Syntheses, Structures, Near-Infrared, and Visible Luminescence of

Apr 29, 2008 - State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuj...
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Syntheses, Structures, Near-Infrared, and Visible Luminescence of Lanthanide-Organic Frameworks with Flexible Macrocyclic Polyamine Ligands Xiandong Zhu,†,‡ Jian Lu¨,† Xiaoju Li,† Shuiying Gao,† Guoliang Li,† Fuxian Xiao,† and Rong Cao*,†

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 6 1897–1901

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fujian, Fuzhou, 350002, P. R. China, and Graduate School, Chinese Academy of Sciences, Beijing, 100039, P. R. China ReceiVed NoVember 7, 2007; ReVised Manuscript ReceiVed December 24, 2007

ABSTRACT: A series of lanthanide-based MOFs with flexible macrocyclic polyamine ligands, formulated as [Ln(H2TETA)]NO3 · 2H2O (Ln ) Nd (1), Eu (2), Tb (3), Dy (4), Y (5); H4TETA ) 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid) have been successfully synthesized and characterized. They possess the same 3D architectures and crystallize in triclinic space group P1j. Their structures are built up from the paddle-wheel SBUs, which lead to 1D inorganic chains and further 3D framework with 13.26 × 10.94 Å2 rhombic channels. The macrocyclic ligand displays special coordination pattern and protonation scheme. Complexes 1-3 exhibit strong fluorescent emissions in the visible and near-infrared region at room temperature. Formation of paddle-wheel structure plays crucial role in realizing efficient fluorescent emissions of complexes 1-3. Introduction Luminescent lanthanide-based metal-organic frameworks (MOFs) are currently of great interest and importance, not only because of their fascinating architectures but also because oftheir technological importance, such as diagnostic tools, luminescence sensors, laser systems, and optical amplification for telecommunications.1 The trivalent lanthanide ions are very attractive luminescence centers for their high color purity and relatively long lifetimes of the excited states as a result of transitions within the partially filled 4f shell of the ions. Complexes of Eu3+ and Tb3+ showing strong luminescence in the visible region have been widely exploited in applications such as fluoroimmunoassays and structural probes.2 More recently, there has been increasing interest in the design of complexes with lanthanide ions that show emissions in the nearinfrared region (800-1550 nm), such as Nd3+, Yb3+, and Er3+. The advantage of signal transmittance of near-infrared radiation makes them attractive for their potential use in luminescence bioassays, laser systems, and optical amplification.3 Because of the weak absorption coefficient of the parity-forbidden transitions, luminescence from lanthanide ions is usually sensitized by suitable organic antenna chromophores.4 There are now a number of reports on designing aromatic ligands with strongly delocalized π-systems5 or using d-block metal chromophores6 that absorb in the near-UV and visible ranges as sensitizers for the near-infrared luminescence. Nevertheless, the requirement for multistep synthetic procedures and the uncertified stability of the complicated systems tends to limit the practical application as luminescence materials. On the other hand, various lanthanide-based MOFs with specific topologies such as cage-type structures or honeycomb networks have been obtained by the self-assembly of lanthanide salts with rigid or flexible organic ligands. Through the elaborate choice of organic linkers and effective synthetic strategy, lanthanide-based MOFs exhibiting intense and long-lived fluorescent emissions with higher stability could be achieved.7 Macrocyclic polyamines or their functionalized derivatives have received increasing interest in the last two decades for * To whom correspondence should be addressed. Fax: 86-591-83796710. E-mail: [email protected]. † Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. ‡ Graduate School, Chinese Academy of Sciences.

