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Showing papers in "Journal of Photochemistry and Photobiology A-chemistry in 1995"


Journal ArticleDOI
TL;DR: In this paper, the photocatalysed oxidation of phenol, 2-chlorophenol and pentachlorophenol was re-examined under conditions in which TiO2 anatase was sensitized by CdS in air-equilibrated aqueous media; this was to assess whether or not the interparticle electron transfer pathway, first discovered a decade ago, could also be applied to photo-oxidative processes.
Abstract: The photocatalysed oxidation of phenol, 2-chlorophenol and pentachlorophenol was re-examined under conditions in which TiO2 anatase was sensitized by CdS in air-equilibrated aqueous media; this was to assess whether or not the interparticle electron transfer pathway, first discovered a decade ago (N. Serpone, E. Borgarello and M. Gratzel, J. Chem. Soc., Chem. Commun., (1984) 342) and subsequently applied to enhance reductive processes on titania, could also be applied to photo-oxidative processes. The results indicate that combinations such as CdS/TiO2 lead to an enhanced rate of disappearance of the initial substrate by a factor greater than two, consistent with the notion that (irradiated) CdS electrons are vectorially displaced onto the non-illuminated TiO2 particulates. Cadmium sulphide is a poorer photo-oxidation catalyst than titania. Other semiconductor materials have also been examined under a variety of conditions of pH and irradiation wavelength. The data also show that when both semiconductors in a coupled system are illuminated simultaneously and their valence and conduction bands are suitably disposed, both electron and hole transfer occur (as in the CdS/TiO2, ZnO/TiO2, TiO2/Fe2O3 and ZnO/Fe2O3 couples), which will influence the efficiency of photo-oxidation. N2O-saturated aqueous dispersions of TiO2 have no effect on the photo-oxidation of phenol, although it was expected that nitrous oxide would scavenge the photogenerated electrons in a manner similar to chemisorbed molecular oxygen, and enhance the efficiency. It is suggested that the role of oxygen in photo- oxidations may be more than just a simple electron scavenger.

641 citations


Journal ArticleDOI
TL;DR: A comparison of the photocatalytic activity for H 2 generation between Au and Pt was made in this paper, where the results showed that the overall activity of Au samples was generally about 30% lower than that of Pt samples.
Abstract: A comparison of the photocatalytic activity for H 2 generation between AuTiO 2 and PtTiO 2 has been made. The deposition of Au and Pt was carried out using TiO 2 powders in aqueous suspensions containing HAuCl 4 ·4H 2 O or H 2 PtCl 6 ·6H 2 O by deposition-precipitation (DP), impregnation (IMP), photodeposition (FD) and, in the case of Au, by mixing TiO 2 with colloidal gold suspensions (MIX). The main reaction products obtained from the irradiation of an aqueous 5 M C 2 H 5 OH suspension containing AuTiO 2 or PtTiO 2 were hydrogen, methane, carbon dioxide and acetaldehyde. Small amounts of acetic acid were also detected. The overall activity of Au samples was generally about 30% lower than that of Pt samples. The activity of Au samples strongly depended on the method of preparation and decreased in the order AuTiO 2 -FD>AuTiO 2 -DP>AuTiO 2 -IMP>AuTiO 2 -MIX. The activities of the platinum samples were less sensitive to the preparation method and decreased in the order PtTiO 2 -FD>PtTiO 2 -DP≈PtTiO 2 -IMP. Gold and platinum precursors calcined in air at 573 K showed the highest activity towards H 2 generation, followed by a decline in activity with increasing calcination temperature. The H 2 yield was found to be dependent on the metal content on TiO 2 and showed a maximum in the ranges 0.3–1 wt.% Pt and 1–2 wt.% Au. The exposed surface area of gold had only a small influence on the rate of hydrogen generation. On the other hand, the rate of H 2 production was strongly dependent on the initial pH of the suspension. pH values in the range 4–7 gave better yields, whereas highly acidic and basic suspensions resulted in a considerable decrease in the H 2 yield.

581 citations


Journal ArticleDOI
TL;DR: In this article, the degradation of methylene blue assisted by TiO2 photocatalysis was investigated in aqueous suspension, and the rate constants for this heterogeneous reaction were evaluated as a function of the concentration of the dye, the amount of TiO 2 and the pH.
Abstract: The degradation (oxidation) of methylene blue assisted by TiO2 photocatalysis (with an irradiation wavelength of 365 nm) was investigated in aqueous suspension. The rate constants for this heterogeneous photocatalysis were evaluated as a function of the concentration of the dye, the amount of TiO2 and the pH. The reaction is found to conform to a Langmuir adsorption isotherm and the rate vs. pH profile exhibits a linear increase with pH in acidic solutions with a maximum at around pH 6.9.

