A promising combination of benzotriazole and quinoxaline units: A new acceptor moiety toward synthesis of multipurpose donor-acceptor type polymers

Article abstract of DOI:10.1039/c2jm16171k

By combining benzotriazole and quinoxaline units in an acceptor unit, three novel monomers, 2-dodecyl-6,7-diphenyl-4,9-di(thiophen-2-yl)-2H-[1,2,3] triazolo[4,5-g]quinoxaline (M₁), 2-dodecyl-4,6,7,9-tetra(thiophen-2- yl)-2H-[1,2,3]triazolo[4,5-g]quinoxaline (M₂) and 6,7-bis(4-tert-butylphenyl)-2-dodecyl-4,9-di(thiophen-2-yl)-2H-[1,2,3] triazolo[4,5-g]quinoxaline (M₃), were synthesized and polymerized electrochemically. All polymers revealed both p- and n-type doping properties under ambient conditions. As found by spectroelectrochemical studies they have small band gaps, ca. 1.00 eV. The polymers revealed two distinct absorption bands as all green coloured neutral state donor-acceptor-donor type polymers should have. Besides this, all have broad absorptions covering the region between 700 nm and 1000 nm which is rarely seen in electrochromic polymers. Due to these promising features, they are considered to be potential materials for optoelectronics.

Full text of DOI:10.1039/c2jm16171k

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PAPER
A promising combination of benzotriazole and quinoxaline units: A new
acceptor moiety toward synthesis of multipurpose donor–acceptor type
polymers
a
Serife Ozdemir, Merve Sendur, Gozde Oktem, Ozdemir Dogan and Levent Toppare
a
a
a
abc
€
*
ꢀ
Received 26th November 2011, Accepted 8th January 2012
DOI: 10.1039/c2jm16171k
By combining benzotriazole and quinoxaline units in an acceptor unit, three novel monomers,
2-dodecyl-6,7-diphenyl-4,9-di(thiophen-2-yl)-2H-[1,2,3]triazolo[4,5-g]quinoxaline (M1), 2-dodecyl-
4,6,7,9-tetra(thiophen-2-yl)-2H-[1,2,3]triazolo[4,5-g]quinoxaline (M2) and 6,7-bis(4-tert-butylphenyl)-
2-dodecyl-4,9-di(thiophen-2-yl)-2H-[1,2,3]triazolo[4,5-g]quinoxaline (M3), were synthesized and
polymerized electrochemically. All polymers revealed both p- and n-type doping properties under
ambient conditions. As found by spectroelectrochemical studies they have small band gaps, ca. 1.00 eV.
The polymers revealed two distinct absorption bands as all green coloured neutral state donor–
acceptor–donor type polymers should have. Besides this, all have broad absorptions covering the
region between 700 nm and 1000 nm which is rarely seen in electrochromic polymers. Due to these
promising features, they are considered to be potential materials for optoelectronics.
level in the structure. In accordance with these strong electron
rich and electron deficient sites on the polymer chain, the
combination of monomer segments with higher HOMO levels
and lower LUMO levels results in band gap lowering due to
interchain charge transfer.8,9
Introduction
Conjugated polymers (CPs) have been investigated over the years
for their potential as advanced materials in many optoelectronic
applications, including electrochromic devices (ECDs),1 organic
light emitting diodes (OLEDs),2 organic photovoltaics (OPVs),3
sensors4 and organic field effect transistors (OFETs).5 Due to
their extensive use in these areas, production of multipurpose
materials mostly relies on newly designed structures. In partic-
ular, smart structures are synthesized through molecular archi-
tecture, to allow the enhancement of the electronic and optical
properties of p conjugated systems via controlling structural
alterations.6
Combining donor and acceptor moieties in the polymer
backbone enhances D–A interactions and helps in developing the
most efficient polymers for OPVs.7 Besides obtaining low band
gap polymers, the D–A approach is also useful in producing
polymers with improved optical, mechanical and electronic
properties.10 The band gap can be tuned by changing the donor
and acceptor units in the repeating unit.11 As a result, the
structural variations in conjugated systems lead to control of not
only the optical and electronic properties of the polymers, but
also allow the creation of low band gap polymers.
