5.5 Conclusions

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.