English

• 0.   Introduction

  Polyoxometalates (POMs), as one type of unique nano-sized metal-oxo clusters, have been used as anionic precursors, not only because of their controllable shape, size, and high negative charges, but also because of their potential advantages in the design of new materials with abundant chemical combinations and multiple functionalities^[1-5]. Especially, the design and synthesis of new coordination polymers (CPs) based on POMs have attracted considerable attention in recent years^[6-12]. In this research field, the POMs can act as “templates” for a variety of structures, and can be regarded as “building blocks”, with their terminal or bridging oxides coordinating to metal cations. In the POM-based CPs, different POMs clusters, including Keggin^[13-14], Dawson^[15-16], Anderson^[17-18], Strandberg^[19-20], and Lindqvist-type^[21-22] clusters, etc., have been employed to build CPs. Among these POM clusters, octamolybdates have received much attention in constructing POM-based CPs due to their structural diversity, that is, octamolybdates exist in eight isomeric forms (α, β, γ, δ, ε, ζ, η, and θ isomers), and these isomers are easily interconvertible under mild environmental modification^[23-26]. Therefore, various topological architectures can be afforded through introducing octamolybdates into metals and organic ligand systems.

  In the synthesis of the POM-based CPs, hydrothermal^[13-26] and solvothermal^[27-28] syntheses have been widely used. Recently, ionothermal synthesis acted as a kind of synthetic method starts to be used, in which ionic liquids (ILs) act as the solvents^[29-31]. Compared to the solvents used in traditional hydro/solvothermal synthesis, ILs possess better solubility to most of the inorganic precursors, lower vapor pressure, higher chemical and thermal stabilities, and lower toxicity. Hence, ionothermal synthesis is considered an environmentally friendly synthetic method^[32]. Additionally, in CPs synthesized by hydrothermal and solvothermal methods, most of the pore channels are occupied by water or traditional organic solvents such as methanol, acetonitrile, and acetone. When using ILs for synthesis, the pore channels can be occupied by organic counter cations, which can be removed by ion exchange, thus restoring the pore structure for relevant property tests.

  The synthetic system of POM-based CPs includes three key factors: transition metal (TM) ions, neutral N-donor bridging ligands (L), and POMs. In this reaction system, the design and selection of organic N-donor bridging ligands play a key role in exploring the POM-based CPs. Among them, bridging ligands formed by the combination of bridging groups and triazole have diverse and flexible coordination modes with TM ions, thus enabling the preparation of POM-based CPs with different structures^[33-35]. Our research group has carried out exploration and research in this regard^[36-38].

  Based on the above considerations and our previous work, we carried out systematic research on the ionothermal reaction system Cu/BBTZ/heptamolybdate (BBTZ=1,4-bis(1,2,4-triazol-1-ylmethyl)benzene) to construct POM-based CPs with the ion exchange properties. Herein, the octamolybdate-based CP, (Emim)₂[Cu(BBTZ)₂(β-Mo₈O₂₆)] (Mo₈-CP), was prepared from this reaction system. Mo₈-CP contains a 3D anionic framework with channels, in which 1-ethyl-3-methylimidazolium ((Emim)⁺) acts as a structure-directing agent and occupies the channels. Furthermore, luminescent materials with different colors can be obtained through the exchange of Eu³⁺ or Tb³⁺.

  1.   Experimental

  1.1   Materials and methods

  All chemicals and organic solvents used for synthesis were of reagent grade without further purification. The ligand BBTZ was synthesized according to the literature procedure^[39]. The 1-ethyl-3-methylimidazolium bromide ((Emim)Br) was prepared according to the literature methods^[40]. Elemental analysis was performed on a Perkin-Elmer 2400 CHN elemental analyzer (C, H, and N) and a Leeman inductively coupled plasma (ICP) spectrometer (Cu and Mo). The FTIR spectra were analyzed on a Mattson Alpha-Centauri spectrometer with KBr pellets in a range of 4 000-400 cm^-1. Thermogravimetry (TG) analysis was carried out on a Pyris Diamond TG instrument in flowing N₂ with a heating rate of 10 ℃·min^-1. The powder X-ray diffraction (PXRD) studies were performed with a Rigaku D/max-IIB X-ray diffractometer operating at 40 kV and 30 mA, using Cu Kα radiation (λ=0.154 18 nm), at a scanning rate of 1 (°)·min^-1 with 2θ ranging from 5° to 50°. Photoluminescence spectra were obtained by using an FLSP 980 Edinburgh instrument (Eng) with a 450 W xenon lamp monochromatized by a double grating. The luminescence photograph was taken under UV excitation by an upright microscope (model Ni-L) equipped with a digital camera.

