Lifetime of charge separation states

The general strategy of multistep electron transfers in triad-type molecules has proven to be a useful one for maximizing the quantum yield of charge separated states formed by photoinitiated electron transfer, the lifetimes of these states, and the amount of energy stored therein. As a result, the strategy has been exploited in a variety of molecular devices, many of which are reviewed below. 

The use of sequential and parallel multistep electron transfer pathways in the pentad has increased the lifetime of charge separation from picoseconds in por-phyrin-quinone dyads to hundreds of microseconds, a factor of 10. By successful molecular engineering, this has been achieved while maintaining a quantum yield near unity and preserving a significant fraction of the photon energy as chemical potential in the final charge-separated state. Such molecules show that biomimetic systems can perform essentially as well as natural photosynthesis in these respects. 

The most simple kind of photosynthetic model systems consists of a chromophore linked to an electron acceptor or donor. For example, the porphyrinquinone diad 1 represents a model of the primary [32] photosynthetic reaction center. Various diads have been synthesized and their photophysical properties have recently been reviewed [33]. The general strategy of multistep ET in triad and multifunctionalized molecular assemblies provides a useful means for maximizing the quantum yields of charge-separated states and the lifetime of the redox intermediate states. Evidently, various molecular assemblies following this strategy have been exploited. 

Quite differently, Pleux et al. tested a series of three different organic dyads comprising a perylene monoimide (PMI) dye linked to a naphthalene diimide (NDI) or C60 for application in NiO-based DSSCs (Fig. 18.7) [117]. They corroborated a cascade electron flow from the valance band of NiO to PMI and, finally, to C60. Transient absorption measurements in the nanosecond time regime revealed that the presence of C60 extends the charge-separated state lifetime compared to just PMI. This fact enhanced the device efficiencies up to values of 0.04 and 0.06% when CoII/m and P/Ij electrolytes were utilized, respectively. More striking than the efficiencies is the remarkable incident photon-to-current efficiency spectrum, which features values of around 57% associated to photocurrent densities of 1.88 mA/cm2. 

The temperature dependence of kET(CR3> revealed only a moderate change (2.6-3.0 sec ) upon varying the temperature between 163 and 203 K [47]. The longest lifetime of the resulting charge-separated state (i.e., ferricenium ion Ceo radical anion pair) in frozen benzonitrile (PhCN) is determined as 0.38 sec [47], which is more than one order of magnitude louger than any other intramolecular... 

Investigation of photoinduced intramolecular ET processes in such systems has in general shown that, compared to analogous porphyrin/quinone systems, i. charge separation occurs with higher efficiency and ii. charge-separated states have longer lifetimes (Scheme 9.4). Such a behavior is the result of the combination of two... 

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