273 citations



Journal ArticleDOI
TL;DR: In this article, the problems raised by the immobilization of a photocatalyst, namely TiO2, on a support are reviewed, from the standpoints of the procedure itself and of the structure, adherence, morphology and photocatalytic activity of the deposit.
Abstract: The problems raised by the immobilization (or anchoring) of a photocatalyst, namely TiO2, on a support are reviewed, from the standpoints of the procedure itself and of the structure, adherence, morphology and photocatalytic activity of the deposit. Some new experimental findings are also reported. It is concluded that the immobilized photocatalyst absorbs all the actinic light that it can, but the immobilization treatment generally lowers its activity. It would therefore be advisable to tailor a treatment which would at least maintain this activity, and even increase it, possibly by size quantization.

159 citations


Journal ArticleDOI
TL;DR: In this paper, it was found that TiO2 can be readily supported on polythene films without inhibiting the photocatalytic activity of phenol and showed that it is an excellent photocatalyst for complete mineralization of organic contaminants.
Abstract: TiO2 is known to be an excellent photocatalyst for complete mineralization of organic contaminants in water. However, the separation of TiO2 particles creates problems in practical application of the process. It is found that TiO2 can be readily supported on polythene films without inhibiting the photocatalytic activity. As an example, data on photocatalytic degradation of phenol by TiO2 supported on polythene films is presented.

142 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that trichloroacetic acid is not formed in the absence of O2; it is suppressed by the addition of hydrogen donors, such as tert-butanol and also by carbonate/bicarbonate ions.
Abstract: In the 254 nm photolysis of air-saturated aqueous solutions of tetrachloroethene, the major products (quantum yields in parentheses) are as follows: chloride ion (2.05), carbon dioxide (0.62), trichloroacetic acid (0.41), dichloroacetic acid (0.08) and hypochlorite (0.08). Trichloroacetic acid is not formed in the absence of O2; it is suppressed by the addition of hydrogen donors, such as tert-butanol and also by carbonate/bicarbonate ions. The quantum yield of dichloroacetic acid remains unaffected by O2 and hydrogen donors. It is concluded that there are two (or three) major primary processes CCl2=CCl2+hv→CCl2=CCl+Cl (1) CCl=CCl2+hv+H2O→CCl2(OH)CCl2H (2) CCl=CCl2+hv→CCL++Cl− (3) From scavenging experiments with alcohols, with and without O2, φ(1) = 0.23 ± 0.03 has been determined. Reactions (2) and (3) will finally both yield dichloroacetic acid and no distinction can be made between the two routes: φ(2+3)=0.08. In the absence of scavengers, the Cl atoms add to tetrachloroethene yielding the pentachloroethyl radical which, in the presence of O2, is converted to the corresponding peroxyl radical, the precursor of trichloroacetic acid. In the presence of O2, a short chain reaction sets in. The chain carrier is the Cl atom which is liberated in the bimolecular termination reactions of the various peroxyl radicals formed in the present system. The rate constants k(Cl.+C2Cl4)=2.8 × 108 dm3 mol−1 s−1, (Cl.CO2−3) = 5.0 × 108 dm3 mol−1 s−1 and k(Cl.+HCO−3)=2.2 × 108 dm3 mol−1 s−1 have been arrived at in this study. Thus bicarbonate prevents the formation of trichloroacetic acid from potentially present tetrachloroethene in the UV disinfection of drinking water.

100 citations


Journal ArticleDOI
TL;DR: In this article, the photocatalyzed degradation of all six isomeric forms of xylenol and the mineralization of 3,4-dimethylphenol were examined in air-equilibrated and oxygenated, illuminated titania dispersions.
Abstract: Dimethylphenols (xylenols) represent an important component of the phenolic substances in coal tar creosote, a mixture used widely as a wood preservative. The photocatalyzed degradation of all six isomeric forms of xylenol and the mineralization of 3,4-dimethylphenol were examined in air-equilibrated and oxygenated, illuminated titania dispersions. Some of the hydroxylated intermediates were identified by high performance liquid chromatography (HPLC). The effects of varying the pH, concentration of TiO2 and xylenol, temperature and light intensity on the process kinetics were also examined. Kinetic considerations emphasize the notion that the oxidative mineralization process follows natural saturation-type kinetics for which the Langmuir-Hinshelwood model, often invoked in photocatalysis, is a special case; however, it is emphasized that this model is not an a priori requirement in heterogeneous photocatalysis.

