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Highly Water-Stable Novel Lanthanide Wheel Cluster Organic Frameworks Featuring Coexistence of Hydrophilic CageLike Chambers and Hydrophobic Nanosized Channels Yuan-Yuan Zhou, Yang Shi, Bing Geng, and Qi-Bing Bo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15613 • Publication Date (Web): 19 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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ACS Applied Materials & Interfaces

Highly Water-Stable Novel Lanthanide Wheel Cluster Organic Frameworks Featuring Coexistence of Hydrophilic Cage-Like Chambers and Hydrophobic Nanosized Channels Yuan-Yuan Zhou1, Yang Shi1, Bing Geng2 and Qi-Bing Bo1* 1

Key Laboratory of Chemical Sensing and Analysis in Universities of Shandong, School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, PR China 2 Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials, School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, PR China

ABSTRACT: In attempts to investigate the potential luminescent sensing materials for sensitive detection of environmental pollutants, a new family of lanthanide wheel cluster organic frameworks (Ln-WCOFs) UJN-Ln4 has been constructed by employing one of the cycloalkane dicarboxylic acid derivatives. Adopting different conformations, the ligand links Ln4 second building units (SBUs) and Ln24 tertiary building unit (TBUs) to form a unique wheel cluster layer-pillared 3D framework featuring the coexistence of hydrophobic nanosized channels and trigonal anti-prism arrays with hydrophilic cage-like chambers. Apart from charming structures, isostructural UJN-Ln4 display interesting porous, water-stable features. Systematic luminescence studies demonstrate that solvent water molecules can enhance the emission intensity of solid state UJN-Eu4. Acting as a recyclable luminescent probe, water-stable luminescent UJN-Eu4 exhibits superior “turn-off” detection for Fe3+ and Cu2+ ions in aqueous solutions. Due to the nanosized hydrophobic channels, UJN-Eu4 also shows highly sensitive sensing of sodium dodecyl benzene sulfonate (SDBS) via luminescence “turn-on” respondence, representing the first example of quantitatively detecting SDBS in aqueous solutions by employing luminescent lanthanide frameworks as fluorescent sensors. The results also open up the exploration of novel luminescent Ln-WCOFs exhibiting unique applications in sensitive detecting of harmful pollutants in aquatic environments. KEYWORDS: wheel cluster metal-organic framework • Lanthanide • water-enhanced luminescence • luminescent sensing • sodium dodecyl benzene sulfonate adsorption

INTRODUCTION Lipid-based synthetic surfactants have been extensively used as detergents, foaming agents and dispersants in wastewater treatment plants. Sodium dodecyl benzene sulfonate (SDBS) is a well-known

1

ACS Paragon Plus Environment


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anionic type surfactant containing a sulfonated aromatic ring attached to a linear alkyl chain at the para position. Considering its low cost, SDBS has been extensively applied in household detergents and entered the environment via wastewater discharge. Due to hardly biodegradable characteristic, the non-degradable SDBS could be accumulated in water body with significant amounts for long periods of time, posing significant adverse health effects on human beings and animals.1,2 Similar with SDBS, high concentrations of the discharged heavy metal ions such as Fe3+, Al3+, Cr3+, Cd2+, Cu2+, Ba2+ and Pb2+ ions from industrial wastes are also undesirable pollutants.3,4 Evidently, with the development of modern society and industry, water contamination and environmental problems have become more and more serious due to improper discharge of wastewater containing high concentrations of SDBS and heavy metal ion pollutants. And the analytical determination and monitoring of these harmful pollutants is crucial to ensure aquatic environment safety. The acid-base titration method (MBAS)

6,7

5

and spectrophotometry of a methylene blue active substance

are widely applied to analyzing anionic surfactants for assay purposes. However, both of

them have many disadvantages, such as high cost, interferences, difficulty of operation, long analysis times, difficulty analyzing colored samples, and the need for large volumes of chloroform. As for heavy metal ions, different analytical methods such as voltammetry,8 spectrophotometry,9 and atomic absorption spectrometry10 can also be used in detecting them accurately. However, these analytical methods are often not easily accessible due to complicated sample preparation, sophisticated instruments and high cost. Recently, fluorescence-based detection methods have been intensively explored owing to their low cost, high sensitivity, short response time, and so on.11 In this regard, fluorescent sensors viewed as one of the most compelling devices for sensing the analytes, have also gained ever-increasing attention due to their high efficiency, portability and simplicity. However, the design and assembly of luminescent sensing materials are the necessary prerequisites to detect and monitor these harmful pollutants such as SDBS and heavy metal ions discharged in aquatic environments. It is known that metal−organic frameworks (MOFs) are assembled from the connectivity of appropriate organic ligands and central metal ions or metal clusters with diverse structural characteristics and functional sites.12 Among them, Eu- and Tb-MOFs can be viewed as potential good candidates as luminescent sensing materials because of their unique optical advantages (large Stokes shift, undisturbed emission, high color purity, visible and bright luminescent colors). For example, through the molecular level interactions between the framework and different analytes, the essentially complete quenching or drastic enhancement of Eu3+ (or Tb3+ ) luminescence suggests specifically the potential use of Eu- or Tb-MOFs for luminescent sensing material. Conceptually, the construction of 2


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luminescent lanthanide MOFs (Ln-MOFs) with structurally porous features immensely depends on the judicious selected organic ligands apart from the lanthanide ions. Lanthanide wheel cluster organic frameworks (Ln-WCOFs) are regarded as another novel kinds of MOFs. To the best of our knowledge, all of the reported Ln-WCOFs are limited to lanthanide-copper heterometallic ones constructed from the ligands of isonicotinic acid

