Benzooxadiazaole-based D-A-D co-oligomers: Synthesis and electropolymerization

Article abstract of DOI:10.1002/pola.26195

Four D-A-D type co-oligomers have been synthesized by Stille condensation between monostannyl derivatives of furan/thiophene/selenophene/3,4- ethylenedioxythiophene (EDOT) and 4,7-dibromo-benzo[1,2,5]oxadiazole. All these co-oligomers were successfully electrochemically polymerized in dichloromethane and characterized by spectroelectrochemistry. All four polymers possess narrow optical band gap. Spectroelectrochemical studies of polymer films on indium tin oxide revealed that the replacement of donor EDOT with furan/thiophene/ selenophene has affected the low-energy charge-carrier (bipolaron) formation significantly. Kinetic studies based on chronoamperometry show that the polymer P5 (EDOT-capped benzo[1,2,5]oxadiazole system) possess better electrochromic property with high transmittance (66%) in visible region than the other copolymers.

Full text of DOI:10.1002/pola.26195

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Benzooxadiazaole-Based D–A–D Co-Oligomers: Synthesis and
Electropolymerization
Palas Baran Pati, Soumyajit Das, Sanjio S. Zade
Department of Chemical Sciences, Indian Institute of Science Education and Research, Kolkata,
P.O. BCKV Campus Main Office, Mohanpur, Nadia, West Bengal 741252, India
Correspondence to: S. S. Zade (E-mail: sanjiozade@iiserkol.ac.in)
Received 31 March 2012; accepted 15 May 2012; published online
DOI: 10.1002/pola.26195
ABSTRACT: Four D–A–D type co-oligomers have been synthe-
(bipolaron) formation significantly. Kinetic studies based on
chronoamperometry show that the polymer P5 (EDOT-capped
benzo[1,2,5]oxadiazole system) possess better electrochromic
property with high transmittance (66%) in visible region than
the other copolymers. VC 2012 Wiley Periodicals, Inc. J Polym
sized by Stille condensation between monostannyl derivatives
of
furan/thiophene/selenophene/3,4-ethylenedioxythiophene
(
EDOT) and 4,7-dibromo-benzo[1,2,5]oxadiazole. All these co-
oligomers were successfully electrochemically polymerized in
dichloromethane and characterized by spectroelectrochemistry.
All four polymers possess narrow optical band gap. Spectroe-
lectrochemical studies of polymer films on indium tin oxide
revealed that the replacement of donor EDOT with furan/thio-
phene/selenophene has affected the low-energy charge-carrier
Sci Part A: Polym Chem 000: 000–000, 2012
KEYWORDS: benzooxadiazole; conducting polymers; conjugated
polymers; EDOT; electrochromic materials; electropolymeriza-
tion; oligomers; spectroelectrochemistry; thin films
INTRODUCTION Planarity with an efficient conjugation in or-
ganic materials plays important role in achieving low band
gap, high conductivity, high mobility, and good electro-optical
response. Thiophene-containing p-conjugated systems have
received considerable attention for the development of or-
ganic electronic devices, such as light-emitting diodes, field-
effect transistors, and solar cells because of their good envi-
studied, in comparison, polyfuran is almost unnoticed, as the
synthesis of polyfuran by direct electrolysis of furan is diffi-
1
5
16
cult (>3 V vs. SCE). Recent discoveries on oligofuran
proved the potential of furan-based conjugated systems as
1
7
promising hole transporting material
band gap polymers for solar cells.
and efficient low
The development of
1
8,19
furan-based conjugated systems relies on its enhanced solu-
tion processability. Compared to sulfur/selenium-based or-
ganic electronic materials, oxygen-based materials are more
1
ronmental and thermal stability. 3,4-Ethylenedioxythiophene
(
EDOT) is one of the most important examples of thiophene
1
7
biodegradable and biocompatible which is desirable as, in
general, organic electronic materials are not biodegradable
and extensive usage of these compounds may generate a sig-
derivatives both academically and industrially, because the
polymers derived from EDOT and its derivatives provide
highly conducting and especially stable doped states, a range
of optical properties with electronic band gaps varying
across the entire visible spectrum, and enhanced redox prop-
erties, making them useful for numerous electronic devi-
2
0,21
nificant amount of hazardous waste.
The donor–acceptor (D–A) concept for band gap reduction
22
was first proposed in 1992, and since then this strategy
2
–5
ces.
