Luminescence Modulation, White Light Emission, and Energy Transfer

Jul 6, 2017 - Luminescence Modulation, White Light Emission, and Energy Transfer in a Family of Lanthanide Metal–Organic Frameworks Based on a ...
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Luminescence Modulation, White Light Emission, and Energy Transfer in a Family of Lanthanide Metal−Organic Frameworks Based on a Planar π‑Conjugated Ligand Xiu-Yuan Li, Wen-Juan Shi, Xiao-Qing Wang, Li-Na Ma, Lei Hou,* and Yao-Yu Wang Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an 710127, P. R. China S Supporting Information *

ABSTRACT: Three isostructural lanthanide metal−organic frameworks (LnMOFs), {[Ln2(BDPO)1.5(DMA)3(H2O)]·5H2O }n (1-Ln, Ln = Eu, Gd, and Tb) were constructed by a less-investigated large delocalized π-electron conjugated ligand N,N′-bis(3,5-dicarboxyphenyl)-oxalamide (H4BDPO). 1-Ln reveals unprecedented trinodal (4,4,6)-connected networks based on binuclear cluster units and planar BDPO linkers. Density functional theory (DFT) and time dependent-DFT calculations were performed on the ground and excited states to provide insight into geometries, the frontier molecular orbitals, and singlet and triplet state energies of H4BDPO ligand. 1-Eu and 1-Tb exhibit strong typical red and green emission of Eu3+ and Tb3+ ions, respectively, originating from the highly efficient energy transfer from BDPO to metal centers, as demonstrated by the comparison of triplet excited energies of ligands and metal ions. Importantly, by doping different concentrations of Eu3+ and Tb3+ ions, a series of dichromatic doped 1-EuxTb1‑x MOFs were fabricated, showing an unusual fluent change of luminescent color among green, yellow, orange, and red. Meanwhile, the trimetallic doped 1-Eu0.0855Gd0.6285Tb0.2860 emits white light, with the correlated color temperature of 5129 K as well as an absolute quantum yield of 22.4%.

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materials.24−27 However, the related reports are presently very sporadic, the development of Ln-MOFs-based WLEDs achieved by controlling the doping amounts of different Ln3+ ions is an appealing and important project. As we know, Laporte forbidden f−f electronic transitions of Ln3+ ions because of low molar absorption coefficients lead to very weak emissions of Ln3+ ions excited directly.28,29 In LnMOFs, the ligands with appropriate chromophores can act as antennas or sensitizers to increase light absorption and transfer energy to Ln3+ ions efficiently through “antenna effect”.30−32 So the design of organic ligands with π-electron conjugated systems is crucial to strengthen the luminescence of LnMOFs.33−37 For this target, a less-investigated rigid N,N′bis(isophthalic acid)-oxalamide (H4BDPO) ligand is noticed for the following considerations:38,39 (1) H4BDPO can be regarded as a diisophthalic acid linker separated by an oxalamide spacer, which would prefer coordinating with multiple Ln3+ ions to produce high-dimensional architectures; (2) both the phenyl rings and oxalamide unit in H4BDPO are typical π-conjugated systems; meanwhile, these two motifs are almost coplanar (Figure S1) and form large delocalized πelectron conjugated system, which endows a strongly absorbing chromophore; (3) TD-DFT calculations confirmed that the

