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Steady-state and time-resolved fluorescence measurements have been made of human and rabbit lens epithelial cells and their total soluble protein. Excitation at 350 nm results in broad fluorescence spectra peaking at 450 nm and stretching into the visible past 650 nm. The fluorescence excitation spectra peak around 350 nm. We assign the species responsible for this absorption and fluorescence as NADPH. Because the absorption of near-UV light (300-400 nm) is responsible for cell damage and death, we postulate that excited states of NADPH are implicated in the mechanisms of cell damage. Preirradiation with 355 nm light leads to a loss of NADPH fluorescence but with no change in decay kinetics. Possible mechanisms for cell damage are explored.

We have studied the influence of electric field and humidity on photoconductivity and fluorescence in particles of highly photoconductive Y-form TiOPc dispersed in a poly(vinylbutyral) polymer matrix. Both integrated and time-resolved fluorescence quenching by electric field were measured. Integrated fluorescence quenching showed a linear dependence at low applied fields and a linear correlation with carrier generation efficiency. Time-resolved fluorescence decays were analyzed by fitting the data to a sum of two exponentials representing fast and slow fluorescence components. Qualitative features of fluorescence decay were the same for low and high humidity levels. The amplitude and lifetime changes of the two fluorescence components upon the application of the electric field are in agreement with theory which describes energy transfer between free and trapped exciton states. These results indicate that carrier generation in Y-TiOPc originates from both relaxed and nonrelaxed intrinsic excited singlet st...

— Much of the emphasis in the development of methods for time-resolved measurements is on increasing experimental time resolution. Equally important, however, is the establishment of methods to apply existing techniques to problems of increasing complexity. Here we describe the application of a combination of picosecond absorption and emission measurements to solve a complex kinetic problem in organic photophysics, namely, the dynamics of interconverting radical-ion pairs. Although fairly routine, the emission measurements require higher time resolution and the absorption measurements require higher signal-to-noise ratios than are often achieved. In the first part of this paper we describe the ways in which these experimental requirements have been met. Next, the radical-ion pair kinetic problem and its solution are described. Finally, we discuss some future directions for the techniques as they relate to studies of radical-ion pair dynamics.

The photophysical behavior of the transition metal complexes {[(bpy)2Ru]2(bphb)}(PF6)4, {[(tpy)(CN)Ru]2(bphb)}(PF6)2, and [(bpy)2Ru(bphb)Ru(tpy)(CN)](PF6)4 (bpy = 2,2‘-bipyridine, tpy = 2,2‘,6‘,2‘‘-terpyridine, bphb = 1,4-bis(2,2‘-bipyrid-4-yl)benzene) was investigated in acetonitrile solution and low-temperature glasses. Luminescence spectra, excitation spectra, and transient absorption decays of the three complexes serve to show that intramolecular electronic energy transfer from the MLCT excited state of the [(bpy)2Ru(bphb) chromophore to the MLCT state of the tpy-containing chromophore occurs in the unsymmetric bimetallic complex. Nearly complete energy transfer from the [(bpy)2Ru(bphb)] chromophore to the tpy-containing chromophore was observed even in 4:1 ethanol:methanol glasses at 20 K. A semiclassical exchange energy transfer mechanism was used to treat the available data; the Franck−Condon weighted density of states (FCWD) was obtained using parameters determined from fits of luminescence spectr...

Fluorescent material generated in the human retina accumulates within lipofuscin granules of the retinal pigment epithelium (RPE) during aging. Its presence has been suggested to contributed to various diseases including age-related macular degeneration. Because this material absorbs light at wave lengths as long as 550 nm, photophysical studies were performed to determine whether lipofuscin could contribute to light damage and to determine if its composition is similar to a synthetically prepared lipofuscin. Time-resolved experiments were performed to monitor (1) fluorescence decay, (2) the UV-visible absorption of longer-lived excited states and (3) the formation and decay of singlet oxygen at 1270 nm. Steady-state and time-resolved fluorescence studies indicate that human and synthetic lipofuscin have fluorophores in common. Time-resolved absorption experiments on human retinal lipofuscin and synthetic lipofuscin showed the presence of at least two transient species, one absorbing at 430 nm (lifetime ca 7 microseconds) and a second absorbing at 580 nm, which decays via second order kinetics. In addition, there is a third absorbing species stable to several hundred milliseconds. The transient species at 430 nm is quenched by oxygen, suggesting that it is a triplet state. Subsequent studies showed the formation of singlet oxygen, which was monitored by its phosphorescence decay at 1270 nm. These studies demonstrate that lipofuscin can act as a sensitizer for the generation of reactive oxygen species that may contribute to the age-related decline of RPE function and blue light damage.

