Luminescent Zn(II) Coordination Polymers for Highly Selective

Article pubs.acs.org/IC

Luminescent Zn(II) Coordination Polymers for Highly Selective Sensing of Cr(III) and Cr(VI) in Water Tian-Yi Gu,†,‡ Ming Dai,†,∥ David James Young,§ Zhi-Gang Ren,*,† and Jian-Ping Lang*,†,‡ †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, People’s Republic of China ‡ State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, People’s Republic of China ∥ Suzhou Clean Environment Institute, Jiangsu Sujing Group Company, Limited, Suzhou 215122, People’s Republic of China § Faculty of Science, Health, Education and Engineering, University of the Sunshine Coast, Maroochydore DC, Queensland 4558, Australia S Supporting Information *

ABSTRACT: Three photoluminescent zinc coordination polymers (CPs), {[Zn2(tpeb)2(2,5-tdc)(2,5-Htdc)2]·2H2O}n (1), {[Zn2(tpeb)2(1,4-ndc)(1,4-Hndc)2]·2.6H2O}n (2), and {[Zn2(tpeb)2(2,3-ndc)2]·H2O}n (3) (tpeb = 1,3,5-tri-4-pyridyl-1,2ethenylbenzene, 2,5-tdc = 2,5-thiophenedicarboxylic acid, 1,4-ndc = 1,4-naphthalenedicarboxylic acid, and 2,3-ndc = 2,3-naphthalenedicarboxylic acid) were prepared from reactions of Zn(NO3)2·6H2O with tpeb and 2,5-H2tdc, 1,4-H2ndc, or 2,3-H2ndc under solvothermal conditions. Compound 1 has a two-dimensional (2D) grid-like network formed from bridging 1D [Zn(tpeb)]n chains via 2,5-tdc dianions. 2 and 3 possess similar one-dimensional (1D) double-chain structures derived from bridging the [Zn(tpeb)]n chains via pairs of 1,4-ndc or 2,3-ndc ligands. The solid-state, visible emission by 1−3 was quenched by Cr3+, CrO42−, and Cr2O72− ions in water with detection limits by the most responsive complex 3 of 0.88 ppb for Cr3+ and 2.623 ppb for Cr2O72− (pH = 3) or 1.734 ppb for CrO42− (pH = 12). These values are well below the permissible limits set by the USEPA and European Union and the lowest so far reported for any bi/ trifunctional CPs sensors. The mechanism of Cr3+ luminescence quenching involves irreversible coordination to free pyridyl sites in the CP framework, while the Cr6+ quenching involves reversible overlap of the absorption bands of the analytes with those of the excitation and/or emission bands for 3.

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applications, and low cost.10 Many luminescent materials, including glutathione-capped CdTe quantum dots,11 nanoparticles,12 carbon dots,13 and some organic molecules,14 have been developed for the detection of Cr(III) and Cr(VI). Much, however, remains to be done, particularly with respect to sensitivity.15 Coordination polymers (CPs) have been developed for the chemical sensing of metal ions and other pollutants.16 A few luminescent CPs have been reported for the detection of Cr(III) or Cr(VI).17−21 The mechanism of the detection for Cr(VI) is related to relatively slow ion exchange17 or intermolecular energy transfer.21c,f−h In the case of Cr(III), responsiveness depends on weak interactions between the Cr3+ cation and the free coordination sites of the CP ligands.22 To our knowledge, there are only two reports of CPs, {[Eu0.3Tb0.7(HL)(H2O)3]·H2O}n (L = 5,5′-(1H-2,3,5-triazole-

INTRODUCTION

Chromium is widely used in industry1 but is a potentially persistent, bioaccumulating, and highly toxic pollutant. Carcinogenic Cr(VI) can induce DNA damage at trace or even ultratrace levels,2−4 while Cr(III) is an essential trace element for human health but is detrimental at high concentrations.5 Chromium waste is generated from burning coal, the operation of cooling towers, electroplating, oxidative dyeing, tanning, and sanitary landfills6 and from the use of corrosion inhibitors in water pipes.7 Many methods have been developed for the detection of Cr(III and VI) ions, including atomic absorption spectrometry (AAS), inductively coupled plasma optical emission spectroscopy (ICP-OES), ion chromatography (IC), and electrochemical detection. Mobile, low-cost detection methods, however, would be valuable for fieldwork and particularly in developing countries8 but necessarily requires high sensitivity in a variety of matrices.9 Chemiluminescent detection is attractive in this respect due to its potential for simplicity of operation, a broad range of © 2017 American Chemical Society

