Electropolymerization of V-shape D-A-D type monomers for efficient and tunable electrochromics

Article abstract of DOI:10.1016/j.dyepig.2021.109615

Five novel donor-acceptor-donor (D-A-D) type monomers (VQ1 ~ VQ5) with a common V-shape configuration have been developed and further electropolymerized for electrochromic applications. The monomers were designed by introducing two triphenylamine groups into the 2- and 3- position of the quinoxaline derivates, respectively. These redox-active monomers could be electrodeposited robustly on the ITO electrodes in an electrolyte solution via the oxidative coupling reactions between triphenylamine radical cations. The obtained thin polymer films (PVQ1 ~ PVQ5) exhibited reversible redox behaviours and obvious color changes upon voltage variation. All these polymers exhibited excellent electrochromic performances involving high optical contrast (over 70%), short response time (less than 2 s) and high coloration efficiency (over 200 cm² C^?1). Moreover, the optical properties of these polymers in both neutral state and oxidation state can be tuned by variation of the additional substituents on the quinoxaline parts, respectively.

Full text of DOI:10.1016/j.dyepig.2021.109615

Dyes and Pigments 194 (2021) 109615
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Electropolymerization of V-shape D-A-D type monomers for efficient and
tunable electrochromics
,
,c,*
Wenyuan Wanga, Hongjin Chena, Youfeng Yueb, Rui Zhanga c, Jian Liua
a College of Chemical Engineering, Jiangsu Key Lab of Biomass-based Green Fuels and Chemicals, Nanjing Forestry University, Nanjing, 210037, China
b Research Institute for Advanced Electronics and Photonics, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi 305-8565,
Tsukuba, Japan
c Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing, 210037, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Five novel donor-acceptor-donor (D-A-D) type monomers (VQ1 ~ VQ5) with a common V-shape configuration
have been developed and further electropolymerized for electrochromic applications. The monomers were
designed by introducing two triphenylamine groups into the 2- and 3- position of the quinoxaline derivates,
respectively. These redox-active monomers could be electrodeposited robustly on the ITO electrodes in an
electrolyte solution via the oxidative coupling reactions between triphenylamine radical cations. The obtained
thin polymer films (PVQ1 ~ PVQ5) exhibited reversible redox behaviours and obvious color changes upon
voltage variation. All these polymers exhibited excellent electrochromic performances involving high optical
contrast (over 70%), short response time (less than 2 s) and high coloration efficiency (over 200 cm2 Cꢀ 1).
Moreover, the optical properties of these polymers in both neutral state and oxidation state can be tuned by
variation of the additional substituents on the quinoxaline parts, respectively.
Electrochromic
Donor-acceptor-donor
V-shape
Quinoxaline
1. Introduction
electrochromic materials, a valid strategy in molecular engineering is
to introduce an electron accepting moiety into the conjugated back-
Electrochromic devices that can change colors under an applied
voltage, have received considerable attention due to their potential
applications in many fields, such as optical displays, smart windows,
antiglare rearview mirrors, and so on [1–4]. During the past decades,
tremendous research efforts have been devoted to the development of
efficient electrochromic materials which play a crucial role in electro-
chromic devices, with the aim of low cost, high optic contrast, long-term
stability and customized color variation [5–13]. So far, various kinds of
electrochromic materials involving metal oxides, viologens, conjugated
conducting polymers, and metal coordination complexes have been re-
ported [14–24].
bones [17]. The state-art linear D-A-D type configuration achieved the
improvement of stability, switch speed and coloration efficiency [31].
However, the intrinsic strong intramolecular charge transfer transition
in D-A-D type conjugated polymers lead to a broad and high absorption
in visible region and reduce their light transmittance in their neutral
states, which limited their practical application in smart window for
energy saving-building [32–34]. On the other hand, the frequent use of
noble metal catalysis for the coupling reaction between donor and
acceptor parts during the synthesis of D-A-D type conjugated com-
pounds also reduced their cost performance [35]. Although the cut of
the conjugation can obtain colorless electrochromic materials, it would
result in the boundedness for color tuning and the probable deteriora-
tion of device performance [29]. Therefore, the development of color
tunable conjugated D-A-D type electrochromic materials among color-
less and colorful is still a challenge [36–41].
