  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 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 non-covalently bound dimers on the surface. The
reaction mechanism and driving forces behind non-covalent association are
discussed in light of density functional theory calculations.
