Yoichi Takanishi

Bent-Core Liquid Crystals: Their Mysterious and Attractive World

Structures and properties of liquid crystalline phases formed by bent-core molecules are reviewed. At least eight phases designated as B1–B8 have been found, being unambiguously distinguished from phases formed by usual calamitic molecules due to a number of remarkable peculiarities. In addition to B1–B8 phases, smectic A-like phases and biaxial nematic phases formed by bent-core molecules are also reviewed. The most attractive aspects of this new class of liquid crystals are in polarity and chirality, despite being formed from achiral molecules. The bent-core mesogens are the first ferroelectric and antiferroelectric liquid crystals realized without introducing chirality. Spontaneous chiral deracemization at microscopic and macroscopic levels occurs and is controllable. Moreover, achiral bent-core molecules enhance system chirality. The interplay between polarity and chirality provides chiral nonlinear optic effects. Further interesting phenomena related to polarity and chirality are also reviewed.

Origin of Helix in Achiral Banana-Shaped Molecular Systems.

Helical structures were confirmed for both the SmAb and SmBlue phases of banana-shaped molecular systems from observations of the microscopic fringe pattern and the selective reflection of blue color, respectively. X-ray and optical microscopy indicate that the helical axes in the SmAb and SmBlue phases are normal and parallel to the smectic layer, respectively. In these two helical phases, 13C NMR spectra show two C=O peaks, suggesting two different configurations of ester group, whereas only one C=O peak appears in the isotropic and crystal phases. This indicates that the two C=O groups in the mesogenic core are not in the same plane but are twisted. The addition of chiral dopant makes the dichroic ratio of the right- and left-circularly polarized scattered light positive or negative in the SmBlue phase, although the wavelength (?430 nm) of the scattering peak does not change significantly. The origin of the helix will be discussed in view of the twisted molecular conformation (conformational chirality) and the escape from macroscopic polarization.

Antiferroelectric chiral smectic liquid crystals

In a brief history of the discovery of antiferroelectricity in liquid crystals, the important role played by tristable switching, i.e. an electric field induced phase transition from antiferroelectric SCA* to ferroelectric SC*, has been emphasised and the antiferroelectric herringbone structure of SCA* has been presented. Then we have explained how to identify the subphases in the SC* region, e.g. SCγ*, SCα*; the clarification of the subphase structures is essential for understanding antiferroelectricity in liquid crystals. After summarizing the evidence for the SCA* structure presented, we have suggested the pair formation of transverse dipole moments in adjacent smectic layers as the cause of its antiferroelectricity, showing that the smectic layer is much closer to the usual picture of molecules lying on equidistant planes; the packing entropy due to the sinusoidal density wave character stabilizes ferroelectric SC*. The competition between the interactions stabilizing SCA* and SC* is responsible for the occurrence of several varieties of ferrielectric and antiferroelectric subphases, which constitutes the Devils staircase. We have further suggested that the essentials of the SCα* phase are its considerably reduced ability to form SCA* and SC*. At least when the spontaneous polarization is zero, SCα* is a smectic C-like phase with molecular tilting that is non-correlated on the visible wavelength scale When the spontaneous polarization is not zero, as suggested by Prost and Bruinsma recently, a novel type of Coulomb interaction between smectic layers due to the collective polarization fluctuations causes the antiferroelectricity in the high-temperature region of SCα*; the competition between this antiferroelectricity and the SC* ferroelectricity may form another staircase, causing the complexity in SCα*. Applications and some future problems have been described in the final section.

Thresholdless antiferroelectricity in liquid crystals and its application to displays

By diminishing the energy barrier between SCA* and SC*, antiferroelectricity has become thresholdless in a three-component mixture. It shows V-shaped switching, realizing attractive display characteristics: extremely wide viewing angle with very large contrast ratio, high speed response and ideal analogue grey scale with no hysteresis. A simplified model of the phase with this property is presented.

