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The phototransformation of 1,4-naphthoquinone (NQ) in aqueous solution has been investigated by means of product studies and nanosecond laser flash photolysis (lambdaexc = 355 nm). The quantum yield of NQ phototransformation at 313 nm is high in acidic media (0.85 +/- 0.08 at pH 2.4), and decreases dramatically when the pH is increased. The main photoproducts are 1,4-dihydroxynaphthalene and 5- and 7-hydroxy-1,4-naphthoquinone. Several transient species were detected by laser flash photolysis. The triplet (lambdamax = 365 nm) yields 1 (lambdamax = 615 nm), which undergoes protonation to give 2 (lambdamax = 485 nm). A stable absorption (lambdamax = 320 nm) is formed upon protonation of 2 at pH5. These data strongly suggest that the mechanism involves (i) addition of water to the triplet state via an heterolytic reaction, (ii) the formation of 1,4,5- and 1,4,7-trihydroxynaphthalene intermediates from 2 via an acid catalysed H-atom migration and (iii) a fast oxido-reduction between the trihydroxynaphthalenes and NQ, leading to the final photoproducts. |
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