Selective Detection of 2,4,6-Trinitrophenol (TNP) by a π-Stacked

Article pubs.acs.org/crystal

Selective Detection of 2,4,6-Trinitrophenol (TNP) by a π‑Stacked Organic Crystalline Solid in Water Soumya Mukherjee, Aamod V. Desai, Arif I. Inamdar, Biplab Manna, and Sujit K. Ghosh* Indian Institute of Science Education and Research (IISER), Dr. Homi Bhabha Road, Pashan, Pune-411008, India S Supporting Information *

ABSTRACT: 2,4,6-Trinitrophenol (TNP), an extremely perilous nitro explosive environmental pollutant, has been detected in aqueous medium with high selectivity and sensitivity. For the first-time, a supramolecular self-assembled crystalline organic solid, originating from a simple discrete molecule has been exploited for selective TNP-detection in the presence of other nitro analytes in water.

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INTRODUCTION Developing chemical sensors for fast detection of harmful explosives, especially those with toxic contaminant features is an exceedingly pressing issue from the standpoint of ecological protection, homeland safety, national security and human health.1−3 Nitro aromatic explosive substances, being the chief constituents of numerous unexploded land mines worldwide, pose a huge threat to mankind.4 Among these nitro explosives, namely, 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4DNT), 2,4,6-trinitrophenol (TNP), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), nitrobenzene (NB), 2,3-dimethyl-2,3dinitrobutane (DMNB), and nitromethane (NM), TNP has superior explosive power even compared to TNT.5 This is frequently used in dyes, fireworks, matches, glass, and leather industries,5 while as an even more dangerous matter of fact; it is a well-known toxic pollutant, considering its mutagenic activities and its unavoidable release in the surroundings during the commercial manufacture and utilization of its byproducts, leading to extensive soil- and aquatic contamination.6−8 Hence, aimed at tracing hidden explosives and ecological monitoring, selective and sensitive detection of TNP in soil and groundwater holds extreme significance, while performing the same in the presence of other nitro explosives in aqua brings in great challenge owing to false response-outcome because of their inherently high electron affinity.9 Realistic scenario involving deployment of trained canines and sophisticated instrumental techniques still prevails with the obvious obstacles of high cost and portability issues; consequently rendering the inexpensive, reliable, and easily portable chemosensors to be proficiently employed for serving the purpose.10,11 Chemosensors being the compounds which noticeably impart significant alterations in magnetic, electrical, electronic, or optical properties, on binding to specific guest molecules or ions;12 fluorescent chemosensors for nitro-explosive sensing © XXXX American Chemical Society

have particularly attracted a great deal of recent attention due to their convenient use and high sensitivity.13−15 Among all the diverse systems studied for nitro-explosive sensing, mostly pertaining to thin films, conjugated polymers; toxicity, instability, and difficulty to command the precise molecular recognitions for such materials are the most frequently encountered drawbacks for explosive-sensing applications.16,17 In fact, the targeted suitable materials functioning as nitroexplosive sensor in aqua comprise of very limited reports of thin films, surfactant micelle, conjugated polymers, mesoporous materials, organic dye and metal−organic frameworks.18−26 Especially, few of the very recent crucial developments in this domain have been done exploiting crystalline solid MOFs.24,25 However, to the best of our knowledge, supramolecular selfassembled crystalline organic solid mediated aqueous-phase detection of nitro-explosives has not been reported yet. The principal aim of this work was to achieve the selectivityefficiency for TNP in aqueous phase by strategic incorporation of suitable motifs in a supramolecular organic material developed from a simple organic precursor. In this regard, hydrophobic water-repellent naphthalene moieties;27 wellrecognized for their π-stacking interactions to form numerous supramolecular architectures, were conceived to be one of the ideally suited motif to start with. Moreover, nitrophenolic TNP being the targeted analyte, H-bond donor fragments in the probe should facilitate strong H-bonding interactions essentially selective for TNP over its congener nitro-explosives. Herein, we have envisioned compound 1, in the form of solidstate crystalline supramolecular assembly, since this fluorescent probe possesses the unique combination of H-bonding donor Received: April 25, 2015 Revised: May 19, 2015

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fragments, requisite hydrophobicity to remain unaffected in water, coupled with π-e− rich naphthalene moieties together in one system (Figure 1).

