Quantum Yield Of Photooxidation Research Articles

Evaluation of the photosensitizer activity of hydrophobic phosphorus(V)porphyrins using the absorption spectral change of 1-benzyl-1,4-dihydronicotinamide

Under visible light irradiation, water-insoluble P(V)porphyrins oxidized 1-benzyl-1,4-dihydronicotinamide (BNADH), a model compound for nicotinamide adenine dinucleotide, and diminished the typical absorption of BNADH at around 340 nm. A singlet oxygen quencher, sodium azide, partially inhibited photosensitized BNADH oxidation. This BNADH oxidation photosensitized by P(V)porphyrins in the presence of sodium azide can be explained by electron transfer oxidation from BNADH to the photoexcited P(V)porphyrins. The quantum yields of BNADH oxidation via electron transfer by these P(V)porphyrins were larger than those of a singlet oxygen mechanism. Redox potential measurements supported the electron transfer mechanism from a thermodynamic point of view, and fluorescence lifetime measurement also suggests this mechanism. The process of this electron transfer oxidation involves the radical formation of BNADH and the further reaction of this radical to the oxidized form (cationic form of BNADH). Analysis of the quantum yields of BNADH photooxidation by P(V)porphyrins suggests that the photoinduced electron transfer from BNADH to photoexcited P(V)porphyrins triggers the radical chain reaction of BNADH oxidation. The electron transfer rate coefficient and this efficiency were increased with an increase in the Gibbs energy of electron transfer from tryptophan to photoexcited P(V)porphyrins (−ΔG). However, the BNADH oxidation quantum yield via electron transfer decreased with an increase in the -ΔG of electron transfer. These results suggest that reverse electron transfer inhibits the decomposition of BNAD radicals. This assay using BNADH can be used to evaluate the photosensitizer activity of water-insoluble compounds. These P(V)porphyrins may be used as photosensitizers for photodynamic therapy in a relatively hydrophobic environment in cancer tissues.

Sensitized photo-oxidation of gadusol species mediated by singlet oxygen

Gadusols are efficient nature UV sunscreens with antioxidant capacity. The kinetics of the quenching reactions of singlet oxygen O2(1∆g) by gadusol species was evaluated in aqueous solution as well as in the presence of direct charged micelles. Time-resolved phosphorescence detection of O2(1∆g) indicated that gadusolate, the main species under biological pH, is a more efficient quencher than the enol form with a rate constant of ca. 1.3 × 108 L mol−1 s−1. The deactivation proceeds via a collisional mechanism with clear dominance of chemical pathways, according to the rates of gadusol and oxygen consumptions, and typical photooxidation quantum yields of ca. 7%. The relative contributions of the chemical and physical quenching steps were not affected by the presence of anionic or cationic micelles emulating simple pseudo-biological environments. The products of the photo-oxidative quenching support a type II mechanism initiated by the addition of O2(1∆g) to the C-C double bond of gadusolate. These results point to the relevance of considering the role of sacrifice antioxidant along with the UV-screening function for gadusol, particularly in the context of potential biotechnological applications of this natural molecule.

The photochemical route to octahedral iron(V). Primary processes and quantum yields from ultrafast mid-infrared spectroscopy.

Recently, the complex cation [(cyclam-ac)Fe(III)(N3)](+) has been used in solid matrices under cryogenic conditions as a photochemical precursor for an octahedral iron nitride containing the metal at the remarkably high oxidation state +5. Here, we study the photochemical primary events of this complex cation in liquid solution at room temperature using femtosecond time-resolved mid-infrared (fs-MIR) spectroscopy as well as step-scan Fourier-transform infrared spectroscopy, both of which were carried out with variable-wavelength excitation. In stark contrast to the cryomatrix experiments, a photooxidized product cannot be detected in liquid solution when the complex is excited through its putative LMCT band in the visible region. Instead, only a redox-neutral dissociation of azide anions is seen under these conditions. However, clear evidence is found for the formation of the highly oxidized iron nitride product when the photolysis is carried out in liquid solution with UV light. Yet, the photooxidation must compete with photoreductive Fe-N bond cleavage leading to azide radicals and an iron(II) complex. Both, redox-neutral and photoreductive Fe-N bond breakage as well as photooxidative N-N bond breakage occur on a time scale well below a few hundred femtoseconds. The majority of fragments suffer from geminate recombination back to the parent complex on a time scale of 10 ps. Upper limits of the primary quantum yield for photooxidation are derived from the fs-MIR data, which increase with increasing energy of the photolysis photon.

