Construction of Pillar-Layer Metal–Organic Frameworks for CO2

Oct 16, 2017 - ... for environmental protection and anti-terrorism activities. However, implementing both of these aspects in metal–organic framewor...
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Research Article pubs.acs.org/journal/ascecg

Construction of Pillar-Layer Metal−Organic Frameworks for CO2 Adsorption under Humid Climate: High Selectivity and Sensitive Detection of Picric Acid in Water S. Senthilkumar,† Ranadip Goswami,† Nnamdi Lawrence Obasi,‡ and Subhadip Neogi*,† †

Inorganic Materials & Catalysis Division, CSIR-Central Salt and Marine Chemicals Research Institute, G. B. Marg, Bhavnagar 364 002, Gujarat, India ‡ Department of Pure and Industrial Chemistry, University of Nigeria, 410001 Nsukka, Nigeria S Supporting Information *

ABSTRACT: Adsorption of CO2 under humid conditions is important, as flue gases contain some degree of moisture, while aqueous-phase sensing of nitroaromatic compounds is critical for environmental protection and anti-terrorism activities. However, implementing both of these aspects in metal−organic frameworks (MOFs) is rare and challenging due to their moisture instability. To this end, we prepared three isostructural, pillar-layer Zn(II) MOFs where criss-cross pillaring by the linkers tunes the pore opening and pore electronic environment that in turn modulate thermal and/or moisture stabilities. While activated 2 (2′), incorporating an azo group in the linker, exhibits excellent CO2/N2 selectivity (>200), 1′, containing a 4,4′-bipyridine linker displays superior hydrolytic stability with minimum loss in CO2 adsorption−desorption cycles up to 10 days of water vapor exposure. However, framework 3, with a bis(4-pyridyl)ethylene linker, is unstable. Importantly, aqueous-phase sensitive detection of picric acid (PA) has been achieved through fluorescence quenching, where the quenching constant for 2′ (3.11 × 104 M−1) is found to be almost double that for 1′ (1.53 × 104 M−1). A combination of experimental and mechanistic studies reveals that the concurrent presence of dynamic and static quenching as well as resonance energy transfer are responsible for such a high fluorescence quenching in 2′. Moreover, strong non-covalent interactions, as observed in the co-crystal of PA and 4-azopyridine linker, provide direct evidence. Together, CO2 adsorption under humid conditions, high selectivity, and very low limit of PA detection in the aqueous phase manifest the present MOFs as potential materials for sustainability. KEYWORDS: Pillar-layer framework, Moisture-stable MOFs, CO2 selectivity, Picric acid sensing, Aqueous-phase detection

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NACs19 typically involve sophisticated instruments20 that are not efficient for in-field application because of limited portability, high cost, and great complexity.21 Fluorescencebased detection of NACs by porous MOFs has recently become very efficient in terms of sensitivity and reproducibility,22 as well as applicability in both solid and solution phases. Generally, NACs being electron-deficient in nature, their precise detection based on fluorescence23,24 can be achieved by MOFs25 comprising electron-rich constituents, by virtue of strong host−guest interactions.26 However, synthesis and functionalization of porous MOFs that can act as stable CO2 adsorbents27 under humid conditions, as well as show efficient sensing of nitro-explosives in the aqueous phase, is still a major challenge.28 For example, although, carboxylate-based structures are adequately strong in stabilizing networks, their poor water stability due to weak M− O coordination is a major obstacle in shaping water-stable

INTRODUCTION The popularity of highly crystalline and porous metal−organic frameworks (MOFs) for potential applications in gas storage,1 separation,2 heterogeneous catalysis,3 sensing,4 and so on5 lies largely in the tunability of these self-assembled materials. Given that massive build-up of carbon dioxide (CO2 ) from anthropogenic activities6 has motivated researchers to develop efficient adsorbents for CO2 from flue gas and other sources,7 MOF materials with optimized pores and different shapes offer great scope. Generally, introduction of open metal sites8 or strongly polarizing Lewis basic groups,9 as well as their combinations,10 can enhance the CO2 adsorption in MOFs by virtue of strong interactions between functional sites and polar CO2 molecules.11 On the other hand, detection of explosive nitro-aromatic compounds (NACs) is of high significance to homeland security,12 environmental protection,13 and anti-terrorism handling.14,15 Among the NACs, picric acid (PA) is severely detrimental and highly reactive to form superior hazardous explosives16 like picramic acid,17 which has motivated the quest for detection of PA18 with very low detection limit. However, the current methods for detection of © 2017 American Chemical Society

