Porphyrin activation on the hydrated surface of MgO nanocubes

Water adsorption can affect the reactivity of functional hybrid nanostructures in unpredictable ways. Moreover, for aqueous environments, the description of interface formation between metal oxides and complex organic molecules, including their interface stabilities, is extremely difficult to assess. In this study, we used MgO nanocubes and combined experimental (optical UV-Vis spectroscopy and photoluminescence emission on metal oxide nanoparticles) and theoretical (density functional theory calculations) approaches to investigate free-base tetraphenyl porphyrin (2HTPP) adsorption on oxide surfaces after prior contact with water vapor. We were able to consistently explain the observation that the hydration-induced formation of charge-compensated hydroxylated MgO(111) planes significantly promotes the metalation reaction of the free-base porphyrin, i.e. the interfacial exchange between protons of the porphyrin and surface Mg²⁺ ions to convert 2HTPP into MgTPP. The observed enhancement of the porphyrin metalation reaction at the hydrated solid-gas interface, together with the supporting DFT calculations, highlights the significant impact of intentional or unintentional H₂O adsorption on metal oxide nanoparticle functionalization with organic dye molecules.