Received: February 3, 2017 Published: March 24, 2017 4668

DOI: 10.1021/acs.inorgchem.7b00311 Inorg. Chem. 2017, 56, 4668−4678

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Inorganic Chemistry Scheme 1. Schematic Illustration of the Synthetic Process of Compounds 1−3

1,4-diyl)diisophthalic acid) 18 and {[Zn 2 (TPOM)(NH 2 BDC)2]·4H2O}n (TPOM = tetrakis(4-pyridyloxymethylene)methane, NH2-BDC = 2-aminoterephthalic acid)20 with potential as bifunctional sensors for detecting Cr3+ and CrO42−/Cr2O72− simultaneously. Again, low sensitivity18/ selectivity19 and the necessity for organic solvents20 could limit the development of this technology. We have a long-standing interest in the use of CPs for detecting or degrading environmental pollutants such as Hg(II), NH4+/NH3, organic dyes, and nitroaromatics in water.16c−g Zinc(II) coordination polymer [Zn(ppvppa)(1,4ndc)]n (ppvppa = dipyridin-2-yl-[4-(2-pyridin-4-yl-vinyl)phenyl]amine) was developed by some of us for the fluorescent sensing of Hg2+ or CH3HgI with high selectivity and sensitivity. In this example, the dipyridinylamine groups of the ppvppa ligand acted as an ionophoric receptor for Hg2+ or MeHg+ ions.16c The related [Cd(ppvppa)(1,4-ndc)]n can detect nitroaromatic compounds by making use of energy transfer between the analytes and the complex, resulting in fluorescence quenching.16d These results encouraged us to prepare some new fluorescent CP materials designed to probe Cr3+ and/or CrO42−/Cr2O72− ions in water. Tridentate π-conjugated ligand 1,3,5-tri-4-pyridyl-1,2-ethenylbenzene (tpeb) (Scheme 1) was reacted with Zn(NO3)2 together with a group of dicarboxylic acids under solvothermal conditions, generating three new CPs {[Zn2(tpeb)2(2,5-tdc)(2,5-Htdc)2]·2H2O}n (1), {[Zn 2 (tpeb) 2 (1,4-ndc)(1,4-Hndc) 2 ]·2.6H 2 O} n (2), and {[Zn2(tpeb)2(2,3-ndc)2]·H2O}n (3) (2,5-H2tdc = 2,5-thiophenedicarboxylic acid, 1,4-H2ndc = 1,4-naphthalenedicarboxylic acid, and 2,3-H2ndc = 2,3-naphthalenedicarboxylic acid). These metal complexes exhibited highly selective fluorescent quench-

ing in response to both Cr(III) and Cr(VI) ions at ppb levels for the latter in water. Described below are the syntheses of 1− 3, their structural characterization, and their excellent luminescent sensing performance of Cr3+, CrO42−, and Cr2O72− ions in water.

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EXPERIMENTAL SECTION

General Procedures. All reagents and chemicals were obtained directly from commercial sources and used without further purification. The ligand tpeb was prepared according to a literature method.23 The 1H NMR spectrum of tpeb was recorded at ambient temperature in DMSO-d6 on a Varian UNITY plus-400 spectrometer with chemical shifts referenced to the solvent signal. Elemental analyses for C, H, and N were performed on a Carlo-Erba CHNO-S microanalyzer. Inductively coupled plasma spectra (ICP) were obtained on a Varian 710-ESICP optical emission spectrometer. Infrared spectra (400−4000 cm−1) were obtained on a Nicolet iS-10 spectrometer with KBr disks. Thermogravimetric analyses (TGA) were obtained on a TA SDT-600 analyzer (heating rate of 5 °C/min). Powder X-ray diffraction (PXRD) measurements were carried out on a PANalytical X’PertPRO MPD system (PW3040/60). UV−vis absorption spectra were obtained on a Varian Cary-50 spectrophotometer. Photoluminescence spectra and quantum yields were obtained on a HORIBA PTI QuantaMaster40 Spectrofluorometer. X-ray photoelectron spectroscopy (XPS) measurements were recorded on a Thermo-Fisher Scientific ESCALAB 250Xi X-ray photoelectron spectrometer (with Al Kα X-ray radiation as the X-ray source for excitation, whose binding energies were referenced to C 1s at 284.7 eV from hydrocarbon to compensate charging effects). Syntheses of 1−3. A mixture of Zn(NO3)2·6H2O (29.7 mg, 0.1 mmol), tpeb (19.4 mg, 0.05 mmol), dicarboxylic acids (0.05 mmol, 2,5-H2tdc, 8.6 mg for 1, 1,4-H2ndc, 10.8 mg for 2, and 2,3-H2ndc, 10.8 mg for 3), 2.4 mL of H2O, and 0.6 mL of MeCN was sealed in a 10 mL glass tube and allowed to react at 150 °C for 24 h. After cooling to 4669


