The effect of para- and meta-substituted fluorine on optical behaviorof benzimidazole derivatives

Article abstract of DOI:10.1016/j.electacta.2013.07.102

Design, synthesis, and electrochemical properties of two novel donor-acceptor-donor type benzimida-zole and thiophene based monomers; 2-(4-fluorophenyl)-4,7-di(thiophen-2-yl)^-1H-benzo[d]imidazole(BIPF) and 2-(3-fluorophenyl)-4,7-di(thiophen-2-y

Full text of DOI:10.1016/j.electacta.2013.07.102

Electrochimica Acta 109 (2013) 214–220
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
The effect of para- and meta-substituted fluorine on optical behavior
of benzimidazole derivatives
Merve Ileria, Serife O. Hacioglua, Levent Topparea,b,c,d,∗
a Department of Chemistry, Middle East Technical University, 06800 Ankara, Turkey
b Department of Biotechnology, Middle East Technical University, 06800 Ankara, Turkey
c Department of Polymer Science and Technology, 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
Article history:
Received 30 May 2013
Received in revised form 11 July 2013
Accepted 11 July 2013
Available online 26 July 2013
Design, synthesis, and electrochemical properties of two novel donor–acceptor–donor type benzimida-
zole and thiophene based monomers; 2-(4-fluorophenyl)-4,7-di(thiophen-2-yl)-1H-benzo[d]imidazole
(BIPF) and 2-(3-fluorophenyl)-4,7-di(thiophen-2-yl)-1H-benzo[d]imidazole (BIMF) were highlighted.
The position of fluorine as a substituent was varied from para- to meta- in order to investigate posi-
tion effects on the electrochemical and optical properties of electrochemically synthesized polymers.
Both polymers were p type dopable and they can be switched between orange and blue color during
p-doping/dedoping. Significant improvements in the percent transmittance for PBIMF were observed
compared to similar molecules in literature.
Keywords:
Electrochromism, Donor–acceptor,
Substitution effect, Fluorine
© 2013 Elsevier Ltd. All rights reserved.
1. Introduction
them, donor–acceptor theory is the most effective and commonly
used method. In donor–acceptor theory, alternating electron-rich
Conjugated donor–acceptor (D–A) type polymers have drawn
considerable interest due to properties like ease of processabil-
ity, lightweight, high optical contrasts, fast switching times and
tuning bandgap via structural modifications. These desirable prop-
erties make them more transcendent than inorganic materials
[1]. On account of these advantages, conducting polymers can
be used in many applications such as organic photovoltaics [2],
electrochromic devices (EC) [3], organic light emitting diodes [4],
organic field effect transistors [5] and sensors [6]. After detailed
investigation, it is proved that bandgap has tremendous effect on
electronic and optical properties of the conjugated polymers. In
order to synthesize a functional polymer which is applicable in
different fields, enhancing several properties with structural modi-
fication is very important. In literature different methods were used
for combining desired properties in one structure such as; bandgap
modification, using different substituents and combining different
donor–acceptor groups [7–9].
(donor) and electron-deficient (acceptor) units were combined in
the polymer backbone to achieve desired bandgap. Besides, the D–A
approach is not only used for obtaining low band gap polymers, but
also for producing polymers with improved optical, mechanical and
electronic properties [11].
Electrochromism is the reversible and visible change in trans-
mittance or reflactance as a result of an applied voltage [12].
Characterization of electrochromic materials can be done by means
of different parameters such as coloration efficiency, stability,
electrochromic contrast, optical memory [13]. More importantly,
an accessibility to a variety of colors by structural modification
makes conjugated polymers one of the prominent EC materi-
als.
In the literature different D–A type polymers were studied
widely in order to investigate the effects of different D and A
groups on the electronic and optical properties [14,15]. Recently,
polymers that consist of benzothiadiazole, benzotriazole and ben-
zimidazole as the acceptor units and derivatives of thiophene as the
donor units were studied in our group [16–20]. However, deriva-
tives of benzimidazole as the acceptor units have not been studied
sufficiently up to now. Although, these benzimidazole derivatives
demostrated promising and enhanced electrochemical properties,
problems related to this unit are their low stability and poor elec-
trochromic contrasts [18–20].
