Capturing Coupled Structural and Electronic Motions During Excited-State Intramolecular Proton Transfer via Computational Multi-edge Resonant Inelastic X-ray Scattering

Proton transfer processes build the foundation of many chemical processes. In excited state intramolecular proton transfer (ESIPT) processes, the ultrafast proton transfer is impulsively initiated through light. Here, we explore the time-dependent coupled atomic and electronic motions during and following ESIPT through computational time-resolved resonant inelastic X-ray scattering (RIXS). Excited-state ab initio molecular dynamics simulations combined with time-dependent density functional theory calculations were performed for a model ESIPT complex, 10-hydroxybenzo[h]quinoline, to obtain transient RIXS signatures. The RIXS spectra at both the nitrogen and oxygen K-edges were computed to resolve the electronic and atomic structural dynamics from both the proton donor and acceptor perspective. The results demonstrate that RIXS provides unprecedented details of the local electronic structure, the coupling between different core and valence excited electronic states, and the reorganization of the electronic structure coupled to the proton transfer process. We also develop a spectroscopic ruler correlating spectral shifts of a RIXS peak to the proton transfer distance during ESIPT. This work highlights the exciting potential of time resolved RIXS experiments at newly commissioned soft X-ray free electron laser facilities in measuring coupled electronic and structural changes during ultrafast chemical processes.