Multifunctional LnâMOF Luminescent Probe for Efficient Sensing of

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Functional Inorganic Materials and Devices

Multifunctional Ln-MOF Luminescent Probe for Efficient Sensing of Fe3+, Ce3+, and Acetone Qiangsheng ZHang, Jun Wang, Alexander M. Kirillov, Wei Dou, Cong Xu, Cai-Ling Xu, Lizi Yang, Ran Fang, and Wei-Sheng Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06103 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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ACS Applied Materials & Interfaces

Multifunctional Ln-MOF Luminescent Probe for Efficient Sensing of Fe3+, Ce3+, and Acetone Qiangsheng Zhang,a Jun Wang,a Alexander M. Kirillov,b Wei Dou,a Cong Xu,a Cailing Xu,a Lizi Yang,*a Ran Fang*a and Weisheng Liu*a a

State Key Laboratory of Applied Organic Chemistry and Key Laboratory of Nonferrous

Metals Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, P. R. China; b

Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Universidade de

Lisboa, Av. Rovisco Pais, 1049-001, Lisbon, Portugal.

KEYWORDS:

Lanthanide

compounds,

Luminescence

properties,

Metal−organic

frameworks, Metal ions, Photoluminescence, Crystal structures, Sensing experiments

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ABSTRACT New series of five three-dimensional Ln(III) MOFs (metal-organic frameworks) formulated as [Ln4(µ6-L)2(µ-HCOO)(µ3-OH)3(µ3-O)(DMF)2(H2O)4]n {Ln3+ = Tb3+ (1), Eu3+ (2), Gd3+ (3), Dy3+ (4), and Er3+ (5)} was successfully obtained via a solvothermal reaction between the corresponding lanthanide(III) nitrates and phenylpyridinetricarboxylic acid, 2-(5-carboxypyridin-3-yl)terephthalic acid (H3L). All the obtained compounds were fully characterized and their structures were established by singlecrystal X-ray diffraction. All products are isostructural and possess porous 3D networks of the flu topological type, which are driven by the cubane-like [Ln4(µ3-OH)3(µ3-O)(µHCOO)]6+ blocks and µ6-L3– spacers. Luminescent and sensing properties of 1−5 were investigated in detail, revealing a unique capability of Tb-MOF 1 for sensing acetone and metal(III) cations (Fe3+ or Ce3+) with high efficiency and selectivity. Apart from a facile recyclability after sensing experiments, the obtained Tb-MOF material features a remarkable stability in a diversity of environments such as common solvents, aqueous solutions of metal ions, and solutions with a broad pH range from 4 to 11. Besides, compound 1 represents a very rare example of the versatile Ln-MOF probe capable of sensing Ce3+ or Fe3+ cations, or acetone molecules.


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1. INTRODUCTION MOFs constitute a relatively novel class of functional materials with tunable pore size and diversity of structural characteristics and applications1-4 including, in particular, selective sensing of diverse analytes.5-8 Combination of a certain transition metal node with a functional organic linker can allow the design of MOFs with specific functional sites. In particular, coordinatively unsaturated metal nodes, various basic or acidic functionalities, and luminescent building blocks can often be served as functional sites for selective sensing.9-11 Compared with transition metals, lanthanide frameworks (Ln-MOFs) feature outstanding luminescence features, namely substantial Stokes shifts, excellent quantum yields, increased purity of colors, and prolonged lifetime of the emission due to an antenna effect, especially for Tb(III) and Eu(III) derivatitves.12-16 Thus, Ln(III) metalorganic frameworks can be used as remarkable fluorescent probes, also owing to their advanced characteristics such as high stability, sensitivity, and selectivity, as well as reproducibility and quick response times. In general, Ln-MOF-based sensors mainly utilize the π-conjugated organic molecules that provide luminescence, backbones and the Lewis basic sites for binding of analytes.17,18 In recent years, a variety of luminescent LnMOFs was designed and applied for sensing of different types of analytes, including small molecules, metal cations, inorganic and organic anions, solvents, gases, and explosives.19-24 The selective sensing of Fe3+ ions attracted a considerable attention25 given a widespread presence of iron in the environment and biological systems.26 On the other hand, Ce3+ species are poisonous pollutants to eco- and biosystems.27 Some traditional detection

methods

for

metal(III)

