Imaging Multistep s‑Triazine Oligomerization via Cobalt-Assisted Deamination and Selective C-C Coupling.

Polymerizing monomer precursors to form monolayer carbon nitrides with determined C/N stoichiometries and tailored properties is intimately related to an in-depth understanding of the reaction paths. However, fundamental details related to molecular-level mechanisms remain largely unexplored because of the challenges associated with real-space visualization of early-stage polymerization processes involving C/N/H activation, stabilization, and evolution of intermediate states. Herein, we characterize the multistep reactions of melamine in the presence of coadsorbed cobalt on Au(111), using bond-resolved scanning probe microscopy (SPM) combined with density functional theory (DFT) calculations. Step-by-step evolution of on-surface-generated intermediate states have been monitored in real space; i.e., the initial step involves the condensation of melamine monomers to afford s-triazine tetramers, which further transform into C-C-linked octamers and longer oligomers. Furthermore, we demonstrate how the formation of the Au-Co-N motif influences the coplanar and out-of-plane configurations of reaction intermediates, therefore affecting C-N cleavage and C-C coupling. Our findings provide mechanistic insights into the preparation of monolayer carbon nitrides with predictable structures.