Generation of Anisotropic Emission by Light-Induced Orientation of Liquid Crystalline Polymers

Abstract

Functionalized polymers containing mesogenic, photosensitive, and fluorescent side groups were synthesized for all-in-one films with anisotropic absorption and emission. Aligned films were prepared by a two-step bulk-alignment procedure combining a photochemical step at room temperature and a thermal aligning step at temperatures of the mesophases. The irradiation with linearly polarized light results in small values of anisotropy via angular-selective photoreactions of cinnamates, stilbenes, and anthracene moieties. The photoinduced anisotropy was significantly amplified by annealing above the glass transition temperature of the liquid crystalline polymers, resulting in a cooperative alignment of all side groups leading to dichroism values up to 0.7 (dichroic ratio up to 6.7) and ratios of anisotropic fluorescence up to 8. The irradiation conditions for the induction of anisotropy were optimized concerning wavelength and dose with respect to the absorption of photosensitive groups and the limited photostability of the dyes. Depending on the distribution of the photoproducts, alignment parallel or perpendicular to the electric field vector of the incident light was induced in dependence on the polymer composition and irradiation conditions.