Figure 1. Formula schemes of some N-functionalized macrocyclic polyamine ligands.

their widespread applications in magnetic resonance imaging (MRI), luminescent probes and in radioimmunotherapy.8 We have recently chosen a tetraaza-functionalized macrocyclic polyamine ligand, 1,4,8,11-tetraazacyclotetradecane-1,4,8,11tetraacetic acid (H4TETA), as the multifunctional organic linker to construct luminescent lanthanide-based MOFs. Previous studies of this class of macrocyclic ligands (Figure 1) mainly focused on the development of simple mononuclear lanthanide complexes based on the 1,4,7,10-tetraazacyclododecane (cyclen) functionalized derivatives.9 Our research involved in using efficient synthetic procedures to synthesize multidimensional MOFs, which bear special structures and exhibit intense and long-lived fluorescent emissions. However, the design and elaborate assembly of multidimensional lanthanide-based MOFs with these kind of flexible ligands are still challenging tasks because of high and variable coordination numbers of lanthanide ions and versatile conformation of the ligands. Herein, we present the systematic investigation of the syntheses, crystal structures, and visible and near-infrared luminescence properties

10.1021/cg701098t CCC: $40.75  2008 American Chemical Society Published on Web 04/29/2008

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of a series of lanthanide-based MOFs with flexible macrocyclic polyamine ligands, generally formulated as [Ln(H2TETA)]NO3 · 2H2O (Ln ) Nd (1), Eu (2), Tb (3), Dy (4), Y (5)). Experimental Section Materials and General Methods. A tetraaza-functionalized macrocyclic polyamine ligand, 1,4,8,11-tetraazacyclotetradecane-1,4,8,11tetraacetic acid (H4TETA), was prepared according to a previously reported procedure.10 All commercially available reagents and starting materials were of reagent-grade quality and used without further purification. Elemental analyses (C, H, N) were carried out on an Elementar Vario EL III analyzer. Infrared (IR) spectra were recorded on PerkinElmer Spectrum One as KBr pellets in the range 4000-400 cm-1. Thermogravimetric analysis was recorded with a NETZSCH STA 449C unit at a heating rate of 10 °C min-1 under nitrogen atmosphere. Solid-state luminescence spectra in the visible range were measured at room temperature with an Edinburgh Instrument FLS920 fluorescence spectrometer. This instrument is equipped with an Edinburgh Xe900 xenon arc lamp as exciting light source. The spectra in the near-infrared region were performed on an Edinburgh FLS920 fluorescence spectrometer with a microsecond flash lamp (µF900, Edinburgh) as the excitation source (resolution 1.0 nm). Synthesis of [Nd(H2TETA)] · NO3 · 2H2O (1). A mixture of Nd(NO3)3 · 6H2O (0.083 g, 0.20 mmol) and H4TETA (0.043 g, 0.10 mmol) in deionized water (10 mL) was adjusted to pH 6 with 25% (C2H5)4NOH. It was then sealed in a 25 mL Teflon-lined stainless steel autoclave, heated at 130 °C for 72 h, and then slowly cooled to room temperature over 48 h. Purple prismatic crystals were recovered by filtration, washed by distilled water, and dried in air at ambient temperature. Yield: 67% (based on H4TETA). Calcd for C18H34NdN5O13 (672.74): C, 32.14; H, 