Anhydrous proton conduction in liquid crystals containing benzimidazole moieties

Article abstract of DOI:10.1039/c6ra03375j

A homologous series of biphenyl benzoate-based compounds with an alkthio chain bearing a benzimidazole moiety at the termini was synthesized by a nucleophilic substitution reaction. The compounds exhibited smectic A and nematic phases over a temperature range of 173-116 °C during the cooling process. A partially interdigitated bilayer order developed in the smectic assemblies and hydrogen-bonding between the benzimidazole moieties extended along the plane parallel to the smectic layer. Electrochemical characterization revealed that the liquid crystal phases favoured anhydrous proton conduction in the benzimidazole compounds and a proton conductivity of 4.4 × 10^-5 S cm^-1 was achieved at 173 °C. The temperature dependence of the proton conductivities approximately followed the Arrhenius law and the proton conduction in the benzimidazole liquid crystals was assumed to be dominated by the proton hopping mechanism.

Full text of DOI:10.1039/c6ra03375j

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Anhydrous proton conduction in liquid crystals
containing benzimidazole moieties†
Cite this: RSC Adv., 2016, 6, 34038
*
Shuai Tan, Bingzhuo Wei, Ting Liang, Xiaohui Yang and Yong Wu
A
homologous series of biphenyl benzoate-based compounds with an alkthio chain bearing
a benzimidazole moiety at the termini was synthesized by a nucleophilic substitution reaction. The
compounds exhibited smectic A and nematic phases over a temperature range of 173–116 ꢀC during the
cooling process. A partially interdigitated bilayer order developed in the smectic assemblies and
hydrogen-bonding between the benzimidazole moieties extended along the plane parallel to the
smectic layer. Electrochemical characterization revealed that the liquid crystal phases favoured
anhydrous proton conduction in the benzimidazole compounds and a proton conductivity of 4.4 ꢁ 10ꢂ5
S cmꢂ1 was achieved at 173 ꢀC. The temperature dependence of the proton conductivities approximately
followed the Arrhenius law and the proton conduction in the benzimidazole liquid crystals was assumed
to be dominated by the proton hopping mechanism.
Received 5th February 2016
Accepted 28th March 2016
DOI: 10.1039/c6ra03375j
www.rsc.org/advances
a triphenylene core with alkyl chains bearing a triazole moiety at
their termini.20 The exact arrangement of the triazole moieties
Introduction
in the liquid crystal phase remained unclear for the absence of
informative X-ray scattering pattern. They found that the liquid
crystal phase lowered the activation energy barrier for proton
Anhydrous proton conductors for fuel cells working at inter-
mediate temperatures (100–200 ꢀC) have attracted much
attention for the potential to promote cell performance.1–3
Hydrogen-bonding networks formed by protonic moieties are
believed to provide pathways for proton conduction in organic
materials.4 Amphoteric nitrogen-heterocycles such as imid-
azole, benzimidazole and triazole are regarded as candidates for
anhydrous proton conductors because they can participate in
hydrogen-bonding as both hydrogen bond donors and accep-
tors.5–11 Jannasch et al. have reported proton conductivities of
10ꢂ5 S cmꢂ1 at 160 ꢀC for ethylene oxide oligomers tethered to
benzimidazole units.7 It was suggested that proton conduction
in the heterocycle compounds occurred through structural
diffusion and local mobility of the heterocycles would facilitate
the process.6–9 Comb polymers, acid–base complexes and
porous coordination polymers have been proposed to construct
well-ordered hydrogen-bonding pathways for the purpose of
conduction and the maximum conductivity reached 1 ꢁ 10ꢂ5
S
cmꢂ1 at 140 ꢀC. Chen et al. have incorporated benzimidazole
moieties into biphenyl mesogens to prepare three series of
smectic liquid crystals.21 However, the rigid mesogens might
restrict reorientation of the nitrogen-heterocycles necessary for
structural diffusion and the authors did not carry out any
electrochemical characterization.