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EXPERIMENTAL SECTION

Materials and Measurements. Materials. TNT and RDX, both were obtained from HEMRL Pune (India). TNP, 2,4-DNT, 2,6-DNT, DMNB, NM were procured from Aldrich; while 4-NT, 1,3-DNB, and NB purchased from local company (Rankem) and used as received. Dry solvents were carefully used during entire analysis and were obtained locally. Caution! TNT, RDX and TNP are extremely explosive in nature and should be handled caref ully and in small amounts. The explosives were handled as dilute solutions and with safety measures to avoid explosion. Synthesis of Compound 1. Synthesis of compound 1 was done according to the synthetic protocol described in Scheme 1. The intermediate triaminoguanidinium chloride salt was prepared according to the previously reported procedure.31 One gram (9.5 mmol) of intermediate triaminoguanidinium chloride salt was dissolved in a hot mixture of ethanol: water (2:1) (total volume: 30 mL), and the pH for this solution was adjusted to pH ∼ 3, by adding aqueous HCl dropwise. 4.7 g (30 mmol) of 1naphthaldehyde was directly added onto this reaction mixture, followed by refluxing at 90 °C under N2 atmosphere for ∼12 h. After the reaction mixture was cooled to room temperature, it was filtered and the residue was washed repeatedly with aqueous MeOH, to get rid of the unreacted reactants. The dark yellow crude product was obtained as pure Compound 1. Characterization of Intermediate: HRMS (ESI) (Supporting Information Figure S1): Calcd for CH9N6 [M]+ 105.0893; Found 105.0896. Characterization of compound 1: 1H NMR (400 MHz, DMSO-d6) (Supporting Information Figure S2) δ 12.4 (S, 3H); 9.8 (S, 3H), 8.6 (d, J = 8.0 Hz, 3H), 8.4 (d, J = 8.0 Hz, 3H), 8.1 (d, J = 8.0 Hz, 3H), 8.0 (d, J = 8.0 Hz, 3H), 7.6−7.7 (m, 6H), 7.6−7.5 (m, 3H), 5.0 (S, 2H), 0.85 (S, 9H), 0.08 (S, 6H); 13C NMR (100 MHz, DMSO-d6) (Supporting Information Figure S3) δ 149.9, 149.3, 133.8, 131.9, 131.4, 129.4, 128.8, 127.9, 126.8, 126.5, 126.0, 123.0; HRMS (ESI) (Supporting Information Figure S4) Calcd for C34H27N6 [M + H]+ 519.2292; Found 519.2294. Elemental Analysis: Anal. Calcd for C34H27ClN6 C, 73.57; H, 4.90; N, 15.14. Found C, 73.01; H, 4.62; N, 15.83. Physical Measurements. X-ray powder pattern was recorded on Bruker D8 Advanced X-ray diffractometer at room temperature using Cu Kα radiation (λ = 1.5406 Å). Thermogravimetric analyses were obtained in the temperature range of 30−800 °C on PerkinElmer STA 6000 analyzer under a N2 atmosphere at a heating rate of 10 °C min−1. Fluorescence measurements were done using Horiba FluoroMax 4 with stirring attachment. X-ray Structural Studies. Single-crystal X-ray data were collected at 100 K on a Bruker KAPPA APEX II CCD Duo diffractometer (operated at 1500 W power = 50 kV, 30 mA) with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å). Crystals ware on nylon CryoLoops (Hampton Research) with Paraton-N (Hampton Research). The data integration and reduction were processed with SAINT32 software. A multiscan absorption correction was applied to the collected reflections. The structures ware solved by the direct method using SHELXTL33 and was refined on F2 by full-matrix leastsquares technique using the SHELXL-9734 program package within the WINGX35 program. All non-hydrogen atoms were refined

Figure 1. (a) Structure of fluorescent probe 1, with the triple blend of π-e− rich, hydrophobic naphthalene groups and H-bond donor fragments. (b) H-bonding arrays between the π-stacked layers of 1 via bridging Cl− anion.

