Triplet states of humic acids studied by laser flash photolysis using different excitation wavelengths

The spectra and kinetics of short-lived intermediates formed from aqueous (0.1 N NaOH) solutions of the natural mixture of humic and fulvic acids (HFA) were studied by laser flash photolysis using excitation wavelengths of 337, 390, 470, and 520 nm. Laser photolysis of HFA with light of 520 and 470 nm results in the formation of triplet excited states (T_HFA) characterized by the broad absorption spectrum with a maximum near 630 nm and lifetimes of 0.15 ms in deoxygenated solutions. The formation of two types of T_HFA with lifetimes of 0.1 and 2 ms and absorption spectra with maxima at 570 nm is observed under photolysis with light of 337 and 390 nm. The estimation of quantum yields of T_HFA gives 1 and 0.3% under photolysis with excitation wavelengths of 337 and 520 nm, respectively. The rate constants of T_HFA quenching by molecular oxygen are equal to (7—8)·10⁸ L mol^–1 s^–1.     

Characterization of reactive oxygen species generated by protoporphyrin IX under X-ray irradiation

As a possible radiosensitizer candidate having biological compatibility, oncotropic property, and X-ray activation capability, contribution of protoporphyrin IX (PpIX) to enhanced generation of reactive oxygen species (ROS) under X-ray and UV irradiations were examined. To identify the kinds of ROS, 2-[6-(4-amino)phenoxy-3H-xanthen-3-on-9-yl] benzoic acid (APF) and dihydroethidium (DHE) were used together with ethanol as a hydroxyl radical (OH) quencher. All of the three species of our interest (OH, superoxide radical (O₂⁻), and singlet oxygen (¹O₂)) were enhanced by PpIX under X-ray and UV irradiations in addition to those by radiolysis and photolysis. Its enhancement factors exceeded 1.7 depending on the concentrations of PpIX from 1.5 to 15.0 μg/ml.     

Photoinduced Formation of Reactive Oxygen Species from the Acid Form of 6‐(Hydroxymethyl)pterin in Aqueous Solution

The photochemistry of 6‐(hydroxymethyl)pterin (HPT; 1 ) in aqueous solution (pH 5–6) was investigated by irradiation at 350 nm at room temperature. The photochemical reactions of the acidic form 1a were followed by UV/VIS spectrophotometry, thin‐layer chromatography (TLC), high‐performance liquid chromatography (HPLC), and enzymatic methods for the determination of the superoxide anion radical (O ) and hydrogen peroxide (H₂O₂). When 1a is exposed to UV‐A radiation, the intermediates 4 and 4′ are formed reacting with O₂ to yield 6‐formylpterin (FPT; 5 ) and 6‐carboxypterin (CPT; 6 ) under formation of O and H₂O₂ (Scheme 3). The quantum yields of the disappearance of HPT ( 1a ) and of the formation of the photoproducts 5 and 6 were determined. HPT was investigated for its efficiency in singlet‐oxygen (¹O₂) production in acidic aqueous solution. The corresponding quantum yield of ¹O₂ production (Φ_Δ) was 0.15 ± 0.02, as measured by the ¹O₂ luminescence in the near‐IR (1270 nm) upon continuous excitation of the sensitizer. However, ¹O₂ does not participate in the actual photooxidation of HPT ( 1a ) to FPT ( 5 ) and CPT ( 6 ).     

Production of reactive oxygen species induced by a new [60]fullerene derivative bearing a tetrazole unit and its possible biological applications

The wide range of physical and chemical properties of modified fullerenes has drawn increasing attention in the past few years. As part of this research, this paper describes the preparation, characterization, and photophysical properties of a new fullerene derivative chemically modified with a tetrazole. The photophysical properties were studied by EPR radical spin-trapping technique and showed that reactive oxygen species (ROS) can be produced through UVA photosensitization. EPR spin-trapping experiments with singlet oxygen (¹O₂) and superoxide (O₂⁻) inhibitors (β-carotene and superoxide dismutase, respectively) revealed also that: (i) the main ROS produced is ¹O₂ and (ii) ¹O₂ is being partially dismutated to O₂⁻. The results suggest that this derivative can be used in biological applications, as for example, in topic photodynamic therapy (PDT) as a photosensitizer.     

