Competition between Reaction and Degradation Pathways in Plasmon-Driven Photochemistry

Plasmonic materials are exciting candidates for driving photochemical reactions, as they couple strongly with light across a wide range of the electromagnetic spectrum and can dramatically impact the photophysical properties of proximal molecular species. Plasmons have been shown to drive a number of photochemical reactions, but a detailed understanding of the mechanism is lacking in many cases. Here we investigate the effects of plasmonic field enhancement of the plasmon-driven conversion of 4-nitrobenzenethiol to 4,4′-dimercaptoazobenzene. By tuning the ensemble-averaged field enhancement of a plasmonic substrate, we quantify how the reaction yield and rate depend on the magnitude of the electric field. Surprisingly, we find no correlation of increased reaction rate or yield with greater field enhancement. Kinetic analysis of the reaction rate constants reveals a wide range of values, indicating that plasmonic excitation is not the rate-limiting step in this system. Additionally, we identify a competing degradation pathway that significantly contributes to the loss of reactant. This work identifies several factors that are critical in determining the overall efficiency of a plasmon-driven process and should help to lead to optimally designed plasmonic photocatalytic systems.