3D lanthanide metal–organic frameworks constructed from lanthanide formate skeletons and 3,5-bis(4′-carboxy-phenyl)-1,2,4-triazole connectors: synthesis, structure and luminescence

Abstract

Three isomorphic lanthanide metal–organic frameworks (Ln-MOFs) have been constructed from lanthanide formate skeletons and 3,5-bis(4′-carboxy-phenyl)-1,2,4-triazole (H₂bct) connectors. Further, by adjusting the co-doping ratio of different Ln³⁺ ions into the framework, two doped Ln-MOFs are synthesized and show tunable luminescence emission including white-light emission.

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Metal–organic frameworks (MOFs) as functional materials are of interest owing to their impressive structural diversity and variety of actual or potential applications.¹ Among them, lanthanide-based MOFs (Ln-MOFs) have received great attention due to their unique luminescence and catalytic properties.^2–5 On the one hand, Ln-MOFs as luminescent materials exhibit high photoluminescence efficiency, unique narrow-band, and long luminescence lifetimes emissions.³ Moreover, the lanthanide based color-tunable and white light-emitting materials can be achieved by incorporating Eu³⁺, Tb³⁺ and Gd³⁺/Ce³⁺ into the isostructural frameworks, and their luminescence can be effectively tuned by varying the stoichiometric ratio of Ln³⁺ ions or the excitation wavelengths.^4,5 However, in comparison with transition metal ions, Ln³⁺ ions possess higher coordination numbers and more flexible coordination geometries, thus to rationally design and construct Ln-MOFs with desired geometries and properties is still a challenge.

As well known, several factors have great influence on the design and synthesis of Ln-MOFs, such as the choice of organic ligands, the metal/ligand ratio and anions etc. Lanthanide ions have a high affinity and prefer to bond to oxygen atoms, thus, the selection or design of ligands containing appropriate oxygen atom coordination sites is crucial to construct Ln-MOFs.⁶ So far, many carboxylic acids, such as benzenepolycarboxylates,⁷ pyridinecarboxylates,⁸ imidazolecarboxylates⁹ and pyrazolecarboxylates¹⁰ have been widely employed to construct Ln-MOFs. More recently, based on mixed organic or inorganic anion ligands, such as carboxylate–oxalate,¹¹ carboxylate–isonicotinate¹² and carboxylate–sulfate,¹³ carboxylate–acetate,¹⁴ various Ln-MOFs have been prepared, in which anions as bridges incorporated in inorganic lattices. Thus, anion not only acts as a contributor to increase complexity but also exerts a synergistic influence in the structural control for self-assembly of Ln-MOFs.¹⁵ Especially, formate anion can be in situ generated by the hydrolysis of dimethylformamide (DMF) solvent under solvothermal conditions.¹⁶ It provides a new strategy for constructing functional Ln-MOFs.

3,5-Bis(4′-carboxy-phenyl)-1,2,4-triazole (H₂bct) endowed with two carboxylate groups and a triazole ring, is a versatile bridging ligand for the construction of multi-dimensional MOFs¹⁷ and has conjugated system possess good energy-transfer ability, which can magnify the luminescence of the f–f transitions of Ln³⁺ ions. In this work, using DMF and H₂O as solvents, a new family of isomorphic Ln-MOFs, namely, [Ln₂(HCOO)₂(bct)₂]·H₂O (Ln = Tb, Eu, Gd) have been constructed from lanthanide formate skeletons and bct²⁻ connectors, exhibiting a 3D framework with 1D oval channels. Luminescence investigation reveals that 1-Tb and 2-Eu show characteristic green and red emissions of the corresponding Tb³⁺ and Eu³⁺ ions, respectively, while 3-Gd exhibits blue emission arising from the bct²⁻ ligand. Then based on their luminescent properties, we have successfully designed and constructed two novel doped Ln-MOFs, Eu³⁺-doped 1-Tb and Eu³⁺, Tb³⁺-doped 3-Gd by adjusting the co-doping ratio of different Ln³⁺ ions into the same framework as that of 1–3. The emission colors of Eu³⁺-doped 1-Tb can be tuned from red to green by varying the molar ratios of Eu³⁺ ion from 10% to 0.02%, while that of Eu³⁺, Tb³⁺-doped 3-Gd can be tuned from yellow to white by changing excitation length.

Solvothermal reaction of H₂bct with Ln(NO₃)₃ in a mixed solvent DMF and H₂O yields isomorphic Ln-MOFs, [Ln₂(HCOO)₂(bct)₂]·H₂O (Ln = Tb (1), Eu (2) and Gd (3)). The corresponding mixed-lanthanide MOF, Eu³⁺-doped 1-Tb and Eu³⁺, Tb³⁺-doped 3-Gd, can be readily synthesized by using mixed lanthanide salts following the same procedure applied to 1–3.

