Atomic-scale visualization of reaction intermediates in surface-confined Ullmann coupling

• Comprehensive visual analysis of Ullmann coupling intermediates achieved via controlled annealing processes. • Formation of unilateral dehalogenated intermediates at low temperature. • Preferential generation of bipyrenes in C–C coupling stage. • Coverage-dependent selective synthesis between C–C bonded dimers and Oligomers. Surface-confined Ullmann coupling is crucial for fabricating covalent nanostructures, while its reaction pathway requires deeper mechanistic understanding. Using scanning tunneling microscopy and density functional theory, we identify a variety of reaction intermediates and their assembly structures of dibromopyrene molecules on a silver substrate over a wide temperature range of 100–600 K, particularly covering the underexplored low-temperature regime. Through stepwise annealing, we track the complete reaction pathway from sequential dehalogenation to organometallic intermediates, followed by preferential C–C bonded dimer formation by controlling the molecular coverage. These findings provide practical guidelines for manipulating reaction intermediates and steering final products, advancing the controlled fabrication of high-quality carbon-based molecular electronic devices.