2D assemblies of ionic liquid crystals based on imidazolium moieties: Formation of ion-conductive layers

Article abstract of DOI:10.1039/c5nj00085h

Ionic liquid-based 2D ion-conductive liquid crystals were obtained by the co-assembly of imidazolium salts and cationic calamitic liquid crystals with an imidazolium moiety at the end of the molecule through a flexible spacer. They exhibit smectic A phase

Full text of DOI:10.1039/c5nj00085h

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2D assemblies of ionic liquid crystals based
on imidazolium moieties: formation of
ion-conductive layers†
Cite this: New J. Chem., 2015,
39, 4471
Junji Sakuda,a Masafumi Yoshio,a Takahiro Ichikawa,b Hiroyuki Ohnob and
Takashi Kato*ac
Ionic liquid-based 2D ion-conductive liquid crystals were obtained by the co-assembly of imidazolium
salts and cationic calamitic liquid crystals with an imidazolium moiety at the end of the molecule through
a flexible spacer. They exhibit smectic A phases. The two components are compatible over a wide range
of compositions for ionic liquids with various anions. This organization of ionic liquids enhances the ionic
conductivities of calamitic ionic liquid crystals. These liquid-crystalline mixtures dissolve lithium salts. The
2D ion-conductive pathways formed by the ionic liquid crystals and ionic liquids act as ion-conductive
media for lithium ions.
Received (in Montpellier, France)
12th January 2015,
Accepted 24th March 2015
DOI: 10.1039/c5nj00085h
www.rsc.org/njc
dye-sensitized solar cells.18 High-temperature stable operation
of the cells was achieved using ionic-liquid-based liquid crystals.
Introduction
Nanostructured liquid crystals1 are emerging as new functional
The aim of the present study is to construct highly mobile
materials for various applications such as ion2 and charge3 trans- ion-conductive pathways using the self-assembly of ionic liquid
port, separation membranes,4 and catalysis.5 Nanosegregation1,6,7 crystals and ionic liquids. Ionic liquids are organized to form
is one approach to control the structures of self-assembled ion-conductive pathways through nanosegregation and inter-
liquid-crystalline (LC) molecules. The incompatible parts actions with ionic LC molecules. The free migration of ions is
segregate at the molecular scale, leading to the formation of expected to provide high ionic conductivity. Diol-based LC mole-
various complex nanostructures. The organization of functional cules have been developed as another approach to the organiza-
molecules into LC nanostructures through this approach is a tion of ionic liquids.15–17 In these diol systems, the interactions
promising strategy for the development of next-generation between the hydroxyl groups and ionic liquids are well-designed
functional materials.
for the co-assembly of the two components. There is a restric-
Ionic liquids8 are organic salts and they melt below room tion on the chemical structures of the ionic liquids that can be
temperature. They have received a lot of attention as electrolyte incorporated into LC nanostructures; for example, a columnar
materials because of their negligible vapor pressure,9 flame assembly of LC structures15 comprising a diol-based fan-shaped
retardancy,10 and high ionic conductivity.11
molecule and an ionic liquid was achieved only for ionic liquids
The development of ionic-liquid-based liquid crystals through with BrÀ anions. The use of ionic liquid crystals as LC molecules
nanosegregation enables the formation of ion-conductive path- is an approach that is applicable to a wide variety of ionic liquids
ways, through which ions are efficiently transported in the and gives more stable LC structures. In the present design,
desired directions.1,2,12–17 These nanostructured ion-conductive the segregation of the ionic and non-ionic moieties of ionic
liquid crystals have potential as new functional electrolytes for liquid crystals and ionic liquids leads to the facile formation of
electrochemical devices;2j,18 for example, smectic liquid crystals LC nanostructures. There have not been many reports on the
containing ioniÀc liquids have been recently found to efficiently ion-conductive LC assemblies obtained using this approach of
transport IÀ/I3 redox couples and have also been used in mixing ionic liquid crystals and ionic liquids.13b More extensive
studies of the LC and ion-conductive properties of the mixtures
a Department of Chemistry and Biotechnology, School of Engineering,
The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
E-mail: kato@chiral.t.u-tokyo.ac.jp
of ionic liquid crystals and ionic liquids for future applications
are needed.2j,18
Herein, we report 2D ion-conductive LC mixtures consisting
of calamitic ionic liquid crystals containing imidazolium cations
and imidazolium ionic liquids. Ionic liquids with various anions
b Department of Biotechnology, Tokyo University of Agriculture and Technology,
Nakacho, Koganei, Tokyo 184-8588, Japan
c CREST, JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nj00085h have been used to induce efficient 2D ion-conductive pathways
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in the ionic liquid crystals. Recently, we reported, for the first
time, that lithium-ion batteries can be operated using 2D ion-
conductive liquid crystals.2j It is therefore important to examine
2D LC ion-conductive materials for the further development of
electrochemical devices.
