Ouroboros: From linear carbons to cyclic carbons, cyclo[25]carbon and cyclo[50]carbon

Carbon allotropes composed solely of sp-hybridized carbon atoms, namely linear and cyclic carbons (Cn), represent a unique class of materials with fascinating chemical structures and long-debated stability among their various isomers. Early gas-phase experiments and theoretical calculations have both suggested that for larger Cn (n ≥ 10) the cyclic form could be more stable than the linear one. This implies that linear Cn could, in principle, serve as precursors for the formation of cyclic carbons. Recently, on-surface synthesis has enabled the generation of a series of linear and cyclic carbons (up to cyclo[30]carbon). However, a direct structural transformation from linear to cyclic carbon has not yet been achieved in real space. Here, we represent a new strategy to generate cyclic carbons from linear ones, inspired by the ancient symbol “Ouroboros”, a snake eating its own tail. Using linear C5 as a precursor (derived from C5Br6) and tip-induced coupling reactions, we achieved the on-surface generation of an odd-numbered cyclic carbon, C25. Electronic characterization and calculations reveal that C25 adopts an open-shell configuration. To demonstrate the generality of this strategy, we employed linear C6 (derived from C6Br6) as the precursor, which led to the formation of the largest cyclocarbon so far, C50, on the surface, and importantly, its first and second ion resonances at two voltage polarities were probed successfully. Our work demonstrates a long-predicated strategy for the synthesis of cyclic carbons.