Luminescent Coordination Polymer with Conjugated Lewis Acid Sites

Article pubs.acs.org/crystal

Luminescent Coordination Polymer with Conjugated Lewis Acid Sites for the Detection of Organic Amines Jian-Jun Liu,† Ying-Fang Guan,† Mei-Jin Lin,*,†,‡ Chang-Cang Huang,*,† and Wen-Xin Dai† †

College of Chemistry, Fuzhou University, Fuzhou 350116, China State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 8, 2015 | http://pubs.acs.org Publication Date (Web): September 8, 2015 | doi: 10.1021/acs.cgd.5b01026

S Supporting Information *

ABSTRACT: A stable three-dimensional coordination polymer, [Eu3(bcbp)3(NO3)7(OH)2]n (1) (H2bcbp·2Cl = 1,1′-bis(4-carboxyphenyl)-(4,4′-bipyridinium) dichloride), was prepared by the solvothermal reaction of Eu(NO3)3·6H2O and H2bcbp·2Cl in methanol, which was characterized by infrared spectroscopy, single-crystal X-ray diffraction, powder X-ray diffraction, and thermogravimetric analyses. In the solid state, 1 consists of an unusual trinuclear Eu(III) unit (Eu3(CO2)6(NO3)7(OH), Eu3− SBU), which further connects to six neighboring ones through six bcbp ligands to form a three-dimensional network. As expected, 1 exhibits a strong red-light emission at ambient temperature. Due to the presence of the electron-deficient bipyridinium moiety in the conjugation, this emission is selectively quenched by electron-rich organic amine compounds with high sensitivity and exhibits a prominent visual color change.

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INTRODUCTION The development of selective sensing materials for the detection of amines has received considerable attention in the fields of environmental and industrial monitoring, quality control of food, and medical diagnosis because organic amines have been widely used in the polymer, dye, and pharmaceutical industries.1−5 However, most of these amines are colorless, making their differentiation via visual inspection difficult. Among the approaches employed toward the detection of amines, such as gas chromatography coupled with mass spectrometry, surface enhanced Raman spectroscopy, nuclear quadruple resonance, energy dispersive X-ray diffraction, and cyclic voltammetry,6−9 there is a requirement for very expensive instruments, and they are not easily accessible in most cases. A sensitive, optical detection method would be simpler, quicker, and less expensive and could be potentially employed both in solution and the solid phase.10,11 Fluorescent sensing or probing has proved to be an expedient detection technique on account of the high signal output and detection simplicity.12−16 Although much success has been achieved in producing fluorescent probes for organic pollutant sensing,17−23 the detection of amines still remains challenging because the sensory materials that meet the requirements for fluorescent probes, including intense emission in the solid state, rapid response, and a high sensitivity and selectivity, are limited.24 Fluorescent coordination polymer (CP) sensors have had a profound influence on modern sensing systems because of their extremely high sensitivity and ease of signal transduction.25−29 For example, fluorescent quenching of CPs via an electrontransfer “turn-off” mechanism showed very high sensitivities to © XXXX American Chemical Society

electron-deficient quenchers, such as 2,4,6-trinitrotoluene (TNT).30−32 However, few reports have dealt with electronrich amines.33−35 Among them, most are based on coordinating amines to unsaturated metal ions.36,37 In addition, although these materials can measure very low concentrations of amines, they are air/water sensitive and subject to degradation at room temperature.38,39 Electron transfer is a common strategy to achieve fluorescent quenching. On the basis of the fact that amines always act as electron donors, luminescent CPs containing electron-deficient building blocks are anticipated to realize fluorescent quenching by amines. 4,4′-Bipyridinium derivatives are an attractive class of electron-deficient (Lewis acid) molecules containing an electrically charged moiety in the conjugation.40−43 Although there are several reports of coordination polymers based on bipyridinium derivatives, none of these CPs exhibits strong emission.44−47 Considering the weak fluorescence of such ligands, we selected luminescent Eu(III) as the metal cation. Moreover, to achieve porous CPs, a rigid pyridinium containing two positive charges, 1,1′-bis(4-carboxyphenyl)-(4,4′-bipyridinium) dichloride (denoted H2bcbp·2Cl, Scheme 1), was used in this article. As expected, the obtained crystalline threedimensional (3D) porous CPs, {Eu3(bcbp)3(NO3)7(OH)2}n (1), exhibited strong red-light emission at ambient temperature. Importantly, this emission is indeed selectively quenched by electron-rich organic amines with high sensitivity. Received: July 19, 2015 Revised: August 22, 2015

