Regulating the mesogenic properties of imidazolium salts by modifying N3-substituents

In this study, a series of imidazolium salts with different N-3-substituents (3-position: methyl, vinyl, ethyl, n-propyl, n-butyl)-based ionic-liquid crystals with iodide as counterion (ILCs) are synthesized and characterized, with the aim of regulating the mesogenic properties of imidazolium salts. The imidazolium salts-based ionic-liquid crystals are characterized by thermal analysis, polarized optical microscopy, X-ray powder diffraction, and single-crystalline diffraction. Liquid crystalline phase with a Smectic A phase interdigitated bilayer structure is observed. The mesophase temperature range decreases with the increase of the alkyl chain length of N-3-substituent. While when vinyl is attached to the N-3 position, intermolecular pi-pi interactions are formed in adjacent layers, which exert a positive effect on the formation of mesophase. The layer-spacing of imidazolium salts keeps increasing with the increase of the alkyl chain length attached to both the N-1 and N-3 positions, and gradually decreases with the increasing temperature in liquid crystalline phase. The 1-alkyl-3-vinylimidazolium salt has the smallest layer-spacing, due to the intermolecular pi-pi interactions between the vinyl and imidazolium rings in adjacent layers.