A Multifunctional Tb-MOF Detector for H2O2, Fe3+, Cr2O7 2-, and TPA Explosive Featuring Coexistence of Binuclear and Tetranuclear Clusters.

A novel three-dimensional microporous terbium(III) metal-organic framework (Tb-MOF) named as [Tb10 (DBA)6(OH)4(H2O)5]·(H3O)4 (1), was successfully obtained by a solvothermal method based on terbium nitrate and 5-di(2',4'-dicarboxylphenyl) benzoic acid (H5DBA). The Tb-MOF has been characterized by single crystal X-ray diffraction, elemental analysis, thermogravimetry, and fluorescence properties, and the purity was further confirmed by powder X-ray diffraction (PXRD) analysis. Structural analysis shows that there are two kinds of metal cluster species: binuclear and tetranuclear, which are linked by H5DBA ligands in two μ7 high coordination fashions into a three-dimensional microporous framework. Fluorescence studies show that the Tb-MOF can detect H2O2, Fe3+, and Cr2O7 2- with high sensitivity and selectivity and can also be used for electrochemical detection of exposed 2,4,6-trinitrophenylamine (TPA) in water. The highly selective and sensitive detection ability of the Tb-MOF might make it a potential multifunctional sensor in the future.

Introduction

Metal–organic frameworks (MOFs) have drawn increasing attention due to their tunable pore sizes, diverse structures, and abundant functional designs.[1] Compared with transition metals, lanthanide metal organic frameworks (Ln-MOFs), especially Eu/Tb-MOFs, have been studied widely due to their outstanding luminescence features, including high-purity color, large Stokes shift, high quantum yields, long decay lifetime, and undisturbed emissive energy. Recently, many Eu/Tb-MOFs were applied as an ideal material for identifying small molecules, metal ions, inorganic anions, organic anions, solvents, gases, and explosives.[2−22] With the rapid growth of the modern industry, more and more harmful chemical pollutants were released into the environment, including heavy metal ions, volatile organic and toxic gases, etc., which have a serious adverse effect on people’s health and life in the past decades.[23] For example, hydrogen peroxide (H2O2) is an essential participant in the energy, food, electrochemistry, enzyme catalysis, and environmental detection as one of the reactive oxygen species (ROS) and is an effective biomarker of several cellular processes, including protein folding, growth, signaling, differentiation, and migration in the cells.[24] Although H2O2 is nontoxic, it produces many free radicals such as a superoxide anion (O2) and hydroxyl radical (·OH) when decomposing, which might induce the initiation of cancer, autoimmunity, neurodegenerative disorders, etc. Aberrant accumulation of H2O2 resulted in oxidative stress, and the level of H2O2 was also connected to aging and some serious diseases, including diabetes, cardiovascular disorder, cancer, and Alzheimer.[2] Therefore, it is urgent and necessary to develop a rapid and sensitive H2O2 detection strategy for the early capture and treatment.

Fe3+ ions are one of the indispensable elements in the human body due to their crucial role in a variety of biochemical processes such as oxygen storage and transport in blood,[25] and they can cause many diseases including anemia, skin ailments, insomnia, kidney damage, dysfunction of organs, and even cancers when they are deficient. However, excessive Fe3+ also causes a series of serious problems to metabolism and homeostasis of human beings, including vomiting, loss of appetite, and diarrhea.[10] Therefore, it is of high importance to develop a rapid, simple, and low-cost approach for sensitive detection of Fe3+ ions for the surveillance of human health.

As we all know, Cr2O72– is a widely used strong oxidant, widely employed in chromium electroplating, pigment production, leather tanning, metallurgy, etc.[26] However, Cr2O72– has been listed as the most harmful anionic contaminant by the U.S. Environmental Protection Agency[27] because it can be absorbed and accumulated by organisms and can do great harm to human health and the environment such as gastrointestinal problems, carcinoma, kidney damage, gene mutation,[26] cardiovascular failure, skin irritation, respiratory infection, and ecological risks, etc.[28,29] Hence, it is important to design and prepare useful sensor materials to detect Cr2O72– precisely for the environmental conservation and health of human beings.[26]

Furthermore, 2,4,6-trinitrophenylamine (TPA) is one of the nitroaromatic compounds (NACs), which can be used in the preparation of large-versus explosives and fur dyes. Since the 1970s, scientists have increasingly paid attention to the environmental effects of nitroaromatic explosives. However, the focus of these studies was mostly on 2,4,6-trinitrotoluene (TNT), 2,4,6-trinitrophenol (TP), 2,4,6-trinitrophenylmethylnitramine (tetryl), various nitrodiphenylamines, and nitronaphthalenes.[30−34] According to the documentation, the special explosive 2,4,6-trinitrophenylamine was never taken into consideration in risk assessment studies of military contaminated sites.

