Ab-initio calculations and STM observations on tetrapyridyl and Fe(II)-tetrapyridyl-porphyrin molecules on Ag(1 1 1)

We modelled the adsorption of tetra-pyridyl-porphyrin (TPyP) and Fe(II)-tetra-pyridyl- porphyrin (Fe-TPyP) molecules on Ag(1 1 1). TPyP adsorbs in a flat geometry at a distance of 5.6 إ from the surface. The dihedral angle of the pyridyl rings is found to be 70°. The molecular structure remains largely unaffected by the adsorption. The structure of the molecular adlayer should thus be determined mainly by lateral intermolecular interactions. These conclusions are confirmed by scanning tunnelling microscopy observations. Upon metalation spin-polarized states with strong Fe d-character appear in Fe-TPyP near the centre of the gap between the highest occupied and the lowest unoccupied states of a TPyP molecule. We find also that the gap is widened in the presence of Fe. This implies a shift of 0.3 eV of unoccupied states as identified by scanning tunnelling spectroscopy simulations and confirmed by experimental data.