Viologen-linked covalent organic frameworks: a charged and redox-active platform for functional materials

Abstract

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.