English

• 1. INTRODUCTION

  The design and construction of metal-organic frameworks (MOFs), as one of the most active research areas, have attracted much attention in recent years, not only because of their intriguing structures but also due to their interesting properties and potential applications^[1-5]. Particularly, the luminescent MOFs arising from the conjugated organic ligands and/or metal ions or clusters building components have emerged as a kind of novel photoluminescence sensors to detect the guest molecules by turn-off and turn-on effects on guest molecules^[6-8]. In crystal engineering, the assembly of MOFs mainly depends on the rational choice of the bridging ligand and metal centers. In addition, other reaction con- ditions such as temperature, solvent, pH value and the nature of anions can affect the resulting framework^[9-12]. It should be mentioned that the design and reasonable use of the characteristic ligand is the most key factor. Generally, two important kinds of ligands including the N- and O-donor organic compounds, are widely employed to construct diverse MOFs due to their various coordination modes and modifiable backbones^[13-15]. More recently, a new type of rigid ligands 1, 4-di(1H-imidazol-4-yl)benzene with 4-imidazolyl group have been designed by our group, and employed to fabricate a series of porous metal-imidazolate complexes with excep- tional gas adsorption properties^[16-18]. Moreover, a series of diverse MOFs have been built based on the mixed system of imidazole and polycarboxylates^{[19, 20]}. Taking their good compatibility for the mixed system including multi-N donor and carboxylate ligands into account, we choose the rigid rod-type 1, 4-di(1H-imidazol-4-yl)benzene ligand (L) together with a semirigid aromatic dicarboxylate ligand 4-carbonyl- phenylacetic acid (H₂cpa) to build novel MOFs as an extension of our work. Here we report the synthesis and crystal structure of {[Zn₂(L)(cpa)₂]·H₂O}_n (1) by reacting zinc sulfate heptahydrate with the mixed ligands of L and H₂cpa.

  2. EXPERIMENTAL

  2.1 Materials and measurements

  The 1, 4-di(1H-imidazol-4-yl)benzene organic ligand was synthesized according to the reported literature^[16]. The reagents were used as commercial sources without further purification. Elemental analyses were performed on a Perkin- Elmer 240C elemental analyzer. The IR spectra were recorded on a Bruker Vector22 FT-IR spectrophotometer using KBr discs. Power X-ray diffraction (PXRD) patterns were measured on a Shimadzu XRD-6000 X-ray diffractometer with CuKα (λ = 1.5418 Å) radiation at room temperature. The UV-vis spectra were recorded using a computer-controlled PE Lambda 900 UV-vis spectrometer. The luminescent spectra for the powdered solid sample was recorded at room tempera- ture on an Aminco Bowman Series 2 spectrofluorometer with a xenon arc lamp as the light source. In the measurements of emission and excitation spectra, the pass width is 5.0 nm. All the measurements were carried out under the same conditions.

  2.2 Synthesis of the compound {[Zn₂(L)(cpa)₂]·H₂O}_n (1)

  Reaction mixture of L (21.2 mg, 0.1 mmol), ZnSO₄·7H₂O (28.7 mg, 0.1 mmol), H₂cpa (18.0 mg, 0.1 mmol) and H₂O (8 mL) was adjusted to pH = 7 with 0.5 mol·L^-1 NaOH solution. The mixture was then sealed into a 16 mL Teflon-lined stainless-steel container and heated at 120 ºC for 3 days. After cooling to room temperature, colorless block crystals of 1 were collected by filtration and washed by water and ethanol for several times with a yield of 65% (based on L ligand). Anal. Calcd. for C₁₅H₁₃N₂O₅Zn (%): C, 49.14; H, 3.57; N, 7.64. Found: C, 48.89; H, 4.09; N, 7.81. IR (KBr pellet, cm^-1): 3439(s), 1610(s), 1551(vs), 1440(m), 1415(m), 1356(s), 1176(w), 1146(m), 1130(m), 1081(w), 1051(w), 976(m), 841(m), 828(m), 783(m), 756(s), 689(m), 646(m), 647(w), 490(w), 419(w).

