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Neutral Luminescent Metal-Organic Frameworks: Structural Diversification, Photophysical Properties, and Sensing Applications Gouri Chakraborty and Sanjay K. Mandal* Department of Chemical Sciences, Indian Institute of Science Education and Research Mohali, Sector 81, Manauli PO, S.A.S. Nagar, Mohali, Punjab 140306, India

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S Supporting Information *

ABSTRACT: Utilizing flexible bis(tridentate)polypyridyl ligands, the two new luminescent 2D metal organic frameworks {Zn2(tpbn)(2,6-NDC)2}n (1) and {[Zn2(tphn)(2,6-NDC)2]· 4H2O}n (2), where tpbn = N,N′,N″,N‴-tetrakis(2-pyridylmethyl)-1,4-diaminobutane, tphn = N,N′,N″,N‴-tetrakis(2-pyridylmethyl)-1,6-diaminohexane, and 2,6-H2NDC = 2,6-naphthalenedicarboxylic acid, have been isolated in good yields under solvothermal conditions. Their solid-state molecular structures have been determined by single-crystal X-ray diffractometry. Both 1 and 2 have pentacoordinated Zn(II) centers with an N3O2 environment from three nitrogen atoms of the tpbn or tphn ligand and two carboxylate oxygen atoms from two different 2,6NDC linkers. However, the binding modes of the tridentate part of polypyridyl ligands to the Zn(II) center are different in 1 and 2meridional (tpbn) vs facial (tphn) due to an increase (1.5 times) in the methylene chain length. Thus, the binding mode of 2,6-NDC to the Zn(II) center differs: bis(monodentate) synanti in 1 and bis(monodentate) syn-syn in 2. This difference in binding modes of the components has a profound effect on the conformation of the six-membered ring (metal centers are considered as the vertices in it) within the 2D framework: honeycomb vs chair form for 1 and 2, respectively. In addition to further characterization by elemental analysis and UV−vis and FT-IR spectroscopy, their framework stabilities in water and thermal properties have been studied by powder X-ray diffraction and thermogravimetric analysis, respectively. On the basis of thermodiffractometry, 1 and 2 retain their crystallinity and overall structure up to 350 and 325 °C, respectively. Their luminescent properties have been utilized to demonstrate sensing of various solvents as well as nitro-aromatic compounds in water, which correlate well with their structural differences. Through the spectral overlap, lifetime measurements, and nature of the Stern−Volmer plots, the fluorescence quenching pathway for the nitroanalytes, particularly 2,4,6-trinitrophenol (TNP), is established for 1 and 2. Their recyclability and stability after sensing experiments are found to be excellent.

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INTRODUCTION In the last few decades, the study of metal-organic frameworks (MOFs) has been one of the key areas of chemical research for various potential applications in catalysis,1,2 chiral separation,1,3 sensing,1,4 gas storage/separation,1,5 magnetism,1,6 ion exchange, luminescent materials,1,7 nonlinear optics,1 and drug delivery.1,8 With a large database of such MOFs reported in the literature, current efforts are focused on the functional MOFs where various modifications in the components, such as nitrogen donor ancillary ligands and multitopic anionic linkers, are carried out to develop correlations between structural diversity and desirable properties. For some time, the detection of nitro-aromatic compounds (NACs), such as 2,4,6-trinitrophenol (TNP), trinitrotoluene (TNT), 2,4-dinitrophenol (2,4-DNP), dinitrotoluene (DNT), p-nitrophenol (4-NP), 1,3-dinitrobenzene (1,3-DNB), nitrotoluene (NT), and nitrobenzene (NB), has been one of the major issues concerning national security and environmental implications because these are the common ingredients of © 2017 American Chemical Society

harmful explosives and essential raw materials for chemical industries.9 These are used in the dye, pesticide, and pharmaceuticals industry and are large contributors to environmental pollution, causing detrimental effects including mutagenic changes in the body. In this respect, luminescent MOFs are highly important due to their use in detecting these harmful NACs with high selectivity and sensitivity. Those with d10 metal ions (Zn2+ and Cd2+) along with fluorogenic linkers containing π-conjugated moieties are found to be good candidates for designing luminescent MOFs. The electrondeficient nature of NACs, which can interact with the electronrich frameworks, results in quenching of the emission intensity.10 Selective detection of NACs in water is preferred over that in other organic solvents,11,12 for which the stability of the framework in water is required. However, examples of luminescent MOFs in the literature for sensing of NACs in Received: September 6, 2017 Published: November 17, 2017 14556

DOI: 10.1021/acs.inorgchem.7b02264 Inorg. Chem. 2017, 56, 14556−14566

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Figure 1. Schematic representation of (a) 2D MOF with fused hexagons consisting six metal nodes as the vertices, synthesized from a 1:1 ratio of M2+ (M = metal) and bifunctional fluorescent linkers and (b) effect of spacer chain length on the conformation of the hexagon.

Figure 2. Structures of the ancillary ligands and linker used in this study.

water are rare.13−16 There are only four examples with 2,6NDC for the sensing of TNP in water:14−16 namely, {Zn4(DMF)(Ur)2(2,6-NDC)4}n (where Ur = urotropine and 2,6-NDC = 2,6-napthalenedicarboxylic acid), {Zn(2,6-NDC)(H2O)}n, {Cd(2,6-NDC)(H2O)}n, and {[Cd(2,6-NDC)L]2· H2O}n (L = 4-amino-3,5-bis(4-imidazol-1-ylphenyl)-1,2,4triazole). However, {[Zn2(2,6-NDC)2(bpy)]·xG}n (bpy = 4,4′-bipyridine, G = guest solvent molecules), {Cd(ppvppa)(1,4-NDC)}n (ppvppa = N-(pyridin-2-yl)-N-(4-(2-(pyridin-4yl)vinyl)phenyl)pyridin-2-amine), and {[H2N(CH3)2]·[Zn(2,6NDC)(atz)·H2O]}n (Hatz = 1H-tetrazole-5-amine) were reported for selective sensing of NT, 2,4-DNP, and NB in ethanol, aqueous medium, and dimethylformamide (DMF), respectively.17 On the other hand, there are few examples where selective fluorescence quenching response toward TNP in ethanol, acetonitrile, and DMSO at the ppm level has been demonstrated.18−20 On the basis of systematic studies, we have recently demonstrated the role of the flexible chain length in the bis(tridentate)polypyridyl ligands that connect the two metal ions21a in the formation of diverse structural architectures of Mn(II) with varied dimensionality and porosity, wherein the aliphatic acetylenedicarboxylic acid (H2adc) has been used. On the other hand, the role of spacer atoms in dicarboxylate linkers in the formation of diverse coordination architectures of Cu(II)

