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

Well-defined monolayer islands of protonated meso-tetrakis(4-sulfonatophenyl)porphine (H2[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 (Hx[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 H2[H4TSPP] is significantly stronger than that of Hx[HyTCPP]. This increased interaction is also apparent in the fact that the H2[H4TSPP] monolayer has a much smaller unit cell. It is suggested that sulfonate groups may be more useful than c...