Stable Lanthanide–Organic Framework Materials Constructed by a

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Stable Lanthanide−Organic Framework Materials Constructed by a Triazolyl Carboxylate Ligand: Multifunction Detection and White Luminescence Tuning Yu Wang,†,§ Shang-Hua Xing,‡,§ Feng-Ying Bai,*,† Yong-Heng Xing,† and Li-Xian Sun⊥ †

College of Chemistry and Chemical Engineering, Liaoning Normal University, Huanghe Road 850#, Dalian 116029, P. R. China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P. R. China ⊥ Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, P. R. China Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/01/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: Under hydrothermal conditions, we have successfully synthesized six isostructural lanthanide coordination polymers, [LnL1.5(H2O)2]·1.75H2O (1−6; Ln = Eu, La, Pr, Nd, Sm, Gd), by the reaction of 5-methyl-1-(4-carboxylphenyl)-1H-1,2,3-triazole-4carboxylic acid (H2L) and Ln(NO3)3·6H2O. Structural analysis shows that polymers 1−6 show novel three-dimensional supramolecular network structures. The luminescent properties for polymer 1 have been investigated at room temperature. The results have shown that polymer 1 can be used as a chemical sensor for multifunctional testing such as UO22+, Fe3+ ion detection, and small organic molecule detection because of its strong fluorescence properties. In particular, polymer 1 exhibits extremely high selectivity and sensitivity for the detection of Fe3+ ions. In addition, white-light emission is achieved through a reasonable tuning proportion by mixing Gd3+ and Eu3+.

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INTRODUCTION With the improvement of people’s living standards, governments and scientists would like to fund and pay attention to environmental issues because they are harmful to human health and environmental sustainability.1−6 Current commercial methods and technology-based instruments used for pollution detection are costly and are not always readily available. Hence, there is an urgent need to find new molecular materials for the development of novel treatment methods that are highly efficient and can easily be applied to various pollutants. Nowadays, metal−organic frameworks (MOFs) are rapidly emerging and being developed because of their unique structural characteristics and attractive application prospects.7−11 MOFs are a family of crystalline porous materials with metal centers and organic linkers, with inherent advantages of ordered and designable structures12,13 and wide applications.14−16 Therefore, it is vital to rationally select organic ligands and create secondary building units for building MOFs with the required functions and properties,17,18 even if directed synthesis is still a challenge.19 In particular, lanthanide coordination polymers have provoked great interest. Lanthanide-based MOF (Ln-MOF) materials have an unusual and interesting porous crystalline structure20,21 and luminous characteristics22 because of their coordination number and the flexible variability of coordination modes of rare-earth ions. Also, the introduction of the organic © XXXX American Chemical Society

poly(carboxylic acid) ligands as linkers will change the electronic characteristics. It is worth mentioning that organic poly(carboxylic acid) ligands can exert an irreplaceable influence in the design and synthesis of lanthanide coordination polymers.23,24 In the process of constructing the lanthanide coordination polymers, different types of the selected organic poly(carboxylic acid) ligands can result in a diversity of structures of the target product, which, in turn, can lead to differences in their application performance.25 Although different coordination modes can affect the function of the polymers, the effect of the ligands on the polymer function is a particularly key action. Usually poly(carboxylic acid) ligands can be divided into two broad categories according to their different configurations: rigid and flexible poly(carboxylic acid) ligands. Flexible poly(carboxylic acid) ligands generally act to further reinforce and modify the overall structure of the crystal in the crystal structure. Common flexible poly(carboxylic acid) ligands are aliphatic poly(carboxylic acid)s, including oxalic acid,26,27 glutaric acid,26 adipate acid, and maleic acid.27 Rigid poly(carboxylic acid) ligands generally act as framework supports in the crystal structure, and it is easy to form a large and stable conjugated system so that the intensity and efficiency of the Received: July 20, 2018

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DOI: 10.1021/acs.inorgchem.8b02050 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

and characterized. Their photophysical properties and fluorescence characteristics were also investigated.

