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

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.