Synthesis and characterization of soluble donor-acceptor type copolymers based on benzotriazole, quinoxaline and benzene units with multicolor electrochromism

Article abstract of DOI:10.1016/j.orgel.2019.105514

Three soluble donor-acceptor (D-A) type copolymers employing benzene (B) as donor, 2-dodecyl-4,7-di(thiophen-2-yl)-2H-benzo [d][1,2,3]triazole (Z) and 2,3-bis((4-(2-octyldodecyl)oxy)phenyl)-quinoxaline (Q) as acceptors were synthesized through chemical po

Full text of DOI:10.1016/j.orgel.2019.105514

Journal Pre-proof
Synthesis and characterization of soluble donor-acceptor type copolymers based on
benzotriazole, quinoxaline and benzene units with multicolor electrochromism
Yan Zhang, Hongmei Du, Yiming Yin, Yunyun Dong, Jinsheng Zhao, Zhen Xu
PII:
S1566-1199(19)30541-5
DOI:
https://doi.org/10.1016/j.orgel.2019.105514
ORGELE 105514
Reference:
To appear in: Organic Electronics
Received Date: 4 September 2019
Revised Date: 16 October 2019
Accepted Date: 17 October 2019
Please cite this article as: Y. Zhang, H. Du, Y. Yin, Y. Dong, J. Zhao, Z. Xu, Synthesis and
characterization of soluble donor-acceptor type copolymers based on benzotriazole, quinoxaline and
benzene units with multicolor electrochromism, Organic Electronics (2019), doi: https://doi.org/10.1016/
j.orgel.2019.105514.
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Graphical Abstract

Synthesis and characterization of soluble donor-acceptor
type copolymers based on benzotriazole, quinoxaline and
benzene units with multicolor electrochromism
1
1
2
2
1,
Yan Zhang , Hongmei Du , Yiming Yin , Yunyun Dong , Jinsheng Zhao *, Zhen
Xu *
3
,
1
Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel
Cell Technology, Liaocheng University, Liaocheng, 252059, P.R. China
2
College of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng,
2
52059, P.R. China
3
School of Chemistry and Chemical Engineering, Shandong University of Technology,
Zibo, 255049, P.R. China
*
Correspondence: j.s.zhao@163.com (J.S.Zhao); xuzhen@sdut.edu.cn (Z. Xu)
Abstract
Three soluble donor-acceptor (D-A) type copolymers employing benzene (B) as
donor, (2-dodecyl-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole (Z) and
,3-bis((4-(2-octyldodecyl)oxy)phenyl)-quinoxaline (Q) as acceptors were
2
synthesized through chemical polymerization. A variety of characterization methods
such as cyclic voltammetry, UV-vis spectroscopy, colorimetry and thermogravimetric
analysis were executed to detect the electrochromic properties of polymers. All the
polymers displayed multicolor in the redox process with medium band gaps, and
different molar ratio of B/Z/Q allowed them to cover diverse color changes,
containing orange-red/brown-yellow/cyan/green ((PBZQ-1), orange-red/yellow/light
grass green (PBZQ-2), and red/black/grey-blue (PBZQ-3). The multichromism of the
polymers involved RGB and black colors. Meanwhile, with the increase of Z unit
ratio and the decrease of Q unit ratio, the polymer demonstrated the reduced onset
oxidation potential and optical band gap, as well as the different kinetic parameters.
Moreover, the three polymers exhibited good solubility, desirable thermal stability,
relatively large optical contrast and high coloration efficiency. The above positive

results implied that these copolymers were expected to be the credible candidates for
the electrochromic devices with commercial values.
Keywords: D-A type copolymer; multichromism; soluble; benzotriazole; quinoxaline
1
. Introduction
Conductive polymer (CP) is a class of macromolecular material which has not
only the optoelectronic property of metal and inorganic semiconductor, but also the
flexible mechanical property and processability of the polymer [1]. Optically
responsive CPs that reveal electrochromism have attracted much attention in recent
years owing to their advantages of multiple color changes, molecular structure
designability, processibility, low cost, and so on [2-4]. Although more and more
electrochromic polymers have been investigated, multichromic polymers remain a
fascinating goal for researchers due to their potential commercial values in smart
window and optical display applications [5,6]. Multicolor polymers presenting green
or black are rare because green color requires two simultaneous absorptions at red and
blue regions and black color needs the complete absorptions in the entire visible
region, and therefore it is necessary to design and develop this species [7-9].
Among different methods of color tuning, preparing donor-acceptor (D-A) type
polymer is considered as the most popular and powerful strategy as introducing
different sorts of donors and acceptors into a same polymer chain can adjust the
intra-molecular charge transfer (ICT) effect availably, thereby regulating the polymer
optical property [2,10,11]. In general, most D-A type electrochromic polymers tend to
show two colors, and only a small number of reported polymers based on triarylamine
[
12,13], naphthalene bisimide [14], benzotriazole [15], benzothiadiazole [16],
quinoxaline [17], etc. exhibit respective impressive multicolor. Moreover, D-A
approach is advantageous to reduce the band gap of the polymer and hence optimizes
the optical and electronic performances [11]. Another property of concern for the
electrochromic polymer is the solubility since the soluble polymer can reduce the
fabrication and processing costs by printing, spraying and coating methods. For the
D-A type polymer, introducing proper long alkyl side chains in the donors and