cations

include


AAS

(atomic

absorption


spectrophotometry),

ICP-AES

(inductively

coupled

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plasma

atomic

emission

spectrometry), and electrochemistry protocols. However, these can be complicated and expensive in certain cases, especially for detecting Fe3+ or Ce3+ under field conditions.28,29 Ln-MOF luminescent probes for the detection of these metal cations are still relatively scarce, especially with regard to Ce3+ cations. Therefore, the development of new materials as well as simple, reliable, and express protocols for sensing cerium(III) and iron(III) cations in different environments is in high demand. Furthermore, with a rapid development of different industries, the release of toxic organic small molecules causes many environmental and health concerns. Acetone is one the most common industrial and household organic solvents with a variety of adverse effects.30-32 In general, the fluorescence detection of analytes based on luminescent MOFs can proceed via the following mechanisms. (A) Effective overlap of the ligand band with adsorption of analyte.33 (B) LMET (ligand-to-metal energy transfer) between ligands and metal cations. Alternatively, weak interactions between analytes and MOFs (e.g., hydrogen bonds, π–π stacking) can also affect a transfer of energy in UV and visible regions.34 (C) On the basis of the hard-soft acid-base theory, pyridine nitrogen atoms can act as binding sites in the luminescent MOFs.35 Given these observations, the structure of the luminescent Ln-MOFs is very crucial for monitoring the luminescence intensity changes and allowing sensing applications of such materials. Despite a variety of LnMOF sensors reported to date,36,37 less effort was focused on the design of materials containing a rigid organic ligand with a pyridine functionality as a potential sensing site. Such a rigid building block can offer a scaffold to overcome the Ln3+ luminescence quenching by solvents and alter the luminescence behavior. Besides, the porosity and


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well-organized structure of Ln-MOFs can enhance the sensitivity of analyte detection. Thus, the fabrication of new and tailored MOF-based luminescent probes that combine rigid organic ligands and lanthanide(III) nodes represents a perspective research direction. Based on the above considerations, for the present study we selected an unexplored phenylpyridine-tricarboxylate building block, H3L {2-(5-carboxypyridin-3-yl)terephthalic acid}, for preparing new Ln-MOFs. Five isostructural metal-organic frameworks, [Ln4(µ6-L)2(µ-HCOO)(µ3-OH)3(µ3-O)(DMF)2(H2O)4]n {Ln3+ = Tb3+ (1), Eu3+ (2), Gd3+ (3), Dy3+ (4), and Er3+ (5)}, was generated using a solvothermal synthetic method. Structural analysis revealed that all the products exhibit the 4,8-connected binodal 3D nets of the fluorite (flu) topological class; the nets are driven by the cubane-like [Ln4(µ3OH)3(µ3-O)(µ-HCOO)]6+ secondary building units and the phenylpyridine-tricarboxylate spacers. Luminescent behaviors of 1-5 were investigated in detail. We found that L3- may effectively sensitize luminescence of lanthanide ions (Eu3+, Tb3+, Dy3+, and Er3+) with a more pronounced effect in the case of Tb3+. Hence, Tb-MOF 1 was chosen to study in detail the sensing behavior of such MOF materials toward different groups of analytes that include transition metal cations, lanthanide ions, and organic solvent molecules. Compound 1 disclosed a high detection ability to sense Fe3+ or Ce3+ cations, or acetone through the quenching of luminescence; possible mechanism of sensing was also proposed.

2 EXPERIMENTAL 2.1 Materials and Methods. Commercially obtained reagents and solvents were used.



Elementar Vario EL elemental analyzer was utilized to run elemental analyses. Bruker EQUINOX 55 spectrometer was applied to measure FT-IR (Fourier transform infrared) spectra in the 4000-400 cm-1 range. Netzsch STA 449 F3 Jupiter® analyzer was utilized for TGA (Thermogravimetric) analyses; these were carried out under N2 atmosphere. Edinburgh FLS920 fluorescence spectrometer (450 W Xe arc lamp) was used to record steady-state luminescence spectra of solid microcrystalline samples. Agilent Cary 5000 spectrophotometer was utilized to run UV/Vis and steady-state excitation spectra. 2.2 X-ray Crytallography. Agilent SuperNova single-crystal diffractometer (a graphitemonochromated Mo-Ka source; λ = 0.71073 Å) was used to collect X-ray data. SADABS was applied for empirical absorption correction. SHELXS-2014 package was applied for structure solution (direct methods) and refinement (full-matrix least-squares protocols on F2).38 Anisotropic refinement of all atoms (except H) was performed. Bruker D8 Advance X-ray diffractometer was applied to measure PXRD (powder X-ray diffraction) patterns of bulk samples at ambient conditions. 2.3 Synthesis of Compounds 1-5 A similar procedure (solvothermal synthesis) was used to obtain all products. For the preparation of 1, Tb(NO3)2·6H2O (90 mg, 0.2 mmol), H3L (28.8 mg, 0.1 mmol), DMF (4 mL), and H2O (2 mL) were added to a stainless steel Teflon-lined reactor (volume: 25 mL). This was sealed and heated at 130 °C for 72 h, followed by gradual (5 °C/h) cooling down to ambient temperature. After opening the reactor, colorless microcrystals of the product were isolated and dried at ambient temperature to furnish MOF 1. Other compounds were prepared by following a similar protocol, using Eu, Gd, Dy, or Er


