Functional Covalent Organic Frameworks: Design Principles to Potential Applications

Covalent organic frameworks (COFs) represent an emerging class of crystalline porous polymers synthesized by linking predesigned organic building units into targeted repetitive networks. The unique features of COFs stem from their modular synthesis, allowing for precise control over the chemical composition and functionalization on both the skeleton and the pore walls. Topologically, COFs are defined not by their chemical nature but by the symmetry and dimensions of the building units, resulting in 2D and 3D structures with distinct surface areas, pore architectures, and arrangements of functional moieties. The combination of predesigned organic units into geometries results in frameworks that can be precisely controlled and modified. This control is vital for applications requiring materials with specific pore sizes, surface areas, and functional group distributions. Particularly, COFs show great potential in the field of gas storage and separation, energy storage and conversion, catalysis, sensing, environmental remediation, and many more. Hence, an effective designed approach to incorporate various functional properties into the structures is pivotal to manipulate the functional properties and potential applications of COFs. In this Account, we summarize our recent contributions to the design and synthesis of functional COFs, focusing on how to develop specific functions within structures to enhance their applications. We highlight the design principles of tailor-made functional COFs through compositional tuning, synthetic and structural modulations, and topological diversification. By precisely integrating building units with specific functions, we have developed various COFs, including charged, optoelectronic tunable, radical, and switch COFs. In particular, the specific functional groups are integrated via either bottom-up or postsynthetic approaches. On the other hand, through synthetic and structural modulations, high-quality COFs with improved crystallinity, porosity, diverse forms, robustness, and enriched functional groups have been achieved. Specifically, elegant strategies to enhance the structural stability of 3D COFs are elaborated. Additionally, adjusting the topological structures creates distinct pore architectures and high surface areas that manipulate host-guest confinement and interactions. From our proposed design strategies, we have resulted in COFs with high gas storage capacities, excellent separation capabilities, efficient energy storage, prominent catalytic activities, and sensitive molecular sensing and capture abilities. More importantly, COFs with enhanced thermal and chemical stabilities were produced. This is particularly important for applications involving harsh conditions, such as high pressure or temperature, where material integrity is crucial. At the end, the key challenges and future perspectives for the development of functional COFs, including efficient synthetic strategies and material characterizations, are put forward to elevate the functional application of COFs. We propose that the future of COFs lies in the continuous exploration of new building blocks and the development of advanced synthesis techniques that will allow for the creation of materials with unprecedented properties. We believe that this Account will inspire innovative research in the field of COFs, particularly their inclusion in emerging interdisciplinary research areas.