Investigation of triplet states and radical anions produced by laser photoexcitation of hypocrellins

Abstract

The transient phenomena in a laser flash photolysis study of two photosensitizing agents for photodynamic therapy, hypocrellin A(HA) and hypocrellin B(HB), are described in solvents of varying polarity at room temperature. The difference between the triplet- and ground-state absorption spectra of HA or HB shows peaks ca. 400, 530, 570 (sh) and 620 nm in Ar-saturated n-hexane solution. The absorption coefficients for the T–T absorption and triplet quantum yield of HA (or HB) have been determined. Electron transfer in the quenching of triplet HA or HB by ground-state molecules of the dye was investigated, and the bimolecular self-quenching rate constant for HA or HB in DMSO, which is 1.2×10⁹ or 0.8×10⁹ d mol^-1 s^-1 respectively, suggested a nearly diffusion-controlled quenching process of the triplet excited state. Meanwhile, the corresponding anion radicals HA^-, HB^-, which are produced by electron-transfer reactions from aromatic amines, have absorption maxima at 640 nm. Triplet-state lifetimes and electron-transfer kinetics are also reported.