Viologen, photoreduction

If the PS-MV layer is deposited from a zero-ionic-strength solution, that is, in a more extended conformation, the quantum yield for viologen photoreduction decreases. Shown in Figure 9 are transient diffuse reflectance decays recorded at 400 and 510 nm for l/2(coiled)/Cab-0-Sil and l/2(uncoiled)/... 

Note that the maximum value 3.3% of the registered quantum yield of the lipophilic viologen photoreduction is evidently not sufficient for the design of an efficient electron-transfer chain of a photocatalytic system. So, there is a necessity for a further improvement of systems based on lipid vesicles sensitized with semiconductor nanoparticles. Possible approaches to this improvement seem to be, e.g., the creation of semiconductor... 

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). 

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,... 

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]. 

The photoreduction of polymer pendant viologen by 2-propanol was reported to proceed by the successive two-electron transfer processes between the adjacent viologen units and the propanol which is a two-electron reducing agent44). Preferential formation of a dimeric cation radical of viologen observed was ascribed to the polymeric structure and the two-electron process. These fundamental studies on polymeric electron mediators contribute to the construction of solar energy conversion systems. 

In cationic micelles, the anionic phthalocyanine, Na4ZnL] (H4L = 167a), in the presence of cysteine, has been shown to sensitize the photoreduction of the l,l -dimethyI-4,4 -bipyridimum dication (methyl viologen, Paraquat) although, in the presence of a surfactant, [Zn(Pc)] was more effective.1167... 

A classic use of photodriven electron transfer is encountered in the photoreduction of pyridinium cations, particularly methyl viologen 15,... 

Movement into a micelle can also be observed in some cases. The photoreduction of an amphiphilic viologen (for example, methyl viologen modified by attachment of a long chain, lipophilic C- 2 fragment), by Ru2+ occurs mainly in the aqueous phase surrounding a micelle, but the hydrophilic reduced product is reasily solubilized into a cationic micelle, eq. 87 ... 

Fig. 14. Schematic representation of light-driven (2e + 2H+) symport across a membrane via the quinone carrier molecule vitamin Kj and its hydroquinone form proflavine (PF)-sen-sitized photoreduction of methyl-viologen MV2+ in the RED phase, yields the reducing species MV+, with simultaneous oxidative decomposition of EDTA used as electron donor the OX phase contains ferricyanide as electron acceptor [6.49].

Porphyrins are excellent sensitizers for the photochemical reduction of vio-logen in the presence of sacrificial electron donors. ZnTPPC intercalated into the interlayer spaces of LDH also acts as a sensitizer toward methyl viologen (MV2+) and PVS-EDTA promotes photoreduction of viologen. However, the rate of radical generation for the neutral viologen is higher than that for MV2+ (Fig. 54) [101d],... 

Photochromic compounds functioning by an oxidation-reduction mechanism (electron transfer), especially a photoreduction mechanism, are known in inorganic materials such as silver halides, which are utilized in eyewear lenses. Although the number of organic photochromic compounds operating via electron transfer is fewer than those by isomerization, heterolytic (or homolytic) cleavage, and pericyclic reactions, several classes of compounds have been reported, such as thiazines,1 viologens,2 and polycyclic quinones.3... 

This chapter relates to the photochromism of viologens and thionine dyes proceeding by electron transfer. The colored species spontaneously disappear in a liquid phase, because the rate of back electron transfer (dotted line in Figure 9.1) is rapid and is accelerated by the existence of oxygen in the system. If the compound is dispersed in a solid-state or layered system, the reversible reaction can be controlled. Since the color development via electron transfer is generally very rapid and the reverse electron transfer can also be carried out electrochemically,4 the combination of both photoreduction and electrooxidation will receive much attention for application in recording devices. 

This section relates to the reversible color change of viologens via a photoreduction mechanism principally in the solid state. The effect of the kind of viologens (bipyridinium rings and counteranions) and polymers on photochromism is discussed. The viologens and their abbreviation under discussion in this section are given in Scheme 3. 

The presence of water in PVP decreases the stability of the radical cation to favor the thermal back electron transfer, because the radical cation has relatively low solubility (affinity) in water. Figure 9.4 shows the extreme energy curves, both in the absence of water and in the presence of a substantial amount of water in PVP matrices. Actual photoreduction of viologen slightly hydrated with water might proceed between both lines in a PVP matrix. 

Since the photoreduction of viologen is induced by electron abstraction (oxidation) by the excited bipyridinium cation from the counteranion, it is difficult to relate the photoelectron transfer to the dark redox potential. However, the photosensitivity for the same skeleton is almost proportional to the first dark reduction potential, as mentioned in the previous section. The degree of solvation... 

Table 9.7. Effect of Chain Length of R on Photoreduction of Viologen Layered Films...

Since the developed color of viologens induced by photoreduction is vivid, and a variety of color species can be obtained by changing their chemical structure, it can be expected that there are several kinds of potential applications using photochromic viologens, especially an erasable-direct-read-after-write (EDRAW) media for recording devices with high storage density and reliability. 

Because the copper complexes with 2,2/-bipyridine, 1,10-phenanthroline, and their derivatives have a triplet MLCT excited state like that of [Ru(bpy)3]2+, the copper complexes exhibit similar photocatalyses to those of [Ru(bpy)3]2 + for instance, the trans-cis isomerization of stylbene through energy transfer [48] and the photoreduction of viologen compounds [9b,c,e,49,50] were successfully carried out with the copper complexes. Also, a Gratzel-type solar cell was constructed with the copper complexes, recently [51,52]. 

Selective production of enzymatically active NADH by visible light and without formation of dimers has been achieved by the photoreduction of NAD using [Ru(bipy>3]S04 and [Ru(bipy)3]2(S04)3 as sensitizers, and triethanolamine as electron donor.The synthesis, photophysical properties, and excited-state redox behaviour of [(CN)(bipy)2Ru(CN)Pt(dien)X 104 and [(dien)PtNC Ru(bipy)2 CNPt(dien)]C104 (dien = diethylenetriamine) have been reported, as have the light-induced electron-transfer reactions between [Ru(bipy) (thpy)3 f [where thpy = 2-(thiazol-2 -yl)pyridine, n=2,1, or 0] and methyl viologen. ... 

The photophysics and photochemistry of gaseous PuFe have been ex-amined. Studies involving zinc porphyrins have been reported and include photo-oxidations in aqueous solution, photoreductions of Zn-TPP with hydrazines, and the role of Zn-TPPSa/ethyl viologen in photoredox processes. The mechanism of the photo-oxidation of water to oxygen with silver chloride has been discussed, and the synthesis of bis(chlorosilyl)-mercury compounds described. Colloidal CdSe has been shown to sensitize the photoreduction of O2 and of methyl viologen by cysteine. ... 

Numerous papers dealing with the photoreduction of viologens have appeared. Rate constants for electron transfer between aromatic hydrocarbon triplets and bipyridinium and pyridinium salts have been measured. The results suggest that the process is both facile and efficient. All the rate constants necessary for the determination of the quantum yield of formation of the viologen cation-radical in the methyl viologen-Eosin-EDTA system are also now available. " The high activity of dihydroxytin(rv) uroporphyrin in the photoreduction of methyl viologen has been ascribed to the failure of a photochemi-cally inactive tt—tt complex to be formed between substrate and sensitizer. 

Photoreduction of viologens in mixed aqueous/organic solvents has been reported but the results compare badly with those obtained in... 

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