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