Hot-Hole-Induced Molecular Scissoring: A Case Study of Plasmon-Driven Decarboxylation of Aromatic Carboxylates

Optically excited plasmonic nanostructures may function as molecular scissors with unique capabilities to cleave specific chemical bonds in molecular adsorbates through either plasmon- enhanced intramolecular electronic excitations or injection of photoexcited hot electrons into adsorbate orbitals. Here we chose plasmon-driven decarboxylation of aromatic carboxylates as a model reaction system to demonstrate that plasmonic hot holes, instead of electrons, could also be effectively harnessed to trigger regioselective bond cleavage in molecular adsorbates. We used surfaceenhanced Raman scattering as an in situ spectroscopic tool to precisely monitor the decarboxylation reactions in real time and further correlate the reaction kinetics to local-field enhancements. The apparent rate constants were proportional to the fourth power of the local-field enhancements and exhibited a superlinear dependence on the excitation powers. Such a non-linear power dependence of reaction rates was a hallmark of hot-carrier-driven reactions involving multiphoton absorption rather than photothermally triggered processes, as inferred by the results of Raman thermometry. The decarboxylation reactions took place only at the surface sites with local-field intensities exceeding a certain threshold value, whereas the molecules experiencing weaker local fields below the threshold remained essentially unreactive. With the aid of density functional theory calculations, we were able to further relate the experimentally observed pH dependence of reaction kinetics to the frontier orbital energies of the hole-accepting adsorbates and the redox potentials of the electron-accepting protons, both of which could be modulated by adjusting the pH of the reaction medium.