Theoretical Study of the Effect of Light on Nematic to Isotropic Phase Transition

Optical response of liquid crystals is very important from the point of view of applications of liquid crystals in display and data storage devices. Experiments have shown that UV rays incident on nematic liquid crystals doped with photoactive azo compounds lower the nematic isotropic transition temperature $(T_{NI})$. This is attributed to the transformation of photoactive trans isomer to cis isomer due to UV rays. We have earlier developed a mean field model involving molecular explanation for two lengths in which the mutual orientation of near neighbor molecules changes from an antiparallel to a parallel configuration as the temperature lowered. In this paper, this model is extended to include the length change from trans to cis isomer. The calculated values of change in $T_{NI}$ qualitatively agree with experimental trends.