No description available.

No description available.

Naphthoxazine-spiro-indolines (NOSIs) can exist in a colourless and a coloured form. These compounds can interconvert between these two forms either photochemically or thermally and are resilient to degradation even after repeated cycles of colouration and bleaching. Absorption spectra of both the stable colourless form and the energetically less stable coloured form have been measured. Several NOSI compounds have been shown to photoconvert to the coloured form with an efficiency of between 0.06–0.74, depending upon the compound conditions under which the conversion is brought about. The factors which have been varied in this work are: the basicity of the 6'-substituent; the size of the N-alkyl group within these compounds; and the nature solvent or matrix in which the reaction occurs. Electron donation from the 6'-group has been shown to increase the quantum yield of the forward reaction whereas solvent polarity increases have been shown to reduce the quantum yield.

Studies have been carried out investigating the photochemical and photophysical properties of anthracene adsorbed on silica gel. The photochemistry and photo physics of anthracene in solution are well reported and known, hence its choice as a probe for the silica gel surface. UV -visible absorption and fluorescence spectra of anthracene adsorbed on silica gel reveal aggregate formation at very low loadings (1 % of a monolayer) indicating preferential adsorption occurs at some surface sites. Laser flash photolysis at 355 nm produces both the triplet and radical cation of anthracene, their production was found to be mono- and multi-photonic respectively. The decays of both these transients were complex and the rates increased with increasing loading. Analysis of the triplet state decay has been carried out by studying the delayed fluorescence which arises from triplet triplet annihilation. Fractal and twodimensional models have been used to describe this bimolecular decay. The coadsorption of anthracene and an electron donor having an oxidation potential below 1.09 V on silica gel causes electron transfer to occur from the electron donor to the anthracene radical cation produced following laser flash photolysis at 355 nm. Studies using a selection of electron donors with varying reduction potentials were carried out. The electron donor transfers an electron to the anthracene radical cation, thus greatly accelerating its rate of decay; for electron donors such as triphenylamine, N,N-dimethylaniline and N,N,N',N'tetramethyl- l,4-phenylenediamine the rise of the donor radical cation is observed as the anthracene cation decays. These systems were studied using fluorescence measurements and laser flash photolysis to study any fluorescence quenching and the rate of decay of both the anthracene triplet and radical cation. A selection of anthracene derivatives adsorbed onto silica gel were also briefly studied to see the effect of substituent group and its position. Symmetrically substituted dialkoxyanthracenes and 9-cyanoanthracene were used. The transient absorption spectra of the 2,3- and 2,6-dialkoxyderivatives and 9-cyanoanthracene revealed spectral similarities with that of unsubstituted anthracene. The spectra of9,10- and I,S-didecyloxyanthracene showed significant differences in the radical cation spectra to those obtained for unsubstituted anthracene.

Photoallergic compounds cause an allergy when applied to the skin and the skin is then exposed to light. Photoallergy is connected with the ability of the allergen to bind to the skin. This thesis describes much basic photochemistry of several photoallergens and has explained any differences in that photochemistry when on surfaces and in the presence of H.S.A.

No description available.

No description available.

t-Butoxyl radical has been used as a chemical model for hydrogen abstractions in many enzymatic and biological systems. However, the question has arisen as to how well this reactive intermediate mimics these systems. In addressing this concern, absolute rate constants and Arrhenius parameters for hydrogen abstraction by t-butoxyl radical were measured for a broad class of substrates including amines, hydrocarbons, and alcohols using laser flash photolysis. Initially, no obvious reactivity relationship between rate constant and substrate structure was observed for these homolytic reactions. However, by closely examining the Arrhenius parameters for hydrogen abstraction, a pattern was revealed. For substrates with C-H bond dissociation energy (BDE) > 92 kcal/mole, activation energy increases with increasing BDE (as expected). However, for substrates with a lower BDE, the activation energy levels out at approximately 2 kcal/mole, essentially independent of structure. Viscosity studies with various solvents were conducted, ruling out the possibility of diffusion-controlled reactions. Entropy rather than enthalpy appears to be the dominating factor at 25°C, contributing to the free energy barrier for these reactions. Laser flash photolysis was also used to study radical anions. Using an indirect photoexcitation method, the properties of radical anions, generated from aryl ketones, were investigated. These radical anions, such as t-butyl phenyl ketone and cyclopropyl phenyl ketone, measured to have decay rate constants of 1.0 x 106 s-1, although they are known to be persistent when studied electrochemically. They also had measured activation energies around 6.0 kcal/mole and log A values close to 9.5. By extending the molecules's conjugation, the decay rate constants increased to greater than 107 s-1, decreased their activation energy by half, and lowered the log A values to 8.0. This trend was observed in aryl ketones such as trans-1-benzoyl-3-phenyl cyclopropane. It is believed that the generation of a benzyl radical during the decay that facilitates the enhancement of the unimolecular decays. These unimolecular decays were also observed with the previously studied hypersensitive SET probes, 5,7-di-tert-butylspiro[2.5]octa-4,7-dien-6-one and 1,1,-dimethyl-5,7-di-tert-butylspiro[2.5]octa-4,7-dien-6-one. The decay rate constants for these radical anions were measured to be greater than 108 s-1, driven by the formation of an aromatic ring. / Ph. D.

