Tuning Mixed‐Valence State in Coordination Polymers for Electronics and Optics

Abstract Coordination‐assembled systems have attracted much attention in physics and chemistry because of their structural tunability and wide variety of building units; however, their functionalities are often limited because of the difficulty in controlling dynamic or static charge‐transfer interactions between metal ions. Mixed valencies have recently provided insights into the promotion of charge transfer in coordination systems, endowing them with unique electronic and optical properties. In particular, coordination polymers (CPs) with mixed valencies are promising for realizing long‐range charge delocalization along extended frameworks, leading to semiconducting and even metallic states. In this review, the origin and characterization of mixed valency based on electron–lattice interactions and their classification is first discussed, as well as the fundamentals of electron transport. It is then highlight the relationship between structure and electronic state of mixed‐valence CPs from the viewpoint of dimensional crossover, including the unique valence ordering and tunable electron conductivity. Finally, switchable optical applications involving the modulation of mixed valency based on the electrochromic phenomenon is presented. It is believed that mixed‐valence CPs have great potential for use in next‐generation materials such as electrical conductors, electronic and optical devices, and high‐performance electrochemical applications related to fast charge transfer based on mixed valencies.