Charge separation/recombination

The scheme indicated in Figure 1 (excitation, charge separation, charge recombination) is most likely to occur when the product state has the same spin multiplicity as the reactant state, or when the product species are held together (e.g., by chemical bonds) for periods that allow intersystem crossing to occur. However, the charge separation-recombination sequence is not the only type of process that photoexcitation can achieve. An example is when the product state is held only loosely by a solvent cage and the entities therein can diffuse apart and have independent reactive identities. Such a scheme would be typified by the sequence... 

Because there are no long-range forces between either the reactants or the products in the charge-shift reaction, D + A D + A , AG° is expected to be independent of distance. Figure 4 demonstrates that this is so. Also plotted in Figure 4 are the calculated estimates of A G° for charge separation-recombination, D + A + A", using the Weller equation. 

Liddell P A, Kuciauskas D, Sumida J P, Nash B, Nguyen D, Moore A L, Moore T A and Gust D 1997 Photoinduced charge separation and charge recombination to a triplet state in a carotene-porphyrin-fullerene triad J. Am. Chem. Soc. 119 1400-5... 

However, not all excitons have sufficiently long lifetimes to reach the interface before recombining. To circumvent this problem and increase device efficiency, heterostmcture devices have been fabricated. In these devices, donors and acceptors are mixed together to create a network that provides many internal interfaces where charge separation can occur. Heterostmcture devices made from the donor polymer... 

Another important factor to determine the charge separation efficiency is the distance between and the mutual orientation of the donor and the acceptor in the geminate ion-pair state. The rate of charge recombination depends on whether... 

Although the electrostatic potential on the surface of the polyelectrolyte effectively prevents the diffusional back electron transfer, it is unable to retard the very fast charge recombination of a geminate ion pair formed in the primary process within the photochemical cage. Compartmentalization of a photoactive chromophore in the microphase structure of the amphiphilic polyelectrolyte provides a separated donor-acceptor system, in which the charge recombination is effectively suppressed. Thus, with a compartmentalized system, it is possible to achieve efficient charge separation. 

Recently, photochemical and photoelectrochemical properties of fullerene (Cto) have been widely studied [60]. Photoinduced electron-transfer reactions of donor-Qo linked molecules have also been reported [61-63]. In a series of donor-Cfio linked systems, some of the compounds show novel properties, which accelerate photoinduced charge separation and decelerate charge recombination [61, 62]. These properties have been explained by the remarkably small reorganization energy in their electron-transfer reactions. The porphyrin-Qo linked compounds, where the porphyrin moieties act as both donors and sensitizers, have been extensively studied [61, 62]. 

Strand cleavage studies have provided relative rate constants for hole transport versus the rate constant for the initial chemical event leading to strand cleavage [18-20]. However, they do not provide absolute rate constants for hole transport processes. Several years ago we introduced a method based on femtosecond time-resolved transient-absorption spectroscopy for investigating the dynamics of charge separation and charge recombination in synthetic DNA hairpins [21, 22]. Recently, we have found that extensions of this method into the nanosecond and microsecond time domains permit investigation of the dynamics of hole transport from a primary hole... 

The dynamics of photoinduced charge separation, kcs, and charge recombination, kcr (Fig. 2a), have been studied in several families of hairpins containing an Sa linker and a single G C base pair by means of femtosecond time-resolved transient absorption spectroscopy [27, 28]. Both the singlet state and anion radical of Sa have strong transient absorption centered at 575 nm. The difference in the independently determined band shapes for Sa ... 

Fig. 2 a Energetics of photo oxidation of G and A by singlet Sa. b Dynamics of charge separation (kcs) and charge recombination (kCY) for Sa-linked hairpins possessing a single guanine... 

Fig. 4 Free energy dependence of the rate constants for charge separation and charge recombination for hairpins in which two A T base pairs separate the linker acceptor from the nucleobase donor. The dashed line is a fit of the charge separation data to the Marcus-Levitch-Jortner equation...

Fig. 5 Dynamics of charge separation and charge recombination for hairpins possessing G, Z, GG, and GGG donors...

Wiberg J, Guo L, Pettersson K et al (2007) Charge recombination versus charge separation in donor-bridge-acceptor systems. J Am Chem Soc 129 155-163... 

---