Surface-assisted photoalignment of discotic liquid crystals by nonpolarized light irradiation of photo-cross-linkable polymer thin films

In this article, we describe the surface-assisted photoalignment of discotic liquid crystals (DLCs) on thin films of photo-cross-linkable polymers with cinnamoyl moieties as the side chains. Oblique irradiation of the polymer thin films with nonpolarized UV light at 313 nm brought about inclined orientation of the cinnamoyl residues as a result of their direction-selective photoisomerization and photodimerization. The DLC molecules on the photoirradiated polymer films were aligned in a tilted hybrid manner. This means that the DLC directors are continuously altered from the substrate to the DLC film surface so as to minimize the elastic free energy. Interestingly, we found that the tilted direction of aligned DLC molecules is clearly influenced by the chemical structures of the cinnamate-containing polymers. When a poly(vinyl cinnamate) thin film was obliquely exposed to nonpolarized UV light, the DLCs were inclined to the direction opposite to the UV light propagation. In a keen contrast, the thin film of poly(methacrylate)s tethering cinnamoyl groups, which was obliquely exposed to nonpolarized UV light in advance, provided the tilting DLC direction in parallel with the light propagation. The results were supported by tilted orientation of calamitic (rod-shaped) liquid crystal on the obliquely irradiated polymer films. Such photoalignment behavior of the DLCs can be rationalized by anchoring balance between intermolecular interaction of the DLC molecules with the photodimers of polymer films and those with the remaining E-isomers of cinnamoyl side chains at the film interface. The present technique of DLC photoalignment opens promising ways not only to understand anisotropic physical properties of DLCs, but also to design and fabricate novel nanodevices for photonics and electronics applications.