  1.2   Synthesis of Mo₈-CP

  The mixture of (NH₄)₆Mo₇O₂₄·4H₂O (0.16 g, 0.13 mmol), CuCl₂·2H₂O(0.15 g, 0.87 mmol), BBTZ (0.1 g, 0.41 mmol), and (Emim)Br (2 g, 4.6 mmol) was stirred at room temperature for 0.5 h. During this period, the pH value of the reaction mixture was adjusted to 4.5 with 1.0 mol·L^-1 NaOH and 1.0 mol·L^-1 HCl. Then, the suspension was sealed into a Teflon-lined autoclave, kept under autogenous pressure at 160 ℃ for 5 d, and then slowly cooled to room temperature at a rate of 10 ℃·h^-1. Blue block crystals of Mo₈-CP thus obtained were isolated via filtration, washed with distilled water, and kept in a vacuum desiccator (55% yield based on Mo). Anal. Calcd. for C₃₆H₄₆N₁₆O₂₆Mo₈Cu(%): C 22.15, H 2.36, N 11.49, Cu 3.28, Mo 39.38. Found(%): C 21.96, H 2.47, N 11.35, Cu 3.12, Mo 39.19. Selected IR data (KBr pellet, cm^-1): 3 450(s), 3 128(s), 3 025(w), 2 962(w), 1 620(w), 1 523(s), 1 423(w), 1 380(w), 1 284(s), 1 209(m), 1 124(s), 1 008(s), 943(w), 904(m), 866(w), 793(w), 761(w), 725(m), 657(m).

  1.3   Ion exchange experiment

  Eu³⁺ exchange: 120 mg of Eu(NO₃)₃·6H₂O was dispersed in 10 mL of ethanol, and then, 100 mg of Mo₈-CP was added and stirred for 24 h. After repeated washing with ethanol and drying, the final product can be collected.

  Tb³⁺ exchange: the preparation method of Tb³⁺ exchange was analogous to that of Eu³⁺ exchange mentioned above; only Eu(NO₃)₃·6H₂O should be replaced by the same amount of Tb((NO₃)₃·6H₂O.

  1.4   X-ray crystallography

  The crystallographic data of Mo₈-CP were collected at 298 K on the Rigaku R-axis Rapid IP diffractometer using graphite monochromatic Mo Kα radiation (λ=0.071 073 nm) and IP techniques. The crystal was maintained at a temperature of 298.15 K throughout the data collection process. The structure was solved by direct methods using the SHELXT program and refined with SHELXL-97 by full matrix least-squares techniques on F². Non-hydrogen atoms were refined anisotropically, while the hydrogen atoms were located geometrically and refined isotropically. During the refinement of Mo₈-CP, non-hydrogen atoms were refined anisotropically. The H atoms on organic C centers were fixed in calculated positions. The detailed crystal data and structure refinement for Mo₈-CP are given in Table 1. Selected bond lengths and angles of Mo₈-CP are listed in Table 2.

  Table 1

  

  Table 1.  Crystal data and structure refinement for Mo₈-CP

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  Parameter	Mo8-CP		Parameter	Mo8-CP
  Formula	C36H46N16O26Mo8Cu		Z	4
  Formula weight	1 949.95		μ/mm-1	1.982
  Crystal system	Monoclinic		F(000)	3 788
  Space group	C2/c		Total reflection	23 785
  a/nm	2.168 0(4)		Rint	0.039 1
  b/nm	1.254 1(3)		GOF	1.025
  c/nm	2.336 0(5)		R1 [I>2σ(I)]a	0.037 2
  β/(°)	102.04(3)		wR2 (all data)b	0.089 1
  V/nm3	6.212(2)
  a R1=∑||Fo|-|Fc||/∑|Fo|; b wR2=∑[w(Fo2-Fc2)2]/∑[w(Fo2)2]1/2.