  2.3 Crystal structure determination

  A colorless crystal of complex 1 was selected for diffrac- tion data collection at 296(2) K on a Bruker Smart Apex II CCD diffractometer equipped with a graphite-monochromatic MoKα radiation (λ = 0.71073 Å). A total of 13516 reflections were collected, of which 3360 (R_int = 0.0551) were inde- pendent in the range of 1.94≤θ≤27.63º by using a φ-ω scan mode. The structure was solved by direct methods with SHELXS-97^[21] program and refined by full-matrix least- squares techniques on F² with SHELXL-97^[22]. All non-hydro- gen atoms were refined anisotropically. All hydrogen atoms except those of water molecules were generated geometrically and refined isotropically using the riding model. For 1, the final R = 0.0385, wR = 0.0945 (w = 1/[σ(F_o) + (0.0653P)² + 0.0000P], where P = (F_o² + 2F_c²)/3), R_int = 0.0551, (Δ/σ)_max = 0.001, S = 1.011, (Δρ)_max = 0.413 and (Δρ)_min = –0.506 e/ų, and its selected bond distances and bond angles are listed in Table 1.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) of {[Zn₂(L)(cpa)₂]·H₂O}_n (1)

  DownLoad: CSV

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Zn(1)–O(1)	1.962(2)		Zn(1)–N(1)	1.977(3)		Zn(1)–O(3)#1	2.017(2)
  Zn(1)–O(4)#2	2.034(2)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Zn(1)–N(1)	129.71(1)		O(1)–Zn(1)–O(3)#1	112.19(1)		N(1)–Zn(1)–O(3)#1	97.09(1)
  O(1)–Zn(1)–O(4)#2	111.96(1)		N(1)–Zn(1)–O(4)#2	104.74(1)		O(3)#1–Zn(1)–O(4)#2	95.02(1)
  Symmetry transformation: #1: x+1/2, –y+1/2, –z+1; #2: x, –y+1/2, z+1/2

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure of {[Zn₂(L)(cpa)₂]·H₂O}_n (1)

  The result of X-ray diffraction analysis revealed that complex 1 crystallizes in orthorhombic system with space group Pbca, and the asymmetric unit consists of one crystal- lographically distinct Zn(II) atom, one cpa^2-, one free lattice water molecule and half of an L ligand (Fig. 1). The Zn(1) atom forms a slightly distorted tetrahedron [ZnO₃N] by one -COO^- oxygen atom (O(1)) and one -CH₂COO^- oxygen atom (O(3A), O(4B)) from three symmetry-related cpa^2- and one imidazolyl nitrogen atom (N(1)) of the L ligand. The Zn–O bonds range from 1.962(2) to 2.034(2) Å, and Zn–N is 1.977(3) Å, while the bond angles around the Zn(1) atom fall in the 95.02(1)~129.71(1)º region (Table 1). In this compound, each cpa^2- ligand acts as a μ₃-bridge to link three Zn(II) atoms. Two carboxylate groups adopt µ₁-η¹: η⁰-mono- dentate and μ₂-η¹: η¹-bridging coordination modes, respec- tively. Without considering the connections of L ligands, Zn(II) atoms are interlinked by cpa^2- ligands to form a 2D [Zn(cpa)] layer structure (Fig. 2), and such 2D layers are pillared by L ligands to complete the layered-pillared 3D framework structure of 1 (Fig. 3). Moreover, rich hydrogen bonding interactions (N(7)···O(5)^#1 2.788(4) Å, N(2)–H(2A)···O(5) 173°; O(5)···O(1)^#2 2.840(4) Å, O(5)–H(5A)···O(1) 163(5)°; O(5)···O(4)^#3 2.817(4) Å, O(5)–H(5B)···O(4) 162(5)°) exist and further stabilize the framework of 1 (Table 2). From the view of topology, each cpa^2- links three Zn(II) atoms, which can be regarded as a 3-connected node. As for each Zn(II) atom, it in turn links three cpa^2- and one L ligand. Hence, it can be treated as a 4-connector. According to the simplification principle, the resulting structure of complex 1 is a binodal (3, 4)-connected net with a Point (Schläfli) symbol (6³·10³)(6³), which has been referred as the tcj/hc notation (Fig. 4)^[23].

