Why do non-symmetric dimers intercalate? The synthesis and characterisation of the -(4-benzylidene-substituted-aniline-4'-oxy)--(2-methylbutyl-4'-(4''-phenyl)benzoateoxy)alkanes

Ten new non-symmetric liquid crystal dimers belonging to the family of compounds -(4-benzylidene-substituted-aniline-4'-oxy)--(2-methylbutyl-4'-(4''-phenyl)benzoateoxy)-alkanes have been synthesised and their transitional properties characterised. The dimers contain either a hexamethylene or octamethylene spacer, while the terminal substituents on the 4-benzylideneaniline fragment are H, CH3, F, Cl and Br. The unsubstituted dimers are not liquid crystalline, while the remaining compounds exhibit enantiotropic nematic behaviour. The trends in the clearing temperatures, according to the chemical nature of the terminal substituent, are largely consistent with those established for conventional low molar mass liquid crystals. Three of the dimers also exhibit an intercalated smectic A phase, specifically the two bromo-substituted dimers and the chloro-substituted dimer containing a hexamethylene spacer. The driving force for the formation of this phase is considered to be, at least in part, the specific anisotropic interaction between the unlike mesogenic units. The absence of smectic behaviour for the isosteric methyl-substituted dimers reveals that steric factors alone cannot stabilise the intercalated smectic A phase.