Photogeneration Of Radicals Research Articles

Photolysis of Isoelectronic Ruthenium Nitrosyl and Diazonium Complexes in Frozen PVC Matrices: Retention of Dinitrogen on Ruthenium Following Photochemical Phenyl Radical Loss

AbstractPhotolysis of RuCl3(η1‐NO)(PPh3)2 in poly(vinyl chloride) (PVC) matrices at 85 K results in reversible linkage isomerism of the nitrosyl ligand to form the isonitrosyl complex RuCl3(η1‐ON)(PPh3)2. Metal‐to‐ligand charge‐transfer (MLCT) excitation of the isoelectronic phenylazo complex, RuCl3(η1‐N2Ph)(PPh3)2, has previously been shown to result in generation of the phenyl radical, presumably through decomposition a photogenerated diazenyl radical. Examination of this photolysis in a PVC matrix at cryogenic temperatures has permitted direct observation of an isotopically sensitive product band that may be assigned to a 17‐electron ruthenium dinitrogen species, suggesting that this photochemical decomposition does not proceed through formation of a diazenyl intermediate but through homolytic cleavage of the parent diazonium complex to give the phenyl radical and the 17‐electron RuCl3(η1‐N2)(PPh3)2.

Design and simple synthesis of composite Bi12TiO20/Bi4Ti3O12 with a good photocatalytic quantum efficiency and high production of photo-generated hydroxyl radicals.

Hybrid Bi12TiO20/Bi4Ti3O12 composites with different Bi : Ti molar ratios were successfully fabricated using a one-step solid-state calcination. These samples were characterized using X-ray diffraction, specific surface area analysis, scanning electron microscopy, high-resolution microscopy, selected-area electron diffraction, X-ray photoelectron spectroscopy, UV-vis diffuse reflection spectroscopy, photoluminescence analyses, and photocurrent and electrochemical impedance spectroscopy. Some specified techniques were also used to investigate the photocatalytic properties and the formation of radical species under UV-vis irradiation. The composites provide an efficient pathway for photo-produced electrons and holes to diffuse and transfer to the exterior of the particles for photodegradation, because of the good contact between hybrid Bi12TiO20 and Bi4Ti3O12. Therefore, the hybrid Bi12TiO20/Bi4Ti3O12 can prolong the carrier lifetime as compared to any individual material (Bi12TiO20 or Bi4Ti3O12). In particular, the hybrid Bi12TiO20/Bi4Ti3O12 composite with a Bi : Ti ratio of 12 : 6 had the longest photo-generated carrier lifetime and produced the maximum amount of hydroxyl radicals during UV-vis irradiation; therefore, it demonstrated the best photo-catalytic properties. A Z-scheme mechanism was proposed to explain the hydroxyl radical yield and photodegradation.

Synthesis and Properties of Novel Photomodulation Magnetic Molecular-Based Magnet Based on Photochromic Biindenylidenedione Derivative

Molecular-based magnetic material can be used for information storage, magnetic imaging, photo-magnetic switching and so on. So the design and synthesis of the molecular-based magnet is a focus of contemporary science research. Sparked by the strategy of molecular magnets, the novel photomodulation magnetic molecular-based magnet photochromic biindenylidenedione derivative with 2-nitropropane radical and 2,2,6,6-tetramethyl-piperidinyloxy (TEMPO) radical (4) was synthesized. The synthetic method of compound 4 is' that starting from photochromic biindenylidenedione derivative 7,7'-dimethyl-[2,2'-bi-1H-indene]-3,3'-diethyl-3,3'-di-hydroxy-1,1'-dione (1), via bromination reaction, 7,7'-bis(bromomethyl)-[2,2'-bi-1H-indene]-3,3'-diethyl-3,3'-dihydroxy-1,1'-dione (2) was obtained. Sodium ethoxide, 2-nitropropane and compound 2 were dissolved in absolute ethanol reaction to obtain biindenylidenedione derivative with 2-nitropropane radical and formyl 3. The structure of compound 3 was confirmed by H-1 NMR, MS and element analysis. The results indicated that 2-nitropropane group bonds with one methylene on the benzene ring of the compound 3. In further analysis to H-1 NMR and electron spin-resonace (ESR) spectra of compound 3 demonstrated that single electron spin exists in 2-nitropropane group. On the basis compound 3, stable nitroxide radical TEMPO was successfully introduced into the benzene rings of photochromic biindenylidenedione derivative to obtain compound 4. The properties of compounds 2, 3 and 4 were investigated. The testing results display that the solid compounds 2 similar to 4 possess photochromic property. ESR spectrum was found when irradiated solid compound 2. ESR spectrum was found and changed before and after photoirradiation in solid state of compound 3 and 4. The magnetic susceptibility measurement shows that the antiferromagnetic interaction for before and after photoirradiation in solid state of compound 4, and the antiferromagnetic interaction becomes stronger after photoirradiation. These testing results demonstrated that after photoirradiation in solid state of compound 3 and 4, there are two or three kinds of spin centres after photoirradiation: one is 2-nitropropane or TEMPO radical, and another is photo-generated radical of two indanione moieties. The color and magnetic of the solid compound 4 can be photomodulated.

