Arsenic tungstate covalently modified with pyrazine-2,3-dicarboxylic acid, preparation method and application thereof as photochromic material

Abstract

The present invention belongs to the technical field of preparation of polyacid-based photochromic materials, and specifically relates to the preparation and photochromic performance research of an example of arsenic tungstate covalently modified with pyrazine-2,3-dicarboxylic acid, whose chemical formula is K3Ba6H3{Ba[AsW 11 O 37 (pzdc)2]2}[pzdc]·22H2O (pzdc = pyrazine-2,3-dicarboxylic acid). This compound exhibits reversible photochromism. After 300 s of UV irradiation, the color changes from pale yellow to blue, and the fading process in the dark takes about 10 h. The color change kinetic half-life of compound 1 (t 1 / 2 ) is 38.5 s. This work provides an important reference for the preparation of Keggin-type polyacids covalently modified with rigid polycarboxylic acid ligands. It has potential and broad prospects in various fields such as optical switches and anti-counterfeiting materials, and also opens up new prospects for the development of photochromic materials.

Description

Technical Field

[0001] The present invention belongs to the technical field of preparation of polyacid-based photochromic materials, and specifically relates to an example of arsenic tungstate covalently modified with pyrazine-2,3-dicarboxylic acid, a preparation method, and studies its photochromic properties. This invention provides important progress in the preparation of Keggin-type polyacids covalently modified with rigid polycarboxylic acid ligands, has potential and broad prospects in different fields such as optical switches and anti-counterfeiting materials, and also opens up new prospects for the development of photochromic materials. Background Art

[0002] Polyoxometalates (POMs) are polynuclear early transition metal oxide octahedrons {MO6} (M = V V , Mo VI , W VI , Nb V or Ta V ) polymers form nanosized metal-oxo clusters. These octahedra are assembled by sharing edges, corners, or faces. Among the reported classical polyanions (Keggin, Wells-Dawson, Anderson-Evans, Lindqvist, Weakley, etc.), Keggin and Dawson polyanions have been extensively studied. Due to their excellent physical and chemical properties, polyanions have shown promising applications in various fields, including sensing technology, catalysis, magnetism, proton conductivity, photochromism, and medicine. Organic carboxylic acid ligands possess carboxyl groups that can coordinate with the coordinating atoms of the polyanion backbone (such as V, Mo, and W). Therefore, by introducing organic carboxylic acid components into the polyanion system, hybrid polyanions covalently modified with organic carboxylic acids can be prepared. In recent years, the covalent modification of polyanions with carboxylic acid groups has been extensively studied. These polyanions exhibit excellent photochromic and thermochromic properties. Compared to carboxylic acid-covalently modified polyvanadates and polymolybdates, the number of reported carboxylic acid-covalently modified polytungstates is relatively small, and research is less in-depth. Previous studies have shown that arsenic tungstates possess a rich variety of structural types and high reactivity, making it easy to construct novel, giant polytungstates via solution assembly. Therefore, research on carboxylic acid-covalently modified polyoxoacid derivatives has mostly been based on organic carboxylic acid-covalently modified arsenic tungstates.

[0003] Currently, most studies on organic carboxylic acid functionalized arsenic tungstates are based on amino acid ligands or flexible carboxylic acid ligands. However, there are few reports on arsenic tungstates covalently modified with rigid carboxylic acid ligands. In 2013, Boskovic et al. reported two examples of arsenic tungstates covalently modified with 5-(methoxycarbonyl)-2-pyridine-carboxylic acid (Hmcpc) and 2,5-pyridine-dicarboxylic acid (H2pdc) [Y{AsW8O 30}2(AsO)2{WO2(pdc)}2] 13- and [{Y(H2O)3}2{As2W 19 O 68}{WO2(L)}2] 8- (L = mcpc or Hpdc) is a typical example of an arsenic tungstate polyacid derivative covalently modified with a rigid carboxylic acid ligand. 19 O 67 (H2O)] 14- It is of great significance to synthesize novel compounds modified with rigid polycarboxylic acid ligands for polyacid precursors.

