Luminescent 3D Lanthanide–Cadmium Heterometal–Organic

Dec 4, 2017 - Crystallographic data for compounds 1a, 1b, 1c, 1d have been deposited in the Cambridge Crystallographic Data Center with CCDC-1565835 (...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Luminescent 3D Lanthanide−Cadmium Heterometal−Organic Frameworks with Chemical Stability and Selective Luminescent Sensing Ling Ding, Le-Hui Liu, Qing Shi, Yan-Qiong Sun,* Yong-Jiang Wang, and Yi-Ping Chen College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, People’s Republic of China S Supporting Information *

ABSTRACT: Four novel three-dimensional (3D) 4d−4f heterometal−organic compounds, [LnCd2(Pbc)4(Meimdc)(H2O)]·3H2O (Ln = Eu, 1a; Tb, 1b; Sm, 1c; Dy, 1d) (HPbc = 4-(4-pyridinyl)benzoic acid; H3Meimdc = 2-methyl1H-4,5-imidazole-dicarboxylic acid), have been successfully prepared by a hydrothermal method. All the compounds are isostructural and show three-dimensional microporous pillarlayered structures with uncoordinated carboxylate sites hung in the channels. Compound 1a possesses excellent chemical stability. The luminescent investigations show that compounds 1a, 1b, 1c, and 1d display the characteristic emission bands of Ln3+ ions. Compound 1a exhibits a good potential as a luminescent sensor material for multi-responsive Ag+, Cu2+, Zn+, Co2+, and Ni2+ cations and some organic amines. Interestingly, 1a can capture Ag+, Cu2+, Zn+, Co2+, and Ni2+ cations and shows cation-dependent colorimetric response, which indicates the potential for naked sensing.

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potential applications in chemical sensors.8 Owing to their strong coordination ability, there are competing reactions between Cd(II) and Ln(III) ions in the formation of metal− organic frameworks, and thus, it is an enormous challenge to synthesize Ln−Cd heterometallic coordination polymers.9 According to the theory of soft−hard acid−base, Ln3+ cations are hard acids and prefer to coordinate with hard basic functional groups such as an O atom from carboxylate, whereas Cd2+ ions are soft acid and have a strong affinity for soft basic functional groups like a N atom. The key to constructing luminescent Ln−Cd heterometallic complexes is the selection of organic structural units. So far, our research group has succeeded in constructing a series of Ln−Cd heterometal− organic frameworks based on N-heterocycle carboxylate ligands, such as 4,5-imidazole dicarboxylic acid, ethylenediamine tetraacetic acid, 2-methyl-1H-4,5-imidazole-dicarboxylic acid, 5,6-benzimidazole-dicarboxylic acid, and so on.8a,9b,10 At the same time, 4-(4-pyridinyl)benzoic acid (HPbc), a multifunctional linear connector with nitrogen and oxygen donors, shows various coordination modes under hydro/solvothermal conditions to build up desired complexes. The nitrogen atoms of HPbc ligand are soft base and have a strong tendency to bond to the transition metal ions, whereas the oxygen atoms from the carboxyl group of HPbc ligand are hard base and prefer to coordinate with the lanthanide ions. To the best of our knowledge, the reported transition-lanthanide (d−f)

INTRODUCTION Recently, the design and construction of metal−organic frameworks (MOFs) has obtained great interest in the view of their intriguing structures, topologies, and potential applications in gas storage, separation,1 catalysis,2 magnetic properties,3 biomedicine,4 and chemical sensors.5 It has been more and more popular to synthesize lanthanide-based organic frameworks to enrich the varieties of structures and improve the properties by introduction of heterometallic ions, especially transition metal ions. The introduction of heterometallic ions not only makes the structures rich but also makes the energy levels of these compounds easy to control. To the best of our knowledge, the researches of most lanthanide-transition (d−f) heterometal−organic frameworks are concentrated on Ln−M (M = Cu, Fe, Zn, Ag, Mn, Ba, etc.) to study their magnetic properties.6 However, luminescent lanthanide-transition (d−f) heterometal−organic frameworks remain less developed.6j−,l Recently, luminescent properties of MOFs have been applied in the fields of chemical sensors, luminescent materials, display materials, and light-emitting diodes (LEDs).7 It is well-known that most rare earth ions emit strong linear spectra under stimulus. However, the f−f transitions of rare earth ions (Ln3+ ions) are partly forbidden, resulting in quite low absorption coefficients, which can be improved by the antenna effect. At the same time, Cd2+ cation with a 4d10 electron configuration could be an excellent building block and serves as a bridge of energy transfer between Ln3+ ions and organic ligands. Therefore, Ln−Cd heterometallic coordination polymers could be good candidates for luminescent materials with © XXXX American Chemical Society