5.09; N, 10.41. Found: C, 31.84; H, 5.46; N, 10.45. IR (KBr, cm-1): 3436 (s, br), 3065 (m), 2956 (m), 2843 (m), 2369 (w), 2344 (w), 1609 (vs), 1553 (s), 1451 (s), 1412 (s), 1383 (s), 1320 (s), 1151 (m), 1111 (m), 1072 (m), 973 (w), 721 (w), 611 (w), 568 (w). Synthesis of [Eu(H2TETA)] · NO3 · 2H2O (2). Compound 2 was prepared in the same way as that for 1 but using Eu(NO3)3 · 6H2O (0.089 g, 0.20 mmol) and H4TETA (0.043 g, 0.10 mmol) as the reactants. Colorless prismatic crystals were obtained in a 45% yield based on H4TETA. Calcd for C18H34EuN5O13 (680.45): C, 31.77; H, 5.04; N, 10.29. Found: C, 31.59; H, 5.21; N, 10.32. IR (KBr, cm-1): 3435 (s, br), 3057 (m), 2955(m), 2842 (m), 2345 (w), 1610 (vs), 1556 (s), 1450 (s), 1414 (s), 1373(s), 1321 (s), 1151 (m), 1111 (m), 1072 (m), 973 (w), 721 (w), 612 (w), 570 (w). Synthesis of [Tb(H2TETA)] · NO3 · 2H2O (3). A mixture of Tb(NO3)3 · 6H2O (0.045 g, 0.10 mmol) and H4TETA (0.022 g, 0.05 mmol) was thoroughly mixed with deionized water (1.5 mL) in a heavy-walled Pyrex tube. It was sealed, heated at 130 °C for 72 h, and then slowly cooled to room temperature over 48 h. Colorless prismatic crystals were recovered by filtration, washed by distilled water, and dried in air at ambient temperature. Yield: 52% (based on H4TETA). Calcd for C18H34TbN5O13 (687.41): C, 31.45; H, 4.99; N, 10.19. Found: C, 31.39; H, 5.15; N, 10.12. IR (KBr, cm-1): 3445 (s, br), 3062 (m), 2954 (m), 2840 (m), 2357(w), 2320 (w), 1618 (vs), 1563 (s), 1449 (s), 1415 (s), 1373 (s), 1321 (s), 1151 (m), 1110 (m), 1072 (m), 974 (w), 721 (w), 570 (w). Synthesis of [Dy(H2 TETA)] · NO3 · 2H2O (4). Compound 4 was prepared in the same way as that for 3 but using Dy(NO3)3 · 6H2O (0.047 g, 0.10 mmol) and H4TETA (0.022 g, 0.05 mmol) as the reactants. Colorless prismatic crystals were obtained in a 56% yield based on H4TETA. Calcd for C18H34DyN5O13 (690.99): C, 31.29; H, 4.96; N, 10.14. Found: C, 31.12; H, 5.19; N, 10.23. IR (KBr, cm-1): 3434 (s, br), 3068 (m), 2959 (m), 2848 (m), 2351(w), 2312 (w), 1620 (vs), 1564 (s), 1451 (s), 1416 (s), 1384 (s), 1322 (s), 1152 (m), 1109 (m), 1072 (m), 974 (w), 724 (w), 571 (w). Synthesis of [Y(H2TETA)] · NO3 · 2H2O (5). Compound 5 was prepared in the same way as that for 3 but using Y(NO3)3 · 6H2O (0.039 g, 0.10 mmol) and H4TETA (0.022 g, 0.05 mmol) as the reactants. Colorless prismatic crystals were obtained in a 60% yield based on H4TETA. Calcd for C18H34YN5O13 (617.39): C, 35.02; H, 5.55; N, 11.34. Found: C, 34.95; H, 5.78; N, 11.22. IR (KBr, cm-1): 3420 (s, br), 3064 (m), 2954 (m), 2841 (m), 2359(w), 2320 (w), 1624 (vs), 1566 (s), 1451 (s), 1417 (s), 1367 (s), 1322 (s), 1152 (m), 1110 (m), 1072 (m), 974 (w), 724 (w), 572 (w).

Zhu et al. Table 1. Crystallographic Data for Complexes 1-5

empirical formula fw cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalcd (g/cm3) µ (mm-1) R1,a wR2b (I > 2σ(I) R1,a wR2b (all data) GOF on F2

1

2

3

C18H34NdN5O13 672.74 triclinic P1j 8.877(2) 11.845(3) 13.626(3) 65.487(9) 72.034(12) 68.491(11) 1192.0(5) 2 1.874 2.257 0.0392, 0.0934 0.0432, 0.1062 1.156

C18H34EuN5O13 680.46 triclinic P1j 8.8994(12) 11.8747(15) 13.5596(19) 65.094(7) 71.784(9) 68.418(9) 1187.3(3) 2 1.903 2.720 0.0306, 0.0766 0.0348, 0.0910 1.172

