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. Cyclo[n]carbons have attracted significant attention owing to their geometric and electronic structures remaining largely unexplored in the condensed phase. Here, the authors extend the on-surface retro-Bergman ring-opening reaction and successfully produce two anti-aromatic cyclocarbons C12 and C20.