Received: August 11, 2017

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DOI: 10.1021/acs.inorgchem.7b02071 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystal Data and Structural Refinement Parameters for 1a−1d empirical formula formula weight temperature (K) wavelength (Å) crystal system space group a b c α β γ volume (Å3) Z Dc (g/cm3) F(000) GOOF on F2 R1a, wR2b (I > 2σ(I)) R1, wR2 (all data) a

1a

1b

1c

1d

C54H43Cd2EuN6O16 1408.7 296.15 0.71073 triclinic P1̅ 13.4612(10) 15.3926(12) 16.8521(13) 63.4790(10) 72.9440(10) 71.8580(10) 2919.8(4) 2 1.602 1392 1.151 0.0298, 0.1073 0.0341, 0.1146

C54H43Cd2TbN6O16 1415.66 296.15 0.71073 triclinic P1̅ 13.4056(13) 15.3620(14) 16.8450(16) 63.4970(10) 72.9080(10) 71.8510(10) 2900.6(5) 2 1.621 1396 1.092 0.0442, 0.1516 0.0556, 0.1714

C54H43Cd2SmN6O16 1407.09 296.15 0.71073 triclinic P1̅ 13.4705(9) 15.4240(11) 16.8302(12) 63.4350(10) 72.9640(10) 71.7740(10) 2922.1(4) 2 1.599 1390 1.038 0.0366, 0.0989 0.0442, 0.1057

C54H43Cd2DyN6O16 1419.24 296.15 0.71073 triclinic P1̅ 13.3800(8) 15.3579(10) 16.8416(11) 63.5430(10) 72.9260(10) 71.8680(10) 2895.2(3) 2 1.628 1398 1.114 0.0406, 0.1520 0.0440, 0.1654

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = {∑[w(Fo2 − Fc)2]/∑[w(Fo2)2]}1/2. Luminescent spectra were collected with an Edinburgh Instruments FS920 TCSPC luminescence spectrometer on powdered crystal material of the compounds. XPS data were obtained using an ESCALAB 250 X-ray photoelectron spectroscopy. ICP data were obtained on an Ultima2 inductively coupled plasma OES spectrometer. The suspensions of 1a were prepared by mixing 1a (5 mg) as powder with different metal salts aqueous solution (5.00 mL) of M(NO3)x or different solvents (each 5.00 mL). The suspensions were shaken for 10 min using ultrasonic waves to ensure uniform dispersion. Syntheses of the Compounds [LnCd2(Pbc)4(Meimdc)(H2O)]· 3H2O (Ln = Eu 1a; Tb 1b; Sm 1c; Dy 1d). A mixture of Ln2O3 (Eu2O3, 0.0351 g, 0.1 mmol (1a); Tb4O7, 0.0374 g, 0.05 mmol (1b); Sm2O3, 0.0348 g, 0.1 mmol (1c); Dy2O3, 0.0373 g, 0.1 mmol (1d)), HPbc (0.0398 g, 0.2 mmol), H3Meimdc (0.0314 g, 0.2 mmol), 3CdSO4·8H2O (0.0300 g, 0.033 mmol), and H2O (8 mL) was added in a 23 mL Teflon-lined stainless steel vessel, and heated at 170 °C for 5 days, and then cooled to room temperature. Orange block crystals were obtained for 1a, 1b, 1c, 1d. Yield: 85% (1a), 85% (1b), 86% (1c), 81% (1d) (based on Ln2O3). Elemental analysis for C54N6O16H43Cd2Eu 1a: C, 46.04; H, 3.08; N, 5.97 wt %, Found: C, 46.10; H, 3.01; N, 5.91 wt %; C54N6O16H43Cd2Tb 1b: C, 45.81; H, 3.06; N, 5.94 wt %, Found: C, 45.75; H, 3.02; N, 5.94 wt %; C54N6O16H43Cd2Sm 1c: C, 46.09; H, 3.08; N, 5.97 wt %, Found: 46.04; H, 3.08; N, 5.97 wt %; C54N6O16H43Cd2Dy 1d: C, 45.70; H, 3.05; N, 5.92 wt %, Found: C, 45.63; H, 3.05; N, 5.89 wt %. Selected IR peaks (cm−1): 3481(w), 1712(m), 1590(m), 1548(s), 1384(m), 1266(m), 775(s), 472(s). X-ray Crystallography. Suitable single crystals of compounds 1a, 1b, 1c, 1d were chosen and mounted on a glass fiber. All the crystallographic data for 1a, 1b, 1c, 1d were collected on a Bruker Apex DUO2 diffractometer (Mo Kα radiation, λ = 0.710 73 Å) radiation using ω scanning mode at room temperature. The structures of 1a, 1b, 1c, 1d were solved using direct methods and refined by fullmatrix least-squares on F2 using the SHELXTL in the Olex2 program package.12 Hydrogen atoms bonding to C were generated geometrically (C−H = 0.93 Å) and refined with fixed isotropic displacement parameters. The unresolvable electron density from the void space in the structure was removed by SQUEEZE program. Crystallographic data for compounds 1a, 1b, 1c, 1d have been deposited in the Cambridge Crystallographic Data Center with CCDC-1565835 (for 1a), −1565837 (for 1b), −1565836 (for 1c), −1565834 (for 1d). These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/

