English

• 0.   Introduction

  Over the past few decades, metal-organic frameworks (MOFs) have attracted widespread attention in the fields of gas storage, catalysis, sensing, and optics due to their unique porous structures, high specific surface areas, and tunable physicochemical properties^[1-5]. The structural diversity of MOFs makes them an ideal platform for the research and development of new functional materials^[6-10]. So far, thousands of MOF materials have been reported^[11-13]. They are typically constructed through self-assembly from metal ions or metal clusters and organic ligands.

  In recent years, an anionic metal-organic cage (Ti₄L₆, L^4-=embonate) with solubility, stability, and rich coordination sites has been proven to be an excellent secondary building unit^[14-18]. The abundant naphthalene rings on its surface facilitate the formation of π-π stacking interactions, and its high negative charge endows it with high binding efficiency for metal ions or cationic units^[19-22]. Using it as a precursor, a series of cage-based MOF materials have been successfully synthesized, some of which exhibited excellent third-order nonlinear optical (NLO) properties^[21-23]. As we all know, π-conjugated organic ligands possess significant advantages in third-order NLO properties^[24-28], which are mainly reflected in enhanced electronic delocalization, efficient charge transfer, narrow band gaps, and low-energy excited states, etc. These characteristics make π-conjugated organic ligands an ideal choice for developing high-performance third-order NLO materials, with broad application prospects. Against this backdrop, combining the Ti₄L₆ cage with π-conjugated organic ligands offers a promising strategy for developing cage-based MOF materials with enhanced NLO properties.

  In this work, we synthesized a cage-based layered MOF (Me₂CH₂)₂[Mg₃(Ti₄L₆)(tipa)(H₂O)₁₂] (PTC-378) using Ti₄L₆ tetrahedral cages as the building units, combined with Mg²⁺ ions and tris[4-(1H-imidazol-1-yl)phenyl]amine (tipa) ligands (Scheme 1). The structure of PTC-378 was well-characterized, and the powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA) were also studied. The structural feature and third-order NLO property of the material were thoroughly investigated to explore its potential application in advanced optical devices.

  Scheme 1

  

  Scheme 1.  Synthetic route of PTC-378

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  1.   Experimental

  1.1   Materials and methods

  All reagents were purchased commercially and used without further purification. PTC-101, as a starting material for Ti₄L₆ cages, was massively synthesized by the method reported in our previous work^[14]. The tipa ligand was synthesized according to the literature^[29]. Thermal stability studies were carried out on a NETSCHZ STA-449C thermoanalyzer with a heating rate of 10 ℃·min^-1 under a nitrogen atmosphere. PXRD patterns were recorded on a Rigaku Dmax/2500 X-ray diffractometer operating at 40 kV and 100 mA, using Cu Kα radiation (λ=0.154 056 nm). The patterns were scanned over an angular range of 3°-45° (2θ) with a step length of 0.05° (2θ).

  1.2   Synthesis of PTC-378

  PTC-101 (80 mg, 0.02 mmol) and magnesium acetate (40 mg, 0.282 mmol) were dissolved in 6 mL of H₂O/1, 4-dioxane (1∶2, V/V) mixed solvents. Then, 1 mL of ethanol solution containing 20 mg of tipa was added. The mixture was heated at 80 ℃ for 3 d, and red block crystals of PTC-378 were obtained (Fig.1). Yield: ca. 63 mg. The experiment has high reproducibility.

  Figure 1

  

  Figure 1.  Photos of PTC-378

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  Scale-up synthesis of PTC-378. PTC-101 (800 mg, 0.02 mmol) and magnesium acetate (400 mg, 0.282 mmol) were dissolved in 60 mL of H₂O/1, 4-dioxane (1∶2, V/V) mixed solvents. Then, 10 mL of ethanol solution containing 200 mg of tipa was added. The mixture was heated at 80 ℃ for 3 d, and red block crystals of PTC-378 were obtained (Fig.1). Yield: ca. 527 mg.

  Note: PTC-101 was first dissolved in a mixed solvent of H₂O/1, 4-dioxane before adding magnesium acetate. The tipa ligand must be added last.

