Structure and luminescence of sodium and lanthanide(III) coordination polymers with pyridine-2,6-dicarboxylic acid

Inorganica Chimica Acta 362 (2009) 3407–3414

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Structure and luminescence of sodium and lanthanide(III) coordination polymers with pyridine-2,6-dicarboxylic acid Taoyun Zhu a, Kazuya Ikarashi a, Tadashi Ishigaki c, Kazuyoshi Uematsu b, Kenji Toda a, Hirokazu Okawa d, Mineo Sato b,* a

Graduate School of Science and Technology, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japan Department of Chemistry and Chemical Engineering, Faculty of Engineering, Niigata University, Ikarashi 2-no-cho, Niigata 950-2181, Japan c Center for Transdiciplinary Research, Niigata University, Ikarashi 2-no-cho, Niigata, 950-2181, Japan d Department of Earth Science and Technology, Faculty of Engineering and Resource Science, Akita University, Tegata Gakuen-machi, Akita 010-8502, Japan b

a r t i c l e

i n f o

Article history: Received 8 October 2008 Received in revised form 16 January 2009 Accepted 28 January 2009 Available online 6 February 2009 Keywords: Lanthanide complexes Crystal structure Pyridine-2,6-dicarboxylic acid Coordination polymer Luminescence

a b s t r a c t A series of coordination polymers constructed by sodium, lanthanide(III), and pyridine-2,6-dicarboxylate (dipic),NaLn(dipic)2  7H2O (Ln = Eu, Gd, Tb), have been prepared under a hydrothermal condition. The crystal structures of the three compounds which are isostructual were determined by single-crystal X-ray diffraction. The two-dimensional layers found in the compounds are built up from six-folded {NaO6} polyhedra and nine-folded {LnN2O7} polyhedra, these being edge-shared each other along the c axis and bridged by carboxylate groups of dipic along the b axis, respectively. This two-dimensional framework provides cavities inside the layer and interlayer spaces outside the layer for accommodation of the two dipic molecules coordinated to a lanthanide(III) ions. The dehydrated materials obtained by heating the as-synthesized crystals at 200 °C held their crystal structure, and absorbed the same amounts of water molecules as those of the as-synthesized crystals upon the exposure of 100% relative humidity at room temperature. The Eu and Tb compounds showed strong red and green emissions, respectively, due to an energy transfer from dipic molecules to trivalent emission ions. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction The metal-organic frameworks (MOFs) based on lanthanidecontaining coordination polymers have received special attention owing to possibilities to combine their variety of magnetic and optical properties and framework structures in order to create novel functionalities [1,2]. The materials having such a sophisticatedly designed structure can be applied potentially in various materials with functionalities as gas storage, ion exchange, and molecular recognition in conjunction with physical properties coming from 4f electrons of lanthanide ions [3–9]. Lanthanide ions, as well known, have a large radius and a high affinity for hard donor atoms and ligands with oxygen or hybrid oxygen–nitrogen atoms, particularly multicarboxylate ligands, which are usually employed in the establishment of desired architectures [10–27]. In the case of design for the MOF structures which intensively utilize luminescent properties of lanthanide ions, it is primarily important what kind of organic ligand should be selected. In general, photophysical properties of lanthanide ions are attributed to f–f transitions, the efficiency of which is usually low due to their spin- and parity-forbidden nature [28]. This drawback can be over* Corresponding author. Tel./fax: +81 25 262 6768. E-mail address: [email protected] (M. Sato). 0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2009.01.036