Layered alkali-metal titanates (Na[sub 2]Ti[sub 3]O[sub 7] and K[sub 2]Ti[sub 4]O[sub 9]), niobates (KNb[sub 3]O[sub 8] and K[sub 4]Nb[sub 6]O[sub 17]), and titanoniobates (KTiNbO[sub 5] and CsTi[sub 2]NbO[sub 7]) were internally platinized, acid-exchanged, sensitized with ruthenium polypyridyl complexes, and studied as photocatalysts for the production of H[sub 2] and I[sub 3][sup [minus]] from acidic alkali-metal iodide solutions. The titanates were inactive as photocatalysts, whereas the niobates and titanoniobates were active with quantum efficiencies up to 0.3% for HI photolysis with visible light. Calculations based on electronegativity showed that the conduction band edge potentials of the acid-exchanged titanates were too positive to prevent semiconductor-mediated recombination of photogenerated H[sub 2] and I[sub 3][sup [minus]]. Laser flash photolysis/transient diffuse reflectance spectra established that iodide reduces the oxidized sensitizer, forming I[sub 2][sup [sm bullet][minus]], which subsequently decays in a bimolecular reaction to form I[sub 3][sup [minus]]. The inefficiency of HI photolysis can be attributed to charge recombination between I[sub 3][sup [minus]] and conduction band electrons for the niobates and titanoniobates. 29 refs., 16 figs., 1 tab.

Methylene blue intercalates more effectively into poly[dGdC] (K = (1.6 [+-] 0.4) [times] 10[sup 6] M[sup [minus]1]) than poly[dAdT] (K = (1.6 [+-] 0.5) [times] 10[sup 4] M[sup [minus]1]); in the latter case, intercalation competes with surface binding (K = (2.3 [+-] 0.6) [times] 10[sup 4] M[sup [minus]1]). Intercalation is accompanied by a dramatic reduction in the lifetime of the first excited singlet state, causing decreases in quantum yields for both fluorescence and formation of the triplet state. In the case of methylene blue intercalated into poly[dGdC], transient absorption spectroscopic measurements have shown that the excited singlet state of the dye abstracts a hydrogen atom (or an electron) from an adjacent nucleic acid base, presumed to be guanine on thermodynamic grounds. The rate constants for forward and reverse transfers, respectively, are (2.5 [+-] 0.3) [times] 10[sup 11] and (3.3 [+-] 0.4) [times] 10[sup 10] s[sup [minus]1]. The rate constant for the forward transfer in poly[dAdT] is (1.4 [+-] 0.5) [times] 10[sup 10] s[sup [minus]1] and is presumed to be slower than that of the reverse transfer. Using thermodynamic arguments, it is concluded that the quenching processes involve hydrogen atom transfer from guanine or adenine to the excited singlet state ofmore » methylene blue. 48 refs., 4 figs.« less

The rates of photoinduced electron transfer (ET) in a series of donor-acceptor molecules (4,4'-R 2 -2,2'-bipyridine) 2 Ru(1-(4-CH 3 -2,2'-bipyridine-4'-yl-(CH 2 ) n )-4,4'-bipyridinediium-1'-R') 4+ (R=H, CH 3 ; R'=CH 3 , CH 2 CN; n=1, 2) were studied by picosecond flash photolysis/transient absorbance techniques. The rate of intramolecular forward ET (MLCT quenching) in acetonitrile varied with -ΔG o according to classical Marcus theory for n=2, but not for n=1