[0004] Compared with amino acid ligands and other flexible carboxylic acid ligands, pyrazine-2,3-dicarboxylic acid has the following advantages, so we chose it as the organic ligand for covalent bonding with the arsenic tungstate component: (a) it contains two carboxyl groups, so it has a greater probability of bonding to the polyacid skeleton through multifunctional coordination with the tungsten center; (b) the presence of a nitrogen-containing heterocycle allows it to coordinate with the tungsten atom through the nitrogen atom on the six-membered unsaturated ring, which further increases the probability of coordination and contributes to increasing the dimensionality of the assembled network; (c) because it can coordinate with metal atoms in a variety of ways to obtain different products, it is possible to It can generate complexes with novel topological structures; (d) it has good rigidity, and the angle and spacing between the coordination teeth basically do not change during the entire coordination process. This feature can effectively improve the stability and rigidity of the target compound; (e) there may be π-π interactions between two adjacent nitrogen heterocycles of pyrazine-2,3-dicarboxylic acid, and these weak interactions can promote their construction into supramolecular systems; (f) due to the presence of carboxyl groups and nitrogen heterocycles, it can form abundant hydrogen bonds with the polyacid skeleton, which may cause the target product to have photochromic properties and proton conductivity.

[0005] In the present invention, an example of a pyrazine-2,3-dicarboxylic acid covalently modified arsenic tungstate compound K3Ba6H3{Ba[AsW 11 O 37 (pzdc)2]2}[pzdc]·22H2O (pzdc = pyrazine-2,3-dicarboxylic acid) was successfully synthesized. The dimeric structural unit of this compound consists of two polyacid anions [AsW 11 O 37 (pzdc)2] 9- Through a Ba 2+ Connected composition, and the polyacid anion [AsW 11 O 37 (pzdc)2] 9- And by a three-missing [AsW9O 33 ] 9-The building block is connected to two {WO2(pzdc)} groups and presents the shape of an official cap. It is worth noting that pyrazine-2,3-dicarboxylic acid can coordinate with tungsten atoms as a chelating ligand, playing an important role in stabilizing the structure. One point that needs to be emphasized is that the nitrogen atom and carboxyl oxygen atom of pyrazine-2,3-dicarboxylic acid are directly coordinated with the tungsten atoms on the polyacid skeleton. This is also rare in previous reports. In further analysis and research, it was found that the compound showed reversible photochromic behavior, opening up new prospects for the development of photochromic materials. Summary of the Invention

[0006] The present invention aims to overcome the defects of the prior art and provides an example of arsenic tungstate covalently modified with pyrazine-2,3-dicarboxylic acid, which is a polyacid-based photochromic material, and studies its photochromic properties.

[0007] The present invention also provides a preparation method of the pyrazine-2,3-dicarboxylic acid covalently modified arsenic tungstate and its application as a photochromic material.

[0008] To achieve the above object, the present invention adopts the following technical solutions:

[0009] An example of an arsenic tungstate covalently modified with pyrazine-2,3-dicarboxylic acid is a polyacid-based photochromic material. The molecular formula of the arsenic tungstate is K3Ba6H3{Ba[AsW 11 O 37 (pzdc)2]2}[pzdc]·22H2O, where pzdc = pyrazine-2,3-dicarboxylic acid.

[0010] The preparation method of the above-mentioned pyrazine-2,3-dicarboxylic acid covalently modified arsenic tungstate comprises the following steps:

[0011] The precursor K 14 [As2W 19 O 67 (H2O)] and pyrazine-2,3-dicarboxylic acid are added to deionized water and stirred until clear. BaCl2·2H2O is added, and the pH of the mixed solution A is measured to determine the conditions for polyacid growth (the pH range of solution A is 2.5-2.8). The solution is then stirred for 0.5-2 h, sealed with plastic wrap, and placed in a constant temperature water bath at 70-90°C, heated for 1-3 h, filtered, and allowed to evaporate slowly at room temperature. After about 1 week, light yellow square lamellar crystals are precipitated.

[0012] Specifically, 0.6-0.7 g of precursor K 14 [As2W 19 O 67(H2O)] and 0.1-0.2 g of pyrazine-2,3-dicarboxylic acid were added to 15-30 mL of deionized water. Furthermore, the amount of BaCl2·2H2O added was 0.06-0.07 g. 14 [As2W 19 O 67 (H2O)] can be prepared by referring to the methods in existing literature.

[0013] The present invention also provides the use of the above-mentioned pyrazine-2,3-dicarboxylic acid covalently modified arsenic tungstate as a photochromic material in the fields of optical switching and anti-counterfeiting.

