Distinct influence of the anion and ether group on the polarity of ammonium and imidazolium ionic liquids
作者:Shiguo Zhang、Zhengjian Chen、Xiujuan Qi、Youquan Deng
DOI:10.1039/c2nj20965a
日期:——
The polarity of ionic liquids (ILs), usually denoted as ET(30) by the solvatochromic probe Reichardt’s dye, is one of the most fundamental properties that remarkably affect the solvation and chemical reaction in ILs. It was generally accepted that the ET(30) of ILs was dominated by the nature of the cation. However, in this work, it was found that the common ammonium-based ILs showed strongly anion-dependent ET(30). For example, the ET(30) value for [N1124][DCA] and [N1124][NTf2] is 49.0 and 59.0 kcal mol−1, respectively, while the corresponding imidazolium ILs bearing the same anions possess nearly identical ET(30), the ET(30) value for [BMIm][DCA] and [BMIm][NTf2] is 51.4 and 51.6 kcal mol−1, respectively. Moreover, introduction of an ether group was found to increase the ET(30) of imidazolium ILs while having no obvious effect on that of ammonium-based ILs. The Kamlet–Taft parameters and density functional theory (DFT) calculations indicated that the distinct result is related to different stabilization of the ground state of Reichardt's dye 30. In imidazolium ILs, the main interactions between ILs and zwitterionic dye involve both coulombic interaction (between the cation and the phenolate oxygen atom) and H-bonding interaction (between the acidic hydrogen on imidazolium ring and the phenolate oxygen atom). However, with the ammonium ILs lack of active hydrogen, the dye is only stabilized by the coulombic interaction between the cation and the phenolate oxygen atom. Interestingly, in both imidazolium and ammonium-based ILs, the spiropyran–merocyanine equilibrium exhibit obvious anion-dependent photochromism, solvatochromism, and thermal relaxation.
离子液体(ILs)的极性通常用溶剂色谱探针Reichardt染料的ET(30)表示,这是影响ILs中溶剂化和化学反应的重要基本属性之一。一般认为,ILs的ET(30)主要由阳离子的性质主导。然而,在这项研究中发现,常见的基于铵的ILs表现出明显依赖阴离子的ET(30)。例如,[N1124][DCA]和[N1124][NTf2]的ET(30)值分别为49.0和59.0 kcal mol−1,而对应的带有相同阴离子的咪唑鎓ILs的ET(30)几乎相同,[BMIm][DCA]和[BMIm][NTf2]的ET(30)值分别为51.4和51.6 kcal mol−1。此外,引入醚基团被发现可以提高咪唑鎓ILs的ET(30),而对铵基ILs则没有明显影响。Kamlet–Taft参数和密度泛函理论(DFT)计算表明,结果的差异与Reichardt染料30基态的不同稳定性有关。在咪唑鎓ILs中,ILs与具有二氨离子的染料之间的主要相互作用包括库仑相互作用(阳离子与酚氧原子之间)和氢键相互作用(咪唑环上的酸性氢与酚氧原子之间)。然而,由于铵ILs缺乏活性氢,染料只能通过阳离子与酚氧原子之间的库仑相互作用来稳定。有趣的是,在咪唑鎓和铵基ILs中,spiropyran-merocyanine平衡表现出明显的阴离子依赖性光致变色、溶剂色谱效应和热弛豫现象。