The odd-number cyclo[13]carbon and its dimer cyclo[26]carbon

Cyclo[N]carbons (CN) are molecular rings of N carbon atoms. These exotic carbon allotropes are excellent benchmarking systems for testing quantum chemical theoretical methods, and they may be valuable precursors to other carbon-rich materials. Odd-N cyclocarbons, elusive to date, are predicted to be even less stable than even-N ones, and their structures are unknown. Here we report the on-surface synthesis of cyclo[13]carbon, C13. We elucidate its properties by scanning probe microscopy and theoretical modelling. C13 adopts an open-shell configuration with a triplet ground state and a kinked geometry, which shows different degrees of pronunciation related to different degrees of carbene localization. Moreover, we prepare and characterize the C13 dimer, yielding cyclo[26]carbon, demonstrating the potential of cyclocarbons as precursors to novel carbon allotropes.