Multicomponent hybrids of surfactant-capped lanthanide polyoxometalates and ZIF-8 with tuneable luminescence

Abstract

Two lanthanide polyoxometalates [Na₉LnW₁₀O₃₆·32H₂O and K₁₃Ln(SiW₁₁O₃₉)₂·30H₂O (Ln = Eu, Tb, Sm, Dy), abbreviated as LnW₁₀ and LnSiW₁₁, respectively] were prepared and functionalized with hexadecyltrimethyl ammonium bromide to give surfactant-capped polyoxometalates (SLnW₁₀/SLnSW₁₁). Zeolitic imidazolate framework (ZIF-8) nanoparticles were prepared and combined with these SLnW₁₀/SLnSiW₁₁ polyoxometalates. Both the inorganic components were formed into multicomponent hybrids via a polymerization reaction between ethyl methacrylate and 4-vinyl pyridine in the presence of a benzoyl peroxide initiator. The physical characterization and photoluminescence of these hybrid materials were investigated in detail. The luminescence colour was tuned by adjusting the building units. These results provide useful data for the preparation of multicomponent hybrid materials for practical optical applications.

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Introduction

Lanthanide inorganic/organic hybrid materials have attracted much attention in recent decades because the organic and inorganic components are combined at the molecular and/or nanometer scale.¹ These materials have both the advantages of the organic compounds (easy processing, flexibility, multi-functionality) and the hardness, thermal and chemical stability of the inorganic components. Studies have focused on chemically bonded hybrids, which are constructed by strong and/or weak interactions such as covalent bonds, ion–covalent interactions, coordination and hydrogen bonding.² A novel direct polymerization process to construct multicomponent hybrid materials has also been reported.³

Polyoxometalates (POMs) are inorganic metal oxide clusters a few nanometers in size composed of transition metal ions connected by oxygen. POMs not only have a well-defined molecular weight and nanostructure, but also chemical, structural and electronic versatility;⁴ they can be used as building blocks to construct large supramolecular systems. These properties mean that they have wide applications in catalysis, electrochemistry, semiconductors and magnets.^5,6 Two lanthanide-doped POMs [10-tungstoeuropate (9-) and bis-(11-tungstosilicato)europate (13-) anions as two different potassium salts] with interesting luminescence properties were first reported by Peacock and Weakley⁷ in 1971. Among the known luminescent POMs, Na₉LnW₁₀O₃₆·32H₂O and K₁₃Ln(SiW₁₁O₃₉)₂·30H₂O have the highest luminescent quantum yield.⁸ The photoluminescent properties of these POMs can help us to clearly understand the charge transfer transition state (LMCT) from oxygen to the metal ion. In these POMs, the inorganic W₅O₁₈⁶⁻ and W₁₁O₃₉⁸⁻ ions affect the coordination geometry of Ln³⁺ and result in a change in its emission properties.

POMs can generally be dissolved in water and pure POMs are difficult to process as crystals or powders. It is necessary to modify POMs and other building blocks to assemble them and this depends on the electrostatic interactions between the extra cations and anions of the POMs themselves.⁹ POMs have been phase-transferred into organic media and functionalized by essential surfactants.¹⁰ Water-soluble POMs can be encapsulated into the organic components of surfactants to obtain surfactant-capped POMs. The counter cations on the surface of the POMs can be completely exchanged by cationic surfactants¹¹ to give surfactant-capped POMs with a hydrophobic surfactant shell and an encapsulated hydrophilic POM core by controlling the stoichiometric ratio in water/organic media. The surface properties of the POMs are thus changed and the surfactant-capped POMs become soluble in organic media such as chloroform, benzene or toluene. Using this effective approach, it is possible to obtain hybrid materials based on POMs.¹² POMs can be introduced into polymer matrices and these hybrids are expected to be easy to process and have good stability.

