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- 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.
- 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: 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.
- 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.