Multicomponent assembly of fluorescent-tag functionalized ligands in metal-organic frameworks for sensing explosives.

DOI: 10.1002/chem.201402791

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& Sensors

Multicomponent Assembly of Fluorescent-Tag Functionalized Ligands in Metal–Organic Frameworks for Sensing Explosives Bappaditya Gole, Arun Kumar Bar, and Partha Sarathi Mukherjee*[a]

Abstract: Detection of trace amounts of explosive materials is significantly important for security concerns and pollution control. Four multicomponent metal–organic frameworks (MOFs-12, 13, 23, and 123) have been synthesized by employing ligands embedded with fluorescent tags. The multicomponent assembly of the ligands was utilized to acquire a diverse electronic behavior of the MOFs and the fluorescent tags were strategically chosen to enhance the electron density in the MOFs. The phase purity of the MOFs was established by PXRD, NMR spectroscopy, and finally by singlecrystal XRD. Single-crystal structures of the MOFs-12 and 13 showed the formation of three-dimensional porous networks with the aromatic tags projecting inwardly into the pores. These electron-rich MOFs were utilized for detection of ex-

Introduction Metal–organic frameworks (MOFs) constructed from functionalized organic linkers and metal ions are becoming a new class of emerging materials for diverse applications such as gas storage,[1] gas separation,[2] drug delivery,[3] imaging,[4] and heterogeneous catalysis.[5] The design and synthesis of luminescent MOFs has gained huge attention in recent times for the selective and sensitive detection of nitroaromatic compounds (NACs)[6] not only because of their explosive nature, but also for their potential as toxic pollutants.[7] The most commonly used nitroaromatic ingredients are picric acid (PA), 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitrobenzene (TNB), dinitrobenzene (DNB), 2,4-dinitrotoluene (DNT), and so on, and they are also categorized as plastic explosives.[8] Apart from their common uses as explosives, they are also found in several unexploded landmines, which were used during World War II.[9] Detection of these landmines has become one of the recent concerns because they can contaminate ground water. It is established that exposure to TNT leads to anemia and abnormal liver function.[10] Based on such health concerns, the US Environmental [a] B. Gole, Dr. A. K. Bar, Prof. P. S. Mukherjee Department of Inorganic and Physical Chemistry Indian Institute of Science Bangalore-560012 (India) Fax: (+ 91) 80-2360-1552 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402791. Chem. Eur. J. 2014, 20, 13321 – 13336

plosive nitroaromatic compounds (NACs) through fluorescence quenching with high selectivity and sensitivity. The rate of fluorescence quenching for all the MOFs follows the order of electron deficiency of the NACs. We also showed the detection of picric acid (PA) by luminescent MOFs is not always reliable and can be misleading. This attracts our attention to explore these MOFs for sensing picryl chloride (PC), which is as explosive as picric acid and used widely to prepare more stable explosives like 2,4,6-trinitroaniline from PA. Moreover, the recyclability and sensitivity studies indicated that these MOFs can be reused several times with parts per billion (ppb) levels of sensitivity towards PC and 2,4,6-trinitrotoluene (TNT).

Protection Agency (EPA) advised a permissible level of TNT in drinking water of below 2 ppb.[11] Among several well-known methods, only metal detectors and canines are used successfully worldwide, however, they are either less sensitive or very expensive.[12] Due to the strong electron-accepting capability of NACs, fluorescence quenchingbased sensors offer many advantages over the above-mentioned methods in terms of their high sensitivity and in-field application.[13] Although several fluorescence quenching-based sensors like metal complexes,[14] supramolecular polymers,[15] carbon nanotubes,[16] conjugated polymers,[17] dendrimers,[18] and so on are known in the literature, MOFs have emerged as potential candidates for this prospect due their simple synthetic procedure, structural tenability, and easy modification of their electronic property.[6] Since NACs are electron deficient in nature, it is noteworthy to mention that MOFs with high electron density are expected to be suitable for detection of NACs. Generally, two methods are used to enhance the electron density of the MOFs, firstly, using conjugated ligands or backbones with metal ions having a non-detrimental nature of fluorescence.[6] Secondly, the use of the fluorescent tag in the backbone of the ligands that also has the ability to enhance the electron density in the MOFs.[19] The first method was used widely, however, the second method has been introduced recently by our group. In this recent report, we have established that the gradual increase of electron density of the MOFs by fluorescent tag modification from phenyl to naphthalene to pyrene enhanced the sensitivity towards TNT detection rapidly. We also demonstrated that the rate of fluorescence quenching

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Full Paper of the MOFs by NACs not only depends on their electron deficiency but also on the porosity of the MOFs.[19] In the present work, we were mainly concerned on whether we would be able to assemble those fluorescent-tag modified ligands with metal ions in a single phase material. This could give us a wide range of materials with diverse electronic property that may enhance the sensing ability towards NACs. Although multicomponent assembly of the organic linkers is well known in directional self-assembly approach involving PdII or PtII metal ions,[20] the multicomponent assembly of MOFs with similar type ligands and identical structural topology is very rare and challenging.[21] An approach using different organic linkers with similar coordinating functionality generally leads to the formation of a multi-phase mixture rather than a single phase. Though a few reports are known with multicomponent assembly of the linkers having single phase, the reason for the observed single phase is still inconclusive. However, one can expect two distinct behaviors to happen in the process of mixed-ligand assembly:[22] 1) Organic linkers with different reactivity may combine together and lead to a thermodynamically more stable single-crystalline phase; 2) In contrast to that, organic linkers with similar reactivity can randomly crystalize to multiple phases. The determination of the single-crystal X-ray structure of multi-ligand (with similar coordinating sites) assembled MOFs has remained a very challenging task to date due to their lower degree of crystallinity.[ 21a–c] In this regard, we attempted to prepare multicomponent MOFs employing the following ligands: 5-(benzyloxy)isophthalic acid (H2L1), 5-(naphthalen-1-ylmethoxy)isophthalic acid (H2L2), and 5-(pyren-1-ylmethoxy)isophthalic acid (H2L3) in combination with 1,4-di(pyridin-4-yl)benzene (dpb) and ZnII ions. The individual reactions of H2L1, H2L2, and H2L3 with ZnII and dpb lead to the formation of MOF-1, MOF-2, and MOF-3, respectively. However, the stoichiometric combination of those ligands with same metal ions resulted in multicomponent MOFs MOF-12, MOF-13, MOF-23, and MOF-123 in a pure single phase. Although all the MOFs are highly crystalline and gave single crystals, we were able to successfully determine the crystal structures of MOF-12 and MOF-13. All the MOFs were found to display remarkable optoelectronic properties and thus their diverse electronic behavior was utilized for the detection of the NACs with extremely high selectivity and sensitivity. Although picric acid (PA) is one of the main constituents of the NAC family, its detection using fluorescent MOFs in most of the cases is not reliable and might be misleading because of its high acidic nature, which often leads to the degradation of MOFs. However, detection of picryl chloride (PC; with an explosive velocity of 7200 ms1 compared to PA (7350 ms1)), which is as explosive as is PA, becomes increasingly significant. In this regard, the synthesized multicomponent MOFs were employed for the sensing of NACs with the main emphasis on PC and TNT. In addition, we have also focused on the mechanisms of the fluorescence quenching of the NACs by determination of the individual collisional and static Stern–Volmer constants, which would help in designing better sensors in the future. Chem. Eur. J. 2014, 20, 13321 – 13336

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Results and Discussion Synthesis We have already demonstrated in our earlier report that employment of individual H2L1–3 ligands with an equivalent amount of 1, 4-di(4-pyridyl)benzene (dpb) and [Zn(NO3)2]·6 H2O at 120 8C in N,N-dimethylformamide (DMF) for 24 h resulted in 3D networks of MOFs-1, 2, and 3.[19] The ligands are composed of two distinct functionalities, isophthalic acid (iph) and a fluorophore. The iph unit was used for the generation of coordination polymer with a combination of dpb as interlinking unit and metal ion connector (ZnII). The fluorophores were strategically used to enhance the electron density in the MOFs. In the present work, our main focus remains the synthesis of MOFs with mixed ligands (H2L1–3) by varying their stoichiometry. Although the preparation of MOFs through multicomponent ligands (with similar coordinating sites) is considered to be a challenging task, we report here the successful synthesis of four new MOFs with different composition of the ligands. The MOFs-12, 13, and 23 were synthesized by solvothermal reaction in dimethyl formamide (DMF) at 120 8C for 24 h using 1:1 stoichiometric ratio of H2L1 and H2L2, H2L1 and H2L3, H2L2 and H2L3, respectively. In every case, dpb and [Zn(NO3)2]·6 H2O were taken as one equivalent to the total carboxylic acid amount. The MOF-123 was synthesized under similar experimental condition combining 0.33:0.33:0.33:1:1 proportion of the reactants: H2L1, H2L2, H2L3, dpb, and [Zn(NO3)2]·6 H2O, respectively (Scheme 1). Characterization All the MOFs were obtained as highly crystalline materials and their bulk purity, composition, and thermal stability were established by using powder XRD, elemental analysis, 1H NMR spectroscopy, and mass spectrometric analysis of the acid-digested solutions of the MOFs, single-crystal XRD, and thermogravimetric analysis, respectively.

