Viologen-inspired functional materials: synthetic strategies and applications

Viologens, functional organic materials composed of conjugated bi-/multi-pyridyl groups, have attracted much attention for decades owing to their properties such as unique electrochromic and radical-rich features under redox manipulation, biosensitive properties, and ionic and localized conjugation. Considerable efforts have been devoted to synthesizing novel viologen-based functional materials to study their properties and develop applications. So far, viologens and viologen-based materials have been widely applied in many fields, including electrochromism, molecular machines, memory devices, gas storage and separation, energy storage, and biochemistry. The rapid development of functional materials in the 21(st) century has inspired extensive integration of viologens into many types of novel and famous materials (e.g., host-guest supramolecules, porous polymers, and metal-organic frameworks), and therefore, viologens seem to no longer be a key player. However, the features of viologens (or bi-/multi-pyridyl-based materials) have always dominated the properties of target functional materials and long been neglected by researchers. In order to provide an overview of this field, this perspective paper focuses on reasonable design and synthesis of viologen-based functional materials and their representative applications. The remaining challenges and future potential applications are also discussed.