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.

The solvatochromic ThTCF probe responds to anion coordination. Correlations of UV/Vis data from ThTCF with various basicity-related polarity scales (Kamlet-Taft β, Freire EHB, Laurence β1) allow an evaluation of their physical significance.

Solvatochromic measurements and 1H NMR spectroscopy have been used to investigate the overall polarity of tetraalkylammonium based ionic liquids and low temperature molten salts.