has been reported.21b Utilizing such observations, we planned to introduce metal ions as well as fluorescent linkers which could provide luminescent MOFs with diversity in their photophysical properties and sensing applications. For such a purpose, a fluorescent linker such as 2,6-H2NDC has been introduced for two reasons: (i) to help in the formation of diversified structure due to the presence of a planar naphthalene moiety between two carboxylates oriented at an angle of 180° which is similar to that for H2adc but with an aromatic ring and (ii) to have sensing applications due to its fluorogenic nature. In addition to the compounds with 2,6NDC mentioned above,14−17,19 several other compounds with 2,6-NDC have been reported in the literature but not utilized for sensing applications.22 For a 2:1 ratio of M2+ (M = metal) to a dicarboxylate, we have shown earlier that either cationic molecular rectangles or cationic 1D coordination polymers are formed.21 On the other hand, it is expected that a 1:1 ratio of M2+ to a dicarboxylate will result in neutral 2D MOFs with fused six-membered ringsmetal centers are considered as the vertices in the sixmembered ring (see Figure 1a). For such 2D frameworks, we can consider three possibilities as shown in Scheme S1 in the Supporting Informationhoneycomb and chair conformers where the fusion of two chairs further results in a cis-decalin or trans-decalin type structure. Due to the flexible chain lengths in 14557

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Synthesis of {[Zn2(tphn)(2,6-NDC)2]·4H2O}n (2). A mixture of Zn(OAc)2·2H2O (29 mg, 0.13 mmol), tphn (31 mg, 0.062 mmol), and 2,6-H2NDC (28 mg, 0.13 mmol) in H2O/DMF (1 mL/1 mL) was heated in a 5 mL Teflon-lined stainless-steel reactor at 120 °C for 48 h and then cooled to room temperature in 24 h at a rate of 5 °C/h. The resultant yellow block-shaped crystals were collected via filtration, washed with water, and dried in air. Yield: 76% (53 mg) based on Zn. Anal. Calcd for C54H60N6O14Zn2 (MW 1147.83): C, 56.50; H, 5.27; N, 7.32. Found: C, 56.06; H, 5.12; N, 7.39. Note that the best fit for elemental analysis was with six water molecules in comparison to the four water molecules found in its crystal structure. Selected FTIR peaks (KBr, cm−1): 3409 (br), 1612 (s), 1579 (s), 1485 (s), 1437 (m), 1398 (s), 1376 (s), 1319 (w), 1053 (s), 1023 (s), 994 (w), 783 (s), 771 (s), 482 (s). Single-Crystal X-ray Data Collection and Refinement. Crystals of the compounds were transferred from the mother liquor to mineral oil for manipulation, selection, and mounting and were then placed inside a nylon loop on a goniometer head. The loop was then placed under a cold stream of nitrogen gas for slow cooling to the desired temperature in each case (100 K for 1 and 200 K for 2; attempts to measure 2 at 100 K failed due to crystal cracking). Initial crystal evaluation and data collection were performed on a Kappa APEX II diffractometer equipped with a CCD detector (with the crystal to detector distance fixed at 60 mm) and sealed-tube monochromated Mo Kα radiation. The diffractometer was interfaced to a PC that controlled the crystal centering, unit cell determination, refinement of cell parameters, and data collection through the program APEX2.23 For each sample, three sets of frames of data were collected with 0.30° steps in ω and an exposure time of 10 s within a randomly oriented region of reciprocal space surveyed to the extent of 1.3 hemispheres to a resolution of 0.85 Å. By using the program SAINT23 for the integration of the data, reflection profiles were fitted, and values of F2 and σ(F2) for each reflection were obtained. Data were also corrected for Lorentz and polarization effects. The subroutine XPREP23 was used for the processing of data that included determination of space group, application of an absorption correction (SADABS),23 merging of data, and generation of files necessary for solution and refinement. The crystal structures were solved and refined using SHELX 97.24 Several full-matrix least-squares/difference Fourier cycles were performed, locating the remainder of the non-hydrogen atoms. Occupancy factors of some atoms were adjusted to have reasonable thermal parameters and converged refinement, resulting in the lowest residual factors and optimum goodness of fit. Crystallographic parameters and basic information pertaining to data collection and structure refinement for all compounds are summarized in Table 1. All figures were drawn using Mercury V 3.025 and OLEX2.26 The final positional and thermal parameters of the non-hydrogen atoms for all structures are given in the CIF files. Powder X-ray Studies. Data were recorded on a Rigaku Ultima IV diffractometer equipped with a 3 kW sealed-tube Cu Kα X-ray radiation (generator power settings: 40 kV and 40 mA) and a DTex Ultra detector using the BB geometry (2.5° primary and secondary solar slits, 0.5° divergence slit with 10 mm height limit slit). For each sample, a fine powder was packed on a glass sample holder that was placed on the sample rotation stage attachment. Data were collected over the angle range 5−50° with a scanning speed of 1°/min with 0.01° step. Sensing Experiments for Solvents and NACs. Finely ground samples of 1 and 2 were used for the detection experiments in making use of the dispersible nature, which will facilitate close contact between the host frameworks and analytes. For the investigation of the luminescent properties of 1 and 2 in different solvents, 1 mg of each sample was added to 2 mL of the respective solvent and their quenching behaviors were recorded. Sensing experiments for NACs were carried as follows: 1 mg of each sample was well dispersed in water (2 mL) by stirring for 10 min to prepare a stable suspension, after which the 1 mM stock solution of NACs (stock solutions of those without a phenolic group were prepared in 9.5 mL of water and 0.5 mL of methanol) was added to the suspension prepared above to monitor the fluorescence quenching.