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Inorganic Chemistry Table 1. Crystal Data and Structure Refinement Parameters for 1−3 empirical formula fw cryst syst space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 ρcalc/g/cm3 Z μ/mm−1 F(000) R1a wR2b GOFc

1

2

3

C72H54N6O14S3Zn2 1454.18 triclinic P1̅ 9.4510(5) 20.0108(7) 20.4706(6) 118.027(4) 91.181(4) 103.093(4) 3292.0(2) 1.4617 2 0.896 1496.0 0.0728 0.2099 1.057

C45H35.2N3O8.6Zn 820.91 monoclinic C2/c 29.287(3) 16.6983(14) 16.7466(15)

C78H56N6O9Zn2 1352.07 triclinic P1̅ 11.780(6) 11.799(5) 24.110(11) 102.896(11) 99.104(12) 102.543(12) 3113(3) 1.442 2 0.839 1396.0 0.1141 0.3261 1.180

101.511(10) 8025.1(13) 1.281 8 0.663 3192.0 0.0976 0.3172 1.041

a R1= Σ∥F0| − |Fc∥/Σ|F0|. bwR2= {Σw(F02 − Fc2)2/Σw(F02)2}1/2. cGOF = {Σw((F02 − Fc2)2)/(n − p)}1/2, where n = number of reflections and p = total number of parameters refined.

ambient temperature at a rate of 7 °C h−1, crystals of each compound were obtained (1, dark yellow prisms; 2, yellow blocks; 3, yellow cubes). These crystals were then washed sequentially with water, EtOH, and Et2O and dried in air. Yield for 1: 32.7 mg (45% based on Zn). Anal. Calcd for C72H54N6O14S3Zn2: C, 59.47; H, 3.74; N, 5.78. Found: C, 60.67; H, 3.61; N, 5.75. IR (KBr disk): 3052 (m), 1782 (m), 1634 (s), 1607 (s), 1529 (s), 1419 (s), 1330 (m), 1202 (s), 1025 (m), 969 (s), 857 (m), 819 (m), 769 (s), 688 (s), 537 (m) cm−1. Yield for 2: 30.28 mg (39% based on Zn). Anal. Calcd for C45H35.2N3O8.6Zn: C, 65.83; H, 4.32; N, 5.12. Found: C, 66.32; H, 4.91; N, 5.86. IR (KBr disk): 3009 (m), 1756 (m), 1610 (s), 1509 (s), 1421 (s), 1384 (s), 1281 (m), 1181 (m), 1064 (s), 1002 (m), 868 (m), 822 (s), 532 (s). Yield for 3: 43 mg (64% based on Zn). Anal. Calcd for C78H56N6O9Zn2: C, 69.29; H, 4.17; N, 6.22. Found: C, 69.30; H, 4.23; N, 6.32. IR (KBr disk): 3050 (m), 1750 (m), 1611 (s), 1502 (s), 1443 (m), 1367 (s), 1204 (s), 1138 (s), 1068 (s), 1026 (s), 966 (m), 905 (s), 846 (s), 798 (m), 739 (s), 668 (m), 536 (s). X-ray Structure Determination. Single crystals of 1−3 suitable for X-ray analysis were obtained directly from the above procedures. Data were collected on an Agilent Technologies Gemini A Ultra CCD (1 and 2) or a Bruker APEX-II CCD diffractometer (3) using graphite-monochromated Mo Kα (λ = 0.71073 Å). The single crystals were mounted on glass fibers at 223 (1) and 298 K (2 and 3). Reflection data were integrated, and unit cell parameters were determined using all observed reflections with the program CrysAlisPro, Agilent Technologies (Version 1.171.36.32, 2013), for 1 and 2 and APEX2 v2012.4-3 (Bruker, AXS) for 3. Empirical absorption correction was applied using the program SADABS.24−26 The reflection data were also corrected for Lorentz and polarization effects. The crystal structures of 1−3 were solved by direct methods and refined on F2 by full-matrix least-squares methods with the SHELXL2013 program.27a For 2, the solvent-accessible void occupied a volume of 245 Å3 per unit cell (3% of the total cell volume calculted by the PLATON program27b). As the disorder model did not give satisfactory results, it was further treated with PLATON/SQUEEZE to find 26 electrons, which corresponded to approximately 2.6 water molecules per chemical formula. One 4-pyridyl-1,2-ethenyl group of tpeb is disordered over two sites with an occupancy factor of 0.6/0.4 for N1− C1−C7 and N1A−C1A−C7A, while a 1,4-ndc molecule (C40−C47− O5−O6) is disordered over two positions by rotating about the center of C41−C41A with an equal occupancy factor. All non-H atoms were refined anisotropically except for those of the disordered 4-pyridyl-1,2-

