Induction of thermotropic bicontinuous cubic phases in liquid-crystalline ammonium and phosphonium salts

Article abstract of DOI:10.1021/ja209010m

Two series of wedge-shaped onium salts, one ammonium and the other phosphonium, having 3,4,5-tris(alkyloxy)benzyl moieties, exhibit thermotropic bicontinuous "gyroid" cubic (Cub_bi) and hexagonal columnar liquid-crystalline (LC) phases by nanose

Full text of DOI:10.1021/ja209010m

Article
pubs.acs.org/JACS
Induction of Thermotropic Bicontinuous Cubic Phases in Liquid-
Crystalline Ammonium and Phosphonium Salts
Takahiro Ichikawa,†,‡ Masafumi Yoshio,† Atsushi Hamasaki,‡ Satomi Taguchi,‡ Feng Liu,§
Xiang-bing Zeng,§ Goran Ungar,§,∥ Hiroyuki Ohno,‡ and Takashi Kato*,†
†Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo
113-8656, Japan
‡Department of Biotechnology, Tokyo University of Agriculture and Technology, Nakacho, Koganei, Tokyo 184-8588, Japan
§Department of Materials Science and Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, United Kingdom
∥WCU Program C2E2, School of Chemical and Biological Engineering, Seoul National University, Seoul, South Korea
S
* Supporting Information
ABSTRACT: Two series of wedge-shaped onium salts, one
ammonium and the other phosphonium, having 3,4,5-
tris(alkyloxy)benzyl moieties, exhibit thermotropic bicontin-
uous “gyroid” cubic (Cubbi) and hexagonal columnar liquid-
crystalline (LC) phases by nanosegregation between ionophilic
and ionophobic parts. The alkyl chain lengths on the cationic
moieties, anion species, and alkyl chain lengths on the benzyl
moieties have crucial effects on their thermotropic phase
behavior. For example, triethyl-[3,4,5-tris(dodecyloxy)benzyl]-
ammonium hexafluorophosphate forms the thermotropic Ia3d
̅
Cubbi LC phase, whereas an analogous compound with trifluoromethanesulfonate anion shows no LC properties. Synchrotron
small-angle diffraction intensities from the Ia3d Cub LC materials provide electron density maps in the bulk state. The resulting
̅
bi
maps show convincingly that the Ia3d Cub structure is composed of three-dimensionally interconnected ion nanochannel
̅
bi
networks surrounded by aliphatic domains. A novel differential mapping technique has been applied successfully. The map of
triethyl-[3,4,5-tris(decyloxy)benzyl]ammonium tetrafluoroborate has been subtracted from that of the analogous ammonium salt
with hexafluorophosphate anion in the Ia3d Cub phases. The differential map shows that the counteranions are located in the
̅
bi
core of the three-dimensionally interconnected nanochannel networks. Changing from trimethyl- via triethyl- to
tripropylammonium cation changes the phase from columnar to Cubbi to no mesophase, respectively. This sensitivity to the
widened shape for the narrow end of the molecule is explained successfully by the previously proposed semiquantitative
geometric model based on the radial distribution of volume in wedge-shaped molecules. The LC onium salts dissolve lithium
tetrafluoroborate without losing the Ia3d Cub LC phase. The Cub LC materials exhibit efficient ion-transporting behavior as a
̅
bi
bi
result of their 3D interconnected ion nanochannel networks. The Ia3d Cub LC material formed by triethyl-[3,4,5-
̅
bi
tris(decyloxy)benzyl]phosphonium tetrafluoroborate shows ionic conductivities higher than the analogous Ia3d Cub material
based on ammonium salts. The present study indicates great potential of Cubbi LC nanostructures consisting of ionic molecules
for development of transportation nanochannel materials.
̅
bi
expectations for their application in nanofiltration mem-
branes9,10 and drug delivery systems.12
INTRODUCTION
■
Bicontinuous cubic (Cubbi) phases1 are a group of nano-
structured liquid-crystalline (LC) phases having three-dimen-
sionally (3D) interconnected nanochannels.2−4 Recently, Cubbi
liquid crystals have attracted attention as transportation
materials5−11 because the 3D channels of Cubbi LC materials
are expected to form successive pathways without the
orientation of the liquid-crystal domains, while decades of
studies have mainly focused only on the molecular assembled
structures in the Cubbi phases.2−4 As an example for application
to transportation materials, we previously succeeded in
development of ion-conductive materials formed by Cubbi LC
ionic compounds.6−8 Since the channel structures of Cubbi LC
materials possess well-defined sizes and shapes, there are great
There are a limited number of reports on the application of
Cubbi LC materials.5−14 Focusing only on thermotropic Cubbi
LC systems, the number is further limited.6−8,14 This is due to
the difficulty in designing LC molecules that would exhibit
thermotropic Cubbi phases. For the design of LC materials, it is
generally thought that control of intermolecular interactions,
such as hydrogen bonding, electrostatic interaction, van der
Waals interaction, and π−π interaction, is important.15−19 On
the basis of this idea, a number of new functional LC materials
Received: September 25, 2011
Published: January 9, 2012
© 2012 American Chemical Society
2634
dx.doi.org/10.1021/ja209010m | J. Am. Chem.Soc. 2012, 134, 2634−2643

Journal of the American Chemical Society
Article
have been developed and the relationships between the
molecular structure and the molecular assembled nanostruc-
tures have been studied.20−26 There has been some progress in
rational design of thermotropic LC molecules intended to
exhibit nematic, smectic, and columnar phases by accumulation
of data on structure/property relationships. However, it is still
difficult to design LC molecules exhibiting thermotropic Cubbi
phases. Therefore, more study is required to obtain Cubbi LC
materials, which have considerable potential as functional
nanochannel materials.
to ammonium cations for design of ionic liquids.30−32
Phosphonium-based ionic liquids generally exhibit more
chemical and thermal stability than the corresponding
ammonium-based ionic liquids.31 Moreover, they show higher
ionic conductivity than the corresponding ammonium salts.32
On the other hand, there have been several reports on
phosphonium salts with LC properties.33,34 For example, Weiss
et al. reported the LC behavior of quaternary phosphonium
salts having two or three long alkyl chains.34 They revealed that
the LC phosphonium salts form molecular assembled structures
similar to the analogous ammonium salts. With this background
in mind, we expected that introduction of phosphonium
structures into our material design would result in an
improvement of the Cubbi LC materials. We also expected
that comparison of the LC behavior of 1−3(n/BF4) with that of
4−6(n/BF4) would improve our knowledge of the design of
thermotropic Cubbi LC molecules.
