Singlet molecular oxygen quantum yield measurements of some porphyrins and metalloporphyrins

Hematoporphyrin IX dimethyl ester (HPDME), tetraphenylporphyrin (TPP), tetra(4-methoxyphenyl)porphyrin (TMPP), tetra(3,4-dimethoxyphenyl)porphyrin (TDMPP), tetra(3,4,5-trimethoxyphenyl)porphyrin (TTMPP), tetraanthrylporphyrin (TAP) and tetraacridylporphyrin (TACP), and their Zn²⁺, SnX ₂ ²⁺ , Pd²⁺ and Pt²⁺ complexes have been prepared and characterized. The singlet molecular oxygen quantum yield (ϕ_Δ) values of the above porphyrins and their metal derivatives in N,N-dimethyl formamide (DMF) have been measured in the presence of 1,3-diphenylisobenzofuran (DPBF) as¹O₂ acceptor using steady state technique after correcting for the intensity of light absorbed by the photosensitizers. The ϕ_Δ values for the free base porphyrins are usually around 0.60. Hematoporphyrin IX dimethyl ester and its metal derivatives follow the order: HPDME (ϕ_Δ = 0.60) > Zn HPDME (0.40) > PdHPDME (0.34) > Sn(OH)₂ HPDME (0.28) > PtHPDME (0.24). Zinc(II) complexes of tetraarylporphyrins show about 65% efficiency in ϕ_Δ values are compared to the ϕ_Δ values of their corresponding free base porphyrins. These results can be explained on the basis of catalyzed intersystem crossing to the ground state. The ϕ_λ values of the above free base porphyrins and their metal complexes in DMF in presence of DPBF using single-pulsed laser excitation technique follow similar trends.     

Singlet oxygen in the heterogeneous catalytic oxidation of benzene

The partial oxidation of benzene on vanadium and molybdenum oxides and the participation of singlet oxygen in this reaction were studied. A correlation between the activity of the catalysts in the partial oxidation of benzene and their ability to generate singlet oxygen was observed. The character of the initiation of oxidative benzene transformations under the action of molecular singlet oxygen was analyzed.     

Singlet oxygen oxidation of 2′-deoxyguanosine. Formation and mechanistic insights

Emphasis was placed in this work on the delineation of mechanistic aspects of the singlet oxygen-mediated oxidation reactions of 2′-deoxyguanosine 1 used as a DNA model compound in aerated aqueous solution. For this purpose a thermolabile naphthalene endoperoxide derivative was used allowing the generation of [¹⁸O]-labeled singlet oxygen for dedicated mechanistic studies. The analysis and characterization of the oxidized nucleosides of the ¹O₂ reactions were achieved on the basis of accurate HPLC-tandem mass spectrometry measurements. Thus it was found that primary oxidation products include, in addition to the previously identified 8-oxo-7,8-dihydro-2′-deoxyguanosine 5 and the two diastereomers of spiroiminodihydantoin 8, two relatively minor nucleosides, namely the two diastereomers of 4-hydroxy-8-oxo-4,8-dihydro-2′-deoxyguanosine 9.     

Singlet oxygen oxidation of isolated and cellular DNA: product formation and mechanistic insights

