Synthesis of triphenylamine copolymers and effect of their chemical structures on physical properties

Article abstract of DOI:10.1021/ma200940v

A series of charge transporting triphenylamine (TPA) polymers containing 2,1,3-benzothiadiazole (BTH) and/or 3,4-ethylenedioxythiophene (EDOT) unit(s) in the backbone were synthesized via C-N coupling polymerization using a palladium catalyst system. The

Full text of DOI:10.1021/ma200940v

ARTICLE
pubs.acs.org/Macromolecules
Synthesis of Triphenylamine Copolymers and Effect of Their
Chemical Structures on Physical Properties
Kousuke Tsuchiya,* Takashi Sakakura, and Kenji Ogino
Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho,
Koganei-shi, Tokyo 184-8588, Japan
S Supporting Information
b
ABSTRACT: A series of charge transporting triphenylamine (TPA)
polymers containing 2,1,3-benzothiadiazole (BTH) and/or 3,4-ethylene-
dioxythiophene (EDOT) unit(s) in the backbone were synthesized via
CꢀN coupling polymerization using a palladium catalyst system. The
preparation of the diblock copolymers based on the TPA-polymers was
achieved by introducing poly(ethylene oxide) (PEO) at the terminal.
Furthermore, the block copolymers selectively functionalized by BTH at
the junction point were also prepared. In order to investigate the structural
and morphological effect on physical properties, the optical and electrical properties were measured and compared among the
homopolymers, random copolymers, and block copolymers. The photoluminescent (PL) spectra of TPA-polymers containing
BTH unit were tunable from blue to red-violet color by the amount of BTH unit. The EDOT-containing polymers showed high hole
drift mobility up to 1 ꢁ 10ꢀ3 cm2 Vꢀ1 sꢀ1. On the other hand, the mobility of the BTH-containing polymers was affected by the
position of the BTH unit in the polymer structure. The polymers with BTH unit randomly involved in the backbone showed low
mobility in the order of 10ꢀ6ꢀ10ꢀ5, whereas no decrease in the mobility was observed for the polymers modified by BTH unit at the
junction point.
’ INTRODUCTION
for materials in future devices. Block copolymers containing arylamine
have been prepared for application to organic light-emitting diode
(OLED) or organic photovoltaic cell.13ꢀ18 We previously prepared
bipolar charge transporting block copolymers consisting of poly
(3-vinyltriphenylamine)19 or poly[2,7-dimethoxy-N-(4-vinylphenyl)
carbazole]20,21 in order to compare the performance of OLED
devices with corresponding random copolymers and polymer
blends. It is revealed that the devices using block copolymers
afforded higher external quantum efficiency compared with
random copolymers or polymer blends. In such devices, the
morphological effect on device performance must be of signifi-
cant importance besides physical properties of the materials.
Hence, sophisticated designs of block copolymers matching to
desired functionality as well as evaluation on the synergistic
performance attributed to the phase-separated structure created
by these architectures are required for further improvement of
the device performance. For example, we prepared the diblock
copolymers consisting of regioregular poly(3-hexylthiophene)
(P3HT) and PEO for a donor material in photovoltaic devices.22
It was demonstrated that the introduction of PEO significantly
changed the phase-separated morphology, where vertically
aligned lamellar structures were observed, in the device using
the block copolymer blended with a fullerene derivative. As a
Arylamine compounds have been widely synthesized and
researched for various possible applications such as electrolumi-
nescent, photovoltaic, and magnetic devices because of their
excellent electrical properties. There are also a large variety of
polymers which contains an arylamine moiety as a backbone of
main chain or a side group.1ꢀ8 Poly(triphenylamine) (PTPA) is
a class of such polymers which is conventionally prepared by
oxidative polymerization from triphenylamine (TPA) derivatives
as monomers.9ꢀ11 PTPA was found to show high hole mobility
and high photoconductivity among amorphous materials such as
poly(9-vinylcarbazole), a well-knownholetransporting polymer.10
In our previous report, a novel synthetic route for the preparation
of PTPA was developed by means of CꢀN coupling polymeriza-
tion using a palladium-based catalytic system.12 We demonstrated
that the modification of each terminal of PTPA could be success-
fully achieved by adding terminal modifying derivatives during
polymerization and that diblock copolymers consisting of PTPA
and poly(ethylene oxide) (PEO) was prepared by our method.
The diblock copolymer poly(4-butyltriphenylamine)-block-poly-
(ethylene oxide) (PTPA-b-PEO) exhibited microphase-separated
structures in thin films, which offers a possibility of new application
of PTPA in bulk.
Microphase separation behavior of block copolymers has been
intensively studied in view of both academic and practical interests.
Block copolymers provide a variety of microphase-separated struc-
tures such as lamellae and cylinder whose domain sizes ranging from
several to tens of nanometers match the technological requirements
Received: April 24, 2011
Revised:
June 2, 2011
Published: June 17, 2011
r
2011 American Chemical Society
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ARTICLE
result, the device provided higher power conversion efficiency
compared with the device based on P3HT homopolymer.
