Enhanced proton conductivity in azole-functionalized three-dimensional crystalline covalent organic frameworks

Proton conduction pathways and mechanisms in covalent organic frameworks (COFs) have long been obscured by polycrystalline disorder. Here we report a solvent-free melt-phase post-synthetic modification (PSM) strategy that enables precise functionalization of three-dimensional single-crystalline COFs while preserving crystallinity. This methodology overcomes the limitations of solvent-mediated PSM by operating above the melting point of azole reagents, ensuring homogeneous pore accessibility without solvent occlusion. Applied to archetypal imine-linked COF-300, the method achieves crystallographically resolved conversion of fragile imine bonds (C = N, 1.245 Å) into robust amine linkages (C-N, 1.415 Å), concurrently covalently anchoring of proton-conductive azoles (C-N, 1.487 Å) on the COFs skeleton. The resulting azole-functionalized COFs (COF-300-1,2,3-triazole, COF-300-1,2,4-triazole, COF-300-pyrazole) exhibit intrinsic anhydrous superprotonic conductivity reaching 4.35 × 10^-3 S cm^-1 at 170 °C, with low activation energies (0.153-0.186 eV). Atomic-resolution crystallography and DFT calculations reveal that rigid hydrogen-bond networks eliminate thermal barriers for proton hopping, establishing a definitive structure-property correlation for proton transport in single-crystal COFs. This work pioneers a versatile platform for functionalizing 3D crystalline porous materials under solvent-free conditions.