acceptors can improve the solubility effectively, and meanwhile, it also can impact the
electronic properties including band gap, oxidation potential, and stability [7,18].
Here, we are committed to exploring soluble multichromic polymers through
designing double acceptor type D-A structure.
In various D-A type molecules, benzotriazole has always been the research
hotspot as an electron-deficient acceptor unit since the benzotriazole-based
electrochromic polymer usually combines multiple desired properties in a single
polymer such as multichromism, transmissive state, dual type dopability, good
solubility and excellent kinetic performance [19,20]. Toppare group synthesized a
series of representative benzotriazole bearing polymers [21-23], among which
poly((2-dodecyl-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole)
(PTBT)
was
considered to be a significant breakthrough in numerous electrochromic materials due
to its multi-color changes including all RGB (red, green, blue) colors, black and
transmissive states [21]. Herein, the structure of PTBT was introduced into the new
polymer main chains in order to utilize its distinctive properties.
Quinoxaline, as another popular acceptor unit in the D-A type polymer, has been
proved to be a desirable building block for a low band gap conjugated polymer which
is attributed to its imine nitrogens (C=N) with strong electron withdrawing character
[
24]. Besides, the quinoxaline structure provides a rigid coplanar backbone which can
form a highly extended π-electron system with strong π-stacking in the polymer [25].
Also, it is convenient for quinoxaline unit to introduce other substituents on its 2 and
3
positions, thus enhancing the optoelectronic property and the solubility of the
corresponding conjugated polymer [24]. With the help of quinoxaline, many high
performance electrochromic polymers were synthesized, and especially some neutral
state green polymers with dual type dupability and narrow band gap (less than 1.5 eV)
were obtained [26-28].
Our group also synthesized a series of D-A type conductive polymers based on
benzotriazole or quinoxaline units which showed different color changes and
favorable photoelectric properties [25,29-31]. However, in this work, we attempted to
incorporate benzotriazole and quinoxaline together as the double acceptor in a same

D-A type copolymer, and expected the novel copolymers could combine the
advantages of the two acceptor units and present desired multichromism, good
solubility and processability, as well as low band gap. Though employing benzene
(
B) as the donor, (2-dodecyl-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole (Z) and
,3-bis((4-(2-octyldodecyl)oxy)phenyl)-quinoxaline (Q) as the acceptors, a kind of
donor-acceptor (1)-donor-acceptor (2) (D-A -D-A ) type copolymer were synthesized
2
1
2
by Suzuki cross coupling reaction.
The two flanking thiophene groups adjacent to benzotriazole in the Z units, as
the π-bridges, played a role of not only improving the planarity of the polymer
backbone but also reducing the steric hindrance between Z and other units [32]. In the
Q units, the two phenyl groups connected on quinoxaline were aimed to expand the
conjugation length and reduce the steric hindrance of the alkoxy long chains [33]. And
the alkyl and alkoxy side chains linked on the benzotriazole and quinoxaline
molecules were used to increase the solubility of the polymer [18,34]. By changing
the molar ratio of different units, three copolymers namely PBZQ-1 (B: Z: Q = 3: 1:
2
), PBZQ-2 (B: Z: Q = 2: 1: 1) and PBZQ-3 (B: Z: Q = 5: 4: 1) were prepared, and as
predicted, these copolymers exhibited different multicolor in the redox process with
good solubility, satisfactory kinetic property and favorable thermal stability. In
particular, the color changes of the three polymers covered RGB and black colors,
which were very important electrochromic features for their actual utilizations.
2
2
. Experimental Section
.1. Materials
4
,7-dibromobenzo[c][1,2,5]thiadiazole
(98%),
and
1
,2-bis(4-methoxyphenyl)ethane-1,2-dione (98%) were purchased from Puyang
HuiCheng electronic material co., Ltd (Puyang, China). Sodium borohydride (NaBH4,
8%), hydrogen bromide (HBr, 40%), bromine (Br , 99.9%), N-bromosuccinimide
9
2
(NBS, 98.0%), 1-bromo-2-octyldodecane, tetrabutylammonium bromide (TBAB),
anhydrous potassium carbonate (K CO ), anhydrous sodium carbonate (Na CO )
2
3
2
3
tributyl(thiophen-2-yl)stannane, 1,2,3-benzotriazole (99%), potassium tert-butoxide

(
t-BuOK, 98%), bromododecane (C H Br, 98%), bis-(triphenylphosphine)
12 25
dichloropalladium (Pd(PPh ) Cl , ≥98%), tetrakis(triphenylphosphine)palladium
3
2
2
(
Pd(PPh ) ,
99.0%),
tetrabutylammonium
fluoride
(N(n-Bu) F),
4
3
4
1
,4-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzene
(>98.0%)
and
tetrabutyl-ammonium hexafluorophosphate (TBAPF , 98%) were purchased from
6
Aladdin Chemical Co., Ltd. (Shanghai, China). Ethanol (EtOH, 99.9%), methanol
(MeOH, 99.7%), toluene (99.9%), glacial acetic acid (HAc, 99.9%), magnesium
sulfate (MgSO ), chloroform, n-hexane and N,N-dimethylformamide (DMF) were
4
bought from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
Dichloromethane (DCM) and acetonitrile (ACN) were bought from Tedia Co., Inc.
(Fairfield, Ohio, USA). Indium-tin-oxide (ITO) coated glass (sheet resistance: < 10
Ω/sq) were bought from Shenzhen CSG Display Technologies (Shenzhen, China).
2
2
.2. Syntheses of the polymers
.2.1. Synthesis of 5,8-dibromo-2,3-bis(4-((2-octyldodecyl)oxy)phenyl)quinoxaline
(3)
The synthetic route of product 3 was illustrated in Scheme 1. The compounds
,6-dibromobenzene-1,2-diamine (1) [26] and
,2-bis(4-((2-octyldodecyl)oxy)phenyl)ethane-1,2-dione (2) [35] were synthesized in
3
1
accordance with the methods reported in the literatures.
The compound 1 acquired in the end was the white flocculent solids. H NMR
1
1
3
(
400 MHz, CDCl ): δ 6.84(s, 2H), 3.89(s, 4H); C NMR (100 MHz, CDCl ) δ 133.71,
3
3
1
23.24, 109.68(see supporting information Fig. S1)
1
The compound 2 acquired in the end was the yellow-green oily liquid. H
NMR(400 MHz, CDCl , δ ppm) δ=7.93 (d, 4H, ArH), 6.95 (d, 4H, ArH), 3.90 (d, 4H),
3
1
3
1
1
2
.80 (m, 2H), 1.26(m, 64H), 0.88 (t, 12H). C NMR (CDCl , 101 MHz, ppm) δ =
3
93.52, 164.69, 132.29, 126.07, 114.70, 71.32, 37.79, 31.88, 31.25, 29.93, 29.56,
9.29, 26.78, 22.65, 14.06 (see supporting information Fig. S2).
As shown in Scheme 1, 0.665 g compound 1 (2.50 mmol) and 2.0 g compound 2
(2.50 mmol) were dissolved by 100 ml HAc in a round-bottom flask. After that, 20 ml
n-hexane was added to this mixture to increase the solubility of compound 2 in HAc.

