And charge separation

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... 

The Poisson-Boltzmann equation is a modification of the Poisson equation. It has an additional term describing the solvent charge separation and can also be viewed mathematically as a generalization of Debye-Huckel theory. 

One possible explanation for the above results is that the transition state for the uncatalyzed reaction is either more ionic or has its charges more highly separated than does the transition state for the catalyzed reaction. A consideration of possible transition state structures makes this explanation improbable, since the transition state for the catalyzed reaction would, in fact, be expected to show the greater charge separation, and this would be equally the case for both the transition state for intermediate formation and the transition state for conversion of intermediate to product. 

The transition state does not involve a large degree of charge separation and hence the relative rates of oxidation of toluene, diphenylmethane and triphenylmethane may be explained in terms of an ionic mechanism. 

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]. 

A number of ab initio molecular orbital calculations have been performed on acyclic and cyclic phosphazenes. These calculations point to a phosphorus-nitrogen bond with a large degree of charge separation and a small but essential contribution from phosphorus d-orbitals. 

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... 

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...

Scheme 2 Schematic representation of the charge separation and charge transport processes in the hairpin 3GAGG. Shading indicates excited state or radical ion species...

The ability to switch a molecular unit on and off is a key component of an efficient molecular device, since it allows modulation of the physical response of such a device by external physical or chemical triggers. A molecular device, based on a trinuclear metal complex, shown in Figure 59, functions as an electroswitchable-photoinduced-electron-transfer (ESPET) device.616 Electrochemical switching of the redox state of a spacer intervening between a donor-acceptor pair can dictate the type of the observable charge separation and the lifetime of the resulting ion pair.616... 

By their very nature, their partial charge separation can make them fairly insoluble and the degree of this charge separation (and hence resultant NMR spectra) tends to be highly dependant on concentration and pH. For these reasons, we recommend dealing with such compounds by pushing them one way or the other, i.e., by adjusting the pH of your NMR solution so that the compound in question is either fully protonated (addition of a drop of DC1) or de-protonated (addition of a drop of saturated sodium carbonate in D20) ... 

Vogel M, Rettig W (1985) Efficient intramolecular fluorescence quenching in triphenyl-methane dyes involving excited states with charge separation and twisted conformations. Ber Bunsenges 89(9) 962-968... 

Tricot, Y.-M. and Fendler, J.H., Colloidal catalyst-coated semiconductors in surfactant vesicles In situ generation of Rh-coated CdS particles in dihexadecylphosphate vesicles and their utilization for photosensitized charge separation and hydrogen generation, /. Am. Chem. Soc., 106, 7359,1984. 

Meyer, M., Wallberg, C., Kurihara, K., and Fendler, J.H., Photosensitized charge separation and hydrogen production in reversed micelle entrapped platinized colloidal cadmium sulphide, /. Chem. Soc., Chem. Commun., 2, 90,1984. 

Paddon-Row MN (2003) Superexchange-mediated charge separation and charge recombination in covalently linked donor-bridge-acceptor systems. Aust J Chem 56 729-748... 

Figure 11. Rates of pho-toinduced charge separation and subsequent charge recombination processes in dyads (in benzonitrile).

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