  Figure 1

  

  Figure 1.  View of the coordination environment of Zn(II) atom with thermal ellipsoids drawn at the 30% probability level for complex 1 (symmetry codes: #1: 0.5 + x, 0.5 – y, 1 – z; #2: x, 0.5 + y, 0.5 – z; #3: 2 – x, – y, 2 – z)

  DownLoad: Full-Size Img PowerPoint

  Figure 2

  

  Figure 2.  (a) 2D network constructed from [Zn(cpa)] in 1, (b) Simplified 2D 6³-hcb net. Turquiose balls and the pink ones represent Zn(II) atoms and the centers of benzene rings of L ligands, respectively

  DownLoad: Full-Size Img PowerPoint

  Figure 3

  

  Figure 3.  3D structure of 1 built from the 2D layers (yellow) pilled by L ligands (blue)

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  Table 2

  

  Table 2.  Intermolecular Hydrogen Bonding Interactions (Å, °)

  DownLoad: CSV

  D–H···A	D–H	H···A	D···A	∠DHA
  N(2)–H(2A)···O(5)#1	0.86	1.93	2.788(4)	173
  O(5)–H(5A)···O(1)#2	0.87(6)	2.00(6)	2.840(4)	163(3)
  O(5)–H(5B)···O(4)#3	0.81(5)	2.03(5)	2.817(4)	162(5)
  Symmetry codes: #1: 1+x, –1–y, z; #2: 1–x, 1/2+y, 3/2–z; #3: 3/2–x, 1–y, 1/2+z

  Figure 4

  

  Figure 4.  Schematic illustration of binodal (3, 4)-connected tcj/hc net with a point (Schläfli) symbol (6³·10³)(6³) in 1

  DownLoad: Full-Size Img PowerPoint

  3.2 IR spectrum, thermal analyses and X-ray powder diffraction analyses

  The infrared spectrum of the complex has been recorded between 4000 and 500 cm^-1. The IR spectra exhibit strong absorption centered at 3439 cm⁻¹ for 1, corresponding to the N−H/O−H stretching vibration of ligand or water molecule (see experimental section). Strong characteristic bands of carboxylic group are observed in the range of 1610~1440 cm⁻¹ for asymmetric vibrations and 1415~1356 cm⁻¹ for symmetric vibrations, respectively. The characteristic bands ranging from 1176 to 756 cm⁻¹ are attributable to the vibra- tions from aromatic nucleus. The absence of absorption peak at 1720 cm^-1 indicates that the H₂cpa ligand is fully deproto- nated and participates in coordination with Zn(II) atoms.

  The stability of complex 1 was evaluated by thermogravi- metric analysis (TGA), and the results are listed in Fig. 5. The TGA data of 1 show a 4.72% weight loss (calcd. 4.90%) around 100 ºC due to the removal of one lattice water molecule, and the residue collapses at about 320 ºC. The diffraction peaks of the as-synthesized 1 are consistent with the simulated PXRD patterns from single-crystal diffraction anyalysis results, which confirms that the as-synthesized crystal of 1 is phase pure, as shown in Fig. 6.

  Figure 5

  

  Figure 5.  Thermal analysis curve of complex 1

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  Figure 6

  

  Figure 6.  Powder X-ray diffraction patterns of complex 1

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  3.3 Optical property

  3.3.1   Diffuse reflectance spectra

  The UV-vis absorption spectra of 1 in solid state are recorded in Fig. 7. The UV-vis diffuse reflectance spectra showed intense absorption peaks in the UV range of 250~300 nm, which corresponds to intraligand n → π* and π → π* transitions^[24]. Furthermore, the diffuse reflectance data obtained were transformed into a Kubelka-Munk function to get their band gap (Eg), which can be employed to evaluate the semiconductivity of the complexes. The value of Eg is estimated as 3.55 ev for compound 1 (Fig. 7), which can be determined by the theory of optical absorption for direct band gap semiconductor: (Ahν)² = B(hν – Eg), indicating that the as-synthesized crystal material is an optical semiconductor^{[25, 26]}.

