Effects of halogen substitution on the photochemical properties of hypericin

Hypericin (hyp) displays photodynamic activity under the influence of visible light, which has opened up new possibilities of utilizing hypericin derivatives as potential photodynamic therapy agents. In the current work, the effects of halogen substitution were considered through a detailed computational study of bromine, chlorine, and fluorine substitution on the geometrical structure, and electronic and phototoxic properties of the hypericin molecule by the use of density functional theory, integral equation formalism polarized continuum models, and a time-dependent formalism (TD-DFT) to the study of singlet and triplet excited states. It is concluded that the effect of halogen substitution is primarily an increase of the electron affinities and thereby increased quantum efficiency of superoxide formation of importance in photodynamic therapy.