Chromatography and its coupled techniques are the most widely used methods in determining environmental contaminants.[35,36] However, chromatography-based techniques often require expensive equipment, complex pretreatment, and long test time. Thus, it is very urgent and necessary to establish simple, easy, effective, and new sensing technologies for environmental pollutants such as H2O2, Fe3+ ions, Cr2O72– inorganic anions, and 2,4,6-trinitrophenylamine.

In this paper, a novel 3D microporous Tb-MOF has been constructed based on a flexible multidentate penta-carboxylate ligand of 3,5-bis(p-carboxyphenyl) benzoic acid (H5L) under the solvothermal condition. Crystal structure and luminescence properties of the Tb-MOF have been investigated. It features two kinds of metal cluster species: binuclear and tetranuclear, which are linked by H5DBA ligands in two μ7 coordination fashions into a 3D microporous framework. It was found that the Tb-MOF has excellent luminescence sensing properties for H2O2, Fe3+, and Cr2O72– and excellent electrochemical activity for sensing recognition of trinitroaniline by a Tb-MOF-doped carbon paste electrode (Tb-MOF/CPE).

Results and Discussion

Crystal Structure Description

Single-crystal X-ray analysis shows that the Tb-MOF is a three-dimensional microporous structure featuring the coexistence of binuclear (Tb1 and Tb4) and tetranuclear (Tb2, Tb2, Tb2, and Tb3) species as the inorganic building unit. There are four crystallographic independent Tb3+ ions and two DBA5– ligands, two hydroxyl groups, and three coordination water molecules in the asymmetric units of the complex (Figure a). Tb1 ions are located at the center of the nine-coordinated single-capped anti-square-prism configuration with the carboxyl oxygen atoms of the surrounding five DBA5– ligands, of which O6b is the capped atom, and O1d, O5b, O22e, and O18c and O3a, O4a, O7a, and O8a atoms occupy four vertices of two planes (Figure S2a). Among these nine carboxyl oxygen atoms, six oxygen atoms (O3a, O4a, O5b, O6b, O7a, and O8a) are chelated and the other three oxygen atoms (O1d, O18c, and O22e) are bidentate bridged coordination with Tb1 ions. The Tb2 ion is located at the center of a twisted nine-coordination tri-capped triangular prism geometry, of which O9, O13b, and O29 and O10, O1a, and O14 constitute two bottoms of the prism, and O12, O11, and O15 are the capped atoms (Figure S2b). In oxygen atoms coordinated to the Tb2 ion, O14 and O15 are chelated, O9 and O10 are bridged carboxyl oxygen atoms, O12, O13a, and O13b are μ3-hydroxyl oxygen atoms, and O11 and O29 are the coordination water molecules. Similar to Tb2 ions, the Tb3 ion also lies in a distorted nine-coordinated triangular prismatic environment (Figure S2c). Among them, three hydroxyl oxygen atoms O13a, O13b, and O13c and three coordination water molecules O30a, O30b, and O30c constitute the two bottoms of the triangular prism, and three μ2-carboxyl O15 atoms are three cap atoms. Also, the Tb4 ion is located at the center of an eight-coordination distorted anti-square-prism geometry (Figure S2d), of which four bridged carboxyl oxygen atoms (O19c, O23e, O6b, and O2d) and four chelated carboxyl oxygen atoms (O16f, O17f, O20f, and O21f) constitute two bottoms. The scope of the Tb–O bond lengths is 2.291–2.892Å, and the range of the O–Tb–O bond angle is 33–149.06°, which conforms to the reported ranges of Tb–O bond length and O–Tb–O bond angle.[20,22] Interestingly, there are two kinds of inorganic building units in the Tb(III) 3D framework: binuclear and tetranuclear cubic metallic clusters. A binuclear metal unit consists of Tb1 and Tb4 ions (the distance of Tb···Tb is 4.0478 Å), surrounded with six DBA5– ligands. Meanwhile, the distorted tetranuclear metal cube cluster comprises three Tb2 ions and one Tb3 ion, together with four μ3-hydroxyl oxygen atoms (the distance of the Tb···Tb range: 3.6964–3.8670 Å). The DBA5– ligand adopts two high coordination fashions: (a) κ1κ2-κ1κ1-κ1κ1-κ1κ1-κ1κ1-μ7 and (b) κ1κ1-κ2κ1-κ1κ1-κ1κ1-κ1κ1-μ7 to link seven Tb ions (Scheme ). One tetranuclear cubic cluster is surrounded by six DBA5– ligands in six directions forming a six-angle group with the angles of binuclear units as shown in Figure b; such six-angle groups extend in six directions into a three-dimensional microporous framework (Figure c), the topology of 3D as shown in Figure d.

