On-surface C-C coupling reactivity of carbon atoms in halogenated azulene

Azulene molecule, consisting of a pair of five and seven-membered rings, represents a promising precursor for the on-surface synthesis of nonbenzenoid, nonalternant carbon nanostructures with exotic properties. However, controlling the selective C-C coupling between azulene molecules remains elusive, undermining the structural uniformity of the attained carbon nanostructures. Here, we report that the on-surface C-C coupling reactivity of different carbon atom sites in azulene relies on the spatial distribution of its frontier orbitals. By performing surface reactions of a tribrominated azulene molecule on Au(111), the probability of C-C coupling between carbon atoms at different sites of azulene has been revealed by scanning tunneling microscopy and non-contact atomic force microscopy. These findings are in accordance with the density functional theory-calculated energy barriers for the corresponding C-C coupling reaction steps.