Combined Photoemission and Scanning Tunneling Microscopy Study of the Surface-Assisted Ullmann Coupling Reaction

The adsorption and reaction of 4,4″-dibromo -para- terphenyl (DBTP) and 1,3,5-tris(4-bromophenyl)benzene (TBB) on Cu(111) surface were studied with X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and density functional theory (DFT) calculations. In addition, complementary scanning tunneling microscopy (STM) data are presented. At submonolayer coverage, scission of C–Br bonds occurs between 170 and 240 K. The estimated activation energy for this process is considerably lower than the C–Br bond energy, indicating that bond scission is assisted by Cu atoms of the substrate. The remaining molecular backbones undergo linkage by C–Cu–C bonds to form organometallic oligomers. Annealing of these oligomers leads to the formation of C–C bonded covalent two-dimensional networks. Above monolayer coverage, complete C–Br cleavage requires higher temperature, confirming the role of the Cu surface in the reaction. The results provide insight into the C–Br bond scission as the initial step of the surface-assisted Ullmann reaction.