Although thiophene-containing p-conjugated systems
was well developed to synthesize small band gap (1.0 eV <
2
3
are heavily explored, their selenium counterparts have lately
drawn decent attention as a novel class of p-conjugated
Eg < 1.9 eV) polymers. By constructing a conjugated chain
of alternating donor and acceptor units, the valence and con-
duction bands could be broadened, which may result in a
narrow band gap polymers. This D–A strategy was exten-
sively studied with electron-deficient systems like ben-
6
materials exhibiting promising optoelectronic properties
7
8,9
ranging from high conductivity to high hole mobility. Just
by a mere change of sulfur for less electronegative and more
polarizable selenium (atomistic approach), one can easily
tune the band gap, and thus the optical and electronic prop-
2
4
zo[1,2,5]thiadiazole
(BDT),
benzo[1,2,5]selenadiazole
2
5
26
(BDS), and benzo[1,2,5]triazole (BTAz). However, their
oxygen counterpart, benzo[1,2,5]oxadiazole (BDO), had
received limited attention until lately.
1
0–13
erties.
Additionally, selenophene system is more vulner-
2
7–31
able to electrophilic substitution than its thiophene counter-
The exciting
1
4
24
part.
While polythiophene and polyselenophene were
report by Toppare and coworkers on EDOT-capped BDT
Additional Supporting Information may be found in the online version of this article.
2012 Wiley Periodicals, Inc.
V
C
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2
5
system and the report by Cihaner and Algi on thiophene/
EDOT-capped BDS systems inspired us to explore the D–A
HITACHI U-4100 UV–vis–NIR spectrophotometer. In spectro-
chemical measurements, the working electrode was an ITO-
coated glass slide, the counter electrode was a platinum
wire, and nonaqueous Ag/AgCl was used as the reference
electrode. All electrochemical potentials were reported
3
1
polymers based on BDO as acceptor. Oxygen over sulfur or
selenium has more electronegativity; hence, it is possible to
stabilize the highest occupied molecular orbital (HOMO)
level of the polymers by introducing oxygen containing
heterocyclic systems into polymers instead of sulfur or
selenium analogs. Thus, replacement of the BTAz, BDT, or
BDS with the more electronegative BDO acceptor may result
against Ag/AgCl taking ferrocene as external standard, the
ferrocence
E
is þ0.35 V.
1
=2
Synthesis of 4,7-Dibromo-benzo[1,2,5]oxadiazole (1)
2
7,28
4,7-Benzo[1,2,5]oxadiazole was prepared starting from o-
in a polymer with more stabilized HOMO level,
which
3
6
nitroaniline following a literature procedure. Diazotization
of o-nitroaniline (4.14 g, 30 mmol) by silica-sulfuric acid
followed by addition of sodium azide afforded the azide deriv-
ative (Supporting Information, Scheme S1). Cyclocondensation
of the azide derivative in boiling toluene gave benzofuroxane
which on treatment with triethylphosphite under reflux condi-
tion in ethanol medium resulted deoxygenation to produce
may be beneficial in increasing the efficiency of polymer
solar cell while maintaining good solubility and planarity of
2
9,30
resulting polymer backbone.
Herein, we report the
synthesis of furan, thiophene, selenophene, and EDOT-capped
BDO co-oligomers anticipating that these would reveal
efficient optical and redox properties as a new type of poly-
3
1
mer electrochromics when polymerized electrochemically.
3
7
BDO. Bromination of BDO in the presence of iron powder
leads to 4,7-dibromo-benzo[1,2,5]oxadiazole (2 g, overall yield
The experimental results are also supported by theoretical
calculations.
3
8 1
25%).
H NMR (400 MHz, CDCl , d, ppm): 7.51 (s, 1H).
3
General Synthesis of 4a, 4b, 4c, and 5
EXPERIMENTAL
4,7-Bibromo-benzo[1,2,5]oxadiazole (0.5 mmol) and the cor-
All reactions were performed under nitrogen atmosphere to
maintain the dry condition. Dry toluene and THF were dis-
tilled from sodium/benzophenone prior to use. Anhydrous
dichloromethane (DCM), furan, thiophene, selenophene,
responding monostannyl derivative (1.0 mmol), that is, tribu-
tyl(furan-2-yl)stannane (2a)/tributyl(thiophen-2-yl)stannane
(2b)/tributyl(selenophen-2-yl)stannane (2c)/tributyl(3,4-eth-
ylenedioxythiophen-2-yl)stannane (3), were dissolved in dry
toluene (30 mL), and the solution was purged with nitrogen
for 15 min. Pd(PPh ) (0.05 mmol) was added at room tem-
EDOT, tributyltinchloride (SnBu Cl), Pd(PPh ) , tetrabutylam-
3
3 4
monium perchlorate (TBAPC), and o-nitroaniline were pur-
chased from Aldrich and used without further purification.
