Introduction

Lanthanide complexes with excellent luminescence properties have attracted considerable attention due to their outstanding emission properties such as long luminescence lifetime, photostability, narrow and easily identifiable emission lines. These distinctive properties make them be suitable for application in lighting devices and biomedical analysis1,2. In such complexes, appropriate organic ligands such as β-diketones3,4,5, phenanthroline6 and heterocyclic carboxylic acid group7,8,9 usually act as antenna or sensitizers which could absorb the light energy and transfer the energy from the lowest triplet state energy level (T1) of these organic groups to the resonance level of the lanthanides. Furthermore, the organic ligands also shield the lanthanides from coordinated solvent molecules (such as water) and these solvent molecules can diminish the luminescence of the complexes because the high energy vibration of O-H will induce the non-radiative transition of lanthanides.

However, the pure lanthanide complexes are not suited for use in optical devices mainly due to their poor thermal stabilities and low mechanical strength. As a result, increasing attention has been focused on the design and synthesis of organic-inorganic hybrid materials by grafting of lanthanide complexes on polymer matrices10,11,12, silica-based materials13,14,15,16,17,18 and liquid crystals19,20, et al. For example, Reddy and coworkers reported a luminescence carbon material by bonding the visible sensitized Eu3+ β-diketonate complex to carboxyl functionalized multi-walled carbon nanotube21. Wang and coworkers prepared a hybrid luminescence material by linking Eu3+ or Tb3+ complexes with a new polydentate amide ligand to mesoporous silica nanospheres (MSNs) for the recognition of fluoride ions22. Lin and coworkers designed a paramagnetic hybrid material by grafting Gd3+ complex to the thiol-functionalized MSNs through a disulfide tether, which could act as a magnetic resonance imaging contrast agent23. Generally, the novel hybrids will exhibit superior physicochemical and optical properties, including high thermal stabilities and mechanical strength, long luminescence lifetimes, as well as promising application. Among all of the lanthanide hybrids, mesoporous silicas with unique properties (large surface area, uniform and adjustable pore sizes, as well as easy surface modification) are frequently used as the scaffolding materials due to their potential applications in sensing, drug delivery, biomedical imaging, etc. Up to now, lanthanide complexes have been incorporated into mesoporous silica matrices, such as MCM-41, SBA-15 by a covalent bond grafting technique or a non-covalent, assembling method24,25.

Ionic liquids functionalized mesoporous materials have drawn considerable attentions over the past decades, owing to theirs application in electrochemistry, catalysis and separation and adsorption26,27. Recently, Mesoporous silicas modified with alkylimidazolium quaternary ammonium salt were used as the novel anion exchange matrixs for preparation of the hybrid mesoporous luminescence materials28,29. A negatively charged lanthanide complexes could be non-covalently immobilized on mesoporous materials grafted with a positively charged group through strong electrostatic interactions. A series of luminescence materials have been synthesized through the incorporation of europium β-diketone complexes into functionalized mesoporous materials, which exhibit a bright red photoluminescence upon illumination with ultraviolet light. However, β-diketones are not the efficient ligands for sensitizing the luminescence of terbium ion, because the lowest triplet state energy level of the β-diketones can not match the emission energy level of terbium ion very well30.

Pyridine-2, 6-dicarboxylic acid (DPA) is an efficient tridentate N, O-chelating ligand for sensitization of terbium or europium ions luminescence. Three DPA ligands are able to coordinate with the lanthanide ion through six oxygens of carboxyl groups and three nitrogen atoms of pyridine units, resulting in the formation of [Ln(DPA)3]3− (Ln: Tb, Eu)31,32,33,34. The coordination sphere of the lanthanide ion is saturated by the ligands, leaving no coordination site for water molecules. Therefore, [Ln(DPA)3]3− exhibit excellent luminescence properties in aqueous solution33. Moreover, [Ln(DPA)3]3− with three negative charges could be incorporated into the positive-charge modified mesoporous materials using an ion exchange method. In addition, the luminescence properties of the complexes formed by lanthanides and DPA derivatives can be adjustable by changing the substituents on the DPA or lanthanides. In this work, we propose a suitable and convenient method to assemble a novel mesoporous luminescence material (SBA-15-IMI-Ln(DPA)3, Ln = Tb or Eu). [Ln(DPA)3]3− was incorporated into the alkylimidazolium quaternary ammonium salt functionalized mesoporous materials by ion-pairing interactions. The coordination feature of Ln3+ and well energy match between the energy levels of DPA and Ln3+ make the hybrids possess excellent luminescent properties such as high luminescent quantum yields, long lifetimes, high color purity and multi-color luminescence (red and green) under the excitation of a single wavelength ultraviolet light.

