Synthesis and Electroluminescent Properties of
Polyfluorene-Based Conjugated Polymers Containing
Bipolar Groups

SHENG-TUNG HUANG,

1

DER-JANG LIAW,

2

LI-GA HSIEH,

1

CHIN-CHUAN CHANG,

3

MAN-KIT LEUNG,

3

KUN-LI WANG,

1

WEN-TUNG CHEN,

4

KUEIR-RARN LEE,

5

JUIN-YIH LAI,

5

LI-HSIN CHAN,

6

CHIN-TI CHEN

6

1

Department of Chemical Engineering and Biotechnology, National Taipei University of Technology,

Taipei 10608, Taiwan

2

Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan

3

Institute of Chemistry, National Taiwan University, Taipei 10607, Taiwan

4

Department of Materials and Yarn Formation, Taiwan Texile Research Institute (TTRI), Taipei Country 23674, Taiwan

5

R&D Center for Membrane Technology, Department of Chemical Engineering, Chung-Yuan University,

Chung Li 32023, Taiwan

6

Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan

Received 13 May 2009; accepted 1 August 2009
DOI: 10.1002/pola.23667
Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT:

A bipolar dibromo monomer, bis-(4-bromophenyl)[4-(3,5-diphenyl-1,2,4-

triazole-4-yl)-phenyl]amine (9), containing electro-rich triphenylamine and electro-
deficient 1,2,4-triazole moieties was newly synthesized and characterized. Two fluo-
rescent fluorene-based conjugated copolymers (TPAF, TPABTF) were prepared via
facile Suzuki coupling from the dibromo bipolar monomer, 4,7-dibromo-2,1,3-benzo-
thiadiazole (BTDZ), and 9,9-dioctylfluorene. They were characterized by molecular
weight determination, IR, NMR, DSC, TGA, solubility, absorption and photolumines-
cence spectra, and cyclic voltammetry. The polymers showed good solubility in com-
mon organic solvents such as dichloromethane, chloroform, tetrahydrofuran, and
dichlorobenzene at room temperature. They had glass transition temperatures (T

g

)

higher than 135

C and 5% degradation temperatures in nitrogen atmosphere were

higher than 428

C. Single layer polymer light-emitting diodes (PLED) of ITO/

PEDOT:PSS/polymer/metal showed a blue emission at 444 nm and Commission
Internationale de I’Eclairage (CIE) 1931 color coordinates of (0.16, 0.13) for TPAF.
The device using TPABTF as emissive material showed electroluminescence at 542
nm with CIE1931 of (0.345, 0.625), low turn-on voltage of 5 V, a maximum electrolu-
minance of 696 cd/m

2

, and a peak efficiency of 2.02 cd/A.

V

V

C

2009 Wiley Periodicals, Inc.

J Polym Sci Part A: Polym Chem 47: 6231–6245, 2009

Keywords: benzothiadiazole; bipolar; conjugated polymers; heteroatom-containing
polymers; light-emiting diodes (LED); triphenylamine; 1,2,4-triazole; synthesis

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 6231–6245 (2009)

V

V

C

2009 Wiley Periodicals, Inc.

Correspondence to: K.-L. Wang (E-mail: klwang@ntut.

edu.tw)

6231

INTRODUCTION

Recently, polymer light-emitting displays (PLEDs)
have been studied extensively and appeared
ready for commercialization in the flat-panel dis-
play market, which is currently dominated by the
liquid-crystal display (LCD), due to its low turn-
on voltage, high brightness, high efficiency, easy
processing, and low-cost fabrication.

1–5

PLEDs

are attractive due to the simplicity and low cost in
fabrication in addition to the possible application
for flexible displays.

4–7

The electroluminescent

properties of PLEDs are predominatly determined
by the intrinsic character of the light-emitting
polymer, such as photoluminescence, thermal sta-
bility, film formability, and chain morphology.

3–7

It is well known that the polyfluorene deriva-

tives (PFs) have emerged as promising blue emit-
ting materials due to their unique combination
of high-thermal-stability, versatile processability,
and high-photoluminescence (PL) quantum yield
in the solid state.

8–15

However, PFs have draw-

backs: they show excimer and/or aggregate forma-
tion upon thermal annealing or the passage of
current and keto effect sites as a result of photo-
or electro-oxidative degradation.

16,17

To overcome

this drawback, several approaches

18–21

have been

developed to use longer and branched side chains
or bulky substituents, copolymerization techni-
ques, dendrimer attachment, end capping with
bulky groups, etc. Another problem encountered
when PFs are used in PLED applications is the
high-ionization-potential (usually 5.8 eV); that is,
a high-energy-barrier exists for holes to travel
into PFs from the indium tin oxide (ITO)/poly(3,4-
rthylene dioxythiophene) (PEDOT) layer (5.2 eV),
and this results in high-driving-voltages.

22

The

incorporation of electron-rich tertiary aromatic
amines into polyfluorene backbones can lead to
copolymers having lower ionization potentials.

23

The injection and transport of hole is much

favored than electron because of high LUMO lev-
els in most conjugated polymers. High injection
and transport barrier for electron leads to imbal-
ance of charge-carrier transport.

24

Therefore, it is

not only important to decrease the barriers of
charge injection from the opposite contacts but
also necessary to balance the injection and trans-
port rates of opposite charges. To achieve more
balanced charge injection and transport, multi-
layer devices with extra functional layers such as
electron-transporting, hole-injecting, hole-block-
ing, or electron-blocking layers have been demon-
strated as an effective method to increase the de-

vice efficiency.

25

However, multilayer devices are

difficult to fabricate because of the interlayer-mix-
ing problem. Therefore, multilayer devices are
still not the ideal configuration for particality.
Another widely employed method is to enhance
electron affinity of the conjugated polymers by
incorporating electron affinitive groups.

26

The ar-

omatic triazole group is an interesting electron-
injection/transport group because of its high-elec-
tron-affinity and thermal stability.