Received: June 26, 2017 Revised: September 6, 2017 Published: October 16, 2017 11307

DOI: 10.1021/acssuschemeng.7b02087 ACS Sustainable Chem. Eng. 2017, 5, 11307−11315

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Figure 1. View of the pillar-layer structures along the b direction in 1 (a), 2 (b), and 3 (c), and the criss-cross positioning of the linkers along the c axis in 1 (d), 2 (e), and 3 (f), showing gradual opening of porous channels with increasing linker length. The layer and linkers are different colors for clarity.

layer framework with 1D channels of dimension ∼5.9 × 8.1 Å2 along the b axis (Figure 1a). Structural analysis suggests that the 2D pillar-layer framework is 3-nodal 2,3,4-c net with Schläfli symbol {63·82·10}2{63}2{8}.33 Further scrutiny of the structure along the crystallographic c axis (Figure 1d) reveals that the distance between the bilayer, considering the closest metal centers, approximates to 8.103 Å. As a consequence, the longer bpy ligand (7.061 Å) cannot fit between the aforesaid distances and rather joins two distant Zn centers (11.11 Å), causing approximately 47° tilt with the 2D layer. This slanted orientation leads to criss-cross pillaring34 and obstructs any void along the c axis. The constructions of MOFs 1−3 are comparable (Figure 1 and Tables S1−S3), as they reveal similar (i) coordination geometry around the Zn(II) center (Figure S1), (ii) twodimensional [Zn(L)]n layer, (iii) non-penetrated pillar-layer structures, with one-dimensional channels along b axis (Figures S2 and S4), (iv) criss-cross pillaring by respective linkers (Figure 1d−f), and (iv) 3-nodal 2,3,4-c net. The neighboring 2D bilayers are packed together by fitting the grooves and extended into 3D supramolecular architecture via hydrogen-bonding interactions between the amino group and carboxylate Oatoms. Moreover, intramolecular π−π interactions (3.407− 3.811 Å) exist between the benzene rings of ligand L within the 2D layers (Figure S3). The intrinsic high disorder did not allow us to establish the lattice solvent molecules inside the channels. Therefore, solvent compositions are calculated from a combination of elemental analysis, IR spectral data (Figure S5), and thermogravimetric weight loss that is consistent with the PLATON35 calculated results. Nevertheless, some structural difference between these MOFs are worth discussion. The interlayer distances, pore shapes, and free volumes of all three frameworks are exclusively governed by the lengths of the N-donor ligands. Accordingly, the dimensions of 1D channels vary from ∼5.9 × 8.1 Å2 in 1, to 5.8 × 10.5 Å2 in 2, to 5.9 × 13.3 Å2 in 3 (Figure 1 and Figure S2), which increases the calculated guest-accessible area per unit cell volume (23.3% in 1, 36.4% in 2, and 46.8% in 3). The values are in line with the systematic increase of linker length from bpy (7.06 Å) to azp (4,4′-azobipyridine, 8.99 Å) to bpe (bis(4-pyridyl)ethylene, 9.37 Å). As described above, pillaring bpy ligands make an approximately 47° angle with the 2D layer, impeding any void along the c axis in 1 (Figure 1d). Interestingly, gradual increase in the linker length progressively