INTRODUCTION Lanthanide metal−organic frameworks (Ln-MOFs) have received growing attention for superior properties and great application prospect in widespread fields, such as luminescence,1−3 molecular magnetism,4 gas storage,5,6 proton conductivity,7 etc. In particular, trivalent lanthanide ions (Ln3+) are well-known as promising candidates for preparing unique luminescent MOF materials owing to their high luminescent quantum yield, narrow and intense band emission, large Stokes shift, long luminescent lifetime, and undisturbed emission wavelength.8−12 White-light-emitting diodes (WLEDs) considered as fourthgeneration light sources have been widely used in displays and lightings,13−16 which profit from their energy saving, friendly to environment, long lifetime, and high efficiency.17 For the sake of attaining white light luminescent modulation, it requires introducing dichromatic or trichromatic (RGB) strategy in bulk materials.18,19 Due to very similar coordination environments of Ln3+ ions, the isostructural frameworks are usually formed for varied Ln3+ ions with the same organic ligands.20 Therefore, not only the different types of Ln3+ ions can be incorporated into the same MOF but also the ratios of Ln3+ ions in a MOF can be tuned by adjusting the adding amounts of Ln3+ ions in reaction.21−23 This fact suggests that the luminescent colors of Ln-MOFs can be tuned. Consequently, the doping of different Ln3+ ions into isostructural MOFs has become a burgeoning and remarkable approach to produce white phosphor © XXXX American Chemical Society

Received: April 14, 2017 Revised: May 19, 2017

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DOI: 10.1021/acs.cgd.7b00530 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Table 1. Crystal Data and Structure Refinement for Complexes

a

complex

1-Eu

1-Gd

1-Tb

molecular formula formula weight temperature crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρ (g/cm3) F (000) reflections collected goodness-of-fit on F2 R1a [I > 2σ(I)] wR2b [I > 2σ(I)]

C39H51N6O24Eu2 1291.70 296(2) monoclinic P21/c 16.472(3) 17.297(3) 20.853(3) 90 109.499(3) 90 5600.4(15) 4 1.425 2380 27559 0.930 0.0562 0.1370

C39H51N6O24Gd2 1302.28 296(2) monoclinic P21/c 16.437(5) 17.223(5) 20.740(7) 90 109.584(5) 90 5532(3) 4 1.456 2388 22205 0.919 0.0608 0.1393

C39H51N6O24Tb2 1305.62 296(2) monoclinic P21/c 16.4258(16) 17.2439(18) 20.743(2) 90 109.465(2) 90 5539.5(10) 4 1.458 2396 27590 1.080 0.0553 0.1442

R1 = ∑(|F0| − |Fc|)/∑|F0|. bwR2 = [Σw(F02 − Fc2)2/Σw(F02)2]1/2. 31G(d) method in density functional theory (DFT). The timedependent (TD)-DFT, B3LYP/6-31G(d) level was adopted to calculate the excitation energy (T1 ← S0). The minimum nature of molecule was determined by frequency calculations. Synthesis of {[Eu2(BDPO)1.5(DMA)3(H2O)]·5H2O}n (1-Eu). A mixture of H4BDPO (20.8 mg, 0.05 mmol) and EuCl3·6H2O (36.6 mg, 0.1 mmol) in DMA (4 mL), MeCN (1 mL), and distilled water (0.5 mL) was placed into a glass vial (10 mL) and heated at 105 °C for 72 h. The vial was then cooled to room temperature at a rate of 5 °C h−1 to afford the colorless block-shaped crystals of 1-Eu in 52% yield. Anal. Calcd for C39H51N6O24Eu2: C, 36.23; H, 3.95; N, 6.50. Found: C, 36.12; H, 4.07; N, 6.63. IR data (KBr, cm−1): 3367(m), 3280(w), 3066(w), 2930(w), 1689(s), 1610(s), 1437(s), 1379(s), 1257(m), 1182(m), 1103(w), 1022(m), 847(w), 785(m), 715(m), 596(w), 499(w), 432(w). Synthesis of {[Gd2(BDPO)1.5(DMA)3(H2O)]·5H2O}n (1-Gd). The procedure was the same as that for 1-Eu, except that EuCl3·6H2O was replaced by GdCl3·6H2O (37.2 mg, 0.10 mmol). Colorless blockshaped crystals of 1-Gd were obtained in 56% yield. Anal. Calcd for C39H51N6O24Gd2: C, 35.94; H, 3.92; N, 6.45. Found: C, 35.69; H, 3.98; N, 6.52. IR data (KBr, cm−1): 3370(m), 3283(w), 3069(w), 2928(w), 1686(s), 1613(s), 1438(s), 1376(s), 1258(m), 1180(m), 1102(w), 1021(m), 845(w), 786(m), 717(m), 595(w), 501(w), 430(w). Synthesis of {[Tb2(BDPO)1.5(DMA)3·(H2O)]·5H2O}n (1-Tb). The procedure was the same as that for 1-Eu, except that EuCl3·6H2O was replaced by TbCl3·6H2O (37.3 mg, 0.10 mmol). Colorless blockshaped crystals of 1-Tb were obtained in 61% yield. Anal. Calcd for C39H51N6O24Tb2: C, 35.85; H, 3.91; N, 6.43. Found: C, 35.66; H, 3.75; N, 6.55. IR data (KBr, cm−1): 3365(m), 3279(w), 3064(w), 2932(w), 1691(s), 1612(s), 1436(s), 1378(s), 1256(m), 1181(m), 1102(w), 1021(m), 845(w), 783(m), 714(m), 598(w), 497(w), 431(w). Synthesis of 1-EuxTb1‑x. The Eu/Tb bimetallic doped frameworks were prepared by the same procedure as that for 1-Eu in which the total molar amounts of Eu3+ and Tb3+ ions were kept the same as that for 1-Eu, and the Eu3+ ion molar amounts in reaction occupy 10−90% with respect to the total molar amounts. Synthesis of 1-Eu0.0855Gd0.6285Tb0.2860. The Eu/Gd/Tb trimetallic doped frameworks were prepared by the same procedure as that for 1-Eu, except that EuCl3·6H2O was replaced by mixed lanthanide salts of 10% EuCl3·6H2O, 60% GdCl3·6H2O, and 30% TbCl3·6H2O, while the total amount of metal ions equals 0.1 mmol. The molar ratio

