Pyrazine-EDOT D-A-D type Hybrid Polymer for Patterned Flexible Electrochromic Devices

Article abstract of DOI:10.1016/j.electacta.2020.136859

Two donor-acceptor type pyrazine-EDOT hybrid polymers are developed as high performance electrochromic materials and employed to fabricate flexible electrochromic devices via electropolymerization patterning. By coupling EDOT with pyrazine at 2,5-positions, the resultant precursor 2,5-BEP can facilely electropolymerize to yield high quality pyrazine-EDOT hybrid polymer films with excellent electrochemical stability. Spectroelectrochemistry and kinetic studies demonstrate that as-formed pyrazine-EDOT hybrid polymer exhibits reversible color-changing nature from purple to gray with high coloration efficiency (134 cm² C^?1), fast response time (0.2 s) and high optical contrast (> 55percent) over a wide wavelength range of 800–2200 nm in near-infrared (NIR) region. To harness its excellent electrochromic performance as well as the ease of electropolymerization patterning, we fabricate patterned flexible electrochromic devices (ECDs) via facile electropolymerization and such ECDs display reversible color changes with decent optical stability (25percent reduction in optical contrast upon 1000 cycling) and excellent mechanical stability (9.6percent reduction in optical contrast upon bending 5000 cycles). The excellent electrochromic performance and stability of pyrazine-EDOT hybrid polymers as well as their-based ECDs make them good candidates towards near infrared electrochromic applications.

Full text of DOI:10.1016/j.electacta.2020.136859

Journal Pre-proof
Pyrazine-EDOT D-A-D type Hybrid Polymer for Patterned Flexible
Electrochromic Devices
Faqi Hu , Yu Xue , Nannan Jian , Kai Qu , Kaiwen Lin ,
Xiukai Zhu , Tao Wu , Ximei Liu , Jingkun Xu , Baoyang Lu
PII:
S0013-4686(20)31252-4
DOI:
Reference:
https://doi.org/10.1016/j.electacta.2020.136859
EA 136859
To appear in:
Electrochimica Acta
Received date:
Revised date:
Accepted date:
6 June 2020
20 July 2020
28 July 2020
Please cite this article as: Faqi Hu , Yu Xue , Nannan Jian , Kai Qu , Kaiwen Lin ,
Xiukai Zhu , Tao Wu , Ximei Liu , Jingkun Xu , Baoyang Lu , Pyrazine-EDOT D-A-D type Hy-
brid Polymer for Patterned Flexible Electrochromic Devices, Electrochimica Acta (2020), doi:
https://doi.org/10.1016/j.electacta.2020.136859
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Highlights
 Two donor-acceptor type pyrazine-EDOT hybrid polymers are
electropolymerized and comparatively investigated.
 High quality polymer films with excellent electrochemical stability
are successfully obtained by low potential electropolymerization.
 Pyrazine-EDOT hybrid polymer exhibits reversible color-changing
nature from purple to gray with fast response time and high optical
contrast in the NIR region.
 Patterned
flexible
ECDs
are
fabricated
by
direct
electropolymerization.
 As-obtained ECDs display reversible color changes with decent
optical stability and excellent mechanical stability.

Pyrazine-EDOT D-A-D type Hybrid Polymer for Patterned Flexible
Electrochromic Devices
Faqi Hua,b, Yu Xue a,b, Nannan Jianb, Kai Qub, Kaiwen Linc, Xiukai Zhua, Tao Wub,
Ximei Liu*,a, Jingkun Xu*,a,b, Baoyang Lu*,a,b
a
School of Pharmacy, Jiangxi Science & Technology Normal University, Nanchang
330013, Jiangxi, PR China
b
Flexible Electronics Innovation Institute (FEII), Jiangxi Science & Technology
Normal University
c Department of Materials and Food, University of Electronic Science and Technology
of China Zhongshan Institute, Zhongshan, 528402, P.R. China
* Corresponding information:
E-mail: lxm5812@163.com; xujingkun@tsinghua.org.cn; lby1258@163.com

Abstract
Two donor-acceptor type pyrazine-EDOT hybrid polymers are developed as high
performance electrochromic materials and employed to fabricate flexible
electrochromic devices via electropolymerization patterning. By coupling EDOT with
pyrazine at 2,5-positions, the resultant precursor 2,5-BEP can facilely
electropolymerize to yield high quality pyrazine-EDOT hybrid polymer films with
excellent electrochemical stability. Spectroelectrochemistry and kinetic studies
demonstrate that as-formed pyrazine-EDOT hybrid polymer exhibits reversible
color-changing nature from purple to gray with high coloration efficiency (134 cm2
C-1), fast response time (0.2 s) and high optical contrast (> 55%) over a wide
wavelength range of 800 ~ 2200 nm in near-infrared (NIR) region. To harness its
excellent electrochromic performance as well as the ease of electropolymerization
patterning, we fabricate patterned flexible electrochromic devices (ECDs) via facile
electropolymerization and such ECDs display reversible color changes with decent
optical stability (25% reduction in optical contrast upon 1000 cycling) and excellent
mechanical stability (9.6% reduction in optical contrast upon bending 5000 cycles).
The excellent electrochromic performance and stability of pyrazine-EDOT hybrid
polymers as well as their-based ECDs make it a good candidate towards near infrared
electrochromic applications.
Keywords: Conjugated polymers; Pyrazine; PEDOT; Electropolymerization
patterning; Flexible electrochromic devices

