A Continuous π-Stacked Starfish Array of Two-Dimensional

Article pubs.acs.org/crystal

A Continuous π‑Stacked Starfish Array of Two-Dimensional Luminescent MOF for Detection of Nitro Explosives Abhijeet K. Chaudhari, Sanjog S. Nagarkar, Biplab Joarder, and Sujit K. Ghosh* Indian Institute of Science Education and Research, Dr. HomiBhabha Road, Pashan, Pune 411008, India S Supporting Information *

ABSTRACT: A highly luminescent two-dimensional metal− organic framework (MOF) containing continuous π-stack cylindrical arrays in a backbone has been designed and synthesized. Here, we demonstrate the “supramolecular wire effect” on a MOF surface for fast and reversible solid-state detection of both aliphatic and aromatic nitro explosives via a fluorescence quenching mechanism. A possible quenching mechanism by a supramolecular wire effect has been demonstrated by structural analysis of host−guest π-stacked interactions and guest sorption studies.

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INTRODUCTION Detection of chemical explosives by simple and inexpensive methods is an issue of international concern, due to the increasing use of explosive materials in terrorism, military operation safety, industrial, and environmental safety control.1 A variety of compounds have been used as chemical explosives, especially nitro organics, as well as nitramines, nitrate ester, and peroxides.1 Nitro-organic compounds are the primary components in many explosives, so the current major focus is on its detection. A wide range of sophisticated instruments like ion mobility spectroscopy (IMS), X-ray dispersion, Raman spectroscopy, etc., as well as canines, are currently being employed to detect nitro explosives.1 The above techniques are not very efficient for explosives detection in the field because of limited portability, high cost, and these instruments need frequent careful calibrations, which restrict them from widespread use. Photoluminescence-based chemosensors have been considered to be the most effective tool for detection of nitro explosives due to their high sensitivity, ease of visualization, and affordability.1 A good sensor must possess the capability of solid-state sensing of nitroaromatic as well as nitro-aliphatic explosives vapor. Nitro aromatic explosives often act as good fluorescence quenchers due to their electron deficient nature. 2,4,6-trinitrotolune (TNT) is a very common nitro-aromatic explosive. Often, detection of TNT is achieved by the detection of 2,4-dinitrotolune (2,4-DNT) due to its comparatively (∼20 times) high vapor pressure, which is one of its inevitable byproducts in the manufacturing process. So far, several fluorescent materials have been used to detect nitro-aromatic explosives with high sensitivity, like conjugated organic and inorganic polymers, organogelators, supramolecular polymers, organic small molecules, nanomaterials, supramolecular cages, and Zn(salicylaldimine)-based sensors.2 On the other hand, detection of nitro-aliphatic explosives remains a great challenge, mainly due their lack of capability to make π−π interactions © XXXX American Chemical Society

with the sensory materials and unfavorable reduction potential (−1.7 V vs standard calomel electrode, SCE). Detection of plastic explosives is often realized by detection of 2,3-dimethyl2,3-dinitrobutane (DMNB),3,4b an aliphatic-nitro compound, which is a required additive in all legally manufactured plastic explosives. Recently, very few reports on luminescent MOF-based nanomaterials, hydro-gel, and MOF itself by our group4a and others have been reported for the detection of nitro explosives.4 MOF materials are comprised of organic ligands and metal ions, and with easy surface functionalization and tunable pore size, they showed great potential as a new type of sensor and for other applications such as gas storage, chemical separation, catalysis, etc.5 Several studies have indicated that a conjugated polymer backbone (“molecular wire” effect)6 or extended onedimensional (1D) molecular packing of discrete compounds with cofacial π−π stacking between the molecules (“supramolecular wire” effect) show unprecedented efficiency in detecting both aliphatic and aromatic nitro explosives, by facilitating the exciton migration along one axis.3a,7 Like conjugated polymers, MOF compounds also have polymer backbones, which can facilitate exciton migration by the molecular wire effect,4b but the supramolecular wire effect has not been reported for nitro explosives detection in MOF materials. MOF materials are generally highly crystalline and have an ordered structure. We hypothesized that if we could design a MOF from conjugated flexible tripodal ligands with aromatic π-rings then it will facilitate exciton migration by forming a π−π stacked 1D molecular packing of ligands. Here, flexibility may help to adjust the position of ligands to provide intraligand interaction, and ligands with extended aromatic πReceived: May 15, 2013 Revised: June 12, 2013

