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

Abstract

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 (IL- IR> 0) at the 5D0→ 7F1transition of Eu3+, which can be changed to a negative sign (i.e., IL- IR> 0 → IL- IR< 0) by the coordination of trifluoroacetic anions (CF3COO-) to the Eu3+center. However, (R)-1 preserved the original positive CPL signal (i.e., IL- IR> 0 → IL- IR> 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 (IL- IR) can distinguish the signals from the competing anions (i.e., IL- IR< 0 vs IL- IR> 0) and eliminate the background emission (i.e., IL- IR= 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., IL- IR= 0 vs | IL- IR| > 0).