Photoinduced phase transition of nematic liquid crystals with donor–acceptor azobenzenes: mechanism of the thermal recovery of the nematic phase

Photoinduced nematic (N)-to-isotropic (I) and thermal I–N phase transition behaviour of liquid crystals (LCs) with donor–acceptor azobenzenes was investigated. Photoirradiation of mixtures of the azobenzenes and host LC resulted in N–I phase transition due to trans–cis photoisomerisation of the azobenzenes, and the N phase recovered when the irradiated sample was kept in the dark because of thermal cis–trans back-isomerisation. Time-resolved measurements with a laser pulse (355 or 532 nm; 10 ns fwhm) revealed that the photoinduced N–I phase transition took place within 200–300 µs in all samples. The rate of the thermal I–N phase transition was affected strongly by the strength of the donor and the acceptor. The response time of the thermal I–N phase transition corresponded closely to the lifetime of the cis isomer of the azobenzenes, hence we can conclude that the rate-determining step of the thermal recovery of the N phase was the cis–trans isomerisation process. The time necessary for the thermal I–N phase transition decreased with increasing donor and acceptor strength, because the thermal cis–trans isomerisation of the azobenzene with a stronger donor–acceptor pair became faster. The N phase recovered within 500 ms in the mixture of the azobenzene (with tertiary amine as the donor and nitro group as the acceptor) and host LC at 37°C.