Introduction

As new crystalline materials have diverse structures and extensive applications in gas adsorption and separation, catalysis, luminescent sensing, and so on [1,2,3,4,5,6], the construction of coordination polymers (CPs) has still attracted much attention. In general, new CPs can be obtained by regulating their building blocks (such as organic ligands, metal ions, and counter anions) or/and assembly conditions (such as temperature, pH, concentration, and solvent). Among them, organic ligands play a main role in the formation of new CPs with desirable structures and properties. Tripodal nitrogen-containing ligands are a group of excellent ligands that have been applied to construct various coordination supramolecules, including CPs and metal-organic cages [7,8,9,10,11,12]. However, the most frequently used tripodal nitrogen-containing ligands are homotopic, and the three coordinating groups are identical. However, those based on heterotopic tripodal nitrogen-containing ligands have been relatively less explored [13].

Recently, we focused on the construction of new CPs based on a series of rigid tripodal nitrogen-containing ligands containing two kinds of donor groups, such as tetrazole-imdazole [13,14,15,16,17], tetrazole-pyridine [18, 19], and pyrazole-imidazole ligands [20, 21]. In this study, a new tetrazole-pyridine ligand, 3-(3,5-di(2H-tetrazol-5-yl)phenoxy)pyridine (H2DTPP), was selected to construct new CPs. A Zn(II) CP with a 2D network structure was successfully obtained and well characterized by single-crystal X-ray diffraction, elemental analysis, and infrared spectroscopy. Additionally, the thermal stability and photoluminescence properties of the selected compounds were investigated in detail.

Experimental

Materials and Methods

The ligand H2DTPP was purchased from Shanghai Kylpharm Co., Ltd., through its customized service. All the other reagents were of analytical grade quality and were obtained from Guangzhou Chemical Reagent Factory without further purification. Infrared (IR) spectroscopy was performed on a Nicolet FT-IR-170SX spectrometer with KBr pellets in the 400−4000 cm−1 region. Powder X-ray diffraction (PXRD) patterns were obtained by an Ultima IV diffractometer with a scan speed of 12°/min at 40 kV and 40 mA with a Cu-target tube and a graphite monochromator. Elemental analysis (C, H, and N) was performed with a Perkin-Elmer 240 C elemental analyzer. Thermogravimetric analysis (TGA) was performed on a NETZSCH STA 449 C thermogravimetric analyzer at a heating rate of 10 °C/min under a nitrogen atmosphere. The luminescent spectra for H2DTPP and compound 1 were determined by using an Edinburgh FLS-900 spectrophotometer with a 150 W xenon lamp as the light source at room temperature.

Synthesis of Compound 1, {[Zn(DTPP)(H 2 O) 2 ]·H 2 O} n

A mixture of H2DTPP (0.03 mmol, 9.2 mg), Zn(NO3)2·6H2O (0.06 mmol, 17.8 mg), methanol (1.5 mL), DMF (1.5 mL), and H2O (3 mL) was stirred for 5 min. Then, the solution was sealed in a conical flask and left for 8 days to obtain pale yellow crystals in solution. The crystals were filtered and washed with deionized water and dried in air (yield: 53% based on H2DTPP). Anal. Calcd. for C104H104N72O32Zn8: C, 36.77; H, 3.09%; N, 29.68%; Found: C, 36.69%; H, 3.12%; N, 29.76%. IR (KBr pellet, cm−1): 3222(s), 1620(m), 1574(m), 1438(s), 1243(s), 1120(w), 1060(w), 934(m), 887(m), 790(m), 691(m).

X−ray Crystallography

Single-crystal X-ray diffraction data for compound 1 were recorded on a Rigaku SuperNova Dual Atlas diffractometer with graphite-monochromated Cu Ka radiation (λ = 1.54184 Å) at 100 K. Data reduction, scaling, and absorption corrections were performed using CrysAlisPro 1.171.41.119a [22]. Empirical absorption correction using spherical harmonics was implemented in the SCALE3 ABSPACK scaling algorithm. Using Olex2 [23], structural solution and refinement based on F2 were performed with the SHELXS-2018 and SHELXL-2018 program packages [24, 25], respectively. Anisotropic atomic displacement parameters were applied to all non-hydrogen atoms during refinement. The hydrogen atoms were added geometrically. The details of the crystal parameters, data collection, and refinement for the title compound are shown in Table 1, and selected bond lengths and angles are displayed in Table 2.

