Anion-Driven Circularly Polarized Luminescence Inversion of Unsymmetrical Europium(III) Complexes for Target...

Anion-responsive sign inversion of circularly polarized luminescence (CPL) was successfully achieved by N₃O₆-type nona-coordinated europium(III) (Eu³⁺) complexes [(R)-1 and (S)-1] composed of a less-hindered unsymmetrical N₃-tridentate ligand (a chiral bis(oxazoline) ligand) and three O₂-chelating (β-diketonate) ligands. Here, (R)-1 exhibited a positive CPL signal (I_L - I_R > 0) at the ⁵D₀ → ⁷F₁ transition of Eu³⁺, which can be changed to a negative sign (i.e., I_L - I_R > 0 → I_L - I_R < 0) by the coordination of trifluoroacetic anions (CF₃COO^-) to the Eu³⁺ center. However, (R)-1 preserved the original positive CPL signal (i.e., I_L - I_R > 0 → I_L - I_R > 0) in the presence of a wide range of competing anions (Cl^-, Br^-, I^-, BF₄^-, ClO₄^-, ReO₄^-, PF₆^-, OTf^-, and SbF₆^-). Thus, (R)-1 acts as a smart target identifiable probe, where the CPL measurement (I_L - I_R) can distinguish the signals from the competing anions (i.e., I_L - I_R < 0 vs I_L - I_R > 0) and eliminate the background emission (i.e., I_L - I_R = 0) from the background emitter (achiral luminescent compounds). The presented approach is also promising in terms of bio-inspired optical methodology because it enables nature's developed chiral sensitivity to use circularly polarized light for object identification (i.e., I_L - I_R = 0 vs | I_L - I_R | > 0).