Charge recombination process

Fig. 12. Dependence of the recombination rate constant k3 on the ground-state M(III)/M(II) reduction potential of complexes 1-5. Also shown is the estimated driving force — AG° for the charge recombination process. Reprinted with permission from Ref. (30). Copyright 2002, American Chemical Society.

Figure 11. Rates of pho-toinduced charge separation and subsequent charge recombination processes in dyads (in benzonitrile).

In contrast to a conventional p-n-junction-type solar cell, the mechanism of the DSSC does not involve a charge-recombination process between electrons and holes because electrons are injected from the dye photosensitizers into the semiconductor, and holes are not formed in the valence band of the semiconductor. In addition, electron transport takes place in the Ti02 film, which is separated from the photon absorption sites (i.e., the photosensitizers) thus, effective charge separation is expected. This photon-to-current conversion mechanism of the DSSC is similar to that for photosynthesis in nature, where chlorophyll functions as the photosensitizer and electron transport occurs in the membrane. 

The charge-recombination process between injected electrons and oxidized dyes must be much slower than the process of electron injection and electron transfer from the I ion into oxidized dyes (i.e., regeneration of dyes) to accomplish effective charge separation. It was reported that charge recombination -... 

The non-coincidence of the slopes of straight lines in Fig. 11(b) for various temperatures for the samples with H20 results, according to refs. 118 and 119, from the participation in the charge recombination process of two (or more) types of P700t -A" pairs with different values of recombination activation energies. 

Table 9.4 represents the calculated AG values for the charge separation and charge recombination processes. Hereby, the charge recombination falls into the inverted regime of the Marcus parabola. With these values in hand, it was possible to place the different possible reaction pathways in a state diagram (Fig. 9.25). 

Since the radical ion pair states are stable on the time-scale of the femtosecond experiments, the charge-recombination rates were analyzed in complementary nanosecond experiments (Fig. 9.46). Therein, the decays of the C o and exTTF + features result in the refurbishment of the singlet ground state of 18a,b lacking any detectable triplet features. The corresponding rate constants for the charge-recombination process are listed in Table 9.5. 

It is clear that much has been learned about fundamental aspects of ET through studies of these covalently linked metal-organic dyads. The survey of photoinduced forward ET rate data reveals that in this process metal-organic dyads are well behaved, i.e., their performance is in accord with modem theories of ET reactions. By contrast, the survey of charge recombination rate data reveals that the kinetics for back ET in the type 2 dyads is unusually slow compared with other systems. This unusual behavior is attributed to effects of electron spin-multiplicity on the charge recombination process. 

Micelles and microemulsions have been explored as membrane mimetic systems since they possess charged microscopic interfaces which act as barriers to the charge recombination process (Fendler et al., 1980 Hurst et al., 1983). Namely, the influence of the location of the sensitizer on photoinduced electron transfer kinetics and on charge separation between photolytic products in reversed micelles has been studied (Pileni etal., 1985). 

Fig. 14 A schematic of the photoinduced charge separation and charge recombination processes in 18(w). A simple orbital diagram is provided which captures the essentials of the ET processes. HD, donor HOMO LD, donor LUMO HA, acceptor HOMO LA, acceptor LUMO. Note that all depicted processes are assumed to take place on the singlet multiplicity manifold.

Fig. 29 Energy diagram for charge separation and charge recombination processes in 18(w). The energy of the CS state varies as a function of the bridge length, n, and the solvent polarity, from slightly above XD A (i.e., for n = 12 in saturated hydrocarbon solvents), to about 1 eV below (i.e., in highly polar solvents). Under all circumstances the locally excited donor triplet (3D A) lies below the CS state.

Charge Separation Process in the Fluorescence Quenching Reaction and the Charge Recombination Process of Ion Pairs Produced in Polar Solvents. 

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