Scheme 1. The synthetic strategy of the MOFs. Ligands H2L1–3 gives MOF-1, 2, and 3, respectively. However, the stoichiometric combination results in MOF-12 (H2L1 and H2L2, 1:1 equivalent), MOF-13 (H2L1 and H2L3, 1:1 equivalent), MOF-23 (H2L2 and H2L3, 1:1 equivalent), and MOF-123 (H2L1, H2L2, and H2L3, 1:1:1 equivalent).

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Full Paper The sharp powder-XRD diffraction pattern of all the MOFs is evident of the highly crystalline nature of the MOFs and the presence of single phase (Figure 1). The diffraction patterns of the MOFs-12, 13, 23, and 123 were closely identical with the MOFs-1, 2, and 3. However, the peaks were slightly shifted from the positions obtained for MOFs-1, 2, and 3, which ruled out the possibility of either copolymerization of these MOFs or present as a statistical mixture of several possible MOFs. The alteration of the peak position with respect to the MOFs-1, 2, and 3 may arise due to the slight expansion of the unit cell length after introduction of different dicarboxylic acids. To confirm the phase purity of the multicomponent MOFs further, we have compared the PXRD of the as-synthesized MOFs with a 1:1 physical mixture of the corresponding MOFs of the individual ligands (the Supporting Information, Figure S4). The appearance of multiple and broad peaks in mixture of the individual MOF samples clearly indicates that the multicomponent MOFs are highly pure. The identical PXRD patterns indicated that the MOFs-12, 13, 23, and 123 possess structural motifs similar to MOFs-1, 2, and 3 with exceedingly high bulk purity. The PXRD of the activated MOFs (the Supporting Information, Figure S3) suggested that the compounds are highly stable even after removal of the solvent molecules. The thermogravimetric analysis (TGA) revealed that the thermal behavior of the multicomponent MOFs are quite similar to the MOFs of individual ligands with an almost constant ( 20 %) weight loss up to 200 8C. The weight loss is attributed to the removal of trapped water and DMF molecules. The stability of the activated samples had been established by TGA, which displayed their high thermal stability similar to that of as-synthesized samples (the Supporting Information). The stoichiometries of the ligands in the MOFs were estimated by elemental analysis, which is different from that of the individual MOFs of the respective ligands themselves. Moreover, C, N, and H proportion indicated that the ligands are present in the MOFs in 1:1 molar ratio (H2L1 and H2L2 in MOF-12, H2L1 and H2L3 in MOF-13, H2L2 and H2L3 in MOF-23) and as 1:1:1 in MOF-123 (H2L1/H2L2/H2L3). Notably, a better idea about the presence of the ligands and their composition was obtained from the 1H NMR spectra of the acid-digested materials of the MOFs. In every case, the activated crystals of each multicomponent MOFs as well as the individual MOFs of the ligands (MOFs-1, 2, and 3) were digested by HCl (conc.). The solid materials obtained were washed several times with water and dried under vacuum before 1H NMR spectroscopic analysis in [D6]DMSO (Figure 2). The predicted coupling constants and peak positions of the ligands observed were similar to MOFs of the individual ligands. Integration of the peak intensities of the 1 H NMR spectra predicted that the ratios of the ligands are the same as expected and match the obtained proportion of the ligands from elemental analyses that confirmed the bulk homogeneity and phase purity of the samples. Notably, in every case, we could not find any proton peak that corresponded to the dpb ligand. It is well known that pyridine-type ligands have a high tendency to form a hydrochloride salt in the presence of HCl, thus we expect that the dpb ligands were removed from the medium in the form of hydrochloride salt Chem. Eur. J. 2014, 20, 13321 – 13336

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Figure 1. Powder X-ray diffraction patterns of the crystalline multicomponent MOFs-12, 13, 23, and 123 compared with MOFs of individual ligands. The identical diffraction pattern show similar molecular structure of these MOFs as obtained for MOFs-1 and 3 earlier. The dotted lines are eye guidelines for the shift of diffraction peaks compared to MOFs-1, 2, and 3. The shifts of the peak represent the formation of distinguished single phase not the statistical mixture of MOFs of individual ligands.

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Full Paper Table 1. Crystallographic parameters of MOF-12 and MOF-13.

Figure 2. 1H NMR spectrum of the acid-digested multicomponent MOFs in comparison with MOFs-1, 2, and 3. The proportion of the ligands present in the MOFs is estimated by integrating relative intensity of the peaks.

during washing with water. Similarly, base digestion of multicomponent MOFs by NaOH retained only the dpb ligand as insoluble material after washing by water (the Supporting Information). In this case, all carboxylic acids were washed out as sodium salts as those are soluble in water. High-resolution mass spectrometry (Figure 3) of the acid-digested solutions of the MOFs also indicated the presence of the ligands in the MOFs. Therefore, the above-mentioned experiments primarily concluded the stoichiometries of carboxylate ligands in the respective MOFs.

Empirical formula FW T [K] Crystal system Space group a [1] b [1] c [1] a [8] b [8] g [8] V [3] Z 1calcd [g cm3] m (MoKa) [mm1] l [] F (000) Collected reflns Unique reflns GOF (F2) R1[a] wR2[b]

MOF-12

MOF-13

C66H46N4O10Zn2

C72H47N4O10Zn2

1185.85 80(2) Triclinic P1¯ 15.654(6) 16.356(6) 15.660(6) 89.987(11) 96.730(12) 90.055(11) 3982(3) 2 0.989

1258.92 90(2) Triclinic P1¯ 15.637(5) 15.691(5) 16.291(5) 90.097(5) 90.067(5) 90.015(5) 3997(2) 2 1.046

0.648

0.650

0.71073 1220.0 184305

0.71073 1294.0 69317

16306

16357

1.333 0.1160 0.3542

0.970 0.0965 0.3220

[a] R1 = S j FoFc j /S j Fo j ; [b] wR2 = S[w(Fo2Fc2)2]/w(Fo2)2]1/2.

Single-crystal XRD of MOFs-12 and -13 (the Supporting Information). The singlecrystal XRD revealed that MOF-12 (Figure 4) and MOF-13 (the Supporting Information) possessed a three-dimensional (3D) porous network. Interestingly, with our expectation, the crystal structure shows both the MOFs composed of two different types of carboxylic acid ligands, however, their molecular structures are similar to our earlier reported structures of MOF-1 and MOF-3. In the frameworks, the iph acid moiety connects metal nodes forming a two-dimensional (2D)-layered network (Figure 4 b). The fluorophores are located above and below the 2D network (Figure 4 c). The dpb ligand connects the neighboring 2D-layered networks by coordinating the axial position of the ZnII ions, which resulted in a 3D structure. The ZnII metal ions are present in the distorted octahedral geometry with four O atoms from three different carboxylic acids coordinated in the equatorial plane and the axial positions are coordinated by N atoms from dpb ligand. Notably, similar to MOF-1 and MOF-3, the fluorophores are located inside the pores in alternate fashion. Topology analysis shows that each Zn2 (bridged by carboxylate) unit connects to six adjacent Zn2 units through isophthalate and dpb ligands to result in the 3D network. Thus, each Zn2 unit can be considered as six-conFigure 3. HRMS spectra of the acid-digested solution of the MOFs, which also indicates nected node (the Supporting Information). Simplifithe formation of the MOFs by the multicomponent assembly of the ligands. The ligand cation of structure on this basis, the obtained net mass is given in the right side. All the values in mass spectra appeared as [MH] , in can be depicted as uniform 6-connected pcu netwhich M is the molecular weight of the ligands.

