English

• 0.   Introduction

  Over the past few decades, transition metal-based coordination polymers (TM-CPs), as important inorganic-organic hybrid materials^[1-5], have received extensive attention and in-depth study due to their promising applications in various fields such as adsorption^[6-8], separation^[9-12], catalysis^[13-17], and sensing^[18-23]. This research interest mainly stems from their unique chemical structure and tunable compositional properties^[24-31], which determine their significant application potential across multiple fields. In the design of novel CPs, many factors are crucial to the material′s dimensionality and topology^[32-34]. Yet, the framework structure of CPs mainly depends on the metal coordination preference and the type of ligand as the connector^[35-40]. Against this research background, carboxylate-based ligands^[41-45] have achieved remarkable success in constructing various CPs. This is mainly due to the ability of carboxylate groups to coordinate with diverse metal ions and their excellent compatibility with various coordination geometries^[46-49].

  When designing appealing CPs, Co(Ⅱ) ions with considerable magnetic anisotropy are a good choice to combine with organic ligands to form porous crystalline materials^[50-53]. Cu(Ⅱ) exhibits a strong affinity for oxygen donors, and polycarboxylate ligands, including aromatic acids and heterocyclic acids, have been widely used in constructing CPs^[54-56]. Among numerous organic ligands, those containing dicarboxylate and dimidazole groups have proven to be ideal candidates for building novel CPs due to their flexible and diverse coordination modes^[57-59]. Among them, 1, 3, 5-triimidazole benzene (L) and 2, 5-furandicarboxylic acid (H₂FDCA), as flexible multidentate ligands, exhibit many fascinating properties. Multidentate ligands can coordinate through oxygen donors in various modes, such as monodentate coordination, chelation, bidentate bridging, monodentate bridging, and chelating bridging.

  We synthesized four CPs by changing the solvent system and ligand concentration through solvothermal method: [Co(L)(FDCA)(H₂O)₂]·0.5H₂O (1), [Co(HL)₂(H₂O)₂](FDCA)₂·6H₂O (2), [Cu(L)(FDCA)(H₂O)]·2H₂O (3) and [Cu(HL)₂(H₂O)₂](FDCA)₂·6H₂O (4). Single- crystal X-ray analysis shows that ligand concentration and solvent systems together control the structure of these CPs. Scanning electron microscopy (SEM) was used to observe the morphological changes of the CPs, and their room-temperature solid-state photoluminescent properties were studied. Besides, Density functional theory (DFT) calculations were performed on the CP materials. The electrostatic potential analysis revealed that oxygen and nitrogen atoms carry most of the negative charge, which facilitates more effective coordination between L, H₂FDCA, and metal ions.

  1.   Experimental

  1.1   Materials and methods

  All the reagents were purchased commercially and used directly without purification. Single-crystal X-ray diffraction analysis was performed on a four-circle diffractometer using a Super Nova-type X-ray single crystal diffractometer (microfocus sealed X-ray tube, Cu Kα, λ=0.154 184 nm) to obtain single-crystal data. Powder X-ray diffraction (PXRD) was performed using a BRUKER D8 ADVANCE diffractometer operated at 40 kV and 40 mA, using Cu Kα radiation (λ=0.154 06 nm), with a 2θ range of 5°-50° and a scan rate of 5 (°)·min^-1. Thermogravimetric (TG) analysis was performed using a Netzsch synchronous thermal analyzer. The surface morphology of the samples was imaged at high resolution by a SEM 3200 scanning electron microscope, while the elemental composition of the samples was analyzed using energy dispersive spectroscopy (EDS). The chemical bonds and functional groups of the samples were characterized using a VERSEX 70 Fourier transform infrared spectrometer (FTIR). To investigate the optical properties of the samples, fluorescence spectra were measured using an F-7000 fluorescence spectrophotometer.

