Photolysis of 2-methyl-1,4-benzoquinone (toluquinone) in aqueous solution results in the oxidation of water to create either hydroxyl radical or some species capable of transferring a hydroxyl radical. Trapping of the latter with dimethyl sulfoxide (DMSO) creates a methyl radical which in turn can be trapped by the stable radical 3-amino-2,2,5,5-tetramethyl-1-pyrrolidinyloxy. Competitive trapping studies using DMSO and either nitrite anion or salicylate anion show that the hydroxylating species is much more selective in its reactions than free hydroxyl radical. Laser flash photolysis experiments on toluquinone in aqueous solution show formation of a transient species immediately (<150 ns) following the excitation pulse that had previously been assigned to the excited triplet state of the quinone. This spectrum differs from the authentic triplet state spectrum generated in less reactive organic solvents (carbon tetrachloride and acetonitrile). The same intermediate is shown to react with the hydroxyl radical traps, cupric ions, and benzoate anion to yield the semiquinone radical. On the basis of these experiments it is argued that this transient species is a hydroxylating intermediate, probably best described as a complex between the semiquinone radical and the hydroxyl radical. It is further argued that this species is responsible for the hydroxyl radical trapping reactions.

Chemical trapping studies combined with optical and electron paramagnetic resonance measurements were employed to examine the mechanisms of the aqueous photochemistry of methyl-benzoquinone (mBQ) at both low and high quinone concentrations. At low [mBQ], dimethylsulfoxide (DMSO) reacted with a photogenerated intermediate to form a methyl radical, but methane did not, thereby unequivocally excluding the hydroxyl radical. DMSO at concentrations between 50 mM and 2 M completely suppressed the formation of the hydroxylated quinone, while only slowing the formation of the hydroquinone, suggesting reaction with either the triplet state or an intermediate arising from the triplet. Addition of Cl-, a putative physical quencher of the triplet, inhibited the DMSO reaction both noncompetitively and competitively in a fashion similar to that observed previously with nitrite, formate, and salicylic acid, thus providing further evidence for a reactive intermediate distinct from the triplet. This intermediate is attributed to a water-quinone exciplex. The relative yield of the methyl radical from the DMSO reaction decreased with increasing [mBQ], suggesting that at high concentrations, a bimolecular reaction of the triplet with the ground-state quinone outcompetes the formation of the quinone-water exciplex.

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A series of ruthenium(II) crown ether complexes containing 4-, 5-, or 6-oxygen crowns tethered to a bipyridine at the 3,3Ј position were prepared and examined for their ability to act as luminescent sensors. The complexes [(bpy) 2 Ru(2,2Ј-bipyridine-3,3Ј-16-crown-4)](PF 6 ) 2 and [(bpy) 2 Ru(2,2Ј-bipyridine-3,3Ј-19-crown-5)](PF 6 ) 2 were characterized by X-ray crystallography. The 4-oxygen crown crystallizes in the P1 space group. The 5-oxygen crown crystallizes in the P1 space group. In both complexes, the inter-pyridine ring angle in