English

• 0.   Introduction

  Metal-organic coordination polymers (MOCPs) of mixed-ligand assembly have become a very attractive research field^[1-4]. Due to its many advantages, it has applications in many fields, such as absorption, heterogeneous catalysis, electrochemistry, ion exchange, and fluorescence sensing^[5-8]. The mixed-ligand strategy by the judicious choice of various organic linkers has been proven to be high-efficient for the construction of MOCPs. Among such systems, the most outstanding is the incorporation of polycarboxylates and N-donors co-ligands, which has successfully been utilized to generate more diverse and interesting polymeric net-works with potential properties and contributes to refin-ing our knowledge of self-assembly processes^[9-11]. Within polycarboxylate ligands, aromatic polycarboxyl compounds have extensively been documented as multifunctional structures, owing to their versatile linking capability by virtue of both covalent bonding and supramolecular interactions^[12-14]. Luminescent metal-organic coordination polymers (LMOCPs) are a very important branch of MOCPs. At present, lots of studies have proved that LMOCPs as fluorescent sensors are feasible and effective in detecting pollutants. LMOCPs generally emit light in the following ways: organic ligands emit light, and the charge transfer between metal ions and ligands can emit light^[15-17]. The tunable structures and properties of cadmium-based coordination polymers provide an important advantage to fluorescence sensing materials.

  In our strategy, multidentate O- or N-donor ligands have also been employed in the construction of coordination polymers (CPs). Among the family of organic carboxylate, tripodal carboxylate shows more superiority, the ligand 4, 4′, 4″-s-triazine-2,4,6-tribenzoic acid (H₃tatb) as a class example of tripodal ligands has been utilized, and some MOCPs based on H₃tatb have also been investigated^[18-21]. To explore the influence of N-donor ligands on achieving different dimensional and topological structures based on tripodal carboxylate ligands, we also employ 1,4-bis(imidazole-1-ylmethyl) benzene(1,4-bimb) and 1,4-bis(1-imidazoly) benzene(1,4-bib) with different conformations as co-ligands. Two cadmium-based CPs, [Cd(Htatb) (1,4-bimb)]·₂O (1) and [Cd(Htatb)(1,4-bib)(H₂O)]·MF (2), were synthesized and characterized. In addition, their fluorescent properties were also investigated.

  1.   Experimental

  1.1   Materials and chemical analysis

  The H₃tatb, 1,4-bimb, and 1,4-bib ligands were purchased in the Jinan Henghua Sci. & Technol. Co., Ltd. All other reagents and solvents employed were commercially available and used without further purification. Elemental analyses were performed with a Perkin-Elmer 2400 CHN Elemental analyzer. Infrared spectra on KBr pellets were recorded on a Nicolet 170SX FT-IR spectrophotometer in a range of 400-4 000 cm^-1. Thermogravimetric (TG) analyses were conducted with a Nietzsch STA 449C micro analyzer under the atmosphere at a heating rate of 5 ℃·min^-1. Powder X-ray diffraction (PXRD) patterns were recorded on a Shimadzu XRD-7000 diffractometer analyzer. The working voltage was 40 kV, the current was 40mA, the radiation source was Cu Kα (λ=0.154 18 nm), and the scanning range was 20°-80° The fluorescence spectra were obtained using a Hitachi F-7100 fluorescence spectrophotometer at room temperature.

  1.2   Synthesis of CP 1

  A mixture of Cd(NO₃)₂·4H₂O (0.1 mmol, 0.031 g), H₃tatb (0.1 mmol, 0.044 g), 1,4-bimb (0.1 mmol, 0.024 g) and 8 mL DMF-H₂O (1:1, V/V) was stirred for 30 min in the air. The mixture was then transferred to a 20 mL airtight glass reactor and kept at 100 ℃ for 5 d under autogenous pressure, and then cooled to room temperature at a rate of 5 ℃·h^-1. Colorless crystals of 1 were obtained and washed with DMF and dried in the air (Yield: 49% based on Cd). Elemental analysis Calcd. for C₃₈H₂₉N₇O₇Cd(%): C, 56.48; H, 3.62; N, 12.13. Found(%): C, 56.62; H, 3.72; N, 12.24. IR data (KBr, cm^-1): 3 424(w), 3 108(s), 1 719(s), 1 656(w), 1 581 (w), 1 522(vs), 1 364(m), 1 122(w), 1 063(m), 1 013(w), 933(w), 822(m), 766(s), 647(w).