heterometal−organic frameworks based on HPbc ligands are mainly restricted to Ln−Cu heterometal−organic frameworks.11 However, Ln−Cd heterometal−organic frameworks based on HPbc ligands have not been reported yet. In accordance with the aforementioned points, our goal is to construct new Ln−Cd heterometal−organic frameworks and study the luminescent sensing for metal cations. Herein, we report the syntheses, structures of four 3D Ln(III)−Cd(II) layer-pillared 4d−4f heterometal−organic frameworks based on HPbc ligand, namely, [LnCd2(Pbc)4(Meimdc)(H2O)]·3H2O (Ln = Eu 1a; Tb 1b; Sm 1c; Dy 1d) (HPbc = 4-(4pyridinyl)benzoic acid; H3Meimdc = 2-methyl-1H-4,5-imidazole-dicarboxylic acid), and the luminescent response of 1a to the different metal cations. The four compounds are isomorphous and constructed by the linkages of 2D heterometallic layers and Pbc− pillars. Interestingly, Compound 1a possesses excellent chemical stability. Uncoordinated carboxylate sites are hung in the channels of 1a, which is first found in the Ln−Cd heterometal−organic frameworks. The uncoordinated carboxylate sites within porous Ln−Cd heterometal−organic frameworks may have potential application value in detection of d-block metal ions. As expected, the as-synthesized 1a displays selective luminescent sensing to metal cations, such as Ag+, Cu2+, Zn+, Co2+, and Ni2+, which quench the emissions of the Eu3+ ions of compound 1a and shows cation-dependent colorimetric response. It indicates that compound 1a possesses the potential for naked sensing. In addition, the mechanism of luminescence quenching has been discussed.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. Commercially available solvents and chemical reagents were used as received. IR spectra were collected using ATR on a Nicolet IS 50 FT-IR spectrometer in the range of 400−4000 cm−1. The elemental analysis of C, H, and N was made on an Elementar Vario EL III elemental analyzer. Powder X-ray diffraction (PXRD) data were collected on a Bruker Apex DUO2 diffractometer (Cu Kα radiation, λ = 1.5418 Å) at room temperature. Thermal gravimetric analysis were carried out on a METTLER TGA/ DSC1 analyzer under nitrogen with a 10 °C·min−1 heating rate. B

DOI: 10.1021/acs.inorgchem.7b02071 Inorg. Chem. XXXX, XXX, XXX−XXX

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Scheme 1. Coordination Modes of Pbc− and Meimdc3− Ligands

cif. Selected crystal data and structural refinements are listed in Table 1.