X-ray analyses of 1-Tb, 2-Eu and 3-Gd reveals that they are isomorphous and exhibit a 3D framework with 1D oval channels constructed via dense [Ln₂(HCOO)₂]_∞ 2D layers and bct²⁻ bridging ligands. Hence, only the crystal structure of 1-Tb is described herein as a representative example. 1-Tb crystallizes in the orthorhombic space group Pnma and there are one unique Tb³⁺ ion, one bct²⁻ ligand, one formate anion, and one lattice water molecule in the asymmetric unit of 1-Tb. As shown in Fig. 1, the central Tb³⁺ ion is eight-coordinated and described as a slightly distorted dicapped trigonal prism coordination mode: four oxygen atoms from the carboxylate moiety of four bct²⁻ anions and four carboxylate oxygen atoms from two formate anions. The Tb–O bond lengths range from 2.279(7) to 2.589(7) Å which are comparable to those reported in the related Tb³⁺ compounds.¹⁸ The bond angles of O–Tb–O are in the range of 50.5(2) to 157.3(2)°. The formate anions adopt the chelating and bridging coordination modes, while bct²⁻ anions exhibit the bis(monodentate) coordination fashion as depicted in Fig. 1c. All carboxyl groups of these ligands are deprotonated, in agreement with the IR data (Fig. S1†) in which no strong absorption peaks around 1700 cm⁻¹ (–COOH) are observed.¹⁹

Fig. 1 (a) The coordination environment of Tb(III) center in 1-Tb. (b) Schematic representation of the distorted TbO₈ dicapped trigonal prismatic coordination polyhedron presents in 1-Tb. (c) The coordination modes of formate anion and the bct²⁻ ligand.

In the structure of 1-Tb, two Tb³⁺ centers, Tb1 and Tb1A (A = symmetry code: −x, −y + 2, −z + 1) are directly bridged by two μ₂-O(4) atoms from two formate anions to form a [Tb₂O₂] rhombic SBU with a Tb⋯Tb distance of 3.8948(9) Å as shown in Fig. 2a. These dinuclear SBUs are connected to each other via O(6) atoms from formate anions forming a dense [Tb₂(HCOO)₂]_∞ 2D network in the ac plane (Fig. 2b). As viewed along the a axis, the bct²⁻ ligands further bridge the adjacent layers in a μ₂-O coordination mode to result in a 3D framework with 1D oval channels (Fig. 2c).

Fig. 2 (a) The [Tb₂O₂] rhombic SBU in 1-Tb. (b) The dense [Tb₂(HCOO)₂]_∞ 2D layer built by [Tb₂O₂] rhombic SBUs and formate anions in 1-Tb. (c) The 3D open-framework constructed by dense [Tb₂(HCOO)₂]_∞ 2D layers and bridging bct²⁻ ligands in 1-Tb.

The luminescent properties of 1-Tb, 2-Eu and 3-Gd and H₂bct ligand in the solid state were investigated at room temperature. As shown Fig. 3, upon excitation with 350 nm, the free H₂bct ligands show a blue emission band centered around 410 nm, which is assigned to the π–π* transition. Upon excitation at 307 nm, 1-Tb exhibits characteristic emission bands of Tb³⁺ ions at 488, 546, 582, and 620 nm, associated with the ⁵D₄–⁷F_J (J = 6–3) transitions. 2-Eu emits red light when excited at 382 nm. The emission peaks at 579, 590, 616, 653, and 700 nm originate from the ⁵D₀–⁷F_J (J = 0–4) transitions. The characteristic emission from H₂bct ligand around 410 nm disappears, indicating that there exists energy transfer from H₂bct ligands to Ln³⁺ ions that “magnifies” the luminescence of the f–f transitions. Under excitation at 380 nm, 3-Gd presents a blue emission with a broad band centered at 430 nm, which is quite similar to that of the free H₂bct ligand, except for a slight red shift. As the lowest excited state of the Gd^{3+ 6}P_7/2 is too high to accept energy from the ligand, its characteristic 4f–4f transition at 311 nm is not visible. Therefore, it is reasonable to assign this emission to the intra ligand π–π* charge transfer. This phenomenon is similar to that previously reported.²⁰

Fig. 3 Emission spectra of 1-Tb (a), 2-Eu (b), 3-Gd (c) and H₂bct ligand (d) in the solid state at room temperature.

The characteristics of luminescent emission in 1-Tb and 2-Eu indicate that the antenna effect occurs, that is, energy migration takes place upon H₂bct ligand absorption, followed by intersystem crossing S₁ → T₁ and antenna T₁ → f transfer, and then generating f–f emissions of Tb³⁺ and Eu³⁺ ions as shown in Fig. S2.† According to Latva's empirical rule, a ligand-to-metal transfer process requires an energy gap of 2500–4500 cm⁻¹ for Tb³⁺ and 2500–4000 cm⁻¹ for Eu³⁺.^21,22 Therefore, the H₂bct ligand can effectively sensitize Tb³⁺ emission, but is not an ideal situation for the sensitization of Eu³⁺ luminescence, due to the energy gap between T₁ and ⁵D₄ state of Tb³⁺ is 2765 cm⁻¹ while the energy gap between T₁ and ⁵D₀ state of Eu³⁺ being 5985 cm⁻¹. The quantum yields of 1-Tb and 2-Eu, measured at room temperature upon excitation at 307 nm and 382 nm respectively, are 19.85% and 17.10%.