Among them, organic electrochromic materials have been demon-
strated to be possessing many advantages including short switching
times, easy color-tuning, and high coloration efficiency [25]. Polyaryl-
amine derivatives have been demonstrated as a kind of active electro-
chromic materials due to their unique reversible redox behaviour of the
arylamine moieties, which can be colorized upon electro-oxidation
process [26–30]. To improve the performance of polyarylamine-based
Herein, we developed five novel D-A-D type monomers with a
common V-shape configuration, in which two triphenylamine groups
were introduced into the 2- and 3- position of the quinoxaline derivates,
* Corresponding author. College of Chemical Engineering, Jiangsu Key Lab of Biomass-based Green Fuels and Chemicals, Nanjing Forestry University, Nanjing,
210037, China.
E-mail address: liu.jian@njfu.edu.cn (J. Liu).
https://doi.org/10.1016/j.dyepig.2021.109615
Received 18 March 2021; Received in revised form 25 June 2021; Accepted 28 June 2021
Available online 29 June 2021
0143-7208/© 2021 Elsevier Ltd. All rights reserved.

W. Wang et al.
Dyes and Pigments 194 (2021) 109615
Preparation of compound 1. To a stirred solution of triphenylamine
(15 g, 61.22 mmol) in anhydrous CH2Cl2 (100 mL) was added anhydrous
aluminum chloride (8.17 g, 10.28 mmol) portion wise at 0 ◦C under N2
atmosphere. Oxalyl chloride (3.95 g, 25.48 mmol) was added dropwise
and maintained the rection system below 0 ◦C. The reaction mixture was
stirred at room temperature for 12 h, and then treated with water (50
mL), extracted twice with CH2Cl2 (40 mL × 2). The combined organic
layers were washed twice with water and once with brine, dried over
anhydrous magnesium sulfate. After evaporation of the solvent under
reduced pressure, The residue was treated with MeOH (15 mL), the solid
was collected by filtration, and dried under vacuum to yield the desired
product 2a as a white powder (7.07 g, 51%). 1H NMR (400 MHz, CDCl3)
δ: 7.80 (d, J = 9.0 Hz, 2H), 7.38–7.34 (m, 4H), 7.21–7.17 (m, 6H), 6.98
(d, J = 9.0 Hz, 2H). 13C NMR (100 MHz, CDCl3) δ: 193.36, 153.43,
145.96, 131.66, 129.75, 126.40, 125.28, 119.00. ESI (m/z): Calcd for
C
38H28N2O2, 544.22 (M)+; found, 544.21.
Preparation of monomer VQ1. To a stirred solution of compound 1
(163 mg, 0.3 mmol) in acetic acid (40 mL) was added benzene-1,2-
diamine (33 mg, 0.3 mmol) at room temperature. The reaction
mixture was stirred at 90 ◦C under N2 atmosphere for 12 h. Evaporation
of the acetic acid under reduced pressure and the residue was treated
with water (50 mL), extracted twice with CH2Cl2 (50 mL × 2). The
combined organic layers were washed twice with water and once with
brine, dried over anhydrous magnesium sulfate. After removing the
solvent under reduced pressure, the residue was purified by chroma-
tography using hexane/EA (10/1, v/v) as an eluent to yield compound
3a as an orange solid (130 mg, 71%). 1H NMR (400 MHz, CDCl3) δ: 8.13
(dd, J1 = 6.4 Hz, J2 = 3.4 Hz, 2H), 7.73 (dd, J1 = 6.4 Hz, J2 = 3.4 Hz,
2H), 7.44 (d, J = 8.8 Hz, 4H), 7.31–7.26 (m, 8H), 7.13 (d, J = 7.6 Hz,
8H), 7.08–7.03 (m, 8H). 13C NMR (100 MHz, CDCl3) δ: 160.15, 156.43,
155.82, 149.82, 149.11, 136.85, 130.46, 128.49, 123.75, 121.38,
118.24, 114.87. ESI (m/z): Calcd for C44H32N4, 616.26 (M+); found,
Scheme 1. Molecular structures and synthetic routes of V-shape monomers
(VQ1 ~ VQ5) and the corresponding D-A-D type electrochromic polymers
(PVQ1 ~ PVQ5).