99 citations


Journal ArticleDOI
TL;DR: In this article, the photocatalytic reaction of xylidine ponceau on zinc oxide power was carried out spectrophotometrically and the effects of the variation of various parameters, such as the concentration of Xylidine Ponceau, pH, amount and particle size of the semiconductor and light intensity, on the rate of photocatallytic bleaching were observed.
Abstract: The photocatalytic reaction of xylidine ponceau on zinc oxide power was carried out. Photocatalytic bleaching of the dye was observed spectrophotometrically. The effects of the variation of various parameters, such as the concentration of xylidine ponceau, pH, amount and particle size of the semiconductor and light intensity, on the rate of photocatalytic bleaching were observed. A tentative mechanism for the photocatalytic bleaching of xylidine ponceau is proposed.

98 citations


Journal ArticleDOI
TL;DR: In this paper, it was concluded that two mechanisms compete, one involving N2O3 and the other the radical adduct phenol…NO2.. This reaction does not occur with pyrocatechol and hydroquinone.
Abstract: The excitation of nitrate and nitrate ions in the presence of phenolic compounds may induce oxidation, nitration and nitrosation. Oxidation occurs whatever the substituents and can be attributed to hydroxyl radicals formed in the photolysis of ions. Nitration and nitrosation are not observed in the presence of electron-withdrawing substituents. When nitrite ions are excited in the presence of phenol or resorcinol the observed para nitrosation can be attributed to nitrogen oxides formed in the photolysis of NO2−. It is enhanced by increasing nitrite concentration. From the influence of oxygen it is concluded that two mechanisms compete, one involving N2O3 and the other the radical adduct phenol…NO2.. This reaction does not occur with pyrocatechol and hydroquinone. With these substrates nitration is observed but mainly in the presence of oxygen. Reactions are more complex when nitrate ions are excited in the presence of phenol or resorcinol. The formation of nitro and nitroso derivatives is attributed to the nitrogen oxides NO2. and N2O4. With pyrocatechol and hydroquinone, nitrosation does not occur and nitration is favoured by deoxygenation. In environmental conditions nitration or nitrosation by excitation of NO2− are highly unlikely. Their occurrence upon excitation of NO3− depends on the concentration of phenolic compounds and on the pH of the solution.

97 citations


Journal ArticleDOI
TL;DR: In this article, the photochemistry of anthracene was studied on dry surfaces of silica, CabOSil (fumes silica) and alumina (neutral) at low coverages (less than 11% of a monolayer).
Abstract: The photochemistry of anthracene ( 1 ) was studied on dry surfaces of silica, CabOSil (fumes silica) and alumina (neutral) at low coverages (less than 11% of a monolayer). The adsorption of 1 onto these surfaces from cyclohexane obeys a Freundlich adsorption isotherm, demonstrating a distribution of adsorption sites for interactions between 1 and the surface. Photolysis of 1 ( λ ex =350 nm) adsorbed on silica, CabOSil or alumina, under deaerated conditions, proceeds slowly to give the anthracene-9,10-photodimer ( 2 ) as the only product. Diffuse reflectance and fluorescence spectroscopy show that a novel process, involving the formation of a stable ground state pair between two molecules of 1 takes place on all three surfaces at coverages of more than 1% of a monolayer. We propose a dimerization mechanism involving a singlet excimer formed from the stable ground state pairs. Photolysis at the solid-air interface, on the other hand, is considerably faster and proceeds to give 2 and photo-oxidation products of 1 . The primary oxidation product is anthracene-9,10-endoperoxide ( 3 ) which undergoes thermal decomposition on the surface to give 9,10-anthraquinone ( 4 ), 9,10-dihydro-9,10-dihydroxyanthracene ( 5 ), bianthronyl ( 6 ), 9-hydroxyanthrone ( 7 ) and 10,10′-dihydroxy-9,9′,10,10′-tetrahydro-9,9′-bianthryl ( 8 ) as the secondary products. Photo-oxidation is mediated by the addition of singlet molecular oxygen to ground state 1 .

