In Situ Observation of Covalent Organic Framework Growth in Solution

Mechanistic studies on the formation of imine covalent organic framework (COF) nanoparticles in solution are limited due to the precipitation of nanocrystals. As a result, a fundamental knowledge about the mechanisms governing the growth of imine COFs is lacking, obstructing the development of efficient synthesis methods and precise control of particle size and morphology through solution synthesis routes. Herein, the formation of imine COF nanoparticles is investigated in situ using dynamic light scattering (DLS) and liquid cell transmission electron microscopy (LCTEM) through a homogenous synthesis strategy that prevents nanocrystal precipitation. DLS provides continuous tracking of particle size evolution, while LCTEM offers direct, real-time visualization of nanoparticle formation throughout the synthesis. Three critical stages in imine COF formation are identified: nucleation, growth, and ripening, along with processes such as precursor accumulation and fluctuation influencing these stages. Furthermore, key mechanistic parameters about COF formation, such as critical size, nucleation rate, and growth rate, are derived. Finally, COF nanoparticle suspensions are processed into crystalline thin films with controllable thickness using spin and flow coating, demonstrating the processing advantages of the homogeneous strategy. The mechanistic insights gained here will aid in developing optimized synthesis methods, scale-up, and processing of imine COFs in the future.