Zinc (II) coordination polymer as well as preparation method and application thereof

Abstract

The invention relates to the field of materials, and discloses a zinc (II) coordination polymer and a preparation method and application thereof.The preparation method comprises the step that zinc salt, a first ligand and a second ligand are subjected to a hydrothermal reaction to obtain the zinc (II) coordination polymer, the first ligand is a bis (imidazole-1-yl methyl) benzene isomer, and the second ligand is terephthalic acid. By simply changing the isomer of the ligand (such as replacement of the isomeride m-bix and p-bix) or introducing the mixed ligand, the structure of the product can be accurately regulated and controlled under the hydrothermal condition, and a diversified structure from a two-dimensional layer to a three-dimensional network is obtained.

Description

Technical Field

[0001] This invention relates to the field of materials, specifically to a zinc(II) coordination polymer, its preparation method, and its applications. Background Technology

[0002] Coordination polymers (also known as metal-organic frameworks) are a class of crystalline materials with tunable structures and diverse functions, whose properties depend on the choice of metal center and organic ligand. Zinc(II) ions possess d¹ 0 Zinc-based coordination polymers exhibit diverse electronic configurations and coordination modes, good biocompatibility, and often demonstrate excellent luminescent properties, thus showing broad application prospects in luminescent materials, chemical sensing, and optoelectronic devices. Currently, regulating the structure and photophysical properties of coordination polymers through ligand design (such as using mixed ligands, isomeric ligands, and auxiliary anions) has become a research hotspot in this field. However, how to precisely control the dimensional structure and luminescent properties of zinc-based coordination polymers through ligand isomerization and mixed ligand strategies, especially to achieve temperature-responsive luminescence behavior, remains a challenge. This invention aims to overcome the shortcomings of existing technologies and solve the problem that existing zinc-based ligand polymers cannot achieve temperature-responsive luminescence behavior, providing a zinc(II) coordination polymer, its preparation method, and its applications. To achieve the above objective, the first aspect of this invention provides a method for preparing a zinc(II) coordination polymer, wherein the preparation method includes: subjecting a zinc salt, a first ligand, and a second ligand to a hydrothermal reaction to obtain a zinc(II) coordination polymer, wherein the first ligand is a bis(imidazol-1-ylmethyl)benzene isomer, and the second ligand is terephthalic acid.

[0003] This invention promotes the deprotonation of the carboxyl group (-COOH) in carboxyl-containing ligands (such as terephthalic acid, 1,4-H2bda) under hydrothermal conditions, converting them into carboxylate ions (-COO⁻), thereby enabling them to coordinate with metal ions. Simultaneously, nitrogen-containing heterocyclic ligands (such as meta / para / ortho-bisimidazolebenzene, m-bix / p-bix / o-bix) coordinate with metal ions through the lone pair electrons on their nitrogen atoms. Furthermore, m-bix / p-bix / o-bix not only act as coordinating groups but also serve as bridging groups, connecting multiple Zn²⁺ ions, thereby forming a two-dimensional layered or three-dimensional network structure.

[0004] A second aspect of the present invention provides a zinc(II) coordination polymer prepared by the preparation method described in the first aspect of the present invention.

[0005] The third aspect of the present invention provides the application of the zinc(II) coordination polymer described in the second aspect of the present invention in luminescent materials. Attached Figure Description

[0006] Figure 1This is a schematic diagram of the reaction principle of the present invention; Figure 2 This is a spatial structure diagram of the coordination polymer obtained in Example 1; Figure 3 This is a spatial structure diagram of the coordination polymer obtained in Example 2; Figure 4 The graphs show the thermal stability test results of the coordination polymers obtained in Examples 1 and 2. Figure 5 XRD patterns of the coordination polymers obtained in Examples 1 and 2; Figure 6 The images show the spectra of the coordination polymers obtained in Examples 1 and 2. Detailed Implementation

[0007] The specific embodiments of this application will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit this application.

[0008] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0009] Furthermore, the term "and / or" in the specification and claims is used to describe the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0010] In the description of this specification, the references to terms such as "an embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0011] The first aspect of the present invention provides a method for preparing a zinc(II) coordination polymer, wherein the preparation method includes: subjecting a zinc salt, a first ligand, and a second ligand to a hydrothermal reaction to obtain a zinc(II) coordination polymer, wherein the first ligand is a bis(imidazol-1-ylmethyl)benzene isomer and the second ligand is terephthalic acid.

[0012] This invention promotes the deprotonation of the carboxyl group (-COOH) in carboxyl-containing ligands (such as terephthalic acid, 1,4-H2bda) under hydrothermal conditions, converting them into carboxylate ions (-COO⁻), thereby enabling them to coordinate with metal ions. Simultaneously, nitrogen-containing heterocyclic ligands (such as meta / para / ortho-bisimidazolebenzene, m-bix / p-bix / o-bix) coordinate with metal ions through the lone pair electrons on their nitrogen atoms. Furthermore, m-bix / p-bix / o-bix not only act as coordinating groups but also serve as bridging groups, connecting multiple Zn²⁺ ions, thereby forming a two-dimensional layered or three-dimensional network structure.

