Liquid crystalline imidazolium salts bearing 5-phenylpyrimidine: dependence of mesomorphic properties on spacer lengths, terminal N-alkyl group and counterions

New imidazolium salts bearing substituted 5-phenylpyrimidine mesogenic groups were synthesised and their mesomorphic properties were studied by differential scanning calorimetry, optical microscopy and powder X-ray diffraction methods (WAXS, SAXS). The parent 5-phenylpyrimidines with the terminal bromide group at the alkoxy spacer exhibited low clearing points and smectic mesophases in narrow temperature ranges. The incorporation of the N-alkylimidazolium cationic core, connected with 5-phenylpyrimidine via an alkoxy spacer, remarkably improved the stability of the mesophases. The imidazolium salts with 5-phenylpyrimidine group formed smectic A phases, stable over broad temperature intervals. The influences of the counteranion, the length of terminal alkyl group and alkoxy spacer on the mesomorphic behaviour of the 5-phenylpyrimidine substituted N-alkylimidazolium salts were investigated in detail.