Cyclo-para-azulenes: Segments of Metallic Nonalternant Carbon Nanotubes

Since the first discovery in 1991, carbon nanotubes (CNTs) have been widely studied, yet, the precise synthesis of structurally defined CNTs remains challenging. [n]Cyclo-para-phenylenes (CPPs), segments of CNTs, have been explored as potential precursors for the synthesis of CNTs. While nonalternant distortions are theoretically predicted to tune the electronic properties and yield metallic CNTs, such structures have remained elusive. Herein, we report the first synthesis of the fully nonalternant [8]- and [10]cyclo-para-azulenes ([8]CPA and [10]CPA) via a platinum-mediated catalyst, which can be considered as the segments of metallic nonalternant carbon nanotubes. Single-crystal X-ray diffraction analysis confirms the fully nonalternant cyclic framework of [8]CPA. Remarkably, this nanoring exhibits rare straight tubular stacking, forming a supramolecular nonalternant nanotube. [8]CPA and [10]CPA display exceptionally small HOMO-LUMO gaps, smaller even than that of [4]CPP (the smallest and highly strained benzenoid nanoring), validating theoretical predictions that nonalternant topologies reduce the energy gap. The successful synthesis of CPA nanorings with narrowed energy gaps demonstrates a key step toward the realization of metallic nonalternant CNTs.