Our intention is to gain further insights into the design of
thermotropic Cubbi liquid crystals and evaluate them as
potential transportation materials. In our previous study we
prepared a variety of ionic compounds showing thermotropic
LC properties for development of a new type of ion-conducting
material.6,7,17,20 In the course of our study on ionic LC
molecules, we found that wedge-shaped ammonium salts form
thermotropic Ia3d Cub phases.6 Moreover, we succeeded in
̅
bi
RESULTS AND DISCUSSION
the preparation of polymer films having 3D ion nanochannels
through in situ photopolymerization of a Cubbi LC ammonium
salt having a polymerizable group.7 On the basis of our previous
results on the ionic compounds showing Cubbi phases, we
focused on the wedge-shaped onium structures consisting of
ionophobic and ionophilic parts as a promising design for Cubbi
LC molecules (Figure 1). Though there have been a lot of
■
Ionic/Nonionic Bicontinuous Cubic Structures
Formed by 2(10/BF4) and 2(10/PF6). In our preliminary
communication,6 the LC behavior of compounds 2(n/BF4) (n
= 10, 12, 14) was reported. We revealed that compounds
2(n/BF4) having a triethyl ammonium moiety form Cubbi
phases, space group Ia3d, depending on the alkyl chain length
̅
n. For example, compound 2(10/BF4) exhibits a Cubbi phase
from 61 to −32 °C on cooling.6
To examine the effects of ionic structures on formation of the
Cubbi phase, we prepared an analogous compound 2(10/PF6)
with an anion different than that in 2(10/BF4). Compound
2(10/PF6) shows a Cubbi LC phase from 34 to −45 °C on
cooling. The detailed LC behavior of 2(10/PF6) will be
discussed further below. We expected that comparison of the
molecular assembly of 2(10/BF4) and 2(10/PF6) would
provide new insight into the Cubbi structures. Synchrotron
small-angle diffraction experiments were performed on
compounds 2(10/BF4) and 2(10/PF6). These experiments
enabled reconstruction of electron density maps of the Cubbi
phases in the bulk state. Figure 3a shows the map of a unit cell
Figure 1. Molecular design in this work for development of
thermotropic liquid-crystalline material forming bicontinuous cubic
phases.
reports on the LC molecules containing ionic moieties,17,20,27
induction of Cubbi phases has been observed only in a handful
of ionic compounds4a,b,28 including our previously reported
molecules.6,7
In the present study, we prepared two series of wedge-shaped
ammonium salts 1−3 and phosphonium salts 4−6 (Figure 2)
of the Ia3d Cub phase in 2(10/BF4). The high electron
̅
bi
density (orange) and the low density (blue) regions represent,
respectively, aromatic/ionic and aliphatic domains rich in chain
ends. The high-density regions form the two continuous
antichiral channel networks; the two networks are separated by
a low-density sheath (blue) that follows the gyroid G-surface of
minimum curvature. Figure 3b is the map of 2(10/PF6) from
which the map of 2(10/BF4) had been subtracted. Prior to
subtraction the maps had been scaled based on electron density
histograms.35 The difference between the two maps is caused
by the extra electrons contained in PF6 as compared to BF4.
The yellow isoelectron surface in Figure 3b encloses the regions
where the maps differ most, i.e., the regions richest in anions;
for details see the Supporting Information. These results
provide compelling evidence that the ionophilic parts occupy
Figure 2. Molecular structures of wedge-shaped ammonium salts 1−
3(n/X) and phosphonium salts 4−6(n/BF4).
the 3D networks of interconnected channels in the Ia3d Cub
̅
bi
structure. It is considered that the localization of anions in the
nanosegregated structures is important in the design of ion-
transporting and nanochannel materials.
Effects of Cationic Moieties of Wedge-Shaped
Ammonium Salts on Mesomorphism. To examine the
effects of the molecular structures of ammonium compounds
to examine the effects of the cationic moieties, alkyl chain
length n, and anion species on their LC properties. We focused
on induction of Cubbi phases. Since phosphonium salts have
similar properties to ammonium salts, they have been studied
for comparison with ammonium salts.29 For example,
phosphonium cations have been recently used as alternatives
2(n/BF ) on induction of Ia3d Cub phases, we designed and
̅
4
bi
synthesized analogous ammonium salts 1(n/BF4) and
2635
dx.doi.org/10.1021/ja209010m | J. Am. Chem.Soc. 2012, 134, 2634−2643

Journal of the American Chemical Society
Article
Figure 4. Wide-angle X-ray diffraction pattern of 1(12/BF4) in the
Colh phase at 150 °C.
indexed as the (10), (11), and (20) reflections of the 2D
hexagonal arrangement of the columns. The Colh mesophases
formed by compounds 1(n/BF4) are stable up to about 200 °C,
whereas 2(n/BF4) exhibits isotropization below 150 °C. The
higher isotropization temperatures of 1(n/BF4) are partly
attributed to stronger electrostatic interactions between the
smaller cationic moiety and the anion in the ionic domain, but
partly it is believed to be the result of a better match between
the geometry of the molecule and that of the space available to
it in the particular LC phase (see below).
In contrast, compounds 3(n/BF4), having a tripropyl
ammonium moiety, which is larger than that of 2(n/BF4),
show no mesomorphic properties except for 3(14/BF4). It is
likely that mesophase formation is disfavored partially by the
fact that the bulky tripropyl ammonium moiety sterically
hinders the counteranion from interacting closely with the
cationic moiety in the ionic domain. Furthermore, increasing
nanosegregation may be partly responsible for the fact that
extending the alkyl chain n beyond 14 induces formation of the
mesophase, e.g., 3(14/BF4) shows a columnar phase between
80 and 91 °C. However, a single unified geometry-based
scheme is capable of explaining, at least qualitatively, most
features of the observed phase behavior of compounds in this
study.
Figure 3. (a) Electron density map ρ obtained from small-angle
diffraction intensities of a powder pattern of 2(10/BF ) in the Ia3d
̅
4
Cubbi phase at 20 °C. High-density regions (aromatic/ionic) are
enclosed within the orange isoelectron surfaces and low-density
regions (alkyl chain ends) within the blue surfaces. (b) Differential
electron density map ρ{2(10/PF6)} − ρ{(2(10/BF4)}, showing the
preferred location of anions enclosed within the yellow isosurfaces. 2D
maps lining the walls of b are cuts in (100), (010), and (001) planes;
refer to the color scale on the right.
3(n/BF4) having different cationic moieties from 2(n/BF4).
The thermal properties of ammonium salts 1−3(n/BF4) are
presented in Table 1. The thermal properties of these
ammonium salts differ significantly between the compounds,
depending on the structures of the cationic moiety.