ethenyl group in 2. The H atoms of the water solvent molecules in 1 and 3 were first located from the Fourier maps, and their O−H bond lengths were fixed to 0.83 Å. Two H atoms at the symmetric C42A and C47A were not located. All other H atoms were placed in the geometrically idealized positions and constrained to ride on their parent atoms. Some very weak reflections or those blocked by the beam stop were not included in refinements. Important crystal data and refinement parameters for 1−3 are given in Tables 1 and S1. Photoluminescence Sensing of Cr3+, CrO42−, and Cr2O72− Ions. Uniform suspensions of compounds 1−3 were prepared by grinding the crystalline solid (3 mg) and suspending the powder in deionized water (3 mL) with ultrasound mixing for 1 h. Aqueous solutions (3 mol/L, 1 μL) of NanX (X = F−, Cl−, Br−, I−, NO3−, ClO4−, IO3−, CO32−, SO42−, P2O74−, CrO42−, Cr2O72−, n = 1, 2, 4) or M(NO3)n (Mn+ = Na+, K+, Ag+, Mg2+, Cd2+, Cu2+, Mn2+, Pb2+, and Cr3+, n = 1, 2, 3) were injected into the suspensions by microsyringe and uniformly mixed at room temperature. The luminescence intensities of these suspensions were then measured on a spectrofluorometer.

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RESULTS AND DISCUSSION

Synthetic, Spectral, and Thermal Aspects. Solvothermal reactions of Zn(NO3)2·6H2O, tpeb, and dicarboxylic acids (2,5H2tdc for 1, 1,4-H2ndc for 2, and 2,3-H2ndc for 3) with a molar ratio of 2:1:1 in H2O/MeCN (v/v = 4/1) at 150 °C for 24 h followed by a standard workup produced crystals of 1−3 in reasonable yields (45% for 1, 39% for 2, and 64% for 3). Compounds 1−3 were the only products regardless of the molar ratios of the three components. The products remained the same when the pH of the reaction systems was adjusted within a wide range of 3−12. Likewise, different temperatures (180 and 100 °C) in a variety of solvent systems (H2O/MeOH, H 2 O/EtOH, and H 2 O/DMF) did not produce other identifiable products. Compounds 1−3 are stable in air and moisture and insoluble in common organic solvents such as DMF, EtOH, MeOH, DMA, DMSO, and CHCl3. Their elemental analyses were consistent with their chemical formulas. PXRD patterns of the bulk samples correlated well with simulated patterns generated from the single-crystal X-ray diffraction data (Figure S1). A strong band at 1607−1623 cm−1 was observed in the IR spectra 4670


