Adsorption of Phosphonic-Acid-Functionalized Porphyrin Molecules on TiO₂(110)

We studied the adsorption geometry and bonding of mono- and tetraphosphonic-acid-functionalized tetraphenylporphyrin molecules on rutile TiO 2 (110) surfaces. The investigation was carried out by means of X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure (NEXAFS) measurements. We found that the molecules bind covalently to the surface in a mixed monodentate and bidentate mode implying deprotonation of one or two phosphonate hydroxyl groups. Our NEXAFS data suggest that molecules containing one functional phosphonic acid group are on average tilted further away from the surface than molecules with four functional groups. The highest occupied electronic state of both molecules is in the band gap at 2.1 eV below the Fermi level. Our results demonstrate that the number of functional phosphonic acid groups determine the adsorption geometry of tetraphenylporphyrins.