Iron thionine

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. 

Figure 9.11 Primitive iron-thionine photogalvanic cell.

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 ... 

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

Disproportionation equilibria have been studied for various systems. Cauquis and co-workers investigated by electrochemical means the matrix of equilibria corresponding to Scheme 2 for 3,7-dimethoxypheno-thiazine and its derivatives, and applied the measurement of the response of the equilibria to different conditions of basicity to the definition of a scale of basicity in acetonitrile. The disproportionation kinetics of the iron-thionine system were measured several years ago solvent effects on the disproportionation rate constant have been examined, and, lately, an indirect measurement of the synproportionation rate constant of thionine and leucothionine has been made. ... 

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 ... 

In spite of the comprehensive theoretical treatments of the iron-thionine cell, progress towards the development of practical cells has slowed noticeably in the last year. Some aspects of the theoretical treatment have been discussed by Brokken-Zijp et alP who have reported that the decay of leucothionine after irradiation is not pseudo-first order, so that it may be necessary to include the synproportionation reaction (1) as an alternative route for the disappearance of... 

The now classical, but in practical terms disappointing, iron-thionine system still has its devotees, including Brokken-Zijp, Dung, and Mesmaeker (yes, really ). 

Table 2 Iron-thionine thin-layer cell performance characteristics 27...

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). 

The fifty-fold to one hundred-fold factor by which the best observed values of S.E.E. for the iron-thionine TI-TL cell with a Sn02 anode and a Pt or ITO cathode fall short of the theoretical maximum can be ascribed, in part, to two relatively simple factors. One of these is ineSicient absorption of light. At the photostationary state, the concentration of unbleached dye in optimized cells was 5 X so that only 25% of the inci-... 

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

Hatchard and Parker [24] further investigated the kinetics of the initial iron-thionine system discovered by Rabinowitch and derived the following mechanism for the collection and conversion of... 

As explained in Table 3, for an ideal cell the absorbance length is approximately 10 " cm, which corresponds to dominance of the k 2 parameter. However, thionine dye has a low solubility - meaning the absorbance length can be as long as 10 cm - and thus the k i/k3 parameter becomes equally important. This is a considerable factor in the relative inefficiencies of iron-thionine PG cells - a high absorbance length means B struggles to disproportionate and C fails to reach the illuminated electrode [12]. 

Various modifications have been attempted in order to increase the solar energy conversion efficiency of the iron-thionine PG cell, including adding sulfonate groups onto the thionine dye in order to increase solubihty [13] and irreversibly coating a thionine electrode with up to twenty thionine monolayers to increase absorbance of solar radiation and thus decrease E [26], However, research has broadened since early innovations by Rabinowitch, Albeiy, Archer, and Foulds, and the common approach is now motivated towards improvisations of the dye and reductant to induce wholesale changes in the efficiencies of PG cells. 

Hall DE, Eckert JA, Lichtin NN, Wildes PD (1976) Multilayer iron-thionine photogalvanic cell. J Electrochem Soc 123 1705... 

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... 

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