On-Surface Synthesis of a Doubly Anti-Aromatic Carbon Allotrope: Cyclo[16]carbon

The synthetic carbon allotropes graphene, carbon nanotubes and fullerenes have revolutionised materials science and led to new technologies. Recently, unconventional synthetic strategies such as dynamic covalent chemistry and on-surface synthesis have been used to create new forms of carbon, including γ-graphyne, covalent fullerene polymers, and biphenylene networks, as well as cyclo[10]carbon, cyclo[14]carbon and cyclo[18]carbon. Here, by using tip-induced on-surface chemistry, we report the synthesis and characterisation of an anti-aromatic carbon allotrope, cyclo[16]carbon. In addition to structural information from atomic force microscopy (AFM), we probed its electronic structure by recording orbital density maps with scanning tunnelling microscopy (STM), which have not been reported previously for any cyclocarbon. 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 allotropes.