Novel photofunctional hybrid materials (alumina and titania) functionalized with both MOF and lanthanide complexes through coordination bonds

Abstract

In this paper, series of luminescent hybrid materials (alumina, titania) functionalized with both lanthanide complexes and metal organic frameworks (MOF, Al-MIL-53-COOH) through coordination bonds. In these hybrid systems, both MOF and lanthanide (Eu³⁺, Tb³⁺) complexes with beta-diketonates (2-thenoyltrifluoroacetonate (TTA), 1,1,1-trifluoroacetylacetone (TAA), 2,4-pentanedionate (AA)) and pyridine-3,5-dicarboxylic acid (PDA) are coordinated to alumina and titania through their carboxylic groups. Al-MIL-53-COOH possesses the active –COOH group and can be linked to metallic alkoxides (Ti[OCH(CH₃)₂]₄ and Al[OCH(CH₃)₂]₃), by a post-synthesis path. Ternary complexes of beta-diketonates and PDA can be further bonded with metallic alkoxides through PDA as a bridge for its carboxylic group coordination. After hydrolysis and a condensation process, the final multi-component hybrid materials can be assembled and characterized. The photophysical properties of these hybrid materials were studied in detail, and their luminescent color can be tuned by controlling the composition of different units in the hybrid system. Especially with the hybrids of europium complexes and MOF, the white luminescence can be obtained by integrating the emission of both europium complex and MOF. Furthermore, the luminescent films that were prepared show both uniformity and transparency. These results provide some useful data for the multi-component assembly and luminescent integration of photofunctional hybrid materials based on MOF and lanthanide units, which can be expected to have some potential applications in luminescent devices for display or lighting.

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Introduction

Photofunctional hybrid materials of both organic and inorganic networks provide a lot of opportunities to exhibit their extraordinary properties,¹ which are favorable for some potential applications in practical fields such as lighting and displays, optical amplifiers, fiber lasers, photophysical sensing, and so on.² In all these applications, luminescent lanthanide hybrid systems are one of the most fascinating fields, which keep the generally luminescent character of the lanthanide complexes, such as narrow and intense emission lines in the visible and near infrared region upon ultraviolet-visible (UV-vis) light irradiation, via an effective intramolecular energy transfer process.³ In recent years, considerable research has been carried out on versatile lanthanide hybrid materials and on some universal strategies or paths to achieve special chemical linkers to construct hybrid systems.^4–6 Among these, it is worth pointing out that the predominant research is focused on the organically modified silica-derived hybrid materials,^5,6 whose sol–gel technology makes it practical to fabricate all kinds of active units in the construction of hybrids. Subsequently, the thermal stabilities and photophysical properties of lanthanide complexes can be improved.^7,8 Furthermore, polymer units can be introduced into the lanthanide hybrid system and this is of significance for their potential application in optical devices.^9,10

Furthermore, the amorphous organically modified silica network hybrids can be transferred to ordered mesoporous silica with special surfactants or other templates, whose regular pore structure, high photostability and thermal stability provide a suitable host for guest–host assembly chemistry of photoactive species. Nowadays, lanthanide hybrids based on a functionalized mesoporous host have been prepared, involving MCM-41(48), mesoporous silica (SBA-15(16)) and polyoxometalates (POMs), and so on.^11–13 The highly ordered structures and uniform tunable pore sizes mean that the applications of these photofunctional hybrids can be expanded into the optical or electronic areas.

But the silica-derived hybrid materials including mesoporous hybrids mainly belong to the group of amorphous-state materials, whose non-crystalline nature makes it very difficult to study their composition and structure in the hybrid system. In order to address this problem, one approach is to introduce a crystalline unit into the hybrid system to keep the exact coordination environment around lanthanide ions, such as lanthanide polyoxometallates as the central species.^14,15 A more extensive strategy is to functionalize the crystalline host, such as traditional microporous zeolite, and some host–guest hybrid materials based on zeolite were developed.¹⁶ Metal organic frameworks (MOFs) are crystalline materials with regular porous networks constructed from organic linker molecules and metal ions.¹⁷ Luminescent MOFs possess high, well-defined porosity and show intense fluorescence, and have gained considerable attention because of their potential applications in chemical sensors, light emitting devices and biomedicine.¹⁸ The abundant groups in the MOFs make it possible for them to be modified by the so-called post-synthesis strategy.¹⁹ This provides great potential for constructing a hybrid system based on MOFs.

