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.