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Multi-component assembly and photophysical properties of europium polyoxometalates and polymer functionalized (mesoporous) silica through a double functional ionic liquid linker† Jing Cuan and Bing Yan* In this paper, we put forward a strategy to assemble a novel series of multi-component photofunctional hybrid materials (named as Eu-Si-P1(2,3)) centered with europium polyoxometalates (Na9EuW10O36· 32H2O, abbreviated as EuW10) and polyester modified silane (P1-Si, P2-Si, P3-Si, P1 = poly glycohol, P2 = bis(2-hydroxyethyl)ether, P3 = 2-hydroxyethyl methacrylate) through an ionic liquid compound (1-methyl-3-(trimethoxysilylpropyl) imidazolium chloride, IM+Cl−) as the double functional linker. Furthermore, using Pluronic P123 surfactant as a template to control the sol–gel process of organically modified siloxane precursors, Eu-SBA15-P1(2,3) hybrids with mesoporous silica are constructed correspondingly. The results reveal that Eu-Si-P1(2,3) hybrids present the lower red/orange intensity ratio, longer lifetime

Received 29th April 2013, Accepted 15th July 2013 DOI: 10.1039/c3dt51113h www.rsc.org/dalton

and higher quantum yield than Eu-SBA15-P1(2,3) hybrids. The luminescent lifetime and quantum efficiency of Eu-Si-P1(2,3) hybrids are comparable with EuW10 compounds in spite of their low concentration of photoactive EuW10, which is important for practical applications. The CIE chromatic coordinates of some systems are close to the cool-white region and can be expected to be utilized as cool white lighting (close to sunlight).

Introduction Lanthanide inorganic/organic hybrid materials have become more attractive in recent years, because of the combination of organic and inorganic components at both the molecular or nanometer scale.1 They bring together the virtues of organic compounds (easy processing, elasticity, and functionalization) and inorganic components (hardness, thermal and chemical stability). The present studies are focused on the chemically bonded hybrids, which are assembled by strong interactions such as covalent, ion-covalent, coordination, and so on.2 Subsequently, lanthanide complexes are grafted onto sol–gel derived host materials, which embody homogeneity and avoid the self-quenching of lanthanide ions to obtain increasing concentrations.3 Until now, many typical lanthanide hybrid materials have been developed, whose research progress is summarized in some reviews.4

Department of Chemistry, Tongji University, State Key Lab of Water Pollution and Resource Reuse (Tongji University), Shanghai 200092, China. E-mail: [email protected]; Fax: +86-21-65981097; Tel: +86-21-65984663 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3dt51113h

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Among all the lanthanide hybrids, the sol–gel derived silica host hybrids are the most popular. which is based on the functional bridge molecule (chemical linker) which can behave in three functions of coordinating and sensitizing lanthanide ions and sol–gel processing to constitute a covalent Si–O network.5–7 The photoactive groups for these bridge molecules are also important for photofunctional hybrids, whose general linkers are organically derived silanes synthesized though the modification reaction between typical photoactive organic ligands and special silane crosslinking reagents. At present, most organic ligands of lanthanide ions have been functionalized to achieve various molecule bridges as linkers.4 Furtherly, lanthanide mesoporous silica hybrids can be prepared in the presence of surfactants or other templates, which combine the luminescence properties of lanthanide complexes and the structural properties of mesoporous materials. These studies involve some kind of typical mesoporous host such as MCM-41(48), SBA-15(16) and periodically organic mesoporous silica (POMs), etc.8–10 Besides, organic polymer units can also be introduced into the lanthanide hybrid system containing silica and mesoporous silica. We have summarized the three methods to construct lanthanide polymeric hybrids. The first is the most common, where the polymer unit behaves as ligands to

