5514 | New J. Chem., 2018, 42, 5514--5522 This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018 Cite this: New J. Chem., 2018, 42, 5514 Solid-state tunable photoluminescence in gadolinium-organic frameworks: effects of the Eu 3+ content and co-doping with Tb 3+ † Jarley Fagner Silva do Nascimento, ab Antonio Marcos Urbano de Arau ´ jo, c Joanna Kulesza, d Arthur Felipe de Farias Monteiro, d Severino Alves Ju ´ nior d and Bra ´ ulio Silva Barros * ae 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 Gd 3+ , Eu 3+ and Tb 3+ ions and terephthalate were synthesized under solvothermal conditions, and the influence of the Eu 3+ 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 Eu 3+ 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 Eu 3+ 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 Eu 3+ ions, based on the results of photoluminescence. The Gd/Eu/Tb-1,4-BDC co-doped sample with 2.5 mol% of Eu 3+ and 2.5 mol% of Tb 3+ obtained as a single phase exhibited both red and green emissions due to the presence of Eu 3+ and Tb 3+ ions, respectively. Furthermore, the spectroscopic analysis indicated the energy transfer from Tb 3+ to Eu 3+ , which allows the color tuning by changing the excitation wavelength. Introduction Metal–organic frameworks (MOFs) have emerged as an important class of hybrid porous materials containing metal clusters linked by organic multidentate ligands. 1 Due to their high crystallinity, chemical stability and versatility of structures and topologies, 2,3 MOFs have received considerable attention in the past two decades due to their applications in magnetism, 4 gas adsorption and separation, 5 drug delivery, 6 catalysis, 7 etc. Owing to the unique luminescence properties of lanthanide ions and inherent porosity of MOFs, lanthanide-organic frame- works (LOFs) have been revealed as promising candidates for light-emitting materials, 8 sensors, 9–11 multimodal image con- trast agents 12 catalysts 13 and luminescent markers for gunshot residues. 14 Recently, considerable attention has been paid to investigate the structure–photoluminescence relationship of LOFs. However, the precise design and prediction are challenging, because of the lability of lanthanide ions combined with the lack of preferential coordination geometry. 15 Therefore, systematic and profound studies on the photophysical properties of LOFs are still required. Regarding the diversity and flexibility of their coordination modes, aromatic polycarboxylate ligands are suitable for con- structing lanthanide-organic frameworks. 16 Linear organic mole- cules such as terephthalic acid (1,4-H 2 BDC) are attractive organic binders since they promote homogeneous network growth and some LOFs based on this ligand have already been reported. 17–21 Organic linkers not only dictate the structure of the network but also act as efficient sensitizers for lanthanide ions. a Universidade Federal do Rio Grande do Norte, Instituto de Quı ´mica, 59078-970, Natal/RN, Brazil b Instituto Federal de Educaça ˜o, Cie ˆncia e Tecnologia Rio Grande do Norte, 59500-000, Macau/RN, Brazil c Universidade Federal do Rio Grande do Norte, Escola de Cie ˆncias & Tecnologia, 59078-970, Recife/PE, Brazil d Universidade Federal de Pernambuco, Departamento de Quı ´mica Fundamental, 50070-901, Recife/PE, Brazil e Universidade Federal de Pernambuco, Departamento de Engenharia Meca ˆnica, Av. Prof. Moraes Rego-1235 – Cidade Universita ´ria, 50070-901, Recife/PE, Brazil. E-mail: braulio.barros@ufpe.br † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c7nj04625a Received 26th November 2017, Accepted 22nd February 2018 DOI: 10.1039/c7nj04625a rsc.li/njc NJC PAPER