Accordingly, optoelectronic properties can be optimized by
structural modification via increasing the quinoidal character of
the conjugated structure, along with the creation of alternating
strong electron donating and withdrawing sites on the polymer
backbone.7 Alternating donor and acceptor units bearing
conjugated polymers are thought to be essential to manipulate
the band gap. The donor–acceptor approach, one of the most
plausible ways for the construction of low band gap systems, is
confirmed by assembling an electron rich unit with a high
HOMO level and an electron deficient unit with a low LUMO
Changing both or either of the donor/acceptor groups is an
effective method to design these alternating systems. Recently,
different fused aromatic heterocyclics containing nitrogen and
sulfur atoms were used as the electron deficient units, such as
benzothiadiazole (BTd), quinoxaline, benzotriazole (BTz), and
benzimidazole (BIm). The electron withdrawing ability of these
acceptor moieties is the key to obtaining a polymer with the
desired band gap and optical properties.12
In the literature, the very first examples of BTz-containing
conjugated polymers were shown by Yamamoto and
coworkers.13 Later on, in 2009, the electrochromic properties of
another BTz derivative (PTBT) were investigated by our group.
This fascinating polymer attracted many researchers and it was
reported as a multipurpose material due to its solubility, proc-
essability and dual type dopability (both p- and n-dopable).14
aDepartment of Chemistry, Middle East Technical University, 06800
Ankara, Turkey. E-mail: toppare@metu.edu.tr; Fax: +90 3122103200;
Tel: +90 3122103222
bDepartment of Biotechnology, Middle East Technical University, 06800
Ankara, Turkey
cDepartment of Polymer Science and Technology, Middle East Technical
University, 06800 Ankara, Turkey
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After this discovery, BTz derivatives were in demand due to the
achievement of resulting desired properties such as multi-
chromism, transmissive states and photoluminescence, all
combined in a single polymer.10
Electrochemistry
Electrochemical polymerization of the monomers on ITO elec-
trodes was performed in 0.1 M TBAPF6 and 0.01 M corre-
sponding monomer solutions in acetonitrile (ACN)–CH2Cl2
(95 : 5, v/v) by applying potentials between 0 V and +1.0 V for
M1 and M3, and 0 V and +1.2 V for M2 at a scan rate of 100 mV
sꢀ1 (Fig. 2). The monomer oxidation potentials were around 1.0
V due to the same electron rich unit, thiophene in their struc-
tures. On the other hand, the polymer oxidation potentials were
distinct from each other due to the different electron densities of
the electrochemically synthesized polymer chains. In all electro-
chemical polymerizations, reversible redox peaks with increasing
current density by repeated cycling were observed.
In the meantime, quinoxaline based conjugated polymers,
which were widely studied in the literature, attracted consider-
able attention. The lack of available neutral state green coloured
polymers with highly transmissive oxidized states was addressed
in 2007 by our group. By virtue of synthesizing these EDOT
bearing quinoxaline derivatives, the missing element of the RGB
(Red, Green, Blue) puzzle was contrived.1d,15
The idea having these two main electron deficient units; ben-
zotriazole and quinoxaline in a single structure leading to the
formation of black coloured polymers inspired us to conduct this
study. Here, we basically aim to synthesize a new acceptor unit
which contains both benzotriazole and quinoxaline moieties.
