Subpicosecond Solvation Response and Partial Viscosity Decoupling of Solute Diffusion in Ionic Acetamide Deep Eutectic Solvents: Fluorescence Up-Conversion and Fluorescence Correlation Spectroscopic Measurements

Abstract

Fluorescence up-conversion (∼250 fs instrumental response) coupled with time correlated single photon counting measurements was performed to explore the complete Stokes shift dynamics of a dipolar solute probe, coumarin 153 (C153), in several ionic acetamide deep eutectic solvents (DESs) that contained lithium nitrate/bromide/perchlorate as electrolyte. Combined measurements near room temperature reflected a total dynamic Stokes shift of approximately 800−1100 cm −1 and triexponential solvation response functions. Interestingly, the average rate of solvation became faster upon successive replacement of bromide by nitrate in these deep eutectics, and a subpicosecond time scale emerged in the measured solvation response when bromide was fully replaced by nitrate. Temperature dependent solute diffusion in these deep eutectics at the single molecule level, monitored by tracking the translational motion of rhodamine 6G (R6G) via fluorescence correlation spectroscopic (FCS) technique, revealed pronounced fractional viscosity dependence of the solute's translational motion. Subsequently, this partial decoupling of solute translation was attributed to the microheterogeneous nature of these ionic DESs after examining the diffusion− viscosity relationship via the FCS measurements of R6G in several normal solvents at room temperature and in a liquid amide solvent at different temperatures.

Evolution of microscopic heterogeneity and dynamics in choline chloride-based deep eutectic solvents

“…Hydrogen bonded networks (HBN) exist in pure HBDs, and these networks are characterized by unique local and supramolecular dynamics 21 – 23 . Moreover, in lithium salt and amide-based DESs, Kaur et al and Subba et al confirmed the existence of spatial and temporal nanoscale heterogeneity, respectively, which likely results from segregated ionic and HBD domains 24 , 25 . However, the evolution of such heterogeneities in DESs with composition as well as their resulting influence on macroscopic transport properties of DESs remain poorly understood.…”

Choline chloride and ethylene glycol based deep eutectic solvent (DES) versus hydroxyl functionalized room temperature ionic liquids (RTILs): assessing the differences in microscopic behaviour between the DES and RTILs

“…[74][75][76] Many researchers have also used FCS exclusively for the measurement of the translational diffusion of different probe molecules in DESs and RTILs, which provides information regarding the extent of solute-solvent association and the local environment of the medium. 29,52,[74][75][76][77][78] Therefore, in the present investigation, FCS studies have also been performed to get a better understanding of the probes' diffusion dynamics and intermolecular solutesolvent interaction at a single molecular level. Fig.…”

Deep eutectics and analogues as electrolytes in batteries

“…Mixtures of Li salts and amides are characterized by a well-studied micro-heterogeneity. 11,130,[138][139][140] The investigation of electrolytes composed Li salts (LiClO4, LiBr, LiNO3) with AA and other short-chain amides (propionamide PA and butyramide BA) via time-resolved fluorescence measurements and all-atom MD simulation revealed for instance pronounced solution heterogeneity. 11,130 In all cases the AA−ion interactions are stronger than the AA−AA ones.…”