  Table 2

  

  Table 2.  Selected bond lengths (nm) and angles (°) of Mo₈-CP

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  Cu1—N1i	0.201 0(5)	Cu1—N1	0.201 0(5)	Cu1—N4i	0.201 1(5)
  Cu1—N4	0.201 1(5)	Cu1—O8i	0.249 9(0)	Cu1—O8	0.249 9(0)
  N1i—Cu1—N1	180.0(2)	N1i—Cu1—N4	89.93(19)	N1i—Cu1—N4i	90.07(19)
  N1—Cu1—N4i	89.9(2)	N1—Cu1—N4	90.07(19)	N4i—Cu1—N4	180.0
  N1—Cu1—O8	88.33(31)	N4—Cu1—O8	94.08(81)	N1i—Cu1—O8	91.66(71)
  N4i—Cu1—O8	85.91(81)	O8—Cu1—O8i	179.98(9)
  Symmetry code: i -x, -y, 1-z.

  2.   Results and discussion

  2.1   Synthesis

  In the construction of porous materials, the synthetic outcomes often fall short of expectations, primarily due to ligand entanglement during the self-assembly process, which leads to pore size reduction or even structural collapse. To address this issue, this study proposes employing bulky organic counterions as pore-filling templates. This strategy effectively suppresses ligand interpenetration during self-assembly by pre-occupying the pore space with organic cations. Although these cations initially block the pores, subsequent ion exchange removes them, restoring porosity and enabling further characterization. The ionothermal synthesis method offers several advantages: (ⅰ) ILs exhibit excellent solubility for inorganic precursors; (ⅱ) they combine low vapor pressure with high thermal/chemical stability and low toxicity; (ⅲ) the intrinsic organic cations of ILs can directly act as pore-filling counterions; (ⅳ) introducing specific organic ligands may provide supplementary cationic species for pore occupation. Based on this rationale, we extended the classical Cu/BBTZ/(Mo₈O₂₆)^4- hydrothermal system to an ionothermal approach, obtaining porous materials with distinct architectures.

  In the hydrothermal synthesis system for preparing CPs using Cu²⁺, organic ligands, and (NH₄)₆Mo₇O₂₄ as raw materials, the pH value is the core factor regulating product formation^[41]. Similarly, in the ionothermal synthesis experiment of Mo₈-CP, 1.0 mol·L^-1 NaOH solution and 1.0 mol·L^-1 HCl solution were used as acid-base regulators to precisely control the pH value of the reaction system. The synthesis effects were systematically investigated when the pH values were 2.5, 3.0, 3.5, 4.0, 4.5, and 5.0. The results showed that: (ⅰ) when the pH value was 2.5, 3.0, or 3.5, no crystals appeared in the system after the reaction was completed, and only amorphous precipitates were formed; (ⅱ) when the pH value increased to 4.0 and 4.5, blue crystals were successfully precipitated from the system, and a comparison revealed that when pH=4.5, the obtained blue crystals had a higher yield and better crystal quality; (ⅲ) when the pH value was further increased to 5.0, the target blue crystals were not generated in the system, and only yellow microcrystals were obtained. Due to the excessively small grain size of these yellow microcrystals, their single-crystal structure could not be successfully resolved.

