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.