Luminescent lanthanide-polyoxometalates assembling zirconia–alumina–titania hybrid xerogels through task-specified ionic liquid linkage

Abstract

This paper is focused on the preparation and characterization of luminescent hybrid materials of zirconia (alumina, titania) xerogels encapsulated with high luminescent lanthanide polyoxometalates via a sol–gel process. Firstly, liquid compound 1-methyl-3-propionyloxy imidazolium bromide (IM⁺Br⁻) is assembled with lanthanide polyoxometalates (NaLnW₁₀O₃₆·32H₂O, abbreviated as LnW10, Ln = Eu, Tb, Sm, Dy) through the ion exchange reaction, resulting in Ln-IL, Then Ln-IL is connected to zirconia (alumina/titania) by the chelating reaction between a propionyloxy group of the IM⁺ component and metallic alkoxides (Zr(OCH₂CH₂CH₂CH₃)₄, Ti(OCH(CH₃)₂)₄, Al(OCH(CH₃)₂)₃) under mild conditions after hydrolysis and condensation. These hybrid materials are characterized by Fourier transform infrared spectroscopy, wide angle X-ray diffraction, thermogravimetric analysis, as well as luminescence. The above measurements indicate that they possess high thermal-stability, amorphous structure features and especially favorable luminescent performances such as long luminescent decay lifetime, high quantum yield. It is found that alumina and zirconia are superior matrices to titania for the luminescence of lanthanide polyoxometalates, and close white-light integration can be realized for hybrids of titania gels.

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Introduction

Lanthanide compounds are known for their narrow-width band emission, large Strokes shift and high color purity which are independent of the surrounding environment. However, as the result of 4f–4f spin-forbidden of lanthanide ions, their absorption coefficient is very low and prevents the effective energy transfer.¹ Wessiman's observation reveals that effective intramolecular energy transfer from the organic ligand to central lanthanide ions with the ultra-violet excitation of europium organic chelates and brings intense f–f* electronic transition to obtain high luminous yields.² This energy transfer mechanism is successfully explained by the so-called antenna effect.³ With the development of material science, the concept of organic-inorganic hybrids is introduced to the fields of lanthanide luminescent materials with better mechanical processability, thermal stability, homogeneous multiphase structure (micro-nanoscale size). Until now all kinds of lanthanide hybrid materials have been reported and the research progress is summarized in some reviews.⁴ Sol–gel process is an effective way to combine inorganic and organic units into nanosized level,⁵ which can improve the mechanical and thermal stability of lanthanide complexes. The homogeneity enables the preparation of high purity materials with tailored microstructures at low temperature,⁶ like the most popular sol–gel derived silica skeleton. In order to fabricate ordered self-assembled optical hybrids, some strategies are tried to find other hosts like silica to encapsulate photoactive lanthanide species. Among titania and zirconia⁷ with high refractive index are supposed to replace the silica framework. Titania is used in many fields like environmental technology for degradation of hazardous substances,⁸ photo-catalysis and energy conversion, suggesting that titanium is a fascinating functional element in preparing novel luminescent materials.

Lanthanide polyoxometalates (Ln-POMs) are a class of compounds built by inorganic metal–oxygen cluster with lanthanide ions. Ln-POMs exhibit wealthy properties such as topologies and physical (optical, electrical, and magnetic) properties,⁹ which have potential application in the fields of optics,¹⁰ catalysis, biology, material and medical science.¹¹ Among Ln-POMs, lanthanide decatungstates (Na₉LnW₁₀O₃₆·36H₂O) possess outstanding luminous natures such as long emission lifetime scale (from μs to ms) and high quantum yields, which is a favorable photoactive species for lanthanide hybrid material. Naturally, Ln-POMs anions can be easily to assemble with ionic liquid compounds through electrostatic force between cations and anions, which are promising luminescent soft materials.¹² Simple carboxylate ligands are known to support mononuclear Ti^IV structures, so carboxyl functional groups of task-specified ionic liquid (TSIL) can graft on the transition metal (zirconium, titanium, aluminum) skeleton by chelating bonds.¹³

Ionic liquid compounds are entirely constructed by inorganic anions with large blocks of organic counterions like phosphonium, quaternary ammonium, imidazolium or pyridinium.¹⁴ They are applied to the fields of solvents, synthesis, catalysis, separations, electrochemistry owing to their environment friendly property, wide electrochemical window, low vapor pressure and tunable physicochemical properties.^14,15 It needs to refer that some ionic liquid derivatives can be expected to behave chemical linkage to construct functional hybrids. Lately, some reports are focused on luminescent soft hybrid materials centered with lanthanide complexes dispersed into ionic liquids or chemically bonded with host through ionic liquids as linkage.^16,17 These soft hybrid systems present excellent luminescent performance such as high quantum yield and superior photostability.¹⁸ In the hybrid system, lanthanide ions still keep the coordination environment in the crystal framework of polyoxometalates.^12,19,20 Especially decatungstoeuropate anion EuW₁₀O₃₆⁹⁻ has been discovered by Peacock and Weakley and shows the best luminescent performance.²¹