  1.2   Synthesis of CP 1

  L (0.006 9 g, 0.025 mmol), H₂FDCA (0.004 3 g, 0.027 mmol), and Co(CH₃COO)₂·4H₂O (0.006 2 g, 0.025 mmol) were dissolved in 10 mL of methanol and water (4∶6, V/V), sonicated for 5 min, and then subsequently transferred to a 25 mL capacity polytetrafluoroethylene (PTFE)-lined automatic pressure vessel. The temperature was increased to 140 ℃ over 3 h and then cooled to room temperature for 3 d. It was washed three times with deionized water to obtain dark red transparent crystals, and then dried under vacuum at 60 ℃ for 24 h. The yield of CP 1 was 60.1%, calculated based on the metal salt Co(CH₃COO)₂·4H₂O. Elemental analysis Calcd. for C₂₁H₁₉N₆O_7.5Co(%): C, 47.20; H, 3.58; N, 15.73. Found(%): C, 46.53; H, 3.46; N, 15.46.

  1.3   Synthesis of CP 2

  L (0.002 8 g, 0.01 mmol), H₂FDCA (0.004 3 g, 0.027 mmol), and Co(CH₃COO)₂·4H₂O (0.006 2 g, 0.025 mmol) were dissolved in 10 mL of water, sonicated for 5 min, and then subsequently transferred to a 25 mL capacity polytetrafluoroethylene (PTFE)-lined automatic pressure vessel. The temperature was increased to 140 ℃ over 3 h and then cooled to room temperature for 3 d. It was washed three times with deionized water to obtain pink transparent crystals and dried under vacuum at 60 ℃ for 24 h. The yield of CP 2 was 29.9%, calculated based on the metal salt Co(CH₃COO)₂·4H₂O. In summary, both CPs 1 and 2 were synthesized using Co(Ⅱ) as the metal center, demonstrating how changes in solvent composition and ligand concentration can lead to different coordination frameworks within the same metal system. Elemental analysis Calcd. for C₂₁H₂₃N₆O₉Co_0.5(%): C, 47.33; H, 4.35; N, 15.77. Found(%): C, 46.49; H, 3.97; N, 16.25.

  1.4   Synthesis of CP 3

  L (0.005 5 g, 0.02 mmol), H₂FDCA (0.003 1 g, 0.02 mmol), and Cu(CH₃COO)₂·H₂O (0.004 g, 0.02 mmol) were dissolved in 10 mL of methanol and water (1∶9, V/V), sonicated for 5 min, and then subsequently transferred to a 25 mL capacity polytetrafluoroethylene (PTFE)-lined automatic pressure vessel. The temperature was increased to 140 ℃ over 3 h and then cooled to room temperature for 3 d. It was washed three times with deionized water to obtain dark blue transparent crystals, and dried under vacuum at 60 ℃ for 24 h. The yield of CP 3 was 55.8%, calculated based on the metal salt Cu(CH₃COO)₂·H₂O. Elemental analysis Calcd. for C₂₁H₂₀N₆O₈Cu(%): C, 46.03; H, 3.68; N, 15.34. Found(%): C, 45.15; H, 4.02; N, 14.98.

  1.5   Synthesis of CP 4

  L (0.006 9 g, 0.025 mmol), H₂FDCA (0.004 3 g, 0.027 mmol), and Cu(NO₃)₂·3H₂O (0.006 g, 0.025 mmol) were dissolved in 10 mL of water, sonicated for 5 min, and subsequently transferred to a 25 mL capacity polytetrafluoroethylene (PTFE)-lined automatic pressure vessel. The temperature was increased to 140 ℃ over 3 h and then cooled to room temperature for 3 d. It was washed three times with deionized water to obtain light-blue transparent crystals and dried under vacuum at 60 ℃ for 24 h. The yield of CP 4 was 20.0%, calculated based on the metal salt Cu(NO₃)₂·3H₂O. Elemental analysis Calcd. for C₄₂H₄₆N₁₂O₁₈Cu(%): C, 47.13; H, 4.33; N, 15.70. Found(%): C, 46.55; H, 4.85; N, 16.06. In summary, both CPs 3 and 4 were synthesized using Cu(Ⅱ) as the metal center, demonstrating the influence of solvent composition and ligand concentration on the formation of distinct coordination frameworks within the same metal system.

