Dual-Knot and Dual-Linkage Design Strategy of 2D COFs: Toward Robust Proton-Conducting Platforms

Covalent organic frameworks (COFs) are an emerging class of crystalline porous materials with promise for diverse functional applications. However, most COFs are limited to architectures constructed from a single knot and a single linker, constraining both their structural complexity and functional tunability. Here, we introduce a dual-knot, dual-linkage design strategy for two-dimensional COFs, in which a conventional C3-symmetric knot and a C2 linker are combined with two distinct C3 knots in a 1:1 ratio. This design simultaneously incorporates two robust linkages-imide/imine or imide/β-ketoenamine-within a single framework, leading to the synthesis of two unprecedented materials: imide-imine COF (II-COF) and imide-β-ketoenamine COF (IK-COF). The dual-knot architecture enriches the framework with multiple heteroatom sites, which act as adsorption centers for proton-conducting media such as phosphoric acid (PA). Upon PA loading, both COFs exhibit high anhydrous proton conductivity and excellent cycling stability, arising from the synergistic contributions of pyridinic nitrogens, imine nitrogens, and carbonyl oxygens in the backbone. This work establishes a generalizable strategy for constructing compositionally complex and symmetry-defined COFs, and highlights their potential as robust platforms for proton conduction and beyond.