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nitrates (Ln(NO3)3·6H2O, 0.2 mmol) in place of terbium(III) nitrate. [Tb4(µ6-L)2(µ-HCOO)(µ3-OH)3(µ3-O)(DMF)2(H2O)4]n·(H2O)4n

(1)：Yield:

69%

relatively to H3L. Calculated (Tb4C35H46N4O28): C, 26.16; H, 2.89; N, 3.49. Found: C, 26.33; H, 3.54; N, 3.25 %. IR (KBr, cm−1): 3435 (vs), 1662 (s), 1575 (s), 1390 (s), 1267 (w), 1093 (w), 1047 (w), 779 (m), 702 (w), 678 (m), 489 (w). [Eu4(µ6-L)2(µ-HCOO)(µ3-OH)3(µ3-O)(DMF)2(H2O)4]n·(H2O)4n

(2)：Yield:

71%

relatively to H3L. Calculated (Eu4C35H46N4O28): C, 26.63; H, 2.94; N 3.54. Found: C, 26.88; H, 3.32; N, 3.17 %. IR (KBr, cm−1): 3435 (vs), 1662 (s), 1593 (s), 1337 (s), 1253 (w), 1093 (w), 1045 (w), 777 (m), 702 (w), 673 (m), 495 (w). [Gd4(µ6-L)2(µ-HCOO)(µ3-OH)3(µ3-O)(DMF)2(H2O)4]n·(H2O)5n

(3)：Yield:

65%

relatively to H3L. Calculated (Gd4C35H48N4O29): C, 25.98; H, 2.99; N, 3.46. Found: C, 26.15; H, 3.56; N, 3.34 %. IR (KBr, cm−1): 3429 (vs), 1658 (s), 1610 (s), 1379 (s), 1253 (w), 1101 (w), 1055 (w), 777 (m), 698 (w), 677 (m), 497 (w). [Dy4(µ6-L)2(µ-HCOO)(µ3-OH)3(µ3-O)(DMF)2(H2O)4]n·(H2O)4n

(4)：Yield:

57%

relatively to H3L. Calculated (Dy4C35H46N4O28): C, 25.93; H, 2.86; N, 3.45. Found: C, 26.38; H, 2.96; N, 3.42 %. IR (KBr, cm−1): 3442 (vs), 1662 (s), 1600 (s), 1377 (s), 1267 (w), 1103 (w), 1055 (w), 777 (m), 702 (w), 680 (m), 493 (w). [Er4(µ6-L)2(µ-HCOO)(µ3-OH)3(µ3-O)(DMF)2(H2O)4]n·(H2O)5n

(5)：Yield:

69%

relatively to H3L. Calculated (Er4C35H48N4O29): C, 25.35; H, 2.92; N, 3.37. Found: C, 25.53; H, 3.47; N, 3.16 %. IR (KBr, cm−1): 3423 (vs), 1664 (s), 1604 (s), 1386 (s), 1260 (w), 1095 (w), 1055 (w), 777 (m), 703 (w), 673 (m), 497 (w).

3 RESULTS AND DISCUSSION



3.1 Structural Description All the obtained products are isostructural (Table 1). The experimental PXRD patterns of 1-5 (Figure S1) are essentially similar and well match those simulated from the singlecrystal CIF files, thus corroborating phase purity and isostructural nature of these MOFs. Structure of MOF 1 is discussed below as a selected example. Structure of 1 (Figure 1) comprises a pair of distinct Tb1 and Tb2 centers, a pair of µ6-L blocks, three µ3-OH and one µ3-O linkers, as well as four terminal H2O ligands per formula unit. Three adjacent µ3-OH groups, one µ3-O and one µ-HCOO linker interconnect four terbium(III) centers (a pair of Tb1 and Tb2 nodes) into the cubane-like [Tb4(µ3-OH)3(µ3-O)(µ-HCOO)(H2O)4]6+ units (Figure 1b), which are then arranged into a complex 3D metal-organic-framework by means of the µ6-L spacers (Figures 1b,c). The eight-coordinate Tb1 ion exhibits a distorted {TbNO7} geometry (Figure S2) which is occupied by a pair of oxygen and one nitrogen donors from three distinct µ6-L blocks, two µ3-OH ligands, one µ3-O ligand, one O donor from µ-HCOO linker, and one terminal H2O moiety. Eight-coordinate Tb2 atom also features a distorted {TbO8} environment taken by three oxygen atoms from three distinct µ6-L spacers, three µ3-OH groups, one O atom from µ-HCOO linker, and two terminal H2O ligands (Figure S2). The Tb–O distances [2.333(14)-2.517(16) Å] are within normal values found for related compounds.39 To get better understanding of a rather complex 3D MOF structure of 1, its simplification and topological analysis was performed taking into consideration the formation of the Tb4 cubane-like cores (secondary building units). These were treated as the 8-connected [Tb4(µ3-OH)3(µ3-O)(µ-HCOO)(H2O)4]6+ cluster nodes (Figure 1d). In