come through the mechanism of a ligand-to-metal energy transfer, termed so-called an antenna effect of highly absorbent ligands [29], which serve as efficient sensitizers. The energy transfer to lanthanide(III) ions can occur from the excited singlet state of the ligand in major systems reported [30]. Typical organic ligands with the antenna effect include polycarboxylate, b-diketone, and carboxamide ligands which contain conjugated C@C double bonds responsible for absorption of ultraviolet or visible light radiations. The ligands of pyridine-2,6-dicarboxylic acid (dipic) has been widely studied for constructing MOFs containing lanthanide ions and shown to be as efficient sensitizer for europium(III) and terbium(III) luminescence in the solid states [22,25,31] as well as in the solutions [32]. We have previously reported the detailed two-dimensional crystal structure of NaSm(dipic)2  7H2O, and demonstrated a unique hydrogen-bonding network located in interlayer spaces [33]. To continue our research for, particularly focused on luminescence as a functionality, another lanthanidebased MOFs, we employed dipic ligands as a component for robust frameworks of MOF and also as a sensitizer for the luminescence of lanthanide(III) ions. Herein, we isolated a series of coordination polymers constructed by sodium, lanthanide(III), and dipic, NaLn(dipic)2  7H2O (Ln = Eu, Gd, Tb), under hydrothermal conditions, which were structurally characterized by single X-ray diffraction. We also

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demonstrate a unique dehydration/hydration behavior of NaEu (dipic)2  7H2O as a representative, which makes a significant influence to its luminescent properties. 2. Experimental 2.1. Syntheses NaLn(dipic)2  7H2O was hydrothermally synthesized at 150 °C for 72 h in a 40 mL Teflon-lined steel autoclave under autogenous pressure. The starting solution was prepared by mixing Na2MoO4  2H2O, Ln2O3, NaCl, pyridine-2,6-dicarboxylic acid, and deionized water with a molar ratio of 2:1:2:1:555 (total volume, 15 mL), and its pH value was adjusted to 3.05 by hydrochloric acid. After the hydrothermal reaction, the autoclave was slowly cooled to room temperature, and colorless crystals were produced. 2.2. Characterization Qualitative and quantitative chemical analyses for the singlecrystal were undertaken by an electron probe microanalysis (EPMA) using a Shimadzu EPMA-8705. The average composition determined for metallic components was confirmed to be a ratio of Na:Ln = 1:1. Thermogravimetric analysis (TG) and differential thermal analysis (DTA) were carried out at a heating rate of 5 °C min1 in air using a Seiko Denshi TG-DTA 6300. The measurements of gas adsorption and specific surface area were carried out using Shimadzu Corporation micrometrics Tristar-3000. The IR spectra were obtained using Shimadzu FT-IR 4200. The excitation and emission spectra were measured on a powder sample using a Shimadzu RF-5500 spectrophotofluorometer at room temperature. 2.3. X-ray crystallography Crystallographic data of the compounds were collected on a Rigaku RAXIS-RAPID single-crystal diffractometer equipped with an imaging plate area detector for the Eu and Tb derivatives and

on a Rigaku AFC7S four circle diffractometer for the Gd derivative with graphite monochromated Mo Ka radiation with 0 k = 0.71073 Å A at a temperature of 23 ± 2 °C. The data processing was accomplished with the Crystal Structure processing program [34]. The structure was solved by direct methods using the program SIR97 [35] and refined by full-matrix least-squares techniques against F2 using the SHELXTL-97 crystallographic software package [36]. All non-H atoms were easily found from the difference Fourier map and refined anisotropically. The hydrogen atoms on the dipic molecules were positioned geometrically and included in the refinement as riding, whereas the hydrogen atoms on the water molecules were first geometrically fixed so that the distance of O–H in water molecules was 0.85 Å, and then in the final refinement stages were refined isotropically without constraint. 3. Results and discussion Since the three compounds are isostructural, the details of the crystal structure are to be described here for the Eu derivatives. The detailed crystallographic data and structure refinement parameters are summarized in Table 1. Selected bond distances, angles, and hydrogen bonds for the Eu derivatives are given in Tables 2 and 3. The asymmetric unit for the compound is shown in Fig. 1. An europium(III) ion is coordinated by two dipic molecules and three water molecules, forming nine-folded coordination environment with four carboxylic oxygen atoms, two dipic nitrogen atoms, and three oxygen atoms of water molecules. The Eu–O(carboxylic) bond distances ranging from 2.406 to 2.431 Å, the Eu–N(pyridinic) bond distances ranging from 2.522 to 2.526 Å, and Eu–O (water) bond distances ranging from 2.460 to 2.548 Å are all comparable to those reported previously. There seems to be two possible geometries for a nine-folded polyhedron. In an ideal case, one is the symmetrical tricapped trigonal prism with D3h symmetry, and the other is the monocapped square antiprism with C4v symmetry. The polyhedron of the europium coordination sphere for the compound is best described as a distorted tricapped trigonal prism (Fig. 1b), but the distortion is so remarkable because three kinds