C-S bond cleavage in the sensitized photooxygenation of tert-alkyl phenyl sulfides. The role of superoxide anion

The N-methylquinolinium tetrafluoroborate (NMQ⁺)-photosensitized oxidation of tert-alkyl phenyl sulfides 1a-c (1a, tert-alkyl=tert-butyl; 1b, tert-alkyl=2-phenyl-2-propyl; 1c, tert-alkyl=1,1-diphenylethyl) and benzyl phenyl sulfide (2) were investigated in CH₃CN by nanosecond laser flash photolysis (LFP) and steady-state irradiation either under nitrogen or in the presence of O₂. By laser irradiation, the formation of sulfide radical cations 1a⁺-c⁺ in the monomeric form (λ_max=520 nm) and of 2⁺ in both the monomeric (λ_max=520 nm) and dimeric form (λ_max=780 nm) were observed within the laser pulse. In both cases, the radical cations decayed by second-order kinetics without any apparent formation of transients attributable to C-S bond rupture. In line with these results, very small amounts of photoproducts were obtained under nitrogen thus suggesting that the sulfide radical cations mainly undergo a back electron transfer process with the reduced N-methylquinolinium (NMQ). A different situation was found in the presence of O₂ since steady-state photolysis produced substantial amounts of C-S bond cleavage products (alcohols, alkenes, and ketones from 1a-c and benzaldehyde from 2), in contrast with LFP experiments. Formation of products was, however, significantly reduced in the presence of benzoquinone, a trap for O₂⁻ generated by NMQ and O₂. For the tert-alkyl phenyl sulfides, 1a-c, these results have been interpreted by suggesting that C-S bond cleavage products in the presence of oxygen mostly derive from the decomposition of a thiadioxirane 6 formed by the reaction of the sulfide radical cation with O₂⁻. In this cleavage a sulfinate and a carbocation formed. The former is oxidized to sulfonate, whereas the carbocation can react with adventitious water to form the alcohol (and the alkene therefrom) and with O₂⁻ to produce the ketone. For 2 (a sulfide with α-CH bonds) probably a different mechanism holds, benzaldehyde coming from the α-phenylthio carbon radical formed from deprotonation by O₂⁻ of 2ⁿ⁺.     

A combined experimental and model analysis on the effect of pH and O2(aq) on γ-radiolytically produced H2 and H2O2

The effects of pH and dissolved O₂ on the γ-radiolysis of water were studied at an absorbed dose rate of 2.5 Gy s⁻¹. Argon- or air-saturated water with no headspace was irradiated and the aqueous samples were analyzed for molecular radiolysis products (H₂ and H₂O₂) as a function of irradiation time. The experimental results were compared with computer simulation results using a comprehensive water-radiolysis kinetic model, consisting of the primary radiolysis production, subsequent reactions and related acid–base equilibria. Both the experimental and computer model results were discussed based on the steady-state kinetic analysis of smaller reaction sets consisting of key production and removal reactions. While the main production path for a water decomposition product is the primary radiolysis, the main removal path varies. For H₂O₂ the main removal path is the reactions with e_aq⁻ and OH, whereas for H₂ it is the reaction with OH. As a result, the presence of a dissolved species, or a change in chemical environment, affects the concentrations of H₂O₂ and H₂ through interaction with radicals e_aq⁻ and OH. Over a wide range of conditions, there exist quantitative but simple relationships between the radical and the molecular product concentrations. The experimental and model analyses show that dissolved oxygen increases the steady-state concentrations of H₂O₂ and H₂ by reacting with OH and e_aq^–, and the impact of oxygen is more noticeable at pH below 8. The steady-state concentrations of water decomposition products are nearly independent of pH in the range 5–8. However, raising pH above the pKa value of the acid–base equilibrium of H (⇆e_aq⁻+H⁺) significantly increases [H₂O₂] and [H₂] at the expenses of [OH] and [e_aq^–]. At pH >10, the radiolytical production of O₂ becomes significant, but at a finite rate. This considerably increases the time for the irradiated system to reach a steady state, and is responsible for different impacts on [H₂O₂] and [H₂] due to radically produced O₂, compared to impacts due to initially dissolved O₂. Model sensitivity analysis has shown that at higher pHs (pH >10) transient species such as O₂⁻ and O₃⁻ play a major role in determining the steady-state concentration of molecular products H₂ and H₂O₂. Further validation of the water radiolysis model, particularly at higher pHs, is also discussed.     