Results and discussion
Molecular design
Calamitic ionic liquid crystals 1(X) (X is an anion) with an
imidazolium moiety at the end of the molecule were designed
and synthesized for the formation of LC layered structures
(Fig. 1). The cyclohexylphenyl mesogen is covalently attached
À
through an alkyl spacer to the imidazolium cation. BF4À, CF3SO3
,
and (CF3SO2)2NÀ were chosen as the counter anions. For the
ionic liquids, 1-ethyl-3-methylimidazolium ionic liquids 2(X)
with various anions [BF4À, CF3SO3À, and (CF3SO2)2NÀ] were
used because of their high ionic conductivities.11,19 Compounds
2(X) are expected to be organized into layered nanostructures
by mixing with 1(X) through nanosegregation, leading to the
formation of highly mobile pathways.
Fig. 2 Phase transition behavior of the mixtures of 1(CF3SO3) and 2(CF3SO3)
on cooling. Iso: isotropic; SmA: smectic A; Cr: crystalline; Cr + Iso: biphasic
mixture of crystalline and isotropic phases. The Iso–SmA transition tempera-
tures for the mixtures containing more than 50 mol% of 2(CF3SO3) were
determined by polarizing optical microscopic observations at a cooling
rate of 1 K minÀ1
.
Liquid-crystalline properties of the ionic liquid crystals
The phase transition behavior of 1(X) is summarized in Table 1.
Compounds 1(X) exhibit smectic A phases; this indicates the
formation of 2D ion-conductive pathways (ESI,† Fig. S1–S6).
The order of isotropization temperature is 1(BF4) 4 1(CF3SO3) 4
1((CF3SO2)2N). This temperature order is inversely related to the
order of the radii of the counter anions.20 These results suggest
that more stable LC structures are formed for 1(X) with smaller
counter anions. The charge density of a smaller anion is higher,
leading to stronger electrostatic interactions between the anions
and cations.
Liquid-crystalline properties of the ionic liquid crystal and
ionic liquid mixtures
The mixtures of ionic liquid crystal 1(CF3SO3) and ionic liquid
2(CF3SO3) were prepared. Fig. 2 shows the phase transition
behavior of the mixtures on cooling. The mixtures show smectic
A phases and form homeotropically aligned monodomains on
glass substrates during cooling from isotropic states. A polariz-
ing optical microscopic image of the mixture of 1(CF3SO3) and
2(CF3SO3) in a 7 : 3 molar ratio is shown in Fig. 3a. A dark image
and a cross-shaped pattern are observed under orthoscopic and
conoscopic conditions, respectively. These observations confirm
the vertical alignment of the LC molecules. This behavior is
attributed to the interactions between the terminal imidazolium
moieties of 1(CF3SO3) and the hydrophilic surfaces of the
unmodified glass substrates.13b–d,16 A macroscopic phase separa-
tion of the two components induced by the crystallization of
1(CF3SO3) is observed at temperatures below those of the smectic
A phases (ESI,† Fig. S9). In the LC states, compounds 1(CF3SO3)
and 2(CF3SO3) are compatible over a wide range of composi-
tions. The mixtures containing up to 80 mol% of 2(CF3SO3) form
smectic LC structures. The high compatibility and high ability to
form LC nanostructures of this system can be attributed to the
similar chemical structures of the ionic moieties in 1(CF3SO3) and
2(CF3SO3). In our previous studies,15,16 based on the mixtures of
hydroxyl-functionalized amphiphilic molecules and ionic liquids,
the components were not compatible when the molar percentage
of the ionic liquid was higher than 70% and ionic liquids
Fig. 1 Molecular structures of the ionic liquid crystals 1(X) and ionic
liquids 2(X).
