT)

offers an opportunity to study electroluminescence in these materials.31

2.3.6. NMR Spectroscopy

The polymer fractions were studied by solution 500-MHz 1H NMR
and 75.4 MHz 13C NMR spectroscopy. The spectra of fraction I and
fraction II with their assignments are shown in Figure 2.8. Fraction I
shows three major peaks in the aromatic region. The weak resonance at
7.30 ppm is attributed to the hydrogen atoms on the a-position of the
terminal thiophene rings. The other two resonances are due to the B-
' hydrogen atoms. This assignment is based on the corresponding NMR
spectra of the DBTT monomer and its dimer (i.e. the hexathiophene

homolog)32. From peak integration of the two types of resonances of the

75

 

 

 

 

 

 

 

 

 

(TIKTIJ
(A) a
f
e c b
"0
TT 1 T l - I ‘fi
7 2 I oo-
‘HzO
(B)
fijkfia A’JLL
r* T I‘ I I T fiT'f"T"r‘IfiTfi‘1"j
s 5 4 3 2 a on-

Figure 2.8. 500 MHz 1H NMR spectra of the two-soluble fractions of
poly(DBTT) at room temperature (in CDC13). (A) fraction I; (B) fraction

II. The peak integration associated with peaks f and e gives 1:1 ratio.

76

terminal OI-hydrogen and the B- hydrogen atoms, a molecular weight
determination by end-group analysis is possible, yielding a value of ~2200,
very close to six DBTT units or ~18 thiophene rings. This number agrees
reasonably well with that estimated from the GPC studies described above.
In fraction II, the peak due to the terminal OL-hydrogen position is almost
totally suppressed and thus no end-group analysis could be done.

The four resonance peaks found in the aliphatic region correspond to
the hydrogen atoms on the side n-butyl groups. For fraction II, the
resonance peaks in the aliphatic region are much broader and less well
resolved, see Figure 2.8B, consistent with its much higher molecular
weight.

The 13C NMR spectrum of fraction I of poly(DBTT) is shown in
Figure 2.9. It shows major resonance lines at 32.9, 28.0, 23.0 ppm assigned
to the three methylene groups of the butyl moiety, while the line at 13.8
ppm is attributed to the methyl groups of same. At least fifteen lines are
observed in the aromatic region and correspond to the carbons on the
thiophene rings. It is known that, in polythiophene, the rings can adopt syn-
or anti- conformation with respect to the position of the S atoms. Ab—initio
Hartree-Fock calculations indicate that the anti- conformation is stable by
only 1.3 Kcal/mol.33 If the conjugated chain in poly(DBTT) existed in the
ideal all anti- conformation, we would expect from symmetry arguments a
total of six resonance lines for the ring carbons. The extra peaks in the
spectrum could arise either from symmetry breaking and adoption, in some
segments, of the syn- conformation, via ring flips, or from the observation
of signals from terminal thiophenes, or even from occasional OI,B—
couplings. Unequivocal assignment of each peak is not available at this

stage, but it appears that several backbone conformations are contributing.

77

('lu'u

 

 

‘ A ‘e .44- J
_ A- ___4 AA “A A..- LA A ‘
v I I I ' I V 'V w‘ V W '— '1 fl '1' w ‘—

[ff-VITWIW—YIVIIIITTW‘IWYTTITI tY'tU'11—'11111111‘1IIVIVVIVITIIIjththivvlvrvtl

140 130 120 I 10 100 90 80 IO 60 50 40 30 20 10 000

 

 

 

Figure 2.9. 13C NMR spectra of fraction I of poly(DBTT) at room
temperature (in CDC13).

78

This is supported by the fact that a mixture of syn and anti conformations
of thiophene rings has been observed even in the solid state in a
hexathiophene derivative (i.e. the dimer of DBTT) by single crystal X-ray
diffraction.15 The most probable site for ring-flipping, or occurrence of
syn-conformation would be where the DBTT units join. At those locations
the absence of butyl groups provides the lowest rotation barrier in the
polymer. Although the electronic absorption spectra discussed above do
not support this possibility, the presence of 0L,B— couplings may not
entirely be ruled out. Scheme III depicts several possibilities of coexisting
syn- and anti- conformations. A satisfactory 13C NMR spectrum of fraction

11 could not be obtained due to low solubility.

 

S/\ S S/\ /\ S/\ s s s
s \/s \ \/s s \/s /\ \/
s s s s s s / s s s s s
\/ /\/\/\/ s \/\l \/\/\l

Scheme 2.2. Possible conformations of poly(DBTT) backbone.

2.3.7. Thermal Analysis
The thermal stability of the polymer in its various forms was
examined by thermogravimetric analysis (TGA) and differential scanning

calorimetry (DSC). Neutral poly(DBTT) exhibits excellent stability in

79

nitrogen up to at least 380 0C, as shown in the TGA data in Figure 2.10. In
oxygen, the material starts to decompose at ~285 0C and loses 98% of its
weight at 580 0C. By comparison, poly(3-octylthiophene) (P3OT) exhibits
lower thermal stability, decomposing in nitrogen at 300 0C and at 250 0C
in oxygen.34 The greater thermal stability of poly(DBTT) is attributed to
the fewer average number of alkyls groups per thiophene ring.

DSC curves of neutral poly(DBTT) show a broad, weak,
endothermic peak at 236 0C, upon heating, and a narrow intense
exothermic peak at 207 0C upon cooling, see Figure 2.11. However, the
polymer shows signs of no melting up to 250 0C as judged by direct visual
observation, suggesting, instead, the presence of an order-disorder
transition in the solid state.23 This is supported by the increased
crystallinity exhibited in the annealed samples. The changes occurring in

the polymer during the heating cycles are repeatable.

2.3.8. Electron Spin Resonance Spectroscopy

Electron spin resonance values for the neutral form and doped form
(with different doping levels of 13,-) of poly(DBTT) are listed in Table 11.
Figure 2.12A shows the ESR spectrum of the neutral poly(DBTT), which
reveals a single, largely symmetric, line centered at nearly the free electron
g value of 2.0033 with a linewidth Apr=8 Gauss. The number of spins
determined, using a diphenylpicrylhydrazide (DPPH) standard, is 3.9x102O

spins/mo] rings, corresponding to approximately 1 spin per 1544 rings.

 

 

 

 

 

 

 

 

 

 

 

110 #41; J I l 1 l l
100d P” I—
... 90- 300 °c .
o\°
5 30- -
I'-
g 70- -
i'fi - .
3 60- -
‘ 50% (A) r
40 I I I I I I I a
O 100 200 300 400 500 600 700 800
TEMPERATURE(°G)
1 l l l L l ’1
100a / , .-
G-I 0 il-
A 00- 285 c __
.\° - ' _
E 60.4 ,_
E . _.
g .01 _.
3 ~ ..
204 (B) T
o I I I f I I I
O 100 200 300 400 500 600 700 800
‘l'EMPERATURE(°C)

Figure 2.10. TGA thermograms of neutral bulk poly(DBTT): (A) under

nitrogen; (B) under oxygen.

81

 

 

 

 

 

 

 

 

 

207 “C
Exo ’
I 0.40
mw 0 20
0.00»
iv» 1””.
Endo 236 °C
1 1 A 1 1 _l L A 4 + L 4 1 L J J g L
0.00 100 00 200.00 300.00
Temperature (°C)

Figure 2.11. DSC thermogram of neutral bulk poly(DBTT) (under

nitrogen).

82

 

 

 

 

 

 

 

 

 

 

 

 

II
E! ( A) I,‘ 8 Gauss
Z I H
D I
g I
5
>*
I:
to
E

FIELD (GAUSS)
{"5 (m
2 . 26 Gauss
D I--I
m’
a:
5
>-
I:
m
E
FIELD (GAUSS)

Figure 2.12. ESR spectra of (A) neutral bulk poly(DBTT) and (B)

polymer doped with iodine (33% mol) at room temperature.

83

Spin quantitation performed on both fractions show a number similar to
the bulk. The number of spins is about one order of magnitude smaller
than that reported for P3HT (2.4x1021 spins/mol“), and suggests a high
level of purity for poly(DBTT) and fewer spin-carrying defects. This is
consistent with our initial expectation that a well designed oligomer, such
as DBTT, could result in high quality conjugated polymer.

Figure 2123 shows the ESR spectrum of iodine doped bulk
poly(DBTT) (33% mol 13'). A broader, single, asymmetric line with
g=2.0030 and Apr=26 Gauss is observed, corresponding to 1.04x1022
spins/mol rings. Assuming that no spin-carrying mobile carriers exist at
this doping level, the larger number of spins in the doped polymer can be
attributed to an increase in defect concentration during the doping process.

Similar effects have been observed for P3AT.24

Table 2.2. ESR Data for Three Different Samples

 

 

Sample g factor AHDD (Gauss) Spins/mol Line Shape
Neutral poly(DBTT) 2.0033 8 3.9x1020 isotropic

{poly(DBTT)(13)o,33} 2.0030 26 3.5x 1021 anisotropic
{poly(DBTF)(I3)0_12 } 2.0036 8 2.1x1021 anisotgric

 

2.3.9. Magnetic Susceptibility Measurements

The magnetic properties of neutral and iodine-doped poly(DBTT)
were investigated as a function of temperature using an applied field of
2000 Gauss. The magnetic data are similar to those of other
polythiophenes. As observed in the ESR spectra, the neutral form shows

residual paramagnetism associated with spin defects on the backbone. The

84

magnetic susceptibility of poly(DBTT) follows Curie-Weiss Law behavior
as a function of temperature. The observed susceptibility (corrected for
diamagnetism) is higher than that implied by ESR spectroscopy, ~9.9x10-5
emu/mole vs. ~2.4x10-6 emu/mole respectively. The higher value is
attributed to the presence of residual dopant or, more likely, to
inaccuracies in the diamagnetic correction, which at these low susceptibility
levels is substantial.21

In agreement with ESR spectroscopy, the magnetic susceptibility of
{poly(DBTT)}(13)0,33 is only slightly larger that the undoped sample,
~3.71x10-4 emu/mole. The small rise may be attributed to an increase in the
spin defects upon doping and is consistent with the presumed spinless
nature of the charge carriers. However, a contribution from Pauli
paramagnetism is also expected for a metallic system (see charge transport
data below). When compared to the calculated magnetic susceptibility from
the spin quantitation of the ESR spectra, the bulk value obtained from
susceptometry is higher, probably for similar reasons given for the neutral

form.

2.3.10. Charge Transport Properties

Electrical Conductivity. The electrical conductivity of bulk
poly(DBTT) (doped with FeCl4- and iodine) has been measured by the
standard four-probe method on pressed pellets as a function of
temperature. Samples from the two soluble fractions doped with ferric
chloride were also studied. At room temperature, fraction II and bulk
poly(DBTT), doped with FeCl4-, have roughly the same conductivity of ~1-
5 S/cm, while fraction I is two orders of magnitude less conductive at

~0.02-0.04 S/cm. The higher conductivity of fraction II is comparable to

85

that of P3HT (~5-15 S/cm) chemically obtained in powder form,24 but
significantly lower than that reported for the ~100% heat-to-tail P3AT.14
Upon cooling these samples, the conductivity decreases only slightly for
fraction II (~0.02 S/cm at 10 K) while it reaches insulator values for
fraction 1, see Figure 2.13. The conductivity measurements of poly(DBTT)
were carried out as a function of iodine (13') doping. Shown in Figure
2.14, the highly doped polymer (33% mol I3',) exhibits higher
conductivity, reaching 2 S/cm at 300 K, while the less doped polymer (12%
mol 13') has a conductivity of 0.05 S/cm at 300 K. The relatively small
temperature dependence of fraction II is very similar to that observed for
doped polyacetylene and typical of granular metals with weak interparticle
contact resistance.35 It is possible that the conductivity of poly(DBTT) is
not higher than that of P3HT, despite its longer conjugation length, because
any gains in mobility from improved conjugation are offset by
corresponding losses in mobility derived from the increased frequency of
carrier hopping caused by the significantly shorter chain lengths in
poly(DBTT). Higher conductivities could be expected if the polymer
synthesis could be improved to yield higher MW material.

In order to gain further insight into the conduction mechanism of
doped fraction I and II, we attempted to fit the experimental variable

temperature data via computer analysis to the analytical expression

C=CoCXP[’(To/T)°‘1
where so and T are constants and oc=1, 1/2, 1/3, or 1/4 based on several
conduction mechanisms suggested for such systems.36s37 Figure 2.15 shows
that single exponential fits of the electrical conductivity data, with 0t=1/2,

are satisfactory for both fractions. However, for 0t=1/4, a satisfactory fit is

 

 

 

 

'- " ' " III In (VIII {Zuni-I: I 10»)
0 _ . -“_“,b§Q.I.I”)‘O..:OZI§_I"III..11".:‘2’:‘:":':'.'.“"m ”~ I' I. 3." .. I
L gum: , ..
. (B)
-2 s- ..............
g _ I '13-"! $970033"71'1”", 111111111111
\ I. H" 8'" I A
U) -4 _ ‘LLLL. ( )
V - “‘
b -— AAA‘
0) P
3 '6' _— A
A
'- A
' A
-8 __ A ‘
-10 g r I 1 l J l J I i l l .
0 50 100 150 200 250 300

Temperature (K)

Figure 2.13. Four-probe variable-temperature electrical conductivity of

poly(DB'I'I‘) doped with ferric chloride: (A) fraction I; (B) fraction II.

87

 

 

 

 

-4 1 l I l l l [J 1 l l l J J J L -l J 14 J J

70 110 150 190 230 270 310
Temperature (K)

 

Figure 2.14. Four-probe variable-temperature electrical conductivity of
bulk poly(DBTT) doped with iodine: (A) poly(DBTT)(I3')o,33; (B)
Poly(DBTT)(I3')0.12.

88

 

 

 

 

 

-3 ._
-4 _
-5 1 1 1 J 1 i 1 l 1
50 70 90 110 130 150

10000"2 (Km)

Figure 2.15. Four probe variable-temperature electrical conductivity of
poly(DBTT), doped with ferric chloride, in a logo vs. T“2 format: (A)

fraction 1; (B) fraction II.

89

 

 

 

 

 

-3 _.
-4 __
_ J L l l l J 1 J l l l l J l
230 260 290 320 350 380

Figure 2.16. Four-probe variable-temperature electrical conductivity of
poly(DBTT), doped .with ferric chloride, in a logo vs. T'“4 format: (A)

fraction 1; (B) fraction II.

90

obtained for fraction II but not for fraction 1, see Figure 2.16. In fact, for
fraction II, the fit is slightly better when (1:1/4. The fact that Ot¢1 rules out
band conduction and activation type diffusion, in agreement with the
behavior of other polythiophenes.38 It must be noted that several
conduction models can be described with 0L=1/2, such as carrier tunneling
between small metallic particles in an insulating matrix,36,37 one-
dimensional variable range hopping (lD-VRH) between localized states37
and the Coulomb gap model for certain disordered systems.39 Three
dimensional variable range hopping (3D-VRH) is described by oc=l/4. We
therefore see that at least fraction I diverges from the 3D-VRH model.
However, using conductivity data alone one cannot distinguish between the
various charge transport models in granular materials. A complementary
probe to address this issue is variable temperature thermoelectric power
(TP) measurements.

Thermoelectric Power Studies. TP measurements are typically
far less susceptible to artifacts arising from the resistive domain boundaries
in the material because they are essentially zero-current measurements and
temperature drops across such boundaries are much less significant than
voltage drops.

The 1D-VRH hopping model and the Coulomb gap disordered model
predict a temperature independent TP, while the carrier tunneling between
small metallic particles model predicts a small TP with a linear
temperature dependence, where TP approaches zero at 0 K.

Thermoelectric power measurements (Seebeck coefficient) for the

two doped (with FeCl4- ) fractions give small positive values in the

91

temperature range of 70-300 K, as shown in Figure 2.17. The

 

 

 

 

 

thermopower
15

2 I
:1 10 _
03 .
3 ..
O -
Q.

0 .
E 5 _
Q)

5 I-
I- _

0 I. J l l l l l I l l l l J l 1' i l l l l I l L l
50 100 150 200 250 300

Temperature (K)

Figure 2.17. Variable-temperature thermoelectric power data for

poly(DBTT) doped with ferric chloride: (A) fraction 1; (B) fraction II.

92

 

80

Thermopower (uV/K)

 

 

 

 

1O 'JllllilllJlJllllllllll

80 120 160 200 240 280 320
Temperature (K)

Figure 2.18. Variable-temperature thermoelectric power data for bulk
poly(DBTT) doped with iodine: (A) poly(DBTT)(I3‘)o.33; (B)
P01Y(DBTT)(I3')0. 1240.

93

data of the corresponding iodine doped samples are shown in Figure 2.18.
It is clear that both samples exhibit a positive Seebeck coefficient at 300 K
which decreases linearly with falling temperature. This is characteristic of
a metal-like systems in which hole conductivity (p-type) is dominant, as
expected from partially oxidized systems. None of the samples show
temperature independent TP and this excludes the possibility of lD-VRH
and Coulomb gap disordered model. Therefore, it appears that only the
carrier tunneling between small metallic particles model is consistent with

the TP data. Similar behavior has been observed in other doped P3AT.38
2.4. Conclusion

We have demonstrated that a judicious choice of monomer can lead
to new highly conductive polythiophene derivatives with long effective
conjugation lengths, small number of spin carrying defects and regular
linear backbone. Despite the relatively short chain length, poly(DBTT) is
one of the most thermally stable poly(alkylthiophenes). In solution and in
the solid state it appears to possess one of the longest chain conjugation
lengths among polythiophenes, a result of fewer steric repulsions exerted

on its backbone.

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94
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large resistance of the sample at low temperature.

CHAPTER 3

Synthesis and Characterization of A New Hybrid
Conjugated Polymer:
Poly[3,4-dibutyl-2,5-bis(2-thienyl)-pyrrole]

100

101
ABSTRACT

A new electrically conducting copolymer of thiophene-pyrrole with
a controlled heteroatom composition (2S : NH) and a known sequence
distribution has been synthesized by chemical oxidative polymerization of
3,4-dibutyl-2,5-bis(2-thienyl)-pyrrole (DBTP), a monomer designed to
yield an ordered, soluble, conjugated polymer. The new polymer,
poly(DBTP), was characterized by X-ray-diffraction, IR, NMR,
UV/Vis/NIR, photoluminescence and ESR spectroscopies and charge
transport measurements. The molecular weights of two soluble fractions of
poly(DBTP) were determined by gel permeation chromatography. The
high molecular weight fraction (Mw~1.1 x 104) has an absorption
maximum of 479 nm in tetrahydrofuran, while the low molecular weight
fraction (Mw~2.2 x 103) has an absorption maximum of 465 nm in
chloroform. Thermal gravimetric analysis studies show that the polymer is
stable in nitrogen up to 361 0C. The doped polymer (with FeCl4’) shows p-
type metallic behavior with a room temperature electrical conductivity of ~
0.1 S/cm. These results are compared to previously characterized

poly(heterocycles).

102

3.1. Introduction

In the field of conjugated conducting polymers, polythiophenes,
polypyrrole and polyaniline are among the best studied systems because of
their high electrical conductivity, thermal and chemical stability,
amenability to chemical modification, reversible redox properties, and
other interesting physical properties},2 Tremendous effort3 has been
focused on the modification of their chemical structures, in order to alter
their electronic structures, and improve their electrical properties,
environmental stability, and processability. Recently, there is increasing
interest in the design and synthesis of novel hybrid polymers as second
generation materials with novel electrical and optical properties.4 Because
of the structural similarity of thiophene and pyrrole, hybrid polymers
containing varying proportions of thiophene and pyrrole are of particular
interest.5'11 From a synthetic point of view, several methods have been
engaged, such as, (a) the direct oxidation of mixtures of thiophene5, or
bithiophene6 or terthiophene7 with pyrrole, (b) the electrochemical
polymerization of the biheterocyclic monomer 2-(2-thienyl)-pyrrole (1)
and some of its N-alkyl derivatives,8 (0) the electrochemical and chemical
polymerization of triheterocyclic monomer 2,5-bis(2-thienyl)-pyrrole (2)
and some of its N-alkyl derivatives?)10 and (d) the electrochemical

polymerization of some higher thienylpyrrole oligomers.11

S/\ S/\3
\/N \/N\/
n FIT
l 2

(R = H, CH3) (R: H, CH3, C2H5, C7H15, C18H37)

103

However, highly irregular structures have been predicted for the polymers
derived from 1 and the direct oxidation of thiophene/pyrrole mixtures7,12.
Meanwhile, electrochemical oxidative polymerization of some higher
thienylpyrrole oligomers (from 4 to 7 heterocyclic units) has not resulted
in the formation of polymer.11 Among the various monomer substrates,
the triheterocyclic monomer system seems to be the best candidate to form
polymers of greater linearity and symmetry due to the symmetrical nature
of the monomer, although a recent electrochemical study of a variety of
substitution products of 2 has provided strong kinetic evidence for
considerable branching.13 The same study also concludes that a
triheterocyclic monomer with dimethyl groups substituted on the [3-
position of the pyrrole ring should lead to predominantly linear, (oc—(x'),
coupling. The polymerization of monoalkyl-substituted triheterocycle has
also been briefly described in the literature without detailed
characterization.9c It is worth noting that N-substituted pyrrole-thiophene
copolymers usually afford materials whose electrical conductivities are
many orders of magnitude inferior to the parent system due to steric
interference with the achievement of coplanarity by the adjacent rings in
the polymer.9

Recently, we reported the synthesis and characterization of a new
polythiophene derivative poly(3',4'-dibutyl-2,2':5',2"-terthiophene)
[poly(DBTT)],14 which is based on the monomer of 3',4'-dibutyl-
2,2':5',2"-terthiophene (3), as an approach to reduce the mislinkage and
steric hindrance and obtain an ordered, soluble, conjugated polymer. In the
poly(DBTT) backbone, every dibutyl-substituted (at both [3 positions)
thiophene unit is separated by two non-substituted thiophene units (acting as

steric diluents). The reduced number of electrophilic coupling sites and

104

side chains leads to predominantly linear (a—oc') coupling and minimizes
extensive steric effects, respectively, and thus results in a more conjugated
polymer. In solution, and in the solid state, the poly(DBTT) appears to
possess one of the longest chain conjugation lengths among polythiophenes.
We also mention here that a significant progress has been reported in the
regiospecific synthesis of ~100% head-to-tail polyalkylthiophenes which
exhibit significantly higher conductivities.15 In addition to monomer 3, we
have successfully synthesize a pyrrole analog, i.e., 3,4-dibutyl-2,5-bis(2-
thienyl)-pyrrole (4).

S/\8 8/\S

\/s\/ \/N\/

H
3 4
3',4'-dibutyl-2,2':5',2"-terthiophene 3,4-dibutyl-2,5-bis(2-thieny1)-pyrrole
(DB'IT) (DBTP)

The use of this new triheterocycle monomer (4) will allow us to
produce a symmetrical copolymers with a controlled heteroatom
composition (25 : NH) and a well-defined sequence of thiophene and
pyrrole rings.13 We also anticipate that the symmetrical substitution of two
butyl groups on the center pyrrole ring will enhance the processability and
the molecular ordering of the resulting copolymer.

In this Chapter, we report the synthesis and characterization of
monomer 3,4-dibutyl-2,5-bis(2-thienyl)-pyrrole and the corresponding

polymer via chemical oxidative polymerization. The properties of this new

105

hybrid poly[3,4-dibutyl-2,5-bis(2-thienyl)-pyrrole] poly(DBTP) are
compared with its sulfur analog, i.e. poly(DBTT).

3.2. Experimental Section

3.2.1. Materials

FeCl3 (anhydrous), CHCl3, CH3CN, MeOH, 12 were purchased from
commercial sources and used as received. Tetrahydrofuran (THF) (HPLC
grade) was used as received from Aldrich Chemical Co., Inc. without
further purification. 3,4-Dibutyl-2,5-bis(2-thienyl)pyrrole was prepared by

Seaver Shieh in Professor LeGoff‘s group and used as received.

3.2.2. Synthesis of 3,4-Dibutyl-2,5-bis(2-thienyl)pyrrole

A mixture of 2,3-dibutyl-1,4-bis(2-thienyl)-1,4-butanedione (2.1 g,
5.8 mmol), anhydrous NH4OAc (1.60 g, 20.8 mmol), acetic anhydride (2.0
mL) in 50 mL acetic acid was kept at gentle reflux for 48 h. Then, the
dark-red reaction mixture was cooled and the acetic acid/acetic anhydride
was removed by vacuum distillation. The residue was extracted with ether
(4x30 mL) and the combined ether was washed with 5% aqueous sodium
bicarbonate solution (3x20 mL), water (2x20 mL), saturated NaCl (2x15
mL) and dried with sodium sulfate. The solvent was removed in a rotary
evaporator. Column chromatography purification over silica gel with
hexane as the eluent gave the 3,4-dibutyl-2,5-bis(2-thienyl)pyrrole as a pale
yellow solid. Yield: 1.60 g (80%). m.p.= 47-48 0C (uncorrected). 1H—
NMR(CDC13): 7.97(s, br., 1H), 7.21(d, J=5.4Hz, 2H), 7.08-7.02(m, 4H),
2.63(t, J=8.6Hz, 4H), 1.56(m, 4H), 1.45(m, 4H), 0.96(t, J=7.8Hz, 6H);
13C-NMR(CDC13): 135.5, 127.4, 123.3, 123.1, 122.8, 122.2, 33.5, 24.7,

106

23.0, 13.9; EI-MS, e/z(relative intensity): 345(M++2, 10.4), 344(M++1,
20.8), 343(M+, base), 301(11.6), 300(50.3), 260(7.6), 259(15.5),
258(76.4), 256(13.2), 244(6.13), 17(5.05), 147(6.33), 129(13.0),
110(6.02), 43(5.05), 41(9.l2); UV-Vis(maximum absorption): 345 nm
(CHC13).

3.2.3. Preparation of Neutral Poly[3,4-dibutyl-2,5-bis(2-

thienyl)-pyrrole]

To a stirred solution of 0.343 g (1.0 mmol) DBTP dissolved in 20
mL of chloroform was added dropwise a mixture of 0.65 g (4.0 mmol)
anhydrous ferric chloride in 75 mL of chloroform. The resultant mixture
first turned to red then formed a blue-black precipitate. The reaction
mixture was stirred at room temperature for ~48 h, then poured into 400
mL of methanol, stirred for 0.5 h and filtered. The black solid was
exhaustively extracted with hot methanol in a Soxhlet extractor for 24 h
until the extract was colorless. The material remained black. It was further
reduced with 20 mL of hydrazine monohydrate under nitrogen for 24 h.
The resulting red material was filtered and Soxhlet extracted with hot
methanol, then dried under a vacuo at 50 0C for 24 h. There was obtained
0.267 g (78% yield) of green (in reflectance) powder of poly[3,4-dibutyl-
2,5-bis(2-thienyl)-pyrrole] poly(DBTP). Elemental analyses by EDS
showed that Fe and Cl impurity was less 0.5%. Anal. Calcd(%) for
C20H23N82 (repeat unit): C, 70.33; H, 6.79; N, 4.10. Found: C, 70.35; H,
7.42; N, 4.02.

107

3.2.4. Isolation of the Chloroform-Soluble, Fraction I

A sample of 0.222 g of poly(DBTP) was extracted with 4 x 50 mL
portions of CHCl3 at 23 0C inside a glove-box under nitrogen. The orange
extracts were combined and reduced to a volume of 25 mL, which was then
added slowly to 250 mL of MeOH. The red precipitate was collected by
filtration and washed with neat MeOH, then dried under vacuum for 24 h.
Green solid (in reflectance) was obtained in 27% yield (0.060 g), as
fraction 1. 1H-NMR (CDC13): 6 (ppm) 7.95 (s) , 7.23-7.21 (m), 7.14-6.90
(m), 2.63(m), 1.64-1.36 (m), 0.97 (m). Peak integration of these
resonances give a ratio of 1/0.4/4/4/8/6.

3.2.5. Isolation of the THF-Soluble, Fraction II

The CHCl3-insoluble solid residue obtained from the above
procedure (ca. 0.158 g) was found to be further partially soluble in THF at
room temperature. It was extracted with 4 x 20 mL portions of THF at 23
0C inside a glove-box filled with nitrogen. The insoluble material (fraction
III) was removed by filtration. The orange-red extracts were combined and
the solvent was removed under vacuum to give 0.022 g of dark red solid

(10% yield), as fraction 11.

3.2.6. Preparation of Doped Poly(DBTP)

(a) Doping with Ferric Chloride in Acetonitrile

To a stirred solution of 0.1M ferric chloride in 50 mL of acetonitrile
at room temperature, was added 0.03 g bulk poly(DBTP). A black product
formed immediately. After stirring for 12 h, the black solid was collected

from the reaction mixture, washed several times with acetonitrile and

108

vacuum dried. The yield was quantitative. Elemental analyses by EDS
showed that S/Fe/Cl= 51.5/9.8/38.7, which corresponded to a doping level
of 0.38 per repeat unit. The doped samples were pressed into pellets for

electrical conductivity measurements.

(b) Doping with Iodine Vapor
Solution (THF or CHCl3) cast films of the poly(DBTP) on a quartz
slide, were put into a closed chamber filled with iodine crystals and stored

for 4 h. The original orange-red films turned to dark-brown.

3.2.7. Physicochemical Measurements

Elemental analyses (S, Fe, Cl) were performed on a JEOL ISM-35C
scanning electron microscope (SEM) equipped with a Tracor Northern
energy dispersive spectroscopy (EDS) detector. Infrared spectra were
recorded on pressed KBr pellets on a N icolet 740 FT IR spectrometer. UV-
visible-NIR absorption spectra were obtained from a Shimadzu UV-
3101PC double beam, double-monochromator spectrophotometer.

X-ray powder diffraction patterns were collected at room
temperature on a Rigaku powder diffractometer using Cu(K0t) radiation
generated by a rotating anode operating at 45 kV and 100 mA. The data
were collected at a rate of l deg/min.

Proton nuclear magnetic resonance spectra were obtained using a
computer controlled Varian Gemini (300 MHz) spectrometer. The
chemical shifts are reported in parts per million (5, ppm) using the residual
solvent resonance peak as reference (CHCl3, 5 7.24 ppm for 1H)

Photoluminescence spectra were measured at room temperature on a

Perkin Elmer LS-5 fluorescence spectrophotometer. Thermogravimetric

109

analysis (TGA) and Differential scanning calorimetry (DSC) were
performed on Shimadzu TGA-50 and DSC-50 under nitrogen or oxygen at
5 OC/min heating rate. Electron spin resonance (ESR) measurements were
conducted on a Varian ESR-E4 spectrometer.

