Independent-systems mechanisms for f–f transition probabilities in lanthanide coordination compounds

Abstract

The Laporte-forbidden f–f electronic transitions of lanthanide coordination compounds acquire an electric-dipole probability by two mechanisms in the general independent-systems model, where overlap between the charge distributions of the metal ion and the ligands is neglected. A first-order electric-dipole transition moment arises, either from the mixing of the f–f with f–d and f–g electron promotions under the electrostatic field of the ligands, or from transient dipoles induced in the ligand groups, by an allowed even-multipole electric moment of the f–f excitation. The electrostatic field and the ligand polarization mechanisms make complementary intensity contributions to the f–f transitions of a given Ln(III) complex, dependent upon the rank of the leading electric multipole moment. The polarization mechanism contributes principally to the intensities of the ligand-hypersensitive 2 2-pole f–f transitions, whereas the electrostatic mechanism is predominant for the 2 6-pole transition intensities, and makes the more important contribution in the 2 4-pole cases. Applied initially to Ln(III) complexes containing monoatomic ligands, which have an effective isotropic polarizability, the ligand polarization mechanism is found to depend, on extension to the corresponding polyatomic ligand cases, upon the anisotropy of the ligand polarizability tensor in complexes belonging to the higher non-centric symmetries.