Figure 1

(a) Coordination environment of Tb1–Tb4 (symmetry codes: a: 4/3-y,2/3+x-y,-1/3+z; b: 1-x+y,1-x,z; c: 1/3+x,-1/3+y,-4/3+z; d: 4/3-y,2/3+x-y,2/3+z; e: 1-x+y,1-x,-2+z; f: 1-x+y,1-x,-1+z). (b) Six-angle group based on one tetranuclear cluster, six DBA ligands, and binuclear units. (c) 3D microporous framework. (d) Topology of 3D.

Scheme 1

Coordination Modes of DBA5– Ligands in Complex 1(a) and (b)

PXRD and TG Analyses

The phase purity of complex 1 has been confirmed by powder X-ray diffraction (PXRD) (Figure ). The peaks on the PXRD of as-synthesized complex 1 are coincident with those on their respective simulated patterns.

Figure 2

PXRD patterns of the Tb-MOF (simulated, determination, and after sensing performances of H2O2, Fe3+, and Cr2O72–).

Thermal gravimetric analysis (TGA) of the Tb-MOF was performed by a Netzsch TG-209 thermogravimetric analyzer in an air atmosphere at a heating rate of 10 °C·min–1 from 30 to 900 °C. As shown in Figure , it was decomposed at the beginning of the TG experiment, which means that the complex is unstable and the whole skeleton collapses when heated.

Figure 3

TGA curve of complex 1.

Luminescence Properties

Solid-State Photoluminescence Spectra

The solid-state fluorescence spectra of the Tb-MOF and H5L have been measured at room temperature under the excitation of 351 nm (slits, 5 nm/10 nm) (Figure ). The ligand displays a broad band emission range from 350 to 400 nm, which centered at about 360 nm, which could be assigned to the intraligand π* → π transitions.[37] The Tb-MOF exhibits four well-resolved characteristic emissions, which are centered at 491, 546, 586, and 621 nm, respectively, which could be attributed to the corresponding transitions from 5D4 → 7F6, 5D4 → 7F5, 5D4 → 7F4, and 5D4 → 7F3 of the Tb ion.[20,38,39] Furthermore, the fluorescence intensity of the H5L ligand at 325 nm excitation is negligible relative to the Tb-MOF, which indicates that the inherent fluorescence emission of the ligand has little interference with the fluorescence properties of the Tb-MOF. We used the intensity change of the strongest peak 5D4 → 7F5 (545 nm) as the detection index for analysis. At the same time, the three-dimensional solid fluorescence determination of the Tb-MOF was measured (Figure inset).

Figure 4

Solid-state fluorescence spectra of the Tb-MOF and H5L (λex = 351 nm) and 3D photoluminescence spectra of the Tb-MOF.

