English

• 0.   Introduction

  Coordination polymers (CPs) are a new class of solids consisting of metal-based nodes bridged by organic linkers, which have attracted much attention due to their excellent prospects in many applications, such as gas storage and separation, drug delivery, and heterogeneous catalysis^[1-4]. Fluorescent CPs showed promise in many applications such as sensing and detecting^[5], light-emitting diode^[6-7], up-conversion luminescence^[8], and bioimaging^[9].

  The choice of ligand is one of the most important factors in building CPs of intriguing network structures and properties. Anthracene and pyrene derivatives^[10] and anthracene and pyrene-based CPs showed interesting prospects in metal ions sensing^[11-12], optical properties modulation^[13-14], CO₂ reduction^[15-16], and fluorophores in living cells^[17], etc. Fluorescent properties of those functional CPs were enhanced by using ligands containing a polycyclic aromatic moiety of a large conjugated system.

  Herein, we designed two highly fluorescent organic ligands based on anthracene and pyrene, 9, 10-di(pyridine-4-yl)-anthracene (DPA) and 1, 6-di(1H-imidazol-1-yl)pyrene (DIP), and synthesized six CPs using transitional metals and DPA/DIP. We report the synthesis, crystal structures, and fluorescent properties of these CPs.

  1.   Experimental

  1.1   Materials and methods

  Both DPA and DIP ligands were synthesized following reported procedures^[18-19]. All reagents and solvents for the synthesis and analysis were of analytical grade and were used without further purification. The NMR experiment was performed using a Bruker Advanced Triple HD 400 Spectrometer. The single-crystal X-ray diffraction data were collected using a Bruker SMART APEX Ⅱ diffractometer with a molybdenum source (Mo Kα, λ=0.071 073 nm). The fluorescence spectrum was recorded using a Hitachi F-4600 fluorescence spectrophotometer equipped with a plotter. The crystal structure was solved by the SHELXT and OLEX2 programs with intrinsic phasing and refined by the SHELXL program. All the H-atoms were generated geometrically with isotropic thermal factors. The residual electron densities belonging to disordered solvent molecules in CPs 1 and 2 were treated with a solvent mask.

  1.2   Synthesis of {[Zn(DPA)Cl₂]·DMF·2H₂O}_n (1) and {[Zn_1.5(DPA)_1.5Cl₃]·5H₂O}_n (2)

  ZnCl₂ (0.14 g, 1.0 mmol) and DPA (0.34 g, 1.0 mmol) were dissolved in a mixed solvent of DMF (2.5 mL) and deionized water (0.5 mL). The reaction mixture was transferred into a 25 mL Teflon-lined autoclave and sealed. The autoclave was heated at 70 ℃ for two days, and crystals were formed. Crystals of 1 were prism-shaped and colorless, and crystals of 2 were plate-shaped and colorless, which were picked out under a microscope for single-crystal X-ray diffraction. 1 and 2 were always obtained as mixtures under many different solvothermal conditions, such as varying temperature (50-120 ℃), concentration of reactants, and changing solvent mixtures.

  1.3   Synthesis of {[Zn₃(DPA)₃Br₆]·2DMF·1.5H₂O}_n (3)

  The synthesis of CP 3 was the same as that of CP 1, except that ZnCl₂ was replaced by ZnBr₂ (0.23 g, 1.0 mmol). Colorless crystals of 3 were formed, collected by filtration, and washed with fresh DMF and acetone (Yield: 55% based on ligand DPA).

  1.4   Synthesis of [Co(DPA)(formate)₂(H₂O)₂]_n (4)

  CoCl₂·6H₂O (0.48 g, 2.0 mmol) and DPA (0.34 g, 1.0 mmol) were dissolved in a mixed solvent of DMF (2.5 mL) and H₂O (0.5 mL), and hydrochloric acid (0.1 mL, 1.0 mol·L^-1) was added. The reaction mixture was transferred into a 25 mL Teflon-lined autoclave and sealed. The reaction took place at 70 ℃, and after two days afforded purple crystals, which were collected by filtration, washed with fresh DMF and acetone (Yield: 30% based on ligand DPA). The formate ions in 4 should be obtained in situ in the solvothermal reaction as described in the discussion section.

