Cycloncarbons

Cycloncarbons are a class of carbon-rich cyclic molecules composed entirely of carbon atoms arranged in closed rings. The term covers cyclocarbons with ring sizes ranging from small to large, where the carbon lattice can incorporate segments that resemble cumulenes (alternating double bonds) or polyynes (alternating single and triple bonds). The hallmark is a carbon ring with no heteroatoms, though stabilized derivatives or metal coordination can aid isolation and study.

Structure and bonding are diverse within cycloncarbons. In smaller rings, high ring strain and the involvement of sp-hybridized carbon units give rise to distinctive bonding patterns that can differ from classical aromatic expectations. Theoretical models often discuss whether certain sizes may exhibit aromatic-like stabilization, while other sizes display non-aromatic or quinoid character. Bond-length patterns may show alternating short and long bonds (bond-length alternation) or appear more uniform under specific experimental conditions.

Synthesis and characterization have progressed through advances in both on-surface chemistry and solution chemistry. Historically, cycloncarbons were difficult to prepare due to their reactivity and tendency to polymerize or dimerize. Recent work has demonstrated on-surface synthesis under ultra-high vacuum, enabling direct imaging with scanning tunneling microscopy and non-contact atomic force microscopy. In solution, stabilization is achieved with substituents, end-capping groups, or coordination to metals, which help preserve the ring during isolation and handling.

Properties and potential applications of cycloncarbons include high strain energies and a range of electronic behaviors from semiconducting to approaching metallic in certain sizes and bonding arrangements. They are primarily of academic interest, contributing to fundamental questions about carbon bonding, aromaticity, and nanoscale electronic structure. Ongoing research aims to refine synthetic routes, stabilize larger rings, and explore practical applications in molecular electronics and materials science.