[0013] This invention allows for precise control of the product structure under hydrothermal conditions by simply changing the isomers of the ligands (such as replacing the isomers m-bix with p-bix or o-bix) or introducing mixed ligands, thus obtaining diverse structures ranging from two-dimensional layers to three-dimensional networks. This demonstrates the efficiency and flexibility of the method in constructing complex topologies.

[0014] The prepared coordination polymer exhibits strong fluorescence emission in the solid state (emission peak at 451-464 nm, in the blue light region).

[0015] For the first time, a significant luminescent thermochromic effect was discovered in this type of material. At low temperatures (77 K), the material's luminescence exhibits a redshift (to 469-496 nm), and its luminescence lifetime is significantly extended. This temperature-sensitive luminescence behavior makes it potentially valuable for applications in low-temperature sensing or temperature indication.

[0016] The obtained coordination polymers also exhibit excellent thermal stability. Thermogravimetric analysis shows that, especially the coordination polymers prepared using para-bisimidazolebenzene, their structures can be stably maintained above 405°C before decomposition. This high thermal stability provides a fundamental guarantee for their application in harsh environments (such as catalysis and optoelectronic devices).

[0017] Preferably, the molar ratio of the zinc salt, the first ligand, and the second ligand is 1:1:0.5-1.5.

[0018] Preferably, the molar ratio of the zinc salt, the first ligand, and the second ligand is 1:1:1.

[0019] In hydrothermal synthesis, the molar ratio of metal salt to organic ligand is one of the core parameters determining the final structure of coordination polymers. It not only affects the yield and purity of the product but also directly determines the crystal nucleation rate, network topology, and material dimensionality. Specifically, in the reaction system of Zn²⁺ with mixed ligands (such as nitrogen-containing ligands m-bix / p-bix / o-bix + carboxylic acid ligand 1,4-bda, and possibly ox), the molar ratio mainly affects the following aspects:

[0020] 1. Coordination competition and saturation: When the ratio of metal ions to specific ligands is different, the coordination sites around the metal center will be competitively occupied by different ligands. The molar ratio determines whether a low-dimensional structure or a high-dimensional cross-linked network is formed.

[0021] 2. Crystal nucleation rate: A suitable molar ratio ensures slow nucleation in the reaction system, resulting in the growth of high-quality, uniformly sized single crystals. An improper ratio can easily lead to microcrystalline precipitation or amorphous products.

[0022] 3. Assembly of secondary structural units (SBU): For example, in the final coordination polymer 1, the bridging effect of oxalate (ox²⁻) depends on its ratio with Zn²⁺, which directly affects the formation of binuclear or multinuclear clusters and thus determines the construction of the two-dimensional layer.

[0023] When the molar ratio is below a certain range (e.g., excess metal ions or insufficient ligands), the number of free metal ions in the solution increases, but there is a lack of sufficient organic linkers, which leads to the formation of low-dimensional or dense phases: simple metal-centered complexes or one-dimensional chain structures are easily formed, making it difficult to expand into the expected two-dimensional or three-dimensional porous framework. Insufficient ligands prevent adequate bridging of all metal nodes; and under high-temperature hydrothermal conditions, excess Zn²⁺ readily combines with water or OH⁻ in the solution to form Zn(OH)₂ or ZnO impurity precipitates, resulting in reduced yield of the target product or even synthesis failure. Furthermore, insufficient ligands lead to incomplete crystal growth, resulting in small crystal particles with many defects, which is detrimental to single-crystal diffraction structure analysis.

[0024] When the molar ratio exceeds the specified range (e.g., ligand excess), a large number of free organic ligands exist in the system. In this case, the excess ligands may self-assemble through hydrogen bonding or π-π interactions, or form mononuclear or polynuclear but structurally dense complexes with metal ions, hindering the formation of long-range ordered network structures and failing to obtain the structure expected in this invention. Alternatively, when a specific ligand (especially a carboxylic acid ligand with strong chelating ability) is in excess, it will saturate all coordination sites of the metal ions, preventing the access of another bridging ligand (such as m-bix), thus preventing dimensional expansion (e.g., unable to further connect from a 2D layered structure into a 3D network). In addition, it increases the difficulty of separation and purification: excess organic ligands are difficult to completely elute during post-processing and are prone to remain in the product channels, increasing the difficulty of separation and purification and affecting the performance of the product (such as luminescence and adsorption properties).

[0025] Therefore, the molar ratio of the zinc salt, the first ligand, the second ligand, and the third ligand can be 1:1:0.5, 1:1:0.6, 1:1:0.7, 1:1:0.8, 1:1:1.0.9, 1:1:1, 1:1:1.1, 1:1:1.2, 1:1:1.3, 1:1:1.4, 1:1:1.5, and any value between any two of these ratios.

