Electron back transfer

Subsequent studies (63,64) suggested that the nature of the chemical activation process was a one-electron oxidation of the fluorescer by (27) followed by decomposition of the dioxetanedione radical anion to a carbon dioxide radical anion. Back electron transfer to the radical cation of the fluorescer produced the excited state which emitted the luminescence characteristic of the fluorescent state of the emitter. The chemical activation mechanism was patterned after the CIEEL mechanism proposed for dioxetanones and dioxetanes discussed earher (65). Additional support for the CIEEL mechanism, was furnished by demonstration (66) that a linear correlation existed between the singlet excitation energy of the fluorescer and the chemiluminescence intensity which had been shown earher with dimethyl dioxetanone (67). 

Peroxyoxalate chemiluminescence is the most efficient nonenzymatic chemiluminescent reaction known. Quantum efficiencies as high as 22—27% have been reported for oxalate esters prepared from 2,4,6-trichlorophenol, 2,4-dinitrophenol, and 3-trif1uoromethy1-4-nitropheno1 (6,76,77) with the duorescers mbrene [517-51-1] (78,79) or 5,12-bis(phenylethynyl)naphthacene [18826-29-4] (79). For most reactions, however, a quantum efficiency of 4% or less is more common with many in the range of lO " to 10 ein/mol (80). The inefficiency in the chemiexcitation process undoubtedly arises from the transfer of energy of the activated peroxyoxalate to the duorescer. The inefficiency in the CIEEL sequence derives from multiple side reactions available to the reactive intermediates in competition with the excited state producing back-electron transfer process. 

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. 

Back Electron Transfer as Studied by Laser Photolysis.77... 

Since the electrostatic potential sharply decreases with increasing distance from the polyelectrolyte cylinder, the degree of reactivity modification by functional groups fixed to the polyion is strongly dependent on the distance from the cylinder surface. Considerable electrostatic potential effects on the photoinduced forward and thermal back electron transfer reactions, which will be discussed in the following chapters, can be attributed to the functional chromophore groups directly attached to the polyelectrolyte back-bone through covalent bonds. 

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. 

A third factor to be considered in this triangular competition (internal deactivation, back electron transfer, chemical reaction) is the possibility of an unfavored chemical reaction followed by the initiation of a long chain39,40. 

Acetylchloride is a trapping agent that allows the reaction to go completion, transforming the product into a less oxidizable compound.The results of other reactions between indole (57) and substituted cyclohexa-1,3-dienes show that the photo-induced Diels-Alder reaction is almost completely regioselective. In the absence of 59 the cycloaddition did not occur the presence of [2+2] adducts was never detected. Experimental data support the mechanism illustrated in Scheme 4.14. The intermediate 57a, originated from bond formation between the indole cation radical and 58, undergoes a back-electron transfer to form the adduct 60 trapped by acetyl chloride. 

FIG. 4 (a) Latimer diagram of the water soluble zinc tetra-Af-methyl-4-pyridium porphyrin (ZnTMPyP ). (Reprinted with permission from Ref. 47.) (b) Schematic representation of a photosynthetic process based on porphyrin sensitized water-organic interface. Dotted line corresponds to the back electron-transfer process. (Reprinted from Ref. 51 with permission from Elsevier Science.)... 

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,... 

FIG. 11 General mechanism for the heterogeneous photoreduction of a species Q located in the organic phase by the water-soluble sensitizer S. The electron-transfer step is in competition with the decay of the excited state, while a second competition involved the separation of the geminate ion-pair and back electron transfer. The latter process can be further affected by the presence of a redox couple able to regenerate the initial ground of the dye. This process is commonly referred to as supersensitization. (Reprinted with permission from Ref. 166. Copyright 1999 American Chemical Society.)... 

Between 0.20 and 0.30 V, a decay of the initial photocurrent and a negative overshoot after interrupting the illumination are developed. This behavior resembles the responses observed at semiconductor-electrolyte interfaces in the presence of surface recombination of photoinduced charges [133-135] but at a longer time scale. These features are in fact related to the back-electron-transfer processes within the interfacial ion pair schematically depicted in Fig. 11. 

A more quantitative description of the photocurrent responses, taking into account the contributions from back electron transfer and RuQi attenuation, was achieved by IMPS measurements [83]. Considering the mechanism in Fig. 11, excluding the supersensitization step, and the equivalent circuit in Fig. 18, the frequency-dependent photocurrent for a perturbation as in Eq. (42) is given by... 

A complex representation of IMPS data obtained for the heterogeneous quenching of ZnTPPC -diferrocenylethane is displayed in Fig. 21(a). The semicircular response in the first quadrant corresponds to the competition between product separation and back electron transfer, while the lower quadrant is determined by the i uQi constant. The i uQi attenuation limited the frequency range to less than 1 kHz. Equation (45) describes the experimental spectra at various Galvani potential differences [solid lines in Fig. 21(a)],... 

FIG. 21 Complex IMPS spectra obtained for the photo-oxidation of DFcET by ZnYPPC" at the water-DCE interface (a). The opposite potential dependencies of the phenomenological ET rate constant and the porph5rin coverage (b) are responsible for the maximum on the flux of electron injection obtained from IMPS responses for DFcET and Fc (c). The potential dependence of the back electron-transfer rate constant is also shown in (d). (From Ref. 83. Reproduced by permission of The Royal Society of Chemistry.)... 

It is clear that one of the major limitations of this analysis is the assumption of constant excited-state coverage. Deviations from the behavior described by Eq. (45) in the low frequency range have been observed at photocurrent densities higher than 10 Acm [50]. These deviations are expected to be connected to excited-state diffusion profiles similar to those considered by Dryfe et al. [see Eq. (38)] [127]. A more general expression for IMPS responses is undoubtedly required for a better understanding of the dynamics involved in back electron transfer as well as separation of the photoproducts. 

Fig. 1 Schematic mechanism for the long-distance oxidation of DNA. Irradiation of the anthraquinone (AQ) and intersystem crossing (ISC) forms the triplet excited state (AQ 3), which is the species that accepts an electron from a DNA base (B) and leads to products. Electron transfer to the singlet excited state of the anthraquinone (AQ 1) leads only to back electron transfer. The anthraquinone radical anion (AQ ) formed in the electron transfer reaction is consumed by reaction with oxygen, which is reduced to superoxide. This process leaves a base radical cation (B+-, a hole ) in the DNA with no partner for annihilation, which provides time for it to hop through the DNA until it is trapped by water (usually at a GG step) to form a product, 7,8-dihydro-8-oxoguanine (8-OxoG)...

On the other hand, oxidation of a DNA base by a triplet state of the an-thraquinone (AQ5"3) generates a contact ion pair in an overall triplet state, and back electron transfer from this species to form ground states is prohibited by spin conservation rules. Consequently, the lifetime of the triplet radical ion pair is long enough to permit the bimolecular reaction of AQ- with 02 to form superoxide (02 ) and regenerate the anthraquinone. 

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]. 

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