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this case, the µ6-L3− spacers simultaneously bind four of these Tb4 units, thus acting as nodes that are 4-connected. As a result, this net is topologically classified as a 4,8connected binodal 3D framework of the fluorite (flu) topological type. It has the (412.612.84)(46)2 point symbol with the (46) and (412.612.84) notations being those of the L3− and [Tb4(µ3-OH)3(µ3-O)(µ-HCOO)(H2O)4]6+ nodes, respectively. 3.2 Thermogravimetric Analyses Thermal analyses of the obtained MOFs 1–5 reveal similar features due to their isostructural nature (Figure S3). For instance, on heating up to 195 °C, MOF 1 displays a ~4.48% weight loss owing to removal of four lattice H2O molecules (calcd. 4.50%). Further release of four coordinated water molecules and two DMF moieties results in the weight loss of 17.52% (calcd. 18.05%) on heating to 330 °C. Further increase of temperature provokes a decomposition of the remaining solid. Compounds 2-5 show essentially similar behavior with the removal of water ligands and solvent molecules up to 360, 340, 350, and 370 °C, respectively (Figure S3). 3.3 Photoluminescent Properties Emission and excitation and spectra of compounds of 1-5 were measured in solid state at ambient temperature. Emission spectrum of compound 1 (Figure 2) displays four typical Tb3+ bands at 488, 544, 588, and 622 nm when excited at 320 nm. These emissions are due to the transitions 5D4→7F6，5D4→7F5, 5D4→7F4, and 5D4 →7F3, correspondingly. Intense band at 544 nm corresponds to a hypersensitive 5D4→7F5 transition due to antenna effect from MOF.39 The decay lifetime of compound 1 is estimated to be 1019 µs (Figure S7, Table 2), which well fits a single-exponential curve; the Ф value (fluorescence quantum yield) attains 26.4%.



For 2, excitation spectra were all recorded by overseeing the most intense emission wavelength (611 nm) for europium(III) ions (Figure 3). Upon an excitation at 320 nm, MOF 2 shows typical emission bands at 592, 611, 651, and 699 nm, ascribed to the transitions 5D0 → 7Fj (j = 1, 2, 3, and 4), correspondingly. Emission intensity of a hypersensitive band (5D0→7F2) is significantly more intense in comparison with the 5D0 →7F1 band; calculated I(5D0→7F2)/I(5D0 →7F1) intensity ratio was 3.68. This demonstrates that Eu3+ is not positioned at a center of inversion.40 Decay curve was fitted with singleexponential loop, resulting in a 531.0 µs luminescence lifetime (Figure S7); the fluorescence quantum yield of 2 was 7.4% (Table 2). Emission behavior of Dy(III) MOF 4 was investigated at room temperature by exciting the sample at 315 nm (Figure S5) Main emission bands are centered at 572, 479, and 410 nm. The first two bands (572 and 479 nm) correspond to typical transitions of Dy3+, namely 4F9/2 → 6H13/2 and 4F9/2 → 6H15/2. Most intense emission band (410 nm) corresponds to the fluorescence of L3-. This also suggests that there is no efficient sensitization of the luminescence of dysprosium ions by L3-. Decay curve was fitted with two-exponential loop, and luminescence lifetime value of 4 was recorded as 3.48 µs (Figure S7); the fluorescence quantum yield of 4 was 3.06% (Table 2). Yellow luminescence was detected in the solid-state for 4, in agreement with prior literature data.41 For MOF 5, emission spectrum (1400-1650 nm range) was measured at ambient conditions by exciting the sample at 524 nm (Figure S6). For Er3+ ion, characteristic transition at 1549 nm is visible and belongs to transition 4I13/2→4I15/2, suggesting near-


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infrared luminescent properties of compound 5.42, 43 To understand the difference in the transfer of energy from phenylpyridinetricarboxylate to terbium and europium atoms in MOFs 1 and 2, we measured a triplet state level of energy for H3L. For gadolinium(III) derivative 3, phosphorescence spectrum was recorded at 77 K by exciting at 320 nm (Figure 4). The spectrum shows a maximum at 410 nm.44 Thus, for the ligand, the lowest triplet-state level of energy equals 24390 cm−1. It is expected that there should be an appropriate gap in energy between Ln3+ and 3