In this paper, a novel series of charge transporting polymers
containing TPA moiety were designed and synthesized by CꢀN
coupling polymerization explained above. To the PTPA back-
bone, 2,1,3-benzothiadiazole (BTH) and 3,4-ethylenedioxythio-
phene (EDOT) were introduced for modifying optical and
electrical properties. Diblock copolymers consisting of TPA-
polymers and PEO were also prepared by terminal modification
during polymerization using PEO derivatives. The optical and
electrical properties were thoroughly examined for these TPA-
polymers. So far, although numerous works on charge transport-
ing property have been done on not only arylamine polymers but
also molecular-doped polymer blend systems,23,24 the charge
drift mobility in the block copolymer films and its dependence on
the morphology have not been profoundly concerned. We
demonstrated in this work that the hole drift mobility was
compared among TPA-containing homopolymer, random co-
polymer, and block copolymer in order to investigate structural
and morphological effects on hole-transporting ability of the
TPA-polymers.
2H), 7.72 (s, 2H), 7.65 (d, J = 8.4 Hz, 2H), 7.18ꢀ7.08 (m, 6H), 5.82
(s, 1H), 2.59 (t, J = 7.5 Hz, 2H), 1.60 (quint, J = 7.5 Hz, 2H), 1.37 (sex, J =
7.5 Hz, 2H), 0.94 (t, J = 7.5 Hz, 3H). 13C NMR (CDCl3): δ154.09, 153.88,
144.51, 139.56, 136.87, 136.39, 133.54, 131.72, 130.86, 130.68, 130.26,
129.29, 128.61, 128.19, 126.62, 122.41, 119.61, 116.08, 34.98, 33.79, 22.35,
13.99. Anal. Calcd for C28H24BrN3S: C, 65.37; H, 4.70; N, 8.17. Found: C,
65.08; H, 4.62; N, 8.17.
Synthesis of2,5-Diphenyl-3,4-ethylenedioxythiophene(6).
To a two-necked flask equipped with a stopcock and a condenser were
added 5 (12.7 g, 42.4 mmol), phenylboronic acid (11.4 g, 93.2 mmol),
Pd(PPh3)4 (0.392 g, 0.339 mmol), and distilled THF (50 mL) under
nitrogen. After 2 M K2CO3(aq) (50 mL, purged by nitrogen for 1 h) was
added, the mixture was vigorously stirred at reflux for 24 h. Chloroform
was added after cooling down to room temperature, and the mixture was
washed with water. The organic layer was dried with MgSO4 and
concentrated by a rotary evaporator. The crude product was recrystallized
from methanol to give a yellow needle crystal. The yield was 7.91 g (63%).
1H NMR (CDCl3): δ 7.79 (d, J = 7.2 Hz, 4H), 7.41 (t, J = 7.2 Hz, 4H),
7.26 (t, J = 7.2 Hz, 2H), 4.36 (s, 4H). 13C NMR (CDCl3): δ 138.56,
132.92, 128.61, 126.58, 126.07, 115.36, 64.56. Anal. Calcd for C18H14O2S: C,
73.44; H, 4.79. Found: C, 73.62; H, 4.93.
Synthesis of 2,5-Bis(40-bromophenyl)-3,4-ethylenedioxy-
thiophene (7). To a mixture of 6 (9.50 g, 32.3 mmol) and chloroform
(200 mL) were added bromine (3.31 mL, 64.6 mmol) and chloroform
(100 mL) dropwise at 0 °C, and the resulting mixture was stirred for 30
min. Additional stirring were carried out for 6 h at room temperature.
The mixture was successively washed with 3% NaOH(aq), saturated
NaHSO3(aq), and water. The organic layer was dried with MgSO4 and
concentrated by a rotary evaporator. The crude product was recrystal-
lized from hexane to give a white needle crystal. The yield was 14.1 g
(97%). 1H NMR (CDCl3): δ 7.61 (d, J = 8.4 Hz, 4H), 7.48 (d, J = 8.4 Hz,
4H), 4.37 (s, 4H). 13C NMR (CDCl3): δ 139.19, 132.00, 131.92, 127.75,
120.59, 114.86, 64.79. Anal. Calcd for C18H12Br2O2S: C, 47.81; H, 2.68.
Found: C, 47.87; H, 2.58.
’ EXPERIMENTAL SECTION
Materials. Toluene was distilled over CaH2 and stored under
nitrogen. Tetrahydrofuran (THF) was used as distilled over sodium
and benzophenone. 4,7-Dibromo-2,1,3-benzothiadiazole (1),25 4,7-bis-
(40-bromophenyl)-2,1,3-benzothiadiazole (3),26 and 2,5-dibromo-3,4-
ethylenedioxythiophene (5)27 were prepared by reported procedures.
