Preparation and application of organic ligands for metals based on metal chain scaffolds and heavy atom effects

Abstract

Preparation and application of organic ligands based on metal chain scaffolds and heavy atom effects to enhance metals, including their properties and applications. A three-dimensional Cd-MOF was synthesized by ingeniously combining organic phosphorescent ligands with metal ions. At room temperature, it exhibits enhanced blue photoluminescence performance, a phosphorescence lifetime of up to 124 ms, and an afterglow duration of up to three seconds. Furthermore, it displays multicolor luminescence characteristics ranging from blue to green to yellow, varying with excitation, time, and temperature. Based on these properties, Cd-MOF shows promising application prospects in areas such as multi-information encryption, display, and anti-counterfeiting, providing a possibility for developing novel MOF platforms for anti-counterfeiting and encryption.

Description

Technical Field

[0001] This invention relates to the field of metal-organic framework long-lasting luminescent materials, specifically to an organic ligand H2L and Cd 2+ Preparation, properties and applications of self-assembly compounds. Background Technology

[0002] Long persistent luminescence (LPL) refers to the ability to continue emitting light for a considerable period after the excitation source is removed. Over the past few decades, LPL materials have attracted considerable attention due to their potential applications in numerous commercial fields, including emergency lighting, interior decoration, traffic signs, and displays, as well as their potential uses in optical recording devices, chemical sensors, bioimaging, and security systems. Currently, commercially available LPL materials are mainly rare-metal inorganic compounds; however, these materials suffer from problems such as demanding preparation conditions, high costs, and severe environmental pollution. In contrast, organic LPL materials have been extensively studied in recent years, offering significant advantages such as ease of design, low cost, good biocompatibility, and low toxicity. However, it is undeniable that the LPL performance of organic materials is currently inferior to that of inorganic materials, and there is still a gap before widespread practical application. Therefore, developing stable, cost-effective, and energy-efficient novel LPL materials has become an urgent priority.

[0003] Organic phosphors, as guest molecules, are doped into various host molecules, such as metal-organic frameworks (MOFs), macrocyclic cavities, steroidal compounds, and polymers, through physical or chemical means. These dopants can induce room-temperature phosphorescence (RTP) through carefully designed host-guest interactions. Among these, luminescent metal-organic frameworks (LMOFs), due to the combination of inorganic units and organic linkers, have recently attracted widespread attention in basic research and industry, showing broad application prospects in displays, drug delivery, and energy storage and conversion. On the one hand, coordination interactions can typically enhance the rigidity of molecular conformations, restricting molecular motion or vibration, thereby further reducing non-radiative losses of triplet excitons and promoting phosphorescence emission. On the other hand, organic molecules containing heteroatoms such as N and O are ideal candidates for phosphorescent ligands, while those with d... 10 Metal ions with specific configurations can increase spin-orbit coupling in the system through heavy atom effects, thereby improving the intersystem crossing efficiency from singlet to triplet states. Nevertheless, reports on MOFs with various easily tunable LPL colors remain very limited. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing Cd-MOF and its application. The coordination compound is obtained by reacting ligand H2L with CdCl2·2.5H2O. Due to the scaffolding of the metal chains and the effect of heavy atoms in the backbone, and with ligands under environmental conditions, it exhibits enhanced blue photoluminescence and a phosphorescence lifetime of 124 ms. Notably, Cd-MOF exhibits interesting excitation, time- and temperature-dependent LPL, lasting 3 s at room temperature, and capable of changing from blue to green and then to yellow at lower temperatures. To achieve the above objectives, the technical solution adopted by this invention is as follows:

[0005] A coordination compound Cd-MOF, characterized in that its molecular formula is C7H5CdNO5, it belongs to the monoclinic crystal system, and its space group is P21 / n.

