Photooxygenations par transfert d'electron sensibilisees par le dicyano-9,10 anthracene. Role du solvant et formation d'oxygene singulet.

Photooxygenation of naphtalenic compounds sensitized by electron acceptors like 9,10 dicyanoanthracene (DCA) is shown to proceed by two distinct ways depending on the solvent polarity. In a polar solvent superoxide ion (O₂^-.) as well as singlet oxygen (¹O₂★) are involved while in a non polar solvent only singlet oxygen is produced.     

The Reaction of Singlet Oxygen with Adamantylideneadamantane Mediated by Rose Bengal

The photo-oxygenation of adamantylideneadamantane ( 1 ) on siliceous supports using admixed granules of ion-exchange resin fixed to methylene blue (MB) and rose bengal (RB) gave exclusively the corresponding dioxetane derivative 2 for the former sensitizer, while the latter gave 2 and traces of the epoxide 3. RB and the charge-transfer complex produced from N-ethylcarbazole and 2,4,5,6-tetranitrofluoren-9-one both reacted with chemically generated singlet oxygen to give superoxide radical anion. Trapping of the latter with 5,5-dimethyl-1-pyrroline 1-oxide gave an adduct exhibiting a characteristic ESR spectrum. The treatment of 1 in MeOH with 30% aqueous H₂O₂ for 22 h at 60° gave 3 in 100% yield. Repetition of this experiment in the presence of 2,6-di(tert-butyl)-p-cresol caused no significant change. These results indicate that singlet oxygen reacts with 1 , in the presence of RB, by two different processes. The first leads to dioxetane formation. The second process involves conversion of singlet oxygen by RB to superoxide radical anion which subsequently gives H₂O₂ so producing epoxide 3 from 1 .     

PHOTOSENSITIZED FORMATION OF ASCORBATE RADICALS BY RIBOFLAVIN: AN ESR STUDY

Abstract— The riboflavin-sensitized photooxidation of ascorbate ion (HA^-) to ascorbate radical (A^-) was followed by electron spin resonance (ESR) spectroscopy in conjunction with oxygen depletion measurements. In air-saturated aqueous media, steady-state amounts of A^- are rapidly established upon irradiation. The ESR signal disappears within a few seconds after the light is extinguished–more slowly under constant irradiation as oxygen is depleted. No photooxidation was observed in deaerated media. Similar results were obtained with other flavins and when ascorbyl palmitate was substituted for HA^-. The effect of added superoxide dismutase, catalase, desferrioxamine, and singlet oxygen scavengers (NaN₃ and tryptophan) was studied, as was replacement of water by D₂O and saturation with O₂. The results are indicative of ascorbate free radical production via direct reaction between ascorbate ion and triplet riboflavin in the presence of O₂. While the presence of superoxide ion tends to reduce the steady-state concentration of A^-, competition from the reaction of HA^- with singlet oxygen is less apparent in this system (at HA^-≥ 1 m M ) than in the previously studied aluminum phthalocyanine tetrasulfonate-photosensitized reaction.     

Reactivity of singlet oxygen generated by the photosensitization of tetraphenylporphyrin in liposomes

A hydrophobic porphyrin derivative, tetraphenylporphyrin (TPP), was used as a sensitizer, and an anionic dye, methyl orange (MO), was employed as a substrate of photooxidation. TPP was incorporated into the hydrophobic environment of phosphatidylcholine (PC) bilayer membranes, liposomes. When oxygen was purged out of the liposome suspension by nitrogen bubbling, the degradation of MO was completely inhibited. A specific superoxide scavenger, superoxide dismutase, had no effect on the MO degradation. The replacement of H₂O by D₂O resulted in a 10 times enhancement in the photodegradation of MO. These results suggested that singlet oxygen was generated by the TPP photosensitization and worked as the mediator of the photoreaction from TPP. Trisulphonated TPP,-phenyl-,, -tri(p-sulphonyl)porphyrin (TPPS), is soluble in aqueous solution. The light irradiation to an aqueous solution of TPPS gave rise to the rapid bleaching (decomposition) of the sensitizer itself. On the other hand, TPP in the hydrophobic environment of liposomes was stable during light irradiation and worked as a sensitizer for the continuous photoreaction. Maximum reactivity was observed at the PC/TPP mole ratio of 50. When TPP molecules were incorporated into liposomes at larger concentrations (PC/TPP<50), a part of the excitation energy of the sensitizer molecules was nonradiatively converted into the lattice energy by the resonance between the closely located TPP molecules. This led to lower efficiency for the photoactivation of oxygen. On the other hand, the increase in liposome concentration resulted in the enhancement of the MO binding to lipid membranes and the retardation of MO degradation. Also, the electrostatic attraction and repulsion between the membrane and the substrate influenced the reaction rate greatly. The oxidative degradations of the substrate by singlet oxygen were considered to be much faster in the polar environment than in the less polar environment. The charge transfer or the polarized transition complex of singulet oxygen and MO are presumed to be stabilized in the polar environment. The distribution of substrate between the less polar membrane surface and the polar bulk aqueous solution was another important factor in the photooxidation.     

