Photoseparated charges

Fig. 1. Schematic representation of molecular structures used for the photoseparation of charges, (a) Molecular monolayers on the base of fatty acids (b) lipid bilayers (c) lipid vesicles (d) micelles (e) microemulsions. The circles and ellipses stand for polar fragments the elongated rectangles, the wavy and straight lines depict hydrophobic fragments. Examples of substances forming the depicted structures are given in Fig. 3.

Fig. 3. Structural formulae of the molecules of (a) donor, (b) acceptor, and (c) fatty acids used in ref. 6 to study the photoseparation of charges in the multilayer structure depicted in Fig. 2.

Other interesting examples of the organized molecular structures used to increase the quantum yield of charge photoseparation are micelles and vesicles. Micelles represent aggregates of surfactant molecules, one end of which is hydrophobic and the other hydrophilic. On reaching a certain critical concentration in a solution, these molecules group into spherical formations in which either the hydrophilic ends of the molecules are turned towards the micelle centre while their hydrophobic ends form its surface or vice versa. Micelles of the former type are usually formed in non-polar solvents and those of the latter type in polar solvents. The micelle is schematically represented in Fig. 1(d). 

Another example of compounds with the fixed mutual location of porphyrin and quinone are the porphyrin-quinone compounds with a rigid bridge. Charge photoseparation in P-L-Q molecules in which L is the trip-ticene bridge, P is tetraphenylporphin, TPP, or its zinc complex, and Q is benzoquinone, BQ, naphthoquinone, NQ, or anthraquinone, AQ, has been studied [55]. The distance between the centres of P and Q fragments in these... 

The charge photoseparation in porphyrin-quinone compounds with a rigid bicyclo[2.2.2]octyl bridge, ensuring a distance between the centres of P and Q of about 16 A, has been studied [57]. The rate constant of intramolecular electron transfer from P to Q was found to depend on the dielectric properties of the medium and reached 3.3 x 107s 1 for a solution of P-L-Q in propionitrile. 

A simple system for modelling the intermediate step of the charge separation process during photosynthesis [the stage of electron transfer from the reduced pheophytin (i.e. chlorophyll deprived of the Mg atom) to quinone] has been advanced and studied [67]. In this work the charge photoseparation process was studied in solutions of P-L-Q compounds in electron-donor, Et3N, solvent at 77 K. The structure of one of the P-L-Q compounds studied is given in Fig. 13. We will consider briefly the main results of ref. 67. 

Charge photoseparation in the covalently linked D-P-Q triad with rigid bridges (see Fig. 21 b) has been studied [105]. The distance between the donor and acceptor... 

Other interesting examples of the organized molecular structures used to increase the quantum yield of charge photoseparation are micelles and vesicles. 

A. I. Burshtein and E. Krissinel. Photoseparation of ion radicals following exd-plex formation and spin conversion. J. Phys. Chem. A, 102(39) 7541-7548, 1998. A. I. Burshtein and K. L. Ivanov. The crucial role of triplets in photoinduced charge transfer and separation. Phys. Chem. Chem. Phys., 4(17) 4115-4125,... 

A cornerstone for the development of such microheterogeneous systems is the choice of an optimal photosensitizer for the process. At present, a great many experimental data have been obtained on the properties of ordered microheterogeneous photocatalytic systems operating with molecular photosensitizers such as complexes of various metals [2,3]. However, some important photochemical properties of the designed systems (e.g., the quantum yields of the charge photoseparation or the rates of the vectorial transmembrane electron transfer) have not allowed, up to now, the design of composite photocatalytic... 

Chapter 8, written by M. G. Kuz min and N. K. Zaitsev, is concerned with the specific case of the photoseparation of charges in micellar systems. The particular importance of this specific case is due to the fact that these systems have an enormous interface but a small total volume. 

In sacrificial systems the oxidized form of D (or the reduced form of A) produced by charge photoseparation reacts with an irreversible consumed reductant (oxidant), e.g. EDTA or triethanolamine [6,160]. Such systems can be of some practical interest, because in principle they enable fuel production from diluted solutions of industrial and other waste. 

Photosynthetic Reaction Centres (EC s). The conversion of solar eneigu to chemical energy with a quantum yield close to unity proceeds by the mechanism of photoseparation of charges in the RC s of photosynthetic bacteria. The following scheme of electron transfer in the Rc of photosynthetic bacteria has been shown to take place ... 

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