Rapid Photooxidation of As(III) through Surface Complexation with Nascent Colloidal Ferric Hydroxide

Contamination of water and soils with arsenic, especially inorganic arsenic, has been one of the most important topics in the fields of environmental science and technology. The interactions between iron and arsenic play a very significant role in the environmental behavior and effect of arsenic species. However, the mechanism of As(III) oxidation in the presence of iron has remained unclear because of the complicated speciation of iron and arsenic. Photooxidation of As(III) on nascent colloidal ferric hydroxide (CFH) in aqueous solutions at pH 6 was studied to reveal the transformation mechanism of arsenic species. Experiments were done by irradiation using light-emitting diodes with a central wavelength of 394 nm. Results show that photooxidation of As(III) and photoreduction of Fe(III) occurred simultaneously under oxic or anoxic conditions. Photooxidation of As(III) in the presence of nascent CFH occurred through electron transfer from As(III) to Fe(III) induced by absorption of radiation into a ligand-to-metal charge-transfer (LMCT) band. The estimated quantum yield of photooxidation of As(III) at 394 nm was (1.023 ± 0.065) × 10(-2). Sunlight-induced photooxidation of As(III) also occurred, implying that photolysis of the CFH-As(III) surface complex could be an important process in environments wherein nascent CFH exists.

UV–visible absorption, fluorescence characteristics and laser activity of (E,E)-2,5-bis(4-methoxystyryl)pyrazine (BMSP)

The electronic absorption and emission spectra of (E,E)-2,5-bis(4-methoxystyryl)pyrazine (BMSP) dye was studied in various solvents at 298K. A bathochromic shift was observed in absorption spectrum(ca. 10nm) and fluorescence (ca. 20nm) spectra upon increase of solvent polarity, which indicates that the transition is π−π⁎ in nature and the singlet excited state of BMSP is more polar than the singlet ground state. The fluorescence quantum yield of BMSP increases as the solvent polarity increases due to solvent of polarity decreasing the vibronic mixing between 1(π, π⁎) and 1(n, π⁎) states of BMSP. BMSP dye is insoluble in water but undergoes solubilization in the hydrocarbon core of micelles such as SDS( anionic surfactant), CTAB(cationic surfactant) and TX-100 (neutral surfactant). Ground and excited state protonation constants of BMSP were calculated as -0.2 and 5.26 respectively which indicates that the excited state of BMSP is more basic than the ground state. The photochemical quantum yield (φc) of photo-oxidation of BMSP was calculated in DMSO, EtOH and CHCl3. The values of φc were found to be 0.004, 0.0052 and 0.009 in DMSO, EtOH and CHCl3, respectively. Dye solutions in DMSO, EtOH and CHCl3 give laser emission in the blue region excitation by a 337.1nm nitrogen pulse. The gain coefficient(α), emission cross section(σe) and fluorescence lifetime have been determined. Excitation energy transfer from BMSP to rhodamine B was studied to improve the laser emission from this dye when excited by 337nm.

