Thieno[3,2-b]thiophene as π-bridge at different acceptor systems for electrochromic applications

Article abstract of DOI:10.1016/j.polymer.2014.05.029

Benzoselenadiazole, quinoxaline and thieno[3,2-b]thiophene are the units preferred in conducting polymers due to their electrochemical properties. There are no reports in the literature on polymers containing both moieties. In this study, novel benzoselen

Full text of DOI:10.1016/j.polymer.2014.05.029

Polymer 55 (2014) 3093e3099
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Polymer
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Thieno[3,2-b]thiophene as
p-bridge at different acceptor systems for
electrochromic applications
, b, c, d
Sinem Toksabay a, Serife O. Hacioglu a, Naime A. Unlu a, Ali Cirpan a
,
, b, c, e, *
Levent Toppare a
a Department of Chemistry, Middle East Technical University, 06800 Ankara, Turkey
b Department of Polymer Science and Technology, Middle East Technical University, 06800 Ankara, Turkey
c The Center for Solar Energy Research and Application (GÜNAM), Middle East Technical University, 06800 Ankara, Turkey
d Department of Micro and Nanotechnology, Middle East Technical University, Ankara 06800, Turkey
e Department of Biotechnology, Middle East Technical University, 06800 Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Benzoselenadiazole, quinoxaline and thieno[3,2-b]thiophene are the units preferred in conducting
polymers due to their electrochemical properties. There are no reports in the literature on polymers
containing both moieties. In this study, novel benzoselenadiazole, quinoxaline and thieno[3,2-b]thio-
phene based monomers; 4-(3a,6a-dihydrothieno[3,2-b]thiophen-2-yl)-7-(thieno[3,2-b]thiophenyl)
Received 7 April 2014
Received in revised form
8 May 2014
Accepted 11 May 2014
Available online 22 May 2014
benzo[c][1,2,5]selenadiazole
(BSeTT)
and
2,3-bis(3,4-bis(decyloxy)phenyl)-5,8-dibromo-2,3-
dihydroquinoxaline (QTT) were synthesized via Stille Coupling and polymerized electrochemically.
These polymers were characterized in terms of their spectroelectrochemical and electrochemical
properties by cyclic voltammetry and UVeViseNIR spectroscopy. Spectroelectrochemistry analysis of
Keywords:
Conjugated polymers
Electrochromism
thieno[3,2-b]thiophene
PBSeTT revealed an electronic transition at 525 nm corresponding to
pep* transition with a band gap of
0.93 eV whereas PQTT revealed electronic transitions at 440 and 600 nm corresponding to
pep*
transitions with a band gap of 1.30 eV. Electrochromic investigations showed that PBSeTT has gray color
PQTT switching between green and gray. Switching time of the polymers was evaluated by a kinetic
study upon measuring the percent transmittance (%T) at the maximum contrast point.
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
polymers [11]. Through regulating the contribution of intra-
molecular charge transfer (ICT), the absorption spectra and energy
A new age had been started in macromolecular science with the
invention of conducting polymers [1] and they have gained
considerable attention due to the their advantages of low cost, high
weight, easy fabrication and compability with flexible substrates
[2e4]. It was found that they can be used in many fields, such as
organic light emitting diodes [5], electrochromic materials [6],
organic solar cells [7], organic field effect transistors [8] and sensors
[9]. Since band gap of the polymer plays an important role to
accomplish desired applications [10], there are many approaches
for manipulating it. However, the donoreacceptor phenomenon,
which provide the band gap alternation due to the electron rich and
electron poor building blocks in the polymer main chain, is one of
the most exploited technique for designing organic conducting
levels of D-A type polymers can be tuned [12]. Based on this,
donoreacceptor conjugated polymers have been extensively used
as a low band-gap donor materials in achieving high performance
polymer solar cells (PSCs) [13,14]. Therefore, many different types
of low band gap polymers have been synthesized and developed for
this purpose. 1,2,3-Benzotriazole [15], diketopyrrolopyrrole [16],
thieno [3,4-b] pyrazine [17], 2,1,3-benzothiadiazole [18], are mostly
preferred acceptor type units to design donoreacceptor type low
band gap molecules. Due to the their good electron-withdrawing
properties benzothiadiazole derivatives are used to generate low
band gap molecules for photovoltaic and electroluminescent de-
vices especially [19]. Owing to the good electron-withdrawing
feature, benzoselenadiazole is one of the mostly used unit in D-A
type low band gap polymer [20]. Benzoselenadiazole (BSe) is
generated by replacing the sulfur atom in the benzothiadiazole (BT)
unit with selenium. Polymers containing benzoselenadiazole (BSe)
building block have lower HOMO energy level than those having
benzothiadiazole (BT) unit due to selenium's larger size and
* Corresponding author. Department of Chemistry, Middle East Technical Uni-
versity, 06800 Ankara, Turkey.