Table 1 Crystallographic data and structure refinement for the title compound.
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Table 2 Selected bond lengths and bond angles for compound 1
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Results and Discussion

Description of the Crystal Structure

Single-crystal X-ray diffraction revealed that compound 1 crystallizes in an orthorhombic system with the space group Ibca. The asymmetric unit contains one Zn(II) ion, one DTPP2- ligand, two coordinated water molecules, and one uncoordinated water molecule. The Zn(II) ion adopts a five-coordinate triangular bipyramidal coordination mode, coordinating with two N atoms from two different tetrazole groups and one N atom from the pyridine group on the triangular plane and coordinating with two coordinated water molecules in the axial direction (Fig. 1). The Zn–N bond lengths range from 2.008(3) to 2.041(3) Å, and the Zn–O bond distances are 2.153(3) and 2.161(3) Å (Table 2), all of which are comparable to those observed for other related Zn(II) CPs based on tetrazole-based and pyridyl-based ligands [26, 27]. The DTPP2- ligand adopts a μ3-coordinate mode in which it binds to three Zn(II) ions with two tetrazole groups and one pyridyl group (Fig. 1). Although the coordination modes of the two tetrazolyl groups in the DTPP2- ligand are the same, the dihedral angles between the two tetrazole groups and the central benzene ring are different: 11.71(14)° and 7.65(14)°, respectively.

Fig. 1
figure 1

The coordination environment of Zn(II) and the coordination mode of H2DTPP drawn at 50% probability ellipsoids. H atoms have been omitted for clarity. Symmetry codes: (i) 1−x, y, −1/2 + z; (ii) 3/2−x, y, −z; (iii) 1−x, y, 1/2 +z

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The combination of Zn(II) ions and DTPP2- ligands gives rise to a 2D coordination network extending along the ac plane, as shown in Fig. 2. From a topological perspective, both Zn(II) ions and DTPP2- can be seen as three connected nodes; thus, the overall 2D network can be simplified as a (4·82) topological network. Such topology is usually observed in 2D coordination polymers [28,29,30].

Fig. 2
figure 2

The 2D network of Compound 1

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Furthermore, the adjacent 2D networks are connected by various O–H···N hydrogen bonds between the coordinated H2O molecule and the uncoordinated N atoms on the tetrazole group (Table 3), resulting in a 3D supramolecular framework, as shown in Fig. 3a and Fig. 3b. Small pores along the b direction were observed, as shown in Fig. 3c. It exhibits a percent effective free volume of 10.9% (a total potential solvent volume of 805 Å3 out of every unit cell volume of 7353 Å3), as calculated by PLATON software [31].

Fig. 3
figure 3

a The hydrogen bonds between two adjacent 2D network in Compound 1. b The 3D supramolecule framework in compound 1. c The small channels in compound 1

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Phase Purity and Thermal Stability

The phase purity of compound 1 was determined by powder X-ray diffraction (PXRD). As shown in Fig. 4, the position of the diffraction peak obtained from the experimental pattern is basically consistent with that from the simulated pattern, indicating that compound 1 has a high phase purity. The dissimilarity in reflection intensity between them may be attributed to the preferred orientation of the compound 1 sample during data collection.

Fig. 4
figure 4

The experimental and simulated PXRD of compound 1.

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Table 3 Selected details of the hydrogen bonds in compound 1
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The thermal stability of compound 1 was measured by thermogravimetric analysis (TGA) from room temperature to 800 °C in a nitrogen atmosphere at a rate of 10 °C min−1. As displayed in Fig. 5, the weight loss of 4.5% from room temperature to 110 °C may be due to the release of uncoordinated water molecules (Calc. 4.2%). The secondary weight loss of 9.0% from 110 to 150 °C may be attributed to the removal of two coordinated water molecules (Calc. of 8.4%). A sharp weight loss is observed above 270 °C, indicating decomposition of the coordination framework.

