Design of twisted conjugated molecular systems towards stable multi-colored electrochromic polymers

Article abstract of DOI:10.1016/j.dyepig.2020.108648

Promising advancement of conjugated polymers in electrochromic devices require to design high-performance electrochromic polymers with rich color conversion and long-term stability under cyclic electrical loads. Here we report a new strategy in developing multi-colored electrochromic polymers with good stability via twisted conjugated molecular engineering. A series of twisted hybrid precursors are synthesized by coupling ortho-alkylenedioxybenzenes with EDOT units, and their corresponding polymers are facilely electrosynthesized at relatively low polymerization potentials. The structure-property relationships of such ortho-alkylenedioxybenzene-EDOT hybrid precursors and polymers are systematically elucidated via DFT calculations, spectral, morphological, electrochemical and spectroelectrochemical analysis, etc. We demonstrate that the dihedral angle between ortho-alkylenedioxybenzenes and EDOT moieties can substantially affect the electrochemical and electrochromic properties of polymers. As the dihedral angle and electron cloud density increases, these hybrid polymers display distinct multiple color switching nature and good overall performance including high coloration efficiency (>200 cm² C^?1), decent optical contrast (>45percent), fast switching (<1 s), and excellent switching stability (96percent of optical contrast after 3500 cycling) under cyclic electrical loads. With these findings, this work will provide novel insights for rational design of stable and highly efficient multi-colored electrochromic polymers.

Full text of DOI:10.1016/j.dyepig.2020.108648

Journal Pre-proof
Design of twisted conjugated molecular systems towards stable multi-colored
electrochromic polymers
Hua Gu, Yu Xue, Faqi Hu, Nannan Jian, Kaiwen Lin, Tao Wu, Ximei Liu, Jingkun Xu,
Baoyang Lu
PII:
S0143-7208(20)31345-0
DOI:
https://doi.org/10.1016/j.dyepig.2020.108648
DYPI 108648
Reference:
To appear in: Dyes and Pigments
Received Date: 7 February 2020
Revised Date: 12 June 2020
Accepted Date: 12 June 2020
Please cite this article as: Gu H, Xue Y, Hu F, Jian N, Lin K, Wu T, Liu X, Xu J, Lu B, Design of twisted
conjugated molecular systems towards stable multi-colored electrochromic polymers, Dyes and
Pigments, https://doi.org/10.1016/j.dyepig.2020.108648.
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition
of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of
record. This version will undergo additional copyediting, typesetting and review before it is published
in its final form, but we are providing this version to give early visibility of the article. Please note that,
during the production process, errors may be discovered which could affect the content, and all legal
disclaimers that apply to the journal pertain.
©
2020 Elsevier Ltd. All rights reserved.

Graphical Abstract
ò Four twisted o-alkylenedioxybenzene-EDOT precursors were synthesized and
electropolymerized.
ò Twisted polymer structures contribute to the multi-color switching nature.
ò o-Alkylenedioxybenzene-EDOT hybrid polymers show excellent electrochromic
performances and stability.

Design of twisted conjugated molecular systems towards stable
multi-colored electrochromic polymers
1
1,2
1,2
1
3
1
Hua Gu †, Yu Xue †, Faqi Hu †, Nannan Jian , Kaiwen Lin , Tao Wu , Ximei
1
,2
1,2
1,2,4
Liu *, Jingkun Xu *, and Baoyang Lu
*
1
Flexible Electronics Innovation Institute, Jiangxi Science & Technology Normal
University, Nanchang 330013, Jiangxi, P.R. China
2
School of Pharmacy, Jiangxi Science & Technology Normal University, Nanchang
3
30013, Jiangxi, P.R. China
3
Department of Materials and Food, University of Electronic Science and Technology
of China Zhongshan Institute, Zhongshan, 528402, P.R. China
4
Soft Active Materials Laboratory, Department of Mechanical Engineering,
Massachusetts Institute of Technology, Cambridge, MA 02139, USA
*
: Corresponding authors. Tel: +86-791-88537967, Fax: +86-791-83823320.
Email: lxm5812@163.com; xujingkun@jxstnu.edu.cn; luby@mit.edu.
: These authors contribute equally to this work.
†
ABSTRACT: Promising advancement of conjugated polymers in electrochromic
devices require to design high-performance electrochromic polymers with rich color
conversion and long-term stability under cyclic electrical loads. Here we report a new
strategy in developing multi-colored electrochromic polymers with good stability via
twisted conjugated molecular engineering. A series of twisted hybrid precursors are
synthesized by coupling ortho-alkylenedioxybenzenes with EDOT units, and their
corresponding polymers are facilely electrosynthesized at relatively low
polymerization potentials. The structure-property relationships of such
ortho-alkylenedioxybenzene-EDOT hybrid precursors and polymers are
systematically elucidated via DFT calculations, spectral, morphological,

electrochemical and spectroelectrochemical analysis, etc. We demonstrate that the
dihedral angle between ortho-alkylenedioxybenzenes and EDOT moieties can
substantially affect the electrochemical and electrochromic properties of polymers. As
the dihedral angle and electron cloud density increases, these hybrid polymers display
distinct multiple color switching nature and good overall performance including high
2
–1
colouration efficiency (> 200 cm C ), decent optical contrast (> 45%), fast
switching (< 1 s), and excellent switching stability (96% of optical contrast after 3500
cycling) under cyclic electrical loads. With these findings, this work will provide
novel insights for rational design of stable and highly efficient multi-colored
electrochromic polymers.
Keywords: Conjugated polymers, PEDOT, electrochemical polymerization, twisted
molecular engineering, multicolor electrochromism
1
. Introduction
The electrochromic polymers have extensively been studied and widely explored
in smart windows, optical memory devices, sensors and flexible display technologies,
owing to their low-power consumption, intrinsic flexibility and facile color tunability
by molecular design.[1−5] However, the construction of high-performance
electrochromic devices (ECDs) require the crucial electrochromic materials
containing with high contrast ratios and coloration efficiency, short switching time
and low operating voltages, as well as rich color conversion and remarkable stability
under cyclic electrical loads.[5−8] In contrast to inorganics and small-molecule
organics, conjugated polymers possess significant advantages in low-cost processing

