Photochemical properties of the triplet pi,pi* state, anion and ketyl radicals of 5,12-naphthacenequinone in solution studied by laser flash photolysis: electron transfer and phenolic H-atom transfer.

Photochemical properties of the lowest triplet pi,pi* state of 5,12-naphthacenequinone (5,12-NQ) have been investigated in solution by means of nanosecond laser flash photolysis at 295 K. The transient absorption spectrum of triplet 5,12-NQ was measured in CCl4 and the molar absorption coefficient was determined. In CCl4, triplet 5,12-NQ is shown to be quenched by the ground-state 5,12-NQ. In acetonitrile, triplet 5,12-NQ is quenched by 1,2,4,5-tetramethoxybenzene (TMB) via electron transfer to produce the TMB cation and 5,12-NQ anion radicals. The absorption spectrum and the molar absorption coefficient of the 5,12-NQ anion radical were determined based on those of the TMB cation radical. Although triplet 5,12-NQ does not abstract H-atom of ethanol or cyclohexane, H-atom transfer occurs from phenol and p-phenylphenol to triplet 5,12-NQ, producing the 5,12-NQ ketyl radical. The absorption spectrum and the molar absorption coefficient of the 5,12-NQ ketyl radical were determined based on those of the p-phenylphenoxyl radical. It is shown that the quenching rate constants by phenol for triplet 5,12-NQ and other triplet paraquinones previously reported obey a Rehm-Weller relationship. The mechanism for the phenolic H-atom transfer to triplet paraquinones is interpreted in terms of coupled electron and proton transfer.