Photochromic Behavior of a Spiro-indolino-oxazine in Reverse-Mode Polymer-Dispersed Liquid Crystal Films

Abstract

Based on the technology of polymer-dispersed liquid crystals (PDLC), this work deals with the preparation of bifunctional films sensitive to electric fields (changing transparency) and to light (changing color). Contrary to our recently proposed films working in the opaque → transparent way on applying an electric field, the new films work in transparent → opaque reverse mode. The spectroscopic and kinetic properties of a suitable photochromic compound of the spirooxazine series were preliminarily investigated in the separated constituents (monomer, polymerized monomer, and liquid crystal) of the PDLC matrix to evaluate the physical and chemical compatibility among components and the kinetic parameters of the color-forming and -bleaching processes. Thereafter, its behavior was studied in the reverse-mode PDLC films, which are naturally transparent in the absence of an electric field and become opaque on applying the field, while their color can be photochromically modulated by light exposure. The prototype of the photochromic reverse-mode film proposed in this work was found to be characterized by quite good electrooptical contrast, long-term stability, uniform droplet size distribution, and low driving fields.