Luminescent Hybrid Materials Based on Laponite Clay Huanrong Li,* Man Li, Yu Wang, and Wenjun Zhang[a] Abstract: The spectroscopic behavior of ionic Eu3 + or Tb3 + complexes of an aromatic carboxyl-functionalized organic salt as well as those of the hybrid materials derived from adsorption of the ionic complexes on Laponite clay are reported. X-ray diffraction (XRD) patterns suggest that the complexes are mainly adsorbed on the outer surfaces of the Laponite disks rather than intercalated within the interlayer spaces. Photophysical data showed that the energy-transfer
Introduction The unique optical properties of lanthanide ions (Ln3 + ), such as narrow and easy to recognize emission bands, large Stoke’s shifts, and long-lived excited states, make them interesting in a variety of technological applications, for example, biomedical imaging and analysis, lighting devices, and solar-energy conversion.[1–7] However, it is difficult to obtain such luminescence by direct excitation of Ln3 + due to the weak absorption caused by the forbidden nature of the f–f transitions. These shortcomings have long been known to be circumventable by coordination of organic ligands with higher molar absorption to Ln3 + ions. The organic ligands can absorb light strongly and transfer it to the lanthanide ions efficiently, as well as protecting the coordinated metal ions from solvent molecules. Luminescent hybrid materials based on Ln3 + complexes in matrices have long been in the focus of intensive study not only because they are of fundamental interest, but also because they show great potential for different applications.[9–15] The hybrid materials typically show enhanced stability and improved luminescence with respect to the pristine materials[16–18] Sol–gel-derived materials are highly promising matrices for designing new luminescent materials due to attractive characteristics such as highly controlled purity, versatile shaping and patterning, mechanical, thermal, and chemical stability, and biocompatibility.[19–21] Zeolites have also been employed to prepare lanthanide-based organic–inorganic hybrid materials because of their characteristic nanoscopic periodicity and their well-defined pore channels.[13, 22–26] Recently, complexes have also been [a] Prof. Dr. H. Li, M. Li, Dr. Y. Wang, Prof. Dr. W. Zhang Hebei Provincial Key Lab of Green Chemical Technology and High Efficient Energy Saving School of Chemical Engineering and Technology Hebei University of Technology Tianjin 300130 (P.R. China) E-mail: [email protected]
Chem. Eur. J. 2014, 20, 10392 – 10396
efficiency from the ligand to Eu3 + ions in the hybrid material is increased remarkably with respect to the corresponding ionic complex. The hybrid material containing the Eu3 + complex shows bright red emission from the prominent 5D0 !7F2 transition of Eu3 + ions, and that containing the Tb3 + complex exhibits bright green emission due to the dominant 5 D4 !7F5 transition of Tb3 + ions.
used to modify clay minerals.[27–34] As opposed to zeolites, the cavities of which are typically rigid, guest species are accommodated in interlayer space between two “hard” layers; the interlayer space can to some extent be flexibly adjusted by the accommodated guest species. Currently, increased interest is being focused on artificial nanosized clays such as Laponite, which is a synthetic smectic clay with very similar structure and composition to the natural clay mineral hectorite. Laponite is composed of layers of [SiO4] tetrahedra sandwiching sheets of Mg2 + . Some of the magnesium atoms are substituted by lithium, and the resulting net negative charge is balanced by interlayer cations, predominantly sodium ions.[35, 36] Modification of Laponite with alkoxysilanes was possible due to the presence of the unsaturated silicate groups (SiOH) at the edge of the clay disks. The insertion of Ln3 + complexes in the interlayer spaces of Laponite were reported by Kynast et al., who also studied the luminescence properties of Ln3 + complexes in aqueous Laponite solutions. TbIII(bpy)2 (bpy = 2,2’-bipyridine) complexes were used in these studies. We recently reported a novel organic salt containing an aromatic carboxylic acid (IL-ACC). Ionogels and complexes, obtained by coordination of IL-ACC with Eu3 + and Tb3 + ions under appropriate experimental conditions, showed luminescence characteristics of Ln3 + due to energy transfer from IL-ACC to both Eu3 + and Tb3 + ions. Herein we replaced the methyl group on the imidazole ring of IL-ACC with a butyl group and obtained a similar organic salt, which we denoted Bu-IL-ACC (Scheme 1). Ionic Ln3 + complexes of the organic salt and the corresponding luminescent hybrid materials based on Laponite were synthesized and their luminescence properties were investigated.
Results and Discussion The organic salt Bu-IL-ACC (Scheme 1), which is capable of coordinating and sensitizing Tb3 + and Eu3 + , was easily synthe10392
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Full Paper were determined to be 20 mol % by titrating the supernatant against EDTA. The presence of the ionic complexes in the hybrid materials is easily visible by the naked eye on near-UV excitation, as shown in Figure 2 which shows bright red and green emission for Laponite-Eu and Laponite-Tb, respectively.
