A novel electrochromic polymer containing triphenylamine derivative and pyrrole

Article abstract of DOI:10.1016/j.electacta.2011.02.103

A novel conducting polymer was successfully synthesized via electropolymerization of N¹,N⁴-bis(4-(1H-pyrrol-1-yl) phenyl)-N¹,N⁴-diphenylbenzene-1,4-diamine (DPTPA). This polymer film exhibited six various colors

Full text of DOI:10.1016/j.electacta.2011.02.103

Electrochimica Acta 56 (2011) 4645–4649
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
A novel electrochromic polymer containing triphenylamine derivative and
pyrrole
Mi Ouyang, Genghao Wang, Cheng Zhang∗
State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering and Materials Science,
Zhejiang University of Technology, Hangzhou, PR China
a r t i c l e i n f o
a b s t r a c t
A novel conducting polymer was successfully synthesized via electropolymerization of N1,N4-bis(4-
(1H-pyrrol-1-yl)phenyl)-N1,N4-diphenylbenzene-1,4-diamine (DPTPA). This polymer film exhibited six
various colors under different potentials. Besides, this polymer film showed high optical contrast (41% at
852 nm, 52% at 617 nm) and fast switching time (1.3 s at 410 nm, 1.4 s at 852 nm and 0.6 s at 617 nm). Cyclic
voltammogram and electro-optical study showed that the polymer film has a stable and well-defined
reversible redox process as well as electrochromic behavior.
Article history:
Received 19 November 2010
Received in revised form 24 February 2011
Accepted 25 February 2011
Available online 4 March 2011
Keywords:
© 2011 Elsevier Ltd. All rights reserved.
Electropolymerization
Conducting polymer
Electrochromism
Multicolor
1. Introduction
and afford stable cationic radicals with lower potential [17]. Since
polypyrrole was first electrochemically synthesized more than two
One of the most attractive properties of electrochromic (EC)
materials would be multicolor display [1]. As a class of impor-
tant EC material, conjugated polymers have received significant
attention throughout the course of the past two decades because
of their promising applications in light-emitting diodes, optical
displays, electrochromic devices and analytical sensors [2–6]. The
most important features of conjugated polymers are the possibili-
ties to fine-tune the color through chemical structure modification
of the conjugated backbone and to get multichromism from the
same material [7].
Recently, triphenylamine (TPA) derivative materials, including
small molecules and macromolecules, have been extensively inves-
tigated for hole transporters [8], light emitters [9–11], and polymer
electronic memories [12]. TPA can also be considered as a good
electrochromic material as it easily oxidizes to form radical cation
with a noticeable change of coloration [13–15]. But Adams and
coworkers reported that TPA could be easily dimerized to form
tetraphenylbenzidine during the anodic oxidation pathway [16].
For electrochromic materials, this dimerization could be considered
as an undesired side reaction, which might cause irreversible defect
after several redox switches. To avoid this behavior, incorporating
electron-donating substituents such as alkyl or alkoxy group at the
para-position of the TPA group could prevent the coupling reactions
decades ago by Diaz et al. [18], it has been intensively investigated.
The superiority of the intrinsic properties of polypyrroles such as
ease of synthesis, good redox properties, stability in the oxidized
form, ability to give high electrical conductivities and useful elec-
trical and optical properties make them certainly one of the most
widely studied organic conducting polymers [19–21]. The combi-
nation of a TPA derivative and pyrrole was expected to obtain an
excellent multicolor polymer containing both the advantages of
TPA and pyrrole.
In this report, we successfully prepared a monomer (DPTPA)
(Scheme 1) containing a TPA derivative and pyrrole. The resultant
polymer was also successfully prepared. The synthesis and redox
behaviors of the monomer and the polymer film are described, and
the electrochemical and electrochromic properties of the polymer
film are also investigated herein.
2. Experimental
All chemicals were purchased from Aldrich and used with-
out further purification. The polymer was synthesized from a
reaction medium containing 3 mM monomer and 0.1 M tetrabuty-
lammonium perchlorate (TBAP) in boron fluoride etherate (BFEE)
via constant potential electrolysis at 1.2 V versus Ag/AgCl. And
the polymer was electrolyzed at 0.0 V again in order to equili-
brate its redox behavior in monomer-free electrolytic solution.
Indium–tin oxide (ITO) was used as working electrode, a plat-
inum (Pt) sheet was used as counter electrode and Ag/AgCl in
∗
Corresponding author.
E-mail address: czhang@zjut.edu.cn (C. Zhang).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.02.103

