Modification of Eu³⁺–beta-diketonate complex-intercalated LAPONITE® with a terpyridine-functionalized ionic liquid

Abstract

A lanthanide complex-based organic–inorganic hybrid material with intense luminescence has been obtained by modifying Eu³⁺–beta-diketonate complex-intercalated LAPONITE® with a terpyridine moiety-functionalized ionic liquid. Remarkable luminescence enhancement as well as improved thermal- and photo-stability were observed after modification with the functional ionic liquid. The ionic liquid is believed to coordinate to and sensitize Eu³⁺ ions as well as decrease proton concentration on LAPONITE® surfaces.

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1. Introduction

To date, lanthanide complex based luminescent inorganic–organic hybrid materials have attracted the attention of researchers across various disciplines due to their rich applications, such as display, lighting, optical devices and medical technology.^1–10 Such luminescent materials typically are prepared by doping lanthanide complexes into various matrices including zeolites,^11–13 polymers,¹⁴ mesoporous silica,¹⁵ titania,^16,17 xerogels,¹⁸ and clay materials.^19,20 As the existing literature illustrates, lanthanide complexes encapsulated in a matrix lead to better optical properties and higher stability compared with the individual lanthanide complexes. As synthesized clay, LAPONITE® has a layered structure with expandable interlayer spaces and favourable cation-exchange capacity, which can be completely exfoliated in water.^21–23 These inherent properties make it a suitable host for incorporation of lanthanide complexes in aqueous solution to form luminescent hybrid materials. However, abundant acidic sites exist on the surface of the individual delaminated platelets of LAPONITE® in water, which have negative influence on the luminescence efficiency.^24,25 For instance, the luminescence efficiency of terbium complexes (Tb(bpy)₂³⁺) on the platelets of LAPONITE® appears to be rather low.²⁵ Fortunately, Yang et al. observed a significant increase in the luminescence efficiency of hybrids consisting of Eu³⁺–beta-dikeonate and LAPONITE® after adding imidazolium salt, which has been ascribed to the decreasing of the proton activity on clay surface by the added imidazolium salt that act through a mechanism of synergic effect of ion exchange and neutralization.²⁶ Other ionic liquids even displaying acidity in water can also enhance the luminescence of such kind of hybrid materials due to their capacity of removing proton via ion exchanges.²⁶

Encouraged by this, herein, we report the luminescence enhancement of hybrid materials consisting of Eu³⁺ (tta_n) complex (tta = 2-thenoyltrifluoroacetone) and the inorganic matrix LAPONITE® upon modification with a terpyridine moiety-containing ionic liquid (tpy-IL),²⁷ which is believed to act both as a sensitizer for Eu³⁺ ions and as an proton exchanger for decreasing the proton concentration on LAPONITE® surfaces. In addition, improved thermal- and photo-stability of the tpy-IL modified hybrid materials can also be observed.

2. Results and discussion

LAPONITE® (chemical composition Na_0.7[Si₈Mg_5.5Li₀O₂₀(OH)₄]) is a layered smectite-type clay with stacked platelets average dimension of 1 nm × 25 nm. It is well known that it is completely delaminated to individual disks to form transparent solution after swelling in water. Each single platelet contains roughly 1500 unitary cells (u.c.). In consequence, counter cations such as Na⁺ are accessible to ion exchange and organic species can be easily inserted in the interlayer.^22,28 According to the supplier, LAPONITE® has a cation exchange capacity (CEC) of 50–60 meq/100 g, i.e. about 55–65% of the existing Na⁺ ions.^20,21,29,30 The mean layer charge of LAPONITE® determined using the n-alkylammonium method is 0.346.^31–33 In this study, the intercalation of Eu³⁺–beta-diketonate complexes within the LAPONITE® was achieved by a two-step procedure according to our previous work. Firstly, material of Eu³⁺@LA were prepared by substituting the positive Na⁺ ions with Eu³⁺ via ion exchanges in aqueous solution, followed by addition of ethanol solution of tta. We find 80% of the available Na⁺ ions are exchanged by Eu³⁺ ions (through analyzing the supernatant titration against EDTA). As a result, the final experimentally determined tta and Eu³⁺ loading per u.c. under this condition is ∼1 and ∼0.352, respectively.²⁶ The presence of the tta ligand can be detected by UV-vis absorption according to the procedure.²⁷ As shown in Fig. 1c, the Eu³⁺(tta_n)@LA exhibited characteristic emission of Eu³⁺ when irradiated by near UV light, this phenomenon indicate the formation of Eu³⁺–beta-diketonate complexes in between the interlayers of LAPONITE®. After the insertion of tpy-IL, the luminescence enhancement was observed by naked eyes in the obtained Eu³⁺(tta_n)@LA/tpy-IL. The influence of the ionic liquid in the hybrid materials is especially obvious, which shows bright red emission (Fig. 1d).