The geometric scheme was originally devised to rationalize
the fact that different wedge-shaped molecules, such as
dendrons, form different 3D-ordered LC phases (cubic,
tetragonal) consisting of spherical clusters and to explain
thermotropic polymorphism in such compounds.18h The
principle was later extended to bicontinuous and tricontinuous
Compounds 1(n/BF4), having a trimethyl ammonium
moiety, which is smaller than that of 2(n/BF4), form hexagonal
columnar phases. Figure 4 shows a typical XRD pattern of
1(12/BF4) in the Colh phase at 150 °C. The intense peak at
30.2 Å and the two weak peaks at 17.5 and 15.2 Å are observed
with the reciprocal d-spacing ratio of 1:√3:2, which can be
Table 1. Thermal Properties of Ammonium Compounds 1−3(n/BF4)
a
phase transition behavior
compound
n
b
c
c
1(n/BF4)
10
12
14
10
12
14
10
12
14
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
60 (8.0)
44 (27)
57 (77)
42 (0.44)
32 (29)
53 (63)
82 (28)
80 (31)
80 (26)
Colh
Cr
194 (−)
Iso
b
c
60 (9.1)
Colh
Iso
206 (−)
Iso
Iso
b
Colh
Cubbi
Cubbi
Colh
Iso
202 (−)
d
d
d
2(n/BF4)
3(n/BF4)
82 (0.83)
Iso
49 (0.80)
Colh
Iso
126 (0.63)
142 (0.99)
Iso
Colh
91 (0.82)
Iso
a
Cr, crystalline; Cubbi, bicontinuous cubic; Colh, hexagonal columnar; Iso, isotropic. Transition temperatures (°C) and enthalpy changes (kJ mol−1,
b
in parentheses) are determined by DSC on the second heating. Transitions are determined by visual observation with a polarizing optical
microscope. No transition enthalpy was detected by the DSC. Reference 7.
c
d
2636
dx.doi.org/10.1021/ja209010m | J. Am. Chem.Soc. 2012, 134, 2634−2643

Journal of the American Chemical Society
Article
cubic phases.2c A bicontinuous cubic, such as Ia3d, can be
zero (dashed curve in Figure 5). At the other extreme are
molecules that are not tapered at all but maintain a constant A
throughout, i.e., where A(r) ∝ dV/dr ∝ r0 = 1 (dashed
rectangle). Molecules whose shape can be described by a
fractional exponent, i.e., where A(r) ∝ dV/dr ∝ rm, with 0 <
m < 1, are likely to form a bicontinuous or tricontinuous cubic
phase. Which of the phase variants will actually appear is
determined by the precise shape of the molecule in its most
probable conformation.2c
̅
represented by networks of branching cylindrical rods (Figure
3). As shown above, the ionic groups, i.e., the narrow ends of
the wedge-shaped molecules, are located near the central axis of
the cylinders. In order to determine the ideal shape of a
molecule that would fit perfectly into a given mesophase, we
start at the central cylinder axis and calculate the rate of
increase in volume of the cylinder network, dV/dr, as the radius
r is increased. The result for the Ia3d Cub phase is shown in
̅
bi
Figure 5 (full line), together with the results for the hexagonal
We can see now why on going from compound 1(12/BF4)
to 2(12/BF4), i.e., by replacing the trimethyl with the triethyl
ammonium group, the structure changes from Col to Ia3d
̅
h
Cub ; close to r = 0 the A(r) curve for the Ia3d Cub phase is
̅
bi
bi
higher than the curve for the Colh phase. The extra cross-
section near the apex of the molecule is provided by the three
extra methylene groups. By the same reasoning one might
expect that moving to compound 3(12/BF4) we might get the
smectic phase. However, this compound does not form a
mesophase. Presumably the equilibrium shape of the
3(12/BF4) molecule does not match sufficiently closely to
any of the curves in Figure 5, and if the length of the alkyls in
the ammonium moiety were increased further, perhaps a
smectic phase would form. However, the fact that compound
3(14/BF4) forms the Colh phase at higher temperatures
suggests that the extra six methylenes at the outer end (large
r) tips the distribution of volume back to the right in Figure 5,
sufficiently close to the Colh (Hex) curve.
These results suggest that the design of the size and shape of
the narrow end of the series of wedge-shaped ammonium salts
is particularly important in directing the type of their
mesophase assembly. This is in agreement with the previous
finding that small changes at the narrow end of wedge-shaped
molecules have a disproportionally large effect on the phase
morphology in 3D LC phases of spherical aggregates.36 The
above geometric model has shown once again its usefulness in
relating molecular architecture to the morphology of the
mesophase.
Figure 5. dV/dr plots for three LC phases: Colh (Hex), smectic (Sm),
and bicontinuous cubic (Ia3d). (Top) Schematic drawing of the
̅
molecular cross-section area profile, A(r), which should match dV/dr
for uniform filling of the volume of a particular phase. Curves are
scaled horizontally to the same radius rar delimiting the “aromatic”
volume fraction (including all but the peripheral alkoxy chains) from
the alkoxy fraction. Vertical scaling normalizes the areas under the
curves.
Comparison of the Mesomorphic Properties of
Ammonium and Phosphonium Wedge-Shaped Salts.
The thermal properties of phosphonium salts 4−6(n/BF4) are
presented in Table 2. It is of interest to compare the LC
properties of these phosphonium salts with those of the
corresponding ammonium salts 1−3(n/BF4). Phosphonium
compounds 4(n/BF4), having a triethyl phosphonium moiety,
form Colh phases, just as in the corresponding ammonium
compounds 1(n/BF4). For the series of 4(n/BF4), the
columnar phase and the smectic (layered) phase with the
molecular axis normal to the layers (dashed lines). For the Colh
phase the cylinders grow in radius unhindered, with dV/dr ∝ r,
until the parallel cylinders clash (around r = 1.5 in Figure 5). As
further growth in volume takes place only where the cylinders
do not overlap, dV/dr drops sharply and falls to 0 when all
space is filled. Thus, the ideal shape of a molecule to fit into the
Colh phase is one for which the cross-sectional area increases
linearly, i.e., A(r) ∝ dV/dr ∝ r, and then drops quite abruptly to
Table 2. Thermal Properties of Phosphonium Compounds 4−6(n/BF4)
a
phase transition behavior
compound
n
4(n/BF4)
10
12
14
10
12
14
10
12
14
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
55 (8.3)
6 (14)
Colh
Cr
164 (0.88)
34 (3.7)
164 (0.55)
0 (18)
Iso
Colh
Iso
181 (0.70)
48 (0.76)
Iso
Iso
40 (29)
−17 (16)
31 (43)
50 (59)
31 (26)
37 (44)
39 (45)
Colh
Cr
5(n/BF4)
6(n/BF4)
Cubbi
Iso
Colh
Colh
Colh
Cr
102 (0.63)
127 (0.53)
66 (3.5)
62 (23)
Iso
Iso
Colh
88 (2.5)
93 (1.2)
Iso
Iso
Cr
64 (29)
Colh
a
Cr, crystalline; Cubbi, bicontinuous cubic; Colh, hexagonal columnar; Iso, isotropic. Transition temperatures (°C) and enthalpy changes (kJ mol−1,
in parentheses) are determined by DSC on the second heating.
