Two novel emissive Pb(II) coordination polymers: syntheses, structures, properties and WLED application

Abstract

Recently, the optical application of coordination polymers (CPs) has received increasing attention. In this paper, two novel Pb(II)-CPs were successfully synthesized under solvothermal conditions. Single-crystal diffraction analysis shows that compound 1 is an orthorhombic crystal system in the Pbca space group, and compound 2 is a monoclinic crystal system in the C2/c space group. Furthermore, we characterized compounds 1 and 2 by powder X-ray diffraction, infrared spectroscopy, solid-state UV-visible absorption spectroscopy and elemental analyses. The optical properties of compounds 1 and 2 were studied in depth, with compound 1 exhibiting yellow luminescence and compound 2 exhibiting blue luminescence. In addition, we mixed compound 1 with a commercial blue phosphor to obtain a white light-emitting diode (WLED) with adjustable correlated color temperature (CCT) and color rendering index (CRI). Therefore, the results suggest that Pb(II)-CPs have potential applications in the WLED field.

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1. Introduction

Coordination polymers (CPs) are a class of complexes with a highly regular infinite network structure formed by organic ligands and metal ions. Metal cation centers coordinate with organic ligands to obtain coordination units, which are repeated to create one-, two- or three-dimensional structures. To date, CPs have attracted significant research interest from scientists due to their functional tunable and potential applications in various fields, such as bioimaging,^1,2 sensors,^3–9 drug delivery,^10,11 catalysis,^12–15 solar energy conversion¹⁶ and luminescence.^17–23 The final structure and properties of coordination polymers are influenced by various factors such as metal ions, organic ligands, solvents, reaction temperature and pH.^24–28 Thus, it is possible to synthesize different CPs by adjusting the above factors and the diversity of the structures and properties of CPs is revealed.

Recently, luminescent CPs have become a new popular research area, widely used in lighting,²⁹ scintillators,^30–37 fluorescence sensing^38–41 and other fields. The luminescence properties of CPs may be the result of surface defects of compounds, ligand centers, metal-to-metal charge transfer (MMCT), ligand-to-metal charge transfer (LMCT), metal-to-ligand charge transfer (MLCT), etc.^42–46 The Pb(II) cation is an important metal ion characterized by a large ionic radius, wide range of coordination numbers and strong coordination ability with oxygen and nitrogen atoms. Currently, Pb(II)-based CPs have been determined to be excellent luminescent materials, and some related studies have been reported.^{17,19,20,22,34,36} Recently, Fei's group reported a series of reticular metal–organic framework (MOF)-type lead halide luminescent ferroelectrics with ultrastability.⁴⁷ On the other hand, the nitrogen-rich ligand 4,5-di(1H-tetrazol-5-yl)-2H-1,2,3-triazole (H₃L) synthesized in our previous work is also one of the well functional linkers due to the intense rigidity of the aromatic moiety.^48,49 Based on the above considerations, we hope that the combination of Pb(II) and H₃L is conducive to forming CPs with novel structures and good luminescence properties.

Therefore, in this work, we have successfully synthesized two new CPs, [Pb₃(L)₂(H₂O)₂] (1) and [Pb(H₂L)₂(H₂O)₂]·H₂O (2), using a nitrogen-rich ligand H₃L and Pb(II) ions under solvothermal conditions (Fig. S1†). Compounds 1 and 2 were characterized by single-crystal X-ray diffraction, powder X-ray diffraction, infrared spectroscopy, solid-state UV-visible absorption and elemental analysis. The time resolved luminescence spectra and luminescence properties of compounds 1 and 2 were also investigated and analyzed, respectively, with compound 1 exhibiting yellow light emission and compound 2 exhibiting blue light emission. Interestingly, under UV light excitation, the CP1-based WLED shows white emissions with excellent CIE color coordinates and a high color rendering index.

2. Experimental section

2.1. Syntheses of compounds 1 and 2

2.1.1. Synthesis of [Pb₃(L)₂(H₂O)₂] (1). A mixture of PbI₂ (0.1 mmol, 0.046 g), H₃L (0.1 mmol, 0.020 g), H₂O (8 mL), and DMA (2 mL) was added to a 15 mL autoclave and heated for 72 hours at 120 °C. After cooling, the mixture was filtered to obtain colorless crystals of 1. The product was washed with a small amount of methanol and dried in air. The yield was 86.9% based on PbI₂. Anal. calcd for C₈H₄N₂₂O₂Pb₃ (Mr = 1061.90): C, 9.05%; H, 0.38%; N, 29.02%. Found: C, 9.65%; H, 0.87%; N, 28.72%. IR (KBr, cm⁻¹): 3434(s), 1596(w), 1413(w), 1130(m), 993(s), 759(w).

