Synthesis, Structure, and Photoluminescence of Color-Tunable and

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Synthesis, Structure, and Photoluminescence of Color-Tunable and White-Light-Emitting Lanthanide Metal−Organic Open Frameworks Composed of AlMo6(OH)6O183− Polyanion and Nicotinate Huanyao Ji, Xiaomin Li, Donghua Xu, Yunshan Zhou,* Lijuan Zhang,* Zareen Zuhra, and Shaowei Yang State Key Laboratory of Chemical Resource Engineering, Institute of Science, Beijing University of Chemical Technology, Beijing 100029, P. R. China S Supporting Information *

ABSTRACT: A series of isostructural compounds Na(HL)(CH3COO)Ln(Al(OH)6Mo6O18)(H2O)6·10H2O [L = nicotinate; Ln = Eu (1), Tb (2)] and Na(HL)(CH3COO)EumTbnLa1−m−n(AlMo6(OH)6O18)(H2O)6· 10H2O (3−8, L = nicotinate), wherein Anderson-type polyanions AlMo6(OH)6O183− as basic inorganic building blocks are connected by Eu(CH3COO)(HL)(H2O)3]24+ and [Na2(H2O)8]2+ cations, resulting in formation of three-dimensional lanthanide metal−organic open frameworks, were synthesized successfully with AlCl3·6H2O, Na2MoO4·2H2O, nicotinic acid, and lanthanide nitrates as starting materials. The compounds were characterized by UV−vis, IR, elemental analysis, powder XRD, and TG−DTA measurements. The single-crystal structures of compounds 1 and 2 show that the two compounds display threedimensional open frameworks with 1D channels along the b and c axes. Investigation of the energy transfer mechanism indicated that the organic nicotinate ligand can transfer energy efficiently to Tb3+ rather than Eu3+. The influence of the POM moiety on the fluorescence of the compounds is also studied. Compounds 1−8 exhibit tunable luminescence color, and emitting of white light was realized through adjusting the molar ratio of Eu:Tb:La within the compounds.

1. INTRODUCTION Color-tunable and white-light-emitting solid materials have attracted considerable attention, owing to their wide applications in flat panel displays, backlighting, illumination, communication, and imaging.1−3 Currently, white-light-emitting materials are mainly inorganic salts,4−6 metal- or transitionmetal-doped oxides,7−9 organic molecules,10−12 and quantum dot.13,14 However, these materials generally more or less lack emission components,15−19 are low in color-rendering index,20−23 and are difficult to obtain as pure materials. Additionally, so far white-light-emitting diodes (WLED) based on the above different phosphors are fabricated mainly through a physical-mixing method,24−26 from which stems a difficulty in uniform mixing due to their different physical and chemical stability and the additional problem of color reabsorption.27 In this context, luminescent lanthanide metal−organic frameworks (Ln-MOFs) as a novel porous crystalline material with rich topology28,29 have attracted intensive interest because of their unique fluorescent properties, such as a long lifetime and a sharp and intense luminescence and emission in the primary colors range.30−32 However, the forbidden f−f transitions of Ln(III) ions exhibit a very small molar absorption coefficient and relatively weak UV absorption with low luminous efficiency,33,34 so organic ligands with strong © XXXX American Chemical Society

absorption (conjugate with rigid structures) in the ultraviolet region were often used as bridge ligands for construction of luminescent Ln-MOFs, which can influence not only the resultant structures but also sensitize Ln3+ luminescence in LnMOFs.35 Till now, although just a few color-tunable and whitelight-emitting Ln-MOFs have been obtained through introducing different Ln3+ ions emitting different color luminescence and adjusting the molar ratio of the Ln3+ ions during the construction of the Ln-MOFs,32,36−39 Ln-MOFs are considered as a new type of WLED material possessing high potential to avoid the problems of an unequal distribution of chromophores encountered in the physical-blending process. On the other hand, polyoxometalates (POMs) are rich in surface oxygen atoms, which act as multifunctional ligands with large active sites40−42 to combine easily with rare-earth-metal ions, resulting in the formation of lanthanide−polyoxometalate (Ln−POM) compounds. The advantages of both POMs and rare-earth ions can be integrated into the resulting Ln−POM compounds with good thermal stability, excellent mechanical properties, and good processability.43 What is noteworthy, the ligand to metal excited states charge transfer (LMCT) from the O → M (M = Received: July 30, 2016

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DOI: 10.1021/acs.inorgchem.6b01852 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