Owing to the increase in imine bonds (two vs. four) with this
single structure, the molecule is expected to yield higher electron
transporting features.13
Both benzotriazole and quinoxaline derivatives have strong
electron accepting ability.1d,10 Hence, the combination of these
two strong acceptor derivatives resulted in stronger acceptor
ability than either benzotriazole or quinoxaline alone. In other
words, increasing the number of imine bonds in a structure yields
a stronger electron acceptor moiety. As a result, all polymers
revealed two reversible peaks as regards to n-doping at ambient
conditions. The HOMO–LUMO energy levels of the polymers
were estimated using the onset of the corresponding oxidation
(Fig. 3) by calibrating the reference electrode against Fc/Fc+ and
calculating the energy levels relative to the vacuum level.3a
The LUMO levels differ from each other as regards to the
electron density on the quinoxaline units which leads to
a decrease in the strength of the acceptor ability with increasing
electron density. Although such an assumption was valid for
LUMO levels of the polymers, experimentally calculated HOMO
levels from their single scan cyclic voltammograms did not reveal
plausible results. This may result from the unequal doping
features of the polymers, especially for P3. When the charge
injection/ejection is compared, P3 revealed the lowest charge, as
seen in Fig. 3. This is most probably due to the presence of the
tertiary butyl group on the phenyl units which obstructs the
ejection/injection of the dopant ions. Since all the polymers were
both n- and p-type doped, the electrochemical band gap Egec and
HOMO/LUMO values were calculated experimentally and all
the related electrochemistry data were summarized in Table 1.
Herein we designed three new monomers namely 2-dodecyl-6,7-
diphenyl-4,9-di(thiophen-2-yl)-2H-[1,2,3]triazole[4,5-g]quinoxa-
line (M1), 2-dodecyl-4,6,7,9-tetra(thiophen-2-yl)-2H-[1,2,3]tri-
azole [4,5-g]quinoxaline (M2) and 6,7-bis(4-tert-butylphenyl)-2-
dodecyl-4,9-di(thiophen-2-yl)-2H-[1,2,3]triazole[4,5-g]quinoxa-
line (M3), in order to have a proper donor–acceptor match.
Electrochemical polymerizations of corresponding monomers
resulted in the desired polymers P1, P2 and P3 (Fig. 1).
Results and discussion
Synthesis
The synthetic route to the monomers was outlined in Scheme 1.
4,7-Dibromo-2-dodecyl-2H-benzo[d][1,2,3]triazole
3
was
synthesized firstly by alkylation of 1H-benzo[d][1,2,3]triazole 1
and then by bromination with HBr and bromine. Nitration of the
resulting brominated product gave 4,7-dibromo-2-dodecyl-5,6-
dinitro-2H-benzo[d][1,2,3]triazole 4. Compound 5 was obtained
by the Stille coupling reaction of 4 and tributyl(thiophen-2-yl)
stannane. The reduction of 5 with iron dust in acetic acid yielded
2-dodecyl-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole-5,6-
diamine 6. Finally, a condensation reaction of 6 with three
different diketones, (benzyl 7, 1,2-di(thiophen-2-yl)ethane-1,2-
dione 8 and 1,2-bis(4-tert-butylphenyl)ethane-1,2-dione 9), in the
presence of PTSA and ethanol gave the desired monomers (M1,
M2, M3).
Electronic and optical studies
To probe the spectral response of the polymers to the doping
processes, in situ UV-Vis-NIR spectra were monitored in
a monomer free, 0.1 M TBAPF6–ACN solution. All three
polymers revealed two absorption maxima, the first one in the
short wavelength region ranging from 300 to 450 nm is due to the
p–p* transition, whereas the second at the long wavelength
region covering 700–1000 nm refers to intramolecular charge
transfer transitions between the donor and the acceptor moieties
(Fig. 4). The absorption of the polymers covered both red and
blue colour regions of the visible spectrum, hence reflecting the
evolution of the green color in their neutral state (Fig. 5). As the
potential was increased, the short and long wavelength absorp-
tion bands were depleted and new transitions in the NIR region
were evolved, indicating the formation of charge carriers. At
further doping stages, the formation of polarons was followed by
the introduction of bipolarons with increasing external potential.
Fig. 1 Electrochemical polymerization of the monomers.
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Scheme 1 Synthetic route for the monomers.
Since polaron and bipolaron formations were in the NIR region,
all polymers were transparent in their oxidized states. As calcu-
lated from the onset of their lowest p–p* transitions, P1 and P2
exhibited optical band gaps of 1.02 eV and 0.95 eV, respectively.
P3 revealed the lowest optical band gap of 0.92 eV due to more
extensive p–p* transitions at around 882 nm. Electronic band
gaps can be higher than the optical ones since electronic band
gaps are calculated using CV data where the polymers contain
charged species like polarons.