Table 1 Crystal data and structure refinement for NaLn(dipic)2  7H2O. Empirical formula Formula weight T (K) Wavelength (Å) Crystal system Space group a (Å) b (A) c (Å) b (°) V (Å3) Z Density (calculated) (g cm3) Absorption coefficient (mm1) F(0 0 0) Crystal size (mm3) 2hmin/2hmax (°) Index ranges Measured reflections Independent reflections [R(int)] Absorption correction Refinement method Data/restraints/parameters Goodness-of-fit (GOF) on F2 Final R indices (I > 2r(I)) R indices (all data) Extinction coefficient Residual electron density (e Å3)

NaEu(C7H3NO4)2  7H2O 631.27 296 0.71069 monoclinic P21/c 11.1770(3) 17.4295(3) 11.3690(2) 98.1130(10) 2192.62(8) 4 1.912 2.958 1248 0.1  0.05  0.03 1.84/30.03 15 6 h 6 15, 24 6 k 6 23, 16 6 l 6 16 21 942 6366 (0.0531) none full-matrix least square on F2 6366/16/341 1.124 R1 = 0.0379, wR2 = 0.0792 R1 = 0.0468, wR2 = 0.0822 0.00136(17) maximum = 0.930, minimum = 0.984

NaGd(C7H3NO4)2  7H2O 636.56 296 0.71069 monoclinic P21/c 11.1770(7) 17.4086(6) 11.3659(8) 98.196(2) 2188.9(2) 4 1.932 3.128 1252 0.25  0.13  0.02 1.84/30.03 15 6 h 6 15, 23 6 k 6 23, 15 6 l 6 15 21 373 6207 (0.0683) none full-matrix least square on F2 6207/16/338 1.185 R1 = 0.0603, wR2 = 0.1314 R1 = 0.0678, wR2 = 0.1351 0.00007(17) maximum = 2.834, minimum = 2.952

NaTb(C7H3NO4)2  7H2O 638.23 296 0.71069 monoclinic P21/c 11.150(6) 17.383(5) 11.359(3) 98.25(4) 2178.9(15) 4 1.946 3.344 1256 0.3  0.1  0.06 2.16/30.05 0 6 h 6 15, 24 6 k 6 0, 15 6 l 6 15 6660 6370 (0.0365) w-scan full-matrix least square on F2 6370/16/338 0.978 R1 = 0.0404, wR2 = 0.0996 R1 = 0.0864, wR2 = 0.1143 0.0021(2) maximum = 2.798, minimum = 2.04

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T. Zhu et al. / Inorganica Chimica Acta 362 (2009) 3407–3414 Table 2 Selected bond distances (Å) and angles (°) for NaEu(C7H3NO4)2  7H2O. Around Eu

Around Na

Distances Eu1–014 Eu1–O12 Eu1–023 Eu1–O21 Eu1–O3W Eu1–N11 Eu1–N21 Eu1–O1W Eu1–O2W

2.406(3) 2.416(3) 2.422(3) 2.432(3) 2.459(3) 2.522(3) 2.526(3) 2.531(3) 2.548(3)

Within dipic

Na1–O21 Na1–O5W Na1–O2W Na1–O24#2 Na1–O14#3 Na1–O1W#3

2.394(3) 2.400(4) 2.438(3) 2.444(4) 2.451(3) 2.612(4)

