Back-electron-transfer rate

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.)... 

The further improvement of the product yield (100%) was achieved by employing AcrPh + instead of AcrH + and this can also be ascribed to the slower back-electron transfer rate for the former than the latter [109]. In the Marcus-inverted region, the back-electron transfer becomes slower with increasing the driving force. Because the EZx value of AcrPh" (E°x versus SCE = - 0.55 V)... 

The absorption ascribable to the radical ions decays after reaching a maximum. The decay of absorption can be attributed to the back-electron-transfer reaction and radical coupling of DTT + yielding dimers. The back-electron-transfer rate (fcbet) was estimated to be 1.1 x 1010 M 1 s-1 using an evaluated s for DNB + at 910 nm (5.9 x 103 M cm-1). 

Table 6 Electrontransfer Rate Constants and Back-Electron Transfer Rate Constants of 3C60 and 3C60-Pyrrolidino Derivatives With DMA in Benzonitrile and Benzonitrile/ Toluene (1 1)...

Changing the solvent from polar to less polar solvents effects not only the electron transfer but also the back-electron transfer. Back-electron transfer rate constants are in less polar solvents larger than those in polar solvents, which can reasonably be interpreted in terms of desolvation process and loose in ion pair formation. The transient absorptions of the pyrrolidino fullerene radical anions are slightly blue-shifted compared to that of Qo (Qo 1076 nm, derivatives radical anions 991-1002 nm) [179],... 

Estimation of the back electron transfer rate (k ). A rough estimate of the rate of the back electron transfer can easily be made using Equation 9 or a modified form as proposed by Gratzel (35) ... 

Back electron-transfer rate, cyanometallates, 116 Band analysis, excited-state structure, 211,25 Base, effect on doublet excited... 

Photo-induced electron transfer between [Ru(bpy)3]2+-like centres covalently bound to positively-charged polymers (N-ethylated copolymers of vinylpyridine and [Ru(bpy)2(MVbpy)]2+) and viologens or Fe (III) has been studied using laser flash photolysis techniques. It is found that the backbone affects the rates of excited state quenching, the cage escape yield, and the back electron transfer rate because of both electrostatic and hydrophobic interactions. The effect of ionic strength on the reactions has been studied. Data on the electron transfer reactions of [Ru(bpy)3]2+ bound electrostatically or covalently to polystyrenesulphonate are also presented. 

Fig. 4. Back electron transfer rates in photogenerated radical ion pairs in acetonitrile. In all cases cyanoanthracenes served as electron acceptors in their excited states, (a) Methylated benzene derivatives [60b] as donors (one-ring compounds), V = 11.5 cm"1, As = 1.63 eV. (b) Methylated biphenyls or naphthalenes [60b] as donors (two-ring compounds), V = 8.5 cm-1, /, = 1.48 eV. (c) Methylated phenanthrenes [60b] as donors (three-ring compounds) V = 8.0 cm-1, Xs = 1.40 eV. (dl Diphenylbutadienes [61] as donors, V = 8 cm-, X, = 1.55 eV. In all cases X, = 0.25 eV and V= 1500 cm-1...

Fig. 5. Back electron transfer rates in photogenerated radical ion pairs in acetonitrile (a) 9,10-Dicyanoanthracene in its excited state served as the acceptor. Aryl, alkyl, methoxy and amino benzene derivatives as well as aliphatic amines served as donors [62] (V = 23 cm-1, 2, = 0.97 eV, Aj = 0.64 eV). (b) Perylene, pyrene, benzperylene, and aromatic amines served as donors. Tetracyanoethylene (TCNE), pyromellitic dianhydride (PMDA), phthalic anhydride (PA), maleic anhydride, pyrene and perylene served as electron acceptors [63], Various combinations of donors or acceptors were excited (V = 20 cm , As = 1.45 eV, A, = 0.07 eV). The parabolas drawn are different from those offered in the original analysis. The parameters that were used were selected to emphasize the similarity to Fig. 4 (in all cases v = 1500 cm-1)...

The hb value consists of the isotropic back electron transfer rate (kiso,b) and the anisotropic one (kani.b). which is due to SOC of the halogen atom. 

Thus, a careful analysis of the rate constants for back electron transfer, hgand substitution, and dimerization leads to the conclusion that ligand exchange in the 17-electron radical (/cgub in Eq. 44) lowers the rate of back electron transfer from the acceptor radical (A ) (/c et in Eq. 43) to such an extent that dimerizations (and other possible follow-up reactions [118]) now become competitive and effect permanent photochemical transformations. The decrease of the back electron transfer rates is due to the attenuated reduction potentials of the phosphine-substituted radicals [176]. 

