Fluorescence Quenching by Nitro Compounds within a Hydrophobic Deep Eutectic Solvent

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-n-butylammonium chloride (TBAC) as H-bond acceptor with n-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 n-decanoic acid. The dynamic quenching constant (K-D) and bimolecular quenching rate constant (k(q)) 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 In k g versus 1/T 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.