The in-depth knowledge about on-surface reaction mechanisms is crucial for the tailor-made design of covalently bonded organic frameworks, for applications such as nanoelectronic or -optical devices. Latest developments in atomic force microscopy, which rely on functionalizing the tip with single CO molecules at low temperatures, allow to image molecular systems with submolecular resolution. Here, we are using this technique to study the complete reaction pathway of the on-surface Ullmann-type coupling between bromotriphenylene molecules on a Cu(111) surface. All steps of the Ullmann reaction, i.e., bromotriphenylenes, triphenylene radicals, organometallic intermediates, and bistriphenylenes, were imaged with submolecular resolution. Together with density functional theory calculations with dispersion correction, our study allows to address the long-standing question of how the organometallic intermediates are coordinated via Cu surface or adatoms.

Molecular self-assembly can be used for the bottom-up fabrication of nanoscale electronic devices. For this, polycyclic aromatic hydrocarbons are particularly suited due to their extended π-system. Besides, these compounds can serve as precursors for fabricating graphene-like material. To design future electronic devices it is essential to be able to reproducibly predict the structure formation of these molecules after adsorption on solid surfaces. Here we studied the self-assembly of triphenylene molecules on a reconstructed Au(111) substrate in the submonolayer regime by scanning tunneling microscopy. Only two different orientations of the planar adsorbed molecules are observed. At intermediate coverages self-assembly is mainly determined by repulsive molecule–molecule interactions, leading to a one-to-one ratio of molecular orientations. At 1.0 monolayer coverage, however, a reorientation into close-packed domains occurs, which significantly shifts the ratio of molecular orientations. It is found that this reorientation is controlled by molecule–subsurface interactions, opening new avenues in assembling molecules on surfaces.

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