the bis(tridentate)polypyridyl ligands that connect the two metal ions, we envisioned that the interplay of the honeycomb and chair conformers of the hexagonal arrangement is possible by pulling two metal centers inward (Figure 1b). Thus, the honeycomb conformer can be expected for a shorter chain length while the chair conformer will be possible as the chain length is increased. Such a structural diversity can be studied in detail by a systematic combination of the variables (metal ion and its preferred coordination geometry, nature and binding of the carboxylate linker, and the flexibility of the chain length of the neutral linker) at hand. To demonstrate our concept, we have first elected to showcase the effect of spacer chain length in bis(tridentate)polypyridyl ligands on the structural diversity. Using the Zn(II) metal center, we have obtained {Zn2(tpbn)(2,6-NDC)2}n (1) and {[Zn2(tphn)(2,6-NDC)2]·4H2O}n (2), where tpbn = N,N′,N″,N‴-tetrakis(2-pyridylmethyl)-1,4-diaminobutane, tphn = N,N′,N″,N‴-tetrakis(2-pyridylmethyl)-1,6-diaminohexane, and 2,6-H2NDC = 2,6-naphthalenedicarboxylic acid (see Figure 2). Herein, we report their synthesis and structural characterization by elemental analysis, FT-IR and UV−vis spectroscopy, thermogravimetric analyses, and powder and single-crystal X-ray diffraction studies. The difference in binding modes of the components has a profound effect on the conformation of the six-membered ring within the 2D framework: honeycomb vs chair form for 1 and 2, respectively. On the basis of thermodiffractometry, 1 and 2 retain their crystallinity and overall structure up to 350 and 325 °C, respectively. Utilizing their excellent stability in water, we have studied 1 and 2 to demonstrate sensing of various solvents as well as nitro-aromatic compounds in water by fluorescence spectroscopy.

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EXPERIMENTAL SECTION

Materials and Methods. All chemicals and solvents used for synthesis were obtained from commercial sources and were used as received, without further purification. All reactions were carried out under aerobic conditions. The tpbn and tphn ligands were prepared following the literature procedure.21 Caution! TNP and TNT are highly explosive and should be handled carefully in small amounts. The explosives should be handled as dilute solutions and with safety measures to avoid an explosion. Physical Measurements. Elemental analysis (C, H, N) was carried out using a TruSpec Micro CHNS analyzer. FT-IR spectra were measured in the 4000−400 cm−1 range on a PerkinElmer Spectrum I spectrometer with samples prepared as KBr pellets. UV− vis spectra of the NACs in water (a typical concentration of 2 mM) were recorded on a Cary 6000 UV−vis spectrophotometer from Agilent Technologies using a quartz cuvette of path length 10 mm. Fluorescence spectra were recorded using a Horiba fluorescence spectrophotometer. Lifetime measurements of 1 and 2 were carried out using a time-resolved Horiba Scientific single photon counting controller. Synthesis of {Zn2(tpbn)(2,6-NDC)2}n (1). A mixture of Zn(NO3)2·6H2O (39 mg, 0.13 mmol), tpbn (28 mg, 0.062 mmol), and 2,6-H2NDC (28 mg, 0.13 mmol) in H2O/DMF (1 mL/1 mL) was heated in a 5 mL Teflon-lined stainless-steel reactor at 120 °C for 72 h and then cooled to room temperature in 36 h at a rate of 5 °C/h to form colorless block-shaped crystals. These were collected via filtration, washed with water to remove the nitric acid byproduct, and dried in air. Yield: 67% (44 mg) based on Zn. Anal. Calcd for C52H44N6O8Zn2 (MW 1011.14): C, 61.73; H, 4.38; N, 8.31. Found: C, 61.08; H, 4.58; N, 7.48. Selected FTIR peaks (KBr, cm−1): 3432 (br), 1613 (s), 1482 (m), 1443 (m), 1381 (s), 1340 (s), 1311 (w), 1094 (m), 1021 (m), 991 (w), 786 (s), 763 (s), 477 (s). 14558

