Comparison of Electron-Transfer Rates for Metal- versus Ring-Centered Redox Processes of Porphyrins in Monolayers on Au(111)
By Jiao, Jieying; Schmidt, Izabela; Taniguchi, Masahiko; Lindsey, Jonathan S. & Bocian, David F.
Published in Langmuir
2008
Abstract
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 porphyrins depend on surface concentration, with higher concentrations resulting in slower rates, consistent with previous studies on porphyrin monolayers. The k0 values for the ring-centered redox process (Zn chelate) are 10-40 times larger than those for the metal-centered process (Fe chelate); the k0 values for the two forms of the Fe porphyrin differ by a factor of 2-4 (depending on surface concentration), the Cl- exchanging form generally exhibiting a faster rate. The faster rates for the ring- versus metal-centered redox process are attributed to the participating molecular orbitals and their proximity to the surface (given that the porphyrins are relatively upright on the surface): a π molecular orbital that has significant electron density at the meso-carbon atoms (one of which is the site of attachment of the linker to the surface anchoring thiol) versus a d-orbital that is relatively well localized on the metal center.