Phototransformation of 5-nitro-2-furaldehyde in aqueous solution. A laser flash photolysis and product analysis study†
Laser flash photolysis of 5-nitro-2-furaldehyde (NFA) in solution shows a short-lived transient absorption with λmax = 475 ± 5 nm, which is relatively insensitive to solvent polarity and is assigned to the lowest triplet state of NFA (3NFA*). In water, the 3NFA* absorption decays to a long-lived absorption, the study of which, at different times after the end of the laser pulse, reveals it to be due to a furyloxyl radical (λmax ≈ 375 nm) and to the radical anion NFA˙− (λmax ≈ 400 nm). These radicals were produced independently to confirm the assignment. The lifetime of 3NFA* depends both on the solvent and the ground-state concentration of NFA. An (n, π*) nature is attributed to 3NFA* on the basis of the propensity of 3NFA* to abstract a hydrogen-atom from the solvent. Kinetic evidence for triplet excimer formation was obtained from the self-quenching of 3NFA* in solvents where the triplet decay is slower. The effect of acidity on the triplet lifetime is discussed with respect to an electron-transfer self-quenching mechanism, assisted by the triplet excimer which is proposed to dissociate into radical ions. Chromatographic and spectroscopic analysis of the photolysed aqueous solution of NFA enabled the identification of 5-hydroxymethylene-2(5H)-furanone, nitrite ion and an unknown substance as the major photoproducts. Conclusive evidence is presented that the observed 5-hydroxymethylene-2(5H)-furanone is formed from the furyloxyl radical. It is shown that the unknown substance can also be obtained from both the photoreduction of NFA in propan-2-ol and chemical reduction of NFA by Fe(s) in water (along with 5-amino-2-furaldehyde). Based on 1H- and 13C-NMR (with 2-D HMQC) and vibrational absorption spectroscopy, a tentative structure is proposed for the substance of tR 3.69 minutes obtained as a photoreduction product of NFA in water. Inorganic anions are shown to be one-electron oxidised by 3NFA* (as indicated by the observation of both the radical anion of NFA and the inorganic radical) with second-order rate constants being dependent on E17 of the inorganic radical. The implications of the results from complete quenching of 3NFA* by inorganic anions, and subsequent product analysis, for the phototransformation mechanism of NFA in water are discussed.