Local electric field in nanocavities dictates the vibrational relaxation dynamics of interfacial molecules

Plasmonic nanocavities enable the generation of strong light–matter coupling and exhibit great potential in plasmon-mediated chemical reactions (PMCRs). Although an electric field generated by nanocavities (En) has recently been reported, its effect on the vibrational energy relaxation (VER) of the molecules in the nanocavities has not been explored. In this study, we reveal the impact of an electric field sensed by molecules (para-substituted thiophenol derivatives) in a nanocavity (Ef) on VER processes by employing advanced time-resolved femtosecond sum frequency generation vibrational spectroscopy (SFG-VS) supplemented by electrochemical measurements. The magnitude of En is almost identical (1.0 ± 0.2 V nm−1) beyond the experimental deviation while Ef varies from 0.3 V nm−1 to 1.7 V nm−1 depending on the substituent. An exponential correlation between Ef and the complete recovery time of the ground vibrational CC state (T2) of the phenyl ring is observed. Substances with a smaller T2 are strongly correlated with the reported macroscopic chemical reactivity. This finding may aid in enriching the current understanding of PMCRs and highlights the possibility of regulating vibrational energy flow into desired reaction coordinates by using a local electric field.