[0006] The aforementioned coordination compound Cd-MOF was prepared by reacting ligand H2L with CdCl2·2.5H2O. Specifically, the preparation method of the coordination compound Cd-MOF is as follows:

[0007] S1. Mix ligand H2L and CdCl2·2.5H2O in a molar ratio of 1:1 to 2 and place them in a 25ml stainless steel container lined with polytetrafluoroethylene. Add DMF, water and ethanol in a volume ratio of 1:1:1 to 1:2:2 to the 25ml stainless steel container lined with polytetrafluoroethylene and gently shake to mix evenly. The ratio of DMF to ligand H2L is 1ml:0.01 to 0.02mmol.

[0008] S2. Place the sealed 25ml stainless steel container lined with polytetrafluoroethylene into an oven, set the program, heat for 1 to 2 hours to raise the temperature from room temperature to 90 to 110°C, maintain this temperature for 40 to 60 hours, and then cool down for 8 to 13 hours.

[0009] S3. Colorless bulk crystals were collected by filtration.

[0010] Furthermore, the molar ratio of ligand H2L and CdCl2·2.5H2O in step S1 is 1:1.

[0011] Furthermore, in step S1, the volume ratio of water to ethanol in the DMF is 1 ml: 1 ml: 1 ml.

[0012] Furthermore, in step S1, the ratio of DMF to ligand H2L is 1 ml: 0.1 mmol.

[0013] Furthermore, in step S2, the oven program is set to heat for 2 hours to raise the temperature from room temperature to 90°C, maintain this temperature for 48 hours, and then cool down to room temperature for 13 hours.

[0014] Specifically, as an alternative embodiment, the preparation method of the coordination compound Cd-MOF includes the following steps: adding a mixture of ligand H2L (0.1 mmol, 16.7 mg), CdCl2·2.5H2O (0.1 mmol, 22.8 mg), DMF (1 ml), water (1 ml), and ethanol (1 ml) into a 25 ml stainless steel container lined with polytetrafluoroethylene (PTFE). The sealed 25 ml PTFE-lined stainless steel container is placed in an oven, and the oven program is set to heat for 2 hours to raise the temperature from room temperature to 90°C. This temperature is maintained for 48 hours, followed by 13 hours of cooling to room temperature. Finally, the mixture is filtered to obtain colorless blocky crystals.

[0015] The present invention has the following beneficial effects:

[0016] (1) The metal chain support and heavy atom effect provided by this invention endow Cd-MOF with tunable LPL

[0017] (2) Coordination compound Cd-MOF can be used for anti-counterfeiting. The materials made from it can accurately and conveniently encrypt and decrypt information by switching ultraviolet irradiation. Attached Figure Description

[0018] Figure 1 a) Infrared spectrum of Cd-MOF, b) X-ray diffraction of simulated and synthesized powders, c) Thermogravimetric analysis.

[0019] Figure 2 Single crystals of H2L. a) Asymmetric unit. bc) One-dimensional chain structure formed by intermolecular hydrogen bonds. d, e) Assembling these 1D chains to form a two-dimensional layer structure. f) Three-dimensional framework structure.

[0020] Figure 3 Structural features of Cd-MOF: a) Asymmetric units. b) One-dimensional metal chains. c) H2L coordination mode. d) Three-dimensional structure assembled from one-dimensional metal chains and ligands. e) Simplified three-dimensional network.

[0021] Figure 4 A simplified 3D network.

[0022] Figure 5 Solid-state UV-Vis spectra of H2L and Cd-MOF.

[0023] Figure 6 Excitation-dependent instantaneous and delayed emission spectra of H2L at room temperature and their corresponding CIE coordinates.

[0024] Figure 7 Excitation-dependent instantaneous and delayed emission spectra of Cd-MOF at room temperature and the corresponding C

[0025] IE coordinates.

[0026] Figure 8 Fluorescence and phosphorescence decay curves of H2L and Cd-MOF at room temperature.

[0027] Figure 9 a, b) Instantaneous and delayed (1 ms) spectra of H2L and Cd-MOF at room temperature. c) Excitation-dependent delayed (1 ms) spectrum of Cd-MOF at room temperature.

[0028] Figure 10 Excitation spectra of H2L and Cd-MOF at room temperature.

[0029] Figure 11 The photoluminescence quantum efficiency and phosphorescence quantum efficiency of H2L and Cd-MOF at room temperature.

