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Efficient Energy Transfer in Terbium Complexes/ Porous Boron Nitride Hybrid Luminescent Materials Xin He, Jing Lin, Wei Zhai, Yang Huang, Qiaoling Li, Chao Yu, Jianli Liang, Zhenya Liu, Lanlan Li, Yi Fang, and Chengchun Tang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07014 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 22, 2017

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The Journal of Physical Chemistry

Efficient Energy Transfer in Terbium Complexes/Porous Boron Nitride Hybrid Luminescent Materials

Xin He1,2, Jing Lin1,2,*, Wei Zhai1,2, Yang Huang1,2, Qiaoling Li1,2, Chao Yu1,2, Jianli Liang1,2, Lanlan Li1,2, Yi Fang1,2, Zhenya Liu1,2, Chengchun Tang1,2

School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, P. R. China. E-mail: [email protected] (J. L.); Hebei Key Laboratory of Boron Nitride Micro and Nano Materials, Hebei University of Technology, Tianjin 300130, P. R. China

Abstract: A novel kind of organic-inorganic hybrid luminescent material has been developed through controllable encapsulation of devise terbium (III) complexes into porous boron nitride (BN) host. The luminescence of the obtained hybrid materials can be modulated by simply changing the type of ligands in the complexes. Especially, intense green-light-emission has been observed in Tb(acac)3/BN (acac=acetylacetone) and Tb(acac)3phen/BN (phen=1, 10-phenanthroline) hybrid materials. A unique synergistic effect between the BN host and Tb complexes has been realized, resulting in efficient energy transfer in the BN-ligands-Tb3+ system. The confinement of terbium complexes into porous BN leads to the hybrid materials demonstrating excellent thermal and photo stabilities. Porous BN host plays an important role for the superior performance of the hybrid materials. The developed organic-inorganic hybrid 1

ACS Paragon Plus Environment


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materials

with

multifunctional

properties are

promising

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for

light-emitting

applications.

Introduction The controllable encapsulation of functional organic components (such as dyes, lanthanide complexes) into porous host materials to form organic-inorganic hybrid materials has attracted great research interest in recent years.1, 2 Porous materials, such as mesoporous silica,3 carbonaceous materials,4 metal oxides,5 metal-organic frameworks (MOFs),6 can not only provide a readily stable chemical environment for the organic molecules, but also effectively prevent their aggregations, making the hybrid materials shown enhanced stability. These hybrid materials possess multifunctional properties of both organic components and porous hosts, thus may find various applications in the fields of white light emitting diodes,7 sensing,8 fluorescence imaging,9 etc. The unique properties of organic-inorganic hybrid materials has inspired the search for new kinds of porous materials to serve as hosts. Porous boron nitride (BN) has features of high specific surface areas, large pore volumes, excellent thermal stability, oxidation resistance and acid corrosion resistance.10-14 In comparison with the widely reported mesoporous silica and/or zeolites hosts,15, 16 the excellent chemical stability, especially the superb corrosion resistance of porous BN is beneficial for protection of organic components, in particular, for those performing at high-temperatures and in hazardous environments. Moreover, unlike zeolite L which often have very small 2


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diameters of less than 2 nm,17 porous BN with adjustable pore structures (pore volumes, pore size distributions) is advantageous for the encapsulation of organic components successfully. These features allow porous BN as a new family of hosts for organic-inorganic hybrid materials. For example, loading of fluorescent dyes within porous BN can efficiently improve the thermal stability and fluorescence efficiency of the organic dyes.18, 19 In addition, encapsulation of lanthanide complexes into porous BN can also readily solve the stability problem issued by the organic complexes.20 Considering the excellent luminescent properties of lanthanide complexes, such as characteristic and strong monochromaticity, full color emission, long

lifetimes,

the

developed

BN-based

hybrid

materials

are

promising

multifunctional materials for light-emitting applications. Moreover, due to the emissive variety of lanthanide complexes, the luminescence properties of BN can be elaborately designed and tuned. It is noteworthy that in such an organic-inorganic hybrid system, one may concern that the effect of the porous host on the properties of hybrid materials. It is believe that the host can provide active sites for the encapsulated organic components and display synergistic functionalities. However, although a number of hybrid luminescent materials have been shown to have enhanced performance, the efforts on the exploration of synergistic properties in the hybrid system have still been limited. In this paper, we report on the encapsulation of terbium (III) complexes into porous BN microfibers to form a hybrid luminescent material. We have designed and synthesized Tb complexes/BN hybrid materials with tunable emission by introducing of devise 3