In our work, lanthanide polyoxometalate Na₉LnW₁₀O₃₆·36H₂O, abbreviated as LnW10, Ln = Eu, Tb, Sm, Dy are embedded into the titania (zirconia, alumina) matrices through special carboxylic derived ionic liquid 1-methyl-3-propionyloxy imidazolium bromide. The physical characterization and especially the photophysical properties of these hybrid materials are studied in details.

Experimental section

Starting materials

Four lanthanide nitrates Ln(NO₃)₃·xH₂O (Ln = Eu, Tb, Sm, Dy) were prepared by dissolving their oxides (Eu₂O₃, Tb₄O₇, Sm₂O₃, Dy₂O₃) into the concentrated nitric acid with heating and stirring to promote the reaction until that the crystal film appeared. Aluminium-isopropylate and 3-bromopropionic acid are from Shanghai Adamas Reagent Co., Ltd. Zirconium(IV) butoxide (ca. 80% in 1-butanol) was from TCI and tetraisopropoxytitanium (ca. 98%) was from Adamas Reagent Co., Ltd.

Synthesis of carboxylic-functionalized ionic liquid (denoted as IM⁺Br⁻)

The carboxylic-functionalized ionic liquid was prepared following the reported method.²² Firstly, 100 mmol solution of 3-bromopropionic acid was dissolved in ethanol (10 mL) and an appropriate amount of 1-methylimidazole was added drop after drop to the solution. The mixture was stirred under refluxing at 75 °C for 8 h. The pale-yellowish viscous ionic liquid was obtained after the crude product was purified by repeated washing with Et₂O for 3 times and the solvent was removed by the rotary vacuum evaporator. The final carboxylic-functionalized ionic liquid was denotes as IM⁺Br⁻. ¹H NMR (400 MHz, DMSO-d⁶) δ(9.17, s, 1H) δ(7.79, s, 1H) δ(7.66, s, 1H) δ(4.36, t, 2H) δ(2.91, t, 2H) δ(3.87, s, 3H).

Synthesis of lanthanide polyoxometalates

(Na₉LnW₁₀O₃₆·36H₂O, abbreviated as LnW10, Ln = Eu, Tb, Sm, Dy) The lanthanide polyoxometalates was obtained according to the method reported by Peacock and Weakly.²¹ Here we took the synthesis of Na₉EuW₁₀O₃₆·32H₂O as an example: 10 mmol Na₂WO₄·2H₂O (3.3 g) was dissolved in 8 mL deionized water and the solution was heated to 85 °C and adjusted the pH to about 7–8 using the glacial acetic acids. Then the solution of Eu(NO₃)₃·xH₂O (containing 1 mmol europium nitrate in 0.8 mL deionized water) was added slowly into the above Na₂WO₄ solution. White precipitate appeared immediately after every drop and the solution was cooled to room temperature. The final crystals were filtered and dried in the air and named as EuW10. Other three lanthanide polyoxometalates Na₉TbW₁₀O₃₆·36H₂O (TbW10), Na₉SmW₁₀O₃₆·36H₂O (SmW10), Na₉DyW₁₀O₃₆·36H₂O (DyW10) were also prepared by replacing Eu(NO₃)₃·xH₂O with Tb(NO₃)₃·xH₂O, Sm(NO₃)₃·xH₂O and Dy(NO₃)₃·xH₂O, respectively.

Synthesis of Eu–IL–Zr hybrid xerogels

0.9 mmol IM⁺Br⁻ was dispersed in the ethanol firstly and then EuW10 was added. After reacting for 24 h, an appropriate amount of Zr(OCH₂CH₂CH₂CH₃)₄ (zirconium(IV) butoxide) solution was added and the whole solution was under refluxing at 70 °C for 6 h in oil bath. Afterwards about 36 mmol deionized water was added into the mixed solution to promote the hydrolysis at room temperature. The molar ratio of EuW10: IM⁺Br⁻: Zr(OCH₂CH₂CH₂CH₃)₄ : H₂O was 1 : 9 : 90 : 360. Zirconia xerogels were formed after sol–gel process by the coordination polymerization of hydroxyl bridges. After being centrifuged at 9000 r min⁻¹ and washed by ethanol for 3 times, the final materials were obtained after dried at 60 °C and denoted as Eu–IL–Zr. Other three materials Tb–IL–Zr, Sm–IL–Zr, Dy–IL–Zr were prepared by the same steps except for replacing EuW10 with their corresponding lanthanide polyoxometalates. The contents of Ln³⁺ ions (Eu³⁺, Tb³⁺) and Zn²⁺ in the hybrids were determined with ICP-OES and N were determined with elemental analysis.

