Electrochemical synthesis and properties of poly[1,4-bis(N-pyrrolylalkoxy)benzene]s with a three-dimensional crosslinked structure

Article abstract of DOI:10.1246/cl.2003.516

The title polymers, which were composed of π-conjugated pyrroles and dialkoxybenzene crosslinkers, were prepared by electrochemical polymerization to afford uniform thin films. These films were characterized by cyclic voltammetry (CV), scanning electron m

Full text of DOI:10.1246/cl.2003.516

516
Chemistry Letters Vol.32, No.6(2003)
Electrochemical Synthesis and Properties of Poly[1,4-bis(N-pyrrolylalkoxy)benzene]s
with a Three-dimensional Crosslinked Structure
Katsuhiko Ono,ꢀ Satsuki Yamada, Masakazu Ohkita, Katsuhiro Saito, Shoji Tanaka,y and Takamasa Hanaichiyy
Department of Applied Chemistry, Nagoya Institute of Technology, Gokiso, Nagoya 466-8555
yInstitute for Molecular Science, Myodaiji, Okazaki 444-8585
yyHanaichi Ultrastructure Research Institute, Daijuji, Okazaki 444-2134
(Received February 26, 2003; CL-030166)
The title polymers, which were composed of p-conjugated
pyrroles and dialkoxybenzene crosslinkers, were prepared by
electrochemical polymerization to afford uniform thin films.
These films were characterized by cyclic voltammetry (CV),
scanning electron microscope (SEM), and redox stability.
OR
(H C)
X O
4 R = H
O (CH2)X
i
RO
2
3 R = Ts
ii
N
(H2C)
N
X O
O (CH2)X
Much research attention has been focused on development
of carrier-transporting materials since they are important for
fabrication of organic electroluminescent (EL) devices, whose
efficiencies and lifetimes are largely governed by facility of car-
rier-injection and transportation.1 Electrical conducting poly-
mers are powerful candidates for these materials, some of which
have been already examined, e.g., poly(p-phenylenevinylene)
(PPV),2 poly(3,4-ethylenedioxythiophene) (PEDOT),3 poly-
aniline,4 and polypyrrole.5 However, applications of p-conju-
gated polymers are very limited because these polymers are in-
soluble in organic solvents without modification by large alkyl
groups6 and are difficult to prepare uniform thin films due to
their highly crystalline properties. In recent papers, poly-
thiophene7 and tris[4-(2-thienyl)phenyl]amine8 films accumu-
lated on ITO (indium-tin-oxide) electrodes in electrolysis were
applied for hole-transporting layers, which exhibited usefulness
of electrochemical synthesis in fabrication of the devices. We
have now prepared poly[1,4-bis(N-pyrrolylalkoxy)benzene]s
(1), which are composed of p-conjugated pyrroles and dialkoxy-
benzene crosslinkers, by electrochemical polymerization of
monomers (2) to afford thin films on ITO electrodes. We report
here the electrochemical synthesis of 1 and properties of the ob-
tained films.
Monomers 2a-c were prepared in 39-60% yields by reac-
tions of pyrrole anions with ditosylates (3a-c), which were de-
rived from alkanediols (4a-c) (Scheme 1). Compounds 2 were
obtained as colorless crystals after recrystallization (2a: mp
98-101 ꢁC, 2b: mp 48–49 ꢁC, 2c: mp 62–63 ꢁC) and are stable
for air. Cyclic voltammograms of 2a-c showed irreversible oxi-
dation waves. The oxidation peak potentials in acetonitrile are
shown in Table 1. These values are similar to that of N-ethyl-
pyrrole (5). Introduction of the hydroquinone moiety into 1,2-
bis(N-pyrrolyl)ethane (6)9 resulted in the higher oxidation po-
tential of 2a.
2
x = 2
x = 6
x = 8
a
b
c
iii
N
O (CH2)X
(H2C)
