Liquid crystallinity-embodied imidazolium-based ionic liquids and their chiral mesophases induced by axially chiral tetra-substituted binaphthyl derivatives

Abstract

We synthesised novel imidazolium-based ionic liquids with thermotropic liquid crystallinity by introducing phenylcyclohexyl and/or cyanobiphenyl mesogenic cores and hexyl or dodecyl methine chains into both sites of imidazolium moieties facing bromide anions. The liquid crystalline ionic liquids (LCILs) thus synthesised showed a nematic or smectic mesophase in both the heating and cooling processes, indicating the enantiotropic nature of the liquid crystallinity. The LCILs bearing the same types of double mesogenic cores [LCIL-2] showed a smectic A phase in the temperature range from 115 to 175 °C, whereas the LCILs with different types of double mesogenic cores [LCIL-3 and LCIL-4] showed nematic phases in the temperature ranges from 58 to 88 °C and 43 to 95 °C, respectively. The axially chiral binaphthyl derivatives substituted by LC groups at the 2,2′, 6,6′ positions of the binaphthyl rings were synthesised and used as chiral dopants with large helical twisting powers. The mixtures of the LCILs and the (R)- and (S)-binaphthyl derivatives exhibited induced chiral nematic phases with right-and left-handed helical senses, respectively. The ionic conductivities of the LCILs were evaluated to be 10⁻⁷–10⁻⁴ S cm⁻¹, depending on the isotropic, LC, and crystal phases. The temperature dependence of the ionic conductivities indicates that the LCILs can be regarded as semi-conducting materials. The LCILs might be used as anisotropic ionic conductors and can even serve as anisotropic solvents and electrolytes in electrochemical polymerisations.