[0026] Preferably, the hydrothermal reaction temperature is 100-170℃ and the time is 100-130h. At high temperatures, the viscosity of the reaction system decreases, and the diffusion rate of the reactants increases, which is beneficial to the nucleation and crystal growth of metal-organic frameworks (MOFs), ultimately yielding a product with a regular structure and high crystallinity. The hydrothermal reaction temperature can be any value between 100℃, 110℃, 120℃, 130℃, 140℃, 150℃, 160℃, and 170℃, and the time can be any value between 100h, 110h, 120h, and 130h.

[0027] Preferably, the bis(imidazol-1-ylmethyl)benzene isomer is selected from one or more of meta-bis(imidazol-1-ylmethyl)benzene, para-bis(imidazol-1-ylmethyl)benzene, and ortho-bis(imidazol-1-ylmethyl)benzene.

[0028] Preferably, the bis(imidazol-1-ylmethyl)benzene isomer is selected from meta-bis(imidazol-1-ylmethyl)benzene or para-bis(imidazol-1-ylmethyl)benzene.

[0029] Preferably, when the first ligand is meta-bis(imidazol-1-ylmethyl)benzene, oxalic acid is used to adjust the pH of the hydrothermal reaction to a preset value, which is 5-7. Acidic solutions commonly used in the art can be used to adjust the pH of the reaction system. However, this invention unexpectedly discovered that when oxalic acid is used as a pH adjuster, it can be unexpectedly coordinated into the polymer, thereby forming a novel coordination polymer. The concentration of oxalic acid can be commonly used in the art, such as 1 mol / L, and the pH can be 5, 5.5, 6, 6.5, 7, or any value between any two of these numbers.

[0030] Preferably, when the first ligand is meta-bis(imidazol-1-ylmethyl)benzene, the hydrothermal reaction is carried out at a temperature of 120°C for 120 hours and at a pH of 6.

[0031] When the first ligand is para-bis(imidazol-1-ylmethyl)benzene, the hydrothermal reaction is carried out at a temperature of 160°C for 120 hours.

[0032] Preferably, the zinc salt is selected from one or more of zinc chloride, zinc nitrate, and their corresponding hydrates.

[0033] A second aspect of the present invention provides a zinc(II) coordination polymer prepared by the preparation method described in the first aspect of the present invention.

[0034] Preferably, the zinc(II) coordination polymer has the structural formula [Zn2(m-bix)(1,4-bda)]. 0.5 (ox) 1.5 ] n Where m-bix is ​​meta-bis(imidazol-1-ylmethyl)benzene, 1,4-bda is terephthalate, ox is oxalate, and n takes the value ≥1, and it has a two-dimensional layered structure.

[0035] The zinc(II) coordination polymer belongs to the monoclinic crystal system, space group P21 / c. Its asymmetric unit contains two independent zinc(II) ions, one of which has a five-coordinate geometry and the other has a six-coordinate geometry. The cell parameters are a = 7.9846(16) Å, b = 15.881(3) Å, c = 18.465(4) Å, α = 90°, β = 94.27(3)°, γ = 90°, and V = 2335.0(8) Å. 3 Z=4.

[0036] When oxalic acid is used to adjust the pH value, oxalic acid can be unexpectedly coordinated. The coordination polymer obtained by this invention can be infinitely repeated from the same structure, the structural formula of which is as follows:

[0037] Preferably, the zinc(II) coordination polymer has the structural formula [Zn 1.5 (p-bix) 0.5 (1,4-bda) 1.5 ] n , where p-bix is ​​para-bis(imidazol-1-ylmethyl)benzene, 1,4-bda is terephthalate, and n takes the value ≥1, and it has a three-dimensional network structure; The zinc(II) coordination polymer belongs to the monoclinic crystal system, space group C2 / c. The zinc(II) center has a four-coordinate geometry with unit cell parameters a = 15.685(3) Å, b = 13.464(3) Å, c = 17.051(3) Å, α = 90°, β = 91.83(3)°, γ = 90°, and V = 3599.0(12) Å. 3 Z=4.

[0038] The coordination polymers obtained by this invention can be infinitely repeated from the same structure, the structural formula of which is as follows:

[0039] The third aspect of the present invention provides the application of the zinc(II) coordination polymer described in the second aspect of the present invention in luminescent materials.

[0040] Preferably, the luminescence temperature of the zinc(II) coordination polymer is 70K-300K. The polymer provided by this invention can exhibit luminescence properties over a wide temperature range.

[0041] Compared with the prior art, the present invention has the following significant advantages: 1. This invention controls the preparation of zinc(II) coordination polymers with different dimensions and topologies by simply changing the substitution position (meta vs. para) of the first ligand (bis(imidazol-1-ylmethyl)benzene) and combining it with the synergistic effect of mixed carboxylic acid ligands (terephthalic acid or further including oxalic acid). Specifically, the use of meta ligands (m-bix) tends to form a two-dimensional layered structure, while the use of para ligands (p-bix) drives the formation of a three-dimensional network structure. This strategy of achieving the evolution from two-dimensional to three-dimensional structures through ligand isomerism provides a simple and efficient way to develop crystalline materials with novel structures.