voltage was 500 V and the scan speed was 240 nm min−1. The photoluminescence quantum yield of the samples was measured using an absolute PL quantum yield measurement system (model C992002). The luminescence decay curves of the samples were acquired with a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz). The excitation (Contimuum Sunlite OPO) was conducted using a tunable laser with a pulse width of 4 ns and a gate of 50 ns. The Commission International de I’Eclairage (CIE) color coordinates were calculated on the basis of the international CIE standards. 2.2. Synthesis of Na(HL)(CH3COO)Eu(Al(OH)6Mo6O18)(H2O)6· 10H2O (1). Five milliliters of glacial acetic acid was added dropwise into the 10 mL aqueous solution of AlCl3·6H2O (0.36 g, 1.5 mmol). NaMoO4·2H2O (0.9 g, 1.5 mmol in 10 mL of H2O) was added dropwise into the above mixture. After 5 min of stirring, a solution of nicotinic acid (0.061 g, 0.5 mmol) and Eu(NO3)3·6H2O (0.223 g, 0.5 mmol) in 10 mL of hot water was added into the reaction mixture, and the mixture stirred for 1 h at 80 °C. The colorless sheet of single crystals was collected after 20 days through filtration with a yield of 42.4% based on Mo. Anal. Calcd for C8H46AlEuMo6NNaO44 (%): C, 5.87; H, 2.83; N, 0.86; Na, 1.40; Al, 1.65; Mo, 35.14; Eu, 9.28. Found: C, 5.67; H, 2.79; N, 0.82; Na, 1.48; Al, 1.54; Mo, 35.16; Eu, 9.18. IR (KBr, cm−1): 3308 (br), 1645 (m), 1624 (m), 1604 (m), 1455 (w), 1401 (m), 1348 (w), 949 (s), 917 (s), 648 (s), 445 (m). 2.3. Synthesis of Na(HL)(CH3COO)Tb(Al(OH)6Mo6O18)(H2O)6· 10H2O (2). The synthetic procedure of compound 2 is the same as that for compound 1, except that Eu(NO3)3·6H2O is replaced by Tb(NO3)3·6H2O (0.2165 g, 0.5 mmol) with a yield of 45.8% based on Mo. Anal. Calcd for C8H46AlTbMo6NNaO44 (%): C, 5.84; H, 2.82; N, 0.85; Al, 1.64; Mo, 34.99; Tb, 9.69; Na, 1.40. Found: C, 5.73; H, 2.79; N, 0.81; Al, 1.71; Mo, 35.06; Tb, 9.58; Na, 1.36. IR (KBr, cm−1): 3308 (br), 1646 (m), 1625 (m), 1604 (m), 1460 (w), 1402 (m), 1349 (w), 949 (s), 914 (s), 646 (s), 445 (m). 2.4. Synthesis of Color-Tunable and White-Light-Emitting Materials: Na(HL)(CH3COO)EumTbnLa1−m−n(Al(OH) 6Mo6O18)(H2O)6·10H2O (3−8). The synthesis process of compounds 3−8 is similar to that of compound 1, except that Eu(NO3)3·6H2O was replaced with a mixture of Eu(NO3)3·6H2O, Tb(NO3)3·6H2O, and La(NO3)3·6H2O. The samples were aimed to produce tunable-color and white-light emission by adjusting the relative molar ratio of Eu, Tb, and La. The Eu, Tb, and La content and chemical compositions of the obtained products were confirmed by elemental analysis and ICP [Table S1, Supporting Information (SI)]. IR for Na(HL)(CH3COO)Eu0.212Tb0.339La0.449(Al(OH)6Mo6O18)(H2O)6·10H2O (3) (the ratio of Eu:Tb:La = 1:1.6:2.12) (KBr, cm−1): 3305 (br), 1641 (m), 1624 (m), 1601 (m), 1458 (w), 1401 (m), 1348 (w), 947 (s), 916 (s), 647 (s), 445 (m). IR for Na(HL)(CH 3 COO)Eu 0.224 Tb 0.359 La 0.417 (Al(OH)6Mo6O18)(H2O)6·10H2O (4) (the ratio of Eu:Tb:La = 1:1.6:1.86) (KBr, cm−1): 3307 (br), 1643 (m), 1624 (m), 1601 (m), 1456 (w), 1400 (m), 1348 (w), 945 (s), 916 (s), 647 (s), 446 (m). IR for Na(HL)(CH 3 COO)Eu 0.232 Tb 0.371 La 0.397 (AlMo 6 (OH) 6 O 18 )(H2O)6·10H2O (5) (the ratio of Eu:Tb:La = 1:1.6:1.71) (KBr, cm−1): 3306 (br), 1646 (m), 1625 (m), 1601 (m), 1458 (w), 1401 (m), 1348 (w), 948 (s), 916 (s), 647 (s), 445 (m). IR for Na(HL)(CH3COO)Eu0.270Tb0.270La0.460(AlMo6(OH)6O18)(H2O)6· 10H2O (6) (the ratio of Eu:Tb:La = 1:1:1.7) (KBr, cm−1): 3306 (br), 1645 (m), 1625 (m), 1602 (m), 1458 (w), 1400 (m), 1350 (w), 947 (s), 914 (s), 646 (s), 444 (m). IR for Na(HL)(CH3COO)Eu0.360Tb0.240La0.400(AlMo6(OH)6O18)(H2O)6·10H2O (7) (the ratio of Eu:Tb:La = 1.5:1:1.7) (KBr, cm−1): 3307 (br), 1644 (m), 1624 (m), 1600 (m), 1456 (w), 1401 (m), 1348 (w), 945 (s), 915 (s), 647 (s), 446 (m). IR for Na(HL)(CH3COO)Eu0.375Tb0.250La0.375(Al(OH)6Mo6O18)(H2O)6·10H2O (8) (the ratio of Eu:Tb:La = 1.5:1:1.5) (KBr, cm−1): 3308 (br), 1647 (m), 1623 (m), 1600 (m), 1458 (w), 1401 (m), 1350 (w), 948 (s), 914 (s), 647 (s), 444 (m). 2.5. Synthesis of (NH4)3Al(OH)6Mo6O18·7H2O (9). Compound 9 was prepared according to the reported reference51 by boiling 33.3 g of (NH4)7Mo7O24·4H2O with 11 g of aluminum sulfate in 500 mL of H2O. The salt crystallized out on cooling. Recrystallization was done in hot water with a yield of 62% (based on Mo).

