The photochemistry of aqueous 9,10-anthraquinone-2-sulfonate (AQS) was investigated by using four different approaches: (1) laser photolysis of AQS in water as a function of concentration and pH; (2) laser photolysis of AQS in CH3CN and CH3CN/H20 mixtures, combined with emission spectroscopy; (3) steady-state photolytic study of the effect of AQS concentration and pH on the yield of photohydroxylation in water; (4) quenching of intermediates and inhibition of photohydroxylation by inorganic anions. Our results lead to identification of triplet AQS ( ~100 ns in water) and two other intermediates (B and C) which are formed by two parallel reactions of triplet AQS with H20. The nature of these intermediates is still uncertain, but evidence is presented to rule out H abstraction or net electron transfer (even in the case of OH") leading to formation of free OH radicals. The possibility that B and C are two different water adducts is discussed. The role of preferential solvation of AQS in CH3CN/H20 mixtures in determining its photochemistry is also examined. Species C (Xmax ~600 nm) is the only transient observed which appears to react with ground-state AQS, and this reaction is considered to be responsible for photohydroxylation. Our results provide direct evidence for the validity of the "3AQS/H20" mechanism proposed by Clark and Stonehill (CS), in which the primary step is reaction of 3AQS with water (and not with ground-state AQS) to produce the hydroxylating agent. However, this mechanism is modified for pH 511 by proposing another hydroxylating agent which may be AQS"OH exciplex (or radical pair) produced by charge-transfer (CT) quenching of 3AQS by OH". Evidence is presented to establish the charge-transfer nature of quenching of triplet AQS by various anions including OH". ACO2-76ER03117).Registry No. 9,10-Anthraquinone-2-sulfonate, 84-48-0.

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The chemical aspects of the interactions between excited 9,lO-anthraquinone-2-sulfonate (AQS) and various inorganic anions are examined. The anions which quench triplet AQS can be divided into two groups: Cl-, Br-, I-and NCS-(group I) photoreduce AQS to AQS-radical anion only at concentrations higher than that required for complete triplet quenching. The effect increases with concentration and passes through a maximum, with highest quantum yields of radical formation reaching N 1 for C1-and NCS-; on the other hand, NOz-, SO3*-, and N; (group 11) give AQS-in parallel to triplet quenching. The nature of the high-concentration effect shown by group I is analyzed. Some results obtained with mixtures of anions support the conclusion that triplet AQS is also responsible for this effect, and it is suggested that triple exciplexes of the type 3(AQS-.X;) are involved. With this view and the recently proposed intraradical spin-orbit-coupling (IRSOC) model, a quantitative interpretation of the results is presented.

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