Electron-transfer Reactions in Micelles

Kinetic Analysis of Electron-transfer Reactions in Micelles... 

Thus, the kinetics of diffusion-controlled bimolecular electron-transfer reactions in the micellar interiors differ from that in the homogeneous solution. Numerous data have shown that Eq. 9 reproduces the dynamics of electron-transfer reactions within micelle interiors [80]. Diffusion coefficients (D) estimated from Eqs. 8 and 9 are very similar to those obtained by independent measurements. For example, Eq. 8 gave ku = 7.5 X 10 s for electron transfer from excited pyrene to CH2I2 in SDS micelles [79b]. One estimates from Eq. 8, with = 20 A and ai = 1.5 (calculated assuming d = 7 A), a value of Z) = 1.3 x 10 cm s, nearly identical with the experimentally determined value of Z) = lO " cm s [45]. 

ELECTROPHILIC AND ELECTRON-TRANSFER REACTIONS IN NON-FUNCTIONAL MICELLES... 

Several other reports of photoinduced electron-transfer reactions in organized assemblies have appeared recently, and the special properties of the crown ethers appear particularly promising.Humphry-Baker et al. have investigated the ability of the Cu crown-ether complex (3) to participate in photoredox processes. The crown-ether complex aggregates to form micelles into which a hydrophobic donor can be incorporated, Le. (4) in Figure 5. Photoexcitation of the donor species then leads to charge transfer to the Cu " ion in the crown ether. Charge transfer is evidently sufficiently rapid that it competes effectively with... 

It must be noted that the dynamic nature of micelles must especially be borne in mind in dealing with electron transfers which invariably are fast processes. The seminal work of Bruhn and Holzwarth [88], an examination of the kinetics of diffusion-controlled electron transfer reactions in micellar sodium dodecyl sulfate solutions, disclosed that sufficient heed must be paid to the continuous disintegration and reconstitution of the micelles in this time range. 

A correlation between the rate constants k and free enthalpy change AG of electron transfer was studied by Hashimoto and Thomas [127] for quenching of excited singlet states of both pyrene and N-ethylcarbazol and of the triplet state of N-methylphenothiazine by a number of metal ions and for back electron transfer reactions in micellar sodium taurocholate and sodium dodecylsulfate solutions. Quenching rate constants were determined from Stern-Volmer plots obtained for lifetimes of excited states at high concentration of micelles, where the exponential decay... 

One of the most important characteristics of ionic micelles is their electrostatic potential (up to hundreds of millivolts at the micellar surface) and their resulting ability to select specific counterion species. The potential also depends on the counterion, so the above two quantities are not necessarily independent. They have a crucial effect on electron transfer reactions in micellar systems. 

Other examples of electron transfer reactions in surfactant assemblies are those between pyrene and dimethylaniline in micelles, between viologen derivative and zinc porphyrin as an electron relay, and between chlorophyll a and methylviologen in microemulsions the photoinduced reduction of duroquinone by zinc porphyrin in micellar solution the photoinduced redox reaction of proflavine in aqueous and micellar solutions retardation of back reactions in micellar systems light-driven electron transfer from tetrathiafulvalene to porphyrin and tris a, a -bipyridine)... 

Last time, electron-transfer reactions were frequently performed in micellar media. Analyzing temperature effects on electron transfer from aromatic amines to coumarins in aqueous Trilon X-100 micelles, Kumbhakar et al. (2006) deduced that the two-dimensional electron-transfer (2DET) model is more suitable to explain the results obtained than the conventional electron-transfer theories. The model is detailed in the article by Kumbhakar et al. (2006) and references therein. 

Although a large number of different possible ways of using micelles to promote electron transfer reactions has been discussed,329 we consider only those in which hydrogen, or a hydrogen precursor, is generated using a metal complex as sensitizer. 

The inhomogeneous structure of a micelle (or inverse micelle) can influence the course of a photoinduced electron transfer. Such a micelle is biphasic, containing a hydrocarbon-like core and a water-like surface. If the photoinduced electron transfer produces a product which has lower solubility in the aqueous phase (a situation which might obtain if a cationic acceptor is reduced to a neutral product), this product will be directed by solubility considerations to move toward the hydrophobic center of the micelle, i.e., remote from the site of the forward electron transfer. This spatial separation, shown conceptually in Scheme 4, in turn will retard the rate of the back reaction compared with that of the forward reaction. 

The combined systems of the photosensitizer with micelle, LB film, protein, and so on are interesting, because we can expect that new functions will be found in such systems. In some pioneering works, such ideas were applied to stereoselective photoinduced electron transfer reactions. We believe that one can construct the new photoreaction with such combined systems and also expect the enhancement of stereoselectivity in such systems. 

