Smectogenic dimeric liquid crystals. The preparation and properties of the α,ω-bis(4-n-alkylanilinebenzylidine-4′-oxy)alkanes

Eleven series of dimeric liquid crystals, namely the α,ω-bis(4-n-alkylanilinebenzylidine-4′-oxy)alkanes, have been synthesized and their transitional properties characterized. The number of methylene groups in the flexible core was varied from one to twelve while the length of the terminal alkyl chains was increased from a methyl group to a decyl substituent. The rich smectic polymorphism of this family of compounds was studied using X-ray diffraction and polarizing microscopy; it includes several examples of rare phase transitions, for example, smectic F-smectic A and crystal G-isotropic, as well as novel modulated hexatic phases based on a two dimensional lattice. The transitional properties of a given series, where the length of the terminal chain is held constant while the length and parity of the flexible spacer is varied, exhibit a pronounced alternation. For example, the entropy change at the nematic-isotropic transition exhibits a strong alternation and this has been interpreted in terms of a corresponding alternation in the long range orientational order at the transition. Increasing the spacer length for a given terminal chain promotes nematic behaviour while increasing the terminal chain length for a given spacer promotes smectic behaviour. A simple relationship relating the occurrence of smectic behaviour to the molecular structure is deduced, namely that for smectic properties to be observed the length of the terminal chains must be greater than half the length of the spacer. We discuss this observation in terms of microphase separation into three distinct regions, the aromatic moieties, the terminal chains and the spacers.