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organic ligands

into the Tb3+/BN precursor.

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The resultant Tb(acac)3/BN

(acac=acetylacetone) and Tb(acac)3phen/BN (phen=1, 10-phenanthroline) composites exhibit significantly enhanced green emission and improved thermal and photo stability. Due to the unique synergistic effect between the BN host and Tb complexes, an efficient energy transfer in the BN-ligands-Tb3+ system occurs, resulting in the excellent performance of the hybrid material.

Experimental Starting materials. Porous BN fibers were synthesized using H3BO3 and C3N6H6 as the source materials.21 Acetylacetone (99%, acac, Aldrich), 1, 10-phenanthroline (99%, phen) and 2, 2'-bipyridyl (99.5%, bpy) were used as received. Synthesis of Tb3+/BN precursor. The Tb3+/BN precursor was prepared by an adsorption method. Firstly, 0.5 g of porous BN was dispersed throughout in 450 ml of distilled water by sonication and stirring. 0.435 g of Tb(NO3).5H2O was dissolved in 50 ml of distilled water. Then the Tb solution was added dropwise into the previous BN suspension. After continuously stirring for 12 h, the mixed solution was filtered and dried at 80°C in air to get the Tb3+/BN precursor. Synthesis of Tb(acac)3/BN, Tb(bpy)3/BN, Tb(phen)3/BN and Tb(acac)3phen/BN samples. Firstly, 0.15 g of the Tb3+/BN precursor was dispersed in 10 ml of distilled water by sonication and stirring. Then 1 ml of acac (liquid) and 1.5 ml of alcohol was added dropwise into the previous Tb3+/BN precursor suspension. After continuously stirring for 2.5 h, the mixed solution was filtered and dried at 80°C in air to get the 4


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Tb(acac)3/BN

samples.

The

synthesis

procedures

for

Tb(phen)3/BN

and

Tb(acac)3phen/BN were similar except that we used a solution of phen or a mixed solution of acac and phen instead of acac. Tb(bpy)3/BN was prepared by a gas diffusion method.20 In detail, 0.15 g of the Tb3+/BN precursor and 0.45 g of bpy were grinded together and placed in a gas diffusion flack. The bottle was kept at 60°C in the oil bath for 1 h under a vacuum condition. Then the temperature was raised up to 100°C for 24 h. Finally, the sample was centrifuged with N,N-Dimethylformamide and dried in air at 80°C. Characterization. The microstructures and compositions of the samples were measured using a transmission electron microscope (TEM, Philips Tecnai F20) equipped with an energy-dispersive X-ray spectrometer (EDS). The excitation and emission spectra, the fluorescent lifetime and the kinetics curves of the samples were measured on the fluorescence spectrophotometer (FL3-22). The luminescent quantum efficiency was determined using an integrating sphere from the F-3000 Fiber-Optic Adapter. Thermal stability was measured at a heating rate of 5°C min-1 in the presence of air using a thermogravimetric analyzer (TG) (SDT Q-600).

