Mechanism of Formation of Benzotrithiophene-Based Covalent Organic Framework Monolayers on Coinage-Metal Surfaces: C-C Coupling Selectivity and Monomer-Metal Interactions

The on-surface synthesis of highly ordered, large-area covalent organic framework (COF) monolayers remains a major challenge in polymer chemistry. Herein, we investigate the polymerization of asymmetric 2,5,8-tribromobenzo[1,2-b:3,4-b':6,5-b '']trithiophene monomers on Au(1 1 1), Ag(1 1 1), and Cu(1 1 1) surfaces, as we have previously predicted that the resulting low-symmetry COF can have promising electronic properties. With scanning tunneling microscopy experiments pointing to the formation of different defective network structures on the three surfaces, we have performed density functional theory calculations and transition-state pathway analyses to address the COF monolayer formation mechanism. Our results highlight that the nature of the substrate significantly affects both the adsorption characteristics and energetic barriers for the s-cis and s-trans couplings between dehalogenated benzotrithiophene radicals. On Au(1 1 1) and Ag(1 1 1), s-trans coupling is energetically favored over s-cis coupling, which is consistent with the more ordered networks experimentally observed on these substrates. However, on Cu(1 1 1), C-C bond formation is far less selective, with similar energy barriers for s-trans and s-cis couplings, which is consistent with the highly disordered networks observed on this surface. These results underline the fact that the s-cis/s-trans selectivity of the C-C coupling reactions strongly depends on the selection of the metal surface used to mediate these reactions. This understanding is an important step toward better control of on-surface 2D monolayer polymerization with asymmetric pi-conjugated monomers.