13-15

and its two derivatives 4-(3-pyridyl)benzoic acid,16 as well as

4-pyridin-4-yl-benzoic acid,17,18 which is probably related to the difficult to find other suitable organic ligands in making sure that their geometric configurations can be immensely flexible and their coordination sites can be accurately controlled to link the central lanthanide clusters. Therefore, linking lanthanide wheel clusters to extended Ln-WCOFs with other ligands still represents a big challenge, and should be further studied. In contrast to widely used ligands of aromatic carboxylic acid and its derivatives to direct many self-assemblies around the given metal ions,19-20 very little attention has been paid to alicyclic carboxylic acid (or its derivative) so far. Among many alicyclic carboxylic acids (or their derivatives), cyclopropane dicarboxylic acid possesses two carboxylic acid groups with the included bend angle of ∼120° and a central cyclopropane spacer. The angular bend makes it possible to form a clip-like ligand due to the rotated cyclopropane spacer and asymmetrically twisted carboxylate groups. Compared with many reported rigid aromatic dicarboxylic acid ligands, the clip-like ligand cyclopropane dicarboxylic acid should present long range pseudo-rigidity with some degree of rotational flexibility and conjugated extent due to its lightweight, which benefits the generation of polynuclear metal clusters and provides new opportunities in the field of coordination-driven self-assembly. Our recent report also confirms that the configuration of two carboxylate groups in one of the cycloalkane dicarboxylic acid derivatives has an intrinsic preference for linking lanthanide ions into unique lanthanide clusters.21 To the best of our knowledge, reports of the homometallic Ln-WCOFs have never been reported so far, except for some of the lanthanide-copper heterometallic ones constructed from the ligands of isonicotinic acid and its derivatives.13-18 Furthermore, compared with usual Ln-MOFs, all of the reported heterometallic Ln-WCOFs have not been applied in the sensitive detection of the environments pollutants such as cations and anions in aquatic solutions. Inspired by our recent studies for the lanthanide clusters, we wonder whether many Ln-WCOFs could be conveniently assembled from cyclopropane dicarboxylic acid and its derivatives, similar to isonicotinic acid and its derivatives mentioned above. If so, the targeted Ln-WCOFs with special interesting properties might form due to the typical characteristics of the lanthanide ions. Taking these into consideration, our main efforts have been centered around the judicious choice of suitable cyclopropane dicarboxylic acid derivatives with different substitution groups, such as methyl, tert-butyl, 3



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hydroxyl, amino, sulfonic and phenyl groups, because each of them plays an important role in the formation of the desired products. Herein, the first example of 3D Ln-WCOFs [Ln4(µ4-dcd)6(H2O)] (Ln=Sm, Eu, Gd and Tb; dcd= 3,3-dimethylcyclopropane-1,2-dicarboxylate dianion) were constructed through in situ ligand-oriented assemble. Apart from the aesthetically charming structures of UJN-Ln4, much effort have been invested into their physical properties, such as porosity, water-stability, photoluminescence and luminescence sensing functions, which are extremely desired for further application in chemical sensing. Specially, a fluorescence sensing method for the determination of SDBS in aqueous solution was developed in this work. As far as we have been able to ascertain, there is no published study on Ln-MOFs as highly sensitive luminescent probes to detect SDBS in aqueous solutions, although porous MOFs with luminescent sensing functions have been widely applied in the determination of many cations and anions.22,23 The results demonstrate that UJN-Eu4 is a promising luminescent sensing material, leading to the potential for determination and monitoring of low level of SDBS, Fe3+ and Cu2+ ions in aqueous solution. EXPERIMENTAL SECTION General Remarks. The syntheses were performed in poly(tetrafluoroethylene)-lined stainless steel autoclaves under the autogenous pressure. Analytical pure reagents were purchased commercially and used without further purification. Ultrapure water used throughout was obtained from a Milli-Q water purification system (Lichun, Jinan, China). Stock standard solution (1000 ug/mL) of SDBS was commercially obtained from Macklin (Shanghai, China), this solution was stored at 4℃ and protected from light. Working standard solutions were freshly prepared by making appropriate dilutions of the stock standard solution with ultrapure water. Physical Techniques. Elemental analyses (C and H) were performed on a Perkin-Elmer 2400 Series II CHNS/O elemental analyzer. Microscopic images of the solid state samples were taken by the Canon digital camera (IXUS 275 HS). FT-IR spectra were recorded in the range of 400-4000 cm-1 on a Perkin-Elmer FTIR spectrometer using KBr pellets. Thermogravimetric / differential thermal analysis (TG/DTA) was performed on a Perkin-Elmer Diamond TG/DTA instrument in flowing air with a heating rate of 10 ℃ min-1. The powder X-ray diffraction patterns (PXRD) were recorded by a Bruker D8-Focus Bragg-Brentano X-ray Powder Diffractometer equipped with a Cu sealed tube (λ = 1.54178 Å) at a scan rate of 0.5 s deg-1. Emission and excitation spectra were obtained on an Edinburgh Instruments analyzer model FLS920 spectrofluorometer. The solid-state photoluminescence

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measurements in different aqueous solutions were performed in the home-made sensor cell (Chart S1). The UV/Vis absorption spectra of the metal ion solutions were obtained on UV-2550 (SHIMADZU). The diffuse-reflectance spectrum of solid state samples were measured on the UV/Vis spectrometer UV-3101PC (SHIMADZU) equipped with an integrating sphere in the wavelength range 200–900 nm. BaSO4 plate was used as a reference (100% reflectance), on which the finely ground powder of the sample was coated. Water vapor sorption studies were performed on Micrometritics 3Flex. Before the sorption measurements, the sample was evacuated again by using the “outgas” function of the analyzer for 6 h at 210 °C .