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Inorganic Chemistry of all three compounds corresponding to the coordinated carboxyl groups. Thermogravimetric analyses of 1−3 revealed that they were stable up to 301 (1), 368 (2), and 336 °C (3), respectively (Figure S2). The two weight losses of 2.27% (33−100 °C) and 89.60% (323−530 °C) for 1 correspond to the loss of two solvated water molecules (calcd 2.48%) and all organic species (calcd 89.23%), respectively. The weight loss of 89.57% (345− 495 °C) for 2 is attributed to the loss of all tpeb and 1,4-ndc ligands (calcd 90.95%),while the three stepwise weight losses of 1.97% (34−89 °C), 33.76% (358−409 °C), and 57.29% (409− 511 °C) for 3 represent the elimination of the solvated water molecule (calcd 1.35%), 2,3-ndc (calcd 32.08%), and tpeb (calcd 58.17%), respectively. The residue species were assumed to be Zn (8.13% for 1, calcd 8.98%) and ZnO (11.02% for 2, calcd 10.51%; 12.18% for 3, calcd 12.14%). Crystal Structure of 1. Compound 1 crystallizes in the triclinic space group P1,̅ and its asymmetric unit contains one independent [Zn2(tpeb)2(2,5-tdc)(2,5-Htdc)2] unit and two water solvent molecules. Each Zn1 is tetrahedrally coordinated by two O atoms from two 2,5-tdc ligands and two N atoms from two separate tpeb ligands, while each Zn2 adopts a distorted tetragonal pyramid geometry which is associated with one O atom from a 2,5-tdc ligand, two O atoms from another 2,5-tdc ligand, and two N atoms from two tpeb ligands (Figure 1a). The average Zn1−N and Zn1−O bond lengths of 2.059(50) and 1.978(47) Å (Table S1) are comparable to those found in {[Zn2(H2O)(1,4-ndc)2(tpcb)]}n (2.046(5) and 1.952(4) Å, respectively, tpcb = tetrakis(4-pyridyl)cyclobutane).16g The mean Zn2−N and Zn2−O bond lengths (2.080(38) and 2.157(50) Å) (Table S1) resemble those found in {[Zn(1,4-ndc)(tpcb)0.5]}n (2.118(3) and 2.186(3) Å).16g In each tpeb ligand, one pyridyl group (N3−C23−C27 and N5− C43−C47) is left uncoordinated. The other two pyridyl groups bridge one four-connected zinc (Zn1) and one five-connected zinc (Zn2), thus producing a 1D [Zn(tpeb)]n chain extending along the a axis (Figure 1b). Such 1D chains are interconnected by the bridging 2,5-tdc dianions to give a 2D grid-like network along the ac plane (Figure 1c), whereas one −COOH group of each terminal coordinated 2,5-Htdc anions stretches above and below the planes. Crystal Structures of 2 and 3. Compound 2 crystallizes in monoclinic space group C2/c, while 3 crystallizes in triclinic space group P1̅. The asymmetric unit of 2 consists of 0.5 [Zn2(tpeb)2(1,4-ndc)(1,4-Hndc)2] unit and 1.3 water molecules, while for 3, its asymmetric unit contains one [Zn2(tpeb)2(2,3-ndc)2] unit and one lattice water molecule. All central Zn atoms in 2 and 3 show a tetrahedral coordination mode (Figures 2a and 3a). Each Zn center is associated with two N atoms from two tpeb ligands and two O atoms from two carboxylate ligands (either 1,4-ndc or 2,3-ndc). The mean Zn− N bond lengths (2.020(56) (2) and 2.064(30) Å (3)) (Table S1) are similar to those of the corresponding ones in 1, while the mean Zn−O bond lengths (1.955(52) (2) and 1.946(8) Å (3)) are shorter than those found in 1. In each tpeb ligand, one pyridyl group (N3−C23−C27 in 2; N2−C16−C20 and N6− C50−C54 in 3) remains uncoordinated. The other two pyridyl groups bridge the neighboring Zn atoms to produce a 1D [Zn(tpeb)]n chain extending along the b axis (Figures 2b and 3b). In 2, the [Zn(tpeb)]n chains are interconnected by the bridging 1,4-ndc dianions to give a 1D double-chain structure, while the terminal coordinated 1,4-Hndc ligands stretch above and below these chains, leaving a pair of free −COOH groups

Figure 1. (a) View of the coordination environments of Zn1 and Zn2 in 1 with a labeling scheme. Symmetry codes: (A) x − 1, y, z − 1; (B) x + 1, y, z − 1. (b) View of a portion of the 1D [Zn(tpeb)]n chain extending along the a axis. (c) View of the 2D network extending along the ac plane. Each cyan tetrahedron represents one Zn atom. All H atoms and water solvent molecules have been omitted for clarity.

(Figure 2b), while in 3 the [Zn(tpeb)]n chains are bridged by pairs of 2,3-ndc ligands, thereby forming a 1D double chain extending along the b axis (Figure 3b). Photoluminescent Properties. Upon excitation at 380 (tpeb), 400 (1), 370 (2), and 378 (3) nm at room temperature, these compounds exhibited solid-state photoluminescence with emission maxima at 416 (tpeb), 468 (1), 452 (2), and 474 (3) nm (Figure 4). The highly π-delocalized planar28 tpeb is emissive with a quantum yield (QY) of 4.70%. The emissions of 1−3 are all red shifted relative to free ligand (Figure 4) with higher quantum yields (QY = 6.87% (1), 7.83% (2), and 12.04% (3)). We assign this red shift to ligand-to-ligand charge transfer (LLCT)29 between the dicarboxylate and the tpeb ligand that are constrained by coordination to Zn2+ with a relatively short distance between adjacent tpeb ligands, improving the weak interactions between them and thus enhancing the emission.30 4671


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Inorganic Chemistry

Figure 4. Solid-state emission spectra of tpeb (black), 1 (red), 2 (green), and 3 (blue).