Results

The synthetic scheme of 1-methyl-3-(3-triethoxysilylpropyl) imidazolium chloride (IMI+Cl−) and the hybrid mesoporous luminescence material (SBA-15-IMI-Ln(DPA)3) is shown in Figure 1.

Figure 1
figure 1

Synthesis procedures of IMI+Cl− and the mesoporous luminescence material SBA-15-IMI-Ln(DPA)3.

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Fourier transform infrared (FT-IR) spectrum is commonly used for structural characterization of hybrid materials because it can provide surface information of samples for identification of chemical functional groups. The FT-IR spectra of SBA-15 and functionalized SBA-15 are shown in Figure 2. As shown in Figure 2B and E, the characteristic absorbance for skeleton vibration of imidazolium ring is found at about 1574 and 1456 cm−1 35, meanwhile the bands corresponding to the vibration of saturated C-H and unsaturated C-H appear at 2935 cm−1 and 3100, 3157 cm−1, respectively, implying that IMI+Cl− has been successfully grafted onto the framework of SBA-1535. Furthermore, the formation of the Si-O-Si framework is evidenced by the bands located at 1080 cm−1 (νas, Si-O), 795 cm−1 (νs, Si-O) and 463 cm−1(δ, Si-O-Si)36, respectively (Figure 2A). The spectra of those lanthanide complex functionalized SBA-15 display two new peaks at 1437 and 1392 cm−1, which can be assigned to the asymmetric and symmetric stretch of the carboxylate group, respectively (Figure 2C and D)37. This means that the lanthanide DPA complexes have been anchored onto the ionic liquid modified SBA-15 frameworks by anion exchange.

Figure 2
figure 2

FT-IR spectra of SBA-15 (A), SBA-15-IMI (B), SBA-15-IMI-Tb(DPA)3 (C), SBA-15-IMI-Eu(DPA)3 (D) and IMI+Cl− (E).

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Figure 3 shows the X-ray powder diffraction (XRD) patterns of pure SBA-15 and modified SBA-15. All the samples clearly display a high-intensity (100) reflection and two weak reflections that can be indexed as the (110) and (200) diffractions respectively, which are characteristic of well-ordered mesoporous material with 2D-hexagonal structure. The hexagonal structures were still clearly observed in the functionalized SBA-15, indicating that the ordered hexagonal mesoporous is maintained during the functionalization of SBA-15 with lanthanide complexes.

Figure 3
figure 3

XRD patterns of pure SBA-15 (A), SBA-15-IMI (B), SBA-15-IMI-Tb(DPA)3 (C) and SBA-15-IMI-Eu(DPA)3 (D).

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The N2 adsorption-desorption isotherms of the SBA-15, SBA-15-IMI-Tb(DPA)3 and SBA-15-IMI-Eu(DPA)3 are displayed in Figure 4. All of the samples exhibit the type IV adsorption isotherms with H1-type hysteresis loops at high relative pressure, which is a typical characteristics of ordered mesoporous materials38. A sharp adsorption step occurs approximately at P/P0 = 0.6–0.8, which reveals that those samples possess a well-defined array of regular mesoporous. The results indicated that the mesoporous structure of SBA-15 has been successfully retained under the functionalization environment. The structure data of these mesoporous materials are summarized in Table S1. As expected, the surface area, pore volume and average pore diameter of the modified SBA-15 decrease considerably compared to pure SBA-15, which is consistent with the presence of anchored lanthanide complexes inside the channels of SBA-1539.

Figure 4
figure 4

N2 adsorption-desorption isotherms of SBA-15, SBA-15-IMI-Tb(DPA)3, SBA-15-IMI-Eu(DPA)3.

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The morphology and the porous structure features of the mesoporous materials can be observed in Figure S1 and Figure 5. The scanning electron microscopy (SEM) images of SBA-15 and modified SBA-15 display the short rod-shaped external morphology whereas the transmission electron microscopy (TEM) images exhibit a well-ordered 2D hexagonal (P6mm) mesostructure, which is the characteristic structure feature of the SBA-15. This result clearly indicates that the mesoporous structure of SBA-15 is not disrupted after modifying with lanthanide complexes. The energy dispersive X-ray spectroscopy (EDS) analysis indicated that Tb or Eu-element has been detected in SBA-15-IMI-Tb(DPA)3 or SBA-15-IMI-Eu(DPA)3. (Figure S2).

Figure 5
figure 5

The selected TEM images of hybrid materials SBA-15-IMI-Tb(DPA)3 (a, b) and SBA-15-IMI-Eu(DPA)3 (c, d).

