Photoreduction system

Figure 23-17 The zigzag scheme (Z scheme) for a two-quantum per electron photoreduction system of chloroplasts. Abbreviations are P680 and P700, reaction center chlorophylls Ph, pheophytin acceptor of electrons from PSII QA, Qg, quinones bound to reaction center proteins PQ, plastoquinone (mobile pool) Cyt, cytochromes PC, plastocyanin A0 and Aj, early electron acceptors for PSI, possibly chlorophyll and quinone, respectively Fx, Fe2S2 center bound to reaction center proteins FA, FB, Fe4S4 centers Fd, soluble ferredoxin and DCMU, dichlorophenyldimethylurea. Note that the positions of P682, P700, Ph, Qa/ Qb/ Ay and A, on the E° scale are uncertain. The E° values for P682 and P700 should be for the (chlorophyll / chlorophyll cation radical) pair in the reaction center environment. These may be lower than are shown.

Since the copper complexes, [Cu(NN)2]+ and [Cu(NN)(PR3)2]+ (NN = 1,10-phenanthroline, 2,2 -bipyridine, and their derivatives) were applied to stoichiometric and catalytic photoreduction of cobalt(III) complexes [8a,b,e,9a,d], one can expect to perform the asymmetric photoreduction system with the similar copper(l) complexes if the optically active center is introduced into the copper(I) complex. To construct such an asymmetric photoreaction system, we need chiral copper(I) complex. Copper complex, however, takes a four-coordinate structure. This means that the molecular asymmetry around the metal center cannot exist in the copper complex, unlike in six-coordinate octahedral ruthenium(II) complexes. Thus we need to synthesize some chiral ligand in the copper complexes. 

Although Ru(bipy)2+ alone will not split water into hydrogen and oxygen, it has been accomplished with Ru(bipy)2+ using various catalysts or radical carriers. Perhaps the most studied system for the photoreduction of water involves using methyl viologen as the quencher, EDTA as an electron donor (decomposed in the reaction) and colloidal platinum as a redox catalyst (Figure 1.19). 

Iron, tris(hexafluoroacetylacetone)-structure, 1,65 Iron, tris(oxalato)-chemical actinometer, 1,409 photoreduction, 1,471 relief-image-forming systems, 6,125 Iron, tris(l,10-phenanthroline)-absorptiometry, 1,549 racemization, 1,466 solid state, 1,467 structure, 1, 64 lron(III) chloride amino acid formation prebiotic systems, 6,871 Iron complexes acetonitrile. 4,1210 acetylacetone, 2,371 amidines... 

ZnO (suspension) sensitizes the photoreduction of Ag" by xanthene dyes such as uranin and rhodamine B. In this reaction, ZnO plays the role of a medium to facilitate the efficient electron transfer from excited dye molecules to Ag" adsortei on the surface. The electron is transferred into the conduction band of ZnO and from there it reacts with Ag. In homogeneous solution, the transfer of an electron from the excited dye has little driving force as the potential of the Ag /Ag system is —1.8 V (Sect. 2.3). It seems that sufficient binding energy of the silver atom formed is available in the reduction of adsorbed Ag" ions, i.e. the redox potential of the silver couple is more positive under these circumstances. 

Photoinduced ET at liquid-liquid interfaces has been widely recognized as a model system for natural photosynthesis and heterogeneous photocatalysis [114-119]. One of the key aspects of photochemical reactions in these systems is that the efficiency of product separation can be enhanced by differences in solvation energy, diminishing the probability of a back electron-transfer process (see Fig. 11). For instance, Brugger and Gratzel reported that the efficiency of the photoreduction of the amphiphilic methyl viologen by Ru(bpy)3+ is effectively enhanced in the presence of cationic micelles formed by cetyltrimethylammonium chloride [120]. Flash photolysis studies indicated that while the kinetics of the photoinduced reaction,... 

Since transfer of a second hydrogen atom from the ether radical is unreasonable, a pathway available to the acetone ketyl radical in the photoreduction in isopropanol is removed in this system and reverse transfer can occur ... 

Product distributions and reaction conversions of several different photochemical systems, irradiated by conventional UV source and by EDL in a MW-UV reactor (Fig. 14.5), were compared to elucidate the advantages and disadvantages of a micro-wave photochemical reactor [90], Some reactions, e.g. photolysis of phenacyl benzoate in the presence of triethylamine or photoreduction of acetophenone by 2-propa-nol, were moderately enhanced by MW heating. The efficiency of chlorobenzene photosubstitution in methanol, on the other hand, increased dramatically with increasing reaction temperature. 

Another possible route for reduction of the iron center is photoreduction. This has been studied in a variety of marine siderophore systems, such as aquachelin, marinobactin, and aerobactin (2), where it was demonstrated that photolytic reduction was due to a ligand-to-metal charge transfer band of the Fe(III)-siderophore complex, eventually resulting in reduction ofiron(III) and cleavage of the siderophore (31,154,155). This suggests a possible role for iron reduction in iron release (71,155). 

Table 12.3 Compounds that have been used as components in successful systems for the photoreduction of water...

The photochemistry of phthalimide systems was thoroughly investigated by many groups over the last two decades. This chromophore shows a broad spectrum of reactivity leading mainly to cycloaddition and photoreduction products by either intermolecular or intramolecular processes. In the presence of electron donors, the electronically excited phthalimide could also undergo electron transfer and act as an electron acceptor. 

Photoreduction of cobalt(III) complexes can occur under a variety of conditions. Irradition of the charge transfer bands of these systems results only in decomposition with production of cobaltous ion and oxidation of one of the ligands. In some instances photoreduction can be initiated by irradiation of the ligand field transitions. Irradiation of ion pairs formed by these complexes with iodide ion with ultraviolet light also leads to reduction of the complexes. Finally, irradiation of iodide ion in the presence of the complexes leads to reduction. 

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