Al-MIL-53-COOH has a high crystalline framework that consists of regular one-dimensional channels (Fig. S1 in ESI†), which show high structural flexibility.²⁰ The presence of a non-coordinating carboxyl group, as well as the high thermal and chemical stabilities makes it a good candidate for assembling hybrid materials with other photoactive units. So in this paper, we report on the fabrication of Al-MIL-53-COOH and lanthanide complexes in inorganic matrices (alumina or titania) using co-ordination bonds to assemble the multi-component hybrid materials, whose physical characterization and luminescent properties are studied in detail.

Experimental section

Starting materials

Al-MIL-53-COOH (MOF) nanoparticles were synthesized according to a synthesis method and conditions (molar ratio, time and temperature) described in the literature²⁰ using AlCl₃·6H₂O and trimellitic acid (1,2,4-benzenetricarboxylic acid). Two lanthanide chlorides, LnCl₃·xH₂O (Ln = Eu or Tb) were prepared by dissolving their oxides (Eu₂O₃ or Tb₄O₇) in concentrated hydrochloric acid (37%) with heating and stirring to promote the reaction until a crystal film appeared. 2-Thenoyltrifluoroacetone (99%, TTA, Adamas), 1,1,1-trifluoroacetylacetone (98%, TAA, Adamas), 2,4-pentanedione (99%, AA, Adamas), aluminum isopropoxide (99%, Adamas), tetraisopropoxytitanium (98%, Adamas) and pyridine-3,5-dicarboxylic acid (98%, PDA, Adamas) were used as received. All the other reagents were analytically pure and used without purification.

Synthesis of nanosized MOF (Al-MIL53-COOH) and MOF-Al/Ti precursors

Al-MIL-53-COOH nanoparticles were synthesized using a solvothermal method²⁰ using aluminum chloride (AlCl₃·6H₂O), trimellitic acid (H₂BDC-COOH) and N,N-dimethylformamide (DMF). The following optimal reaction conditions were determined: (1) the molar ratio of Al³⁺ : H₂BDC-COOH was 1 : 1; (2) the temperature was kept at 180 °C for 12 h. The reactants were stirred for a few minutes before transferring the resulting suspension to a Teflon-lined steel autoclave. The resulting nanocrystals were separated by centrifugation at 16 000 rpm for 10 min, then washed three times with ethanol to remove the excess of unreacted aluminum chloride species and trimellitic acid and then dried at 200 °C for 6 h to remove the adsorbed DMF molecules. The scheme for its crystal structure and the X-ray powder diffraction (XRD) pattern has been shown in Fig. S1 and S2† in the ESI. The MOF-Al/Ti precursors were prepared by mixing MOF and Al[OCH(CH₃)₂]₃ (or Ti[OCH(CH₃)₂]₄) in a molar ratio of 1 : 2 after refluxing at 70 °C for 6 h.

Synthesis of lanthanide hybrid hydrogels

Preparation of the L-Ln-PDA-Al/Ti system (Ln = Eu, L = TTA, TAA; Ln = Tb, L = TAA, AA). In this step, we prepared eight specimens of lanthanide hybrid hydrogels. Considering the synthesis of TTA-Eu-PDA-Ti as an example: (1) 3 mmol TTA was dissolved in an appropriate amount of anhydrous ethanol using 3 mmol NaOH to destroy the molecular structure of the methylene proton in the reaction system. The solution was kept at 60 °C for 2 h and then 1 mmol EuCl₃·6H₂O was added dropwise to the solution. The mixture continued to react at 60 °C for 1 h and was then cooled, filtered to remove the white precipitate (NaCl) and the resulting product was Eu-TTA. (2) 2 mmol PDA was dispersed in the ethanol first and then an appropriate amount of aluminum isopropoxide (Eu³⁺ : Al[OCH(CH₃)₂]₃ = 1 : 20) was added. After stirring for a while, NH₄OH was added and whole solution was refluxed at 70 °C for 6 h in an oil bath. The two solutions were mixed together, and then a mixed solution of Eu-TTA-Ti was formed.

