Thermally Stable White Emitting Eu3+ Complex@Nanozeolite

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Thermally Stable White Emitting Eu Complex@Nanozeolite@Luminescent Glass Composite with High CRI for Organic-Resin-Free Warm White LEDs Jinhui Zhang, Shuming Gong, Jinbo Yu, Peng Li, Xuejie Zhang, Yuwei He, Jianbang Zhou, Rui Shi, Huanrong Li, Ai-Yun Peng, and Jing Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15739 • Publication Date (Web): 08 Feb 2017 Downloaded from http://pubs.acs.org on February 9, 2017

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ACS Applied Materials & Interfaces

Thermally Stable White Emitting Eu3+ Complex@Nanozeolite@Luminescent Glass Composite with High CRI for Organic-Resin-Free Warm White LEDs Jinhui Zhang,a Shuming Gong,a Jinbo Yu,a Peng Li,b Xuejie Zhang,a Yuwei He a, Jianbang Zhou a, Rui Shi, Huanrong Li b,*, Aiyun Peng a,*, Jing Wanga,* Ministry of Education Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, KLGHEI of Environment and Energy Chemistry, School of Chemistry, Sun Yat-sen University, Guangzhou, Guangdong 510275, PR China b School of Chemical Engineering and Technology, Hebei University of Technology, Guangrong Dao 8, Hongqiao District, Tianjin 300130, China a

*Corresponding

author

ABSTRACT: Nowadays, it is still a great challenge for lanthanide complexes to be applied in solid state lighting, especially for high-power LEDs because they will suffer severe thermal-induced luminescence quenching and transmittance loss when LEDs are operated at high current. In this paper, we have not only obtained high efficient and thermally chemical stable red emitting hybrid material by introducing europium complex into nanozeolite (NZ) functionalized with the imidazolium based stopper but also abated its thermal-induced transmittance loss and luminescence quenching behavior via coating it onto a heat resistant luminescent glass (LG) with high thermal conductivity (1.07 W/mK). The results show that the intensity at 400 K for Eu(PPO)n-C4Si@NZ@LG remains 21.48% of the initial intensity at 300 K, which is virtually 153 and 13 times the intensity of Eu(PPO)3·2H2O and Eu(PPO)n-C4Si@NZ, respectively. Moreover, an organic-resin-free warm white LEDs device with a low CCT of 3994K, a high Ra of 90.2 and R9 of 57.9 was successfully fabricated simply by combining NUVChip-On-Board with a warm white emitting glass-film composite, i.e, yellowish-green emitting luminescent glass coated with red emitting hybrid film. Our method and results provide a feasible and promising way for lanthanide complexes to be used for general illumination in the future. KEYWORDS: high thermal stability, high CRI, white LEDs, nanozeolite, luminescent glass, Europium composite coworkers synthesized white light-emitting materials via first embedding

1. INTRODUCTION Lanthanide complexes have been attractive for decades due to their fascinating photophysical properties, including large Stokes Shifts, high luminescence quantum efficiencies, sharp emission lines, etc.1-7 Among them, lanthanide β-diketonates are the most popular and intensive to be studied.8-10 However, the instability towards long-term UV irradiation, the inability of lanthanide β-diketonates complexes to withstand high temperature as well as moisture, the poverty in mechanical strength and heat conduction, and the tendency to aggregate severely hinder their full exploitation in practical applications such as tunable solid-state lasers or high-power phosphorconverted white-light-emitting diodes (pc-WLEDs).11-14 One successful alternative to circumvent these shortcomings is to encapsulate the complexes in a stable inorganic matrix. As have been reported, organic-inorganic hybrid material integrates certain advantages of organic complex like easy processing, elasticity with the merits of inorganic host like high thermal and photochemical stabilities.15-17 Up to now, lanthanide complexes have been successfully introduced into sol-gel, mesoporous silica, ionic liquids, zeolites and so forth.18-23 Recently, zeolites as hosts of lanthanide complexes have attracted increasing interest. In 2013, Yan and his

Eu(TTA)n or Tb(TTA)n into zeolite A, and then grafting another lanthanide complex onto the surface.21 Hereafter, our group reported a host-guest material with brilliant enhancement in luminescence efficiency by taking advantage of an imidazolium based stopper molecule which can not only prevent guests from leaving the host, but protect center ions from outside quenchers resulted from vibration coupling of the hydroxyl groups.22 However, these hybrid materials are only proved to be thermally stable in chemical structure.24 In fact, thermalinduced luminescence quenching behavior and transmittance loss play a more significant role in high-power WLEDs application for future general lighting. More attention has been paid to luminescent intensity variation at low temperature (10-350 K) for lanthanide complexes. Studies on thermal quenching behavior of lanthanide complexes at high temperature (300-500 K) are rare.25-26 To date, the input current of high-power InGaN-based LED chips with a power of more than 3 W used as pump excitation source in pc-WLEDs has typically been in the 350/750/1000 mA range, which leads to high local heat flux where junction temperature can reach up to 400 K or more.27-31 Besides, such an enormous amount of heat is accumulating when LEDs are in operation. Inevitably, this heat then causes serious degradation to the chips, phos-

