Mostrando recursos 1 - 20 de 86

  1. Probing the photoreactivity of aryl chlorides with oxygen

    Da Silva, JP; Jockusch, S; Turro, NJ
    Molecular oxygen was used to probe the mechanism of the phototransformation of chlorobenzene and 4-chloroanisole in organic solvents. Laser. ash photolysis, electron paramagnetic resonance and product distribution studies clarified the reaction mechanisms of these compounds under a wide range of conditions. The main primary photochemical reaction step is the homolytic cleavage of the C-Cl bond to produce a triplet radical pair in the solvent cage. In non-polar solvents hydrogen abstraction, after radical diffusion, leads to reduction. In polar solvents, in addition to H-abstraction, electron transfer within the caged radical pair occurs and leads to an ion pair (phenyl cation and...

  2. A diffuse reflectance comparative study of benzil inclusion within p-tert-butylcalix[n]arenes (n=4, 6, and 8) and silicalite

    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...

  3. Surface photochemistry: benzophenone within nanochannels of H+ and Na+ ZSM-5 zeolites

    Ferreira, Luís F. Vieira; Costa, A. I.; Machado, I. Ferreira; Silva, José P. da
    This 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...

  4. Novel laser-induced luminescence resulting from benzophenone/O-propylated p-tert-butylcalix[4]arene complexes. A diffuse reflectance study

    Ferreira, Luís F. Vieira; Ferreira, Margarida R. Vieira; Silva, José P. da; Machado, Isabel Ferreira; Oliveira, A. S.; Prata, José V.
    Laser-induced room temperature luminescence of air-equilibrated benzophenone/O-propylated p-tert-butylcalix[ 4] arene solid powdered samples revealed the existence of a novel emission, in contrast with benzophenone/p-tertbutylcalix[ 4] arene complexes, where only benzophenone emits. This novel emission was identified as phosphorescence of 1-phenyl-1,2-propanedione, which is formed as the result of an hydrogen atom abstraction reaction of the triplet excited benzophenone from the propoxy substituents of the calixarene. Room temperature phosphorescence was obtained in air-equilibrated samples in all propylated hosts. The decay times of the benzophenone emission vary greatly with the degree of propylation, the shortest lifetimes being obtained in the tri- and tetrapropylated...

  5. Self Aggregation of Supramolecules of Nitroxides@Cucurbit[8]uril Revealed by EPR Spectra

    Jayaraj, N; Porel, M; Ottaviani, MF; Maddipatla, MVSN; Modelli, A; Da Silva, JP; Bhogala, BR; Captain, B; Jockusch, S; Turro, NJ; Ramamurthy, V
    Supramolecular complexation behavior of cucurbiturils with paramagnetic nitroxide spin probes was examined by H-1 NMR, X-ray diffraction studies of crystals, computation, and EPR. Both cucurbit[7]uril (CB7) and cucurbit(S]uril (CB8) form a 1:1 complex with 4-(N,N,N-trimethylammonium)-2,2,6,6-tetramethylpiperidinyl-N-oxy bromide (CAT 1). The structure of the complex in the solid state was inferred by X-ray diffraction studies and in the gas phase by computation (B3LYP/6-31G(d)). Whereas ESI-MS data provided evidence for the existence of the complex in solution, indirect evidence was obtained through H-1 NMR studies with a structural diamagnetic analogue, 4-(N,N,N-trimethylammonium)-2,2,6,6- tetramethyl-N-methylpiperidine iodide (DCAT1). The EPR spectrum of the CAT1@CB7 complex consisting of...

  6. Aqueous photochemistry of pesticides triadimefon and triadimenol

    Silva, José P. da; Ferreira, Luís F. Vieira; Silva, Abílio M. da
    The 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...

  7. Release of Guests from Encapsulated Masked Hydrophobic Precursors by a Phototrigger

    Jayaraj, N; Jagadesan, P; Samanta, SR; Da Silva, JP; Ramamurthy, V
    Examples of release of organic acids from encapsulated p-methoxyphenacyl esters provided here demonstrate the value of a phototrigger strategy to release chemicals of interest in water from hydrophobic precursors. In this study, a photochemical beta-cleavage process centered on the p-methoxyphenacyl group is exploited to release the acid of interest from a water-soluble capsule made up of octa acid.