This survey focuses on recent aspects of the singlet oxygen oxidation of the guanine moiety of nucleosides, oligonucleotides, isolated and cellular DNA that has been shown to be the exclusive DNA target for this biologically relevant photogenerated oxidant. A large body of mechanistic data is now available from studies performed on nucleosides in both aprotic solvents and aqueous solutions. A common process to both reaction conditions is the formation of 8-oxo-7,8-dihydroguanine by reduction of 8-hydroperoxyguanine that arises from the rearrangement of initially formed endoperoxide across the 4,8-bond of the purine moiety. However, in organic solvent the hydroperoxide is converted as a major degradation pathway into a dioxirane that subsequently decomposes into a complex pattern of oxidation products. A different reaction that involved the formation of a highly reactive quinonoid intermediate consecutively to the loss of a water molecule from the 8-hydroperoxide has been shown to occur in aqueous solution. Subsequent addition of a water molecule at C5 leads to the generation of a spiroiminodihy-dantoin compound via a rearrangement that involves an acyl shift. However, in both isolated and cellular DNA the latter decomposition pathway is at the best a minor process, because only 8-oxo-7,8-dihydroguanine has been found to be generated. It is interesting to point out that singlet oxygen has been shown to contribute predominantly to the formation of 8-oxo-7,8-dihydroguanine in the DNA of bacterial and human cells upon exposure to UVA radiation. It may be added that the formation of secondary singlet-oxygen oxidation products of 8-oxo-7,8-dihydroguanine, including spiroiminodihydantoin and oxaluric acid that were characterized in nucleosides and oligonucleotide, respectively, have not yet been found in cellular DNA.     

Molecular recognition at the gas-solid interface: a powerful tool for chemical sensing

This tutorial review deals with the design of molecular receptors capable of molecular recognition at the gas-solid interface, to be used as selective layers in gas sensors. The key issue of specific versus nonspecific binding in the solid layer is discussed in terms of cavity inclusion and layer morphology. The combined use of mass spectrometry and crystal structure analysis provide accurate information on type, number, geometry and strength of receptor-analyte interactions in the gas phase and in the solid state. From these data, the gas sensing properties of a given receptor toward a single class of analytes can be anticipated.     

Singlet oxygen in the environmental sciences

This review of the part played by the singlet states of molecular oxygen in the environment deals with atmospheric aspects. There are five bound excited states of molecular oxygen that correlate with two ground state, ³P, oxygen atoms. Of these, three are singlets, although the other two states (triplets) are closely associated with singlet oxygen processes, especially in the mesosphere. A weakly bound quintet state has been invoked, as well, in explaining some aspects of the physical chemistry of the singlet species. Of the three singlet states, the a¹Δ_g is the most familiar. It has a low excitation energy, a long radiative lifetime, and is rather resistant to collisional deactivation in the gas phase. As a consequence, its chemistry has been susceptible to detailed study in the laboratory. These investigations, coupled with estimates of production rates, suggest that O₂(a¹Δ_g) is probably not important in initiating much chemical change in the lower atmosphere, at least in the gas phase; excited molecules dissolved in water droplets may promote chemical change under special circumstances. In the stratosphere and mesosphere, each of the bound excited states gives rise to characteristic emission features of the airglow, both by day and by night. The observational data, obtained from the ground, and from balloons, high-flying aircraft, rockets and satellites is surveyed as a background to examining the chemical and photochemical mechanisms by which the different states become excited. These mechanisms clearly differ by day and by night, and they also depend on the altitude from which the emission comes. The most intense feature of the oxygen dayglow, the Infrared Atmospheric Band, comes from O₂(a¹Δ_g) that is produced in the photolysis of ozone. Because dayglow measurements are sometimes used to derive ozone concentrations and altitude profiles in the atmosphere, the efficiency of production of the species in the photolysis of ozone is examined critically, and some unexpected laboratory findings are reported. The b¹Σ⁺ _g state of oxygen is excited during the day largely by resonance scattering, although some is also populated by energy transfer from O(¹D) to O₂. At night, recombination of O(³P) atoms is the most likely source of excitation of all the states of oxygen. Laboratory experiments that bear on these processes are reviewed, and theoretical estimates of the partitioning of recombination events between the different states are presented. Direct recombination into the a¹Δ_g and b¹Σ⁺ _g states is unlikely to be efficient enough to produce the observed concentrations of these species, and some indirect process is thus implicated. Laser excitation experiments show that quenching of the three higher excited (ungerade) states of oxygen by O₂ and, especially, N₂, can generate O₂(b¹Σ⁺ _g) with high efficiency; similar experiments demonstrate explicitly that the quenching of O₂(b¹Σ⁺ _g) by the atmospheric gases yields O₂(a¹Δ_g). A consistent excitation scheme for the nightglow emissions is presented; this scheme also pays attention to the “auroral green” line produced by the ¹S state of atomic oxygen, the intensities of which in the atmospheres of Earth and Venus provide some clues about the excitation of the molecular states. Finally, the laboratory studies are shown to indicate that the formation of excited molecular oxygen from vibrationally rich hydroxyl (OH) radicals is unlikely to be of major importance in the atmosphere.     