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of Zn(II), a central one with an octahedral environment coordinated from six carboxylate oxygen atoms from three different L and the other two with tetrahedral geometries surrounded by three carboxylate oxygen atoms from three L and one oxygen atom from coordinated water. The resultant 2D sheets contain hexagonal pores with approximate dimensions of 12.2 × 13.4 Å2. Trimeric Zn(II) units of alternate sheets are positioned at the center of the hexagonal pores of the middle sheet from above and below, forming a starfish array of the framework. It results in small apertures of an ∼7.5 × 4.2 Å2 dimension along the c axis (Figure 1) of the three-dimensional (3D) packing diagram. In another way, the arrangements of the 2D sheets are in A−B−A−B−A fashion with exactly the same compositions of A and B, except for their alternate stacked confirmations to each other in a slide pattern. The uniqueness of this 2D Zn(II) MOF is the π−π stacked 1D molecular packing of the ligands giving a supramolecular wirelike arrangement. There is a continuous π-interaction of each sheet with the next one, which makes the 1D π-stacked cylindrical units of the central aromatic ring of L. The πinteraction of two central moieties of L in the same bilayer sheet is stronger (3.357 Å) over the same present between two consecutive sheets (3.857 Å). Guest benzene (BN) molecules trapped inside the framework also make π-interactions with the host 1. During the preparation of our manuscript, we found that the framework with different guest molecules recently has been reported by others, but luminescent properties have not been explored.9b,c Platon10 analysis revealed 3453.7 Å3 of total solvent accessible area per unit cell (i.e., a 39.5% solvent accessible area per unit cell of guest omitted compound 1). The framework guests were exchanged with acetone, and the exchanged compound was pretreated by applying vacuum and gentle heat for 24 h to get the guest free MOF (1′). Powder Xray diffraction (PXRD) patterns were checked for phase purity and thermogravimetric analysis for its stability. Complete evacuation of the guests was also confirmed by studying the benzene sorption (at 298 K) with a 1′, where the uptake amount exactly matched with the amount of benzene present in the parent as-synthesized compound 1 (6 benzene molecules per unit cell, Figures S11 and S12 of the Supporting Information). Furthermore, it has been elucidated that as the guest size increases from benzene to cyclohexane to toluene, uptake also decreases from 6 to 4 and 2 molecules per unit cell, respectively. During desorption of these guest molecules, hysteresis was observed, which suggests strong noncovalent electronic interaction of guest molecules with the framework.11 Low-temperature adsorption (N2 at 77 K and CO2 at 195 K) measurements were performed over 1′ to check its surface area and pore size. From the Langmuir plot, the total surface area calculated is 374.5 cm2/g and a 0.71 nm pore size by the HK (Horvath−Kawazoe) size distribution plot (Figure 2). Compound 1′ displayed strong solid-state fluorescence at 390 nm upon excitation at 280 nm at room temperature. Increase in fluorescence intensity in the MOF compared to the ligand fluorescence was observed. Absolute fluorescence quantum yields were calculated for both ligand and MOFs, using the Brill equation12 as follows,

conjugation provide highly luminescent character. In addition, if we use Zn(II) to connect the ligands then the resulting compound should be highly luminescent.8 Harnessing this design principle, we have synthesized a highly luminescent MOF [Zn1.5(L)(H2O)]·1.5benzene (1) (Figure 1) using a

Figure 1. (a) Perspective view of the starfish array of the MOF and (b) part of 1 shown along with the schematic visualization of cylindrical units of the π-stacked L moiety in a framework along the c axis (orange and blue color indicates two different 2D sheets).

flexible tripodal carboxylate ligand based on four aromatic rings, where the central one is connected with the other three by conjugatable ether linkage. The work reported herein demonstrates the “supramolecular wire effect” in a MOF for the fast and reversible detection of rarely reported solid-state detection of both aliphatic (DMNB) and aromatic (DNT) nitro explosives. To the best of our knowledge, only one example for detection of DMNB by a MOF compound is reported in literature.4b

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RESULTS AND DISCUSSION Ligand H3L used for MOF synthesis was synthesized according to the reported procedure.9a Compound 1 was obtained solvothermally at 90 °C, using H3L and Zn(NO3)2 solution in DMF/benzene (1:1 v/v). Single crystal X-ray diffraction (SCXRD) studies revealed a two-dimensional (2D) framework with strong π-stacked cylindrical units of orderly arranged L moiety. Compound 1 consists of bilayer sheets of the tripodal ligand connected by linear trimeric Zn(II) clusters at each end and all three carboxylate groups are coordinated to Zn(II) in a bidentate fashion. The linear trimeric cluster unit has two types

⎡ 1 − rst ⎤⎡ A x ⎤ ϕ=⎢ ⎥ ⎢ ⎥ϕ ⎣ 1 − rx ⎦⎣ A st ⎦ st

where rst and rx are the diffuse reflectance (at the fixed wavelength) of standard sodium salicyalate and the compound, B

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Figure 2. Low temperature N2 (77 K) sorption profile of 1′. Inset: A Haworth−Kawazoe (H−K) plot for micropore analysis, showing an average pore diameter.