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S2 in the Supporting Information), a series of bands from 1612 to 1311 cm−1 is present, out of which some of the features are due to the ancillary ligands, tpbn or tphn, with a shift of a few wavenumbers. The asymmetric and symmetric stretching frequencies of the carboxylate groups of 2,6-NDC dianion appear at 1613 and 1381 cm−1 (1) and 1612 and 1398 cm−1 (2), respectively. Values for the difference between the asymmetric and symmetric stretching frequencies, 232 and 214 cm−1 for 1 and 2, respectively, are indicative of the monodentate binding mode of the carboxylate groups, as is clearly evident in their crystal structures (vide infra). Description of Structures. Crystals of 1 and 2 suitable for single-crystal X-ray studies were selected from the solvothermal reactions. Compound 1 crystallizes in the monoclinic P21/c space group. Each Zn(II) center in 1 exhibits a pentacoordinate geometry with an N3O2 environment from three nitrogen atoms of the tpbn ligand and two carboxylate oxygen atoms from two different 2,6-NDC groups, as shown in Figure 3a. Each tridentate part of the tpbn ligand binds to the Zn(II) centers in a meridional fashion, which is evident from the N1− Zn1−N3 angle of 154.52(14)°. On the other hand, the 2,6NDC binds to the Zn(II) centers in a bis(monodentate) synanti fashion. The coordination environment of Zn(II) in 1 with labeled atoms is provided in Figure S3 in the Supporting Information. The Zn(II)···Zn(II) distances are 8.194 Å (connected by tpbn) and 13.467 Å (connected by 2,6-NDC). The Zn−Npy distances (2.123(5)−2.166(5) Å) are shorter than the Zn−Nalkyl distance (2.211(5) Å). The two Zn−Ocarb distances are found to be almost similar, Zn−O1 (1.990(4) Å) and Zn−O3 (2.017(4) Å), and are in a range similar to that reported in the literature.14,15 The O−Zn−Npy angles are 105.48(17)° (O1−Zn1−N1), 97.59(18)° (O3−Zn1−N1), 91.99(18)° (O1−Zn1−N3), and 95.58(17)° (O3−Zn1−N3). The O−Zn−Nalkyl angles are in the range of 125.86(15)− 128.67(15)°. Compound 2 crystallizes in the monoclinic P21/n space group. Similar to the case for 1, all Zn(II) centers in 2 are pentacoordinated with an N3O2 environment. Each Zn(II) center is coordinated to three nitrogen atoms of the tphn ligand and two carboxyl oxygen atoms from two different 2,6-NDC groups (see Figure 3c). Each tridentate part of the tphn ligand binds to the Zn(II) centers in a facial fashion, which is evident from the N1−Zn1−N3 angle of 114.09(5)°. Due to this change in the binding mode of tphn in comparison to tpbn, the 2,6NDC binds to the Zn(II) centers in a bis(monodentate) synsyn fashion. The coordination environment of Zn(II) in 2 with labeled atoms is provided in Figure S3 in the Supporting Information. The Zn(II)···Zn(II) distances are 11.001 Å (connected by tphn) and 13.013 Å (connected by 2,6-NDC). As is the case for 1, the Zn−Npy distances (2.0906(14)− 2.1119(13) Å) are shorter than the Zn−Nalkyl distance (2.3337(13) Å), which is longer than that in 1. The two Zn−Ocarb distances are found to be almost similar, Zn−O1 (2.0067(2) Å) and Zn−O3 (2.0119(12) Å). The O−Zn−Npy angles are 109.71(5)° (O1−Zn1−N1), 110.53(6)° (O3−Zn1− N1), 129.44(5)° (O1−Zn1−N3), and 89.53(5)° (O3−Zn1− N3). The O−Zn−Nalkyl angles are 90.55(5) and 165.47(5)°. The selected bond distances and angles for 1 and 2 are summarized in Table S1 in the Supporting Information. By variation of the methylene chain length from tpbn to tphn, the coordination geometry of Zn(II) in 1 and 2 has been greatly affected, as shown in Figure 3b,d, respectively: in 1 it is

Table 1. Crystallographic Data and Structure Refinement Parameters for 1 and 2 chem formula formula wt temp (K) wavelength (Å) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) Z V (Å3) density (g/cm3) μ(mm−1) F(000) θ (deg) range for data collecn (deg) no. of rflsn collected no. of indep rflns no. of rflns with I > 2σ(I) Rint no. of params refined GOF on F2 final R1a/wR2b (I > 2σ(I)) R1a/wR2b (all data) largest diff peak and hole (e Å−3)

1

2

C26H22N3O4Zn 505.83 100(2) 0.71073 monoclinic P21/c 9.0705(18) 12.146(3) 19.737(4) 90 94.509(4) 90 4 2167.7(8) 1.550 1.174 1044 1.97−25.00 13322 3812 2825 0.0608 307 1.054 0.0507/0.1185 0.0744/0.1315 1.093/−0.434

C27H28N3O6Zn 555.89 200(2) 0.71073 monoclinic P21/n 9.7994(12) 19.134(2) 13.5922(16) 90 94.010(2) 90 4 2542.3(5) 1.452 1.014 1156 1.84−25.00 14458 4484 4083 0.0157 340 1.034 0.0242/0.0637 0.0271/0.0657 0.269/−0.377

R1 = Σ∥Fo| − |Fc∥/Σ|Fo|. bwR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2, where w = 1/[σ2(Fo2) + (aP)2 + bP], P = (Fo2 + 2Fc2)/3. a

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RESULTS AND DISCUSSION Synthesis. Compounds 1 and 2 were solvothermally (H2O/ DMF, 1/1) synthesized in good yields from a Zn(II) salt, 2,6H2NDC, and the respective hexadentate ligand (in a 2:2:1 ratio) as summarized in Scheme 1. If the ratio of Zn(II) and Scheme 1. Solvothermal Synthesis of 1 and 2

2,6-H2NDC is changed to 2:1, the same products are formed with lower yields based on Zn(II). In comparison to the systems with a 2:1 ratio of metal ion to a dicarboxylate, where either cationic molecular rectangles or cationic 1D coordination polymers are formed, in the current study a 1:1 ratio has resulted in neutral 2D MOFs with fused six-membered rings (vide infra). This confirms a preference for the formation of neutral species over cationic species with such dicarboxylates. For 2, Zn(NO3)2·6H2O was also used to check the effect of anion; however, it led to the formation of the same compound, as confirmed by PXRD. Compounds 1 and 2 are highly stable in air and water and are insoluble in common solvents, such as CH3OH, DMSO, and DMF. On the basis of the FT-IR spectra of 1 and 2 recorded at room temperature in the form of KBr pellets (Figures S1 and 14559

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Figure 3. Coordination environment around (a, c) the Zn(II) centers in the asymmetric units of 1 and 2, respectively, and (b, d) the Zn(II) centers in 1 and 2, respectively.

Figure 4. Schematic representations of 2D structures in (a) 1 with a honeycomb conformation and (b) 2 with a fused chair (cis-decalin) conformation.

chairlike cis-decalin conformation. Both of them belong to the uninodal 3-connected periodic Shubnikov hexagonal plane net with the point symbol {6∧3}, considering the metal center as a three-coordinated node with both naphthalenedicarboxylate and tpbn or tphn as ditopic linkers. Effect of Spacer Chain Length. As the naphthalenedicarboxylate length is constant in 1 and 2 (8.052 and 8.043 Å, respectively), their structural difference mentioned above can be correlated to the spacer chain length in the two bis(tridentate) polypyridyl ligands, tpbn and tphn. As the chain length is varied from four to six methylene groups (6.332 to 8.915 Å), the angle between the two pyridyl nitrogens

more toward square pyramidal, whereas in 2 it is more toward trigonal bipyramidal on the basis of the τ parameters which are 0.43 and 0.60 for 1 and 2, respectively.27 The distances between the metal centers and uncoordinated oxygen atoms (Zn1···O2 and Zn2···O4) are 3.055 and 2.823 Å in 1 and 2.821 and 3.290 Å in 2, respectively. This reverse trend for the Zn···O distances is related to different coordination geometries around the metal center. The overall packing diagrams of 1 and 2 along the a axis are shown in Figure S4 in the Supporting Information. From a topological viewpoint, both 1 and 2 have the hcb topology;28 however, 1 has a honeycomb-like conformation and 2 exhibits a 14560