[0014] The polyacid-based photochromic material of the present invention is an example of arsenic tungstate covalently modified with pyrazine-2,3-dicarboxylic acid. During the preparation process, pyrazine-2,3-dicarboxylic acid can act as a chelating ligand to coordinate with tungsten atoms, playing an important role in stabilizing the structure. In addition, the nitrogen atom and carboxyl oxygen atom of pyrazine-2,3-dicarboxylic acid are directly coordinated with the tungsten atoms on the polyacid skeleton. The dimerized structural unit in the prepared arsenic tungstate compound is composed of two official cap-shaped polyacid anions [AsW 11 O 37 (pzdc)2] 9- Through a Ba 2+ ions connected, and this cap-shaped polyacid anion [AsW 11 O 37 (pzdc)2] 9- And by a three-missing [AsW9O 33 ] 9- The building blocks are connected with two {WO2(pzdc)} groups. Under ultraviolet irradiation, the color of the sample changes from light yellow to blue and reaches a saturation level detectable by the human eye after 300 seconds of irradiation. In addition, the photochromic kinetic half-life of the compound (t 1 / 2 ) value is 38.5 s.

[0015] The present invention provides the preparation of the aforementioned polyacid-based materials as photochromic materials and a synthesis strategy for these materials. Research has revealed that pyrazine-2,3-dicarboxylic acid, as a chelating ligand, can coordinate with tungsten atoms, playing a significant role in stabilizing the structure. Both its nitrogen and carboxyl oxygen atoms directly coordinate with the tungsten atoms on the polyacid backbone. Furthermore, the presence of carboxyl groups and nitrogen heterocycles allows for the formation of abundant hydrogen bonds with the polyacid backbone, potentially resulting in the target product possessing photochromic properties and proton conductivity. Under ultraviolet irradiation, the sample's color shifted from pale yellow to blue, reaching a saturation level detectable by the human eye after 300 seconds. This suggests the importance of selecting an excellent rigid carboxylic acid ligand to effectively enhance the stability and rigidity of the target compound while simultaneously achieving excellent photochromic properties.

[0016] The polyacid-based photochromic material of the present invention is synthesized through a one-pot process, mixing a ligand, a metal salt, and a precursor to produce a single crystal sample of the compound. Compared with existing photochromic materials, the present invention has the following advantages:

[0017] 1) The present invention uses single crystal X-ray diffraction technology to accurately determine the molecular structure of polyacids;

[0018] 2) The present invention adopts a one-pot method and solution evaporation self-assembly strategy, and the reaction is carried out at room temperature and pressure, making the experimental operation safe and environmentally friendly;

[0019] 3) This invention uses pyrazine-2,3-dicarboxylic acid as a ligand to explore the role of the rigid carboxylic acid ligand structure in the structure of arsenic tungstate compounds;

[0020] 4) This invention utilizes pyrazine-2,3-dicarboxylic acid to covalently modify arsenic tungstate compounds, providing important progress in the preparation of Keggin-type polyacids covalently modified with rigid polycarboxylic acid ligands.

[0021] 5) Compared with existing photochromic materials, the arsenic tungstate compound of the present invention has a clearer mechanistic relationship between its structure and photochromism, and exhibits attractive reversible photochromic behavior. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 In the figure, (a) molecular structure of compound 1, (b) coordination environment of tungsten in compound 1, (c) coordination environment of W11 without polyacid fragment, (d) coordination configuration of W11, (e) coordination configuration of W10;

[0023] Figure 2 In the figure, (a) the plane where the four W atoms are located; (b) the distance between the four W atoms;

[0024] Figure 3In the figure, (a) the plane where the four N atoms from four pyrazine-2,3-dicarboxylic acids coordinate with the four W centers in the dimer structure unit; (b) the distance between the four N atoms;

[0025] Figure 4 In the figure, (a) the plane where the four N atoms on the four pyrazine-2,3-dicarboxylic acid ligands that do not participate in coordination are located; (b) the distance between the four N atoms;

[0026] Figure 5 is the angle between the three planes;

