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 elec. field vector of the incident light was generated upon irradiating films of a liq.-cryst. side-chain polymer with linearly polarized red light. The polyester is characterized by smectic and nematic phases g24SX26SA34N46i and a strong tendency to form J-aggregates. The process requires a photochem. pretreatment by irradn. with UV light or exposure to visible light of high power d. to produce a certain concn. 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. 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 irradn. 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 were characterized by FTIR polarization spectroscopy and exhibit also varying intermediate thermal stability.
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