Visible light-driven water oxidation using a covalently-linked molecular catalyst-sensitizer dyad assembled on a TiO2 electrode.

The combination of porphyrin as a sensitizer and a ruthenium complex as a water oxidation catalyst (WOC) is promising to exploit highly efficient molecular artificial photosynthetic systems. A covalently-linked ruthenium-based WOC-zinc porphyrin (ZnP) sensitizer dyad was assembled on a TiO2 electrode for visible-light driven water oxidation. The water oxidation activity was found to be improved in comparison to the reference systems with the simple combination of the individual WOC and ZnP as well as with ZnP solely, demonstrating the advantage of the covalent linking approach over the non-covalent one. More importantly, via vectorial multi-step electron transfer triggered by visible light, the dye-sensitized photoelectrochemical cell (DSPEC) achieved a broader PEC response in the visible region than DSPECs with conventional ruthenium-based sensitizers. Initial incident photon-to-current efficiencies of 18% at 424 nm and 6.4% at 564 nm were attained under monochromatic illumination and an external bias of -0.2 V vs. NHE. Fast electron transfer from the WOC to the photogenerated radical cation of the sensitizer through the covalent linkage may suppress undesirable charge recombination, realizing the moderate performance of water oxidation. X-ray photoelectron spectroscopic analysis of the photoanodes before and after the DSPEC operation suggested that most of the ruthenium species exist at higher oxidation states, implying that the insufficient oxidation potential of the ZnP moiety for further oxidizing the intermediate ruthenium species at the photoanode is at least the bottleneck of the system.

Sulfate Radical Photogeneration Using Fe-EDDS: Influence of Critical Parameters and Naturally Occurring Scavengers.

In this study, the activation of persulfate induced by Fe(III)-ethylenediamine-N,N′-disuccinic acid (EDDS) under dark and irradiation conditions and the reactivity of the generated sulfate radical (SO4•–) under a wide range of experimental conditions were investigated by means of experimental kinetic analyses and modeling. The Fe(III)-EDDS induced activation of persulfate was found to be efficient across a wide range of pH value (3–7), whereas the second order rate constant of SO4•– with 4-tert-butylphenol (4tBP) kSO4•–,4tBP = (4.21 ± 0.22) × 109 M–1 s–1 was found to be unchanged between pH 2.5 and 8.5. Experimental and theoretical investigations showed clearly that the 4tBP degradation was enhanced in the presence of chloride (10 mM), whereas an almost complete inhibition was observed in the presence of carbonates (10 mM). For the first time, second order rate constants evaluated by laser flash photolysis experiments revealed that SO4•– has a similar reactivity with EDDS (6.21 × 109 M–1 s–1) and 4tBP (4....