[0042] 2. Mild one-step hydrothermal method with high yield and good crystal quality: This invention adopts a one-pot hydrothermal synthesis process with mild reaction conditions (temperature 100-170°C, pH 5-7) and simple operation. The obtained products are all colorless and transparent bulk single crystals with high purity (XRD patterns are in high agreement with simulated patterns), which facilitates subsequent structural characterization and application research, and is conducive to industrial-scale production.

[0043] 3. Significantly improved thermal stability: By introducing a rigid aromatic carboxylic acid (terephthalic acid) and a small molecule acid with strong coordination ability (oxalic acid) to construct a mixed ligand system, the rigidity of the polymer backbone is effectively enhanced. Thermogravimetric analysis shows that the obtained coordination polymers exhibit excellent thermal stability, especially coordination polymer 2 (three-dimensional structure), which has a decomposition temperature as high as 405°C, far exceeding that of many known coordination polymers, making its application in extreme environments possible.

[0044] 4. It achieves stable light emission over a wide temperature range (77K-300K) and exhibits a luminescent thermochromic effect: The invention broadens the luminescence temperature range: the polymer overcomes the problems of luminescence quenching at low temperatures or luminescence instability caused by increased thermal vibration at high temperatures in traditional materials. Experiments have confirmed that the prepared zinc(II) coordination polymer exhibits significant luminescence performance over a wide temperature range from 77K (liquid nitrogen temperature) to 300K (room temperature), covering the range from deep cryogenics to room temperature, and has application potential in low-temperature environment monitoring or multi-level temperature sensing.

[0045] Excellent low-temperature luminescence performance: At 77K, the polymer exhibits strong luminescence with a significant redshift compared to room temperature (coordination polymer 1: 459 nm → 479 nm; coordination polymer 2: 464 nm → 496 nm). This not only demonstrates that it maintains good luminescence ability at low temperatures (without low-temperature quenching), but also that the low temperature suppresses non-radiative transitions, resulting in a sharper emission peak and lower energy, exhibiting a unique "luminescent thermochromic" behavior.

[0046] The present invention will be further described in detail below with reference to specific embodiments, but the implementation of the present invention is not limited thereto. Unless otherwise specified, the test methods and testing equipment used in the following embodiments are conventional test methods and testing equipment in the art.

[0047] meta-bis(imidazol-1-ylmethyl)benzene was purchased from Jinan Henghua Technology Co., Ltd. para-bis(imidazol-1-ylmethyl)benzene, purchased from Jinan Henghua Technology Co., Ltd.; ortho-bis(imidazol-1-ylmethyl)benzene was purchased from Jinan Henghua Technology Co., Ltd.

[0048] Example 1

[0049] Zinc nitrate hexahydrate (30.8 mg, 0.1 mmol), meta-bis(imidazol-1-ylmethyl)benzene (m-bix, 23.8 mg, 0.1 mmol), and terephthalic acid (1,4-H₂bda, 17.2 mg, 0.1 mmol) were dissolved in 5 mL of distilled water in a 1:1:1 molar ratio. The pH was adjusted to 6 by adding 2 mL of 1.0 mol·L⁻¹ oxalic acid solution. After stirring for 30 min, the mixture was transferred to a 15 mL stainless steel high-pressure reactor and heated at 120ºC for 120 h. After cooling, colorless and transparent blocky crystals were obtained. The crystals were removed, rinsed with pure water, and dried. This was designated as coordination polymer 1. The reaction principle is as follows: Figure 1 As shown in the figure, this embodiment uses Zn(NO3)2·6H2O and meta-bis(imidazol-1-ylmethyl)benzene (m-bix) as raw materials, introduces terephthalic acid (1,4-H2bda), and uses oxalic acid (H2ox) as a pH adjuster. A coordination polymer with a two-dimensional layered structure is obtained through a hydrothermal reaction. Oxalic acid and 1,4-bda jointly bridge zinc ions to form a mixed ligand framework. The molecular formula of coordination polymer 1 is [Zn2(m-bix)(1,4-bda)]. 0.5 (ox) 1.5 ] n For any given element n≥1, it can grow infinitely in space. The structural formula of a single structural unit is as follows:

[0050] Example 2

[0051] Zinc nitrate hexahydrate (30.8 mg, 0.1 mmol), p-bis(imidazol-1-ylmethyl)benzene (p-bix, 23.8 mg, 0.1 mmol), and terephthalic acid (1,4-H2bda, 17.2 mg, 0.1 mmol) were dissolved in 5 mL of distilled water at a molar ratio of 1:1:1. After stirring for 30 minutes, the mixture was transferred to a 15 mL stainless steel high-pressure reactor and heated at 160ºC for 120 h. After cooling, colorless and transparent blocky crystals were obtained. The crystals were removed, washed with pure water, and dried. This was designated as coordination polymer 2. The reaction principle is as follows: Figure 1 As shown in the figure, the polymer obtained in this embodiment has a three-dimensional network structure. 1,4-bda connects zinc ions through multiple coordination modes to form rectangular channels, and p-bix further bridges these channels to form a dense three-dimensional framework. The molecular formula of coordination polymer 2 is [Zn...]. 1.5 (p-bix)(1,4-bda) 1.5 ] n For any given element n≥1, it can grow infinitely in space. The structural formula of a single structural unit is as follows:

