Nonradiative Deactivation of Lanthanoid Luminescence by Multiphonon Relaxation in Molecular Complexes

Nonradiative deactivation of metal-centered excited state by multiphonon relaxation (MR) is one of the most important processes in the photophysics of trivalent lanthanoid ions. In molecular complexes, the dominant drivers for this phenomenon are strongly anharmonic, high-energy local-mode vibrations such as O–H, N–H, or C–H stretching modes in surrounding ligands and solvent molecules. This chapter discusses MR of lanthanoid luminescence with a special focus on the unique situation for molecular luminophores, in particular highlighting the differences with solid-state lanthanoid materials. Various aspects relevant to this topic are covered, from the properties of vibrational local-mode overtones, the comparison of different models for the quantitative description of nonradiative deactivation, to MR isotope effects.