For the preparations of 4,7-diphenyl-2,1,3-benzothiadiazole (2) and 2,5-
diphenyl-3,4-ethylenedioxythiophene (6), the modified procedure re-
ported by Fang et al. was applied.28 A monomer for poly(4-butyl-
triphenylamine), 4-(40-bromophenyl)-400-butyldiphenylamine (9), and
poly(ethylene oxide) with a 4-bromophenyl terminal (PEO-Br) were
prepared according to previous report.12 All the other reagents were used
as purchased without any further purification.
Synthesis of 2-(40-Bromophenyl)-5-[400-(4000-butylphenylamino)
phenyl]-3,4-ethylenedioxythiophene (8). To a two-necked flask
equipped with a stopcock and a condenser were added 7 (4.78 g,
10.6 mmol), 4-butylaniline (1.67 mL, 10.6 mmol), Pd(dppf)Cl2 (0.086 g,
0.106 mmol), sodium tert-butoxide (1.42 g, 14.8 mmol), and dry toluene
(75 mL) under nitrogen. The mixture was stirred at reflux for 12 h. After
cooling down to room temperature, the mixture was washed with brine.
The organic layer was dried with MgSO4 and concentrated by a
rotary evaporator. The crude product was purified by column chroma-
tography eluted with toluene/hexane (2/1 in volume ratio) to give a
Synthesis of 4,7-Diphenyl-2,1,3-benzothiadiazole (2). To a
two-necked flask equipped with a stopcock and a condenser were added 1
(10.0 g, 34.0 mmol), phenylboronic acid (9.12 g, 74.8 mmol), tetrakis-
(triphenylphosphine)palladium(0) (Pd(PPh3)4) (0.078 g, 0.068 mmol),
and distilled THF (50 mL) under nitrogen. After 2 M K2CO3(aq)
(50mL, purgedbynitrogenfor 1 h) wasadded, themixture wasvigorously
stirred at reflux for 24 h. Chloroform was added after cooling down to
room temperature, and the mixture was washed with water. The organic
layer was dried with MgSO4 and concentrated by a rotary evaporator. The
crude product was recrystallized from hexane to give a yellow needle
crystal. Yield was 7.31 g (75%). 1H NMR (CDCl3): δ 7.97 (d, J = 7.2 Hz,
4H), 7.80 (s, 2H), 7.56 (t, J = 7.2 Hz, 4H), 7.47 (t, J = 7.2 Hz, 2H). 13C
NMR (CDCl3): δ 154.11, 137.42, 133.37, 129.24, 128.62, 128.36, 128.12.
Anal. Calcd for C18H12N2S: C, 74.97; H, 4.19; N, 9.71. Found: C, 75.01;
H, 4.29; N, 9.96.
1
yellow powder. The yield was 2.34 g (43%). H NMR (acetone-d6):
δ 7.69ꢀ7.58 (m, 4H), 7.51 (d, J = 8.7 Hz, 2H), 7.27 (s, 1H), 7.14ꢀ
7.05 (m, 6H), 4.38 (s, 4H), 2.57 (t, J = 7.5 Hz, 2H), 1.59 (quint,
J = 7.5 Hz, 2H), 1.37 (sex, J = 7.5 Hz, 2H), 0.93 (t, J = 7.5 Hz, 3H). 13C
NMR (acetone-d6): δ 144.70, 141.67, 140.75, 138.77, 136.40, 133.64,
132.52, 129.98, 128.12, 127.98, 125.07, 120.06, 119.56, 117.30, 117.08,
112.29, 65.74, 65.45, 35.61, 34.67, 23.01, 14.25. Anal. Calcd for
C28H26BrNO2S: C, 64.61; H, 5.04; N, 2.69. Found: C, 64.82; H, 5.09;
N, 2.42.