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Scheme 1. Synthesis of DPTPA.
saturated KCl(aq) solution as reference. After polymerization, the
films were rinsed with electrolysis solvent and transferred into
another cell involving 0.1 M TBAP dissolved in BFEE solution. Elec-
trochemical synthesis and cyclic voltammetric measurements were
performed using CHI-660C electrochemical working station. An
UV-1800 spectrophotometer was applied to conduct spectroelec-
trochemical studies. For spectroelectrochemical tests, ITO coated
with the polymer film was applied as working electrode, a gyroidal
Pt wire was used as counter electrode and Ag/AgCl in saturated
KCl(aq) solution as reference. The transmittance and absorbance
spectra of PDPTPA were recorded in situ under various applied
potentials. All experiments were carried out at room temperature.
reversible redox couples were observed, and the current inten-
sity of these reversible peaks increased after each successive cycle,
which clearly indicated the formation of electroactive polymer film
(PDPTPA) on the surface of the working electrode. The first electron
removal could be assumed to occur at pyrrole group, and the second
one should due to the TPA group.
DPTPA can also be electrochemically synthesized on ITO elec-
trode in acetonitrile (ACN) solution containing 3 mM monomer
and 0.1 M TBAP, and the polymer film is electrochemically stable.
Two polymer films were synthesized from ACN and BFEE solution
respectively. The redox behavior of the polymer film obtained from
ACN solution was investigated in monomer-free 0.1 M TBAP/ACN,
and the film got from BFEE solution was investigated in 0.1 M
TBAP/BFEE. The CVs of the two polymer films were exhibited in
Fig. 2. The two curves both displayed two couples of reversible
redox peaks. The oxidation potential in BFEE was lower than that
in ACN about 0.2 V. But only a very thin polymer film could be
obtained in ACN solution, and the electrolyte solution around work-
ing electrode turned blue gradually in the electrolytic process. The
oligomers formed afterward were dissolved or dispersed in the
solution might be the reason [23].
2.1. Synthesis of N1,N4-bis(4-(1H-pyrrol-1-yl)phenyl)-N1,N4-
diphenylbenzene-1,4-diamine
(DPTPA)
4 mM 1-(4-bromophenyl)-1H-pyrrole (1) (1 was synthesized
recording to reference [22]), 2 mM N1,N4-diphenylbenzene-1,4-
diamine and 4 mM NaOtBu were dissolved in 15 mL toluene, 0.02 g
Pd(OAc)2 and 2 mL PtBu3 were added. The mixture was stirred in Ar
atmosphere at 80 ◦C for 10 h. The final solution was extracted with
CH2Cl2 and water. Then CH2Cl2 was removed under reduced pres-
sure. The residue was recrystallized with ethanol for three times.
The product was obtained as white powder in a 72% yield. M.P.
205 ◦C. 1H NMR (CDCl3): ı 7.30–7.27 (8H, t); 7.17–7.15 (8H, m);
7.06–7.05 (6H, t); 7.04 (4H, s); 6.35–6.34 (4H, t). 13C NMR (CDCl3):
ı 129.3, 127.9, 125.3, 124.5, 123.8, 122.7, 121.5, 120.7, 119.4, 116.6,
110.6, 110.1. FT-IR (KBr, cm–1): 3436m, 3039w, 1592m, 1517s,
1502s, 1311m, 1274s, 1118w, 1071m, 828w, 725m, 695m, 540w.
MS: m/z (EI) 542 (M+).
3. Results and discussion
3.1. Electropolymerization of DPTPA and electrochemical
characterization
The redox behavior of DPTPA was investigated via cyclic voltam-
metry (CV) in 0.1 M TBAP/BFEE electrolyte solution. As seen in Fig. 1,
during the first anodic scan, two reversible oxidation peaks were
observed at 0.54 and 0.85 V around. During the other scans, new
Fig. 1. Repeated cyclic voltammogram of DPTPA on ITO electrode in 0.1 M
TBAP/BFEE vs Ag/AgCl at a scan rate 100 mV/s.