Role of reactive intermediates in the radiolytic degradation of Acid Red 1 in aqueous solution

The role of reactive intermediates of water radiolysis (e_aq⁻, H, HO, O₂⁻/HO₂) in decoloration and mineralization of aqueous solutions of Acid Red 1 dye was investigated. The decoloration is highly effective in the reactions of e_aq⁻ and H, and less effective in HO reactions. The O₂⁻/HO₂ pair does not take part in decoloration. For mineralization, which is an oxidative degradation, HO radicals are needed: the efficiency increases with the dissolved oxygen concentration. The reactions of the O₂⁻/HO₂ radical pair slightly increase the rate of mineralization. Iron and copper ions (possible constituents of waste waters) in low concentration do not influence the reactions.     

Electrochemical characterization of a new system for detection of superoxide ion in alkaline solution

A new medium system containing dimethyldistearylammonium bromide (DSAB) was developed for the electrochemical detection of superoxide ion in alkaline solution. The reductions of molecular oxygen in alkaline media as a function of the electrode material were evaluated for Pt, Ag, Au and glassy carbon (GC) electrode. And a quasi-reversible redox process for the O₂/O₂⁻ couple was only obtained at the Pt electrode. The electrochemical responses of the O₂/O₂⁻ couple at a platinum electrode and a platinized platinum electrode were compared, which suggesting that the electrochemical behavior of the O₂/O₂⁻ couple was improved greatly at a platinized Pt electrode. The frequency change (mass change) on the surface of Pt electrode was characterized by the electrochemical quartz crystal microbalance. The reduction of the dissolved oxygen at a platinized Pt electrode in the presence of DSAB was also studied by using chronocoulometry and the result indicated that a one-electron reduction was involved. In addition, the scavenging activity of cysteine towards superoxide ion was evaluated by cyclic voltammetry.     

Evaluation of chemiluminescence reagents for selective detection of reactive oxygen species

In order to evaluate the chemiluminescence (CL) reagents for selective detection of reactive oxygen species (ROS), we comprehensively measured the CL responses of 20 CL reagents (three luminol derivatives, two imidazopyrazinone derivatives, eight lophine derivatives, six acridinium ester derivatives and lucigenin) against six types of ROS (superoxide anion: O₂⁻, hydroxyl radical: OH, hydrogen peroxide: H₂O₂, hypochlorite anion: ClO⁻, singlet oxygen: ¹O₂, and nitric oxide: NO). As a result of the screening, it was found that nine CL reagents selectively detected O₂⁻ while one CL reagent selectively detected OH. However, no CL reagent had selectivity on the detection of H₂O₂, ClO⁻, ¹O₂ and NO. Our screening results could help to select the most suitable CL reagent for selective determination of different ROS.As an application study, 4-methoxyphenyl-10-methylacridinium-9-carboxylate (MMAC), one of the acridinium ester derivatives, showed high selectivity on the detection of O₂⁻, and thus was applied to the assay of superoxide dismutase (SOD) activity. The dynamic range and detection limit of the developed CL assay were 0.1-10 and 0.06 U mL⁻¹, respectively. Significant correlation (r = 0.997) was observed between the results by the CL assay using MMAC and the spectrophotometric assay using 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium monosodium salt.     

Femtosecond relaxation of photoexcited states in nanosized semiconductor particles of iron oxides

Relaxation of photoexcited states in nanosized semiconductor particles of iron oxides was studied by femtosecond laser photolysis techniques: (1) in an aqueous colloidal solution of -Fe₂O₃; (2) in Fe₂O₃ particles in the Nafion^® cation-exchange polymeric membrane; (3) in an aqueous colloid of -Fe₂O₃; and (4) in nanocrystals of ferrihydrite 5Fe₂O₃·9H₂O, which are contained in the protein shell of ferritine. The photoinduced excited states relax at the femtosecond and picosecond time scale. The spectra of photoinduced absorption of photoexcited states and the relaxation dynamics in the studied iron oxides weakly depend on the structure and surface environment of a nanoparticle.     