As evidenced by this symposium, the use of micelles and other organized assemblies to control the selectivity of chemical reactions has recently attracted much attention. In most of these cases, micelles or vesicles have been used as a means of separating charged intermediates formed by electron transfer reactions, thereby preventing the back reaction. The effects of the micelle or vesicle are usually dramatic. 

We have prepared and studied a number of surfactant, hydrophobic and water soluble luminescent metal complexes. These can serve as excited substrates in light-induced electron transfer reactions. Both the quenching processes and subsequent reactions can be strongly affected by incorporation of the substrate and/or quencher in an organized assembly. This paper focuses mainly on studies in micelles. 

Photochemical electron transfer reactions have been examined in micellar systems as probes for the diffusion and location of quenchers, and as environments for solar energy storage 2 3>90 95 96>. The relative rates of quenching will depend on the location of the donor and acceptor (Scheme XXXII). For example, the rate of quenching of a hydrophobic donor located inside the micelle by Cu2+ is much faster in anionic micelles compared to cationic micelles. Similarly a hydrophobic excited state is quenched faster by a hydrophobic donor or acceptor than by a hydrophilic one in micellar systems. 

The construction and properties of monolayers has been well documented by Kuhn (1979) and the photochemical reactions which occur in such systems reviewed (Whitten et al., 1977). Molecules in monolayers are usually ordered and in the case of rru/i -azastilbenes irradiation of the ordered array produces excimer emission and dimers (Whitten, 1979 Quina et al, 1976 Quina and Whitten, 1977). This contrasts with what is found when the fra/jj-isomers of such compounds are incorporated into micelles. In such systems the predominant reaction is cis-trans isomerisation excimer emission is lacking. It is suggested that the lack of isomerisation in the fatty acid monolayers is due to the tight packing and consequent high viscosity of such systems. Styrene also dimerises in a fatty acid monolayer. Interestingly, the products formed on photo-oxidation of protoporphyrins are dependent upon whether the reaction is carried out in a monolayer or a micelle (Whitten et al., 1978). Zinc octa-ethylporphyrin exhibits excimer emission in monolayers (Zachariasse and Whitten, 1973). Porphyrins are photoreduced by amines in monolayers (Mercer-Smith and Whitten, 1979). Electron-transfer reactions have been carried out with monolayers of stearic acid containing chlorophyll and electron acceptors such as quinones (Janzen et al., 1979 Janzen and Bolton, 1979). 

Charge transfer between amphipathic ruthenium(II) complexes and N-butylphenothiazine in micelles, synthetic bilayers and liposomes has been studied by flash photolysis (Takayanagi et al., 1980). It was shown that the energy wasting back electron-transfer reaction is less efficient in the vicinity of the charged surface and that it is disfavoured by an increase in temperature. 

One of the main factors influencing the activation barrier in fast electron-transfer reactions is the change in the polarization of the immediate space surrounding the activated complex in solution. The more-well-known salt effects as well as the relatively new field of micellar effects can be used as mechanistic probes in this context. Since micelles have a hydrophobic as well as a hydrophilic part, this creates two different kinds of interfaces where electron transfer can occur if one of either the oxidant or reductant is contained or associated with these molecular aggregates. A futuristic approach could be that studies of this kind may serve as models for enzymatic reactions with complex bioaggregates such as membranes. 

Quantitative approaches to describing reactions in micelles differ markedly from treatments of reactions in homogeneous solution primarily because discrete statistical distributions of reactants among the micelles must be used in place of conventional concentrations [74], Further, the kinetic approach for bimolecular reactions will depend on how the reactants partition between micelles and bulk solution, and where they are located within the microphase region. Distinct microphase environments have been sensed by NMR spectrometry for hydrophobic molecules such as pyrene, cyclohexane and isopropylbenzene, which are thought to lie within a hydrophobic core , and less hydrophobic molecules such as nitrobenzene and N,N-dimethylaniline, which are preferentially located at the micelle-water interface [75]. Despite these complexities, relatively simple kinetic equations for electron-transfer reactions can be derived for cases where both donors and acceptors are uniformly distributed inside the micelle or on its surface. 

Consider the diffusion-controlled electron transfer reaction D - - A —> D+ -I- A in a spherical micelle of radius R with donors (D) and acceptors (A) located in the micelle interior. Electron transfer is presumed to require collisional interaction of the reactants. When one of the reactants, e.g., the donor, is immobile and placed at the center of the micelle and N acceptor molecules are initially uniformly distributed throughout the micelle interior, the kinetics of the donor decay are given by the expression [76]... 

Experimental studies of electron-transfer reactions occurring on the surface of micelles confirm the exponential character of the kinetics results from some of these studies are summarized in Table 4. The values of the surface diffusion coefficients estimated from Eq. 12 using the measured rate constants are close to values determined by using NMR relaxation and fluorescence measurements [44]. 

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]... 

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