A study of photoinduced electron transfer and redox properties of hypericin

Hypericin (HYP), is a derivative of polyhydroxy substituted perylenequinones. In acetonitrile, the fluorescence of HYP can be quenched by electron donor: N,N-diethylaniline (DEA) and accepters: methyl viologen (MV), anthraquinone (AN) (less than 1.2 x 10(-3) mel l(-1)), and the quenching rate constants are K-q (DEA) = 3.52 x 10(10) l mol(-1) s(-1), K-q (MV) = 7.16 x 10(11) l mol(-1) s(-1), K-q (AN) = 1.05 x 10(11) l mol(-1) s(-1) respectively, indicating that hypericin can act either as an electron acceptor like quinones or as an electron donor like phenones. Hypericin in ground state maybe forms charge transfer complex with the higher concentrated anthraquinone which has a plane configuration. The stoichiometry is 1:2.6 (HYP:AN) determined by Job's photometric titration plots. The methyl viologen cation radical formed in the photoinduced interaction between hypericin and methyl viologen was also detected by the absorption spectrum. (C) 1998 Elsevier Science S.A. All rights reserved.