M. Emília Azenha

A comparative study of water soluble 5,10,15,20-tetrakis(2,6-dichloro-3-sulfophenyl)porphyrin and its metal complexes as efficient sensitizers for photodegradation of phenols

5,10,15,20-Tetrakis(2,6-dichloro-3-chlorosulfophenyl)porphyrin and its tin and zinc complexes were synthesized with high yields and fully characterized. The corresponding water-soluble 5,10,15,20-tetrakis(2,6-dichloro-3-sulfophenyl)porphyrins were obtained by hydrolysis with water. An extensive photophysical study of the new water soluble porphyrinic compounds was carried out including absorption and fluorescence spectra, fluorescence quantum yields, triplet absorption spectra, triplet lifetimes, triplet and singlet oxygen quantum yields. These sensitizers were successfully used in the photodegradation of 4-chlorophenol and 2,6-dimethylphenol. A comparison is made of their efficiencies, and some mechanistic considerations are highlighted.

Aqueous Solutions of Uranium(VI) as Studied by Time-Resolved Emission Spectroscopy: A Round-Robin Test

Results of an inter-laboratory round-robin study of the application of time-resolved emission spectroscopy (TRES) to the speciation of uranium(VI) in aqueous media are presented. The round-robin study involved 13 independent laboratories, using various instrumentation and data analysis methods. Samples were prepared based on appropriate speciation diagrams and, in general, were found to be chemically stable for at least six months. Four different types of aqueous uranyl solutions were studied: (1) acidic medium where UO22+aq is the single emitting species, (2) uranyl in the presence of fluoride ions, (3) uranyl in the presence of sulfate ions, and (4) uranyl in aqueous solutions at different pH, promoting the formation of hydrolyzed species. Results between the laboratories are compared in terms of the number of decay components, luminescence lifetimes, and spectral band positions. The successes and limitations of TRES in uranyl analysis and speciation in aqueous solutions are discussed.

Deactivation processes of the lowest excited state of [UO2(H2O)5]2+ in aqueous solution

A detailed analysis of the photophysical behaviour of uranyl ion in aqueous solutions at room temperature is given using literature data, together with results of new experimental and theoretical studies to see whether the decay mechanism of the lowest excited state involves physical deactivation by energy transfer or a chemical process through hydrogen atom abstraction. Comparison of the radiative lifetimes determined from quantum yield and lifetime data with that obtained from the Einstein relationship strongly suggests that the emitting state is identical to that observed in the lowest energy absorption band. From study of the experimental rate and that calculated theoretically, from deuterium isotope effects and the activation energy for decay support is given to a deactivation mechanism of hydrogen abstraction involving water clusters to give uranium(v) and hydroxyl radicals. Support for hydroxyl radical formation comes from electron spin resonance spectra observed in the presence of the spin traps 5,5-dimethyl-1-pyrroline N-oxide and tert-butyl-N-phenylnitrone and from literature results on photoinduced uranyl oxygen exchange and photoconductivity. It has previously been suggested that the uranyl emission above pH 1.5 may involve an exciplex between excited uranyl ion and uranium(v). Evidence against this mechanism is given on the basis of quenching of uranyl luminescence by uranium(v), together with other kinetic reasoning. No overall photochemical reaction is observed on excitation of aqueous uranyl solutions, and it is suggested that this is mainly due to reoxidation of UO2+ by hydroxyl radicals in a radical pair. An alternative process involving oxidation by molecular oxygen is analysed experimentally and theoretically, and is suggested to be too slow to be a major reoxidation pathway.

Photophysics of the excited uranyl ion in aqueous solutions. Part 6.—Quenching effects of aliphatic alcohols

The quenching of the excited state of the uranyl ion by aliphatic alcohols has been studied under conditions where the luminescence decay is bi-exponential. In agreement with previous reports, the quenching is strongly dependent on the type of alcohol, and shows a marked deuterium isotope effect. This is consistent with the major process for quenching being α-hydrogen-atom abstraction by (UO2+2)*. Support for this comes from agreement between the isotope effects observed for quenching and for product formation. In an earlier study, the bi-exponential decay was interpreted in terms of a reversible crossing between two states, U* and X*. Analysis of the quenching data indicates that these have very similar reactivity towards alcohols. Results are also given for the decay of the intermediate hydroxyalkyl radicals in the presence of uranyl ion, and suggest that formation of the final organic product normally involves metal-ion oxidation.

Luminescence from cerium(III) acetate complexes in aqueous solution: considerations on the nature of carboxylate binding to trivalent lanthanides

The luminescence of cerium(III) has been studied in aqueous solutions in the presence of acetate ion. In contrast to previous reports that cerium(III) carboxylate complexes do not luminesce, a weak emission is observed, which has a similar spectrum to the aquo cerium(III) species but a much shorter lifetime. Results from a variety of studies on complexation of various trivalent lanthanide ions with carboxylates, including NMR spectral and relaxation measurements, molecular mechanics calculations, deuterium isotope effects on Tb(III) luminescence lifetimes, together with literature data strongly suggest that the dominant mode of complexation between carboxylates and trivalent lanthanides involves a weak, predominantly bidentate, binding to the metal cation. We suggest that the observed emission arises from a 1 : 1 cerium acetate species involving such bidentate coordination.