n-BuLi (1.6 M solution in hexane) was purchased from Neo-
3
4
perature under argon atmosphere. The mixture was stirred
ꢁ
at 100 C for 24 h, cooled, and solvent was evaporated on
3
2
Synth. Tributyl(furan-2-yl)stannane,
yl)stannane,
tributyl(thiophen-2-
the rotary evaporator. The residue was subjected to column
chromatography to isolate the desired compound.
3
3
34
tributyl(selenophen-2-yl)stannane,
tributyl(3,5-ethylenedioxythiophen-2-yl)stannane were syn-
thesized according to previously described methods. All the
and
3
5
Compound 4a
1
13
After purification by column chromatography (hexane), pure
product was obtained as yellow solid in 72% yield. mp:
new compounds were characterized by H NMR, C NMR,
1
13
and HRMS. H NMR and C NMR spectra of the compounds
were taken on Bruker Avance 500 MHz and Jeol ECS 400
MHz spectrometer with either CDCl or DMSO-d as the sol-
ꢁ
18–122 C. H NMR (500 MHz, DMSO-d , d, ppm): 6.78 (m,
6
1
1
1
1
3
H), 7.38 (d, J ¼ 3.5 Hz, 1H), 7.89 (s, 1H), 7.99 (m, 1H).
C
3
6
NMR (125 MHz, DMSO-d , d, ppm): 112.62, 113.04, 116.48,
vent, and chemical shifts (d) are reported in parts per mil-
lion (ppm) relative to tetramethylsilane as the internal
standard. Electrochemical studies were carried out with a
Princeton Applied Research 263A potentiostat using plati-
num (Pt) disk electrode as the working electrode, a platinum
wire as counter electrode, and an AgCl-coated Ag wire,
which was directly dipped in the electrolyte solution, as the
reference electrode. Nonaqueous Ag/AgCl wire was prepared
6
1
24.97, 145.14, 146.09, 147.83. HRMS (m/z): calcd for
þ
C H N O , 252.0535; found, 252.0541 [M] .
14
8 2 3
Compound 4b
After purification by column chromatography (DCM:hexane,
1
6
:20), pure product was obtained as orange-yellow solid in
ꢁ
1
8% yield. mp: 123–125 C. H NMR (400 MHz, DMSO-d , d,
6
ppm): 6.78 (m, 1H), 7.45 (d, J ¼ 3.5 Hz, 1H), 7.62 (s, 1H),
by dipping silver wire in a solution of FeCl and HCl. Pt disk
13
3
8
.12 (m,1H). C NMR (100 MHz, DMSO-d , d, ppm): 112.07,
6
electrodes were polished with alumina, water, and acetone
and were dried with nitrogen gas before use to remove any
incipient oxygen. The electrolyte used was 0.1 M TBAPC in
DCM. Electropolymerization was done on indium tin oxide
1
26.36, 128.62, 128.72, 137.87, 147.84. HRMS (m/z): calcd
þ
for C14
H
8
N
2
OS
2
, 284.0078; found, 284.0065 [M] .
Compound 4c
(
ITO)-coated glass as a working electrode. The polymer films
After purification by column chromatography (DCM:hexane,
1:20), pure product was obtained as orange-yellow solid in
were deposited on ITO-coated glass electrode with dimen-
2
ꢁ
1
sions of 5 ꢀ 0.7 cm at monomer’s first oxidation potential
78% yield. mp: 145–148 C. H NMR (500 MHz, DMSO-d , d,
6
(
V) versus Ag/AgCl and a passing charge of 50–100 mC
ppm): 7.5 (m, 1H), 7.89 (s, 1H), 8.25 (m, 1H), 8.43 (m, 1H).
1
3
in 0.1 M TBAPC/DCM. Before examining the optical proper-
ties of polymer films, the films were rinsed with DCM.
UV–vis–near infrared (NIR) spectra were recorded on a
C NMR (125 MHz, DMSO-d , d, ppm): 112.76, 127.89,
6
129.99, 130.87, 135.15, 142.25, 147.49. HRMS (m/z): calcd
þ
for C H N OSe , 379.8967; found, 379.8963 [M] .
1
4
8
2
2
2
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SCHEME 1 Synthetic route to furan, thiophene, selenophene, and EDOT-capped 2,1,3-benzooxadiazole based D–A–D type
copolymers.