  1.3   Synthesis of CP 2

  The preparation of 2 was the same as that of 1 except using 1,4-bib ligand (0.1 mmol, 0.021 g) instead of 1,4-bimb. Colorless crystals of 2 were obtained and washed with DMF and dried in the air (yield: 51% based on Cd). Elemental analysis Calcd. for C₃₉H₃₂N₈O₈Cd(%): C, 54.91; H, 3.78; N, 13.13. Found (%): C, 54.83; H, 3.87; N, 13.21. IR data (KBr, cm^-1): 3 429(w), 3 122(w), 1 713(m), 1 655(w), 1 540(m), 1 516 (vs), 1 401(m), 1 362(s), 1 237(m), 1 112(m), 1 088(m), 1 016(m), 939(w), 833(m), 771(s), 650(w).

  1.4   X-ray crystallographic studies

  Diffraction intensities for CPs 1 and 2 were collected at 293 K on a Bruker SMART 1000 CCD diffractometer employing graphite-monochromated Mo Kα radiation (λ=0.071 073 nm). A semi-empirical absorption correction was applied using the SADABS program^[22]. The structures were solved by direct methods and refined by full-matrix least-squares on F² using the SHELXS 2014 and SHELXL 2014 programs, respectively^[23-24]. Non-hydrogen atoms were refined anisotropically and hydrogen atoms were placed in geometrically calculated positions and refined using a riding model. The crystallographic data for CPs 1 and 2 are listed in Table 1, and selected bond lengths and angles are listed in Table S1 (Supporting information).

  Table 1

  

  Table 1.  Crystal data and structural refinement summary of CPs 1 and 2

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  Parameter	1	2
  Empirical formula	C38H29CdN7O7	C39H32CdN8O8
  Formula weight	808.08	853.12
  Crystal system	Triclinic	Triclinic
  Space group	P1	P1
  a/nm	0.884 21(6)	0.852 66(17)
  b/nm	1.174 70(9)	1.306 8(2)
  c/nm	1.695 04(12)	1.761 1(3)
  α/(°)	90.573 0(10)	107.122(3)
  β/(°)	102.532 0(10)	93.698(3)
  γ/(°)	94.004 0(10)	90.617(3)
  V/nm3	1.713 9(2)	1.870 5(6)
  Dc/(g·m-3)	1.566	1.515
  Z	2	2
  μ/mm-1	0.700	0.649
  θ range for data collection/(°)	2.366-25.000	2.395-25.000
  Reflection collected, unique	8 649, 5 975 (Rint=0.014 0)	9 356, 6 533 (Rint=0.024 8)
  Data, restraint, number of parameters	5 975, 2, 487	6 533, 15, 515
  Goodness-of-fit on F2	1.053	1.030
  Final R indices[I > 2σ(I)]	R1=0.029 3, wR2=0.074 1	R1=0.049 1, wR2=0.120 0
  Largest difference in peak and hole/(e·m-3)	861 and -469	1 102 and -579

