Photoreduction water

The philosophy of membrane-mimetic chemistry may be illustrated by a comparison of plant photosynthesis with sacrificial water photoreduction in artificial systems the process has been mediated by metal-catalyst-coated semiconductor colloids supported on polymerized vesicles (Fig. 4a) [59-64]. 

AOT-isooctane-H20 reversed micelles 50-A-diameter CdS particles generated in situ in reversed micelles from CdCl2 or Cd(N03)2 by H2S Reversed-micelle-entrapped CdS was fluorescence quenched by methylviologen band-gap excitation in the presence of Rh as catalyst and PhSH as sacrificial electron donor resulted in water photoreduction 611... 

Fig. 101. Idealized model for the CdS-sensitized water photoreduction by PhSH in AOT reversed micelles in isooctane. VB = valence band, CB = conduction band [611]...

Visible-light irradiation of DHP-vesicle-incorporated, 626, 627 Rh-coated CdS in the presence of PhSH as a sacrificial electron donor resulted in water photoreduction... 

Rh-coated CdS resulted in sacrificial water photoreduction a thiol-functionalized surfactant acted as the sacrificial electron donor, system could be recycled... 

Almost all photoreactions of porphyrins and phthalocyanines occur via the triplet state whose lifetime (ca. 1 ms at T = 300K) and yields of formation are greatest for porphyrins. The redox potentials of porphyrins are also more amenable to water photoreduction via oxidative or reductive cycles and some selected values are found in Table 11. 

This system was further extended to the photofixation of carbon dioxide [151], Small ZnS nanoclusters with low density of defects were found to be effective in catalyzing photoreduction of C02 in water at pH 7 with NaH2P02 in the coexistence of Na2S. A large quantity of HC02 accompanied by a small quantity of CO is formed, with a simultaneous evolution of H2 as a result of water photoreduction. Total quantum yield for the formation of HO and H2 is as high as 0.84 in the presence of 0.7 MH2PO and 0.24 M of SH [151]. 

Positively charged vesicles prepared from dioctadecyldimethylammonium chloride (DODAC) [Ci5H3iC02(CH2)2N+(CH3)(CH2C6H4CH = CH2Cr [Fig. 5(b)], as well as from its polymerized counterpart, were found to be better media for CdS-mediated water photoreduction than those prepared from DHP [Fig. 5(c)] [11,12]. Unfortunately, the quantum efficiency of the hydrogen formation was lower than 0.5% and the accumulated oxidized electron donor diphenyl disulfide PhSSPh led rapidly to the ultimate destruction of the vesicles. 

Caged Amine complexes of Co(III) The reduced form of Co(III) complexes such as Co(III)(NH3)5X2+ are substitutionally labile. A number of caged cobalt(II) compounds have been found to be fairly inert to substitution. The sar, sep or the sulfur-containing capten cage complexes have [Co(III)/Co(II)] potentials > -0.2V vs NHE are and efficient quenchers of Ru(bpy)3 [41-43]. Some of them have been found use as relays for water photoreduction. 

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). 

FIG. 11 General mechanism for the heterogeneous photoreduction of a species Q located in the organic phase by the water-soluble sensitizer S. The electron-transfer step is in competition with the decay of the excited state, while a second competition involved the separation of the geminate ion-pair and back electron transfer. The latter process can be further affected by the presence of a redox couple able to regenerate the initial ground of the dye. This process is commonly referred to as supersensitization. (Reprinted with permission from Ref. 166. Copyright 1999 American Chemical Society.)... 

Within the potential range where Ru(bpy)3 remains in the aqueous phase, photocurrent responses are clearly observed with a slow rising time of the order of 10 s as shown in Fig. 14(a). According to the convention employed by these authors, positive currents correspond to the transfer of a negative charge from water to DCE. No photoresponses were observed in the absence of either the dye in the aqueous phase or TCNQ in DCE. Further analysis of the interfacial behavior of the product TCNQ revealed that the ion transfer occurred outside of the polarizable window [cf. Fig. 14(d)], confirming that these photoresponses are not affected by coupled ion-transfer processes. An earlier report also showed photoeffects for the photoreduction of the viologen under similar conditions [131]. 

Maldotti (96) studied the kinetics of the formation of the pyrazine-bridged Fe(II) porphyrin shish-kebab polymer by means of flash kinetic experiments. Upon irradiation of a deaerated alkaline water/ethanol solution of Fe(III) protoporphyrin IX and pyrazine with a short intense flash of light, the 2 1 Fe(II) porphyrin (pyrazine)2 complex is formed, but it immediately polymerizes with second-order kinetics. This can be monitored in the UV-Vis absorption spectrum, with the disappearance of a band at 550 nm together with the emergence of a new band due to the polymer at 800 nm. The process is accelerated by the addition of LiCl, which augments hydrophobic interactions, and is diminished by the presence of a surfactant. A shish-kebab polymer is also formed upon photoreduction of Fe(III) porphyrins in presence of piperazine or 4,4 -bipyridine ligands (97). 

Complex 7 is stable in water, does not react with 5 -GMP in the dark, and only 5% of Pt(IV) was reduced by GSH after 21 days. Irradiation of the complex alone in water yielded little reduction to Pt(II) and photosubstitution of one or both of the azides was observed. In the presence of 5 -GMP, however, rapid photoreduction resulted in the formation of the mono-adduct (SP-4-4)-[Ptn (5 -GMP-Ar7)(N3)(NH3)(py)]+ (7rG) and bis-adduct trans-... 

A quick survey of the photochemistry of the different complexes described above shows that the mechanism of photoactivation and the subsequent nature of the observed photoproducts varies from complex to complex and from one geometric isomer to another. Photochemical pathways often involve a combination of photosubstitution, photoisomerization, and photoreduction steps. In general, photolysis is rather slow in water and many different products are obtained if the complex is irradiated alone. The presence of nucleophilic biomolecules, on the other hand, can have a major influence, as photoreduction is usually rapid and accompanied by simpler reaction pathways. NMR methods... 

Figure 1.17 (a) ESR spectrum from a vanadium silicate catalyst after photoreduction with H2 at 77 K [138], The ESR data obtained indicate the existence ofV4+ ions in tetrahedral coordination, (b) Addition of a small amount of water leads to a new ESR trace identified with distorted octahedral V02+ ions, indicating the easy accessibility of the vanadium surface species. (Reproduced with permission from Elsevier.)... 

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