Synthesis and electrochemical properties of a new benzimidazole derivative as the acceptor unit in donor-acceptor-donor type polymers

Article abstract of DOI:10.1016/j.electacta.2012.02.047

A new benzimidazole unit, 4′-(tert-butyl)spiro[benzo[d]imidazole-2, 1′-cyclohexane] was synthesized and coupled with different donor units like 3-hexylthiophene and 3,4-ethylenedioxythiophene (EDOT) via Stille coupling. The donor-acceptor-donor (D-A-D) ty

Full text of DOI:10.1016/j.electacta.2012.02.047

Electrochimica Acta 67 (2012) 224–229
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Synthesis and electrochemical properties of a new benzimidazole derivative as
the acceptor unit in donor–acceptor–donor type polymers
Ali Can Ozelcaglayana , Merve Sendura , Naime Akbasoglua , Dogukan Hazar Apaydinb , Ali Cirpana,b,d
,
Levent Topparea,b,c,d,∗
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 Department of Biotechnology, Middle East Technical University, 06800 Ankara, Turkey
d The Center for Solar Energy Research and Application (GÜNAM), 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
A new benzimidazole unit, 4ꢀ-(tert-butyl)spiro[benzo[d]imidazole-2,1ꢀ-cyclohexane] was synthesized
and coupled with different donor units like 3-hexylthiophene and 3,4-ethylenedioxythiophene
(EDOT) via Stille coupling. The donor–acceptor–donor (D–A–D) type monomers, 4ꢀ-(tert-butyl)-
4,7-bis(4-hexylthiophen-2-yl)spiro[benzo[d]imidazole-2,1ꢀ-cyclohexane] (BIHT) and 4ꢀ-(tert-butyl)-4,7-
bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)spiro[benzo[d]imidazole-2,1ꢀ-cyclohexane] (BIED) were
electrochemically polymerized, their electrochemical and optical properties were investigated by cyclic
voltammetry, UV–vis-NIR spectroscopy techniques. Effect of donor groups on the optical and electronic
properties of polymer was studied.
Article history:
Received 29 November 2011
Received in revised form 13 February 2012
Accepted 15 February 2012
Available online 25 February 2012
Keywords:
Benzimidazole
Electrochromism
Multichromic
Conjugated polymers
Donor effect
© 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Recently, low band gap D–A–D type conjugated polymers
have attracted great attention with their organic electronic
Applications of conjugated polymers, such as electrochromic
devices [1–7], organic light emitting diodes [8–10], organic photo-
voltaic devices [11–13], and organic field effect transistors [14–16]
made them outstanding topics in polymer science. Properties like
ease of processability, lightweight and ease of changing the struc-
tural properties to alter the absorption, solubility and electrical
properties make them superior to their inorganic counterparts [17].
Among these band gap is the most important criterion for the opti-
cal and electronic properties of the conjugated polymers. Therefore,
most studies focus on tuning the band gap of polymer in order
to obtain desired electronic and optical properties of conjugated
polymers. There are several parameters affecting the band gap of
conjugated polymers, namely resonance effect, planarity, inter-
chain alternation and bond length alternation. These effects can
be monitored by donor–acceptor (DA) approach [18–20]. DA the-
ory is the mostly used one in order to tune band gap of a conjugated
polymer [21,22].
applications [22]. Polymers containing benzotriazole, ben-
zothiadiazole and benzimidazole as the acceptor groups and
thiophene derivatives; such as, thiophene, 3-hexyl thiophene,
3,4-ethylenedioxythiophene as the donor groups were synthe-
sized by our group [23–29]. The optical and electronic properties
of these polymers are accompanied with low band gaps, low
switching times and high optical contrasts. In the literature, there
are many examples of benzotriazole and benzothiadiazole moi-
eties used as the electron deficient groups [30,31]. Nevertheless,
the properties of donor–acceptor–donor type benzimidazole
compounds are still in an early stage of research. In this study,
4ꢀ-(tert-butyl)spiro[benzo[d]imidazole-2,1ꢀ-cyclohexane] accep-
tor unit was synthesized for the first time and coupled with
strong donor units; 3-hexylthiophene and EDOT to generate
donor–acceptor–donor type benzimidazole containing monomers.
These new monomers were electrochemically polymerized and
their electrochemical and spectroelectrochemical properties were
investigated. The new benzimidazole derivative was used as the
electron deficient group for the first time where two thiophene
derivatives were used as the donor groups. In order to obtain
conjugated polymers with good electrical and optical properties,
two monomers, namely 4ꢀ-(tert-butyl)-4,7-bis(4-hexylthiophen-
∗
Corresponding author at: Department of Chemistry, Middle East Technical Uni-
versity, 06800 Ankara, Turkey. Tel.: +90 312 210 3251; fax: +90 312 210 3200.
2-yl)spiro[benzo[d]imidazole-2,1ꢀ-cyclohexane]
(BIHT)
and
E-mail address: toppare@metu.edu.tr (L. Toppare).
0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2012.02.047