frameworks.29 On the other hand, the counter-ions required for charge compensation often block the resulting channels in pyridine-based MOFs. In this context, the “pillar-layer” strategy is considered as one of the most rational and effective ways to design mixed-ligand frameworks,30 offering ways to control channel functionalization.31 Furthermore, mixed coordination also benefits framework stability and the display of emergent properties.32 In light of the aforesaid annotations, we envisaged that 5aminoisophthalic acid (H2L) should result a two-dimensional (2D) layered structure with a transition metal ion, where further coordination by an N,N′-donor linker may lead to the formation of pillar-layer MOFs. Herein, we describe the synthesis of three isostructural Zn(II) MOFs, using the combination of H2L, diverse N,N′-donor linker (Scheme S1), and Zn2+ metal center. Increasing lengths of linkers modify the size of one-dimensional (1D) channels along the b axis, with concomitant opening of the voids along the c axis. Diverse functionalities in the linker control the pore electronic environment that in turn modulates the CO2 adsorption properties and water stabilities. The results demonstrate that, although the presence of polar functional groups in the framework improves the affinity and selectivity for CO2, water stability is weakened. More importantly, the present MOFs show excellent aqueous-phase fluorescence quenching of electron-deficient NACs, specifically PA. The high quenching efficiency and low limit of detection for PA sensing render these materials as potential candidates for highly sensitive infield detection of PA in aquatic system.

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RESULTS AND DISCUSSION Crystal Structures and Characterization. Colorless, block-shaped crystals of 1 were grown by solvothermal reaction of H2L, 4,4′-bipyridine (bpy), and Zn(NO3)2·6H2O in 1:1:2 molar ratio. Single-crystal X-ray data (Table S1) revealed that the asymmetric unit contains one ZnII ion, one ligand L (L = L2−), and one-half of the bpy linker. Each ZnII center shows distorted square pyramidal geometry with ligation from three carboxylate O-atoms of two different L ligands, one amine nitrogen from a third L ligand, and one N-atom of the bpy linker. Such coordination allows a 2D [Zn(L)]n layer to form (Figure S1) with small aperture dimensions of 3.4 × 4.9 Å2 (distance refers to atom-to-atom connection). These 2D layers are further connected by the bpy linkers to form a 2D pillar11308

DOI: 10.1021/acssuschemeng.7b02087 ACS Sustainable Chem. Eng. 2017, 5, 11307−11315

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Figure 2. Time-variable PXRD curves for 1 (a) and 2 (b) without and with exposure of water vapor at different time intervals.

Figure 3. CO2, CH4, and N2 adsorption isotherms (273 K) for 1′(a) and 2′ (b), along with time variable CO2 adsorption−desorption curves of 1′ (c) and 2′ (d) at 273 K, after exposure to the water vapor.

Thermal and Moisture Stabilities. Thermogravimetric analysis (TGA) under N2 atmosphere shows (Figure S6) weight losses of 14.3% for 1 (calcd 14.5%) and 19.8% for 2 (calcd 19.6%) in the temperature range 25−150 °C that correspond to the release of lattice solvent molecules. Thereafter, the TGA curve remains unchanged up to 350 °C, above which decomposition occurs. In contrast, framework 3 is thermally unstable and starts losing its lattice guest molecules from the beginning without showing any distinct plateau. The high thermal stabilities of 1 and 2 were further supported from

releases the strain that leads to concomitant opening of the channel along the c axis. For instance, linker azp in 2 makes an approximately 54° angle with the 2D layer, still facilitating π−π stacking interactions (3.310−3.886 Å) between the pyridyl ring and benzene ring of L (Figure S3). However, framework 3, incorporating the longest bpe linker and acquiring the largest interlayer distance, exhibits 74° slanting of the linker with the 2D layer that leads to the largest opening of the channels along the c axis (Figure 1f) and obscures any π−π stacking interaction between the pyridyl ring and layer, as observed for 1 and 2. 11309