00
Synthesis of 4-(40-Bromophenyl)-7-[4 -(4000-butylphenylamino)
phenyl]-2,1,3-benzothiadiazole (4). To a two-necked flask equipped
with a stopcock and a condenser were added 3 (8.00 g, 17.9 mmol),
4-butylaniline (2.83 mL, 17.9 mmol), [1,10-bis(diphenylphosphino)
ferrocene]dichloropalladium(II) (Pd(dppf)Cl2) (0.14 g, 0.18 mmol),
sodium tert-butoxide (2.41 g, 25.1 mmol), and dry toluene (50 mL) under
nitrogen. The mixture was stirred at reflux for 12 h. After cooling down to
room temperature, the mixture was washed with brine. The organic layer
was dried with MgSO4 and concentrated by a rotary evaporator. The crude
product was purified by column chromatography eluted with toluene/
hexane (4/1 in volume ratio) to give an orange crystal. The yield was 2.50 g
(27%). 1H NMR (CDCl3): δ 7.89 (d, J = 8.4 Hz, 2H), 7.84 (d, J = 8.4 Hz,
Synthesis of Poly(ethylene oxide) with a Diphenylamine
Terminal (PEO-DPA). To a two-necked flask equipped with a stop-
cock and a condenser were added PEO-Br (Mn = 2000, 4.30 g,
2.15 mmol), aniline (0.39 mL, 4.30 mmol), sodium tert-butoxide
(0.413 g, 4.30 mmol), Pd(OAc)2 (0.0097 g, 0.043 mmol), tri-tert-
butylphosphine (41.7 μL, 0.172 mmol), and dry THF (10 mL) under
nitrogen. The mixture was stirred at reflux for 24 h. After cooling to
room temperature chloroform was added, and the mixture was washed
with brine. The organic layer was dried with MgSO4 and concentrated
by a rotary evaporator. The crude product was washed with diethyl ether
1
and dried. The yield was 3.55 g (83%). H NMR (CDCl3): δ 7.20
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Scheme 1. Synthesis of BTH- and EDOT-Containing Monomers
(t, J = 8.1 Hz, 2H), 7.05 (d, J = 8.1 Hz, 2H), 6.93ꢀ6.72 (m, 5H), 5.62
(s, 1H), 4.10 (t, J = 4.5 Hz, 2H), 3.89ꢀ3.51 (m, 200H), 3.37 (s, 3H).
Synthesis of Poly(ethylene oxide) with a BTH Terminal
(PEO-BTH). To a two-necked flask equipped with a stopcock and a
condenser were added PEO-DPA (3.50 g, 1.75 mmol), 3 (1.17 g,
2.63 mmol), sodium tert-butoxide (0.185 g, 1.93 mmol), Pd(OAc)2
(0.0079 g, 0.035 mmol), tri-tert-butylphosphine (34.0 μL, 0.14 mmol),
and dry THF (15 mL) under nitrogen. The mixture was stirred at reflux
for 24 h. After cooling to room temperature, the mixture was poured in
water. An insoluble solid was filtered off, and the filtrate was extracted
with chloroform. The organic layer was dried with MgSO4 and
concentrated by a rotary evaporator. The resulting solid was washed
with diethyl ether. The yield was 3.35 g. 1H NMR (CDCl3): δ 7.86 (d,
J = 8.7 Hz, 2H), 7.75 (s, 2H), 7.67 (d, J = 8.7 Hz, 2H), 7.22ꢀ6.79 (m,
14H), 4.12 (d, J = 4.5 Hz, 2H), 3.90ꢀ3.52 (m, 200H), 3.38 (s, 3H).
General Procedure for CꢀN Coupling Polymerizations. To
a two-necked flask equipped with a stopcock and a condenser were
added monomer (2.50 mmol), sodium tert-butoxide (0.264 g, 2.75
mmol), and dry THF (5.0 mL) under nitrogen. A solution of 4-bromo-
toluene (0.0086 g, 0.05 mmol), Pd(OAc)2 (0.0112 g, 0.05 mmol), and
tri-tert-butylphosphine (48.5 μL, 0.2 mmol) in THF (5 mL) was added
to the flask, and the mixture was stirred at reflux for 24 h. To the reaction
mixture was added a solution of diphenylamine (0.042 g, 0.25 mmol) in
THF (1 mL), and then stirring was continued for 1 h. After cooling to
room temperature, the mixture was poured into methanol. The pre-
cipitate was filtered, dissolved in THF, and reprecipitated in acetone.
The polymer was collected and dried. A similar procedure was applied to
the preparation of block copolymers with PEO segment, where PEO-Br
or PEO-BTH was used instead of 4-bromotoluene.
thickness of the polymer layer was in the range 10ꢀ20 μm. After the
polymer layer was dried in vacuum, the semitransparent gold layer was
sputtered on the polymer layer as a cathode. The pulsed light was
illuminated on the device to generate charges in the phthalocyanine layer
by a xenon lamp, and the photocurrent was recorded on an oscilloscope.
1
Measurements. H and 13C NMR spectra were obtained on a
JEOL ALPHA400 instrument at 300 and 75 MHz, respectively. Deu-
trated chloroform or acetone was used as a solvent with tetramethylsi-
lane as an internal standard. Number- and weight-average molecular
weights (Mn and Mw) were determined by gel permeation chromatog-
raphy (GPC) analysis with JASCO RI-2031 and UV-970 detectors
eluted with chloroform at a flow rate of 0.5 mL minꢀ1 and calibrated by
standard polystyrene samples. Differential scanning calorimetry (DSC)
analyses were performed on a Rigaku DSC-8230 under a nitrogen
atmosphere at heating and cooling rate of 10 °C minꢀ1. The cyclic
voltammetry (CV) was measured at room temperature using a typical
three-electrode system with a working (carbon wire), a reference
(Ag/AgCl), and a counter electrode (Pt spiral) under a nitrogen
atmosphere at a sweep rate of 10 mV sꢀ1. The block copolymer film
was spin-coated on the carbon electrode from chloroform solution. A 0.1
M solution of tetra-n-butylammonium perchlorate in anhydrous acet-
onitrile was used as an electrolyte. UVꢀvis and photoluminescent (PL)
analyses were conducted on a JASCO V-570 spectrophotometer and a
JASCO FP-6500 spectrofluorometer, respectively.