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Paper coordinate to central lanthanide ions.11,12 The second is to assemble the lanthanide polymeric hybrids with a covalent bond between the organically modified silica and polymer unit while no direct interaction (coordination bonds) exists between the lanthanide ions and polymer unit.13 The third is the so-called direct polymerization process.14 Here the main interaction is a covalent bond with the host and a coordination bond with lanthanide ions. Besides, the ion exchange can also achieve the linking of lanthanide species and other functional components or hosts.15 Room temperature ionic liquid compounds are a good choice for this process as their salts consist of anion organic and cations stabilized by ionic interaction through hydrogen bonds and van der Waals forces.16 Some functionalized ionic liquids can be expected to have the potential to assemble lanthanide hybrids for their reactive groups.17 For example, luminescent lanthanide β-diketonate complexes are doped into ionic liquids to prepare hybrids, which show a favorable luminescent performance like high quantum yield and stationary photostability.18 For lanthanide hybrid materials themselves, the tendency is shifted to introduce the inorganic crystalline building block into the hybrid system to further improve their physical and chemical properties. Some works have realized the introduction of some semiconductor compounds to luminescent lanthanide hybrid systems.19 Besides, polyoxometalates are inorganic metal oxide clusters with dimensions of several nanometers, whose crystalline framework structure and versatile function are favorable for the hybrid materials. In particular the decatungstoeuropate (9-) anion as a potassium salt is worth paying attention to,20 whose sodium salt (Na9EuW10O36·32H2O, EuW10) possesses a long luminescent lifetime and high luminescent quantum yield for its crystalline structure.21 In this paper, we make efforts to extend the building units to construct multi-component hybrid systems. EuW10 behaves as the photoactive crystalline species, which is assembled with polymer functionalized silane through the special linkage of an ionic liquid compound with an alkoxy group (1-methyl-3(trimethoxysilylpropyl) imidazolium chloride, IM+Cl−). Subsequently, a series of silica hybrids and mesoporous silica (SBA-15) hybrids based around EuW10 are prepared, whose physical characterization and particularly photoluminescent performance are discussed in detail.

Experimental section

Dalton Transactions Synthesis of polymer modified silane precursors (P1-Si, P2-Si and P3-Si) A typical adopted procedure for the synthesis of P1-Si and P2Si was similar to the procedure in ref. 13a,b, whose details are shown in the ESI.† P3-Si was prepared by the procedure below: 2-hydroxyethyl methacrylate (10 mmol, 1.302 g) was dissolved into 15 mL dehydrated toluene solvent in a covered flask. Then 10 mmol (2.474 g) 3-(triethoxysilyl)-propyl isocyanate (TEPIC) was added drop by drop into the above solution, stirred and heated to 85 °C for 12 h under the protection of nitrogen atmosphere. The gelatinous substance was obtained through removing the solvent by rotary vacuum evaporator. Then the crude products were purified with hexane three times. Finally P3-Si was obtained. 1H NMR (DMSO-d6, 400 MHz): P3-Si: δ 6.78(1H, s), δ 6.82(1H, s), δ 2.34(3H, s), δ 4.37(4H, m), δ 7.79(1H, t), δ 3.41(2H, m), δ 2.68(2H, m), δ 3.41(6H, m), δ 1.06(9H, t), δ 2.30(2H, t). Synthesis of Na9EuW10O36·32H2O (EuW10) EuW10 was synthesized following the method of Peacock and Weakley.20a Firstly, Na2WO4·2H2O (6.6 g, 2 mmol) was dissolved in 16 mL deionized water and the solution was heated to 85 °C. Then glacial acetic acid was used to adjust the pH to 7.5, which produced the highest yield. After that 1.6 mL Eu (NO3)3·xH2O (2 mmol) was added dropwise with stirring, and simultaneously, a large amount of white precipitate appeared. The solution was cooled in an ice water bath and large amounts of colorless crystals emerged. The sodium salts were washed with ethanol two times and dried in air. Elemental analyses data: Found (Calcd) %: Eu 4.40 (4.54); W 55.10 (54.88). Synthesis of ionic liquid linkage (1-methyl-3(trimethoxysilylpropyl) imidazolium chloride, IM+Cl−) IM+Cl− was synthesized by the reported method through the reaction between the mixture of 1-methyl imidazole (0.820 g, 10 mmol) and (3-chloropropyl) trimethoxysilane (1.987 g, 10 mmol). A little amount of toluene was added to the mixture to isolate water and oxygen and the reaction was carried out under the protection of a nitrogen atmosphere. The solution was heated to 70 °C in an oil bath for 3 days to obtain a paleyellowish and transparent vicious liquid. Then the ionic liquid was washed with anhydrous ethyl acetate three times and the excess ethyl acetate and toluene were removed by rotary evaporator. 1H NMR: δ 4.08(2H, t), 3.86(3H, s), 3.28(9H, s), 7.59(1H, s), 7.31(1H, s), 1.78(2H, m), 0.99(2H, t), 10.09(1H, s).

Starting materials

Synthesis of polymer functionalized silica hybrid materials containing EuW10 through IM+ linkage (Eu-Si-P1(2,3))

Europium nitrate (Eu(NO3)3·xH2O) was prepared by directly dissolving its oxide (Eu2O3, 99.9%) into concentrated nitric acid. The solvent toluene and N,N-dimethyl formamide (DMF) were used after desiccation with anhydrous calcium chloride. Other reagents were used as received without further purification.

