Abstract
Covalent organic frameworks (COFs) have emerged as one of the most promising classes of crystalline and porous materials as they offer precise control over structural, catalytic, and optical properties via the design of monomers and their connectivity. While most COFs are rigid materials with fixed geometry, porosity, and properties, recently developed dynamic COFs can change these properties in response to external stimuli such as solvents, vapours, temperature, or light. This research field is rapidly developing and new generations of both 2D and 3D dynamic COFs now offer large and fully reversible changes of unit cell volumes and porosities as well as full control over the expansion/contraction along pre-defined crystal directions. They hold great promise for various applications ranging from economically important uses in gas separation and monomer purification to future technologies including adaptive sensing and spin-optoelectronics. While the design, synthesis and applications of rigid COFs have been the subject of several recent reviews, a comprehensive summary of the current state of the art, challenges and opportunities of dynamic COFs has remained elusive. In this review, we provide an overview of the dynamic COFs reported to date and analyse their structures and underlying design principles. We discuss the different origins of structural flexibility, and summarise first applications and future challenges and opportunities of these intriguing materials.