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 different influence of the -alkyl chain length of imidazolium- and tetraalkylammonium-based ILs on the dipolarity and HBA strength is confirmed by comparison with the N isotropic hyperfine coupling constants () of a positively () and negatively charged spin probe () of TEMPO-type [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl] and quantum chemically derived dipoles of the cations. The values correlate with the absorption energy of ThTCF and , but in different ways depending on the anion or charge of the spin probe. In a final discussion of the , , and scales in relation to ThTCF, the importance of the molar concentration of ionic liquids for the physical significance of the respective parameters is discussed.