Aromatic annular carbon allotropes: cumulenic cyclo[10]carbon and Peierls-transition-intermediate cyclo[14]carbon

Abstract Cyclo[ n ]carbon allotropes built from sp-hybridized carbon atoms have aroused substantial interests, but until now even their structures (i.e., polyynic or cumulenic) and stabilities (i.e., cyclic or linear) are still under debate. Due to their high reactivity, it is rather difficult to synthesize in condensed phase, which hampers further real-space characterizations. Recently, isolated cyclo[18]carbon (C 18 ) and cyclo[16]carbon (C 16 ) molecules were generated on the surface by atom manipulation, which allowed to identify the polyynic structure of C 18 and C 16 . More interestingly, for the smaller cyclo[ n ]carbon, questions arise: is it still stable in cyclic form? If so, what is the structure? Here, we successfully produce two aromatic cyclocarbons, that is, cyclo[10]carbon (C 10 ) and cyclo[14]carbon (C 14 ), via tip-induced dehalogenation and followed by retro-Bergman reaction of fully chlorinated naphthalene (C 10 Cl 8 ) and anthracene (C 14 Cl 10 ) molecules, on a bilayer NaCl/Au(111) surface at 4.7 kelvin. The cumulenic structure of C 10 and Peierls-transition intermediate of C 14 , different from the polyynic structure of C 18 and C 16 , are revealed by bond-resolved atomic force microscopy. Our results demonstrate a universal synthetic strategy to generate aromatic cyclo[ n ]carbons on the surface, opening an avenue for characterizing a series of annular carbon allotropes for the structure and stability.