A new multicolored and near-infrared electrochromic material based on triphenylamine-containing poly(3,4-dithienylpyrrole)

Article abstract of DOI:10.1016/j.orgel.2014.10.033

A new compound containing both 3,4-dithienylpyrrole (DTP) and triphenylamine (TPA) groups, namely, 4′-(2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl)-N,N-diphenylbiphenyl-4-amine (DTP-Ph-TPA), was designed and synthesized. The polymer poly-DTP-Ph-TPA (PDTP-Ph-TPA)

Full text of DOI:10.1016/j.orgel.2014.10.033

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Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
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A new multicolored and near-infrared electrochromic material
based on triphenylamine-containing poly(3,4-dithienylpyrrole)
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⇑
Jian-Cheng Lai, Xin-Rong Lu, Bo-Tao Qu, Feng Liu, Cheng-Hui Li , Xiao-Zeng You
7 Q1
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State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures Nanjing
University, Nanjing 210093, People’s Republic of China
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a r t i c l e i n f o
a b s t r a c t
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Article history:
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A new compound containing both 3,4-dithienylpyrrole (DTP) and triphenylamine (TPA)
groups, namely, 40-(2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl)-N,N-diphenylbiphenyl-4-amine
(DTP-Ph-TPA), was designed and synthesized. The polymer poly-DTP-Ph-TPA (PDTP-Ph-
TPA) was prepared by electropolymerization from DTP-Ph-TPA. When the applied poten-
tial circulates from 0.0 V to 1.4 V, the polymer not only exhibits reversible multicolor in the
visible region (yellow, light green, magenta and blue), but also shows excellent electro-
Received 18 September 2014
Received in revised form 20 October 2014
Accepted 21 October 2014
Available online xxxx
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Keywords:
Electrochromic
Near-IR
Poly(3,4-dithienylpyrrole)
Triphenylamine
Device
chromic properties in the NIR region with high contrast ratio (DT = 70.5% in 1550 nm,
DT = 67.9% in 1310 nm) and a very short response time (about 1.4 s for 1550 nm, 0.9 s
for 1310 nm). A single layer electrochromic device (ECD) based on polymer PDTP-Ph-
TPA was constructed and characterized.
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Ó 2014 Published by Elsevier B.V.
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1. Introduction
(PProDOTs) which showed essentially no color change in
the visible region [9]. Wang and Wan reported a quinone-
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Electrochromic materials, which exhibit reversible
absorption spectral changes by changing the applied poten-
tials, have attracted much attention in recent years [1–8].
Originally, the research of electrochromic materials was
mainly focused on the optical changes in the visible region
(e.g., 350–750 nm), which had variable applications such as
smart windows, E-paper, optical switching devices, vari-
able reflectance mirrors, and camouflage materials. Later,
efforts has also been made on near-infrared region (NIR;
e.g., 750–2000 nm), which are useful in optical fiber-based
telecommunication (specifically, 850 nm, 1310 nm and
1550 nm), optical attenuator, data storage, thermal control
and aerospace and military camouflage [9–16]. Reynolds
demonstrated color-to-transmissive NIR electrochromic
conjugated polymers poly(3,4-propylenedioxythiophene)s
containing electrochromic materials, which showed high
absorption in the NIR region in the range of 700–1100 nm
by electrochemical reduction [10]. Liou made efforts on
the triarylamine-containing electroactive aramids, which
revealed highly stable electrochromism and high contrast
ratio between NIR and visible light region [11–14]. How-
ever, so far there is less work on electrochromic materials
that can function in both the UV–Vis and near-IR region
[4], which makes it possible to monitor the near-IR electro-
chromic process through naked eye. Actually, it is a chal-
lenge to obtain a material that can be switched from
highly transmissive state to highly absorbed state in a wide
range from UV–Vis to near-IR region.
Typical electrochromic materials are coordination com-
plexes, transition-metal oxides, organic molecules and
conjugated polymers [4–6]. Among these different types
of electrochromic materials, conjugated polymers have
several advantages, such as multiple hues within the same
material, absorption spectral tunability through structural
⇑
Corresponding authors.
E-mail addresses: chli@nju.edu.edu (C.-H. Li), youxz@nju.edu.cn
(X.-Z. You).
http://dx.doi.org/10.1016/j.orgel.2014.10.033
1566-1199/Ó 2014 Published by Elsevier B.V.
Please cite this article in press as: J.-C. Lai et al., A new multicolored and near-infrared electrochromic material based on triphenylamine-
containing poly(3,4-dithienylpyrrole), Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.10.033

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modification, flexibility, easy processability, fast switching,
high optical contrast ratio and coloration efficiency [2].