27-29

To achieve

a good balance of holes and electrons, both hole-
and electron-transporting functions should be
incorporated into a single bipolar material.

30–32

On the other hand, Jen and coworkers reported
benzothiadiazole-based

conjugated

copolymers

possess high-electron-affinity and preferential
electron-transporting properties compared with
those of the PF homopolymers.

33

Accordingly, we incorporated a dipolar moiety,

consisting of an electron-rich triphenylamine
group and an electron-deficient 1,2,4-triazole
group in conjugated polymers, with the aim of
improving the charge-injection/transport charac-
teristics of PFs. In addition, we hope that the
introduction of bulky 2,4,6-triphenyltriazole side
chain substitute would help to prevent p stacking
between the polymer chains and suppress the in-
formation of excimers in the solid state, which, in
turn, would enhance the thermal stability of the
resultant polymers. Moreover, color tuning can be
achieved by incorporating an electron-deficient
monomer, benzothiadiazole, into the polymer. The
optical, thermal properties, and electrolumines-
cent devices of the conjugated polymers were
investigated.

EXPERIMENTAL

Materials

The materials, bromine (Acros), benzoyl chloride
(Alfa Aesor), phosphorus pentachloride (Riedel-de
Hae¨n), 4-nitrofluorobenzene (Acros), N-bromosuc-
cinimide

(Acros),

tin(II)

chloride

anhydrous

(Showa), hydrazine monohydrate (Alfa Aesar),
heptanoyl chloride (Acros), tetrakis (triphenyl-
phosphine)palladium (0) (Acros), 9,9-dioctylfluor-
ene-2,7-diboronic acid bis(1,3-propanediol) ester
(Aldrich), aluminum chloride (Acros), and 2,1,3-
benzothiadiazole (Aldrich) were used as received.
The 4,7-dibromo-2,1,3-benzothiadiazole was pre-
pared from 2,1,3-benzothiadiazole (BTDZ) with
molecular bromine in hydrobromic acid according
to the published procedures.

34

The solvents, N-

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HUANG ET AL.

Journal of Polymer Science: Part A: Polymer Chemistry

DOI 10.1002/pola

methyl-2-pyrrolidinone (NMP), N,N-dimethylace-
tamide (DMAc), and dimethyl sulfoxide (DMSO)
were purchased from TEDIA and tetrahydrofuran
(THF) and N,N-dimethylformamide (DMF) were
purchased from ECHO. These solvents were dis-
tilled after drying with appropriate drying agents
and stored over 4 A

˚ molecular sieves. The surfactant

trioctylmethylammonium chloride (Aliquat336

V

R

)

was purchased from TCI and used as received.

Measurements

Infrared (IR) spectra were recorded on a Perkin-
Elmer GX FTIR spectrometer. NMR spectra were
recorded using a BRUKER DRX-500 NMR (

1

H at

500 MHz and

13

C at 125 MHz). Elemental analy-

ses were made on a Perkin-Elmer 2400 instru-
ment. Weight-average (M

w

) and number-average

(M

n

) molecular weight were determined by gel

permeation chromatography (GPC). Five water
(Ultrastyragel) columns 300

 7.7 mm (guard, 500,

10

3

, 10

4

, 10

5

A

˚ in a series) were used for GPC anal-

ysis with THF (1 mL

min

1

) as an eluent. The elu-

ents were monitored with a refractive index detec-
tor (RI 2000). Polystyrene was used as a standard.
Thermogravimetric data were obtained on a Perkin-
Elmer Pyris 6 TGA under nitrogen flowing condi-
tion at a rate of 20 cm

3

min

1

and a heating rate of

20

C

min

1

. Differential scanning calorimetric

analysis was performed on a Perkin Elmer Pyris
DSC 6 under nitrogen flowing condition at a rate
of 20 cm

3

min

1

and a heating rate of 10

C

min

1

.

Absorption spectra were measured with a UV 500
UV–visible spectrometer at room temperature in
air. Photoluminescence (PL) spectra were mea-
sured with a PerkinElmer LS 55 Luminescence
spectrometer. Cyclic voltammetry (CHI model
619A) was conducted with the use of a three-elec-
trode cell in which ITO was used as a working elec-
trode. A platinum wire was used as an auxiliary
electrode. All cell potentials were taken with the
use of a homemade Ag/AgCl reference electrode.
Ferrocene was used for potential calibration.

EL Device Fabrication and Characterization

The PLED structure in this study is ITO glass/
HTL/polymer/Metal. ITO-coated glass, with a
sheet resistance of 15

X/sq, was purchased from

Applied Film Corp. Glass substrates with pat-
terned ITO electrodes were well washed and
cleaned by oxygen plasma treatment. A thin film
(850 ˚

A) of hole-transporting material poly(3,4-

ethylenedioxythiophene):poly(styrene

sulfonate)

(PEDOT:PSS; Bayton PVPAI 8000) was formed on
the ITO layer of a glass substrate by the spin-
casting method as hole-transporting layer. The
polymer film was then spin-coated from the 10
mg/mL of chlorform solution onto the hole-trans-
porting layer and was dried at 80

C for 1 h in a

glove box. Before film casting, the polymer solu-
tion was filtered through a Telfon filter (0.45 lm).
Subsequently, a cathode was thermally deposited
onto the polymer thin film, followed by the deposi-
tion of metal as the top layer, in a high-vacuum
chamber. After the electrode deposition, the
PLED was transferred from the evaporation
chamber to a glove box purged by high-purity
nitrogen gas to keep oxygen and moisture levels
below 1 ppm. The device was then encapsulated
by glass covers, which was sealed with UV-cured
epoxy glue in the glove box. The decomposition
rate of cathode was determined with a quartz
thickness monitor (STM-100/MF, Sycon). The
thickness of the thin film was determined with a
surface texture analysis system (3030ST, Dektak).
Current-voltage characteristics were measured on
a programmable electrometer with current and
voltage sources (Keithley 2400). Luminance was
measured with a BM-8 luminance meter (Topcon).

