Abstract
The strong molecule-surface interaction, followed by charge transfer at the interface, plays a fundamental role in the thermal stability of the layer by rigidly anchoring the porphyrin molecules to the copper substrate.
Highlights
Many applications of molecular layers deposited on metal surfaces, ranging from single-atom catalysis to on-surface magnetochemistry and biosensing, rely on the use of thermal cycles to regenerate the pristine properties of the system
The molecules undergo an irreversible transition at 420 K, which is associated with an increase of the charge transfer from the substrate, mostly localized on the phenyl substituents, and a downward tilting of the latters without any chemical modification
Whereas in the gasphase the nickel ion is in the formal 2+ oxidation state (Ni(II)), the charge transfer taking place upon the adsorption of Ni(I)-containing tetraphenyl porphyrin (NiTPP) on the copper surface leads to a stabilization of the Ni(I) species[27,28] and to a partial filling of the former lowest unoccupied molecular orbitals (LUMOs) up to the LUMO+3.29 The uncommon Ni(I) oxidation state can be found in the Ni-containing porphyrinoid core of the biological coenzyme F430.27 This coenzyme plays a key role in both the methanogenesis and the anaerobic methane oxidation and owes its reactivity to the 1+ metal oxidation state.[30]
Summary
Many applications of molecular layers deposited on metal surfaces, ranging from single-atom catalysis to on-surface magnetochemistry and biosensing, rely on the use of thermal cycles to regenerate the pristine properties of the system. Binding of a ligand produces significant changes in the overall electronic properties of the porphyrin, the molecular layer becomes, to all intents and purposes, a gas sensor.[4] In the field of on-surface magnetochemistry,[5] these effects have been exploited to control exchange coupling between a molecular layer and a ferromagnetic substrate.[6,7] The added ligand competes with the surface underneath the porphyrin for the stronger bond at the metal center, giving rise to the so-called surface trans effect,[8,9,10,11] a very intriguing method to control the spin, electronic and chemical properties of metal/molecular interfaces in the field of molecular spintronics[12] and catalysis All these applications require thermally and chemically stable interfaces that can be regenerated at will.[13] Temperature annealing is one of the most effective ways to restore the molecular layer to its pristine configuration by removing anchored ligands that alter the activity of the catalytic center. Whereas in the gasphase the nickel ion is in the formal 2+ oxidation state (Ni(II)), the charge transfer taking place upon the adsorption of NiTPP on the copper surface leads to a stabilization of the Ni(I) species[27,28] and to a partial filling of the former lowest unoccupied molecular orbitals (LUMOs) up to the LUMO+3.29 The uncommon Ni(I) oxidation state can be found in the Ni-containing porphyrinoid core of the biological coenzyme F430.27 This coenzyme plays a key role in both the methanogenesis and the anaerobic methane oxidation and owes its reactivity to the 1+ metal oxidation state.[30]
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