The well-characterized, monodispersed nature of reverse micelles formed by sodium bis(2-ethylhexyl)sulfosuccinate/heptane and their usefulness in approximating a membrane-like environment have been exploited to investigate the effect of pH and water pool size on the photophysical properties of hypericin (Hyp). Our measurements reveal two titratable groups of pKa ∼1.5 and ∼12.5. These are assigned to the HypH /Hyp equilibrium (the deprotonation of a carbonyl group) and the Hyp−/Hyp2− equilibrium (the deprotonation of a peri hydroxyl group). The low-energy absorbance maxima of HypH , of Hyp and Hyp− and of Hyp2− are 583, 594 and 613 nm, respectively. Neither at pH 13 nor at 1 M HCl is the system entirely in the Hyp2− or the HypH forms. Ours is the first study of Hyp in reverse micelles as well as the first time-resolved study of Hyp as a function of pH.
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Vol. 72 • No. 5