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Chemistry Letters Vol.36, No.9 (2007)
Novel Photoinduced Phase Transition
Observed in Three-dimensional Liquid-crystalline Phase of Azobenzene Compound
Takahiro Yamamoto,ꢀ1;2 Isa Nishiyama,3 and Hiroshi Yokoyama1;2
1Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology,
Central 5, 1-1-1 Higashi, Tsukuba 305-8565
2JST ERATO/SORST Liquid-Crystal Nano-System Project, 5-9-9 Tokodai, Tsukuba 300-2635
3Liquid Crystal Materials Technical Department, Dainippon Ink and Chemicals Inc.,
4472-1 Komuro, Ina-machi, Saitama 362-8577
(Received June 12, 2007; CL-070635; E-mail: takahiro.yamamoto@aist.go.jp)
A novel dichiral azobenzene compound which shows two
temperature mesophases are described as ML and MH phases,
respectively. Both mesophases exhibit a ‘‘mosaic’’ texture, as
shown in Figure 2. At the transition from the ML phase to the
MH phase transition, the birefringence drastically decreases.
Figure 3 shows a DSC thermogram of (S,S)-AZO-PP in a
heating process (scan rate = 5 ꢁC/min). The emergence of clear
DSC peaks on the phase transitions indicates that the ML and MH
phase are thermodynamically different phases in nature. It
should be noted that both of the ML-to-MH and MH-to-Iso phase
transitions show only tiny peaks with enthalpies of 0.23 and
0.82 kJ/mol, respectively. Generally, the mosaic textures are
obtained for the smectic B, E, F, and so on, in which molecules
are highly ordered.8 Therefore, the enthalpy changes on the
phase transition are expected to be large. However, we observed
kinds of highly ordered liquid-crystalline phases was prepared.
One of the mesophases was found, by miscibility studies, to be
a chirality-induced three-dimensional (3D) liquid crystal phase.
We, for the first time demonstrate the photoinduced phase
transition of the liquid-crystalline 3D structure.
Liquid-crystal phases are so soft that varieties of structures
can be generated by the induction of chirality; helical molecular
assembly is one of the most typical effects. Macroscopic helical
structures along a certain one direction are found in chiral nem-
atic and twist grain boundary phases, whereas, for more strongly
chiral liquid crystal systems, the helical nature sometimes gener-
ates an over-all three-dimensional (3D) structures.1 Blue phases
are the 3D structures seen in nematic liquid crystals, of which
temperature range can be dramatically enhanced by polymer-
stabilization of the defects,2 which enable us to obtain room-
temperature blue phase at ease that is important from the practi-
cal point of view. For smectics, various 3D structures are
induced by chirality, i.e., ‘‘smectic’’ blue,3 cubic,4 and rather
mysterious smectic Q (SmQ)5,6 phases, of which stability is quite
sensitive to the optical purity. The molecular orientations are not
so sensitive, however, to the external electric field due to an
intricate smectic ordering, thus it has been difficult to imagine
practical applications of these structures.
Figure 1. Chemical structure of a novel dichiral azobenzene
compound.
We design a novel dichiral azobenzene compound possess-
ing two chiral moieties at both peripheral ends of the molecular
structure, of which structure is shown in Figure 1 ((S,S)-AZO-
PP). This chiral architecture is known to be efficient to generate
chirality-induced liquid-crystalline phases.1 We report here that
the azobenzene compound ((S,S)-AZO-PP) shows chirality-
induced liquid-crystalline 3D structures and that the resulting
3D structure can be manipulated by ‘‘light’’ as an external stim-
ulus instead of ‘‘electric field.’’ Although photochromic reac-
tions of azobenzene derivatives have so far been employed for
optical control of physical properties and structures of common
two-dimensional liquid-crystalline phases,7 the first example of
the photoinduced phase transition of liquid-crystalline 3D phase
is reported in this paper.
Figure 2. ‘‘Mosaic texture’’ observed for (S,S)-AZO-PP, (a)
in the ML phase at 145 ꢁC, and (b) in the MH phase at 152 ꢁC.
(S,S)-AZO-PP was prepared from p-bromoaniline by four
steps (see Supporting Information for the detailed preparation
procedures and characterization of materials).6 Liquid-crystal-
line properties were investigated using a polarizing optical mi-
croscope and a differential scanning calorimetry (DSC). (S,S)-
AZO-PP shows two kinds of mesophases as follows: Cryst.
125 ꢁC ML 153 ꢁC MH 165 ꢁC Iso., where the lower and higher
Figure 3. A DSC thermogram obtained for (S,S)-AZO-PP on
heating (scan rate = 5 ꢁC/min). An inset indicates an expanded
view around ML-to-MH phase transition.
Copyright Ó 2007 The Chemical Society of Japan