The typical synthesis procedure for Eu-Si-P1 hybrids is set out as an example below: 4.5 mmol (3.124 g) P1-Si was dissolved in DMF solvent, and IM+Cl− was added to the solution. The mixed solution was put under ultrasonic dispersing for 5 min and then was stirred and refluxed at 85 °C for 2 h. Then stoichiometric amounts of 4.5 mmol TEOS (0.937 g) and 36 mmol

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Dalton Transactions (0.648 g) water were added to the above mixed solution. A drop of dilute hydrochloric acid was used to promote hydrolysis. The reaction was kept for 6 h. After the treatment of hydrolysis, a little amount of hexamethylenetetramine was added to adjust the pH to 6–7. Then 0.5 mmol EuW10 was added into the solution to react for another 12 h. The molar ratio of EuW10 : IM+Cl− : P1-Si : TEOS : H2O was 1 : 9 : 9 : 90 : 180). The mixture was agitated magnetically to achieve a homogenous phase in a covered Teflon beaker, and then it was left at 85 °C until the onset of gelation for about 5 days. The other two hybrids were prepared with similar procedures except different polymer modified silanes (P2-Si, P3-Si) were used. The contents of W and N in the hybrids were determined. For Eu-SiP1: W 18.30%, N 2.75%; for Eu-Si-P2, W 17.22%, N 2.47%; for Eu-Si-P3: W 18.75%, N 2.88%. According to the content of W and N, it could be predicted the molar ratio of W : N was close to 1 : 2 (W10 : N18 in hybrid system).

Synthesis of polymer functionalized mesoporous silica hybrid materials containing EuW10 through IM+ linkage (Eu-SBA15-P1(2,3)) The typical synthesis procedure for Eu-SBA15-P1 hybrids is set out as an example below: firstly, 1.0 g P123 was dissolved in 7.5 mL deionized water. 30 g (2 mol L−1) dilute hydrochloric acid was added into the solution and the mixed solution was stirred at 38 °C. The mixtures of 2.041 g (9.8 mmol) TEOS and 0.139 g (0.2 mmol) P1-Si were ultrasonically dispersed for 5 min. When P123 was completely dissolved and the temperature was stable, the uniformly dispersed solution of mixed TEOS and P1-Si was added drop by drop into the above solution. The reaction was kept at 38 °C for 24 h. Then the solution was transferred into a Teflon autoclave and calcined at 100 °C for 48 h. The pale yellowish precipitates were filtered and washed by the deionized water until the pH is about 7. The crude products were dried at 60 °C for 12 h. To remove the excess template P123, the anhydrous product was refluxed at 78 °C for 48 h and filtered, and washed with hexane three times. The precipitates were dried in a vacuum to obtain the P1-SBA15. The anhydrous P1-SBA15 was dissolved in 10 mL toluene solvent in a covered flask and 4.5 mmol IM+Cl− was added into the solution under refluxing for 12 h. 0.5 mmol EuW10 was added to the solution which was refluxed for another 24 h to carry on the ion-exchange reaction. The excess toluene solvent was distilled off through a rotary evaporator. After being washed in hexane three times and dried in a vacuum at 60 °C for 6 h, the final Eu-SBA15-P1 material was obtained. The other two mesoporous hybrids Eu-SBA15-P2 and Eu-SBA15-P3 were also prepared using the above method by replacing P1-Si with P2-Si or P3-Si. The contents of W and N in the hybrids were determined. For Eu-SBA15-P1: W 8.80%, N 1.170%; for Eu-SBA15-P2, W 8.03%, N 1.02%; for Eu-SBA15P3: W 9.15%, N 1.29%. According to the content of W and N, it could be predicted the molar ratio of W : N was close to 1 : 2 (W10 : N18 in hybrid system).

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Paper Physical measurements The elemental analyses of N for the hybrids were measured with a CARIO-ERBA 1106 elemental analyzer. The analysis of W was performed on the Optima 7000 DV ICP-OES. Fourier transform infrared (FTIR) spectra were measured within the 4000–400 cm−1 region on an infrared spectrophotometer with a KBr pellet as the blank. The ultraviolet-visible diffuse reflection (UV-vis) spectra were recorded by the BWS003 spectrophotometer. Scanning electronic microstructure (SEM) was used to give a comprehensive description of the silica microspheres on a Philip XL30 at room temperature. Thermogravimetric analysis (TGA) was obtained on a Netzsch STA 409 under nitrogen atmosphere at the speed of 15 °C min−1. X-ray diffraction patterns (XRD) were recorded on a Rigaku D/max-Rb diffractometer equipped with a Cu anode over the 2θ range from 10 to 70°. Luminescence excitation and emission spectra were obtained on a RF-5301 spectrophotometer. Luminescent lifetimes of the hybrid materials were obtained with an Edinburgh Instruments FLS 920 phosphorimeter. The outer luminescent quantum efficiency was determined using an integrating sphere (150 mm diameter, BaSO4 coating) from Edinburgh FLS920 phosphorimeter. The spectra were corrected for variations in the output of the excitation source and for variations in the detector response. The quantum yield was defined as the integrated intensity of the luminescence signal divided by the integrated intensity of the absorption signal.

