Two Dynamic ABW-Type Metal Organic Frameworks Built of

Article pubs.acs.org/crystal

Two Dynamic ABW-Type Metal Organic Frameworks Built of Pentacarboxylate and Zn2+ as Photoluminescent Probes of Nitroaromatics Ling Di,† Jian-Jun Zhang,*,† Shu-Qin Liu,† Jun Ni,† Huajun Zhou,*,‡ and Ying-Ji Sun† †

Chemistry College, Dalian University of Technology, Dalian 116024, China High Density Electronics Center, University of Arkansas, Fayetteville, Arkansas 72701, United States

‡

S Supporting Information *

ABSTRACT: Self-assembly reactions of Zn2+ and L5− (H5L = 2,5-(6-(4-carboxyphenylamino)-1,3,5-triazine-2,4-diyldiimino) diterephthalic acid) lead to the formation of two new ABWtype zeolitic metal−organic frameworks (Z-MOFs): (Me2NH2)[Zn2L]·3.5DMF (1) and (Me2NH2)[Zn2L(H2O)]· 2DMF·8H2O (2) (DMF = N,N-dimethylformamide). They are the first two Z-MOFs which are built of the same pentacarboxylate ligand and metal ion but have two configurations and channel shapes (distorted honeycomband herringbone-shaped channels for 1 and 2 respectively). They can demonstrate interesting structural transformations triggered by vacuum heating or soaking in different solvents. While direct transformations between 1 and 2 were revealed to be not feasible, 2 could be first transformed to a crystalline intermediate 3 and then into 1. Furthermore, while transformations between 2 and 3 are irreversible, those between 1 and 3 are reversible, accompanied by a 26 nm shift of their emission peak positions. In comparison to the ligand, 1, 2, and 3 exhibit blue shifts in their luminescent emission peaks and have intensive blue emission in both solid and solution phases. The efficient and selective quenching of their photoluminescence by a series of nitroaromatics (NACs) solutions phase and by nitrobenzene (NB) vapor makes them promising probes for detecting NACs. 1− 3 represent the first series of MOFs as promising photoluminescent probes for detecting dinoseb down to 2.4 ppm. The electron transfer, long-range energy transfer, and/or electrostatic interactions between the frameworks and NACs mainly contribute to the quenching mechanisms.

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INTRODUCTION Zeolites, characterized by interconnected tetrahedra of SiO4/ AlO4 and non-interpenetrating porosity, are among the most important porous solid-state materials. However, their applications have generally been limited to small molecules due to small pore and channel sizes.1,2 In contrast, metal organic frameworks (MOFs) have more tunable pores, channels, and functionalities. The functionalities can come from individual inorganic and organic components and synergic interactions between them.3−5 Their porosities and functionalities can jointly help MOFs to find niche applications.6−8 Among them, responsive MOFs (dynamic frameworks or “breathing” MOFs) with flexible frameworks can reversibly deform their pores or channels under various external stimuli9−13 such as light, temperature, and guest molecules, making them intriguing candidates for selective capture, storage, release, and separation of molecules, and thus chemical sensing.14−17 Zeolitic metal organic frameworks (Z-MOFs), a unique subset of MOFs, represent a perfect union of zeolites and MOFs.18−22 Among the over 170 natural or synthetic zeolites, ABW (sra, SrAl2, or CeCu2) represent a topology in which 4-, © XXXX American Chemical Society

6-, and 8-membered rings are used as building blocks. A number of ABW-type Z-MOFs based on tetrahedral molecular building blocks (MBBs) and organic linkers have been reported.23−28 Interestingly, depending on the structures of organic ligands and MBBs, the channels of these ABW Z-MOFs can adopt three kinds of configurations and shapes of pores and channels, namely, honeycomb,26 brick-wall,27,28 and herringbone23 (Scheme S1). Up to now while most of Z-MOFs utilize imidazolate, pyrimidine, and their derivatives as ligands,29−38 Z-MOFs using pure polycarboxylate ligands have remained largely unexplored.39−41 Herein, we report the syntheses of two new ABW-type responsive Z-MOFs, namely, (Me2NH2)[Zn2L]· 3.5DMF (1) and (Me2NH2)[Zn2L(H2O)]·2DMF·8H2O (2) (H5L = 2,5-(6-(4-carboxyphenylamino)-1,3,5-triazine-2,4-diyldiimino) diterephthalic acid) through reactions between the pentacarboxylate ligand and different zinc salts. 1 and 2 represent the first series of Z-MOFs using a pentacarboxylate Received: May 1, 2016 Revised: June 10, 2016

