The Eu(III) ion as luminescent probe: Laser-spectroscopic investigation of the metal ion sites in an 18-crown-6 complex

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Abstract
The structure of [Eu(NO3)2(18-6)]3[Eu(NO3)6] has been probed by high-resolution excitation and emission spectra at 77 and 4.2 K. The compound contains centrosymmetric [Eu(NO3)6]3− ions with D2h, C2h, or Ci symmetry and three types of complex [Eu(NO3)2(18-6)]+ cations with D2 symmetry slightly distorted towards C2v. The luminescence spectra of Ln2Mg3(NO3)12 ⋅ 24H2O (Ln=Eu and La doped with 1% Eu) have also been recorded and are compared to the spectrum of the hexanitrato ion. The high-resolution study of the 5D0→7F1 transition at 4.2 K, both of polycrystalline samples and of monocrystals of the Eu-doped Gd complex, revealed that each metal site does not generate a single emission spectrum but, rather, a series of very similar spectra. These arise from molecules experiencing slightly different crystal field effects, that is, having slightly different conformations. The Eu(III) ion is therefore a quite sensitive luminescent probe responding to faint chemical differences often overseen in x-ray diffraction studies; it could be used to probe the conformational distribution of molecules in polycrystalline solids.