Deep eutectic solvents (DESs) have shown promise as environmentally benign and inexpensive media with superior properties. Because of their structural features, they have potential to be versatile alternatives to the conventional electrolytes in various applications in science and technology. A mixture of a common and popular DES Reline, composed of salt choline chloride and H-bond donor urea in a 1:2 mole ratio, and lithium salt LiCl is investigated in a 298.15−358.15 K temperature range using a well-known multidimensional fluorescence probe pyrene. The band 1-toband 3 emission intensity ratio (Py I 1 /I 3 ) at a given temperature reveals no change in the dipolarity of the pyrene cybotactic region of Reline on addition of up to 2.093 m(LiCl). Decrease in dipolarity of the medium on increasing temperature becomes less pronounced in the presence of LiCl. Excited-state intensity decay of pyrene fits best to a two-exponential decay equation irrespective of the temperature and LiCl concentration. While the shorter of the decay times does not vary with temperature, the longer one shows decrease with increasing temperature that is independent of the LiCl concentration. At a given temperature, addition of LiCl results in a slight decrease in the longer decay time because of the presence of Cl − , which facilitates nonradiative decay pathways of excited pyrene. Decrease in the pyrene decay time by an electron/charge acceptor quenching agent nitromethane is found to obey the Stern−Volmer equation, implying the quenching to be purely dynamic in nature. The estimated bimolecular quenching rate constant (k q ) first increases as LiCl is added to Reline before decreasing monotonically on further addition of LiCl. The decrease in k q with increasing LiCl is attributed to the exponential increase in the dynamic viscosity of Reline with LiCl addition. The initial increase is due to the stabilization of the partial positive charge that develops on excited pyrene during the electron/charge transfer to nitromethane by the added Cl − during the quenching process. The Stokes−Einstein equation is not obeyed within LiCl-added Reline, but it is found to be followed at a given LiCl concentration. Iso-viscous LiCl-added Reline mixtures are found to have significantly different k q values, suggesting the important role of LiCl in controlling the bimolecular quenching process within the system.

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Deep eutectic solvents (DESs) have emerged as nontoxic and inexpensive alternatives not only to the common organic solvents but to the ionic liquids as well. Some of the common and popular, and perhaps the most investigated, DESs are the ones comprising an ammonium salt and an appropriate hydrogen bond (HB) donor in a predetermined mole ratio. The formation of the DES is attributed to the H-bonding interaction(s) present between the salt and the HB donor. Consequently, addition of a predominantly HB donor or a predominantly HB acceptor cosolvent to such DESs may result in intriguing features and properties. We present investigation of two DESs constituted of salt choline chloride along with HB donors urea and glycerol, respectively, in 1:2 mol ratio, named reline and glyceline as the cosolvent of very high HB donating acidity and no HB accepting basicity 2,2,2-trifluoroethanol (TFE) and of very high HB accepting basicity and no HB donating acidity hexamethylphosphoramide (HMPA), respectively, is added. TFE shows up to 0.25 mole fraction miscibility with both reline and glyceline. While up to 0.25 mole fraction HMPA in glyceline results in transparent mixtures, this cosolvent is found to be completely immiscible with reline. From the perspective of the solvatochromic absorbance and fluorescence probes, it is established that the cybotactic region dipolarity within up to 0.25 mole fraction TFE/HMPA-added DES strongly depends on the functionalities present on the solute. Fourier transform infrared absorbance and Raman spectroscopic investigations reveal no major shifts in vibrational transitions as TFE/HMPA is added to the DES; spectral band broadening, albeit small, is observed nonetheless. Excess molar volumes and excess logarithmic viscosities of the mixtures indicate that while TFE may interstitially accommodate itself within H-bonded network of reline, it does appear to form H-bonds with the constituents of the glyceline. Increase in overall net repulsive interactions as HMPA is added to glyceline is suggested by both positive excess molar volumes and excess logarithmic viscosities. The addition of HB donor/acceptor cosolvent appears to disturb the salt-HB donor equilibria within DES via complex interplay of interactions within the system.

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 ln k q 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.

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