DOI: 10.1002/chem.201405267
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& Sensors
Highly Selective Detection of 2,4,6-Trinitrophenol and Cu2 + Ions Based on a Fluorescent Cadmium–Pamoate Metal–Organic Framework Junwei Ye, Limei Zhao, Raji Feyisa Bogale, Yuan Gao, Xiaoxiao Wang, Xiaomin Qian, Song Guo, Jianzhang Zhao, and Guiling Ning*[a]
Abstract: A luminescent cadmium–pamoate metal–organic framework, [Cd2(PAM)2(dpe)2(H2O)2]·0.5(dpe) (1), has been synthesized under hydrothermal conditions by using p-electron-rich ligands 4,4’-methylenebis(3-hydroxy-2-naphthalenecarboxylic acid) (H2PAM) and 1,2-di(4-pyridyl)ethylene (dpe). Its structure is composed of both mononuclear and dinuclear CdII building units, which are linked by the PAM and dpe ligands, resulting in a (4,8)-connected 3D framework. The p-conjugated dpe guests are located in a 1D channel of 1. The strong emission of 1 could be quenched efficiently by trace amounts of 2,4,6-trinitrophenol (TNP), even in the presence of other competing analogues such as 4-nitrophenol,
Introduction Nitroaromatic compounds (NACs), such as 2,6-dinitrotoluene (2,6-DNT), 2,4,6-trinitrotoluene (TNT), and 2,4,6-trinitrophenol (TNP), are primary ingredients of industrial explosives, environmentally deleterious substances, and many unexploded land mines.[1] Rapid detection of these nitroaromatic explosives is attracting increasing attention due to their broad range of applications in security, military operations, environmental management, mine-field analysis, and in forensic and criminal investigations.[2] Among such NACs, TNP is an extremely hazardous chemical with strong explosive capacity and a low safety coefficient. The compound is widely used in dyes, explosives, leather, fireworks, matches, and pharmaceutical industries,[3] and it can directly contaminate the soil and aquatic systems if it is released to the surrounding environment by some industrial processes. Furthermore, the leakage of TNP can also trig[a] Dr. J. Ye, L. Zhao, R. F. Bogale, Y. Gao, X. Wang, X. Qian, S. Guo, Prof. J. Zhao, Prof. G. Ning State Key Laboratory of Fine Chemicals School of Chemical Engineering Faculty of Chemical, Environmental Biological Science and Technology Dalian University of Technology 2 Linggong Road, Dalian 116024 (China) Fax: (+ 86) 411-84986065 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405267. Chem. Eur. J. 2015, 21, 2029 – 2037
2,6-dinitrotoluene, 2,4-dinitrotoluene, nitrobenzene, 1,3-dinitrobenzene, hydroquinone, dimethylbenzene, and bromobenzene. The high sensitivity and selectivity of the fluorescence response of 1 to TNP shows that this framework could be used as an excellent sensor for identifying and quantifying TNP. In the same manner, 1 also exhibits superior selectivity and sensitivity towards Cu2 + compared with other metal ions such as Zn2 + , Mn2 + , Mg2 + , K + , Na + , Ni2 + , Co2 + , and Ca2 + . This is the first MOF that can serve as a dual functional fluorescent sensor for selectively detecting trace amounts of TNP and Cu2 + .
ger skin irritation, anemia, abnormal liver functions, and male infertility.[4] Curiously, less effort has focused on detecting TNP than other hazardous substances,[5] so it is highly desirable to develop rapid, convenient and specific analytical methods for detecting this compound. On the other hand, as an essential biological metal ion, Cu2 + plays a critical role in a variety of fundamental processes in living biological systems.[6] However, at higher concentrations, Cu2 + can become toxic, leading to metabolism disorders such as Wilson’s disease and neurodegenerative disorders such as Alzheimer’s disease.[7] Therefore, the efficient detection of trace amounts of Cu2 + is also very important for assessing human health. Until now, various analytical and spectroscopic methods have been employed to detect NACs and metal ions,[8] but high cost, intricate operation, low sensitivity, and high levels of interference by other analytes associated with these methods have restricted their widespread use.[9] Recently, fluorescencebased detection of NACs or metal ions by using luminescent polymers, small organic molecules, and nanomaterials has attracted much attention because of the sensitivity, selectivity, portability, short response time, and convenient visual detection.[10] In our continuing research on luminescent metal–organic frameworks (MOFs),[11] we focused our study on the development of MOF-based fluorescent sensors for the detection of NACs or metal ions. MOFs are a novel class of inorganic-organic hybrid materials that show structural diversity and potential applications for gas storage, separation, catalysis, molecular magnets, and sensors.[12] Tunable porosity, functional
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Full Paper surfaces, and the bulk conjugated backbone of MOFs can induce transduction of host-guest chemistry leading to detectable changes in light-emitting color or luminescent characteristics, which can make them promising candidates for sensing applications.[13] In fact, some luminescent MOFs have been reported for detecting NACs or metal ions based on fluorescence quenching process,[14, 15] but details of the mechanism are not clear. In particular, the rapid and selective detection of trace amounts of TNP or Cu2 + remains limited.[16] Herein, we report a new three-dimensional (3D) MOF, [Cd2(PAM)2(dpe)2(H2O)2]·0.5(dpe) (1) constructed from p-electron-rich rigid-flexible ligand pamoic acid (H2PAM, 4,4’-methylene bis(3-hydroxy-2-naphthalenecarboxylic acid)) and rigid ligand 1,2-di(4-pyridyl)ethylene (dpe) under hydrothermal conditions. The strong emission of 1 could be quenched efficiently by trace amounts of TNP or Cu2 + at room temperature, even in the presence of other competing analogues, which indicated that 1 can serve as an excellent sensor for specific selective identification and quantification of TNP and Cu2 + through a fluorescence quenching process.