Table 1 Thermal properties of compounds 1(X)
Phase transition behaviorb,c
X
Radius of XÀ a (Å)
BF4
CF3SO3
2.29
2.70
250d
SmA 35e (10) Cr
Iso 148 (1.1) SmA 35 (22) Cr
(CF3SO2)2N 3.24
Iso
64 (1.6) SmA
a
b
Ref. 20. Transition temperatures (1C) and transition enthalpies
(kJ molÀ1, in parentheses) are determined by differential scanning
calorimetry (DSC) on the first cooling at a scan rate of 10 K minÀ1
.
c
d
Iso: isotropic; SmA: smectic A; Cr: crystalline. Thermal degradation
occurs around 250 1C before the isotropization was observed.
e
Observed on the 1st cooling from 100 1C.
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Fig. 4 Layer spacings of smectic LC structures formed by the mixtures of
1(CF3SO3) and 2(CF3SO3) at temperatures 30 1C below the isotropization
temperature.
Fig. 5 shows the conductivities obtained on cooling from the
isotropic states. In this process, a homeotropically aligned
monodomain is spontaneously formed and this enables the
measurement of the conductivities parallel to the ion-conductive
pathways.
An increase in the molar percentage of 2(CF3SO3) increases the
conductivity (Fig. 5). The ionic conductivities at 41 1C for 1(CF3SO3)
and the mixture of 1(CF3SO3) and 2(CF3SO3) in a 6 : 4 molar ratio
are 2.6 Â 10À5 S cmÀ1 and 2.9 Â 10À4 S cmÀ1, respectively. The
conductivity is enhanced 10-fold by the incorporation of 2(CF3SO3).
To obtain more information on the ion-transport behavior of
these materials in the smectic A phase, the activation energy for
Fig. 3 (a) Polarizing optical microscopy image of the mixture of 1(CF3SO3)
and 2(CF3SO3) in a 7 : 3 molar ratio at 100 1C on cooling. The inset shows
the conoscopic image. (b) X-ray diffraction pattern of the mixture at 58 1C
on cooling. (c) Schematic illustration of the self-assembled structure in
the mixture.
À
with anions larger than BF4 were not examined. The isotropi-
zation temperature of the mixture decreases with an increasing
molar percentage of 2(CF3SO3).
The incorporation of an ionic liquid gives a more fluid LC
nanostructure. The assembled structures of the mixtures of
1(CF3SO3) and 2(CF3SO3) were examined using X-ray diffrac-
tion. The pattern of the mixture of 1(CF3SO3) and 2(CF3SO3) in a
7 : 3 molar ratio at 58 1C has two peaks at 50.2 and 24.3 Å, which
are assigned to the (001) and (002) diffractions, respectively, of
a layered structure with a spacing of 49 Å (Fig. 3b). The layer
spacing is twice the molecular length of 1(CF3SO3), which is
estimated to be 26 Å by molecular mechanics calculations. This
observation indicates the formation of a bilayered structure
(Fig. 3c). The layer spacings of the structures are shown in
Fig. 4; they increase with an increasing molar percentage of
2(CF3SO3). This observation indicates that 2(CF3SO3) is organized
into ion-conductive layers formed by the imidazolium moiety
in 1(CF3SO3).
Ion-conductive properties of the ionic liquid crystals and ionic
liquids mixtures
The ionic conductivity of 1(CF3SO3) (ESI,† Fig. S10) and the
mixtures of 1(CF3SO3) and 2(CF3SO3) were determined using
comb-shaped gold electrodes deposited on a glass substrate.
Fig. 5 Ionic conductivities of the mixtures of 1(CF3SO3) and 2(CF3SO3).
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ion conduction was calculated from the slopes of the Arrhenius
plots shown in Fig. 5 using the Arrhenius conductivity equation
(ESI,† eqn (S1)). The activation energy is defined as the energy
barrier that needs to be overcome for ions to migrate in the
presence of intermolecular interactions. As the molar percen-
tage of 2(CF3SO3) in the mixture increases, the activation energy
gradually decreases from 50 kJ molÀ1 for 1(CF3SO3) to 36 kJ molÀ1
for the mixture of 1(CF3SO3) and 2(CF3SO3) in a 6 : 4 molar ratio.
