The role of covalency and bridging ligands in two‐ion cooperative optical transitions in lanthanide ion‐coupled systems

Abstract

AbstractThe influence of covalency and bridging ligands on cooperative optical effects in dimers with closely coupled lanthanide ions in insulating crystals and binuclear complexes is theoretically treated. It is shown that there is a specific interaction between Ln3+ ions (which may be called covalent coupling) in tight ion pairs originating from virtual electron hoppings between lanthanide ions via common bridging ligands. This interaction can be regarded as a generalization of the superexchange interaction mechanism. A general microscopic model and the corresponding formalism for the covalent coupling mechanism are developed. A general expression to estimate the intensity of electric dipole two‐ion cooperative optical transitions is derived. Both the nature of bridging ligands and the local geometry of the Ln3+ (A)–ligandLn3+ (B) dimer are found to affect strongly the intensities of cooperative transitions. Our model predicts rapid increase in the contribution of this mechanism to the intensities of two‐ion transitions when going from small “ionic” ligands (F− and O2−) to large “covalent” ligands (Br−, I−). Possible manifestations of the covalent coupling mechanism in fast energy transfer processes and unusually strong up‐conversion fluorescence observed in CsCdBr3 : Ln3+ systems and as well as in electronic spectra of the U2Cl10 binuclear compound are discussed.