Deuterium isotope effect on the excited-state photophysics of hypocrellin: Evidence for proton or hydrogen atom transfer

A deuterium isotope effect of 1.4 is observed in the excited-state proton or atom transfer of hypocrellin A in methanol and methanol-O-d. This is the most direct and convincing evidence to date that excited-state intramolecular proton or atom transfer occurs in hypocrellin. It is consistent with previous proposals (e.g., Chem. Rev. 1996, 96, 523) that such excited-state intramolecular processes occur generally in the class of polycyclic quinones possessing light-induced antiviral activity, of which hypocrellin and the related compound hypericin are members. The transfer reaction in these compounds is discussed in terms of being in the adiabatic limit in the proton stretch coordinate. We discuss the possibility of presenting a unified picture of the hypericin and hypocrellin photophysics.