Circular Fluorescence Polarization of Achiral Molecules in Cholesteric Liquid Crystals

Abstract

In liquid crystalline matrices the molecular axes of solute molecules adopt an orientation which is given by the director of the liquid crystal. Consequently, the physical properties of the solute become anisotropic, governed by the director distribution within the matrix. An example of this general effect is the circularly polarized fluorescence emission of achiral molecules dissolved in cholesteric liquid crystals. The degree of circular polarization (CPF), given by a dissymmetry factor g_e = 2(I_L - I_R)/(I_L + I_R), is rather high (I_L, I_R intensity of left or right circularly polarized fluorescence, respectively). The sign of g_e was found to depend (i) on the screw sense of the cholesteric helical structure, and (ii) on the direction of the electric transition moment within the emitting molecule. The absolute value of g_e is a function of the helical pitch g. The CPF can be understood as an ensemble effect of helically oriented emitting chromophores (e.g. perylene, pyrene, azulene, diphenyl-hexatriene). Based on the theory of Mauguin and de Vries relations have been derived describing quantitatively the dependence of CPF on the structure of the emitting molecules as well as on the helix structure of the cholesteric solvent.