Jean-Claude Bunzli - Metal-centered photoluminescence as a tool for detecting phase transitions in Eu-III- and Tb-III-containing metallomesogens

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  Suarez,S., Imbert,D., Gumy,F., Piguet,C., Bunzli,J.C.G.   Chemistry of Materials 2004 16  (17) 3257-3266

  Abstract: The non-mesogenic ligand L-1, derived   from a 1,7-diaza-18-crown ether, forms mesogenic complexes with   Eu-III and Tb-III nitrates, Eu(NO3)(3)L-1.0.25 H2O (EuL1) and   Tb(NO3)(3)L-1.l THF (TbL1). Thermal analyses and polarized light   microscopy data indicate the formation of a liquid-crystalline   phase at 86 (Eu) and 81 degreesC (Tb), while isotropization   occurs around 198200 degreesC, immediately followed by   decomposition. In the case of Eu-III, the mesogenic phase is   unambiguously identified as being a hexagonal columnar mesophase   by SAXD. According to a detailed luminescence study, the   crystalline form of EuL1 features two different metal ion   environments: W a C-2-symmetrical site without coordinated water   molecule (population: 79 +/- 6%) and (ii) a low-symmetry site   with one water molecule coordinated onto the metal ion   (population: 21 +/- 6%). For EuL1, the variation of both the   relative intensity of the D-5(0)-->F-7(2) transition,   ln(I-T/I-295), and of the EU(D-5(0)) lifetime,   ln(tau(T)/tau(295)), over the Cr-Col(h) transition have a S-shape   dependence versus 1/T, allowing a precise determination of the   transition temperature. In the case of the luminescence   intensity, corrections for the expected decrease versus   increasing temperature (determined on a reference compound EuL2   with L-2 devoid of mesogenic side chains) and for the refractive   index change over the transition still leave a sigmoid   dependence, pointing to a genuine effect generated by the phase   transition. Consequently, luminescence intensity and lifetime   switching can be used to signal Cr-LC transitions in   Eu-containing mesogenic compounds. A similar effect is observed   for TbL1, as far as the intensity of the D-5(4) --> F-7(5)   transition is concerned

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