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Thermogravimetric analysis is one common used thermal analysis technique, which is particularly useful for observing the thermal decomposition of organic-inorganic hybrid materials. The thermogravimetric (TG) curves and the corresponding derivative thermogravimetric (DTG) curves of SBA-15-IMI-Tb(DPA)3 and SBA-15-IMI-Eu(DPA)3 are shown in Figure 6. The weight loss observed for all the functionalized samples is at two temperatures range. The first important weight loss below 200°C is related to physically adsorbed and coordinated water molecules. The relatively large weight loss occurs between 300 and 550°C, which is assigned to the decomposition of organic species. Beyond this temperature, the weight loss is less obvious. The total weight losses for SBA-15-IMI-Tb(DPA)3 and SBA-15-IMI- Eu(DPA)3 are about 35.2% and 30.5%, respectively, which indicates that the organic groups have been anchored onto the surface of SBA-1511,40.

Figure 6
figure 6

Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of SBA-15-IMI-Tb(DPA)3 (A) and SBA-15-IMI-Eu(DPA)3 (B).

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The excitation and emission spectra of SBA-15-IMI-Tb(DPA)3 and SBA-15-IMI-Eu(DPA)3 are shown in Figure 7. The excitation spectra were recorded by monitoring the 5D4 → 7F5 transition at 543 nm for Tb3+ and the 5D0 → 7F2 transition at 615 nm for Eu3+. Both of the spectra display a broad excitation band between 240 nm to 350 nm (λmax = 280 nm), which can be ascribed to the absorption of the complexes. Meanwhile, the maximum absorption of the lanthanide complexes could also be observed at 280 nm in the UV-Vis diffuse reflection spectra (Figure S3). Upon excitation with 280 nm ultraviolet light, the obtained SBA-15-IMI-Tb(DPA)3 material gives characteristic emission of Tb3+ and the emission peaks at 492, 543, 583 and 622 nm can be assigned to the 5D4 → 7FJ (J = 6, 5, 4, 3) transitions, respectively, with the 5D4 → 7F5 transition as the most prominent one41,42. Under the excitation of ultraviolet light with the same wavelength, SBA-15-IMI-Eu(DPA)3 reveal the characteristic Eu3+ emission peaks centered at 594, 615, 649 and 695 nm, corresponding to the 5D0 → 7FJ (J = 1, 2, 3, 4) transitions respectively43,44. Among these transitions, the 5D0 → 7F2 transition (the strongest one) is a typical electric dipole transition which is fairly sensitive to the local symmetry of Eu3+ ions, whereas the 5D0 → 7F1 corresponds to a magnetic dipole transition that is independent of the host material. According to the transition probabilities, 5D0 → 7F2 electron dipole transition becomes the strongest emission when Eu3+ is in a noninversion symmetry site, while in a center with inversion, the 5D0 → 7F1 magnetic dipole transition is dominant45. Therefore, the relative intensity ratio (RI) of 5D0 → 7F2 (red light) to 5D0 → 7F1 (orange red light) depends strongly on the local symmetry around the Eu3+ ion. The value of RI for SBA-IMI-Eu(DPA)3 is 7.6, suggesting that the local surrounding of Eu3+ is highly asymmetric without an inversion center. In addition, the full width at half maximum of the strongest band is less than 4 nm, which shows that the SBA-15-IMI-Eu(DPA)3 exhibits a high color purity. As observed by the luminescence photos (Figure 7 b and d, insets), the hybrid materials SBA-15-IMI-Tb(DPA)3 and SBA-15-IMI-Eu(DPA)3 exhibit bright green and red color luminescence respectively upon irradiation with a single wavelength (280 nm) ultraviolet light.

Figure 7
figure 7

The excitation (a, c) and emission (b, d) spectra of hybrid material SBA-15-IMI-Tb(DPA)3 (a, b) and SBA-15-IMI-Eu(DPA)3 (c, d) and insets of Figure 7b and 7d are photos of SBA-15-IMI-Tb(DPA)3 (green) and SBA-15-IMI-Eu(DPA)3 (red) excited by ultraviolet light at 280 nm.

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The chromaticity diagram (CIE) shown in Figure S4 indicates that the emission of both hybrids lie in green and red region with the chromaticity coordinates (x = 0.35, y = 0.59) for SBA-15-IMI-Tb(DPA)3 and (x = 0.66, y = 0.33) for SBA-15-IMI-Eu(DPA)3, respectively. Moreover, the chromaticity coordinate of the SBA-15-IMI-Eu(DPA)3 is close to the edge of the CIE diagram, which also shows this sample with high color purity.

