Center for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China.
J Phys Chem B. 2010 Mar 25;114(11):3912-20. doi: 10.1021/jp911430t.
The polarity of a series of ionic liquids (ILs) based on hydroxyethyl-imidazolium moiety with various anions ([PF(6)], [NTf(2)], [ClO(4)], [DCA], [NO(3)], [AC], and [Cl]) and their corresponding nonhydroxyl ILs was investigated by solvatochromic 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: (Iota) acetate-based hydroxyl ILs [HOEMIm][AC] exhibit polarity scale (E(T)(30) = 51.2 kcal/mol) similar to short chain alcohol and fall in the range of the nonhydroxyl ILs; (II) Hydroxyl ILs containing anions [NO(3)], [DCA], and [Cl] exhibit comparable polarity (E(T)(30) = 55.5-56.9 kcal/mol), moderately higher than those of their nonhydroxyl ILs; (III) Hydroxyl ILs containing anions [PF(6)], [NTf(2)], and [ClO(4)] 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 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.
基于羟乙基-咪唑鎓部分的一系列离子液体（ILs）及其相应的非羟 ILs 的极性通过溶剂化染料和荧光探针分子进行了研究。大多数非羟 ILs 表现出与阴离子无关的极性，在 50.7-52.6 kcal/mol 的狭窄范围内具有相似的 E(T)(30)，除了 [EMIm][AC]（49.7 kcal/mol）。然而，羟基 ILs 的极性范围相当广泛（E(T)(30) = 51.2-61.7 kcal/mol），并且强烈依赖于阴离子。根据它们的 E(T)(30)或 E(T)(33)值，羟基 ILs 可以进一步分为以下三组：（Iota）乙酸盐基羟基 ILs [HOEMIm][AC] 表现出与短链醇相似的极性范围（E(T)(30) = 51.2 kcal/mol），属于非羟 ILs 范围；（II）含有阴离子 [NO(3)]、[DCA] 和 [Cl] 的羟基 ILs 表现出可比的极性（E(T)(30) = 55.5-56.9 kcal/mol），略高于它们的非羟 ILs；（III）含有阴离子 [PF(6)]、[NTf(2)] 和 [ClO(4)] 的羟基 ILs 具有不寻常的“超极性”（E(T)(30) = 60.3-61.7 kcal/mol），接近于质子性 ILs 和水。Kamlet-Taft 参数和密度泛函理论计算表明，羟基 ILs 极性范围的大大扩展与离子氢键 HBD/HBA 复合物的分子内协同溶剂效应有关，该效应是由阴离子与阳离子上的羟基之间的溶剂内 HBD/HBA 缔合产生的，其中羟基对氢键强度表现出显著的区分作用，从而影响极性。螺吡喃-变色酸平衡确实作为一种模型极性敏感反应表现出明显的与溶剂有关的变色、光致变色和热反转，在羟基 ILs 中。
