Preliminary Work on Orientation of Nematic Liquid-Crystalline Molecule by Electronic Spectra

Abstract

Absorption spectra of a nematic liquid-crystalline molecule (4-butyl-4′-ethoxyazobenzene) were measured in sandwich-type cells with very thin gaps which ranged from 0.2 to 1.0 μm when the cells were empty. The intensity ratio of the first (354 nm) to second (240 nm) band in the liquid state (86 °C) decreases as the state changes into liquid-crystalline (LC) states (70 and 48 °C). According to results of dichroism analysis and MO calculation, the 354 nm and 240 nm bands are due to π-π* transitions polarized along the molecular long and short axes, respectively (with an angle of about 66°to each other). This finding coupled with an assumption that the molecular orientation of the liquid state is isotropic, allows a qualitative discussion on the orientation of molecule in the LC state, leading to a conclusion that the molecular long axis against the cell surface takes two kinds of orientation with respective tendencies to be parallel in the interface region and perpendicular in the bulk region.