Surfactants electron transfer systems

An attractive way to overcome this problem is to use microheterogeneous photocatalytic systems based on lipid vesicles, i.e. microscopic spherical particles formed by closed lipid or surfactant bilayer membranes (Fig. 1) across which it is possible to perform vectorial photocatalytic electron transfer (PET). This leads to generation of energy-rich one-electron reductant A" and oxidant D, separated by the membrane and, thus, unable to recombine. As a result of such PET reactions, the energy of photons is converted to the chemical energy of spatially separated one electron reductant tmd oxidant. 

In the artificial system Figure 4b, a polymerized surfactant vesicle is substituted for the thylakoid membrane. Energy is harvested by semiconductors, rather than by PSI and PSII. Electron transfer is rather simple. Water (rather than C02) is reduced in the reduction half cycle to hydrogen, at the expense of benzyl alcohol. In spite of these differences, the basic principles in plant and mimetic photosyntheses are similar. Components of both are compartmentalized. The sequence of events is identical in both systems energy harvesting, vectorial charge separation, and reduction. 

From the point of view of light stability and range of absorptivity, inorganic redox systems might be more interesting. Photoinduced electron transfer in an aqueous solution of tris-(2, 2 -bipyridine) ruthenium (II) has been found to decompose water in to a mixture of H2 and 02. The Complex can serve both as an electron donor and electron acceptor in the excited state. The efficiency is low because of barrier to electron transfer. SVhen spread as a monolayer on glass slides after attaching to a surfactant... 

DBS) are employed. The rate of electron transfer and its dependence upon chromophore concentration suggest that the electron tunnels from a chromophore on the inner surface of the vesicle to one on the outer surface rather than being carried by flipping of the surfactant chromophore from one surface to the other. In the system involving amphiphilic zinc porphyrin... 

Photodecomposition of azo compounds can be brought about by single electron transfer. Thus sensitization of (14) (Scheme 6) by 9,10-dicyanoanthra-cene (DCA) results in the formation of the trityl cation and a phenyl radical. Photocleavable surfactants have several potential applications, e.g. in water-based paints and coatings and in advanced drug-delivery systems. The azo group can be used to provide a photocleavable link between the tail and headgroup of a surfactant molecule, and a number of such surfactants, based on the azosulfonate functionality, have been synthesized (15, R = hexyl, octyl, decyl and dodecyl). When aqueous solutions of these surfactants are photolysed, N2 and the sulfite... 

Fluorescence lifetimes of diphenylhexatriene in molecules located in both flat and bent bilayer liquid membranes show the effect of changes both in exposure to water and burial within the nonpolar membrane . The effect of hydrostatic pressure on the system confirms the interpretation put forward to account for these effects. Photochemical electron transfer across surfactant bilayers has been shown to be mediated by the presence of 2,l,3-benzothiadiazole-4,7-dicarbonitrile . 

Photoinduced electron transfer in functional surfactant system... 

NaLS) by copper(II) yields assemblies in which Cu2+ ions constitute the counter ion atmosphere of the micelle (Fig. 4.8). These may be photoreduced to the monovalent state by suitable donor molecules incorporated in the micellar interior. An illustrative example is that where D = N,N -dimethyl 5,11-dihydroindolo 3,3-6 carbazole(DI). When dissolved in NaLS micelles, DI displays an intense fluorescence and the fluorescence lifetime measured by laser techniques is 144 ns. Introduction of Cu2+ as counterion atmosphere induces a 300 fold decrease in the fluorescence yield and lifetime of DI. The detailed laser analysis of this system showed that in Cu(LS) micelles there is an extremely rapid electron transfer from the excited singlet to the Cu2+ ions. This process occurs in less than a nanosecond and hence can compete efficiently with fluorescence and intersystem crossing165. This astonishing result must be attributed to a pronounced micellar enhancement of the rate of the transfer reaction. It is, of course, a consequence of the fact that within such a functional surfactant unit regions with extremely high local concentrations of Cu2+ prevail. (Theoretical estimates predict the counterion concentration in the micellar Stem layer to be between 3 and 6 M). 

With the growth of PTC, various new technologies have been developed where PTC has been combined with other methods of rate enhancement. In some cases, rate enhancements much greater than the sum of the individual effects are observed. Primary systems studied involving the use of PTC with other rate enhancement techniques include the use of metal co-catalysts, sonochemistry, microwaves, electrochemistry, microphases, photochemistry, PTC in single electron transfer (SET) reactions and free radical reactions, and PTC reactions carried out in a supercritical fluid. Applications involving the use of a co-catalyst include co-catalysis by surfactants (Dolling, 1986), alcohols and other weak acids in hydroxide transfer reactions (Dehmlow et al., 1985,1988), use of iodide (traditionally considered a catalyst poison, Hwu et... 

The redox potentials of short-lived silver clusters have been determined through kinetics methods using reference systems. Depending on their nuclearity, the clusters change behavior from electron donor to electron acceptor, the threshold being controlled by the reference system potential. Bielectronic systems are often used as electron donors in chemistry. When the process is controlled by critical conditions as for clusters, the successive steps of monoelectronic transfer (and not the overall potential), of which only one determines the threshold of autocatalytical electron transfer (or of development) must be separately considered. The present results provide the nuclearity dependence of the silver cluster redox potential in solution close to the transition between the mesoscopic phase and the bulk metal-like phase. A comparison with other literature data allows emphasis on the influence of strong interaction of the environment (surfactant, ligand, or support) on the cluster redox potential and kinetics. Rela-... 

The photoinduced reduction of some quinones by zinc porphyrin and also by its tetraphenyl derivative has been studied in micellar systems. The mean time for intramicellar electron transfer has been established as 0.2 ps, and for duroquinone the rates of entry and exit from the micelle have been found to be 5 x 10 m s and6 x 10 m s respectively. Quinones possessing long chains are less mobile and partial charge separation could be achieved. Irradiation of anthraquinone in aqueous sodium dodecyl sulphate leads to anthraquinol and the surfactant-anthrahydroquinone ether as major products via the triplet state of the anthraquinone. ... 

Generally reactions in normal micelles in water occur in one micelle, and intermicellar reactions do not seem to be important. However, such intermicellar reactions have been observed in reverse micelles. They have been observed in imidazole-catalyzed hydrolysis of carboxylic esters [133], and in reactions involving energetic species generated photochemically [139] or involving electron transfer [140]. In these systems there is rapid transfer of reactants between water pools, so that the so-called reverse micelles may best be regarded as submicroscopic water droplets stabilized by surfactant. 

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