Distance dependence charge separation/recombination

Another important factor to determine the charge separation efficiency is the distance between and the mutual orientation of the donor and the acceptor in the geminate ion-pair state. The rate of charge recombination depends on whether... 

Distance-Dependent Rates of Photoinduced Charge Separation and Dark Charge Recombination in Fixed-Distance Porphyrin-Quinone Molecules... 

Fig. 3, and therefore reduce the quantum yield of C-P+-QT. However, this does not necessarily mean that the yield of the final C+-P-QT state will be similarly reduced. Indeed, it was found that addition of a second methylene spacer as in 5 increased the quantum yield of the final state by a factor of 1.44 [56], With 3 and 4 methylene groups (6 and 7) the yield decreased to 0.65 and 0.56 that of 4, respectively. This complicated distance dependence for the yield of the final state is in part a consequence of the fact that increasing the porphyrin-quinone separation not only reduces the rate of step 2, but also that of charge recombination (step 3). The rate of the second forward electron transfer step 4 is essentially unaffected, since this step does not involve porphyrin-quinone electron transfer. Thus, increasing the separation will decrease the quantum yield of step 2, but increase the ratio kjk3, which determines the efficiency of the second electron transfer step. With 5, the loss in quantum yield of step 2 is more than compensated for by the increase of efficiency of step 4, and the overall quantum yield increases. In 6 and 7, any increase in the efficiency of step 4 evidently cannot compensate for the decrease in quantum yield for step 2, and the overall quantum yield decreases. 

Finally, relating the charge-separation and charge-recombination dynamics with the distance between electron donor and acceptor (i.e. center-to-center distance Rcc), allows the evaluation of the attenuation factor / . The corresponding relationships in THF are represented in Fig. 9.8. Both reveal linear dependencies and the derived attenuation factors are in perfect agreement with each other ... 

Fig. 9.8 Centre-to-centre distances (Rex ) dependence of charge-separation (In A cs) and charge-recombination (In A cr) rate constants in eTITF-oPPE-Cgo in nitrogen-saturated THF at room temperature. The slope represents /(. The dashed line represents the singlet lifetime of C60. In blue, the extrapolated charge-separation rate for the trimer 9d is represented...

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... 

If one studies only the fluorescence quenching by irreversible bimolecular ionization (3.52), there is seldom any need to trace the fate of the charged products. On the contrary, those who are interested in photoinduced geminate recombination (3.188) rarely care about the kinetics of ionization, its quenching radius, and all the rest studied in Section III. All that they need to obtain the charge separation yield is the initial ion distribution mo(r), prepared by photoionization. However, the latter is scarcely so simple as in Eq. (3.201), which is usually favored. Even so, the initial separation ro is not a fitting parameter but the characteristic interion distance, which is dependent on the precursor reaction of photoionization. 

In general such distributions are neither contact nor infinitely thin, as in Eq. (3.202), and their subsequent evolution in the course of geminate recombination can be essentially different. In particular, when the backward transfer occurs in the remote recombination layer (see Fig. 3.27), the starting positions of ions can be either inside or outside it and their separation crucially depends on this initial distance. The initial distribution (3.299) allows one to specify the fractions of ions that are in and out and estimate their true contribution to the total charge separation yield. 

By varying the stilbene-G separation it was possible to obtain a distance dependence for both the photoinduced rate of charge separation (CS) and the subsequent rate of charge recombination (CR) for each series of DNA hairpins.167... 

Am. Chem. Soc., 115, 5665 (1993). Intramolecular Photoinduced Charge Separation and Charge Recombination of the Product Ion Pair States of a Series of Fixed-Distance Dyads of Porphyrins and Quinones Energy Gap and Temperature Dependences of the Rate Constants. 

Figure 14. Distance-dependence of the rate constants for charge separation (open symbols) and charge recombination (filled symbols) for hairpin families in which guanine is either in the polyT arm (e.g., 3G C, O, , solid lines) or in the polyA arm (e.g., 3C G, A, A, dashed lines) [110b].

Wasielewski M. R., NiemczykM. P., Svec W. A. andPewittE. B. (1985), Dependence of rate constants for photoinduced charge separation and dark charge recombination on the free energy of reaction in restricted-distance porphyrin-quinone molecules , J. Am. Chem. Soc. 107, 1080-1082. 

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