Physical & Materials Chemistry Division, CSIR-National Chemical Laboratory , Pune 411008, India.
J Phys Chem B. 2017 Dec 21;121(50):11367-11375. doi: 10.1021/acs.jpcb.7b08240. Epub 2017 Dec 12.
To carry out a chemical reaction between the reactants with largely different polarities, it becomes important to have a reaction medium that possesses both the polar and nonpolar solvation environments. In an attempt to explore the reaction media with such unique polarity properties, the present study provides a thorough understanding of the bipolar solvent media using the aqueous mixtures of amino acid anion-based ionic liquids. The highly polar behavior of the binary mixtures used in the study has been ascribed to the pure ionic liquid state. However, the less polar solvation shells have been attributed to the presence of a neutral form of the anions. Addition of water in the amino acid anion-based ionic liquids causes the protonation of a certain fraction of the anions of the ionic liquids, resulting into the formation of a less polar nonionic protonated form along with the highly polar natural anionic form. This results into the formation of two solvation spheres with different polarities, which can be seen very clearly from the presence of two absorption bands (lower wavelength absorption band and higher wavelength absorption band) in the UV-vis absorption spectrum of Reichardt's E(30) dye and two emission bands (lower wavelength emission band and higher wavelength emission band) in the fluorescence emission spectrum of C481 dye. The values of the E polarity parameter corresponding to the two solvation shells having different polarities have been calculated from the deconvoluted absorption spectra of the Reichardt's E(30) and were analyzed in three amino acid anion-based ionic liquids. Generation of the neutral form of anions in the aqueous mixtures formed via a protonation transfer reaction has been confirmed by the H NMR spectroscopy and UV-vis absorption spectrum of 18DHAQ dye. The study also establishes that the Reichardt's E(30) dye can be used as a valid polarity probe to study the solvatochromic behavior of the binary mixtures of amino acid-based ionic liquids.
为了使反应物之间发生极性差异很大的化学反应，具有同时具有极性和非极性溶剂环境的反应介质就变得非常重要。为了探索具有这种独特极性性质的反应介质，本研究使用基于氨基酸阴离子的离子液体的水合混合物，彻底了解了双极溶剂介质。研究中使用的二元混合物的高极性行为归因于纯离子液体状态。然而，较少极性的溶剂化壳归因于阴离子的中性形式的存在。在基于氨基酸阴离子的离子液体中添加水会导致离子液体的一部分阴离子质子化，从而形成具有较少极性的非离子质子化形式以及高度极性的天然阴离子形式。这导致形成两个具有不同极性的溶剂化球，可以从 Reichardt 的 E(30)染料的紫外可见吸收光谱中存在两个吸收带（低波长吸收带和高波长吸收带）和 C481 染料的荧光发射光谱中存在两个发射带（低波长发射带和高波长发射带）非常清楚地看出。从 Reichardt 的 E(30)的解卷积吸收光谱中计算了对应于具有不同极性的两个溶剂化壳的 E 极性参数的值，并在三种基于氨基酸阴离子的离子液体中进行了分析。质子转移反应形成的水合混合物中阴离子的中性形式的生成已通过 18DHAQ 染料的 H NMR 光谱和紫外可见吸收光谱得到证实。该研究还表明，Reichardt 的 E(30)染料可用作研究基于氨基酸的离子液体的二元混合物的溶剂化变色行为的有效极性探针。
