title: 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) D0 →(7) F1 transition. The proportion of the s-trans-D-1 against s-cis-D-1 increases upon changing the solvent from [D3 ]acetonitrile to [D6 ]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 (τ1 ) decrease and instead the shorter lifetime components (τ2 ) increase upon changing the solvent from [D3 ]acetonitrile to [D6 ]acetone.
authors: Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192 (Japan), Fax: (+81) 743-72-6179. yuasaj@ms.naist.jp.
authors: Junpei
authors: J
authors: Yuasa
authors: Hiroshi
authors: H
authors: Ueno
authors: Tsuyoshi
authors: T
authors: Kawai
mesh_terms: D002942:Circular Dichroism; D008024:Ligands; D008163:Luminescent Measurements / Q000379:methods*; D008968:Molecular Conformation; D015394:Molecular Structure; D013237:Stereoisomerism
chemical_list: D008024:Ligands
keywords: chirality; europium; ligand design; luminescence; twisting motion