energy gap between the singlet and triplet excited states of H4BDPO falls within the range of efficient energy transfer for the intersystem crossing (ISC) process, which satisfies the first prerequisite for sensitizing the luminescence of Ln3+ ions. These intrinsic features suggest H4BDPO to be an excellent linker and luminescent sensitizer in Ln-MOFs. In this presentation, three isostructural Ln-MOFs, {[Ln2(BDPO)1.5(DMA)3(H2O)]·5H2O}n (1-Ln, Ln = Eu, Gd, and Tb), based on H4BDPO have been constructed by solvothermal procedure. Furthermore, a class of novel bimetallic Eu/Tb and trimetallic Eu/Gd/Tb doped MOFs bearing modulated luminescence and white light emission were obtained by tuning the ratios of different Ln3+ ions in MOFs. The o btained white light emission ma terial 1Eu0.0855Gd0.6285Tb0.2860 shows the high absolute quantum yield of 22.4%, the CIE color coordinate value of (0.34, 0.33), and the correlated color temperature (CCT) of 5129 K. The crystal structures and luminescent mechanism and color tuning of complexes were investigated in detail.

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EXPERIMENTAL SECTION

Materials and General Methods. All reagents and solvents employed were commercially available and used without further purification. H4BDPO was synthesized by a modified literature method.39 Fourier transform infrared spectrum was determined with a Nicolet FT-IR 170 SX spectrophotometer in the range 4000−400 cm−1. Elemental analyses for C, H, and N were performed with a PerkinElmer 2400C Elemental analyzer. Solid-state photoluminescence measurement was performed on an Edinburgh FLS920 fluorescence spectrometer. Quantum efficiency was measured using the integrating sphere on a FluoroMax-4 spectrophotometer. Inductively coupled plasma (ICP) spectroscopy was performed on an Agilent 725 ICP-OES spectrometer. Thermogravimetric analysis (TGA) was carried out with a NETZSCH TG 209 thermal analyzer under a nitrogen atmosphere with a heating rate of 10 °C min−1. Powder X-ray diffraction (PXRD) pattern was recorded on a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα, 1.5418 Å). Electronic Structure Calculations. Single-crystal structure data of H4BDPO was used for theoretical calculations using Gaussian 09 program.40 The geometry optimization and calculation of the HOMO and LUMO energy levels of molecules were performed by B3LYP/6B