Results and Discussion Description of crystal structure Single-crystal X-ray diffraction studies revealed that 1 is composed of a 3D framework crystallizing in the triclinic space group P1¯, with two CdII ions, two PAM ligands, two dpe ligands, two coordinated water molecules, and half dpe guest molecule in the asymmetric unit. Two crystallographic independent CdII centers possess different coordination environments, as depicted in Figure 1 A. The Cd1 center adopts a seven-coordinate mode (CdO5N2) by bonding to four oxygen atoms from two PAM ligands, two nitrogen atoms from two dpe ligands, and one oxygen atom from a coordinated water molecule, forming a pentagonal bipyramidal geometry. The
Figure 1. a) View of the coordination environments of Cd2 + centers. b) View of the (4,8)-connected topological net. c) 3D framework showing the guest dpe ligands with space-filling representation of 1. d) The p–p interaction between naphthyl rings of PAM and guest dpe ligand. Chem. Eur. J. 2015, 21, 2029 – 2037
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Cd2 shows a distorted octahedral geometry [CdO4N2] by coordinating to three carboxyl oxygen atoms from three different PAM ligands, two nitrogen atoms from two dpe ligands, and one oxygen atom from a coordinated water molecule. The CdO and CdN bond lengths are in the range of 2.194(2)– 2.684(2) and 2.292(2)–2.451(2) , respectively, which are comparable to those found in the related CdII-pamoate coordination polymers.[17–20] Two crystallographic equivalent Cd2 atoms are bridged by four carboxylate groups in bis-monodentate fashion to give a dinuclear cadmium molecular building block (MBB), [Cd2N4(CO2)4], with a separation Cd···Cd distance of 4.375 . Each mononuclear [CdO5N2] MBB serves as a four-connected node and dinuclear [Cd2N4(CO2)4] MBB acts as an eightconnected node, which are linked to each other by PAM and dpe ligands, forming a (4,8)-connected 3D network with the Schlfli symbol of {4.52.63}2{46.55.615.82} (Figure 1 B and Figure S1 in the Supporting Information). It should be noted that 1 has a small 1D channel along the c-axis, which is lined with p-electron-rich naphthyl moieties of bent PAM ligands. The guest dpe ligand with coplanar p-conjugated structures is located in channels (Figure 1 C). The p-p interactions (distance of the closest carbon atoms is 3.489 ) occur between naphthyl rings of PAM and guest dpe ligands (Figure 1D). This supramolecular wire-like arrangement and rich p-electron aggregated structures are considered to be a beneficial feature because they lead to additional luminescent properties for fluorescencebased detection of NACs.[21, 22] Thermogravimetric analysis (TGA) indicated framework 1 has excellent stability. As shown in Figure S2 (in the Supporting Information), the first weight loss of 2.19 % from 30 to 178 8C corresponds to the loss of coordinating water molecules (calcd: 2.42 %). Then it is stable to approximately 296 8C, when a second weight loss starts with the decomposition of dpe and PAM ligands. Luminescent sensing of nitroaromatic compounds The photoluminescence (PL) spectrum of compound 1 in dimethyl sulfoxide (DMSO) exhibits a strong emission peak at 502 nm upon excitation at 371 nm at room temperature, which is redshifted (34 nm) in comparison with that of 1 in the solid state (Figures S3 and S4). A possible explanation for this redshift involves guest-dependent interactions between the framework and solvent molecules.[23] However, in solution, intermolecular rotation and nonradiative relaxation pathways are increased through the collision between framework and guest molecules.[24] The fluorescence quantum yield of 1 (FMOF = 0.463) is higher than that of free H2PAM ligand (FL = 0.063), which is probably due to the electronic coupling between neighboring ligands through the CdII centers, reducing the nonradiative decay of ligands.[24c] To explore the potential application of 1 in the detection of TNP, fluorescence-quenching titrations were performed by gradual addition of TNP (1 mm in DMSO) to a solution of 1 in DMSO. Very high levels of fluorescence quenching were observed upon incremental addition of TNP (Figure 2). The quenching efficiencies were estimated by using the formula
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Figure 2. The fluorescence emission spectra of 1 in DMSO upon incremental addition of TNP (1 mm) (lex = 371 nm). Inset shows solutions of 1 in DMSO under UV irradiation before and after titrating with TNP.
(I0I)/I0 100 %, where I0 is the maximum fluorescence intensity of 1 and I is the intensity of 1 containing TNP solution. The initial fluorescence intensity of 1 decreased by 98.4 % when only 11.5 ppm TNP was added. The fluorescence quenching phenomenon could be easily detected even at very low concentration of TNP (0.76 ppm). Good linear dependencies (R2 = 0.9945) of luminescence quenching efficiencies on the TNP concentration were obtained, as shown in Figure S5 in the Supporting Information. It is noticeable that the calibration curve plot of 1 toward TNP showed a detection limit of 1.76 108 g L1. This system therefore represents a rare case of luminescent MOF materials that can be used to determine the concentration of TNP quantitatively.[16, 25] The powder X-ray diffraction (XRD) patterns of the samples before and after immersing in TNP solution are shown in Figure 3. Most peak positions of both samples were in good agreement with those of a simulated pattern generated from the single-crystal structure analysis, suggesting that the framework remains intact even immersed in TNP solution. To compare the sensing selectivity of 1 toward TNP, the emission spectra of 1 were also recorded upon addition of other NACs, including 4-nitrophenol (4-NP), 1,3-dinitrobenzene (1,3-DNB), 2,6-dinitrotoluene (2,6-DNT), 2,4-dinitrotoluene (2,4DNT), nitrobenzene (NB), dimethylbenzene (DMB), bromobenzene (BB), and hydroquinone (h-DQ), with the same concentration (1 mm) (Figure 4 and Figures S6–S9). The quenching efficiencies (%) and corresponding Stern–Volmer (S–V) plots of different NACs are plotted in Figure 5 and Figure S10 (in the Supporting Information), respectively. The electron-deficient 4-NP, 1,3-DNB, 2,6-DNT, 2,4-DNT, and NB showed little effect on the fluorescence intensity, with 14.8, 8.05, 8.30, 5.80, and 4.41 % of the quenching ability, respectively. Interestingly, a slight fluorescence enhancement of 1 was observed when electron-rich DMB, BB, and h-DQ were used as analytes. Such luminescence quenching or enhancement response may be attributed to Chem. Eur. J. 2015, 21, 2029 – 2037
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Figure 3. XRD patterns of 1, a) simulated, b) experimental, c) immersed in TNP solution.
electron donor-acceptor transfer between 1 and the analytes. Similar change in the fluorescence intensity of MOFs towards guest molecules was detected previously.[14a, 26] To examine the potential of the selective detection by 1 of TNP in the presence of other interfering NACs, a series of competition experiments were performed by gradual addition of a solution of different nitroaromatic analogues followed by TNP into 1, and the corresponding fluorescence spectra were monitored (Figure 6 and Figure S11–S15 in the Supporting Information). Taking addition of electron-deficient 1,3-DNB as an example (Figure 6 A), initially, the addition of 1,3-DNB showed negligible effect on the luminescent intensity of 1, but a significant fluorescence quenching occurred after the addition of TNP to a mixed solution of 1 and 1,3-DNB. A similar quenching phenomenon was observed when electron-rich DMB and BB were taken as interfering substance, respectively. The stepwise decrease in luminescent intensity (Figure 6 d) demonstrates the high ability of 1 for selectively detecting TNP even in the presence of other nitroaromatic compounds. In addition, the fluorescence quenching of 1 dissolved in N,N-dimethylformamide (DMF) was observed upon incremental addition of TNP (Figure S16 in the Supporting Information). The effect of water as a solvent on the detection of TNP was also monitored for practical applications. Due to poor solubility of 1 in pure aqueous solution, compound 1 was dissolved in H2O–DMSO mixed solvents and then its fluorescence spectrum was recorded by the dropwise addition of an aqueous solution of TNP. Compared with that of 1 in only DMSO, the fluorescent intensity of 1 in H2O–DMSO was slightly lowered with a slight redshift (Figure S17 in the Supporting Information), which may be attributed the effect of water polarity on charge transfer;[16a] nevertheless, significant quenching of the fluorescent intensity could be observed upon addition of TNP. These experimental results demonstrate that 1 constitutes a reliable and efficient fluorescence sensor for TNP with high sensitivity and selectivity compared with the response to other small nitroaromatic analogues.