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phors and organic resin, which will bring about total loss in output luminous efficiency and shift in emission peaks. Moreover, this accumulation of heat will result in stronger phononelectron interaction and can remarkably reduce the luminescent intensity of lanthanide complexes. Although lanthanide β-diketonates are incorporated into a thermally chemical stable matrix, thermal-induced luminescence quenching and transmittance loss still occur. To solve these problems, one should either change the chemical structure to be thermally stable or reduce the heat from the chips conducted to the lanthanide β-diketonates. Many efforts have been devoted to increase the decomposition temperature of lanthanide complexes32-34 but thermally optical stability including luminescence intensity and transmittance is still low due to the increased electron-phonon interaction and thermally chemical bond-breaking as temperature increases.35-38 Therefore, heat dissipation might be a more effective way. Owing to its versatile properties, luminescent glass has received immense attention in high-power LEDs. It can not only act as luminescent convertor and encapsulating material but also have excellent thermal resistance and conduction. Prompted by our previous works,28, 29 we conceived a new and more effective method to increase the chemical stability as well as the optical stability when exposed to heat by introducing europium complex into nanozeolite functionalized with the imidazolium based stopper and then coating it onto a luminescent glass. Once the heat is conducted away effectively, thermally activated non-radiative rates and thermally chemical bond-breaking will be less, which results in the enhancement in luminescent intensity. In this paper, this strategy has been proved to be effective and feasible. Furthermore, an organic-resin-free warm white LEDs device with a low CCT and a high CRI was also successfully demonstrated.

2. EXPERIMENTAL SECTION 2.1. Materials. Solution of EuCl3·xH2O in ethanol was obtained by dissolving Eu2O3 (4N) in hydrochloric acid (37%), evaporating solvent water and re-dissolving in ethanol. Unless otherwise noted, the other materials mentioned below were procured commercially and used without subsequent purification: 3acetylphenanthrene (98%, Aldrich), methyl pentafluoropropionate (99%, Aldrich), Lithium diisopropylamide (2M, LDA, Acros), 1-Butylimidazole (99%, J&K) and (Chloropropyl)triethoxysilane (97%, J&K), LUDOX® HS-40 colloidal silica (40 wt. % suspension in H2O, Aldrich). 2.2. Synthesis of the Ligand, Hpfppd (HPPO). The ligand 4,4,5,5,5-pentafluoro-3-hydroxy-1-(phenanthren-3-yl)pent-2en-1-one(Hpfppd, HPPO for short) was synthesized according to the procedure reported previously by M.L.P. Reddy with a little alteration.39 In a typical procedure, lithium diisopropylamide (LDA) was initially added into a solution of 3acetylphenanthrene (0.2202g, 1 mmol) in dry tetrahydrofuran (2 ml) (THF) in an inert atmosphere and vigorously stirred at -40 oC for 2 h. Methyl pentafluoropropionate (190 μL, 1.5 mmol) was then added via a syringe and stirred at 0 oC for additional 4 h. To the resulting solution, saturated ammonium chloride solution was added followed by further extracting twice with dichloromethane (2 × 30 ml). The organic extract was separated and dried over Na2SO4. Finally, solvent was removed under reduced pressure and the crude residue was purified by column chromatography (SiO2, petroleum

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ether/ethyl acetate = 15:1) to give pure product as a yellow solid. The observed results of 1H NMR and 13C NMR of the ligand HPPO are shown in Figure S1 and S2, respectively. Yield = 60.6%, elemental analysis (%): calcd. for C19H11F5O2 (366.28): C 62.30, H 3.03; found: C 62.26, H 2.82; 1H NMR (400 MHz, CDCl3) δ/ppm: 15.49 (s, 1H), 9.34 (s, 1H), 8.79 (d, J = 8.2 Hz, 1H), 8.07 (dd, J = 8.4, 1.6 Hz, 1H), 8.02 – 7.89 (m, 3H), 7.78 (dd, J = 10.7, 4.9 Hz, 2H), 7.71 (t, J = 7.1 Hz, 1H), 6.87 (s, 1H); 13C NMR (101 MHz, CDCl3) δ/ppm: 185.85, 179.17, 178.92, 178.66, 135.72, 132.23, 130.57, 130.34, 130.18, 130.00, 129.31, 128.96, 127.62, 127.57, 126.16, 124.26, 123.48, 122.70, 93.86; LCMS (ESI) calcd. for C19H11F5O2 [M]- 365.28, found:365.3. 2.3. Synthesis of Nanozeolite L (NZ). Nanozeolite L was synthesized according to the procedure reported previously by using a starting mixture with the composition 9.34 K2O-1.00 Al2O3-20.20 SiO2-41.84 H2O.40 Firstly, a clear solution (A) and a silica suspension (B) were prepared separately as follows. For a clear solution (A), 4.9387 g potassium hydroxide and 1.5622 g aluminium hydroxide were added into 16.1397 g deionized water and refluxed for 15 h in an oil bath at 115 oC. For a silica suspension (B), 10.1262 g deionized water was added into 30.3394 g colloidal silica solution and stirred vigorously at 100 oC for 1 h, followed by adding a solution of potassium hydroxide (7.4056 g KOH dissolved in 25.8003 ml deionized water) and stirring at 100 oC for another 15 h. After cooling to room temperature, (A) solution and (B) solution were mixed and fiercely stirred for 10 min. The resulting solution was then transferred to the PTFE vessel for crystallization at 170 oC for 6 h. After crystallization, the pressure vessel was cooled in ice for 1 h. The precipitate was then collected by centrifugation (10000 rpm, 10 min) and washed with boiling deionized water until the pH of the supernatant became neural. The as-obtained crystals were finally dried at 80 oC for approximately 24 h and its final yielding amount is about 2.0 g. 2.4. Synthesis of 1-butyl-3-trimethoxysilane imidazolium chloride (C4Si). The ionic liquid (C4Si) was synthesized according to the procedure reported in ref.41 To a solution of 1Butylimidazole (1.490 g, 12 mmol) in DMF was added (Chloropropyl)triethoxysilane (2.408 g, 10 mmol) in an inert environment at 80 oC for 48 h. The reaction solution was finally purified via liquid extraction in ether with subsequent evaporation of solvent to produce the 1-butyl-3-trimethoxysilane imidazolium chloride (C4Si). 2.5. Synthesis of Eu2+ doped SiO2-Li2O-SrO-Al2O3-K2O-P2O5 Silicate Luminescent Glass (SLSAKP: 0.05% Eu2+ Glass). The green emitting glass was synthesized according to our previous paper without any modification28. 2.6. Modification of the Host-Guest Materials with C4Si. The host-guest materials were synthesized according to the paper previously reported.22 Briefly, a certain amount of NZ was initially added into an ethanol solution of EuCl3·xH2O to obtain Eu3+-exchanged NZ (Eu@NZ) by ion-exchange reaction. Then a ship-in-a bottle method was used to prepare the hostguest sample with in-situ coordination reaction, as illustrated in Scheme 1. Thereafter, protons in NZ were transferred by reacting Eu(HPPO)n@NZ with C4Si. The final product Eu(PPO)n-C4Si@NZ was collected by centrifugation, washed with deionized water several times and dried at 50 oC overnight. 2.7. Treatment with Trimethylamine (Et3N). Sample of Eu(HPPO)n@NZ in tube was exposed to Et3N vapor for 1h in a sealed container of 5 ml Et3N liquid till it was at gas-liquid equilibrium. Real time sensing of Et3N was carried out by re-