Bisgold(III) bisporphyrins, in which a 1,10-phenanthroline spacer imposes a constrained geometry, have been synthesized. Mash photolysis studies indicate that, at high laser intensity, triplet exciton annihilation occurs with a diffusion coefficient of 4×10 -5 cm 2 s -1 . Two such molecules coordinate to a copper(I) ion, via the 1,10-phenanthroline spacers, to form a porphyrin ensemble in which triplet exciton annihilation competes with electron transfer from the copper(I) complex to a gold porphyrin triplet

The effect of substituted ferrocene complexes on the surface of quantum-confined (Q) cadmium sulfide clusters in inverse micelles was examined by steady-state and time-resolved photoluminescence (PL) spectroscopy. Addition of (dimethylamino)methylferrocene (DMAMF) to Q-CdS enhanced cluster PL peak areas by 120%, in contrast to the carboxylic acid derivatives ferrocenecarboxylic acid (FCA) and ferrocenedicarboxylic acid (FDCA), which quenched PL by 60% and 80%, respectively

— The human lens is continually under photooxidative stress from ambient radiation. In the young lens the major absorbing (between300–400 nm) species is the glucoside of 3-hydroxy kynurenine. Using time resolved fluorescence spectroscopy on both the isolated compound and the intact human lens, the first excited singlet state of this compound is shown to have fast (ps) decay processes. This would tend to minimize damage to lens constituents because there would be little time for energy transfer into more harmful channels. Thus, this compound appears to act as a protection for the retina. With aging, human lens proteins become yellow with absorptions out to 450 nm. Time resolved diffuse reflectance spectroscopic studies on intact older human lenses showed that excitation (355 nm) resulted in the formation of long lived (microseconds) transient species with an absorption maximum at ca 490 nm. Similar spectra were obtained from two model systems used to explain age related changes in human lens proteins.

Pulse radiolysis has been used to study the reaction of ethidium bromide with Br 2 - , (SCN) 2 - and N 2 , both in aqueous solution and in aqueous solutions of DNA. Rate constants of 1.9 ± 0.2, 1.8 ± 0.2 and 5.7 ± 0.6 x 10 9 M -1 s -1 were measured for Br 2 - , (SCN) 2 - and N 2 respectively in aqueous solution. The rate constant for reaction of Br 2 - and (SCN) 2 - with ethidium bromide is reduced by a factor of approximately 1000 when the dye is intercalated in DNA, whilst the reduction is only a factor of 50 for N 2 . The product of these reactions is one electron oxidised ethidium bromide, which is shown to undergo a deprotonation reaction in alkaline solution. A pK value of 8.2 ± 0.3 is measured for oxidised ethidium bromide in aqueous solution and this value does not change appreciably in the presence of DNA. Possible implications of these observations are discussed.

Previous fluorescence resonance energy transfer (FRET) measurements, using BIDS (4-benzamido-4'-isothiocyanostilbene-2,2'-disulfonate) as a label for the disulfonic stilbene site and FM (fluorescein-5-maleimide) as a label for the cytoplasmic SH groups on band 3 (AE1), combined with data showing that the cytoplasmic SH groups lie about 40 A from the cytoplasmic surface of the lipid bilayer, would place the BIDS sites very near the membrane's inner surface, a location that seems to be inconsistent with current models of AE1 structure and mechanism. We reinvestigated the BIDS-FM distance, using laser single photon counting techniques as well as steady-state fluorescence of AE1, in its native membrane environment. Both techniques agree that there is very little energy transfer from BIDS to FM. The mean energy transfer (E), based on three-exponential fits to the fluorescence decay data, is 2.5 +/- 0.7% (SEM, N = 12). Steady-state fluorescence measurements also indicate

Previous studies have shown that the triplet state lifetimes of various porphyrins are increased by several orders of magnitude when they are bound to lens protein. Flash photolysis studies of mesotetra (p-sulfonatophenyl)porphyrin (TPPS) on intact bovine lenses indicated a biexponential decay of the triplet state with lifetimes of 160 microsecond and 1.6 ms. Here we extend those measurements to TPPS associated with intact human lenses. Steady-state fluorescence measurements indicate that TPPS binds to both young and old human lenses. In an intact young human lens, the TPPS triplet state is observed to decay biexponentially with lifetimes of 50 and 680 microsecond. As the age of the lens increases, the lifetime of the shorter-lived component lengthens while that of the longer-lived component decreases slightly. In order human lenses, the two lifetimes coalesce and the triplet decay exhibits purely monoexponential behavior. These photophysical characteristics apparently are due to age-related modification(s) of the protein in the human lens resulting in an increasingly more homogeneous environment around the porphyrin.

