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Chemistry Letters Vol.37, No.12 (2008)
Anti-ferroelectric Banana Phase in a Bent-shaped Molecule with a Low Bend Angle of 60ꢀ
Seng Kue Lee,ꢀ Lu Shi, Ryohei Ishige, Sungmin Kang, Masatoshi Tokita, and Junji Watanabe
Department of Organic and Polymeric Materials, Tokyo Institute of Technology,
2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552
(Received September 12, 2008; CL-080878; E-mail: sklee@polymer.titech.ac.jp)
This paper presents the first example of an antiferroelectric
banana phase in a bent molecule with a low bend angle of about
60ꢁ.
Since the discovery of the spontaneous formation of polar-
ordered superstructures and chiral symmetry breaking in bent-
shaped (or banana) molecules, the unconventional mesomorphic
properties of these molecules have opened up a highly innova-
tive area in liquid crystal research, providing new science into
the soft materials.1–3 A number of bent-shaped molecules have
been prepared and characterized, and at least eight different ba-
nana (B) phases have been identified.4,5
Figure 1. Photomicrographs for the (a) homeotropic smectic A,
(b) non-tilted polar smectic A (SmAPA), and (c) B4 phases.
White arrows indicate the directions of polarity.
banana phase similar to the typical B2 phase was formed here
in addition to the solid B4 phase.
One of the major research targets in the banana liquid crystal
field is to clarify how the molecular structure relates to meso-
phase properties such as polarity, chirality, and frustration.
Though a variety of bent-shaped molecules have been synthe-
sized, most of the central cores used are resorcinol derivatives
with bent angles of about 120ꢁ. This bent angle is significant,
as banana phases have been absent in bent-shaped molecules
with lower and higher angles. Matsuzaki and Matsunaga have re-
ported,6 for example, that bent-shaped molecules based on 1,2-
phenylene and 2,3-naphthylene groups, molecules with bent an-
gles of about 60ꢁ, form only conventional nematic and smectic
phases. This occurs simply because the molecules assume U-like
shapes and behave like calamitic molecules rather than bent
ones.
Watanabe et al.7 recently prepared six bent-shaped mole-
cules with classical side wings containing Schiff-base moieties
at different positions of the central naphthalene core, in order
to study and discuss the effects of the substituent position. Intri-
guingly, the derivative with the 1,7-naphthalene core forms a
typical chiral B4 phase irrespective of its low bend angle of
around 60ꢁ; however, no switchable banana phases have been
found. In this study we prepared a homologous bent-shaped
molecule with the same central naphthalene core and long side
wings in which one phenyl ring is attached to the classic one
via an ester linkage, as shown in Scheme 1.17 As a result of the
expanded mesophase temperature region, an antiferroelectric
The transition behavior is as follows: isotropic (251.5 ꢁC/
ꢀH ¼ 0:31 kJ molꢂ1)–nematic (N) (244.7 ꢁC/ꢀH ¼ 0:59 kJ
molꢂ1)–smectic A (SmA) (240.1 ꢁC/ꢀH ¼ 12:7 kJ molꢂ1)–non-
tilted polar smectic (SmAPA) (205.6 ꢁC/ꢀH ¼ 23:6 kJ molꢂ1)–
B4 upon decreasing temperature. The N phase was identified
by the observation of a typical schlieren texture. On cooling
from the nematic, the SmA phase appears with a black homeo-
tropic texture (Figure 1a) and quickly transforms to the SmAPA,
which in turn shows a highly birefringent schlieren-like homeo-
tropic texture (Figure 1b). The texture darkens with further cool-
ing to the B4 phase, but with chiral segregation domains in
which the brightness is interchanged by the positive and negative
rotations of the polarizer from the cross polarization position
(Figure 1c).
Though electric switching behavior is observed for the
SmAPA, no electrooptical measurements are possible for the
higher-temperature SmA phase because of its very narrow tem-
perature span. The application of a rectangular-wave field results
in the development of fan-shaped domains, even when the sam-
ple in the cell is prepared with homeotropic alignment (Figure 2).
This means that the polar direction is parallel to the layer. A bi-
refringent color changes from red in the off-state to blue the on-
state, whereas the extinction directions parallel and perpendicu-
lar to the layer remain unchanged in both the on- and off-states.
Thus, the molecules lie perpendicular to the layer and the polar-
O
O
O
O
N
N
O
O
O
O
Figure 2. Photomicrographs illustrating the switching behavior
in the SmAPA phase under an electric field. A sample of 5.0 mm
in thickness was sandwiched between glass substrates with ITO
electrodes.
l
OC12H25
C12H25
O
Scheme 1.
Copyright ꢀ 2008 The Chemical Society of Japan