Mass transport and heterogeneous electron transfer of a ferrocene derivative in a room-temperature ionic liquid

Electrochemical and mass-transport properties of a redox-active ionic liquid, 1-(ferrocenylmethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (FcEmiTFSI), were investigated for the pure compound and for solutions in acetonitrile (ACN) and in a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmiTFSI). Cyclic voltammetry data were compared with those obtained for the oxidation of ferrocene (Fc) in EmiTFSI and ACN. The half-potential of FcEmiTFSI/Fc+EmiTFSI is higher than that of Fc/Fc+ in both media. The apparent heterogeneous rate constants kapp° of FcEmiTFSI and Fc were determined in both solvents. The values of kapp° for these two redox probes in the same solvent are of the same order of magnitude. However, kapp° is lower in EmiTFSI than in ACN. Comparison of kapp° in these solvents indicates that the decrease in kapp° cannot be explained in terms of Marcus–Hush theory. The electrochemical and mass-transport properties of FcEmiTFSI in EmiTFSI were investigated by means of scanning electrochemical microscopy (SECM). The tip current depends upon the tip velocity. With high tip velocity, one observes that the tip current increases as the distance between the tip and insulating substrate decreases. This behavior is not due to a positive feedback mechanism. It is interpreted in terms of mixed diffusion and convective mass transfer control.