First Observation of a Photo-Induced Transition to a More Ordered Phase in a System Exhibiting Reentrant Nematic - Smectic A Phase Sequence

Abstract

Materials composed of photoactive molecules like azobenzenes are known to show photo-induced isothermal phase transitions. However, in all the cases investigated so far, the trans-cis conversion brought about by photoisomerization always leads to an order to disorder transition, i.e., the ordering decreases in the process. In this paper we report observation of the reverse behavior, namely, a photo-induced disorder to order transition. Experiments have been done on a binary liquid crystalline system doped with a photoactive azo molecule, showing a reentrant nematic-smectic A phase transition. In the reentrant nematic phase UV illumination of the sample and the consequent photoisomerization leads to the transformation to the smectic A phase. A possible explanation for this behavior is suggested on the basis of the recently proposed photo controlled nanophase segregation mechanism.

Key Words:

• photo-induced transition
• reentrant nematic
• nanophase segregation