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The CO2 uptake values for 1′ and 2′ are comparable with those of recently reported frameworks,38 and much higher than that of pillar-layer structures with differently substituted isophthalic acid.39,40 The reasonably high adsorption capacity of 2′ can be ascribed to the azo group decorated pore surface of this framework that exhibits stronger interaction32,41 with polar CO2 molecules (quadrupole moment = 13.4 × 10−40 C·m2; polarizability = 26.3 × 10−25 cm3). This assumption was further substantiated by the calculation of isosteric heat of CO2 adsorption (Qst), using Clausius−Clapeyron equation, from the isotherms at 273 and 298 K.42 The Qst values at zero loading for 1′ and 2′ were found to be 26.9 and 34.9 kJ mol−1 respectively (Figure S13). Thereafter, the Qst value decreases slightly due to filling of the maximum affinity sites, and shows steady maintenance even at higher coverages. This moderate enthalpy of adsorption provides not only a good affinity for CO 2 capture but also points to the facile adsorbent regeneration without heating, which is a desired property for applications. Although, the values are lower than well-known MOFs such as, bio-MOF-11 (45 kJ/mol),43 CAU-1 (48 kJ/ mol),44 NH2-MIL-53(Al) (38.4 kJ/mol),45 USO-1-Al-A (50 kJ/ mol),46 but higher than that of MAF-26 (23 kJ/mol),47 CuBTTri (21 kJ/mol),48 IRMOF-3 (19 kJ/mol),49 UMCM-1 (12 kJ/mol)50 and NOTT-140 (25 kJ/mol).51 Taking the advantage of moisture stabilities, we next performed the CO2 adsorption of water vapor exposed samples at different time intervals.29 The activation of the individual moisture exposed samples was accomplished by heating at 120 °C for 4 h. In each case, TGA was measured after activation (Figure S6d) to make sure that no water is remaining in the pores. The results for 1′ and 2′ at 273 K are depicted in Figure 3c,d, while results at 298 K are provided in Figure S14. Clearly, CO2 adsorption isotherms remain unaltered for 1′, even after 10 days of water vapor exposure. However, the values gradually drop for 2′, with adsorption and desorption curves exhibiting profound hysteresis upon increasing the number of exposure days. This finding is in line with the relative water stability of 1′ and 2′ as observed in the preceding section. Overall, although 1′ exhibits less porosity than 2′, the greater stability to a natural air environment renders the former framework for possible applications toward CO2 capture from post-combustion flue gases that contain some moisture. Additionally, the high pressure (40 bar) CH4 adsorption measurements for 1′ and 2′ at 303 K divulges (Figure S15a) type-I adsorption behavior with uptake of 3.2 mmol/g for 1′ and 3.9 mmol/g for 2′. Moreover, high pressure CO2 storage capacity at 298 K and 15 bar pressure (Figure S15b) revealed uptake of 3.5 mmol/g for 1′ and 5.0 mmol/g for 2′. The results are in agreement with larger pore size as well as polar azo functionalization in the 1D channels of 2. Adsorption Selectivity. In order to gain complete understanding of the gas separation phenomena, the CO2 gas selectivity (S) over N2 and CH4 were analyzed by the ideal adsorbed solution theory (IAST) based on single-component isotherms52 at 273 K (Figure 3a,b). The CO2 selectivity over N2 (CO2/N2 = 15:85 mol ratios) for 1′ and 2′ are calculated to be 113.3 and 210, respectively (Figure S16a,b and Table S4). To the best of our knowledge, only a handful frameworks have shown the CO2/N2 selectivity of >200.7 Such a high CO2 selectivity over N2, as observed for 2′, is rarely reported and among the best CO2 selectivity values in MOFs to date.53 The high selectivity for CO2/N2 in 2′ is mainly ascribed to the