and good overall elcctrochromic performance. Moreover, the chemical structure of
electrochromic polymers can be purposeful modified in backbone or side chains,
which provides the opportunity to achieve unique performance, such as multi-colored
switching.[4,7−13] Despite recent progress in multi-colored electrochromic polymers
(e.g. polyaniline (PANI), ploy(triphenylamine) (PTPA), polycarbazole, and
quinones-based polymers), most of above electrochromic polymers are the lack of
long-term stability under cyclic electrical loads, which significantly limit their
practical applications.[14−16] Therefore, rational design of suitable polymer
structures achieving excellent electrochromic performance and stable multi-colored
conversion remains a challenging task up to date.
Considerable studies have proved that structural modification on the geometry of
the conjugated backbone by the design of precursor structure provides a feasible route
to adjust the electronic and optical properties.[17] For decades, a special kind of
materials with non-planar or helical conformation (e.g. acene-based,
p-phenylene-based molecules) has long been the focus of chemists, because of their
unique spatial structure and interesting physicochemical and optical properties.[18−24]
However, their typical helical structures generally lead to weak π–electron
delocalization, which could reduce the electrical performances of conjugated
polymers.[25−29]
Recently,
our
group
reported
a
series
of
o–
alkylenedioxybenzene-based polymers by electropolymerization, and the as-obtained
polymers exhibited preferable electrochemical properties and good electrochemical
stability.[30−34] However, the elcctrochromic properties of above polymers are

unsatisfactory, such as lower optical contrast and monotonous color switching. For a
long time, conducting poly(3,4-ethylenedioxythiophene) (PEDOT) with
optoelectronic advantages and outstanding stability was considered as a favoured
platform for developing high-performance electrochromic polymers.[35−45] More
recently, we synthesized a promising electrochromic polymer based on
1
,2-methylenedioxybenzene (MDOB) and 3,4-ethylenedioxythiophene (EDOT),
namely poly(4-(benzo[d][1,3]dioxol-5-yl)-2,3-dihydrothieno[3,4-b][1,4]dioxine), and
such hybrid polymer with twisted structure presents significant improved
electrochemical and electrochromic performance.[46] Since a systematic and deeper
investigation about the influence of ortho-alkylenedioxybenzene-EDOT based
conjugated polymers with special molecular structures on the electrochromic
performance may help us to design high-performance electrochromic polymers.
In order to reveal such significant effects generated by special conformation on
the
physicochemical
performance,
herein
we
synthesize
four
ortho-alkylenedioxybenzenes-EDOT hybrid polymers with twisted conjugated
structure via electrochemical polymerization of their precursors. The
structure-property relationships of such precursors and polymers are systematically
explored by the DFT calculations, spectral characterization, surface morphology, and
electrochemistry
and
electrochromic
tests.
We
found
that
these
ortho-alkylenedioxybenzene-EDOT hybrid polymers exhibit multiple color switching
nature with preferable electrochromic performance and excellent switching stability.
2
. Experimental

2
.1 Materials
,2-Methylenedioxybenzene
,2-ethylenedioxybenzene (EDOB, 98%; Energy Chemical), N-bromosuccinimide
1
(MDOB,
98%;
Energy
Chemical)
1
(
NBS, 98%; J&K Chemical Ltd), bromine (Br , 98%; J&K Chemical Ltd),
2
3
,4-ethylenedioxythiophene
(EDOT,
98%;
Sigma-Aldrich),
tetrakis(triphenyphosphine)palladium (0) (Pd(PPh ) , 99%; Energy Chemical),
3
4
–
1
n-butyllithium (n–BuLi, 1.6 mol L in hexanes; J&K Chemical Ltd) and
chlorotributyltin (SnBu Cl, AR; Energy Chemical) were purchased commercially and
3
used as received. Acetonitrile (MeCN, AR; J&K Chemical Ltd), toluene (AR; Xilong
Chemical), tetrahydrofuran (THF, AR; Xilong Chemical), N,N-dimethylformamide
(
DMF, AR; Xilong Chemical) and dichloromethane (CH Cl , AR; Xilong Chemical)
2 2
were refined before use. Anhydrous magnesium sulfate (MgSO , AR; Shanghai Vita
4
Chemical), tetrabutylammonium hexafluorophosphate (Bu NPF , 98%; Energy
4
6
Chemical), propylene carbonateas (PC, 99.5%; Extra Dry, with molecular sieves,
Energy Chemical), together with other chemicals and reagents were used without
further treatment.
2
.2 Synthesis of precursors
The synthesis of tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)stannane, and
4
-(benzo[d][1,3]dioxol-5-yl)-2,3-dihydrothieno[3,4-b][1,4]dioxine
(M1)
was
successfully performed following the methods as described in our previous work.[46]
2
.2.1. 4,5-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)benzo[d][1,3]dioxole (M2)
NBS (2.90 g, 16.37 mmol) and MDOB (2.29 g, 8.18 mmol) were added into a

two-necked reaction flask containing CHCl /AcOH (v/v = 1:1) solution and refluxed
3
for 6 h. The crude product was filtered off and washed with CH Cl , NaHCO (aq),
2
2
3
and H O. Then, 4,5-dibromobenzo[d][1,3]dioxole (1.39 g, 4.95 mmol) and Pd(PPh3)4
2
(0.23 g, 2% mmol) were added into the anhydrous toluene (60 mL) containing
tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)stannane (6.35 g, 14.84 mmol)
under the N atmosphere and refluxed for 24 h. After that, the resulting mixture was
2
washed with NaCl (aq), extracted by CH Cl and then dried with anhydrous MgSO .
2
2
4
After removing solvent by rotary evaporation, the crude products were generated and
purified by column chromatography to get a white solid powder (yield: 80%). Tm:
−
1
2
6
2
22-227 °C, FT-IR (KBr cm ): 2976, 2889, 1386, 1362, 1232, 1154, 1053, 874, 759,
1
56. H NMR (400 MHz, CDCl , ppm) δ 7.02 (s, J = 4.4 Hz, 2H), 6.28 (s, J = 4.3 Hz,
3
1
3
H), 5.98 (s, J = 4.3 Hz, 2H), 4.12–4.09 (m, 4H), 4.04–4.00 (m, 4H). C NMR (400
MHz, CDCl , ppm) δ 147.48, 141.12, 138.24, 125.77, 111.25, 101.69, 98.72, 64.72,
3
6
4.63. Found: C, 56.93; H, 3.68; S, 15.71%; molecular formula C H O S requires
19 14 6 2
C, 56.71; H, 3.51; S, 15.93.
2
.2.2. 4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)benzo[d][1,3]dioxole (M3)
The refined DMF solution (20 mL) containing NBS (2.90 g, 16.37 mmol) was
dropwise added into the mixture of EDOB (2.28 g, 16.37 mmol) and DMF (20 mL) in
two-necked reaction flask. The mixture was stirred at 0 °C for 24 h under N2
atmosphere and the resulting crude product was purified by extraction and desiccation
to give 4-bromo-2,3-dihydrobenzo[b][1,4]dioxine (EDOB-Br) as a light-yellow
colloidal liquid. To a mixture solution of EDOB-Br (1.06 g, 4.95 mmol) and

tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)stannane (6.35 g, 14.84 mmol) in
anhydrous toluene (60 mL) and Pd(PPh ) (0.23 g, 2% mmol) used as catalyst was
3
4
also added. The mixture was refluxed and stirred magnetically for 24 h under the N2
atmosphere. After washed and dried by NaCl (aq) and MgSO , respectively, the crude
4
products were generated and purified by column chromatography to get a pale yellow
−
1
solid powder (yield: 76%). T : 186-189 °C, FT-IR (KBr cm ): 2986, 2986, 1567,
m
1
1
052, 1280, 1161, 885, 804, 742, 704, 676. H NMR (400 MHz, CDCl , ppm) δ 7.29
3
(d, J = 2.1 Hz, 1H), 7.17 (d, J = 2.1 Hz, 1H), 6.86 (d, J = 8.5 Hz, 1H), 6.24 (s, 1H),
1
3
4
.29–4.22 (m, 8H). C NMR (400 MHz, CDCl , ppm) δ 148.02, 146.52, 142.36,
3
1
37.59, 127.50, 119.97, 108.69, 107.33, 101.22, 96.88, 64.93, 64.67. Found: C, 61.13;
H, 4.65; S, 15.71%; molecular formula C H O S requires C, 60.86; H, 4.38; S,
1
4
12
4
1
5.53%.
2
.2.3.
4,5-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2,3-dihydrobenzo[b][1,4]
di-oxine (M4)
Br (2.60 g, 16.2 mmol) was dropped into the mixture of EDOB (1.01 g, 7.4
2
mmol) and CH Cl (20 mL), and reacted at room temperature for 12 h. The solvent
2
2
was removed by rotary evaporation and poured into water to recrystallize to obtain
,5-dibromo-2,3-dihydrobenzo[b][1,4]dioxine (Br-EDOB-Br) (1.45 g, 4.95 mmol) as
an white solid, which was followed by reaction with
4
a
tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)stannane (6.35 g, 14.84 mmol) and
Pd(PPh ) (0.23 g, 2% mmol) at 100 °C for 24 h. After the solution is cooled, and then
3
4
extracted with CH Cl and dried with anhydrous MgSO . After removing the solvent
2
2
4

by rotary evaporation, the crude products were generated and purified by column
chromatography to get a pale yellow solid powder (yield: 85%). T : 211-215 °C,
m
−
1
FT-IR (KBr cm ): 2971, 2871, 1519, 1427, 1060, 1245, 1213, 1167, 808, 714, 666.
1
H NMR (400 MHz, CDCl ) δ 7.07 (s, J = 4.6 Hz, 2H), 6.27 (s, J = 4.4 Hz, 2H), 4.28
3
1
3
(
s, 4H), 4.12–4.10 (m, 4H), 4.03–4.00 (m, 4H). C NMR (400 MHz, CDCl , ppm) δ
3
1
43.28, 141.14, 138.21, 125.32, 119.91, 116.37, 98.56, 64.70, 64.65, 64.61. Found: C,
5
7.96; H, 3.98; S, 15.27%; molecular formula C H O S requires C, 57.68; H, 3.87;
2
0
16
6 2
S, 15.40%.
2
.3 Electrochemistry
The electrochemical experiments of precursors/polymers were carried out by
using a three-electrode-in-one-cell system on a Model 263 potentiostat/galvanostat
EG&G Princeton Applied Research, Oakridge, TN) controlled by a computer. An
(
Ag/AgCl wire, which was prepared in 6 M HCl solution and N atmosphere, was
2
directly employed as the reference electrode. The working and counter electrode (i.e.
Pt wires, Φ = 1 mm) was polished by using 1500 mesh abrasive paper and washed
with acetone. The oligomers were electropolymerized in CH Cl -THF (v/v = 1:1)
2
2
solution, and the electrochemistry and spectroelectrochemistry of the resulting
polymers were tested in blank propylene carbonateas (PC) containing 0.1 M Bu NPF .
4
6
In order to obtain sufficient polymers, a Pt sheet with an effective area of 2.5 cm × 2.5
cm was used as the working electrode for electropolymerization. The resulting
polymers were washed repeatedly with refined MeCN to remove the electrolyte,
precursors, and oligomers for next characterizations.

2
.4 Kinetic electrochromic studies of polymer films
A three-electrode-in-one-compartment system consisting of ITO conductive
glass (working electrode), Cu wire (counter electrode), and Ag/AgCl (reference
electrode) was applied for electrochromic tests of the polymer films. The films were
deposited on ITO glass with an area of 0.8 cm × 2.5 cm, and their UV-vis spectra and
optical features were recorded in a quartz cell by combining a SPECORD 200 PLUS
UV-vis spectrophotometer with a V3 potentiostat controlled by a computer.
Optical band gap (Eg,opt) is an important parameter to evaluate the optical
properties of conducting polymers. Generally, conducting polymer with lower optical
band gap contributes to larger wavelength range absorption and smaller power
consumption. The optical band gap (Eg,opt) is calculated according to the onset
absorption of π–π* transition (λonset) with the following formula[4]:
Eg,opt = 1240/λonset
(1)
The optical density (ΔOD) at the specific wavelength (λmax) can be determined
by %T value of electrochemically oxidized and reduced polymer film, using the
formula[4]:
ΔOD = log(T /T )
(2)
ox red
The coloration efficiency (CE) is defined as the relation between the
injected/ejected charge as a function of electrode area (Q ) and the change in optical
d
density (ΔOD) at the specific dominant wavelength (λmax) as illustrated by equation
(3)[4]:
CE =ΔOD/Qd
(3)