1. Introduction
Conjugated polymer-based flexible ECDs as an advance of traditional
rigid ECDs have been attracting tremendous interests over the last decade,
which possess the characterizations such as long-term redox stability, high
contrast ratio and coloration efficiency, low power consumption, as well as
good processability. All these advantages make them promising towards diverse
applications such as wearable displays or sensors, advertising signage,
biomimicry, and adaptive camouflage, etc.[1-4] Despite rapid progress most
recently, the challenges on the lack of high performance electrochromic
materials and facile fabrication techniques still hinders further development of
conjugated polymer-based flexible ECDs.
D-A type conjugated polymers, with a push-pull electronic structure
consisting of donor units (D) and acceptor moieties (A) which can enhance the
intramolecular charge transfer by forming massive π-bonds, have attracted great
attention owing to their tunable structure, low bandgap, together with intriguing
electrochromic properties like high optical contrast and rapid response time, etc.
This type of materials have been widely used as active layers in many
electronic devices, such as supercapacitors,[5, 6] organic solar cells,[7, 8] and
electrochromic devices.[9-11] Previous reports demonstrate that the electron
affinity of heterocycles increase with increasing the number of nitrogen atoms,

from -0.62 eV for pyridine to 0 eV for pyrimidine, and further to +0.25 eV for
pyrazine.[12] Pyrazine contains two imine nitrogen carbons (C=N) at 1,4
positions symmetrically, leading to its strong electron affinity as an acceptor
moiety.[13] Therefore, pyrazine and its fused ring derivatives such as
quinoxaline,[14] thieno[3,4-b]pyrazine,[15] and pyrido[3,4-b]pyrazine[16] have
been widely used as effective electron-withdrawing building blocks to construct
high performance D-A type conjugated polymers for organic photovoltaics.
Unfortunately, such a promising acceptor are rarely studied for rational design
of advanced electrochromic materials.
Herein, to explore the performance of pyrazine-based conjugated polymer
systems towards organic electrochromics, we design a pair of isomers by coupling
pyrazine with 3,4-ethylenedioxythiophene (EDOT), which is one of the best donor
units with superior electrochromic performance and stability in the electrochromic
community.[17-19]
Pyrazine-EDOT
isomers,
(2,5-BEP)
(2,6-BEP)
i.e.,
and
2,5-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)pyrazine
2,6-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)pyrazine
were
successfully electopolymerized and their corresponding polymers were systematically
studied. We find that varying EDOT substitution sites on the pyrazine unit changes the
π-conjugation structure and intramolecular charge transfer, leading to significant
difference in physicochemical and electrochromic properties for P(2,5-BEP) and
P(2,6-BEP). We further fabricate flexible electrochromic devices (ECDs) via facile
electropolymerization patterning, and the resultant ECDs exhibit excellent optical and

mechanical stability against potential cycling and mechanical bending.
2. Experimental
2.1 Materials
Building
blocks
like
2,5-dibromopyrazine
(98%;
J&K),
2,6-dibromopyrazine (95%; Alfa Aesar) and 3,4-ethylenedioxythiophene
(EDOT, 98%; Sigma-Aldrich) were used directly as received without further
purification. Tetrahydrofuran (THF, AR; General-Reagent) as the solvent for
the mono-stannylation of EDOT, was purified by distillation with sodium under
nitrogen atmosphere before use. Tetrabutylammonium hexafluorophosphate
(Bu4NPF6, 99%; Energy) was dried under vacuum at 100 °C for 12 h and
employed
as
the
electrolyte
for
the
electropolymerization.
Tetrakis(triphenylphosphine)palladium (0) (Pd(PPh3)4, 99%; J&K) was used as
the catalyst for Stille coupling. Lithium perchlorate (LiClO4, AR; J&K),
propylene carbonate (PC, AR; J&K), acetonitrile (ACN, AR; J&K) and
polymethyl methacrylate (PMMA; TCL) were used directly without further
purification to prepare the gel electrolyte. Other reagents were purchased from
guaranteed chemical plants and directly used for the synthetic procedure, such
as n-butyllithium (n-BuLi, 1.6 M in hexanes; Adamas-Beta), chlorotributyltin
(SnBu3Cl, 98%; Energy), toluene (AR; Xilong) and dichloromethane (CH2Cl2,
with molecular sieves; J&K), and so on.

2.2 Synthesis of D-A-D type Pyrazine-EDOT Isomers
The synthetic procedure of two D-A-D type pyrazine-EDOT isomers,
2,5-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)pyrazine
2,6-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)pyrazine
(2,5-BEP)
(2,6-BEP),
and
is
illustrated in Fig. 1a. 2,5- and 2,6-substituted pyrazines were employed as the
acceptor units, while EDOT was used as the donor, and both of the isomers
2,5-BEP and 2,6-BEP were synthesized by Stille coupling. The
mono-stannylation of EDOT to 2-tributylstannyl-3,4-ethylenedioxythiophene,
was achieved according to our previous work and the product was employed for
the next step without further purification.[20, 21]
Detailed procedure for 2,5-BEP and 2,6-BEP: To a solution of
2-tributylstannyl-3,4-ethylenedioxythiophene (7.3 g, 16.8 mmol) and
2,6-dibromopyrazine or 2,5-dibromopyrazine (0.5 g, 2.1 mmol) in toluene (30
mL), tetrakis(triphenylphosphine)palladium (Pd(PPh3)4, 0.3 g, 0.2 mmol) was
added, and then the mixture was stirred for 24 h at 110 °C under a nitrogen
atmosphere. The mixture was cooled to room temperature and extracted with
CH2Cl2, and the organic layer was purified by silica column chromatography.
1
2,5-BEP was obtained as a yellow solid with a yield of 74%. H NMR
(400 MHz, CDCl3, Fig. S1 in Supplementary Materials): δ 9.11 (s, 2H), 6.50 (s,
2H), 4.41 (s, 4H), 4.31 (s, 4H); 13C NMR (400 MHz, DMSO-d6, Fig. S2): δ
144.41, 142.52, 141.34, 140.48, 115.33, 103.48, 65.80, 64.59.

