Direct Electron Transfer of Thiol-Derivatized Tetraphenylporphyrin Assembled on Gold Electrodes in an Aqueous Solution

Self-assembled monolayers (SAMs) of thiol-derivatized tetraphenylporphyrin (SH-TPP) and cobalt tetraphenylporphyrin (SH-CoTPP) have been prepared to examine the structure and direct electron transfer (ET) on a gold electrode in an aqueous solution. The cyclic voltammetry peaks of the porphyrin were obviously observed in aqueous solution. An interesting phenomenon was that direct ET cannot occur in phosphate-buffered solutions when a highly oriented and tightly packed monolayer is in existence, but the relatively low-density SH-TPP SAMs exhibited good electrochemical response. The electron transfer rate constants were obtained from the impedance spectra (Cole-Cole plot), upon application of different biasing potentials to the modified electrode. We propose a new kinetic model of the ET processes of SAMs of thiol-derivatized porphyrin and apply the density functional theory (DFT) to study the ET of porphyrin molecules on Au surfaces.