Novel Structures and Luminescence Properties of Lanthanide

Nov 16, 2009 - molecules and a coordinated water molecule with each metal center are thermally unstable, but the orders of the framework structures of...
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DOI: 10.1021/cg900518s

Novel Structures and Luminescence Properties of Lanthanide Coordination Polymers with a Novel Flexible Polycarboxylate Ligand§

2009, Vol. 9 5128–5134

Qilong Zhu,†,‡ Tianlu Sheng,† Ruibiao Fu,† Shengmin Hu,† Jianshan Chen,† Shengchang Xiang,† Chaojun Shen,†,‡ and Xintao Wu*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, The Chinese Academy of Sciences, Fuzhou, Fujian 350002, China, and ‡Graduate University of the Chinese Academy of Sciences, Beijing 100039, China Received May 13, 2009; Revised Manuscript Received October 26, 2009

ABSTRACT: A series of lanthanide coordination polymers with a novel flexible hexapodal ligand, formulated as [Ln2(TTHA)(H2O)4] 3 9H2O (TTHA = 1,3,5-triazine-2,4,6-triamine hexaacetic acid; Ln = 1, Eu; 2, Tb; 3, Gd; 4, Dy) and [Yb2(TTHA)(H2O)2] (5) have been hydrothermally synthesized and characterized. Complexes 1-4 possess the same three-dimensional 4-connected 4284 networks constructed from Ln2(CO2)4 units with two different shapes of three-dimensional channels determining certain adsorption for H2 and CH4. The thermogravimetric analyses and XRD data show that the guest water molecules and a coordinated water molecule with each metal center are thermally unstable, but the orders of the framework structures of complexes 1-4 are thermally stable to 400 °C. Complex 5 shows a 2D network with 1D inorganic chains bridged by the ligands, and the 3D network was extended through π 3 3 3 π stacking interactions between triazine rings. Photoluminescence measurements show that complexes 1 and 2 are highly emissive at room temperature with quantum yields of 31% and 89%, which even amount to unexpectedly 77% and 96% after thermal treatments at 394 and 343 nm, respectively, and may be good candidates for light-emitting diodes (LEDs) and light applications. Introduction Exploring highly symmetrical multitopic ligands and suitable metal salts to construct supramolecular architectures has attracted much interest.1 Lanthanide coordination frameworks are of great importance currently, not only because of their fascinating architectures but also for their technological applications, such as diagnostic tools, luminescence sensors, and sensors over light-emitting devices (LEDs, OLEDs).2 Efficient lanthanide luminescence in organometallic complexes is typically accomplished by the use of antenna linkers, which can efficiently transfer the energy gained through photon absorption to the Ln ions in the complexes.3 Therefore, searching for efficient antenna complexation with high absorption in the UV/near-UV spectral region is an attractive task. Recently, many papers have been concerned with structures, adsorptions, magnetic and luminescent properties, and so on, based on transition metal ions such as zinc- and coppercontaining carboxylate triazine-based ligands.4 However, the lanthanide coordination polymers constructed from carboxylate triazine-based ligands have been less studied,5 particularly the flexible polycarboxylate ligands.6 Flexible ligands can adopt versatile conformations according to the geometric requirements of different metal ions and may afford unpredictable and interesting supramolecular networks. Previously, some flexible polypodal ligands, such as 1,3,5tri(carboxymethyl)benzene (TCMB),1d N,N0 ,N00 -1,3,5-triazine2,4,6-triyltris-glycine (TTG),4e and 2,20 ,200 -[1,3,5-triazine-2,4,6triyltris(thio)]tris-acetic acid (TTTA),4c leading to novel complexes with unusual characters and topologies have been reported. Herein, we introduce a novel polycarboxylate ligand, 1,3,5-triazine-2,4,6-triamine hexaacetic acid (TTHA), to the investigation, which holds flexibility because of the §

Dedicated to Prof. Xin-Tao Wu on the occasion of his 70th birthday. *To whom correspondence should be addressed. Fax: 86-591-83714947. Tel: 86-591-83719238. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 11/16/2009