Molecular weight measurements of the soluble fractions of
poly(DBTT) were carried out using gel-permeation chromatography
(GPC) using a Shimadzu LC-lOAS high pressure liquid chromatograph
(HPLC) equipped with a PL-GEL 5p (MIXED-B) column of length 300
mm. Chromatographic grade tetrahydrofuran (THF) was used as an eluent.
Calibration was made with a series of polystyrene standards (Mw in the

range of 3250 to 500800).

3.3. Results and Discussion

3.3.1. Oxidative Polymerization

Based on the fact that FeCl3 serves as useful oxidant in the synthesis
of poly(3-alkylthiophene)16 and poly(DBTT)”, we utilized this reagent in
chemical polymerization of 3,4-dibutyl-2,5-bis(2-thienyl)pyrrole (DBTP).
The product was then reduced with an aqueous solution of hydrazine.
Neutral poly[3,4-dibutyl-2,5-bis(2-thienyl)pyrrole] [poly(DBTP)] was
obtained as green powder in 78% yield. The elemental analysis data is in
good agreement with the expected structure.

Bulk neutral poly(DBTP) is partially soluble in chloroform at room
temperature, giving a orange solution from which fraction I can be
isolated. The room temperature chloroform-insoluble residue of
poly(DBTP) is further partially soluble in THF, giving a orange-red

solution from which fraction II can be isolated. This difference in

110

solubilities indicates the presence of a wide molecular weight distribution
as will be discussed later. It is worth noting that solutions of poly(DBTP)
are not stable in air as judged by the observation of color change of the
solutions, and changes in the electronic absorption spectra (see below).
Thus, spectroscopic characterization in solution was carried out under
nitrogen. Free-standing films can be cast from solution under nitrogen by
slow solvent evaporation. The films can be doped with iodine vapor in a
closed chamber.

The expectation that the DBTP monomer would yield an ordered
product was confirmed by X-ray scattering which shows that neutral
poly(DBTP) is polycrystalline, see Figure 3.1. The most notable feature is
the intense low angle reflection at 18.72 A and the very broad high angle
reflection at 4.51 A. The first reflection arises from the packing of
poly(DBTP) chains which is dominated by the length and density of the
butyl side-chains. The broad peaks at 4.51 A are attributed to interlayer
spacing arising from a lamellar packing of the polymer backbone. These
X-ray diffraction patterns are similar to that of poly(DBTT).14

The chemical structure and properties of the polymer were further
examined with vibrational, optical, NMR and ESR spectroscopies.
Photoluminescence spectroscopy was also used to explore the excited state

properties of the neutral poly(DBTP).

3.3.2. Chromatographic Molecular Weight Studies

Molecular weights of the soluble fractions of poly(DBTP) were
determined by gel permeation chromatography (GPC) in THF. Figure 3.2
shows typical chromatograms representing the molecular weight

distribution for fraction I and fraction II. From the Figure 3.2 it is

111

 

 

 

 

 

 

 

5. 10.

Figure 3.1.

poly(DBTP).

15. 20. 25. 30. 35. 40. 45. 50.

2 Theta

>5
d—b
0—
m
I:
a)
H
c:
I-It
. I‘ll!IIIIIYIII‘IIIII[IIIIIIIIIIUIUI‘IITIY‘III]IIVIIIIIIIIIIIIIIIIII[I'IIIlilIlYlIFV'IIIIIIIII[III'VII‘IIIIIIIIIWIII
55.

60.

X-ray powder diffraction profile of neutral bulk

 

 

Intensity (arb. units)

 

Irrlrrrlrrrlrrrlrrr
IIIlIII'IrIIIrIIIIT

 

 

 

 

 

 

 

I I I I I I T I T. T I T I T I I I I i I I I I I T I
O 10 20 3O 4O 50 50
Retention Tlme (min)
t 1 I 1 1 I r ' ° 1 I - 1 I I 24 1 r I 1 L' L1 '
a? -; (B) _—
.E _ _
S " r—
45 i T
3—1
CU _.
v
>1
vb: "‘ I.—
'm - I-
C.‘ ..
8 - .
I:
H "' I—
7 I I o I I I I I—T I I I O . I I I 1 ' o I I I

 

- I . . I
O l O 20 30 4O 50 60
Retention Time (min)

Figure 3.2. Typical GPC trace of the two soluble-fractions of

poly(DBTP) at room temperature.

113

apparent that complete separation of the two soluble-fractions has not been
achieved. The molecular weights were obtained from the common method
of a retention time calibration curve using a series of polystyrene standards
with a UV-Vis detector set at 450 nm. The weight-average molecular
weight (Mw) of fraction I is ~2.2 x 103 and number-average molecular
weight (Mn) is ~2.0 x 103 giving a polydispersity index (PD) of 1.10. The
Mn corresponds to an average of 18 rings/chain. This is in good agreement
with the value obtained for this fraction by end-group analysis using NMR
spectroscopy (see below). For fraction 11, MW is ~1.1 x 104 and Mn is ~6.1
x 103 with PD of 1.80. The Mn corresponds to an average of 54
rings/chain. The measured molecular weights of poly(DBTP) are
comparable to those of poly(DBTT). We note that the GPC studies on the
two soluble fractions of poly(DBTP) place a lower limit on the molecular
weight of the bulk polymer. The presence of insoluble fraction III is

expected to have a MW much higher than 104.

3.3.3. Infrared Spectroscopy

Figure 3.3 shows the infrared spectra of the monomer DBTP and the
neutral bulk poly(DBTP). The principal IR absorption bands observed in
poly(DBTP) and their assignments, together with the corresponding results
for poly(DBTT) are listed in Table 3.1.

The C-H out-of-plane bending modes for the DBTP monomer appear
at 698 cm“1 (OI—hydrogens) and 843-833 cm‘1 (fi-hydrogens) as shown in
Figure 3.3A. Upon polymerization, a new dominant absorption band
appears at 782 cm"1, which is due to the B-hydrogens out-of—plane
vibration of all internal thiophene rings and is characteristic of 0L,0I-

coupled polythiophene rings, while the band at 698 cm'1 virtually

114

 

 

 

J

(A)

 

l

7. THflNSH I TTflNCE

l

(B)

 

 

41000 aaao also 2370 2260 1350 1550 1130 $20 210
HRVENUHsER

Figure 3.3. FT-IR transmission spectra (KBr pellets) of (A) monomer
DBTP, (a) neutral bulk poly(DBTP).

115

disappears. In addition to that, the Ca-H stretching mode, observed in the
spectrum of monomer DBTP at 3098 cm'l, is absent in the spectrum of the
polymer. These results suggest a linear polymer chain structure with a
predominance of thiophene ring (Lev-couplings in the polymer backbone.17
The single broad peak at 3062 cm‘l, which is due to the CB-H stretching
mode, as well as the band in the vicinity of 3440 cm'1 (N-H stretching),
presents in the IR spectra of both monomer and polymer as would be

expected.

Table 3.1. Comparison of Infrared Band Positions (cm—1) and Their

Assignments for Poly(DBTP) and Some Related Polymers.

 

 

arom aliph. methyl arom C-H
sample (313-11 811 GB str ring str def out-of-plane
POIJ(DBTP) 3062 2951 2924 2855 1495 1456 1377 782
Poly(DBTT) 3062 2951 2925 2856 1492 1456 1377 788
VIII“ 3063 1491 1453 1441 1377 788
P3BTa 3055 2955 2928 2858 1512 1458 1439 1377 829
P3HTa 3055 2959 2930 2858 1512 1458 1439 1377 825

 

a) see ref. 17: PTH-polythiophene, P3BT-poly(3-butylthiophene), P3HT-
poly(3-hexylthiophene)

3.3.4. NMR Spectroscopy
Figure 3.4 shows the 1H NMR spectrum of fraction I of poly(DBTP)

in CDCl3 at room temperature. The strong and broad resonance at 7.95

ppm is attributed to the hydrogen atoms on the N-position of the pyrrole

116

 

 

 

Figure 3.4. 1H NMR spectrum of fraction 1 of poly(DBTP) in CDCl3 at

room temperature.

117

rings. The weak multiple peaks at ~7.22 ppm are attributed to the hydrogen
atoms on the (II-position of the terminal thiophene rings. The other three
multiple resonances at 7.14-6.90 ppm are due to the fi-hydrogen atoms on
thiophene rings. The four resonance peaks found in the aliphatic region
correspond to the hydrogen atoms of the side n-butyl groups on the
polymer backbone. These assignments are based on the corresponding
NMR spectrum of the DBTP monomer.18 The number of protons
corresponding to each resonance, based on peak integration, are in good
agreement with the proposed polymer structure. From the ratio of the
terminal a-hydrogen atoms to B-hydrogen atoms or N-hydrogen atoms,
one obtains a molecular weight of ~1700, very close to five DBTP units or
~15 rings. This number agrees reasonably well with that estimated from
the GPC studies described above. A satisfactory 1H NMR spectrum of
Fraction II was not available at this time due to low solubility and the

presence of some impurity.

3.3.5. Electronic Spectroscopy, UV-Visible-NIR

The optical absorption spectra of fraction I and fraction II in CHCl3
and THF are shown in Figures 3.5 and 3.6, respectively. The as-prepared
CHC13 solution of fraction I has a maximum absorption at 466 nm, while
the as-prepared THF-solution of fraction II has a maximum absorption at
479 nm. The longer wavelength of maximum absorption of the freshly-
prepared solution of fraction 11, compared to that of fraction 1, is in good
agreement with its higher molecular weight. The strong optical absorbance
decreases in intensity when the CHCl3-solution exposed in air for 0.5 h,
meanwhile a board peak appears in the region of 700-800 nm. The CHC13

solution changes color from orange to brown. Stored in air for 20 h, the

118

 

 

Absorbance (arb. units)

111111111111111111111111111

 

 

 

 

200 300 '400 500 600 700 800 9001000
Wavelength (nm)

Figure 3.5. Solution UV-vis absorption spectra of fraction I of

poly(DBTP) in CI-lCl3 at room temperature.

119

 

T -.

I

IIIITI

IIIIIIII

 

Absorbance-(arb. units)
111111111111111111111111

IIIIIIII

 

 

 

200 300 400 500 600 700 800 9001000
Wavelength (nm)

Figure 3.6. Solution UV-vis absorption spectra of fraction II of

poly(DBTP) in THF at room temperature.

120

THF-solution changes color from red-orange to yellow, and the maximum
absorption shifts toward high energy. In contrast to the sulfur analog
poly(DBTT), which is quite stable in air, the instability of the poly(DBTP)
solutions is attributed to possible oxidation of the pyrrole rings in the
polymer backbone by oxygen to give rise to oxidative doping or even
decomposition. It has been reported that undoped polypyrrole can be easily
oxidized by molecular oxygen. 19

The solid state electronic spectra of the solution-cast films of fraction I
and II are shown in Figures 3.7 and 3.8, respectively. The optical band-
gaps of fraction I and II are close to 2.0 eV, as estimated from absorption
edges of the spectra, which are comparable to those of poly(DBTT). The
most notable feature is the presence of a low energy absorption band at
0.72 for fraction I and 0.72 fraction II, in addition to a shoulder ~ 1.5 eV
in both electronic spectra of as-prepared films. These low energy
absorption bands indicate that the so-call "neutral" films of poly(DBTP)
have been somewhat oxidized (possibly by exposure to oxygen). When the
"neutral" films were doped with iodine vapor, two low energy absorption
bands appear, which can be attributed to the transitions associated with the
two localized bipolaron energy levels within the bandgap, as has been
observed in many polythiophenes.2C However, oxygen doping of neutral
polymer films was not observed in the case of poly(DBTT).14 Thus, it
appears that the substitution of the central thiophene ring in DBTT with a
pyrrole ring causes a significant change of its electronic properties, and

gives rise to a significantly more oxidation-sensitive polymer.

121

 

11111111

11111101111111

 

Absorbance (arb. units)

1111111
9
\1
LA

0
00
O
)._I
O\
\D
II TIIIIIIIIIIIIIIIjIIIIl IIIIII

 

Eg = 2.0 eV

Ill

 

l

 

IIIIITITII.1III’IIIIlItleIIITI I711

3 4 5 I
Energy (eV)

 

O
_.1 -
N
\1

Figure 3.7. UV-vis-NIR absorption spectra of the CHCl3-solution cast
thin films of fraction 1 of poly(DBTP) in (A) "neutral form" (B) iodine

doped form.

122

 

0.72

   

 

Absorbance (arb. units)

   

E z: 2.0 eV

.9,
U

IIIliIIITIIIIIIIIIIIIIIIIIIIIIIIIIIIIII

LIIIILIIIIIllllllJlILlllIllllIlll21111l

 

 

 

I'QIIIIIIIIITIIj‘ITOtlIIII|IIIT

3 4 5 6
Energy( eV)

0
—J
N
\3

Figure 3.8. UV-vis-NIR absorption spectra of the THF-solution cast thin
films of fraction II of poly(DBTP) in (A) "neutral form" (B) iodine doped

form.

123

3.3.6. Photoluminescence Spectroscopy

Figure 3.9 shows the photoluminescence spectra of the fraction I and
fraction II of poly(DBTP) in chloroform and THF at 23 0C, respectively,
using excitation wavelength of 400 nm. Photoexcitation of the dilute
solutions results in broad band luminescence with a peak maximum at 565
nm for fraction I and 576 nm for fraction II. By comparison, poly(DBTT)
shows emission peaks at 557 and 620 nm for fraction I and II in

chloroform, respectively.

3.3.7. Thermal Analysis

The thermal stability of the polymer was examined by
thermogravimetric analysis (TGA) and differential scanning calorimetry
(DSC). Neutral bulk poly(DBTP) exhibits excellent stability in nitrogen up
to at least 361 0C, as shown in Figure 3.10. In oxygen, the polymer starts
to decompose at ~274 OC and loses 94% of its weight at 600 0C. These
values are comparable to those of neutral poly(DB'I'I‘).14

The DSC curve of bulk poly(DBTP) shows a broad endothermic
peak at 252 oC, upon heating, and a broad exothermic peak at 167 0C upon
cooling during the first DSC scan, see Figure 3.11. Further heating cycles
between 30-300 oC reveal that the endothermic peak shifts toward lower
temperature at ~232 OC and the exothermic peak remains at 167 0C.
However, the polymer shows signs of no melting up to 250 0C as judged by
direct visual observation, suggesting, instead, the presence of an order-
disorder transition in the solid state.20 A similar behavior has been

observed in the case of poly(DBTT).14

124

 

 

 

 

1 1 J 1 L 1 ' - ' 1 ' ' ' '
: 576 nm 3
i 565 nm / ~—
A . .
£9. 1 . \ . T
g 3 Fraction I Fractron II :
.o' ’: :
g d -
(U _I _
V _ .
z: : 3
'33 T. ?
I: - .
.93 L :.
C - -
r—d .-
Z/ I :
r I I I I . 1 o l I T I ° I ‘ I ' I
500 550 600 650 700

Wavelength (nm)

Figure 3.9. Photoluminescence spectrum of the two soluble-fractions of

poly(DBTP) at room temperature. (A) fraction 1 in CI-ICl3 (B) fraction II
in THF.

 

 

 

 

 

 

 

 

110 a 1 LI 4 l [U I . 1.11.1. I I P

: 361 °C :

1005 r fl/ 3
A 90.: 5.
5° : :
5 80—: E-
:5.» 7‘o—: E-
d) : :
3 60-3 :—
50; (A) E’-

I 40 .1 T I l I . I i i I . I7 10' I n I I I I I To I I '1' I I I I I T I l I I I I p
0 100 200 300 400 00 600 700 800

Temperature (°C)

120 v:4_:i'-'v|'-v-lzoo-lrr'v'v'..|:--Llu.o-L

; 274 °C Z

1001 f / e
83 805 r
E :
2..“ 60-4 _
_c - ._
.29 " ‘
s 40- ~
20.: L

: (B) :

O‘TTaI].afa‘.1-u[.ltr[FTt..rrt...rrrftrrt

 

 

 

O 100 200 300 400 500 600 700 800
Temperature (°C)

Figure 3.10. TGA thermograms of neutral bulk poly(DBTP) (A) under

nitrogen flow and (B) under oxygen flow.

126

 

 

 

 

 

0.5
. I *
3 -05 T
E
U 4 .
m -1~ *-
CD
I 252 0C
-155 l
(A)
-zl
TTIIIIITI]ITTIITIIIIIIII|1Il1]TITl

0 50 100 150 200 250 300 350
Temperature (°C)

 

0.8
0.6

‘l
0'4 167 0C

 

DSC (mW)
O

 

  

 

 

 

~O.2
-o.4-

l L
-O.6: (B) r.
’0-8 1TI1UI'11TT'IIVTIthrIIITTIIIIIvI

0 50 100 150 200 250 300 350
Temperature (°C)

Figure 3.11. DSC thermograms of neutral bulk poly(DBTP) under

nitrogen flow (A) first heating cycle (B) second heating cycle.

127

3.3.8. Electron Spin Resonance Spectroscopy

Figure 3.12 shows a typical ESR spectrum of the neutral bulk
poly(DBTP), which reveals a single, largely symmetric, line centered at
nearly the free electron g value of 2.0025 with a linewidth Apr=4.5
Gauss. The number of spins determined, using a diphenylpicrylhydrazide
(DPPH) standard, is ~ 4.5 x 1021 spins/mol rings, corresponding to
approximately 1 spin per 133 rings. The number of spins is about one
order of magnitude larger than that of poly(DBTT) (3.9 x 1020 spins/mol),
and suggests the presence of more spin-carrying radicals in the former.
This is possibly due to the fact that some amount of poly(DBTP) has been
oxidized by oxygen.

3.3.9. Charge Transport Properties

The electrical conductivity of bulk poly(DBTP) (doped with 0.38
FeCl4' per repeat unit) was measured by the standard four-probe method
on pressed pellets as a function of temperature. (Figure 3.13) At room
temperature, the doped poly(DBTP) has a conductivity of ~0.1 S/cm, which
is comparable of that of poly(DBTT) doped with FeCl4'. 14

Thermoelectric power measurements (Seebeck coefficient) for the
doped poly(DBTP) (with FeCl4' ) give small positive values in the
temperature range of 50-300 K, as shown in Figure 3.14. It is clear that the
samples exhibit a positive Seebeck coefficient at 300 K which decreases
linearly with falling temperature. This is characteristic of metallic systems
in which hole conductivity (p-type) is dominant. This behavior is expected

from partially oxidized conjugated polymers.

F1;

[Cl

 

 

 

 

 

 

 

g=2.0025
-' Apr=4.5G
ZR
":75
53— LA u a
,8 v v
.5
l l l T l 7
3300 3320 3340 3360 3380 3400 3420 3440
FIELD(G)

Figure 3.12. ESR spectra of neutral bulk pOly(DBTP) at room

temperature.

129

 

10'1
10‘2
10'3

10‘4 °

1 0‘5

1 0'6 g

1 0'7

o (S/cm)

O

1 0‘8 °

0

 

10-9 1 1£L J l l l J. i J l l 1 1 14 1 1 l l l J l 1 [J l l L

50 100 150 200 250 300
Temperature (K)

Figure 3.13. Four-probe variable-temperature electrical conductivity of

bulk poly(DBTP) dOped with ferric chloride.

130

 

 

 

 

 

20
18 1
g I
> 16 2'
.3 _
<32 14 r
3 .
8 '”
E 12 _f
2 C
1— 1o -
8 _-
6 -J I l 1_1 l. i l l l J l 1 l l l l L 1 l l l l I
so 100 150 200 250 300

Temperature (K)

Figure 3.14. Variable-temperature thermoelectric power data for bulk

poly(DBTP) doped with ferric chloride.

131

3.4. Conclusion

In summary, we have prepared electrically conducting copolymers
of thiophene-pyrrole with a controlled heteroatom composition (ZS : NH)
and a formally known sequence distribution. In contrast to its sulfur
analog, poly(DBTT), neutral poly(DBTP) is not stable in solution in the
presence of oxygen, which is attributed to the presence of easily oxidized
pyrrole rings on the copolymer backbone. Nevertheless, poly(DBTP) bears
strong similarity in its physicochemical properties with poly(DBTT).

(1)

(2)

(3)

132
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(a) Ruiz, J. P.; Dharia, J. R.; Reynolds, J. R.; Buckley, L. J.
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Gningue, D.; Horowitz, G.; Garnier, F. J. Electrochem. Soc. 1988,
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Inganas, 0.; Liedberg, B.; Chang-Ru, W.; Wynberg, H. Synth. Met.
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Naitoh, S. Synth. Met. 1987, 18, 237-240.

(a) Ferraris, J. P.; Skiles, G. D. Polymer 1987, 28, 179-182. (b)
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Niziurski-Mann, R. E.; Scordilis-Kelley, C.; Liu, T-E.; Cava, M. P.;
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Souto Maior, R.M.; Hinkelmann, K.; Eckert, H.; Wudl, F.
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135

Shieh, S. Ph.D. Dissertation. Michigan State University, 1995.

(a) Lei, J.; Martin, C. R. Chem. Mater. 1995, 7. 578-584 and
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Carlsen, P. H. Synth. Met. 1992, i8, 363-380.

CHAPTER 4

Synthesis and Characterization of Conjugated and
Non-Conjugated Copolymers Containing 3',4'-
Dibutyl-a-Terthiophene Moiety via Wittig
Reactions

136

137
ABSTRACT

A series of soluble copolymers containing the rigid conjugated block
3',4'-dibutyl-2,2':S',2"-terthiophene spaced by aromatic and aliphatic
segments has been designed and prepared through the Wittig reaction. Four
copolymers poly(3',4'-dibutyl-0t-terthiophene-vinylene-1,4-phenylene-
vinylene) (PBTPV-p), poly(3',4'-dibutyl-a-terthiophene-vinylene-1,3-
phenylene-vinylene) (PBTPV-m), poly(3',4'-dibutyl-a-terthiophene-
vinylene-methylene-vinylene) (PBTMV), and poly(3',4'-dibutyl-a-
terthiophene-vinylene-ethylene-vinylene) (PBTEV) were characterized by
X-ray diffraction, gel permeation chromatography, FTIR, NMR,
UV/vis/NIR, photoluminescence and ESR spectroscopies. The solid state
bandgaps of the copolymers are between 2.05 and 2.47 eV. Films of
PBTPV-p are electroactive and turn reversibly and rapidly from red to
green-blue upon doping and undoping electrochemically. Among the
iodine-doped copolymers, PBTPV-p has the highest conductivity achieving
an electrical conductivity of ~3.2x10'2 S/cm at room temperature. The
properties of these copolymers are compared to previously characterized
polymers such as poly(3',4'-dibutyl-2,2':5',2"-terthiophene) (PDBTT).and
poly(p-phenylenevinylene) (PPV).

4.1.

138

4.1. Introduction

Conjugated polymers such as poly(arylenevinylene)s, polythiophenes
and poly(p-phenylene)s1 exhibit interesting electrical, nonlinear optical and
electrooptical properties which make them good candidates for applications
in rechargeable batteries,2 supercapacitors,3 field-effect transistors,4
sensors,5 electrochromic display devices6 and light-emitting diodes
(LEDs).7 Since the discovery of polymer-based electroluminescent (EL)
devices,8 tremendous effort has been focused on the design and synthesis of
new soluble electro-active polymers to gain control of color and efficiency
of light emission. Among a variety of polymer systems, p-
phenylenevinylene-based polymers are the most studied, possibly due to the
fact that poly(p-phenylenevinylene) (PPV) was the first to demonstrate
electroluminescence.8 Different synthetic approaches have since emerged
from several research groups, such as controlling the extent of conjugation
length,9 incorporating isolated chromophors and noncoplanar structures
into the polymer structure,10 producing saturated chain polymers with
pendant conjugated side-groups,11 and synthesizing main chain polymers
with intermittent sequences of conjugated and nonconjugated segments. 12

From a design and synthesis point of view, the Wittig reaction13 is
especially applicable to copolymer systems such as PPV-based
copolymers.14 Recently, two papers have demonstrated the use of
appropriate Wittig reactions to synthesize PPV derivatives in order to vary
the color of emitted light.15

Poly(3',4'-dibutyl-2,2':5',2"-terthiophene) (PDBTT) is soluble in
common organic solvents and shows interesting electrical and

photoluminescent (PL) properties.16 This polymer is based on the

139
monomer 3',4'-dibutyl-2,2':5',2"-terthiophene (DBTT) 1 shown below.

Now we have successfully synthesized a dialdehyde derivative of 1, namely
2,5"-diformyl-3',4'-dibutyl-2,2':5',2"-terthiophene 2 (DFDBT),

s /\ s
\/ s \/ s
1 2

S S

which affords us a great opportunity to exploit the Wittig reaction to
synthesize interesting new copolymers by employing the appropriate bis-
ylides. The resulting tailor-made copolymers should lead to materials with
"tunable" properties while maintaining the enhanced solubility imparted by
the DBTT building block. We have already reported preliminary results
from this work in the case of a copolymer consisting of 3',4'-dibutyl-
2,2':5',2"-terthiophene and 1,4-phenylene vinylene units.17 Here, we
describe the details of the synthesis and characterization of four
copolymers containing DBTT building block in conjugated and non-

conjugated polymer backbone.
4.2. Experimental Section

4.2.1. Materials

0t,OL'-Dimromo-p-xylene (97% purity), 01,01'-dimromo-m-xylene
(97% purity), 1,3-dibromopropane (99% purity), 1,4-dibromobutane (99%
purity), and n-butyllithium (2.5 M solution in hexane) were used as
received from Aldrich Chemical Co., Inc. Triphenylphosphine was used as
received from J. T. Baker Inc. Tetra-n-butylammonium perchlorate was

purchased from GFS Chemicals Inc. and used without further purification.

140
Acetonitrile (HPLC grade) and l-methyl-Z-pyrrolidinone (NMP) (HPLC

grade) were used as received from Aldrich Chemical Co., Inc.
Tetrahydrofuran (THF) (HPLC grade, Aldrich) was freshly distilled from
lithium aluminum hydride under nitrogen. Dimethylforrnamide (DMF) was
stored over 4-A Linde molecular sieves for several days and then distilled
under reduced pressure at ~34 0C. Other solvents were used as received
from commercial sources. All reactions were performed under an
atmosphere of nitrogen or argon using either standard Schlenk or glove

box techniques.

4.2.2. Synthesis of 2,5"-Diformyl-3',4'-dibutyl-2,2':5',2"-

terthiophene, (DFDBT)

The synthesis of DFDBT was carried out by Seaver Shieh in
Professor LeGoff's laboratory as described in the following:

BuLi (2.8 mL of 1.6M in hexane solution, 2.2 eq) was added to a
cooled (-50 0C) mixture of 3',4'-dibutyl-2,2',5',2"-terthiophenel6 (720
mg, 2.0 mmol), tetramethylethenediamine (TMEDA, 400 mg) and 40 mL
hexane by a syringe under N2. The reaction mixture was then stirred at -50
0C for 30 min and then again at room temperature for another 30 min.
This was followed by cooling to -78 0C and the slow addition of 2.2 eq. of
DMF in 5 mL of ether with a syringe. Then, the reaction mixture was
slowly warmed to room temperature, stirred for one hour and then poured
into 200 mL of 2% ice cold aqueous HCl solution with vigorous stirring.
The mixture was extracted with CH2C12 (4x30 mL) and the combined
organic layers were washed with water, saturated sodium bicarbonate and
brine. Dried with MgSO4. Removal of the solvent, with a rotary

evaporator, gave a dark-colored residue as the crude product.

141

Chromatographic column purification over silica gel with
hexane/ether/CH2C12 (65/20/15) as the eluent gave the expected 2,5"-
diformy1-3',4'-dibutyl-2,2':5',2"-terthiophene (DFDBT), as orange
needles. Yield, 0.674 g (81%). mp. = 92-93 0C (uncorrected). 1H-
NMR(CDC13): 9.91(s, 2H), 7.73(d, J=4.5Hz, 2H), 7.27(d, J=4.5Hz, 2H),
2.78(t, J=8.9Hz, 4H), 1.54(m, 4H), 1.48(m, 4H), 0.97(t, J=7.9Hz, 6H); 13C-
NMR(CDC13): 13.8, 22.9, 28.0, 32.5, 126.7, 130.5, 136.7, 142.8, 142.9,
145.6, 182.6; EI-MS (e/z, relative intensity): 418(M++2, 16.4, 417(M++1,
25.2), 416(M+, base), 345(23.9), 331(7.3), 303(42.0), 288(6.1), 275(5.6),
270(5.5), 240(6.6), 227(5.6), 127(6.6), 41(7.1). UV-vis(CDCl3): maximum
absorption (lmax) 404 nm.

4.2.3. Synthesis of Di-Wittig Salts

4.2.3.1. Synthesis of p-Xylenebis(triphenylphosphonium

bromide), (I)

A solution of p-xylene dibromide (5.28 g, 0.020 mol) and
triphenylphosphine (13.11 g, 0.050 mol) in 15 mL of DMF was stirred and
heated at reflux under nitrogen for 24 h in a 100-mL flask. The resulting
solid was filtered with suction, washed with ether, reprecipitated from
ether-methanol, and dried in vacuo at 110 0C to give 13.41 g (85%) of p-
xylenebis(triphenylphosphonium bromide) as a white powder. NMR
(CDC13) 5 (ppm) 5.29 (d, 4 H, -CH2-), 6.89 (s, 4 H, -C6H4-), and 7.51-
7.53 (m, 30 H, -C6H5).

142

4.2.3.2. Synthesis of m-Xylenebis(triphenylphosphonium

bromide), (11)

A solution of a,a'-dimromo-m-xylene (2.64 g, 0.010 mol) and
triphenylphosphine (6.56 g, 0.025 mol) in 15 mL of DMF was stirred and
heated at reflux under nitrogen for 18 h in a 100-mL flask. The resulting
solid was filtered with suction, washed with ether, reprecipitated from
ether-methanol, and dried in vacuo at 120 0C for 24 h to give 4.50 g
(57%) of m-xylenebis(triphenylphosphonium bromide) as a white powder.
NMR (CDC13) 5 (ppm) 5.18 (d, 4 H, -CH2-), 6.85 (s, 3 H, -C6H4-), 7.35
(s, 1 H, -C6H4-), and 7.58-7.80 (m, 30 H, -C6H5).