C18H34TbN5O13 687.42 triclinic P1j 8.9441(3) 12.0863(4) 13.5695(4) 101.594(1) 108.780(1) 111.399(1) 1205.72(7) 2 1.893 3.010 0.0489, 0.1276 0.0559, 0.1437 1.276

empirical formula fw cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalcd (g/cm3) µ (mm-1) R1,a wR2b (I > 2σ(I)) R1,a wR2b (all data) GOF on F2 a

4

5

C18H34DyN5O13 691.00 triclinic P1j 8.9432(12) 12.114(2) 13.550(2) 101.844(10) 108.954(8) 111.228(7) 1204.3(3) 2 1.906 3.180 0.0320, 0.0872 0.0354, 0.0954 1.170

C18H34YN5O13 617.41 triclinic P1j 8.9506(5) 12.1202(6) 13.5301(7) 101.7800(10) 108.9730(10) 111.2600(10) 1204.12(11) 2 1.703 2.500 0.0566, 0.1486 0.0651, 0.1617 1.141

R ) ∑|Fo| - |Fc|/∑|Fo|. b wR(F2) ) [∑w(Fo2 - Fc2)2/∑w(Fo2)2]1/2.

X-ray Crystallographic Study. Data collection was performed on a SIEMENS SMART CCD (complexes 3, 5) or Rigaku Mercury CCD (complexes 1, 2, 4) diffractometer with graphite-monochromated Mo KR (λ ) 0.71073 Å) radiation at room temperature. The structures were solved by direct methods and refined by the full-matrix leastsquares on F2 using the SHELXTL-97 program.11 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 0.99 Å). Idealized positions of H atoms belonging to nitrogen atoms and water molecules were located from Fourier difference maps and refined isotropically. Crystallographic data and structure determination summaries are listed in Table 1. Selected bond lengths and angles of complex 1 are listed in Table 2, and the bond lengths and angles of the other complexes are listed in Table S1-S4 in the Supporting Information.

Results and Discussion Prismatic crystals of 1-5 have been successfully synthesized through the preheating and cooling-down crystallization approach. Single-crystal X-ray diffraction, elemental analysis, and vibrational spectroscopic studies preformed on complexes 1-5 reveal that they are isomorphous. Therefore, the structure of complex 1 is selected and described in detail to represent their frameworks. Crystal Structure of Complex 1. X-ray diffraction study of complex 1 reveals that the asymmetric unit contains one crystallographically unique Nd(III) motif. As shown in Figure 2, the Nd(III) atom is coordinated by eight carboxylate oxygen atoms from six different macrocyclic ligands. The Nd-O bond distances range from 2.387(3) to 2.550(3) Å, and the O-Nd-O bond angles range from 69.40(11) to 146.25(12)°, all of which

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Table 2. Selected Bond Lengths (Å) and Angles (deg) for Complex 1a Nd(1)-O(1) Nd(1)-O(3)#1 Nd(1)-O(7)#2 Nd(1)-O(6)#2 O(3)#1-Nd(1)-O(1) O(3)#1-Nd(1)-O(7)#2 O(1)-Nd(1)-O(7)#2 O(3)#1-Nd(1)-O(6)#2 O(1)-Nd(1)-O(6)#2 O(7)#2-Nd(1)-O(6)#2 O(3)#1-Nd(1)-O(8)#3 O(1)-Nd(1)-O(8)#3 O(7)#2-Nd(1)-O(8)#3 O(6)#2-Nd(1)-O(8)#3 O(3)#1-Nd(1)-O(4)#4 O(1)-Nd(1)-O(4)#4 O(7)#2-Nd(1)-O(4)#4 O(6)#2-Nd(1)-O(4)#4

2.393(3) 2.387(3) 2.439(3) 2.467(3) 146.25(12) 139.43(12) 73.84(11) 72.59(11) 140.57(11) 69.40(11) 78.14(11) 75.21(11) 124.16(11) 138.76(12) 114.48(12) 77.79(12) 69.64(11) 76.56(12)

