Electron transfer process annihilation

V. Nucleophilic and Electron-Transfer Processes in Ion-Pair Annihilation.. 96... 

NUCLEOPHILIC AND ELECTRON-TRANSFER PROCESSES IN ION-PAIR ANNIHILATION... 

The generalized mechanism for ion-pair annihilation as presented in Scheme 11 involves the rather circuitous route for radical-pair production [involving Eqs. (55) and (56), certainly in comparison with the direct electron-transfer pathway (Scheme 8)]. In other words, why do ion pairs first make a bond and then break it, when the simple electron transfer directly from anion to cation would achieve the same end The question thus arises as to whether electron transfer between Fe(CO)3L+ and CpMo(CO)3 is energetically disfavored. The evaluation of the driving force for the electron transfer process obtains from the separate redox couples, namely,... 

Yoshida et al. reported that Qo forms the l 2-complex with y-CD (Fig. 16), which shows good solubility in water [74]. Andersson et al. reported that C70 also forms the complex with y-CD [75]. Since the spectral shapes of the ground state and triplet excited state are not changed by the inclusion in y-CD, the interaction between fullerenes and y-CD is quite weak. It was revealed that the bimolec-ular excitation-relaxation and electron-transfer processes of the inclusion complex of fullerene in y-CD are changed in comparison to the pristine fullerenes [29]. For example, the rate constants for the triplet-triplet annihilation processes of C6q and C70 in y-CD are much smaller than... 

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)...

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... 

At this point it is necessary to consider the mechanism of electron-transfer luminescence in solutions which cannot involve ion-radical annihilation because both cation and anion of the fluorescer are not formed. Such emission can be achieved by treating anion radicals with chemical oxidants or electrochemically under conditions where the corresponding cation cannot be produced, and it may also be achieved by electrochemical reduction of cations without producing the corresponding anion. In addition to triplets, three types of processes and pathways have been proposed to help explain why such emission occurs. These may be described as (7) impurities, (2) ion-radical aggregates, and (5) heterogeneous electron transfer. It is evident63 that impurities,... 

Contrary to the electron transfer in organic solvents, the reduction process of functionalized fullerenes in aqueous solutions is very complex. Although some adducts are sufficiently water-soluble (Fig. 22) no reduction could be observed [181,183,187], This is attributed to the irreversible formation of fullerene clusters in aqueous media, which seem to prevent electron transfer. Consequently, efficient triplet-triplet annihilation within the clusters is observed resulting in short triplet lifetimes (< 0.1 (xs compared to microseconds for their monomeric analogue) [182,187],... 

Durrant and co-workers have compared the electron injection and recombination processes of 28 or Zn-28 with that of N3 (a famous ruthenium polypyridyl complex with very high IPCE). Their experiments revealed that the electron injection and recombination kinetic for these three dyes on the surface of Ti02 are almost identical. The high IPCE for N3 dye probably originates from the electron transfer from the iodide redox couple to the dye cations. It is also possible that the lower efficiency of porphyrin sensitizers was caused by the annihilation of the excited states between the neighboring porphyrin molecules because of the closed proximity [70],... 

The latter process results in an overall second-order annihilation of the radicals as observed in the complete decays of the transient absorption to the spectral baseline on the microsecond time scale (see Figures 12 and 13). Since dimerization of the 17-electron radicals is orders of magnitude slower than the highly exothermic back electron transfer, no net photochemical transformations are observed even after prolonged charge-transfer irradiation. 

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