Results and discussion The whole multi-component hybrids belong to complicated systems which contain the following building units: europium polyoxometalates EuW10, an ionic liquid linker, a polymer unit and its functionalized silica or mesoporous silica. So it is impossible to determine the exact structure like a simple crystalline material. Here we only predict the basic composition and chemical linking mode among different units. As shown in Fig. 1, the coordination environment around the europium ion can not be changed because the EuW10 crystal framework is still kept in the hybrid system. TEPIC firstly is grafted onto the three polymers (P1, P2 and P3) through an addition reaction between the inter ester group of TEPIC and the hydroxyl groups of the polymers under the reaction conditions described in the experimental procedure. On the other hand, IM functionalized EuW10 are prepared with an ion exchange reaction between IM+Cl− and EuW10. Finally, the whole multi-component hybrid systems consist of EuW10 and polymer functionalized silanes (P1-Si, P2-Si and P3-Si) which are linked by Si–O bonds after the co-hydrolysis and copolymerization process between their alkoxy groups (TEPIC unit and IM+). Here we name the hybrids as Eu-Si-P1(2,3). For the functionalized mesoporous silica (SBA15) hybrids, the reaction process adopts a similar procedure to that above except for the presence of P123 as a template. We name these hybrids as Eu-SBA15-P1(2,3). The scheme in Fig. 1 can be verified to a certain degree by the elemental data

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Fig. 1 Synthesis process of the precursor and composition of the hybrid systems. Here we set up P1 as an example, for the hybrids of P2 and P3 polymers, it shows a similar scheme.

and IR spectra. The content of W and N suggests a W10 : N8 ratio in the hybrid system, which is the key content for the ratio of EuW10 and IM functionalized silica (IM-Si) and mesoporous silica (IM-SBA-15). Here it needs to be referred that the concentration of EuW10 in Eu-Si-P1(2,3) is more than that in Eu-SBA15-P1(2,3), which will have an influence on the corresponding luminescent properties. Fig. S1† exhibits the FT-IR spectra of three polymer modified silane precursors. In the spectra of P1-Si, the single broad peak at 2880 cm−1 derived from the associated methylene groups has shifted to some multiple peaks with a dominant peak at 2970 cm−1 for the introduction of methylene groups from TEPIC.13a New peaks of the bending vibration of –NH– in a –CONH– group at 1540 cm−1 and 1650 cm−1 for P1-Si can be observed. The peak at 1730 cm−1 corresponds to the

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Dalton Transactions asymmetric stretch of a carbonyl group in P1-Si. Moreover, the peaks at 1090, 1250 cm−1 for P1-Si indicate the absorption of the stretching vibrations of C–Si and Si–O groups. The above spectra data provide the evidence that TEPIC has been grafted onto the polymer P1. Similarly, in the spectra of P2-Si, the single broad peak at 2973 cm−1 changes to some multiple peaks with a dominant peak at 2874 cm−1, which is due to the increased methylene groups from the grafted TEPIC in P2-Si.13b New peaks of the bending vibration of –NH– from the amide groups at 1537 cm−1 and 1651 cm−1 for P2-Si can be observed. The peak at 1724 cm−1 originates from the asymmetric stretch of a carbonyl group in P2-Si. Furthermore, the peaks at 1078, 1248 cm−1 for P1-Si suggest the stretching vibration absorption of C–Si and Si–O groups. These results prove that TEPIC has been grafted onto the polymer P2. For the P3-Si precursor, some multiple peaks with a dominant peak at 2976 cm−1 appear in the spectrum of P3-Si representing the methylene groups of TEPIC. The peak at 1724 cm−1 is ascribed as the asymmetric stretch of a carbonyl group in P3-Si. New peaks of the bending vibration of –NH– (–CONH–) at 1531 cm−1 and 1639 cm−1 for P3-Si can be checked. In addition, the peaks at 1080 and 1248 cm−1 for P3-Si reveals the stretching vibration absorption of C–Si and Si–O. The above spectra data demonstrate that TEPIC has been grafted onto P3. In addition, there exists wide bands for the double multiple absorption of CvO (3500–3400 cm−1) and N–H (3500–3200 cm−1) for all three spectra. Fig. 2(a) presents the FTIR spectra of polymer functionalized silica hybrids based with EuW10 through IM+ linkage. All three spectra show an identical character. It is shown that the formation of the Si–O–Si framework is evidenced by the broad bands located at about 1050–1150, 833 and 472 cm−1, which is ascribed to the asymmetric stretching vibration, symmetric stretching vibration and planar bending vibration of the group Si–O–Si, respectively. This verifies the formation of a Si–O–Si network in the hybrid system during the process of hydrolysis and co-poly-condensation. The stretching vibrations of an O–H group appear at about 3500–3000 cm−1 as broad bands, corresponding to the H2O molecule produced during the hydrolysis/ condensation reaction. The bands at around 1639 and 1564 cm−1 are assigned to the vibration of a –CONH– group originating from the modified organic ligands. Fig. 2(b) shows the FTIR spectra of polymer functionalized mesoporous silica hybrids based with EuW10 through IM linkage. It can be clearly observed that the broad band is centered at 1078 cm−1 and 880 cm−1, which originates from the asymmetric stretching vibration (Si–O) and symmetric stretching vibration (Si–O– Si) respectively. This fact verifies that Si–O–Si groups locate inside or on the surface of mesoporous SBA15. The framework of the mesoporous structure can also be evidenced by the peaks at about 3404 cm−1, 3477 cm−1 and 3549 cm−1, respectively, belonging to the Si–OH stretching vibration of mesoporous silica for the hydrolysis and co-poly-condensation reaction. Meanwhile, the disappearance of the peak at 2273 cm−1 ascribed to the vibration of –NvCvO of the isocyanate group and the emergence of new peaks at 1639 cm−1