2.1.2. Synthesis of [Pb(H₂L)₂(H₂O)₂]·H₂O (2). The preparation process of compound 2 was similar to that of compound 1 except that H₂O (10 mL) and HCl (0.2 mL) were used as solvents and heating was done for 72 hours at 160 °C. Colorless crystals of 2 were obtained in a 78.9% yield based on PbI₂. Anal. calcd for C₈H₁₀N₂₂O₃Pb (Mr = 669.57): C, 14.35%; H, 1.51%; N, 46.03%. Found: C, 14.77%; H, 1.98%; N, 45.68%. IR (KBr, cm⁻¹): 3433(s), 3028(w), 1623(w), 1417 (w), 1337(w), 1117(w), 964(s).

3. Results and discussion

3.1. Description of the crystal structures

3.1.1. Structure of [Pb₃(L)₂(H₂O)₂] (1). Single-crystal diffraction analysis indicates that compound 1 crystallizes in an orthorhombic crystal system with the Pbca space group (Table 1). Each asymmetric unit consists of one and a half Pb(II) ions, one deprotonated L³⁻ ligand and one water coordinated molecule. As shown in Fig. 1a, Pb1 reveals an eight-coordination with N1, N11, N4^#1, N5^#1, N3^#4, N7^#5, N10^#2 and O1W from five different coordination units, showing a slightly distorted dodecahedron geometrical configuration. Besides, Pb1 and the L³⁻ ligand are interconnected to form a planar two-dimensional layer (Fig. 1b). Pb2 is four-coordinated to four nitrogen atoms (N8, N9, N6^#3 and N2^#6) from three L³⁻ ligands, giving a distorted tetrahedron. The bond length deviation (Δd) was calculated using the formula in the ESI† to assess the extent of distortion of the coordination geometry. The values of Δd₁ (Pb1) and Δd₂ (Pb2) are 0.0029 and 0.0060, respectively. The lower value of Δd indicates a smaller distortion of the coordination geometry. The result suggests that the coordination geometry of Pbl is less distorted than that of Pb2. The adjacent layers are extended to a three-dimensional framework by Pb2–N2 bonds (Fig. 1c). In the whole framework, there are C–H⋯π interactions in the aromatic rings. The distance between the centers of the aromatic ring (N5, N6, N7, N8, and C4) and the aromatic ring (N5, N6, N7, N8, and C4) of the other asymmetric unit is 3.81 Å. Meanwhile, the slip is 2.1 Å, and the dihedral angle is 26.8°. The presence of C–H⋯π interactions increases the overall stability of the structure. The whole structure can be simplified by topological analysis. Pb1 and Pb2 are considered to be 5-connected and 3-connected nodes, respectively, while the organic ligand is an 8-connected node. The result reveals a 3,5,8-connected framework with symbols of {4¹⁰·6¹³·8⁵}{4³}{4⁸·6²} (Fig. 1d).

Table 1 Crystal data and structure refinement details of compounds 1 and 2

Empirical formula	C8H4N22O2Pb3	C8H10N22O3Pb
Formula weight	1061.90	669.57
Crystal system	Orthorhombic	Monoclinic
Space group	Pbca	C2/c
a/Å	7.3616(4)	11.0514(6)
b/Å	14.2632(6)	8.2260(6)
c/Å	19.3471(8)	19.8646(12)
β/°	90	90.055(2)
Volume/Å^3	2031.44(16)	1805.9(2)
Z	4	4
D c (g cm^−3)	3.472	2.463
μ/mm^−1	24.867	9.420
F(000)	1872.0	1272.0
R(int)	0.0556	0.0329
GOF on F^2	1.088	1.219
R 1 [I ≥ 2σ(I)]	0.0523	0.0384
wR2 [all data]	0.1696	0.0985

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Fig. 1 (a) Coordination environment diagram of Pb(II) ions. (b) View of the 2D layer. (c) View of the 3D framework. (d) Topological structure.