Synthesis of Monomers

1,2-Bis(benzoyl)-hydrazine (2)

To a one-necked flask containing benzoyl chloride
(1, 1 g, 7 mmol) and 17 mL of NMP, hydrazine
monohydrate (0.18 g, 3.5 mmol) was added slowly
(Scheme 1). The mixture was allowed to react at
room temperature for 5 h. After reaction was com-
pletely promoted, it was precipitated from dis-
tilled water. The appearing products were col-
lected by filtration, washed with ethyl acetate,
and dried under vacuum to give white solid (2)
(66%). m.p. 145

C (by DSC). FTIR (KBr, cm

1

):

3203 (

ACONHA).

1

H NMR (DMSO-d6, 60

C,

ppm): d10.52 (s, 2H, ANHA); d7.93–7.95 (d, 4H,
Ar-H, J

¼ 7.37 Hz); d7.57–7.60 (t, 4H, Ar-H, J ¼

7.34 Hz); d7.50–7.53 (t, 4H, Ar-H, J ¼ 7.54 Hz).

13

C NMR (DMSO-d6, ppm): d127.5, 128.5, 132.1,

132.6, 166.0; Anal Calac for C

14

H

12

N

2

O

2

: C,

69.66; N, 11.66; H, 5.03. Found C, 69.60; N, 11.70;
H, 5.15.

1,4-Dichloro-1,4-bisphenyl-2,3-diaza-1,3-butadiene (3)

A mixture of 2 (5 g, 21 mmol) and phosphorus
pentachloride (9.5 g, 45 mmol) was dissolved in

CONJUGATED POLYMERS CONTAINING BIPOLAR GROUPS

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Journal of Polymer Science: Part A: Polymer Chemistry
DOI 10.1002/pola

50 mL of toluene and stirred at 120

C for 3 h in a

nitrogen atmosphere. Toluene was stripped off
under vacuum, and the residue was washed with
distilled water. The appearing solids were col-
lected by filtration, dried, and recrystallized from
ethanol to afford yellow crystals of 3 (47.7%). m.p.
121

C (by DSC). FTIR (KBr, cm

1

): 1571

(

AC¼¼N).

1

H NMR (CDCl

3

, ppm): d7.47–7.50 (t,

2H, Ar-H, J

¼ 7.48 Hz); d7.53–7.56 (t, 4H, Ar-H,

J

¼ 7.39 Hz); d8.15–8.16 (d, 4H, Ar-H, J ¼ 8.85

Hz).

13

C NMR (CDCl

3

, ppm): d128.6, 128.7, 131.7,

133.3, 144.1; Anal Calac for C

14

H

10

C

l2

N

2

: C,

60.67; N, 10.11; H, 3.64. Found: C, 60.66; N,
10.19; H, 3.92.

N,N-Bis-(4-bromophenyl)-4-nitrophenylamine (7)

The compound 6 was prepared according to the
published paper.

28

Into a three-necked flask

equipped with a nitrogen inlet and a magnetic
stirrer was added (6, 6 g, 0.021 mol) and dissolved
with 42 mL of DMF. A solution containing NBS
(8.09 g, 0.045 mol) in 24 mL of DMF was added
dropwise very slowly and the reaction mixture
was stirred for 48 h. After reaction was com-
pletely promoted, it was precipitated from dis-

tilled water. The appearing products were col-
lected by filtration and dried under vacuum to
give orange solid 7 (97.7%). m.p. 214

C (by DSC).

FTIR (KBr, cm

1

): 1075 (C

ABr); 1337–1579

(

ANO

2

).

1

H NMR (CDCl

3

, ppm): d6.94–6.96 (d,

2H, Ar-H, J

¼ 9.22 Hz); d7.02–7.06 (d, 4H, Ar-H,

J

¼ 8.66 Hz); d7.46–7.48 (d, 4H, Ar-H, J ¼ 8.66

Hz); d8.03–8.05 (d, 2H, Ar-H, J ¼ 9.24 Hz).

13

C

NMR (CDCl

3

, ppm): d118.8, 119.3, 125.4, 126.4,

127.6, 133.1, 141.2, 144.5, 152.5; Anal Calac for
C

18

H

12

Br

2

N

2

O

2

: C, 48.25; N, 6.25; H, 2.70. Found:

C, 48.24; N, 6.79; H, 2.60.

N,N-Bis-(4-bromophenyl)-4-aminophenylamine (8)

A mixture 7 (9 g, 0.02 mol) and tin chloride (39 g,
0.12 mol) were mixed in 300 mL of ethanol. The
mixture was refluxed for 24 h and the major part
of the ethanol was evaporated under vacuum. To
the residue, cooled in an ice bath, 0.1 M aqueous
NaOH was added. The solids were collected by
filtration and extraction with ethyl acetate. The
ethyl acetate part was dried over anhydrous
sodium sulfate and concentrate. The crude prod-
uct was dried under vacuum to give compound 8
(91%). m.p. 133

C (by DSC). FTIR (KBr, cm

1

):

1071 (C

ABr); 3374, 3462 (ANH

2

).

1

H NMR

Scheme 1.

Synthetic route of dibromo compound (9) containing donor and acceptor

moieties.

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HUANG ET AL.

Journal of Polymer Science: Part A: Polymer Chemistry

DOI 10.1002/pola

(DMSO, ppm): d5.14 (s, 2H, NH): d6.56–6.58 (d,
2H, Ar-H, J

¼ 10 Hz); d6.76–6.78 (d, 2H, Ar-H, J

¼ 8.62 Hz); d6.80–6.83 (d, 4H, Ar-H, J ¼ 8.61 Hz);
d7.29–7.32 (d, 4H, Ar-H, J ¼ 7.07 Hz).

13

C NMR

(DMSO, ppm): d112.9, 115.2, 123.3, 128.1, 131.9,
134.2, 146.7, 146.8; Anal Calac for C

18

H

14

Br

2

N

2

:

C,51.71; N, 6.70.; H, 3.37. Found: C, 51.83; N,
6.60; H, 3.38.