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RESULTS AND DISCUSSION Description of Crystal Structures. Single-crystal X-ray diffraction studies reveal that [LnCd2(Pbc)4(Meimdc)(H2O)]· 3H2O (Ln = Eu 1a; Tb 1b; Sm 1c; Dy 1d) are isomorphous and crystallize in the triclinic space group of P1̅. Thus, only the structure of 1a is depicted in detail. The asymmetric unit of 1a consists of one independent Eu(III) ion, two independent Cd(II) ions, one Meimdc3− anions, four Pbc− anions, one coordinated water molecule, and three free water molecules (Figure 1). The coordination polyhedron for nine-coordinated

Interestingly, in the structure of compound 1a, two center symmetric Eu3+ ions are bridged by two μ2-O atoms from a Meimdc3− ligand to form an edge-sharing Eu2O2 cluster unit which is further linked by two Cd(1)2+ ions through two μ2-O atoms to generate a zigzag-like Ln−Cd heterometal tetranuclear [Eu2Cd2O4] cluster unit (Figure 2a). The two central

Figure 1. Coordination environments of Eu3+ and Cd2+ cations in compound 1a. Atoms having “A”, “B”, “C”, “D”, “E” in their labels are symmetry-generated. A: 1 − x, 2 − y, −z; B: x, y − 1, z; C: 1 − x, 2 − y, 1 − z; D: 1 + x, y, z; E: 2 − x, 1 − y, −z. Hydrogen atoms are omitted for clarity. Color code: Eu, purple; Cd, cyan; O, red; N, blue; C, gray.

Eu(III) is close to tricapped trigonal prism: five OCOO− atoms from three Pbc− anions and four OCOO− atoms from two Meimdc3− anions (Figure S1). Moreover, the coordination polyhedron of two six-coordinated Cd(II) can be viewed as octahedrons. Cd1(II) are coordinated to one OCOO− and two N atoms from three Pbc− ligands, one N and one OCOO− atom from one Meimdc3− ligand, and one coordinated water (Figure S2). Cd2(II) are connected with one OCOO− and two N atoms from three Pbc− ligands, and one N and two OCOO− atoms from two Meimdc3− ligands. The Eu−O bond lengths range from 2.325(5) to 2.543(4) Å. The Cd−O and Cd−N bond lengths are in the ranges of 2.226(5)−2.386(4) Å and 2.273(5)− 2.465(6) Å, respectively, which are within the range of reported bond distances in the previous researches.5d,13 The Pbc− ligand adopts three kinds of distinctly different coordination modes: (a) μ3-η1:η2: one behaves as μ3-Pbc− mode linking one Cd atom and one Eu atom in a monodentate and bidentate way, respectively (Scheme 1b-I). (b) μ3-η1:η1:η1: the second serves as μ3-Pbc− mode linking one Eu and two Cd atoms in monodentate modes (Scheme 1b-II). (c) μ2-η1:η1: the third acts as μ2-Pbc− mode connecting two Cd atoms in monodentate modes with one uncoordinated OCOO− atom (Scheme 1b-III). The Meimdc3− ligand adopts only one kind of mode: μ9-η2:η2:η2:η2:η1, linking two Eu(III) and three Cd(II) centers (Scheme 1a).

Figure 2. (a) Stick and polyhedral views of zigzag-like Ln−Cd heterometal tetranuclear [Eu2Cd2O4] cluster unit and binuclear Cd2O2 cluster unit in 1a. (b) Polyhedral view of a 1D ribbon-like structure constructed from [Eu2Cd2O4] and binuclear Cd2O2 cluster units linked by Meimdc3− ligands in 1a. Color code: Eu, purple; Cd, cyan; O, red; N, blue; C, gray.

