Energy Migration in Lanthanide Chelates

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.