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Fig. 2 FT-IR spectra of multi-component hybrids: (a) Eu-Si-P1(2,3); (b) EuSBA15-P1(2,3).

originating from the vibration of OvC–NH group prove the grafting reaction between the TEPIC and polymer units. Besides, the featured peak at around 1385 cm−1 for the C–N stretching vibration also reveals the modification of TEPIC. Fig. S2† shows the IR spectra of these hybrids heat-treated (annealed to 250 °C), which show similar features as Fig. 2. Fig. S3† show the X-ray diffraction patterns of the hybrid materials, which shows the main amorphous feature of the hybrid materials. Besides, some X-ray diffraction peaks suggest the EuW10 crystal framework. Small angle X-ray diffraction analysis (SAXRD) (Fig. 3) is used to characterize the highly uniform mesoporous materials. In Fig. 3(a) for polymer functionalized mesoporous silica without introduction of EuW10, a strong diffraction peak centered at 0.8° can be observed, representing the (100) Bragg diffraction peaks of SBA15. There are also two obvious weak shoulder peaks located at 1.2 and 1.6°, respectively, which correspond to the (110) and (200) featured diffraction peaks of the mesostructured SBA15. These characteristics provide evidence for the existence of two dimensional cubic pore structures and hexagonal symmetry of the space group P6mm in these three hybrids. Compared with Fig. 3(a), the locations of Bragg diffraction peaks in Fig. 3(b) for functionalized mesoporous silica with the introduction of EuW10 and IM+ don’t change too much, demonstrating that the entrances of organic compounds don’t influence the three dimensional structures of the mesoporous SBA15 too much and the reserved characteristics of mesoporous SBA15 can do much to

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Fig. 3 The small-angle X-ray diffraction patterns of multi-component hybrids: (a) Eu-Si-P1(2,3); (b) Eu-SBA15-P1(2,3).

Fig. 4 The selected TEM images of Eu-SBA15-P1 hybrids along the [100] (left), [110] (right) zone axes.

help improve the thermal stabilities, chemical stabilities and mechanical properties of luminescent materials. But the tiny peaks at 1.2, 1.6° both disappear; it is believed that the declination of volumes and spaces in the mesostructured SBA15 is owing to the entrance of the EuW10 and IM+ linkage, which occupies too much space. Fig. 4 presents the selected TEM micrographs of Eu-SBA15P1 hybrids along the [100] and [110] zone axes, respectively. It can be found that the highly ordered mesostructure remains and the hexagonal symmetry of the space group P6mm is shown clearly. No apparent phase separation phenomenon can be observed in the TEM micrographs, indicating that homogeneous and uniformally ordered materials are obtained for the covalent bonds between the functionalized units and the

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Fig. 5 The N2 adsorption and desorption isotherms of multi-component mesoporous hybrids Eu-SBA15-P1(2,3).

