Uncovering the Role of Chemical and Electronic Structures in Plasmonic Catalysis: The Case of Homolysis of Alkoxyamines

Abstract

The local surface plasmon resonances of gold nanoparticles have the potential to create alternative pathways for organic chemical reactions. These transformations depend on various physical factors, such as the temperature, illumination regime, and nanoparticle type. However, the role of chemical factors associated with organic reactants, including the molecular structure, electronic effects, and bonding with the metal surface, is often underestimated. To explore the role of these chemical factors, we synthesized five alkoxyamines (AAs) with different chemical and electronic structures and used electron paramagnetic resonance spectroscopy to study the kinetics of plasmon-induced homolysis. The kinetic data revealed that the rate constant (k d) for plasmon-assisted homolysis is dependent on the highest occupied molecular orbital (HOMO) energy of the AAs, which cannot be described by the kinetic parameters or activation energies observed in thermal homolysis experiments. The proximity of the HOMO to the Fermi energy (E f) of Au led to a more active decrease in the energy required to excite the adsorbate. The observed trend in k d indicates that the intramolecular excitation mechanism plays a key role instead of other commonly accepted mechanisms, which is supported by DFT calculations, spectroscopic characterization, and numerous control experiments. The intramolecular excitation mechanism is the most relevant explanation for the plasmon-induced homolysis of AAs. This observation suggests that the electronic structures of the organic molecules may play a key role in other related reactions used to study the mechanisms of plasmon catalysis.

Plasmon-Assisted Chemistry Using Chiral Gold Helicoids: Toward Asymmetric Organic Catalysis

“…The preparation, separation, and characterization of the alkoxyamines, including the optically active and inactive forms (used as the control), are described in the SI (Figures S4–S12 and Table S1, with corresponding discussions). The mechanism of plasmon-induced radical generation via C–ON bond homolysis follows first-order reaction kinetics through the excitation of inner electrons induced by plasmon resonance, a phenomenon previously demonstrated in our previous study. − Plasmon excitation employs both circularly polarized light with opposite rotations and nonpolarized light. Notably, our main goal centers on exploring how different combinations of experimental conditions (alkoxyamine chirality, plasmon-active nanoparticles, and light characteristics) impact the potential of plasmon-assisted enantioselective chemistry in organic transformations.…”

“…Conversely, our previous experiments and quantum mechanics calculations revealed the great possibility of "direct" plasmon catalysis in the case of C−ON bond homolysis in alkoxyamines. 41,42 In particular, our previous work involved plasmon-triggered homolysis in various alkoxyamines with distinct chemical structures, revealing a strong correlation between the reaction kinetics and the position of the HOMO relative to the Au Fermi level. Notably, the reaction kinetics practically did not depend on the position of LUMO, indicating that plasmon catalysis proceeds through a HOMO−LUMO excitation of the inner electron rather than the injection of a hot electron from AuNPs to the LUMO.…”

“…The former implies an “indirect” electron transfer mechanism, where an external hot electron is injected from the plasmon-excited nanostructure into the organic probe. The “direct” pathways proceed through a plasmon-assisted excitation from the highest occupied molecular orbital (HOMO) to the least unoccupied molecular orbital (LUMO) within the probe molecule. , Despite its feasibility, the “indirect” mechanism is considered less plausible. , However, it cannot be completely excluded, as evidenced by the observed surface adsorption of probe molecules on nanoparticles, which enable electron transfer (Figure S14, Tables S3 and S4, and related discussions).…”

“…57,58 Despite its feasibility, the "indirect" mechanism is considered less plausible. 41,42 However, it cannot be completely excluded, as evidenced by the observed surface adsorption of probe molecules on nanoparticles, which enable electron transfer (Figure S14, Tables S3 and S4, and related discussions).…”

Plasmonics: A Versatile Toolbox for Heterogeneous Photocatalysis

“…Importantly, and separating those mechanisms from the aforementioned near-field enhancement effects, hybridization of the molecular orbitals leads to a decrease of their intramolecular bandgap, allowing the generation of an electronic transition driven by visible photons. [60] Some of the first reports describing this process present the photoinduced dissoaciation of dimethyl disulfide molecules adsorbed onto single-crystal Cu and Ag surfaces. [61,62] The authors discuss that for such interaction to take place, a weak hybridization is preferred in order to suppress fast relaxation of the excited state.…”