Journal ArticleDOI
TL;DR: In this paper, the photoreduction behavior of CO2 on cadmium-loaded ZnS microcrystals and solid solution micro-crystals was investigated, and the highest quantum efficiency of production was 32.5%.
Abstract: The photoreduction behaviour of CO2 on cadmium-loaded ZnS microcrystals and ZnSCdS solid solution microcrystals was investigated. The photoreduction product on Cd-loaded ZnS was formate, and the highest quantum efficiency of production was 32.5%, which was twice as large as that obtained on bare ZnS microcrystals. However, solid solutions of ZnSCdS microcrystals did not exhibit high activities for the photoreduction of CO2. With increasing mole fraction of CdS, the activities for formate production decreased, and the production of CO was observed for a CdS mole fraction of 0.5–0.67.

Journal ArticleDOI
TL;DR: In this paper, the authors studied the degradation of carbetamide in water suspensins of TiO2 on illumination at λ ⩾ 310 nm and determined the initial rate of degradation and the effects of the mass of catalyst and radiant flux.
Abstract: The kinetics of disappearance of carbetamide in water suspensins of TiO2 on illumination at λ ⩾ 310 nm (to avoid direct phoyolysis), were determined. The photocatalytic reaction was studied by measurement of the initial rate of degradation and the effects of the mass of catalyst and radiant flux were determined. The value of the quantum yield is approximately 0.01. A Langmuir-type relationship between the initial rate and the initial concentration indicates that reaction occurs with the adsorbed substrate. The main intermediates in the degradation are of two types: (a) hydroxylated products and derivatives; (b) products of cyclization of the side chain. The structure of carbetamide allows the formation of these two types of compound, which are explained by the reaction with two different oxidant species: hydroxyl radicals and holes.

Journal ArticleDOI
TL;DR: In this paper, a mass transfer model was proposed for the photocatalytic degradation of water contaminants with the adsorption phenomena, taking both the equilibrium state and kinetics into consideration.
Abstract: The purpose of this study was to correlate the photocatalytic degradation of water contaminants with the adsorption phenomena, taking both the equilibrium state and kinetics into consideration. 1,2-Dichloroethane (DCE) was chosen as a test pollutant. The adsorption isotherm was found to follow the Langmuir model. The kinetics of adsorption were studied and interpreted as a mass transfer process. The kinetics of photocatalytic degradation were analysed with special attention given to the influence of the reactant concentration. A kinetic model is proposed, which includes mass transfer, leading to an apparent Langmuir-Hinshelwood equation for the degradation rate, even when the kinetics are controlled by mass transfer. The experimental results show an agreement with the model at concentrations above 0.5 mmol l −1 , but at lower concentrations the rate is lower than expected, possibly due to a loss of DCE or hydroxyl radicals.

Journal ArticleDOI
TL;DR: In this paper, the photocatalytic oxidation of dilute (10−750 mg m−3) mixtures of toluene and 1,1,3-trichloropropene (TCP) or perchloroethylene (PCE) in air was investigated.
Abstract: At 1–2 ms residence time, the photocatalytic oxidation of dilute (10–750 mg m−3) mixtures of toluene and 1,1,3-trichloropropene (TCP) or perchloroethylene (PCE) in air increased the toluene conversion to 100% from the pseudo-steady single-component toluene feed levels of 10%–20%. A chain mechanism involving chlorine radical attack of the toluene and halogenate, previously proposed by Luo and Ollis (J. Catal., 00 (1994) 00-00) for toluene-trichloroethylene mixtures, is extended to rationalize the current results. The conversion enhancement occurred at low toluene concentrations (up to 22 mg m−3) and high TCP/PCE levels. Increasing the toluene concentration to 43 mg m−3 completely quenched the TCP oxidation chain mechanism, giving the same toluene conversion as observed for the toluene-only feeds. The possible enhancement of toluene conversion in toluene-halomethane (methylene chloride, chloroform, carbon tetrachloride) mixed feeds was examined. Carbon tetrachloride and chloroform were ineffective as toluene promoters, and were not photo-oxidized themselves, either alone for mixtures. Methylene chloride photo-oxidation occurred at very modest rates and conversion, and toluene-methylene chloride mixtures exhibited only a slight enhancement of toluenece conversion. Catalyst deactivation was observed with all toluene feeds. Both chlorocarbon addition (TCP, PCE) and humidification slowed, but did not eliminate, deactivation.

