Back electron transfer, suppression

Although the electrostatic potential on the surface of the polyelectrolyte effectively prevents the diffusional back electron transfer, it is unable to retard the very fast charge recombination of a geminate ion pair formed in the primary process within the photochemical cage. Compartmentalization of a photoactive chromophore in the microphase structure of the amphiphilic polyelectrolyte provides a separated donor-acceptor system, in which the charge recombination is effectively suppressed. Thus, with a compartmentalized system, it is possible to achieve efficient charge separation. 

All the surface recombination processes, including back reaction, can be incorporated in a heavy kinetic model [22]. The predicted, and experimentally observed, effect of the back reactions is the presence of a maximum in the donor disappearance rate as a function of its concentration [22], Surface passivation with fluoride also showed a marked effect on back electron transfer processes, suppressing them by the greater distance of reactive species from the surface. The suppression of back reaction has been verified experimentally in the degradation of phenol over an illuminated Ti02/F catalyst [27]. 

Considering salt-cage effect interplay, it is necessary to go into a possibility of suppressing back-electron transfer during the formation of ion-radicals. One of the most prominent manifestation of this interplay consists in the oxidation of 1,2-diarylcyclopropane (DACP) with oxygen in the presence of photoexcited 9,10-dicyanoanthracene (DCNA) in acetonitrile (Mizuno et al. 1987) (Scheme 5.23). 

If the reaction proceeds in the presence of Mg(C104)2, the product yield becomes significantly better. The added salt suppresses back-electron transfer ... 

It is important to differentiate between the effects of a nonnucleophilic salt such as Mg(C104)2 on one hand, and a weak nucleophilic salt such as Et4NOAc on the other. The effect of nonnucleophilic salts on photo-oxygenation via electron transfer can be understood as the stabilization of ion-radicals by coulombic interaction, resulting in the suppression of a back electron transfer between ion-radicals. The weak nucleophilic salts cause unusual effects. The addition of the anionic nucleophile to the cation-radical and complexation of the weak nucleophilic salt with the ion-radicals bring about these effects. 

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],... 

The advantages in applying mixed-valence compounds are due to the fact that the energy of the IT transition may be varied by convenient synthetic procedures, in addition, the photochemical behavior may be predicted using the theoretical treatment proposed by Hush. However, the most serious restriction for the application of mixed-valence compoun ds in the static spectral sensitization arises from the fast back electron transfer. Therefore, very efficient scavenging reactions are required in order to suppress back electron transfer. 

A kinetic scheme can be written, Scheme 1, in which the rates of back electron transfer from the contact, solvent-separated, and free ions must each be individually considered. Clearly, the greater the tendency of a given solvent to separate the ions, the less significant will be the electrostatic interaction between them, and the greater tendency to suppress back electron transfer for a period sufficiently long to allow radical ion chemistry to ensue. A major concern of this article will be in defining the efficacy of supramolecular assemblies which create inhomogeneous arrays to controll the secondary chemistry of radical ions formed via photoinduced electron transfer. 

The problem of differentiating the rates of back electron transfer and of chemical reaction of radical ions can be addressed in one of two ways either the rate of reaction of the radical ions can be increased, or the rate of back electron transfer between the components of the radical ion pair can be suppressed. Although the reactivity of the component radical ions can be manipulated by standard physical organic techniques (which alter the electron density and steric access to sites of electron sufficiency or deficiency), it is very difficult to change the chemical reactivity of fixed members of a donor-acceptor pair. Yet the rate of chemical reaction, rearrangement, or trapping of the individual radical ions must be competitive with the rate of back electron transfer (the reverse of Eq. 1) if net chemistry is to be observed. Obviously, if the rate of reaction of these radical ions is extremely fast, the relative rate of back electron transfer may be slow enough to obviate this problem. 

The interception of the radical anion by add or the formation of 28 should have the same effect. In fact, the back electron-transfer reaction from (TCA ) to (DPA ) is suppressed, or at least reduced, maximizing the cage escape efliriency of the radical cation and its further reaction with molecular oxygen. In these reactions benzil 26 is probably formed via the dioxete intermediate, also suggested by de Mayo and co-workers [100], whereas the mechanism leading to benzoic add is still uncertain. Other examples of photoinduced oxygenations, preceded by nucleophilic addition, have been also reported. 

The DCA-sensitized photooxygenation of less electron-rich cyclopropanes in the absence of Mg(C104)2 does not afford 1,2-dioxolanes. However, the photoreaction in the presence of MgfClOJj gives 1,2-dioxolanes in moderate yields [157], In this photoreaction, Mg(C104)2 suppresses not only a back electron transfer from DCA to the radical cations of cyclopropanes, but also the decomposition of the 1,2-dioxolanes produced. 

As a result, the energy-wasting back electron transfer (to regenerate the original charge-transfer salt) is partially suppressed, and the reactive phenyl radical couples with the 17-electron carbonylmetal radical within the solvent cage to form the phenyl-substituted metal complex (Eq. 54). 

The kinetics of the oxidation of iodide by the oxidized state of c -[Ru (dcbpy)2-(NCS)2] sensitizer adsorbed on nanocrystalline Ti02 films was measured by transient laser spectroscopy [92]. Figure 16 shows the transient absorption kinetics recorded in propylene carbonate with various electrolytes added. In all cases, the recovery of the ground-state absorption of the dye, after the fast electron injection into the solid, does not follow a simple kinetic law. In the absence of any electrolyte (trace a), the time needed to reach half of the initial absorbance (/1/2) through back electron transfer is 2 ps. Total recovery of the initial absorption, however, requires several hundreds of microseconds to milliseconds. Traces b, c, and d were recorded after addition of a common concentration of 0.1 m of iodide in the form of tetra-butylammonium (TBA+), Li+, and Mg + salts, respectively. Addition of the electrolyte in all three cases led to a considerable acceleration of the dye regeneration with ti/2 < 200 ns and complete suppression of the slow kinetic tail. 

Apart from minor complications, discrepancy between fluorescence quenching representing the forward reaction of Py l and photoreactivity including back reaction and subsequent processes is the manifestation that suppression of back electron transfer is the central subject to improve quantum efficiency of MVt formation. 

The role of added magnesium perchlorate is probably suppression of the intramolecular back-electron transfer as weU as stabilization of VIII and IX. Such effects of metal ions added to enhance PET reactions are well known. ... 

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