Lanthanide ions charge transfer

In view of the magnitude of crystal-field effects it is not surprising that the spectra of actinide ions are sensitive to the latter s environment and, in contrast to the lanthanides, may change drastically from one compound to another. Unfortunately, because of the complexity of the spectra and the low symmetry of many of the complexes, spectra are not easily used as a means of deducing stereochemistry except when used as fingerprints for comparison with spectra of previously characterized compounds. However, the dependence on ligand concentration of the positions and intensities, especially of the charge-transfer bands, can profitably be used to estimate stability constants. 

The detection of aromatic carboxylates via the formation of ternary complexes using lanthanide ion complexes of functionalised diaza-crown ethers 30 and 31 has been demonstrated [134]. Like the previous examples, these complexes contained vacant coordination sites but the use of carboxylic acid arms resulted in overall cationic 2+ or 1+ complexes. Furthermore, the formation of luminescent ternary complexes was possible with both Tb(III) and Eu(III). A number of antennae were tested including picolinate, phthalate benzoate and dibenzoylmethide. The formations of these ternary complexes were studied by both luminescence and mass spectroscopy. In the case of Eu-30 and Tb-30, the 1 1 ternary complexes were identified. When the Tb(III) and Eu(III) complexes of 30 were titrated with picolinic acid, luminescent enhancements of 250- and 170-fold, respectively, were recorded. The higher values obtained for Tb(III) was explained because there was a better match between the triplet energy of the antenna and a charge transfer deactivation pathway compared to the Eu(III) complex. 

Very few calculations have so far been performed for lanthanides and not much is known about the choice of the active space. However, most lanthanide complexes have the metal in oxidation state 3+. Furthermore, are the 4/ orbitals inert and do not interact strongly with the ligands. It is therefore likely that in such complexes only the 4/ orbitals have to be active unless the process studied includes charge transfer from the ligands to the metal. In systems with the metal in a lower oxidation state, the choice of the active space would show similar problems as in the actinides, in particular because the 5d orbitals may also take part in the bonding. As an example we might mention a recent study of the SmO molecule and positive ion where 13 active orbitals where shown to produce results of good accuracy [42],... 

The influence of the ligand field on the electronic states of lanthanides is small and generally of the order of 200 cm-1. Because the ligand field perturbation of J states are minimal, the f-f electronic transitions are sharp. In addition to f-f transitions, both 4f —> 5d and charge transfer transitions are also observed in the spectra of lanthanides [92]. Lanthanide ions exhibit emission in the solid state, and in some cases in aqueous solutions. Energy transfer from the ligand or intermolecularly from an excited state can give rise to the emission from lanthanide ions. 

Multicolored luminescence is the most attractive property of rare earth-based compounds. Lanthanide ions possess many sharp emission lines that cover the visible and near infrared (NIR) region due fo fhe abundanf fransifions of f-orbital configurations. However, the forbidden f-f fransi-fions induce narrow excitation lines for mosf rare earfh ions. This low absorbency cross-section is the bottleneck in practical application, so host-sensitized emission mode is commonly employed by rare earth phosphors. The vanadate matrix is one of fhe candidafes, which excifes lanthanide ions via charge-transfer energy migration. 

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