Reversible Plasmonic Switch in a Molecular Oxidation Catalysis Process

Abstract

Currently, plasmonic nanoparticles (PNPs) are considered highly efficient enhancers of catalytic processes. Herein, we report a concept where plasmonic Ag⁰@SiO₂ nanoparticles can reversibly switch-off an oxidation catalytic process under light-excitation. The catalytic process recommences when illumination is stopped. The catalytic system under study is a well-characterized molecular LMn^II catalyst that performs alkene oxidation, with H₂O₂ as the oxidant. Three types of plasmonic core–shell Ag⁰@SiO₂ nanoparticles, with a SiO₂ shell of varying thickness (0.1–5 nm), were utilized in this study. Using electron paramagnetic resonance spectroscopy, we have identified the reversible inhibition of the transient LMn^IV═O intermediate formation, to be the key-step of the photoinduced pause of the catalytic process by the Ag⁰@SiO₂ PNPs. Surface-enhanced Raman spectroscopy (SERS) and redox potential data show that the plasmonic Ag⁰@SiO₂ NPs exert a moderate SERS effect on the LMn^II catalyst, and a considerable lowering of the solution redox potential E_h. Our data show that near-field generation is not the sole origin of inhibition of LMn^IV═O formation, while plasmonic heating was insignificant. We suggest that the generation of hot electrons by the Ag⁰@SiO₂ PNPs is implicated, along with near-field generation, in the reversible switch-off of the catalytic process.