Polythiophene, polypyrrole, polyaniline, poly(3,4-ethyl-
enedioxythiophene) (PEDOT), poly(3,4-dithienylpyrrole)
(PDTP), and their derivatives are the typical electrochromic
conjugated polymers reported in literature [4,7]. Particu-
larly, PDTP and its derivatives have become one of the
most important electrochromic materials during the past
20 years [17–21]. They have many advantages over other
conjugated polymers such as lower oxidation potentials
(about 0.7 V vs SCE), sufficient electron density and good
hole-transporting ability, and better film-forming proper-
ties. If the substituent group on the N-position were mod-
ified, they can show more electrochromic hues ranging
from yellow [21,22], green [23], magenta [24], and cyan
[25], to blue [23]. So far, most of the PDTP derivatives
can show electrochromism only in the UV–Vis region,
electrochromic PDTP polymers which can exhibit revers-
ible absorption changes in both the near-IR region and
UV–Vis region are scarce [4].
In the literature, two groups were typically introduced
to induce near-IR electrochromic properties. One is
triphenylamine (TPA) and its derivatives, which are well-
known electron-rich compounds and have been widely
used as electron-donating and hole-transporting materials
in the fields of photovoltaics [26] and electrochromism
[27]. Because triphenylamines can easily be oxided to form
TPA cationic radicals with an obvious change of absorption
spectrum in the NIR region, it has been reported that TPA
can be used to improve the electrochromic performance
in the NIR regions [28]. The other one is p-diphenylenedi-
amine-containing molecules, which are also excellent ano-
dic electrochromic systems for NIR applications because of
its particular low energy intramolecular charge transfer
under the oxidized states. Moreover, the p-diphenylenedi-
amine cation radical has been reported as a symmetrical
delocalized class III structure with a strong electronic cou-
pling, leading to an intervalence charge transfer absorption
band in the NIR region [29–31].
In this work, we designed and synthesized a new electro-
chromic material by substitution of N-position of DTP unit
with Ph-TPA group. The novel TPA-containing DTP derives
monomer, 40-(2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl)-N,N-
diphenylbiphenyl-4-amine (DTP-Ph-TPA; see Scheme 1),
was synthesized by Knorr–Paal reaction and SUZUKI cou-
pling reaction, and the corresponding polymer was obtained
by electrochemical polymerization. The polymer PDTP-Ph-
TPA is expected to have excellent electrochromic properties
in the visible and NIR region because the DTP section has the
variable hues while the TPA section can enhances the NIR
electrochromic performance due to its electron sufficiency
and hole-transporting properties. Moreover, the two nitro-
gen atoms in the DTP and TPA groups are connected by
diphenylene moiety thus forming a p-diphenylenediamine
unit, which could further strengthen the absorption spectral
in NIR region. Here we demonstrate that the polymer PDTP-
Ph-TPA exhibits reversible multicolor in the visible region
(yellow, light green, magenta and blue) when the applied
potential circulates from 0.0 V to 1.4 V. Furthermore, it also
shows excellent electrochromic properties in the NIR region
in 1310 nm) and a very short response time (about 1.4 s for
1550 nm, 0.9 s for 1310 nm). Such features are very intrigu-
ing in electrochromic materials for wide applications like
data storage, optical attenuators, and thermal control (heat
gain or loss) in buildings and spacecrafts [32,33].
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2. Experiments
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2.1. Materials and instrumentation
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All reagents and solvents were purchased from com-
mercial sources and used as received without further
purification. The solvent used for the electrochemical mea-
surements were purchased from Sigma Aldrich with the
purity of HPLC, which include dichloromethane (DCM)
and acetonitrile (ACN). The ITO substrates as the working
electrode were purchased from Kaivo with the resistance
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(X) below 10 X/sq. Mass spectra were acquired on a time
of flight mass spectrometer. 1H NMR and 13C NMR spectra
were recorded using a Bruker spectrometer at 500 MHz
using TMS as an internal standard. Elemental analyses of
the compounds were performed on a Perkin-Elmer 240
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analyzer. UV–Vis–NIR spectra were measured on
a
UV-3600 spectrophotometer. The absorption to the mono-
mer was recorded in DCM with the concentration of
10ꢀ5 M, and the optical band gap (Eg) of the monomer
and polymer was calculated from their low-energy absorp-
tion edge (konset) (Eg = 1241/konset). The cyclic voltammetry
(CV) measurement was performed by CHI-660D electro-
chemical workstation. The measurements were carried
out under an argon atmosphere, and the electrochemical
three-electrode cell includes a Pt disk (d = 0.2 cm) or ITO
(Kaivo, <10
X
/sq, 9 mm ꢁ 50 mm) as the working elec-
trode, Pt wire as counter electrode and the Ag wire as the
quasi-reference electrode which was calibrated vs Fc/Fc+
to be 0.62 V in DCM solution. 0.1 M Bu4NClO4 dissolved
in DCM was used as electrolyte solution. HOMO and LUMO
energy levels of the polymer were calculated according to
the Ferroence/Ferrocenium standard redox couple
E(Fc/Fc+) = 0.62 V (vs Ag wire) in DCM by using the formula
EHOMO = ꢀe(EOX1 ꢀ EFc) + (ꢀ0.62 eV) [23], and LUMO =
HOMO ꢀ Eg.