Bis-(4-bromophenyl)-[4-(3,5-diphenyl-1,2,4-tria-
zole-4-yl)-phenyl]amine (9)

The mixture of 3 (2 g, 7.2 mmol) and 9 (2.394
g, 14.4 mmol) were mixed in 60 mL of p-xylene.
The mixture was heated with stirred at 160

C

for 3 days. p-Xylene was stripped off under
vacuum, and the crude product was purified by
recrystallization from ethanol to afford white
crystals 9 (71%). m.p. 247

C (by DSC). FTIR

(KBr, cm

1

): 1071 (C

ABr); 1509 (C¼¼N).

1

H

NMR (CDCl

3

, ppm): d6.92–6.94 (d, 4H, Ar-H, J

¼ 8.73 Hz); d6.96–6.97 (d, 2H, Ar-H); d6.99–
7.00 (d, 2H, Ar-H); d7.31–7.34 (t, 4H, Ar-H, J

¼ 7.35 Hz); d7.36–7.39 (t, 6H, Ar-H, J ¼ 7.45
Hz); d7.47–7.48 (d, 4H, Ar-H, J ¼ 7.20 Hz).

13

C

NMR (CDCl

3

, ppm): d116.9, 122.9, 126.2, 126.7,

128.2, 128.6, 128.7, 129.5, 132.6, 145.3, 147.7,
154.6; Anal Calac for C

32

H

22

Br

2

N

4

: C, 61.26;

N, 9.00; H, 3.56. Found: C, 61.26; N, 8.94; H,
3.61.

Preparation of the Fluorene-Based
Conjugated Polymers

The conjugated polymers were prepared via facile
Suzuki coupling reaction (Scheme 2). The stand-
ard procedure for polymer, TPAF, is described
below.

To a mixture of 9 (1 g, 1.61 mmole), 9,9-di-

octylfluorene-2,7-diboronic acid bis(1,3-propane-
diol) ester (10) (0.925 g, 1.65 mmole), several
drops of Aliquat336

V

R

, and Pd(pph

3

)

4

(0.037 g,

0.032 mmole) under argon was added 2 M aque-
ous K

2

CO

3

(6.7 mL) and THF (10 mL). The mix-

ture was heated to 65

C and stirred for 96 h,

followed by pouring into deionized water and
methanol. The precipitate was collected by filtra-
tion and dried. The precipitate was further puri-
fied by dissolving in chloroform and reprecipi-
tated into deionize water and methanol several
times. The polymer was purified by a Soxhlet
extraction in acetone for 24 h. The polymer
(TPAF) was obtained after drying under vacuum
at 60

C overnight (1.37 g; yield, 94%). FTIR

(KBr, cm

1

): 1508 (C

¼¼N); 2851, 2924 (alkyl

group).

1

H NMR (CDCl

3

, ppm): d0.67–0.81 (m,

10H); d1.07–1.19(m, 20H); d2.05 (s, 4H); d7.04–
7.05 (d, 2H, J

¼ 8.46 Hz); d7.18–7.19 (d, 2H, J ¼

8.50 Hz); d7.26–7.28 (t, 4H); d7.33–7.49 (m, 6H);
d7.57–7.60 (t, 8H); d7.65–7.67 (d, 4H, J ¼ 8.29
Hz); d7.77–7.79 (d, 2H, J ¼ 7.87 Hz).

13

C NMR

(CDCl

3

, ppm): d14.0, 22.5, 23.7, 29.1, 29.9, 31.7,

40.4, 55.2, 120.0, 120.9, 122.6, 125.3, 125.7,
126.5, 127.1, 127.8, 128.2, 128.4, 128.6, 128.7,
128.9, 129.8, 137.4, 139.0, 139.9, 145.6, 148.7,
151.6, 154.7; Anal Calac for C

61

H

64

N

4

: C, 85.87;

N, 6.57; H, 7.56. Found: C, 84.85; N, 6.49; H,
7.47.

The TPABTF copolymer was prepared in the

similar procedure instead of compound 9 with
equal ratio of dibromo compounds (9 and 11).
The yield is 75%. FTIR (KBr, cm

1

): 1508

(C

¼¼N); 2845, 2924 (alkyl group).

1

H NMR

(CDCl

3

, ppm): d0.80–1.26 (m, 60H); d2.06–2.16

(d, 8H); d7.06 (s, 2H); d7.19–7.28 (m, 6H); d7.42–
7.46 (m, 6H); d7.59–7.68 (m, 12H); d7.78–8.08
(m, 10H). Anal Calac for C

96

H

106

N

6

S: C,83.80;

N,6.11; H, 7.76. Found for : C, 83.42; N, 6.21; H,
7.89.

RESULTS AND DISCUSSION

The bipolar dibromo monomer (9) containing elec-
tron-donor triphenylamine and electro-acceptor
1,2,4-triazole moieties was prepared from chlori-
nated azine compound (3) and triphenylamine-
substituted aniline compound (8) via a facile
cyclocondensation reaction as shown in Scheme 1.
The benzoyl chloride (1) was reacted with hydra-
zine hydrate to give hydrazide (2) and then chlo-
rinated in the presence of phosphorus penta-
chloride to give chlorinated azine compound (3).
The dibromo triphenylamine-substituted aniline
compound (8) was prepared via reduction (by
tin chloride) and bromination reaction (by NBS;
N-bromosuccinimide)

of

4-nitrotriphenylamine

(6), which was prepared by the condensation of
diphenylamine (4) and 4-fluoronitrobenzene (5).
The structures of all intermediates and the
desired bipolar dibromo compound (9) were con-
firmed by NMR, FTIR, and elemental analysis.
The cyclocondensation of chlorinated azine com-
pound (3) and dibromo triphenylamine-substi-
tuted aniline compound (8) to 1,2,4-triazole ring
could be monitored by the change of characteristic
band in IR spectra. The characteristic band of
chlorinated azine compound (3) at 1571 cm

1

CONJUGATED POLYMERS CONTAINING BIPOLAR GROUPS

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Journal of Polymer Science: Part A: Polymer Chemistry
DOI 10.1002/pola

disappeared and a new characteristic band of
1,2,4-triazole ring appeared at 1509 cm

1

in the

FTIR spectra. Figure 1 shows the

1

H NMR [Fig.

