Photo charge separation

The physical appearance of pure gum and its complexes were often found to exhibit colour therefore their optical absorption characteristics in the UV-VIS region were studied in order to investigate their optical absorption, absorbance region, and the photo-charge separation therein. The optical absorption in the UV-VIS region was measured for coloured complex of gum Arabica, and when and where it was important. 

Clusters Fa tmd Fb can be (photo-)reduced by light-induced charge separation within the RC complex at cryogenic temperatures. Since, under these conditions, only one electron is injected into the Fa/Fb-protein, and since furthermore the EPR spectra of Fa and Fb are significantly different, a straightforward assignment of individual clusters to the cluster-binding motifs in the primary sequence was... 

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... 

Figure 1. Schematic representation of the artificial photosynthetic reaction center by a monolayer assembly by A-S-D triad and antenna molecules for light harvesting (H), lateral energy migration and energy transfer, and charge separation across the membrane via multistep electron transfer (a) Side view of mono-layer assembly, (b) top view of a triad surrounded by H molecules, and (c) energy diagram for photo-electric conversion in a monolayer assembly.

SSPM of unidirectionally oriented A-S-D triads in alternate mono-layers for detection of photo-induced charge separation... 

These workers employed a more elaborate system than Wasielewski et al., which included the presence of a membrane to assist in maintaining the initial photo-induced charge separation. Although the complexity of the system did generate a number of technical problems, it probably comes closer to an actual, viable prototype of a workable biomimetic system. 

Light absorption causes formation of an electron/hole (e h ) pair in the interfacial region of the solid and, in the presence of an electric field (e. g. when the solid is held in an electrolyte), the electrons migrate inwards towards the bulk of the solid and the holes move towards the surface and react with the FeOH groups, i.e. the charges separate. The surface reaction is, Fe-OH + hye Fe(OH)s where s = surface and hvB is a hole. A feature of the iron oxides is electron/hole pair recombination - many electrons recombine with the holes and are neutralized - which decreases the photo-activity of the solid. The extent of recombination depends to some extent on the pH of the solution and its effect on the proportion of FeOH groups at the surface (see Chap. 10 and Zhang et al., 1993). 

Since the reduction potential of MV2+/MV is low enough (—0.44 V at pH 7) to reduce protons, the presence of platinum as a catalyst in the solution containing MV 7 brings about hydrogen formation. Scheme 1 is a typical model of photo-induced charge separation and electron relay to yield H2. It also represents the half reaction cycles of the reduction site for the photochemical conversion shown in Fig. 3. 

Fig. 3. Like a photoelectrochemical cell, such a powder includes sites for photo-induced oxidation and reduction, but no external current flow accompanies these transformations. Photoactivity is also maintained as the size of the particle decreases to the colloidal range although the absorption characteristics, the quantum efficiency of charge separation, and the kinetics of interfacial electron transfer may be influenced by the particle size. On sufficiently small particles, for example, the calculated space-charge width necessary for effective band bending may exceed the dimensions of the particle.

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]. 

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