Pore Wall-Functionalized Luminescent Cd(II) Framework for Selective

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Pore Wall Functionalized Luminescent Cd(II) Framework for Selective CO Adsorption, Highly Specific 2,4,6Trinitrophenol Detection and Colorimetric Sensing of Cu Ions 2

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S Senthilkumar, Ranadip Goswami, Vincent J Smith, Hari C Bajaj, and Subhadip Neogi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01646 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 29, 2018

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ACS Sustainable Chemistry & Engineering

Pore Wall Functionalized Luminescent Cd(II) Framework for Selective CO2 Adsorption, Highly Specific 2,4,6-Trinitrophenol Detection and Colorimetric Sensing of Cu2+ Ions S. Senthilkumar,†ǁ Ranadip Goswami,†ǁ Vincent J. Smith,‡ Hari C. Bajaj,† and Subhadip Neogi*,† †

Inorganic Materials & Catalysis Division, CSIR-CSMCRI Bhavnagar, Gujarat-364002, India Department of Chemistry, Rhodes University, Grahamstown, 6140, South Africa ǁThese authors contributed equally *E-mail: [email protected] ‡

ABSTRACT: Astute combination of basic functionality and luminescence property can pursue multifunctional metal-organic frameworks (MOFs) with assorted applications such as selective CO2 adsorption, specific detection of explosive nitro compounds and toxic metal ion sensing. The bifunctional ligand 4-(4-carboxyphenyl)-1,2,4-triazole (HL) is used to build the framework [Cd(L)2]·(DMF)0.92 (1) (L = L–1, DMF = N,N′-dimethylformamide), having free N-atom decorated porous channel. The solvothermal synthesis is extended to produce three isoskeletal frameworks in diverse solvents, where pore size maximizes in 2 by employing N,N′-diethylformamide solvent. The activated framework [Cd(L)2] exhibits strong CO2 affinity with good CO2/N2 selectivity, and shows minimum CO2 loss during five adsorptiondesorption cycles. Sensing studies for nitro-aromatic compounds in DMF reveals highly specific detection of 2,4,6-trinitophenol (TNP) with remarkable quenching (KSV = 9.3 × 104 M–1), and low limit of detection (LOD: 0.3 ppm). The quenching mechanism is ascribed to the combined existence of static and dynamic quenching plus resonance energy transfer. The activated framework further shows highly selective luminescent detection of Cu2+ ions with a quenching constant of 4.4×103 M-1 and very low LOD of 3.9 ppm. The detection of Cu2+ ions accompanies a visible colour change in solution and solid phase, which validates the present system as a potential colorimetric Cu2+ sensor. Of note is that bifunctional sensor shows excellent reusability towards TNP and Cu2+ detection. Overall, selective and multicycle CO2 adsorption, together with efficient sensing of both TNP and Cu2+ ion manifest this pore functionalized MOF as a versatile material for sustainability. KEYWORDS: Isoskeletal framework; CO2 selectivity; Luminescent MOF; TNP sensing; Colorimetric Cu2+ detection INTRODUCTION Capture and separation of the major green-house gas carbon dioxide (CO2) from a mixture of gases, which are primarily released from anthropogenic activities, have become important from an energy and environmental standpoint.1-6 The current method of amine-based CO2 capture from flue gas uses chemisorption that suffers from high regeneration cost.7,8 A better approach is to use porous materials that adsorb CO2 in a physisorption process as the cost of regeneration is lower. In this milieu, metal–organic frameworks (MOFs),9-13 also known as porous coordination polymers (PCPs),14-16 represent one of the most promising functional materials owing to the enormous modularity, which revolves around their designed syntheses. Most importantly, principles of coherent pore surface engineering endorse multifarious opportunities to systematically fine-tune the voids1720 and control pore performances.21-26 This modus operandi includes a plethora of synthetic strategies that can be used to modify pore apertures. Besides, inclusion of guest solvent 1 ACS Paragon Plus Environment