1(A)] and

13

C NMR [Fig. 1(B)] spectra of the

desired dibromo monomer (9) in d-chloroform as
well as the peak assignments assisted by 2-dimen-
sional COSY [Fig. 1(C)] and HMQC techniques
[Fig. 1(D)]. When a resonance electron-withdraw-
ing group is present on a phenyl ring, a partial
positive charge develops at the ortho and para
positions through resonance interactions. In the

1

H NMR spectrum, the peaks for the para- (peak

H

g

; 7.37 ppm) and ortho- (peak H

e

; 7.47 ppm) pro-

tons at the phenyl groups attached to C

3

and C

5

position of 1,2,4-triazole unit suggest the electro-
withdrawing ability of 1,2,4-triazole moiety. How-
ever, the peak of ortho-proton (peak H

d

) at the

phenyl group attached to N

4

position of 1,2,4-tria-

zole unit exhibited at up-field (7.00 ppm) due to
the elecro-donating ability of triphenylamine
moiety. Therefore, the dibromo compound (9) is a
bipolar compound containing electro-donor and
electro-acceptor moieties. In the

13

C NMR spec-

trum [Fig. 1(B)], the carbons (C

9

) in the 1,2,4-tria-

zole moiety shows at 154.6 ppm. Assignments of
each proton and carbon were assisted by COSY

Figure 1.

(A)

1

H NMR (B)

13

C NMR (C) COSY, and (D) HMQC spectra of the bipo-

lar dibromo compound (9).

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Journal of Polymer Science: Part A: Polymer Chemistry

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[Fig. 1(C)] and two-dimensional (2D) HMQC [Fig.
1(D)] techniques. The basic COSY and HMQC pro-
cedures give 2D spectra from which almost all of
the

1

H-

1

H and

1

H-

13

C connectivity can be easily

determined. The overlapped carbon peaks of C

7

,

C

10

, and C

11

at 128.6 ppm were observed and

easily confirmed by the 2D spectra. Elemental
analysis, IR, and NMR spectra clearly confirmed
that the desired dibromo compound (9) synthe-
sized herein is fully consistent with the proposed
structure.

Two fluorene-based conjugated polymers shown

in Scheme 2 were readily prepared via facile
Suzuki cross-coupling from 2,7-bis(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctyl-fluorene
(10), the bipolar dibromo monomer (9), and 4,7-
dibromo-2,1,3-benzothiadiazole (11), which was
prepared according the published paper.

34

The po-

lymerization was carried out in a biphasic mixture
of aqueous K

2

CO

3

and THF with Pd(PPh

3

)

4

as the

catalyst precursor. The polymers were obtained in
high yields (94–95%). Figure 2 illustrates the

1

H

NMR spectra of the resulted conjugated polymer-
sTPAF [Fig. 2(A)] and TPABTF [Fig. 2(B)] as well
as the peak assignments. The protons of the ali-
philic n-octyl chain appear at 0.8–2.2 ppm. The
proton peak assignments of the polymer are easy
to be decided based on the peak assignments of the

bipolar monomer and fluorene monomer as well
as the integral ratios. From the characteristic
peak integrals of H

d

and H

h

in Figure 2(B), the

segment ratio (p/q) (as shown in Scheme 2) of
triphenylaminotriazole to 2,1,3-benzothiadiazole
units in TPABTF [Fig. 2(B)] was calculated to
be about 3/2. In the IR spectra, the characteris-
tic band of 1,2,4-triazole exhibited at 1508 cm

1

and the characteristic bands of alkyl chains
showed at 2851, 2924 cm

1

. Moreover, the char-

acteristic stretching C

ABr band of the dibromo

monomer (9) at 1070 cm

1

disappeared. These

results suggest the conjugated polymers were
successfully prepared.

The molecular weight and thermal properties

of the conjugated polymers are summarized in Ta-
ble 1. The number-average molecular weight (M

n

)

and weight-average molecular weight (M

w

) of the

polymers were 2.05

 10

4

and 3.25

 10

4

for poly-

mer TPAF and 2.40

 10

4

and 3.80

 10

4

for poly-

mer TPABTF, respectively, and the polydispersity
index (PDI

¼ M

w

/M

n

) were about 1.60, as meas-

ured by gel permeation chromatography (GPC),
relative to polystyrene standards. The thermal
properties of the conjugated polymer were eval-
uated by differential scanning calorimeter (DSC)
and thermogravimetric analysis (TGA). The glass
transition temperatures (T

g

) of the polymers were

Scheme 2.

Preparation of the conjugated polymers from the bipolar monomer (9).

CONJUGATED POLYMERS CONTAINING BIPOLAR GROUPS

6237

Journal of Polymer Science: Part A: Polymer Chemistry
DOI 10.1002/pola

found to be 135 and 155

C, respectively, which

are much higher than that of poly-(9,9-diotylfluor-
ene) (POF; T

g

¼ 75

C).

35

It is evident that incor-

porating the rigid triphenylamine with pendant
1,2,4-triazole units into the polymer backbone
enhances the rigidity of the chain and increases
T

g

. A relatively high value of T

g

, which can pre-

vent morphological change and suppress the for-
mation of aggregates and excimers upon exposure
to heat, is an essential characteristic of polymers
if they are to be used as emissive materials in
light-emitting applications. The DSC traces of the
two polymers did not show any absorption peak
after annealing at 200

C for 30 min. The result

Figure 2.

1

H NMR spectrum of the conjugated polymers (A) TPAF and (B)

TPABTF.

