Evaluation of circularly polarized luminescence in a chiral lanthanide ensemble

Abstract

This work demonstrates a methodology for the evaluation of circularly polarized luminescence of a chiral europium(III) (Eu^III) complex species in an ensemble system. The chiral Eu^III complex species consists of chiral bis(oxazolinyl)pyridine [(R)- or (S)-iPr-Pybox] and β-diketonate ligands with a pendant nitro group (DK-NO₂). The pendant NO₂ group in [iPr-Pybox](Eu^III)(DK-NO₂)₃ (monometallic Eu^III species) coordinates to the Eu^III center of another Eu^III complex, giving rise to the generation of a chiral Eu^III ensemble consisting of mono-, di-, and the other oligomeric Eu^III species, {[iPr-Pybox](Eu^III)(DK-NO₂)₃}_n. The luminescence dissymmetry factors (g_lum) of the chiral Eu^III ensemble have been successfully determined by using a commercially available fluorescence spectrophotometer attached with a rotatable λ/4 filter and a fixed linearly polarized plate. This study suggests that the chiral Eu^III ensemble in solution displays a large circularly polarized luminescence (|g_lum| = 0.19) as compared to that of a reference monometallic complex [iPr-Pybox](Eu^III)(DK-CN)₃ (|g_lum| = 0.11–13). The larger g_lum is primarily attributed to the contribution of the dimetallic Eu^III species exhibiting a high degree of circular polarization in luminescence (g_lum2 = 0.27) in the ensemble. Conversely, a large linearly polarized component was observed in luminescence from the chiral Eu^III ensemble in the solid state (KBr pellet).