Reaction Kinetics of Phenolic Antioxidants toward Photoinduced Pyranine Free Radicals in Biological Models.

8-Hydroxy-1,3,6-pyrenetrisulfonic acid (pyranine, PyOH) free radicals were induced by laser excitation at visible wavelengths (470 nm). The photochemical process involves photoelectron ejection from PyO- to produce PyO• and PyO•- with maxima absorption at 450 and 510 nm, respectively. The kinetic rate constants for phenolic antioxidants with PyO•, determined by nanosecond time-resolved spectroscopy, were largely reliant on the ionic strength depending on the antioxidant phenol/phenolate dissociation constant. Further, the apparent rate constant measured in the presence of Triton X100 micelles was influenced by the antioxidant partition between the micelle and the dispersant aqueous media but limited by its exit rates from the micelle. Similarly, the rate reaction between ascorbic acid and PyO• was markedly affected by the presence of human serum albumin responding to the dynamic of the ascorbic acid binding to the protein.

Facile photochemical synthesis and characterization of highly fluorescent silver nanoparticles.

Highly fluorescent silver nanoparticles (AgFNP) have been prepared by a facile photochemical method, yielding these materials in just a few minutes and with excellent long-term stability. The method makes use of photogenerated ketyl radicals that reduce Ag(+) from silver trifluoroacetate in the presence of amines. While as functional materials these AgFNP can be described as of nanometer dimensions, we believe that the luminescence arises from particle-supported small metal clusters (predominantly Ag(2)). The materials have been characterized by electron microscopy, fluorescence and absorption spectroscopy, fluorescence lifetime studies, and (19)F NMR spectroscopy. Exploratory work shows that the fluorescence from AgFNP can be efficiently quenched by paramagnetic quenchers, and these studies have been combined with electron paramagnetic resonance work.

Raising the ceiling of diastereoselectivity in hydrogen transfer on acyclic radicals.

We report here that the monodentate complexation of Me2AlCl to an ester group significantly enhances the selectivity of hydrogen transfer on acyclic radicals flanked by both an ester functionality and a stereogenic center, leading to C-2,C-3-anti products with high diastereoselectivity. In certain cases improved ratios were obtained using bulkier aluminum Lewis acid such as MAD (methylaluminum-di(di-2,6-tert-butyl-4-methylphenoxide). Electron spin resonance studies on these acyclic radicals indicate that Lewis acid complexation leads to conformational changes in the radicals. The stereochemistry of the preferred enolate radicals complexed with Lewis acids and their impact on the acyclic transition states involved are suggested.

Long-lived (minutes) photoinduced charge separation in a structured periodic mesoporous titania containing 2,4,6-triphenylpyrylium as guest.

2,4,6-Triphenylpyrylium (TP(+)), an electron acceptor, has been included inside the channels of two ordered mesoporous titania (mpTiO(2)). MpTiO(2) contain anatase nanoparticles (3-5 nm) templated by cetyltrimethylammonium and differ by the presence or absence of silica domains as binders of the structure. UV irradiation of TP-mpTiO(2) gives rise to a strong EPR signal. This behaviour was not observed for related materials in which TP(+) was included inside the channels of MCM-41 or the cavities of zeolite Y. Also, transient spectroscopy shows remarkable differences between TP(+) included in mpTiO(2) (charge separation) and in porous silicates (triplet excited state). Based on EPR and laser flash spectroscopic evidence, the occurrence of photo-induced electron transfer from TiO(2) as the donor to TP(+) as the acceptor leading to a long-lived (minutes at room temperature) charge separated state is proposed.

Photophysical properties of the prefluorescent nitroxide probes QT and C343T.

The photophysical properties of the nitroxide prefluorescent probes 4-(3-hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethyl-piperidin-4-yl) ester free radical (QT) and 2,3,4,6,7,8-hexahydro-quinolizino [1,10,9-gh] coumarin-3-carboxylic acid (1-oxyl-2,2,6,6-tetramethyl-piperidin-4-yl) ester free radical (C343T) were evaluated as a function of pH and solvent properties. The absorbance of QT showed high pH sensitivity. The pKa values for the different ionization forms involved in the acid-base equilibrium of the quinoline chromophore were determined in the ground and excited states. The fluorescence lifetimes of QT, and N-hydroxylamine (QTH) and quinoline methyl ester (QMe) derivatives, showed that the intramolecular quenching efficiency by the nitroxide moiety is independent of the quinoline ionization form. The fluorescence and absorbance of C343T were highly sensitive to solvent polarity in agreement with a charged transfer excited state of the chromophore. However, we noted a decrease in the intramolecular fluorescence quenching efficiency by the nitroxide moiety when changing the polarity of the solvent from hexane to water. This behavior has been attributed to a suppression of an energy transfer mechanism in the nitroxide quenching process in very polar solvents. The results obtained in micelles allow us to propose QT and C343T as sensors for pH and micropolarity, respectively, in addition to their role as monitors for free radicals or hydrogen transfer from phenols.

Photolysis of an alkoxyamine using intramolecular energy transfer from a quinoline antenna--towards photo-induced living radical polymerization.