Fig. 1 Digital photos of the samples; (a) Eu³⁺(tta_n)@LA (day light), (b)Eu³⁺(tta_n)@LA/tpy-IL (day light); (c) Eu³⁺(tta_n)@LA (UV light), (d)Eu³⁺(tta_n)@LA/tpy-IL (UV light).

2.1 X-ray diffraction

The formation of luminescent hybrid materials based on LAPONITE® was confirmed by the powder X-ray diffraction (XRD) (Fig. 2). All the samples exhibit a somewhat broad diffraction pattern, indicating a rather pronounced stacking disorder and the small crystallites particle size of the samples.³⁴ The broad diffraction pattern at approximately 2θ = 5.89° is attributed to the (001) crystal plane or the basal spacing of LAPONITE®.¹⁹

Fig. 2 XRD patterns of (a) LA, (b) Eu³⁺(tta_n)@LA and (c) Eu³⁺(tta_n)@LA/tpy-IL (left), idealized structure formula of LAPONITE® (right).

As shown in Fig. 2b, the basal spacing of LAPONITE® expanded from approximately 15 Å to 16.4 Å after the accommodation of Eu³⁺–beta-diketonate complexes, and implied that at least a proportion of tta complexes were intercalated within the interlayers. The basal spacing of LAPONITE® in Eu³⁺(tta_n)@LA/tpy-IL increased from approximately 15 Å to 18.5 Å (Fig. 2c), indicating that tpy-IL has been located the interlayer space of LAPONITE®. Nevertheless, the location of guest complexes on external adsorption sites on LAPONITE® could not be excluded.

2.2 FT-IR spectra and SEM

The modification of tpy-IL with Eu³⁺(tta_n)@LA can be further confirmed by the FTIR spectra. Fig. 3 displays the FTIR spectra of Eu³⁺(tta_n)@LA (a) and Eu³⁺(tta_n)@LA/tpy-IL (b). Bands centered at about 1607, 1587, and 1563 cm⁻¹ are assigned to the imidazole ring and the pyridine ring of tpy-IL.^35,36 Upon modification of tpy-IL with the hybrid material Eu³⁺(tta_n)@LA, these bands appears in spectrum (b) and the shift of some absorption bands corresponding to the terpyridine moieties also can be observed, which indicates a successful formation of Eu³⁺(tta_n)@LA/tpy-IL. The morphology of the Eu³⁺(tta_n)@LA and Eu³⁺(tta_n)@LA/tpy-IL was characterized by SEM experiment. As shown in Fig. 4a, a number of uniform size nanoparticles with an average diameter of 40–50 nm were observed in SEM image. Similar morphology with larger size nanoparticles was observed in the SEM image of Eu³⁺(tta_n)@LA/tpy-IL (Fig. 4b).

Fig. 3 FTIR spectra of (a) Eu³⁺(tta_n)@LA and (b) Eu³⁺(tta_n)@LA/tpy-IL.

Fig. 4 The scanning electron micrographs for (a) Eu³⁺(tta_n)@LA and (b) Eu³⁺(tta_n)@LA/tpy-IL.

2.3 Optical properties

We find that the luminescence performance of Eu³⁺(tta_n)@LA/tpy-IL is significant influenced by the initial addition amount of tpy-IL. The luminescence intensity of Eu³⁺(tta_n)@LA/tpy-IL increased gradually with increasing amount of tpy-IL, which reached its maximum when the quality ratio of tpy-IL to Eu³⁺(tta_n)@LA was set as 1 : 1 (Fig. 5). Therefore, the initial amount of tpy-IL to Eu³⁺(tta_n)@LA was maintained at 1 : 1 in the following experiments. The amount of tpy-IL actually loaded on the Eu³⁺(tta_n)@LA/tpy-IL hybrid materials was determined by elemental analysis. We found that the loaded amount of tpy-IL is about 0.225 per u.c.

