Methyl viologen cation photoreduction

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,... 

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 ... 

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. 

To demonstrate the ability of this system to achieve sensitized photocatalysis, the authors measured the electron transfer from the zinc porphyrin to methyl viologen. Adding p-mercaptoethanol as a sacrifidal reductant enabled regeneration of the resulting porphyrin radical cations, thus dosing the catalytic cycle and allowing for continued photoreduction. When capsids that were modified with donor dyes inside and porphyrins outside were illuminated at the peak donor exdtation (where the porphyrin has a very low extinction coeffident), an almost fourfold increase in the photocatalytic efficiency of the porphyrin was observed. 

In vivo electron transport assays demonstrate that the site of Ca /Na action is Photosystem II. Ascorbate/DAD to methyl viologen rates (measured as O2 uptake) in the presence of DCMU are not affected by depletion of Ca " and Na" " while water to benzoquinone rates (measured as 2 evolution) in the presence of DBMIB show a decline concommitent with the loss of complete photosynthetic electron transport. Jji vitro examination of partial reactions within PS II reveals that the site of the cation effect excludes the water-splitting enzyme and occurs prior to the site of DCMU inhibition diphenylcarbazide-mediated silicomolybdate photoreduction after heat inactivation of the water-splitting enzyme shows diminished rates in membranes prepared from cells partially depleted of calcium and sodium. 

---