Although all the MOFs were highly crystalline and single crystals were obtained (established by polarizing microscope, see the Supporting Information), due to their highly instability, we were only able to determine single-crystal structure of MOF-12 and MOF-13 (crystallographic parameters in Table 1). The diffraction patterns of MOFs-23 and 123 were too poor to evaluate their structure. Therefore, our several attempts to obtain single-crystal structures of these two remained unsuccessful. The comparison of the PXRD patterns indicated that the assynthesized MOF-23 and MOF-123 are isostructural with that

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Figure 4. Crystal structure of MOF-12. a) 3D Porous network view indicates that the fluorophores are located insides the channels. The phenyl and naphthalene moieties of the ligands (H2L1 and H2L2) are highlighted in black and grey (inside pore), respectively; b) Two-dimensional layered network involving iph units of H2L1 and H2L2 with ZnII ions; c) The functionalized phenyl and naphthalene moieties are residing alternately above and below the layer.

work with long topological (4.4.4.4.4.4.4.4.4.4.4.4.*.*.*).

vertex

symbol

of

Photophysical property The characteristic fluorescence emission maxima of the ligands H2L1–3 at 335, 340, and 375 nm upon excitation at 306, 283, and 345 nm, respectively, closely resemble the absorption of the corresponding tags. This indicates that the luminescence behavior of the MOFs has their origin in these fluorescent tags of the ligands when they were connected through ZnII ions (d10). We have already shown the absorption peaks at 369 and 365 nm for MOF-1 and MOF-2, respectively (Figure 5). However, MOF-3 absorbs at two distinct regions of UV/Vis spectrum at 399 and 575 nm. The peak at 399 nm is the characteristic absorption due to the pyrene moiety and the latter peak represents the corresponding excimer absorption. Interestingly, the multicomponent MOFs showed completely different absorption behavior compared with the MOFs of the individual ligands. This observation also confirms the formation multicomponent MOFs rather than physical mixture of individual MOFs. Several absorption peaks observed were characteristic to the ligands present within MOFs (Table 2). Notably, MOFs-13, 23, and 123 showed an absorption peak at 590 nm due to the pyrene excimer absorption as observed in MOF-3, indicating the presence of the H2L3 ligands along with other co-ligands. The fluorescence spectra of the MOFs were recorded using ethanol dispersion of the MOFs at room temperature. The spectra showed strong emission bands at lem = 417 (12), lem = 471 (13), lem = 476 (23), and lem = 473 nm (123) upon excitation at 330 (12), 347 (13), and 350 nm (23 and 123), respectively (Table 2). The substantial electronic coupling between the neighboring ligands through ZnII ions leads to the strong redshift of the emission maxima of the MOFs compared with the parent ligands. It is noteworthy to mention that the emission maxima of the multicomponent MOFs appeared in between Chem. Eur. J. 2014, 20, 13321 – 13336

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Figure 5. a) Solid-state UV/Vis spectra of the MOFs; b) Normalized photoluminescence spectra of the MOFs in ethanol when they were excited at 310 (MOF-1), 300 (MOF-2), 360 (MOF-3), 330 (MOF-12), 347 (MOF-13), and 350 nm (MOFs-23 and 123).

Table 2. Photophysical data of the MOFs. Compounds MOF-1 MOF-2 MOF-3 MOF-12 MOF-13 MOF-23 MOF-123

lmax [nm]

399, 317, 582, 344, 267, 592, 390, 348, 278, 585, 375, 329, 268,

369 365 575 224 219 215 220

lem [nm]

[%][a]

t [s]

415 420 480 417 471 476 473

25.36 16.74 14.67 21.45 14.10 15.45 14.80

9.35 109 1.27 108 2.17 108 8.78 109 6.51 108 6.91 108 7.16 108

[a] The quantum yields were estimated in solid state.

those of the MOFs with the individual ligands. This also suggested that we are able to tune the electronic property of the MOFs by the gradual incorporation of different functionalized ligands with similar reactive groups without altering their basic structural motifs. To better understand the emission behavior of the multicomponent MOFs, the band structure and density of states (DOS) calculations were carried out by using the CASTEP module in Material studio 6 for MOF-12 and MOF-13. The obtained valence band (VB) and conduction band (CB) can be assigned according to the total and partial density of states (PDOS) of each element or by the individual functional moieties. The PDOS analysis of the individual elements revealed that the VB and CB are mainly composed of C, N, and O (the Supporting Information). The lower part of the VB mainly originates from the C-2s, N-2s, and O-2s states, however, the upper

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Full Paper part of VB as well as the CB have more contribution from the C-2p, N-2p, and O-2p states. As expected, Zn does not have much contribution in VB; a small contribution was observed in the energy range 7.5 to 2.5 eV (for both MOFs) and it mainly originated from Zn-3d, but in the CB it does not have any contribution at all (the Supporting Information). The DOS analyses of different functional moieties, such as individual phenyl and naphthalene tags, a combination of phenyl and naphthalene tags (for MOF-12), pyrene tags, combination of phenyl and pyrene tags (for MOF-13) and dpb ligands in both cases were carried out to understand the type of charge-transfer process occurring in the observed emission behavior (Figure 6). The dpb ligands mostly contributed to the VB in the energy range 20.5 eV to 0.97 eV compared with

Figure 7. Fluorescence microscope images of the crystals of MOFs showing their highly luminescent behavior when excited under UV light.

Sensing of nitroaromatics

Figure 6. Total density of states of MOF-12 and MOF-13 with contribution from different functional groups such as phenyl, naphthalene, pyrene tags, and dpb ligands.

the fluorescent tags. However, the top of the VB (1.62 to 0.0 eV) was mainly composed of the combination of both fluorescent tags in MOF-12. On the other hand, for MOF-13 the top of the VB was composed by fluorescent tags as well as by the dpb ligand. The CB between 1.61 to 3.75 eV mainly originates from the combination of both fluorescent tags and dpb ligands. Accordingly, the origin of the emission band in the MOFs was mainly ascribed to a ligand-to-ligand charge transfer (LLCT), in which the electrons are transferred from fluorescent tag moieties to the unoccupied band of either the fluorescent tag or the dpb ligand. Interestingly, fluorescence microscopy of the crystals of MOFs indicated their highly luminescent behavior when excited by UV light. The blue fluorescence arises in MOF-12 is due to the presence of phenyl and naphthalene moieties as tags, however the introduction of pyrene tag along with phenyl and naphthalene in MOFs-13, 23, and 123 shifted the emission to the green region (Figure 7). The fluorescence lifetime measurements in ethanol revealed that all the MOFs have almost similar excited state lifetime in the order of nanoseconds. All the absorption, emission wavelengths, and lifetimes of the excited state parameters are compiled in Table 2. Chem. Eur. J. 2014, 20, 13321 – 13336

