Excitation energy effect on the early photophysics of hypericin in solution

Picosecond transient absorption spectra of hypericin in solution have been recorded over the 350-850 nm spectral range under excitation fluences, respectively, 3/4 and four times the saturation fluence. At low fluence only a weak sub-100 ps decay component is detected, superimposed to the main nanosecond decay of the excited state. At high fluence a sub-10 ps rising component, comparable to that of earlier reports, is observed. This novel observation is discussed within the context of the state-of-the-art theory of hypericin photophysics, involving intramolecular excited-state H atom transfer. A cooling process in the S1 state after biphotonic absorption is proposed as an alternative explanation.