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The singlet and triplet excited states of 4-hydroxybenzophenone (4BPOH) undergo deprotonation in the presence of water to produce the anionic ground-state, causing fluorescence quenching and photoactivity inhibition. The same process does not take place in an aprotic solvent such as acetonitrile. In acetonitrile, 4BPOH is fluorescent (interestingly, one of its fluorescence peaks overlaps with peak C of humic substances), it yields singlet oxygen upon irradiation and induces the triplet-sensitised transformation of phenol (with a rate constant of (6.6 ± 0.3) × 107 M−1 s−1 (μ±σ) between phenol itself and a triplet 4BPOH). The 4BPOH shows an intermediate behaviour in a partially protic solvent such as 2-propanol, where some deprotonation of the excited states is observed. In acetonitrile/2-propanol mixtures (at least up to 50% of 2-propanol) there is also some evidence of alcohol oxidation by the 4BPOH triplet state, while the experimental data are silent concerning such a possibility in pure 2-propanol. Considering that benzophenones are important components of chromophoric dissolved organic matter (CDOM) in surface waters, the present findings could have significance for the photoactivity of the hydrophilic surface layers vs. the hydrophobic cores of CDOM. [ABSTRACT FROM AUTHOR]

Photo-Fenton processes activated by biodegradable Fe(III)–EDDS complexes have attracted huge attention from the scientific community, but the operative mechanism of the photo-activation of H2O2 in the presence of Fe(III)–EDDS has not been fully clarified yet. The application of the Fe(III)–EDDS complex in Fenton and photo-Fenton (mainly under UV-B light) processes, using 4-chlorophenol (4-CP) as a model pollutant was explored to give insights into the operative mechanism. Furthermore, the potential synergistic contribution of soybean peroxidase (SBP) was investigated, since it has been reported that upon irradiation of Fe(III)–EDDS the production of H2O2 can occur. SBP did not boost the 4-CP degradation, suggesting that the possibly produced H2O2 reacts immediately with the Fe(II) ion with a quick kinetics that does not allow the diffusion of H2O2 into the bulk of the solution (i.e., outside the solvent cage of the complex). So, a concerted mechanism in which the photochemically produced H2O2 and Fe(II) react inside the hydration sphere of the Fe(III)–EDDS complex is proposed. [ABSTRACT FROM AUTHOR]

Anthraquinone-2-sulphonate (AQ2S) is a triplet sensitiser that has recently been used to model the photoreactivity of chromophoric dissolved organic matter (CDOM). We show that the photolysis quantum yield of AQ2S under UVA irradiation varies from (3.4 ± 0.2) × 10−3 at μM AQ2S levels to (1.8 ± 0.1) × 10−2 at 3 mM AQ2S (μ±σ). This trend is consistent with a combination of direct phototransformation and transformation sensitised by a photogenerated reactive species. In both cases a transient water adduct of AQ2S would be involved. Depending on the initial quinone concentration, the adduct could undergo transformation, give back ground-state AQ2S or react with it. The prevalence of the latter process at high AQ2S concentration would account for the increased values of the photolysis quantum yield. When using AQ2S as a triplet sensitiser, one should not exceed an initial concentration of 0.1 mM. Under the latter conditions the sensitised process is negligible compared to the direct photolysis, providing a simpler system to be studied, and the photolysis quantum yield is independent of the initial AQ2S concentration. This paper also shows, by adoption of density functional theory calculations, that the triplet state of AQ2S has most of the spin density localised on C=O, analogous to other photoactive quinones, which accounts for the oxidising character of the triplet state that tends to be reduced to a semiquinone radical. [ABSTRACT FROM AUTHOR]

The excited triplet state of 1-nitronaphthalene (31NN*) reacts with OH−with a second-order reaction rate constant of (1.66 ± 0.08)×107M−1s−1(μ ± σ). The reaction yields the OH radical and the radical anion 1NN−. In aerated solution, the radical 1NN− would react with O2to finally produce H2O2upon hydroperoxide/superoxide disproportionation. The photolysis of H2O2is another potential source of OH, but such a pathway would be a minor one in circumneutral (pH 6.5) or in basic solution ([OH−] = 0.3–0.5 M). The oxidation of H2O by 31NN*, with rate constant 3.8 ± 0.3 M−1s−1, could be the main OH source at pH 6.5. [ABSTRACT FROM AUTHOR]

The excited triplet state of 1-nitronaphthalene, 1NN, (31NN) is able to oxidise nitrite to NO2, with a second-order rate constant that varies from (3.56 ± 0.11) × 108M−1s−1(μ ± σ) at pH 2.0 to (3.36 ± 0.28) × 109M−1s−1at pH 6.5. The polychromatic quantum yield of NO2photogeneration by 1NN in neutral solution is ΦNO21NN≥ (5.7 ± 1.5) × 107× [NO2−]/{(3.4 ± 0.3) × 109× [NO2−] + 6.0 × 105} in the wavelength interval of 300–440 nm. Irradiated 1NN is also able to produce OH, with a polychromatic quantum yield ΦOH1NN= (3.42 ± 0.42) × 10−4. In the presence of 1NN and NO2−/HNO2under irradiation, excited 1NN (probably its triplet state) would react with NO2to yield two dinitronaphthalene isomers, 15DNN and 18DNN. The photonitration of 1NN is maximum around pH 3.5. At higher pH the formation rate of NO2by photolysis of NO2−/HNO2would be lower, because the photolysis of nitrite is less efficient than that of HNO2. At lower pH, the reaction between 31NN and NO2is probably replaced by other processes (involving e.g.31NN-H+) that do not yield the dinitronaphthalenes. [ABSTRACT FROM AUTHOR]