Sign Reversal of a Large Circularly Polarized Luminescence Signal by the Twisting Motion of a Bidentate Ligand

Abstract This work demonstrates sign reversal of large circularly polarized luminescence (CPL) signal based on the hinge‐like twisting motion of a bidentate ligand, 3,3‐bis(diphenylphosphoryl)‐2,2‐bipyridine (BIPYPO), in a cis – trans isomerization of chiral europium(III) complexes. X‐ray diffraction analysis revealed that twisting motion of BIPYPO provides s ‐ cis and s ‐ trans geometries of a chiral Eu III complex containing either tris[3‐(trifluoromethylhydroxymethylene)‐(+)‐camphorate] ( D ‐ 1 ) or tris[3‐(heptafluoropropylhydroxymethylene)‐(+)‐camphorate] ( D ‐ 2 ). The s ‐ cis Eu III complexes show eight‐coordinate geometry around the Eu III ion, in which the chelate between the phosphoryl oxygen and the Eu III ion forces the s ‐ cis geometry of BIPYPO. In contrast, the phosphorus–nitrogen interaction provides a conformational lock for the s ‐ trans geometry of the BIPYPO ligand, inducing a quasi‐seven‐coordinate Eu III complex. The difference in coordination geometry causes the sign change of the CPL signals between the s ‐ cis and s ‐ trans isomers, whereby the s ‐ cis and s ‐ trans isomers of Eu III complexes exhibit the positive and negative CPL signals, respectively, for the 5 D 0 → 7 F 1 transition. The proportion of the s ‐ trans ‐ D ‐ 1 against s ‐ cis ‐ D ‐ 1 increases upon changing the solvent from [D 3 ]acetonitrile to [D 6 ]acetone, inducing a sign change of the CPL signals. The complexes D ‐ 1 and D ‐ 2 show a biexponential decay with two different lifetimes, suggesting two emitting species, that is, the s ‐ cis and s ‐ trans isomers of Eu III complexes. In both cases, the proportions of the longer lifetime components ( τ 1 ) decrease and instead the shorter lifetime components ( τ 2 ) increase upon changing the solvent from [D 3 ]acetonitrile to [D 6 ]acetone.