Quantum Yields Of Disappearance Research Articles

Mechanistic Aspects Regarding the Ultraviolet Degradation of Polychlorinated Biphenyls in Different Media: Insights from Carbon and Chlorine Isotope Fractionation.

In this study, the carbon and chlorine isotope fractionation during ultraviolet-photolysis of polychlorinated biphenyls (PCBs, including PCB18, PCB77, PCB110, and PCB138) in n-hexane (Hex), methanol/water (MeOH/H2O), and silica gel was first investigated to explore their mechanistic processes. We observed a significant variation in ΛCl-C (εCl/εC) for the same PCBs in different photochemical systems, implying that PCB degradation processes in various photoreaction systems could differ. Although all substrates showed normal apparent carbon/chlorine kinetic isotope effects (C-/Cl-AKIE >1), the putative inverse C-AKIE of nondechlorinated pathways was suggested by 13C depletion of the average carbon isotope composition of PCB138 and corresponding dechlorinated products in MeOH/H2O, which might originate from the magnetic isotope effect. Significant negative correlations were found between C-AKIE and relative disappearance quantum yields ("Φ") of ortho-dechlorinated substrates (PCB18, PCB110, and PCB138) in Hex and MeOH/H2O. However, the C-AKIE and "Φ" of PCB77 (meta/para-dechlorinated congener) obviously deviated from the above correlations. Furthermore, significantly different product-related carbon isotope enrichment factors of PCB77 in Hex were found. These results demonstrated the existence of dechlorination position-specific and masking effects in carbon isotope fractionations.

Kinetic study of photocatalytic degradation of the emerging contaminant bisphenol A using N–TiO2 in visible light: a study of the significance of dissolved oxygen

This study compares the tracking of bisphenol A (BPA) degradation by three analytical techniques: liquid chromatography mass spectrometry (HPLC–MS), ultraviolet/visible (UV–Vis) spectroscopy and dissolved oxygen (DO) content. In each evaluation method, the photochemical behavior of TiO2 and N–TiO2 under visible light confirmed the relationship between the adsorption and photodegradation processes, with the reaction analysis indicating the possibility of two photocatalytic mechanisms: conventional photocatalyzed radical oxidation and lattice oxygen driven oxidation. The first one concerns HPLC, where the average half-lives (t1/2) were 38 min (5% N–TiO2), 60 min (1% N–TiO2) and 64 min (TiO2), these values vary in 12% if compared with the values obtained by UV–Vis spectroscopy. The second mechanism showed a decrease in the initial concentration by more than 50% (8 mg L−1) after 3 h. The solution of doped photocatalysts is tracked best by DO measurement. The results presented here confirm that the efficiency of photocatalytic oxidation (EPO) of a reaction is highly related to the DO content, given that dissolved O2 proactively causes the release of radicals on the surface of excited materials under the action of visible light, thus increasing the BPA degradation rate. The quantum yield of BPA disappearance was below 0.1 for all the materials.

Photochemical transformation of the thyroid hormone levothyroxine in aqueous solution

The direct aqueous photolysis of the thyroid hormone levothyroxine (T(4)) has been studied. One of the major photoproducts, i.e., 4-[4-(2-amino-2-carboxy-ethyl)-2,6-diiodo-phenoxy]-penta-2,4-dienoic acid (P1), was isolated by liquid chromatography and structurally assigned by mass spectrometric (MS) and nuclear magnetic resonance spectroscopic methods. The identity of a second major product, i.e., 3,5-diiodo-L: -thyrosine (P3), was confirmed through access to a commercially available standard. Furthermore, the structures of three additional transformation products are proposed on the basis of data obtained by high-resolution MS analyses. UV absorption spectra were determined for T(4) and the two photoproducts P1 and P3. Disappearance quantum yields were calculated for T(4) (ϕ = 0.014 at pH12) and P3 (ϕ = 0.024 at pH12 and ϕ = 0.010 at pH8.5), whereas the compound P1 was found to be stable under the studied conditions (T(1/2) = 600min). The results indicate that solar UV light may have a significant impact on the fate of T(4) in the aquatic environment.

ChemInform Abstract: The Photochemistry of α-Phenoxyacetophenones Investigated by Flash-CIDNP-Spectroscopy.

