Proton transfer-driven intersystem crossing in apigenin

The 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 process that converts the normal tautomer to the proton-transferred tautomer. The intersystem crossing is another event that can occur at the Franck-Condon region and beyond, i.e., along the proton-transfer coordinate, and can 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-lying triplets in intersystem crossing. These findings suggest that proton transfer promotes triplet formation in apigenin. Further simulations reveal an ultrafast internal conversion in the triplet manifold.