Mechanisms for f-f transition probabilities in lanthanide coordination compounds

Abstract

The Laporte-forbidden transitions of Ln(III) coordination compounds acquire a first-order electricdipole probability from transient dipoles induced in the ligand groups by an allowed even-multipole electric moment of the f-f excitation, and by the mixing of the f-f with f-d and f-g electron promotions under the electrostatic field of the ligands. 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. The electrostatic field and the ligand polarization mechanisms make complementary intensity contributions to the f-f transitions of a given Ln(III) complex type, 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.