Electron Transfer and Energy Transfer in Heterogeneous Photocatalysis

When an electronically excited species A* interacts with a species B in its ground state, both an energy transfer and an electron transfer may occur due to an initial orbital overlap or to dipole-dipole interactions. These fundamental reactions are closely related both conceptually and mechanistically. However, despite the qualitative similarities at the initial stage of interaction, the two processes become quantitatively different along with the reaction coordinate, and different factors may address the occurrence of one or the other of them. Nevertheless, distinguishing between energy and electron transfer is often a hard task in heterogeneous photocatalytic complex reactions. In fact, the two mechanisms can afford the same products, and consecutive reactions or adsorptive phenomena makes it difficult to differentiate them. Heterogeneous photocatalysis is generally considered based on electron transfer reactions. However, rare examples such as singlet oxygen generation or some isomerization reactions are reported in literature to be energy transfer induced processes. This work will report some of these cases, and in particular will focus on the importance of the energy transfer mechanism in photocatalysis.