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Fig. 2. CVs of PDPTPA films in monomer-free 0.1 M TBAP/ACN and 0.1 M TBAP/BFEE
vs Ag/AgCl at a scan rate 100 mV/s.
Fig. 4. FT-IR spectra of DPTPA and PDPTPA, PDPTPA film was prepared from a reac-
tion medium containing 3 mM monomer and 0.1 M TBAP in BFEE.
Fig. 3 displayed the CV curves of PDPTPA film at different scan
rates. The scan rate dependence experiments showed a linear rela-
tionship between the peak current and the scan rate, indicating
non-diffusional redox process and the well-adhered electroactive
polymer film [24].
3.3. Spectroelectrochemical characterization
Spectroelectrochemical analysis is a powerful way to investi-
gate the optical switches and contrasts of EC conducting polymers
upon potential change, which provides insights into the electronic
structure of the conducting polymer. The spectroelectrochemical
and electrochromic properties of the resultant films were studied
in 0.1 M TBAP/BFEE solution.
The electrochromic absorption spectra were monitored by an
UV–vis spectrometer at different applied potentials (Fig. 5). When
the applied potential high up to 0.5 V, the peak at around 345 nm
decreased gradually, while the peak at 410 nm around intensi-
fied and a very intense broad with a new peak at 852 nm around
appeared due to the first stage oxidation. When the potential was
adjusted to more positive values, it was according to the sec-
ond electron oxidation [25]. The characteristic peak at 852 nm
decreased, and a new band at 617 nm around appeared.
The colors of PDPTPA film at different potentials were presented
in Fig. 6. From 0.0 V to 1.2 V the PDPTPA film presented six colors
from brown to blue. These different colors corresponding to vari-
ous doped and neutral states had also been confirmed by the CV
tests, and were remained even after voltage off, indicating that the
film has memory effect. Furthermore, after applying a voltage for
a long time the polymer film could hold its color also. This multi-
3.2. FT-IR spectra
Fig. 4 exhibited the FT-IR spectra of monomer DPTPA and the
resultant polymer film PDPTPA. PDPTPA film was prepared via con-
stant potential electrolysis at 1.2 V versus Ag/AgCl, then dedoped
at negative potential and washed with clean CH2Cl2 for three times
to remove the residual supporting electrolyte and the monomers.
Finally, the film was dried in vacuum at 60 ◦C for 10 h. As to DPTPA,
the wide peak at 3436 cm−1 was attributed to N–H stretching in
pyrrole ring, while the peaks at 1300 cm−1 around due to the
C–N stretching. The disappearance of the peak at 3436 cm−1 in
the spectrum of PDPTPA film demonstrates the polymerization of
pyrrole group. The peak at 3039 cm−1 around in the spectrum of
DPTPA should be ascribed to the C–H stretching in phenyl ring,
the C–C stretching located at 1502 cm−1, and the C–H bending of
phenyl ring presented at 1071 and 725 cm−1. For PDPTPA film, the
degradation of peaks at 3039 and 725 cm−1 around implies the
polymerization of TPA group.
Fig. 3. Scan rate dependence of PDPTPA film on ITO electrode in 0.1 M TBAP/BFEE
Fig. 5. Spectroelectrochemical spectra of PDPTPA film as applied potentials between
solution at scan rates 50, 100, 200, 300, 400 and 500 mV/s.
0.0 and 1.2 V in 0.1 M TBAP/BFEE.

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Fig. 6. Colors of PDPTPA film at different potentials.
Fig. 9. Electrochromic switching, optical absorbance monitored for PDPTPA at
852 nm in 0.1 M TBAP/BFEE between 0.0 and 0.8 V with a residence time of 10 s.
Fig. 7. Electrochromic switching, optical absorbance monitored for PDPTPA at
410 nm in 0.1 M TBAP/BFEE between 0.0 and 0.8 V with a residence time of 10 s.
color property possesses significant potential applications in smart
windows or displays and so on.
percentage transmittance changes (ꢀT%) between the neutral and
oxidized states were 20% for 410 nm, 52% for 617 nm and 41% for
852 nm. And the switching times were 1.3 s at 410 nm, 0.6 s at
617 nm and 1.4 s at 852 nm. The good stability of the ꢀT% in time,
fast switching property and high contrast ratio make this polymer
a promising material for EC devices.
3.4. Electrochromic switching
The optical switching studies of PDPTPA film were carried out
in the electrolyte solution with a residence time of 10 s. At 410
and 852 nm, the film was switched between 0.0 and 0.8 V, while at
617 nm was between 0.0 and 1.2 V. The switching time was defined
as the time required for reach 95% of the full change in absorbance
after the switching of the potential. As seen from Figs. 7–9, the
Fig. 8. Electrochromic switching, optical absorbance monitored for PDPTPA at
Fig. 10. CVs of PDPTPA film as a function of repeated scans 500 mV/s in 0.1 M
617 nm in 0.1 M TBAP/BFEE between 0.0 and 1.2 V with a residence time of 10 s.
TBAP/BFEE.

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3.5. Stability of PDPTPA film
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In summary, a novel multicolor polymer (PDPTPA) was elec-
trochemically synthesized. The polymer film showed excellent
multicolor electrochromism (six colors). Otherwise, the polymer
film exhibited high optical contrast, fast switching time and good
stability. These prominent features make PDPTPA an excellent can-
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