Although all monomers and polymers contain bulky groups
attached to quinoxaline units, P3 has an additional tertbutyl
bulky group on the quinoxaline unit. As a result it may affect the
degree of polymerization and arrangement of repeating units in
the polymer chains. This may be the reason why the spectral
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Fig. 2 Electrochemical polymerizations of (a) M1 (b) M2 and (c) M3 at 100 mV sꢀ1 in 0.1 M TBAPF6/CH2Cl2/ACN on ITO electrodes.
absorption, broadening of absorptions covering between 550 nm
and 650 nm were observed.
During the n-doping process, all polymers showed multi-
chromic behaviour which is rarely seen in donor–acceptor type
polymers (Fig. 6). This multichromic behaviour of the polymers
during the n-doping process may arise from two features: firstly,
the presence of two reversible redox couples as given in Fig. 2 and
secondly the presence of alkyl chains bonded to triazole units
which affects the doping rate.
Electrochromic switching studies
To investigate switching studies, the polymer films on ITO
surfaces were studied via switching the potentials corresponding
to their doped and dedoped states. During the switching, the
absorbance at the specific wavelengths determined from the
spectra of polymers, were monitored as a function of time with
a UV-Vis-NIR spectrophotometer. Due to their high absorption
in the NIR region, all polymers showed a high optical contrast in
NIR region; P1 showed 93% contrast at 2000 nm with 2 s of
switching time, P2 revealed 71% with 1.7 s at the same wavelength
and P3 presented 69% with 1.6 s at 1800 nm (Fig. 7). In the
studies as the wavelength concerned decreased, the optical
contrasts revealed by the polymers were increased; on the other
hand, switching times were decreased. Optical contrasts,
switching times and percent loss in optical contrasts were
summarized in Table 2.
Fig. 3 Single scan cyclic voltammograms of P1, P2 and P3 films using
ACN as the solvent and 0.1 M TBAPF6 as the supporting electrolyte at
a scan rate of 100 mV sꢀ1 and reduction peaks.
behaviours of P1 and P2 are different from that of P3. Recently
both p- and n-dopable polymers made several application areas
feasible under atmospheric conditions such as light emitting
diodes, field-effect transistors and organic solar cells.2,3,5
Although P1, P2 and P3 revealed reversible n-type redox couples,
there should be additional evidence such as considerable struc-
tural and especially optical differences after the introduction of
charge carries to the conjugated system. Accretion of bands in
the NIR absorption region upon n-type doping of P1, P2 and P3 is
clear evidence for the formation of negative charge carriers
(Fig. 4).
Experimental
The normalized absorbance spectra of monomers in both
CH2Cl2 and thin film forms are shown in Fig. 5. In the solution,
monomers revealed absorption in visible region centered at 550
nm, 567 nm and 548 nm for M1, M2 and M3, respectively. As
regards the ones in thin film form, reduction in conformational
freedom, solvent–monomer interactions and a tendency to
aggregate in thin film form mostly causes a red shift absorption
compared to the ones taken in solutions.16 In addition to red shift
Materials
All chemicals and reagents were obtained from commercial
sources and used without further purification. THF was dried
over sodium and benzophenone. 2-Dodecyl-2H-benzo[d][1,2,3]
triazole (2),14 4,7-dibromo-2-dodecyl-2H-benzo[d][1,2,3]triazole
(3)14 and tributyl(thiophen-2-yl)stannane17 were synthesized
Table 1 Summary of electrochemical and spectroelectrochemical properties of P1, P2 and P3
Emonox/V
Ep-doping/V
Ep-dedoping/V
En-doping/V
En-dedoping/V
HOMO/eV
LUMO/eV
lmax/nm
Egop/eV
Egec/eV
P1
P2
P3
0.95
1.00
0.94
0.54
0.70
0.78
0.36
0.52
0.70
ꢀ1.38/ꢀ1.75
ꢀ1.13/ꢀ1.60
ꢀ1.38/ꢀ1.91
ꢀ0.88/ꢀ1.46
ꢀ0.60/ꢀ1.11
ꢀ1.04/ꢀ1.54
ꢀ5.00
ꢀ5.07
ꢀ5.37
ꢀ3.77
ꢀ3.89
ꢀ3.88
450/950
432/1020
400/886
1.02
0.98
0.92
1.23
1.18
1.49
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Fig. 4 p-Type doping spectra for P1 (a.1), P2 (b.1) and P3 (c.1) and n-type doping spectra for P1 (a.2), P2 (b.2) and P3 (c.2).