Indole substituted zinc phthalocyanine: Improved photosensitizing ability and modified photooxidation mechanism

The photophysical and photochemical processes within a novel photosensitizer (PS), zinc phthalocyanine (ZnPc) modified by indole units, were explored. The properties related to photodynamic therapy of tumor (PDT) were studied by time-resolved transient UV–vis absorption spectra, steady state and time-resolved fluorescence spectra, and chemical trapping of singlet oxygen by diphenylisobenzofuran (DPBF). Intra-molecular photoinduced electron transfer (PET) within the conjugate from the indole subunits (donor A), to S 1 (excited singlet state) of ZnPc moiety (acceptor D), is featured by the significant decrease of fluorescence quantum yield and lifetime of ZnPc moiety, and the occurrence of transient absorption bands of ZnPc − at 570 and 630 nm. The triplet state, on the other hand, was not quenched by indole units. The kinetics and thermodynamics of PET were analyzed quantitatively, and the quantum efficiency of PET is computed to be 38%, almost double of the emission efficiency (20%). The quantum efficiency of triplet (T 1) formation is 0.50 and the quantum yield of DPBF photooxidation is 0.72. Both are larger than the expected value of 0.32. The evolution of transient absorption spectra showed that the charge separation state (ZnPc −–indole +) recombined to triplet state ZnPc(T 1)–indole, which is responsible for the high yield of T 1 formation. In the presence of oxygen, both T 1 and ZnPc − were quenched efficiently, which forms singlet oxygen and superoxide anion, respectively. DPBF is therefore photo-oxidized by both singlet oxygen (Type II reaction, 46%) and superoxide anion radical (Type I reaction, 54%), which led to the high yield of photooxidation. This is in contrast to free ZnPc PS, in which only singlet oxygen is responsible for the photooxidation. The result suggests that the reaction mechanism is changed upon conjugation so that the importance of Type I reaction is greatly enhanced, and the indole-conjugated ZnPc is an even better PS than the free ZnPc.

Unraveling the flavin-catalyzed photooxidation of benzylic alcohol with transient absorption spectroscopy from sub-pico- to microseconds

Flavin-mediated photooxidations have been described for applications in synthetic organic chemistry for some time and are claimed to be a route to the use of solar energy. We present a detailed investigation of the involved photophysical and photochemical steps in methoxybenzyl alcohol oxidation on a timescale ranging from sub-picoseconds to tens of microseconds. The results establish the flavin triplet state as the key intermediate for the photooxidation. The initial step is an electron transfer from the alcohol to the triplet state of the flavin catalyst with (3)k(ET)≈ 2 × 10(7) M(-1) s(-1), followed by a proton transfer in ∼6 μs. In contrast, the electron transfer involving the singlet state of flavin is a loss channel. It is followed by rapid charge recombination (τ = 50 ps) without significant product formation as seen when flavin is dissolved in pure benzylic alcohol. In dilute acetonitrile/water solutions of flavin and alcohol the electron transfer is mostly controlled by diffusion, though at high substrate concentrations >100 mM we also find a considerable contribution from preassociated flavin-alcohol-aggregates. The model including a productive triplet channel and a competing singlet loss channel is confirmed by the course of the photooxidation quantum yield as a function of substrate concentration: We find a maximum quantum yield of 3% at 25 mM of benzylic alcohol and significantly smaller values for both higher and lower alcohol concentrations. The observations indicate the importance to perform flavin photooxidations at optimized substrate concentrations to achieve high quantum efficiencies and provide directions for the design of flavin photocatalysts with improved performance.

The influence of surface fluorination in the photocatalytic behaviour of TiO2 aqueous dispersions: An analysis in the light of the direct–indirect kinetic model