Comparison of intermediates of tryptophan, tyrosine and their dipeptide induced by UV light and SO. 4 –

The chemical processes of tryptophan (Trp), tyrosine (Tyr) and a dipeptide Trp-Tyr, which are induced by UV radiation and one-electron oxidation of SO^. ₄ ^–, have been investigated in aqueous solution by KrF (248 nm) laser flash photolysis. On the basis of optical studies, the photoionization of Trp and Tyr produces the tryptophan indolyl radical and tyrosine phenoxyl radical, respectively, and these are different from the intermediates resulting from interaction of Trp and Tyr with SO^. ₄ ^–. In the case of Trp, SO^. ₄ ^– would attack the indole moiety to produce a C(2)-yl sulphate radical adduct, and Tyr is oxidized to produce mainly the corresponding one-electron oxidized radical, which deprotonates rapidly to form the phenoxyl radical in neutral solution, and a possible sulphate radical adduct. From transient absorption spectra of photoionization of Trp-Tyr, an intramolecular electron transfer, Trp/N^.-Tyr Trp-Tyr/O^., has been observed, but there was no observation of the process of one-electron oxidation of Trp-Tyr by SO^. ₄ ^–.     

Significant roles of oxygen and unbound <Superscript>•</Superscript>OH radical in phenol formation during photo-catalytic degradation of benzene on TiO<Subscript>2</Subscript> suspension in aqueous system

Studies on photo-catalytic degradation of benzene using TiO₂ photo-catalyst as a suspension in water is reported. Degradation studies have been carried out using 350 nm UV light. Phenol, a photo-catalytic product of benzene, was monitored under varying experimental conditions such as amount of TiO₂, concentration of benzene, photolysis time, ambient (air, O₂, Ar, N₂O and N₂O–O₂ mixture), etc. The phenol yields in both aerated and O₂-purged systems increased with the photolysis time. In contrast to oxygenated systems, the yields of phenol in deoxygenated (viz. Ar-purged and N₂O-purged) systems were quite low (~30 μM) and remained steady. H₂O₂ yields in all these systems were also monitored, and found lower than an order of magnitude as compared to phenol yields for the respective systems. The rate of phenol production in aerated 1 mM benzene solution containing 0.05% TiO₂ suspension was evaluated at 12.3 μM min⁻¹ which is lower than the rate obtained in an O₂-saturated system (22.4 μM min⁻¹). The low yields of phenol in both Ar- and N₂O-purged systems, and also the increasing trends in oxygenated systems, together reveal that, for the phenol formation with an enhanced rate, oxygen is essential. In the present study, it is implied that the photo-generated hole, which is mainly an ^•OH radical, is either freely available in the aqueous phase or migrates to the aqueous phase from the catalyst surface, to react with benzene to produce HO-adduct radical. Later, following reaction with oxygen, the adduct produces phenol. On the other hand, h⁺ and surface adsorbed ^•OH radical, being trapped/bonded due to rigid association with the catalyst surface, were not able to generate phenol under similar experimental conditions. The mechanism of phenol formation with TiO₂ photolysis in an aqueous system is rechecked, on the basis of present results on h⁺/surface adsorbed ^•OH radical/unbound ^•OH radical scavenging by benzene, collectively with previous reports on various systems.     

