Photoorientation of a liquid-crystalline polyester with azobenzene side groups: Effects of irradiation with linearly polarized red light after photochemical pretreatment

In contrast to the conventional photoorientation process with blue light, an orientation of 4-cyano-4'-alkoxyazobenzene side groups parallel to the electric field vector of the incident light is generated upon irradiating films of a liquid-crystalline side-chain polymer with linearly polarized red light. The polyester is characterized by smectic and nematic phases g24S(x)26S(A)34N46i and a strong tendency to form J-aggregates. The process requires a photochemical pretreatment by irradiation with UV light or an exposure to visible light of high power density to produce a certain concentration of the Z-isomer, which destroys any initial orientational order and J-aggregates. The orientation process is cooperative, whereas the light-induced orientation of the photochromic moiety causes an ordering of the alkylene spacers and even of the main-chain segments into the same direction. The most probable mechanism of this two-step process is the angular-selective transformation of the bulky Z-isomers to the rodlike E-isomeric formed by the red light. The aligned E-azobenzene side groups become strongly J-aggregated. Very high values of dichroism of about 0.8 and birefringence of about 0.3 were generated as a result of this combination of the photoinduced orientation process and the thermotropic self-organization, which take place simultaneously under the irradiation conditions. The process results in a uniaxial prolate order of the film, whereas conventional photoorientation leads to a biaxial oblate order. These two different three-dimensional orders have been characterized by FTIR polarization spectroscopy and exhibit also varying intermediate thermal stabilities.