To perform the spectroelectrochemical measurements
and electrochromic switching studies, the polymer films
were deposited on the ITO by electrochemical polymeriza-
tion. The measurements were carried out by a spectro-
electrochemical cell which consists of a quartz cell with
an Ag wire, a Pt wire, and an ITO as the transparent work-
ing electrode. 0.1 M Bu4NClO4 dissolved in DCM was used
as electrolyte solution during all the measurement.
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2.2. Synthetic procedure
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The synthesis of the monomer DTP-Ph-TPA and corre-
sponding polymer P-DTP-Ph-TPA is outlined in Scheme 1.
The monomer DTP-Ph-TPA was prepared from Paal–Knorr
reaction [34,35] and subsequent Suzuki coupling reaction.
Succinyl chloride, 4-bromoaniline, Pd(PPh3)4, 4-(diphenyl-
amino) phenylboronic acid was purchased from Alfa-Aesar,
and used as received. Polymer PDTP-Ph-TPA was prepared
by electrochemical polymerization.
with high contrast ratio (DT = 70.5% in 1550 nm, DT = 67.9%
Please cite this article in press as: J.-C. Lai et al., A new multicolored and near-infrared electrochromic material based on triphenylamine-
containing poly(3,4-dithienylpyrrole), Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.10.033

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Scheme 1. Synthetic route to the monomer DTP-Ph-TPA and polymer PDTP-Ph-TPA and the characters of the compound DTP-Ph-TPA: (a) (CH2COCl)2,
AlCl3, DCM, rt, 6 h; (b) 4-bromoaniline, p-toluenesulfonic acid monohydrate, toluene, Na2SO4, reflux, 36 h; and (c) 4-(diphenylamino)phenylboronic acid,
Pd(PPh3)4, toluene, reflux, 24 h.
2.2.1. Synthesis of 1,4-bis(20-thienyl)-1,4-butanedione (1)
To a suspension of AlCl3 (32 g, 0.24 mol) in 90 mL of
DCM, a solution of 11 mL (0.1 mol) succinyl chloride and
19.2 mL (0.24 mol) of thiophene in 30 mL DCM was added
dropwise at room temperature. The suspension was stirred
for 6 h and poured into a mixture of 10 mL of hydrochloric
acid and 400 g of ice. The color of the suspension turned
from red to dark green after further stirring for 30 min.
The organic phase was collected and the aqueous phase
was extracted with DCM. The organic phases was combined
and washed with water and saturated aqueous NaHCO3,
dried over NaSO4, concentrated in vacuo, and purified by
column chromatography (eluent:DCM:hexane = 1:1) to
afford 14.5 g (58%) of 1 as pale blue crystals. 1H NMR
(500 MHz, CDCl3, d, ppm): 7.82 (dd, 2H), 7.65 (dd, 2H),
7.15 (dd, 2H), 3.40 (s, 4H) (Fig. S1). MS (EI) (m/z): [M]+ calcd
for C12H10O2S2, 250.0; found, 250.1.
d, ppm): 7.54 (dd, 2H), 7.17 (d, 2H), 7.10 (dd, 2H), 6.85 (dd,
2H), 6.56 (dd, 2H), 6.54 (s, 2H) (Fig. S2). MS (EI) (m/z): [M]+
calcd for C18H12NS2Br, 385.0; found, 385.1.
