| Abstract
| - The structure and photophysical properties of heterobimetallic Ln(III)−Zn(II) iminophenolate cryptates are examined. A comparative study of the solution and solid-state structures reveals that the helical structure is essentially maintained in solution, but the conformation of the ligand is somewhat different. The photophysical properties of the cryptates depend on the nature of the Ln(III) ion, and the anionic cryptand receptor is revealed to be a good sensitizer for Eu(III) and Tb(III) at low temperatures.
- Lanthanide complexes with the Schiff base axial macrobicyclic ligand L1 react with Zn(II) nitrate in the presence ofCaH2 to yield Ln(III)−Zn(II) heterodinuclear cryptates with the formula [Ln(NO3)(L1-3H)Zn](NO3)·xH2O·yMeOH. Themacrobicyclic receptor L1 is an azacryptand N[(CH2)2NCH−R−CHN−(CH2)2]3N (R = 1,3-(2-OH-5-Me-C6H2)).The crystal structures of the Pr(III), Yb(III), and Lu(III) complexes, chemical formulas [Ln(NO3)(L1-3H)Zn](NO3)·xSolv (monoclinic, C2/c, Z = 8), as well as that of [Zn2(L1-3H)](NO3)·H2O (15) (triclinic, P1̄, Z = 2), have beendetermined by X-ray crystallography. The ligand is helically wrapped around the two metal ions, leading to pseudo-C3 symmetries around the metals. The Ln(III)−Zn(II) distances lie in the range 3.3252(13) to 3.2699(14) Å, whilethe Zn(II)−Zn(II) distance in 15 amounts to 3.1037(18) Å. The three five-membered chelate rings of the ligandbackbone coordinating the Ln(III) ion adopt a (λλδ)5 (or (δδλ)5) conformation while the three pseudochelate ringsformed by the coordination of the ligand to the Zn(II) ion adopt a (λ‘λ‘λ‘)5 (or (δ‘δ‘δ‘)5) conformation. Thus in thesolid state the conformation of the three cations is Λ(δδλ)5(δ‘δ‘δ‘)5 or its enantiomeric form Δ(λλδ)5(λ‘λ‘λ‘)5. Insolution, the helicates present a time-averaged C3 symmetry, as shown by 1H NMR, and the conformation of thecations is described as Λ(δδδ)5(δ‘δ‘δ‘)5 (or Δ(λλλ)5(λ‘λ‘λ‘)5). The photophysical properties of the cryptatesdepend 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) atlow temperatures, but the emission at room temperature is limited by the low energy of the ligand 3ππ* state.While Eu(III) is most effectively sensitized by the ligand triplet state, the Tb(III) (5D4) sensitization occurs via thesinglet 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 3ππ* state allows efficient sensitization of the Nd(III) andYb(III) cryptates, which emit in the near-infrared.
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