Journal ArticleDOI
TL;DR: In this article, the spectroscopy and photophysics of several substituted enones with different bridged structures were studied in a variety of solvents, and it was shown that the non-radiative and radiative decay processes of the excited states are strongly influenced by the molecular structure and the solvent polarity.
Abstract: The spectroscopy and photophysics of several substituted enones with different bridged structures were studied in a variety of solvents. Results show that, in all cases, the non-radiative and radiative decay processes of the excited states are strongly influenced by the molecular structure and the solvent polarity. The “negative solvatokinetic effect” and “positive solvatokinetic effect” are observed for all four compounds. Compounds 3 and 4, with bridged double bond structures, have high fluorescence quantum yields and (especially compound 4 with a rigid structure even in non-polar solvents (cyclohexane, Φf=0.5)), indicating that radiative decay is predominant during the decay of the excited states of these compounds. In contrast, the enone compounds with a free double bond have very low fluorescence quantum yields, indicating that non-radiative decay is important for these compounds.

Journal ArticleDOI
TL;DR: In this article, the fully deuterated photosensitizer, zinc phthalocyanine-d16, was synthesized and its excited singlet and triplet state photophysics determined.
Abstract: The fully deuterated photosensitizer, zinc phthalocyanine-d16, was synthesized and its excited singlet and triplet state photophysics determined. Surprisingly, no differences in the excited state kinetics or quantum yields were observed compared with the undeuterated material. Furthermore, deuteration of the axial pyridine ligand of the metallophthalocyanine produced no changes in its photophysical behaviour. These results are compared with recent work (A. Beeby, A.W. Parker, M.S.C. Simpson and D. Phillips, J. Photochem. Photobiol. B: Biol., 16 (1992) 73–81) in which the photophysical properties of aluminium phthalocyanines were altered in deuterated solvents, and the effects of deuteration on the non-radiative relaxation processes of phthalocyanines were discussed. Implications for the use of such compounds in photodynamic therapy are also mentioned.

Journal ArticleDOI
TL;DR: In this paper, a treatment of the spectroscopic data from thermoreversible photochromic systems is described which allows the molar absorption coefficient of the metastable photomerocyanine, the quantum yield of the colour-forming reaction and the kinetic parameters of the thermal bleaching to be determined.
Abstract: In this paper, a treatment of the spectroscopic data from thermoreversible photochromic systems is described which allows the molar absorption coefficient of the metastable photomerocyanine, the quantum yield of the colour-forming reaction and the kinetic parameters of the thermal bleaching to be determined. The results obtained for five spiroindoline-oxazines indicate that “colourability” on steady state irradiation depends on the photomerocyanine molar absorption coefficient, quantum field and bleaching rate coefficient. For the molecules investigated, the “colourability” markedly depends on temperature, due to the temperature dependence of the bleaching reaction. The results obtained are compared with literature “colourability scales”, determined empirically by comparison of different systems excited by pulsed light.

Journal ArticleDOI
TL;DR: In this article, the absolute photoabsorption cross sections at room temperature are reported for Cl2 (190-500 nm), Br2 (200-600 nm), and BrCl (190−560 nm) using a semi-logarithmic Gaussian distribution function.
Abstract: Absolute photoabsorption cross sections at room temperature are reported for Cl2 (190–500 nm), Br2 (190–600 nm), and BrCl (190–560 nm). In addition, cross sectional data for BrCl were deconvoluted using a semi-logarithmic gaussian distribution function into a UV band centered at 228.3 nm, a C1Π1 band centered at 376.4 nm, a B3Π0+ band centered at 454.9 nm, and an A3Π1 band centered at 533.1 nm. The BrCl band maximum at approximately 228 nm of 6.0×10−20 cm2 molecule−1 was assigned to a transition from the X1Σ0+ ground state to a repulsive 0+ electronic state by analogy with similar UV bands present in ICl and IBr. Finally, an absorbance maximum was also detected for Br2 at 227 nm of 0.96×10−20 cm2 molecule−1.

Journal ArticleDOI
TL;DR: In this article, the absorption and emission characteristics of bis(dialkylamino)benzopyrylium dyes are compared, and their very different fluorescence quantum yields are correlated with the calculated energies of the lowest excited states of twisted cyanine dyes.
Abstract: The absorption and emission characteristics of several bis(dialkylamino)benzopyrylium dyes are compared, and their very different fluorescence quantum yields are correlated with the calculated energies of the lowest excited states of twisted cyanine dyes. The latter are models for selectively bridged compounds. The laser performance of the dyes is described and compared with well-known laser dyes. The 4-(2-carboxyphenyl)-substituted pyrylium salts are considerably more stable than the unsubstituted compounds.