Scheme 1. Chemical structure of Bu-IL-ACC and schematic of the hybrid materials.
sized from the commercially available 4-(bromomethyl)benzoic acid and 1-butylimidazole. The structure and purity were verified by 1H NMR spectroscopy. Reaction of Bu-IL-ACC with Eu(NO3)3·6 H2O and Tb(NO3)3·6 H2O in ethanolic solution produced the ionic complexes, which we abbreviate as Bu-IL-ACC-Eu and Bu-IL-ACC-Tb, respectively. The products are highly soluble in DMF. Since efforts to obtain their crystals failed; we do not know the exact structure and composition of the ionic complexes. However, the amounts of Eu3 + and Tb3 + ions in the ionic complexes were determined as 16.43 and 16.36 %, respectively, by the ethylenediaminetetraacetic acid (EDTA) titration method, which are in good agreement with the values of 16.19 % for Bu-IL-ACC-Eu and 16 % for Bu-IL-ACC-Tb determined by thermogravimetric (TG) analysis (Figure 1). The functionalization of clays with ionic complexes was carried out in DMF under heating at 80 8C for 24 h. The obtained luminescent hybrid materials were recovered by centrifugation, washed with DMF several times, and denoted Laponite-Eu and Laponite-Tb, respectively. The concentrations of ionic complexes BuIL-ACC-Eu and Bu-IL-ACC-Tb in Laponite-Eu and Laponite-Tb
Figure 2. Digital photos of the samples.
Pristine Laponite shows a somewhat broad XRD pattern due to the small size and low crystallinity of the Laponite disks (Figure 3 a). The broad peak at approximately 5.98 is attributed to the (001) crystal plane or the basal spacing of the clay. Similar XRD patterns were observed for Laponite-Eu (Figure 3 b)
Figure 3. XRD patterns of a) Laponite, b) Laponite-Eu, and c) Laponite-Tb.
Figure 1. TG curves of the ionic complexes a) Bu-IL-ACC-Eu and b) Bu-ILACC-Tb. Chem. Eur. J. 2014, 20, 10392 – 10396
and Laponite-Tb (Figure 3 c), with only the reflection at 2 q = 28 8C showing some loss of definition. No remarkable differences were observed in the basal spacing of the Laponite after modification with the ionic complexes, and this implies that the complexes are mainly adsorbed on the outer surfaces of the Laponite disks rather than intercalated in the interlayer spaces. It is well documented that the interlayer region of clay mineral shows significant changes after intercalation of molecules. Adsorption of the ionic complexes on the outer surface of the Laponite disks possibly proceeds by exchange of the countercations (Na + ions) of the clay with positively charged parts of the complexes (Scheme 1). The excitation spectrum (Figure 4 a, solid line) of Bu-IL-ACCEu obtained by monitoring the 5D0 !7F2 transition consists of 10393
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Full Paper line). However, the intensity of f–f transitions with respect that of the broad band is much weaker in comparison with that of the ionic complex Bu-IL-ACC-Eu. This implies much more efficient energy transfer from the ligand to Eu3 + ions in the hybrid material compared with that in the ionic complex. Similarly, excitation into the broad band results in five sharp emission lines characteristic of Eu3 + ions and is dominated by the 5 D0 !7F2 line at 618 nm, which accounts for the red emission color shown in Figure 2. A shorter lifetime of 0.62 ms was obtained from the mono-exponential decay curve of Laponite-Eu (Figure 4 c, dotted line), which implies that depopulation of the excited states of Eu3 + ions in Laponite-Eu by nonradiative processes becomes much more efficient than that in the ionic complex. The overall (absolute) quantum yields of the ionic complex and Laponite-Eu were determined with a calibrated sphere system and were found to be 0.34 and 0.21, respectively. The 5D0 radiative (kr) and nonradiative (knr) transition probabilities, the 5D0 intrinsic quantum efficiency (FLn), and the energy-transfer efficiency (Fsen) were calculated according to the reported method and are listed in Table 1 Table 1 shows
Table 1. Photophysical data.