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molecule offers distinct pore size tuning.27-33 The topical observation is that astute incorporation of polar functionality in the linker plays critical role in the manifestation of selective adsorption of targeted gas molecules.34-40 Another candid methodology is to introduce ample N-containing groups in MOF pores to effect stronger interaction of the CO2 quadrupole with localized dipoles of Lewis basic nitrogen atoms. This promotes adsorption and separation capabilities.41-44 However, the strategic incorporation of N-donor sites in MOF pores to effectively exploit the accessible nitrogen atoms of organic linkers is challenging. For instance, when the exposed nitrogen atoms do not precisely line through the channels or considerably block the void space, they have a trivial effect on the improvement in CO2 sorption capacity. Also, the combination of functionalization as well as π-conjugation in organic ligands, together with the presence of spectroscopically silent metal ions render MOFs45-49 suitable for fluorescence-based detection of harmful chemicals.50-57 In fact, the widespread use of nitroaromatics (NACs), volatile organic com-pounds (VOCs), metal ions,58,59 and their adverse consequences to environment as well as human life has set acute goals to identify such chemicals as a part of sustainable development agendas. However, most instruments and methods used to detect NACs and metal ions are not easily portable, expensive and lack simplicity. In this respect, the characteristics that make luminescent MOFs as efficient sensors are their accessibility, sensitivity, reproducibility, and applicability in both the solid and solution phase.60-63 Amongst NACs, picric acid or trinitrophenol (TNP) is an important components of explosive64 and even powerful than trinitrotoluene (TNT). Moreover, when TNP is metabolised, it forms extremely hazardous and mutagenic picramic acid byproduct.65 However, the selective and rapid detection of TNP in the presence of other nitro analytes66-69 is quite challenging because of competing interactions, which lead to false responses.68,69 Likewise, copper is the third most-abundant essential trace element found in biological systems and plays a pivotal role in diverse biological, and chemical functions.70 With the increasing use of copper materials in the manufacturing of medicinal and chemical products, it has become one of the most common metal pollutants, posing serious health and environmental threats.71-72 Nevertheless, present methods for detecting Cu2+ are expensive, laboursome, time-consuming, and lack sensitivity.73-75 In this scenario, the proper amalgamation of basic functionality and luminescence properties can lead to the development of a multifunctional MOF with diverse properties such as preferential CO2 adsorption, sensitive luminescent detection of TNP, and selective sensing of Cu2+ ions. However, examples of such all-in-one MOFs are rare.76-79 Thus, bearing in mind the aforementioned challenges and our aim of utilizing a single framework for multiple applications,80 we used the bifunctional ligand 4-(4-carboxyphenyl)-1,2,4-triazole (HL, Scheme S1) to build a Cd(II) framework (1) for this purpose. 1 has an N-functionalized one dimensional channel that exhibits inclusion of diverse solvent molecules through de-novo synthesis, producing a set of isoskeletal frameworks with slight modification in pore size. The activated framework shows preferential uptake of CO2 over N2 and CH4, with only minor losses of CO2 over five adsorption–desorption cycles. The framework also demonstrates selective and sensitive detection of picric acid (TNP) through fluorescence quenching in DMF, with a significant quenching constant and a low detection limit. The multifunctionality of the present system is further validated by the detection of Cu2+ in the presence of mixed metal ions using the fluorescence quenching effect. It is noteworthy that the framework shows excellent reusability towards both TNP and Cu2+ sensing. RESULTS AND DISCUSSION

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Crystal Structures and Characterizations. The solvothermal reaction of Cd(NO3)2·6H2O with ligand HL (1:2) at 120 °C for 36 h afforded colourless, needle-shaped crystals of [Cd(L)2]·(DMF)0.92 (1) in DMF (L= L–). From single crystal X-ray diffraction analysis (Table S1) we determined that the asymmetric unit consists of half a Cd(II) ion, a single ligand molecule L and approximately half a molecule of DMF (0.46). Each Cd(II) ion is six-coordinated through ligation with four carboxylate O atoms, and two triazolyl N atoms from different L ligands, exhibiting a distorted octahedral geometry. The organic linker has a dihedral angle of 16.8° between triazole and benzene moieties, and bridges three different Cd(II) ions (Figure S1). This coordination allows the formation of Cd2(CO2)4 chains along the a axis, with a Cd···Cd spacing of 4.605 Å (excluding van der Waals radii), bridged by carboxylate groups (Figure 1b). Framework 1 consists of two dimensional (2D) square grids that are connected by the carboxylate coordination bridges along the c axis forming a 3D network. The network topology of 1 can be described as a 3,6-c binodal flu net with the Schlafli symbol {42.6}2{44.62.87.102} (Figure S1).81 The 2D grids are parallel to (101) while consecutive planes are offset. Such a structural arrangement leads to the formation of 1D channel, parallel to the c axis (Figure 1c and Figure 2), having the pore size ∼7.30 ×7.83 Å2. It should be noted that the structure of 1 is different from the previously reported interpenetrated structures.82-84 Importantly, the pore walls of the 1D channels are decorated by uncoordinated nitrogen atoms of the triazole moiety, bestowing the channels Lewis base character. However, the structure reveals existence of no pores along the b axis (Figure 1c and Figure S2). As a positive outcome, the framework should favour increased interactions with CO2 molecules and/or electron deficient nitroaromatics or specific metal ions. The included DMF molecules are highly disordered and are involved in weak hydrogen bonding (C14-H14…O1, D…A = 3.172(8) Å, 129.0°) with the host framework. The host displays π–stacking interactions between the benzene and triazole rings of the ligand (Figure S3).

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Figure 1. (a) Perspective view of 1 along the c axis, (b) The infinite Cd2(CO2)4 chains along the a axis, (c) A tilted view of the structure along the b axis, displaying existence of unidirectional pores. For clarity, H-atoms are omitted and guest DMF molecules are shown only in the central pore. The presence of appreciable solvent accessible void volume (26.4%)85 together with weakly held DMF guest molecules inside the pores provides scope for solvent exchanging. Thus, employing a larger solvent molecule may expand the channel dimensions. With this rationale in mind, we successfully synthesized three isoskeletal frameworks, in DEF (diethylformamide), DBF (dibutylformamide) and NMF (N-methylformanilide) solvents (Figure 2). Single crystal X-ray data of these structures reveal retention of the space group 3 ACS Paragon Plus Environment


(C2/c) with only minor variation in cell parameters (Table S1). Furthermore, the binding modes of ligand L as well as metal coordination are identical to 1 (Figure S4). However, the guest molecules differ (Figure 2) in 2 (DEF), 3 (DBF), and 4 (NMF). Analogous to 1, the solvent molecules are highly disordered in 2, 3, and 4. Nevertheless, in this modus operandi, the revised channel dimensions along the c axis are 7.63×7.90 Å2 in 2, with a slight increase in solvent accessible void volume (28.6%) compared to rest of the structures. Clearly, inclusion of the DEF molecule increases the channel opening, although not in a huge extent.

