A Series of Lanthanide-Based Metal–Organic Frameworks: Synthesis

Feb 6, 2017 - To the best of our knowledge, this is the first example for Eu-MOFs to yield SC-WLE stemming from EWDP property. Three isomorphic lantha...
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A Series of Lanthanide-Based Metal−Organic Frameworks: Synthesis, Structures, and Multicolor Tuning of Single Component Dandan Yang,† Yun Tian,† Wenlong Xu,† Xiaowei Cao,† Shaojun Zheng,‡ Qiang Ju,† Wei Huang,*,† and Zhenlan Fang*,† †

Key Laboratory of Flexible Electronics & Institute of Advanced Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, 30 South Puzhu Road, Nanjing 211816, P. R. China ‡ School of Environmental and Chemical Engineering, Jiangsu University of Science and Technology, 2 Mengxi Road, Zhenjiang 212003, Jiangsu, P. R. China S Supporting Information *

ABSTRACT: The single component (SC) white-light emitting (WLE) metal-organic frameworks based on europium (EuMOFs), which could be applied in lighting and display, have drawn great attention but have rarely been exploited. In this work, we dedicated to design and synthesize SC-WLE Eu-MOFs via a dichromatic strategy on the balance of simultaneous ligand-based and Eu-based emissions. The Eu-MOF {[Eu4(obb)6(H2O)9]·(H2O)}∞ (IAM16−3) generated via the self-assembly of the flexible ligand 4,4′-oxybisbenzoic acid (H2obb) and europium ions displays fascinating excitation-wavelength-dependent photoluminescence (EWDP) property. Upon different excitation wavelengths, tunable WLE through manipulating the intensity ratio of characteristic emissions of Eu3+ ions and ligand-based emissions was performed. To the best of our knowledge, this is the first example for Eu-MOFs to yield SC-WLE stemming from EWDP property. Three isomorphic lanthanide-based MOFs (LnMOFs), that is, {[Ln4(obb)6(H2O)9]·(H2O)}∞ (Eu3+: IAM16−3; Tb3+: IAM16−4; Dy3+: IAM16−5) based on the flexible bridging linker, that is, 4,4′-oxybisbenzoic acid (H2obb), were obtained. The Eu-MOF, showing with EWDP property, is the first example of SC WLE Eu-MOFs via a dichromatic strategy on the balance of the simultaneous ligand-based and Eu(III)-based emissions at different excitation wavelengths.

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INTRODUCTION White-light-emitting diodes (WLED) have attracted widespread attention for their potential applications in general lighting,1−4 displays,5 and backlights.6 Metal−organic frameworks (MOFs) offer a unique platform for the development of luminescence devices and optical displays due to their infinite diversity of combining metal centers with organic ligands.7−10 Lanthanide-based metal−organic frameworks (LnMOFs) have been reported with high luminescence efficiency, narrow bandwidth, and long luminescence lifetimes,9,11 resulting in potential applications in sensing,11−14 phosphors,15 lighting,15 and integrated optics.16 Generally, white light emitting (WLE) can be obtained by adjusting the mixed ratio of red, green, and blue (RGB) light-emitting components in materials.4 In contrast to the traditional materials, LnMOFs are good candidates for highly efficient WLE materials17 by codoping © 2017 American Chemical Society

of different lanthanide ions with colorful emissions into one system. For example, codoping Dy/Eu or Dy/Sm18 or Eu(III)/ Tb(III) ions into the LnMOFs9,11,19−23 with blue emission ligands have been implemented to generate pure white-light emission. Besides this codoping of different lanthanide ions strategy, in situ synthesis of mixed-lanthanide MOFs11,24 and encapsulating luminescent species, such as dyes25 and lanthanide ions/nanoparticles,26,27 in pores of MOFs have also been regarded as efficient strategies for synthesis of WLE materials. Single component (SC) color-tunable WLE materials have attracted growing interests due to their characteristics of uniform and well-balanced color, easy-manipulation, as well as Received: January 10, 2017 Published: February 6, 2017 2345