  1.6   Crystal structure analysis of CPs 1-4

  CPs 1-4, which had an appropriate crystallographic size and were transparent and free of cracks, were selected and placed on a single-crystal diffractometer equipped with a single-wavelength X-ray source (Cu Kα radiation, λ=0.154 184 nm) for data collection, and the crystal structures were analyzed and refined using SHELX-97. The crystal structures were solved using SHELXL-2018/3 installed in Olex2 and anisotropically refined using F²-based full-matrix least squares. Table 1 lists the key crystallographic data and the results of the structure refinement. Table S1 (Supporting information) shows the main bond length and bond angle information for the single-crystal structure in detail.

  Table 1

  

  Table 1.  Crystallographic data and structural refinement of 1-4

  下载: 导出CSV

  Parameter	1	2	3	4
  Formula	C21H19N6O7.5Co	C21H23N6O9Co0.5	C21H20N6O8Cu	C42H46N12O18Cu
  Formula weight	534.35	532.92	547.97	1 070.45
  Temperature / K	293	293	292	292
  Crystal system	Orthorhombic	Triclinic	Triclinic	Triclinic
  Space group	Pbca	P1	P1	P1
  a / nm	1.559 03(3)	0.795 26(7)	0.848 56(4)	0.794 04(4)
  b / nm	1.579 61(3)	1.054 24(6)	1.051 85(4)	1.044 59(4)
  c / nm	1.778 85(3)	1.425 13(13)	1.952 0(7)	1.437 54(8)
  α / (°)		90.327(6)	88.978(4)	87.454(4)
  β / (°)		104.825(8)	71.497(4)	75.049(5)
  γ / (°)		93.341(6)	78.681(3)	88.869(4)
  V / nm3	4.380 70(14)	1.152 82(17)	1.073 73(9)	1.150 82(10)
  Z	8	2	2	1
  Dc / (g·cm-3)	1.620	1.535	1.695	1.542
  μ / mm-1	6.687	3.709	2.019	1.462
  F(000)	2192	553	562	553
  Crystal size / mm	0.02×0.04×0.04	0.03×0.04×0.04	0.03×0.04×0.04	0.02×0.04×0.04
  θ range for data collection / (°)	5.6-69.8	4.2-69.7	3.6-69.7	4.2-69.8
  Limiting indices	-18 ≤ h ≤ 13, -19 ≤ k ≤ 13, -21 ≤ l ≤ 21	-6 ≤ h ≤ 9, -12 ≤ k ≤ 12, -17 ≤ l ≤ 16	-7 ≤ h ≤ 10, -8 ≤ k ≤ 12, -15 ≤ l ≤ 15	-9 ≤ h ≤ 9, -12 ≤ k ≤ 10, -14 ≤ l ≤ 17
  Reflection collected, unique	9 558, 4 072 (Rint=0.043)	7 455, 4 269 (Rint=0.023)	6 785, 3 945 (Rint=0.015)	7 504, 4 265 (Rint=0.015)
  Completeness to θ / %	98.6	97.9	97.4	98.1
  Data, Nr, Np*	4 072, 3, 338	4 269, 0, 357	3 945, 0, 336	4 265, 0, 341
  GOF on F2	1.040	1.04	1.04	1.05
  R1 [I > 2σ(I)]	0.067 9	0.036 4	0.031 1	0.034 7
  wR2 [I > 2σ(I)]	0.189 4	0.094 6	0.083 6	0.096 0
  Largest diff. peak and hole / (e·nm-3)	-1 290 and 910	-550 and 200	-490 and 290	-410 and 660
  *Nr: number of restraints, Np: number of parameters.

  2.   Results and discussions

  2.1   Crystal structure description of CPs 1-4

  Single-crystal X-ray diffraction analysis showed that 1 presents a 2D layered neutral skeleton. Its crystal structure belongs to the orthorhombic crystal system and is characterized by the Pbca space group (Table 1). The asymmetric unit of 1 contains one Co(Ⅱ) center, a deprotonated FDCA^2- ligand, a L ligand, two coordinated water molecules, and half of a lattice water molecule (Fig.1a). Table S1 shows that the bond distances of Co1—O6 [0.216 0(4) nm] and Co1—N1 [0.211 6(3) nm] are not equal, so the coordination geometry of the Co(Ⅱ) center in 1 is a slightly distorted octahedral geometry configuration (Fig.1b). The central metal Co(Ⅱ) ions were bridged by FDCA^2- and L to form a 2D AB interspersed layer structure (Fig.1c).