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2.2.3. Synthesis of DTP-Ph-TPA
Compound
2 (0.386 g, 1 mmol), 4-(diphenylamino)
phenylboronic acid (0.347 g, 1.2 mmol) and Pd(PPh3)4
(40 mg) were added to a mixture of 2 M K2CO3 (15 mL)
and toluene (20 mL) under argon. The reaction mixture
was heated to reflux for 10 h under stirring. After cooled
to room temperature and extracted with DCM
(3 ꢁ 50 mL), the combined organic phase was washed with
water (50 mL) and saturated brine (50 mL). The organic
phase was separated and dried over Na2SO4. The solvent
was removed under reduced pressure and the residue was
purified by column chromatography with DCM:hex-
ane = 1:4 as eluent. The final product was obtained as pale
yellow crystals (0.41 g, 74%). 1H NMR (500 MHz, CDCl3, d,
ppm): 7.65 (d, 2H), 7.56 (d, 2H), 7.33 (d, 2H), 7.28 (dd,
4H), 7.14 (d, 2H), 7.12 (d, 2H), 7.10 (d, 2H), 7.05 (dd, 4H),
6.81 (dd, 2H), 6.60 (d, 2H), 6.53 (s, 2H) (Fig. S3); 13C NMR
(500 MHz, CDCl3, d, ppm): 135.00, 133.45, 130.28, 129.37,
127.77, 127.03, 126.97, 124.55, 124.29, 124.00, 123.85,
123.18, 109.88 (Fig. S4); MS (EI) (m/z): [M]+ calcd for
C36H26N2S2, 550.1; found, 550.0. Anal. calcd for
C36H26N2S2: C, 78.51; H, 4.76; N, 5.09; S, 11.64. Found: C,
78.15; H, 4.98; N, 4.96; S, 11.76.
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2.2.2. Synthesis of 1-(4-bromo-phenyl)-2,5-di-thiophene-2-
yl-1H-pyrrole (2)
A three-neck round-bottomed flask equipped with a
Dean-Stark trap, an argon inlet and a reflux condenser,
was charged with 1.5 g (6 mmol) compound 1, 3.01 g
(18 mmol) of 4-bromoaniline, 1.25 g (6.6 mmol) of p-tolu-
enesulfonic acid monohydrate, 2.5 g Na2SO4, and 50 mL tol-
uene. The reaction mixture was stirred and refluxed for
36 h under argon. After cooling to room temperature, tolu-
ene was removed by evaporation under reduced pressure.
The residue was dissolved in DCM and washed with water
and saturated brine. The organic phase was then collected
and evaporated under reduced pressure, followed by flash
column chromatography (SiO2 column, elution with ethyl
acetate:hexane = 1:5). The desired product was obtained
as pale yellow solid (1.8 g, 78%). 1H NMR (500 MHz, CDCl3,
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2.2.4. Synthesis of polymer PDTP-Ph-TPA
The polymerization of the monomer was performed in
an electrochemical cell with the presence of 20 mg
DTP-Ph-TPA and 0.1 M Bu4NClO4 in 10 ml DCM. The elec-
trochemical cell was equipped with Pt disk (d = 0.2 cm) or
ITO glass (whole area was 9 mm ꢁ 50 mm and the active
Please cite this article in press as: J.-C. Lai et al., A new multicolored and near-infrared electrochromic material based on triphenylamine-
containing poly(3,4-dithienylpyrrole), Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.10.033

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area was 9 mm ꢁ 30 mm) as the working electrode, Pt wire
as counter electrode and the Ag wire as the pseudo-refer-
ence electrode (calibrated vs Fc/Fc+). Before the polymeri-
zation, argon was passing through the electrochemical
cell for 30 min. Cyclic voltammetry was run between
0.0 V and 1.4 V (vs Fc/Fc+) for 30 cycles at room tempera-
ture under argon atmosphere. As the polymerization pro-
ceeded, the solution became colorful and the uniform
film PDTP-Ph-TPA whose color was reversible changing
with the change of potential was deposited on the working
electrode. After the polymerization, the polymer film
deposited on the Pt disk or ITO glass was washed with
DCM for three times to remove unreacted monomer and
residue electrode and then stored in argon.
After washed with DCM and dried by argon, the gel electro-
lyte based on PC/PMMA/LiClO4/ACN in a 20:7:3:70 weight
ratio was coated on the electrochromic layer (Scheme 2c).
The electrochromic and gel electrolyte layers were then
surrounded by the Surlyn film (Scheme 2d) and capped
with another ITO-coated glass. The two pieces of glass
were stuck to each other with Surlyn film by heat sealing,
thus forming the typical sandwich structured electrochro-
mic device (Scheme 2e). The cross-section of the electro-
chromic device was show in Scheme 2f.
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3. Results and discussions
3.1. Synthesis and characterization
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2.3. X-ray structure determination
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The new electroactive monomers DTP-Ph-TPA which
contains TPA as hole transport segment was successfully
synthesized by SUZUKI cross coupling reaction of the
bromo-compound 2 and 4-(diphenylamino) phenylboronic
acid in the presence of Pd(PPh3)4 as catalyst (Scheme 1).
