Photolysis Time Research Articles

Significant roles of oxygen and unbound •OH radical in phenol formation during photo-catalytic degradation of benzene on TiO2 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.

Ultraviolet-induced surface modification of polyurethane films in the presence of oxygen or acrylic acid vapours

An efficient surface functionalization of polyurethane (PU) films has been obtained by ultraviolet (UV)-assisted modification in the presence of oxygen or acrylic acid (AA) vapours. Film analyses were carried out by water contact angle measurements, X-ray photoelectron spectroscopy (XPS) and Near-edge X-ray absorption fine structure (NEXAFS). Film hydrophilicity increased with photolysis time in the presence of oxygen or AA vapours. Incorporation of COO and C O functional groups at the polymer surface after the UV-assisted treatments was observed. In addition, High resolution XPS and NEXAFS results showed that a thin film of poly (acrylic acid) (PAA) is formed over the PU films during the UV irradiation with AA vapours. The obtained results are compared with previous published oxygen and AA low-power plasma treatments. Similarity between both treatment methodologies is shown. UV surface functionalization and polymerization of PAA can be used instead of a traditional plasma treatment with the advantage of set-up simplicity and lower costs.

Cationic polymerization of cyclohexene oxide by pentamethoxytrityl salts

Abstract—Polymerization of cyclohexene oxide (CHO) has been investigated in dichloromethane, using crystalline, soluble, and nonhygroscopic methoxy substituted trityl carbocationic salts having nonnucleophilic anions such as SbF 6 − and PF 6 − without cocatalysts or promoters at 25°C. The effects of salt counterion, photolysis time, and variation in salt concentration on the polymerization of CHO are discussed.

Photoinitiated Cationic Polymerization of p-methylstyrene by Methoxy Trityl Salts

Photoinitiated cationic polymerization of p-methylstyrene has been investigated in dichloromethane at 25°C, using stable, soluble, and nonhygroscopic di, tri and pentamethoxy trityl carbocationic salts, having non-nucleophilic anions such as SbF6 −, AsF6 −, PF6 −. The reactivity of these salts falls in the order: trimethoxy > dimethoxy > pentamethoxy. The effects of counter ion structure, salt concentration, photolysis time and photolysis wavelength on the polymerization rate are presented.

Technical note: Measuring tropospheric OH and HO<sub>2</sub> by laser-induced fluorescence at low pressure. A comparison of calibration techniques

Abstract. The hydroxyl radical (OH) is one of the most important oxidants in the atmosphere, as it is involved in many reactions that affect regional air quality and global climate change. Because of its high reactivity, measurements of OH radical concentrations in the atmosphere are difficult, and often require careful calibrations that rely on the production of a known concentration of OH at atmospheric pressure. The Indiana University OH instrument, based on the Fluorescence Assay by Gas Expansion technique (FAGE), has been calibrated in the laboratory using two different approaches: the production of OH from the UV-photolysis of water-vapor, and the steady-state production of OH from the reaction of ozone with alkenes. The former technique relies on two different actinometric methods to measure the product of the lamp flux at 184.9 nm and the photolysis time. This quantity derived from N2O actinometry was found to be 1.5 times higher than that derived from O2 actinometry. The water photolysis and ozone-alkene techniques are shown to agree within their experimental uncertainties (respectively 17% and 44%), although the sensitivities derived from the ozone-alkene technique were systematically lower by 40% than those derived from the water-vapor UV- photolysis technique using O2 actinometry. The agreement between the two different methods improves the confidence of the water-vapor photolysis method as an accurate calibration technique for HOx instruments. Because several aspects of the mechanism of the gas phase ozonolysis of alkenes are still uncertain, this technique should be used with caution to calibrate OH instruments.