E-mail address: toppare@metu.edu.tr (L. Toppare).
http://dx.doi.org/10.1016/j.polymer.2014.05.029
0032-3861/© 2014 Elsevier Ltd. All rights reserved.

3094
S. Toksabay et al. / Polymer 55 (2014) 3093e3099
stronger electron affinity [21]. In addition to this, selenium
comprising polymers have red shift in the absorption spectrum
compared to sulfur containing polymers [22e24]. The selenophene
unit could also enhance inter-chain interactions between polymer
chains via strong SeeSe interactions [25].
Also, among an extensive range of acceptor units, quinoxaline
units (Qx) have been demonstrated to be an admirable building
block for synthesis of low band gap conjugated polymers due to the
their electron withdrawing feature of two imine nitrogens in the
Qx. In terms of providing the utility of introducing substituents
easily on the 2 and 3 positions of itself, the Qx unit has a great
structure for controlling the electronic structure of the ending
polymers [26].
0.071 mmol) was added at room temperature under inert atmo-
sphere. The mixture was stirred at 100 ꢁC under argon atmosphere
for 15 h, cooled and concentrated on the rotary evaporator. The
residue was subjected to column chromatography to afford dark
purple solid (silica gel, CHCl3:hexane, 1:1).
1H NMR (400 MHz, CDCl3,
J ¼ 5.2 Hz, 2H), 7.23 (d, J ¼ 5.3 Hz, 2H).
13C NMR (100 MHz, CDCl3,
):162.6, 146.7, 146.1, 138.4, 134.9,
d):8.33 (s,2H), 7.73 (s,2H), 7.38(d,
d
131.2, 131.1, 130.9, 127.4, 127.3, 118.4.
2.1.2. Synthesis of 2,3-bis(3,4-bis(decyloxy)phenyl)-5,8-di(thieno
[3,2-b]thiophen-2-yl)-2,3-dihydroquinoxaline
2,3-Bis[3,4-bis[decyloxy]phenyl]-5,8-dibromoquinoxaline
(200 mg, 0.2 mmol) and tributyl[thieno[3,2-b]thiophen-2-yl]stan-
nane (171.7 mg, 0.4 mmol) were dissolved in dry THF (100 mL). The
mixture was degased with argon for 30 min and Pd(PPh3)2Cl2
On the other hand, photoactive semiconductor materials using
in PSCs are recently developed to enhance their performance. For
this purpose, the incorporation of
p-conjugated, rigidly fused
thiophene rings are used in a conjugated polymer main chain, since
fused thiophene rings can make the molecular backbone more rigid
and coplanar [27]. In addition, materials having fused thiophene
rings remarkably attractive owing to the their great optoelectronic
properties consist of aromatic coupled structures with extending
(50 mg, 0.071 mmol) was added at room temperature under an
inert atmosphere. The mixture was stirred at 100 C under argon
ꢁ
atmosphere for 15 h, cooled and concentrated on the rotary evap-
orator. The residue was subjected to column chromatography to
afford an orange solid (silica gel, CHCl3:hexane 1:1).
