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Construction of Pillar-Layer MOFs for CO Adsorption under Humid Climate, High Selectivity and Sensitive Detection of Picric Acid in Water S Senthilkumar, Ranadip Goswami, Nnamdi Lawrence Obasi, and Subhadip Neogi ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 16, 2017

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ACS Sustainable Chemistry & Engineering

Construction of Pillar-Layer MOFs for CO2 Adsorption under Humid Climate, High Selectivity and Sensitive Detection of Picric Acid in Water S. Senthilkumar,† Ranadip Goswami,† Nnamdi L. 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, Nsukka, Nigeria *E-mail: [email protected] ABSTRACT: Adsorption of CO2 under humid condition is important as flue gas contains some degree of moisture, while aqueous phase nitroaromatics sensing is critical concerning environmental protection and anti-terrorism activities. However, implementing both these aspects in MOFs is rare and challenging due to their moisture instability. To this end, we prepared three isostructural, pillar–layer Zn(II) metal–organic frameworks (MOFs) where criss–cross pillaring by the linkers tune the pore opening and pore electronic environment that in turn modulate thermal and/or moisture stabilities. While activated 2 (2ʹ), incorporating azo group in the linker exhibits excellent CO2/N2 selectivity (˃200), bpy linker containing 1′ displays superior hydrolytic stability with minimum loss in CO2 adsorption–desorption cycles up to 10 days of water vapour exposure. However, framework 3 with bpe linker is unstable. Importantly, aqueous phase sensitive detection of picric acid (PA) has been achieved through fluorescence quenching, where quenching constant for 2ʹ (3.11×104 M-1) is found almost double to that of 1ʹ (1.53×104 M-1). A combination of experimental and mechanistic studies reveal that concurrent presence of dynamic and static quenching as well as resonance energy transfer (RET) are responsible for such a high fluorescence quenching in 2ʹ. Moreover, strong non-covalent interactions, as observed in the co-crystal of PA and azp linker, provide direct proof of evidence. Together, CO2 adsorption under humid condition, high selectivity and very low limit of PA detection in 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 INTRODUCTION The popularity of highly crystalline and porous metal–organic frameworks (MOFs) for potential applications in gas storage,1 separation,2 heterogeneous catalysis,3 sensing4 and so on,5 much lies in the tunability of these self–assembled materials. Given that massive build−up of carbon dioxide (CO2) from anthropogenic activities 6 has initiated 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 sites,8 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 sties and polar CO2 molecules.11 On the other hand, detection of explosive nitro-aromatics (NACs) are of high significance to homeland security,12 environmental protection,13 and antiterrorism handling.14,15 Among other NACs, picric acid (PA) is severely detrimental and highly reactive to form superior hazardous explosives16 like picramic acid,17 which has raised the quest for detection of PA18 with very low detection limit. However, the current methods of detection19 for NACs typically involve sophisticated instruments20 that are not efficient for in-field application because of limited portability, high cost and great complexity.21 In this regard, fluorescence based detection of NACs by porous 1 ACS Paragon Plus Environment


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MOF has recently become very efficient in terms of sensitivity, reproducibility22 as well as applicability in both solid and solution phase. Generally, NACs being electron deficient in nature, their fluorescence based precise detection23,24 can be achieved by MOFs,25 comprising of electron rich constituents, by virtue of strong host-guest interactions.26 However, synthesis and functionalization of porous MOFs that can act as stable CO2 adsorbent27 under humid conditions, as well as show efficient sensing of nitroexplosives in 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 frameworks.29 On the other hand, the counter–ions, required for charge compensation, often blocks 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 framework,30 offering ways to control channel functionalization.31 Furthermore, mixed coordination also benefits framework stability and display emergent properties.32 In light of the aforesaid annotations, we envisaged that 5–aminoisophthalic acid (H2L) should result a 2D layered structure with transition metal ion, where further coordination by 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) with Zn2+ metal centre. Increasing the lengths of linker modify the size of one dimensional channels along 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, presence polar functional groups in the framework improve the CO2 affinity and selectivity, chances of water stability are weakened. More importantly, the present MOFs show excellent aqueous phase fluorescence quenching of electron-deficient nitroaromatic compounds, specifically PA. The high quenching efficiency and low limit of detection for PA sensing renders these materials as potential candidates for highly sensitive in-field detection of PA in aquatic system. RESULTS AND DISCUSSION Crystal Structures and Characterization. Colourless, block shaped crystals of 1 were grown by solvothermal reaction of H2L, bpy and Zn(NO3)2·6H2O in 1:1:2 molar ratio. Single crystal X-ray data (Table S1) revealed that asymmetric unit contains one ZnII ion, one ligand L (L= L2-) and one half of the bpy linker. Each ZnII centre 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 to form two−dimensional [Zn(L)]n layer (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 2D pillar−layer framework with one dimensional channels of dimension ∼5.9 × 8.1 Å2 along b axis (Figure 1a). Structural analysis suggests that 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 crystallographic c axis (Figure 1d) reveals that distance between the bilayer, considering the closest metal centres, approximates to 8.103 Å. As a consequence, longer bpy ligand (7.061 Å) cannot fit between the aforesaid distances and rather joins two distant Zn centres (11.11 Å), causing approximately 47° tilt with the 2D layer. This slanted orientation leads to criss−cross pillaring34 and obstruct any void along the c axis.