Fig. 5 Luminescence intensity at 612 nm versus the amount of tpy-IL initially added to Eu³⁺(tta_n)@LA.

Further support is gained from the optical spectra as shown in Fig. 6. Both Eu³⁺(tta_n)@LA and Eu³⁺(tta_n)@LA/tpy-IL materials show similar excitation and emission spectra, the excitation spectra are composed of a broad band from 200 to 400 nm resulting from the absorption of ligands. The excitation of the Eu³⁺(tta_n)@LA is rather low over the monitored spectral range. Nevertheless, introduction of tpy-IL into the Eu³⁺(tta_n)@LA materials leads to a remarkable excitation enhancement peaked at approximately 370 nm. Characteristic luminescence of Eu³⁺ located at 579 (⁵D₀ → ⁷F₀), 592 (⁵D₀ → ⁷F₁), 612 (⁵D₀ → ⁷F₂), 652 (⁵D₀ → ⁷F₃) and 698 (⁵D₀ → ⁷F₄) nm was observed both in Eu³⁺(tta_n)@LA and Eu³⁺(tta_n)@LA/tpy-IL under excited at 350 nm (Fig. 6b). However, Eu³⁺(tta_n)@LA show relatively weak characteristic luminescence of Eu³⁺, obvious improvement in luminescence intensity is observed upon the incorporation of tpy-IL. Apparently, the spectrum is dominated by the hypersensitive transition ⁵D₀ → ⁷F₂, resulting in bright red luminescence. The integrated intensity of ⁵D₀ → ⁷F₂ transition of Eu³⁺(tta_n)@LA/tpy-IL was 10 times than that of Eu³⁺(tta_n)@LA.

Fig. 6 (a) Excitation, (b) emission spectra and (c) decay curves of Eu³⁺(tta_n)@LA (dotted), Eu³⁺(tta_n)@LA/tpy-IL (solid). The excitation spectra were monitored at 612 nm and the emission spectra were obtained on excitation at 350 nm.

The remarkable luminescent enhancement in Eu³⁺(tta_n)@LA/tpy-IL can be explained as follows. The addition of tpy-IL can fully protect Eu³⁺ ions from the water molecules quenching and remove the abundant protons on the platelets. The number of water molecules in the coordination sphere of Eu³⁺ was evaluated based on the emission spectra and the life time of the ⁵D₀ state of the Eu³⁺ ions according to the method described elsewhere.³⁷ The coordinated water molecules in Eu³⁺(tta_n)@LA was ∼3, while the number of water molecules coordinated to the europium ions in the hybrid material Eu³⁺(tta_n)@LA/tpy-IL was calculated to be 0.8. This means that most of the water molecules have been shielded from the first coordination sphere of the Eu³⁺ ions by the synergistic coordination of organic ligand combined with the functionalized ionic liquid.

In addition, the intensity ratio of I(⁵D₀ → ⁷F₂)/I(⁵D₀ → ⁷F₁) is increased from 6 to 12, indicating the asymmetric coordination field around Eu³⁺ ions, which is due to the strong coordination interaction appearing between the ligands and the Eu³⁺ ions.³⁸ Moreover, obvious stark splitting (3 lines ⁵D₀ → ⁷F₂) can be seen, which further indicates the low-symmetry site occupied by Eu³⁺ ion and a strong ligand field around Eu³⁺ ion. On the other hand, it has been well-documented that the acid environment has significant influence on the luminescence performances of Eu³⁺–beta-diketonate complexes.²⁵ TTA ligand can be protonated under acidic environment, which competes with full coordination to Eu³⁺ ions.²⁶ We propose that the protons on the platelets can be removed by addition of tpy-IL through a mechanism of synergic effect of ion exchange and neutralization. This can be supported by the following observations, the suspension of Eu³⁺(tta_n)@LA shows a decrease in pH from ∼7.3 to ∼6.6 after addition of tpy-IL, indicating the releasing of protons from the surface of LAPONITE® by exchanging H⁺ with positively charged of tpy-IL. In order to further exploring the luminescence properties of both hybrid materials, the typical decay curves were measured (Fig. 6c). The observed lifetime of Eu³⁺(tta_n)@LA/tpy-IL (0.61 ms) is longer than Eu³⁺(tta_n)@LA (0.23 ms), which is in coincidence with the results of emission spectra. As expected, the modified materials yield about 7-fold increase in luminescence quantum efficiency.