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Figure 4. The fluorescence emission spectra of 1 in DMSO upon incremental addition of 1 mm a) 4-NP, b) 1,3-DNB, c) DMB, and d) BB (lex = 371 nm). Insets are enlarged images of the segment of the emission spectra.
To date, although molecule recognition derives from the interaction between host and specific guest substrates,[27] the mechanism of selective sensing of explosives based on MOFs is not very clear. From the crystal structure (Figure 1) and adsorption isotherm (Figure S18 in the Supporting Information) of 1, it is confirmed that the lower porosity and small pore size of 1 reduces its ability to encapsulate analytes, so the fluorescence response mechanism in our experiments might be attributed to light absorption and an electron transfer from electron-donor to electron-withdrawer adsorbed on the surface of 1. This sensing mechanism differs from most of the known guest-induced fluorescence quenching or enhancement mechanisms,[28] in which the analytes act as guest molecules that occupy the channels or cages of MOFs. To better understand the mechanism of detection, the absorption spectra of 1 upon incremental addition of TNP were recorded (Figure 7 A and Figure S19 in the Supporting Information). Two distinct absorption bands of UV/Vis spectra of 1 were observed at 291 and 366 nm. The absorption band intensity at 291 nm reduced rapidly with increased TNP concenChem. Eur. J. 2015, 21, 2029 – 2037
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tration, indicating a strong interaction between the MOF and the TNP at ground state. Notably, the absorption band of 1 at 366 nm is buried in the strong absorption band at 379 nm of TNP, which indicated the light absorption by TNP is the main source for the observed fluorescence quenching. The spectral overlap between the absorption spectra of analytes and the emission spectrum of 1 were also checked. Figure 7 B shows that the absorption spectrum of only TNP has a weak overlap with the emission spectrum of 1. So the resonance energy transfer between 1 and TNP should be negligible. The variation of the fluorescence lifetime of compound 1 was checked in the absence and presence of TNP. The emission lifetime of compound 1 showed a biexponential feature with t1 = 2.70 ns (3.01 %) and t2 = 12.12 ns (96.99 %) and this remained essentially unchanged in the presence of TNP (Figure 7 C and 8 D), which supports the previously indicated absence of significant levels of electron transfer.
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Figure 5. Bar charts showing fluorescence reduction upon the addition of different analytes. Inset is a bar chart of fluorescence enhancement upon the addition of h-DQ, DMB, and BB.
Luminescent sensing of metal ions To investigate the sensing properties of 1 toward Cu2 + , PL spectra of 1 were recorded upon addition of different volumes of CuCl2 aqueous solution (1 102 mol L1) into a solution of 1 in DMSO (5 104 mol L1). Figure 8 shows the relationship between the luminescence quenching of 1 and the amount of Cu2 + . The maximum fluorescence intensity of 1 was reduced by 77.6 % at a Cu2 + concentration of 372 ppm. The emitted visible green light of 1 under UV-light was becoming significantly darker due to the incorporation of Cu2 + in 1. As depicted in Figure S20 (in the Supporting Information), a good linear dependency of luminescence quenching efficiencies on the Cu2 + concentration was obtained. The absorption spectra recorded upon addition of metal ions to 1 is shown in Figure S21 (in the Supporting Information). The absorption band intensity at 366 nm decreased rapidly upon addition of only Cu2 + among other metal ions, which implied an interaction between the MOF and Cu2 + . Other metal ions such as Zn2 + , Mn2 + , Mg2 + , K + , Na + , Ni2 + , Co2 + , and Ca2 + as their chloride salts were used to further establish the selectivity of 1 by fluorescence spectra. The fluorescent intensity of 1 with different metal ions is plotted in Figure 9 a. Upon the addition of ten equivalents of each metal ion, only Cu2 + induced a clear fluorescence quenching; other metal ions had no significant effect on the luminescence intensity. Furthermore, to determine effect of the counter anions on Cu2 + recognition based on 1, a range of aqueous solutions of Cu2 + salts containing the anions Cl , NO3 , SO42, and (OAc) were analyzed. Figure 9 b shows that the luminescence intensity of 1 decreased upon addition of Cu2 + salt solutions containing different counter anions, which demonstrates that the nature of the anion had a negligible effect on the luminescence intensity of 1. Chem. Eur. J. 2015, 21, 2029 – 2037
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This luminescence quenching mechanism of 1 towards Cu2 + might be similar to that observed in {Mg(DHT)(DMF)2} (DHT = 2,5-dihydroxyterephthalate),[29] the desolvated microporous framework of which was exploited for the binding and specific sensing of metal ions through Lewis acid-base interactions. In further experiments, 1 was immersed in an aqueous solution containing CuCl2 (1 mm), and the solution was continuously stirred for 10 h to prepare Cu2 + -incorporated product Cu2 + @1. The latter powder was filtered and washed with water until the washing was colorless. The XRD pattern of Cu2 + @1 showed similar peaks to those of 1, suggesting the framework structure remains intact even after metal immobilization in aqueous solution (Figure S22 in the Supporting Information). ICP studies on Cu2 + @1 indicated that a small amount (0.64 %) of Cu loading was present on the framework. It is evident that the functional sites including pendent hydroxyl groups of PAM and pyridyl sites of dpe in 1 could have an effect on Cu2 + recognition. The interaction between the Cu2 + and 1 may induce energy or charge transfer through the partially filled d-orbital based on ligand field transitions, which has been demonstrated previously.[6, 15, 29] The differential quenching effects of metal ions on the luminescence intensity of 1 may be attributed to their electronic nature. For example, Mg2 + , Na + , and K + , having a closed-shell electron configuration, display essentially no quenching effects. Only with Cu2 + is the quenching effect significant, which demonstrates that 1 could be regarded as a potential material for selective sensing of Cu2 + .
Conclusion We have successfully synthesized a novel 3D luminescent cadmium–pamoate metal–organic framework, [Cd2(PAM)2(dpe)2(H2O)2]·0.5(dpe) (1) by employing p-electronrich rigid-flexible ligand 4,4’-methylenebis(3-hydroxy-2-naphthalenecarboxylic acid) and rigid ligand 1,2-di(4-pyridyl)ethylene. The material exhibits unique fluorescence quenching behavior upon addition of 2,4,6-trinitrophenol (TNP) or Cu2 + . The high sensitivity and selectivity to TNP and Cu2 + shows that 1 could be used as an excellent sensor for detecting these analytes, even in the presence of other competing analogues. This work presents a promising approach for preparing luminescent MOFs for detecting nitroaromatic explosives and metal ions by assembled p-electron-rich organic ligands and d10 metal ions.
