Molecular orbital shift of perylenetetracarboxylic-dianhydride on gold

Scanning tunneling microscopy and spectroscopy reveal that the energy of orbitals of individual perylenetetracarboxylic-dianhydride molecules depends on their adsorption geometry. Internal molecular structure appears at characteristic energies for each of the coexisting adsorption domains on Au(7 8 8) and Au(1 1 1). Tunneling spectroscopy on single molecules belonging to different adsorption domains exhibits an energy shift of the lowest and second-to-lowest unoccupied molecular orbital of ≈0.35 eV. On the basis of density functional theory calculations a possible contribution to this shift can be traced back to hydrogen bond-mediated intermolecular interaction. Hence, tunneling spectroscopy can be used as an efficient tool to discriminate between phases of organic molecular layers.