The dichroic ratio of pleochroic dyes dissolved in nematic liquid crystals

Abstract

We discuss the factors which determine the dichroic ratio R of pleochroic dyes dissolved in nematic liquid crystals. For cylindrical dye molecules the order parameter and the dichroic ratio can be calculated from a generalization of the Maier–Saupe theory. In planar dyes such as anthraquinones the order tensor strongly deviates from cylindrical symmetry. The direction of the optical transition moment does in general not coincide with one of the principal axes of the order tensor. We calculate the reduction of R by this effect. The direction of the optical transition moment can be calculated and we discuss the optimization of substituents in anthraquinones with respect to angle between transition moment and molecular axis. Vibrational effects on the dichroic ratio are shown to be restricted to thermally excited vibrations in the electronic ground state and are small in rigid molecules.