Solvent Molecule Sensing Experiments

Tb-MOF powder (3 mg) was immersed in 3 mL of different solvents, including methanol (MT), ethanol (EA), water (H2O), n-butanol (NBA), dimethylsulfoxide (DMSO), ethyl acetate (EAC), isopropanol (IPA), formaldehyde (HCHO), hexamethylene (CYH), N,N-dimethylformamide (DMF), acetic acid (HAc), trichloromethane (CHCl3), normal hexane (NH), acetone (CP), aldehyde (AH) (40%), and H2O2 (30%), or Tb-MOF powder (3 mg) was dispersed in H2O solution containing different concentrations of H2O2 (0.1–300 μM). Then, the sample was treated with ultrasonication for 30 min and then aged for 2 days to form a stable emulsion before the fluorescence experiments (Figure a). Interestingly, the fluorescence intensity depends largely on the solvent molecules. It is worth noting that the complex has a strong fluorescence intensity in H2O, showing the potential capability of the Tb-MOF as an ideal fluorescent probe for the detection of pollutants in water. However, the emission in H2O2 is the weakest, and the quenching effect is the most remarkable, indicating that this Tb-MOF can be used highly selectively as a sensor to detect H2O2 molecules in water, and the stability of the Tb-MOF after sensor performances of H2O2 was checked by PXRD (Figure ). Then, the luminescence intensity of the Tb-MOF gradually decreases with increasing concentration of H2O2. It was found that there is a good double exponential relationship between the concentration of H2O2 at 0.1–300 μM and the fluorescence intensity, with a nonlinear equation of y = 1961.11 × exp(−x/0.27) + 3340.74 × exp(−x/0.02) + 43.51 (R2 = 0.9916) (Figure b inset). The quenching mechanism may be due to the presence of unstable O–O bonds in H2O2, which makes its bi-electron electrophilic. Moreover, H2O2 is also an excellent nucleophile with the existence of the α-effect of adjacent nonbonding orbitals on its oxygen atoms.[24] So, when the complex is immersed in H2O2 aqueous solution, the electrophilicity and nucleophilicity of H2O2 lead to weak hydrogen bonds and other forces between H2O2 and the organic ligand, which weaken the partial energy transfer from the organic ligand H5L to Tb.[10−13,20]

Figure 5

(a) Luminescence spectra and histogram of the Eu-MOF introduced into various solvents, λex = 351 nm. (b) Luminescence spectra of the Tb-MOF with H2O2 at different concentrations in a water solution.

Metal Cation Sensing

Tb-MOF powder (3 mg) was immersed in M(NO3) (M = Na+, Al3+, Ag+, Cd2+, Ga2+, Mg2+, K+, Sr3+, Zn2+, Pb2+, Bi3+, Hg+, Ni2+, Co2+, Cu2,+ or Fe3+ (10–2 M)). Also, 3 mg of Tb-MOF powder was dispersed in H2O solutions containing different concentrations of Fe3+. Before photoluminescence measurements, the suspensions were ultrasound for 30 min and stood at room temperature for 2 days, Interestingly, the Tb-MOF exhibited significantly different luminescence properties in different metal ion solutions (Figure a). It is noted that when Tb-MOF samples are immersed in an aqueous solution containing Fe3+ ions, the luminescence of the Tb-MOF is obviously quenched. It shows that the Tb-MOF can be used as a highly selective fluorescent sensor for Fe3+; furthermore, the Tb-MOF is very sensitive to Fe3+, and the lowest detectable concentration can reach 10–9 M; the stability of the Tb-MOF after sensor performances of Fe3+ ions was checked by PXRD (Figure ). The luminescence intensity of the system decreases gradually when the concentration of Fe3+ ions increases. It was found that there is a good nonlinear relationship between the luminescence intensity and Fe3+ concentration in the range of 10–9–0.01 M, with a nonlinear equation of y = 3841.80 × exp(−x/4.86 × 10–4) + 2575.51 × exp(−x/4.71 × 10–7) + 27.20 (R2 = 0.9614) (Figure b inset). This quenching effect can be expressed by the Stern–Volmer quenching coefficient in the equation I0/I = 1+ Ksv[M] (where I0 and I are the luminescence intensity of the Tb-MOF and the suspension of the Tb-MOF mixed with metal ions, respectively, [M] is the molar concentration of metal ions, and Ksv is the quenching coefficient of metal ions).[16−18] The quenching coefficient Ksv for Fe3+ mixed with the Tb-MOF in H2O (0.01 M) is 9580 M–1.

Figure 6

(a) Luminescence spectra and histogram of the Tb-MOF introduced into different metal ions (λex = 351 nm). (b) Luminescence spectra of the Tb-MOF in different concentrations of Fe3+ ions.

In order to understand the fluorescence quenching mechanism of Fe3+ to the Tb-MOF, the quenching mechanism of different metal ion aqueous solutions was studied by recording the UV–visible absorption spectrum of the metal ion system (Figure S3). Among them, the absorption spectrum of Fe3+ has an observable peak in the range of 260–398 nm, and there is no other metal ion absorption in this wavelength range. Under light conditions, the competitive absorption of Fe3+ and the Tb-MOF at 325 nm results in fluorescence quenching of the system.[23]

To further understand the interference of other metal cations on the Tb-MOF system, adding nitrates of the 16 metals to the Fe3+ solution (10–2 M) containing 3 mg of Tb-MOF powder so that the nitrate concentration is 10–2 M too, the results are shown in Figure . It can be seen that as long as Fe3+ is 10–2 M, the addition of other ions almost does not affect its fluorescence quenching effect on Tb-MOF aqueous solution. This may be because Fe3+ can completely shift the LUMO from Tb3+ to the ligand; in other words, photo-excited electrons are hardly injected to the Tb center.[12]

Figure 7

Interference of metal ions with Tb-MOF fluorescence sensing Fe3+.