Table 1. Molecular Weights and Thermal Properties
of the Conjugated Polymers

Polymer

GPC Data

a

Thermal

Properties

b

M

w

M

n

PDI

T

g

Td

5

Td

10

TPAF

32,500

20,500

1.58

135

428

455

TPABTF

38,000

24,000

1.60

155

433

456

a

The molecular weights are relative to polystyrene

standard.

b

By DSC and TGA measurements, respectively. Td

5

and

Td

10

are 5% and 10% degradation temperature, respectively.

6238

HUANG ET AL.

Journal of Polymer Science: Part A: Polymer Chemistry

DOI 10.1002/pola

means the polymers are amorphous. The poly-
mers show multistep degradation in the TGA
spectra in nitrogen atmosphere as shown in Fig-
ure 3. The 1st degradation begins at about 420

C

for both polymers and ends at about 500 and
513

C with weight losses of 27.0% and 33.0% for

polymers TPAF and TPABTF, respectively. The
weight losses at 1st stage are analogous to the
alkyl content in the polymers (26.6% and 32.9%,
respectively). These results suggest that the 1st
degradation was due to the decomposition of weak
n-octyl groups attached on fluorene.

36

The tem-

peratures for 5% and 10% weight loss of the poly-
mers were observed at 428

C and 455

C for

TPAF, and at 433

C and 456

C for TPABTF,

respectively.

The solubility of the conjugated polymers is

summarized in Table 2. Although the polymer has
extended conjugation along the rigid polymer back-
bone, aliphatic long n-octyl substituent render the
polymer highly soluble in most common organic
solvents, such as dichloromethane, chloroform,
THF, dichlorobenzene, and 1,4-dioxane at room
temperature, which make the structural charac-
terization very convenient, but only partially solu-
ble in toluene. The aliphatic n-octyl groups in the

polymers play an important role for solubility but
decrease the thermal stability of the polymers.

The optical properties of the polymers were

investigated by UV–vis and photoluminescence
spectroscopy in solid state and in chloroform with
a polymer concentration of 1 mg/10 mL. The UV–
vis and photoluminescence spectroscopy of the
polymers are shown in Figure 4 and the results
are summarized in Table 3. The polymer film of
TPAF [Fig. 4(A)] exhibits a major absorption at
364 and a minor absorption at 280 nm. Similar
absorption spectrum was observed at 275 and 371
nm for chloroform solution. By comparing the
spectra with the absorption spectra of POF and
the dibromo compound 9, we ascribe peaks at the
364 and 371 nm to a p–p* transition derived from
the conjugated polymer backbone. It is blue shift
by 20–27 nm relative to that observed for POF
(391 nm)

35

due to the propeller-shape structure of

triphenylamine. The peaks at 275 and 280 nm are
assigned to the absorption originating from the
pendant 3,4,5-triphenyl-1,2,4-triazole groups. The
cut-off of the absorption exhibited at 425 nm for
the solid state, and energy gap (E

g

) was calculated

to be 2.92 eV. The polymer emits blue fluorescence
in chloroform solution with a maximum emission
(k

PL
max

) at 421 nm and a shoulder peak at 435 nm,

whereas it exhibits a maximum emission (k

PL
max

) in

film state at 435 nm with a FWHW (full width at
half wavelength) of 50 nm. The PL quantum yield
(

U

f

) in chloroform as excited at 350 nm was mea-

sured to be 0.92 relative to a 9,10-diphenylan-
thrancene (

U

f

¼ 0.9) standard.

36

The quantum

yield in chloroform is higher than the fluorescence
yield measured for POF (

U

f

¼ 0.85). The spectral

changes observed in thin film are the result of ag-
gregate formation in the solid state. The quantum
yield of the conjugated polymer in solid state was
estimated to be 0.18. For the copolymer TPABTF
containing 2,1,3-benzothiadiazole (BTDZ), a peak
locating at 346 nm attributed to the p–p* elec-
tronic transition of the polymer backbones [Fig.
4(B)]. The additional shoulder absorption peak at
ca. 450 nm is responsible for the BTDZ unit in

Figure 3.

TGA spectra of the conjugated polymers

in nitrogen atmosphere.

Table 2. Solubility of the Conjugated Polymers

a

Polymer

CH

2

Cl

2

CHCl

3

THF

Toluene

Dichlorobenzene

1,4-Dioxane

TPAF

þþ

þþ

þþ

þ

þþ

þþ

TPABTF

þþ

þþ

þþ

þ

þþ

þ

a

The solubility was measured in the concentraction of 1 mg/mL.

þþ, soluble at room temperature; þ, partially soluble at room temperature.

CONJUGATED POLYMERS CONTAINING BIPOLAR GROUPS

6239

Journal of Polymer Science: Part A: Polymer Chemistry
DOI 10.1002/pola

the copolymer.

37

The polymer emits green–yellow

fluorescence in chloroform solution with a maxi-
mum emission (k

PL
max

) at 543 nm, whereas it exhib-

its a maximum emission (k

PL
max

) in film state at

546 nm. The emission is red shift by 111 nm rela-

tive to that observed for TPAF due to the BTDZ
unit. Different from the emission of TPAF, the
emission of TPABTF shows a small peak at ca.
425 nm in the solutions, which depends on the
polymer concentration.

Figure 5 shows the concentration dependence

of PL spectra of TPABTF in chloroform. Figure 5
indicates that emission at around 425 nm respon-
sible for the TPAF segment increases with
decreasing copolymer concentration and that
TPAF emission can be completely quenched only
in highly concentrated solutions or film state. The
ratio of the integrated intensities of 550 nm peaks
to that of 425 nm peaks can be considered as a
measure of energy transfer efficiency from fluo-
rene segment to BTDZ unit.

37

The result shows

the energy transfer efficiency from fluorene seg-
ment to BTDZ increases with decreasing copoly-
mer concentration.

The electrochemical behavior of the polymers

was investigated by cyclic voltammetry (CV). The
cyclic voltammograms are shown in Figure 6.

Figure 4.