After argon was injected to eliminate residual oxygen, the mixture was heated and
refluxed for 24 hours. Then，the solution was extracted with deionized water and
n-hexane. After the solvent of organic phase was removed by distillation under
reduced pressure, the crude product was obtained and further refined by column
chromatography using hexane/DCM (3:1, v/v) as eluent. The pure product 3 was the
1
yellow-green waxy solid. H NMR(400 MHz, CDCl , δ ppm) δ=7.84 (s, 2H, ArH),
3
7
.66 (d, 4H, ArH), 6.89 (d, 4H, ArH), 3.87 (d, 4H), 1.78 (m, 2H), 1.27 (m, 64H), 0.88
1
3
(
t, 12H)。 C NMR (CDCl , 101 MHz, ppm) δ = 160.74, 153.55, 139.00, 132.41,
3
1
2
31.60, 130.30, 123.43, 114.42, 71.07, 37.92, 31.89, 31.36, 30.00, 29.62, 29.31, 26.84,
2.66, 14.08 (see supporting information Fig. S3).
S
N
N
H N
NH2
NaBH , EtOH
2
4
o
0
C, 24h
Br
Br
Br
Br
1
H CO
3
OCH3
HO
OH
RBr, TBAB, K CO
RO
OR
HAc, HBr
2
3
reflux, 8h
DMF, reflux, 1.5h
C C
C C
C C
2
O
O
O
O
O
O
RO
OR
HAc
reflux, 24h
1
+
2
N
N
R =
Br
Br
3
Scheme 1. The synthetic route of 5,8-dibromo-2,3-bis(4-((2-octyldodecyl)oxy)phenyl)quinoxaline.
2
4
.2.2.
Synthesis
of
,7-bis(5-bromothiophen-2-yl)-2-dodecyl-2H-benzo[d][1,2,3]triazole (6)
The synthetic route of product 6 was exhibited in Scheme 2. The compound
,7-dibromo-2-dodecyl-2H-benzo[d][1,2,3]triazole (4) [22,23] was synthesized
4
according to the literatures.
1
The compound 4 acquired in the end was the yellow-white solid powder. H
NMR(400 MHz, CDCl , δ ppm) δ=7.42 (s, 2H, ArH), 4.76 (t, 2H), 2.13 (m, 2H), 1.24
3
1
3
(
m, 18H), 0.86 (t, 3H). C NMR (CDCl , 101 MHz, ppm) δ = 143.68, 129.44, 109.96,
3

5
7.43, 31.86, 30.15, 29.55, 29.44, 29.28, 26.93, 26.47, 22.64, 14.07 (see supporting
information Fig. S4).
As shown in Scheme 2, firstly, 2.0 g compound 4 (4.5mmol), 4.2 g
tributyl(thiophen-2-yl)stannane (11.25 mmol) and 0.2 g Pd(PPh ) Cl (0.285 mmol)
3
2
2
were added to 100 ml toluene. After 30 min of argon flow, the mixture was refluxed
for 24 h to acquire 2-dodecyl-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole (5).
Secondly, weigh accurately 1.2 g compound 5 (2.64 mmol) and 1.1 g NBS (6.2 mmol)
to a round-bottomed flask equipped with 35ml HAc and 35ml DCM. Then the
solution was heated to reflux for 12 h avoiding light. After completion, the mixture
was extracted with DCM and the crude product was purified by silica gel column
1
chromatography. The pure product 6 was the bright yellow solid. H NMR (400 MHz,
CDCl , δ ppm) δ=7.78 (d, 2H, ArH), 7.50 (s, 2H), 7.12(d, 2H), 4.79 (t, 2H), 2.18 (m,
3
1
3
2
1
2
H), 1.25 (m, 18H), 0.87 (t, 3H). C NMR (CDCl , 101 MHz, ppm) δ = 141.72,
3
41.22, 130.84, 126.91, 123.00, 122.18, 113.14, 56.92, 31.88, 30.00, 29.59, 29.51,
9.40, 29.30, 28.98, 26.55, 22.65, 14.08 (see supporting information Fig. S5).
C H
C H
12 25
1
2
25
N
N
N
HN
N
+
-
N
N
N
N
K (OtBu) , C H Br
HBr, Br
2
1
2
25
o
MeOH
135 C, 12h
Br
Br
4
C H
C H
12 25
1
2
25
N
N
N
N
N
N
Pd(
), toluene
reflux, 24h
NBS, DCM, HAc
reflux, 12h
4
+
SnBu3
S
S
S
S
S
Br
Br
5
6
Scheme
2.
The
synthetic
route
of
4
,7-bis(5-bromothiophen-2-yl)-2-dodecyl-2H-benzo[d][1,2,3]triazole.
2
.2.3. Syntheses of PBZQ-1, PBZQ-2 and PBZQ-3
As shown in Scheme 3, Suzuki cross-coupling reaction was used to synthesize
the copolymer. 0.2396 g 1,4-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzene
0.7259 mmol), 0.5000 g compounds 3 (0.4839 mmol), 0.1475g compounds 6 (0.2420
mmol), 0.042 g Pd(PPh ) (0.0360 mmol), 0.0150 g N(n-Bu) F (0.0570 mmol) and 60
(
3
4
4

