Duroquinone triplet reduction, in cyclohexane, ethanol and water, and by durohydroquinone

Abstract

Laser flash spectroscopy has been used to study the lifetime, extinction coefficient, oscillator strength and quantum yield of formation (ϕ_T) of duroquinone triplet ³Q in solution. Reaction of this triplet with solvent leads in ethanol and cyclohexane to the neutral semiquinone QH. Reaction of this triplet with itself leads in water to the anion Q·^– and in cyclohexane and ethanol to the excited singlet ¹Q* followed by ³Q. Transient spectra and kinetic measurements demonstrate that triplet-triplet annihilation is the only path for duroquinone photoreduction in water. ϕ_T in the solvents used is unity (±0.15), ϕ(QH) is 0.4 (±0.1) in ethanol and 0.09 (±0.03) in cyclohexane.

Laser flash spectroscopy of the duroquinone + durohydroquinone system gave rise to spectra of the transient species ³Q and QH linked by isobestic points in ethanol and cyclohexane. In water ³Q and Q·^– were observed, again linked by isobestic points. The observed reaction ³Q + QH₂→ 2 QH allowed a determination of the QH extinction in ethanol and cyclohexane. The formation of a triplet exciplex ³(Q, QH₂) is suggested.