Interaction Of Molecular Oxygen Research Articles

Porphyrin-layered double hydroxide/polymer composites as novel ecological photoactive surfaces

Nanocontainer and nanofiller aspects of layered double hydroxides (LDH) were combined to prepare porphyrin-LDH/polymer composites for photoactive coatings. The suggested properties are derived from cytotoxicity of singlet oxygen, O2(1Δg), produced by interaction of molecular oxygen with excited porphyrin molecules located within the interlayer space of ZnRAl and MgRAl LDH. Porphyrins, Pd(II)-5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin (PdTPPC) and 5,10,15,20-tetrakis(4-sulfonatophenyl) porphyrin (TPPS) photoproducing O2(1Δg), were successfully intercalated into LDH hosts using the co-precipitation procedure and then used as fillers in two eco-friendly polymers, namely polyurethane (PU) and poly(butylene succinate) (PBS). Porphyrin-LDH/polymer composites were prepared either by the solvent cast/cross-linking technique or melt-compounding with different porphyrin-LDH filler loadings (up to 1.3 wt%). Both X-ray diffraction and transmission electron microscopy measurements indicate a good dispersion of porphyrin-LDH fillers into the polymer matrices and that the porphyrin molecules remain intercalated within LDH layers. The polymers do not block the diffusion of oxygen and the triplet states of the intercalated porphyrins in the composite films have enough long lifetimes to produce O2(1Δg) upon irradiation by visible light. The present composites allow the elaboration of photoactive surfaces with a precise control of the O2(1Δg) concentration depending on porphyrin-LDH filler loadings.

Interaction of molecular oxygen with oxygen vacancies on reduced TiO 2: Site specific blocking by probe molecules

A heterogeneity of surface sites for superoxide ( O 2 - ) stabilization on thermally reduced TiO 2 (P25) have been identified by cw EPR. Two main groups of adsorption sites exist; site I due to O 2 - adsorbed at oxygen vacancies, [ Vac ⋯ O 2 - ] , and sites II–III due to O 2 - adsorbed at five co-ordinate Ti 4+ centres. Electron transfer from the precursor Ti 3+ centres to O 2 (forming O 2 - ) can be prevented at each site through selective site blocking with probe molecules (CO 2 and Ar). CO 2 preferentially adsorbs at the five co-ordinate Ti non-vacancy 3 + centres, while Ar preferentially adsorbs at the Ti 3+ centres associated with the oxygen vacancies ( Ti vacancy 3 + ) .

Resonance enhancement of photoluminescence from silicon nanocrystallites

AbstractAn interaction of molecular oxygen and p‐Si nanocrystallites in HF–ethanolic solution with simultaneous illumination has been investigated. It is shown that after such treatment with white‐light exposure the photoluminescence from nanocrystallites enhances substantially caused by photogenerated singlet forms of molecular oxygen. In the case of monochromatic illumination with photon energy in the range 2.5–0.95 eV dependence of photoluminescence intensity after the treatment on photon energy used shows resonant peaks corresponding to the absorbance peaks of excited molecular oxygen. To our knowledge such photoluminescence peaks in porous silicon spectroscopy have been observed for the first time. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Singlet oxygen production in photosystem II and related protection mechanism

High-light illumination of photosynthetic organisms stimulates the production of singlet oxygen by photosystem II (PSII) and causes photo-oxidative stress. In the PSII reaction centre, singlet oxygen is generated by the interaction of molecular oxygen with the excited triplet state of chlorophyll (Chl). The triplet Chl is formed via charge recombination of the light-induced charge pair. Changes in the midpoint potential of the primary electron donor P(680) of the primary acceptor pheophytin or of the quinone acceptor Q(A), modulate the pathway of charge recombination in PSII and influence the yield of singlet oxygen formation. The involvement of singlet oxygen in the process of photoinhibition is discussed. Singlet oxygen is efficiently quenched by beta-carotene, tocopherol or plastoquinone. If not quenched, it can trigger the up-regulation of genes, which are involved in the molecular defence response of photosynthetic organisms against photo-oxidative stress.

