Multiphotonic excitation and solvation dynamics effects on the femtosecond transient absorption of O-hexamethoxyhypericin

The hypothesis of a photoinduced intramolecular H-atom transfer in hypericin is reexamined by comparison with O-hexamethoxyhypericin, a derivative bearing no labile proton. In both cases, femtosecond transient absorption spectroscopy reveals an additional picosecond rising component in the kinetics under high excitation fluences. For the hexamethoxy derivative this effect is mixed with solvation dynamics but we succeeded in decoupling it. The delayed rise is ascribed to bi/multiphotonic excitation to upper excited states, followed by internal conversion to a hot S1 state that subsequently cools down. The present observations are not consistent with the intramolecular H-atom transfer interpretation.