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

Abstract

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.