fluorescence emission of the coordination polymer can be enhanced. This feature is particularly important for fluorescence detection. Furthermore, the rigid poly(carboxylic acid) ligands include conjugated benzene ring ligands and conjugated heterocyclic ligands. Conjugated benzene ring ligands are mainly divided into the following categories: (1) binary rigid poly(carboxylic acid)s such as phthalic acid, isophthalic acid, and terephthalic acid; (2) ternary rigid poly(carboxylic acid)s such as trimesic acid;28,29 (3) quaternary rigid poly(carboxylic acid)s such as pyromellitic acid.30 Conjugated heterocyclic ligands are primarily nitrogen- or sulfur-containing ligands. Nitrogen heterocycles and their derivatives play a crucial role in all heterocyclic compounds and are widespread in the synthesis of many compounds as well as in nature.31,32 Of these, triazole and tetrazole as well as their derivatives have received the most attention because of their good application in many fields such as bioactivity, fluorescence, electrochemistry, and medicine. Some polymers containing triazole ligands have been reported, such as H2L-Cd, H2L-Co, H2L-Zn, H2L-Ni, and H2L-Cu33 (H2L = 5-methyl-1-(4-carboxylphenyl)-1H-1,2,3-triazole-4-carboxylic acid). It is worth noting that the centers of these polymers are mostly transition metals. The lanthanide coordination polymers based on the H2L ligand have not been widely explored. The H2L ligand has six binding sites and flexible and various connections, wherein the oxygen atoms of the carboxylic group could coordinate to the lanthanide ions with monodentate or bidentate coordination modes or chelate Ln3+ by N atoms from the triazole group. However, there are few polymers with triazolyl carboxylate ligands reported so far. The introduction of stable nitrogen heterocyclic units in coordination polymer systems could be advantageous for stability and also in terms of electronic state regulation of the polymers. Taking advantage of their good p-electron mobility, crystallizing coordination polymers have been applied in fluorescence sensing of organic compounds and metal ions and other applications. Because the presence of nitrogen-rich heterocyclic moieties in MOF networks significantly improves their molecular binding affinity and enhances the interaction between the polymer and detected molecules, Ln-MOFs possess high molecular recognition and capacities at ambient pressure. Recently, luminescent MOFs that contains nitrogen heterocyclic moieties have attracted great attention of many scientists because their rich and varied light-emitting properties for the detection of target analysts without complicated sample preparation.34−37 Therefore, it is considered to be one of the most promising technologies in biological and chemical detection applications. In particular, fluorescent Ln-MOFs containing nitrogen heterocycles are widely used in luminescent sensing because of their unique photophysical properties. Herein, assembly of the H2L ligand and Ln3+ ions produced three-dimensional (3D) supramolecular network MOF porous materials by linking alternatively between Ln3+ and the ligand moiety as well as hydrogen bonding, which includes Lewis basic triazole motifs as well as exposed Lewis acidic Ln3+ ions. In addition, the supramolecular network structures show good stability and realize effective recognition of metal ions, and it was found that light-emitting-diode fluorescence is tuned first by reasonable doping ions under ambient conditions. In this contribution, six new 3D supramolecular network structure LnMOFs based on H2L ligands, [LnL1.5(H2O)2]·1.75H2O [Ln = Eu (1), La (2), Pr (3), Nd (4), Sm (5), Gd (6)], were prepared

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EXPERIMENTAL SECTION

Materials and Methods. All chemicals purchased were of reagent grade or better and were used without further purification. Caution! Uranyl nitrate used in the study possessed radioactive activity, so standard precautions were taken for handling radioactive materials, and all studies were conducted in a laboratory dedicated to studies of actinide elements! The preparation of the starting materials lanthanide nitrate salts and all detailed measurement parameters and methods are listed in the Supporting Information. Crystal details of the data collection and structure refinement are given in Table S1. Because the crystal quality of polymer 6 was not good, no crystal data suitable for the measurement of X-rays in the single-crystal structure was obtained. However, polymer 6 was characterized by elemental analysis, IR, and X-ray diffraction (XRD) to prove that its structure is consistent with the structures of polymers 1−5. The main bond lengths and hydrogen bond lengths and angles of polymers 1−5 are given in Tables S2 and S3. Synthesis of 5-Methyl-1-(4-carboxylphenyl)-1H-1,2,3-triazole-4-carboxylic Acid (H2L). The ligand H2L was successfully synthesized according to the modified method in the literature.33,38 The synthetic route of the ligand H2L is shown in Scheme 1. p-

Scheme 1. Synthetic Route of the Ligand H2L

Aminobenzoic acid (6.86 g, 0.05 mol) was dissolved in a concentrated hydrochloric acid solution, and the resulting solution was then cooled to 0 °C. Sodium nitrite (3.45 g, 0.05 mol) was dissolved in 10 mL of water (H2O), and the resulting solution was added to the above reaction system with stirring at 0 °C. After filtration, sodium azide (3.25 g, 0.05 mol) dissolved in 10 mL of H2O was added to the solution, and stirring was continued for 2 h. The white solid (M) was obtained by vacuum filtration. A total of 0.30 g of metal sodium (0.30 g, 0.01 mol) was added to 10 mL of cold anhydrous ethanol (EtOH). The resulting solution was mixed with ethyl acetoacetate (1.31 g, 0.01 mol), and then the intermediate M (1.63 g, 0.01 mol) was added. After stirring for 30 min at 0 °C, the mixture was heated and refluxed to completely dissolve the precipitate. The pale-yellow product was precipitated by adjusting the pH to 5−6. The H2L (2.05 g) was obtained with a yield of 83.02% after filtration under reduced pressure and purification by recrystallization. Anal. Calcd for C11H9N3O4 (247.21): C, 53.44; H, 3.67; N, 17.00. Found: C, 53.72; H, 3.43; N, 17.03. IR data (KBr, cm−1): 3420, 2993, 2880, 1696, 1511, 1116, 1179, 1064, 860. 1H NMR (400 MHz, DMSO-d6): δ 13.285 (s, 2H), 8.19 (d, J = 8.53 Hz, 2H), 7.80 (d, J = 8.66 Hz, 2H), 2.57 (s, 3H). 13C NMR (101 MHz, DMSO-d6): δ 166.8, 162.9, 139.6, 139.0, 137.3, 132.5, 131.1, 125.9, 125.9, 10.3. ESI-QTOF-HRMS ([C11H9N3O4Na(M + Na)]+). Calcd: m/z 270.0491. Found: m/z 270.0486. The 1H and 13C NMR and high-resolution mass spectrometry (HRMS) spectra of the ligand H2L are shown in Figures S1−S3. Preparation of the Polymers. Polymer 1 was prepared by the reaction of Eu(NO3)3·6H2O (0.1 mmol, 0.043 g) and H2L (0.05 mmol, 0.01 g) dissolved in a mixed solution of H2O (2 mL) and EtOH (5 mL). The pH value of the solution was adjusted to 3−4 by nitric acid (3.8 mol/L). After being stirred for 2 h, the resulting solution was sealed in a 23 mL Teflon-lined stainless steel autoclave. It was heated at 100 °C for 4 days under self-generated pressure and then cooled to B