’ RESULTS AND DISCUSSION
Previously, we have demonstrated that poly(4-butyltriphenylamine)
was synthesized by CꢀN coupling polymerization of self-
condensing monomer using palladium catalyst.12 In a similar
regime, novel photoconductive polymers based on TPA back-
bone were designed. The monomers containing BTH (4) or
EDOT (8) units were prepared in four-step reactions as shown in
Scheme 1. One equivalent of 4-butylaniline was coupled with 4,
7-bis(40-bromophenyl)-2,1,3-benzothiadiazole (3) or 2,5-bis
Measurement of Hole Mobility (Time-of-Flight Method).
Aluminum (100 nm) was vacuum-deposited on a well-cleaned glass slide
as an anode. Thin charge generating layer was laminated on aluminum
by spin-coating from dispersion of titanium phthalocyanine at a rate of
2500 rpm for 30 s. Sequentially, the polymer layer was deposited by bar-
coating from 10 wt % of 1,1,2,2-tetrachloroethane solution. The
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Scheme 2. CꢀN Coupling Polymerization of Monomers
Table 1. Characteristics of TPA-Polymers Prepared by CꢀN Coupling Polymerizationa
PDIb
mole ratio in polymerc
Tg (°C)d
b
b
polymer
monomer
yield (%)
Mn
Mw
P1
P2
P3
P4
P5
P6
4
99 (10e)
99
1 600e
9 500
3 900e
25 000
38 000
40 000
38 000
23 000
2.48e
2.58
5.39
1.95
1.90
1.85
4 and 9 (1/99)
4 and 9 (5/95)
4 and 9 (20/80)
8
98.1/1.9
94.7/5.3
80.5/19.5
207
225
238
234
208
99
7 000
93
21 000
20 000
13 000
94
4 and 8 (20/80)
99
79.0/21.0
a Reaction conditions: 2.5 mmol of monomer, 1.1 equiv of t-BuONa, 2 mol % of Pd(OAc)2, 8 mol % of P(t-Bu)3, and 2 mol % of 4-bromotoluene in
10 mL of THF at reflux for 24 h. b Determined by GPC with polystyrene standards. c Estimated from 1H NMR spectra. d Determined by DSC profiles in
the second heating. e Soluble part in chloroform.
(40-bromophenyl)-3,4-ethylenedioxythiophene (7) using palla-
dium catalyst in order to obtain the monomers with bromophe-
nyl and diphenylamine moieties. Because these reactions gave
the mixture of the desired product and oligomeric byproduct, the
amount of sodium tert-butoxide was optimized at 1.4 equiv to
4-butylaniline to maximize the yield of the monomers. The
isolation of 4 and 8 from the mixture was carried out by column
chromatography. The structures of monomers were confirmed
by 1H and 13C NMR spectra, in which the characteristic signals at
7.72 and 4.38 ppm derived from BTH and EDOT units were
observed, respectively. The signal assignable to the proton of
secondary arylamine was also observed for both monomers.
The CꢀN coupling polymerization of monomers was con-
ducted using palladium(II) acetate and tri-tert-butylphosphine as
a catalytic system in THF at reflux for 24 h (Scheme 2). For end-
capping of terminals, 4-bromotoluene (2 mol % to monomer)
was initially added to modify the diphenylamine terminal, whereas
excess amount of diphenylamine (10 mol % to monomer)
was added at the end of polymerization to modify the bromo-
phenyl terminal. The characteristics of the polymers are sum-
marized in Table 1. At first, polymerizations of 4 and 8 were
carried out in order to obtain homopolymers (P1 and P5) whose
backbone composed of alternative sequence of TPA with BTH or
EDOT, respectively. Homopolymerization of 4 gives the insoluble
polymer in any common solvents, with small amount of soluble
part. On the other hand, the homopolymer of 8 shows high Mn up
to 20 000 and good solubility in chloroform and THF. Random
copolymerizations of 4 with 9, a monomer for PTPA component,
were conducted with various feeding ratios. The polymers with
different BTH unit contents ranging from 1 to 20 mol % were
synthesized as shown in Table 1 (P2 to P4). The molecular weights
of the random copolymers are high up to 21 000 in Mn for P4.
Polydispersity indices of P2 and P3 are broadened compared with
P4, probably because of the formation of cyclic oligomers as
reported in the previous work.12 In addition, the random copolymer
of 4 and 8 was also prepared only for 20 mol % of BTH unit (P6).
