Aqueous and Surfactant‐Free Synthesis of Nanoscale Covalent Organic Frameworks

The synthesis of nanosized covalent organic frameworks (nanoCOFs) with good dispersibility is vital for optical and optoelectronic applications. However, conventional methods rely on surfactants and organic solvents, limiting scalability and water compatibility. Here, we report a facile, scalable, and surfactant‐free strategy to synthesize imine‐linked nanoCOFs incorporating porphyrin units in aqueous acetic acid. By tuning monomer and catalyst concentrations, we modulated polymerization kinetics to obtain highly crystalline nanoCOFs with controlled sizes (∼50 nm–5 µm), tunable morphologies (nanocubes, nanorods, nanofibers), and good dispersibility. Protonation of porphyrin units during synthesis introduced surface charges, preventing aggregation and enabling excellent water dispersibility. The nanoCOFs exhibited strong and broad light absorption along with high colloidal stability. A proof‐of‐concept study demonstrated their remarkable photocatalytic activity for oxidative coupling of benzylamines in water, even at an ultra‐low loading (0.0074 mol%). This sustainable approach offers a versatile route to high‐performance nanoCOFs, advancing COF‐based photocatalysis and fundamental studies of their photophysical properties.