[0052] Test Example 1

[0053] Using the polymers prepared in Examples 1 and 2 as examples, single-crystal X-ray diffraction analysis was performed on the crystals using a Rigaku diffractometer (Japan) with Mo-Kα rays (λ = 0.071073 nm). The collected data were analyzed using the direct method in SHELXTL-2014 crystallography software. The crystal structure was optimized using the full-matrix least squares method, and anisotropic thermal parameters were corrected for all non-hydrogen atoms. The coordinates of hydrogen atoms were obtained through theoretical calculations. The results are shown in Table 1.

[0054] Table 1

[0055] Note: a R1= ||F o |–|F c || / |F o ; b wR2=[ [w(F o 2 –F c 2 ) 2 ] / [w(F o 2 ) 2 ]] 1 / 2 .

[0056] Test Example 2

[0057] Single-crystal X-ray diffraction analysis was performed using Mo-Kα rays (λ = 0.071073 nm) on a Rigaku diffractometer in Japan. The collected data were analyzed using the direct method in SHELXTL-2014 crystallography software. The crystal structure was optimized using the full-matrix least squares method, and anisotropic thermal parameters were corrected for all non-hydrogen atoms. The coordinates of hydrogen atoms were obtained through theoretical calculations. The resulting CIF files were then plotted using Diamond software.

[0058] Coordination polymer 1 crystal belongs to the monoclinic crystal system, space group P21 / c, and its space structure is as follows: Figure 2 As shown, where Figure 2 (a) represents the asymmetric unit of coordination polymer 1. Figure 2 (b) is a polyhedral representation of the metal-centered coordination mode of coordination polymer 1. Figure 2 (c) shows the two-dimensional layered structure of coordination polymer 1. Figure 2(d) is a one-dimensional chain structural unit of coordination polymer 1. In the figure, blue represents N, gray represents C, yellow represents Zn, black represents terephthalate ligand, and red represents O.

[0059] The asymmetric unit of coordination polymer 1 comprises two crystallographically independent Zn²⁺ ions, an m-bix ligand, a half 1,4-bda²⁻ anion, and a half ox²⁻ anion. The Zn1 center adopts a five-coordinate geometry, forming a slightly distorted trigonal bipyramidal [ZnO4N] coordination environment by coordinating with three oxygen atoms (O3, O5, and O8) from the half ox²⁻ anion, one oxygen atom (O1) from the half 1,4-bda²⁻ anion, and a nitrogen atom (N4) from the m-bix ligand. In contrast, the Zn2 center exhibits a slightly distorted octahedral [ZnO5N] configuration with six coordination atoms, which are derived from four oxygen atoms (O4, O6, O7, and O8) of a half-ox²⁻ anion, one oxygen atom (O2) of a half-1,4-bda²⁻ anion, and a nitrogen atom (N2) of an m-bix ligand. The Zn–O bond lengths range from 1.988(2) Å to 2.220(2) Å, while the Zn–N bond lengths are 2.038(2) Å and 2.047(2) Å, respectively.

[0060] In coordination polymer 1, the organic carboxylic acid ligands H2ox and 1,4-H2bda are completely deprotonated, thus enabling coordination with the central metal ion. Their carboxyl groups play a crucial role in the crystal structure formation process. One of the ox²⁻ anions bridges two adjacent zinc ions through its carboxyl oxygen atom, forming a one-dimensional chain structure. These chains are further interconnected by 0.5 ox²⁻ anions, constructing a one-dimensional beaded double-chain structure, where the Zn···Zn distances are 3.651(1) Å, 5.401(2) Å, and 5.321(1) Å, respectively. Two adjacent one-dimensional ladder-like double chains are interconnected, forming a two-dimensional layered structure. Within the asymmetric unit, three ligands with different conjugated systems simultaneously coordinate with Zn²⁺ ions, forming a crowded coordination environment. The dihedral angle between the two imidazole rings in the m-bix ligand is 24.48°, and they are nearly parallel. This allows the two coordinating nitrogen atoms on the m-bix ligand to form a closed structure with the Zn²⁺ ion, without any additional coordination sites. Therefore, the m-bix ligand does not contribute to the dimensional extension of the overall structure. Four zinc ions are connected by four oxygen atoms from 0.5 ox²⁻ anions, forming a secondary structural unit (SBU). These SBUs are further connected by 0.5 1,4-bda²⁻ anions, generating another type of one-dimensional chain structure. Adjacent chains are connected in an alternating manner, forming an infinitely extended two-dimensional layered structure.

[0061] Coordination polymer 2 crystals belong to the monoclinic crystal system, space group C2 / c, and the space structure is as follows: Figure 3 As shown, where Figure 3 (a) represents the asymmetric unit of coordination polymer 2. Figure 3 (b) is a polyhedral representation of the coordination mechanism of the coordination polymer 2 with the metal center. Figure 3 (c) shows the two-dimensional layered structure of coordination polymer 2 and the dihedral angle of the ligand. Figure 3 (d) shows the three-dimensional network structure of coordination polymer 2.