Synthesis of Eu–IL–Ti hybrid xerogels

0.9 mmol IM⁺Br⁻ was dissolved in 20 mL ethanol, and then the lanthanide polyoxometalates EuW10 was added. After ion-exchange reaction for 24 h, an appropriate amount of the tetraisopropoxytitannium solution was mixed and the whole solution was under refluxing for 6 h at 70 °C. Finally, 36 mmol deionized water was dropped into the above solution to promote the hydrolysis of tetraisopropoxytitannium and the formation of hydrogel was constructed by Ti–O network. Then the final materials Eu–IL–Ti was obtained after centrifuge and washed with ethanol for three times and dried at 60 °C. The synthesis of Tb–IL–Ti, Sm–IL–Ti and Dy–IL–Ti was also obtained following the same procedures only to substitute TbW10, SmW10 and DyW10 for EuW10. And the four aluminum colloid materials Eu–IL–Al, Tb–IL–Al, Sm–IL–Al, and Dy–IL–Al were prepared in the same way.

Physical measurements

Fourier transform infrared (FTIR) spectra were measured within the 4000–400 cm⁻¹ region on the infrared spectrophotometer with the KBr pellet technique. The contents of Ln³⁺ ions (Eu³⁺, Tb³⁺, Sm³⁺, Dy³⁺) in the hybrids were determined with ICP-OES and the molar ratio of LnW10 in the hybrids could be estimated. The room temperature X-ray powder diffraction patterns (XRD) were recorded on a Rigaku D/max-rB diffractometer equipped with a Cu anode in a 2θ range from 10–70°. The luminescent spectrum and luminescence lifetime were measured on an Edinburgh FLS920 phosphorimeter using a 450 W xenon lamp as excitation source. The outer luminescent quantum efficiency was measured using an integrating sphere (150 mm diameter, BaSO₄ coating) with the Edinburgh FLS920 phosphorimeter. The luminescence 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. Thermogravimetric analysis (TG) was carried out on a Netzsch STA 409 under nitrogen atmosphere in the Al₂O₃ crucibles at a heating rate of 15 °C min⁻¹ from 30 to 1000 °C. ¹H NMR spectra were carried on a BRUKER AVANCE-500 spectrometer with tetramethylsilane (TMS) as internal reference using DMSO-d⁶ as solvent.

Results and discussion

Fig. 1 shows the selected overview of the composition and the predicted frameworks are depicted in details. In fact, the exact characterization of such complicated system is hardly possible for they involve several kinds of building blocks through different linking modes of lanthanide polyoxometalates (LnW10 unit), ion liquid linkage (IM⁺) and inorganic polymeric gel matrices (alumina, zirconia, titania). Here we only can simply predict the basic composition and chemical environment among different units. The coordination environment around lanthanide ion (Eu³⁺, Tb³⁺, Sm³⁺, Dy³⁺) can not change for LnW10 crystal framework is still kept in the hybrid system. IL (IM⁺) behaves as the linkage to both interact with EuW10⁹⁻ ion through ion exchange interaction and coordinates to zirconia, alumina, titania matrices with carboxylate groups after the hydrolysis and copolymerization process. Here we name these hybrids as Ln–IL–M (Ln = Eu, Tb, Sm, Dy, M = Ti, Zr, Al). Besides, it needs to be referred that the hydrolysis and polycondensation reaction of tetraisopropoxytitanium is much faster than other two alkoxyl precursors (aluminium-isopropylate and zirconium butoxide), which is too quick to produce the lower encapsulating content of LnW10 in titania than other two systems (alumina and zirconia) (See Table 1). This will affect the luminescent performance of the corresponding hybrid xerogels.

Fig. 1 The composition and the predicted structure of Eu–IL–Ti hybrids, for Ln–IL–Al and Ln–IL–Zr hybrids, they show the similar composition except for titania is replaced by zirconia and alumina.

