Picosecond linear dichroism and absorption anisotropy of hypocrellin: Toward a unified picture of the photophysics of hypericin and hypocrellin

The photophysics of hypocrellin is examined by means of transient absorption spectroscopy. On the shortest time scale investigated, an similar to 10 ps component is observed in the isotropic ("magic angle") transient absorption, Delta A(t), and in the anisotropy decay, r(t), for hypocrellin in viscous solvents such as octanol and ethylene glycol. Because the excited-state intramolecular proton-transfer time of the related compound hypericin is similar to 10 ps, the observation of a transient of similar duration in hypocrellin is interpreted as evidence that the photophysics of hypericin and hypocrellin have common features. In particular, it is suggested that their ground- and excited-state potential energy surfaces are similar and that what distinguishes the observed kinetic behavior, which can differ significantly, is the ground-state heterogeneity, that is, the number and population of different forms of hypericin or hypocrellin, which may be tautomers, ions, or conformational isomers.