Electron transfer kinetics in ferrocene-terminated self-assembled monolayers: The effect of ferrocene surface coverage variation

Herein, we report the synthesis and electrochemical characterization of seven ferrocene-terminated self-assembled monolayers (SAMs) on fluorine-doped tin oxide (FTO). The ferrocene surface coverage (Γ_Fc) of SAMs was systematically varied and its impact on the kinetics of interfacial electron transfer (ET) was investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It was found that as Γ_Fc gradually declined from ∼8.0 molecules/nm² (its maximum value) to ∼2.4 molecules/nm², the standard ET rate constant ($k_{\text{ET}}^0$, extracted from CV data) slowly declined from 4.9 s^-1 to 4.1 s^-1. However, with a further decrease in Γ_Fc, $k_{\text{ET}}^0$ abruptly dropped by ∼2.0 s^-1 and remained in the range of 2.2–2.7 s^-1. The charge transfer resistance (R_ct) of SAMs (determined from EIS data) was also found to follow a trend consistent with the one observed for $k_{\text{ET}}^0$. Density functional theory (DFT) calculations confirmed that the observed behavior of ET kinetics as a function of Γ_Fc can be attributed to the favorable interaction between adjacent Fc and Ph groups and the stabilization of the Fc HOMO. Besides exploring ET kinetics, Γ_Fc data was also employed to evaluate the kinetics of imine condensation reaction at the surface. The corresponding bimolecular rate constant for PhCHO ($k_{\text{PhCHO}}$) was found to be ∼6.2 times greater than that of FcCHO ($k_{\text{FcCHO}}$).