Table 1 The luminescent data of twelve obtained materials

Materials	LnW10 (mol %)	τ ^a/ms	η ^b/%
a The error for decay time is ±10%. b The error for quantum yield is ±10%.
Eu–IL–Al	9.5	2.93	96
Eu–IL–Ti	3.1	2.30	11
Eu–IL–Zr	8.5	2.84	89
Tb–IL–Al	8.8	0.84	13
Tb–IL–Ti	2.6	0.70	1.1
Tb–IL–Zr	7.0	0.81	4.5
Sm–IL–Al	8.3	0.091	10
Sm–IL–Ti	2.2	0.082	0.61
Sm–IL–Zr	7.0	0.090	6.3
Dy–IL–Al	9.0	0.083	47
Dy–IL–Ti	2.9	0.059	1.3
Dy–IL–Zr	8.5	0.076	35

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Fig. 2 shows the FT-IR information of the 1-methyl-3-propionyloxy imidazolium bromide (IM⁺Br⁻) and four obtained luminescent materials Ln (Eu, Tb, Sm, Dy)–IL–Al. As shown in Fig. 2(a), the vibration bands centered about 3100 cm⁻¹ can be reckoned as the C–H stretching vibration of the ring and of the aliphatic chain. The peaks whose absorption bands locating at 1633, 1570, 1455 cm⁻¹ are ascribed to ring stretching of the imidazolium ring. The bands corresponding to the v(C–O, C=O) for COOH of IM⁺Br⁻ are situated at 1168 and 1730 cm⁻¹ respectively.^21,23 However, in Fig. 2(b) these two bands disappear and 1636, 1558 cm⁻¹ are observed in these twelve materials which can be assigned to the asymmetric stretching vibration of carboxylate group. The result is caused by the coordination reaction between –COOH group grafted on the ionic liquid and metal alkoxides. The same phenomena are observed in the FTIR spectra (shown in Fig. S1†) of other eight hybrid materials of zirconia (alumina, titania) xerogels. This indicates the formation of chelating bonds between the carboxyl group of the ionic liquid IM⁺Br⁻ and metal oxide networkds of zirconium, titanium and aluminum ions.

Fig. 2 The Fourier transform infrared spectra of 1-methyl-3-propionyloxy imidazolium bromide (IM⁺Br⁻) (a) and selected Ln–IL–Al hybrid xerogels (Ln = Eu, Tb, Sm, Dy) (b).

The representative XRD patterns for the samples are shown in Fig. 3. As displayed from the XRD pattern (Fig. 3a), the samples Eu–IL–Al, Tb–IL–Al, Sm–IL–Al, Dy–IL–Al exhibits four broad diffraction peaks with low intensity, suggesting that amorphous Al₂O₃ phase was formed. Fig. 3(b) depicts the wide angle X-ray diffraction patterns of four lanthanide polyoxometalates hybrids based on the matrix of zirconia. The previous works done by Zhang et al.²⁴ pointed out that the transition temperature of ZrO₂ from amorphous to tetragonal phase begin above 400 °C. The annealed ZrO₂ powder will show obvious peaks at 2θ = 30.2, 50.3 and 60.2° which can be assigned to the (0 1 1), (1 1 2), (1 2 1) reflections of tetragonal ZrO₂ respectively. In our work, the reaction temperature is below 100 °C and can't generate the phase of calcined tetragonal ZrO₂. The uncalcined matrix ZrO₂ still demonstrates a weak broad peak centered at 30°, which indicate the homogenous amorphous phase of ZrO₂. The results are consistent with that of Eu–IL–Ti, Tb–IL–Ti, Sm–IL–Ti and Dy–IL–Ti (shown in Fig. S2†). The lack of crystalline peaks in these obtained samples indicates the presence of organic chains in the inorganic framework and suggests that no free lanthanide polyoxometalates salts exist.

Fig. 3 The wide angle X-ray diffraction patterns of hybrid xerogels Ln–IL–Al (a) and Ln–IL–Zr (b) (Ln = Eu, Tb, Sm, Dy).

To investigate the thermal stabilities of the obtained materials, the thermogravimetric (TG) and differential thermogravimetry (DTG) are measured. Fig. 4(a) and (b) present the TG–DTG curves of Eu–IL–Al and Eu–IL–Ti hybrid xerogels, respectively. The TG curves of Eu–IL–Zr are shown in Fig. S3.† In Fig. 4(a), the selected TG curve of Eu–IL–Al hybrids is shown and three main distinct thermal weight loss processes are pointed out. The first weight loss (about 8.7%) from 30 to 230.5 °C is attributed to the evaporation of absorbed water and residual organics from the preparation.¹⁸ The second weight loss of 10.8% in the temperature range of 230–439.7 °C is ascribed to the decomposition of 1-methyl-3-propionyloxy imidazolium skeleton, with the destruction of chemical bonds.²⁵ At last, the slowest weight loss of 7.9% from 439.7 to 1000 °C is corresponding to the degradation of remaining organic groups, and the final residues is metal oxides (Al₂O₃, POM). Fig. 4(b) gives the representative thermal information of Eu–IL–Ti, and includes three mass changes in the temperature range from 30–1000 °C. The first weight loss of 9.3% below 238 °C is caused by the evaporation of physically absorbed water and crystal water in POMs and residual organics during preparation. The second weight loss from 238 to 585 °C is ascribed to the decomposition of ionic liquid and some of the organic groups, at last followed by the last mass change of 5.4% above 585 °C which is caused by the burning of residual organic components. Fig. S3† shows three similar weight loss process of Eu–IL–Zr as the above two materials. The weight loss happened from 90 °C and completed at 1000 °C. The remainder of the solid is mainly ZrO₂ and EuW10. From the thermal analysis data, we conclude that the thermal stability of the luminous sphere Ln-POMs is improved on a large scale by embedding in matrices of the ionic liquid and metal oxide.