2
.5 Instruments and characterization
By using a Bruker AV 400 NMR spectrometer, H and C NMR spectra of all
1
13
precursors dissolving in CDCl were recognized with tetramethylsilane (TMS) as the
3
internal standard. The melting point (T ) of monomers was tested by using American
m
Optimelt MPA 100 full automatic melting point meter. Elemental analysis (C, H, O)
was carried out using the Elemental analyzer Vario EL III (Elementar). Infrared
spectra were recorded using a Bruker Vertex 70 Fourier transform infrared (FT-IR)
spectrometer with samples in KBr pellets. The optimized precursor geometry and
charge-density isosurfaces for the lowest unoccupied molecular orbital (LUMO) and
the highest occupied molecular orbit (HOMO) levels were investigated by density
functional theory (DFT) calculations at the B3LYP/6-31G(d) level. The
micromorphological images of all the polymer film were explored on a Tescan JSM–
6
701F scanning electron microscope (SEM).
3
. Results and discussion
3
.1 Precursor synthesis
Overall, these precursors with twisted structure are synthesized via the Stille
coupling between brominated MDOB/EDOB and tinbutyl-EDOT (Fig. 1). Starting
from 4-bromobenzo[d][1,3]dioxole (MDOB-Br), M1 (yield: 58%) was synthesized
according to previously reported procedures.[46] Similarly, three other novel
precursors were synthesized by Stille coupling reaction between brominated
MDOB/EDOB (i.e. Br-MDOB-Br, EDOB-Br, Br-EDOB-Br) and tinbutyl-EDOT
1
13
(
yields: 80%, 76% and 85% for M2, M3 and M4). H and C NMR spectra of those

precursors were exhibited in the Fig. S4-S9 (Supporting Information).
3
.2 Computational studies of precursors
In order to verify the influence of spatial structure on electrochemical and optical
properties and understand the minimum-energy conformation and frontier molecular
orbitals distribution of precursors, density functional theory (DFT) calculations were
carried out at the B3LYP/6-31G(d) level. As shown in Fig. 1. (B), M1/M3 are slightly
twisted with a dihedral angle of 23.94°/21.69° between phenyl of MDOB/ EDOB and
thiophene ring of EDOT. While the dihedral angles of M2/M4 increase to
5
1.53°/51.85° when introducing another EDOT unit. The slightly different dihedral
angles between M1 and M3, M2 and M4 could be attributed to the different electron
cloud density of MDOB and EDOB.
The understanding of frontier molecular orbital distribution is important for
illuminating the electropolymerization mechanism. According to the frontier orbital
theory, the chemical reaction between active molecules occurs primarily in the
frontier orbitals, thus the atom with large HOMO contribution will become the
polymerization site for the electropolymerization. From the calculated frontier
molecular orbital distribution (Fig. 1(C) and Table 1), it can be seen that the HOMO
of M2/M4 is mainly distributed in the α-positions in the terminal EDOT moieties,
indicating that the electropolymerization will happen preferentially on these sites. On
the contrary, the HOMO of M1/M3 is localized on the MDOB/EDOB and EDOT, thus
the electropolymerization will happen preferentially on these sites.
3
.3 Electrochemical behaviors of oligomers and polymers

The anodic oxidation behaviors of these precursors (M1, M2, M3 and M4) were
−
1
performed in CH Cl /THF–Bu NPF (0.1 mol L ) system. As depicted in Fig. S11
2
2
4
6
(Supporting Information) and Table 1, the anodic polarization potential of all
precursors were lower than EDOT (1.24 V vs. Ag/AgCl in CH Cl ), MDOB (1.35 V
2
2
vs. Ag/AgCl in CH Cl ) and EDOB (1.32 V vs. Ag/AgCl in CH Cl )[46] in same
2
2
2
2
electrolyte solution, which may be due to the elongation of π−conjugated chain
lengths with electron-rich group of EDOT. The onset oxidation potential (Em,onset) of
those precursors presented the following trend: M1 (0.80 V vs. Ag/AgCl) < M2 (0.84
V vs. Ag/AgCl), M3 (0.81 V vs. Ag/AgCl) < M4 (0.88 V vs. Ag/AgCl). However,
such results were obviously contrary to the general rule, i.e. the onset oxidation
potentials will decrease as the increase of π-conjugated degree of precursors.[39] We
believe that such phenomenon is mainly attributed to the nonplanar π-conjugated
structures caused by the specific 2-fold-symmetric conformation (easy to form a
stacked helix) of M2 and M4 [18−24] in comparison with M1 and M3.
Further, the electrochemical behavior of precursors was investigated by using
cyclic voltammetry (CV), as presented in Fig. S13(A). The reversibility of electron
transfer and electroactivity of polymer films could be examined by the CV method
qualitatively. The anodic and peak current density can achieve progressive growth
between the state of repetitive doping and dedoping, which manifest the formation of
corresponding electroactive polymers on the electrode. In contrast to MDOB analogs,
M3 and M4 present higher current densities and more uniform deposition rates that
reveal them with more electrochemical reaction activities.

