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

The Kamlet-Taft dipolarity/polarizability parameters pi* for various ionic liquids were determined using 4-tert-butyl-2-((dicyanomethylene)-5-[4-N,N-diethylamino)-benzylidene]-Delta 3-thiazoline and 5-(N,N-dimethylamino)-5 '-nitro-2,2 '-bithiophene as solvatochromic probes. In contrast to the established pi*-probe N,N-diethylnitroaniline, the chromophores presented here show excellent agreement with polarity measurement using the chemical shift of Xe-129. They do not suffer from additional bathochromic UV/vis shifts caused by hydrogen-bonding resulting in too high pi*-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 pi* and the comparison to quantum-mechanical methods. We find that pi* correlates strongly with the ratio of molar refractivity to molar volume, and thus with the refractive index.