Photoluminescence Sensing of Cr3+, CrO42−, and Cr2O72− Ions. The PXRD patterns of 1−3 after treating them in acidic and basic aqueous solutions (Figure S3) demonstrated that these three compounds possessed high water stability in a wide pH range from 3 to 12. This phenomenon suggested the possibility of directly sensing Cr3+, Cr2O72−, and CrO42− in water. Zn−CPs 1−3 were suspended in water, and changes to their emission spectra were observed on exposure to different cations (1 × 10−3 mol/L, Na+, K+, Ag+, Mg2+, Cd2+, Cu2+, Mn2+, Pb2+, and Cr3+) or anions (1 × 10−3 mol/L, F−, Cl−, Br−, I−, NO3−, ClO4−, IO3−, CO32−, SO42−, P2O74−, CrO42−, Cr2O72−). The luminescence curves of these mixtures all showed the characteristic emission peaks of 1−3 (Figure S4). The sensing effects were explored by monitoring their intensities at emission λmax. Among the various cations tested (Figure 5a), only Cr3+ caused a significant emission decrease (1 and 2) and nearly total quenching (3). Similarly, in anion solutions, 1−3 showed decreases in photoluminescence in the presence of Cr2O72− and CrO42− to varying degrees (Figure 5b). The fluorescent quenching of 1−3 is similar to that reported for other fluorescent CPs.20 Since compounds 1− 3 can respond to chromium ions of different valence (III and VI) at the same time, there is a potential for these compounds to be used as trifunctional chemical sensors for the detection of the common chromium pollutants Cr3+, Cr2O72−, and CrO42− in industrial wastewater. Compound 3 displayed the most significant quenching in the presence of both Cr(III) and Cr(VI) (Figure 5) and thus was chosen as a representative sample for further investigation of selectivity and sensitivity. The selectivity of 3 was examined by measuring the luminescent intensities of its suspensions in the mixture of the above-mentioned cations with and without Cr3+ or anions with and without Cr2O72− and CrO42− (1 × 10−3 mol/L for each ion). As observed in Figure 6, the intensities were less influenced by other cations or anions but evidently weakened with addition of Cr3+, Cr2O72−, and CrO42−, which demonstrated the good selectivity of 3 for all three states of chromium relative to other common cations and anions found in industrial wastewater. The detection limits of 3 toward Cr3+, CrO42−, and Cr2O72− were calculated by the fluorescence titration. A suspension of 3 was titrated with Cr(NO3)3 solution and demonstrated outstanding sensing behavior toward Cr3+ with 86.7% quenching of the initial fluorescence at 52 ppm (1 × 10−3 mol/L) Cr3+. The dose−response graph fitted the relationship

Figure 2. (a) View of the coordination environment of Zn1 in 2 with a labeling scheme. Symmetry codes: (A) x, y − 1, z; (B) −x + 1, y, −z + 1.5. (b) View of a section of the 1D double chain in 2 extending along the b axis. Each cyan tetrahedron represents one Zn atom. All disordered motifs and H atoms have been omitted for clarity.

Figure 3. (a) View of the coordination environments of Zn1 and Zn2 in 3 with a labeling scheme. Symmetry code: (A) x − 1, y + 1, z. (b) View of a segment of the 1D double chain in 3 extending along the b axis. Each cyan tetrahedron represents one Zn atom. All H atoms and the water solvent molecule have been omitted for clarity. 4672


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Figure 5. Luminescence intensities of compounds 1 (at 468 nm), 2 (at 452 nm), and 3 (at 474 nm) treated with different (a) cations (1 × 10−3 mol/ L) and (b) anions in aqueous phase at ambient temperature.

Figure 6. Emission spectra of 3 in the (a) mixture of cations (Na+, K+, Ag+, Mg2+, Cd2+, Cu2+, Mn2+, Pb2+, 1 × 10−3 mol/L for each) with/without Cr3+ and (b, c) mixture of anions (F−, Cl−, Br−, I−, NO3−, ClO4−, IO3−, CO32−, SO42−, P2O74−, 1 × 10−3 mol/L for each) with/without Cr2O72− and CrO42−.