C3-Symmetric organic molecular materials comprising of triaminoguanidinium cationic species have been comprehensively studied by Müller et al. in the past decade,28−30 and is well-regarded as a superb triangular building block, apposite for supramolecular solid-state crystalline assembly to develop intriguing tectons, designed to experience facile H-bonding interactions with their neighbors in specific directional pathways. Keeping these features in mind, triaminoguanidine motif has been strategically engaged for imparting a nitrogenrich environment to favor strong hydrogen bonding, whereas the simultaneous use of naphthalene moiety assisted to furnish excellent luminescence characteristics to the fluorophore probe 1, with strong intermolecular π−π interactions essential for facile crystallization. A multimolecular solid-state supramolecular assembly has been fabricated out of a predesigned basic precursor, following a simple one-step synthetic protocol involving the starting materials triaminoguanidinium chloride and 1-naphthaldehyde (Scheme 1). Use of the tripodal schiffbase 1, constituted of water-repellent 1-naphthaldehyde and triaminoguanidinium moiety helped behind coherent attainment of the desired amalgamation of strong hydrogen bonding interactions along with adequate π-stacking influence, crucial for selective interplay with electron-withdrawing nitrophenolic analyte TNP in water.

Scheme 1. Synthesis Protocol for the Preparation of Fluorescent Sensor Probe 1

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anisotropically. All hydrogen atoms were located in successive difference Fourier maps and they were treated as riding atoms using SHELXL default parameters. The structures were examined using the Adsym subroutine of PLATON,36 to ensure that no additional symmetry could be applied to the models. Crystallization Procedure of Compound 1. Compound 1 (0.1 mmol) was dissolved in a mixture of three different solvents, consisting of of DCM, acetonitrile, and benzene (2:3:1 v/v) and then kept for ∼10 days with no mechanical disturbance for allowing the crystals of compound 1 to grow, suitable for X-ray diffraction mediated characterization. Preparation of Guest-Free Compound 1D. Given the fact that the crystals of compound 1 registered a thermogravimetric analysis plot (Supporting Information Figure S9) indicating presence of low-boiling solvents removable until a temperature ∼75 °C, followed by a phase possessing clear thermal stability up to ∼200 °C; the crystalline phase of compound 1 was heated at 80 °C under vacuum, to obtain the guest-free desolvated crystalline phase compound 1D. Fluorescence Study. One milligram of desolvated guest-free crystals of 1D is weighed and added to a fluorescence cuvette (path length of 1 cm), containing 2 mL of Milli-Q water under constant stirring. The photoluminescence spectral response in 350−650 nm range upon excitation at 340 nm was measured in situ, after incremental addition of freshly prepared aqueous analyte solutions (1 mM each; 20−200 μL) and corresponding fluorescence intensities were monitored at 468 nm. The solution was stirred at constant rate during fluorescence measurement, to maintain homogeneity of solution. Crystallographic data for compund 1 is available at CCDC 1049489.

Figure 2. PXRD profiles confirming the stability of 1D in water, aqueous TNP solution, and 1 N HCl solution.