Preparation of the L-Ln-PDA-Al/Ti-MOF hybrid system (Ln = Eu, L = TTA, TAA; Ln = Tb, L = TAA, AA). The solutions of MOF-Al/Ti and L-Ln-PDA-Al/Ti with a molar ratio of 10 : 1 were mixed and then 160 mmol of deionized water was added dropwise to the solution to promote the hydrolysis of Al[OCH(CH₃)₂]₃ (Ti[OCH(CH₃)₂]₄) and the formation of a hydrogel was constructed using an Al–O or Ti–O network. Then the final material, L-Ln-PDA-Al/Ti-MOF, was obtained after centrifugation and washed three times with ethanol and then dried at 60 °C. The contents of the metal ions (Al³⁺, Eu³⁺, Tb³⁺, Ti⁴⁺) and N in the hybrids were determined. It is hard to determine the exact composition of them within the complicated hybrid system, but the ratio among the different units based on their contents in the whole hybrid materials can be predicted. For TTA-Eu-PDA-Al-MOF: Eu 2.49%, Al 21.30%, N 0.43%; for TAA-Eu-PDA-Al-MOF: Eu 2.58%, Al 22.05%, N 0.45%; for TAA-Tb-PDA-Al-MOF: Tb 2.61%, Al 22.00%, N 0.44%; for AA-Tb-PDA-Al-MOF: Tb 2.77%, Al 22.81%, N 0.48%; for TTA-Eu-PDA-Ti-MOF: Eu 2.11%, Al 3.65%, Ti 26.82%, N 0.38%; for TAA-Eu-PDA-Ti-MOF: Eu 2.19%, Al 3.74%, Ti 27.60%, N 0.40%; for TAA-Tb-PDA-Ti-MOF: Tb 2.29%, Al 3.75%, Ti 27.64%, N 0.39%; for AA-Tb-PDA-Ti-MOF: Tb 2.33%, Al 3.82%, Ti 28.20%, N 0.40%. Depending on the content of Ln³⁺ and N, it can be predicted that the molar ratio of Ln : N is close to 1 : 2, suggesting that the two PDA ligands were co-ordinated to Ln³⁺. Also, for alumina, the molar ratio of Ln³⁺ : Al³⁺ is close to 1 : 50, whereas for titania, the molar ratio of Ln³⁺ : Al³⁺ : Ti⁴⁺ is close to 1 : 10 : 40, both of which are in agreement with the reaction molar ratio. Therefore, the previously described preparation reaction can be controlled, and the scheme in Fig. 1 is acceptable for the preparation of these hybrid systems.

Fig. 1 The selected scheme for the synthesis process, composition and proposed structure of TTA/TAA-Eu-PDA-Ti-MOF; other hybrid systems (except for different beta-diketonates or alumina) show a similar reaction scheme.

Preparation of thin films of lanthanide hybrid hydrogels (TTA/TAA-Eu-PDA-Ti-MOF)

The lanthanide hybrid hydrogel thin films of Al-MIL-53-COOH assemblies were prepared by direct dip-coating of their colloidal solution at room temperature and under ambient pressure, using a pre-cleaned 1 cm × 1 cm indium tin oxide glass which was fixed on a Laurell spin coater. The spin rate and spin time were kept at 1000 rpm for 60 s. The solvent adsorbed was removed by drying the thin films at room temperature after spin coating.

