Initial charge separation

MV2 + or SPV as an acceptor [125, 126], They have concluded that the steric protection of chromophores from the quencher crucially affects the efficiency of the initial charge separation. This conclusion is essentially the same as that reached by Morishima et al. [119, 120],... 

Dye-sensitized solar cells (DSSCs) are photoelectrochemical solar devices, currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. DSSCs are based upon the sensitization of mesoporous nanocrystalline metal oxide films to visible light by the adsorption of molecular dyes.5"7 Photoinduced electron injection from the sensitizer dye (D) into the metal oxide conduction band initiates charge separation. Subsequently, the injected electrons are transported through the metal oxide film to a transparent electrode, while a redox-active electrolyte, such as I /I , is employed to reduce the dye cation and transport the resulting positive charge to a counter electrode (Fig. 17.4). 

DSSCs convert sunlight to electricity by a different mechanism than conventional p-n junction solar cell. Light is absorbed directly at the solid/liquid interface by a monolayer of adsorbed dye, and initial charge separation occurs without the need of exciton transport.42,43 Following the initial charge separation, electrons and holes are confined in two different chemical phases electrons in the nanocrystalline... 

The rate of the initial charge separation in 4 and similar molecules may be estimated from the fluorescence lifetime of the porphyrin moiety. It is assumed that the electron transfer rate constant is given by ... 

A relative of 12 has been prepared by Sanders, van der Plas, and coworkers [64], Triad 13 features an N,A-dimethylaniline-type donor and an anthraquinone acceptor. These moieties are linked to the ortho positions of the porphyrin aryl groups, and this leads to a folded conformation for the molecule, as determined from NMR studies. Both the free base and zinc derivatives of 13 were prepared. The folded conformation might be expected to facilitate electron transfer among the components of the triad, and while this could enhance the quantum yield of the initial charge separated state, it might unfavorably affect the yield of the final D+-P-QT state and its lifetime. Unfortunately, photochemical or spectroscopic studies were not reported. 

Figure 3.31. The normalized distribution over the initial charge separations under (K) kinetic control (D = oo), (D) diffusional control (D = 10 6cm2/s) and for (S) static ionization (D = 0). The remaining parameters are the same as in Figure 3.30. (From Ref. 24.)...

Moser, C. C., Sension, R. J., Szarka, A. Z., Repinec, S. T., Hochstrasser, R. M., and Dutton, P. L., 1995, Initial charge separation kinetic of bacterial photosynthetic reaction centers in oriented Langmuir-Blodgett films in an applied electric field. J. Chem. Phys. 197 3439354. 

Finkele, U., Lauterwasser, C., Zinth, W., Gray, K. A., and Oesterhelt, D., 1990, Role of tyrosine M210 in the initial charge separation of reaction centers of Rhodobacter sphaeroides. Biochemistry, 29 8517-8521. 

The initial charge separation in PS I and PS II can be followed by what are known as ultrafast optical spectroscopy techniques. Several variations on this method exist, but they can be grouped into pump-probe absorbance difference and transient fluorescence methods (25, 26). In the first instance, the sample is irradiated with a pump pulse to initiate the electron transfer and the absorbance is measured using a probe pulse at a... 

A Special Pair of Chlorophylls Initiates Charge Separation... 

Zinc chlorins are more easily oxidized than their free base counterparts, and this fact has been used in the design of some chlorin-porphyrin-imide triads that demonstrate multistep electron transfer [16]. For example, triad 47 consists of a zinc chlorin linked to a free base porphyrin that also bears a pyromellitimide acceptor moiety. Phenyl linkers join all active constituents. Excitation of the zinc chlorin moiety of 47 in tetrahydrofuran gives CZ-P-Im, which decays with a 20-ps time constant to yield CZ +-P -Im. This initial charge-separated state evolves into CZ +-P-Im with a lifetime of 120 ps. The CZ -P-Im " state is formed with an overall quantum yield of 0.90 and decays biphasically with time constants of 110 ns and 400 ns. 

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