Synthesis of Compounds. Synthesis of UJN-Sm4. 0.5 mmol Sm2O3 and 2.0 mmol 6,6-dimethyl-3-oxabicyclo[3.1.0]hexane-2,4-dione were mixed in 12.0 ml H2O, and stirred for 15 min at room temperature, generating an aqueous suspension. Then, the aqueous suspension was transferred into a Teflon-lined stainless-steel vessel (23 mL), and heated at 210 ℃ for five days under the autogenous pressure. After the reaction mixture was slowly cooled down to room temperature, yellowish block-like single crystals of UJN-Sm4 were filtered off (Figure S1-S2), washed with ultrapure water and dried in air (yields: ~15%). Elemental analysis (%) calcd. for C42H50Sm4O25 (1556.22): C 32.41, H 3.24; found: C 32.38, H 3.21. IR (KBr pellet, cm-1): 3404 s, 2969 w, 2951 w, 1612 m, 1535 s, 1471 w, 1405 m, 1322 m, 1303 m, 1260 w, 1218 w, 1118 s, 1034 m, 1001 w, 977 m, 861 m, 825 w, 814 m, 761 w, 670 m, 664 m, 551 s, 456 w. Synthesis of UJN-Eu4. The synthesis of UJN-Eu4 is similar to that of UJN-Sm4 by using Sm2O3. Yellowish block-like single crystals of UJN-Eu4 were filtered off (Figure S1-S2), washed with ultrapure water and dried in air (yields: ~90%). Elemental analysis (%) calcd. for C42H50Eu4O25 (1562.66): C 32.28, H 3.23; found: C 32.26, H 3.20. IR (KBr pellet, cm-1): 3428 vs, 2969 w, 2952 w, 1613 w, 1537 s, 1470 w, 1404 m, 1324 m, 1303 m, 1260 w, 1218 w, 1118 s, 1035 w, 999 w, 978 m, 862 w, 815 w, 666 m, 651 w, 551 s, 456 w. Synthesis of UJN-Gd4. The synthesis of UJN-Gd4 is similar to that of UJN-Sm4 by using Gd2O3. Yellowish block-like single crystals of UJN-Gd4 were filtered off (Figure S1-S2), washed with ultrapure water and dried in air (yields: ~90%). Elemental analysis (%) calcd. for C42H50Gd4O25 (1583.82): C 31.85, H 3.18; found: C 31.88, H 3.21. IR (KBr pellet, cm-1): 3413 vs, 2969 w, 2951 w, 1613 w, 1537 s, 1472 s, 1405 m, 1325 m, 1302 m, 1260 w, 1218 w, 1118 s, 1036 w, 1001 w, 978 m, 861 w, 814 w, 666 m, 651 w, 554 s, 456 w.

5



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Synthesis of UJN-Tb4. The synthesis of UJN-Tb4 is similar to that of UJN-Sm4 by using 0.25 mmol Tb4O7. Yellowish block-like single crystals of UJN-Tb4 were filtered off (Figure S1-S2), washed with ultrapure water and dried in air (yields: ~15%). Elemental analysis (%) calcd. for C42H50Tb4O25 (1590.50): C 31.72, H 3.17; found: C 31.69, H 3.20. IR (KBr pellet, cm-1): 3404 s, 2969 w, 2950 w, 1614 w, 1539 s, 1472 m, 1404 m, 1326 m, 1302 m, 1260 w, 1218 w, 1118 s, 1036 w, 1001 w, 978 m, 862 w, 824 w, 814 w, 762 vw, 669 m, 651 m, 554 s, 456 w. Single Crystal X-ray Crystallography. Suitable single crystals of UJN-Ln4 (Ln= Sm, Eu, Gd and Tb) were selected for single-crystal X-ray diffraction analysis. Crystal data were collected on an Xcalibur, Eos, Gemini diffractometer (MoKα radiation, λ=0.71073 Ǻ). Data reduction was accomplished by the CrysAlisPro (Oxford Diffraction Ltd., Version 1.171.33.55) program. The structures were solved by direct method and refined by a full matrix least-squares technique based on F2 using SHELXL 97 program.24 All of the non-hydrogen atoms were refined anisotropically. The organic hydrogen atoms were generated geometrically, the aqua hydrogen atoms were located from difference maps and refined with isotropic temperature factors. The structural pictures for the compounds were drawn with the program Diamond.25 Crystallographic data for the reported structures in this paper have been deposited with the Cambridge Crystallographic Data Centre. CCDC 1484196 (UJN-Sm4), 1484197 (UJN-Eu4), 1484198 (UJN-Gd4) and 1484199 (UJN-Tb4) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. RESULTS AND DISCUSSION Description of Crystal Structures. Single-crystal X-ray diffraction studies reveal that Ln-WCOFs [Ln4(µ4-dcd)6(H2O)]

(UJN-Ln4,

Ln

=

Sm,

Eu,

Gd

and

Tb;

dcd=3,3-dimethylcyclopropane-1,2-dicarboxylate dianion) are isostructural. Their phase purities are supported by PXRD patterns of the corresponding bulk samples (Figure S3-S6). It can be seen that the patterns of the bulk samples match those calculated from their single-crystal structure data well, which confirms that each of the WCOFs is presented as a single phase. The selected crystallographic data and refinement parameters for UJN-Ln4 are presented in Table 1.

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Table 1. Selected crystallographic data and refinement parameters for UJN-Ln4

Empirical formula Formula weight Temperature/ K Wavelength/ Å Crystal system Space group a/Ǻ b/Ǻ c/Ǻ α / deg β /deg γ / deg Volume/ Ǻ3 Z ρcal /g·cm-3 µ /mm-1 F (000) Reflections collected Independent refl. GOF Final R indices [I>2σ(I)] R indices All data

UJN-Sm4 C42H50Sm4O25 1556.22 295(2) 0.71073 Trigonal P-3c1 18.3094(5) 18.3094(5) 19.8946(8) 90 90 120 5775.8(3) 4 1.790 4.081 3000 29329 3935 [R(int)= 0.0804] 1.122 R1 = 0.0459 wR2 = 0.0962 R1 = 0.063 wR2 = 0.1062

UJN-Eu4 C42H50Eu4O25 1562.66 295(2) 0.71073 Trigonal P-3c1 18.2874(4) 18.2874(4) 19.8488(6) 90 90 120 5748.7(2) 4 1.806 4.379 3016 22280 3924 [R(int)= 0.0360] 1.152 R1 = 0.0234 wR2 = 0.0479 R1 = 0.0322 wR2= 0.0528

UJN-Gd4 C42H50Gd4O25 1583.82 295(2) 0.71073 Trigonal P-3c1 18.2451(6) 18.2451(6) 19.8056(7) 90 90 120 5709.7(3) 4 1.842 4.661 3032 21758 3906 [R(int)= 0.0361] 1.104 R1 = 0.0219 wR2 = 0.046 R1 = 0.0287 wR2 = 0.0491