DOI: 10.1021/acs.inorgchem.7b02264 Inorg. Chem. 2017, 56, 14556−14566

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Additionally, insight into the crystalline behavior of 1 and 2 over a temperature range was sought through variabletemperature powder diffraction patterns with a heating profile from −100 to 350 °C for 1 and −100 to 340 °C for 2. As shown in Figure S9a in the Supporting Information, 1 retains its crystallinity up to 350 °C, which corroborates well with the thermogravimetric analysis described above. Similarly, for 2 a shift in peaks is observed at 50 °C due to loss of lattice solvents, after which the crystallinity is maintained up to 325 °C (see Figure S9b). Photoluminescent Properties. In order to study the photophysical properties of 1 and 2, their solid-state reflectance spectra were recorded, in which both of them exhibited a strong and broad absorption with λmax 250 and 340 nm corresponding to π−π* and n−π* transitions, respectively, as shown in Figure S10 in the Supporting Information. On the basis of these data, emission spectra of 1 and 2 exhibit strong emission at 365 and 372 nm, respectively, on excitation at 320 nm (see Figure S11 in the Supporting Information), which is attributed to the linker-based emission due to the fluorogenic nature of the naphthalene moiety present in 2,6-H2NDC. Another band present at 450 nm in 2,6-H2NDC is suppressed in 1 and 2 due to strong electronic coupling of the 2,6-NDC dianion with the metal centers. When the emission maxima of 1 and 2 are compared with that of 2,6-H2NDC, a slight red shift is observed in the MOFs, which can be attributed to the strong connectivity between the neighboring tpbn or tphn ligands through the Zn(II) ions. Sensing of Solvents and Nitro-Aromatic Compounds. The influence of different solvents such as water (H2O), methanol (CH3OH), ethanol (EtOH), acetonitrile (CH3CN), dimethylformamide (DMF), tetrahydrofuran (THF), and dimethyl sulfoxide (DMSO) on the emission intensity of 1 and 2 was studied. It is significant to observe that 1 and 2 behave differently toward these solvents. However, no shift in emission wavelength was observed due to the change in polarity of the solvents.17b 1 shows the maximum emission intensity in DMF (5.70 × 107) followed by CH3OH, THF, and DMSO, and moderate emission is observed for other solvents. On the other hand, for 2 the maximum emission intensity is observed in EtOH (3.14 × 107) followed by MeOH. However, both 1 and 2 have very strong emission intensities in H2O, 2.92 × 107 and 1.68 × 107, respectively, and are therefore suitable for sensing studies in this solvent. The variation in the luminescence response observed for different solvents can be attributed to electron transfer processes resulting from the interaction between the solvents and the 2D frameworks in 1 and 2.29a Among all the solvents, in nitrobenzene the emission intensity of 1 and 2 was totally quenched (see Figure 5). This is because of the electron-withdrawing ability of the nitro group that facilitates an electron transfer from the excited state of MOFs to nitrobenzene, restricting the self-relaxation to the ground state.29b Inspired by the above results for nitrobenzene, the photoluminescence experiments of NACs were performed due to their electron-deficient nature. The quenching efficiency of the NACs has been evaluated on the basis of the formula (I0 − I)/I0 × 100% (Figure 6 and Figure S12 in the Supporting Information). It is noteworthy to observe that hydroxysubstituted NACs, such as 4-NP, 2,4-DNP, and especially TNP, preferentially quench the emission of 1 over the others and show an order based on the increase in the number of nitro groups. On the other hand, almost all the nitro-analytes

around the Zn(II) center is found to be 154.52(14)° for 1 and 114.09(5)° for 2 with a difference of 40.43°. This is due to the fact that each tridentate part of the tpbn ligand binds to the Zn(II) centers in a meridional fashion in 1 while each tridentate part of the tphn ligand binds to the Zn(II) centers in a facial fashion in 2. Thus, in 1 two pyridyl rings of the tpbn ligand do not come inside the six-membered ring with no hindrance to the pore; however, in 2 the two pyridyl rings block the pore. This change in pore size can be easily visualized from the spacefilling models of 1 and 2, as shown in Figure S5 in the Supporting Information. When the chain length is changed from tpbn to tphn, the dihedral angle between the planes containing two naphthalenedicarboxylates gets widened from 51.64° to 77.18° with a difference of 25.54°. This angle widening enables the insertion of a tphn chain between the naphthalene rings connected to two different Zn(II) centers in 2, forming an angle of 44.24° between the planes containing the hexane spacer and each naphthalenedicarboxylate (see Figure S6 in the Supporting Information). In 1, out of the three metal nodes on each side of a sixmembered ring, two are connected by the methylene spacer of tpbn and one is linked by one naphthalenedicarboxylate in one direction (along the x axis) and one naphthalenedicarboxylate in another direction (along y axis). Two metal nodes at the corner of a six-membered ring are further linked by the dicarboxylate and the methylene chain of tpbn alternately, leading to the formation of a six-membered ring with dimensions of 15.32 Å × 16.75 Å (excluding van der Waals radii). Each six-membered ring is further connected by the neighboring dicarboxylates and methylene chains of tpbn to form a 2D framework resembling a honeycomb-like conformation (see Figure 4a). In 2, three metal nodes present on each side of a six-membered ring provide a perfect chairlike conformation, which is anticipated from the longer spacer chain length in tphn (vide supra). In this case, each chairlike sixmembered ring is fused with another similar ring via the neighboring dicarboxylates and methylene chains of tphn to form a 2D framework, resembling the cis-decalin conformation as shown in Figure 4b. Framework Stabilities and Thermal Properties of 1 and 2. In addition to verification of the formulas of 1 and 2 by CHN analysis and SCXRD (vide supra), their bulk phase purity was confirmed by powder X-ray diffraction, in which the experimental patterns are in good agreement with the simulated patterns from single-crystal data (see Figure S7 in the Supporting Information). Furthermore, the high stability of 1 and 2 in water is verified by matching the powder patterns of each compound immersed in water for 24 h with the respective experimental and simulated powder patterns (Figure S7). This property of retaining crystallinity in water is the key to utilizing these compounds for sensing experiments with NACs (vide infra). To study the thermal stability as a function of temperature, thermogravimetric analyses (TGA) were conducted for the single-phase polycrystalline samples of 1 and 2 between 25 and 500 °C under a dinitrogen atmosphere (see Figure S8 in the Supporting Information). From the multistep decomposition process observed for these compounds, it is clearly evident that 1 and 2 are highly stable up to 350 and 340 °C, respectively. There is no loss of weight in 1 prior to this temperature, indicating an absence of solvent molecules, corroborated by its crystal structure. For 2, there is an initial weight loss of four lattice water molecules (calcd, 7.44%; found, 6.92%) at 100 °C. 14561

