Substrate charge transfer drives the adsorption site of metal-phthalocyanines and porphyrins on coinage metal surfaces

The Frontier electronic structure of tetraphenylporphyrinato (TPP^2-) and phthalocyaninato (Pc^2-) square planar transition metal complexes (MTPP and MPc; M = V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) has been revisited through DFT calculations. The different ground state symmetry and spin multiplicity between MPc and MTPP of the same M is shown to originate from the different Pc^2- and TPP^2- ligand field, stronger in the former ligand than in the latter. The corresponding spatial localization and symmetry of the unoccupied molecular orbitals postulate unescapable geometric constraints to their overlap with the electron cloud of a crystalline metal surface. From comparison with literature experimental evidence, we show that the adsorption geometry (atomic site and azimuthal orientation) of MTPPs and MPcs on the low index crystal planes of coinage metals (CM = Au, Ag, Cu) may be predicted when two conditions are satisfied: (i) evidence of a surface → adsorbate charge transfer, (ii) absence of significant distortion of the macrocycle upon adsorption. In this regard, the overall susceptibility to charge transfer is determined by the strength of the molecular ligand field (<i>i.e.</i>, charge transfer to MPc is more favoured than to MTPP) and inversely linked to the electronegativity of the surface atoms (being Au the most inert CM substrate thanks to its highest electronegativity).