DOI: 10.1021/acs.inorgchem.7b00074 Inorg. Chem. 2017, 56, 2345−2353

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Inorganic Chemistry

and controlled by a personal computer. The samples were ground into fine powder and then pressed onto a thin glass slide holder. A BaSO4 plate was used as a standard (100% reflectance). X-ray Crystallography. The single crystals of these three isomorphic LnMOFs in present work were mounted on a glass fiber for the X-ray diffraction analysis. Diffraction data were collected at 298 K on a Bruker SMART APEX II CCD area-detector diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å) from a rotating anode generator, and intensities were corrected for Lorentz and polarization (LP) factors and empirical absorption using the ψ scan technique. The structures were solved by direct methods, developed by subsequent difference Fourier syntheses, and refined using the Siemens SHELXTL version 5 software package.38 All nonhydrogen atoms were refined with anisotropic displacement parameters. All H atoms on carbons were placed in calculated positions and refined using the riding model. Only the hydrogen atoms in the hydrogen bonds could be placed in calculated positions and refined using the riding model, while the other H atoms of water molecules could not be placed. Crystal data as well as details of data collection and refinement for them are summarized in Table 1. The selected interatomic distances and bond angles are given in Table S1.

low-cost. Recently, SC-WLE of Dy-MOF has been achieved through a BY dichromatic strategy on the balance of the simultaneous blue (B, ligand-based) and yellow (Y, Dy-based) emissions.28 Because of the low ability of Dy ions in harvesting light, the ligand of Dy-MOF should contain the following two functions: (a) harvesting excitation light and sensitizing Dy(III) ions through the “antenna effect”; (b) showing blue ligandbased emission. Different with the case of Dy(III), Eu(III) ions can absorb light by themselves owing to their bigger absorption cross section and then exhibit their characteristic red emission. However, to the best of our knowledge, studies on the colortunable SC-WLE Eu-MOFs have rarely been reported.29 Herein, we dedicated to design and synthesize SC-WLE EuMOFs via the dichromatic strategy. The research interests of excitation-wavelength-dependent photoluminescence (EWDP) property stem from their potential applications in photoluminescence (PL),30,31 optoelectronic and chemical sensor devices,15,32 and biological labeling.11,33,34 Proper organic bridging linkers play a critical role in constructing WLE LnMOFs via tuning the emissions of lanthanide and ligand upon different excitation.28 Multiple bridging modes of carboxylates result in structural diversity and interesting luminescence of generated LnMOFs.35 Recently, we found that MOFs constructed by flexible ligand could possess EWDP property.36 4,4′-Oxybisbenzoic acid (H2obb), which is a wellstudied carboxylate ligand, has been employed to synthesize LnMOFs. Its flexible −O− group can bend and rotate freely, and meanwhile, its two phenyl rings could freely twist along the −O− atom to meet the demands of the coordination geometries of lanthanide centers in the assembly process.37 We expected that the resulting LnMOFs generated by H2obb could show fascinating EWDP property, and finally exhibit WLE via manipulating intensity ratio of emissions from lanthanide and ligand at different excitation wavelengths. As anticipated, the targeted color-tunable SC-WLE LnMOF {[Eu4(obb)6(H2O)9]·(H2O)}∞ (IAM16−3) was successfully synthesized via the self-assembly of Eu(III) ions with the flexible ligand H2obb. To better study the photoluminescence of IAM16−3, we also investigated that of the other two isomorphic LnMOFs (Tb3+: IAM16−4; Dy3+: IAM16−5), synthesized under the same synthesis conditions.

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Table 1. Crystal Data and Structure Refinement Results for IAM16-3, IAM16-4, and IAM16-5 LnMOFs temp (K) empirical formula formula weight crystal system space group Z a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) ρcalcd (g/cm3) μ (mm−1) GOF R1 (I > 2σ(I))a wR2 (I > 2σ(I))a R1 (all data)b wR2 (all data)b

EXPERIMENTAL SECTION

Materials and General Methods. The original bridging ligand H2obb was bought from Jinan Henghua Sci. & Tec.Co. Ltd. Other chemicals were obtained from commercial sources and used without further purification. The IR spectra were recorded on an ALPHA Brochure FT-IR spectro-photometer in the range of 400−4000 cm−1. C, H, and O elemental analyses were determined on an EA1110 CHNS-0 CE element analyzer. Powder X-ray diffraction data were recorded on a PANaytical X′pert pro X-ray diffractometer with graphite-monochromatized Cu Kα radiation (λ = 1.542 Å). Thermal stability studies were performed on a NETSCHZ STA 449C thermoanalyzer under N2 (30−800 °C range) at a heating rate of 10 K/min. Fluorescence spectra were measured on F-4600 FL Spectrophotometer Instrument. The photoluminescence lifetimes and the absolute emission quantum yields, determined by the absolute method using a BaSO4-coated integrating sphere as sample chamber, were recorded at room temperature with Edinburgh Instruments FLS920 spectro-fluorimeter. X-ray photoelectron spectroscopy (XPS) analysis was performed in a VG instrument equipped with CLAM2 analyzer using un-monochromatized Al Kα photons (1486.6 eV). Solid-state optical absorption spectrum of H2obb was measured at room temperature with a PerkinElmer Lambda 950 UV−vis spectrophotometer. The instrument was equipped with an integrating sphere

a

IAM16−3 298 Eu4O39C84H66 2307.18 monoclinic P21/c 2 8.694(2) 29.112(7) 18.738(4) 90 117.644(8) 90 4201.2(17) 1.817 3.041 1.033 0.0571 0.1650 0.0852 0.1878