  2.   Results and discussion

  2.1   Description of the structure

  2.1.1   Crystal structures of CP 1

  Single-crystal X-ray analysis reveals that CP 1 exhibits a 4-fold interpenetrating 3D structure. The asymmetric unit of 1 comprises one Cd^Ⅱ ion, one Htatb^2- ion, one 1,4-bimb molecule, and one free water molecule. Each five-coordinated Cd^Ⅱ center is surrounded by two nitrogen atoms coming from two 1,4-bimb molecules, and three oxygen atoms from two Htatb^2- ions, taking a distorted square pyramidal geometry (Fig. 1). The bond lengths of Cd—O/N are in a range of 0.220 25(18)-0.244 1(2) nm, and the O/N—Cd—O/N bond angles cover a range of 53.80(8)°-134. 41(9)°. In CP 1, the tripodal carboxylate ligands are partly deprotonated and one carboxylate group adopts μ₁-η¹-η⁰ to link one Cd^Ⅱ ion, and another one adopts μ₁-η¹-Ⅱ¹ chelating mode to link one Cd^Ⅱ ion, resulting in a 1D chain structure, further through 1,4-bimb ligand bridging, the adjacent chains are connected to generate a 2D layer structure (Fig. 2). These 2D layers are further joined by O—H⋯O hydrogen bonding (O6⋯O2 distance: 0.257 8(3) nm, O6—H6⋯O2 angle: 162.3(4)°) to produce a 3D architecture (Fig. 3). To simplify the 3D framework, we considered the Htatb^2- anion as a 3-connected node and Cd^Ⅱ ion as a 5-connected node, 1,4-bimb ligand as linkers, and topological analysis by TOPOS program suggests that the 3D framework can be simplified as a 3, 5-connected net with a point symbol of (3·7²) (3²·7⁵·8³) (Fig. S1). However, due to the absence of large guest molecules to fill the void space, the potential voids are filled via mutual interpenetration of three independent equivalent frameworks, generating a four-fold interpenetrating 3D architecture (Fig. 4).

  Figure 1

  

  Figure 1.  Coordination environment of Cd(Ⅱ) ion in CP 1

  The free H₂O molecules are omitted for clarity; Displacement ellipsoids are drawn at the 40% probability level; Symmetry codes: #1: x+1, y+1, z; #2: x+1, y, z+1

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  Figure 2

  

  Figure 2.  Two-dimensional layer structure of CP 1

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  Figure 3

  

  Figure 3.  Three-dimensional architecture of CP 1

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  Figure 4

  

  Figure 4.  Four-fold interpenetrating 3D architecture of CP 1

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  2.1.2   Crystal structures of CP 2

  CP 2 shows a 2-fold interpenetrating 3D structure. Each seven-coordinated Cd^Ⅱ ion is located in a [CdO₅N₂] distorted pentagonal bipyramid geometry and is coordinated to five oxygen atoms of two Htatb^2- ions and a coordination water molecule, and two nitrogen atoms of two 1,4-bib molecules, as shown in Fig. 5. The bond lengths of Cd—O and Cd—N are in a range of 0.225 1(4)-0.263 7(3) nm, these bond lengths are similar to those found in related cadmium-based coordination polymers^[25]. Compared with CP 1, the carboxylic groups of 2 adopt μ₁-η¹-η¹ and μ₁-η¹-η¹ chelating mode to link Cd^Ⅱ ions, resulting in a 1D chain structure(Fig. S2), and through 1,4-bib ligand bridging, the adjacent chains are connected to generate a 2D layer structure (Fig. 6). These 2D layers are further joined through O—H⋯O hydrogen bonding (O5⋯O2 distance: 0.259 7 (5) nm, O5—H5⋯O2 angle: 161.76(4)° to produce a 3D architecture (Fig. S3). Topological analysis by the TOPOS program suggests that the 3D framework can be simplified as a 3, 5-connected net with a point symbol of (3·7²) (3²·7⁵·8³) (Fig. 7). It is noteworthy that there are large open channels in CP 2. It is apt to form interpenetrating frameworks, accordingly, the final structure of CP 2 is a two-fold interpenetrating 3D framework (Fig. 8).