Absorption and PL spectra of the fluorene-

based conjugated polymers (A) TPAF (B) TPABTF.
[Color figure can be viewed in the online issue, which
is available at www.interscience.wiley.com.]

Table 3. Optical Properties of the Conjugated Polymers

Polymer

k

UV
max

(nm)

k

PL
max

(nm)

a

U

f

(%)

b

Chloroform

Film

Chloroform

Toluene

Film

Chloroform

Film

TPAF

275, 371

280, 364

421, 442

–

435

91.8

18.4

TPABTF

346, 438

346, 453

543

526

546

98.8

33.1

a

The maximum wavelength (nm) of photoluminescence was excited by the maximum wave-

length (nm) of absorption.

b

The PL quantum yields (

U

f

) were measured relative to a 9,10-diphenylanthrancene (

U

f

¼

0.9) standard.

Figure 5.

PL spectra of polymer TPABTF polymer

in chloroform solution with different concentrations.
[Color figure can be viewed in the online issue, which
is available at www.interscience.wiley.com.]

6240

HUANG ET AL.

Journal of Polymer Science: Part A: Polymer Chemistry

DOI 10.1002/pola

From the oxidation potential relative to ferrocene/
ferrocenenium one, which can correspond to 4.8
eV below the vacuum level,

38

we can approxi-

mately calculate the HOMO energy level of the
conjugated polymers. The LUMO level of the poly-
mer was calculated from the HOMO energy and
optical band gap estimated from the absorption
onset according to the equation: LUMO

¼ HOMO

þ E

g

. The results as well as the data of POF

reported in the literature

39

are summarized in Ta-

ble 4. The HOMO and LUMO values of the TPAF
polymer were calculated be

5.31 and 2.39 eV,

respectively. The HOMO and LUMO values of the
TPABTF polymer were calculated be

5.38 and

3.02 eV, respectively. Both prepared polymers
exhibited higher HOMO lever than POF.

The investigation of electroluminescent proper-

ties was conducted by fabricating devices with the
configuration of ITO/HTL/polymer/metal. The
PEDOT:PSS or poly(vinyl carbazole) (PVK) were
used as hole-transporting materials. The hole-

transporting material PVK was blended with the
emissive polymer due to the solubility of PVK in
chloroform, which is likely that the interface
between the PVK and copolymer layers is diffuse
rather than being flat and well defined. The EL
performance of all devices obtained from the cur-
rent density (J)-Voltage (V)-Luminance (L) char-
acteristics is summarized in Table 5. An EL device
(I) with an emission maximum (k

EL

max

) at 444 nm

and a FWHM (full width at half wavelength) of
96 nm was observed for the polymer TPAF. When
the radiometric EL spectrum was converted into a
chromaticity coordinated on a CIE (Commission
International de I’Eclairage) 1931 diagram, an
indication (0.16, 0.13) of the blue light emitting
from the device was obtained. The emission
started at around 6 V and reached a maximum
luminescence of 84 cd/m

2

at 11.5 V. The devices

(II–IV) using polymer TPABTF as emissive layer
exhibited green–yellow emission with maximum
emission at 538–544 nm due to the BTDZ moiety.
The emission of the device III with configura-
tion of ITO/PEDOT:PSS/TPABTF/Ca:Ag stared
at 5 V and reached a maximum luminescence of
696 cd/m

2

at 13 V with a maximum emission

Figure 6.

Cyclic voltammograms of monomer 9 and

prepared polymers.

Table 4. Electrochemical Properties of the
Conjugated Polymers

Polymer

E

g

(eV)

a

E

onset

(V)

b

HOMO

(eV)

c

LUMO

(eV)

c

TPAF

2.91

1.11

5.31

2.39

TPABTF

2.36

1.18

5.38

3.02

POF

39

2.95

1.4

5.80

2.85

a

Energy gap (E

g

)

¼ 1240/k

onset

(nm).

b

Initial oxidation potential in film state.

c

HOMO

¼ (E

ox,onset

 E

Fc

ox

;onset

)

 4.8; LUMO ¼ E

g

HOMO.

Table 5. EL Performance of Devices with the Configuration of ITO/HTL/Polymer/Metal

Device

HTL

a

Polymer

Cathode

V

on

(V)

b

L

max

(cd/m

2

)

c

E

max

(cd/A)

d

k

EL

max

(nm)

CIE (x,y)

I

PEDOT

TPAF

LiF:Al

6

84

0.044

444

0.160, 0.130

II

PEDOT

TPABTF

Mg:Ag

7

516

1.24

544

0.355, 0.618

III

PEDOT

TPABTF

Ca:Ag

5

696

2.02

542

0.345, 0.625

IV

–

TPABTF

þ5%PVK

Ca:Ag

14

629

0.06

538

0.306, 0.628

V

–

TPABTF

þ10%PVK Ca:Ag

–

–

–

–

–

a

Hole-transporting layer.

b

Turn-on voltage observed at 1 cd/m

2

.

c

Maximum luminance.

d

Maximum luminance efficiency.

CONJUGATED POLYMERS CONTAINING BIPOLAR GROUPS

6241

Journal of Polymer Science: Part A: Polymer Chemistry
DOI 10.1002/pola

coordinated on CIE (0.345, 0.625) and a peak effi-
ciency of 2.02 cd/A.

The energy level diagrams of the devices are

sketched in Figure 7. For device I fabricated with
TPAF, there is a small energy gap between the
PEDOT-PSS and the TPAF layers, which implies
efficient hole-injection process across this inter-
face. The matching of energy-levels is much better
than that of PEDOT-PSS with poly(9,9-dioctyl-
fluorene), which has an estimated HOMO level
of –5.8 eV.

39

Similar tactics of introducing hole-

transporting triarylamines onto the polyfluorene
backbone

have

been

previously

adopted

to

enhance the hole-injection properties.

22,40–42

On

the other hand, due to the presence of large
energy gap of 0.51 eV between the LiF/Al cathode
and TPAF, high-energy-barrier for electron injec-
tion is expected. This led to a contradictory pre-
diction that against the observation of the low
turn-on voltage of 6.0 V. This dilemma arises from
the optical LUMO assignment based on the UV–
vis spectral data.