X O
N
n
m
1
Scheme 1. Reagents and conditions: (i) TsCl, Et3N, Et3NHCl,
CH2Cl2; (ii) pyrrole, KOH, DMSO, 65 ꢁC, 5 h; (iii) electroche-
mical polymerization.
Table 1. Oxidation potentialsa of monomers 2 and polymers 1
and onset potentialsa for polymerization
Monomer
Epa
Eonset
Polymer
Epa
2a (x = 2)
2b (x = 6)
2c (x = 8)
5
6
0.88
0.80
0.81
0.83
0.70
0.66
0.61
0.61
0.66
0.47
1a
1b
1c
7
0.19
0.19
0.23
0.24
8
À0:17
a0.1 M n-Bu4NClO4 in MeCN, Pt electrode, scan rate
100 mV sÀ1, V vs Fc/Fcþ.
method and the potentiostatic electrolysis. The oxidation poten-
tials of polymers 1 are not dependent on the used electrodes (Pt,
ITO) and are summarized in Table 1. The values are compar-
able to that of poly(N-ethylpyrrole) (7)10 and are higher than
that of poly[1,2-bis(N-pyrrolyl)ethane] (8).9 Dependence of
the oxidation potentials with chain length (x) was not observed
in contrast to those of bis(N-pyrrolyl)alkane homopolymers.
The cyclic voltammograms of polymer 1b measured at various
scan rates are displayed in Figure 1. The peak current increased
depending on the scan rates with a linear relationship, indicating
that the electrode reactions were attributed to a surface-attached
redox species.11 The linear relationship was also observed in the
CVs of 1a and 1c.
Polymers 1a-c were prepared by electrochemical synthesis
using a Pt disk (f 1.6mm) or ITO (10 ꢀ=Ã, 12 Â 8 mm2) as a
working electrode in acetonitrile containing 0.1 M tetrabutylam-
monium perchlorate as supporting electrolyte. Minimum poten-
tials (Eonset) for the polymerization were surveyed (Table 1) and
the shifts from the peak potentials of monomers (Epa) were al-
most À0:2 V. These conditions were employed for the CV
The potentiostatic electrolysis in 1.0 mM solutions of
monomers 2 afforded thin films of 1a-c with fine smooth sur-
faces on the ITO electrodes although the surface of poly(N-
Copyright ꢀ 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.6(2003)
517
0.8
0.7
0.6
0.5
0.4
0.3
0.2
poly[1,2-bis(N-pyrrolyl)ethane]
8
poly(N-ethylpyrrole)
polymer 1b
7
20
40
60
80
100
Cycle number
Figure 3. Peak current densities at oxidation peak poten-
tials of dedoped polymers with increasing number of CV
cycle at scan rate of 100 mV sÀ1
.
of crosslinker in the network structure was highly effective for
stabilization of the redox cycles in the application as electronic
materials.12
Figure 1. Cyclic voltammograms of polymer 1b mea-
sured in a monomer-free MeCN/0.1 M n-Bu4NClO4
electrolyte at various scan rates under a nitrogen atmo-
sphere. Pt electrode, Pt wire, Ag/Agþ were used as the
working, counter, and reference electrodes, respectively.
Conductivities of doped polymers 1a-c measured by a two-
probe method as a compressed pellet were 8:4 Â 10À8
,
3:0 Â 10À7, and 1:7 Â 10À7 S cmÀ1, respectively. These values
were lower than that of poly[1,2-bis(N-pyrrolyl)ethane] (8)
(4:7 Â 10À4 S cmÀ1).9 This was due to increase of an insulating
moiety by introduction of the hydroquinone unit as well as mor-
phological change. Improvement for highly conducting materi-
als is under study.
This work was supported by a grant from the NITECH 21st
Century COE Program for Environmental Ceramics.
References and Notes
1
2
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3
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´
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Published on the web (Advance View) May 13, 2003; DOI 10.1246/cl.2003.516