Dye-sensitized photo-oxygenation of 1,2-cyclohexanediones

The dye-sensitized photo-oxygenation of the enols of 1,2-cyclohexanedione (1) has been carried out in various solvents at -70°-40°. Singlet oxygen is involved in the reaction as evidenced by quenching and rate enhancement observed in deuterated methanol. The reaction proceeds by an ene reaction with singlet oxygen to afford the hydroperoxide, 4, which closes to a five-membered endoperoxide, 5, as a major path or to dioxetane (6) as a minor one. The endoperoxide, 5, decomposed to 5-oxoalkanoic acid (2) with evolution of carbon monoxide or was trapped by the solvent (MeOH or EtOH) to give methyl or ethyl 5-carboxy-2-hydroxypentanoate (3). Competition between the enol of 3-methyl-1,2-cyclohexanedione (1a) and 2,3-dimethyl-2-butene (TME) has shown that the enol is as reactive as TME toward singlet oxygen.     

THE ROLE OF SUPEROXIDE AND SINGLET OXYGEN IN LIPID PEROXIDATION

Abstract— An investigation into the mechanism of lipid peroxidation catalyzed by xanthine oxidase showed a dependence upon superoxide, singlet oxygen and adenosine 5'-diphosphate chelated iron (ADP-Fe³⁺). In the absence of ADP-Fe³⁺ or in the presence of superoxide dismutase there is complete inhibition of enzymatic peroxidation. Initiation of peroxidation likely occurs through an ADP-perferryl ion complex formed by ADP-Fe³⁺ and superoxide. Use of the singlet oxygen trapping agent 2,5-diphenylfuran showed that singlet oxygen does not participate in the initiation of peroxidation but rather in the propagation of peroxidation. The mechanisms of NADPH-cytochrome P450 reductase-catalyzed and ADP-Fe²⁺ catalyzed lipid peroxidation parallel that of xanthine oxidase in that initiation occurs through a superoxide dismutase-sensitive reaction and that singlet oxygen is present during propagation of lipid peroxidation.     

Singlet Oxygen and Superoxide: Experimental Differentiation and Analysis

Superoxide produced by either pulse radiolysis or from KO₂ undergoes no reaction with furfuryl alcohol competitive to the corresponding endo-peroxide formation with singlet oxygen. Thus, furfuryl alcohol can be used as a specific chemical quencher of singlet oxygen. Superoxide can be quantitatively analyzed by its electron transfer reaction with 1, 4-benzoquinone. The differentiation of the two activated species of oxygen has been realized in microemulsions. Singlet oxygen is known to penetrate the interfaces of surfactant aggregates and may be analyzed in the hydrophobic core by known methods. Charged surfaces on such aggregates, specially those formed by anionic surfactants, prevent the passage of superoxide and restrict its presence and the need for analysis to the bulk aqueous phase. Hydrated electrons may penetrate the interface depending on the dose per pulse applied. 2, 5-Di (t-butyl)-1, 4-benzoquinone has been taken as an acceptor for electron transfer reactions within the hydrophobic core.     

Synthesis and excited-state photodynamics of perylene-porphyrin dyads. 4. Ultrafast charge separation and charge recombination between tightly coupled units in polar media