their variable-temperature (VT) PXRD measurements (Figures S7 and S8), which revealed that individual PXRD patterns of the as-synthesized framework exactly match to that of the simulated one, confirming bulk purity. For both the frameworks, the PXRD patterns remain unchanged at least up to 300 °C. However, 3 is unstable beyond 100 °C, as realized from its VTPXRD pattern (Figure S9). Given that coordinate bonds are prone toward moisture attack, leading to decomposition of the overall framework,36 we next probed the hydrothermal stabilities of these structures. Since compound 3 already proved to be a less-stable framework, moisture stabilities of 1 and 2 were monitored by allowing water vapor to diffuse into the respective crystals, kept in a glass vial at 298 K. Time-variable PXRD pattern of 1 up to 10 days of water vapor exposure showed no observable change in the peak positions (Figure 2a), corroborating the moisture stability of this framework. However, 2 indicated broadening of peaks since tenth day followed by loss of crystallinity in day 12 (Figure 2b). The aforesaid observations were further cross checked by CO2 adsorption under humid condition (vide inf ra). At the onset, the water stability can be explained by the fact that Zn− N(pyridyl) coordination, being stronger than Zn−O (carboxylate) bonds,9 ensures better resistance while attack by the water molecules at the metal center. Alternatively, a thorough structural comparison between 1 and 2 along c direction reveals that voids are completely blocked by the criss-cross-linking of the bpy linker in the former, while some degree of opening exists in 2. Although, the narrow window aperture may partially shield the incoming water molecule to pass through the void in 1,37 larger opening along b and c axis, as well as presence of polar −NN− groups in 2 may permit H2O molecules to penetrate the pores and disrupt the framework upon long exposure to moisture. This rationale was further substantiated by H2O vapor adsorption of the individual activated frameworks, which revealed superior water uptake for framework 2 (9.0 mmol/g), compared to 1 (3.4 mmol/g) (Figure S10). Gas Adsorption Studies. The presence of 1D channel, together with high thermal stabilities of 1 and 2 motivated us to check their gas adsorption properties. Prior to the adsorption measurements, each framework was activated by exchanging the guest solvent molecules with ethanol, followed by outgassing under vacuum at 120 °C to afford the desolvated compounds (1′ and 2′). The N2 adsorption experiments at 77 K up to a relative pressure (P/P0) of 1.0 bar resulted only surface adsorption for both the frameworks. To our understanding, presence of 1D channel along crystallographic b axis allows the larger sized N2 molecules (kinetic diameter = 3.6 Å) to block the windows and hinder passing other molecules. Given the importance to develop porous materials for CO2 uptake, we explored the CO2 (kinetic diameter = 3.3 Å) adsorption potential of the activated frameworks at different temperatures, up to 1 bar pressure. Quite in contrast to the N2 adsorption, both 1′ and 2′ exhibit a typical type-I isotherm, suggesting microporous nature of the frameworks. Adsorption and desorption curves display no hysteresis, and show a rapid uptake of CO2 until the full range of pressure. As depicted in Figure 3a,3b, the CO2 adsorption capacities of 1′ and 2′ at 273 K are 2.4 and 3.6 mmol/g respectively, while those values at 298 K amounted to be 1.9 and 2.7 mmol/g, respectively (Figure S11). However, framework 3′ shows only negligible CO2 uptake with pronounced hysteresis both at 273 K (1.1 mmol/g) and 298 K (0.7 mmol/g) (Figure S12), owing to its poor stability. 11310

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Figure 4. Emission spectra of 1′ (a) and 2′ (b) dispersed in water, upon incremental addition of 2,4,6-trinitrophenol (PA) solution (0.5 mM) in water. Inset shows visual color change of 1′ (a) and 2′ (b) before and after titrating with PA under UV light.

Figure 5. Stern−Volmer (SV) plots for various nitro analytes in water suspensions of 1′ (a) and 2′ (b) (2.0 mg/2 mL). Conditions: λem ca. 403 nm for 1′ and ca. 401 nm for 2′; slit widths 5 nm. Analyte sequence: (1) PA, (2) 4-NP, (3) 3-NP, (4) 2-NP, (5) 2,4-DNT, (6) 4-NT, and (7) 2-NT.