UJN-Tb4 C42H50Tb4O25 1590.50 295(2) 0.71073 Trigonal P-3c1 18.1712(5) 18.1712(5) 19.7581(5) 90 90 120 5649.9(3) 4 1.870 5.021 3048 21633 3861 [R(int)= 0.0310] 1.093 R1 = 0.0205 wR2 = 0.0468 R1 = 0.0239 wR2 = 0.0483

As shown in Figure 1, representative sample of UJN-Eu4 was selected to describe its structural assembly processes in detail. The asymmetric unit for UJN-Eu4 contains one and one third crystallographically independent Eu3+ ions, two dcd ligands and one third coordinated water molecules. Each Eu1 ion is composed of eight carboxylate oxygen donors from six dcd ligands, resulting in a square-antiprismatic configuration. While Eu2 center is coordinated by ten oxygen atoms belonging to six dcd ligands and one water molecule, affording a bicapped square-antiprismatic geometry. The bond lengths of Eu-Ocarboxylate are in the range of 2.3012(1)-2.6731(0) Å . And the Eu2-Owater bond length is 2.5006(1) Å. Acting as µ4-bridging linkers, two crystallographically independent dcd ligands possess anti-and gauche- conformations, and adopt the coordination modes of µ4-η1:η1:η2:η2 (denoted as L1) and µ4-η1:η1:η1:η1(L2), respectively (Figure 1b). The second building unit (SBU) in UJN-Eu4 can be viewed as a triangular pyramidal [Eu4(COO)3] (denoted as Eu4) core (Figure 1a). With the distance of about 3.9 Å (Eu1•••Eu1) and a reverse orientation, six vertex-sharing triangle units of Eu4 SBUs are alternately linked by eighteen dcd ligands adopting L1 coordination mode, to form a wheel cluster [Eu24(H2O)6(dcd)18]36+ (denoted as Eu24) that can be viewed as a centrosymmetrical tertiary building 7



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unit (TBU).26 The single Eu24 wheel cluster viewed as a TBU is depicted in Figure S7a. Each of the TBUs is surrounded by six equivalent neighbors with a distance of about 1.83 nm to form a wheel cluster layer with highly ordered honeycomb arrays (Figure 1c). Then, the parallel Eu24 wheel cluster layers seem able to slide with -AB- alternations along the c axis, and are further pillared by a bunch of eighteen dcd ligands adopting L2 coordination mode to give rise to a finally porous 3D lanthanide WCOF formulated as [Eu24(H2O)6(dcd)36]n, as shown in Figure 1d and Figure S7b-S7c. Structurally speaking, two types of synergistic coordination linkages are simultaneously observed in assembling the wheel cluster layer-pillared structure for UJN-Ln4: one is between the triangular pyramidal Eu4 SBUs and dcd ligands adopting mode L1; the other is between the Eu24 wheel cluster layers and dcd ligands adopting mode L2. These two synergistic coordination linkages work together to promote the formation of aesthetically porous 3D framework.

Figure 1. Structural assembly process for UJN-Eu4. (a) triangular pyramidal SBU; (b) two different bridge-linking modes for the ligand; (c) Eu24 wheel cluster layer with highly ordered honeycomb arrays; (d) porous 3D structure of UJN-Eu4 containing one-dimensional trigonal anti-prism arrays and 12-membered-ring channels. Symmetry codes: A(-x+y, 1-x, z), B(1-y, 1+x-y, z), C(y, x, 1.5-z), D(1-x, 1-y, 1-z), E(y, 1-x+y, 1-z), F(1-y, 1-x, 0.5+z). mauve: hydrophilic cage-like chamber ; sky-blue: hydrophobic channels.

Remarkably, the porous 3D framework presents a one-dimensional trigonal anti-prism array with the separation distance (Eu1•••Eu1) and cross-section triangle size of 10.0 Å and 6.8 × 6.8 × 6.8 Å3, 8


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respectively (Figure 2b-2c). It is noteworthy that one coordinated water molecule points inward to the center of each of the trigonal anti-prisms, and provides a hydrophilic environment inside the cavity, resulting in a hydrophilic cage-like chamber (atom-to-atom distance 6.3 Å; 3.3 Å when considering van der Waals radii) (Figure S8a).27 At the same time, every six of the trigonal anti-prism arrays are connected roundedly with a distance of 3.9 Å ( Eu1•••Eu1) (Figure 2c), generating many hydrophobic 12-membered-ring channels (Eu1•••Eu1, cross-section diameter size of 1.73 nm) filled with disordered methyl groups and cyclopropane rings (Figure 2a, 2d). Taking into the steric hindrance of the methyl groups, the free channels (atom-to-atom distance 9.8 Å; 6.8 Å when considering van der Waals radii) can also be found in the porous 3D structure (Figure S8b). Even if the coordinated water molecules, disordered methyl groups and cyclopropane rings are all included, the effective free volumes of the porous 3D structure are calculated with the program PLATON to be about 1037.2 Å3 , comprising 18.0% of the crystal volume, being also larger than 17.4 % in the case of the activated porous coordination polymer upon solvent removal with heating under vacuum.28

Figure 2. Schematic representation of 3D metallic skeleton assembly for UJN-Eu4. (a) Porous 3D structure of UJN-Eu4 simplified as a metallic skeleton containing the 12-membered-ring channels with a cross-section diameter size of 17.3 Å (the ligands are simplified as different colored lines); (b) one-dimensional trigonal anti-prism array with a cage-like chamber of around 6.3 Å; (c) two adjacent one-dimensional trigonal anti-prism arrays around the 12-membered-ring channels; (d) a hydrophobic 12-membered-ring channel filled with disordered methyl groups and cyclopropane rings.