p-conjugated length, facilitating the achievement of closely packed
1H NMR (400 MHz, CDCl3,
d
):8.06 (d, J ¼ 6.9 Hz, 2H), 7.47(s, 2H),
conjugated backbones and consequently more effective charge
intermolecular hopping and transport [28]. Among large number of
fused-ring structures, thieno[3,2-b]thiophene derivatives with
fused two thiophene rings have attracted scientific interest due to
the their symmetrical, planar structure and their potential to pro-
vide high charge carrier mobility due to its planar structure
7.35 (d, J ¼ 5.2 Hz, 2H), 7.22 (d, J ¼ 5.2 Hz, 2H), 7.16(s, 2H), 6.77 (d,
J ¼ 8.4 Hz, 2H), 3.97 (t, J ¼ 6.6 Hz, 8H), 3.92(d, J ¼ 6.6 Hz, 8H),1.77(m,
J ¼ 7.9 Hz, 30H), 1.44(m, J ¼ 7.6 Hz, 8H), 0.82(t, J ¼ 4.4 Hz, 30H).
13C NMR (100 MHz, CDCl3,
d):163.6, 154.5, 149.2, 141.5, 132.5,
126.5, 122.5, 118.5, 117.8, 114.7, 111.8, 68.22, 68.15, 30.92, 29.88,
28.78, 28.71, 28.65, 28.60, 28.47, 28.45, 28.39, 28.35, 28.28, 25.16,
25.07, 21.68, 13.09.
enhancing
p-stacking ability of resulting polymer [29]. Also, dis-
cussion on the effect of thieno[3,2-b]thiophene on the properties of
polymers containing Bse and Qx acceptors lacks in literature. With
this aim, we designed and synthesized polymers consisting of BSe,
Qx and thieno[3,2-b]thiophene moieties.
3. Results and discussion
3.1. Synthesis
2. Experimental section
Synthetic route of monomers was presented in Scheme 1. Pre-
viously published procedure was utilized for the syntheses of 4,7-
dibromobenzo[c][1,2,5]selenadiazole [32], 2,3-bis[3,4-bis[decy-
loxy]phenyl]-5,8-dibromoquinoxaline [33], tributyl[thieno[3,2-b]
thiophen-2-yl]stannane [31]. Syntheses of QTT and BSeTT were
performed via Stille coupling in the presence of Pd(PPh3)2Cl2 and
THF.
2.1. General
All reagents were obtained from commercial sources and used
without further purification unless otherwise mentioned. THF was
dried over sodium and benzophenone. 3,6-Dibromobenzene-1,2-
diamine [30] tributyl(thieno[3,2-b]thiophen-2-yl)stannane [31]
4,7-dibromobenzo[c][1,2,5]selenadiazole [32] and 2,3-bis(3,4-
3.2. Electrochemical properties
bis(decyloxy)phenyl)-5,8-dibromo-2,3-dihydroquinoxaline
[33]
were synthesized according to previously published procedures.
1H NMR and 13C NMR spectra were recorded in CDCl3 on Bruker
Spectrospin Avance DPX-400 Spectrometer with TMS as the inter-
nal reference. Electropolymerization was performed in a three
electrode cell consisting of an Indium Tin Oxide doped glass slide
(ITO) as the working electrode, platinum wire as the counter elec-
trode, and Ag wire as the pseudo reference electrode under
ambient conditions using a Gamry potentiostat. Before each mea-
surement, argon gas was purged into the dichloromethane solution
for 5 min. The reference electrode was subsequently calibrated to
Fc/Fcþ and the band energies were calculated relative to the vac-
uum level taking the value of SHE as ꢀ4.75 eV [23]. Spectroelec-
trochemical studies of polymers were carried out using Varian Cary
5000 UVeVis spectrophotometer.
To get a deeper perspective on the electrochemical and spec-
troelectrochemical properties of conjugated polymers, different
methods have been conducted as a further characterization and
results are summarized in this section. Cyclic voltammetry (CV)
studies were performed both for electrochemical polymerization of
monomers and investigation of the redox properties of the elec-
trochemically synthesized polymers. The system consists of a
potentiostat and a cell bearing indium tin oxide (ITO) coated glass
plate as working electrode, platinum wire as counter and Ag wire
pseudo reference electrodes.
