Coupled nuclear and electronic dynamics of proton transfer observed with combined experimental and computational resonant inelastic x-ray scattering

In excited-state intramolecular proton transfer (ESIPT) systems, UV or visible excitation initiates an ultrafast proton transfer that gives rise to distinctive fluorescence properties relevant to optoelectronic applications. We investigate ESIPT in 10-hydroxybenzo[h]quinoline (HBQ) using time-resolved X-ray spectroscopy. Proton transfer in HBQ occurs within ~13 fs and couples strongly to specific vibrational modes. The element-specific nature of the X-ray probe enables direct observation of the local electronic structure and coupling of the core (1s) and valence electronic states at the proton donor (O) and acceptor (N) atoms. The calculated transient resonant inelastic X-ray scattering (RIXS) spectra at the N and O K-edges predict the spectral changes linked to the coupled nuclear and electronic motion of HBQ along its excited-state trajectory. The state-of-the-art measurements of transient RIXS spectra presented here have only recently become possible with the advent of high-repetition-rate soft X-ray pulses at LCLS-II. The experimental spectra allow a detailed mapping of electronic changes following proton transfer. Our results shed light on structure-spectra relationship and the coupling of vibrational modes to the ultrafast proton transfer as viewed through advanced core-level soft X-ray RIXS spectroscopy.