In order to modify the bandgap of these functional mate-
rials, bond length alternation, increasing planarity, interchain
effects, increasing stability of polymer with using resonance
and donor–acceptor (D–A) theory can be utilized [10]. Among
∗
Corresponding author at: Department of Chemistry, Middle East Technical Uni-
versity, 06800 Ankara, Turkey.
In this study, we have demonstrated a new class of elec-
trochemically synthesized polymers, consisting of para- and
E-mail address: toppare@metu.edu.tr (L. Toppare).
0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2013.07.102

M. Ileri et al. / Electrochimica Acta 109 (2013) 214–220
215
meta-fluorine substituted benzimidazole unit as an acceptor
2.3. Synthesis of
group and thiophene as a donor unit. Fluorine atom as a sub-
stituent has both inductive and mesomeric effects. Inductive
effect will have the same influence for both position whereas,
mesomeric effect will be dominant over inductive effect in
para-position. In this case, depending upon the position of
fluorine atom on the ring (para- or meta-) oxidation barri-
ers will probably change. 2-(4-Fluorophenyl)-4,7-di(thiophen-
2-yl)-1H-benzo[d]imidazole (BIPF) and 2-(3-fluorophenyl)-4,7-
di(thiophen-2-yl)-1H-benzo[d]imidazole (BIMF) were synthe-
sized. These two novel monomers were polymerized electrochem-
ically to obtain (PBIPF) and (PBIMF). Additionally, electrochemical,
spectroelectrochemical and kinetic studies of polymers were per-
formed.
It is worth to consider that both monomers reported in this arti-
cle are novel and also the discussion on the position of fluorine
atom on electronic properties will clarify substitution concept. In
addition, this fluorine subtituent makes these functional polymers
applicable in different fields. Herein, we introduce an approach to
alter and get desired electronic properties of D–A type polymers
simply by changing the position of substituents.
2-(4-fluorophenyl)-4,7-di(thiophen-2-yl)-1H-benzo[d]imidazole
(BIPF)
A
solution of tributyl(thiophen-2-yl)stannane (1.010 g,
2.70 mmol) and 4,7-dibromo-2-(4-fluorophenyl)-1H-
benzo[d]imidazole (4) (200 mg, 0.54 mmol) in tetrahydrofuran
(40.0 mL, 0.49 mmol) was refluxed overnight under argon
atmosphere with bis(triphenylphosphine)palladium (II) dichlo-
ride. The mixture was cooled and concentrated on the rotary
evaporator before purification. So as to purify 2-(4-fluorophenyl)-
4,7-di(thiophen-2-yl)-1H-benzo[d]imidazole
(BIPF),
column
chromatography technique was performed (silica gel, hex-
ane:EtOAc 2:1) effectively. After that, the residue was subjected
to preparative thin layer chromatography (silica, hexane:ethyl
acetate 4:1) to purify BIPF completely as a yellow solid in 37%
yield.
1H NMR (400 MHz, CDCl3): ı 9.57 (s, 1H), 8.16 (dd, J = 1.2, 3.7 Hz,
1H), 8.04 (dd, J = 3.7, 5.2, 8.9 Hz, 2H), 7.53–7.55 (m, 1H), 7. 37–7.31
(m, 4H), 7.16–7.12 (m, 4H). 13C NMR (100 MHz, CDCl3): ı 163.9,
161.4, 149.0, 138.7, 127.4, 127.3, 126.6, 124.2, 123.9, 120.3, 114.8,
114.6. HRMS (EI) for C21H13FN2S2, calculated 377. 0582, found 377.
0557.
