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Chemistry Letters Vol.37, No.5 (2008)
Ionic Liquid Crystals: Self-assembly of Imidazolium Salts Containing an L-Glutamic Acid Moiety
Sanami Yazaki,1 Yuko Kamikawa,1 Masafumi Yoshio,1 Atsushi Hamasaki,2 Tomohiro Mukai,2 Hiroyuki Ohno,2 and Takashi Katoꢀ1
1Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656
2Department of Biotechnology, Tokyo University of Agriculture and Technology, Nakacho, Koganei, Tokyo 184-8588
(Received February 12, 2008; CL-080149; E-mail: kato@chiral.t.u-tokyo.ac.jp)
Liquid-crystalline (LC) behavior and ionic conductivities of
imidazolium salts containing an L-glutamic acid moiety have
been studied. The ionic conductivities of the (CF3SO2)2Nꢁ salt
forming the columnar LC phase are higher than those of the Brꢁ
salt in the columnar LC state. The ionic conductivity shows a
sudden decrease at the columnar–micellar cubic LC transition
of the Brꢁ salt.
Self-assembly of liquid-crystalline (LC) molecules with
functional moieties is one of the versatile approaches to the
Figure 2. Schematic illustration of an ideal on–off switch of
ion conduction using the structural change from columnar to
micellar cubic LC structures.
preparation of functional soft materials with dynamic and aniso-
tropic nature.1–4 Ionic liquid crystals5–8 have attracted attention
because they have potentials for applications as ion conductors,6
redox-active materials,7 and ordered solvents.8 They form a
Table 1. Thermal properties of 1 and 2
variety of LC nanostructures1,2,9 such as micelles, cylinders, lay-
Compound
Phase-transition behaviora
ers, and bicontinuous cubic structures through ionic interactions
and nanosegregation between ionic and non-ionic moieties.
Recently, we have reported on one-dimensional ion conduc-
tion in ionic columnar liquid crystals,6a–6d 1-methyl-3-[3,4,ꢁ5-
1
2
Cr
Cr
51 (48)
64 (60)
Colh
Colh
136 (1.0)
99 (1.2)
Cubb
Iso
aTransition temperatures (ꢂC) and enthalpies (kJ molꢁ1, in
parentheses) were determined by DSC on the second heating
at 2 ꢂC minꢁ1. The transition temperatures were taken at the
tris(alkyloxy)benzyl]imidazolium salts containing BF4ꢁ, PF6
,
CF3SO3ꢁ, and (CF3SO2)2Nꢁ (TFSIꢁ). The LC properties and
ionic conductivities were strongly dependent on the properties
of counter anions. To obtain highly ion-conductive materials,
the use of TFSIꢁ is considered to be desirable because of its
negative charge delocalization and weak electrostatic interac-
tions with cations.10 However, the preparation of LC imidazoli-
um TFSIꢁ salts that form columnar phases in wide temperature
ranges has not yet been achieved.6a–6c For the preparation of
self-assembled materials that exhibit stable mesophases, it
should be important to control intermolecular interactions
and the balance of volume fraction of ionic and non-ionic parts
of the molecules.2,9,11
b
onset points of transition peaks. The isotropization temper-
ature could not be determined because thermal degradation
occurred above 180 ꢂC before reaching the isotropic liquid
state. Cr: crystalline, Colh: hexagonal columnar, Cub: cubic,
Iso: isotropic liquid.
molecules6a–6c into imidazolium salts may induce a structural
change from columnar to micellar cubic phases, leading to the
on–off switch3d of ion conduction (Figure 2). Herein, we report
on self-assembly of imidazolium-based ionic liquid crystals 1
and 2 exhibiting stable columnar and cubic LC phases and
their ionic conductivities.
Our molecular design here is to attach a fan-shaped L-gluta-
mic acid derivative bearing bis(alkyloxy)phenyl moieties12 to
imidazolium salts (Figure 1). The glutamic acid derivative
can be used as a molecular building block to produce stable
columnar and micellar cubic LC materials upon hydrogen
bonding.12 In addition, the incorporation of a more bulky
lipophilic part than those of previously reported ionic
The LC properties of 1 and 2 are summarized in Table 1.
Compound 1 exhibits columnar and cubic phases, while com-
pound 2 shows only a columnar phase in the temperature range
that is wider than those of previously reported imidazolium-
based LC salts with TFSIꢁ.6b The X-ray diffraction (XRD)
patterns confirm that the LC phases of 1 and 2 at 80 ꢂC are
hexagonal columnar phases. The intercolumnar distances for
˚
both of 1 and 2 are 50 A. The XRD pattern of 1 shows three
˚
peaks at 45.5, 26.0, and 22.4 A with a reciprocal d-spacing ratio
pffiffi
OC11H23
of 1: 3:2. These peaks correspond to the (100), (110), and (200)
reflections, respectively. The cubic LC phase of 1 has been
determined as a micellar cubic phase with Pm3n symmetry
from the XRD pattern taken at 150 ꢂC.13
O
O
O
OC11H23
+
N
N
O
N
H
X
O
OC11H23
OC11H23
The macroscopic orientation of the LC columns parallel to
the surface of a glass substrate having comb-shaped gold elec-
trodes was achieved by mechanical shearing of the polydomain
samples of 1 and 2.6a–6c Anisotropic ionic conductivities of the
aligned columnar liquid crystals on heating were measured by
1:X = Br
2:X = (CF3SO2)2N
Figure 1. Imidazolium salts having an L-glutamic acid moiety.
Copyright Ó 2008 The Chemical Society of Japan
Yazaki, Sanami