IAM16−4 298 Tb4O39C84H66 2334.98 monoclinic P21/c 2 8.621(1) 29.011(2) 18.632(1) 90 117.562(2) 90 4131.3(5) 1.870 3.479 1.053 0.0514 0.1492 0.0786 0.1683

IAM16−5 298 Dy4O39C84H66 2349.30 monoclinic P21/c 2 8.655(2) 29.042(6) 18.670(3) 90 117.618(6) 90 4158.1(14) 1.870 3.649 1.007 0.0603 0.1734 0.0942 0.1947

R1 = ∑∥Fo| − |Fc∥/∑|Fo|. bwR2 = {∑[w(Fo2 − Fc2)2/∑[w(Fo2)2]]1/2.

Synthesis of {[Eu4(obb)6(H2O)9]·(H2O)}∞ (IAM16−3). Eu(NO3)3· 6H2O (47.10 mg, 0.106 mmol), Ce(NO3)3·6H2O (11.46 mg, 0.026 mmol) and H2obb (30 mg, 0.12 mmol) were mixed in 5 mL of water and added to an aqueous solution of sodium hydroxide (0.01 mol/L, 1 mL). After that the mixture was placed in a Parr Teflon-lined stainless steel vessel (20 mL) under autogenous pressure and stirred at room temperature for 5 h. Then, the mixture was heated at 180 °C for 72 h. This was followed by slow cooling to room temperature at a rate of 2 °C h−1. After being washed with water and ethanol and air-dried, the colorless and transparent sheet crystals of IAM16−3 were obtained in 64.6% yield based on H2obb. Anal. Calcd (%) for Eu4O40C84H68: C, 43.69; O, 27.04; H, 2.86; Found: C, 43.46; O, 27.38; H, 2.87. FT-IR for IAM16−3 (v/cm−1): 3360(bs), 3051(bs), 2341(w), 1590(s), 1518(s), 1497(s), 1391(vs), 1299(w), 1252(s), 1202(shoulder, w), 1158(m), 1096(m), 1010(w), 877(m), 859(m), 805(w), 783(s), 760(m), 711(w), 693(w), 649(w), 622(m), 546(m), 510(m), 485(m), 412(w). Synthesis of {[Tb4(obb)6(H2O)9]·(H2O)}∞ (IAM16−4). Tb(NO3)3· 6H2O (29.9 mg, 0.066 mmol), Ce(NO3)3·6H2O (28.66 mg, 0.066 2346

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Figure 1. For IAM16−3: perspective view of coordination environments of (a) obb2− ligand and (b) Eu(III); (c) perspective view of the 2-D infinite layer with the 1-D infinite ···Eu−O−C−O−Eu··· inorganic chains, hightlighted by blue rectangle. mmol) and H2obb (30 mg, 0.12 mmol) were mixed in 5 mL of water and then added to an aqueous solution of sodium hydroxide (0.01 mol/L, 1 mL). Followed by the same synthesis processes of IAM16−3, the colorless and transparent sheet crystals of IAM16−4 were obtained in 53.9% yield based on H2obb. Anal. Calcd (%) for Tb4O40C84H68: C, 43.17; O, 26.72; H, 2.83. Found: C, 43.38; O, 27.07; H, 2.97. FT-IR for IAM16−4 (v/cm−1): 3360(bs), 3051(bs), 2341(w), 1590(s), 1518(s), 1497(s), 1391(vs), 1299(w), 1252(s), 1202(shoulder, w), 1158(m), 1096(m), 1010(w), 877(m), 859(m), 805(w), 783(s), 760(m), 711(w), 693(w), 649(w), 622(m), 546(m), 510(m), 485(m), 412(w). Synthesis of {[Dy4(obb)6(H2O)9]·(H2O)}∞ (IAM16−5). Dy(NO3)3· 6H2O (30.12 mg, 0.066 mmol), Ce(NO3)3·6H2O (28.66 mg, 0.066 mmol), and H2obb (30 mg, 0.12 mmol) were mixed in 5 mL of water and added to an aqueous solution of sodium hydroxide (0.01 mol/L, 1 mL). Followed by the same synthesis process of IAM16−3, the colorless and transparent sheet crystals of IAM16−5 were obtained in 64.6% yield based on H2obb. Anal. Calcd (%) for Dy4O40C84H68: Dy4O40C84H68: C, 42.91; O, 26.56; H, 2.81. Found: C, 42.79; O, 27.01; H, 2.79. FT-IR for IAM16−5 (v/cm−1): 3360(bs), 3051(bs), 2341(w), 1590(s), 1518(s), 1497(s), 1391(vs), 1299(w), 1252(s), 1202(shoulder, w), 1158(m), 1096(m), 1010(w), 877(m), 859(m), 805(w), 783(s), 760(m), 711(w), 693(w), 649(w), 622(m), 546(m), 510(m), 485(m), 412(w).

