Reverse photoinduced charge transfer

We note that the model presented here in- and reverse photoinduced charge-transfer as... 

TTF-based D-A systems have been extensively used in recent years to play around photoinduced electron transfer processes. Typically, when an electron acceptor moiety that emits fluorescence intrinsically is linked to TTF (D), the fluorescence due to the A moiety may be quenched because of a photoinduced electron transfer process (Scheme 15.1). Accordingly, these molecular systems are potentially interesting for photovoltaic studies. For instance, efficient photoinduced electron transfer and charge separation were reported for TTF-fullerene dyads.6,7 An important added value provided by TTF relies on the redox behavior of this unit that can be reversibly oxidized according to two successive redox steps. Therefore, such TTF-A assemblies allow an efficient entry to redox fluorescence switches, for which the fluorescent state of the fluorophore A can be reversibly switched on upon oxidation of the TTF unit. 

Photoinduced electron transfer rates can be considerably reduced when the counterion X- is changed from chloride to bromide. Charge transfer between the cationic part of a molecule and the bromide ion may be responsible to the reduction of photoinduced electron-transfer rates. Such a counterion effect on the photoinduced electron transfer and the reverse process has been demonstrated for examples of porphyrin-viologen-linked compounds (Mitsui et al. 1989). 

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],... 

Photoinduced electron transfer [1, 2] represents the simplest way of achieving charge reversal in chemical reagents. As shown in Eq. 1, an electron-rich reagent D can interact... 

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). 

In summary, the stereoselectivity was certainly observed in the photoin-duced electron transfer reactions of chiral ruthenium(II) complexes with chiral viologen and Co(III) complexes. However, not only the photoinduced electron transfer reaction but also the charge separation in the encounter complex and the reverse reaction between the ruthenium(III) complex and Co(acac)2 + acac participate in the stereoselection. In the reactions between the ruthenium(II) and Co(III) complexes, the energy transfer also contributes to the quenching reaction, which makes difficult the observation of stereoselectivity in the quenching reaction. 

Intervalence charge transfer Electron transfer (thermal or photoinduced) between two metal sites differing only in oxidation state. Quite often such electron transfer reverses the oxidation states of the sites. The term is frequently extended to the case of metal-to-metal charge transfer between non-equivalent metal centers. 

The sequence of events taking place in this system is (i) charge transfer excitation (process 7 in Fig. 4, eq 30), (ii) intercomponent electron transfer (process 13 in Fig. 4, eq 31), (iii) localized emission (process 5 in Fig. 4, eq 32). As shown in Figure 4, this is essentially the reverse sequence with respect to photoinduced electron transfer, except for the fact that... 

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