Revisiting the Dynamics of Triplet Formation in Anthraquinones.

Triplet formation pathways in 9,10-anthraquinone (AQ) and its hydroxy derivative, 1-hydroxyanthraquinone (HAQ), are studied theoretically. Dynamics simulations on the model singlet-triplet potential energy surfaces within the linear vibronic coupling framework are performed to elucidate possible internal conversion (IC) and intersystem crossing (ISC) pathways in these molecules. An ultrafast IC decay from the "bright" S to S followed by efficient ISC via S-T and S-T pathways fosters a high triplet quantum yield (Φ = 0.90) in AQ. In HAQ, a new nonradiative channel of "barrierless" excited-state intramolecular proton transfer (ESIPT) opens up and competes with the IC decay to S upon photoexcitation to the "bright" S. Extremely fast ESIPT on S reduces the efficiency of triplet formation via possible ISC pathways involving S and S, resulting in a low Φ (=0.17).