Table 1 The structural parameters of multi-component mesoporous hybrids Eu-SBA15-P1(2,3)

Hybrids

d100 (nm)

a0 (nm)

SBET (m2 g−1)

V (cm3 g−1)

DBJH (nm)

Eu-SBA15-P1 Eu-SBA15-P2 Eu-SBA15-P3

10.86 10.54 10.09

12.54 12.17 11.65

255.15 334.95 346.99

0.421 0.540 0.565

5.95 5.83 6.06

d100 represents the d(100) spacing, a0 is the cell parameter (a0 = 2d100/ √3), SBET represents the BET surface area, V is the total pore volume, DBJH stands for the average pore diameter.

framework of SBA15. The results of the selected TEM images are consistent with XRD diffraction patterns and the N2 adsorption and desorption isotherms (shown as follows). Therefore, it can be concluded that polymer functionalized SBA15 containing EuW10 almost keeps the same parent SBA15 structures through organic modification. The nitrogen adsorption–desorption was conducted at liquid nitrogen temperature to further investigate the mesoporous structure of these hybrids Eu-SBA15-P1(2,3), whose nitrogen adsorption/desorption isotherms are shown in Fig. 5. All the three hybrids exhibit type IV isotherms with H1-type hysteresis loops under high relative pressure, which is the special characteristic of the SBA15 mesoporous structure. Compared with pure SBA15 according to ref. 9, the surface area, pore size, and volume data have decreased correspondingly after involving the organic group, whose data are shown in Table 1. The surface area of pure SBA-15 can reach 1197 m2 g−1, but the surface area of polymer modified SBA15 drops to around 255 m2 g−1 in Eu-SBA15-P1, 355 m2 g−1 in Eu-SBA15-P2 and 347 m2 g−1 in Eu-SBA15-P3, respectively because polymer, IM+ and EuW10 units are introduced into the pure SBA15 host and they are all big molecules with large molecular mass. So this can prove the existence of EuW10 in some extent. Fig. S4† is the nitrogen adsorption–desorption isotherm of the parent EuW10, which shows no porous structural information. Fig. 6 shows the selected thermogravimetry (TG-DTG) traces of Eu-Si-P1 (a) and Eu-SBA15-P3 (b) hybrid materials, which is measured with a heating rate at 15 °C min−1 under a nitrogen

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Fig. 6 The selected thermogravimetric (TG-DTG) curves of Eu-Si-P1 (a) and Eu-SBA15-P3 (b) hybrid materials.

atmosphere and are described in Fig. 5. In Fig. 6(a) three main degradation steps can be observed in the heating process. The first weight loss of 8.9% from 30–260 °C can be attributed to desorption of physically absorbed water and residual solvents. The second weight loss of 26.4% between 260–450 °C is ascribed to the degradation of the skeleton structures of P1 and the alkyl side-chain of the IM parts. The third weight loss of 10.7% from 450–900 °C is estimated as the decomposition of the remaining organic ingredients. Fig. 6(b) also demonstrates three decomposition ranges of Eu-SBA15-P3. The biggest weight loss of 49.1% from 30 to 156 °C is caused by the physically absorbed water and residual solvents. The second weight loss of 9.1% between 156 and 553 °C corresponds to the decomposition of incompletely removed template P123 and the organic ingredients.22 The slight weight loss of 3.3% in the range of 553–963 °C is the result of the collapse of the mesoporous structure. To have a clear look at the thermal stability of the hybrids, we further checked the selected TG-DTG plots of Eu-Si-P1 (Fig. S5(b)†) and Eu-SBA15-P3 (Fig. S5(b)†), respectively. Fig. S5(a)† shows three main degradation steps in the heating process. The first weight loss of 34.8% from 167–454 °C can be ascribed to the degradation of P1 and the alkyl chain of IM+ parts. The second weight loss of 8.3% from 454–652 °C is the decomposition of the remaining organic ingredients. The last weight loss of 10.3% can be estimated as the breaking of the whole skeleton of the materials. Fig. S5(b)† also demonstrates three decomposition ranges of