1
2,6-BEP was obtained as a gray white solid with a yield of 70%. H NMR
(400 MHz, CDCl3, Fig. S3): δ 9.11 (s, 2H), 6.50 (s, 2H), 4.40 (s, 4H), 4.31 (s,
4H); 13C NMR (400 MHz, DMSO-d6, Fig. S4): δ 146.01, 141.94, 141.45,
137.49, 114.32, 103.79, 65.39, 64.63.
(Insert Fig. 1 here)
2.3 Electropolymerization and electrochemical tests
The electropolymerization of pyrazine-EDOT precursors and
electrochemical tests of the electrodeposited polymer films were investigated
by using a Versa Stat 3 electrochemical workstation (EG&G Princeton Applied
Research). For electropolymerization, the working electrodes were Pt wires
with diameter of 1 mm or indium tin oxide (ITO)-coated glass (0.8 × 5 cm2),
and the counter electrodes were Pt wires (diameter: 1 mm) or Pt sheets (0.8 × 5
cm2), respectively. An Ag/AgCl wire was used as the reference electrode,
which was prepared by submerging an Ag wire into 6 mol L-1 hydrochloric acid
aqueous solution for the electrodeposition of AgCl layer at a constant potential
of 1.5 V for 100 s. Moreover, the electrolytic system of CH2Cl2-Bu4NPF6 (0.10
mol L-1) was bubbled by nitrogen for 30 min before experiments to minimize
the oxygen effect. For electrochemical tests, both the working and counter
electrodes were Pt wires with diameter of 1 mm, which were carefully polished
with 1500 mesh sandpaper and burnt with alcohol lamp to remove the organic
substance attached on the electrode. All electrodes were rinsed sequentially

with water, ethanol, acetone, and then dried before use. Note that the
pyrazine-EDOT polymer films employed for all the following characterizations,
namely P(2,5-BEP) and P(2,6-BEP), were prepared by potentiostatic
electrolysis at the optimal constant potentials of 1.16 V and 1.27 V in
CH2Cl2-Bu4NPF6 (0.10 mol L-1), respectively.
2.4 Characterization
1H NMR and 13C NMR spectra were recorded to identify the chemical
structure of pyrazine-EDOT precursors with a Bruker AV 400 MHz NMR
spectrometer (Bruker AV 400, Germany), with trimethylsilane (TMS) as the
internal standard and CDCl3 or DMSO-d6 as the solvent. The UV-vis spectra of
pyrazine-EDOT precursors dissolved in CH2Cl2 were performed with a
ultraviolet-visible spectrophotometer (SPECORD 200, Germany).
A
fluorescence spectrophotometer (F-4600, Hitachi, Japan) was employed to take
the emission spectra of pyrazine-EDOT precursors in THF.
Computational calculations of pyrazine-EDOT precursors were studied by
density functional theory (DFT) with the Gaussian 09 program package.
Becke's three-parameter exchange functional and the gradient-corrected
functional of Lee, Yang, and Paar (B3LYP) together with 6-31G (d, p) basis in
vacuum was set to optimize the structure geometry and to calculate the
electronic structures at the minimum form.

Infrared spectra were tested with the samples pressed in KBr pellets by a
Fourier-transform infrared (FT-IR) spectrometer (Bruker Vertex 70, Germany).
Scanning electron microscopy (SEM) images were taken by using a scanning
electron microscope (acceleration voltage of 10.0 kV; S4800, Hitachi, Japan).
These polymer films were obtained by constant potential method on
ITO-coated glass and doped/ de-doped before characterization.
Spectroelectrochemical and electrochromic kinetic studies were performed
by combining a UV-vis spectrophotometer (SPECORD 200, Germany) and an
electrochemical workstation (Versa Stat 3, EG&G Princeton Applied Research).
In a transparent cuvette, an Ag/AgCl electrode was employed as the reference
electrode, ITO-coated glass (sheet resistance: 7-10 Ω/square) with
electrodeposited pyrazine-EDOT polymer films (0.8 ×5 cm2) as the working
electrode, and a Pt wire (diameter: 1 mm) as the counter electrode, respectively.
All tests were performed in blank CH2Cl2-Bu4NPF6 (0.10 mol L-1).
2.5 Flexible Electrochromic Devices (ECDs)
The patterning of electrochromic layer was carried out via potentiostatic
electrolysis of pyrazine-EDOT polymer films on flexible ITO-coated
polyethylene terephthalate (PET) substrate (6×4 cm2; 182 μm) with a stencil,
and then the film was rinsed with CH2Cl2 to remove precursors, oligomers and
the
residual
electrolyte.
For
the
electropolymerization,
one-compartment-three-electrode system was employed with ITO-coated PET,

stainless steel (6×6 cm2) and Ag/AgCl as the working, counter and reference
electrodes, respectively.
As a vital component of flexible ECDs, gel electrolyte displays unique
advantages over liquid and solid electrolyte, such as no risk of leakage, high
chemical stability and fast switching time.[22] To prepare the gel electrolyte in
this work, chemicals including LiClO4 (1.35 g), ACN (1.5 ml) and PC (15 ml)
were added in the flask. Then, PMMA (1.25 g) was slowly added in the mixture,
followed by stirring of 12 hours along with reflux condensation at 65 °C. The
gel electrolyte was obtained with a composition of ACN: PC: LiClO4: PMMA
(5.4 wt%: 82.7 wt%: 6.2 wt%: 5.7 wt%).[23]
The flexible ECDs based on pyrazine-EDOT polymer film were assembled
according to the following configuration: ITO-coated PET/ patterned
electrochromic layer/gel electrolyte/ITO-coated PET. As illustrated in Fig. 2,
after electrodepositing patterned pyrazine-EDOT polymer film on the
ITO-coated PET, a double-sided tape (3M) was attached as the spacer layer
along the periphery of the pattern polymer film to control the uniform thickness
of the gel electrolyte layer (ca. 185 μm). The gel electrolyte were then dropped
on the pattern and covered by another ITO-coated PET sheet to afford the
flexible electrochromic device.
The edge of the device was completely sealed with single component
room temperature vulcanized silicone rubber (RTV; Ausbond) in order to

