Ultrafast excited-state proton transfer dynamics using linearized pair-density functional theory

Accurate simulation of excited-state bond-rearrangement dynamics remains a major challenge since photoinduced reactions can often involve significant changes in electronic structure along excited-state reaction pathways. Describing such processes requires an electronic structure method that provides balanced descriptions of all electronic states across the nuclear configuration space, while remaining computationally feasible for molecular dynamics. Linearized pair-density functional theory (L-PDFT) provides an efficient multireference framework for excited-state simulations by enabling an accurate multistate treatment of excited-state potential energy surfaces. In this work, we assess the performance of L-PDFT for excited-state bond-rearrangement dynamics using excited-state intramolecular proton transfer (ESIPT) as a stringent benchmark. Ab initio molecular dynamics simulations are performed for 10-hydroxybenzo[h]quinoline, a prototypical ESIPT system that undergoes ultrafast proton migration following photoexcitation. L-PDFT predicts that ESIPT for the molecule occurs within 16 fs, in close agreement with latest ultrafast time-resolved fluorescence experiments. Trajectory analysis reveals an active role of the proton in driving the ESIPT. These results demonstrate that L-PDFT can describe excited-state photodynamics involving bond rearrangements, highlighting its potential for broader light-driven chemical processes, including excited-state reactivity in photocatalytic transition metal-based systems.