  1.5   Synthesis of [Cu(DPA)(formate)₂(H₂O)₂]_n (5)

  The synthesis of CP 5 was the same as that of CP 4, except that CoCl₂·6H₂O was replaced by CuCl₂·6H₂O (0.49 g, 2.0 mmol). Dark blue crystals of 5 were collected by filtration and washed with fresh DMF and acetone (Yield: 30 % based on ligand DPA). The formate ions in 5 should also be obtained in situ in the solvothermal reaction as described in the discussion section.

  1.6   Synthesis of {[Zn(DIP)₂Cl]ClO₄}_n (6)

  ZnCl₂ (6 mg, 0.02 mmol), Zn(ClO₄)₂ (8 mg, 0.02 mmol), and DIP (12 mg, 0.04 mmol) were dissolved in DMF (8 mL) and H₂O (4 mL). The reaction mixture was transferred into a 25 mL Teflon-lined autoclave and sealed. The reaction was carried out at 90 ℃ for two days to obtain colorless crystals. The crystals were collected by filtration and washed with pure DMF and acetone to obtain 25 mg of crystals (Yield: 70%, based on ligand DIP). Caution! Perchlorate is a highly corrosive oxidizing agent. Avoid contact with reducing agents and flammable materials, and take protective measures to prevent harm to human health and the environment.

  2.   Results and discussion

  2.1   Crystal structure

  The crystallographic data of the CPs are listed in Table 1. CP 1 crystallizes in space group C2/m. The asymmetric unit of 1 contains two halves of a Zn²⁺ ion, two halves of a DPA ligand, and two Cl^- anions (Fig. 1a). The two Zn²⁺ ions are both tetrahedrally coordinated by two Cl^- ions and two N atoms from DPA ligands. The Zn—N bond lengths are 0.204 9(3) and 0.205 1(2) nm, and Zn—Cl bond lengths are in a range of 0.219 7(5)-0.221 8(6) nm.

  Table 1

  

  Table 1.  Crystallographic data of CPs 1-6

  下载: 导出CSV

  Parameter	1	2	3	4	5	6
  Formula	C24H16Cl2N2Zn	C24H16Cl2N2Zn	C78H62Br6N8O2Zn3	C26H22CoN2O6	C26H22CuN2O6	C176H112Cl8N32O16Zn4
  Molecular weight	468.66	468.66	1 818.92	517.38	521.99	3 476.05
  Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Tetragonal
  Space group	C2/m	P21/m	P21/m	P21/c	P21/c	P4/ncc
  Temperature / K	293	287.21	294.92	296.30	297(2)	243.0
  a / nm	1.981 7(2)	1.2213 7(4)	1.220 6(1)	1.333 22(5)	1.328 2(10)	1.695 66(8)
  b / nm	2.498 4(3)	2.519 05(8)	2.533 5(4)	0.771 31(3)	0.766 4(6)	1.695 66(8)
  c / nm	1.389 9(3)	1.364 27(4)	1.361 1(1)	1.154 12(4)	1.150 0(8)	1.304 46(10)
  β / (°)	130.660(2)	116.048 0(10)	115.681(3)	99.869(2)	99.86(2)
  V / nm3	5.220 4(13)	3.771 1(2)	3.793 3(9)	1.169 25(8)	1.153 4(14)	3.750 7(5)
  Z	4	4	2	2	2	1
  F(000)	1 904	1 428	1 804	534	538	1 776
  μ / mm-1	1.156	1.200	4.151	0.779	0.993	0.856
  Dc / (g·cm-3)	1.193	1.238	1.592	1.470	1.503	1.539
  Rint	0.043 6	0.039 5	0.062 8	0.028 7	0.021 7	0.097 0
  GOF on F 2	1.094	1.039	1.061	1.083	1.091	1.006
  R1 [I > 2σ(I)]	0.053 0	0.042 4	0.054 1	0.034 6	0.028 7	0.047 1
  wR2 [I > 2σ(I)]	0.147 6	0.114 9	0.165 4	0.093 1	0.082 4	0.132 6

