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).