The polymer structures were characterized by 1H NMR
spectra, and all the signals were reasonably assigned as shown
in Figure 1. Besides the signals derived from 4-butyltriphenyla-
mine backbone, the characteristic signals assignable for BTH and
EDOT units were observed at 7.70ꢀ7.87 and 4.31 ppm,
respectively. Because no signal at around 8.0 ppm derived from
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the unreacted bromophenyl terminal in 1H NMR spectra, it was
indicated that the diphenylamine unit was quantitatively intro-
duced at the bromophenyl terminal of the polymers. For the
random copolymers, the mole ratios of 4 and 9 (or 8) units in the
polymers were estimated from H NMR spectra. The signal at
1
7.87 ppm derived from BTH unit was compared with the signal at
2.58 ppm assignable to the methylene protons of butyl group
attached to TPA. As the results are listed in Table 1, all the
random copolymers showed similar mole ratios in the polymers
with feeding ratios. The thermal property of the polymers was
investigated by DSC analysis. A glass transition temperature (Tg)
was observed at over 200 °C for all the polymers except for P1, as
listed in Table 1. The more BTH unit is introduced in the
random copolymers, the higher the Tg becomes up to 238 °C.
Introduction of PEO as the second block at the diphenylamine
terminal of the polymers containing BTH and/or EDOT units
was achieved by end-capping reaction. As terminal modifiers, two
kinds of PEO derivatives with Mn of 2000 were used instead of
4-bromotoluene. One is PEO with a bromophenyl group at a
terminal (PEO-Br) as reported before.12 The other was synthe-
sized from PEO-Br in two steps. PEO-Br was treated with aniline
to prepare PEO derivative with a diphenylamine terminal (PEO-
DPA). The coupling reaction between PEO-DPA and 3 gave
PEO derivative containing a BTH unit at a terminal (PEO-BTH,
shown in Chart 1). Random copolymerization of 4 and 9
(5/95 in molar feed ratio) or homopolymerization of 8 was
performed in the presence of 5 mol % of PEO-Br (B1 and B2 in
Chart 1). On the other hand, 9 and 8 were polymerized in the
presence of 5 mol % of PEO-BTH (B3 and B4 in Chart 1). After
the polymerization, fractional reprecipitations from methanol
and acetone were carried out for the removal of unreacted PEO
terminal modifiers. All the polymerizations provided diblock copo-
lymers containing PEO as the second segment. B3 and B4 possess
BTH unit only at the junction point between TPA-polymers and
PEO segments. The contents of BTH unit in the diblock copoly-
mers (B1, B3, and B4) were constant around 5 mol % in feed ratios.
The results are summarized in Table 2. The diblock copolymers
Figure 1. 1H NMR spectra of (a) P4, (b) P5, and (c) P6.
Chart 1. Chemical Structures of Diblock Copolymers Consisting of TPA-Polymers and PEO
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Table 2. Characteristics of Block Copolymers Based on TPA-Polymersa
b
b
polymer
monomer
terminal modifier
yield (%)
Mn
Mw
PDIb
Tmc (°C)
Tgc (°C)
B1
B2
B3
B4
4 and 9 (5/95)
PEO-Br
86
94
98
88
7 500
11 000
8 000
19 000
21 000
16 000
14 000
2.55
1.83
2.03
1.71
75
73
74
73
172
172
167
195
8
9
8
PEO-Br
PEO-BTH
PEO-BTH
8 100
a Reaction conditions: 2.5 mmol of monomer, 1.1 equiv of t-BuONa, 2 mol % of Pd(OAc)2, 8 mol % of P(t-Bu)3, and 5 mol % of terminal modifier in
10 mL of THF at reflux for 24 h. b Determined by GPC with polystyrene standards. c Determined by DSC profiles in the second heating.
Figure 2. UVꢀvis absorption spectra of (a) BTH- and (b) EDOT-containing polymers and PL spectra of (c) BTH- and (d) EDOT-containing
polymers.
showed the Mns from 7500 to 11 000 with reasonable PDI values
from 1.71 to 2.55. In the 1H NMR spectra of block copolymers, a
new signal derived from the methylene protons of PEO segment
appeared at 3.65 ppm (see Supporting Information). No signal
assignable to the terminal secondary amine of undesired homo-
polymer was observed at around 5.7 ppm. The mole ratio of
ethylene oxide monomer unit to TPA monomer unit was around
33/67, which is comparable to the feeding ratio, confirming that
almost all of PEO terminal modifier was introduced at the end of
TPA-polymers. From these results, the formation of diblock
copolymers without undesired homopolymers was confirmed.
DSC analysis was conducted for the block copolymers, and the
results are listed in Table 2. The melting point (Tm) of PEO
segment at 75 °C as well as the Tg of TPA-polymer was clearly
observed for all the block copolymers. This may be an evidence
for microphase separation of the block copolymers. However, the
Tgs of TPA-polymer segments decreased because the Mns were
lower than those of corresponding homopolymers.