DOI: 10.1021/acs.inorgchem.8b02050 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry room temperature with a rate of 2 °C/h under ambient conditions. After washing with EtOH several times, colorless crystals were obtained and collected. The synthetic method of polymers 2−6 was similar to that of polymer 1. Detailed analysis data are as follows. [EuL1.5(H2O)2]·1.75H2O (1). Yield: 50% (based on H2L). Anal. Calcd for C16.5H14.5N4.5O10.25Eu (591.79): C, 33.45; H, 2.47; N, 10.65. Found: C, 33.34; H, 2.56; N, 10.42. IR data (KBr, cm−1): 3404, 2925, 2854, 1605, 1575, 1424, 1399, 1314, 1243, 1176, 864. [LaL1.5(H2O)2]·1.75H2O (2). Yield: 60% (based on H2L). Anal. Calcd for C16.5H14.5N4.5O10.25La (578.74): C, 34.24; H, 2.53; N, 10.89. Found: C, 34.44; H, 2.63; N, 10.88. IR data (KBr, cm−1): 3406, 2924, 2852, 1605, 1573, 1423, 1398, 1314, 1243, 1176, 863. [PrL1.5(H2O)2]·1.75H2O (3). Yield: 55% (based on H2L). Anal. Calcd for C16.5H14.5N4.5O10.25Pr (580.74): C, 34.13; H, 2.52; N, 10.85. Found: C, 34.28; H, 2.68; N, 10.75. IR data (KBr, cm−1): 3412, 2922, 2849, 1605, 1576, 1424, 1400, 1313, 1243, 1176, 865. [NdL1.5(H2O)2]·1.75H2O (4). Yield: 55% (based on H2L). Anal. Calcd for C16.5H14.5N4.5O10.25Nd (584.07): C, 33.93; H, 2.50; N, 10.79. Found: C, 33.88; H, 2.66; N, 10.89. IR data (KBr, cm−1): 3423, 2923, 2858, 1605, 1575, 1424, 1399, 1314, 1240, 1177, 864. [SmL1.5(H2O)2]·1.75H2O (5). Yield: 45% (based on H2L). Anal. Calcd for C16.5H14.5N4.5O10.25Sm (590.18): C, 33.58; H, 2.48; N, 10.68. Found: C, 33.68; H, 2.55; N, 10.67. IR data (KBr, cm−1): 3415, 2926, 2864, 1603, 1574, 1424, 1398, 1312, 1241, 1178, 864. [GdL1.5(H2O)2]·1.75H2O (6). Yield: 50% (based on H2L). Anal. Calcd for C16.5H14.5N4.5O10.25Gd (597.07): C, 33.58; H, 2.48; N, 10.68. Found: C, 33.62; H, 2.52; N, 10.67. IR data (KBr, cm−1): 3405, 2924, 2854, 1605, 1575, 1425, 1397, 1315, 1241, 1176, 865. Preparation of x%Eu3+@6. The synthesis method of multirare mixed MOFs samples x%Eu3+@6 is consistent with that of polymer 6. It is only necessary to add Eu(NO3)3·6H2O and Gd(NO3)3·6H2O to the reaction as the starting material Ln(NO 3 )3 ·6H 2 O in a stoichiometric ratio. The concentration of the rare-earth-ion Eu3+ replacement is from 0.5 to 10 mmol %. The corresponding multirare mixed MOF samples x%Eu3+@6 can be obtained after the same reaction time.