Photocatalytic Hydrogen Production from Water Employing A Ru, Rh, Ru Molecular Device for Photoinitiated Electron Collection

A molecular device for photoinitiated electron collection at a metal center, [{(bpy)2Ru(dpp)}2RhCl2](PF6)5, has been studied and shown to photocatalytically produce H2 from H2O. This system absorbs visible light and in the presence of an electron donor is reduced by multiple electrons. In the presence of water, the [{(bpy)2Ru(dpp)}2RhCl2](PF6)5 system photocatalytically produces H2 that varies linearly with photolysis time. The photocatalyst functions in the presence of chloride, but the yield of H2 is lower, consistent with chloride loss being important in the photocatalysis cycle

Luminescence decay of broadband emission from CdS quantum dots

The stretched exponential photoluminescence decay of the energy-resolved broadband emission of purified and unpurified CdS quantum dots (QDs) made in reverse micelles is characterized as a function of photolysis time and thiol addition. Photolysis is found to proportionately increase both the lifetime and quantum yield of these QDs. This proportionality is consistent with a simple parallel channel model of the decay of the excited states. The ultimate QY of the purified sample is found to be as high as 24%, which is twice that previously reported for this preparation. At −70 °C both the QY and the lifetime increase by more than a factor of two, indicating that thermal quenching limits the QY at room temperature. Finally, the addition of alkanethiols is shown to red-shift and quench the emission while only modestly altering the lifetime. These thiolated QDs show an extremely large temperature dependence of QY, demonstrating stronger thermal quenching than the unfunctionalized QDs.

Aqueous photochemistry of triclosan: Formation of 2,4‐dichlorophenol, 2,8‐dichlorodibenzo‐p‐dioxin, and oligomerization products

The photochemical fate of the antimicrobial agent triclosan is presented. Experiments performed in both natural and buffered deionized water show that triclosan rapidly photodegrades by direct photolysis (t(1/2) = 5 h, pH 8, noon summer sunlight, 45 degrees N latitude). Both 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD) and 2,4-dichlorophenol (2,4-DCP) are produced. The 2,8-DCDD and 2,4-DCP also are photolabile and, thus, are intermediates. The yields for 2,8-DCDD and 2,4-DCP ranged from 3 to 12% depending on the conditions employed. When triclosan is photolyzed in the presence of Suwannee River (GA, USA) fulvic acid, a portion of the initial mass is recovered as insoluble material. Based on experiments in which the formation of insoluble material was monitored with photolysis time, it is postulated that photolysis in natural waters leads to some of the triclosan being coupled to humic matter. Triclosan also reacts rapidly with both singlet oxygen (k(rxn) = 1.07 +/- 0.03 x 10(8) M(-1) s(-1) in water of pH 10) and hydroxyl radical (k(*OH) = 5.4 +/- 0.3 X 10(9) M(-1)(s-1). Indirect photolysis pathways, however, are not expected to be important because of low steady-state concentrations of reactive oxygen species in natural waters and the efficiency of the direct photolysis of triclosan.

Higher oxides of chlorine: absorption cross-sections of Cl 2O 6 and Cl 2O 4, the decomposition of Cl 2O 6, and the reactions of OClO with O and O 3

The absorption cross-sections of Cl 2O 6 and Cl 2O 4 have been obtained using a fast flow reactor with a diode array spectrometer (DAS) detection system. The absorption cross-sections at the wavelengths of maximum absorption ( λ max) determined in this study are those of Cl 2O 6: (1.47±0.15)×10 −17 cm 2 molecule −1, at λ max=276 nm and T=298 K; and Cl 2O 4: (9.0±2.0)×10 −19 cm 2 molecule −1, at λ max=234 nm and T=298 K. Errors quoted are two standard deviations together with estimates of the systematic error. The shapes of the absorption spectra were obtained over the wavelength range 200–450 nm for Cl 2O 6 and 200–350 nm for Cl 2O 4, and were normalized to the absolute cross-sections obtained at λ max for each oxide, and are presented at 1 nm intervals. These data are discussed in relation to previous measurements. The reaction of O with OClO has been investigated with the objective of observing transient spectroscopic absorptions. A transient absorption was seen, and the possibility is explored of identifying the species with the elusive sym-ClO 3 or ClO 4, both of which have been characterized in matrices, but not in the gas-phase. The photolysis of OClO was also re-examined, with emphasis being placed on the products of reaction. UV absorptions attributable to one of the isomers of the ClO dimer, chloryl chloride (ClClO 2) were observed; some Cl 2O 4 was also found at long photolysis times, when much of the ClClO 2 had itself been photolysed. We suggest that reports of Cl 2O 6 formation in previous studies could be a consequence of a mistaken identification. At low temperatures, the photolysis of OClO leads to the formation of Cl 2O 3 as a result of the addition of the ClO primary product to OClO. ClClO 2 also appears to be one product of the reaction between O 3 and OClO, especially when the reaction occurs under explosive conditions. We studied the kinetics of the non-explosive process using a stopped-flow technique, and suggest a value for the room-temperature rate coefficient of (4.6±0.9)×10 −19 cm 3 molecule −1 s −1 (limit quoted is 2 σ random errors). The photochemical and thermal decomposition of Cl 2O 6 is described in this paper. For photolysis at λ=254 nm, the removal of Cl 2O 6 is not accompanied by the build up of any other strong absorber. The implications of the results are either that the photolysis of Cl 2O 6 produces Cl 2 directly, or that the initial photofragments are converted rapidly to Cl 2. In the thermal decomposition of Cl 2O 6, Cl 2O 4 was shown to be a product of reaction, although not necessarily the major one. The kinetics of decomposition were investigated using the stopped-flow technique. At relatively high [OClO] present in the system, the decay kinetics obeyed a first-order law, with a limiting first-order rate coefficient of 0.002 s −1.