Physical characterization

Fourier transform infrared (FTIR) spectra were measured within the 4000–400 cm⁻¹ region on an infrared spectrophotometer using the KBr pellet technique. The contents of Al³⁺, Eu³⁺, Tb³⁺ and Ti⁴⁺ in the hybrids were determined using an inductively coupled plasma optical emission spectrometer. The elemental analyses of the nitrogen element were measured with a CARLO-ERBA 1106 elemental analyser. The room temperature XRD patterns were recorded on a Rigaku D/Max-rB diffractometer equipped with a Cu anode in a 2θ range from 10–70°. The scanning electron microscopy (SEM) images were obtained with a Philips XL30 field emission scanning electron microscope. Nitrogen adsorption–desorption isotherms were measured at the temperature of liquid nitrogen, using a Nova 1000 surface area analyzer. Surface areas were calculated by the Brunauer–Emmett–Teller (BET) method. The luminescent spectra were measured on an Edinburgh Instruments 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 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.

Results and discussion

The scheme for the synthesis process, composition and proposed structure of TTA/TAA-Eu-PDA-Ti-MOF is shown in Fig. 1. Metallic alkoxides (Ti[OCH(CH₃)₂]₄, Al[OCH(CH₃)₂]₃) can be modified by carboxylic group reagents such as PDA, a technique which has been proved in previous research.²¹ Then the lanthanide beta-diketonates can be further coordinated to the PDA-modified metallic alkoxides to give a high coordination number and with a nitrogen atom remaining in the PDA, resulting in the eight coordinated complexes.^21,22 However, Al-MIL-53-COOH (MOF) can also be introduced in the course of the sol–gel reaction process to form alumina and titania for its coordination of COOH group. Subsequently, after the hydrolysis and polycondensation process of metallic alkoxides, the final hybrid materials based on alumina or titania were prepared and the photoactive units of MOF and lanthanide complexes were chemically linked. Here we only used Eu-TTA/TAA-Ti-1 as an example; other hybrids (except for different beta-diketonates or alumina) show a similar reaction scheme.

Fig. 2a depicts the FTIR spectrum of compound MOF. The peak at 3454 cm⁻¹ is in a high frequency region and corresponds to the adsorbed water molecules. The absorption peaks at 1675 cm⁻¹ (ν_as) and 1453, 1419 cm⁻¹ (ν_s) suggest the presence of carboxylic acids and carboxylate. The absorption peaks at 1170 and 785 cm⁻¹ are ascribed to the C–H deformation vibrations of the aromatic ring. Fig. 2b shows the selected FTIR spectra of the final hybrids: Eu-TTA/TAA-Ti/Al-MOF. The characteristic stretching vibrations of the coordinating carboxylate groups are easily visible, ν_as at 1613 and ν_s at 1402 or 1379 cm⁻¹, revealing that the further coordination to Ti or Al in the hybrids produces the red-shift of frequency of the COO⁻ group in MOF. The presence of water molecules can be shown by the broad band in a high frequency region over 3200 cm⁻¹.

Fig. 2 The FTIR spectra of Al-MIL-53-COOH (MOF) (a) and selected hybrid materials (b).

Fig. 3 shows the selected SEM images of the hybrids TAA-Tb-PDA-Al-MOF (a) and TTA-Eu-PDA-Ti-MOF (b), respectively. Both of them show little difference between their morphology. Compared to the bulk Tb-TAA-Al-MOF hybrids, Eu-TTA-Ti-MOF shows a softer and finer microstructure. This may be related to the different sol–gel reaction behaviour of Al and Ti precursors.

Fig. 3 The selected SEM images of hybrid materials TAA-Tb-PDA-Al-MOF (a) and TTA-Eu-PDA-Ti-MOF (b).