Figure 4. a) Excitation and b) emission spectra and c) decay curves of Bu-ILACC-Eu (solid), Laponite-Eu (dotted). The excitation spectra were observed at 612 nm and the emission spectra were obtained on excitation at 280 nm. The decay curves measured at room temperature with 280 nm excitation and monitored around the most intense emission line at 612 nm are well fitted by mono-exponential functions.
a broad band in the range of 200–350 nm attributed to the absorption of the Bu-IL-ACC ligand and a series of sharp lines as a result of transitions between 7F0 and 5H6, 5D4, 5G2, 5L6, 5D3, and 5D2 levels. The latter transitions are much less intense than the broad band, that is, population of the Eu3 + excited state through excitation of the ligand is much more efficient than by direct excitation of the Eu3 + absorption level. The emission spectrum (Figure 4 b, solid line), excited at 280 nm, shows five sharp lines at 578, 590, 612, 652, and 701 nm, assigned to the 5D0 !7FJ (J = 0–4) transitions. The 5D0 !7F2 line at 612 nm dominates the whole spectrum and is responsible for the red emission color shown in Figure 2. The luminescence decay shown in Figure 4 c is well fitted by a mono-exponential function, and the decay time was determined to be 1.0 ms. The hybrid material Laponite-Eu shows a similar excitation spectrum to the ionic complex Bu-IL-ACC-Eu (Figure 4 a, dotted Chem. Eur. J. 2014, 20, 10392 – 10396
that knr of the ionic complex increases from 0.36 to 1.10 ms1 after hybridization with the clay, whereas Fsen increases from 0.52 to 0.65. This result indicates that the adsorption of ionic complexes on the nanoclay is beneficial for the energy-transfer efficiency from the ligand to Eu3 + ions, despite the negative influence on the quantum yield and the radiative transition process. A broad band ranging from 200 to 350 nm is similarly observed in the excitation spectrum of Bu-IL-ACC-Tb (Figure 5 a, solid line) obtained by monitoring the 5D4 !7F5 transition of Tb3 + at 544 nm. Only very weak absorptions of Tb3 + are observed, which indicate that energy transfer from the ligand to Tb3 + is more efficient than that to Eu3 + . The emission spectrum on excitation at 280 nm (Figure 5 b, solid line) consists of four sharp emission lines at 488, 544, 583, and 620 nm, assigned to the transitions from the 5D4 level to the 7FJ levels (J = 6, 5, 4, 3). The luminescence decay was measured under excitation at 280 nm. It follows a single-exponential function with a lifetime of the 5D4 state of 1.43 ms (Figure 5 c, solid line). The ionic complex Bu-IL-ACC-Tb shows an absolute quantum yield of 0.43. In the excitation spectrum of hybrid material LaponiteTb (Figure 5 a, dotted line), the complete disappearance of the absorption lines attributed to f–f transitions of Tb3 + indicates that energy transfer from the ligand to the Tb3 + ion in the ionic complex becomes more effective after hybridization with the nanoclay. The hybrid material Laponite-Tb displays a similar emission spectrum (Figure 5 b, dotted line) composed of four sharp lines when excited into the ligand absorption, with the
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Full Paper determined from the lower-wavelength emission edges of the phosphorescence spectra of the corresponding gadolinium complex according to the method reported previously. The experimentally observed T1 energy level (ca. 2.45 104 cm1) lies above the Eu3 + 5D0 level (17 300 cm1) and the Tb3 + 5D4 level (20 500 cm1). The energy gaps between Bu-IL-ACC and the metal-centered level for Eu3 + and Tb3 + are 7500 and 4300 cm1, respectively. An optimal ligand-to-metal energytransfer process is available when the energy gaps between the ligand and Eu3 + or Tb3 + levels DE(T1–5DJ) are 2500– 4000 cm1 or 2500–4500 cm1, according to Latva’s empirical rule.[44, 45] We therefore can conclude that the Bu-IL-ACC ligand much more efficiently sensitizes Tb3 + than Eu3 + . This is in good agreement with the fact that obvious Eu3 + absorption lines are presented in the excitation spectrum of Bu-IL-ACC-Eu, whereas much less absorption of Tb3 + is found in the excitation spectrum of Bu-IL-ACC-Tb and the absolute quantum yield of the ionic Eu3 + complex is much lower than that of the ionic Tb3 + complex.
Figure 5. a) Excitation, b) emission spectra and c) decay curves of Bu-IL-ACCTb (solid), Laponite-Tb (dotted). The excitation spectra were observed at 544 nm and the emission spectra were obtained on excitation at 280 nm. The decay curves measured at room temperature at an excitation of 280 nm and monitored around the most intense emission line at 544 nm are well fitted by mono-exponential functions.
We designed and synthesized a new organic salt capable of coordinating and sensitizing both Eu3 + and Tb3 + ions. Highly luminescent ionic complexes exhibiting strong red and green emission with quantum yields of 0.34 and 0.43 were obtained by complexion of Eu3 + and Tb3 + with the organic salt, respectively. Luminescent hybrid materials prepared by electrostatic adsorption of the ionic complex on Laponite showed bright visible emission on UV illumination. The excitation spectra for both hybrid materials showed a broad band in the range of 220–350 nm, which indicates energy transfer from the ligand to metal ions in the hybrid materials. The emission spectra show bands characteristic of the electronic transitions from the excited state 5D0 to the 7FJ (J = 0–4) for Eu3 + and from the excited state 5D4 to 7FJ (J = 3–6) for Tb3 + .The method used in this study for designing and preparing luminescent hybrid materials is simple and applicable to other hybrid materials based on clays.