Experimental Section Materials and methods All solvents, organic reagents, and metal salts were used without further purification. Powder X-ray diffraction (XRD) data were collected with a Rigaku d-Max 2400 diffractometer working with CuKa radiation (l = 1.5418 ). Thermogravimetric analysis (TGA) was performed under air atmosphere from RT to 800 8C with a heating rate of 10 8C min1 with a PerkinElmer TGA/SDTA851e instrument. Element analysis was carried out with a PerkinElmer 240C elemental analyzer. Infrared (IR) spectra were recorded from 400 to 4000 cm1 with a Nicolet Impact 4100 FT/IR spectrometer by using KBr pellets. Absorption spectra were recorded with a HITACHI U-
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Figure 6. Fluorescence emission spectra of 1 in DMSO, with competing analytes (300 mL, 1 mm) a) 1,3-DNB, b) DMB, and c) BB added first, followed by incremental addition of TNP (1 mm). d) Decrease in percentage of fluorescence intensity of 1 upon the addition of different competing compounds (300 mL, 1 mm) followed by incremental addition of TNP (1 mm).
4100 UV/Vis spectrophotometer. The photoluminescence (PL) studies were carried out with a JASCO FP-6300 spectrofluorimeter with a 150 W Xe lamp. Fluorescence lifetimes were measured with an Edinburgh Instruments OB920 fluorescence spectrometer.
Synthesis of [Cd2(PAM)2(dpe)2(H2O)2]·0.5(dpe) (1) A mixture of Cd(NO3)2·4H2O (0.062 g, 0.20 mmol), H2PAM (0.039 g, 0.10 mmol), dpe (0.036 g, 0.20 mmol), H2O (10 mL), and DMF (2 mL) was sealed in a 25 mL Teflon-lined stainless steel autoclave and heated at 120 8C for 72 h. After cooling to RT, yellow crystals were obtained in a yield of 49 % based on Cd. IR (KBr): n˜ = 3387 (w), 1643 (m), 1608 (m), 1558 (vs), 1499 (w), 1452 (m), 1390 (m), 1354 (m), 1201 (w), 1011 (w), 822 (s), 751 (w), 543 (w) cm1; elemental analysis calcd (%) for C76H57N5O14Cd2 (1489.09): C 61.30, H 3.86, N 4.70; found: C 61.54, H 3.90, N 4.62.
crystals glued at the end of a glass fiber. Data collections were carried out at 293 K using w-scan and Mo-Ka radiation (l = 0.71073). Empirical absorption correction was applied for all data. The structure was solved by direct methods and further refined by fullmatrix least-square refinements on the basis of F2 using SHELXTL program. The heaviest atoms were found first, and O, N, and C atoms were subsequently located in difference Fourier maps. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms of 1 were calculated by geometrical models. Experimental details for the structural analysis of 1 are given in Table 1. Selected bond distances and bond angles are given in Table S1. Crystallographic data for 1 have been deposited at the Cambridge Crystallographic Data Center with deposition number CCDC-991620. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif.
Single-crystal structure determination
Fluorescence measurements
The crystal structures of 1 were determined by single-crystal X-ray diffraction experiments. The reflection data were collected with a Bruker-AXS SMART CCD area detector diffractometer with the
A 1 105 mol L1 stock solution of 1 was prepared in DMSO solvent. The relative fluorescence quantum yields were estimated relative to solutions of 1 104 mol L1 quinine sulfate in 0.1 N sulfuric
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Figure 7. a) Absorption spectra of 1 in DMSO upon incremental addition of TNP. b) Spectral overlap between the absorption spectra of analytes and the emission spectrum of 1 in DMSO. Fluorescence lifetime of c) 1 and d) a mixture of 1 and TNP in DMSO.
the aromatic analyte solution (1 103 mol L1 in DMSO) were added to 1 (3 mL stock solution) in DMSO. The mixed solution was equilibrated for 10 min before the spectral measurements. The emission spectra of these solutions were recorded at RT. Furthermore, competition experiments were also carried out to validate the selectivity of the sensor. Aromatic analogue solution (300 mL) was added into a solution of 1 (3 mL) in DMSO, then different volumes of TNP solution (1 103 mol L1) were added to the system for the luminescent detection experiment. Analogously, for the experiment of sensing metal ions, different volumes of metal ions (M = Zn2 + , Mn2 + , Mg2 + , K + , Na + , Ni2 + , Co2 + , Ca2 + , and Cu2 + ) as aqueous solution (1 103 mol L1) were added to 1 (1 105 mol L1) in either DMSO or DMSO–H2O mixed solvents for the luminescence studies.
Table 1. Crystallographic data and structure refinements for 1. formula Mr crystal system space group a [] b [] c [] a[8] b [8] g[8] V [3]
C76H57N5O14Cd2 1489.09 triclinic P1¯ 14.605(3) 14.931(4) 16.946(4) 112.160(3) 103.198(4) 100.390(4) 3183.2(13)
Z 1calcd [mg m3] m [mm1] F(000) Rint GOF on F2 R1 [I > 2s(I)][a] wR2 [I > 2s(I)][a] R1 (all data)[a] wR2 (all data)[a]
2 1.554 0.743 1512 0.0191 1.009 0.0382 0.0864 0.0591 0.0969
[a] R1 = S j j Fo j j Fc j j /S j Fo j ; wR2 = {S[w(F 2oF 2c)2]/S[w(F 2o)]2}1/2.
acid with FF = 0.55 as a standard sample.[30] For investigations on sensing NACs including 4-nitrophenol, 2,6-dinitrotoluene, 2,4-dinitrotoluene, nitrobenzene, 1,3-dinitrobenzene, hydroquinone (hDQ), dimethylbenzene and bromobenzene, different volumes of
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Full Paper Acknowledgements This work was supported by the National Natural Science Foundation of China (51003009), the Fundamental Research Funds for the Central Universities of China (DUT14LK32) and the Science and Technology Research Foundation of Education Department of Liaoning Province (L2014033). Keywords: analytical methods · copper · fluorescent probes · metal–organic frameworks · sensors
Figure 8. The fluorescence emission spectra of 1 in DMSO (5 104 mol L1) upon incremental addition of Cu2 + aqueous solution (1 102 mol L1) (lex = 371 nm). Inset shows 1 under UV irradiation before and after titration with Cu2 + .
