Hydrogenation/oxidation triggered highly efficient reversible color switching of organic molecules

Abstract

Catalytic hydrogenation and oxidative dehydrogenation reactions are fundamental and significant processes in organic transformation, and reversible color switching of organic redox dyes finds potential applications in rewritable paper, sensing devices, data recording and security feature technologies. In this study, we report an interesting result of reversible hydrogenation and oxidative dehydrogenation of a redox dye over a Pd–ZnO_1−x hybrid nanocatalyst under ambient conditions. Thionine (TH⁺) is used as a model compound to evaluate the catalytic performance. The reversible color switching between purple thionine (TH⁺) and colorless leuco-thionine (LTH) depends on the reducing or oxidizing environments. Our newly developed Pd–ZnO_1−x nanocatalyst exhibits high catalytic activity for the hydrogenation of TH⁺ with a turnover frequency (TOF) as high as 397 h⁻¹ under H₂ (1 bar). The oxidative dehydrogenation of LTH is performed under 1 bar O₂ flow in the same reaction system. The Pd–ZnO_1−x nanocatalyst readily adsorbs and subsequently dissociates O₂ to oxidize LTH to the original purple colored TH⁺ with higher efficiency. The abundant oxygen vacancies on ZnO_1−x nanorods and strong metal–support interaction (SMSI) promote the adsorption and subsequent dissociation of molecular hydrogen and oxygen leading to high catalytic activity. This novel reversible color switching of organic dyes can be performed successively for more than 10 cycles in a one pot-fashion using a Pd–ZnO_1−x nanocatalyst with a small loss in performance. The highly efficient reversible color switching of TH⁺/LTH over the Pd–ZnO_1−x nanocatalyst provides a state-of-the-art protocol to find practical applications as printing inks for rewritable paper and in sensing and security feature devices.