On-Surface Synthesis of B₃N₃-Substituted Two-Dimensional Covalent Organic Frameworks with Distinct Pore Sizes and Kagome Band Structures

Covalent organic frameworks (COFs) are a versatile class of materials whose structural, electronic and functional properties can be tailored by selecting the geometry and chemical composition of their linker and spacer moieties. We have synthesized single-layer B₃N₃-linked 2D COFs with biphenyl and quaterphenyl spacers on Ag(111) and Au(111) under ultra-high vacuum conditions. The comprehensive experimental characterization combined scanning tunneling microscopy, bond-resolved atomic force microscopy and photoemission spectroscopy. Density functional calculations revealed differences between the two BN-substituted COFs and their carbon-based analogues. The conduction bands of the COFs are primarily derived from electronic states of the spacer units. Introducing B₃N₃ linkers into the COFs increases the band gap and reduces frontier band dispersion, effects that can be further modulated by the length of the spacers. Additionally, the site-selective dehydrogenation of B₃N₃ nodes is shown to locally modify the COF's electronic properties. We thus demonstrate the effect of atomically precise B₃N₃ substitution on the electronic structure of two distinct kagome systems, through a comparative analysis of isostructural BN and CC substituted COFs. These results establish a new strategy for developing stable, metal-free COFs with structural diversity and programmable band structures, offering insights into the controlled BN-doping of low-dimensional carbon nanostructures.