  Figure 5

  

  Figure 5.  Coordination environment of Cd(Ⅱ) ion in CP 2

  Hydrogen atoms are omitted for clarity except for the carboxyl group and coordination water; Displacement ellipsoids are drawn at the 40% probability level; Symmetry codes: #1: x, y+1, z; #2: x, y, z-1

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  Figure 6

  

  Figure 6.  Two-dimensional layer structure of CP 2

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  Figure 7

  

  Figure 7.  3, 5-connected net topology of CP 2

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  Figure 8

  

  Figure 8.  Two-fold interpenetrating 3D architecture of CP 2

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  2.2   TG analysis

  The experimental diffraction patterns featured peaks that are almost consistent with the simulated patterns, indicating that the products are almost pure phases (Fig. S4 and S5). To study the thermal stability of CPs 1 and 2, TG analyses were performed on the polycrystalline samples under a nitrogen atmosphere (Fig. S6 and S7). TG curve of 1 revealed that the first weight loss of 2.4% from 50 to 110 ℃ corresponds to the loss of the lattice water molecules (Calcd. 2.23%), and then the larger weight loss (Obsd. 84.3%) occurred in a range of 240-490 ℃, corresponding to the decomposition of the Htatb^2- and 1,4-bimb ligands (Calcd. 83.86%). The TG curve of 2 showed two-step weight losses. The first weight loss in a range of 60-170 ℃ (Obsd. 10.9%, Calcd. 10.68%) is assignable to the loss of DMF and coordination water molecules. The second weight loss of 76.4% in a temperature range of 240-460 ℃ corresponds to the release of the Htatb^2- and 1,4-bib ligands (Calcd. 76.14%). The final decomposition products of 1 and 2 were confirmed to be CdO, which has also been further confirmed by the PXRD patterns of the CPs.

  2.3   Infrared spectra of CPs 1 and 2

  IR spectra of CPs 1 and 2 showed features attributable to compositions of the coordination polymers (Fig. S8 and S9). The observed strong characteristic peaks appearing around 3 424 and 3 429 cm^-1 in spectra are attributed to the O—H stretching vibrations, respectively. Due to partial deprotonation of carboxylate in 1 and 2, the absorptions of about 1 719 and 1 713 cm^-1 can be attributed to the stretching vibrations of the ν_COOH in the carboxylate. The presence of the characteristic bands at 1 656 and 1 655 cm^-1 for 1 and 2 suggests the ν_—C=N— stretching vibrations of Htatb^2- ion. The intense characteristic peaks appearing around 1 581 and 1 522, 1 364 cm^-1 for 1, 1 540 and 1 516, 1 401 cm^-1 for 2 in the IR spectra correspond to asymmetric and symmetric stretching vibrations of carboxylic groups, respectively. The presence of the characteristic bands at 1 122 and 1 112 cm^-1 for 1 and 2 suggests the ν_C—O stretching vibrations. The presence of the characteristic bands at 1 063 cm^-1 for 1 and 1 088 cm^-1 for 2 suggests the ν_C—N stretching vibrations of the imidazole ring. The absorptions of 640-850 cm^-1 of 1 and 2 can be attributed to the γ_C—H out-of-plane bending vibration of the phenyl ring.

  2.4   Photoluminescence properties

  The luminescent emission spectra of CPs 1 and 2 were examined in the solid state at room temperature as shown in Fig. 9. The main emission peak of the free H₃tatb appeared at 454 nm (λ_ex=364 nm), which can be assigned to the intra-ligand π*-π transitions^[26]. CP 1 showed a strong emission peak at 409 nm (λ_ex=369 nm), however, the intense emission of 1,4-bimb was observed at 473 nm (λ_ex=400 nm), respectively. Relative to their ligands, 1 showed a blue shift, probably owing to ligand-to-metal charge transfer (LMCT) ^[27-28]. CP 2 showed an emission peak at 394 nm (λ_ex=334 nm), in comparison with that of free H₃tatb and 1,4-bib (an intense emission at 398 nm with λ_ex=306 nm), which are attributed to H₃tatb or 1,4-bib ligand-based charge transfer^[29-30].

