Reactions of Excited Molecules at Electrodes

In the previous section discussion has centered on electrode reactions stimulated by light absorption in the semiconductor electrode. In the present section, electron transfer processes between electrodes and excited molecules will be treated. In order to avoid any light absorption by the electrode itself, only large band gap semiconductors can be used. 

The first investigations in this field were made with organic crystals (anthracene) as electrodes and electrolytes containing rhodamine B as an 

In this case photocurrents were measured upon 

Before discussing the experimental results, we have to derive an energy scheme, which is actually an extension of the usual model (Section 3) to excited molecules. 

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In the following we shall first discuss the theoretical basis for redox reactions of excited molecules at electrodes in comparison with such reactions in the ground state. We shall then present and discuss some typical examples for the best studied types of reactions in the case of semiconductor and insulator electrodes. Since the properties of these two materials are sufficiently different to need different techniques for the investigation and rather distinct models for the theoretical interpretation, we shall deal with reactions in the case of semiconductors and insulators in separate chapters. 

These reactions have a characteristic free energy which implies the minimal voltage required. As discussed in section 4.1, an excited molecule is at the same time more easily oxidized and reduced than the ground state species. Reactions of excited molecules at electrodes are however practically unknown because their short lifetimes preclude the contact with the electrode when irradiation takes place in the bulk of the liquid. In practice the photoelectro-chemical reactions at non-excited electrodes are simply the thermal reactions of photoproducts. We shall give here two examples of such reactions. 

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