Chlorine nitrate photolysis by a new technique: very low pressure photolysis

Very low pressure photolysis (VLPØ) of chlorine nitrate was performed in a quartz Knudsen cell. The light source was a 2500 W high-pressure xenon lamp, and a modulated molecular-beam mass spectrometer was used to monitor the concentration of ClONO₂ and photolysis products. Because of the low pressures used (? 10^?3 torr) and the short residence time in the cell (≈1 s), secondary reactions were unimportant and the primary products could be directly identified. The primary photolysis products (λ ≈ 2700 Å) are atomic chlorine and NO₃ free radical. Chlorine atoms were identified both by the appearance of Cl₂ (wall recombination product; the walls were not poisoned) and by HCl produced when C₂H₆ was added to the cell. Nitrate free radical was directly identified as a mass peak at m/e = 62, as well as by chemical titration with nitric oxide: NO₃ + NO → 2NO₂. It was verified by direct tests that the peak at m/e = 62 did not arise from possible HNO₃ contamination or from N₂O₅, a possible secondary product. This titration reaction was used to measure quantitatively a lower limit to the primary quantum yield, φ ? 0.5 ± 0.3. This represents a lower limit because of the unknown extent of the secondary photolysis of NO₃ under our conditions. We believe this to be the first observation using mass spectrometry of the NO₃ free radical. The quantum yield for atomic chlorine is φ = 1.0 ± 0.2. N₂O was used to test for O(¹D) according to the reaction, O(¹D) + N₂O → products; none was observed. Triplet oxygen, O(³P) was observed to the extent of ≈ 10% by the reaction O(³P) + NO₂ → NO + O₂, but this yield can also be due to the photolysis of NO₃ free radical produced in the primary step. We conclude that the predominant reaction pathway is

.     

Photosensitized Oxidation of Tetrabromobisphenol A by Humic Acid in Aqueous Solution†

The brominated flame retardant 3,3′,5,5′‐tetrabromobisphenol A (TBBPA) may accumulate in the environment, including surface waters, and degrade there to potentially toxic products. We have previously shown that singlet oxygen (¹O₂), produced by irradiation of rose bengal with visible light, oxidizes Triton X‐100‐solubilized TBBPA to yield the 2,6‐dibromo‐p‐benzosemiquinone anion radical while consuming oxygen (Environ. Sci. Technol. 42 , 166, 2008). Here, we report that a similar ¹ O ₂ ‐induced oxidation can be initiated in aqueous solutions by the irradiation of TBBPA dissolved in a humic acid (HA) solution. HA is a known weak ¹ O ₂ photosensitizer and we indeed detected the infrared ¹ O ₂ phosphorescence from HA preparations in D ₂ O. When an aqueous preparation of HA was irradiated (λ > 400 nm) in the presence of TBBPA, oxygen was consumed, and the 2,6‐dibromo‐ p ‐benzosemiquinone anion radical was generated and detected using electron paramagnetic resonance. Radical formation and oxygen consumption were inhibited by sodium azide, a singlet oxygen quencher. Our results suggest that solar radiation, in the presence of HA, may play an important role in the photodegradation of TBBPA in the aquatic environment.     

IR spectroscopic study of allylperoxy radical and products of its phototransformations in the Ar matrix

Matrix IR spectra of an allylperoxy radical (1) obtained by co-condensation of allyl radicals and molecular oxygen into an argon matrix at 12 K have been studied. The bands observed in the matrix IR spectrum are assigned to normal vibrations on the basis of ab initio calculations of radical1 and its isotope-substituted analog CA 18_{O 2} (1a). The band at I128.6 cm^–1 is assigned to the stretching vibration of the 0-0 bond. The products of photodecomposition of radical1 under UV irradiation ( > 248 nm) have been studied. A vinylacyl radical (2) with a characteristic high vibrational v(C=0) frequency of 1823.I cm -1 has been observed among the products of photolysis of1. According to the data of quantum chemical calculations, some delocalization of a free electron between an oxygen atom and two C atoms is observed in radical 2.Preliminary results were presented at the XII International Conference on Physical Organic Chemistry (Padova, Italy) in 1994.¹⁵ Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. I, pp. 109–116, January, I996.     