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DOI: 10.1021/acs.cgd.5b01026 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Scheme 1. Synthesis of Ligand H2bcbp·2Cl


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assumed to contain 10 MeOH molecules (calcd: C, 40.47; H, 3.62; N, 7.46%). IR data (KBr, cm−1): 3419 (m), 3113 (w), 3049 (w), 1632 (m), 1611 (s), 1569 (m), 1405 (s), 1300 (s), 1221 (m), 1030 (m), 853 (m), 782 (s), 703 (m), 650 (m), 507 (w), 470 (m). Fluorescence Experiments. In a 1 cm quartz cuvette, 2 mg of compound 1 was added to 3 mL of MeOH and sonicated for 1 h; then, the fluorescence response upon excitation at 395 nm was measured in situ after incremental addition of freshly prepared analyte solutions in the range 550−750 nm. To maintain homogeneity, the fluorescence measurement was performed while the analyte was added and stirred after another 1 min. X-ray Diffraction Analysis. A suitable single crystal of complex 1 was mounted on a glass fiber for X-ray measurements. Diffraction data was collected on a Rigaku-AFC7 equipped with a Rigaku Saturn CCD area-detector system. The measurement was made using graphite monochromatic Mo Kα radiation (λ= 0.71073 Å) at 293 K. The frame data were integrated, and absorption correction was calculated the Rigaku CrystalClear program package. All calculations were performed with the SHELXTL-97 program package,49 and structures were solved by direct methods and refined by full-matrix least-squares against F2. All non-hydrogen atoms were refined anisotropically, and hydrogen atoms of the organic ligands were generated theoretically onto the specific atoms. The diffraction data were treated by the SQUEEZE method, as implemented in PLATON, to remove diffuse electron density associated with these badly disordered solvent molecules.50 The crystal data and structure refinements are summarized in Table 1. Crystallographic data have been deposited with the Cambridge Crystallographic Data Center as supplementary publication number CCDC 1412259. These data can be obtained free of charge from The

EXPERIMENTAL SECTION

Materials and Measurements. All chemicals and reagents were used as received unless otherwise stated. NMR spectra were recorded with a Bruker Avance 400 MHz NMR spectrometer. Chemical shifts are given in parts per million (ppm) and referred to TMS as an internal standard. 1H coupling constants J are given in hertz (Hz). Fourier transform infrared (FT-IR) spectra were recorded from KBr discs on a Perkin-Elmer Spectrum 2000 FT-IR spectrometer. Powder X-ray diffraction (PXRD) intensities were recorded on a Rigaku MiniFlex-II X-ray diffractometer, and high-resolution mass spectra were recorded on a Thermo Fisher Scientific Exactive Plus system. TGA measurements were performed on a TG-209 system with a heating rate of 10 °C/min under a N2 atmosphere. UV−vis diffuse reflectance spectra were recorded at room temperature on a Varian Cary 500 UV−vis spectrophotometer equipped with an integrating sphere using BaSO4 as a white standard in the range 200−800 nm. Luminescence measurements were made with an Edinburgh Instrument FLS 980 luminescence spectrometer on powdered crystal material of the compound. Elemental analyses of C, H, and N were carried out on a Vario EL III elemental analyzer. The electron spin resonance (ESR) measurements were obtained on a Bruker A300 instrument operating in the X-band at room temperature. The gas sorption studies were performed on Micromeritics ASAP 2020 adsorption equipment. Synthesis. Preparation of 1,1′-Bis(4-carboxyphenyl)-(4,4′-bipyridinium) Dichloride (H2bcbp·2Cl). Ligand H2bcbp·2Cl was prepared in two steps using modified literature procedures.48 As shown in Scheme 1, a solution of 4,4′-bipyridine (3.6 g, 23 mmol) and 2,4dinitrochlorobenzene (16.5 g, 81 mmol) in acetonitrile (70 mL) was heated under reflux for 72 h. The hot reaction mixture was filtered, and the filtered cake was refluxed with ethanol (300 mL). After the solid was dried under vacuum, 1,1′-bis(2,4-dinitrophenyl)-(4,4′-bipyridinium) dichloride was collected as a white solid (6.0 g, 50%). 1H NMR (400 MHz, CD3OD): 9.68 (d, J = 6.4 Hz, 2H), 9.34 (d, J = 2.4 Hz, 1H), 9.12 (d, J = 6.4 Hz, 2H), 8.97 (dd, J1 = 8.4 Hz, J2 = 2.4 Hz, 2H), 8.43 (d, J = 8.8 Hz, 1H). A suspension of 1,1′-bis(2,4-dinitrophenyl)-(4,4′-bipyridinium) dichloride (2.80 g, 5 mmol) and 4-aminobenzoic acid (1.50 g, 11 mmol) in anhydrous ethanol (60 mL) was heated under reflux for 84 h. After being cooling to room temperature, the obtained solid was filtered and recrystallized from H2O. The collected brown crystals were washed with ice water and dried, affording H2bcbp·2Cl (1.97 g, 85%). IR data (KBr, cm−1): 3345 (m), 3213 (m), 3128 (m), 3059 (m), 1643 (m), 1611 (s), 1533 (s), 1374 (s), 1215 (m), 1167 (m), 851 (m), 776 (s); 1H NMR (400 MHz, CD3OD): 9.65 (d, 2H, J = 4.0 Hz), 8.96 (d, 2H, J = 4.0 Hz), 8.38 (d, 2H, J = 8.0 Hz), 7.99 (d, 2H, J = 8.0 Hz); HRMS (ESI) calcd for C24H18ClN2O4+ [M − Cl], 431.0950; found, 431.0803. Preparation of {Eu3(bcbp)3(NO3)7(OH)2}n (1). A mixture of Eu(NO3)3·6H2O (0.1 mmol, 44.6 mg), H2bcbp·2Cl (0.05 mmol, 24 mg), and methanol (5 mL) was stirred for 30 min in air and then transferred to and sealed in a 23 mL Tefon-lined autoclave, which was heated in an oven at 100 °C for 72 h and cooled to room temperature at a rate of 10 °C/h to form yellow block-shaped crystals, which were collected by filtration, washed with MeOH, and dried in air (ca. 63% yield based on ligand). Elemental analyses of C, H, and N are 40.45, 3.53, and 7.54%, respectively. On the basis of the results of TGA and PLATON from crystallographic data, each smallest structural unit is