  1.3   X-ray crystallography

  Crystallographic data of PTC-378 were collected on a Supernova single-crystal diffractometer equipped with graphite-monochromatic Ga Kα (λ=0.134 05 nm) at 100 K. Absorption correction was applied using SADABS. The structure was solved by the direct method and refined by full-matrix least-squares on F² using SHELXTL. In the structure, cations and free guest molecules were highly disordered and could not be located. The diffused electron densities resulting from these residual cations/anions and guest molecules were removed from the data set using the SQUEEZE routine of PLATON and refined further using the resulting data. Crystal data and details of data collection and refinement of PTC-378 were summarized in Table 1.

  Table 1

  

  Table 1.  Crystallographic data and structure refinement details for compound PTC-378

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  Parameter	PTC-378		Parameter	PTC-378
  Formula	C164H116O48Mg3Ti4		μ / mm-1	1.429
  Formula weight	3 203.63		F(000)	19 900
  Crystal system	Trigonal		θ range / (°)	4.214-97.708
  Space group	R3c		Total and unique reflections	45 462 and 9 092
  a / nm	2.188 21(2)		Observed data [(I > 2σ(I)]	7 536
  b / nm	2.188 21(2)		Rint	0.035 8
  c / nm	13.394 75(16)		Data, restraint, number of parameters	9 092, 49, 695
  V / nm3	55.544 8(12)		R1, wR2 [I > 2σ(I)]	0.099 5, 0.277 6
  Z	12		R1, wR2 (all data)	0.113 3, 0.290 1
  Dc / (g·cm-3)	1.155		Goodness-of-fit on F 2	1.038

  1.4   Preparation of PTC-378 dispersed PDMS film

  The crystals of PTC-378 need to be dried in the air before sampling. Firstly, 1 mg crystals were ground into fine powder in an agate mortar, and then mixed with 1.5 g PDMS (Sylgard 184, polydimethylsiloxane), and the sample was evenly dispersed by magnetic stirring for several hours. Secondly, 1/10 mass of the specific curing agent was added into the mixture and continued to stir evenly for about 10 min. Thirdly, 1 g of the mixture was taken and put into a specific membrane. Under the action of gravity, the mixture was poured into the mold and then placed at room temperature for 0.5 h to eliminate bubbles. Finally, the membrane utensil was put into a 60 ℃ oven for 5 h to obtain films for testing. In addition, we also tested PTC-378 in different dispersion concentrations. Using the same preparation method, 1 mg, 2 mg, and 3 mg crystals were used to make a film, respectively.

  2.   Results and discussion

  2.1   Crystal structure

  Herein, the simple cage compound PTC-101 ([(Me₂NH₂)₈(Ti₄L₆)]·Guests) reported earlier^[14] serves as a unique ligand in the synthesis of PTC-378. The tipa ligand and Mg(CH₃COO)₂·4H₂O were added to the H₂O/1, 4-dioxane solution of PTC-101, which was heated at 80 ℃ for 3 d, forming the red block crystals of PTC-378. Single crystal structural analysis reveals that PTC-378 crystallizes in the trigonal space group R$ \stackrel{-}{3} $c. As shown in Fig.2a and 2b, the asymmetric unit of PTC-378 possesses one third of the Ti₄L₆ cage, one third of the tipa ligand, one Mg²⁺ ion, and four coordinated H₂O molecules (Solvents and (Me₂NH₂)⁺ countercations could not be located because of high disorder). In PTC-378, each Mg center is six-coordinated by one carboxyl O atom from the Ti₄L₆ cage, one N atom from one tipa ligand, and four coordinated H₂O molecules to build a distorted tetrahedral coordination geometry. The Mg—O bond distances vary from 0.204 2 to 0.215 0 nm, and the Mg—N bond distance is 0.214 0 nm. In PTC-378, each tipa ligand adopts a saturated triangular coordinated mode, and each Ti₄L₆ cage captures three Mg²⁺ ions by one vertex of the tetrahedron. The connectivity between Ti₄L₆ cages, Mg²⁺ ions, and tipa ligands creates a 2D layer structure (Fig.2c and 2d). Such layers further pack into a 3D dense superstructure (Fig.2e and 2f), and adjacent layers are stabilized by the π⋯π (ca. 0.36 nm), C—H⋯π (C⋯π: ca. 0.39 nm), and O—H⋯O (O⋯O: 0.26-0.29 nm) interactions (Fig.2g).

