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Abstract Plasmon-driven photocatalysis offers a unique means of leveraging nanoscale light–matter interactions to convert photon energy into chemical energy in a chemoselective and regioselective manner under mild reaction conditions. Plasmon-driven bond cleavage in molecular adsorbates represents a critical step in virtually all plasmon-mediated photocatalytic reactions and has been identified as the rate-determining step in many cases. This review article summarizes critical insights concerning plasmon-triggered bond-cleaving mechanisms gained through combined experimental and computational efforts over the past decade or so, elaborating on how the plasmon-derived physiochemical effects, metal–adsorbate interactions, and local chemical environments profoundly influence chemoselective bond-cleaving processes in a diverse set of molecular adsorbates ranging from small diatomic molecules to aliphatic and aromatic organic compounds. As demonstrated by several noteworthy examples, insights gained from fundamental mechanistic studies lay a critical knowledge foundation guiding rational design of nanoparticle–adsorbate systems with desired plasmonic molecule-scissoring functions for targeted applications, such as controlled release of molecular cargos, surface coating of solid-state materials, and selective bond activation for polymerization reactions. 

Plasmon-driven photocatalysis has emerged as a paradigm-shifting approach, based on which the energy of photons can be judiciously harnessed to trigger interfacial molecular transformations on metallic nanostructure surfaces in a regioselective manner with nanoscale precision. Over the past decade, the formation of aromatic azo compounds through plasmon-driven oxidative coupling of thiolated aniline-derivative adsorbates has become a testbed for developing detailed mechanistic understanding of plasmon-mediated photochemistry. Such photocatalytic bimolecular coupling reactions may occur not only between thiolated aniline-derivative adsorbates but also between their nonthiolated analogs. How the nonthiolated adsorbates behave differently from their thiolated counterparts during the plasmon-driven coupling reactions, however, remains largely unexplored. Here, we systematically compare an alkynylated aniline-derivative, para-ethynylaniline, to its thiolated counterpart, para-mercaptoaniline, in terms of their adsorption conformations, structural flexibility, photochemical reactivity, and transforming kinetics on Ag nanophotocatalyst surfaces. We employ surface-enhanced Raman scattering as an in situ spectroscopic tool to track the detailed structural evolution of the transforming molecular adsorbates in real time during the plasmon-driven coupling reactions. Rigorous analysis of the spectroscopic results, further aided by density functional theory calculations, lays an insightful knowledge foundation that enables us to elucidate how the alteration of the chemical nature of metal–adsorbate interactions profoundly influences the transforming behaviors of the molecular adsorbates during plasmon-driven photocatalytic reactions.