D4 !7F5 line as the prominent one that is responsible for the green emission shown in Figure 2. The absolute quantum yield increased from 0.43 to 0.50, although the decay time decreased from 1.4 to 1.2 ms, for complexes in Laponite matrices. The hybrid material reported here shows a promising quantum yield comparable with those reported for Tb3 + complexes in nanoclay matrices. The lifetime is the reciprocal of the total deactivation rate, including the radiative and nonradiative rates. It is therefore reasonable that a highly luminescent material may have a short lifetime. This only means that the radiative rate is high after the hybridization of the ionic complex Bu-IL-ACC-Tb with the nanoclay. The luminescent properties of Ln3 + complexes are significantly dependent on the intramolecular energy transfer from the triplet state of the ligand to the resonance level of Ln3 + ions. The energy levels of the triplet state of Bu-IL-ACC were Chem. Eur. J. 2014, 20, 10392 – 10396
Materials The nanoclay (Laponite RD), 1-butylimidazole (99 %, Aldrich), and 4-(bromomethyl)benzoic acid (95 %, TCI) were used as received without further purification. Eu(NO3)3·6 H2O and Tb(NO3)3·6 H2O were obtained by dissolving Tb4O7 and Eu2O3, respectively, in nitric acid. The excess of the acid was removed by addition of water followed by evaporation, and this process was repeated several times.
Synthesis of aromatic carboxyl-functionalized organic salt Bu-IL-ACC 1-Butylimidazole (0.2073 g, 4 mmol) was added to an ethanolic solution of (4-bromomethyl)benzoic acid (0.5660 g, 4 mmol), and the mixture was heated to reflux for 17 h with vigorous stirring. Evaporation of the solvent under vacuum yielded the crude organic salt,
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Full Paper which was purified by repeated washing with Et2O and dried under vacuum at 70 8C overnight before use. 1H NMR: (400 MHz, DMSO): d = 13.10 (s, 1 H, COOH), 9.35 (s, 1 H, ring CH), 8.00 (d, 2 H, ArH), 7.85 (s, 2 H, ring CH), 7.50 (d, 2 H, ArH), 5.52 (s, 2 H, CH2), 4.20 (t, 2 H, CH2), 1.78 (m, 2 H, CH2), 1.23 (m, 2 H, CH2), 0.90 (t, 3 H, CH3); IR (KBr): ~ n = 3502 (OH), 3325 (OH), 1720 (C=O), 1701 (C=O), 1612 (C=C), 1575, 1555 (CN), 1424 (OH), 1271 (CO).
Synthesis of the ionic complexes Ionic complexes Bu-IL-ACC-Eu and Bu-IL-ACC-Tb were obtained by treating Bu-IL-ACC (0.9 mmol, 0.3053 g) and Ln(NO3)3·6 H2O (0.3 mmol; Ln = Eu, Tb) in 10 mL of absolute ethanol. The pH was then adjusted to 7 with dilute NaOH solution. The reaction mixture was stirred at 70 8C for 5 h. The precipitate was filtered off, washed with absolute ethanol, and dried at 70 8C under reduced pressure yielding the complex in quantitative yield. ~ = 3425 (ns OH), 2963 (nas CH3), 2935 (nas Bu-IL-ACC-Eu: IR (KBr): n CH2), 2869 (ns CH2), 1713 (ns C=O), 1612 (nas COO), 1559 (d NH), 1410 (ns COO), 1313 (CO); Eu: 16.43 % Bu-IL-ACC-Tb: IR (KBr): ~ n = 3423 (ns OH), 2963 (nas CH3), 2933 (nas CH2), 2868 (ns CH2), 1713 (ns C=O), 1615 (nas COO), 1560 (d NH), 1412 (ns COO), 1313 (CO); Tb: 16.36 %
Preparation of organic–inorganic hybrid materials Laponite (0.2932 g) was added to a DMF solution of Bu-IL-ACC-Eu or Bu-IL-ACC-Tb. The mixture was heated at 80 8C overnight. The precipitate was filtered off, washed with DMF three times, and dried at 80 8C under reduced pressure to yield the hybrid material.
Acknowledgements This work was financially supported by National Key Basic Research Program (2012CB626804), the National Natural Science Foundation of China (21171046, 21271060, 21236001), the Tianjin Natural Science Foundation (13JCYBJC18400), Hebei Province Natural Science Foundation (no. B2013202243), and Educational Committee of Hebei Province (2011141, LJRC021). Keywords: clays · energy transfer · hybrid materials · lanthanides · luminescence            
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Received: March 26, 2014 Published online on July 15, 2014
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