  Figure 1

  

  Figure 1.  (a) Molecular structure and coordination environment of the Zn²⁺ ion in CP 1 (Ellipsoid probability level: 50%); (b) Layer structure formed by linking zigzag chains with C—H…Cl interactions (C: gray, H: light gray, Zn: yellow, N: blue, Cl: green); (c) Diagrams showing the interpenetration between two hydrogen-bonded layers of different orientations (Interpenetrated layers: blue and yellow, respectively, C—H…Cl hydrogen bonds: dotted lines); (d) 2D to 3D interpenetration view along the b-axis

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  The Zn²⁺ ions are linked by DPA ligands into a zigzag chain, and adjacent chains are further linked by C—H…Cl interactions between pyridine rings of DPA ligands and coordinated Cl^- ions (Fig. 1b). The H…Cl and C…Cl distances are 0.275 8(3)-0.280 2(3) nm and 0.368 5(2)-0.371 6(3) nm, and the C—H…Cl angles are 167.7(2)° and 175.0(1)°. Individual C—H…Cl interaction is relatively weak as indicated by long interatomic H…Cl and C…Cl distances, while the synergy effect of many C—H…Cl interactions stabilize the relative positions of the zigzag chains and link them into a layer structure (Fig. 1b). The layer structure contains rectangular-shaped windows of dimensions of 0.40 nm×0.90 nm considering van der Waals radii. The C—H…Cl hydrogen-bonded layers in 1 are of two different orientations, which are perpendicular to each other (Fig. 1c). And the perpendicular layers interpenetrate into each other, exhibiting a rare 2D to 3D interpenetrating mode (Fig. 1d).

  CP 2 crystallizes in space group P2₁/m, which is a CP isomer of 1. The asymmetric unit of 2 contains three halves of Zn²⁺ ions, three halves of DPA ligands, and three Cl^- anions (Fig. 2a). The Zn²⁺ ions in 2 are also tetrahedrally coordinated by two Cl^- ions and two N atoms from DPA ligands. The Zn—N bond length is 0.205 2(1) nm, and Zn—Cl bond lengths are 0.218 8(3) and 0.221 0(2) nm, which are also comparable to those in 1.

  Figure 2

  

  Figure 2.  (a) Molecular structure and coordination environment of the Zn²⁺ ion in 2 (Ellipsoid probability level: 50%); (b) A diagram showing the zigzag chains running along the b-axis (C: gray, H: light gray, N: blue, Zn: yellow, Cl: green)

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  The DPA ligands in 2 also connect the Zn²⁺ ions into zigzag chains (Fig. 2b). Different from the hydrogen-bonded layer structure and 2D to 3D interpenetration mode in 1, the zigzag chains in 2 are arranged in parallel, running along the b-axis (Fig. 2b).

  The two isomers 1 and 2 contain Zn²⁺ centres of similar coordination geometries, and similar zigzag chain structures, but 1 and 2 were composed of zigzag chains of different orientations. The DPA ligands can take part in constructing polymeric zigzag chains through coordination interactions, but can also take part in many other supramolecular interactions, such as C—H…π, C—H…Cl interactions. These supramolecular interactions stabilize the relative positions of the zigzag chains of different orientations and lead to the two isomers. CP 3 is isostructural to 2. 3 was obtained using ZnBr₂ instead of ZnCl₂.

  CPs 4 and 5 are isostructural and both crystallize in space group P2₁/c. The asymmetric unit of 4 contains one Co²⁺ ion, two formate ions, two water molecules, and one DPA ligand. The Co²⁺ ion is in an octahedral coordination environment with two formate oxygen atoms and two nitrogen atoms from DPA ligands in the equatorial plane and two water molecules in the axial positions [Fig. 3a, Co—O1: 0.196 0(3) nm, Co—O3: 0.246 5(2) nm, Co—N1: 0.203 3(2) nm]. The formate ions are in a monodentate coordination mode. It was reported that the DMF molecule can be hydrolyzed into formic acid and dimethylamine under acidic conditions in the presence of some metal cations, such as Co²⁺, Cu²⁺, or Ni²⁺, and the hydrolysis process can be accelerated with an increase in temperature^[20-24]. Therefore, the formate ions in 4 and 5 should be obtained in situ in the solvothermal reaction. The DPA ligands link Co²⁺ ions into a linear chain (Fig. 3b).

