In summary, STM, XAS, core-level XPS and DFT were used to study the self-assembly and the central ligand transformation of the surface-supported monolayer of (5,10,15,20-tetraphenylporphyrinato)Mn(III)Cl (MnClTPP).
When deposited onto the Ag(111) surface, MnClTPP molecules self-assemble into large well-ordered molecular domains. Each molecule adopts a saddle conformation with the axial Cl-ligand pointing out of the plane of the molecule. One phenyl ring of each molecule is rotated slightly closer to the perpendicular, giving rise to a ×R45° molecular Bravais unit cell.
Annealing the Mn(III)ClTPP layer up to 510 K causes the chlorine ligand to desorb from the porphyrin while leaving the supramolecular order unaffected. The Mn(III)TPP is stabilised by the Ag(111) surface acting as a fifth ligand for the metal centre.
When the Mn(III)TPP molecules are then exposed to molecular oxygen, the oxidation state of the central Mn changes from Mn(III) to Mn(IV), and O2 is bound to the central Mn ion in peroxide form. Further annealing at 445 K reduces the Mn(IV)O2TPP complex back to Mn(III)TPP/Ag(111). However, exposure of the MnClTPP layer to O2 exhibits no direct substitution of the Cl ligand by O2.
The activation energies for Cl and O2 removal, derived from the XPS data, were found to be 0.35 ± 0.02 eV and 0.26 ± 0.03 eV, respectively.
In conclusion, it has been shown that STM, together with spectroscopic surface science techniques such as XAS and XPS can shed light on the chemical make-up of single molecules and even oxidation states of individual atoms. When combined with DFT, structures observed by high-resolution STM can be explained with a high degree of confidence.