Optical Sensors Using Solvatochromic Metal–Organic Frameworks

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Cite This: Inorg. Chem. 2017, 56, 14164-14169

Optical Sensors Using Solvatochromic Metal−Organic Frameworks Philipp Müller, Florian M. Wisser,† Pascal Freund, Volodymyr Bon, Irena Senkovska,* and Stefan Kaskel* Department of Chemistry, Technische Universität Dresden, Bergstraße 66, D-01062 Dresden, Germany

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S Supporting Information *

ABSTRACT: A series of copper and 1,3-phenylebis(azanetriyl)tetrabenzoate based MOFs were obtained by postsynthetic modification of DUT-71 (DUT = Dresden University of Technology) using various nitrogen containing, neutral ligands to afford the compounds DUT-74, DUT-95, DUT-112, and DUT-114. The structure of the new MOFs DUT-112 and DUT-114 was solved from synchrotron X-ray single-crystal diffraction data. Both structures are tetragonal (P4/mnc) but differ slightly in the lattice parameters. All materials show specific shifts in absorption bands in solid state UV/vis spectra as a response to the exposure to various analytes. Analyzing this shift, it was possible to distinguish between solvents differing in polarity. Moreover, the determination of the polar analyte content in the excess of lower polarity solvent at low concentrations of 0.01 wt % is feasible.

1. INTRODUCTION Chemical testing and analysis are essential to understand the quality and composition of chemical substances and materials that are used in products, industrial processes, and manufacturing. However, analysis, detection, and identification of impurities (for example in pharmaceutical or food industry) or additive traces (in fuel or polymer chemistry) are often very complex. To design an efficient sensor, several detection principles can be used, e.g., electrochemical detection,1−3 changes in mass,4,5 or changes in optical properties. Detection of optical changes can be divided in different subgroups, such as changes in luminescence, UV/vis, or IR spectra.6 The efficient sensor should be selective and sensitive over a wide concentration range of the analyte. Because of their modular building block principle, metal− organic frameworks represent a versatile platform for functionalization, making them promising for many applications.7,8 High porosity combined with the presence of specific functional groups, capable of sensitive detection of desired molecules, is a useful concept, widely applied already. Especially optical detection of changes in luminescence properties is one promising approach.9−13 Often lanthanide-based compounds are used as luminescent reporter or organic building blocks as typical organic chromophore.14−19 Such luminescent active MOFs can be used, for example, for detection of explosives or relatively small organic compounds.20−22 Here often the quenching rate was monitored and detection to very small concentrations was possible.10,23−25 Also the determination of heavy metals26−28 or impurities in water29−31 by changes in MOF luminesce has been reported. One the other hand, one of the most simple sensing principles is based on colorimetric analysis, since the content of an analyte can be determined by a visual color change.17,32 For example, Banerjee and co-workers have shown that the as-made © 2017 American Chemical Society

brown Mg-NDI (NDI = naphthalenediimide) MOF is able to change the color to yellow upon exposure to ethanol.33 Exposure to other solvents results in different colors: for example, contact with DEF yields dark red color, and with MeCN, orange. Zheng et al. reported about an impressive, solvent dependent color change of the coordination polymer (WS4Cu4)I2(bpta) (bpta = 3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine). The material appears red in MeCN, and the color turns toward black in CHCl3.34 Both examples indicate that analyte−MOF interactions can result in a pronounced visible color change.35 It was also shown by us that it is possible to use reactivity of MOF for optical sensing. So, yellow dihydro1,2,4,5-tetrazine-3,6-dicarboxylate linker in a UiO-66-like structure turns to pink after exposure to oxidizing agents.18 Recently, we also have shown the possibility to stabilize and functionalize DUT-71 with a series of nitrogen containing neutral ligands (DUT = Dresden University of Technology).36,37 After two postsynthetic modification steps DUT-71 based materials were robust enough to be activated by supercritical drying. During this process the solvated green crystals change their color to a dark blue. The color change can be reverse by exposure to the solvents or solvent vapors. The behavior was utilized for the detection of ethanol vapors in the low ppm region.36 In this contribution we report DUT-71 based materials showing characteristic color changes as a response toward solvents differing in polarity. The color change can be noticed visually, as well as using solid state UV/vis. Received: September 8, 2017 Published: November 7, 2017 14164

