Impact and Interplay of Quantum Coherence and Dissipative Dynamics for Isotope Effects in Excited-State Intramolecular Proton Transfer

The quantum dynamics of excited-state intramolecular proton transfer (ESIPT) is studied using a non-Markovian open quantum system perspective. Models of 2-(2′-hydroxyphenyl) benzothiazole (HBT) and 10-hydroxybenzo[h]quinoline (HBQ) are adapted from Zhang et al., ACS Phys. Chem. Au, 2023, 3, 107–118 and simulated via the numerically exact TEDOPA matrix product-state formalism, using a newly developed framework for continuous degrees of freedom subject to dissipation. The quantum treatment of the proton wave packet shows a counterintuitive kinetic isotope effect, with strong isotope dependence for the barrierless potential surface of HBQ and no isotope effect in the double-well energy landscape of the HBT, in accordance with experimental results. Strikingly, for HBQ we find that changing laser pulse durations can even reverse the isotope effect on the proton transfer rate, revealing the role of vibration-assisted absorption in ESIPT. This study highlights the often neglected effect of excitation conditions on ESIPT, as well as the role of entangled, vibrationally assisted absorption processes that can be directly visualized in our multidimensional treatment of the full electro-vibronic-environment wave function.