Heteromolecular metal-organic interfaces: Electronic and structural fingerprints of chemical bonding

Beside the fact that they attract highest interest in the field of organic electronics, heteromolecular structures adsorbed on metal surfaces, in particular donor-acceptor blends, became a popular field in fundamental science, possibly since some surprising and unexpected behaviors were found for such systems. One is the apparent breaking of a rather fundamental rule in chemistry, namely that stronger chemical bonds go along with shorter bond lengths, as it is, e.g., well-known for the sequence from single to triple bonds. In this review we summarize the results of heteromolecular monolayer structures adsorbed on Ag(1 1 1), which - regarding this rule - behave in a counterintuitive way. The charge acceptor moves away from the substrate while its electronic structure indicates a stronger chemical interaction, indicated by a shift of the formerly lowest unoccupied molecular orbital toward higher binding energies. The donor behaves in the opposite way, it gives away charge, hence, electronically the bonding to the surface becomes weaker, but at the same time it also approaches the surface. It looks as if the concordant link between electronic and geometric structure was broken. But both effects can be explained by a substrate-mediated charge transfer from the donor to the acceptor. The charge reorganization going along with this transfer is responsible for both, the lifting-up of the acceptor molecule and the filling of its LUMO, and also for the reversed effects at the donor molecules. In the end, both molecules mutually enhance their respective donor and acceptor characters. We argue that this effect is of general validity for π-conjugated molecules adsorbing on noble metal surfaces.