Linear dichroism of perylene derivatives in a nematic liquid crystal

The polarized absorption spectra of derivatives of 3,4,9-perylenetricarboxylic acid and of 3,4,9,10-perylenetetracarboxylic acid dissolved in the nematic liquid crystal mixture ZLI 1695 have been recorded as a function of temperature. On the basis of these spectra, the directions of the absorption transition moments with respect to the principle axes of the order tensor, and the order parameters S* and D* for perylene derivatives have been determined assuming a C-s or C-2 point symmetry group for the dyes molecules. From a generalized equation describing properties of anisotropic phases, the conditions necessary to determine a complete set of the order parameters and the pieces of possible information that can be obtained from the polarized optical spectroscopy are discussed.