Tunable Dual-emission of Mixed-lanthanide MOFs with no Antenna Effect.

Abstract

Heteronuclear zinc(II):mixed-lanthanide(III) metal organic frameworks (MOFs) bearing the formula [ ${Ln}_x^A{Ln}_y^B$ Zn₃(oda)₆(H₂O)₆]⋅nH₂O (Ln = lanthanide(III) ion, ${Ln}^B$ is smaller than ${Ln}^A$ , x+y=2, n=13 or 14) were obtained by direct reaction of the corresponding metal ion salts and oxydiacetate (^-OOC-CH₂-O-CH₂-COO^-) in aqueous solution, followed by slow solvent diffusion. This strategy led to the isolation of thirteen pure crystalline samples of isostructural hexagonal compounds (space group P6/mcc). Characterization by single-crystal X-ray diffraction was complemented by the analytical determination of the final ${Ln}^B/{Ln}^A$ molar ratio by microwave induced plasma atomic emission spectrometry. Results show that the experimentally determined ${Ln}^B/{Ln}^A$ molar ratio is systematically higher than that set out in the synthesis, especially for combinations of Ln that have larger size differences. Dual emission luminescence is observed for the compounds containing a combination of the more efficient visible-emitting Ln (Sm, Eu, Tb, Dy). The selected aliphatic ligand does not promote antenna effect. Thus, tunable color luminescence can be achieved by selectively targeting each Ln preferential excitation wavelength. Dual emission is also observed for CeEu mixed-MOF since the simultaneous excitation of both Ln is possible. Interestingly, GdEu mixed-MOF also shows dual emission when only Gd is directly excited.