Noncovalent Dimerization after Enediyne Cyclization on Au(111)

We investigate the thermally induced cyclization of 1,2-bis(2-phenylethynyl)benzene on Au(111) using scanning tunneling microscopy and computer simulations. Cyclization of sterically hindered enediynes is known to proceed via two competing mechanisms in solution: a classic C¹-C⁶ (Bergman) or a C¹-C⁵ cyclization pathway. On Au(111), we find that the C¹-C⁵ cyclization is suppressed and that the C¹-C⁶ cyclization yields a highly strained bicyclic olefin whose surface chemistry was hitherto unknown. The C¹-C⁶ product self-assembles into discrete noncovalently bound dimers on the surface. The reaction mechanism and driving forces behind noncovalent association are discussed in light of density functional theory calculations.