Molecularly Precise Triangular Termination of Kagome Covalent Organic Framework Crystals Enabled by Side-Chain Engineering.

High-resolution structural characterization of two-dimensional (2D) kagome covalent organic frameworks (COFs) remains limited, often leaving critical questions about lattice ordering and surface-terminating functionality largely unanswered. To address this challenge, we designed a series of acceptor-donor-acceptor linear linkers based on benzo[1,2-b:4,5-b']dithiophene and 2,1,3-benzothiadiazole units with systematically varied alkoxy side chains. Diverse 2D kagome COFs bearing methoxy, ethoxy, and propoxy side chains were synthesized by imine condensation of these linkers with a dibenzo[g,p]chrysene-based node. While featuring identical backbone architecture, the resulting COFs exhibit remarkably different degrees of crystallinity, demonstrating the critical role of side-chain length in regulating framework ordering. Owing to its superior crystallinity and large pore apertures, the methoxy-substituted COF (OMe COF) enables direct real-space visualization of extended kagome pore lattices by high-resolution transmission electron microscopy (HRTEM). Furthermore, highly crystalline and preferentially oriented OMe COF thin films were synthesized, allowing for detailed HRTEM analysis. Strikingly, HRTEM could clearly resolve triangular terminations of the kagome lattice, thereby establishing the half-condensed dibenzochrysene nodes as the triangular pores terminating the crystal facets and providing unprecedented real-space evidence of exposed amino groups. These findings create a structural basis for the future rational design of surface functionalization and potential interfacial engineering in 2D kagome COFs.