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cording the emission spectra of Eu(PPO)n-Et3N@NZ for different exposed time (t = 10, 90, 160, 250, 370, 625, 910, 1030 s) after a bottle of Et3N vapor was injected into a sealed cuvette. 2.8. Fabrication of Red Phosphor Converted LEDs (pcLEDs) with Eu(PPO)n-C4Si@NZ. Red LEDs were fabricated by combining a NUV chip (VQ-P003UV380, Vanq) with assynthesized Eu(PPO)n-C4Si@NZ material. Initially, 0.01 g of Eu(PPO)n-C4Si@NZ and 0.05g of silicone were thoroughly mixed. Then the slurry was kept in vacuum to remove air bubbles, coated on the lead frame of NUV chip and cured under 100 oC for 4 h. The obtained red pc-LEDs were operated at 3.0 V with drive current 100 mA. 2.9. Preparation of Transparent Films on Quartz and SLSAKP: 0.05% Eu2+ Luminescent Glass (LG). The quartz (D= 2 cm) and SLSAKP: 0.05% Eu2+ glass (D = 2 cm) were cleaned with alcohol and acetone and then dried at 80 oC overnight. 10 mg of Eu(PPO)n-C4Si@NZ was dispersed into 10 ml of ethanol and sonicated for 1 h to obtain a colloidal suspension. Finally, the colloidal suspension solution was dropped onto the pre-cleaned quartz and SLSAKP glass, directly spin-coating at 500 rpm for 20 s. 2.10. Fabrication of LG LEDs and Warm White LEDs (WLEDs) based on LG Coated with Red Emitting Film. The LG LEDs and warm WLEDs devices were constructed simply by placing the LG coated without and with red-emitting film on the n-UV Chip-On-Board (n-UV COB) LED modules consisted of a 5 ×2 array of n-UV (～390 nm) chips, which was fixed on an aluminium sink, respectively. The obtained LEDs were operated at 16 V under a forward bias current of 100 mA. 2.11. Characterization. 1H NMR and 13C NMR spectra were measured in CDCl3 and recorded on Brucker ARX 400 spectrometer. CHN elemental analyses were performed on an elemental analyzer (Vario EL) and the contents of Eu3+ ion were obtained by inductively coupled plasma-mass spectra (ICP-MS, iCAPQc). LCMS was obtained on LCQ DECA XP mass spectrometer. The morphology and elemental composition of the as-prepared samples were inspected using a transmission electron microscopy (FEI Tecnai G2 Spirit). The IR spectra were conducted on a Fourier transform infrared spectrometer coupled with infra-red microscopy (EQUINOX 55). The phase assemblages were identified by laboratory X-Ray powder diffraction (XRD) using a D8 ADVANCE powder diffractometer operating with Cu Kα radiation at 40 kV and 40 mA. The photoluminescence excitation (PLE) and emission spectra (PL) at room temperature together with the decay curves were recorded on an Edinburgh Instruments FSP920 Time Resolved and Steady State Fluorescence Spectrometers equipped with a 450 W Xe lamp, a 60 W μF900 μs flash lamp and thermoelectric cooled red-sensitive PMT. The quantum yield (QY) of the samples was measured using a barium sulfate coated integrating sphere (150 mm in diameter) attached to the FSP920. The absorption spectra were measured with a Cary 5000 UV-visNIR spectrophotometer (Varian) equipped with double out-ofplane Littrow monochromator. The thermal stability was studied by thermogravimetric analysis (TGA), performed under an N2 protect gas flow on TGA-50 Thermogravimetric Analyzers with a heating rate of 20 oC/min. The thermal conductivity was determined by the Laser Flash technique which was performed on Netzsch LFA 447 NanoFlash apparatus.

