The dipolarity/polarisability of 1-alkyl-3-methylimidazolium ionic liquids as function of anion structure and the alkyl chain length

Based on the developed tool to measure Kamlet-Taft polarity parameters alpha (hydrogen bond donating ability), beta (hydrogen bond accepting ability), and pi* (dipolarity/polarisability) for ionic liquids (ILs), it is now possible to precisely determine the influence of both cation and anion structure on pi*. alpha, beta, and pi* values of 38 1-alkyl-3-methylimidazolium ILs ([Rmim]X) are presented in this work to give a solid background for the discussion. The dipolarity/polarisability of [Rmim]X determined by means of the established pi*-sensitive solvatochromic probe 4-tert-butyl-2-(dicyanomethylene)-5-[4-(diethylamino)benzylidene]-Delta(3)-thiazoline is a blend of dipolarity and polarisability in equal parts. The large pi* parameters of ionic liquids are attributed to two cumulative effects: the ion pairing strength and the individual polarisabilities of both the cation and anion. This is shown by the influence of anion and cation structures (alkyl chain length of R) on the dipolarity/polarisability. For all studied ionic liquids we observed a general trend. The stronger the ion pairing effect, the greater is the determined pi* value.