Electron transfer charge separation

The observation of selective fluorescence quenching by G C base pairs is consistent with the energetics of electron transfer (charge separation) from the bases to the singlet excited state Sa, which can be estimated using Weller s equation [26] ... 

Figure 1. Schematic energy level diagram for a typical photoinduced electron transfer. Charge separation and recombination routes are shown—see text.

Figure 9.13 Compounds studied for ability to undergo photoinduced electron transfer (charge separation).

Of much significance is the realization of long-lived photo generated tautomeric states and long-range proton transfer (LRpT) processes. The latter could lead to proton transfer charge separated states to be put in parallel with the extensively studied charge separation by photoinduced electron transfer (see Section 8.2.3). A number of systems present photochromism on the basis of photoinduced proton transfer [8.229]. 

Fig. 16.8 Charge recombination lifetimes in the compounds shown in the inset in dioxane solvent. (J. M. Warman, M. P. de Haas, J. W. Verhoeven, and M. N. Paddon-Row, Adv. Chem. Phys. 106, Electron transfer—from isolated molecules to bio-molecules, Part I, edited by J. JortnerandM. Bixon (Wiley, New York, 1999). The technique used is time-resolved microwave conductivity (TRMC), in which the change in dielectric response of a solution is monitored following photoinduced electron transfer—a charge separation process that changes the solute molecular dipole. The lifetimes shown as a function of bridge length (number of a-bonds separating the donor and acceptor sites in the compounds shown in the inset) are for the back electron transfer (charge recombination) process.

We have seen the Z-scheme for the two photosystems in green-plant photosynthesis and the electron carriers in these photosystems. We have also described how the photosystems of green plants and photosynthetic bacteria all appear to function with basically the same sort ofmechanisms of energy transfer, primary charge separation, electron transfer, charge stabilization, etc., yet the molecular constituents of the two reaction centers in green plants, in particular, are quite different from each other. Photosystem I contains iron-sulfur proteins as electron acceptors and may thus be called the iron-sulfur (FeS) type reaction center, while photosystem 11 contains pheophytin as the primary electron acceptor and quinones as the secondary acceptors and may thus be called the pheophytin-quinone (0 Q) type. These two types of reaction centers have also been called RCI and RCII types, respectively. 

Williams R M, Koeberg M, Lawson J M, An Y-Z, Rubin Y, Paddon-Row M N and Verhoeven J W 1996 Photoinduced electron transfer to Cgg across extended 3- and 11 a-bond hydrocarbon bridges creation of a long-lived charge-separated state J. Org. Chem. 61 5055-62... 

The proposed scenario is mainly based on the molecular approach, which considers conjugated polymer films as an ensemble of short (molecular) segments. The main point in the model is that the nature of the electronic state is molecular, i.e. described by localized wavefunctions and discrete energy levels. In spite of the success of this model, in which disorder plays a fundamental role, the description of the basic intrachain properties remains unsatisfactory. The nature of the lowest excited state in m-LPPP is still elusive. Extrinsic dissociation mechanisms (such as charge transfer at accepting impurities) are not clearly distinguished from intrinsic ones, and the question of intrachain versus interchain charge separation is not yet answered. 

This review article attempts to summarize and discuss recent developments in the studies of photoinduced electron transfer in functionalized polyelectrolyte systems. The rates of photoinduced forward and thermal back electron transfers are dramatically changed when photoactive chromophores are incorporated into polyelectrolytes by covalent bonding. The origins of such changes are discussed in terms of the interfacial electrostatic potential on the molecular surface of the polyelectrolyte as well as the microphase structure formed by amphiphilic polyelectrolytes. The promise of tailored amphiphilic polyelectrolytes for designing efficient photoinduced charge separation systems is afso discussed. 

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. 

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