“Inversion substitution” reactions with participation of molecular oxygen: ZH3I + O2 → O2ZH3 + I (Z = C, Si). The potential energy surface and rate constants

A new class of reactions of molecular oxygen O2 + ZH3I → O2ZH3 + I (Z = C, Si) proceeding by the mechanism of “inversion substitution” was investigated by quantum chemistry methods and the transition state theory (TST). The profiles of the potential energy surfaces (PES) along the reaction coordinate and the characteristics of transition states were calculated using the DFT approach with the B3LYP hybrid functional and the DZVP basis set. The characteristics of the transition states were then used for TST calculations of the rate constants for the direct and reverse “inversion substitution” reactions and their temperature dependences in the temperature interval 273–2000 K. The activation barriers to the substitution reactions under study were found to be substantially lower than the barriers to the abstraction reactions O2 + ZH3I → ZH2I + HO2 (by 16.3 kcal mol−1 for Z = C and by 7.2 kcal mol−1 for Z = Si). The results obtained show that the “inversion substitution” reactions dominate over the abstraction reactions in the interaction of molecular oxygen with carbon- and silicon-centered iodides as well as (probably) many other substrates.

Formation of phenols from the low-temperature fast pyrolysis of radiata pine ( Pinus radiata) : Part II. Interaction of molecular oxygen and substrate water

In Part I, the influence of molecular oxygen on phenolic compound formation from the low-temperature fast pyrolysis of Radiata pine ( Pinus radiata) was reported. This work is elaborated upon and includes the interaction, or dual effect, of molecular oxygen and substrate water on phenolic compound formation. Experimentation was performed on a bench scale fluidised bed system and gas chromatography/mass spectroscopy was used for product analysis. Oven-dried and ‘as received’ (9.7% moisture content) Radiata pine (150–250 μm) was processed at 290–295 °C using air, nitrogen and oxygen enriched nitrogen as the fluidising gases. The inclusion of both molecular oxygen, in the form of air, and substrate moisture in the reaction system resulted in an increase in the yield of monomeric phenols from 0.07 to 5.03% (m/m). However, the inclusion of either molecular oxygen or substrate moisture resulted in a phenolic compound yield of only 0.60 and 0.27% (m/m), respectively. It was proposed that the molecular oxygen promoted the homolytic, free radical induced, aryl ether cleavage through facilitation of free radical formation as well as through hydroperoxide formation on the aliphatic carbon associated with aryl ether linkages. This explanation was consistent with the significant increase in carbonyl content of the phenolic product when molecular oxygen was included in the reaction atmosphere. However, such carbonyl groups are quite reactive and may participate in cross-linking reactions at elevated temperatures. The positive influence of water on the yield of monomeric phenolic compounds was believed to be indirect. That is, under fast pyrolysis conditions, in which the residence time of both substrate and volatile decomposition products is short, the evaporation of water would have suppressed substrate temperature elevation, thereby suppressing the rate of cross-linking reactions, resulting in the increased preservation of carbonyl containing low molecular weight products.

The aqueous reference for ESR oximetry

The interaction of molecular oxygen with derivatives of nitroxide EPR spin labels has been investigated using nuclear spin-relaxation spectroscopy in aqueous and nonaqueous solvents. The proton spin–lattice relaxation rate induced by oxygen provides a measure of the local concentration of oxygen, which we find is dependent on solvent. In water, the hydrophobic effect increases the local concentration of oxygen in the nonpolar portions of solute molecules. For nitroxides reduced to the hydroxylamine in aqueous solutions, we find that the local concentration of oxygen is approximately twice that associated with a free diffusion hard sphere limit, while in octane, this effect is absent. These results show that nitroxide based ESR oximetry may suffer a reference concentration shift of order a factor of two if the aqueous nitroxide spectrum or relaxation is used as the reference.