Better insight into the porous 3D framework for UJN-Eu4 can also be achieved by topology analysis. 9



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From the topological viewpoint, the triangular pyramidal Eu4 clusters serve as 8-connected nodes, and the dcd ligands can be considered as the linkers. Therefore, the equivalent 3D topology framework for UJN-Eu4 can be simplified as a non-interpenetrating uninodal eca-type topological motif with a Schlafli symbol of (39.412.57), being first example among the reported WCOFs (Figure S9).13-18 TG/DTA and Framework Stability. To study the thermal stability of the crystalline samples, TG/DTA was carried out in dry-air atmosphere (Figure S10). Because of the similarity of thermal decomposition behaviors, a representative example of UJN-Eu4 is discussed here. The weight loss of UJN-Eu4 is mainly divided into two steps in the range of 30-750℃. Below 220℃, the first weight loss of 1.17 % (calcd: 1.15) corresponds to the release of one coordinated aqua ligand. Accompanied with the exothermic peaks, a tremendous weight loss can be found above 300℃, which is ascribed to burning of the organic groups. The final inorganic remnant matches reasonably well with a deposition of Eu2O3 (calcd: 45.11 %; found: 45.04 %). The TG-DTA trace of UJN-Eu4 demonstrates that the coordinated water molecule can be removed above 220℃. In addition, the thermal stability of UJN-Eu4 is further confirmed through temperature-dependent PXRD patterns. As shown in Figure S11, the experimental PXRD patterns for the samples heating from room temperature to 260℃ correspond well to the simulated ones in position, supporting the notion that the framework for UJN-Eu4 also remains intact even if the coordinated water molecules are removed. However, the diffraction peaks on 280℃ reveal the collapse of the framework, indicating the framework is stable up to 260℃. It should be pointed that even on heating single crystal of UJN-Eu4 to 260℃ overnight in an oven, they also remain their crystalline state suitable to be selected for single-crystal X-ray diffraction analysis. Considering the results of the TGA-DTA study, other single-crystal X-ray diffraction experiments have been performed to investigate the crystal structures of sample UJN-Eu4 that was heated to 220, 240 and 260 ℃ ( hereafter named UJN-Eu4-220, UJN-Eu4-240 and UJN-Eu4-260 ) overnight. However, the accurate structures for UJN-Eu4-220, UJN-Eu4-240 and UJN-Eu4-260 can’t be obtained under the normal conditions of single-crystal X-ray diffraction measurements, because these anhydrous samples are extremely hygroscopic in air under ambient conditions. This is confirmed by the fact that the collected crystal cell parameters for UJN-Eu4-220, UJN-Eu4-240 and UJN-Eu4-260 are very similar to UJN-Eu4. Evidently, the process of dehydration /rehydration for UJN-Eu4 is reversible due to its porous structural feature, which can also be seen from the water vapor sorption isotherm measured at 298 K (Figure S12). Very interestingly, Figure S12 shows that the water vapor uptake can steeply climb to near 13.6 cm3 g-1 ( 0.6 mmol g-1 ) at very low pressure ( P/P0=1.25×10-3 ). Where, one water molecule should be quickly adsorbed onto an open Eu3+ site per formula. This is consistent with 10


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the fact that the freshly isolated UJN-Eu4 contains one coordinated water molecule in its formula of Eu4(µ4-dcd)6(H2O). Then, the water uptake gradually increased as the water vapor pressure increased, and finally reached to the highest of 28 cm3 g-1 (1.25 mmol g-1) at P/P0=0.97. At the same time, a slight hysteresis between the adsorption and desorption isotherms can also be found for the activated UJN-Eu4. Here, it should be noted that the PXRD patterns for the rehydrated sample vacuumly activated at 210℃ and the freshly isolated sample of UJN-Eu4 are very similar to each other (Figure S13), further confirming that the framework transformation between the as-synthesized and dehydrated samples is reversible in the process of adsorption/desorption. At the same time, the porous crystalline solid for UJN-Eu4 is very stable in water due to the hydrophobic channels presented in its structure, which is supported by the retention of its crystalline shape and single-crystal X-ray diffraction analysis after immersing the crystalline solid in aqueous solution for more than 2 weeks.

Photoluminescence, Water-Enhanced Luminescence and Luminescent Sensing. Improved Method for Measuring Solid-State Photoluminescence in Aqueous Solution. An in situ measuremental system to investigate the potential of solid state luminescent MOFs for the additive-sensing function in aqueous solutions has been constructed in our research group.29 As shown in Chart S1, solid crystal UJN-Eu4 was ground into powder, which was further pressed into a circular sheet. The two arc edges of the circular sheet were removed by gently scraping with a drawknife. Then, the suitable sheet sample was sticked on the surface of a piece of quartz slide (dimension: 8.0 mm × 40.0 mm) with the help of the cyanoacrylate adhesive (also called 502 adhesive). The quartz slide coated with solid-state sample suitable for the photoluminescence measurement was finally pushed into a 1 cm suprasil cuvette, which was positioned on the universal solid sample chamber appended in an Edinburgh FLS920 phosphorimeter. The emission and excitation spectra of sample UJN-Eu4 before and after exposure to pure water or different aqueous solution, were carried out resemble for the solid-state samples as described universally. It is noticeable that to avoid the refraction effect of aqueous solution, the sensor cell positioned on the solid-state sample bracket should be adjusted to a suitable position and firmly fixed with the rubber bands before measuring the fluorescence data. Furthermore, to decrease the measurement errors, the adding or removing of H2O and different aqueous solutions should be performed on the fixed suprasil cuvette through an injector equipped with a long pinhead. All the experimental conditions, such as solid-state sheet sample, excitation wavelength, light source power, excitation/emission slit, were kept constant during the measurements. In order to make the analytes contact with sheet samples more sufficient between the solid/liquid surfaces, all the fluorescence data

11



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were collected after 10 min. The effects of quartz slide and adhesive were evaluated and found to be negligible as shown in Figure S14. Photoluminescence and Water-Enhanced Luminescence. As shown in Figure 3, UJN-Eu4 displays well resolved red-emitting characteristic of Eu3+ ion from the transition 5D0→7FJ (J=0-4). Excitation and emission spectra of UJN-Eu4 before and after exposure to pure water show that the most intense emission line of 620 nm can be found among the three splitted emission peaks of Eu3+ (5D0→7F2) upon excitation at 394 nm, and the strongest excitation line of 394 nm appears among some sharp lines due to the intraconfigurational 4f→4f transitions of Eu3+ ions (7F0→5H6, 7F0→5D4, 7F0→5G2, 7F0→5L6 and 7

