Solid-state tunable photoluminescence in gadolinium-organic frameworks: effects of the Eu3+ content and co-doping with Tb3+

Abstract

Mixed lanthanide-organic frameworks (MLOFs) are an interesting class of hybrid materials with unique luminescence properties. The detailed structure–property relationship studies are still insufficient and therefore, the precise design and synthesis of these materials are still required. With this view, MLOFs based on Gd³⁺, Eu³⁺ and Tb³⁺ ions and terephthalate were synthesized under solvothermal conditions, and the influence of the Eu³⁺ dopant concentration on the photophysical properties of Gd/Eu-1,4-BDC-MOFs was studied. Moreover, the effect of the excitation wavelength on color tuning in a Gd/Tb/Eu-1,4-BDC-MOF co-doped sample is also discussed here. Analyses of X-ray diffraction data indicated the Gd/Eu-1,4-BDC single phase formation in samples doped up to 7 mol% of Eu³⁺ ions. By increasing the europium content, a second crystalline phase was formed. Both crystalline phases with a metal–organic structure exhibited a red luminescence due to the characteristic 4f–4f transitions of Eu³⁺ ions. Although not observed in the X-ray diffraction patterns, most probably the second phase was also present in the sample with 7 mol% of Eu³⁺ ions, based on the results of photoluminescence. The Gd/Eu/Tb-1,4-BDC co-doped sample with 2.5 mol% of Eu³⁺ and 2.5 mol% of Tb³⁺ obtained as a single phase exhibited both red and green emissions due to the presence of Eu³⁺ and Tb³⁺ ions, respectively. Furthermore, the spectroscopic analysis indicated the energy transfer from Tb³⁺ to Eu³⁺, which allows the color tuning by changing the excitation wavelength.