Generation of anisotropic emission by light-induced orientation of liquid crystalline polymers

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 photosens itive 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

Light-induced orientation of liquid crystalline terpolymers containing azobenzene and dye moieties

Films of liquid crystalline copoly(methacrylate)s with photochromic azobenzene and benzanilide side groups and related terpolymers with additional dye side groups were oriented by the irradiation with linearly polarized visible or, alternatively, UV light. The orientational behaviors of the terpolymers with benzoxazole, stilbene, and anthracene moieties were compared. The irradiation conditions for the photoorientation process were optimized concerning wavelength and dose with respect to the absorption of the dyes and their limited photostability. Caused by the cooperativity of the photoorientation process, the light-induced ordering of the azobenzene group is connected with the alignment of the nonphotochromic co-mesogenic and the dye side groups even below the glass transition temperature. The light-induced orientational order generated in the glassy state was significantly amplified by the subsequent annealing of the irradiated films at temperatures within the mesophase of the co- and terpolymers. Amplification factors up to 30 were found in this series of polymers. The photoinduction process and the subsequent amplification by thermotropic self-organization were investigated in dependence of the polymer composition, the irradiation dose, and the wavelength of the incident light. The required dose or the irradiation time was significantly reduced by the optimization of light-induced and thermal processing, respectively. In this way, dichroic polymer films containing different types of dyes were created