On-surface synthesis of a doubly anti-aromatic carbon allotrope

Synthetic carbon allotropes such as graphene¹, carbon nanotubes² and fullerenes³ have revolutionized materials science and led to new technologies. Many hypothetical carbon allotropes have been discussed⁴, but few have been studied experimentally. Recently, unconventional synthetic strategies such as dynamic covalent chemistry⁵ and on-surface synthesis⁶ have been used to create new forms of carbon, including γ-graphyne⁷, fullerene polymers⁸, biphenylene networks⁹ and cyclocarbons^10,11. Cyclo[N]carbons are molecular rings consisting of N carbon atoms^12,13; the three that have been reported to date (N = 10, 14 and 18)^10,11 are doubly aromatic, which prompts the question: is it possible to prepare doubly anti-aromatic versions? Here we report the synthesis and characterization of an anti-aromatic carbon allotrope, cyclo[16]carbon, by using tip-induced on-surface chemistry⁶. In addition to structural information from atomic force microscopy, we probed its electronic structure by recording orbital density maps¹⁴ with scanning tunnelling microscopy. The observation of bond-length alternation in cyclo[16]carbon confirms its double anti-aromaticity, in concordance with theory. The simple structure of C₁₆ renders it an interesting model system for studying the limits of aromaticity, and its high reactivity makes it a promising precursor to novel carbon allotropes¹⁵.