Ultrafast nonadiabatic excited-state intramolecular proton transfer in 3-hydroxychromone: A surface hopping approach.

We employ the ab initio molecular dynamics within the surface hopping method to explore the excited-state intramolecular proton transfer taking place on the coupled "bright" S1 (ππ*) and "dark" S2 (nπ*) states of 3-hydroxychromone. The nonadiabatic population transfer between these states via an accessible conical intersection would open up multiple proton transfer pathways. Our findings reveal the keto tautomer formation via S1 on a timescale similar to the O-H in-plane vibrational period (<100 fs). Structural analysis indicates that a few parameters of the five-membered proton transfer geometry that constitute the donor (hydroxyl) and acceptor (carbonyl) groups would be adequate to drive the enol to keto transformation. We also investigate the role of O-H in-plane and out-of-plane vibrational motions in the excited-state dynamics of 3-hydroxychromone.

O-H vibrational motions promote sub-50 fs nonadiabatic dynamics in 3-hydroxypyran-4-one: interplay between internal conversion and ESIPT.

A theoretical study is used to explore the involvement of O-H vibrational motions in the S0 → S2 photoinduced dynamics of 3-hydroxypyran-4-one (3-HOX). Two transitions, S0 → S1 and S0 → S2, are attributed to the experimentally observed electronic absorption spectral features in the range of 3.5-5.5 eV. We compute model potential energy surfaces of vibronically coupled S1 (nπ*) and S2 (ππ*) states with the aid of extensive electronic structure calculations. The S1-S2 conical intersection is characterized in the O-H bend and O-H stretch vibrational coordinate space. Quantum wavepacket dynamics simulations reveal an ultrafast S2 → S1 internal conversion decay, where about 90% of the S2 population disappears within the first 50 fs of the propagation time. The participation of O-H vibrational motions in the early events of nonadiabatic dynamics is analyzed based on the time evolution of nuclear densities on S2. We discuss the implications of these observations to provide fundamental insights into the nonadiabatic excited-state intramolecular proton transfer in 3-HOX and its derivatives.

Excited-state intramolecular proton transfer driven by conical intersection in hydroxychromones.

Nonradiative decay pathways associated with vibronically coupled S1 (ππ*)-S2 (nπ*) potential energy surfaces of 3- and 5-hydroxychromones are investigated by employing the linear vibronic coupling approach. The presence of a conical intersection close to the Franck-Condon point is identified based on the critical examination of computed energetics and structural parameters of stationary points. We show that very minimal displacements of relevant atoms of intramolecular proton transfer geometry are adequate to drive the molecule toward the conical intersection nuclear configuration. The evolving wavepacket on S1 (ππ*) bifurcates at the conical intersection: a part of the wavepacket moves to S2 (nπ*) within a few femtoseconds while the other decays to S1 minimum. Our findings indicate the possibility of forming the proton transfer tautomer product via S2 (nπ*), competing with the traditional pathway occurring on S1 (ππ*).

Nonadiabatic excited-state intramolecular proton transfer in 3-hydroxyflavone: S2 state involvement via multi-mode effect.

The excited-state intramolecular proton transfer process in 3-hydroxyflavone is investigated based on the computed structural parameters and energetics of stationary points of vibronically coupled S1-S2 potential energy surfaces. A conical intersection close to the Franck-Condon point on S1 is identified. The minimum energy of the conical intersection is found to be near-degenerate with the equilibrium minimum of S2. Quantum nuclear wavepacket simulations revealed a small population transfer from the "bright" S1 to "dark" S2 on a time scale shorter than the O-H stretching vibrational period. Such a nonadiabatic transition opens up the possibility of new photophysical and photochemical pathways, including the proton transfer via S2.