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The molecular structures of the MOFs revealed that they are composed of different fluorescent tags located in the channels. Their high electron density and photoluminescent nature are appropriate for the detection of the NACs (which are electron deficient in character) by the fluorescence quenching method. Moreover, their electron-donation ability could be different from our earlier reported MOFs (1, 2, and 3)[19] because of the different proportion of fluorescent tags present in the MOFs. Thus, their sensing applicability towards electron-deficient NACs was thoroughly investigated in dispersion in ethanol. Since PA, TNT, TNB, and DNT are toxic environmental pollutants and the main constituents of commercially available plastic explosives, our main emphasis remained detection of these compounds. However, detection of PA or other phenolic NACs using MOFs is always challenging, and is liable to misinterpret the observed fluorescence quenching phenomenon. MOFs are generally very sensitive towards acid and decompose even in the presence of dilute acids. The pKa (0.38) value of PA indicates that its acidity is comparable to many mineral acids and may cause degradation of MOFs. The fluorescence quenching in the presence of PA does not give any idea about the decomposition of the MOFs. To our surprise, in the present case, the addition of quite a small amount of PA solution to the dispersion of the MOFs in ethanol causes abrupt quenching of the fluorescence intensity with shifting of the lem maxima (Figure 8). In contrast, addition of the same amount of other NACs solutions does not alter the initial fluorescence intensity of the MOFs appreciably. To understand whether the fluorescence quenching really takes place through charge transfer or by simply decomposition of MOFs, we carried out a 1H NMR spectroscopic analysis of the MOFs in the presence of PA in [D6]DMSO. As expected, the MOFs did not give any proton signals as they are not soluble in DMSO. However, on addition of

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Figure 9. 1H NMR spectra of MOF-12 in [D6]DMSO at different time intervals after addition of 100 equivalents of PA. Initially, due to insolubility of the MOF-12 in [D6]DMSO no proton signals were observed. The appearance of ligands peaks after addition of PA indicates degradation of MOF. Figure 8. Rapid reduction in the fluorescence-emission intensity of MOF-12 upon gradual addition of 10 mm solution of PA. The shift of the emission maximum peak (by 43 nm) provides an indication about the decomposition of the MOF.

100 equivalents of PA to the dispersion of MOF, the peaks of the ligands started to appear. In Figure 9, we have given a representative 1H NMR spectrum of MOF-12 in [D6]DMSO before and after the addition of PA. The similar 1H NMR spectra of the other MOFs are given in the Supporting Information. Such observations clearly indicated the decomposition of MOFs in the presence of PA. In this degradation process it is expected that PA protonates the carboxylates and regenerates carboxylic acids. To be sure about this, we have compared the pKa of isophthalic acid, which functionally mimics the used carboxylic acid ligands, with that of PA in 80 % ethanol (the Supporting Information). The higher pKa2 of isophthalic acid (7.28) with respect to PA (pKa = 2.25) supports our hypothesis. Thus, PA could not be detected using these MOFs, however, replacement of the phenolic group of PA by chloride leads to the picryl chloride (PC), which has almost similar explosive nature with explosive velocity of 7200 ms1 compared to PA (7350 ms1). Moreover, PC is used as intermediate in the preparation of most stable explosive, 2,4,6-trinitroaniline from PA. Therefore, we concentrated our attention on the detection of PC along with other nitroaromatic explosives such as TNT, TNB, 2,4-dinitrotoluene (2,4-DNT), 3,4-dinitrotoluene (3,4-DNT), 3,5dinitrobenzoic acid (3,5-DNBA), 3-nitrobenzoic acid (3-NBA), 4nitrobenzoic acid (4-NBA), 4-nitrotoluene (4-NT), and nitrobenzene (NB). We chose a few electron-rich aromatic compounds (p-xylene and p-cresol) along with a few electron-deficient non-nitroaromatic compounds like chlorobenzene, dichlorobenzene, and benzoic acid to judge the selectivity of sensing towards the nitroaromatics. To explore the ability of these MOFs for sensing NACs, fluorescence quenching titrations were performed by gradual addition of a 10 mm solution of different analytes to the dispersions of MOFs in ethanol. The fluorescence quenching titration also gives an idea about their relative quenching efficiency and quenching rate, which are essentially needed to estimate the binding constants of the analytes towards MOFs. Representative fluorescence quenching titrations of PC with MOFs-12, Chem. Eur. J. 2014, 20, 13321 – 13336

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13, 23, and 123 are shown in Figure 10. Other titrations can be found in the Supporting Information. The observed fluorescence quenching behavior also encouraged us to employ fluorescence microscopy, which indicated a considerable decrease of the initial intensity after addition of PC solution for all the MOFs (Figure 11). Selectivity experiments revealed that the initial fluorescence of the MOFs significantly quenched upon addition of a constant amount of a solution of the NACs (500 mL, 10 mm), however, upon addition of the same amount of other aromatic compounds, no appreciable changes in intensity were noticed (Figure 12). Therefore, the inability to quench the fluorescence emission of MOFs by non-nitro analytes indicates that the MOFs are highly selective towards NACs, thus offering their usefulness for the detection of nitro-explosives. The ability of quenching efficiency is in accordance with their order of electron deficiency, with PC having the highest response towards all of the MOFs. The fluorescence quenching phenomenon can be explained by the donor–acceptor electron-transfer mechanism through interaction between MOFs and the analytes. Due to the extended framework nature and highly localized electronic system (especially in the case of MOFs of ZnII (d10)), they possess a very narrow band gap. For an extended system, calculation of the electronic band structure is much more appropriate over simple estimation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which is feasible for discrete analogues. As we depicted earlier, fluorescence quenching can take place by electron-deficient analytes (e.g., nitroaromatics), only if their LUMO occupies a position in between the valence band (VB) and conduction band (CB) of the electron-rich MOFs. Thus, upon excitation, an effective electron transfer occurs from the CB of the MOFs to the LUMO of the electron-deficient species with reduction of initial fluorescence intensity of the MOFs (Figure 13). Therefore, for more electron-deficient analytes with a more stabilized LUMO, the electron transfer becomes thermodynamically more favorable, which may result in higher quenching rate. Thus, the observed quenching efficiency for all the MOFs towards NACs is in accordance with their electron deficiency. Thermodynamical feasibility of the electron transfer can be understood by estimating the free energy change (DG0) for the

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Full Paper interaction process of MOFs and analytes. The DG0 can be estimated from following Rehm– Weller equation;[23]

oðoxÞ

0ðredÞ

DG0 ¼ EMOF EAnalyte DE00 ðMOFÞ oðoxÞ

in which EMOF is the oxidation 0ðredÞ potential of the MOFs, EAnalyte is the reduction potential of various analytes, and DE00 represents the difference in energy between the lowest vibrational levels of the excited state and the ground state of the MOFs. oðoxÞ 0ðredÞ Values of EMOF and EAnalyte were calculated by electrochemical method with respect to the standard calomel electrode (SCE). DE00 was calculated from the absorption and emission spectra of the MOFs. The reduction potentials of the MOFs along with few analytes are given in the Supporting Information. The calcuFigure 10. Reduction in the photoluminescence emission intensity of MOFs upon gradual addition of PC solution. lated DG0 values (Table 3) indicated that the electron-transfer processes were highly feasible and led to fluorescence quenching.

Figure 11. Fluorescence microscope images of the MOFs before and after addition of PC under UV light.

Figure 12. Quenching of the initial fluorescence intensity (plotted as quenching efficiency %) observed upon constant addition of a solution (500 mL 10 mm) of several quenchers to the dispersion of MOFs (1 mg in 2 mL) in ethanol. The abbreviations used have their usual names: picryl chloride (PC), 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitrobenzene (TNB), 2,4-dinitrotoluene (2,4-DNT), 3,4-dinitrotoluene (3,4-DNT), 3,5-dinitrobenzoic acid (3,5-DNBA), 3-nitrobenzoic acid (3-NBA), 4-nitrobenzoic acid (4-NBA), 4-nitrotoluene (4NT), nitrobenzene (NB), benzoic acid (BA), chlorobenzene (CB), and 1,2-dichlorobenzene (1,2-DBC). Chem. Eur. J. 2014, 20, 13321 – 13336

