Intramolecular Energy Transfer in Rare Earth Chelates. Role of the Triplet State

Results of luminescence studies on chelates of La3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Tm3+, Yb3+, and Lu3+ are reported. It is shown that the intramolecular energy transfer efficiency is a sensitive function of the relative positions of the resonance energy levels of the ions and the metastable triplet states of the complexes. In the case of Eu3+ it is demonstrated that, by a proper choice of ligands, it is possible to selectively excite only the lowest resonance level of the two existing resonance levels of this ion. Coupling of the rare earth ions to the ligands is discussed, and a comparison is made between the optical properties of rare earth ions in chelates and in inorganic salts.