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

[Image: see text] 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).