Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India.
J Phys Chem B. 2020 May 21;124(20):4164-4173. doi: 10.1021/acs.jpcb.0c02231. Epub 2020 May 6.
Hydrophobic deep eutectic solvents (DESs) exhibit immense potential as viable environmentally benign inexpensive alternatives to both nonpolar organic solvents as well as hydrophobic ionic liquids. Pyrene fluorescence and its quenching by five different nitro compounds are used as a tool to examine structural features and solute dynamics within a prototypical hydrophobic DES formed by mixing salt tetra--butylammonium chloride (TBAC) as H-bond acceptor with -decanoic acid (DA) as H-bond donor in 1:2 mol ratio, named TBAC-DA, in the temperature range 298.15-358.15 K. Changes in fluorescence emission intensity, empirical polarity scale, and excited-state intensity decay of pyrene with change in temperature within TBAC-DA are compared and contrasted with those reported within common and popular hydrophilic DESs and water miscible and immiscible ionic liquids. All five nitro compounds-nitromethane, nitrobenzene, 4-nitrobenzaldehyde, 1-chloro-4-nitrobenzene, and 4-nitroanisole-quench the fluorescence from pyrene in TBAC-DA; the quenching follows a simplistic Stern-Volmer relation and is purely dynamic in nature. Quenching of pyrene fluorescence by nitromethane is more within TBAC-DA as compared to the hydrophilic DES reline and ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. It is attributed to the possible stabilization of partial positive charge that develops on the excited pyrene during electron/charge transfer to the quencher by partially/completely dissociated acid groups of the H-bond donor -decanoic acid. The dynamic quenching constant () and bimolecular quenching rate constant () within TBAC-DA are significantly higher for nitrobenzene in comparison to the other four quenchers. Conformity to the empirical Arrhenius expression is exhibited by the linear behavior of ln versus 1/ for all five nitro compounds. While overall pyrene-quencher data does not comply with the Stokes-Einstein relation, each of the pyrene-quencher pair data does. This suggests the dependence of diffusion behavior on the structure of the quencher within TBAC-DA. Pyrene fluorescence is established as an effective tool to characterize such DESs; the DESs can be used as solubilizing media to detect and assess the important class of nitro compounds.
疏水深共晶溶剂 (DESs) 作为非极性有机溶剂和疏水性离子液体的可行、环保、廉价替代品，具有巨大的潜力。使用芘荧光及其被五种不同硝基化合物猝灭作为工具，研究了在由盐四丁基氯化铵 (TBAC) 作为氢键受体与癸酸 (DA) 作为氢键供体以 1:2 摩尔比混合形成的典型疏水 DES 中，结构特征和溶质动力学，该 DES 命名为 TBAC-DA，温度范围为 298.15-358.15 K。比较了在 TBAC-DA 中温度变化时芘的荧光发射强度、经验极性标度和激发态强度衰减的变化，并与常见和流行的亲水性 DES 和水溶性和不溶性离子液体中的变化进行了对比。五种硝基化合物——硝基甲烷、硝基苯、4-硝基苯甲醛、1-氯-4-硝基苯和 4-硝基苯甲醚——猝灭 TBAC-DA 中的芘荧光；猝灭遵循简单的 Stern-Volmer 关系，本质上是纯动力学的。与亲水性 DES reline 和离子液体 1-乙基-3-甲基咪唑双(三氟甲基磺酰基)亚胺相比，硝基甲烷在 TBAC-DA 中猝灭芘荧光的程度更大。这归因于部分/完全离解的氢键供体癸酸的酸基团在电子/电荷转移到猝灭剂时，可能会稳定激发芘上的部分正电荷。与其他四种猝灭剂相比，TBAC-DA 中硝基苯的动态猝灭常数 () 和双分子猝灭速率常数 () 显著更高。对于所有五种硝基化合物，ln 与 1/ 的线性关系都符合经验 Arrhenius 表达式。虽然整体芘-猝灭剂数据不符合 Stokes-Einstein 关系，但每个芘-猝灭剂对数据都符合。这表明在 TBAC-DA 中，扩散行为取决于猝灭剂的结构。芘荧光被确立为表征此类 DES 的有效工具；DES 可用作增溶介质，以检测和评估重要的硝基化合物类。