Figure 2. View of the one dimensional channels in isoskeletal frameworks with free N (triazolyl) atom decorated pore (left). Chemical structures of included solvent molecules in the pore (right) (a) DMF, (b) DEF, (c) DBF, and (d) NMF. The integrity and phase purity of the bulk samples 1–4 were independently established from strong co-relations of the peaks in their individual powder X-ray diffraction (PXRD) profiles to that of the simulated ones, derived from respective X-ray structure (Figures S6S7). Although, peak overlap complicates the assignment of IR bands of the framework, the >C═O (carbonyl) band of the included solvents could be easily detected in the FT-IR spectrum. The carbonyl peak of the guest solvent arises at 1683 cm–1 for 1, 1653 cm–1 for 2, 1615 cm–1 for 3, and 1680 cm–1 for 4 (Figures S8-S9).86 Thermogravimetric analysis of 1–4 display (Figures S10-S11) an initial weight loss up to 150 ºC, probably due to the expulsion of included solvent molecules. The plateaus ranging from 150−290 ºC indicate thermally stable frameworks. The de-solvated frameworks were achieved by replacing the included DMF guests with dichloromethane (DCM) for 3 days, followed by heating at 100 °C for 6 h under vacuum. The TGA curves, recorded for the individual de-solvated frameworks, show no weight loss up to 300 °C. Alongside, variable temperature powder X-ray diffraction (VTPXRD) experiments for 1 and 2 exhibit retention of structural integrity until 300 °C (Figure S12). Both the experiments equally corroborate the high thermal robustness of the structures. Gas Adsorption Studies. The high thermal stability along with presence of 1D porous channel in the isoskeletal frameworks prompted us to pursue gas adsorption studies in the activated state. The activation was accomplished as specified in the preceding section, to afford the solvent free form 1′. At the onset, N2 adsorption for 1′ was conducted at 77 K, which results only surface adsorption (Figure S13) up to 1.0 relative pressure (P/P0). To our knowledge, the lack of N2 adsorption could be a consequence of the 1D channel existing along the c axis, with no substantial opening in other direction. This possibly prevent the N2 molecules, having a kinetic diameter of 3.6 Å, from entering the unidirectional pore.87-89 The accessible N-donor sites from free nitrogen atoms of triazole ring are properly oriented towards the pore, and expected to enhance interactions between CO2 and framework.90,91 We therefore, next performed CO2 sorption studies in 1′ at different temperatures. As opposed to N2 gas, the 4 ACS Paragon Plus Environment

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CO2 (kinetic diameter = 3.3 Å) adsorption revealed a typical type–I isotherm at 273 K (Figure 3), displaying the framework nature to be microporous. Noticeably, no hysteresis could be found between adsorption and desorption, and a maximum of 27.8 cm3/g (1.24 mmol/g, 5.5 wt %) CO2 uptake was achieved. The uptake value at 295 K under similar measurement conditions (Figure 3) approximates to 18.66 cm3/g (0.83 mmol/g, 3.7 wt %). Though, the uptake value for 1′ is not exceptional, as compared to famous nitrogen rich MOFs,92-96 it does fairly equivalent or better than MOFs such as SNU-110 (23.6 cm3/g, 1.05 mmol/g, 4.6 wt %), [Cd(ipa)(L(NH2)2)] (24.3 cm3/g, 1.08 mmol/g, 4.8 wt %), [Zn2(HDDCBA)] (26.1 cm3/g, 1.17 mmol/g, 5.1 wt %), Pcp (29.1 cm3/g, 1.30 mmol/g, 5.7 wt %), UCY-1 (31.4 cm3/g, 1.40 mmol/g, 6.2 wt %), ZIF-100 (32.6 cm3/g, 1.46 mmol/g, 6.4 wt %).97-102 The CO2 adsorption capacity of 1′ is ascribed to the nitrogen rich pore environment. Further insight on the interaction of the framework with the adsorbate was gained from the isosteric heat of adsorption (Qst), assessed by employing the Clausius– Clayperon equation, using the CO2 isotherms at 273 and 295 K.103-106 As shown in Figure 3, the Qst value at zero loading is 29.05 kJ mol-1, which increases afterwards due to enhanced interaction between the polar CO2 molecules (polarizability = 26.3 × 10-25 cm3; quadrupole moment = 13.4 × 10–40 C m2) and the basic N atoms in the framework. Thereafter, the value drops as the sites for maximum affinity become saturated, and the curve maintains the steady nature even until the full coverage. The Qst value is lower than for other known MOFs containing –NH2 groups such as, bio–MOF–11 (45 kJ/mol), CAU–1 (48 kJ/mol), NH2–MIL–53(Al) (35 kJ/mol), USO–1–Al–A (50 kJ/mol), but higher than that of PCN-88 (27 kJ/mol), MAF–26 (23 kJ/mol), NOTT–140 (25 kJ/mol), CuBTTri (21 kJ/mol), MIL-53-Al (20.1 kJ/mol), IRMOF–3 (19 kJ/mol), MOF-5 (17 kJ/mol), and UMCM–1 (12 kJ/mol). Evidently, the adsorption enthalpy is moderate that benefits not only good CO2 adsorption, but also implies the easy regeneration of the adsorbent, which is crucial and important standard to evaluate an adsorption material. In this regard, the regenerative feature of 1′ was also investigated. To our delight, five cycles of CO2 uptake at 273 K show almost equal capacities with minimal loss (Figure 4), revealing sorption recurrence. Moreover, the PXRD profile of 1′ after fifth adsorption-desorption cycle shows excellent correspondence of peaks to that of the as synthesized structure, demonstrating uptake and release are non-destructive (Figure S15). The sorption behaviours of 1′ with respect to CH4 and N2 were also studied at 273 and 295 K. The result shows (Figure 4) a maximum CH4 uptake of 8.3 cm3/g (0.37 mmol/g, 0.59 wt %) and 5.7 cm3/g (0.25 mmol/g, 0.41 wt %) at 273 and 295 K, respectively. However, the adsorption values for N2 are only negligible. The results clearly demonstrate that the cavity of 1′ is mainly reachable to CO2.