  8. Photochemistry of 4-Chlorophenol and 4-Chloroanisole Adsorbed on MFI Zeolites: Supramolecular Control of Chemoselectivity and Reactive Intermediate Dynamics

    Da Silva, JP; Jockusch, S; Martinho, JMG; Ottaviani, MF; Turro, NJ
    The phototransformation of aryl chlorides adsorbed on MFI zeolites is markedly different from that observed in solution and other solid surfaces such as silica. The formation of radical cations and the constraints imposed by the channels shift the reactivity from the C-CI bond to the O-R bond. Irradiation generates kinetically stabilized intermediates that can be characterized using conventional steady-state spectroscopic techniques, and these intermediates can be used as ordinary chemical reagents.

  9. Photochemistry of 4-chlorophenol on cellulose and silica

    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,...

  10. Differences in Cucurbit[7]uril: Surfactant Complexation Promoted by the Cationic Head Group

    Pessego, M; Da Silva, JP; Moreira, José A.; Garcia Rio, L
    The formation of host/guest complexes between cucurbit[7]uril (CB7) and tetradecyltrialkylammonium bromide surfactants with trimethyl (C(14)TMA(+)), triethyl (C(14)TEA(+)), and tripropyl (C(14)TPA(+)) head groups was studied. Complexation was studied by ESI-MS, isothermal titration calorimetry (ITC), and NMR spectroscopy. The structure of the complexes is strongly dependent on the surfactant head group. Both 1:1 and 2:1 complexes are formed with all cationic surfactants. The magnitude of the K-1:1 binding constants decreases with the alkyl length of the head group, whereas that of the K-2:1 binding constants is independent of the nature of the surfactant head group. Distinct structures were found for the 1:1...

  11. Surface photochemistry of pesticides containing 4-chlorophenoxyl chromophore

    Silva, José P. da; Ferreira, Luís F. Vieira; Osipov, Ivan; Machado, Isabel Ferreira
    The 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...

  12. A diffuse reflectance comparative study of benzil inclusion within microcrystalline cellulose and beta-cyclodextrin

    Ferreira, Luís F. Vieira; Machado, Isabel Ferreira; Silva, José P. da; Oliveira, A. S.
    Diffuse reflectance and laser-induced techniques were used to study photochemical and photophysical processes of benzil adsorbed on two solid powdered supports, microcrystalline cellulose and beta-cyclodextrin. In both substrates, a distribution of ground-state benzil conformers exists, largely dominated by skew conformations where the carbonyl groups are twisted one to the other. Room temperature phosphorescence was observed in air-equilibrated samples in both cases. The decay times vary greatly and the largest lifetime was obtained for benzil/beta-cyclodextrin, showing that this host's cavity accommodates benzil well, enhancing its room temperature phosphorescence. Triplet - triplet absorption of benzil entrapped in cellulose was detected and benzil...

  13. Deep-Cavity Cavitand Octa Acid as a Hydrogen Donor: Photofunctionalization with Nitrenes Generated from Azidoadamantanes

    Choudhury, R; Gupta, S; Da Silva, JP; Ramamurthy, V
    1-azidoadamantane and 2-azidoadamantane form a 1:1 complex with hosts octa acid (OA) and cucurbit[7]uril (CB7) in water. Isothermal titration calorimetric measurements suggest these complexes to be very stable in aqueous solution. The complexes have been characterized by H-1 NMR in solution and by ESI-MS in gas phase. In both phases, the complexes are stable. Irradiation of these complexes (lambda > 280 nm) results in nitrenes via the loss of nitrogen from the guest azidoadamantanes. The behavior of nitrenes within OA differs from that in solution. Nitrenes included within octa acid attack one of the four tertiary benzylic hydrogens present at...

  14. Preparation of novel distinct highly aromatic liquors using fruit distillates

    Galego, Ludovina; Da Silva, JP; Almeida, VR; Bronze, MR; Boas, LV
    P>This work describes the preparation of aromatised liquors using deodorised and concentrated fruit distillates. The raw spirits were improved by making a partial deodorisation, using activated charcoal, followed by concentration, using a distiller. The liquors were prepared by a maceration process. The procedure is exemplified using fig distillates to prepare myrtle berry liquors. The acidity, copper, polyphenol and anthocyanin indexes and volatile and anthocyanin profiles were monitored in each preparation step. The concentration process increased the ethanol proof to 75% v/v and decreased the acidity and the copper content. The partial deodorisation decreased the levels of high molecular weight volatiles,...

