Azo-Linked Covalent Organic Frameworks.

Covalent organic frameworks (COFs) linked by azo bonds (-NN-), termed as azo-linked COFs, represent a novel class of crystalline porous polymers, in which small organic units are covalently connected into ordered frameworks via azo bonds. Endowed with the inherent stability and photoactive/electroactive properties of azo units, azo-linked COFs hold significant promise for high-performance applications. However, their development has been substantially hindered by their synthesis, for which the low reversibility of the azo bonds typically results in amorphous polymers rather than crystalline frameworks. Recently, by employing the "COF-to-COF" structural transformation strategy developed in 2017, this approach has successfully overcome this limitation, enabling the construction of crystalline azo-linked COFs with exceptional optoelectronic properties and photocatalytic efficiency. This concept introduces the emerging azo-linked COFs, focusing on their synthesis and photocatalytic applications. Furthermore, future efforts to accelerate the development of azo-linked COFs are discussed.