Mechanisms of Covalent Coupling Reaction of Dibromofluoranthene on Au(111)

The reaction mechanism of on-surface coupling of 7,10-dibromofluoranthene (Br₂FL) on Au(111) was studied on the basis of density functional theory calculations, as a possible route for fabricating graphene nanoribbons (GNRs) including pentagonal rings. The reaction pathways and energy barriers of debromination, radical coupling, and diffusion processes were investigated. The results indicate that the reaction mechanism is substantially different for Br₂FL compared to that for the phenyl radical, which has been extensively studied as a model system. The rate-limiting step was radical coupling associated with the formation of a gold-metallic intermediate, which is rarely observed on Au(111), because of steric repulsion between the two radicals and that between the radicals and the substrate. The energy barriers were comparable with those for cyclohexa-<i>m</i>-phenylene on Ag(111), and the reaction rate estimated using transition state theory was consistent with the experimental results. These results suggest that Br₂FL is preferred as a precursor of the coupling reaction to fabricate fluoranthene polymers. Well-defined GNRs including pentagonal rings would be formed by further cyclodehydrogenation

Solvation Effects on OH Adsorbates on Stepped Pt Surfaces

Density functional theory calculations were applied to OH formations on stepped Pt electrodes of Pt­[<i>n</i>(111) × (111)] (<i>n</i> = 3, 4, and 6) for examining solvation effects on the OH adsorbates. Results indicated that OH adsorbates at terrace sites are slightly destabilized by water molecules adsorbed at step sites forming 1-dimensional water chains whereas OH adsorbates at step sites are significantly destabilized by water molecules adsorbed at terrace sites forming 2-dimensional honeycomb structures. On stepped Pt surfaces with narrow terrace widths, water molecules cannot exist at terrace sites, and therefore, the solvation effects on OH adsorbates at step sites disappear. Hence, OH adsorbates are formed at step sites at a low potential region, ca. 0.3 V (standard hydrogen electrode (SHE)). When high-coverage CO adsorbates are present on the stepped Pt surfaces, water molecules cannot exist at the terrace sites either because strongly bound CO molecules exclude the water molecules. In such conditions, OH formation potentials decrease significantly, too. Thermodynamic stabilities of OH adsorbates are, therefore, controlled not only by the local surface morphology but also by long-ranged interfacial solvation environments. In other words, the stability and presence/absence of OH adsorbates should be considered to be totally different with water adsorbates (like in inert conditions) and without them (like in the CO oxidation)