The photolysis of an alkoxyamine triad comprised of the 1-phenylethyl moiety, 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) and 2-methyl-3-hydroxy quinoline leads to energy transfer from the quinoline moiety (antenna) and cleavage of the C-O alkoxyamine bond, suggesting its possible applications in low temperature living free radical polymerization.

Spin coupling in the supramolecular structure of a new tetra(quinoline-TEMPO)yttrium(III) complex.

The newly synthesized tetra(quinoline-TEMPO)yttrium(III) potassium salt shows interesting structural features at the molecular and supramolecular levels, revealed by the analysis of the X-ray diffraction data. The magnetic susceptibility and EPR data corroborated with structural considerations showed that the exchange and dipolar spin coupling interactions are taking place at the nodes assembling the supramolecular 2D structure. The Y(III) center shows antiprismatic octacoordination, close to the idealized D2 symmetry. The diamagnetic transition metal plays no role in mediating the radical interactions since the TEMPO-type fragments are remote from the chelating moieties of the ligand. In turn, significant interaction occurs on the nodes consisting in the quasi-rectangular coordination of potassium counterions by the spin-bearing TEMPO groups coming from four distinct complex units. The antiferromagnetic susceptibility was consistently modeled by a spin Hamiltonian based on the rectangle topology of four spins S = 1/2. The fitted exchange parameters are Ja = -5.1 cm-1 and Jb = -3.4 cm-1 for the edges, imposing Jd = 0 for the diagonal. These values are in excellent agreement with the ab initio results Ja = -4.83 cm-1, Jb = -3.44 cm-1, Jd = -0.07 cm-1 obtained in a CASSCF(12,8) calculation. Based on the reliability of the ab initio results we were able to select the presented J parameters among several versions of multiple solutions with acceptable goodness of the fit. A methodological caveat about the artifacts of the automatic use of best fit parameters, in the absence of supplementary criteria, in the context of relative blindness of magnetic susceptibility modeling, is raised. The details of the EPR spectrum at 10 K are also consistent, in the frame of dipolar approximation, with the model of four interacting spins at the nodes of the supramolecular assembling.

Novel excited quintet state in porphyrin: bis(quinoline-TEMPO)-yttrium-tetraphenylporphine complex.

New mono- and bis[4-(3-hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethylpiperidin-1-oxyl](meso-tetraphenylporphyrinato)yttrium(III) complexes have been synthesized, and the properties of the excited states generated by photoexcitation of porphyrin were studied by time-resolved (TR) and pulsed two-dimensional electron paramagnetic resonance (EPR) spectroscopy. A TR-EPR spectrum was observed in the quartet (S=3/2) or quintet (S=2) states generated from interactions of one or two radicals with the photoexcited triplet state of the porphyrin. The zero-field splitting D values of these states were analyzed in terms of those of the triplet and the radical-triplet pair. The spin states of the excited states were definitely assigned by measuring the mutation frequencies with pulsed EPR.

Monitoring photodecomposition of dibenzyl ketone within NaY zeolite with a pre-fluorescent nitroxide compound.

Benzyl radicals derived from photodecomposition of dibenzyl ketone have been monitored in NaY zeolite using the pre-fluorescent probe 4-(3-hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethylpiperidine-1-oxyl free radical (QT). The steady-state and time-resolved fluorescence show a strong interaction of QT within the zeolite cavities. Furthermore, the increase in the fluorescence with the irradiation time reveals the efficient trapping of benzyl radicals by QT under vacuum. Dispersion of the zeolite composites in liquid polymers results in an improvement in the fluorescence measurement and faster kinetics in comparison with the solid samples. These results are interpreted in terms of partial refractive index matching as a consequence of polymer reptation into the zeolite cavities. Analysis of the fluorescence lifetime distribution shows the presence of different fluorescence ensembles with different fluorescence quantum yields, depending on the microenvironment within the zeolite particle. Pre-fluorescent probes offer a novel and useful methodology for studying radical processes in zeolites.

TiO2-promoted mineralization of organic sunscreens in water suspension and sodium dodecyl sulfate micelles.

The photostability of the widely used UVB sunscreen agents 2'-ethylhexyl-2-cyano-3-phenylcinnamate (1), 2-hydroxy-4-methoxybenzophenone (2), octyl salicilate (3), and 2'-ethylhexyl-4-methoxycinnamate (4) has been investigated under UVA irradiation in the absence and presence of TiO2, an inorganic filter commonly employed in combination with organic filters in sunscreen preparations. In the absence of TiO2, 1-3 are photostable and 4 undergoes the expected E-Z isomerization; the presence of TiO2 caused mineralization of the organic filters and, surprisingly, the process is noticeably faster in the presence of surfactant than in sunscreen and water suspensions. The results indicate that in water suspension, mineralization is likely to occur on or near the TiO2 particle surface; when the organic sunscreens are segregated in the micelle core, reactive radicals, produced during TiO2-promoted degradation of the micellar system, may participate in sunscreen degradation. In addition, a pre-fluorescent probe for carbon-centered radical detection, 4-(3-hydroxy-2-methyl-4-quinolineoxy)-2,2,6,6-tetramethylpiperidine-1-oxyl free radical or QT (5), was employed to demonstrate that carbon-centered radicals are evolved during micelle degradation and may participate in the mineralization of sunscreens.