ISOTHERMAL PHASE-TRANSITION OF LIQUID-CRYSTALS INDUCED BY PHOTOISOMERIZATION OF DOPED SPIROPYRANS

The phase-transition behaviour of mixtures of liquid crystals (LCs) and photochromic spiropyran derivatives, induced by isomerization of the guest molecules, has been investigated. Two types of spiropyran derivatives were employed: normal spiropyrans which are stable in the closed form (spiropyran form; SPF) and open-form (merocyanine form; MCF) stabilized derivatives. These spiropyran derivatives were doped into two LC compounds: 5-cyanobiphenyl (5CB) and 4-butoxyphenyl-4'-ethylcyclohexanecarboxylate (ECH 204). SPF-Stabilized derivatives exhibited photoisomerization behaviour from SPF to MCF in LC media which was accompanied by an isotropic (I) to nematic (N) phase transition of the mixtures. On the other hand, an N --> I phase transition of the mixtures was observed when the MCF-stabilized spiropyran doped in both LCs was photoisomerized from MCF to SPF. The result was interpreted in terms of shape anisotropy of the guest molecule associated with each isomer of the spiropyrans.