2. Experimental
2.1. General
2.4. Synthesis of
4,7-dibromo-2-(3-fluorophenyl)-1H-benzo[d]imidazole (5)
All chemicals and reagents were obtained from commercial
sources and used without further purification. THF was dried over
sodium and benzophenone. 4,7-Dibromo-2.1.3-benzothiadiazole
(2) and 3,6-dibromo-1,2-phenylenediamine (3) were synthesized
according to previously published procedures [21,22]. 1H NMR and
13C NMR spectra were recorded on a Bruker Spectrospin Avance
DPX-400 Spectrometer with TMS as the internal standard and
CDCl3 as the solvent. All shifts were given in ppm. Electrochem-
ical studies were performed in a three-electrode cell consisting of
an indium tin oxide (ITO) doped glass slide as the working elec-
trode, platinum wire as the counter electrode, and Ag wire as the
pseudo reference electrode using a Voltalab 50 potentiostat. The
reference electrode was subsequently calibrated to Fc/Fc+ (0.30 V)
and the band energies were calculated relative to the vacuum level
taking the value of SHE is −4.75 eV vs. vacuum. To perform the
spectroelectrochemical studies of polymers Cary 5000 UV–vis spec-
trophotometer was used.
4,7-Dibromo-2-(3-fluorophenyl)-1H-benzo[d]imidazole
(5)
was synthesized by the modification of a previously published
procedure [23]. 3,6-Dibromo-1,2-phenylenediamine (3) (600 mg,
2.25 mmol) was dissolved in 4 mL ACN. H2O2 (1.2 mL, 12 mmol)
was added to the mixture directly. 3-Fluorobenzaldehyde (0.3 mL,
0.225 mmol) was added to the mixture dropwise. After that,
ammonium cerium (IV) nitrate (120 mg, 0.21 mmol) was added to
the mixture and stirred overnight at room temperature. Recrystal-
lization with cold water and hexane was used in order to purify
the compound 5. As a result, compound 5 was obtained as a bejge
solid.
1H NMR (400 MHz, CDCl3): ı 8.20–8.15 (m, 4H), 7.64 (dd, J = 6.6,
7.6, 14.2 Hz, 2H). 13C NMR (100 MHz, CDCl3): ı 131.1, 131.0, 130.9,
126.4, 123.7, 123.6, 117.5, 117.3, 114.1, 113.9.
2.5. Synthesis of
2-(3-fluorophenyl)-4,7-di(thiophen-2-yl)-1H-benzo[d]imidazole
2.2. Synthesis of
4,7-dibromo-2-(4-fluorophenyl)-1H-benzo[d]imidazole (4)
(BIMF)
4,7-Dibromo-2.1.3-benzothiadiazole (2) and 3,6-dibromo-
1,2-phenylenediamine (3) were synthesized using previously
stated procedures [21,22]. 4,7-Dibromo-2-(4-fluorophenyl)-
1H-benzo[d]imidazole (4) was synthesized by the modification
A
solution of tributyl(thiophen-2-yl)stannane (867 mg,
2.32 mmol) and 4,7-dibromo-2-(3-fluorophenyl)-1H-
benzo[d]imidazole (5) (170 mg, 0.46 mmol) in tetrahydrofuran
(40.0 mL, 0.49 mmol) was refluxed overnight under argon atmo-
sphere. Bis(triphenylphosphine)palladium (II) dichloride was
added to the mixture as the catalyst. The reaction was concen-
trated on the rotary evaporator and for purification of BIMF the
residue was subjected to column chromatography technique
(silica gel, hexane:EtOAc 3:1). After that, preparative thin layer
chromatography (silica, hexane:ethyl acetate 4:1) was carried out
to get BIMF totally as a yellow solid in 42% yield.
of
a previously published procedure [23]. 3,6-Dibromo-1,2-
phenylenediamine (3) (400 mg, 1.50 mmol) was dissolved in 4 mL
acetonitrile. After hydrogen peroxide (0.8 mL, 8 mmol) was added
to the mixture directly, 4-fluorobenzaldehyde (0.2 mL, 1.50 mmol)
was added to the mixture drop wise. After that, ammonium
cerium (IV) nitrate (90 mg, 0.16 mmol) was added to the mixture
and stirred overnight at room temperature. 4,7-Dibromo-2-(4-
fluorophenyl)-1H-benzo[d]imidazole (4) was purified by means of
recrystallization with cold water and hexane. Compound 4 was
obtained as beige solid.