[0027] Figure 6 In the figure, (a) the three-dimensional stacking of compound 1 on the bc plane; (b) the three-dimensional stacking structure of compound 1 arranged in a regular –ABAB- manner on the bc plane; (c) the simplified stacking structure on the bc plane; (d) the three-dimensional stacking on the ab plane; (e) the three-dimensional stacking structure of compound 1 arranged in a regular –ABAB- manner on the ab plane; (f) the simplified stacking structure of compound 1 along the c-axis direction;

[0028] Figure 7 In the figure, (a) the three-dimensional stacking of compound 1 along the b-axis; (b) the three-dimensional stacking structure of compound 1 arranged in a regular –ABAB- manner on the ac plane; (c) the simplified stacking structure of compound 1 along the b-axis;

[0029] Figure 8 (a) Dimeric structural unit of compound 1; (b) Simplified model of the dimeric structural unit of compound 1; (color marking: As: pink; W: green; N: blue; O: red; C: black; Ba: gray; {WO6}: green;)

[0030] Figure 9 For the precursor {As2W 19}, infrared spectra of pyrazine-2,3-dicarboxylic acid ligand and compound 1;

[0031] Figure 10 In the figure, (a) X-ray powder diffraction pattern of compound 1 and (b) thermogravimetric curve of compound 1;

[0032] Figure 11 The color of compound 1 changes after irradiation with UV light for 20 s, 60 s, 150 s, and 300 s, and gradually returns to its original color after being placed in the dark for 3 h, 6 h, and 10 h.

[0033] Figure 12 is the infrared spectrum of compound 1 under light irradiation;

[0034] Figure 13Figure 3. Relationship between the reflectance value R(t) and t of compound 1 measured at 700 nm after irradiation with UV light for 20 s, 40 s, 60 s, 90 s, 120 s, 150 s, 180 s, 240 s, and 300 s. DETAILED DESCRIPTION

[0035] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0036] The present invention is described in detail below with reference to specific embodiments.

[0037] In the following examples, unless otherwise specified, all raw materials used are commercially available products that can be purchased directly or prepared according to conventional methods in the art. Room temperature refers to 25±5°C.

[0038] Example 1:

[0039] Polyacid-based photochromic material K3Ba6H3{Ba[AsW 11 O 37 A method for preparing (pzdc)2]2}[pzdc]·22H2O (Compound 1) comprises the following steps:

[0040] 1) The precursor K 14 [As2W 19 O 67 (H2O)] (0.660 g, 0.125 mmol) and pyrazine-2,3-dicarboxylic acid (0.182 g, 0.541 mmol) were dissolved in deionized water (20 mL) and stirred until clear. BaCl2·2H2O (0.061 g, 0.25 mmol) was then added. The resulting solution was stirred for approximately 1 h, and the pH was measured with a pH meter, indicating a range of 2.5-2.8.

[0041] 2) After sealing with plastic wrap, heat the solution in a constant-temperature water bath at 80°C for 2 hours, then cool to room temperature and filter. The filtrate is allowed to slowly evaporate at room temperature. After approximately one week, pale yellow square flaky crystals of compound 1 are obtained. These crystals are filtered, air-dried, and collected. This is compound 1.

[0042] Example 2:

[0043] Single crystal samples of complexes 1 and 2 were collected and subjected to a series of photochromic tests. Compound 1 exhibits reversible photochromism. After 300 seconds of UV irradiation, the color of sample 1 changes from pale yellow to blue, and the fading process in the dark takes about 10 hours. The kinetic half-life of the color change of compound 1 (t 1 / 2 ) is 38.5 s. This indicates that the selection of excellent rigid carboxylic acid ligands can effectively improve the stability and rigidity of the target compound while obtaining good photochromic properties.

[0044] The present invention uses single crystal X-ray diffraction technology to determine the single crystal structure of compound 1 prepared in Example 1 above.

[0045] The unit cell parameters of complex 1 are as follows: monoclinic system, P 21 / c space group, unit cell parameters a = 18.264 (10) Å, b = 34.751 (18) Å, c = 25.526 (14) Å, α = 90.00, β = 107.705(2), γ= 90.00, V = 15433 (14) Å 3 , Z = 4, R 1= 0.1748, wxya 2= ​​0.3109.