New perylene-porphyrin dyads that have excellent light-harvesting and energy-utilization capabilities in nonpolar media are found to exhibit efficient, ultrafast and tunable charge-transfer activity in polar media. The dyads consist of a perylene-monoimide dye (PMI) connected to a porphyrin (Por) via an ethynylphenyl (ep) linker. The porphyrin constituent of the PMI-ep-Por arrays is either a zinc or magnesium complex (Por = Zn or Mg) or a free-base form (Por = Fb). Following excitation of the perylene in each array in acetonitrile, PMI* decays in ≤0.4 ps by a combination of energy transfer to the ground-state porphyrin (forming Por*) and hole transfer (forming PMI^-Por⁺). The excited porphyrin formed by energy transfer (or via direct excitation) then undergoes effectively quantitative electron transfer back to the perylene (τ = 1, 1, 700 ps for Por = Mg, Zn, Fb). Subsequently, charge recombination within PMI^- Por⁺ returns each dyad quantitatively to the ground state (τ = 2, 4, 8 ps for Por = Mg, Zn, Fb). The dynamics of the PMI Por* → PMI^-Por⁺ and PMI^- Por⁺ → PMI Por charge-transfer processes can be modulated by altering the type of polar solvent (acetonitrile, benzonitrile, tetrahydrofuran and 2,6-lutidine). The charge-separation times for PMI-ep-Zn are 1, 6, 9 and 22 ps in these solvents, while the charge-recombination times are 4, 24, 38 and 34 ps. The efficient, rapid and tunable nature of the charge-transfer processes in polar media makes the PMI-ep-Por dyads useful units for performing molecular-switching functions. These properties when combined with the excellent light-harvesting and energy-transfer capabilities of the same arrays in nonpolar media afford a robust perylene-porphyrin motif that can be tailored for a variety of functions in molecular optoelectronics.     

CHEMISTRY OF SINGLET OXYGEN—XXVI. PHOTOOXYGENATION OF PHENOLSy†

Abstract— The aerobic dye-sensitized photooxygenation of monohydric phenols proceeds by way of singlet oxygen under the conditions studied. Various phenols give different proportions of reaction with and quenching of singlet oxygen. Para-substituted 2,6-di-t-butylphenols show a linear correlation between the log of the total rate of singlet oxygen removal and their halfwave oxidation potentials; the same correlation is given for certain phenol methyl ethers. A Hammett plot using s?⁺ gives ρ - 1.72 ± 0.12, consistent with development of some charge in the quenching step. Reaction of photo-chemically generated singlet oxygen with 2,4,6-triphenylphenol gives 2,4,6-triphenylphenoxy radical as an intermediate in singlet oxygen quenching, although no overall reaction occurs. Kinetic analysis indicates that the radical is derived exclusively from the interaction of 2,4,6-triphenylphenol with singlet oxygen. A charge-transfer mechanism for quenching of singlet oxygen by phenols is proposed.     

Quantification of singlet oxygen production in the reaction of superoxide with hydrogen peroxide using a selective chemiluminescent probe

A sensitive chemiluminescent probe that selectively reacts with singlet oxygen in the presence of superoxide and hydrogen peroxide has been used to quantify the production of singlet oxygen in the reaction of superoxide with hydrogen peroxide. The yield of singlet oxygen from this reaction was found to be low (0.2% relative to the initial superoxide concentration). No evidence for the formation of hydroxyl radical was observed in this reaction, ruling out the Haber-Weiss mechanism as a major singlet oxygen formation pathway. No singlet oxygen production was observed in the reaction of superoxide with 2-nitrobenzoic acid, which has a pKa similar to that of hydrogen peroxide, rendering the protonation of superoxide, followed by its disproportionation, an unlikely explanation for the formation of singlet oxygen in this system. The low yields of singlet oxygen and hydroxyl radical suggest that their formation in this reaction should be relatively unimportant in biological systems.     

The oxygen reduction reaction at silver electrodes in high chloride media and the implications for silver nanoparticle toxicity

The oxygen reduction reaction (ORR) at neutral pH in various aqueous chloride-containing solutions was investigated voltammetrically. In particular, the ORR was performed in high chloride containing aqueous media including authentic and synthetic seawater under oxygen saturated conditions and compared with that in aqueous nitrate and perchlorate media. The experimental voltammograms revealed a two-electron process forming hydrogen peroxide in low chloride media. In contrast, high concentration chloride solutions, including both synthetic and authentic seawater showed an increase of overpotential, accompanied by a splitting of the voltammetric peak into two one-electron features indicating the formation of superoxide in the first step and its release from the silver-solution interface. The implications for silver nanoparticle toxicology are discussed given the markedly greater toxicity of superoxide over peroxide and the high levels of chloride in biological media as well as in seawater.

Superoxide produced at silver electrode in seawater.     

CHEMISTRY OF SINGLET OXYGEN—XXVIII*. STERIC AND ELECTRONIC EFFECTS ON THE REACTIVITY OF SULFIDES WITH SINGLET OXYGEN

Abstract—The rates of reaction of singlet oxygen with a number of alkyl and aryl sulfides have been determined in CH₃OH. The rate for diethylsulfide is 1.71 ± 0.06 × 10⁷M^-1 s^-1. The addition of methyl groups α to the sulfur causes an approximate tenfold decrease in rate for each symmetrical pair. A similar effect is caused by replacement of alkyl by phenyl groups. The rates of substituted thioanisoles correlate well with σ(ρ= -1.6). but poorly with E_P/2 (half-peak oxidation potentials). A mechanism involving nucleophilic reaction of the sulfide with oxygen, rather than charge transfer is suggested.     