XRD patterns of the NZ host, Eu@NZ, Eu(HPPO)n@NZ and Eu(PPO)n-C4Si@NZ are shown in Figure 1a. The diffraction peaks of NZ host are quite similar to the result previously reported,42 indicative of the successful preparation of NZ. Usually, nanozeolite L is a microporous crystalline aluminosilicate with one-dimensional channels in the free diameter range of ca. 0.71 nm to 1.26 nm, which are formed by [AlO4]5- and [SiO4]4- tetrahedrons linked via bridging oxygen atoms.43 Figure 1b shows TEM image of as-prepared NZ. Its length is about 30 nm and it has many onedimensional channels inside with an inner diameter of 13 Å, close to the largest free diameters of ca. 1.26 nm. This further confirms the successful preparation of NZ. For Eu@NZ, Eu(HPPO)n@NZ and Eu(PPO)n-C4Si@NZ, it is clearly seen in Figure 1a that there are no significant changes, compared with the NZ host. This indicates that ion- exchange reaction with Eu3+ ion, in-situ complexation with HPPO and modification with C4Si show no damage to the framework of NZ host, which is strongly supported by TEM images of Eu@NZ (c), Eu(HPPO)n@NZ (d) and Eu(PPO)n-C4Si@NZ (e) as shown in Figure 1. The main features of NZ host remain. To confirm whether Eu3+ ions can exchange with K+ ions in NZ and ligand HPPO can enter into the channels of NZ by a method called “ship in a bottle” and complex with Eu3+ or not, energy dispersive X-ray spectroscopy (EDS) analysis, as shown in Figure S4, ICP-MS and CHN element analyses were carried out. In our case, the loading of Eu3+ content and HPPO in the samples were determined to be ~1.955/u.c. and ~0.202/u.c., respectively.

Figure 1. XRD patterns (a) of as-prepared NZ, Eu@NZ, Eu(HPPO)n@NZ, Eu(PPO)n-C4Si@NZ and NZ reported and TEM images of (b) NZ, (c) Eu@NZ, (d) Eu(HPPO)n@NZ and (e) Eu(PPO)n-C4Si@NZ.

3. RESULTS AND DISCUSSION

3.1. Properties



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Figure 2. FTIR spectra of C4Si (black line), Eu(HPPO)n@NZ (red line), Eu(PPO)n-C4Si@NZ (green line).

FTIR spectra were used to characterize the successful modification of Eu(HPPO)n@NZ. As shown in the spectrum of Eu(HPPO)n@NZ in Figure 2, there is a broad band dominating at 1142-1015 cm-1, which is assigned to the υas(T-OT) stretching vibration (T=Si4+ or Al3+) in TO4 tetrahedra. The weak bands at 1529 cm-1 and 1639 cm-1 are attributed to the stretching vibration of C=O belonging to the ligand HPPO. Compared to Eu(HPPO)n@NZ and C4Si, it can be clearly seen in the FTIR spectrum of Eu(PPO)n-C4Si@NZ (Figure 2) that the characteristic imidazole ring bands at 1464 and 1564 cm-1 appear and the SiOCH2CH3 bands at 956 and 1167 cm-1 disappear. This implies the terminal triethoxy groups of C4Si reacted with the surface OH groups of NZ, revealing the successful attachment of C4Si to the NZ.

Figure 3. Images (A) of the as-prepared NZ (a), Eu@NZ (b), Eu(HPPO)n@NZ (c), Eu(PPO)n-C4Si@NZ (d) taken under natural daylight (A, upper) and near UV excitation (A, below). Excitation spectra (B) monitored at 611 nm and emission spectra (C) excited at 365 nm.

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C4Si@NZ. Under natural daylight (Figure 3A, a-d, upper), both NZ and Eu@NZ exhibit pure white body color while Eu(HPPO)n@NZ and Eu(PPO)n-C4Si@NZ show yellow and pale-yellow body color. Under near UV excitation (Figure 3A, a-d, bottom), only Eu(PPO)nC4Si@NZ gives an intense red emitting light. The excitation and emission spectra of Eu(HPPO)n@NZ and Eu(PPO)n-C4Si@NZ are shown in Figure 3B and C. Both of them give the same characteristic spectroscopy features except the intensity. Monitoring at 611 nm, they all exhibit a broad band extending from 250 to 420 nm, assigned to the π-π*electron transition of HPPO, strongly supported by the absorption spectrum (Figure S5). Under excitation at 365 nm, there are five characteristic sharp lines at 579, 593, 611, 650, 698 nm in PL spectra, attributed to 5D0 → 7FJ (J=0-4) transitions of Eu3+ ion, respectively. It’s worthy to note that the 5D0 → 7F2 transition of 611 nm dominates the emission spectra, which is induced by the electric dipole moment of Eu3+ ion and hypersensitive to the site symmetry. Additionally, the absent intrinsic emission of the ligand in Eu(PPO)n-C4Si@NZ indicates that energy transfer from HPPO to central Eu3+ ion (antenna effect) is efficient. The experimental absolute quantum yields for Eu(HPPO)n@NZ and Eu(PPO)n-C4Si@NZ were determined to be 0.18% and 10.80%, respectively, which increases by ca. 60–fold. The effect of C4Si on the red emission enhancement of Eu3+ in NZ is investigated as follows. It is seen in Figure 4a that no obvious shift or broadening of the characteristic emission peaks of Eu3+ could be observed except for intensity enhancement upon gradual addition of C4Si, whose tendency is apparently seen in Figure 4b, upper. Interestingly, as the nominal amount of C4Si added increases, the luminescence intensity of Eu3+ increases sharply at first and then levels off, which is almost consistent with the phenomenon of actual amount of C4Si bound.