Metal–organic frameworks are a class of porous coordination polymer materials with a well-defined porous structure.¹³ Porous zeolitic imidazolate frameworks (ZIFs), metal–organic frameworks that have exceptional chemical and thermal stability, have attracted attention for use in energy, storage and chemical applications.^13–15 In particular, ZIF-8 has a sodalite zeolite-type structure with small apertures (3.4 Å) and large cavities (11.6 Å)¹⁴ and its nanoparticles show unique properties distinct from both the molecules and bulk solids. ZIF-8 has visible emission bands covering the blue-green region. Luminescent units may be introduced into hybrid systems and are expected to integrate with POMs to realize tuneable and even white luminescence. ZIF-8 nanoparticles are popular as they are cheap, non-toxic and chemically stable. ZIF-8 can also be easily modified with polymers and connected with surfactant-capped POMs and polymers through polymerization.¹⁶

In the work reported here, we assembled hybrid systems of lanthanide POMs, ZIF-8 and polymers. We carried out detailed characterizations of the obtained materials and determined their photophysical properties.

Experimental section

Materials

Four lanthanide nitrates, Ln(NO₃)₃·xH₂O (Ln = Eu, Tb, Sm, Dy), were synthesized by dissolving their oxides (Eu₂O₃, Tb₄O₇, Sm₂O₃, Dy₂O₃) in concentrated nitric acid with heating and stirring to accelerate the rate of the reactions until a crystal film appeared. Na₂WO₄·2H₂O and H₄[Si(W₃O₁₀)₄]·xH₂O were purchased from Aldrich. Hexadecyltrimethyl ammonium bromide (HTAB) and AcOK were purchased from Shanghai Adamas Reagent Co. Ltd. The solvent tetrahydrofuran (THF) was used after desiccation with anhydrous magnesium sulfate. All other reagents were analytically pure and were used as received.

Synthetic procedures

Preparation of Na₉LnW₁₀O₃₆·32H₂O (LnW₁₀). LnW₁₀ (Ln = Eu, Tb, Sm, Dy) was synthesized using the method reported by Peacock and Weakly.⁷ Na₉EuW₁₀O₃₆·32H₂O (EuW₁₀) is given here as an example: 3.3 g (10 mmol) of Na₂WO₄·2H₂O were dissolved in 8 mL of deionized water and the solution was heated to 85 °C and the pH adjusted to 7–8 with glacial acetic acid. Europium nitrate (1 mmol) was then also dissolved in 0.8 mL of deionized water and the solution of Eu(NO₃)₃·xH₂O was added slowly to the solution of Na₂WO₄. A white precipitate appeared immediately and the solution was cooled to room temperature. The final powder products (EuW₁₀) were filtered and dried in a vacuum drying oven. The other three lanthanide tungstates [Na₉TbW₁₀O₃₆·32H₂O (TbW₁₀), Na₉SmW₁₀O₃₆·32H₂O (SmW₁₀) and Na₉DyW₁₀O₃₆·32H₂O (DyW₁₀)] were prepared by replacing Eu(NO₃)₃·xH₂O with Tb(NO₃)₃·xH₂O, Sm(NO₃)₃·xH₂O and Dy(NO₃)₃·xH₂O, respectively.

Preparation of K₁₃Ln(SiW₁₁O₃₉)₂·30H₂O (LnSiW₁₁). The lanthanide-doped tungsten silicates LnSiW₁₁ (Ln = Eu, Sm, Dy) were prepared using the method reported by Peacock and Weakly.⁷ A typical procedure is as follows: H₄SiW₁₂O₄₀ (5.7 g, 2 mmol) was dissolved in 20 mL of deionized water and successively heated to 90 °C. Warm concentrated mixed solutions of Eu(NO₃)₃·6H₂O (0.4 g, 1 mmol) and AcOK (8.0 g, 80 mmol as the source of potassium, about pH 7) were added dropwise to these H₄SiW₁₂O₄₀ solutions. The light yellow solution was stirred for about 5 min at 90 °C until the suspension of K₇SiW₁₁O₃₉ dissolved completely. A colourless oil separated and crystallized from the solution when the temperature was set at 5 °C. The compound was recrystallized three times from hot water. The final powder products were filtered and dried in a vacuum drying oven and named as EuSiW₁₁. Two other lanthanide-doped tungsten silicates [K₁₃Sm(SiW₁₁O₃₉)₂·30H₂O (SmSiW₁₁) and K₁₃Dy(SiW₁₁O₃₉)₂·30H₂O (DySiW₁₁)] were also prepared by replacing Eu(NO₃)₃·xH₂O with their nitrates.