4.2.3.3. Synthesis of l,3-Bis(triphenylphosphonium)-

propane Dibromide, (III)

A solution of 1,3-dibromopropane (4.04 g, 0.020 mol) and
triphenylphosphine (13.11 g, 0.050 mol) in 15 mL of DMF was stirred and
heated at reflux under nitrogen for 24 h in a 100-mL flask. The resulting
solid was filtered with suction, washed with ether, reprecipitated from
ether-methanol, and dried in vacuo at 110 0C for 24 h to give 13.80 g
(95%) of 1,3-bis(triphenylphosphonium) propane dibromide as a white
powder. NMR (CDC13) 5 (ppm) 1.71-2.00 (m, 2 H, -CH2-), 4.39-4.75 (m,
4 H, -CH2-), and 7.45-8.06 (m, 30 H, -C6H5).

4.2.3.4. Synthesis of l,4-Bis(triphenylphosphonium)butane

Dibromide, (IV)

A solution of 1,4-dibromobutane (4.32 g, 0.020 mol) and
triphenylphosphine ( 13.11 g, 0.050 mol) in 15 mL of DMF was stirred and
heated at reflux under nitrogen for 24 h in a 100-mL flask. The resulting

143

solid was filtered, washed with ether, reprecipitated from ether-methanol,
and dried in vacuo at 110 0C for 24 h to give 12.03 g (81%) of 1,4-
bis(triphenylphosphonium)butane dibromide as a white powder. NMR
(CDC13) 5 (ppm) 2.11-2.33 (m, 4 H, -CH2-), 3.90-4.12 (m, 4 H, -CH2-),
and 7.60-7.96 (m, 30 H, -C6H5).

4.2.4. Synthesis of Copolymers via Wittig Reactions

4.2.4.1. Synthesis of Poly(3',4'-dibutyl-oc-terthiophene-

vinylene-1,4-phenylene-vinylene), (PBTPV-p)

A 250-mL three-necked flask equipped with two rubber septa, a
magnetic stirring bar, a condenser and a source of dry nitrogen was
charged with p-xylenebis(triphenylphosphonium bromide) (0.946 g, 1.2
mmol) and 30 mL distilled anhydrous THF. The flask was placed under
nitrogen and stirred at room temperature for 0.5 h. To the white
suspension was added 1.0 mL of n—butyllithium (2.5 mmol) via syringe.
The resulting deep-red mixture was stirred for an additional 0.5 h, and
then was added dropwise a solution of the dialdehyde (DFDBT) (0.416 g,
1.0 mmol) in 10 mL of THF via a plastic syringe at ambient temperature.
The mixture was stirred at room temperature for 2 h, and then heated to
reflux for 6 h. After removal of the THF, the product was washed with
250 mL MeOH, filtered and dried in vacuo at 50 0C for 24 h to give 0.378
g (77.8%) of copolymer PBTPV-p as a red solid. Anal. Calcd for
C30H3()S3 (repeat unit): C, 74.07; H, 6.17. Found: C, 71.53; H, 6.35.
Elemental analyses by SEM/EDS showed the ratio of SIP was larger than
99/1. 1H NMR (CDC13) 8 (ppm) 7.43, 7.16, 7.11, 7.02-6.84, 2.72, 1.60-

1.40, 0.97.

144

4.2.4.2. Synthesis of Poly(3',4'-dibutyl-01-terthiophene-

vinylene-1,3-phenylene-vinylene), (PBTPV-m)

A 250-mL three-necked flask equipped with two rubber septa, a
magnetic stirring bar, a condenser and a source of dry nitrogen was
charged with m-xylenebis(triphenylphosphonium bromide) (0.473 g, 0.6
mmol) and 40 mL distilled anhydrous THF. The flask was placed under
nitrogen, and stirred at room temperature for 0.5 h. To the white
suspension was added 0.5 mL of n-butyllithium (1.25 mmol) via syringe.
The resulting red-orange mixture was stirred for an additional 1 h, and
then was added dropwise a solution of dialdehyde (DFDBT) (0.208 g, 0.5
mmol) in 10 mL of THF via a plastic syringe at ambient temperature. The
mixture turned to orange and kept stirred at room temperature for 1 h
after the addition, and then heated at reflux for 24 h under nitrogen. The
reaction mixture was cooled to room temperature, poured into 200 mL of
anhydrous ethanol, filtered and washed with more ethanol to remove the
byproducts triphenylphosphine oxide and LiBr. The orange solid was then
completely dissolved in 20 mL of THF, reprecipitated into 200 mL
anhydrous ethanol, filtered, and washed with copious hexane and dried
under vacuum at 50 0C for 24 h to give 0.18 g (74%) of copolymer
PBTPV-m as a orange-red solid. Anal. Calcd for C30H30$3 (repeat unit):
C, 74.07; H, 6.17. Found: C, 73.16; H, 6.55. Elemental analyses by
SEM/EDS showed the ratio of SIP was larger than 99/ 1. 1H NMR (CDC13)
5 (Ppm) 7.50, 7.42, 7.33, 7.18, 7.13, 7.11, 7.02-6.84, 2.72, 1.55-1.46,
0.97.

145

4.2.4.3. Synthesis of Poly(3',4'-dibutyl-a-terthiophene-

vinylene-methylene-vinylene), (PBTMV)

A 250-mL three-necked flask equipped with two rubber septum, a
magnetic stirring bar, a condenser and a source of dry nitrogen was
charged with 1,3-bis(triphenylphosphonium)propane dibromide (0.436 g,
0.6 mmol) and 20 mL distilled anhydrous THF. The flask was placed under
nitrogen, and stirred at room temperature for 0.5 h. To the white
suspension was added 0.5 mL of n-butyllithium (1.25 mmol) via syringe.
The resulting red-orange mixture was stirred at for 0.5 h, and then was
added dropwise a solution of dialdehyde DFDBT (0.208 g, 0.5 mmol) in 2
mL of THF via‘ a plastic syringe at ambient temperature. The mixture
turned to dark-green immediately and kept stirred at room temperature for
2 h, and then heated at reflux for 6 h. After removal of the THF, the
product was washed with 250 mL MeOH and filtered. The red-orange solid
was further extracted with copious THF at room temperature. The orange
THF extracts were combined and reduced to a volume of 25 mL, and then
precipitated into 250 mL MeOH, filtered, washed with MeOH, and dried
under vacuum at 50 C’C for 24 h to give 0.070 g (33%) of copolymer
PBTMV as orange solid. Anal. Calcd for C25H2883 (repeat unit): C,
70.75; H, 6.60;. Found: C, 67.27; H, 6.74. Elemental analyses by SEM/EDS
showed the ratio of SIP was about 96/4. 1H NMR (CDC13) 5 (ppm) 9.83,
7.80-7.40, 7.00-6.70, 6.58, 626, 5.88, 3.62, 2.62, 1.60-1.23, 0.97.

146

4.2.4.4. Synthesis of Poly(3',4'-dibutyl-oc-terthiophene-

vinylene-ethylene-vinylene), (PBTEV)

A 250-mL three-necked flask equipped with two rubber septum, a
magnetic stirring bar, a condenser and a source of dry nitrogen was
charged with 1,4-bis(triphenylphosphonium)butane dibromide (0.444 g,
0.6 mmol) and 30 mL distilled anhydrous THF. The flask was placed under
nitrogen, and stirred at room temperature for 0.5 h. To the white
suspension was added 0.5 mL of n-butyllithium (1.25 mmol) via syringe.
The resulting red-orange mixture was stirred for an additional 1 h, and
then was added dropwise a solution of dialdehyde (DFDBT) (0.208 g, 0.5
mmol) in 15 mL of THF via a plastic syringe at ambient temperature. The
mixture turned to dark and kept stirred at room temperature for 1 h after
the addition, and then heated at reflux for 24 h. After removal of the THF,
the product was washed with 250 mL EtOH and filtered. The orange-
yellow solid was completely dissolved in 20 mL THF, and then precipitated
into 250 mL EtOH, filtered, washed with EtOH, hexane, and dried under
vacuum at 50 0C for 24 h to give 0.065 g (30%) of copolymer PBTEV as
orange-yellow solid. Anal. Calcd for C26H3083 (repeat unit): C, 71.23; H,
6.85;. Found: C, 63.04; H, 5.99. Elemental analyses by SEM/EDS showed
the ratio of SIP was about 93/7. 1H NMR (CDC13) 5 (ppm) 9.85, 7.71,
7.45, 7.01-6.79, 6.52, 6.11-6.04, 5.50, 2.67, 2.24-2.46, 1.60-1.30, 0.97

4.2.5. Synthesis of Iodine-Doped Copolymers
(a) Doping with iodine vapor. The film of copolymer PBTPV-p,
solution-cast (THF) on a quartz slide, was put into a closed chamber filled

with iodine crystal and stored for 4 h. The original red film of PBTPV-p

147

turned to dark-brown. Thin films of the other three copolymers were
doped by iodine vapor in a similar manner.

(b) Doping with iodine in acetonitrile. To a stirred 45 mL 0.1 M
iodine acetonitrile solution, 0.020 g red solid of the copolymer PBTPV-p
was added. The red powder turned black immediately. After stirring for
10 h, the black solid (dark green) was collected, washed several times with
acetonitrile and vacuum dried overnight. The yield was quantitative. The
solids of the other three copolymers were doped by iodine in acetonitrile
similarly. The iodine doped samples were pressed into pellets for the
electrical conductivity measurements. The doping levels are estimated from
the sulfur to iodine ratio as obtained by SEM/EDS semiquantitative

elemental analysis.

4.2.6. Physicochemical Methods

Carbon, hydrogen and nitrogen elemental analyses were performed
by Oneida Research Services Inc., Whitesboro, NY. Elemental analyses
(semiquantitative) for sulfur, phosphorus and iodine were performed on a
JEOL JSM-6400V scanning electron microscope (SEM) equipped with a
Tracor Northern energy dispersive spectroscopy (EDS) detector. Infrared
spectra were recorded as KBr pressed pellets on a Nicolet 740 FT-IR
spectrometer. UV-Visible-NIR absorption spectra were obtained from a
Shimadzu UV-3101PC double beam, double-monochromator
spectrophotometer. Proton nuclear magnetic resonance spectra were
obtained using a computer controlled Varian Gemini-NMR (300 MHz)
spectrometer. The chemical shifts are reported in parts per million (5,
ppm) using the residual solvent resonance peak as reference (CHCl3, 5 7.24

ppm for 1H). Solution photoluminescence spectra were measured in dilute

148

THF solution on a Hitachi F-4500 fluorescence spectrophotometer at room
temperature. Solid state photoluminescence spectra were obtained on a
SPEX fluorolog-2 (Model F 1 1 1A1) spectrofluorometer at 23 OC.
Powdered samples were loaded in 3 mm quartz tubing and sealed under
vacuum (approx. 1.0 x 10'4 torr). X-ray powder diffraction patterns were
collected at room temperature on a Rigaku powder diffractometer, Rigaku-
Denki/RW400F2 (Rotaflex), using Cu(Ka) radiation generated by a
rotating anode operating at 45 kV and 100 mA. The data were collected at
a scan rate of 1 deg/min. Thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) were performed on Shimadzu
TGA-50 and DSC-50 under nitrogen or oxygen at 5 OC/min heating or
cooling rate. Electron spin resonance (ESR) spectra were recorded with a
Varian EPR-E4 spectrometer with diphenylpicrylhydrazyl radical as g
marker (g=2.0037). Cylindrical quartz tubes were employed for powders.
The conductivity data were measured by the standard four-probe method
on pressed pellets as a function of temperature as described elsewhere18.
Molecular weight of the copolymers was estimated by gel permeation
chromatography (GPC) (relative to polystyrene standards, Mw in the range
of 3,120 to 500,800) with Shimadzu LC-10AS liquid chromatograph
equipped with a PL-GEL 101.1 (MIXED-B) column of length 300 mm,
using THF as an eluent. Cyclic voltammetry were performed with a PAR
273 potentiostat/galvanostat equipped with a PAR RE0091 X-Y recorder.

149

4.3. Results and Discussion

4.3.1. Polymer Synthesis

Four copolymers were synthesized through Wittig reaction by
condensation of the dialdehyde (DFDBT) and four different bis-ylides in
THF as shown in Scheme 4.1. The color of the copolymers varies from red
to orange-yellow depending on the copolymer. The elemental analysis data
of PBTPV-p and PBTPV-m are in good agreement with their proposed
structures, while the low carbon contents of PBTMV and PBTEV are due
to the presence innegligible amount of P in the copolymers. All of the
copolymers are polycrystalline as indicated by their X-ray powder patterns
as shown in Figure 4.1. The most notable feature is the broad high angle
reflection at 4.15-4.42 A, which is attributed to an interlayer spacing
arising from a lamellar packing of polymer chains. A similar broad
reflection peak at 4.4 A has also been observed in the X-ray diffraction
pattern of the homopolymer PDBTT.16 The copolymers are soluble in
common organic solvents such as THF and CHCl3, giving orange-yellow
solutions.

The chemical structures and properties of the copolymers were
further examined with FTIR, UV/vis/NIR, NMR and ESR spectroscopies.
Photoluminescence spectroscopy was used to explore the excited state
properties of the copolymers. The electrochemistry of solution-cast thin
films of the copolymers was examined by cyclic voltammetry. The
molecular weights of the copolymers were determined by the gel

permeation chromatography (GPC) technique.

150

p — Ph3P= CH— C6H4— CH=PPh3 ‘

m — Ph3P= CH— C6H4'— CH= PPh3
? +1.2 OHC

/\
\

S

.---»|-.---
CD
\
/
/
(D
\l/
----O.----
I
O

Ph3P= CH- CH2 — CH= PPh,

 

Ph3P= CH— (CH2);- CH= PPh3 J DBTT

r -€DBTT— CH=C CH—O—CH= (21+)- PBTPV- -p
DB'IT— CH=C CH—Q
CH=C PBTPV- -m

-€DB’IT— CH= CH" CH2 _CH= C115: PBTMV
n

THF

¥
7

N 2, reflux

 

\ +DBTI— CH= CH— (CHM—CH= CH)- PBTEV
11

Scheme 4.1. Synthesis of copolymers via the Wittig reaction.

151

 

 

 

I
‘ m-
' Y A‘ A “A; 4
~— - e

W“
V, ‘4...—
.. vmhrm
---.~

”7".

' _ v r A - .A AA. A-; -AA
ALAAA u._ - AM ( )
W' I vr “4‘ v-v-v m ' * 4-
AA A

w

 

Intensity

 

 

 

_‘ A A
W

.' I'UIIIYIVIIIVIVIUIUIUIIIUIYIIIIIIII'IIIIUII'Y'VIY'I[Illl.'l‘U‘IllI;-IWUI;VVITAT

5. 10. 15. 20. 25. 30. 35. 4o.
2 Theta

 

 

Figure 4.1. X-ray powder diffraction patterns of the copolymers at 23 0C
(A) PBTPV-p. (B) PBTPV-m, (C) PBTMV, and (D) PBTEV.

152

4.3.2. Chromatographic Molecular Weight Studies

Figure 4.2 shows the typical GPC chromatograms representing the
molecular weight distribution of the four different copolymers. The
molecular weights were obtained from the common method of a retention
time calibration curve using a series of polystyrene standards with a UV-
vis detector. The weight-average molecular weight (Mw), number-average
molecular weight (Mn) and the corresponding number of repeat units,
along with polydispersity index (PD), of these four copolymers are
summarized in Table 4.1. As we can see, PBTPV-m has the highest Mn in
this series, which correspond to about 6 repeat units (4 x 6 = 24 rings) per
chain. It has been reported that polymers prepared via Wittig reaction
usually have low molecular weight.153 This can be attributed to slow
reactions between bulky substrates and the stepwise polymerization nature

of the Wittig reaction.

Table 4.1. Molecular Weight Data for Four Copolymers Determined by
the GPC Method.

 

 

polymer Mw Mn PD index Number of repeat units
PBTPV-P 1.89 x 103 1.43 x 103 1.32 ~ 3
PBTPV-m 4.85 x 103 3.01 x 103 1.61 ~ 6

PBTMV 4.15 x 103 2.29 x 103 1.81 ~ 5

PBTEV 2.45 x 103 1.72 x 103 1.42 ~ 4

153

 

 

 

 

 

 

 

Fir—Y—rr I

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3 (A) M - 3 (C)
A " " A T
2 - 5’ 1
.E I . ‘E .
=3 - L ‘3 L
.6 - .o' .
.- L-
“ ‘1 C!
v . l- V .
.>.~ - — b -
'8 _ a; 4
c I:
3 8
s. - - s -
r- /
.nr.[.r.r‘....,.rr.‘.rr1,.... r...|..r.| tttttttttt .r.1.r.
o 10 20 . 30. 4o 50 so 0 10 20 30 40 so
Retention Time (mm) Retention Time (min)
..1..,,1.,,I,..1,.,1,.. ...1..,‘1.,,I...,1.,,.1,._,
(B) r (D)
’u? . T A T
.: , 1’3
: 1~ ‘
3 ‘ C .
a .' v— : .1
.e . .. ' .
a . . '2
v N
A d b V d
.2 . ' >~ ‘
a, - .: 1
= ‘ b In 1
3 . . C .
c .1 I— 3 d
... . . t: .
I1 b ~ 1
- 1
n a . . T I' Y I | l a I . . T T u n . I I a . I F a b - I T l l . I 1T . e ‘ Y o T . 1 . u | I . l’ q I I u o I
o 10 20 . 30. 40_ so so 0 10 20 30 40 so
Retention Time (mm) Retention Time (min)

Figure 4.2. Typical GPC traces of the copolymers in THF solution at
room temperature (A) PBTPV-p, (B) PBTPV-m, (C) PBTMV, and (D)

PBTEV.

154

4.3.3. Infrared spectroscopy

Figure 4.3 shows the infrared spectra of the starting dialdehyde
DFDBT, and the conjugated copolymers PBTPV-p and PBTPV-m. Figure
4.4 shows the infrared spectra of the non-conjugated copolymers PBTMV
and PBTEV. The principal IR absorption bands observed in these
copolymers and their assignments, together with the corresponding results
for DFDBT and PDBTT, are listed in Table 4.2.

In Figure 4.3. the strong, sharp absorption band at 1655 cm'1 in
spectrum (A) is due to the aldehyde group in the starting material. In the
spectra of (B) and (C), this band is significantly reduced in intensity, while

two new bands

Table 4.2. Comparison of Infrared Band Positions (cm-1) and Their

Assignments for Copolymers and Related Polymers.

 

 

polymer vinylene arom. aliph. ring methyl arom C-H
str CB-H Stf- C-H stretch stretch def out-of-plane
DFDBT 3063 2955 2934 2853 1539 1508 1431 1381 795

PBTPV-p 1617 3063 2953 2926 2851 1549 1513 1441 1378 804
PBTPV-m 1618 3062 2951 2926 2855 1592 1574 1444 1377 790
PBTMV 1664 3065 2953 2926 2857 1513 1458 1437 1385 797
PBTEV 1660 3063 2952 2925 2857 1554 1511 1436 1378 795
PDBTTa 3062 2951 2925 2856 1492 1456 1377 788

a) see ref 16 PDBTT-poly(3,4-dibutyl-Ot-terthiophene)

155

(W

L

 

 

 

 

~11 l

1

 

 

l

TFIPINSM I TTHNCE

3

X

153/1 ’ /l

 

 

 

 

 

 

 

lmoo 3é7o 2540 1510 1080 1£150
NHVENUMBEH

Figure 4.3. FT-IR transmission spectra (KBr pellets) of the dialdehyde
DFDBT and copolymers (A) DFDBT, (B) PBTPV-p, and (C) PBTPV-m.

156

appear at ~1618 and ~943 cm'1 indicating the formation of trans-alkene in
PBTPV-p and PBTPV-m copolymer backbone.15,19 The strong, sharp
absorption band at 790-804 cm'1 (C-H out-of-plane vibration) is
characteristic of an 01,0t-coupled alkyl-substituted polythiophene ring, as
has been observed in PDBTT.16 By comparison of spectra B and C, the
sharp peak at 683 cm"1 in PBTPV-m is characteristic of the meta-
disubstituted benzene. For the non-copolymers PBTMV and PBTEV
(Figure 4.4 A and B) the bands due to trans-alkene vibrations are shifted to
higher energy at 973 and 950 cm'l, respectively. The band at ca. 1716 cm-
1 is possibly due to the terminal aldehyde group. The three bands in the
region of 744-694 cm“1 indicate the presence of phosphorus-related group
in PBTMV and PBTEV, as supported by the presence of ~ 4-7 % P
(relative to S) in SEM/EDS results.

The single broad peak at ~3062 cm'1 present in the IR spectra of all
copolymers is due to the CB-H stretching vibration on the unsubstituted
thiophene rings. The other single peak at 3018-3020 cm‘l, shown in the IR
spectra of all copolymers, is due to the CH stretching vibrations of the

vinylene-linkages.

4.3.4. NMR Spectroscopy

Figure 4.5 and 4.6 show the 1H NMR spectra for the conjugated and
non-conjugated copolymers in CDC13, respectively. For the conjugated
copolymers PBTPV-p and PBTPV-m, the resonances at the aliphatic region
correspond to the butyl groups on the polymer backbone. The resonances
between 6.80-7.20 ppm are attributed to the aromatic protons on the
unsubstituted thiophene rings and vinylene groups, while the resonances

beyond 7.24 ppm correspond to aromatic protons on the benzene rings.

157

 

 

 

 

 

 

 

 

 

1
cu? (A)
Uta
=1
:5.
33.:
g:
:1
:3-
a..-
E":
5":
aooor ‘mbo‘"'aliofr'jioliort
-. Wavenumber (cm-1) W
8 l
g. (B)
33 N
a l
g l U
*- l
E“. |
5°
.1

 

 

 

l100 3590 also 2170 2560 1550 1550 than Tao 510
Wavenumber (cm-1)

Figure 4.4. FT-IR transmission spectra (KBr pellets) of copolymers (A)
PBTMV and (B) PBTEV.

158

(A)

 

 

 

 

12 11 10 9 B 7 6 5 4 3 2 1 0 com

(B)

 

 

WTI’TTTIIIUTTIVYITIVTYTIYUIUIUYTTTYU"[3111']
12 11 10 9 8 7 6 5 l 3 2 .1

Figure 4.5. 300 MHz 1H NMR spectra of the copolymers (A) PBTPV-p
and (B) PBTPV-m at room temperature (in CDC13).

(A)

_ I. I IMLAL V,

TWITIVT"FTITIIVYIIYITIIYUUTTW—YffilTYTIIT—IWTTfi—TIVUFYIU'FI T'UVIIIUVI'IITI'I

12 ll 10 9 8 7 6 5 4 3 2 i 0 00m

 

 

 

 

 

(B)

 

 

———-——

.011 ,JUL

w.

 

tr'I'jry‘tTr‘rfiT—IIIITIT"1‘1'1 IT'I1'YYV.IYY011"1jTjUV'i'I‘UI‘TKV‘IUj'fi'T‘ITY‘

12 51 :0 9 8 7 6 5 4 3 2 l 0 com .

Figure 4.6. 300 MHz 1H NMR spectra of the copolymers (A) PBTMV
and (B) PBTEV at room temperature (in CDC13).

160

Only one doublet resonance of the benzene protons in the 1H NMR
spectrum of PBTPV-p was observed, compared to at least three doublet
resonances in 1H NMR spectrum of PBTPV-m. This result indicates the
presence of higher symmetry in the chemical structure of PBTPV-p than
PBTPV-m, as would expected. The number of protons corresponding to
each resonance based on integration of the resonances of the NMR spectra
are in good agreement with the proposed structure. For the non-conjugated
copolymers PBTMV and PBTEV, the resonances at the aliphatic region
correspond to the butyl groups and methylene or ethylene units of polymer
backbone, respectively. In the aromatic region, three resonances at 5.50-
6.60 ppm are attributed to the protons on the vinylene linkage units, while
the other three resonances at 6.70-7.00 ppm are assigned to the protons on
the unsubstituted thiophene rings. The notable features of the 1H NMR
spectra of the non-conjugated copolymers are the presence of small low
field resonance at ~9.85 ppm and two multiple resonances at ca. 7.40-7.80
ppm. The former peak can be reasonably assigned to the terminal aldehyde
groups. In conjunction with their FI‘IR spectra and the presence of P in the
EDS/SEM analysis, the later ones are tentatively assigned to organic

phosphorus-related materials as possible end-groups of polymer chains.

4.3.5. Electronic Spectroscopy, UV-Visible-NIR

The solution optical absorption spectra of all four copolymers in
THF are shown in Figure 4.7. As we can see, the wavelength of the
maximum absorption (kmax) for the 1t—1t* transition decreases in the order
of PBTPV-p, kmax(450 nm) > PBTPV-m, Xmax(4l9 nm) > PBTMV,
hmax(389 run) > PBTEV, kmax(381 nm). The decrease of Kmax is expected

since the copolymers change structures from the most conjugated backbone

161

lllllLilll't'll';1"ll"11'11

 

 

lllllljlll

 

tlilrtltrirlrullllrvliirr
IIIIIIIIIII

Absorbance (arb. units)

Lilli!!!

 

 

IIII'IIIIIIII

 

 

 

jilf[lltuattiiritl'utntlleT

l
200 300 400 500 600 700 800
Wavelength (nm)

Figure 4.7. Solution UV-Vis absorption spectra of copolymers in THF
solution at room temperature (A) PBTPV-p, (B) PBTPV-m, (C) PBTMV,

and (D) PBTEV.

162

to the least-conjugated backbone. Figures 4.8 and 4.9 show the optical
absorption spectra of the corresponding solution-cast thin films of the
copolymers. The optical bandgap (Eg) of the copolymers are estimated

from the edges of the solid state absorption spectra. The bandgap increases
in the order of PBTPV-p, Eg(2.05 eV) < PBTPV-m, Eg(2.25 eV) <

PBTMV, Eg(2.36 eV) < PBTEV, Eg(2.47 eV). The data of the optical
absorption spectra are summarized in Table 4.3. The band-gaps of the
copolymers are between those of PDBTT (Eg ~ 2.0 eV)16 and PPV (Eg ~
2.5 eV).20

Table 4.3. Electronic Absorption Data of Four Copolymers in Solution
and as Solid State Thin Films.

 

polymer Xmax, (soln)a (nm) kmax. (film)b (nm) Eg(fi1m)b(eV)

 

PBTPV-p 450 450 2.05
PBTPV-m 419 434 2.25
PBTMV 389 387 2.36
PBTEV 381 381 2.47

 

a THF solution. b THF solution-cast thin film on a quartz slide.

The UV-vis-NIR absorption spectra of thin films doped with iodine
or FeCl3 are compared with their corresponding neutral films in Figures
4.8 and 4.9. For the conjugated copolymers PBTPV-p and PBTPV-m, the
1t—1t* absorption bands lose intensity upon oxidation and shift to slightly
higher energy, while two new subgap absorption bands appear at 0.81 and
1.71 eV, and 0.95 and 1.79 eV, respectively. These midgap absorptions

developed in both cases correspond to transitions associated with the two

Absorbance (arb. units)

Absorbance (arb. units)

163

 

 

 

 

 

 
     

.. F.
3 (A) dOped :
E / E
: If t
; 1.71 eV \ \ i
- l-
‘f 0.81 eV ;’
T \ C
_ \ - _
I \ :
_3 neutral ;
3 Eg = 2.05 eV 3
2 2
I T I a | v I a 1 I I I t I I I 1 I T I I l o I I T I I I h-
0 l 2 3 4 5 6
Energy (eV)
1 1 l y l l 1
E E
E (B) E
\
5 0.95 eV neutral

 

IITI:IIIIII1TIITIIllIIIIIIII

 

 

 

II ID I I a—i I I I II

3
Energy (eV)

3 T I I I I I a 1 I I

Figure 4.8. UV-vis-NIR absorption spectra of solution-cast films of the

copolymers (A) PBTPV-p and (B) PBTPV-m at neutral and iodine doped

states.

164

I «
L41 L;L’Jk'l‘1141Lil".'LLJ

d0pcd with /“

(A) FeCl3 \ L

1.22 eV
{1.25 eV/ /~. \

2.14 eV doped with

neutral IOdmC
E Eg=2.36 eV T

7‘71.oyTjY7.7TTi.Yf7ITTYIY‘YI'UI

O 1 2 3 4 S 6 7

 

11111
YI'TYT

Absorbance (arb. units)

 

 

 

 

 

Energy (eV
J L i - 1 . - - 1 1 L L 1 LL - 1 l J l 4 . l l L . i

A- (B) , ;
£3} 3 dOped wrth FeCl3 :
x: « :
s d o o o ' '-
3. : dOped wrth lOdlnC \ / :
_ . . :
3 j \ e.

- \ C
0 ‘ ,.
o ; .
c: . \ :
_S : 2.19 eV :
l-u "‘ ..
g -_ \ _—
—D J , neutral 2'
< : r-

; / Eg=2.47 eV

7 T 7 1"] o T r 7 Ti I T Yj T i I I I I U T I I Y T 1 IT} I I T

 

 

O 1 2 3 4 5 6 7
Energy (eV)

Figure 4.9. UV-Vis-NIR absorption spectra of solution-cast films of the
copolymers (A) PBTMV and (B) PBTEV at neutral, iodine and Fer

doped states.

165

localized bipolaron energy levels that appear within the band gap upon
oxidative doping},21 These results are consistent with charge storage
predominantly in bipolaronsl,21 and follow the same trends as many
conjugated polymers.20 Further support for the conclusion that PBTPV-p
possesses a longer effective average conjugation length than PBTPV-m
derives from the lower energies associated with the midgap transitions of
the former. The copolymer PBTMV forms one new subgap absorption
bands at 1.22 eV upon doing with iodine, and two bands at 1.25 and 2.12
eV when doped with 0.1 M FeCl3 acetonitrile solution, these two bands
may indicate the formation of bipolaron transitions. This is surprising
because, given the non—conjugated nature of the polymer backbone, no
oxidative doping is expected. Therefore, these spectral changes imply that a
fully conjugated backbone may be forming upon oxidation of the -CH2-
unit which is then followed by the familiar type of oxidative doping, which
is known for other conjugated polymers (e. g. polythiophene, polyacetylene,
PPV, etc.), see Scheme 4.2. However, more experiments are needed to
confirm this process. In contrast, PBTEV does not form any new subgap
bands upon doping with iodine, and only one band at ~ 2.19 eV when
doped with 0.1 M FeCl3 acetonitrile solution. These results indicate that
copolymers become increasingly difficult to oxidize as the saturated carbon
units in the polymer backbone become longer. Thus, we have demonstrated
that one can tune the electronic properties of copolymers such as polymer

bandgap by selective choice of the building block.