The photochemistry of 14 different substituted α-phenoxyacetophenones (1) was studied by CIDNP-spectroscopy using an XeCl excimer laser (308 nm) and a 250 MHz 1H-NMR spectrometer. To characterize the excited states the electronic absorption and emission spectra as well as disappearance quantum yields were investigated also. – Two reaction channels were established: 1) β-cleavage from the singlet state; and 2) the formation of acetophenones via the respective enol forms where the origin of the polarization remains unclear. It is concluded that these are the main reaction channels, since all products formed show polarizations in the CIDNP-spectra. – In contrast to the current assumptions for the typical acetophenone reaction of (1) leading to the β-cleavage, we are led to interprete the CIDNP-spectra of (1) as a result of a reaction due to the phenoxy part with all characteristics of a photo-Claisen reaction.

Photoinduced reduction of chromium(vi) by iron aminopolycarboxylate complex (FeNTA)

Chromium(VI) reduction photoinduced by iron(III) nitrilotriacetate (FeNTA) was investigated under monochromatic excitation. 313 nm was used as the irradiation wavelength in order to minimize the absorption of the light by chromium(VI): 91% of the photons were absorbed by FeNTA at pH = 7.0. Quantum yields of FeNTA and chromium(VI) disappearance and iron(II) formation were measured at pH 2.0, 4.0 and 7.0. In all the cases, chromium(VI) reduction follows first order kinetics with maximum efficiency at pH 2.0. The observed rate constant is proportional to FeNTA concentrations up to a maximum of 6.0 x 10(-4) mol L(-1). The effect of oxygen was also investigated. If there is no large pH effect and no effect of oxygen on the quantum yields of chromium(VI) and FeNTA disappearance, the pH strongly influences the nature of the reduced chromium species. At pH 2.0, only chromium(III) was detected, whereas at pH 4.0 and 7.0 no chromium(III) resulting from chromium(VI) reduction was observed. Chromium(V) is supposed to be formed and stabilized by the chelating groups of NTA or NTA photoproducts.

A Photochemical Study of Diclofenac and Its Major Transformation Products

The photolytic transformation of 2-(2-(2,6-dichlorophenylamino)phenyl)acetic acid (diclofenac) and its transformation products (TPs) (8-chloro-9H-carbazol-1-yl) acetic acid (Cz1), 2-(2-chloro-phenylamino)-benzaldehyde (Ald) and (1,4-dioxo-4,9-dihydro-1H-carbazol-8-yl) acetic acid (Cz4) in aqueous solutions have been studied. The previously unreported TP (Cz4) was isolated by LC and completely characterized by NMR and MS. UV-absorption spectra of diclofenac and three of its TPs were determined and used to calculate disappearance quantum yields. The dominating transformation pathway of diclofenac occurs via initial formation of Cz1 and proceeds to form mainly (Cz4) over 200 min of UV-irradiation. A second minor transformation pathway, which yields Ald as the sole product, was observed only under deaerated conditions.

A comparative study on the aqueous photodegradation of two organophosphorus pesticides under simulated and natural sunlight

Aqueous solutions of fenitrothion and methyl parathion were photochemically degraded in an Atlas Suntest solar simulator (500 W m(-2)) as well as under ambient sunlight at Lancaster University (June & August 2007, 54 degrees N) and the degradation kinetics and disappearance quantum yields are reported. Fenitrothion degradation confirmed to first order kinetics (r(2) = 0.90-0.99) with a half-life range of 4.9 h-5.3 h, shorter than previously reported studies. In contrast, methyl parathion did not show significant degradation over the duration of these experiments. Light irradiances were monitored with a spectroradiometer during the Suntest simulator and outdoor experiments. The filtered-xenon arc lamp of the Suntest yielded spectral irradiances comparable to natural sunlight in the UVB and UVA wavelength range (280-400 nm), but with higher irradiances in the visible region (400-750 nm). Nonetheless, as both compounds have light absorption spectra at wavelengths < 400 nm, then the half-lives and disappearance quantum yields were similar between the Suntest and natural sunlight, and demonstrated that the Suntest is suitable for environmentally-relevant photochemical degradation experiments.