A.C. Ozelcaglayan et al. / Electrochimica Acta 67 (2012) 224–229
225
4ꢀ-(tert-butyl)-4,7-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-
5-yl)spiro[benzo[d]imidazole-2,1ꢀ-cyclohexane]
(BIED)
synthesized and electrochemically polymerized. BIHT and BIED
were compared with respect to their different donor groups and
their effects on electronic and optical properties of the polymers.
removed under reduced pressure and the residue was purified
by column chromatography using dichloromethane:hexane (1:3)
as the eluent. 5,5ꢀ-(4-(Tert-butyl)spiro[cyclohexane-1,2ꢀ-indene]-
4ꢀ,7ꢀ-diyl)bis(3-hexylthiophene) was obtained as a red solid with a
yield of 55%.
were
1H NMR (400 MHz, CDCl3): ı (ppm) 7.92 (d, J = 1.2 Hz, 1H),
7.76(d, J = 1.2 Hz, 1H), 7.20 (s, 2H), 7.15 (s, 2H), 2.60 (t, 4H), 1.60
(m, 4H), 1.50 (m, 2H), 1.48 (m, 2H), 1.38 (m, 1H), 1.30 (m, 4H), 1.29
(m, 8H), 0.94 (s, 9H), 0.88 (t, 6H).
2. Experimental
2.1. General
13C NMR (100 MHz, CDCl3): ı (ppm) 159.00, 158.50, 145.30,
145.20, 139.55, 139.45, 130.50, 130.30, 129.61, 129.59, 129.20,
129.10, 128.40, 128.30, 122.3, 48.50, 34.85, 34.75, 33.50, 32.67,
31.60, 31.55, 31.40, 31.35, 30.40, 30.30, 28.80, 28.70, 28.60, 27.20,
27.10, 23.61, 23.59, 15.10, 15.00.
All chemicals and reagents were obtained from commercial
sources and used without any further purification unless men-
tioned otherwise. THF (tetrahydrofuran) was dried over sodium
and benzophenone. 1H NMR and 13C NMR spectra were recorded
on a Bruker Spectrospin Avance DPX-400 Spectrometer with TMS
(tetramethylsilane) as the internal standard and CDCl3 as the
solvent and chemical shifts were given in ppm. HRMS studies
were done with a Waters SYNAPT MS system. Electrochemical
studies were performed in a three-electrode cell consisting of
an ITO (indium tin oxide) coated glass slide as the working
electrode, platinum wire as the counter electrode, and Ag wire
as the pseudo reference electrode under ambient conditions
using a VoltaLab PST-50 potentiostat. HOMO-LUMO values were
calculated taking the value of SHE as −4.75 eV vs. vacuum.
Varian Cary 5000 UV–vis-NIR spectrophotometer was used to
perform the spectroelectrochemical studies of the polymers.
4,7-Dibromobenzothiadiazole (1) [32], 3,6-dibromobenzene-1,2-
diamine (2) [33], tributyl(4-hexylthiophen-2-yl)stannane (4) [34]
and tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)stannane
(5) [35] were synthesized according to previously published
procedures.
HRMS-ESI+ (m/z): [M+] C36H51N2S2 calculated: 575.3494,
found: 575.3495.
2.3.1. 4ꢀ-(tert-Butyl)-4,7-bis(2,3-dihydrothieno[3,4-
b][1,4]dioxin-5-yl)spiro[benzo[d]imidazole-2,1ꢀ-cyclohexane]
(BIED)
A
solution of 5,5ꢀ-(4-(tert-butyl)spiro[cyclohexane-1,2ꢀ-
indene]-4ꢀ,7ꢀ-diyl)bis(2,3-dihydrothieno[3,4-b][1,4]dioxine)
(400 mg, 0.70 mmol) and tributyl(2,3-dihydrothieno[3,4-
b][1,4]dioxin-5-yl)stannane (151 mg, 3.5 mmol) was refluxed
in 41 ml THF at 100 ◦C under argon atmosphere. After 12 h, solvent
was removed under reduced pressure and the residue was purified
by column chromatography using dichloromethane:hexane (3:1)
as the eluent. 5,5ꢀ-(4-(tert-butyl)spiro[cyclohexane-1,2ꢀ-indene]-
4ꢀ,7ꢀ-diyl)bis(2,3-dihydrothieno[3,4-b][1,4]dioxine) was obtained
as a purple solid with a yield of 80%.
1H NMR (400 MHz, CDCl3):
ı (ppm) 7.8 (dd, J1 = 7.7 Hz,
