Viologen-based electrochromic devices: A review of structural and functional modifications

Electrochromic technology has garnered increasing attention from both academia and industry due to its expanding practical applications across diverse fields. Viologen (4,4′-bipyridinium salts), a small-molecule electrochromic material, exhibits exceptional performance and holds significant promise in electrochromic technology and devices. However, viologen-based materials are prone to degradation of properties during electrochemical reactions. Recent research has increasingly concentrated on viologen-based electrochromic devices (ECDs). This review comprehensively examines modification strategies—including structural modifications of viologen molecules, construction of viologen-containing polymers, and formation of viologen-based complexes—to enhance electrochromic performance. These advancements have not only improved the functional characteristics of viologen materials but also broadened the application potential and functional versatility of ECDs. Furthermore, this work prospects the future development of viologen-based ECDs, highlighting critical technical bottlenecks and unresolved challenges that demand urgent attention. By summarising current progress and outlining research directions, this review provides valuable insights and guidance for researchers in related fields.