Figure 3A shows the digital photos of the asprepared NZ, Eu@NZ, Eu(HPPO)n@NZ, Eu(PPO)n-


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Figure 4. a) PL spectra of Eu(PPO)n-xC4Si@NZ (x=12.1-19.9). The x values indicate the actual amount of C4Si bound to Eu(HPPO)n@NZ. b) Normalized emission intensity of Eu3+ at 611 nm (black line) and actual amount of C4Si covalently loaded to the composite (red line) versus the nominal amount of C4Si added to the reaction mixture.

As is well known, deprotonation of diketones can contribute to the increase in its coordination numbers of oxygen atom coordinated with Eu3+, leading to the enhancement of luminescence intensity. In our case, diketones can hardly be deprotonated under the acidic environment of hydrated nanozeolite. Addition of C4Si, served as a weak base, promotes deprotonation reaction of diketones HPPO inside the NZ channels (Scheme 1 and Scheme 2), thereby boosting luminescence intensity of Eu3+. These results are in accordance with and strongly proved by our previous work.22 Moreover, it is theoretically expected that every single C4Si is preferentially attached to each channel entrance to exchange one of the protons nearby showing remarkable increase in intensity at low dosage.44, 45 Consequently, the luminescence intensity does not become saturated until one dense monolayer has been achieved. It’s obviously seen in Figure 4b that the luminescence intensity of Eu3+ reaches the maximum and becomes saturated when the nominal amount of C4Si is over 221/u.c. where the actual amount of C4Si covalently loaded keep almost constant at 19/u.c..

As is reported, there is a strong proton activity inside the channels of NZ resulting from dissociation of the adsorbed water.46 To further verify the key role of C4Si in promoting deprotonation reaction of HPPO and consequently enhancing red emission intensity of Eu3+, another organic base Et3N was used to monitor the luminescence intensity of Eu(HPPO)n@NZ in real time as shown in Figure 5. Obviously, luminescence intensity initially experiences a significant enhancement by a factor of 4.95 within 10 s and slightly increases to maximum up to 250 s upon exposure to Et3N vapor, which is similar to the phenomenon of that modified with C4Si (Figure 4b upper). Apparently, Et3N vapor serves as a base to neutralize the acid leading to the larger Eu3+-ligand coordination numbers, which give rise to the enhanced intensity, similar to C4Si as discussed above. This consistency supports that the hydrated NZ is in an acidic environment where β-diketonate is inclined to exist in keto form going against full complete coordination to Eu3+.47,48



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Scheme 1. Schematic illustration of assembling Eu(PPO)n in NZ channels and its selective modification with C4Si. that it can pass through the 7.1 Å opening to enter the NZ channel while silane moiety is so large that its ethoxysilyl group can only react with a silanol group at the channel entrance resulting in a covalent bond. No more C4Si can enter into the NZ when all of the channels are stuffed with C4Si. It’s proved in Figure 4b that when an amount of C4Si loaded (ca. 19/u.c.) saturates the NZ, an excess of C4Si makes little contribution to the luminescence intensity. However, Et3N, a smaller gas molecular, can easily and quickly weave through the inside channels to neutralize more unprotonated HPPO (Figure 6 (f)). In summary, both high basicity and great steric hindrance of organic base Et3N make the deprotonation reaction of diketone HPPO complete and consequently enhance the emission intensity of Eu3+ greater than that of C4Si. The luminescence decays shown in Figure 6 (c) are compatible with what is displayed in Figure 6 (a) and (b). Scheme 2. Formation of Eu(PPO)n Complex. Comparatively, as shown in Figure 6a and 6b, Et3N enhances the emission intensity of Eu3+ by a factor of 424, higher than 110 for C4Si. What’s the difference they make in this system? Obviously, the stronger basicity of Et3N vapor (pH = 10.8) overtakes that of C4Si (pH = 7.83, 2 mg/mL). The former shows more ability than the latter to neutralize the protons in NZ and contributes to greater increase in intensity. Moreover, as clearly illustrated in Figure 6 (d) and (e), the imidazole moiety is small enough


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Figure 5. a) Real Time-dependent emission of 5D0 → 7F2 transition at 611 nm upon exposure to Et3N vapor. b) Corresponding emission spectra before and after exposure of Et3N vapor within 10 and 160 s.

Figure 6. Excitation spectra (a) monitored at 611 nm and emission spectra (b) excited at 365 nm. Decay curves (c) of asprepared samples with excitation at 365 nm and emission monitored at 611 nm. (d) Side view of the main channel with smallest and largest free diameter of about 7.1 and 12.6 Å, respectively. (e) Insertion and covalent bond formation in the modification of NZ with C4Si. (f) Diffusion of Et3N vapor into the channels of NZ.

