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 inve stigated. Photoirradiation of mixtures of the azobenzenes and host LC resul ted in N-I phase transition due to trans-cis photoisomerisation of the azob enzenes, and the N phase recovered when the irradiated sample was kept in t he dark because of thermal cis-trans back-isomerisation. Time-resolved meas urements with a laser pulse (355 or 532 nm; 10 ns fwhm) revealed that the p hotoinduced N-I phase transition took place within 200-300 mu s in all samp les. 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 therma l I-N phase transition corresponded closely to the lifetime of the cis isom er 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 pro cess. The time necessary for the thermal I-N phase transition decreased wit h increasing donor and acceptor strength, because the thermal cis-trans iso merisation of the azobenzene with a stronger donor-acceptor pair became fas ter. 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 degrees C.