Observation of Excited-State Tautomerization in the Antiviral Agent Hypericin and Identification of Its Fluorescent Species

Gai, F.
Fehr, Michael
Petrich, Jacob
Petrich, Jacob
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The absorption spectra, fluorescence spectra, and fluorescence lifetimes of hypericin, an analog lacking hydroxyl groups, mesonaphthobianthrone, and hexamethylhypericin are obtained in aprotic and protic solvents. In aprotic solvents, mesonaphtobianthrone is nonfluorescent. In strong acids such as sulfuric or triflic acids, it becomes fluorescent. Furthermore, its spectrum is very similar to that of hypericin. Similarly, only in sulfuric acid does hexamethylhypericin afford absorption and emission spectra resembling those of hypericin. We therefore conclude that the fluorescent species of hypericin has one or both of its carbonyl groups protonated. The protonation equilibrium in both the ground and the excited state is discussed. The first detailed measurements of the primary processes in the antiviral agent, hypericin, are performed with picosecond resolution and a white-light continuum. Transient absorption measurements of hypericin with - 1-ps resolution indicate that upon optical excitation a new species is created that absorbs in the range of roughly 580-640 nm. This species exhibits a 6-1 2-ps decay, depending on the solvent. It is also observed that the stimulated emission signal, which arises from the fluorescent state, grows in with a time constant of 6-12 ps. Based upon the identification of the fluorescent species as hypericin with one or both carbonyl groups protonated, the rise time for the appearance of the stimulated emission signal is attributed to excited-state tautomerization.

<p>Reprinted (adapted) with permission from <em>Journal of Physical Chemistry</em> 98 (1994): 5784, doi: <a href="http://dx.doi.org/10.1021/j100073a036" target="_blank">10.1021/j100073a036</a>. Copyright 1994 American Chemical Society.</p>