DOI: 10.1021/acs.inorgchem.7b02241 Inorg. Chem. 2017, 56, 14164−14169

Article

Inorganic Chemistry

Figure 1. Topological and structural representation of single-crystal to single-crystal transformations of DUT-71 to DUT-74, DUT-95, DUT-112, and DUT-114. Å3, Z = 4, Dc = 0.642 g cm−3, 11086 independent reflections observed, R1 = 0.0560 (I > 2σ(I)), wR2 = 0.2114 (all data), and GooF = 1.083. CCDC 1437368 and 1437369 contain the supplementary crystallographic data for DUT-112 and DUT-114, respectively. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. 2.4. Crystal Preparation for Solvatochromic Experiment. The as-made, solvated crystals of DUT-74 and DUT-95 were soaked with the solvent of choice, except when n-heptane was used. Here the solvent was exchanged with ethanol first (six times over a period of 2 days). The solvent exchange was performed three times per day over a period of 4 days. 2.5. Estimation of Solvent Mixture Composition. The solvated crystals of DUT-74 or DUT-95 (DMF/EtOH 1:1) were exchanged with ethyl acetate/acetonitrile mixtures (of the following compositions: 3:1; 1:1; 1:3) three times per day over a period of 4 days. 2.6. Detection of Impurities. To supercritical dried DUT-114 crystals were added (10 mg) n-heptane/analyte mixture (2 mL) (analyte = acetone or isopropanol). The following concentrations were used: 0.01, 0.1, 1, and 10 wt %.

2. EXPERIMENTAL SECTION 2.1. Synthetic Procedures. Ligands used for the synthesis of DUT-71, DUT-74, and DUT-95 as well as MOFs were synthesized as described before (for more details see Supporting Information section 1).36 2.2. Synthesis of DUT-112: Cu4(mpbatb)2(AzoBIPY)0.5. To the DUT-71 crystals was added 4,4′-azobispyridine (62.6 mg, 0.34 mmol) dissolved in a mixture of DMF (N,N-dimethylformamide) and ethanol (1:1, 6 mL), and the reaction mixture was heated to 80 °C for 7 days. After cooling down to room temperature the solvent was replaced with a fresh mixture of DMF and ethanol (1:1) (Figure S4). DUT-112 was directly used for the synthesis of DUT-114. Anal. Calcd (%) for Cu4(m-pbatb)2(AzoBIPY)0.5(H2O)3(EtOH)1.5(DMF) 2 [Cu 4 (C 68 O 16 N 4 H 40 )(C 10 N 4 H 10 ) 0.5 (H 2 O) 3 (C 2 H 6 O) 1.5 (C3H7NO)2]: C 55.2, H 3.8, N 5.7, found C 55.3, H 3.8, N 5.7. Crystal data for DUT-112: C73H44Cu4N6O19, Mr = 1563.30, tetragonal P4/mnc, a = 26.720(4) Å, c = 26.640(5) Å, V = 19020(7) Å3, Z = 4, Dc = 0.546 g cm−3, 10599 independent reflections observed, R1 = 0.0568 (I > 2σ(I)), wR2 = 0.2173 (all data), and GooF = 1.048. 2.3. Synthesis of DUT-114: Cu4(mpbatb)2(AzoBIPY)0.5(dabco)X. To the DUT-112 crystals was added dabco (1,4diazabicyclo[2.2.2]octane) (57.4 mg, 0.51 mmol) dissolved in a mixture of DMF and ethanol (1:1, 6 mL), and the reaction mixture was heated to 80 °C for 7 days. After cooling down to room temperature the solvent was replaced with a fresh mixture of DMF and ethanol (1:1) (Figures S5 and S6). Anal. Calcd (%) for Cu4(m-pbatb)2(AzoBIPY)0.5(dabco)1.4(H2O)1.3 [Cu4(C68O16N4H40)(C10N4H10)0.5(C6N2H12)1.4(H2O)1.3]: C 57.6, H 3.8, N 7.3, found C 57.5, H 3.6, N 7.3. Crystal data for DUT-114: C88H74Cu4N11O16, Mr = 1795.74, tetragonal P4/mnc, a = 26.830(4) Å, c = 25.800(5) Å, V = 18572(6)

3. RESULTS AND DISCUSSION Following the strategy reported earlier,36,37 postsynthetic incorporation of linear neutral ligands of appropriate length (4,4′-azobispyridine (azobipy) and dabco) into DUT-71 was performed, giving two new compounds, DUT-112 and DUT114 (Figure 1). According to the synchrotron single-crystal Xray diffraction analysis, the structures are isoreticular to, e.g., DUT-74 and DUT-95, correspondingly,36 showing slight deviation in lattice parameters. DUT-71 structure crystallizes in the tetragonal space group P4/mnc with three independent 14165

