Photoluminescence of europium and terbium trifluoroacetylacetonates. Participation of LMCT state in processes of the energy transfer to Eu3+ ion

Abstract The effect of fluorination on spectroscopic properties of europium and terbium trifluoroacetylacetonates Ln(TFAA)3·2H2O, Ln(TFAA)3·Phen and Ln(TFAA)3·Bpy in the solid state have been investigated. A threefold increase in the luminescence intensity of europium compounds at -CF3 for −CH3 substitution in the β-diketonate ligand was obtained at 77 and 295 K. Different mechanisms of the excitation energy transfer to Ln3+ ions in europium and terbium compounds, which result in a drop in the luminescence efficiency at elevated temperatures, have been analyzed. A key role of ligand-metal charge transfer (LMCT) states in the processes of sensitization of the europium trifluoroacetylacetonates is demonstrated. Expansion of the energy gap between the LMCT state and the lowest triplet state T1 at fluorination gives rise to an increase in the excitation efficiency of the Eu3+ luminescence at 295 K. The back-transfer energy 5D4-T1 in the terbium trifluoroacetylacetonates is the main reason for shortening the 5D4 lifetime and decreasing the luminescence efficiency. The energy dissipation processes in the lanthanide TFAA adducts with heterocyclic diimines are mainly related to the β-diketonate ligand.