2.4 Thermal- and photo-stability

Drawbacks such as low thermal^39,40 and photochemical stability⁴¹ and poor mechanical limit the full exploitation of lanthanide complex in practical application. Hybridize the complexes with stable matrices provide an effective way to overcome the drawbacks.^{12,18,42–45} In the present work, the thermal-and photo-stability of both materials were studied.

We performed thermogravimetric analysis (TGA) to compare the thermal stability of the Eu³⁺(tta_n)@LA and the Eu³⁺(tta_n)@LA/tpy-IL. Three decomposition stages can be observed in the TG curve of Eu³⁺(tta_n)@LA (Fig. 7a). The first weight loss (6%) below 150 °C could be attributed to the removal of water. The second step, there is a 5% mass loss between 150–330 °C, which corresponds to the dissociation of the tta component. Beyond 330 °C, the LAPONITE® structure also decomposes with a gradual weight loss of 7% and a plateau is developed above 1000 °C. For comparison, the thermal analysis profile of Eu³⁺(tta_n)@LA/tpy-IL is shown in Fig. 7b. Below 150 °C, no obvious weight loss is observed for the TG curve, implying less water molecules coordinated in the complex. This is probably due to the formation of Eu³⁺(tta_n) with tpy-IL. The weight loss (28%) between 200 and 480 °C is possibly associated with the degradation of organic moieties from tta and tpy-IL. The LAPONITE® decomposes process is concentrated at narrow temperature range of 480–800 °C, this phenomenon reflected a more regular and orderly structure in Eu³⁺(tta_n)@LA/tpy-IL, which is believed to be ascribed to the enhanced luminescence properties after addition of tpy-IL. As concluded, Eu³⁺(tta_n)@LA/tpy-IL exhibited a higher thermal stability than Eu³⁺(tta_n)@LA.

Fig. 7 TG curves of (a) Eu³⁺(tta_n)@LA and (b) Eu³⁺(tta_n)@LA/tpy-IL.

In addition, the thermo-stability of the hybrid materials with and without tpy-IL also was examined by heating at 150 °C under air. The time-dependence profiles of the ⁵D₀/⁷F₂ transition integrated intensity are shown in Fig. 8. We observed drastic decrease of emission intensity of Eu³⁺(tta_n)@LA without tpy-IL protection (36%) after 20 h. In contrast, few decrease of the emission intensity of Eu³⁺(tta_n)@LA/tpy-IL was observed, 90% of its original integrated intensity was remained. Photo-stability was investigated by exposing the hybrid materials under ultraviolet lamp. The emission spectra curve of Eu³⁺(tta_n)@LA and Eu³⁺(tta_n)@LA/tpy-IL exposed to UV irradiation for 10 h are shown in Fig. 9. A ∼45% decrease in the ⁵D₀/⁷F₂ transition integrated intensity is observed for Eu³⁺(tta_n)@LA while the value is only ∼8% for Eu³⁺(tta_n)@LA/tpy-IL. In summary, pronounced improved thermal- and photo-stability is achieved by incorporation tpy-IL to the hybrid materials. As we can see from the above results, not only because unique properties of the ionic liquid (tpy-IL), but also coordinating ability between europium(III) ions and the ligand plays a crucial role in improving the stability of the hybrid materials.

Fig. 8 Changes of the integrated intensity of ⁵D₀/⁷F₂ transition after 150 °C heat treatment (holding 0 h, 5 h, 10 h, 15 h, 20 h); —Eu³⁺(tta_n)@LA; —Eu³⁺(tta_n)@LA/tpy-IL.

Fig. 9 Emission spectra of (a) Eu³⁺(tta_n)@LA and (b) Eu³⁺(tta_n)@LA/tpy-IL after exposed to UV irradiation (λ_ex = 365 nm) (holding 0 h, 10 h).

