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The relative fluorescence output and Eu3+ level lifetime, T , of crystalline Eu(hfaa)4ButNH3(hfaa = hexafluoroacetylacetonate anion) have been measured over the temperature range 90-300 K.

Andrews KY16 9ST, Scot I a n d Investigations of tris-and tetrakis-hexafluoroacetylacetonato-and trifluoroacetylacetonato-terbium(iii) complexes have established that the temperature-dependence of the lifetime is caused by thermal depopulation of the ion level to a ligand triplet state. Ligand triplet to ground state deactivation rates, A, have been determined and range from 1 O4 to l O7 s-l. Neither variation in thermal depopulation rates norA i s sufficient to explain the observed differences in quantum efficiencies and it is suggested that significant losses may occur before the triplet state in the energy transfer process.LANTHANIDE ions form two general types of complex with p-diketoenolate anions (a) neutral tris complexes, LnL,, which may be solvated, and (b) anionic tetrakis complexes, [LnL,]-, which are associated with a cationic species.l Many of these complexes, especially those of Eu3+, Tb3+, and Sm3+, emit narrow line fluorescence owing to radiative transitions within the 4f electron shell of the lanthanide ion when they are irradiated in the first excited singlet absorption band of the ligand. The energy-transfer mechanism leading to this emission involves intersystem crossing from the ligand singlet, S,,

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