Jean-Claude Bunzli - Tuning the emission color of europium-containing ionic liquid-crystalline phases

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  Guillet,E., Imbert,D., Scopelliti,R., Bunzli,J.C.G. Chemistry   of Materials 2004 16 (21)   4063-4070

  Abstract: Luminescent liquid-crystalline phases   are produced by introducing trivalent europium salts, EuY3 (Y =   Cl, NO3, ClO4, CF3SO3) in room-temperature ionic liquids (RTIL)   derived from 1-alkyl-3-methylimidazolium, [C-n-mim]X (X = Cl,   NO3; n = 12-18). Four new ionic liquids are synthesized (X =   NO3-, n = 12, 14, 16, 18) and characterized, and the structure of   [C-12-mim]Cl is elucidated by X-ray diffraction. Differential   scanning calorimetry and polarized light microscopy demonstrate   that the liquid-crystalline properties of europium-containing   [C-12-mim](Cl) are not much affected up to a salt concentration   of 10 mol %, except for the mesogenic window which is enlarged.   The RTIL displays a blue fluorescence and its intensity decreases   substantially upon the introduction of Eu-III salts, pointing to   energy transfer from the RTIL to the metal ion. A high-resolution   luminescence study conducted both at room and low (10 K)   temperature unambiguously demonstrates that the 5 mol % solutions   contain a single solvated Eu-III species; when the counterion is   Cl-, ClO4-, or CF3SO3-, it appears to be a polychloro complex   with a low symmetry derived from an idealized cubic symmetry. In   the case of nitrate, a stronger anion-Eu-III interaction results   in an emission spectrum in which the hypersensitive   metal-centered red transition (D-5(0) --> F-7(2))   predominates. As a matter of fact, the emission color of the   liquid-crystalline phases can be easily turned from blue to red,   depending on the excitation wavelength and the counterion Y, as   demonstrated by the trichromatic coordinates of these materials

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