Confined growth of ordered organic frameworks at an interface.

Given their modular synthesis, unique structural features and rich functionality, structurally ordered covalent organic frameworks (COFs) and covalent monolayers have shown great potential in a broad range of applications, such as catalysis, molecular separation, energy storage, light harvesting, etc. The synthesis of COF thin films and covalent monolayers mainly utilizes dynamic covalent chemistry (DCvC), which relies on the reversible formation and breaking of rather strong covalent bonds within molecules under certain external stimuli. Such reversible reaction conditions enable a self-correction mechanism, which can selectively resolve defect sites leading to the formation of highly ordered COF films under thermodynamic control. Novel techniques to obtain single-layer covalent nanosheets have spread throughout recent literature. Emerging interfacial polymerization techniques (e.g., air-water, liquid-liquid, liquid-solid, etc.) have been employed to successfully synthesize crystalline COF thin films from a variety of starting building blocks. Although the growth of ordered frameworks at the interface represents a rapidly developing field, the reversible reactions suitable for the synthesis of thin films or monolayers are still very limited. The identification and development of new dynamic reactions and interfacial polymerization conditions would be critical for the further development of COF thin films and covalent monolayer materials. This review covers the recent design and synthesis of COF thin films and covalent monolayers as well as their property study. The fundamental working mechanisms of different surface and interfacial polymerization and the current challenges and opportunities in this rapidly growing field are presented.