Hiromoto Sato

Study of Internal Electric Field of Liquid-Crystal Cell Effected by Space-Charge Polarization

The distribution of the local electric field in a liquid-crystal cell containing mobile ions was simulated in accordance with the Poisson equation in the steady state under a DC voltage application. The distribution of the local field was determined to be markedly unbiased due to the contribution of the bound charges on the electrodes induced by ions. This result suggests that the intensity of the local field acting on ions can be approximated by that of the external field, for analyzing the space-charge polarization under a small AC voltage application even in a very low frequency region, where the diffusion distance of ions during the period of the external AC field becomes greater than the thickness of the cell.

Correlation Between Order Parameter of Liquid Crystal Mixture and Dichroic Ratio of its Guest-Host System

Good correlation was found between the order parameter S of liquid crystal (LC) host-mixture and the dichroic ratio of its guest-host system. We investigated temperature dependence of the S value of several LC substances, focusing on its index β. The index β was determined by measuring the temperature dependence of refractive index. LC substances with a cyano terminal group have smaller β values than those with an alkoxy or a fluoro-substituted terminal. This means that the S value is expected to be larger for the cyano substances than the alkoxy or fluoro substances at a fixed reduced-temperature. There exists good correlation between the calculated S value of a host LC mixture and the measured dichroic ratio of its guest-host system. This enables us to design highly ordered LC mixtures for high contrast guest-host LC displays based on β values of LC substances.

DIELECTRIC RELAXATION OF NEMATIC LIQUID CRYSTALS WITH DIFFERENT MOLECULAR SHAPES

Abstract The parallel dielectric constants of several liquid crystal compounds having different core structures and polar substituents have been investigated as a function of frequency and temperature using the dilute solution by an unpolar nematic host mixture. The dielectric relaxation frequencies were found to depend roughly on the molecular length. With regard to the compounds showing a large deviation from the correlation between the relaxation frequency and the molecular length, the disagreement was explained by considering the intermolecular interaction estimated from the molecular structure.

16.1: Influence of lonsin LCMaterials on Optical Threshold Voltage of Passively Addressed LCDS

Frequency dependence of the optical threshold voltage of LCDS is successfully explained by considering the microscopic internal-field, induced by ions distributed in a LC layer. The effective-voltage is calculated from the local internal-field, and is found to agree fairly well with experimental results. This study enables to develop cross-talk free LC materials for use in passively addressed LCDS.

Light-emitting materials based on liquid crystals

Liquid crystal (LC) materials with negative dielectric anisotropy have been successfully applied to light emitting diode. Even for a nematic phase, the Fredericks transition was avoided under such a strong electric field as used in organic light emitting diode (OLED). The LC materials having shallower highest occupied molecular orbital (HOMO) levels have been emissive due to their higher probability for hole injection. The LC materials incorporated three fluorescent dyes for each color (R,G,B) show corresponding color emission. However, only blue dye shows extraordinarily small light emission. Relative fluorescence quantum yield measurement reveals similar fluorescence efficiency for all dyes. Only blue dye’s HOMO level lies very close to that of the matrix LC. This fact may lead to very small hole trapping probability for the blue dye molecule. It is suggested that in the OLED system using LC materials studied here, the recombination of the electrons and the holes trapped at the dye molecules is dominant for light emission rather than Foerster energy transfer.

Optical properties of advanced liquid crystalline materials for uses in modern displays

Absorption spectrum, optical anisotropy, and photostability of liquid crystalline compounds are reviewed in connection with their molecular structures, providing explanation of superior optical properties of advanced materials. A novel class of substances is introduced to exhibit both high birefringence and good photostability. These features are indispensable to liquid crystalline materials for uses in modern displays.