respectively. The synthesis of V-shape monomers VQ1 ~ VQ5 can be
easily achieved by condensation reactions between diketone and di-
amines, respectively. The corresponding D-A-D polymers PVQ1 ~ PVQ5
were successfully prepared by electropolymerization, which exhibited
efficient electrochromic properties and device performances. Further-
more, the color of the present D-A-D type electrochromic polymers in
both neutral and oxidation states can be easily tuned by introducing
addition substitutes on the quinoxaline parts, which guaranteed their
potential applications in smart windows.
2. Experimental
Table 1
Optical and redox parameters of monomers VQ1 ~ VQ5a.
All chemicals and reagents were used as received from chemical
companies without further purification. Column chromatography was
performed using silica gel as a stationary phase. UV–Vis spectra were
measured in dichloromethane (DCM) solution or thin film on ITO glass
using LAMBDA 365 Spectrophotometer (PerkinElmer). Cyclic voltam-
metry (CV) was performed on a CHI 760E potentiostat/galvanostat
system. Platinum electrode was used as a working electrode, Ag/AgCl in
saturated KNO3 (aq.) as a reference electrode, and a platinum wire as a
counter electrode. The NMR measurements were performed by a DRX-
400 spectrometer (Bruker BioSpin). Mass spectra were measured on a
Shimadzu Biotech matrix-assisted laser desorption ionization (MALDI)
mass spectrometer. TGA were characterized by TG 209F-1 (NETZSCH).
Monomer
aλmax (nm)
bS+/0 (eV)
cS+/* (eV)
dE0-0 (eV)
VQ1
VQ2
VQ3
VQ4
VQ5
401
399
410
420
460
0.93
0.91
0.95
0.96
0.97
ꢀ 1.65
ꢀ 1.72
ꢀ 1.55
ꢀ 1.47
ꢀ 1.18
2.58
2.63
2.50
2.43
2.15
a
The absorption spectra were measured in CH2Cl2 solution.
b
The S+/0 corresponding to the ground-state oxidation potential (vs NHE) in
CH2Cl2 internally calibrated with ferrocene.
c
S+/* = S+/0 – E0-0, where E0-0 is the zero-zero transition energy.
d
E0-0 values were estimated from the onset of the absorption spectra in
CH2Cl2.
Fig. 1. a) Absorption spectra of V-shape D-A-D type monomers VQ1 ~ VQ5 in CH2Cl2; b) Cyclic voltammograms of monomers VQ1 ~ VQ5 in CH2Cl2/TBAPF6 (0.1
M), [c] = 1 × 10ꢀ 4 mol Lꢀ 1, 293 K, scan rate = 100 mV sꢀ 1
.
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W. Wang et al.
Dyes and Pigments 194 (2021) 109615
Fig. 2. The HOMO and LUMO of monomers VQ1 ~ VQ5 calculated at B3LYP/6-31G** level.
616.26.
= 8.5 Hz, 4H). 13C NMR (100 MHz, CDCl3) δ: 155.06, 154.51, 149.08,
149.01, 147.16, 142.10, 140.00, 131.60, 130.84, 130.76, 130.12,
129.44, 127.04, 125.21, 123.76, 121.77. ESI (m/z): Calcd for
Preparation of monomer VQ2. To a stirred solution of compound 1
(163 mg, 0.3 mmol) in acetic acid (40 mL) was added 4,5-dimethoxy-
benzene-1,2-diamine (50 mg, 0.3 mmol) at room temperature. The re-
action mixture was stirred at 90 ◦C under N2 atmosphere for 12 h.