presence of a, -N(CH2-)2 spacer between the triazine ring and the two carboxylates that can bend to meet the requirement of the coordination conformation. The multifunctional coordination sites have good prospects for generation of structures with high dimensions. Meanwhile, this ligand is proved to be a efficient UV/near-UV photosensitization of the Tb(III) and Eu(III) luminescence. To the best of our knowledge, no crystal structures and luminescence properties of Ln3þ complexes with flexible hexacarboxylate ligands based on the triazine group have been reported. Herein, we report four novel 3D four-connected 4284 networks with the general formula [Ln2(TTHA)(H2O)4] 3 9H2O (Ln = 1, Eu; 2, Tb; 3, Gd; 4, Dy) and a 2D ytterbium coordination polymer [Yb2(TTHA)(H2O)2] (5) containing the flexible ligand TTHA. The organic ligand shows two conformations with the metal centers, demonstrating the flexibility of its six arms. The former 3D complexes can be described as 3D networks constructed from Ln2(CO2)4 units possessing two different shapes of large three-dimensional channels. The latter one exhibits an interesting 2D framework with Yb-( μ2-O)2-Yb-(OCO)2 chains bridged by the completely deprotonated ligands, and the π 3 3 3 π interactions between the triazine rings result in a 3D network. The orders of the framework structures of complexes 1-4 can be retained upon even complete removal of the guest water molecules and parts of the coordination water molecules, which indicates their high thermal stability. In addition, the encouraging photoluminescence properties with high quantum fields (Φ > 90%) of these complexes are investigated in detail. Experimental Section Materials and Measurements. All reagents and solvents were used as received from commercial suppliers without further purification. Elemental analyses (C, H, and N) were performed with a Vario MICRO CHNOS elemental analyzer. The infrared spectra of KBr r 2009 American Chemical Society

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Table 1. Crystallographic Data for Complexes 1-5 complex emprical formula formula weight space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z Dcalcd (g/cm3) μ (mm-1) θ range (deg) h, k, l, ranges

1 2 Tb2C15H38N6O25 Eu2C15H38N6O25 1006.43 1020.01 C2/c C2/c 12.762(4) 12.750(4) 16.866(5) 16.808(6) 19.373(4) 19.318(5) 90.00 90.00 130.191(13) 130.350(14) 90.00 90.00 3185.4(15) 3155.0(17) 4 4 2.099 2.147 8.128 4.555 2.00-27.46 2.00-27.47 -16 to 16, -17 to 21, -12 to 16, -21 to 21, -25 to 24 -25 to 20 0.0489, 0.0956 0.0285, 0.0750 R1,a wR2b (I > 2σ(I )) 0.0645, 0.1044 0.0304, 0.0764 R1,a wR2b (all data) 1.092 1.078 GOF on F2 P P P P a R = (||Fo| - |Fc||)/ |Fo|. b Rw = { w[(Fo2 - Fc2)2]/ w[(Fo2)2]}1/2.

pellets were recorded in the range of 4000-400 cm-1 on a PerkinElmer Spectrum One FT-IR spectrometer. Thermal analyses were performed on a NETZSCH STA 449C instrument from room temperature to 1100 °C with a heating rate of 10 °C min-1 under nitrogen flow. The solid-state luminescence emission and excitation spectra were recorded on a FLS920 fluorescence spectrophotometer. Powder X-ray diffraction (PXRD) data were collected on a X’Pert Pro diffractometer with Cu KR. Synthesis. The ligand 1,3,5-triazine-2,4,6-triamine hexaacetic acid (TTHA) was synthesized according to the method described in the literature.7 1,3,5-Triazine-2,4,6-triamine Hexaacetic Acid (TTHA). A solution of iminodiacetic acid (12.64 g, 95 mm) and sodium hydroxide (12 g, 300 mm) in 40 mL of water was added dropwise into cyanuric chloride (5.53 g, 30 mm) in 40 mL of water at 0-5 °C under stirring. After 1 h, the mixture was warmed to room temperature and reacted with stirring for 3 h. Then the mixture was allowed to reflux at 110 °C for another 12 h. After cooling, the pH value was adjusted to about 2 using concentrated HCl. The white solid was collected by filtration, washed with alcohol and water, and dried under vacuum at 60 °C, giving the product in 76% yield. 1H NMR (400 MHz, D2O): δ 4.18 (s, 2H). Elemental Anal. Calcd for TTHA (C15H18N6O12): C, 37.98; H, 3.82; N, 17.72. Found: C, 38.04; H, 3.86; N, 17.64. IR (KBr, cm-1): 3000 (vs,br), 2551 (vs),1735 (s), 1651 (s), 1621 (vs), 1481 (s), 1392 (s), 1337 (m), 1319 (m), 1180 (m), 1091 (m), 985 (w), 965 (w), 928 (m), 811 (m), 716 (w), 689 (w), 627 (m), 603 (m), 557 (w), 524 (w), 475 (w). [Eu2(TTHA)(H2O)4] 3 9H2O (1). Complex 1 was synthesized hydrothermally in a 23 mL Teflon-lined autoclave by heating a mixture of 0.3 mmol of TTHA, 0.6 mmol of NaOH, and 0.6 mmol of Eu(ClO4)3 at 140 °C for 3 days and then slowly cooling to room temperature over 24 h. Large, colorless, block single crystals were collected in 75% yield on the basis of Eu(ClO4)3. Anal. Calcd for Eu2C15H38N6O25: C, 17.90; H, 3.81; N, 8.35. Found: C, 17.97; H, 3.62; N, 8.33. IR (KBr, cm-1): 3395 (vs, br), 1614 (s), 1554 (vs), 1494 (s), 1436 (s), 1405 (m), 1348 (m), 1308 (s), 1208 (m), 995 (m), 974 (w), 937 (w), 908 (w), 820 (w), 760 (m), 613 (s). [Tb2(TTHA)(H2O)4] 3 9H2O (2). Complex 2 can also be obtained by the same synthetic procedure as that for 1 except using TbCl3 instead of Eu(ClO4)3 as the starting material. Large, colorless, block single crystals were collected in 72% yield on the basis of TbCl3. Anal. Calcd for Tb2C15H38N6O25: C, 17.66; H, 3.75; N, 8.24. Found: C, 17.81; H, 3.62; N, 8.23. IR (KBr, cm-1): 3400 (vs, br), 1614 (s), 1554 (vs), 1493 (s), 1437 (s), 1406 (m), 1348 (m), 1309 (s), 1208 (m), 996 (m), 974 (w), 937 (w), 908 (w), 820 (w), 760 (m), 614 (s). [Gd2(TTHA)(H2O)4] 3 9H2O (3). Complex 3 can also be obtained by the same synthetic procedure as that for 1 except using Gd(ClO4)3 instead of Eu(ClO4)3 as the starting material. Large, colorless, block single crystals were collected in 85% yield on the basis of Gd(ClO4)3. Anal. Calcd for Gd2C15H38N6O25: C, 17.72; H, 3.77; N, 8.26. Found: C, 17.75; H, 3.61; N, 8.25. IR (KBr, cm-1): 3397