TiO2 surface fluorination is known to produce an enhancement of the photocatalytic oxidation rate of some organic compounds such as phenol, acid orange, azo dye Acid Red 1 and benzoic acid, while leads to a clear decrease of the photooxidation quantum yield of other compounds like formic acid, and dichloroacetate. Here we show that these differences in behaviour cannot be explained in the framework of the classical “Langmuir–Hinshelwood” (L–H) kinetic model, although they are compatible with the predictions of the alternative, recently developed “Direct–Indirect”(D–I) model [19]. For weak electronic interaction of the TiO2 surface with dissolved substrate species, as it is for instance the case for phenol, the D–I model predicts that photooxidation takes place mainly via an interfacial, indirect transfer (IT) mechanism involving inelastic trapping of valence band free holes by terminal, twofold coordinated oxygen ions, and further adiabatic transfer to dissolved substrate species. On the basis of open-circuit voltage measurements, the origin of the photooxidation quantum yield increase observed in these cases when the TiO2 surface becomes fluorinated is attributed to the partial substitution of water molecules adsorbed on terminal Ti sites by fluoride ions, leading to: (1) an interfacial decrease of the electron-hole recombination rate as the surface concentration of recombination centres associated with terminal Ti sites diminishes; (2) an increase of the IT rate of surface trapped holes to reduced dissolved species, because of the upward shift experimented by the semiconductor energy levels with respect to filled electrolyte energy levels. In contrast, for strong electronic interaction of substrate species with the naked TiO2 surface, as it is for instance the case of formate ions at low enough pH, photooxidation mainly take place via an interfacial, direct transfer (DT) mechanism, involving inelastic trapping of valence band free holes by specifically adsorbed substrate species. The photooxidation quantum yield decrease observed in this case is explained to be due to a diminution of the density of TiO2 surface sites active for adsorption of formate ions. The enhanced photogeneration of free OHS radicals and/or photooxidation rate of water molecules in the liquid phase with free VB holes, assumed by some authors as the origin of the observed photooxidation quantum yield increase at the fluorinated TiO2 surface, should be disregarded as far as both processes appears to be thermodynamically and kinetically hindered.

The photooxidative destruction of C.I. Basic Yellow 2 using UV/S 2O 82− process in a rectangular continuous photoreactor

The photooxidative decolorization of C.I. Basic Yellow 2 (BY2), was investigated using UV radiation in the presence of peroxydisulfate (S 2O 8 2−) in a rectangular photoreactor at experimental condition. S 2O 8 2− and UV-light showed negligible effect when they were used independently. Removal efficiency of BY2 was sensitive to the operational parameters such as initial concentrations of S 2O 8 2−, BY2, light intensity, flow rate and pH. The conversion ratios of BY2 at the volumetric flow rates of 330, 500 and 650 ml/min were 84%, 79%, 51% in 30 min, respectively. Our results showed that light intensity was a beneficial parameter for dye removal. The results showed that in the presence of S 2O 8 2−, the photooxidation quantum yield obtained was higher than direct photolysis quantum yield, suggesting that photodecay of BY2 was dominated by photooxidation. The electrical energy per order ( E EO) values for decolorization of BY2 solution was calculated. Results show that applying a desired peroxydisulfate concentration can reduce the E EO.

The photo-oxidative destruction of C.I. Basic Yellow 2 using UV/S2O8 2− process in an annular photoreactor

The photo-oxidative decolorization of C. I. Basic Yellow 2 (BY2), was investigated using UV radiation in the presence of peroxydisulfate (S2O8 2−) in an annular photoreactor at different conditions. S2O8 2− and UV-light showed negligible effect when they were used independently. Removal efficiency of BY2 was sensitive to the operational parameters such as initial concentrations of S2O8 2−, BY2 and pH. The conversion ratios of BY2 at the volumetric flow rates of 330, 500 and 650 mLmin− 1 were 84%, 78%, 69% in 1 h, respectively. The results showed that in the presence of S2O8 2−, the photooxidation quantum yield obtained higher than direct photolysis quantum yield, suggesting that photodecay of BY2 was dominated by photooxidation. The electrical energy per order (EE/O) values for decolorization of BY2 solution was calculated. Results show that applying an optimum peroxydisulfate concentration can reduce the EE/O.

