Plasmon-Driven Protodecarboxylation of Mercaptobenzoic Acids: A Comparative Study of Regioisomers

Localized plasmons sustained by metallic nanostructures can be judiciously exploited to trigger photocatalytic reactions through unconventional molecule-transforming channels typically inaccessible to heterogeneous catalysis under thermal conditions. With the aid of plasmons, chemisorbed para-mercaptobenzoic acid on nanostructured metal surfaces may selectively transform into thiophenol through a protodecarboxylation process in aqueous environments at temperatures substantially lower than those required for thermal catalytic decarboxylation reactions. Although the decarboxylation behaviors of para-mercaptobenzoic acid in response to plasmonic stimuli have been investigated on a broad range of nanostructured surfaces under a diverse set of reaction conditions, plasmon-driven decarboxylation of ortho-mercaptobenzoic acid and meta-mercaptobenzoic acid has only been reported sporadically in the literature. How the three regioisomers differ in their reactivity and chemoselectivity toward plasmon-driven protodecarboxylation under specific reaction conditions remains an interesting open question that deserves detailed comparative studies. Here, we systematically compare the decarboxylation rates, yields, and selectivity of the three mercaptobenzoic acid regioisomers triggered by the gap-mode plasmons of Ag nanoparticle arrays under visible and near-infrared laser illuminations in the pH range of 1–13. Using surface-enhanced Raman scattering as an in situ molecular fingerprinting tool, we have been able to extract detailed kinetic information about the photocatalytic molecular transformations occurring within the gap-mode plasmonic hot spots. The results presented in this work provide a critical knowledge foundation for understanding the regioselective substitution effects involved in plasmon-driven protodecarboxylation of aromatic carboxylate adsorbates.