N11–C12 N11–C16 O11–C11 O12–C11 O13–C17 O14–C17 C11–C12 C12–C13 C13–C14 O14–C17 C11–C12 C12–C13 C13–C14 C14–C15

Around Eu

1.334(5) 1.335(4) 1.241(4) 1.277(5) 1.243(4) 1.263(5) 1.504(5) 1.383(6) 1.393(7) 1.263(5) 1.504(5) 1.383(6) 1.393(7) 1.382(7)

C15–C16 C16–C17 N21–C22 N21–C26 O21–C21 O22–C21 O23–C27 O24–C27 C21–C22 C22–C23 C23–C24 C24–C25 C25–C26 C26–C27

1.384(6) 1.515(5) 1.329(5) 1.341(4) 1.265(4) 1.237(4) 1.270(5) 1.243(4) 1.516(5) 1.385(5) 1.382(6) 1.395(7) 1.386(5) 1.503(5)

Around Na

Angles O14–Eu1–O12 O14–Eu1–O23 O12–Eu1–O23 O14–Eu 1–O21 O12–Eu 1–O21 O23–Eu 1–O21 O14–Eu1–O3W O12–Eu1–O3W O23–Eu1–O3W O21–Eu1–O3W O14–Eu1–N11 O12–Eu1–N11 O23 –Eu1–N11 O21–Eu1–N11 O3W–Eu1–N11 O14–Eu1–N21 O12–Eu1–N21 O23–Eu1–N21

127.58(9) 84.85(10) 82.18(10) 143.48(9) 79.55(9) 126.93(9) 73.92(9) 137.91(9) 139.53(9) 69.89(9) 63.99(9) 63.68(9) 78.50(10) 132.50(10) 119.37(9) 139.90(9) 74.93(9) 64.03(10)

O21–Eu1–N21 O3W–Eu1–N21 N11–Eu1–N21 O14–Eu1–O1W O12–Eu1–O1W O23–Eu1–O1W O21–Eu1–O1W 03W–Eu1–01W N11–Eu1–O1W N21–Eu1–O1W O14–Eu1–O2W O12–Eu1–O2W O23–Eu1–O2W O21–Eu1–O2W O3W–Eu1–O2W N11–Eu1–O2W N21–Eu1–O2W O1W–Eu1–O2W

63.19(9) 113.34(9) 127.10(10) 72.63(10) 145.05(9) 70.77(10) 99.07(10) 70.15(9) 128.34(10) 73.52(10) 88.51(9) 75.94(9) 146.32(10) 73.92(9) 68.36(9) 68.96(10) 131.42(9) 137.74(9)

O21–Na1–O5W O21–Na1–O2W O5W–Na1–O2W O21–Na1–O24#2 O5W–Na1–O24#2 O2W–Na1–O24#2 O21–Na1–O14#3 O5W–Na1–O14#3 O2W–Na1–O14#3 O24#2–Na1–O14#3 O21–Na1–O1W#3 O5W–Na1–O1W#3 O2W–Na1–O1W#3 O24#2–Na1–O1W#3 O14#3–Na1–O1W#3

165.08(13) 76.60(10) 96.27(13) 82.92(11) 83.98(12) 89.78(11) 102.81(11) 89.22(12) 83.77(11) 170.08(13) 83.55(10) 109.10(13) 143.03(11) 118.58(12) 70.52(10)

Symmetry transformations used to generate equivalent atoms: #1, x, y + 1/2, z + 1/2; #2, x, y + 1/2, z + 3/2; #, x, y + 1/2, z  1/2; #4, x, y  1/2,z + 3/2.