ml NaCO aqueous solution (2 mol/L) were added to 90ml toluene, and the mixture
3
was heated to refluxed for 48h in the protection of argon. After completion of the
reaction, toluene was distilled off under reduced pressure and then the crude product
was recrystallized with methanol. The acquired solid powder still needed to be
extracted in adipose extractor with methanol and acetone as the solvent in sequence
1
until the solvent was colorless. The resulting PBZQ-1 was bright red solid. H
NMR(400 MHz, CDCl , δ ppm) δ=8.12 (d, 2H), 8.05-7.99 (m, 2H), 7.90 (s, 2H),
3
7
.80-7.34 (m, 8H), 7.21-7.00 (m, 2H), 6.84 (s, 5H), 3.81 (s, 4H), 2.17 (s, 2H), 1.74 (s,
H), 1.50-0.98 (m, 120H), 0.86 (s, 27H) (see supporting information Fig. S6a). The
2
gel permeation chromatography (GPC) data: M = 24.5 kDa, M = 15.3 kDa,
w
n
polydispersity index = 1.60.
The synthetic methods of PBZQ-2 and PBZQ-3 were identical with PBZQ-1
except for the different ratio of monomers. For PBZQ-2, the amounts of reagents were
0
.3194 g 1,4-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzene (0.9678 mmol),
.5000 g compounds 3 (0.4839mmol), 0.2949 g compounds 6 (0.4839 mmol), 0.060 g
0
Pd(PPh ) (0.0520 mmol), 0.015g N(n-Bu)4F (0.0570 mmol), 60 ml NaCO (2 mol/L)
3
4
3
aqueous solution and 90 ml toluene. The product PBZQ-2 was also bright red solid.
1
H NMR (400 MHz, CDCl , δ ppm) δ=8.13-7.90 (m, 4H), 7.67-7.38 (m, 6H),
3
7
1
1
.12-7.08 (m, 2H), 6.83 (s, 4H), 3.81 (s, 2H), 2.17 (s, 2H), 1.51-0.95 (m, 70H), 0.85 (s,
5H) (see supporting information Fig. S6b). The GPC data: M = 23.9 kDa, M =
w
n
5.1 kDa, polydispersity index = 1.58.
For PBZQ-3, the amounts
of
reagents
were
0.3383
g
1
,4-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzene (1.025 mmol), 0.2118 g
compounds 3 (0.205 mmol), 0.5000 g compounds 6 (0.8200 mmol), 0.0600 g
Pd(PPh ) (0.0520 mmol), 0.0150 g N(n-Bu)4F (0.0570 mmol), 60 ml NaCO
3
3
4
aqueous solution (2 mol/L) and 90 ml toluene. The product PBZQ-3 was dark red
1
solid. H NMR(400 MHz, CDCl , δ ppm) δ=7.87-7.72 (d, 2H), 7.70-7.42 (m, 3H),
3
7
1
.23-6.99 (m, 4H), 6.98-6.65 (d, 5H), 3.87 (s, 4H), 2.77 (s, 3H), 1.95-1.63 (m, 4H),
.49-1.05 (m, 52H), 0.87 (s, 15H) (see supporting information Fig. S6c). The GPC
data: M = 22.9 kDa, M = 15.9 kDa, polydispersity index = 1.44.
w
n

RO
OR
C H
1
2
25
N
N
N
O
O
O
O
N
N
+
+
B
B
Br
Br
S
S
Br
Br
3
6
Pd(PPh3)4 Toluene
reflux
8h
Na CO
N(n-Bu)4F
2
3
4
RO
OR
C H
25
1
2
N
N
N
N
N
S
S
x
x+y
y
n
PBZQ-1: x=1, y=2
PBZQ-2: x=1, y=1
PBZQ-3: x=4, y=1
R =
Scheme 3. The synthetic route of PBZQ-1, PBZQ-2 and PBZQ-3.
2
.3. Instrument and characterization
1
13
The H NMR and C NMR spectra measurements were carried out on a Varian
AMX 400 spectrometer (Varian Inc., Santa Clara, CA, USA) with tetramethylsilane
(TMS) as the internal standard. The GPC analysis was performed by a Waters 2414
0
system (Waters Inc., Milford, MA, USA) with PSS SDV 100000 A 5 µm 8 × 300 mm
column, and the UV-vis detector. The fourier transform infrared (FT-IR) spectra were
recorded on a Nicolet 6700 FTIR spectrometer (Thermo Fisher Scientific Inc.,
Waltham, MA, USA). The X-ray photoelectron microscopy (XPS) tests were executed
by a Thermo Escalab Xi+ (Thermo Fisher Scientific, Waltham, MA, USA) using a
monochromated Al X-ray resource at 1486.6 eV.
Before the electrochemical and spectroelectrochemical experiments, the polymer
solutions (5 mg/mL) were prepared with chloroform as solvent, and then spray-coated
onto the ITO glasses to create the thin films (active area of 0.9 cm × 3.0 cm). The
electrochemical experiments were performed in a one-compartment cell under air
atmosphere by a CHI 760 C Electrochemical Workstation (Shanghai Chenhua
Instrument Co., Shanghai, China), using a ITO glasses as working electrode (WE), a

platinum ring as counter electrode (CE), and a silver wire (0.03 V vs. saturated
calomel electrode, SCE) as pseudo-reference electrode (RE).
The UV-Vis-NIR spectra were recorded on
a
Varian Cary 5000
spectrophotometer (Varian Inc., Santa Clara, CA, USA), and the required applied
potentials were controlled synchronously by the aforementioned electrochemistry
workstation. The kinetic and colorimetric data were also obtained by the Varian Cary
5
000 spectrophotometer. The thicknesses of the polymer films were detected on a
KLA-Tencor D-100 step profiler (KLA-Tencor Inc., Milpitas, CA, USA).
All digital photographs showing polymer colors were taken by a Canon Power
Shot A3000 IS digital camera (Canon Inc., Tokyo, Japan). The thermogravimetric
analysis (TGA) was executed by a Netzsch STA449C TG/DSC simultaneous thermal
analyzer (Netzsch Inc., Bavaria, Germany) in nitrogen atmosphere with 15 ºC/min
heating rate.
3
3
. Results and discussion
.1. FT-IR spectra
FT-IR spectra were measured to further confirm the structures of the
copolymers, and the results were shown in Fig.1. The following peaks were identified
-1
-1
in the spectrum of PBZQ-1: 2923 cm and 2852 cm (aliphatic C-H stretching
-1
-1
-1
-1
vibrations), 1605 cm , 1577 cm , 1509 cm and 1492 cm (skeletal vibrations in
-1
-1
benzene, thiophene, benzotriazole and quinoxaline rings), 1465 cm , 1400 cm and
-
1
-1
1
1
377 cm (C-H in-plane bending vibrations in methylene and methyl), 1336cm ,
-1
-1
-1
289 cm , 1172 cm and 1111 cm (C-S, C-N and N-N stretching vibrations in
-1
−1
thiophene, benzotriazole and quinoxaline rings), 1247 cm and 1030 cm (C-O-C
-1
stretching vibrations), 987 cm (C-H in-plane bending vibrations in benzene,
-1
-1
-1
thiophene, benzotriazole and quinoxaline rings), 831 cm , 801 cm , 721 cm (C-H
out-of-plane bending vibrations in benzene, thiophene benzotriazole and quinoxaline
rings). The FT-IR spectra of PBZQ-2 and PBZQ-3 were basically the same as that of
PBZQ-1 except for the slight difference in the peak intensities, which resulted from
the different proportion of B, Z, Q units in the polymer backbones.