1H NMR (400 MHz, CDCl3): ı 9.64 (s, 1H), 8.19 (dd, J = 1.1, 2.5,
3.6 Hz, 1H), 7.85–7.82 (m, 1H), 7.81–7.78 (m, 1H), 7.59–7.56 (m, 1H),
7.42–7.34 (m, 5H), 7.17–7.13 (m, 3H). 13C NMR (100 MHz, CDCl3):
ı 163.4, 160.9, 149.2, 149.1, 139.3, 129.7, 129.6, 127.1, 124.5, 121.1,
116.4, 116.2, 113.0, 112.8. HRMS (EI) for C21H13FN2S2, calculated
377. 0582, found 377. 0558.
1H NMR (400 MHz, CDCl3): ı 13.33 (s, 1H), 8.39 (dd, J = 3.1, 5.5,
8.6 Hz, 2H), 7.45–7.39 (m, 4H). 13C NMR (100 MHz, CDCl3): ı 130.0,
129.9, 126.5, 125.9, 116.0, 115.8, 111.2.

216
M. Ileri et al. / Electrochimica Acta 109 (2013) 214–220
Scheme 1. Synthetic route for the monomers.
3. Results and discussion
described in literature [23]. Finally, the Stille coupling of these
compounds with tributyl(thiophen-2-yl)stannane [24] gave the
3.1. Synthesis
desired monomers BIPF and BIMF (Scheme 1).
The general synthetic routes toward the monomers are
depicted in Scheme 1. Bromination of 1,3-benzothiadiazole
with Br2/HBr was afforded 4,7-dibromo-2.1.3-benzothiadiazole
(2) with high yield [21]. After bromination, the reduction
of this dibrominated compound (2) by NaBH4 gave 3,6-
dibromo-1,2-phenylenediamine (3) as described in literature
[22]. In order to synthesize desired acceptor units; 4,7-
3.2. Electrochemical properties
The monomers BIPF and BIMF were electrochemically poly-
merized by cyclic voltammetry (CV) in order to probe the
electrochemical characters of polymers. The CV was per-
formed on indium tin oxide (ITO, 1.5 cm2) coated glass
slides with 0.1 M acetonitrile (ACN)/dichloromethane (CH2Cl2)
(95/5, v/v) solution at several scan rates where best results
were obtained with 100 mV/s. Sodium perchlorate (NaClO4)
and lithium perchlorate (LiClO4) were used as the supporting
electrolytes during electropolymerization with repeated scan
dibromo-2-(4-fluorophenyl)-1H-benzo[d]imidazole
(4)
and
4,7-dibromo-2-(3-fluorophenyl)-1H-benzo[d]imidazole (5), the
condensation of compound (3) with 4-fluorobenzaldehyde and
3-fluorobenzaldehyde were performed from an adapted procedure

M. Ileri et al. / Electrochimica Acta 109 (2013) 214–220
217
Fig. 1. Repeated potential scan polymerization of (a) BIPF and (b) BIMF at 100 mV s−1 in 0.1 M LiClO4/NaClO4 CH2Cl2/ACN (5:95, v:v) solution on an ITO electrode.
Fig. 2. Single scan cyclic voltammograms of (a) PBIPF and (b) PBIMF in a monomer free 0.1 M LiClO4/NaClO4 ACN solution.
intervals (10 scans) between 0 V and 1.3 V for BIPF and 0 V and
1.4 V for BIMF (Fig. 1).
affects electrochemical and spectroelectrochemical properties sig-
nificantly.
Fluorine atom as a substituent has both inductive effect
The irreversible monomer oxidation peaks were appeared in
the first cycles at 1.0 V for BIPF and at 1.05 V for BIMF. After the
first cycle, observation of a new reversible redox couple with an
increasing current intensity after each successive cycle proves the
formation of electroactive polymer films PBIPF and PBIMF (Fig. 1).
