English

• Coordination polymers (CPs) have caused extensive attention from researchers as a new kind of functional hybrid materials because they have fascinating structures and extensive application in gas storage and separation^[1-4], optics properties^[5-10], magne-tism^[11-12], catalysis^[13-14] and drug sustained release^[15], etc. At present, there have also been a lot of reports on the research of CPs in luminescence probe. Compared with traditional instrument methods, luminescence sensing is considered as a promising method due to its advantages of quick response, high sensitivity, low cost, simple operation, and so on^[16-18].

  In the past few years, with the rapid development of economy and the explosive growth of the popula-tion, the problems such as environmental pollution and public health are arousing more and more concerns^[19]. Fe³⁺ is not only necessary for metabolism, but also widely used in industry^[20]. However, excessive amounts of Fe³⁺ in the human body are harmful, and Fe³⁺ can contaminate the environment by being carelessly discarded^[21]. While Cr⁶⁺ is widely used in diverse industrial applications, improper disposal of Cr⁶⁺ will pollute the living conditions of people^[22]. So, it is very urgent to synthesize materials that can selectively and sensitively identify these ions. In recent years, researchers have agreed that CPs are one of the promising alternatives for luminescent probe. One of the biggest challenges is the design and construction of CPs with stable structure and ideal function. In the process of self-assembly, the construc-tion of the desired CPs depends on organic ligands and metal ions^[23]. In terms of ligands, studies have shown that aromatic polycarboxylic acids are extensi-vely used to construct CPs due to the following advantages: firstly, their multiple binding nodes; secondly, their carboxylate groups can be partially or completely deprotonated to form multiple structures; furthermore, aromatic conjugated systems can coor-dinate with d¹⁰ center ions forming CPs materials with excellent luminescence properties. At the same time, the introduction of the second nitrogen-containing ligand is helpful to construct CPs with novel structures.

  So, based on the ligands of p-terphenyl-3, 3″, 5, 5″-tetracarb oxylic acid (H₄tptc), 1, 4-bis(imidazol-1-ylmethyl) benzene (1, 4-bimb) or 1, 2-bis(imidazol-1-ylmethyl) benzene (1, 2-bimb) (Scheme 1), two novel CPs, namely {[Zn₂(tptc)(1, 4-bimb)₂]·H₂O}_n (1) and {[Ni(tptc)_0.5(1, 2-bimb)(H₂O)]·H₂O}_n (2), have been synth-esized under solvothermal method and characterized by luminescence properties (1) and magnetic properties (2).

  Scheme 1

  

  Scheme 1.  Structures of the ligands

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  1.   Experimental

  1.1   Materials and physical measurements

  All chemicals were purchased commercially and used without further purification. IR (KBr pellet) spectra were recorded under a FTIR-8400S spectro-meter in a range of 4 000~400 cm^-1. Thermogravi-metric analyses (TGA) were collected on a METTLER TGA analyzer at a heating rate of 10 ℃·min^-1 under N₂ atmosphere from room temperature to 750 ℃. Elemental analyses (C, H, and N) were performed by using a PerkinElmer 2400C elemental analyzer (EA). Powder X-ray diffraction (PXRD) was performed using a Rigaku D/Max-2500 PC diffractometer (Mo Kα radiation, λ=0.154 06 nm) at 50 kV, 30 mA with the 2θ range of 5°~50°. Luminescence spectra were performed on Hitachi F4600 spectrophotometer. Magnetic properties were measured by Quantum Design MPMS-XL-7 SQUID magnetometer.

  1.2   Synthesis of the complexes

  1.2.1   Synthesis of {[Zn₂(tptc)(1, 4-bimb)₂]·H₂O}_n (1)

  Zn(NO₃)₂·6H₂O (0.015 mmol, 4.5 mg), H₄tptc (0.005 mmol, 2.1 mg), 1, 4-bimb (0.005 mmol, 1.2 mg), 0.15 mL NaOH aqueous solution (0.5 mol·L^-1) and 1 mL H₂O/DMF (1:1, V/V) were mixed in a stainless steel vessel (25 mL), kept at 130 ℃ for 72 h and then naturally cooled to ambient temperature to obtain colorless crystals. Yield: 41% (based on Zn). Elemental analysis Calcd. for C₅₀H₄₀N₈O₉Zn₂(%): C, 58.48; H, 3.91; N, 10.85. Found(%): C, 58.54; H, 3.95; N, 10.89. IR (KBr, cm^-1): 3 455 (m), 1 630 (s), 1 524 (vs), 1 393 (s), 742 (s), 733 (s), 683 (m), 678 (m) (Supporting information, Fig.S1).

