Excited-state intermolecular proton transfer and competing pathways in 3-hydroxychromone: a non-adiabatic dynamics study.

Excited-state intramolecular proton transfer (ESIPT) is a fundamental photochemical process in which photoexcitation induces proton transfer within a molecule, leading to the formation of a tautomeric excited state. It was observed experimentally that the 3-hydroxychromone (3-HC) system exhibits two distinct proton-transfer time scales upon excitation to the lowest bright singlet excited state: an ultrafast component on the femtosecond time scale and a slower one on the picosecond time scale, largely insensitive to solvent effects. Up to now, the microscopic origin of the second time constant has only been hypothesised. Here, using mixed quantum-classical non-adiabatic dynamics simulations, we explicitly observe the two ESIPT time constants and we rationalise the origin of the second time scale by the presence of a competitive out-of-plane hydrogen torsional motion. Comprehensive analysis of the excited-state potential energy surfaces and non-adiabatic trajectories enables us to construct an explicit reaction network for 3-HC, delineating the interplay between direct ESIPT and torsion-mediated pathways. This unified mechanistic framework reconciles the coexistence of ultrafast and slower ESIPT components, offering new insights into the non-adiabatic excited-state dynamics of the system.