π-Extended viologen derivatives and their related macrocycles: from design and synthesis to photofunctional applications

Viologen derivatives (RV²⁺) have attracted much attention due to their unique redox processes, which can be used to catalyze organic reactions and fabricate photoelectric devices. However, the inherent defects such as broad optical bandgaps, weak luminescence and unstable radicals severely restricted their photofunctional applications. To solve these problems, the π-extension strategy was utilized to modulate the electron cloud delocalization and molecular π–π stacking pattern. Various functional substituent groups have been incorporated into the RV²⁺ skeleton, achieving excellent photoelectric properties. With the development of radical and macrocyclic chemistry, the formation of discrete macrocycles has provided a new platform for further configuration regulation and performance optimization of RV²⁺. In this review, the chemical structures and synthesis methods of representative π-extended RV²⁺ are reviewed at the beginning, followed by their related cyclophanes, covalent macrocyclic hosts and non-covalent macrocyclic supramolecular coordination complexes. Meanwhile, the recent advances in their applications are discussed, helping to comprehend the relationship between the structures and properties of molecules. Looking ahead, incorporating RV²⁺ within macrocycles should be given great importance to create more valuable photofunctional materials. We hope this review will not only summarize the recent advances in π-extended RV²⁺ but also inspire novel designs of radical-based macrocyclic architectures.