Back electron transfer systems

This review article attempts to summarize and discuss recent developments in the studies of photoinduced electron transfer in functionalized polyelectrolyte systems. The rates of photoinduced forward and thermal back electron transfers are dramatically changed when photoactive chromophores are incorporated into polyelectrolytes by covalent bonding. The origins of such changes are discussed in terms of the interfacial electrostatic potential on the molecular surface of the polyelectrolyte as well as the microphase structure formed by amphiphilic polyelectrolytes. The promise of tailored amphiphilic polyelectrolytes for designing efficient photoinduced charge separation systems is afso discussed. 

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. 

For systems such as these, which consist of electron transfer quenching and back electron transfer, it is in general possible to determine the rates both of quenching and of the back reaction. In addition to these aspects of excited state chemistry, one can make another use of such systems. They can be used to synthesize other reactive molecules worthy of study in their own right. The quenching reaction produces new and likely reactive species. They are Ru(bpy)3+ and Ru(bpy)j in the respective cases just shown. One can have a prospective reagent for one of these ions in the solution and thereby develop a lengthy and informative series of kinetic data for the transient. 

Photoinduced ET at liquid-liquid interfaces has been widely recognized as a model system for natural photosynthesis and heterogeneous photocatalysis [114-119]. One of the key aspects of photochemical reactions in these systems is that the efficiency of product separation can be enhanced by differences in solvation energy, diminishing the probability of a back electron-transfer process (see Fig. 11). For instance, Brugger and Gratzel reported that the efficiency of the photoreduction of the amphiphilic methyl viologen by Ru(bpy)3+ is effectively enhanced in the presence of cationic micelles formed by cetyltrimethylammonium chloride [120]. Flash photolysis studies indicated that while the kinetics of the photoinduced reaction,... 

With the site-selective hole injection and the hole trapping device established, the efficiency of the hole transport between the hole donor and acceptor, especially with respect to the distance and sequence dependence, were examined. Our experiments showed that hole transport between two guanines was extremely inefficient when the intervening sequence consisted of more than 5 A-T base pairs [1]. Hole injection into the DNA n-stack using photoexcited dCNBPU was accompanied by the formation of dCNBPU anion radical. Therefore, hole transport would always compete with the back electron transfer (BET). To minimize the effect of BET, we opted for hole transport between G triplets, that are still lower in oxidation potential than G doublet. With this experimental system, we researched the effect of the bridging sequence between two G triplets on the efficiency of hole transport [2]. 

The previous examples of eel were interpreted on the basis of a relatively simple mechanism. In these cases the back electron transfer generates directly the emitting excited state (annihilation). However, in more complicated systems back electron transfer and formation of an emitting state may be separate processes... 

J.R. Bolton In solution most photochemical electron transfer reactions occur from the triplet state because in the collision complex there is a spin inhibition for back electron transfer to the ground state of the dye. Electron transfer from the singlet excited state probably occurs in such systems but the back electron transfer is too effective to allow separation of the electron transfer products from the solvent cage. In our linked compound, the quinone cannot get as close to the porphyrin as in a collision complex, yet it is still close enough for electron transfer to occur from the excited singlet state of the porphyrin Now the back electron transfer is inhibited by the distance and molecular structure between the two ends. Our future work will focus on how to design the linking structure to obtain the most favourable operation as a molecular "photodiode . 

Let us begin with the one-mode electron-transfer system. Model IVa, which still exhibits relatively simple oscillatory population dynamics [205]. SimUar to what is found in Fig. 5 for the mean-field description, the SH results shown in Fig. 13 are seen to qualitatively reproduce both diabatic and adiabatic populations, at least for short times. A closer inspection shows that the SH results underestimate the back transfer of the adiabatic population at t 50 and 80 fs. This is because the back reaction would require energetically forbidden electronic transitions which are not possible in the SH algorithm. Figure 13 also shows the SH results for the electronic coherence which are found to... 

