Charge-transfer interaction in liquid-crystalline materials and their application to photonics

Photochemical phase transition of push-pull type azobenzene liquid crystal (LC), which contains both an electron donor and an acceptor in both ends of the azobenzene moiety, has been explored. Polymer azobenzene LCs were prepared, which show nematic (N) LC behavior in the trans form while no LC phase in the cis isomer. Photoirradiation of a very thin film of the azobenzene LCs (similar to 200 nm) in the trans form resulted in disappearance of the N phase due to trans-cis photoisomerization of each mesogen, and the N phase recovered quickly when the irradiated sample was kept in the dark because of cis-trans thermal isomerization and reorientation of trans-azobenzenes. Time-resolved measurements by the use of a laser pulse (355 nm; 10 ns FWHM) revealed that the N to isotropic (I) phase transition took place in 200 mu s The thermal I-N phase transition of the push-pull type azobenzene LC occurred in 800 ms at 135 degrees C. This response is faster by one order of magnitude than the response of azobenzene LCs without charge-transfer interaction.