Intense multi-colored luminescence in a series of rare-earth metal-organic frameworks with aliphatic linkers.

Two series of highly luminescent yttrium(iii), europium(iii) and terbium(iii) metal-organic frameworks containing diimine aromatic ligands and the dicarboxylate linker trans-1,4-cyclohexanedicarboxylate (chdc) which can be described by the general formulas [M(bpy)(chdc)], where M = Y (1), Eu (2), and Tb (3) and bpy = 2,2'-bipyridyl, and [M(phen)(chdc)], where M = Y (4), Eu (5), and Tb (6) and phen = 1,10-phenanthroline, were synthesized and characterized. All compounds are based on the same dinuclear {M(L)(OOCR)} building blocks and possess a similar topology of the 3D framework with narrow pores. The chelate aromatic ligands act as efficient light-harvesting antennas for subsequent energy transfer to the emitting metal center (M = Eu, Tb) or intraligand photoemission (M = Y). As a result, the reported compounds display intense emission in the red (Eu), green (Tb) or blue (Y) regions representing three basic colors (RGB) of visible light. The measured quantum yields (QYs) of the solid-state luminescence for individual compounds were found to be 63% (1), 46% (2), 59% (3), 2.3% (4), 55% (5) and 49% (6). The drastic reduction of the luminescence efficiency for 4 is explained by the strong disorder of phen ligands. The high thermal stability (up to 300 °C) and exceptional moisture resistance of the bpy-based frameworks 1-3 were confirmed by TG and PXRD measurements. Various bimetal or trimetal compositions were also prepared for the bpy-series. The luminescence properties of these mixed-metal compounds depend on both the chemical composition and excitation wavelength (λ). Remarkably, pure white emission with color temperature = 6126 K was achieved for [YEuTb(bpy)(chdc)] at λ = 360 nm with QY = 20%. The reported results suggest that the obtained coordination framework series is a convenient platform for the design of highly efficient light emitting materials with tunable properties.