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Figure 3. (a) The CO2 adsorption isotherms for 1′ at 273 and 295 K, (b) The isosteric heat of adsorption (Qst) curve. 5 ACS Paragon Plus Environment


Encouraged by relatively high CO2 uptake compared to other gases, we investigated the selective CO2 adsorption capacity of 1′ over N2 and CH4. The selectivity (S) of CO2 over other gases were determined based on single–component adsorption isotherms, by using the ideal adsorbed solution theory (IAST) (Figures 3a,b).107 At 273 K, the CO2/N2 selectivity is calculated as 116.6, while CO2/CH4 selectivity is found to be 12.9 (Figure S14). Although, the kinetic diameter of CH4 (3.8 Å) is larger than N2 (3.64 Å), the greater polarizability of the former (26 × 10-25 cm3) over N2 (17.6 × 10-25 cm3) attributes the favourable CH4 adsorption, and not to any consequence of size exclusion. Thus, the π–electron availability in the framework facilitates the CH4 molecules to interact with the structure and get adsorbed. The moderate enthalpy of CO2 adsorption as well as good CO2 selectivity over CH4 and N2 may be reasonable considering the ample uncoordinated N-heteroatom on the inner surface of the narrow cavities (vide supra). This benefits the interaction with CO2 that has greater polarizability and a larger quadrupole moment than CH4 and N2.

(a) (b) Figure 4. (a) Adsorption isotherms of different gases for 1′ at 273K, (b) Diagram for five adsorption-desorption cycles of CO2, demonstrating negligible loss in uptake capacities. Photoluminescence Study. As discussed above, MOFs comprising of electronically inert d10 metal ions and electron-rich multidentate ligands exhibit strong luminescent properties that promise their advantages for in field photoactive applications. On this basis, we investigated the photoluminescence behavior of guest free 1 at room temperature. At the onset, the apohost 1′ was dispersed in a series of organic solvents (1 mg in 20 mL of each solvent) and individual suspensions were excited at the excitation wavelength to determine the best suited solvent for fluorescence experiment. As evidenced in Figure S16, both DMF and THF dispersions show maximum emission intensity upon excitation at 271 nm and 258 nm, respectively. Nevertheless, the DMF dispersion of 1′ displays humps at 334 nm and 400 nm due to the n-π* or π-π* transition, which correlates the presence of free nitrogen atom and π–π stacking interaction between the ligands in the framework. The THF dispersion however, does not comprise any such features. Furthermore, UV-vis spectra of 1′ in DMF shows a blue shift of 20 nm to that of free ligand (Figure S18a) owing to the strong interaction of Cd(II) ions with bifunctional ligands.108-110 A comparison of fluorescence emission between 1′ and the free ligand at respective λmax of individual compound shows close proximity of the peaks, which confirmed ligand centered photoluminescence in 1′. The slight red shift (25 nm) of the framework compared to free ligand can be ascribed to metalligand interactions. Noticeably, 1′ exhibits large 86% intensity enhancement compared to the