[0030] Figure 12 Instantaneous and delayed emission spectra of Cd-MOF in air and vacuum at room temperature.

[0031] Figure 13 Decay curves of Cd-MOF at 515, 540, and 570 nm in air and vacuum at room temperature.

[0032] Figure 14 Excitation-dependent gating spectra of Cd-MOF at 77 K at room temperature, temperature-dependent emission spectra of Cd-MOF in the range of 300 to 77 K, and corresponding CIE coordinates.

[0033] Figure 15 Temperature-dependent decay curves of Cd-MOF at wavelengths of 515, 540, and 570 nm in the range of 300 to 77 K.

[0034] Figure 16 Temperature-dependent delayed spectra of Cd-MOF under excitation at 311, 385, 405, and 450 nm in the range of 300 to 77 K.

[0035] Figure 17 Excitation-related instantaneous and delayed emission spectra of Cd-MOF at room temperature and their corresponding CIE coordinates.

[0036] Figure 18LPL of Cd-MOF. a, b) Excitation-related LPL spectra at room temperature and 77 K after removing 275, 311, 365, 385, 405, and 450 nm flashlight (8 ms) respectively. c) Time-resolved LPL spectrum after removing 365 nm flashlight at 100 K. d) Temperature-related LPL spectrum after removing 311 nm flashlight. e, f) LPL images after removing 450 and 311 nm flashlight from 300-77 K respectively.

[0037] Figure 19 The photo shows an anti-counterfeiting application based on Cd-MOF.

[0038] Figure 20 The encryption pattern for the number "8" was manually created using CdCl2, H2L, and Cd-MOF. Positions ①, ②, and ⑥ are engraved with Cd-MOF, while positions ③, ④, and ⑦ are engraved with H2L. Additionally, position ⑤ is marked with CdCl2. Detailed Implementation

[0039] The present invention will be further described below with reference to the accompanying drawings and specific examples, but the examples do not limit the present invention in any way. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the art.

[0040] Unless otherwise specified, all reagents and materials used in the following examples are commercially available.

[0041] Example 1: Preparation of the Cd-MOF complex

[0042] A mixture of ligand H₂L (0.1 mmol, 16.7 mg), CdCl₂·2.5H₂O (0.1 mmol, 22.8 mg), DMF (1 ml), water (1 ml), and ethanol (1 ml) was added to a 25 ml PTFE-lined stainless steel container. The sealed 25 ml PTFE-lined stainless steel container was placed in an oven, and the oven program was set to heat for 2 hours to raise the temperature from room temperature to 90°C. This temperature was maintained for 48 hours, followed by 13 hours of cooling to room temperature. Finally, the mixture was filtered to obtain colorless blocky crystals.

[0043] Example 2 Crystallographic Data

[0044] X-ray diffraction intensity data were obtained at room temperature using a Bruker Apex IICCD diffractometer with graphite monochromatic Mo radiation. The structures were collected. The direct method was used to solve for the structures, which were then refined using the SHELXL-2014 package via full matrix least squares. All hydrogen atoms were located at their calculated positions and anisotropically refined. Detailed crystallographic data are shown in Table 1.

[0045] Table 1 Crystallographic data

[0046]

[0047]

