Jean-Claude Bunzli - Structural and photophysical properties of heterobimetallic 4f-Zn iminophenolate cryptates

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  Rodriguez-Cortinas,R., Avecilla,F., Platas-Iglesias,C.,   Imbert,D., Bunzli,J.C.G., de Blas,A., Rodriguez-Blas,T.   Inorganic Chemistry 2002 41  (21) 5336-5349

  Abstract: Lanthanide complexes with the Schiff   base axial macrobicyclic ligand L(1) react with Zn(II) nitrate in   the presence of CaH(2) to yield Ln(III)-Zn(II) heterodinuclear   cryptates with the formula   [Ln(NO(3))(L(1)-3H)Zn](NO(3)).xH(2)O-yMeOH. The macrobicyclic   receptor L(1) is an azacryptand   N[(CH(2))(2)N=CH-R-CH=N-(CH(2))(2)](3)N (R =   1,3-(2-OH-5-Me-C(6)H(2))). The crystal structures of the Pr(III),   Yb(III), and Lu(III) complexes, chemical formulas   [Ln(NO(3))(L(1)-3H)Zn](NO(3)). xSolv (monoclinic, C2/c, Z = 8),   as well as that of [Zn(2)(L(1)-3H)](NO(3)).H(2)O (15) (triclinic,   P (1) over bar, Z = 2), have been determined by X-ray   crystallography. The ligand is helically wrapped around the two   metal ions, leading to pseudo-C(3) symmetries around the metals.   The Ln(III)-Zn(II) distances lie in the range 3.3252(13) to   3.2699(14) Angstrom, while the Zn(II)-Zn(II) distance in 15   amounts to 3.1037(18) Angstrom. The three five-membered chelate   rings of the ligand backbone coordinating the Ln(III) ion adopt a   (lambdalambdadelta)(5) (or (deltadeltalambda)(5)) conformation   while the three pseudochelate rings formed by the coordination of   the ligand to the Zn(II) ion adopt a (lambda'lambda'lambda')(5)   (or ((delta'delta'delta')(5)) conformation. Thus in the solid   state the conformation of the three cations is   Lambda(deltadeltalambda)(5)(delta'delta'delta')(5) or its   enantiomeric form   Delta(lambdalambdadelta)(5)(lambda'lambda'lambda')(5). In   solution, the helicates present a time-averaged C(3) symmetry, as   shown by (1)H NMR, and the conformation of the cations is   described as Lambda(deltadeltadelta)(5)(delta'delta'delta')(5)   (or Delta(lambdalambdalambda)(5)(lambda'lambda'lambda')(5)). The   photophysical properties of the cryptates depend on the nature of   the Ln(III) ion, and (L-3H)(3-) is revealed to be a good   sensitizer for Eu(III) and Tb(III) at low temperatures, but the   emission at room temperature is limited by the low energy of the   ligand (3)pipi(*) state. While Eu(III) is most effectively   sensitized by the ligand triplet state, the Tb(III) ((5)D(4))   sensitization occurs via the singlet state. The quantum yield of   the metal-centered luminescence in the Eu-Zn cryptate amounts to   1.05% upon ligand excitation. The low energy of the ligand   (3)pipi(*) state allows efficient sensitization of the Nd(III)   and Yb(III) cryptates, which emit in the near-infrared

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