Mo or W) in Ln−POM compounds44−46 can occur under the irradiation of UV light, and the produced energy can transfer to Ln3+ ion and thus sensitize Ln3+ ion. In this context, it is speculated that construction of Ln-MOFs composed of POMs (inorganic ligands) and suitable organic bridging ligands can sensitize luminescence of Ln3+ ions35 and result in the formation of new materials that possess not only high luminous intensity but also show full spectra of luminescence through adjusting the ratio of Ln3+ ions in the mixture. However, until now, though some POM-based Ln-MOFs were synthesized successfully,30,47−51 only one paper44 reported the color-tunable and emitting properties of Ln-POMs, namely, {[Ln2(DMF)8(H2O)6][ZnW12O40]}·4DMF (Ln = La, Eu, and Tb), which present interesting color-tunable and white-lightemitting properties. However, in the structure of {[Ln2(DMF)8(H2O)6][ZnW12O40]}·4DMF, only solvent molecules DMF and H2O enter into the Ln3+ coordination environment, which can lead to nonradiative deactivation of the excited states of the Ln3+ via O−H vibrations and low luminescence emission of Ln3+.52−54 Therefore, the design and preparation of POMs-based Ln-MOFs with suitable organic ligands revealing high-efficiency color-tunable and white-lightemitting properties are of great concern.47 On the basis of the above analysis, in this work, Andersontype polyoxoanion [AlMo6(OH)6O18]3− was chosen as benign inorganic building blocks because each Mo atom has two terminal oxygen atoms with high reactivity.55 Nicotinic acid was used as the organic linker to enhance the luminescence intensity of Ln-MOFs based on POMs due to its ability to sensitize the luminescence of Ln3+ ions.56,57 Two Ln-MOFs based on [AlMo6(OH)6O18]3−, namely, Na(HL)(CH3COO)Ln(Al(OH)6Mo6O18)(H2O)6·10H2O (L = nicotinate, Ln = Eu, Tb), and a series of mixed-lanthanide analogues were synthesized by a simple mixture of AlCl3·6H2O, Na2MoO4· 2H2O, nicotinic acid, and lanthanide nitrates under acidic conditions. The energy transfer mechanism of the compounds was also studied systematically. The compounds exhibit tunable luminescence color, and white-light-emitting properties with good luminescence were successfully obtained with careful adjustment of the concentration of Eu:Tb:La cations within the compounds.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Ln(NO3)3·6H2O (Ln = Eu, Tb, Sm, Dy, and Gd) were prepared from their commercially available lanthanide oxides (99.99% purity) by dissolving them in nitric acid, followed by recrystallization and drying, respectively. All the other chemicals and reagents obtained commercially were of reagent grade and were used directly for the corresponding experiments. Elemental analyses for C, H, and N were conducted on a PerkinElmer 240C analytical instrument, while analyses for Na, Al, Mo, Eu, Tb, and La of the samples were conducted on a model ICPS-7500 inductively coupled plasma emission spectrometer (ICP-ES) after the samples were dissolved in dilute hydrochloric acid. IR spectra of the samples in the range of 4000−400 cm−1 were recorded on a Nicolet FTIR-170SX spectra photometer as KBr pellets. Thermogravimetric analyses of the samples were done on a SHIMADZU DTG-60A unit at a heating rate of 10 °C/min under air atmosphere. Powder X-ray diffraction (PXRD) measurements of the samples were conducted on a Rigaku-Dmax 2500 diffractometer at a scanning rate of 15°/min in the 2θ range from 3° to 55° using graphite-monochromatized Cu Kα radiation with a wavelength λ = 0.154 05 nm. The luminescence spectra at room temperature and at 77 K for the solid samples were recorded on a Hitachi F-7000FL luminescence spectrophotometer with both excitation and emission slits of 5.0 nm. The photomultiplier tube B


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Inorganic Chemistry Table 1. Crystal Data and Structure Refinement for Compounds 1 and 2 empirical formula formula weight temp/K crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg vol/Å3 Z ρcalc/mg mm−3 μ/mm−1 F(000) crystal size/mm3 θ range for data collection index ranges

reflections collected independent reflections data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices [all data] largest diff. peak/hole/e Å−3 CCDC No.

1

2

C8H46EuAlMo6NaO44N 1638.03 93(2) K triclinic P1̅ 12.932(6) 14.338(6) 14.510(6) 104.7430(10) 108.981(5) 108.119(4) 2220.6(17) 2 2.450 3.181 1584 0.40 × 0.30 × 0.27 3.01°−25.50° −14 ≤ h ≤ 15 −17 ≤ k ≤ 17 −17 ≤ l ≤ 17 15 489 8081 [R(int) = 0.0371] 8081/60/565 1.019 R1 = 0.0836 wR2 = 0.2333 R1 = 0.0915 wR2 = 0.2382 5.211/−3.014 1495545

C8H46TbAlMo6NaO44N 1644.99 97.5 K triclinic P1̅ 12.980(3) Å 14.381(3) Å 14.506(3) Å 104.50(2) 108.88(2) 108.14(2) 2241.4(9) 2 2.437 3.330 1588 0.70 × 0.45 × 0.40 3.22°−26.00° −16 ≤ h ≤ 16 −17 ≤ k ≤ 17 −17 ≤ l ≤ 17 19 050 8780 [R(int) = 0.0493] 8780/24/29 1.134 R1 = 0.1359 wR2 = 0.2938 R1 = 0.1426 wR2 = 0.2975 10.294/−6.625 1495544

2.6. X-ray Crystallography. Single-crystal X-ray diffraction data were collected on a Bruker APEX 2 X-Diffraction instrument with Mo Kα radiation (λ = 0.710 73 Å) in the ω scans mode. The structures of compounds 1 and 2 were solved by direct methods and subsequent successive difference Fourier syntheses and refined with the full-matrix least-squares technique with the SHELX 97 program package.59,60 H atoms bonded on C atoms were added to their geometrically ideal positions, while attempts to locate the H atoms of water molecules were unsuccessful. All of the non-H atoms for compound 1 were refined anisotropically, while isotropical refinement was only made on the heavy atoms for compound 2, due to low quality of the single crystals. A summary of the crystallographic data and structural determination parameters of compounds 1 and 2 is given in Table 1, and the selected bond lengths and bond angles of compounds 1 are listed in Table S2 (SI).