[0048] Example 3: Structural Analysis of Cd-MOF Crystals

[0049] H₂L crystals were obtained by supersaturating powdered ligands and crystallizing in ethanol solvent. Cd-MOF was synthesized by hydrothermal reaction of H₂L and CdCl₂ in a mixture of DMF, EtOH, and H₂O at 90 °C for 2 days. Infrared spectra are shown below. Figure 1 As shown in figure a, the powder X-ray diffraction peaks of Cd-MOF perfectly match the simulated data obtained from single-crystal data, proving its high phase purity. Figure 1 b). Thermal stability was studied using thermogravimetric analysis. Figure 1 As shown in c, structural integrity can be maintained up to approximately 350°C. Specifically, when the temperature rises from room temperature to 190°C, coordinated water molecules escape. However, when the temperature exceeds 350°C, H₂L liquefies, subsequently inducing framework collapse. Single-crystal X-ray analysis indicates that H₂L crystallizes in an orthorhombic crystal system, belonging to space group P212121. The asymmetric units of H₂L contain ligands with proton transfer, where carboxyl protons transfer to pyridine nitrogen atoms, leading to the formation of intramolecular salts. Figure 2 a). For example Figure 2 As shown in b and 2c, H₂L forms 1D chains through NH hydrogen bonds or OH hydrogen bonds along different directions. These chains further extend through CH hydrogen bonds, forming two-dimensional frameworks (…). Figure 2 d and 2e). Ultimately, H2L assembles into a three-dimensional hydrogen-bonded organic framework driven by these non-covalent interactions (d and 2e). Figure 2 f). For Cd-MOF, it exhibits a 3D network structure and crystallizes in a monoclinic system with the P21 / n space group. The asymmetric unit consists of an L2 - Ligand, a coordinated water molecule, and a Cd 2+ composition( Figure 3 a). Specifically, Cd1 is hexacoordinated, influenced by four L2 groups. - The ligand consists of four oxygen atoms (O1, O2, O3, and O4), one water molecule (O1w), and one L2 molecule. - The nitrogen atoms (N1) surrounding the ligands exhibit a distorted octahedral geometry. Notably, these octahedrons are linked by μ2-COO carboxyl groups to form a one-dimensional metal chain. Figure 3 b). Furthermore, the ligand employs a five-toothed (μ5-η) design. 1 η 1 η 1η 1 η 1 Coordination modes connect these metal chains to create a three-dimensional framework structure. Figure 3 (c and 3d). Simplified ligands are represented in a five-connected manner, with metal ions acting as connecting nodes, presenting an understandable and simplified network. Figure 3 e and 4).

[0050] Example 4: Photophysical properties of ligands and coordination compounds

[0051] The solid-state UV-Vis spectra of H₂L and Cd-MOF show similar absorption shapes in the 200-400 nm range, with Cd-MOF exhibiting a broader absorption band. Figure 5 The absorption in the 200-300 nm range can be attributed to π-π* transitions, while the absorption in the 300-400 nm range is attributed to n-π* transitions induced by heteroatoms in H₂L. To better understand the luminescence properties at room temperature, photoluminescence spectra were investigated. The instantaneous and delayed spectra of H₂L and Cd-MOF under various excitations at room temperature are shown below. Figure 6 and 7 As shown, similar emission was revealed. Transient spectra showed that, regardless of the excitation wavelength, H₂L and Cd-MOF exhibited single emission peaks at 500 and 505 nm, respectively. The emission lifetime of H₂L was 5.75 ns, and that of Cd-MOF was 5.47 ns, thus highlighting the fluorescence properties. Figure 8 However, H₂L and Cd-MOF exhibit excitation-dependent delayed emission. Under 360 nm excitation, the emission of H₂L and Cd-MOF peaks at 535 and 515 nm, respectively. Figure 9 (a and 9b). As the excitation wavelength increases from 310 nm to 430 nm, the delayed emission spectra of H2L and Cd-MOF gradually redshift ( Figure 6 and 9 c). H2L shifts from 520 nm to 545 nm, while Cd-MOF changes from an initial 515 nm to 540 nm, and then to 570 nm. When excited at 440 and 450 nm, their delayed spectra both show bimodalities near 500 and 570 nm. Notably, H2L and Cd-MOF exhibit multiple excited triplet states near 500, 540, and 570 nm, respectively, a combination of single-molecule and aggregated states induced by non-covalent interactions. The phosphorescent properties exhibited by delayed emission are further supported by time-resolved spectroscopy. The fitted phosphorescent lifetimes of H2L at 500 nm, 540 nm, and 570 nm are 42.53 ms, 48.19 ms, and 40.40 ms, respectively. Similarly, the lifetimes of Cd-MOF at 515 nm, 540 nm, and 570 nm are 124.01 ms, 122.14 ms, and 106.5 ms, respectively. Figure 8After assembly into MOFs, the phosphorescence lifetime increased by 3 times, indicating a strong stabilizing effect of the framework. The fluorescence and phosphorescence excitation spectra of H₂L and Cd-MOF were recorded, showing consistent matching excitation wavelengths at 360-370 nm, revealing that the photoemission of H₂L and Cd-MOF originates from the same source. Figure 10 Under 360 nm excitation, the photoluminescence quantum yield and phosphorescence quantum yield of H2L were 4.20% and 0.79%, respectively, while those of Cd-MOF were 4.95% and 1.04%, respectively. Figure 11 The phosphorescence lifetime and quantum yield of Cd-MOFs were both improved, reflecting the role of coordination in enhancing phosphorescence performance.