  2.2   Description of crystal structure

  Mo₈-CP crystallizes in the monoclinic space group C2/c, with the basic structural unit comprising one polyoxometalate anion (β-Mo₈O₂₆)^4-, one Cu²⁺ cation, two BBTZ ligands, and two (Emim)⁺ counter cations (Fig.1). In Mo₈-CP, the (β-Mo₈O₂₆)^4- anion consists of eight distorted {MoO₆} octahedra, containing four types of oxygen atoms: six μ₂-O, four μ₃-O, two μ₅-O, and twelve terminal O (O_t) atoms, exhibiting characteristic β-type octamolybdate structural features. The metal-organic fragment contains only one crystallographically independent Cu²⁺ center. Each Cu²⁺ center adopts a six-coordinated geometry, ligated by four nitrogen atoms from triazole groups of BBTZ ligands and two terminal oxygen atoms from the (β-Mo₈O₂₆)^4- polyanion (Fig.2). The Cu—N bond lengths are 0.201 0(5) and 0.201 1(5) nm, with N—Cu—N bond angles of 90° and 180°. The Cu—O8 bond distance of 0.249 9(0) nm suggests a weak coordination interaction between the Cu²⁺ center and the surface oxygen atom of (β-Mo₈O₂₆)^4-. Structurally, each BBTZ ligand in Mo₈-CP adopts a cis-configuration, bridging two Cu²⁺ centers to form binuclear [Cu-BBTZ]₂ metallacycles (Fig.2). These [Cu-BBTZ]₂ units are further interconnected via Cu²⁺ centers to generate loop-containing 1D chain (Cu-BBTZ chain) (Fig.2). Concurrently, octamolybdate (Mo₈) clusters coordinate to Cu²⁺ centers, forming 1D Cu-Mo₈ chains (Fig.3a). Notably, the two Cu²⁺ centers in each [Cu-BBTZ]₂ unit coordinate to Mo₈ clusters, resulting in two sets of 1D Cu-Mo₈ chains extending orthogonally (Fig.3b). Through this connection, the 1D Cu-BBTZ chains and Cu-Mo₈ chains collectively assemble into a 3D anionic framework via shared Cu²⁺ centers, featuring 1D channels along the b-axis (Fig.4a). (Emim)⁺ occupy these channels, serving dual roles: preventing framework entanglement through steric effects and acting as structure-directing agents for channel formation (Fig.4b).

  Figure 1

  

  Figure 1.  ORTEP diagram of the basic structural units in Mo₈-CP with thermal ellipsoids at 30% probability displacement

  All H atoms are omitted for clarity; Symmetry code: ⁱ -x, -y, 1-z.

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  Figure 2

  

  Figure 2.  Structure of loop-containing 1D chains in Mo₈-CP

  Symmetry codes: ⁱ 1/2-x, 1/2+y, 3/2-z; ⁱⁱ 1/2+x, 1/2+y, z; ⁱⁱⁱ 1/2-x, 1/2+y, 1/2-z; ^iv 1/2+x, 1/2+y, -1+z.

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  Figure 3

  

  Figure 3.  Two 1D-Mo₈ chains possessing two different extended directions in Mo₈-CP

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  Figure 4

  

  Figure 4.  (a) Structural illustration of 3D POMs-based anionic framework with the pore along the b-axis in Mo₈-CP; (b) Structural illustration of the 3D POMs-based CP with (Emim)⁺ cations occupying the 1D channels in Mo₈-CP

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  2.3   IR spectrum

  The IR spectrum (Fig.5) of Mo₈-CP showed characteristic peaks of the polyoxoanion (Mo₈O₂₆)^4- in a range of 667-943 cm^-1, which are ascribed to the vibrations of ν(Mo—O_t) and ν(Mo—O—Mo). In addition, the peaks at 3 025-3 128 cm^-1 are attributed to the vibrations of the C—H in phenyl/triazole/imidazole rings of the BBTZ ligand or (Emim)⁺. The peaks at 2 960 and 1 380 cm^-1 are assigned to the ν(C—H) and δ(C—H) in the CH₃ group of (Emim)⁺. Peaks in the regions of 1 423-1 620 cm^-1 may belong to the vibrations of the C=C, C=N, and N=N in phenyl/triazole/imidazole rings of the BBTZ ligand or (Emim)⁺. The peaks at 3 450 cm^-1 is attributed to the vibrations of H₂O.

  Figure 5

  

  Figure 5.  IR spectrum of Mo₈-CP

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  2.4   TG analysis

  The TG curve of Mo₈-CP showed one weight loss step (Fig.6). The whole weight loss of 36% occurring from 255 to 560 ℃ is mainly ascribed to the decomposition and loss of BBTZ ligands and counter cation (Emim)⁺ in Mo₈-CP (Calcd. 35.67%). Additionally, a weight loss of approximately 11.74% occurring between 255 and around 320 ℃ can be attributed to the loss of counter cation (Emim)⁺. Thereafter, the decomposition and loss of BBTZ ligands commence, leading to the collapse of the framework.