prevent the possible leakage of the gel electrolyte upon mechanical bending.
Commercial copper strips (double-sided conductive copper foil tape, thickness:
0.1 mm) were used to form ohmic contact with the electrodes of the ECDs. The
fabrication process of ECDs was performed at ambient conditions and the
thickness of the whole device is around 550 μm. The spectroelectrochemistry
and electrochromic kinetic tests of the patterned ECDs were carried out by
coupling a UV-vis spectrophotometer with an electrochemical workstation. The
switching and mechanical stability of ECDs was monitored by recording the
changes in optical contrast under repeated electrochemical scanning and/or
bending.
(Insert Fig. 2 here)
3. Results and discussion
3.1 Molecular design and synthesis
A pair of D-A-D type pyrazine-EDOT isomers (2,5-BEP and 2,6-BEP)
have been designed and synthesized to explore the structure-property
relationship. The synthetic procedure of target precursors is depicted in Fig. 1a.
With EDOT as the donor unit and 2,5-/2,6-substituted pyrazine as the acceptor
moiety, 2,5-BEP and 2,6-BEP, are synthesized with relatively high yields (up to
74%) via Stille coupling catalyzed by Pd(PPh3)4. 1H NMR and 13C NMR
spectra of these two pyrazine-EDOT are given in Figs. S1-S4 in the
supplementary materials.

3.2 Optical property
To illustrate their optical properties, UV-vis absorption and fluorescence
emission spectra of the 2,5-BEP and 2,6-BEP isomers are examined in CH2Cl2
(Figs. 1b-c). Corresponding optical parameters are summarized in Table 1.
From the UV-vis absorption spectrum, both 2,5-BEP and 2,6-BEP display
two wide absorption bands in the range of 300~450 nm (Fig. 1b and Table 1), a
characteristic absorption commonly observed for D-A-D type compounds.[9,
20, 24] The absorption at the higher energy band in the range of 300~340 nm
can be attributed to the intramolecular π-π* transition of the conjugated
system,[9, 25] while the broader low energy band in the range of 350~400 nm
can be ascribed to charge transfer between EDOT donor units and pyrazine
acceptor moieties.[26, 27] Compared to 2,6-BEP, 2,5-BEP displays a red-shift
of around 16 nm in the absorption peak wavelengths, owing to its higher
π-electron delocalization from the improved conjugated structure, which can be
confirmed by the lower dihedral angles of 2,5-BEP from the computational
results.
In good agreement with the UV-vis absorption spectra, the isomerization
of pyrazine-EDOTs also influences their fluorescence properties. Due to its
better π-conjugation, the emission peak of 2,5-BEP (463 nm) exhibits a 20 nm
redshift compared to that of 2,6-BEP (443 nm), as shown in Fig. 1c and Table 1.
Moreover, both 2,5-BEP and 2,6-BEP reveal a blue/purple light-emitting

phenomenon in dilute solution (Inset of Fig. 1c). The absolute fluorescence
quantum yields of 2,5-BEP and 2,6-BEP in THF solution with same
concentration (4.72 × 10-5 M) are determined to be 0.085 and 0.185,
respectively.
(Insert Table 1 here)
3.3 Electrochemical polymerization of pyrazine-EDOT isomers
From the anodic oxidation curve (Fig. S5, Supporting Information), the
oxidation onset potential (Em,ox) of 2,6-BEP is measured to be around 1.17 V,
while that of 2,5-BEP is reduced to 1.06 V due to its better conjugated structure
(Table 1 and Fig. S5). Significant difference in cyclic voltammograms (CVs,
Fig. 3a,b) between 2,5-BEP and 2,6-BEP can be found during the potential
scanning process. The CVs of 2,5-BEP show broad redox waves with
uniformly increasing current densities (Fig 3a) and deep colored polymer film
is gradually electrodeposited on the working electrode surface,[28] which is
commonly observed for the electropolymerization EDOT-based D-A-D type
precursors.[18] The redox peaks of the deposited film shift to higher potentials
because of the gradual resistance increment. On the other hand, the
electropolymerization of 2,6-BEP exhibits unsatisfactory cycling behavior
without obvious redox waves (Fig. 3b) in a series of solvent systems such as
CH2Cl2, CHCl3, MeCN, etc. We believe that this phenomenon is mainly

attributed to the interrupted conjugated structure in 2,6-BEP, which makes it
very difficult to electropolymerize.
(Insert Fig. 3 here)
3.4 Electrochemistry of pyrazine-EDOT hybrid polymers
The redox activity and stability of the pyrazine-EDOT hybrid polymer
film-modified electrodes are examined by cyclic voltammetry in blank
CH2Cl2-Bu4NPF6 (0.10 mol L-1), as displayed in Fig. 3c,d and Fig. S6-S8.
P(2,5-BEP) film electrodeposited onto both Pt or ITO working electrodes
reveals two reversible broad redox waves with almost same peak potentials and
current densities upon potential sweeping, while no obvious redox peaks can be
found for P(2,6-BEP). For P(2,5-BEP), the peak current density exhibits good
linear fitting with R2 close to 1 versus potential scanning rate, indicating that its
redox process is non-diffusional and the P(2,5-BEP) film adheres well on the
working electrode surface.[14, 18] Moreover, P(2,5-BEP) also reveals good
electrochemical stability against long term cycling (Fig. S9 and Table 2), with
more than 70% of its electroactivity remaining after 5000 cycles. Since the long
term redox stability of conducting polymers is a significant challenge for most
organic electronic devices, P(2,5-BEP) can be a good candidate for device
fabrication like electrochromics, capacitors, etc.
3.5 Theoretical calculations

