Observation of the Marcus inverted region of electron transfer reactions at a liquid/liquid interface

Scanning electrochemical microscopy was used to probe the influence of a driving force on the heterogeneous electron transfer (ET) processes at the externally polarized water/1,2-dichloroethane interface. Being a part of the driving force, the Galvani potential difference at the interface, Δ_o^wφ, can be quantitatively controlled in a wide range, allowing the precise measurements of the rate constants of the ET reactions. Two opposite systems were chosen in this work. One was 5,10,15,20-tetraphenyl 21H,23H-porphyrin zinc (ZnPor, O)/Fe(CN)₆^4- (W), and the other was TCNQ (O)/Fe(CN)₆^3- (W). For both systems studied, the relations between the rate constant and the Δ_o^wφ were of parabolic shape; that is, the rate constants increased initially with the Δ_o^wφ until reaching a maximum and then decreased steadily as the Δ_o^wφ increased further. This is in accordance with the prediction of the Marcus theory. To our knowledge, this is the first report that the Marcus inverted region can be observed electrochemically at an unmodified liquid/liquid interface with only one redox couple at each phase. The effect of the concentrations of the organic supporting electrolyte has also been discussed in detail.