Spectroscopic Studies of Trans-1,6-Diphenyl-1,3,5-Hexatriene in Ordered Liquid Crystal Solutions

A detailed study has been made of the absorption spectrum of all trans-1,6 diphenyl-1,3,5-hexatriene dissolved in a nematic liquid crystal mixture (1.95:1 cholesteryl chloride and cholestryl laurate (by mass)) using polarized light. Apart from two transitions with moments directed along the molecular axis a transition between these two perpendicular to the main axis has been discovered. This is attributed to a a $^{1}$A-$^{1}$G transition, predicted by theory. The application of ordered systems to the resolution of transitions in the absorption spectrum has been checked by a comparison with fluorescence polarization methods. A theory has been developed for the treatment of absorption data in ordered media and the effects of distribution of molecular orientations examined. The technique presented is found to be better in several respects than fluorescence methods for identification of electronic transitions.