Fig. 1. FT-IR spectra of PBZQ-1, PBZQ-2 and PBZQ-3.
3
.2. XPS characterization
The element composition of the polymer surface was analyzed by the XPS
experiments to substantiate the constituent unit in the polymer backbone, and the
spectra of C, N, O and S elements of PBZQ-1 were shown in Fig. 2. The C (1s) peak
3
at 284.4 eV manifested the presence of sp hybridized carbon atoms which belonged
to the alkyl and alkoxy chains in Z and Q units. The peak at 285.8 eV was derived
from C-S (Z units) and C-O-C (Q units). C=N gave a peak at 287.1 eV for the C (1s).
The N (1s) peak at 398.7 eV was originated from N=C in triazole and pyrazine (Z and
Q units), and that at 401.6 eV was attributed to N-N in triazole (Z units). For the O (1s)
spectrum, the peak at 532.3 eV corresponded to C-O-C, suggesting the existence of Q
units in the polymer chain. The two S (2p) peaks centered at 163.5 eV (2p ) and
3
/2
1
64.6 eV (2p ) were ascribed to thiophene-S, implying the presence of Z units in the
1/2
polymer chain. Besides, the XPS spectra of PBZQ-2 and PBZQ-3 were also recorded
see supporting information Fig. S7 and Fig. S8), which presented the similar peaks as
PBZQ-1 and confirmed the constitution of the polymers.
(

Fig. 2. XPS spectra of PBZQ-1: (a) C1s, (b) N1s, (c) O1s, and (d) S2p.
3
.3. Electrochemical properties
After the polymers were sprayed onto the ITO glasses, the cyclic voltammetry
CV) experiments were carried out in ACN solution containing 0.1 M TBAPF as
(
6
supporting electrolyte at 100 mV/s scan rate to investigate their electrochemical
properties, and the results were illustrated in Fig. 3. Besides, the CV measurements
with 50 mV/s scan rate were also performed, and the redox peaks at 50 mV/s scan rate
were similar with those at 100 mV/s scan rate (see supporting information Fig. S9).
As shown in Fig. 3, all the three polymers exhibited obvious p-type doping process
and the redox peaks were centered at 1.61 V/1.01 V for PBZQ-1, 1.57 V/1.00 V for

PBZQ-2 and 1.45 V/0.75 V for PBZQ-3. The onset oxidation potentials (Eonset) of
PBZQ-1, PBZQ-2 and PBZQ-3 were measured as 1.14 V, 1.13 V and 1.06 V,
respectively. Both the oxidation peak potentials and Eonset displayed the same order as
PBZQ-1 > PBZQ-2 > PBZQ-3, and especially the values of PBZQ-3 were obviously
lower than those of PBZQ-1 and PBZQ-2, which resulted from the different
proportions of the two acceptors (Z/Q) in the polymer backbones. By comparing the
composition of the copolymer, it was easily found that PBZQ-1 possessed the highest
content of Q units, followed by PBZQ-2 and PBZQ-3 in turn. The steric hindrance
effect derived from the two (2-octyldodecyl)oxy groups in Q units would compel the
adjacent rings to twist out of coplanarity, hence reduce the conjugated length and
impede the transfer of donor electrons during the doping process [36]. As for Z units,
the π-bridge flanking thiophene groups effectively strengthened the π-π interaction
and reduced the steric hindrance. On the other aspect, the thiophene groups, also as
the electron rich donor units, would be beneficial to enhance the polymer highest
occupied molecular orbital (HOMO) energy level, thus lowering the potential
required for the formation of radical cation [37]. In summary, polymer with fewer Q
units and more Z units presented the lower oxidation peak potential and Eonset
.
In addition, it could be recognized from Fig. 3 that PBZQ-1 and PBZQ-2 did not
show obvious n-type doping process, while PBZQ-3 possessed a slight n-type
reduction peak at -0.45V which should be attributed to the high content of Z units in
PBZQ-3 backbone as benzotriazole-based polymers usually displayed double-doping
features (p-type and n-type doping) [23]. For most polymers, the presence of
dissolved oxygen and little water in the electrolyte solution will make it difficult for
the inherently unstable n-type doping to exist.

Fig. 3. CV curves of the polymer films: (a) PBZQ-1, (b) PBZQ-2, and (c) PBZQ-3.
.4. Optical properties
3
The UV-Vis absorption spectra of the three polymers in the liquid state
(dissolved in DCM) and in the solid state (spray-coated on ITO electrodes and
de-doped) were examined and analyzed. As shown in Fig. 4, each polymer in the
different phase state revealed the similar spectra with two absorption peaks which
stemmed from the high energy and the low energy π-π* transitions [38]. In the DCM
solution, the maximum absorption wavelengths (λmax) were 398 nm/485 nm for
PBZQ-1, 397 nm/484 nm for PBZQ-2 and 520 nm/561 nm for PBZQ-3, as well as in
the solid film, the λmax were 403 nm/492 nm for PBZQ-1, 403 nm/491 nm for
PBZQ-2 and 521 nm/561 nm for PBZQ-3. It could be observed that the λmax of the

three polymers in the solid state exhibited small bathochromic shifts compared with
those in the solution state which suggested the solid films possessed the improved
π-π* stacking and stronger intra molecular interaction [39,40].
By comparing among the three polymers, both in the solution and in the solid
film, PBZQ-1 and PBZQ-2 displayed essentially the same absorption peaks and had
obvious hypsochromic shifts in contrast with PBZQ-3. In general, quinoxaline
possessed the stronger electron-accepting ability and bandgap-lowering ability than
benzotriazole [41], but in the main chains of PBZQ-1 and PBZQ-2, the more content
Q units with the alkoxybenzene substitutions caused the larger steric hindrance
between (2-octyldodecyl)oxy and adjacent units which offset the powerful
electron-withdrawing effect of quinoxaline [36]. In addition, as mentioned earlier, the
larger proportion of Z units in PBZQ-3 backbone carried more thiophene groups
arranged on both sides of benzotriazole, which could availably reduce the steric
hindrance between Z and benzene/Q units, as well as improve the planarity and D-A
conjugation of the polymer chains [32]. Thus, the UV-Vis absorption spectra of
PBZQ-3 showed bathochromic shifts compared with those of PBZQ-1 and PBZQ-2.
Besides, corresponding to the absorption bands in the visible region, PBZQ-1 and
PBZQ-2 showed orange-red color both in liquid and solid phases, while PBZQ-3
displayed strawberry red in the solution and tomato red in the solid film.
Fig. 4. UV-Vis absorption spectra of PBZQ-1, PBZQ-2 and PBZQ-3: (a) in solution and (b) in