Figure 9. a) Comparison of the luminescence intensity of 1 interacting with different metal ions. Insets are photographs of 1 under UV irradiation after mixing with metal ions. b) The fluorescence emission spectra of 1 in DMSO upon addition of Cu2 + salt aqueous solutions (lex = 371 nm). Chem. Eur. J. 2015, 21, 2029 – 2037
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Received: September 14, 2014 Published online on November 27, 2014
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& Sensors
Highly Selective Detection of 2,4,6-Trinitrophenol and Cu2 + Ions Based on a Fluorescent Cadmium–Pamoate Metal–Organic Framework Junwei Ye, Limei Zhao, Raji Feyisa Bogale, Yuan Gao, Xiaoxiao Wang, Xiaomin Qian, Song Guo, Jianzhang Zhao, and Guiling Ning*[a]
Abstract: A luminescent cadmium–pamoate metal–organic framework, [Cd2(PAM)2(dpe)2(H2O)2]·0.5(dpe) (1), has been synthesized under hydrothermal conditions by using p-electron-rich ligands 4,4’-methylenebis(3-hydroxy-2-naphthalenecarboxylic acid) (H2PAM) and 1,2-di(4-pyridyl)ethylene (dpe). Its structure is composed of both mononuclear and dinuclear CdII building units, which are linked by the PAM and dpe ligands, resulting in a (4,8)-connected 3D framework. The p-conjugated dpe guests are located in a 1D channel of 1. The strong emission of 1 could be quenched efficiently by trace amounts of 2,4,6-trinitrophenol (TNP), even in the presence of other competing analogues such as 4-nitrophenol,
Introduction Nitroaromatic compounds (NACs), such as 2,6-dinitrotoluene (2,6-DNT), 2,4,6-trinitrotoluene (TNT), and 2,4,6-trinitrophenol (TNP), are primary ingredients of industrial explosives, environmentally deleterious substances, and many unexploded land mines.[1] Rapid detection of these nitroaromatic explosives is attracting increasing attention due to their broad range of applications in security, military operations, environmental management, mine-field analysis, and in forensic and criminal investigations.[2] Among such NACs, TNP is an extremely hazardous chemical with strong explosive capacity and a low safety coefficient. The compound is widely used in dyes, explosives, leather, fireworks, matches, and pharmaceutical industries,[3] and it can directly contaminate the soil and aquatic systems if it is released to the surrounding environment by some industrial processes. Furthermore, the leakage of TNP can also trig[a] Dr. J. Ye, L. Zhao, R. F. Bogale, Y. Gao, X. Wang, X. Qian, S. Guo, Prof. J. Zhao, Prof. G. Ning State Key Laboratory of Fine Chemicals School of Chemical Engineering Faculty of Chemical, Environmental Biological Science and Technology Dalian University of Technology 2 Linggong Road, Dalian 116024 (China) Fax: (+ 86) 411-84986065 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405267. Chem. Eur. J. 2015, 21, 2029 – 2037
2,6-dinitrotoluene, 2,4-dinitrotoluene, nitrobenzene, 1,3-dinitrobenzene, hydroquinone, dimethylbenzene, and bromobenzene. The high sensitivity and selectivity of the fluorescence response of 1 to TNP shows that this framework could be used as an excellent sensor for identifying and quantifying TNP. In the same manner, 1 also exhibits superior selectivity and sensitivity towards Cu2 + compared with other metal ions such as Zn2 + , Mn2 + , Mg2 + , K + , Na + , Ni2 + , Co2 + , and Ca2 + . This is the first MOF that can serve as a dual functional fluorescent sensor for selectively detecting trace amounts of TNP and Cu2 + .
ger skin irritation, anemia, abnormal liver functions, and male infertility.[4] Curiously, less effort has focused on detecting TNP than other hazardous substances,[5] so it is highly desirable to develop rapid, convenient and specific analytical methods for detecting this compound. On the other hand, as an essential biological metal ion, Cu2 + plays a critical role in a variety of fundamental processes in living biological systems.[6] However, at higher concentrations, Cu2 + can become toxic, leading to metabolism disorders such as Wilson’s disease and neurodegenerative disorders such as Alzheimer’s disease.[7] Therefore, the efficient detection of trace amounts of Cu2 + is also very important for assessing human health. Until now, various analytical and spectroscopic methods have been employed to detect NACs and metal ions,[8] but high cost, intricate operation, low sensitivity, and high levels of interference by other analytes associated with these methods have restricted their widespread use.[9] Recently, fluorescencebased detection of NACs or metal ions by using luminescent polymers, small organic molecules, and nanomaterials has attracted much attention because of the sensitivity, selectivity, portability, short response time, and convenient visual detection.[10] In our continuing research on luminescent metal–organic frameworks (MOFs),[11] we focused our study on the development of MOF-based fluorescent sensors for the detection of NACs or metal ions. MOFs are a novel class of inorganic-organic hybrid materials that show structural diversity and potential applications for gas storage, separation, catalysis, molecular magnets, and sensors.[12] Tunable porosity, functional
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Full Paper surfaces, and the bulk conjugated backbone of MOFs can induce transduction of host-guest chemistry leading to detectable changes in light-emitting color or luminescent characteristics, which can make them promising candidates for sensing applications.[13] In fact, some luminescent MOFs have been reported for detecting NACs or metal ions based on fluorescence quenching process,[14, 15] but details of the mechanism are not clear. In particular, the rapid and selective detection of trace amounts of TNP or Cu2 + remains limited.[16] Herein, we report a new three-dimensional (3D) MOF, [Cd2(PAM)2(dpe)2(H2O)2]·0.5(dpe) (1) constructed from p-electron-rich rigid-flexible ligand pamoic acid (H2PAM, 4,4’-methylene bis(3-hydroxy-2-naphthalenecarboxylic acid)) and rigid ligand 1,2-di(4-pyridyl)ethylene (dpe) under hydrothermal conditions. The strong emission of 1 could be quenched efficiently by trace amounts of TNP or Cu2 + at room temperature, even in the presence of other competing analogues, which indicated that 1 can serve as an excellent sensor for specific selective identification and quantification of TNP and Cu2 + through a fluorescence quenching process.
