Excited-State Intermolecular Proton Transfer and Competing Pathways in 3-Hydroxychromone: A Non-adiabatic Dynamics Study

TL;DR

This study investigates ESIPT in 3-Hydroxychromone using on-the-fly non-adiabatic dynamics (SHARC with TD-PBE0) to reveal two proton-transfer time scales. It maps the excited-state potential energy surfaces, identifying a low barrier on S1 and a large barrier on S2, plus a nearby S1/S2 MECI and S1 torsional minima that enable a competing torsion-mediated pathway. Kinetic analysis shows a fast component around tens of femtoseconds and a slower component on the sub-picosecond to picosecond scale, with the slower rate arising from out-of-plane hydrogen torsion after seam-crossing. The authors construct an explicit eight-state reaction network that unifies canonical ESIPT with torsion-driven channels, offering a mechanistic framework to interpret dual ESIPT time constants and guiding design of 3-HC–based photonic probes.

Abstract

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 nonadiabatic trajectories enables us to construct an explicit reaction network for 3-HC, delineating the interplay between canonical 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.

Figures (12)

• Figure 1: (a) Enol and keto tautomeric forms of the 3-hydroxychromone (3-HC). (b) (cis-)enol to trans-enol conformational equilibrium.
• Figure 2: TD-PBE0/cc-pVDZ vertical absorption spectrum of 3-HC obtained via the nuclear ensemble approach. The gray band represents the energy window used for the initial condition selection. No energy shift was applied to the computed spectrum. The digitalised experimental absorption spectrum (band C) from Chevalier et al. chevalier2013esipt is reported as a blue dashed line.
• Figure 3: Linear interpolation in internal coordinates (LIIC) along the critical points that characterise the ESIPT coordinate at TD-PBE0/cc-pVDZ level of theory. The energy barrier on the S$_1$ potential energy surface between the ($cis$-)enol minimum to the TS is indicated by the two arrows.
• Figure 4: Linear interpolation in internal coordinates (LIIC) between the MECI and the S$_1$ torsional minimum (left) and relaxed scan from the S$_1$ torsional minimum to the planar trans-enol conformer (right). Both the energy profiles were obtained at TD-PBE0/cc-pVDZ level.
• Figure 5: Electronic state population evolutions upon excitation to the S$_1$ (top) and S$_2$ (middle) electronic state. Combined S$_1$ and S$_2$ ensembles (bottom).