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Figure 1. View of the pillar-layer structures along b direction in 1 (a), 2 (b), and 3 (c), and the criss−cross positioning of the linkers along 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 differently coloured for clarity. The constructions of MOFs 1−3 are comparable (Figure 1 and Table S1−S3) as they reveal similar (i) coordination geometry around Zn(II) centre (Figure S1), (ii) two−dimensional [Zn(L)]n layer, (iii) non-penetrated pillar−layer structures, with one dimensional channels along b axis (Figure S2 and Figure S4), (iv) criss−cross pillaring by respective linkers (Figure 1d, e and f), and (iv) 3−nodal 2,3,4−c net. The neighbouring 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 oxygen atoms. 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 should be of worth discussion. The interlayer distances, pore shapes, and free volumes of all the three frameworks are exclusively governed by the lengths of the N donor ligands. Accordingly, the dimensions of 1D channels varies from ∼5.9 × 8.1 Å2 in 1, to 5.8×10.5 Å2 in 2, and 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 (8.99) to bpe (9.37 Å). As described above, pillaring bpy ligands make approximately 47° tilt with the 2D layer, impeding any void along c axis in 1 (Figure 1d). Interestingly, gradual increase in the linker length progressively releases the strain that leads to concomitant opening of the channel along c axis. For instance, linker azp in 2 makes approximately 54° angle with the 2D layer, still facilitating π–π stacking interaction (3.310−3.886 Å) between the pyridyl ring and benzene ring of L (Figure S3). However, framework 3, incorporating longest bpe linker and acquiring largest inter layer distance, exhibits 74° slanting of the linker with the 2D layer that leads to largest opening of the channels along c axis (Figure 1f), and obscure any π–π stacking interaction between the pyridyl ring and layer, as observed for 1 and 2.



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Thermal and Moisture Stabilities. Thermogravimetric analysis (TGA) under N2 atmosphere show (Figure S6) weight loss of 14.3 % for 1 (Calc. 14.5 %) and 19.8 % for 2 (Calc. 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 their variable temperature PXRD (VTPXRD) measurements (Figures S7, S8), which revealed that individual PXRD patterns of the as synthesized framework exactly matches 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 towards 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 vapour to diffuse into the respective crystals, kept in glass vial at 298 K. Time variable PXRD pattern of 1 up to 10 days of water vapour 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).

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Figure 2. Time variable PXRD curves for 1 (a) and 2 (b) without and with exposure of water vapour at different time intervals. The aforesaid observations were further cross checked by CO2 adsorption under humid condition (vide infra). 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 ensure better resistance while attack by the water molecules at the metal centre. 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 vapour 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).


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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 77K up to a relative pressure (P/Po) 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 = 3.3Å) adsorption potential of the activated frameworks at different temperatures, up to 1 bar pressure. Quite in contrast to the N2 adsorption, both 1ʹ and 2ʹ exhibit 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 Figures 3a and 3b, the CO2 adsorption capacities of 1′ and 2′ at 273K are 2.4 mmol/g, 3.6 mmol/g respectively, while those values at 298K amounted to be 1.9 mmol/g, and 2.7 mmol/g, respectively (Figure S11).

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Figure 3. CO2, CH4, and N2 adsorption isotherms (273 K) for 1ʹ (a) and 2ʹ (b), along with time variable CO2 adsorption-desorption curves of 1ʹ (c) and 2ʹ (d) at 273K, after exposure to the water vapour. 5 ACS Paragon Plus Environment