  Figure 9

  

  Figure 9.  Emission spectra of the ligands and CPs 1, 2

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  2.5   Detection of nitroaromatic compounds

  The luminescent responses of CPs 1 and 2 were investigated by treating suspensions (2 mg dispersed in 2 mL aqueous solution) with 50 μmol·L^-1 different analytes such as p-nitrobenzoic acid (p-NBA), m-nitroani-line (m-NA), o-nitroaniline (o-NA), o-nitrophenol (o-NP), p-nitrophenol (p-NP), p-nitrophenylhydrazine (p-NPH), nitrobenzene (NB), 2, 4-dinitrophenylhydra-zine (2, 4-DNPH), 2,4,6-trinitrophenol (2,4,6-TNP), and 2,4,6-trinitrophenyl hydrazine (2,4,6-TNPH), respec-tively. Among these nitroaromatic compounds, NB almost quenched the luminescent intensity of 1 (Fig. 10). The result indicates that 1 may be regarded as a potential luminescent sensor for detecting NB. The luminescent intensities gradually decreased with the increasing concentration of NB. The best quenching efficiency observed for NB was calculated to be 99.17% upon incremental addition of 0-400 μL 1 mmol·L^-1 NB solution (Fig. 11). To further analyze the luminescent titration results, the Stern-Volmer equa-tion: I₀/I=1+K_svc_NB was used to calculate the luminescence quenching constant, in which I₀ and I are the luminescence intensities before and after the addition of NB, K_sv is the quenching constant (L·mol^-1), and c_NB is the concentration of NB (mmol·L^-1), respectively^[31-32]. At low concentrations, the Stern-Volmer curves displayed an almost linear relationship, and the linear equation was I₀/I=0.851 58+148.684 9c_NB. The K_sv for NB was calculated to be 1.49×10⁵ L·mol^-1 (Fig. 12). The Stern-Volmer curve deviated from the linear correlation when the concentration increased, demonstrating the simultaneous involvement of both the static and dynamic quenching process. Further detailed analysis denoted that the LOD (limit of detection) was 0.197 μmol·L^-1 according to 3σ/k (σ and k represent the standard error and slope, respectively)^[33-34].

  Figure 10

  

  Figure 10.  Fluorescent spectra (left) and fluorescent intensities (right) of CP 1 dispersed in aqueous solutions of different nitroaromatic analytes

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  Figure 11

  

  Figure 11.  Fluorescence response of CP 1 upon incremental addition of 1 mmol·L^-1 NB in aqueous solutions

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  Figure 12

  

  Figure 12.  Stern-Volmer plot of I₀/I vs c_NB in the aqueous dispersion of CP 1 (left); The area enlarged view for linearity of the plot at lower concentrations of NB (right)

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  In the fluorescence titration, emission profiles of CP 2 showed selective and significant quenching for 2, 4, 6-TNP, and relatively low quenching was observed for other nitroaromatic analytes (Fig. 13). The best quenching efficiency observed for 2,4,6-TNP was calculated to be 99.76% upon incremental addition of 0-80 μL 1 mmol·L^-1 2,4,6-TNP solution (Fig. 14). When the concentration of 2,4,6-TNP is as low as 0.074 mmol·L^-1, the luminescent intensity of 2 is completely quenched by 2,4,6-TNP. As shown in Fig. 15, good linearity of the plot at low concentrations of 2,4,6-TNP was observed which fitted well with the Stern-Volmer equation (I₀/I=0.536 92+468.319 7c_TNP). High fluorescence quenching efficiency was proved by the high Stern-Volmer quenching constant (K_sv=4.68×10⁵ L·mol^-1), and further detailed analysis denoted that the LOD was 0.062 6 μmol·L^-1. However, a nonlinear curvature at higher concentrations of 2,4,6-TNP was obtained. The nonlinear nature of the Stern-Volmer plot of 2,4,6-TNP can be attributed to self-absorption, a combination of static and dynamic quenching, or an en-ergy-transfer process between 2,4,6-TNP and 2^[35-36].

