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

Abstract

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.

Probing the Role of Atomic Defects in Photocatalytic Systems through Photoinduced Enhanced Raman Scattering

“…These measurements were repeated at different random positions across each respective sample to determine an average SERS decay. It should be noted that for the experimental conditions used here no evidence of MBA decarboxylation , or other competing processes was observed in any of the measured Raman series.…”

Regioselective Plasmon-Driven Decarboxylation of Mercaptobenzoic Acids Triggered by Distinct Reactive Oxygen Species

“…Time-dependent SERS spectra of functionalized AuNPs (Figure S5) during plasmon-driven decarboxylation are displayed in Figure A,B. In contrast to that observed for catalytic experiments over AgNPs, the SERS spectrum of 2-MBA changes as the laser exposure time increases, and the appearance and steady growth of two new Raman peaks at 999 and 1072 cm –1 , assigned to the plasmon-driven production of TP, ,− is observed in the first seconds of the reaction (Figure C). The removal of carboxylate groups exhibited similar laser-power- and wavelength-dependence in comparison to the TP formation over AgNPs and indicated that the photocatalytic process on AuNPs is plasmon-driven as well (Figures S6 and S7).…”

“…While SERS spectra of 2-MBA did not exhibit any noticeable changes upon laser illumination (Figure C), time series SERS spectra of 4-MBA adsorbed on AgNPs show that the intensity of the COO – stretching mode of vibration − at 1430 cm –1 diminishes, accompanied by the rise of new Raman peaks at 999 and 1020 cm –1 (Figure D). Additionally, the intensity of the Raman peak at 1585 cm –1 , assigned to a ring mode of 4-MBA, also decreases over time, together with the appearance of a shoulder peak at 1572 cm –1 . , These new Raman peaks are assigned to spectral features of TP (Figure D, top panel) and indicate that 4-MBA molecules adsorbed to AgNPs undergo decarboxylation to produce TP through plasmon-driven photocatalysis. ,− …”

“…Additionally, the intensity of the Raman peak at 1585 cm −1 , assigned to a ring mode of 4-MBA, also decreases over time, together with the appearance of a shoulder peak at 1572 cm −1 . 34,35 These new Raman peaks are assigned to spectral features of TP (Figure 1D, top panel) and indicate that 4-MBA molecules adsorbed to AgNPs undergo decarboxylation to produce TP through plasmon-driven photocatalysis. 14,34−36 The decarboxylation reaction on AgNPs exhibited powerdensity-and wavelength-dependence characteristics of plasmon-induced photocatalysis (Figures S2 and S3).…”

Ultrasensitive detection of pectin based on the decarboxylation reaction and surface-enhanced Raman spectroscopy

“…Briey, the uronic acid loses the carboxylic acid (carbon dioxide) group (-COOH) and the three molecules of water are removed, resulting in the formation of the corresponding furan compound under conditions of strong acidity and high temperature. 45,46 The three acid-derived components of the uronic acids, namely, 2-furan aldehyde (furfural), 5-formyl-2-furanformic acid, and reductive acid (2,3-dihydroxy-2-cyclopentene-1-one) are essentially endproducts in the reaction. 22,47 Surprisingly, during the reaction, the C-2 of uronic acid forms furfural, and the aldehyde carbon is completely from the aldehyde group of the parent uronic acid.…”