With a view to obtaining enhanced anhydrous proton
conduction through nitrogen-heterocyclic liquid crystals, we
synthesized a homologous series of benzimidazole compounds
enhancing
proton
conduction
in
the
heterocycle
compounds.12–15
Liquid crystals have been adopted to obtain regular proton
conducting pathways in some sulfonic compounds by virtue of
their intrinsic order and uidity,16–19 but there are hardly any
reports on nitrogen-heterocyclic liquid crystals for proton
conduction. Basak et al. obtained a liquid crystal phase using
College of Chemical Engineering, Sichuan University, No. 24 South Section 1, Yihuan
Road, Chengdu 610065, China. E-mail: wuyong@scu.edu.cn; Fax: +86 28 85403397;
Tel: +86 28 85467527
† Electronic supplementary information (ESI) available: 1H NMR spectra, FT-IR
spectra, DSC curves, XRD patterns, impedance spectra and current relaxation
curves. See DOI: 10.1039/c6ra03375j
Fig. 1 Synthetic route of 1.
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as shown in Fig. 1. A rod-shaped biphenyl benzoate mesogen procedures analogous to that described below for 4a. 3a (1.9 g,
was employed to induce liquid crystal phases. A exible alkthio 8.6 mmol) was dissolved in thionyl chloride and stirred at room
spacer between a terminal benzimidazole moiety and the temperature for 30 min. The excess thionyl chloride was
mesogen assured local mobility of the nitrogen-heterocycle. A removed under reduced pressure and 4-hexoxy benzoyl chloride
terminal alkoxy chain (containing carbon number from 6 to 12) was obtained. Triethylamine (1.8 mL, 12.9 mmol) was added
on the other side of the mesogen harmonized rigidity and into a ask containing 2 (3 g, 8.6 mmol) in dichloromethane.
exibility of the molecule. The objective benzimidazole The solution was stirred at room temperature for 30 min. 4-
compounds exhibited thermotropic nematic and smectic A Hexoxy benzoyl chloride dissolved in dichloromethane was
phases during the cooling process. Hydrogen-bonding between added dropwise into the solution. The mixture was stirred at
the benzimidazole moieties formed lamellar networks owing to room temperature for 5 h. Aer the reaction, the mixture was
the bilayer smectic order. Electrochemical impedance spec- concentrated under reduced pressure. The residue was crystal-
troscopy (EIS) measurements revealed that the liquid crystal lized from methanol for twice and puried by ash column
phases favoured anhydrous proton conduction in the benz- chromatography (silica) using dichloromethane as an eluent. 4a
1
imidazole compounds. The smectic benzimidazole liquid crys- (3.0 g) was obtained in 63% yield. H NMR (400 MHz, CDCl3):
tals provided an optional strategy to develop anhydrous proton 8.16, 7.57, 7.51, 7.23, 6.97 (m, 12H, Ph), 4.01 (m, 4H, PhOCH2),
conductors from nitrogen-heterocycles.
3.44 (t, 2H, CH2Br), 1.75–1.24 (m, 16H, CH2CH2), 0.87 (t, 3H,
–CH3). FT-IR n: 2932, 2861, 1725, 1606, 1513, 1498, 1470, 1389,
1258, 1212, 1163, 1072, 996, 880, 846, 791, 761, 728, 692, 651,
551 (C–Br), 513. For 4b: yield: 61%. For 4c: yield: 63%. For 4d:
yield: 62%.