Stability of Antiferroelectricity and Causes for its Appearance in SmCα* and SmCA* Phases of a Chiral Smectic Liquid Crystal, MHPOBC

The smectic layer thickness and the switching behavior in MHPOBC have been studied as a function of temperature; SmCα* is a tilted phase and is antiferroelectric at least in the higher-temperature region but becomes ferrielectric with decreasing temperature. The gradual appearance of ferrielectric characteristics in SmCα* suggests the emergence of the Devils staircase. To explain the antiferroelectricity in SmCα* and SmCA*, two types of molecular interactions are proposed: one is the interaction between the spontaneous polarizations in the neighboring layers which is effective in SmCα*, and the other is the pairing of transverse dipole moments responsible for the antiferroelectricity in SmCA*.

A racemic layer structure in a chiral bent-core ferroelectric liquid crystal

A fluid smectic phase of a chiral bent-core liquid crystal was found to have a ground state structure that is anticlinic in tilt and ferroelectric in polar order, SmCAPF*. The layer chirality of this structure alternates from layer to layer despite their being composed of chiral mesogens. Observations of the optical second harmonic generation signal from well-aligned domains confirm that the ground state of this phase is bistable ferroelectric. In addition to the ground state two types of metastable domains are also observed.

A binaphthyl derivative with a wide temperature range of a blue phase

We prepared a binaphthyl derivative possessing two 2,3-difluoro-1,4-diphenylbenzene units, and then investigated the phase transition behaviour. On cooling, the compound was found to exhibit a phase sequence of iso liq–blue phase–smectic phase. The temperature of the blue phase with a cubic structure was about 30 K. An electric-field-induced phase transition was observed in the blue phase. On heating, the compound exhibited marked phase separation. Some regions showed a phase sequence of cryst–chiral nematic (N*)–blue phase (BP)–iso liq, whereas others showed that of cryst–iso liq. The temperature range of coexisting crystal and iso liq phases was about 29 K. The phase separation is quite unusual for a single compound. The phase-transition behaviour is discussed in terms of the molecular shape of the binaphthyl derivative.

Ferroelectric Properties in Banana-Shaped Achiral Liquid Crystalline Molecular Systems

We investigated the ferroelectric behavior in novel ferroelectric liquid crystals consisting of banana-shaped achiral molecules. Under the application of a triangular-wave electric field, we observed a single peak corresponding to the switching current in the higher-order smectic ( SmX2) phase as well as the smectic Ab ( SmAb) phase, where ferroelectricity had been already reported. This result proves that the spontaneous polarization is switchable with a single hysteresis loop in SmX2 as well as in SmAb. We also discussed the dielectric behavior in both the phases.

Spontaneous Formation of Quasi-Bookshelf Layer Structure in New Ferroelectric Liquid Crystals Derived from a Naphthalene Ring

A quasi-bookshelf layer structure has been obtained by a conventional rubbing method at room temperature in a FLC mixture composed of naphthalene-derived liquid crystals. This layer structure is spontaneously formed due to a unique temperature dependence of the interlayer spacing; X-ray diffraction indicates that the layer spacing decreases from 34 to 33.5 A and then increases to 33.8 A with decreasing temperature in SmC*. Contrary to the unusual behavior of the layer spacing, the director tilt angle determined optically shows ordinary temperature dependence and attains a relatively large value, 17°, at room temperature.

Ideal Liquid Crystal Display Mode Using Achiral Banana-Shaped Liquid Crystals

We have demonstrated a totally new liquid crystal display (LCD) mode using a smectic A-like phase of banana-shaped molecules. An in-plane electric field was applied to homeotropically aligned cells, resulting in fast polarization reorientation and the associated birefringence. The reported LCD mode has all the advantages of the existing LCD modes, such as vertical alignment (VA), in-plane switching (IPS), ferroelectric LC (FLC) or antiferroelectric LC (AFLC) and V-shaped switching (VS) modes; namely, fast response of the order of 100 µs, high contrast ratio (3000:1), wide viewing angle, continuous gray level, and small threshold voltage. These performances originate from the cooperative motion of bent molecules with quasi-long-range order of dipoles based on a two-dimensional Langevin process.