Photochemical and photophysical properties of lumazine in aqueous solutions

Lumazine (pteridine-2,4(1 H,3 H)-dione, LU) was investigated for its efficiency of singlet oxygen ( 1O 2) production and quenching in aqueous solution. The quantum yield of 1O 2 production ( Φ Δ) was determined by measurements of the 1O 2 luminescence in the near-infrared upon continuous excitation of the sensitizer. Values of Φ Δ are sensitive to the pH and were found to be 0.44 ± 0.01 and 0.080 ± 0.004 in acidic and alkaline media, respectively. The photochemical stability of LU was investigated under different pH conditions, in the presence and in the absence of O 2. The photochemical consumption of LU in aqueous solution at room temperature under irradiation at 350 nm was followed by UV–vis spectrophotometry and HPLC. Values of the quantum yields of LU disappearance are low, indicating that LU is rather photostable under physiological conditions.

PHOTOLYSIS OF PHENANTHRENEQUINONE IN PHOSPHOLIPID VESICLES

— Phospholipid vesicles of saturated and unsaturated phosphatidylcholines have been prepared with phenanthrenequinone incorporated into the hydrocarbon region of the lipid bilayer. Blue or ultraviolet light exposure of these vesicles causes a loss of quinone absorbance and the corresponding formation of new absorbance bands due to addition products. In vesicles composed of unsaturated phospholipids, cycloaddition is preferred over R-H addition. The quantum yield for quinone disappearance is -0.25 in vesicles which contain unsaturated phospholipids and decreases to 0.08 when the photolysis is conducted below the lipid characteristic temperature.

Photochemical Ligand Substitution Reactions of fac-[Re(bpy)(CO)3Cl] and Derivatives

Excitation by high-energy light, such as that of 313 nm wavelength, induces a photochemical ligand substitution (PLS) reaction of fac-[Re(bpy)(CO)3Cl] (1a) to give the solvento complexes (OC-6-34)- and (OC-6-44)-[Re(bpy)(CO)2(MeCN)Cl] (2 and 3) in good yields. The disappearance quantum yield of 1a was 0.01+/-0.001 at 313 nm. The products were isolated, and X-ray crystallographic analysis was successfully performed for 2. Time-resolved IR measurements clearly indicated that the CO ligand dissociates with subpicosecond rates after excitation, leading to vibrationally hot photoproducts, which relax within 50-100 ps. Detailed studies of the reaction mechanism show that the PLS reaction of 1a does not proceed via the lowest vibrational level in the 3MLCT excited state. The PLS reaction gives 2 and (OC-6-24)-[Re(bpy)(CO)2(MeCN)Cl] (5) as primary products, and one of the products, 5, isomerizes to 3. This type of PLS reaction is more general, occurring in various fac-rhenium(I) diimine tricarbonyl complexes such as fac-[Re(X2bpy)(CO)3Cl] (X2bpy=4,4'-X2-bpy; X=MeO, NH2, CF3), fac-[Re(bpy)(CO)3(pyridine)]+, and fac-[Re(bpy)(CO)3(MeCN)]+. The stable photoproducts (OC-6-44)- and (OC-6-43)-[Re(bpy)(CO)2(MeCN)(pyridine)]+ and (OC-6-32)- and (OC-6-33)-[Re(bpy)(CO)2(MeCN)2]+ were isolated. The PLS reaction of rhenium tricarbonyl-diimine complexes is therefore applicable as a general synthetic method for novel dicarbonyls.

