The Iron Thionine Photogalvanic Cell

THE IRON THIONINE PHOTOGALVANIC CELL Using the typical values of , = 0.5... 

Figure 9. The compounds thionine (Th), semithionine (S), and leukothionine (L) proposed to exist in iron-thionine photogalvanic cells.

A regenerative photogalvanic cell with oxidative quenching (Fig. 5.58b) is based, for example, on the Fe3+-Ru(bpy)2+ system. In contrast to the iron-thionine cell, the homogeneous photoredox process takes place near the (optically transparent) cathode. The photoexcited Ru(bpy)2+ ion reduces Fe3+ and the formed Ru(bpy)3+ and Fe2+ are converted at the opposite electrodes to the initial state. 

The first attempts to understand the mechanism of operation of photogalvanic cells was performed on the iron-thionine system [22, 23], Here we summarize the main observations. Thionine (Th) is a cationic purple dye extremely soluble as the chloride salt. Two other forms of the compound, Leukothionine (L) and Semithionine (S ), are also important in the sensitization process and their structures are shown in Figure 9. According to Albery, the light driven redox reactions that lead to a photocurrent are as follows ... 

Earlier light-to-electricity conversion cells were often based on the photogalvanic effect, whereby light was absorbed in a dye in solution or adsorbed on an elecnode surface. The iron-thionine cell of Murphy (1978) provides an example of a photogalvanic device with storage. The forward reaction is ... 

Iron-thiazine photogalvanic cells use the photoredox reactions of Fe(II) with thiazine dyes, represented for thionine by Reactions 1, 2, 3, 4, and 5, to convert photon energy into chemical potential. The spontaneous ground state reactions represented by Reactions 6, 7, 8, and 9 also occur in homogeneous solution during illumination. Photogalvanic action results when homogeneous Reactions 7, 8, and 9 are replaced by anodic oxidation of and TH2 coupled with cathodic reduction of... 

Photogalvanic Cells, Principles and Perspectives, Fig. 5 The three structures of the thionine dye within the iron-thionine PG system... 

Fig. 1. A photogalvanic cell and a schematic representation of the iron thionine system.

In lecture 1 we discussed the ideal theoretical A, B, Y, Z system. In this lecture we examine/ in more detail/ the most successful candidate for a photogalvanic cell/ the iron thionine system. Thionine/ Th/ Semi-thionine. S , and Leucothionine/ h, are... 

Photogalvanic transduction in the TI-TL iron-thionine cell or similar cells using other thiazines (4) can be analyzed in terms of five basic processes (5) (I) absorption of incident light (2) conversion of absorbed radiant energy into chemical potential of charge carriers (3)... 

Sensitization of photogalvanic action by dyes which are themselves not capable of photoredox action has been demonstrated (7). Action spectra of solutions containing rhodamine 6G and two coumarin dyes in addition to thionine and methylene blue closely parallel absorption spectra, corresponding to the possible use of about 50% of the insolation spectrum and a theoretical maximum sunlight engineering efBciency of 7 %. It has been demonstrated that rhodamine 6G does, in fact, increase the power output of iron-thionine (or other thiazine) cells under white-light illumination an approximately 40% increase has been observed under illumination with 35 mWcm" (8). 

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