[0052] Considering the sensitivity of triplet excitons to their surroundings, which could potentially affect phosphorescence properties, the photoluminescence of Cd-MOFs was investigated under vacuum and ambient conditions, such as... Figure 12 As shown, when in a vacuum environment, the emission intensity of both instantaneous and delayed spectra increases while maintaining the spectral shape. More importantly, their phosphorescence lifetimes are prolonged, indicating that molecular oxygen influences and disrupts triplet excitons (…). Figure 13 As is well known, temperature affects phosphorescence. Therefore, temperature-dependent photoluminescence measurements were performed between 300 and 77 K. With decreasing temperature, the instantaneous emission intensity under 370 nm excitation increased, and the emission peak showed a slight redshift, as shown... Figure 14 As shown. Decay curves monitored at emission points of 515, 540, and 570 nm show similar changes. Figure 15 As shown, the phosphorescence lifetime gradually increases with decreasing temperature. This phenomenon is attributed to the suppression of nonradiative transitions of triplet excitons at low temperatures. Since Cd-MOFs exhibit wavelength-dependent phosphorescence emission, phosphorescence spectra can be similarly collected at low temperatures using representative excitation wavelengths. Figure 16 Under excitation at wavelengths of 311 and 405 nm, phosphorescent emission is predominantly located at 515 and 540 nm, respectively, with a slight blue shift near these wavelengths at low temperatures due to energy level splitting. Under excitation at 450 nm, the multi-peak emission gradually red-shifts and merges into a single peak at 580 nm. At 77 K, a clear red shift from blue to yellow in phosphorescent emission is observed as the excitation wavelength increases from 300 nm to 450 nm. The CIE coordinates corresponding to the instantaneous and delayed emission spectra at various excitation wavelengths at 77 K are shown below. Figure 17 As shown.

[0053] Due to its extremely long phosphorescence lifetime, Cd-MOF exhibits bright LPL emission after the excitation source is removed. Based on the presence of multiple phosphorescence emission, five representative wavelengths—275, 311, 365, 385, and 405 nm—were selected as excitation sources. With increasing excitation wavelength, LPL emission shifted from 520 nm to 542 nm at room temperature, and then to 575 nm. Figure 18 a). At 77K, as the excitation wavelength increases, LPL emission redshifts from 450nm to 600nm ( Figure 18 b). Interestingly, after the 365nm radiation source was turned off, the time evolution of LPL changed from green to cyan, as shown in the photograph and spectrum. Figure 18 c) This is caused by inconsistent decay rates of multiple triplet states and phosphorescent lifetimes. Intriguingly, after removing the 311 nm excitation from 300 to 77 K, LPL emission changed from 540 nm to 470 nm. Figure 18 d). As the temperature decreases, the LPL color changes from green to blue, and the corresponding LPL image is as follows. Figure 18 As shown in f. At low temperatures, removing the 450nm excitation wavelength can gradually capture the yellow LPL ( Figure 18 e). Therefore, due to the presence of multiple triplet states, Cd-MOF exhibits an attractive multicolor LPL.