Evaporation of the acetic acid under reduced pressure and the residue
was treated with water (50 mL), extracted twice with CH2Cl2 (50 mL ×
2). The combined organic layers were washed twice with water and once
with brine, dried over anhydrous magnesium sulfate. After removing the
solvent under reduced pressure, the residue was purified by chroma-
tography using hexane/EA (10/1, v/v) as an eluent to yield compound
3a as an orange solid (155 mg, 76%). 1H NMR (400 MHz, CDCl3) δ:
7.48–7.42 (m, 6H), 7.29 (t, J = 7.8 Hz, 8H), 7.15 (d, J = 7.6 Hz, 8H),
7.09–7.05 (m, 8H), 4.09 (s, 6H). 13C NMR (100 MHz, CDCl3) δ: 152.63,
150.75, 148.10, 147.44, 138.17, 133.14, 130.69, 129.33, 124.79,
123.27, 122.58, 106.70, 56.40. ESI (m/z): Calcd for C46H36N4O2, 676.28
(M+); found, 676.28.
C
45H31F3N4, 684.25 (M+); found, 684.25.
Preparation of monomer VQ5. To a stirred solution of compound 1
(163 mg, 0.3 mmol) in acetic acid (40 mL) was added 4-nitrobenzene-
1,2-diamine (46 mg, 0.3 mmol) at room temperature. The reaction
mixture was stirred at 90 ◦C under N2 atmosphere for 12 h. Evaporation
of the acetic acid under reduced pressure and the residue was treated
with water (50 mL), extracted twice with CH2Cl2 (50 mL × 2). The
combined organic layers were washed twice with water and once with
brine, dried over anhydrous magnesium sulfate. After removing the
solvent under reduced pressure, the residue was purified by chroma-
tography using hexane/EA (10/1, v/v) as an eluent to yield compound
3a as an orange solid (130 mg, 66%). 1H NMR (400 MHz, CDCl3) δ: 9.00
(d, J = 2.4 Hz, 1H), 8.46 (dd, J1 = 9.0 Hz, J2 = 2.4 Hz, 1H), 8.20 (d, J =
9.0 Hz, 1H), 7.50 (dd, J1 = 10.6 Hz, J2 = 8.7 Hz, 4H), 7.30 (t, J = 7.6 Hz,
8H), 7.15 (d, J = 7.8 Hz, 8H), 7.13–7.09 (m, 4H), 7.04 (dd, J = 8.6, 6.8
Hz, 4H). 13C NMR (100 MHz, CDCl3) δ: 155.88, 155.30, 149.50, 149.32,
147.48, 147.04, 146.97, 143.55, 139.73, 131.04, 130.94, 130.77,
130.34, 129.49, 125.43, 125.36, 124.00, 123.91, 122.84, 121.47,
121.29. ESI (m/z): Calcd for C44H31N5O2, 661.25 (M+); found, 661.24.
Preparation of monomer VQ3. To a stirred solution of compound 1
(163 mg, 0.3 mmol) in acetic acid (40 mL) was added 4,5-difluoroben-
zene-1,2-diamine (43 mg, 0.3 mmol) at room temperature. The reac-
tion mixture was stirred at 90 ◦C under N2 atmosphere for 12 h.
Evaporation of the acetic acid under reduced pressure and the residue
was treated with water (50 mL), extracted twice with CH2Cl2 (50 mL ×
2). The combined organic layers were washed twice with water and once
with brine, dried over anhydrous magnesium sulfate. After removing the
solvent under reduced pressure, the residue was purified by chroma-
tography using hexane/EA (10/1, v/v) as an eluent to yield compound
3a as an orange solid (143 mg, 73%). 1H NMR (400 MHz, CDCl3) δ: 7.87
(t, J = 9.2 Hz, 2H), 7.45 (d, J = 8.6 Hz, 4H), 7.30 (t, J = 7.8 Hz, 8H), 7.16
3. Results and discussion
The synthesis routes of five D-A-D type monomers VQ1 ~ VQ5 and
corresponding polymers PVQ1 ~ PVQ5 were shown in Scheme 1.