The reaction was carried in 2 M K2CO3 and toluene under
inertia condition. Compound 2 was synthesized according
to the reference via Paal–Knorr reaction of compound 1
with 4-Bromoaniline with the presence of p-toluenesul-
fonic acid as catalyst and dehydrant [34]. In order to accel-
erate the reaction speed, anhydrous sodium sulfate was
added to the reaction mixture. Moreover, a small amount
of anhydrous cupric sulfate was added in the Dean-Stark
apparatus to track the process of the reaction. Compound
1 was synthesized according to the reference as mentioned
above.
The chemical structures of the compound 1, 2 and
DTP-Ph-TPA were elucidated by 1H NMR and EI-MS. The
crystal structure of DTP-Ph-TPA was determined by sin-
gle-crystal X-ray diffraction measurements. DTP-Ph-TPA
crystallizes in orthorhombic crystal system with space
group of Pbcn. The molecular structure and crystal packing
diagram of DTP-Ph-TPA are shown in Fig. 1. The crystallo-
graphic data was summarized in Table S1, and the bond
distances and bond angles were summarized in Table S2.
The thienyl moieties are disorder due to their rotation
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Yellow needle crystals of complex DTP-Ph-TPA were
grown in acetonitrile/dichloromethane (v/v = 1:1) solution
at room temperature. Single-crystal X-ray diffraction mea-
surements were carried out on a Bruker SMART APEX CCD
based on a diffractometer operating at room temperature.
Intensities were collected with graphite monochromatized
Mo Ka radiation (k = 0.71073 Å) operating at 50 kV and
30 mA, in
x/2h scan mode. The data reduction was made
with the Bruker SAINT package [36]. Absorption correc-
tions were performed using the SADABS program [37].
The structures were solved by direct methods and refined
on F2 by full-matrix least-squares using SHELXL-97 with
anisotropic displacement parameters for all non-hydrogen
atoms in all two structures. Hydrogen atoms bonded to the
carbon atoms were placed in calculated positions and
refined as riding mode, with C–H = 0.93 Å (methane)
or 0.96 Å (methyl) and Uiso(H) = 1.2 Ueq(Cmethane
) or
Uiso(H) = 1.5 Ueq(Cmethyl). The water hydrogen atoms were
located in the difference Fourier maps and refined with
an O–H distance restraint [0.85(1) Å] and Uiso(H) = 1.5
Ueq(O). All computations were carried out using the
SHELXTL-97 program package [38]. CCDC 1022982 contain
the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
along the C2–C3 single bond (Fig. 1a). There is no
p–p
stacking interaction between the neighboring molecules
due to the large steric hindrance of the nonplanar triphen-
ylamine groups.
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2.4. Fabrication of the electrochromic devices
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The structure of the electrochromic device is similar to
the literature as reported previously [39–42]. First, a piece
of ITO glass (50 mm ꢁ 15 mm) was etched with zinc pow-
der and 2 M HCl to get the desired pattern. The etched ITO
glass was then washed carefully with DI-water, isopropa-
nol and acetone (Scheme 2a). Oxygen plasma was subse-
quently used to treat the ITO glass for 10 min. The
uniform film PDTP-Ph-TPA was deposited electrochemi-
cally on the ITO substrate with a DCM solution containing
2 mg/mL DTP-Ph-TPA and 0.1 M Bu4NClO4 in an electro-
chemical cell. Pt wire was used as counter electrode and
the Ag wire as the pseudo-reference electrode. The cyclic
voltammetry experiment was taken at the potential
between 0.0 V and 1.4 V (vs Fc/Fc+) for several cycles at
room temperature under argon atmosphere (Scheme 2b).
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3.2. Electrochemical properties of the DTP-Ph-TPA
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The electrochemical properties of the monomer
DTP-Ph-TPA was investigated by CV (Fig. 2a) and differen-
tial pulse voltammetry (DPV) (Fig. S5). The CV data was
listed in Table 1. During the anodic scan, the monomer
DTP-Ph-TPA exhibits two oxidation peaks at Eox1 = 0.98 V
and Eox2 = 1.27 V vs Ag wire. The first oxidation peak at
0.98 V was attributed to the radical cation (TPA+) forma-
tion which was consistent with the reported TPA deriva-
tives [43]. If the anodic potential window was limited to
1.1 V, only one couple of stable quasi-reversible redox peak
appeared. The intensity of the peak showed no increase
under repetitive cycling. Furthermore, no film was formed
Please cite this article in press as: J.-C. Lai et al., A new multicolored and near-infrared electrochromic material based on triphenylamine-
containing poly(3,4-dithienylpyrrole), Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.10.033

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Scheme 2. Process for preparing the single layer solid-state electrochromic devices.