  Figure 13

  

  Figure 13.  Fluorescent spectra (left) and fluorescent intensities (right) of CP 2 dispersed in aqueous solutions of different nitroaromatic analytes

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  Figure 14

  

  Figure 14.  Fluorescence response of CP 2 upon incremental addition of 1mmol·L^-1 2,4,6-TNP in aqueous solutions

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  Figure 15

  

  Figure 15.  Stern-Volmer plot of I₀/I vs c_2,4,6-TNP in the aqueous dispersion of CP 2 (left); The area enlarged view for linearity of the plot at lower concentrations of 2,4,6-TNP (right)

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  2.6   Detection of metal ions

  To examine the potential of CP 1 for sensing metal ions, changes in the fluorescence intensity of 1 dispersed in water on the addition of different metal ions, including Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺, Al³⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Ag⁺, Pb²⁺, and Hg²⁺ in aqueous solutions, were investigated (Fig. 16). Among these metal ions aqueous solutions, high fluorescence quenching of the luminescent intensity of 1 was observed in Fe³⁺ aqueous solution. To further study how the presence of non-Fe³⁺ metal cations affected the recognition of 1 to Fe³⁺ ions, all previously tested metal cations were analyzed again by adding to the solution of 1 containing Fe³⁺. The fluorescence spectra produced by these new mixtures are shown in Fig. 17. From these results, it can be seen that the emission intensity of 1 and Fe³⁺ was subject to fluctuation when in the presence of the testing cations. However, these fluctuations were deemed mostly minor concerning relative percent change, therefore, the conclusion shows that 1 still retains significant selectivity for the recognition of Fe³⁺ ions even in the matrix containing all cations tested^[37]. The luminescent intensities gradually decreased with the increasing concentration of Fe³⁺, and the best quenching efficiency observed for Fe³⁺ was calculated to be 97.92% upon incremental addition of 0-160 μL 1 mmol·L^-1 Fe³⁺ solution (Fig. 18). As shown in Fig. 19, good linearity of the plot at low concentrations of Fe³⁺ was observed which fitted well with the Stern-Volmer equation (I₀/I=1.063 76+82.839 7c_Fe³⁺). High fluorescence quenching efficiency was proved by the high Stern-Volmer quenching constant (K_sv=8.28×10⁴ L·mol^-1), and further detailed analysis denoted that the LOD was 0.354 μmol·L^-1. However, a nonlinear curvature at higher concentrations of Fe³⁺ was obtained, revealing that both dynamic and static quenching take place.

  Figure 16

  

  Figure 16.  Fluorescent spectra (left) and fluorescent intensities (right) of CP 1 dispersed in aqueous solutions with various metal cations

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  Figure 17

  

  Figure 17.  Fluorescent spectra (left) and fluorescent intensities (right) of CP 1 and Fe³⁺ in the presence of other cations

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  Figure 18

  

  Figure 18.  Fluorescence response of CP 1 upon incremental addition of 1 mmol·L^-1 Fe³⁺ in aqueous solutions

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  Figure 19

  

  Figure 19.  Stern-Volmer plot of I₀/I vs c_Fe³⁺ in the aqueous dispersion of CP 1 (left); The area enlarged view for linearity of the plot at lower concentrations of Fe³⁺ (right)