Figure 1. (a) Connections of the H2L ligand. (b) Coordination environment of the central metal. (c) 2D network structure. (d) 3D supramolecular network structure.

metal Eu has two coordination environments. As depicted in Figure 1b, one is nine-coordinated by five oxygen atoms (O1, O2#1, O3#2, O4#2, and O5), two nitrogen atoms (N3 and N6) from the H2L ligands, and two oxygen atoms (O9 and O10) from coordinated water molecules to form a distorted tricapped trigonal-prismatic geometry (Figure S4a). The other is ninecoordinated by six oxygen atoms (O1, O2#1, O3#2, O4#2, O5, and O7), one nitrogen atom (N3) from the H2L ligands, and two oxygen atoms (O9 and O10) from coordinated water molecules to form a distorted tricapped trigonal-prismatic geometry (Figure S4b). The Eu−O (from H2L) distances range from 2.381(4) to 2.56(2) Å, and the Eu−N (from H2L ligand) bond lengths are within the scope of 2.480(17)−2.719(5) Å, with 2.425(5)−2.453(5) Å for the Eu−Owater distance, all of which are similar to that in the reported Eu−O donor polymers.39 In order to thoroughly understand the structure framework and molecular constitution, it is important to investigate the connections by which metal centers and carboxylate ligands are attached. In polymer 1, as depicted in Figure S5, the building blocks [EuO7N2] and [EuO8N] of polymer 1 are linked into a one-dimensional (1D) double-chain structure by the H2L ligand. The 1D double chain is linked into a two-dimensional (2D) network structure by the C4 atom of the carboxylate groups (Figure 1c). 2D network structure is linked by hydrogen-bonding O(9)−H(9A)···O(1)#7 (#7, −x + 1/2, y + 1 /2, −z + 1/2) into a 3D supramolecular network structure (Figure 1d). In addition, it was found that there are two kinds of pores in the structure. The pore size is 13.85 × 13.05 Å2 for a small pore and 23.6 × 12.1 Å2 for a large pore (Figure S6). IR Spectra. The IR spectra of the H2L ligand and polymers 16 are shown in Figure S7. The IR spectra of polymers 1−6 are similar to each other. For polymer 1, a broad absorption band appearing at 3404 cm−1 should be attributed to the stretching vibrations of the unassociated O−H in the water molecules. The characteristic vibration modes of νC−H of the −CH3 group appear at 2925 and 2854 cm−1, which are weak absorption bands. The peaks at 1604 and 1398 cm−1 are the characteristic absorptions for the asymmetric (νasCOO−) and symmetric stretching (νsCOO−) vibrations of of CO bond. The peak at 1549 cm−1 is the characteristic stretching vibration of νCN. The peak at 1098 cm−1 belongs to the stretching vibration of νC−N, and the peak at 1032 cm−1 belongs to the stretching vibration of νC−O. A single absorption band at 864 cm−1 could

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RESULTS AND DISCUSSION Synthesis, Structure, and Spectral Characteristics. Synthesis. In the reaction system, it is particularly important to find the optimal reaction conditions for the H2L ligand and rare-earth ions. We attempted to change the pH of the reaction system. After doing experiments several times in which we only adjusted the pH of the reaction system to weak acid (3−4) and the other reaction factors remained constant, well-formed single crystals Ln-H2L (Ln = Eu, La, Pr, Nd, Sm, Gd) with the desired yields were obtained. In addition, the effect of temperature change on the reaction system was studied. We experimented with reaction temperatures of 100, 120, 140, and 160 °C, respectively. These experimental results show that the best single crystals for suitable single-crystal XRD can only be achieved at 100 °C. The effect of the molar ratio of metal to ligand on the experiment was also studied. The results show that the molar ratio has little effect on the morphology of the crystals. The above experimental results indicated that the pH and reaction temperature of the reaction system may be the main factors affecting the desired polymer. Structural Description. Single-crystal X-ray structural analysis demonstrated that polymers 1−5 crystallized in a monoclinic system with space group C2/c. Because polymers 1−5 have the same structural features, here we chose polymer 1 as an example to discuss in detail. In the structure of polymer 1, there are two types of connections for the H2L ligand: one is the μ2-η1Oη1Oη1Oη1N terdentate chelation mode and the other is the μ2-η1Oη1Oη1N tridentate chelation mode (Figure 1a). Because of the crystallographic symmetry disorder of the ligand, the central C

DOI: 10.1021/acs.inorgchem.8b02050 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Detailed Attributes of the IR Data of H2L and Polymers 1−6 (cm−1) νO−H νC−H νas (COO)− νs (COO)− νCN νC−N νC−C νC−O δ(Ar−H)

H2L

1

2

3

4

5

6

3420 2993, 2880 1696 1511 1570 1116 1179 1064 860

3404 2925, 2854 1604 1398 1549 1098 1177 1032 864

3410 2924, 2849 1603 1396 1546 1095 1178 1031 864

3416 2925, 2854 1604 1400 1544 1095 1176 1045 864

3419 2925, 2856 1603 1398 1548 1092 1175 1046 864

3418 2926, 2871 1605 1396 1545 1095 1177 1041 865

3405 2924, 2854 1605 1397 1547 1095 1176 1036 865

There are also two strong high-energy absorption bands at 267 and 389 nm for polymer 1, which are similar to the peaks of the H2L ligand. Therefore, the absorption bands of the polymer are mainly derived from the H2L ligand. The f−f transition of the Eu3+ ion in polymer 1 is a spin-forbidden transition, so no f−f absorption peak appears in the UV−vis spectrum of the polymer. Fluorescence Behavior and Sensing Properties. Fluorescence Behavior. The excitation and emission spectra of polymer 1 are shown in Figure 3. The widths of the slit for