ππ* emissions, especially given that emissive states of Ln3+ ions have to be populated to

display emission. Following the rule of Latva,45 an efficient energy transfer from ligand to lanthanide(III) requires the following parameters: ∆E [= E(3ππ*)−E(5DJ)] = 2500−4500 cm−1 in case of terbium(III) and ∆E = 2500−4000 cm−1 in case of europium. Calculated values of ∆Ε cm−1 are 3890 cm−1 for 1 and 5390 cm−1 for 2. Therefore, we can expect a more efficient transfer of energy (absorbed by L3–) to terbium(III) vs. europium(III). According to Judd–Ofelt Theory, 46, 47 the efficiency of sensitization in compounds 1 and 2 (ηsens) can be estimated with regard to phenylpyridine-tricarboxylate ligand. For MOF 1, the ΦTb (intrinsic quantum yield for terbium ions) of 92.37% was calculated using equation 1.48 For compound 2, the values of ηsens and Φln are 54.86% and 13.49%, respectively. Further details can be found in Table 2. On the basis of the obtained results, it can be concluded that an insufficient efficiency of the sensitization in 2 is primarily related to a bigger difference in energy between the 3ππ* and excited 5D0 level. Φ = τ( ) /τ(

) (1) 3.4 Sensing of Organic Solvent Molecules Given the unique structural features and interesting luminescent properties of 1, we



selected it as a representative example to study its sensing properties, namely toward different solvents. Hence, dispersions of MOF 1 were prepared using different organic solvents (CH3OH, C2H5OH, C3H7OH, isopropanol, 1-hexanol, ethylene glycol, CHCl3, CH2Cl2, acetonitrile, acetone, 1,4-dioxane, THF, DMF, and ethyl acetate), followed by ultrasonication for 30 min before the luminescence measurements. Emission spectra of 1 (after being dispersed in organic solvents) show typical bands for terbium(III) ions (Figure 5a). However, intensity of the main transition (5D4→7F5) is significantly affected by solvent, especially for acetone that provokes a notorious luminescence quenching. Furthermore, the effect of the acetone amount on the luminescence quenching of 1 was investigated in DMF-acetone suspensions (Figure 6). The emitted visible green light of the suspension of 1 in DMF fades upon an addition of acetone, followed by a gradual decrease of the 5D4→7F5 transition intensity. We subsequently studied a possible interference of various organic solvents on selective quenching of the luminescence of 1 caused by acetone (Figure S8). It was found that there is no significant impact of other solvent on the quenching of the luminescence caused by the presence of acetone. 3.5 Metal Ion Sensing Potential of MOF 1 toward detecting different metal cations was studied by preparing a series of 14 samples. A quantity of 1 (2 mg) was dispersed in DMF solution (2 mL) containing a metal nitrate (concentration 10−2 M; sodium, potassium, magnesium, calcium, barium, copper(II), silver(I), cobalt(II), nickel(II), zinc(II), cadmium(II), aluminum(III), iron(III), or chromium(III) nitrate), followed by ultrasonic treatment for 30 min. Then, the luminescence spectra of the obtained dispersions of 1 were recorded.


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Interestingly, a very pronounced emission quenching of 1 was observed in the samples containing Fe3+ cations (Figure 7), while other metal cations showed less pronounced quenching (Cu2+, Ag+, Cr3+) or only minor decrease of the luminescence intensity. This observation suggests that MOF 1 acts as a luminescent probe toward iron(III) cations. As shown in Figure 8, the 5D4–7F5 transition intensity (545 nm) declines on augmenting the Fe3+ concentration; a limit of the detection attains 10−6 M. It should be mentioned that the Environmental Protection Ministry (P. R. China) established the standard (5.4 µM) for the presence of iron ions in drinking H2O.49 Hence, compound 1 can potentially be used to probe toxic metal ions in drinking water. Effect of the luminescence quenching can be approximatively calculated by applying the equation of Stern-Volmer {I0/I = 1 + Ksv[M]}.50 Herein, I0 and I refer to emission intensity of 1 prior and post-incorporation of extra metal cation, correspondingly; [M] represents the Fe3+ concentration. Obtained Stern-Volmer plot for Fe3+ (Figure 8c) reveals a linear fit at low amounts of iron(III) ions (0-0.3 mM Fe3+, R2 = 0.99). The Ksv value of 16590 M-1 indicates the remarkable influence of Fe3+ on quenching the emission of 1. It was also found that the quenching of the luminescence of 1 due to the presence of Fe3+ does not influenced, to a considerable extent, by other metal cations that might be present in the medium (Figure S9). 3.6 Sensing of Lanthanide Ions Inspired by a notable sensing behavior of 1 toward Fe3+ cations, we investigated a sensing ability of 1 for different lanthanide cations. Hence, samples of 1 (2 mg) were prepared by its dispersion in DMF medium (2 mL) that also comprises an additional lanthanide(III) nitrate (concentration: 10−2 M; lanthanum, cerium, praseodymium,