Mechanism of protection of adenosine from sulphate radical anion and repair of adenosine radicals by caffeic acid in aqueous solution

The photooxidation of adenosine in presence of peroxydisulphate (PDS) has been studied by spectrophotometrically measuring the absorbance of adenosine at 260 nm. The rates of oxidation of adenosine by sulphate radical anion have been determined in the presence of different concentrations of caffeic acid. Increase in [caffeic acid] is found to decrease the rate of oxidation of adenosine suggesting that caffeic acid acts as an efficient scavenger of SO •4 and protects adenosine from it. Sulphate radical anion competes for adenosine as well as for caffeic acid. The quantum yields of photooxidation of adenosine have been calculated from the rates of oxidation of adenosine and the light intensity absorbed by PDS at 254 nm, the wavelength at which PDS is activated to sulphate radical anion. From the results of experimentally determined quantum yields (Φexpt¹) and the quantum yields calculated (Φcal) assuming caffeic acid acting only as a scavenger of SO •4 show that Φexpt¹ values are lower than Φexpt¹ values. The ǵf values, which are experimentally found quantum yield values at each caffeic acid concentration and corrected for •4 scavenging by caffeic acid, are also found to be greater than Φexpt¹ values. These observations suggest that the transient adenosine radicals are repaired by caffeic acid in addition to scavenging of sulphate radical anions.

Photooxidation quantum yield efficiencies of naphthalene diimides under concentrated sun light in comparisons with perylene diimides

Thermal stabilities, solubilities redox potentials and photophysical parameters of 10 synthesized N-alkyl(aryl) derivatives of naphthalene diimides were studied for selecting the best derivatives at concentrated sun light experiments. N-aryl substitutions created better thermal stabilities than the N-alkyl substitution in naphthalene diimides. LUMO energy levels of NDIs, calculated from redox potentials, in the range of −3.55/−3.68 ev, proved that the naphthalene diimides can inject electrons efficiently in titania based organic dye sensitized solar cells. Fluorescence lifetimes of naphthalene diimides, τ f = 11–34 ps, are faster than that of perylene diimides, τ f = 4 ns. Calculated photooxidation quantum yields with a naphthalene diimide photosensitizer for the direct sun light oxidation reaction of anthracene is ϕ = 0.014, and for the concentrated sun light of 10.2 sun is ϕ = 0.044. In general photooxidation quantum yield efficiencies of naphthalene diimides under concentrated sun light irradiations are found to be higher with respect to the perylene dimide photosensitizers, due to production of both singlet oxygen and superoxide anion radical and enhancement of low wavelength solar radiation with aluminium surfaced mirror under concentrated sun light.

True quantum yields and adsorption constants as tools for a mechanistic study of the TiO 2-sensitized photooxidation of benzylic derivatives

The quantum yields ( Φ) of the colloidal TiO 2-sensitized photooxidation of 4- ( 1) and 3-methoxybenzyl alcohol ( 2) together with 4-methoxybenzyltrimethyl- ( 3) and 4-methoxybenzyltriisopropylsilane ( 4) were determined in CH 3CN, in the presence of HClO 4 for 3 and 4. The true quantum yields ( Φ 0) of 1, 2 and 3, obtained from a Langmuir–Hinshelwood isotherm treatment of Φ at different substrate concentrations, are linearly correlated with I A −1/2, where I A is the light intensity. According to the previously suggested mechanisms for alcohols and silanes, a kinetic scheme that justifies this correlation is suggested. It is shown that the ratio of the slopes (from the Φ 0 versus I A −1/2 plots) for 1 and 2 is equal to the Φ 0 1 /Φ 0 2 ratio at any I A; this ratio depends on the rate constants in the kinetic scheme, in this case principally on the electron transfer constant, k et. On the contrary, the Φ 0 3 /Φ 0 4 ratio depends on k p, the cation radical desilylation rate constant, confirming a steric hindrance to nucleophylic assistance in the CSi fragmentation by the bulky isopropyl group in the SiR 3 moiety. Differently from Φ 0, the adsorption constants on the semiconductor under irradiation ( K D, obtained from the above isotherm treatment) are independent of I A. Moreover, as K D 1 =K D 2 and K D 3 =K D 4 , the structural modifications within the two alcohols and within the two silanes should be far enough away from the adsorption site. For all the substrates, K (the dark adsorption constant) is five times greater than K D, showing that this change does not depend on the substrate structure but is the result of different experimental conditions.

