Five-membered vs. six-membered ESIPT in 3,5-dihydroxychromone

Abstract

3,5-dihydroxychromone (3,5-DHC) possesses five-membered and six-membered ring hydrogen-bonding structures that would facilitate an excited-state intramolecular proton transfer upon photoexcitation to the dipole-allowed S1 (ππ∗) state. Our investigation based on surface hopping trajectory-based simulations show that proton transfer in 3,5-DHC happens efficiently via the six-membered ring structure with an average timescale of 29 fs. The proton transfer via this pathway is a barrierless process, whereas a barrier of ∼0.46 eV exists for the other pathway that proceeds via a five-membered ring structure. The proton-transferred tautomer occurring via the former pathway is more stable than the one originating from the latter. Hence, the former tautomer exhibits a relatively larger Stokes-shifted fluorescence emission than the latter one.