3.1.2. Structure of [Pb(HL)₂(H₂O)₂]·H₂O (2). Single-crystal diffraction analysis indicates that compound 2 crystallizes in a monoclinic crystal system with the C2/c space group. Each asymmetric unit consists of half a Pb(II) ion, one partially deprotonated H₂L⁻ ligand, one coordinated water molecule and half a free water molecule. As shown in Fig. 2a, Pb1 shows an eight-coordinated environment with N1, N1^#1, N3^#2, N3^#3, N5, N5^#1, O1^#4 and O1^#5 from five different asymmetric units. The value of Δd is 0.0024, which suggests that the coordination geometry of Pbl is a slightly distorted dodecahedron geometrical configuration. The Pb1–N bond lengths are all within the normal range. N3 in one H₂L⁻ ligand is connected to Pb1, then Pb1 is connected to N1 and N5 in the other H₂L⁻ ligand. As described above, the chain structure is formed in this manner (Fig. 2b). The chain structure is expanded to form a planar two-dimensional layer by interconnecting (Fig. 2c), and the layer structure shows a 4-connected network with symbols of {4⁴·6²} (Fig. 2d). There are H-bond interactions (O1–H1A⋯N8, O1–H1B⋯N10 and O2–H2A⋯N9) between the water molecules and two-dimensional layer, and these H-bond interactions can stabilize the network effectively. Moreover, there are multiple π⋯π interactions between the adjacent two-dimensional layers of compound 2. Fig. 2e shows the π⋯π interactions between aromatic rings. The distance between the center of the aromatic ring (N8, N9, N10, N11 and C4) and the other aromatic ring (N5, N6, N7, C2 and C3) is 3.58 Å. Meanwhile, the slip is 1.58 Å and the dihedral angle is 0.7°. The planar two-dimensional layer of compound 2 can be extended to be a three-dimensional two-periodic structure through this weak interaction.

Fig. 2 (a) Coordination environment diagram of the Pb(II) ion. (b) View of the chain structure. (c) View of the 2D layer. (d) Topological structure. (e) View of the 3D supramolecular structure.

3.2. Powder X-ray diffraction, IR spectra and solid UV-vis absorption spectra

The phase composition of compounds 1 and 2 was tested by powder X-ray diffraction to determine the phase purity of the as-synthesized crystal samples. As shown in Fig. S2,† the diffraction peaks in the PXRD pattern of the as-synthesized samples match the peaks in the simulated results, indicating that the as-synthesized compounds 1 and 2 are pure phases.

The FT-IR spectra of the synthesized compounds 1 and 2 are shown in Fig. S3.† Compounds 1 and 2 reveal similar infrared absorption peaks. The strong peak around 3434 cm⁻¹ for compounds 1 and 2 can be attributed to the stretching vibration frequency of the O–H bond of the water molecule. The peak at 3028 cm⁻¹ for compound 2 can be ascribed to the stretching vibration of the N–H bond in the aromatic ring. Several bands near 1620 cm⁻¹ for compounds 1 and 2 are due to the stretching vibration of the aromatic ring in the organic ligand. The band near 1420 cm⁻¹ for compounds 1 and 2 can be attributed to the bending vibration of the aromatic ring in the organic ligand. The bands near 1350 cm⁻¹ and 1117 cm⁻¹ for compounds 1 and 2 are all caused by the stretching vibrations of the C–N bond.

In addition, solid-state UV-vis absorption spectra were obtained for compounds 1 and 2 and the H₃L ligand. We have utilized the Kubelka–Munk functional equation to obtain the energy band gap values for compounds 1 and 2, which are 2.91 eV and 3.05 eV, respectively (Fig. S4†).⁵⁰

3.3. Luminescence properties

It is of great practical significance to develop new photofluorescent materials. Therefore, in this work, we researched the photofluorescence properties of compounds 1 and 2. The solid-state luminescence properties of compounds 1 and 2 were measured at room temperature. As shown in Fig. 3a, the maximum emission peak of compound 1 is at 560 nm (λ_ex = 375 nm), and there is a small shoulder peak at 470 nm. The main emission peak of compound 1 is located in the yellow-green region with a Stokes shift of 185 nm. For the excitation spectrum of compound 2, we can see that there are two excitation centers located at 305 nm and 375 nm. When excited by 305 nm light, the main emission peak of compound 2 is at 472 nm and is located in the blue light region with a Stokes shift of 97 nm. With the excitation wavelength of 375 nm, the main emission peak of compound 2 is located at 454 nm and in the blue light region with a Stokes shift of 79 nm. In addition, there is a distinct shoulder peak at 560 nm, which shows a Stokes shift of 145 nm (Fig. 3b).

Fig. 3 (a) Excitation and emission spectra of compound 1. (b) Excitation and emission spectra of compound 2.