A

DOI: 10.1021/acs.cgd.6b00656 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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mixture was allowed to stabilize to afford a stable PL spectrum. The quenching efficiency (%) was estimated using the formula [(I0 − I)/I0] × 100%. Thin layers of 1−3 were prepared for detecting NACs vapors. A total of 0.5 mL of solvent (DMF for 1 and 2, acetonitrile for 3) and 3 mg of finely grounded powders were mixed on a quartz plate to get a slurry. In parallel, a 5 mL open glass vial containing 100 mg of solid NAC (1,3-DNB, 1,4-DNB, 2,4-DNT, 2,6-DNT, TNT, or TNP) or 1 mL of NB was put into a sealed glass cuvette for a few days to ensure that the NAC’s vapor pressure reached equilibrium. When the slurry became dry, the quartz plate was put into the glass cuvette which was sealed for a specific period of time. Then the slide was taken out for immediate PL measurements. Crystal Structure Determination. Intensity data of 1 were collected at 220(2) K on a Bruker SMART APEX II CCD area detector system. Data reduction and unit cell refinement were performed with Smart-CCD software. Intensity data of 2 were collected at 100(2) K on a Mar CCD 165 mm detector by ω-scan techniques on the beamline 3W1A of Beijing Synchrotron Radiation Facility (BSRF) at the wavelength of 0.80000 Å. Data were corrected for absorption effects using the spherical harmonics technique. Both structures were solved and refined using the Bruker SHELXTL (version 6.1) Software Package.55 The hydrogen atoms were included in the structural model as fixed atoms (using idealized sp2-hybridized geometry and C−H bond lengths of 0.95 Å) “riding” on their respective carbon atoms. No attempts were made to locate the hydrogen atoms on coordinated water molecules. Since the (Me2NH2)+ countercations and the disordered H2O and DMF solvent molecules could not be unambiguously modeled, the PLATON/ SQUEEZE56 program was utilized to calculate the solvent disorder area and remove its contribution to the overall intensity data. The residual electron densities amounted to 158 and 178 e per formula for 1 and 2, respectively, which roughly correspond to one (Me2N)+ and three and a half DMF molecules for 1 while one (Me2N)+, two DMF, and eight H2O molecules for 2. A summary of the most important crystal and structure refinement data is given in Table 1.