  1.2.2   Synthesis of {[Ni(tptc)_0.5(1, 2-bimb)(H₂O)]·H₂O}_n(2)

  Ni(NO₃)₂·6H₂O (0.03 mmol, 8.8 mg), H₄tptc (0.01 mmol, 4.2 mg), 1, 2-bimb (0.02 mmol, 4.8 mg) and 8 mL CH₃CN/H₂O (1:1, V/V) were placed in a 25 mL autoclave and heated to 130 ℃ for 72 h. After slowly being cooled to ambient temperature, green crystals were gained. Yield: 46% (based on Ni). Elemental Analysis Calcd. for C₂₅H₂₃N₄NiO₆(%): C, 56.16; H, 4.30; N, 10.48. Found(%): C, 56.24; H, 4.25; N, 10.68. IR (KBr, cm^-1): 3 402 (vs), 1 613 (vs), 1 546 (vs), 1 526 (s), 1 402 (vs), 1 371 (s), 986 (m), 977 (m). 870 (s), 782 (s), 773 (s), 685 (m), 653 (m) (Fig.S1).

  1.3   X-ray crystallographic study

  All crystallographic data were collected on a Bruker APEX Ⅱ CCD diffractionmeter using Mo Kα radiation (λ=0.071 073 nm) at 25 ℃. The structures were determined by direct methods and refined by the full-matrix least-squares method based on F ² using SHELXL program and OLEX 2^[24-25]. All nonhydrogen atoms were refined with anisotropic displacement parameters and hydrogen atoms were placed geometri-cally and refined using a riding model. Crystal structural parameters, some selected bond lengths and angles are listed in Table 1 and Table S1, respectively. The topology of CPs was analyzed by using TOPOs 4.0 program package^[26].

  Table 1

  

  Table 1.  Crystal structure data and refinement parameters of 1 and 2

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  	1	2
  Empirical formula	C50H40N8O9Zn2	C25H21N4NiO5
  Formula weight	1 027.64	516.17
  Temperature/K	296.15	298(2)
  Crystal system	Monoclinic	Triclinic
  Space group	P21	P1
  a/nm	0.735 9(7)	0.907 7(3)
  b/nm	1.901 7(18)	1.009 8(4)
  c/nm	1.610 0(15)	1.717 9(6)
  α/(°)		94.964(3)
  β/(°)	94.683(11)	103.998(3)
  γ/(°)		108.727(3)
  Volume/nm3	2.246(4)	1.423 7(9)
  Z	2	2
  Dc/(g·cm-3)	1.520	1.204
  μ/mm-1	1.137	0.718
  F(000)	1 056.0	534.0
  2θ range for data collection/(°)	5.078~49.998	4.944~55.426
  Reflection collected	11 712	19 215
  Independent reflection	4 988	6 581
  Rint	0.092 8	0.053 1
  Data, restraint, parameter	4 988, 1, 623	6 581, 0, 336
  Goodness-of-fit on F2	1.051	1.023
  R1, wR2 [I≥2σ(I)]	0.061 9, 0.113 2	0.040 7, 0.093 0
  R1, wR2 (all data)	0.110 3, 0.132 8	0.058 5, 0.099 4

  CCDC: 1911100, 1; 1911101, 2.

  2.   Results and discussion

  2.1   IR spectra

  The absorption spectrum at 3 440~3 460 cm^-1 corresponds to the characteristic peak of the stretching vibration of the O-H group in water molecules. The peaks at 1 393 cm^-1 (1) or 1 376 cm^-1 (2) and 1 630 cm^-1 (1) or 1 546 cm^-1 (2) are attributed to the symm-etric and asymmetric stretching vibration of the carboxylate group, respectively. For 1~2, the lack of strong peak in a range of 1 690~1 730 cm^-1 demon-strates that the H₄tptc ligand is completely deprotonated (Fig.S1).

