Liquid Crystalline Materials for Photonics: Optical Switching by Means of Photochemical Phase Transition of Liquid-Crystalline Azobenzene Films

Isothermal phase transition of liquid crystals (LCs) can be induced reversibly by photochemical reaction of guest molecules incorporated into the LC phase at concentrations of 1 ~ 5 mol%. Such photoresponsive molecules as azobenzene and spiropyran derivatives have been proved to be effective guest molecules to bring about the photochemical phase transition. 1-5 The mechanism of the photochemical phase transition is interpreted in terms of the change in the molecular shape of the guest molecules by the photochemical reaction. For example, trans-azobenzenes are rod-like shape, stabilizing the LC phase, while cis-azobenzenes are bent, destabilizing the LC phase. When the trans-azobenzene/nematic (N) LC mixtures are irradiated to cause trans-cis photoisomerization of the guest molecules, the LC phase of the mixtures is destabilized in accumulation of the cis form and the N to isotropic (I) phase transition temperature (tNI) is lowered. When tNI is lowered below the irradiation temperature, N-I phase transition of the guest/host mixture is induced isothermally. This process is reversible, and cis-trans back isomerization restores the initial N phase. Time-resolved measurements by the use of a pulsed laser have revealed that the photochemical N-I phase transition takes place in the time region of 50 ~ 200 ms for the nematic hosts of low-molecular-weight (LMW) as well as polymeric LCs.6,7 Propagation of perturbation in the form of the trans-cis isomerization of the guest molecules may require a relatively long time in the LC systems.