The cumulenic linear C₅ and its coupling-reaction products

Linear carbon (C_n), an elusive sp-hybridized carbon allotrope, has long attracted interest for its remarkable properties and debated structures. Here, we report the synthesis of an uncapped linear C₅ via tip-induced dehalogenation and ring-opening reaction of C₅Br₆, with its cumulenic structure confirmed by atomic force microscopy. We further demonstrate the tip-induced coupling of linear C₅ to form longer chains (e.g., C₁₀, C₁₅). By applying higher voltage pulses to C₅Br₆ (also C₆Br₆), various carbon chains, including C₉, C₁₀, C₁₃, C₁₄, C₁₅, C₁₇, C₁₈, C₂₁, C₂₃, could be synthesized. Even-numbered chains (C₁₀, C₁₄ and C₁₈) adopt polyynic structures due to a Peierls transition on NaCl, whereas for odd-numbered chains, C₉ adopts a cumulene-like structure, and longer ones exhibit hybrid structures with terminal triple bonds and cumulene-like interiors. Scanning tunneling spectroscopy reveals smaller transport gaps for odd- than even-numbered chains, consistent with Peierls theory, and decreasing gaps with increasing chain length.