Quantum chemical calculation can be effectively applied to study the
geometry and electronic structure of pyrazine-EDOT molecules. From the
optimized ground-state geometries of two precursors (Fig. 4a), the dihedral
angles between the pyrazine ring and EDOT units (Table 1) for 2,5-BEP are
smaller than those of 2,6-BEP, indicating its more planar ground-state geometry
from better π-conjugation. Moreover, based on the frontier orbital theory, atoms
in these pyrazine-EDOT molecules with higher HOMO proportion are more
likely to lose electrons to generate radical cations, so the electropolymerization
between pyrazine-EDOT molecules tends to occur on these sites.[29] In the
calculated frontier molecular orbital distribution results (Fig. 4a), the HOMO
proportion at α-positions of the thiophene ring in EDOT is much higher than
other atoms for both 2,5-BEP and 2,6-BEP, indicating that the electrochemical
polymerization preferentially occurs at the α-positions of end-capping EDOT
moieties. Note that the HOMO-LUMO gap of 2,5-BEP is smaller than
2,6-BEP, suggesting its higher reaction activity to afford radical cations.
3.6 Structural characterization and electropolymerization mechanism
FT-IR spectra of pyrazine-EDOT precursors and their corresponding
hybrid polymers are recorded to elucidate their structures and further interpret
the electropolymerization mechanism. As illustrated in Fig.4b-c, the
characteristic peaks of pyrazine and thiophene rings at around 1470-1440/1360
cm-1
(
C=C and/or C=N/C-S stretching vibration), 1366/1000 cm-1 and 800/683
cm-1 (C-H/C-S and ring deformation vibration) can be observed in the spectra

of both isomers, indicating that these structures are intact after
electropolymerization. On the other hand, the absorption peaks at 3104 cm-1
and 718 cm-1 of EDOT units (=C-H stretching vibration and out-of-plane
deformation vibration) disappear in the spectra of pyrazine-EDOT hybrid
polymers (Fig. 4c), confirming that 2,5-BEP and 2,6-BEP electropolymerize
though the coupling at α positions of terminal EDOT moieties.[30-32]
Moreover, the absorption peaks of the polymers (Fig. 4c) are significantly
broadened compared to those of precursors, mainly resulting from their
complex composition of oligomers/polymers with varying chain dispersity. The
vibrational peaks of pyrazine-EDOT hybrid polymer with different chain
lengths display various infrared shifts. These peaks overlap one another and
thus produce such broad peaks with hyperstructures for the hybrid
polymers.[18, 33-35]
Based on the computational results and FT-IR spectral analysis, we
propose the electropolymerization mechanism of pyrazine-EDOT isomers
according to the well known radical cation mechanism for typical aromatic
compounds, as depicted in Fig. 4d exemplified by 2,5-BEP. The initiation step
involves the monomeric precursors being oxidized to their radical cations in the
vicinity of the working electrode, and then the dual-cations form though the
dimerization of radical cations and further yield the dimers via deprotonation of
two protons. The dimers are further oxidized and couple with another radical
cation. The polymer chains continue to grow by such a repeated process of

re-oxidation, coupling, and deprotonation. Once the polymer chain length
exceeds the limitation of the solvent solubility, nuclei precipitation happens on
the anode and finally the polymer film is electrodeposited on the surface of the
electrode.[36-38]
(Insert Fig. 4 here)
3.7 Surface morphology
Macroscopically, the as-electrodeposited pyrazine-EDOT hybrid polymer
films are smooth and uniform. Surface morphological examination of these
films by SEM (Fig. 5) demonstrates that the P(2,5-BEP) film shows a relatively
large globular surface, while the surface of the P(2,6-BEP) film is much more
flat and compact even at a high magnification of 50,000 ×. We hypothesize that
such a morphology difference probably results from polymerization process
with varying rates. Namely, 2,6-BEP electropolymerize slowly due to its higher
onset oxidation potential and difficulty in film electrodepostion. In contrast
P(2,5-BEP) is deposited at a faster polymerization rate, thus forms relatively
condensed aggregation structure. Additionally, no obvious difference is
observed between the doped and de-doped polymer films, demonstrating that
the immigration/emigration of counterions during the doping/de-doping
processes do not destroy their original surface morphology of polymer films.
This is commonly observed for EDOT-based conjugated polymers prepared by
electrochemical polymerization.[18, 31, 35]

