Prototypes of bifunctional photochromic and electro-optical systems

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Abstract
Two types of bifunctional films having both electro-optical and photochromic properties have been prepared and investigated. These films are able to modulate the light transmission by means of suitable electric fields and to change their color under light irradiation. One of the films is fluid-like, being an emulsion of a liquid crystal and a photochrome in an organic monomer. It is defined as a photochromic emulsion dispersed liquid crystal (PEDLC). The other film is solid, having the structure of a polymer dispersed liquid crystal and containing a small percentage of photochrome; it is defined as a photochromic polymer dispersed liquid crystal (PPDLC). The solid films were obtained by photopolymerizing the fluid one. The electro-optical and photochromic properties of these films were investigated. To choose suitable photochromic components, the color-forming and color-bleaching kinetics of some compounds of the classes of spiro-oxazines and spiropyrans were preliminarily investigated in the components of the matrix (liquid crystals and monomers). They generally exhibited a better colorability in the matrix, as compared with organic solvents, due to decreasing of the bleaching rate. Their presence did not alter the electro-optical properties of the films with respect to those of PDLC and EDLC systems. Some significant variations in the color-forming and color-bleaching processes, observed for some of the photochromes dispersed in PDLC films, are discussed. Two of them were found to have the prerequisites of colorability, reversibility, and durability to be used in the bifunctional devices. The results are promising in view of the future involvement of these films in technology.