Carbon allotropes built from rings of two-coordinate atoms, known as cyclo[n]carbons, have fascinated chemists for many years, but until now they could not be isolated or structurally characterized, due to their high reactivity. We generated cyclo[18]carbon (C18) using atom manipulation on bilayer NaCl on Cu (111) at 5 Kelvin by eliminating carbon monoxide from a cyclocarbon oxide molecule C24O6. Characterization of cyclo[18]carbon by high-resolution atomic force microscopy revealed a polyynic structure with defined positions of alternating triple and single bonds. The high reactivity of cyclocarbon and cyclocarbon oxides allows covalent coupling between molecules to be induced by atom manipulation, opening an avenue for the synthesis of other carbon allotropes and carbon-rich materials from the coalescence of cyclocarbon molecules.

Cyclo[18]carbon (C 18 , a molecular carbon allotrope) can be synthesized by dehalogenation of a bromocyclocarbon precursor, C 18 Br 6 , in 64% yield, by atomic manipulation on a sodium chloride bilayer on Cu(111) at 5 K, and imaged by high-resolution atomic force microscopy. This method of generating C 18 gives a higher yield than that reported previously from the cyclocarbon oxide C 24 O 6 . The experimental images of C 18 were compared with simulated images for four theoretical model geometries, including possible bond-angle alternation: D 18 h cumulene, D 9 h polyyne, D 9 h cumulene, and C 9 h polyyne. Cumulenic structures, with ( D 9 h ) and without ( D 18 h ) bond-angle alternation, can be excluded. Polyynic structures, with ( C 9 h ) and without ( D 9 h ) bond-angle alternation, both show a good agreement with the experiment and are challenging to differentiate.

The cyclocarbons constitute a family of molecular carbon allotropes consisting of rings of two-coordinate atoms. Their high reactivities make them difficult to study, but there has been much progress towards understanding their structures and properties. Here we provide a short account of theoretical and experimental work on these carbon rings, and highlight opportunities for future research in this field.

The on-demand delivery of drug molecules from nano-scale carriers with spatio-temporal control is a key challenge in modern medicine. Here we show that lipid bilayer vesicles (liposomes) can be triggered to release an encapsulated molecular cargo in response to an external control signal by employing an artificial transmembrane signal transduction mechanism. A synthetic signal transducer embedded in the lipid bilayer membrane acts as a switchable catalyst, catalyzing the formation of surfactant molecules inside the vesicle in response to a change in external pH. The surfactant permeabilises the lipid bilayer membrane to facilitate release of an encapsulated hydrophilic cargo. In the absence of the pH control signal, the catalyst is inactive and the cargo remains encapsulated within the vesicle.

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.