Electron-transfer Reactions in Vesicles and Membranes

The separation of photoproducts formed in photosensitized electron transfer reactions is essential for efficient energy conversion and storage. The organization of the components involved in the photo-induced process in interfacial systems leads to efficient compartmentalization of the products. Several Interfaclal systems, e.g., lipid bllayer membranes (vesicles), water-in-oil mlcroemulslons and a solid SIO2 colloidal Interface, have been designed to accomplish this goal. 

In membranes, the motional anisotropies in the lateral plane of the membrane are sufficiently different from diffusion in the transverse plane that the two are separately measured and reported [4b, 20d,e]. Membrane ffip-ffop and transmembrane diffusion of molecules and ions across the bilayer were considered in a previous section. The lateral motion of surfactants and additives inserted into the lipid bilayer can be characterized by the two-dimensional diffusion coefficient (/)/). Lateral diffusion of molecules in the bilayer membrane is often an obligatory step in membrane electron-transfer reactions, e.g., when both reactants are adsorbed at the interface, that can be rate-limiting [41]. Values of D/ have been determined for surfactant monomers and probe molecules dissolved in the membrane bilayer typical values are given in Table 2. In general, lateral diffusion coefficients of molecules in vesicle... 

These examples should serve to underscore the difficulty in predicting the effects that interfacial potentials, membrane structure and microphase organization will have on electron-transfer reactions across the membrane interface and within the bilayer itself. The principles involved are common to micelles and vesicles, but the more anisotropic and highly ordered vesicles provide a more complex reaction environment for solubilized or adsorbed reactants. 

Reactions that occur between components in the bulk solution and vesicle-bound components, i.e., reactions occurring across the membrane interface, can be treated mathematically as if they were bimolecular reactions in homogeneous solution. However, kinetic analyses of reactions on the surface of mesoscopic structures are complicated by the finiteness of the reaction space, which may obviate the use of ordinary equations of chemical kinetics that treat the reaction environment as an infinite surface populated with constant average densities of reactant molecules. As was noted above, the kinetics of electron-transfer reactions on the surface of spherical micelles and vesicles is expressed by a sum of exponentials that can be approximated by a single exponential function only at relatively long times [79a, 81], At short times, the kinetics of the oxidative quenching of excited molecules on these surfaces are approximated by the equation [102]... 

Electron-transfer Reactions - Light-induced electron transfer from a donor to a suitable acceptor has been described for numerous bimolecular systems. The reagents have been dispersed in a polar solvent,at microscopic or macroscopic interfaces, in latex dispersions, in nematic liquid crystals, in reverse micelles, in vesicles, and in lipid bilayer membranes. Additional studies have been concerned with electron transfer... 

Bilayer membranes and vesicles provide not only charged surfaces but also two phases, separating the reaction sites and products. It was first demonstrated that photoinduced electron transfer occurs from EDTA in the inner water phase of vesicles incorporated with surfactant Ru(bpy)2+ to MV2+ in the outer water phase22 (Eq. (14)). 

For the systems with photoactive membranes discussed in the previous section the photosensitizer embedded into the vesicle membranes not only participated in photochemical and dark redox reactions with substances which are located in water phases on both sides of the membrane, but also served as the carrier of the electron across the membrane. In the presence of the appropriate electron carrier which is capable of penetrating through the membrane core it is also possible to perform electron transfer between membrane-separated water phases when photosensitizers are located in these phases rather than in the membrane. Membranes containing no photosensitizers can be called photopassive ones since no photophysical and photochemical processes occur in them, and their role is only to (i) provide electron transfer from one water phase to the other leading to the formation of spatially separated oxidant and reductant and (ii) to suppress recombination reactions. 

Sometimes electron transfer via the transport of electron carrier and via electron exchange reactions occur simultaneously. Co-existence of both these channels was observed for dark electron transfer across the viologen-containing vesicle membrane [169, 201]. To illustrate this let us turn back to the experiments shown schematically in Fig. 5 a. In accordance with the reaction sequence (34)-(37) and... 

For this purpose an electron transfer across the bilayer boundary must be accomplished (14). The schematic of our system is presented in Figure 3. In this system an amphiphilic Ru-complex is incorporated Into the membrane wall. An electron donor, EDTA, is entrapped in the inner compartment of the vesicle, and heptylviolo-gen (Hv2+) as electron acceptor is Introduced into the outer phase. Upon illumination an electron transfer process across the vesicle walls is initiated and the reduced acceptor (HVf) is produced. The different steps involved in this overall reaction are presented in Figure 3. The excited sensitizer transfers an electron to HV2+ in the primary event. The oxidized sensitizer thus produced oxidizes a Ru located at the inner surface of the vesicle and thereby the separation of the intermediate photoproducts is assisted (14). The further oxidation of EDTA regenerates the sensitizer and consequently the separation of the reduced species, HVi, from the oxidized product is achieved. In this system the basic principle of a vectorial electron transfer across a membrane is demonstrated. However, the quantum yield for the reaction is rather low (0 4 X 10 ). 

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