[0046] Figure 1 The molecular structure of compound 1 is given. Figure 1 As can be seen from (a), the molecular structure of compound 1 consists of an organic-inorganic hybrid polyacid anion {Ba[AsW 11 O 37 (pzdc)2]2} 16- (pzdc = pyrazine-2,3-dicarboxylic acid), 1 free [pzdc] 2- Anion, 3 K + Counter cation, 6 Ba 2+ Counter cation, 3 H + protons and 22 crystal water molecules. Among them, the dimer structure unit of compound 1 {Ba[AsW 11 O 37 (pzdc)2]2} 16- Composed of two official cap-shaped polyacid anions [AsW 11 O 37 (pzdc)2] 9- By Ba 2+ The bonding effect connects the composition, and this cap-shaped polyacid anion [AsW 11 O 37 (pzdc)2]9- And by a three-missing [AsW9O 33 ] 9- The building block is connected with two {WO2(pzdc)} groups ( Figure 1 b). It is particularly interesting that the polyacid anions are not induced by RE 3+ Instead of coordinating with pyrazine-2,3-dicarboxylic acid, it directly coordinates with W11 and W10 through the N3, O40 (from the carboxyl group) and N2, O23 (from the carboxyl group) of two pyrazine-2,3-dicarboxylic acids. The W–O–C–C–N five-membered chelate ring ( Figure 1 c), and the five-membered chelate ring formed makes the pyrazine-2,3-dicarboxylic acid ligand play an important role in stabilizing the skeleton structure of compound 1. In addition, both W11 and W10 adopt a hexacoordinated octahedral geometric structure. Both W11 and W10 centers are coordinated with a pyrazine-2,3-dicarboxylic acid ligand, which adopts a bidentate coordination mode to W through O40, N3 and O23, N2 atoms respectively. 6+ Center to connect ( Figure 1 d in the figure and e in the figure). At the same time, the coordination structure centered on the W10 ion also presents an octahedron. In addition, a free pyrazine-2,3-dicarboxylic acid is connected to the Ba 2+ ions and [AsW9O 33 ] 9- The building blocks are connected and coordinated to W10.

[0047] The dimeric structural unit ( Figure 1 In (a), the four W atoms (W11, W10, W18, and W2) are chelated and coordinated by four pyrazine-2,3-dicarboxylic acid ligands to form an octahedral configuration. Further analysis revealed that the four W atoms are on the same plane (plane 1), and the W centers form a quadrilateral ( Figure 2 (a). The side lengths of the quadrilateral, i.e. the distances between W···W, are 5.009(3)Å, 7.260(4)Å, 5.145(3)Å and 7.198(4)Å respectively ( Figure 2 In addition, the four N atoms (N9, N1, N3, and N2) coordinated with the W atom in the four pyrazine-2,3-dicarboxylic acid ligands are also on the same plane (plane 2), and the distances between the four N atoms in the quadrilateral are 8.561 (41) Å, 5.164 (52) Å, 8.697 (31) Å, and 4.932 (40) Å, respectively. Figure 3More interestingly, the four N atoms (N4, N6, N5, and N10) in the four pyrazine-2,3-dicarboxylic acid ligands that are not coupled to the W atom are also in the same plane (plane 3), and the distances between the four N···N atoms in the quadrilateral are 13.121(40)Å, 3.563(51)Å, 13.390(36)Å, and 3.200(47)Å, respectively. Figure 4 ). It is worth noting that the dihedral angle between plane 1 composed of four W atoms and plane 2 composed of four N atoms is 51.80(57)°, while the dihedral angle between plane 1 and plane 3 composed of another four N atoms is 14.66(36)°. At the same time, the angle between plane 1 and plane 2 composed of eight N atoms is 37.16(64)° ( Figure 5 ).

[0048] Finally, a three-dimensional network structure is obtained by stacking the dimer structure unit as the minimum repeating unit. The dimer units are arranged in a cross-shaped manner instead of overlapping, which greatly reduces the energy and makes the structure more stable. The dimer structure unit of compound 1 is arranged in a three-dimensional stack along the a, b and c axes as shown in FIG. Figure 6 a, d and Figure 7 As shown in a. The dimeric structural unit of compound 1 presents a similar –ABAB- arrangement on the bc, ac, and ab planes ( Figure 6 b, e and Figure 7 Taking the two As atoms in the dimer unit as reference points, a simplified short stick view can be obtained ( Figure 8 ). Figure 6 c, f and Figure 7 Figure c shows a simplified three-dimensional stacking diagram of the dimerization unit of compound 1. It is not difficult to find that in the A and B layers, the dimerization unit of compound 1 is regularly arranged in a -AAA- manner. However, at the same time, we can observe that compared with the ac and ab planes, the adjacent layers of the bc plane are arranged in a staggered manner. This arrangement is conducive to reducing steric hindrance to a certain extent. In addition, it is interesting that the dimerization unit of compound 1 along the a-axis is windmill-shaped in the simplified stacking diagram ( Figure 6 In (c), the dimerization unit of compound 1 along the b axis appears to be a ladder in the simplified stacking structure ( Figure 7 In the simplified stacking diagram, the dimer unit of compound 1 along the c-axis shows a bow-tie shape ( Figure 6 f).