Briefly, the preparation of five monomers were achieved in high yields
via condensation reactions between corresponding benzene-1,2-diamine
derivates and a same intermediate diketone 1, respectively, which was
synthesized by Friedel crafts acylation of triphenylamine. The structures
of desired V-type monomers were characterized by NMR spectroscopy as
well as mass spectrometry (Fig. S1 ~ S12). All these monomers exhibited
high thermal stability, with decomposition temperatures around 410 ◦C
at 5% weight loss (Fig. S13), which is higher than most of the triphe-
nylamine derivatives [42,43].
(d, J = 7.8 Hz, 8H), 7.09 (t, J = 7.2 Hz, 4H), 7.06 (d, J = 8.6 Hz, 4H). 13
C
NMR (100 MHz, CDCl3) δ: 153.39, 150.96, 150.83, 148.84, 147.22,
138.35, 138.29, 138.19, 131.78, 130.74, 129.45, 125.16, 123.68,
121.90, 114.71, 114.65, 114.59, 114.53. ESI (m/z): Calcd for
C
46H36N4O2, 652.24 (M+); found, 652.24.
Preparation of monomer VQ4. To a stirred solution of compound 1
(163 mg, 0.3 mmol) in acetic acid (40 mL) was added 4-(tri-
fluoromethyl)benzene-1,2-diamine (53 mg, 0.3 mmol) at room tem-
perature. The reaction mixture was stirred at 90 ◦C under N2 atmosphere
for 12 h. Evaporation of the acetic acid under reduced pressure and the
residue was treated with water (50 mL), extracted twice with CH2Cl2
(50 mL × 2). The combined organic layers were washed twice with
water and once with brine, dried over anhydrous magnesium sulfate.
After removing the solvent under reduced pressure, the residue was
purified by chromatography using hexane/EA (10/1, v/v) as an eluent
to yield compound 3a as an orange solid (142 mg, 69%). 1H NMR (400
MHz, CDCl3) δ: 8.43 (s, 1H), 8.22 (d, J = 8.8 Hz, 1H), 7.88 (dd, J1 = 8.8
Hz, J2 = 1.7 Hz, 1H), 7.47 (dd, J1 = 8.8 Hz, J2 = 7.0 Hz, 4H), 7.29 (t, J =
8.0 Hz, 8H), 7.15 (d, J = 8.2 Hz, 8H), 7.10 (d, J = 7.4 Hz, 4H), 7.04 (d, J
The absorption spectra of five monomers VQ1 ~ VQ5 in dichloro-
methane were shown in Fig. 1a, and the data were summarized in
Table 1. All these monomers exhibited two or three distinct absorption
bands: the longer wavelength absorption bands around the visible re-
gion (400–480 nm) that can be assigned to intramolecular charge
transfer (ICT) transitions from the triphenylamine donating groups to
the quinoxaline-based accepting moieties, [19]; the shorter and broad
wavelength absorption bands in the UV region (300–400 nm) is mainly
originated from the π-π* electron transitions of the conjugated mole-
cules. The absorption peak originated from ICT transition of VQ1
located in 401 nm. It was noted that a linear D-A-D isomer with two
triphenylamine group substituted in 5,8-positions of quinoxaline moiety
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W. Wang et al.
Dyes and Pigments 194 (2021) 109615
electrolyte to investigate the electrochemical properties of five new D-A-
D type monomers VQ1 ~ VQ5 (Fig. 1b). As shown in Table 1, these five
monomers exhibited similar first oxidation potentials due to the same
electron donor parts in them. In order to gain further insight into the
configurations of these five monomers and their frontier molecular or-
bitals, DFT calculations of monomers VQ1 ~ VQ5 are carried out at the
B3LYP/6-31G** level. As shown in Fig. 2, LUMOs of these V-type
monomers are mainly located on the quinoxaline parts, whereas their
HOMOs show delocalized electron distributions through the two tri-
phenylamine moieties and their adjacent pyrazine group, respectively.
