Direct visualization of surface-assisted two-dimensional diyne polycyclotrimerization.

Cyclotrimerization of alkynes to aromatics represents a promising approach to two-dimensional conjugated networks due to its single-reactant and atom-economy attributes, in comparison with other multicomponent coupling reactions. However, the reaction mechanism of alkyne cyclotrimerization has not yet been well understood due to characterization challenges. In this work, we take a surface reaction approach to study fundamental polymerization mechanism by using a diyne monomer named 4,4'-diethynyl-1,1'-biphenyl as a test bed. We have succeeded in directly characterizing reactants, intermediates, and their reaction products with the aid of scanning tunneling microscope, which allows us to gain mechanistic insights into the reaction pathways. By combining with density functional theory calculation, our result has revealed for the first time that the polycyclotrimerization is a two-step [2+2+2] cyclization reaction. This work provides an in-depth understanding of polycyclotrimerization process at the atomic level, offering a new avenue to design and construct of single-atom-thick conjugated networks.