The 5D0 emission decay curves were monitored within the 5D0 → 7F2 transition under an excitation wavelength of 365 nm. The mean duration is defined by the formula given in the following:

∅ ∅ (1) Table 1. Luminescence data of the hybrid materials Sample

τ (ms)

kr (ms-1)

knr(ms-1)

q (%)

PLQY (%)

nw

R

Eu(HPPO)n@NZ

0.15

0.32

6.38

4.80

0.18

6.74

3.8

Eu(PPO)n-C4Si@NZ

0.37

1.14

1.60

41.7

10.80

1.43

18.3

Eu(PPO)n-Et3N @NZ

0.50

1.41

0.59

70.6

24.26

0.31

23.3

τ, experimental 5D0 lifetimes; kr and knr, calculated radiative and non-radiative 5D0 rate constants, respectively; q, 5D0 quantum efficiency; PLQY, photoluminescence quantum yield; nw, the number of water molecules around Eu3+; R, intensity ratio of I(5D0 → 7F2)/I(5D0 → 7F1).

The average lifetime values of Eu(PPO)n-C4Si@NZ and Eu(PPO)n-Et3N@NZ are 0.42 and 0.57 ms, respectively, longer than that of Eu(HPPO)n@NZ (0.15 ms). The prolonged lifetime of the Eu3+ excited state indicates a displacement of water molecules and/or OH oscillators from the first coordination sphere, which is confirmed by the empirical formula that Supkowski and Horrocks used to calculate the number of water molecules (nw) coordinated to the Eu3+, as estimated as follow:

nw = 1.11 × (kexp– kr – 0.31) (2) where kexp is the reciprocal value of the 5D0 lifetime. On the basis of eq (2), the results were calculated and shown in table 1. Obviously, nw decreases from 6.74 for Eu(HPPO)n@NZ to 1.43 for Eu(PPO)n-C4Si@NZ and 0.31 for Eu(PPO)n-Et3N@NZ, which lead us to deduce that at least

five and six water molecules around Eu(HPPO)n were removed from the first coordination shell of Eu3+ after modification with C4Si and Et3N. Comparatively, modification with Et3N, to greatest extent, gives a removal of almost all water molecules resulting in the highest 5D0 quantum efficiency (70.6%). In addition, what’s mentioned above is in good agreement with the ratio of the integral intensity of I(5D0 → 7F2)/I(5D0 → 7F1) which is often used to measure the degree of Eu3+ symmetry variation in different local environment. The larger the R value, the lower the symmetry of the Eu3+ site. Therefore, Eu(PPO)n-Et3N@NZ with the highest R value of 22.2 indicates more complete complexation of Eu3+ to β-diketonate, due to large interaction of Eu3+ with its neighbors. Although NZEu(PPO)n-Et3N exhibits brighter red light than NZEu(PPO)n-C4Si does, it’s luminescence intensity, however, will suffer a constant loss when kept under ambient atmosphere.



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stability

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high

temperature.

3.2. Thermal Behaviors of Eu(PPO)3· 2H2O, Eu(PPO)nC4Si@NZ and Eu(PPO)n-C4Si@NZ@LG

Figure 7. Comparison of the thermal stability of Eu(PPO)3 ·2H2O, NZ and Eu(PPO)n-C4Si@NZ in presence of nitrogen.

Usually, europium complexes are susceptible to heat and light which hampers their application in high-power LEDs. Herein we have successfully incorporated the europium complex into the channels of NZ framework with higher thermal stability. The chemical stability of Eu(PPO)3· 2H2O, and Eu(PPO)nC4Si@NZ were studied by means of thermogravimetric analysis (TGA) under nitrogen atmosphere and the results are depicted in Figure 7. It’s clearly seen from the data that Eu(PPO)3 · 2H2O undergoes a mass loss of about 2.49% in the first step up to 150 oC, corresponding to the elimination of coordinated water molecules. Subsequently, the decomposition of the complex begins by three weight loss steps, finishing at about 810 oC. It’s known that NZ possesses a good thermal stability which is obviously seen in Figure 7 that no obvious weight loss is observed after the absorbed water is removed at 200 oC. Compared with NZ, Eu(PPO)n-C4Si@NZ suffers a similar loss in weight of water in the initial temperature ranging from 30 to 200 oC whereas it suffers continuous weight losses which are attributed to europium complex and C4Si. And finally it maintains 80% weight at 700 oC which is supposed to belong to NZ framework. In summary, encapsulating the complexes into the cavities of zeolites will help improve the chemical

Figure 8. PL spectra of the (a) Eu(PPO)3 ·2H2O, (b) Eu(PPO)nC4Si@NZ and (c) Eu(PPO)n-C4Si@NZ@LG upon 390 nm excitation in the temperature range from 300 to 500 K. (d) PL integrated intensity of Eu(PPO)n·2H2O, Eu(PPO)n-C4Si@NZ and Eu(PPO)n-C4Si@NZ@LG at T= 300-500 K.

