A Universal Strategy for Reversible Photochromism of Viologen Derivatives in Solutions

Abstract Viologens are important members of organic photochromic (PC) materials. However, the photochromism of the viologen derivatives in solutions, especially in water, can be hardly detected due to the restricted photo‐induced electron transfer (PET) caused by the solvation of counter anions and fast rapid back electron transfer upon oxidation by the dissolved oxygen. Herein, a universal strategy is successfully developed for reversible photochromism of viologen derivatives in various solvents by simply adding an electron donor triethylamine (TEA). Upon the formation of a charge transfer complex, PET process can smoothly occur, resulting in the formation of stable viologen radicals. The PC kinetic analysis reveals that a viologen derivative with narrower bandgap exhibits faster coloration process upon ultraviolet (UV) light irradiation. Moreover, incorporating 1‰ eq. of a conjugation‐extended viologen derivative as a photosensitizer significantly shortens the coloring time of a conventional viologen without affecting its bleached and colored states. Based on the PC properties of viologen derivatives, distinct patterns are realized in their aqueous solutions and pre‐soaked erasable papers upon UV light irradiation. Overall, the findings not only develop a universal strategy for reversible photochromism of viologen derivatives in various solvents but also open a new avenue for information storage and anticounterfeiting applications.