ligand and two ABW-type Z-MOFs built of the same ligand and metal ion but with different configurations and channel shapes. Their reversible and/or irreversible transformations triggered by vacuum heating or soaking in solvents, i.e., relatively rare “breathing” behaviors with 3 as a crystalline intermediate, were investigated. Recently photoluminescent MOFs have been proven to be good candidates for sensing of cations, anions, small molecular organics, and biomacromolecules.17,42−45 Thus, the photoluminescence (PL) of 1−3 and the quenching of their PL spectra upon exposure to both vapor phases and lowconcentrations of nitroaromatics (NACs) solutions were also investigated. Among these widely used but toxic and explosive NACs,46−53 dinoseb, an important polymerization inhibitor in petrochemical industries and a pesticide in agriculture, currently requires sophisticated instruments for its detection. Our work here represents the first attempt to use MOFs for its detection. Details of their crystal structures, dynamic “breathing” behaviors and mechanisms, and photoluminescent detection of NACs and the underlying mechanisms are investigated.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. All reagents of analytical grades were purchased and used as received without further purification. H5L was synthesized following a reported method54 and characterized by electrospray ionization (ESI) and infrared spectra (IR) (Figure S1). ESI mass spectra were recorded by a Q-TOF Micro MS mass spectrometer. IR of KBr pellets in the range of 4000−400 cm−1 were recorded on a ThermoFisher 6700 spectrometer. Thermogravimetric analysis (TGA) were performed under N2 atmosphere at a heating rate of 10 °C/min using a TA-Q50 thermogravimetric analyzer. NMR spectra were recorded at ambient temperature on a Bruker Avance II 400 M spectrometer. Powder X-ray diffraction (PXRD) data over the 2θ range of 3−50° were collected on D/MAX-2400 at the scan rate of 5°/min at room temperature. Elemental analyses (C, H, N) were determined on a Vario EL elemental analyzer. Photoluminescence spectra were recorded at room temperature by a Hitachi F-7000 luminescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The emission decay lifetime was measured on an Edinburgh instrument FLS920 fluorescence spectrometer. Synthesis of (Me2NH2)[Zn2L]·3.5DMF (1). A 20 mL scintillation vial containing a mixture of H5L (11.5 mg, 0.02 mmol) and Zn(NO3)2· 6H2O (5.9 mg, 0.02 mmol) in 2.5 mL of DMF/H2O (5:1 v/v) was heated at 115 °C for 24 h and allowed to cool to room temperature. The colorless crystals were collected, washed with DMF, and dried in air (yield: 12.38 mg, 61.8%). Elemental analysis calcd for C38.5H45.5N10.5O13.5Zn2: C 46.14, H 4.58, N 14.68: found C 45.93, H 4.58, N 14.73. IR (KBr pellet, cm−1): 3441 (vs), 3303 (m), 3157 (m), 1566 (s), 1503 (s), 1364 (s),770 (m). Synthesis of (Me2NH2)[Zn2L(H2O)]·2DMF·8H2O (2). The above procedure was followed while Zn(Ac)2·2H2O was used instead. The colorless crystals were collected, washed with DMF, and dried in air (yield: 11.07 mg, 53.2%). Elemental analysis calcd for C34H53N9O21Zn2: C 38.72, H 5.06, N 11.95; found C 38.98, H 4.68, N 11.85. IR (KBr pellet, cm−1): 3433 (vs), 3280 (m), 1549 (s), 1506 (s), 1370 (s), 769 (m). Detecting NACs in Dispersions and Vapor Phases. Each detection was repeated at least three times, and consistent results were obtained, and the excitation slit width and emission slit width in PL measurements were 5 and 2.5 nm, respectively. Finely ground MOF samples (10 mg) were dispersed in 10 mL of solvent (DMF for 1 and 2, acetonitrile for 3) or 0.01 M 10 mL solutions of a series of analytes, treated by ultrasonication for 4 h and then aged for 1 day to form a stable emulsion before the fluorescence studies. To study the concentration-dependent quenching efficiencies, the NAC solution was added stepwise into the Z-MOF dispersion, and the resulting

Table 1. Crystal Data and Structure Refinement for Compounds 1 and 2 formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3)/Z Dcalcd (g/cm3) μ(mm−1) F(000) θ range (deg) Reflections collected/unique R(int) GOF on F2 R1a, I > 2σ(I) (all) wR2b, I > 2σ(I) (all) max/mean shift in final cycle

1

2

C38.5H45.5N10.5O13.5Zn2 1002.15 orthorhombic P2(1)2(1)2(1) 16.7533(3) 17.1585(3) 21.5052(5) 90 90 90 6181.9(2)/4 0.752 0.807 1404 1.89−25.00 45428/10874 0.0653 1.049 0.0421 (0.0905) 0.0530 (0.0929) 0.001/0.000

C34H53N9O21Zn2 1054.65 orthorhombic P2(1)2(1)2(1) 14.261(3) 16.664(3) 26.160(5) 90 90 90 6217(2)/4 0.767 0.804 1444 2.44−29.36 23866/15307 0.0449 1.091 0.0600(0.0615) 0.1711(0.1734) 0.003/0.000

a R = ∑(||Fo| − |Fc||)/∑|Fo|. bRw = {∑w[(F2o − F2c)]/∑w[(F2o)2]}0.5, w = [σ2(F2o) + (aP)2 + bP] −1, where P = (F2o + 2F2c)/3 ]. 1, a = 0.0374, b = 0.0000; 2, a = 0.1356, b = 0.0000.