be attributed to the bending vibration of the benzene ring. Compared with the ligand, the characteristic peak of the stretching vibration of νas(COO)− is shifted from 1696 to 1604 cm−1. The significant red shift indicates that the metal ion is coordinated by the ligand H2L. Detailed attributes of the IR data of H2L and polymers 1−6 are given in Table 1. Thermogravimetric Analysis (TGA). To verify the thermal stability of the coordination polymer, TGA was performed at a heating rate of 10 °C/min under a N2 atmosphere within the temperature range from 30 to 800 °C. The TGA curves of polymers 1−5 are shown in Figure S8. The TGA curves of the polymers are similar to each other. For polymer 1, the first weight loss is displayed from 30 to 110 °C, which is consistent with the loss of 1.75 lattice water molecules (obsd 5.44%; calcd 5.33%). The second weight loss is displayed at 110 °C and completed at 200 °C, which can be due to the loss of two coordinated water molecules (obsd 6.12%; calcd 6.09%). The stage from 200 to 800 °C is a slow decomposition process, corresponding to the collapse of the polymer skeleton. Powder XRD (PXRD). In order to confirm that the phase of the polymer is pure, the PXRD patterns of the coordination polymer 1 were recorded, as shown in Figure 2. Because all of

Figure 3. Photoluminescence excitation and emission spectra for polymer 1 in the solid state at room temperature.

measuring the excitation and emission spectra are 1:0.5 and 1:1.5 nm, respectively. Under the monitoring of 615 nm, the excitation spectra of polymer 1 include a wide band at 373−384 nm attributed to the π−π* electron transition of an organic ligand. Several narrow bands at 393, 463, and 531 nm are attributed to energy-level transitions of the Eu3+ ion. This shows that the excitation spectra of polymer 1 include both the π−π* transition of an organic ligand and the strong transition of the energy level of the Eu3+ ion. The luminescent emission spectrum of polymer 1 was considered with an excitation wavelength of 390 nm. On the basis of the above analysis, for corresponding excitation spectra of polymer 1, it is found that the contribution of emission spectra about the polymer is not only from the electronic transition of the Eu3+ ion itself but also partly from the organic ligand “antenna effect” transmission. In the emission spectra, the characteristic 5D0 → 7FJ (J = 1−4) transition of Eu3+ at about 592, 615, 640, and 694 nm reveals effective energy transfer between the ligand and Eu3+ ion. The quite weak emission band 5D0 → 7F0 in the position of 578 nm is due to the symmetry-forbidden transition of the europium ion in the polymer. A particularly weak emission band appearing at 640 nm belongs to the transition of 5D0 → 7F3. The 5D0 → 7F1 emission band is the prominent magnetic-dipole

Figure 2. PXRD spectra of polymers 1−6.

the peaks in the measurement patterns match the simulated patterns generated by the single-crystal XRD data, it is shown that polymer 1 is in the pure phase. Polymers 2−6 are also in the pure phase. In addition, the stability of polymer 1 in H2O and EtOH was also confirmed by PXRD. The PXRD pattern indicated that polymer 1 can maintain its crystallinity after soaking in H2O or EtOH for 48 h. UV−Vis Spectra. The UV−vis absorption spectra of the ligand H2L and polymer 1 (Figure S9) were recorded in the form of solid samples. There are two strong high-energy absorption bands at 276 and 369 nm for H2L, which could be ascribed to the π−π* and n−π* transitions of the H2L ligand. D

DOI: 10.1021/acs.inorgchem.8b02050 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry transition, and it is almost uninfluenced by the coordination environment. Also, the outstanding 5D0 → 7F2 emission bands have strong electric-dipole character, which is related to the coordination environment. Herein luminescence concerning an europium polymer can act as a sensitive probe with the lanthanide coordination environment. The intensity ratio of the 5 D0 → 7F2 and 5D0 → 7F1 transitions is very sensitive to the surrounding environment of the Eu3+ center. In the emission spectrum of polymer 1, it can be seen that the intensity of the transition 5D0 → 7F2 is obviously much stronger than that of the transition 5D0 → 7F1. This implies that the Eu3+ ion of polymer 1 is in a lower symmetric coordination environment. This also can be verified by the results of single-crystal XRD data analysis. The 5D0 → 7F2 transition is the most intense emission with red luminescence. For a further study on the photophysical behaviors of polymer 1, the fluorescent lifetime values for polymer 1 were measured at room temperature from the respective luminescent decay profiles by fitting with monoexponential decay curves, and the correlation values are given in Table S4. As can be seen from Figure 4, the characteristic emission of the Eu3+ ion shows

Figure 5. (a) Comparison of the luminescence intensity of polymer 1 interacting with different metal ions under the same conditions. (b) Fluorescent quenching percentage of different metal ions (inset: photographs taken under UV-light excitation).

adding a Fe3+ ion aqueous solution to polymer 1 dispersed in EtOH. As shown in Figure 6, with a gradual increase in the

Figure 4. Luminescence lifetime curve of polymer 1.