3 Gd2C15H38N6O25 1016.61 C2/c 12.767(4) 16.867(5) 19.354(5) 90.00 130.276(14) 90.00 3179.7(16) 4 2.124 4.243 2.41-27.49 -16 to 16, -19 to 21, -25 to 24 0.0298, 0.0896 0.0404, 0.1115 1.047

4 Dy2C15H32N6O14 1027.21 C2/c 12.748(4) 16.844(6) 14.802(6) 90.00 90.989(7) 90.00 3178(2) 4 2.147 4.774 2.00-27.51 -16 to 15, -21 to 21, -19 to 17 0.0561, 0.1692 0.0657, 0.1844 1.036

5 Yb2C15H16N6O14 850.21 P-1 9.249(5) 11.311(5) 12.162(6) 62.333(14) 87.39(2) 77.24(2) 1096.6(9) 2 2.575 8.564 2.26-27.49 -10 to 12, -12 to 14, -15 to 15 0.0585, 0.1463 0.0833, 0.1601 0.992

(vs, br), 1610 (s), 1555 (vs), 1494 (s), 1438 (s), 1406 (m), 1347 (m), 1310 (s), 1209 (m), 996 (m), 974 (w), 937 (w), 908 (w), 820 (w), 760 (m), 614 (s). [Dy2(TTHA)(H2O)4] 3 9H2O (4). Complex 4 can also be obtained by the same synthetic procedure as that for 1 except using Dy(ClO4)3 instead of Eu(ClO4)3 as the starting material. Large, colorless, block single crystals were collected in 79% yield on the basis of Dy(ClO4)3. Anal. Calcd for Dy2C15H38N6O25: C, 17.53; H, 3.73; N, 8.18. Found: C, 17.96; H, 3.64; N, 8.31. IR (KBr, cm-1): 3389 (vs, br), 1610 (s), 1554 (vs), 1493 (s), 1436 (s), 1405 (m), 1348 (m), 1308 (s), 1208 (m), 995 (m), 974 (w), 936 (w), 908 (w), 820 (w), 760 (m), 614 (s), 542 (w). [Yb2(TTHA)(H2O)2] (5). Complex 5 can also be obtained by the same synthetic procedure as that for 1 except using Yb(ClO4)3 instead of Eu(ClO4)3 as the starting material. Colorless, needle-like single crystals were collected in 53% yield on the basis of Yb(ClO4)3. Anal. Calcd for Yb2C15H16N6O14: C, 21.19; H, 1.90; N, 9.88. Found: C, 21.17; H, 2.62; N, 9.90. IR (KBr, cm-1): 3430 (vs, br), 1566 (vs), 1536 (s), 1481 (s), 1448 (s), 1394 (m), 1309 (m), 1202 (m), 1148 (w), 990 (w), 811 (w), 730 (w), 617 (w), 570 (w). X-ray Crystallographic Study. Data collection was performed on Rigaku Mercury CCD diffractometer with graphite-monochromated Mo KR (λ = 0.710 73 A˚) radiation at room temperature. The structures were solved by direct methods and refined by the fullmatrix least squares on F2 using the SHELXTL-97 program.8 All non-hydrogen atoms were refined with anisotropic displacement parameters. The positions of hydrogen atoms attached to carbon atoms were generated geometrically (C-H bond fixed at 0.97 A˚). Crystallographic data and structure determination summaries are listed in Table 1. Selected bond lengths and angles of the complexes are listed in Tables S1-S5 in the Supporting Information.