  Figure 3

  

  Figure 3.  (a) Coordination environment of the Co²⁺ ion in CP 4 (Symmetry code: a: -x, 2-y, 1-z; Ellipsoid probability level: 50%); (b) Chain structure of 4 (C: gray, H: light gray, N: blue, Co: purple, O: red)

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  CP 6 crystallizes in space group P4/ncc. The Zn²⁺ ion was in a quadrangular pyramid coordination environment with four imidazole N atoms from DIP ligands in the equatorial plane and the Cl^- ion at the axial position (Fig. 4). The Zn—N bond length is 0.212 6(1) nm, and Zn—Cl bond length is 0.225 2(2) nm, which are slightly longer than those in 1 and 2. The Zn²⁺ ions are linked by four DIP ligands with four adjacent Zn²⁺ ions into a layer structure (Fig. 5a). Interestingly, the four DIP ligands form a bowl-shaped cavity (Fig. 5b).

  Figure 4

  

  Figure 4.  Coordination environment of the Zn²⁺ ion in CP 6

  Ellipsoid probability level: 50%; Symmetry codes: a: 0.5-x, y, z; b: 0.5-x, 0.5-y, z; c: x, 0.5-y, z.

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  Figure 5

  

  Figure 5.  (a) Top-view of the layer structure in CP 6; (b) Cavity formed between DIP ligands within the layer

  C: dark gray, N: blue, Zn: yellow, Cl: green, O: red; Large yellow balls show the open space between the ligands.

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  2.2   Fluorescent property

  Fluorescent spectra for organic ligands DPA and DIP in dilute solution, and spectra for CPs 3 and 6 in solid state were collected at room temperature (Fig. 6). The DPA ligand in dilute DMF solution showed a broad peak at 418 nm, which should be attributed to the π-π^* transition of DPA molecule (Fig. 6a). The emission of solid sample 3 were similar to DPA ligand in DMF solution and red-shifted by 30 nm (448 nm). The fluorescent emission of 3 should be the DPA ligand-centred emission, and the red shift of the emission should be attributed to the coordination of the metal cation. Metal coordination may increase the rigidity of organic ligands and limit the vibrational or rotational degrees of freedom of ligands, and reduce non-radiative relaxation pathways of excited states. Rigid structures may reduce the geometric differences between excited and ground states, thereby altering the energy level difference.

  Figure 6

  

  Figure 6.  Fluorescent emissions for CPs 3 (a) and 6 (b), and ligands DPA (a) and DIA (b)

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  The DIP ligand in dilute DMF solution showed two peaks and a shoulder at 381, 399, and 424 nm under excitation at 365 nm (Fig. 6b). The emission of solid sample 6 were similar to DIP ligand in DMF solution and red-shifted by 25 nm, respectively (406, 426, and ca. 450 nm). The fluorescent emission of 6 should also be the DIP ligand-centred emission, and the red shift of the emission should be attributed to the coordination of the metal cation. This observation of the emission indicated that there are not many intermolecular interactions involving the organic DIP ligands in 6, which was consistent with the crystal structure analysis.

  3.   Conclusions

  We report six CPs based on highly fluorescent anthracene and pyrene-derivative ligands. CPs 1 and 2 are framework isomers, which both contain zigzag chains formed by DPA, Zn²⁺, and Cl^-. The zigzag chains in 1 are further assembled by C—H…Cl interactions into layers, and these layers are of two different orientations and show a rare 2D to 3D interpenetration mode. The zigzag chains in 2 are parallelly arranged. CP 3 is isostructural to 2. CPs 4 and 5 are isostructural and contain chain structures formed by DPA, Cu²⁺/Co²⁺, and formate ions, which were formed in situ in the solvothermal reaction. CP 6 contains a layer structure formed by DIP and Zn²⁺ ions. Free DPA and DIP ligands are highly fluorescent at room temperature, and CPs 3 and 6 showed enhanced fluorescent emissions.