2637
dx.doi.org/10.1021/ja209010m | J. Am. Chem.Soc. 2012, 134, 2634−2643

Journal of the American Chemical Society
Article
isotropization temperatures are lower than those for the
1(n/BF4) series. This behavior can be explained by the
weakening of electrostatic interactions as the cation atom
changes from nitrogen to phosphorus. This assumption is
further supported by the comparison of the isotropization
temperatures of ammonium salts 2, 3(n/BF4) and phospho-
nium salts 5, 6(n/BF4).
Induction of Cubbi phase is observed for phosphonium
compound 5(10/BF4), having a triethyl onium moiety, as well
as the corresponding ammonium compound 2(10/BF4).
Compound 5(10/BF4) exhibits a Cubbi phase from 0 to
48 °C on heating. Figure 6 shows the small-angle X-ray
density maps of 5(10/BF4) with that of 2(10/BF4) (see
Supporting Information) reveals that the self-assembled Ia3d
̅
Cubbi nanostructures formed by 2(10/BF4) and 5(10/BF4) are
very similar.
Figure 7 compares the thermal properties of ammonium salts
2(n/BF4) and phosphonium salts 5(n/BF4) having triethyl
onium moieties. It is noteworthy that 2(n/BF4) and 5(n/BF4)
show similar variation in phase sequence and transition
temperatures on alkyl chain length n. For example, only
2(10/BF ) and 5(10/BF ) exhibit the single Ia3d Cub phases
̅
4
4
bi
which transform directly to the isotropic phases. In addition, it
is common to both 2(n/BF4) and 5(n/BF4) that increasing the
alkyl chain length n leads to induction of Colh phases. These
results suggest that induction of Cubbi phases is very sensitive
to the length n of the alkyl chains tethered to the molecules.
In contrast to the similarity in the phase behavior between 1,
2(n/BF4), and 4, 5(n/BF4), ammonium compounds 3(n/BF4)
and phosphonium compounds 6(n/BF4), having tripropyl
onium moieties, differ considerably in their phase behavior.
For example, 6(10/BF4) forms a Colh phase from 31 to 66 °C
on heating, whereas 3(10/BF4) exhibits no mesomorphism.
Another difference is observed in the enthalpy of isotropization.
The enthalpy values of compounds 6(n/BF4) (n = 10, 12, 14)
are 3.5, 2.5, and 1.2 kJ mol−1, which are larger than those of
compounds 1−5(n/BF4). We attribute the mesophase
formation in compounds 6(n/BF4) to van der Waals
interactions between the phosphonium cation moieties. This
assumption is consistent with the general tendency of
phosphonium salts to form stronger van der Waals interactions
compared to the corresponding ammonium salts.32−34
Figure 6. Small-angle X-ray scattering pattern of 5(10/BF4) in the
Ia3d Cub phase at 35 °C.
̅
bi
scattering pattern for 5(10/BF4) in the Cubbi phase at 35 °C.
Two sharp peaks at 32.3 Å and 27.7 Å and four weak peaks at
20.7, 17.5, 16.7, and 15.4 Å are observed. The reciprocal
s p a c i n g r a t i o o f t h e s e s i x p e a k s i s
√6:√8:√14:√20:√22:√26; hence, the peaks can be
indexed as (211), (220), (321), (420), (332), and (510)
The fact that the Cubbi phase is observed only in compound
5(10/BF4) among the series of phosphonium salts 4−6(n/BF4)
further supports the conclusion that the triethyl onium moieties
play an important role in inducing the Cubbi phase.
1H NMR Studies of Ammonium and Phosphonium
reflections of the gyroid Cubbi phase with Ia3d̅ symmetry. The
1
Salts. H NMR studies of CDCl3 solutions of these salts have
lattice parameter (a) of the cubic structure is calculated to be
79 Å, and the number (N) of molecules for the unit cell is
estimated to be 4.6 × 102 from N = NAa3ρ/M, where NA is
Avogadro’s number (6.02 × 1023 mol−1) and M is the molecular
weight (764.89 g mol−1). The density (ρ) of the material is 1.2
g cm−3, which has been determined by the floatation method in
D-(+)-sucrose/H2O at 20 °C. The self-assembled nanostructure
formed by 5(10/BF4) has also been examined by synchrotron
small-angle diffraction experiments. Comparison of the electron
been performed in order to compare the ammonium and
phosphonium salts with regard to ionicity of the cationic
1
moiety. Figure 8 shows the H NMR spectra of compounds
2(10/BF4) and 5(10/BF4). A coupling peak corresponding to
the benzyl phosphonium proton for 5(10/BF4) appears at 3.62
ppm (Figure 8b), whereas a corresponding singlet peak for
analogous ammonium salt 2(10/BF4) is observed at 4.30 ppm
(Figure 8a). These results suggest that the phosphonium cation
Figure 7. Comparison of LC phase sequences of 2(n/BF4) and 5(n/BF4): Cr, crystalline; Cubbi, bicontinuous cubic; Colh, hexagonal columnar; Iso,
isotropic.
2638
dx.doi.org/10.1021/ja209010m | J. Am. Chem.Soc. 2012, 134, 2634−2643

Journal of the American Chemical Society
Article
evaporation of the tetrahydrofuran solution of 2(10/BF4) or
5(10/BF4) and the requisite amounts of LiBF4.
The LC properties of the mixtures were examined by
polarizing optical microscopy, differential scanning calorimetry
(DSC), and X-ray diffraction measurements. These mixtures
show not only Ia3d Cub phases but also Colh phases. For
̅
bi
example, upon cooling the isotropic liquid mixture 5(10/BF4)/
LiBF4 with a 4:1 molar ratio a focal conic fan texture is
observed at 105 °C under crossed Nicols (see Supporting
Information). This indicates formation of a Colh phase. On
further cooling, the birefringence disappears entirely following a
phase transition to a Ia3d Cub phase (see Supporting
̅
bi
Information). The DSC thermogram of the mixture shows
two exothermic peaks at 88 and 43 °C upon cooling (Figure
Figure 8. 1H NMR spectra of (a) ammonium-based compound 2(10/
BF4) and (b) phosphonium-based compound 5(10/BF4) in CDCl3
(50 mM) at 20 °C.
Figure 10. DSC thermograms of the mixture of 5(10/BF4)/LiBF4
with a 4:1 molar ratio: Cr, crystalline; Cubbi, bicontinuous cubic; Colh,
hexagonal columnar; Iso, isotropic.
is less positively charged than the ammonium cation and
therefore forms weaker electrostatic interaction with counter-
anion. This speculation is consistent with the tendency in the
bulk state for the phosphonium compounds 4−6(n/BF4) to
turn isotropic at lower temperatures than the corresponding
ammonium compounds 1−3(n/BF4).
