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

Anion-responsive sign inversion of circularly polarized luminescence (CPL) was successfully achieved by N ₃ O ₆ -type nona-coordinated europium(III) (Eu ³⁺ ) complexes [( R )- 1 and ( S )- 1 ] composed of a less-hindered unsymmetrical N ₃ -tridentate ligand (a chiral bis(oxazoline) ligand) and three O ₂ -chelating (β-diketonate) ligands. Here, ( R )- 1 exhibited a positive CPL signal ( I _L - I _R > 0) at the ⁵ D ₀ → ⁷ F ₁ transition of Eu ³⁺ , 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 (CF ₃ COO ^- ) to the Eu ³⁺ 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 ^- , BF ₄ ^- , ClO ₄ ^- , ReO ₄ ^- , PF ₆ ^- , OTf ^- , and SbF ₆ ^- ). 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).