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

Synthetic carbon allotropes such as graphene 1 , carbon nanotubes 2 and fullerenes 3 have revolutionized materials science and led to new technologies. Many hypothetical carbon allotropes have been discussed 4 , but few have been studied experimentally. Recently, unconventional synthetic strategies such as dynamic covalent chemistry 5 and on-surface synthesis 6 have been used to create new forms of carbon, including γ-graphyne 7 , fullerene polymers 8 , biphenylene networks 9 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 chemistry 6 . In addition to structural information from atomic force microscopy, we probed its electronic structure by recording orbital density maps 14 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 C 16 renders it an interesting model system for studying the limits of aromaticity, and its high reactivity makes it a promising precursor to novel carbon allotropes 15 .