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However, framework 3ʹ shows only negligible CO2 uptake with pronounced hysteresis both at 273K (1.1 mmol/g) and 298K (0.7 mmol/g) (Figure S12), owing to its poor stability. The CO2 uptake values for 1ʹ and 2ʹ 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 2ʹ 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 1ʹ and 2ʹ 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 CO2 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 vapour 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 1ʹ and 2ʹ at 273K are depicted in Figures 3c and 3d, while results at 298K are provided in Figure S14. Clearly, CO2 adsorption isotherms remain unaltered for 1ʹ, even after ten days of water vapour exposure. However, the values gradually drop for 2ʹ, 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 1ʹ and 2ʹ as observed in the preceding section. Overall, although 1ʹ exhibits less porosity than 2ʹ, the greater stability to a natural air environment renders the former framework for possible applications towards CO2 capture from post-combustion flue gases that contain some moisture. Additionally, the high pressure (40 bar) CH4 adsorption measurements for 1ʹ and 2ʹ at 303K divulges (Figure S15a) type–I adsorption behavior with uptake of 3.2 mmol/g for 1ʹ and 3.9 mmol/g for 2ʹ. Moreover, high pressure CO2 storage capacity at 298 K and 15 bar pressure (Figure S15b) revealed uptake of 3.5 mmol/g for 1ʹ and 5.0 mmol/g for 2ʹ. 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 analysed by the ideal adsorbed solution theory (IAST) based on single–component isotherms52 at 273 K (Figures 3a and 3b). The CO2 selectivity over N2 (CO2/N2 = 15:85 mol ratios) for 1ʹ and 2ʹ 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 2ʹ, is rarely reported and amongst the excellent CO2 selectivity values in MOFs to date.53 The high selectivity for CO2/N2 in 2ʹ is mainly ascribed to the strong electrostatic interactions between the porous surface and adsorbates. To further investigate the gas separation ability of 1ʹ and 2ʹ, IAST was extended to calculate the CO2/CH4 selectivity, which shows the selectivity for 1ʹ and 2ʹ as 8.65 and 11.8, respectively (Figure S16c,d). Although, CH4 has larger kinetic diameter (3.8 Å) compared to that of N2 6 ACS Paragon Plus Environment

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(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. Presence of d10 metal ions as well as π-electron rich environment in the the present MOF’s provide a platform to be utilized for sensing of nitroaromatic explosives.54 The extreme importance to detect NACs in aqueous medium, together with the moderate hydrolytic stability of 1ʹ and 2ʹ 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 behaviour 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 centred around 320 nm), as well as n-π* band (cis; at about 440 nm), upon irradiation with UV-light.55 Although, this phenomena is absent for bpy linker, both linkers show a broad region centred at 250 nm due to the pyridyl ring.56 The quenching experiments were performed with stable aqueous suspension of desolvated frameworks (2mg in 2mL Mili-Q water). To maintain homogeneity, individual MOF solution was stirred at constant rate during fluorescence emission measurements. For 1ʹ, strong fluorescent peak was observed at 403 nm upon excitation at 314 nm, while 2ʹ 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 1ʹ and 2ʹ by aqueous 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 solution 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), 2,4-dinitrotoluene (2,4-DNT) were prepared in 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 solution. A successive quenching of the fluorescence intensity was observed upon incremental addition of the analytes in both 1ʹ and 2ʹ (Figures S19-S32). To our delight, 1ʹ and 2ʹ show very high quenching efficiency for PA (83% for 1ʹ and 89.8 % for 2ʹ) by incremental addition up to 400 µL (starting from 0 µL, 0 mM - 0.1 mM) at 25°C. In a separate experiment, the individual visible light blue emission of 1ʹ and 2ʹ under UV light completely disappeared after addition of 400 µL of 0.5 mM PA solution (Figure 4).



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Figure 4. Emission spectra of 1ʹ (a) and 2' (b) dispersed in water, upon incremental addition of 2,4,6-trinitro phenol (PA) solution (0.5mM) in water. Inset shows visual colour change of 1' (a) and 2' (b) before and after titrating with PA under UV-light. Evidently, the aqueous phase quenching efficiency of 2ʹ is better than 1ʹ. On the other hand, compared to nitro phenols, addition of same equivalent of other nitro aromatics (4NP, 3NP, 2NP, 2,4-DNT, 2NT, 4NT) revealed minor effect on the fluorescence intensity of 1ʹ and 2ʹ (Figures S35 and S36). This in turn made a good agreement with selective detection of nitro phenols only. In case of nitro phenols, 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 nitro aromatics, and in accordance with the reported data.57 Importantly, both 1ʹ and 2ʹ maintain their structural integrity, even after immersing individual MOF to 0.5 mM aqueous solution of PA for one week, as confirmed by their time variable PXRD patterns (Figures S37a, 37b). This experiment imply high reusability of the present MOFs, which in turn render them potentially applicable for long-term in-field detection of nitro-explosives, specifically PA. Detection of Quenching Behavior. To investigate the rationlae behind such efficient sensing of PA, as well as to evaluate the quenching mechanism, we analysed quenching efficiencies of all the analytes by using the Stern–Volmer equation: (I0/I) = KSV [Q] + 1.58 The quenching rate is determined by the 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).

