On-surface Ullmann coupling has been intensely utilized for the tailor-made fabrication of conjugated frameworks towards molecular electronics, however, reaction mechanisms are still limitedly understood. While manipulating the noncovalent intramolecular interaction and cage effect on surface have been proven to be effective in steering Ullmann coupling reactions, reaction mechanisms are still under intensive investigation nowadays.
Motivated by such a challenge, a research team led by Prof. Fei Song at Shanghai Advanced Research Institute (SARI), along with collaborators from Shanghai Institute of Applied Physics, Soochow University, Central South University and Harbin Institute of Technology, reported that the Ullmann reaction path is unique regardless of predesigned assembled structures, dominated by the fact that weak intermolecular interaction in assembled nanostructures is suppressed by strong covalent bonding during reactions. The research results were published in Nano Research entitled "Identifying the convergent reaction path from predesigned assembled structures: Dissymmetrical dehalogenation of Br2Py on Ag(111)”.
Surface Ullmann coupling of 2,7-dibromopyrene (Br2Py) on Ag (111) has been elucidated by scanning tunnelling microscopy (STM), X-ray photoelectron, spectroscopy (XPS) and density function theory (DFT). By manipulating deposition conditions, diverse assembled architectures have been constructed for Br2Py on Ag (111). Stepwise annealing leads to an identical reaction diagram for the surface Ullmann coupling from individual assembled structures convergent into the brick-wall-pattern OM dimers first, and then into elongated OM chains in order and eventually long-range polymers with direct C–C coupling. While the reaction mechanism is demonstrated to be dominated by the metal coordinated and halogen bonding motifs, interestingly, it has also been revealed that surface adatoms and dissociated Br atoms play a crucial role in coupling reactions.
Although previous reports have claimed that pre-self-assembly strategy can be utilized to steer Ullmann reaction paths and intermediate species, however, for Br2Py on Ag (111) herein, distinct phenomenon is observed while no apparent relationship is found between predesigned nanostructures and reaction path (reactants). Thus, this report proposes essential insights on fundamental understanding of surface Ullmann coupling towards high-yield surface synthesis of functional nanostructures for nono-electronics.
Convergent reaction path from diverse assembled structures. (Image by Prof. SONG’s group)