Hydrogen peroxide formation in the interaction of oxygen with boron-containing Pd catalysts prereduced by hydrazine in aqueous acidic medium containing bromide anions

Interaction of molecular oxygen with Pd/BPO4 or ZrO2 (or Al2O3, CeO2, TiO2) - B2O3 catalysts, prereduced by hydrazine hydrate, in an aqueous acidic (H2SO4 or H3PO4) reaction medium containing bromide ions leads to the formation of H2O2. However, in the absence of boron in the catalyst and also in the absence of acid and/or bromide ions in the reaction medium, almost no H2O2 is formed.

The interaction of molecular oxygen with active sites of graphite: a theoretical study

The adsorption of molecular oxygen at defective edge sites of zigzag and armchair graphite surfaces has been investigated by adopting cluster models in conjunction with density functional theory. Several different types of chemisorbed O 2 species are identified. It was found that the defect edge sites exhibit the significant catalytic role toward the adsorption and activation of molecular oxygen. The O 2 molecule is not only able to strongly bind to these edge sites, but the O–O bond strength is obviously weakened. Moreover, the calculated adsorption energy for O 2 adsorbed on the clean graphite basal surface is fairly consistent with the weak interaction nature of O 2 with the surface observed in the experiment, indicating one-layer cluster model is an effective way to study O 2 adsorption on graphite surface in terms of accuracy and computational cost, which is in agreement with previous experience. Whereas, we note that the local detailed arrangement of edge carbon atoms can play an important effect on the adsorption of O 2 on defect surfaces.

Architecture of the Qo site of the cytochrome bc1 complex probed by superoxide production.

Although several X-ray structures have been determined for the mitochondrial cytochrome (cyt) bc(1) complex, none yet shows the position of the substrate, ubiquinol, in the quinol oxidase (Q(o)) site. In this study, the interaction of molecular oxygen with the reactive intermediate Q(o) semiquinone is used to probe the Q(o) site. It has been known for some time that partial turnover of the cyt bc(1) complex in the presence of antimycin A, a Q(i) site inhibitor, results in accumulation of a semiquinone at the Q(o) site, which can reduce O(2) to superoxide (O(2)(*)(-)). It was more recently shown that myxothiazol, which binds close to the cyt b(L) heme in the proximal Q(o) niche, also induces O(2)(*)(-) production. In this work, it is shown that, in addition to myxothiazol, a number of other proximal Q(o) inhibitors [including (E)-beta-methoxyacrylate-stilbene, mucidin, and famoxadone] also induce O(2)(*)(-) production in the isolated yeast cyt bc(1) complex, at approximately 50% of the V(max) observed in the presence of antimycin A. It is proposed that proximal Q(o) site inhibitors induce O(2)(*)(-) production because they allow formation, but not oxidation, of the semiquinone at the distal niche of the Q(o) site pocket. The apparent K(m) for ubiquinol at the Q(o) site in the presence of proximal Q(o) site inhibitors suggests that the "distal niche" of the Q(o) pocket can act as a fully independent quinol binding and oxidation site. Together with the X-ray structures, these results suggest substrate ubiquinol binds in a fashion similar to that of stigmatellin with H-bonds between H161 of the Rieske iron-sulfur protein and E272 of the cyt b protein. When modeled in this way, mucidin and ubiquinol can bind simultaneously to the Q(o) site with virtually no steric hindrance, whereas progressively bulkier inhibitors exhibit increasing overlap. The fact that partial turnover of the Q(o) site is possible even with bound proximal Q(o) site inhibitors is consistent with the participation of two separate functional Q(o) binding niches, occupied simultaneously or sequentially.

