also highly important in biology and me- cine. Most of the applications mentioned are directly related to the properties of the electronic ground state and the lower-lying excited states. Metal complexes with organic ligands or organometallic compounds exhibit outstanding features as compared to purely organic molecules. For instance, metal compounds can often be prepared and applied in different oxidation states. Furthermore, various types of low-lying electronic excitations can be induced by a suitable choice of ligands, for example, such as metal-centered transitions (MC, e. g. d-d tran- tion), ligand-centered (LC, e. g. n-n), metal-to-ligand-charge transfer (MLCT, e. g. d-7r*), intra-ligand-charge- transfer (ILCT) transitions,etc. In particular,the orbitals involved in the resulting lowest excited states determine the photoph- ical and photochemical properties and thus the specific use of the compound.