Figure 1. Powder XRD patterns of compounds 1−8 and the simulated single-crystal X-ray diffraction pattern of compound 2.

3. RESULTS AND DISCUSSION 3.1. Powder XRD Patterns. The powder XRD patterns of the compounds 1−8 are almost the same and match well with the simulated pattern obtained from X-ray single-crystal data of compound 1, except that the low-angle peaks diappeared and a slight shift appeared for some of the Bragg peak positions. This result indicates that the final bulky products are homogeneous phases, as shown in Figure 1. The disappearing of the low-angle peaks and a slight shift in some of the Bragg’s peak positions is frequently observed in other MOF systems, which probably arose due to removal of the solvent molecules.54,61 No other peaks being found in the pattern revealed that the product has no impurities. That they have almost the same powder XRD patterns indicates the isomorphic nature of compounds 1−8.

3.2. Structure of Compounds 1 and 2. Known from the results of powder X-ray diffraction analysis and single crystal Xray diffraction analysis of compounds 1 and 2, the seven coordination polymers are isostructural and crystallize in space group P1̅ of the triclinic system. Herein, compound 1 is selected as representative for the description of the structural details. The BVS calculation55 result shows that all Al atoms have an oxidation number of +3, all Mo atoms +6, Eu atom +3, and Na atom +1. The oxidation number of O(6), O(8), O(11), O(17), O(18), and O(23) is +1, representing −OH groups; the oxidation number of O(1W)−O(8W) is less than +0.43, indicating that they are water molecules; and the other oxygen C


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Inorganic Chemistry

Figure 2. ORTEP plot of the asymmetry unit in compound 1 showing the atom-labeling scheme with thermal ellipsoids given at 50% probability (a) and the coordination environment of Eu3+ in compound 1 (b). All H atoms and lattice waters are omitted for clarity (symmetry transformations used to generate equivalent atoms: #1, 1 − x, 2 − y, 1 − z). Color code: C, gray; O, red; N, blue; Eu, olive green; Na, teal; Al, dark yellow; Mo, aqua.

atoms have an oxidation number of about 2. The N(1) atom is protonated for charge balance. As shown in Figure 2a, the asymmetric unit of compound 1 includes [Al(OH)6Mo6O18]3− polyoxometalate anions, [Eu(CH3COO)(HL)(H2O)3]24+ complexes, [Na2(H2O)8]2+, and the lattice water molecules. In the structure of compound 1, the building block [Al(OH)6Mo6O18]3− belongs to the B-type Anderson structure,56,57 which consists of seven edge-sharing octahedral, six of which are {MoO6} arranged hexagonally around the central {Al(OH)6} octahedron. Four kinds of oxygen atoms exist in the cluster according to the oxygen coordination mode: the terminal oxygen (Ot) that does not connect any other atoms, the other terminal oxygen (Ot′) that is further linked to Eu(1) and/or Na(1), the double-bridging oxygen (Ob), and the central oxygen (Oc). Thus, the Mo−O distances can be grouped into four sets: Mo−Ot, 1.693(12)− 1.731(11) Å; Mo−Ot′, 1.701(12) and 1.734(11) Å; Mo−Ob, 1.896(11)−1.952(11) Å; and Mo−Oc, 2.229(10)−2.303(11) Å. The central Al−Oc distances vary from 1.888(10) to 1.907(11) Å. The asymmetric unit in the crystal structure of compound 1 consists of two crystallographically independent “one-half” Anderson anions, in which both Al3+ ions [Al(1) and Al(2)] occupy special positions. As shown in Figure 2b, each Eu3+ ion is nine-coordinate bonding to three water oxygen atoms [O(1W), O(2W), O(3W)], one terminal oxygen atom [O(5)] of one Andersontype anion, three oxygen atoms [O(3), O(3)#1, and O(4)#1] from two acetates, and two oxygen atoms [O(1), O(2)#1] from two HL ligands, leading to a distorted tricapped-trigonal prism coordination geometry. The bond lengths of Eu−O are in range from 2.362(10) to 2.539(11) Å, and the bond angles of O−Eu−O is in range from 50.9(4)° to 146.3(4)°, which are within the normal ranges and confirmed by those described in the literature.58,59 Each Na+ is six-coordinated, with one of the terminal oxygen atom [O(25)] of one Anderson-type anion and five water oxygen atoms [O(1W), O(4W), O(5W), O(6W), and O(4W)#1] with Na−O distances within 2.40(3)−2.57(3) Å and O−Na−O bond angles varying from 80.3(12)° to 173.3(11)°. Both the acetate and HL ligands adopt bridging coordination modes (Figure 3a,b) linking a pair of Eu3+ ions, forming a [Eu(CH3COO)(HL)(H2O)3]24+ complex cation. Two [Na-

Figure 3. Ball-and-stick presentation of coordination modes adopted by the organic ligands and [AlMo6(OH6)O18]3− anions in compound 1: the coordination mode of nicotinic acid (a), the coordination mode of acetic acid (b), the coordination mode of [AlMo6(OH6)O18]3− containing Al(1) (c), and the coordination mode of [AlMo6(OH6)O18]3− containing Al(2) (d). Symmetry transformations used to generate equivalent atoms: #1, −x + 1, −y + 2, −z + 1; #2, −x, −y + 1, −z + 1; #3, −x + 1, −y + 1, −z + 2.