[0049] like Figure 9 Infrared spectra show that the wavelength range is 700–1000 cm -1 The characteristic vibration of the polyacid anion skeleton exists in the low wavenumber region of W–O t The stretching vibration absorption peak is at 943 cm-1 , 717 cm -1 The peak near 861 cm corresponds to the stretching vibration of W–O(–As), while the peak at 861 cm -1 and 790 cm -1 There are strong vibration absorption bands near 1625 cm, which correspond to the stretching vibrations of W–O(–W) and As–O bonds. -1 The strong absorption peak is the antisymmetric stretching vibration of the carboxyl group ν as (COO – ), at 1395 cm -1 The strong absorption peak at is from the symmetric stretching vibration of carboxyl group ν s (COO – ) caused by 1451 cm -1 The strong absorption peak at 3429 cm corresponds to the stretching vibration of the C–N bond in pyrazine-2,3-dicarboxylic acid. Obviously, due to the coordination effect between the pyrazine-2,3-dicarboxylic acid ligand and the W atom, the stretching vibration of the carboxyl group has a certain degree of red shift compared with the infrared absorption peak of the free pyrazine-2,3-dicarboxylic acid ligand. In addition, the infrared spectrum of compound 1 shows a wavelength of 3429 cm -1 There is a strong and broad absorption peak in the high frequency region, which is caused by the stretching vibration ν(O–H) of water molecules. The bending vibration δ(O–H) of water molecules causes the absorption peak at 1623 cm -1 There is a peak with higher intensity near . Both vibration modes are related to lattice water molecules. Figure 10 As shown in Figure a, the positions of the powder diffraction peaks of compound 1 are consistent with those of the simulated powder diffraction peaks, which indicates that the sample is relatively pure. Figure 10 As shown in b, the first weight loss in the range of 30-180 °C is about 4.8% (theoretical value is 5.3%), corresponding to the loss of 22 lattice water molecules in the molecule. The second weight loss in the range of 180-465 °C is attributed to the collapse of the arsenic tungstate framework.

[0050] Example 3:

[0051] Photochromic experiment: Under ultraviolet irradiation, the color of sample 1 gradually changed from light yellow to blue with increasing irradiation time, and reached a saturation level detectable by the human eye after 300 seconds of irradiation. Interestingly, under ambient conditions, after about 10 hours of dark treatment, the sample returned to its original light yellow color ( Figure 11 At the same time, the polyoxometalate skeleton did not change during the entire process, and the skeleton structure of compound 1 maintained good stability. This conclusion can be confirmed by comparing the infrared spectra of the sample before irradiation, during irradiation, and after restoration to its original color, and observing the intensity and position of the characteristic vibration absorption band ( Figure 12). These phenomena indicate that compound 1 has reversible photochromic behavior.

[0052] Color change mechanism of compound 1: Under irradiation, electron transfer (LMCT) from the low-energy 2p orbital of O atom to the high-energy d orbital of W atom occurs in the induced polyacid component. 6+ Restored to W 5+ The color gradually changes to a heteropoly blue. Hydrogen bonds form between the pyrazine-2,3-dicarboxylic acid ligand and the oxygen atom at the photoreduction site of the coordinated octahedron centered on the W atom. When the nitrogen atom in the pyrazine-2,3-dicarboxylic acid ligand interacts with the hole left by the charge transfer, a charge transfer compound is generated, increasing the stability of the color-changing product.

