Toyokazu Ogasawara

Structures in optically isotropic and bluish colored cubic phases formed by enantiomeric association in an (R,S) dichiral compound and a stereoisomeric (R,R) and (S,S) mixture

The structures of an optically isotropic cubic phase and a bluish colored phase exhibited in an (R,S) dichiral compound with fluorines have been investigated. Not only the bluish colored phase but also the cubic phase exhibit circular dichroism, being confirmed to be chiral phases. Based on the analysis of several X-ray diffraction peaks observed in the small angle range, we find that the optically isotropic chiral cubic (IsoX) phase belongs to the symmetry group I432. It is also found that the cubic phase emerging in the equimolar mixture of (R,R) and (S,S) stereoisomers also has I432 symmetry, suggesting the formation of the cubic phase due to chiral recognition between chiral end chains. The molecular recognition upon forming the IsoX phase is also discussed by mixing (R,R) and (S,S) stereoisomers in the crystalline phase. The other important finding is the exotic phase with a blue colored and foggy texture below the isotropic liquid. X-Ray diffraction results in this phase clearly show a diffraction ring corresponding to a layer structure with a quite long correlation length. We assigned this phase to a smectic blue phase (BPsm).

Dynamic Behaviour of the Local Layer Structure of Antiferroelectric Liquid Crystals under a High Electric Field Measured by Time-resolved Synchrotron X-Ray Microbeam Diffraction

The local layer structure response to a triangular electric field in an antiferroelectric liquid crystal cell has been measured using synchrotron X-ray diffraction with 3 ms time resolution and a few µm spatial resolution. The initially coexisting vertical and horizontal chevron structures are irreversibly transformed to the layer structure with a rearranged molecular orientation at the surface (so-called vertical bookshelf structure). After the irreversible transformation, the rearranged layer structure shows a reversible transition between the horizontal chevron (high field, ferroelectric state) and the combination of the modified vertical and horizontal chevron (low field, antiferroelectric state) associated with the field-induced antiferroelectric-ferroelectric transition. The reversible layer structure has a smaller horizontal chevron angle (a few degrees) than that in the initial state (about 17°). The detailed microbeam diffraction revealed that the layer structure at a low electric field consists of a broad vertical chevron with a small chevron angle and a bent bookshelf in combination with a horizontal chevron, depending on the analyzing position. The stripe texture is related to the modified horizontal chevron structure.

Cubic and blue phases in a fluorine-containing dichiral compound

The structure and dynamics in an optically isotropic liquid crystalline phase exhibited in a dichiral compound with fluorines have been investigated using X-ray, 19F-NMR and dielectric measurements. Results in the dielectric measurement and 19F-NMR measurements clearly show that molecular motion in this phase is just like in the isotropic phase as observed by texture. However, several X-ray diffraction peaks observed in a small angle range indicate akind of cubic phase (Cub). Based on the X-ray diffraction patterns together with the existence of circular dichroism, the phase is assigned to a chiral cubic structure belonging to crystallographic spacegroupI432. Moreover, the exotic phase with blue colored and foggy texture was found in the lower temperature range of isotropic liquid. X-ray diffraction in this phase clearly shows a diffraction ring corresponding to a layer structure, indicating that this blue-colored phase is assignable to a smectic blue phase (BPsm). It was also found that the layer correlation length is remarkably long compared with the previously reported BPsm phases. We want to emphasize that the present material is quite unique in the sense that both cubic phases, BPsm and Cub, appear. The influence of cell thickness and incubation time on the emergence of the smectic C* phase is also discussed based on dielectric measurements and AC calorimetry.

Dynamic layer response under electric field in antiferroelectric liquid crystal cells measured by synchrotron microbeam time resolved X-ray diffraction

Abstract Direct observation of the local layer response of an antiferroelectric liquid crystal to the step form electric field has been carried out by a time resolved synchrotron X-ray micro diffraction measurement. When an electric field was changed from high voltage to OV, corresponding to the ferroelectric to antiferroelectric phase transition, the local layer transformed from the bookshelf to the quasi-bookshelf structure within 0.3 ms. The horizontal chevron structure was found in both the phases, though the decrease in the horizontal chevron angle was observed during a period of 0.2 ms after turning off the electric field. In the antiferroelectric to ferroelectric phase transition process (from OV to high voltage), the layer structure transformed to the bookshelf within 0.04 ms.

Simulation of Conoscopic Figures Using 4 × 4 Matrix Method

Abstract Simulation of conoscopic figures was made using 4 × 4 matrix method. After summarizing the 4 × 4 matrix method, the process for simulating conoscopic figures was briefly described. The conoscopic figures were simulated for the subphases exhibited in antiferroelectric liquid crystals, SmC*, SmCγ* and SmCA*. The simulation was made for more complicated structures, in which SmC* and SmCγ* coexist, and for a hypothetical phase, in which directors randomly distribute on smectic cones.

X-Ray Analysis of the Layer Structure in the Chiral Smectic Phase Showing V-shaped Switching

The layer structure of the chiral smectic phase showing V-shaped switching was investigated using the X-ray diffraction method. In thin planar cells of a three-component mixture in which V-shaped switching was first reported, the chevron layer structure was confirmed. The layer structure almost kept chevron even in the application of an electric field, and additional small diffraction peaks suggesting the existence of a substructure during V-shaped switching were observed. Preliminary time-resolved results are also reported.

Surface behavior of V-shaped switching in smectic-C*-like liquid crystals

V-shaped switching in the smectic-C* like phase of a three- component mixture has been studied from the viewpoint of surface behavior. Two alignment layers, polyvinylalcohol (PVA) and polyimide (PI), were used to study the effect of alignment layer. Electro-optic, switching current and dielectric measurements were conducted using cells with various cell thicknesses. The PI cells exhibit V-shaped switching, though the PVA cells show bistable switching, clearly indicating the importance of alignment layer to realize V-shaped switching. The specialty of the three-component mixture was confirmed in the cell thickness dependence of the dielectric strength (Delta) (epsilon) and the relaxation time (tau) r. Namely, (Delta) (epsilon) and (tau) r show linear dependences on the cell thickness and does not saturate even in 50 micrometer cells, which are quite different from those in conventional ferroelectric liquid crystals. Attenuated total internal reflection ellipsometry was made to evaluate the molecular orientation and switching behavior near the surfaces. It was found that the orientation and the switching characteristics are almost the same as in the bulk, i.e., (1) molecules are aligned along the layer normal at 0 V, which is deviated from the rubbing direction by about 10 degrees, and (2) switching occurs collectively without threshold.

Molecular Motion in A Smectic Liquid Crystal Showing V‐shaped Switching As Studied by Optical Second‐harmonic Generation

The molecular motion during the V-shaped switching in a homogeneously aligned smectic C*-like liquid crystal (LC) cell has been investigated by measuring second-harmonic generation (SHG) interferometry. It was shown that observed SHG behaviors were successfully explained using the collective model and that azimuthal rotation of LC molecules is limited within half a cone and that LC molecules undergo counter azimuthal rotations between the top and bottom halves of the LC chevron layer structure. Possible molecular orientation within a layer was also discussed using the 4 × 4 matrix method.