English

• 1. INTRODUCTION

  Metal organic frameworks (MOFs), a class of crystalline materials constructed by metal clusters and organic ligands, have been one of the most interesting materials due to their tailorability and porosity^[1]. They have a vital sub-category named Luminescent MOFs (LMOFs) which can adsorb radiation excitation energy and produce photo emission. To sever as heterogeneous sensors, LMOFs have been largely designed and synthesized. The materials can be applied to detect metal ions, nitroaromatic compounds and other small organic molecules^[2]. While most of the detections are performed in water but the majority of MOFs are not stable in it, designing and synthesizing multifunction LMOFs which have water stability are highly demanded^[3].

  Thermooxidatively stable compounds with unique rigid aromatic bicyclic framework, e.g., thiazolo{5, 4-d}thiazole (TTz) and some of its derivatives, which can be simply synthesized through the double condensation reaction of aromatic aldehyde and dithiooxamide^{[4, 5]}, can be used as donor-acceptor-donor (D-A-D) molecules^[6], whose central Lewis-basic nitrogen atoms prompt it to be responsive to strong acids^[7]. Some literatures reveal that 2, 5-bis(4-pyridyl) thiazolo[5,4]thiazole (Py₂TTz), a molecule with rigid π-conjugated system^[8], is a long life-time photo-luminescent material with excellent luminescence properties^[9]. Meanwhile, it has multiple coordination sites, not only on the pyridine nitrogen, but also on the thiazole nitrogen, a peculiarity which can be flexibly applied on MOFs design. However, MOFs based on Py₂TTz are extremely rare, and most of them are based on the strategy of mixing with carboxylic acid ligands^[10]. Herein, we synthesized a MOF named CdPyTTz with single Py₂TTz and investigated its structure and properties.

  2. EXPERIMENTAL

  2.1 Materials and instruments

  All chemicals were obtained from commercial sources and used without further purification. Elemental analyses (EA) for C, H, and N were carried out on a German Elementary Vario EL III instrument. The infrared (IR) spectra (KBr pellets) were recorded on a Nicolet Magna 750 FT-IR spectrometer in the range of 400~4000 cm^-1. Powder X-ray diffraction (PXRD) data were recorded on a Rigaku MiniFlex II using CuKα radiation and diffractometer with a scan speed of 5 º·min^-1. Thermogravimetric analyses (TGA) were carried out on a NETSCHZ STA-449C thermoanalyzer under N₂ (range, 25~800 ºC) at a heating rate of 10 ºC·min^-1.

  2.2 Synthesis of CdPyTTz

  CdPyTTz was obtained by mixing Py₂TTz (55 mg, 0.19 mmol) and Cd(NO₃)₂·4H₂O (30 mg, 0.24 mmol) in the solution of DMSO (5 mL) with the addition of 5 drops of HF (40%). Then the mixture was transferred into a 25 mL Teflon-lined stainless-steel vessel, and heated at 150 ºC for 3 days. After cooling down to room temperature, block yellow crystals were obtained. Yield: 60% (based on Py2TTz). FT-IR (4000~400 cm^-1): 3780 (w), 3761 (w), 3633 (w), 3567 (w), 2397 (w), 2318 (w), 1717 (w), 1601 (s), 1397 (s), 1363 (s), 1128 (vs), 671 (w), 613 (s), 458 (w). EA calcd. (C₁₄H₈CdN₄O₄S₃): C, 25.30; H, 1.20; N, 8.43%. Found: C, 28.90; H, 1.60; N, 9.21%.