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tpbn vs tphn resulted in the pore size difference in 1 and 2. The result has been analyzed using the Stern−Volmer (SV) equation: I0/I = 1 + KSV[A], where I0 and I are the luminescent intensities of the compound before and after addition of the nitro-analytes, respectively, [A] is the molar concentration of the nitro-analyte, and KSV is the quenching constant (M−1). The quenching constants (KSV) for TNP are found to be 5.907 × 103 and 2.464 × 103 M−1 for 1 and 2, respectively, indicating stronger electrostatic interaction between TNP and 1 in comparison to 2 (see Figures S13 and S14 in the Supporting Information). The detection limit of TNP for compound 1 at low concentration is 11 ppm, while that for 2 is 19 ppm (as shown in Figures S15 and S16 in the Supporting Information, respectively). As shown in Table S4 in the Supporting Information, only two similar Zn(II) compounds (containing both dicarboxylate and neutral linkers), in addition to {Zn(2,6NDC)(H2O)}n, have been used for sensing of TNP prior to this study. Considering TNP as the analyte, the KSV value for 1 is higher than that of {[Zn2(2,6-NDC)2(bpy)]·xG}n, where EtOH was used as the solvent, but less than that for {Zn4(DMF)(Ur)2(2,6-NDC)4}n, which is structurally different from 1. However, further modification of N-donor ligands can be done to make numerous derivatives of 1 to improve the detection limit of TNP at the ppb level. This will provide an opportunity to add many more examples to the limited study so far. One can consider the hydrogen bonding between uncoordinated oxygen atoms of the carboxylate groups and the acidic phenolic proton of TNP in 1 and 2. On the basis of the structures of 1 and 2 (Figure 4), the space between two electron-rich naphthalene rings in 1 is 8.9 Å, which allows strong π−π interactions between the naphthalene rings and electron-deficient nitro-analytes (aggregation caused quenching), while there is no such space available in 2. For this purpose, approximate sizes of nitro-analytes are compared in Table S3 in the Supporting Information, as these can accumulate inside the MOF matrix according to interactions and consequent quenching response is observed.29c This seems to be a good correlation with respect to the quenching observed in the presence of TNP with respect to other analytes in 1. On the other hand, in the case of 2 an incoherent behavior of different nitro-analytes is observed without such interactions, although other phenomena such as energy transfers are involved (vide infra). Although our effort was directed toward performing the sensing experiments in water, it was worthwhile to investigate the same in different proportions of water and the respective solvent in which the maximum fluorescence intensity was observed for 1 (DMF) and 2 (EtOH). This provided an opportunity to show that the KSV values for 1 and 2 in aqueous DMF or EtOH, respectively, are better than or comparable to those reported in the literature (see Table S4 in the Supporting Information). Thus, several experiments with TNP were performed in two solvent ratios for 1 and 2. For 1 dispersed in 1:1 or 4:1 DMF:H2O (v:v), the KSV values increased to 2.19 × 104 and 2.89 × 104 M−1 with the corresponding detection limits of 4 and 0.7 ppm, respectively (see Figures S17c,d and S18a,b in the Supporting Information). The detection limit for 1 obtained in DMF:H2O (4:1) is higher than that of {Cd(2,6NDC)(H2O)}n (0.92 ppm) but lower than that of {Zn(2,6NDC)(H2O)}n (0.23 ppm).15 Similarly, for 2 dispersed in 1:1 or 4:1 EtOH:H2O (v:v), the KSV values increased to 7.25 × 103 and 2.2 × 104 M−1 with the corresponding detection limits of 7 and 1.6 ppm, respectively (see Figures S19c,d and S20a,b). The

Figure 5. Effect of solvents on the emission spectra of 1 (top) and 2 (bottom) on excitation at 320 nm.

Figure 6. Comparison of quenching percentages of different nitroanalytes in aqueous medium by 1 and 2.

moderately quench the emission of 2, showing less selectivity in comparison to 1. Consequently, a significant quenching effect was observed for TNP in the case of 1 but not for 2, as shown in Figure 7. It can be correlated to the structural difference observed for the two MOFs, where the modulation of methylene chain length in 14562

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Figure 7. Change in emission spectra of (a) 1 and (b) 2 dispersed in aqueous medium upon incremental addition of TNP solution (1 mM) in water. (c) Comparison of 3D Stern−Volmer plots for TNP in 1 and 2.