4.3.6. Photoluminescence Spectroscopy
Figure 4.10 shows the photoluminescence (PL) spectra of the

copolymers in THF at 23 0C using excitation wavelength of 400 nm.

166

 

oxidation
- 3+
w1th Fe

 

 

 

Scheme 4.2. Proposed structural change of PBTMV upon oxidation with
FeCl3.

167

Photoexcitation of these copolymers results in broad band luminescence
with a peak maximum at 550 nm for PBTPV-p, 521 and 545 nm for
PBTPV-m, 523 nm for PBTMV, and 501 nm for PBTEV. These results
are in good agreement with the decrease of conjugation length from
PBTPV—p to PBTEV as discussed in the above section. Figure 4.11 shows
the photoluminescence spectra of the copolymers in the solid state at 23 0C
when excited at the wavelength of 400 nm. The PL data are summarized in
Table 4.4. There is a significant red shift of the emission maximum in
going from the solution state to the solid state. This red shift is presumably
due to the achievement of longer effective conjugation length in the solid
state (rod-like conformation) than in solution (coil-like and non-coplanar
conformation). It is notable that the red shift of PBTPV-p is the largest one
(96 nm), while that of PBTPV-m is the smallest one (40 nm). Similar red
shift has been observed in the case of homopolymer PDBTT and other
P3ATs.la,l6 The photoluminescent property of the copolymers offers an
opportunity to explore the potential application of these materials in the

study of electroluminescence devices.22

Table 4.4. Photoluminescence Emission Maxima in Solution and Solid

State of Four Copolymers at 23 0C.

 

polymer Kmax, ($01!!)a (nm) Kmax, (solid)b (nm) red shift (nm)

PBTPV-p 555 651 96
PBTPV-m 521, 545 585 40
PBTMV 523 607 84
PBTEV 501 558 57

 

a THF solution. b solid powders

168

 

I!
I ‘

rTlllTIlllltlllltlTll‘kl

Intensity (arb. units)
I I i I I T ‘ I l I I l I l i I I I

lllllLllLllllll'llll

Irtl

 

 

 

d

400 450 500 550 600 650 700
Wavelength (nm)

Figure 4.10. Photoluminescence emission spectra of the copolymers in
THF solution at room temperature (a) PBTPV—p, (b) PBTPV-m, (c)
PBTMV, and (d) PBTEV.

169

 

_ 607 nm
1; -' 558 nm .651 nm
‘E i a -
3 c
{3 d
«3 . .
" I i
Z? “ - 585 nm :'
.5 ;// y .
.1
C u .
8 - -
E. b

WIIT‘II’II

 

 

.i l I l '

ltlllfialryirrultllalryrrl

500 550 600 650 700 750 800
Wavelength (nm)

 

Figure 4.11. Photoluminescence emission spectra of the copolymers in

the solid state at room temperature (a) PBTPV-p, (b) PBTPV-m, (c)
PBTMV, and (d) PBTEV.

170
4.3.7 . Thermal Analysis

The thermal stability of the copolymers were examined by
thermogravimetric analysis (TGA) and differential scanning calorimetry
(DSC). Figures 4.12 and 4.13 show the TGA thermograms of PBTPV-p,
PBTPV-m, PBTMV and PBTEV under flowing nitrogen and oxygen,
respectively. The thermal analysis data is tabulated in Table 4.5. The
PBTPV-p has the highest thermal stability in nitrogen among the four
copolymers and is comparable to that of the homopolymer PDBTT, while
the PBTEV is the lest stable in nitrogen. Based on the chemical structures
of the copolymers, the thermal stability is largely controlled by the
polymer backbone since each repeat unit has same number of pendent butyl
groups. Thus, the insertion of flexible saturated carbon units in polymer

backbone causes a reduction of in thermal stability of the material.

Table 4.5. Summary of Thermal Analysis Data for Four Copolymers and
PDBTTa.

 

 

polymer Tdb (0C) Tdb (0C) TmC (0C)
(under N2) (under ()2) (under N 2)
PBTPV-P 378 260 no
PBTPV-m 333 289 1 19
PBTMV 327 25 1 no
PB TEV 210 130 no
PDBTT 380 285 no

 

a PDBTT, see ref 16b. b Td: thermal decomposition temperature. C Tm:

melting point.

171

DSC curves of the neutral copolymers are shown in Figure 4.14. All
the copolymers exhibit broad endothermic transitions. In conjunction with
direct melting point measurements, only the endothermic peak ca. 119 0C
as shown in Figure 4.14B corresponds to the melting transition of PBTPV-
m. Upon cooling, no corresponding exothermic peak is observed,
indicating the absence of crystallization in the PBTPV-m. The broad
endothermic transitions observed in the other three copolymers do not
correspond to melting transition, they are possibly due to some kind of
order-disorder transitions which are not clear at this moment. It is
noteworthy that repetitive DSC scans of all materials give only featureless

CUI'VCS.

 

120 JKLKLLLLILILLIIALilJAL|..A'[LillLLllLL

(A) i
/ L

 

5...
l

    

l J l

. 9.3

Weight (wt%)
9

 

/
(D)

 

 

 

O TTTI I IIrjIIIT1I71TIIIITIT71ITII II T7

0 100 200 300 400 500 600 700 800
Temperature (°C)

Figure 4.12. TGA thermograms of the copolymers under nitrogen (A)
PBTPV-p, (B) PBTPV-m, (C) PBTMV, and (D) PBTEV.

173

 

120 '11L'40111111111LL111LJJI'LL111L4111411
q .

 

Weight (wt%)
i8

 

 

‘C

TFII

 

 

 

O ITYTIIIIThTITTIjIIchTVIIIIIITIITII

0 100 200 300 400 500 600 700 800
Temperature (°C)

Figure 4.13. TGA thermograms of the copolymers under oxygen (A)
PBTPV-P. (B) PBTPV-m, (C) PBTMV, and (D) PBTEV.

DSC (mW) #1

DSC (mW)

ILLAJALAIJJLLL

 

 

q l-
‘ D
.1
4 A)

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G D
I .

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- 270 c»
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100 ISO 200 250

Temperature (°C)

SO

1L11J11L1JJJ 111

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300

 

I

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11141111111111111 111111

0
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III'V'U'Y'IYI'V'YIUIVVIYYYT

 

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50 100 1 50 200 255
Temperature (° C )

rrvv

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DSC (mW)

Io '-

DSC (mW)
.<'>

24 1 1 1 1 L 1 1 1 .
.5: (D) :-
1'3 5'
.1 ° :
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.55 l l E.
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'2.l 1 v t I Ij’ 1 ffi I’ I 1 v v‘r

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50

T V I I I T V U V

I I U r ‘ I t
100 150 200

Temperature (°C)

 

 

 

 

 

'1
50

100 ‘ISO 200

Temperature (°C)

Figure 4.14. DSC thermograms of the copolymers under nitrogen (A)
PBTPV-p, (B) PBTPV-m, (C) PBTMV, and (D) PBTEV.

175

4.3.8. Electron Spin Resonance Spectroscopy

Figure 4.15 shows the ESR spectra of the PBTPV-p and PBTPV-m,
both spectra reveal a single line centered at nearly the free electron g value
of 2.0030 and 2.0031, with a linewidth (Apr) of 6.0 and 6.4 Gauss,
respectively. Figure 4.16 shows the ESR spectra of the PBTMV and
PBTEV. Electron spin resonance values for the copolymers are listed in
Table 4.6. The number of spins/mol is determined for the copolymers by
using a diphenylpicrylhydrazide (DPPH) standard. Spin quantitation
performed on these copolymers show a number in the order of 1020
spins/mol, corresponding to approximately 1 spin per 6000 repeat units.
This results suggests a high level of purity for the copolymers and fewer

spin-carrying defects.

Table 4.6. ESR Data for Four Copolymers at 23 oC.

 

 

copolymers g factor Apr (Gauss) spins/mol
PBTPV-P 2.0030 6.0 1.6 x 1020
PBTPV-M 2.0031 6.4 7.8 x 1019

PBTMV 2.0026 6.2 1.2 x 1020

PBTEV 2.0037 6.5 1.2 x 1020

 

176

)—
..

 

 

lJldLLLJ

I I I I I I | I I I I I I I

 

Intensity (arb. units)

§ (A)

T r I I I

 

 

 

I I I . T I l I I I

3300 3320 3340 3360 3380 3400 3420
Field (G)

L . l i y . o I I 1 1 l 0 1 ‘ t l l. J ' 1 1

.t

.1

.r

.4

.4

.1

.1

-

.1 l
.r

.r

-

—1

.1

.t

.4

« (B)

.1

.4

.“‘lr'rt‘ ‘T"'l"‘l‘7‘
3300 3320 3340 3360 3380 3400 3420
Field(G)

 

IIIIIITIII

l

Intensity (arb. units)

jIII'ITIIIIIII

 

 

 

Figure 4.15. ESR spectra of the copolymers at room temperature (A)
PBTPV—p and (B) PBTPV—m.

177

 

 

 

 

 

 

 

 

 

 

 

fig .3: i
'E E E
_5 ; :
8—1 " "
53 i 3
3‘ : E
’5 t r
‘3 - E
8 - -
s. ‘- 3

(A) f.

i E

3300 3320 3340 3360 3380 3400 3420

Field (G)
v 1 I . 1 1 l . . . 114 . l u . . I 1 1 n
33 .1 _
'E q -
3. 1 .—
-é : -
3 _ _
Z? - I
C q _.
8 .. _
c: _ .
"" (B) .
I—r . I T l I I I ‘ t I 7*. 7i r . I I
3300 3320 3340 3360 3380 3400 3420
Field (G)

Figure 4.16. ESR spectra of the copolymers at room temperature (A)
PBTMV and (B) PBTEV.

178

4.3.9. Polymer Electrochemistry

The solution-cast (THF solution) films of the copolymers PBTPV-p
and PBTPV-m were studied by cyclic voltammetry. Figure 4.17A shows a
representative cyclic voltammogram (CV) of a PBTPV-p film on a Pt
electrode in 0.1 M Bu4NC104/CH3CN solution at a scan rate of 20 mV/S.
The PBTPV-p film turns green-blue upon anodic oxidation in a process
which shows a current maximum at 0.84 V vs. SCE. The color returns to
red in the reverse scan which shows a current maximum at 0.72 V vs. SCE.
At higher scan rates (up to 1 V/sec), the CV curves are less well resolved,
but the observed color changes continued to respond well to the potential
changes. The electrochemical properties of PBTPV-p suggest that it may
have potential application in electrochromic devices. Figure 4.17B shows a
representative cyclic voltammogram (CV) of a PBTPV—m film on a Pt
electrode in a similar condition. It shows an irreversible reduction peak at
ca. 0.64 V vs. SCE. The film was orange in the neutral state and dark-
green in the oxidized state. However, the color change of the PBTPV-m
film during the CV scans was not as fast as that of the PBTPV—p film. The
poor redox behavior of PBTPV-m film may be due to its low conjugation.
Figure 4.18 shows a typical CV of a film of PBTMV on an ITO electrode.
The anodic scan from 0.0 to 1.4 V vs. SCE shows a rapid current increase
with an onset of 1.04 V vs. SCE. The color of the film changed from
yellow to blue (or purple). The following cathodic scan was featureless.
The film of PBTMV stayed blue at 0.0 V. Further cycles from 0.0 to -1.5

V did not show any redox waves.

179

 

 

 

 

 

 

 

 

1t
0.65 V

 

 

Potential (V) vs. SCE

Figure 4.17. Typical cyclic voltammograms of cast films of the
copolymers (A) PBTPV—p and (B) PBTPV—m on Pt electrode in
CH3CN/0.1 M (Bu4N)ClO4 at room temperature. Scan rate 20 mV/s.

180

 

 

an
<

Current

1.40 V

 

 

 

Potential (V) vs. SCE

Figure 4.18. A typical cyclic voltammogram of a cast film of the
copolymer PBTMV on ITO electrode in CH3CN/0.l M (Bu4N)ClO4 at

room temperature. Scan rate 20 mV/s.

181

4.3.10. Charge Transport Properties

The electrical conductivity of iodine doped PBTPV-p (ca. 0.61 13',
per repeat unit) has been measured by the standard four-probe method on
pressed pellets as a function of temperature. (Figure 4.19A) It shows a
room temperature conductivity of ~3.2 x 10'2 S/cm. The conductivity of
other three copolymer has only been measured at room temperature, the
corresponding values are in the order of magnitude of 104-10‘5 S/m. The
lower conductivities of PBTPV-m, PBTMV and PBTEV are due to their
decreases in the conjugation length as compared to PBTPV-p. The
thermoelectric power data of iodine doped PBTPV-p is shown in Figure
4.193. The positive value indicates p-type conductivity. The thermopower

data of the other three doped copolymers are not available at this stage.

182

 

 

 

 

 

 

 

 

01
2~ (A)
: 00°
-4 — 0°
rs _ O
E o
g ' O
‘3 o
o '5 ”
O) O
O .
-’ L.
-8 #- O
r o
-10 .—
~00
12b§111141'114411L4111J111111111
0 50 100 150 200 250 300
80 Temperature (K)
0
7o :- (B) o
A h a
x e
\ o
i 50 :. o G’M%°
V > go (a? 6°06
3 : o 0‘" 0:90 a
g .. 80 o q? 0 0° 0 o
8 50:- 00 O 000 (lb 0
o .. W O O 0 00
E :er 0 0° 0°
2 40 F &0° $1055ng 0°°
1'- " poo 800°
‘ 0 ° 0
.. 6’ 3° 8 o
30 — ° °
r- O
20 h J l ‘ L 1 l L 1 L l L 1 1 1 l 1 4 1 1
100 150 200 250 300

Temperature (K)

Figure 4.19. (A) Four-probe variable-temperature electrical conductivity

of PBTPV-p doped with iodine. (B) Variable-temperature thermoelectric

power data for PBTPV-p doped with iodine.

183

4.4. Conclusion

A series of new soluble copolymers containing the rigid conjugated
block 3',4'-dibutyl-2,2':5',2"-terthiophene spaced by aromatic and aliphatic
segments has been designed and prepared through the Wittig reaction. We
have demonstrated that a judicious choice of bis-ylide can lead to electronic
tunability of the resulting copolymers. The most conjugated copolymer
PBTPV-p shows the longest wavelength of absorption maximum in solution
and lowest bandgap in solid state. PBTPV-p emits red light when excited.
while the least conjugated copolymer PBTEV emits orange light when
excited. Solution-cast films of PBTPV—p shows interesting and fully
reversible electrochromic behavior with reasonable electrical conductivity
in the doped form. The less conjugated copolymer PBTPV-m and the non-
conjugated copolymers PBTMV and PBTEV have higher bandgaps and low

electrical conductivities. They emit light at higher frequencies.

(1)

(2)

(3)

(4)

(5)

184
LIST OF REFERENCES

(a) Roncali, J. Chem. Rev. 1992, fl, 711-738 and references
therein. (b) Skotheim, T. A., Ed. Handbook of Conducting
Polymers; Marcel Dekker: New York, 1986; Vols.1 and 2. (c)
Skotheim, T. A., Ed. Electroresponsive Molecular and Polymeric

System; Marcel Dekker: New York, 1991; Vols.1 and 2.

(a) Nakajima, T.; Kawagoe, T. Synth. Met. 1989, 2_8, C629. (b)
Mizumoto, M.; Namba, M.; Nishimura, S.; Miyadera, H.; Kosehi,
M.; Kobayshi, Y. Synth. Met. 1989, 28, C639.

(a) Huang, S. C.; Huang, S. M.; Ng, H.; Kaner, R. B. Synth. Met.
1993, 11:51, 4047. (b) Baughman, R. H. Makromol. Chem,
Macromol. Symp. 1991, Q, 193. (c) Conway, B. E. J. Electrochem.
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Chem. Mater. 1994, 6, 1437-1443.

Horowitz, G.; Fichou, D.; Peng, X.; Garnier, F. Synth. Met. 1991,

_4_1_, 1127 and references therein.

(a) Galal, A.; Mark, Jr., H. 3.; Bishop, P. L. PMSE (Am. Chem.
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Eng. Div.) 1994, 7_1_, 381-382.

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(7)

(8)

(9)

(10)

185

(a) Panero, S.; Passerini, S.; Scrosati, B. Mol. Cryst. Liq. Cryst.
1993, a, 97. (b) Nguyen, M. T.; Dao, L. H. J. Electrochem. Soc.
1989, 116, 2131.

(a) Braun, D.; Gustafsson, G.; McBranch, D.; Heeger, A. J. J. Appl.
Phys. 1992, Q, 564-568. (b) Gustafsson, G.; Cao, Y.; Trracy, G.
M.; Klavetter, F.; Colaneri, N.; Heeger, A. J. Nature 1992, m,
477. (c) Ohmori, G.; Uchida, M.; Yoshino, K. Solid State Commun.
1991, 80, 605. (d) Vestweber, H.; Greeeiner, A.; Lemmer, U.;
Mahrt, R. F.; Richert, R.; Heitz, W.; Bassler, H. Adv. Mat. 1992, 4,
661.

Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.;
Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature
1990, £11, 539-541.

(a) Burn, P. L.; Holmes, A. B.; Kraft, A.; Bradley, D. D. C.;
Brown, A. R.; Friend, R. H. J. Chem. Soc., Chem. Commun. 1990,
32. (b) Zhang, C.; Braun, D.; Heeger, A. J. J. Appl. Phys. 1993,
fl, 5177-5180.

Seggern, H. V.; Schmidt-Winkel, P.; Zhang, C.; Pakbaz, K.;
Kraabel, B.; Heeger, A. J.; Schmidt, H.-W. Polymer Preprints
1993, 34(2), 532-533.

(11)

(12)

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186

(a) Trial, T. T.; Khanna, R. K. Polymer Preprints 1993, £111.
869-870. (b) Khanna, R. K.; Cui, H. Macromolecules 1993, 26,
7076-7078. (c) Khanna, R. K.; Bhingare, N. Chem. Mater. 1993, 5,
899-901. (d) Ying, J.; Khanna, R. K. Polymer Preprints 1994,
_3_5_(2), 753-754.

(a) Burn, P. L.; Holmes, A. B.; Kraft, A.; Bradley, D. D. C.;
Brown, A. R.; Friend, R. H.; Gymer, R. W. Nature 1992, 356, 47
and references therein. (b) Malliaras, G. G.; Herrema, J. K.;
Wildeman, J.; Wieringa, R. H.; Gill, R. E.; Lampoura, S.;
Hadziioannou, G. Adv. Mater. 1993, 5, 721-723.

March, J., Ed. Advanced Organic Chemistry; 3rd, John Wiley &
Sons: New York, 1985, 845-854 and references therein.

(a) Jen, K.-Y.; Cava, M. P.; Huang, W. S.; MacDiarmid, A. G. J.
Chem. Soc., Chem. Commun. 1983, 1502-1503. (b) Huang, W. S.;
Jen, K.-Y.; Angelopoulos, M.; MacDiarmid, A. G.; Cava, M. P. Mol.
Cryst. Liq. Cryst. 1990, 189, 237-254.

(a) Hay, M. F.; Yang, Y.; Klavetter, F. L. Polymer Preprints 1994,
35( l ), 293-294. (b) Yang, 2.; Sokolik, I.; Karasz, F. E.
Macromolecules 1993, 26, 1188-1190.

(16)

(17)

(18)

(19)

(20)

(21)

(22)

187

(a) Wang, C.; Benz, M. E.; LeGoff, E.; Schindler, J. L.; Kannewurf,
C. R.; Kanatzidis, M. G. Polymer Preprints 1993, 34(2), 422-423.
(b) Wang, C.; Benz, M. E.; LeGoff, E.; Schindler, J. L.; Albritton-
Thomas, J.; Kannewurf, C. R.; Kanatzidis, M. G. Chem. Mater.
1994, 6, 401-411.

Wang, C.; Xie, X.; LeGoff, E.;J. L.; Albritton-Thomas, J.;
Kannewurf, C. R.; Kanatzidis, M. G. Synth. Met. 1995, 14, 71-74.

(a) Lyding, J.W.; Marcy, H. 0.; Marks, T. J.; Kannewurf, C. R.
IEEE Trans. Instrum. Meas. 1988, 31, 76-80. (b) Marcy, H. 0.;
Marks, T. J.; Kannewurf, C. R. IEEE Trans. Instrum. Meas. 1990,
.32, 756-760.

Socrates, G. Infrared Characteristic Group Frequencies, John Willey
& Sons: New York, 1994.

(a) Bradley, D. D. C.; Colaneri, N. F.; Friend, R. H. Synth. Met.
1989,29, E121-127. (b) Friend, R. H.; Bradley, D. D. C.;
Townsend, P. D. J. Phys. 1987, D20, 1367.

Patil, A.O.; Heeger, A. J .; Wudl, F. Chem. Rev. 1988, 88, 183-200.

Future work is needed to explore the applications of these

copolymers in EL devices.

CHAPTER 5

Synthesis and Characterization of A New

Conjugated Aromatic Poly(azomethine) Derivative
Containing 3',4'-Dibutyl-0t-Terthiophene Linkages

188

189
ABSTRACT

A new conjugated aromatic poly(azomethine) derivative, poly(3',4'—
dibutyl-0t-terthiophene-azomethine- 1,4-phenylene-azomethine) (PB TPI),
has been prepared by polycondensation of 2,5"-diformyl-3',4'-dibutyl-
2,2':5',2"-terthiophene (DFDBT) with 1,4-phenylene diamine under the
ethanothermal conditions. The polycrystalline PBTPI was characterized by
X-ray diffraction, NMR, FT IR, UV/vis/NIR, photoluminescence and ESR
spectroscopies. PBTPI is partially soluble in tetrahydrofuran, giving an
orange solution with an absorption maximum (hmax) of 457 nm. The
polymer is also completely soluble in concentrated sulfuric acid and
nitromethane solution containing Lewis acids (e.g. A1C13), giving blue
solutions with hmax of 656 and 638 nm, respectively. Neutral red PBTPI
has an optical band-gap of 2.06 eV, which is one of the lowest among
poly(azomethines), and is highly sensitive to strong acid environment.
Protonation yields a blue polymer with an optical band-gap of 1.61 eV.
The iodine doped PBTPI shows low electrical conductivity at the order of
107-10'8 S/cm. The properties of PBTPI are compared to other previously

characterized related polymers.

190

5.1. Introduction

Conjugated polymers have gained widespread interest during the last
two decades, because of their useful electronic, optoelectronic,
electrochemical, and nonlinear optical properties.1,2 Among conjugated
polymers, those with extended 1: systems involving alternating C=C and C-
C bonds are predominant. This is the case for many prototypical
conjugated polymers, such as polyacetylene,3 poly(p-phenylene),4 poly(p-
phenylene-vinylene).5 Because the -CH=N- group is isoelectronic with the
-CH=CH- group, the incorporation of nitrogen atoms into the conjugated
system is another approach to form classes of materials with equally
interesting electronic and optical properties. Polyaniline is a well-known
example, which is constructed by the alternating p-phenylene rings and
nitrogen atoms in the polymer backbone.6 Polyaniline shows unique
features such as non—oxidizing doping by protonation7 and the dependence
of the degree of oxidation and properties on the fraction of two types
nitrogen atoms, i.e., imine (=N-, sp2) and amine (>N-, sp3).

Polyazines, [-N=C(R)-C(R)=N-]x (R = H, alkyl), have also received
attentions recently.8‘10 The unsubstituted polyazine, [-N=CH-CH=N-]x, is
formally isoelectronic with polyacetylene, [-CH=CH-CH=CH-]x, but unlike
polyacetylene, the presence of nitrogen in polyazines stabilizes them in
air.8 Polyazines are synthesized via acid-catalyzed condensation reaction
between an 0t,B—dihydrazone and an 0t,|3-dicarbonyl. They can be doped
with iodine to give air-stable electrically conducting materials with room
temperature conductivity as high as 1.3 S/cm.8

Conjugated poly(azomethines), polyimines or poly(Schiff bases) are

another interesting class of conjugated polymers containing nitrogen atoms

191

in polymer backbone. The first poly(azomethines) were prepared by
Adams and co-workers from terephthalaldehyde and benzidine and
dianisidine in 1923.11 The basic aromatic conjugated poly(azomethine) is
poly( 1 ,4-phenylenemethylidynenjtrilo-1 ,4-phenylene-nitrilomethylidyne) 12
(PPI), which is isoelectronic with poly(p-phenylene-vinylene) (PPV), as
shown in Scheme 5.1. Unlike PPV, the imine nitrogen of the PPI backbone
introduces novel features and chemical flexibility,12‘19 including the high
solubility in concentrated sulfuric acid and the complexation with Lewis

acids.

_QCH=HC_©_CH= H..-
.0... = N_©_N= Hc_

Scheme 5.1. Chemical structures of PPV and PPI.

Recently, conjugated poly(azomethines) have been shown to possess
high thermal stability,12-l3 good mechanical strength and fiber-forming
properties,l3 thermotropic liquid crystallinity,13 lyotropic liquid
crystallinity in sulfuric or methanesulfonic acid,13-14 third-order nonlinear
optical properties,15 and electrical properties.16 It has been reported that
the electrical conductivity of conjugated polyazomethines can be increased
by about eight orders of magnitude (up to the level of semiconductors)
when they are doped by iodine.l6 However, a major obstacle to
characterizing and developing most conjugated aromatic poly(azomethines)

has been their intractability and insolubility in common organic solvents.

192

Several methods have been reported to improve the processability of
conjugated poly(azomethines) by modification and selection of polymer
structure, for example, unsymmetrical17 or symmetrical18 substitutions of
mainchain aromatic benzene ring units with flexible alkyl or alkoxy side-
chains. A recent approach based on the reversible Lewis acid-base
complexation has also been successfully applied to the processing of non-
substituted and substituted poly(azomethines).19 Scheme 5.2. shows the
chemical structures of some previously investigated poly(azomethines).17'
19 The known structures of the poly(azomethines) include p-phenylene, p-
biphenylene, 1,5-naphthalene, and vinylene, ether linkages in the polymer
backbone, and alkyl, alkoxy, and hydroxyl side-group substitutions.17'19
Poly(azomethines) possess optical bandgaps in the range 2.03-2.83 eV.
Poly(azomethines) usually can be prepared either by conventional
solution or solid melt methods.l3 According to the solution method, the
reaction between aromatic diamines and dialdehydes is initiated in a water-
free solvent, such as dimethylacetamide, ethanol, or benzene. No catalysts
are necessary but removal of water expedites polymerization. The
poly(azomethines) may be obtained in high yield with low to intermediate
molecular weight. High molecular weights are attained by conducting the

reaction at elevated temperature in the solid or molten state.

193
Rik R2 0 \ M R
{-0- flflfih :QWN}

1. R 1=R2=H (PPI) 10. R=H (PSPI)
2, R1=CH3, R2=H (PMPI) ll. R=OCH3 (PSMOPI)
3. Rle, R2=OCH3 (PMOPI)

4. R1=H, R2=OH (PHOPI)

5. R1=OCH3, R2=H (MO-PPI)

6. R1=R2=OCH3 (P3MOPI)

7. R1=OCH3, R2=OH (MO-PHOPI) O R
0 kg}-
\
R N9”

12. R=H (1,5-PNI)

F€O_ N\\ O 13. R=OCH3 (1,5-PMONI)
@-HN%\% N9;

 

8. PPI/PMPI

@‘O—O‘N\\—( >—\\N_>_

H— N\ S \9— 14. PBEPI

9. PBPI

Scheme 5.2. Chemical structures of previously investigated

poly(azomethines). (reprinted from ref. 19 b)

194

Recently, we reported the synthesis and characterization of a new
soluble and dopable conjugated copolymer consisting of 3',4'-dibutyl-
2,2':5',2"-terthiophene and phenylene vinylene units,20a i.e., poly(3',4'—
dibutyl-2,2':5',2"-terthiophene-1,2-ethenylene-1,4-phenylene-1 ,2-
ethenylene) (PBTPV).

\S/ /s\ \S/ CH=HC—O—CH=CH

PBTPV
Exploiting the Wittig reactions of 2,5"-diformyl-3',4'-dibutyl-2,2':5',2"-
terthiophene (DFDBT) with various appropriate bis-ylides gives several
other interesting new copolymers.20b The resulting tailor-made
copolymers show "tunable" physicochemical properties while maintaining
the enhanced solubility imparted by the 3',4'—dibutyl-2,2':5',2"-
terthiophene (DBTT) building block. As an natural extension of this
research, we have synthesized a new conjugated aromatic
poly(azomethines), i.e. poly(3',4'-dibutyl-(it-terthiophene-azomethine-1,4-
phenylene-azomethine) (PBTPI), by using the polycondensation reaction
between the dialdehyde derivative of the terthiophene (DFDBT) and 1,4-
phenylenediamine under ethanothermal condition.21 PBTPI is formally

isoelectronic with the PBTPV.

"' S /\ S CH=N N=CH>'
\/ s \/ O n

PBTPI

195
To our knowledge, this is the first alkyl-substituted oligothiophene-linked

conjugated poly(azomethine).22 It is notable that the replacement of a
phenylene linkage by a thiophene linkage in the polyquinolines or
polyanthrazoline backbone has been reported recently,23 and there shows a
significant reduction of solid state band gap by 0.3-0.5 eV. We expect that
by substitution of benzene ring with 3',4'-dibutyl-2,2':5',2"-terthiophene
building block in conjugated poly(azomethine) systems will not only
improve the solubility but also further reduce the band gap of the resulting
polymer. The preparation and physicochemical characterization including
XRD, FT IR, UV/vis/NIR, NMR of this new conjugated poly(azomethine)
are described herein. The properties of the new poly(azomethines) are

compared with other known related polymers.