Dediazoniation of arenediazonium ions in homogeneous solution. Part VIII. Reaction kinetics and products in dimethyl sulfoxide

p-Nitrobenzenediazonium tetrafluoroborate dissolved in dimethylsulfoxide (DMSO) at 50° forms p-nitrophenol in 88–90% yield. The phenolic oxygen atom originates exclusively from the oxygen atom of DMSO as demonstrated by the use of ¹⁸O-labelled DMSO. The first-order rate of dediazoniation is the same under N₂ as it is in the presence of air. The rate is little influenced by the addition of benzene or iodobenzene. However, the products formed in the presence of these additives are significantly different. UV. spectra and the reactivity of diazonium salt solutions in DMSO when mixed with reagents in aqueous solution demonstrate that a relatively stable charge-transfer complex is formed between the diazonium ion and DMSO. The product analyses and the kinetic and spectral results of dediazoniation in DMSO with and without additives are consistent with a mechanism in which the rate-limiting step is the formation of a p-nitrophenyl radical from the charge-transfer complex. p-Nitrophenol and the products with benzene and iodobenzene are formed in subsequent fast competition steps. In the presence of small amounts of pyridine the dediazoniation is much faster and follows a different kinetic law. Pyridine effectively competes with DMSO in the reaction with diazonium ions.     

Synthesis of endoperoxides derived from cyclooctatetraenes via singlet oxygenation

Cyclooctatetraene (1) reacted with photo-generated singlet oxygen to give the endoperoxide 7,8-dioxabicyclo[4.2.2]deca-2,4,9-triene (1a), which was further transformed to the cis-diepoxide 1b by catalytic rearrangement with Co-TTP to the unsaturated cis-diol 1c and the saturated cis-diol 1d by catalytic hydrogenation, to the saturated endoperoxide 1e by reaction with diimide, and to the epoxycyclooclatetraene If by deoxygenation with dimethylphosphine. Similarly, the methoxy-, phenyl- and methyl-substituted cyclooctatetraenes 3-5, respectively, gave the corresponding endoperoxides with the substituents located at the 1-position (3a, 5a), the 2-position (5b) and the 9-position (3b, 4a). Their structures were determined on the basis of their ¹H- and ¹³C-NMR data and by means of chemical transformation to the corresponding syn-diepoxides, i.e. 5,10-dioxatricyclo[7.1.0.0^4,6]deca-2,7-dienes. The formation of the endoperoxides is postulated to involve an electron transfer mechanism to give the radical cation of cyclooctatetraene and the superoxide ion. The latter couples into the homotropylium-type zwitterionic intermediate and subsequent cyclization leads to the endoperoxides.     

Photoinduced Electron‐Transfer Oxidation of Olefins with Molecular Oxygen Sensitized by Tetrasubstituted Dimethoxybenzenes: A Non‐Singlet‐Oxygen Mechanism

α‐Methylstyrene ( 1 ) was photo‐oxidized in the presence of a series of alkylated dimethoxybenzenes as sensitizers in an oxygen‐saturated MeCN solution to afford the cleaved ketone 2 , epoxide 3 , as well as a small amount of the ene product 4 in ca. 1 : 1 : 0.04 ratio. The relative rate of conversion was well‐correlated with the fluorescence quantum yield of sensitizers. Thus, a non‐singlet‐oxygen mechanism is proposed, in which an excited sensitizer is quenched by (ground‐state) molecular oxygen to produce a sensitizer radical cation and a superoxide ion (O), the former of which oxidizes the substrate, while the latter reacts with the resulting olefin radical cation ( 1 ^{+ .}) to give the major oxidation products. Photodurability of such electron‐donating sensitizers is dramatically improved by substituting four aromatic H‐atoms in 1,4‐dimethoxybenzene with Me or fused alkyl groups, which provides us with an environmentally friendly, clean method of photochemical functionalization with molecular oxygen, alternative to the ene reaction via singlet oxygenation.     

