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

UV/Vis absorption data of (E)-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 (e.g. of N(CN)₂, BF₄, PF₆, N(Tf)₂, CF₃COO) as a function of the cation structure of ionic liquids. Several 1-n-alky-3-methylimidazolium- and tetraalkylammonium CH₃-NR₃⁺-based ILs with different n-alkyl chain lengths (R = -C₄H₉, -C₆H₁₁, -C₈H₁₇, -C₁₀H₂₁) are considered. UV/Vis absorption data of ThTCF show subtle correlations with hydrogen bond accepting (HBA) ability-related measurands such as Kamlet-Taft β, Freire's E_HB, and Laurence β₁ parameter as a function of anion and cation structure. The different influence of the n-alkyl chain length of imidazolium- and tetraalkylammonium-based ILs on the dipolarity and HBA strength is confirmed by comparison with the ¹⁴N isotropic hyperfine coupling constants (A_iso) of a positively (CATI) and negatively charged spin probe (TSKCr) of TEMPO-type [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl] and quantum chemically derived dipoles of the cations. The A_iso values correlate with the absorption energy of ThTCF and E_HB, but in different ways depending on the anion or charge of the spin probe. In a final discussion of the β, E_HB, and β₁ scales in relation to ThTCF, the importance of the molar concentration N of ionic liquids for the physical significance of the respective parameters is discussed.