Orientations of a Nematic Liquid Crystal in Sandwich-Type Cells Studied by Polarized Electronic Absorption Spectra

Abstract

Polarized electronic absorption spectra of a nematic liquid crystal (4-butyl-4′-ethoxyazobenzene, BEAB) were measured in sandwich-type cells to which no rubbing treatment was applied. These polarized spectra were utilized to evaluate the mean squares of the direction cosines of the transition moment, those of the molecular orientation axis, and the three-dimensional absorption components as a function of the cell gap. It has been shown that the inclinations of the transition moment for the 354 nm band (long-axis polarized) and the molecular orientation axis to the cell plane increase with the decrease in the cell gap while the absorption component for the 354 nm band normal to the cell plane decreases. These observations provide a quantitative support for the molecular orientation model proposed previously that the molecular long axis tends to be oriented parallel to the cell plane in the interface region and perpendicular in the bulk region. The thickness of the interface region has been estimated to be about 0.02 μm by a model calculation based on a discrete model for the orientation of the BEAB molecules in the cell.