Treatment of landfill leachate using electrochemically assisted UV/chlorine process: Effect of operating conditions, molecular weight distribution and fluorescence EEM-PARAFAC analysis

Leachate, a highly polluted wastewater, is generated from landfill containing complex compositions which have great impact on the environment. The advanced treatment of biochemically treated landfill leachate using electrochemically assisted UV/chlorine process (UEC) in a flow reactor was investigated. The effects of pH and current density on COD and NH3-N removal by UEC process were evaluated. At the pH value of 5.0, current density of 60mA/cm2, the removal efficiencies for COD and NH3-N were observed as 77% and 87%, respectively. The electrochemical process alone (EC) could obtain relatively high NH3-N removal but very low COD removal, compared with which, the UEC process significantly promoted the removal efficiency of COD while that of NH3-N increased to a very limited extent. Most of NH3-N was converted into N2 and little nitrogen compounds and chloramines were remained in the leachate after 8h treatment. Moreover, the variations of active chlorine and chloride ions were monitored in the EC and UEC systems, and a possible mechanism was proposed for the UEC process. The in situ electro-generated active chlorine and photo-generated radical species should be responsible for the efficient degradation of landfill leachate. The total energy consumption and current efficiency were 216.5 (kWh/kg COD) and 17.5%, respectively, for 8h treatment. In addition, the molecular weight (MW) percentage distribution analysis and PARAFAC analysis of excitation emission matrix (EEM) fluorescence spectroscopy indicated that the organic matters in the leachate were significantly degraded into the fractions with small molecular weight.

ORGANIC CHEMISTRY. Enchained by visible light-mediated photoredox catalysis.

Radical-chain processes can dominate the kinetics of photogenerated radical catalysis

Protein Corona Prevents TiO2 Phototoxicity.

Background & AimTiO2 nanoparticles have generally low toxicity in the in vitro systems although some toxicity is expected to originate in the TiO2-associated photo-generated radical production, which can however be modulated by the radical trapping ability of the serum proteins. To explore the role of serum proteins in the phototoxicity of the TiO2 nanoparticles we measure viability of the exposed cells depending on the nanoparticle and serum protein concentrations.Methods & ResultsFluorescence and spin trapping EPR spectroscopy reveal that the ratio between the nanoparticle and protein concentrations determines the amount of the nanoparticles’ surface which is not covered by the serum proteins and is proportional to the amount of photo-induced radicals. Phototoxicity thus becomes substantial only at the protein concentration being too low to completely coat the nanotubes’ surface.ConclusionThese results imply that TiO2 nanoparticles should be applied with ligands such as proteins when phototoxic effects are not desired - for example in cosmetics industry. On the other hand, the nanoparticles should be used in serum free medium or any other ligand free medium, when phototoxic effects are desired – as for efficient photodynamic cancer therapy.

Antioxidant capacity automatic assay based on inline photogenerated radical species from l-glutathione-capped CdTe quantum dots

This work aimed at the development of a methodology implemented in an automatic flow system for determination of the antioxidant capacity in food samples, based on the luminol oxidation by inline photogenerated radical species from cadmium telluride nanoparticles capped with l-glutathione. Radical species were generated inline by a high-power visible light obtained by Light Emitting Diodes (LEDs) assembled in a multipumping flow system (MPFS). The use of visible light instead of UV radiation allowed the development of a new methodology for antioxidant capacity determination, more environment friendly and to circumvent the risk for UV photo-induced degradation of sample antioxidant compounds. Additionally, the formation of superoxide radical species was theoretically predicted considering the variation of the redox potential with the size of CdTe QDs and the values of redox potential of the oxidizing and oxidable species present in the irradiated medium.The obtained results of trolox equivalent antioxidant capacity (TEAC) from the analysis of commercial beverages were compared with the results of ABTS and DPPH batch assays through Spearman's-Rho correlation coefficients and no correlation was found (for ABTS: ρ=0.2, p<0.6 and for DPPH: ρ=0.5, p<0.1) since the mechanism of action of the proposed methodology was based on the scavenging capacity of ROS species rather than the reduction of a colored oxidant.An analytical linear response range between 0.0001 and 0.005mmolL−1 of trolox and a limit of detection of 0.00005mmolL−1 was found. The QDs based MPFS methodology allowed a determination rate of about 79h−1, a total waste generation of 20.5mLh−1 and the consumption of 0.100mgh−1 of QDs and 2.1mgh−1 of luminol.

Photogenerated lophyl radicals in 1-alkyl-3-vinylimidazolium bis(trifluoromethylsulfonyl)imides.

