Sign Reversal of a Large Circularly Polarized Luminescence Signal by the Twisting Motion of a Bidentate Ligand |
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Authors: | Dr Junpei Yuasa Hiroshi Ueno Prof?Dr Tsuyoshi Kawai |
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Institution: | Graduate School of Materials Science, Nara Institute of Science and Technology, 8916‐5 Takayama, Ikoma, Nara 630‐0192 (Japan), Fax: (+81)?743‐72‐6179 |
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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 EuIII complex containing either tris3‐(trifluoromethylhydroxymethylene)‐(+)‐camphorate] (D ‐ 1 ) or tris3‐(heptafluoropropylhydroxymethylene)‐(+)‐camphorate] (D ‐ 2 ). The s‐cis EuIII complexes show eight‐coordinate geometry around the EuIII ion, in which the chelate between the phosphoryl oxygen and the EuIII 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 EuIII 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 EuIII complexes exhibit the positive and negative CPL signals, respectively, for the 5D0→7F1 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 EuIII 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. |
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Keywords: | chirality europium ligand design luminescence twisting motion |
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