Nd(1)-O(8)#3 Nd(1)-O(4)#4 Nd(1)-O(5) Nd(1)-O(2)#1 O(8)#3-Nd(1)-O(4)#4 O(3)#1-Nd(1)-O(5) O(1)-Nd(1)-O(5) O(7)#2-Nd(1)-O(5) O(6)#2-Nd(1)-O(5) O(8)#3-Nd(1)-O(5) O(4)#4-Nd(1)-O(5) O(3)#1-Nd(1)-O(2)#1 O(1)-Nd(1)-O(2)#1 O(7)#2-Nd(1)-O(2)#1 O(6)#2-Nd(1)-O(2)#1 O(8)#3-Nd(1)-O(2)#1 O(4)#4-Nd(1)-O(2)#1 O(5)-Nd(1)-O(2)#1

2.500(3) 2.530(4) 2.531(3) 2.550(3) 143.36(12) 73.20(11) 78.84(11) 142.33(11) 124.57(11) 71.19(11) 79.69(11) 72.66(12) 116.44(12) 83.10(11) 72.99(11) 70.93(11) 144.70(11) 132.99(11)

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

Figure 2. ORTEP representation of the coordination environment of the eight-coordinated Nd(III) motif in complex 1, with thermal ellipsoids at the 30% probability level.

Figure 3. (a) View of 2D framework in complex 1 constructed from 1D inorganic chains cross-linked via the macrocyclic rings of TETA. (b) View of the formation of 1D inorganic chains in complex 1, in which the paddle-wheel SBUs {Nd2(CO2R)4} have been shown.

are comparable to those reported for other neodymium-oxygen donor complexes.7b,12 The resulting coordination polyhedron of Nd(III) adopts a slightly distorted square-antiprism geometry, which can be deduced by calculating the least-squares equation of the mean planes and torsion angles between pairs of adjacent faces. In turn, each macrocyclic ligand connects six Nd(III) atoms with its four acetate groups adopting µ2-η1:η1 bridging binding mode. The 14-memberred macrocyclic ring adopts a rectangular [3434] conformation by calculating the sequence of torsion angles.13 It should be mentioned that the organic linkers coordinate to metal centers just using pendant carboxylate groups, whereas the nitrogen atoms on the ring are free of coordination. To the best of our knowledge, this is a rare example in macrocyclic chemistry in which the metal ion does not actually enter the internal cavity of the chelating ligand but left empty coordination site.10,14 The unique coordination pattern of the macrocyclic ligand in the complex gives us new conception to explore their specific applications, such as binding of small guests in the cavity while maintaining the porosity of the solid state structure. In the polymeric structure of complex 1, two crystallographically equivalent Nd(III) atoms are bridged by four COOgroups in bis-monodentate syn-anti fashion to give a paddlewheel secondary building units (SBUs) {Nd2(CO2R)4}. As illustrated in Figure 3, the paddle-wheel SBUs connect each other along the [100] direction leading to one-dimensional (1D)

inorganic chains arrayed in the · · · ABAB · · · fashion, with alternate intrachain Nd · · · Nd distances of 4.411 and 4.486 Å. The 1D infinite chains are further cross-linked via the macrocyclic rings of TETA in the [010] and [001] directions respectively, to form a 3D framework (Figure 4). Noticeably, the flexible macrocyclic rings in complex 1 act as pillar linkers extending the structure into multidimensional MOFs. It is also noteworthy that the 3D framework contains 13.26 × 10.94 Å2 rhombic channels along the [100] direction (calculated from the distances of metal centers), which are occupied by the guest water molecules and dissociative nitrate anions. Protonation Scheme of the Macrocyclic Ligand. The macrocyclic ligand exhibiting peculiar protonation scheme gives the compounds formula in the form of [Ln(H2TETA)]NO3 · 2H2O. The carboxylate groups are fully deprotonated while half of the amino groups are protonated in the asymmetrical unit. This is clearly indicated by the longer C-N distances in the case of protonated nitrogen atoms N2 and N4 (mean 1.498 Å) than in the case of unprotonated atoms N1 and N3 (mean 1.463 Å). The µ2-η1:η1 bridging coordination mode of the ligand results in fully deprotonated carboxylate groups. As shown in Figure S1 in the Supporting Information, the similar FT-IR spectra of complexes 1-5 are consistent with the protonation scheme. The spectrum of complex 1 clearly exhibits the characteristic vibrational bands for carboxylate groups (COO- · · · Ln3+) at 1609 cm-1 for the asymmetric stretching and at 1553 cm-1 for