DOI: 10.1021/acs.inorgchem.7b02241 Inorg. Chem. 2017, 56, 14164−14169

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Inorganic Chemistry copper atoms and half of an mpbatb linker in the asymmetric unit. The combination of two independent copper paddlewheel nodes with organic ligand in tetrahedral conformation leads to a formation of 3D framework with nou topology. The resulting framework has two different microporous cages, which can be additionally tuned in size by introduction of suitable neutral cross-linkers for improvement of the framework stability (Figure 1). The postsynthetic incorporation of AzaBipy causes slight shortening of the c-axis of the tetragonal unit cell in DUT-112. Further embodiment of the dabco during the second PSM step leads to the further shortening of the c-axis (Table S3). Both postsynthetic modification steps proceed without breaking the tetragonal symmetry of the structure. The crosslinkers introduced postsynthetically could be localized in the least-squares refinement of the corresponding structures (Figure S21). The solvent accessible void calculated by SOLV routine of PLATON amounts to 73.8% for DUT-112 and 66.9% for DUT-114. DUT-112 could not be activated without significant loss of crystallinity in contrast to DUT-114 (Figure S7). After solvent removal by supercritical drying, DUT-114 changes color from green to blue and shows permanent porosity in nitrogen adsorption experiments. DUT-114 has a BET area of 2900 m2 g−1 and an accessible pore volume of 1.2 cm3 g−1 (Figures S7 and S8), rendering this material as a highly porous adsorbent.36 Most important, after resolvation of DUT-95 and DUT-114, a reversible, solvent dependent color change was observed. Depending on their chemical composition, solvents can be classified into polar and nonpolar. One of the common polarity scales is an ET(30) scale based on the change of Reichardt’s dye color in response to the polarity of solvent.38 The ability of Reichardt’s dye to change the color is based on the solvent dependent charge transfer absorption. ET reflects the transition energy between the ground state and the lowest excited state of Reichardt’s dye. For example, polar solvents stabilize the ground state of the dye and have high ET values. One important limitation to the broad application of these indicator dyes is that ET(30) values cannot always be determined for low-polarity solvents, such as alkanes, due to the poor solubility of the dye in them. Therefore, the development of alternative sensitive systems is still attractive. Complexes based on transition metal and organic linkers containing a π-electron system often exhibit solvent dependent charge transfer absorption of two different types (metal-toligand or ligand-to-metal), depending on the relative electrondonor/electron-acceptor properties of metal and ligand.39 A strong solvatochromism was observed for both types of charge transfer absorption complexes.40−42 A similar effect is also observed in DUT-71 derivatives. DUT-74 and DUT-95 contain 3,6-di(pyridin-4-yl)-1,2,4,5tetrazine (bpta) ligand, known to support the solvatochromic behavior, as reported by Zheng et al. for (WS4Cu4)I2(bpta).34 Ligands in this compound are paired up with strong π−π interactions between tetrazine rings (3.578 Å). The authors claim that the solvatochromism of (WS4Cu4)I2(bpta) is ligand based and originates from π → π* transition for bpta. The bpta ligands in DUT-74 and DUT-95 are separated from each other, and such a drastic solvotachromic effect was not observed for DUT-materials. In n-heptane, the most nonpolar solvent investigated in this study, DUT-95 has a minimum in the absorption spectrum at 505 nm, and in 1,2-ethyleneglycol,

as the most polar solvent, at 542 nm, corresponding to a solvent-induced solvatochromic minimum shift of Δλ = 37 nm (i.e., Δν = 1352 cm−1, respectively ΔE = 0.17 eV). Aprotic nonpolar, aprotic polar, and protic solvents cause slightly different changes in the spectrum of DUT-95 (Figures S13−S16 and S18). Nevertheless, in each group of solvents the increasing solvent polarity induces a larger bathochromic shift of the minimum (Figure 2a and Table S3).

Figure 2. Plot of ET(30) value versus characteristic minima of absorption UV/vis spectra for DUT-95: ◆ = aprotic nonpolar, ■ = aprotic polar solvents, and ▲ = protic solvents (a); and oxygen containing aprotic solvents (b).

According to Reichardt et al., primary linear alcohols show a nonlinear increase in their polarity.42 Interestingly, the same behavior was observed in the experiments using DUT-95 (Figure 2a). Not only the discrimination of different solvent classes was possible but also arranging of oxygen bearing analytes for aprotic solvents (Figure 2b). The bathochromic shift of the characteristic minimum can be correlated to the polarity according to ET(30). DUT-74 shows a similar effect by adding different solvents to the crystals (Figures S9−S12, Table S2). The solvent sensitivity of DUT-74 and DUT-95 could be also used for the estimation of the composition of solvent mixtures. As model system, ethyl acetate and acetonitrile was chosen. Ethyl acetate is a representative of aprotic nonpolar solvents and acetonitrile of aprotic polar solvents. The 14166

DOI: 10.1021/acs.inorgchem.7b02241 Inorg. Chem. 2017, 56, 14164−14169

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Inorganic Chemistry wavelength shift caused by this particular solvent is 20 nm for DUT-74 and 16 nm for DUT-95. The shift value is linearly dependent on the solvent mixture composition (Figure 3). Therefore, it was possible to determine