Experimental
General
Synthesis of 40-(6-(benzimidazolethio)hexoxy)-biphenyl-4-yl
4-(alkoxy) benzoate (1). All of these compounds were prepared
by procedures analogous to that described below for 1a. 2-
Mercaptobenzimidazole (1.0 g, 6.7 mmol) in aqueous NaOH
solution (3.4 mL, 2 mol Lꢂ1) was stirred at room temperature for
30 min. 4a (3.7 g, 6.7 mmol) dissolved in tetrahydrofuran (5 mL)
was then added dropwise into the solution. The mixture was
stirred at reux for 8 h. Aer cooling, the mixture was evapo-
rated under reduced pressure and the residue was extracted
with water. The crude product was recrystallized from ethanol
for twice and puried by ash column chromatography (silica)
using dichloromethane as an eluent to give 1a (3.8 g) as a white
The 1H nuclear magnitude resonance (NMR) spectra were
measured by using a Bruker AV II-400 spectrometer. The Fourier
transform infrared (FT-IR) spectra during the cooling process
were obtained by a Nicolet 6700 spectrometer equipped with
a Linkam hot stage. The sample was sandwiched between two
ungreased ZnSe disks. Elemental analyses were done by using
a Euro EA3000 CHNS/O Elemental Analyzer. The differential
scanning calorimetry (DSC) measurements were performed by
a TA DSC Q20 modulated instrument under a nitrogen atmo-
sphere. The heating and cooling rates were 10 C minꢂ1. Polar-
ꢀ
ized optical microscopies (POM) of thin lms were performed
using a Weitu XPL-30TF equipped with a WT-3000 hot-stage. The
X-ray diffraction (XRD) analyses were conducted on a Bruker AXS
D8 discovery diffractometer equipped with a Hi-Star 2D detector,
1
solid. Yield: 91%. H NMR (400 MHz, CDCl3): 8.16, 7.57, 7.51,
7.23, 6.97 (m, 16H, Ph), 4.01 (m, 4H, PhOCH2), 3.37 (t, 2H,
CH2S), 1.75–1.24 (m, 16H, CH2CH2), 0.87 (t, 3H, –CH3). FT-IR n:
3070, 2932, 2861, 1725, 1606, 1513, 1498, 1470, 1440 (C–S), 1389,
1344 (C]N), 1258, 1212, 1163, 1072, 996, 880, 846, 791, 761,
735, 682, 651, 519. Elemental analysis calcd (%) for
¨
using Cu-Ka radiation ltered by cross-coupled Gobel mirrors at
40 kV and 40 mA. Sample's temperature was controlled by an
Anton Parr hot-stage. The EIS measurements were carried out
using an electrochemical workstation consisting of an EG&G
Princeton Applied Research (PAR) potentiostat/galvanostat
model 273A and PAR lock-in-amplier model 5210 connected
to a PC running electrochemical impedance soware (frequency
range: 100 kHz to 0.1 Hz, applied voltage: 10 mV). The Wagner's
direct current (DC) polarization22 was measured using the same
electrochemical workstation with direct current DC power supply
(applied voltage: 0.85 V).
C
38H42N2O4S requires: C, 73.28; H, 6.80; N, 4.50; S, 5.15. Found:
C, 73.31; H, 6.78; N, 4.48; S, 5.12. For 1b: yield: 94%. Elemental
analysis calcd (%) for C40H46N2O4S requires: C, 73.81; H, 7.12;
N, 4.30; S, 4.93. Found: C, 73.77; H, 7.09; N, 4.33; S, 4.89. For 1c:
yield: 95%. Elemental analysis calcd (%) for C42H50N2O4S
requires: C, 74.30; H, 7.42; N, 4.13; S, 4.72. Found: C, 74.28; H,
7.38; N, 4.12; S, 4.74. For 1d: yield: 91%. Elemental analysis
calcd (%) for C44H54N2O4S requires: C, 74.75; H, 7.70; N, 3.96; S,
4.54. Found: C, 74.71; H, 7.71; N, 3.93; S, 4.57.
Materials
All commercially-available starting materials, reagents and
solvents were used as supplied and were obtained from TCI,
Acros and Chengdu Changzheng. All reactions were carried out
Results and discussion
Mesomorphic properties
under a dry nitrogen atmosphere. The synthetic route of 40-(6- Mesomorphism of 1 was characterized by POM observation,
(benzimidazolethio)hexoxy)-biphenyl-4-yl 4-(alkoxy)benzoate (1) DSC and XRD measurements. Multiple peaks detected in DSC
is presented in Fig. 1. Compounds 2 and 3 were synthesized traces of 1 (Fig. S4 see ESI†) indicated appearance of meso-
according to the procedures described previously.23,24
phases. The mesophase types were identied by textures of
Synthesis of 40-(6-bromohexoxy)-biphenyl-4-yl 4-(alkoxy) POM observation. When 1 was cooled from isotropic liquid,
benzoate (4). All of these compounds were prepared by nematic (N) phase was recognized from schlieren textures. On
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Fig. 2 Polarized optical micrographs of 1d at 155 ꢀC (a) and at 130 ꢀC
(b) during cooling (ꢁ400). Direction of A: analyzer; P: polarizer.