about 440 nm), upon irradiation with UV light.55 Although, this phenomena is absent for bpy linker, both linkers show a broad region centered at 250 nm due to the pyridyl ring.56 The quenching experiments were performed with stable aqueous suspension of desolvated frameworks (2 mg in 2 mL of Milli-Q water). To maintain homogeneity, individual MOF solution was stirred at constant rate during fluorescence emission measurements. For 1′, strong fluorescent peak was observed at 403 nm upon excitation at 314 nm, while 2′ shows a peak at 401 nm upon excitation at 277 nm (Figure S17). The slit widths for both excitation and emission were set to 5 nm. The primary fluorescence quenching of 1′ and 2′ by aqueous nitrobenzene (NB, 0.5 mM) (Figure S18) motivated us to further inspect the recognizing abilities of these two MOFs toward other NACs. All titration were carried out three times to maintain consistent results. A series of different NACs solutions, viz. 2,4,6-trinitrophenol (TNP, or picric acid (PA)), 4-nitrophenol (4NP), 3-nitrophenol (3NP), 2-nitrophenol (2NP), 2-nitrotoluene (2NT), 4-nitrotoluene (4NT), and 2,4dinitrotoluene (2,4-DNT), were prepared in the aqueous phase (0.5 mM), and fluorescence titration experiments were performed by gradual addition of 0.5 mM stock solutions of analytes to individual MOF solutions. A successive quenching of the fluorescence intensity was observed upon incremental addition of the analytes in both 1′ and 2′ (Figures S19−S32). To our delight, 1′ and 2′ show very high quenching efficiency for PA (83% for 1′ and 89.8% for 2′) by incremental addition up to 400 μL (starting from 0 μL, 0−0.1 mM) at 25 °C. In a separate experiment, the individual visible light blue emission of

strong electrostatic interactions between the porous surface and adsorbates. To further investigate the gas separation ability of 1′ and 2′, IAST was extended to calculate the CO2/CH4 selectivity, which shows the selectivity for 1′ and 2′ as 8.65 and 11.8, respectively (Figure S16c,d). Although, CH4 has larger kinetic diameter (3.8 Å) compared to that of N2 (3.64 Å), favorable adsorption of the former can be correlated to the larger polarizability of CH4 (26 × 10−25 cm3) than N2 (17.6 × 10−25 cm3). Thus, availability of π-electron in the framework facilitates the CH4 molecules to interact with the structure and get adsorbed. Photoluminescence Properties and Explosive Nitroaromatic Sensing. The presence of d10 metal ions as well as a π-electron-rich environment in the present MOFs provide a platform to be utilized for sensing of nitro-aromatic explosives.54 The extreme importance to detect NACs in aqueous medium, together with the moderate hydrolytic stability of 1′ and 2′ encouraged us to check their photoluminescence (PL) properties in water. It should be mentioned that framework 3 being both thermally and hydrolytically unstable (vide supra), its aqueous-phase luminescence behavior was not investigated. Ligand H2L possesses two types of transitions, where forbidden transition outcomes as n→π* transition around 330 nm, originating from the lone pair on N-atom and π cloud of benzene moiety. The allowed π→π* transition at around 250 nm (Figure S17a, b) results due to the influence of two electron withdrawing group. For azp linker, photo switching ability of azo group (trans→cis) in solution causes concurrent presence of the π−π* band (trans; broad region centered around 320 nm), as well as n−π* band (cis; at 11311

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Figure 6. Upward bending nature of Stern−Volmer plot for picric acid in water suspensions of 1′ (a) and 2′ (b) (2.0 mg/2 mL; 0−0.1 mM). Insets show linear region of each plot at lower concentration (0−0.05 mM).

0.05 mM). The LOD was calculated by using the equation: LOD = (3σ/m), where σ is the standard deviation of initial intensity of MOF without analyte for five consecutive blank measurements at fixed time intervals, and m denotes the slope of the above-mentioned linear curve. The detection limit of 1′ for PA sensing was found to be 0.00348 mM (2.62 ppm), while the value for 2′ is 0.00182 mM (1.52 ppm) (Tables S5 and S6). The results are comparable with other reported (Table S7) MOF-based sensor materials. For both 1′ and 2′, the observed nonlinearity of the SV plot for PA essentially suggests the concurrent presence of dynamic and static quenching processes and/or a resonance energy transfer (RET) mechanism.60,61 It should be mentioned that small pore apertures of both 1′ and 2′ eliminate any possible encapsulation of aforesaid NACs (Table S8) into their cavities.17 For high sensing of NACs by the present MOFs, a photoinduced electron transfer (PET) mechanism can be hypothesized. In principle, the electronic features of the analytes should be helpful to rationalize the PET process. Since conduction band (CB) of MOFs lie above the lowest unoccupied molecular orbital (LUMO) of these electron deficient analytes, PET occurs upon excitation from the CB of MOF to LUMO of the analyte, leading to the quenching effect.62 The lower the extent of LUMO energy of analyte, greater will be the quenching ability. The highest occupied molecular orbital (HOMO)−LUMO energies of the analytes, used herein, were calculated by density functional theory using the B3LYP/6-31+G(d) method with Gaussian 09. The results firmly support the maximum quenching for PA (Figure S40, Table S9). Nevertheless, the PL-quenching for all other nitro analytes, except PA, are not in perfect accordance with their LUMO energy trend, suggesting the synchronous presence of other quenching mechanisms may be involved. In principle, effective spectral overlap between absorption band of analytes and emission band of the fluorophore may ground RET from the fluorophore of organic ligand to the non-emissive analytes. As depicted in Figure 7, indeed a substantial spectral overlap occurs between the absorption spectrum of PA and the emission spectrum of 1′ and 2′, while negligible overlap ensues for all other analytes.63 These observations clearly support the highest quenching efficiency by PA compared to the other nitro analytes. Additionally, the higher quenching efficiency of PA for 2′,