Result and Discussion Syntheses of the Complexes. All the complexes were obtained under the same conditions by the hydrothermal reactions of lanthanide nitrates with the ligand in water at 140 °C. In the syntheses, variations of starting materials, reaction temperature, pH value, and molar ratio have been investigated in detail. In order to synthesize complexes 2 and 3, salts with different anions, such as Cl- and ClO4-, have been used, and the same crystals were obtained. For complexes 1-4, the ratios of the base as well as temperature from 120 to 150 °C had no effect on the products, which may indicate the stability of complexes. Structure Description. The X-ray crystal structure analyses of 1-4 reveal that they are isostructural, possessing 3D frameworks with the general formula [Ln2(TTHA)(H2O)4] 3 9H2O. Complex 5 possesses a 2D framework, and the

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Zhu et al. Scheme 1. Coordination Modes of the Ligands in Complexes 1-4 and 5

Figure 1. Coordination environment of the Gd(III) ion in 3. Hydrogen atoms have been omitted for clarity. Symmetry codes: A, 1 x, 1 - y, 0.5 - z; B, -x, 1 - y, -z; C, 0.5 - x, 0.5 þ y, 0.5 - z; D, -0.5 þ x, 0.5 - y, -0.5 þ z.

resulting 3D network was extended through π 3 3 3 π interactions between the triazine rings. [Ln2(TTHA)(H2O)4] 3 9H2O (Ln = 1, Eu; 2, Tb; 3, Gd; 4, Dy). The structures of these four isostructural complexes were represented by complex 3, 3 Gd, which can be described as a 3D network constructed from Gd2(CO2)4 units. As illustrated in Figure 1, the fundamental building unit of complex 3 consists of one crystallographically independent Gd(III) center and half of the ligand TTHA, in which N1, C3, and N5 were shared by two independent fundamental building units. The Gd1 possesses nine coordinations with seven oxygen atoms (O1D, O2D, O3, O4B, O5, O6, O6B) from five carboxylate groups of three separated TTHA ligands and two aqua molecules. The distances between Gd and coordinated oxygen atoms from ligands are in the range from 2.359(4) to 2.599(4) A˚, and the aqua molecules are coordinated to metal ions with Gd1-O7D and Gd1-O8D distances of 2.429(4) and 2.477(4) A˚, respectively, all of which are comparable to those reported for other gadolinium-oxygen donor complexes.9 The ligand is completely deprotonated as depicted in Scheme 1a. Because of the flexibility, six of the arms show significant deviation from the central triazine ring, and none of them is coplanar with the triazine ring. The two arms connecting to the same nitrogen atom are arranged above and below the triazine plane, respectively. The adjacent arms bend in the same direction when they bridge the same two Gd atoms, while in other conditions the arms bend in opposite directions to the triazine plane, which may reduce the stereospecific blockade. In addition, three different coordination modes of the TTHA ligand are observed: the first one is chelating/bridging tridentate, whereas the other two are syn-syn bridging and chelating bidentate. Thus, the whole ligand acts as a μ14bridge to link six Gd(III) centers. The Gd1-O6 distance (2.599 A˚) is quite long, while the other eight Gd1-O distances are typical. The reason is probably the stronger strain in the four-membered ring of chelating/bridging coordination. Importantly, as it can be seen, one oxygen atom of the bridging carboxylate presents shorter interatomic distance than those involving the chelate bridging oxygen atoms. This behavior is similar to that shown in other Ln3þ

complexes, which present carboxylate groups coordinated to the metal ion as both chelate-bridging and bridging fashions.10 The chelating/bridging tridentate and syn-syn bridging bidentate coordination modes of carboxylate groups result in [Gd2(η2-CO2)2( μ2-CO2)2] units (Figure 2b). The neighboring metal centers Gd1 and Gd1C (symmetry codes C, 0.5 - x, 0.5 þ y, 0.5 - z) are separated with a distance of 3.9627(8) A˚, and the Gd1-O6-Gd1C angle is 106.04(13)°, which is close to those of similar bridges.6 The nearest metal-metal distance bridged by the TTHA ligand is ca. 7.638(8) A˚. As shown in Figure 2, each TTHA ligand is connected with four [Gd2(CO2)4] units, while each [Gd2(CO2)] unit links four TTHA ligands. As a consequence, the final three-dimensional networks are formed (Figure 3). The networks can be viewed as special frameworks with two different shapes of three-dimensional channels, which are intersecting (Figures 4 and 5). In Figure 5a, large rhombic channels are formed with dimensions of 13.612(2) A˚  10.805(2) A˚ based on the distances of the opposite metal centers, while the effective sizes are about 5.2 A˚  3.2 A˚. In Figure 5b, the diameter of circular channels is calculated to be about 7.0 A˚, while the effective size is about 3.6 A˚. Different channels in a single crystal are not reported usually.11 In order to identify the connectivity between the ligands and the metals, we investigated the topology of the whole network. If the dimer unit is viewed as a node, the simplified structural representation of the complex is illustrated in Figure 6, in which each ligand in this complex adopts a tetrahedral conformation, bridging four adjacent nodes, and each dimer unit connects four coplanar geometry centers of the ligands. In this case, the hexapodal ligand functions as a tetradentate ligand, and the dimer unit also acts a four-connected node.