Photophysics and photochemistry of tetrahydrocurcuminoids

The photochemical properties of 10 tetrahydrocurcuminoids (THCs) have been studied for the first time. They display very low fluorescence in ethanol solution at room temperature with quantum yields between 0.9 and 13 × 10 −3 whereas their phosphorescence quantum yields in ethanol glass at 77 K are between 0.025 and 0.5. Their disappearance quantum yield measured in ethanol solution, between 5 × 10 −4 and 10 −2, follows the same trend as their antiradical power efficiency (ARP) previously measured. The reactivity of THCs is enhanced if they include a phenol group in meta or para of the linking chain and a phenol or methoxy group as neighbor. UV absorption difference spectra of the irradiated solutions, HPLC and GC–MS analyses performed on 4-propylguaiacol (4PG), which displays similar behavior as tetrahydrocurcumin (THC2) indicates the formation of quinone methides and ortho-quinone. The formation of a phenoxy radical by photooxidation appears to be the first reaction step. This demonstrates that the general photodegradation mechanism of phenols found for guaiacyl lignins applies also to simplest molecules of natural origin such as THCs. The irradiation of the THCs in carboxymethylcellulose films follows the same trends as the solution studies, except that the quinone methides and ortho-quinones are not detected, probably because they react with the cellulosic matrix. The knowledge obtained in this study probably will help to promote the use of tetrahydrocurcuminoids in various applications where it is necessary to develop antioxidant properties in presence of solar light.

Diastereoselectivity and Site Dependency in the Photochemistry of Ketoprofen in the Bovine Serum Albumin Matrix†

The photodegradation of the S(+)- and R(-)-ketoprofen (KP) enantiomers in the bovine serum albumin matrix was studied by steady-state photolysis with the use of lambda(irr) > 320 nm and transient absorption spectroscopy with lambda(exc) = 355 nm, at 1/1 and 2/1 KP/BSA molar ratios. R(-)-KP was found to be more labile than S(+). Triplet ketoprofen species were evidenced with lifetimes of 400 ns for S(+) and 600 ns for R(-)-KP. Further longer-lived transients with lifetimes of 2.6 and 6.0 mus for S(+) and R(-), respectively, were detected. On the basis of the binding constants of the drug enantiomers to the two main binding sites of the protein, obtained from circular dichroism experiments, the individual disappearance quantum yields of the 1:1 and 2:1 diastereomeric KP:BSA complexes could be estimated. The photoreactivity in the BSA matrix was rationalized on the basis of diastereoselective photodecarboxylation in the two main protein sites.

Photochemical Behavior of 6‐Methylpterin in Aqueous Solutions: Generation of Reactive Oxygen Species¶

ABSTRACTPterins are a family of heterocyclic compounds present in a wide range of living systems that participate in relevant biological functions and are involved in different photobiological processes. 6‐Methylpterin (MPT) was investigated for its efficiency of singlet‐oxygen (1O2) production and quenching in aqueous solution. The quantum yields of 1O2 production (φΔ) was determined by measurements of the 1O2 luminescence in the near‐infrared upon continuous excitation of the sensitizer. Values of φΔ were found to be 0.10 ± 0.02 and 0.14 ± 0.02 in acidic and alkaline media, respectively. Studies of the photo‐oxidation of MPT in acidic (pH = 5.0–6.0) and alkaline (pH = 10.2–10.8) aqueous solutions at 350 nm and room temperature have been performed. The photochemical reactions were followed by UV‐visible spectrophotometry, high‐performance liquid chromatography and an enzymatic method for H2O2 determination. MPT is not light sensitive in the absence of oxygen, but it undergoes a photooxidation reaction in the presence of oxygen, yielding several nonpteridinic products. The quantum yields of MPT disappearance were determined and values of 2.4 (±0.5) × 10−4 and 8.1 (±0.8) × 10−4 were obtained in acidic and alkaline media, respectively. H2O2 was detected and quantified in irradiated solutions of MPT. The rate constant of the chemical reaction between 1O2 and MPT (kr) was determined to be 4.9 × 106M−1 s−1 in alkaline medium and the role of 1O2 in the photooxidation of MPT is discussed.

Photochemical behaviour of 6-methylpterin in Aqueous Solutions: Generation of Reactive Oxygen Species

