Electron-Transfer Studies of Model Redox-Active Species (Cationic, Anionic, and Neutral) in Deep Eutectic Solvents

The redox potentials of electroactive species are significantly influenced by the solvation characteristics of the medium. This is manifested in the shift of half-peak potentials with the change in the solvent medium. There have been many approaches till date, both experimental and theoretical to understand the role of molecular solvents in the peak potentials of redox species. The electrochemical studies reported here are aimed at understanding the effect of deep eutectic solvents (DESs) which is distinct from conventional solvents in terms of highly concentrated ionic composition, on the half-peak potentials of some standard redox reactions. The redox species selected for this study are distinct either in terms of their charge [Fe(CN)(6)(4-/3-), Ru(NH3)(6)(2+/3+), and ferrocene methanol, FcMeOH(0/+)] or their hydrophilic/hydrophobic properties [methyl viologen and ferrocene]. The redox potentials are compared with the values obtained in the aqueous medium which is very well characterized in terms of solvent reorganization energy and free-energy changes. The cyclic voltammetric behavior of the redox species in DES is significantly different from that of aqueous medium. The diffusion coefficients of the redox species in DES measured by EIS and cyclic voltammetry showed significant deviations from that predicted by Stokes-Einstein equation, indicating the dominant effect of Coulombic interactions within the components of DES.