1-Alkyl-3-vinylimidazolium bis(trifluoromethylsulfonyl)imides were investigated as a matrix for photogenerated lophyl radicals obtained by irradiation of o-chlorohexaarylbisimidazole (o-Cl-HABI). Photoinduced polymerization of ionic liquid monomers using the photoinitiator system composed of o-Cl-HABI and 3-mercapto-1,2,4-4H-triazole was investigated by photo-DSC. Selected thermal properties and viscosity of these ionic liquid monomers are important to understand the lophyl radical kinetics after exposure. Solvent cage effects and viscosity of the ionic liquid monomers strongly affect radical recombination in the dark. This was investigated at different temperatures. The rate constant for radical recombination (krec) decreases from the methyl to the butyl substituted ionic liquid monomer. This may be attributed to an increasing viscosity with increasing size of the alkyl substituent. However, further increase in the size of the alkyl substituent from a butyl to a heptyl group bound at the imidazolium ion results in an increase of krec although the viscosity does further increase. Therefore, a minimum in krec was found for the butyl substituted ionic liquid monomer. Furthermore, the Eyring parameters indicated a dependence on the chain length of the alkyl substituent bound at the imidazolium ion while the activation energy of the viscous flow only slightly changes. Furthermore, the size of the alkyl substituent bound at the cation of the ionic liquid monomers strongly influences both solvent cage and viscosity, and therefore, the concentration of lophyl radicals during photoinduced generation. Photo-induced polymerization of the ionic liquid monomers is affected by viscosity at low conversion and by vitrification at higher conversion. The latter is important for application of the ionic liquid monomers and the polymers made from them by photoinduced polymerization.

ChemInform Abstract: Interactions Between Ionic Liquids and Radicals

AbstractReview: 102 refs.

Visible-light-mediated synergistic photocatalytic antimicrobial effects and mechanism of Ag-nanoparticles@chitosan–TiO2 organic–inorganic composites for water disinfection

By coupling chitosan bioaffinity adsorption, molecular imprinting, and nano TiO2 photocatalysis technologies, Ag nanoparticles@chitosan–TiO2 organic–inorganic composite (Ag-NPs@CTA) was prepared and found to be an efficient visible-light-mediated photocatalytic antimicrobial agent useful for water disinfection. Ag-NPs@CTA exhibited favorable antimicrobial activity for Escherichia coli, Staphylococcus aureus and Candida albicans, with the inactivation rate in excess of 99.0% in 2h under visible light irradiation. Ag-NPs@CTA could be reused for five consecutive cycles directly without regeneration treatment and showed rather low cytotoxicity toward mammalian cells. Characterization of E. coli cells using TEM, AFM, Raman spectroscopy, FTIR and intracellular protein leakage illustrated that Ag-NPs@CTA could not only destroy the cell wall and membrane but also cause metabolic dysfunction. The synergistic antimicrobial mechanism was studied by Ag release evaluation, radical scavengers study, and Ag-NPs@CTA characterization by XPS and UV–vis absorption. The enhanced antimicrobial ability originated from the synergistic activities of the three components and the proposed mechanisms contained electrostatic interaction followed by two pathways: released Ag species and oxidative stress caused by photo-generated reactive radicals. The present work provided an efficient water disinfection technology of realistic potential and also opened a new idea in designing visible-light-mediated photocatalytic antimicrobial agents and in studying the antimicrobial mechanism.

Direct and indirect photolysis of triclocarban in the presence of dissolved organic matter

Abstract Photolysis is an important attenuation pathway for the removal of wastewater effluent organic micropollutants from surface waters. In this work, direct and indirect processes leading to the degradation of the disinfectant, triclocarban were studied. Photo-irradiation experiments were conducted in water collected from Old Woman Creek (OWC) a tributary of Lake Erie near Huron, OH, USA and in solutions of fulvic acids isolated from the Suwannee River, Georgia, USA (SRFA), Old Woman Creek (OWCFA) and Pony Lake, Antarctica (PLFA). Photodegradation of triclocarban proceeded faster in the presence of all three fulvic acids relative to deionized water. PLFA, an autochthonous dissolved organic matter (DOM) was found to be more reactive than the other fulvic acids, while the mostly allochthonous SRFA exhibited the lowest reactivity toward triclocarban. The later observation can be in part explained by anti-oxidant moieties present in SRFA. Photosensitized triclocarban degradation in whole water DOM from OWC was entirely attributable to the fulvic acid fraction and suggests that this component is the most photo-reactive fraction of the DOM. Anoxic and methanol-quenched experiments revealed unexpected results whereby the former suggests oxidation through reaction with triplet DOM, while the later is indicative of reaction with photo-generated hydroxyl radicals. It is possible that methanol can quench excited DOM species, which would shut down the triplet oxidation pathway. Finally, we observed no enhancement of triclocarban-photosensitized degradation through the addition of iron.