Structure alterations of perfluorinated sulfocationic membranes under the action of ethylene glycol (SAXS and WAXS studies)

Small-angle X-ray scattering and wide-angle X-ray scattering studies were carried out for perfluorinated sulfocationic membranes MF-4SK (membranes of a Nafion type) in the normal ‘as-manufactured’ state and soaked in ethylene glycol at 110 °C and subsequently washed with water. It was shown that such a treatment of MF-4SK membranes resulted in alterations of their nanostructure and these alterations were stable for a long time. On the basis of the paracrystalline layered model of the perfluorinated membrane nanostructure these structure alterations were interpreted as an increase in the thickness of the liquid layers (ionic channels) separating clusters of ionomer groups in MF-4SK membranes. The importance of these structure alterations was confirmed by the twofold growth of the quantum yield of anthracene photo-oxidation catalyzed by tetraphenylporphyrin (TPP) immobilized on MF-4SK membranes treated with ethylene glycol.

Kinetics and mechanism of protection of thymine from sulphate radical anion under anoxic conditions

The rates of photooxidation of thymine in presence of peroxydisulphate (PDS) have been determined by measuring the absorbance of thymine at 264 nm spectrophotometrically. The rates and the quantum yields (φ) of oxidation of thymine by sulphate radical anion have been determined in the presence of different concentrations of caffeic acid. Increase in [caffeic acid] is found to decrease the rate of oxidation of thymine suggesting that caffeic acid acts as an efficient scavenger of SO •-4 and protects thymine from it. Sulphate radical anion competes for thymine as well as for caffeic acid. The rate constant of sulphate radical anion with caffeic acid has been calculated to be 1.24 x 1010 dm3 mol-1s-1. The quantum yields of photooxidation of thymine have been calculated from the rates of oxidation of thymine and the light intensity absorbed by PDS at 254 nm, the wavelength at which PDS is activated to sulphate radical anion. From the results of experimentally determined quantum yields (φexpt1) and the quantum yields calculated (φcl) assuming caffeic acid acting only as a scavenger of SO •-4 radicals show that φexpt1 values are lower than φcl values. The φ ’ values, which are experimentally found quantum yield values at each caffeic acid concentration and corrected for SO •-4 scavenging by caffeic acid, are also found to be greater than φexpt1 values. These observations suggest that the thymine radicals are repaired by caffeic acid in addition to scavenging of sulphate radical anions.

Kinetics and mechanism of protection of thymine from phosphate radical anion under anoxic conditions

The rates of photooxidation of thymine in the presence of peroxydiphosphate (PDP) have been determined by measuring the absorbance of thymine at 264 nm spectrophotometrically. The rates and the quantum yields (φ) of oxidation of thymine by phosphate radical anion have been determined in the presence of different concentrations of dithiothreitol (DTT). An increase in DTT is found to decrease the rate of oxidation of thymine, suggesting that DTT acts as an efficient scavenger of PO4·2− and protects thymine from it. Phosphate radical anion competes for thymine as well as DTT; the rate constant for the phosphate radical anion with DTT has been calculated to be 2.21 × 109 dm3 mol−1 s−1, assuming the rate constant of phosphate radical anion reaction with thymine as 9.6 × 107 dm3 mol−1 s−1. The quantum yields of photooxidation of thymine have been calculated from the rates of oxidation of thymine and the light intensity absorbed by PDP at 254 nm, the wavelength at which PDP is activated to phosphate radical anion. From the results of experimentally determined quantum yields (φexptl) and the quantum yields calculated (φcl), assuming DTT acts only as a scavenger of PO4·2− radicals, show that φexptl values are lower than φcl values. The φ′ values, which are experimentally found quantum yield values at each DTT concentration and corrected for PO4·2− scavenging by DTT, are also found to be greater than φexptl values. These observations suggest that the thymine radicals are repaired by DTT in addition to scavenging of phosphate radical anions. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 271–275, 2001

Kinetics and mechanism of protection of thymine from phosphate radical anion under anoxic conditions