Preparation of zeolitic imidazolate frameworks. Solid-state zinc nitrate hexahydrate [Zn(NO₃)₂·6H₂O, 1.5 g (5 mmol)] and 0.4 g (5 mmol) of 2-methylimidazole were successively dissolved in 75 mL of DMF. The solution was transferred into a 100 mL vial and heated to 140 °C in a programmable oven, kept at this temperature for 24 h and then cooled to room temperature. The mother liquor was removed from the solution and 20 mL of chloroform were added to the vial. Colourless polyhedral crystals were separated from the upper layer, washed three times with DMF and dried in air for 10 min to remove the excess DMF.

Synthesis of surfactant-capped POM (LnW₁₀/LnSiW₁₁) clusters (SLnW₁₀/SLnSiW₁₁). The two kinds of newly prepared POMs were dissolved in aqueous solution and a chloroform solution of the surfactant HTAB was added slowly with stirring as reported previously.¹⁷ The initial molar ratio of HTAB to LnW₁₀ was set as 9 : 1 and the initial molar ratio of HTAB to LnSiW₁₁ was 13 : 1. The two POMs were extracted and transferred into chloroform solution after ion exchange. The organic phase was separated using a funnel and dry SLnW₁₀/SLnSiW₁₁ was obtained by evaporating the organic phase using a rotary evaporator. The products were washed three times with deionized water to remove the NaBr/KBr salts. After re-crystallization with ethanol, the final products were obtained as white powders.

Preparation of multicomponent hybrid SLnW₁₀/SLnSiW₁₁/ZIF-8/PEMA polymers (SLnW₁₀–ZIF-8–PEMA/SLnSiW₁₁–ZIF-8–PEMA)¹⁸. The sample of ZIF-8 was degassed and dried for 10 h at 426 K under vacuum to remove the solvent molecules from their channels. ZIF-8 (200 mg) was then added into 25 mL of the THF solution of the lanthanide complexes SLnW₁₀/SLnSiW₁₁ (9 : 1 molar ratio of ZIF-8/SLnW₁₀ or ZIF-8/SLnSiW₁₁). The reaction was dispersed uniformly by magnetic agitation for 5 h at 313 K. 4-Vinylpyridine (VPD) was the added dropwise to the suspension and the reaction continued for another 5 h (the final molar ratio of ZIF-8/SLnW₁₀/VPD or ZIF-8/SLnSiW₁₁/VPD was 9 : 1 : 2). A 1 mL volume of the liquid monomer EMA was then added to the mixture, followed by the aggregation initiator benzoyl peroxide (BPO). The amount of initiator added was 0.4–0.5% of the monomer. The mixed solution was agitated magnetically for approximately 6 h at 353 K under an argon atmosphere to obtain the hybrids via addition polymerization between VPD and EMA. The obtained materials were concentrated below the boiling point of THF to remove excess solvent using a rotary vacuum evaporator and the resulting product was a viscous liquid. The data for the elemental analyses are given in the ESI.†

Preparation of SLnW₁₀–ZIF-8–PEMA/SLnSiW₁₁–ZIF-8–PEMA thin films. The luminescent polymer thin films of SLnW₁₀–ZIF-8–PEMA/SLnSiW₁₁–ZIF-8–PEMA were obtained by a direct spin-coating method. A 1 mL volume of the colloid was dissolved in the correct amount of THF and dropped onto a cleaned 1 × 1 cm ITO glass fixed on a Laurell spin-coater. The spin rate and spin time were 1000 rpm. The solvent was removed by drying the thin films at room temperature after spin-coating.

Physical measurements¹⁹

The powder X-ray diffraction (PXRD) patterns were acquired on a Bruker D8 diffractometer using Cu Kα radiation at 40 mA and 40 kV; the data were collected in the 2θ range 5–65°. Fourier transform infrared (FTIR) spectra were measured in the range 4000–400 cm⁻¹ on a Nexus 912 AO446 spectrophotometer using KBr pellets. The elemental analyses (C, H, N) of the ternary lanthanide hybrids were carried out on a CARIO-ERBA 1106 elemental analyser and the metal concentrations (Ln, Zn) were determined using a Perkin Optima 2100DV inductively coupled plasma optical emission spectrometer. Photoluminescence spectra and luminescence lifetimes were detected using an Edinburgh FLS920 phosphorescent instrument. The outer absolute luminescent quantum efficiency was determined using an integrating sphere (150 mm diameter, BaSO₄ coating) on an Edinburgh FLS920 phosphorimeter. The fluorescence spectra were corrected for variations in the output of the excitation source (a 450 W xenon lamp) and for variations in the detector response. The quantum yield can be defined as the integrated intensity of the luminescence signal divided by the integrated intensity of the absorption signal. The absorption intensity was calculated by subtracting the integrated intensity of the light source with the sample in the integrating sphere from the integrated intensity of the light source with a blank sample in the integrating sphere.