Fig. 4 The thermogravimetric and differential thermogravimetry curves of Eu–IL–Al (a) and Eu–IL–Ti (b).

The excitation and emission spectra of the hybrid materials are shown in Fig. 5. Fig. S4(a)† depicts the excitation spectra of the parent lanthanide polyoxometalates EuW10. The excitation spectrum of EuW10 is monitored within 614 nm of the ⁵D₀ → ⁷F₂ transition at room temperature. The broad band centered at 325 nm is ascribed to the O → W charge transfer (CT) states.²⁶ The charge transfer band plays an important part in the luminescence of EuW10. Excitation of LMCT brings energy transfer from [W₅O₁₈]⁶⁻ to Eu³⁺ which lead to series of Eu³⁺ characteristic emission. The series of lines in the excitation spectra are caused by the intra-4f⁶ transitions and the characteristic peaks of Eu³⁺ situate at 362 (⁷F₀ → ⁵D₄), 374 (⁷F₀ → ⁵G₄), 381 (⁷F₀ → ⁵G₃), 384 (⁷F₀ → ⁵G₂) and 394 (⁷F₀ → ⁵L₆), respectively.²⁷ The ⁷F₀ → ⁵L₆ transition at 394 nm and ⁷F₀ → ⁵D₂ transition at 465 nm are the strongest intensity of excitation bands. This indicates that intra 4f⁶ lines show higher intensity than intensity of the LMCT states. Thus, for this compound of EuW10, direct excitation into the Eu³⁺ levels is comparable to the sensitization process via the LMCT states. Other excitation spectra of TbW10, SmW10, DyW10 are shown in Fig. S4(b) and S5(a) and (b)† resemble that of EuW10, also displaying broad peaks centered at 325 nm in 250–350 nm region and series of lanthanide inner electron transitions. The differences exist in the relative intensity about LMCT states and characteristic excitation lines of lanthanide ions. In EuW10, TbW10, the intensity of characteristic excitation lines of lanthanide ions is stronger than LMCT states, whereas comparisons of the relative intensity of the latter two polyoxometalates are opposite. The reason may be due to that Eu/Tb ions belong to a pair of conjugate elements on the both sides of Gd while Sm/Dy ions are another pair of conjugate elements. In the tungstate matrix, conjugate elements with the similar inner electron configuration present analogous luminescent behaviors.

Fig. 5 The excitation and emission spectra of Eu–IL–M (a) and Tb–IL–M (b) (M = Ti, Zr, Al) hybrid xerogels.

Fig. S4 and S5† also show the information of emission spectra of four parent lanthanide polyoxometalates LnW10, (Ln = Eu, Tb, Sm, Dy). In the emission spectra of EuW10 (Fig. S4(a)†), there are four main sharp emission bands from 550 to 750 nm which exhibit the characteristic transitions of Eu³⁺ ions and they are attributed to ⁵D₀ → ⁷F₁ (589 nm), ⁵D₀ → ⁷F₂ (615 nm), ⁵D₀ → ⁷F₃ (652 nm), ⁵D₀ → ⁷F₄ (700 nm) respectively. As is known, red luminescence of ⁵D₀ → ⁷F₂ is assigned to the electric-dipole transition which is closely related with the europium coordination environment, and the orange luminescence of ⁵D₀ → ⁷F₁ transition is ascribed to magnetic-dipole type, which is independent of the surrounding coordination environment. The ratio (R/O) of relative intensity of ⁵D₀→⁷F₂ transition to ⁵D₀ → ⁷F₁ transition is the criterion to measure the symmetry of the europium coordination environment. The larger the value of R/O indicates the lower symmetry of europium site, demonstrating the stronger intensity of red luminescence in the whole material. Seen in the emission spectra of EuW10, the intensity of ⁵D₀ → ⁷F₂ transition is weaker than that of ⁵D₀ → ⁷F₁ transition, suggesting europium is located at a higher symmetry in the coordination environment. The reasons are reckoned as europium is coordinated by two [W₅O₁₈]⁶⁻ segments, so the metal ions are completely hidden from interactions with solvent molecules.^26a The introduction of tungstate changes the lanthanide ion local symmetry which brings different appealing properties and important modifications in the surroundings of Eu³⁺. The research of Ballardini and his coworkers discloses the photophysical properties of EuW₁₀O₃₆⁹⁻ and makes a conclusion that EuW₁₀O₃₆⁹⁻ is of D_4d symmetry, in which Eu³⁺ is at a rather high symmetric site.²⁸ Compared to the emission spectra of pure lanthanide salts, another difference can be observed from the spectra of parent lanthanide polyoxometalates. ⁵D₀ → ⁷F₄ transition is abnormally high compared with the ordinary hybrids doping europium nitrate. These abnormal phenomena are analyzed by Ferreira et al.^26a In their research, the analysis about the abnormal ⁵D₀ → ⁷F₄ intensity as the consequence of the behavior of the Ω_λ intensity parameters (calculated by γ^t_P (ligand field parameters) and Γ^t_P (ligand polarizability-dependent term)) is valuable.

