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 pH < 5. 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.
The phototransformation of 1-naphthol in aerated aqueous solution was investigated by means of product studies and laser flash photolysis. The quantum yield as measured at 313 nm was found to be equal to 3.2 x 10(-2) in oxygen-saturated medium while being 10-fold lower in deoxygenated solution. The main photoproducts in aerated medium were 1,4-naphthoquinone, 2-hydroxy-1,4-naphthoquinone and 6-hydroxy-1,4-naphthoquinone. Traces of 1,2-naphthoquinone and 5-hydroxy-1,4-naphthoquinones were detected too. Solvated electrons were detected by laser flash photolysis. The quantum yield of monophotonic ionisation was found to be lower than that of 1-naphthol photolysis indicating that other reaction pathways took place. The mechanisms of naphthoquinones and hydroxynaphthoquinones formation are discussed.
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