Inorganic Anion Sensing

Tb-MOF powder (3 mg) was immersed in H2O solution containing 13 potassium salts KX (X = SCN–, Cl–, IO3–, CH3COO– (Ac–), I–, SO42–, Br–, NO2–, SO32–, S2–, C2O42–, PO43–, and Cr2O72– (10–2 M)) or 3 mg of Tb-MOF powder was dispersed in H2O solutions containing different concentrations of Cr2O72–. Before photoluminescence measurements, the suspensions were ultrasound for 30 min and stood at room temperature for 2 days. Luminescence emissions are recorded in Figure a. Noteworthy is that most of the inorganic anion ions possess varying degrees of luminescence quenching effects. Especially, Cr2O72– ions could almost completely quench the luminescence of the Tb-MOF, which shows that the Tb-MOF has high selectivity for sensing Cr2O72– ions, and the stability of the Tb-MOF after sensor performances of Cr2O72– ions was checked by PXRD (Figure ). The luminescence intensity of the Tb-MOF decreased gradually with the increase in Cr2O72– ion concentration. It was found that there is a good nonlinear relationship between the luminescence intensity and Cr2O72– ion concentration in the range of 0.01–10–9 M with a nonlinear equation of y = 3690.01 × exp(−x/5.28 × 10–4) + 2718.03 × exp(−x/5.44 × 10–7) + 17.04 (R2 = 0.9867) (Figure b inset). This quenching effect can be expressed by the Stern–Volmer quenching coefficient in the equation I0/I = 1+ Ksv[M] (where I0 and I are the luminescence intensity of the Tb-MOF and the suspension of the Tb-MOF mixed with Cr2O72– ions, respectively, [M] is the molar concentration of Cr2O72– ions, and Ksv is the quenching coefficient of Cr2O72– ions).[16−18] The quenching coefficient Ksv for Cr2O72– mixed with the Tb-MOF in H2O (0.01 M) is 6543 M–1. Noteworthy, the Tb-MOF is a very sensitive sensor to Cr2O72– ions, the lowest detectable concentration of Cr2O72– ions can reach 10–9 M (10 ppb), and the method has a lower detection limit compared with literature reports.[15] The traditional methods for the determination of Cr2O72– such as atomic spectroscopy and ICP generally have the disadvantages of expensive equipment, complicated sample processing, and low selectivity. However, the method has the characteristics of high sensitivity, a low detection limit, and simple operation and effects.

Figure 8

(a) Luminescence spectra and histogram of the Tb-MOF introduced into different inorganic anions (λex = 351 nm). (b) Luminescence spectra of the Tb-MOF in different concentrations of Cr2O72–.

In order to know the luminescence quenching mechanism of the Tb-MOF toward Cr, different inorganic anion ionic aqueous solution were researched by UV–vis absorption spectroscopy (Figure S4), which shows that the strong absorption bands of only Cr2O72– aqueous solution were found to be between 230–312 nm and 312–500 nm, whereas other anionic aqueous solutions did not overlap with these absorption bands. The luminescence quenching of the Tb-MOF is due to the competition of Cr2O72– aqueous solution and carboxylic acid ligands for the absorption of energy at an excitation wavelength (325 nm).

Electrochemical Sensing Experiment (Carbon Paste Electrode)

Due to the excellent electrochemical sensing properties of MOF materials, many researchers tried to use them for ionic sensing (including metal cations and inorganic anions),[40] drug sensing,[21] and organic compound sensing (including volatile organic pollutants and explosives);[16−18,32,41] however, there are several electrochemical sensing studies of 2,4,6-trinitrophenyl amine (TPA) in the study of explosives.