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free ligand (Figure S18b),which indicates the strong electronic communication between adjacent coordinated ligand molecules through the Cd(II) ions in the framework. Fluorescence Studies for Nitroaromatic Detection. The strong emission of 1′, together with its stability and enhanced intensity in DMF satisfies the conditions to be utilized in liquid phase fluorescence detection of harmful nitro compounds. The preliminary quenching experiment with nitrobenzene dispersion of 1′ divulges lowest PL spectra that motivated us further to explore other toxic nitro compounds such as 2,4,6-trinitrophenol (TNP), 2,4-dinitrophenol (2,4-DNP), 2-nitrophenol (2-NP), 3nitrophenol (3-NP), 4-nitrophenol (4-NP), 4-nitrotoluene (4-NT), 2,4-dinitrotoluene (2,4DNT), and 2,3-dimethyl-2,3-dinitrobutane (DMDNB). Sensing was investigated by the incremental addition of 20 µL aliquots (up to 200 µL) from 0.5 mM stock solution of different nitro compounds to DMF suspension of 1′ (1 mg in 2 mL DMF) at room temperature with constant stirring. Each titration was performed in triplicate to maintain the consistency of results. The supporting information contains all the particulars about quenching experiments. It was noticed that the fluorescence intensity of 1′ quenches successively depending on the type of analytes. Noticeably, the addition of 200 µL (Figure 5a) TNP significantly quenches the luminescent intensity with a high 85.6% quenching efficiency (Figure S19), without showing any significant spectral changes (such as shifts in λmax). We used the equation (I0 − I)/I0 × 100% to calculate the quenching efficiency of 1′, where I and I0 stands for the emission intensities after and before the addition of nitro compounds, respectively. In the case of nitro phenols, quenching competence was found in the order TNP > 2,4-DNP > NP (Figure S20) that is in line with the increasing number of nitro groups in the analysts.111 However, other nitro analytes show moderate to poor quenching effects (Figures S21-S27), demonstrating high sensitivity of 1′ towards phenolic nitroaromatics over potentially interfering non-phenolic nitro compounds. We speculate that such selective sensing of phenolic nitroaromatics might stem from strong intermolecular interactions between the –OH group and free Lewis basic N atoms of the triazole moieties.

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(c) (d) Figure 5. (a) Emission spectra of 1′ upon incremental addition of TNP solution (0.5 mM), (b) The curvature of the S–V plot for TNP (0–0.05 mM). Inset shows the linear portion of the S– V plot at concentration 0–0.04 mM; the colour change of 1′ with and without TNP is also shown in UV and day light, (c) S–V plots for different nitro compounds in a DMF suspension of 1′; Conditions: λmax for 1′ ca. 400 nm; 5 nm slit width, (d) The plot of competitive analyte test, showing decrease in fluorescence intensities upon addition of diverse nitro congeners (0.5 mM), followed by TNP (0.5 mM) to 1′. With the aim of gaining insight about quenching phenomena, the gradual decrease in fluorescence intensity of 1′ by the addition of every nitro analytes at different concentrations were measured. We examined the quantitative fluorescent quenching efficiency of 1′ to TNP by using the Stern−Völmer (S−V) equation (I0/I) = 1 + KSV[Q], where I is the fluorescence intensity at TNP concentration of [Q], and I0 signifies the initial fluorescence intensity of the MOF. The quenching constant is indicated by KSV (M-1).112 All other nitro analytes maintains linear S-V plot, however; TNP shows a linear curve at relatively low concentration that bend upwards as the concentration increases (Figure 5b). The observed non-linearity for TNP can be credited to the concurrent existence of static and dynamic quenching phenomena as well as energy relocation between TNP and 1′.113 A linear fit of the plot between 0 and 0.04 mM produced a remarkable quenching constant of 9.3×104 M-1,(Figure S28), which is many times higher than for the rest of the analytes. It is noteworthy that the observed KSV is one of the highest reported values for TNP sensitive MOFs (Table S5).

(a) (b) Figure 6. (a) The HOMO-LUMO energies for ligand HL as well as the studied nitro analytes, (b) Spectral overlap between the absorption spectra of the nitro analytes and emission spectrum of 1′. To gain supplementary understanding into the PL-quenching of 1′ by TNP in the presence of other nitro analytes, selective nitro-analytes test was performed.114 Initially, the emission intensity of 1′ was recorded in the presence of 4-NP (20 µL of 0.5 mM in each successive addition for two batches), maintaining standard protocol (vide supra), which resulted in minor quenching. In sharp contrast, consecutive additions of TNP (40 µL of 0.5 mM evenly distributed in two batches) shows extremely rapid quenching of emission intensity. Such sequential addition cycles were continued up to 200 µL to check the selective sensing of TNP by 1′ in presence of 4-NP. This observation motivated us to check the discrimination of TNP in the presence of related nitro congeners. The results show (Figure 5d) high TNP selectivity of the framework in the concurrent presence of other NACs is maintained throughout.


