Revisiting the Dynamics of Triplet Formation in Anthraquinones

Abstract

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 4 to S 1 followed by efficient ISC via S 1 −T 4 and S 1 −T 5 pathways fosters a high triplet quantum yield (Φ T = 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 1 upon photoexcitation to the "bright" S 2 . Extremely fast ESIPT on S 2 reduces the efficiency of triplet formation via possible ISC pathways involving S 1 and S 2 , resulting in a low Φ T (=0.17).

Anthraquinone-Centered Type I Photosensitizer with Aggregation-Induced Emission Characteristics for Tumor-Targeted Two-Photon Photodynamic Therapy

“…On the basis of this consideration, a basic building block named anthraquinone (AQ) comes into our sights because of its unique photophysical properties. 47 Upon excitation, the triplet quantum yield of AQ is reported as high as 90%, which attributes to the significantly large spin−orbit coupling (SOC) effect caused by its fast n−π* transition. 48,49 More importantly, because of its distinct structural character, an oxidation− reduction reaction cycle among the triplet AQ, the semiquinone anion radical (AQ •− ), and 9,10-dihydroanthracene-9,10-diol (AQH2) can be steadily operated while also continuously capturing and releasing electrons.…”

“…After revisiting the processes of type I ROS generation that 3 PS* first captures one electron from its surroundings to produce PS •– followed by transfer of the electron to molecular oxygen to yield O 2 •– or further conversion to highly oxidative OH • through the well-known Haber–Weiss and Fenton reactions, , it can be noted that the beneficial release of the captured electrons from PS •– plays a vital role in the subsequent chemical reactions for radicals generation. On the basis of this consideration, a basic building block named anthraquinone (AQ) comes into our sights because of its unique photophysical properties . Upon excitation, the triplet quantum yield of AQ is reported as high as 90%, which attributes to the significantly large spin–orbit coupling (SOC) effect caused by its fast n−π* transition. , More importantly, because of its distinct structural character, an oxidation–reduction reaction cycle among the triplet AQ, the semiquinone anion radical (AQ •– ), and 9,10-dihydroanthracene-9,10-diol (AQH2) can be steadily operated while also continuously capturing and releasing electrons.…”