The CVs of these polymers exhibit a proportional change under different
potential scan rates (Fig. S13B), which indicate good adherence of polymer film to
the electrode.[47,48] Obviously, such electrochemical behaviour of polymers is
similar to the precursor’s electropolymerization, and both P1 and P3 have a pair of
redox peaks, while P2 and P4 possess two redox couples because of the occurrence of
polaron and bipolaron.[49] The redox peaks are more remarkable in magnitude
accompanying by no changes in peak shape with the increasing scan rates, which
indicated these polymers have satisfied stability.[50] In addition, the electrochemical
behaviour of polymers is non-diffusion controlled process according to the linear
dependences of current density versus scan rate (Fig. S13(C)), especially for P4 with
the linear regression fitting of 0.9998, and it also suggested the superior redox
reversibility for these polymers.
The long-term electrochemical stability between the repeatedly doped and
dedoped state is significant factor to evaluate the performance of polymers. The
long-term redox stability of polymer film coating on the surface of the working
electrode was investigated in monomer-free PC–Bu NPF , as shown in Fig. S16.
4
6
P1[46] and P3 revealed satisfied electrochemical stability, retaining electroactivity of
1.80% after 100 cycles of P1[46] and 80% after 1000 cycles of P3 (Table 2). Upon
9
increasing the conjugate length of precursors, the electroactivity of P2 was weakened
than P1, retained 79% after 500 cycles of P2 and 75% after 1000 cycles of P4. On the
contrary, P4 exhibited more prominent stability than that of the other three polymers
(Table 2), retained up to 87% of electroactivity after 1000 cycles. These results are

consistent with the above electrochemical behavior. The improved electrochemical
stability for P3 and P4 could benefit from the more stable ethylenedioxy bridge.
3
.4 Optimization geometries of polymers
In order to understand the curious phenomenon that the smaller dihedral angle
tend to be more planar in theory but less stable, the optimization geometry of
polymers was performed. From the Fig. 2, the dihedral angle of PMDOB or PEDOB
was 60°. If return to the original plane, the PMDOB or PEDOB requires a minimum
of six repeating units. In a similar way, P1 or P3 needs a minimum of four repeating
units and the P2 or P4 only requires two repeat units to return to the original planar.
Also, from left view of optimization geometry, we could find that the P2 and P4 tend
to be more regularization because they needed less repeating units. Take into account
these evidences, we can understand that P2 and P4 have better electrochemical
properties and better stability than P1 and P3 in theory.
3
.5 Surface morphology
The micromorphology of P3 and P4 film coated on ITO glass in de-/doped states
were studied by SEM to assess its electrochemical activity and stability as gave in Fig.
. The doped P3 film exhibited rough surface morphology, and the roughness level
3
was more pronounced at dedoped state. Compared with P3, P4 film in the doped state
appeared distinct lines like a fine crack but uniform in appearance compact. However,
the films displayed obvious solid particles and uneven in the dedoped state. This
phenomenon can be attributed to the immigration/emigration of counterions that
destroyed oligomers, precursors or electrolyte particles existing between the polymers

and the surface during the doping and dedoping process.[51] In addition, the surface
morphology of P1/P2 film was more uniform whether in doped or dedoped state than
P4.
3
.6 Spectroelectrochemistry
UV-vis spectroelectrochemistry was employed to acquire the optical and color
changes of polymer films during the redox process and the results were presented in
Fig. 4 (A) and Table 2. The dominant absorption of four polymer appears at 400~500
nm and presents obviously red-shifted with the extension of conjugated structure of
the precursors. According to the onset of the lower energy π–π* transition, the Eg,opt of
polymers were calculated as the following trend P4 (1.38 eV) < P2 (1.59 eV) = P1
(1.59 eV)[46] < P3 (1.82 eV), which were lower than PMDOB (2.12 eV) and PEDOB
(2.45 eV)[30,31]. Low Eg,opt could bring about a low activation barrier and efficient
charge separation and charge transport, desirable properties for high-performance
opto-electronic materials. With increasing effective conjugated chain length of those
polymers by introducing EDOT as end-group, the optical band gaps were significantly
reduced.[52] In contrast to the P4, the absorption maxima of P2 film (Fig. S17) arises
obviously blue-shift because of the effect of the methylene group on
ortho-dioxybenzene structure.[46] In dedoped state, P3 and P4 possessed an intensive
absorption bands at 445 nm and 483 nm, respectively, which revealed clay-bank color
for P3 and gray-rose-red for P4 (see Table 3 and Fig. S18). By stepwise oxidation, the
intensity of π–π* transitions was gradually weakened and accompanied with the color
changing from clay-bank to gray for P3 and from gray-rose-red to cyan color for P4.

And then, the π–π* transitions peak was finally annihilated. Meanwhile, new
dominant absorption band of polaron was formed between 600 nm and 800 nm. And
this process accompanied by obviously color changing about P3 of blue (0.90 V) and
P4 of sky-blue (0.90 V). Finally, only one absorption peak was presented. By further
enhance the potentials, P3 transformed into purple color at 1.20 V and P4 film
switched to mazarine blue at 1.50 V, respectively. The concrete CIE color coordinates
and the pictures of the actual polymer films were summarized up in Table 3. Thus,
P3/P4 could be stable in the appropriate potential range and carried rich color changes,
while P1/P2 does not possess any color changes during the redox process and the
subsequent electrochromic properties are failed to test.
3
.7 Electrochromic properties
Transmittance changes are probed by stepping potentials repeatedly between the
dedoped and doped state within 10 s time intervals. Electrochromic parameters of P3
and P4, including optical contrast (ΔT), coloration efficiency (CE) and response time
are summarized in Table 4. P3 revealed excellent transmittance ratios (46%) at 750
nm and at 994 nm (28%), while P4 showed satisfied transmittance ratios as 21% at
7
89 nm and 36% at 1000 nm (Fig. 4 B)). For another 3200s cycling time, the optical
transmittance of P3 at 750 nm became instability (Fig. S19). The optical stability of
P4 at 1000 nm was impressive and only slightly damaged (less than 3.30%) with
another 3500 s switching times (Fig. 4 (C)), which was consistent with
electro-activity stability. On the other hand, the satisfied electrochromic performances
of P4 appeared at 789 nm with faster response time about 2.94 s during decolorization

2
–1
process and higher CE of 104 cm C . Overall, P3/P4 presented unique advantages in
electrochromic aspect in contrast to conventional electrochromic polymers, which
could originate from their twisted structure. Previous studies have manifested that
ethylene group on ortho-dioxybenzene structure results in larger steric hindrance than
methylene group causing more nonplanar π-conjugation.[30−34] That is, the distorted
configuration may help to reduce intermolecular interactions and tends to be more
stable of polymer film.
3
.8 Open circuit stability
The optical memory can reflect the ability of polymers to maintain oxidation or
reduction state (color or transmittance), and it’s worthy to judge the polymers’
practical application.[4] Initially, we chose 750 nm in P3 with voltage between –0.35
V and 0.95 V, and 1000 nm for P4 with voltage between –0.05 V and 0.95 V as
monitored wavelength because they are outstanding electrochromic properties in the
above wavelength. And then, the open circuit stability of both polymers was measured
with applied potential for 2 s and potential-free for 100 s. As shown in Fig. 4 (D), P4
and P3 exhibited better optical memory in dedoped state with little fluctuation (0.50%
for P4; 1.56% for P3). What’s more, the similar situation also occurred in the
oxidative state (1.00% fluctuation for P4 and 2.62% fluctuation for P3). The optical
memory studies exhibited that P3 and P4 could adhere well to the ITO electrode and
possessed lower energy consumption, minimizing the exchange of electrons without
the presence of an applied potential. Thus, both electrochromic polymers were
promising EC material for application in optical or electrochromic memory.[53]