In our experiment, we accurately obtained 0.01 g of Tb-MOF powder (using an agate mortar), 3 g of graphite powder, and 0.5 g of liquid paraffin and thoroughly mixed them. Then, the Tb-MOF/CPE electrode was prepared by filling the PVC pipe with an inner diameter of 3 mm with compacting and inserting a copper wire as the conductor. Leave it at room temperature for a week and polish the CPE electrode to a mirror surface on the weighing paper before use. In the buffer solution of PBS (pH = 7), the electrochemical activity of the Tb-MOF/CPE was investigated by cyclic voltammetry (CV, sweep rate of 0.02 mV/s) to detect the nitro-explosive compound of 2,4,6-trinitrophenylamine (TPA) in water with different concentrations (c = 0.1–0.8 mM) (Figure ). It can be seen that there is a sharp reduction peak of TPA at −0.8 V compared with the Tb-MOF bare electrode. At the TPA concentration of 0.1 mM, the current change of the electrode increases by 10 μA, indicating that this material has good electrochemical activity. On the other hand, as shown in the inset of Figure , with the increase in the concentration of TPA, the current response increases gradually. A good linear relationship is obtained with the equation of y = −2.733x – 6.142 (R2 = 0.9987). These results indicate that the Tb-MOF has a very broad application prospect in the field of electrochemical sensing. With the slope of this fitting line and the measurement error of the current intensity with blank samples, the detection limit for TNA was calculated to be 1.24 μM (3σ/k). In addition, the carbon paste electrode has the advantages of simple fabrication and easy updating of the electrode surface, which can be further studied to establish a highly selective electrochemical sensor for the detection of TPA compounds.

Figure 9

Tb-MOF/CPE by cyclic voltammetry-detected TNP.

Conclusions

A three-dimensional porous multifunctional Tb-MOF fluorescent probe has been successfully constructed for sensing small molecules, metal ions, and inorganic anions. The structural feature exhibits two kinds of inorganic building units: binuclear and tetranuclear cubic clusters and high coordination organic building units of flexible μ7-DBA ligands. Fluorescence analysis shows that the Tb-MOF could act as a fluorescent probe for highly sensitive induced aqueous solutions of Fe3+ (10–9 M), Cr2O72– (10–9 M), and H2O2 molecules (10–7 M) as well. In addition, the electrochemical performance of the Tb-MOF/CPE detection of 2,4,6-trinitrophenylamine (TPA) (1.24 μM) shows that the Tb-MOF also had good electrochemical activity (10 μA/0.1 mM) to detect TPA. In a word, the Tb-MOF is a multifunctional fluorescent detection probe for Fe3+ ions, Cr2O72–, and H2O2 molecules and the electrochemical sensing of TPA, which makes it potentially useful for analyzing these pollutants in wastewater.

Experimental Section

Materials and Methods

All reagents are commercially purchased without further purification. Single-crystal data of the Tb-MOF were collected on a SHIMADZU XRD-7000 X-ray diffractometer on a Bruker D8 Venture system with Mo Kα radiation (λ = 0.71073 Å) at 150 K. Fluorescence spectra has been recorded by a fluorescence spectrophotometer (Hitachi F-7000). Infrared spectra have been recorded by an FT-IR spectrometer (Nicolet Avatar 3600) from KBr pellets in the range of 4000–400 cm–1. Elemental analyses (C, H, and N) were performed using a Vario EL elemental analyzer. Electrochemical properties were performed by a CHI660D electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd.). Thermal gravimetric analysis (TGA) was performed by a Netzsch TG-209.

Synthesis of the Tb-MOF

A mixture of 3,5-di(2′,4′-dicarboxylphenyl) benzoic acid (H5L) (0.075 mmol), Tb(NO3)3·6H2O (0.05 mmol), and DMF (1 mL), water (4 mL), and NaOH (0.1 moL/L, 5D) was placed in a Teflon-lined stainless steel autoclave and heated at 160 °C for 72 h then dropped until 30 °C with the rate of 4 °C·h–1. After being filtrated and washed with H2O, some colorless bulk crystals (Tb-MOF) were obtained and dried in air (83% yield based on the Tb element). Anal. Calcd for C138H76O73Tb10 (%): C, 36.88; H, 1.69. Found (%): C, 36.90; H, 1.68. IR (KBr, cm–1) (Figure S1): 3405 (m), 1621 (s), 1605 (s), 1557 (s), 1493 (m), 1441 (s), 1392 (s), 1095 (w), 1019 (w), 912 (w), 823 (w), 787 (m), 755 (m), 695 (m), 667 (w).

X-ray Crystallography

X-ray crystallographic treatment of the Tb-MOF in the same literature,[17] crystallographic data and structure refinements, and the selected bond lengths and bond angles for the Tb-MOF are summarized in Tables S1 and S2.