Two-dimensional sub-stoichiometric covalent organic frameworks: from synthesis to applications

Sub-stoichiometric covalent organic frameworks (ss-COFs) are an emerging class of crystalline porous materials featuring periodic unreacted sites derived from organic building blocks. While maintaining the crystallinity, porosity, and stability of stoichiometric COFs, these frameworks exhibit enhanced functionality due to their uncondensed sites. These reactive sites not only modulate key structural properties such as charge transport and light absorption but also enable post-synthetic modifications for tailored applications. This review highlights recent advances in the design and synthesis of two-dimensional ss-COFs, emphasizing precise stoichiometric control. Their distinctive properties and potential applications in gas separation, catalysis, and sensing are discussed, along with key challenges and future research directions in this rapidly evolving field.