The physical significance of the Kamlet-Taft: π ∗ parameter of ionic liquids

Abstract

The Kamlet-Taft dipolarity/polarizability parameters π∗ for various ionic liquids were determined using 4-tert-butyl-2-((dicyanomethylene)-5-[4-N,N-diethylamino)-benzylidene]-Δ3-thiazoline and 5-(N,N-dimethylamino)-5′-nitro-2,2′-bithiophene as solvatochromic probes. In contrast to the established π∗-probe N,N-diethylnitroaniline, the chromophores presented here show excellent agreement with polarity measurement using the chemical shift of 129Xe. They do not suffer from additional bathochromic UV/vis shifts caused by hydrogen-bonding resulting in too high π∗-values for some ionic liquids. In combination with large sets of various ionic liquids, these new chromophores thereby allow for detailed analysis of the physical significance of π∗ and the comparison to quantum-mechanical methods. We find that π∗ correlates strongly with the ratio of molar refractivity to molar volume, and thus with the refractive index. This journal is