Table 1 Phase transition temperatures and corresponding enthalpies
derived from DSC tracesa
DTLC (ꢀC)
Transition temperatures (ꢀC)
Fig. 3 XRD pattern of 1d at 130 ꢀC (inset is the proposed molecular
packing mode in the SA phase).
Comps
1a
(enthalpy changes (kJ molꢂ1))
SA
N
C1st: I 173(0.8) N 133(5.9) SA 125(26.6) C
H2nd: C 166(33.8) N 177(0.7) I
8
40
11
41
13
16
6
—
10
—
28
—
30
—
1b
C1st: I 170(0.9) N 129(0.5) SA 119(39.1) C
H
2nd: C 156(37.7) N 169(0.8) I
1c
C1st: I 162(0.8) N 146(0.9) SA 118(39.1) C
H2nd: C 159(36.7) N 165(0.8) I
1d
C1st: I 159(0.8) N 146(1.0) SA 116(38.2) C
13
5
H
2nd: C 157(37.0) N 162(1.0) I
a
Note: C, solid; I, isotropic liquid; C1st, rst cooling; H2nd, second
heating; DTLC, temperature range of a liquid crystal phase.
further cooling, smectic A (SA) phase was discerned from focal
conic textures. Only N phase was observed when 1 was heated
from solid to isotropic liquid state. Representative POM textures
observed for 1 are presented in Fig. 2. Phase transition
temperatures and enthalpy changes derived from the DSC
traces are listed in Table 1. The longer alkoxy chain resulted in
the broader temperature range of the SA phase because of the
enhanced laterally intermolecular forces of attractions.25
Fig. 4 FT-IR spectra of 1d at 160, 140 and 30 ꢀC during the cooling
process.
Temperature dependent XRD measurements of 1 further
conrmed the presence of the SA phase. The XRD pattern of 1d at
130 ꢀC during cooling is shown in Fig. 3. A sharp peak centered at
2q ¼ 1.1ꢀ indicated a lamellar structure with a d-spacing of 7.8
nm. The spacing was longer than the length L of 1d in the
extended conformation (4.5 nm) and shorter than 2L. Accord-
ingly, a partially interdigitated bilayer order was assumed to
develop in the smectic assembly. Essentially identical XRD
patterns were also observed for 1a, 1b and 1c (Fig. S5†). Mean-
while, intermolecular hydrogen-bonding between the benzimid-
azole moieties26 was recognized from the broad absorption
centered at 3070 cmꢂ1 in the FT-IR spectrum of 1d at tempera-
tures up to 160 ꢀC (Fig. 4). Extended hydrogen-bonding networks
have ever been observed in some benzimidazole self-assem-
blies13,21 because the imidazole moieties participated in
hydrogen-bonding both as donors and acceptors. Taking the XRD
pattern and FT-IR spectrum into consideration, we inferred that
side-by-side hydrogen-bonding between the benzimidazole
moieties extended along the plane parallel to the smectic layer.
The proposed molecular packing mode is schematically illus-
trated in Fig. 3. As a result, well-ordered pathways for proton
conduction arose in the smectic assembly.
Electrochemical characterization
EIS measurements were performed to determine proton
conductivities (detailed measurement information was
described in ESI†). The impedance responses obtained for 1
were characterized by the semicircles at high frequencies
(Fig. S6 see ESI†), indicating occurrence of ionic conduction.