aromatic nitro-explosives like TNP, 2,4-DNT, 2,6-DNT, TNT, 1,3-dinitrobenzene (1,3-DNB), 4-nitrotouene (4-NT), NB, and nitro-aliphatic explosives, such as RDX, DMNB, and NM (Figure 3 and Supporting Information Figures S11−S19). From the desired TNP-selectivity standpoint, Figure 3a showed drastic photoluminescence (PL)-intensity quenching (88.45% on 200 μL addition), with steadily increasing amounts of added TNP. The PL-quenching for TNP was exactly determined at as low as 15.2 μM concentration, that is, 3.48 ppm (parts per million) (Supporting Information Figure S24 and Tables S1− S2). On the contrary, all the other nitro analytes came up with only minute changes (Figure 3b). To get an insight into the observed selective TNP-quenching mechanism, the quenching efficiency for all the studied analytes were analyzed using Stern−Volmer (SV) equation, and quenching constant (KD) for TNP was calculated to be as high as 3.43 × 103 M−1 (Figure 4 and Supporting Information Figures S24−S26). An upward bending only in case of incremental addition of TNP, in comparison to the linear increasing responses registered for all other nitro analytes (Figures 4 and Supporting Information Figure S24), validates exclusive nonlinearity of the SV plot for TNP. This indeed offers strong evidence behind the unification of static and dynamic quenching processes. To verify the mechanism operative behind the remarkably selective TNPmediated quenching observation, the spectral overlap extent between the analyte’s absorption band and the emission band of the luminophore (1D) was checked (Supporting Information Figure S23). The large overlap extent between the emission spectrum of 1D and exclusively that of analyte TNP’s absorption band (strikingly no overlap for other nitro analytes) was in absolute accord to the TNP-selectivity encountered. Since resonance energy transfer is a long-range phenomenon, the fluorescence emission quenching can transcend well to the adjacent fluorophore molecules and consequently, by means of energy-transfer process, the quenching efficiency undergoes a drastic increase, intensifying the ensuing detection selectivity and sensitivity as monitored in this case. To verify whether phenolic moiety in the NAC indeed play a crucial role for induction of the selective quenching response, PL-intensity changes were also monitored in cases of adding aqueous solutions of 4-nitrophenol (4-NP) and 2,4-dinitrophenol (2,4-DNP). The fluorescence quenching performance of phenolic analytes is in accordance with acidity of phenolic protons TNP > 2,4-DNP > NP. As a befitting outcome, declining quenching efficiency with decreasing acidity of the

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RESULTS AND DISCUSSION The as-synthesized crude solid compound (C34H27N6+Cl−) was crystallized to form a supramolecular molecular assembly (1) (Ortep view, Supporting Information Figure S5), from a ternary mixture solvent comprising of DCM, benzene and acetonitrile (2:1:3 v/v), which revealed the dual features of luminescence and H-bonding, as rationally anticipated in its crystalline assemblage of multimolecular packing (Figure 1b, Supporting Information Figures S4 and S6). High-quality yellow colored rod-shaped crystals suitable for further studies were obtained by adopting slow evaporation technique. Singlecrystal X-ray diffraction (SC-XRD) study revealed that the compound crystallized in monoclinic C2/c space group, other details of which are enlisted in Supporting Information Table S4. Powder-X-ray diffraction (PXRD) studies revealed its excellent crystalline feature, along with its phase purity (Supporting Information Figure S7), while thermogravimetric analyses came up with the indication of a thermally stable phase, obtained on heating at ∼80 °C under vacuum, and consequently obtaining the guest-free crystalline phase 1D, possessing thermal robustness until ∼200 °C with similar PXRD profile as observed for 1 (Supporting Information Figure S7). This evidently suggests the crystal-structure of 1D retained exactly similar to crystalline phase 1, although it has transformed to a desolvated phase devoid of any guest species, thereby facilitating host−guest interactions more in the crystalline state. Interestingly, the compound 1D was found to have excellent water and acid stability as anticipated (Figure 2), which led us to comprehensively investigate its traceamount selective TNP-sensing competence in water. Guest-free crystals of 1D when dispersed in water, presented a strong luminescence-signature upon excitation at 340 nm (Supporting Information Figure S10). To ascertain the nitroexplosive sensing ability of 1D in water, fluorescence intensities of 1D dispersed in water were monitored upon addition of aqueous solutions (1 mM; 20−200 μL each) of different C

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Figure 3. (a) Emission spectra of luminophore probe (1D) dispersed in water, upon incremental addition of aqueous TNP solution (1 mM). (b) Quenching-efficiency plot (bar diagram) for different nitro analytes in case of compound 1D.