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process by referring to the Reinhoudt’s empirical rule.3,44 Therefore, an energy transfer from singlet and triplet excitation states in H4BDPO can easily take place. This fact associating with good π-electron conjugated system demonstrate the potential of H4BDPO as an excellent luminescent sensitizer in Ln-MOFs. Crystal Structure of {[Ln2(BDPO)1.5(DMA)3(H 2O)]· 5H2O}n (1-Ln, Ln = Eu, Gd, and Tb). Three 1-Ln MOFs are isostructural with the same monoclinic P21/c space group and show the dinuclear cluster-based 3D networks (Figure S3). The structure of 1-Eu is herein discussed. The asymmetric unit of 1-Eu contains two Eu3+ ions, one and a half fully deprotonated BDPO ligands, and three coordinated DMA molecules (Figure 1a). Eu1 is eight-coordinated by six carboxylate oxygen atoms from four BDPO ligands [Eu−O = 2.488(6)−2.307(5) Å], one water molecule [Eu−O = 2.395(7) Å], and one DMA ligand [Eu−O = 2.408(7) Å], forming a distorted dodecahedron coordination geometry. Eu2 also adopts a distorted dodecahedron coordination sphere defined by eight oxygen atoms including six carboxylate oxygen atoms from four BDPO [Eu−O = 2.342(7)−2.561(5) Å] and two oxygen atoms from two DMA molecules [Eu−O = 2.301(8)− 2.361(9) Å]. In 1-Eu, the phenyl, oxalamide, and four carboxylate groups of BDPO are almost coplanar with the dihedral angles of 6.3°−15.9° for adjacent groups, which suggests a delocalized π-electron conjugated system, similar to the situation of free H4BDPO molecule. BDPO in 1-Eu exists two kinds of coordination modes: BDPO-I connects six Eu3+ ions through four carboxylate groups with bidentate chelating and bidentate bridging modes, respectively (Figure S4a); while BDPO-II links four Eu3+ ions by four carboxylate groups with the same bidentate chelating modes (Figure S4b). One Eu1 and one Eu2 are bridged by two carboxylates from two BDPO-I to form a dinuclear cluster, which is connected by BPDO-I to produce a wave-like layer paralleling to the ac plane (Figure 1b). The neighboring layers are further extended by BDPO-II to give rise to a 3D framework (Figure 1c). Topologically, one cluster linking six BDPO can be regarded as a 6-connected node, while BDPO serves as a 4-connected node. Thus, 1-Eu forms an unprecedented trinodal (4,4,6)-connected network with the point symbol of (42.84)(45.6)2(46.66.83)2 calculated by TOPOS (Figure 1d).45 PXRD and TGA. The experimental PXRD patterns of 1-Ln match well with those simulated, indicating the phase purities of samples (Figure S5). Meanwhile, PXRD patterns of all bimetallic and trimetallic doped samples were found very similar to the simulated pattern of 1-Eu (Figure S6), demonstrating that they are isostructural with 1-Eu (Figure S7). 1-Ln shows the gradual weight loss corresponding to all coordinated and lattice water molecules (calcd. 8.4%, obsed. 9.0% for 1-Eu; calcd. 8.3%, obsed. 8.5% for 1-Gd; calcd. 8.3%, obsed. 7.6% for 1-Tb) before 155, 150, and 165 °C, respectively. The second weight loss appears before 300, 300, and 340 °C, respectively, resulting from the removal of coordinated DMA molecules (calcd. 20.2%, obsed. 19.2% for 1Eu; calcd. 20.1%, obsed. 19.0% for 1-Gd; calcd. 20%, obsed. 18.8% for 1-Tb). The further rising temperature leads to the framework decomposition. Luminescence Properties. The solid-state luminescence of free H4BDPO ligand and complexes were examined at ambient temperature. The excitation spectra for H4BDPO and 1-Ln are given in Figure S8. H4BDPO displays luminescence at λem = 485 nm (λex = 352 nm), attributing to the intraligand π