A potential gradient along the layer-by-layer architecture for electron transfer rectification

Electrochemical and photochemical measurements demonstrated that dendritic phenylazomethines, which can make complexes with SnCl2 by a stepwise process, only permit an outbound electron transfer. The unique dendrimer effect allows efficient production of photo-generated radical ion pairs by suppressing their charge recombination. In sharp contrast, the phenylenevinylene or benzylether dendrimers, which lack a heteroatom or π-conjugation, did not exhibit such non-symmetric characters.

High Performance Photoinitiating Systems for Holography Recording: Need for a Full Control of Primary Processes

Optimization of holography recording in photopolymers was studied from the point of view of a quite general process, that is, the photogeneration of radicals. On the basis of a dye/coinitiator photoinitiating system, the effect of primary events and their relative efficiency was investigated with respect to the final overall properties, such as the diffraction efficiency. Quenching of the dye excited states by the borate salts coinitiators exhibits important differences depending on the dye used (Rose Bengal or Safranine O). Keeping in mind that both singlet and triplet states of the dyes can react, and taking into account the viscosity of the matrix, a method to evaluate the overall quantum yield of radicals released is proposed. It is found that this quantum yield well correlates with the maximum rate of photopolymerization. More interestingly, the dose required to obtain a given diffraction efficiency was found to be also governed by the radical quantum yield, showing that the final property is directly governed by primary events. This shed some light on the efficiency of photochemical pathway to generate radicals for use in organic or polymer areas.

Trading oxidation power for efficiency: Differential inhibition of photo-generated hydroxyl radicals versus singlet oxygen

The ability of reactive oxygen species (ROS) to interact with target pollutants is crucial for efficient water treatment using advanced oxidation processes (AOPs), and inhibition by natural organic matter (NOM) can significantly reduce degradation efficiency. We compare OH-based degradation (H2O2-UV) to 1O2-based degradation (Rose Bengal) of several probe compounds (furfuryl alcohol, ranitidine, cimetidine) interacting in water containing background constituents likely to be found in treatment water such as natural organic matter (NOM) and phosphate, as well as in effluent from a waste-water treatment plant (WWTP). Hydroxyl radicals were much more susceptible to hindrance by all three background matrices (NOM, phosphate and WWTP effluent) tested, while 1O2 was only slightly inhibited by NOM and not by phosphate or WWTP effluent. A mechanistic model accounting for this inhibition in terms of radical scavenging and inner filter effects was developed, and accurately simulated the results of the NOM interactions. These results underscore the importance of considering the effect of background constituents in the selection of photocatalysts and in the design of AOPs for emerging applications in tertiary treatment of wastewater effluent and disinfection of natural waters.

Detecting free radicals in sunscreens exposed to UVA radiation using chemiluminescence

One of the current concerns with the application of nanoparticles in sunscreens, and in particular nano-TiO2 and ZnO, is their potential to photogenerate free radicals and reactive oxygen species (ROS) when they absorb ultraviolet wavelengths from sunlight. Free radicals and ROS are known to be associated with UV-induced skin damage and oxidative stress, from which sunscreens are expected to offer significant protection. Here we describe a simple method, based on chemiluminescence emission, for detecting free radicals generated in commercial sunscreens alone, and when applied to various substrates, following exposure to UVA (320–400nm) radiation. This photo-induced chemiluminescence (PICL) technique could be used to optimise sunscreen formulations so as to minimise free radical photogeneration during exposure to sunlight.