Journal ArticleDOI
TL;DR: In this paper, negative photochromic properties of spiropyran are investigated in various solvents and in solutions of different acidity, and the results indicate that the polarity of the solvent has a prominent effect on λ max of the coloured merocyanine form (MC), whereas its effects on the closed form (SP) and on the coloration rates after visible bleaching are very weak.
Abstract: The negative photochromic properties of a spiropyran are investigated in various solvents and in solutions of different acidity The results indicate that the polarity of the solvent has a prominent effect on λ max of the coloured merocyanine form (MC), whereas its effects on λ max of the closed form (SP) and on the coloration rates after visible bleaching are very weak From the effect of acid concentration on the coloration rates the p K a of the cis intermediate Y in the coloration process is determined to be about 11 lower than that of the protonated merocyanine (MCH + ) The structures and configurations of MC, SP, MCH + and Y are assigned via their 1 H NMR spectra

Journal ArticleDOI
TL;DR: In this article, the photoelectronic function of the porphyrin polymers connected by oligothienyl bridges was investigated and a microscopic polymer junction chip was constructed by electrochemical polymerization of symmetrical bis(oligothienelalkoxy) phosphorus(V)-porphyrin.
Abstract: The nanofabrication of molecular semiconductors for the construction of molecular photoelectronic devices is an important of research; to this end, we have constructed porphyrin polymers possessing a tailor-made structure. In this paper, we report the photoelectronic function of the porphyrin polymers connected by oligothienyl bridges. One-dimensional porphyrin polymers with various lenghts of oligothienyl bridge in the axial direction were synthesized by the electrochemical polymerization of symmetrical bis(oligothienylalkoxy) phosphorus(V)-porphyrin. The conductivity of the polymer was strongly enhanced by photoirradiation, indicating photoinduced carrier formation. The layered sandwich cell of the polymer with polybithiophene showed electric rectifying properties in the dark. Interestingly, photovoltage and photo-current were observed on photoirradiation. On the basis of these results, a microscopic polymer junction chip was constructed. Two-dimensional porphyrin polymers containing oligothienyl bridges in the lateral direction were prepared by similar electrochemical oxidation of meso-tetrakis (oligothienyl) porphyrin metal complexes and were fabricated as layered diodes.

Journal ArticleDOI
TL;DR: In this article, both zinc phthalocyanine and naphthaocyanines (ZnPcS4 and ZnNcS 4 ) were aggregated and photochemically inactive in aqueous buffer of pH 7.4, while in 10 mM cetyl pyridinium chloride in buffer they were monomeric and active.
Abstract: Phthalo- and naphthaocyanines are of interest as sensitizers for the photodynamic therapy of tumors because of their strong absorption in the 680 and 760 nm ranges respectively. Both zinc phthalocyanine and naphthalocyanine tetrasulfonates (ZnPcS 4 and ZnNcS 4 ) were aggregated and photochemically inactive in aqueous buffer of pH 7.4, while in 10 mM cetyl pyridinium chloride in buffer they were monomeric and active. Therefore all these studies were carried out using the buffered detergent. The triplet lifetimes of ZnPcS 4 and ZnNcS 4 under argon were 490 and 110 μs respectively, with oxygen bimolecular quenching constants of 4.2 × 10 9 and 2.0 × 10 8 M −1 s −1 respectively. Triplet decay curves in argon, air and 100% oxygen were first order, suggesting that there was little back reaction of the triplet states with oxygen as has been observed with some naphthalocyanines. The quantum yield of singlet oxygen generation by ZnPcS 4 was 0.70 and that for ZnNcS 4 was 0.25. Both compounds sensitized the photo-oxidation of furfuryl alcohol, cysteine, histidine, methionine, tryptophan, tyrosine and guanosine; ZnPcS 4 was three times more efficient than ZnNcS 4 . These reactions were 50% inhibited by about 0.5 mM azide, suggesting the involvement of singlet oxygen. Both sensitizers photobleached on illumination, with quantum yields of 1.7 × 10 −5 for ZnPcS 4 and 4.2 × 10 −3 for ZnNcS 4 .