neodymium, samarium, europium, gadolinium, dysprosium, erbium, or ytterbium nitrate), followed by sonication for 30 min. The luminescent behavior of the obtained dispersions was investigated (Figure 9), revealing a remarkable effect of Ce3+ on quenching the luminescence of 1. In contrast, other lanthanide ions almost no influence luminescence intensity of DMF dispersion of 1. These data suggest that MOF 1 acts as a remarkable luminescent probe to detect Ce3+ cations. We therefore investigated a Ce3+ concentration influence (Figure 10) on the emission quenching of 1. There is a linear dependence between the Ce3+ concentration and quenching efficiency (Figure 10b). This quenching behavior can be rationalized with the equation of Stern-Volmer, which resulted in Ksv value of 279.4 M-1 for Ce3+ ions. There is also no significant effect (interference) of other lanthanide ions on the luminescence quenching caused by Ce3+ ions (Figure S10). 3.7 Luminescence Quenching Mechanism For acetone, the effect of quenching on the emission properties of Tb(III) MOF can be explained by prior literature data.6 In the solvents we tested, acetone has an observable absorption in the 225-325 nm regions, while other solvents do not show an absorption band at 320 nm (Figures S11). Analysis of the emission and absorption spectra suggests that there is an energy transfer between L3- and acetone, indicating a sort of competitiveness between the compound excitation and the acetone absorption. This causes a decrease in the luminescence intensity of 1. To explain a possible sensing mechanism of 1 toward Fe3+ or Ce3+ cations, their luminescence quenching effects were analyzed further. Firstly, compound 1 was suspended in the DMF solutions of Fe3+ or Ce3+ nitrates, followed by powder X-ray diffraction analysis of recovered solids. The obtained results (Figures S12 and S13) prove


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that a three-dimensional structure of Tb(III) MOF is preserved; hence, the observed quenching of the emission is not associated with a disintegration of 1. According to previous studies on the Fe3+ sensing by lanthanide derivatives, the behavior observed for compound 1 toward Fe3+ cations can be attributed to the following factors. (1) The porous MOF structure can facilitate the Lewis acid Fe3+ to bind to the pyridine Lewis base functionality of 1 despite its coordination to Tb3+ center. (2) Smaller ion radius and higher charge density of Fe3+ can lead to weak Fe3+-O interactions, which probably come from oxygen atoms of coordinated water molecules inside the cavity (Figure S14).51-53 To verify the above speculation, the XPS (X-ray photoelectron spectroscopy) and the UV/vis studies were performed. For Tb-MOF 1 before and after the treatment with Fe3+, the N 1s peaks in the XPS spectra were assigned to N atoms from L3- moieties. The N 1s peak shifts to 396.05 eV from 398.9 eV, whereas the O 1s peak shifts to 529.2 eV from 529.7eV as a result of the weak Fe3+ interaction with N and O atoms of the L3- ligand in Tb-MOF 1 (Figure 11). Similarly, before and after the treatment of 1 with Ce3+ solution, the N 1s peaks shift to 396.05 eV from 398.9 eV, while the O 1s peaks shift to 529.15 eV from 529.5eV, respectively; this may also result from a weak interaction of Ce3+ with N and O atoms of phenylpyridine-tricarboxylate ligand (Figure 12). After the treatment of 1 with Fe3+ or Ce3+, a slight shift in the solid-state UV/vis spectra appears (Figure S15), indicating a weak interaction of 1 with Fe3+ or Ce3+ cations. To further prove the above hypothesis, the photographs of 1 before and after the treatment with Fe3+ ions were taken (Figure S16-S17). We also found that after the treatment with Fe3+, the color of compound 1 essentially recovers to the original one



upon washing. In addition, the Fe3+ or Ce3+ solutions exhibit an absorption in the 250-400 nm range, which overlaps with an excitation pattern of Tb(III) MOF. This indicates that energy of excited light is taken by Fe3+ or Ce3+ cations. As a result, the transfer of energy from L3- to Ln3+ diminishes, giving rise to an emission quenching effect.54-57 A possible mechanism is suggested on the basis of the suppression of LRET (luminescence resonance energy transfer) and the enhancement of intermolecular electron transfer. Moreover, compound 1 maintains a high stability in diverse media (different organic solvents, aqueous or DMF solutions of various metal cations, and solutions with a broad pH ranging from 4 to 11) (Figure S18). Owing to the requirements of recyclability for potential practical applications, we investigated the recycling performance of compound 1 as the Fe3+ or Ce3+ luminescent probe. Hence, we dispersed compound 1 in the DMF solution (10−3 M) of Fe3+ or Ce3+ nitrates to obtain the 1-Fe3+ or 1-Ce3+ samples with a quenched luminescence. In the next step, the obtained solids were treated by DMF to wash out the Fe3+ or Ce3+ cations. The emission intensity of recovered MOF 1 (Figure 13) is well comparable to that of the parent sample. The same behavior is observed upon repeating the recycling procedure for five times, thus indicating that 1 can act as a stable, reusable, and versatile luminescent probe toward Fe3+ and Ce3+ ions, allowing their detection via fast and simple procedure.