1′ and 2′ under UV light completely disappeared after addition of 400 μL of 0.5 mM PA solution (Figure 4). Evidently, the aqueous-phase quenching efficiency of 2′ is better than 1′. On the other hand, compared to nitrophenols, addition of the equivalent amount of other nitro-aromatics (4NP, 3NP, 2NP, 2,4-DNT, 2NT, 4NT) revealed minor effects on the fluorescence intensity of 1′ and 2′ (Figures S35 and S36). This in turn made a good agreement with selective detection of nitrophenols only. In case of nitrophenols, the order of the quenching competence follows the order, PA > 2,4-DNP > NP, which is in agreement with the order of acidity of these NACs, and in accordance with the reported data.57 Importantly, both 1′ and 2′ maintain their structural integrity, even after immersing individual MOF to 0.5 mM aqueous solution of PA for 1 week, as confirmed by their time-variable PXRD patterns (Figure S37a,b). This experiment imply high reusability of the present MOFs, which in turn render them potentially applicable for long-term in-field detection of nitroexplosives, specifically PA. Detection of Quenching Behavior. To investigate the rationale behind such efficient sensing of PA, as well as to evaluate the quenching mechanism, we analyzed quenching efficiencies of all the analytes by using the Stern−Volmer equation: (I0/I) = KSV[Q] + 1.58 The quenching rate is determined by KSV, which was derived from the plot of relative fluorescence intensities of different nitro analytes [(I0/I) − 1] vs molar concentration of added nitro analytes [Q] (Figure 5). While other NACs show almost linear SV plots for both 1′ and 2′, the plot for PA are linear at low concentrations and show upward bending at higher concentrations.59 From the linear fitting of the plot, the quenching constant for PA (from 0 mM to 0.05 mM) was found to be 1.53 × 104 M−1 for 1′, while the value for 2′ is almost double (3.11 × 104 M−1) (Figure 6 and Figures S33 and S34). Clearly 2′ indicates the super quenching ability toward PA. To the best of our knowledge, this is the one of the highest quenching constant values among the reported MOFs for sensing of PA in the aqueous phase (Table S7). In order to obtain the limit of detection (LOD) , 0.5 mM PA solution was added incrementally to individual dispersed MOFs in water and the fluorescence quenching was monitored. The plot of change in fluorescence intensity versus concentration of PA (Figures S38 and S39) revealed a linear curve (from 0 to 11312