Results and Discussion Description of crystal structure Single-crystal X-ray diffraction studies revealed that 1 is composed of a 3D framework crystallizing in the triclinic space group P1¯, with two CdII ions, two PAM ligands, two dpe ligands, two coordinated water molecules, and half dpe guest molecule in the asymmetric unit. Two crystallographic independent CdII centers possess different coordination environments, as depicted in Figure 1 A. The Cd1 center adopts a seven-coordinate mode (CdO5N2) by bonding to four oxygen atoms from two PAM ligands, two nitrogen atoms from two dpe ligands, and one oxygen atom from a coordinated water molecule, forming a pentagonal bipyramidal geometry. The
Figure 1. a) View of the coordination environments of Cd2 + centers. b) View of the (4,8)-connected topological net. c) 3D framework showing the guest dpe ligands with space-filling representation of 1. d) The p–p interaction between naphthyl rings of PAM and guest dpe ligand. Chem. Eur. J. 2015, 21, 2029 – 2037
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Cd2 shows a distorted octahedral geometry [CdO4N2] by coordinating to three carboxyl oxygen atoms from three different PAM ligands, two nitrogen atoms from two dpe ligands, and one oxygen atom from a coordinated water molecule. The CdO and CdN bond lengths are in the range of 2.194(2)– 2.684(2) and 2.292(2)–2.451(2) , respectively, which are comparable to those found in the related CdII-pamoate coordination polymers.[17–20] Two crystallographic equivalent Cd2 atoms are bridged by four carboxylate groups in bis-monodentate fashion to give a dinuclear cadmium molecular building block (MBB), [Cd2N4(CO2)4], with a separation Cd···Cd distance of 4.375 . Each mononuclear [CdO5N2] MBB serves as a four-connected node and dinuclear [Cd2N4(CO2)4] MBB acts as an eightconnected node, which are linked to each other by PAM and dpe ligands, forming a (4,8)-connected 3D network with the Schlfli symbol of {4.52.63}2{46.55.615.82} (Figure 1 B and Figure S1 in the Supporting Information). It should be noted that 1 has a small 1D channel along the c-axis, which is lined with p-electron-rich naphthyl moieties of bent PAM ligands. The guest dpe ligand with coplanar p-conjugated structures is located in channels (Figure 1 C). The p-p interactions (distance of the closest carbon atoms is 3.489 ) occur between naphthyl rings of PAM and guest dpe ligands (Figure 1D). This supramolecular wire-like arrangement and rich p-electron aggregated structures are considered to be a beneficial feature because they lead to additional luminescent properties for fluorescencebased detection of NACs.[21, 22] Thermogravimetric analysis (TGA) indicated framework 1 has excellent stability. As shown in Figure S2 (in the Supporting Information), the first weight loss of 2.19 % from 30 to 178 8C corresponds to the loss of coordinating water molecules (calcd: 2.42 %). Then it is stable to approximately 296 8C, when a second weight loss starts with the decomposition of dpe and PAM ligands. Luminescent sensing of nitroaromatic compounds The photoluminescence (PL) spectrum of compound 1 in dimethyl sulfoxide (DMSO) exhibits a strong emission peak at 502 nm upon excitation at 371 nm at room temperature, which is redshifted (34 nm) in comparison with that of 1 in the solid state (Figures S3 and S4). A possible explanation for this redshift involves guest-dependent interactions between the framework and solvent molecules.[23] However, in solution, intermolecular rotation and nonradiative relaxation pathways are increased through the collision between framework and guest molecules.[24] The fluorescence quantum yield of 1 (FMOF = 0.463) is higher than that of free H2PAM ligand (FL = 0.063), which is probably due to the electronic coupling between neighboring ligands through the CdII centers, reducing the nonradiative decay of ligands.[24c] To explore the potential application of 1 in the detection of TNP, fluorescence-quenching titrations were performed by gradual addition of TNP (1 mm in DMSO) to a solution of 1 in DMSO. Very high levels of fluorescence quenching were observed upon incremental addition of TNP (Figure 2). The quenching efficiencies were estimated by using the formula
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Figure 2. The fluorescence emission spectra of 1 in DMSO upon incremental addition of TNP (1 mm) (lex = 371 nm). Inset shows solutions of 1 in DMSO under UV irradiation before and after titrating with TNP.
(I0I)/I0 100 %, where I0 is the maximum fluorescence intensity of 1 and I is the intensity of 1 containing TNP solution. The initial fluorescence intensity of 1 decreased by 98.4 % when only 11.5 ppm TNP was added. The fluorescence quenching phenomenon could be easily detected even at very low concentration of TNP (0.76 ppm). Good linear dependencies (R2 = 0.9945) of luminescence quenching efficiencies on the TNP concentration were obtained, as shown in Figure S5 in the Supporting Information. It is noticeable that the calibration curve plot of 1 toward TNP showed a detection limit of 1.76 108 g L1. This system therefore represents a rare case of luminescent MOF materials that can be used to determine the concentration of TNP quantitatively.[16, 25] The powder X-ray diffraction (XRD) patterns of the samples before and after immersing in TNP solution are shown in Figure 3. Most peak positions of both samples were in good agreement with those of a simulated pattern generated from the single-crystal structure analysis, suggesting that the framework remains intact even immersed in TNP solution. To compare the sensing selectivity of 1 toward TNP, the emission spectra of 1 were also recorded upon addition of other NACs, including 4-nitrophenol (4-NP), 1,3-dinitrobenzene (1,3-DNB), 2,6-dinitrotoluene (2,6-DNT), 2,4-dinitrotoluene (2,4DNT), nitrobenzene (NB), dimethylbenzene (DMB), bromobenzene (BB), and hydroquinone (h-DQ), with the same concentration (1 mm) (Figure 4 and Figures S6–S9). The quenching efficiencies (%) and corresponding Stern–Volmer (S–V) plots of different NACs are plotted in Figure 5 and Figure S10 (in the Supporting Information), respectively. The electron-deficient 4-NP, 1,3-DNB, 2,6-DNT, 2,4-DNT, and NB showed little effect on the fluorescence intensity, with 14.8, 8.05, 8.30, 5.80, and 4.41 % of the quenching ability, respectively. Interestingly, a slight fluorescence enhancement of 1 was observed when electron-rich DMB, BB, and h-DQ were used as analytes. Such luminescence quenching or enhancement response may be attributed to Chem. Eur. J. 2015, 21, 2029 – 2037
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Figure 3. XRD patterns of 1, a) simulated, b) experimental, c) immersed in TNP solution.
electron donor-acceptor transfer between 1 and the analytes. Similar change in the fluorescence intensity of MOFs towards guest molecules was detected previously.[14a, 26] To examine the potential of the selective detection by 1 of TNP in the presence of other interfering NACs, a series of competition experiments were performed by gradual addition of a solution of different nitroaromatic analogues followed by TNP into 1, and the corresponding fluorescence spectra were monitored (Figure 6 and Figure S11–S15 in the Supporting Information). Taking addition of electron-deficient 1,3-DNB as an example (Figure 6 A), initially, the addition of 1,3-DNB showed negligible effect on the luminescent intensity of 1, but a significant fluorescence quenching occurred after the addition of TNP to a mixed solution of 1 and 1,3-DNB. A similar quenching phenomenon was observed when electron-rich DMB and BB were taken as interfering substance, respectively. The stepwise decrease in luminescent intensity (Figure 6 d) demonstrates the high ability of 1 for selectively detecting TNP even in the presence of other nitroaromatic compounds. In addition, the fluorescence quenching of 1 dissolved in N,N-dimethylformamide (DMF) was observed upon incremental addition of TNP (Figure S16 in the Supporting Information). The effect of water as a solvent on the detection of TNP was also monitored for practical applications. Due to poor solubility of 1 in pure aqueous solution, compound 1 was dissolved in H2O–DMSO mixed solvents and then its fluorescence spectrum was recorded by the dropwise addition of an aqueous solution of TNP. Compared with that of 1 in only DMSO, the fluorescent intensity of 1 in H2O–DMSO was slightly lowered with a slight redshift (Figure S17 in the Supporting Information), which may be attributed the effect of water polarity on charge transfer;[16a] nevertheless, significant quenching of the fluorescent intensity could be observed upon addition of TNP. These experimental results demonstrate that 1 constitutes a reliable and efficient fluorescence sensor for TNP with high sensitivity and selectivity compared with the response to other small nitroaromatic analogues.
