Probing Adsorption Configurations and Reaction Selectivity in Plasmon-Induced Reactions at the Nanoscale via Tip-Enhanced Raman Spectroscopy.

Plasmon-induced chemical reactions, particularly those probed via surface-enhanced Raman spectroscopy and tip-enhanced Raman spectroscopy (TERS), have long served as model systems for investigating photocatalytic processes at the nanoscale. However, the influence of molecular orientation on reaction selectivity down to the molecular level remains elusive. Here, we investigate the role of molecular adsorption orientation in the plasmon-driven chemical reactions of 4-BTP on polycrystalline silver (Ag), polycrystalline gold (Au), and single-crystal Au(111) surfaces. Using high-resolution TERS, we observe pronounced differences in reaction pathways across substrates and between ordered versus disordered self-assembled monolayers. These variations in selectivity─between hydrodebromination to thiophenol (TP) and intermolecular C-C coupling to 4,4'-biphenyldithiol (BPDT)─are primarily governed by steric constraints imposed by molecular packing and orientation. Our results establish a clear structure-selectivity relationship at the molecular level and demonstrate that steric engineering of reactant assembly offers a powerful strategy for tailoring surface reaction pathways, complementing traditional electronic control paradigms.