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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.
- Surface photochemistry: dibenzo-p-dioxin adsorbed onto silicalite, cellulose and silicaPublication . Ferreira, Luís F. Vieira; Silva, José P. da; Machado, I. Ferreira; Branco, T. J. F.; Moreira, J. C.Laser-induced luminescence of argon purged and air-equilibrated samples of dibenzo-p-dioxin adsorbed onto three different solid powdered supports, silicalite, cellulose and silica, revealed the existence of both fluorescence and phosphorescence emissions at room temperature. A remarkable difference in transient absorption spectra was found when dibenzo-p-dioxin was included within the narrow internal channels of silicalite: triplet-triplet absorption of dibenzo-p-dioxin was detected in the silicalite case as a host, simultaneous with radical cation formation immediately after pulsed laser excitation (in the hundreds of nanoseconds time scale) while in the case of cellulose and silica as adsorbents, absorption transients arising from 2,2'-biphenylquinone and possibly from the spiroketone were found. For all hosts dibenzo-p-dioxin exhibits a transient band peaking at 330-340 nm in the microsecond and millisecond time scales, which we assigned to the biradical of dibenzo-p-dioxin. Diffuse reflectance laser flash photolysis and chromatographic techniques provided complementary information, the former about transient species and the latter regarding the final products formed after laser irradiation at 266 nm. Product analysis and identification clearly show that the photodegradation products are dependent on the host, the photochemistry being much more rich and complex in the cellulose and silica cases, where the main detected photoproduts were 2,2'-dihydroxybiphenyl and 1-hydroxydibenzofuran. In the case of silicalite as host, which has a channel-like internal structure and reduced space available for the guest dioxin, photodegradation reactions are highly reduced or even inhibited and no photodegradation products were detected. (c) 2006 Elsevier B.V. All rights reserved.
- Surface photochemistry: benzophenone within nanochannels of H+ and Na+ ZSM-5 zeolitesPublication . Ferreira, Luís F. Vieira; Costa, A. I.; Machado, I. Ferreira; Silva, José P. daThis work reports a surface photochemistry study of benzophenone within nanochannels of MFI zeolites, in this case a series of H+ and Na+ ZSM-5 zeolites where the acidity of the host is varied systematically. Laser-induced room temperature luminescence of air equilibrated or argon purged solid powdered samples of benzophenone adsorbed onto the two sets of zeolites, which we will name HZSM-5 and NaZSM-5 zeolites, revealed the existence of three different benzophenone emissions. In the nanosecond time range a broad band centred at about 430 nm was detected and its origin was hydrogen bonded benzophenone. A second broad band peaking at about 470 nm and in the millisecond time range was also detected, which was the emission of the protonated benzophenone. Finally a vibrationally structured emission existed (448 nm), the normal room temperature phosphorescence emission of benzophenone. Triplet-triplet transient absorption of benzophenone was obtained for all zeolites under study, and clear evidence for the formation of hydroxybenzophenone radicals was obtained for the NaZSM-5 series (only Lewis acid sites). The more acidic the internal surface the more radical was detected. Other products resulting from a-cleavage of benzophenone were also identified. When the host was HZSM-5 (Bronsted and Lewis acid sites) benzophenone seemed to be less reactive, and no ketyl radicals from benzophenone were detected. All these findings clearly show that benzophenone is an excellent probe which is able to provide important information regarding the internal surfaces of MFI zeolites, namely the different photocatalytic activity of the different active sites of these zealites. The present study is paradigmatic in what regards the host influence on the photochemistry of the guest. (C) 2008 Elsevier Inc. All rights reserved.
- Photochemistry of benzophenone adsorbed on MCM-41 surfacePublication . Silva, José P. da; Machado, I. F.; Lourenço, J. P.; Ferreira, Luís F. VieiraThe photochemistry of benzophenone adsorbed on a channel-type solid support MCM-41 was studied by the use of diffuse reflectance and chromatographic techniques. For comparison purposes parallel studies were also conducted on silicalite and H-ZSM-5. The observed ground state absorption, luminescence and transient absorption spectra and photoproduct distribution strongly depend on the substrate under study. Triplet state luminescence was observed in all solids, hydrogen-bonded excited benzophenone emission was detected on MCM-41 while protonated excited benzophenone luminescence was observed only on H-ZSM-5. Transient absorption showed the triplet state in all solids and formation of the OH benzophenone adduct on MCM-41 and H-ZSM-5. Ketyl radical formation occurs after 355 nm irradiation but is a minor degradation pathway. The formation of transients resulting from the reaction of benzophenone with molecular oxygen was also detected on MCM-41, in air-equilibrated conditions and benzoic acid is the main degradation product. 2-Hydroxybenzophenone is one of the major photodegradation products on MCM-41 and H-ZSM-5. Products resulting from the a-cleavage were also detected under 266 nm irradiation. Benzophenone was found to be very stable in silicalite under all used conditions.
