Photoinduced self-organization and photoorientation of a LC main-chain polyester containing azobenzene moieties

Light-induced ordering processes were investigated in spin-coated films of a smectic main-chain polyester with mesogenic 4,4'-dioxyazobenzene groups in the backbone. Spin-coating results in aggregation due to pi-pi stacking and out-of-plane orientation. Both are destroyed by UV exposure. However, they are rebuilt by subsequent thermal Z/E isomerization of the azobenzene groups, forming a lamellar structure parallel to the substrate. The irradiation with linearly polarized visible light results in the orientation of the azobenzene moieties perpendicular to the electric field vector, preserving the lamellar structure with values of dichroism up to 0.43. However, the efficiencies of the E/Z photoisomerization and that of the photoorientation process strongly depend on the history of the sample. The scanning force microscopy (SFM) histogram indicates two types of domains in the top layer, one oriented in the normal and the second one formed by tilted azobenzene groups with an average tilt angle of about 50 degrees to the normal. The last spacing is erased by annealing, but is rebuilt due to linearly polarized exposure. The photoinduced anisotropy of a "wet" dewetted ultrathin film of 5 nm is 1 order of magnitude smaller. It demonstrates that the photoreaction and the self-organization influence each other mutually.