Compound 5
Under repeated CV cycles, these D–A–D co-oligomers have
undergone smooth polymerization to produce insoluble D–A
copolymer deposit at the surface of the platinum working
electrode. Meanwhile, an increase in the current intensity
was observed, which is due to an increase in the active area
of the working electrode owing to electroactive polymer
(P4a, P4b, P4c, and P5) coating on the Pt electrode (Fig. 1).
The inset pictures show the scan rate dependence of the cur-
rent response during redox cycling of P4a [Fig. 1(b)], P4b
After purification by column chromatography (ethyl acetate:-
hexane, 1:10) pure product was obtained as red solid in
1
8
5% yield. H NMR (400 MHz, DMSO-d , d, ppm): 4.32 (d,
6
1
3
2H), 4.43 (d, 2H), 6.95 (s, 1H), 8.19 (s, 1H). C NMR (100
MHz, DMSO-d , d, ppm): 64.06, 65.18, 103.10, 110.78,
6
118.42, 127.32, 141.22, 141.79, 147.16. HRMS (m/z): calcd
þ
for C H N O S , 400.0188; found, 400.0193 [M] .
1
8 12 2 5 2
RESULTS AND DISCUSSION
[
Fig. 1(d)], P4c [Fig. 1(f)], and P5 [Fig. 1(h)] as a function of
different scan rates. It is evident that both the anodic and ca-
thodic peak currents scale linearly with increasing scan rate,
which is as expected for an electrode-supported nondiffusive
electroactive film.
Synthesis
To demonstrate the effect of the donor and acceptor units on
the structure–property relationships four different BDO-
based D–A–D co-oligomers, 4a, 4b, 4c, and 5 were studied.
Synthesis of 4a, 4b, 4c, and 5 was accomplished by Stille
coupling between stannyl derivative of donor (furan, thio-
phene, selenophene, and EDOT) and 4,7-dibromo-ben-
zo[2,1,3]oxadiazole 1 in toluene medium as shown in
Scheme 1. The selenophene analog 4c revealed stronger
intermolecular interactions (including Se/Se interactions)
than their furan (4a) or thiophene (4b) counterparts, as evi-
denced by its significantly higher melting point. All the co-
Electrochemical measurements are important for evaluation
of HOMO and lowest unoccupied molecular orbital (LUMO)
ox
onset
energy levels. The onset oxidation potential (E
) of the
polymers increases in the order P5 (ꢂ0.18 V) < P4a (0.43
V) < P4b (0.60 V) < P4c (0.78 V), and the HOMO of the
polymers P4a, P4b, P4c, and P5 are calculated as ꢂ4.88,
3
1
39
ꢂ5.05, ꢂ5.23, and ꢂ4.27 eV, respectively. Similar to previ-
26
ous observation, we found that HOMO of D–A type copoly-
1
13
oligomers were fully characterized by H NMR, C NMR,
and high resolution mass spectroscopy analysis.
mer comprising furan as donor is more destabilized than
their thiophene or selenophene counterpart. This trend may
be generalized by the fact that oxygen in a donor destabilizes
HOMO, whereas in acceptor would stabilize the HOMO. This
indicates that the delicate balance between the donor and
acceptor properties in a D–A polymer plays the key role in
deciding electronic characteristic of resulting polymers. The
LUMO level was estimated relative to the HOMO from the
absorption onset values as ꢂ3.3, ꢂ3.43, ꢂ3.6, and ꢂ3.02 eV
for P4a, P4b, P4c, and P5, respectively.
Electrochemistry and Polymerization
Electrochemical synthesis of conducting polymers offers
many advantages over chemical synthesis including the in
situ deposition of the polymer at the electrode surface and,
hence, eliminating processability problems. Cyclic voltamme-
try experiment on 4a [Fig. 1(a)], 4b [Fig. 1(c)], 4c [Fig. 1(e)],
and 5 [Fig. 1(g)] in DCM/TBAPC solvent/electrolyte couple
indicated irreversible cyclic voltammograms revealing their
first oxidation potentials at 1.2, 1.32, 1.22, and 0.95 V,
respectively. The EDOT-based co-oligomer 5 showed the low-
est oxidation potential among the four BDO-based co-oligom-
ers, which may be credited to the more electron-rich nature
of EDOT. Replacing BTAz, BDT, and BDS with BDO makes it
more difficult to extract an electron from the D–A–D systems
Spectroelectrochemistry
UV–vis absorption spectra of the co-oligomers were recorded
in THF solvent (Fig. 2). In all the cases, two absorption
bands were found. The kmax corresponds to the higher
energy band for 4a, 4b, and 4c is less than 300 nm; but for
5 only, it is 314 nm due to increase in conjugation.