  2.2   Descriptions of crystal structures

  2.2.1   Crystal structure of {[Zn₂(tptc)(1, 4-bimb)₂]· H₂O}_n (1)

  Complex 1 crystallizes in the monoclinic system with the P2₁ space group. Its asymmetric unit contains two Zn²⁺ ions, one tptc^4- linker, two 1, 4-bimb linkers and one lattice water molecule. Both Zn²⁺ ions are four-coordinated and exhibit distorted tetrahedron geometries (Fig. 1a). Each Zn²⁺ ion is bound to two oxygen atoms of two distinct tptc^4- linkers and two nitrogen atoms provided by two 1, 4-bimb ligands. The bond angles around Zn²⁺ range from 97.7° to 129.1°, and the bond lengths of Zn-O and Zn-N vary from 0.193 4 to 0.197 1 nm and 0.200 4 to 0.202 1 nm, respectively.

  Figure 1

  

  Figure 1.  (a) Coordination environment of Zn²⁺ ions in 1; (b) 2D network of 1 based on Zn²⁺ ion and tptc^4- observed along a-axis; (c) 3D structure of 1 viewed along a-axis; (d) Overall topological network for complex 1

  Symmetry codes: ^ⅰ 2-x, -0.5+y, 1-z; ^ⅱ 3+x, y, 1+z; ^ⅲ 1-x, 0.5+y, -z; ^ⅳ-3+x, 1+y, z

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  The H₄tptc ligands in 1 are completely depro-tonated. All carboxylate groups adopt monodentate bridging coordination modes to link Zn²⁺ ions forming a 2D network (Fig. 1b), which are further expanded by 1, 4-bimb linkers to construct 3D structures (Fig. 1c). Topologically, complex 1 reveals a 2-nodal (4, 4)-c network with the topology of (8⁶) by denoting Zn²⁺ ions and H₄tptc ligands to be 4-c nodes, respectively.

  2.2.2   Crystal structure of {[Ni(tptc)_0.5(1, 2-bimb)(H₂O)]·H₂O}_n (2)

  Complex 2 crystallizes in the triclinic system P1 space group. There are one Ni²⁺ ion, half of tptc^4- linkers, one 1, 2-bimb, and one coordinated water molecule in the asymmetric unit of 2. As exhibited in Fig. 2a, each Ni²⁺ ion is coordinated by three carboxy-late O atoms (Ni1-O1 0.214 26 nm, Ni1-O2 0.214 44 nm, Ni1-O3^ⅰ 0.202 80 nm), a lattice water O atom (Ni1-O5W 0.207 47 nm), and two N atoms of two 1, 4-bimb (Ni1-N1 0.205 47 nm and Ni-N4^ⅱ 0.206 35 nm), presenting a pseudo-octahedral geometry. The bond angles around Ni1 range from 61.46° to 179.23°.

  Figure 2

  

  Figure 2.  (a) Coordination environment of Ni²⁺ ions in 2; (b) Coordination mode of the H₄tcpb ligand in complex 2

  Symmetry codes: ^ⅰ x, -1+y, z; ^ⅱ 1+x, 1+y, z; ^ⅴ 1-x, 3-y, 2-z

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  As shown in Fig. 2b, four carboxylate groups of the H₄tcpb ligand adopt two different coordination patterns (bridging mode and chelating mode). The H₄tcpb ligands link the Ni²⁺ ions to get an interesting 1D rectangle chain (Fig. 3a), while 1, 2-bimb ligands link with the Ni²⁺ ions to form a 1D [Ni(1, 2-bimb)]_n linear chain (Fig. 3b). These two chains intertwine each other to form a 2D sheet by sharing metal centers (Fig. 3c). Finally, 3D supramolecular structure is formed through O5W-H5WA…O2 and O5W-H5WB…O4 hydrogen bonds interactions between the adjacent sheets (Fig. 4). Topological analysis indicates that the framework of 2 can be simplified to a new (4, 4)-c network with the point symbol of {4.6⁴.8}₂{4².6⁴}, where H₄tptc ligands and Ni²⁺ ions are taken as 4-connected nodes, respectively.