- Surface photochemistry of pesticides containing 4-chlorophenoxyl chromophorePublication . Silva, José P. da; Ferreira, Luís F. Vieira; Osipov, Ivan; Machado, Isabel FerreiraThe surface photochemistry of pesticides containing 4-chlorophenoxyl chromophore was accessed on the model surfaces cellulose and silica, using diffuse reflectance and chromatographic techniques. 4-Chloroanisole was chosen as the model compound and its phototransformation was followed under lamp (254 nm) and sunlight irradiation. The photodegradation rates are faster on cellulose than on silica. The main photodegradation products on cellulose are anisole and dimerization compounds while on silica the formation of 4-hydroxyanisole and dimerization are the main reaction pathways. Transient absorption studies showed the formation of the 4-methoxyphenylperoxyl radical only on cellulose and in the presence of oxygen. The homolytic cleavage of the C-Cl bond, and consequent formation of a triplet radical pair, is the main primary reaction step on both surfaces. The radical pair then diffuses apart or undergoes electron transfer to form phenyl radicals and phenyl cations, respectively. These two reaction intermediates account for reduction, addition, and formation of bonded residues, the main reactions observed on the studied surfaces and also expected to prevail under natural conditions. (c) 2010 Elsevier B.V. All rights reserved.
- Surface photochemistry: diffuse reflectance studies of thioketones included into p-tert-butylcalix[6 and 8]arenesPublication . Ferreira, Luís F. Vieira; Machado, I. Ferreira; Oliveira, A. S.; Silva, José P. da; Krawczyk, A.; Sikorski, M.This paper presents time resolved laser induced luminescence (LIL) and diffuse reflectance laser flash-photolysis (DRLFP) studies of solid powdered samples of xanthione (XT) and 4H-1-benzopyran-4-thione (BPT). These thioketones form inclusion complexes with ptert-butylcalix[6]arene (CLX[6]) and p-tert-butylcalix[8]arene (CLX[8]). Room temperature air equilibrated LIL studies for XT/CLX[8] inclusion complex have shown the existence of both fluorescence and delayed fluorescence emission S-2 -> S-0 emissions in the nanosecond and microsecond time ranges, respectively) peaking at ca. 460 nm while phosphorescence peaks at about 670 nm with a half life of about 4 mu s. For the BPT/CLX[8] inclusion complex, the emissions now peak at ca. 450 and 630 nm, respectively. When these two thioketones are included into the CLX[6] nanocavities similar luminescence spectra were detected. The phosphorescence spectra in all cases present a clear vibrational structure characteristic of non-polar environments, in accordance with the non-polar character of the calixarene's cavities. Diffuse reflectance transient absorption spectra revealed in all cases the presence of the triplet state of the thioketones and also of longer lived species. The use of chromatographic methods (GC-MS) allowed us to identify the formation of xanthone, chromone and hydroxy-thio ketones following laser irradiation at 355 or 337 nm. (c) 2006 Elsevier B.V. All rights reserved.
- 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.
- Photochemistry of benzophenone on Ti-MCM-41 surfacesPublication . Silva, José P. da; Machado, I. F.; Lourenço, J. P.; Ferreira, Luís F. VieiraDiffuse reflectance and chromatography techniques were used to study three Ti-MCM-41 solid powders, containing 1%, 2.5% and 8% of Ti, and to investigate the photochemistry of benzophenone adsorbed on their surfaces. The observed ground state absorption, luminescence and transient absorption spectra depend on the Ti content of the solid support. The luminescence intensity of the pure powders and of the solids with adsorbed benzophenone decreases with the Ti content. The benzophenone triplet emission was detected in all solids but its half-lives decrease with the Ti content of the support. Hydrogen-bonded excited benzophenone luminescence is also quenched but was detected only on supports with low Ti percentage. Transient absorption reveals the formation of trapped electrons and the presence of ligand-to-metal charge transfer in the pure solids, under 266 nm excitation. Ti aggregates are present in all used solid supports. Triplet–triplet absorption of benzophenone was observed in all solids under argon atmosphere. Transient absorption due to the OH–benzophenone radical was detected in all conditions and increases with the Ti content of the powder. 2-Hydroxybenzophenone and benzoic acid are the major photodegradation products in air-equilibrated conditions.