A Self-Assembled Two-Dimensional Zwitterionic Structure: H6TSPP Studied on Graphite

Well-defined monolayer islands of protonated meso-tetrakis(4-sulfonatophenyl)porphine (H-2[H4TSPP]) self-assembled on highly oriented pyrolytic graphite (HOPG) from 0.75 M HCl solutions are studied in ultrahigh vacuum using scanning tunneling microscopy (STM), orbital-mediated tunneling spectroscopy (OMTS), ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS). HOPG proved to be a superior substrate because it does not form a thin surface chloride in the presence of HCl, as does Au(111). Unlike meso-tetrakis(4-carboxyphenyl)porphine (H-x[HyTCPP]), the carboxylate analog, monolayers are stable on HOPG and can be studied at room temperature without the addition of a second stabilizing compound. Thus, the surface organizational energy for H-2[H4TSPP] is significantly stronger than that of Hx[HyTCPP]. This increased interaction is also apparent in the fact that the H-2[H4TSPP] monolayer has a much smaller unit cell. It is suggested that sulfonate groups may be more useful than carboxylates for creating self-assembled structures on HOPG. Protonation of the porphyrin nitrogens in the surface species is confirmed by XPS. High-resolution images of single molecule layers show a well-defined deformation of the porphyrin ring, as expected with complete protonation of the central nitrogen atoms. OMTS and UPS were used to identify the HOMO and LUMO of the H-2[H4TSPP] monolayer species, and results are contrasted to those of nickel(II) tetraphenylporphyrin (NiTPP). H-2[H4TSPP] previously has been shown to form well-defined nanorods on HOPG. I-V curves of single and stacked rods taken by STM are consistent with conduction in a band formed from the LUMO of H-2[H4TSPP].