On-surface synthesis of aromatic cyclo[10]carbon and cyclo[14]carbon

All-carbon materials based on sp²-hybridized atoms, such as fullerenes¹, carbon nanotubes² and graphene³, have been much explored due to their remarkable physicochemical properties and potential for applications. Another unusual all-carbon allotrope family are the cyclo[n]carbons (C_n) consisting of two-coordinated sp-hybridized atoms. They have been studied in the gas phase since the twentieth century^4-6, but their high reactivity has meant that condensed-phase synthesis and real-space characterization have been challenging, leaving their exact molecular structure open to debate^7-11. Only in 2019 was an isolated C₁₈ generated on a surface and its polyynic structure revealed by bond-resolved atomic force microscopy^12,13, followed by a recent report¹⁴ on C₁₆. The C₁₈ work trigged theoretical studies clarifying the structure of cyclo[n]carbons up to C₁₀₀ (refs. ^15-20), although the synthesis and characterization of smaller C_n allotropes remains difficult. Here we modify the earlier on-surface synthesis approach to produce cyclo[10]carbon (C₁₀) and cyclo[14]carbon (C₁₄) via tip-induced dehalogenation and retro-Bergman ring opening of fully chlorinated naphthalene (C₁₀Cl₈) and anthracene (C₁₄Cl₁₀) molecules, respectively. We use atomic force microscopy imaging and theoretical calculations to show that, in contrast to C₁₈ and C₁₆, C₁₀ and C₁₄ have a cumulenic and cumulene-like structure, respectively. Our results demonstrate an alternative strategy to generate cyclocarbons on the surface, providing an avenue for characterizing annular carbon allotropes for structure and stability.