(H2O)3]+ ions are joined together by two μ2-bridging water oxygen atoms (O4W), forming a [Na2(H2O)8]2+ dimer. In the structure of compound 1, there are two kinds of [Al(OH)6Mo6O18]3− anions, and both act as bidentate ligands linking [Eu(CH3COO)(HL)(H2O)3]24+ or [Na2(H2O)8]2+ through their terminal oxygen atoms [O(5) or O(22)] of two opposite MoO6 octahedra (Figure 3c,d), respectively. The 3D open framework based on polyoxoanions can be descripted as follows: the polyoxoanions [Al(OH)6Mo6O18]3− containing Al(1) (Figures 3c and 4a) and complex fragments [Eu(CH3COO)(HL)(H2O)3]24+ (Figure 4b) link each other alternatively to form a 1D [Al(OH) 6 Mo 6 O 18 ] 3− −[Eu(CH3COO)(HL)(H2O)3]24+ infinite chain along the a axis (Figure 4c). The distance between the neighboring polyoxoanions within the 1D chain is about 13.41 Å in terms of the distance measurement of Al···Al. The adjacent 1D infinite chains join together through [Na2(H2O)8]2+ dimers by sharing aqua oxygen (O1W) with Eu3+ ions from the adjacent 1D D


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Inorganic Chemistry

Figure 5. IR spectra of compounds 1−8. Figure 4. View of [AlMo6(OH6)O18]3− containing Al(1) in the 1D infinite chain in compound 1 (a), the Eu3+ cation pair and ligand (b), the 1D infinite chain in compound 1 consisting of a pair of Eu3+ and [AlMo6(OH6)O18]3− containing Al(2) (c), the 2D layer in compound 1 (d), and the 3D framework of compound 1 along the b direction (e). All the hydrogen atoms, lattice waters, and organic ligands were deleted for clarity. Color code: Eu, green; Al, yellow; Mo, cyan; Na, pink.

ring stretching vibration of CC and CN.65 Compound 1 displays the characteristic patterns of the Anderson-type anion, the bands between 400 and 600 cm−1 are attributed to the Mo−Oc stretching vibrations, the 640 and 800 cm−1 region belonged to the Mo−Ob stretching vibrations, and the range between 890 and 950 cm−1 is assigned to the Mo−Ot characteristic vibrational bands.66−68 3.4. UV−Vis Spectra of Nicotinic Acid and the Anderson-type (NH4)3Al(OH)6Mo6O18·7H2O. The free ligand nicotinic acid in ethanol solution (c = 1 × 10−3 mol· L−1) displays UV absorption bands at 261 nm (Figure S2, SI), which are assigned to the π → π* transition of the ligand.69 The molar extinction coefficient value 4.58 × 103 L·mol−1·cm−1 (261 nm) for the ligand indicates that it is an effective lightharvesting chromophore to sensitize lanthanide luminescence.70 The singlet state energy level 1ππ* of nicotinic acid was estimated to be 35714 cm−1 from the absorbance edge (280 nm) of its UV−vis spectrum.71The UV−vis absorption spectrum of the aqueous (NH4)3Al(OH)6Mo6O18·7H2O (c = 2 × 10−5 mol·L−1) is shown in Figure S3 (SI). Two UV absorption bands appear at 208 and 242 nm, which are assigned to O → Mo charge transfer72,73 in (NH4)3Al(OH)6Mo6O18· 7H2O. The large molar extinction coefficient values 2.93 × 104 L·mol−1·cm−1 (208 nm) and 3.28 × 104 L·mol−1·cm−1 (242 nm) for (NH4)3Al(OH)6Mo6O18·7H2O indicate that the ligand is a suitable light-harvesting chromophore to sensitize lanthanide luminescence.70 The singlet state energy level of (NH4)3Al(OH)6Mo6O18·7H2O was estimated by referencing its absorbance edge, which was 32 680 cm−1 (306 nm) and 31 546 cm−1 (317 nm), known from its UV−vis spectrum.71 3.5. Diffuse Reflectance Spectroscopy. As shown in Figure 6, the free ligand nicotinic acid displays a UV absorption band at 278 nm due to the π → π* transition,45,74 and the spectra of compounds 1 and 2 also show a large broad absorption band in the UV region at 250−400 nm, which is almost the same as that of the free ligands, indicating that the complexation of the Ln3+ ion does not significantly affect the singlet excited state of the ligand.75 In the meantime, a slight bathochromic shift is observed in the maximum absorption of the compounds owing to an effective interaction between the Ln3+ and the ligands.76 Therefore, the observed band of the UV spectra of compounds 1 and 2 and the ligand can be attributed to electronic transitions from the ground-state level S0 of the organic ligand to its excited-state level S1. 3.6. Thermal Stability Analysis. For thermal stability analysis, the thermogravimetric (TG) curves of compounds 1