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Although all the MOFs showed considerably high selectivity towards NACs over other aromatic compounds, it is important to determine their quenching rates, which may give an idea about their applicability as sensors. Most of the earlier reported articles were mainly concerned about the selectivity, but a material could be categorized as a good sensor depending on how low it can sense a particular analyte or a group of analytes. In this regard, the rate of fluorescence quenching was plotted as quenching efficiency (%) with respect to addition of analyte solution (Figure 14). It shows that the rate of quenching was in accordance with the electron deficiency of the analytes. In every case, PC and TNT have much higher rates than the others. Thus, it is essential to estimate Stern–Volmer binding constants (KSV) by using all the MOFs to judge their applicability to detect PC and TNT in solution.[24] To determine the Stern–Volmer binding constants, the complete elaboration of the quenching mechanism is essential. In most of the cases, the KSV values are estimated from the steady-state fluorescence quenching experiment, although the observed quenching can take place in two different ways: either collisional quenching through excited-state charge transfer or static quenching due to groundstate electron transfer. However, in most of the cases, 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper in which I0 is the initial fluorescence intensity of the MOFs and I is the intensity after addition of analytes at a particular concentration [Q]. Only the linear variation of I0/I versus [Q] allows one to estimate KSV accurately. However, in many cases, such variation deviates from linearity, particularly at higher concentration of analytes due to excited-state charge transfer (collisional quenching). In static quenching, a nonemissive complex formed through electron transfer from MOFs to the quencher leads to quenching of the fluorescence, however, in collisional quenching, Figure 13. The calculated density of states of MOF-12 (left). Schematic representation of the charge transfer takes place in the excited state fluorescence quenching mechanisms (right). The LUMO of PC as well as that of TNT is lothrough collision. These two mechanisms can be difcated in between the VB and CB of the MOFs leading to the quenching. ferentiated from each other with the help of time-resolved fluorescence decay in the presence of several analytes. If the quenching takes place through a colliTable 3. The change of free energy (DG0) in the fluorescence quenching process. sional pathway, a reduction of the fluorescence lifetime of the MOFs is expected because it offers an 0 0 0 0 0 DGTNT DG2;4DNT DGNB DG4NT MOFs Reduction DGPC extra relaxation pathway in the presence of the 1 1 1 1 1 potential [V] [kcal mol ] [kcal mol ] [kcal mol ] [kcal mol ] [kcal mol ] quencher. Otherwise, it is assumed that there is no MOF-12 1.16 28.29 25.53 18.63 15.18 14.03 such effect of excited-state charge transfer in the MOF-13 1.18 20.01 17.25 10.35 6.90 5.75 fluorescence quenching process. MOF-23 1.21 18.75 15.98 9.08 5.64 4.48 MOF-123 1.19 19.50 16.74 9.84 6.39 5.24 To explore the quenching mechanism in the present case, the change in the excited state lifetime of [a] Reduction potential estimated with respect to standard calomel electrode (SCE). the all four MOFs was estimated in the presence of a few analytes (PC, TNT, DNTs, and 4-NT). Although in all the cases a small change in lifetime was observed, a considcombination of both the quenching processes is responsible erable change was noticed upon addition of PC to all the for the overall quenching. If the quenching process occurs MOFs, which implies a significant charge transfer in the excited only through ground-state electron transfer, KSV can be evaluatstate (Figure 15). The collisional quenching constant (KC) was ed by using the Stern–Volmer equation (1): estimated by using Equation (2) and the values are given in I0 the Table 4. ð1Þ ¼ 1 þ KSV ½Q I

Figure 14. Rate of fluorescence quenching plotted in terms of quenching efficiency (%) after gradual addition of selected nitroaromatic analytes to the dispersions of MOF-12, MOF-13, MOF-23, and MOF-123 in ethanol. Chem. Eur. J. 2014, 20, 13321 – 13336

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Figure 15. Time-resolved Stern–Volmer plots for MOFs with a few selected NACs such as PC, TNT, DNTs, and 4-NT. The solid lines represent linear fits into the time resolved data using Equation (2).

t0 ¼ 1 þ KC ½Q t

ð2Þ

in which t0 is the initial fluorescence lifetime of the MOFs and t is the lifetime after addition of particular analytes of concentration [Q]. The calculated KC values for PC with all the MOFs are 1241.7, 223, 193, and 250.9 m1 for MOFs-12, 13, 23, and 123, respectively, which are much higher values than with any of the other analytes. If the quenching takes place through both the mechanisms, the change of fluorescence quenching is given by the following equation in which both the collisional and static quenching terms are involved. I0 ¼ ð1 þ KC ½QÞð1 þ KS ½QÞ I

Expansion of the above equation revealed the following non-linear Equation (3) I0 ¼ 1 þ KC ½Q þ KS ½Q þ KC KS ½Q2 I

When the analyte concentration is very low, the contribution of the [Q]2 term is negligible. However, when the concentration is high, it becomes more prominent at a given value of KC. Alternatively, if KC has a higher magnitude, there is a chance that the third component becomes more prominent and could lead to nonlinearity in steady-state fluorescence quenching. The KS values were calculated by fitting steady-state quenching data using Equation (3) with the help of KC ([Eq. (2)] and Figure 16). The calculated values of KS were assembled in the Table 4. It is now noteworthy to mention that the combination of the KC

Table 4. The estimated values of collisional (KC) and static (KS) quenching constants. A combination of both leads to overall Stern–Volmer binding constant (KSV) given at the bottom of each value. MOFs

MOF-12 MOF-13 MOF-23 MOF-123

Picryl chloride KS KC [m1] [m1]

KC [m1]

1241.7 912 2153.7 223 1822.4 2045.4 193 1569.1 1762.1 250.9 1434.4 1685.3

285.9 1484.9 1770.8 194.6 601.6 796.2 66.6 1046.7 1113.3 105.2 1208 1313.2

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TNT KS [m1]

3,4-DNT KC KS [m1] [m1]

2,4-DNT KC KS [m1] [m1]

4-NT KC KS [m1] [m1]

118.3 917.5 1035.8 29.4 876.7 906.1 27.8 658.2 686 125.6 664.3 789.9

57.7 755.8 813.5 56.2 326.6 382.8 26.9 526.3 553.2 61.6 606.6 668.2

14.1 643 657.1 63.4 279.3 342.7 23.2 360.7 383.9 5.8 429.6 435.4

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ð3Þ

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and KS resulted in overall Stern–Volmer binding constant KSV (Table 4). The calculated KSV values indicated that the MOFs are sensitive towards NACs in accordance with their electron deficiency. As expected, the binding constants for PC and TNT for all MOFs were much higher than the other NACs used in this study. In our earlier report we established that in the case of MOF-1 and MOF-2, which are porous in nature and possess channel-like pores, the fluorescence quenching was observed by the competition between size selectivity and electron deficiency of the analytes.[19] In those cases, small molecules (such as 4-NT and DNT) undergo encapsulation and had a better interaction with the fluorescent tag, which led to very 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper

Figure 16. Steady-state Stern–Volmer plots for MOFs with a few selected NACs. The solid lines represent fits to the steady-state data using Equation (3).

high KSV values. However, in case of MOF-3, which is significantly less porous, the quenching takes place through surface adsorption of the analytes and the order of electron deficiency is prominent in the quenching process. In contrast to that, in the present case we found the quenching takes place in order of electron deficiency of the analytes. The lower KSV value compared with MOF-1 and MOF-2 also indicated that the quenching took place through surface adsorption of the MOFs particles as observed in the case of MOF-3. In a similar way, the presence of a pyrene moiety inside channels could prevent the encapsulation of analytes in the pores of MOF-13, MOF-23, and MOF-123. However, MOF-12, which is composed of both phenyl and naphthalene as fluorescent tags, is expected to show size-dependent fluorescence quenching of the analytes. To our surprise, the binding constants are in the order of electron deficiency. This may be due to the relative arrangement of the phenyl and naphthalene inside channels that does not allow encapsulation of the analytes molecules (Figure 17). In MOF-1 the tags are oriented in one particular direction leaving the maximum porosity in channels. However, in MOF-12 the fluorescent tags are arranged alternatively up and down, resulting in considerably less porosity inside the channels, which is reflected in its inability to encapsulate small analytes. Sensitivity and reusability The sensitivity of the MOFs towards PC and TNT solutions are estimated in the range of the parts per billion (ppb) level (Table 5). The sensitivity order follows the electron-donating abilities of the MOFs. As expected, MOF-3 is considered as highly electron-rich due to the presence of the pyrene tag. Chem. Eur. J. 2014, 20, 13321 – 13336

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Figure 17. Representation of relative orientation of fluorescence tags in MOF-1 and MOF-12. a) and b) 3D arrangement of the MOF-1 showing the fluorescent tags are oriented in one direction; c) and d) In MOF-12 those are arranged alternatively up and down blocking the effective pore dimension.

