A Viologen Derivative Transition from Photosilence to Sensitive Photochromism through Ion Exchange

Viologen has attracted much attention due to its sensitive chromic properties in response to external stimuli, especially its photochromic behavior. Herein, a viologen derivative, 4PV·2Br, was prepared based on our previous studies, which exhibited strong photoluminescence due to the introduction of the thiazolothiazole group but was silent to light, even under 300 W UV light irradiation. By anion exchange with tetraphenylborate, a new viologen derivative, 4PV·2B(Ph)4, was obtained, which not only exhibits strong fluorescence with a quantum yield of 69% and a lifetime of 1.93 ns in solution but also exhibits sensitive photochromism to UV light, even under sunlight within 5 s. It is very sensitive to light and may become a potential excellent solar chromic material. It should be emphasized that the colored 4PV·2B(Ph)4 radical is particularly stable and can maintain its coloring state in air for a long time, while vinegar vapor can cause it to fade in 2 s, giving it good color reversibility. The single-crystal structure before and after UV light excitation shows that UV light shortens the π···π distances between B(Ph)4 anions and 4PV cations, which is beneficial to faster electron transfer between them and the formation of stable viologen radicals. Sensitive photochromism accompanied by strong photoluminescence may endow 4PV·2B(Ph)4 with potential application in information security and provide a good strategy to regulate the functionality of materials by regulating the counteranions of new viologen derivatives.