  Figure 1

  

  Figure 1.  (a) Coordination environment of Co(Ⅱ) in 1; (b) Geometric configuration of Co(Ⅱ); (c) 2D interspersed structure

  The hydrogen atoms are omitted for clarity; Symmetry codes: a: 1-x, 1-y, 1-z; b: x, -1+y, z; c: 1-x, 2-y, 1-z.

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  CP 2 presents a 1D chain cationic skeleton. Its crystal structure belongs to the triclinic crystal system and is characterized by the P1 space group (Table 1). The asymmetric unit of 2 contains one Co(Ⅱ) center, two protonated HL⁺ ligands, two coordinated water molecules, two deprotonated FDCA^2- ions for balanced charge, and six lattice water molecules (Fig.2a). Table S1 shows that the bond distances of Co1—O6 [0.212 9(2) nm] and Co1—N1 [0.213 9(2) nm] are not equal, so the coordination geometry of the Co(Ⅱ) center in 2 is also a slightly distorted octahedral geometry configuration (Fig.2b). As shown in Fig.2c, N1 and N4 of the two L ligands connect each Co(Ⅱ) ion via the μ₂-bridging mode, forming an infinitely extended 1D looped chain structure.

  Figure 2

  

  Figure 2.  (a) Coordination environment of Co(Ⅱ) in 2; (b) Geometric configuration of Co(Ⅱ); (c) 1D chain structure

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  The Co1 ion in 2 is six-coordinated with four nitrogen atoms from the HL⁺ ligands and two oxygen atoms from the coordinated water molecules. And the Co1 ion in 1 is six-coordinated with two oxygen atoms in the FDCA^2- anion, one oxygen atom in the coordinated water molecule, and three nitrogen atoms in the L ligands. In the presence of varying ligand concentrations and solvents, the difference in the CPs structures from 2D (1) to 1D (2) is attributed to the fact that the deprotonated FDCA^2- is uninvolved in the coordination in 2. This also leads to a change in the CPs from a neutral backbone (1) to a cationic backbone (2).

  3 shows a 1D neutral skeleton. It is a triclinic crystal system in the P1 space group (Table 1). The asymmetric unit of 3 contains one Cu(Ⅱ) center, one deprotonated FDCA^2- ligand, one L ligand, one coordinated water molecule, and two lattice water molecules. Both carboxylates in the FDCA^2- are bridge and chelate two Cu(Ⅱ) ions through three oxygen atoms, and it is worth noting that the furan oxygen atom is also involved in the coordination (Fig.3a). The Cu(Ⅱ) center was six-coordinated with three oxygen atoms from the FDCA^2- anion, which includes two oxygen atoms from the carboxylate groups and one furan oxygen atom, as well as an oxygen atom from the coordinated water molecule (Fig.3b). Table S1 shows that the bond distances of Cu1a—O1a [0.197 96(2) nm] and Cu1a—O4 [0.198 67(2) nm], which are not equal, so the coordination geometry of the Cu(Ⅱ) center in 3 is a slightly distorted octahedral configuration (Fig.3b). As shown in Fig.3c, N1 and N6 of the two L ligands connected each Cu(Ⅱ) ion via the μ₂-bridging mode, thus forming an infinitely extended 1D looped chain structure.

  Figure 3

  

  Figure 3.  (a) Coordination environment of Cu(Ⅱ) in 3 (30% probability level of ellipsoids); (b) Geometric configuration of Cu(Ⅱ) (30% probability level of ellipsoids); (c) 1D chain structure

  The hydrogen atoms are omitted for clarity; Symmetry codes: a: 2-x, -y, -z; b: 1-x, -y, 1-z; c: 1+x, y, -1+z.