of ligands coordinate the europium atom. Not as understood directly by insight from the Fig. 1 but discussed later in detail, a six-folded coordination environment occurs around a sodium ion. The sodium atom is coordinated by three oxygen atoms (carboxylic) with the bond distances ranging from 2.394 to 2.451 Å and by three oxygen atoms of water molecules with the bond distances ranging from 2.400 to 2.612 Å. The polyhedron of the sodium coordination sphere is a slightly distorted octahedron. The asymmetric unit involves seven water molecules, which are classified into two

Table 3 Hydrogen bond distances (Å) and angles (°) for NaEu(C7H3NO4)2  7H2O. D–H  A #1

01W–H1WA  O22 01W–H1WB  O5W#4 O2W–H2WA  O11#1 O2W–H2WB  O13#3 O3W–H3WA  O24#2 O3W–H3WB  O12#1 O4W–H4WA  O22#4 O4W–H4WB  023 O5W–H5WA  O4W#3 O5W–H5WB  07W O6W–H6WA  O13#3 O6W–H6WB  O4W#5 O7W–H7WA  011#1 O7W–H7WB  06W

d(D–H)

d(H  A)

d(D  A)

\(DHA)

0.850 0.850 0.846 0.850 0.849 0.849 0.854 0.853 0.850 0.851 0.852 0.852 0.853 0.855

1.916 1.955 1.99 1.885 2.035 1.869 2.16 1.943 1.95 1.97 1.95 2.01 2.16 1.94

2.750(4) 2.798(5) 2.782(4) 2.724(4) 2.866(4) 2.717(4) 2.911(5) 2.782(5) 2.782(6) 2.799(7) 2.787(6) 2.850(8) 2.992(6) 2.740(9)

167 172 155 169 166 177 147 168 164 164 167 171 165 155

Symmetry transformations used to generate equivalent atoms: #1, x, y + 1/2, z + 1/ 2; #2, x, y + 1/2, z + 3/2; #3, x, y + 1/2, z  1/2; #4, x, y  1/2, z + 3/2; #5, x + 1, y + 1/2, z + 3/2.

groups; one is the molecules coordinating metal ions (O1W, O2W, O3W, and O5W) and the other the molecules isolated as a water of crystallization (O4W, O6W, and O7W) with relatively large thermal vibration ellipsoids. The structural architecture constructed in the compound is of particular interest (Fig. 2). The structure can be described as a layered structure, which consists of metallic coordination polymer layers, separated by an interlamellar region populated by water molecules of crystallization (not shown in Fig. 2 for clarity). One of the two dipic ligands coordinating an europium ion acts as a pendant arm vertically extending to the interlamellar region, whereas the other one is embedded in the layer block with an angle of 51.60° between the dipic plane and the layer plane. In the layer block, chains are constructed by the nine-folded europium polyhedra and the sodium octahedra with edge-sharing fashion, running along the direction parallel to the c axis (Fig. 3). Each chain is bridged by carboxylate groups of the embedded dipic molecules to adjacent chains, thus forming a two-dimensional network. Relatively large cavities occur within the layer block to accommodate the pyridine rings of the embedded dipic molecules. The geometrical parameters of the hydrogen bonds involved in the waters of crystallization are given in Table 3. Within the interlayer, the oxygen atoms (O1W and O5W) form a six-membered ring, together with two sodium atoms, such like cyclohexane (Fig. 4b). The interlayer water molecules form unique octamer clusters by hydrogen-bonding, giving eight-membered rings. The atoms O4W, O5W, O6W, and O7W are related to those of the symmetrically equivalent opposite side by the center of symmetry, as shown in Fig. 4c. These clusters can be regarded as the simplest

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Fig. 1. ORTEP view of the asymmetric unit of NaEu(dipic)2  7H2O (a) and the environment around Eu (b), showing 50% probability displacement ellipsoids. H atoms are omitted for clarity.

Fig. 2. Overview of layered structure nearly along the [0 0 1] direction of NaEu(dipic)2  7H2O. Water molecules located in the interlayer are omitted for clarity.

