On-surface synthesis and characterization of linear and cyclic C6

Abstract

Cyclo[n]carbons, belonging to carbon clusters (Cn), have attracted substantial attention from both experimentalists and theoreticians because of their chemical structures and the controversial stabilities of their different isomers. Recently, C26, C20, C18, C16, C14, C13, C12 and C10 have been synthesized and characterized on thin insulating NaCl surfaces. However, synthesis of smaller cyclocarbons (n < 10) remains challenging due to their inherent high reactivity and increased strain. In particular, C6 has been a subject of long-standing theoretical debate because the energy difference between its linear and cyclic forms is very small. Here we successfully generate both linear and cyclic C6 by modulating the thickness of NaCl layers deposited on a Au(111) surface, using tip-induced dehalogenation of hexaiodobenzene (C6I6) molecules at 4.7 K. The linear C6 was generated on 1-monolayer (1-ML) or 2-ML NaCl surfaces and identified as a polyynic structure through bond-resolved atomic force microscopy, verifying the theoretical calculations of a Peierls transition for linear C6 from gas phase to on-surface adsorption. Meanwhile, cyclic C6 could be generated on a 2-ML NaCl surface and its cumulenic nature is confirmed both experimentally and theoretically. Additionally, voltage pulses can induce cyclic C6 to undergo either an adsorption configuration transformation from a nearly planar to a tilted geometry, or a structural transformation to the linear form. Linear C6 and its isomer, cyclic C6 (the smallest aromatic cyclocarbon), are successfully generated on NaCl surfaces by tip-induced dehalogenation of hexaiodobenzene molecules at 4.7 K, and their polyynic and cumulenic structures, respectively, are characterized by bond-resolved atomic force microscopy.