a single-exponential decay process, implying the presence of a single chemical environment around the Eu3+ ion. The fluorescent lifetime of polymer 1 is similar to those of some common Eu3+ polymers, and a comparison of their photoluminescence properties (lifetime and quantum yield) is shown in Table S4. Fluorescence Detection Applications. Detection of Metal Ions. In order to examine the ability of polymer 1 to detect trace metal ions, a powder sample of 1 (20 mg) was soaked in 50 mL of EtOH and ultrasonicated for 40 min to make up a suspension of polymer 1. Then, 3 mL of the MOF suspension was added to a cuvette. Next, 160 μL each of 10 mM aqueous solutions of 15 kinds of metal ions such as Ag+, Al3+, Ba2+, Ca2+, Cd2+, Co2+, Cr2+, Cu2+, K+, Mg2+, Na+, Ni2+, Pb2+, Zn2+, and Fe3+ (as a Cl− or NO3− salt aqueous solution) were added to make up the metal-ion-included MOF-M n+ suspensions, which were tested under the same conditions. The fluorescence intensities of these suspensions were recorded at room temperature and compared. Interestingly, as shown in Figure 5, their fluorescence intensity was found to depend to a large extent on the type of metal ion. Fe3+ ions have a particularly obvious fluorescence quenching effect. Ag+, Al3+, Ca2+, and K+ show moderate quenching, while the other metal ions have a negligible effect on the fluorescence. Further experiments of the sensitivity were conducted by gradually

Figure 6. Change of the fluorescent spectra of polymer 1 dispersed in EtOH upon the gradual addition of a Fe3+ aqueous solution.

concentration of the Fe3+ aqueous solution, the emission intensity of polymer 1 at 615 nm gradually decreases. The quenching percentage of polymer 1 toward Fe3+ ions was evaluated to be 91.59%. The fluorescent quenching efficiency can be explained by the Stern−Volmer equation I0/I = 1 + Ksv[Q], in which I0 and I are the fluorescent intensities of the MOF suspensions without and with Fe3+, Ksv is the quenching constant (M−1), and [Q] represents the molar concentration of Fe3+ ions (μM). As depicted in Figure S10, there is a good E

DOI: 10.1021/acs.inorgchem.8b02050 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry linear relationship (R 2= 0.997) between I0/I and the concentration of Fe3+ ions. The Ksv value of polymer 1 for Fe3+ ions was calculated to be 1.30 × 104 M−1. Compared with the Ksv values of the previously reported lanthanide-based luminescent sensors, the Ksv value of polymer 1 is higher than those of [Tb(L)(DMA)]·DMA·0.5H2O (where L = L3−, H3L = 3′-hydroxybiphenyl-3,4′,5-tricarboxylic acid; Ksv = 1.91 × 103 M−1)40 and [Eu(IMS1)2]Cl·4H2O (where IMS1 = 1,3-bis(4carboxylbenzyl)-imidazolium; Ksv = 5.87 × 103 M−1).41 According to the detection limit formula 3σ/Ksv (σ is the standard deviation of the fluorescence intensity of the blank solution), the detection limit of polymer 1 toward Fe3+ ions was calculated to be 3.45 μM, which is much lower than that of [Eu(IMS1)2]Cl·4H2O (2.3 × 10−5 mol/L).41 In order to test the effect of other metal ions on the detection of Fe3+, interference experiments were performed. A total of 160 μL each of 10 mM aqueous solutions of different metals were separately added to the EtOH suspension of polymer 1, and then 160 μL of an aqueous solution of Fe3+ was added. It was interesting to note that the fluorescence intensity was significantly quenched after the addition of Fe3+ ions (Figure 7). The experimental results showed that the presence of other competitive metal ions does not affect the detection of Fe3+ by polymer 1. Detection of UO22+. Uranium is an important natural radioactive element.42 Uranium radioactive and heavy-metal toxicity can lead to irreversible damage to biological systems.43,44 Therefore, it is vital and urgent to develop an effective method for detecting UO22+ ions in aqueous systems.

To explore the detection ability of polymer 1 toward UO22+, the emission spectra of polymer 1 toward the detection of UO22+ were also recorded. The solution preparation method is similar to the method for detecting metal ions to make a steady suspension (Figure 8). Then, 3 mL of the MOF suspension was

Figure 8. Change of the fluorescent spectra of polymer 1 dispersed in EtOH upon the gradual addition of an UO22+ aqueous solution.

