Plasmon-Driven Reaction Mechanisms: Hot Electron Transfer versus Plasmon-Pumped Adsorbate Excitation

Abstract

Photochemistry that can be driven at low incident photon flux on optically excited plasmonic nanoparticles is attracting increasing research interest because of the fundamental need to combine surface reaction and in situ spectroscopy as well as the opportunity that plasmon-driven reactions may offer a pathway for efficient conversion of solar energy into fuel. In mechanistic studies of plasmon-driven reactions to date, a great deal of emphasis is given to hot electron transfer. The results summarized in this Feature Article indicate that photochemistry on plasmonic nanoparticles can be induced by hot electron transfer from the nanoparticle to an unoccupied orbital of the adsorbate and/or by plasmon-pumped electron transition from an occupied molecular orbital to an unoccupied molecular orbital of the adsorbate. The branching photochemical reaction of para-aminothiophenol on the plasmonic gold surface depending on the presence of a cetyltrimethylammonium bromide surface ligand that influences the hot electron concentration is used to highlight reactions driven by hot carriers. The importance of plasmon-pumped electronic excitation of adsorbates in initiating surface photochemistry is demonstrated based on the N-demethylation of methylene blue (MB) on gold nanostructures depending on excitation wavelengths. At excitation wavelength that overlaps with the resonances of MB and the gold nanoparticles, conversion of MB to thionine is observed in the presence of oxygen in the atmosphere and water in the surface−molecule complex. Considering that MB is a well-known photosensitizer, this observation suggests that the photochemical N-demethylation reaction involves singlet oxygen that can be generated via energy transfer from the MB triplet excited state to the O 2 triplet ground state.

Mechanistic Insights into Photocatalyzed H2 Dissociation on Au Clusters

“…This is also known as desorption (or dissociation) induced by electronic transitions (DIETs) in the surface science community. , (2) An alternative is the direct charge transfer (CT) mechanism, where the electrons are directly excited from metal states near the Fermi level to the lowest unoccupied molecular orbital (LUMO) of the adsorbed molecule, which circumvents the thermalization of HEs, , but requires matching between the plasmon energy and the energy gap between the metal states and the molecular LUMO. (3) The excitation may also take place within the adsorbate, in the direct intramolecular excitation mechanism, in which the photon promotes the electron from the highest occupied molecular orbital (HOMO) to the LUMO of the adsorbed molecule without the participation of the metal electrons . This mechanism requires the LSPRs to overlap the molecular electronic transition energy .…”

“…(3) The excitation may also take place within the adsorbate, in the direct intramolecular excitation mechanism, in which the photon promotes the electron from the highest occupied molecular orbital (HOMO) to the LUMO of the adsorbed molecule without the participation of the metal electrons. 43 This mechanism requires the LSPRs to overlap the molecular electronic transition energy. 44 (4) Finally, reaction can be activated by local heating, resulting from the hot-carrier relaxation.…”

Can Plasmon Change Reaction Path? Decomposition of Unsymmetrical Iodonium Salts as an Organic Probe

“…[27][28][29][30][31] As a result, three main hypotheses have been proposed: (1) reaction acceleration through 4 plasmonic heating, (Figure 1A); [32][33][34] (2) transfer of a hot electron to the organic molecule, followed by the formation and relaxation of the excited state (Figure 1B); and (3) intramolecular excitation of an electron to the LUMO orbitals via the decay of the optically excited surface plasmon (SP) (Figure 1B). 28,35,36 The all proposed mechanisms have been probed by theoretical and experimental studies in a range of (organic) transformations, but the final point in these discussions have not been set. One possible key issue into understanding the mechanism of plasmon-induced organic reactions involves analyzing their regioselectivity, which may act as a reporter of the reaction path of the interaction of the plasmon with the organic molecule.…”

Сan Plasmon Change Reaction Path? An Unprecedented Regioselective Decomposition of Unsymmetrical Iodonium Salts

“…[27][28][29][30][31] As a result, three main hypotheses have been proposed: (1) reaction acceleration through plasmonic heating, ( Figure 1A); [32][33][34] (2) transfer of a hot electron to the organic molecule, followed by the formation and relaxation of the excited state ( Figure 1B); and (3) intramolecular excitation of an electron to the LUMO orbitals via the decay of the optically excited surface plasmon (SP) ( Figure 1B). 28,35,36 The all proposed mechanisms have been probed by theoretical and experimental studies in a range of (organic) transformations, but the final point in these discussions have not been set. One possible key issue into understanding the mechanism of plasmon-induced organic reactions involves analyzing their regioselectivity, which may act as a reporter of the reaction path of the interaction of the plasmon with the organic molecule.…”