ABSTRACT

We shall now turn our attention to semiconductor systems and discuss the salient features of electron-transfer processes in this type of heterogeneous medium. Semiconductors are a very important class of solids and will be in the focus of our considerations. With regard to light-induced charge separation, they have several advantages over the molecular assemblies dealt with in the previous chapters. For example, the diffusion of mobile charge carriers in semiconductors is very fast. Even for a material such as TiO,, which is characterized by a heavy effective electron mass, the diffusion constant of the electron is at least 104 times larger than that of a molecular charge carrier in a micelle or vesicle. This presents an important advantage for heterogeneous photoreactions, where the achievement of high efficiency requires rapid displacement of photogenerated species from the interior of the light harvesting unit to the surface. Furthermore, since the chemical transformations usually involve redox reactions at the surface, they can be mediated by deriving the semiconductor with suitable functional groups or by deposition of catalysts. Before addressing these issues in more detail, we shall first discuss the basic properties of semiconductor particles and give an overview of the experimental techniques that have been developed to investigate lightinduced redox reactions in this type of assembly.