T-T absorption and semiquinone formation in solutions of 5,8-dihydroxy-1,4-naphthoquinone (naphthazarin): a nanosecond laser flash photolysis study

Abstract

The triplet properties of naphthazarin (5,8-dihydroxy-1,4-naphthoquinone) have been investigated in cyclohexane, methanol and isopropanol solutions using the technique of nanosecond laser flash photolysis. The T-T absorption maxima are at around 280, 335, 377, 410 and 440 nm, while the transients formed after the disappearance of the triplets in alcohols absorb at around 265, 365, 395 and 440 nm. The second transient has been shown to be a semiquinone free radical of naphthazarin, produced by abstraction of a hydrogen atom from the alcohols. The extinction coefficient at many wavelengths and the quantum yields of both triplet and semiquinone formation have been estimated. Kinetic parameters associated with the decay of both the triplet and the semiquinone have been evaluated. The effect of laser intensity and the presence of triplet scavengers on the triplet properties have been studied. A comparison has been made with the corresponding transients produced from juglone (5-hydroxy-1,4-naphthoquinone).