Role of the Central Metal Atom in Substrate-Mediated Molecular Interactions in Phthalocyanine-Based Heteromolecular Monolayers

摘要

Molecular monolayer films containing two different types of molecules (socalled heteromolecular films) are promising candidates for the controlled functionalization of metal organic hybrid interfaces. This is particularly true for blends formed by charge donor and acceptor molecules. Here we study heteromolecular monolayer systems containing 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) as charge acceptor, and either copper(II) or tin(II) phthalocyanine (CuPc or SnPc) as charge donor, adsorbed on Ag(111). We find that both systems exhibit structural phases with identical lateral ordering (isostructural phases), which is an important prerequisite for studying the role of the central metal atom without competing effects caused by different lateral structures. Using normal incidence X-ray standing waves and photoemission tomography we find distinct differences in the (vertical) geometric and electronic structure for the heteromolecular systems under study: While the vertical structure of CuPc is essentially unaffected by mixing with PTCDA, the SnPc clearly reacts to the formation of a blend by reducing its adsorption height by approximately 0.2 angstrom. Also, the vertical structure of the PTCDA anhydride groups changes strongly: While the anhydride oxygen atoms are located below the perylene core for most mixed phases, for one of the PTCDA + CuPc phases it is lying above the perylene core. Regarding the electronic structure we find that while mixing with PTCDA causes a complete depletion of the CuPc former lowest unoccupied molecular orbital (FLUMO), the SnPc FLUMO is pinned to the Fermi level instead, and thus it remains partially filled. We demonstrate that all these differences are driven by the rearrangement of the substrate electron density in the vicinity of the PTCDA molecules, which are caused by the interaction with the metal phthalocyanine molecules.