The photochemistry of electron donor–acceptor (EDA) complexes in micellar solutions. I. The stilbene–methylviologen system

Abstract

M. HAMITY and R. H. LEMA. Can. J. Chem. 66, 1552Chem. 66, (1988.The association constants for the electron donor-acceptor (EDA) complexes formed between both cis-stilbene (cS) and trans-stilbene (tS) as donors and methylviologen (MV+2) as acceptor were determined in ethanol and micellar sodium dodecyl sulfate (SDS) solutions in the range of SDS concentration 0.05-0.1 M. The values obtained in micellar solutions were much higher than those in ethanol and were heavily dependent upon SDS concentration. This effect is due to an increase in the local concentration of the reactants in the micellar pseudophase. The tS fluorescence quenching by MV+2 was also studied in the same solvent media. In ethanol, the Stem-Volmer plot was found to be linear, with quenching constant (Ksv) similar to the association constant deteimined by the absorption method. In micellar solutions, although upward curvature of the Stem-Volmer plots was observed, a reaction scheme based on static quenching via ground state EDA complex is proposed, which explains the experimental results. Irradiation in the absorption band of the EDA complexes formed by tS or cS and MV+2 was carried out in ethanol and SDS solutions, in the absence of oxygen. Only cis-trans isomerization of cS in SDS solution was observed, with a quantum yield value of @ , , = 0.012.

The photochemistry of EDA complexes in micellar solutions. II. The stilbene–methylviologen system in the presence of oxygen

“…The results obtained in this work and those reported in a previous paper (17) indicate the occurrence of stilbene radical cation S'+ in the photoisomerization and photooxygenation of stilbene by excitation of the stilbene/methylviologen EDA complex.…”

“…Experiments with the cS EDA complex were carried out under experimental conditions similar to those for the tS complex. As shown in Table 3, oxygen inhibits the photoreaction of cS (17). An increase of medium acidity decreases this inhibition.…”

“…This is indicative that the reaction scheme presented to explain the experimental results in ethanol can also be used in SDS solutions. For micelle-bound reactants, as with MV+' and the stilbenes (17) The medium acidity is given through the bulk water proton activity aHw = 10-pH = [H+],, where pH is measured by a glass electrode. The local interfacial proton activity a,<" - [H+],, at the surface of charged micelles, can be expressed by the following equation (26,27):…”

“…In a previous work we carried out the photolysis, exciting the absorption charge transfer band, of the EDA complexes formed by cS and tS with MV+2 in micellar SDS solution and, for the sake of comparison, in homogeneous ethanolic solutions, in the absence of oxygen (17). We report here the results obtained by photolysis of the same systems in the presence of oxygen and the influence exerted on quantum yields by methylviologen concentration and the acidity of the media.…”