The lower energy absorption band of 5 (470 nm) is signifi-
cantly red shifted than that of 4a (444 nm), 4b (441 nm),
4a, 4b, 4c, and 5 resulting in higher first oxidation potential
that could be attributed to the higher electron-acceptance
nature of BDO stabilizing the HOMO.
2
4–26
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FIGURE 1 CV of 4a (a), 4b (c), 4c (e), and 5 (g) in DCM and Bu NClO (TBAPC) on a Pt electrode at a scan rate of 50 mV/s versus
Ag/AgCl wire. Multisweep electrochemical polymerization (ECP) of 4a (b), 4b (d), 4c (f), and 5 (h) on a Pt electrode in DCM and 0.1
M TBAPC at 50 mV/s versus Ag/AgCl wire. (Inset) CV of D–A copolymers P4a (b), P4b (d), P4c (f), and P5 (h) produced in DCM and
0.1 M TBAPC as a function of scan rate.
and 4c (450 nm). It is in the order of the calculated dipole
moment of ground state geometry, which is significantly
higher for 5 (8.11 D) than that of 4a (3.70 D), 4b (3.39 D),
and 4c (3.24 D).
metrically (i.e., the electrode current is recorded over time)
deposited on ITO-coated transparent glass electrode, and the
spectroelectrochemical properties were investigated. As
shown in Figure 3, the absorption spectra in the UV–vis–NIR
region of P4a, P4b, P4c, and P5 films revealed good electro-
chromic nature at different applied potentials in DCM/TBAPC
solvent/electrolyte couple. As seen from the Figure 3, beyond
To find relation between redox processes and UV–vis absorp-
tion, polymers P4a, P4b, P4c, and P5 were chronoampero-
4
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FIGURE 1 Continued
a certain potential (0.8 V for P4a, 0.9 V for P4b, 1.0 V for
P4c, and 0.05 V for P5), the UV–vis absorption band (arises
due to the p–p* transition of the neutral polymer) started to
lose its intensity gradually as the applied potential was
increased. Meanwhile, a new absorption band was intensified
extending from 700 nm to NIR region owing to the formation
of polaron (cation radical). The formation of other charge car-
riers (bipolaron band) was not observed upon further oxida-
tion for P4a, P4b, and P4c, which could be attributed to the
strong electron-accepting nature of BDO that does not allow
withdrawal of further electron from the resulting polaronic
structures. P5 revealed two well-separated absorption max-
ima at 406 nm and 746 nm, which are indispensable for
green color to be observed. Upon oxidation, the intensities of
the two p–p* transition bands of P5 were depleted simultane-
ously, and formations of two new bands at 960 and 1400 nm
indicate the formations of polarons and bipolarons, respec-
tively, which is accompanied by a color change from green to
transparent light blue. Replacement of more electron-rich sys-
tems like EDOT with furan/thiophene/selenophene afforded
polymers with high oxidative stability. The optical band gaps
can be tuned with the energy levels of the acceptor such as
fullerene to improve the performance of the photovoltaic
devices.
Density Functional Theory Calculations
The density functional theory (DFT) using periodic boundary
condition (PBC) with B3LYP functional and the 6-31G(d) basis
set is applied to determine the geometric and electronic struc-
tures and the corresponding energies of HOMO, LUMO, and
band gap. The PBC-DFT method, implemented in the Gaussian
4
1
0
3 program package, is a reliable method for predicting the
4
2
energy gaps of D–A copolymers. The BDO substituent as an
acceptor between two donor units (furan, thiophene, seleno-
phene, or EDOT) of the resulting polymer does not create any
geometrical twist in the polymer backbone, which results in
uninterrupted p-conjugation along polymer chain that results
into small to low band gap polymers. The optimized planar
(
side view) D–A copolymer backbone is shown in Table 1.
The calculated HOMO levels for P4a, P4b, P4c, and P5 at k ¼
0
are ꢂ4.75, ꢂ4.92, ꢂ4.87, and ꢂ4.12 eV, respectively. The
theoretical band gaps are as 1.66, 1.62, 1.51, and 1.44 eV,
which are in good agreement with the optical band gap.
