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*).