A Quenchofluorometric Investigation Reveals New Insights into Lithium Salt-Based Deep Eutectic Solvents

Lithium salt-based deep eutectic solvents (DESs) have been a topic of intense investigation for their imminent application potential in various electrochemical applications. DESs constituted of lithium <i>bis</i>(trifluoromethylsulfonyl)imide (LiTf₂N) as the H-bond acceptor (HBA) and urea, 2,2,2-trifluoroacetamide, and acetamide, respectively, as H-bond donors (HBDs) are investigated utilizing fluorescence from the probe pyrene and its quenching by nitromethane in the temperature range of 288 to 348 K. Both constitution (each of the three DESs in 1:2 mol ratio) and composition (LiTf₂N/acetamide DES in 1:2, 1:3, and 1:4 mol ratios) dependences are explored. Band 1-to-band 3 emission intensity ratios imply that the cybotactic region dipolarity is the highest for LiTf₂N/urea DES followed by LiTf₂N/acetamide and then LiTf₂N/2,2,2-trifluoroacetamide DESs. Increasing the relative amount of acetamide results in a decrease in dipolarity offered to pyrene by LiTf₂N/acetamide DES. Dipolarity offered by LiTf₂N DESs is relatively lower than that offered by choline chloride-based DESs as well as imidazolium-based ionic liquids. The fluorescence quenching data for the pyrene-nitromethane fluorophore-quencher pair for LiTf₂N-based DESs are in stark contrast to those for other common and popular polar hydrophilic DESs, ionic liquids, and isotropic solvent media. Lifetime quenching data within LiTf₂N-based DESs adhere to a simplistic Stern-Volmer formulation; quenching of steady-state intensity shows the presence of a quencher close to the excited fluorophore, forming a "dark" transient complex hinting at an apparent static quenching component. The outcomes highlight the solvation characteristics of LiTf₂N-based DESs that are different from those of other solvents, including other DESs and ionic liquids. The findings highlight the possible presence of anomalous solute-solvent interactions within LiTf₂N-based DESs.