Exploring Nonemissive Excited-State Intramolecular Proton Transfer by Plasmon-Enhanced Hyper-Raman Scattering and Two-Photon Excitation Fluorescence

A large anomalous Stokes shift is one of the most important pieces of evidence for excited-state intramolecular proton transfer (ESIPT). It is a great challenge to confirm ESIPT, when molecular tautomers are nonemissive. In this paper, we provide a proposal to observe ESIPT with very weak emissions in which ESIPT can be experimentally measured by plasmon-enhanced hyper-Raman scattering and two-photon excitation fluorescence (TPEF). Because of the large anomalous Stokes shift in fluorescence, plasmon can strongly simultaneously enhance the resonance absorption and fluorescence by nonlinear and linear (double frequency and fundamental frequency) local surface plasmon resonances. Also, though plasmon-enhanced TPEF may be strong, it cannot interfere with measurement on plasmon-enhanced hyper-Raman scattering, because of the large anomalous Stokes shift in fluorescence. Our methods can provide a potential candidate for measuring ESIPT with weak emissions.

Automated summary

The study proposes a method to measure ESIPT using plasmon-enhanced hyper-Raman scattering and two-photon excitation fluorescence, allowing observation of very weak emissions. Plasmon can enhance resonance absorption and fluorescence simultaneously.