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.