The optical and thermal behaviors of a chiral europium(III) β-diketonate complex, Eu(D-facam)3 (facam: 3-(trifluoromethylhydroxymethylene)-(+)-camphorate), were examined in the presence of imidazolium-based ionic liquid 1-ethyl-3-methylimidazolium acetate (EMImOAc). The addition of EMImOAc to Eu(D-facam)3 butanol solutions enhanced their luminescence intensity by up to 74-fold and induced clear circularly polarized luminescence (gCPL = −0.28 for the 5D0 → 7F1 transition). When Eu(D-facam)3 was dissolved directly in EMImOAc, the Eu(III) complex also exhibited distinct circularly polarized luminescence (gCPL = −0.22). In addition, compared with the thermal stability of luminescence in 1-butanol, the ionic liquid solution exhibited superior thermal robustness, retaining approximately 30% of its room-temperature emission intensity even at 100 °C. Arrhenius analysis of the solutions was performed using their emission intensity and lifetime to evaluate the emission stability at higher-temperature regions near 70–100 °C. In EMImOAc, the thermal acceleration of the nonradiative decay of the ligands was suppressed; thus, the energy transfer from the ligand to the Eu(III) ion was stabilized even at higher temperatures. These results highlight the role of ionic liquids as effective media toward achieving thermally robust and highly emissive chiral Eu(III) systems.
Suzuki et al. (Wed,) studied this question.