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 (C_n), 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 C_n (n ≥ 10), the cyclic form could be more stable than the linear one. This implies that linear C_n could, in principle, serve as a precursor 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 present 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-predicted strategy for the synthesis of cyclic carbons.