  Figure 2

  

  Figure 2.  Crystal structure of PTC-378: (a) the asymmetric unit and (b) coordinated environment; (c, d) layer structure; (e, f) 3D packing framework; (g) interlayer interactions

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  It can be seen that Ti₄L₆ cages, as an emerging class of building units, will become an important choice for constructing high-performance MOFs due to their excellent chemical stability and rich coordination chemistry characteristics. By effectively connecting Ti₄L₆ cages with other metal ions or auxiliary organic ligands, MOFs with ideal pore structures and functional properties can be constructed. We expect that through this study, new ideas and methods for the application of Ti₄L₆ cages in the field of MOFs will be provided, and the practical application of such materials in multiple fields will be promoted.

  2.2   TGA and PXRD characterization

  In order to evaluate the thermal stability and phase purity of PTC-378, TGA and PXRD were performed. The TGA curve of PTC-378 was measured in a N₂ atmosphere, with the temperature ranging from room temperature to 800 ℃ at a heating rate of 10 ℃·min^-1. The TGA curve of PTC-378 indicated an approximate 20% weight loss below 400 ℃ (Fig.3a), attributed to the removal of guest and coordinated H₂O molecules, after which the structure began to decompose. The residuals following the decomposition of the compound were likely to be titanium dioxide (TiO₂) and magnesium oxide (MgO).

  Figure 3

  

  Figure 3.  TGA curve (a) and PXRD patterns (b) of PTC-378

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  The sample was allowed to dry naturally, followed by PXRD analysis. The PXRD pattern of PTC-378 (Fig.3b) confirmed its phase purity, as it closely matches the pattern simulated from single-crystal data. The test result also demonstrates that PTC-378 is stable in air. Additionally, it is found to be stable in CH₂Cl₂ solvent.

  2.3   Third-order NLO properties

  To eliminate the potential impact of solvents on PTC-378 and to maintain the stability of its structure, the crystals of PTC-378 were uniformly dispersed into PDMS. Subsequently, flexible and transparent film was fabricated and designated as PDMS-PTC-378 (Fig.4a). The film showed high transparency, similar thickness, and good surface uniformity. The third-order NLO property of PDMS-PTC-378 film was measured by a typical open-aperture Z-scan system with a nanosecond laser at 532 nm. The experimental result indicates that PDMS-PTC-378 film displayed a characteristic reverse saturation absorption (RSA) behavior, which is indicative of a significant optical limiting property. At Z=0, the minimum normalized transmittance (T_min) of PDMS-PTC-378 was 0.85. For better comparison, we also tested third-order NLO properties of Ti₄L₆ starting material (PDMS-PTC-101) using the same thin film preparation method. Apparently, it does not possess a notable optical limiting effect. Obviously, PTC-378 displayed an obvious optical limiting effect as compared to the Ti₄L₆ cage, which can be attributed to the strong π-π stacking interactions in its structure and the introduction of π-conjugated tipa ligands. In addition, we also studied the effect of dispersion concentration on the optical limiting performance (Fig.4b). The experimental result indicates that the optical effects of PDMS-PTC-378 were distinctly influenced by its concentration. The concentration-dependent third-order NLO response may be related to the concentration of the sample in the thin film. Within a certain range, the higher the concentration, the greater the number of molecules per unit volume participating in nonlinear interactions, thereby enhancing the third-order nonlinear susceptibility. To quantitatively evaluate the NLO properties of the sample, the nonlinear absorption coefficient (β) of the sample was calculated based on the results of the fitted open-aperture Z-scan experiment. The β of PDMS-PTC-378 was 1.6×10^-10 m·W^-1. The optical limiting curves confirmed that the normalized transmittance of the samples depended on the input laser pulse energy according to the equations (Fig.4c). Moreover, the output fluence of PTC-378 linearly increased at low-incident fluence. However, it deviated from the linearity at high-incident fluence, which indicates the typical optical limiting effect (Fig.4d). A comparison table is given for how the performance of PTC-378 compares with that of some materials that have been used for NLO research, as shown in Table 2. It can thus be seen that the synergistic interaction between Ti₄L₆ cages and π-conjugated ligands significantly enhances the NLO performance of PTC-378. The aforementioned findings indicate that PTC-378 holds potential as a candidate for optical limiting applications.