Electrochemically synthesized polymers were subjected to
CV in a monomer free solution in order to investigate the p-
type and n-type doping properties of the polymers. In 0.1 M
LiClO4/NaClO4/ACN solution both polymers are p-dopable with a
reversible redox couple at 1.0 V/0.78 V for PBIPF and at 1.1 V/0.84 V
for PBIMF vs. the Ag wire pseudo reference electrode.
(electron-withdrawing) and mesomeric effect (electron-donating).
Inductive effect will have the same influence for both position
whereas, mesomeric effect will be dominant over inductive effect
in para-position [25]. In this case, depending upon the position
of fluorine atom on the ring (para- or meta-) oxidation barriers
will probably change. When resonance structures are considered,
it is seen that para-substituted fluorine (PBIPF) could stabilize the
possible cationic charge. As a result of higher ring stabilization of
para-substituted one (PBIPF), its oxidation will be easier and its
oxidation potential will be lower compared to meta-substituted
derivative (PBIMF) which supports our results (Table 1).
The HOMO energy levels of each polymers were calculated from
onset of the corresponding oxidation potentials vs. Fc/Fc+ refer-
ence electrode (Fig. 2). Both polymers have only p-doping property
hence, LUMO energy levels were calculated by using optical band
gap values. As seen in Table 1 the HOMO levels were reported as
−5.46 eV (PBIPF) and −5.54 eV (PBIMF) respectively.
3.3. Spectroelectrochemistry
The optical changes upon stepwise doping and the elec-
trochromic properties of conjugated polymers were investigated
by conducting spectroelectrochemistry studies of PBIPF and
PBIMF. Both polymers were electrochemically synthesized on ITO
and spectral changes were explored by UV–vis-NIR spectrom-
eter in a monomer-free 0.1 M NaClO4–LiClO4/ACN solution via
The HOMO levels differ from each other slightly, this difference
indicates the ease of oxidation of polymer films in this electrolytic
medium (Table 1). This behavior can be explained by position of
fluorine atom. Changing the position of fluorine unit from para-
to meta- on the phenyl ring has a tremendous effect on kinetic
properties as depicted in Table 2. In addition, this position change
Table 2
Summary of kinetic and optic studies of PBIPF and PBIMF.
Optical contrast (ꢁT%)
Switching times (s)
Table 1
PBIPF
16%
17%
41%
475 nm
685 nm
1220 nm
0.4
0.4
0.3
Summary of electrochemical and spectroelectrochemical properties of PBIPF and
PBIMF.
HOMO (eV)
LUMO (eV)
ꢀmax (nm)
Egop (eV)
PBIMF
29%
28%
73%
460 nm
770 nm
1265 nm
0.7
1.3
1.7
PBIPF
PBIMF
−5.46
−5.54
−3.55
−3.58
471
440
1.91
1.96

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M. Ileri et al. / Electrochimica Acta 109 (2013) 214–220
Fig. 3. Electronic absorption spectra for (a) PBIPF and (b) PBIMF films in 0.1 M NaClO4–LiClO4/ACN solution between 0.0 V and 1.2 V for PBIPF, 0.0 V and 1.15 V for PBIMF.
incrementally increasing applied potential between 0.0 V and 1.2 V
for PBIPF, 0.0 V and 1.15 V for PBIMF. These ranges are in accor-
dance with the CV results as given in Fig. 1.
respectively. The ꢀmax of the PBIPF signifies 31 nm a red-shift when
compared with ꢀmax of the PBIMF. To sum up, better packing for
PBIPF will result lower energy and higher ꢀmax
.
Initially, in order to remove any trapped charge and dopant
ion during electrochemical polymerization, polymer coated ITO
films were reduced to their neutral state, and then stepwise oxida-
tion was performed. During oxidation while the absorption in the
visible region reached a minimum value, new absorption bands
(polaron, bipolaron bands). For PBIPF polaronic and bipolaronic
bands appear at 720 and 1180 nm respectively. These are revealed
at 705 nm, 1170 nm for PBIMF. The normalized spectral changes
occurring upon electrochemical oxidation of PBIPF and PBIMF
were depicted in Fig. 3a and b in the range of 300−1650 nm.