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Figure 4. The three-dimensional channel packing along the [101] orientation. The blue circular cylinder perpendicular to the paper shows the circular channel, and the green and yellow elliptical cylinders show the rhombic channels along the [110] and [-110] orientations.

Figure 2. The environment of the TTHA ligand (a) and the [Gd2(CO2)4] unit (b) in complex 3.

Figure 5. View of (a) large channels packing along the [110] orientation and (b) channels packing along the [101] orientation with the balland-stick and space-filing representations of complex 3. Hydrogen atoms and lattice aqua molecules are omitted for clarity. The [-110] orientation possesses the same channels as those in panel a.

Figure 3. View of the 3D network constructed from Gd2(CO2)4 units (green polyhedrons) along the a-axis in complex 3. Hydrogen atoms and lattice aqua molecules are omitted for clarity.

Accordingly, the highly symmetrical form of this net represents the structure of PtS (cooperite), and the Schl€ afli symbol of the whole topology is presented as 4284.12

[Yb2(TTHA)(H2O)2] (5). X-ray single crystal diffraction reveals that complex 5 is a 2D network, showing inorganic Yb-( μ2-O)2-Yb-(OCO)2 alternating chains bridged by organic ligands. As shown in Figure 7, the Yb(III) center binds to eight oxygen atoms, four from chelating carboxylate groups of two distinct ligands, three from bridging carboxylate groups of three discrete ligands, and one from the coordinated water molecule. The bond lengths of Yb-O, where the oxygen atoms come from the carboxylate groups,

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Figure 6. The topology of the complex 3. Green and gray balls represent the [Gd2(CO2)] dimer unit and the geometry centers of the TTHA, respectively.

Zhu et al.

Figure 8. Perspective view of 2D layer structure along the a-axis in complex 5. The dotted lines show the π 3 3 3 π stacking interactions between triazine rings.

Figure 7. Coordination environment of the Yb(III) ion in 5. Hydrogen atoms have been omitted for clarity. Symmetry codes: A, -x, 2 - y, -z; B, 1 þ x, y, z; C, x, -1 þ y, 1 þ z.

are in the range of 2.161(10)-2.548(10)A˚ with a mean value of 2.336(4) A˚. The water molecules coordinate to the Yb(III) with distances of 2.271(10) and 2.235(10)A˚. The Yb-O bond lengths are much shorter than complexes 1-4 because of the smaller ion radius of ytterbium likely.13 Thus, each of Yb(III) ions possesses a distorted triangular dodecahedron. The whole ligand is completely deprotonated in this complex and functions as a μ14-bridge, connecting six different metal centers, and the coordination mode is illustrated in Scheme 1b. Interestingly, the ligand in complex 5 exhibits more flexibility than it does in the former complexes. All six arms of the ligand are located above the plane of the triazine ring, restraining it from extending in the other direction, which may be the cause of the 2D network (Figure 8). However, it also presents a good configuration for π 3 3 3 π interactions between the triazine rings of different layers. In addition, three types of coordination modes of TTHA ligands exist in the structure similar to those in the former structures. It is interesting that the metal ions are connected to onedimensional metal chains, in which two different rings are arranged alternately. The 2D framework can be described as inorganic chains interconnected by flexible hexapodal TTHA ligands (Figure 9). The carboxylate groups connecting the metal centers in syn-syn mode result in eightmembered rings {Yb2(OCO)2} with a Yb-Yb distance of

Figure 9. View of (a) the 2D framework in complex 5 constructed from 1D inorganic chains cross-linked via the TTHA ligand and (b) the formation of the 1D inorganic chain in complex 5.