  
  
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• 

  Figure 1  (a) Molecular structure and coordination environment of the Zn²⁺ ion in CP 1 (Ellipsoid probability level: 50%); (b) Layer structure formed by linking zigzag chains with C—H…Cl interactions (C: gray, H: light gray, Zn: yellow, N: blue, Cl: green); (c) Diagrams showing the interpenetration between two hydrogen-bonded layers of different orientations (Interpenetrated layers: blue and yellow, respectively, C—H…Cl hydrogen bonds: dotted lines); (d) 2D to 3D interpenetration view along the b-axis

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  Figure 2  (a) Molecular structure and coordination environment of the Zn²⁺ ion in 2 (Ellipsoid probability level: 50%); (b) A diagram showing the zigzag chains running along the b-axis (C: gray, H: light gray, N: blue, Zn: yellow, Cl: green)

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  Figure 3  (a) Coordination environment of the Co²⁺ ion in CP 4 (Symmetry code: a: -x, 2-y, 1-z; Ellipsoid probability level: 50%); (b) Chain structure of 4 (C: gray, H: light gray, N: blue, Co: purple, O: red)

  下载: 全尺寸图片 幻灯片
  

  Figure 4  Coordination environment of the Zn²⁺ ion in CP 6

  Ellipsoid probability level: 50%; Symmetry codes: a: 0.5-x, y, z; b: 0.5-x, 0.5-y, z; c: x, 0.5-y, z.

  下载: 全尺寸图片 幻灯片
  

  Figure 5  (a) Top-view of the layer structure in CP 6; (b) Cavity formed between DIP ligands within the layer

  C: dark gray, N: blue, Zn: yellow, Cl: green, O: red; Large yellow balls show the open space between the ligands.

  下载: 全尺寸图片 幻灯片
  

  Figure 6  Fluorescent emissions for CPs 3 (a) and 6 (b), and ligands DPA (a) and DIA (b)

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  Table 1.  Crystallographic data of CPs 1-6

  Parameter	1	2	3	4	5	6
  Formula	C24H16Cl2N2Zn	C24H16Cl2N2Zn	C78H62Br6N8O2Zn3	C26H22CoN2O6	C26H22CuN2O6	C176H112Cl8N32O16Zn4
  Molecular weight	468.66	468.66	1 818.92	517.38	521.99	3 476.05
  Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Tetragonal
  Space group	C2/m	P21/m	P21/m	P21/c	P21/c	P4/ncc
  Temperature / K	293	287.21	294.92	296.30	297(2)	243.0
  a / nm	1.981 7(2)	1.2213 7(4)	1.220 6(1)	1.333 22(5)	1.328 2(10)	1.695 66(8)
  b / nm	2.498 4(3)	2.519 05(8)	2.533 5(4)	0.771 31(3)	0.766 4(6)	1.695 66(8)
  c / nm	1.389 9(3)	1.364 27(4)	1.361 1(1)	1.154 12(4)	1.150 0(8)	1.304 46(10)
  β / (°)	130.660(2)	116.048 0(10)	115.681(3)	99.869(2)	99.86(2)
  V / nm3	5.220 4(13)	3.771 1(2)	3.793 3(9)	1.169 25(8)	1.153 4(14)	3.750 7(5)
  Z	4	4	2	2	2	1
  F(000)	1 904	1 428	1 804	534	538	1 776
  μ / mm-1	1.156	1.200	4.151	0.779	0.993	0.856
  Dc / (g·cm-3)	1.193	1.238	1.592	1.470	1.503	1.539
  Rint	0.043 6	0.039 5	0.062 8	0.028 7	0.021 7	0.097 0
  GOF on F 2	1.094	1.039	1.061	1.083	1.091	1.006
  R1 [I > 2σ(I)]	0.053 0	0.042 4	0.054 1	0.034 6	0.028 7	0.047 1
  wR2 [I > 2σ(I)]	0.147 6	0.114 9	0.165 4	0.093 1	0.082 4	0.132 6

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