1,720,987 research outputs found
Porphyrin-based electrostatically self-assembled multilayers as fluorescent probes for mercury(II) ions: a study of the adsorption kinetics of metal ions on ultrathin films for sensing applications
No full textThe electrostatic multilayer self-assembly technique was employed to prepare fluorescent probes for
mercury(II) ions. Films showing an appreciable fluorescence emission were obtained by alternate
adsorption of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) with polyelectrolytes. Fluorescence
intensity quenching, arising from the coordinative interaction between the porphyrin and mercury ions,
was monitored as a function of time. The response mechanism was studied on the basis of kinetic and
equilibrium models for adsorption. The adsorption process of mercury ions on porphyrin-based films
was described by the pseudo-second-order equation and the kinetic parameters were correlated with
the initial concentrations of the metal ions in aqueous solution. The kinetic approach was evaluated as
an alternative to monitoring the sensing responses at equilibrium
Light-induced degradation of metsulfuron-methyl in water
No full textPhotodegradation of metsulfuron-methyl, a sulfonylurea herbicide, has been investigated in aqueous solution at different pH and excitation wavelengths. The efficiency of the process has been evaluated through quantum yield determinations. The identification of the photoproducts indicates that the major photochemical pathway is initiated by C-S bond dissociation followed by involvement of water to yield the main final products; the behaviour in water is shown to differ markedly from that in an organic environment. © 2004 Elsevier Ltd. All rights reserved
Photodegradation of organic pesticides in the environment
No full textReactions induced by absorption of light contribute to the degradation of many chemical compounds in the environment. A critical overview on the current research activity concerning direct and indirect photodegradation processes of organic pesticides in the aquatic environment and on soil is reported
Consequences of H-dimerization on the photophysics and photochemistry of oxacarbocyanines
No full textThe photophysical/photochemical behaviour of the monomers and the H dimers of four oxacarbocyanines (dyes D-G in the scheme) was investigated in water. In contrast with the usually observed effect of H dimerization, the dimers of dyes D-G were found to fluoresce with efficiencies comparable to or larger than those of the corresponding monomers. Analysis of the decay paths of the lowest excited singlet state showed, however, that dimerization causes a decrease of the radiative rate constants and an enhancement of intersystem crossing to the triplet manifold, as expected from application of exciton theory to a model H dimer. Twisting about one of the polymethine bonds contributes to the decay from the spectroscopic minimum of monomers, though yielding a rather small amount of a distorted cis isomer. The process is inhibited in dimers, likely due to a pronounced increase of activation energy connected with a loss of van der Waals attractive energy at the twisted geometry
Irradiation-wavelength dependent photochemistry of the bichromophoric sulfonylurea chlorsulfuron
No full textThe photochemistry of the bichromophoric sulfonylurea chlorsulfuron has been investigated at different irradiation wavelengths and environmental conditions. The identified products indicate that different reaction paths are followed, depending whether the benzene or the triazine component chromophores are excited. In the first case, the chlorine atom is substituted by hydrogen or hydroxyl in water; in the second one three competitive photodegradation paths may be followed, the most efficient one being the cleavage of the S-N bond in the sulfonylurea bridge. Photodegradation quantum yield measurements, combined with laser flash photolysis experiments have shown that, while both singlet and tripler states contribute to all photolytic paths in deaerated solution, quenching by oxygen makes the triplet contribution negligible in samples equilibrated with air. (C) 2001 Elsevier Science B.V. All rights reserved
Modelling of the cis-trans partitioning in the photoisomerizations of cyanines and stilbene derivatives
No full textIn the course of photoisomerization, polymethine cyanines as well as stilbene and its derivates decay from the S1 potential energy minimum, corresponding to the perpendicular geometry, to yield either cis or trans ground-state molecules. The fraction of cis isomers obtained, alpha, spans a larger range of values for symmetric cyanines than for stilbene derivatives. It is argued that such different behaviour for the two classes of compounds should be traceable to the electronically different nature of their S1 perp species. Making use of radiationless transition theory results, it is shown that the relative location of the S1 minimum and S0 maximum along the internal rotation coordinate is crucial to the evaluation of alpha: even small differences between these critical twisting angles, which are more reasonably expected for polymethine cyanines than for stilbene-like compounds, may cause strong deviations from equipartitioning (alpha = 0. 5 )
PHOTOISOMERIZATION DYNAMICS OF 3,3'-DIETHYL OXACARBOCYANINE - INTRAMOLECULAR AND SOLVENT VISCOSITY EFFECTS
No full textThe dynamics of photoisomerization and of ground-state back isomerization of 3,3'-diethyl oxacarbocyanine (DOC) in linear alcohols are analyzed. Intramolecular activation energies are obtained and compared with those of the corresponding dicarbocyanine (DODCI) in an attempt at establishing the consequence of the polymethine chain lengthening on the isomerization dynamics. The activation energy for the photoisomerization of DOC is slightly lower than that of DODCI, whereas the reverse is found for the ground-state back isomerization. The coupling of the internal twisting coordinate with solvent drag is analyzed within the framework of the hydrodynamic Kramers model. Although this approach does not provide a fully satisfactory description of the experimental points, it has been taken as a reference model: the parameters obtained from the data fitting have been assigned a semi-quantitative significance and a number of possible reasons of breakdown of the hydrodynamic Kramers description have been considered and qualitatively discussed
Electronic spectra and fluorescence properties of multichromophoric sulfonylureas
No full textThe multichromophoric character of two sulfonylurea herbicides, SMT and BNS, has been investigated in its manifestations in the electronic absorption spectra and in some fluorescence properties through a combined experimental and theoretical approach. After a theoretical analysis of the most stable structures of these flexible systems, the UV absorption spectra of the two multichromophoric compounds have been analysed both experimentally and theoretically, and most transitions have been assigned to individual chromophores, also by comparison with four suitable reference compounds (5-8). Finally, some experimental information concerning the fluorescence spectra and quantum yields have been analysed with reference to the contributions from single fluorophores and the role of low-lying n -> pi* states. (c) 2006 Elsevier B.V. All rights reserved
Induction of cytochrome P450 enzyme activity by UVB and xenobiotics in normal human keratinocites.