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supramolecular analogue of cyclooctane. In the ring, O4W behaves as hydrogen acceptors while O5W behaves as hydrogen donors, in the hydrogen-bonding scheme. Both the atoms show tetracoordination. On the other hand, the atoms O6W and O7W behave both as hydrogen donors and acceptors with tricoordination. The average O–O distance in the ring is 2.806 Å, somewhat longer than those observed in HCOOH (2.67 Å) and ice (2.76 Å) [37]. The twodimensional structure of the compound is largely a consequence of hydrogen-bonding interactions among water molecules themselves and the metal-organic framework. The thermal stability of the compound has been investigated using TG and powder X-ray diffraction measurements, as shown in Figs. 5 and 6, respectively. The TG, performed from room temperature to 800 °C, shows two types of major weight losses in this temperature range. The first weight loss initiated at about 40 °C, accompanied with a small endothermic heat, implying the elimination of water molecules. The step accompanied with a weight loss of 19.2 wt% may correspond to the loss of all water molecules (seven water molecules) involved in the compound (calc. 19.98 wt%). Based on the results of crystal structure determination of the compound in which five water molecules (O1W, O2W, O3W, and O5W) are directly coordinated to metal ions and three water molecules (O4W, O6W, and O7W) are waters of crystallization, it may be somewhat curious that not only the coordinated water molecules but also isolated waters of crystallization are simultaneously released at the same temperature. On the hand, the high temperature weight loss with an initiation temperature of 420 °C and an end temperature of 520 °C is attributed to the elimination due to the combustion of two dipic molecules per unit formula. The observed weight loss is 60.2 wt%, good consistent with the calculated one

Fig. 3. View of the framework structure drawn by (a) wireframe and (b) polyhedra for the two-dimensional layer of NaEu(dipic)2  7H2O.

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(60.61 wt%). The fact that the compound show an high thermal stability is noteworthy, in comparison with those of many of MOFs involving 3d transition metals and main group metals, usually below 400 °C [38–41]. Employment of oxophilic metals such as lanthanides provides potential for construction of MOFs with high thermal stabilities, probably due to their high coordination number and ionic bond character against multidentate ligands. The powder XRD studies were performed for the as-synthesized sample, the sample heated at 200 °C for 3 h, and the samples obtained after the heated sample was exposed to 100% relative humidity for 24 h (Fig. 6). Interestingly, the dehydrated compound still holds a crystalline state even upon the release of water molecules even though their diffraction intensities decrease a little (Fig. 6b). The decreases in the diffraction intensity are probably due to the decrease in the crystalline size given by cleavage or cracks during the dehydration. Additionally, partial breakdowns of the crystal lattice may occur. However, this is thought to be recovered easily upon the following re-hydration process of the exposure of water vapor at room temperature, as indicated, as shown in Fig. 6c, by the fact that the diffraction intensities of the re-hydrated sample again increase. The results indicate that the framework architecture was able to survive the heat treatment and proved to be thermally stable. It should be noted, as shown in Fig. 7, that the sample exposed to the humidity shows actually the same thermal behavior as that of the as-synthesized sample. This result implies that the dehydrated sample still has micro-pores which can accommodate water molecules, keeping the main framework almost unchanged. Although such thermal behavior was also observed in many lanthanide-base MOFs, NaNa(dipic)2  7H2O exhibits a remarkable

Fig. 4. Views of hydrogen-bonding of (a) six-membered ring found in the interlayer and (b) eight-membered ring located in the interlayer space. Dotted lines represent hydrogen-bonding interactions. Symmetry transformations used to generate equivalent atoms: #1, x, 1/2  y, 1/2 + z; #2, 1  x, 1/2 + y, 1/2  z; #3, 1  x, 1  y, 1  z; #4, x, 1/2  y, 1/2 + z; #5, x, 1/2 + y, 1.5  z; #6, 1 + x, 1/2  y, 1/ 2 + z; #7, 1  x, 1/2 + y, 3/2  z; #8, x, 1  y, 1  z; #9, x, 1/2  y, 1/2 + z; #10, 1 + x, y, z.