Complexation of the Cubbi LC Salts with Lithium
Tetrafluoroborate. One interesting potential application of
these ionic nanostructured liquid crystals is their use as
electrolytes in lithium batteries.5 Figure 9 shows our material
design for a new class of lithium ion conductors having a Cubbi
LC structure. It is important to examine whether the wedge-
shaped onium salts show compatibility with lithium salts and
form Cubbi phases in the presence of lithium salts. We mixed
the wedge-shaped onium salts and lithium tetrafluoroborate
(LiBF4) in various ratios. As LC samples, compounds
10). These correspond to the Iso−Colh and Colh−Cubbi phase
transitions, respectively. The above results show that the ionic
LC compounds 2(10/BF4) and 5(10/BF4) are compatible with
lithium salts and form homogeneous mixtures with them.
Binary phase diagrams of the mixtures 2(10/BF4)/LiBF4 and
5(10/BF4)/LiBF4 are presented in Figure 11. It is noteworthy
that these ionic compounds maintain the Cubbi phase in the
presence of the lithium salts. With increasing mole fraction of
the lithium salt, both mixtures show the Colh phases at
temperatures above those of the Cubbi phase. Moreover, the
isotropization temperature increases with increasing mole
fraction of the lithium salt. These results suggest the following
conclusions. One is that the hard lithium cations, being
incorporated in the ion channels composed of the ionophilic
parts, enhance electrostatic interactions in the ion channels and
stabilize the nanosegregated structures. The second conclusion
2(10/BF ) and 5(10/BF ), which exhibit only the Ia3d Cub
phases, were selected. The mixtures were prepared by slow
̅
4
4
bi
Figure 9. Material design for a new class of lithium ion conductors having a Cubbi LC structure.
2639
dx.doi.org/10.1021/ja209010m | J. Am. Chem.Soc. 2012, 134, 2634−2643

Journal of the American Chemical Society
Article
S cm−1 at 48 °C, which is about 30 times higher than that of
2(10/BF4) at the same temperature. The higher ionic
conductivities of phosphonium salt 5(10/BF4) can be explained
by the idea that the weaker electrostatic interaction between the
phosphonium cationic moiety and the BF4 anion may form ion
channels in which ions can migrate more freely. This
explanation is consistent with the known fact that phospho-
nium-based ionic liquids have lower viscosity and higher ionic
conductivity than analogous ammonium-based ionic liquids.32
It has been established that the difference of the physical
properties between phosphonium-based ionic liquids and
ammonium-based ionic liquids results from the difference in
ionicity between phosphonium and ammonium cations.31,32 In
the case of ionic liquids, it is generally accepted that the weaker
the electrostatic interaction within a cation−anion pair is, the
lower the viscosity is and the higher ionic conductivity is.37
The ionic conductivity of the mixture of 5(10/BF4) with 0.25
mol of LiBF4 is also plotted against T−1 in Figure 12 (blue
triangles). The mixture also shows efficient ion conduction in
Figure 11. Phase transition temperatures for (a) mixtures of
2(10/BF4) with LiBF4 and (b) 5(10/BF4) with LiBF4 on heating as
a function of mole fraction of LiBF4: Cr, crystalline; M, unidentified
mesophase; Cubbi, bicontinuous cubic; Colh, hexagonal columnar; Iso,
isotropic.
is that the volume balance between the ionophilic and
ionophobic parts, which is influenced by the thermal motion
and conformation of alkyl chains, significantly affects the
mesophase behavior of these materials.
the Ia3d Cub phase. In this phase the conductivity of the
̅
bi
mixture is comparable to that of the single component
5(10/BF4). Interestingly, above the Cubbi−Colh transition
temperature the ion conductivity of the mixture decreases.
Ionic Conductivities of the LC Materials. Ionic
conductivities of the Ia3d Cub LC materials have been
̅
bi
For example, the value at 44 °C in the Ia3d Cub phase is
̅
bi
1.3 × 10−4 S cm−1 and that at 50 °C in the Colh phase is
studied by the alternating current impedance method using a
comb-shaped gold electrode.20b Figure 12 shows the ionic
5.5 × 10−5 S cm−1. The higher ionic conductivity in the Ia3d
Cubbi phase can be attributed to its 3D channel network
̅
structure. In the Ia3d Cub phase, ions can migrate efficiently
̅
bi
through the ion channels even in the presence of structure
defects where the channels do not continue because the 3D
branched channels in the Ia3d Cub phase form alternative
̅
bi
pathways, bypassing the structure defects. However, in the Colh
phase, the transportation of ions may be blocked by structural
defects such as channel ends because ions cannot bypass the
defects due to the straight 1D channel structure.
Effect of the Counteranion. For ionic liquids, the
counteranions greatly influence their thermal and chemical
properties.38 In order to study the effects of the counteranion
on the thermal properties and self-organization behavior of
these wedge-shaped ammonium salts, we prepared analogous
ammonium salts having other perfluorinated anions, such as
PF6 and CF3SO3, which are representative anions for the design
of conventional ionic liquids. A comparison of electron density
maps for compounds with different anions also allowed us to
perform isomorphous replacement X-ray analysis and locate the
anions in the structure, as described above (Figure 3b). The
thermal properties of compounds 2(n/PF6) and 2(n/CF3SO3)
are collected in Table 3. It is apparent that the series of
ammonium salts 2(n/X) show some significant differences in
phase behavior depending on their anion species.
Figure 12. Arrhenius plots of ionic conductivities of ammonium salt
2(10/BF4) (black circles), phosphonium salt 5(10/BF4) (red squares),
and the complex of phosphonium salt 5(10/BF4) and LiBF4 with a 4:1
molar ratio (blue triangles).
conductivities of ammonium salt 2(10/BF4) and phosphonium
salt 5(10/BF4) as a function of temperature. The ionic
conductivities for 2(10/BF4) (black circles) were reported in
our preliminary communication.6 Upon heating, the ionic
conductivities for phosphonium salt 5(10/BF4) (red squares)
increase with increasing temperature up to the isotropization
temperature. At that temperature the conductivities drop
abruptly. We attribute this to the break up of the 3D
nanochannel network. This behavior is similar to that of
compound 2(10/BF4). These results further confirm the
Figure 13 compares the phase transition behavior of the
series of ammonium salts 2(n/X). It is notable that induction of
Ia3d Cub phases is observed in compounds 2(n/PF ) as well
̅
bi
6
as in 2(n/BF4). For example, 2(10/PF6) forms a monotropic
Ia3d Cub phase from 34 to −45 °C. Upon increasing the alkyl
̅
bi
chain lengths from 10 to 12, an enantiotropic Ia3d Cub phase
̅
bi
efficiency of the Ia3d̅ Cubbi channel structure for transportation
is observed. As the alkyl chains are extended to n = 14 the Ia3d
̅
materials. It is noteworthy that phosphonium salt 5(10/BF4)
(red squares) shows higher ionic conductivity than ammonium
salt 2(10/BF4) (black circles) at the same temperature,
although the molecular assembled structures formed by
2(10/BF4) and 5(10/BF4) are very similar. The highest
conductivity achieved for compound 5(10/BF4) is 2.3 × 10−4
Cubbi phase is replaced by the Colh phase. Compound
2(14/PF6) forms a Colh phase from 72 to 114 °C on heating.
