Photocontrolled gel-to-sol-to-gel phase transitioning of meta-substituted azobenzene bisurethanes through the breaking and reforming of hydrogen bonds.

An azobenzene derivative substituted at its meta positions with two urethane moieties linked to two cholesteryl ester units forms a gel that exhibits sol-gel phase transitions upon photoirradiation as a result of trans-cis isomerization of the azobenzene unit. During the sol-gel phase transitions, hydrogen bonds, which are partly responsible for stabilizing the gels, are broken or reformed. Using circular dichroism spectroscopy, we detected asymmetrical fibrous networks in the aggregates of these gelators.