Results and discussion

The PXRD patterns indicate that all the POMs with different lanthanide ions have the same crystal structure (Fig. S1†). HTAB is a cationic surfactant that has an excellent ability to coordinate with cationic, non-ionic and amphoteric surfactants. We outline here the modification used for the specific case of Na₉LnW₁₀O₃₆·32H₂O (LnW₁₀); this procedure was followed for all the POMs. The surfactant-capped LnW₁₀ (SLnW₁₀) was prepared by ion exchange between the HTAB surfactant and LnW₁₀ as a result of the electrostatic attraction force between their different charges (Fig. 1a).²⁰ A weighed amount of LnW₁₀ was dissolved in water and a chloroform solution of surfactant HTAB (molar ratio HTAB to LnW₁₀ 9 : 1) was added slowly with stirring. After ion exchange, the LnW₁₀ was extracted and transferred into the organic phase. The chloroform solution was separated via a funnel and the excess chloroform was evaporated to obtain SLnW₁₀. Energy dispersive spectrometry was used to check the full replacement of Na⁺ by HTAB in the SEuW₁₀ (Fig. S2 and Table S1†). The key to understanding the polymerization process of these lanthanide inorganic/organic hybrid materials is the aggregation reaction between the different units. Although VPD is added to the HTAB-modified LnW₁₀ and ZIF-8 to aid blending, VPD can also coordinate to the lanthanide ions through its nitrogen atoms. EMA, via its unsaturated bonds, can aggregate with VPD to construct the polymer hybrids. The final multicomponent hybrid systems were assembled through the copolymerization reaction between SLnW₁₀, ZIF-8 and the polymer.

Fig. 1 Scheme for the synthesis of (a) SLnW₁₀ and (b) SLnW₁₀–ZIF-8–PEMA. The process for the LnSiW₁₁ system is similar.

Fig. 2 shows the FTIR spectra of HTAB, EuW₁₀, SEuW₁₀ and SEuW₁₀–ZIF-8–PEMA. In the spectrum of SEuW₁₀, the bands at 2917 and 2848 cm⁻¹ are the asymmetric and symmetric stretching vibrations of CH₂ in the HTAB long alkyl chains, respectively. The C–H bending vibration is located at 1486 cm⁻¹. SEuW₁₀ has almost the same bands as HTAB and the partial bands of EuW₁₀, suggesting that the HTAB surfactant has completely replaced the sodium ions in EuW₁₀. The broad band at 3450 cm⁻¹ is related to the stretching mode of O–H, indicating the presence of water molecules; it is a common phenomenon that some water molecules surround the SEuW₁₀ in their long alkyl chains. In addition, the stretching vibration absorption bands ν(C=O, 1620 cm⁻¹) and ν(C–O, 1276 and 1147 cm⁻¹) are representative of the –COO– group of EMA. The band centred at 1470 cm⁻¹ for SEuW₁₀–ZIF-8–PEMA is related to the asymmetric stretching vibration of C–N, revealing the presence of VPD. Compared with SEuW₁₀–ZIF-8–PEMA, the band for HTAB and EuW₁₀ in the spectrum of SEuW₁₀ is observed as a result of the polymerization reaction. In contrast with the LnW₁₀ material, they have the nearly same FTIR spectra in LnSiW₁₁ (Fig. S3†).

Fig. 2 Selected FTIR spectra of HTAB, EuW₁₀, SEuW₁₀ and SEuW₁₀–ZIF-8–PEMA.

The thermal stability of the products was examined using thermal gravimetric analysis (TGA) performed in air at a heating rate of 5 °C min (Fig. S4†). The TGA data revealed that the hybrid materials released their organic components (EMA, VPD and BPO) in the temperature range 45–220 °C. SEuW₁₀–ZIF-8–PEMA is thermally stable to 350 °C, above which a further mass loss of 70 wt% before 500 °C is ascribed to the decomposition of the framework forming Zn(MeIM)₂. No mass loss was observed between 220 and 500 °C, showing that ZIF-8 does not contain any other small molecules inside the channels. The TGA results show that the thermal stability of the hybrid materials is much better than that of the lanthanide complexes.