Interaction of molecular oxygen with single-wall carbon nanotube bundles and graphite

The adsorption of oxygen on highly oriented pyrolytic graphite (HOPG) and bundles of single-wall carbon nanotubes (SWNTs) at 28 K is studied using thermal desorption spectroscopy and by a measurement of sticking probabilities. The low-coverage binding energy of oxygen adsorbed on SWNT bundles, 18.5 kJ/mol, is 55% higher than the low-coverage binding energy on HOPG, 12.0 kJ/mol. Molecular mechanics calculations reveal that such an increase can be attributed to the higher effective coordination of binding sites on SWNT bundles. The character of the oxygen-SWNT interaction should therefore be van der Waals type which suggests that the observed oxygen species is physisorbed and does not facilitate bulk doping of SWNT samples.

Interaction of molecular oxygen with single wall nanotubes: Role of surfactant contamination

The interaction of molecular oxygen with single wall nanotubes in the form of a commercial bucky paper was investigated by high resolution photoemission spectroscopy. Sodium contamination was found in the sample, which was completely removed only after prolonged heating at 1250 K. The C 1s core level spectrum measured on the sample annealed to 1020 K dramatically changed upon exposure to molecular oxygen. On the contrary, when exposing the Na-free SWNTs to several KL of O 2, the sample remained oxygen free and no modification in the C 1s core level was observed. Therefore the observed sensitivity of the sample to O 2 was due to a Na mediated oxidation, determining a charge transfer from the C tubes to the Na–O complex.

S 1 ∼>T 1 intersystem crossing in π-conjugated organic polymers

Quantum yields for triplet formation have been determined for seven common π-conjugated polymers in benzene solution using time-resolved photoacoustic calorimetry (PAC) in conjunction with fluorescence quantum yields, singlet and triplet energies. The polymers studied include three poly(thiophenes), poly(2-methoxy,5-(2′-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV), a cyano derivative of MEH-PPV, a ladder type poly(p-phenylene) (MeLPPP), and a poly(fluorene). Yields of singlet oxygen formation have also been determined for these polymers in benzene by time-resolved phosphorimetry, and are in reasonable agreement with triplet yields obtained by PAC. Polythiophenes show the highest intersystem crossing yields, which are suggested to result from extensive spin-orbit coupling. Where singlet oxygen yields are less than triplet yields, it is suggested that interaction of molecular oxygen with the ground state of the polymers may be involved.

Feasibility of the solar-pumped fullerene-oxygen-iodine laser (Sun-light FOIL)

The feasibility of the optically pumped (in particular, solar-pumped) fullerene-oxygen-iodine laser (FOIL) is considered. The singlet oxygen is assumed to be formed through the interaction of molecular oxygen with a mixture of (both lower and higher) fullerenes in a photogenerated triplet metastable state. The estimates based on a photokinetic model show that the solar-pumped FOIL efficiency can reach several tens of percent. The singlet-oxygen yield resulting from the interaction of optical pumping radiation with fullerenes in solutions is studied both theoretically and experimentally. As the optical pumping, we used laser radiation at 532 nm and broad-band emission of a lamp with a solarlike spectrum.

Chemisorption of molecular oxygen on Cu(1 0 0): a Hartree–Fock and density functional study

The interaction of molecular oxygen with the Cu(1 0 0) surface has been studied by using both Hartree–Fock and density functional methods in the framework of the cluster model approach. In this study, we have used the Cu 8(6,2) cluster in order to simulate the O 2 molecular adsorption on different high symmetry chemisorption sites (top–top, bridge–fourfold, bridge–top, fourfold–fourfold) on the Cu(1 0 0) surface. High level non-local density functional (NLSD) computations indicate that the more stable chemisorption site is the bridge–bridge followed by the top–top, bridge–top and bridge–fourfold ones. The calculated 1s O XPS shifts are in good agreement with the experimental indications.

Molecular oxygen interaction with Bi2O3: a spectroscopic and spectromagnetic investigation

The interaction of molecular oxygen with polycrystalline Bi2O3 was investigated by infrared (IR) and thermogravimetric (TGA) analyses, X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). The results indicate the formation of “end-on” and “bridging” superoxide species, which are trapped in the β-Bi2O3 vacant oxygen sites, Bi(IV) and Bi(V) centres also being produced. O2− interacts with Bi(IV) centres giving two superoxide adducts with temperature-dependent relative amounts, Bi(IV)–O2− and Bi(IV)–O2−–Bi(III), both having a triplet spin state.