Photophysical properties of the polymers were investigated by
UVꢀvis absorption and PL measurements in chloroform solu-
tion. The UVꢀvis and PL spectra of the polymers are shown in
Figure 2. In addition to the absorption band around 350 nm
derived from PTPA backbone, the polymers containning BTH
unit (P1 to P4) showed a maximum absorption wavelength at
480 nm. The absorption edge of the polymers with BTH unit
reached 590 nm. Increasing BTH unit in the PTPA backbone
gave a gradual increase in the absorption at 480 nm. On the other
hand, the polymers containing EDOT unit (P5 and P6) showed
a bathochromic shift to 420 nm compared with PTPA. In the
UVꢀvis spectrum of P6, a shoulder was observed at 500 nm
derived from BTH unit.
The PL spectra excited at each maximum absorption wave-
length were obtained for the polymers. Figure 2c shows the PL
spectra of BTH-containing polymers (P1 to P4). When the
content of BTH unit was below 1 mol % (P2), a blue emission
was observed with a peak top at 424 nm, which corresponds to
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Table 3. Electrical Properties of TPA-Polymers
polymer Eonset (V) Epaa (V) Epca (V) Eo a (V) HOMO (eV)
0
P2
P3
P4
P5
P6
B1
B2
B3
B4
0.935
0.905
0.930
0.745
0.780
0.915
0.755
0.880
0.775
1.95
1.87
2.34
1.65
1.47
1.65
1.46
1.98
1.27
0.135
0.215
1.04
ꢀ5.69
ꢀ5.69
ꢀ5.64
ꢀ5.41
ꢀ5.49
ꢀ5.61
ꢀ5.36
ꢀ5.61
ꢀ5.45
1.04
ꢀ0.335
ꢀ0.115
0.215
0.990
0.768
0.843
0.965
0.710
0.960
0.803
0.280
ꢀ0.035
ꢀ0.060
0.335
a Formal potential Eo0 was determined by the following equation: Eo
=
0
(Epa + Epc)/2, where Epa and Epc are the anodic and cathodic peak
potentials, respectively.
Figure 3. (a) UVꢀvis absorption spectra and (b) PL spectra of block
copolymers.
the emission of PTPA. A new emission band at 600 nm appeared
by increasing BTH unit for P3 and P4, and the color of the
emission turned to yellow and reddish orange, respectively.
Homopolymer P1 showed only the emission band at 600 nm
colored red-violet. The energy transfer from TPA to BTH
partially occurred during excitation at maximum absorption
wavelengths around 350 nm. Therefore, the emission color can
be tuned for the BTH-containing polymers by changing the
content of BTH unit. On the other hand, an emission band at
475 nm with a shoulder at 507 nm was observed in the spectra of
P5 and P6 (Figure 2d) excited at 420 nm. Although P6 contains
20 mol % of BTH unit, the emission band around 600 nm derived
from BTH moiety was not observed.
The UVꢀvis absorption and PL spectra of block copolymers in
chloroform solution were also measured as shown in Figure 3. All
the block copolymers showed similar spectra to those of corre-
sponding homopolymers. In the UVꢀvis absorption spectra of B3
and B4, a weak peak or a shoulder at 500 nm was attributed to BTH
unit between blocks. In the PL spectra excited at the maximum
absorption wavelengths, B1 showed the yellow emission similar to
that of P3, whereas the blue emission was observed for B3 even with
almost same the BTH content as B1. This resulted from the
difference of the chemical structure. Energy transfer from TPA unit
to BTH unit effectively took place in B1 where BTH unit was
randomly dispersed in the TPA-polymer backbone. In contrast,
energy transfer rarely occurred in B3 in which BTH unit located far
from the excited TPA unit because the intrachain energy transfer is
assumed to occur only on short segment length.29
Figure 4. Cyclic voltammograms of (a) P3 and (b) P5 in film vs Ag/
AgCl.
Cyclic voltammetry was conducted for the thin polymer films
on a glassy carbon electrode using Ag/AgCl as a reference
electrode. All the polymers except P1 showed the redox peaks
in the anodic sweep whereas no noticeable peak was observed in
the cathodic sweep. The oxidation potentials and the highest
occupied molecular orbital (HOMO) levels were estimated and
are summarized in Table 3. The first set of redox peaks is assumed
to be derived from the oxidation of TPA moiety. The HOMO
levels of the BTH-containing polymers are slightly deeper than
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Figure 6. Dependence of the hole drift mobility on the applied field for
block copolymers.
in the presence of electrically inert PEO block. B1 and B2
showed comparable μ values to those of corresponding homo-
polymers P3 and P5, respectively. These results revealed that the
existence of PEO block does not impede the transport of the
generated hole. It is assumed that the generated hole transported
through the TPA-polymer domain created by the microphase
separation of block copolymers. The morphology of the surface
of the polymer devices was confirmed by AFM measurement.
