Mechanisms of non-radiative energy transfer involving lanthanide ions revisited

In this work, the theoretical background of the Coulomb direct and exchange interactions in non-radiative energy transfer involving lanthanide ions is critically reviewed. Emphasis is given to the shielding effect produced by the filled 5s and 5p sub-shells of the lanthanide ion and the appropriate use of the so-called forced electric dipole intensity parameters of 4f-4f transitions, two aspects that have been often overlooked in the literature. An effective exchange hamiltonian, based on Mulliken-type approximations, and an expression for the resonant or quasi-resonant energy mismatch spectral overlap factor are proposed. Both cases of ion-to-ion and intramolecular energy transfer processes are discussed. Numerical estimates are compared with previous calculations, showing that order of magnitude differences appear. The analysis indicates that the donor-acceptor distance dependence of transfer rates by the exchange mechanism goes beyond the single exponential behavior adopted from Dexter's approach. (c) 2008 Elsevier B.V. All rights reserved.