Internal-Energy-Transfer Efficiencies in Eu3+ and Tb3+ Chelates Using Excitation to Selected Ion Levels

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Abstract
Fluorescence yields of solutions of several chelates of europium and terbium have been measured: (a) upon excitation in the ligand absorption bands and (b) upon selective excitation to individual upper levels of the rare‐earth ion. Yields with upper‐ion‐level excitation are lower than when the emitting level is excited directly and yields with ligand excitation are lower still. Efficiencies of energy transfer to the emitting level from upper ion levels and from the ligands, for the solutions studied, are calculated from the data. The fluorescence yield observed when the emitting level is excited directly, together with the fluorescence lifetime, allows both radiative and radiationless rate constants for deactivation of the emitting level to be calculated. The enhanced fluorescence yields of chelates compared to unchelated rare‐earth ions are found to be due primarily to the enhancement of the radiative transition by the chelate environment rather than to a protective influence against quenching provided by the ligands. For terbium chelates the strong temperature dependence of the fluorescence intensity is explained in terms of quenching of the emitting level via thermal excitation to the lowest triplet state of the ligand. This is indicated by the agreement between the activation energy for quenching and the energy difference between the emitting level and the triplet state.