The photochemical phase-transition behavior of polymer liquid crystals (LCs) containing azobenzene moieties with strong donor-acceptor substituents has been investigated. Copolymers of a mesogen and a donor-acceptor azobenzene were prepared. Photoirradiation of a thin film of the polymer LCs (100-200 nm thickness) in the nematic (N) phase caused an N-to-isotropic (I) phase transition due to trans-cis photoisomerization of the azobenzene moiety, and the N phase recovered rapidly when photoirradiation was ceased because of cis-trans thermal back-isomerization. Time-resolved measurements with a laser pulse (532 nm; 10 ns, fwhm) revealed that the N-I phase transition took place in 300 mu s. The thermal recovery of the N phase in the polymer LC with a donor-acceptor azobenzene, which stabilized the LC phase, occurred in 8 ms at 146 degrees C. This response of the recovery is 10(3) times faster than that of non-donor-acceptor azobenzene LCs. In a sample that showed a smectic (S) phase, however, no photochemical phase transition was observed. These phenomena are interpreted in terms of the stability of the LC phase.