Columnar liquid crystal with a spontaneous polarization along the columnar axis

Article abstract of DOI:10.1021/ja101866e

A fan-shaped molecule (2), carrying hydrogen-bonding amide groups in proximity to its polar aromatic core, self-assembles into a columnar liquid crystalline mesophase, which unprecedentedly possesses a spontaneous macroscopic polarization along the columnar axis. Due to its polar nature, the mesophase displays a signal of second harmonic generation (SHG), which disappears on phase transition to an isotropic melt and then retrieves its original intensity on subsequent cooling.

Full text of DOI:10.1021/ja101866e

Published on Web 06/08/2010
Columnar Liquid Crystal with a Spontaneous Polarization along the
Columnar Axis
Daigo Miyajima,† Fumito Araoka,‡ Hideo Takezoe,*,‡ Jungeun Kim,§ Kenichi Kato,| Masaki Takata,§,|
and Takuzo Aida*,†
School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, Department of
Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan,
Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan, and
RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
Received March 5, 2010; E-mail: takezoe.h.aa@m.titech.ac.jp; aida@macro.t.u-tokyo.ac.jp
Ferroelectric materials are those having a spontaneous polariza-
tion that can be reversed by an applied electric field. Ferroelectric
liquid crystals (FLCs) have attracted much attention in material
sciences because of their potential utility for piezoelectric, pyro-
electric, and second-order nonlinear optical (NLO) devices as well
as displays.1 However, it is essentially difficult to realize polar
orders in fluidic media, because molecules prefer to align sym-
metrically so as to cancel their dipole moments. While a variety of
FLCs have so far been reported since the pioneering work of Meyer
et al.,2 they are smectic and composed of chiral rod-shaped or bent-
core molecules3 capable of breaking the symmetry of the systems.
Because of the presence of an in-layer macroscopic polarization in
such FLCs, each layer in principle may serve as a 1-bit memory if
the in-layer polarization can be switched individually. However,
this idea might not be realistic. A more feasible approach would
be to make use of the geometry of columnar liquid crystals. If each
column has a spontaneous polarization along its column axis, and
the columns are oriented in such a way that their polarizations align
unidirectionally, the resulting LC materials are promising for the
development of high-density ferroelectric memory devices, given
that the polarization switching is possible in each column.4 Despite
such a prospect for columnar FLCs, the absence of a rational design
strategy has made progress extremely difficult. As the first step
toward this goal, we report here fan-shaped 2 (Figure 1a) is the
first LC molecule that forms a columnar mesophase (Figure 1b)
with a spontaneous polarization along the columnar axis.
distances of the LC mesophases of 2 and 3 were evaluated as 4.2
and 3.4 nm, respectively, suggesting that four and three molecules
of 2 and 3 assemble concentrically into expected umbrellas.
Note that the structural difference between 2 and 3 lies only in
that the former has a one-carbon shorter spacer than the latter
between the Ar-S and amide groups. In regard to a question why
2 and 3 assemble into such different mesophase structures, we found
that their H-bonding modes are totally different from one another.
In variable-temperature infrared (VT-IR) spectroscopy, 2 at 120
°C showed characteristic stretching vibrational bands due to
H-bonded amide CO and NH groups at 1644 and 3330 cm-1
,
respectively (Figure 2c). Of interest, these bands shifted discontinu-
ously to lower wavenumbers at the isotropic melt-to-LC phase
transition,6 indicating that the molecules of 2 are tightly connected
together by a strong H-bonding interaction in each column. In sharp
contrast, 3 displayed a much less explicit H-bonding nature (Figure
Percec et al. have reported that certain fan-shaped molecules self-
assemble conically into an umbrella, which stacks up to form a
columnar LC structure (Figure 1b).5 We expect that introduction
of a large dipole into the focal core of such fan-shaped molecules
might result in spontaneous macroscopic polarization in the resulting
LC mesophase. For this purpose, 4,5-dithiaphthalonitrile derivatives
2 and 3 were synthesized and unambiguously characterized by NMR
and MALDI-TOF-MS analyses.6 These compounds commonly
possess two H-bonding amide groups with a view to ensure their
umbrella-shaped assemblies, essential for polarized columns. By
means of differential scanning calorimetry (DSC) and polarizing
optical microscopy (POM), we confirmed that 2 and 3 both form
an LC mesophase (Figure 1c, d).6 Although compound 3 in its LC
state showed an X-ray diffraction (XRD) pattern typical of a
hexagonal columnar (Colh) geometry (Figure 2b), that of 2
suggested the existence of a 3D lattice, in which 10.7 nm long
columns form a 2D hexagonal lattice (Figure 2a, e).6,7 Intercolumnar
† The University of Tokyo.
Figure 1. (a) Structures of compounds 1-3 (phase transition temperatures
in °C) and (b) illustration of their columnar assembly. POM images of (c)
2 and (d) 3 at 90 and 80 °C, respectively.
‡ Tokyo Institute of Technology.
§ Japan Synchrotron Radiation Research Institute.
| RIKEN SPring-8 Center.
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8530 J. AM. CHEM. SOC. 2010, 132, 8530–8531
10.1021/ja101866e  2010 American Chemical Society

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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