Thus, the sensitivity towards TNT is highest in MOF-3 ( 0.9 ppb) compared to others. However, the current study suggests that when phenyl tag is introduced gradually into MOF-3 (in case of MOF-13) the electron density is also gradually decreased. A similar behavior can also be expected if naphthalene tag (in case of MOF-23) or both the phenyl and naphthalene tags (in case of MOF-123) are introduced. Hence, the reduction of electron-donating ability compared with MOF-3 is reflected in their low sensitivities. Although these multicomponent MOFs have less sensitivity, the current study

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Full Paper gands resulted in electron-rich fluorescent MOFs, which have been utilized for the selective and efficient sensing of electrondeficient explosive NACs and the present observations may enable the discovery of improved sensors for real-time monitoring.

Table 5. Sensitivity[a] of the MOFs towards PC and TNT. Compounds

PC

TNT

MOF-1 MOF-2 MOF-3 MOF-12 MOF-13 MOF-23 MOF-123

– – – 16.43 8.68 3.11 0.39

3.63 2.27 0.89 13.74 12.95 2.52 2.31

Experimental Section Materials and methods

[a] The sensitivities are given in ppb.

further establishes a few basic points: 1) Sensing of the electron-deficient analytes (here NACs) highly depends on porosity of the MOFs and 2) the sensitivity of the MOFs depends on their electron-donating capability. Moreover, the recyclability test revealed that all the MOFs can be reused for a significant number of cycles by washing several times with ethanol. The appearance of almost initial fluorescence intensity after each cycle of all the MOFs indicated their high photostability could be applicable for real time infield explosive detection and environment monitoring purposes.

Conclusion Four new luminescent Zn-MOFs have been synthesized by the multicomponent assembly of the ligands functionalized with fluorescent tags. The ligands with similar coordinating functionality does not alter the basic molecular structures compared to their individual MOFs (MOFs-1, 2, and 3) during the assembly process. The phase purity of the MOFs was established by PXRD, NMR spectroscopy, and finally by single-crystal XRD. The multicomponent assembly of the ligands was strategically employed to obtain a diverse electronic property of these materials. Unlike other reported methods using conjugated linkers, we were able to enhance the electron density into MOFs by the incorporation of fluorescent tags, which led to their applicability towards detection of NACs. The strong fluorescence emissions of the MOFs in ethanol dispersion were gradually quenched upon addition of a small amount of nitroaromatic explosives. We established that these MOFs are not suitable for the detection of PA due to its high acidic nature, which causes degradation of the MOFs. However, this phenomenon attracts our attention to detect PC, which has similar explosive behavior and is widely used for the preparation of more stable explosive 2,4,6-trinitroaniline. All the fluorescence quenching studies indicated that the MOFs were highly selective towards NACs over the other electron-deficient aromatics. In contrast to MOFs-1 and 2 reported earlier, the rate of quenching follows the order of electron deficiency of NACs. The fluorescence quenching mechanisms were investigated by estimating their collisional (KC) and static (KS) quenching constants. Importantly, the recyclability and very high sensitivity of the MOFs towards PC and TNT revealed their potential applicability as sensors for the detection of NACs. Therefore, the multicomponent assembly of the fluorescent tag-incorporated liChem. Eur. J. 2014, 20, 13321 – 13336

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All the chemicals/solvents were obtained from different commercial sources and used without further purification, unless otherwise mentioned. 1-(Bromomethyl)naphthalene, 1-, (bromomethyl)pyrene, and 1,4-di(pyridin-4-yl)benzene were prepared according to the literature procedures.[25] NMR spectra were recorded on a Bruker 400 MHz spectrometer. The chemical shifts (d) in the 1 H NMR spectra are reported in ppm relative to tetramethylsilane (TMS) as internal standard (0.0 ppm). IR spectra were recorded on a Bruker ALPHA FT-IR spectrometer used in the 4000–400 cm1 range. Elemental analyses were carried out on PerkinElmer 240C CHNS analyser. Powder X-ray diffraction (PXRD) data were recorded on a Philips X’pert Pro using CuKa radiation (l = 1.5406 ). Thermogravimetric (TGA) analyses of the MOFs were carried out on a Metler–Toledo thermal gravimetric analyzer under nitrogen flow. Electronic absorption spectral measurements were performed by using PerkinElmer LAMBDA 750 UV/Visible spectrophotometer and fluorescence emission studies were carried out on HORIBA JOBIN YVON Fluoromax-4 spectrometer. The lifetime analyses were carried out on HORIBA Scientific DAS6 using spectroscopic grade solvents.

Synthesis Synthesis of MOF-12: [Zn(NO3)2]·6 H2O (6 mg, 0.02 mmol), H2L1(2.7 mg, 0.01 mmol), H2L2 (3.2 mg, 0.01 mmol), and 1,4-di(pyridin-4-yl)benzene (4.6 mg, 0.02 mmol) were taken in a 8 mL scintillation vial. DMF (3 mL) was added and the mixture was stirred for 10 min at room temperature. The reaction vial was placed in a programmable oven after capping tightly and heated at 120 8C for 48 h followed by slow cooling to room temperature at a cooling rate of 15 8C h1. Rectangular colorless crystals (10.6 mg, 89 %, the yield was calculated with respect to H2L1) of the product were collected by filtration and washed with fresh DMF (two times, 3 mL). IR: n˜ = 2346.6 (w), 2217.6 (w), 2064.6 (w), 1997.9 (w), 1704.7 (w), 1611.2 (m), 1555.9 (s), 1451 (w), 1407.1 (m), 1368.8 (s), 1224 (w), 1125.1 (w), 1027.3 (m), 912.6 (w), 779.4 (s), 718.4 (s), 481.8 cm1 (w); elemental analysis calcd (%) for C66H46N4O10Zn2 [L1L2Zn2(dpb)2] (activated sample): C 66.85; H 3.91; N 4.72; found: C 67.14; H 3.70; N 4.89. Synthesis of MOF-13: [Zn(NO3)2]·6 H2O (6 mg, 0.02 mmol), H2L1 (2.7 mg, 0.01 mmol), H2L3 (3.9 mg, 0.01 mmol) and 1,4-di(pyridin-4yl)benzene (4.6 mg, 0.02 mmol) were taken in a 8 mL scintillation vial. DMF (3 mL) was added subsequently to that and the mixture was stirred for 10 min at room temperature. The reaction vial was capped and heated at 120 8C for 48 h in an oven. Subsequent cooling to room temperature at a cooling rate of 15 8C h1 yielded rectangular light brown crystals (9.4 mg, 74 %), which were washed with fresh DMF (2 times, 3 mL) to remove unreacted reactants. IR: n˜ = 2237.5 (w), 2003.6 (w), 1703.4 (w), 1605.5 (w), 1558.7 (s), 1446.8 (w), 1407.1 (w), 1368.8 (s), 1222.9 (w), 1123.7 (w), 1027.3 (m), 848.8 (w), 779.4 (s), 718.4 (s), 483.2 cm1 (w); elemental analysis calcd (%) for C72H50N4O10Zn2 [L1L3Zn2(dpb)2] (activated sample): C 68.53; H 3.99; N 4.44; found: C 69.21; H 4.16; N 4.70.