symmetric Cd(2)2+ ions also are bridged via two μ2-O atoms to form a central symmetric binuclear Cd2O2 cluster unit. The [Eu2Cd2O4] and binuclear Cd2O2 cluster units are linked by Meimdc3− ligands into a one-dimensional ribbon-like structure running along the a-axis direction (Figure 2b). These onedimensional ribbons are connected side-by-side by Pbc− ligands to generate a two-dimensional ladder-like layer (Figure 3). Then, 2D layers are pillared by Pbc− ligands to produce a threedimensional open pillar-layered architecture with 1D channels running along the b axis and c axis, respectively (Figure 4a). The uncoordinated OCOO− atoms of Pbc− ligands point to the channels. From the result of topological analysis, the threedimensional architecture of 1a is an eight-connected hex-type net. Each Eu2Cd2(μ2O4)] and Cd2O2 cluster unit acts as eightconnected node, and the Meimdc3− and Pbc− ligands function as linkers. The topological point symbol of 1a is (36.418.53.6), C

DOI: 10.1021/acs.inorgchem.7b02071 Inorg. Chem. XXXX, XXX, XXX−XXX

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Luminescence Properties. For the Ln−Cd heterometal− organic compounds, the most fascinating property is their excellent luminescent property. The luminescence spectra of the 1a−1d were recorded in the solid state at room temperature. Compounds 1a−1d exhibit the characteristic emissions spectra of Ln3+ ions. The excitation and emission spectra of compounds 1a−1d are depicted in Figure S6 and Figure 5. Compound 1a emits red light when excited at 394 nm and shows the characteristic transition of 5D0 → 7F1, 5D0 → 7 F2, 5D0 → 7F4 of the Eu3+ ion at the bands of 592, 617, and 696 nm, respectively (Figure 5a). The intensity of the 5D0 → 7 F2 transition (electric dipole) is much stronger than that of the 5 D0 → 7F1 transition (magnetic dipole), showing that the Eu3+ cations in 1a are situated at a low-symmetry site without inversion centers, which is in accordance with the single-crystal structure analysis. Complex 1b emits green light, due to the dominating Tb3+ emission at 543 nm (5D4−7F5). Complex 1b shows the characteristic transition emission spectrum of Tb3+ with an excitation spectrum at 312 nm. The bands at 488, 543, 585, and 620 nm are assigned to the characteristic emission of 5 D4 → 7FJ (J = 6−3) transitions of the Tb3+ ion, respectively (Figure 5b). In addition, we tested their emission decay lifetimes of 1a and 1b. The luminescent decay curves were fitted by a double-exponential decay function (Figure S7). The emission decay lifetimes for 1a are τ1 = 673 μs, τ2 = 1147 μs; for complex 1b, τ1 = 495 μs, τ2 = 895 μs (Table S1). In the emission spectrum of 1c, three bands at 562, 599, 641, and 699 nm at excitation of 320 nm correspond to 4G5/2 → 6H5/2, 4G5/2 → 6H7/2, 4G5/2 → 6H9/2, and 4G5/2 → 6H11/2 transitions of Sm3+ ions (Figure 5c). The emission spectra of 1d exhibits the characteristic transition of Dy3+ at the bands 488, 543, and 572 nm at excitation of 325 nm, attributed to 4F9/2 → 6H15/2, 4F9/2 → 6H13/2, 4F9/2 → 6H11/2, respectively (Figure 5d). Chemical Stability. In order to examine the chemical stability of 1a, their samples were treated in water, concentrated HCl aqueous solution (pH = 2), and NaOH aqueous solution (pH = 11) at room temperature. After being soaked in these solutions for 48 h, the measured PXRD patterns of 1a show retained crystallinity and original structures, implying their excellent chemical stability (Figure S8). The treated samples of 1a in (pH = 2−11) solutions still exhibit the characteristic emissions of Eu3+ (Figure S9). The solid-state emission intensity of the treated samples gradually decreased in acidic and basic conditions, and the highest value of emission intensity was observed in pH = 7 solution, which could result from the uncoordinated carboxylate groups in the framework of compound 1a. Luminescent Sensing. As there are uncoordinated carboxylate sites in compound 1a, we investigate their sensing ability for metal ions and small organic solvent molecules. The results of fluorescent sensing measurements show that compound 1a has a good selectively sensing for Cu2+, Ag+, Co2+, Ni2+, Zn2+, and organic amine molecules. For the sake of investigating the sensing of 1a for small organic solvent molecules, its suspension-state emission spectra were measured. The powder of 1a (5.00 mg) was dispersed into ethylene glycol, ethanol, dichloromethane, dimethylformamide (DMF), n-butyl alcohol, chloroform, acetone, H2O, acetonitrile, dimethylacetamide (DMA), methylbenzene, nbutane, methanol, ethylenediamine, and diethylenetriamine. As shown in Figure 6, the intensity of luminescent spectra is closely related to the organic solvent molecules. Ethylene glycol