(
Eg) were obtained from absorption onsets of the polymer
film spectra (onset value) as 1.66 eV (P4a, 746 nm), 1.62 eV
P4b, 765 nm), 1.59 eV (P4c, 779 nm), and 1.25 eV (P5, 992
(
nm), which indicate that the band gap of D–A–D type copoly-
mer can nicely be tuned by only replacing the donor in a
polymer backbone. The smaller band gap of the D–A polymer
2
5
based on EDOT-capped BDS compared to that based on
EDOT-capped BDO may be rationalized by the degree of bond
length alternation (EAN bond, E ¼ O, S, Se) in the benzene
ring that increases in the order O < S < Se trending toward a
decrease in aromaticity, which affects the conjugation between
donor and acceptor unit resulting in destabilization of the
4
0
HOMO and stabilization of the LUMO. Low band gap mate-
rial (to harness more solar photon) with low lying HOMO
level (to prevent aerial oxidation) is the prerequisite for an ef-
ficient organic photovoltaic devices. EDOT containing poly-
mers generally possess low band gap but high HOMO level,
which hampered their application as a light-absorbing donor
in organic solar cells. This strategy can be extended to pre-
pare the polymers in which the frontier orbital energy levels
FIGURE 2 Normalized absorption spectra of co-oligomers 4a–
4c and 5 recorded in THF solution.
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FIGURE 3 Spectroelectrochemistry of P4a (a), P4b (b), P4c (c), and P5 (d) thin film, prepared on ITO-coated glass slides, as a func-
tion of various applied potentials in co-oligomer-free DCM/TBAPC solvent/electrolyte couple.
Theoretically, we found that the HOMO of selenium (P4c) or
thiophene (P4b) based system is more stabilized by 0.12–
ment with our experimental observations but contradicts the
fact that HOMO of furan containing polymers is more stabi-
lized than their sulfur or selenium analogs.
0.17 eV than their oxygen counterpart (P4a) that is in agree-
TABLE 1 Optimized Geometry of the D–A Copolymers at DFT-PBC/B3LYP/6-31G(d)
Polymer
Front View
Side View
HOCO (eV)
LUCO (eV)
P4a
ꢂ4.75
ꢂ3.09
P4b
ꢂ4.92
ꢂ3.30
P4c
P5
ꢂ4.87
ꢂ4.12
ꢂ3.36
ꢂ2.68
6
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absorption maxima (Fig. 4). As illustrated in Figure 4, P4a,
P4b, and P5 were switched between oxidized and reduced
states with a switching interval of 3 s in a 0.1 M DCM/
TBAPC solvent/electrolyte system while measuring their
transmittance at their respective absorption maxima (visible
region) and NIR region. The contrast was given as the differ-
ence between %T in the reduced and oxidized states and
reported as D%T. P4a and P4b show quite similar perform-
ance, where 19% (P4a) and 16% (P4b) of the transmittance
modulation are maintained at 1600 nm when the time of the
applied potential is switched between ꢂ0.1 and þ1.3 V at 3-
s interval. At a similar condition, P4a revealed 29% optical
contrast at 1000 nm and P4b revealed only 8% at 1100 nm.
However, their performance was poor near their wavelength
maximum even P4c also failed to perform better (see Sup-
porting Information). To evaluate the electrochromic proper-
ties of P5, the P5 polymer film was electrodeposited onto
2
ITO-coated glass having dimensions of 3 ꢀ 0.7 cm at a con-
stant potential of 0.95 V versus Ag/AgCl and passing charge
of 50 mC. While the films were switched between ꢂ0.5 and
þ1.3 V, the percentage transmittance at visible absorption
maximums and NIR regions were simultaneously monitored
as a function of time (Fig. 4). The P5 films showed 95%
transmittance at the oxidized and a 29% transmittance at
the neutral state with a rapid switching time of less than 2 s
and noteworthy optical contrast of 66% at 405 nm (the first
p–p* transition), which is undoubtedly higher than analogous
BDT and BDS based green D–A copolymers. The optical con-
trast for P5 near kmax (750 nm, the second p–p* transition)
was calculated as 14% with a switching time of 0.9 s. In NIR
region, polymer P5 showed 36% (1100 nm) to 34% (1600
nm) transmittance change in less than 1 s. The coloration ef-
2
ꢂ1
ficiency of the P5 film was found to be 101 cm C for 405
nm. P4a, P4b, and P4c exhibited lower optical contrast val-
ues in visible region compared to P5 due to their extant
absorption at around 400–600 nm.
