Charge recombination, reversible

Charge recombination Reverse of charge separation. In using this term it is important to specify the resulting electronic state of the donor and acceptor. 

Here the summation index i covers both traps and recombination centers. We suppose initially that there is only one trap and that the rest of the impurities or defects act as recombination centers. A trap, of course, is by definition more likely to release its charge to the appropriate band than to hold it until annihilation by subsequent capture of the opposite charge. The reverse is true for recombination centers. We can often lump the sum effect of all the recombination centers into a single lifetime t ... 

The second process that can reduce the yield of useful ions is charge recombination between any reduced electron acceptor and the original positive hole left in the initially excited molecule (shown as and in Figure 5.8). This can be prevented only by the spatial separation of the positive and negative charges. The reduction potentials of the successive electron acceptors decrease as their spatial separation increases, so that both the reverse electron transfer and the charge recombination become very slow. 

The dynamics of electron-transfer at the interfaces of Ti02/Dye 2/electrolyte are summarized in Fig. 20.7. Two forward electron-transfer steps are much faster than the corresponding reverse electron-transfer (charge recombination) on the order of 103 to 106. The results well explain the high rjei value due to the efficient and vectorial electron-transfer in Dye 2-sensitized DSC. 

Mg(DOPMR)2-H2(DOP) [Mg(DOP )+-(R)2-[H2(DOP )] - Solvent acetone, CH2C12, DMF or alkyl-acetates X, = 532 or 588 nm the charge-recombination rate constant correlates with the reverse of the solvent relaxation times [196]... 

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°. 

After [9], numerous papers were published related to the low-temperature photooxidation of P700 in the PS1 reaction centres of plants and the reverse process of recombination of P700+ and a reduced electron acceptor (see e.g. Refs. [208-211]). In these works controversial data on the kinetics of the dark decay of P700 + were obtained. Therefore in Ref. [212] the kinetics of charge recombination in the PS1 reaction centres was investigated in detail over a broad range of times and temperatures. 

Temperature dependences of the rate for direct photoinduced electron transfer process and reverse charge recombination reaction were studied in some works. As a rule both processes were found to be temperature dependent. However for [p(MP), a(Fe(III)P hemoglobin hybrid (M = Zn(II), Mg(II)) the rate constants of both processes were found to be temperature independent in the temperature interval 273-293 K [285],... 

As a result of intramolecular PET between the structural units of such bridging molecules one can obtain within 10 9 s spatially separated charges, localized at different ends of the molecule. Due to the rigidity and large size of the molecules used and also to a higher potential barrier for back electron tunneling, the reverse intramolecular recombination proceeds relatively slowly. For the best triad molecules the characteristic time of the intramolecular charge recombination... 

The border between the irreversible and reversible ionization lies at a AG, value where the rate of the backward transfer to the ground state equals the rate of reverse transfer to the excited state kb(AG°) = kc(AG°). As a rule AG° is negative and not small, so that the border (indicated by the dashed line in Fig. 3.47) is far below the resonance reached at AG, = 0. To shift it up another channel of charge, recombination should be opened and its rate must be faster than kc. This ionic reaction may be parallel to that included in scheme (3.90) or recombination through the triplet channel proposed in Refs. 107 and 150. The latter is discussed in Section XI among other reactions affected by the spin conversion. 

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). 

The distanee dependence for the reverse process, thermal charge recombination back to the ground state of 34(ii), which occurs in the Marcus inverted region, was also determined and was foimd to be similar to that for the forward photoinduced ET reaetion ... 

Fig. 16. A photo- and electrochemically controllable molecular shuttle. The unperturbed rotaxane 116+ exists preferentially in the translational isomer in which the BPP34C10 crown ether resides around the bipyridinium unit, a Photochemical excitation of the Ru(bipy)3 unit results in PET to the bipyridinium site, and consequent translation of the crown ether to the 3,3dimethylbipyridinium unit, which is a less efficient recognition site for the cyclophane CBPQT4+ than a bipyridinium system. This process occurs only in the presence of a sacrificial reductant which reduces the Ru(III) center back to its Ru(II) state in order to prevent charge recombination, b Conversely, upon electrochemical reduction of the bipyridinium unit, the crown ether takes up residency around the 3,3 -dimethylbipyridi-nium site. This process is reversed through electrochemical oxidation of the bipyridinium radical cation back to the dication...

---