Tailoring Near Fermi-Level Topological Flatbands in Clar’s Goblet Graphene Nanoribbons through Regioselective Cyclization of Five-Membered Rings

On-surface synthesis via metal-surface-catalyzed C-C bond formation presents unique advantages for the design of graphitic nanomaterials with atomic precision. Following this approach, the coimplantation of nontrivial topology and flatband structures in graphene nanoribbons (GNRs) has emerged as a compelling pursuit, serving as platforms for realizing exotic quantum phases of matter through the interplay of topological states and strong correlations. However, the exploration of these intriguing properties has been largely constrained by the limited known on-surface reactions capable of creating topological flatbands in GNRs. In this work, we promote the intermolecular oxidative coupling of concealed non-Kekuléan nanographenes to construct topological flatband GNRs and GNR heterojunctions on the Au(111) surface. Utilizing Clar's goblet as a proof of concept, we demonstrate repetitive intermolecular cyclodehydrogenation with high regioselectivity to form pentagon-embedded GNRs. The coupling of the zero modes in Clar's goblets generates extended electronic states with evident nodes between them, arising from inherent topological frustration, thus resulting in topological flatbands close to the Fermi level and topologically protected end states. Our atomically resolved measurements obtained using scanning tunneling microscopy and noncontact atomic force microscopy, complemented by density functional theory and tight-binding model calculations, illustrate the on-surface reaction cascade and the electronic properties of the designed products. These findings open significant opportunities for the on-surface construction of low-dimensional carbon-based quantum materials.