Fig. 1. The molecular structure (a) and crystal packing diagram (b) of DTP-Ph-TPA.
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395
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398
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400
401
402
403
404
405
406
373
374
375
376
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379
380
381
382
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384
385
386
Fc/Fc+ is 4.80 eV. The results are listed in Table 1. The
HOMO energy level of the DTP-Ph-TPA was 5.01 eV.
To evaluate the adhesive ability of the polymer PDTP-
Ph-TPA film deposited on the electrode, the peak currents
intensities were recorded as a function of scan rates by
cyclic voltammetry between 0.0 V and 1.5 V (Fig. 2b).
Evidently, the peak currents intensities of the polymer
were increased steadily by promoting the scan rates. A lin-
ear dependence of the peak currents of anodic and cathodic
as a function of scan rates was obtained, and the least
square fit were R = 0.9942, R = 0.9915, respectively
(Fig. 2c), indicating that the polymer PDTP-Ph-TPA film
can firmly adhere to the electrode.
on the electrode surface via either repetitive cycling or
constant potential electrolysis. All these observation are
proofs that this couple of peaks belongs to the redox of
the TPA unit. When the anodic potential increased to
1.4 V,
a new couple of quasi-reversible redox peaks
appeared at 1.27 V which can be ascribed to the radical
cation formation of DTP unit [22,23]. The negative shift
of the potential as compared to other N-substituted DTP
derivatives is due to the electron-donating character of
the TPA unit [17]. Under repetitive cycling, the intensity
of the peaks increased rapidly and there was a colorful film
deposited on the electrode surface (Fig. 3a). The energy
levels of the HOMO and LUMO of the investigated mono-
mer and polymers can be estimated from the oxidation
onset Eonset according to the following equation
387
388
390
407
3.3. Electrochemical polymerization of the DTP-Ph-TPA
HOMO ¼ Eonset ꢀ EonsetðFc=FcþÞ þ HOMOðFc=FcþÞ
where the Eonset(Fc/Fc+) refers to the Ferroence/Ferroce-
nium standard redox couple and has a value of 0.62 V (vs
Ag wire) in DCM, the standard HOMO energy level of
408
409
410
411
The electrochemical polymerization of the homopoly-
mer was performed via cyclic voltammetry technique
between 0.0 V and 1.4 V, ꢀ0.2 V and 1.5 V (vs Ag wire).
The polymerization process was carried out in a reaction
391
392
393
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(a)
(b)
(c)
Fig. 2. (a) Cyclic voltammograms of DTP-Ph-TPA in 0.1 M TBAP/DCM on a platinum electrode vs Ag wire at a scan rate of 100 mV/s (the applied potential of
dash line was between 0.0 V and 1.1 V, solid line was between 0.0 V and 1.4 V); (b) scan rate dependence of PDTP-Ph-TPA film on a platinum electrode in
0.1 M TBAP/DCM at different scan rates vs Ag wire: 20 mV, 40 mV, 60 mV, 80 mV, 100 mV, 120 mV, 140 mV; and (c) relationship of anodic (Ia, the black one)
and cathodic (Ic, the red one) peaks as a function of scan rate for PDTP-Ph-TPA film in 0.1 M TBAP/DCM. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
Table 1
Electrochemical properties of monomer and polymer.
Compound
Absorption spectra
Cyclic voltammetry (vs Ag wire)
Oxidation potential (V)b
Eogpt (eV)a
kmax (nm)
kedge (nm)
Energy levels (eV)
Eonset
Eox1
Eox2
HOMOc
LUMOd
DTP-Ph-TPA
PDTP-Ph-TPA
341
360, 440
410
572
3.03
2.17
0.83
0.65
0.98
0.92
1.27
1.33
5.01
4.83
1.98
2.66
a
b
c
The data were calculated by the equation: Eg = 1241/kedge of monomer and polymer films.
Oxidation potentials from cyclic voltammograms vs Ag wire in DCM.
The HOMO energy levels were calculated from cyclic voltammetry and referenced to ferrocene (4.80 eV).
LUMO = HOMO ꢀ Eg.
d
medium containing 2.0 ꢁ 10ꢀ3 M DTP-Ph-TPA and 0.1 M
the electrode. The intensities of the redox peaks become
higher and higher under repetitive cycling, indicating that
an electroactive polymer formed on the electrode surface.
