Molecular Nanocarbons with Diverse Connectivity and Topologies Enabled by Bismuth(III) Triflate-Mediated Benzannulation Reaction

Conspectus: Molecular nanocarbons have attracted growing interest as atomically precise π-conjugated carbon frameworks, owing to their diverse connectivity patterns, structural complexity, and tunable properties. By introducing topology as an additional structural dimension, these systems enable access to π-conjugated architectures that cannot be described solely by conventional planar or curved frameworks, thereby providing new opportunities for regulating electronic structures and optical properties at the molecular level. From a topological perspective, molecular nanocarbons can be classified into topologically trivial and nontrivial systems. Topologically trivial molecular nanocarbons–including planar or curved nanographenes, closed yet topologically simple macrocycles, and helicene-based frameworks–exhibit conventional connectivity and can undergo continuous deformation without changing their topological invariants. In contrast, topologically nontrivial molecular carbons exhibit intrinsic global topological constraints, as represented by molecular links (e.g., catenane), Möbius belts, and molecular knots, in which the topological framework is preserved unless covalent bonds are broken. Despite their conceptual appeal and structural diversity, the bottom-up construction of systematic families of molecular nanocarbons remains highly challenging. In particular, topologically nontrivial molecules often involve significant structural strain, rigid connectivity requirements, and strict conformational control, making their synthesis substantially more demanding than that of topologically trivial systems. Consequently, existing strategies are largely structure-specific and developed on a case-by-case basis, making it difficult to establish general and broadly applicable synthetic methodologies. Therefore, developing general and designable bottom-up synthetic strategies that allow access to molecular nanocarbons with diverse connectivity and topologies through the incorporation of similar functional groups is of central importance.In recent years, leveraging a bismuth(III) triflate (Bi(OTf)₃)-mediated benzannulation reaction strategy for efficient C–C bond formation, we have realized the precise construction of a series of fully fused molecular nanocarbons and investigated their distinctive (chiro-)optical properties. We employed macrocycles bearing vinyl ether units, readily accessible via Suzuki coupling, as key precursors to extend the Bi(OTf)₃-catalyzed benzannulation to cycloarenes of various sizes, including kekulene, expanded kekulenes, and octulene. Through rational precursor design, this approach further enabled the precise synthesis of highly strained, fully fused twisted carbon nanobelts, most notably the triple Möbius nanobelt, with remarkable chiroptical properties. Beyond macrocycle architecture, the strategy was also expanded to highly fused expanded helicenes and zigzag nanographenes, affording helicenes with large dissymmetry factors and the first [4]rhombene for near-infrared organic lasing applications. In this Account, we summarize our efforts to develop a versatile cyclization strategy for constructing diverse molecular nanocarbons, and we anticipate that this overview will inspire researchers across disciplines to further explore these structurally well-defined and functionally rich platforms for a broad range of applications.