  A coupling-limited approach for the Ullmann reaction-like on-surface
synthesis of a two-dimensional covalent organic network starting from a
halogenated metallo-porphyrin is demonstrated.
Copper-octabromo-tetraphenylporphyrin molecules can diffuse and self-assemble
when adsorbed on the inert Au(111) surface. Splitting-off of bromine atoms
bonded at the macrocyclic core of the porphyrin starts at room temperature
after the deposition and is monitored by X-ray photoelectron spectroscopy for
different annealing steps. Direct coupling between the reactive carbon sites of
the molecules is, however, hindered by the molecular shape. This leads
initially to an ordered non-covalently interconnected supramolecular structure.
Further heating to 300{\deg}C and an additional hydrogen dissociation step is
required to link the molecular macrocycles via a phenyl group and form large
ordered polymeric networks. This approach leads to a close-packed covalently
bonded network of overall good quality. The structures are characterized using
scanning tunneling microscopy. Different kinds of lattice defects and,
furthermore, the impact of polymerization on the HOMO-LUMO gap are discussed.
Density functional theory calculations corroborate the interpretations and give
further insight into the adsorption of the debrominated molecule on the surface
and the geometry and coupling reaction of the polymeric structure.
