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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.
- Surface photochemistry: benzophenone as a probe for the study of silica and reversed-phase silica surfacesPublication . Ferreira, Luís F. Vieira; Machado, I. Ferreira; Silva, José P. da; Branco, T. J. F.This work reports the use of benzophenone, a very well characterized probe, to study new hosts: two reversed-phase silicas. Laser-induced room temperature luminescence of argon purged solid powdered samples of benzophenone adsorbed onto the two different reversed-phase silicas, RP-18 and RP-8, revealed the existence of a low energy emission band in contrast with the benzophenone adsorbed on 60 A pore silica, where only triplet benzophenone emits. This low energy emission band was identified as the fluorescence of the ketyl radical of benzophenone, which is formed as the result of a hydrogen atom abstraction reaction of the triplet excited benzophenone from the alkyl groups of the surface of the reversed silicas. Such emission does not exist for benzophenone adsorbed onto 60 A pore silica. Room temperature phosphorescence was obtained in argon purged samples for all the surfaces under use. The decay times of the benzophenone emission vary greatly with the alkylation of the silica surface when compared with "normal" silica surface. A lifetime distributions analysis has shown that the shortest lifetimes for the benzophenone emission exist in the former case. Triplet-triplet absorption of benzophenone was detected in all cases and is the predominant absorption in the case of 60 A pore silica, while benzophenone ketyl radical formation occurs in the case of the reversed silicas. Diffuse reflectance laser. ash photolysis and gas chromatography-mass spectrometry techniques provided complementary information, the former about transient species and the latter regarding the final products formed after laser irradiation, both at 266 nm or 355 nm. Product analysis and identification show that the degradation photoproducts are dependent on the excitation wavelength, the photochemistry being much more rich and complex in the 266 nm excitation case, where an alpha-cleavage reaction occurs. A detailed mechanistic analysis is proposed.
- A diffuse reflectance comparative study of benzil inclusion within p-tert-butylcalix[n]arenes (n=4, 6, and 8) and silicalitePublication . Ferreira, Luís F. Vieira; Machado, I. Ferreira; Oliveira, A. S.; Ferreira, M. R. Vieira; Silva, José P. da; Moreira, J. C.Diffuse reflectance and laser-induced techniques were used to access photochemical and photophysical processes of benzil in solid supports, namely p-tert-butylcalix[n]arenes with n = 4, 6, and 8. A comparative study was performed using these results and those obtained with another electronically inert support, silicalite, which is a hydrophobic zeolite. In the latter substrate, ground-state benzil has the two carbonyl groups in an s-trans planar conformation while in the calixarenes a distribution of conformers exists, largely dominated by skew conformations where the carbonyl groups are twisted one to the other. In all substrates, room-temperature phosphorescence was obtained in air-equilibrated samples. The decay times vary greatly and the largest lifetime was obtained for benzil/p-tert-butylcalix[6]arene, showing that this host cavity well accommodates benzil, enhancing its room-temperature phosphorescence. p-tert-Butylcalix[6] and [8]arene molecules provide larger hydrophobic cavities than silicalite, and inclusion complexes are formed with these hosts and benzil as guest; p-tert-butylcalix[4]arene does not include benzil. This probe is deposited outside the calix[41 cavity, in the form of microcrystals. Triplet-triplet absorption of benzil was detected in all cases and is predominant in the silicalite channel inclusion case. Benzil ketyl radical formation occurs with inclusion in calix[6]arene and calix[8]arene. In the three cases, benzoyl radical was detected at long times (in the millisecond time scale). Product analysis and identification clearly show that the main detected degradation photoproducts in all substrates are benzoyl radical derivatives. Calix[6] and [8]arenes are able to supply hydrogen atoms that allow also another reaction, the reduction to benzoin through benzil ketyl radical formation.
- 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.
- 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.