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0

60.2%(calc. 60.61%)

40

DTA

Weight loss / wt%

19.2%(calc. 19.98%)

20

60 80 -100 0

200

400

600

800

1000

Temperature / ºC Fig. 5. TG and DTA curves for as-synthesized crystal of NaEu(dipic)2  7H2O.

Intensity / a.u.

Simulated pattern

(a) (b)

(c) 20

10

30

40

2θ / deg

extending to the interlamellar region. The location of the dipic ligand seems to be crucial in avoiding the destruction of the crystalline state upon the release of water molecules, assisted by a change in coordination modes of dipic molecules [12] to adjust the coordination site vacancy resulted from the elimination of coordinated water molecules (Fig. 8). Nitrogen gas adsorption measurements were performed at 77 K on the as-synthesized sample and the dehydrated sample. Only a quite small amount of N2 uptake was observed for both the samples. The calculated BET surface areas of the samples were both less than 2 m2 g1. Considering the kinetic diameters of N2 (3.65 Å) and H2O (2.64 Å) [44], the micropores created in the dehydrated sample may be too small for accommodation of N2 gas. Otherwise, small polar molecules such as H2O may be adsorbed into the micro-pores because the formation of hydrogen-bonding network is indispensable to the H2O uptake in interlamellar space. We are now planning to do the gas adsorption measurements using such polar molecules. Emission and excitation spectra measured at room temperature for the powder samples of NaEu(dipic)2  7H2O and NaTb(dipic)2  7H2O are displayed in Figs. 9 and 10, respectively. The emission spectra of the Eu derivative, obtained with excitation wavelength of 280 nm, are composed of well known emission lines typical for europium(III), which are assigned to transitions between the first excited state (5D0) and the ground multiplet 7FJ (J = 0–4). The transitions of 5D0 ? 7F0 (forbidden in inversion center) and 5D0 ? 7F3 (magnetic and electric dipole transitions) are very weak while those of 5D0 ? 7F1 (magnetic dipole transition), 5 D0 ? 7F2 and 5D0 ? 7F4 (electric dipole transition) are strong. The fact that hypersensitive 5D0 ? 7F2 transition is the most intensive indicates a highly polarizable chemical environment around the europium site. For a careful insight of the emission for the 5 D0 ? 7F1 transition, it can be noted that the peak is split into three lines. This transition must be triplet in a C1 symmetry, consistent with the observation of the compound. This fact is in good agreement with the results of the crystal structure determination which

Fig. 6. Powder X-ray diffraction patterns for (a) the as-synthesized sample, (b) the sample heated at 200 °C for 3 h, and (c) the sample obtained after the heated sample was exposed to 100% relative humidity for 24 h.

2,6-dipic

Weight loss / a.u.

H2O

(a)

H2O

heating

(b) 0

200

400

600

800

Temperature / ºC Fig. 7. TG-DTA curves for (a) as-synthesized sample and (b) the sample obtained after the heated sample was exposed to 100% relative humidity for 24 h.

contrast in comparison with one- and two-dimensional MOFs, crystalline state of which are usually destroyed upon the removal of water of crystallization [26]. As stated before, NaLn(dipic)2  7H2O has two kinds of crystallographically independent dipic molecules, one of which acts as a pendant arm vertically

Fig. 8. Simple image for dehydration and re-hydration process for NaLn(dipic)2  7H2O (Ln = Eu, Gd, Tb).

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5D

80

0

7F 2

60 40

5D

7F 3

0

5D 5D

20 0 200

5D

0

7F 1

0

7F 4

0

(a) 300

400

500

600

700 500

300

5D

4

7F 5

200

100 5D 5D

300

400

4

4

7F 6

500

550

600

650

700

Wavelength / nm

Fig. 9. Luminescence spectra of NaEu(dipic)2  7H2O at room temperature. The emission spectrum was obtained under 280 nm excitation and the excitation spectrum was obtained by monitoring 617 nm emission.

