Linear dichroism of perylene derivatives in a nematic liquid crystal

The polarized absorption spectra of derivatives of 3,4,9-perylenetricarboxy lic acid and of 3,4,9,10-perylenetetracarboxylic acid dissolved in the nema tic liquid crystal mixture ZLI 1695 have been recorded as a function of tem perature. On the basis of these spectra, the directions of the absorption t ransition moments with respect to the principle axes of the order tensor, a nd the order parameters S* and D* for perylene derivatives have been determ ined assuming a C-s or C-2 point symmetry group for the dyes molecules. Fro m a generalized equation describing properties of anisotropic phases, the c onditions 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.