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- A comparative study of the photophysics and photochemistry of 4-chlorophenol adsorbed on silicalite and beta-cyclodextrinPublication . Da Silva, José Paulo; Ferreira, Luís F. Vieira; Silva, Abílio M. da; Oliveira, A. S.The photochemistry and photophysics of 4-chlorophenol (4-CP) were studied onto two model solid supports, silicalite and beta-cyclodextrin (beta-Cl)), using time resolved diffuse reflectance techniques and product degradation analysis. The results have shown that the photochemistry and photophysics of 4-CP are different from solution and depend on the solid. Ground state diffuse reflectance and time resolved luminescence demonstrated the inclusion of the probe in both substrates. 4-CP exhibits room temperature luminescence in both hosts, being structured and much more intense in beta-CD. The emission was assigned to phosphorescence of the inclusion complex. Transient absorption demonstrated the formation of the unsubstituted phenoxyl radical and of 4-chlorophenoxyl radical in beta-CD. In silicalite only the later was detected. The studies of the photodegradation products indicate that phenol is the main photoproduct in beta-CD. In silicalite the chromatographic analysis indicates the presence of products that involve the ring cleavage. (C) 2002 Elsevier Science B.V. All rights reserved.
- Photolysis of 4-chloroanisole in the presence of oxygen: formation of the 4-methoxyphenylperoxyl radicalPublication . Silva, José P. da; Ferreira, Luís F. Vieira; Machado, Isabel Ferreira; Silva, Abílio M. daThe photochemistry of 4-chloroanisole was studied in cyclohexane, with and without molecular oxygen. The triplet state is the main transient in argon purged solutions. In air equilibrated conditions the triplet state was not detected and a new broad absorption band was observed above 450 nm. This absorption was assigned to the 4-methoxyphenylperoxyl radical. The formation of phenylperoxyl radicals after direct photolysis of the parent chlorinated compound in the presence of oxygen is reported here for the first time. Anisole is one of the main photoproducts in both conditions. Bicyclohexane was detected only in argon purged conditions while cyclohexyl ether is only formed in the presence of molecular oxygen. The formation of phenylperoxyl radicals after direct photolysis of chloroaromatics can be used to detect the homolyfic cleavage of the C-Cl bond. (c) 2006 Elsevier B.V. All rights reserved.
- Aqueous photochemistry of pesticides triadimefon and triadimenolPublication . Silva, José P. da; Ferreira, Luís F. Vieira; Silva, Abílio M. daThe photochemistry of pesticides triadimefon and triadimenol was studied in aqueous solution and in methanol/water mixtures, in controlled and natural conditions. The photodegradation kinetics and product distribution are strongly dependent on the solvent and on the irradiation wavelength. The degradation rates are faster at 254 nm than at 313 nm. The kinetics is faster in water than methanol. Direct photoreaction is an important dissipation pathway of triadimefon in natural water systems while triadimenol is stable in these conditions. 1,2,4-Triazole and 4-chlorophenol are two of the major photodegradation products. The formation of the 4-chlorophenoxyl radical was detected for both pesticides in methanol and methanol/water mixtures. In methanol/water mixtures the reaction of both pesticides also occurs with 4-chlorophenolate formation, which increases with the water content. The photochemical studies of pesticides and other pollutants should be made in conditions as similar as possible to those observed in environmental systems. (C) 2003 Elsevier Science B.V. All rights reserved.
- Comparative study of the dissipation of triadimefon in greenhouse and field conditionsPublication . Silva, José P. da; Silva, Abílio M. daThe dissipation of triadimefon, {1-(4-chlorophenoxy)-3,3-dimethyl-1-(1H-1,2,4-triazol-1-yl)butanone}, was studied after its application to melon leaves, glass and paper, both in greenhouse and field conditions. The dissipation rate of triadimefon in its commercial formulation Bayleton 5 was found to be lower in greenhouse than field. The results for different samples in the same conditions show that the dissipation of triadimefon was found to be biphasic. This result can be accounted by a semi-empirical model which assumes an initial fast decline of the dissipation rate, attributed to an exponential decay of the volatilization rates, followed by a second phase where the dissipation is due to a first order degradation processes.The dissipation of triadimefon, {1-(4-chlorophenoxy)-3,3-dimethyl-1-(1H- 1,2,4-triazol-1-yl)butan-one}, was studied after its application to melon leaves, glass and paper, both in greenhouse and field conditions. The dissipation rate of triadimefon in its commercial formulation Bayleton 5 was found to be lower in greenhouse than field. The results for different samples in the same conditions show that the dissipation of triadimefon was found to be biphasic. This result can be accounted by a semi-empirical model which assumes an initial fast decline of the dissipation rate, attributed to an exponential decay of the volatilization rates, followed by a second phase where the dissipation is due to a first order degradation processes.