J2 = 13.3 Hz, 2H), 6.45 (s, 1H), 6.38 (s, 1H), 4.3 (t, 4H), 4.2 (t, 4H),
1.6 (m, 1H), 1.4 (t, 4H), 1.35 (m, 4H), 0.95 (s, 9H).
2.2. Synthesis of monomers
13C NMR (100 MHz, CDCl3): ı (ppm) 156.60, 156.58, 143.50,
143.40, 140.50, 140.40, 139.50, 139.40, 134.20, 134.15, 124.5,
106.60, 106.55, 102.50, 101.50, 64.10, 63.90, 63.35, 63.30, 45.80,
40.31, 40.29, 31.46, 26.65, 26.60, 26.55, 21.65, 21.60.
HRMS-ESI+ (m/z): [M+] C28H31N2O4S2 calculated: 523.1725,
found: 523.1733.
2.2.1. 4,7-dibromo-4ꢀ-(tert-butyl)spiro[benzo[d]imidazole-2,1ꢀ-
cyclohexane]
(3)
A
solution of 3,6-dibromobenzene-1,2-diamine (300 mg,
1.13 mmol), 4-tert-butylcyclohexanone (174 mg, 1.13 mmol) and
8 ml toluene was stirred at 130 ◦C for 24 h under argon (Ar) atmo-
sphere. Then the solvent was removed under reduced pressure. The
crude product was dissolved in 11 ml dichloromethane and stirred
at room temperature under argon atmosphere. To this solution, 85%
activated manganese (IV) oxide (800 mg, 9.16 mmol) was added.
After stirring at room temperature for 4 h, the reaction mixture
was filtered, diluted with dichloromethane (80 ml), washed with
distilled water (3 × 80 ml) and dried over MgSO4. After removing
the solvent under reduced pressure, the residue was purified
by column chromatography using hexane:ethylacetate (10:1) as
the eluent. 4,7-Dibromo-4ꢀ-(tert-butyl)spiro[benzo[d]imidazole-
2,1ꢀ-cyclohexane] was obtained as a yellow solid with a yield of
55%.
3. Results and discussion
3.1. Synthesis
Synthetic route was outlined in Scheme 1. Benzimidazole group
with an electron deficient
C
N
moiety was used as the elec-
tron acceptor while electron rich thiophene groups were used
as the electron donating moieties [36,37]. Benzothiadiazole was
brominated, and then reduced to brominated diamine according
to the previously published procedures [32,33]. After that in the
presence of manganese dioxide, benzimidazole compound was
formed. For the coupling reactions, 3-hexylthiophene and 2,3-
dihydrothieno[3,4-b][1,4]dioxine were stannylated in the presence
of n-BuLi and SnBu3Cl [34,35]. All monomers were synthesized via
Stille coupling in the presence of palladium catalyst. The structure
of monomers was confirmed by NMR and HRMS.
1H NMR (400 MHz, CDCl3): ı (ppm) 7.12 (s, 2H), 2.25 (t, 2H), 2.10
(m, 1H), 1.70 (t, 2H), 1.60 (t, 2H), 1.30 (t, 2H), 0.90 (s, 9H).
13C NMR (100 MHz, CDCl3): ı (ppm) 157.00, 156.70, 135.80,
135.60, 119.30, 107.50, 107.50, 107.45, 47.80, 47.85, 47.80, 32.50,
35.80, 35.75, 27.75, 25.7.
3.2. Cyclic voltammetry
2.3. 4ꢀ-(tert-butyl)-4, 7-bis(4-hexylthiophen-2-
yl)spiro[benzo[d]imidazole-2,1ꢀ-cyclohexane]
(BIHT)
To investigate the electrochromic behaviors of polymers,
monomers (BIHT and BIED) were electrochemically polymerized
by applying potentials between 0.2 V to 1.1 V and −0.3 V to 1.5 V
respectively on indium tin oxide (ITO) coated glass in 0.1 M of
TBAPF6/DCM/ACN (5:95, v:v). In order to enhance the solubility
of BIHT and BIED 5% dichloromethane was used in the electrolytic
medium due to poor solubility of the monomers in acetonitrile.
A
solution of 4ꢀ,7ꢀdibromo-4-(tert-butyl)spiro[cyclohexane-
1,2ꢀ-indene] (300 mg, 0.75 mmol) and tributyl(4-hexylthiophen-
2-yl)stannane (1715 mg, 3.75 mmol) was refluxed in 41 ml THF
at 100 ◦C under argon atmosphere. After 12 h, solvent was