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Furthermore, the limit of detection (LOD) was obtained by monitoring the fluorescence quenching via incremental addition of TNP (0.5 mM) to the DMF dispersion of 1′. The fluorescence intensity change was plotted with corresponding TNP concentration, which produced a linear curve in the concentration range 0 – 0.04 mM (Figure S29). From the slope (K) of this linear calibration curve, we estimated the LOD from the equation 3σ/K wherein, σ denotes the standard deviation of the initial emission intensity of the MOF in the absence of any analyte (Table S2). The detection limit being calculated as low as 0.3 ppm (1.30 µM). The value is superior to reported MOF-based sensor materials (Table S5), and clearly indicates the excellent potential of present system in the precise detection of TNP. Given the recyclability and stability of the framework is a vital issue for the repeated in-field scrutiny of nitro compounds, sensing reproducibility of 1′ towards TNP was investigated. The sample was recovered via centrifugation after every fluorescence titration experiments in the presence of TNP, followed by thorough washing with methanol and acetone. To our delight, the recovered compound maintains 92% of the initial emission intensity even after five sensing-recovery cycles (Figure S30). Moreover, The PXRD pattern of the recovered framework exhibits well matched diffraction peaks to that of 1′, indicating that integrity and crystallinity is well maintained (Figure S31). These results suggest outstanding reusability of the material for TNP sensing. Such efficient sensing of TNP with a pronounced non-linear S-V plot demonstrates the concurrent existence of static as well as dynamic quenching phenomena along with a resonance energy transfer mechanism (RET), as mentioned previously.115,116 The small aperture of one dimensional channels in 1 excludes any possibility of encapsulation of nitro explosives into the pores. Interaction between nitro analytes and 1′ can therefore only be correlated to surface adsorption. For high sensing ability of TNP by 1′, a photoinduced electron transfer (PET) process could be anticipated.117 To rationalize such PET mechanism, the understanding of electronic aspects of the aforementioned nitro analytes should be invaluable. It is well-known that the extended structure of MOFs, composed of d10 metal ions, endow them as giant “molecules” that often possess narrow band gap energies between valence band (VB) and conduction band (CB), owing to highly confined electronic states.118,119 The conduction band (CB) of MOFs lie at higher energies than the lowest unoccupied molecular orbital (LUMO) of the electron deficient analytes. Thus, efficient electron transfer takes place upon excitation, from the conduction band of MOF to the LUMO of electron deficient analyte that generate the quenching event. Accordingly, better quenching is anticipated when the LUMO energy of a particular analyte is lower. To this end, we calculated energies of the ligand, as well as the highest occupied molecular orbital (HOMO)–LUMO of all the studied nitro analytes by DFT applying the B3LYP/6-31+G(d) level of theory in Gaussian 09 (Table S3).80 The results obtained are depicted in Figure 6a and firmly supports the highest quenching effect by TNP. The LUMO energy of ligand (2.280 eV) is found to be greater than all of the nitro congeners (range: −2.627 to −4.320 eV) that in turn signifies effective electron transfer in the excited state from 1′ to the respective nitro analytes. Nevertheless, PET may not be the sole pathway for sensing, as the observed PL-quenching for some of the nitro congeners does not properly follow their respective LUMO energy order. Another valid reason for the quenching is resonance energy transfer, which is an effective energy transfer process between the fluorophore and the analyte that transpires when the absorption band of the analyte exhibits effective overlap with the emission band of the MOF. Indeed, the absorption spectrum of TNP shows a substantial overlap with the emission spectrum of 1′, while almost negligible overlap is observed for the other nitro congeners (Figure 6b). These findings clearly corroborate the highest quenching efficiency for TNP.



Fluorescence Studies for Cu2+ Detection. Encouraged by the excellent luminescent property, coupled with the presence of Nfunctionalized porous channels, we investigated the ability of 1′ to detect metal ions in solution. Initially, a 10-2 (M) solution was prepared by dissolving diverse sets of metalchloride (MCl2) (M = Zn2+, Cu2+, Co2+, Mn2+, Ca2+, Hg2+, Ni2+, Fe3+, Pb2+, Ln3+) salts in DMF. The photoluminescence behaviour of a well dispersed solution of 1′ (1mg in 2 mL DMF) was observed by the incremental addition of 200 µL of the individual metal solution in chronological manner. Interestingly, among all other metal ions (Figures S32-S40), only Cu2+ shows drastic quenching of emission intensity of the framework upon addition of 200 µL of the CuCl2 solution (Figure 7a). The quenching efficiency was calculated in a similar manner as previously discussed, and the results demonstrate highest quenching value for Cu2+ (89%) compared to the other cationic species (Figure S41). The trend in quenching follows the order: Cu2+> Fe3+>Ni2+>Pb2+ >Co2+> Ca2+> Hg2+> Zn2+>Ln3+> Mn2+.

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Figure 7. (a) Emission spectra of 1′ upon incremental addition of Cu2+ solution (10 mM) in DMF, (b) Non-linear nature of S–V plot for Cu2+ (0 – 1 mM). The inset shows linear plot at lower concentration range (0 – 0.5 mM), (c) S-V plots of the metal cations in a DMF suspension 1′; the conditions were: λmax ca. 400 nm for 1′; slit widths 5 nm, (d) Colour change of CuCl2 solution (10 mM) before and after adding 1′along with solid state colour change of 1′ before and after being dipped in CuCl2 solution (10 mM). Furthermore, to quantify the quenching constant (KSV), S-V plots were obtained by plotting relative fluorescence intensity [(I0/I)] against molar concentration [Q] of Cu2+ ion. A linear curve was obtained in lower concentration range (0 - 0.5 mM). Based on the linear fitting, the KSV value of 1′ for Cu2+ was found to be 4.4×103 M-1(Figure S42), which is considerably larger than the value for other metal ions. However, the curve deviates from 10 ACS Paragon Plus Environment