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  2.7   Detection of anions

  To examine the potential of CP 2 for sensing anions, changes in the fluorescence intensity of 2 dispersed in water on the addition of different anions, including I^-, Br^-, Cl^-, IO₃^-, ClO₃^-, NO₃^-, CO₃^2-, SO₄^2-, CrO₄^2-, Cr₂O₇^2-, and PO₄^3- in aqueous solutions, were investigated (Fig. 20). Among these anions, high fluorescence quenching of the luminescent intensity of 2 was observed in CrO₄^2- aqueous solution. To further study how the presence of non-CrO₄^2- anions affects the recognition of 2 to CrO₄^2- anions, all previously tested anions were analyzed again by adding to the CP 2 solu-tion containing CrO₄^2-. The fluorescence spectra produced by these new mixtures are shown in Fig. 21. From these results, it can be seen that the emission intensity of 2 and CrO₄^2- was subject to fluctuation when in the presence of the testing anions. However, these fluctuations were deemed mostly minor with respect to relative percent change, so the conclusion shows that 2 still retains significant selectivity for the recognition of CrO₄^2- anions even in the matrix containing all anions tested. The luminescent intensities gradually decreased with the increasing concentration of CrO₄^2-. The best quenching efficiency observed for CrO₄^2- was calculated to be 97.45% upon incremental addition (0-200 μL) of 1 mmol·L^-1 CrO₄^2- solution (Fig. 22). As shown in Fig. 23, good linearity of the plot at low concentrations of CrO₄^2- was observed which fitted well with the Stern-Volmer equation (I₀/I= 0.950 28+70.055 8c_CrO4^2-). High fluorescence quenching efficiency was proved by the high Stern-Volmer quenching constant (K_sv=7.01×10⁴ L·mol^-1). Further detailed analysis denoted that the LOD was 0.418 μmol·L^-1. A nonlinear curvature at higher concentrations of CrO₄^2- was obtained, revealing both dynamic and static quenching take place.

  Figure 20

  

  Figure 20.  Fluorescent spectra (left) and fluorescent intensities (right) of CP 2 dispersed in aqueous solutions of different anions

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  Figure 21

  

  Figure 21.  Fluorescent spectra (left) and fluorescent intensities (right) of CP 2 and CrO₄^2- in the presence of other anions

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  Figure 22

  

  Figure 22.  Fluorescence response of CP 2 upon incremental addition of 1 mmol·L^-1 CrO₄^2- in aqueous solutions

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  Figure 23

  

  Figure 23.  Stern-Volmer plot of I₀/I vs c_CrO4^2- in the aqueous dispersion of CP 2 (left); The area enlarged view for linearity of the plot at lower concentrations of CrO₄^2- (right)

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  3.   Conclusions

  In summary, two cadmium-based coordination polymers have been synthesized and characterized by the self-assembly of Cd(Ⅱ) salts with H₃tatb and imidazolyl ligands. CPs 1 and 2 display the 2D layer structure, further these layers are joined by O—H⋯O hydrogen bonding to generate the interpenetrating 3D architecture. The fluorescent properties of CPs 1 and 2 have been investigated. CP 1 was highly selective and sensitive towards NB and Fe³⁺ through different detection mechanisms, while CP 2 was highly selective and sensitive towards 2,4,6-TNP and CrO ₄^2- through different detection mechanisms. This work demonstrates the potential application of fluorescent CPs 1 and 2 as multi-responsive probes for the detection of nitroaromatic compounds and anions or cations in the aqueous phase.

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

  
  
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  Figure 1  Coordination environment of Cd(Ⅱ) ion in CP 1

  The free H₂O molecules are omitted for clarity; Displacement ellipsoids are drawn at the 40% probability level; Symmetry codes: #1: x+1, y+1, z; #2: x+1, y, z+1

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  Figure 2  Two-dimensional layer structure of CP 1

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  Figure 3  Three-dimensional architecture of CP 1

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  Figure 4  Four-fold interpenetrating 3D architecture of CP 1

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  Figure 5  Coordination environment of Cd(Ⅱ) ion in CP 2

  Hydrogen atoms are omitted for clarity except for the carboxyl group and coordination water; Displacement ellipsoids are drawn at the 40% probability level; Symmetry codes: #1: x, y+1, z; #2: x, y, z-1

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  Figure 6  Two-dimensional layer structure of CP 2

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  Figure 7  3, 5-connected net topology of CP 2

