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 → 7F1 transition 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).