Other lanthanide polyoxometalates possess similar structures and their excitation spectra are alike. Their emission spectra are lanthanide ion typical emission without much discrepancy. In emission spectrum of TbW10 (Fig. S4(b)†), four sharp peaks in the range of 450–700 nm are due to f–f* transitions of Tb³⁺ and are ascribed to ⁵D₄ → ⁷F_J (J = 6–3) transitions at about 489, 544, 582 and 621 nm respectively. Among these emission peaks, the prominent green luminescence ⁵D₄ → ⁷F₅ was observed in their emission spectra. The characteristic narrow emission bands of SmW10 in 550–750 nm range are clearly observed in Fig. S5(a). But the peaks split into some parallel shoulder peaks which centered about 567 (560), 607 (597), 652 (645), 702 nm. They are assigned to ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2, 9/2, 11/2) respectively. Among these peaks, the orange luminescence ⁴G_5/2 → ⁶H_7/2 is the strongest. Fig. S5(b)† shows the emission spectrum of DyW10. The typical transition of dysprosium is seen splitting at 479 (489) and 573 (585) nm which represent ⁷F_9/2 → ⁶H_15/2 (479 nm, blue luminescence) and ⁷F_9/2 → ⁶H_13/2 (573 nm, yellow luminescence) correspondingly. The blue emission is more efficient than the yellow emission by comparing their relative intensities.

Photoluminescence comparisons of polyoxometalates encapsulated hybrids in different inorganic matrices are discussed below. Fig. 5(a) shows the luminescence spectra of europium hybrid xerogels Eu–IL–Al, Eu–IL–Zr, Eu–IL–Ti. The excitation spectra of these three hybrids are obtained by monitoring the strongest emission band of the Eu³⁺ ion at 590 nm, also with large broad band ranging from 250–350 nm and in 350–500 nm range following the typical intra-4f⁶ lines of Eu³⁺ serial transitions. In the excitation spectra, the broad band is ascribed to the O → W charge transfer of the tungstate affected by ionic liquid and inorganic matrices. The center of broad band in Eu–IL–Al, Eu–IL–Zr, Eu–IL–Ti blue-shift to about 300 nm from the central 325 nm of the parent EuW10, and hybrids Eu–IL–Ti shifts farthest. And the excitation spectrum of Eu–IL–Ti showing characteristic peaks of Eu³⁺ is much stronger than the LMCT band. The differences of excitation spectra are probably caused by different environments around EuW10. In Fig. 5(a), the emission spectra of Eu–IL–Al, Eu–IL–Zr, and Eu–IL–Ti are obtained by monitoring their maximum excitation wavelength. The relative intensity of Eu–IL–Ti is expanded 10 times to match that of Eu–IL–Al, Eu–IL–Zr. The R/O of three EuW10-assembled hybrids Eu–IL–Al, Eu–IL–Zr, Eu–IL–Ti are 0.73, 1.04, 1.21 respectively compared with 0.68 of EuW10. This indicates the chemical environment of Eu³⁺ is affected to some degree and the red luminescence is changed by inorganic host through the IL linkage. The interaction between EuW10 and ionic liquid functionalized matrix probably has influence on EuW10 through the static electronic force. From the above discussion, it is found that zirconia and alumina matrices are more suitable matrices for the luminescence of EuW10 than titania. This is not only depends on the encapsulating content (as shown in Table 1 for mol% EuW10 in hybrids) of EuW10 but also is related to the different chemical environment of inorganic xerogels.