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  Figure 8  Two-fold interpenetrating 3D architecture of CP 2

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  Figure 9  Emission spectra of the ligands and CPs 1, 2

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  Figure 10  Fluorescent spectra (left) and fluorescent intensities (right) of CP 1 dispersed in aqueous solutions of different nitroaromatic analytes

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  Figure 11  Fluorescence response of CP 1 upon incremental addition of 1 mmol·L^-1 NB in aqueous solutions

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  Figure 12  Stern-Volmer plot of I₀/I vs c_NB in the aqueous dispersion of CP 1 (left); The area enlarged view for linearity of the plot at lower concentrations of NB (right)

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  Figure 13  Fluorescent spectra (left) and fluorescent intensities (right) of CP 2 dispersed in aqueous solutions of different nitroaromatic analytes

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  Figure 14  Fluorescence response of CP 2 upon incremental addition of 1mmol·L^-1 2,4,6-TNP in aqueous solutions

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  Figure 15  Stern-Volmer plot of I₀/I vs c_2,4,6-TNP in the aqueous dispersion of CP 2 (left); The area enlarged view for linearity of the plot at lower concentrations of 2,4,6-TNP (right)

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  Figure 16  Fluorescent spectra (left) and fluorescent intensities (right) of CP 1 dispersed in aqueous solutions with various metal cations

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  Figure 17  Fluorescent spectra (left) and fluorescent intensities (right) of CP 1 and Fe³⁺ in the presence of other cations

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  Figure 18  Fluorescence response of CP 1 upon incremental addition of 1 mmol·L^-1 Fe³⁺ in aqueous solutions

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  Figure 19  Stern-Volmer plot of I₀/I vs c_Fe³⁺ in the aqueous dispersion of CP 1 (left); The area enlarged view for linearity of the plot at lower concentrations of Fe³⁺ (right)

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  Figure 20  Fluorescent spectra (left) and fluorescent intensities (right) of CP 2 dispersed in aqueous solutions of different anions

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  Figure 21  Fluorescent spectra (left) and fluorescent intensities (right) of CP 2 and CrO₄^2- in the presence of other anions

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  Figure 22  Fluorescence response of CP 2 upon incremental addition of 1 mmol·L^-1 CrO₄^2- in aqueous solutions

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  Figure 23  Stern-Volmer plot of I₀/I vs c_CrO4^2- in the aqueous dispersion of CP 2 (left); The area enlarged view for linearity of the plot at lower concentrations of CrO₄^2- (right)

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  Table 1.  Crystal data and structural refinement summary of CPs 1 and 2

  Parameter	1	2
  Empirical formula	C38H29CdN7O7	C39H32CdN8O8
  Formula weight	808.08	853.12
  Crystal system	Triclinic	Triclinic
  Space group	P1	P1
  a/nm	0.884 21(6)	0.852 66(17)
  b/nm	1.174 70(9)	1.306 8(2)
  c/nm	1.695 04(12)	1.761 1(3)
  α/(°)	90.573 0(10)	107.122(3)
  β/(°)	102.532 0(10)	93.698(3)
  γ/(°)	94.004 0(10)	90.617(3)
  V/nm3	1.713 9(2)	1.870 5(6)
  Dc/(g·m-3)	1.566	1.515
  Z	2	2
  μ/mm-1	0.700	0.649
  θ range for data collection/(°)	2.366-25.000	2.395-25.000
  Reflection collected, unique	8 649, 5 975 (Rint=0.014 0)	9 356, 6 533 (Rint=0.024 8)
  Data, restraint, number of parameters	5 975, 2, 487	6 533, 15, 515
  Goodness-of-fit on F2	1.053	1.030
  Final R indices[I > 2σ(I)]	R1=0.029 3, wR2=0.074 1	R1=0.049 1, wR2=0.120 0
  Largest difference in peak and hole/(e·m-3)	861 and -469	1 102 and -579

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