On‐Surface Synthesis of B ₃ N ₃ ‐Substituted Two‐Dimensional Covalent Organic Frameworks with Distinct Pore Sizes and Kagome Band Structures

ABSTRACT 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 B ₃ N ₃ ‐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 B ₃ N ₃ 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 B ₃ N ₃ nodes is shown to locally modify the COF's electronic properties. We thus demonstrate the effect of atomically precise B ₃ N ₃ 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.