In order to investigate the film quality of the polymer,
the SEM micrographs of polymer films were obtained and
shown in Fig. 4. The polymer films were synthesized by
electrochemical polymerization on the ITO substrates and
peeled off carefully for the SEM measurement. Fig. 4a
shows that the polymer film exhibits a uniform and
smooth surface with small pit on it. Fig. 4b was the
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422
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431
TBAP in DCM via repetitive cycling at
a scan rate
100 mV/s (Fig. 3). At the beginning, there were two
oxidation peaks at 0.98 V and 1.27 V in the first process
of oxidation which is in agreement with the CV measure-
ment. When the scan was reversed, two reduction peaks
with half wave potentials (Ep1/2) of 0.84 V and 1.16 V,
respectively, were observed. After the second cycle, there
was
a new oxidation peak at 0.82 V which can be
attributed to the oxidation of the polymer deposited on
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494
showed a hypsochromic shift compared with the polymer
PDTP-Ph [17]. The absorptions were corresponded to the
p–
p⁄ and n–p⁄ transitions of the PDTP backbones and the
TPA subunits [17]. The band gap Eg of PDTP-Ph-TPA was
calculated to be 2.17 eV from the onset of the
p
–
p⁄ transi-
tion at 572 nm, which was lower than the precedent PDTP
analogs [4]. Upon oxidation of the PDTP-Ph-TPA film, the
absorption of the
p–
p⁄ transition at 360 nm gradually
decreased while two new absorption at 494 nm and
810 nm as well as a new broad band from 1000 nm to
1650 nm grew up. When the potential was increased to
0.9 V, the absorption at 440 nm decreased to the lowest
and the color of the film became green. This potential
was associated with the onset of first anodic process in
the CV measurements, indicating that the electrons were
gradually transporting into the polymer [44]. Upon
increasing the potentials to 1.1 V, which corresponds to
the first anodic process of PDTP-Ph-TPA, the absorption
at 494 nm and the broad band at the NIR region became
the highest, while the color of the film changed into pink.
It is known that the formation of PDTP-Ph-TPA+ radical
cation arises from the oxidation of the TPA unit of the poly-
mer [45]. The positive charge of the PDTP-Ph-TPA+ radical
cation can transport between the different amino centers
of the TPA and DTP through p-diphenylenediamine group
(Scheme 1), which leads to an intervalence charge transfer
of the polymer and exhibits strongest absorption band in
the NIR region when the potential increased to 1.1 V
[14,31]. Finally, when the potential reached the highest
applied potential of 1.4 V, the absorption at 360 nm
became the lowest and the new absorption at 810 nm
increased to the top, and the color of the film stayed at
blue. Meanwhile, the absorption at the NIR region was
decreasing, which was consistent with the second anodic
process and indicates the further oxidation and formation
of PDTP-Ph-TPA2+ dication [45–47]. The absorption band
of the PDTP-Ph-TPA was reversible when the applied
potential was circulated. Fig. 5c illustrates the dynamic
spectral changes of PDTP-Ph-TPA in % transmittance at
1550, 1310, 810, 494, 440, and 360 nm by changing the
applied potentials, which show the maximum optical
changes of the electrochromic polymer PDTP-Ph-TPA at
Fig. 3. Electrochemical polymerization of 2.0 ꢁ 10ꢀ3 M DTP-Ph-TPA in
0.1 M TBAP/DCM via repetitive cycling at a scan rate 100 mV/s.
432
433
434
enlargement of the cross section of the film which shows
the stratified structure of the polymer with the thickness
about 1.0 lm.
435
3.4. Spectroelectrochemical properties
436
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439
440
441
442
443
444
445
446
447
448
449
450
451
452
Spectroelectrochemical measurements were conducted
by UV–Vis–NIR spectrophotometer upon incrementally
changing the applied potential (from 0.0 V to 1.4 V) of
the polymer films. In order to remove the doping ion and
the residual charge, the polymer films on the ITO were
reduced to their neutral state before the measurements
of spectroelectrochemical properties. From these measure-
ments, both the optical behavior and electronic structure
of the polymer films on the transparent electrode ITO upon
p-doping can be explained [22].
Fig. 5a and b illustrates the UV–Vis–NIR absorption
profiles correlated with applied potentials and the three-
dimensional wavelength-potential-absorbance correlation
of PDTP-Ph-TPA polymer film, respectively. At neutral
state, PDTP-Ph-TPA polymer is yellow in the visible region
and transparent in the NIR regions, with two strong
absorption maxima at around 360 nm and 440 nm which
Fig. 4. The SEM micrographs of PDTP-Ph-TPA polymer films.