B

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Figure 1. Crystal structure of 1. (a) The coordination environment of the binuclear Zn2+ center. Symmetry codes: A = −x, y − 1/2, −z + 3/2; B = −x + 1/2, −y + 1, z + 1/2; C = −x + 1, y − 1/2, −z + 3/2. (b) The quasi-paddlewheel {Zn2(CO2)5} as a MBB; (c) the bridging mode of the ligand. (d, e) The 3D frameworks viewed along the b (d) and a (e) axes.

Figure 2. Schematic representation of the 4-connected net in 1.

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RESULTS AND DISCUSSION Crystal Structure of 1. Single-crystal X-ray diffraction analysis of 1 reveals that it crystallizes in P212121 space group and bears a 4-connected anionic 3D framework chargebalanced by (Me2NH2)+ generated through the decomposition of DMF molecules. The asymmetric unit contains two independent Zn2+ (Zn1 and Zn2) and one independent ligand. While each Zn2+ adopts a four-coordinated distorted tetrahedral O4 donor set, four carboxylate O atoms from three and four L5− ligands coordinate to Zn1 and Zn2, respectively (Figure 1a). The two Zn2+ ions are bridged by three carboxylates with Zn··· Zn separation of 3.40 Å, and each Zn2+ ion is also monocoordinated by a carboxylate group. The resulted quasipaddlewheel {Zn2(CO2)5} MBB can be regarded as a 4connected node as the five carboxylate groups belong to four L5− ligands (Figure 1b). Both the Zn−O bond lengths

(1.867(3)−1.972(2) Å) and the O−Zn−O bond angles (93.99(8)−123.96(10)°) are close to those reported for other complexes containing Zn2+-carboxylate units.57 Each ligand can also be considered as a 4-connected linker (Figure 1c). It adopts a μ7-κO1: κO1: κO2: κO1: κO1: κO1: κO1 coordination mode and uses its five carboxylate groups to coordinate to seven Zn2+ from four MBBs. The 4-connected MBBs and 4-connected ligands connect to each other in a 1:1 ratio to afford a 3D anionic framework (Figure 1d and Figure S3) with disordered (Me2NH2)+ cations and DMF molecules located in the distorted honeycomb-shaped channels (21.3 × 13.4 Å2) along the a axis (Figure 1e). The solvent accessible volume of 1 without guest molecules is calculated by PLATON56 to be about 67.1% of the unit cell volume, i.e., 4146.3 Å3 out of 6181.9 Å3. A topological analysis using the TOPOS58 reveals that its framework is a zeolitic uninodal 4C

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Figure 3. Crystal structure of 2. (a) The coordination environment of the binuclear Zn2+ center. Symmetry codes: A = x − 1/2, −y + 5/2, −z; B = x, y + 1, z; C = −x, y + 1/2, −z − 1/2; (b) the quasi-paddlewheel {Zn2(CO2)5} as the MBB; (c) the 3D framework viewed along the b axis.