  2.3 Structure determination

  A yellow block single crystal was selected and mounted on a glass fiber. Single-crystal X-ray diffraction (SCXRD) data were collected on a MM007 CCD diffractometer equipped with graphite-monochromatic MoKα radiation (λ = 0.71073 Å) at 293 K. A total of 6301 reflections were collected at 293 K in the range of 3.20≤θ≤27.47º by using an ω-scan mode, of which 1057 were unique with R_int = 0.0589. The structure was solved by direct methods with SHELXS and refined by full-matrix least-squares methods with SHELXL2014 program package^[11]. Non-hydrogen atoms were located with successive difference Fourier technique and refined anisotropically, while hydrogen atoms were added in the idealized positions. The final R = 0.0434, wR = 0.0948 (w = 1/[σ²(F_o²) + (0.0333P)² + 0.0412P], where P = (F_o² + 2F_c²)/3), S = 1.072, (Δ/σ)_max = 0.001, (Δρ)_max = 1.092 and (Δρ)_min = –0.471 e·Å^-3. Selected bond lengths and bond angles from X-ray structure analysis are listed in Table 1.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

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  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Cd(1)–N(4)	2.343(9)		C(6)–S(2)	1.734(10)		C(4)–N(2)	1.30(2)
  Cd(1)–O(1)	2.279(5)		C(6)–N(3)	1.33(2)		S(3)–C(5)	1.619(12)
  Cd(1)–N(1)d	2.420(9)		C(4)–S(3)	2.747(9)		N(2)–C(5)	1.38(2)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)d–S(1)–O(1)	110.37(19)		O(1)c–Cd(1)–O(1)a	91.8(6)		S(1)–O(1)–Cd(1)	134(3)
  O(1)a–Cd(1)–O(1)b	131.2(11)		O(1)–Cd(1)–N(1)	80.4(12)		C(6)–N(3)–C(5)	115.1(19)
  O(1)c–Cd(1)–O(1)b	86.2(6)		O(1)–Cd(1)–N(4)	99.6(12)		C(5)–S(2)–C(6)	86.2(6)
  Symmetry transformation: a: –1/2 – y, –1/2 – x, z; b: –1/2 + y, 1/2 + x, z; c: x, y, –1 + z; d: –x, –y, z

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure description

  The compound's structure resembles the 'organic-inorganic' polymeric species reported by Zubieta et al., in which sulfate anions act as bridge to link metal ions, like [Cu(bpe)(MoO₄)] (MOXI-1), rather than being counterions^[12]. Though some compounds using sulfuric acid to bridge the structure have been reported, they used ordinary N-containing ligands as its body (4, 4-bipyridine for instance), not Py₂TTz in their structures^[13]. The single-crystal X-ray structure analysis indicates the compound's three-dimensional framework structure (Fig. 1a) and its coordination mode of the metal ion and the ligand (Fig. 1b). It crystalizes in the tetragonal with P42₁m space group. Hexa-coordinated central metal atom Cd(1) locates in a distorted octahedral environment. Its coordination with the bridge oxygen (O(1)) from the SO₄^2- forms an ab-plane, in which average Cd–O bond distance is 2.27 Å and Cd–Cd shortest contacts 4.9 Å. The bond angle of S(1)–O(1)–Cd(1) is 134°. It should be noted that SO₄^2– comes from the decomposition of dimethyl sulfoxide rather than the raw material. To form the 3D framework, the ligand Py₂TTz, acting as a pillar, connects the plane by coordinating with the Cd(1). The distance between Cd(1) and N(1) is 2.4 Å, and that between layers is about 17 Å. The bond angle of N(4)– Cd(1)–O(1) is 99.6°, and N(1)–Cd(1)–O(1) is 80.4°.

  Figure 1

  

  Figure 1.  (a) Compound's 3D framework structure (b) Compound's coordination mode, symmetry transformation: a: –1/2–y, –1/2–x, z; b: –1/2+y, 1/2+x, z; c: x, y, –1+z; d: –x, –y, z. e: –1–x, –y, z; f: y, –x, –z; g: –y, x, –z (c) View of the compound's structure along the c axis (d) View of the compound's space-filing mode

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  Although the distance between Cd and its neighbor is very short, there is still a certain gap between the ligands. Interestingly, there is a side-to-side pattern between thiophenes with a distance about 4 Å. The distance between thiophenes has been shown to play a key role in materials' electrochromic property^{[5, 14]}. This material provides a good platform for further research on electrochromic and other properties.

