Singlet oxygen produced by quasi-continuous photo-excitation of hypericin in dimethyl-sulfoxide

Singlet oxygen (O2(1Δg)) production by photo-excited hypericin (Hyp) dissolved in dimethyl-sulfoxide (DMSO) was studied by means of time-resolved phosphorescence measurements. In order to minimize photo-bleaching, the samples were excited in quasi-continuous mode using long-pulse (35 μs) laser excitation. The measured lifetime of singlet oxygen is τΔ=5.5±0.3 μs. This result helps to resolve the discrepancy existing in the literature concerning singlet oxygen lifetime in DMSO. The obtained quantum yield of singlet oxygen photosensitized by Hyp in air-saturated DMSO is ΦΔ=0.4±0.03. The rate constant for Hyp triplet state depopulation in reaction with ground state molecular oxygen is measured to be kq=1.6±0.3×109 M−1 s−1.