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  CP 4 shows a 1D cationic skeleton. Its crystal structure belongs to the triclinic crystal system and is characterized by the P1 space group (Table 1). The asymmetric unit of 4 contains one Cu(Ⅱ) center, two protonated HL⁺ ligands, two coordinated water molecules, and two deprotonated FDCA^2- ions for balanced charge, and six lattice water molecules (Fig.4a). The Cu(Ⅱ) center was six-coordinated with two nitrogen atoms each from two HL⁺ ligands, and two oxygen atoms from the coordinated water molecules. From Fig.4b, it can be seen that the coordination geometry of the Cu(Ⅱ) center in 4 is octahedral. As shown in Fig.4c, N1 and N4 of the two HL⁺ ligands connected each Cu(Ⅱ) ion via the μ₂-bridging mode, thus forming an infinitely extended 1D looped chain structure.

  Figure 4

  

  Figure 4.  (a) Coordination environment of Cu(Ⅱ) in 4; (b) Geometric configuration of Cu(Ⅱ); (c) 1D chain structure

  The hydrogen atoms are omitted for clarity; Symmetry codes: a: x, 1+y, z; b: 1-x, 1-y, -z; c: 1-x, 2-y, -z.

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  Although 3 and 4 are both 1D looped chains, 3 is a bimetallic [(L)₂-Cu₂]_n neutral skeleton consisting of a deprotonated FDCA^2- bridged and chelated by three oxygen atoms, whereas 4 is a monometallic [(HL)-Cu]_n cationic backbone formed by the attachment of N atoms (HL⁺ ligand). In addition, the water molecules are involved in the coordination of 1-4, which shows that water molecules also have a strong coordination ability. Adjusting the solvent to water and increasing the concentration of H₂FDCA, based on 1 and 3, the deprotonated FDCA^2- ligands are uninvolved in the coordination in 2 and 4. This is a result of the presence of more water molecules occupying some of the sites when the pure solvent is water. Therefore, solvent molecules may have been involved in crystal growth as templating or structure-directing agents during the experiment, leading to the formation of specific crystal morphologies and structures. The presence of coordination sites for the water molecule allows for a change in the coordination mode, resulting in a different structure, which is consistent with our experimental results.

  2.2   Morphology of CPs 1-4

  The morphology of 1-4 was observed by SEM. As shown in Fig.5a-5b, SEM shows that 1 and 2 had irregular rectangular morphology. As shown in Fig.5c-5d, SEM shows that 3 and 4 had regular parallel tetrahedral morphology. The EDS indicates that the elements contained in 1-4 were well distributed (Fig.S1-S4). The SEM comparisons revealed that by adjusting the solvent system and ligand concentration, the change in crystal structure does not produce a large change in the morphology.

  Figure 5

  

  Figure 5.  SEM images of CPs (a) 1, (b) 2, (c) 3, and (d) 4

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  2.3   PXRD and TG analysis

  The experimental PXRD data for samples 1, 2, 3, and 4 were compared with the corresponding simulated data from single-crystal X-ray diffraction (Fig.6). The comparison revealed that the diffraction peaks of the experimental and simulated data were in good agreement, indicating that the respective crystal phases of the complexes are pure.

  Figure 6

  

  Figure 6.  PXRD patterns of CPs (a) 1, (b) 2, (c) 3, and (d) 4

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  TG analysis was conducted on samples 1, 2, 3, and 4 in an N₂ environment to assess their thermal stability (Fig.7). The first weightless stage of 1 was from room temperature to198.08 ℃, which is the loss of two coordination water and 0.5 lattice water molecules, and the loss in weight was 8.3% (Calcd. 8.4%). The second stage of weightlessness was from 288 to 373 ℃, attributed to the loss of FDCA^2-, with a weight loss of about 24.4% (Calcd. 28.8%). Above 373 ℃, the decomposition attributed to the remaining ligand L led to the complete collapse of the skeleton. At 800 ℃, the residual weight was 48.1% (Fig.7a).

  Figure 7

  

  Figure 7.  TG curves of CPs (a) 1, (b) 2, (c) 3, and (d) 4

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  The first weightless stage of 2 was from room temperature to 176 ℃, which is the loss of two coordinated water molecules and six lattice water molecules, and a weight of 14.2% (Calcd. 13.5%). The second stage of weightlessness was from 244 to 345 ℃, attributed to the loss of FDCA^2-, with a weight loss of about 28.0% (Calcd. 28.9%). Above 345 ℃, the decomposition attributed to the remaining ligand HL⁺ led to the complete collapse of the skeleton. At 800 ℃, the residual weight was 28% (Fig.7b).