F0→5D2 ).30

Figure 3. Excitation (a) and emission (b) spectra for UJN-Eu4 before and after exposure to pure water (excited and monitored at 394 and 620 nm, respectively). It is known that OH- oscillators in the water molecules usually act as quenching species for the lanthanide luminescence. However, UJN-Eu4 after exposure to pure water gives more than onefold enhancement of luminescence compared with the original sample, which means that water molecules in fact enhance the luminescence but not quench it during the process of the luminescence measurements. On the other hand, we wonder that whether the enhanced fluorescence of the sample after exposure to water can be reversible reverted to the former weak fluorescence of the original sample upon drying the wet sheet sample. Therefore, the quartz slide coated with solid-state sheet sample was drawn out of the suprasil cuvette, and air-dried overnight in room temperature. Then the sample was used to perform the next rotated fluorescence test (Here, it is noticeable that all the experimental conditions, such as excitation wavelength, excitation / emission slit and the position of the suprasil cuvette should be kept same with the first fluorescence measurement). The results show that the fluorescence intensity of the air-dried sample is nearly same with that of original one (Figure S15), demonstrating that the

12


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luminescent signal becomes weaker once the wet sheet sample is air-dried in room temperature, and the characteristic of the reversible fluorescence response can also be found between the original and air-dried samples. Here, it should be pointed out that the fluorescence of the adhesive film attached to quartz slide was also measured under the same experimental conditions (Figure S14). Taking the emission spectrum of UJN-Eu4 as a comparison (Figure 3), the fluorescence of the quartz slide coated with the adhesive can be negligible. We know that the 5D0→7F2 transition for Eu3+ is particularly sensitive to the nature of Eu3+ environment. Thus, the luminescence of Eu3+ can provide valuable information about the local environment and make it very suitable for acting as a structural probe deciphering the symmetry of the chemical environment. Especially, the ratio of the intensities for 5D0→7F2 and 5D0→7F1 transitions is very sensitive to the symmetry of the Eu3+ centers, because the 5D0→7F1 emission is attributed to the magnetic dipole and independent of the environment, while the 5D0→7F2 emission is due to the electric dipole, sensitive to the crystal field symmetry. As shown in Figure 3b, for the original sample UJN-Eu4, the intensity ratio of the 5D0→7F2 and 5D0→7F1 transitions is 7.7. While the intensity ratio of UJN-Eu4 exposed to pure water is determined to be 9.1. The larger increased value in the intensity ratio of 5D0→7F2 and 5D0→7F1 transitions, suggests that the environment of Eu3+ for sample exposed to solvent aqueous molecules is probably less symmetric and much more strained than that of original sample, thus triggering enhancement of Eu3+ emission.31 This consequence can be exemplified by the crystal structure of UJN-Eu4 as mentioned below. Crystal structure for UJN-Eu4 shows that one coordinated water molecule is presented independently in the axial position of trigonal anti-prism. Upon exposure to solvent water, the coordinated water molecule will interact with the solvent aqueous molecules through one face of the trigonal antiprism. Therefore, the environment of Eu3+ becomes less symmetric, resulting in some strain between the coordinated water and some solvent aqueous molecules (Figure S16). When the wet sample was air-dried in room temperature, the solvent aqueous molecules were removed, which would again give rise to a coordinated water molecule lonely resided in the axial position of trigonal anti-prism. It should be pointed out that the related behavior of water-enhanced luminescence for lanthanide MOFs has also been discovered by Daniel et. recently.32 As proposed by them, this interesting reverse is attributed to the band sensitization of a network solid instead of the traditional antenna effect. Luminescent Sensing. The unique luminescent property for Eu3+ ion, the high porosity and excellent water stability of UJN-Eu4, make us to explore its ability of detecting a trace analysis for analytes in aqueous solutions. When UJN-Eu4 is soaked in MClx ( M = K+, Ca2+, Zn2+, Cd2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+ and Mn2+ ) aqueous solutions, it emits the visible red light with characteristic emission peaks 13



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of Eu3+ ion, and only the highest emission peak at 620 nm is monitored. The emission spectra reveal that the various metal ions display markedly different effects on the luminescence of Eu3+ ion (Figure S17-S18). Obviously, alkaline metal and alkaline-earth metal ions have basically no effect on the emission intensity at 620 nm for UJN-Eu4, while others have different degrees of quenching effects on the emission intensity, especially for Fe3+ and Cu2+ ions with more significant quenching effects. For a better

understanding

of

the

fluorescence

response

of

UJN-Eu4

to

Fe3+

and

Cu2+,

concentration-dependent luminescent measurements were further performed in detail. As depicted in Figure 4a and Figure 4c, UJN-Eu4 indeed exhibits an easily distinguished “turn-off” detection of Fe3+ and Cu2+ ions in water solutions. Upon adding the concentration of Fe3+ solution to 0.9 mM, the emission intensity at 620 nm is more than 3.9 times weaker than that of pure water, which means that UJN-Eu4 possesses relatively high sensitivity in the detection of Fe3+ ion through fluorescence quenching (turn-off). Furthermore, the decrease of luminescence intensity is still clearly observed when UJN-Eu4 is soaked in a 5×10-6 M Fe3+ solution, indicating that the detection limit of UJN-Eu4 as a Fe3+ probe is much more lower. Comparatively, most of the reported MOFs can detect Fe3+ with concentrations from 10-3 to 10-5 M,33-37 and the lowest detected concentration of Fe3+ (0.9×10-6 M)is reported in MIL-53(Al).38 Evidently, the detection limit of Fe3+ using UJN-Eu4 as a sensor, is comparable to or better than some previously reported fluorescence sensors.