chains to lead to a 2D layer (Figure 4d). The distance between the adjacent 1D chains within the 2D layer is about 14.51 Å in terms of the distance Al···Al. The adjacent 2D layers were linked by [Al(OH6)Mo6O18]3− polyoxoanions containing Al(2) (Figure 3d) via the coordination bonds between Na+ ions of [Na2(H2O)8]2+ dimers and the terminal oxygen atom [O(22)] of the polyoxoanions to lead to a 3D open framework with [Al(OH6)Mo6O18]3− polyoxoanions as basic building blocks (Figure 4e). The distance between the neighboring layers of the 3D open framework is 14.34 Å based on the Al···Al distance measurements. The 1D channels are along the b and c axes in the 3D open framework with the dimensions being ca. 12.93 × 14.51 Å (Figure 4e) and 12.93 × 14.34 Å (Figure S1, SI), respectively. A large amount of water molecules fill in the 1D channels (Figure 4e). In addition, the extensive hydrogen bonds formed between the nitrogen atoms of nicotinates and the terminal oxygen atoms [N(1)···O(10), 2.8612(10)Å; N(1)···O(16), 2.8307(10)Å] and between the lattice water molecules and the polyoxoanions (in the range of 2.569−3.074 Å) make the 3D open framework more stable. The total potential solvent volume is 622.1 Å3, accounting for ca. 28.0% of the cell volume of 2220.6 Å3 in compound 1 calculated by the Platon program.27,62 3.3. IR Spectra. The IR spectra of compounds 1−8 are nearly the same (Figure 5) due to their isomorphic nature; thus, only the spectrum of compound 1 is interpreted. The broad and intense vibrations in the range of 3100−3500 cm−1 are assigned to the characteristic peaks of OH vibration of water molecules.60 The IR spectra of compound 1 is different in the range of 1650−4000 cm−1 from that of free nicotinic, indicating that the nicotinic ligands has coordinated with Eu3+ ion. The intense vibrations located at 1641, 1624, 1604, and 1401 cm−1 correspond to the asymmetric and symmetric stretching vibrations of the carboxylate group of the nicotinic ligands and acetates.63,64 The absence of intense bands at 1690−1730 cm−1 indicates that the nicotinic ligands are deprotonated and coordinated to Eu3+ ion. Weak peaks between 1100 and 1300 cm−1 correspond to the pyridine E


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Inorganic Chemistry

For an organic ligand, the intersystem crossing process becomes effective when ΔE(1ππ*−3ππ*) is more than 5000 cm−1,79 so the energy gap of 11 791 cm−1 between the 1ππ* (35 714 cm−1) and 3ππ* (23 923 cm−1) levels79 for nicotinic acid shows an effective intersystem crossing process of nicotinic acid in the resulting product compounds. An optimal energy difference for Ln(III) needs 2500−3500 cm−1 for Eu(III) and 2500−4500 cm−1 for Tb(III),78 and here the energy differences between the lowest triplet state of nicotinic acid and the resonant energy levels of Eu3+ (5D1, 18 674 cm−1) and Tb3+ (5D4, 20 500 cm−1) are 5249 and 3243 cm−1, respectively. This means that the transitions from the lowest triplet energy level of nicotinic acid to the resonant energy level of Tb3+ is more effective, i.e., nicotinic acid can sensitize more efficiently the luminescence of Tb3+ than that for Eu3+ (Scheme 1).

Figure 6. Solid-state diffuse reflectance spectra of compounds 1 and 2 and the free ligand nicotinic acid.

Scheme 1. Schematic Energy Level Diagram and Energy Transfer Process in Compounds 1 and 2

and 2, selected as representatives, were obtained in the temperature range from 40 to 800 °C. Figure 7 shows the

Figure 7. TG−DTA curves of compounds 1 and 2.

3.8. Photoluminescence Properties of Compounds 1 and 2. Compounds 1 and 2 emitted intense Eu(III)-based red and Tb(III)-based green luminescence under the irradiation of UV light (λ = 254 nm). The luminescence behavior of compounds 1 and 2 at 77 K and at room temperature was investigated in order to understand the effect of nicotinic ligands and POM ligands on sensitizing the lanthanide(III) luminescence (Figure 8). The 77 K and room-temperature excitation spectra of compound 1 (Figure 8a) were obtained by monitoring the more intense emission line of Eu3+ at λ = 617 nm. A strong, broad band composed of two peaks at 280 and 311 nm in the region of 200−350 nm can be ascribed to the electronic transitions of nicotinate (peak at 280 nm) and [Al(OH)6Mo6O18]3− ligands (peak at 311 nm), respectively.46,47,80−83 In addition, the excitation spectrum of compound 1 also exhibits the 7F0 → 5D4 (361 nm), 7F0 → 5 G2 (380 nm), 7F0 → 5L6 (392 nm), 7F0 → 5D3 (415 nm), and the 7F0 → 5D2 (465 nm) transitions of Eu3+ ions.47 The 77 K and room-temperature emission spectra for compound 1 are shown in parts b and c of Figure 8 with excitation wavelengths of 280 and 311 nm corresponding to excitation through the nicotinate and POM ligands, respectively. The 77 K emission spectrum (red lines in Figure 8b,c) of compound 1 exhibited a series of emission lines assigned to intra-4f6 (5D0 → 7F0, 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3, and 5 D0 → 7F4) transitions of Eu3+ ion with the maximum centered at 580, 592, 617, 653, and 797 nm.84 The detection of the 5D0 → 7F0 transition implies only one emitting site in the europium