Figure 3. Polyhedral view of a 2D ladder-like layer in 1a. Color code: Eu, purple; Cd, cyan; O, red; N, blue; C, gray.

Figure 4. (a) Polyhedral view of 3D open pillar-layered framework with 1D channels along c axis in 1a. Color code: Eu, purple; Cd, cyan; O, red; N, blue; C, gray. (b) Representation showing the topologies of the 3D framework for 1a. Each Eu2Cd2(μ2O4)] and Cd2O2 cluster unit as eight-connected node is denoted by pink spheres and the Meimdc3− and Pbc− ligands as linkers.

and the extension of the topological symbol is [3.3.3.3.3.3.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.52.52.52.66] (Figure 4b). PXRD, IR Spectra, and Thermal Properties. In order to confirm the purity of the compounds 1a, 1b, 1c, and 1d, powder X-ray diffraction (PXRD) data were recorded for compounds 1a, 1b, 1c, and 1d at room temperature (Figure S3). The peak positions of as-synthesized samples are almost in agreement with the simulated PXRD patterns, which confirms the phase purity of each compound. As shown in Figure S4, the similarity of the IR spectra of 1a− 1d suggests that 1a−1d are isostructural, respectively. For 1a, the IR spectra exhibit a broad absorption peak at 3385 cm−1 and a sharp absorption peak at 3054 cm−1 that are associated with the ν(C-H) and ν(O-H), respectively. The strong bands at 1596 and 1536 cm−1 for 1a are associated with ν(Ar-H). The strong bands at 1607, 1395 cm−1 for 1a are associated with νas(CO) and νs(CO), respectively. The sharp peaks at 865 and 852 cm−1 are associated with γ(C-H) of the benzene functional group. These facts are in agreement with the singlecrystal diffraction results. The thermal gravimetric analysis (TGA) data of compounds 1a−1d were recorded in a dry N2 atmosphere from 30 to 1200 °C (Figure S5). In the TGA curves of 1a−1d, the weight losses of 3.27% (calcd: 3.83%) for 1a, 3.72% (calcd: 3.92%) for 1b, 3.65% for 1c (calcd: 3.84%), and 3.65% for 1d (calcd: 3.84%) were observed from 30 to 205 °C, attributed to the successive removal of all free and coordinated water molecules. The decomposition of HPbc and H3Meimdc was observed from 205 to 1086 °C. D

DOI: 10.1021/acs.inorgchem.7b02071 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Emission spectra of 1a (excitation at 394 nm) (a), 1b (excitation at 312 nm) (b), 1c (excitation at 320 nm) (c), and 1d (excitation at 325 nm) (d) in the solid state at room temperature.

concentration of Cu2+ solution to 1 × 10−3 mol·L−1, the emission intensity at 617 nm is more than 5 times weaker than that of pure water, which means that compound 1a possesses relatively high sensitivity in the detection of the Cu2+ ion through luminescent quenching (Figure S11). Furthermore, the reduction of luminescence intensity is still clearly observed when compound 1a is soaked in a 1 × 10−5 mol·L−1 Cu2+ solution, suggesting that the detection limit of compound 1a as a Cu2+ probe is much lower. On the other hand, when the concentrations of Ag+, Ni2+, Zn2+, Co2+ were higher than 0.01 mol·L −1, respectively (Figures S12−S15), the luminescence quenching effects of metal ions on compound 1a were obvious. The sensitivity of the fluorescent probe was Cu2+ > Ag+ ≈ Co2+ ≈ Ni2+ ≈ Zn2+. Interestingly, the process of soaking is accompanied by visible changes in color. When the compound 1a was immersed in these aqueous solutions of 0.1 mol·L−1 (M(NO3)x) (M = Cu2+, Ag+, Ni2+, Co2+, Zn2+) for 3 days, respectively, the surfaces of the crystals of 1a were covered with the powder of metal cations and the crystals turn blue in Cu(NO3)2 solution (1a-Cu), green in Ni(NO3)2 solution (1a-Ni), light yellow in Zn(NO3)2 solution (1a-Zn), dark brown in AgNO3 solution (1a-Ag), and yellow in Co(NO 3 ) 2 solution (1a-Co) respectively, which makes it easy to distinguish by the naked eye (Figure 8). Mechanism of Luminescent Sensing. There are several possible mechanisms to explain this luminescence quenching. The quenching on fluorescent MOFs by metal ions is basically attributed to the collapse of MOF structures by the metal ion, ion exchange occurring between the framework metal centers and the targeted cations, competition absorption between metal ions and Ln-MOFs, as well as strong interaction between metal ions and the MOFs.14 In order to elucidate the possible