  The first weightless stage of 3 was from room temperature to 152 ℃, which is the loss of one coordination water and two lattice water molecules, and a weight of 9.7% (Calcd. 9.9%). The second stage of weightlessness was from 235 to 290 ℃, attributed to the loss of FDCA^2-, with a weight loss of about 24.7% (Calcd. 28.1%). Above 290.38 ℃, the decomposition attributed to the remaining ligand L led to the complete collapse of the skeleton. At 800 ℃, the residual weight was 49.1% (Fig.7c).

  The first weightless stage of 4 was from room temperature to 166 ℃, which is the loss of two coordination water molecules and six lattice water molecules, and the loss in weight was 13.3% (Calcd. 13.5%). The second stage of weightlessness was from 234 to 287 ℃, attributed to the loss of FDCA^2-, with a weight loss of about 28.7% (Calcd. 28.8%). Above 287 ℃, the decomposition attributed to the remaining ligand HL⁺ led to the complete collapse of the skeleton. At 800 ℃, the residual weight was 31.7% (Fig.7d).

  2.4   FTIR spectra

  Comparison of the FTIR spectra of the ligands (H₂FDCA and L) with CPs 1-4 provides insight into the coordination interactions between the ligand and metal ions. Infrared spectroscopic measurements showed that the major characteristic peaks of the ligand and 1-4 were successfully identified in the wave number range of 500-4 000 cm^-1 (Fig.8).

  Figure 8

  

  Figure 8.  FTIR spectra of CPs (a) 1, (b) 2, (c) 3, and (d) 4

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  For the H₂FDCA ligand, the absorption peak located near 1 670 cm^-1 is thought to be due to stretching vibrations occurring at the carbonyl (C=O) group in the carboxylic acid moiety^[43-45]. The absorption peak observed at 1 565 cm^-1 can be attributed to the C=C bond stretching vibration in the furan ring. In CPs 1 and 3, the metal ions form a coordination bond with the ligand H₂FDCA in C=O (1 670 cm^-1), resulting in a displacement in the infrared absorption peak of C=O. This displacement is due to the fact that coordination changes the electron density distribution of the C=O double bond, thus affecting its vibrational frequency. Chemical change of deprotonation of H₂FDCA to form carboxylate ions (FDCA^2-) was found in the single crystal structures of CPs 2 and 4. This change may alter the vibrational properties of the C=O bond, resulting in a displacement of the infrared absorption peak of C=O. The generation of absorption peaks near 1 614 and 1 070 cm^-1 for ligand L is attributed to the C=C and C—H bonding vibrations in the benzene ring, respectively. For ligand L, the absorption peak located near 1 494 cm^-1 is usually attributed to the stretching vibration of the C—N bond in the imidazole ring^[57]. It is important to note that in 2 and 4, although the carboxylate groups do not participate in coordination, they form multiple hydrogen bonds. These hydrogen bonds have a significant impact on the vibrational characteristics of the C=O bonds. As a result, it is challenging to distinguish the C=O absorption peaks of coordinated and uncoordinated carboxylate groups in the infrared spectra. This is because the hydrogen bonding alters the vibrational frequency of the C=O bonds, thus masking the subtle differences that might be caused by coordination. This phenomenon demonstrates that hydrogen bonds play a crucial role in modulating the vibrational properties of carboxylate ions, which in turn affects the characteristic absorption peaks in the infrared spectra. However, in 1-4, the intensity of the absorption peak at 1 494 cm^-1 was weakened compared to L, suggesting that the coordination of the metal ion to the ligand may have led to a change in this absorption peak. This change may be due to the coordination of the metal ions with the nitrogen atoms in the imidazole ring, which affects the electron density distribution of the C—N bond, thus altering its vibrational frequency and intensity. This spectral variation is an important basis for the study of metal-ligand interactions in CPs.

