Bidirectional Photoregulated Chromism in Pyridinium Derivatives via Secondary Excitation-Driven Electron Transfer

The key challenge for photochromic pyridinium derivatives is the difficulty in precisely modulating their reverse process. To overcome this limitation, we have developed a secondary excitation strategy to achieve bidirectional photoswitching between the pyridinium cation and its radical state, thus permitting precise control over the dynamic chromic behavior. Owing to the electron-deficient nature of the pyridinium derivative, the intensely colored radical state is generated via a photoinduced electron transfer process under UV light irradiation. The extended absorption band of this radical, originating from the π-stacked molecular structure, endows it with the capability to respond to both visible and near-infrared (NIR) light. Subsequent excitation within this band elevates the radical to a higher-energy state, facilitating single-electron transfer to O₂ molecule and enabling rapid, light-driven recovery to the initial state. This work marks the first demonstration of UV–vis–NIR light-mediated dynamic chromism between pyridinium redox states. Notably, the strong NIR responsiveness and unique radical quenching mechanism endow the material with distinctive environment-sensitive photothermal properties, and establish a novel platform for erasable photoprinting and advanced information encryption/decryption systems.