Hydroxyl Ionic Liquids: The Differentiating Effect of Hydroxyl on Polarity due to Ionic Hydrogen Bonds between Hydroxyl and Anions

The polarity of a series of ionic liquids (ILs) based on hydroxyethyl-imidazolium moiety with various anions ([PF6], [NTf2], [ClO4], [DCA] [NO3], [AC], and [CI]) and their corresponding nonhydroxyl ILs was investigated by solvatochronlic dyes and fluorescence probe molecules. Most of the nonhydroxyl ILs exhibit anion-independent polarity with similar E-T(30) in the narrow range of 50.7-52.6 kcal/mol, except [EMIm][AC] (49.7 kcal/mol). However, the polarity of the hydroxyl ILs covers a rather wide range (E-T(30) = 51.2-61.7 kcal/mol) and is strongly anion-dependent. According to their E-T(30) or E-T(33) values, the hydroxyl ILs can be further classified into the following three groups: (1) acetate-based hydroxyl ILs [HOEMI][AC] exhibit polarity scale (ET(30) = 5 1.2 kcal/mol) similar to short chain alcohol zinc] fall in the range of the nonhydroxyl ILs; (11) Hydroxyl ILs containing anions [NO3], [DCA], and [Cl] exhibit comparable polarity (E-T(30) = 55.5-56.9 kcal/mol), moderately higher than those of their nonhydroxyl ILs; (111) Hydroxyl ILs containing anions [PF6], [NTf2], and [ClO4] Possess unusual "hyperpolarity" (E-T(30) = 60.3-61.7 kcal/mol) close to protic ILs and water. Kamlet-Taft parameters and density functional theory calculations indicated that the greatly expanded range of polarity of hydroxyl ILs is correlated to all intramolecular synergistic solvent effect of the ionic hydrogen-bonded HBD/HBA complexes generated by intrasolvent HBD/HBA association between the anions and the hydroxyl group Oil Cations, Wherein hydroxyl group exhibits a Significant differentiating effect on the strength of H-boning and thus the polarity. Spiropyran-merocyanine equilibrium acted as a model polarity-sensitive reaction indeed Shows Obviously polarity-dependent solvatochromism, photochromism, and thermal reversion in hydroxyl ILs.