Photochemistry of 9,10-anthraquinone-2-sulfonate in solution. 1. Intermediates and mechanism

Abstract

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.

Could triplet-sensitised transformation of phenolic compounds represent a source of fulvic-like substances in natural waters?

“…Indeed, the evolution of AQ2S triplet state upon reaction with water produces transient adducts with water (where water is attached to either the aromatic ring or an oxygen atom), which either yield back ground-state AQ2S, or evolve into AQ2S-OH isomers (Maurino et al, 2008;Maddigapu et al, 2010;Bedini et al, in press). To support this, we observed the formation of a broad absorption band in the 350-550 nm range (that is, right in the range of EEM excitation wavelengths), which is fully compatible with the occurrence of α and β-hydroxy-AQ2S (Loeff et al, 1983; see also Figure SM1). …”

Modelling the photochemical fate of ibuprofen in surface waters

“…The initial transformation rate of FFA under the adopted conditions was Rate FFA = (1.26±0.06)⋅10 (Loeff et al, 1983;Maurino et al, 2008, Maddigapu et al, 2010c. at pH 2 and (1.59±0.04)⋅10 2 [IBP] at pH 8.…”

“…It has a formation quantum yield Φ 3AQ2S* = 0.18 and a deactivation rate constant k 3AQ2S* = 1.1⋅10 7 s −1 (Loeff et al, 1983). The formation rate of 3 AQ2S* would be R 3AQ2S* = Φ 3AQ2S* P a…”

Photohydroxylation of 1,4‐Benzoquinone in Aqueous Solution Revisited

“…[61] Photohydroxylation is suppressed by the addition of halide ions. [37,59,62,63] As short-lived intermediates CT-complexes have been suggested that can either decay to the ground state or, at elevated halide concentrations, form dihalide radical anions and a semiquinone radical anion, QC À . [62] Oxidation of carbonate to CO 3 C À has also been reported.…”