(Supporting Information Figure S25) reiterated that merely the capacity of phenolic moiety to accept protons from guanidinium core is not behind the observed quenching, rather the combined effect of nitro and phenolic groups are responsible behind the selective interplay of probe 1D with TNP. Enthused from these; the selectivity of 1D toward TNP even in the presence of other nitro-analytes was comprehensively investigated by competing nitro-analyte test (CNA test).24,25,37 Following a planned investigational protocol, the PL spectrum for water-dispersed 1D was examined upon the incremental consecutive addition of a 1 mM aqueous solution of RDX (40 μL in two evenly distributed batches), to allow high-affinity basic sites in the surface of 1D to get available to RDX, which led to just a petite photoluminescence quenching. Much intriguingly, subsequent TNP (40 μL, 1 mM aqueous solution) addition resulted in a strikingly prompt and noteworthy PL-quenching response, with a precisely alike trend observed in the following repeat-cycles of RDX and TNP addition performed in the same sequential manner (Figure 4b). Addition of aqueous solutions of all the congener nitro analytes followed by TNP to 1D also presented similar trend, reaffirming TNP-selectivity. Not only in water, even to substantiate the applicability of the present probe 1D for real-time TNP-sensing, a typical experiment involving the visual response of it encrusted over black TLC paper-strips to the assorted nitro analytes was performed.24,38 As anticipated, the strip for TNP solution within 1 min showed marked darkening, with much reduced PL-response (Figure 5), while for all other analytes, they remained nearly unaltered. To extrapolate this observation, other nitro analytes were added to similar MOF loaded strips. The distinct signature of the crystals of 1D toward aqueous

Figure 4. (a) Stern−Volmer plot for all the nitro analytes in water. (b) Photoluminescence quenching response of 1D, upon addition of aqueous solutions of different nitro-explosive analytes followed by TNP.

nitrophenolic analytes viz. for 2,4-DNP and 4-NP (Supporting Information Figures S20 and S21; bar diagram at Supporting Information Figure S22) suggested that the decisive role behind the selective interplay of such nitrophenolic aromatics can only be ascribed to the distinct presence of electrostatic and Hbonding interactions between TNP and probe (1D), which is entirely absent for other nitro-analytes. To introspect more into the origin of PL-quenching with TNP, phenol was employed as an analyte; the absence of any noteworthy quenching

Figure 5. Response of compound 1D-coated black paper strips to different aqueous nitro-explosives, monitored under 365 nm UV illumination. D

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TNP in such contact mode on black strips simply reinforced the exceptional TNP-selectivity (Figure 5). This prompt nakedeye recognition of TNP under UV light (λ = 365 nm) exploiting 1D, paves the way for its practical application to exhibit selective response to aqueous toxin TNP.

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CONCLUSION In conclusion, the exceptional strategic combination of three convergent factors in a single supramolecular π-stacked solidstate crystalline probe (1D), namely, hydrophobic pendant naphthalene groups allowing π-stacking interactions with electron-withdrawing nitro-explosive TNP in water, with the deliberate incorporation of triaminoguanidine fragment rendering specific intermolecular H-bonding interactions with TNP, leads to the extremely selective and sensitive aquatic TNPdetection, even in concurrent presence of all other congener nitro analytes. The designed strategic rationale to attain such unprecedented crystalline-state organic small fluorescent molecule-based selective TNP sensing phenomena might indeed prove imperative from the perspectives of national safety and environmental applications.

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing HRMS, 1H NMR, 13C NMR, FTIR, and emission spectra, crystal structure of compound 1, ORTEP diagrams, π-stacking and H-bonding assisted supramolecular channel packing, powder X-ray diffraction patterns, TGA profiles, quenching efficiency plot, spectral overlap, Stern− Volmer plots, linear region of fluorescence intensity, tables showing standard deviation for probe, detection limit for proble, HOMO and LUMO energies, and crystal data and refinement, detection limit calculations, and a CIF of compound 1. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.cgd.5b00578.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +91 20 2590 8186. E-mail: [email protected]. Home page: http://www.iiserpune.ac.in/~sghosh/. Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.M. and A.V.D. are thankful to IISER Pune for research fellowships. B.M. and A.I.I. thank CSIR and SERB (Project No. 30112083), respectively, for the same. We are grateful to IISER Pune for research facilities. DST (Project No. GAP/DST/ CHE-12-0083) is acknowledged for the financial support. DSTFIST (SR/FST/CSII-023/2012) is acknowledged for microfocus SC-XRD facility.

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REFERENCES

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DOI: 10.1021/acs.cgd.5b00578 Cryst. Growth Des. XXXX, XXX, XXX−XXX