Fig. 3 shows the room temperature PXRD patterns of HTAB, EuW₁₀, SEuW₁₀, ZIF-8 and SEuW₁₀–ZIF-8–PEMA from 5 to 60°. The PXRD pattern of surfactant-capped EuW₁₀ (SEuW₁₀) showed slightly different peaks from the original HTAB and EuW₁₀ materials, suggesting that the structure and morphology are completely changed after ion exchange between the surfactant and the POMs (Fig. S5† shows the PXRD patterns of HTAB, EuSiW₁₁, SEuSiW₁₁, ZIF-8 and SEuSiW₁₁–ZIF-8–PEMA). HTAB can replace the Na⁺ ions and surround them to form a hydrophobic material. In the upper trace (Fig. 3), the diffraction peaks of SEuW₁₀–ZIF-8–PEMA partially agree with each value for the as-synthesized EuW₁₀ and ZIF-8, indicating that the products had two main components of ZIF-8 and EuW₁₀. However, there is a slight difference in the product, which may be influenced by the excess HTAB surfactant sticking to the external surface of EuW₁₀. The intensities of the ZIF-8 peaks are much stronger than the other component in SEuW₁₀–ZIF-8–PEMA, which means that the amount of doping of lanthanide ions is less than the amount of ZIF-8 in the product and that ZIF-8 is the major component in these compounds. We predict that the intensity of the lanthanide diffraction peaks increases with the increase in the SEuW₁₀ content.

Fig. 3 Wide-angle X-ray diffraction patterns of HTAB, EuW₁₀, SEuW₁₀, ZIF-8 and SEuW₁₀–ZIF-8–PEMA.

The luminescence performance of the lanthanide hybrid materials were studied at room temperature. Fig. S6a† shows the luminescent excitation and emission spectra of the parent lanthanide EuW₁₀ POMs. The excitation spectrum of EuW₁₀ is monitored at the characteristic ⁵D₀ → ⁷F₂ Eu³⁺ transition at 614 nm. The broad band located at 300 nm is attributed to the O–W charge transfer (CT) states.²¹ The excitation of the ligand to metal charge transfer (LMCT) state accompanied by energy transfer from the ligand [W₅O₁₈]⁶⁻ to Eu³⁺ leads to a series of characteristic Eu³⁺ emissions. The various lines are caused by interior 4f–4f transitions in the excitation spectra and the characteristic peaks of Eu³⁺ at 362 (⁷F₀ → ⁵D₄), 374 (⁷F₀ → ⁵G₄), 381 (⁷F₀ → ⁵G₃), 384 (⁷F₀ → ⁵G₂) and 394 nm (⁷F₀ → ⁵L₆), respectively. The ⁷F₀ → ⁵L₆ transition at 394 nm has the strongest intensity of the excitation bands, indicating that the 4f–4f transitions have a higher intensity than the LMCT states. The excitation spectra of the other LnW₁₀ (TbW₁₀, SmW₁₀ and DyW₁₀) are shown in Fig. S6b–d,† respectively; they are all similar to the spectra for EuW₁₀, showing a broad band with a nearby peak at 300 nm and lanthanide inner electron transitions. Fig. S5† also shows the emission spectra of the four parent lanthanide LnW₁₀ POMs (Ln = Eu, Tb, Sm and Dy); the emission spectra of the lanthanide LnSiW₁₁ POMs (Ln = Eu, Sm and Dy) are shown in Fig. S7.† In the emission spectra of EuW₁₀ and EuSiW₁₁ (Fig. S6a and S7a†), there are four main sharp emission bands between 550 and 750 nm, which are the characteristic transitions of Eu³⁺ ions and are ascribed to ⁵D₀ → ⁷F_J (J = 1–4) transitions at about 594, 614, 652 and 703 nm, respectively. In the emission spectrum of TbW₁₀ (Fig. S6b†), four sharp peaks in the range 450–700 nm are attributed to the f–f transitions of Tb³⁺ and are due to ⁵D₄ → ⁷F₆ (488 nm), ⁵D₄ → ⁷F₅ (545 nm), ⁵D₄ → ⁷F₄ (583 nm) and ⁵D₄ → ⁷F₃ (621 nm), respectively. Among these emission peaks, prominent green luminescence ⁵D₄ → ⁷F₅ was observed. The characteristic narrow emission bands of SmW₁₀ and SmSiW₁₁ located from 550 to 750 nm are clearly shown in Fig. S6c and S7b.† The peaks that are split into parallel shoulder peaks are centred at about 567 (561), 607 (597), 652 (646) and 703 nm, respectively. These belong to the ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2, 9/2, 11/2) transitions of Sm³⁺. Among these peaks, the orange luminescence of ⁴G_5/2 → ⁶H_7/2 is the strongest. Fig. S6d and S7c† show the emission spectrum of DyW₁₀ and DySiW₁₁. The typical transitions of the dysprosium ion are obviously split at 479 (490) and 574 (585) nm, corresponding to ⁷F_9/2 → ⁶H_15/2 (479 nm, blue luminescence) and ⁷F_9/2 → ⁶H_13/2 (574 nm, yellow luminescence). The blue emission is more efficient than the yellow emission with respect to the relative intensities.