Both BSeTT and QTT were polymerized potentiodynamically on
ITO coated glass slide. While electrochemical polymerization of
BSeTT was performed in a 0.1 M NaClO4/LiClO4 (1:1)/ACN:DCM
(acetonitrile: dichloromethane) (95:5) solution, the system for
electropolymerization of QTT as NaClO4/LiClO4 (1:1)/ACN:DCM
(1:1) is preferred due to the low solubility of this derivative. The
cyclic voltammograms for electrochemical polymerizations scan-
ned between 0 V/1.6 V and between 0 V/1.1 V at a scan rate of
100 mV/s, respectively are reported Fig. 1.
2.1.1. Synthesis of 4,7-di(thieno[3,2-b]thiophen-2-yl)benzo[c][1,2,5]
selenadiazole
4,7-Dibromobenzo[c][1,2,5]selenadiazole (200 mg, 0.6 mmol)
and tributyl[thieno[3,2-b]thiophen-2-yl]stannane (515.2 mg,
1.2 mmol) were dissolved in dry THF (100 mL). The mixture was
degased with argon for 30 min and PdCl2(PPh3)2 (50 mg,
In the first cycle of CV irreversible monomer oxidation peaks
wererecordedat 1.39 V andat 0.83 V for BSeTTandQTT, respectively.

S. Toksabay et al. / Polymer 55 (2014) 3093e3099
3095
Scheme 1. Synthetic pathway for monomers.
Then, after each successive cycle formation of new reversible redox
couples with an increasing current intensity were observed which
prove the deposition of electroactive polymer films on ITO surface.
(Fig. 1) After electrochemical polymerization, polymer coated ITO
electrodes were subjected to CV in a monomer free 0.1 M LiClO4/
NaClO4/ACN solution to investigate the redox behaviors of PBSeTT
and PQTT. As illustrated in Fig. 2, reversible p-doping processes at
1.17 V for PBSeTT and at 0.7 V for PQTT were recorded.
Fig. 1. Repeated potential scan electropolymerization of a) BSeTT in 0.1 M NaClO4/LiClO4 (1:1)/ACN:DCM (95:5) and b) QTT in 0.1 M NaClO4/LiClO4 (1:1)/ACN:DCM (1:1) on an ITO
electrode at a scan rate of 100 mV/s.

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S. Toksabay et al. / Polymer 55 (2014) 3093e3099
Fig. 2. Single scan cyclic voltammograms of electrochemically synthesized polymers a) PBSeTT and (b) PQTT in 0.1 M LiClO4/NaClO4 (1:1)/ACN solution.
When the shapes of single scan cyclic voltammograms are
quinoxaline derivatives were compared with previous examples,
lower oxidation potentials of latter easily observed (Fig. 2), which is
consistent with the literature.
HOMOeLUMO energy levels of conducting polymers are very
important for their application areas and should be discussed. Us-
ing the onset of the corresponding oxidation potentials (Fig. 2)
compared, it can be predicted that PQTT has a more charge trap-
ping capacity than that of PBSeTT. In addition, in terms of the
oxidation potentials of PBSeTT and PQTT, the lower oxidation po-
tential of PQTT can be attributed to the different electron density of
acceptor units in the structure. When benzoselenathiazole and
Fig. 3. Scan rate dependence of (a,b) PBSeTT and (c,d) PQTT in a 0.1 M NaClO4eLiClO4 (1:1)/ACN solution.