  15. Surface photochemistry: photodegradation study of pyrene adsorbed onto microcrystalline cellulose and silica

    Oliveira, A. S.; Ferreira, Luís F. Vieira; Silva, José P. da; Moreira, J. C.
    Ground-state diffuse reflectance, time resolved laser-induced luminescence, diffuse reflectance laser flash-photolysis transient absorption and chromatographic techniques were used to elucidate the photodegradation processes of pyrene adsorbed onto microcrystalline cellulose and silica. Ground-state diffuse reflectance showed that on both substrates low concentrations display absorption of pyrene monomers. At high concentrations spectral changes attributed to aggregate formation were observed. Laser induced fluorescence showed that pyrene onto microcrystalline cellulose mainly presents fluorescence from monomers, while for silica, excimer-like emission was observed from low surface loadings (greater than or equal to 0.5 mumol g(-1)). Transient absorption and photodegradation studies were performed at concentrations where...

  16. Polyphenol and volatile profiles of pomegranate (Punica granatum L.) fruit extracts and liquors

    Galego, Ludovina; Jockusch, S; Da Silva, JP
    This work reports on the preparation of new aromatic pomegranate liquors by maceration of pomegranate juice and arils in Arbutus unedo L. distillate. The volatile, anthocyanin and ellagitannin profiles were monitored after each step. Aromatic compounds of pomegranates, such as limonene, 1-hexenol and trans-caryophyllene, were detected in trace levels and showed little contribution to the liquor aroma. The main volatiles of the liquors, namely isobutanol, isopentanols, vitispiranes and volatile esters, are given by the fruit distillate. The anthocyanin contents of the pomegranate juice were around 135mgL1 but decreased to about 5% of this value in the final liquors. Anthocyanin diglucoside...

  17. A photoinduced pH jump applied to drug release from cucurbit[7]uril

    Parente Carvalho, CP; Uzunova, VD; Da Silva, JP; Nau, WM; Pischel, U
    A proof-of-principle for the application of a photoinduced pH jump for delivery of the Hoechst 33258 drug by disassembly of its host-guest complex with cucurbit[7]uril is described.

  18. A comparative study of the photochemistry of biphenyl adsorbed on cellulose and silicalite

    Da Silva, JP; Ferreira, LFV
    The photochemistry of biphenyl (BP) was studied on two model solid supports, silicalite and cellulose, using time resolved diffuse reflectance techniques and product degradation analysis. The results showed that the photochemical behaviour of BP depends on the solid support. Ground state absorption spectra indicated a near planar configuration in the ground state. BP triplet state was the only species detected on cellulose, while the radical cation (BP.+) was observed in silicalite. BP is relatively stable in both supports. Prolonged irradiations in cellulose lead to the formation of the three monohydroxybiphenyls, while in silicalite photooxidation products prevailed.

  19. Benzoin type photoinitiator for free radical polymerization

    Esen, Duygu Sevinc; Arsu, Nergis; Silva, José P. da; Jockusch, Steffen; Turro, Nicholas J.
    Benzoin, a popular photoinitiator for free radical polymerization of vinyl monomers, was improved by introduction of two methyl thioether substituents. This new benzoin derivative showed an about 50 times higher light absorption in the near-UV spectral region and performed better than the unsubstituted benzoin in polymerization experiments in bulk solutions or films of acrylate monomers when low initiator concentrations are used. Laser flash photolysis, low temperature luminescence experiments and photoproduct studies by mass spectrometry suggest that a slow -cleavage mechanism (k = 2.2 x 105 s1) from the electronic triplet state with a quantum yield of 0.1 is the primary...

  20. Supramolecular photocatalysis: insights into cucurbit[8]uril catalyzed photodimerization of 6-methylcoumarin

    Pemberton, BC; Singh, RK; Johnson, AC; Jockusch, S; Da Silva, JP; Ugrinov, A; Turro, NJ; Srivastava, DK; Sivaguru, J
    Guest induced shape change of the cucurbit[8]uril cavity is likely rate limiting in the supramolecular photocatalytic cycle for CB8 mediated photodimerization of 6-methylcoumarin.

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