These results suggest that more fluid ion-conductive pathways
in which ions migrate with high mobility are formed in the
mixtures containing an ionic liquid. In our previous report16b
on non-covalent systems based on diol LC molecules with the
same mesogenic moiety as 1(X), the mixture containing 20 mol%
of an imidazolium ionic liquid showed a conductivity of 4.8 Â
10À4 S cmÀ1 at 122 1C. In the present study, a conductivity of
2.0 Â 10À3 S cmÀ1 at the same temperature was achieved for a
mixture of 1(X) with the same molar percentage of an ionic
liquid.16b The present system offers ionic conductivity one order
of magnitude higher than that of the previous diol system.16b
The activation energies for ion conduction in these systems are
also almost the same.
Fig. 6 Ionic conductivities of the mixtures of 1(CF3SO3), 2(CF3SO3), and
LiCF3SO3.
Liquid-crystalline properties of the mixtures containing lithium
salts
Ion-conductive properties of the mixtures containing lithium
salts
The mixtures consisting of 1(CF3SO3), 2(CF3SO3), and LiCF3SO3
were prepared to evaluate the properties of these materials as
lithium-ion conductors. The presence of 2(CF3SO3) keeps the
viscosity of the material low, resulting in enhanced lithium-ion
transport. To examine the effect of the lithium salt concentration,
a fixed molar ratio of 7 : 3 for 1(CF3SO3) and 2(CF3SO3) was used.
Table 2 summarizes the phase transition behavior of the mixtures
of 1(CF3SO3), 2(CF3SO3), and LiCF3SO3. These mixtures also
show smectic A phases with bilayered structures (ESI,† Fig. S8).
The homeotropic alignment of the LC molecules on glass
substrates on cooling from isotropic states was confirmed by
the polarizing optical microscopic observations (ESI,† Fig. S8).
The isotropization temperature is not significantly changed by
the addition of LiCF3SO3. However, the crystallization tempera-
ture of 1(CF3SO3) in the mixture decreases slightly with an
increasing concentration of LiCF3SO3. For the mixture with a
molar ratio of 7 : 3 : 1.5, no crystallization of 1(CF3SO3) occurs
on cooling to À30 1C.
The ion-conductive properties of the mixtures containing
LiCF3SO3 were examined. Fig. 6 shows the Arrhenius plots
of the ionic conductivity during cooling from the isotropic
state. The addition of LiCF3SO3 results in a decrease in the
conductivity; this is attributed to the lower mobility found
in the ion-conductive pathways derived from the formation
of aggregated species21 through the coordination of the
anions to the lithium cations (ESI,† Fig. S11). It has been
reported for isotropic ionic liquids22 that binary mixtures of
ionic liquids and lithium salts show lower conductivities and
higher viscosities than the ionic liquids themselves; this is also
explained by a decrease in the ionic mobility as a result of ion
association.
Liquid-crystalline properties of the ionic liquid crystals and
ionic liquids mixtures with different counter anions
The effects of the co-existence of two types of counter anions
on the LC behavior were studied by mixing 1(CF3SO3) and 2(X)
[X = BF4 or (CF3SO2)2N]. The phase diagrams of the mixtures
on cooling are shown in Fig. 7. The mixtures show smectic A
phases. The two components are compatible in the LC states
over the entire composition range, which indicates that
1(CF3SO3) has high compatibility with 2(X), regardless of the
counter anion. When the molar percentage of 2(X) is 30% or
higher, 1(CF3SO3) does not crystallize on cooling to À30 1C,
although crystallization is observed for the mixtures of
1(CF3SO3) and 2(CF3SO3). The isotropization temperature
decreases with an increasing radius of the counter anion of
2(X) (ESI,† Fig. S12).20 The addition of 2(X) with large counter
anions destabilizes the LC phase.
