Thionine, photoreduction

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. 

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. 

C. A. Parker, Photoreduction of methylene blue and thionine by ferrous sulfate, Nature, 182, 245-246 (1958). 

Various reports of the photoreduction of organic dyes have appeared. The photoreduction of methylene blue in the presence of several electron donors,93 thionine in the presence of allylthiourea,94 fluorescein,95 Rhodamine B, and other xanthene dyes have all been investigated by spectrophotometric or e.s.r. techniques. 

Hatchard CG, Parker CA (1961) The photoreduction of thionine by ferrous sulphate. Trans Faraday Soc 57 1093... 

Kamogawa H, Katsuta S, Masato N (1982) Organic solid photochromism by photoreduction mechanism thionine-reductant system. J Appl Polym Sci 27 1621-1628... 

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