and ethanol can significantly improve the emission intensity of 1a with remarkable enhancement effects. As for dichloromethane, dimethylformamide (DMF), n-butyl alcohol, chloroform, acetone, H2O, and acetonitrile, the luminescence spectra exhibit no differences from each other. However, for solvents like ethylenediamine and diethylenetriamine, the emission spectra of la show complete fluorescence quenching (Figure S10). These results indicate that la could be a promising fluorescent probe for ethylenediamine and diethylenetriamine. This may be owing to that UV−vis absorption spectra of ethylenediamine and diethylenetriamine fall in the range of the excitation spectra of Eu3+ ions (λex = 395 nm).10a Similarly, the powder sample of 1a was immersed in an aqueous solution containing different 0.1 M M(NO3)x (M = K+, Er3+, Li+, Na+, Ca2+, Ag+, Mg2+, Zn2+, Cd2+, Ba2+, Pb2+, Al3+, Cu2+, Co2+, Cr3+, Fe3+, and Ni2+) and the emission spectra of Eu3+ ions were measured (Figure 7). The corresponding suspension-state solution of 1a in different metal solutions still shows the four characteristic emission bands of Eu3+ ions, so only the highest emission bands at 617 nm were monitored under the perturbation of various metal cations. In stark contrast, most metal cations exhibit an enhancement effect for the luminescent intensities. Especially, the interaction with Mg2+ drastically enhanced the luminescent intensity, with a maximum of more than 3.9 times as much as that of the parent MOF. This obvious enhancement were also found in the Ln− Zn heteronuclear metal−organic framework.6j However, the Co2+, Ni2+, Cu2+, Zn2+, and Ag+ cations exhibit a complete quenching effect on the luminescence of 1a. In order to further verify their luminescence quenching effect, the crystals of compound 1a were dispersed into different concentrations of M (NO3)x (M = Co2+, Ni2+, Cu2+, Zn 2+ , and Ag + ) aqueous solution. Upon adding the E

DOI: 10.1021/acs.inorgchem.7b02071 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. (a) Emission spectra of 1a introduced into various 0.1 M aqueous cations solution. (b) Luminescent intensities at 616 nm of 1a treated with various 0.1 M aqueous cation solutions.

Figure 6. (a) Emission spectra of 1a introduced into various organic solvents. (b) Luminescent intensities at 616 nm of 1a treated with various organic solvents.