Investigation of the photoelectronic properties of nanocrystalline carbon- and nitrogen-doped titanium dioxide

This paper reports the results of the study of photoelectronic properties of nanocrystalline titanium dioxide doped with carbon and nitrogen. Photo-generated paramagnetic radicals were detected and identified in nanocrystalline titanium dioxide by the electron paramagnetic-resonance method. The concentrations of the radicals versus the carbon and oxygen content were determined in the dark and under light exposure. The presence of C and CO 2 − -radicals in carbon-doped titanium dioxide and mainly N and NO radicals in nitrogen-doped titanium dioxide is shown. The concentration of the radicals increases under light exposure, which indicates the recharging processes of electronic states in the bandgap, due to the presence of a dopant, accompanied by generation of paramagnetic particles. The electron paramagnetic resonance data correlate with the kinetics of formaldehyde decomposition on the titanium dioxide samples under study.

Exploring Photovoltaic Feasibility of Pentaaryl [60]Fullerene in Bulk Heterojunction Architecture

The photochemical process of pentaaryl [60]fullerenes (PAC60) has been investigated by fluorescence and transient absorption spectroscopies, aiming at their use in bulk heterojunction organic photovoltaic (OPV). Fluorescence quenching was observed by adding C60 into PAC60, where the intensity was decreased in an exponential manner. The photo-generated radical cation of PAC60 was identified at 925 nm in the presence of electron acceptor. Motivated from the observed shallow molecular orbital levels of PAC60, we performed conceptual study of all-fullerene OPV, where PAC60 and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were presumably used as a p-type and n-type semiconductor, respectively. The power conversion efficiency was 0.007%, much smaller than 0.05% found for the conventional combination of P3HT (p-type) and PAC60 (n-type). The mechanism is discussed on the basis of photoconductive and spectroscopic results.

Interactions between ionic liquids and radicals

Ionic liquids were investigated with both stable radicals on the basis of 2,2,6,6-tetramethylpiperidine-1-yloxyl (TEMPO) and photogenerated lophyl radicals. The ionic liquids are composed either of bis(trifluoromethylsulfonyl)imide (NTf2) as anion and various cations or they contain an imidazolium ion in combination with various anions. The cations include imidazolium, pyrrolidinium, piperidinium, polymethine or ammonium ions. Furthermore, BF4−, PF6−, triflate, camphorsulfonate, lactate, tosylate or tris(pentafluoroethyl)trifluorophosphate (FAP) are the counter ions in the imidazolium salts. The structural variation of the ionic liquids results in differences in glass formation, semicrystallinity, or crystallinity, as well as in viscosity differences. Furthermore, a vinyl substituent at the imidazolium ion and a methacryloyloxyethyl substituent at the ammonium ion result in polymerizable ionic liquids that were converted via a radical mechanism in amorphous polymerized ionic liquids with a glass transition temperature, which is significantly higher compared to the ionic liquids. An additional substituent at TEMPO causes additional hydrogen bond formation or additional Coulomb interactions with the individual ions of the ionic liquids compared to TEMPO. This influences the mobility of these radicals in the ionic liquid expressed by differences in the average rotational correlation time (τrot). The mobility of the radicals in the ionic liquids as function of the temperature describes ionic liquids either as continuum in analogy to molecular solvents using the Stokes–Einstein model, that is the case for 1-butyl-3-methylimidazolium NTf2, or as medium where free volume effects are important for the mobility of a solute in the ionic liquid using the model of Spernol, Gierer, and Wirtz. The 1-butyl-3-methylimidazolium BF4− fits well into the latter. Furthermore, the isotropic hyperfine coupling constant (Aiso(14N)) of the stable radicals gives information about micropolarity of the ionic liquids only if the mobility of the radical is high enough in the ionic liquid. In addition to the rotational mobility of the stable radicals, the photogenerated lophyl radicals give information about translational diffusion of radicals and solvent cage effects in the ionic liquids. The application of the Eyring equation results mostly in the expected negative values of the activation entropy for the transition state that is typical for bimolecular reactions. Only few examples show a less negative or positive activation entropy for the bimolecular reaction, which may be attributed to radical recombination within the solvent cage to a high extent. The results obtained during investigation of radicals in ionic liquids are important to understand the radical processes in ionic liquids that may occur for example in dye sensitized solar cells, photo or thermally induced reactions or radical polymerizations in ionic liquids.