Dual-Excited-State Intramolecular Proton Transfer Pathways Observed in Fisetin

Flavonoids represent a major class of phenolic secondary metabolites in the plant kingdom. Their intrinsic fluorescence and diverse pharmacological activities grant them significant application value in the fields of medicine and food. Although excited-state intramolecular proton transfer (ESIPT) has been reported in several hydroxyl-substituted flavonoids, its underlying mechanism remains unclear, particularly when higher electronic states are involved. In this work, the excitation-wavelength-dependent ESIPT mechanism of fisetin, a 3-hydroxyflavone derivative, was investigated by using femtosecond transient absorption (TA) spectroscopy combined with time-resolved infrared spectroscopy (TRIR) and quantum chemical calculations. Dual-ESIPT pathways were directly observed when fisetin was dissolved in acetone and acetonitrile. Furthermore, even when only the S1 state is initially excited in these solvents, the S2-mediated ESIPT channel remains the dominant route for fisetin, though its proportion progressively decreases as the excitation wavelength shifts to the blue. The underlying mechanism is proposed and is discussed.