(a) (b) Figure 5. Stern−Volmer (SV) plots for various nitro analytes in water suspensions of 1ʹ (a) and 2ʹ (b) (2.0 mg/2 mL). Conditions: λem ca. 403 nm for 1ʹ 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 7) 2-NT. While other NACs show almost linear S–V plots for both 1ʹ and 2ʹ, 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 1ʹ, while the value for 2ʹ is almost double (3.11×104 M-1) (Figures 6a, 6b and Figures S33, S34). Clearly 2ʹ indicates the super quenching ability towards 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 aqueous phase (Table S7).


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(a) (b) Figure 6. Upward bending nature of Stern−Volmer (SV) plot for Picric acid (PA) in water suspensions of 1ʹ (a) and 2' (b) (2.0 mg/2 mL) (0 mM – 0.1mM). Inset shows linear region of the plot at lower concentration (0 mM to 0.05 mM) (inset) for individual framework In order to obtain the limit of detection, 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 revealed (Figures S38 and S39) a linear curve (from 0 mM to 0.05 mM). The limit of detection (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 1ʹ for PA sensing was found to be 0.00348 mM (2.62 ppm) while the value for 2ʹ is 0.00182 mM (1.52 ppm) (Table S5 and S6). The results are comparable with other reported (Table S7) MOFbased sensor materials. For both 1′ and 2′ the observed nonlinearity of the S-V plot for PA essentially suggests the concurrent presence of dynamic and static quenching processes and/or a resonance energy transfer mechanism (RET).60,61 It should be mentioned that small pore apertures of both 1′ and 2′ eliminate any possible encapsulation of aforesaid nitroaromatic compounds (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 band 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 PLquenching 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 nonemissive 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



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Figure 7. Spectral overlaps between absorption bands of different NACs with the emission bands of 1′ and 2′ These observations clearly support the highest quenching efficiency by PA compared to the other nitroanalytes. Additionally, the higher quenching efficiency of PA for 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 attemps, 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′. CONCLUSIONS In conclusion, pillar–layer Zn(II) metal–organic frameworks are rationally constructed, where criss–cross pillaring by pyridine based 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 linker, 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 incorporation of polar functional groups improves the affinity of the framework towards some specific gases, the chances of water stability is weakened. For instance, framework 2′, incorporating 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 1ʹ and 2ʹ allows for aqueous phase detection of electron-deficient nitroaromatic compounds through fluorescence quenching. Among all other nitro aromatics, PA shows significantly high quenching efficiency, with quenching constant for 2ʹ being almost double to that of 1ʹ. Experimental and mechanistic studies reveal that PET, 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 coligand (azp-PA), is 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 presence of moisture 10 ACS Paragon Plus Environment

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imply their practical usability, while extreme low limit of detection for PA allows for highly sensitive in-field detection of PA in aquatic system. EXPERIMENTAL SECTION 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 (20 mg; 0.074 mmol), 4,4′–bipyridine (bpy) (13 mg; 0.074 mmol) was dissolved in a DMF/H2O (3:1 v/v) solvent mixture in 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 to that of 1 except, azp (14 mg; 0.076 mmol) was taken 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 compound 1 except, bpe (13 mg; 0.074 mmol) was taken 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%. ASSOCIATED CONTENT Supporting Information Materials and physical measurements, schemes, additional structural figures, selected bond lengths, bond angles, crystal data, PXRD patterns, TGA curves, FT-IR, fitting for sorption isotherms, photoluminescent spectra, and tables (PDF). Report of checkcif (CIF), CIF file for 1, 2, 3 and azp-PA co-crystal (CIF) AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] ORCID Subhadip Neogi: 0000-0002-3838-4180 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS S. N. and R. G. acknowledge the financial support from DST-SERB (Grant No. ECR/2016/000156), while S. S. acknowledges Network project (Grant No. CSC-0122). The 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. CSMCRI Communication No. 006/2017. REFERANCES (1) Wilmer, C. E.; Farha, O. K.; Yildirim, T.; Eryazici, I.; Krungleviciute, V.; Sarjeant, A. A.; Snurr, R. Q.; Hupp, J. T. Gram–scale, high–yield synthesis of a robust metal-organic framework for storing methane and other gases. Energy Environ. Sci. 2013, 6, 1158–1163.



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Synopsis Isostructural pillar-layer MOFs show CO2 adsorption under humid condition with high selectivity and sensitive detection of picric acid in water.



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Isostructural pillar-layer MOFs show CO2 adsorption under humid condition with high selectivity and sensitive detection of picric acid in water. 231x190mm (150 x 150 DPI)

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Adsorption under Humid Climate - ACS Publications - American

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