  3.2 PXRD, TGA and chemostability

  Experiments demonstrated that the compound is very stable and insoluble in common solvents such as DMF, MeOH, EtOH, H₂O, MeCN, CHCl₃, etc. Besides, we focus on its stability in water. Powder X-ray diffraction results show that it remains stable after being immersed in aqueous solution (pH = 5) for 1 day, also fumed by concentrated hydrochloric acid (37%) (Fig. 2a). The thermogravimetric curve shows that the material can be stabilized to 470 ℃ (Fig. 2b). The excellent stability of the material provides good conditions for performance exploration.

  Figure 2

  

  Figure 2.  (a) Compound's PXRD pattern under water condition via pH and (b) TGA curve of CdPyTTz

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  3.3 Fluorescence studies

  Considering rigid coplanar molecule Py₂TTz with strong π-conjugated effect has been proved to have good luminescence, and the central metal ion Cd²⁺ has d¹⁰ electron configuration, so the compound is expected to present fine luminescence properties^[15]. As reported, the pristine ligand Py₂TTz has two emission peaks, located at 395 and 409 nm, assigned to π*-π transition, upon excitation at 409 nm. The emission peak of the compound appears at 455 nm, which is slightly red-shifted compared to the pristine ligand, upon excitation at 370 nm (Fig. 3a). According to the literature, the redshift can be attributed to the coordination of Cd(II) with the organic linkers which may decrease the π*-π energy gaps of the ligands^[10]. Moreover, the commission internationale de I'Eclairage (CIE) coordinate of the compound is calculated, whose value locates at the blue area (Fig. 3b).

  Figure 3

  

  Figure 3.  (a) Excitation and emission spectra of the compound. (b) CIE chromaticity coordinate of the compound (CIE value, x = 0.1611, y = 0.1625)

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  4. CONCLUSION

  This study reports a new luminescent metal-organic framework, CdPyTTz, synthesized under solvothermal conditions. And its structure was determined by single-crystal X-ray structure analysis. The compound shows good luminescent property and excellent water stability, suggesting it may have potential to be a luminescent sensor in detecting small molecule in water phase. In addition, due to the unique molecular structure, which has the characteristics of electron transmission and high free charge carrier mobilities, TTz has been widely used in the solar/optoelectronic applications. With its outstanding stability, TTz and its derivates may have great potential in further exploration.

  
  
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• 

  Figure 1  (a) Compound's 3D framework structure (b) Compound's coordination mode, symmetry transformation: a: –1/2–y, –1/2–x, z; b: –1/2+y, 1/2+x, z; c: x, y, –1+z; d: –x, –y, z. e: –1–x, –y, z; f: y, –x, –z; g: –y, x, –z (c) View of the compound's structure along the c axis (d) View of the compound's space-filing mode

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  Figure 2  (a) Compound's PXRD pattern under water condition via pH and (b) TGA curve of CdPyTTz

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  Figure 3  (a) Excitation and emission spectra of the compound. (b) CIE chromaticity coordinate of the compound (CIE value, x = 0.1611, y = 0.1625)

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Cd(1)–N(4)	2.343(9)		C(6)–S(2)	1.734(10)		C(4)–N(2)	1.30(2)
  Cd(1)–O(1)	2.279(5)		C(6)–N(3)	1.33(2)		S(3)–C(5)	1.619(12)
  Cd(1)–N(1)d	2.420(9)		C(4)–S(3)	2.747(9)		N(2)–C(5)	1.38(2)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)d–S(1)–O(1)	110.37(19)		O(1)c–Cd(1)–O(1)a	91.8(6)		S(1)–O(1)–Cd(1)	134(3)
  O(1)a–Cd(1)–O(1)b	131.2(11)		O(1)–Cd(1)–N(1)	80.4(12)		C(6)–N(3)–C(5)	115.1(19)
  O(1)c–Cd(1)–O(1)b	86.2(6)		O(1)–Cd(1)–N(4)	99.6(12)		C(5)–S(2)–C(6)	86.2(6)
  Symmetry transformation: a: –1/2 – y, –1/2 – x, z; b: –1/2 + y, 1/2 + x, z; c: x, y, –1 + z; d: –x, –y, z

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