Fotoionizacao e fotoprotonacao de sistemas aromaticos em solucao:Um estudo de transientes por fotolise laser de um e dois pulsos

The laser flash photolysis technique is used to study the formation and reactivity of short lived transients, namely radicals, cations and radical cations, in solution. The photoionization of neutral radicals in polar solvents is reported, using two-pulse laser flash photolysis. Following this approach, the triphenylmethyl radical was photoionized to the corresponding triphenylmethyl cation in water (#tau#_1/2=4#mu#s). A series of diphenylmethyl systems was photolyzed (#lambda#=248-nm). Following this process two types of transients were observed, the electronically excited diphenylmethyl radicals and their related cations. Finally, and still within the framework of radical ionization processes using the double pulse technique, employing conductimetric detection, the reactivity of several #alpha#-substituted benzyl cations was studied in aqueous solution. The relevance of this last group of substrates for polymerization processes is briefly discussed. All together, the results obtained from these studies lead to the formulation of a novel ionization mechanism, which was tested under different conditions (influence of an electron scavenger, nature of the solvent and radical structure). With the aid of conventional laser flash photolysis, two types of studies were undertaken, direct photoionization of organic substrates to yield short lived radical cations, and photoprotonation of electronically excited aromatic systems to yield short lived carbocations. The studies on the photolysis of several tetraarylethanes in polar solvents showed that photoionization competes efficiently with homolysis of the central C-C bond. The radical cation generated after electron photoejection to the solvent undergoes sidechain fragmentation to the corresponding diphenylmethyl radicals and cations. In the case of less conjugated systems such as the tetraalkyl-diarylethanes, a similar mechanism was found to be operative in polar solvents. However in the protic solvent 1,1,1,3,3,3-hexafluoroisopropanol, efficient photoprotonation of the aromatic ring was the major observed process. As result, a short lived (<20ns) cyclohexadienyl cation is formed which..Available from Fundacao para a Ciencia e a Tecnologia, Servico de Informacao e Documentacao, Av. D. Carlos I, 126, 1200 Lisboa / FCT - Fundação para o Ciência e a TecnologiaSIGLEPTPortuga