[0054] Example 5: Application of the coordination compound Cd-MOF

[0055] Multi-color anti-counterfeiting work examples

[0056] The excellent and tunable LPL properties of Cd-MOF enable its application in fields such as displays, anti-counterfeiting, and advanced encryption technologies. For example... Figure 19 As shown, the snowflake material is prepared from Cd-MOF used in display technology. It appears white in sunlight and emits blue fluorescence when exposed to 365nm UV light. After removing the UV light source, the snowflake exhibits a green light luminescence (LPL) lasting for 1 second. Considering that CdCl2, H2L, and Cd-MOF have different luminescence lifetimes and exhibit different LPL durations, time-resolved information security is further developed. Furthermore, an encrypted pattern of the number "8" was manually created on a non-fluorescent paper using the three materials. Positions ①, ②, and ⑥ are engraved with Cd-MOF, and positions ③, ④, and ⑦ are engraved with H2L (…). Figure 20Furthermore, position ⑤ is marked with CdCl2. Similarly, the numerals inscribed using different materials display a white pattern under sunlight, while emitting blue light under 365nm UV illumination. Notably, the numeral immediately exhibits a bright green LPL after exposure is turned off. However, due to the difference in LPL lifetime between CdCl2, H2L, and Cd-MOF, the numeral "9" becomes visible within 1 second, while only the numeral "7" becomes discernible after more than 3 seconds. Therefore, ingenious and multifaceted anti-counterfeiting / encryption techniques can be achieved through the integration of these materials.

[0057] Finally, it should be noted that the above embodiments are intended only to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Those skilled in the art will readily appreciate that other variations or modifications may be made based on the above descriptions and concepts. It is not necessary and impossible to provide an exhaustive list of all possible implementations. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing organic ligands based on metal chain scaffolds and heavy atom effects to facilitate metal synthesis, characterized in that, The organic ligand has the molecular formula C7H5CdNO5, belongs to the monoclinic crystal system, and has the space group [missing information]. P The coordination compound Cd-MOF was prepared using ligand 3,4-pyridinedicarboxylic acid (H2L) and CdCl2·2.5H2O as raw materials, including the following steps: S1. Mix ligand H2L and CdCl2·2.5H2O in a molar ratio of 1:1~2 and place them in a 25 ml stainless steel container lined with polytetrafluoroethylene. Add DMF, water and ethanol in a volume ratio of 1:1:1~1:2:2 to the 25 ml stainless steel container lined with polytetrafluoroethylene and gently shake to mix evenly. The ratio of DMF to ligand H2L is 1 ml : 0.01~0.02 mmol. S2. Place the sealed 25 ml stainless steel container lined with PTFE into an oven, set the program, and heat for 1-2 hours, raising the temperature from room temperature to 90-110°C. o C, maintain this temperature for 40-60 hours, then cool down for 8-13 hours; S3. Colorless blocky crystals were collected by filtration.

2. The method for preparing organic ligands based on metal chain scaffolds and heavy atom effects of metals according to claim 1, characterized in that, The molar ratio of ligand H2L and CdCl2·2.5H2O in step S1 is 1:

1.

3. The method for preparing organic ligands based on metal chain scaffolds and heavy atom effects of metals according to claim 1, characterized in that, The DMF in step S1 has a volume ratio of water to ethanol of 1 ml: 1 ml: 1 ml.

4. The method for preparing organic ligands based on metal chain scaffolds and heavy atom effects of metals according to claim 1, characterized in that, In step S1, the ratio of DMF to ligand H2L is 1 ml : 0.1 mmol.

5. The method for preparing organic ligands based on metal chain scaffolds and heavy atom effects of metals according to claim 1, characterized in that, Step S2: Set the oven program to heat for 2 hours to raise the temperature from room temperature to 90°C. o C, maintain this temperature for 48 hours, then cool to room temperature for 13 hours.

6. The method for preparing organic ligands based on metal chain scaffolds and heavy atom effects of metals according to claim 1, characterized in that, The procedure includes the following steps: A mixture of 0.1 mmol of ligand, 16.7 mg of H₂L, 0.1 mmol of CdCl₂·2.5H₂O, 1 ml of DMF, 1 ml of water, and 1 ml of ethanol is added to a 25 ml stainless steel container lined with PTFE. The sealed 25 ml PTFE-lined stainless steel container is placed in an oven, and the oven program is set to heat for 2 hours until the temperature rises from room temperature to 90°C. o C, maintain this temperature for 48 hours, then cool to room temperature for 13 hours, and finally filter to obtain colorless block crystals.

Images