Preparation and In-Plane Anisotropic Photobleaching Properties of Polyimide Langmuir-Blodgett Films Having a Photoresponsive p-Nitroazobenzene Pendant Group

Polyamic acid 4 possessing a p-nitroazobenzene pendant group was synthesized from freshly prepared tetracarboxylic acid and 4,4′-diaminodiphenyl ether. A Langmuir-Blodgett (LB) film of the polyamic acid/long alkyl amine salt 5, which was prepared by mixing a solution of 4 with the long alkyl amine N-icosylpyrrolidine, was then prepared by a usual LB technique. Treatment of the LB film of 5 with a mixture of acetic anhydride and pyridine afforded the polyimide LB films. The UV spectrum of the polyimide exhibited a λ_max of 480 nm, which suggested a trans configuration of the azobenzene unit. Isotropic orientation of the azobenzene group in a plane of the polyimide LB film was observed by using polarized UV spectroscopy. After irradiation of linearly polarized argon ion laser beam at 458 nm to the LB film, photobleaching of the azo dye at 480 nm was observed only in the direction parallel to the incident beam. The absorption ratio between parallel and perpendicular planes of the irradiated LB film was more than 2.