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(a)
(b)
(c)
(d)
Fig. 5. (a) Spectroelectrochemical properties of homo-polymer PDTP-Ph-TPA thin film on the ITO in 0.1 M TBAP/DCM at various applied potentials (from
0.0 to 1.4); (b) 3-D spectroelectrochemical behavior from 0.0 to 1.4 V (vs Ag wire) of PDTP-Ph-TPA thin film on the ITO in 0.1 M TBAP/DCM; (c) optical
change in T% as a function of applied potential for six absorption bands at 1550, 1310, 810, 494, 440 and 360 nm; and (d) the illustration of the chemical
structure with color changes of PDTP-Ph-TPA as a function of applied potentials. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
495
496
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504
505
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510
511
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513
514
the appropriate step potentials. Fig. 5d illustrates the
chemical structure with color changes (from yellow to
light green to magenta and blue) of PDTP-Ph-TPA as a
function of applied potentials.
the polymer films at the given wavelength was monitored
as a function of time with UV–Vis–NIR spectroscopy when
the film was switched between different potential. Switch-
ing data of PDTP-Ph-TPA at 1550 nm and 1310 nm were
shown in Fig. 6a and b. The switching time of the polymer
was calculated at 90% of the ultimate contrast because it
is difficult to perceive any further color change with naked
eye beyond this limit. As showed in Fig. 5, the applied
potential of PDTP-Ph-TPA was switched between 0.0 and
1.1 V. When the detective wavelength was 1550 nm
(Fig. 6a), PDTP-Ph-TPA revealed switching time of 2.8 s
499
3.5. Electrochromic switching
500
501
502
503
For electrochromic switching studies, PDTP-Ph-TPA
was deposited on ITO glass by electrochemical polymeriza-
tion as described above. Chronoamperometric and absor-
bance measurements were performed. The absorbance of
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(a)
(b)
Fig. 6. Potential step absorptiometry of the polymer film PDTP-Ph-TPA on the ITO (coated area: 2 cm2) in 0.1 M TBAP/DCM. kmax = 1550 nm (a) and
1310 nm (b) with the applied potential stepped between 0.0 V and 1.1 V at a cycle time of 20 s (tc = coloring time, tb = bleaching time).
Table 2
Summary of kinetic studies of PDTP-Ph-TPA.
k (nm)
Optical contrast
tc (s)
tb (s)
D
OD
Qd (C/cm2)
CE (cm2/C)
Tb
Tc
DT%
PDTP-Ph-TPA
1550
1310
810
87.44
95.29
61.83
16.93
25.47
8.65
70.51
69.82
53.18
2.8
2.2
3.0
1.4
0.9
1.1
0.6196
0.5730
0.8542
0.004966
0.003083
0.008307
125.72
185.86
102.83
(a)
(b)
Fig. 7. (a) Optical change in T% as a function of applied potential for six absorption bands at 1550, 1310, 810, 494, 440 and 360 nm; and (b) the photo shows
the color change of electrochromic devices at different applied potentials.
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521
522
523
524
for coloring process (tc) and 1.4 s for bleaching (tb) at the
transmittance of 70.5%. When the detective wavelength
was 1310 nm (Fig. 6b), PDTP-Ph-TPA requires 2.2 s for
coloration and 0.9 s for bleaching at the transmittance of
67.9%. The polymer showed a faster bleaching process in
comparison with the coloring process, which was because
the polymer has a faster kinetics of dopant ion diffusion in
the reduction process than for the reverse process [25,48].
Moreover, PDTP-Ph-TPA shows reversible optical changes
upon switching between its neutral and oxidation states.
Please cite this article in press as: J.-C. Lai et al., A new multicolored and near-infrared electrochromic material based on triphenylamine-
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The switching time and transmittance for other wave-
lengths of the polymer film PDTP-Ph-TPA were listed in
Table 2, and the details were showed in supplementary data
(Fig. S6).
The coloration efficiency (CE) is one of the important
parameters for the electrochromic materials. The CE of
the polymer films is equal to the ratio of optical density
Doctoral Fund of Ministry of Education of China
(20120091130002).
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Appendix A. Supplementary material
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Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.orgel.2014.10.033.
change (DOD) to the amount of injected/ejected charge
(Q d), and it can be calculated by utilizing the following
534
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537
equations [49]
585
References
CEðkÞ ¼
D
ODðkÞ=Qd and
D
ODðkÞ ¼ log½TcðkÞ=TbðkÞꢂ
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This work was supported by the National Natural
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containing poly(3,4-dithienylpyrrole), Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.10.033

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Please cite this article in press as: J.-C. Lai et al., A new multicolored and near-infrared electrochromic material based on triphenylamine-
containing poly(3,4-dithienylpyrrole), Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.10.033