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

Room temperature ionic liquids (RTILs) are of growing interest due to their outstanding solventproperties. The high conductivity and large electrochemical window of RTILs have enabled theiruse in electrochemistry without adding supporting electrolyte. Heterogeneous electron transferrate constants (khet) and diffusion coefficients (D) of ferrocene, 2,6-dimethylbenzoquinone, bro-manil, tetracyanoethylene, tetrathiofulvalene, methylviologen, and ethylviologen were determinedin several RTILs such as [emim][BF4], [bmim][OTf], [bmim][BF4] and [bmim][PF6] using cyclicvoltammetry. The results obtained forkhetandD, range from 0.25–29.6×10−4cm s−1and1.27–25.5×10−8cm2s−1respectively. Both were significantly lower than those found in organicsolvents like acetonitrile (MeCN), dimethylformamide (DMF),etc.It was found thatkhetandDwere two to three orders of magnitude lower in more viscous RTILs. Diffusion coefficientswere inversely proportional to the viscosity of the RTILs for all substances under investigation.Marcus theory was applied to compare thekhet. The main problem arising is to understand therole of solvent reorganization energy (λo). Whereas Marcus theory describesλoin two parts ofpolarization, a fast electronic and a slower orientational contribution both expressed by the Pekarfactorγ=(1/n2−1/εs), the solvent is treated as a continuum having a dielectric constant (εs)anda refractive index (n). Such a concept seems to be not applicable to ionic liquids