Scheme 1. Schematic Diagram Showing Transformationsa

connected ABW network with Schläfli symbol of (42)(63)(8) (Figure 2). The vertex symbols of both the MBB and ligand nodes are 4·6·4·6·62·86. Crystal Structure of 2. 2 has the same ABW-type anionic framework as that of 1. Its 4-connected {Zn2(CO2)5} MBB is almost the same as that of 1 except that an additional coordinated H2O makes Zn2 bear a five-coordinated distorted pyramidal O5 donor set (Figure 3a,b). Though the 4-connected ligands in 1 and 2 adopt the same coordination mode (Figure S4a), their configuration modes differ since they are near mirror images of each other (Figure S5). The MBBs and ligands connect to each other in a 1:1 ratio to afford an anionic 3D framework (Figure S4b,c) with the same topology as that of 1. 2 has rectangular-shaped channels of 17.1 × 13.6 Å2 along the b axis (Figure 3c) which host disordered (Me2NH2)+ cations and DMF solvent molecules. Its solvent accessible volume without guest molecules is calculated by PLATON56 to be about 66.1% of the unit cell volume, i.e., 4106.3 Å3 out of the 6217.0 Å3. Up to now although many organic ligands have been used to construct ABW-type zeolitic frameworks,23−28 our work represents the first example in which L5− is used as a pentacarboxylate ligand to build up such frameworks and the same ligand can lead to ABW-type frameworks with differentshaped channels. Syntheses and Structural Transformations. The syntheses of 1 and 2 were sensitive to the anions of Zn2+ salts and metal-to-ligand ratios in reactants, but insensitive to temperatures and solvents. When NO3− and AC− were used, 1 and 2 were obtained respectively though NO3− and AC− were not included in the products. No crystalline products were obtained when Cl− or SO42− salt was used. While high-yields of 1 and 2 were obtained when the metal-to-ligand ratio in the reactants was 1:1, extremely low-yields of 1 and 2 were obtained when the ratio was 2:1. In contrast, reactions undertaken at 85 to 115 °C led to the same products but 115 °C led to the highest yield. Both of them can be prepared in three kinds of solvent systems including DMF, DMF/H2O, and DMF/H2O/ethanol with the use of DMF/H2O/ethanol affording the highest yield. Replacement of DMF by DMA (DMA = N,N-dimethylacetamide) led to the same products but with lower yields. The purity and crystallinity of the bulk samples were confirmed by powder X-ray diffraction (PXRD) analysis (Figure S6). Solvent- and vacuum heating-induced structural transformations of 1 and 2 into 3 were observed (Scheme 1) and supported by PXRD, elemental analysis, and IR. While 1 and 2

a

Low boiling solvents include methanol, acetone, dichloromethane, acetonitrile, tetrahydrofuran, and cyclohexane.

retained their frameworks when being soaked in solvents with higher boiling points, e.g., DMF, DMF/H2O, DMF/H2O/ ethanol, DMA, DMA/H2O, and N-methyl-2-pyrrolidone at either room temperature or 115 °C (Figure S7), both were transformed into a new crystalline phase 3 when being soaked for 24 h in solvents with lower boiling points, e.g., methanol, acetone, dichloromethane, acetonitrile, tetrahydrofuran, and cyclohexane. Similar transformations were also achieved when they were vacuum-heated at 150 °C for 30 min (Figure S8). 3 remained intact when it was soaked in the above low boiling point solvents either at room temperature or 45 °C. However, it was transformed into 1 when it was soaked into the above solvents with higher boiling points either at room temperature or 115 °C (Figure S9), indicating the reversible transformations between 1 and 3. In contrast, 3 could not be transformed back to 2, possibly because it is very difficult, if not impossible, for 2 to recover its lost coordinated water molecule and original framework. Therefore, 2 could be transformed to 3 and then to 1. However, many attempts at direct transformations between 2 and 1 were proven unsuccessful. Although 1 and 2 have very similar porosities (67.1% for 1 and 66.1% for 2) and calculated densities (0.752 g/cm3 for 1 and 0.767 g/cm3 for 2 without guest solvents and counteractions), 2 is less thermodynamically stable than 1 possibly because various external stimuli can lead to the removal of coordinated H2O molecules and thus the irreversible collapse of the framework. The reasons why 1 and 2 remain stable in solvents with higher boiling points while 3 remains stable in solvents with lower boiling points are still being investigated. D

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Figure 4. (a) Histograms of the percentages of remaining emission intensities after suspensions of 1 were dispersed in 0.01 M DMF solution of different organics. (b) The PL spectra of 1/DMF upon an incremental addition of dinoseb solution. Inset: a photograph showing the original and the decreased luminescence upon addition of dinoseb. (c) Plots of quenching efficiencies vs concentrations of different NACs. (d) The time-dependent PL quenching of 1 upon exposure to NB vapor at 35 °C. Insets: results of emission intensities before (purple) and after (blue) for three continuous cycles of PL quenching on exposure to NB vapor for 15 min.

