Stable hydrogen-bonded organic frameworks and their photo- and electro-responses

Hydrogen-bonded organic frameworks (HOFs) are a recent class of porous materials that have garnered considerable research interest owing to their distinctive characteristics. HOFs can be constructed through judicious selection of H-bonding motifs, which are further enforced by other weak intermolecular interactions such as π–π stacking, van der Waals forces, and framework interpenetration. Taking advantage of these interactions, we can expand the functional field of HOFs by introducing active molecules. Recently, researchers have made substantial advancements in using HOFs for chemical sensing, catalysis, proton conduction, biological applications, and others. The low bonding energy of H-bonds allows for precise control over the concentration of ligands in solvents, forming diverse HOF structures. These varied structures offer significant advantages for producing HOFs with photo-responsive and electro-responsive properties. However, the presence of H-bonds in HOFs results in their inherent lower stability compared to metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) formed via coordination and covalent bonds, respectively. As a result, the pursuit of stable and innovative HOF materials with novel functional sites remains an ongoing challenge. This review provides an overview of recent research progress in the development of new strategies for stable HOF synthesis and applications of HOFs with stimuli-responsive properties. We first classified all synthetic methods reported to date and discussed the stable HOFs synthesized, as well as their unique properties and applications. In addition, we summarized the applications of HOFs utilizing their synergistic responses to external stimuli, including photo, electrical, pressure, and chemical stimuli. We systematically reviewed stable HOF synthesis and applications, which may lead to a deeper understanding of the structure–activity relationship for these materials and guide future HOF design.