Figure 3. Linear dependence of characteristic minimum wavelength shift of DUT-74 and DUT-95 from acetonitrile/ethyl acetate ratio.

the composition of three different ethyl acetate/acetonitrile mixtures with the unknown ratio of components measuring the spectra of DUT@solvent composite only. The most precise estimation was possible using DUT-95 due to the best linear regression of the calibration curve. The maximal deviation in estimated concentration lies within 2%. Azo dyes are known to show a positive solvatochromism. Incorporation of further spectroscopically active linker, 4,4′azobispyridine (AzoBIPY), into DUT-71 was also possible, resulting in DUT-112 and DUT-114 (Figure 1). DUT-114 was used for determination of analytes in the low concentration region (down to 170 ppm). This concept allows acetone (as polar aprotic solvent) or isopropanol (as protic solvent) to be detected in n-heptane (Figure S17). The n-heptane loaded DUT-114 has dark green color, and the characteristic minimum in UV/vis spectra is located at 502 nm. If n-heptane contains 1 wt % of acetone or isopropanol, even a visible color change occurs (Figure S19). This observation was also confirmed by solid state UV/vis measurements. If DUT-114 is exposed to n-heptane containing only 0.01 wt % (172 ppm) of acetone, the characteristic minimum shifts to 505 nm, and for 0.01 wt % (166 ppm) isopropanol in n-heptane to 507 nm (Figure 4). If n-heptane contains 0.1 wt % acetone or isopropanol, the minimum shifts to 511 or 516 nm, respectively. By increasing the concentration to 1 or 10 wt %, the characteristic shift in the UV/vis spectra is not so pronounced any more (514 nm [1 wt %] and 515.5 nm [10 wt %] for acetone, 529 nm [1 wt %] and 533 nm [10 wt %] for isopropanol). The UV/vis shift approaches a plateau, similar to the adsorption isotherm behavior (Figure 4). Therefore, reliable data can be obtained in the concentration range of 0.01−1 wt %. Li et al. reported that the detection of ketone or alcohol based side products can be used for the detection of the most commercially used RDX explosive (perhydro-1,3,5-trinitro1,3,5-triazine, cyclonit, T4).43 An advantage of this strategy is that handling of explosive agents itself can be avoided. Therefore, the ability of DUT-114 to detect similar compounds was tested. Solutions of n-heptane containing cyclohexanone or cyclohexanol were used for this purpose. As

Figure 4. Wavelength of characteristic minimum in UV/vis spectrum of DUT-114 vs concentration of acetone or isopropanol in n-heptane. (a) Linear scale and (b) semilogarithmic scale.

expected, DUT-114 shows visible color shift as response on the presence of mentioned analytes as low as 1 wt % (Figure S20) and thus may be regarded as a promising detection material for cyclic ketones and alcohols.

4. CONCLUSION In summary, the porous MOFs based on nitrogen containing ligands and copper paddle wheels, namely, DUT-74, DUT-95, or DUT-114, can be advantageously used for solvent polarity determination (within aprotic nonpolar, aprotic polar, and protic solvent groups) as well as for detection of analytes in different concentration ranges. The compounds show solvatochromic shifts of characteristic minimum in UV/vis spectrum sensitive to different aspects of solvent polarity and solvent− MOF interactions (originating from electrostatic and dispersive interactions or hydrogen-bond donating/accepting ability). DUT-95 enables the detection of additives at relatively high concentration, and DUT-114 enables the detection of trace impurities in binary mixtures down to a concentration of 0.01 wt %.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02241. Materials and methods, PXRDs, physisorption isotherms, and solid state UV/vis spectra (PDF) 14167

DOI: 10.1021/acs.inorgchem.7b02241 Inorg. Chem. 2017, 56, 14164−14169

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Inorganic Chemistry Accession Codes

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CCDC 1437368−1437369 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +49 351 463-33632. Fax: +49 351 463-37287. *E-mail: [email protected]. Tel: +49 351 463-32564. Fax: +49 351 463-37287. ORCID

Florian M. Wisser: 0000-0002-5925-895X Irena Senkovska: 0000-0001-7052-1029 Stefan Kaskel: 0000-0003-4572-0303 Present Address †

F.M.W.: Université de Lyon, Université Claude Bernard Lyon 1, CNRS, IRCELYONUMR 5256, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge BESSY II/HZB for granting beam time and travel costs. REFERENCES

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DOI: 10.1021/acs.inorgchem.7b02241 Inorg. Chem. 2017, 56, 14164−14169

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DOI: 10.1021/acs.inorgchem.7b02241 Inorg. Chem. 2017, 56, 14164−14169