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 ∼10 ps component is observed in the isotropic (“magic angle”) transient absorption, Δ 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 ∼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.