The influence of the cation structure on the basicity-related polarity of ionic liquids.

UV/Vis absorption data of ()-4-(2-[5-{4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl}thiene-2-yl]vinyl)-2-(dicyano-methylene)-3-cyano-5,5-dimethyl-2,5-dihydrofuran (ThTCF) as a solvatochromic probe is applied to examine the anion coordination strength ( of N(CN), BF, PF, N(Tf), CFCOO) as a function of the cation structure of ionic liquids. Several 1--alky-3-methylimidazolium- and tetraalkylammonium CH-NR-based ILs with different -alkyl chain lengths (R = -CH, -CH, -CH, -CH) are considered. UV/Vis absorption data of ThTCF show subtle correlations with hydrogen bond accepting (HBA) ability-related measurands such as Kamlet-Taft , Freire's , and Laurence parameter as a function of anion and cation structure.

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

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.