was obtained by applying H2obb to directly react with Ln(NO3)3 (Ce3+, Dy3+, and Tb3+) under the same synthesis conditions of IAM16−3 (Eu-MOF). Eu3+, Dy3+, and Tb3+ ions can be quickly coordinated by H2obb ligands to form uniform polycrystalline powder. However, the H2obb ligand tends to crystallize by itself from the reaction solution containing Ce(NO3)2. These results indicate that H2obb ligand prefers to coordinate to Eu3+, Dy3+, and Tb3+ ions rather than coordinate to Ce3+ ions. This is consistent with the Cambridge Crystallographic Data Centre data showing a few compounds constructed by H2obb and Eu3+, Dy3+, and Tb3+ ions, while no example of Ce3+ and H2obb is seen. Starting from this point, we added the second lanthanide nitrate Ce(NO3)3 as modulator to slow the reaction speed of the H2obb and lanthanide ions (Ln = Eu3+, Tb3+, and Dy3+) to get desirable isomorphic Ln-MOFs. As anticipated, the purity and yield of IAM16−3 (Eu3+) were greatly improved, and IAM16−4 (Tb3+) and IAM16−5 (Dy3+) were successfully yielded via adding the second lanthanide nitrate Ce(NO3)3 as modulator. To check whether the cerium ions are incorporated into IAM16−3, IAM16−4, and IAM16−5 samples and determine their oxidation state, the X-ray photoelectron spectroscopy (XPS) experiments were conducted. The XPS spectra show that neither Ce3+ nor Ce4+ ion is present in these samples (Figure S1). The single-crystal X-ray diffraction studies of these three samples show their data completeness achieving 99.6%. It further demonstrates that Ce3+/Ce4+ ions are not incorporated into these samples. Structure Descriptions of IAM16−3, IAM16−4, and IAM16−5. The single-crystal X-ray diffraction studies reveal that IAM16−3, IAM16−4, and IAM16−5 are isomorphic and crystallize in the centrosymmetric monoclinic space group P21/ c with six 4,4′-oxybis(benzoate) (obb2−) ligands, four Ln(III) atoms, and nine coordinating water molecules in the crystallo-

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RESULTS AND DISCUSSION Synthesis of IAM16−3, IAM16−4, and IAM16−5. The IAM16−3 can be synthesized through applying H2obb to react with Eu(NO3)3 in water by adjusting pH values under hydrothermal conditions, but the product yield is low and is calculated to be ∼5.2% based on H2obb. To increase the yield of IAM16−3 and to well investigate the photoluminescence property of IAM16−3, we tried to synthesize other isomorphic LnMOFs based on well-studied luminescent lanthanide(III) ions, such as yellow-emitting Ce(III), blue-emitting Dy(III), and green-emitting Tb(III). However, no anticipated product 2347

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Figure 2. (a) The infinite ···Eu−O−C−O−Eu··· inorganic chains bridged via hydrogen bonds (the hydrogen atoms only in the hydrogen bonds are shown); (b) schematic view of the topology framework of IAM16−3 (the atoms with gold, rose, orange, light blue, and sky blue colors represent 4connected obb2−-O3, 2-connected obb2−-O8, 4-connected obb2−-O13 ligands, 5-connected Eu1, and 5-connected Eu2 metal nodes, respectively); (c) illustration showing the stacking fashion of the layers.

