Viologen-Linked Covalent Organic Frameworks: A Charged and Redox-Active Platform for Functional Materials

Viologen-linked covalent organic frameworks (Vio-COFs) are an emerging class of charged, redox-active porous materials that effectively combine long-range structural order with tunable electronic and ionic functionalities. The incorporation of viologen units imparts intrinsic positive charge, reversible redox activity, and well-defined ion-accessible channels, enabling precise control over charge density, pore architecture, and transport behavior. This review provides a comprehensive overview of recent advances in the design and synthesis of Vio-COFs, encompassing both direct strategies using cationic building blocks and post-synthetic modifications of pre-formed frameworks. Emphasis is placed on structure–property relationships that govern crystallinity, porosity, stability, and redox performance. The diverse applications of Vio-COFs, including photocatalytic H₂ evolution and CO₂ reduction, electrochemical energy storage, electrochromic and optoelectronic devices, antimicrobial systems, and ion exchange and separation, are critically analyzed. Key challenges remain, including achieving high crystallinity with enhanced charge densities, improving long-term chemical and electrochemical stability, and developing scalable synthetic routes. Future directions emphasize rational molecular design, advanced characterization techniques, and interdisciplinary approaches to accelerate the translation of Vio-COFs into practical applications in energy conversion, catalysis, optoelectronics, environmental and biomedical fields.