Fig. 4a shows the luminescence spectra the SEuW₁₀–ZIF-8–PEMA hybrids. The excitation spectra of the hybrids were monitored by selecting the characteristic emission band of the Eu³⁺ ion at 613 nm and the large broad band ranging from 250 to 350 nm. The strong and sharp lines in the range 330–500 nm are attributed to the narrow interior 4f–4f transitions of Eu³⁺ in the host lattice. In the excitation spectra, the first broad band is ascribed to the O–W CT of tungstate. The centre of the broad band in SEuW₁₀–ZIF-8–PEMA is blue-shifted to about 290 nm from the central 300 nm of the parent EuW₁₀ (the SEuSiW₁₁–ZIF-8–PEMA hybrids show the same and further blue-shift phenomena from 350 to 330 nm in Fig. S8a†). The excitation spectra of the two hybrids show that the characteristic peaks of Eu³⁺ are much stronger than the LMCT band. Comparison of the two main excitation spectra of ZIF-8 and EuW₁₀ shows that they have the same excitation band at approximately 390 nm (the luminescent excitation and emission spectra of ZIF-8 are shown in Fig. S9†). When monitoring the emission wavelength at less than 395 nm, both materials appeared in the emission luminescent spectra of SEuW₁₀–ZIF-8–PEMA. The digital photograph of the sample was obtained by excitation at 395 nm (Fig. 4b). The weak blue emissions between 430 and 500 nm were assigned to ZIF-8 and a dominant red emission originating from EuW₁₀ was observed between 580 and 710 nm. The integrated luminescence appears pink in colour as a result of a combination of the blue and red emissions from the two materials. The SEuSiW₁₁–ZIF-8–PEMA hybrid displays a blue–white colour (digital photograph in Fig. S8b†).

Fig. 4 (a) Excitation and emission spectra of SEuW10–ZIF-8–PEMA hybrid and (b) digital photograph of SEuW10–ZIF-8–PEMA hybrid obtained by excitation at 395 nm in the dark.

Fig. 5a shows the photoluminescence spectra of the terbium hybrid STbW₁₀–ZIF-8–PEMA. The excitation spectrum shows the characteristic emission wavelength of 545 nm of the Tb³⁺ ion and a broad band between 320 and 400 nm; the maximum peak at approximately 377 nm originates from direct excitation of the Tb³⁺ 4f–4f transition. The emission spectrum also shows the maximum excitation wavelength of the characteristic peaks of Tb³⁺. Although the broad band O–W ligand–metal CT transition has a strong intensity, it does not excite the emission of ZIF-8 at the same time. In the emission spectrum of the hybrids, the blue luminescence at 485–495 nm is due to the ⁵D₄ → ⁷F₆ transition and the strongest peak at 545 nm is ascribed to the ⁵D₄ → ⁷F₅ transition. Two other weak peaks in the orange luminescence region at 585 nm and the red luminescence at 623 nm are related to the ⁵D₄ → ⁷F₄ and ⁵D₄ → ⁷F₃ transitions, respectively. In addition to the characteristic emission of Tb³⁺, a broad band located in the range 420–470 nm in the blue region is assigned to the emission of ZIF-8. Fig. 5b shows a digital photograph of the sample under the excitation conditions. The final luminescence displays a light blue colour as a combination of the green and blue emissions from TbW₁₀ and ZIF-8, respectively.

