On-surface synthesis and characterization of anti-aromatic cyclo[12]carbon and cyclo[20]carbon

Cyclo[n]carbons have recently attracted significant attention owing to their geometric and electronic structures remaining largely unexplored in the condensed phase. In this work, we focus on two anti-aromatic cyclocarbons, namely C₁₂ and C₂₀. By designing two fully halogenated molecular precursors both including 4-numbered rings, we further extend the on-surface retro-Bergman ring-opening reaction, and successfully produce C₁₂ and C₂₀. The polyynic structures of C₁₂ and C₂₀ are unambiguously revealed by bond-resolved atomic force microscopy. More importantly, subtly positioning the C₂₀ molecule into an atomic fence formed by Cl clusters allows us to experimentally probe its frontier molecular orbitals, yielding a transport gap of 3.8 eV measured from scanning tunneling spectroscopy. Our work may advance the field by easier synthesis of a series of cyclocarbons via on-surface retro-Bergman ring-opening strategy.