solid films.
3
.5. Spectroelectrochemical properties
Spectroelectrochemistry is a useful method for studying the optical changes in
the absorption spectra and gaining the information about the electronic structures of
conjugated polymers as a function of the applied potential difference. After the
polymers were spray-coated onto the ITO glasses, the spectroelectrochemical
experiments were performed in a 0.1 M TBAPF /ACN solution with applied
6
potentials ranging from 0 V to 1.5 V. Fig. 5 revealed the absorption spectra and the
corresponding colors of the three polymer films in the neutral and doped states.
As shown in Fig. 5, PBZQ-1, PBZQ-2 and PBZQ-3 exhibited two absorption
peaks in the neutral states due to the π-π* transitions in the polymer main chains,
which accorded with the feature of D-A type conjugated polymers [42]. And the
absorption peaks were regarded as the transitions from benzene and thiophene based
valence bands to their antibonding counterparts (high-energy transition) and to the
substituent localized narrow conduction bands (low-energy transition) [43]. The peaks
were located at 403 nm/492 nm for PBZQ-1, 403 nm/491 nm for PBZQ-2 and 521
nm/561 nm for PBZQ-3, which made their color manifest orange-red for PBZQ-1 and
PBZQ-2, and red for PBZQ-3 in the neutral states (0 V).
As the potentials continued to increase irregularly, the absorption spectra of the
three polymers demonstrated the similar changes. It could be observed from Fig. 5a
that the two absorption peaks of PBZQ-1 in the neutral state declined gradually,
especially the absorption band at 492 nm vanished when the potential reached 1.25 V.
Meanwhile, the other two new absorption bands centered at 695 nm and 1500 nm
emerged and heightened, which corresponded to the formations of polarons and
bipolarons respectively. The absorption peak located at 695nm reached its maximum
at 1.35 V, and then decreased gradually as the oxidation continued. The other peak in
the near infrared region around at 1500 nm enhanced continually with the augment of
applied potential until PBZQ-1 was completely oxidized at 1.5 V. The decline of
polaron band from 1.35 V to 1.5 V could be explained that the successively improving

bipolaron band (around 1500 nm) restrained the polaronic population which was also
called the repression of interband transition [44]. Some other polymers also
demonstrate the similar phenomenon that is they would lead to spin-1/2 polarons at
light doping and blossom into spinless bipolarons at heavy doping [29,45]. Moreover,
the gradually weakened π-π* transition bands and the significantly enhanced polaron
band were both situated in the visible region, so PBZQ-1 exhibited several different
colors in the oxidation process as observed in Fig. 5a. Upon stepwise oxidation, the
neutral orange-red (0 V) PBZQ-1 changed to brown-yellow at 1.14 V, cyan at 1.35 V,
and green at 1.5 V in turn. It was clear that PBZQ-1 showed multichromism and the
color changes covered red, yellow, cyan and green, in which red and green belonged
to the RGB colors. As previously mentioned, multichromic polymers have important
application values in smart window and optical display technology [5,6,46], and
particularly, the multicolor polymers with green color and good solubility are rarely
reported in the literatures [7,8,47]. Therefore, the design and synthesis of PBZQ-1 had
great significance for the development of electrochromic polymers.
As the oxidation degree of PBZQ-2 raised (Fig. 5b), the two π-π* transition peaks
weakened gradually and the peak at 491 nm disappeared absolutely at 1.25 V. At the
same time, the polaron band at 690 nm enhanced to maximum at 1.35 V, and the
bipolaron band at about 1470 nm steadily increased until 1.50 V. The color changes of
PBZQ-2 were from orange-red (0 V), to yellow (1.15 V), and to light grass green (1.5
V) which illustrated in Fig. 5b.
During the oxidation process of PBZQ-3 as Fig. 5c suggested, the peaks at 521
nm and 561 nm attenuated gradually until both of them faded away at 1.30 V. While
the polaron band at 702 nm improved to the maximum at 1.40 V and then decreased,
and the bipolaron band at 1318 nm maintained the trend of growth with the increase
of the applied potentials. It was noteworthy that when PBZQ-3 was oxidized to 1.13
V, its absorption band uniformly covered the entire visible region between 400 nm
and 760 nm, which indicated that PBZQ-3 displayed black color. At the potential of
1
.5 V for complete oxidation, the only absorption peak of 702 nm in the visible region
made PBZQ-3 show grey-blue color. As shown in Fig. 5c, PBZQ-3 underwent red

(0V), black (1.13V) and grey-blue (1.5 V) with the increase of applied potentials as
expected. A polymer owning black color in any state is very important in the
application of smart window [21]. In consideration of the difficulty in gaining black
color, PBZQ-3 offered a new kind of structure design for black electrochromic
polymer and deserved further research and development [9,48]. From the above
analysis of PBZQ-1, PBZQ-2 and PBZQ-3, it was concluded that adjusting the
proportion of the two acceptors was a useful method to tailor the polymer color.

