Photo-responsive doped cholesteric liquid crystals

This review deals with recent developments in the design of switchable dopants capable of altering the pitch of cholesteric liquid crystals under the influence of light. Cholesteric liquid crystals possess many potentially useful properties owing to the helical organisation of their mesogens. For many of these applications, dynamic control over the cholesteric pitch can be achieved using chiral dopants capable of changing their shape in response to an external stimulus. The first attempts at developing such responsive systems stem from the 1970s, but major advances have been reported in recent years (2003-present), which is the subject of this review. Efficient dopants showing large changes in helical twisting power upon photo-switching have been developed, often capable of inverting the sign of the helical organisation of the surrounding liquid crystalline host. Overcrowded alkene-based molecular motors and binaphthylazobenzene-based switches have emerged as two classes of highly efficient dopants, enabling manipulation of cholesteric pitch over wide ranges using low concentrations of dopant.