Thermal Stabilities of 1−3. Thermogravimetric analyses (TGA) under N2 atmosphere were used to study the thermal stabilities of compounds 1−3 (Figure S11a). When 1 was heated from 30 °C to about 151 °C, 25.9% weight was lost, corresponding to the loss of guest DMF molecules (cal. 25.5%); the framework remained stable from 151 to 348 °C, as indicated by the long plateau in this temperature range; the subsequent continuous weight loss indicated the complete decomposition of the structures. In 2, the first weight loss (27.1%) in 30−153 °C corresponded to the loss of guest H2O and DMF molecules (cal. 27.5%); then there was a pseudo plateau in 153−350 °C and thereafter a continuous weight lost up to 850 °C. 2 is less stable than 1 in the range of 150−350 °C possibly due to the slow collapse of the framework after the removal of coordinated H2O molecules during heating. 3 lost its guest solvent molecules (4.2% in weight) at 30−145 °C which are lower than temperatures in the cases of 1 and 2 possibly due to its solvent molecules having low boiling points. After experiencing a quasi-plateau in 96−380 °C, 3 went through a complete structural decomposition during the subsequent heating. Interestingly, 3 was revealed to be able of absorbing water after its desolvated form was exposed to air at room temperature for 1 h. (Figure S11b). In the first cycle, 3 lost its 2.6% weight which corresponded to the loss of one H2O

The transformations of 1 or 2 into 3 via vacuum heating rules out a possible dissolution−recrystallization mechanism, i.e., solvent-mediated transformation, but strongly suggests a solidstate process as observed in many dynamically flexible frameworks.59 3 is polycrystalline but not single-crystalline, indicating the transformation of 1 or 2 to 3 is not via a singlecrystal-to-single-crystal (SCSC) route. Contrary to the conventional rigid and stable zeolites, here the two ABW-type Z-MOFs show remarkably dynamic behaviors possibly due to three reasons: first, the three C− NH−C connections are flexible enough for the ligand to adopt different conformations to meet different coordination requirements (Figure S5); second, L5− has five carboxylate groups and is used in a “3 + 2” 4-connected node in which three carboxylate groups connect to three {Zn2(CO2)5} MBBs respectively while the remaining two carboxylate groups connect to the fourth MBB. The separation between the triazine ring and the fourth MBB can be adjusted through adjusting the conformation of the ligand’s two related arms (Figure S10) to further support flexible structural transformation; third, Zn2+ can adopt either tetrahedral O4 or distorted pyramidal O5 coordination polyhedrons in 1 and 2, in comparison with the rigid tetrahedral N4 coordination polyhedron in imidazolate-based zeolitic frameworks.60 E

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molecule (cal. 2.35%). ∼3.6% weight loses were observed in the subsequent three cycles, indicating that 3’s desolvated form can reversibly and repeatedly absorb 1.5 H2O molecules (cal. 3.49%) from air. All these results indicate 3’s framework is stable up to 145 °C and holds less solvent molecules than 1 and 2. Photoluminescent Sensing of NAC Dispersions and Vapors. The solid-state photoluminescence (PL) spectra of H5L, 1, 2, and 3 (λex = 360 nm) were collected at room temperature (Figure S12). An intense emission peak at 470 nm observed in the PL spectrum of H5L should correspond to a linker-localized n → π* or π → π* transition. In contrast, 1, 2, and 3 exhibit obvious blue shifts with strong peaks at 436, 440, and 462 nm respectively possibly due to the Zn2+-L 5− coordination which can increase the ligand conformational rigidity and reduce nonradioactive decay.61,62 Their different emission blue shifts may be attributed to the different configurations of ligands and coordination modes of the metal ions (Figure S5).61,62 The emission decay lifetimes of 1− 3 are 1.8580, 1.7593, and 1.8311 μs, respectively. Here guest molecules induced the reversible structural transformation between 1 and 3 accompanied by the emission peak shift, which has been rarely reported in dynamic frameworks.9−13 Their PL spectra in solvents (DMF for 1 and 2, acetonitrile for 3) where they remained stable were measured (Figure S13). Strong emissions at 434 nm for 1, 430 nm for 2, and 455 nm for 3 are close to those of their solid-state samples. Both strong emissions and stabilities in organic solvents make them excellent candidates for photoluminescent sensing of NACs in solutions. Photoluminescent sensing experiments were carried out using suspensions of 1 and 2 in DMF and 3 in acetonitrile. Significant emission intensities, above 80% for 1, 90% for 2, and 60% for 3 were quenched by a series of NACs including nitrobenzene (NB), 1-nitronaphthalene (1-NP), 4-nitrotoluene (4-NT), 4-nitrobromobenzene (4-NB), dinoseb, and 3dinitrobenzene (3-DNB) (Figure 4a and Figure S14−15). Among them, 1-NP and dinoseb could completely quench the emissions of 1−3 (quenching efficiencies >99%). In contrast, the addition of non-nitro-containing aromatics such as aniline, bromobenzene, styrene, benzene, toluene, acetophenone, and benzaldehyde has negligible effect. All the observations suggest that 1−3 are selectively quenched by NACs though they were revealed by PXRD analyses to retain their original frameworks when exposed to NACs (Figures S16−17). To further examine the sensitivity of detecting NACs, PL quenching titrations were performed with the incremental addition of NACs to suspensions of 1−3. As the amount of dinoseb (Figure 4b) and other five NACs (Figures S18−23) increases, the quenching efficiencies increases. 1.0 × 10−5 M, i.e., 2.4 ppm, dinoseb was found to quench 19.2%, 16.7%, and 12.2% of the emissions from 1, 2, and 3 respectively, indicating their extremely high sensitivities toward dinoseb detections. The quenching efficiency (%) was quantitatively explained by the Stern−Volmer equation: I0/I = 1 + KSV × [Q], where I0 and I are the fluorescence intensity before and after the addition of NACs respectively, while KSV and [Q] are the quenching constant (M−1) and the molar concentrations of NACs (Figure 4c and Figures S24−25). The SV plots for NB, 4-NT, 4-NB, and 3-DNB at concentrations below 2 mM were nearly linear. In contrast, the plots for dinoseb and 1-NP at lower concentrations (0.05 mM and 0.2 mM respectively) were almost linear, and subsequently tended to saturate at higher