Such behavior is to be expected, since a larger aliphatic volume
increases the effective cross-section at the wide end of the
wedge-shaped molecule, thus favoring the Colh phase. By
contrast, Ia3d Cub phases are favored when the wedge curves
̅
bi
2640
dx.doi.org/10.1021/ja209010m | J. Am. Chem.Soc. 2012, 134, 2634−2643

Journal of the American Chemical Society
Article
Table 3. Thermal Properties of Ammonium Compounds
2(n/PF6) and 2(n/CF3SO3)
CONCLUSION
■
Two series of wedge-shaped ammonium and phosphonium
salts have been prepared, and their mesomorphic properties
have been successfully examined. These ammonium and
a
phase transition behavior
compound
n
phosphonium salts exhibit Ia3d Cub and Colh phases
̅
bi
2(n/PF6)
10
12
14
10
12
14
G
34 (7.7)
64 (48)
72 (64)
45 (22)
53 (40.4)
65 (68)
Cr
58 (10)
Iso
Iso
Iso
depending on the cationic moiety, alkyl chain length n, and
anion species. The phase transition behavior of the
phosphonium salts is similar to that of the corresponding
ammonium salts. To the best of our knowledge, the
phosphonium-based compound 5(10/BF4) is the first example
of a phosphonium salt showing a thermotropic Cubbi phase,
although a few phosphonium salts exhibiting lyotropic Cubbi
Cr
Cr
Cr
Cr
Cr
Cubbi
Colh
Iso
76 (0.54)
114 (0.63)
2(n/CF3SO3)
Cr
64 (2.6)
Iso
Iso
a
Cr, crystalline; Cubbi, bicontinuous cubic; Colh, hexagonal columnar;
phases have been reported.10,33b The structure of the Ia3d
Iso, isotropic. Transition temperatures (°C) and enthalpy changes (kJ
̅
mol−1, in parentheses) are determined by DSC on the second heating.
Cubbi phase has been further confirmed by electron density
mapping based on synchrotron diffraction experiments. More-
over, the location of the anions in the structure has been
determined using a novel differential mapping technique. The
phase behavior of these salts suggests that the chemical
structures of the ionic moiety and alkyl chain length n are
important for induction of mesomorphic properties.
Ionic conduction measurements have revealed that the Ia3d
̅
Cubbi structures with 3D interconnected ion channels are
indeed effective in ion transportation. The phosphonium-based
Cubbi LC materials show ionic conductivities in the order of
10−4 S cm−1 at room temperature, which are higher than those
of the analogous ammonium-based Cubbi LC materials. The
ionic conductivities of conventional amorphous solvent-free
solid polymer electrolytes based on poly(ethylene oxide) range
from 10−8 to 10−4 S cm−1.39 Introduction of well-designed LC
nanostructures into these poly(ethylene oxide)-based materials
increases the conductivity up to values in the order of 10−3
S
cm−1 at room temperature.21,26b On the other hand, for liquid
electrolytes, ionic liquids show ionic conductivities ranging
from 10−3 to 10−1 S cm−1 at room temperature.37 Development
of solid polymer electrolytes keeping high ionic conductivities
using LC nanostructures7 is our next target.
One of the most interesting aspects of the Ia3d Cub
̅
bi
structures is that their 3D interconnected channels function
as transport pathways without the macroscopic orientation of
the liquid-crystal domains. In the present study, their
application as ion-transporting channels has been demon-
strated. However, the utility of these nanochannels may not be
limited to ion transportation. Owing to their nano-ordered
channel structures, they have great potential to be applied as
size-selective filtration membranes. In the near future, the
striking innovation of nanochannel materials will be exploited
in the development of novel Cubbi LC materials. We believe
that the presented research focusing on induction of Cubbi LC
phases will help improve the basic knowledge required for
designing a new generation of Cubbi LC molecules.
Figure 13. Comparison of LC properties for a series of ammonium
salts with different anions: 2(n/BF4), 2(n/PF6), and 2(n/CF3SO3). Cr,
crystalline; Cubbi, bicontinuous cubic; Colh, hexagonal columnar; Iso,
isotropic.
in at the wide end.3a In the same fashion the transition from
Ia3d̅ Cubbi to Colh phase with increasing temperature can be
attributed to the increased wedge cross-section at the wide
aliphatic end. On the other hand, compounds 2(n/CF3SO3),
having a bulky anion, never exhibit mesomorphic properties.
The bulky anion may sterically disturb their packing into
nanosegregated assemblies. Examining the isotropization
temperature of compounds 2(n/X) it is seen to decrease as
the size of the counteranion increases in the order BF4 < PF6 <
CF3SO3. For example, the isotropization temperatures of
2(12/BF4), 2(12/PF6), and 2(12/CF3SO3) are 126, 76, and
64 °C, respectively. The trend is similar to that observed in a
series of LC imidazolium salts reported in our previous
report.17 These results suggest that the electrostatic interaction
exerts significant effects on the stability of the mesophases.
ASSOCIATED CONTENT
■
S
* Supporting Information
Syntheses, polarizing optical microscopic images, DSC charts,
and X-ray diffraction and scattering patterns of compounds 1−
6; detailed procedure for construction of the electron density
maps for 2(10/BF4), 2(10/PF6), and 5(10/BF4). This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
kato@chiral.t.u-tokyo.ac.jp
2641
dx.doi.org/10.1021/ja209010m | J. Am. Chem.Soc. 2012, 134, 2634−2643

Journal of the American Chemical Society
Article
Ungar, G.; Balagurusamy, V. S. K. Science 1997, 278, 449−452.
(d) Gin, D. L.; Gu, W.; Pindzola, B. A.; Zhou, W.-J. Acc. Chem. Res.
2001, 34, 973−980. (e) Seddon, J. M.; Robins, J.; Gulik-Krzywicki, T.;
Delacroix, H. Phys. Chem. Chem. Phys. 2000, 2, 4485−4493. (f) Bisoyi,
H. K.; Kumar, S. Chem. Soc. Rev. 2010, 39, 264−285. (g) Saez, I. M.;
Goodby, J. W. J. Mater. Chem. 2005, 15, 26−40. (h) Ungar, G.; Liu, Y.;
Zeng, X.; Percec, V.; Cho, W.-D. Science 2003, 299, 1208−1211.
