Photoreduction, water-promoted

As can be seen from Table 9.12, photochromism is also strongly affected by humidity. It is well known that thiazine and its derivatives undergo photoreduction smoothly in the presence of such activated surfaces as silica gel and alumina with water molecules,37 and that methylene blue is also photochemically reduced in acid solution, even with only available water as the reducing agent38 Therefore the water present in polymer films must produce a thionine-water hydration system, which accelerates the rate of the photoreduction of thionine, as well as promoting the contact of reductants by a plasticizer effect. PVA, used as the matrix, can also play the role of reductant, but its extent may be minor as compared with the added reductant. The effect of water is supported by the fact that a less hydrophilic polymer matrix such as poly(methyl methacrylate) does not exhibit photochromism even though the system contains an appreciable amount of reductants. 

Activators increase the quantum yield for the photoreduction of ribo-flavine, and appear to facilitate the cleavage of the 0—H bond of water. Their role may be, first, to provide sites for hydrogen bonding within a complex consisting of riboflavine, water, and activator, and, second, to promote reaction by accepting the oxygen moiety from the water. 

A short report from Kometani etal. [226] describes the same synthetic method, using the system water/sc CO2 and adding a perfluorinated surfactant (PFPE) and AgC104 in it to obtain the microemulsion. The conditions of supercritical CO2 were rather mild (see Section 1.3). 2-propanol was added for promoting photoreduction (UV light, X = 254 nm) of Ag. The size of the silver particles was 10-15 nm, though the microemulsion core radius was only 2 nm. 

Ag clusters CO2 AgCI04, 2-propanol was added to promote the photoreaction Photoreduction, Irradiation of water-in-scC02 PFPE-1, PFPE-2, PFPE-3 ... 

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