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Published on the web July 2, 2011
Effect of Lateral Fluorine Substituent on Mesophase Behavior of Bent-shaped Molecules
with Asymmetric Central Naphthalene Core
Xiaodong Li, Maosheng Zhan,* Kai Wang, and Huazhen Zhou
School of Material Science and Engineering, Beijing University of Aeronautics and Astronautics,
Beijing 100191, P. R. China
(Received May 19, 2011; CL-110424; E-mail: zhanms@buaa.edu.cn)
Novel lateral fluorinated bent-shaped molecules based on 1,7-
and 1,6-naphthalene central core are prepared. Their mesomorphic
properties are discussed in comparison with those of conventional
unsubstituted bent-shape molecules.
O
O
O
O
N
N
X
X=F N(1,6)-F12
X=H N(1,6)-O12
X
X=F N(1,7)-F12
X=H N(1,7)-O12
Bent-shaped liquid crystals have attracted considerable atten-
tion over the past few years since its first discovery by Watanabe
et al. in 1996.1 Due to the occurrence of novel polar-ordered
structures, supramolecular chirality induced by symmetry breaking
in achiral molecules, and the noticeable optical, ferroelectric, or
antiferroelectric reponse,2 extensive research has been carried out in
this intriguing field.3,4 A large number of bent-shaped molecules
have been synthesized and characterized, and at least eight types of
banana phases (B1-B8) have been identified.5 One of the major
research targets in bent-shaped liquid crystals is to clarify the
structure-property relationships. Generally, a bent-shaped molecule
consists of three parts: the angular central core, side arms, and
terminal chains. Outstanding work where modifications on different
parts of the molecule has been analyzed and reported.6 It is of
particular interest to note that the introduction of a fluorine atom to
the external phenyl ring has a great effect on the mesomorphic
behavior in molecules based on symmetric central core such as 1,3-
benzene or 2,7-naphthalene unit.7,8 Most of the chiral mesophases
exhibited by bent-shaped compounds have an antiferroelectric
interlayer structure. But the introduction of a fluorine substituent
ortho to the terminal n-alkoxy chains often results in a ferroelectric
switching phase. The mesophase with synclinic tilt of molecules
in adjacent layers exhibiting ferroelectric polarization, namely,
SmCSPF phase, is formed instead of the original SmCSPA phase in
unsubstituted molecules. However, the effect of the lateral fluorine
substituent on mesomorphic behaviors of bent-shaped molecules
with asymmetric central core has been rarely reported.
C12H25
O
OC12H25
Scheme 1.
Table 1. Transition temperatures and enthalpies (on cooling at a rate of
¹1
10 °C min
)
¹1
Transition temperature/°C (enthalpy/kJ mol
)
N(1,7)-F12
N(1,7)-O12
N(1,6)-F12
N(1,6)-O12
B4 87.6 (11.8) Colh 147.5 (2.8) Iso
B4 145.7 (27.4) Iso
Cr 66.5 (6.3) B2 (SmCSPA) 178.2 (23.1) Iso
Cr 95.8 (2.8) B2 (SmCSPA) 193.5 (25.4) Iso
(a)
(c)
(b)
Thus, we have synthesized bent-shape molecules with asym-
metric central core based on 1,7- or 1,6-naphthalene units and side
arms containing Schiff base moieties, designated as N(1,7)-F12 and
N(1,6)-F12 (Scheme 1). Polarized optical microscope observation
and electrooptical and X-ray diffraction measurements have been
carried out to investigate their mesomorphic properties and phase
structures, in comparison with those of unsubstituted molecules.
All compounds show enantiotropic transitions. The transition
temperatures and associated enthalpy changes were collected from
DSC and are listed in Table 1.
It is of interest that for molecules based on 1,7-naphthalene
central core, the introduction of the lateral fluorine substituent induces
a polar columnar hexagonal mesophase with a phase sequence of
Iso-Colh-B4. On cooling the isotropic liquid of N(1,7)-F12, Colh
phase appears as a fan-shaped texture (Figure 1a), characteristic of
hexagonal molecular arrangement.9 In some areas, the textures exhibit
an optically isotropic zone, indicating columns align vertically to the
substrate. On further cooling to B4 phase, two optically active
domains with opposite chirality were observed (Figure 1b).
Figure 1. Microphotographs observed in N(1,7)-F12: (a) fan texture of
Colh phase (at 135 °C), (b) chiral B4 phase under uncrossed polarizers
(at 65 °C), and (c) X-ray diffractions.
The columnar nature in Colh phase is identified by a character-
istic X-ray pattern. The broad diffuse scattering, corresponding to
the liquid-like disorder of molecules with a mean distance of 4.5 ¡,
is detected in the wide angle region. In the small-angle region, three
sharp reflections are observed (Figure 1c). They have spacings in
the ratio of 1, (1/3)1/2, and 2 and are further indexed to (100),
(110), and (200) in the two-dimensional hexagonal lattice with
p6mm symmetry.10 The hexagonal lattice edge, a, is calculated to be
66.9 ¡. B4 phase shows only one inner layer reflection, the spacing
of which is 35.2 ¡. It approximates the molecular length calculated
on the most extended conformation.
By applying an electric field to the Colh phase, the fan-shaped
texture (Figure 2a) immediately disappears and rebuilds to give a
zero birefringence (Figure 2b). This indicates the columns reorient
their axis vertically to ITO substrates.9 A single current peak in
Chem. Lett. 2011, 40, 820-821
© 2011 The Chemical Society of Japan
Li, Xiaodong