  Figure 7

  

  Figure 7.  (a) UV-Vis spectra for 1 and (b) Eg value for 1 treated with Kumble-Munk function

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  3.3.2   Photoluminescent property

  Inorganic-organic hybrid coordination polymers comprising of π-conjugated organic ligands and d¹⁰ metal centers have potential photoluminescent properties because of their interaction between metal and ligands^{[27, 28]}. The fluorescence property of complex 1 has been tested in the solid state (Fig. 8). As shown in Fig. 8, compound 1 shows strong broad emission band at 430 nm upon 368 nm excitation on complexation of the ligands with Zn(II) atoms, which may be attributable to the coordination interactions between the ligand and central metal Zn(II) atom^[29]. Because the crystalline material 1 exhibits good luminescence property, the decay lifetime and quantum yield (QY) were further investigated (Fig. 9). The QY value of compound 1 is 2.53%, which is probably attributed to the immobilization of the L ligand as it strongly coordinates to the metal Zn(II) ions, which effectively increases the rigidity of the ligands^[30]. The exponential func- tion as I(t) = Aexp(−t/τ) was employed to fit the luminescence decay curves^{[31, 32]}. The luminescence lifetime of complex 1 is 14.23 ns, indicating the characteristic of a singlet state for the emissions because of their shorter luminescence lifetime than a triplet state (> 10⁻³ s)^[33]. Therefore, the good photolu- minescence property of 1 indicates it could be a potential luminescent material.

  Figure 8

  

  Figure 8.  Solid-state photoluminescent spectra of 1 at room temperature

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  Figure 9

  

  Figure 9.  QY (a) and decay curve (b) of compound 1

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• 

  Figure 1  View of the coordination environment of Zn(II) atom with thermal ellipsoids drawn at the 30% probability level for complex 1 (symmetry codes: #1: 0.5 + x, 0.5 – y, 1 – z; #2: x, 0.5 + y, 0.5 – z; #3: 2 – x, – y, 2 – z)

  下载: 全尺寸图片 幻灯片
  

  Figure 2  (a) 2D network constructed from [Zn(cpa)] in 1, (b) Simplified 2D 6³-hcb net. Turquiose balls and the pink ones represent Zn(II) atoms and the centers of benzene rings of L ligands, respectively

  下载: 全尺寸图片 幻灯片
  

  Figure 3  3D structure of 1 built from the 2D layers (yellow) pilled by L ligands (blue)

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  Figure 4  Schematic illustration of binodal (3, 4)-connected tcj/hc net with a point (Schläfli) symbol (6³·10³)(6³) in 1

  下载: 全尺寸图片 幻灯片
  

  Figure 5  Thermal analysis curve of complex 1

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  Figure 6  Powder X-ray diffraction patterns of complex 1

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  Figure 7  (a) UV-Vis spectra for 1 and (b) Eg value for 1 treated with Kumble-Munk function

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  Figure 8  Solid-state photoluminescent spectra of 1 at room temperature

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  Figure 9  QY (a) and decay curve (b) of compound 1

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) of {[Zn₂(L)(cpa)₂]·H₂O}_n (1)

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Zn(1)–O(1)	1.962(2)		Zn(1)–N(1)	1.977(3)		Zn(1)–O(3)#1	2.017(2)
  Zn(1)–O(4)#2	2.034(2)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Zn(1)–N(1)	129.71(1)		O(1)–Zn(1)–O(3)#1	112.19(1)		N(1)–Zn(1)–O(3)#1	97.09(1)
  O(1)–Zn(1)–O(4)#2	111.96(1)		N(1)–Zn(1)–O(4)#2	104.74(1)		O(3)#1–Zn(1)–O(4)#2	95.02(1)
  Symmetry transformation: #1: x+1/2, –y+1/2, –z+1; #2: x, –y+1/2, z+1/2

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  Table 2.  Intermolecular Hydrogen Bonding Interactions (Å, °)

  D–H···A	D–H	H···A	D···A	∠DHA
  N(2)–H(2A)···O(5)#1	0.86	1.93	2.788(4)	173
  O(5)–H(5A)···O(1)#2	0.87(6)	2.00(6)	2.840(4)	163(3)
  O(5)–H(5B)···O(4)#3	0.81(5)	2.03(5)	2.817(4)	162(5)
  Symmetry codes: #1: 1+x, –1–y, z; #2: 1–x, 1/2+y, 3/2–z; #3: 3/2–x, 1–y, 1/2+z

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