TG spectra of compounds 1 and 2 obtained under air flow. The results of thermogravimetric analysis showed that compounds 1 and 2 mainly experienced a two-step gradual weight loss reaction. The first weight loss of 17.57% (calcd 17.60%) and 17.67% (calcd 17.52%) for compounds 1 and 2 in the range of 40−265 °C corresponds to the removal of lattice water molecules and coordinated water molecules, respectively. A sharp weight loss of 11.37% (calcd 11.13%) in the range of 385−504 °C for compound 1 and 11.35% (calcd 11.08%) in the range of 385−532 °C for compound 2 is ascribed to the decomposition of the compounds and release of organic moieties. From the TGA curves, it is known that the compounds are not stable in air. The low thermal stability is also proved by the XRD measurement for the samples of compound 1 selected as a representive which were treated under different temperatures (Figure S4, SI). It is found that the characteristic X-ray diffraction peaks of compound 1 disappear after treatment at 120 °C, indicating its amorphous character. 3.7. Mechanism of Energy Transfer in the Compounds. It is accepted43,77,78 that an effective intersystem crossing process of the ligands and the optimal energy differences between the lowest triplet energy level of ligands and the resonant energy levels of Ln3+ ion is required for efficient luminescence of lanthanide compounds. F


Article

Inorganic Chemistry

ground level (7F6−3) are observed. The emissions at 490, 544, 584, and 621 nm are assigned to the characteristic transitions of 5 D4 → 7F6, 5D4 → 7F5, 5D4 → 7F4, and 5D4 → 7F3 of the Tb3+ ion, respectively.35 The room-temperature excitation spectrum of compound 2 (black line in Figure 8a′) obtained by monitoring around λ = 545 nm for Tb3+ showed two broad and strong bands at 225 and ca. 268 nm, which are ascribed to the electronic transitions of nicotinate ligands, and some peaks in the range of 300−400 nm correspond to the 7F6 → 5L10, 5D2−3 (at 352, 369, and 378 nm) intraconfigurational forbidden 4f8 → 4f8 transitions of Tb3+ ion also being detected.87 The absence of an excitation peak of [Al(OH)6Mo6O18]3− indicates that in contrast to the Eu analogue, the sensitization of the Tb3+ ion at room temperature is solely via the organic ligand and does not involve POM-centered LMCT states.81 This is due to radiationless deactivation involving Tb(III)−Mo(VI) charge transfer states leading to inefficient sensitization for Tb3+ ion, which is consistent with results from Yamase and Boskovic and their co-workers.46,47,81 As shown in Figure 8b′,c′, under the excitation at 309 nm wavelength, the emission intensity of compound 2 at room temperature decreased 20 times compared to the 77 K luminescence intensity but only decreased 1.7 times at 268 nm excitation wavelength light. These results indicate more significant temperature-dependent Tb(III) sensitization for excitation through the POM compared with excitation through the nicotinate ligand.

Figure 8. Solid-state excitation spectra for compounds 1 (λem = 617 nm) (a) and 2 (λem = 545 nm) (a′) and emission spectra for compound 1 with λex = 280 nm (b) and λex = 311 nm (c) and for compound 2 with λex = 268 nm (b′) and λex = 309 nm (c′) at 77 K (red line) and room temperature (black line), respectively.

compound 1.47 The intensity ratio of 3.8 and 4.0 for I( 5D0→7F 2):I(5D0→7F1) with λex = 280 and 311 nm, respectively, indicated the low symmetry of the Eu3+ ion site85 in compound 1.86 The most intense transition in compound 1 is 5D0 → 7F2, indicated by intense red luminescence. The intensity of all transitions decreases as the temperature is increased from 77 K to room temperature, with very similar I77K/Irm intensity ratios of about 2.6 and 2.8 for the 5D0 → 7F1 transitions for λex = 280 and 311 nm, respectively. No obvious shift of the peak positions indicated that the structure of compound 1 does not change with the increase of temperature. The temperature dependence of the luminescence intensity for all excitation wavelengths is similar, possibly because of the thermal relaxation of the O → M LMCT state.46,47,81 The Eu− O−Mo bond angle (152.789o) allows effective d1 hopping through fπ−pπ−dπ orbital mixing, resulting in inefficient energy transfer at room temperature.46,47,81 The 77 K excitation spectrum of compound 2 (red line in Figure 8a′) monitored with the more intense emission line, 545 nm for Tb3+, shows a strong band composed of three peaks at 225, 268, and 309 nm ascribed to the electronic transitions of nicotinic ligands and [Al(OH)6Mo6O18]3−, and some peaks appearing in the range of 300−400 nm correspond to the 7F6 → 5L10, 5D2−3 (at 352, 369, and 378 nm) intraconfigurational forbidden 4f8 → 4f8 transitions of Tb3+ ion in compound 2.87 The luminescence emission spectra at 77 K were measured upon excitation at 268 and 309 nm, respectively. As shown in Figure 8b′ and 8c′ (red lines), four characteristic multiplet transitions of Tb(III) ion from the emitting level (5D4) to the

Figure 9. Room-temperature emission decay curves for compounds 1 and 2 excited at 280 and 268 nm and monitored at 617 and 544 nm, respectively.

The room-temperature 5D0 (Eu3+) and 5D4 (Tb3+) emission decay profiles (Figure 9) were monitored at 617 nm (5D0 → 7 F2) and 544 nm (5D4 → 7F5), corresponding to the more intense lines for compounds 1 and 2, respectively. The decay curves of compounds 1 and 2 can be well-fitted by a singleexponential function, showing that the compounds in the excited states have only one coordination site.88 The experimentally determined luminescence lifetimes (τobs) are 344 and 662 μs for compounds 1 and 2, respectively. The pure radiative lifetime (τR) of Eu(III) 5D0 → 7FJ (J = 0−4) transitions in compound 1 is calculated to be 7080 μs by using the equation89 1/τR = AMD,0n3(Itot /IMD) G