Table 1. Crystal Data and Structure Refinement Parameters for 1

a

B

complex

1

empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z T (K) ρcalc (g/cm3) μ (Mo Kα) (mm−1) F(000) collected reflections unique reflections no. of observations goodness-of-fit on F2 R1a, wR2b (I > 2σ(I)) R1a, wR2b (all data)

Eu3C72H48N13O35 2111.11 monoclinic C2/c 23.068(5) 19.934(4) 22.714(5) 115.85(3) 9400(3) 4 173(2) 1.492 2.062 4160 39 322 10 777 (0.0706) 9018 1.112 0.0706, 0.1676 0.0873, 0.1776

R1 = ∑∥Fo| − |Fc∥/∑|Fo|. bwR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)]1/2. DOI: 10.1021/acs.cgd.5b01026 Cryst. Growth Des. XXXX, XXX, XXX−XXX



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Figure 1. (a) View of the coordination environment of the Eu(III) ions in 1; symmetry codes: (i) −x, y, 0.5 − z. (b) View of the framework of 1. (c) Schematic representation of six-connected topology, with the trinuclear europium cluster as the six-connected node.

Three Eu(III) ions in Eu3−SBU are triply bridged by six carboxylate groups from six bcbp ligands with an Eu···Eu distance of 5.217(2) Å. Other coordination sites of Eu(III) ions are completed by O atoms from the nitrate ions or hydroxyl ions. All Eu−O bond distances fall in the normal range of 2.374(5)−2.850(5) Å. Eu3−SBU further connects to six neighboring ones through six bcbp ligands, forming a 3D framework. From the viewpoints of topology, 1 features a uninodal six-connected network with a Schläfli symbol of 412.63 (Figure 1c), where the Eu3−SBU units are considered to be sixconnected nodes and the bcbp ligands act as linkers. Such a network had been identified as pcu-net according to the RCSR symbol.51 Significantly, some methanol molecules can be detected, but most of them are disordered and could not be well-located during the refining of the crystal structure, which is, indeed, supported by thermogravimetric analysis (Figure S6, Supporting Information). Accordingly, the contribution of all of the methanol molecules is subtracted from the data using SQUEEZE during the refinement. After subtracting the methanol contribution, PLATON calculations indicate that the effective pore volume is 2310.7 Å3 per unit cell, which is 24.6% of the crystal volume. However, the small diameter of this pore implies that it is inaccessible for bulky guest molecules. Photoluminescence. The photoluminescent properties of 1 in the solid state at room temperature were investigated (Figure 2). As expected, 1 shows characteristic emission bands of the Eu(III) ion centered at 594, 616, 651, and 694 nm, which can be attributed to the f−f 5D0 → 7Fj (j = 1−4) transitions, respectively. The spectrum is dominated by the intense band of the 5D0 → 7F2 electron dipole transition, which is the so-called hypersensitive transition and is responsible for the brilliant red emission of the complex. The photoluminescence properties of 1 were also investigated after dispersing it in different solvents. Excitingly, the

Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.