The ionic transference numbers determined by the Wagner's
DC polarization technique were 0.98–0.99 (Fig. S7 see ESI†),
which suggested that ions were the predominant contribution
to the conduction. Since diffusible ions other than protons did
not exist in the compounds, the observed conductivities were
reasonably ascribed to proton conductivities which could be
calculated from the EIS responses and the cell constant (eqn
(S2) see ESI†).
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Table 2 Activation energies of 1 in the mesophases obtained by the
least-squares fitting
Activation energy Ea (kJ molꢂ1) (regression
coefficient (R2))
Comps
SA
N
1a
1b
1c
1d
71.0(0.99)
70.8(0.99)
65.1(0.99)
69.0(0.97)
38.4(0.99)
38.2(0.99)
36.0(0.99)
34.6(0.98)
versus Tꢂ1 (Kꢂ1) is a straight line), which was quite different
from the Vogel–Tamman–Fulcher mode27 observed for some
amorphous nitrogen-heterocycle compounds.6–8 The proton
conduction in these amorphous heterocycle conductors was
assumed to be mediated by segmental motion of exible chains.
The Arrhenius behavior observed for 1 suggested that proton
conduction in the benzimidazole liquid crystals was rather
dominated by the proton hopping mechanism.28–30
The activation energies (Ea) estimated from the Arrhenius
plots (eqn (S3) see ESI†) are listed in Table 2. Ea for proton
conduction in the SA phase was approximately twice the value in
the N phase. The positional restriction on molecules in the SA
phase might bring about additional energy barrier for proton
exchange between the neighbouring benzimidazole moieties.
Fig. 5 Temperature dependent anhydrous proton conductivities of 1a
(a), 1b (b), 1c (c) and 1d (d) during the cooling process (- solid; A SA
phase; : N phase; C isotropic liquid).
The variation of the proton conductivity with temperature is
shown in Fig. 5. The conductivities decreased with the decrease
in temperature during the cooling process except for the I / N
and N / SA phase transition points. At the I / N transition
point, conductivities of 1 increased by 11% in average. The
slight increase in proton conductivities should originate from
the nematic order. Uniaxial arrangement of the benzimidazole
molecules probably promoted formation of proton pathways. At
the N / SA transition point, conductivities of 1 increased by
24% in average. The elevated proton conductivities were
ascribed to the increased lamellar proton pathways induced by
the smectic order. The abrupt increase in conductivities at the I
/ N and N / SA phase transition points revealed that liquid
crystal phases favoured anhydrous proton conduction in the
benzimidazole compounds. Nevertheless, proton conductivities
of 1 decreased by 85% in average at the SA / C transition point.
The signicant decrease in conductivities should be caused by
the loss of both the bilayer arrangement of the molecules and
local mobility of the benzimidazole moieties in solid state. Mass
fraction of the benzimidazole moiety in the molecule also
affected the resultant conductivities. At the same temperature,
conductivities of 1 decreased slightly with the increase in the
length of the alkoxy chains. For example, s values of smectic 1a,
1b, 1c and 1d at 125 ꢀC were 1.7 ꢁ 10ꢂ5, 1.6 ꢁ 10ꢂ5, 1.4 ꢁ 10ꢂ5
and 1.3 ꢁ 10ꢂ5 S cmꢂ1, respectively. The maximum conductivity
achieved in the mesomorphic benzimidazole compounds was
4.4 ꢁ 10ꢂ5 S cmꢂ1, which was higher than that of the reported
mesomorphic triazole compound.20
Conclusions
Mesomorphic biphenyl benzoate-based compounds with an
alkthio chain bearing a benzimidazole moiety at the termini
were synthesized. The compounds exhibited thermotropic N
and SA phases during the cooling process. Lamellar hydrogen-
bonding networks benecial to anhydrous proton conduction
developed in the smectic assemblies. The liquid crystal phases
favoured anhydrous proton conduction in the benzimidazole
compounds and conductivity of 4.4 ꢁ 10ꢂ5 S cmꢂ1 was achieved
at 173 ꢀC. The benzimidazole liquid crystals provided an
optional strategy to prepare anhydrous proton conductors from
nitrogen-heterocycles.
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