Evaluation of the intramolecular energy transfer rate constants in crystalline Eu(hfaa)4ButNH3

Abstract

The relative fluorescence output and Eu^{3+ 5}D₀ level lifetime, τ, of crystalline Eu(hfaa)₄Bu^tNH₃(hfaa = hexafluoroacetylacetonate anion) have been measured over the temperature range 90–300 K. The observed decrease in τ with increasing temperature is attributed to a back donation of energy from the ⁵D₀ level to the ligand triplet, T_Eu. These data in conjunction with data from crystalline Gd(hfaa)₄Bu^tNH₃ and a 1 % europium doped mixed complex have been used to evaluate the T_Eu[graphic omitted]→⁵D₀ transfer rate, k₂, as ca. 1.1 × 10⁶ s^–1 from 170 to 270 K. This value is considerably lower than those previously reported for Eu³⁺ complexes in solution i.e. ca. 10¹⁰ s^–1. Possible causes of the apparent decrease in k₂ at higher and lower temperatures are discussed. The rate of deactivation of T_Euby routes other than via the ⁵D₀ level has been calculated and shown to closely follow an Arrhenius function. The results indicate that two ligand triplet levels occur only one of which is involved in intermolecular energy transfer.