2.562(5) Å, and the O−Eu1−O bond angles range from 51.7(2) to 158.8(2)°, while the Eu2−O bond lengths vary from 2.929(6) to 2.663(2) Å, and the O−Eu2−O bond angles range from 59.3(4) to 159.8(3)°. Table S1 demonstrates that the Ln(III)−O bond lengths and the O−Ln(III)−O bond angles of IAM16−3, IAM16−4, and IAM16−5 are comparable. O6 and O9 atom of obb2−-O8 are hydrogen-bound to H2wa and H5wa of coordinated water molecules (O2w and O5w), respectively. The formation of these hydrogen bonds enables IAM16−3 to possess the most stable conformation. The carboxylic groups of obb2−-O3 and obb2−-O13 alternately bridge adjacent Eu1 dimer and Eu2 dimer to generate an infinite [Eu2(COO)4]n chain (Figure 1c and Figure 2a), in which the adjacent Eu1···Eu1, Eu1···Eu2, and Eu2···Eu2 separations are 4.189(1), 5.284(1), and 4.637(1) Å, respectively. The resulting chains are further interconnected by the obb2− ligands to yield 2-dimensional (2-D) framework (Figure 1c,b viewed along a and b axes, respectively). If each Ln(III) ion acts as a 5-connected node, both obb2−-O3 and obb2-O13 ligands are regarded as 4-connected nodes, while obb2−-O8 ligand is considered as 2-connected node, and then the whole framework can be topologically represented as a typical (2,4,4,5,5)-connected net with a vertex symbol of: {4}{44· 62}{45·6}{45·64·8}2 (Figure 2b). Notably, the 2-D layers are packed in an ABAB fashion (Figure 2c), generating a threedimensional supramolecular framework via the strong hydrogen bonds between the hydrogen atom H1wa of coordinated water molecule and O4 of the adjacent nearest chains. Although H2obb has been widely used in the creation of various coordination polymers through mixed-ligands strategy by incorporating nitrogen-based ligands, to the best of our knowledge, the LnMOFs (Eu, Tb, and Dy) constructed from

graphic minimum asymmetric unit. We discuss the structure of IAM16−3 in detail and only mention the pertinent points of IAM16−4 and IAM16−5 for comparison. The coordination environments of obb2− and Eu(III) are presented in Figure 1a,b, respectively. These three crystallographically independent obb2− ligands display three different coordinating fashions. The obb2− containing O3 (obb2−-O3) adopts μ4−bridging mode by one μ2-η2:η1 chelate-bridging carboxylate group and the other μ2-η1:η1 bis(monodentate) coordination carboxylate group. The μ2-η2:η1 and μ2-η1:η1 carboxylate groups coordinate to two crystallographic identified Eu1(III) and two Eu2(III) ions, respectively. The obb2− containing O8 (obb2−-O8) adopts a μ2−bridging mode with two μ1-η1 monodentate coordination carboxylate groups that coordinate to Eu1(III) and Eu2a(III), respectively. The μ4−bridging obb2−-O13 (obb2− containing O13) encompasses two μ2-η1:η1 bis(monodentate) coordination carboxylate groups that are conjugated to one Eu1(III) and three crystallographically identified Eu2(III), respectively. As shown in Figure 1b, both Eu1(III) and Eu2(III) ions adopt dodecahedron geometric configuration. Eu1(III) ion is eight-coordinated by six oxygen atoms (O4, O5, O5a, O7, O11, O15) from five obb2− ligands and two oxygen atoms (O1w and O2w) of water molecules, while Eu2(III) ion is coordinated by five oxygen atoms (O1a, O2, O10, O12, O14) of five obb2− ligands and three oxygen atoms (O3w, O4w, and O5w) from water molecules. The crystallographic independence of Eu1(III) and Eu2(III) could be further confirmed by the highresolution emission spectra. Normally, under the excitation at 394 nm, only one peak at ∼579 nm can be observed for one Eu(III) ion; herein, two peaks centered at 578.1 and 578.8 nm, assigned to 5D0→7F0 transition of Eu3+, were clearly observed, which demonstrates two distinct Eu(III) ions in IAM16−3 (Figure S2). The Eu1−O bond lengths vary from 2.324(5) to 2348

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Figure 3. Solid-state emission spectra of IAM16−3 at various excitation wavelengths (a) from 254 to 330 nm and (b) from 365 to 420 nm at room temperature. (inset) Photoluminescent photos of IAM16−3 in the solid state excited at 254 and 405 nm by fluorescence spectrometer.

Figure 4. (a) The normalized solid-state emission spectra of IAM16−3 upon excitation at various wavelengths from 254 to 420 nm at room temperature, (b) the CIE chromaticity diagram [A (0.6400,0.3481): λex = 254 nm; B (0.5210, 0.3517): λex = 365 nm; C (0.4856, 0.3542): λex = 400 nm; D (0.4243, 0.3574): λex = 403 nm; E (0.3693, 0.3609): λex = 405 nm; F (0.3170, 0.3629): λex = 410 nm; G (0.2951, 0.3612): λex = 420 nm].