added to a cuvette. A total of 5−320 μL of a UO22+ aqueous solution at a concentration of 10 mM was added to the system of a dispersion of polymer 1 in EtOH, respectively, to form a MOF−UO22+ suspension at different concentrations for fluorescence sensing studies. Also, the fluorescence intensity was measured and compared under the same conditions. We can observe that as the concentration of the UO22+ solution increased, the fluorescence intensity was significantly quenched. Interestingly, that is to say their fluorescent intensities are highly dominated by the concentrations of the uranium solution. The quenching percentage of polymer 1 toward UO22+ was evaluated to be 82.51%. According to the Stern− Volmer equation I0/I = 1 + Ksv[Q], the Ksv value of polymer 1 for UO22+ ions was calculated to be 3.71 × 103 M−1 (Figure S11), which is lower than those of the previously reported lanthanide-based luminescent sensors.42 Therefore, the effect of polymer 1 detection of UO22+ needs to be improved, and we need to study this further. Detection of Small Organic Molecules. The potential application of polymer 1 in the detection of small organic molecules (phenol, resorcinol, and catechol) was further discussed based on the outstanding luminescence properties of polymer 1. The solution preparation method is similar to the method for detecting metal ions. Then, 3 mL of the MOF suspension was taken and 20−320 μL of small organic molecules (phenol, resorcinol, and catechol) in aqueous solution was added at a concentration of 10 mM to the system of dispersion of polymer 1 in EtOH, respectively, to form suspensions at different concentrations for fluorescence-sensing studies. Fluorescence emission spectra of this suspension were recorded at an excitation wavelength of 390 nm. As shown in Figure 9, the fluorescence intensity of polymer 1 in the suspension gradually diminishes at 614 nm as the small organic molecule (phenol, resorcinol, and catechol) concentration is increased. The quenching efficiency of polymer 1 toward small organic molecules (phenol, resorcinol, and catechol) was evaluated as follows: phenol, 72.34%; resorcinol, 74.24%; catechol, 70.06%. The Ksv values of polymer 1 for the detection of small organic molecules (phenol, resorcinol, and catechol) were evaluated according to the Stern−Volmer equation I0/I =

Figure 7. (a) Change of the fluorescent spectra of polymer 1 interacting with different metal ions with and without Fe3+ ions. (b) Change of the luminescence intensities at 615 nm upon the addition of different metal ions. F

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305 μs after Fe3+ ion incorporation (Table S4 and Figure 10), which indicates that energy transfer from the ligand to the Eu3+

Figure 10. Change of the lifetime curves before and after detection.

center has been reduced. The change of the lifetime can confirm that the process is dynamic quenching.45,46 The fluorescence lifetime of the MOF samples after the addition of metal ions is shorter than that of the MOF samples without metal ions. This indicates that a dynamic quenching process occurred. In the same way, the fluorescent lifetime of polymer 1 is shortened from 325 to 309 μs after UO22+ ion incorporation. As shown in Figure S14, the UV−vis absorption spectra of polymer 1 before and after the addition of Fe3+ ions were also recorded. It was found that the solution only with polymer 1 and a Fe3+ ion showed strong absorption in the position of 200−410 nm. This overlap of the corresponding spectra showed that the excited-state photons were absorbed by Fe3+ ions, so that energy transfer from the ligand to the Eu3+ ions was reduced.47,48 Meanwhile, the quantum yield of polymer 1 is 2.08% at room temperature but almost undetectable after detection. White-Light Emission Regulation. One method for preparing MOFs with white-light emission is to prepare MOFs that can directly emit white light,49,50 and the other is to construct multimetal hybrid white-light-emitting MOFs based on the principle of two primary colors and three primary colors.51−53 White-light emission can be obtained by reasonably matching and combining blue (400−490 nm), green (510−570 nm), and red (600−720 nm) light emission based on the principle of three primary colors for preparing white luminescent materials.54−56 We know that polymer 6 has blue-light emission of the ligand at 460 nm (Figure S15). Also, the organic ligand H2L can transmit energy to the rare-earth ions Eu3+. Therefore, we propose to partly replace nonluminous Gd3+ ions with luminescent Eu3+ ions and introduce red emission centers in the polymer 6 host material, thereby realizing regulation of the luminescent color of the polymer 6 materials and obtaining white-light emission. Figure S16 shows the excitation and emission spectra with a slit width of 2:2 nm for 5%Eu3+@6. Under the monitoring of 614 nm, the excitation spectrum of 5%Eu3+@6 includes a broad band at 374−385 nm attributed to the π−π* electron transition of the organic ligand. Several narrow bands at 393, 463, and 531 nm are attributed to energy-level transitions of the Eu3+ ion. The strong transition of the energy level of the Eu3+ ion is clearly stronger than that of the organic ligand in the excitation spectrum of 5%Eu3+@6. This indicates that the contribution of the Eu3+ ion itself to the emission spectrum is much greater than the transmission of the

Figure 9. Luminescence intensity of polymer 1 interacting with small organic molecules: phenol (a), resorcinol (b), and catechol (c) under the same conditions.

1 + Ksv[Q]: phenol, 2.55 × 103 M−1; resorcinol, 3.81 × 103 M−1; catechol, 2.27 × 103 M−1 (Figure S12). It can be seen that polymer 1 can detect small organic molecules (phenol, resorcinol, and catechol) in an EtOH solvent and has high sensitivity. Therefore, polymer 1 can be used as a potential fluorescence sensor to detect environmental pollutant small organic molecules (phenol, resorcinol, and catechol). Attempting to investigate the mechanism involved in our work, the PXRD spectrum of polymer 1 after detection of the Fe3+ ions was measured. It was found that the PXRD pattern of polymer 1 remained unchanged after soaking in a Fe3+ aqueous solution for 24 h (Figure S13), ruling out collapse of the MOF. Also, the fluorescence lifetime was measured before and after detection. Each of these characteristic emissions exhibits a single-exponential decay process. It was found that the fluorescent lifetime of polymer 1 is shortened from 325 to G

