C O M M U N I C A T I O N S
that in mind, we inveastigated the ferroelectric response of 2 in its
LC state. However, the polarity did not switch in response to an
applied electric field, most likely due to an insufficient flexibility
of the LC mesophase. Nevertheless, fan-shaped 2 is the first LC
molecule whose columnar mesophase possesses a spontaneous
macroscopic polarization and, therefore, provides useful design
strategies toward the realization of columnar FLCs. Considering
the VT-IR profile (Figure 2c), the polar aromatic core of 2 is highly
constrained due to a short spacing from the tightly H-bonded amide
groups, so that the umbrella-shaped conical assembly of 2, essential
for the polar columnar assembly, seems to be quite stable against
thermal fluctuations. The absence of spontaneous polarization in
the mesophase of loosely H-bonded 3 supports the importance of
a strong H-bonded network around the assembled aromatic cores.
We also consider that the larger intercolumnar distance in the LC
mesophase of 2 than that of 3 may be advantageous for polarized
columns to align unidirectionally.
In summary, we developed a fan-shaped 2 having H-bonding
amide groups in proximity to its polar aromatic core as the first
LC molecule that self-assembles into a columnar geometry with a
spontaneous macroscopic polarization along the columnar axis.
Although this LC is not ferroelectric because its polarization does
not respond to the applied electric field, further elaboration of this
molecular design may lead to the realization of ferroelectric
columnar LC materials.
Acknowledgment. The synchrotron radiation experiments were
performed at BL02B2 and BL44B2 in SPring-8 under the Budding
Researchers Support Programs and the Priority Nanotechnology
Support Program administrated by JASRI (Proposal Nos. 2008B1777,
2009A1651, and 2009A1699) and with the approval of RIKEN
(Proposal No. 20090021).
Figure 2. XRD patterns of (a) 2 and (b) 3 at 90 and 80 °C, respectively.
IR spectral change profiles of the NsH and CdO stretching vibrational
bands of (c) 2 and (d) 3 on cooling from 120 to 40 °C, where the pink- and
yellow-colored regions represent temperature ranges for the isotropic melt
and columnar mesophase, respectively. (e) Schematic representation of the
columnar assembly of 2. (f) SHG signal intensity profiles of 2 on cooling
(blue) followed by heating (red) in a temperature range of 60-150 °C
(broken curves represent the corresponding DSC thermograms).
Supporting Information Available: Details of synthesis and
characterization, POM, XRD, DSC, and IR spectral profiles of 1-3.
This material is available free of charge via the Internet at http://
pubs.acs.org.
2d), where its CO and NH stretching vibrations did not display an
abrupt change upon phase transition.
Second harmonic generation (SHG), observable for molecular
assemblies with a noncentrosymmetric geometry, has been utilized
as a probe for the existence of polar structures.8 Compound 3 in
its LC state as well as in other temperature ranges showed no
detectable SHG signal. In sharp contrast, the columnar mesophase
of 2 clearly displayed an SHG signal with a hysteresis nature in
accord with that of the phase transition profile (Figure 2f).
Noteworthy, the SHG signal was more pronounced as the material
was allowed to cool more slowly from its isotropic melt. Further-
more, use of a substrate covered with an aligned polyimide film
did not cause any change in the signal intensity. Considering the
hexagonal symmetry, all the above observations allow us to
conclude that the columnar LC mesophase of 2 possesses a
spontaneous macroscopic polarization along the columnar axis
(c axis). The observed polarization originates from the large dipole
in the aromatic core of 2, since reference compound 1 without cyano
groups (Figure 1a), though adopting an LC geometry identical to
that of 2,6 was SHG-silent over a wide temperature range.
Spontaneous polarization is a prerequisite for ferroelectric liquid
crystals. Although a few LC materials with a columnar geometry
have been claimed to be ferroelectric based on their switchable
current profiles,4b reinvestigations using SHG as a probe have
revealed that they do not possess a spontaneous polarization.8a With
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