Spectral Signatures of Ultrafast Excited-State Intramolecular Proton Transfer from Computational Multi-edge Transient X‑ray Absorption Spectroscopy

Excited-state intramolecular proton
transfer (ESIPT) is a fundamental
chemical process with several applications. Ultrafast ESIPT involves
coupled electronic and atomic motions and has been primarily studied
using femtosecond optical spectroscopy. X-ray spectroscopy is particularly
useful because it is element-specific and enables direct, individual
probes of the proton-donating and -accepting atoms. Herein, we report
a computational study to resolve the ESIPT in 10-hydroxybenzo­[h]­quinoline (HBQ), an intramolecularly hydrogen bonded compound.
We use linear-response time-dependent density functional theory (LR-TDDFT)
combined with ab initio molecular dynamics (AIMD)
and time-resolved X-ray absorption spectroscopy (XAS) computations
to track the ultrafast excited-state dynamics. Our results reveal
clear X-ray spectral signatures of coupled electronic and atomic motions
during and following ESIPT at the oxygen and nitrogen K-edge, paving
the way for future experiments at X-ray free electron lasers.