The zinc(II) porphyrin of 1 is particularly suitable for electron transfer studies because it is a photoactivated electron donor with easily monitored spectral properties. Forward and back electron transfer rate constants for la ( f = 5.0 x lO ... 

Overall the forward electron-transfer step must be much faster than all other photophysical decay mechanisms of the dye s excited state. The forward electron-transfer time should be >10 faster than the back-electron-transfer dynamics. The regeneration of the dye must be faster than both back electron transfer and the rate of photoinduced surface charging. The mechanism of regeneration and ensuing dynamics determines the required conditions for the back-electron-transfer rates. 

Photoinduced electron transfer is a subject characterised, particularly at the present time, by papers with a strongly theoretical content. Solvent relaxation and electron back transfer following photoinduced electron transfer in an ensemble of randomly distributed donors and acceptors, germinate recombination and spatial diffusion a comparison of theoretical models for forward and back electron transfer, rate of translational modes on dynamic solvent effects, forward and reverse transfer in nonadiabatic systems, and a theory of photoinduced twisting dynamics in polar solvents has been applied to the archetypal dimethylaminobenzonitrile in propanol at low temperatures have all been subjects of very detailed study. The last system cited provides an extended model for dual fluorescence in which the effect of the time dependence of the solvent response is taken into account. The mechanism photochemical initiation of reactions involving electron transfer, with particular reference to biological systems, has been discussed by Cusanovich. ... 

The general kinetic expression for this reaction mechanism is quite involved, but, clearly, the overall velocity will depend on the detailed kinetics of all three steps. If the back electron transfer rate is neglected (i.e., k 3[Ar][Bo] = 0), and the steady-state approximation is applied to the two central complexes, then the reaction velocity, v, may be derived ... 

The presence of p-CD leads to stabilization of the photoproducts formed by electron transfer from zinc(II)meso-tetrakis[l-(3-sulfonatopropyl)-4-pyridino]porphyrin (120) to anthraquinone-2-sulfonate (121, AQS ). This is due to the ability of CD to prevent ground-state association between the sensitizer and 121 by complexation of the latter. Moreover, the interaction of the reduced form AQSH " (formed in presence of cysteine) with the cavity lowers the back-electron transfer rate by one order of magnitude, further contributing to the accumulation of the radical anion [327]. 

In aqueous solution at 25 C except as noted. Energies in 10 cm absorptivity in cm M Reorganizarional energy in cm from reference noted in column 2 except as indicated. Free energy change for back electron transfer from reference noted in column 2 except as indicated. Electronic matrix element in cm Vlfr from reference noted in column 2 except as indicated. Back electron transfer rate constant observed after MM CT photoexcitation in M U from reference noted in column 2 except as indicated. Calculated from + Vr. see alsoTable 2. bb = l,4-bis-[4-(4 -methyl-2,2 -... 

The recombination reaction between the oppositely charged particles is a bimolecular electron transfer reaction, and the back electron transfer rate may be described by the Marcus-type electron transfer rate [69]. 

Cytochrome P-450 Low temperatures have been used to examine the sequence of oxygen reduction and hydroxylation in bacterial cyt P-450. Anaerobic reduction of cyt P-450, with camphor (RH) as substrate, by putidaredoxin (put) proceeds through an intermediate complex which is formed with a second-order rate constant of 3.5 x 10 M s extrapolated to 25 °C (0.1 M phosphate, pH 7.0) and assuming that the ratio of forward to back electron-transfer rate constants, Ara/Ar-2, is 0.4 ... 

Besides, the photoexcited complex has also been found to react with a series of pyridinium acceptors such as MV [129]. The electron transfer nature of the photoreaction mechanism has been established by the appearance of the characteristic MV cation radical absorption in the transient absorption difference spectrum. The reaction has been shown to be reversible with a back-electron transfer rate constant of 1.5 x 10 dm mol" s" . From the oxidative quenching experiments with a series of structurally related pyridinium acceptors, an excited state reduction potential of [Au2 /AU ] of -1.6(1) V vs. SSCE [/fT In KV = 0.58(10) V vs. SSCE, = 0.9(K10) eV] has been estimated by three-parameter, nonlinear, least-squares fits to the equation ... 

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