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Figure 4. Perspective view of 3D framework structure of complex 1, showing the guest water molecules and dissociative nitrate anions distributed in the rhombic channels.

symmetric stretching. Meanwhile, the lack of the band at 1693-1730 cm-1 (υCdO for COOH) demonstrates the complete deprotonation of the carboxylate groups. Typical vibrational bands at 2369 and 2344 nm (NH+/OH intercombination bands) suggest the presence of protonated amino groups. The analogous protonation scheme was also observed in complex [Mg(H2TETA)(H2O)4] · 4H2O reported by Maurya.10 The strong and broad absorption centered at 3436 cm-1 is attributed to the presence of water molecules in the complex. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) of complex 1 (see Figure S2 in the Supporting Information) reveals that the first weight loss of 5.18% occurred from 99 to 169 °C, corresponding to the loss of two crystallization guest water molecules (Calcd 5.35%). No further weight loss was observed until it reached 300 °C, at which the decomposition of the complex occurred. TGA data shows the enhanced stability of coordination framework, which is a very significant value given that only flexible macrocyclic ligands are involved. Photoluminescence Properties. Taking into account the excellent luminescence properties of Eu(III) and Tb(III) ions in the visible region and Nd(III) ions in the near-infrared region, the luminescence properties of complexes 1-3 were investigated in the solid state at room temperature. Complex 2 exhibits an intense, characteristic transition spectrum of Eu(III) ions upon excitation with a wavelength of 394 nm. As shown in Figure 5a, transitions from the excited 5D0 state to the different J levels of the lower 7F state were observed in the emission spectrum (J ) 0-4), i.e., 5D0 f 7F0 at 579 nm, 5D0 f 7F1 at 592 nm, 5 D0 f 7F3 at 613 nm, 5D0 f 7F3 at 651 nm, and 5D0 f 7F4 at 698 nm. It is well-known that the 5D0 f 7F3 transition of Eu(III) is of electric-dipole (ED) nature and very sensitive to site symmetry, while the 5D0 f 7F1 transition is of magnetic-dipole (MD) nature and insensitive to site symmetry. The emission intensity of the so-called hypersensitive 5D0 f 7F2 transition is much stronger than that of 5D0 f 7F1 transition, indicating that Eu(III) ions locate at a low-symmetry site without inversion center, in agreement with the crystal structural analysis. Moreover, the 5D0 f 7F0 transition of Eu(III) induced by crystalfield J mixing is present in the emission spectrum, which is only allowed for the following 10 site symmetries, Cs, C1, C2, C3, C4, C6, C2V, C3V, C4V, and C6V, according to the ED selection rule.15 The one line observed for 5D0 f 7F0 transition and three lines splitting for 5D0 f 7F1 transition of Eu(III) suggests C2V, C2, Cs, or C1 site symmetry. Combined with the diffraction study

Figure 5. Solid-state emission spectra for complexes (a) 2, (b) 3, and (c) 1 at room temperature (excitation at 394, 369, and 340 nm, respectively).