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Figure 4. Emission spectra (excited at 394 nm) of UJN-Eu4 soaked in different concentrations of FeCl3 (a), CuCl2 (c) and SDBS (e) aqueous solutions, and the linear correlations for the plots of (I0/I)-1 vs. different concentrations of FeCl3 (b), CuCl2 (d) and SDBS (f) aqueous solutions, respectively. (Taking the emission spectrum of the sample soaked in pure water as a comparison). On the other hand, compared with the Fe3+ concentration (0.9 mM), only 2.6 times of the initial 15



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emission intensity at 620 nm is quenched even the concentration of Cu2+ increase to 10.0 mM. The result indicates that UJN-Eu4 should exhibit relatively high selectivity to Cu2+ ion in the mixed solution of Fe3+ and Cu2+ ions. Apart from the obvious “turn-off” detection of Fe3+ and Cu2+ ions in water solution, a high sensitive fluorescence enhancement is also observed upon taking UJN-Eu4 as a fluorescent sensor for SDBS in aqueous solutions as shown in Figure 4e. Compared with the initial pure water, the emission intensity at 620 nm can be enhanced by almost 3.4 times when the SDBS solution with a concentration of 400 ug/mL is added to the sensor cell, implying the great potential of UJN-Eu4 for SDBS sensing in aqueous solution. Furthermore, the luminescence “turn-on” respondence in the presence of SDBS aqueous medium can be clearly detected to the low concentration 5ug/mL (Figure S19), which is equivalent to 5 ppm of SDBS in water. The observed detection limit can satisfy the highest requirements of SDBS (5-20 ppm) for synthetic detergent industry (seeing Integrated Wastewater Discharge Standard: GB 8978-1996, P.R.China). On the other hand, the decreased emission intensity at 620 nm can be curiously observed when the concentration of SDBS further increases to larger than its critical micelle concentration (CMC) value (1.2 mM or 418 ug/mL). It is known that the surfactants in aqueous solutions should be presented in their monomeric forms below their CMCs, while the micelles in aqueous solutions can be found above their CMCs. The fluorescence response of UJN-Eu4 to SDBS demonstrates that the effect of SDBS in the monomeric forms on the emission intensity at 620 nm is higher than the micellar ones. Therefore we speculated that the excessive SDBS molecules in the micellar forms would block the excitation path of the incoming UV light and optically distort the intensity of the emission spectrum. Based on experimental data of the emission intensity, the fluorescence quenching efficiency can be calculated by using the Stern−Volmer (SV) equation, I0 / I = KSV [A] + 1, where I0 is the initial emission intensity before the addition of the analyte, I is the emission intensity after the addition of the analyte, [A] is the molar concentration of the analyte, and KSV is the quenching coefficient. As shown in Figure 4b and Figure 4d, the SV plots are nearly linear at the concentration ranges of 0.0−0.9 and 0.0−10.0 mM for FeCl3 and CuCl2 aqueous solutions, respectively. The KSV values for the response of UJN-Eu4 to Fe3+ and Cu2+ ions are accurately estimated to be of 3139 and 153 M−1, respectively. However, the SV plots for SDBS aqueous solutions are nearly linear at the concentration range of 0.0−400 ug/mL with an obviously negative slope, and subsequently deviate from linearity above the concentration of 400ug/mL (Figure 4f). The nonlinear nature may be attributed to the formation of micelle-like structure by taking into account the CMC for SDBS. The KSV value for SDBS is found to be -650 M−1, as verified by linear fitting of the SV plot at the concentration range of 0.0-400 ug/mL. As 16


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far as we have been able to ascertain, the detection of SDBS in aqueous solutions through fluorescence enhancement (turn-on) by employing luminescent lanthanide frameworks as fluorescent sensors, has never been reported previously. It should be pointed out that the fluorescence intensity of the water-washed sample after treatment with metal ion (or SDBS) solution is nearly same with that of original sample after exposure to pure water (Figure S20-S22). The results demonstrated that the used solid-state sheet sample of UJN-Eu4 can be regenerated by simple water-washing, and reused in the aforementioned experiments without notable pollution by the former adding of the analyte solutions. Mechanism of Luminescence “Turn-Off” Detection for Fe3+ and Cu2+. In order to elucidate the possible mechanism for such photoluminescence quenching by different metal ions, the UV/Vis absorption spectra for the corresponding metal ion solutions, together with the UV-Vis diffuse-reflectance and emission spectrum for solid state UJN-Eu4 were further measured. As shown in Figure S23, the solution of Fe3+ exhibits a strong absorption band from 250 to 550 nm, which is completely overlapped by the UV-Vis diffuse reflectivity spectrum of solid state UJN-Eu4. The result demonstrates that the luminescent quenching caused by Fe3+ ion is absolutely attributed to the competition absorption between Fe3+ ion and solid sample. On the other hand, as show in Figure S24, the UV-Vis absorption spectrum of Cu2+ ion solution displays a gradual broadening of the absorption band in the visible region of 550-900 nm (red), being well consistent with its solution color (green). Comparatively, the UV-Vis absorption spectrum of Cu2+ ion solution shows much overlap with the emission spectrum of UJN-Eu4. It is evident that the competition between the absorption of Cu2+ ion and the emission of solid state sample results in the decreased emission intensity at 620 nm for UJN-Eu4. Mechanism of Luminescence “Turn-On” Detection for SDBS. Surfactants are known to enhance the emission of luminescent metal complexes in water solution.39-41 According to the previous reports, high luminescence intensity characteristic of Eu3+ or Tb3+ is often achieved due to spontaneous formation of micelle-like structure in which the hydrophobic core prevents luminescence-quenching by water molecules.42-45 However, as mentioned above, the effect of SDBS concentration on the emission intensity at 620 nm for UJN-Eu4 shows that the emission intensity rapidly increases with the increasing of SDBS concentration up to its CMC value (418 ug/mL), and thereafter decreases with the increase of concentration. The results confirm that the formation of micelle-like structure between solid sample and SDBS is impossible since the micelle solutions can only be formed above the CMC value of SDBS. Furthermore, it should be pointed out all the previous reported photoluminescence analysis between the metal complex and surfactant are based on the liquid/liquid surfaces. However, our 17