S. Toksabay et al. / Polymer 55 (2014) 3093e3099
3097
Table 1
530 nm for PBSeTT started to decrease, new bands were appeared
at around 900 nm and 1300 nm due to the formation of charge
carriers on the polymer backbone namely polarons (radical cation)
and bipolarons (dication) (Fig. 4a). While PBSeTT revealed only one
absorption maxima in the visible region, two absorption maxima
were recorded for PQTT centered at 442 nm and 600 nm which is
similar to its quinoxaline based counterparts. Similar to PBSeTT,
successive oxidation resulted in a decrease in neutral state ab-
sorptions with the formation of new bands at around 800 nm and
1200 nm for PQTT(Fig. 4b). Polymer PQTT could not be fully
undoped since at negative voltages leakage of the film occurs from
the electrode surface. From spectroelectrochemical studies, besides
lmax values, optical band gaps of these type of materials could be
calculated which is a crucial parameter for their applications in
Electrochemical and optical properties of PBSeTT and PQTT.
HOMO(eV)
LUMOa (eV)
lmax (nm)
Eg (eV)
PBSeTT
PQTT
ꢀ5.88
ꢀ5.47
ꢀ4.95
ꢀ4.17
530
442/600
0.93
1.30
a
LUMO energy levels calculated using optical band-gap values and HOMO energy
levels.
against Fc/Fcþ reference electrode (NHE was taken as 4.75 eV vs.
vacuum) HOMO energy levels were calculated as ꢀ5.88 eV
and ꢀ5.47 eV for PBSeTT and PQTT. As mentioned before these
polymers have only p-doping characteristics, hence LUMO energy
levels were calculated using HOMO energy and optical band gap
values and found as ꢀ4.95 eV and ꢀ4.17 eV, respectively.
The scan rate dependence of dopingededoping processes is
important to investigate whether this process is non-diffusion
controlled or not. For this purpose single scan cyclic voltammo-
grams of both polymer films were recorded at different scan rates
and the scan rate dependence of both PBSeTT and PQTT (Fig. 3)
were investigated. Linear relationship between the current density
and the scan rate proves that the electroactive polymer films were
well adhered and the redox processes were non-diffusion
controlled. Results of electrochemical studies were reported in
Table 1.
different fields such as organic solar cells. Optical band gaps (Egop
of both polymer films were determined from the lowest energy
* transitions (530 nm and 600 nm for PBSeTT and PQTT), and
)
pep
the following values were found as 0.93 eV and 1.30 eV respectively
(Table 1). Calculated Egop values state that both polymers can be
regarded as a low band gap polymers. While Fig. 4 reveals spec-
troelectrochemistry results of polymers, corresponding colors at
the neutral and oxidized states were reported in Fig. 5.
From Figs. 4 and 5 it can be clearly pointed out that spec-
troelectrochemical studies are also consistent with the colors of
polymer films in their neutral and oxidized states. Both polymers
revealed different colors in their neutral and oxidized states. As
seen in Fig 4, PBSeTT has a wide range of absorption (nearly full
visible absorption) in the visible region which makes it dark gray in
In the light of the points mentioned in this part, it can be clearly
pointed out that both electrochemically synthesized polymers have
only p-type doping character.
both
states.
Similar
to
other
quinoxaline
bearing
donoreacceptoredonor type polymers two absorption peaks in the
red (in the web version) and blue (in the web version) regions of the
visible spectrum resulted in a neutral state green (in the web
version) polymer, PQTT. After stepwise oxidation, PQTT revealed a
gray colored oxidized state where almost all visible light was har-
vested by the polymer film.
3.3. Spectroelectrochemical studies
Spectroelectrochemical studies mainly focus on optical changes
and electronic transitions as a result of applied potentials. For these
studies polymer films were prepared via electrochemically as
described before and all these studies were conducted in 0.1 M
NaClO4eLiClO4/ACN using UVeViseNIR spectrophotometer via
incrementally increasing applied potential between 0.0 V and
0.95 V for PBSeTT, 0.0 V and 1.2 V for PQTT as decided from CV
results reported in Fig. 2.