[0062] The basic structural unit of coordination polymer 2 comprises 1.5 crystallographically independent Zn²⁺ ions, 0.5 p-bix ligands, and 1.5 1,4-bda²⁻ anions. Zn1 adopts a four-coordinate geometry, bonded to three oxygen atoms from the carboxyl groups of the 1.5 1,4-bda²⁻ anions and one nitrogen atom from the 0.5 p-bix ligands. Zn2 also exhibits a four-coordinate geometry, coordinated through two oxygen atoms from the 1,4-bda²⁻ carboxyl groups and their symmetrical equivalent oxygen atoms. Both Zn1 and Zn2 form a slightly distorted triangular pyramidal coordination environment. In this basic structural unit, the dihedral angle between the imidazole ring and the benzene ring of the p-bix ligand coordinated to the central Zn²⁺ ion is 65.80°, while the two imidazole rings are completely parallel, exhibiting better coplanarity than the m-bix ligand. In the crystal structure of coordination polymer 2, due to the two coordination modes of the 1,4-bda²⁻ ligand, μ2=η¹:η¹ and μ4=η¹:η¹:η¹:η¹, adjacent metal ions are interconnected through the 1,4-bda²⁻ ligand, forming a quasi-rectangular three-dimensional channel structure [Zn1.5(1,4-bda²⁻)1.5] with a size of approximately 19.5 Å × 13.4 Å. These adjacent quasi-rectangular three-dimensional channels are further connected through p-bix ligands, constructing a complex and dense three-dimensional framework structure.

[0063] Test Example 3

[0064] Thermal stability test: The coordination polymer crystal sample was ground into powder using a quartz mortar and pestle. The weight loss data of the coordination polymer from room temperature to 700℃ was recorded using a ZPY-2 thermogravimetric analyzer in an air atmosphere. The curve was plotted with temperature as the abscissa and weight loss percentage as the ordinate.

[0065] The results are as follows Figure 4As shown, the TG curve of coordination polymer 1 reveals a dominant weight loss phase between 277 and 519°C, with a mass loss of 78.84%, attributed to the decomposition of an m-bix ligand, 0.5 equivalents of 1,4-H₂bda, and 1.5 equivalents of oxalic acid ligand in the crystal structure (theoretical value: 78.27%). Coordination polymer 2 exhibits excellent thermal stability, maintaining its structural integrity below 405°C. A single weight loss phase was observed in the range of 405°C to 496°C, with a mass loss of 78.17% (theoretical value: 77.98%), corresponding to the removal of 0.5 equivalents of p-bix ligand and 1.5 equivalents of 1,4-H₂bda ligand from the crystal.

[0066] This indicates that coordination polymer 1 has excellent thermal stability below 277°C, while coordination polymer 2 has excellent thermal stability below 405°C. It is evident that they can maintain their original morphology at higher temperatures, providing a fundamental guarantee for their application in harsh environments (such as catalysis and optoelectronic devices).

[0067] Test Example 4

[0068] The phase purity of coordination polymers 1 and 2 was tested at room temperature (25°C) by X-ray powder diffraction (PXRD). Figure 5 As shown, the left side displays the XRD pattern of coordination polymer 1, where a is the curve of the test sample and b is the calculated standard spectrum (simulated in Hg software based on the CIF file obtained from single-crystal test data). The right side displays the XRD pattern of coordination polymer 2, where a is the curve of the test sample and b is the calculated standard spectrum. It can be seen that the PXRD patterns of the two coordination polymers are in high agreement with the experimentally obtained spectra. Based on the comparison and analysis of the sample curves and standard spectra, the purity of coordination polymer 1 is approximately 98.5%, and the purity of coordination polymer 2 is approximately 98.2%. This demonstrates that the coordination polymers prepared by the method provided in this invention can be crystallized with very high purity simply by washing with water after crystallization.

[0069] Test Example 5

[0070] Luminescence performance testing methods: The photoluminescence of the samples in both solid and liquid states was measured using an Edinburgh FLS920 fluorescence spectrometer. A Xenon lamp (Xe900 450 W) was used as the excitation source, and a Typer 928 detector was employed to record the luminescence signal of the samples in the wavelength range of 200-800 nm. Steady-state testing at 77 K involved encapsulating the solid powder of the test sample in a quartz NMR tube, placing it in a cryostat, and maintaining the temperature with liquid nitrogen.

[0071] Because coordination polymers 1 and 2 are composed of d¹ 0Constructed from electronically configured Zn²⁺ ions and the fluorescent ligand m-bix, we investigated the luminescent properties of these two coordination polymers. Single-crystal ground powder samples of coordination polymers 1-2 were subjected to spectroscopic measurements at 298 K and 77 K, respectively. The results are as follows: Figure 6 As shown, the left side is the fluorescence spectrum of coordination polymer 1, and the right side is the fluorescence spectrum of coordination polymer 1. The black line represents solid state at room temperature (298 K), the red line represents solid state at low temperature (77 K) (both are solid state), and the green line represents solid state in DMSO solution.