5.174(9) A˚. The neighboring four-membered rings {Yb2( μ2O)2} are different; in these, ytterbiums are bridged by O2 and O10 alternately. Within the unit, the metal centers are separated by distances of 3.858(2) and 3.918(7) A˚, respectively. Each inorganic chain is connected by two others through four organic ligands, while every ligand bridges two such chains through the six arms, resulting in a 2D framework. The centroid-centroid distance of triazine rings in complex 3 is 3.644(0) A˚, indicating a strong π 3 3 3 π stacking, which further extends to a 3D network. Thermogravimetric Analyses. The thermal stability of these compounds was investigated through TGA experiments in the temperature range of 30-1100 °C under a flow of nitrogen. Because complexes 1-4 are isostructural, their TGA curves are similar; complex 2 is selected to be discussed. A sharp weight loss in the temperature range 30-110 °C was attributed to the loss of the nine free and two coordinated water molecules per formula (calcd 19.43%, found 19.04%). Each central metal lost a coordinated water molecule and formed an eight coordination mode. From then on, almost no loss was observed until 400 °C, at which temperature the complex began to decompose. Powder X-ray diffraction studies under different temperatures further indicated that

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Figure 10. Isotherms for H2 adsorption (9) and desorption (b) at 78 K (a) and CH4 adsorption at 183 K (b) measured for complex 3.

Figure 11. Solid-state emission spectra of (a) complex 1 and (b) complex 2 at room temperature upon excitation at 394 and 343 nm, respectively.

the orders of the framework structures of these complexes were retained upon complete removal of the guest water molecules and partial removal of the coordinated water molecules (see Supporting Information). Gas Absorption. To examine the pore characteristics and storage capability, the sorption properties of the desolvated complex 3 have been performed. By PLATON/SOLV14 analysis, the accessible void in the desolvated structure of complex 3 was estimated to be 33.0% of the total cell volume. Based on thermogravimetric analyses, the crystal sample was dried under a high vacuum at 120 °C for 10 h to remove the lattice water molecules. The nitrogen adsorption behavior has been examined; the complex exhibits hardly any adsorption for N2 at 77 K under low pressure; thereby the N2 molecules perhaps do not diffuse into the micropores efficiently. We also have examined gas storage behaviors under high pressure; the gas uptakes are calculated to be 0.31 wt % for H2 and 0.82 wt % for CH4 at 21 bar (Figure 10). Luminescent Properties. Complexes Eu2(TTHA)(H2O)4 3 9H2O (1) and Tb2(TTHA)(H2O)4 3 9H2O (2) emit intense red and turquoise fluorescence under UV light, respectively. Their solid-state excitation and emission spectra were measured at room temperature. The excitation spectra for 1 and 2 exhibiting the absorptions are in the region from 230 nm to nearly 400 nm (see Supporting Information). Compound 1 exhibits several strong characteristic emission bands for f-f transitions of isolated europium(III) ions in the visible region

excited at 394 nm (Figure 11a). These emission bands are 579 (5D0 f 7F0), 591 and 592 (5D0 f 7F1), 614 and 619 (5D0 f 7 F2), 687 (5D0 f 7F4), and 698 nm (5D0 f 7F4). The strongest emission attributes to the hypersensitive 5D0 f 7F2 transition at 614 and 619 nm, which is typical of Eu(III). The emission spectrum presents a weak 5D0 f 7F0 transition. It is noted that the intensity of the transition 5D0 f 7F1 is much weaker than that of 5D0 f 7F2. The former transition is a magnetic dipole, while the latter one is an electric dipole whose intensity is strongly affected by crystal field.15 The quantum yield of complex 1 was determined by means of an integrating sphere and was found to be 31% under excitation at 350 nm. Nevertheless, the quantum yield even amounts to 77% after the annealing at 120 °C under air condition for 4 h, which might be thanks to the elimination of energy absorption of the guest water molecules. On the other hand, under excitation of 343 nm, complex 2 displays the characteristic emission bands for isolated terbium(III) ions. The spectrum (Figure 11b) shows four emission bands at 488 (5D4 f 7F6), 545 (5D4 f 7F5), 584 (5D4 f 7F4), and 622 nm (5D4 f 7F3) with the strongest emission corresponding to the 5D4 f 7F5 transition. It is noteworthy that there is no apparent residual ligand-based emission in the 400-480 nm region, indicating an efficient energy transfer from the ligand p-excited states to the terbium f-excited states.16 This possibility is further confirmed by the remarkably high quantum yield of 89%. Surprisingly, the quantum

