Freedericksz transition in an anticlinic liquid crystal

Pseudopolar smectic-C phases of azo-substituted achiral bent-core hockey-stick-shaped molecules

We report experimental studies on an azo-substituted compound consisting of bent-core hockey-stick-shaped molecules. The experimental results establish two pseudopolar tilted smectic phases, which are characterized by an in-plane axial-vector order parameter in addition to tilt order in the smectic layers. Electro-optical measurements in the mesophases indicate that the birefringence of the sample strongly depends on the applied electric field. We develop a theoretical model to account for this observation. The change in the birefringence of the sample arises from the field-induced reorientation of the tilt plane of the molecules in the layer above a threshold field. The effect is analogous to the field-induced Freedericksz transition which is quadratic in the applied electric field.

Demonstration of the antiferroelectric aspect of the helical superstructures in Sm-C*, Sm-C*alpha, and Sm-C*a liquid crystals

We show that the helical superstructure in chiral smectic-C-type liquid crystal phases can, if the pitch is short enough, give rise to the tristate switching characteristic of antiferroelectrics even if the local commensurate order is synpolar as in the ordinary Sm-C* phase. Since the field-induced helix unwinding exhibits a distinct threshold, in contrast to mere helix distortion, two unwinding / rewinding peaks per half cycle of an applied triangular wave voltage can be seen in the current response. By considering this antiferroelectric aspect of the helical modulation we give a simple explanation of why the ultrashort-pitch Sm-C*alpha phase exhibits antiferroelectric switching although its dielectric spectroscopy response is qualitatively identical to that of the synpolar Sm-C* phase. Using data from the compound MHPOCBC we show that the Sm-C*alpha dielectric response is well described by continuum theory. We also demonstrate that, if the pitch is very short as in MHPOCBC, helix unwinding/rewinding leave characteristic traces in the electrooptic response even in the commensurately antiferroelectric (antipolar) Sm-C*(a) phase, distinguishable from the switching between the antipolar and synpolar states of this phase.

Bent-core liquid crystals forming two- and three-dimensional modulated structures

Two columnar phases of bent-core molecules have been observed, with a two-dimensional (2D) structure modulated in the plane perpendicular to the direction of the spontaneous polarization vector. The phases are switchable under an applied electric field, contrary to the commonly observed 2D modulated B1 phase. These new phases are built from broken smectic layers with orthogonal or tilted molecules. The evidence for a 3D structure in which the density modulations along and perpendicular to the spontaneous polarization vector co-exists is also given.

Dynamics of electro-optical switching in the antiferroelectric B2 phase of an achiral bent-core shape compound

A detailed study of the dynamics of electro-optical response has been carried out over the whole temperature range of the antiferroelectric B2 phase of a compound with bent-core shape molecules, a homolog (n=14) of the series 4-chloro-1,3-phenylene bis[4-(4-n-alkylphenylimino)benzoates]. Two types of stripe domains were observed with opposite handedness and simultaneous clock and anticlock motion of the director in the neighboring domains. The temperature dependence of the interlayer potential has been found from the threshold of the transition from the ground antiferroelectric (AF) state to the field-induced ferroelectric (F) state. The rotational viscosity gamma(phi) has been calculated from the dynamics of the field-induced azimuthal director switching between F-F and AF-F states and free relaxation of the director from F to AF state. The electro-optical response was also observed below the AF-F threshold. The latter was attributed to the soft-mode distortion of the molecular tilt angle in the vicinity of the transition from the B2 phase to the isotropic phase.

Tilt plane orientation in antiferroelectric liquid crystal cells and the origin of the pretransitional effect

The optic, electro-optic, and dielectric properties of antiferroelectric liquid crystals (AFLCs) are analyzed and discussed in terms of the local tilt plane orientation. We show that the so-called pretransitional effect is a combination of two different electro-optic modes: the field-induced antiphase distortion of the antiferroelectric structure and the field-induced reorientation of the tilt plane. In the presence of a helix, the latter corresponds to a field-induced distortion of the helix. Both electro-optic modes are active only when the electric field has a component along the tilt plane. Thus, by assuring a horizontal surface-stabilized condition, where the helix is unwound by surface action and the tilt plane is everywhere parallel to the cell plates, the pretransitional effect should be suppressed. We also discuss the dielectrically active modes in AFLCs and under which circumstances they contribute to the measured dielectric permittivity.

Orientational distributions in smectic liquid crystals showing V-shaped switching investigated by polarized Raman scattering

Molecular orientational order parameters have been obtained by Raman scattering in two types of liquid crystal materials showing the V-shaped switching in thin homogeneous cells. One is the Mitsui mixture and the other is one component of the Inui mixture. The antiferroelectric phase exists in the bulk of both materials but, in thin homogeneous cells, the stability is distinct from each other. The obtained distribution of the local in-plane directors at the tip of the V is considerably broad in the former, while it is narrow in the latter. These differences have been explained by the barrier between the ferroelectric and antiferroelectric orderings, the chiral twisting power, and the interface induced destruction of the antiferroelectric ordering.