The AFM image of B1 and B2 showed microphase-separated
structures compared with that of homopolymer (see Supporting
Information).
Furthermore, there is no influence of BTH unit on the drift
mobility of the block copolymers with BTH unit at the junction
point (B3 and B4). Although B3 contains BTH unit of 5 mol % at
the junction point between PTPA and PEO, the μ of B3 was from
4 ꢁ 10ꢀ5 to 5 ꢁ 10ꢀ5 cm2 Vꢀ1 sꢀ1, which is slightly higher than
that of PTPA. Similarly, the μ of B4 was almost equivalent to that
of P5 in spite of BTH unit. The improved mobility of B3
compared with P3 or B1, the BTH contents of which are almost
same of 5 mol %, may be attributed to the structural difference
that BTH unit exists randomly in the polymer backbone or at the
junction point. In the latter case, it is suggested that BTH unit lies
only at the interface of the microphase-separated structure, i.e.,
between TPA-polymer and PEO domains. Therefore, the gen-
erated hole is assumed to pass through the TPA-polymer domain
without being trapped at BTH unit.
Figure 5. Dependence of the hole drift mobility on the applied field for
(a) BTH- and (b) EDOT-containing polymers.
that of PTPA10 because of the electron deficient nature of BTH
unit. On the other hand, the EDOT-containing polymers
(P5 and B2) give the HOMO levels of around ꢀ5.4 eV, which
is comparable to the HOMO level of PTPA.
The photocurrent on the exposure of a pulsed light was
measured for the polymer films with a thickness ranging from
10 to 20 μm, and the hole drift mobility (μ) was estimated from
the transit time (τ) represented as the knee point of the photo-
current. The logarithm of drift mobility in the polymers was
plotted versus the square root of electric field in Figure 5. The
random copolymers containing BTH unit (P2 to P4) showed
almost constant drift mobility of around 1 ꢁ 10ꢀ5 cm2 Vꢀ1 sꢀ1
independent of the amount of BTH unit (Figure 5a). This value is
slightly lower than the hole drift mobility of PTPA (around 3 ꢁ
’ CONCLUSIONS
10ꢀ5 cm2 Vꢀ1 ꢀ1) reported in the literature.10 Especially,
s
The hole transporting polymers involving TPA unit in com-
bination with BTH and/or EDOT unit(s) were synthesized via
CꢀN coupling polymerization using palladium catalytic system.
By chain-end modification using PEO terminal modifiers, the
diblock copolymers consisting of TPA-polymer and PEO were
prepared. Furthermore, the junction point between the segments
of diblock copolymers was selectively functionalized using the
BTH-modified PEO derivative. The optical and electrical mea-
surements of the TPA-polymers revealed that the physical
properties were influenced by the position where BTH unit
was modified in the polymer backbone. High hole mobility up to
1.0 ꢁ 10ꢀ3 cm2 Vꢀ1 sꢀ1 was obtained by introduction of EDOT
unit in PTPA backbone. Whereas the hole drift mobility of BTH-
containing polymer films lowered because of hole trapping
nature of BTH unit, the selective modification by BTH unit at
although only 1.9% of BTH unit was involved in the polymer
backbone of P2, the μ decreased compared with PTPA. This is
probably due to the existence of dipoles resulting from the
combination of electron deficient BTH unit with electron-rich
TPA unit. It is reported that doping of polar molecules to hole
transporting materials decrease the drift mobility.30,31 In the mean-
time, the introduction of EDOT unit in the polymer backbone (P5
and P6) increased drift mobility up to 1 ꢁ 10ꢀ3 cm2 Vꢀ1 sꢀ1
(Figure 5b). The μ of P6 decreased to the values ranging from
5 ꢁ 10ꢀ5 to 1 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1 because of the existence of
BTH unit but was still higher value compared with PTPA.
The hole drift mobility of block copolymers films was also
measured as shown in Figure 6. All the block copolymers
exhibited the drift mobilities over 1 ꢁ 10ꢀ5 cm2 Vꢀ1 sꢀ1 even
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the junction point of block copolymers does not impede the hole
transporting ability.
(26) Liu, J.; Bu, L. J.; Dong, J. P.; Zhou, Q. G.; Geng, Y. H.; Ma,
D. G.; Wang, L. X.; Jing, X. B.; Wang, F. S. J. Mater. Chem. 2007,
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(27) Zhu, Y. B.; Wolf, M. O. J. Am. Chem. Soc. 2000, 122,
10121–10125.
’ ASSOCIATED CONTENT
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(29) Rothberg, L. J.; Yan, M.; Papadimitrakopoulos, F.; Galvin,
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S
Supporting Information. Figures S1ꢀS28. This material
b
is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Phone: +81-42-388-7212. Fax: +81-42-388-7404. E-mail:
ktsuchiy@cc.tuat.ac.jp.
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