  Figure 6

  

  Figure 6.  TG curve of Mo₈-CP

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  2.5   Ion exchange property

  Microporous CPs with negatively charged groups can achieve precise replacement of cations in the channels through ion exchange strategies, thereby constructing functional composite materials^[42-44]. The advantage of this strategy lies in its ability to directionally introduce a variety of functional cationic species into the channel structure, providing an efficient approach for regulating material functions. Notably, Mo₈-CP not only has a stable negatively charged framework, but its crystal structure can remain stable and insoluble in water and various organic solvents. This excellent chemical stability provides a reliable guarantee for the structural integrity during the ion exchange process. Therefore, the counterion (Emim)⁺ filled in its channels can be exchanged through a mild ion exchange process, creating conditions for expanding the material′s functions. Considering that the BBTZ organic ligand contained in Mo₈-CP may have luminescent properties, we systematically investigated its luminescent performance. The experimental results showed that under an excitation wavelength of 375 nm, the maximum emission peak of Mo₈-CP was located at 423 nm (Fig.7). A comparison with the luminescent spectrum of the free BBTZ ligand (whose maximum emission peak was at 427 nm, Fig.8) reveals that the emission characteristics of the two are homologous, both originating from electronic transitions within the aromatic system of the BBTZ ligand—specifically, π-π* or π-n ligand-centered transitions^[45]. This further confirms that the emission peak is a characteristic emission of the organic ligand. Furthermore, through observation with an upright fluorescence microscope, blue fluorescence emitted by Mo₈-CP could be directly observed (inset of Fig.7). The above results indicate that this polyoxometalate-based coordination polymer has potential application value in the field of fluorescent materials.

  Figure 7

  

  Figure 7.  Emission spectrum of Mo₈-CP

  The upper left inset shows the corresponding fluorescence microscopy image of the sample; The upper right inset indicates the excitation wavelength (λ_ex=375 nm)

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  Figure 8

  

  Figure 8.  Emission spectrum of free BBTZ excited at 368 nm

  The upper right inset indicates the excitation wavelength (λ_ex=368 nm).

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  Based on the structural characteristics (capable of ion exchange) and luminescent properties (intrinsically emitting blue light) of Mo₈-CP, we further designed rare-earth ion exchange experiments with the aim of regulating its luminescent performance. The specific operation was as follows: the crystals of Mo₈-CP were immersed in an ethanol solution containing Eu³⁺. After a mild exchange for 24 h, the Eu³⁺-exchanged composite material was obtained through multiple centrifugation, washing, and drying steps. The characterization of its fluorescent properties showed that under excitation at 332 nm, the composite material had characteristic emission peaks at 592, 616, 653, and 697 nm, which correspond to the ⁵D₀→⁷F₁, ⁵D₀→⁷F₂, ⁵D₀→⁷F₃, and ⁵D₀→⁷F₄ transitions of Eu³⁺, respectively (Fig.9)^[46-47]. Meanwhile, obvious red fluorescence could be observed under an upright fluorescence microscope (inset of Fig.9), confirming that a red fluorescent material was successfully constructed through ion exchange. Using the same experimental approach, the Tb³⁺-exchanged composite material was obtained by replacing the exchange ion with Tb³⁺. Under excitation at 248 nm, the emission peaks at 490, 544, 584, and 622 nm in its fluorescence spectrum (Fig.10) correspond to the ⁵D₄→⁷F₆, ⁵D₄→⁷F₅, ⁵D₄→⁷F₄, and ⁵D₄→⁷F₃ transitions of Tb³⁺, respectively^[49]. In addition, green fluorescence could be observed under an upright fluorescence microscope (inset of Fig.10), indicating the successful preparation of a green fluorescent material.

  Figure 9

  

  Figure 9.  Characteristic luminescence spectrum of Eu(Ⅲ) in Mo₈-CP after cation exchange, excited at 332 nm

  The upper right inset indicates the excitation wavelength (λ_ex=332 nm); The upper left inset shows the fluorescence microscopy image of Mo₈-CP after Eu³⁺ exchange.

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  Figure 10

  

  Figure 10.  Characteristic luminescence spectrum of Tb(Ⅲ) in Mo₈-CP after cation exchange, excited at 248 nm

  The upper right inset indicates the excitation wavelength (λ_ex=248 nm); The upper left inset shows the fluorescence microscopy image of Mo₈-CP after Tb³⁺ exchange.

