  We report an example that demonstrates the clear interdependence between
surface-supported reactions and molecular adsorption configurations. Two
biphenyl-based molecules with two and four bromine substituents, i.e.
2,2-dibromo-biphenyl (DBBP) and 2,2,6,6-tetrabromo-1,1-biphenyl (TBBP), show
completely different reaction pathways on a Ag(111) surface, leading to the
selective formation of dibenzo[e,l]pyrene and biphenylene dimer, respectively.
By combining low-temperature scanning tunneling microscopy, synchrotron
radiation photoemission spectroscopy, and density functional theory
calculations, we unravel the underlying reaction mechanism. After
debromination, a bi-radical biphenyl can be stabilized by surface Ag adatoms,
while a four-radical biphenyl undergoes spontaneous intramolecular annulation
due to its extreme instability on Ag(111). Such different chemisorption-induced
precursor states between DBBP and TBBP consequently lead to different reaction
pathways after further annealing. In addition, using bond-resolving scanning
tunneling microscopy and scanning tunneling spectroscopy, we determine the bond
length alternation of biphenylene dimer product with atomic precision, which
contains four-, six-, and eight-membered rings. The four-membered ring units
turn out to be radialene structures.
