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).

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.

On the low-lying excited states of sym-triazine-based herbicides.

We report a joint computational and luminescence study on the low-lying excited states of sym-triazines, namely, 1,3,5-triazine (1) and the ubiquitous herbicides atrazine [6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine (2)] and ametryn [6-methylthio-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine (3)]. Geometrical structures, energetics, and transition and state properties of I and 2 were computed at the TD-DFT, CASSCF, and CASPT2 levels of theory. The fluorescence and phosphorescence emission spectra, lifetimes, and fluorescence quantum yields were measured for the three compounds, and from these, the energies of the lowest excited states and their corresponding radiative rates were determined. The predictions from CASPT2 calculations are in good agreement with the experimental results obtained from the luminescence studies and allow the interpretation of different absorption and emission features.

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 UO2(2+)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.