The electronic properties of the polymers were

studied by PM5 method with CAChe package
version 6.1. Figure 8 demonstrated the results
of our semiempirical calculations, in which we
adopted A-C as models for theoretical investiga-
tion. The first HOMO of A mainly contains the
lone pair of the triarylamine segment, whereas
second and third HOMO and HOMO consist of
the fluorene components. Their calculated orbital

energies are –8.319, –8.558, and –8.693 eV,
respectively.

On the other hand, the first LUMO (

0.428

eV) mainly arises from the triazole unit, whereas
the second LUMO (

0.408 eV) arises from the

fluorene units. Our results revealed that the
HOMO–LUMO electronic transitions from the
nitrogen lone-pair or from the p bonds of the PF
backbone to the LUMO of the triazole unit is less
effective due to poor orbital overlap integrals.
Therefore, the major absorption band observed in
the UV–vis spectrum is originating from the elec-
tronic transition of the fluorene segment.

This prediction is further supported by our

ZINDO calculations; (1) The major absorption
band of TPAF is mainly arising from the elec-
tronic transition between the HOMO/LUMO of
the polyfluorene backbone. (2) The transition
from the triphenylamine core to the triazole com-
ponent is much weaker. (3) The allowed HOMO/
LUMO transition of the triazole component
appears at higher energy region due to the low-
lying HOMO level. Even though the LUMO of the
triazole unit is indeed lower than that of the fluo-
rene unit by 0.2 eV, it could not be reflected by the
optical data. After correction of the LUMO, the
energy gap between the LiF/Al cathode and TPAF
should be reduced to 0.31 eV. More important is
the fact that the calculated electron-affinity of
28.6 kcal/mol for the triazole unit B is much
stronger than that of 16.5 kcal/mol for the fluo-
rene unit C, where calculated electron-affinity
(EA) was defined by the equation: EA

¼ the heat

of formation of radical anion of B (or C) – Heat of
formation of B (or C). This makes the electron-
deficient triazole component a good electron
acceptor. The presence of the triazole segments
would lead to big contribution to the electron-
transport properties. All these explain the role of
the triazole units on TPAF.

43

It is noteworthy to mention that device I dem-

onstrates relatively low-electroluminescent cur-
rent-efficiency. Since the PL quantum yield of

Figure 7.

Relative energy level diagrams for the

devices.

6242

HUANG ET AL.

Journal of Polymer Science: Part A: Polymer Chemistry

DOI 10.1002/pola

TPAF is high, we attribute the low-current-effi-
ciency to the imbalanced charge-transport proper-
ties of TPAF. It is known that the polyfluorene
has higher hole-mobility than the electron-mobil-
ity.

44

Introduction of the triphenylamine units

conjugated with the polyfluorene backbone would
further enhance the bias.

To improve the PLED’s current efficiency,

materials with balanced charge-injection and
transport properties are highly desired. It has
been known that poly(fluorene-benzothiadiazole)
had been used to reduce the electron-injection

barrier

between

Al

and

the

light-emitting

layers.

45–48

Therefore, introduction of benzothia-

diazole onto the TPFA backbone to form a copoly-
mer of TPABTF should provide a reasonable
solution. Indeed TPABTF shows much better
PLED performance either on the turn-on voltage
as well as the current efficiency. Instead of having
a relatively poor apparent energy-level matching
of

DE ¼ 0.68 eV on the cathode, between the

metallic Mg cathode and the TPABTF layer,
device II shows the low turn-on voltage of 7 V
with the high-current-efficiency of 1.24 cd/A.
When the calcium-silver metallic cathode, which
has even lower work function, was employed, the
turn-on voltage was further reduced to 5 V with
the current efficiency of 2.02 cd/A which is 50
times higher than that of device I.

To further verify the role of the charge-carrier

balance conditions on the PLED performance,
devices IV and V were fabricated and tested. In
these devices, PVK, a hole-transport polymer, was
mixed with TPABTF and spin-coated on top of
ITO/PEDOT-PSS for device fabrication. Adding
PVK would help in blocking electrons from
entering the light-emitting layer while holes are
allowed to penetrate.

49

In these cases, the turn-on

voltage significantly increases while the electrolu-
minescent efficiency undoubtfully drops. These
results indicated that prohibiting the electron-
injection process would significantly retard the
PLED process. The role of the triazole and benzo-
thiadiazole units are clearly on increasing the
electron-injection and electron-transport proper-
ties of the polymer matrix.

Our results suggest that introduction of the

bipolar moiety on to the PF backbone would
significantly help in balancing charge injection
and transport through the polymer matrix;
Being further helped by the benzothiadiazole
units, the PLED efficiency of TPABTF further
boosted up. The study of multilayer PLED de-
vices in which higher quantum efficiency is
expected is ongoing.

CONCLUSIONS

A bipolar dibromo monomer (9) containing a elec-
tro-donating triphenylamine unit and a electro-
deficient 1,2,4-triazole unit was synthesized and
used to prepare fluorene-based conjugated poly-
mers. Two conjugated polymers containing the
bipolar moiety were prepared. The two conjugated
polymers exhibited good thermal properties due

Figure 8.

(A) A plot that combines the first three

HOMO’s of A. (B) The first LUMO of A. (C) The sec-
ond LUMO of A.

CONJUGATED POLYMERS CONTAINING BIPOLAR GROUPS

6243

Journal of Polymer Science: Part A: Polymer Chemistry
DOI 10.1002/pola

to the rigid triphenylamine and triazole groups.
The bipolar moiety on to the PF backbone would
significantly help in balancing charge injection
and transport through the polymer matrix. The
electroluminescent properties of the polymers
showed low turn-on voltages due to the bipolar
moiety.

Financial support through project NSC98-2221-E-027-
002 by National Science Council of Republic of China,
Taiwan, is gratefully acknowledged.

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