of the triclinic crystal system, the actual symmetry of Eu(III) site could be the lowest site symmetry C1. An interesting characteristic feature of complex 2 can be found in the 5D0 f 7 F4 transition, which has the strongest intensity in the visible region. This indicates the hypersensitivity to the low site symmetry in the 5D0 f 7F4 transition rather than 5D0 f 7F3 transition.16 An enhancement of the 5D0 f 7F4 luminescence intensity has also been observed for Eu(TETA) and Eu(terpyridyl)3.17 Complex 3 gave typical Tb(III) emission spectrum containing the expected sequence of 5D4 f 5D J (J ) 3-6) transitions upon excitation at 369 nm (Figure 5b). The spectrum is dominated by the 5D 4 f 5D5 transition at 545 nm, which gave an intense green luminescence output for the sample. The other three bands at 492, 584, and 620 nm correspond to transition

MOFs with Flexible Macrocyclic Polyamine Ligands

from 5D4 state to 7F6, 7F4, and 7F3 levels, respectively. Under an excitation of 340 nm, complex 1 displays a strong emission band at 1062 nm (4F3/2 f 4I11/2), an emission band at 895 nm (4F3/2 f 4I9/2) with a much lower intensity, and a very weak band at 1349 nm (4F3/2 f 4I13/2) in the near-infrared region (Figure 5c). The profiles of the three characteristic bands and the relative intensity for complex 1 are consistent to the previously reported spectra of Nd(III) complexes.6f,7b The luminescence decay curves of complexes 2 and 3 were obtained at room temperature. The decay curves are well fitted into a single-exponential function as I ) I0exp(-t/τ), indicating the occupation of the same average local environment of Ln(III) sites in the structure.18 The corresponding lifetime for complex 2 is about 2.14 ms, whereas that for complex 3 is about 2.60 ms (determined by monitoring 5D0 f 7F3 and 5D4 f 7F5 line, respectively). Both of them have long luminescence lifetimes at millisecond order, which are comparable to other corresponding Eu(III) and Tb(III) complexes.19 As described above, the structures are constructed from the paddle-wheel SBUs that lead to 1D inorganic chains. Thus the lanthanide ions are encapsulated in a rigid architecture, shielding them from interactions with the environment, especially from interactions with small molecules like water. Radiationless decay pathways associated with the close proximity of high-energy C-H, N-H, or O-H oscillators to the lanthanide centers can be eliminated. Efficient fluorescent emissions for complexes 1-3 were then achieved, which may act as good candidates for efficient luminescence materials and fluorescent probes.

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Conclusion In summary, a series of lanthanide-based MOFs formulated as [Ln(H2TETA)]NO3 · 2H2O (Ln ) Nd (1), Eu (2), Tb (3), Dy (4), Y (5)) have been successfully synthesized through the preheating and cooling-down crystallization approach. Complexes 1-5 represent the first systematic investigation of the syntheses, crystal structures, visible and near-infrared luminescent properties of lanthanide-based MOFs with flexible macrocyclic polyamine ligands. Their structures are built up from the paddle-wheel SBUs, which lead to 1D inorganic chains and further 3D framework with 13.26 × 10.94 Å2 rhombic channels. The macrocyclic ligand displays special coordination pattern and protonation scheme. Complexes 1-3 exhibit strong fluorescent emissions in the visible and near-infrared region at room temperature. Formation of paddlewheel structure plays crucial role in realizing efficient fluorescent emissions of complexes 1-3. Thus, these complexes could be anticipated as good candidates for efficient luminescent materials and fluorescent probes. Acknowledgment. This work was financially supported by the 973 Program (2006CB932900/03), NSFC (90206040, 20325106, 20521101, 20731005), NSF of Fujian Province (2005HZ01-1, E0520003), Fujian Key Laboratory of Nanomaterials (2006L2005), “The Distinguished Oversea Scholar Project”, “One Hundred Talent Project”, and Key Project from CAS. Supporting Information Available: Crystallographic file for compounds 1-5 in CIF format; selected bond lengths and angles for 2-5 (Table S1-S4); FT-IR spectra for 1-5 (Figure S1); TGA curve for 1 (Figure S2) (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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CG701098T