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RESULTS AND DISCUSSION Crystal Structure of 1. Single-crystal X-ray analysis reveals that 1 crystallizes in space group C2/c. There are one and a half

Figure 2. Photoluminescence spectra (λex = 395 nm) of 1 in the solid state.

Eu(III) ions, one and a half bcbp ligands, three and a half nitrate ions, and one hydroxyl ion in the asymmetric unit. As shown in Figure 1a, all of the deprotonated bcbp ligands exhibit μ4-η1-η1-η1-η1 coordination modes. One prominent structural feature of 1 is the presence of an unusual trinuclear Eu(III) unit (Eu3(CO2)6(NO3)7(OH), Eu3−SBU). Two Eu(III) ions at the ends of Eu3−SBU have a common ten-coordinate geometry, and the central Eu(III) ion is in eight-coordinate geometry. C




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Figure 5. Percentage of luminescence quenching obtained in solutions of 0.03 mM organic amines in methanol. The 5D0 → 7F2 transition intensities (616 nm) were selected.

Figure 3. (a) Photoluminescence spectra of 1 dispersed in various pure solvents. (b) 5D0 → 7F2 transition intensities (616 nm) of 1 in various pure solvents.

Figure 6. Photoluminescence spectra of 1 dispersed in MeOH with incremental addition of ethylamine. Inset: Photograph showing the changes in fluorescence under UV light (365 nm) upon incremental addition of ethylamine (left: 1 in methanol; right: 1 in methanol containing ethylamine).

3) when a finely ground sample of 1 (2 mg) was suspended in 3 mL of various solvents (CH2Cl2, MeCN, H2O, THF, DMF, EtOH, and MeOH) and sonicated for 1 h. The PXRD patterns of the samples after immersion in various organic solvents are consistent with that of the original sample (Figure S7, Supporting Information), which suggests that the frameworks of 1 is stable in organic solvents. In addition, due to 1 having the strongest emission in methanol, we selected methanol as the solvent in the following measurements. Sensing of Organic Amines in Solution. Because of the possible electron transfer between the bipyridinium moieties of 1 and aromatic molecules, we first investigated its luminescent responses to aromatic molecules, including benzene (Ph), toluene (PhCH3), chlorobenzene (PhCl), bromobenzene (PhBr), phenol (PhOH), 4-nitrophenol, nitrobenzene (PhNO2), and aniline (PhNH2). After the samples had been suspended in MeOH solution (0.1 M) containing various

Figure 4. Photoluminescence spectra of 1 obtained after introducing different guest molecules into the MeOH emulsion of 1. Inset: Photograph of samples immersed in solutions containing different guest molecules.

photoluminescence intensities of 1 were observed to be independent of the solvent, exhibiting limited changes (Figure D




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Figure 7. (a) Photograph showing the color changes of 1 treated in different organic amines; (b) Solid state UV−vis spectra of amine@1.

propanediamine, diethylamine, and cyclohexylamine, were selected as the investigated reagents. The results show that all organic amines can weaken the fluorescence intensities of 1 gradually by increasing the concentration of organic amines (Figures S12 and S13, Supporting Information), but the percent of quenching that results differs (Figure 5). At a concentration of 0.3 mM, the aniline, diethylamine, and cyclohexylamine solutions caused relatively little change in the fluorescence intensity. However, the addition of ethylamine and 1,3-propanediamine significantly quenched the fluorescence intensity. The fluorescence quenching efficiency was calculated using the Stern−Volmer (SV) equation,52 (I0/I) = Ksv[A] + 1, where I0 is the initial fluorescence intensity before the addition of analyte, I is the fluorescence intensity after adding the analyte, [A] is the molar concentration of analyte, and Ksv is the quenching coefficient. According to the equation, the quenching efficiency for the organic amines in MeOH was found to follow the order ethylamine > 1,3-propanediamine > diethylamine > cyclohexylamine > aniline (Figure S8, Supporting Information), which is consistent with the order of their sizes and can be attributed to their diffusion abilities toward the pores of the framework. The small size of the ethylamine molecule leads to it having easy access to the pore and causes a more efficient electron transfer with the pyridinium moieties within the framework. In addition, the detection sensitivities toward amine compounds, especially toward ethylamine with its small size, are very important. Hence, the dependence of the degree of luminescence quenching on the concentration has also been examined, which reveals that 1 is indeed very sensitive to ethylamine even at very low concentrations (Figures 6 and S9). At around 500 ppm, the luminescence intensity is nearly completely quenched with a yield of 94%. This result demonstrates that 1 has good sensitivity for detecting small