Fig. S3† shows the XRD patterns (from 10 to 70°) of hybrid materials L-Eu/Tb-PDA-Al/Ti-MOF (a) alumina system and (b) titania system (Eu³⁺, L = TTA, TAA; Tb³⁺, L = TAA, AA). Both of the alumina and titania hybrids present in the dominant broad band belong to alumina or titania gels (Al–O network and Ti–O network) in the amorphous state. Because there is a limited amount of MOF in the hybrids for fabrication, the crystalline diffraction peaks for MOF cannot be identified clearly in these XRD patterns. Comparing the two figures, the Eu-TTA/TAA-Al-MOF hybrids seem to show slightly better crystallization than the Eu-TTA/TAA-Ti-MOF systems, which is consistent with the SEM of the product obtained. Considering the same Al³⁺ ion component in both MOF and alumina, the corresponding crystalline state may be good. It needs to be made clear that in the hybrid system, the host is titania or alumina and the lanthanide complex or MOF are all active units in the titania or alumina host. Therefore, MOF and the lanthanide complex are both present in a small amount in the larger amount of the host titania or alumina. So the final powder X-ray diffraction (PXRD) patterns present the main features of the host titania or alumina and it is hardly worth checking the characteristics of the lanthanide complex or the MOF. Naturally, it is impossible to show clearly the crystalline character of MOF from the PXRD pattern. In addition, for the previously mentioned reason, the structure of TTA-Eu-PDA-Ti@MOF and TTA-Tb-PDA-Ti@MOF, TAA-Tb-PDA-Al@MOF and TAA-Eu-PDA-Al@MOF obtained from XRD analysis should show mainly the characteristics of amorphous titania for TTA-Eu-PDA-Ti@MOF and TTA-Tb-PDA-Ti@MOF and of alumina for TAA-Tb-PDA-Al@MOF and TAA-Eu-PDA-Al@MOF, respectively, but no characteristics are present from MOF as it is limited in its content.

Fig. 4 presents the N₂ adsorption–desorption isotherms of the compounds: Al-MIL-53-COOH (MOF) and TTA-Eu-PDA-Al/Ti-MOF hybrids. It indicates that the final hybrid material can show porosity toward N₂. Unlike other microporous materials, both show a type I adsorption curve, and these samples display typical type IV curves with hysteresis loops at high relative pressure. The steric hindrance of the free non-coordinated carboxyl groups in the channels of MOF leads to the atypical hysteresis of the N₂ adsorption isotherms, which reduces the access of N₂ molecules. TTA-Eu-PDA-Al/Ti-MOF samples exhibit similar sorption behavior with MOF. The BET surface area of Al-MIL-53-COOH(MOF) is 394 m² g⁻¹, whereas the hybrid TTA-Eu-PDA-Al/Ti-MOF samples exhibit similar sorption behavior to Al-MIL-53-COOH (MOF). The BET surface areas of TTA-Eu-PDA-Al-MOF and TTA-Eu-PDA-Ti-MOF are 345 m² g⁻¹ and 339 m² g⁻¹, respectively, and show a reasonable reduction in comparison with the surface area of Al-MIL-53-COOH (MOF); this may be because the uploading of some small molecules exists in the hybrid alumina or titania host. MOF is fabricated in an alumina or titania host and can easily be affected by other components within the host.

Fig. 4 N₂ adsorption–desorption isotherms of compound Al-MIL-53-COOH (MOF) and TTA-Eu-PDA-Al/Ti-MOF.

The excitation and emission spectrum and the International Commission on Illumination (CIE) chromaticity diagram of MOF (Al-MIL-53-COOH) are shown in Fig. S4.† The maximum excitation and emission wavelengths are 372 nm and 420 nm, respectively. These can be ascribed to the excitation and emission of the trimellitic acid ligand. The corresponding CIE chromatic coordinate of MOF is located in the light blue region, which can be composed of the luminescent species in the yellow-orange-red region to white emission. This predicts what can be proved from the following hybrids of fabricated Eu³⁺ complexes.