Figure 5. Solid-state emission spectra of IAM16−4 at various excitation wavelengths (a) from 254 to 290 nm and (b) from 310 to 390 nm at room temperature. (inset) Photos of IAM16−4 in the solid state excited at 254 and 365 nm with a 4 W hand-held UV lamp.

solid state at room temperature are shown in Figures 3−5, respectively. It is well-known that the nonradiative O−H, N− H, and C−H vibrations from organic ligands and water or other solvent molecules could deactivate the emissive states of Ln(III) ions, resulting in an outstandingly lower luminescence intensity for Ln(III) centers.42−48 Herein, both IAM16−3 and IAM16−4 contain O−H from water molecules; however,

H2obb as single ligand are scarce, according to the Cambridge Crystal Structural Database.39−41 Photoluminescence Properties. To well investigate the photoluminescence properties of IAM16−3 (Eu3+), we also studied that of the as-synthesized isomorphic LnMOFs, that is, IAM16−4 (Tb3+) and IAM16−5 (Dy3+). The photoluminescent spectra and photos of IAM16−3 and IAM16−4 in the 2349

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Figure 6. (a) Solid-state excitation spectra of IAM16−3 and IAM16−4 when monitoring the emission bands at ∼616 and ∼549 nm, respectively, at room temperature; (b) solid-state emission spectra of H2obb ligand at ambient temperature upon excitation at various wavelengths from 277 to 420 nm at room temperature.

demonstrate that this Eu-MOF contains two kinds of emissive centers. The intensity changes of the characteristic emission of Eu(III) ions, exicted at different wavelengths, are consistent with the excitation spectrum of IAM16−3, exhibiting a broad band ranging from 250 to 360 nm and a sharp band peaked at ∼397 nm overlapped with two weak bands peaked at 384 and 417 nm when monitoring the emission band at ∼616 nm (Figure 6a). To the best of our knowledge, the EWDP property of Eu-based LnMOFs is first reported up to now. Interestingly, the normalized photoluminescent spectra of IAM16−3 show that the intensity of the broad emission band gradually enhances at increasing excitation wavelengths ranging from 254 to 420 nm (Figure 4a). The corresponding CIE coordinate of each emission profile goes through red to green (Figure 4b). Remarkably, proper balance of the relative intensity of these four emissions results in white-light output, offering a CIE coordinate of (0.37, 0.36), which is close to that of pure white light (0.33, 0.33). This is the rare example of SCWLE of Eu-MOFs via a dichromatic strategy on the balance of simultaneous ligand-based and Eu-based emissions. The photoluminescence spectra of IAM16−4 at room temperature show the characteristic emission bands of Tb(III) ions without any shift at different excitation wavelengths from 254 to 390 nm. The intensity of these characteristic emission bands enhances along with the increasing excitation wavelengths until the excitation wavelength is up to 290 nm, and then it decreases quickly. It is consistent with its excitation spectrum when monitoring the emission band at ∼549 nm, which shows a broad band peaked at 294 nm (Figure 6a). The bands peaked at 493, 549, 591, and 625 nm are attributed to the 5D4→7FJ (J = 6, 5, 4, 3) transitions of Tb(III) ions, respectively. The CIE chromaticity diagram of IAM16−4 illustrates that IAM16−4 shows green emission with ignorable disparity upon different excitation at room temperature (Figure S3). As demonstrated in Figure 6b, H2obb ligand possesses interesting EWDP properties. At room temperature, the PL spectra of H2obb upon different excitation wavelengths (λex = 277, 290, 310, 330, 365, 420 nm) shift to longer wavelength when the excitation wavelength increases. As shown in Figure 7, the absorption spectra of H2obb ligand in ethanol solution and in solid state show that H2obb ligand can absorb light from 200 to 300 nm and from 200 to 360 nm, respectively. Noticeably, the solid-state optical absorption spectrum of IAM16−4 shows the light absorption band ranging from 200 to 650 nm, which should be assigned to the deprotonated H2obb (obb2−) ligand,