We studied the influence of electric field and humidity on photoconductivity and fluorescence in particles of Y-form TiOPc dispersed in polyvinyl butyral (PVB) polymer matrix. Increasing relative humidity from 0% to 42% at room temperature lead to a decrease of the total fluorescence by about 45%. Fluorescence time decays were analyzed by fitting the data to a sum of two exponentials. Qualitative features of fluorescence decay were the same for low and high humidity levels. The fast fluorescence component, assigned to mobile singlet excitons, shows a decrease in both its amplitude (“amplitude quenching”) and lifetime (“rate quenching”) decrease with increasing electric field. The slow component of fluorescence shows mostly electric field induced amplitude quenching and is assigned to extrinsic fluorescence from a trapped exciton. The relative intensities of the two fluorescence components, fast and slow, are in agreement with a theory which describes energy transfer between free and trapped exciton states. These results indicate that carrier generation in Y-TiOPc originates from both relaxed and non-relaxed intrinsic excited singlet states, while the trapped excitons do not lead to significant carrier production. The presence of water strongly influences the amplitude quenching by the electric field of the fast fluorescence component. This indicates that the decay probability of non-relaxed precursor excitons is influenced by water present in the sample. We propose that the non-relaxed excitons are charge transfer excitons which have been previously observed in electroabsorption measurements. Relaxed excitons, which emit fast component of fluorescence, are then identified as first excited singlet states which do not have significant charge transfer character. The overall picture of carrier generation in Y-TiOPc therefore encompasses three different processes: sensitization of carrier generation by water molecules, field assisted dissociation of charge transfer excitons, and finally field assisted dissociation of first excited singlet state excitons.

The first excited triplet state of DNA-intercalated ethidium bromide is produced with a quantum yield of 0.01 +/- 0.002 on irradiation at 532 nm. A difference extinction coefficient of 1.5 +/- 0.2 x 10(3) m2 mol-1 is measured for the triplet state at 380 nm. Mercuric ions quench the first excited singlet state of DNA-intercalated ethidium bromide via induced spin orbit coupling to give an increased yield of ethidium triplet states. The same mercuric ion that quenches the singlet state then quenches the triplet state, via the same mechanism, with a rate constant of ca 3.5 x 10(3) s-1. An upper limit for the rate of detachment of Hg2+ from its binding site in DNA may be fixed at ca 10(3) s-1.

Previous steady state and time resolved spectroscopic studies on porphyrins have shown that the triplet lifetimes of those sensitizers that bind to lens proteins are lengthened by several orders of magnitude. Presented here is an extension of this experiment to measure these transients in an intact bovine lens. As demonstrated by steady state fluorescence spectroscopy and flash photolysis, mesotetra (p-sulfonatophenyl)porphyrin (TPPS) binds to lens proteins. In air-saturated aqueous solution, TPPS has a triplet lifetime of 2 microseconds. In an intact bovine lens the triplet state decayed via biexponential kinetics with lifetimes of 0.16 and 1.6 microseconds. In addition to a lengthening of the lifetime there was a red shift in the triplet transient spectra of 10-20 nm of the porphyrin in the intact lenses.

