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

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 D-5(0)-> F-7(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 (tau(1)) decrease and instead the shorter lifetime components (tau(2)) increase upon changing the solvent from [D-3]acetonitrile to [D-6]acetone.