Cyclic Voltammetric Study of Heterogeneous Electron Transfer Rate Constants of Various Organic Compounds in Ionic liquids: Measurements at Room Temperature

Abstract

Room temperature ionic liquids (RTILs) are of growing interest due to their outstanding solvent properties. The high conductivity and large electrochemical window of RTILs have enabled their use in electrochemistry without adding supporting electrolyte. Heterogeneous electron transfer rate constants (k_het) and diffusion coefficients (D) of ferrocene, 2,6-dimethylbenzoquinone, bromanil, tetracyanoethylene, tetrathiofulvalene, methylviologen, and ethylviologen were determined in several RTILs such as [emim][BF₄], [bmim][OTf], [bmim][BF₄] and [bmim][PF₆] using cyclic voltammetry. The results obtained for k_het and D, range from 0.25–29.6 × 10⁴ cm s^-1 and 1.27–25.5 × 10⁸ cm² s^-1 respectively. Both were significantly lower than those found in organic solvents like acetonitrile (MeCN), dimethylformamide (DMF), etc. It was found that k_het and D were two to three orders of magnitude lower in more viscous RTILs. Diffusion coefficients were inversely proportional to the viscosity of the RTILs for all substances under investigation. Marcus theory was applied to compare the k_het. The main problem arising is to understand the role of solvent reorganization energy (λ_o). Whereas Marcus theory describes λ_o in two parts of polarization, a fast electronic and a slower orientational contribution both expressed by the Pekar factor γ = (1/n² - 1/ɛ_s), the solvent is treated as a continuum having a dielectric constant (ɛ_s) and a refractive index (n). Such a concept seems to be not applicable to ionic liquids.