(19) (a) Chen, B.; Baumeister, U.; Pelzl, G.; Das, M. K.; Zeng, X.;
Ungar, G.; Tschierske, C. J. Am. Chem. Soc. 2005, 127, 16578−16591.
(b) Percec, V.; Holerca, M. N.; Uchida, S.; Cho, W.-D.; Ungar, G.;
Lee, Y.; Yeardley, D. J. P. Chem.Eur. J. 2002, 8, 1106−1117.
(c) Choi, J.-W.; Ryu, M.-H.; Lee, E.; Cho, B.-K. Chem.Eur. J. 2010,
16, 9006−9009. (d) Jin, L. Y.; Bae, J.; Ryu, J.-H.; Lee, M. Angew.
Chem., Int. Ed. 2006, 45, 650−653.
ACKNOWLEDGMENTS
■
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, the
Global COE Program for Chemistry Innovation through
Cooperation of Science and Engineering (T.K.), and a Grant-
in-Aid for Scientific Research (A) (No. 19205017) from the
Japan Society for the Promotion of Science (JSPS). We also
acknowledge support from EC 7th Framework Programme,
under contracts NANOGOLD, grant no. 228455. T.I. is
grateful for financial support from the JSPS Research
Fellowship for Young Scientists and G.U. for support from
the WCU program through the National Research Foundation
of Korea (R31-10013). For help with the synchrotron
experiments we thank Drs. Nick Terrill and Jen Hiller of
Diamond Light Source.
(20) (a) Yoshio, M.; Mukai, T.; Kanie, K.; Yoshizawa, M.; Ohno, H.;
Kato, T. Adv. Mater. 2002, 14, 351−354. (b) Yoshio, M.; Mukai, T.;
Ohno, H.; Kato, T. J. Am. Chem. Soc. 2004, 126, 994−995. (c) Kato,
T.; Yoshio, M. In Electrochemical Aspects of Ionic Liquids; Ohno, H.,
Ed.; Wiley: Hoboken, NJ, 2005; Chapter 25, pp 307−320. (d) Yoshio,
M.; Kagata, T.; Hoshino, K.; Mukai, T.; Ohno, H.; Kato, T. J. Am.
Chem. Soc. 2006, 128, 5570−5577. (e) Shimura, H.; Yoshio, M.;
Hoshino, K.; Mukai, T.; Ohno, H.; Kato, T. J. Am. Chem. Soc. 2008,
130, 1759−1765. (f) Yoshio, M.; Mukai, T.; Ohno, H.; Kato, T. ACS
Symp. Ser. 2007, 975, 161−171.
REFERENCES
(1) Luzzati, V.; Spegt, P. A. Nature 1967, 215, 701−704.
■
(2) (a) Bruce, D. W. Acc. Chem. Res. 2000, 33, 831−840.
(b) Kutsumizu, S.; Morita, K.; Ichikawa, T.; Yano, S.; Nojima, S.;
Yamaguchi, T. Liq. Cryst. 2002, 29, 1447−1458. (c) Zeng, X.; Ungar,
G.; Imper
376.
(3) (a) Zeng, X.; Ungar, G.; Imper
́
or-Clerc, M. Curr. Opin. Colloid Interface Sci. 2005, 9, 370−
(21) Kishimoto, K.; Suzawa, T.; Yokota, T.; Mukai, T.; Ohno, H.;
Kato, T. J. Am. Chem. Soc. 2005, 127, 15618−15623.
́
or-Clerc, M. Nat. Mater. 2005, 4,
(22) Yasuda, T.; Ooi, H.; Morita, J.; Akama, Y.; Minoura, K.;
Funahashi, M.; Shimomura, T.; Kato, T. Adv. Funct. Mater. 2009, 19,
411−419.
562−567. (b) Ozawa, K.; Yamamura, Y.; Yasuzuka, S.; Mori, H.;
Kutsumizu, S.; Saito, K. J. Phys. Chem. B 2008, 112, 12179−12181.
(4) (a) Donnio, B.; Heinrich, B.; Gulik-Krzywicki, T.; Delacroix, H.;
Guillon, D.; Bruce, D. W. Chem. Mater. 1997, 9, 2951−2965.
(b) Fuchs, P.; Tschierske, C.; Raith, K.; Das, K.; Diele, S. Angew.
Chem., Int. Ed. 2002, 41, 628−631. (c) Ichihara, M.; Suzuki, A.;
Hatsusaka, K.; Ohta, K. Liq. Cryst. 2007, 34, 555−567.
(23) (a) Simpson, C. D.; Wu, J.; Watson, M. D.; Mullen, K. J. Mater.
̈
Chem. 2004, 14, 494−504. (b) Funahashi, M.; Zhang, F.; Tamaoki, N.
Adv. Mater. 2007, 19, 353−358. (c) Percec, V.; Glodde, M.; Bera, T.
K.; Miura, Y.; Shiyanovskaya, I.; Singer, K. D.; Balagurusamy, V. S. K.;
Helney, P. A.; Schnell, I.; Rapp, A.; Spiess, H.-W.; Hudson, S. D.;
Duan, H. Nature 2002, 419, 384−387. (d) Boden, N.; Bushby, R. J.;
Clements, J.; Movaghar, B. J. Mater. Chem. 1999, 9, 2081−2086.
(24) Yazaki, S.; Funahashi, M.; Kagimoto, J.; Ohno, H.; Kato, T. J.
Am. Chem. Soc. 2010, 132, 7702−7708.
(5) Kato, T. Angew. Chem., Int. Ed. 2010, 49, 7847−7848.
(6) Ichikawa, T.; Yoshio, M.; Hamasaki, A.; Mukai, T.; Ohno, H.;
Kato, T. J. Am. Chem. Soc. 2007, 129, 10662−10663.
(7) Ichikawa, T.; Yoshio, M.; Hamasaki, A.; Kagimoto, J.; Ohno, H.;
Kato, T. J. Am. Chem. Soc. 2011, 133, 2163−2169.
(25) Sagara, Y.; Kato, T. Nat. Chem. 2009, 1, 605−610.
(26) (a) Ikkala, O.; ten Brinke, G. Chem. Commun. 2004, 2131−
2137. (b) Zheng, Y.; Lui, J.; Ungar, G.; Wright, P. V. Chem. Rec. 2004,
4, 176−191.
(8) Frise, A. E.; Ichikawa, T.; Yoshio, M.; Ohno, H.; Dvinskikh, S. V.;
́
Kato, T.; Furo, I. Chem. Commun. 2010, 46, 728−730.
(9) Lu, X.; Nguyen, V.; Zhou, M.; Zeng, X.; Jin, J.; Elliott, B. J.; Gin,
D. L. Adv. Mater. 2006, 18, 3294−3298.
(10) Zhou, M.; Nemade, P. R.; Lu, X.; Zeng, X.; Hatakeyama, E. S.;
Noble, R. D.; Gin, D. L. J. Am. Chem. Soc. 2007, 129, 9574−9575.