Fig. 5(b) is the photoluminescence spectra of terbium hybrid xerogels Tb–IL–Al, Tb–IL–Zr, Tb–IL–Ti. Their emission spectra are all excited by the maximum excitation wavelength of broad band O → W ligands–metal charge transfer transition and show Tb³⁺ characteristic emission peaks. The blue luminescence at 484–495 nm is ascribed to ⁵D₄ → ⁷F₆ transition and the most prominent peaks 545 nm is due to ⁵D₄ → ⁷F₅ transition. Two tiny peaks in the orange and red luminescence region at 583 nm and 621 nm are assigned to ⁵D₄ → ⁷F₄, ⁵D₄ → ⁷F₃ transition respectively. The relative intensity of both excitation and emission spectra of zirconia hosts are more intense than that of alumina and titania. This demonstrates zirconia is a more suitable matrix for TbW10 than alumina and titania. This is both related to the encapsulating content (as shown in Table 1 for mol% TbW10 in hybrids) of TbW10 but also is related to the different chemical environment of inorganic xerogels.

We investigated the photoluminescence property of hybrid xerogels Sm–IL–Al, Sm–IL–Zr and Sm–IL–Ti. Fig. 6(a) shows their excitation spectra which are monitored at 597 nm and their emission spectra are measured by the excitation of LMCT at room temperature. All emission spectra show three bands of emission transitions as the result of the introduction of tungstate. Tungstate leads to the split of ⁴G_5/2 → ⁶H_5/2, ⁴G_5/2 → ⁶H_7/2, ⁴G_5/2 → ⁶H_9/2 transitions. The transition ⁴G_5/2 → ⁶H_5/2 at 570 nm divide from 561 to 570 nm, another transition ⁴G_5/2 → ⁶H_7/2 at 608 nm split from 597 nm to 608 nm and the last observed transition ⁴G_5/2 → ⁶H_9/2 at 653 nm break into emission region from 644 nm to 654 nm. This suggests the [W₅O₁₈]⁶⁻ segment, IM⁺Br⁻, inorganic matrix have an important influence on width, position and shapes of the peaks. Among three emission bands, the transition ⁴G_5/2 → ⁶H_7/2 is of the highest intensity. These hybrid xerogels of SmW10 are weaker than other lanthanide system, which is due to the luminescence nature of Sm³⁺ itself. Because of the existence of internal energy level (⁶F_11/2, ⁶F_9/2, …⁶H_11/2etc.) between the first excited state ⁴G_5/2 and ground state ⁶H_9/2 of Sm³⁺, the non-radiative energy transfer process takes place easily to loss the excited energy.

Fig. 6 The excitation and emission spectra of Sm–IL–M (a) and Dy–IL–M (b) (M = Ti, Zr, Al) hybrid xerogels.

Fig. 7 The CIE chromaticity diagrams of lanthanide polyoxometalates and their twelve hybrid xerogels.

The luminescence features of alumina, zirconia, and titania xerogels incorporating DyW10 are presented in Fig. 6(b). The excitation spectra are obtained by monitoring the ⁷F_9/2 → ⁶H_15/2 transition at 484 nm. In the excitation spectra of Dy–IL–Ti, the relative intensity of characteristic excitation lines of the Dy³⁺ f–f transitions is more intense than the O → W ligands–metal charge transfer transition. Whereas in excitation spectra of Dy–IL–Al, Dy–IL–Zr, the broad band of O → W ligands–metal charge transfer transition contributes more than lanthanide characteristic excitation lines, showing high intensity of the LMCT. Comparison about the intensities of excitation spectra suggests that zirconia is more advantageous in sensitizing the luminescence of DyW10 and in the longer wavelength region of Dy–IL–Zr, a broad band appears instead of lanthanide narrow excitation peaks in this region, which is not observed in pure DyW10. The appearance of such broad band around 390 nm may be probably related to a superposition with host broad emission. Their emission spectra assemble the characteristic emission of DyW10 in positions, shapes, width, with division peaks of ⁷F_9/2 → ⁶H_15/2, ⁷F_9/2 → ⁶H_13/2 transitions in the range of 479–489 nm, 573–585 nm, correspondingly. The blue luminescence is stronger than the yellow luminescence. The Y/B values (Y and B stand for the integrated intensities of the ⁷F_9/2 → ⁶H_15/2, ⁷F_9/2 → ⁶H_13/2 transitions) of Dy–IL–Al, Dy–IL–Zr, and Dy–IL–Ti are 0.76, 0.73 and 0.87 respectively, and the calculated Y/B value of parent DyW10 is 0.73. As is known, ⁷F_9/2 → ⁶H_13/2 transition is hypersensitive and changes with the influence of the external surrounding environments (alumina, zirconia and titania), resulting in the change of emission intensity ratio among emission spectra of three DyW10-incorporating hybrids. This is not only depends on the encapsulating content (as shown in Table 1 for mol % EuW10 in hybrids) of EuW10 but also is related to the different chemical environment of inorganic xerogels.

