An sp-sp2-hybridized molecular carbon allotrope C16 flake.

The molecular carbon allotropes have an enduring attraction to chemists and physicists for their elusive structures and extraordinary properties. Cyclo[16]carbon has been produced on the surface and is well characterized, while it is interesting that molecular carbon allotrope, like C16, referring to molecules composed of 16 carbon atoms, presents a fascinating realm of isomeric possibilities. Except for cyclo[16]carbon, C16 isomers with other structures have been predicted only by theory. Here, we report the synthesis and structural characterization of a graphene-shaped isomer, i.e., C16 flake on a bilayer NaCl surface grown on Au(111), using an atom-manipulation strategy by eliminating chlorine from a fully chlorinated pyrene molecule, C16Cl10. The sp- and sp2-hybridized structure of C16 flake is well characterized by bond-resolved atomic force microscopy. Theoretical calculations reveal an open-shell singlet ground state of C16 flake.