Comparison of Electron-Transfer Rates for Metal- versus Ring-Centered Redox Processes of Porphyrins in Monolayers on Au(111)

The standard electron-transfer rate constants (k0) are measured for redox processes of Fe versus Zn porphyrins in monolayers on Au(111); the former undergoes a metal-centered redox process (conversion between FeIII and FeII oxidation states) whereas the latter undergoes a ring-centered redox process (conversion between the neutral porphyrin and the π-cation radical). Each porphyrin contains three meso-mesityl groups and a benzyl thiol for surface attachment. Under identical solvent (propylene carbonate)/electrolyte (1.0 M Bu4NCl) conditions, the ZnII center has a coordinated Cl− ion when the porphyrin is in either the neutral or oxidized state. In the case of the Fe porphyrin, two species are observed—a low-potential form (El0 ≈ −0.6 V) wherein the metal center has a coordinated Cl− ion when it is in either the FeII or FeIII state and a high-potential form (Eh0 ≈ +0.2 V) wherein the metal center undergoes ligand exchange upon conversion from the FeIII to FeII states. The k0 values observed for all of the ...