Electrostatic interaction and commensurate registry at the heteromolecular FCuPc-CuPc interface

Tailoring the properties of molecular thin films and interfaces will have decisive influence on the success of future organic electronic devices. This is equally true for metal-organic and hetero-organic contacts as they occur, for example, in donor-acceptor systems. Here, we report on the structure formation and interaction across such a heteromolecular interface. It is formed by monolayers of FCuPc and CuPc stacked on a Ag(111) surface. We investigated the lateral and vertical structure using spot-profile analysis low energy electron diffraction and normal incidence X-ray standing waves, and performed pair potential calculations to understand the driving forces for the structure formation. Most surprisingly, for one phase we found a commensurate registry between the two organic layers, usually a sign for a strong (chemisorptive) interaction often involving metallic states of the surface. However, because the organic bilayer is not commensurate with the underlying Ag substrate in our case, the dominating factor must be the intermolecular interaction. Pair potential calculations suggest a site-specific adsorption that leads to a commensurate registry at the heteromolecular interface. The adsorbate system was further characterized by measuring adsorption heights, indicating flat-lying molecules and a CuPc-FCuPc layer spacing of 3.06 Å. © 2013 American Chemical Society.