concentrations possibly due to self-absorption or energy transfer.63,64 The calculated quenching constants (Ksv) of the six NACs (Table 2) are comparable with those of known Table 2. Quenching Constants (KSV) of Different NACs toward Quenching Emissions of 1−3

1 2 3

NB [M−1]

3-DNB [M−1]

4-NT [M−1]

4-NB [M−1]

1-NP [M−1]

Dinoseb [M−1]

318.6 504.6 194.0

243.8 364.2 262.8

396.0 277.7 237.8

400.7 232.4 283.6

9653.4 3954.2 2862.6

35573.1 19036.5 11184.9

MOFs.63,64 The quenching constant of dinoseb toward quenching 1’s emission is ∼4 times greater than that of 1-NP and 30−50 times greater than those for the other four NACs, further confirming its best quenching effect. The same trend also applies to the cases for 2 and 3. In contrast, among all six NACs only NB can use its vapor as a fluorescence quencher of 1−3 possibly due its extremely high vapor pressure (0.2416 mmHg at 25 °C) as well as the presence of electron-withdrawing groups. 91.5%, 96.3%, and 42.5% quenching efficiencies were observed after 1, 2 and 3 were exposed to NB vapors for 15 min (Figure 4d and Figure S26) and are comparable to the highest quenching efficiencies previously reported when NB vapors were used to quench other MOFs’ emissions.64 Interestingly, the emissions of 1−3 can be regenerated after being quenched. For 1, 86.7% and 80.1% of the emission intensities could be recovered for the second and third cycles respectively after the quenched forms were heated at 60 °C for 15 min. In contrast, the regeneration of 2 took 12 h of heating at 60 °C and only 71.9% and 58.2% emission intensities could be recovered for the second and third cycles, respectively. For 3, a much lower percentage of emission intensities was recovered despite longer regeneration time (24 h). Mechanisms for the Photoluminescent Sensing of NACs. Different factors could help explain the observed photoluminescent sensing behaviors. First, the photoinduced electron transfer from the conduction band (CB) of the electron-rich Z-MOFs to the LUMO orbitals of the electrondeficient NACs can help quench the photoluminescence of 1− 3 by inhibiting the relaxation to the ground state.63,64 Since the CB of frameworks lies higher than the LUMO energies of the NACs, the energy levels of NACs’ LUMO are expected to represent how easily an electron can be transferred to the NACs in the quenching process. The HOMOs and LUMOs of H5L and all the NACs were calculated with density functional theory (DFT) at the B3LYP/6-31G* level (Figure 5a), and the LUMO energy of dinoseb was revealed to be the lowest among all six NACs, in good agreement with its highest quenching efficiencies. Second, the overall nonlinear SV plots for dinoseb and 1-NP and the spectral overlap between NACs’ absorption and 1−3’s emission (Figure 5b) suggest a simultaneous resonance energy transfer from frameworks to the NACs during the quenching. The energy transfer was also evidenced by the discriminatory quenching of the emissions (434, 430, and 455 nm for 1, 2, and 3) over the peak around 495 nm in the PL titration experiments of dinoseb (Figure 4b and Figure S22). Efficient quenching of the emission occurs through energy transfer process due to the suitable spectral overlap, while poor quenching of the emission peaked around 495 nm through F