according to previously published procedures. All condensation
reactions were conducted with respect to the known
procedures.18
stirred at room temperature for 3 h and then poured into an ice-
bath. The precipitate was collected by filtration and recrystallized
with an acetone-water mixture to yield 4,7-dibromo-2-dodecyl-
5,6-dinitro-2H-benzo[d][1,2,3]triazole (4) as a yellow solid (1.0 g,
56% yield).
1H NMR (400 MHz, CDCl3) d 4.93 (t, J ¼ 7.3 Hz, 2H), 2.31
(m, 2H), 1.35–1.02 (m, 18H), 0.88 (t, J ¼ 6.8 Hz, 3H). 13C NMR
(101 MHz, CDCl3) d 141.7, 140.5, 105.7, 33.2, 30.4, 28.5, 28.2,
28.1, 27.9, 27.8, 27.7, 27.3, 24.9, 21.2, 12.6.
Synthesis of 4,7-dibromo-2-dodecyl-5,6-dinitro-2H-benzo[d]
[1,2,3]triazole (4)
Compound 4 was synthesized by the modification of a previously
published procedure.19 A mixture of concentrated sulphuric acid
(15 ml) and fuming nitric acid (15 ml) was used for the nitration
of 4,7-dibromo-2-dodecyl-2H-benzo[d][1,2,3]triazole (3). The
acid mixture was cooled to 0 ꢁC and compound 3 (1.5 g, 3.37
mmol) was added to ꢁthe mixture in small portions to keep the
temperature below 5 C. This heat control prevents over-nitra-
tion. After the addition was completed, the reaction mixture was
Synthesis of 2-dodecyl-5,6-dinitro-4,7-di(thiophen-2-yl)-2H-
benzo[d][1,2,3]triazole(5)
4,7-Dibromo-2-dodecyl-5,6-dinitro-2H-benzo[d][1,2,3]triazole
(4) (1.5 g, 2.80 mmol) and tributyl(thiophen-2-yl)stannane
Fig. 5 Absorption spectra of monomers in solution (a) and thin film form (b).
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Fig. 6 Structures of the polymers and their colors under different applied potentials.
(5.22 g, 13.97 mmol) were dissolved in dry THF (60 ml).
PdCl2(PPh3)2 catalyst was added after the reaction mixture was
refluxed for 1 h. Then it was stirred at 100 ꢁC under argon
atmosphere for 18 h. At the end of this period, the residue was
subjected to column chromatography (silica gel, CHCl3–hexane,
1 : 2) to afford compound 5 in 45% yield (660 mg, 1.22 mmol).
1H NMR (400 MHz, CDCl3) d 7.67 (dd, J ¼ 5.1 Hz, 2H), 7.54
(dd, J ¼ 3.7 Hz, 2H), 7.21 (dd, J ¼ 5.0 Hz, 2H), 4.81 (t, J ¼ 7.2
Hz, 2H), 1.62 (m, 2H), 1.38–1.29 (m, 18H), 0.94 (t, 3H).13C NMR
(101 MHz, CDCl3) d 141.9, 140.1, 130.4, 130.4, 130.1, 127.9,
119.8, 32.1, 30.0, 29.6, 29.4, 29.3, 29.0, 28.3, 26.7, 26.5, 22.7, 17.3,
13.5.
mmol) and benzil 7 (230 mg, 1.09 mmol) in EtOH (40 ml) was
refluxed overnight with a catalytic amount of p-toluene sulfonic
acid. The mixture was cooled to 0 ꢁC and concentrated on
a rotary evaporator. The residue was purified by flash column
chromatography (silica gel, CHCl3–hexane, 1 : 1) to yield
a purple oily product in 40% yield (110 mg, 0.17 mmol).