3. Experimental

3.1 Materials

LAPONITE® and the alkylamines were purchased from Rockwood Additives Ltd and used as received without further purification. 2-Thenoyltrifluoroacetone (tta) was purchased from Aldrich. Europium chloride (EuCl₃·6H₂O) was prepared by dissolving Eu₂O₃ into concentrated hydrochloride acid (37%). The excess acid was removed by addition of ethanol followed by successive fuming. The organic salt containing terpyridine moieties (tpy-IL) was synthesized according to the reported procedure.²⁷

3.2 Characterizations

X-ray powder diffraction studies were completed with an X-ray powder diffractometer (BRUKER D8 Focus) employing Cu Kα radiation (λ = 1.5418 Å), operating at 40 kV and 40 mA. Infrared (IR) spectra were obtained with a Bruker Vector 22 spectrometer by using KBr pellets for solid samples ranging from 400 to 4000 cm⁻¹. The UV-Vis spectra were recorded on a VARIAN CARY 50 UV-Vis spectrophotometer. Elemental analysis was performed on a Flash EA 1112. SEM images were obtained from a Nova Nano SEM450 at an acceleration voltage of 15 kV. Luminescence spectra were measured on an Edinburgh Instruments model FLS920P spectrometer, with a 450 W Xe lamp as the steady-state excitation source, a double-excitation monochromator (1800 lines mm⁻¹), an emission monochromator (600 lines mm⁻¹), and a semiconductor cooled Hamamatsu model RMP928 photomultiplier tube. Layer charges were estimated from the n-alkylammonium ion-exchange technique³² and the exchange procedure were operated according to this procedure.³³

3.3 Preparation of Eu³⁺(tta_n)@LA

On addition of 5 mL EuCl₃·6H₂O (0.1 mol L⁻¹) solutions to 5 wt% aqueous dispersions of LAPONITE® (0.5 g), the mixture was stirred at 80 °C for 24 h. The product (denoted as Eu³⁺@LA) was collected by centrifugation and washed with deionized water several times. The Eu³⁺@LA was dispersed in 10 mL water; 0.15 g of tta dissolved in 5 mL of EtOH was then added. After vigorous sonication of the mixture for 2 h, the product was collected by centrifugation, washed with EtOH, and dried at 80 °C overnight. The product was denoted as Eu³⁺(tta_n)@LA. CHN analysis of Eu³⁺(tta_n)@LA: C 9.00%, H 3.68%.

3.4 Modification of Eu³⁺(tta_n)@LA with tpy-IL

The Eu³⁺(tta_n)@LA was mixed with aqueous solution of tpy-IL in accordance with suitable molar ratio, the mixture was sonicated for 1 h. The product (Eu³⁺(tta_n)@LA/tpy-IL) was then recovered by centrifugation, washed three times with water and dried at 80 °C. Characteristic bands of tpy-IL was observed in the FTIR spectra of Eu³⁺(tta_n)@LA/tpy-IL, which indicate the coordination of tpy-IL with Eu³⁺ ions. CHN analysis of Eu³⁺(tta_n)@LA/tpy-IL: C 14.15%, N 1.43%, H 3.61%.

4. Conclusions

In conclusion, we have observed a remarkable increase of luminescence efficiency of a inorganic–organic hybrid material, obtained by loading of Eu³⁺–beta-diketonate complexes into the LAPONITE® interlayers by a two-step procedure, after modification with tpy-IL. The present of tpy-IL can fully protect Eu³⁺ ions from the water molecule quenching together with remove the abundant protons on the platelets are believed to be ascribed to the luminescence enhancement. The unique luminescence properties of the hybrid material, together with its good stability and processability, make it highly promising candidate for fabrication of different kinds of optical devices.

Acknowledgements

Financial support by the National Key Basic Research Program (2012CB626804), the National Natural Science Foundation of China (20901022, 21171046, 21271060, and 21236001), the Tianjin Natural Science Foundation (13JCYBJC18400), the Natural Science Foundation of Hebei Province (No. B2013202243), the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, IRT1059), and Educational Committee of Hebei Province (2011141, LJRC021) is gratefully acknowledged.

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