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Figure 5. (a) The HOMO and LUMO energy levels for all the NACs and H5L; (b) spectral overlaps between the absorption spectra of NACs and the emission spectra of 1−3.

design and synthesis of Z-MOFs, dynamic “breathing” behavior of soft porous crystals, and applications of Z-MOFs as promising probes for in-field detection of NACs.

weak energy transfer was due to the poor spectral overlap. A higher degree of spectral overlap results in higher probability of energy transfer and hence higher quenching efficiency.63,64 Thus, the large, small, and almost zero spectral overlap of dinoseb, 1-NP, and other four NACs can well explain the high and medium quenching efficiencies of dinoseb and 1-NP observed among the six NACs. The order of the other four NACs’ slightly different quenching efficiencies does not match that of their corresponding LUMO energy levels possibly due to their different configurations. Third, the electrostatic interactions between the basic sites in MOFs and the phenolic groups in NACs have been reported to help quench MOFs’ photoluminescence.63,64 For 1 and 2, the channels (distorted honeycomb for 1 or herringbone for 2) and three free secondary amino groups in each L5− ligand favor strong electrostatic interactions between dinoseb and the frameworks, leading to sensing dinoseb better than other non-phenolic-containing NACs. The electrostatic interactions were supported by gradual blue shifts (27, 22, and 29 nm for 1−3 respectively) of the emissions, gradual red shifts of the peak around 495 nm upon the addition of dinoseb (Figure 4b and Figure S22), and the absence of such shifts for other NACs. Such spectral shifts could also be used to differentiate dinoseb from other NACs. Last, other factors such as dispersed solvents and coordination environment of the metal ions may also contribute to the different quenching efficiencies of 1−3.

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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.cgd.6b00656. Table S1 and Figures S1−S26 (PDF) Accession Codes

CCDC 1477413−1477414 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 Authors

*(J.-J.Z.) E-mail: [email protected]. *(H.Z.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation of China (21071025, 21471024), the Fundamental Research Funds for the Central Universities (DUT15ZD118 and DUT15LK20). BSRF (Beijing Synchrotron Radiation Facility) is also acknowledged for the crystal structure determination using synchrotron radiation X-ray diffraction analysis.

CONCLUSIONS In summary, two 3-D ABW-type Z-MOFs, namely, (Me2NH2)[Zn2L]·3.5DMF (1) and (Me2NH2)[Zn2L(H2O)]·2DMF· 8H2O (2) with distorted honeycomb- and herringbone-shaped channels, were constructed from a pentacarboxylate ligand and Zn2+. The appealing structural transformations between them can be triggered by vacuum-heating or guest molecule exchanges through a crystalline intermediate 3. A series of low-concentration NACs solutions can quench the photoluminescence of 1−3, and they show higher sensitivities toward detecting dinoseb than other NACs and represent the first series of Z-MOFs to function as promising photoluminescent probes for detecting dinoseb down to 2.4 ppm. The electron transfer, long-range energy transfer, and/or electrostatic interactions between the frameworks and NACs could contribute to the quenching efficiencies. Furthermore, NB vapor can quench the photoluminescence of 1−3 due to its high vapor pressure. This work provides a new insight into the

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DOI: 10.1021/acs.cgd.6b00656 Cryst. Growth Des. XXXX, XXX, XXX−XXX