π-Extended Viologen Derivatives and Their Related Macrocycles: From Design Synthesis to Photofunctional Applications

Abstract

Viologen derivatives (RV2+) have attracted much attention due to their unique redox processes, which can be used to catalze 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+ were overviewed at the beginning, followed by their related cyclophanes, covalent macrocyclic hosts and non-covalent macrocyclic supramolecular coordination complexes. Meanwhile, the recent advances of their applications were discussed, helping to comprehend the relationship between structures and properties of molecules. Looking ahead, incorporating RV2+ within macrocycles should be attached great importance to create more valuable photofunctional materials. We hope this review will not only summarize the recent advances of π-extended RV2+ but also inspire the novel design of radical-based macrocyclic constructions.