Pterins are a family of heterocyclic compounds present in a wide range of living systems that participate in relevant biological functions and are involved in different photobiological processes. 6-Methylpterin (MPT) was investigated for its efficiency of singlet-oxygen (1O2) production and quenching in aqueous solution. The quantum yields of 1O2 production (phi(delta)) was determined by measurements of the 1O2 luminescence in the near-infrared upon continuous excitation of the sensitizer. Values of phi(delta) were found to be 0.10 +/- 0.02 and 0.14 +/- 0.02 in acidic and alkaline media, respectively. Studies of the photooxidation of MPT in acidic (pH = 5.0-6.0) and alkaline (pH = 10.2-10.8) aqueous solutions at 350 nm and room temperature have been performed. The photochemical reactions were followed by UV-visible spectrophotometry, high-performance liquid chromatography and an enzymatic method for H2O2 determination. MPT is not light sensitive in the absence of oxygen, but it undergoes a photooxidation reaction in the presence of oxygen, yielding several nonpteridinic products. The quantum yields of MPT disappearance were determined and values of 2.4 (+/-0.5) x 10(-4) and 8.1 (+/-0.8) x 10(-4) were obtained in acidic and alkaline media, respectively. H2O2 was detected and quantified in irradiated solutions of MPT. The rate constant of the chemical reaction between 1O2 and MPT (k(r)) was determined to be 4.9 x 10(6) M(-1) s(-1) in alkaline medium and the role of 1O2 in the photooxidation of MPT is discussed.

Generation of Reactive Oxygen Species during the Photolysis of 6‐(Hydroxymethyl)pterin in Alkaline Aqueous Solutions

AbstractPhotochemical studies of the reactivity of 6‐(hydroxymethyl)pterin (=2‐amino‐6‐(hydroxymethyl)pteridin‐4(1H)‐one; HPT) in alkaline aqueous solutions (pH 10.2–10.8) at 350 nm and room temperature were performed. The photochemical reactions were followed by UV/VIS spectrophotometry, thin‐layer chromatography (TLC), high‐performance liquid chromatography (HPLC), and an enzymatic method for H2O2 determination. In the presence of O2, 6‐formylpterin (=2‐amino‐3,4‐dihydro‐4‐oxopteridine‐6‐carboxaldehyde; FPT) was the only photoproduct detected. In the absence of O2, we observed a compound with an absorbance maximum at 480 nm, which was oxidized very rapidly by O2 in a dark reaction to yield FPT. The quantum yields of substrates disappearance and of photoproducts formation were determined. The formation of H2O2 during photooxidation was monitored, and the number of mol of H2O2 released per mol of HPT consumed corresponded to a 1 : 1 stoichiometry. HPT was also investigated for efficiency of singlet‐oxygen (1O2) production and quenching in aqueous solution. The quantum yield of 1O2 production (ΦΔ=0.21±0.01) was determined by measurements of the 1O2 luminescence in the near‐IR (1270 nm) upon continuous excitation of the sensitizer. The rate constant of 1O2 total quenching by HPT was determined (kt=3.1⋅106 M−1 s−1), indicating that this compound was able to quench 1O2. However, 1O2 did not participate in the photooxidation of HPT to FPT.

Studies on the photochemistry of 1,7-diphenyl-1,6-heptadiene-3,5-dione, a non-phenolic curcuminoid model.

The comparative photostability of curcumin 1, and two non-phenolic curcuminoids: 1,7-diphenyl-1,6-heptadiene-3,5-dione 2 (unsubstituted curcumin) and dimethylcurcumin 3 in non-degassed dilute solutions (approximately 3-5 x 10(-5) mol l(-1)) has been established by UV-visible absorption spectroscopy; disappearance quantum yields were measured. The similar behavior of the three studied curcuminoids is indicative of only a moderate role of phenol groups in the photodegradation process. Structural analysis of the photodegradation products of compound 2 in more concentrated solution (approximately 3.6 x 10(-3) mol l(-1)) shows formation of benzaldehyde, cinnamaldehyde, 2'-hydroxy-5',6'-benzochalcone 4, flavanone 5 and some other unidentified photoproducts. Flavanone 5 is formed by irradiation of chalcone 4. It represents a unique example of photochemical conversion of a diarylheptanoid molecule into a flavonoid, another very important class of natural products.