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home-made sensor cell used as the solid-state photoluminescence measurements in aqueous solution is completely different from any of these approaches, since it takes advantage of the liquid/solid surface as a platform to integrate the lanthanide frameworks and the analyte of SDBS in close proximity. We know that the amphiphilic SDBS molecules can be viewed as the conjugation of a C12 hydrophobic chain with a hydrophilic benzene sulfonate core. When UJN-Eu is soaked in amphiphilic SDBS solution below its CMC value, the monomeric SDBS molecules will be formed. Inspired by the specially porous structure with coexistence of hydrophilic microsized cage-like chambers and hydrophobic nanosized channels for UJN-Eu, as well as the universal theory of ‘like dissolves like’, we presume that the monomeric SDBS analyte should be trapped in the hydrophobic nano-sized channels, forming the weak interactions between the sulfonate groups and Eu3+ skeletons to sensitize europium-based luminescent frameworks. In order to further elucidate the enhanced luminescence mechanism, the fluorescence spectra for the native SDBS solution was also measured as shown in Figure S25. Excitation and emission spectra of 10 ug/mL SDBS solution show that the maximal excitation and emission peaks appear at 250 and 292 nm, respectively. Under excitation of 250 nm, a weak emission band at ca. 578 nm can be found besides the intense ultraviolet emission (ca. 292 nm and 340 nm). Upon excitation at 394 nm (which is the excitation condition for UJN-Eu4), SDBS solution can only emit weakly (λmax = 578 nm). Considering the intense absorption of the aromatic moieties, we tentatively assign the intense and weak emission peaks in the SDBS solution to the dodecyl benzene and sulfonate groups, respectively. Combined with the emission of UJN-Eu4, it is most likely to lead to energy transfer from sulfonate groups to Eu3+ skeletons under excitation, resulting in the increase of the luminescent intensity finally. As for the fluorescent enhancer, previous researchers have proposed a light-producing pathway originated from the oxidation of sulphite.46-50 Based on the presence of sulfonate group emission (578 nm ) upon excitation at 394 nm, as well as the similarities between the sulphite and the sulfonate groups presented in the SDBS, the possible enhanced luminescence mechanism for the response of UJN-Eu4 to SDBS may be the direct excitation of UJN-Eu4 associated with indirect energy transfer between the excited sulfonate groups and the Eu3+ skeletons, as shown in Figure S26 and Chart S2. CONCLUSIONS We chose one of the cycloalkane dicarboxylic acid derivatives and successfully prepared a new family of 3D lanthanide WCOFs [Ln4(µ4-dcd)6(H2O)] (UJN-Ln4; Ln = Sm, Eu, Gd and Tb; dcd=3,3-dimethylcyclopropane-1,2-dicarboxylate dianion) through in situ ligand-oriented assembly process. Two types of synergistic coordination linkages are simultaneously observed in constructing the 18


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wheel cluster layer-pillared structure: one is between the triangular pyramidal Ln4 SBUs and dcd ligands adopting anti-conformations; the other is between the Ln24 wheel cluster layers and dcd ligands adopting gauche-conformations. These two synergistic coordination linkages work together to promote the formation of finally porous Ln-WCOFs featuring hydrophobic nanosized channels surrounded by 1D trigonal anti-prism arrays with hydrophilic cage-like chambers. This is the first observation that the nanosized lanthanide wheel cluster layers formed by Ln24 TBUs are further pillared by dcd ligands in Ln-WCOFs. Moreover, these Ln-WCOFs demonstrate exceptional water stability attributed to the hydrophobicity of nanosized channels. Photoluminescent studies reveal that solvent water molecules can enhance the fluorescence intensity of solid state UJN-Eu4 in our home-made sensor cell used as the solid-state photoluminescence measurements in H2O and different aqueous solution. Due to the competitive absorption and emission responses between different metal ions and solid state sample, water-stable UJN-Eu4 is highly sensitive towards Fe3+ and Cu2+ ions in aqueous solutions through fluorescence quenching (turn-off). Most importantly, UJN-Eu4 also demonstrates highly sensitive sensing of SDBS in aqueous medium through fluorescence enhancement (turn-on). The enhanced luminescence response of UJN-Eu4 to SDBS likely originates from the direct excitation of UJN-Eu4 associated with indirect energy transfer between the excited sulfonate groups and the Eu3+ skeletons. To our knowledge, this is the first example of quantitatively detecting SDBS in aqueous solutions by employing luminescent lanthanide frameworks as fluorescent sensors. Moreover, used as a recyclable luminescent sensor, UJN-Eu4 can also be reused by simple water-washing. We anticipate that many Ln-WCOFs would be synthesized through other cycloalkane dicarboxylic acid derivatives, strongly corroborating a less-investigated but feasible strategy by employing cyclopropane dicarboxylic acid derivatives as ligands to design and assemble new luminescent Ln-WCOFs applied in sensitive detection of harmful pollutants in aquatic environments. ASSOCIATED CONTENT Supporting Information Schematic representation of home-made sensor cell used as the solid-state photoluminescence measurements in H2O and different aqueous solution, mechanism speculation on the luminescent enhancement for SDBS, block-like single crystal pictures, FT-IR spectra, the experimental and simulated PXRD patterns, the single Eu24 wheel cluster viewed as TBU, the single hydrophobic 12-membered-ring channel surrounded by six trigonal anti-prism arrays, perspective view of the hydrophilic cage-like chamber and the free hydrophobic channel, the equivalent 3D topology framework, the TG/DTA curves, temperature-dependent PXRD patterns, the PXRD patterns for the 19



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rehydrated and freshly isolated samples, water vapor sorption isotherm for UJN-Eu4 measured at 298 K, emission spectra for the adhesive film before and after exposure to pure water, emission spectra of UJN-Eu4 before and after exposure to pure water, schematic representation of the interactions between the coordinated water and solvent aqueous molecules, emission spectra for UJN-Eu4 soaked in different concentrations of metal ions and SDBS aqueous solutions, UV-Vis absorption spectrum of FeCl3 in aqueous solution and UV-Vis diffuse reflectivity spectrum of solid state UJN-Eu4, UV-Vis absorption spectrum of CuCl2 in aqueous solutions, excitation and emission spectra of SDBS solution, schematic representation of the indirect energy transfer between the excited sulfonate groups and the Eu3+ skeletons, an X-ray crystallographic file (CIF). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (grant no. 21171068) and the Shandong Provincial Natural Science Foundation of China (grant no. ZR2010BM036).

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Gu,

X.;

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D.

3D

Coordination

Framework

[Ln4(µ3-OH)2Cu6I5(IN)8(OAc)3]

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