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Inorganic Chemistry “antenna effect” of organic ligands. The luminescent emission spectra of the 5%Eu3+@6 solid were studied at an excitation wavelength of 390 nm. Also, it has characteristic 5D0 → 7FJ (J = 1−4) transitions of Eu3+ at about 592, 614, 640, and 694 nm. The emission spectra have both blue emission of the organic ligand and red emission characteristic of the Eu3+ ion, which also shows that the organic ligand only uses resonance coupling to transfer part of the energy to the Eu3+ ion under the excitation light conditions. Figure 11 shows the emission

coordinates are (0.3358, 0.3474) and (0.3371, 0.3475), and their CIE chromaticity diagrams are shown in Figure 13. They

Figure 13. CIE chromaticity diagrams excited at 390 nm: (a) 2.5% Eu3+@6; (b) 5%Eu3+@6.

are located in the white light emitting region due to the synergistic effect of Eu3+ and the H2L ligand that can be used as common lighting sources. Therefore, this type of rare earth mixed polymers can be applied to potential white light emitting materials.

Figure 11. Emission spectra of x%Eu3+@6 excited at 390 nm.

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spectra with a slit width of 4:4 nm for x%Eu3+@6 in the dualrare-earth MOFs when substituted with different ratios of Eu3+ ions. It can be seen that the characteristic luminescence intensity of the corresponding rare-earth Eu3+ ion in x%Eu3+@6 of the rare-earth MOF samples increases regularly with an increase of the proportion of Eu3+-ion substitution under the excitation of 390 nm light irradiation. At the same time, the intensity of the emission peak of the ligand remains basically unchanged. As shown in Figure 12, the PXRD patterns of the

CONCLUSIONS In summary, six new 3D Ln-MOFs based on the H2L ligand, [LnL1.5(H2O)2]·1.75H2O [Ln = Eu (1), La (2), Pr (3), Nd (4), Sm (5), Gd (6)], were successfully synthesized, which include a highly luminescent europium-based coordination polymer 1. We studied its application in multifunction detection: fluorescence-sensing detection of metal ions, UO22+, and small organic molecules. We found that polymer 1 has good detection capabilities for Fe3+, UO22+, and small organic molecules. It is important that polymer 1 exhibits excellent performance for the detection of Fe3+ ions, even with the existence of the other metal ions. Therefore, polymer 1 can be potential candidates for detecting UO22+ and Fe3+ ion. However, the effect of polymer 1 detection of UO22+ needs to be improved, and the long-term stability and detection repeatability of polymer 1 remain to be further studied. The method for preparing MOF white-light-emitting materials by mixing multiple rare-earth ions provides new ideas for the design and development of new white-light materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02050. Experimental materials, preparation of the starting materials lanthanide nitrate salts, all detailed measurement parameters and methods, crystallographic data of polymers 1−5 (Table S1), selected bond lengths and hydrogen bond lengths and angles of polymers 1−5 (Tables S2 and S3), 1H and 13C NMR and HRMS of the ligand H 2L (Figures S1−S3), distorted tricapped trigonal-prismatic polyhedral structures of Eu (Figure S4), 1D double-chain structure (Figure S5), packing pore structures (Figure S6), IR spectra of the ligand H2L and polymers 1−6 (Figure S7), thermal analysis of polymers

Figure 12. PXRD patterns of the samples x%Eu3+@6.

dual-rare-earth MOFs are consistent with the PXRD patterns of the pure-phase sample 6. This shows that the dual-rare-earth MOF sample has the same structure as the original 6 sample and that the phase of polymer 6 does not change after the replacement of Gd3+ with a small amount of Eu3+. Therefore, a tunable luminescence property can be easily obtained by combining the blue-light emission from the ligand and the redlight emission of Eu3+. Especially, the MOFs of 2.5%Eu3+@6 and 5%Eu3+@6 with near-white-light emission were prepared by reasonably combining the blue-light emission of the ligand and the red-light emission of rare-earth ions. The CIE H

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1−5 (Figure S8), UV−vis spectra of the ligand H2L and polymer 1 (Figure S9), photoluminescence properties (lifetime and quantum yield) of some Eu-MOFs (Table S4), fitting of the SV plots of the detection of Fe3+ (Figure S10), fitting of the SV plots of the detection of UO22+ (Figure S11), fitting of the SV plots of the detection of small organic molecules (Figure S12), PXRD spectra of polymer 1 before and after socking in a Fe3+ solution (Figure S13), UV−vis absorption spectra of polymer 1 before and after the addition of Fe3+ ions (Figure S14), emission spectra of polymer 6 (Figure S15), and excitation and emission spectra of 5%Eu3+@6 (Figure S16) (PDF) Accession Codes

CCDC 1849037−1849041 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.-Y.B.). ORCID

Feng-Ying Bai: 0000-0001-6202-054X Yong-Heng Xing: 0000-0002-7550-2262 Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant 21571091), Commonweal Research Foundation of Liaoning Province in China (Grant 20170055), and Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, P. R. China (Project 151002-K).

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