Ionic liquids and ionic liquid crystals of vinyl functionalized imidazolium salts

Abstract

1-Vinyl-3-alkylimidazolium salts ([C_nVIm]X, (where, C_nVIm = vinylimidazolium cation with alkyl chains of C_nH_2n+1; n = 4, 6, 8, 10, 12, 14, 16 and 18 for X = Br and I, and n = 12, 14, 16 and 18 for X = BF₄ and PF₆), were prepared and characterized. Ethyl substituted congeners [C₁₆EIm]Br and [C₁₆EIm]I were also prepared to understand the unique property of the vinyl substitution. Salts with shorter alkyl chain lengths (n ≤ 12) are either room temperature or close to room temperature ionic liquids, whereas those with longer alkyl chain lengths are ionic liquid crystals with SmA mesophase. Diffractograms from powder X-ray diffraction studies suggest that in the solid state the [C_nVIm]Br series salts adopt a double bilayer structure, whereas the ethyl substituted analogues and salts in the I⁻, BF₄⁻ and PF₆⁻ series have a simple bilayer structure. The thermal behavior of [C₁₆VIm]Br and [C₁₆VIm]I was compared with their saturated congeners. The vinyl functionalized salts, have slightly higher melting points and much higher clearing points than those of the saturated congeners, and therefore wider mesophases are found for the formers. Nuclear magnetic resonance spectroscopic studies suggest that vinyl functionalization provides additional hydrogen bonding interactions between the cations and anions.