Journal ArticleDOI
TL;DR: In this paper, the half-life time of solar photomineralization in the 100 ppb to 50 ppm concentration range was less than 1 h for most of the pesticides.
Abstract: Results are presented on the solar, titanium-dioxide-photocatalyzed degradation of pesticides in rinse waters of agricultural sprayers and in heavily polluted well water. The half-life time of solar photomineralization in the 100 ppb to 50 ppm concentration range was less than 1 h for most of the pesticides. Comparison of the reaction in open and polyethylene-film-covered vessels showed that efficient photodegradation also occured in sealed systems. Samples of polluted waters prepared from liquid commercial pesticide formulations were also purified effectively by the titanium-dioxide-catalyzed solar photo-mineralization The photodegradation of anionic detergents within the formulations was also observed. These results demonstrate the potential of solar photocatalytic oxidation in the removal of organic pollution in water and its practical value in the purification of highly polluted agricultural rinse waters.

Journal ArticleDOI
TL;DR: In this article, the authors studied the solvatochromism, Lewis acid-base equilibrium and photochromism of an indoline spiropyran in neat and mixed organic solvents by UV-visible spectroscopy.
Abstract: The solvatochromism, Lewis acid-base equilibrium and photochromism of an indoline spiropyran in neat and mixed organic solvents were studied by UV-visible spectroscopy. The variation of the transition energy E T of the coloured form (PC) in different solvents was used as an empirical parameter to measure the polarity of the solvents and as a reference process to predict the shift of the Lewis acid-base equilibrium between PC and the colourless form (SP) and the thermal decolouration rates of the PC form in neat and mixed organic solvents. From the linearity of the plot of In K e or In k dc vs. E T , the Lewis acid-base equilibrium, photochromism and solvatochromism of the spiropyran were found to be similar solvent-dependent processes. The changes in K e and k dc were interpreted in terms of the differences in solvation of PC in solvents of different polarity. The relatively small influence of solvent polarity on the colouration rate k op serves as evidence supporting the view that the rate-determining ring-opening process is a concerted electrocyclic reaction.

Journal ArticleDOI
TL;DR: Barros et al. as discussed by the authors investigated the conjugative effects of π electrons between imide groups vicinal to arene moieties and showed that the extent of mixing between these states is a function of the molecular geometry.
Abstract: Conjugation of π electrons between imide groups vicinal to arene moieties can drive the photophysical and photochemical properties to a mixed transition of close lying states having n , π ∗ and π,π ∗ characters (Almeida et al., J. Photochem. Photobiol. A: Chem., 58 (1991) 289). Furthermore, the extent of mixing between these states is a function of the molecular geometry (Barros et al., J. Photochem. Photobiol. A: Chem., 76 (1993) 55). New imides were prepared (1,8-N-propyldinaphthalimide (C3) and 1,8-N-butyldinaphthalimide (C4) to investigate the n , π ∗ and π,π ∗ conjugative effects through space. The steady state and time-resolved fluorescence emission of C3 and C4 in solvents of high polarity shows the presence of a new emitting species at longer wavelength. This emission arises from a rotation in the alkyl chain, resulting in an intramolecular energy transfer between the two chromophores during the excited state lifetime of the extended form.

Journal ArticleDOI
TL;DR: In this paper, the rate of H2 evolution from aqueous suspensions of Pt/TiO2 with deposited zinc porphyrin in the presence of an electron donor (EDTA, triethanolamine, ascorbic and oxalic acids) during irradiation by visible light (λ > 520 nm) was optimized with respect to the concentrations of platinum, zinc pmorphyrin and electron donor.
Abstract: The rate of H2 evolution from aqueous suspensions of Pt/TiO2 with deposited zinc porphyrin in the presence of an electron donor (EDTA, triethanolamine, ascorbic and oxalic acids) during irradiation by visible light (λ > 520 nm) was optimized with respect to the concentrations of platinum, zinc porphyrin and electron donor. The relationship between the rate of hydrogen evolution and pH has a maximum at pH 4–5 for EDTA, ascorbic and oxalic acids, as does the adsorption of these donors on TiO2. In the case of triethanolamine, H2 production increases sharply in alkaline solutions when the triethanolamine molecules are deprotonated. The photostability of the photosensitizer increases on the semiconductor. Turn-over numbers for the zinc porphyrins varied from 50 for water-soluble zinc porphyrin up to 150–185 for water-insoluble sensitizers.

