Characterization of the odd-number cyclo[13]carbon and its dimer C26

Recently, the family of carbon allotropes was expanded by the generation and on-surface characterization of cyclo[N]carbons (CN), molecular rings composed of N carbon atoms. Several even-N cyclocarbons have been structurally characterized [1-5]. Odd-N cyclocarbons, elusive to date, are predicted to be even less stable than even-N ones [6, 7], and to have distorted low-symmetry structures, localized carbene centres and small singlet-triplet gaps [8], making them excellent systems for benchmarking theoretical approaches [9]. Here we report the synthesis and characterization of cyclo[13]carbon, C13, on a NaCl surface. We elucidate its geometry and electronic structure by atomic force microscopy (AFM), scanning tunnelling microscopy (STM), and theoretical modelling. C13 adopts a geometry with a kink that is observed with different degrees of pronunciation indicating different degrees of carbene localization. The ground state is a triplet with 13 electrons in both in-plane and out-of-plane π-system. The extremely high reactivity of odd-N cyclocarbons facilitates the synthesis of larger carbon allotropes, as indicated by the formation of the C13 dimer [8], cyclo[26]carbon.