Hence, both orbitals provide overlap between donor and acceptor to
guarantee an intramolecular charge transfer transition.
The polymer thin films PVQ1 ~ PVQ5 were successfully deposited
on ITO/glass via electrochemical polymerization of these new mono-
mers (see Fig. 3), which was carried out by a conventional three-
electrode system in the solution of monomers in a mixed solvent of
methylbenzene/acetonitrile (4/1, v/v), using TBAPF6 as a supporting
electrolyte, respectively [45]. As seen in Fig. 3a and 3e, a reversible
oxidation peak at 1.2 V was observed for each sample during the first
anodic scan, which was attributed to the oxidation of the corresponding
triphenylamine moiety [46]. In the second scan an additional shoulder
oxidation peak at 1.05 V appeared, which shifted to a higher potential
after each successive cycle. The additional peak may be a typical
oxidation wave of the tetraphenylbenzidine (TPB) group, suggesting the
occurrence of the oxidative coupling between the triphenylamine units
(Fig. S14) [47]. In the subsequent voltammetric cycles, the anodic peaks
gradually shifted to higher potentials and the cathodic peaks shifted to
lower potentials with the increasing intensity of the oxidation peaks.
The increase of redox current intensities by successive CV scan implied
the formation of the electrochemically active polymeric film on the
electrode surface [48]. After ten repeated CV scans between 0 and 1.8 V,
an electrochemically deposited thin film was observed on the electrode
surface. All five monomers exhibited a similar phenomenon during the
electrochemical polymerization process.
Cyclic voltammograms (CVs) of the polymer films PVQ1 ~ PVQ5 on
ITO was carried out in a three-electrode electrochemical system, in
which the polymer films on ITO as working electrode, Pt wire as the
counter electrode, Ag wire as the reference electrode, respectively. The
cyclic voltammetry (CV) curves of electrodes at different scan rates
between 20 and 200 mV sꢀ 1 were recorded in 0.1 M TPAPF6/PC solu-
tions. As shown in Fig. 3f–3j, a pair of redox peaks of PVQ1 ~ PVQ5
electrodes were observed, respectively. The plot of scan rate against the
maximum current (I) shows the linear relationship (Fig. S15), indicating
that the charge transfer process of the redox reaction of these electrode is
not limited by the diffusion of counter ions. The result also suggested a
good contact between the electro-generated polymeric films and ITO
electrode.
Fig. 3. a~e) CV curves of five monomers in ACN:toluene (1:4, v/v, 2 mM)
containing 100 mM TBAPF6 between 0 and 1.8 V for 10 cycles at scan rate of
100 mV sꢀ 1, respectively; f~j) CV of PVQ1 ~ PVQ5 thin films on ITO in
TPAPF6/PC (0.1 M) at scan rate from 20 to 200 mV sꢀ 1, respectively.
Then, we investigated the electrochromic properties of the present
polymer PVQ1 ~ PVQ5 on ITO electrode by spectroelectrochemistry
measurements, which were performed in a bath of 0.1 M TBAPF6/PC.
For example, PVQ1 film in neutral form exhibited a broad absorption
band with a peak value around 340 nm corresponding to the over-
exhibited a maximum ICT absorption peak around 413 nm reported by
Zhou et al. previously [44]. The result implied that a blue-shift was
achieved by constructed V-shape D-A-D configuration as compared to
that with linear D-A-D type configuration. With the introduction of two
methoxy groups on 6- and 7- positions of its quinoxaline moiety, the
corresponding ICT absorption peak of VQ2 exhibited slight hypo-
chromatic shift, probably due to the weakened electron accepting ability
of the quinoxaline-based acceptor. In contrast, with the introduction of
one or more electron withdrawing groups in quinoxaline moiety, VQ3,
VQ4 and VQ5 exhibited different degrees of redshift in comparison to
VQ1. As a result, the absorption onset of VQ5 with additional strong
electron-deficient group (-NO2) extent up to 580 nm.
