Electron transfer rates recombination

The donor-acceptor complexes [Ir(/r-dmpz)(CO)(PPh2 0(CH2)2R )]2 exhibit photo-induced electron-transfer rate constants of 1012s—1 and charge recombination rates slower than 2 x 10los-1 when R = pyridine and 4-phenylpyridine.534 Further studies on these complexes revealed that recombination reactions were temperature dependent and slower for the deuterated acceptors.535... 

Fig. 16.3 Quantum yield (QY) for electron and hole transfer to solution redox acceptors/donors as a function of the reduced variables y (related to the surface properties of the catalyst, i.e., ratio between interfacial electron transfer rate and surface recombination rate) and w (related to the ratio between surface migration currents of hole and electrons to the rate of bulk recombination), according to the proposed kinetic model [23],...

Since the values of i/ depend on several factors noted above, in the absence of additional data such as the temperature dependence of the electron transfer rate constants for i-2 it is difficult to analyze the apparent difference between i/ for the charge separation reaction and that of the radical ion pair recombination reaction. However, the difference between these two values of u is not unreasonable given that the charge separation involves oxidation of an excited state of the donor, while radical ion pair recombination involves two ground state radicals. Small changes in the nuclear coordinates of the donor and acceptor for these two reactions should be sufficient to produce the observed difference in i/. The electronic coupling factor between ZnTPP and AQ should be different than that between ZnTPP " and AQ". 

The effect of the FC term on ICT and MLCT-based chemosensors appears when the electron transfer rate constant is generalized within the context of nonradiative decay theory [191-193], MLCT excited states are produced directly upon excitation whereas ICT states are produced by a surface crossing from an initially prepared localized excited state (see Fig. 9). Return of the system from the charge transfer excited state to ground state has the overall form of an electron transfer recombination problem that is described by the inverted Marcus curve of Fig. 13. As described by the FC term of Eq. (5), the rate constant for... 

In Ref. 142, a detailed analysis of the forward and reverse electron transfer rates for capped P-L2-Q in a variety of solvents was given. The results show that forward electron transfer is in the normal region Er > — AG° and charge recombination is in the Marcus inverted region, Er < — AG°. 

Thus, from fluorescence lifetime and transient absorption measurements we gathered the electron-transfer rate constants, i.e. both for charge-separation and for charge-recombination. Next, we plotted these rate constants as a function of donor-acceptor distance. From the resulting linear dependence (Fig. 9.26) it is possible to determine the attenuation factors p for the presented donor-acceptor... 

The rate constants for electron transfer and recombination are readily separated because in the limit (w- 0), equation (8.31) tends to kir/(ktr + krec), and the maximum of the semicircle occurs when ca = 2ir f=kt + krec. In the absence of RC attenuation effects, the high frequency intercept of the IMPS plot (minority carriers. Measurements of gac as a function of potential (band bending) can be used to determine the minority carrier lifetime and absorption coefficient [46]. The main advantage of using the IMPS data rather than dc measurements of... 

The transient Q-band EPR experiments provide direct evidence for sequential electron transfer from the primary to the secondary radical pair of the triplet channel in a triad consisting of a zinc-9-desoxo-meso-methylpyrochlorophyllide donor (ZC), a pyromellitimide primary acceptor (PI), and a naphthalene-1,8 4,5-diimide secondary acceptor oriented in a liquid crystal (Heinen et al., 2002). At room temperature this process occurs with an exponential time constant of tT = 50 + 1 ns. In the singlet-initiated channel, the intramolecular electron-transfer rates are too fast for direct EPR detection. The species decay with a time constant of tS = 36 1 ns by charge recombination to the singlet ground state. 

The most widely used electron acceptors in inorganic chromophore-quencher systems have been bipyridinium dications, often called viologens (quatemarized derivatives of 4,4 -bipyridine) or diquat (cyclic quatemarized derivatives of 2,2 -bipyridine). The classical studies of Elliott, Schmehl, and Mallouk have been concentrated on dyads of types (4) and (5). For dyads (4) [168, 169], oxidative PET takes place, with forward processes in the 80 to 1700-ps time scale and very fast (<30 ps) charge recombination. The main observations are that (i) electron transfer to the diquat quencher occurs from the directly linked bipyridine ligand (ii) fast equilibration between the MLCT excited states on the three bipyridine ligands precedes electron transfer (iii) the electron transfer rates are in the normal Marcus... 

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. ... 

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