bright luminescence of the polycrystalline solid samples under moderate excitation were observed by naked eyes. As demonstrated in the photoluminescent photos in the solid state, IAM16−3 exhibits red and warm white luminescence, excited at 254 and 405 nm by fluorescence spectrometer (Figure 3), respectively. While IAM16−4 exhibts different bright green luminescence without color disparity (Figure 5), when excited at 254 and 365 nm with a 4 W hand-held UV lamp, respectively. This result indicates that IAM16−3 exhibits EWDP property, while IAM16−4 does not. Generally, for isomorphic LnMOFs, the luminescence of Dy-MOF (IAM16− 5) is weak and even invisible when excited both at 254 and 365 nm with a 4 W hand-held UV lamp. This is mainly induced by the narrow energy gap of Dy(III) (7384 cm−1), while the energy gap for Eu(III) and Tb(III) was estimated to be 12 260 and 14 842 cm−1, respectivley. The characteristic emission bands of IAM16−3 assigned to 5 D0→7FJ (J = 1, 2, 4) transitions of Eu(III) ions are presented in the emission profiles, while no emission peaks corresponding to the 5D0→7F0 and 5D0→7F3 transition of Eu(III) ions are clearly detected (Figure 3). The predominant emission band peaked at 616 nm is assigned to the electric dipole transition of 5 D0→7F2, which is hypersensitive to the coordination environment of the Eu(III) ions. The second strongest band centered at 593 nm is corresponding to the magnetic dipole transition of 5 D0→7F1, which is fairly insensitive to the environment of the Eu(III) ions, while the weakest emission peaked at 705 nm is attributed to the transition of 5D0→7F4. Obviously, the 5 D0→7F2 transition is stronger than the 5D0→7F1 transition, confirming that Eu(III) ions locate at a low symmetric coordination environment without an inversion center, which is in agreement with the result of the single-crystal X-ray diffraction of IAM16−3. Upon the increasing excitation wavelengths ranging from 254 to 330 nm, only the characteristic emission bands peaked at 593, 616, and 705 nm of Eu(III) ions were observed with their intensity gradually enhanced. However, when excited at 365 nm, the intensity of characteristic emission bands of Eu(III) ions declines, accompanied by the appearance of a broad emission band covering the range from 420 to 575 nm. The intensity of all emission bands increases along with the increasing excitation wavelengths from 365 to 400 nm. Noticeably, when excited at increasing wavelengths from 400 to 420 nm, the intensity of characteristic emission band significantly reduces, while the broad emission band demonstrates no intensity variation. These results 2350

DOI: 10.1021/acs.inorgchem.7b00074 Inorg. Chem. 2017, 56, 2345−2353

Article

Inorganic Chemistry

transferred to Tb3+ ions to comply the characteristic emission of Tb3+ ions, inducing no emission of ligand detected. The time decay curves of the strongest emission bands peaked at 616 nm (5D0→7F2 transition of Eu3+) and 549 nm (5D4→7F5 transition of Tb3+) for the as-synthesized IAM16−3 and IAM16−4, respectively (Figure S5), can be fitted to diexponential decay (correlation constants > 0.98) with long multicomponent fluorescent lifetimes (τ1 = 14.99 μs, 1%, τ2 = 3.03 μs, 99% for IAM16−3 and τ1 = 16.37 μs, 1.3%, τ2 = 3.63 μs, 98.7% for IAM16−4). For IAM16−3, the absolute luminescence quantum yield of the red light emission excited at a wavelength of 365 nm was measured to be 48.6%, while for IAM16−4, that of the green light emission at an excitation wavelength of 290 nm was measured to be 43.3%. Thermogravimetric Analysis (TGA) and X-ray Powder Diffraction (XRPD). The structures of IAM16−3, IAM16−4, and IAM16−5 were characterized via XRPD (Figures S6−S8), FT-IR spectra (Figure S9), and TGA (Figure S10), respectively. The XRPD patterns measured for the as-synthesized samples and the dry sample obtained from as-synthesized crystals of IAM16−3 after being heated at 523 K under vacuum for 3 h are in good agreement with the XRPD patterns simulated from the single-crystal X-ray data. This result illuminates that the frameworks of these three samples can be maintained after removal of coordinating water molecules. For the thermogravimetric analysis (TGA), it was performed in a flow of nitrogen atmosphere. As shown in Figure S10, the TGA curves are quite similar, which is consistent with the isomorphic configuration of these samples. They are thermally stable up to 131 °C and lost a total weight in two distinct stages in the range from 131 to 800 °C. The first mass loss of 7.45, 7.13, and 7.56 in the temperature range of 131−186 °C, corresponding to the removal of lattice water and coordinated water, is comparable to the calculated values 6.84, 6.88, 6.96% for IAM16−3, IAM16−4, and IAM16−5, respectively. When the temperature reaches 492 °C, the second weight loss attributed to the decomposition of organic obb2− ligand was observed.

