Proton Transfer-Driven Intersystem Crossing in Apigenin

Abstract

Energetics and dynamics of excited-state events that originate from the S1 state of the apigenin molecule are studied theoretically. The dominant event is the barrierless excited-state intramolecular proton transfer that converts the normal tautomer to the proton-transferred tautomer. The intersys- tem crossing is another event that can occur at the Franck-Condon region and beyond, i.e., along the proton transfer coordinate, allowing it to compete with proton transfer. Aided by the small energy gap and a noticeable spin-orbit coupling, the T4 would be an energetically feasible receiver triplet state at the Franck-Condon geometry of the S1 state compared to the other lower triplet states. The S1-T3 and S1-T2 pathways become relevant beyond the Franck-Condon point, highlighting the role of higher triplets in the intersystem crossing. These findings suggest that proton transfer promotes triplet formation in apigenin. Further simulations reveal an ultrafast internal conversion in the triplet manifold