  Figure 4

  

  Figure 4.  (a) Open-aperture Z-scan (points) and theoretical fit (solid lines) curves of PDMS-PTC-101 and PDMS-PTC-378 films at 532 nm (inset: photos of the films); (b) Effect of dispersion concentration of PTC-378 on NLO response of PDMS-PTC-378; (c) Optical limiting curves and (d) the curves of output fluence vs input fluence of the samples dispersing in DMF solution at 532 nm (input energy: 100 μJ)

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  Table 2

  

  Table 2.  Comparison of NLO properties on Z-scan measurements of transmittance for open-aperture conditions at 532 nm

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  Compound	Tmin / %	β / (cm·GW-1)	Medium	Ref.
  [Al16(OnPr)16(μ2-OH)8(2-NA)24] (AlOC-134)	84	22	PDMS	[30]
  {[NH4][WS4Cu4(tpb)Cu2(CN)5]·DMF}n	92	0.12	DMF	[31]
  [In(FcDCA)(Im)(H2O)]	90	7	PDMS	[32]
  BIF-141@PDMS	20	900	PDMS	[33]
  ZnTPyP-1/PDMS-0.15%	17	1 350	PDMS	[34]
  (Me2CH2)2[Mg3(Ti4L6)(H2O)12(tipa)] (PTC-378)	85	16	PDMS	This work

  3.   Conclusions

  In this work, we successfully synthesized a cage-based 2D layered material (PTC-378) using Ti₄L₆ cages combined with Mg²⁺ and triphenylamine-based ligand. The third-order NLO property of PTC-378 was systematically investigated using Z-scan techniques, revealing a significant enhancement in NLO performance compared to the Ti₄L₆ raw material. Our findings demonstrate that the synergistic interaction between Ti₄L₆ cages and π-conjugated organic ligands plays a crucial role in enhancing the NLO properties of the materials. This work not only highlights the potential of Ti₄L₆ cages as versatile building units for constructing functional materials but also provides valuable insights into the design and synthesis of advanced 2D materials with tailored optical properties. Additionally, PDMS films possess high optical transparency, good stability, and low cost, making them promising candidates for integration with functional ligands to develop advanced third-order nonlinear optical materials.

  
  
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• 

  Scheme 1  Synthetic route of PTC-378

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  Figure 1  Photos of PTC-378

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  Figure 2  Crystal structure of PTC-378: (a) the asymmetric unit and (b) coordinated environment; (c, d) layer structure; (e, f) 3D packing framework; (g) interlayer interactions

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  Figure 3  TGA curve (a) and PXRD patterns (b) of PTC-378

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  Figure 4  (a) Open-aperture Z-scan (points) and theoretical fit (solid lines) curves of PDMS-PTC-101 and PDMS-PTC-378 films at 532 nm (inset: photos of the films); (b) Effect of dispersion concentration of PTC-378 on NLO response of PDMS-PTC-378; (c) Optical limiting curves and (d) the curves of output fluence vs input fluence of the samples dispersing in DMF solution at 532 nm (input energy: 100 μJ)

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  Table 1.  Crystallographic data and structure refinement details for compound PTC-378

  Parameter	PTC-378		Parameter	PTC-378
  Formula	C164H116O48Mg3Ti4		μ / mm-1	1.429
  Formula weight	3 203.63		F(000)	19 900
  Crystal system	Trigonal		θ range / (°)	4.214-97.708
  Space group	R3c		Total and unique reflections	45 462 and 9 092
  a / nm	2.188 21(2)		Observed data [(I > 2σ(I)]	7 536
  b / nm	2.188 21(2)		Rint	0.035 8
  c / nm	13.394 75(16)		Data, restraint, number of parameters	9 092, 49, 695
  V / nm3	55.544 8(12)		R1, wR2 [I > 2σ(I)]	0.099 5, 0.277 6
  Z	12		R1, wR2 (all data)	0.113 3, 0.290 1
  Dc / (g·cm-3)	1.155		Goodness-of-fit on F 2	1.038

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  Table 2.  Comparison of NLO properties on Z-scan measurements of transmittance for open-aperture conditions at 532 nm

  Compound	Tmin / %	β / (cm·GW-1)	Medium	Ref.
  [Al16(OnPr)16(μ2-OH)8(2-NA)24] (AlOC-134)	84	22	PDMS	[30]
  {[NH4][WS4Cu4(tpb)Cu2(CN)5]·DMF}n	92	0.12	DMF	[31]
  [In(FcDCA)(Im)(H2O)]	90	7	PDMS	[32]
  BIF-141@PDMS	20	900	PDMS	[33]
  ZnTPyP-1/PDMS-0.15%	17	1 350	PDMS	[34]
  (Me2CH2)2[Mg3(Ti4L6)(H2O)12(tipa)] (PTC-378)	85	16	PDMS	This work

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