Unraveling the sub-100 fs ESIPT in 5-hydroxychromone using surface hopping simulations

We explore the structural, energetic and dynamic aspects of the excited-state intramolecular proton transfer process of 5-hydroxychromone using the trajectory surface hopping method. Gas-phase dynamics simulations show that the proton transfer on the “bright” S1 (ππ∗) is essentially a barrierless event and occurs on a timescale of about 24 fs. Our findings also demonstrate the possible proton transfer via “dark” S2 (nπ∗) after a successful hop from S1 to S2 via an accessible conical intersection that lies energetically and spatially nearby the Franck–Condon point of S1. An intrinsic barrier on S2 would delay the proton transfer dynamics (∼ 100 fs). The proton transfer occurs entirely via S1 in the presence of acetonitrile as a solvent. Computed energetics and geometries associated with key stationary points in the solvent environment are similar to gas-phase estimates, suggesting negligible solvation effects on the rapidity of the proton transfer via S1 in 5-hydroxychromone. [Display omitted] •Barrierless proton transfer on S1.•Extremely rapid ESIPT (∼25 fs).•Negligible implicit solvation effects on the excited-state dynamics.•Predicted single emission with large Stokes shift.