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Scheme 1. Synthetic route to monomers.
In the first cycles, the irreversible monomer oxidation of
indicating that corresponding insoluble polymer film was well
adhered on the ITO surface. As seen in Fig. 1, while PBIHT shows one
redox couple, PBIED revealed two redox couples during the elec-
tropolymerization process, which proved higher electron donating
ability of EDOT moiety than 3-hexylthiophene. p- and n-doping
abilities of both polymer films were investigated in a monomer
free 0.1 M TBAPF6/ACN solution by cyclic voltammetry technique
as given in Fig. 2.
monomers revealed at 1.04 V for BIHT, 1.26 V for BIED at a bare
ITO glass as given in Fig. 1. Difference in their monomer oxi-
dation potentials was found to be 0.22 V, which may be due to
the different electronic nature of the donor units. After the first
cycle, oxidation peaks and their reverse cathodic peaks emerged at
lower potentials than the monomer oxidation potentials and their
current densities increased with the increasing number of cycles

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227
Fig. 1. Repeated potential scan polymerization of (a) 0.01 M BIHT between 0.2 and +1.1 V and (b) 0.01 M BIED between −0.3 and 1.5 V at 100 mV s−1 in 0.1 M TBAPF6
CH2Cl2/ACN (5:95, v:v) on an ITO electrode.
Table 1
Summary of electrochemical studies of PBIHT and PBIED.
Emox
Epox
Epred
En-dope
En-dedope
HOMO
LUMO
Egop
Egec
PBIHT
PBIED
1.04 V
1.26 V
0.93 V
0.55 V/0.82 V
0.79 V
0.34 V/0.59 V
−1.9 V
−1.04 V
−0.6 V
−5.56 eV
−5.31 eV
−3.83 eV
−4.42 eV
1.19 eV
1.15 eV
1.73 eV
0.89 eV
−1.49 V
Sharp reversible peaks corresponding to reversible n-doping
of PBIHT and PBIED appear at −1.9 V/−1.04 V and −1.49 V/−0.6 V,
respectively. The current associated with the p- and n-type dop-
ing processes for both polymers were nearly same, indicating that
PBIHT and PBIED have nearly equal p- and n- type doping ability.
Their HOMO and LUMO energy levels were calculated from onset
of the corresponding oxidation and reduction potentials vs. Fc/Fc+
reference electrode taking the value of SHE as −4.75 eV vs. vac-
uum [23] and reported in Table 1. Although both polymers contain
same acceptor unit, their LUMO levels were quietly different than
each other. PBIED has higher n-doping capacity than PBIHT due to
its high capacity of stabilizing negative charges on polymer chains,
which leads to lower LUMO energy level. LUMO energy levels of
resulting polymers were in accordance with the donor–acceptor
match of donor and acceptor units. EDOT is known as a more
efficient donor unit than 3-hexylthiophene [2,38], hence PBIED
yields in a better donor–acceptor match which lowers the LUMO
level.
3.3. Spectroelectrochemistry and color studies
Spectroelectrochemistry studies were performed in situ by
measuring the absorption behavior of electrochemically deposited
polymer films onto ITO coated glass slides in 0.1 M TBAPF6/ACN.
During p-doping, the evolution of the free charge carrier bands of
the polymers was measured (Fig. 3). These pre-determined poten-
tials were applied to the polymers with 10 mV increments at each
step.
In their neutral states, PBIHT and PBIED revealed blue (L: 49.92,
a: 1.02, b: −7.41) and green colors (L: 55.54, a: −49.86, b: 35.04)
respectively owing to the broad absorption range of PBIHT at
around 640 nm and two distinct absorptions of PBIED at 430 nm
and 837 nm. In most donor–acceptor–donor type polymers there
are two distinct absorption bands since ␲–␲* transition of the poly-
mer and intramolecular charge transfer transitions between donor
and acceptor units absorb at different wavelengths [1,2]. Although
PBIHT has two absorption maxima similar to PBIED, the short
Fig. 2. Single scan cyclic voltammograms of (a) PBIHT between 1.1 V and −2.0 V and (b) PBIED between 1.4 V and −2.0 V using ACN as the solvent and 0.1 M TBAPF6 as the
supporting electrolyte at a scan rate of 100 mV s−1
.