Tuning the singlet oxygen quantum yield of near-IR-absorbing porphyrazines

We report the quantum yields for singlet oxygen production by a series of porphyrazines (pz) of the form M[pz(An;B4-n)] (Scheme 1), where the peripheral substituent A is [S-R]2 with R = (CH2CH2O)3H, B is a fused alpha,alpha'-dialkoxybenzo group and M = 2H, Mg or Zn. These compounds show intense near-IR absorbance/emission (longest wavelength emission, approximately 830 nm). Their solubilities vary with R, whereas their optical properties do not. We show that singlet oxygen sensitization by these luminescent compounds can be "tuned" from essentially off to on by varying n and selection among M = 2H, Mg or Zn. The quantum yields vary ca 60-fold within the set of compounds studied, from phidelta = 0.007 for compound 3 to phidelta = approximately 0.4 for compound 11.     

Singlet oxygen oxidation of enol ethers; the synthesis of optically active 1,2-dioxetanes-II

Several chiral enol ethers, among which two derived from (+)-fenchone have been converted to (chiral) 1,2-dioxetanes by photosensitized oxygenation.     

Incommensurate modulation in the microporous silica SSZ-24

A detailed investigation of the structure of microporous silica, SSZ-24, is presented. It is shown by X-ray powder diffraction and (29)Si MAS NMR experiments that the structure deviates from the previously proposed AlPO(4)-5-type structure. At room temperature, electron diffraction (ED) patterns exhibit extra diffraction spots, which can be attributed to an incommensurate structural modulation along the c axis. This in turn results in a pleat pattern in real space with two different intervals arranged aperiodically along the c axis, as observed with high-resolution electron microscopy (HREM). The modulated structure may easily turn into a disordered one through excessive electron irradiation or heat-treatment. In order to understand the origin of the modulation, soft phonon-modes of the ideal premodulated structure were analyzed by the use of the rigid-unit-mode model. The distribution of soft modes in reciprocal space might correspond roughly to diffuse streaks that could be observed in the diffraction patterns at higher temperatures. It was found that several phonon branches soften at specific wave vectors, which are incommensurate with respect to the original period and might be responsible for the modulation. We present a simple analytic treatment to deduce the wave vectors and associated displacement eigenvectors for the incommensurate soft-modes.     

High-density storage of H2 in microporous crystalline silica at ambient conditions

Molecular hydrogen was encapsulated in the cages of clathrasil decadodecasil 3R (DD3R) during the hydrothermal synthesis of this microporous silicate. The crystalline structure of DD3R facilitates high-density hydrogen storage at ambient conditions. Prompt gamma activation analysis (PGAA) revealed that on average about one molecule of H2 is trapped in each (5(12)) cage of DD3R. The presence of molecular hydrogen inside the DD3R framework was confirmed by solid-state 1H NMR spectroscopy. Temperature-programmed decomposition (TPD) in combination with mass spectrometry showed that the encapsulated hydrogen is released upon decomposition of the clathrasil structure. This release can be promoted by the presence of water.     

