Theoretical Analysis of the Effect of Surface Defects on Porphyrin Adsorption and Self-Assembly on Graphite

Previous scanning tunneling microscopy (STM) observations of porphyrin submonolayers deposited onto highly ordered pyrolytic graphite (HOPG) repeatedly detected the formation of self-assembled porphyrin ribbons. An important observation was that these nano-sized ribbons were frequently found in contact with topological defects in graphite; in particular, this was the case of meso-tetraphenylporphine (H₂TPP), its nickel(II) and cobalt(II) complexes, and hemin. These STM results led us to suggest that the modification in the electronic structure of HOPG caused by the presence of topological (e.g., surface curvature) and oxidized defects, might result in the preferential adsorption of porphyrin molecules, therefore serving as the starting points or nucleation sites for the self-assembled ribbons. In order to support this possibility, we performed first-approach theoretical calculations with both molecular mechanical (MM + force field) and density functional theory (DFT; PWC/DN theoretical level) methods to simulate two different kinds of defects on the graphene surface, and analyze their effects on the adsorption of H₂TPP as a model porphyrin molecule.