Figure 8. Recyclability of 1 dispersed in MeOH in the presence of 500 ppm ethylamine.

aromatic molecules, their photoluminescence spectra were measured. As shown in Figure 4, except for PhNH2, none of these aromatic molecules shows any influence on the luminescence intensity of 1, indicating that 1 can be considered to be a probe to detect PhNH2. The quenching mechanism is likely derived from electron transfer from the electron-rich PhNH2 to the electron-deficient bipyridinium moieties (Figure S22, Supporting Information). More importantly, the PhNH2treated sample undergoes a color change from yellow to brown, whereas the remaining samples remain yellow, which suggests that 1 can also serve as a sensor for PhNH2 because of this visible color change. In order to further explore the sensing potentials of 1 toward other organic amines, four analogues, e.g., ethylamine, 1,3E




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amounts of ethylamine in a MeOH solution, which is superior to that of previously reported sensors for detecting ethylamine.53 Meanwhile, PXRD patterns of 1 after being immersed in organic amine solutions for 12 h confirmed that the original framework structure is retained (Figure S11, Supporting Information). UV−vis Spectra. As mentioned above, 1 treated with PhNH2 undergoes a color change from yellow to brown. However, 1 treated with 0.1 M of different amines in MeOH exhibits different colors (the treated samples are denoted amine@1, Figure 7a), and this color change is extremely rapid and very prominent and can be easily detected by the naked eye. The solid-state UV−vis spectra of 1 showed a strong absorption band at about 380 nm (Figure 7b), which corresponds to the n−p* and π−π* transitions of the aromatic carboxylate ligands.54−56 The UV−vis spectra of amine@1 (Figure 7b) displayed a gradual broadening of the absorption band in the region 400−800 nm after exposure to the different organic amines, which may arise from an intermolecular electron-transfer transition from the organic amines to the bcbp linker. These results again supported the ability of 1 to sense organic amines through distinct color changes, a characteristic that has been rarely observed in previous reports. Sensing of Organic Amine Vapors. The significant ability of 1 to sense organic amines in a MeOH suspension prompted us to study its capabilities to detect vapor-phase organic amines at room temperature. In a typical experiment, vacuum-dried 1 was exposed to the above organic amine vapors for a period of time at room temperature, and we then measured their photoluminescence properties. Due to larger vapor pressures of ethylamine, 1,3-propanediamine, and diethylamine at room temperature, the fluorescence of 1 was completely quenched within a few minutes (Figures S14−S16). However, the fluorescence quenching abilities of aniline and cyclohexylamine toward 1 are rather low. Only when 1 was exposed to aniline or cyclohexylamine vapors for 1 day at room temperature was its emission remarkably quenched (Figures S17 and S18). These results indicate that 1 is also able to sense vapors for some organic amines. Importantly, 1 can be regenerated by centrifuging the samples, washing with MeOH several times, and further heating at 80 °C for 2 h. Remarkably, limited quenching efficiency losses occur even after five cycles, which suggests that 1 is highly recyclable and supports its use for detection applications (Figure 8).

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AUTHOR INFORMATION

Corresponding Authors

*(M.-J.L.) E-mail: [email protected]. *(C.-C.H.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Jun-Dong Wang and Prof. Xin Fang for useful discussions. This work was supported by the National Natural Science Foundation of China (21202020 and 21273037), the Doctoral Fund of the Ministry of Education of China (20123514120002), the Natural Science Foundation of Fujian Province (2014J01040 and 2014J01045), and the Science & Technical Development Foundation of Fuzhou University (2012-XQ-10 and 2013-XQ-14).

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REFERENCES

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CONCLUSIONS In summary, a porous and luminescent 3D lanthanide coordination polymer containing a rigid pyridinium moiety and two electrical charges at its conjugated tectons was synthesized, and it exhibited selective colorimetric and fluorescent sensing toward trace amounts of organic amine solutions and vapors. More importantly, due to the small pores of the coordination polymer, such sensing is size-dependent. Further studies of analogous coordination polymers in various applications are under way.

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FTIR, 1H NMR, HRMS, and photoluminescence spectra; TGA and powder XRD data; and other photographs (PDF). X-ray crystallographic data (CIF).

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01026. F




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Luminescent Coordination Polymer with Conjugated Lewis Acid Sites

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