  2.5   Photoluminescence spectra of CPs 1-4

  Solid-state fluorescence spectra at room temperature were determined for 1-4 and the ligands (Fig.9). The solid-state fluorescence spectra of H₂FDCA and L were measured at an excitation wavelength of 278 nm. It can be found that the ligand H₂FDCA showed an obvious emission peak at 372 nm, which may be due to the transition between the π-electrons on the furan ring and the lone pair of electrons on the carboxyl group of the ligand. The ligand L showed a distinct emission peak at 351 nm, which may be due to the π-π* jump within the ligand. The solid-state fluorescence spectra of CPs 1-4 were measured at an excitation wavelength of 255 nm. In the synthesis of CPs, metal ions are coordinated to organic ligands. During this process, excited electrons may be transferred from the organic ligand to the metal center, and this charge transfer leads to a decrease in the energy of the luminescent emission peak, resulting in a redshift phenomenon. Compared to the ligand, 1-4 showed a lower fluorescence intensity, a phenomenon likely due to ligand-to-metal charge transfer (LMCT). The photoluminescence spectra of 1-4 showed different absorption patterns compared to L and H₂FDCA, reflecting the effect of interaction or complexation between CPs single crystals and ligands on optical properties^{[51, 57]}.

  Figure 9

  

  Figure 9.  Photoluminescence spectra of CPs (a) 1, (b) 2, (c) 3, and (d) 4

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  2.6   Theoretical calculation

  To investigate the mechanism of the CPs′ single-crystal formation from ligands and transition metals, DFT calculations were carried out. The B3LYP/6-31G(D) method basis set in the Gaussian 09 program was employed for the calculations. As depicted in Fig.10a, the electron clouds of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are predominantly concentrated in the central region of the ligand. This area typically manifests as a denser region of the electron cloud, which facilitates more efficient coordination with metal ions. To explore the electronic properties of the ligands in CPs under different conditions, an electrostatic potential analysis of the ligands was performed. The results indicate that the ligand possesses a notable low/high potential region at (-0.025 50, 0.022 57), which may influence its coordination behavior with the metal center (Fig.10b). Further investigations revealed that the negative charge distribution is primarily focused on the O and N atoms of the ligand. This distribution favors the coordination of the ligand with the metal ion^[60-62].

  Figure 10

  

  Figure 10.  (a) HOMO and LUMO energy levels of L and H₂FDCA; (b) Electrostatic potentials of L and H₂FDCA

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  3.   Conclusions

  Four CPs were synthesized via solvothermal synthesis, and the effects of ligand concentration, solvent system, and metal ions on coordination modes were investigated. Results show that ligand concentration and solvent can cause changes in crystal structure. However, in the four studied CPs, metal ions maintained a consistent coordination mode with ligands, exhibiting an octahedral configuration. Thermogravimetric analysis indicated that all CPs demonstrated good thermal stability. Studies on the solid-state fluorescence photoluminescence properties of 1-4 revealed that the interaction between CPs and ligands affects optical properties. Theoretical calculations and electrostatic potential analyses showed that the L ligand has superior coordination ability. Ligands with higher energy possess optimal spatial and geometric structures, enabling metal ions to form more stable coordination environments with ligands.

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

  
  Acknowledgments: We thank the Analytical Testing Center of Lanzhou Jiaotong University for support in characterization testing. Declaration of competing interest: The authors declare that they have no conflicts of interest.
  
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• 

  Figure 1  (a) Coordination environment of Co(Ⅱ) in 1; (b) Geometric configuration of Co(Ⅱ); (c) 2D interspersed structure

  The hydrogen atoms are omitted for clarity; Symmetry codes: a: 1-x, 1-y, 1-z; b: x, -1+y, z; c: 1-x, 2-y, 1-z.

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  Figure 2  (a) Coordination environment of Co(Ⅱ) in 2; (b) Geometric configuration of Co(Ⅱ); (c) 1D chain structure

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  Figure 3  (a) Coordination environment of Cu(Ⅱ) in 3 (30% probability level of ellipsoids); (b) Geometric configuration of Cu(Ⅱ) (30% probability level of ellipsoids); (c) 1D chain structure

  The hydrogen atoms are omitted for clarity; Symmetry codes: a: 2-x, -y, -z; b: 1-x, -y, 1-z; c: 1+x, y, -1+z.