Fig. 5 shows the luminescent excitation and emission spectra of Tb hybrids: TAA-Tb-PDA-Al-MOF (A), AA-Tb-PDA-Al-MOF (B), TAA-Tb-PDA-Ti--MOF (C) and AA-Tb-PDA-Ti-MOF (D). Their excitation bands are similar with wide range of 250–350 nm, corresponding to the excitation of the beta-diketonates and the carboxylic acid unit of MOF. No excitation peaks to the f–f transitions of Tb³⁺ can be detected. In the titania hybrid system, the excitation bands become narrow and another wide excitation band in the visible region can be observed. This may be because the titania is not so favourable as alumina for luminescence of Tb³⁺ complexes, which were found in many previous studies.²⁰ Organic ligands absorb the energy and then transfer it to the accepting energy level of the lanthanide ions. The wide excitation bands are favourable for the energy transfer and luminescence of Tb³⁺. The corresponding emission spectra of all terbium hybrids present the characteristic luminescence of Tb³⁺, while no emission of MOF could be detected. The emission lines are assigned to the ⁵D₄ → ⁷F_J transitions of Tb³⁺, locating at around 490, 545, 587 and 623 nm, for J = 6, 5, 4 and 3, respectively. The most striking green luminescence is for the ⁵D₄ → ⁷F₅ transition. In addition, it can be seen that no excitation and emission bands for MOF can be detected, suggesting that there is no overlap and energy transfer between the MOF and terbium complexes.

Fig. 5 The excitation and emission spectra of terbium hybrid materials TAA-Tb-PDA-Al-MOF (A), AA-Tb-PDA-Al-MOF (B), TAA-Tb-PDA-Ti-MOF (C) and AA-Tb-PDA-Ti-MOF (D).

Fig. 6 shows the luminescent excitation and emission spectra of hybrids TAA-Eu-PDA-Al-MOF (a) and TTA-Eu-PDA-Al-MOF (b), respectively. Different from the previous terbium hybrids, the excitation spectra by monitoring the emission of Eu³⁺ at 614 nm show both excitation of organic ligands and the f–f transition of Eu³⁺. A broad excitation band from 250 to 380 nm centered at about 320 nm (TAA-Eu-PDA-Al-MOF) and 340 nm (TTA-Eu-PDA-Al-MOF) in the UV region, which is attributed to the absorption from the organic ligands to form the charge transfer state (CTS) of Eu–O. The shoulder excitation bands at a low wavelength of 270 or 280 nm suggests the excitation of TTA or TAA ligands. At the same time, there exist peaks at around 396 and 467 nm because of the narrow f–f absorption transition (⁷F₀ → ⁵L₆ and ⁷F₀ → ⁵D₂) of the Eu³⁺ ion, whose intensity is much weaker than the absorption from organic ligands. The emission bands of the two hybrid materials are assigned to the ⁵D₀ → ⁷F_J (J = 0–4) transitions of Eu³⁺ at 580, 591, 614, 652, and 700 nm under two different kinds of excitation wavelengths (320/340 nm, 330/348 nm). It is worth noting that different excitation wavelengths may have an influence on the emission spectra, which is apparently shown in Fig. 6a. At excitation of 340 nm, both emission of MOF and the Eu-TAA complex can be observed and the different luminescence spectral position in the visible region may be expected to adjust the final luminescence color of the hybrids. This will be shown clearly in the following CIE diagrams. For the excitation at 320 nm, there is no apparent luminescence of MOF observed but the red emission of europium TAA complexes is present. From these results, it can be found that the excitation at 320 nm is mainly from the ligands of TTA forming the CTS, however, this is not true for MOF. The absorption at 340 nm does not produce the strong emission of Eu³⁺ and it overlaps with the excitation of MOF in Fig. S4.† So the emission of MOF can be observed. For TTA-Eu-PDA-Al-MOF hybrids, no distinction appears for the two different excitation wavelengths, where two emissions of MOF and Eu-TTA are both observed.

Fig. 6 The excitation and emission spectra of the hybrids TAA-Eu-PDA-Al-MOF (a) and TTA-Eu-PDA-Al-MOF (b).

