Polycyclic aromates on close-packed metal surfaces: functionalization, molecular chemisorption and organic epitaxy

In this paper we present a detailed analysis of the molecular mechanisms of perylene and PTCDA bonding to two close-packed noble metal surfaces, Ag(111) and Au(111), with the aim of elucidating the relation between bonding and interface structure. Our analysis is based on a combination of high-resolution electron energy-loss spectroscopy and first-principles quantum chemical calculations ( density functional theory) of molecular orbitals and vibrations. Experimentally, the key observation is the activation of certain totally symmetric modes by symmetry breaking at the surface. Analysing calculated vibrational eigenvectors and the induced dynamic distortions of the lowest unoccupied molecular orbital in detail, we propose an interaction model according to which the central carbon ring in both molecules constitutes the molecular chemisorption centre. In the case of perylene, its reactivity is rather small. By the application of functional groups to the aromatic backbone, however, the reactivity can be tuned over a wide range, leading to a dramatic enhancement of the molecule - substrate interaction in the case of PTCDA/Ag(111), as testified by the remarkable interface epitaxy. The example of the weakly interacting PTCDA/Au(111) interface illustrates the limitations of the present scheme of chemisorption strength engineering. Finally, the implications of our results for a detailed determination of the adsorption site are discussed.