Columnar thermotropic mesophases formed by dimeric liquid-crystalline ionic liquids exhibiting large mesophase ranges

Abstract

A series of 16 symmetric, dimeric, dicationic liquid-crystalline ionic liquids with mesogenic 3,4,5-tris(alkyloxy)benzyl moieties tethered to two bridged imidazolium cations were designed and synthesised. As a comparison, 12 monocationic imidazolium liquid crystals with the same mesogens were also prepared. The mesomorphic properties of these ionic liquids were characterised in terms of polarised optical microscopy (POM) and differential scanning calorimetry (DSC), while thermal stabilities were obtained by thermogravimetric analysis (TGA). All compounds having chloride (Cl⁻) and tetrafluoroborate (BF₄⁻) anions exhibit hexagonal columnar (Col_h) liquid crystal mesophases, with the dimeric materials showing mesophases over an extended temperature range. The effect of the linking chain, alkyl substituents, and anion type on the thermal properties of these dimers was examined and showed a significant influence. For example, Col_h mesophases were only observed for ionic liquids having Cl⁻ and BF₄⁻ anions, whereas systems with the bis(trifluoromethylsulfonyl)imide (Tf₂N⁻) anion melted directly to an isotropic liquid. A shorter spacer paired with longer alkoxy chains tends to give rise to broad-temperature-range Col_h ionic liquid crystal phases.