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Figure 4. The fluorescence emission spectra of 1 in DMSO upon incremental addition of 1 mm a) 4-NP, b) 1,3-DNB, c) DMB, and d) BB (lex = 371 nm). Insets are enlarged images of the segment of the emission spectra.
To date, although molecule recognition derives from the interaction between host and specific guest substrates,[27] the mechanism of selective sensing of explosives based on MOFs is not very clear. From the crystal structure (Figure 1) and adsorption isotherm (Figure S18 in the Supporting Information) of 1, it is confirmed that the lower porosity and small pore size of 1 reduces its ability to encapsulate analytes, so the fluorescence response mechanism in our experiments might be attributed to light absorption and an electron transfer from electron-donor to electron-withdrawer adsorbed on the surface of 1. This sensing mechanism differs from most of the known guest-induced fluorescence quenching or enhancement mechanisms,[28] in which the analytes act as guest molecules that occupy the channels or cages of MOFs. To better understand the mechanism of detection, the absorption spectra of 1 upon incremental addition of TNP were recorded (Figure 7 A and Figure S19 in the Supporting Information). Two distinct absorption bands of UV/Vis spectra of 1 were observed at 291 and 366 nm. The absorption band intensity at 291 nm reduced rapidly with increased TNP concenChem. Eur. J. 2015, 21, 2029 – 2037
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tration, indicating a strong interaction between the MOF and the TNP at ground state. Notably, the absorption band of 1 at 366 nm is buried in the strong absorption band at 379 nm of TNP, which indicated the light absorption by TNP is the main source for the observed fluorescence quenching. The spectral overlap between the absorption spectra of analytes and the emission spectrum of 1 were also checked. Figure 7 B shows that the absorption spectrum of only TNP has a weak overlap with the emission spectrum of 1. So the resonance energy transfer between 1 and TNP should be negligible. The variation of the fluorescence lifetime of compound 1 was checked in the absence and presence of TNP. The emission lifetime of compound 1 showed a biexponential feature with t1 = 2.70 ns (3.01 %) and t2 = 12.12 ns (96.99 %) and this remained essentially unchanged in the presence of TNP (Figure 7 C and 8 D), which supports the previously indicated absence of significant levels of electron transfer.
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Figure 5. Bar charts showing fluorescence reduction upon the addition of different analytes. Inset is a bar chart of fluorescence enhancement upon the addition of h-DQ, DMB, and BB.
Luminescent sensing of metal ions To investigate the sensing properties of 1 toward Cu2 + , PL spectra of 1 were recorded upon addition of different volumes of CuCl2 aqueous solution (1 102 mol L1) into a solution of 1 in DMSO (5 104 mol L1). Figure 8 shows the relationship between the luminescence quenching of 1 and the amount of Cu2 + . The maximum fluorescence intensity of 1 was reduced by 77.6 % at a Cu2 + concentration of 372 ppm. The emitted visible green light of 1 under UV-light was becoming significantly darker due to the incorporation of Cu2 + in 1. As depicted in Figure S20 (in the Supporting Information), a good linear dependency of luminescence quenching efficiencies on the Cu2 + concentration was obtained. The absorption spectra recorded upon addition of metal ions to 1 is shown in Figure S21 (in the Supporting Information). The absorption band intensity at 366 nm decreased rapidly upon addition of only Cu2 + among other metal ions, which implied an interaction between the MOF and Cu2 + . Other metal ions such as Zn2 + , Mn2 + , Mg2 + , K + , Na + , Ni2 + , Co2 + , and Ca2 + as their chloride salts were used to further establish the selectivity of 1 by fluorescence spectra. The fluorescent intensity of 1 with different metal ions is plotted in Figure 9 a. Upon the addition of ten equivalents of each metal ion, only Cu2 + induced a clear fluorescence quenching; other metal ions had no significant effect on the luminescence intensity. Furthermore, to determine effect of the counter anions on Cu2 + recognition based on 1, a range of aqueous solutions of Cu2 + salts containing the anions Cl , NO3 , SO42, and (OAc) were analyzed. Figure 9 b shows that the luminescence intensity of 1 decreased upon addition of Cu2 + salt solutions containing different counter anions, which demonstrates that the nature of the anion had a negligible effect on the luminescence intensity of 1. Chem. Eur. J. 2015, 21, 2029 – 2037
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This luminescence quenching mechanism of 1 towards Cu2 + might be similar to that observed in {Mg(DHT)(DMF)2} (DHT = 2,5-dihydroxyterephthalate),[29] the desolvated microporous framework of which was exploited for the binding and specific sensing of metal ions through Lewis acid-base interactions. In further experiments, 1 was immersed in an aqueous solution containing CuCl2 (1 mm), and the solution was continuously stirred for 10 h to prepare Cu2 + -incorporated product Cu2 + @1. The latter powder was filtered and washed with water until the washing was colorless. The XRD pattern of Cu2 + @1 showed similar peaks to those of 1, suggesting the framework structure remains intact even after metal immobilization in aqueous solution (Figure S22 in the Supporting Information). ICP studies on Cu2 + @1 indicated that a small amount (0.64 %) of Cu loading was present on the framework. It is evident that the functional sites including pendent hydroxyl groups of PAM and pyridyl sites of dpe in 1 could have an effect on Cu2 + recognition. The interaction between the Cu2 + and 1 may induce energy or charge transfer through the partially filled d-orbital based on ligand field transitions, which has been demonstrated previously.[6, 15, 29] The differential quenching effects of metal ions on the luminescence intensity of 1 may be attributed to their electronic nature. For example, Mg2 + , Na + , and K + , having a closed-shell electron configuration, display essentially no quenching effects. Only with Cu2 + is the quenching effect significant, which demonstrates that 1 could be regarded as a potential material for selective sensing of Cu2 + .
Conclusion We have successfully synthesized a novel 3D luminescent cadmium–pamoate metal–organic framework, [Cd2(PAM)2(dpe)2(H2O)2]·0.5(dpe) (1) by employing p-electronrich rigid-flexible ligand 4,4’-methylenebis(3-hydroxy-2-naphthalenecarboxylic acid) and rigid ligand 1,2-di(4-pyridyl)ethylene. The material exhibits unique fluorescence quenching behavior upon addition of 2,4,6-trinitrophenol (TNP) or Cu2 + . The high sensitivity and selectivity to TNP and Cu2 + shows that 1 could be used as an excellent sensor for detecting these analytes, even in the presence of other competing analogues. This work presents a promising approach for preparing luminescent MOFs for detecting nitroaromatic explosives and metal ions by assembled p-electron-rich organic ligands and d10 metal ions.