Fig. 5
figure 5

TGA curve for compound 1

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Fluorescence Properties

Zn(II) CPs generally exhibit photoluminescent properties [32, 33]. Therefore, the photoluminescence of compound 1 was evaluated. As shown in Fig. 6, compound 1 exhibited strong blue luminescence with a maximum emission band at 345 nm when excited at 276 nm in the solid-state at room temperature. For comparison, the photoluminescence of H2DTPP was also measured. The results showed an emission band centered at 350 nm when excited at 248 nm. Compound 1 only exhibited a slight blueshift of 5 nm compared with that of H2DTPP. In addition, the shapes of the emission peaks for compound 1 and H2DTPP are similar. Thus, the emission of compound 1 may be attributed to intraligand transitions.

Fig. 6
figure 6

Excitation and emission of H2dDTPP and compound 1 in the solid-state at room temperature

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Properties of Metal Ion Sensing

The ability of compound 1 to sense metal ions was investigated. The selected metal ions were added to the aqueous suspension of compound 1, and their photoluminescence spectra were recorded. The results showed that the emission of the compound 1 suspension strongly quenched after the addition of Fe3+, Cu2+, and Ag+, indicating that compound 1 may have the ability to sense these ions and may be most sensitive to Fe3+ (Figs. 7a and S1). The sensitivity of the compound 1 suspension for detection in water was further determined. The photoluminescence intensities of the compound 1 suspensions with different Fe3+, Cu2+, and Ag+ concentrations are shown in Figs. 7b and S2-S4. As shown in Fig. 7b, the luminescence intensity at 345 nm decreased as the concentration of Fe3+ increased. The linear relationship between I0/I and the Fe3+ concentration is in the range of 5×10−4–1×10−3 M. Consequently, the LOD is calculated to be 1.30×10−5 M. These LODs are more moderate than those of some reported MOF sensors [34, 35]. For Cu2+ and Ag+, the LODs are 5.64×10−2 and 5.06×10−2, respectively, which are higher than those for Fe3+.

Fig. 7
figure 7

a Luminescence spectra of compound 1 in aqueous solutions containing various cations (0.5 M). b Luminescence spectra of compound 1 with various concentrations of Fe3+ ions in aqueous solutions. c The PXRD of compound 1 before and after immersing in aqueous solutions containing Fe3+, Cu2+, and Ag+. d Compare the UV‒vis spectra of Fe3+, Cu2+, and Ag+ with the excitation and emission spectra of compound 1

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The quenching of the fluorescence emission of MOFs by metal ions may be due to structural collapse, energy transfer, competition, etc. [35,36,37,38,39]. The PXRD pattern of compound 1 after immersion in solutions containing 0.1 M Fe3+, Cu2+, and Ag+ ions showed that the crystal structures were retained (Fig. 7c); thus, quenching was not caused by structural collapse. In addition, the FT-IR spectra (Fig. S5) of the ion-treated sample remained unchanged, further revealing that compound 1 has a stable crystal structure. To determine whether energy transfer and/or competition occurred. The liquid UV‒Vis spectra of Fe3+, Cu2+, and Ag+ were measured. The results showed that they have a broad absorption between 250 and 350 nm for Cu2+ and Ag+ and between 250 and 375 nm for Fe3+. These bands covered the majority of the absorption band of compound 1. Consequently, these metal ions competed to absorb the energy of the light source when excited light passed, resulting in quenching [39]. For the Fe3+ ion, there is also some overlap between the absorption spectrum of Fe3+ and the emission spectrum of compound 1; therefore, energy transfer from compound 1 to the Fe3+ ion also contributed to fluorescence quenching.

Conclusions

In conclusion, we successfully constructed and structurally characterized a new Zn(II) CPs synthesized from a new tripodal N-containing heterotopic ligand. The compound displays a 2D network structure with a (4·82) topology and exhibits blue photoluminescent emission. In addition, it showed the ability to sense Fe3+ via photoluminescence quenching. This work indicates that a tripodal N-containing heterotopic ligand with two pyridyl and tetrazolyl groups is an effective organic ligand for the construction of new CPs.