No full textThe function of the skin is to provide a barrier for protection against the external environment. Relatively little is known about the overall role of CYP450 in the metabolism of xenobiotics or endogenous cellular compounds in the skin. The aim of this study was to analyse the expression and induction of several drugs metabolizing enzymes involved in either phase I or phase II reactions, in proliferanting human keratinocytes after exposure to UVB radiation and to three classical cytochrome inducers such as: β-naphthoflavone (BNF), 3-methylcholanthrene (MC), phenobarbital (PB). We investigated 7-ethoxyresorufìn O-deethylase (EROD) and 7-pentoxyresorufìn O-depenthylase activities (PROD). Normal human keratinocytes were cultured with mitomycin-treated 3T3 cells in Dulbecco’s modified Eagle’s medium/Ham’s F12. At confluency cells were incubated with inducers or irradiated with different doses of UVB. The microsomal fraction was studied by western-blot analysis. The MC-induced EROD activity was up to 4 fold higher when compared with BNF induced activity. UVB exposure resulted in a dose-dependent (25-75 mJ) and time dependent (6-24 h) induction of CYP450 1A1. Immunoblotting assay showed expression for CYP450 1B1 for both keratinocytes and melanocytes. Proadifen, an inhibitor of CYP450-monooxygenase, led to a significant decrease in EROD activity. The results of the present study clearly show that irradiation with UVB is capable of modifying the activity of CYP450 isoenzymes in keratinocytes. The phase II enzyme gluthatione S transferase activity (GST) was induced by UVB and PB. These experimental findings stress the value of epidermal cell culture for pharmaco-toxicological studies of topical agents used in dermatology
Dimerization of green sensitizing cyanines in solution. A spectroscopic and theoretical study of the bonding nature
No full textThe bonding nature in cyanine-dye aggregates has been investigated by studying dimerization in solution of 3, 3'-disulfopropyl-4,5,4',5'-dibenzo-9-ethyloxacarbocyanine (D) and three other oxacarbocyanine analogues (E, F, G) used as photographic sensitizers in the green spectral region. Quantitative information on the monomer-dimer equilibrium of dye D in different solvents and of its analogues (E, F, G) in water was obtained by measurement of the absorption spectrum as a function of dye concentration and of temperature. Dimerization was found to be generally driven by enthalpic factors traceable to strong attractive van der Waals interactions between the two large and highly polarizable dye molecules. Entropic contributions to DeltaG(0) usually favour dissociation but are smaller than the enthalpic ones. The visible absorption spectrum of the dimer consists in a classic two-branched exciton band with a marked splitting (1600 cm(-1) in water). The experimental observations were the subject of a theoretical study including classical molecular dynamics (MD) and Monte Carlo (MC) calculations of the dimer structure and comparative analysis of monomer and dimer spectra by the CS INDO CI method. Computer simulations led to three similar H-type structures, the most stable of which is characterized by a distance of 4.7 Angstrom between the planes of the chromophores and an endoendo configuration of the sulfopropyl substituents. The calculated dimer spectrum was clearly interpreted in terms of exciton model but a quantitative agreement with the two-maximum exciton band could be obtained only by assuming substantial deviation of the long molecular axes from parallelism. On the basis of normal coordinate calculations it is suggested that such configurations may occur with a high probability in virtue of twisting vibrational motions of extremely low frequency. (C) 2002 Elsevier Science B.V. All rights reserved
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