Figure 8. Stern−Volmer plots of different nitro-analytes for (a) 1 and (b) 2.

between the absorbance spectra of different nitro-analytes and emission spectra of 1 and 2 were recorded (see Figures S21 and S22 in the Supporting Information). It is found that the maximum extent of overlap takes place in TNP and 2,4-DNP for 1 and in TNP, 2,4-DNP, and 4-NP for 2. This means that there is an energy transfer taking place from electron-rich MOFs 1 and 2 to the electron-deficient analytes due to the presence of electron-withdrawing nitro group(s). Notably, the

detection limit for 2 in EtOH:H2O (4:1) is comparable to that of {Zn4(DMF)(Ur)2(2,6-NDC)4}n (1.63 ppm).14a In the case of 1 and 2, the S-V plots for NACs were linear at lower concentration, which deviated from linearity with a bent upward curve as the concentration was increased (Figure 8). This nonlinear nature can be attributed to the energy transfer mechanism that takes place during fluorescence quenching.29 In order to know the reason behind it, the spectral overlap 14563

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Figure 9. Quenching and recyclability test of 1 (left) and 2 (right). The upper dots represent the initial luminescence intensities, and the lower dots represent the intensities upon addition of 520 μL of the aqueous solution of TNP.

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fluorescence spectrum of 1 is broader in comparison to that of 2, which is attributed to effective spectral overlap and a greater extent of energy transfer with the emission band of 1 in the presence of TNP. The extent of energy transfer was evaluated by calculating the overlap integral J(λ) values using the equation30a J (λ ) =

∫0

∞

CONCLUSIONS In this article, we have demonstrated the effect of methylene chain length acting as a spacer between the alkyl nitrogen of bis(tridentate)polypyridyl ligands on the formation of luminescent 2D frameworks with varied topological conformations from honeycomb to chairlike cis-decalin and their subsequent effect on the sensing of nitro-aromatic compounds. Solid-state molecular structures of {Zn2(tpbn)(2,6-NDC)2}n (1) and {[Zn2(tphn)(2,6-NDC)2]·4H2O}n (2) determined by single-crystal X-ray diffractometry have revealed that the polypyridyl ligands bind to the Zn(II) center differently, meridional for tpbn as opposed to facial for tphn, due to an increase in the methylene chain length by 1.5 times; similarly, the binding mode of 2,6-NDC to the Zn(II) center has been found to be bis(monodentate) syn-anti in 1 and bis(monodentate) syn-syn in 2. Their thermal properties and framework stabilities established by thermogravimetry and thermodiffractometry have been well correlated to their crystal structures. Due to the structural diversity in 1 and 2, it was possible for us to compare their sensing of various solvents as well as nitroaromatic compounds in water. First, 1 and 2 behave differently toward these solvents with the maximum emission intensity in DMF and EtOH, respectively. However, no shift in emission wavelength was observed for 1 and 2 due to the change in polarity of the solvents. This can be attributed to electron transfer processes resulting from the interaction between the solvents and the 2D frameworks in 1 and 2. Second, hydroxysubstituted NACs, such as 4-NP, 2,4-DNP, and especially TNP, preferentially quench the emission of 1 over the others and show an order based on the increase in the number of nitro groups. On the other hand, almost all of the nitro-analytes moderately quench the emission of 2, showing less selectivity in comparison to 1. Consequently, a significant quenching effect was observed for TNP in the case of 1 which is not the case for 2. The detection limits of TNP for 1 and 2 in water have been found to be 11 and 19 ppm, respectively. This can be correlated to the structural difference observed for the two MOFs with pore size differences. Exploiting the fact that the maximum fluorescence intensity of 1 and 2 was observed in DMF and EtOH, respectively, we established an improvement in the detection limits (0.7 ppm in 4:1 DMF:water for 1 and 1.6 ppm in 4:1 EtOH:water for 2). Through the spectral overlap,

FD(λ)εA (λ)λ 4 dλ

where FD(λ) is the corrected fluorescence intensity of donor in the range λ to λ + dλ with the total intensity normalized to unity and εA is the extinction coefficient of the NACs at λ in M−1 cm−1. The maximum overlap integrals for TNP were calculated as 3.8 × 1014 and 3 × 1014 M−1 cm−1 nm4 for 1 and 2, respectively, which are comparable with that for {[Cd(NDC)0.5(PCA)]·xG}n.19 Furthermore, this deviation from linearity indicates a combination of static and dynamic quenching processes between TNP and the host frameworks, as confirmed from the lifetime decay curves of 1 and 2, before and after addition of TNP (see Figures S23 and S24 in the Supporting Information). The average lifetimes of 1 are 8.52 ns (before TNP addition), 8.75 ns (after 80 μL TNP addition), and 13.17 ns (after 520 μL of TNP addition), whereas for 2, they are 6.10 ns (before TNP addition), 7.22 ns (after 80 μL TNP addition), and 8.53 ns (after 520 μL TNP addition) (see Table S5 in the Supporting Information). This observation of change in fluorescence lifetime after TNP addition also shows that photoinduced electron transfer (PET) takes place from the excited state of the MOF to the lowest unoccupied molecular orbital (LUMO) of the electron-deficient trinitro-analyte.30,31 Thus, both PET and energy transfer processes contribute to the fluorescence quenching in the presence of TNP. In order to study the recyclability and stability of 1 and 2, quenching experiments of TNP were carried out by reusing the samples that were regenerated by centrifugation and washing several times with water. For up to five cycles, the detection ability was renewed (Figure 9) and the structural integrity was maintained on the basis of the PXRD patterns of the samples after immersion in TNP for 24 h (Figure S25 in the Supporting Information). This implies that both 1 and 2 are very suitable for long-term in-field detection of strongly acidic TNP due to their high photostability. 14564