In addition to chemical stability, thermal quenching behavior, i.e., photoluminescent stability at high temperature plays a more important role in high-power WLEDs application. The temperature-dependent emission spectra of Eu(PPO)3·2H2O, Eu(PPO)n-C4Si@NZ and Eu(PPO)nC4Si@NZ@LG at different temperatures ranging from 300 K to 500 K are presented in Figure 8(a, b, c). With the temperature increasing, the integrated emission intensity of each material gradually decreases. Compared to Eu(PPO)3·2H2O, Eu(PPO)n-C4Si@NZ with improved chemically thermal stability, shows a little higher luminescent intensity below 400 K. Unfortunately, at 400 K, close to the joint- temperature of LED chips, the emission intensity of Eu(PPO)3·2H2O and Eu(PPO)n-C4Si@NZ sharply slides to 0.14% and 1.67% of that observed at 300 K, respectively. Importantly, the intensity at 400 K for Eu(PPO)nC4Si@NZ@LG remains 21.48% of the initial intensity at 300 K, which is virtually 153 and 13 times the intensity of Eu(PPO)3·2H2O and Eu(PPO)n-C4Si@NZ, respectively. Obviously, such a great enhancement cannot be ascribed to the slight difference in chemical stability between those three samples at the temperature range of 300-500 K as shown in Figure 7.In general, the more serious thermal luminescent quenching the sample suffers, the stronger the electron-phonon interaction becomes. This is most likely due to the reason that Eu(PPO)n-C4Si@NZ@LG with higher emission intensity undergoes weak electron-phonon interaction at high temperature, compared to Eu(PPO)3·2H2O or Eu(PPO)n-C4Si@NZ. Therefore, the phonon-induced nonradiative luminescence quenching processes speed ups slowly in Eu(PPO)n-C4Si@NZ@LG but greatly in Eu(PPO)3·2H2O or Eu(PPO)n-C4Si@NZ as the temperature elevates. This explanation is strongly supported by the data of the thermal conductivity. It’s about 1.07 W/mK for the luminescent glass, 6.7 times as large as that of


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Eu(PPO)n-C4Si@NZ (0.159 W/mK) and 9.6 times larger than Eu(PPO)3·2H2O (0.101 W/mK), respectively. These results indicate that it’s easier for LG to conduct heat away, consequently leading to weak electron-phonon interaction in Eu(PPO)n-C4Si@NZ@LG. Apart from chemical stability and thermal behavior, thermal-induced transmittance loss exerts a considerable influence on luminous efficacy. Traditionally, LED devices were fabricated via combining a NUV chip with Eu(PPO)nC4Si@NZ and epoxy resin mixture. It should be noted that this kind of fabrication experienced a process of heat solidification at 100 oC for 4 h, during which both the Eu(PPO)nC4Si@NZ and organic resin had suffered degradation more or less. As we know, the tendency for epoxy resin to be aged due to the irreversible oxidation when exposed to heat will bring about loss in transparency. Hence, the difference in refractive index between the phosphor powder and organic resin will become larger and it gives weaker and inhomogenously emitted light. Similarly, Eu(PPO)nC4Si@NZ, like an organic complex, will oxidize or decompose when it’s heated which can lead to the enhancement in opacity after 500 K treatment, as demonstrated by Figure S5. Since the transparency decreases, it’s more likely for light to be absorbed or scattered, generating a reduction in luminescent intensity. On the contrary, Eu(PPO)nC4Si@NZ@LG composite exhibits almost no change in body color and no obvious shift in the whole wavelength range (200-800 nm) of the transmittance spectra after 500 K treatment, as shown in Figure 9, further confirming that the LG-film composite has an excellent heat-resistant and heat-conductive performance. To sum up, Eu(PPO)n-C4Si@NZ@LG not only shows better chemical stability but also maintains higher luminescent intensity and high transmittance at high temperature compared with Eu(PPO)n-C4Si@NZ power. Therefore, the combination of the LG with the Eu(PPO)n-C4Si@NZ film is strongly expected to be a better way for Eu3+ complexes to be applied in high-power WLEDs. 3.3. Potential Applications of Eu(PPO)n-C4Si@NZ As discussed above, we have obtained stable and bright red-emitting hybrid material by embedding europium complex into nanozeolite functionalized with the imidazolium based stopper. Furthermore, the red LED device (Figure 10a) was traditionally fabricated by combining a NUV chip with as-synthesized Eu(PPO)n-C4Si@NZ material. It gives bright red light with the chromaticity coordinates of (0.6289, 0.3403) and the luminous efficacy of 8.61 lm/W under an operating current of 50 mA (Figure 10b, Figure 11a and 11d). Based on our previous paper, transparent film was easily prepared by dropping an aqueous suspension of NZ hybrid material on the micro-slide glass before drying. Herein, we used spin-coating to obtain more uniform film on a round quart followed by drying. As shown in Figure 10 (c) and (d), the as-prepared red-emitting film shows high transparency under natural light and gives bright red luminescence under 365 nm UV lamp. Previously, we have synthesized the highly thermally stable LG applied in high-power LEDs. It emits yellowishgreen light (Figure 10f) whose CIE chromaticity coordi-

nates are (0.3144, 0.4210) with a high CCT of 6136 K, a low Ra of 70.0 and R9 of -67 due to the lack of red emission, as shown in Figure 11b and 11d. Combination of Eu(PPO)nC4Si@NZ and LG can be a promising method to not only improve the thermal stability of Eu(PPO)n-C4Si@NZ but obtain a warm white LG-LEDs.

Figure 9. Transmittance spectra of Eu(PPO)n-C4Si@NZ@LG before and after 500 K treatment.