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linearity at higher concentration range (Figure 7b), indicating the synchronous presence of static and dynamic quenching mechanisms.120 Furthermore, the limit of detection (LOD) of 1′ toward Cu2+ was calculated to be 3.9 ppm (16.9 µM), following the equation: LOD = 3σ/K (Table S4), where each term bears its own significance (vide supra). To the best of our knowledge, these values are comparable or even higher than literature values for well-known MOFs used for Cu2+ detection (Table S6). Such efficient Cu2+ sensing could be a joint consequence of strong metal ion interaction with the framework as well as presence of a suitably sized cavity. The later justification has been well established towards ensuring the accessibility and enhancing the efficiency of metal ion detection.121 In fact, a close look inside the structure of 1 reveals that the uncoordinated nitrogen sites in the triazole moiety of the bifunctional ligand are projected towards the pores of the one dimensional channels, and excludes the restriction of weak interactions. In an attempt to corroborate this rationale, 1′ was immersed in a CuCl2 solution to result Cu2+ incorporated framework. Remarkably, a visible colour change of the solution from deep yellow to pale green was observed (Figure 7d). In addition, the pictures of Cu2+ incorporated 1′ in solid state showed a clear colour change from pale brown to dark green, confirming MOF–Cu2+ interaction. These results clearly show that 1′ changes colour upon metal encapsulation and can be considered as a naked-eye detector for Cu2+. The inductively coupled plasma (ICP) analysis results indicate a 1:5.22 ratio of Cu2+ and Cd(II) in the Cu2+ included framework (Table S7), which confirms that Cd(II) in the framework was not exchanged by Cu2+ during the quenching process. To the best of our knowledge, the strong affinity of free N atoms towards Cu2+ reduces the energy-transfer efficiency from ligands to the Cd(II) ions, thus decreasing the luminescent intensity and eventually conferring high sensitivity on the framework. In this modus operandi, however; the counter anions may also influence specific metal ion sensing. Therefore, a range of DMF solutions of Cu2+ salts containing various anions (NO3–, SO42–, and OAc–) were analysed. The results show (Figures S43-S45) that the luminescence intensity of 1′ decreases in an almost equal extent upon addition of Cu2+ solutions with different counter anions. Thus, the type of anion has an insignificant effect on luminescence quenching. Given the selective identification of a specific analyte from a mixed system is imperative for sensing applications, we next checked the competitive metal ion detection (CMD) of Cu2+ by 1′ in the presence of other metal ions. This investigation was carried out in an identical manner to the competitive detection of nitro analytes. While addition of other metal ion solutions (Ni2+, Co2+, Ca2+, Hg2+, Zn2+, Mn2+) marginally affect the emission intensity of 1′ (20 µL of 10 mM solution, added in two consecutive aliquots), a radical alteration is observed when the Cu2+ solution (40 µL of a 10 mM solution, evenly distributed in two aliquots) was subsequently added (Figures S46-S51). The results clearly manifest the framework as a favourable candidate for the selective as well as sensitive Cu2+ ion detection even in the minute presence of different other metal ions. To assess the reproducibility of Cu2+ detection, 1′ was dispersed in DMF solution containing 200 µL of Cu2+. As expected, it results in the turn-off mode. The resultant solid containing Cu2+ was then washed with fresh DMF to remove the metal ions from the framework channels. Surprisingly, the original metal-ion-free luminescence was restored. This reversible process was continued for five cycles, which showed that the emission intensity as well as quenching ability was retained with only negligible deviation (Figure S52). Importantly, 1ʹ maintains its structural integrity during the sensing-recovery cycles, as confirmed by the unaltered PXRD patterns (Figures S53). Thus, removal of the Cu2+ ions from the framework was readily achieved by simple washing with DMF, which is a necessary attribute for sensory materials, and emphasizes the potential of the present MOF for repeated use in real time applications.