5.2. Experimental Section

5.2.1. Materials

1,4-Phenylenediamine (97% purity) was used as received from
Aldrich Chemical Co., Inc. Ethanol was distilled from CaH2 and stored
over 4-A Linde molecular sieves before use. Acetone was used as received
from commercial sources. Tetrahydrofuran (THF) (HPLC grade) and 1-
methyl-2-pyrrolidinone (NMP) (HPLC grade) were used as received from
Aldrich Chemical Co., Inc. without further purification. Protic acids such
as HCl, HNO3 and H2SO4 were purchased from commercial sources and
used as received. Deuterated nitromethane (CD3NOz-d3, 99%) was used as
received from Cambridge Isotope Laboratories. Anhydrous aluminum

trichloride (AlCl3) was purchased from EM Science, Gibbstown, NJ, used

and stored under dry-glove box filled with nitrogen.

196

5.2.2. Synthesis of 2,5"-Diformyl-3',4'-dibutyl-2,2':5',2"-

terthiophene (DFDBT)

Dialdehyde 2,5"-diformyl-3',4'-dibutyl-2,2':5',2"-terthiophene
(DFDBT) was prepared by Seaver Shieh in Professor LeGoff‘s group. The

detailed synthesis is described in reference 20b.

5.2.3. Synthesis of Poly(3',4'-dibutyl-0t-terthiophene-

azomethine-p-phenylene-azomethine) (PBTPI)

In a heavy-wall (4 mm thickness) pyrex tube (~3 mL) were loaded
0.052 g (0.125 mmol) of the dialdehyde DFDBT, 0.017 g (0.150 mmol) of
1,4-phenylene-diamine and ~ 0.5 mL of anhydrous ethanol. The loaded
tube was frozen in liquid nitrogen and flame-sealed under a vacuum of ~
1.0 x 10'3 torr. The sealed tube was heated in an oven at 125 0C for 24 h.
The red solid formed inside the tube was isolated and washed exhaustively
with acetone to remove residual starting materials and oligomers, and then
dried in a vacuo at 50 0C for 24 h. Yield 0.060 g, 98 %. Elem. anal.
Calcd(%) for C28H28N283 (repeat unit): C, 68.81; H, 5.78; N, 5.73.
Found: C, 66.87; H, 5.96; N, 5.92. 1H NMR of PBTPI (in CD3NOz-d3
containing 2 wt% AlCl3) 5 (ppm) 11.23, 9.39, 8.53, 7.93, 3.09, 1.66, 1.05;
13C NMR of PBTPI (in CD3NOz-d3 containing 2 wt% AlCl3) 8 (ppm)
157.96, 155.72, 149.55, 138.61, 134.83, 132.11, 131.55, 127.15, 123.63.
32.93, 29.66, 24.04, 14.23. The density of PBTPI is ca. 1.26 g/cm3, as
determined by flotation density measurements using a mixed-solvent system
of CC14 (d=1.583 g/cm3) and cyclohexane (d=0.774 g/cm3). We note,

however, that this value most likely represents a lower bound since the

197

polymer particles contain pores which may not be filled completely by

solvent during the flotation experiment.

5.2.4. Synthesis of Protonated PBTPI

(3) Reaction with HCl vapor. 0.020 g of the red PBTPI on a glass
substrate was exposed to the HCl fume from a concentrate HCl solution
(12N) by bubbling nitrogen for 20 minutes. The red solid became dark-
blue instantly. The yield was quantitative.

(b) Reaction with concentrate HCl solution. 0.020 g of the red
PBTPI was stirred with 20 mL of concentrate HCl solution (12N) for 4 h.
The red solid turned to dark-blue instantly. It was collected by suction

filtration and air dried at room temperature. The yield was quantitative.

5.2.5. Synthesis of Iodine-Doped PBTPI

(a) Doping with iodine vapor. A solution (THF) cast film of the
PBTPI on a quartz slide, was put into a closed chamber filled with iodine
crystals and stored for 4 h. The original orange film turned to dark-brown.

(b) Doping with iodine in acetonitrile. To a stirred 25 mL 0.1 M
iodine acetonitrile solution, 0.012 g red solid of the PBTPI was added. The
red powder turned black immediately. After stirring for 10 h, the black
solid was collected, washed several times with acetonitrile and vacuum
dried at room temperature overnight. The yield was quantitative. The

sample was pressed into pellets for electrical conductivity measurements.

5.2.6. Physicochemical Methods
Carbon, hydrogen and nitrogen elemental analyses were performed

by Oneida Research Services Inc., Whitesboro, NY. Elemental analyses

198

(semiquantitative) for sulfur and iodine were performed on a J EOL J SM-
6400V scanning electron microscope (SEM) equipped with a Tracor
Northern energy dispersive spectroscopy (EDS) detector. Molecular weight
of the polymer was estimated by Gel Permeation Chromatography (GPC)
method (relative to polystyrene standards, Mw in the range of 1,320 to
500,800) with Shimadzu LC-10AS liquid chromatography equipped with a
PL-GEL 511 MIXC column of length 300 m, using THF as an eluent at
room temperature. Infrared spectra were obtained in the transmission
mode with Nicolet IR-44 FT-IR spectrometer in the form of pressed KBr
pellets. UV-Visible-NIR absorption spectra (either in absorption or diffuse
reflectance mode) were obtained from a Shimadzu UV-3101PC double
beam, double-monochromator spectrophotometer. Nuclear magnetic
resonance spectra (1H and 13C) were obtained at room temperature using a
computer controlled Varian Gemini-NMR (300 MHz) spectrometer. The
chemical shifts are reported in parts per million (5, ppm) using the residual
solvent resonance peak as reference (CD3NOz, 5 4.33 ppm for 1H and 62.8
ppm for 13C). Solution photoluminescence spectra were measured in dilute
THF solution on a Hitachi F-4500 fluorescence spectrophotometer at room
temperature. Solid state photoluminescence spectra were obtained on a
SPEX fluorolog-2 (Model F111A1) spectrofluorometer at both 298 K (23
0C) and 77 K (-196 0C, liquid nitrogen). Powdered samples were loaded in
3 mm quartz tubing and sealed under vacuum (approx. 1.0 x 10'4 torr). X-
ray powder diffraction patterns were collected at room temperature on a
Rigaku powder diffractometer, Rigaku-Denki/RW400F2 (Rotaflex), using
Cu(K0t) radiation generated by a rotating anode operating at 45 kV and
100 mA. The data were collected at a scan rate of 0.5 deg/min. The

observed d-spacing was corrected according to an internal standard of Si

199

metal. Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) were performed on Shimadzu TGA—50 and DSC-50
under nitrogen or oxygen at 5 OC/min heating and cooling rate. Electron
spin resonance (ESR) spectra were recorded with a Varian EPR-E4
spectrometer with diphenylpicrylhydrazyl radical as g marker (g=2.0037).
Cylindrical quartz tubes were employed for powders. The conductivity
data were measured by the standard four-probe method on pressed pellets
at room temperature. Electrochemistry and cyclic voltammetry were
performed with a PAR 273 potentiostat/galvanostat equipped with a PAR
RE0091 X-Y recorder.

5.3. Results and Discussion

5.3.1. Polymer Synthesis and Characterization

Polycondensation of the dialdehyde (DFDBT) with 1,4-pheny1ene
diamine was first carried in anhydrous ethanol or dimethylsulfoxide
(DMSO) at room or elevated temperature (e.g. ~120 0C). However, the
resulting materials had low molecular weight, were obtained in low yield,
and were accompanied with a lot of unreacted starting materials. To
achieve high quality polymer and higher yield, we adopted a relatively new
method, i.e., solvothermal method,21 to carry out the polycondensation
reaction. Recently, hydrothermal24 or solvothermal methods have been
extensively and successfully used in the synthesis of unusual, high
crystalline, and high yield new inorganic materials.24 Similarly, the
polycondensation of the dialdehyde (DFDBT) with 1,4-phenylenediamine

can be carried out rapidly without any catalyst in a sealed tube under

200

ethanothermal conditions to give nearly quantitative yield, according to

equation (1).

s /\ s

 

0.5mLEtOH S /\ 8 CH N CH HO
sealedtube, \/ s \/ _ < > —' + 2

125 0C, 24 h eq.]

The resulting conjugated poly(azomethine) PBTPI is a red powder,
stable in air and water. The elemental analysis data of the polymer are in
good agreement with its expected structure (repeating units). PBTPI shows
a highly polycrystalline character as determined by the X-ray powder
diffraction (XRD) pattern, see Figure 5.1. Table 5.1 lists the observed d
spacings and relative intensities corresponding to ten reflection peaks of
PBTPI at 23 OC. Although the lattice constants cannot be unambiguously
determined with these few reflections, we have attempted to index the
peaks to obtain a reasonable unit cell of PBTPI. The observed d spacings
were corrected using internal standard of Si powder. Indexing of the
reflections was carried out using the TREOR90 and PIRUM programs

included in the Cerius2 molecular simulation software package.25

201

 

Intensity

 

 

'|.
A
l‘ A.*‘n_. A“
'W L‘-

A; A‘

‘7‘..-
‘ ' _v'

 

 

 

a I'TIUIUIF‘III‘IIIIIIIIII‘II‘TIUIIIIIIIIIIIIIVI'IIIII,[YIYII‘IIU‘VIVIIIUIII‘IIIIII'YIUIYIUIIII1IIIIU'IlIIU‘III‘I

-5. 10. 15. 20. 25. 30. 35. 4o. 45. 50. 55. so.
2 Theta

Figure 5.1. X-ray powder diffraction profile of pristine PBTPI.

202
Table 5.1. X-ray Diffraction Data for PBTPI at 23 OC.

 

 

hkl d(obsd) (A) d(ca1cd) (A) Intensitymbsd) (%)
(A) Orthorhombic Unit Cell(a)
001 14.82 14.81 62.3
101 11.84 11.83 100.0
111 9.36 9.36 17.7
120 7.24 7.24 28.6
310 5.86 5.86 34.4
022 5.381 5.382 28.0
130 5.034 5.035 46.6
410 4.528 4.532 49.0
303 3.889 3.889 77.7
510 3.682 3.682 30.0
(B) Monoclinic Unit Cell(b)
-101 14.82 14.81 62.3
-201 11.84 11.83 100.0
-111 9.36 9.37 17.7
210 7.24 7.25 28.6
-1 12 5.86 5.86 34.4
-221 5.381 5.384 28.0
220 5.034 5.028 46.6
400 4.528 4.528 49.0
-131 3.889 3.889 77.7
230 3.682 2.682 30.0

 

(a) a = 18.93 A, b = 15.67 A, c = 14.81 A, [3 = 90.00, v = 4393.14 A3, 2
= 8, dcalcd = 1.48 g/cm3.

(b) a = 24.11 A, b = 12.09 A, c = 14.82 A, (3: 131.370, v = 3243.83 A3,
Z = 8, dcalcd = 2.00 g/cm3.

203

Two tentative unit cells are derived from the indexed reflections: (A)
an orthorhombic unit cell with a = 18.93 A, b = 15.67 A, c = 14.81 A, v =
4393.14 A3 and (B) a monoclinic unit cell with a = 24.11 A, b = 12.09 A, c
= 14.82 A, and [3 = 131.370, V = 3243.83 A3. The calculated and observed
d spacings and the corresponding indices are given in Table 5.1. With the
above lattice parameters, the calculated density is approximately 1.48
g/cm3 and 2.00 g/cm3 for the cell A and B, respectively, assuming eight
chains per unit cell. It has been reported that the calculated density is 1.33
g/cm3 for 3',4'-dibuty1-pentathiophene and 1.26 g/cm3 for 3',3"",4',4""-
tetrabutyl-hexathiophene.26 We expect PBTPI to have a slightly higher
density than those of these related-oligothiophenes due to its polymeric
condensed nature. Furthermore the PBTPI is expected to possess a lower
density than that of polythiophene itself (dcalcd = 1.55 g/cm3)27 because of
the presence of n-butyl groups on the PBTPI backbone. The experimentally
measured density of PBTPI is about 1.26 g/cm3, which gives a low limit,
given the porosity of the powder sample and even the presence of a certain
fraction of amorphous material. Thus, according to the polymer density,
model A is a more reasonable unit cell than model B. In model A, the a-
axis of 18.93 A is close to the length of the repeat unit ~18.6 A, which is
calculated from the energy minimized structure of the repeat unit.25 The
polymer chains could lie in parallel to the a-axis.

Interestingly, neutral PBTPI is highly sensitive to strong acid
environment. Upon exposing to either acid fumes (HCl, HNO3) or strong
acid solution, the red PBTPI (neutral from) changes color rapidly to dark-
blue (referred to as the "protonated form"). The XRD powder patterns of
the dark-blue materials are very similar to that of pristine PBTPI. The

204

dramatic change of color is accompanied by significantly spectroscopic
changes as will be discussed later.

In contrast to PPI, pristine PBTPI is partly soluble in THF,
chloroform and NMP to give orange solutions and a red solid residue. The
molecular weight of the soluble fraction can be estimated by gel
permeation chromatography (GPC) using polystyrene standards and THF
as an eluent. Figure 5.2 shows a typical GPC chromatogram of PBTPI
representing the molecular weight distribution. The weight-average
molecular weight (Mw) is ~ 3.86 x 103 and number-average molecular
weight (Mn) is ~ 2.93 x 103 giving a polydispersity index (PD) of 1.32.
The Mn corresponds to an average of 6 repeat units (24 rings per chain).
Because the molecular weight of most poly(azomethines) are reported in
their intrinsic viscosities in concentrated H2804,19 the direct comparison of
the molecular weight of the new polymer with others is not available at this
moment. However, we notice that the soluble fraction has an intermediate
molecular weight, while the red, insoluble, residue is expected to have a
much higher average molecular weight.

Similar to PPI and other poly(azomethines),19 PBTPI is completely
soluble in concentrated sulfuric acid and nitromethane containing ca 2 wt%
aluminum trichloride. The resulting blue solution is stable in air when
stored in capped vials. The enhanced solubility of poly(azomethines) is
based on the Lewis acid-base complexation, which has been extensively
investigated.28 The imine nitrogen on the backbone of poly(azomethines)
provides a Lewis base site for effecting reversible complexation and
solubilization in organic solvents.

The chemical structures and properties of the polymers (neutral and

protonated forms) were investigated by NMR, FTIR, UV/vis/NIR, and ESR

[‘0
0
LI]

 

l

 

 

 

Intensity (arb. units)
11111111111111111

1 I I I I I I I I I I I I I I I I I I

l

I I 'I l I I I I I I I I I I

 

 

I I I l I 0 T I I I I I I

' I
0 10 20 30 40 50 60
Retention Time (min)

 

Figure 5.2. Typical GPC trace of the THF-soluble fraction of PBTPI at

room temperature.

206

spectroscopies. Photoluminescence spectroscopy was also used to explore

the excited state properties of the neutral polymer.

5.3.2. NMR Spectroscopy

The 1H NMR and 13C NMR spectra of PBTPI were obtained in
deuterated nitromethane containing aluminum trichloride. Figure 5.3
shows the 1H NMR spectra of PBTPI and its assignment. The number of
protons corresponding to each resonance based on integration of the
resonances of the NMR spectrum are in good agreement with the proposed
structure. The resonance peak at 11.22 ppm is tentatively attributed to the
terminal aldehyde and/or to small amount of residual dialdehyde monomer.
This is consistent with the presence of small peak due to aldehyde groups in
the FTIR spectrum of the neutral polymer (see below). If we assumed that
polymer chains have only aldehyde groups as their end-groups, we
estimate, from the ratio of the integration of the resonances of the aliphatic
and aldehyde group, about 3~4 repeat units (12~16 rings) per polymer
chain This corresponds to the molecular weight of 2000, which is lower
than the molecular weight of the polymer determined by the GPC methods.

The 13C NMR spectra of PBTPI is shown in Figure 5.4. It shows
four resonance lines in the aliphatic region at 32.9, 29.6, 24.0, and 14.2
ppm assigned to the four different carbons of the butyl group. There are
nine lines in the aromatic region as would be expected, which correspond
to the aromatic carbons on the polymer backbone. The number of
resonance peaks in the 13C NMR spectrum in conjunction with the 1H NMR
spectrum clearly confirm the proposed structure of the conjugated

aromatic PB TPI.

€0,1902

 

 

 

 

 

 

c-h a
be
CHO d
'TVVlvjvtrvtwvVY'TIYT[IVYIIIVIVIVVVYTTIrT‘TYVVIVIIfiTWII1'TI111111111171'1
12 11 10 9 8 7 6 5 4 3 2 1 O ‘1 99.

Figure 5.3. 1H NMR spectrum of PBTPI in CD3N 02/A1C13 and its

assignment.

208

 

 

 

 

 

 

 

. , . I .
u‘er—[II‘II‘ITITIIIIIITIIIII‘IIIIIIIIIIITIIIIT'IoTIII-IT‘TIIIIIIIIIITITIIIIIIIIIIIIIIIIIIIIIIOII'I.'III

1m 160 140 120 100 80 60 40 2.0 0 pom

Figure 5.4. 13C NMR spectrum of PBTPI in CD3N02/A1Cl3.

209
5.3.3. Infrared Spectroscopy

Figure 5.5 shows the Fourier transform infrared (FTIR) spectra of
the dialdehyde DFDBT, the neutral polymer and the protonated polymer as
KBr pellets (spectra A, B, C, respectively). The principal FI‘IR absorption
bands observed in the monomer and polymers, their assignments, and data
from other known related polymers are listed in Table 5.2. In spectrum A,
an absorption band at 1653 cm"1 clearly indicates the existence of the
aldehyde group in the DFDBT monomer, while in spectrum B its intensity
is drastically decreased while at the same time a new strong absorption
band at 1597 cm'1 appears which is consistent with the formation of imines
(C=N stretching). The strong, sharp absorption band at 1597 cm‘1 is
characteristic of extended conjugated polyazomethines.19 Also the new
bands at 1194 and 955 cm‘1 (in spectrum B) are due to the aromatic C-H
in-plane deformation vibrations, while the band at 843 cm'1 (arom C-H
out-of-plane deformation) is characteristic of 1,4-disubstituted (para)
benzene.29 The band at 799 cm‘1 (arom C-H out-of-plane deformation) is
characteristic of 2,5-coupled thiophene rings as has been observed in
PDBTT3O and other alkyl-substituted polythiophenes.31 Upon exposing to
hydrochloride acid (either vapor or solution), the red polymer turns to
blue immediately, and this is associated with a profound change in the IR
spectrum, as shown in spectrum C. The presence of a broad absorption of
medium intensity at 2573 cm'1 (N-H+ stretching) and 1638 cm"1 (C=NH+
stretching) suggests the formation of imine hydrochloride C=NH“',29,32
and thus, consistent with protonation by HCl, see Scheme 5.3. The shift of
C=N stretching band of azomethine or poly(azomethine) toward higher
energy, upon protonation or complexation with Lewis acids, has been

previously reported in the literature.19r32 The protonation or complexation

210

 

 

 

 

 

 

 

 

 

 

 

 

.11 l

/ 1f 0””

m/M/L/‘i
i; (B)

F .

Li
(C) r f

/\/ v

4015 T T 1301001 I T #20003 I 110100‘ 1400

Wavenumber (cm'l)

Figure 5.5. FP-IR transmission spectra (KBr pellets) of (A) dialdehyde
DFDBT, (B) pristine PBTPI and (C) prontonated (with HCl) PBTPI.

211

with Lewis acids causes a significant change in electronic structure, by the
means of coordination of a strong electron-withdrawing group to the imine
nitrogen (C=N), inducing of a redistribution of electron density throughout
the material.

The neutral polymer could be regenerated from the protonated form
by heating in a vacuo at 100 0C for 24h or by treating with a base solution.
The IR spectrum of the regenerated polymer (red color) is identical to that
of the pristine neutral polymer. The protonation and deprotonation
processes are completely reversible and can be repeated many times
without significant degradation. The level of protonation of the polymer is
about 1.5 per repeating unit (each repeat unit has 2 possible protonation
sites), as estimated from the results of the TGA studies and EDS/SEM
microprobe analysis (see below). Additional experimental evidence of this
reversible facile protonation and deprotonation include the dramatic

changes in the electronic spectra which will be discussed below.

212
(A)

red 3 /\ S CH=N N=CH>'
\/ s \/ _Q’ n

Heating HCl
or NaOH

H+ H+
Cl

(B)

red \8/ /S\ \S/ CH=N—ON=CH>;

H20 AlC13
[1103
S /\ S _ _
blue \ I S \ ICH_N—O—II—CH>;

A1C13

Scheme 5.3. Proposed structures of PBTPI: (A) protonated with HCl and
(B) complexed with AlCl3.

213

Table 5.2. Comparison of Infrared Band Positions (cm'l) and Their

Assignments for PBTPI and Related Polymers.

 

 

polymer C=N arom. aliph. ring methyl arom C-H
str CB-H str. C—H stretch stretch def out-of-plane
DFDBTa 3063 2955 2934 2853 1539 1508 1431 1381 795

PBTPI-Nb 1597 3067 2953 2926 2851 1505 1469 1441 1385 798

PBTPI-Pb 1638 3063 2951 2926 2863 1491 1426 802
PPIC 1610 850
PMOPIC 1605 840
PHOPIC 1600 840-870
PBTPVd 3063 2953 2926 2851 1459 1513 1441 1378 804
PDB'I'I‘e 3062 2951 2925 2856 1492 1456 1377 788

 

a) DFDBT-2,5"-diformyl-3',4'-dibutyl-2,2':5',2"-terthiophene

b) PB TPI-poly(3 ',4'-dibutyl-oc-terthiophene-azomethine- 1 ,4-phenylene-
azomethine), N-neutral polymer, P-protonated polymer.

C) see ref 19. PPI-poly(1,4-phenylenemethylidynenitn'lo-1,4-phenylene—
nitrilomethylidyne), PMOPI-poly( 1 ,4-phenylenemethylidynenitrilo-2,5-
dimethoxy- 1 ,4-phenylene-nitrilomethylidyne), PHOPI-poly( 1 ,4-
phenylenemethylidynenitrilo-Z,5-dihydroxy- l ,4-phenylene-
nitrilomethylidyne).

d) see ref 20. PBTPV-poly(3',4'-dibutyl-2,2':5',2"-terthiophene-1,2-
ethenylene- 1,4-pheny1ene- 1 ,2-ethenylene)

6) see ref 30. PDBTT-poly(3,4-dibutyl—0t-terthiophene)

214

5.3.4. Electronic Absorption Spectra of the Neutral

PBTPI and Complexes

The neutral PBTPI is partly soluble in THF, CHC13 and NMP, giving
orange solutions. The electronic absorption spectra of the soluble fraction
of the polymer in THF or NMP are shown in Figure 5.6. The absorption
maximum (Amax) of the n—n’“ transition in the THF and NMP solution is
457 nm and 464 nm, respectively. Figure 5.7 shows the electronic spectra
of PBTPI in sulfuric acid and AlCl3/CH3N02. The solution absorption
spectra of PBTPI in sulfuric acid and AlCl3/CH3N02 show the Amax of the
lowest energy 7t-7t* transition at 656 nm (with an onset of the absorption
edge at 1.66 eV) and 638 nm (with an onset of the absorption edge at 1.72
eV), respectively. Those absorption maxima occur at the longest
wavelengths among the known conjugated poly(azomethines).19 For
example, the solution electronic spectra of PPI, PMPI, PPI/PMPI, PMOPI
and PHOPI in sulfuric acid show the Amax of the lowest energy 7t-7t*
transition is in the range of 438—565 nm.19 Thus, our polymer contains the
longest conjugation length in concentrated sulfuric acid.

One notable aspect of the electronic spectra of the PBTPI in sulfuric
acid or AlCl3/CH3N02 is their significant red shift compared to the
solution spectra of neutral polymers in THF or NMP, which indicates a
dramatic change of the polymer electronic structure upon complexation.
These optical changes are likely due to excited-state perturbations by the
resonance effect of a strong electron-withdrawing group (e.g. AlCl3), as
has been reported in the protonation of Schiff bases.32 Theoretical
calculations show protonation lowers the energy of 11:* orbitals localized on
the C=N of the Schiff bases,32 but leaves the 71: orbitals essentially
unaltered, and as a result, decreases the HOMO-LUMO gap.32 It

 

Absorbance (arb. units)

Jllllllllllllllllllllllllll'llll

 

 

 

 

200 300 400 500 600 700 800
Wavelength (nm)

Figure 5.6. Solution UV-vis absorption spectra of PBTPI in THF (solid
line, Amax = 457 nm) and NMP (dash line, Amax = 464 nm).

 

lllllll

lllllll!

l J

Absorbance (arb. units)

328 nm

llllllllLLl

IjIIIIIIITIIIIIIIIIIIIIIIIIIIIIIII

 

 

 

TTIIIIFIIIIITIIIIII|TITIIIIIIIIIIIIIIII

200 300 400 500 600 700 800 9001000
Wavelength (nm)

 

 

 

 

 

 

I I L I I . I I I I 1
i"? - 638 nm 1
'E‘: (B) :
3 L L
43 : :
S J :
v - .
o . _
C .2 L'.
CU . ..
.o _
I... _ -
o _ _
VJ _ _-
.o _
<3 _, E.
I I I I I I I I I I I I I I I I—I I l I I I I I I I I I I I I I I | I I T I
200 300 400 500 600 700 800 900 1000
Wavelength (nm)

Figure 5.7. Solution UV-vis absorption spectra of PBTPI in (A) H2SO4
and (B) CH3N02 containing ca 2 wt% AlCl3.

217

is notable that a similar red shift of electronic spectra of some known
poly(azomethines) upon complexation has been previously attributed to
complexation-induced change in polymer conformation.19a However,
conformational changes are not as likely to cause such dramatic shifts in the
electronic spectra.

Figure 5.8 shows the electronic absorption spectra of thin films
casting from the THF solution of PBTPI. The lowest energy absorption
maximum (Amax), the optical absorption edge, and the corresponding solid
state bandgap of the polymer are summarized in Table 5.3 as compared to

the related conjugated polymers.
The bandgap of PBTPI is 2.10 eV with the Amax of 475 nm, as

compared to the 2.50 eV (Amax = 405 nm) bandgap of the PPI.l9 Recently,
Agrawal and Jenekhe have reported that the replacement of a phenylene
linkage by a thiophene linkage in the polyquinoline or polyanthrazoline
backbone produced a significant red shift of Amax by 64-106 nm and a
reduction of band gap by 0.3-0.5 eV.23 The authors attributed the effect to
the reduction of steric hindrance and the higher delocalized n-electron
density in the thiophene-linked conjugated polymer, which may involve the
possible delocalization of the lone pair electrons of the sulfur atom in a
thiophene ring. It has been known from our previous studies that the
dibutyl-substituted terthiophene building block is highly conjugated and less
steric hindered.30 Thus, the incorporation of the terthiophene units in the
conjugated poly(azomethine) backbone would dramatically enhance the
coplanarity of the flexible terthiophene rings with the rigid phenylene
azomethine (CH=N-C6H4) units, and improve the n-electron delocalization
along the polymer backbone. It has been suggested from theoretical

calculations on periodic conjugated block copolymers that the band gap and

218

 

l 1

l1
Irrllrllllrrlr

111411
A/

'1_ll

. V
e \ neutral

Absorbance (arb. units)

IIIIIIIIjl

 

 

 

ITI-liIIIIIITT‘IIII'IIIIIIII

O l 2 3 4 5 6 l .
Energy (eV)

\1

Figure 5.8. UV-vis-NIR absorption spectra of the THF-solution cast thin
films of PBTPI in (A) neutral form (B) iodine doped form.

219

electronic states of such copolymers are controlled more by the
contributions from the moieties of the lower band-gap homopolymer.33
Since the band gap of PBTPI is close to that of the homopolymer, PDBTT
(Eg ~ 2.0 eV), our experimental results seem to support such a theoretical
argument.

Upon doping with iodine, the solution cast films turn dark brown.
The UV—vis-NIR absorption spectra of films at the neutral and doped state
are compared in Figure 5.8. The n—n* absorption band at 2.69 eV
disappears upon oxidation and two new absorption bands appear at higher
energy (3.11 eV and 4.22 eV), while one new low energy absorption
appears at 1.94 eV. These results are different from those of PBTPV and
PDBTT, which forms two new subgap absorption bands (formation of
bipolaron transitions) upon oxidation by iodine.20’3O

The optical properties of the bulk polymer (in neutral or protonated
form) were also assessed by studying the UV/vis/Near-IR diffuse
reflectance spectra of these materials in the solid state. Absorption data
were calculated from the reflectance data using the Kubelka-Munk
function.34 The spectra of the neutral (red) and protonated (blue) forms of
bulk PBTPI are shown in Figure 5.9. From the steep absorption edges, the
band-gap (Eg) of the neutral and protonated PBTPI is estimated at 2.06 and
1.61 eV, respectively, by extrapolating the linear portions of the square of
the absorption coefficient, (on/S)2 vs. E plots to (a/S)2—->0. As we can
see, the band-gap of the neutral bulk polymer is only slightly smaller than
that of the soluble fraction of the same polymer. Thus, we believe that the
polymer has a similar electronic structure in solution as well as solid state.
The quick reversible response of this material to acid suggests its potential

application as a solid state pH indicator. To test the sensitivity of the

220

polymer to acid, we prepared a thin film (THF solution-cast) of the
polymer on a quartz slide, and then dipped it into aqueous hydrochloride
solution with a series pH values from 7 to 0 for the same amount of time (4
h). Figure 5.10 shows the change of optical absorbance (monitored at 1.8
eV) as a function of pH. It appears that the polymer only responds to
highly acidic media (pH 2 2), which is consistent with the low basicity of

the azomethine nitrogen atom.

Table 5.3. Electronic Absorption Maxima, Absorption Edge, and Solid-
State Bandgap of Conjugated Poly(azomethines).

 

 

polymer Xmax, nm absorption edge, nm Eg, eV
PBTPI 461 590 2.100
PPIb 405 496 2.50
PMPIb 406 497 2.49
PMOPIb 447 529 2.34
PHOPIb 494 600 2.07
PBPIC 490 2.53
PSPIC 520 2.38
1,5-PNIC 495 2.51
vad 405 512 2.43
PBTPVe 450 605 2.05

 

a Value from the THF-solution cast film. Bulk PBTPI has a smaller Eg ~
2.06 eV.
b Values from ref19 a .0 Values from ref 19 b.

d Values from ref 35. e Values from ref 20.