The luminescent spectra of titania hybrids are shown in Fig. 7. The excitation spectra by monitoring the emission of Eu³⁺ at 614 nm from europium hybrids shows both excitation of the organic ligands and the f–f transition of Eu³⁺, which causes the apparent distinction. For Eu-TAA-Ti-MOF hybrids, the excitation spectrum consists of a broad excitation UV band from 250 to 350 nm centered at about 300 nm and a series of sharp excitation peaks from 350 to 500 nm in the UV-vis region. The former is ascribed to the absorption from the organic ligands forming the CTS of Eu–O. The latter belongs to the f–f absorption transition (327 nm, ⁷F₀ → ⁵H₆; 363 nm, ⁷F₀ → ⁵D₄; 383 nm, ⁷F₀ → ⁵G₂; 394 nm, ⁷F₀ → ⁵L₆, strongest; 415 nm, ⁷F₀ → ⁵D₃; 467 nm, ⁷F₀ → ⁵D₂) of the Eu³⁺ ion.²³ It is worth pointing out that the intensity of them is strong, even stronger than the absorption from organic ligands, whereas for Eu-TTA-Ti-MOF hybrids, the CTS excitation from the organic ligand is much stronger than the f–f transition so that only two narrow f–f absorption transition (⁷F₀ → ⁵L₆, 396 nm and ⁷F₀ → ⁵D₂, 466 nm) of Eu³⁺ ion can be seen. The excitation at 396 nm appears as a shoulder peak in the broad excitation bands of CTS (300–450 nm range, centered at 370 nm). Both of the two hybrids show the similar shape of emission spectra consisting of the Eu³⁺ ion and MOF. The emission to the ⁵D₀ → ⁷F_J (J = 0–4) transitions of Eu³⁺ is located at 580, 591, 614, 652, and 700 nm. The appearance of a wide emission of MOF at 400 to 580 nm may be expected to integrate with the emission of Eu³⁺, which is shown in the following CIE diagrams.

Fig. 7 The excitation and emission spectra of hybrid materials TTA-Eu-PDA-Ti-MOF (a) and TAA-Eu-PDA-Ti-MOF (b).

Fig. 8 shows, from left to right, photographs of four hybrid materials TAA-Tb-PDA-Al-MOF, TAA-Eu-PDA-Al-MOF, TTA-Eu-PDA-Al-MOF and TAA-Eu-PDA-Ti-MOF, respectively. Of these, three hybrids are based on alumina, and the TAA-Tb-PDA-Al-MOF and TAA-Eu-PDA-Al-MOF hybrids show the typical bright green for Tb³⁺ and the red emission for Eu³⁺, whereas Eu-TTA-Al-MOF hybrids present an emission that is a pink-white color. For Eu-TAA-Ti-MOF hybrids, there is a luminescence with a color which is close to white. So these hybrids can realize the multi-color light output by adjusting the component and excitation wavelengths.

Fig. 8 Photographs of the photoluminescence color using a Xe lamp as the UV excitation source. The images from left to right are the hybrid materials TAA-Tb-PDA-Al-MOF, TAA-Eu-PDA-Al-MOF, TTA-Eu-PDA-Al-MOF and TAA-Eu-PDA-Ti-MOF, respectively.

Fig. S5† shows the CIE diagrams of the spectra in Fig. 6 and 7, TAA-Eu-PDA-Al-MOF (a), TTA-Eu-PDA-Al-MOF (b), TTA-Eu-PDA-Ti-MOF (c) and TAA-Eu-PDA-Ti-MOF (d), respectively. From these four diagrams, it can be found that selecting the excitation with a suitable wavelength can realize the output of luminescence with a color which is close to white. For TAA-Eu-PDA-Al-MOF hybrids, the white colored luminescence can be obtained using a λ_ex of 320 nm, whereas the blue-white emission is obtained using a λ_ex of 340 nm. For TTA-Eu-PDA-Al-MOF hybrids, the two excitation wavelengths show the CIE coordinates in a color which is close to white (330 nm) and pink-white (348 nm) region. For the two kinds of hybrid materials based on titania (TTA-Eu-PDA-Ti-MOF, TAA-Eu-PDA-Ti-MOF), they both show the typical white luminescence. The parent MOF and the other hybrid systems present blue and green colored emission, respectively, and their CIE diagrams of spectra are shown in Fig. S6.† The detailed CIE coordinates are summarized in Table 1.