[0072] like Figure 6 As shown in the spectra and internal CIE chromaticity diagrams, both coordination polymers 1 and 2 exhibit strong solid-state luminescence at room temperature (298 K), as detailed below:

[0073] Under 330 nm UV excitation, the emission peak of coordination polymer 1 is located at 459 nm, and that of coordination polymer 2 is located at 464 nm. According to CIE chromaticity coordinates, the coordinates of coordination polymer 1 are (0.20, 0.23) and those of coordination polymer 2 are (0.20, 0.25), indicating that both emit light in the blue light region. Compared to the free m-bix ligand (whose emission peak is located at 422 nm under the same excitation), the emission peaks of both coordination polymers show a significant redshift (coordination polymer 1 redshifted by 37 nm, and coordination polymer 2 redshifted by 42 nm). This phenomenon is mainly attributed to the following two points: First, coordination enhances the rigidity of the ligands and reduces energy loss caused by molecular vibrations; second, through the bridging effect of oxalic acid (ox²⁻) and terephthalic acid (1,4-bda²⁻), a binuclear or multinuclear structure is formed, which enhances the conjugation degree of the ligands, thereby effectively reducing the HOMO-LUMO band gap and leading to a decrease in transition energy. This decrease in transition energy results in a redshift of the emission spectrum.

[0074] Furthermore, the emission peak (464 nm) of coordination polymer 2 (using the para-ligand p-bix) is broader and slightly red-shifted than that of coordination polymer 1 (using the meta-ligand m-bix, 459 nm). This is because the two imidazole rings in the p-bix ligand are in a completely parallel configuration. This highly coplanar structure promotes the delocalization of electrons within the molecule, resulting in a longer emission wavelength and a red-shifted emission spectrum.

[0075] The emission spectra of coordination polymers 1 and 2, measured in a solid state at 77 K (liquid nitrogen environment, sample chamber maintained under vacuum), showed a significant redshift compared to the spectra at 298 K (coordination polymer 1: 459 nm → 479 nm; coordination polymer 2: 464 nm → 496 nm). Correspondingly, the CIE chromaticity coordinates of coordination polymers 1 and 2 at 77 K were (0.26, 0.35) and (0.26, 0.38), respectively, indicating that the emission was located in the green light region. This temperature-dependent redshift phenomenon from 298 K to 77 K is termed "luminescent thermochromism," demonstrating the material's high temperature sensitivity.

[0076] In addition, after the low-temperature (77K) spectral testing, the sample was re-tested using room-temperature X-ray powder diffraction (PXRD). The diffraction pattern after the test was compared with the original pattern before the test (e.g., Figure 5 The peak positions and shapes of the two samples are compared. If they match perfectly, it proves that the structure of the sample has not changed after low-temperature cycling, and the test process does not damage the integrity of the sample. The samples of Examples 1 and 2 of this application both passed this verification after low-temperature testing, which shows that the coordination polymer prepared by this invention can still maintain its complete structure at low temperatures.

[0077] Compared to solid-state spectroscopy, the prepared coordination polymers were dissolved in dimethyl sulfoxide (DMSO) solution, and their luminescence properties were then tested. The CIE coordinates of coordination polymers 1 and 2 at 298 K were (0.18, 0.15) and (0.18, 0.16), respectively, with emission peaks less than 440 nm. A significant blue shift was observed in the emission spectra in DMSO solution, with the emission color changing from light blue to dark blue. This is because the hydrogen bonding interactions between molecules are stronger in the solid state than in solution. These hydrogen bonds effectively reduce the HOMO-LUMO band gap, affecting the π*→π electron transition. This characteristic, where the material's emission color changes when exposed to the vapor of a specific organic solvent (such as DMSO), makes it suitable for detecting the presence of specific organic solvent molecules (such as DMSO) in the environment, thus enabling its application in the fabrication of chemical sensors.

[0078] Example 3

[0079] The procedure was carried out in accordance with Example 1, except that the molar ratio of zinc nitrate hexahydrate, meta-bis(imidazol-1-ylmethyl)benzene (m-bix), and terephthalic acid (1,4-H2bda) was 1:1:0.5.

[0080] Example 4

[0081] The procedure was carried out in accordance with Example 1, except that the molar ratio of zinc nitrate hexahydrate, meta-bis(imidazol-1-ylmethyl)benzene (m-bix), and terephthalic acid (1,4-H2bda) was 1:1:1.5.

[0082] Example 5

[0083] The reaction was carried out in the same manner as in Example 1, except that the reaction temperature was 160°C.

[0084] Example 6

[0085] The reaction was carried out in the same manner as in Example 1, except that the reaction temperature was 100°C.

[0086] Example 7

[0087] The reaction was carried out in the same manner as in Example 1, except that the reaction time was 130 hours.

[0088] Example 8

[0089] The reaction was carried out in the same manner as in Example 1, except that the reaction time was 100 hours.