Fig. 5 (a) Excitation and emission spectra of STbW10–ZIF-8–PEMA hybrid and (b) digital photograph of STbW10–ZIF-8–PEMA hybrid obtained by excitation at 377 nm in the dark.

We studied the photoluminescence properties of the SSmW₁₀–ZIF-8–PEMA and SSmSiW₁₁–ZIF-8–PEMA hybrids. Fig. S10a† shows the excitation spectra at 600 nm and the emission spectra obtained by the excitation of the LMCT at room temperature. The emission spectra show three emission transition bands as a result of the introduction of tungstate. Tungstate leads to splitting of the ⁴G_5/2 → ⁶H_5/2, ⁴G_5/2 → ⁶H_7/2 and ⁴G_5/2 → ⁶H_9/2 transitions. The transition ⁴G_5/2 → ⁶H_5/2 at 568 nm divides from 561 to 568 nm, another transition ⁴G_5/2 → ⁶H_7/2 at 608 nm splits from 600 to 608 nm and the last observed transition ⁴G_5/2 → ⁶H_9/2 at 653 nm breaks in the emission region from 646 to 653 nm. Among these three emission bands, the transition ⁴G_5/2 → ⁶H_7/2 has the highest intensity. The emission spectrum of SSmW₁₀–ZIF-8–PEMA only shows the red colour of the matrix resulting from the excitation of the LMCT (Fig. S10b†). When the emission wavelength of the characteristic peak of ZIF-8 was monitored under 395 nm, another matrix peak appeared in the emission luminescent spectrum of SSmW₁₀–ZIF-8–PEMA (blue photograph in Fig. S10c†). SSmW₁₀–ZIF-8–PEMA can thus show different luminescence behaviour under different excitation conditions. In addition, the hybrid SSmSiW₁₁–ZIF-8–PEMA shows further different photoluminescence features as a result of the different POM. The excitation spectrum of SSmSiW₁₁–ZIF-8–PEMA was collected at 600 nm, which is a characteristic emission wavelength of Sm³⁺ ions, and in a narrow band between 390 and 420 nm with a maximum peak of approximately 403 nm attributed to the interior narrow 4f–4f transitions of Sm³⁺. Therefore both materials can be observed in the emission spectrum of SSmSiW₁₁–ZIF-8–PEMA; the digital photograph of the hybrids is shown in Fig. 6b with excitation at 403 nm. A dominant red emission located between 580 and 710 nm originates from SmSiW₁₁ and the blue emission between 430 and 490 nm is ascribed to ZIF-8. The integrated luminescence displays a white colour as a result of the combination of the blue and red emissions from the two materials.

Fig. 6 (a) Excitation and emission spectra of SSmSiW11–ZIF-8–PEMA hybrid and (b) digital photograph of SSmSiW11–ZIF-8–PEMA hybrid excited at 403 nm in the dark.