The luminescence decay curves of SBA-15-IMI-Tb(DPA)3 and SBA-15-IMI-Eu(DPA)3 are shown in Figure 8. Both of the curves can be well fitted by a single exponential function described as equation (1)

Where τ stands for the luminescence lifetime and I0 is the corresponding fitting parameter. The values of τ are determined to be 2.38 ms for SBA-IMI-Tb(DPA)3 and 2.34 ms for SBA-IMI-Eu(DPA)3, respectively. The results indicate that the average chemical environments of Tb3+ or Eu3+ are uniform. The luminescence lifetime of lanthanides is greatly affected by the vibration of the nearby hydroxyl groups and ligands. The vibrations of hydroxyl groups and ligands will absorb the excitation energy of Tb3+ or Eu3+, leading to decrease the lifetime of the lanthanides. As for our hybrid luminescence materials, the strong electrostatic interaction between the matrix and the lanthanide complexes limits the vibration of the ligands around lanthanides. Furthermore, [Ln(DPA)3]3− have no coordinated water molecules, leading to long luminescence lifetimes of lanthanides. The quantum efficiencies (η) of the SBA-15-IMI-Tb(DPA)3 and SBA-15-IMI-Eu(DPA)3 are about 63% and 79%, respectively. The photoluminescence data including excitation wavelength, emission wavelength, quantum efficiency and lifetime of our samples and other analogous materials are listed in Table 1, it appears that the lifetime in SBA-15-IMI-Ln(DPA)3 (Ln = Tb, Eu) is longer than that in other hybrid materials containing a similar lanthanide complexes. The high quantum efficiencies and long luminescence lifetimes suggest that this method is a preferable way for constructing hybrid mesoporous materials with bright green or red emission.

Table 1 Photoluminescence data of the SBA-15-IMI-Ln(DPA)3, (Ln = Tb, Eu) and previously reported hybrid luminescence mesoporpous materials. excitation wavelength (λ (ex)), emission wavelength (λ (em)), lifetime (τ), quantum efficiency (η), Reference (ref)
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Figure 8
figure 8

Luminescence decay curves of the hybrid materials, SBA-15-IMI-Tb(DPA)3 (a), SBA-15-IMI-Eu(DPA)3 (b).

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The reason that the excellent luminescent properties of SBA-15-IMI-Ln(DPA)3 is mainly attributed to two aspects. On the one hand, As can be seen in Figure S5, three DPA ligands are able to coordinate with the Ln3+ through six oxygen atoms of carboxyl groups and three nitrogen atoms of pyridine units and the coordination sphere of Ln3+ is saturated by the ligands, leaving no coordination site for water and other solvent molecules. On the other hand, the energy levels between DPA ligand and Ln3+ are well matched and there exist efficient energy transfer between them. It has been confirmed that the matching degree between the lowest triplet state energy level of ligand and emission energy level of Ln3+ is a critical factor which affects the luminescence properties of the lanthanide complexes47. Generally, the energy transfer between the organic ligand and the center lanthanide may be proceeded when the lowest energy of the triplet state of the ligands is higher than the energy of the resonance levels of the lanthanides. For the DPA ligand, the lowest energy of the triplet state of the DPA (26600 cm−1)30 is higher than the resonant energy level of Tb3+ (5D4, 21500 cm−1) and Eu3+ (5D0, 17250 cm−1), indicating that DPA would be an efficient ligand for sensitizing the luminescence of Tb3+ and Eu3+. The simplified energy levels, electron transition schematic diagram and the luminescent mechanism are illustrated in Figure 9. First, the DPA complex is excited from the singlet ground state (S0) to excited singlet states by absorbing the light energy and then relaxed to the lowest excited singlet states (S1) through non-radiative transitions. Secondly, the energy of excited singlet state (S1) of the ligand is transferred to its excited triplet state (T1) through intersytem crossing, Thirdly, the intramolecular energy transfer from the excited triplet state of the ligands to the excited 4f states of the Tb3+ ions (5D4) or Eu3+ ions (5D0) through non-radiative transition. Finally, the radiative transition of Ln3+ produces the characteristic luminescence of Ln3+.

Figure 9
figure 9

Simplified energy level and electron transition diagram of [Ln(DPA)3]3− system.

Non-rad = nonradiative transitions, ISC = intersystem crossing, ET = energy transfer, S = Singlet, T = triplet.

