Electrochemistry of self-assembled monolayers of iron protoporphyrin IX attached to modified gold electrodes through thioether linkage

Iron(III) protoporphyrin IX (Fe(III)PP) and iron(III) hematoporphyrin (Fe(I II)HP) were selectively and covalently attached to dimercaptoalkane-modifie d gold electrodes. Reaction of their vinyl or hydroxyethyl groups with the surface-immobilized thiols produced thioether linkages, reminiscent of the heme macrocycle attachment in c-type cytochromes. Cyclic voltammetry reveal ed reversible electrochemistry of self-assembled monolayers (SAMs) of FePPs and FeHPs on the thiol-modified gold substrates. The surface coverage esti mated from the charges transferred corresponds to 30% of a monolayer. The h eterogeneous rate constant of electron transfer between the Fe(III)PPs and the gold substrate decreases exponentially with the length of the spacer, d emonstrating a value of 1.0 Angstrom(-1) for the tunneling length coefficie nt, beta. At pH 8, a linear increase of the formal redox potential (E-o') w ith the Length of the linker was also observed. This suggests that in the f ilm, there is a dose contact between the porphyrins and the alkane SAM: the E-o' is affected by the drop of the electrostatic potential from the elect rode to the surface of the alkane SAM, and also that there is a strong ion pairing of the Fe(III)PPs in the film with the anions of the solution. The E-o' of Ft (III)PPs in the SAM shows a strong and complex dependency on the pH of the solution, explained by variations in the coordination of the iro n, involving hydroxyl ions, water, and eventually dioxygen molecules. Inter actions of the iron with either functional groups present at the surface of the substrate or with the propionate groups attached to the porphyrin ring , do not appear to be involved in the electron-proton transfer coupling mec hanisms.