Effects of alkyl chain length and anion size on thermal and structural properties for 1-alkyl-3-methylimidazolium hexafluorocomplex salts (CxMImAF6, x = 14, 16 and 18; A = P, As, Sb, Nb and Ta)
作者:Fei Xu、Kazuhiko Matsumoto、Rika Hagiwara
DOI:10.1039/c2dt11693f
日期:——
A series of 1-alkyl-3-methylimidazolium hexafluorocomplex salts (CxMImAF6, x = 14, 16 and 18, A = P, As, Sb, Nb and Ta) have been characterized by thermal analysis, X-ray diffraction and polarized optical microscopy. A liquid crystalline mesophase is observed for all the C16MIm and C18MIm salts. The C14MIm+ cation gives a liquid crystalline mesophase only with PF6−. The temperature range of the liquid crystalline mesophase increases with an increase in alkyl chain length or with decrease in anion size. Single-crystal X-ray diffraction revealed that all the C18MImAF6 salts (A = P, As, Sb, Nb and Ta) are isostructural with each other in the crystalline phase and have a layered structure. The interdigitated alkyl chain of the cation has a bent shape like a spoon near the imidazolium ring in the crystalline phase at −100 °C and is tilted with respect to the sheets of the imidazolium headgroups and anions. An increase of temperature increases the ratio of an all-trans conformation to the bent conformation in the crystalline phase. X-ray diffraction and polarized optical microscopy suggested that the liquid crystalline mesophase has a smectic A2 structure. The interlayer distance increases with a decrease in the anion size since the smaller anion has a stronger coulombic interaction with the imidazolium headgroup, resulting in the decrease of the interdigitated part to give a larger layer spacing.
一系列 1-烷基-3-甲基咪唑鎓六氟复合物盐(CxMImAF6,x = 14、16 和 18,A = P、As、Sb、Nb 和 Ta)已通过热分析、X 射线衍射和偏振光学显微镜进行了表征。在所有 C16MIm 和 C18MIm 盐中都观察到了液晶介相。C14MIm+ 阳离子只有在与 PF6- 反应时才会产生液晶介相。液晶介相的温度范围随着烷基链长度的增加或阴离子尺寸的减小而增大。单晶 X 射线衍射显示,所有 C18MImAF6 盐(A = P、As、Sb、Nb 和 Ta)在结晶相中彼此同构,并具有层状结构。在零下 100 ℃ 的结晶相中,阳离子的相互咬合的烷基链在咪唑环附近呈勺状弯曲,相对于咪唑头基和阴离子的薄片呈倾斜状。温度升高会增加全反式构象与结晶相中弯曲构象的比例。X 射线衍射和偏振光学显微镜表明,液晶介相具有共晶 A2 结构。由于较小的阴离子与咪唑头基团的库仑相互作用较强,层间距离随阴离子尺寸的减小而增大,从而导致相互咬合的部分减小,层间距增大。