Electric-field-dictated phase diagram and accelerated dynamics of a reentrant nematic liquid crystal under photostimulation.

In a system consisting of photoactive molecules that exhibit light-driven isomerization transformations, actinic light can diminish or enhance ordering to the extent that transitions from the equilibrium to a more disordered phase can be brought about isothermally. This feature enables light to be used as a thermodynamiclike parameter to investigate phase behavior and adds another dimension to the studies owing to the nonequilibrium character of the isothermal transitions. We have carried out experiments which exploit the combination of two recent findings, viz., an electric field can accelerate the return to the nematic liquid crystalline phase from a photodriven isotropic phase; and in a reentrant mesogen, the photoinduced phase can be more ordered. To photostimulate the nonequilibrium transitions a low power uv radiation (0.1 mW cm(-2)) has been used. Unique temperature-electric-field phase diagrams of a liquid crystal exhibiting isotropic-nematic-smectic- A -reentrant nematic sequence, mapped using light transmission as probe reveal that the electric field influences all the transitions, but the effect is maximum on the equilibrium reentrant nematic to the photoinduced smectic- A transition. Temporal measurements have been performed under nonequilibrium conditions to study the dynamics of both the photochemical and the back relaxation processes across the different transitions. The electric field is indeed observed to accelerate the thermal back relaxation in each case, and especially the recovery of the reentrant phase is hastened by three orders of magnitude in time. We explore possible causes for the acceleration and present a finding which can be associated with one of the predictions of density-functional calculations for isomerization of azobenzenes.