Electron donor concentration, decay

Figure 7. Decay of electron donor concentration as measured at pH 4.8 and as calculated by numerical simulation with dependence on n. Experimental signal-same conditions as in Figure 4. Because the concentration of silver ions after the puke is smaller than that of hydroquinone, the ordinate of the experimental plot is (OD-OD j/ODt=o- Numbers next to simulation curves correspond to n. The value of Is 2.25 X JO mol The best adjustment unth numerical simula-...

Figure 8. Decay of electron donor concentration as measured at pH 4.8 and as calculated by numerical simulation of dependence on (Same conditions as in...

The decay kinetics of excited electron donor molecules (the intensity of fluorescence is proportional to the concentration of excited molecules at any given time) can be interpreted in two ways. First, one may try to approxi-... 

The decay kinetics of excited electron donor molecules (the intensity of fluorescence is proportional to the concentration of excited molecules at any given time) can be interpreted in two ways. First, one may try to approximate it with the sum of two exponents, one of which refers to the decay of the fluorescence of free donor molecules and the other to that of the complex between the donor and the acceptor. This interpretation is similar to the description of the two-exponential decay of the fluorescence observed in the presence of two compounds containing heavy atoms [40]... 

The kinetic studies make use of the unequally sized reaction partners (e.g. a large electron donor and a small electron acceptor couple) and benefit from the low viscosity of dichloromethane (DCM), both of which elevate the diffusion-controlled limit. To study the electron transfer, deoxygenated DCM solutions of, for example, w-terphenyl at high concentrations (0.02 m) were irradiated in the presence of different concentrations of fullerene (ca 10 m) [62]. This resulted in accelerated decay of (arene) + UV-Vis absorption, with rates linearly depending on fullerene concentration [62]. Formation of the electron-transfer product, fullerene", was confirmed spectroscopically by measurement of the NIR fingerprint (Amax = 1080 nm) [62, 65]. 

The low quantum yield observed for the formation of the long lived chlorophyll cation suggests poor binding of the quinone. At high DBMIB concentrations, the Chl decay becomes multiexponential either due to the quinone binding to the reaction centre in more than one way, or to reduced DBMIB acting as an electron donor. A rough estimate of the proportion of photochemically active, bound DBMIB, at a concentration lOpM, is 0.25%. 

When the concentrations of reagents have comparable values, it is necessary to pay attention to the correlation effect in the decay of different donors, i.e. to consider the fact that the spatial distribution of acceptors near the chosen donor can change as a result of the decay of the acceptors in the reactions with other donors neighbouring the chosen one. The rigorous derivation of kinetic equations with the consideration of such a correlation is, as far as we know, unavailable. The approximate description of the kinetics of a biomolecular electron tunneling reaction at n(t) = N t) can be given in terms of the pair density method with the help of eqn. (19) in which, however, N is not a constant quantity but depends on time in the same way as n(t), i.e. 

Time profiles of the formation of fullerene radical anions in polar solvents as well as the decay of 3C o obey pseudo first-order kinetics due to high concentrations of the donor molecule [120,125,127,146,159], By changing to nonpolar solvents the rise kinetics of Go changes to second-order as well as the decay kinetics for 3C o [120,125,133,148], The analysis of the decay kinetics of the fullerene radical anions confirm this suggestion as well. In the case of polar solvents, the decay of the radical ion absorptions obey second-order kinetics, while changing to nonpolar solvents the decay obey first-order kinetics [120,125,127,133,147]. This can be explained by radical ion pairs of the C o and the donor radical cation in less polar and nonpolar solvents, which do not dissociate. The back-electron transfer takes place within the ion pair. This is also the reason for the fast back-electron transfer in comparison to the slower back-electron transfer in polar solvents, where the radical ions are solvated as free ions or solvent-separated ion pairs [120,125,147]. However, back-electron transfer is suppressed when using mixtures of fullerene and borates as donors in o-dichlorobenzene (less polar solvent), since the borate radicals immediately dissociate into Ph3B and Bu /Ph" [Eq. (2)][156],... 

In Fig. 2, there are two features worth noting. First, the decay of the 0-H peak and the decay of the free-electron concentration are correlated, providing evidence that the 0-H complexes are shallow donors. Second, the data were fit using a biexponential decay model. 

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