(11) Kerr, R. L.; Miller, S. A.; Shoemaker, R. K.; Elliott, B. J.; Gin, D.
L. J. Am. Chem. Soc. 2009, 131, 15972−15973.
(12) Shah, J. C.; Sadhale, Y.; Chilukuri, D. M. Adv. Drug Delivery Rev.
2001, 47, 229−250.
(13) Yang, D.; Armitage, B.; Marder, S. R. Angew. Chem., Int. Ed.
2004, 43, 4402−4409.
(14) (a) Cho, B.-K.; Jain, A.; Gruner, S. M.; Wiesner, U. Science 2004,
305, 1598−1601. (b) Dobbs, W.; Heinrich, B.; Bourgogne, C.;
Donnio, B.; Terazzi, E.; Bonnet, M.-E.; Stock, F.; Erbacher, P.;
Bolcato-Bellemin, A.-L.; Douce, L. J. Am. Chem. Soc. 2009, 131,
13338−13346.
(27) (a) Binnemans, K. Chem. Rev. 2005, 105, 4148−4204.
(b) Bazuin, C. G.; Guillon, D.; Skoulios, A.; Nicoud, J.-F. Liq. Cryst.
1986, 1, 181−188. (c) Ujiie, S.; Iimura, K. Chem. Lett. 1990, 995−998.
(d) Kosaka, Y.; Kato, T.; Uryu, T. Liq. Cryst. 1995, 18, 693−698.
(e) Bravo-Grimaldo, E.; Navarro-Rodríguez, D.; Skoulios, A.; Guillon,
D. Liq. Cryst. 1996, 20, 393−398. (f) Gordon, C. M.; Holbrey, J. D.;
Kennedy, A. R.; Seddon, K. R. J. Mater. Chem. 1998, 8, 2627−2636.
(g) Kouwer, P. H. J.; Swager, T. M. J. Am. Chem. Soc. 2007, 129,
14042−14052.
́
(28) (a) Neve, F.; Imperor-Clerc, M. Liq. Cryst. 2004, 31, 907−912.
(b) Boydston, A. J.; Pecinovsky, C. S.; Chao, S. T.; Bielawski, C. W. J.
Am. Chem. Soc. 2007, 129, 14550−14551. (c) Alam, M. A.;
Motoyanagi, J.; Yamamoto, Y.; Fukushima, T.; Kim, J.; Kato, K.;
Takata, M.; Saeki, A.; Seki, S.; Tagawa, S.; Aida, T. J. Am. Chem. Soc.
2009, 131, 17722−17723.
(15) (a) Kato, T.; Mizoshita, N.; Kishimoto, K. Angew. Chem., Int. Ed.
2006, 45, 38−68. (b) Kato, T. Science 2002, 295, 2414−2418.
(c) Kato, T.; Yasuda, T.; Kamikawa, Y.; Yoshio, M. Chem. Commun.
2009, 729−739.
(29) Hatakeyama, E. S.; Wiesenauer, B. R.; Gabriel, C. J.; Noble, R.
D.; Gin, D. L. Chem. Mater. 2010, 22, 4525−4527.
(30) (a) Arce, A.; Earle, M. J.; Katdare, S. P.; Rodríguez, H.; Seddon,
K. R. Chem. Commun. 2006, 2548−2550. (b) Fukumoto, K.; Ohno, H.
Angew. Chem., Int. Ed. 2007, 46, 1852−1855.
(31) Vega, J. A.; Zhou, J.; Kohl, P. A. J. Electrochem. Soc. 2009, 156,
A253−A259.
(16) Kato, T.; Frec
8534.
́
het, J. M. J. J. Am. Chem. Soc. 1989, 111, 8533−
(17) Yoshio, M.; Ichikawa, T.; Shimura, H.; Kagata, T.; Hamasaki, A.;
Mukai, T.; Ohno, H.; Kato, T. Bull. Chem. Soc. Jpn. 2007, 80, 1836−
1841.
(32) Seki, S.; Hayamizu, K.; Tsuzuki, S.; Fujii, K.; Umebayashi, Y.;
Mitsugi, T.; Kobayashi, T.; Ohno, Y.; Kobayashi, Y.; Mita, Y.;
Miyashiro, H.; Ishiguro, S. Phys. Chem. Chem. Phys. 2009, 11, 3509−
3514.
(18) (a) Demus, D.; Goodby, J. W.; Gray, G. W.; Spiess, H.-W.; Vill,
V. Handbook of Liquid Crystals; Wiley-VCH: Weinheim, Germany,
1998. (b) Tschierske, C. J. Mater. Chem. 1998, 8, 1485−1508.
(c) Hudson, S. D.; Jung, H.-T.; Percec, V.; Cho, W.-D.; Johansson, G.;
2642
dx.doi.org/10.1021/ja209010m | J. Am. Chem.Soc. 2012, 134, 2634−2643

Journal of the American Chemical Society
Article
(33) (a) Kanazawa, A.; Ikeda, T.; Abe, J. J. Am. Chem. Soc. 2001, 123,
1748−1754. (b) Pindzola, B. A.; Jin, J.; Gin, D. L. J. Am. Chem. Soc.
2003, 125, 2940−2949.
(34) Abdallah, D. J.; Robertson, A.; Hsu, H.-F.; Weiss, R. G. J. Am.
Chem. Soc. 2000, 122, 3053−3062.
(35) Dukeson, D. R.; Ungar, G.; Balagurusamy, V. S. K.; Percec, V.;
Johansson, G.; Glodde, M. J. Am. Chem. Soc. 2003, 125, 15974−15980.
(36) Ungar, G.; Percec, V.; Holerca, M. N.; Johansson, G.; Heck, J. A.
Chem.Eur. J. 2000, 6, 1258−1266.
(37) (a) Pringle, J. M.; Golding, J.; Forsyth, C. M.; Deacon, G. B.;
Forsyth, M.; MacFarlane, D. R. J. Mater. Chem. 2002, 12, 3475−3480.
(b) Ohno, H. Bull. Chem. Soc. Jpn. 2006, 79, 1665−1680. (c) Tokuda,
H.; Tsuzuki, S.; Susan, M. A. B. H.; Hayamizu, K.; Watanabe, M. J.
Phys. Chem. B 2006, 110, 19593−19600.
(38) (a) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.;
Scrosati, B. Nat. Mater. 2009, 8, 621−629. (b) Wasserscheid, P.; Keim,
W. Angew. Chem., Int. Ed. 2000, 39, 3772−3789.
(39) Meyer, W. H. Adv. Mater. 1998, 10, 439−448.
2643
dx.doi.org/10.1021/ja209010m | J. Am. Chem.Soc. 2012, 134, 2634−2643