of different Ln3+ ions in 1-Eu0.0855Gd0.6285Tb0.2860 was confirmed by ICP spectroscopy, matching well with that of the starting LnCl3 reactants. Model of White Light-Emitting LED. The coating solution was prepared by dispersing samples of 1-Eu0.0855Gd0.6285Tb0.2860 in ethyl acetate under ultrasonication until a homogeneous suspension was obtained. A commercial ultraviolet LED (385 nm) was dipped into the suspension solution several times until a uniform film formed on the surface of bulb.41 Crystallography. Single crystal X-ray diffraction analyses were carried out on a Bruker SMART APEXII CCD diffractometer equipped with a graphite monochromated Mo Kα radiation source (λ = 0.71073 Å) at 296(2) K. The structures were solved by direct methods and refined on F2 by full-matrix least-squares methods using the SHELXTL program package.42 All non-hydrogen atoms were refined anisotropically with the hydrogen atoms added to their geometrically ideal positions and refined isotropically. The disordered lattice water molecules in 1-Ln cannot be well identified, so the SQUEEZE routine of PLATON program43 was adopted in structural refinement. Crystal data and structure refinements for 1-Ln and selected bond distances and angles are listed in Tables 1 and S1, respectively.

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RESULTS AND DISCUSSION Ligand Calculation. The sensitization of Ln3+ ions triggered by organic “antenna” ligands is a prime and important procedure to increase the luminescence of Ln-MOFs. A good match between the energies of singlet (S1) and triplet (T1) excited states in organic ligands is required for an efficient ISC process that is the necessary first step for energy transfer from ligands to Ln3+ centers in “antenna effect” process (Scheme 1). Therefore, the design of ligands with approximate energy levels of excited states is crucial for luminescent Ln-MOFs. Scheme 1. Schematic Representation of the Energy Adsorption, Transfer, and Emission Processes of Lanthanide Complexes

In H4BDPO, both the two outer isophthalic acid units and the central oxalamide unit possess good planarity; meanwhile, the adjacent units form a small dihedral angle of 29.4° (Figure S1), and therefore, the whole molecule shows a large and planar delocalized π-electron conjugated system. DFT and TD-DFT calculations revealed that in H4BDPO the HOMO − 2 → LUMO and HOMO − 2 → LUMO + 4 transitions dominantly contribute to the electronic excitations (98.5%) (Figure S2 and Table S2). The singlet excitation energy gap ΔE(H−L) of ligand was calculated to be 4.69 eV, which corresponds to the difference between HOMO and LUMO energy levels (−6.83 and −2.14 eV). The triplet excitation energy (T1 ← S0) was determined to be 2.29 eV. Accordingly, the energy difference between ΔE(H−L) and (T1 ← S0) is 2.40 eV (1.94 × 104 cm−1), which is close to the 4-fold value (5000 cm−1) of ISC C

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Figure 1. (a) Coordination environment of Eu3+ ions in 1-Eu; (b) 2D layer; (c) 3D framework; (d) (4,4,6)-connected topology of 1-Eu.

Figure 2. Solid state emission spectra of 1-Gd (a), 1-Eu (b), and 1-Tb (c) at room temperature. The inset shows the corresponding luminescence picture under UV-light irradiation at 365 nm.

