Mechanistic insight into circularly polarized photoluminescence from a chiral-nematic film

Circularly polarized photoluminescence (CPPL) was characterized for rod-lik e molecules of Exalite 428 helically arranged in a chiral-nematic liquid cr ystalline film. With an unpolarized excitation at 370 nm, CPPL intensities measured at 428 nm, and a selective reflection wavelength ranging from 7.2 to 72.6 mu m, the observed dissymmetry factor, g(e), was found to be in exc ellent agreement with theoretical prediction, without resorting to adjustab le parameters. As a result, new insight into liquid crystal-induced CPPL ha s emerged. Specifically, it was found that circular dichroism and circular polarization of the excitation beam prior to inducing linearly polarized ph otoluminescence (LPPL) at quasi-nematic layers play an insignificant role. The relatively large g(e) value in the spectral region far removed from the selective reflection band was attributed to the circular polarization of L PPL emanating from all quasi-nematic layers comprising the chiral-nematic f ilm. In the absence of a fluorescent dye, the propagation of unpolarized li ght through the chiral-nematic film under otherwise identical conditions re sults in no circular polarization. The present study has provided a foundat ion for generating circularly polarized light by way of photo-excitation wi th unpolarized light of a chiral-nematic him containing a fluorescent dye.