Interaction of Oxygen with Poly(paraphenylene vinylene): A Theoretical Study

The electronic properties of conjugated polymers can be modified upon oxygen exposure. Such modifications influence the operation of conjugated polymer-based electrooptic devices, such as light-emitting diodes and photodiodes. In this contribution, the interaction of molecular oxygen with poly(paraphenylene vinylene) is investigated theoretically on the basis of quantum-mechanical calculations carried out on molecular model systems.

Reactive intermediates formed upon electron transfer from the surface of oxide catalysts to adsorbed molecules

The basic steps of catalytic oxidation processes at the surface of metal oxides consist in redox interactions involving the solid and one or more reactants (in particular molecular oxygen). The general redox properties of a metal oxide can be investigated by means of the electron paramagnetic resonance (EPR) technique which is capable of detecting the paramagnetic centers often formed upon one electron redox processes. The principles of the EPR technique are briefly described in the first part of this paper. In the second part routine methods to measure the electron donor and the electron acceptor ability of a surface (using particular probe molecules) are described. Finally the paper deals with the interactions of molecular oxygen with the surfaces of oxides leading to electron transfer towards the adsorbed molecule. Various examples of electron transfer are discussed in relation to the nature and activation conditions of the solid. In conclusion the particular case of surface intermolecular electron transfer (SIET) is briefly illustrated.

Intrinsic Point Defects on a TiO2(110) Surface and Their Reaction with Oxygen: A Scanning Tunneling Microscopy Study

ABSTRACTThe interaction of molecular oxygen, at room temperature, with a reduced TiO2(110) surface has been studied in situ by scanning tunneling microscopy (STM). Oxygen vacancies (point defects) were created on a clean TiO2(110) surface by annealing in ultra-high vacuum and successfully imaged on the atomic scale. These point defect sites were stable under ultrahigh vacuum conditions. During exposure to molecular oxygen, new point defects appear at different locations on the surface although their overall number is reduced. A mechanism for this dynamic healing process is proposed.

Photoinhibition of Photosystem II from Higher Plants: EFFECT OF COPPER INHIBITION

Strong illumination of Cu(II)-inhibited photosystem II membranes resulted in a faster loss of oxygen evolution activity compared with that of the intact samples. The phenomenon was oxygen- and temperature-dependent. However, D1 protein degradation rate was similar in both preparations and slower than that found in non-oxygen evolving PSII particles (i.e. Mn-depleted photosystem II). These results seem to indicate that during illumination Cu(II)-inhibited samples do not behave as a typical non-oxygen evolving photosystem II. Cytochrome b559 was functional in the presence of Cu(II). The effect of Cu(II) inhibition decreased the amount of photoreduced cytochrome b559 and slowed down the rate of its photoreduction. The presence of Cu(II) during illumination seems to protect P680 against photodamage as occurs in photosystem II reaction centers when the acceptor side is protected. The data were consistent with the finding that production of singlet oxygen was highly reduced in the preparations treated with Cu(II). EPR spin trapping experiments showed that inactivation of Cu(II)-treated samples was dominated by hydroxyl radical, and the loss of oxygen evolution activity was diminished by the presence of superoxide dismutase and catalase. These results indicate that the rapid loss of oxygen evolution activity in the presence of Cu(II) is mainly due to the formation of .OH radicals from superoxide ion via a Cu(II)-catalyzed Haber-Weiss mechanism. Considering that this inactivation process was oxygen-dependent, we propose that the formation of superoxide occurs in the acceptor side of photosystem II by interaction of molecular oxygen with reduced electron acceptor species, and thus, the primarily Cu(II)-inhibitory site in photosystem II is on the acceptor side.