Table 2. Luminescent Lifetimes, Absolute Quantum Yield, and CIE Color Coordinate of 1-Eu, 1-Gd, 1-Tb, and Doped LnMOFs τ1 (μs) 1-Eu 1 2 3 4 5 6 7 8 9 1-Tb 1-Gd 1-Eu0.0855Gd0.6285Tb0.2860

723.19 892.66 835.49 784.67 749.55 721.75 693.52 660.84 626.11 629.85 943.29 1.94 × 10−3 297.48

τ2 (μs)

φ (%) 23.2

5.0 7.11 × 10−3 749.54

→ π* charge transfer (Figure 2a). 1-Gd presents a broadly ligand-based emission band centered at 485 nm (λex = 412 nm) (Figure 2a), which possesses a double-exponential luminescent decay with the lifetimes of 1.9 and 7.1 ns (Table 2).

22.4

CIE color coordinates (0.67, (0.38, (0.47, (0.54, (0.57, (0.59, (0.62, (0.63, (0.64, (0.65, (0.33, (0.25, (0.34,

0.33) 0.53) 0.46) 0.43) 0.40) 0.37) 0.35) 0.34) 0.33) 0.33) 0.56) 0.43) 0.33)

λex (nm) 394 378 378 378 378 378 378 378 378 378 378 412 390

Upon excitation at 394 nm, 1-Eu emits the bright characteristic red color of Eu3+ ions with the CIE color coordinate of (0.67, 0.33). The emission peaks at 579, 595, 615, 651, and 703 nm are assigned to 5D0 → 7FJ (J = 0, 1, 2, 3, 4) D

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Figure 3. (a) Solid state emission spectra of doped 1-EuxTb1‑x with different molar ratios when excited at 378 nm; (b) the CIE chromaticity diagram for 1-Eu, 1-Tb, and doped 1-EuxTb1‑x; (c) the luminescence picture of 1-EuxTb1‑x under UV-light irradiation at 365 nm; (d) the energy transfer efficiency from Tb3+ to Eu3+ ions in 1-EuxTb1‑x.

transitions of Eu3+ ions, respectively (Figure 2b). The electric dipole transition 5D0 → 7F2 dominates the red color emission. 1-Tb excited at 378 nm displays narrow peaks at 491, 543, 583, and 619 nm, corresponding to the characteristic transitions of 5 D4 → 7FJ (J = 6, 5, 4, 3) of Tb3+ ions (Figure 2c). The strongest peak at 543 nm produces the green color emission for 1-Tb with the CIE color coordinate of (0.33, 0.56). Notably, the ligand-centered emission is completely quenched in the emission spectra of 1-Eu or 1-Tb, implying the energy transfer from BDPO to the Eu3+ or Tb3+ ions via “antenna effect”. 1-Eu and 1-Tb display the similar single-exponential luminescent decay with the lifetimes of 723.19 and 943.29 μs, respectively (Table 2), and the corresponding absolute quantum yields were found to be 23.2% and 5.0%. Energy Transfer. The luminescent intensity of Ln-MOFs is closely dependent on the energy transfer efficiency involving not only the ISC process in ligands but also the triplet state of ligand to resonance level of Ln3+ ions. The efficient ISC process in H4BDPO has been demonstrated by theoretical calculation. To further interpret the energy transfer process in 1-Ln, the triplet excited energy-level T1 of H4BDPO was calculated by solid-state phosphorescence of 1-Gd measured at 77 K. As the lowest excited state of Gd3+6P7/2 is too high to accept energy from the triplet excited energy-level T1 of ligand, the phosphorescence of 1-Gd in fact results from ligand.46 The phosphorescent spectrum of 1-Gd at 77 K presents the maximum emission peak at 480 nm (20833 cm−1) excited at 335 nm (Figure S9), corresponding to the triplet excited energy-level T1 of H4BDPO. This observed energy level of H4BDPO significantly exceeds the 5D0 level of Eu3+ ion (17300 cm−1) while slightly surpasses the 5D4 level of Tb3+ ion (20500 cm−1). Basing on Latva’s empirical rule,47 energy transfer