4
. Conclusion
In summary, four novel o−alkylenedioxybenzene-EDOT hybrid precursors (M1,
M2, M3 and M4) with distorted spatial conformation have been synthesized and
electropolymerized into twisted electrochromic conjugated polymers. The distorted
spatial conformation exerts significant effect on electrochemical polymerization, M2
and M4 with specific 2-fold-symmetric conformation (easy to form a stacked helix)
presents high initial oxidation potential than M1 and M3. Also, such twisted
molecular systems provide excellent electrochromic performances including fast
response time during the oxidative process, high CE values, and excellent open circuit
stability. Furthermore, the multi-color conversion feature brings great potential for
P3/P4 film to develop into electrochromic polymer display devices. With these
findings, we believe that rational molecular engineering of twisted conjugated
polymers is a feasible and promising strategy to develop high-performance
multi-colored electrochromic materials.
Supporting Information
NMR of precursors and intermediates, FT-IR characterization, and additional
electrochemical and electrochromic data were shown in Supporting Information.
Notes
The manuscript was written through contributions of all authors. All authors have
given approval to the final version of the manuscript. And the authors declare no
competing financial interest.
Acknowledgments

This work was supported by the National Natural Science Foundation of China
51763010 and 51963011), Technological Expertise and Academic Leaders Training
(
Program of Jiangxi Province (20194BCJ22013), Double Thousand Talents Plan in
Jiangxi Province – Youth Program, and Research Project of State Key Laboratory of
Mechanical System and Vibration (MSV202013).
References
[
1] Q. Zhang, C.Y. Tsai, L.J. Li, D.J. Liaw, Colorless-to-colorful switching
electrochromic polyimides with very high contrast ratio, Nat. Commun. 2019,
0:1239.
2] G.F. Cai, J.X. Wang, P.S. Lee, Next-generation multifunctional electrochromic
devices, Acc. Chem. Res. 2016, 49, 1469−1476.
3] P.M. Beaujuge, C.M. Amb, J.R. Reynolds, Spectral engineering in π–conjugated
polymers with intramolecular donor-acceptor interactions, Acc. Chem. Res. 2010, 43,
396–1407.
4] P.M. Beaujuge, J.R. Reynolds, Color control in π–conjugated organic polymers for
use in electrochromic devices, Chem. Rev. 2010, 110, 268–320.
5] C.G. Granqvist, Electrochromics for smart windows: oxide-based thin films and
devices, Thin Solid Films 2014, 564, 1−38.
6] R.J. Mortimer, Electrochromic materials, Annu. Rev. Mater. Res. 2011, 41,
41−268.
7] A.A. Argun, P.H. Aubert, B.C. Thompson, I. Schwendeman, C.L. Gaupp, J.
Hwang, N.J. Pinto, D.B. Tanner, A.G. MacDiarmid, J.R. Reynolds, Multicolored
1
[
[
1
[
[
[
2
[

electrochromism polymers: structures and devices, Chem. Mater. 2004, 16, 4401–
412.
8] S Lin, J Liu, X Liu, X Zhao, Muscle-like fatigue-resistant hydrogels by
mechanical training, PNAS, 2019 116 (21) 10244-10249.
9] X.J. Lv, W.J. Li, M. Ouyang, Y.J. Zhang, D.S. Wright, C. Zhang, Polymeric
electrochromic materials with donor-acceptor structures, J. Mater. Chem. C. 2017, 5,
2–28.
10] C.M. Amb, A.L. Dyer, J.R. Reynolds, Navigating the color palette of
solution-processable electrochromic polymers, Chem. Mater. 2011, 23, 397–415.
11] C. Yang , W. Shi, X. Chen , K.S Zhang , X.Y. Zhou , X. Sun , S. Ding, S. Liu, Z.F.
4
[
[
1
[
[
Xie, Novel triphenylamine-based polyamides: effcient preparation via
benzoxazine-isocyanide-chemistry at room temperature and electrochromic properties
investigation, Dyes and Pigments 2020, 176, 108206.
[
12] M.T. Otley, Y.M. Zhu, X.Z. Zhang, M.F. Li, G.A. Sotzing, Color-tuning
neutrality for flexible electrochromics via a single-layer dual conjugated polymer
approach, Adv. Mater. 2014, 26, 8004–8009.
[
13] T.M.S.K. Pathiranage, D.S. Dissanayake, C.N. Niermann, Y.X. Ren, M.C. Biewer,
M.C. Stefan, Role of polythiophenes as electroactive materials, J. Polym. Sci. Part: A,
Polym. Chem. 2017, 55, 3327–3346.
[
14] K.L. Zhou, H. Wang, J.T. Jiu, J.B. Liu, H. Yan, K. Suganuma, polyaniline films
with modified nanostructure for bifunctional flexible multicolor electrochromic and
supercapacitor applications, Chem. Eng. J. 2018, 345, 290–299.