I0/I = 1.64 × exp(1057 × [Cr3+]) + 0.124 in the range from 1 × 10−6 to 1 × 10−3 mol/L (Figure 7a) (I0 and I are the

luminescent intensities of 3 in the absence and presence of Cr3+, respectively), and so the concentration of Cr3+ ion can be 4673


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Inorganic Chemistry

Figure 7. Dose−response graphs during detection of (a) Cr3+ in the range from 1 × 10−6 to 1 × 10−3 mol/L and (c) Cr2O72− and (e) CrO42− in the range from 1 × 10−6 to 2 × 10−3 mol/L (I0 and I are the luminescent intensities of 3 in the absence and presence of Cr3+, Cr2O72−, and CrO42−, respectively), and linear relationships at low concentrations of (b) Cr3+, (d) Cr2O72−, and (f) CrO42− in the range from 1 × 10−7 to 1 × 10−6 mol/L.

The aqueous suspension of 3 could also detect Cr2O72− at pH = 3 and CrO42− at pH = 12. These dose−response graphs fitted the relationship I0/I = 2.582 × exp(1630 × [Cr2O72−]) − 1.078) (for Cr2O72−, Figure 7c) and I0/I = 1.962 × exp(1767 × [CrO42−]) − 0.007) (for CrO42−, Figure 7e) in the range from 1 × 10−6 to 2 × 10−3 mol/L, while linear correlations were also observed for (I0 − I)/I0 = 70913 × [Cr2O72−] + 0.054 (R2 = 0.990, Figure 7d) and (I0 − I)/I0 = 73481 × [CrO42−] + 0.053 (R2 = 0.991, Figure 7f) within the range from 1 × 10−7 to 1 × 10−6 mol/L. Detection limits were calculated to be 2.623 (for Cr2O72−) and 1.734 ppb (for CrO42−). The intact framework of 3 after sensing was confirmed by PXRD (Figure S6). The

accurately quantified by the quenching of luminescence intensity.31 A good linear correlation was observed for the plot of (I0 − I)/I0 vs [Cr3+] in the range from 1 × 10−7 to 1 × 10−6 mol/L ((I0 − I)/I0 = 50972 × [Cr3+] + 0.003, R2 = 0.990) (Figure 7b). The slope of this graph indicates a very low detection limit of 0.88 ppb for Cr3+ (3δ/slope, where δ is the standard deviation (2.87 × 10−7) calculated by measuring the luminescent intensity of a blank solution 10 times).21f This value is much lower than that reported for MOFs used for fluorescence-based sensing of Cr3+ (21.3 ppb for {[Eu(HL)(H2O)3]·H2O}n18 and 94.2 ppb for [Zn2(TPOM)(NH2BDC)2]·4H2O20). 4674


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Figure 8. (a) Schematic of the luminescent quenching mechanism experiment. Comparison of the emission spectra of 3 (excited at 378 nm) with (b) Cr2O72−, (c) CrO42−, and (d) Cr3+. Black curves, 3 solely in position A; blue curves, mixture of 3 and chromium ions (1 × 10−3 mol/L) in position A; red curves, 3 in position A and chromium ions in position B; green curves, 3 in position A and chromium ion in position C.

maximum contaminant level (MCL) of drinking water for chromium set is 100 ppb by the United States Environmental Protection Agency (USEPA) and 50 ppb by the European Union (Council directive 9/83/EC).32 The detection limits of 3 toward both Cr(III) and Cr (VI) are well below these levels and much lower than some other reported CPs-based singlefunction sensors of either Cr(III) or Cr (VI).21 Proposed Mechanism. Quenching of luminescence for a detecting material usually involves the so-called energy transfer or the interaction of substrates with the luminescent group. Researchers have found that overlap of the absorption by the substrate with the excitation and/or emission of the detector might cause the intermolecular energy transfer. Both Cr2O72− and CrO42− have wide absorption bands in the UV−vis region, from 275 to 476 nm for Cr2O72− and 230 to 450 nm for CrO42−, which overlap with the excitation and/or emission bands of 3 (Figure S7). Hence, we performed an experiment to investigate whether the energy transfer caused the observed quenching effect. As shown in Figure 8a, a suspension of compound 3 in water was placed in a quartz cell (position A), while another one containing the solution of Cr2O72−, CrO42−, and Cr3+ ions (2 × 10−3 mol/L) was placed in the excitation path (position B) or emission path (position C). Under excitation at 378 nm, the luminescent intensities of 3 (black curves) and 3 with the chromium ions inserted into the excitation path (red curves) or the emission path (green curves) were compared to that of the mixture of 3 with the chromium ions (blue curves). When Cr2O72− and CrO42−