processes happened in 1-Eu and 1-Tb, moreover, which is more effective in 1-Eu than 1-Tb. This result can be further confirmed by the higher absolute quantum yield of 1-Eu (23.2%) relative to 1-Tb (5.0%). The energy gap ΔE(T1−5D4) = 333 cm−1 for Tb3+ ions is too small to avoid a back energy transfer from the 5D0 level of Tb3+ ions to the triplet state of H4BDPO, leading to a low absolute quantum yield of 1-Tb.48 In addition, the highly efficient energy transfer in 1-Eu can also be attributed to the good planarity of BDPO ligands, which behaving as a delocalized π-electron conjugated system sensitizes the emission of Eu3+ ions. Tuning of Luminescent Color for Bimetallic Doped Eu/Tb-MOFs. 1-Eu, 1-Gd, and 1-Tb are isostructural, which provides possibility to synthesize heterometallic frameworks. A series of bimetallic-doped 1-Eux/Tb1‑x frameworks were obtained successfully by tuning different molar ratios of Eu3+ and Tb3+ reactants, and the molar ratio of Eu3+ and Tb3+ ions was determined by means of ICP spectroscopy (Table S3). The heterometallic complexes display the dual emissions of Eu3+ and Tb3+ ions in luminescent spectra. With the increase of Eu3+/Tb3+ molar ratios from 1:9 to 9:1 in reaction, the emission intensity of Tb3+ ion at 5D4 → 7F5 (543 nm) descends gradually, while that of Eu3+ ion at 5D0 → 7F2 (615 nm) ascends stepwise (Figure 3a). Importantly, the mixture of red and green emissions resulted from Eu3+ and Tb3+ ions, respectively, gives rise to a fluent change of emission colors from green to yellow, orange, and red for the doped 1-Eux/Tb1‑x (Figure 3b). Under UV 365 nm lamp, this color change is very obvious through naked-eye observation (Figure 3c), indicating the luminescent colors of 1-Eux/Tb1‑x can be systematically modulated by adjusting different contents of Tb3+ and Eu3+ ions, this unique change of luminescent color was extremely rarely reported in E

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Figure 4. Solid state emission spectra of 1-Eu0.0855Gd0.6285Tb0.2860 with the excitation wavelength at 378 nm (a) and varying from 350 to 400 nm (b); (c) the CIE chromaticity diagram for 1-Eu0.0855Gd0.6285Tb0.2860 excited from 350 to 400 nm, the inset shows the 385 nm ultraviolet LED coated with 1-Eu0.0855Gd0.6285Tb0.2860 as phosphor.

Ln-MOFs.49−51 In Figure 3b, points 1−9 represent CIE color coordinates ranging from (0.38, 0.53) to (0.65, 0.33) for the doped Eu/Tb-MOFs (Table 2). As for the Eu/Tb doped MOFs, the energy transfer efficiency from Tb3+ to Eu3+ ions can be calculated by equation: ηET = 1 − τ/τ0 in which τ and τ0 are the excited-state lifetimes of a donor in the presence and absence of Eu3+ acceptor, respectively (Table 2).52 The values of ηET raise with the increase of Eu3+ molar amounts in the range of 10−80%. When the Eu3+ molar amount is increased to 90%, the value of ηET is almost constant (Figure 3d). WLED for Trimetallic Doped Eu/Gd/Tb-MOF. WLEDs have recently been intensively explored because of their unique applications as optical devices. The above-achieved strong and tunable emission colors of Eu/Tb-doped MOFs inspire us to generate a white luminescent material. To achieve this goal, a blue light is required to adjust the mixed red and green lights. Owing to the lowest excited state of Gd3+6P7/2 is too high to accept energy from the ligand, 1-Gd shows the broad blueemission stem from the BDPO ligand. So the Gd3+, Eu3+, and Tb3+ ions with varied ratios were added into the reaction system, obtaining the three-component Eu/Gd/Tb-MOFs, in which the Gd3+ ions dilute the emission of Eu3+ and Tb3+ ions, meanwhile, enhance the intensity of ligand emission peak. As expected, the emission output of Eu/Gd/Tb-MOFs can be tuned precisely through compositional adjustment of three metal ions. Remarkably, by optimizing the molar ratio of Eu3+, Gd 3+ , and Tb 3+ ions, the mixed-lanthanide MOF 1Eu0.0855Gd0.6285Tb0.2860 was synthesized successfully. On excitation at 378 nm, this three-component framework simultaneously features both the characteristic emissions of Eu3+ and Tb3+ ions, as well as the broad blue emission of BDPO ligand with the CIE color coordinate of (0.34, 0.37)