According to the conceptual and actual fabrication process in Figure 10g, we obtained an organic-resin-free warm WLEDs by coating Eu(PPO)n-C4Si@NZ red-emitting film onto a plate of yellowish-green emitting glass placed on a n-UV-COB (5 × 2 multiple n-UV-LED chips) which is fixed on an aluminum sink. It’s clearly seen that the device prototype emits reddish white light (Figure 10g and 11c) under 50 mA forward-bias current (IF). The spectrum of warm WLEDs is composed of three parts (Figure 11c): nUV COB chip at around 390 nm, yellowish green emitting LG characterized by a broadband emission centering at 530 nm and covering from 450 to 750 nm and Eu(PPO)nC4Si@NZ red-emitting film dominating at 611 nm with some other weak f-f emission peaks of Eu3+. Compared with LG LEDs, the CIE color coordinates (Figure 11d) of the as-fabricated warm WLEDs are (x=0.4155, y=0.3900), with a lower CCT of 3277K, a higher Ra of 82.8 and R9 of 63, due to the enhancement of red emission from the redemitting film.



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Figure 10. The fabricated Red LED out (a) and in operation (b), respectively; red-emitting film taken under natural light (c) and 365 nm lamp (d); the LG LEDs devices (e) and the corresponding photograph of device driven by 50 mA current (f). The conceptual and actual fabrication process (g) of Warm WLEDs by combination of red-emitting film and luminescent glass onto COB.

Figure 11. (a,b,c) EL spectra and (d) CIE chromaticity coordinates of the as-fabricated red LED, LG LEDs and warm WLEDs devices under an operating current of 50 mA.

indicates that both Eu(PPO)n-C4Si@NZ and epoxy resin are thermally unstable when LEDs are operated at high current, which is also congruent with the thermal properties we mentioned above (Figure 8) that Eu(PPO)n-C4Si@NZ suffers severe luminescence quenching and thermalinduced transmittance loss at high temperature resulted from high-power LEDs. One the other hand, there is a gradual increase in the overall intensity of the warm WLEDs from 380 to 750 nm (Figure 12a). The intensity of the warm WLEDs at 612nm driven at 120 mA is 178.8% as intense as that at 50 mA while the intensity of the red LED driven at 120 mA is only 94% of that at 50 mA, as shown in Figure 12b. This is due to the differences in the thermal quenching between Eu(PPO)n-C4Si@NZ and Eu(PPO)nC4Si@NZ@LG and the thermal induced transmittance loss between organic resin and LG. In addition, the as-obtained warm WLEDs show a slight chromaticity shift with increasing current (Figure 12c). The CIE chromaticity coordinates, correlated color temperature (CCT) and color rendering index (CRI, Ra) are summarized in Table S1. The CCT varies from 3200 to 4000 K and the CRI value changes from 82.8 to 90.2. In summary, we have successfully fabricated the thermally stable warm WLEDs with low CCT and high CRI. 4. CONCLUSION

Figure 12. Electroluminescence (EL) spectra (a) of warm WLEDs under different operating currents from 50 to 120 mA. (b) Normalized emission intensity at 612 nm of red LED (red line) and warm WLEDs (blue line) versus the operating currents. (c) CIE chromaticity coordinates of the as-fabricated Warm WLEDs driven by different currents.

However, the as-fabricated red LED suffers a continuous decline at the intensity of 612 nm as the operating current increases from 50 to 120 mA (Figure 12b, upper). This

In conclusion, we have obtained more stable and brighter red-emitting hybrid material by embedding europium complex into nanozeolite functionalized with the imidazolium based stopper. The effect of two bases, C4Si and Et3N, on the red emission enhancement of Eu3+ in NZ is investigated. And strongly luminescent transparent films were prepared by spin coating. Furthermore, we have successfully fabricated the warm WLEDs device by coating redemitting film on the yellowish green-emitting luminescent glass. Such a combined prototype shows a red-dish white light with lower CCT of 3994 K and higher CRI value of 90.2. More importantly, it shows better thermal stability as well as higher thermal conductivity than Eu(PPO)3·2H2O and Eu(PPO)n-C4Si@NZ. Therefore, the combination of the LG with the Eu(PPO)n-C4Si@NZ film is strongly expected to


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be a better and promising way to be applied in high-power WLEDs.

8.

ASSOCIATED CONTENT

9.

Supporting Information. 1H

(Figure S1) and 13C NMR (Figure S2) spectrum; EDS of Eu@NZ (Figure S3); absorption spectra of NZ series (Figure S4); reflection spectra of Eu(PPO)n-C4Si@NZ before and after 500 K treatment (Figure S5); comparison of variation in optical parameters of warm WLEDs (Table S1). The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

10.

11.

AUTHOR INFORMATION 12.

Corresponding Author * E-mail: [email protected], Tel: +86-20-84112112, Fax: +86-20-84111038. Author Contributions All authors have given approval to the final version of the manuscript.

13.

Notes

14.

The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the NSFC (51572302 and 21271191), the “973” programs (2014CB643801), the Joint Funds of the National Natural Science Foundation of China and Guangdong Province (U1301242), Teamwork Projects of Guangdong Natural Science Foundation (S2013030012842), Guangzhou Science & Technology Project (2013B090800019 and 2015B090926011), Natural Science Foundation of Guangdong Province (2014A030313114), and the Natural Science Foundation of Hebei Province (Nos. B2016202147).

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Table of Contents

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Figure 1 170x80mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2 84x59mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3 330x250mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4 170x70mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5 70x32mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6 170x140mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7 150x119mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8 175x150mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9 150x119mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 11 340x199mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 12 170x119mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1 338x230mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 2 183x142mm (300 x 300 DPI)


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Thermally Stable White Emitting Eu3+ Complex@Nanozeolite

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