Ultrafast Intersystem Crossing in Naturally Occurring Plant Pigments 5-Hydroxyflavones under Direct UV Excitation

Flavonoids, a group of natural pigments, have attracted notable attention for their intrinsic fluorescent bioactive properties and potential therapeutic implications. Recent studies have suggested that the photoexcitation of specific flavonoids can also lead to the formation of triplet states, thereby potentially enhancing their applications in photoactivated antioxidant mechanisms. However, the crucial mechanism details about triplet state formation are still poorly understood. In this Letter, the ultrafast excited state relaxation mechanism for a series of 5-hydroxyflavone derivatives was studied by femtosecond time-resolved spectroscopy combined with quantum chemical calculations. Our results reveal the ultrafast ISC (k_ISC ≈ 10¹¹ s^-1) channel, which is sensitive to molecular structure and solvent environment, in 5-hydroxyflavones for the first time. Notably, the triplet excited state quantum yield of 4′,7-dimethoxy-5-hydroxyflavone can reach up to 8% in acetonitrile solution. These results are essential for understanding the triplet state generation mechanism in 5-hydroxyflavone derivatives and could help the further development of 5-hydroxyflavone scaffold antioxidant agents.