First Identification of an Aliphatic cis-α,β-Dinitroso Compound: A Combined Experimental and Quantum Chemical Study

A combination of experimental and quantum chemical results has been used to characterize dimethylfuroxan, (CH3)2C2N2O2, isolated in a matrix of solid Ar atoms at 12 K, and then to study its photoinduced isomerization and decomposition. The less-stable isomer 2,3-dinitrosobut-2-ene can be generated by UV photolysis (λmax = 254 nm). However, this species is photolabile and decomposes upon prolonged photolysis times with light having λmax = 254 nm or with IR radiation (λmax = 700 nm) to give CH3CNO as the final, photostable product. The two photoproducts were characterized using a combination of experimental and quantum chemical results. The work presented herein sheds light on the physical and chemical properties of furoxans, which may gain considerable importance as a source of NO, which itself plays a significant role as a biological messenger.

Photo electrochemical study of ferrioxalate actinometry at a glassy carbon electrode

An alternative photo electrochemical approach of potassium ferrioxalate actinometry has been investigated via the amperometric detection of product (Fe 2+) formation at a glassy carbon electrode. The corresponding oxidation current response of [Fe 2+] formed due to the photolysis of ferrioxalate, was monitored as a function of illumination time, which allowed the intensity of the illumination source to be determined. To validate this approach a parallel measurement to calculate the light flux was also carried out via monitoring the reactant (i.e. Fe 3+) disappearance with photolysis time.

Matrix removal for the ion chromatographic determination of some trace elements in milk

An ion chromatographic method has been developed for the determination of some microelements in different types of milk. It involves oxidative photodegradation of the organic matrix with H 2O 2 in a UV digester, equipped with a high-pressure mercury lamp. The temperature of the sample is maintained at 85±5 °C by a combined air/water cooling system. This procedure provides an efficient alternative to traditional dry ashing and wet digestion methods. Milk degrades in less than 2 h, while inorganic constituents, except for iodide, nitrate, nitrite, sulfite and manganese (II), are unaffected by UV radiation. Depending upon the type of milk (whole, skimmed, powdered, evaporated, etc.) to be analysed, the amount of sample and the UV photolysis time can be adjusted as per requirements. The clear solution resulting from the UV digestion is diluted, filtered and injected onto an ion chromatograph equipped with both conductivity and variable-wavelength UV-Vis detectors. The method has been tested with standards and real milk samples and has been found to be satisfactory for the determination of total chloride, bromide, phosphorus (as phosphate) and sulfur (as sulfate), and of copper, nickel, zinc, cobalt, iron and lead.

