Cracking Adsorbate–Substrate Interactions through Local Orbital Symmetry: Titanyl and Vanadyl Phthalocyanines on Low-Index Planes of Coinage Metals

The adsorption of MOPc (M = Ti, V) complexes on the low-index crystal surfaces of coinage metals (Cu, Ag, and Au) is examined by using a streamlined framework that integrates elementary symmetry arguments with readily accessible frontier-orbital information on the isolated adsorbates. We show that the surface -> adsorbate charge transfer, confined to the delocalized MOPc 37e pi* LUMO and decreasing with increasing electronegativity of the surface atoms, provides a reliable basis for determining both the preferred adsorption geometry and the adsorbate-substrate grasping mechanism for MOPc in its up configuration, where the oxo ligand of the TiO2+/VO2+ fragment points away from the surface. Although this charge transfer is weaker when MOPc adopts the down (dp) configuration, with the oxo group directed toward the surface, we successfully predicted the adsorption site and the azimuthal orientation of dpMOPc on Cu surfaces. This is achieved by analyzing repulsive interactions between the occupied, TiO2+-/VO2+-based 27a1 and 33e molecular orbitals and the symmetry-adapted linear combinations of the surface Cu 4s atomic orbitals. The study demonstrates that a minimal, physically transparent model can guide and complement computationally demanding simulations of adsorption phenomena on coinage metal surfaces.