(2) DOI: 10.1021/acs.inorgchem.6b01852 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry where AMD,0 = 14.65 s−1 is the spontaneous emission probability of the 5D0 →7F1 transition of Eu(III), n is the refractive index of the medium, and Itot/IMD is the ratio of the integrated total area of the corrected Eu(III) emission spectrum to the area of the magnetic dipole 5D0 → 7F1 transition. The intrinsic quantum yield of the Eu(III) luminescence step (ΦLn) in compound 1 is evaluated to be 4.86% by using the formula90 ΦLn = τobs/τR. Thus, the ligand sensitization efficiency (ηsens) is the product of two processes involving intersystem crossing (ISC) from the first excited singlet state of the ligand to the triplet state. The energy transfer (LET) to the lanthanide is calculated as 72.84% for compound 1 by using ηsens = ΦtotΦLn, where the absolute emission quantum yield (Φtot) of ligandsensitized lanthanide emission is obtained experimentally to be 3.54%, which is moderate as compared to those of the compounds reported in the literature.27,47,49,50,91,92 3.9. Color-Tuning of the Emission of the Compounds. From the emission spectra of the compounds 3−8 shown in Figure S5 (SI), it can be seen that several narrow-band emission peaks from Eu3+ and Tb3+ and a broad-band emission from nicotinate ligand can be tuned by adjusting the relative molar ratio of Eu3+:Tb3+:La3+, and as a consequence, the emission of the compounds 3−8 can be well-tuned, which provides the basis to compensate the red color from Eu3+, the green color from Tb3+, and the blue color of the organic ligand under excitation at 264 nm ultraviolet light. From the corresponding CIE coordinates listed in Table 2 and the CIE luminescent

Figure 10. CIE luminescent color coordinates of compounds 3−8 [for compound 3 (a), x = 0.202, y = 0.209; for compound 4 (b), x = 0.240, y = 0.312; for compound 5 (c), x = 0.268, y = 0.389; for compound 6 (d), x = 0.274, y = 0.355; for compound 7 (e), x = 0.254, y = 0.278); for compound 8 (f), x = 0.294, y = 0.295] excited at 264 nm.

through the POM ligand compared with excitation through the nicotinate ligand. Compounds 1−8 showed color-tunable emission under UV excitation. With careful adjustment of the molar ratio of Eu, Tb, and La, the optimized composition for white-light emission was determined as Na(HL)(CH3COO)Eu0.375Tb0.250La0.375(Al(OH)6Mo6O18)(H2O)6·10H2O. The present paper provides important insight into the structure− property relationship of the resulting compounds. Given the large number of organic linkers and polyoxometalates of various structures, constituents, and properties reported in the literature, the present results will guide us to design and explore new polyoxometalate-based metal−organic open frameworks possessing excellent color-tunable luminescence and white-light-emitting performance with better thermal stability.

Table 2. Molar Ratio of Eu:Tb:La and CIE Coordinate of Compounds 3−8 Excited at 264 nm compd

molar ratio of Eu:Tb:La

3 4 5 6 7 8

1:1.6:2.12 1:1.6:1.86 1:1.6:1.71 1:1:1.7 1.5:1:1.7 1.5:1:1.5

CIE coordinate (0.202, (0.240, (0.268, (0.274, (0.254, (0.294,

0.209) 0.312) 0.389) 0.355) 0.278) 0.295)

color coordinates shown in Figure 10 for compounds 3−8, it is known that compound Na(HL)(CH3COO)Eu0.375Tb0.250La0.375(AlMo6(OH)6O18)(H2O)6·10H2O (8) with the molar ratio of Eu:Tb: La = 1.5:1.0:1.5 emits white color when excited at 264 nm. The calculated chromaticity coordinates of light emission of compound 8 fall within the white-light region of the CIE chromaticity diagram (0.294, 0.295), which is very close to the value for ideal white light (0.333, 0.333). As expected, compound 8 can produce white light at a reasonably broad excitation wavelength.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01852. The elemental analysis and ICP for compounds 3−8 (Table S1), selected bond lengths and angles for compound 1 (Table S2), view of the 3-D structure of compound 1 along the b direction (Figure S1), UV−vis spectra of nicotinic acid in a H2O/ethanol mixture (1:1 in volume) (Figure S2) and aqueous (NH4)3AlMo6O24H6·7H2O (Figure S3), XRD patters of compound 1 at different temperatures (Figure S4), and photoluminescence spectra of compounds 3−8 excited at 264 nm (Figure S5) (PDF) X-ray crystallographic files of 1 in CIF format (CIF) X-ray crystallographic files of 2 in CIF format (CIF)

4. CONCLUSIONS A series of three-dimensional lanthanide metal−organic open frameworks composed of AlMo6(OH)6O183− polyanions which were connected by Eu(CH 3COO)(HL)(H2 O) 3 ]2 4+ and [Na2(H2O)8]2+ cations were synthesized successfully. The investigation of the luminescent properties and energy transfer mechanism of compounds 1 and 2 indicated that nicotinate ligand is a good luminescent sensitizer of Tb3+ rather than Eu3+. A variable-temperature study of compound 1 implied that the luminescence intensity is temperature-dependent for all excitation wavelengths, due to thermal relaxation of the O → M LMCT state, while compound 2 shows more significant temperature-dependent Tb(III) sensitization for excitation

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AUTHOR INFORMATION

Corresponding Authors

*Y.Z. e-mail: [email protected]. H


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Inorganic Chemistry *L.Z. e-mail: [email protected].

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ORCID

Yunshan Zhou: 0000-0003-0143-7370 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. H.J. and X.L. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China is greatly acknowledged. Prof. Xue Duan of Beijing University of Chemical Technology is greatly acknowledged for his kind support.

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