Sensitized PET reactions are often very slow and have low quantum yields due to dominating back-electron transfer. In these cases, the addition of cosubtrates (e.g., biphenyl or phenanthrene to DCA- or DCN-sensitized reactions) is useful. The use of such an additive is called cosensitization. In these reactions, the substrate is not oxidized (or reduced) by the excited sensitizer but by the radical ion of the cosensitizer (ET, ). This is a thermal electron-transfer step without the problems of back-electron transfer. The key step is the primary PET process (ETJ in which the cosensitizer radical ion is formed. The main characteristic of cosensitization systems is the high quantum yield of the free-radical ion (e.g., overall quantum yield is high and the reaction is fast (Scheme 7). 

Nevertheless, there are two highly efficient CL systems which are believed to involve the CIEEL mechanism in the chemiexcitation step, i.e. the peroxyoxalate reaction and the electron transfer initiated decomposition of properly substituted 1,2-dioxetanes (Table 1)17,26 We have recently confirmed the high quantum yields of the peroxyoxalate system and obtained experimental evidence for the validity of the CIEEL hypothesis as the excitation mechanism in this reaction. The catalyzed decomposition of protected phenoxyl-substituted 1,2-dioxetanes is believed to be initiated by an intramolecular electron transfer, analogously to the intermolecular CIEEL mechanism. Therefore, these two highly efficient systems demonstrate the feasibility of efficient excited-state formation by subsequent electron transfer, chemical transformation (cleavage) and back-electron transfer steps, as proposed in the CIEEL hypothesis. 

The peroxyoxalate system is the only intermolecular chemiluminescent reaction presumably involving the (71EEL sequence (Scheme 44), which shows high singlet excitation yields (4>s), as confirmed independently by several authors Moreover, Stevani and coworkers reported a correlation between the singlet quantum yields, extrapolated to infinite activator concentrations (4> ), and the free energy involved in back electron-transfer (AG bet), as well as between the catalytic electron-transfer/deactivation rate constants ratio, ln( cAx( i3), and E j2° (see Section V). A linear correlation of ln( cAx( i3) and E /2° was obtained for the peroxyoxalate reaction with TCPO and H2O2 catalyzed by imidazole and for the imidazole-catalyzed reaction of 57, both in the presence of five activators commonly used in CIEEL studies (anthracene, DPA, PPO, perylene and rubrene). A further confirmation of the validity of the CIEEL mechanism in the excitation step of... 

In the photochemical conversion model (Fig. 3), the most serious problem is the undesired and energy-consuming back electron transfer (shown as dotted arrows) as well as side electron transfer, e.g., the electron transfer from (Q) to (T2)ox. It is almost impossible to prevent these undesired electron transfers, if the reactions are carried out in a homogeneous solution where all the components encounter with each other freely. In order to overcome this problem, the use of heterogeneous conversion systems such as molecular assemblies or polymers has attracted many researchers. The arrangement of the components on a carrier, or the separation of the Tj—Q sites from the T2—C2 ones in a heterogeneous phase must prevent the side reactions of electron transfer. 

The photoinduced electron relay systems in the solid phase and at the solid-liquid interface containing Ru(bpy) + in the solid phase are summarized in Fig. 15. The electron transfer from Ru(bpy)2 + to MV2+ is very facile and occurs even at the solid-liquid interface. The back electron transfer of the products (Ru(bpy) + + MV--> Ru(bpy) + + MV2+) is so rapid, however, that MVt can be accumulated only when Ru(bpy)3+ is rapidly reduced by a reducing agent such as EDTA. If the reduction of Ru(bpy)j+ by EDTA shall compete with that by MV"t, both EDTA and Ru complex, or at least both EDTA and MV2+ should exist in the phase. The systems (a) and (b) of Fig. 15 reduce Ru(bpy) + by EDTA with the Ru complex... 

The system is reversible in the absence of an added electron donor but undergoes irreversible reaction at the reduced rhenium bipyridine center in the presence of added triethylamine. The observation of reaction at the rhenium site upon excitation in the absorption band of the metalloporphyrin site is compatible with an ultrafast back electron transfer, provided that the triethylamine coordinated to the magnesium prior to absorption and that the electron transfer from the metalloporphyrin to the bipyridine was followed rapidly by irreversible electron transfer from the triethylamine to the metalloporphyrin. The experiments graphically demonstrated the benefits of the incorporation of carbonyl ligands at the electron acceptor as they allowed a tracking of the sequence of charge separation and back electron transfer via time-resolved IR data . ... 

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