Before performing stepwise oxidation, both polymers (PBSeTT
and PQTT) were reduced to their neutral states to get a real neutral
state absorption and to remove any trapped charge and dopant ion
remained from electrochemical polymerization. Upon stepwise
oxidation, while the absorption in the visible region centered at
3.4. Electrochromic contrast and switching studies
Both percent transmittance and switching times are crucial
parameters for electrochromic materials. Switching time can be
defined as the time required for the color change of a polymer
between two extreme states (neutral and oxidized/reduced states).
How fast the electrochromic materials change their colors affects
the switching time. Percent transmittance values and switching
Fig. 4. Electronic absorption spectra of a) PBSeTT switching between 0.0 V and 0.95 V, b) PQTT between 0.0 V and 1.2 V in 0.1 M NaClO4/LiClO4(1:1)/ACN solution.

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S. Toksabay et al. / Polymer 55 (2014) 3093e3099
Fig. 5. Structures and colors of the polymers at neutral and oxidized states.
times for both polymers were monitored in visible and NIR regions
by applying square-wave potential steps. Results were calculated
from Fig. 6 and summarized in Table 2.
In this study two different groups (benzoselenathiazole and
quinoxaline) were chosen as the acceptor units and coupled with
fused donor unit, thieno[3,2-b]thiophene. The optical contrasts
were observed for PQTTas 13% at 440 nm,12% at 800 nm and 34% at
1180 nm with low switching times as 0.3 s, 0.3 s and 0.5 s respec-
tively. Although the optical contrasts are not so high, the polymer
film was stable upon repetitive cycles. PBSeTT revealed 13% optical
contrast upon doping/de-doping processes with a switching time
of 3.1 s in NIR region. It showed 4% optical contrast at 525 nm and
13% at 830 nm with 3.1 s and 1.4 s switching times at corresponding
wavelengths.(Table 3)
When benzoselenathiazole and quinoxaline based polymers
PBSeTT and PQTT are compared in terms of their electrochromic
contrast and switching abilities, it was clearly seen that PQTT
has shorter switching times and higher percent transmittance
values than that of PBSeTT which is also consistent with the
literature.
Fig. 6. Optical contrasts and switching times for (a) PBSeTT and (b) PQTT recorded at different wavelengths in 0.1 M NaClO4/LiClO4 (1:1)/ACN solution.

S. Toksabay et al. / Polymer 55 (2014) 3093e3099
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Table 2
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Two novel donoreacceptoredonor type monomers comprising
benzoselenadiazole and quinoxaline units as the acceptor and
thieno[3,2-b]thiophene as the donor group were successfully syn-
thesized by Stille Coupling reaction (BSeTT and QTT). After the
characterization of monomers, polymerizations were carried out
electrochemically on ITO coated glass slides to examine their
electrochemical and optical properties. Both polymers revealed
different colors in their neutral and oxidized states. PBSeTT has a
wide range of absorption (nearly full visible absorption) in the
visible region which makes it dark gray in both states. Similar to
other quinoxaline bearing donoreacceptoredonor type polymers
two absorption peaks in the red and blue regions of the visible
spectrum resulted in a neutral state green polymer, PQTT. After
stepwise oxidation, PQTT revealed a gray colored oxidized state
where almost all visible light was harvested by the polymer film.
The band gap of the polymers were calculated to be 0.93 eV and
1.3 eV for PBSeTT and PQTT which is a proper value keeping in mind
that polymers with DA units in general demonstrate band gaps
between 0.9 eV and 1.3 eV. Comparison among monomers showed
that increasing electron density of donor units decreases the band
gap of the monomers where donoreacceptor couple is a substantial
effect on the electronic properties of donoreacceptoredonor ma-
terial. PQTT has lower oxidation potential and band gap. PQTT will
be used as active materials in future display technology. Moreover,
PBSeTT which has broadest absorption than others which makes it
a good candidate for solar cell applications.
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