Photochemistry of 1-nitronaphthalene: a potential source of singlet oxygen and radical species in atmospheric waters

1-Nitronaphthalene (1NN) was used as a model of nitro-PAHs to investigate photosensitized reactions in aqueous solution in the presence of oxygen and halides. Laser flash photolysis (LFP) was employed to investigate electron transfer between halide anions and the triplet state of 1NN, leading to the formation of dihalogen radical anions (X(2)(*-)) in solution. The experiments were performed in the absence or presence of oxygen, showing a bimolecular quenching rate constant for the triplet state of 1NN ((3)1NN) by oxygen of (1.95 +/- 0.05) x 10(9) M(-1) s(-1). The decay of (3)1NN was observed to strongly depend on the pH of the medium. At pH > 2, (3)1NN decayed with a pseudofirst order rate constant close to 6.0 x 10(5) s(-1). The rate constant was markedly enhanced at lower pHs, reaching 2.0 x 10(6) s(-1) at pH approximately 0.1, which suggests formation of the protonated triplet state ((3)1NNH(+)) at low pHs. Furthermore, we showed that the photoreactions of (3)1NN in the presence of oxygen are potential sources of HO(2)(*), (1)O(2), and possibly (*)OH in aqueous media. In Milli-Q water (pH approximately 6.5) and in the presence of halide anions, the quenching rate constant of (3)1NN was evaluated to be (2.9 +/- 0.4) x 10(4) M(-1) s(-1) for chloride, (7.5 +/- 0.2) x 10(8) M(-1) s(-1) for bromide, and (1.1 +/- 0.1) x 10(10) M(-1) s(-1) for iodide. Also in this case a pH-dependent reactivity was evidenced, and the quenching rate constant was (7.7 +/- 1.2) x 10(5) M(-1) s(-1) with chloride at pH 1.1.