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extremely low limit of detection for PA allows for highly sensitive in-field detection of PA in aquatic systems.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Synthesis of {[Zn2(L)2(bpy)]·(DMF)·(H2O)2}n (1). A mixture of Zn(NO3)2·6H2O (44 mg; 0.148 mmol), 5-aminoisophthalic acid (13.4 mg; 0.074 mmol), and 4,4′-bipyridine (bpy) (11.6 mg; 0.074 mmol) was dissolved in a DMF/H2O (3:1 v/v) in a Teflon-lined autoclave and heated under autogenous pressure at 90 °C for 72 h. Colorless block-shaped crystals were isolated in 60% yield. Anal. Calcd for C29H29N5O11Zn2: C, 46.18; H, 3.88; N, 9.28%. Found: C, 46.39; H, 3.59; N, 9.14%. Synthesis of {[Zn2(L)2(azp)]·(DMF)2·(H2O)}n (2). The synthetic procedure for 2 was the same as that of 1, except azp (14 mg; 0.076 mmol) was used in place of bpy. Orange plate-like crystals were isolated in 57% yield. Anal. Calcd for C32H34N8O11Zn2: C, 45.90; H, 4.09; N, 13.38%. Found: C, 46.11; H, 4.16; N, 13.14%. Synthesis of {[Zn2(L)2(bpe)]·(DMF)2.5·(H2O)2}n (3). The synthetic procedure for 3 was the same as that of 1, except bpe (13 mg; 0.074 mmol) was used in place of bpy. Colorless block crystals were isolated in 48% yield. Anal. Calcd for C35.5H41.5N6.5O12.5Zn2: C, 47.91; H, 4.70; N, 10.23%. Found: C, 48.07; H, 4.59; N, 10.14%.

Figure 7. Spectral overlaps between absorption bands of different NACs with the emission bands of 1′ and 2′.

compared to 1′, may also be related to the better interactions between PA and azo group in the linker, of the former MOF via non-covalent bonding interactions. In spite of numerous attempts, in the absence of any convincing evidence of PA loaded crystal of 2, the aforesaid hypothesis was substantiated via formation of a co-crystal of PA and azp co-ligand (azp-PA). As anticipated, the azo group of the linker is involved in strong interaction with PA (Figure S41, Table S10). Such interaction in the MOF should amplify the feasible host−guest interactions, leading to a better turn-off fluorescence response for PA in 2′.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02087. Materials and physical measurements, Scheme S1, Figures S1−S41, selected bond lengths, bond angles, crystal data, PXRD patterns, TGA curves, FT-IR, fitting for sorption isotherms, photoluminescent spectra, and Tables S1−S10 (PDF) X-ray crystallographic data for 1, 2, 3, and co-crystal (CIF)

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CONCLUSIONS In conclusion, pillar-layer Zn(II) metal−organic frameworks are rationally constructed, where criss-cross pillaring by pyridinebased linkers of diverse lengths modifies the shape and size of the porous channels that in turn affect the water stability and/ or CO2 adsorption properties. While frameworks 1 and 2, incorporating bpy and azp linkers, are stable, framework 3, with bpe linker, is fragile. The results compare and contrast the performance of these isostructural MOFs and demonstrate that, although increasing the pore sizes and/or incorporating polar functional groups improves the affinity of the framework toward some specific gases, the water stability is weakened. For instance, framework 2′, incorporating an azo group in the linker, exhibits excellent selective CO2 adsorption, while framework 1′ displays moisture stability with minimum loss in multiple CO2 adsorption−desorption cycles under humid conditions. More importantly, the hydrolytic stability of 1′ and 2′ allows for aqueous-phase detection of electron-deficient nitroaromatic compounds through fluorescence quenching. Among all other NACs, PA shows significantly high quenching efficiency, with the quenching constant for 2′ being almost double that for 1′. Experimental and mechanistic studies reveal that PET and RET, along with better interactions between PA and azo group in the linker, as evidenced through formation of a co-crystal between PA and azp co-ligand (azp-PA), are responsible for such a high fluorescence quenching in the former. The MOFs described herein truly manifest potential materials for sustainability, where excellent CO2 adsorption selectivity with multiple adsorption−desorption cycles in the presence of moisture imply their practical utility, while the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Subhadip Neogi: 0000-0002-3838-4180 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.N. and R.G. acknowledge financial support from DST-SERB (Grant No. ECR/2016/000156), while S.S. acknowledges Network project (Grant No. CSC-0122). Analytical support from ADCIF is gratefully acknowledged. N.L.O. is thankful to University of Nigeria. S.N. acknowledges Dr. E. Suresh and Dr. V. Smith for their kind help in crystallography. This work is CSMCRI Communication No. 006/2017.

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