solutions were placed in the excitation path, the luminescence of 3 was greatly weakened. When put in the emission path, Cr2O72− also absorbed much of the emission light whereas the effect of CrO42− was somewhat minor. These results indicated that the luminescent quenching of 3 by CrO42− is mainly due to the energy transfer from the excitation light to the CrO42− anion, while that by Cr2O72− is probably due to the energy transfers from both excitation and emission processes to the dissolved Cr2O72− anion.34 In contrast to the Cr(VI) anions, the results for Cr3+ luminescence quenching were quite different. When the Cr3+ solution was put in the excitation or emission path, the photoluminescence of 3 did not get decreased obviously because the absorption of Cr3+ has only a minor overlap with the excitation or emission bands of 3 (Figure S7). Moreover, as mentioned before, the luminescence of 3 was evidently quenched when a suspension of 3 was mixed with Cr3+ solution. Therefore, quenching by Cr3+ should not be due to energy transfer. Considering that one pyridyl group of the tpeb ligand in 3 remains uncoordinated (as in 1 and 2), we propose that the mechanism behind the sensing selectivity of Cr3+ is the weak coordination between Cr3+ and the uncoordinated pyridyl groups. An EDS test indicated that the ratio of Cr:Zn on the surface of solid 3 after sensing Cr3+ (1 × 10−3 mol/L) was 1:15, showing the existence of coordinated Cr3+. However, ICP analysis showed the ratio of Cr:Zn was 1:50 (1.829 × 10−5 and 8.996 × 10−3 mol/L, respectively) for the bulk phase, illustrating that Cr3+ only interacted with the surface of solid 4675


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3. To further verify the interaction between the backbone and Cr3+, XPS spectra of 3 before and after exposure to Cr3+ solution were compared (Figure S8). A characteristic peak for Cr3+ was found in its XPS spectrum of the post-treated sample of 3. The energy transfer (for Cr2O72− and CrO42−) and the weak coordination (for Cr3+) mechanisms for each analyte were confirmed by check experiments. When compound 3 was separated from a solution of Cr(VI) anion by centrifugation, its original luminescent intensity might get recovered (Figure S9). Very little efficiency decay was observed when 3 was reused in detecting Cr(VI) anions after three cycles (Figure S10). However, the photoluminescence of 3 cannot be recovered by washing after exposure to Cr3+ (Figure S9), which supports our hypothesis of relatively tight binding. The blue shift (∼50 nm) of the highest emission peak at 474 nm (Figure S5) is also an evidence of a weak coordination interaction.20,33 The reusability of 3 by removal of Cr3+ was investigated by flushing with a Na2Y (Y = ethylene diamine tetraacetate) solution (0.1 mol/L). The photoluminescence of 3 could only be recovered to 60% after the first run and decreased steadily during 3 cycles (Figure S11) but could still be fully quenched by using Cr3+ solution afterward.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 86-512-65880328. *E-mail: [email protected]. Fax: 86-512-65880328. Phone: 86-512-65882865. ORCID

Jian-Ping Lang: 0000-0003-2942-7385 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (21271134, 21373142, 21531006, and 21671144) and State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences (2015kf-07) for financial support. J.P.L. also highly appreciates financial support from the “Qing-Lan” Project, the “Priority Academic Program Development” of Jiangsu Higher Education Institutions, and the “SooChow Scholar” Program of Soochow University. We are very grateful to the useful comments of the editor and the reviewers.

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CONCLUSIONS A family of strongly luminescent Zn−CPs 1−3 was successfully constructed using the π-conjugated ligand tpeb. These compounds have a 2D grid-like network (1) or 1D doublechain structures (2 and 3). Highly emissive photoluminescence coupled with exposed, uncoordinated pyridyl group active sites and good water stability over a wide pH range proved to be an ideal combination of features for the efficient and selective sensing of Cr3+, Cr2O72−, and CrO42− ions. The detection limit for the most responsive 3 was 0.88 ppb for Cr3+ and 2.623 ppb for Cr2O72− (pH = 3) or 1.734 ppb for CrO42− (pH = 12), which are well below the permissible limits set by the USEPA and European Union and the lowest so far reported for any bi/ trifunctional CPs sensor materials. Our present work demonstrates the potential of fluorescent CP-based materials to be practical probes for the detection of hazardous metal ions in different states.

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Article

REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00311. Additional information regarding the synthetic procedure of tpeb ligand, TGA, selected bond distances, emission spectra in the solid state, PXRD patterns, XPS spectra of 1−3, emission spectra during the sensing procedures, and the proposed mechanisms (PDF) X-ray crystallographic data in CIF format (CIF) Accession Codes

CCDC 1529426, 1529427, and 1529428 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, by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. 4676


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Luminescent Zn(II) Coordination Polymers for Highly Selective

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