221

 

j /“\ ‘/ protonated
3 Eg=1.6l eV

Iljrrrlrllifil

 

neutral
Eg:2.06 eV

:rrllifirlrrlr

 

 

 

a/S Absorption Coeff. (arb. units)

IIFI1III|IIIIIIIIIIIT11|quTIIIII

O l 2 3 4 S 6
' Energy (eV)

\1

Figure 5.9. Optical absorption spectra of pristine and protonated (with

HCl) bulk PBTPI.

 

1 \\ pH=3

IIIIIIIII'IIIITIIIIj’IIII'II‘IIITIII

O I 2 3 4 S 6 7
Energy (eV)

0 1 F o I 1 I I 1 1 I I ' I I

Absorbance (arb. units)
III'IIIT'IIWIIITIITITIIIIIIYIIIII

[I

 

VIII

 

 

 

 

Absorbance (arb. units)

 

 

 

 

Figure 5.10. (A) UV-vis-NIR absorption spectra of THF-solution cast
thin films of PBTPI treated with HCl solution at various pH values. (B) A

plot of absorbance at 1.8 eV vs. pH values.

223

5.3.5. Photoluminescence Spectroscopy

Figure 5.11 shows the emission spectrum of the PBTPI in THF
solution at 23 0C when excited at 400 nm. Photoexcitation of the polymer
in THF solution results in broad band luminescence (halfwidth ~ 0.22 eV)
with a peak maximum at 2.26 eV (548 nm). In the solid state, the bulk
polymer emits red-light at 608 nm when excited at 400 nm at 23 0C, as
shown in Figure 5.12A. The broad tail toward longer wavelength indicates
the presence of longer polymer chains resulting from molecular weight
polydispersity. The excitation spectrum shows a peak at 576 nm when
emission wavelength monitored at 655 nm, see Figure 12B. At 77 K the
emission spectrum shows two peaks at 605 and 647 nm, while the excitation
spectrum shows a peak at 582 nm, when monitored at 655 nm. The
resolution of two emission peaks at low temperature is possibly due to
vibrational narrowing of the emission lines and is consistent with a
significant molecular weight distribution in the sample (see GPC results
above and difference in solubility). The PL data of the PBTPI in solution at
room temperature are comparable to those of PBTPV20 (555 nm), PPV35
(550 nm), and PDBTT3O (557 nm).

5.3.6. Thermal Analysis

The thermal stability of the polymer in its various forms was
examined by thermogravimetric analysis (TGA). The TGA thermograms
of PBTPI in nitrogen or oxygen atmosphere are shown in Figure 5.13. As
we can see, the onset of thermal decomposition of PBTPI is 370 0C in
flowing nitrogen and 330 0C in flowing oxygen. By comparison, the onset
of thermal decomposition of PPI is 504 0C in flowing nitrogen.19 The
observed thermal stability of PBTPI is lower than that of PPI as would be

224

 

548nm ”

I I I I I I

1111

I I

11_Ili11l11

Intensity (arb. units)

rtrlrrrlrrrlr

 

I .

 

 

I I I

450 500 550 600 650

Wavelength (mm)

Figure 5.11. Photoluminescence spectrum of PBTPI in THF solution at

room temperature.

 

L.
608 nm j
(A) _
12 /\
.t: 5/6 nm ,
c: _ -
D o
_0' EM .
a -
v ‘ EX “
3» _ -
.2; .
c - ..
8 .
C . .
I—vI ‘1 _
\Nh
ITTT'II IIIIIITIITTIII—I I I‘ II I III IITI'II I

 

 

 

400 450 500 550 600 650 700 750 800
Wavelength (nm)

 

i 605nm ;
A -8 nm .
3 r :
.._. J _,
C 2 I
:f ' 647nm t.
.0 r
:5 1 EX E‘
V .
22 EM 5
.5 . _
C '. L
o - _.
H 1-
C . .
o—a .4 l I-

'qIIIIlIITI‘IIrI[IIWTII[ITIIIIIITVTI'TTYT’Z

 

 

 

400 450 500 550 600 650 700 750 800
Wavelength (nm)

Figure 5.12. Photoluminescence spectra of bulk PBTPI solid at (A) 300
K and (B) 77 K.

226

expected, given the presence of the flexible pendant butyl groups in the
former. In spite of this, PBTPI has a reasonably good thermal stability,
comparable to its isoelectronic analog PBPT V (378 0C under nitrogen)20
and to the homopolythiophene poly(DBTT) (380 0C under nitrogen).30
Figure 5.14 shows the thermogram of the protonated polymer under
nitrogen. By comparison with the neutral polymer, the weight loss from 54
to 200 0C is assigned to the weight-loss of HCl (deprotonation process).
Above 200 0C, the TGA profile is very similar to that of the neutral
polymer. From the weight-loss due to HCl, we estimate the level of
protonation of the polymer to be about 1.5 per repeating unit (each repeat
unit has 2 possible protonation sites). This result is in good agreement of
the EDS/SEM microprobe analysis of the sulfur to chlorine ratio of 4.3.
These results suggest that 25% of the nitrogen sites are not protonated.
Differential scanning calorimetry (DSC) analysis of the neutral
polymer showed no evidence of a measurable glass transition or melting

point in the range of 30 to 330 0C.

5.3.7. Electron Spin Resonance (ESR) Spectroscopy

The structure homogeneity of the neutral and protonated polymers
was probed by ESR spectroscopy. Figure 5.15 shows the ESR spectra of
the neutral and protonated polymers at room temperature (23 OC). Both
materials give very weak ESR signals with the same g value of 2.0040 ,and
Apr of 7.6 and 8.5 gauss for the neutral and protonated polymers,
respectively. The spectra also show that the protonation of PBTPI did not
increase the number of spins in the polymer. Because of the extreme
weakness of the signals, we judge that both forms of the polymer contain

few structural defects.

 

 

 

 

 

 

 

..... I W01 I :L
100i /— 4” 370 C —

: (A) :
of 80— . L
5 i I
1.. - )-
ii 60.1 -
'3 . _
3 q :
40~ -

. i

:20 IIII II III I III II ' IIIIIIIIIIIII
0 100 200 300 400 500 600 700 800

Temperature (° C)

100:- 4/ 330°C ;

A 80; (B) __
59 ‘ C
E 603 L
E j I
.9.” .
a) 40- _
3 :
20~ \v’ i

0 III Ii|0|I|IIIIIIIIItIIII.I II [IIIIITIII

 

 

 

O 100 200 300 400 500 600 700 800
Temperature (0 C)

Figure 5.13. TGA thermograms of pristine bulk PBTPI (A) under

nitrogen flow and (B) under oxygen flow.

228

 

 

 

110_-.11:-.-.n....n.........c..., ,,,,,,,,,

100‘: '\ -10wt% 371 OC :—

.: \ L —

90— f3 __

8:) 80~ / f_
3 2 o :

if 70; 00 C _
.E‘.’ ., ..
0.0 I :

g3 6O‘: _

50% 5.

40% .-
30:,,4.,....,....,...T,....,.,,, M.,...

 

0 ’100 200 300 400 500 600 700 800
Temperature (°C)

Figure 5.14. TGA thermogram of protonated (with HCl) bulk PBTPI

under nitrogen flow.

228

 

110-..,.l....1....t.,,.l....
100g \ -10 wt% 371°C Z—

': \\ -

90': W; E-

98 804:? / i
3 : 0 E
I: 70—j 200 C ~_
41" - :
0.0 3 -
g 60: E
50% g.
40-3 E
30:.17.]..--|u....r1rr‘.r.r‘1...,....l111;

 

 

 

O .100 200 300 400 500 600 700 800
Temperature (°C)

Figure 5.14. TGA thermogram of protonated (with HCl) bulk PBTPI

under nitrogen flow.

229

 

60- . . . l . , . I r . v I . . .
40‘. (A) :
20: l
0-‘ _ _

-20: L

Intensity (arb. units)

.40; '—

 

 

 

—60
3300 3320 3340 3360 3380 3400 3420
Field (Gauss)

 

30‘---

' I

-16;

Intensity (arb. units)
0
l

 

 

 

 

-20.: 1
‘30 r I I r I - . I I I 1 n ‘
3300 3320 3340 3360 3380 3400 3420

Field (Gauss)

Figure 5.15. ESR spectra of (A) pristine PBTPI and (B) protonated
PBTPI at room temperature. Experimental setting: frequency, 9.425 GHz;
power, 1 mW; gain, 2 x 104; center field, 3360 G; scan range, 100 G; time

constant, 0.3 sec; scan time, 16 min.

230

5.3.8. Electrochemistry

The redox properties of both neutral and protonated PBTPI were
investigated by cyclic voltammetry. PBTPI thin films were solution-cast
onto a Pt-plate as a working electrode. Figure 5.16 shows a typical cyclic
voltammogram (CV) of the neutral polymer film in acetonitrile solution
containing 0.1 M Bu4NClO4. The first negative scan from 0.0 to -1.0 V vs.
SCE was featureless while the film stayed orange. During the first cycle
from 0.0 to +1.4 V, the as-deposited orange thin film on the Pt electrode
turned blue after oxidation at 1.23 V and remained blue after reduction at
0.90 V. Further scans between -1.0 to 1.4 V did not show well-defined
redox waves. Figure 5.17 shows a CV of the blue protonated film
(prepared by exposing the red neutral film to HCl vapor) in acetonitrile
solution containing 0.1 M Bu4NClO4. The first negative scan from 0.0 to
-1.0 V shows a broad reduction wave at -0.69 V vs. SCE, which may be
due to the reduction of proton to hydrogen. After the negative scan, the
film turned to orange. Further repetitive cathodic scans from 0.0 to -1.0 V
result a decrease of the cathodic current, while the film stays orange. Then,
the following first positive scan from 0.0 to 1.2 V turned the film to blue,
and showed an anodic peak at 1.10 V and a cathodic peak at 1.06 V.
Further scans from 0.0 to 1.4 V did not show well-defined redox waves.
The detailed elucidation of the electron or proton-transfer process of the

neutral and protonated forms of PBTPI needs further investigation.

231

~l.00V

 

Potential (V) vs. SCE

11.23 V
1.30 V

Figure 5.16. Typical cyclic voltammogram of a solution-cast PBTPI film
on a Pt electrode in CH3CN/0.1 M (Bu4N)ClO4. Scan rate, '20 mV/s.

 

-O.69 v
i
1.06 V ?
i / -100 V
W +—
i 0.00 v

Potential (V) vs. SCE

 

 

<——-—>—
-—>

1.10V

Figure 5.17. Typical cyclic voltammogram of a solution-cast PBTPIgfilm
protonated with HCl on a Pt electrode in CH3CN/01 M (Bu4N)ClO4. Scan

rate, 20 mV/s.

233
5.3.9. Electrical Conductivity

.The electrical conductivity of iodine d0ped polymers (both the
neutral and protonated materials) and the undoped materials were
measured by the standard four-probe method on pressed pellets at room
temperature. Both the neutral and protonated polymers are insulators (6 ~
10'9-10‘10 S/cm). Both the iodine-doped materials have conductivities of
10'7~10‘8 S/cm. These conductivities are much lower than those of
reported poly(Schiff bases) (0 ~ 10'3-10'4 S/cm).16b The low conductivities
are probably due to the fact that the polymer is difficult to oxidize, as

suggested by the CV studies mentioned above.
5.4. Conclusion

A new derivative of conjugated aromatic poly(azomethine)
containing the alkyl-substituted oligothiophene was prepared under the
ethanothermal reaction conditions. The resulting polymer, PBTPI, has
improved solubility in organic solvents as imparted by the dibutyl-
substituted terthiophene linkages on the polymer backbone. Replacement of
a phenylene linkage by an oligothiophene linkage in the poly(azomethine)
backbone also improved the coplanarity and the n-electron delocalization
of the polymer and resulted in a significant reduction of band gap by ~ 0.4
eV. The new poly(azomethine), PBTPI, has an optical band-gap of 2.06
eV, which is the lowest among conjugated aromatic poly(azomethines), and
comparable to its isoelectronic counterpart PBTPV. However, unlike
PBTPV, the incorporation of imine nitrogen on the polymer backbone
provides a Lewis base site for effecting reversible protonation and

complexation. Thus, PBTPI is completely soluble in concentrated sulfuric

234

acid and nitromethane containing Lewis acids (e.g., AlCl3). Protonation is
reversible and causes the color of the polymer to change from red to blue,
and reduce the band-gap to 1.61 eV. This band-gap narrowing is attributed
to the strong electron-withdrawing effect of the protons attached to the
imine nitrogen. The electrical conductivity of the iodine doped PBTPI
remains low in the order of 107-10'8 S/cm. The reactivity of the polymer
with strong acids and the low band gap of the protonated form may have

potential applications in solid state acid indicators and photonic materials.

(1)

(2)

(3)

(4)

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235
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238

During our preparation of this manuscript, a poly(azomethine)
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Macromolecules 1993, fl, 1188-1190.

CHAPTER 6

Poly(3,4-ethylenedithiathiophene). A New Soluble
Conductive Polythiophene Derivative

242

ABSTRACT

A new polythiophene derivative has been synthesized by both
chemical and electrochemical oxidative polymerization of the monomer
3,4-ethylenedithiathiophene (EDTT). Both methods produce a polymer
which is completely soluble in 1-methyl-2-pyrrolidinone (NMP) and partly
soluble in tetrahydrofuran (THF) and chloroform. The FT-IR spectra of
the neutral polymer indicate a regular structure formed via 0t,0t coupling
of thiophene rings. The new polymer shows two absorption bands at 341
nm and 413~419 nm in NMP solution in the UV-vis region.
Photoexcitation of the polymer in dilute NMP solution results in a broad
band luminescence with peak at ca. 552 nm. The redox behavior of the
polymer was studied by cyclic voltammetry in 0.1 M (Bu4N)ClO4
acetonitrile solution. The average molecular weights have been determined
by gel permeation chromatography (GPC) to be Mn~ 3.03x103 and Mn~
4.75x103 for the chemically and electrochemically synthesized polymer,
respectively. Electron spin resonance data are reported. Thermal
gravimetric analysis studies show that the new polymer is stable in nitrogen
up to 276 0C. The chemically doped (with FeC14') polymer and the
electrochemically doped (with C104') polymer show electrical conductivity
of 0.1 S/cm and 0.4 S/cm at room temperature, respectively. These results

are compared to some previously characterized polythiophenes.

243

6.1. Introduction

Polythiophenes and their derivatives have been investigated intensely
because of their interesting electrical and electronic properties and
relatively good environmental stability.l Significant experimental and
theoretical effort2 has been focused on the modification of their chemical
structures, in order to alter their electronic structures, and improve their
electrical properties, environmental stability, and processability. For
example, substituting long alkyl chains on the 3-position of thiophene
affords a series of processable and highly conductive poly(3-
alkylthiophenes).3 This effect has been attributed to the higher solubility of
polymers with long alkyl chains and suppression of unfavorable 2,4'-
couplings that interrupt the conjugation system along the polymer
backbone. Furthermore, using regiochemically defined oligothiophenes can
lead to more regular conjugated polymers, due to the reduced number of
2,4-mislinkages.4 Fusing a benzene ring on the 3,4-position of thiophene
leads to a low band-gap (1.0 eV) "transparent" conducting
poly(isothianaphthene).5 This happens because the more stable planar
quinoid form of the thiophene is favored with the presence of the benzene
ring.6 Recently, the synthesis of nearly 100% head-to-tail poly-
alkylthiophene has been reported, which shows great improvement of
conjugation chain length and high electrical conductivity.7 In addition to
the numerous studies on poly(3-alkylthiophenes), polymers generated from
3,4-dia1kylthiophene monomers also attracted considerable interest due to
the fact that no possibility exists for (1,13' or [3,[3' coupling of monomers
during the polymerization process. This approach has been drastically

limited by the large steric hindrance of the two substituents,8 which forces

244

the polymer backbone out of planarity and results in the loss of conjugation
and reduced electrical conductivity. However, cyclization between 3,4-
positions of thiophene considerably reduces the steric hindrance.9 Recently,
we reported the synthesis and properties of a new polythiophene derivative
poly(3',4'-dibutyl-2,2':5',2"-terthiophene) [poly(DBTT)],10 as another
approach to reduce the steric hindrance and yield an ordered, soluble,
conjugated polymer. In the poly(DBTT) backbone, every dibutyl-
substituted thiophene unit is separated by two non-substituted thiophene
units (acting as steric diluents). The reduced number of side chains
provides more space for the extension of the two butyl groups, and
minimizes extensive steric effects, and thus leads to a more coplanar
conjugated polymer. In solution and in the solid state the poly(DBTT)
appears to possess one of the longest chain conjugation lengths among
polythiophenes.

Relative to mono-substituted polythiophenes, di-substituted
polythiophenes are less well explored. To date, there have been only a few
reports on the study of mercapto-substituted polythiophenes, in addition to
alkyl, alkoxy and mixed alkyl, alkoxy disubstituted polythiophenes.11
Poly[(3-methy1mercapto)thiophene] (PMMT) has been prepared both
chemically and electrochemically,8a,12 and shows conductivity in the order
of 10'1~10'2 S/cm. Poly[3-(ethylmercapto)thiophene] (PEMT) and
poly[3,4-bis(ethylmercapto)thiophene] (PBEMT) have been synthesized via
a nickel catalyzed Grignard coupling reaction of the corresponding 2,5-
dihalogeno monomers.13 Both of the polymers are soluble in common
organic solvents and semiconducting (10'3 S/cm and 10'7 S/cm ,
respectively) in the oxidized state. However, both the 3-

(ethylmercapto)thiophene (EMT) and 3,4-bis(ethylmercapto)thiophene

245

(BEMT) monomers fail to be electrochemically polymerized because
neither monomer has significant positive spin density at both a-carbons on
the thiophene rings according to the theoretical calculations.13 Recently, a
new polythiophene derivative-poly(3,4-ethylenedioxythiophene) (PEDT)
has been briefly reported.l4 PEDT can be synthesized by either chemical
or electrochemical polymerization of the corresponding 3,4-
ethylenedioxythiophene (EDT) monomer and results in the conductivity of
15~19 S/cm and 200 S/cm, respectively. However, PEDT is insoluble and
infusible, which limits its further characterization. When a solution of the
3,4-ethylenedioxythiophene (EDT) monomer reacts with a thin layer of
polyvinylacetate containing an iron(III) salt, a transparent film (1 um
thickness) of PEDT has been claimed to form. In this paper we report on
the synthesis and characterization of the sulfur analog of PEDT i.e.
poly(3,4-ethylenedithiathiophene) [poly(EDTT)], which represents a new
type fused-ring mercapto-disubstituted polythiophene. In contrast to PEDT,
the poly(EDTT) is completely soluble in NMP and partly soluble in THF,
CHCl3, and other common organic solvents. This advantage lends this
polymer to a more detailed spectroscopic, physicochemical and charge
transport characterization than PEDT. The properties of poly(EDTT) are

compared with some known related polythiophenes.
6.2. Experimental Section

6.2.1. Materials
1,2-Dibromoethane, n-butyl-lithium, carbon disulfide and hydrazine
hydrate were used as received from Aldrich Chemicals Inc. 3,4-

dibromothiophene was used as received from Lancaster Synthesis. Tetra-n-

246

butylammonium perchlorate was purchased from GFS Chemicals Inc. and
used without further purification. Acetonitrile (HPLC grade) and 1-
methyl-Z-pyrrolidinone (NMP) (HPLC grade) were used as received.
Other solvents were distilled and degassed prior to use. All reactions were
performed under an atmosphere of nitrogen or argon with either standard

Schlenk or dry box techniques.

6.2.2. Synthesis of Thieno[3,4-d]-1,3-dithiole-2-thione

A solution of 3,4-dibromothiophene (4.95 g, 20.5 mmol) in
anhydrous diethyl ether (30 mL) was cooled to -78 OC (dry ice and acetone
bath) under nitrogen. To this stirred pre-cooled solution, n-butyl lithium
(8.2 mL, 20.5 mmol, 2.5 M in hexane) was added via syringe. The solution
was stirred for 0.5 h, then sulfur (0.66 g, 20.6 mmol) was added and
stirred for 1 h. Another portion of n-butyl lithium (8.2 mL, 20.5 mmol,
2.5 M in hexane) was added via syringe and stirred for another 0.5 h. To
the reaction mixture, additional sulfur (0.66 g, 20.6 mmol) was added and
stirred for an additional 1 h. Finally, the mixture was allowed to come to
room temperature and the solvent was removed under vacuum to get a
yellow solid. To the yellow solid, 2N sodium hydroxide solution (50 mL)
and carbon disulfide (20 mL) were added. The mixture was refluxed under
nitrogen for 6 h and then allowed to stand at room temperature overnight.
The excess carbon disulfide was removed under vacuum, and the dark
reaction mixture was filtered and washed with 2X30 mL of water to give a
yellow solid. Recrystallization of the solid from dichloromethane-hexane
(1:5 (v/v)) gave 0.85 g (22% yield) of thieno[3,4-d]-1,3-dithiole-2-thione
as amber needles. (lit.15 33% yield) mp: 142 0C; GC-MS (in ether

247

solution) (m/z) (relative intensity): 190 (100), 146 (71.5), 126 (15.6),
82(10). lH-NMR (CDC13): 5 7.21 (s, 2H).

6.2.3. Synthesis of 3,4-Ethylenedithiathiophene (EDTT)

Potassium metal (0.284 g, 7.28 mmol) was added in one portion into
stirred, freshly distilled, methanol (75 mL) under nitrogen. After the
potassium dissolved completely, thieno[3,4-d]-1,3-dithiole-2-thione (0.380
g, 2.0 mmol) was added to the solution. The reaction mixture was left to
react for 1~2 h at 50 0C under nitrogen, and gave a clear yellow solution.
To this yellow solution, 1,2-dibromoethane (0.2 mL, 2.32 mmol) was
added via syringe. After 24 h at room temperature, the methanol was
removed under dynamic vacuum, and anhydrous ether (30 mL) was added.
A yellow liquid of 3,4-ethylenedithiathiophene was obtained in 87.6% yield
(0.305 g) at room temperature when the ether was removed from the
yellow filtered extract. GC-MS: (m/z) (relative intensity) 174 (87), 159
(100), 146 (38), 82 (22); 1H NMR (CDC13): 5 6.95 (s, 2H, aromatic), 3.21
(s, 4H, methylene); 13C NMR (CDC13): 5 125.08 (aromatic, B-C), 118.06
(aromatic, 0t-C), 27.94 (methylene); FT-IR (KBr pellet): 3091(m),
2956(m), 2915(m), 1472(m), 1411(m), 1385(w), 1324(m), 1287(m),
857(3) and 772(s). UV-Vis ( CHCl3): kmax=283 nm.

6.2.4. Chemical Polymerization of 3,4-Ethylenedithia-

thiophene
Anhydrous FeCl3 (0.57 g, 3.5 mmol) was dissolved in 80 mL of

CH3CN and stirred for 10 min. To this red-orange solution, a solution of
3,4-ethylenedithiathiophene (0.15 g, 0.86 mmol) in 20 mL of CH3CN was

added dropwise. Dark-green precipitate formed immediately. The reaction

248

mixture was stirred for about 24 h under nitrogen at room temperature.
The dark-green solid was filtered through a glass frit, and washed with
3X20 mL of fresh CH3CN, and dried under vacuum for 24 b. An amount
of 0.076 g of product was obtained in doped form ( ~ 0.25 FeCl4“ per
repeat unit). The neutral form of the polymer was obtained by first
Soxhlet-extraction of the green solid with methanol, followed by acetone,
and then treated with 20 mL of N2H4-H20 for 12h. The resultant mixture
was filtered and Soxhlet-extracted again with methanol, then dried under
vacuum for 12 h. Brown neutral poly(3,4-ethylenedithiathiophene)
[poly(EDTT)-C] was obtained in 43% yield (0.065 g). Elemental analysis
by EDS showed almost no impurity of Fe and C1 (<0.5%). Calcd (%) for
C6H4S3 C 41.9; 2.30. Found (%), C 40.38; H 2.34.

The neutral polymer can be redoped by reaction with either 0.1 M
FeCl3/CH3CN solution or 0.1 M Iz/CH3CN solution. The solution-cast

films (using NMP solution) can be doped with 12 vapor in a closed

chamber.

6.2.5. Electrochemical Polymerization of 3,4-Ethylenedi-

thiathiophene

Electrochemical polymerization was carried out in potentiostatic
conditions at the oxidation potential of the EDTT monomer (Ea, mm). at
ambient temperature in a three-electrode single-compartment cell
containing 5 mM monomer and 0.1 M (Bu4N)ClO4 in 20 mL acetonitrile
(HPLC grade; Aldrich). The working electrode was either a platinum disc
electrode of 0.015 cm2 area or a platinum plate electrode of 1.8 cm2 area.
The counter electrode was a platinum wire. A saturated calomel electrode

(SCE) was used as the reference electrode. The solution was degassed by

249

argon bubbling for 20 min prior to use and maintained under an argon
blanket throughout each experiment. Thin films for electrochemical
characterization were deposited on the small area platinum disc electrode at
the oxidation potential of the EDTT for 1 min. The polymer-covered
working electrode was then removed and washed with fresh acetonitrile
and dried, then transferred to another cell containing monomer-free 0.1 M
(BU4N)ClO4 acetonitrile solution for cyclic voltammetric analysis. Bulk
doped polymer films were prepared on the larger area platinum plate
electrode under the similar conditions by using ~ 20 mM (0.070g)
monomer in 0.1 M (Bu4N)ClO4 acetonitrile solution and holding at the
oxidation potential of the EDTT for 45 min. After deposition, the films
were rinsed with neat acetonitrile and dried under vacuum overnight. The
doped polymer has ~ 0.33 C104" per repeat unit. Neutral (undoped)
polymer poly(EDTT)-E were obtained by electrochemically reducing the
doped films at -0.4 V/SCE until the residual cathodic current reached a
constant value, then rinsed with neat acetonitrile, and were further
chemically reduced by hydrazine hydrate under nitrogen for 12 h. They
were washed with methanol, dried under vacuum overnight. Brown
powder of neutral poly(3,4-ethylenedithiathiophene) [poly(EDTT)-E] was
obtained in 13% yield (0.009 g). EDS showed no chloride impurity. Calcd
(%) for C6H4S3 C 41.9; 2.30. Found (%), C 39.80; H 2.75.

6.2.6. Physicochemical Measurements

Elemental analyses (semiquantitative) were performed on a JEOL
ISM-35C scanning electron microscope (SEM) equipped with a Tracor
Northern energy dispersive spectroscopy (EDS) detector. Infrared spectra

were recorded as KBr pressed pellets on a Nicolet 740 FT-IR

250

spectrometer. Carbon and hydrogen elemental analyses were performed by
Oneida Research Services Inc., Whitebor NY. UV-Visible-NIR absorption
spectra were obtained from a Shimadzu UV-3101PC double beam, double-
monochromator spectrophotometer. Nuclear magnetic resonance spectra
(1H and 13C) were obtained using a computer controlled Varian Gemini-
NMR (300 MHz) spectrometer. The chemical shifts are reported in parts
per million (5, ppm) using the residual solvent resonance peak as reference
(CHCl3, 8 7.24 ppm for 1H and 77.00 ppm for 13C). Fluorescence emission
and excitation spectra were measured on a Perkin Elmer LS-5 fluorescence
spectrophotometer. Thermogravimetric analysis(TGA) and differential
scanning calorimetry(DSC) were performed on Shimadzu TGA-50 and
DSC-50 under nitrogen or oxygen at 5 0C/min heating rate. Electron spin
resonance (ESR) spectra were recorded with a Varian EPR-E4
spectrometer with diphenylpicrylhydrazyl radical as g marker (g=2.0037).
Cylindrical quartz tubes were employed for powders. The conductivity
data were measured by the standard four-probe method on pressed pellets
as a function of temperature as described elsewhere16. Molecular weight of
the poly(EDTT)-C and poly(EDTT)-E was estimated by Gel Permeation
Chromatography (GPC) (relative to polystyrene standards, Mw in the
range of 3,120 to 500,800) with Shimadzu LC-lOAS liquid chromatograph
equipped with a PL-GEL 10u (MIXED-B) column of length 300 mm,
using 1-methyl-2-pyrrolidinone (NMP) with and without containing 0.5%
by weight LiCl as an eluent. Electrochemical polymerization and cyclic
voltammetry were performed with a PAR 273 potentiostat/galvanostat

equipped with a PAR RE0091 X-Y recorder.

251

6.3. Results and Discussion

6.3.1. Monomer and Polymer Syntheses

The precursor thieno[3,4-d]-1,3-dithiole-2-thione was prepared from
3,4-dibromothiophene using a slightly modified procedure reported by
Chiang et al.15 Potassium reacts with the thione in methanol to form the
dipotassium salt intermediate via the C-S bond cleavage. The dipotassium
salt of the thiophene-dithiolate intermediate (2) reacts with dibromoethane
to give the new monomer 3,4—ethylenedithiathiophene (EDTT) in good
yield (87.6%), as shown in Scheme 6.1.

Br
=8

s ‘ S
\ , MeOH \ SK Br \
s >=s +3.6 K s
/ 50 °C / RT, 1d /
S 1h ‘ SK 3

Scheme 6.1. Synthetic route of 3,4-ethylenedithiathiophene.

 

The structure and purity of the monomer were confirmed by FT -IR, 1H
NMR, and 13C NMR results as described in the experimental section.
Oxidative Polymerization. Both chemical and electrochemical
methods have been used in the oxidative polymerization of aromatic
heterocycles.17 In order to distinguish the origin of the polymer we will
use the designation poly(EDTT)-C and poly(EDTT)-E for the chemically

and electrochemically prepared material respectively. For chemical

polymerization we used FeC13 as a convenient oxidant to polymerize
EDTT, followed by reduction with hydrazine hydrate (N2H4-H20) to
obtain the undoped polymer.