respectively, Ax and Ast denotes the area under the compound and the standard phosphor emission (i.e., sodium salicyalate), respectively. ϕst and ϕx are the quantum yield for the standard and the compound, respectively. With consideration of the sodium salicylate as a standard phosphor with ϕst = 0.60 for the excitation range between 220 to 380 nm (λem: 420 nm),13 ϕL (quantum yield of ligand) was found to be 0.043, whereas that for the MOF compound ϕMOF was 0.2323. This highly increased MOF fluorescence is mainly due to the decrease in nonradiant decay after the complexation with the d10 Zn(II) metal centers,8 which provides a stable and ordered π-stacked spacial arrangement of L within the MOF.14 Making a practical sensing device fluorescence sensing in the vapor phase is desirable. To check the potential of the current MOF for detecting nitro explosives, we performed solid-state time-dependent fluorescence-quenching studies, by exposing a thin layer of 1′ to the vapor of the series of nitro-compounds. Immense quenching was observed in the case of nitrobenzene (NB, Figure 3a). Quenching does occur without any shift in emission maximum at 390 nm after exposure. The effect with 15.89% of quenching was observed in just a span of two seconds, and the nature of the quenching profile tends toward the maximum as the vapor exposure time increased. Structural evidence of π−π-stacked benzene guest molecules inside the framework gives an idea of how the electron-deficient nitro-aromatic compounds can interact with the π-electron cloud from the highly ordered π-stacked array of the framework on the surface. The current compound with super π-electronrich supramolecular 2D sheets and a continuous π−π-stacked 1D molecular packing of the ligands gives a “supramolecular wire” effect, which makes it possible for nitrobenzene to interact strongly with the framework surface (Figure 3b). It is unfavorable for NB to go inside the pores of the solventaccessible area inside the MOF. Additionally, the sorption amount of toluene is almost one-third that of BN, thus nitrobenzene [a bigger guest than toluene, Figures 3 (panels c and d) and 4a] plausibly interacts on the π-stacked surfaces of 1′. Due to the large band gap of MOF compounds, the ease of transfer of the electrons from the conduction band of the MOF to the low-lying LUMO of the nitro analytes in the fluorescence

Figure 3. (a) A quenching profile of 1′ by NB. (b) Supra-molecular wire effect showing a flow of π-electrons from sheet to sheet, and the plausible diffusion of benzene and the surface interaction of nitrobenzene molecules in 1′. (c) Structure of 1 along the ab axis with sandwiched benzene molecules between the π-electron cloud of two different sheets, making the π-complex. (d) A void diagram showing four different orientations of benzene molecules inside the framework. It is impossible for nitrobenzene to enter inside the framework (two alternate sheets shown in pink and blue colors).

quenching process is expected.4b The experimental band gap value for the MOF compound using the Kubelka−Munk plot was found to be ∼3.0 eV (Figure S17 of the Supporting Information). For all the nitro analytes, the band gap values were calculated using the DFT/LSDA/6-311G (d, p) basis set in Gaussian 03 (Figure 4b, see details in Table S2 of the Supporting Information). Theoretical band gap values were found to be lower than those of the experimental band gap for MOF (i.e., NT (66%) > DMNB (57%) > 2,4-DNT (53%) > 2,6-DNT (46%) > NP (39%) is observed (Figure 6). Vapor exposure of

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ASSOCIATED CONTENT

S Supporting Information *

Details about structural analysis, SCXRD data (CIF), TGA, PXRD, sorption, and luminescence studies of compound 1, HOMO−LUMO band gap, and ESP surface plots for nitro compounds; CCDC 898460 contains the supplementary crystallographic data for this paper. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank IISER Pune and DAE (Grant 2011/20/37C/06/ BRNS) for financial support and Prof. K. N. Ganesh for encouragement.

Figure 6. Descending sequential quenching effects of NB, NM, NT, DMNB, 2,4-DNT, 2,6-DNT, and NP on luminescence of 1′ in 10 min.

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the nitro analytes to the thin film does not affect the structure of the MOF, which was confirmed by the PXRD analysis of the separately exposed activated MOF (i.e., 1′ to the vapor of different nitro analytes) (Figure 7).

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Figure 7. Powder X-ray diffraction pattern of the activated MOF 1′ compound when exposed to the different nitro analytes.

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CONCLUSIONS In conclusion, we successfully designed and synthesized a novel 2D MOF with a starfishlike array and a super π-electron-rich surface, showing the supramolecular wire effect. Due to its unique structure, it exhibited a high efficiency for the quenching of both aromatic as well as aliphatic nitro explosives. Herein, several factors decide the quenching effects, like the electronrich and deficient regions in the host or guest, their maximum possible suitable orientations for interactions, vapor pressures of the nitro compounds, their sizes, HOMO−LUMO energy E

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