(Insert Fig. 5 here)
3.8 Spectroelectrochemical analysis
Spectroelectrochemistry of pyrazine-EDOT hybrid polymer films upon
doping/dedoping is systematically investigated in order to evaluate their
electrochromic performances, as shown in Fig. 6a and Fig. S10. P(2,5-BEP)
film reveals an obvious electrochromic phenomenon with reversible color
changes from saturated purple (L*=51.32, a*=9.28, b*=-18.21) in the neutral
state to gray-green (L*=67.74, a*=5.41, b*=-5.75) in the oxidized state (Table
2). In agreement with two broad redox peaks in the CV of P(2,5-BEP) modified
ITO electrode, as the applied potential increases, the stepwise oxidation of the
P(2,5-BEP) film leads to the spectral absorption decrement centered at 550 nm
in the visible region and the emergence of two new broader absorption bands
with peaks at 750 nm and 1300 nm in the near-infrared region.[39] In contrast,
the electrodeposited P(2,6-BEP) film displays two narrow absorption bands at
450 nm and 800 nm in its neutral state (Fig. S10), yet no obvious color changes
can be observed during the transition from the neutral state (L*: 85.58, a*: 7.82,
b*: 18.49) to its oxidized state (L*: 83.20, a*: 7.88, b*: 18.86) upon oxidation
(Table 2). Note here that in comparison with the pyrazine-EDOT precursors
(Fig. 1b), the absorption peaks of both polymers in the neutral state red-shift to
the low energy region due to the increased conjugated polymer backbone (Fig.
6a). The optical bandgap (Eg,opt) of P(2,5-BEP) is calculated to be 1.55 eV,
lower than that of P(2,6-BEP) (1.65 eV). This is likely attributed to the more

effective intramolecular charge transfer resulting from better conjugated
structure in 2,5-BEP-based molecular systems.[9, 21]
(Insert Table 2 and Table 3 here)
3.9 Kinetic studies
Systematic kinetic studies are performed to further evaluate the
electrochromic performance of pyrazine-EDOT hybrid polymers. Note here that
the kinetic results of P(2,6-BEP) are not systematically investigated due to its
inferior electrochromic performances with low optical contrast (2% for 450 nm
and 1% for 800 nm) (Fig. S11) and poor stability, which is in agreement with
the spectroelectrochemical results (only slight color changes between the
neutral state and the oxidized state). From the time-transmittance profile (Fig.
6b), the optical contrast of P(2,5-BEP) film is determined to be 26% at 550 nm
and 22% at 750 nm in the visible region, while in the near infrared region it
reveals much higher contrast of up to 55% at 1300 nm. Additionally, P(2,5-BEP)
can remain high optical contrast against switching time decrement from 10 s
down to 0.5 s (33% at 1300 nm, Fig. 6c). Interestingly, P(2,5-BEP) displays
high optical contrast values and excellent stability against multiple switching
with no obvious fluctuation over the whole near infrared wavelength range (800
~ 2200 nm, Fig. 6 a, b), demonstrating that P(2,5-BEP) can be employed as a
promising candidate for near-infrared electrochromic applications.
Various electrochromic parameters of pyrazine-EDOT hybrid polymers are

calculated based on the time-transmittance profiles. Coloration efficiency (CE),
which is regarded as a valuable standard for evaluating the electrochromic
performances of conjugated polymers, is calculated to be 107 cm2 C-1 at 750
nm and 134 cm2 C-1 at 1300 nm for P(2,5-BEP), while P(2,6-BEP) shows a
much lower CE of only 30 cm2 C-1 at 800 nm (Table 3) due to its poor
performance. The switching time for P(2,5-BEP) to achieve 95% of its optical
contrast is very short (0.2 s from the neutral state to the oxidized state at 1300
nm and 0.4 s from the oxidized state to the neutral state at 750 nm), also much
faster than P(2,6-BEP) (0.7 s in the 450nm) (Table 3) and even comparable to
the best values of conjugated polymers reported so far.[40, 41]
Open-circuit memory tests, a parameter for electrochromic materials to
maintain their oxidized state or neutral state without external potentials,
demonstrate that the transmittance of P(2,5-BEP) at varying wavelengths (1300
nm, Fig. 6d; 750 nm, Fig. S12; 550 nm, Fig. S13) remains almost unchanged in
the neutral state -0.2 V, while a mild fluctuation occurs in the oxidized state 1.4
V (about 2% transmittance lost under the open-circuit state for 100 s), revealing
that P(2,5-BEP) has excellent optical memory in both neutral states and
oxidized state. Overall, compared to its analog P(2,6-BEP), P(2,5-BEP) exhibits
better electrochromic performances with high optical contrast, decent coloration
efficiency, faster switching time, as well as applicable in a wide wavelength
range in the near infrared region.
(Insert Fig. 6 here)

3.10 Flexible ECD with patterns
To further evaluate the electrochromic performance of P(2,5-BEP) in real
applications, we fabricate patterned flexible electrochromic devices with the
configuration of: PET/ITO/patterned P(2,5-BEP) electrochromic layer/gel
electrolyte/ITO/PET. The colored elephant can be facilely patterned via direct
electropolymerization of 2,5-BEP with high resolution. After sealing,
as-fabricated flexible ECD can realize reversible color changing between purple
and gray with a decent optical contrast of 26%. Upon repetitive potential
cycling between the neutral and oxidized states, these devices exhibit good
stability with less than 25% optical contrast reduction after 1000 successive
cycling (Fig. 7a). Furthermore, such flexible ECDs can also withstand
long-term mechanical torturing with around 9.6% reduction after bending 5000
cycles (Fig. 7b). The excellent optical stability against both potential switching
and mechanical bending of these flexible ECDs can guarantee their real
applications as NIR electrochromic devices.
(Insert Fig. 7 here)
4. Conclusions
In this work, we design two novel isomeric pyrazine-EDOT precursors by
Stille coupling and comparatively investigate the structure and optoelectronics
properties of the precursors as well as their corresponding hybrid polymers via
electropolymerization, including electrochemical, spectral, morphological, and

electrochromic properties. We find that P(2,5-BEP), which can be facilely
electropolymerized at lower oxidation potentials, exhibits excellent redox
stability (remaining 70% of its electroactivity even after 5000 cycles) and much
better electrochromic performance with higher CE value (up to 134 cm2 C-1),
superior optical contrast (up to 55%) in the near-infrared region (1300 nm).
Based on these findings, we successfully fabricate flexible ECDs with direct
electrochemical patterning of P(2,5-BEP) film, and the resultant devices display
good switching stability (25% reduction after scanning 1000 cycles) and
superior mechanical stability under repeatedly bending (only 9.6% reduction
after bending 5000 cycles). This work provides an effective method to design
high performance isomeric electrochromic polymers and presents a simple way
to fabricate flexible ECDs via electropolymerizationn patterning that may better
satisfy more sophisticated and futuristic needs for deformable electrochromic
products and applications.
Declaration of Competing Interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science Foundation of China
(grant numbers 51763010, 51963011); the Technological Expertise and Academic
Leaders Training Program of Jiangxi Province (grant number 20194BCJ22013),
Double Thousand Talents Plan in Jiangxi Province-Domestic Youth Program; and the

