On-surface synthesis of anti-aromatic cyclo[12]carbon and aromatic cyclo[6]carbon

Abstract A novel family of carbon allotropes, known as cyclo[n]carbons (Cn), have attracted significant attention from both experimentalists and theoreticians because of their remarkable properties. Recently, certain aromatic cyclocarbons, such as cyclo[18]carbon (C18), cyclo[14]carbon (C14), and cyclo[10]carbon (C10), as well as an anti-aromatic cyclo[16]carbon (C16), have been successfully synthesized and characterized on the thin insulating NaCl surface. However, the characterization of even smaller cyclocarbons in real space, particularly those with anti-aromatic characteristics, still remains challenging. The substantial difficulties in synthesizing smaller anti-aromatic cyclocarbons are due to their inherent high reactivity, and on the other hand, stemming from the increased strain and reduced stability owing to their anti-aromatic nature. Here, we successfully produce a smaller anti-aromatic cyclocarbon, cyclo[12]carbon (C12), via tip-induced dehalogenation and accompanied retro-Bergman ring-opening reaction of a fully halogenated biphenylene (C12Br4I4), on a bilayer NaCl/Au(111) surface at 4.7 kelvin. Moreover, the smallest aromatic cyclocarbon, cyclo[6]carbon (C6), is also generated by atom manipulation of hexachlorobenzene (C6Cl6) molecule. The polyynic structure of C12 and cumulenic structure of C6 are revealed by bond-resolved atomic force microscopy (AFM) and theoretical calculations. Our work further demonstrates the feasibility of such an on-surface synthesis strategy for the generation of cyclo[n]carbons in the condensed phase, and to further characterize their precise structures with bond-resolved resolution.