Time-resolved electronic absorption, infrared, resonance Raman, and magnetic circular dichroism spectroscopies are applied to characterization of the intermediate which is formed within 20 ps after photodissociation of CO from cytochrome a3 of reduced cytochrome oxidase. This intermediate decays with the same halflife (ca. 1 microsecond(s)) as the post- photodissociation CuB+-CO species previously observed by time-resolved infrared. The transient UV-Vis spectra, kinetics, infrared, and Raman evidence suggest that an endogenous ligand is transferred from CuB to Fea3 when CO binds to CuB, forming a cytochrome a3 species with axial ligation which differs from the reduced unliganded enzyme. The time-resolved magnetic circular dichroism results suggest that this transient is high spin and therefore five coordinate. Thus it is inferred that the ligand from CuB binds on the distal side of cytochrome a3 and displaces the proximal histidine imidazole. This remarkable mechanistic feature is an additional aspect of the previously proposed 'ligand shuttle' activity of the CuB/Fea3 pair. The authors suggest that the ligand shuttle may play a functional role in redox-linked proton translocation by the enzyme. More detail on this work is presented elsewhere.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

— Photooxidation, whether initiated by an endogenous or exogenous sensitizer, is an important mechanism in light induced damage to the lens. One of the substrates for this damage is lens protein. A porphyrin sensitizer which binds to lens proteins [mesotetra(p-sulfonatophenyl) porphyrin (TPPS)] was found to photooxidize Skh-2 pigmented mice lens protein in vivo. Uroporphyrin, a model for a non-binding photosensitizer, did not induce photooxidative damage to the mouse lens. The radioprotector 3-amino-2-hydroxypropyl phosphorothioate (WR-77913) was investigated as an agent to retard or negate in vivo photooxidative damage to the lens. Intraperitoneal injections of WR-77913 prior to irradiation reduced the TPPS induced photodestruction of lens protein in Skh-2 pigmented mice. The mechanism of protection was also investigated. Thiols were found to quench both the triplet state of porphyrins and the reactive intermediate singlet oxygen on the order of 105 and 106M-1 s1 respectively. These are probably not fast enough to explain most of the protection afforded by thiols. An additional mechanism may be the accelerated photobleaching of porphyrins by thiols which protects tissue by reducing the absorptions due to the porphyrins.

The young human lens contains species (3-hydroxy kynurenine; 3-HK and its glucoside; 3-HKG) which absorb most light between 300 and 400 nm. Photochemical studies have indicated that these compounds are relatively inefficient sensitizers of lens proteins. An investigation of the fluorescent properties of 3-HKG indicate that it contains a fast deactivation pathway (ps) which would be expected to have minimal photochemical effect on the integrity of the lens. Further phot physical studies on 3-HK indicates that it has an even faster fluorescent lifetime (less than 10 ps) with a much lower quantum yield of fluorescence (0.001 vs 0.03 for 3-HKG). With aging, the human lens proteins undergo numerous changes including a generalized yellowing. These chromophores exhibit a higher quantum yield of fluorescence, an increase in the fluorescent lifetime by 2 orders of magnitude and the formation of two long lived transient species (microsecond). These species might be expected to drastically increase the susceptibility of the human lens to ambient radiation. Based upon quantitative experimental comparisons with 3-HK this does not seem to be the case. Further time resolved studies on old lens proteins indicate that the two transient species are interconnected in that the first transient species is the precursor to the second. The implications of this mechanism on the integrity of the lens and origin of those chromophores is discussed.

Previous studies have shown that mesotetra(p-sulfonatophenyl)porphine (TPPS) binds to lens proteins. This characteristic should increase the residence time of the sensitizer in the lens and therefore enhance the probability of inducing photooxidative damage to that tissue in vivo. Subsequent in vivo studies have verified that contention. The present studies were performed to determine the effect of such binding on the spectroscopy and photophysics of the porphyrins. It was found that the binding of TPPS (1) quenches the fluorescence of lens proteins, (2) causes a shift in the ground state absorption spectra, fluorescence excitation spectra and the triplet excited state spectrum of TPPS to longer wavelengths and (3) results in an increase in the triplet state lifetime of TPPS. In the presence of the isolated crystallins the average triplet lifetime increases in the following order: gamma less than beta less than alpha.