(Figure 4a), which is close to the ideal coordinate for pure white light (0.33, 0.33). Therefore, the emission output of 1Eu0.0855Gd0.6285Tb0.2860 was further investigated by varying excitation wavelength (Figure 4b). As the excitation wavelength augments from 350 to 400 nm, the CIE color coordinate (0.34, 0.33) excited at 390 nm is very close to that of pure white light (0.33, 0.33) (Figure 4c), and the correlated color temperature (CCT) was calculated to be 5129 K; thus, this material can be considered as a cold white-light source.53 The measured luminescent decay lifetimes are 297.48 and 749.54 μs (Table 2). The absolute quantum yield of 22.4% is relatively higher compared to the most reported white-light-emitting doped MOFs (Table S4). Interestingly, the white-light-emitting 1Eu0.0855Gd0.6285Tb0.2860 can be coated as phosphor on surface of the commercial 385 nm ultraviolet LED, as shown in the inset of Figure 4c. In contrast, the mixture of 1-Eu, 1-Gd, and 1-Tb according to the molar ratio in 1-Eu0.0855Gd0.6285Tb0.2860 displays the mixed emission colors of red, blue, and green light rather than white light under 365 nm UV-light irradiation (Figure S10), demonstrating 1-Eu0.0855Gd0.6285Tb0.2860 is an effective white-light-emitting phosphor material but not only a physical mixture of three sole 1-Ln.

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CONCLUSIONS In summary, three isostructural luminescent Ln-MOFs have been built by H4BDPO ligand. H4BDPO with good planarity behaves as a large delocalized π-electron conjugated system to sensitize the red and green luminescence of 1-Eu and 1-Tb, respectively. TD-DFT calculations confirmed H4BDPO as a sensitizer to efficiently absorb and transfer energy to Ln3+ ions. On the basis of similar structures of 1-Ln, the bimetallic and trimetallic Ln3+ ions strategy were implemented to successfully fabricate a series of Ln3+ ions-doped isostructural frameworks. F

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The doped Eu/Tb-MOFs display an uncommon fluent change of the luminescent color among green, yellow, orange, and red. Furthermore, by tuning the molar ratios of Eu3+, Gd3+, and Tb3+ ions in MOFs, a white light emission framework 1Eu0.0855Gd0.6285Tb0.2860 was obtained with the CIE color coordinate of (0.34, 0.33), which is very close to that of the pure white light. This white light emission material possesses high absolute quantum yield of 22.4% and is a promising candidate for LED phosphor material. The present work affords an important strategy to not only tune the emission colors but also prepare the white-emitting devise based on Ln-MOFs by employing a large delocalized π-electron conjugated ligand.

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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.cgd.7b00530. Additional figures, TGA, PXRD, excitation spectra, selected bond length/angle table, and detailed calculation results from TD-DFT (PDF) Accession Codes

CCDC 1523604−1523606 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

Lei Hou: 0000-0002-2874-9326 Yao-Yu Wang: 0000-0002-0800-7093 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSFC (21471124 and 21531007) and NSF of Shannxi province (15JS113 and 15JK1731). REFERENCES

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DOI: 10.1021/acs.cgd.7b00530 Cryst. Growth Des. XXXX, XXX, XXX−XXX