Table 2 The thermal properties of the mixtures of 1(CF3SO3), 2(CF3SO3),
and LiCF3SO3
Molar ratio of 1(CF3SO3),
2(CF3SO3), and LiCF3SO3
Phase transition behaviora,b
7 : 3 : 0
Iso
Iso
Iso
Iso
107
109
109
109
SmA
SmA
SmA
SmA
13
10
6
Cr + Iso
Cr + Iso
Cr + Iso
7 : 3 : 0.375
7 : 3 : 0.75
7 : 3 : 1.5
a
Transition temperatures (1C) are determined using differential scan-
ning calorimetry (DSC) on the first cooling at a scan rate of 10 K minÀ1
.
b
Iso: isotropic; SmA: smectic A; Cr: crystalline; Cr + Iso: biphasic
mixture of crystalline and isotropic phases.
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assemblies based on ionic liquids, which could be used as new
functional electrolytes for energy devices.2j,18
Experimental section
General procedures
1H and 13C NMR spectra were obtained using a JEOL JNM-
LA400 at 400 and 100 MHz in CDCl3, respectively. Chemical
shifts of the 1H and 13C NMR signals are quoted with respect to
Me4Si (d = 0.00) and CDCl3 (d = 77.0) as internal standards,
respectively; the chemical shifts in parts per million (d), multi-
plicities, coupling constants (Hz), and relative intensities are
reported. FTIR measurements were conducted using a JASCO
FT/IR-660 Plus spectrometer. Matrix-assisted laser desorption
ionization time-of-flight mass spectra were obtained using a
PerSeptive Biosystems Voyager-DE STR spectrometer with
dithranol as the matrix. Elemental analyses were performed
using an Exeter Analytical CE440 elemental analyzer. The thermal
properties of the materials were examined by differential scanning
calorimetry using a Netzsch DSC204 Phoenix. The heating and
cooling rates were 10 K minÀ1. Transition temperatures were
taken at the onsets of the transition peaks. A polarizing optical
microscope Olympus BX51 equipped with a Linkam LTS350 hot
stage was used to study liquid crystallinity. X-ray diffraction
patterns were obtained using a Rigaku RINT-2500 diffractometer
with CuKa radiation.
Ionic conductivity measurements
The temperature dependence of the ionic conductivities were
investigated by the alternating current impedance method using
a Solartron 1260 impedance/gain-phase analyzer (frequency
range: 100 Hz–1 MHz, applied voltage: 0.6 V) equipped with a
temperature controller. The heating and cooling rates were fixed
at 2 K minÀ1. The ionic conductivities were calculated as
the product of 1/Rb (OÀ1) times the cell constants (cmÀ1) for
the electrode cells, which were calibrated with KCl aqueous
solution (1.00 mmol LÀ1) as a standard conductive solution.
The impedance data were modeled as a connection of two RC
circuits in series.
Fig. 7 Phase transition behavior of the mixtures of (a) 1(CF3SO3) and
2(BF4), and (b) 1(CF3SO3) and 2((CF3SO2)2N) on cooling. Iso: isotropic;
SmA: smectic A; Cr: crystalline; Cr + Iso: biphasic mixture of crystalline and
isotropic phases. The Iso–SmA transition temperature for the mixture
containing 90 mol% of 2(BF4) was determined by polarizing optical
microscopic observation at a cooling rate of 1 K minÀ1
.
Acknowledgements
Conclusions
This study was partially supported by the Funding Program for
World-Leading Innovative R&D on Science and Technology
(FIRST) from the Cabinet Office, Government of Japan. J. S. is
grateful for the financial support from the JSPS Research
Fellowships for Young Scientists.
Calamitic ionic liquid crystals containing an imidazolium
cation form 2D ion-conductive pathways; moreover, the imid-
azolium ionic liquids with various counter anions are organized
at high molar fractions. The structural similarities of the ionic
moieties in the two components provide high compatibility in
forming liquid-crystalline layered nanostructures. An enhance-
ment in the ionic conductivity is achieved by their co-existence
with ionic liquids. The liquid-crystalline mixtures of ionic liquid
crystals, ionic liquids, and lithium salts were also prepared as
lithium-ion conductors. Materials designed using the non-covalent
self-assembly of ionic liquid crystals and ionic liquids have
potential in the development of functional liquid-crystalline
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