Synthesis of new photosensitive H2BBQ2+[ZnCl4]2-/[(ZnCl)2(µ-BBH)] complexes, through selective oxidation of H2O to H2O2.

A new two-electron photosensitizer, H2BBQ2+[ZnCl4]2-/[(ZnCl)2(µ-BBH)] (BBQ stands for 2,5-bis[bis(pyridin-2-ylmethyl)amino]-1,4-quinone and BBH stands for 2,5-bis[bis(pyridin-2-ylmethyl)amino]-1,4-hydroquinone), has been synthesized and the oxidation state of the ligand was determined by X-ray crystallography and NMR spectroscopy. Under light illumination the H2BBQ2+[ZnCl4]2- + ZnCl2 is reduced quantitatively to [(ZnCl)2(µ-BBH)] (pH ∼ 5) oxidizing H2O to H2O2 as is evident by trap experiments. Electrochemistry gave a reversible two-electron ligand-centered redox wave for [(ZnCl)2(µ-BBH)]. UV-Vis, luminescence and EPR spectroscopies reveal the slow formation of a stable quinone diradical, intermediate of the reaction. DFT calculations are in agreement with the proposed mechanism. Based on this property an aqueous {[(ZnCl)2(µ-BBH)]||H2O2} solar rechargeable galvanic cell has been constructed.

Photophysical Behavior of a New Gemini Surfactant in Neat Solvents and in Micellar Environments.

A novel fluorescent gemini surfactant, 1,4-bis-(2'-(N-dodecyl pyridinio-4"-yl)ethenyl)benzene dibromide, abbreviated BDPEBB, has been synthesized and its photophysical properties have been studied in different environments. BDPEBB has a limited solubility in alcohols where it is found in aggregate form at concentrations>/=1 mM. In other solvents, e.g., water, it is only found in aggregate form, even at much lower concentrations. Solvent polarity has a small and insignificant solvatochromic effect but alcohols give a specific interaction with BDPEBB, causing a significant hypsochromic shift in absorption maxima and a large increase in relative fluorescence efficiency. Pyrene fluorescence is effectively quenched by BDPEBB. Pyrene also forms associative complexes with BDPEBB in water. These complexes are partly dissociated in the presence of surfactant micelles. Triton X-100 micelles provide a favorable environment for BDPEBB solubilization well distinguished from the behavior of ionic surfactants. Small quantities of BDPEBB have a large influence on the behavior of aqueous sodium dodecylsulfate (SDS) and sodium decylsulfate (SDeS) micelles, inducing the formation of large aggregates, visible by the naked eye. These large aggregates are most probably microcrystals of BDPEBB(2+)/2DS(-) or BDPEBB(2+)/2DeS(-). The aggregation number of SDS and SDeS micelles in the absence and in the presence of BDPEBB has been calculated by exploitation of the static luminescence quenching kinetics of Ru(bpy)(3)(2+) by 9-methylanthracene, both solubilized in the micellar phase. It has been observed that Ru(bpy)(3)(2+) inhibits the precipitation of SDeS micelles in the presence of BDPEBB. Our results suggest that double-chain surfactant chromophores should be employed with particular care if they are to be used as probes of the micellar phase. Copyright 2000 Academic Press.

Structural and Dynamic Properties of Lecithin-Alcohol Based w/o Microemulsions: A Luminescence Quenching Study

Water-in-oil microemulsions have been made with lecithin in the presence of some alcohols. The structure of the microemulsions has been studied by steady-state and time-resolved analysis of the luminescence quenching of Ru(bipy)32+ by Fe(CN)63-. We found that well-defined microemulsions can be made only in the presence of short-chain alcohols such as propanol-1 and butanol-1. There exists a threshold for water content in order to obtain typical reverse micelles. Thus in the case of propanol-1, water/surfactant ratio wo should be above 20. By varying water content in the range 20 < wo

Photophysical Studies in AOT Films Deposited on Fused Silica Slides

Thin surfactant films were made by dip-coating a fused-silica slide in reverse micelles of bis(2-ethylhexyl) sulfosuccinate sodium salt [AOT] in cyclohexane. The fluorescence quenching analysis of charge transfer between Ru(bpy)$^{2+}_3$ and MV2+, of resonance energy transfer between pyrene and coumarin-153, and of pyrene excimer formation has revealed that the original micellar structure is to a large extent transferred into the structure of the film. The film consists of a hydrophilic and a hydrophobic subphase. Nondried films contain entrapped cyclohexane which allows diffusion of the hydrophobic probes in the film and a rather random distribution of them. Drying leads to solvent evaporation and hydrophobic probe aggregation. The kinetics of the hydrophilic probes reflect the structure of the hydrophilic domain which is not affected by drying procedure. The above films are transparent matrices and make excellent hosts for photophysical studies.

Reactions in lipid vesicles. Pyrene excimer formation in restricted geometries. Effect of temperature and concentration.

The decay of pyrene in the presence of excimers in small unilamellar vesicles of 3-sn-phosphatidyl glycerol, dipalmitoyl and 3-sn-phosphatidylglycerol from egg yolk has been analyzed with the use of models appropriate for reactions in restricted geometries. Results are presented with emphasis on probe concentration and temperature. The reaction rate in an organized lipid phase is redefined in a simple manner which allows for a simple treatment of any reaction in such environments. The analysis allows detection of pyrene aggregation in the vesicle lipidic core.

A subnanosecond-pulse fluorometric study of the CA2+ and MG2+ induced conformational changes on S-100a protein.

The fluorescence parameters of the single tryptophan residue (Trp 90 alpha) in S-100a (alpha beta) protein have been studied by steady state fluorimetry and by subnanosecond fluorescence decays excited by pulsed-picosecond laser system. At pHs 7.1 and 8.5, double exponential decays were consistently observed. At both pHs, Ca2+ and to a less extent Mg2+ ions proved to modify the percentage contribution of the two decaying species. The interest of the finding is discussed.