Planar Viologen-Based Crystalline Compounds Showing Heat-Induced Electron Transfer and Thermochromism

Heat-induced electron transfer (ET) is desirable for some applications, such as thermal/photothermal catalysis, thermochromic antifake, and thermal control of molecular recognition, but needs to be avoided for photochromic or electrochromic windows and other devices. However, to date, the key structural factors of heat-induced ET for viologen compounds are still unclear. This work reveals that the planar structures of viologen moiety may be an important factor for heat-induced ET and thermochromism, through the summarization of a structural law from established thermochromic viologen compounds and the experimental verification using three known compounds with near-planar viologen moieties. Hence, this work contributes to enhance the probability of finding thermochromic viologen compounds with heat-induced electron transfer process.