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  To confirm the replacement of (Emim)⁺ cations in the framework by Eu³⁺ and Tb³⁺ through ion exchange, C, H, and N elemental analyses were performed on Mo₈-CP after the ion exchange process. The results indicated a significant decrease in the C, H, and N contents of the exchanged samples compared to the original material, as shown in Table 3. Based on the assumption that each Eu³⁺/Tb³⁺ replaces three (Emim)⁺ to maintain charge balance, the elemental analysis data were used to calculate the extent of exchange. It was found that approximately 1.447 (Emim)⁺ per Mo₈-CP molecule were displaced in the case of Eu³⁺ exchange, corresponding to an exchange ratio of 72.35% (based on (Emim)⁺). For Tb³⁺ exchange, about 1.323 (Emim)⁺ per Mo₈-CP molecule were replaced, giving an exchange ratio of 66.15%. These results strongly support the occurrence of ion exchange. Furthermore, energy-dispersive X-ray spectroscopy (EDS) elemental mapping confirmed the uniform distribution of Eu and Tb elements in Mo₈-CP after ion exchange (Fig.11 and 12). To further evaluate the structural stability of the material during ion exchange, the IR spectra (Fig.13) and PXRD patterns (Fig.14) of Mo₈-CP before and after exchange were compared. The results showed that the position and intensity of the absorption peaks and the diffraction peaks remained largely unchanged, indicating that the framework structure was preserved without collapse and exhibited good stability. This study not only clearly demonstrates the excellent ion exchange capability of Mo₈-CP but also reveals its potential for modulating luminescence properties through ion exchange, providing an effective strategy for the design and preparation of multifunctional fluorescent materials.

  Table 3

  

  Table 3.  C, H, and N elemental analysis of Mo₈-CP before and after ion exchange with Eu³⁺/Tb³⁺

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  Material*	Mass fraction/%
  	C	H	N
  Mo8-CP	21.96	2.47	11.35
  Mo8-CP-Eu3+	17.73	1.69	9.85
  Mo8-CP-Tb3+	18.12	1.74	9.98
  *Mo8-CP-Eu3+: Mo8-CP after Eu3+ exchange; Mo8-CP-Tb3+: Mo8-CP after Tb3+ exchange.

  Figure 11

  

  Figure 11.  SEM images and the EDS elemental mappings of C, N, O, Cu, Mo, and Eu/Tb for Mo₈-CP after Eu³⁺ exchange (a) or Tb³⁺ exchange (b)

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  Figure 12

  

  Figure 12.  EDS profiles of Mo₈-CP after Eu³⁺ exchange (a) or Tb³⁺ exchange (b)

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  Figure 13

  

  Figure 13.  IR spectra of Mo₈-CP: as-synthesized (black) and recovered after cation exchange (red)

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  Figure 14

  

  Figure 14.  PXRD patterns of Mo₈-CP: simulated (black), measured (red), and recovered after cation exchange (blue)

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  3.   Conclusions

  In summary, we successfully prepared a POM-based microporous CP based on Cu²⁺, bridging ligands (BBTZ), and octamolybdate polyoxoanions (Mo₈O₂₆)^4- by ionicothermal synthesis. In Mo₈-CP, the interlocking 1D chains connect with (Mo₈O₂₆)^4- polyoxoanions extending in two directions to form a 3D CP. Given that the formation of entangled networks during the construction of CP is unpredictable and unavoidable, introducing a large-sized organic cationic template agent into the reaction system can partially prevent entanglement in POM-based microporous CPs. Therefore, the ionic liquid (Emim)Br serves dual roles in the reaction system: as a reaction solvent and as a large organic cationic template agent, providing an effective strategy for the design and synthesis of POM-based microporous CPs. Based on this research, it is expected that novel POM-based CPs with intriguing structural topologies, cavities, and functional properties can be obtained. This work is ongoing in our group.

  
  
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  Figure 1  ORTEP diagram of the basic structural units in Mo₈-CP with thermal ellipsoids at 30% probability displacement

  All H atoms are omitted for clarity; Symmetry code: ⁱ -x, -y, 1-z.

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  Figure 2  Structure of loop-containing 1D chains in Mo₈-CP

  Symmetry codes: ⁱ 1/2-x, 1/2+y, 3/2-z; ⁱⁱ 1/2+x, 1/2+y, z; ⁱⁱⁱ 1/2-x, 1/2+y, 1/2-z; ^iv 1/2+x, 1/2+y, -1+z.