1H NMR (400 MHz, CDCl3) d 8.92 (dd, J ¼ 3.9 Hz, 2H), 7.76–
7.70 (m, 4H), 7.56 (dd, J ¼ 5.2, 2H), 7.37–7.32 (m, 6H), 7.23 (dd,
J ¼ 5.1 Hz, 2H), 4.91 (t, J ¼ 7.3 Hz, 2H), 1.48–1.45 (m, 2H),
1.21–1.16 (m, 18H), 0.80–0.78 (m, 3H). 13C NMR (101 MHz,
CDCl3) d 151.7, 142.9, 138.8, 136.2, 134.8, 133.0, 131.7, 130.7,
129.9, 129.0, 128.1, 126.5, 57.9, 47.1, 32.0, 30.1, 30.0, 29.6, 29.5,
29.4, 29.4, 29.1, 26.7, 22.8. HRMS (EI) for C40H41N5 S2 calcu-
lated 656.2882, found 656.2880.
Synthesis of 2-dodecyl-6,7-diphenyl-4,9-di(thiophen-2-yl)-2H-
[1,2,3]triazolo[4,5-g]quinoxaline (M1)
Synthesis of 2-dodecyl-4,6,7,9-tetra(thiophen-2-yl)-2H-[1,2,3]
triazolo[4,5-g]quinoxaline (M2)
Iron dust (350 mg) in acetic acid (25 ml) was used for the
reduction of 2-dodecyl-5,6-dinitro-4,7-di(thiophen-2-yl)-2H-
benzo[d][1,2,3]triazole (240 mg, 0.44 mmol) at 50 ꢁC. The mixture
was poured into cold NaOH solution and then extracted three
times with ether. 2-Dodecyl-4,7-di(thiophen-2-yl)-2H-benzo[d]
[1,2,3]triazole-5,6-diamine (6) was used without any purification.
A previously published procedure was modified for compound
6.20 Then a condensation reaction was performed to get the
desired monomer. A solution of compound 6 (200 mg, 0.42
2-Dodecyl-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole-5,6-
diamine was synthesized according to a previously described
procedure.20 A solution of compound 6 (250 mg, 0.52 mmol) and
1,2-di(thiophen-2-yl)ethane-1,2-dione (8) (115 mg, 0.52 mmol) in
EtOH (40 ml) was refluxed overnight with a catalytic amount of
p-toluene sulfonic acid. The mixture was cooled to 0 ꢁC and
concentrated on the rotary evaporator. The residue was purified
Fig. 7 Optical contrasts and switching times of (a) P1, (b) P2 and (c) P3 at different wavelengths.
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Table 2 Summary of kinetic and optic studies of P1, P2 and P3
P1
P2
P3
945 nm
1350 nm
2000 nm
900 nm
1650 nm
2000 nm
810 nm
1560 nm
1800 nm
%Ta
29
0.1
1.7
52
0.5
4.9
93
2
2.8
18
0.8
3.1
70
1.2
0.8
71
1.7
1.5
20
0.3
3.7
62
1.4
7.2
69
1.6
1.9
t/sb
%T lostc
a
%T is the optical contrast difference revealed by polymers during square-wave voltammetry technique at certain wavelengths. b t is the switching time
at certain wavelengths. c %T lost is the percent contrast of the polymers lost after 30 full switches.
by flash column chromatography (silica gel, CHCl3–hexane,
1 : 1) to yield a bluish purple solid in 26% yield (90 mg, 0.13
mmol).
measurement, solutions were purged with nitrogen gas for 5 min.
A Cary 5000 UV–Vis-NIR spectrophotometer was used to
perform the spectroelectrochemical studies of polymers. HRMS
studies were done with a Waters SYNAPT MS system.