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Conclusions

In this work, two novel luminescence mesoporous hybrid materials have been prepared by incorporating [Ln(DPA)3]3− into ionic liquid modified SBA-15 through strong electrostatic interactions. The mesoporous structures of SBA-15 can be retained after functionalization with [Ln(DPA)3]3− (Ln: Tb, Eu). Under the excitation of ultraviolet light with the same wavelength (280 nm), the hybrid materials show the characteristic emission of Tb3+ (green) and Eu3+ (red) with long luminescence lifetimes and high quantum efficiencies, which indicates that [Ln(DPA)3]3− are very suitable as the luminescence center for construction of the hybrid luminescence materials.

Methods

Synthesis of 1-methyl-3-(3-triethoxysilylpropyl) imidazolium chloride (IMI+Cl−)

1-methyl-3-(3-triethoxysilylpropyl) imidazolium chloride was synthesized from (3-Chloropropyl)triethoxysilane and 1-methylimidazole as previously reported48.

Synthesis of mesoporous material SBA-1549

Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (Pluronic P123, EO20PO70EO20, Mav = 5800, 4.0 g) was dissolved in hydrochloric acid (120 mL, 2 mol/L) and distilled water (30 mL) under stirring. Then tetraethoxysilane (TEOS, 8.4 g) was added dropwise to the above mixture solution under vigorous stirring at 313 K. After TEOS was completely added, the mixture was left to stirred at 313 K for 24 h and then transferred into a hermetically sealed Erlenmeyer flask and kept at 353 K for 48 h. Finally the product was recovered by filtration, washed with distilled water and air-dried. The template was removed by Soxhlet extraction with hydrochloric acid and ethanol solution under reflux for 2 d.

Synthesis of hybrid mesoporous material SBA-15-IMI50

1-methyl-3-(3-triethoxysilylpropyl) imidazolium chloride (IMI+Cl−, 3 mL) and dry triethylamine (3 mL) were added dropwise to a suspension of SBA-15 (2 g) in dry toluene (60 mL). The resulting mixture was refluxed at 115°C for 24 h under an atmosphere of argon. After cooling, the suspension was centrifuged and the adsorptive ionic liquid was removed by Soxhlet extraction with dry dichloromethane under reflux for 24 h. The resulting powder, denoted as SBA-15-IMI, was dried under vacuum at 60°C for 10 h.

Synthesis of hybrid mesoporous luminescence material SBA-15-IMI-Ln(DPA)3

Pyridine-2, 6-dicarboxylic acid (DPA, 1.5 mmol) was added to an aqueous solution of Tb(NO3)·6H2O or Eu(NO3)·6H2O (0.5 mmol, 20 mL) and the pH was adjusted to be neutral by adding NaHCO3 (According to the above procedure, the crystals of the lanthanide complexes (Na3[Ln(DPA)3], Ln = Tb, Eu) were obtained by the solvent evaporation method). The mixture was left to stir for 2 h and then SBA-15-IMI (0.5 g) was added to the above solution. After stirred at room temperature for about 24 h, the resulting product, denoted as SBA-15-IMI-Tb(DPA)3 or SBA-15-IMI-Eu(DPA)3, was filtered out and washed with water, air-dried.

Fourier transform infrared (FT-IR) spectrum was measured using a Nicolet 5700 infrared spectrometer by the KBr tablet method. X-ray powder diffraction patterns were recorded on Bruker D8 FOCUS diffractometer using Cu target (voltage: 40 kV, current: 40 mA) equipped with a scintillation crystal detector. Single crystal X-ray diffraction data were collected on a Bruker SMART APEX II diffractometer. Data reductions and absorption corrections were performed with the SAINT and SADABS programs, respectively. The structures were solved by direct methods and refined with full-matrix least squares on F2 using the SHELXS-97 program. N2 adsorption-desorption isotherms were obtained on a Micromeritics Tristar Π 3020 sorptometer. The pore size distribution calculated from adsorption isotherms based on the Barrett-Joyner-Halenda (BJH) method. The structure and composition of the samples were inspected using a JEOL-2100F transmission electron microscope (TEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) with an accelerating voltage of 200 kV. The scanning electron microscopy (SEM) was obtained from a FEI Quanta 200 scanning electron microscope. Thermogravimetric (TG) analysis was performed on a STA 6000 (Perkin Elmer) instrument. Ultraviolet-Visible (UV-Vis) diffused reflection spectra measurements were carried out on a TU-1901 (Purkinje) UV-Vis spectrophotometer. Photoluminescence (PL) spectrum and luminescence lifetime measurements were carried out on an FLS920P Edinburgh instruments apparatus equipped with a 450 W xenon lamp and a high-energy micro-second flash lamp as the excitation sources. The quantum efficiencies were determined using an integrating sphere whose inner face was coated with BenFlect from FLS920P Edinburgh spectrofluorometer.