The adsorption of porphyrin derivatives
on a Au(111) surface was
studied by scanning tunneling microscopy and spectroscopy at low temperatures
in combination with density functional theory calculations. Different
molecular appearances were found and could be assigned to the presence
of single gold adatoms bonded by a coordination bond underneath the
molecular monolayer, causing a characteristic change of the electronic
structure of the molecules. Moreover, this interpretation could be
confirmed by manipulation experiments of individual molecules on and
off a single gold atom. This study provides a detailed understanding
of the role of metal adatoms in surface–molecule bonding and
anchoring and of the appearance of single molecules, and it should
prove relevant for the imaging of related molecule–metal systems