Porphyrin Metalation at MgO Surfaces: A Spectroscopic and Quantum Mechanical Study on Complementary Model Systems

Abstract

We show that both single-crystalline and nanostructured MgO surfaces convert free-base tetraphenyl porphyrin (2HTPP) into magnesium tetraphenyl porphyrin (MgTPP) at room temperature. The reaction can be viewed as an ion exchange between the two aminic protons of the 2HTPP molecule with a Mg(2+) ion from the surface. The driving force for the reaction is the strong stability of the formed hydroxyl groups along the steps and at defects on the MgO surface. We have used an integrated characterization approach that includes UV/Vis diffuse reflectance measurements on nanostructured powders, X-ray photoelectron spectroscopic investigation of atomically clean MgO(100) single-crystalline thin films, and density functional theory (DFT) calculations on model systems. The DFT calculations demonstrate that MgTPP formation is strongly exothermic at the corners, edges and steps, but slightly endothermic on terrace sites. This agrees well with the UV/Vis diffuse reflectance, which upon adsorption of 2HTPP shows a decrease in the absorption band associated with corner and edge sites on MgO nanocube powders.

Porphyrin Metalation at the MgO Nanocube/Toluene Interface

“…29 We assume that the Consequently, a significantly higher surface fraction is available for the physisorption of the free base on larger cubes, which seems to take place at extended (100) terraces. This is consistent with our observation that on the large MgO cubes only a minor fraction of the adsorbed porphyrin is metalated whereas the majority exists as the free base.…”

Thermally Activated Self‐metalation of Carboxy‐functionalized Porphyrin Films on MgO Nanocubes

“…In that work we have shown that the low‐coordinated surface cations at edge and corner sites of the MgO nanocubes play an important role for the reactivity towards the porphyrin film. We have shown that the low coordinated Mg 2+ ions are the key species in porphyrin metalation ,. We could demonstrate that 2H‐TPP adsorbs in a flat‐lying orientation on the nanocubes, limiting the amount of porphyrin that is bound per cube.…”

“…In our recent work we investigated the anchoring of organic films to oxide surfaces combining surface science work and studies on well‐defined oxide nanoparticles . Among the most commonly used linkers for oxides is the carboxylic acid group .…”

“…Thermal behavior of the materials was only marginally addressed. Previous works that focused on the aspect of film formation, structure and stability have been mostly performed on two‐dimensional model systems, such as single crystal surfaces and thin oxide films under UHV conditions ,. In our present work, we intend to bridge the gap between these surface science studies and conventional nano‐particulate systems.…”

Zinc Porphyrin Metal‐Center Exchange at the Solid–Liquid Interface

“…In these reactions either the two aminic protons in the central cavity of the molecule are exchanged by a metal ion from solution (metalation) or vice versa (demetalation), or the metal center of a metalloporphyrin is exchanged by another metal ion (metal center exchange). A significant number of studies also exists addressing metalation of porphyrins on surfaces under ultrahigh vacuum with codeposited metal atoms, substrate metal atoms, oxide lattice ions, and recently also coordination of ions from aqueous solution . However, to our knowledge, the work of Doyle et al., showing the exchange of nickel metal centers with copper substrate atoms, is the only available study on metal center exchange of adsorbed porphyrins under ultrahigh vacuum.…”