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  Figure 4  (a) Coordination environment of Cu(Ⅱ) in 4; (b) Geometric configuration of Cu(Ⅱ); (c) 1D chain structure

  The hydrogen atoms are omitted for clarity; Symmetry codes: a: x, 1+y, z; b: 1-x, 1-y, -z; c: 1-x, 2-y, -z.

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  Figure 5  SEM images of CPs (a) 1, (b) 2, (c) 3, and (d) 4

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  Figure 6  PXRD patterns of CPs (a) 1, (b) 2, (c) 3, and (d) 4

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  Figure 7  TG curves of CPs (a) 1, (b) 2, (c) 3, and (d) 4

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  Figure 8  FTIR spectra of CPs (a) 1, (b) 2, (c) 3, and (d) 4

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  Figure 9  Photoluminescence spectra of CPs (a) 1, (b) 2, (c) 3, and (d) 4

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  Figure 10  (a) HOMO and LUMO energy levels of L and H₂FDCA; (b) Electrostatic potentials of L and H₂FDCA

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  Table 1.  Crystallographic data and structural refinement of 1-4

  Parameter	1	2	3	4
  Formula	C21H19N6O7.5Co	C21H23N6O9Co0.5	C21H20N6O8Cu	C42H46N12O18Cu
  Formula weight	534.35	532.92	547.97	1 070.45
  Temperature / K	293	293	292	292
  Crystal system	Orthorhombic	Triclinic	Triclinic	Triclinic
  Space group	Pbca	P1	P1	P1
  a / nm	1.559 03(3)	0.795 26(7)	0.848 56(4)	0.794 04(4)
  b / nm	1.579 61(3)	1.054 24(6)	1.051 85(4)	1.044 59(4)
  c / nm	1.778 85(3)	1.425 13(13)	1.952 0(7)	1.437 54(8)
  α / (°)		90.327(6)	88.978(4)	87.454(4)
  β / (°)		104.825(8)	71.497(4)	75.049(5)
  γ / (°)		93.341(6)	78.681(3)	88.869(4)
  V / nm3	4.380 70(14)	1.152 82(17)	1.073 73(9)	1.150 82(10)
  Z	8	2	2	1
  Dc / (g·cm-3)	1.620	1.535	1.695	1.542
  μ / mm-1	6.687	3.709	2.019	1.462
  F(000)	2192	553	562	553
  Crystal size / mm	0.02×0.04×0.04	0.03×0.04×0.04	0.03×0.04×0.04	0.02×0.04×0.04
  θ range for data collection / (°)	5.6-69.8	4.2-69.7	3.6-69.7	4.2-69.8
  Limiting indices	-18 ≤ h ≤ 13, -19 ≤ k ≤ 13, -21 ≤ l ≤ 21	-6 ≤ h ≤ 9, -12 ≤ k ≤ 12, -17 ≤ l ≤ 16	-7 ≤ h ≤ 10, -8 ≤ k ≤ 12, -15 ≤ l ≤ 15	-9 ≤ h ≤ 9, -12 ≤ k ≤ 10, -14 ≤ l ≤ 17
  Reflection collected, unique	9 558, 4 072 (Rint=0.043)	7 455, 4 269 (Rint=0.023)	6 785, 3 945 (Rint=0.015)	7 504, 4 265 (Rint=0.015)
  Completeness to θ / %	98.6	97.9	97.4	98.1
  Data, Nr, Np*	4 072, 3, 338	4 269, 0, 357	3 945, 0, 336	4 265, 0, 341
  GOF on F2	1.040	1.04	1.04	1.05
  R1 [I > 2σ(I)]	0.067 9	0.036 4	0.031 1	0.034 7
  wR2 [I > 2σ(I)]	0.189 4	0.094 6	0.083 6	0.096 0
  Largest diff. peak and hole / (e·nm-3)	-1 290 and 910	-550 and 200	-490 and 290	-410 and 660
  *Nr: number of restraints, Np: number of parameters.

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