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 NO-type nona-coordinated europium(III) (Eu) complexes [()- and ()-] composed of a less-hindered unsymmetrical N-tridentate ligand (a chiral bis(oxazoline) ligand) and three O-chelating (β-diketonate) ligands. Here, ()- exhibited a positive CPL signal ( - > 0) at the D → F transition of Eu, which can be changed to a negative sign (i.e., - > 0 → - < 0) by the coordination of trifluoroacetic anions (CFCOO) to the Eu center. However, ()- preserved the original positive CPL signal (i.e., - > 0 → - > 0) in the presence of a wide range of competing anions (Cl, Br, I, BF, ClO, ReO, PF, OTf, and SbF). Thus, ()- acts as a smart target identifiable probe, where the CPL measurement ( - ) can distinguish the signals from the competing anions (i.e., - < 0 vs - > 0) and eliminate the background emission (i.e., - = 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., - = 0 vs | - | > 0).