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yield is up to 96% after the annealing at 120 °C under air for 4 h. These studies have indicated that the TTHA ligands are capable of converting energy efficiently to strong Tb(III) green emission in complex 2, acting as an efficient sensitizer of the Tb(III) luminescence. In some cases, relatively high quantum yields (Φ > 50%) have been obtained.17 However, to the best of our knowledge, a better result has not appeared in microporous metal-organic frameworks up until now. The luminescence decay curves of complexes 1 and 2 were obtained at room temperature. The decay curves are well fitted into a single-exponential function as I = I0 exp(-t/τ), indicating the occupation of the same average local environment of Ln(III) sites in the structure.18 The corresponding lifetime for complex 1 is about 0.34 ms, whereas that for complex 2 is about 1.33 ms (determined by monitoring the 5 D0 f 7F2 line excited at 614 nm and the 5D4 f 7F5 line excited at 263 nm, respectively). Both of them have long luminescence lifetimes at millisecond order, which are comparable to other corresponding Eu(III) and Tb(III) complexes.19 The strong emission in the fluorescence spectra of the two complexes could be accounted for by the supramolecular interactions in such structures, which suggest that energy transferred from the ligand to the metal centers is quite effective and can sensitize the lanthanide emission to a large extent. Because the excitations now fall in the range of those commercially available, complexes 1 and 2 could be good candidates for light-emitting diodes (LEDs) and light applications. Conclusion A series of lanthanide coordination polymers formulated as [Ln2(TTHA)(H2O)4] 3 9H2O (Ln= 1, Eu; 2, Tb; 3, Gd; 4, Dy) and [Yb2(TTHA)(H2O)2] (5) have been hydrothermally synthesized and structurally characterized from an unexplored flexible hexapodal ligand, 1,3,5-triazine-2,4,6-triamine hexaacetic acid (TTHA). The isostructural complexes 1-4 exhibit novel 3D four-connected 4284 networks constructed from Ln2(CO2)4 units with two kinds of channels in three orientations determining certain adsorption for H2 and CH4, whereas [Yb2(TTHA)(H2O)2] shows a 2D framework with 1D inorganic chains, further extended to a 3D network through π 3 3 3 π stacking interactions between triazine rings. The thermogravimetric analyses and XRD data indicate that the orders of the framework structures of complexes 1-4 are thermally stable to 400 °C. Complexes 1 and 2 exhibit strong fluorescent emissions in the visible region at room temperature with quantum yields of 31% and 89%, which even amount to unexpectedly 77% and 96% after thermal treatments, respectively. These preliminary studies have indicated that TTHA ligands are capable of converting efficiently UV/ near-UV light to strong Tb(III) and Eu(III) emission, acting as efficient sensitizers of the Tb(III) and Eu(III) luminescence. In summary, our research demonstrates that the new flexible ligand TTHA could be a potential building block to construct novel supramolecular architectures with interesting physical properties. Further investigation is ongoing. Acknowledgment. This work was supported by grants from the 973 Program (Nos. 2007CB815301 and 2006CB932904) and the National Science Foundation of China (Nos. 20333070, 20673118, 20821061).

Zhu et al. Supporting Information Available: Crystallographic files for compounds 1-5 in CIF format, selected bond lengths and angles for 1-5 (Tables S1-S5), FT-IR spectra for 1-5 (Figure S1); X-ray powder diffraction for 1-4 (Figures S2 and S3), TGA curves for 1-4 (Figure S4), and the excitation spectra for 1 and 2 (Figure S5). This material is available free of charge via the Internet at http:// pubs.acs.org.