  Figure 3

  

  Figure 3.  (a) One dimensional rectangle chain based on H₄tcpb and Ni²⁺ ions; (b) 1D [Ni(1, 2-bimb)]_n linear chain constructed by 1, 2-bimb ligands and Ni²⁺ ions; (c) View of the 2D network of 2

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  Figure 4

  

  Figure 4.  Hydrogen bonds between adjacent 2D layers of 2

  Hydrogen bonds are showed by dotted lines

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  Figure 5

  

  Figure 5.  Schematic diagram of the 4, 4-connected net of 2 with the point symbol of{4.6⁴.8}₂{4².6⁴}

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  2.3   Powder X-ray diffraction analyses and TGA analyses

  To evaluate the phase purity of CPs, the PXRD patterns of the as-synthesized samples were analyzed at ambient temperature (Fig.S2). The key peaks of the experimental PXRD patterns were almost consistent with the simulated ones, indicating that the phase purity of CPs is good. The difference in strength may be caused by the preferred orientation of crystal powder samples.

  As shown in Fig.S3, complex 1 exhibits a weightlessness of 1.76% (Calcd. 1.75%) below 159 ℃, which is attributed to the loss of a lattice water molecule. After that, its framework is stable below 400 ℃. As for 2, the first weight loss of 6.88% (Calcd. 6.74%) corresponds to the release of a lattice water and a coordinated water molecule under 196 ℃. The framework of 2 started to break down after 396 ℃.

  2.4   Luminescence properties

  The luminescence properties of ligands (H₄tptc and 1, 4-bimb) and complex 1 were measured under room temperature (Fig. 6). The emission spectra of H₄tptc and 1, 4-bimb ligands were observed at 408 and 457 nm (λ_ex=280 nm), respectively, which can be attributed to the π-π^* and π^*-n transition^[27-31]. Further-more, compared with the emission bands of H₄tptc and 1, 4-bimb, complex 1 was blue-shifted and exhibited an obvious emission maximum at 359 nm (λ_ex=280 nm). This is due to structural changes in ligands because of coordination with metal ions, which greatly enhances the rigidity of CP and decreases the energy loss through radiation less decay^[32].

  Figure 6

  

  Figure 6.  Solid-state luminescence emissions of H₄tptc, 1, 4-bimb and complex 1

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  In addition, from the perspective of practical application, the luminescence sensing properties of 1 in common solvents are also investigated. The ground samples were dispersed in different solvents (2 mL), including H₂O, DMA, DMF, methanol, acetonitrile, ethanol, acetone, DMSO, etc., by ultrasonic treatment for 30 min to obtain uniform 1@solvent suspensions. As exhibited in Fig. 7, the luminescence intensity of 1 depends on the types of the solvent. It is worth noting that 1 exhibited the strongest emission peak in H₂O and the weakest emission in acetone, which may be the result of interaction between the network of CPs and solvent molecules with disparate polarities^[33].

  Figure 7

  

  Figure 7.  Luminescence intensities of complex 1 scattered in different organic solvents

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  Furthermore, because of water stability of complex 1, luminescence sensing properties of 1 towards metal ions in aqueous solution were carried out. The finely ground sample of 1 was dispersed in M(NO₃)_x aqueous solutions (0.01 mol·L^-1, M=Na⁺, Cd²⁺, K⁺, Cu²⁺, Pb²⁺, Ag⁺, Fe³⁺, Mn²⁺, Cr³⁺, Zn²⁺, Co²⁺, Ni²⁺, Ba²⁺, Al³⁺, Hg²⁺, Ni²⁺) to form 1@M suspensions treated by the ultrasonic for 30 min. As presented in Fig. 8, the luminescence intensity of 1 presents ion-dependent changes. Al³⁺ and Na⁺ ions enhanced its luminescence intensity, while the other cations reduced its luminescence intensities. Particularly, the luminescence intensity of 1 was almost completely quenched by Fe³⁺ ion, implying that 1 can be one of the luminescent probes sensing Fe³⁺ ion.

