Molecular Mechanism of Anomalous Increase in the Helical Pitch of Cholesteric Liquid Crystals Induced by Achiral Dopants

Abstract

A dopant-induced anomalous increase in the helical pitch of dicholesteryl 10,12-docosadiynedioate 1 is explained by generation of smectic clusters in the cholesteric phase where the dopants act as promoters of smectic domains. The dopants, having a mesogenic core and the proper length of alkyl chains on both their molecular sides, such as 4,4‘-dialkylazobenzenes, 4,4‘-dialkylazoxybenzenes and 4,4‘-dialkylbiphenyls, considerably increase the pitch. However, dopants having benzene or a binaphthyl structure in the molecular center, an alkyl chain at one side of the molecules and Z-isomers of azobenzene and azoxybenzene derivatives, slightly decrease the pitch by acting as impurities that decrease the transition temperature of the host. An X-ray diffraction study of the cholesterics revealed that the pitch increases with an increase in size and amount of the smectic domains induced by the appropriate dopants.