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

Carboxylic acid functionalized imidazolium salts, their deprotonated carboxylate zwitterions and acid-zwitterion adducts have been studied, and are denoted as [N-Cn,N′-CO2H-Im][X] (Cn = CnH2n+1; X = Cl−, Br−, BF4− and PF6−), [N-Cn,N′-CO2-Im] and [(N-Cn,N′-CO2-Im)2H][anion] (anion = NO3− and Br−), respectively. Crystal structures of [N-C10,N′-CO2-Im]Br and [N-Cn,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-Cn,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.