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Full Paper Synthesis of MOF-23: This was synthesized in an identical manner as applied for MOF-13 replacing H2L1 by H2L2. Rectangular lightbrown crystals (11.7 mg, 89 %) of the product were collected by filtration and washed with fresh DMF (two times, 3 mL) to remove unreacted reactants. IR: n˜ = 2005.1 (w), 1929.9 (w), 1701.8 (w), 1606.9 (m), 1554.5 (s), 1446.8 (m), 1407.1 (w), 1367.4 (s), 1225.7 (w), 1123.7 (w), 1065.6 (w), 1025.9 (w), 915.4 (w), 845.9 (w), 782.2 (s), 714.2 (s), 483.2 cm1 (w); elemental analysis calcd (%) for C76H52N4O10Zn2 [L2L3Zn2(dpb)2] (activated sample): C 69.57; H 3.99; N 4.27; found: C 69.89; H 3.78; N 4.42. Synthesis of MOF-123: [Zn(NO3)2]·6 H2O (8.9 mg, 0.03 mmol), H2L1 (2.7 mg, 0.01 mmol), H2L2 (3.2 mg, 0.01 mmol), H2L3 (3.9 mg, 0.01 mmol), and 1,4-di(pyridin-4-yl)benzene (6.9 mg, 0.03 mmol) were taken in a 8 mL scintillation vial. DMF (3 mL) was added to the above mixture and stirred for 10 min at room temperature. The reaction vial was capped tightly and then placed in a programmable oven and heated at 120 8C for 48 h followed by slow cooling to room temperature at a rate of 15 8C h1. Rectangular light-brown crystals (17.6 mg, 93.6 %) of the product were collected by filtration and the unreacted reactants were washed out with fresh DMF (two times, 3 mL). IR: n˜ = 2179.4 (w), 2053.3 (s), 2012.2 (w), 1704.7 (w), 1608.3 (m), 1554.5 (s), 1448.2 (m), 1407.1 (m), 1367.4 (s), 1222.9 (w), 1125.1 (w), 1025.9 (m), 919.6 (w), 848.8 (w), 789.3 (s), 718.4 (s), 569.6 (w), 484.6 cm1 (w); elemental analysis calcd (%) for C107H74N6O15Zn3 [L1L2L3Zn3(dpb)3] (activated sample): C 68.36; H 3.97; N 4.47; found: C 68.72; H 4.17; N 4.63.

C15H12O5 : 271.0685 [MH] ; found: 271.0607; m/z calcd for H2L2 : C19H14O5 : 321.0841 [MH] ; found: 321.0762. MOF-13: 1H NMR ([D6]DMSO): d = 8.434 (m, 3 H, H2L3), 8.357–8.075 (m, 7 H, H2L3), 8.092 (s, 1 H, H2L1), 7.885 (s, 2 H, H2L3), 7.729 (d, 2 H, H2L1), 7.464 (d, 2 H, H2L1), 7.403 (m, 2 H, H2L1), 7.331 (d, 1 H, H2L1), 5.984 (s, 2 H, H2L3) and 5.236 ppm (s, 2 H, H2L1); HRMS (ESI): m/z calcd for H2L1; C15H12O5 : 271.0685 [MH] ; found: 271.0615; m/z calcd for H2L3 ; C25H16O5 : 395.0998 [MH] ; found: 395.0921. MOF-23: 1H NMR ([D6]DMSO): d = 8.434 (m, 3 H, H2L3), 8.357–8.091 (m, 7 H, H2L3), 8.149 (d, 1 H, H2L2), 8.116 (d, 1 H, H2L2), 7.998 (m, 2 H, H2L2), 7.884 (s, 2 H, H2L3), 7.813 (d, 2 H, H2L2), 7.702 (d, 1 H, H2L2), 7.600 (m, 3 H, H2L2), 5.984 (s, 2 H, H2L3) and 5.692 ppm (s, 2 H, H2L2); HRMS (ESI): m/z calcd for H2L2 ; C19H14O5 : 321.0841 [MH] ; found: 321.0758; m/z calcd for H2L3 ; C25H16O5 : 395.0998 [MH] ; found: 395.0912. MOF-123: 1H NMR ([D6]DMSO): d = 8.434 (m, 3 H, H2L3), 8.337–8.079 (m, 7 H, H2L3), 8.134 (d, 1 H, H2L2), 8.110 (d, 1 H, H2L2), 8.104 (s, 1 H, H2L1), 7.980 (m, 2 H, H2L2), 7.885 (s, 2 H, H2L3), 7.815 (d, 2 H, H2L2), 7.726 (d, 1 H, H2L2), 7.729 (d, 2 H, H2L1), 7.600 (m, 3 H, H2L2), 7.484 (d, 2 H, H2L1), 7.399 (m, 2 H, H2L1), 7.337 (d, 1 H, H2L1), 5.984 (s, 2 H, H2L3), 5.692 (s, 2 H, H2L2) and 5.236 ppm (s, 2 H, H2L1); HRMS (ESI): m/z calcd for H2L1; C15H12O5 : 271.0685 [MH] ; found: 271.0603; m/z calcd for H2L2 ; C19H14O5 : 321.0841 [MH] ; found: 321.0758; m/z calcd for H2L3 ; C25H16O5 : 395.0998 [MH] ; found: 395.0913.

X-ray crystallographic data collection and refinements Characterization of MOFs The presence and composition of the carboxylic acid in the MOFs were confirmed by 1H NMR spectroscopy. In a typical process, the MOFs were separately digested with dilute HCl. The precipitates obtained were centrifuged and washed several times with distilled water. The solid materials were then dried under vacuum and the 1 H NMR spectra recorded. In all the cases, we did not find the presence of 1,4-di(pyridin-4-yl)benzene in the NMR spectra. This is due to the hydrochloride salt forming during acid digestion, which has already been removed in the washing process because of its high solubility in water. The proportion of the ligands was measured by integrating the relative peak intensities corresponding to the ligands. The presence of several carboxylic acids in the MOFs was also confirmed by high-resolution mass spectrometry (HRMS). The digested materials were dissolved in DMSO and finally diluted with methanol before taking the measurements. The obtained peak positions in both NMR spectroscopy and HRMS for all the MOFs are given below. MOF-1: 1H NMR ([D6]DMSO): d = 8.091 (s, 1 H), 7.738 (d, 2 H), 7.490 (d, 2 H), 7.391 (m, 2 H), 7.343 (d, 1 H), 5.244 ppm (s, 2 H) for H2L1; HRMS (ESI): m/z calcd for H2L1 C15H12O5 : 271.0685 [MH] ; found: 271.0610. MOF-2: 1H NMR ([D6]DMSO): d = 8.141 (d, 1 H), 8.124 (d, 1 H), 7.999 (m, 2 H), 7.830 (d, 2 H), 7.710 (d, 1 H), 7.620 (m, 3 H), 5.698 ppm (s, 2 H) for H2L2. HRMS (ESI): m/z calcd for H2L2 ; C19H14O5 : 321.0841 [MH] ; found: 321.0755. MOF-3: 1H NMR ([D6]DMSO): d = 8.433 (m, 3 H), 8.356–8.082 (m, 7 H), 7.882 (s, 2 H), 5.982 (s, 2 H) for H2L3 ; HRMS (ESI): For H2L3 : m/z calcd for C25H16O5 : 395.0998 [MH] ; found: 395.0919. MOF-12: 1H NMR ([D6]DMSO): d = 8.132 (d, 1 H, H2L2), 8.107 (d, 1 H, H2L2), 8.080 (s, 1 H, H2L1), 7.980 (m, 2 H, H2L2), 7.815 (d, 2 H, H2L2), 7.729 (d, 2 H, H2L1), 7.704 (d, 1 H, H2L2), 7.613 (m, 3 H, H2L2), 7.506 (d, 2 H, H2L1), 7.399 (m, 2 H, H2L1), 7.343 (d, 1 H, H2L1), 5.692 (s, 2 H, H2L2) and 5.236 ppm (s, 2 H, H2L1); HRMS (ESI): m/z calcd for H2L1: Chem. Eur. J. 2014, 20, 13321 – 13336

