Scanning Tunneling Microscopy of Porphyrin-Based Molecules on TiO2 Surfaces

The conformational features of bis(3,5-di-t-butyphenyl)(4-ethynylphenyl)(methylthiophenyl)porphyrin (EMTBPP) molecules on rutile TiO2(110)-(1 ×1) surfaces were investigated by scanning tunneling microscopy under ultrahigh vacuum conditions. The EMTBPP molecules are successively observed at single-molecule resolution. At lower coverage, the EMTBPP molecules seem to be selectively adsorbed to oxygen vacancy places in the oxygen row due to an attractive force between the center of the porphyrin ring and the oxygen vacancy. At higher coverages, periodical and orientational ordering of the EMTBPP molecules along the (1 ×1) surface structure occurs. The effects of molecule–substrate and intermolecular interactions on the conformational features are discussed.