  Figure 8

  

  Figure 8.  Luminescence intensity of complex 1 in different cationic water solutions

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  To test the sensitivity of 1 for sensing Fe³⁺ ion, the titration experiments were carried out. 2 mg crystal samples were dispersed in 3 mL aqueous solution to form 1@H₂O suspensions, and then 30 μL Fe(NO₃)₃ solution (0.01 mol·L^-1) was gradually added into the above suspensions at a time. As can be seen in Fig. 9, the luminescence intensity of 1@H₂O suspension decreases gradually with the addition of Fe³⁺ ion. When the concentration of Fe³⁺ ion was 0.7 mmol·L^-1, the luminescence intensity of 1 was almost completely quenched. The relationship between I₀/I and the concentration of Fe³⁺ ion can be expressed by the equation of I₀/I=0.74exp(c_M/0.26)+0.44, where c_M represents the concentration of Fe³⁺, I₀ and I stand for luminescence intensities of 1@H₂O and 1@Fe³⁺ suspen-sion, respectively. When the concentration of Fe³⁺ ion is low, the Stern-Volmer (S-V) curve can be expressed as a linear equation of I₀/I=1+K_svc_M (Fig. 9)^[34]. The K_sv value of Fe³⁺ was 5.29×10³ L·mol^-1. The detection limit was calculated by 3σ/K_sv to be as low as 4.63×10^-4 mol·L^-1 (σ is the standard deviations by measuring the blank solution for 5 times at room temperature).

  Figure 9

  

  Figure 9.  Influence of the addition of Fe³⁺ ions on the emission spectra of 1 dispersed in water solution

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  The luminescence experimental procedure of anions is similar to that of cations except substituting M(NO₃)_x with K_nX (0.01 mol·L^-1, X=Cr₂O₇^2-, HPO₄^2-, SCN^-, CO₃^2-, H₂PO₄^-, I^-, HCO₃^-, PO₄^3-, Br^-, SCN^-, SO₄^2-, H₂PO₄^- and Cl^-). As exhibited in Fig. 10, compared to other anions, Cr₂O₇^2- ion has the remarkable quenching effect to the luminescence of 1. Similarly, the titration experiment of Cr₂O₇^2- anion is shown in Fig. 11, and the S-V curve is linear at low concentration and gradually deviated from linearity with the increasing of Cr₂O₇^2- concentration. The quenching constant (K_SV) was calculated to be 6.15×10³ L·mol^-1, which was higher than that of the reported MOFs for sensing Cr₂O₇^2- ions (Table S2)^[35-36].

  Figure 10

  

  Figure 10.  Luminescence intensity of complex 1 in aqueous solution containing different anions

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  Figure 11

  

  Figure 11.  Influence of the addition of Cr₂O₇^2- ions on the emission spectra of 1 dispersed in water solution

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  2.5   Quenching mechanism

  In order to explore the mechanism of lumine-scence quenching, PXRD patterns of samples were measured before and after luminescence experiment, and the results showed that PXRD patterns of samples after luminescence experiment were almost identical with those of the original ones, which indicates that the structural collapse of complex is not the cause of luminescence quenching (Fig.S4). Furthermore, the UV-Vis absorption spectra (Fig.S5) showed that there were the partial overlap between the excitation band of 1 and the absorption band of Fe³⁺/Cr₂O₇^2- ions, which shows the competitive absorption of energy between the frameworks and that Fe³⁺/Cr₂O₇^2- ions is responsible for luminescence quenching^[37].

  2.6   Magnetic properties

  The direct-current (dc) magnetic susceptibility of 2 was measured in a temperature range of 2~300 K under a 1 000 Oe applied field. As exhibited in Fig. 12, the χ_MT value is 1.08 cm³·K·mol^-1 at room temperature, which is close to the theoretical value of 1.0 cm³·K·mol^-1 for one isolated Ni²⁺ ion (S=1, g=2.0). The χ_MT value decreased slowly to 0.49 cm³·K·mol^-1 at 30 K, and then decreased sharply, which may be attributed to the antiferromagnetic interaction between Ni²⁺ ions. Furthermore, the curve of the reciprocal susceptibilities ( χ_M^-1) vs T was well fitted by the Curie-Weiss law: χ_MT=C/(T-θ), which gave C=1.07 cm³·K·mol^-1 and θ=-2.68 K in a temperature range of 50~300 K, confirming the antiferromagnetic interaction between Ni²⁺ ions.

  Figure 12

  

  Figure 12.  Temperature properties of χ_MT vs T and χ_M^-1 vs T for 2

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  3.   Conclusions

  In short, two new CPs have been successfully synthesized by utilizing the mixed ligands strategy. Complex 1 shows a 3D network with the point symbol of (8⁶), and complex 2 is a 3D supramolecular archit-ecture formed by the H-bond interaction between the adjacent 2D layers. Furthermore, the luminescence properties show that 1 has good sensing selectivity for Fe³⁺/Cr₂O₇^2- ions in aqueous solution, which indicates 1 has potential application in the detection of Fe³⁺ and Cr₂O₇^2- ions. Moreover, magnetic measurements indicate that there is the antiferromagnetic interaction between Ni²⁺ ions in 2.