Table 1 The luminescence data of MOF and the lanthanide hybrid materials

Materials	λem (nm)	λex (nm)	Color (CIE-X, Y)	η (%)
Al-MIL-53-COOH (MOF)	420	372	Blue (0.1669, 0.1398)	17.0
TTA-Eu-PDA-Al-MOF	614	330	Close white (0.3456, 0.2604)	3.3
	614	348	Pink-white (0.3160, 0.2261)	3.7
TAA-Eu-PDA-Al-MOF	614	320	White (0.3602, 0.3186)	11.1
	614	340	Blue-white (0.2692, 0.2379)	13.2
TAA-Tb-PDA-Al-MOF	545	321	Green (0.2972, 0.6030)	8.2
AA-Tb-PDA-Al-MOF	545	315	Green (0.3062, 0.5984)	15.6
TTA-Eu-PDA-Ti-MOF	614	330	White (0.3525, 0.2844)	0.9
TAA-Eu-PDA-Ti-MOF	614	360	White (0.3272, 0.2804)	4.5
TAA-Tb-PDA-Ti-MOF	545	302	Green (0.2807, 0.5061)	0.7
AA-Tb-PDA-Ti-MOF	545	314	Green (0.2901, 0.5358)	0.4

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Furthermore, 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. The luminescent quantum yields of the hybrids based on alumina are higher than those with titania, which is in agreement with the results reported for other lanthanide hybrids functionalized with alumina and titania.^21,22 Alumina is more favourable for the luminescence of lanthanide species than tiania.^21,22 The rule of the η values of them is not apparent for the complicated hybrid system, TAA-Eu-PDA-Al-MOF hybrid material shows white color luminescence and 11.1% quantum efficiency (λ_ex = 320 nm), which would benefit from further study. For comparison, we selectively measured the luminescent quantum yields of four lanthanide hybrid (alumina or titania) materials themselves without MOF: they are 21.4% (TAA-Eu-PDA-Al), 6.2% (TAA-Eu-PDA-Ti), 17.0 (TAA-Tb-PDA-Al) and 2.1% (TAA-Tb-PDA-Ti). These values are all higher than those of hybrids with both lanthanide complexes and MOF, which is because of the much lower content of lanthanide complexes in the hybrids with MOF than those of hybrids without MOF.

Furthermore, we then tried to prepare the hydrogel thin films based on TTA-Eu-PDA-Ti-MOF (a, left) and TAA-Eu-PDA-Ti-MOF (b, right) hybrids, whose pictures are shown in Fig. S7.† Both of the two hybrid thin films were uniform and transparent, suggesting that it is feasible to prepare such hybrid thin films for further optical application. Fig. S8† shows the excitation and emission spectra of hybrid thin films fabricated with TTA-Eu-PDA-Ti-MOF (a) and TAA-Eu-PDA-Ti-MOF (b), which present similar features as the spectra of their powder samples. Their luminescent spectra consist of the emission from both europium beta-diketonates (TTA, TAA) and a MOF functionalized titania host in the red region and blue-green one, respectively, integrating the white color luminescence. Fig. 9a and (b) show the photograph and CIE diagrams of the two hybrid thin films.

Fig. 9 The photograph (left) and CIE chromaticity diagram (right) of the thin films prepared with TTA-Eu-PDA-Ti-MOF (a, λ_ex = 330 nm) and TAA-Eu-PDA-Ti-MOF (b, λ_ex = 360 nm) hybrids.

Conclusions

In this research, multi-component sol–gel derived alumina and titania hybrid materials were assembled by the functionalization of both lanthanide complexes and a special metal organic framework compound Al-MIL-53-COOH using the coordination bonds. Al-MIL-53-COOH can be fabricated with alumina or titania using post-synthesis for its free carboxylic groups. Lanthanide beta-diketonates can be introduced into alumina and titania using a special chemical linker pyridine-3,5-dicarboxylic acid. After hydrolysis and a condensation process, the multi-component hybrid materials can be prepared. The multi-color luminescence can be tuned by controlling the different units in the hybrid system. The white color light input can be realized by integrating the emission of both the europium complex and the MOF. In addition, uniform luminescent and transparent films were prepared. These results provide some useful data for further practical applications in photofunctional devices.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (91122003) and Developing Science Funds of Tongji University.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra05704j

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