Experimental Section Materials and methods All solvents, organic reagents, and metal salts were used without further purification. Powder X-ray diffraction (XRD) data were collected with a Rigaku d-Max 2400 diffractometer working with CuKa radiation (l = 1.5418 ). Thermogravimetric analysis (TGA) was performed under air atmosphere from RT to 800 8C with a heating rate of 10 8C min1 with a PerkinElmer TGA/SDTA851e instrument. Element analysis was carried out with a PerkinElmer 240C elemental analyzer. Infrared (IR) spectra were recorded from 400 to 4000 cm1 with a Nicolet Impact 4100 FT/IR spectrometer by using KBr pellets. Absorption spectra were recorded with a HITACHI U-
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Figure 6. Fluorescence emission spectra of 1 in DMSO, with competing analytes (300 mL, 1 mm) a) 1,3-DNB, b) DMB, and c) BB added first, followed by incremental addition of TNP (1 mm). d) Decrease in percentage of fluorescence intensity of 1 upon the addition of different competing compounds (300 mL, 1 mm) followed by incremental addition of TNP (1 mm).
4100 UV/Vis spectrophotometer. The photoluminescence (PL) studies were carried out with a JASCO FP-6300 spectrofluorimeter with a 150 W Xe lamp. Fluorescence lifetimes were measured with an Edinburgh Instruments OB920 fluorescence spectrometer.
Synthesis of [Cd2(PAM)2(dpe)2(H2O)2]·0.5(dpe) (1) A mixture of Cd(NO3)2·4H2O (0.062 g, 0.20 mmol), H2PAM (0.039 g, 0.10 mmol), dpe (0.036 g, 0.20 mmol), H2O (10 mL), and DMF (2 mL) was sealed in a 25 mL Teflon-lined stainless steel autoclave and heated at 120 8C for 72 h. After cooling to RT, yellow crystals were obtained in a yield of 49 % based on Cd. IR (KBr): n˜ = 3387 (w), 1643 (m), 1608 (m), 1558 (vs), 1499 (w), 1452 (m), 1390 (m), 1354 (m), 1201 (w), 1011 (w), 822 (s), 751 (w), 543 (w) cm1; elemental analysis calcd (%) for C76H57N5O14Cd2 (1489.09): C 61.30, H 3.86, N 4.70; found: C 61.54, H 3.90, N 4.62.
crystals glued at the end of a glass fiber. Data collections were carried out at 293 K using w-scan and Mo-Ka radiation (l = 0.71073). Empirical absorption correction was applied for all data. The structure was solved by direct methods and further refined by fullmatrix least-square refinements on the basis of F2 using SHELXTL program. The heaviest atoms were found first, and O, N, and C atoms were subsequently located in difference Fourier maps. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms of 1 were calculated by geometrical models. Experimental details for the structural analysis of 1 are given in Table 1. Selected bond distances and bond angles are given in Table S1. Crystallographic data for 1 have been deposited at the Cambridge Crystallographic Data Center with deposition number CCDC-991620. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif.
Single-crystal structure determination
Fluorescence measurements
The crystal structures of 1 were determined by single-crystal X-ray diffraction experiments. The reflection data were collected with a Bruker-AXS SMART CCD area detector diffractometer with the
A 1 105 mol L1 stock solution of 1 was prepared in DMSO solvent. The relative fluorescence quantum yields were estimated relative to solutions of 1 104 mol L1 quinine sulfate in 0.1 N sulfuric
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Figure 7. a) Absorption spectra of 1 in DMSO upon incremental addition of TNP. b) Spectral overlap between the absorption spectra of analytes and the emission spectrum of 1 in DMSO. Fluorescence lifetime of c) 1 and d) a mixture of 1 and TNP in DMSO.
the aromatic analyte solution (1 103 mol L1 in DMSO) were added to 1 (3 mL stock solution) in DMSO. The mixed solution was equilibrated for 10 min before the spectral measurements. The emission spectra of these solutions were recorded at RT. Furthermore, competition experiments were also carried out to validate the selectivity of the sensor. Aromatic analogue solution (300 mL) was added into a solution of 1 (3 mL) in DMSO, then different volumes of TNP solution (1 103 mol L1) were added to the system for the luminescent detection experiment. Analogously, for the experiment of sensing metal ions, different volumes of metal ions (M = Zn2 + , Mn2 + , Mg2 + , K + , Na + , Ni2 + , Co2 + , Ca2 + , and Cu2 + ) as aqueous solution (1 103 mol L1) were added to 1 (1 105 mol L1) in either DMSO or DMSO–H2O mixed solvents for the luminescence studies.
Table 1. Crystallographic data and structure refinements for 1. formula Mr crystal system space group a [] b [] c [] a[8] b [8] g[8] V [3]
C76H57N5O14Cd2 1489.09 triclinic P1¯ 14.605(3) 14.931(4) 16.946(4) 112.160(3) 103.198(4) 100.390(4) 3183.2(13)
Z 1calcd [mg m3] m [mm1] F(000) Rint GOF on F2 R1 [I > 2s(I)][a] wR2 [I > 2s(I)][a] R1 (all data)[a] wR2 (all data)[a]
2 1.554 0.743 1512 0.0191 1.009 0.0382 0.0864 0.0591 0.0969
[a] R1 = S j j Fo j j Fc j j /S j Fo j ; wR2 = {S[w(F 2oF 2c)2]/S[w(F 2o)]2}1/2.
acid with FF = 0.55 as a standard sample.[30] For investigations on sensing NACs including 4-nitrophenol, 2,6-dinitrotoluene, 2,4-dinitrotoluene, nitrobenzene, 1,3-dinitrobenzene, hydroquinone (hDQ), dimethylbenzene and bromobenzene, different volumes of
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Full Paper Acknowledgements This work was supported by the National Natural Science Foundation of China (51003009), the Fundamental Research Funds for the Central Universities of China (DUT14LK32) and the Science and Technology Research Foundation of Education Department of Liaoning Province (L2014033). Keywords: analytical methods · copper · fluorescent probes · metal–organic frameworks · sensors
Figure 8. The fluorescence emission spectra of 1 in DMSO (5 104 mol L1) upon incremental addition of Cu2 + aqueous solution (1 102 mol L1) (lex = 371 nm). Inset shows 1 under UV irradiation before and after titration with Cu2 + .
Figure 9. a) Comparison of the luminescence intensity of 1 interacting with different metal ions. Insets are photographs of 1 under UV irradiation after mixing with metal ions. b) The fluorescence emission spectra of 1 in DMSO upon addition of Cu2 + salt aqueous solutions (lex = 371 nm). Chem. Eur. J. 2015, 21, 2029 – 2037
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Received: September 14, 2014 Published online on November 27, 2014
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