[0090] Example 9

[0091] The reaction was carried out in the same manner as in Example 1, except that the pH of the reaction was 5.

[0092] Example 10

[0093] The reaction was carried out in the same manner as in Example 1, except that the pH of the reaction was 7.

[0094] Example 11

[0095] The procedure was carried out as in Example 1, except that meta-bis(imidazol-1-ylmethyl)benzene was replaced with ortho-bis(imidazol-1-ylmethyl)benzene.

[0096] Comparative Example 1

[0097] The reaction was carried out in accordance with Example 1, except that meta-bis(imidazol-1-ylmethyl)benzene was missing from the reaction raw materials.

[0098] Comparative Example 2

[0099] The reaction was carried out in accordance with Example 1, except that terephthalic acid was not present in the reaction raw materials.

[0100] Comparative Example 3

[0101] The reaction was carried out in the manner described in Example 1, except that oxalic acid was not present in the reaction raw materials.

[0102] The crystals obtained in Examples 3-11 and Comparative Examples 1-3 were tested according to Test Examples 3-4, and the results are shown in Table 2.

[0103] Table 2

[0104] Note: "-" in the table indicates that a qualified crystal sample could not be obtained or that a valid luminescence signal could not be detected.

[0105] The contents not described in detail in this specification are existing technologies known to those skilled in the art, and will not be elaborated upon here.

[0106] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combining the various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing a zinc(II) coordination polymer, characterized in that, The preparation method includes: subjecting a zinc salt, a first ligand, and a second ligand to a hydrothermal reaction to obtain a zinc(II) coordination polymer, wherein the first ligand is a bis(imidazol-1-ylmethyl)benzene isomer and the second ligand is terephthalic acid.

2. The preparation method according to claim 1, wherein, The molar ratio of the zinc salt, the first ligand, and the second ligand is 1:1:0.5-1.

5.

3. The preparation method according to claim 2, wherein, The molar ratio of the zinc salt, the first ligand, and the second ligand is 1:1:

1.

4. The preparation method according to claim 1 or 2, wherein, The hydrothermal reaction is carried out at a temperature of 100-170℃ for a duration of 100-130 hours. And / or, the bis(imidazol-1-ylmethyl)benzene isomer is selected from one or more of meta-bis(imidazol-1-ylmethyl)benzene, para-bis(imidazol-1-ylmethyl)benzene, and ortho-bis(imidazol-1-ylmethyl)benzene, preferably meta-bis(imidazol-1-ylmethyl)benzene or para-bis(imidazol-1-ylmethyl)benzene; The zinc salt is selected from one or more of zinc chloride, zinc nitrate, and their corresponding hydrates.

5. The preparation method according to claim 4, wherein, When the first ligand is meta-bis(imidazol-1-ylmethyl)benzene, oxalic acid is used to adjust the pH value of the hydrothermal reaction to a preset value of 5-7. Preferably, the temperature of the hydrothermal reaction is 120°C, the time is 120h, and the pH is 6. Alternatively, when the first ligand is para-bis(imidazol-1-ylmethyl)benzene, the hydrothermal reaction is carried out at a temperature of 160°C for 120 hours.

6. A zinc(II) coordination polymer prepared by the method according to any one of claims 1-5.

7. The zinc(II) coordination polymer according to claim 6, wherein, The zinc(II) coordination polymer has the structural formula [Zn2(m-bix)(1,4-bda). 0.5 (ox) 1.5 ] n Where m-bix is ​​meta-bis(imidazol-1-ylmethyl)benzene, 1,4-bda is terephthalate, ox is oxalate, and n takes a value ≥1, and it has a two-dimensional layered structure. The zinc(II) coordination polymer belongs to the monoclinic crystal system, space group P21 / c. Its asymmetric unit contains two independent zinc(II) ions, one of which has a five-coordinate geometry and the other has a six-coordinate geometry. The cell parameters are a = 7.9846(16) Å, b = 15.881(3) Å, c = 18.465(4) Å, α = 90°, β = 94.27(3)°, γ = 90°, and V = 2335.0(8) Å. 3 Z=4.

8. The zinc(II) coordination polymer according to claim 6, wherein, The zinc(II) coordination polymer has the structural formula [Zn 1.5 (p-bix) 0.5 (1,4-bda) 1.5 ] n , where p-bix is ​​para-bis(imidazol-1-ylmethyl)benzene, 1,4-bda is terephthalate, and n takes the value ≥1, and it has a three-dimensional network structure; The zinc(II) coordination polymer belongs to the monoclinic crystal system, space group C2 / c. The zinc(II) center has a four-coordinate geometry with unit cell parameters a = 15.685(3) Å, b = 13.464(3) Å, c = 17.051(3) Å, α = 90°, β = 91.83(3)°, γ = 90°, and V = 3599.0(12) Å. 3 Z=4.

9. The use of a zinc(II) coordination polymer according to any one of claims 6-8 in a luminescent material.

10. The application according to claim 9, wherein, The luminescence temperature of the zinc(II) coordination polymer is 70K-300K.

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