Carboxylic acid functionalized imidazolium salts: sequential formation of ionic, zwitterionic, acid-zwitterionic and lithium salt-zwitterionic liquid crystals

Abstract

Carboxylic acid functionalized imidazolium salts, their deprotonated carboxylate zwitterions and acid-zwitterion adducts have been studied, and are denoted as [N-C_n,N′-CO₂H-Im][X] (C_n = C_nH_2n+1; X = Cl⁻, Br⁻, BF₄⁻ and PF₆⁻), [N-C_n,N′-CO₂-Im] and [(N-C_n,N′-CO₂-Im)₂H][anion] (anion = NO₃⁻ and Br⁻), respectively. Crystal structures of [N-C₁₀,N′-CO₂-Im]Br and [N-C_n,N′-CO₂-Im]·2H₂O studied by single-crystal X-ray diffraction indicate that carboxylic acid and carboxylate groups provide rich C–H⋯Br and/or C–H⋯O hydrogen bonding interactions in the construction of helical architecture. Except the PF₆⁻ salts, all the other types of compounds with long alkyl chains exhibit wide range SmA mesophase. Zwitterions and adducts represent two new classes of imidazolium based liquid crystals. The adducts [(N-C_n,N′-CO₂-Im)₂H]Br display wider mesophase range than the other series of compounds. Mixtures of lithium salts with zwitterion of [N-C₁₀,N′-CO₂-Im] form lithium containing liquid crystals at or near room temperature. A preliminary study of the mixtures of [N-C₁₀,N′-CO₂-Im]/[LiClO₄] (3 : 1) and [N-C₁₀,N′-CO₂-Im]/[LiBF₄] (3 : 1) placed between two ITO glasses shows that lithium ion conductivities increase upon increasing the temperature in the mesophase, yet decrease beyond the clearing temperature.