Role of axial coordination in the adsorption configuration of M(II)-tetraphenylporphyrins (M = Co, Ni, Cu, Zn) on r-TiO2(110)

The efficiency of interfacial charge transfer for photovoltaic and photocatalytic applications of metal-tetraphenylporphyrins (M-TPPs) on rutile TiO(110) is governed by the molecular adsorption site and configuration. Although nitrogen-chelated metals in oxidation state (II) display a different tendency to axial coordination at the porphyrin centre, we found that M(II)-TPPs with M = Co, Ni, Cu, Zn adsorb on the O rows with the same molecular conformation and they aggregate into a commensurate phase with the same symmetry. According to density functional theory (DFT) calculations, the tendency to axial coordination determines the hierarchy of the molecular adsorption energy (where Co-TPP displays the largest one) and the preference for adsorption either atop one O atom (Co- and Zn-TPP) or on bridge between two O atoms (Ni- and Cu-TPP). In the latter case, the adsorption site is driven by the optimization of the molecular backbone H-bonds to the O rows rather than by metal coordination to the substrate.