Journal ArticleDOI
TL;DR: In this paper, the solvation characteristics of the ground and excited states of a ketocyanine dye were studied by monitoring the solvatochromic absorption and fluorescence bands.
Abstract: The solvation characteristics of the ground and excited states of a ketocyanine dye were studied by monitoring the solvatochromic absorption and fluorescence bands. Multiple linear regression analysis indicates that both non-specific dipolar interaction and specific hydrogen-bonding interaction play an important role in the positions of the absorption and fluorescence maxima in pure solvents. In mixed binary solvents, the solute is preferentially solvated. The preferential solvation characteristics of the excited state are determined predominantly by the hydrogen-bond donating nature of the two solvents, while the preferential solvation of the ground state is determined by both dipolar and hydrogen-bonding interactions. In mixed aqueous solvents, the solute acts as a ‘probe’ for studying the ‘microheterogeneity’ of the mixture.

Journal ArticleDOI
TL;DR: In this article, the reactivity of phenol under oxic and anoxic conditions in irradiated (350 nm) aqueous suspensions of TiO2 as a function of pH was studied.
Abstract: The reactivity of phenol under oxic and anoxic conditions in irradiated (350 nm) aqueous suspensions of TiO2 as a function of pH was studied. The initial rate of disappearance of phenol is significantly influenced by solution pH. The initial disappearance rates and Langmuir-Hinshelwood kinetic parameters are comparable over a range from pH 3 to 9 and at pH 13.7, while substantial changes are observed at pH 1 and 11. These differences are attributed to speciation of the reactants and changes in the surface of the TiO2 as a function of solution pH. The involvement of oxide radical anion .O− is proposed at pH ⩾ 12. Under highly acidic or basic conditions, colored oligomers are formed as a results, at least in part, of direct photolysis. In the presence of TiO2 the colored oligomers are readily degraded, while in the absence of TiO2 the color continues to develop upon extended irradiation.

Journal ArticleDOI
TL;DR: In this article, a photolysis of deoxygenated aqueous solutions of tyrosine (TYROH, 10 −3 mol dm −3 ) yielded a quantum yield of 9.1 × 10−3 while the observed products (quantum yields in parentheses) are 2,2′-bityrosyl (BITYR, 3.7 × 10 −2 ), 3,4-dihydroxyphenylalanine (DOPA), 3.6 × 10-2 ) and 2-amino-4-
Abstract: In the 254 nm photolysis of deoxygenated aqueous solutions of tyrosine (TYROH, 10 −3 mol dm −3 ) TYROH is consumed with a quantum yield of 9.1 × 10 −3 while the observed products (quantum yields in parentheses) are 2,2′-bityrosyl (BITYR, 3.1 × 10 −3 ) and 2-amino-4-ethenyl-hex-4-enic acid (AEHEA, 1.6 × 10 −3 ). In N 2 O-saturated solutions, N 2 (1.8 × 10 −2 ), in the presence of 2-propanol, H 2 (1 × 10 −3 ) are formed. In air-saturated solutions the tyrosine consumption is increased (9.7 × 10 −2 ); in addition to increased yields of BITYR ({if1.4 × 10 −2 }) but unaltered yields of AEHEA (1.6 × 10 −2 ), 3a-hydroxy-6-oxo-2,3,3a,6,7,7a-hexahydro-1H-indol-2-carboxylic acid (HOHICA,1.6 × 10 −2 ), 3,4-dihydroxyphenylalanine (DOPA, 3.8 × 10 −2 ) and 2,4-dihydroxyphenylalanine (2,4-DHPhe, 2.3 × 10 −3 ) are observed. The sum of the quantum yields of the O 2 -dependent products increases with increasing O 2 and TYROH concentrations, approaching a plateau value of approximately 7 × 10 −2 at high reactant concentrations. In D 2 O the quantum yields of BITYR, HOHICA and DOPA are increased considerably while those of AEHEA are decreased. Irradiation of Rose Bengal with visible light ( 1 O 2 formation) in the presence of TYROH yields HOHICA (90%) and BITYR (10%). It is concluded that from the excited singlet state ( 1 TYROH) photoionization and splitting of the phenolic OH bond occurs giving rise to tyrosine-derived phenoxyl radicals (TYRO) which are the precursors of BITYR. Also, from 1 TYROH a 1,3-H-shift of the phenolic OH and concomitant ring-opening occurs. Addition of water and loss of carbon dioxide in subsequent thermal reactions yields AEHEA as the final product. Quenching of the TYROH triplet state ( 3 TYROH) by O 2 yields 1 O 2 which reacts with TYROH by H abstraction. The resulting intermediates TYRO . and HO 2 . are the precursors of BITYR and HOHICA. In addition, a reaction of O 2 with 3 TYROH possibly yields 1,4- and 3,6-endoperoxides which are the precursors of further HOHICA and of DOPA and 2,4-DHPhe, respectively.