4 CONCLUSIONS New porous lanthanide(III) metal-organic frameworks with a general formula [Ln4(µ6-L)2(µ-HCOO)(µ3-OH)3(µ3-O)(DMF)2(H2O)4]n (Ln = terbium (1), europium (2), gadolinium (3), dysprosium (4), and erbium (5)), were assembled and completely


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characterized. All MOFs are isostructural and show a porous 3D network of flu topological type, which is assembled from cubane-like [Ln4(µ3-OH)3(µ3-O)(µ-HCOO)]6+ secondary building units and the phenylpyridine-tricarboxylate L3– spacers. The luminescent properties of Ln-MOFs samples were measured and investigated in detail. The sensing ability of compound 1 (as a representative example) toward different types of analytes was also studied, revealing its high detection ability to sense Fe3+ or Ce3+ cations or acetone via luminescence quenching. Good selectivity, high stability, and excellent response of 1 toward the above-mentioned analytes is most likely associated with a weak interaction of the framework 1 with Fe3+ or Ce3+ cations, wherein there is also a competition between the absorption of Fe3+ or Ce3+ cations and the excitation of TbMOF. It should be mentioned that the luminescent Tb-MOF probes for detecting Ce3+ ions are still largely unknown. Good correlation of the emission intensity of 1 and concentration of Ce3+ cations thus furnishes a simple method for the Ce3+ detection. Hence, the present study also demonstrated that Tb-MOF 1 can be applied as a practical and multi-responsive probe with potential significance in environmental areas.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Crystallographic data for 1-5 in CIF format (CIF). Experimental details, selected bonding parameters, additional spectra, figures, and tables



with structural, PXRD, TGA, photoluminescence and other data (Tables S1-S3, Figures S1-S18) (PDF). Accession Codes CCDC 1836807-1836811 contain the supporting crystallographic data.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest.

Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant 21301080, 21672090, 21431002, and 21673105), the Fundamental Research Funds for the Central Universities (Grant lzujbky-2017-108, lzujbky-2016-44, lzujbky-2016-K09 and lzujbky-2015-k04), as well as the Foundation for Science and Technology (FCT) and Portugal 2020 (LISBOA-01-0145-FEDER-029697, UID/QUI/00100/2013).


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Table 1. Crystal Data and Structure Refinements for 1-5. compound

1

2

3

4

5

formula

C32H33N3O24Tb4

C32H33N3O24Eu4

C35H35N4O24Gd4

C35H35N4O24Dy4

C35H35N4O24Er4

formula weight

1479.33

1451.45

1524.67

1547.69

1564.71

crystal system

Monoclinic

Monoclinic

Monoclinic

Monoclinic

Monoclinic

space group

C12/c1

C12/c1

C12/c1

C12/c1

C12/c1

a, Å

26.00 (3)

25.810 (15)

25.809(5)

25.6353(16)

25.594(4)

b, Å

10.644 (15)

10.557 (5)

10.2855(16)

10.4616(7)

10.3060(15)

c, Å

22.23 (3)

22.042(14)

21.999(4)

21.7893(15)

21.680(3)

α, deg

90

90

90

90

90

β, deg

110.537 (18)

111.294(13)

110.400(3)

110.4380(10)

109.638(2)

γ, deg

90.00

90.00

90

90

90

Z

4

4

4

8

8

V, Å

5761 (13)

5596 (6)

5473.5(16)

5475.7(6)

5385.9(14)

T, K

100.15

100.15

100.15

296.15

296.15

1.706

1.723

1.850

1.877

1.930

µ(Mo Kα), mm

4.918

4.491

4.858

5.470

6.245

refl. collected

11502

18413

19743

14621

19428

independent refl.

