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.