Long-alkyl-chain-derivatized imidazolium salts and ionic liquid crystals with tailor-made properties

Abstract

A series of new 1,3-dialkylimidazolium salts with the general formula [C_nC_nIM][A] (for n = 12; A = PF₆⁻, OTf⁻, NTf₂⁻; for n = 10, 14, 16, and 18; A = BF₄⁻, ClO₄⁻) has been synthesized. The [C₁₂C₁₂IM][A] salts are ionic liquids at room temperature, whereas all [C_nC_nIM][BF₄] and [C_nC_nIM][ClO₄] (n = 10, 14, 16, 18) salts demonstrate a liquid crystalline phase at elevated temperatures that have a large mesophase window, which varies from 10 to 80 °C with increasing alkyl chain length. In particular, [C₁₀C₁₀IM][BF₄] shows liquid crystalline behavior at room temperature and therefore is potentially suitable for application as a pre-organized reaction medium in synthesis and catalysis. Viscosity studies of [C₁₆C₁₆IM][BF₄] and the corresponding perchlorate salt demonstrate strong non-Newtonian viscosity behavior for the liquid crystalline state of these ionic liquids.