CONCLUSIONS
A new series of D–A–D polymers, comprising the least
explored BDO unit as acceptor, have been synthesized and
characterized electrochemically. Our experimental investiga-
tions indicated that BDO has the highest electron-acceptance
property than its sulfur, nitrogen, or selenium counterpart,
as incorporation of acceptor like BDO has affected each co-
oligomer’s oxidation potential and thus the oxidative stabil-
ity. However, it is not always necessary that HOMO of oxygen
containing polymers would stabilize more as inclusion of ox-
ygen (as BDO unit) instead of sulfur or selenium has affected
each co-oligomer in a different way. As a result, 4a oxidizes
at almost same potential where its selenium counterpart
does, which might be credited to the more extended conjuga-
tion. The EDOT-BDO based polymer P5 films exhibit better
electrochromic property compared to other BDO-based
copolymers with a high contrast ratio (66%) and moderate
coloration efficiency at 405 nm. The combination of furan
and BDO can make an important contribution in producing
environmentally sustainable organic electronic materials.
FIGURE 4 Percent transmittance changes of polymer films P4a
(
a), P4b (b), and P5 (c) in monomer-free 0.1 M TBAPC/DCM
solution at visible and NIR region.
Chronoabsorptometry
Switching properties of the ITO-coated polymers were inves-
tigated by chronoamperometry to monitor the changes in
transmittance as a function of time while sweeping the
potentials between neutral and fully oxidized states at local
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ACKNOWLEDGMENTS
24 Durmus, A.; Gunbas, G. E.; Camurlu, P.; Toppare, L. Chem.
Commun. 2007, 3246–3248.
PBP thanks UGC for his fellowship and SSZ thanks DRDO, India
for funding.
2
5 Cihaner, A.; Algi, F. Adv. Funct. Mater. 2008, 18, 3583–3589.
6 Lu, N. A. O.; Balan, A.; Baran, D.; Cirpan, A.; Toppare, L. J.
2
Polym. Sci. Part A: Polym. Chem. 2010, 48, 5603–5610.
7 Caputo, B. J. A.; Welch, G. C.; Kamkar, D. A.; Henson, Z. B.;
Nguyen, T.-Q.; Bazan, G. C. Small 2011, 7, 1422–1426.
8 Jiang, J.-M.; Yang, P.-A.; Chen, H.-C.; Wei, K.-H. Chem.
Commun. 2011, 47, 8877–8879.
9 Ding, P.; Zhong, C.; Zou, Y.; Pan, C.; Wu, H.; Cao, Y. J. Phys.
Chem. C 2011, 115, 16211–16219.
0 Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.;
2
REFERENCES AND NOTES
1
Perepichka, I. F.; Perepichka, D. F. Handbook of Thiophene-
2
Based Materials: Applications in Organic Electronics and Pho-
tonics; Wiley: New York, 2009.
2
2
Groenendaal, L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reyn-
olds, J. R. Adv. Mater. 2000, 12, 481–494.
Groenendaal, L.; Jotti, G.; Aubert, P.; Waybright, S. M.; Reyn-
olds, J. R. Adv. Mater. 2003, 15, 855–879.
3
3
Plesu, R. N.; Durocher, G.; Tao, Y.; Leclerc, M. J. Am. Chem.
Soc. 2008, 130, 732–742.
4
1
Roncali, J.; Blanchard, P.; Frere, P. J. Mater. Chem. 2005, 15,
589–1610.
3
1 During our research work on the manuscript, Cihaner et al.
have reported compound 4b and 5 and their electropolymeriza-
tion. I_ c¸ li- O¨ zkut, M.; Algi, M. P.; O¨ zta ꢀs , Z.; Algi, F.; O¨ nal, A. M.;
5
Spencer, H. J.; Skabara, P. J.; Giles, M.; McCulloch, I.;
Coles, S. J.; Hursthouse, M. B. J. Mater. Chem. 2005, 15,
Cihaner, A. Macromolecules 2012, 45, 729.
4
6
7
783–4792.
3
2 Liebeskind, L. S.; Wang, J. Tetrahedron 1993, 49, 5461–5470.
Patra, A.; Bendikov, M. J. Mater. Chem. 2010, 20, 422–433.
Patra, A.; Wijsboom, Y. H.; Zade, S. S.; Li, M.; Sheynin, Y.;
3
3 Manuela, M.; Raposo, M.; Maur ı´ cio, A.; Fonseca, C.; Kirsch,
G. Tetrahedron 2004, 60, 4071–4178.
Leitus, G.; Bendikov, M. J. Am. Chem. Soc. 2008, 130,
3
4 Udum, Y. A.; Tarkuc, S.; Toppare, L. Synth. Met. 2009, 159,
6734–6736.