Considering that the emissions of 1–3 consist of three primary colors and they are isostructural, we expect to obtain tunable luminescent emission including white-light emission by doping different Ln³⁺ ions into one framework. By carefully choosing the starting lanthanide salts and optimizing the molar ratio of different Ln³⁺ ions, we successfully designed and constructed two novel doped Ln-MOFs, Eu³⁺-doped 1-Tb and Eu³⁺, Tb³⁺-doped 3-Gd. As shown in Fig. 4, the luminescent behaviors of a series of Eu³⁺-doped 1-Tb with different doping molar ratios were investigated when excited at 307 nm. It was found that their emission colors can be tuned from red to green by varying the molar ratios of Eu³⁺ ion from 10% to 0.02%. Indeed, the luminescent intensities of Tb³⁺ characteristic peak decrease abruptly as soon as some Eu³⁺ ions are present. Moreover, the red component to the color emission keeps strong even for low Eu³⁺/Tb³⁺ ratios. It indicates the emission of the Eu³⁺ ions in Eu³⁺-doped 1-Tb is further sensitized by the Tb³⁺ ions within the same frameworks. In addition, the luminescent decay test shows the values of lifetime for 1-Tb and 2-Eu are 0.57(1) ms and 0.56(1) ms, while ⁵D₄ (Tb³⁺) and ⁵D₀ (Eu³⁺) lifetime values in Eu³⁺-doped 1-Tb being 0.16(01) ms and 1.28(3) ms, respectively (Fig. S3†). This further approves the phenomenon of Tb³⁺ to Eu³⁺ energy transfer in Eu³⁺-doped 1-Tb.

Fig. 4 (a) Emission spectra of Eu³⁺-doped with different Eu³⁺ molar ratios. (b) CIE chromaticity diagram for Eu³⁺-doped 1-Tb when excited at 307 nm. Inset: the optical photographs of Eu³⁺-doped 1-Tb products with different Eu³⁺ molar ratios under irradiation of UV light.

Due to the blue emission of the bct²⁻ ligand in 1-Tb and 2-Eu is almost completely suppressed by the strong green and red luminescence of the Tb³⁺ and Eu³⁺ ions, an excess amount of non-luminescent Gd³⁺ ion is required to dilute these two Ln³⁺ ions in the solid state. By optimizing the molar ratio of Eu³⁺, Tb³⁺ and Gd³⁺ ions being 0.0006 : 0.2 : 0.7994, Eu³⁺, Tb³⁺-doped 3-Gd is synthesized, which can simultaneously show the characteristic emissions of the Eu³⁺ and Tb³⁺ ions, as well as the broad blue emission of the ligand. The emission spectra changes of Eu³⁺, Tb³⁺-doped 3-Gd as a function of the excitation wavelength were investigated (Fig. 5). As the excitation wavelength increases from 307 nm to 393 nm, the emission intensities of the red and blue components gradually increase, while the emission intensity of green component decreases, thus the emission color of Eu³⁺, Tb³⁺-doped 3-Gd can be tuned from yellow to white. A white emission with CIE coordinates of (0.313, 0.312), which fall within the white-light region, emerges upon excitation at 377 nm. The quantum yield of Eu³⁺, Tb³⁺-doped 3-Gd is 10.7%, which is measured under λ_ex = 377 nm at room temperature.

Fig. 5 (a) The emission spectra of Eu³⁺, Tb³⁺-doped 3-Gd by changing the excitation wavelength. (b) CIE chromaticity diagram showing the emission of Eu³⁺, Tb³⁺-doped 3-Gd varying from yellow to white by changing different excitation wavelength. x = 0.313 and y = 0.312 for A.

Conclusions

In summary, by reacting H₂bct ligand with lanthanide ions, three isostructural Ln-MOFs 1-Tb, 2-Eu and 3-Gd have been synthesized under the solvothermal conditions. They present 3D framework with 1D oval channels built by dense lanthanide formate skeletons and bridging bct²⁻ ligands. Based on their luminescent properties, two novel doped Ln-MOFs, Eu³⁺-doped 1-Tb and Eu³⁺, Tb³⁺-doped 3-Gd have been successfully prepared by adjusting the co-doping ratio of different Ln³⁺ ions into the same framework as that of 1–3. The emission colors of Eu³⁺-doped 1-Tb can be tuned from red to green by varying the molar ratios of Eu³⁺ ion from 10% to 0.02%, while Eu³⁺, Tb³⁺-doped 3-Gd displays white light emission upon excitation at 377 nm and its emission can be tuned from yellow to white by the change of excitation wavelength. Further studies will focus on investigating the differences arising upon exchange of H₂bct ligand for other N-heterocyclic carboxylate ligands toward the synthesis of novel functional Ln-MOFs.

Acknowledgements

We acknowledge financial support from the National Natural Science Foundation of China (21171065 and 21201077).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional structural figures, PXRD patterns, TGA and luminescent decay curves, IR, excitation spectra of 1–3 and H₂bct and energy levels of H₂bct, Eu³⁺ and Tb³⁺. CCDC 1011116 and 1428248. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra21557a

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