COMPLEXES OF LANTHANOID SALTS WITH MACROCYCLIC LIGANDS .42. ENERGY-TRANSFER PROCESSES IN LANTHANIDE DINUCLEAR COMPLEXES WITH P-TERT-BUTYLCALIX[8]ARENE - AN EXAMPLE OF DIPOLE-DIPOLAR MECHANISM

The study of energy-transfer processes between Tb(III) and various Ln(III) ions (Ln = Ho, Nd, Eu) in dinuclear complexes of p-tert-butylcalix[8]arene, [(Tb(x)Ln(2-x)) (LH(2)) (DMF)(5)]-4DMF, is reported. Easily available crystals allow an extended work on these transfers. The compounds contain isolated molecules, so that a simple model accounts for the observed luminescence decays. Only two types of interacting Tb-Ln pairs need to be taken into account, one at 3.7 Angstrom and the other at 11 Angstrom. A rate equation is developed to quantify the energy transfer, and the analyzed data give an interaction strength sequence proportional to the overlap integral, F-Tb-Ho > F-Tb-Nd > F-Tb-Eu. Moreover, the good fits obtained point unambiguously to a dipole-dipolar character of the energy-transfer processes. The low-lying ligand-to-metal charge-transfer state (LMCT) increases the absorption probabilities of Eu(III) f-f transitions considerably, and the resulting quasi-allowed D-5(0)<--(CF0)-F-7 t an important role in the energy-transfer process. This feature permits us to assign a dipolar character to the interaction responsible for the energy transfer. The low-lying LMCT state also induces a strong intramolecular Eu-Eu interaction.