Photocatalytic generation of a non-heme Fe(iii)-hydroperoxo species with O2 in water for the oxygen atom transfer reaction

Coupling a photoredox module and a bio-inspired non-heme model to activate O₂ for the oxygen atom transfer (OAT) reaction requires a vigorous investigation to shed light on the multiple competing electron transfer steps, charge accumulation and annihilation processes, and the activation of O₂ at the catalytic unit. We found that the efficient oxidative quenching mechanism between a [Ru(bpy)₃]²⁺ chromophore and a reversible electron mediator, methyl viologen (MV²⁺), to form the reducing species methyl viologen radical (MV˙⁺) can convey an electron to O₂ to form the superoxide radical and reset an Fe(iii) species in a catalytic cycle to the Fe(ii) state in an aqueous solution. The formation of the Fe(iii)-hydroperoxo (Fe^III–OOH) intermediate can evolve to a highly oxidized iron-oxo species to perform the OAT reaction to an alkene substrate. Such a strategy allows us to bypass the challenging task of charge accumulation at the molecular catalytic unit for the two-electron activation of O₂. The Fe^III–OOH catalytic precursor was trapped and characterized by EPR spectroscopy pertaining to a metal assisted catalysis. Importantly, we found that the substrate itself can act as an electron donor to reset the photooxidized chromophore in the initial state closing the photocatalytic loop and hence excluding the use of a sacrificial electron donor. Laser Flash Photolysis (LFP) studies and spectroscopic monitoring during photocatalysis lend credence to the proposed catalytic cycle.

A photoinduced iron(III)-hydroperoxo intermediate (Fe^III-OOH) was trapped by bypassing the charge accumulation process, that triggers the oxygen atom transfer reaction to an alkene with O₂ as sole oxygen source in water.     

Spectra and reactions of acetyl and acetylperoxy radicals

The ultraviolet absorption spectra of the acetyl and acetylperoxy radicals have been characterized in the range 195–280 nm. The acetyl radical was generated by the flash photolysis of Cl₂ in the presence of CH₃CHO and was converted to the acetylperoxy radical in the presence of excess O₂. The extinction coefficient of the acetylperoxy radical was measured to be 2300 L/mol cm at the maximum at 207 nm and the rate constant for the reaction was evaluated to be k₅ = (4.8 ± 0.8) × 10⁹ L/mol s.     

Enzymatic oxidation of lignin and compounds modeling it. III. Formation of singlet oxygen in the peroxidase oxidation of lignin

The main emitters of radiation in the aerobic oxidation of lignin are the carbonyl groups in an excited state and singlet oxygen. It has been shown that the main source of O₂(¹) may be the radical anion O^–·₂. Singlet oxygen and the radical anion are by-products of the radical oxidation of lignin.Siberian Scientific-Research Institute of Cellulose and Board, Bratsk. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 510–514, July–August, 1984.     

A mechanistic investigation of (Me3Si)3SiH oxidation

The interaction of (Me₃Si)₃SiH with O₂ is known to afford (Me₃SiO)₂Si(H)SiMe₃ in which the two oxygen atoms arise from the same oxygen molecule. In order to investigate the mechanism of this unusual reaction, the oxidation rates were measured in the temperature range 30-70 °C by following oxygen uptake in the presence and absence of hydroquinone as inhibitor. The rate constant for the spontaneous reaction of (Me₃Si)₃SiH with O₂ was determined at 70 °C to be ∼3.5 × 10⁻⁵ M⁻¹ s⁻¹. A sequence of the propagation steps is proposed by combining the previous and present experimental findings with some theoretical results obtained at the semiempirical level. These calculations showed that the silylperoxyl radical (Me₃Si)₃SiOO undergoes three consecutive unimolecular steps to give (Me₃SiO)₂Si()SiMe₃. Evidence has been obtained that the rate determining step is the rearrangement of silylperoxyl radical to a dioxirand-like pentacoordinated silyl radical. Our findings are of considerable importance for the understanding of the oxidation of hydrogen-terminated silicon surfaces.     

EPR of gamma irradiation induced radicals in NaHCO3, CsHCO3 and Na2CO3

In this study gamma irradiated NaHCO₃, CsHCO₃ and Na₂CO₃ were investigated at room temperature. The radicals induced by gamma irradiation in NaHCO₃ were found to be CO⁻₃, HCO₃ and CO⁻₂; in CsHCO₃ the species were attributed to HCO₃; and in Na₂CO₃ to CO⁻₃ and CO⁻₂ radicals. The hyperfine parameters for the hydrogen in HCO₃, and the ¹³C nucleus in CO⁻₂ radical have been determined. The results were compared with literature data for similar compounds and the EPR properties of the CO⁻₂ radical were discussed.