CONCLUSIONS The bifunctional ligand 4-(4-carboxyphenyl)-1,2,4-triazole (HL), containing both triazole and carboxylic acid moieties, was used to form a Cd(II) framework 1 that possesses uncoordinated N-atom functionalized channels, without sacrificing the internal pore. Three isoskeletal frameworks were also synthesized by changing the solvent medium. The pore size maximizes in 2 by employing N,N′-diethylformamide (DEF) solvent. Owing to the presence of surface functionalized channel, the activated framework 1′ exhibits a strong affinity towards CO2 molecules and has good CO2/N2 and CO2/CH4 selectivity. Interestingly, the framework shows negligible CO2 loss even after five adsorption−desorption cycles, demonstrating robustness of the structure in multiple uptake and release. Moreover, 1′ reveals extremely selective and sensitive fluorescence quenching by 2,4,6-trinitophenol (TNP) in DMF, with a remarkable quenching constant (9.3 × 104 M–1) and extreme low detection limit (0.3 ppm). The combined inputs from mechanistic as well as experimental studies validate that aforesaid quenching can be credited to the synchronous presence of static and dynamic quenching, along with resonance energy transfer mechanism. Further studies on metal-ion detection reveal highly sensitive luminescent quenching of the framework by Cu2+ in the presence of other metal ions. The attractive quenching constant (4.4×103 M-1) and very low limit of detection (3.9 ppm), together with the obvious colour change in solution and solid phase, upon metal encapsulation, validates 1′ as a potential naked-eye detector for Cu2+. The excellent reusability towards of the framework with respect to both TNP and Cu2+ sensing is certainly noteworthy. EXPERIMENTAL SECTION Materials and Methods. All the solvents were procured from S. D. Fine Chemicals, India and purified by following standard conventional methods prior to use. Reagent grade N,N′−dimethylformamide (DMF), N,N′−diethylformamide (DEF), and Cd(NO3)2·4H2O were purchased from Sigma−Aldrich and used without further purification. All the solvents such as methanol, dichloromethane (DCM) and various metal salts were procured from S. D. Fine Chemicals, India. All metal salts, 1, 2, 4–Triazole, 4–Fluoroethyl benzoate and the nitro aromatics, used in this study, were purchased from Sigma−Aldrich and used without further purification. Caution!!! Being hazardous and explosive, all the nitro aromatics should be handled carefully. Physical Measurements. All spectroscopic and crystallographic studies are provided in the Supporting Information. Synthesis of the ligand. Ligand 4-(1H-triazole-1-yl) benzoic acid was prepared by following a literature method122,123 and characterized by elemental, 1H and 13C NMR, ESI-MS analyses (see Supporting Information). Synthesis of 1. A mixture of Cd(NO3)2.4H2O (30.8mg, 0.1mmol) and 4-(4-carboxyphenyl)1,2,4-triazole (HL) (37.8mg, 0.2 mmol) was dissolved in 7 mL DMF, and heated in a 15 mL glass vial at 120 °C for 36h. Colourless, needle shaped crystals were isolated in 62% yield, washed with DMF and dried in air. FT-IR (KBr pellets, cm–1): 3468 br, 2934 w, 1683 s, 1382 s, 1252 m, 970 s, 867 s, 784 s, 660 s. Anal. calcd. For CdC18H12N6O4·0.92(C3H7NO) = ([Cd(L)2]·0.92DMF): C: 44.84; H: 3.34; N: 17.42%. Found: C, 44.95; H, 3.40; N, 17.30%. Synthesis of isoskeletal frameworks (2-4). Compound 2-4 were synthesized following a similar procedure as that of 1, except using solvent DEF for 2, DBF for 3 and NMF for 4. 12 ACS Paragon Plus Environment

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Colourless, needle shaped crystals were isolated in 52%, 40%, and 35% yield, respectively. The crystals were washed with solvent, used for the synthesis, and finally dried in air. All the isoskeletal frameworks upon activation (vide supra) provided the similar molecular formula [Cd(L)2]. Anal. calcd. for C18H12N6O4Cd [Cd(L)2]: C, 44.24; H, 2.47; N, 17.20%. Found: C, 44.18; H, 2.55; N, 17.12%. ASSOCIATED CONTENT Supporting Information Synthesis of HL, additional structural figures, PXRD patterns, TGA, FT-IR, adsorption isotherms and calculation detalils for IAST selectivity, photoluminescent spectra and diagrams (Figure S1-S54), tables for X-ray structural data, selected bond lengths and bond angles, LOD, theoretical calculation results and ICP analysis (Table S1-S15) (PDF). X-ray crystallographic data for 1–4 (CIF). CCDC 1834619-1834622 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. AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] ORCID Subhadip Neogi: 0000-0002-3838-4180 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS SN acknowledge the financial support from DST-SERB (Grant No. ECR/2016/000156), and analytical support from ADCIF. SK and RG are thankful to CSIR and DST, India, respectively, for finnancial support. VJS thanks Rhodes University for financial support. S.N. acknowledges Dr. B. Ganguly for his kind help in theoretical computation. CSMCRI Communication No. 067/2018. REFERENCES (1) Li, J. R.; Sculley, J.; Zhou, H. C. Metal-Organic Frameworks for Separations. Chem. Rev. 2012, 112, 869– 932. (2) D'Alessandro, D.M.; Smit, B.; Long, J. R. Carbon Dioxide Capture: Prospects for New Materials. Angew. Chem. Int. Ed. 2010, 49, 6058–6082. (3) Jacobson, M. Z.; Review of solutions to global warming, air pollution, and energy security. Energy Environ. Sci. 2009, 2, 148–173. (4) Yu, K. M. K.; Curcic, I.; Gabriel, J.; Tsang, S. C. E. Recent Advances in CO2 Capture and Utilization. ChemSusChem. 2008, 1, 893–899. (5) Petron, G.; Tans, P.; Frost, G.; Chao, D.; Trainer, M. High-resolution emissions of CO2 from power generation in the USA. J. Geophys. Res. 2008, 113, 1–17. (6) Smit, B. J.; Reimer, A.; Oldenburg, C. M.; Bourg, I. C. Introduction to Carbon Capture and Sequestration; Imperial College Press, 2014. (7) Wang, M.; Lawal, A.; Stephenson, P.; Sidders, J.; Ramshaw, C. Post-combustion CO2 capture with chemical absorption: a state-of-the-art review. Chemical Engineering Research and Design. 2011, 89, 1609–1624. 13 ACS Paragon Plus Environment


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For Table of Contents Use Only

Synopsis Pore wall functionalized, isoskeletal frameworks show selective and recyclable CO2 adsorption, highly selective luminescent detection of TNP and colorimetric Cu2+ sensing



For Table of Contents Use Only

Synopsis Pore wall functionalized, isoskeletal frameworks show selective and recyclable CO2 adsorption, highly selective luminescent detection of TNP and colorimetric Cu2+ sensing


Page 22 of 22

Pore Wall-Functionalized Luminescent Cd(II) Framework for Selective

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