Intramolecular energy transfer in lanthanide chelates can occur through different paths depending on the nature of the chelate. Contrary to prevailing notions, it is shown that energy transfer does not require the participation of the lowest triplet level of the chelate. The experimental results from a study of solutions of more than 600 chelate systems are consistent with a transfer mechanism via the ligand-excited singlet state, with rate constants ket of about 1011 sec−1 and higher. Transfer via the lowest triplet state may predominate, however, when the rate of energy transfer from the singlet excited level is smaller than the rate of intersystem crossing to the triplet level. Whatever the mechanism, intramolecular energy transfer is generally a very efficient process, regardless of the fluorescence quantum yield of the chelated lanthanide ion. Low quantum yields of sensitized fluorescence are usually the result of radiationless decay processes following intramolecular energy transfer. This quenching is most pronounced in chelates in which the lowest triplet level lies below the emissive level of the lanthanide ion.

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Efficient long-range energy transfer takes place from the ligand 1,10-phenanthroline to Tb3+ and Eu3+ in lightly doped crystalline chelates of gadolinium (1,10-phenanthroline) triacetate, GdPhenAc3, and from Tb3+ to Eu3+ in TbPhenAc3 doped with various concentrations of EuPhenAc3. A self-consistent interpretation of the observation is given in terms of a mechanism involving triplet exciton migration. In Eu3+-doped TbPhenAc3 the measured temperature dependence and the europium concentration dependence of the fluorescence decay time of the D45 level of Tb3+ and the buildup time of the Eu3+ fluorescence agree rather well with our theoretical prediction. The activation energy of the lifetime of the D45 level of Tb3+ can be identified with the difference of the ligand triplet level and the D45 level.

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