  Supporting information is available at http://www.wjhxxb.cn

  
  Acknowle dgements: The authors sincerely thank the National Natural Science Foundation of China (Grant No.21676258), the international Scientific and Technological Coop-eration Projects of Shanxi Province (Grant No.201803D421080). Meanwhile, the authors honestly acknowledge the support of innovative investigated group of inorganic-organic mixed functional materials in North University of China.
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  Scheme 1  Structures of the ligands

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  Figure 1  (a) Coordination environment of Zn²⁺ ions in 1; (b) 2D network of 1 based on Zn²⁺ ion and tptc^4- observed along a-axis; (c) 3D structure of 1 viewed along a-axis; (d) Overall topological network for complex 1

  Symmetry codes: ^ⅰ 2-x, -0.5+y, 1-z; ^ⅱ 3+x, y, 1+z; ^ⅲ 1-x, 0.5+y, -z; ^ⅳ-3+x, 1+y, z

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  Figure 2  (a) Coordination environment of Ni²⁺ ions in 2; (b) Coordination mode of the H₄tcpb ligand in complex 2

  Symmetry codes: ^ⅰ x, -1+y, z; ^ⅱ 1+x, 1+y, z; ^ⅴ 1-x, 3-y, 2-z

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  Figure 3  (a) One dimensional rectangle chain based on H₄tcpb and Ni²⁺ ions; (b) 1D [Ni(1, 2-bimb)]_n linear chain constructed by 1, 2-bimb ligands and Ni²⁺ ions; (c) View of the 2D network of 2

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  Figure 4  Hydrogen bonds between adjacent 2D layers of 2

  Hydrogen bonds are showed by dotted lines

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  Figure 5  Schematic diagram of the 4, 4-connected net of 2 with the point symbol of{4.6⁴.8}₂{4².6⁴}

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  Figure 6  Solid-state luminescence emissions of H₄tptc, 1, 4-bimb and complex 1

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  Figure 7  Luminescence intensities of complex 1 scattered in different organic solvents

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  Figure 8  Luminescence intensity of complex 1 in different cationic water solutions

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  Figure 9  Influence of the addition of Fe³⁺ ions on the emission spectra of 1 dispersed in water solution

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  Figure 10  Luminescence intensity of complex 1 in aqueous solution containing different anions

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  Figure 11  Influence of the addition of Cr₂O₇^2- ions on the emission spectra of 1 dispersed in water solution

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  Figure 12  Temperature properties of χ_MT vs T and χ_M^-1 vs T for 2

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  Table 1.  Crystal structure data and refinement parameters of 1 and 2

  	1	2
  Empirical formula	C50H40N8O9Zn2	C25H21N4NiO5
  Formula weight	1 027.64	516.17
  Temperature/K	296.15	298(2)
  Crystal system	Monoclinic	Triclinic
  Space group	P21	P1
  a/nm	0.735 9(7)	0.907 7(3)
  b/nm	1.901 7(18)	1.009 8(4)
  c/nm	1.610 0(15)	1.717 9(6)
  α/(°)		94.964(3)
  β/(°)	94.683(11)	103.998(3)
  γ/(°)		108.727(3)
  Volume/nm3	2.246(4)	1.423 7(9)
  Z	2	2
  Dc/(g·cm-3)	1.520	1.204
  μ/mm-1	1.137	0.718
  F(000)	1 056.0	534.0
  2θ range for data collection/(°)	5.078~49.998	4.944~55.426
  Reflection collected	11 712	19 215
  Independent reflection	4 988	6 581
  Rint	0.092 8	0.053 1
  Data, restraint, parameter	4 988, 1, 623	6 581, 0, 336
  Goodness-of-fit on F2	1.051	1.023
  R1, wR2 [I≥2σ(I)]	0.061 9, 0.113 2	0.040 7, 0.093 0
  R1, wR2 (all data)	0.110 3, 0.132 8	0.058 5, 0.099 4

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