Photochemical control of helical pitch of glass-forming dimeric cholesteric liquid crystals by isomerization of embedded di-mesogenic compounds with both cholesterol and azobenzene groups

We synthesized di-mesogenic liquid crystals with both cholesterol and azobenzene groups by esterification of 10,12-docosadiynedioic acid or docosanedioic acid with 4-alkyl-4′-hydroxyazobenzene (or 4-hydroxyazobenzene) and cholesterol. Two compounds without an alkyl chain on the azobenzene unit show cholesteric and smectic phases, while five compounds with an alkyl chain from butyl to hexadecyl show only a smectic phase. When the compounds were doped in 2-10 w% to a dicholesteryl ester forming stable cholesteric glass, the mixture became photo-responsive. Upon irradiation at 366 nm, the cholesteric band of the mixtures shifted toward shorter wavelength caused by the photo-isomerization from trans to cis of the azobenzene unit on the dopant. The extent of the shift depends on irradiation energy, temperature and the length of the alkyl chain on the azobenzene unit. When a thin film of the dicholesteryl compounds containing 5w% of the dopant having a heptyl substituent on the azobenzene unit was irradiated at 90°C, the reflection band shifted from 730 nm (infrared) to 490 nm (blue). The mixture went into the glassy state keeping the shifted cholesteric reflection band after rapid cooling from the liquid crystalline temperature to 0°C. The cholesteric glassy state was stable up to 80°C. Since the molecular arrangement of the dicholesteryl compound and the conformation of the doped azobenzene derivatives completely returned to the initial state upon reheating to isotropic temperature, the photo-responsibility and glass-forming property of the mixture is applicable to full-color rewritable recording materials.