Alkyl halides RI (R = Me, Et, n-Pr, i-Pr, n-pentyl, CF3, CH2I2) undergo a thermal reaction with Pt2(pop)44− (pop = pyrophosphite) to give the axially substituted “lantern” complexes Pt2(pop)4RI4−. For R = Me, the pure complex can be isolated The solution structure has been characterized by a combination of 1H, 13C, 31P and 195Pt NMR spectroscopy. With the higher homologues, (R = Et, n-Pr, i- Pr, n-pentyl) the reaction gives a mixture of Pt2(pop)4RI4− and Pt2(pop)4I24−. A radical pathway is proposed. Aryl halides ArX (X = Cl, Ar = Ph; X = Br, Ar = Ph, p-FC6H4, p-HOC6H4, p-CH3OC6H4, p-HO2CCH4, p-CH3C6H4; X = I, Ar = Ph) photochemically add to the triplet excited state Pt2(pop)44−★ to give Pt2(pop)4ArX4−. For the photochemical reaction with C6F5Br, CCl4, CHCl3, CH2ClCO2H, CH2BrCO2H and p-BrC6H4NH3+ the product is the dihalo complex Pt2(pop)4X24− (X = Cl, Br).

Photoionisation de Ru(bpy) 2 (CN) 2 et Ru(bpy) 3 2+ par rayonnement laser (355 nm). Rendements en electrons hydrates en fonction de l'energie laser en presence et absence de micelle de SDS. Mecanismes de formation. Constantes de vitesse

Obtention de derives monoradical trication et diradical de cation par reduction de 3 dimethyl-1'1''' polymethylene-1,1'' bis-bipyridylium-4,4'. Etude des caracteristiques spectrales et electrochimiques d'equilibre de ces composes et cinetique de leurs reactions de reduction, oxydation et dismutation

La spectrometrie de fluorescence est utilisee pour etudier la formation d'agregats entre des ions d'ethidium, de metaux divalents et de dodecylsulfate de sodium

Two (d/sup 8/.d/sup 8/)psigma binuclear complexes, Pt/sub 2/(pop)/sub 4//sup 5 -/ (pop = P/sub 2/O/sub 5/H/sub 2//sup 2 -/) and Rh/sub 2/(TMB)/sub 4//sup +/ (TMB = 2,5-dimethyl-2,5-diisocyanohexane), have been generated by pulse radiolysis of aqueous and acetonitrile solutions, respectively, of the corresponding d/sup 8/.d/sup 8/ species. An intense absorption system attributable to the allowed dsigma* ..-->.. psigma transition is observed in the spectrum of each of the transients (420 nm, Pt/sub 2/(pop)/sub 4//sup 5 -/; 570 nm, Rh/sub 2/(TMB)/sub 4//sup +/).

The work which follows may be thought of in terms of photosensitization of simple one electron transfer reactions and of the way in which the DNA environment effects both their yields and rates. Electron transfer reactions in DNA have important biological consequences; for example, they are involved in the formation and repair of thymine dimers which are the major damaging species produced in the cell by UV light (Jorns, 1987). Also, abnormally high mobilities for electrons in DNA have been reported (Van Lith et al., 1986), a phenomenon which may be of considerable importance in the understanding of many biological processes. The following studies make no pretense of being actual biological processes which commonly occur in living systems. The reactions studied are, however, electron transfer reactions which are caused to take place in the DNA matrix, and as such are valid reflections of the way DNA mediates such processes.

— Laser flash photolysis studies of DNA-complexed ethidium bromide were undertaken. We have observed a singlet-singlet (S1-Sn) absorption process for DNA-complexed ethidium bromide. The observed lowest singlet excited state lifetime was 21 ± 2 ns. The molar difference extinction coefficient was measured to be 2.4 ± 0.4 × 103M-1 cm-1 at 370 nm. The assignment of this transition was confirmed by time resolved fluorescence measurements.