Amphiphilic silica nanoparticles at the decane-water interface: insights from atomistic simulations

The properties of 3 nm-diameter silica nanoparticles with different surface chemistry were systematically investigated at the decane-water interface using molecular dynamics simulations. Our results show that the decane-water interfacial tension is not much influenced by the presence of the nanoparticles. The three-phase contact angle increases with nanoparticle surface hydrophobicity. Contact angles observed for the nanoparticles at 300 and at 350 K differ very little. The contact angle of the nanoparticle with randomly dispersed hydrophobic groups is smaller than that observed in Janus nanoparticles of equal overall surface chemistry composition. The energy necessary to desorb Janus nanoparticles from the interface is usually higher than that required to desorb the corresponding homogeneous nanoparticles. Desorption from the interface into the aqueous phase is preferred over that into the organic phase for all except one of the nanoparticles considered. Structural and dynamic properties including nanoparticle rotational relaxation, solvent density profiles, and solvent residence autocorrelation functions near the nanoparticles are also presented. The data are useful for designing Pickering emulsions.     

Modification of silica surfaces by grafting of alkyl chains. II-Characterization of silica surfaces by inverse gas-solid chromatography at finite concentration

Summary Model compounds, resulting from the surface esterification of fumed and precipitated silicas with methanol or hexadecanol, have been characterized by inverse gassolid chromatography at finite concentration. Surface treatments are shown to result in a decrease of adsorbate-adsorbent interactions (hexadecylated silicas are indeed virtually unable to exchange any specific interaction with a solute) and with an improvement of the energetic homogeneity of the materials. Results obtained suggest moreover that the distribution of surface silanol groups on precipitated silicas is not the same as on fumed silicas of identical surface area.     

Polydiacetylene-supported silica films formed at the air/water interface

Mesostructured silica films have attracted interest as potential platforms for sensing, molecular sieving, catalysis, and others. The fabrication of free-standing silica films on water, however, is challenging due to the need for scaffolding agents that would constitute effective templates. We describe the assembly of thin film at the air/water interface comprising quaternary silicates and polydiacetylene (PDA), a unique chromatic polymer forming two-dimensional conjugated networks, and the use of these films for biological sensing. PDA exhibits a dual role in the system-both as the amphiphilic matrix facilitating immobilization of the silicate colloidal units at the air/water interface and additionally a chromatic reporter that undergoes visible blue-red transitions, accompanied by fluorescence transformations, in the presence of analytes. We demonstrate the application of the silicate/PDA thin films for the detection of bacterial proliferation.     

单重态氧化学VII. 迷迭香中主要抗氧化成分对单重态氧的猝灭作用

本文研究了迷迭香酚和鼠尾草酚对单重态氧的猝灭作用。实验证明迷迭香酚和鼠尾草酚在甲醇中猝灭单重态氧的总速率常数K~A分别为2.51×10^7和7.19×10^6mol^1.dm^-3.s^-1, 其中化学猝灭的速率常数k~r分别为1.27×10^7和9.82×10^5mol.dm^-3.s^-1, 相应的半猝灭浓度β值为4.6×10^-3和1.6×10^-2mol.dm^-3。这是一类有实用价值的高效抗氧化剂。     

Adsorption of cetyltrimethylammonium bromide at the silica gel/water interface

The adsorption isotherms of cetyltrimethylammonium ion (CTA+) together with that of the Br counterion on silica gel, and the effects of pH and added salts (NaF, NaCl and NaBr) have been systematically determined at 25°C. Electrophoretic mobilities of the silica gel particles have also been measured in the same conditions. The adsorption isotherm of CTA⁺ consists of four regions. Region I, at low concentrations of surfactant, the adsorption results primarily from electrostatic force between CTA⁺ and the negatively charged silica surface. Region II (first plateau), at medium concentrations, the adsorption is due to both the electrostatic force and the specific attraction (vdW forces) between CTA⁺ and the surface. Region III, characterized by an abrupt increase in the slope of the isotherm when the concentration reaches a particular point known as hemimicelle concentration (HMC). The abrupt increase in the adsorption is due to the hydrophobic interaction between hydrocarbon chains. Region IV (second plateau), at or above CMC, the limiting adsorption is reached as the micelle is not adsorbed. Based on this model, the experimental results can be explained reasonably. The results show that the HMC is about half of the CMC. According to the assumption that each adsorbed CTA⁺ ion in the first plateau is an active center for surface aggregation, the average aggregation number of hemimicelle have been calculated.     

