Theoretical investigation on the interaction between RhIII octaethylporphyrin and a graphite basal surface: a comparison study of DFT, DFT-D, and AFM

Using density functional theory based calculations and atomic-force-microscopy observations, we investigated the interaction between [Rh-III(OEP)(Cl)] (OEP = octaethylporphyrin) and a graphite basal surface, and the electronic structure of [Rh-III(OEP)(Cl)]/graphite. The [Rh-III(OEP)(Cl)] complex has an electronic structure effective for CO activation, possessing a closed singlet structure as its ground state; hence, both sigma-donation from the CO molecule (anode-reaction reactant) to Rh-III, and -back-donation from Rh-III to CO, occur, because the [Rh-III(OEP)(Cl)] complex does not have a singlet occupied molecular orbital on the porphyrin ring, the - stacking interaction between porphyrin and graphite is not present and their interaction is dominated by dispersion forces. The [Rh-III(OEP)(Cl)] complex easily diffused on the graphite basal surface, and an aggregated structure of [Rh-III(OEP)(Cl)] was observed by atomic force microscopy. The difference of the electronic structures of [Rh-III(OEP)(Cl)] before and after its adsorption is very small, the dispersion force being the dominant force for the adsorption. However, the lowest unoccupied molecular orbital of [Rh-III(OEP)(Cl)]/graphite is a sigma bonding orbital between Rh-III and graphite that will cause fast electron transfer from [Rh-III(OEP)(Cl)] to graphite during the CO electro-oxidation; this would be a reason why the carbon-supported [Rh-III(OEP)(Cl)] has high catalytic activity for CO electro-oxidation.