Singlet oxygen production in the reaction of superoxide with organic peroxides

[reaction: see text] A selective chemiluminescent probe for singlet oxygen has been employed to detect and quantify singlet oxygen in the reactions of superoxide with organic peroxides. The production of singlet oxygen has been quantified in the reaction of superoxide with benzoyl peroxide (BP). No singlet oxygen was detected in the reactions of superoxide with cumyl peroxide, tert-butyl peroxide, or tert-butyl hydroperoxide. On the basis of these results and on the temperature dependence of the reaction, we proposed a mechanism for singlet oxygen formation in the reaction of superoxide with BP.     

Possible singlet oxygen generation from the photolysis of indigo dyes in methanol, DMSO, water, and ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate

We suggest that singlet molecular oxygen [¹O₂ (¹Δ_g)] is formed upon irradiation of indigo 1 [in air or O₂-saturated DMSO and DMSO (0.5% H₂SO₄)] and indigo carmine 2 [in air or O₂-saturated CH₃OH, D₂O, and 1-butyl-3-methylimidazolium tetrafluoroborate (BmIm-BF₄)]. The quantum yield for production of ¹O₂ is estimated to be 0.6 for 1 and 0.3-0.5 for 2. The rates of reaction of ¹O₂ with 1 and 2 were determined by monitoring the emission of ¹O₂ at 1270 nm over time. Low molar absorptivities (at 532 nm) and rapid physical quenching caused by 1 and 2 limit their utility as ¹O₂ photosensitizers in solution. Compounds 1 and 2 degrade slowly during the photolysis due to a self-sensitized (type I or II) photooxidation reaction. Oxidative cleavage of 1 by singlet oxygen and superoxide, and 2 by superoxide has been noted before (Kuramoto, N.; Kitao, T. J. Soc. Dyers Color. 1979, 95, 257-261; Kettle, A. J.; Clark, B. M.; Winterbourn, C. C. J. Biol. Chem. 2004, 279, 18521-18525).     

Synthesis,structure and biological properties of several binary and ternary complexes of copper(II) with ciprofloxacin and 1,10 phenanthroline

In this study, a new binary complex [Cu(HCip)₂](NO₃)₂ · 6H₂O (1) has been synthesized and then characterized by X-ray structure analyses. In this compound, each ciprofloxacin acts as a bidentate ligand resulting in a crystallographically planar configuration; the nitrate anions are located in apical positions with an axial distance significantly larger than the equatorial distances, which would be consistent with a very weak metal ion interaction due to the Jahn–Teller effect. In addition, both the synthesis and characterization of two new ternary complexes of ciprofloxacin–copper(II)–1,10-phenanthroline, [Cu(phen)(Cip)](NO₃) · 4H₂O (2) and Cu(phen)(HCip)(NO₃)₂ · H₂O (3), have been accomplished. We have also undertaken the single crystal structural determination of 2, in which the ciprofloxacin acts as tridentate bridging ligand; the complex exhibits a five-coordinated motif in a distorted square pyramidal environment around the metal center. The chemical nuclease activity of compounds 2 and 3 has also been studied, revealing that both compounds behave as efficient chemical nucleases in the presence of ascorbate. Mechanistic studies, with various radical oxygen scavengers, indicate that the DNA cleavage reaction is mediated by hydroxyl radicals, singlet oxygen, and the superoxide anion.     

Transient ion pairs from OH-radical reactions: Pulse radiolysis of aqueous alkyl iodide solutions

Pulse radiolysis studies of aqueous alkyl iodide solutions reveal the existence of the transient ion pair (HOI^?.R⁺)_aq. This species is formed by OH-radical reaction with the alkyl iodide followed by a rapid solvent-assisted rearrangement from the outer to the inner charge-transfer complex.     

A novel sensitized chemiluminescence: Infrared emissions from thiazine dyes sensitized by singlet oxygen

An unusual infrared chemiluminescence emission (8130Å) of methylene blue, and other thiazine dyes, sensitized by singlet molecular oxygen is reported. This chemiluminescence does not correspond to the ordinary fluorescence of the dye and cannot be explained by previously proposed mechanisms for singlet oxygen sensitized emissions of dyes. From energetic considerations singlet molecular oxygen in its ¹Σ_g⁺ state is postulated as the sensitizing agent for the thiazine dye chemiluminescences. Schemes in which ¹Σ_g⁺ oxygen transfers electronic excitation energy (a) to the lowest triplet state of the dye, (b) to a combined multiplicity state of the lowest triplet state of the dye, and triplet molecular oxygen, or (c) to a charge-transfer state between the dye and oxygen, are compared. The chemiluminescence of methylene blue in aqueous solution may be used as a luminescent probe for ¹Σ_g⁺ oxygen.