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X-ray diffraction quality single crystals of MOF-12 and MOF-13 were obtained by solvothermal reaction of corresponding mixed ligands and [Zn(NO3)2]·6 H2O. The single crystals slowly lose their crystalline nature once taken out of their mother liquors. Though we collected the diffraction data at low temperature (80 K for MOF-12 and 90 K for MOF-13), the quality of diffraction data is not up to the satisfaction. The MOF-12 and MOF-13 (CCDC-992362 and CCDC-992363, respectively, 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) crystallized in a triclinic crystal system with the P1 space group. The diffraction data of both the complexes were collected using a BRUKER KAPPA CCD diffractometer with MoKa radiation 0.71073 . Data reduction was carried out using SMART/SAINT program.[26] The SADABS program was used for empirical absorption correction.[27] The structures were solved by direct methods (SHELXS-97) and standard Fourier techniques, and refined on F2 using full matrix least squares procedure (SHELXL-97) incorporated in WinGX.[28–29] All the hydrogen atoms were assigned idealized positions and given thermal parameters equivalent to either 1.5 (methyl hydrogen atoms) or 1.2 (all other hydrogen atoms) times the thermal parameter of the carbon atoms to which they were attached. Due to extreme disorder in the phenyl, naphthalene, and pyrene groups, PART commands along with AFIX 69 were amended to restrict the displacement of the corresponding electron density. Several thermally disordered atoms are refined with partial anisotropy. In both the cases, refinements were carried out constraining a few bond distances fixed using DFIX and DANG commands. Due to unconstrainable disorder of several solvent molecules, they are squeezed out using PLATON software. All the atoms in the phenyl and pyrene tags in MOF-13 are refined isotropically due to high thermal disorder. Several atoms in structures have high ADP max/min ratio due to thermal vibration. Several fragments in the main structure deviate from their ideal geometry owing to high thermal vibration which lead to high non-solvent H Uiso(max)/Uiso(min) ratio (10.0) upon affixa-

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Full Paper tion of H atoms to complete the models of the electronic structures. Due to such thermal disorder in the case of MOF-13, the refinement could not reach global minima of satisfactory level. The SQUEEZE refinement using PLATON revealed the presence of high solvent accessible voids of 1836.5 3 with 430 electrons and 1495.0 3 with 587 electrons for the complexes MOF-12 and MOF13, respectively. The squeezed out solvents are expected to be H2O or DMF or both. The relatively high values of R(int.) and R(sigma) are also due to weak diffraction of the crystals. The alerts in the chekCIF originate from such thermal disorder in the structure and weak diffraction strength. The weak diffraction and high thermal distortion can be attributed to fact that the crystals decay via losing crystallization solvents when taken out of mother liquor even at low temperature. Moreover, large framework-cavity allows dislocation of the crystallization solvent molecules and flipping of the conformationally flexible fragments (especially the fluorescent tags) of the structures. The crystallographic data and refinement parameters for the complexes are given in the Table 1.

Activation of MOFs All the as-synthesized crystalline MOFs (about 150 mg) were soaked in methanol and the supernatant methanol was discarded every 8 h (3 ) and fresh methanol was added subsequently each time. After methanol exchange, the sample was treated further in the same way with acetone and dichloromethane to remove methanol and acetone, respectively. Finally, the dichloromethane was decanted and the sample was dried under a dynamic vacuum at 110 8C for 6 h.

Fluorescence quenching titrations in dispersion The fluorescence properties of MOFs-12, 13, 23, and 123 were investigated in ethanol dispersions of MOFs at 293 K. The dispersions were prepared by adding 3 mg of corresponding MOFs powder in ethanol (3 mL). For fluorescence measurement, the stock solution (1 mL) was diluted to 2 mL using fresh ethanol and sonicated for 30 min. The solution subsequently placed in a quartz cell of 1 cm width. All titrations were carried out by gradually adding several aromatic analyte (10 103 m) solutions in methanol in an incremental fashion. Each titration was repeated at least three times to get concomitant values. For all measurements, excitation wavelength (lex) 330 nm for MOF-12, 347 nm for MOF-13, 350 nm for both MOF-23 and MOF-123 when their corresponding emission wavelength (lem) was monitored from 340–600 nm for MOF-12, 380–650 nm for 13, 23, and 123. Both excitation and emission slit width were 2 nm for all measurements. There was no change in the shape of the emission spectra, only quenching of the initial fluorescence emission intensity took place upon titration with electron-deficient nitroaromatic quenchers whereas shifting of the emission maxima were observed during titration with PA. To check the selectivity of the MOFs 500 mL of (10 103 m) solution of each quencher was added to the dispersed solution of the MOFs in ethanol. The fluorescence efficiency was calculated using [(I0I)/I0] 100 %, in which I0 is the initial fluorescence intensity and I is the fluorescence intensity after addition of particular amount of analyte. For the estimation of fluorescence lifetime the fluorescence decays were fitted with double exponential convoluted with instrumental reference. The recyclability of the MOFs towards PC and TNT sensing has been checked. The initial fluorescence of the MOFs was recorded by dispersing in ethanol. Subsequently 500 mL TNT (10 103 m) solution was added to the dispersed solutions of MOFs and fluorescence was measured. The material was recovered by centrifuge after each quenching experiment and washed severChem. Eur. J. 2014, 20, 13321 – 13336

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al times with ethanol. That material was dried and used for several other cycles. For the sensitivity measurement, 10 mm of TNT solution was added gradually to the dispersed solution of MOFs in ethanol. For a few initial additions, the fluorescence intensity of the MOFs does not change, after that it started to quench. The detection limit was calculated from the intercept of the two linear fits of the data.

Electrochemistry Reduction potentials of the three MOFs and selected analytes were measured using a three-electrode cell at room temperature. The indium tin oxide (ITO) electrode was used for the working electrode whereas platinum and standard calomel electrodes (SCE) were used for counter and reference electrodes, respectively. Electrochemical measurements of the analytes were carried out using 0.01 mm solution of each in a mixture of 50 % acetonitrile and 50 % 1.0 m tetrabutylammonium nitrate aqueous solution. However, the powder MOFs materials were coated on ITO electrode. The reduction potentials of the compounds were obtained from the cyclic voltammograms and corrected with respect to the SCE.

Computational details First principles DFT calculations were carried out using a DMOL3 module implemented in Accelrys[30–32] to understand the electrontransfer mechanism. The unit cell of the MOF-12 and MOF-13 was optimized with local density approximation with Perdew–Wang correlation (LDA/PWC) functional. A double numerical basis set with d polarization (DND), which is comparable to 6–31G* basis set, is used for all calculations. A Fermi smearing of 0.005 Hartree was employed to improve computational performance. The convergence criteria for energy, gradient, and displacement are 2.0 105 Hartree, 4.0 103 Hartree 1, and 5.0 103 , respectively. The self-consistent field convergence criterion for all calculations is 1.0 105. The projected density of states (PDOS) was calculated from the optimized structures for quenching mechanism analysis. Energy of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital of PC and TNT were calculated by using same method. To understand the contribution of fluorescent tags in emission behavior of the multicomponent MOFs, electronic band structures along with density of states (DOS) was carried out utilizing earlier optimized geometry with density functional theory (DFT) using one of the three nonlocal gradient corrected exchange-correlation functionals (GGA-PBE) and performed with the CASTEP module. The band structures were calculated for MOF-12 and MOF-13 along high symmetry points of the first Brillouin zone, where the labelled k-points are present as G (0.0, 0.0, 0.0), F (0.0, 0.5, 0.0), Q (0.0, 0.5, 0.5), and Z (0.0, 0.0, 0.5). The contribution of each elements and different functional groups in the valence band (VB) and conduction band (CB) of MOF-12 and MOF-13 was also calculated. The number of plane waves included in the basis was determined by a cut-off energy of 300 eV. All the pseudoatomic calculations were performed for H-1s1, C-2s22p2, N-2s22p3, O-2s22p4, and Zn-3d104s2. The parameters used and convergence criteria were set for the calculations by the default values of the CASTEP code, such as: SCF convergence tolerance of 1 105 eV, Gaussian smearing Scheme with the smearing width of 0.1 eV and reciprocal space pseudopotentials representations. In both the cases we have considered only 15 empty bands.

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Received: March 26, 2014 Published online on August 27, 2014

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Doping of metal-organic frameworks towards resistive sensing.

Encapsulation of strongly fluorescent carbon quantum dots in metal-organic frameworks for enhancing chemical sensing.

Pore size dynamics in interpenetrated metal organic frameworks for selective sensing of aromatic compounds.

3D microporous base-functionalized covalent organic frameworks for size-selective catalysis.

DNA hydrogel by multicomponent assembly for encapsulation and killing of cells.

A luminescent microporous metal-organic framework with highly selective CO₂ adsorption and sensing of nitro explosives.

An in situ self-assembly template strategy for the preparation of hierarchical-pore metal-organic frameworks.