Fig. 5. Spectroelectrochemical spectra of the polymer films and their color changes at different
applied potentials: (a) PBZQ-1, (b) PBZQ-2, and (c)PBZQ-3.
In addition, some electrochemical and optical parameters of the polymers such as
the λmax in the liquid state and in the solid state, the onset wavelengths of the optical
absorption spectra in the neutral states (λonset), the Eonset, the optical band gaps (Eg,op),
and the HOMO/LUMO values were all illustrated in Table 1. The λonset of PBZQ-1,
PBZQ-2 and PBZQ-3 films were 588 nm, 593 nm and 624 nm, and the Eg,op of them
were calculated as 2.11 eV, 2.09 eV and 1.99 eV, respectively. The order of Eg,op was
PBZQ-1 > PBZQ-2 > PBZQ-3, which was due to the gradually incremental molar
ratio of Z units to Q units in the polymer backbones. As previously discussed, the two
thiophene groups connected on benzotriazole improved the electron-accepting ability
of Z units, but the long-chain alkoxy substitutions adjacent to quinoxaline diminished
the electron-accepting ability of Q units [32,36,41]. Consequently, the Eg,op of
polymers decreased gradually with the increase of Z unit content, which could also be
reflected by the HOMO and LUMO energy levels. PBZQ-3 had the largest molar ratio
of Z to Q (4:1) in contrast with PBZQ-1 (1:2) and PBZQ-2 (1:1), which not only
made the polymer have the most donor units (thiophene groups were also regarded as
donors) and hence show the highest HOMO energy level, but also made the polymer
have the most Z acceptor units with stronger electron-deficient ability and thus

display the lowest LUMO energy level [37] Besides, the other three copolymers
.
based on benzotriazole and quinoxaline units reported in the literatures were
compared with PBZQ-1, PBZQ-2 and PBZQ-3, and their molecular structures were
shown in Fig. 6 [49,50]. As list in Table 1, CoP1, CoP2 and P2 displayed two colors
in the redox process, though they had the relatively lower Eg,op. PBZQ-1, PBZQ-2 and
PBZQ-3 exhibited impressive multicolor, and the more alkyl side chains caused
higher Eg,op, but also increased the solubility of polymers.
Table 1 Electrochemical and optical parameters of PBZQ-1, PBZQ-2, PBZQ-3, and
several other copolymers with similar structures.
λmax
λmax
(film)
nm
λonset
(film)
nm
a
b
c
Eonset Eg,op
eV
HOMO
LUMO
Polymer (solution)
color
V
eV
eV
nm
PBZQ-1 398/485
403/492
588
593
624
-
1.14 2.11
1.13 2.09
1.06 1.99
-5.54
-5.53
-5.46
-5.22
-5.41
-5.67
-3.43
-3.44
-3.47
-3.55
-3.65
PBZQ-2 397/484
403/491
PBZQ-3 520/561
521/561
d
CoP1
CoP2
-
-
-
-
-
-
-
1.72
1.48
1.62
d
-
e
P2
-
-
-3.55
a
b
c
d
Eg,op = 1241/λonset; E
= -e(Eonset + 4.4); ELUMO = EHOMO + Eg,op; Data were taken from
HOMO
e
reference [49]; Data were taken from reference [50].

S S
C H
C H
12 25
1
2
25
N
N
N
N
N
N
N
N
N
N
S
m
S
n p
S
m
S
n p
CoP1
CoP2
C H
1
2
25
N
N
N
S
S
n
N
N
S
P2
Fig. 6. Molecular structures of the polymers reported in the literatures.
3
.6. Electrochromic switching properties
In order to study the kinetic properties of polymers, electrochromic switching
tests were carried out using the double potential step chronoamperometry technique.
The polymers were sprayed onto the ITO glasses beforehand, and then the
experiments were performed in ACN solution containing 0.1 M TBAPF electrolyte
6
under the repeated potential stepping between reduced and oxidized states with a
residence time of 4 s. The average thicknesses of PBZQ-1, PBZQ-2 and PBZQ-3
films were estimated as 348 nm, 381 nm and 378 nm, respectively. The kinetic curves
and the calculated kinetic parameters at the definite wavelengths were manifested in
Fig. 7-9, Fig. S10-S12, Fig. S13-S15 (in supporting information) and Table 2
respectively.
Optical contrast (∆T%) refers to the percentage transmittance change between
oxidized and neutral states, and the ∆T% values of PBZQ-1 at 490 nm, 690 nm and
1
500 nm were 28.4% (Fig. 7a), 27.2% (Fig. 8a) and 38.9% (Fig. 9a) respectively.
While the ∆T% values of PBZQ-2 at 490 nm, 690 nm and 1500 nm were 21.1%,
6.8% and 41.5% respectively (see supporting information Fig. S10-S12), and those
3
of PBZQ-3 at 520 nm, 720 nm and 1320 nm were 20.6%, 22.8% and 33.7%
respectively (see supporting information Fig. S13-S15). From the data it was found

that all the three polymers exhibited higher ∆T% in the near-infrared region compared
with in the visible region, which was in agreement with many D-A type
electrochromic polymers [8]. In addition, by contrasting the three polymers, PBZQ-1
and PBZQ-2, which possessed more
Q
units connected with more
(2-octyldodecyl)oxy groups, revealed relatively higher ∆T% than PBZQ-3 at all
corresponding wavelengths. The long-chain alkoxy groups could expand the distance
of chain segments and improve the charge/discharge capacity in the redox process,
thus raising the optical transmittance to a certain degree [51].
Switching time (t95%) is defined as the time required for reaching 95% of the
maximum transmittance change between bleached and colored states, and this value
can represent the switching speed of the polymer. As listed in Table 2, the t95% of
PBZQ-1 at 490 nm, 690 nm and 1500nm were 3.39 s, 1.70 s and 2.46 s, the t95% of
PBZQ-2 at 490 nm, 690 nm and 1500nm were 3.61 s, 2.04 s and 2.12 s, and the t95%
of PBZQ-3 at 520 nm, 720 nm and 1320nm were 3.00 s, 0.91 s and 1.81 s. Except at
the wavelengths of high energy absorption peaks, the t95% values of the three polymers
at other wavelengths were less than 3 s. And in contrast to PBZQ-3, PBZQ-1 and
PBZQ-2 showed the longer response time at the corresponding wavelengths, which
also resulted from the more Q units with more long-chain alkoxy groups in their
backbones.
Coloration efficiency (CE) denotes the ratio between optical density change
(∆OD) and charge consumed per unit electrode area (∆Q), which can be used to
evaluate the polymer power efficiency during the electrochromic switching process.
The formula was shown below [52]:
∆
OD
CE =
∆
Q



Tb 
∆OD = log


Tc
Q
∆
Q=
A
Wherein, T and T are the transmittance in the bleaching state (neutral state) and
b
c

in the colored state (oxidation state). Q is the total charge acquired by integrating of
the multi-potential step curve. A is the effective area of ITO glass. As shown in Table
2
, all the polymers demonstrated the relatively larger CE values, and in particular,
2
PBZQ-1 deserved focused attention with a CE value as high as 498.26 cm /C at 490
nm. In conclusion, the three polymers displayed satisfactory kinetic performances
which also could be adjusted through optimizing Z-to-Q proportion.
Fig. 7. (a) Transmittance-time curve of PBZQ-1 at 490 nm. (b) The coloring time (t ) and the
c
bleaching time (t ) of PBZQ-1 at 490 nm. (c) Current-switching time curve of PBZQ-1 between 0
b
V and 1.5 V at 490 nm. (d) Charge density consumed during the dedoping process at 490 nm.