1H NMR (400 MHz, CDCl3) d 8.81 (dd, J ¼ 3.8 Hz, 2H), 7.56
(dd, J ¼ 9.6 Hz, 2H), 7.50 (dd, J ¼ 5.0 Hz, 2H), 7.41 (dd, J ¼ 3.7
Hz, 2H), 7.19 (dt, J ¼ 7.1 Hz, 2H), 6.98 (dd, J ¼ 5.0 Hz, 2H), 4.83
(t, J ¼ 7.3 Hz, 2H), 2.24–2.16 (m, 2H), 1.23–1.07 (m, 18H), 0.80
(t, J ¼ 4.7 Hz, 3H). 13C NMR (101 MHz, CDCl3) d 144.1, 141.7,
140.7, 134.7, 131.9, 130.7, 129.7, 129.5, 128.8, 126.3, 125.6, 118.3,
30.9, 29.0, 28.9, 28.6, 28.5, 28.4, 28.3, 28.0, 26.0, 24.3, 21.9, 13.5.
HRMS (EI) for C36H37N5S4 calculated 668.2010, found
668.1979.
Conclusion
Three new donor–acceptor type monomers bearing combined
benzotriazole and quinoxaline units as the acceptor and thio-
phene as the donor group were synthesized. Although both
quinoxaline and benzotriazole derivatives were widely studied in
the literature, their combination in one acceptor unit, in order to
have a more electron deficient acceptor unit, was designed for the
first time. After the characterization of the monomers, their
polymers were electrochemically synthesized on ITO coated glass
slides to investigate their electrochemical and optical properties.
All polymers are both p- and n-dopable. Furthermore, the
multicoloured n-doped state of all polymers is a rare property
that widens their use in several applications. Spectroelec-
trochemical studies illustrate the broad absorptions between 700
nm and 1000 nm which in return refer to low band gap polymers
with about 1.00 eV. Another attractive character of the prepared
polymers is their high optical contrast in the NIR region, espe-
cially for P1 which has 93% contrast at 2000 nm which makes it
a promising candidate for NIR electrochromic applications.
Synthesis of 6,7-bis(4-tert-butylphenyl)-2-dodecyl-4,9-
di(thiophen-2-yl)-2H-[1,2,3]triazolo[4,5-g]quinoxaline (M3)
2-Dodecyl-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole-5,6-
diamine was synthesized according to the previously described
procedure. A solution of compound 6 (250 mg, 0.52 mmol) and
1,2-bis(4-tert-butylphenyl)ethane-1,2-dione
9 (170 mg, 0.52
mmol) in EtOH (40 ml) was refluxed overnight with a catalytic
amount of p-toluene sulfonic acid. The mixture was cooled to
ꢁ
0 C and concentrated on a rotary evaporator. The residue was
purified by flash column chromatography (silica gel, CHCl3–
hexane, 1 : 2) to give a purple oily product in 20% yield (80 mg,
0.10 mmol).
1H NMR (400 MHz, CDCl3) d 8.90 (dd, J ¼ 3.9 Hz, 2H), 7.71
(d, J ¼ 8.4 Hz, 4H), 7.57 (dd, J ¼ 5.1 Hz, 2H), 7.35 (d, J ¼ 8.5 Hz,
4H), 7.23 (dd, J ¼ 5.1 Hz, 2H), 4.90 (t, J ¼ 7.3 Hz, 2H), 2.25–2.19
(m, 2H), 1.36–1.24 (m,18H), 1.30 (s, 18H), 0.78 (t, 3H). 13C NMR
(101 MHz, CDCl3) d 152.3, 151.8, 142.3, 135.9, 135.7, 131.5,
130.5, 130.4, 126.5, 125.0, 124.0, 122.6, 34.8, 33.7, 33.5, 33.4,
31.9, 31.3, 30.1, 29.6, 29.5, 29.4, 29.3, 29.0, 27.0, 26.7, 26.6, 26.5.
HRMS (EI) for C48H57N5S2 calculated 768.4134, found
768.4134.
Acknowledgements
The authors thank TUBA and State Planning Organization;
DPT2009K121030 for financial support and also METU Central
Laboratory for HRMS data.
Notes and references
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