[
15] J.C. Lai, X.R. Lu, B.T. Qu, F. Liu, C.H. Li, X.Z. You, A new multicolored and
near-infrared electrochromic material based on triphenylamine-containing
poly(3,4-dithienylpyrrole), Org. Electron. 2014, 15, 3735–3745.
[
16] K. Wang, T. Zhang, Y.H. Hu, W.G. Yang, Y. Shi, Synthesis and characterization
of a novel multicolored electrochromic polymer based on a vinylene-linked
EDOT-carbazole monomer, Electrochim. Acta 2014, 130, 46–51.
[
17] Y. Xue, Z. X. Xue, W. W. Zhang, W. N. Zhang, S. Chen, K. W. Lin, J. k. Xu,
Thieno[3,2-b]Thiophene end-Capped all-Sulfur analog of 3,4-ethylenedioxythiophene
and its eletrosynthesized polymer: is distorted conformation not suitable for
electrochromism?, J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1041–1048.
[
18] J.D. Luo, Z.L. Xie, J.W.Y. Lam, L. Cheng, H.Y. Chen, C.F Qiu, H.S. Kwok, X.W.
Zhan, Y.Q. Liu, D.B. Zhu, B.Z. Tang, Aggregation-induced emission of
-methyl-1,2,3,4,5-pentaphenylsilole, Chem. Commun. 2001, 18, 1740–1741.
1
[
[
19] R. A. Pascal, Twisted acenes, Chem. Rev. 2006, 106, 4809−4819.
20] S.J. Zhen, S.W. Wang, S.W. Li, W.W. Luo, M. Gao, L.G. Ng, C.C. Goh, A.J. Qin,
Z.J. Zhao, B. Liu, B.Z. Tang, Efficient red/near-infrared fluorophores based on
benzo[1,2-b:4,5-b’]dithiophene 1,1,5,5-tetraoxide for targeted photodynamic therapy
and in vivo two-photon fluorescence bioimaging, Adv. Funct. Mater. 2018, 1706945.
[
21] J. Mei, N.L. Leung, R.T. Kwok, J.W.Y. Lam, B.Z. Tang, Aggregation-induced
emission: together we shine, united we soar! Chem Rev, 2015, 115, 11718–11940.
22] R.G. Clevenger, B. Kumar, E.M. Menuey, K. V. Kilway, Synthesis and structure
of a longitudinally twisted hexacene, Chem. Eur. J. 2018, 24, 3113–3116.
[

[
23] P. Kovacic, J.T. Uchic, L. C. Hsu, Polymerization of aromatic nuclei. XII.
oligomerization of halobenzenes by aluminum chloride-cupric chloride, J. Polym. Sci.,
Part A: Polym. Chem. 1967, 5, 945–964.
[
24] S. Malzkuhn, X.W. Guo, D. Häussinger, O.S. Wenger, Electron transfer across
o-phenylene wires. J. Phys. Chem. A 2019, 123, 96–102.
25] E. Ohta, H. Sato, S. Ando, A. Kosaka, T. Fukushima, D. Hashizume, M.
[
Yamasaki, K. Hasegawa, A. Muraoka, H. Ushiyama, K. Yamashita, T. Aida,
Redox-responsive molecular helices with highly condensed π-clouds, Nat. Chem.
2
010, 3, 68–73.
[
26] J. He, J. L. Crase, S. H. Wadumethrige, K. Thakur, L. Dai, S. Z. Zou, R. Rathore,
C. S. Hartley, ortho-Phenylenes: Unusual Conjugated Oligomers with a Surprisingly
Long Effective Conjugation Length, J. Am. Chem. Soc. 2010, 132, 13848–13857.
[
[
27] C.S. Hartle, Folding of ortho-Phenylenes, Acc. Chem. Res. 2016, 49, 646–654.
28] S. Mathew, L.A. Crandall, C.J. Ziegler, C.S. Hartley, Enhanced helical folding of
ortho–Phenylenes through the control of aromatic stacking interactions, J. Am. Chem.
Soc. 2014, 136, 16666–16675.
[
29] S.M. Mathew, J.T. Engle, C.J. Ziegler, C.S. Hartley, The role of arene–arene
interactions in the folding of ortho-phenylenes, J. Am. Chem. Soc. 2013, 135, 6714–
722.
30] J.K. Xu, H.T. Liu, S.Z. Pu, F.Y. Li, M.B. Luo, A novel electrochromic
6
[
polyacetylene derivative with good fluorescence properties electrodeposited by direct
anodic oxidation of 1,2-methylenedioxybenzene, Macromolecules 2006, 39, 5611–

5
616.
31] B. Dong, L.Q. Zeng, J.K. Xu, H.T. Liu, S.Z. Pu, Electropolymerization of
,2-methylenedioxybenzene in 1-butyl-3-methylimidazolium hexafluorophosphate
room temperature ionic liquid, Polymer 2007, 48, 5548–5555.
32] M.L. Ma, H.T. Liu, J.K. Xu, Y.Z. Li, Y.Q. Wan, Electrochemical polymerization
[
1
[
of o–dihydroxybene and characterization of its polymers as polyacetylene derivatives,
J. Phys. Chem. C 2007, 111, 6889–6896.
[
33] H.T. Liu, Y.Z. Li, J.K. Xu, Z.G. Le, M.B. Luo, B.S. Wang, S.Z. Pu, L. Shen,
Electrosyntheses and characterization of polyalkylenedioxybenzenes, Eur. Polym. J.
008, 44, 171–188.
34] Y. Huang, B.Y. Lu, J.K. Xu, G. Zhang, L.Q. Qin, S. Chen, F.F. Kong, Novel
2
[
cross-linking poly(ethylene oxide) grafted poly(1-hydroxy-2-methoxyphenol)
copolymers by secondary polymerization, Electrochim. Acta 2012, 77, 163–170.
[
35] B.Y. Lu, H.W. Yuk, S.T. Lin, N.N. Jian, K. Qu, J.K. Xu, X.H. Zhao. Pure
PEDOT:PSS hydrogels. Nat. Commun. 2019, 10: 1043.
36] H.W. Yuk, B.Y. Lu, X.H. Zhao. Hydrogel bioelectronics. Chem. Soc. Rev. 2019,
8, 1642–1667.
37] K.W. Lin, S. Chen, B.Y. Lu, J.K. Xu, Hybrid π–conjugated polymers from
[
4
[
dibenzo pentacyclic centers: precursor design, dlectrosynthesis and electrochromics,
Sci. China Chem. 2017, 60, 38–53.
[
38] D.Z. Mo, J.K. Xu, B.Y. Lu, Alkyl chain engineering in the hybrid
bithiophene-3,4-ethylenedioxythiophene: synthesis, electronic properties, and