The luminescence of Pb(II) compounds may be caused by surface defects, charge transfer of ligands, metal-to-metal charge transfer (MMCT), ligand-to-metal charge transfer (LMCT), and metal-to-ligand charge transfer (MLCT). In order to determine the luminescence mechanism of compounds 1 and 2, some experiments were done. By comparing the photoluminescence emission spectra of the bulk crystals and powder crystals of compounds 1 and 2, it was found that the maximum emission centers were essentially the same (Fig. S5†). This indicates that the luminescence properties of compounds 1 and 2 are not due to the defects on the surface. As shown in Fig. S6,† the luminescence spectrum of the H₃L ligand reveals two excitation centers located at 305 nm and 375 nm, respectively. When excited at 305 nm and 375 nm, respectively, nearly identical emission peaks occur, which can be attributed to the π*–π or π*–n charge transfer of ligands.

Compared to the emission peak of the H₃L ligand, the small shoulder peak of compound 1 is the same as the main emission peak of the ligand, suggesting that the small shoulder peak of compound 1 can be attributed to the π*–π or π*–n charge transfer of the ligand. The main emission peak of compound 2 is almost identical to that of the H₃L ligand, suggesting that the main emission peak of compound 2 can be attributed to the ligand. The emission peak formed by Pb-to-Pb charge transfer (metal-to-metal charge transfer) is usually located above 600 nm. In addition, the distance between Pb(II) ions in the single-crystal structure is too far. Thus, the possibility of luminescence due to MMCT can be ruled out for compounds 1 and 2. Pb(II) has difficulty losing electrons and is difficult to oxidize, so the luminescence of compounds 1 and 2 is not caused by MLCT. Therefore, it can be speculated that the main emission peak of compound 1 is due to LMCT (Fig. 4). The small shoulder peak (560 nm) of compound 2 can also be attributed to the organic ligand to Pb(II) charge transfer. The differences between these two compounds mainly stem from different components and structures. The L³⁻ anion in compound 1 contains a more negative charge than H₂L⁻ in compound 2. Therefore, it is easier to achieve LMCT in compound 1.

Fig. 4 Possible mechanisms of compounds 1 and 2.

To better study the optical properties of compounds 1 and 2, we investigated their 3D fluorescence spectra. Compound 1 has a strong emission center ca. 560 nm excited from 350 to 400 nm light (Fig. 5a). Compound 2 shows two strong emission centers ca. 472 nm and 453 nm excited from 310 to 400 nm, respectively (Fig. 5b).

Fig. 5 (a) 3D luminescence spectra of compound 1. (b) 3D luminescence spectra of compound 2.

As shown in Fig. S7,† compound 1 shows the CIE chromaticity coordinates of (0.41, 0.47), corresponding to yellow light. The coordinates of compound 2 are (0.24, 0.26), corresponding to blue luminescence.

In addition, the time-resolved PL curves of compounds 1 and 2 at room temperature were determined. The average lifetimes were calculated to be 2.19 ns for the emission peak at 560 nm (λ_ex = 375 nm) for compound 1 (Fig. S8a†) and 2.30 ns for the emission peak at 460 nm (λ_ex = 375 nm) for compound 2 (Fig. S8b†), which indicated that both compounds 1 and 2 possessed fluorescence properties.

To verify the potential applications as WLED devices, as shown in Fig. 6, we continued to fabricate the dual-component WLED by mixing compound 1 with commercial phosphor BaMgAl₁₀O₁₇:Eu as the blue component and encapsulating it onto a 365 nm UV LED chip. Finally, white light with the CIE chromaticity coordinates of (0.3178, 0.3259) was obtained, which is very close to (0.33, 0.33). The correlated color temperature (CCT) and color rendering index (CRI) were 6248 K and 78.4, respectively, resulting in a “cold” white-light.

Fig. 6 (a) The PL spectrum of the WLED device; (b) CIE chromaticity coordinates of the WLED device.

4. Conclusions

In this work, we have synthesized two novel CPs under solvothermal conditions using nitrogen-rich organic ligands H₃L and Pb(II). Single-crystal X-ray diffraction analyses show that compounds 1 and 2 exhibit different structures due to the difference in the coordination mode of the organic ligands. Moreover, we have thoroughly investigated the optical properties of compounds 1 and 2. The results suggest that these two compounds are both suitable optical materials. Therefore, a WLED is prepared by doping compound 1 with a commercial phosphor. The WLED reveals a “cold” CCT of 6248 K, an applicable CRI of 78.4 and the CIE chromaticity coordinates of (0.3178, 0.3259) corresponding to pure white light. We believe more CPs can have potential application in WLEDs.

Data availability

Data will be made available on request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Innovation Training Program of Qufu Normal University (No. XJ202307) and the Natural Science Foundation of Shandong Province (No. ZR2022MB070).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2360850 and 2360851. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4ce00607k

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