Revising Model Reactions in Plasmonic Chemistry: From Nitrothiophenol Coupling to Alkoxyamine Homolysis

The progress in plasmonic chemistry requires research on energy transfer, mechanisms, and materials discovery. In this pursuit, there are >3000 papers applying the azo coupling of 4-nitrothiophenol (PNTP) as a model reaction. Here, we challenge the status of this reaction as a model due to experimental evidence of thiol desorption during plasmon excitation using laser irradiation monitored by X-ray photoelectron spectroscopy (XPS) as an analytic technique. The azo coupling was performed on commonly used Au nanoparticles (NPs) coated with PNTP and confirmed by Raman spectroscopy and XPS. Changes in the N 1s and S 2p spectral regions indicated the cleavage of the Au-S bond, accompanied by thiol oxidation. Based on XPS data, we hypothesized a chemical pathway and a kinetic model that surpasses previously used simple models in complexity, making it challenging to draw reliable conclusions. The dissociation of the Au-S bond is triggered by plasmonic heating, supported by experimentally and theoretically determined local temperatures exceeding the thiol desorption temperature. The azo coupling reaction does not fit within the requirements of the model one, which should be simple and proceed with structurally evidenced products. As one of the alternative reactions, we suggest alkoxyamine homolysis tracked by electron paramagnetic resonance spectroscopy because of known products and the simple kinetic model. Applications of suitable model reactions accelerate discoveries in plasmon catalysis.