The polarity of a series of ionic liquids (ILs) based on hydroxyethyl-imidazolium moiety with various anions ([PF6], [NTf2], [ClO4], [DCA], [NO3], [AC], and [Cl]) and their corresponding nonhydroxyl ILs was investigated by solvatochromic dyes and fluorescence probe molecules. Most of the nonhydroxyl ILs exhibit anionindependent polarity with similar ET(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 (ET(30) ) 51.2-61.7 kcal/mol) and is strongly anion-dependent. According to their ET(30) or ET(33) values, the hydroxyl ILs can be further classified into the following three groups: (Ι) acetate-based hydroxyl ILs [HOEMIm][AC] exhibit polarity scale (ET(30) ) 51.2 kcal/mol) similar to short chain alcohol and fall in the range of the nonhydroxyl ILs; (II) Hydroxyl ILs containing anions [NO3], [DCA], and [Cl] exhibit comparable polarity (ET(30) ) 55.5-56.9 kcal/mol), moderately higher than those of their nonhydroxyl ILs; (III) Hydroxyl ILs containing anions [PF6], [NTf2], and [ClO4] possess unusual “hyperpolarity” (ET(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 an 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 on cations, wherein hydroxyl group exhibits a significant differentiating effect on the strength of H-bonding 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.

★★★☆☆ Zhang SG,Qi XJ,Ma XY,et al. Hydroxyl Ionic Liquids: The Differentiating Effect Of Hydroxyl On Polarity Due To Ionic Hydrogen Bonds Between Hydroxyl And Anions[J]. J. Phys. Chem. B,2010,114:3912-3920.