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the annealed material Eu-SBA15-P3. The first weight decrease of 11.7% in the range of 167–454 °C is the decomposition of incompletely removed surfactant.23 The second weight loss (approximately 6.4%) in the range of 454–653 °C can be assigned to the degradation of organic ligands and the ionic liquid. Finally, the weight loss of 5.4% above 653 °C is due to the breaking of the skeleton of the mesoporous structures. Therefore, the annealed hybrids show similar thermal decomposition behaviors, which also verifies the above thermal stability performance of these hybrids. The ultraviolet-visible diffuse reflection absorption spectra of the hybrid materials are given in Fig. S6(a) for silica hybrids and Fig. S6(b) for mesoporous silica hybrids (ESI†). Both of the two series of spectra exhibit similar broad absorption bands in the UV-vis range (200–800 nm), which originate from the absorption of the polytungstate framework. Besides they partially overlap with the luminescent excitation spectra (as shown in the following). The polytungstate framework absorbs abundant energy in the UV region to transfer the energy to Eu3+ within the hybrid system.19–21 Subsequently, the final hybrid materials can be expected to have excellent luminescence properties after energy transfer processes, which has been proved by the luminescence emission spectra. Besides, obvious inverse peaks in the visible region can be observed owing to the characteristic emission of Eu3+ excited by the UV component in the incident ray during the measurement. These inverse peaks are located at the same position as the characteristic emission lines of europium ions, which are due to the self-absorption of hybrids (Fig. S6†). Similarly, the selected ultraviolet-visible diffuse reflection absorption spectra of Eu-SBA15-P1(2,3) hybrid materials annealed to 250 °C also show the similar feature (see Fig. S7†). The luminescent excitation and emission spectra of Eu-SiP1(2,3) hybrids are measured at room temperature and presented in Fig. 7. The excitation spectra of them monitored at 614 nm are dominated by the broad band at about 250–325 m, which are ascribed to the charge-transfer state (CTS) of Eu–O in the EuW10 framework (EuW10O369−). The host absorption from EuW10 may overlap with CTS bands. The effective wide excitation bands are favorable for the effective energy transfer

to Eu3+ ions within the whole hybrid system. Besides, there are some weak sharp excitation bands in the long wavelength region at the range of 350 to 450 nm, which are ascribed to f–f transitions of Eu3+ ions (365 nm, 7F0 → 5D4; 385 nm, 7F0 → 5 G2; 397 nm, 7F0 → 5L6, strongest; 419 nm, 7F0 → 5D3).24 For the emission spectra of the three hybrids in Fig. 7, five narrow emission peaks at 580, 589 (594), 614 (621), 652 and 692 (700) nm are observed and assigned to the characteristic 5D0 → 7F0, 5 D0 → 7F1, 5D0 → 7F2, 5D0 → 7F1 and 5D0 → 7F2 transitions of Eu3+, respectively. Among them the magnetic dipole transition 5 D0 → 7F1 at 589 (594) nm is the strongest and stronger than that of the electronic dipole transition 5D0 → 7F2 transition at 614 (621) nm, suggesting that Eu3+ is situated in a chemical environment with an inversion center.25 Besides, the emission intensities of 5D0 → 7F4 at 692 (700) nm are stronger than general europium compounds. These results take agreement with the luminescent performance of the parent EuW10. No other emission is observed, revealing efficient energy transfer takes place in the EuW10 framework of the hybrid system under the broad band excitation. Since the symmetry of the coordination sphere decided the intensity ratio of the electric dipole transition to magnetic dipole transition, so the integration ratio (I02/I01) of 5D0 → 7F2/5D0 → 7F1 transition is widely taken as an indicator of Eu3+ site symmetry. Subsequently, the intensity ratio of 5D0–7F2/5D0–7F1 is less than 1.0 but more than that of the parent EuW10, suggesting that Eu3+ is located in a similar chemical environment of the EuW10 framework and the polymer functionalized silica hosts have a slight influence on the local structure and luminescence of Eu3+. Fig. 8 exhibits the room-temperature luminescent excitation and emission spectra of the Eu-SBA15-P1(2,3) hybrids. The excitation spectra of them show similar wide excitation bands at a range of 250–325 nm as the excitation spectra in Fig. 7, corresponding to the CTS absorption and host self absorption of EuW10O369− in the EuW10 unit. But the line absorption to the f–f transition of Eu3+ is present at a lower intensity than those in Fig. 7, suggesting the wide absorption in the modified silica system is not as effective as that in the modified mesoporous silica system. The characteristic emission for the 5D0 →

Fig. 7 The luminescent excitation and emission spectra of three polymer functionalized silica hybrids containing EuW10.

Fig. 8 The luminescent excitation and emission spectra of three polymer functionalized mesoporous silica hybrids containing EuW10.