Co-ordination behaviour of dioxouranium(VI) nitrate in water-acetone mixtures

Potentiometry using nitrate ion-selective electrodes, Raman, 1H n.m.r. and u.v.–visible absorption spectroscopy have been used to study the co-ordination behaviour of dioxouranium(VI) nitrate in water–acetone mixtures over the complete solvent composition range. At the concentrations studied (up to 0.14 mol dm–3) there was no evidence of UO22+–NO3– complexing in pure water, and the co-ordination species is believed to be [UO2(H2O)5]2+. Complexing starts to occur above 60%(v/v) acetone–water, probably via a two-step equilibrium, and at high acetone concentrations the only significant complex species present is suggested to be [UO2(NO3)2(H2O)2].

Understanding the interaction between trivalent lanthanide ions and stereoregular polymethacrylates through luminescence, binding isotherms, NMR, and interaction with cetylpyridinium chloride.

Complexation of isotactic, syndiotactic, and atactic poly(methacrylic acid), PMA, with trivalent lanthanide ions has been studied in water at a degree of neutralization 0.5. Metal ion binding is shown by quenching of cerium(III) fluorescence, enhancement of Tb(III) luminescence, and lanthanide-induced line broadening in the PMA (1)H NMR spectra. Comparison with lanthanide-acetate complexation suggests carboxylate binds in a bidentate fashion, while Ce(III) luminescence quenching suggests an ≈3:1 carboxylate:metal ion stoichiometry, corresponding to charge neutralization. The presence of both free and bound Ce(III) cations in PMA solutions is confirmed from luminescence decays. Studies of Tb(3+) luminescence lifetime in H2O and D2O solutions show complexation is accompanied by loss of 5-6 water molecules, indicating that each bidentate carboxylate replaces two coordinated water molecules. The behavior depends on pH and polyelectrolyte stereoregularity, and stronger binding is observed with isotactic polyelectrolyte. Binding of cetylpyridinium chloride, CPC, in these systems is studied by luminescence, NMR, and potentiometry. NMR and Tb(3+) luminescence lifetime studies show the strongest binding with the isotactic polymer. Binding of surfactant to poly(methacrylate) in the presence of lanthanides is noncooperative, i.e., it binds to the free sites; binding isotherms in the presence of lanthanides are shifted to higher free surfactant concentrations, compared with sodium ions, have lower slopes and show a clear two-step binding mechanism. While CPC readily replaces the Na(+) ions of poly(methacrylate) and binds very strongly (low critical association concentrations), exchange is much more difficult with the strongly bound trivalent lanthanide ions. Effects of tacticity are seen, with surfactant interacting most strongly with isotactic chains in the initial stages of binding, while in the final stages of binding the interaction is strongest with atactic poly(methacrylate).

Photodegradation of Pesticides and Photocatalysis in the Treatment of Water and Waste

A brief overview on the main photoprocesses applied to the treatment of water and wastewater is presented. The photodegradation methods that have been applied to the oxidation of organic pollutants are described. A review on advanced oxidation processes (AOP’s) and photooxidation mechanisms in homogeneous and heterogeneous solution is presented and some practical applications discussed. Combinations of biological and chemical treatments are considered to be a good approach to improve the removal efficiencies and reduce costs.

Synthesis and photophysical properties of a covalently bonded palladium meso-sulfophenylporphyrin-poly(vinyl alcohol)polymer with potential applications as an oxygen sensor

The synthesis and photochemical characterization of a covalently bonded palladium meso-sulfophenylporphyrin-poly(vinyl alcohol)polymer (PVA-PdTPPS), made via reaction of the chlorosulfonyl derivative of Pdmeso-tetrakis-phenylporphyrin with the alcohol group of the polymer to form the sulfonate ester, is described. Absorption, emission and phosphorescence lifetime data are reported. In both the solid film and in aqueous solution phosphorescence decays in the absence of oxygen are non-monoexponential and suggest heterogeneity in the decay and oxygen quenching kinetics. As a solid film there is very little phosphorescence quenching by oxygen at moderate pressures of O2 (0-30 Torr), but in aqueous solution oxygen quenches at a rate of 7.5 (±0.5) × 108M.s−1. The synthetic route described is versatile enough to be used with a variety of metalloporphyrins and polymers and this allows the possibility of extensive tuning in terms of lumophore lifetime, polarity, solubility, location in heterogeneous media, and compatibility with other polymers.

Synthesis of vinylated 5,10,15,20-tetraphenylporphyrins via Heck-type coupling reaction and their photophysical properties

The direct coupling reaction between substituted olefins and 5,10,15,20-tetrakis(4-bromophenyl)porphyrin, via a Heck-type reaction, constitutes a versatile method for the vinylation of 5,10,15,20-tetrakis(4-bromophenyl)porphyrin to yield new vinylated tetraphenylporphyrins quantitatively. Another strategy for the vinylporphyrin synthesis has been developed. A phosphapalladacycle has been used as catalyst for the coupling reaction between 4-bromoaryl aldehydes and olefins to yield vinyl aldehydes quantitatively. These aldehydes have been condensed with pyrrole, by the one-step nitrobenzene method, to give the corresponding vinylated tetraphenylporphyrins. Photophysical studies of these new porphyrins are also reported.