Reversible Redox Switching of Concurrent Luminescence and Visual Color Change Based on Lanthanide Metallogel

The development of reversible redox supramolecular gels capable of concurrent luminescence switch and visible color change with the facile redox process has always been an intriguing challenge. A redox-responsive supramolecular lanthanide metallogel with strong luminescence and yellow color is obtained via coordination interaction between 3,5-dinitrosalicylic acid (DNSA) and europium (Eu³⁺). Upon the addition of TiO₂ to the prepared gel (DNSA/Eu³⁺ gel), the oxidation process of the gel (DNSA/Eu³⁺/TiO₂ gel) can be easily achieved by UV irradiation. The DNSA/Eu³⁺/TiO₂ gel exhibits a concurrent reversible "on-off" luminescence and color change in response to redox stimuli. The DNSA/Eu³⁺/TiO₂ gel shows a concurrent quench of luminescence and a color change from yellow to red when the gel was stimulated by the reductant. Upon UV irradiation, the luminescence and color of the reduced DNSA/Eu³⁺/TiO₂ gel restored to its initial state due to the strong oxidation ability of hydroxyl radicals derived from photocatalytic oxidation of TiO₂. The results of UV-vis and mass spectroscopy indicated that the reversible redox responsiveness of DNSA/Eu³⁺/TiO₂ gel depends on the reversible oxidation-reduction reactions of DNSA. Moreover, DNSA/Eu³⁺/TiO₂ gel remains stable because the morphology of the gel had no change during the redox process. Exemplarily, the application of DNSA/Eu³⁺/TiO₂ gels to achieve luminescent patterning was investigated. The results demonstrated that the prepared metallogel has potential applications in the fields of writable materials, anticounterfeiting, sensors, and others.