Polyynes are chains of sp1 carbon atoms with alternating single and triple bonds. As they become longer, they evolve towards carbyne, the 1D allotrope of carbon, and they become increasingly unstable. It has been anticipated that long polyynes could be stabilized by supramolecular encapsulation, by threading them through macrocycles to form polyrotaxanes, but, until now, polyyne polyrotaxanes with many threaded macrocycles have been synthetically inaccessible. Here we show that masked alkynes, in which the CC triple bond is temporarily coordinated to cobalt, can be used to synthesize polyrotaxanes, up to the C68 [5]rotaxane with 34 contiguous triple bonds and four threaded macrocycles. This is the length regime at which the electronic properties of polyynes converge to those of carbyne. Cyclocarbons constitute a related family of molecular carbon allotropes, and cobalt-masked alkynes also provide a route to [3] and [5]catenanes built around cyclo[40]carbon and cyclo[80]carbon, respectively.

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.

Cyclocarbons are rings of carbon atoms, often formed as gas-phase carbon clusters. The only cyclocarbons yet to be well characterized are C10 and C18, which are doubly aromatic with 4n+2 carbon atoms (where n is an integer), resulting in enhanced thermodynamic stability. Cyclocarbons with 4n atoms have been predicted to be less stable and doubly anti-aromatic. Here we report the first structural characterization of such a cyclocarbon, C16, generated from C16(CO)4Br2 on a NaCl surface. Atomic force microscopy (AFM) and scanning tunneling microscopy (STM) provide insight into the geometry and electronic structure, respectively, of neutral C16 and anionic C16–. We find that neutral C16 is circular, with significant bond-length alternation. This geometry confirms that it has an anti-aromatic ground state.