Chiral Nematic Copolymers with Photoreversible and Irreversible Changing of Helical Supramolecular Structure Pitch

A new cholesteric copolymer consisting of nematogenic phenyl benzoate groups and two different types of photosensitive chiral groups, azobenzene- and benzilidene- p -menthan-3-one, was synthesized. Planarly oriented films of copolymer selectively reflect light in the near-infrared region of spectrum. It was shown that under UV irradiation (365 nm) irreversible untwisting of cholesteric helix takes place and selective light reflection maximum is shifted to the long-wavelength spectral region. This phenomenon is explained by the E − Z isomerization of benzilidene- p -menthan-3-one chiral groups and the decrease of helix twisting power. On the contrary, under irradiation with visible light (>450 nm), a thermally reversible shift of selective light reflection peak was observed due to the E − Z isomerization of azobenzene-containing chiral units. In both cases, cholesteric helix untwisting is explained by the decrease of the helix twisting power of chiral units during the photoisomerization process. As was demonstrated, such ternary copolymer containing chiral groups sensitive to the light of different wavelengths may be used for reversible as well as irreversible recording of optical information.