AbstractThe rates of photooxidation of thymine in the presence of peroxydiphosphate (PDP) have been determined by measuring the absorbance of thymine at 264 nm spectrophotometrically. The rates and the quantum yields (φ) of oxidation of thymine by phosphate radical anion have been determined in the presence of different concentrations of dithiothreitol (DTT). An increase in DTT is found to decrease the rate of oxidation of thymine, suggesting that DTT acts as an efficient scavenger of PO4·2− and protects thymine from it. Phosphate radical anion competes for thymine as well as DTT; the rate constant for the phosphate radical anion with DTT has been calculated to be 2.21 × 109 dm3 mol−1 s−1, assuming the rate constant of phosphate radical anion reaction with thymine as 9.6 × 107 dm3 mol−1 s−1. The quantum yields of photooxidation of thymine have been calculated from the rates of oxidation of thymine and the light intensity absorbed by PDP at 254 nm, the wavelength at which PDP is activated to phosphate radical anion. From the results of experimentally determined quantum yields (φexptl) and the quantum yields calculated (φcl), assuming DTT acts only as a scavenger of PO4·2− radicals, show that φexptl values are lower than φcl values. The φ′ values, which are experimentally found quantum yield values at each DTT concentration and corrected for PO4·2− scavenging by DTT, are also found to be greater than φexptl values. These observations suggest that the thymine radicals are repaired by DTT in addition to scavenging of phosphate radical anions. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 271–275, 2001

New polymeric photosensitizers

Abstract Novel polymeric photosensitizers for singlet oxygen production in water are described. The polymers contain rose bengal (4,5,6,7-tetrachloro-2',4',5',7'­tetraiodo-fluorescein) chromophores covalently attached to the polymer chain. The quantum yields of singlet oxygen formation and photooxidation of phenol in water have been determined. The methods of separation of photosensitizer from the reactants after completion of reaction have been established.

Quantum Efficiency of Photooxidation of Porphyrins by Halomethanes in Solutions

The quantum yields of photooxidation of porphyrins, chlorophyll a, and β–carotene in diethyl ether in the presence of different concentrations of CCl4 in irradiation into the S0 → S1 absorption bands are determined. The quantum yields for deoxygenated solutions are in the range (0.001–4.200)·10−3. In the presence of oxygen, the quantum yield is increased by more than an order of magnitude and is related to the formation of the peroxyl radical CCl3O2•. It is shown that in the initial stage of the photochemical reaction, an electron from the excited molecules of tetrapyrrole pigments in the S1 state is transferred to the molecules of halogen derivatives of methane.

Kinetics of the dye sensitized photooxidation of 2-amino-4-hydroxy-6-methylpyrimidine, a model compound for some fungicides

The kinetics of the dye-sensitized photooxidation of 2-amino-4-hydroxy-6-methyl-pyrimidine (AHMPD), a compound with the basic molecular structure of some systemic pyrimidine fungicides, has been studied in solution in water and in the mixture acetonitrile–water 4:1 v/v. Rate constants in the range 9×10 4–2.7×10 7 M −1 s −1 for both the overall and the reactive singlet molecular oxygen [O 2( 1 Δ g )] quenching processes were determined by time-resolved O 2( 1 Δ g )-phosphorescence detection and polarographic methods. Photooxidation quantum yields were in the range 0.005–0.41, with values significantly higher in alkalinized media. On the contrary, pyrimidine, 2-aminopyrimidine, 4-hydroxypyrimidine and 6-methylpyrimidine do not react with O 2( 1 Δ g ), in neutral or alkalinized D 2O solutions. The presence of a 4-hydroxy group in pyrimidines plays a key role in the photooxidative process, because the formation of the tautomeric 4-oxo form, the predominant one in aqueous solution, diminishes or suppresses the interaction with O 2( 1 Δ g ). However, in the presence of alkali the OH-ionization greatly enhances the photooxidation. The experimental evidence suggests a charge-transfer mediated mechanism involving an initial encounter excited complex. These results also show that the sensitized photooxidation is an alternative pathway for the environmental or programmed degradation of these colorless N-heteroaromatic compounds, in special in OH-ionized form.