Knowledge Bank

A large number of crystalline chelates of lanthanum, gadolinium and lutetium, doped with small amounts of europium or terbium show a disproportionately strong luminescence from the dopant when irradiated in the ultraviolet. In most chelates thus far investigated energy transport to the luminescent centers occurs via triplet excitons, whereas in some systems signlet exciton migration predominates. The crystalline chelates of anthranilic acid are particularly interesting in that they show energy migration by both singlet and triplet excitons in the same sample. When the dopant is $Tb3+$, appreciable luminescence from its $\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}D4$ level occurs, especially in the gadolinium anthranilate, for dopant concentrations down to a few parts per million. When the dopant is $Eu3+$, quenching of the ligand luminescence occurs, without the appearance of the characteristic europium luminescence. It will be shown that, although energy transport to the $Eu3+$ centers is relatively efficient, the transferred energy is dissipated by a radiationless process from higher excited levels of $Eu3+$, thus preventing the population of the $\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}D6$ level. The relative efficiencies of singlet and triplet exciton migration processes will be discussed.