- Photophysics and photochemistry of azole fungicides: triadimefon and triadimenolPublication . Silva, José P. da; Silva, Abílio M. da; Khmelinskii, Igor; Martinho, J. M. G.; Ferreira, Luís F. VieiraPhotophysics and photochemistry of pesticides triadimefon {1-(4-chlorophenoxy)-3,3-dimethyl-1-(1H-1,2,4-triazol-1-yl) butanone} and triadimenol {1-(4-chlorophenoxy)-3,3-dimethyl-1-(1H-1,2,4-triazol-1-yl) butan-2-ol} were studied in the solution. The excited singlet states were identified by comparison with the absorption spectra of adequate model compounds, in several solvents. The first excited singlet state of triadimefon is an n, pi* state localized on the carbonyl group, while higher excited states are localized on the chlorophenoxy group and have a pi, pi* character. The lowest singlet state of triadimenol is pi, pi* state, since a methoxyl group replaces the carbonyl group of triadimefon. Triadimefon shows a weak fluorescence from the n, pi* state, upon excitation at both 310 and 250 nm. This suggests a fast intramolecular energy transfer process from the localized pi, pi* state of the chlorophenoxy group to the n, pi* state of the carbonyl group. The photodegradation quantum yield of triadimefon in cyclohexane at 313 run is 0.022. Triadimenol is photostable, under the same conditions. Two major photodegradation products of triadimefon and triadimenol were identified: 4-chlorophenol and 1,2,4-triazole. 4-Chlorophenoxyl radicals were detected by flash photolysis, suggesting a homolytic cleavage of the C-O bond of the asymmetric carbon. (C) 2001 Elsevier Science B.V. All rights reserved.
- Photochemistry of 4-chlorophenol on cellulose and silicaPublication . Silva, José P. da; Ferreira, Luís F. Vieira; Silva, Abílio M. da; Oliveira, A. S.The photochemistry of 4-chlorophenol (4-CP) was studied on silica and cellulose, using time-resolved diffuse reflectance techniques and product degradation analysis. The results have shown that the photochemistry of 4-CP depends on the support, on the concentration, and also on the sample preparation method. Transient absorption and photoproduct results can be understood by assuming the formation of the carbene 4-oxocyclohexa-2,5-dienylidene in both supports. On cellulose, at concentrations lower than 10 mumol g(-1), the carbene leads to the unsubstituted phenoxyl radical, and phenol is the main degradation product. At higher concentrations a new transient resulting from phenoxyl radicals coupling was also observed, and dihydroxybiphenyls are also formed. The reaction of the carbene with ground-state 4-CP was also detected through the formation of 5-chloro-2,4'-dihydroxybiphenyl. 4-Chlorophenoxyl radical and degradations products resulting from its coupling were also detected. Oxygen has little effect on the photochemistry of 4-CP on cellulose. On silica the transient benzoquinone O-oxide was formed in the presence of oxygen. Benzoquinone and hydroquinone are the main degradation products. In well-dried samples the formation of hydroquinone is reduced. At higher concentrations the same products as detected on cellulose were observed. 4-CP undergoes slow photochemical decomposition under solar radiation in both supports. The same main degradation products were observed in these conditions.
- Solution and surface photochemistry of fenarimol: a comparative studyPublication . Silva, José P. da; Mateus, Maria Da Conceição; Silva, Abílio M. da; Ferreira, Luís F. Vieira; Burrows, Hugh D.The photochemistry of the pesticide fenarimol was studied in methanol and on a cellulose surface. The ground state absorption and fluorescence spectra are similar in solution and at the solid/gas interface. Room temperature phosphorescence emission was observed in cellulose. Transient absorption of fenarimol and of its chromophoric units, chlorobenzene and pyrimidine, showed a common absorption band centered at 320 nm in methanol. An absorption band above 350 nm was observed only for fenarimol and was assigned to a recombination intermediate. On cellulose the transient absorption is broader and extends up to 600 nm. The main photodegradation pathway in methanol involves the homolytic cleavage of the bond to the pyrimidyl ring, followed by a fast in-cage recombination of the initially formed pyrimidine and ketyl radicals. On cellulose surfaces, dechlorination is also one of the important photodegradation pathways. (c) 2006 Elsevier B.V. All rights reserved.
- Dissipation of triadimefon on the solid/gas interfacePublication . Silva, José P. da; Silva, Abílio M. da; Khmelinskii, IgorThe dissipation or triadimefon, as pure solid and in the Bayleton 5 commercial formulation, was studied under controlled and natural conditions. Volatilization and photodegradation were shown to be the main dissipation processes. The volatilization results can be described by an empirical model assuming exponential decay of the volatilization rate. The filler of the commercial formulation is determinant for the volatilization but has little effect on the photodegradation rates. The main photoproducts were identified and a reaction mechanism proposed. (C) 2001 Elsevier Science Ltd. All rights reserved.