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(4) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Metal-Organic Framework Materials as Chemical Sensors. Chem. Rev. 2012, 112, 1105−1125. (5) (a) Han, S. S.; Mendoza-Cortés, J. L.; Goddard, W. A., III Recent advances on simulation and theory of hydrogen storage in metal− organic frameworks and covalent organic frameworks. Chem. Soc. Rev. 2009, 38, 1460−1476. (b) Murray, L. J.; Dincă, M.; Long, J. R. Hydrogen storage in metal-organic frameworks. Chem. Soc. Rev. 2009, 38, 1294−1314. (6) Kurmoo, M. Magnetic metal-organic frameowrks. Chem. Soc. Rev. 2009, 38, 1353−1379. (7) (a) Cui, Y.; Yue, Y.; Qian, G.; Chen, B. Luminescent Functional Metal-Organic Frameworks. Chem. Rev. 2012, 112, 1126−1162. (b) Allendorf, M. D.; Bauer, C. A.; Bhakta, R. K.; Houk, R. J. T. Luminescent metal−organic frameworks. Chem. Soc. Rev. 2009, 38, 1330−1352. (8) (a) Della Rocca, J.; Liu, D.; Lin, W. Nanoscale Metal Organic Frameworks for Biomedical Imaging and Drug Delivery. Acc. Chem. Res. 2011, 44, 957−968. (b) Horcajada, P.; Serre, C.; Vallet-Regí, M.; Sebban, M.; Taulelle, F.; Férey, G. Metal-Organic Frameworks as Efficient Materials for Drug Delivery. Angew. Chem., Int. Ed. 2006, 45, 5974−5978. (9) (a) Chaudhari, A. K.; Nagarkar, S. S.; Joarder, B.; Ghosh, S. K. A Continuous π-Stacked Starfish Array of Two-Dimensional Luminescent MOF for Detection of Nitro Explosives. Cryst. Growth Des. 2013, 13, 3716−3721. (b) Germain, M. E.; Knapp, M. J. Discrimination of Nitroaromatics and Explosives Mimics by a Fluorescent Zn(salicylaldimine) Sensor Array. J. Am. Chem. Soc. 2008, 130, 5422− 5423. (c) Li, L.; Zhang, S.; Xu, L.; Han, L.; Chen, Z.-N.; Luo, J. An Intensely Luminescent Metal−Organic Framework Based on a Highly Light-Harvesting Dyclo-Metalated Iridium(III) Unit Showing Effective Detection of Explosives. Inorg. Chem. 2013, 52, 12323−12325. (d) Kim, T. K.; Lee, J. H.; Moon, D.; Moon, H. R. Luminescent LiBased Metal−Organic Framework Tailored for the Selective Detection of Explosive Nitroaromatic Compounds: Direct Observation of Interaction Sites. Inorg. Chem. 2013, 52, 589−595. (10) Hu, Z.; Deibert, B. J.; Li, J. Luminescent metal−organic frameworks for chemical sensing and explosive detection. Chem. Soc. Rev. 2014, 43, 5815−5840. (11) Gole, B.; Bar, A. K.; Mukherjee, P. S. Modification of Extended Open Frameworks with Fluorescent Tags for Sensing Explosives: Competition between Size Selectivity and Electron Deficiency. Chem. Eur. J. 2014, 20, 2276−2291. (12) Wang, G. Y.; Song, C.; Kong, D. M.; Ruan, W. J.; Chang, Z.; Li, Y. Two luminescent metal−organic frameworks for the sensing of nitroaromatic explosives and DNA strands. J. Mater. Chem. A 2014, 2, 2213−2220. (13) (a) Nagarkar, S. S.; Desai, A. V.; Ghosh, S. K. A fluorescent metal−organic framework for highly selective detection of nitro explosives in the aqueous phase. Chem. Commun. 2014, 50, 8915− 8918. (b) Song, X.-Z.; Song, S.-Y.; Zhao, S.-N.; Hao, Z.-M.; Zhu, M.; Meng, X.; Wu, L.-L.; Zhang, H.-J. Single-Crystal-to-Single-Crystal Transformation of a Europium(III) Metal−Organic Framework Producing a Multi-responsive Luminescent Sensor. Adv. Funct. Mater. 2014, 24, 4034−4041. (c) Wang, B.; Lv, X.-L.; Feng, D.; Xie, L.-H.; Zhang, J.; Li, M.; Xie, Y.; Li, J.-R.; Zhou, H.-C. Highly Stable Zr(IV)-Based Metal−Organic Frameworks for the Detection and Removal of Antibiotics and Organic Explosives in Water. J. Am. Chem. Soc. 2016, 138, 6204−6216. (14) (a) Mukherjee, S.; Desai, A. V.; Manna, B.; Inamdar, A. I.; Ghosh, S. K. Exploitation of Guest Accessible Aliphatic Amine Functionality of a Metal−Organic Framework for Selective Detection of 2,4,6-Trinitrophenol (TNP) in Water. Cryst. Growth Des. 2015, 15, 4627−4634. (b) Sapchenko, S. A.; Samsonenko, D. G.; Dybtsev, D. N.; Melgunov, M. S.; Fedin, V. P. Microporous sensor: gas sorption, guest exchange and guest-dependant luminescence of metal−organic framework. Dalton Trans. 2011, 40, 2196−2203. (15) Ghosh, P.; Saha, S. K.; Roychowdhury, A.; Banerjee, P. Recognition of an Explosive and Mutagenic Water Pollutant, 2,4,6-

lifetime measurements, and nature of the Stern−Volmer plots, it was established that both PET and energy transfer processes contribute to the fluorescence quenching in the presence of TNP. The quenching ability and selectivity observed for 1 can be ascribed to resonance energy transfer with large spectral overlap and electrostatic interaction. Both 1 and 2 were found to be highly photostable in the presence of acidic TNP as well as recyclable without much loss of sensitivity up to five cycles. These results have provided an insight into the construction of new MOFs of several metal ions based on further modification of N-donor ligands and incorporation of various other fluorogenic linkers to improve the detection limit of TNP at the ppb level. Over two dozen such MOFs are being compiled into several papers to be published in the near future.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02264. Crystallographic data of 1 and 2 and additional figures and tables related to characterization and sensing experiments (PDF) Accession Codes

CCDC 1572295−1572296 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for S.K.M.: [email protected]. ORCID

Sanjay K. Mandal: 0000-0002-5045-6343 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this work was provided by IISER Mohali. G.C. is grateful to the MHRD of India for a research fellowship. The use of the X-ray facility at IISER Mohali is gratefully acknowledged.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b02264 Inorg. Chem. 2017, 56, 14556−14566

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DOI: 10.1021/acs.inorgchem.7b02264 Inorg. Chem. 2017, 56, 14556−14566