252

 

s 's S
(Ham y+
+4FeC|
/ \ 3 24mm 88/ \n [FeC'4 1y
s
In order to determine he proper conditions for

electropolymerization, we first examined the cyclic voltammetry (CV) of
the EDTT monomer. During the first anodic scan, an solution of EDTT
exhibited a rapid increase in current at the electrode with an onset of 1.15
V vs. SCE as illustrated in Figure 6.1A. Application of repetitive potential
scans (between 0.0 V and 1.2 V vs. SCE) to the monomer solution resulted
in a new anodic process well below the onset of oxidation of the monomer.
The intensity of this new anodic process (at ~ 0.91 V vs. SCE, scanning at
100 mV/s) increases, as shown in Figure 6.1B. This is consistent with
conducting polymer deposition on the anode surface, and is confirmed by
the formation of small amount of dark-green deposit on the electrode. The
oxidation potential of the EDTT monomer is Ea mom: 1. 15 V vs. SCE
which 1S lower than that of thiophene (T, Ea, mon = 1.65 V/SCE) and 3-
(methylmercapto)thiophene (MMT, Ea, mon = 1.30 V/SCE).8a The trend
in oxidation potentials is consistent with the presence of two electron-
donating sulfur atoms on the [5 positions of the thiophene ring. The low
oxidation potential of the EDTT monomer demonstrates the ease of
formation of the radical cations and suggests polymerization will occur
with fewer side reactions. Two of the possible side reactions avoided here
include the formation of [3 linkages and overoxidation of the polymer. By
holding the working electrode potential at 1.15 V/SCE for 0.5 h, a thick
dark green, smooth deposit was obtained which could not be peeled off as a
free-standing film because of its somewhat brittle nature. By stripping off

the outer layer with adhesive tape it was possible to measure the electrical

253

 

(A) 1,“,

Current

 

0.0— ::

 

 

l l l l l l l
0.0 0.2 0.4 0.6 0.8 1.0 1.2

Potential (V) vs. SCE

 

 

Current

 

 

 

m 1 1 1 1 1 1
0.0 0.2 0.4 0.5 0.8 1.0 1.2

Potential (V) vs. SCE

Figure 6.1. Cyclic voltammogram of 3,4-ethylenedithiathiophene
(EDTT) monomer (5 mM) in CH3CN/0.1M (Bu4N)ClO4 on a Pt disk
electrode. Scan rate, 100 mV/s. (A) First scan showing nucleation loop on
the Pt electrode. (B) Fourth to eighth scan showing polymer deposition on

the Pt electrode.

254

conductivity of the polymer. A pressed pellet was used for the four-probe
variable temperature electrical conductivity measurements (see below).

The successful oxidative polymerization of the EDTT monomer
stands in contrast to EMT and BEMT which cannot polymerize under the
same conditions.13 Based on the spin density argument advanced for EMT
and BEMT radical cations, it can be concluded that in the EDTT radical
cation, the positive spin density is mainly localized on both 01 positions of
the thiophene ring. However, theoretical calculations of the cation-radical
spin densities for the EDTT monomer are needed to confirm this assertion.

Regardless of the polymerization method, poly(EDTT) is completely
soluble in N MP and partly soluble in THF and CHC13. This is in contrast to
its oxygen analog, PEDT, which is totally insoluble. The solubility of
poly(EDTT) allows us to do a detailed spectroscopic characterization of
this new polythiophene derivative. The powder X-ray diffraction (XRD)
pattern of the neutral poly(EDTT)-C shows that it is amorphous.

6.3.2. Polymer Electrochemistry

The electrochemistry of both poly(EDTT)-C and poly(EDTT)-E was
studied by cyclic voltammetry. Figure 6.2A shows a representative cyclic
voltammogram(CV) of a directly electrogenerated poly(EDTT)-E film on
a Pt electrode in 0.1 M Bu4NClO4/CH3CN solution at scan rate of 20
mV/S. The Epa and Epc values are linearly depended on scan rate. The
CV of electrogenerated poly(EDTT)-E exhibits a broad, weak, first anodic
peak at +0.54 V/SCE and a sharp strong second anodic peak at + 0.86 V vs.
SCE. The latter is associated with one broad cathodic peak at ca + 0.70V/
SCE. These two oxidation steps seem to be related to the formation of

polarons and bipolarons, as has been seen in several polythiophene

255

derivatives.18 The high symmetry and narrowness of the second anodic
peak (half-height width = 84 mV) suggests relatively homogeneous
conjugated chains. The second anodic peak currents vary linearly with scan
rates in the range of 0.02~0.1 V/ S as shown in Figure 6.23, which indicate
the surface-confined nature of the species, as has been observed in many
polythiophenes.19 No significant loss of electroactivity was seen after 50
cycles as the applied potential swept continuously between 0.0 V and 1.2 V
vs. SCE. The film was brown-yellow in the neutral state and dark-green in
the oxidized state.

For comparison, the electrochemical behavior of the chemically
synthesized poly(EDTT)-C in the solid state was also investigated with
solution-cast films (using NMP solution) on a Pt electrode. Figure 6.3(A)
shows a representative cyclic voltammogram of a cast film of poly(EDTT)-
C. It exhibits a reversible redox process with an anodic peak potential
(Epa) at 0.92 V and a cathodic peak potential (Epc) at 0.69 V vs. SCE and
at 20 mV/s. The anodic peak is noticeably sharper than the cathodic peak
and the narrow peak width at half-height of about 84 mV indicates a
homogeneous and relatively narrow distribution of conjugation lengths
among the polymer chains. This is in agreement with the relatively small
polydispersity found from the GPC molecular weight measurements (see
below). A large potential hysteresis (AEp=0.23 V) is attributed to a
number of factors including the ease of diffusion of dopant ions in and out
of the film, film thickness, and conformational relaxation of polymer
chains between the rigid planar oxidized and flexible neutral states. The

film was brown-yellow in the neutral state and dark-green in the oxidized

 

 

 

 

 

 

 

 

 

 

(A)
>
.‘L‘.’
(D
C
G)
C3
d—J
C
Q)
L
L.-
:3
00.0-
I . l l l l l L
0.0 02 0.4 0.5 0.: 1.0 1.2
Potential (V) vs. SCE
25 #1 1 1 l 1
s ‘8’ .
2..
U
2
E
V 1.5-1 r-
5‘
2 o
a 1. -
‘a
E 0.5- .
:3
L)
V0 0.32‘ a; a}; o;- 0T1 0.12

ScaanWlsec)

Figure 6.2. (A) Typical cyclic voltammogram of a directly
electrogenerated poly(EDTT)-E film on Pt electrode in CH3CN/0.1M
(Bu4N)ClO4. Scan rate, 20 mV/s. (B) Plot of peak current density vs. the
scan rate for a directly electrogenerated poly(EDTT)-E film on Pt
electrode in CH3CN/0.1M (Bu4N)ClO4.

257

 

(A)

   

Y

10.05 mA . cm‘2

  

Anodic

 

p
o
l

Current Densit
Cathodic

 

 

 

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Potential (V) vs. SCE

 

Current Density (mA/cml)

 

 

 

 

1 1 fi I
0 0.02 0.04 0.06 0.08 0.1 0.12

Sam Rate (Vlsec)

Figure 6.3. (A) Typical cyclic voltammogram of a cast poly(EDTT)-C
film on Pt electrode in CH3CN/0.1M (Bu4N)ClO4. Scan rate, 20 mV/s. (B)
Plot of peak current density vs. the scan rate for a cast poly(EDTT)-C film
on Pt electrode in CH3CN/0.1M (Bu4N)ClO4.

258

state. No significant loss of electroactivity was seen after 30 cycles. The
peak currents vary linearly with scan rate (see Figure 6.33), as was
observed in the electrochemically prepared polymer. We also observe that
the Epc is relatively independent of scan rate, while the Epa shifts to higher
positive values with increased scan rates.

The cyclic voltammetry of poly(EDTT)-C differs slightly from that
of poly(EDTT)-E, in that the first anodic redox wave occurs at a slightly
higher oxidation potential, suggesting smaller average molecular weight
for poly(EDTT)-C relative to poly(EDTT)-E. Thus, as has been found in
other conjugated polymers,20 the method of synthesis yields a slightly
different polymer. Both the oxidation potentials of poly(EDTT)-C
(Epa=1.00 V/SCE) and poly(EDTT)-E (Epa=0.90 V/SCE) are lower than
those of PBEMT (Epa=1.27 V/SCE) but comparable to PEMT (Epa=0.98

V/SCE).13

6.3.3. Gel Permeation Chromatography (GPC) Molecular

Weight Analysis

The average molecular weights of both poly(EDTT)-C and
poly(EDTT)-E were determined by gel permeation chromatography
(GPC). The molecular weights were estimated from a retention time
calibration curve constructed using a series of polystyrene standards. This
commonly used technique assumes that, in solution, the conjugated polymer
and polystyrene behave similarly, which we recognize may not be entirely
appropriate. Thus, the numbers reported here are only relative. Both neat
NMP and NMP containing ~0.5% by weight LiCl were used as eluents with
a flow rate of 0.2 mL/min. The UV-Vis detector was set at 400 nm with a

temperature of 23 0C. A dramatic difference in the GPC results is

259
observed when ~0.5% wt LiCl is added to the NMP solvent. In the absence

of LiCl, a multimodal distribution of polymer species was observed in the
GPC trace corresponding to very high molecular weights. When LiCl is
added to the solution, this behavior disappears and only one peak is
observed. For example, a representative GPC trace of the poly(EDTT)-C is
presented in Figure 6.4, where the molecular weight distribution is clearly
trimodal in character in neat NMP. It basically consists of a high and a low
molecular weight fraction with area of ca 38% and 62%, respectively. The
high molecular weight fraction corresponds to number average molecular
weight (Mn) ~3.93x105 and weight average molecular weight (Mw)
~5.30x105 with polydispersity (PD) of 1.35 and is due to polymer chain
aggregates in solution. The low molecular weight fraction corresponds to
Mn ~ 2.64x103 and Mw ~ 5.38x103 with PD of 2.0. Figure 6.4B shows
the single peak obtained when the NMP/0.5 wt% LiCl was used as an
eluent. This peak has a longer retention time and corresponds to a
molecular weight fraction (Mn=3.03x103 , Mw=5.75x103, PD=1.9). The
dramatic change in the GPC trace upon addition of LiCl suggests that in
neat NMP the polymer does not disperse into single chains but forms
intermolecular aggregates, which give rise to the large molecular weight.
Li+ ions help dissolve these aggregates, probably via Lewis acid-base
interactions, into single chains and allows for the estimation of the correct
molecular weight. Such aggregation phenomena in NMP have been
observed in previous GPC studies of polyanilines.21 Similar behavior was
observed in the GPC measurements of the poly(EDTT)-E. A single GPC
trace was observed, corresponding to the Mn=4.75x103 , Mw=8.08x103,
PD=1.7, when NMP/0.5 wt% LiCl was used as an eluent. (see Figure 6.4C
and 6.4D).

INTENSITY (ARE. UNITS)

INTENSIW (ARE. UNITS)

260

 

 

 

 

 

 

 

 

 

 

 

I
1 1} 1 1
10 20 30 40 50 60
RETENTIONTMENIN)
1 1 1 1 1
(B)
1 l 1 I 1
10 20 30 4O 50 60
RETENTmmENIN)

wrensnv (ARE. UNITS)

INTENSITY (ARB. UNITS)

 

 

(C)

 

 

1 1
20 30 40 50

 

 

 

 

10 60
HETENTIONTNENIN)
m 1 1 1 1
(D)
T f 1 A 1
10 20 30 40 50 60
RETENTIONTIMENIN)

Figure 6.4. GPC traces of neutral poly(3,4-ethylenedithiathiophene) in

NMP solution at room temperature. (A) poly(EDTT)-C, without LiCl; (B)
poly(EDTT)-C, with addition of LiCl; (C) poly(EDTT)-E, without LiCl;
(D) poly(EDTT)-E, with addition of LiCl. The Li'l‘ ions are thought to

serve as Lewis acid centers probably complexing the sulfur atoms on the -

polymer chains, interfering with chain-to-chain interactions and helps

dissolve the large agglomerated particles to single chains.

261

In comparison with poly(3-ethylmercapto)thiophene (PEMT)13
(Mn=2200, Mw =13000, DP=5.9), and poly[3,4-
bis(ethylmercapto)]thiophene (PBEMT) (Mn=2600, Mw=9000, DP=3.46),
the Mn of poly(EDTT)-C and poly(EDTT)-E are slightly higher though in
the same order of magnitude. The Mn corresponds to about 17 repeat units
in the polymer chain for poly(EDTT)-C and 27 repeat units for
poly(EDTT)-E. The relatively small polydispersity indexes for both
poly(EDTT)-C and poly(EDTT)-E indicate a more homogeneous
distribution of polymer chain lengths.

6.3.4. Infrared Spectroscopy

Figure 6.5 shows the infrared spectra of the EDTT monomer and the
corresponding neutral polymers poly(EDTT)-C and poly(EDTT)-E. The
principal IR absorption bands for the monomer and polymer and their
assignments, including data for mono- and poly-(3-
ethylmercapto)thiophene (EMT and PEMT), mono- and poly-[3,4-
bis(ethylmercapto)]thiophene (BEMT and PBEMT), poly(3',4'-dibutyl-
2,2':5',2"-terthiophene) [poly(DBTT)], poly polythiophene (PT) and
poly(3-hexylthiophene) (P3HT) are compared in Table 6.1. The absence of
absorbance at 820 cm"1 (due to CB-H bending vibration) in the spectra of
PBEMT, poly(EDTT)-C and poly(EDTT)-E is consistent with the lack of
thiophene ring hydrogen atoms in these polymers. In the thiophene ring
vibration region, only two peaks are present in the EDTT monomer and
the poly(EDTT)-C. The reduced number of vibrational modes, compared
to the other listed polymers, could be attributed to the existence of
additional symmetry in the chemical structure of the repeating units.

Similar effects were observed in the IR spectra of poly(DB'I'I‘).10 The

(A)

W\ “MM

 

l

 

TRANSMI'I'I‘ANCE (%)

.l

 

L

W WWW/”M

 

 

Iloo 3390 also aim 2360 1350 15110 1130 #20 910
wavsuumssa (cm‘l)

Figure 6.5. FT-IR transmission spectra (KBr pellets) of (A) monomer
3,4-ethylenedithiathiophene (EDTT); (B) neutral poly(EDTT)-C; (C)
neutral poly(EDTT)-E.

263

Table 6.1. Comparison of Infrared Band Positions (cm'l) and Their

Assignments for Some Monomeric and Polymeric Polythiophene

 

 

Derivativesa
arom C-H aliph C-H ring methyl arom C-H

sample (1 B stretch stretch def out-of-plane
EDTT 3091 2956 2915 1472 1411 1385 970 857 772
poly(EDTT)-C 2951 2911 1442 1409 1384
poly(EDTT)-E 2962 2909 1409 1373
EMTb 31013066 2974 2924 2870 14921446 1427 1373 1099 852 779
PEMTb 3090 3070 2974 2916 2867 1508 1481 1423 1373 825
BEMTb 3101 2970 2924 2870 1473 1446 1427 1373 960 856 787
PBEMTb 3090 2968 2922 2865 1508 1489 1446 1373
poly(DB'I'l‘)C 3062 2951 2925 2856 1492 1456 1377 788
PThd 3063 1491 1453 1441 1377 788
P3HTd 3055 295929302858 1512 1458 1439 1377 825

 

a all polymers are in the undoped state- b see ref 13- C see ref 10. d

see ref 3(a) PTH-polythiophene, P3HT-poly(3-hexy1thiophene).

264

absence of the aromatic Cfi-H stretch at 3070 cm"1 is consistent with the
3,4-disubstitution in the monomer and in the corresponding polymers. The
band due to Cor-H stretch at ca 3091 cm'1 is nearly absent in the IR spectra
of poly(EDTT)-C and poly(EDTT)-E in contrast to the IR spectra of
PEMT and PBEMT, and this can be attributed to the higher molecular
weights for the poly(EDTT)-C and poly(EDTT)-E, in agreement with the
GPC results. However, there are slight differences in the IR spectra of
poly(EDTT)-E and poly(EDTT)-C. This may be because the two different
synthetic methods yield products which may differ in the degree of
crystallinity, cis/trans conformation ratio, and the number of defects.

The IR spectra of the oxidized polymer, doped with FeCl4‘, 13‘
(chemically) and C104' (electrochemically), are all similar and shown in
Figure 6.6. The doping process causes a profound change in the IR spectra
presumably due to the change in the electronic structure of the neutral
polymer. Surprisingly, the doped polymers (regardless of the dopant
anion) have an additional intense band at 831 ~ 818 cm‘l. In other
polythiophene derivatives, this band has been assigned to the CB-H
deformation vibration of the 2,3,5-trisubstituted thiophene ring.22
However, in the polymers described here such a CB-H group does not exist.
We note that this band is not observed in the IR spectrum of the neutral
polymer and it disappears when the doped polymers are reduced back to
the undoped state. This precludes the possibility of a C-S cleavage and
formation of a C-H group on the thiophene ring upon doping. Thus, the
origin of this band is uncertain at this stage. The bands at 1110 and 625
cm"1 in Figure 6.6C are due to the presence of ClO4' dopant. The higher
background absorbance levels at the high energy region of the spectra,

compared to the undoped polymer, are characteristic of conducting

265

 

 

 

A (A)

2‘;

III.

U

2

<

l:

E-

(I)

2

E

q (B)

.J W
(C)

11000 3390 3180 2170 2360 1350 13:10 1130 520 310

WAVENUMBER (cm'l)

Figure 6.6. FT-IR transmission spectra (KBr pellets) of (A)
poly(EDTT)-C doped with FeCl4'; (B) poly(EDTT)-C doped with 13"; (C)
poly(EDTT)-E doped with ClO4‘ .

266

polymers and is attributed to the tailing of the electronic bipolaronic

absorption from oxidatively doped regions in the polymer.

6.3.5. UV-Vis-NIR Spectroscopy

The electronic spectra of both poly(EDTT)-C and poly(EDTT)-E in
NMP solution display two absorption bands in the UV-Vis region as shown
in Figure 6.7, in contrast to the single absorption observed in the case of
PEMT and PBEMT, and some other polythiophenes. The low energy
peaks, both with an onset around 2.39 eV (518 nm), have an absorption
maximum at 413 nm for poly(EDTT)-C and 419 nm for poly(EDTT)-E,
which is a result of the 1t—1t* absorption transition.23 The higher energy
peaks at 341 nm are attributed to n—1t* transitions involving the excitation
of non-bonding electrons from the periphery substituents (i.e. S atoms) to
the antibonding tt* orbitals of the heterocyclic rings. The onset of UV-Vis
absorption in poly(EDTT)-C and poly(EDTT)-E, in solution, is close to
that of PBEMT (2.4 eV) but slightly larger than that of PEMT (2.2 eV),
suggesting enhanced ring rotation in poly(EDTT).

Figure 6.8A shows electronic spectra of neutral and doped solid
films of poly(EDTT)-C. In the solid state, the absorption maximum of
poly(EDTT)-C appears at 434 nm, indicating a greater degree of planarity
than in solution.24 The band gap of the poly(EDTT)-C of ca 2.19 eV
(onset of the 1t—1t* transition), which is also confirmed by optical diffuse
reflectance measurements, lies between that of PEMT (Eg ~ 2.0 eV) and
PBEMT (Eg ~ 2.24 eV). Upon doping with iodine vapor, the doped solid
film shows two new low energy absorptions with peak maxima at ~ 0.80
eV and 1.47 eV. This is qualitatively similar to other polythiophenes and

consistent with charge storage primarily in bipolarons.25 The electronic

 

  
  

 

 

 

 

1 1 1 1
(73‘
t".
Z 1
3 1
Ed 413 nm
1:: poly(EDTT)-C
SE, /
E”)
\
Z ‘, 419 nm poly(EDTT)-E
< \ I I ‘~ ’ T I
m \
CE
0
U)
m
<
I T T 1
250 400 550 700 850 1000
WAVELENGTH (nm)

Figure 6.7. UV-Visible absorption spectra of poly(EDTT)-C and
poly(EDTT)-E in NMP solution at room temperature.

 

ABSORBANCE (ARB. UNITS)

 

 

 

 

l T I I I I

 

 

 

 

 

ENERGY (eV)
1 l L J I l
63‘
1:.
Z
I)
m'
[I
33,
LL!
0
Z
<
CD
[I
o
(D
[D
<
I T j I I I
0 1 2 - 3 4 5 6 7
ENERGY (eV)

Figure 6.8. UV-Visible-NIR absorption spectra of (A) neutral and doped
poly(EDTT)-C films (B) neutral and doped poly(EDTT)-E films at room

temperature.

269

spectra of cast films (from NMP solution) of poly(EDTT)-E in the neutral
and doped state show similar features to those of poly(EDTT)-C, as
depicted in Figure 6.8B. The band gap of the poly(EDTT)-E is slightly
lower at ~2.l4 eV. Upon doping with iodine vapor, the doped solid film
shows two new lower energy absorptions with peak maxima at ~ 0.74 eV
and 1.41 eV. These results are consistent with the slightly higher molecular
weight of poly(EDTT)-E. However, neither poly(EDTT)-C nor
poly(EDTT)-E are optically transparent in the doped state, as has been
claimed for the oxygen analog PEDT.l4

6.3.6. Photoluminescence Spectroscopy

The photoluminescence spectra of the polymers were studied both in
solution and in the solid state. Figure 6.9A shows the emission spectra of
both poly(EDTT)-C and poly(EDTT)-E in dilute NMP solution (~1 mg/30
mL) at room temperature when excited at 400 nm. Photoexcitation of these
polymers results in broad band luminescence with a peak at ca. 552 nm
(2.25 eV), and half-height width of 0.33 eV for both poly(EDTT)-C and
poly(EDTT)-E. Both emission spectra exhibit a broad tail at low energy,
presumably a result of emission from longer conjugated segments.26
Figure 6.9B shows the excitation spectra of the poly(EDTT)—C and
poly(EDTT)-E in dilute NMP solution at room temperature with emission
monitored at 552 nm. Both spectra show two peaks at ca 340 nm and 394
nm, which correspond to the two absorptions in the UV-Vis spectra
(Figure 6.7). In the solid state, the poly(EDTT)-C emits light at lower
energy (at 611 run) when excited at 350 nm (see Figure 6.10). This is in
good agreement with the decrease in band gap observed in going from

solution to the solid state. The photoluminescence data suggest that these

270

 

. 552 nm
(A) poly(EDTT)-C

INTENSITY (ARB. UNITS)

 

 

 

 

1 1 1 1 1
430 480 530 580 630 680 730

WAVELENGTH (nm)

 

344 nm 396 nm poly(EDTT)-C

a? (B) /

F2: / ’\ / \ \

E /\

E / poly(EDTT)-E \

/ \
E / \

 

 

7 1 1 r T r 1
250 290 330 370 410 450 490 530

WAVELENGTH (nm)

 

Figure 6.9. (A) Fluorescence emission spectra of poly(EDTT)-C and
poly(EDTT)-E in dilute NMP solution at room temperature (Xex=400 nm)

(B) excitation spectra of poly(EDTT)-C and poly(EDTT)-E in. dilute NMP

solution at room temperature.

271

 

 

I I I I I
611 nm
a?
t
2
D
:15
[I
55,
a
Z
LU
I...
Z
I l I I 1

 

 

500 550 600 650 700 750 800
WAVELENGTH (nm)

Figure 6.10. Fluorescence emission spectra of poly(EDTT)-C in the solid

state at room temperature (lex=350 nm).

272

materials may be good candidates for application in electroluminescence

devices.

6.3.7. Thermal Analysis

The thermal properties of the neutral poly(EDTT)-C were examined
by thermogravimetric analysis (TGA), and the results are depicted in
Figure 6.11. The poly(EDTT)-C under nitrogen atmosphere starts to
decompose at 276 0C and loses about 66% of its weight by 800 0C. The
decomposition in oxygen starts at ~ 245 OC and results in 92% weight loss
by 400 0C. By comparison, poly(3-octylthiophene) exhibits thermal
stability in nitrogen up to 300 0C and about 250 0C in oxygen.27

Figure 6.12 shows the DSC thermogram of poly(EDTT)-C under
nitrogen atmosphere. At the first heating and cooling cycle it displays an
irreversible sharp exothermic peak at ca 253 0C with the heating and
cooling rate of 5 0C/min. The exothermic peak is an irreversible event and
could be due to energy released from ring opening of the fused ring

substituted on the thiophene.

6.3.8. Electron Spin Resonance Spectroscopy

The nature of localized and itinerant spins in the poly(EDTT)
materials was probed by ESR spectroscopy. Figure 6.13 shows the ESR
spectrum of the neutral poly(EDTT)-C at room temperature (RT) of 23 0C
and low temperature (LT) of -157 0C. A largely anisotropic signal with g
values of 2.0062(g1) and 2.0035 (g2) was well resolved at 23 0C, while it
was less resolved at -157 OC. The number of spins corresponding to this
signal was calculated (using a DPPH standard) to be ~5.89x1020 spins/mol

and ~3.40x1020 spins/mol for the room temperature and low temperature

 

Weight (Wt%)

(A)

 

 

 

O

l T I l T l l

100 200 300 400 500 600 700 800
Temperature(°C)

l l l l l l l

 

120

.100-

Weight (Wt%)

4?- O) (D

o o o
I I I

N
O
l

247 °c
/

 

(B)

j

 

 

O

0

Figure 6.11. TGA thermograms of neutral poly(EDTT)-C (A) Under

 

I 7 I T I I I
100 200 300 400 500 600 700 800
Temperature (° C)

nitrogen and (B) under oxygen.

274

 

 

 

 

 

 

1.5 I L I I I I
Exo
"l 1_1 2530c _

0.5-T -
_J *
mW 0- _
l"

~0.5" -
I. -1 _ _
Endo

-1.5 I I I I I I‘

 

 

 

O 50 100 150 2000 250 300 350
Temperature (°C)

Figure 6.12. DSC thermogram of neutral poly(EDTT)-C (under

nitrogen). (* due to the baseline of empty Al pan).

275

measurements, respectively. The origin of these spins lies in defects along
the polymer backbone. At room temperature there is approximately 1 spin
per ~1022 rings (corresponding to a bulk susceptibility of 95,“: 1.2x10'6
emu/mol), which is a very small number compared to other conjugated
polymers and indicative of a high quality polymer.

In contrast to the generally observed single symmetrical line of
polyalkylthiophenes,33928 the larger g1 and anisotropic character of the
ESR spectra of the neutral poly(EDTT)-C indicates that the orbitals of the
sulfur atoms in the thiophene ring periphery are significantly involved in
the highest occupied molecular orbital (HOMO) of the polymer where the
unpaired spins reside. It appears that the spins localize part of the time on
the ethanedithiolate sulfur atoms, instead of just along the conjugated
backbone, which would imply a significant degree of mixing of the 3s and
3p orbitals on the sulfur atoms with the thiophene carbon p-orbitals as has
been proposed in the related PEMT and PBEMT.13

The anisotropic ESR spectrum becomes more symmetric and more
intense when poly(EDTT)-C is iodine doped, see Figure 6.14. The broad
nearly symmetrical line (Apr~19 G) at g=2.0058 at 23 0C, corresponds to
~1.16x1021 spins/mol. At -157 0C, the linewidth of the ESR spectrum
decreases to 7.2 G at g=2.0061. The dramatic changes of the ESR line
shapes and intensities in going from the neutral to the doped state of
poly(EDTT)-C, are attributed to the significant changes in electronic
structure, the increase in the number of defects in the polymer backbone
during the doping process, and perhaps the appearance of some itinerant
spins. The contribution of the ethane-dithiolate ring substitutent to the ESR
response is less significant in the doped poly(EDTT)-C, possibly due to

either the greater delocalization of spins in the oxidized conducting state or

276

 

 

 

 

 

 

 

 

 

 

 

 

 

6 Gauss
(A) H
r J)
an _
2 F
LU
i-
Z i/
FIELD (GAUSS)
. 6 Gauss
(B) I———I
E
m
2
li‘
_Z_
FlELD(GAUSS)

Figure 6.13. ESR spectra of neutral poly(EDTT)-C (A) at 23 0C and (B)
-157 OC.

 

 

 

 

 

 

 

19 Gauss
E
U)
2
E /’
Z L/
FIELD (GAUSS)
3 Gauss
(B) /l H
t
:73 J
2 V
LIJ
j.—
Z

 

 

 

 

FIELD (GAUSS)

Figure 6.14. ESR spectra of (A) doped poly(EDTT)-C (13") and (B)
doped poly(EDTT)-E (ClO4‘) at 23 OC

278

Table 6.2. ESR Data for Various Poly(EDTT) Samples

 

 

sample g factor Apr (Gauss) spins/mol line shape
At 23 0C
poly(EDTT)-C 2.0062 6.0 5.89x1020 anisotropic
(neutral) 2.0035
poly(EDTT)-E 2.0062 6.0 4.64x1021 anisotropic
(neutral) 2.0035
poly(EDTT)-C 2.0058 19.3 1.16x1021 isotropic
(13‘ doped)
poly(EDTT)-E 2.0047 3.2 8.36x1021 isotropic
(ClO4' doped)
At -157 0C
poly(EDTT)-C 2.0059 6.0 3.40x1020 anisotropic
(neutral) 2.0035
poly(EDTT)-E 2.0060 6.0 3.39x1021 anisotropic
(neutral) 2.0035
poly(EDTT)-C 2.0061 7.2 1.21x1021 isotropic
(13' doped)
poly(EDTT)-E 2.0046 2.8 1.31x1022 isotropic

(ClO4' doped)

279

the formation of a bipolaron band which contains virtually no contributions
from the substituted sulfur orbitals in the fused six-membered ring. The
creation of additional spins whose spectrum masks that of those present in
the undoped material cannot be ruled out.

The ESR spectra of neutral and doped (ClO4') poly(EDTT)-E reveal
similar features as the neutral and doped poly(EDTT)-E at 23 OC and -157
OC, i.e. anisotropic and isotropic line shape for the neutral and doped
polymer, respectively. The detailed ESR data for all versions of

poly(EDTT) are summarized in Table 6.2.