Comparisons of fluorescence lifetimes and quantum yield about poloxometalates-incorporating alumina, zirconia and titania: to further investigate the influence of different inorganic host matrix on the luminescence properties of polyoxometalates, the typical decay curves of obtained hybrids are measured at room temperature. The decay times of excited states ⁵D₀ (EuW10), ⁵D₄ (TbW10), ⁴G_5/2 (SmW10), ⁷F_9/2 (DyW10) are measured on the basis of emission decay curves monitored with the more intense Eu³⁺ (⁵D₀ → ⁷F₁), Tb³⁺ (⁵D₄ → ⁷F₅), Sm³⁺ (⁴G_5/2 → ⁶H_7/2), Dy³⁺ (⁷F_9/2 → ⁶H_15/2) transitions, under the excitation at their maximum excitation wavelength, respectively. The luminescence decay time of EuW10-incorporating hybrids follows a single exponential function in the formula ln[S(t)/S₀] = −k₁τ = −t/τ, indicating that only one sort of Eu³⁺ center exist in these hybrids and Eu³⁺ occupies the same average coordination environment (see Table 1).²⁹ Other obtained materials present similar properties.

The lifetimes of EuW10-incorporating alumina, zirconia, and titania can reach to millisecond time scale which is above 2 ms. At the same time, hybrids Eu–IL–Al, Eu–IL–Zr display longer decay lifetime than the Eu–IL–Ti, which is consistent with the analysis results of their emission spectra. Decay lifetime values of Tb–IL–Al, Tb–IL–Zr, and Tb–IL–Ti are 0.84 ms, 0.81 ms, 0.704 ms correspondingly. The values are lower than these inorganic host matrices encapsulating EuW10. The lifetime values of Sm–IL–Al, Sm–IL–Zr, Sm–IL–Ti, Dy–IL–Al, Dy–IL–Zr, and Dy–IL–Ti are 91, 90, 82, 83, 76, and 59 μs, respectively, far below the lifetime of EuW10 hybrids.

Moreover, the absolute luminescence quantum yield data are obtained with an integrating sphere and a calibrated detector setup for solid materials. The resulting absolute luminescence quantum yields are listed in Table 1. In parent lanthanide polyoxometalates, η of EuW10 is 45%. However, for Eu-IL–Ti, the quantum yield (η) of ⁵D₀ emitting level is 10.7%. When EuW10 is encapsulated into matrices of alumina and zirconia, the values of η have increased significantly which can approach to 96%, 89% correspondingly. Considering the measurement error, these value are matchable to the parent EuW10 compound at the least, here we consider it may due to that the ionic liquid functionalized alumina and zirconia affect the chemical environment of EuW10, even Eu³⁺ ion, which further decreases the non-radiative energy loss to be benefit for the luminescence of Eu³⁺.

For TbW10-incorporating hybrids, the η value (1.1%) of Tb–IL–Ti is the smallest, less than that (4.5%) of Tb–IL–Zr and the η value (13%) of Tb–IL–Al presents the maximum. The η value of TbW10 is 5.9%, so by comparison, only when TbW10 is assembled in alumina, the quantum yield can be effectively improved. In SmW10 assembled hybrid system, it seems the overall quantum yield is not high. The pure lanthanide polyoxometalates SmW10 shows 3.1% quantum yield. The quantum yield η of Sm–IL–Al, Sm–IL–Zr, and Sm–IL–Ti are 10%, 6.3% and 0.61% respectively. Through the analysis of these data, we can presume zirconia and alumina is superior host matrices to titania for incorporating SmW10. The quantum yield η of Dy–IL–Al, Dy–IL–Zr, Dy–IL–Ti are 47%, 35, %, 1.3% comparing with the value 17% of parent lanthanide polyoxometalates DyW10. On the basis of the above discussion about these polyoxometalates assembled hybrids, we may conclude that the modifications with the alumina and zirconia matrix are beneficial for the luminescent performances of lanthanide polyoxometalates.

Furthermore, the CIE chromaticity diagrams of polyoxometalates, the derived hybrids are shown in Fig. 7 and their CIE coordinates are listed in Table S1.† Among the obtained materials, Eu–IL–Ti, Tb–IL–Zr, Tb–IL–Ti, Dy–IL–Ti, Dy–IL–Zr, Dy–IL–Al locate at warm white light regions, displaying favorable luminescent performance for practical applications.

Conclusion

In summary, twelve derived hybrids are prepared through sol–gel process at mild conditions possessing remarking luminous performances. The effect of the incorporation of lanthanide polyoxometalates into different inorganic host matrices and their photoluminescence properties, characteristic emission behaviors are studied. On the basis of above results, we may presume that different matrices have influence on the luminescence of hybrid system. Alumina/zirconia matrices are more benefit for the luminescence polyoxometalates than titania. Besides, and hybrid titania xerogels are favorable for the potential to synthesize white-light luminous materials.

Acknowledgements

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

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra45281f

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