Figure 7. Optical absorption spectra of H2obb ethanol solution, H2obb in solid state, IAM16−3 and IAM16−4 crystalline samples.

as Tb3+ cannot absorb light by itself. These results clearly demonstrate that the absorption of H2obb ligand is dependent on its aggregation states and coordination environments. Consequently, the broad light absorption band of IAM16−3 ranging from 200 to 750 nm can also be assigned to obb2−, while the sharp peak at 394 nm is assigned to the inherent absorption of Eu3+ ions (7F0→5L6). The differences between the absorption spectra of IAM16−3 and IAM16−4 should be induced by different coordination Ln3+ ions and/or a trace amount of organic impurity, that is, H2obb incorporated in the IAM16−3 sample. And the broad bands peaked at 294 and 322 nm in the excitation spectra of IAM16−3 and IAM16−4, respectively, should be attributed to the excitation of obb2− ligand. These results indicate that the sharp characteristic Eu3+ emission of IAM16−3, excited at the wavelengths smaller than 360 nm, should be induced by the sensitizing Eu(III) ions through the “antenna effect” of obb2− ligand. And the characteristic Tb(III) luminescence of IAM16−4 originate from sensitizing Tb(III) ions through the “antenna effect” of obb2− ligand. The broad emission bands in Eu3+-MOF (IAM16−3) are quite similar to that of ligand (Figure 6b), under the excitation ranging from 365 to 420 nm. Furthermore, when monitoring the emission at ∼488 nm, the broad band ranging from 315 to 430 nm was detected in the excitation spectra of both H2obb and IAM16−3, and there was a peak centered at 376 nm observed in that of H2obb (Figure S4). The similar broad emission bands and the comparable range of the broad excitation band of IAM16−3 and H2obb, as well as the broad light absorption ability of IAM16−3 (from 200 to 750 nm, Figure 7), all demonstrate that the broad excitation-wavelengthdependent photoluminescence emission bands of IAM16−3 can be assigned to ligand-based emission. Importantly, the sharp band peaked at ∼397 nm overlapped with two weak bands peaked at 384 and 417 nm in the excitation spectra of IAM16−3 is assigned to the characteristic excitations 7F0→5G6 (384), 7F0→5L6 (397), and 7F0→5D3 (417) of Eu3+ ions, respectively. Therefore, when IAM16−3 is excited at the wavelengths larger than 360 nm, the characteristic emissions of Eu3+ ions mainly originate from the energy harvested by Eu3+ ions themselves, while the energy absorbed by obb2− ligand primarily emits intrinsic ligand-centered luminescence. Differently, in IAM16−4, the absorbed energy of obb2− upon different excitation wavelengths ranging from 254 to 390 nm is

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CONCLUSIONS

The first example of the corlor-tunable SC-WLE Eu-MOF has been successfully synthesized via self-ssembly of the flexible ligand H2obb and Eu(III) ions. It is noteworthy that this EuMOF possesses fascinating EWDP property; accordingly, it shows WLE via tuning the intensity ratio of emissions from Eu(III) ions and metal-to-ligand charge transfer transtion at different excitation wavelengths. This work illuminates that the LnMOFs constructed by flexible ligand can possess EWDP property, further enabling LnMOFs to show WLE via a dichromatic strategy on the balance of the simultaneous ligandbased and lanthanide-based emissions. Unlike the Eu-MOF (IAM16−3), the isomorphic Dy-MOF (IAM16−5) displays very weak photoluminescence. However, the broad band (440 to 575 nm) that is attributed to ligand-based emission in the photoluminescent spectra of IAM16−3 is absent in IAM16−4 (Tb-MOFs). These results demonstrate that the H2obb ligand plays different roles on the photoluminescence properties of these three isomorphic LnMOFs. This work paves a new way for exploitation of SC-WLE LnMOFs via rational design strategy. 2351

DOI: 10.1021/acs.inorgchem.7b00074 Inorg. Chem. 2017, 56, 2345−2353

Article

Inorganic Chemistry

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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.7b00074. Experimental and simulated X-ray powder diffraction patterns, Ce(3d) XPS spectra of IAM16−3, IAM16−4, and IAM16−5, the CIE chromaticity diagram of IAM16−4, the high-resolution solid-state emission spectrum of IAM16−3 at excitation wavelength 394 nm at room temperature, FT-IR spectra, time decay curves, TGA profiles of crystalline IAM16−3, IAM16−4, and IAM16−5 (PDF) X-ray crystallographic information (CIF) X-ray crystallographic information (CIF) X-ray crystallographic information (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (Z.F.) *E-mail: [email protected]. (W.H.) ORCID

Qiang Ju: 0000-0002-8521-845X Wei Huang: 0000-0001-7004-6408 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (973 Program, No. 2015CB932200), National Natural Science Foundation of China (61136003, 51173081, 21501089, and 61505077), and Natural Science Foundation of Jiangsu Province, China (BM2012010, BK20150936, and BK20150939).

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