mechanism for such photoluminescence quenching by different metal ions, the PXRD, IR, UV−vis diffuse-reflectance, and emission spectra for solid-state 1a-Cu, 1a-Ag, 1a-Ni, 1a-Co, and 1a-Zn were further measured. The PXRD patterns and IR spectra for 1a and 1a-Cu, 1a-Ag, 1a-Ni, 1a-Co, and 1a-Zn are almost identical, and thus, it suggests that the overall structure remained intact (Figures S16 and S17). The framework of 1a is neutral, so it is very difficult to ion exchange between the framework metal centers and the targeted cations. The emission spectra in solid state reveal that 1a-Cu, 1a-Ag, 1aNi, 1a-Co, and 1a-Zn have different degrees of quenching effects on the emission intensity, especially for 1a-Cu with more significant quenching effects, which were consistent with luminescent sensing in the Cu2+ solution (Figure S18). The UV−vis absorption spectra showed that compounds of 1a-Cu, 1a-Ag, 1a-Ni, 1a-Co, and 1a-Zn have UV−vis absorption in the range at 200−400 nm, which is assigned to π−π* transition of the ligands. At the same time, there are broad absorption bands in the range of 450−700 nm that originated from d−d transition for compounds 1a-Cu, 1a-Ni, and 1a-Co (Figure S19). As the excitation wavelength of 1a is 396 nm and the emission spectra of 1a is in the range from 550 to 700 nm, the luminescence decrease of 1a in the solutions of Co2+, Ni2+, Cu2+, Zn2+, and Ag+, respectively, might be attributed to partial absorption of the light source energy and some emission energy of 1a. The ICP results for 1a-Cu, 1a-Ag, 1a-Ni, 1a-Co, and 1aZn are shown in Table S2; the metal cations (Co2+, Ni2+, Cu2+, Zn2+, and Ag+) concentrations were found from 1.35% to

Figure 8. Crystal photographs of compound 1a dispersed into various 0.1 M aqueous M(NO3)x solutions.

26.81%, especially for 1a-Cu, 1a-Ag, and Cu2+ and Ag+ concentrations reached 7.17% and 26.81%, respectively. From the point of view of crystal structures, there are onedimensional channels along the b axis and c axis, respectively. The uncoordinated OCOO− atoms of Pbc− ligands point to the channels and provide binding sites for metal ions. The distance between the uncoordinated OCOO− atoms is 4.2622 Å, so appropriate ionic radii of metal cations, such as Cu2+, Ag+, Ni2+, Co2+, and Zn2+, lead to the easy combination of metal cations and uncoordinated OCOO− atoms. The interaction between uncoordinated OCOO− atoms and metal cations is expected to perturb the singlet and triplet excited states of ligands. There is F

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Inorganic Chemistry no doubt that this will affect antenna efficiency and further decrease the energy transitions from ligands to lanthanide ions, leading to the luminescence quenching. To confirm this speculation, X-ray photoelectron spectroscopy (XPS, Figure 9) of the 1a-Cu, 1a-Ag indicated the presence of Cu2+ and Ag+

vis spectra for 1a-Cu, 1a-Ag, 1a-Ni, 1a-Co, 1a-Zn; and XPS spectra for O 1s of the 1a, 1a-Cu, 1a-Ag (PDF) Accession Codes

CCDC 1565834−1565837 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yan-Qiong Sun: 0000-0001-9480-9721 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21473030 and 21003020) and the Natural Science Fund of Fujian Province (No. 2017J01579).

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Figure 9. XPS spectra of the 1a (red) and Mnn+-incorporated 1a-M (black): (a) Cu 2p, (b) Ag 3d.

cations, respectively. Studies of O 1s XPS were tested on 1a, 1a-Cu, and 1a-Ag (Figure S20). The O 1s peak of O atoms at 531.25 eV in 1a is shifted to 531.55 eV, 531.35 eV after addition of Cu2+ or Ag+, which further verifies the weak binding between uncoordinated OCOO− atoms and Cu2+ or Ag+.

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CONCLUSIONS In conclusion, four 3D Ln−Cd heterometal−organic frameworks with the similar pillar-layered structures were designed and synthesized. Complexes 1a, 1b, 1c, and 1d exhibit the characteristic emission bands of Ln3+ ions. Luminescent sensing measurements indicate that compound 1a with uncoordinated carboxylate sites shows selective and sensitive sensing for Cu2+, Ag+, Ni2+, Co2+, and Zn2+ ions and ethylenediamine and diethylenetriamine organic molecules. Compound 1a exhibits a cation-dependent colorimetric response for Cu2+, Ag+, Ni2+, Co2+, and Zn2+ ions, which indicates the potential for naked sensing. This work provides a useful insight into the design and synthesis of Ln−Cd heterometal−organic frameworks-based sensors in the future.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02071. Structure figures for 1a; the plots of IR, XRD, TG, excitation spectra for 1a−d; emission spectra and UV− G

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