Shifting Interfacial Nucleation from Turbulent to Laminar by Diffusion-Directed Growth to Yield Crystalline COF Nanofilms

Covalent organic frameworks (COFs) hold immense promise for next-generation molecular separation membranes, owing to their precise nanochannels and exceptional porosity. Interfacial polymerization offers a powerful route for fabricating such membranes; however, achieving structural uniformity remains challenging due to poorly controlled growth kinetics, often resulting in anisotropic polycrystalline domains. Herein, we demonstrate a novel strategy to overcome these growth disorders by harnessing co-directional molecular diffusion during interfacial crystallization. Monomer transport dynamics were visualized through in situ particle image velocimetry. Our results show that conventional reverse diffusion induces fluid instability and turbulent nucleation, leading to disordered crystallite orientation and thick and mechanically fragile films. In contrast, imposing a co-directional diffusion regime facilitates the preassembly of well-oriented macromolecular oligomers in the solution phase, which serve as ordered precursors for heterogeneous nucleation. This diffusion-directed preorganization promotes the growth of ultrathin, continuous, and highly crystalline COF membranes with preferred crystal orientation. This work establishes a direct link between fluidic transport conditions and the microstructure of COF films, highlighting the critical role of a coordinated molecular supply in minimizing kinetic disorders and enabling epitaxial-like growth at the interfaces. The insights offer a foundational theory for the physical design of high-performance COF membranes and advance interfacial growth as a precision-manufacturing platform for molecularly ordered thin films.