Research Project of State Key Laboratory of Mechanical System and Vibration (grant
number MSV202013). F. H. thanks Jiangxi Science & Technology Normal University
for a Postgraduate Innovation Program grant (grant number YC2019-X16).
Notes and references
[1] P.M. Beaujuge, C.M. Amb, J.R. Reynolds, A side-chain defunctionalization
approach yields a polymer electrochrome spray-processable from water, Adv Mater,
22 (2010) 5383-5387.
[2] P.M. Beaujuge, S. Ellinger, J.R. Reynolds, The donor-acceptor approach allows a
black-to-transmissive switching polymeric electrochrome, Nature Materials, 7 (2008)
795-799.
[3] M.A.I. Michael T Otley, Gregory A Sotzing, Fabric Electrochromic Displays for
Adaptive Camouflage, Biomimicry, Wearable Displays and Fashion, (2015) 503-524.
[4] A.L.-S. Eh, A.W.M. Tan, X. Cheng, S. Magdassi, P.S. Lee, Recent Advances in
Flexible Electrochromic Devices: Prerequisites, Challenges, and Prospects, Energy
Technology, 6 (2018) 33-45.
[5] X. Ju, L. Kong, J. Zhao, G. Bai, Synthesis and electrochemical capacitive
performance of thieno 3,4-b pyrazine-based Donor-Acceptor type copolymers used as
supercapacitor electrode material, Electrochimica Acta, 238 (2017) 36-48.
[6] W. Li, Y. Guo, J. Shi, H. Yu, H. Meng, Solution-Processable Neutral Green
Electrochromic Polymer Containing Thieno 3,2-b thiophene Derivative as

Unconventional Donor Units, Macromolecules, 49 (2016) 7211-7219.
[7] B.P. Leliege A, Rousseau T, et al., Triphenylamine/tetracyanobutadiene-based
DAD π-conjugated systems as molecular donors for organic solar cells, Organic
letters, 13 (2011) 3098-3101.
[8] J. Han, Y. Chen, W. Chen, C. Yu, X. Song, F. Li, Y. Wang, High Performance
Small-Molecule Cathode Interlayer Materials with D-A-D Conjugated Central
Skeletons and Side Flexible Alcohol/Water-Soluble Groups for Polymer Solar Cells,
ACS Appl Mater Interfaces, 8 (2016) 32823-32832.
[9] S. Ming, S. Zhen, X. Liu, K. Lin, H. Liu, Y. Zhao, B. Lu, J. Xu,
Chalcogenodiazolo 3,4-c pyridine based donor-acceptor-donor polymers for green and
nearinfrared electrochromics, Polymer Chemistry, 6 (2015) 8248-8258.
[10] C.K. Lo, D.E. Shen, J.R. Reynolds, Fine-Tuning the Color Hue of π-Conjugated
Black-to-Clear Electrochromic Random Copolymers, Macromolecules, 52 (2019)
6773-6779.
[11] S.C. Kaiwen Lin, Baoyang Lu, Jingkun Xu, Hybrid π-conjugated polymers from
dibenzo pentacyclic centers: precursor design, electrosynthesis and electrochromics,
Science China Chemistry, 60 (2017) 38-53.
[12] K.P. Dillow G W, Electron affinities of aza-substituted polycyclic aromatic
hydrocarbons, Canadian journal of chemistry,, 67 (1989) 1628-1631.
[13] X. Song, L. Kong, H. Du, X. Li, H. Feng, J. Zhao, Y. Xie, Effects of Pyrazine

Derivatives and Substituted Positions on the Photoelectric Properties and
Electromemory Performance of D-A-D Series Compounds, Materials, 11 (2018).
[14] S. Ming, S. Zhen, K. Lin, L. Zhao, J. Xu, B. Lu, Thiadiazolo 3,4-c pyridine as an
Acceptor toward Fast-Switching Green Donor-Acceptor-Type Electrochromic
Polymer with Low Bandgap, Acs Applied Materials & Interfaces, 7 (2015)
11089-11098.
[15] S. Tarkuc, E.K. Unver, Y.A. Udum, C. Tanyeli, L. Toppare, The effect of changes
in pi-conjugated terthienyl systems using thienyl and ethylenedioxybenzene
functionalized thieno 3,4-b pyrazine precursors: Multicolored low band gap polymers,
Electrochimica Acta, 55 (2010) 7254-7258.
[16] H. Zhao, Y. Wei, J. Zhao, M. Wang, Three donor-acceptor polymeric
electrochromic materials employing 2,3-bis(4-(decyloxy)phenyl)pyrido 4,3-b
pyrazine as acceptor unit and thiophene derivatives as donor units, Electrochimica
Acta, 146 (2014) 231-241.
[17] C.L. Chochos, S.A. Choulis, How the structural deviations on the backbone of
conjugated polymers influence their optoelectronic properties and photovoltaic
performance, Progress in Polymer Science, 36 (2011) 1326-1414.
[18] B. Lu, S. Zhen, S. Zhang, J. Xu, G. Zhao, Highly stable hybrid
selenophene-3,4-ethylenedioxythiophene
as
electrically
conducting
and
electrochromic polymers, Polym. Chem., 5 (2014) 4896-4908.