Photochemical reactivity of sulfamethoxazole and other sulfa compounds with photodiode array detection

The photochemical activities of six sulfa compounds [sulfacetamide (CET), sulfadiazine (DIA), sulfaguanidine (GUA), sulfamerazine (MER), sulfamethoxazole (SMX), and sulfamethizole (MET)] under different experimental conditions such as photolysis time, solvent and buffer pH are investigated by photodiode array (PDA) spectrophotometry. With no photolysis, the sulfa drugs CET and DIA show no absorbance at 332 nm and the other compounds only modest absorbance. Upon photolysis for 4 min, absorbance enhancements at 332 nm of three to four times for GUA and MET and 12–15 times for SMX and MER are observed. For CET and DIA after photolysis, the (absorbance) l/mg is now approximately 0.01–0.02 units. Although two pH optima of approximately 3–4 and 7 are noted, the optimum solvent for photolysis is ethanol without pH adjustment. For flow injection (FI) with on-line photolysis and PDA detection, a mobile phase of 100% ethanol with a step flow rate from 0.1 to 1 ml/min is used providing a 4-min reaction time. The FI detection limit for SMX with photolysis at 330 nm is 1 mg/l. The relative standard deviation data ( n=4) of seven individual points in a calibration curve from 5 to 150 mg/l are 0–4%. The recovery of SMX from pharmaceutical tablets is 99.7% indicating no interference from trimethoprim which is not photochemically active.

A highly efficient photochemical bromination as a new method for preparation of mono, bis and fused pyrazole derivatives.

N-Bromosuccinimide (NBS) was found to afford photochemical bromination of N-substituted 3-methyl-2-pyrazolin-5-one in chloroform solution. The nature of the products formed was found to be highly dependent on the photolysis time and on the type of N-substituted 3-methyl-2-pyrazolin-5-one. The reaction of substituted 2-pyrazolin-5-ones with NBS in the absence of light yielded in all cases a mixture of N-substituted 3-methyl-4-bromo- (and -4,4-dibromo-) 2-pyrazolin-5-ones. The mechanism of photobromination was illustrated. Separation of all of the products was achieved using column chromatography. The chemical structure of all of the products was assigned by elemental analysis, IR, 1H NMR and GC-MS measurements.

CO + OH → CO2 + H: The relative reaction rate of five CO isotopologues

The reaction of carbon monoxide with the hydroxyl radical (CO + OH) plays a central role in tropospheric chemistry. While the analysis of stable isotope enrichment has been used to refine models of the sources and sinks of atmospheric CO and CO2, less is known about the mechanism behind the enrichment. We have initiated the present work to provide a larger set of experimental observations of the effect. We report the OH radical reaction rate of 12C16O relative to 13C16O, 12C18O and 13C18O and the reaction rate of 12C17O relative to 12C18O at 295 K. At 1013 mbar the relative rates are 0.960 ± 0.014, 0.943 ± 0.019, 0.948 ± 0.024 and 0.996 ± 0.0067 respectively (klight/kheavy, 2σ errors indicated); results at 506 and 66 mbar are also reported. The hydroxyl radical was generated by the UV photolysis of ozone in the presence of water. The concentration of the carbon monoxide isotopologues as a function of photolysis time was determined using a global fit of the rovibrationally resolved FTIR spectrum of the gas mixture in the reaction cell. The observed inverse kinetic isotope effect is best understood in terms of the effect of isotopic substitution on the relative rate of unimolecular dissociation of the HOCO intermediate to reform reagents versus dissociation to products.

PHOTODEGRADATION OF St-MMA COPOLYMERS: CORRELATION OF DEGRADATION, STYRENE CONTENT, AND PHOTOPHYSICS

A series of styrene(St)-methyl methacrylate(MMA) copolymers with compositions ranging from 0 to 100% St were synthesized and exposed to UV light for various periods of time. The changes in yellowing (yellowness index—YI) and absorbance of exposed films were evaluated as a function of photolysis time and St content. Both the changes in YI and absorbance at 400 nm were found to increase dramatically for polymer films with styrene (St) content greater than about 50%. A direct correlation between increases in the YI and absorbance at 400 nm with mean styrene sequence length was found with a maximum change being obtained for copolymers with mean sequence length of approximately 5 St units or greater. A small absorbance shoulder at 270–290 nm was observed for copolymers with St content less than 30% after relatively short irradiation times (e. g. 2 hours). Excimer formation of copolymers in dilute solution was found to increase with an increase in St content and St sequence length; in films, a plateau in the relative excimer emission was attained with a St content of 60%, corresponding to a mean St sequence length of 2.

