Electron transfer quenching separation efficiency

Application of the energy gap law to the energy conversion mechanism in Scheme 1 leads to a notable conclusion with regard to the efficiency for the appearance of separated redox products following electron transfer quenching. From the scheme, the separation efficiency, sep> is given by eq. 18. Diffusion apart of the... 

When using DCE as quencher (Fig.3a), the second deactivation channel remains open until Ati=l ns. The small free ion yield for this system indicates that charge recombination is much more efficient and therefore much more rapid than charge separation. Furthermore, we know from time resolved fluorescence decay measurements that electron transfer quenching is still not finished at At =lns. Consequently the absorbance should be dominated by fresh geminate ion pairs, over the whole timescale investigated. Thus, the GSR dynamics of Pe + in presence of DCE should be independent of Ati, which is confirmed by our experimental observations. 

In order to achieve high efficiencies for photoinduced electron transfer, it is necessary that the rate constant fcx quenching exceed that for excited state decay and that the rate constant for separation be greater than that for back electron transfer. Tlie separation of the photoproduced r ox equivalents relies on the existence of a fiee energy... 

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. 

Triad 25 is another example of this general type [75]. As was the case with the previously discussed triads 15—18, the absorption spectrum of 25 indicates some degree of excitonic interaction between the porphyrins. The fluorescence quantum yield of 25 is 5 5 x 10-6, which indicates efficient quenching of the porphyrin singlet states, presumably by electron transfer. No information concerning the lifetime of any charge separated state was presented, but one would predict that it would be extremely short. 

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