Electron–Vibration Couplings Open New Channels for Energy Redistribution of Self-Assembled Monolayers on Plasmonic Nanoparticles

Unveiling how the interaction between self-assembled monolayers and plasmonic nanoparticles (PNPs) impacts molecular vibrational energy redistribution (VER) is crucial for optimizing plasmon-mediated chemical reactions (PMCRs). However, direct experimental evidence for molecule-PNP interactions opening new energy channels, such as up-pumping energy transfer and self-trapping of vibrational excitation, for VER has yet to be validated. Here, we demonstrate that electron-vibration coupling (EVC) induced by molecule-PNP interactions can open these new pathways for VER by utilizing femtosecond time-resolved sum-frequency generation vibrational spectroscopy. Using self-assembled 4-nitrothiophenol (4-NTP) monolayers on PNPs as a model, we observed that EVC opens a "forbidden" up-pumping energy transfer channel from 4-NTP nitro symmetric stretching (ν_NO2) to phenyl ring C═C stretching (ν_C═C) modes. The self-trapped state of excited ν_C═C modes is found, which originates from EVC-driven intermolecular coupling. These findings contribute to a better understanding of PMCR mechanisms and help guide the design of plasmonic catalysts with excellent performance.