5310

4867

4808

4408

4334

Rint

0.0808

0.1166

0.0391

0.0327

0.0389

0.0767, 0.2404

0.0971, 0.2182

0.0373,0.0875

0.0278,0.0685

0.0287,0.0778

0.978

1.208

1.070

1.031

1.035

3

Dc, g cm−3 −1

R1 a, wR2 b [I ≥ 2σ(I)] 2

GOF on F a

R1 = Σ||Fo| – |Fc||/Σ|Fo|. b wR2 = [Σ[w(Fo2 – Fc2)2]/Σ[w(Fo2)2]]1/2



(a)

(c)

(b)

(d)

Figure 1. Structural fragments of 1. (a) Coordination environment of Tb1 and Tb2 centers. (b) Cubane-like [Tb4(µ3-OH)3(µ3-O)(µ-HCOO)(H2O)4]6+ unit (right) and µ6-L3– spacer (left). (c) 3D metal-organic framework. (d) Topological representation of a simplified binodal 4,8-connected 3D net with the flu (fluorite) topology and point symbol of (412.612.84)(46)2. Further details: (a, b) H atoms are omitted for clarity except those of µ3-OH and µ-HCOO moieties in (b); (c) H atoms are omitted for clarity, view along the b axis; (d) view along the c axis; centroids of 8-connected [Tb4(µ3-OH)3(µ3-O)(µHCOO)(H2O)4]6+ cluster nodes (green balls), centroids of 4-connected L3– nodes (gray).


Page 28 of 38

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Figure 2 Solid-state excitation (black, λex = 320 nm) and emission (green, λem = 544 nm) spectra of 1.

Figure 3 Solid-state excitation (black, λex = 320 nm) and emission (red, λem = 611 nm) spectra of 2.



Page 30 of 38

Figure 4 Phosphorescence spectrum of compound 3 at 77K (λex = 320 nm).

Table 2. Photoluminescence data for compounds 1 and 2 in the solid state. a Sample

τ1 (µs)

1 (Tb)

1019 (100%)

τ2 (µs)

(μs) b

Фsen (%)/

ФLn (%)

Фoverall (%)

28.57

92.37

26.4 a

54.86

13.49

7.4 a

414.37 (10.74%)c 1133.17 (89.26%) c1103.06c

a

2 (Eu)

531.0 (100%)

4 (Dy)

1.02 (10.54%)

3.57 (89.46%)

3.48

3.06 a

5 (Er)

0.05 (9.14%)

1.41 (90.86%)

2.66

0.95a

Lifetimes (τ) and corresponding relative weightings and absolute quantum yields Ф. b Calculated

using the equation ∑ /∑

c

τobs at 77K.


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(a)

(b)

5

7

Figure 5 (a) Luminescence spectra (a) and intensities of the D4→ F5 transition (b) for 1

dispersed in various organic solvents (2 mL, 2 mg 1).

(a)

(b)

Figure 6 (a) Luminescence spectra of 1 dispersed in DMF-acetone solution (2 mL total volume; 2 mg 1) with different volume content of acetone; inset shows color of DMF (left) and acetone (right) solutions under UV light. (b) Luminescence intensities of the 5

D4→7F5 transition (544 nm) of 1 with an increasing volume content of acetone in DMF-

acetone solution.



(a)

(b)

Figure 7 Luminescence spectra (a) and intensities of the 5D4→7F5 transition (b) of 1 dispersed in DMF solutions of different metal ions (10-2 M metal nitrate, 2 mL DMF, 2 mg 1).


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(a)

(b)

(c)

Figure 8 (a) Luminescence spectra of 1 dispersed in DMF solutions of Fe(NO3)3 (10-110-6 M, 2 mL DMF, 2 mg 1); inset shows a color of solutions under UV light before (left) and after (right) addition of Fe3+ (10-2 M). (b) Plot of the quenching efficiency of 1 dispersed in DMF at different concentrations of Fe3+. (c) Linear dependence between the quenching efficiency and the concentration of Fe3+ in the range of 0-0.3 mM.



(a)

(b)

Figure 9 Luminescence spectra (a) and intensities of the 5D4→7F5 transition (b) of 1 dispersed in DMF solutions of different lanthanide ions (10-2 M lanthanide nitrate, 2 mL DMF, 2 mg 1).


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(b)

(a)

(c)

Figure 10 (a) Luminescence spectra of 1 dispersed in DMF solutions of Ce(NO3)3 (10-110-6 M, 2 mL DMF, 2 mg 1); inset shows color of solutions under UV light before (left) and after (right) addition of Ce3+ (10-2 M). (b) Plot of the quenching efficiency of 1 dispersed in DMF at different concentrations of Ce3+. (c) Linear dependence between the quenching efficiency and the concentration of Ce3+ in the range of 0.1-5 mM.



(a)

(b)

Figure 11 (a) XPS spectra of 1 (Tb-MOF) and (b) 1 treated with Fe3+. Overall spectra (left), N 1s (center), O1 s (right).


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(a)

(b)

Figure 12 (a) XPS spectra of 1 (Tb-MOF) and (b) 1 treated with Ce3+. Overall spectra (left), O1 s (center), N 1s (right).

(a)

(b)

Figure 13 Original and quenched luminescence intensity (544 nm) of 1 recycled after sensing experiments for Fe3+ (a) and Ce3+ (b) cations; five recycling tests were investigated.



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