When we compare the electronic absorption spectra of PBIPF
and PBIMF, the effect of position of fluorine atom on spectroelec-
trochemical properties can be seen. The difference on the ꢀmax
values and optical band gaps can be explained by the parameters
that affect absorption spectra of conjugated polymers. In literature,
it is known that these properties are widely affected by effective
conjugated length and packing of resulting polymers [26].
In addition, spectroelectrochemical studies of the polymer films
illustrate that the film’s color changed during the p-doping (oxida-
tion) processes. Structures of PBIPF, PBIMF and their colors at their
neutral and different oxidized states were demonstrated in Fig. 4a
and b.
At the neutral state, both PBIPF and PBIMF have orange color.
Following successive oxidation, intensity of absorption bands in
the visible region steadily decrease while new absorption bands
appear, and polymers have green color during oxidation. Finally,
further oxidation results blue color for both polymers at fully oxi-
dized state.
As seen in Fig. 4 there is little difference between the colors
of PBIPF and PBIMF, which means that the changing position of
flourine atom from para- to meta-position has a little effect on
colors of corresponding polymers.
3.4. Electrochromic contrast and switching studies
When the position of fluorine atom was changed from meta to
para, better packing was achieved in the polymer backbone which
causes a shift in absorption maxima from 440 nm to 471 nm. Optical
band gaps (Egop) of the PBIPF and PBIMF were calculated from the
onsets of lowest energy ␲–␲* transitions as 1.91 eV and 1.96 eV,
The capability of a polymer to change its color rapidly between
two states (neutral state and oxidized state) and illustrating a
significant change are crucial properties for an electrochromic
polymer. Switching time is defined as the period required for
Fig. 4. Structures of the polymers and their colors at their neutral and different oxidized states.

M. Ileri et al. / Electrochimica Acta 109 (2013) 214–220
219
Fig. 5. Optical contrasts and switching times monitored at different wavelengths for (a) PBIPF and (b) PBIMF in 0.1 M NaClO4–LiClO4/ACN solution.
the color change of a material between its neutral and oxidized
states. The percent transmittance changes and switching times of
PBIPF and PBIMF between their fully oxidized and reduced states
were investigated by applying potentials within 5 s time inter-
vals. The kinetic studies were performed in a monomer free 0.1 M
NaClO4–LiClO4/ACN solvent-electrolyte couple. The wavelengths
at which kinetic studies performed were determined from the max-
imum absorbance at the spectra of polymer films.
As summarized in Table 2, PBIPF showed 41% transmittance
change upon doping/de-doping process at 1220 nm, 17% at 685 nm
and 16% at 475 nm. PBIMF revealed 73% transmittance at 1265 nm,
28% at 770 nm and 29% at 460 nm. Switching times were reported
as 0.3 s, 0.4 s and 0.4 s for PBIPF and 1.7 s, 1.3 s and 0.7 s at corre-
sponding wavelengths (Fig. 5).
fluorine containing benzimidazole derivatives are multipurpose
materials due to improved kinetic results and band gaps. These
materials will be subject of further research interest in our group.
Acknowledgements
The authors appreciate the financial support of BAP-01-03-
2013-013 and Prof. Dr. Cihangir Tanyeli for valuable discussions.
We thank Cem Ozcan for the preparation of the graphical abstract.
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4. Conclusion
In this study, two new benzimidazole and thiophene based
polymers; PBIPF and PBIMF were synthesized, purified and char-
acterized successfully. In order to discuss the effect of position of
substituents on electrochemical properties, both para- and meta-
substituted derivatives were designed. Different switching times,
optical contrasts and HOMO/LUMO energy levels were observed
for these two derivatives from electrochemical and spectroelectro-
chemical studies. Both polymer films have p-type doping property
and showed electrochromic properties under applied different
potentials. This makes them applicable for use in electrochromic
devices. In addition, optical contrasts were enhanced by changing
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