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Fig. 3. Spectroelectrochemistry of (a) PBIHT and (b) PBIED films on an ITO coated glass slide in monomer-free, 0.1 M TBAPF6/ACN electrolyte-solvent couple.
wavelength absorptions are at UV region, hence the polymer
revealed blue color. On the other hand, PBIED has two distinct
absorption maxima at around 400 nm and 700 nm; as a result the
neutral state color was observed as green. The optical band gaps
(Egop) were calculated from the onset of their long wavelength
absorptions as 1.19 eV and 1.15 eV for PBIHT and PBIED, respec-
tively. The broad absorption of PBIHT leads to the onset of ␲–␲*
transition in longer wavelength, hence the optical band gap of the
polymer is approximately that of the band gap of its EDOT moi-
ety containing one. As the potentials increased, the absorption in
the NIR begun to arise, whereas absorptions in the visible region
were depleted, indicating the creation of lower energy polaron and
bipolaron charge carriers for both PBIHT and PBIED. When the poly-
mers are fully oxidized, strong absorptions in the NIR region were
observed. Absorption tail of the polaron band for PBIED is adja-
cent to the visible region, resulting a gray color (L: 72.99, a: −3.03,
b: 27.34) and a blue color (L: 59.87, a: −1.36, b: −6.59) in doped
state, whereas PBIHT revealed transmissive-gray color (L: 59.43, a:
−10.57, b: 14.06) (Fig. 4).
PBIHT revealed transmissive-gray color (L: 58.35, a: −4.39, b:
8.56) in n-doped state, whereas PBIED showed multichromism dur-
ing n-doping; light green (L: 58.67, a: −40.01, b: 74.03), orange (L:
53.99, a: 44.01, b: 58.97) and brick-red (L: 20.99, a: 21.99, b: 24.89)
(Fig. 4). Both polymer films turned back to their neutral state colors
after dedoping.
3.4. Electrochromic contrast and switching studies
Electrochromic switching studies were carried out in order
to monitor the percent transmittance changes as a function of
time and to calculate the switching times of the polymers at
their maximum absorptions (ꢀmax) by stepping potentials repeat-
edly between their neutral and fully oxidized states within 5 s
time intervals. The switching time was calculated from the kinetic
Fig. 4. Chemical structures of PBIHT and PBIED and their color changes. under p- and n-type doping processes. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of the article.)

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229
Fig. 5. ꢁT% change monitored at different wavelengths for (a) PBIHT and (b) PBIED in 0.1 M TBAPF6/ACN.
studies to determine the time required for 95% of full switches
between their colored and bleached states.
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At their dominant wavelengths in the visible region, PBIHT
showed 27% percent transmittance change with a high switch-
ing time (4.6 s) at 595 nm, whereas PBIED revealed ꢁT% of 44%
at 440 nm with a sub second switching time (0.9 s) (Fig. 5). Also
PBIED loses 9% of its original contrast in visible region; on the
other hand PBIHT loses ca. 32% of the initial contrast upon sequen-
tial 30 full switches. The alkyl chain group on the donor moiety
leads to a difficulty in charge injection/ejection processes for PBIHT.
PBIED revealed notable switching properties in visible region. In
NIR region, PBIHT and PBIED revealed 46% (1555 nm) and 46%
(1820 nm) transmittance with high switching times (Fig. 5).
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Two benzimidazole containing donor–acceptor–donor type
polymers were synthesized successfully and polymerized elec-
trochemically. Their electrochemical and optical properties were
characterized by cyclic voltammetry and spectroelectrochemistry.
For PBIED, switching time was less than a second. Both PBIHT and
PBIED showed high optical contrast in NIR region and both of them
have a broad absorption band making them a possible candidate for
solar cell applications. The solar cell applications of these polymers
will be the subject of an incoming study.
Acknowledgments
The authors are grateful to METU and TUBA grants.
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