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'-dibromobiphenyl (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 biradical 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 with atomic precision the bond-length alternation of the biphenylene dimer product, which contains 4-, 6-, and 8-membered rings. The 4-membered ring units turn out to be radialene structures.

Centro de Fisica de Materiales CFM MPC CSIC UPV EHU San Sebastián 20018 Spain

Department of Biological Systems Engineering Washington State University Pullman Washington 99164 United States

Department of Chemistry Washington State University Pullman Washington 99164 United States

Department of Physics and Astronomy Washington State University Pullman Washington 99164 United States

Donostia International Physics Center San Sebastián 20018 Spain

Ikerbasque Basque Foundation for Science 48013 Bilbao Spain

Institute for Integrated Catalysis Pacific Northwest National Laboratory Richland Washington 99352 United States

Institute of Physics of the Czech Academy of Sciences Cukrovarnická 10 16200 Prague 6 Czechia

National Synchrotron Radiation Laboratory Department of Chemical Physics and Key Laboratory of Surface and Interface Chemistry and Energy Catalysis of Anhui Higher Education Institutes University of Science and Technology of China Hefei 230029 P R China

School of Physical Science and Technology ShanghaiTech University Shanghai 201210 P R China

The Gene and Linda Voiland School of Chemical Engineering and Bioengineering Washington State University Pullman Washington 99164 United States

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