New azobenzene-based chiral-photochromic substances with thermally stable Z-isomers and their use for the induction of a cholesteric mesophase with a phototunable helix pitch

Novel photosensitive azobenzene-containing substances with sorbitol- and lactate-based chiral fragments were synthesized to serve as chiral-photochromic dopants in cholesteric mixtures. Owing to the presence of chlorine or methyl lateral substituents in the azobenzene fragment of the dopants, the photoinduced Z-form of their molecules is highly stable. The photooptical properties of the prepared cholesteric mixtures were studied and it was shown that UV-irradiation of the planarly-oriented films results in a shift of the selective light reflection wavelength to a long-wavelength spectral region due to E–Z isomerization of the azobenzene moieties. This process is thermally and photooptically reversible and the rate of blue-light-induced helix twisting is comparable with the rate of the untwisting process, whereas the thermal relaxation completes only after ca. 40 days at room temperature (for a chlorine-substituted chiral-photochromic dopant). It was found that this value is almost two orders of magnitude less in comparison with the non-substituted chiral-photochromic dopant also studied in this work. The obtained results demonstrate the promising properties of the synthesized compounds for creation of novel photoswitchable cholesteric materials.