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Table 2 The luminescent data of Eu-Si-P1(2,3) and Eu-SBA15-P1(2,3) hybrid materials

Hybrids

I02/I01

τa (ms)

ηb (%)

Eu-Si-P1 Eu-Si-P2 Eu-Si-P3 Eu-SBA15-P1 Eu-SBA15-P2 Eu-SBA15-P3

0.81 0.98 0.74 0.89 0.62 1.30

3.021 2.017 3.114 0.321 0.288 0.402

60.1 45.7 64.0 14.2 10.5 17.9

The luminescent lifetimes of the 5D0 → 7F1 transition for Eu3+. b The absolute luminescent quantum yields using an integrating sphere.

a

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Fig. 9 The CIE chromaticity diagrams of Eu-Si-P1(2,3) and Eu-SBA15-P1(2,3) hybrid materials. 7

FJ (J = 0–4) transitions of Eu3+ are located at about 580, 589 (594), 613 (621), 652 and 692 (701) nm, respectively, which shows a similar performance to the parent EuW10 and EuSi-P1(2,3) hybrids, indicating the same chemical environment around Eu3+ as the former. Checking the integration ratio (I02/I01) of the 5D0 → 7F2/5D0 → 7F1 transition, it is found that the values of I02/I01 are slightly higher than that of Eu-Si-P1 (2,3), especially the I02/I01 value of Eu-SBA15-P1 which is more than 1.0, suggesting the chemical environment of Eu3+ in this hybrid system is changed more severely than the other two mesoporous hybrids, Eu-Si-P1(2,3) hybrids and parent EuW10 compound. To further investigate the luminescence efficiency of these multi-component hybrid materials, the typical decay curves are measured and can be described as single exponential decay curves, indicating that all Eu3+ ions occupy the same average coordination environment. Table 2 shows the resulting lifetime data of the Eu3+ hybrid materials. It can be seen that the polymer derived silica hybrid materials show longer lifetimes (5D0 for Eu3+) than the polymer derived mesoporous hybrid ones. This maybe because the effective concentration of Eu3+ species in the SBA15 system is less than that in the silica system (see the data in Experimental section). Moreover, the absolute luminescence quantum yield data can be obtained on condition that they are accurately performed with an integrating sphere and a calibrated detector setup for solid materials (the data shown in Table 2). It can be clearly seen that the quantum yields of the Eu-Si-P1(2,3) hybrids are higher than the Eu-SBA15-P1(2,3) hybrids, which is in agreement with the order of their lifetimes for the distinction of the effective concentration of EuW10. Among the Eu-Si(SBA15)-P1(2,3) hybrids, some possess quantum efficiencies over 50%, which is comparable to the data of the parent EuW10 compound. But the effective concentration of EuW10 species is much lower than that of EuW10 itself. The favorable luminescent performance (long lifetimes and high quantum yields) are favorable for further practical applications. Furthermore, the CIE chromaticity diagrams of these hybrid materials are checked, which is shown in Fig. 9 and Table 3, respectively. It is found that the four hybrid systems (Eu-Si-P2(3) and Eu-SBA15-P2(3)) emit close to white luminescence, whose CIE coordinates ((0.3941, 0.4035), (0.3767, 0.4061), (0.3847, 0.4039) and (0.3799, 0.4046)) are in the

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Table 3 Color coordinates and color of Eu-Si-P1(2,3) and Eu-SBA15-P1 (2,3) hybrid materials

Hybrids

x

y

Color

Eu-Si-P1 Eu-Si-P2 Eu-Si-P3 Eu-SBA15-P1 Eu-SBA15-P2 Eu-SBA15-P3

0.6099 0.3941 0.3767 0.5747 0.3847 0.3799

0.3728 0.4035 0.4061 0.3639 0.4039 0.4046

Orange Nearly white Nearly white Yellow Nearly white Nearly white

cool-white region, close to sun light, which is useful for lighting. Combined with the long luminescent lifetimes and higher quantum yields, the two kinds of multi-component hybrids Eu-Si-P2(3) are expected to have potential application value.

Conclusions In summary, multi-component hybrids (Eu-Si-P1(2,3) and EuSBA15-P1(2,3)) are assembled with polymer functionalized silica and mesoporous materials based EuW10 through an IM+ ionic liquid linkage. The photoluminescent properties of these hybrids show that Eu-Si-P1(2,3) hybrids possess longer lifetimes and higher quantum yields than Eu-SBA15-P1(2,3) hybrids, which are comparable to the parent EuW10 compounds. Some of the hybrid systems offer almost cool-white emission. Both Eu-Si-P2 and Eu-Si-P3 hybrids in particular can be expected to have potential value in the practical cool-white lighting (close to sunlight).

Acknowledgements This work is supported by the National Natural Science Foundation of China (20971100, 91122003) and Program for New Century Excellent Talents in University (NCET-08-0398).

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Multi-component assembly and photophysical properties of europium polyoxometalates and polymer functionalized (mesoporous) silica through a double functional ionic liquid linker.

In this paper, we put forward a strategy to assemble a novel series of multi-component photofunctional hybrid materials (named as Eu-Si-P1(2,3)) cente...
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