References (1) (a) Ma, S.; Zhou, H.-C. J. Am. Chem. Soc. 2006, 128, 11734. (b) Hee, K. C.; Jaheon, K.; Olaf, D. F.; Michael, O.; Omar, M. Y. Angew. Chem., Int. Ed. 2003, 42, 3907. (c) Kepert, C. J.; Rosseinsky, M. J. Chem. Commun. 1999, 375. (d) Wang, S.-N.; Xing, H.; Li, Y.-Z.; Bai, J.-F.; Pan, Y.; Scheer, M.; You, X.-Z. Eur. J. Inorg. Chem. 2006, 15, 3041. (2) (a) Parker, D. Coord. Chem. Rev. 2000, 205, 109. (b) Kuriki, K.; Koike, Y. Chem. Rev. 2002, 102, 2347. (c) Kido, J.; Okamoto, Y. Chem. Rev. 2002, 102, 2357. (d) Adachi, N.; Imanaka, S.; Tamura Chem. Rev. 2002, 102, 2405. (3) (a) Allendorf, M. D.; Bauer, C. A.; Bhakta, R. K.; Houk, R. J. T. Chem. Soc. Rev. 2009, 38, 1330. (b) Montalti, M.; Prodi, L.; Zaccheroni, N.; Charbonniere, L.; Douce, L.; Ziessel, R. J. Am. Chem. Soc. 2001, 123, 12694. (c) Parker, D.; Dickins, R. S.; Puschmann, H.; Crossland, C.; Howard, J. A. K. Chem. Rev. 2002, 102, 1977. (d) Jean-Claude, G. B.; Claude, P. Chem. Soc. Rev. 2005, 34, 1048. (4) (a) Sun, D.-F.; Ma, S.-Q.; Ke, Y.-X.; Collins, D. J.; Zhou, H.-C. J. Am. Chem. Soc. 2006, 128, 3896. (b) Wang, X.-S.; Ma, S.-Q; Sun, D.-F.; Parkin, S.; Zhou, H.-C. J. Am. Chem. Soc. 2006, 128, 16474. (c) Wang, S.-N.; Xing, H.; Bai, J.-F.; Li, Y.-Z.; Pan, Y.; Scheer, M.; You, X.-Z. Chem. Commun. 2007, 2293. (d) Wang, S.-N.; Bai, J.-F.; Li, Y.-Z.; Pan, Y.; Scheer, M.; You, X.-Z. CrystEngComm 2007, 9, 1084. (e) Wang, S.-N.; Bai, J.-F.; Xing, H.; Li, Y.-Z.; Song, Y.; Pan, Y.; Scheer, M.; You, X.-Z. Cryst. Growth Des. 2007, 7, 747. (5) Wei, Y.-Q.; Wu, K.-C.; Broer, R.; Zhuang, B.-T.; Yu, Y.-F. Inorg. Chem. Commun. 2007, 10, 910. (6) Wang, S.-N.; Sun, R.; Wang, X.-S.; Li, Y.-Z.; Pan, Y.; Bai, J.-F.; Scheerb, M.; You, X.-Z. CrystEngComm 2007, 9, 1051. (7) Hoog, P. De.; Gamez, P.; Driessen, W. L.; Reedijk, J. Tetrahedron Lett. 2002, 43, 6783. (8) SHELXTL, version 5.10; Siemens Analytical X-ray Instruments Inc.: Madison, WI, 1994. (9) Rizzi, A.; Baggio, R.; Garland, M. T.; Pe na, O.; Perec, M. Inorg. Chim. Acta 2003, 353, 315. (10) (a) Teotonio, E. E. S.; Felinto, M. C. F. C.; Brito, H. F.; Malta, O. L.; Trindade, A. C.; Najjar, R.; Strek, W. Inorg. Chim. Acta 2004, 357, 451. (b) Teotonio, E. E. S.; Brito, H. F.; Felinto, M. C. F. C.; Thompson, L. C.; Young, V. G.; Malta, O. L. J. Mol. Struct. 2005, 751, 85. (11) (a) Abrahams, B. F.; Moylan, M.; Orchard, S. D.; Robson, R. Angew. Chem. 2003, 115, 1892. (b) Ohmori, O.; Kawano, M.; Fujita, M. Angew. Chem. 2005, 117, 1998. (c) Zang, S.-Q.; Su, Y.; Duan, C.-Y.; Li, Y.-Z.; Zhu, H.-Z.; Meng, Q.-J. Chem. Commun. 2006, 4997. (12) Wells, A. F. Structural Inorganic Chemistry, 4th ed.; Clarendon Oxford University Press: New York, 1975. (13) Wan, Y.-H.; Zhang, L.-P.; Jin, L.-P.; Gao, S.; Lu, S.-Z. Inorg. Chem. 2003, 42, 4985. (14) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7. (15) (a) Wang, S.-N.; Sun, R.; Wang, X.-S.; Li, Y.-Z.; Pan, Y.; Bai, J.-F.; Scheer, M.; You, X.-Z. CrystEngComm 2007, 9, 1051. (b) Vicentini, G.; Zinner, L. B.; Zukerman-Schpector, J.; Zinner, K. Coord. Chem. Rev. 2000, 196, 353. (16) Benisvy, L.; Gamez, P.; Fu, W.-T.; Kooijman, H.; Spek, A. L.; Meijerinkc, A.; Reedijk, J. Dalton Trans. 2008, 3147. (17) (a) Ma, L.; Evans, O. R.; Foxman, B. M.; Lin, W.-B. Inorg. Chem. 1999, 38, 5837. (b) Fiedler, T.; Hilder, M.; Junk, P. C.; Kynast, U. H.; Lezhnina, M. M.; Warzala, M. Eur. J. Inorg. Chem. 2007, 291. (18) (a) Zhu, X.-D.; Lu, J.; Li, X.-J.; Gao, S.-Y.; Li, G.-L.; Xiao, F.-X.; Cao, R. Cryst. Growth Des. 2008, 8, 1897. (b) Yang, J.; Yue, Q.; Li, G.-D.; Cao, J.-J.; Li, G.-H.; Chen, J.-S. Inorg. Chem. 2006, 45, 2857. (19) (a) Zhang, Z.-H.; Song, Y.; Okamura, T.; Hasegawa, Y.; Sun, W.-Y.; Ueyama, N. Inorg. Chem. 2006, 45, 2896. (b) Huang, Y.-G.; Wu, B.-L.; Yuan, D.-Q.; Xu, Y.-Q.; Jiang, F.-L.; Hong, M.-C. Inorg. Chem. 2007, 46, 1171.