electropolymerization, Synth. Met. 2014, 198, 19–30.
39] Y. Xue, Z.X. Xue, W.W. Zhang, W.N. Zhang, S. Chen, K.W. Lin, J.K. Xu, Effects
[
on optoelectronic performances of EDOT end-capped oligomers and electrochromic
polymers by varying thienothiophene cores. J. Electroanal. Chem. 2019, 834, 150–
1
60.
[
40] Y. Xue, Z.X. Xue, W.W. Zhang, W.N. Zhang, S. Chen, K.W. Lin, J.K. Xu,
Enhanced electrochromic performances of Polythieno[3,2-b]thiophene with
multicolor conversion via embedding EDOT segment, Polymer 2018, 159, 150–156.
[
41]
selenophene-3,4-ethylenedioxythiophene
electrochromic polymers, Polym. Chem. 2014, 5, 4896–4908.
42] S.J. Zhen, J.K. Xu, B.Y. Lu, Tuning the optoelectronic properties of polyfuran by
B.Y.
Lu,
J.K.
Xu,
G.Q.
Zhao,
Highly
stable
hybrid
as
electrically
conducting
and
[
design of furan-EDOT monomers and free-standing films with enhanced redox
stability and electrochromic performances, Electrochim. Acta 2014,146, 666–678.
[
43] S.L. Ming, S.J. Zhen, K.W. Lin, L. Zhao, J.K. Xu, B.Y. Lu,
Thiadiazolo[3,4-c]pyridine as an acceptor toward fast-switching green
donor-acceptor-Type electrochromic polymer with low bandgap, ACS Appl. Mater.
Interfaces 2015, 7, 11089−11098.
[
44] H. Gu, S.L. Ming, K.W. Lin, S. Chen, X.M. Liu, B.Y. Lu, J.K. Xu. Isoindigo as
an electron-deficient unit for high-performance polymeric electrochromics,
Electrochim. Acta 2018, 260, 772−782.
[
45] Z.X. Xue, S. Chen, N. Gao, Y. Xue, B.Y. Lu, O. Waston, L. Zang, J.K. Xu.

Structural design and applications of stereoregular fused Thiophenes and Their
Oligomers and Polymers, Polym. Rev. 2019, 1−41.
[
46] B.Y. Lu, K.W. Lin, S.J. Zhen, S.L. Ming, H.T. Liu, H. Gu, S. Chen, J.K. Xu,
Fluorescent and electrochromic poly(5-(benzo[d][1,3]dioxol-5-yl)-2,3-
dihydrothieno[3,4-b][1,4]dioxin), Synth. Met. 2016, 220, 202–207.
47] C.S.Y. Tan, J. Liu, A. S. Groombridge, S. J. Barrow, C. A. Dreiss, O. A.
[
Scherman, Controlling Spatiotemporal Mechanics of Supramolecular Hydrogel
Networks with Highly Branched Cucurbit[8]uril Polyrotaxanes, Adv. Funct. Mater,
2
018, 28, 1702994.
48] J. Liu, C.S.Y. Tan, O.A. Scherman, Dynamic Interfacial Adhesion through
Cucurbit[n]uril Molecular Recognition, Angew. Chem. Int. Ed, 2018 130 (29), 8992–
996.
49] L.R. Savagian, A.M. Österholm, D.E. Shen, D.T. Christiansen, M. Kuepfert, J.R.
[
8
[
Reynolds, Conjugated polymer blends for high contrast black-to-transmissive
electrochromism, Adv. Opt. Mater. 2018, 1800594.
[
50] S. I. Cho, S. B. Lee, Fast Electrochemistry of conductive polymer nanotubes:
synthesis, mechanism, and application, Acc. Chem. Res. 2008, 41, 699–707.
51] E. Poverenov, M. Li, A. Bitler, M. Bendikov, Major effect of
[
electropolymerization solvent on morphology and electrochromic properties of
PEDOT films, Chem. Mater. 2010, 22, 4019–4025.
[
52] F.Q. Hu, Y. Xue, J.K. Xu, B.Y. Lu. PEDOT-based conducting polymer actuators,
Front Robot AI 2019, 6: 114.

[
53] J.Z. Low, W.T. Neo, Q. Ye, W.J. Ong, I.H.K. Wong, T.T. Lin, J.W. Xu, Low
band-gap diketopyrrolopyrrole-containing polymers for near infrared electrochromic
and photovoltaic applications, J. Polym. Sci. A Polym. Chem. 2015, 53, 1287–1295.

Table captions:
Table 1 The calculated and experimental parameters of precursors.
Em,onset HOMO LUMO Dihedral angles
Precursors
(
V)
(eV)
−5.07
−5.03
−5.05
−4.98
(eV)
−2.16
−2.18
−2.12
−2.13
(°)
M1
M2
M3
M4
0.80
0.84
0.81
0.88
23.94
51.53, 51.53
21.69
51.85, 51.85
Table 2 The optical and electrochemical parameters of four polymers.
b
HOMO LUMO λabs,max
Eg,opt
(eV)
1.59
1.59
1.82
1.32
Stability
Sample
th
th
(
eV)
(eV)
−3.08
−3.20
–3.10
–3.13
nm
514
541
449
483
500
1000
–
a
P1
−4.43
–4.79
–4.92
–4.86
–
P2
P3
P4
79%
89%
94%
75%
80%
87%
a
Our previous results from Ref. 30.
b
Eg,opt of polymer is calculated by formula (1).
Table 3 Photographs at different applied potentials and CIE color coordinates of P3 and P4.
Polymers
P3
P4
Color
Potential (V) –0.30
0.30
0. 90
1.20
86.80 56.84 55.91
4.60 7.65 0.43
0.00
0.20
0.90
1.50
38.16
32.59
L*
a*
b*
89.96 90.54 88.19
59.14
3.38
1.31
−5.00
−2.53
13.12
8.01 −10.96 −11.79 3.81 −13.89 −34.87 −39.92

Table 4 Electrochromic properties of P3 and P4.
λ
Tred
(%)
36
Tox
(%)
25
6
ΔT
(%)
11
46
28
5
Response time (s)
CE
2
Polymer
P3
–1
(
nm)
tox
tred
(cm C )
48
4
55
750
1.04
0.87
5.58
7.8
5.06
2.15
8.06
6.3
52
211
108
22
9
4
94
83
53
25
56
38
43
61
P4
789
000
59
21
36
4.94
8.83
2.94
8.69
104
92
1
79