Excited-state processes in polycyclic quinones: The light-induced antiviral agent, hypocrellin, and a comparison with hypericin

Hypocrellin is a naturally occurring perylene quinone that possesses light-induced antiviral activity, most notably against the human immunodeficiency virus (HIV), as does the related molecule, hypericin. White-light continuum is employed to examine the excited-state processes in hypocrellin from the picosecond to the nanosecond time scales. These processes are assigned to intramolecular proton transfer, intersystem crossing, and interconversion between different conformations of hypocrellin, which is constrained to be nonplanar in its ground-state owing to its bulky side chains. The ground state of hypocrellin is suggested to be heterogeneous and to be comprised of an equilibrium between at least two tautomeric forms. The results are discussed in terms of the properties of hypericin, which bears marked similarities and differences with respect to hypocrellin, both in terms of its excited-state properties as well as its mode of induced antiviral activity.