Tuning aromaticity of cyclocarbons by heteroatom doping: C₁₂S and C₁₂N

Cyclo[n]carbons (Cn) have sparked substantial interest among experimentalists and theoreticians owing to their elusive geometric structures and unique aromaticity. Composed of two-coordinated sp-hybridized carbon atoms, Cn thus forms two perpendicular conjugated π-electron systems, i.e. out-of-plane and in-plane. Till now, on-surface generated cyclocarbons are either doubly aromatic or doubly anti-aromatic, as the number of electrons within out-of-plane and in-plane π systems was equal. Doping with heteroatoms allows one to create two π systems with different numbers of electrons, and to tune the aromaticity. Herein, we successfully generated two heteroatom-doped cyclocarbons, C12S and C12N, and characterized their chemical and electronic structures. Calculations show that C12S exhibits an out-of-plane (14 e) aromatic and in-plane (12 e) anti-aromatic character, resulting in a total non-aromaticity. For C12N, the out-of-plane (14 e) aromatic and in-plane (13 e) non-aromatic characters lead to total aromaticity. Doping with heteroatoms may open up the field of aromaticity engineering within cyclocarbons.