[0053] Fitting of half-life: The photochromic kinetics of compound 1 were further evaluated by analyzing the change of the reflectance value R(t) of compound 1 in the range of 200–800 nm with irradiation time (t). According to the theory proposed by Dessapt's group (Inorganic Chemistry, 2009, 48(2): 574-580, Journal of Materials Chemistry C, 2014, 2(24): 4748-4758), at the maximum absorption wavelength (λ max = 700 nm)Reflectance value (R λmax ) with irradiation time (t) can be fitted by the following function:

[0054]

[0055] from Figure 13 It can be seen that the R λmax The value first drops sharply with the extension of irradiation time, and then gradually stabilizes. This is because W 6+ The rate of photoinduced color change is expressed as the half-life t 1 / 2 To characterize, t 1 / 2 Approximately 1 / b. t is obtained by curve fitting 1 / 2 = 38.5 s, which is consistent with the narrow optical band gap of compound 1.

[0056] The present invention utilizes the rigid carboxylic acid ligand pyrazine-2,3-dicarboxylic acid, K 14 [As2W 19 O 67 (H2O)] precursor and BaCl2·2H2O salt were used as raw materials, and pyrazine-2,3-dicarboxylic acid covalently modified arsenic tungstate K3Ba6H3{Ba[AsW 11 O37 (pzdc)2]2}[pzdc]·22H2O (pzdc = pyrazine-2,3-dicarboxylic acid). Structural analysis shows that the dimeric structural unit consists of two official cap-shaped polyacid anions [AsW 11 O 37 (pzdc)2] 9- Through a Ba 2+ ions connected, and this cap-shaped polyacid anion [AsW 11 O 37 (pzdc)2] 9- And by a three-missing [AsW9O 33 ] 9- The building blocks are connected by two {WO2(pzdc)} groups. We conducted a relatively detailed physical and chemical characterization of compound 1, which showed that compound 1 exhibited efficient and reversible photochromic properties. Under ultraviolet irradiation, the color of sample 1 changed from light yellow to blue and reached a saturation level detectable by the human eye after 300 seconds of irradiation. In addition, the photochromic kinetic half-life of compound 1 (t 1 / 2 ) value is 38.5 s. This work provides important progress in the preparation of Keggin-type polyacids covalently modified with rigid polycarboxylic acid ligands, opening up new prospects for the development of photochromic materials.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate and not to limit the technical solutions of the present invention. Any equivalent substitutions of the present invention and any modifications or partial substitutions that do not depart from the spirit and scope of the present invention should be included in the scope of protection of the claims of the present invention.

Claims

1. An example of an arsenic tungstate covalently modified with pyrazine-2,3-dicarboxylic acid, characterized in that: The molecular formula of the arsenic tungstate is K3Ba6H3{Ba[AsW 11 O 37 (pzdc)2]2}[pzdc]·22H2O, where pzdc = pyrazine-2,3-dicarboxylic acid; The unit cell parameters of the arsenic tungstate are as follows: monoclinic system, P 21 / c space group, unit cell parameters a = 18.264 (10) Å, b = 34.751 (18) Å, c = 25.526 (14) Å, α = 90.00, β = 107.705(2), γ= 90.00, V = 15433 (14) Å 3 , Z = 4, R 1 = 0.1748, wxya 2 = 0.3109.

2. The method for preparing the pyrazine-2,3-dicarboxylic acid covalently modified arsenic tungstate according to claim 1, characterized in that: The preparation steps are as follows: The precursor K 14 [As2W 19 O 67 (H2O)] and pyrazine-2,3-dicarboxylic acid are added to deionized water and stirred until clear. BaCl2·2H2O is added and stirred for 0.5-2 h. After sealing, the mixture is heated in a 70-90°C water bath for 1-3 h, filtered, and allowed to evaporate at room temperature to precipitate light yellow square flaky crystals.

3. The method for preparing arsenic tungstate covalently modified with pyrazine-2,3-dicarboxylic acid according to claim 2, characterized in that: 0.6-0.7 g of precursor K 14 [As2W 19 O 67 (H2O)] and 0.1-0.2 g of pyrazine-2,3-dicarboxylic acid were added to 15-30 mL of deionized water.

4. The method for preparing arsenic tungstate covalently modified with pyrazine-2,3-dicarboxylic acid according to claim 3, characterized in that: The amount of BaCl2·2H2O added is 0.06-0.07 g.

5. Use of the arsenic tungstate covalently modified with pyrazine-2,3-dicarboxylic acid according to claim 1 as a photochromic material.

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