lapping π-π* transition and ICT, of which the intensity decreased grad-
ually under the increasing working potential. Meanwhile, a new
absorption peak around 500 nm was observed accompanying the in-
crease of the absorption intensity, which may be attributed from the
formation of TPB radical cation [49]. Notably, when a working potential
over 1.04 V was applied, the dication of TPB may be formed [50],
resulting in a new intense absorption band extend to the near IR region
with a peak around 750 nm was appeared, and its intensity reached a
maximum value at around 1.2 V (Fig. 4a). As a result, The polymer
PVQ1 exhibited multicolor change from light yellow to orange, and
further to blue under certain applied voltage. As shown in Fig. 4b–4f,
other four analogues PVQ2 ~ PVQ5 exhibited similar electrochromic
behaviour.
Cyclic voltammetry (CV) was carried out in CH2Cl2 containing 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) as a supporting
We further investigated the optical eletrochromism by
4

W. Wang et al.
Dyes and Pigments 194 (2021) 109615
Fig. 4. Electronic absorption spectra of PVQ1 ~ PVQ5 thin films on ITO in TPAPF6/PC (0.1 M), respectively.
chronoamperometry with putting on square-wave potential between
0 and the corresponding highest oxidation potentials for PVQ1 ~ PVQ5,
respectively (see Fig. 5) [51]. While the films were switched, the
transmittance at selected wavelengths was monitored as a function of
time with UV–vis–NIR spectroscopy. The response time was calculated
at 90% of the full-transmittance change, because it is difficult to
perceive any further color change with the naked eye beyond this point
[52]. As shown in Fig. 5a and 5e, the optical transmittance of PVQ1 ~
PVQ5 film at the longer wavelength peak (around 750 nm) was detected
by applying potential between 0 and the corresponding highest oxida-
tion (around 1.2 V) potentials with a residence time of 10 s. As sum-
marized in Table 2, all these five polymers performed high optical
contrast over 70% with coloration time (tc) less than 2 s and bleaching
time (tb) less than 1s. Especially, the tc and tb of PVQ4 was 0.9 s and 0.7
s, which is superior to the reported electrochromic materials based on
triphenylamine derivatives [16,28,34,45,46,50]. The faster response
time for these five films may be attributed to its D-A-D structure, which
might be more favorable for electron transfer within molecules. As
shown in Fig. 5f–5j, all these five electrochromic materials exhibited
good stability with a little decrease of optical contrast about 5% after 20
cycles switching.
Chrono-absorptometric and chronoamperometric techniques were
employed to evaluate the coloration efficiencies of these five novel D-A-
D type polymers. The charge/discharge amount (Qd) was recorded by
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W. Wang et al.
Dyes and Pigments 194 (2021) 109615
where Tb and Tc are the percent transmittance of bleaching and coloring
state at the corresponding wavelength around 750 nm, respectively. As
listed in Table 2, CE of these five new electrochromic materials were
calculated to be 200 cm2/C for PVQ1, 224 cm2/C for PVQ2, 251 cm2/C
for PVQ3, 232 cm2/C for PVQ4, 312 cm2/C for PVQ5, respectively. The
electrochromic switch properties around 495 nm of these five polymer
were also investigated by the same way, and the results were shown in
Fig. S16 and Table S1.
Finally, we fabricated electrochromic devices based on PVQ1 ~
PVQ5 films with a sandwich configuration. Briefly, a gel electrolyte
containing 0.1 M TBAPF6 in PC solution and 20 wt% poly(methyl-
methacrylate) (PMMA) was spread on the top of the PVQ1 ~ PVQ5 films
that were pre-deposited on ITO glass, respectively. Then, a bare ITO
glass was covered on the top of electrolyte and sealed by a commercial 3
M tape to prevent leakage [54]. As shown in Fig. 6 and Fig. S17-20, EC
devices based on polymers PVQ1 ~ PVQ5 exhibited similar electro-
chromic phenomena with multicolor change from pale yellow to orange,
and further to blue upon the increase of applied potentials, respectively.
