Hydroxyl-induced partial charge states of single porphyrins on titania rutile

The adsorption of Cu–porphyrin derivatives terminated with peripheral carboxyphenyl side groups on hydroxylated TiO_{2}(110) surfaces is investigated by combined scanning tunneling microscopy (STM), atomic force microscopy (AFM), and density functional theory (DFT). Two distinct contrasts of the molecules are revealed by STM and tunneling spectroscopy. Via single-molecule manipulations, the origin of these peculiar contrasts is found to arise from the presence or not of hydroxyl groups below the molecules. Hence, the electronic coupling of the molecule with the underlying TiO_{2} surface is locally modified altering the amount of charge transfer and thus their charge state at the molecular scale. Our results particularly underline the fundamental role of hydroxyls of TiO_{2} on the charge state of adsorbed organic molecules potentially used in dye-sensitized solar cells.