Effect of lithium salt on fluorescence quenching in glycerol: a comparison with ionic liquid/deep eutectic solvent

A comparison of physical properties and fluorescence quenching behavior between glycerol with a deep eutectic solvent (DES) and an ionic liquid (IL) in the presence of lithium salt as media is presented. It is reported earlier that addition of LiCl to the DES ChCl:Urea (composed of salt choline chloride and H-bond donor urea in 1 : 2 mole ratio) and addition of LiTf2N [Tf2N: (CF3SO2)2N] to the IL [C2C1im][Tf2N] ([C2C1im]: 1-ethyl-3-methylimidazolium), respectively, results in increase in the dynamic viscosity of the medium. However, as the concentration of the Li salt is increased, instead of decreasing, the bimolecular quenching rate constant (k_q) for quenching of pyrene fluorescence by nitromethane is observed to first increase and only then decreases within both the media. This unusual initial increase in quenching is hypothesized to the structural changes in the DES ChCl:Urea and the IL [C2C1im][Tf2N], respectively, as the Li salt was added. We tested this hypothesis by comparing this outcome with that within 1 wt% water in glycerol solution having similar viscosity as that of the DES ChCl:Urea. In complete contrast, irrespective of the temperature, k_q is found to decrease monotonically with increasing concentration of LiCl within 1 wt% water in glycerol media. The structural reorganization leading to anionic clustering within the media dominates in DES ChCl:Urea and IL [C2C1im][Tf2N] and overcomes the effect of increasing viscosity. Such anionic clustering is either absent or takes place at relatively lesser extent in LiCl added 1 wt% water in glycerol such that the charge transfer quenching process is dominated entirely by the viscosity in this media. The ionic nature of the IL and the high concentration of ions in the DES are deemed responsible for these outcomes. These findings highlight the unusual characteristics of ILs and DESs as solubilizing media.