Femtosecond Multicolor Pump−Probe Spectroscopy of Ferrous Cytochrome c

Ligand photolysis and subsequent electronic and structural relaxation, followed by ligand recombination, in ferrous cytochrome c (cyt c) have been studied using ultrafast laser spectroscopy. A broad-band white-light continuum, generated by amplified pulses from a Ti:sapphire laser, was used to monitor the transient absorption spectra of cyt c in the Soret and Q bands following 50-fs pulsed photoexcitation at 400 nm. The reconstructed photoproduct absorption spectrum is found to closely resemble that of a model pentacoordinate histidine-ligated complex, microperoxidase (MP-8), suggesting methionine photolysis. Vibrational modes at ∼40, ∼80, and ∼220 cm-1 are observed in femtosecond coherence spectroscopy (FCS) measurements, which also indicates photodissociation of the methionine ligand. The quantum yield of ligand photolysis is found to be ≥80%, which is consistent with the ultrafast photolysis time constant (≤40 fs) needed to induce coherent oscillations in the FCS measurements. The combination of high quantum yield and short time constant helps to resolve the longstanding question of the origin of the short lifetime (τe) and large Soret-state electronic damping factor (Γe) previously found in cytochrome c. We propose a simple multilevel model to describe the observed experimental data. The global analysis of the measured kinetics leads to a characterization of the major kinetic rates, including the 6.2-ps geminate rebinding of methionine to the heme iron.

Photochromic Mechanism of Heterofused Benzopyran Derivatives

ABATRACT The photochromic behavior of 2-phenyl-2-(thien-2-yl)-2H-benzo [b]furano[3,2-f]benzopyran (1) and 2,2-diphenyl-2H-benzo[b] furano[3,2-f]benzopyran (2) was examined by the nanosecond laser photolysis and normal time scale analysis. The results showed that the thienyl group could stabilize the colored forms as compared with the phenyl group and both singlet and triplet states were involved in the formation of the colored forms.

Photodegradation of metolachlor: isolation, identification, and quantification of monochloroacetic acid.

The photolysis of metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] in a sunlight simulator under actinic radiation was investigated. The focus of the study was to determine the extent of monochloroacetic acid (MCA) production. MCA was concentrated and derivatized from photolysate as the n-propyl ester using propanol and sulfuric acid and then identified as the ester using GC/MS and GC/ECD. On the basis of regression analysis, it was shown that the direct photodegradation of approximately 10 microM metolachlor followed pseudo-first-order kinetics with respect to the metolachlor concentration, and the half-life of the herbicide ( approximately 74 h) was independent of the pH of the medium. Photolysis in synthetic field water (SFW) resulted in a significant reduction of photolysis time (t(1/2) approximately 9 h). Direct photolysis experiments indicate a 5.19 +/- 0.81% (n=3) conversion of metolachlor to MCA, while photolysis in synthetic field water and in a Don River water sample resulted in 29.8 +/- 4.6% (n = 3) and 12.6 +/- 4.1% (n = 3) conversion, respectively; MCA was shown to be hydrolytically stable over the time course of the photoreaction. The photodegradation of alachlor, butachlor and a model chloroacetanilide, 2-chloro-N-methylacetanilide, in SFW were also investigated.

Photoinitiated polymerization of cyclohexene oxide by pentacarbonyl rhenium(I) halides

Pentacarbonylrhenium(I) halides, Re(CO)5X(X = Cl, Br or I), initiate the ring-opening polymerization of cyclohexene oxide photochemically without a co-catalyst at room temperature. The rate of polymerization is faster for X = Cl than it is for X = Br or I under comparable conditions. Replacement of the carbonyl ligand by pyridine or triphenylphosphine deactivates the catalyst. The effects of the halide ligand (X), photolysis times, photolysis light wavelength, catalyst concentration, and solvent polarity on the polymerization yield are reported and the mechanism of polymerization is suggested.