The luminescence features of SDyW₁₀–ZIF-8–PEMA and SDySiW₁₁–ZIF-8–PEMA incorporating DyW₁₀ and DySiW₁₁ are presented in Fig. 7a and S11a,† respectively. The two excitation spectra were both obtained by monitoring the ⁷F_9/2 → ⁶H_15/2 transition at 479 nm. In the excitation spectrum of SDyW₁₀–ZIF-8–PEMA, the broad band of the O–W ligand–metal CT transition contributed more than the characteristic lanthanide excitation lines, showing the high intensity of the LMCT as well as SSmW₁₀–ZIF-8–PEMA. However, in the excitation spectrum of SDySiW₁₁–ZIF-8–PEMA, the relative intensity of the characteristic excitation lines of the Dy³⁺ 5f–5f transitions was more intense than that of the O–W ligand–metal CT transition. The emission spectrum of SDyW₁₀–ZIF-8–PEMA has similar features to that of SSmW₁₀–ZIF-8–PEMA and only shows white luminescence on excitation of the LMCT (Fig. 7b). The emission resembles the characteristic emission of DyW₁₀ in position, shape and width, with split peaks of the ⁷F_9/2 → ⁶H_15/2 and ⁷F_9/2 → ⁶H_13/2 transitions in the corresponding ranges of 479–490 and 574–585 nm. The blue luminescence is stronger than the yellow luminescence. ZIF-8, the other component, was monitored at the emission wavelength of its characteristic peak at 397 nm (Fig. 7c). The luminescence phenomenon of the SDySiW₁₁–ZIF-8–PEMA hybrid is different from that of SDyW₁₀–ZIF-8–PEMA as a result of the different matrix. The excitation spectrum of SDySiW₁₁–ZIF-8–PEMA was monitored at 479 nm, one of the characteristic emission wavelengths of Dy³⁺ ions, and showed series bands at 340–420 nm with a maximum value of nearly 366 nm, which is ascribed to the interior 5f–5f transitions of Dy³⁺. Two main components can be observed in the emission spectrum of SDySiW₁₁–ZIF-8–PEMA (Fig. S11a†); the digital photograph of the hybrid is shown in Fig. S11b† with excitation at 366 nm. There are two dominant blue and yellow luminescences in the ranges 479–490 and 574–585 nm, respectively, derived from DySiW₁₁. A tiny blue emission between 420 and 470 nm was attributed to ZIF-8. The integrated luminescence displays a blue-white colour, which is a combination of the light blue (trace amounts of ZIF-8 in the hybrids) and white emissions from the two components.

Fig. 7 (a) Excitation and emission spectra of SDyW₁₀–ZIF-8–PEMA and digital photographs of SDyW₁₀–ZIF-8–PEMA irradiated at (b) 297 nm and (c) 397 nm in the dark.

Transmission electron microscopy images are provided in Fig. S12.† These images show that the process of ion exchange turns the LnW₁₀/LnSiW₁₁ into nanoparticles and that the inorganic components of SLnW₁₀/SLnSiW₁₁ and ZIF-8 are homogeneously diffused into the organic matrices. The photographs of the thin films are given in Fig. S13.† These show the transparency of the SEuW₁₀–ZIF-8–PEMA and SEuSiW₁₁–ZIF-8–PEMA films. Table 1 lists the excitation wavelengths, luminescence lifetimes and emission quantum yields of the hybrids. These data show that the hybrid films fabricated with EuW₁₀ and EuSiW₁₁ have longer lifetimes and higher quantum yields than the other hybrid systems.

Table 1 Luminescence lifetimes and absolute quantum yields of SLnW₁₀–ZIF-8–PEMA and SLnSiW₁₁–ZIF-8–PEMA hybrids

LnW10/LnSiW10	τ (μs)	η (%)	λex (nm)	λem (nm)
EuW10	1017	24.3	395	613
TbW10	936	17.7	377	545
SmW10	54	10.6	295	600
DyW10	55	9.1	297	479
EuSiW11	938	19.9	394	613
SmSiW11	100	11.4	403	600
DySiW11	74	9.2	366	479

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Conclusions

We prepared two kinds of lanthanide POMs [Na₉LnW₁₀O₃₆·32H₂O, K₁₃Ln(SiW₁₁O₃₉)₂·30H₂O (Ln = Eu, Tb, Sm, Dy)] and encapsulated POM into a surfactant to obtain surfactant-encapsulated POM clusters (SLnW₁₀/SLnSiW₁₁) and zeolitic imidazolate frameworks (ZIF-8). In the presence of BPO as the initiator, the synthesized surfactant-encapsulated POMs, ZIF-8 and polymer units are formed by additional polymerization reactions. The photoluminescent properties of these hybrids show that the hybrids containing EuW₁₀ or EuSiW₁₁ both have longer lifetimes and higher quantum yields than the other lanthanide hybrids. Some of the hybrid systems have an almost cool white emission. SSmW₁₀–ZIF-8–PEMA and SDyW₁₀–ZIF-8–PEMA emit different luminescence colours under different excitation wavelengths. The SEuW₁₀–ZIF-8–PEMA, SEuSiW₁₁–ZIF-8–PEMA and SSmSiW₁₁–ZIF-8–PEMA hybrids in particular have potential value in practical cool white lighting applications. This technique provides a new strategy to obtain white luminescence hybrid materials.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (91122003), Science & Technology Commission of Shanghai Municipality (14DZ2261100) 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/c4ra12615g

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