7. Charge Transfer States

Up to now, the electronic transitions that have been discussed are intra-atomic, i.e. the involved electrons reorganize in orbitals that can be associated with the luminescence activator itself. However, some important processes in luminescent materials involve the interatomic transfer of an electron. From the computational point of view, this is a nontrivial complication, involving enlarged active spaces, lower symmetries and a significantly larger number of excited states. In this chapter, the basics of several types of charge transfer (CT) states and the corresponding transitions are discussed. Special attention is paid to the construction of diabatic electron transfer configurational coordinate diagrams from single-center ab initio calculations or empirical data. The diabatic approximation circumvents computational demands that can be enormous and eventually make the calculations unfeasible and, although it comes together with some quantitative and qualitative limitations, it provides much significant information on luminescence related processes. The application of the diabatic approximation recipes to luminescence is discussed in Chap. 11.