6.3.9. Charge Transport Properties

The electrical conductivities of doped poly(EDTT)-C (with FeCl4‘)
and doped poly(EDTT)-E (with ClO4‘) and also the neutral materials were
measured by the standard four-probe method on pressed pellets as a
function of temperature. In the neutral state, the polymers have
conductivities ~10-10 S/cm at room temperature . With doping, the room
temperature conductivities increase to ~01 S/cm for poly(EDTT)-C and
0.4 S/cm for poly(EDTT)-E. These samples maintain these conductivities
for several months. These conductivity values are comparable to that of the
poly(DBTT), and among the highest obtained in mercapto-substituted
polythiophenes. They are several orders of magnitude higher than those of
the related polymer PBEMT (6:10'7 S/cm) and PEMT (0:10'3 S/cm)13
and about 2 orders of magnitude lower than that of PEDT (o=15~19
S/cm).14 The slightly higher conductivity of poly(EDTT)-E compared to
poly(EDTT)-C is in good agreement with its higher molecular weight.
Given the comparable molecular weights of PBEMT, PEMT, poly(EDTT)-
C and poly(EDTT)-E, one reason for the high conductivity of the latter

280

two is that the charge carriers in the latter two materials, are less localized
on the peripheral sulfur atoms compared to PEMT and PBEMT13, which
could give rise to higher carrier mobilities. A contributing factor may be
the decreased steric interactions in poly(EDTT) due to the cyclic di-
substitution pattern on the thiophene ring which eliminates OI,B couplings
and gives rise to a more planar backbone.

The electrical conductivity of poly(EDTT)-C and poly(EDTT)-E was
also measured as a function of temperature and show a thermally activated
behavior in which the conductivity drops with falling temperature. The
data in Figure 6.15 that the temperature dependence does not follow the
typical semiconductor behavior for a single activation energy. This
suggests that the charge transport in these materials is affected by scattering
mechanisms that are not dominant in classical semiconductor samples. A
significant factor affecting charge flow in these samples is boundaries
between adjacent polymer grains as well as other activation barriers
associated with chain-to-chain transport. In order to gain further insight
into the conduction mechanism of poly(EDTT)-C and poly(EDTT)-E, we
attempted to fit the experimental variable temperature data to the analytical

expression
G=GOCXP[-(To/T)°‘]

where 0'0 and T0 are constants and on: 1/2, 1/3, or 1/4 based on several
conduction mechanisms suggested for such systems.29r30 It must be noted
that several conduction models, such as carrier tunneling between small
metallic particles in an insulating matrix,29»30 one-dimensional variable

range hopping (lD-VRH) between localized states30 and the Coulomb gap

281

 

 

 

 

O poly(EDTT)-E
A poly(EDTT)-C
(B)
O
O
A O 0
AA OOOO¢
A O O
A
A
A A
1 I 1 l 1 l l 1 i l l
50 100 150
1000rr (K")

Figure 6.15. Four-probe variable-temperature electrical conductivity of
(A) poly(EDTT)—C doped with FeCl4' and (B) poly(EDTT)-E doped with
C104“. The temperature range is 5-300 K. i

282

model for certain disordered systems31 can be described with oc=l/2.
Three-dimensional variable range hopping (3D-VRH) is described by
0t=1/4. Single exponential fits of the electrical conductivity data, with
OI=1/2 and OI=1/4 are shown in Figure 6.16. It is clear from these plots that
the fits over the entire temperature range are not entirely satisfactory
except for poly(EDTT)-C when (1:1/2, which excludes the 3D-VRH model
for this material. The poly(EDTT)-E shows a definite change in slope at 25
K below which it gives a good fit for 0t=1/2. This slope change may be due
to a change in charge transport mechanism. In any case it would be
difficult to separate the effect of grain boundaries from that of interchain
hopping with the data at hand. Given the one-dimensional nature of the
conducting species in these polymers it is reasonable to envision a charge
transport model favoring a lD-VRH.

Thermoelectric power measurements are less susceptible to grain
boundary effects and can better probe the intrinsic transport properties of
materials. That is because it is a zero-current technique. Such
measurements were performed on doped pressed pellets of both the
chemically and electrochemically derived polymer. The values of the
thermopower varied from —5 ItV/K to +5 LIV/K and in all samples this
property trended toward zero at lower temperatures, see Figure 6.17. The
magnitude of the thermopower suggests a metallic state for doped
poly(EDTT) as has been found in other highly conducting polythiophenes.
The vacillation between a negative and a positive sign to the thermopower,
in what appears to be similarly doped samples, is more difficult to assess.
Ideally, oxidatively doped poly(EDTT) should be a p-type conductor if the
polaron/bipolaron model proposed for these materials is correct. All other

polythiophene derivatives have found to possess small positive

283

 

O poly(EDTT)-E
A poly(EDTT)-C

 

 

(B)
O
0
AA 0
A O
0
AA 0%
A Cbo
A O
A
A
10.12JllllllJlIlllllllllllllliglallllllll

 

50 100 150 200 250 300 .350 400
1000/T°'5 (K°°'5)

 

 

 

10°
10.2 0 poly(EDTT)-E
(B) a poly(EDTT)-C
.10'4 00
g (A) 0
g 106 AAA 0
o a o O O
10 AA QQ225
A
10-10 A O
A
A
10.121111J1111111111411AIIIIL1
200 300 400 500 600 700

1 ooonQZS (K-0.25)

Figure 6.16. Four probe variable-temperature electrical conductivity of
(A) poly(EDTT)-C , doped with FeCla' and (B) poly(EDTT)-E, doped with
C10; : (top) in a logo vs. T'”2 format; (bottom) in a logo vs. T~1"4 format.

The temperature range is 5-300 K.

 

10.0 I I I I I T T j fi' I

Thermopower (IIV/K)

 

 

 

 

 

_100 I 4 1 I I l I l I
100 150 200

Temperature (K)

Figure 6.17. Variable temperature thermopower data for several pressed

pellet samples of doped poly(EDTT).

Regardless of chemical or

electrochemical origin of the sample, both positive and negative

thermopower values have been observed.

285

thermopowers between 300 and 5 K.10 Although such changes in
thermopower sign have been previously observed in polyaniline samples,
they have been attributed to variations in protonation levels. This
possibility does not exist in poly(EDTT) and thus a satisfactory explanation
cannot be advanced at this state. The findings reported here with respect to
the thermopower of poly(EDTT) may imply that currently accepted views
of charge transport in some conjugated polymers may need further
refinement. Work to identify the factors that affect the sign of the

thermopower from sample to sample is continuing.

6.4. Concluding Remarks

The incorporation of the ethylenedimercapto group at the 3,4-
positions on thiophene allows for a better control of polymer synthesis and
gives the resultant polymer high solubility, unusual optical absorption and
anisotropic ESR spectra. In contrast to its oxygen analog, PEDT,
poly(EDTT) is not optically transparent in the doped state. The doped
polymer shows metallic conductivity with G~ 0.1-0.4 S/cm at room
temperature about 2 to 6 orders of magnitude higher than that of PEMT
and PBEMT, two polythiophenes with mercapto substitutents on the
thiophene ring. The dramatic enhancements in the conductivity of
poly(EDTT) can be attributed to improved conjugation and better
interchain contacts compared to other poly(mercapto-thiophenes). We note,
however, that these conductivity values are still much lower than those

reported for PEDT. The reasons for this difference are not clear.

(1)

(2)

286
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288

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Wang, C.; Benz, M.; LeGoff, E.; Schindler, J. L.; Albritton-Thomas,
J .; Kannewurf, C. R.; Kanatzidis, M. G. Chem. Mater. 1994, 6, 401-
411.

(a) Feldhues, M.; Kampf, G.; Litterer, H.; Mecklenburg, T.;
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Leclerc, M. Macromolecules, 1991, 2_4, 455-459.

(a) Elsenbaumer, R. L.; Jen, K.Y.; Oobodi, R. Synth. Met. 1986,
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(a) Ruiz, J.P.; Gleselman, M. B.; Nayak, K.; Marynick, D.S;
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Nayak, K.; Marynick, D.S.; Reynolds, J.R. Macromolecules 1989,
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Chiang, L-Y.; Shu, P.; Holt, D.; Cowan, D. J. Org. Chem. 1983, 4_18,
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(a) E. Waltman, R.T.; Bargon, J.; Diaz, A.F. J. Phys. Chem. 1983,
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L49-L51.

CHAPTER 7

3,4-Vinylenedithiathiophene Perchlorate. A New
Conductive Charge Transfer Compound

292
ABSTRACT

A new n-electron donor, 3,4-vinylenedithiathiophene (VDTT), has
been prepared by fusion 1,3-dithiole on the 3,4-positions of a thiophene
ring. The electrochemistry of this electron donor shows a reversible one-
electron redox process, with anodic and cathodic peak potentials at 0.91
and 0.85 V vs. SCE, respectively. A corresponding radical cation
perchlorate salt was obtained by controlled potential electrolysis and
characterized by X-ray single crystal diffraction, IR, UV-vis-NIR, and
ESR spectroscopies. The crystal structure of the VDTT-C104 salt shows
two types of short intermolecular S---S contacts of 2.97 and 3.53 A,
respectively. The material has an optical band gap of 1.0 eV. It has a room

temperature electrical conductivity of 103-10'5 S/cm on pressed pellets.

 

293

7 .1. Introduction

During the past two decades, both the fields of organic charge-
transfer (CT) salts and electrical conducting polymers have attracted
considerable interest due to their unusual solid state properties such as
metallic conductivity and/or superconductivity},2 Among a variety of It-
electron donors, the analogs and derivatives of tetrathiafulvalene (TTF)
have played a major role.1 Over fifty superconducting charge transfer salts
have been discovered which contain a TTF radical moiety.3 On the other
hand, thiophene and its derivatives have been extensively investigated to

achieve environmentally stable and processable conducting polymers.4,5

[:>=<:] ‘ [3

S
TTF thiophene

Recently, the incorporation of both thiophene and tetrachalcogenafulvalene
units has been developed as a useful approach to form new hybrid It-donor
systems.6'10 Several known examples are shown in Scheme 7.1.
Thiophene6 (1) and oligothiophene7 (2) have been used in composing 1:-
extended TTF systems, resulting in a significant decrease in oxidation
potentials. Thiophene moieties have also been fused through either b-bond
or c-bond of thiophene to two TTF based donor systems (3,8 49) and one
TTeF (tetratellurafulvalene) (5) based donor system“),11

[:)=ai—©— CH=<:]

1

(IZFC‘HWHD

2

b-bond fusion c-bond fusion

/ \ M63 / \ SMe
CC:>=<:I> s:[:>=<:j:s

c-bond fusion
Me / \ Me
TC TC /
\
s >=< 3
Me Me

Scheme 7.1. Some known hybrid tt-electron donor systems.

295

/\ /\

S S

EDTT VDTT

In the course of our investigation of new interesting polythiophene
derivatives, we have successfully synthesized two new thiophene derived
monomers, i.e. 3,4-ethylenedithiathiophene12 (EDTT) and 3,4-
vinylenedithiathiophene (VDTT). The former monomer can be chemically
and electrochemically oxidatively polymerized to yield a soluble conductive
poly(3,4-ethylenedithiathiophene), poly(EDTT).12 In contrast, the latter
monomer VDTT can not be polymerized under similar conditions. Instead,
a stable radical cation salt is obtained by the electrolysis of the monomer in
0.1 M Bu4NClO4 acetonitrile solution. The VDTT monomer can be viewed
as a new type of n-electron donor in which the 1,3-dithiole group, i.e. the
basic moiety of TTF is grafted to the B-positions of thiophene ring through
c-bond fusion. Here we report the detailed synthesis and characterization of
the VDTT monomer and its corresponding charge-transfer salt with

perchlorate.
7.2. Experimental Section

7.2.1. Materials
1,2-Dibromoethylene (98% purity), n-butyl lithium, carbon
disulfide, hydrazine hydrate were used as received from Aldrich Chemicals

Inc. 3,4-Dibromothiophene was used as received from Lancaster Synthesis.

296

Tetra-n-butylammonium perchlorate was purchased from GFS Chemicals
Inc. and used without further purification. Acetonitrile (HPLC grade) was
used as received from Aldrich Chemicals. Other solvents were distilled and
degassed prior to use. All reactions were performed under an atmosphere

of nitrogen or argon using either standard Schlenk or dry box techniques.

7.2.2. Synthesis of Thieno[3,4-d]-1,3-dithiole-2-thione
The synthesis and characterization of thieno[3,4-d]-1,3-dithiole-2-

thione are described in reference 12, also see chapter six.

7.2.3. Synthesis of 3,4-Vinylenedithiathiophene, (VDTT)

Potassium metal (0.120 g, 3.07 mmol) was added in one portion into
stirred, freshly distilled, ethanol (100 mL) under nitrogen. After the
potassium dissolved completely, the thieno[3,4-d]-1,3-dithiole-2-thione
(0.190 g, 1.0 mmol) was added to the solution. The reaction mixture was
left to react for l h at 50 0C under nitrogen, and gave a clear yellow
solution. To this yellow solution, 1,2-dibromoethylene (0.1 mL, 1.21
mmol) was added via syringe. After 24 h at room temperature, the ethanol
was removed from the mixture under dynamic vacuum, and the resulting
solid was extracted with 20 mL of distilled hexane. The extract was dried
and re-extracted with 10 mL of hexane, and then filtered, dried under
vacuum to give a yellow liquid of 3,4-vinylenedithiathiophene in 29% yield
(0.050 g) at room temperature. GC-MS: (m/z) (relative intensity) 172
(100), 140 (22), 127 (38), 108(12), 96(62), 82 (12); 1H NMR (CDC13): 5
6.88 (s, 2H, thiophene ring), 6.32 (s, 2H, ethylene); 13C NMR (CDC13): 5
127.28 (aromatic, B-C), 118.43 (aromatic, OI-C), 119.08 (ethylene); FT-IR

297

(KBr pellet): 3091(m), 2956(m), 2915(m), 1472(m), 1411(m), 1385(w),
1324(m), 1287(m), 857(s) and 772(s).
UV-VIS (CHCl3). Amax = 305 nm.

7.2.4. Preparation of 3,4-Vinylenedithiathiophene

Perchlorate, (VDTT-C104)

Controlled potential electrolyses were carried out at ambient
temperature in a three-electrode single-compartment cell containing 5 mM
monomer and 0.1 M (Bu4N)ClO4 in 20 mL acetonitrile (HPLC grade;
Aldrich). The working electrode was an indium-tin oxide coated glass
(ITO) electrode of 2.3 cm2 area. The counter electrode was a platinum
wire. A saturated calomel electrode (SCE) was used as the reference
electrode. The electrolyte solution was degassed by argon bubbling for 20
min prior to use and maintained under an argon blanket throughout each
experiment. Purple, rectangular platelet-shaped, small crystals were
deposited on the surface of the ITO electrode after applying an anodic
potential of 1.0 V vs. SCE for about 4 h. The crystals of VDTT-C104 was
washed with fresh acetonitrile and dried under vacuum at room
temperature. The yield was about 11%. Semi-quantitative microprobe

analysis (EDS/SEM) on purple crystals gave an average ratio of S/Cl = 3/ 1.

7.2.5. Physicochemical Methods

Elemental analyses (semiquantitatvie) were performed on a JEOL
ISM-35C scanning electron microscope (SEM) equipped with a Tracor
Northern energy dispersive spectroscopy (EDS) detector. Infrared spectra
were recorded as KBr pressed pellets on a Nicolet 740 FT-IR

spectrometer. UV-Visible-NIR absorption spectra were obtained from a

298

Shimadzu UV-3101PC double beam, double-monochromator
spectrophotometer. Nuclear magnetic resonance spectra (1H and 13C) were
obtained using a computer controlled Varian Gemini-NMR (300 MHz)
spectrometer. The chemical shifts are reported in parts per million (5,
ppm) using the residual solvent resonance peak as reference (CHCl3, 5 7.24
ppm for 1H and 77.00 ppm for 13C). X-ray powder diffraction (XRD)
patterns were collected at room temperature on a Rigaku powder
diffractometer, Rigaku-Denki/RW400F2 (Rotaflex), using Cu(K0t)
radiation generated by a rotating anode operating at 45 kV and 100 mA.
The data were collected at a scan rate of 1 deg/min. Electron spin
resonance (ESR) spectra were recorded with a Varian EPR-E4
spectrometer with diphenylpicrylhydrazyl radical as g marker (g=2.0037).
Cylindrical quartz tubes were employed for powders. The conductivity
data were measured by the standard four-probe method on pressed pellets
at room temperature. Electrochemical synthesis and cyclic voltammetry
were performed with a PAR 273 potentiostat/galvanostat equipped with a
PAR RE0091 X-Y recorder.

7.2.6. Crystallographic Studies

A purple, platelet-like crystal of VDTT-C104 with approximate
dimensions of 0.100 x 0.060 x 0.030 mm3 was selected for the X-ray
analysis and mounted on the tip of a glass fiber, and collected on a Rigaku
AFC6S diffractometer with graphite monochromated Mo KOL radiation by
using an (0—20 scan mode at 23 0C. Accurate unit cell parameters for the
radical cation salt were obtained from the least-squares refinement of the
20, o), x, (1) values of 20-25 machine-centered reflections. The intensities of

three check reflections were monitored every 150 reflections to detect

299

possible decay during the data collection period. No significant decay was
observed. An empirical absorption correction based on \V scans was applied
to all data, followed by a DIFABS13 correction to the isotropically refined
data. The structure of the VDTT-C104 was solved by direct methods using
SHELXS86143 and refined by full-matrix least-squares techniques with the
TEXSAN package of crystallographic programs.”b All calculations were
performed on a VAX station 3100/76 computer. The hydrogen atom
positions were calculated and were included in the structure factor
calculations but were not refined. They were given an arbitrary
temperature factor which is 1.2 times that of the carbon they are attached
to. The complete data collection parameters and details of the structure
solution and refinement for the VDTT-C104 are given in Table 7.1. The
final atomic coordinates, average temperature factors, and their estimated
standard deviations for the non-hydrogen atoms of the VDTT-C104 are
shown in Table 7.2. It is notable that anisotropic refinement of the crystal

structure was not carried out due to insufficient unique data.

300

Table 7.1. Summary of Crystallographic Data and Structure
Analysis for the VDTT-C104 Salt

 

formula

fw

a, A

b, A

c, A

OI, deg

13, deg

7, deg

z; V, A3

it, A

space group
DCfllC.’ g/cm3

u, cm'l

crystal dimens (mm3)
29max, deg
temperature, 0C
total data measured
unique data

data with F02>2.500(F02)
no. of variables
final R/Rw,a %

C6H4S3O4Cl
271.73
13.030(3)

9.51 1(2)
7.538(2)

90.00

90.00

90.00

4; 934.2(4)
0.71069 (Mo KOI)
Pbam (No.55)
1.932

10.32 (Mo KOI)
0.10 x 0.06 x 0.03
49.9 (Mo Ken)
23

1493

822

188

32

5.9/5.8

a R=2IIFol-IFcll/ZIFOI. Rw={Zw(IF0|-|Fc|)2/2wlF0|2}1/2.

301

Table 7.2. Fractional Atomic Coordinates and B eq Values for the Non-
Hydrogen Atoms in the VDTT-C104 Salt with Estimated Standard

Deviations in Parentheses

 

 

atom x y z B eq, a A2
Cl 0.6368(6) 0.3524(7) 1.0000 2.2(1)
S(1) 0.886(1) 0.1017(7) 1/2 2.6(2)
S(2) 0.8861(5) 0.5044(5) 0.7330(6) 2.5(1)
O(1) 0.730(2) 0.2738 1.0000 5.7(8)
O(2) 0.554(2) 0.249(2) 1.0000 4.2(6)
0(3) 0.625(2) 0.433(1) 0.846(2) 5.2(3)
C(l) 0.882(3) 0.223(2) 0.664(2) 2.6(4)
C(2) 0.881(2) 0.359(2) 0.593(2) 2.2(4)
C(3) 0.883(2) 0.643(1) 0.592(2) 2.3(4)

a B values for isotropically refined atoms.

302

7.3. Results and Discussion

7.3.1. Synthesis and Electrochemistry

The 3,4-vinylenedithiathiophene (VDTT) was prepared according to
the synthetic route shown in Scheme 7.2. Potassium reacts with the thione
(l) in ethanol to form the dipotassium salt intermediate (2) via C-S bond
cleavage. The dipotassium salt intermediate (2) reacts with 1,2-

dibromoethylene to give VDTT in 29% yield

SK
::\ EtOH, N2, 3 \

/C= S +3K
50°C, 1h /2

 

SK

Intermediate
BrCH=CHBr
RT, 1 D

Scheme 7.2. Synthetic route to 3,4-vinylenedithiathiophene.

 

The structure and purity of the compound were confirmed by FT-
IR, 1H NMR, 13C NMR, and GC-MS results as shown in the experimental
section.

The electrochemistry of the VDTT was examined by cyclic
voltammetry. Figure 7.1 shows a typical cyclic voltammogram (CV) of the
VDTT monomer in 0.1 M Bu4NClO4/CH3CN solution using a Pt-disk
(0.015 cmz) working electrode. The solution of VDTT exhibits a highly

reversible redox couple with an anodic peak potential (Epa) at 0.91 V vs.

303

0.91 V

1.20 V

 

 

0.00 V

l

0.85 V

Potential (V) vs. SCE

Figure 7.1. Typical cyclic voltammogram of 3,4-vinylenedithiathiophene
(VDTT) (5 mM) in CH3CN/0.1 M (Bu4N)ClO4 on a Pt-disk electrode.

Scan rate, 100 mV/s.

304

SCE and a cathodic peak potential (Epc) at 0.85 V vs. SCE. The difference
in the redox peak potentials (Epa-Epc) is 0.06 V vs. SCE, which indicates a
near perfectly reversible one-electron transfer process. It is notable that the
redox peak potentials are independent of scan rates in the range of 10 to
200 mV/s. No loss of electroactivity was seen after 50 cycles. Figure 7.2
shows a typical CV of the VDTT monomer in 0.1 M BU4NCIO4/CH3CN
solution using an ITO (area of 2.3 cm2) working electrode. During the
first anodic scan, the VDTT monomer acetonitrile solution revealed a rapid
increase in current at the electrode with an onset of 0.86 V/SCE. thus, the
oxidation potential (Ea, mm) of the VDTT monomer is about 0.25-0.29 V
lower than that of EDTT (Ea, mon = 1.15 V). The CV exhibits an anodic
peak at 1.20 V vs. SCE and a cathodic peak at ca 0.81 V vs. SCE. The ITO
electrode turned to pink as the applied potential above 0.86 V vs. SCE and
faded as the potential scanned negatively. No significant loss of
electroactivity was seen after 20 cycles as the applied potential was swept
continuously between 0.0 V and 1.3 V vs. SCE. By holding the anodic
potential at 1.0 V vs. SCE for 4 h, small shiny purple crystals were
obtained on the surface of the ITO electrode. Quantitative microprobe
analyses of the purple material give an average ratio of S/Cl=3/1. It is
noteworthy that a similar electrolysis of VDTT using Pt-plate as a working

electrode did not yield crystals.

7.3.2. Spectroscopic Characterization

Figure 7.3 shows the IR spectra of the VDTT monomer and the
purple material. In the IR spectrum of the purple material (spectrum B),
three strong absorption bands at 1106, 1082, and 625 cm'1 are assigned to
the ClO4' group, which is in good agreement with the presence of Cl

305

 

 

I

0.81 V

Potential (V) vs. SCE

Figure 7.2. Typical cyclic voltammogram of 3,4-vinylenedithiathiophene
(VDTT) (5 mM) in CH3CN/0.1 M (Bu4N)ClO4 on an ITO electrode. Scan

rate, 100 mV/s.

306

 

«Fig/WM mar—T

 

“a
U

2

I

g.

y.

3:" B
2 < )
C

I.
I-"—_\.__.”1
.\.d

‘a

zéso Iéso 1350 {I30 izo —§10
HHVENUHBEH

 

 

I.000 35290 also 2270‘

Figure 7.3. FT-IR transmission spectra (KBr pellets) of (A) donor
VDTT and (B) the VDTT-C104 salt. (* peaks are due to C104")

307

element in the material as determined by the microprobe analyses. Thus, in
conjunction with the electrochemical study mentioned above, the
composition of the purple material is expected to be C6H4S3-CIO4, which is
a formally radical cation salt. The chemical composition and structure of
this material was further examined and confirmed by the single crystal X-
ray diffraction analysis as will be discussed later. The radical cation salt is
stable in air and insoluble in common organic solvent such as CHCl3,
MeOH, and CH3CN. The optical property of the salt was assessed by
studying the UV-vis-NIR reflectance spectrum of the material. Absorption
data were calculated from the reflectance data using the Kubelka-Munk
function.15 Figure 7.4 shows an optical absorption spectrum of the radical
cation salt. The spectrum of the material exhibits a steep absorption edge
from which the band-gap, Eg, can be estimated at ca. 1.0 eV. This small
bandgap of the material indicates that it may be a good semiconductor. The
material also shows high crystallinity as indicated by its X-ray powder
diffraction (XRD) pattern as shown in Figure 7.5. The strongest reflection
corresponds to a d-spacing of 3.25 A. Suitable single crystals of this

material were used for the X-ray single crystal structure determination.

308

 

 

 

 

a/S Absorption Coeff. (arb. units)

 

Energy (eV)

Figure 7.4. Optical absorption spectrum of the VDTT-C104 salt.

309

 

Intensity

 

 

 

 

“MM‘A AA A
—v *— A‘ 4A

Ij'l'll‘V‘YWUIU‘UII‘I‘I'IIY'II‘I’WY'V'YIUl1]I'Il"UIYIV‘UI'IIIYIU'U"IUIIIFIVIIIUIUl"ll“"'l"IIYI‘—FI""III‘I

5. 10. 15. 20. 25. 30. 35. 40. 45. so. 55. so.
2Theta

 

 

Figure 7.5. X-ray powder diffraction pattern of the VDTT-C104 salt at

room temperature. Note: the broad peaks are due to the background of

supportive plastic tape.

310
7.3.3. Description of the Structure of the VDTT-C104 Salt

Figure 7.6 shows the molecular geometry of VDTT+ cation and
ClO4' anion in the radical cation salt. The sulfur atom of the thiophene ring
has a special half-occupancy on a mirror plane perpendicular to the
thiophene ring, while the two sulfur atoms on the peripheral ring have full-
occupancy above and below the mirror plane. The six-member ring and the
thiophene ring are coplanar. Selected bond distances and angles for the salt
are given in Table 7.3. The C-S bond distances are in the range of 1.69-
1.74 A, which are comparable to the C-S distances found in thiophenel6
and TTF systems”. It is noteworthy that the C-C bond distance of 1,3-
dithiole in the VDTT+ cation is significantly longer than that of TTF
(~1.31 A),17 and close to the C-C bond of the disubstituted thiophene ring
(1.40 A) in the same cation (see Figure 7.6). This result indicates that the
electron density is highly delocalized in the fused ring system, which in
turn minimizes the difference of electron population between C=C and C-C
bonds. The crystal packing diagram for the salt is shown in Figure 7.7 . The
VDTT-C104 crystal structure consists of a loose packing of VDTT+
cations separated by ClO4’ anions. The cations are parallel to b-axis. There
are two kinds of short intermolecular S---S contacts within the sum (3.7A)
of the van der Waals radii of sulfur atoms.18 The distance between two
thiophene S atoms is 3.53 A, while the shortest distance between two S
atoms on different periphery rings is 2.97 A. The presence of such short
intermolecular S---S distances indicates a sizable intermolecular interaction
between the cations through a 2-D networks (zig-zag fashion) of
intermolecular S---S contacts, which may considerably facilitate charge

transport.

Table 7.3. Distances (A) and Angles (deg) in the VDTT-C104 salt with

Estimated Standard Deviations in Parentheses

 

Cl-O(1)
Cl-O(2)
Cl-O(3)
Cl-O(3)'
S(1)-C( 1)
S(1)-C(l)'
S(2)-C(2)

O(1)-Cl-O(2)
O(1)-Cl-O(3)
O( l )-Cl-O(3)'
O(2)-Cl-O(3)
O(2)-CI-O(3)'
O(3)-Cl-O(3)'

143(2)
146(2)
1.40( 1)
140(1)
1.70(2)
1.70(2)
1.74(2)

106(1)
112.5(9)
112.5(9)
106.6(9)
106.6(9)
112(1)

S(2)-C(3)

C(1)-C(2)
C(2)-C(2)'
C(3)-C(3)'

C(1)-H(1)
C(3)-H(2)

C(1)-S(1)-C(1)'
C(2)-S(2)-C(3)
S(1)-C(1)-C(2)
S(2)-C(2)-C(1)
S(2)-C(2)-C(2)'

C(1)-C(2)-C(2)'

S(2)-C(3)-C(3)'

169(2)
140(2)
1.40(3)
1.39(3)

0.994
0.953

94(1)
105.5(8)
110(1)
120(1)
127.4(5)
112.7(9)
128.9(6)

312

0(1) 0(2)

 

Figure 7.6. Molecular geometry and atom labeling scheme of the cation
and anion in the VDTT-C104 salt.

313

 

 

 

 

 

 

 

 

 

Figure 7.7. ORTEP packing diagram of the VDTT-C104 salt looking

down the c-axis.

314

7.3.4. Electron Spin Resonance (ESR) Spectroscopy

The cation radical salt of VDTT-C104 was characterized by ESR
spectroscopy. Figure 7.8 shows a room temperature EPR spectrum of the
material in powder form (not a monocrystal). The hyperfine splitting
shows the coupling of 2 set of 2 equivalent hydrogens (pattern of nine
lines), with a coupling constant of ~ 3 gauss. The corresponding g-factors

are g1=2.0131, g2=2.0082, and g3=2.0022. The hyperfine splitting can be

attributed to a S-radical based proton-hyperfine interaction.19

7.3.5. Electrical Conductivity

A preliminary room temperature electrical conductivity
measurement of the material indicated a value in the range of 103-10'5
S/cm. Large single crystals were not available for variable temperature
measurements. A higher electrical conductivity is expected from large

single crystals due to the elimination of grain boundary effects.

315

 

 

 

 

 

- g1: 2.0131
4 i ..
: g2: 2.0082
29 ‘ /
Q) _, ..
5:: ~ ..
I L
— A , r
. g3: 2.0022 »
3320 ' .3320‘ I 33o0 3380' 13400
Field (G)

Figure 7.8. ESR spectrum of the VDTT-C104 salt at room temperature.

316
7.4. Concluding Remarks

The incorporation of the 1,3-dithiole group at the 3,4-positions of
thiophene gives a new n-electron donor, 3,4-vinylenedithiathiophene
(VDTT). Controlled potential electrolysis of VDTT in 0.1 M (Bu4N)ClO4
yields a stable radical cation salt. The salt shows short intermolecular S---S
contacts of 2.97 and 3.53 A. The optical band gap of this material is about
1.0 eV, which confirms its semiconductor nature. The material has a room

temperature electrical conductivity of 103-10"5 S/cm.

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317
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