A comparative study of photo-optical behaviour of photosensitive chiral copolymers with cholesteric mesophases induced in nematogenic and smectogenicmatrices

The photo-optical behaviour of two series of chiral photochromic acrylic co polymers with a chiral nematic phase has been studied. These copolymers con tain identical chiral photochromic units, but have different structures of the phenyl benzoate mesogenic side groups which are responsible for the dev elopment of LC phases. This approach allowed us to examine specific feature s of the photo-optical behaviour of the copolymers as a function of the nat ure of the LC 'matrix' in which the cholesteric phase was induced. The acti on of UV irradiation was shown to lead to the E-Z isomerization of the chir al side groups and, as a consequence, to untwisting of the cholesteric heli x of the copolymers. For copolymers of both series, the effective quantum y ields of this photochemical process were calculated. In the case of copolym ers in which the cholesteric mesophase is induced in a smectogenic matrix, the corresponding values of the quantum yield are lower and depend slightly on temperature. A plausible explanation of the above phenomena is suggeste d.