[19] F. Hu, W. Zhang, Y. Xue, K. Lin, B. Lu, J. Xu, G. Zhao, Tuning optoelectronic
performances for 3-methylselenophene-EDOT hybrid polymer, Materials Chemistry
and Physics, 244 (2020) 122699.
[20] H. Gu, S. Ming, K. Lin, S. Chen, X. Liu, B. Lu, J. Xu, Isoindigo as an
electron-deficient unit for high-performance polymeric electrochromics,
Electrochimica Acta, 260 (2018) 772-782.
[21] K. Lin, S. Ming, S. Zhen, Y. Zhao, B. Lu, J. Xu, Molecular design of DBT/DBF
hybrid thiophenes pi-conjugated systems and comparative study of their
electropolymerization and optoelectronic properties: from comonomers to
electrochromic polymers, Polymer Chemistry, 6 (2015) 4575-4587.
[22] O. Bohnke, Gel Electrolyte for Solid-State Electrochromic Cell, Journal of The
Electrochemical Society, 139 (1992) 1862.
[23] Y. Bar-Cohen, S.E. Legenski, C. Xu, L. Liu, M.O. Le Guilly, M. Taya, Gel
electrolyte candidates for electrochromic devices (ECDs), Smart Structures and
Materials 2004: Electroactive Polymer Actuators and Devices (EAPAD). International
Society for Optics and Photonics, (2004), 319.
[24] T.T. Steckler, K.A. Abboud, M. Craps, A.G. Rinzler, J.R. Reynolds, Low band
gap EDOT-benzobis(thiadiazole) hybrid polymer characterized on near-IR
transmissive single walled carbon nanotube electrodes, Chem Commun, (2007)
4904-4906.

[25] B. Lu, S. Ming, K. Lin, S. Zhen, H. Liu, H. Gu, S. Chen, Y. Li, Z. Zhu, J. Xu,
1,2,5 Chalcogenodiazolo 3,4-c pyridine and selenophene based donor-acceptor-donor
electrochromic polymers electrosynthesized from high fluorescent precursors, New
Journal of Chemistry, 40 (2016) 8316-8323.
[26] G.L. Gibson, T.M. McCormick, D.S. Seferos, Atomistic band gap engineering in
donor-acceptor polymers, J Am Chem Soc, 134 (2012) 539-547.
[27] N. Jian, H. Gu, S. Zhang, H. Liu, K. Qu, S. Chen, X. Liu, Y. He, G. Niu, S. Tai, J.
Wang, B. Lu, J. Xu, Y. Yu, Synthesis and electrochromic performances of
donor-acceptor-type polymers from chalcogenodiazolo 3,4-c pyridine and alkyl
ProDOTs, Electrochimica Acta, 266 (2018) 263-275.
[28] J.K.E. Kerszulis J A, Kuepfert M, et al., Tuning the painter's palette: subtle steric
effects on spectra and colour in conjugated electrochromic polymers, Journal of
Materials Chemistry C, 3 (2015) 3211-3218.
[29] X. Zhao, C. Chen, Q. Sun, Y. Li, H. Yu, Molecular structure optimization design
of inhibitors based on frontier orbitals theory, Applied Surface Science, 494 (2019)
895-907.
[30] Z. Feng, D. Mo, Z. Wang, S. Zhen, J. Xu, B. Lu, S. Ming, K. Lin, J. Xiong,
Low-potential electrosynthesis of a novel nitrogen analog of PEDOT in an ionic
liquid and its optoelectronic properties, Electrochimica Acta, 160 (2015) 160-168.
[31] B. Lu, S. Zhang, L. Qin, S. Chen, S. Zhen, J. Xu, Electrosynthesis of

poly(3,4-ethylenedithiathiophene) in an ionic liquid and its electrochemistry and
electrochromic properties, Electrochimica Acta, 106 (2013) 201-208.
[32] I.T. Li C, Electrochemical and optical properties of the poly (3,
4-ethylenedioxythiophene) film electropolymerized in an aqueous sodium dodecyl
sulfate and lithium tetrafluoroborate medium, Macromolecules, 37 (2004) 2411-2416.
[33] J. Xu, J. Hou, S. Zhang, G. Nie, S. Pu, L. Shen, Q. Xiao, Electrosyntheses of
high quality freestanding polyselenophene films in boron trifluoride diethyl etherate,
Journal of Electroanalytical Chemistry, 578 (2005) 345-355.
[34] B. Dong, Y. Xing, J. Xu, L. Zheng, J. Hou, F. Zhao, Electrosyntheses of
free-standing and highly conducting polyselenophene films in an ionic liquid,
Electrochimica Acta, 53 (2008) 5745-5751.
[35] B. Lu, J. Yan, J. Xu, S. Zhou, X. Hu, Novel Electroactive Proton-Doped
Conducting Poly(aromatic ethers) with Good Fluorescence Properties via
Electropolymerization, Macromolecules, 43 (2010) 4599-4608.
[36] G. Sabouraud, S. Sadki, N. Brodie, The mechanisms of pyrrole
electropolymerization, Chemical Society Reviews, 29 (2000) 283-293.
[37] B. Lu, L. Zeng, J. Xu, Z. Le, H. Rao, Electrosynthesis of highly conducting
poly(1,5-dihydroxynaphthalene) in BF3·Et2O, European Polymer Journal, 45 (2009)
2279-2287.
[38] J.X. Baoyang Lu, Changli Fan, Huaming Miao, and Liang Shen, Electrochemical