Photochemical transformations of tetrabromobisphenol A and related phenols in water

A method was developed for studies of the phototransformation at UV irradiation of aqueous solutions of tetrabromobisphenol A (TBBPA), tribromobisphenol A (TriBBPA), tetrachlorobisphenol A (TCBPA), 2,4-dichlorophenol at various pHs as well as 2-chlorophenol, 2-bromophenol, 3,4-dichlorophenol and bisphenol A at pH 11. The absorbance spectra of the compounds and the emission spectra of the light-source were determined and used to calculate disappearance quantum yields of the photochemical reactions that were taking place. No major differences between the disappearance quantum yields of TBBPA and TCBPA were observed at pH 10, while the disappearance quantum yield of TriBBPA was approximately two times higher. The rate of decomposition of TBBPA was six times higher at pH 8 than at pH 6. Identification of the degradation products of TBBPA and TriBBPA, by GC–MS analysis and by comparison to synthesised reference compounds, indicated that TBBPA and TriBBPA decompose via different mechanisms. Three isopropylphenol derivatives; 4-isopropyl-2,6-dibromophenol, 4-isopropylene-2,6-dibromophenol and 4-(2-hydroxyisopropyl)-2,6-dibromophenol, were identified as major degradation products of TBBPA while the major degradation product of TriBBPA was tentatively identified as 2-(2,4-cyclopentadienyl)-2-(3,5-dibromo-4-hydroxyphenyl)propane.

Degradation of amitrole by excitation of iron(III) aquacomplexes in aqueous solutions

The induced degradation of amitrole (3-amino-1,2,4-triazole or aminotriazole) by excitation of iron(III) aquacomplexes was investigated under irradiation at λ=365 nm. The process mainly involved the predominant monomeric species of iron(III), namely Fe(OH) 2+, which leads to the formation of hydroxyl radicals upon excitation in the UV-Vis region. Competitive reactions experiments using nanosecond laser flash photolysis were undertaken for the determination of the second-order rate constant for the reaction of amitrole with hydroxyl radical. A value of 1.5×10 9 mol −1 l s −1 was obtained. The quantum yield of amitrole disappearance upon irradiation of the mixture amitrole/iron(III) increased with increasing oxygen concentration and was found to be independent on iron(III) and amitrole concentration. The high performance liquid chromatography (HPLC) analyses showed the formation of two main photoproducts: 5-hydroxy amitrole and urazole (2-aminothiazole). The latter product accumulated in the solution while the former rapidly disappeared with the irradiation time. Efficient mineralisation of the solutions was obtained by artificial as well as by sunlight irradiations.

Photodegradation of chlortoluron sensitised by iron(III) aquacomplexes

The degradation of chlortoluron photoinduced by Fe(III) aquacomplexes has been investigated under continuous irradiation at 365 nm. The determined quantum yields of chlortoluron disappearance give evidence for the involvement of the most photoactive species Fe(OH) 2+, which leads to the formation of hydroxyl radicals and Fe(II); the importance of oxygen in the degradation process was also demonstrated. According to the nature of the obtained photoproducts, the attack of OH radicals on two sites of chlortoluron are favoured: the aromatic ring and the methyl group of the urea function. A homogenous photocatalytic process based on the oxidation of Fe(II) into Fe(III) is responsible for the continuous production of hydroxyl radicals in such a system. This photocatalytic cycle allows the total mineralisation of compounds like chlortoluron using conditions compatible with a safe aquatic environment.

Near UV quantum yields for rotenone and piperonyl butoxide.

Rotenone and piperonyl butoxide (PBO) mixtures, so-called "synergized" rotenone, are invaluable in fisheries management where they are used to protect the habitat of endangered, native species and promote desirable gamefish populations. Continued use of synergized rotenone is threatened by inadequate control of persistence in surface water, especially where drinking water supplies are impacted. The photochemical kinetics of these chemicals were studied in the laboratory with a goal to better understand their fate in natural water. Disappearance quantum yields (phi) were determined in polychromatic light from fluorescent lamps emitting maximally at 350 nm. Rotenone, PBO and trifluralin, an actinometer, were irradiated as aqueous solutions at 25 or 50 microg L(-1) and the piscicides were determined by electrospray-liquid chromatography-mass spectrometry (ESI-LC-MS). In the photoreactor rotenone and PBO photodegraded with first-order half-lives of 500 and 220 min, respectively, and corresponding quantum yields of 0.00015 and 0.034. Rotenone absorbs sunlight strongly, while PRO does not. Differences in spectal overlap tended to counteract the disparities in phi and, in general, mathematical modeling indicates moderately rapid direct photolysis rates for both substances in surface water.