Electric‐Field‐Assisted Acceleration of the Photostimulated Nematic—Isotropic Transition

Abstract

Photoisomerization is a molecular property in which selective photoexcitation of the molecules causes transformation between isomers that are structurally different. The property has been of tremendous practical implication, for instance, in dye molecules employed in optical data storage devices such as CDs, DVDs, etc. More recently they are being investigated as candidates for molecular devices, like molecular switches, motors, and electronic components. The combination of this property with the self-assembling nature of liquid crystalline systems possessing large birefringence values is valuable from both application and fundamental points of view, especially in photostimulated non-equilibrium phase transitions. Here we report a novel method of an accelerated means of recovering the equilibrium from the photo-driven state. The employed guest-host system consists of the photoactive guest azobenzene and host nonphotoactive molecules exhibiting a nematic mesophase. By irradiating the sample with UV radiation the system is photo-driven from the equilibrium nematic phase to the isotropic phase via an isothermal phase transition. The recovery of the equilibrium phase takes place spontaneously, but is very slow. We demonstrate here that by application of an electric field the recovery occurs at least two orders of magnitude faster. The rapid recovery is associated with the faster reverse isomerization and also observed in the isotropic phase, and therefore is applicable to all systems connected with such isomerization processes. Photonics, in which light can be controlled by light as a stimulus, is a field of growing interest with numerous promising applications. [1‐3] Owing to the large photoinduced birefringence, liquid crystals (LC) are attractive candidates in this field, having promising features for a variety of applications including optical data storage, a topic that has generated a lot of activity. [4‐7] Of particular interest to the work presented in this article is the phenomenon of reversible photo-induced shape transformation of the molecules containing chromophoric azo groups. [8] Upon UV irradiation (around 360 nm,