We have modified the total internal reflectance fluorimetry (TIRF) method to determine, indirectly, quantum fluorescence yield and thus infer structural changes of adsorbed plasma proteins. We employed a frequency multiplied Nd:YAG pulse laser to excite tryptophan residues of bovine serum albumin (BSA), measuring the resulting fluorescence lifetimes. BSA adsorbed on quartz was investigated at normal pH and under acidic (pH 3.8) conditions at which albumin shape change has been established in bulk solution. Albumin adsorbed at pH 7.2 has a slightly lower lifetime than the bulk solution value, reflecting a population of both native and isomerized forms. Gently washing the adsorbate surface significantly reduces fluorescence lifetime, suggesting retention of more tenacious, conformationally altered molecules. Vigorous washing further reduces fluorescence lifetimes to values close to those obtained in bulk under acidic conditions. Similar alterations in fluorescence lifetimes were observed for heat-denatured albumin. These results support the hypothesis that adsorbed albumin exists in the form of a two-layer deposit, each with a possibly different structure. A loosely held layer, consisting of BSA microaggregates and including partially unfolded molecules, exists on the undisturbed surface and is partially removed by gentle washing. Vigorous washing removes all but the most tightly held protein, which exists in a possibly further unfolded configuration. The decrease in fluorescence lifetime of adsorbed BSA implies a corresponding change in quantum yield. A change in quantum yield of BSA tryptophan fluorophors implies that measuring adsorbed protein concentration by intrinsic fluorescence is unreliable.

Etude de la reaction photochimique de Pt 2 (μ-P 2 O 5 H 2 ) 4 4− avec C 6 H 5 Br, C 6 F 5 Br et 1,2-C 2 H 4 Br 2 . Dans les 3 cas, le produit principal est Pt 2 (μ-P 2 O 5 H 2 ) 4 Br 4− . Mecanisme

The kinetics of the fluorescence quenching of Ru(dipy)2+3 and Ru(dipy)3(CN)2 by Cu2+ and MV2+ are reported. At SDS concentrations above the c.m.c. the kinetics of Ru(dipy)2+3 quenching by both quenchers are simple first order with constants which increase linearly with quencher concentration and which decrease with increasing SDS concentration. The results quantitatively agree with those predicted by the model in which the emitter is completely micellised and the quenching is slow enough to allow equilibration of the quencher between solution and micelles. With Ru(dipy)2(CN)2 above the c.m.c., the quenching by MV2+ has fast and slow components, the latter being that of the natural emission decay. The results are shown to be consistent with almost complete micellisation of the quencher and a high micellar quenching rate constant. With Cu2+ the micellar quenching is too fast to follow so that ‘quasi-static quenching’ is seen, i.e. there is a decrease in the initial emission intensity followed by the slower unquenched decay. However, analysis shows that specific interaction of Cu2+ with the emitter on the micelle must be assumed to account for the abnormally high efficiency of Cu2+.Enhanced quenching of Ru(dipy)2+3 emission by Cu2+ and MV2+ is also observed below the c.m.c. This is explained in terms of the formation of clusters of the emitter and SDS with which the quenchers associate. With Ru(dipy)2(CN)2 only Cu2+ gives enhanced quenching below the c.m.c. It is suggested that Cu2+ complexes with SDS to form a micelle-like species with which the emitter can associate.

The p-nitrobenzyl carbanion is a photoelimination product from various p-nitrobenzyl (p-NO2C6H4CH2R, R = CO2–, PO32–, CH2OH, etc.) compounds in aqueous media. Following millisecond flash photolysis of N2-saturated p-nitrophenylacetate (pH > 6) solutions, we have shown that the carbanion decays via mixed first-(solvent protonation) and second-(dimerisation) order processes. Rate constants k1= 1.3 ± 0.1 × 10–2 s–1 and 2k2= 1.7 ± 0.3 × 103 dm3 mol–1 s–1, respectively, are determined. Reactions of the anion with O2, H+, and an efficient one-electron oxidant, IrCl62–, are also examined. Studies in dimethyl sulphoxide–water mixtures indicate that the anion exists as a hydrated form (λmax 356 nm) in water and an ‘unspecifically’ solvated species in dimethyl sulphoxide (λmaxca. 440 nm). A reaction mechanism is developed which accounts for transformation of the carbanion to p,p′-dinitrobibenzyl and p-nitrotoluene, the major isolable nitro products from these systems.