Intensity / a.u.

(b)

7F 0

Wavelength / nm

0 200

Intensity / a.u.

Intensity / a.u.

100

7F 4 5D

600

4

7F 3

700

Wavelength / nm Fig. 10. Luminescence spectra of NaTb(dipic)2  7H2O at room temperature. The emission spectrum was obtained under 280 nm excitation and the excitation spectrum was obtained by monitoring 543 nm emission.

Fig. 11. Emission spectra of (a) the as-synthesized sample and (b) the dehydrated sample of NaTb(dipic)2  7H2O.

drated sample which was obtained after the dehydrated sample was exposed to 100% relative humidity for 24 h exhibits almost the same spectra as that of the as-synthesized sample, while the dehydrated sample shows some distinctive features: (1) all the emission peaks become sharp, (2) a peak at the wavelength corresponding to the transition of 5D0 ? 7F0 is observed, and (3) the number of peaks in the region for the transition of 5D0 ? 7F1 is more than three, apparently impossible supposed only one crystallographic emission center exists. The first feature is easily understood by accounting for the release of water molecules coordinated to an emission center of europium(III). The O–H vibrational modes of coordinated water molecules is well known to bring about significant deactivation of the excited states of europium(III) [42,43]. The facts (2) and (3) may be appreciated by assuming that at least two kinds of crystallographically independent sites for europium exist in the dehydrated sample although its crystal structure is not completely determined at present.

4. Summary shows a single crystallographic site for europium. The excitation spectrum of the compound was obtained by monitoring the emission wavelength of 617 nm (transition of 5D0 ? 7F2). Not only a broad excitation band in the range 200–350 nm but also several sharp line spectra are observed. The former band can be assigned to the p ? p* transition of the dipic ligand from the measurement of diffuse reflection spectra of pyridine-2,6-dicarboxylic acid, which has a maximum absorption at 280 nm. This observation suggests that the ligand-to-metal energy transfer from dipic molecules to europium emission centers is moderately efficient, The latter excitation lines can be assigned to the direct energy transitions from 7F0 to upper excited states of europium(III). The emission spectra of the Tb derivative, obtained with excitation wavelength of 280 nm, are composed of emission lines typical for terbium(III), which are assigned to transitions between the first excited state (5D4) and the ground multiplet 7FJ (J = 6–3) (Fig. 10). The emission mechanism of the Tb derivative is substantially the same as that of the Eu derivatives, i.e., not only the direct excitation of lanthanide emission centers but also the energy transfer from the p ? p* excitation of dipic ligand to emission centers are important. The vibronic coupling of the carboxylic group and the lanthanide f levels may play a key role in the ligand-to-metal energy transfer, as pointed out in [42]. The variation in emission spectra accompanied with dehydration was investigated for the Eu derivative (Fig. 11). The re-hy-

A series of two-dimensional coordination polymers based on lanthanide-based MOFs, NaLn(dipic)2  7H2O (Ln = Eu, Gd, Tb), have been synthesized under mild conditions. XRD analysis reveals that they are isostructural and have the same type as that of the Sm derivatives previously reported. The compounds show a reversible dehydration/hydration behavior, without destruction of framework of the crystal structure, upon the heating and the exposure of humidity at room temperature. The dehydration/hydration process also gave a significant influence to the luminescence properties of NaEu(dipic)2  7H2O, i.e., the dehydrated sample show higher and more sharpened emission spectra together with emergence of the 5D0 ? 7F0 transition. This behavior may be applied to the development of functional materials with a sensing prove for humidity. Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research (No. 440, Panoscopic Assembling and High Ordered Functions for Rare Earth Materials, and No. 17042012) from the Japanese Ministry of Education, Culture, Sports, Science, and Technology. A part of this work was also supported by the Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Science, Sports and Culture (No. 18350104).

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