Fig. 8. (a) Transmittance-time curve of PBZQ-1 at 690 nm. (b) The coloring time (t ) and the
c
bleaching time (t ) of PBZQ-1 at 690 nm. (c) Current-switching time curve of PBZQ-1 between 0
b
V and 1.35 V at 690 nm. (d) Charge density consumed during the dedoping process at 690 nm.

Fig. 9. (a) Transmittance-time curve of PBZQ-1 at 1500 nm. (b) The coloring time (t ) and the
c
bleaching time (t ) of PBZQ-1 at 1500 nm. (c) Current-switching time curve of PBZQ-1 between
b
0
V and 1.5 V at 1500 nm. (d) Charge density consumed during the dedoping process at 1500 nm.
Table 2 The ∆T%, t95% and CE values of the polymers at different wavelengths.
λ
∆
T%
t95%
s
CE
Polymer
2
nm
%
cm /C
4
90
28.4
27.2
38.9
21.1
3.39
1.70
2.46
3.61
498.26
237.50
316.8
PBZQ-1
PBZQ-2
690
1
500
490
291.50

6
90
500
20
720
320
36.8
41.5
20.6
22.8
33.7
2.04
2.12
3.00
0.91
1.81
206.62
359.37
262.40
155.08
122.83
1
5
PBZQ-3
1
Furthermore, for studying the effect of different residence time on the
performance of electrochromic switching, the T% of the polymers were detected at
the residence times of 10 s, 4 s, 2s and 1 s respectively. As shown in Fig. 10, when the
retention time decreased from 10s to 1s, the T% of PBZQ-1 declined by 17.5% at
90 nm, 16.6% at 690 nm and 18.3% at 1500 nm. The T% of PBZQ-2 reduced by
0.1% at 490 nm, 33.6 % at 690 nm and 35.8% at 1500 nm, and the T% of PBZQ-3
∆
∆
4
3
∆
∆
declined by 11.5% at 520 nm, 21.1% at 720 nm and 28.2% at 1320 nm (see
supporting information Fig. S16). It was easy to find that the T% of the three
∆
polymers all expressed obvious attenuation with the decreasing of residence time
which was due to the fact that the polymer could not be fully doped (oxidized) or
dedoped (reduced) when the retention time was short.

Fig. 10. Electrochromic switching of PBZQ-1 with interval of 10 s, 4 s, 2 s and 1 s: (a) 490 nm, (b)
90 nm, and (c) 1500 nm.
6
3
.7. Colorimetry
Based on the spectroelectrochemical experiments, it was observed that the color
changes of the three polymer films were obviously different during the
oxidation-reduction process. If we want to investigate the color changes in depth,
however, the first one to be used was CIE 1976 L*a*b* Color Space to detect the
variations of lightness, chroma and hue of the polymers, in which L* denotes the
luminance of color from black to white, a* denotes red-green balance, and b* denotes
yellow-blue balance [24,48]. The L*, a*, and b* values of the polymer films with
different thicknesses (represented by the maximum optical absorption values) were
determined by spectrophotometer at the scanning potential from 0 to 1.6 V, and the
data were depicted in Fig. 11 and Fig. S17 (in supporting information).
As shown in Fig. 11a, regardless of the thickness of PBZQ-1, the L* values had
the same change trend with the raise of the applied potentials. Starting from 0 V, the
L* values remained unchanged until the potential exceeded the Eonset, and then began
to augment by degrees. On the other aspect, at the same applied potential, with the
thickness of PBZQ-1 increased, that is, the maximum optical absorption value
increased from 0.34a.u to 0.55a.u., and to 0.77a.u., its L* values decreased gradually.
The thicker the polymer film was, the lower intensity the light passed through, which

caused the weakening of the luminance and the decline of the L* value. Besides, it
was found that PBZQ-2 (see supporting information Fig. S17a) and PBZQ-3 (Fig. 11c)
exhibited the similar tendency on the L* values as PBZQ-1.
In the a*-b* graphs of PBZQ-1, as demonstrated in Fig. 11b, the a* values
changed from positive to negative with the color changing from red to green with the
steady increasing of applied potentials, and meanwhile, the b* values always changed
in positive range with the yellow component. The a*-b* curves gradually declined
and rebounded to a certain extent from the first quadrant to the second quadrant. This
tendency was consistent with the conclusion of spectroelectrochemical experiments
with the color transformations from orange-red to brown-yellow, to cyan, and to green
in the process of polymer oxidation. In particular, it was observed from the
spectroelectrochemical spectra (Fig. 5a) that the peak intensities of PBZQ-1 in the red
and blue regions (at 403 nm and 695 nm) at 1.35 V were both higher than those at
1
.50 V, which corresponded to the inflection points in the a*-b* curves (Fig. 11b), that
is, PBZQ-1 had more green color component when oxidized to 1.35 V. For PBZQ-2
see supporting information Fig. S17b), the change of a* and b* values was similar
(
with PBZQ-1, and the overall trend of a*-b* curves also changed from the first
quadrant to the second quadrant with the color changes from orange-red, to yellow,
and to light grass green as the applied potential increasing. When it came to PBZQ-3
(Fig. 11d), the a* values changed from positive to negative with the applied potentials
raising, but the b* values varied in negative range with the blue component. The
overall trend of a*-b* changed from the fourth quadrant to the third quadrant
accompanied by the color changes from red to black, and to grey-blue. Significantly,
the a* and b* values were close to 0 at the intermediate potentials, especially a set of
a*/b* values was measured as 0.52/-3.32 at the maximum optical absorption value
0
.87 a.u., the closest to an achromic black accompanying with the corresponding low
L* value.