Electrochromic devices based on these five polymers exhibited highest
optical contrast over 80% around 750 nm, and exhibited good redox
stability after 20 cycles switching, respectively. Moreover, the diversi-
form color in both bleaching and coloring states of these five electro-
chromic devices suggested the successful molecular design of the present
V-type D-A-D compounds toward tunable electrochromism.
4. Conclusion
In summary, we developed five novel D-A-D type monomers with a
common V-shape configuration, in which two triphenylamine groups
were introduced into the 2- and 3- position of the quinoxaline derivates.
Five polymer films PVQ1 ~ PVQ5 were successfully deposited onto the
ITO electrodes via electro-polymerization, which exhibited large optical
contrast, short response time, high coloration efficiency and good redox
stability, respectively. Electrochromic devices based on the present D-A-
D polymers were also prepared and exhibited efficient electrochromic
performance, which further demonstrated the rational structure design
of V-shape monomers. Moreover, the multiple and tunable color change
among the visible and near IR region guaranteed their potential appli-
cations in smart windows.
CRediT authorship contribution statement
Wenyuan Wang: Investigation, Formal analysis, Writing – original
draft. Hongjin Chen: Synthesis, Formal analysis. Youfeng Yue: Formal
analysis, Writing – review & editing. Rui Zhang: Project administration,
Writing – review & editing. Jian Liu: Conceptualization, Writing – re-
view & editing, Supervision, Funding acquisition.
Fig. 5. a~e) Optical contrast and response time of PVQ1 ~ PVQ5 thin films
around 750 nm, respectively; (f~j) Electrochromic switching stability for
PVQ1 ~ PVQ5 thin films, respectively.
Declaration of competing interest
monitoring the change in current with time (Fig. 5f–5j). Then, we
calculated the coloration efficiency (CE) of these five polymers by using
the following equations [53].
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
CE = [log (Tb/Tc)]/Qd
Table 2
Optical and electrochromic properties of PVQ1 ~ PVQ5 thin films monitored around 750 nma.
Polymers
Operation voltage
(V)
Transmittance change (ΔT,
Bleaching time (tb,
s)
Coloration time (tc,
s)
Charge/discharge amount (Qd,
Coloration efficiency (CE,
%)
mC⋅cmꢀ 2
)
cm2⋅Cꢀ 1
)
PVQ1
PVQ2
PVQ3
PVQ4
PVQ5
0–1.22 V
0–1.2 V
0–1.25 V
0–1.25 V
0–1.2 V
76.5
71.4
78.7
73.4
76.2
0.5
0.7
0.5
0.7
0.7
2.0
1.4
1.3
0.9
1.8
2.47/1.76
2.28/1.51
2.35/1.24
2.28/1.51
2.35/1.24
200
224
251
232
312
6

W. Wang et al.
Dyes and Pigments 194 (2021) 109615
Fig. 6. a) Absorption spectra of a EC device based on PVQ1 at 0 V, 2.2 V and 2.6 V; b) The transmittance change for the EC device based on PVQ1 at 751 nm; c)
Current consumption and transmittance changes monitored of the EC device based on PVQ1 at 755 nm during 20 cycles of electrochromic switching between 0 and
2.6 V.
active tetraphenylethylene for highly integrated electrochromic/
Acknowledgements
electrofluorochromic performances. J Mater Chem C 2019;7(30):9308–15.
[17] Dai Y, Li W, Chen Z, Zhu X, Liu J, Zhao R, Wright DS, Noori A, Mousavi MF,
This work was supported in part by the Natural Science Foundation
of Jiangsu Province (BK20191385), the National Natural Science
Foundation of China (21631006), and the High Level Talent Project of
Nanjing Forestry University (GXL2018003).
Zhang C. An air-stable electrochromic conjugated microporous polymer as an
emerging electrode material for hybrid energy storage systems. J Mater Chem
2019;7(27):16397–405.
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Giannuzzi R, Beneduci A, Gigli G. Arylamino-fluorene derivatives: Optically
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