Singlet oxygen formation in monomeric and aggregated porphyrin c

The absorption and fluorescence spectra of monomeric and aggregated species present in aqueous solutions of porphyrin c have been resolved by steady-state and time-resolved spectroscopy. The dependence of the singlet oxygen formation yield (phi delta) on excitation wavelength has also been determined. In the Q-band spectral region, the aggregate absorption and emission spectra are shifted to longer wavelengths with respect to the monomer spectrum with phi delta (monomer) = 0.59 and phi delta (aggregate) = 0.33. The relevance of these findings to the optimization of irradiation conditions in tumour phototherapy using porphyrin c are discussed.     

Adsorption of polymers at the solution-solid interface. VIII. Competitive effects in the adsorption of polystyrenes on silica

The adsorption behavior on silica of some polystrenes of moderate molecular weight distribution, both singly and in mixtures, has been examined. The adsorption isotherms indicate that, in both a good solvent (trichloroethylene) and under theta conditions, the species of higher molecular weight is preferentially adsorbed at or near full surface coverage, but that the smaller adsorbate has an improved opportunity for adsorption at low surface coverage. The use of tritiated adsorbates substantiate the isotherm data in cyclohexane solution.     

Temperature dependence of the initial quantum yield of oxygen photoadsorption on zinc oxide

The initial quantum yield of photoadsorption at =365–577 nm is shown to decrease with the temperature increase. Experimental data are partly accounted for by the temperature shift of the long wavelength edge of the fundamental absorption band that is of low activity in oxygen photoadsorption.

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Nanostructured aniline oxidation products: self-assembled films at the air/liquid interface

With the use of the "falling pH" approach, the free floating films obtained at the air/liquid interface during aniline oxidative polymerization in the presence of hydrochloric acid have been reported and characterized for the first time. The surface of the films is compact and consists of small densely packed particles (around 10-20 nm in diameter). In addition to the free floating films the precipitation of various self-assembled nanostructures was also observed. The thickness of the films depends on the concentration of the reactants and ranges from 40 to 50 nm. Scanning electron microscopy (SEM) and transmission electron microscopy results imply that 3D nanostructures are physically adsorbed onto the film and that they represent two separated phases. Fourier transform infrared (FTIR) and UV-vis measurements have confirmed the presence of polyaniline and branched oligoanilines. On the basis of an orientation distribution of H(2)O and aniline molecules at the air/liquid interface and with the use of the cooperative effect of hydrogen-bond formation, a mechanism for film growth has been proposed. Three samples are synthesized in the presence of different acids-nitric, sulfuric, and acetic. The SEM and FTIR data showed similar morphologies and structural characteristics. This implies that the morphology and structure of the final products depend on the acidity conditions (pH) during the reaction rather than on the nature of the dopant acid.     

Singlet oxygen and polymer photooxidations. I. Sensitizers,quenchers, and reactants

A study designed to ascertain the role of singlet molecular oxygen in the photodegradation of plastics established that most classes of dye chromophores are sensitizers in polymer films, absorbing light and transferring the absorbed energy to ambient triplet ground state molecular oxygen, generating metastable reactive singlet molecular oxygen. Unsaturated polymers containing polybutadiene, polyisoprene, etc. are highly reactive to singlet oxygen produced through photosensitization, generating hydroxylic and carbonyl derivatives and losing their rubbery properties as consequences of such reactions. Many types of transition metal chelates are singlet oxygen quenchers. The relationships of the structures and spectroscopic properties of these chelates to their efficiency in quenching singlet oxygen are examined and discussed.