Pulsed laser flash photolysis with both visible and infrared detection has been applied to the study of the displacement of weakly coordinating ligands (Lw) by strongly "trapping" nucleophiles (Ls) containing either an olefinic functionality (Ls = 1-hexene, 1-decene, 1-tetradecene) or nitrogen (Ls = acetonitrile, hydrocinnamonitrile) from the photogenerated 16 electron pentacarbonylchromium (0) intermediate. 5-Chloropent-l-ene (Cl-ol), a potentially bidentate ligand, has been shown to form (ol-Cl) pentacarbonylchromium (0), in which Cl-ol is bonded to Cr via a lone pair on the chlorine, and isomerize to (Cl-ol) pentacarbonylchromium (0), in which Cl-ol is bonded to the olefinic functionality on the submillisecond time scale. This process has been studied in both the infrared and visible region employing both fluorobenzene or n-heptane as the "inert" diluent. Parallel studies employing 1-chlorobutane and 1-hexene were also evaluated and showed great similiarity with the Cl-ol system. The data supported a largely dissociative process with a possibility of a small interchange process involving the H's on the alkyl chain. Studies were also carried out for various Cr(CO)6/arene/Ls systems (arene = various alkyl or halogenated substituted benzenes). The data indicated that for both C6H5R (R=various alkyl chains) or multi-alkyl substituted arenes (i.e. o-xylene, 1,2,3-trimethylbenzene) containing an "unhindered" ring-edge, bonding to the the Cr(CO)5 moiety occurs "edge on" via a partially delocalized center of unsaturation on the ring. The data indicated that both electronic and steric properties of the arenes influence the kinetics, and that an interchange pathway takes place at least, in part, through the alkyl chains on both the arenes and "trapping" nucleophiles. Moreover, halogenated arenes bond through the lone pair on the halogen for both CI- and Br- derivatives but "edge-on" for the fluorinated arenes. Finally, in the case of arene complexes without and "unhindered" ring-edge (i.e., 1,2,3,4,5-pentamethylbenzene) bonding can occur either "edge-on" or through the ring center of the arene or combination of the two. Carbonyl stretching frequencies for the arenes are also indicative of the type of bonding.

Três macroiniciadores foram sintetizados baseados em tioxantona; MMa-co-TXA, BMa-co-TXA e HMa-co-TXA. Estes macroiniciadores possuem vantagens em relação aos compostos de baixa massa molar apesar do seu alto custo. A fotoatividade destes macroiniciadores e da tioxantona para a polimerização de monômeros monofuncional (MMA) e multifuncionais, 2,2-bis[4-(2-hidroxi-3-metacriloxipropoxi)fenil]propano (Bis-GMA) e dimetacrilato de trietilenoglicol (TEGDMA) foi examinada com as técnicas de Fotocalorimetria Exploratória Diferencial e Espectroscopia de Infravermelho por Transformada de Fourier com acessório de ATR. Os co-iniciadores usados foram as aminas EDB, TEA e DMAEMA. Todos os sistemas foram estudados na ausência de solvente. Os resultados mostraram que os macroiniciadores são mais eficientes do que o composto de baixa massa molar. Medidas de fotólise por pulso de laser permitiram obter o espectro de absorção de transientes dos compostos estudados, bem como a constante de supressão do estado triplete pelas aminas e pelo monômero. A polimerização fotoiniciada pelos macroiniciadores na presença de aminas e dos monômeros foi estudada com o objetivo de se determinar o mecanismo que leva à formação dos radicais iniciadores. Expressões para o rendimento quântico de radicais ativos foram deduzidas a partir do mecanismo proposto. Resultados mostraram que a produção de radicais ativos para os macroiniciadores é maior do que a tioxantona. / In this work three macroinitiators based on thioxanthone were synthesized and characterized; MMA-co-TXA, BMA-co-TXA and HMA-co-TXA. Macroinitiators offer some advantages when compared with their corresponding low molecular weight analog. The photopolymerization initiated by macroinitiators and thioxanthone of methyl methacrylate (MMA) and mixtures of 2,2-bis[4-(2-hydroxy- 3-metacryloxipropoxi)phenyl]propane (Bis-GMA) and triethyleneglycol dimethacrylate (TEGDMA) was studied through Photocalorimetry (Photo-DSC) and Fourier Transform Infrared Spectroscopy with ATR accessory. All systems were studied in the absence of solvent (bulk) using EDB, TEA and DMAEMA, as co-initiators. Measures of Laser Flash Photolysis determined the transient absorption spectra of the compounds and also the bimolecular rate constants for the triplet quenching of the compounds. The polymerization reaction of the macroinitiator in presence of amine and monomer was studied in order to determine the mechanism leading to the formation of radical initiators. Expressions for the quantum yield of radicals assets were deducted from the proposed mechanism. Results showed that the radical production assets to macroinitiator is greater than the thioxanthone, making macroinitiators more efficient than the low molecular weight compound.