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 

Thionine is a deep purple dye with an absorption band well placed to trap solar radiation leucothionine is colourless and together they constitute a two electron redox couple - 

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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 ... 

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. ... 

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

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

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... 

Since for any system B and Y are energetic species, and furthermore, to be formed, they have to undergo rapid electron transfer reactions one will need a rather special sort of electrode to prevent the recombination. Semi-conductor electrodes (n type Sn02) have been used with limited success for the iron thionine cell. 

The classical example of snch a device is a cell where thionine dye is used. Thionine is the oxidizing agent in the reaction T -r e + f TH. Thionine itself is hard to reduce electrochemically. Therefore, the mediating redox system Fe /Fe is used, which functions as an electron shnttle. The excited form of thionine, T, produced under illumination is readily rednced by divalent iron ions ... 

The thionine-iron(II) photoinduced redox system operates in a similar manner. Rabinowitch (72, 73) noted that, at low concentrations of thionine, rate of photobleaching is proportional to the first power of the light intensity and inversely proportional to the concentration of the iron(III). In the absence of oxygen the reaction appears to be completely reversible but in the presence of oxygen the dye is partially reoxidized by oxygen and the iron(II) is not completely regenerated. When re-... 

E. Rabinowitch, The photogalvanic effect I. The photochemical properties of the thionine-iron system, J. Chem. Phys., 8, 551-559 (1940). 

Following on from Becquerel s aforementioned discovery [3], the PG effect was initially recognized by Rideal and Williams [14] before Rabinowitch [9, 10] subsequently examined the photochemical and photogalvanic properties of a thionine (Th)-iron system. The scheme is described below, while the mechanism and kinetics are described later. 

Rabinowitch discovered this redox equilibrium was the most light-sensitive known -thionine dyes absorb at 500-700 nm, comparable with values of maximal solar radiation - and as a result would possess a high level of PG sensitivity. As explained in the later sections, the thionine-iron (a dye and reductant, respectively) redox cell yielded a low energy conversion efficiency (<1 %), but the results were somewhat promising due to the ability to manipulate the dye and/or reductant - and subsequently established much research into optimization of the necessary components within the system [9, 15]. 

This two-electron thionine couple (A, C) and the one-electron outer sphere iron (reductant) couple (T, Z) manifests itself as an ability to obtain the required electrode selectivity of B and Y on the illuminated and dark electrode, respectively. However, this mechanism also complicates the method of establishing the system s homogeneous kinetics, as explained further on. 

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