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  Figure 3  Two 1D-Mo₈ chains possessing two different extended directions in Mo₈-CP

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  Figure 4  (a) Structural illustration of 3D POMs-based anionic framework with the pore along the b-axis in Mo₈-CP; (b) Structural illustration of the 3D POMs-based CP with (Emim)⁺ cations occupying the 1D channels in Mo₈-CP

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  Figure 5  IR spectrum of Mo₈-CP

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  Figure 6  TG curve of Mo₈-CP

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  Figure 7  Emission spectrum of Mo₈-CP

  The upper left inset shows the corresponding fluorescence microscopy image of the sample; The upper right inset indicates the excitation wavelength (λ_ex=375 nm)

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  Figure 8  Emission spectrum of free BBTZ excited at 368 nm

  The upper right inset indicates the excitation wavelength (λ_ex=368 nm).

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  Figure 9  Characteristic luminescence spectrum of Eu(Ⅲ) in Mo₈-CP after cation exchange, excited at 332 nm

  The upper right inset indicates the excitation wavelength (λ_ex=332 nm); The upper left inset shows the fluorescence microscopy image of Mo₈-CP after Eu³⁺ exchange.

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  Figure 10  Characteristic luminescence spectrum of Tb(Ⅲ) in Mo₈-CP after cation exchange, excited at 248 nm

  The upper right inset indicates the excitation wavelength (λ_ex=248 nm); The upper left inset shows the fluorescence microscopy image of Mo₈-CP after Tb³⁺ exchange.

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  Figure 11  SEM images and the EDS elemental mappings of C, N, O, Cu, Mo, and Eu/Tb for Mo₈-CP after Eu³⁺ exchange (a) or Tb³⁺ exchange (b)

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  Figure 12  EDS profiles of Mo₈-CP after Eu³⁺ exchange (a) or Tb³⁺ exchange (b)

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  Figure 13  IR spectra of Mo₈-CP: as-synthesized (black) and recovered after cation exchange (red)

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  Figure 14  PXRD patterns of Mo₈-CP: simulated (black), measured (red), and recovered after cation exchange (blue)

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  Table 1.  Crystal data and structure refinement for Mo₈-CP

  Parameter	Mo8-CP		Parameter	Mo8-CP
  Formula	C36H46N16O26Mo8Cu		Z	4
  Formula weight	1 949.95		μ/mm-1	1.982
  Crystal system	Monoclinic		F(000)	3 788
  Space group	C2/c		Total reflection	23 785
  a/nm	2.168 0(4)		Rint	0.039 1
  b/nm	1.254 1(3)		GOF	1.025
  c/nm	2.336 0(5)		R1 [I>2σ(I)]a	0.037 2
  β/(°)	102.04(3)		wR2 (all data)b	0.089 1
  V/nm3	6.212(2)
  a R1=∑||Fo|-|Fc||/∑|Fo|; b wR2=∑[w(Fo2-Fc2)2]/∑[w(Fo2)2]1/2.

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  Table 2.  Selected bond lengths (nm) and angles (°) of Mo₈-CP

  Cu1—N1i	0.201 0(5)	Cu1—N1	0.201 0(5)	Cu1—N4i	0.201 1(5)
  Cu1—N4	0.201 1(5)	Cu1—O8i	0.249 9(0)	Cu1—O8	0.249 9(0)
  N1i—Cu1—N1	180.0(2)	N1i—Cu1—N4	89.93(19)	N1i—Cu1—N4i	90.07(19)
  N1—Cu1—N4i	89.9(2)	N1—Cu1—N4	90.07(19)	N4i—Cu1—N4	180.0
  N1—Cu1—O8	88.33(31)	N4—Cu1—O8	94.08(81)	N1i—Cu1—O8	91.66(71)
  N4i—Cu1—O8	85.91(81)	O8—Cu1—O8i	179.98(9)
  Symmetry code: i -x, -y, 1-z.

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  Table 3.  C, H, and N elemental analysis of Mo₈-CP before and after ion exchange with Eu³⁺/Tb³⁺

  Material*	Mass fraction/%
  	C	H	N
  Mo8-CP	21.96	2.47	11.35
  Mo8-CP-Eu3+	17.73	1.69	9.85
  Mo8-CP-Tb3+	18.12	1.74	9.98
  *Mo8-CP-Eu3+: Mo8-CP after Eu3+ exchange; Mo8-CP-Tb3+: Mo8-CP after Tb3+ exchange.

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