3
61–365.
8
Shahid, M.; McCarthy-Ward, T.; Labram, J.; Rossbauer, S.;
3
5 Turbiez, M.; Fr e` re, P.; Allain, M.; Videlot, C.; Ackermann, J.;
Domingo, E. B.; Watkins, S. E.; Stingelin, N.; Anthopoulosb, T.
D.; Heeney, M. Chem. Sci. 2012, 3, 181–185.
Roncali, J. Chem. Eur. J. 2005, 11, 3742–3752.
6 Zarei, A.; Hajipour, A. R.; Khazdooz, L.; Aghaei, H. Tetrahe-
dron Lett. 2009, 50, 4443–4445.
7 Deghati, P. Y. F.; Borghini, A.; Nieuwendijk, A. M. C. H.;
Groote, M. D.; Jzerman, A. P. I. Bioorg. Med. Chem. 2003, 11,
3
9
Mohammed, A.-H.; Mohammed, A. B.; Florian, C.; Scott, E.
W.; Thomas, D. A.; Natalie, S.; Heeney, M. Macromolecules
3
2
1
011, 44, 5194–5199.
0 Chung, D. S.; Kong, H.; Yun, W. M.; Cha, H.; Shim, H.-K.;
8
99–908.
Kim, Y.-H.; Park, C. E. Org. Electron. 2010, 11, 899–904.
1 Haid, S.; Mishra, A.; Uhrich, C.; Pfeiffer, M.; B a€ uerle, P.
Chem. Mater. 2011, 23, 4435–4444.
3
8 Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.;
1
Neagu-Plesu, R.; Belletete, M.; Durocher, G.; Tao, Y.; Leclerc,
M. J. Am. Chem. Soc. 2008, 130, 732–742.
1
2 Das, S.; Zade, S. S. Chem. Commun. 2010, 46, 1168–1170.
3 Das, S.; Bedi, A.; Krishna, G. R.; Reddy, C. M.; Zade, S. S.
ox
ferrocene
1=2
3
9 Experimental HOMO ¼ ꢂe(E
ꢂE
þ 4.8) (eV).
onset
1
4
0 Gibson, G. L.; Mc-Cormick, T. M.; Seferos, D. S. J. Am.
Org. Biomol. Chem. 2011, 9, 6963–6972.
Chem. Soc. 2012, 134, 539–547.
1
2
4 Das, S.; Dutta, P. K.; Panda, S.; Zade, S. S. J Org. Chem.
010, 75, 4868–4871.
4
1 Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Men-
nucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.;
Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnen-
berg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hase-
gawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.;
Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov,
V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell,
A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.;
Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.;
Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J.
W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.;
Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D.
J. Gaussian 09, Revision A.02; Gaussian, Inc.: Wallingford CT,
2009.
1
5 Glenis, S.; Benz, M.; LeGoff, E.; Schindler, J. L.; Kannewurf,
C. R.; Kanatzidis, M. G. J. Am. Chem. Soc. 1993, 115,
1
2519–12525.
1
6 Gidron, O.; Diskin-Posner, Y.; Bendikov, M. J. Am. Chem.
Soc. 2010, 132, 2148–2150.
7 Gidron, O.; Dadvand, A.; Sheynin, Y.; Bendikov, M.; Pere-
pichka, D. F. Chem. Commun. 2011, 47, 1976–1978.
8 Beaujuge, P. M.; Holcombe, T. W.; Lee, O. P.; Fr eꢀ chet, J. M.
J. J. Am. Chem. Soc. 2010, 132, 15547–15549.
9 Wang, X.; Chen, S.; Sun, Y.; Zhang, M.; Li, Y.; Li, X.; Wang,
H. Polym. Chem. 2011, 2, 2872–2877.
1
1
1
20 Gandini, A. Macromolecules 2008, 41, 9491–9504.
21 Binder, J. B.; Ronald, R. T. J. Am. Chem. Soc. 2009, 131,
1979–1985.
2
1
2 Havinga, E. E.; Ten-Hoeve, W.; Wynberg, H. Polym. Bull.
992, 29, 119–126.
4
2 Pappenfus, T. M.; Schmidt, J. A.; Koehn, R. E.; Alia, J. D.
23 Roncali, J. Chem. Rev. 1997, 97, 173–205.
Macromolecules 2011, 44, 2354–2357.
8
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