Results and Discussion The porous BN used in this work shows specific surface area of ~1093 m2/g and a wide pore size distribution (Figure S1, Supporting Information). The synthesis of organic-inorganic hybrid material includes two steps. As shown in Figure 1a, Firstly, porous BN microfibers with high porosity were used as host and adsorbed with Tb3+ 5



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ions to get a Tb3+/BN precursor. Then the organic ligands, i.e. acac, were introduced into the BN host to coordinate with Tb3+ ions. Finally, the hybrid luminescent materials Tb(acac)3/BN were obtained. Diverse BN-based hybrid materials, such as Tb(bpy)3/BN and Tb(phen)3/BN, can be obtained by loading of different ligands, as shown in Figure 1b. Then the photoluminescence (PL) of three hybrid materials were studied and shown in Figure 1c and 1d. In addition, PL spectrum of porous BN was also measured for comparison. Pure BN has a strong emission peak in the UV region, which is attributed to the defect-related centers or intrinsic impurities in BN.22 After introducing of bpy ligands, the PL of Tb(bpy)3/BN shows similar UV emission peak from porous BN and a broad emission band at ~400 nm, which belongs to the π*→π transitions of ligands bpy. Besides, the characteristic peaks of Tb3+ at 488, 546, 582 and 619 nm can be observed, belonging to 5D4→7F6, 5D4→7F5, 5D4→7F4 and 5D4→7F3 transitions in Tb3+ respectively.23, 24 Interestingly, the PL of Tb(phen)3/BN exhibits much stronger Tb3+ characteristic peaks and a broad emission band in the range of 350-475 nm, which can be attributed to the π*→π transitions of the ligands phen. Very surprisingly, in the Tb(acac)3/BN system, the emissions of BN host and acac ligands have almost disappeared, only very strong and sharp Tb3+ peaks can be observed. The acac ligand produces π→π* transitions by assimilating the energy of the excitation light, then transfer the assimilated energy to Tb3+ and lead to the strong green emission.25,

26

The significantly enhanced green emission indicates that the

Tb(acac)3/BN hybrid material to be very promising in lighting-emitting device applications. 6


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Figure 1. (a) Schematic illustration of the procedure for synthesis of Tb complex/porous BN hybrid material. (b) Structure of different ligands. (c) PL spectra of BN, Tb(bpy)3/BN, Tb(phen)3/BN, Tb(acac)3/BN. (d) Partial enlargement of (c).

Inspired by the excellent green emission of hybrid Tb(acac)3/BN product, we explored

its

microstructure

and

composition

by

TEM.

Representative

low-magnification TEM image in Figure 2a reveals that the material maintains fibrous morphology as pristine porous BN. The diameter of the fiber is ~0.7 µm. Figure 2b shows many bright spots on the fiber, revealing some pores existing in the fiber. The microstructural details of the Tb(acac)3/BN were investigated by high-resolution TEM (HRTEM). Figure 2c is the HRTEM image of the edge of the fiber, indicating that the BN fiber contains both amorphous and crystalline phase.27,

28

HRTEM image in 7



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Figure 2d derived from the center of the fiber reveals an amorphous phase. The amorphous phase may be attributed to the Tb(acac)3 on BN fibers. The chemical stoichiometry of the fiber was investigated with EDS.29 The EDS spectrum derived from a single Tb(acac)3/BN fiber clearly shows the peaks of B, N, O, C and Tb elements, implying the existence of terbium complexes in the product.

Figure 2. (a) TEM image of Tb(acac)3/BN hybrid material, showing fibrous morphology. (b) Enlarged TEM image of the material. (c, d) HRTEM images derived from the edge and center of the fiber, respectively. (e) The EDS spectrum derived from a single Tb(acac)3/BN fiber. (f) FTIR spectra of porous BN, Tb3+/BN and Tb(acac)3/BN.

Then we used Fourier transform infrared (FTIR) to study the chemical bonds and functional groups existing in the hybrid Tb(acac)3/BN. Figure 2f shows typical absorption peaks (~1383 and ~805 cm-1) in the spectrum of pristine porous BN, corresponding to B-N stretching and B-N-B bending vibrations, respectively.30, 31 The 8


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curve of Tb3+/BN is similar with pure BN, implying that the Tb3+ ions adsorbed on BN through weak interaction, i.e., electrostatic interaction, rather than strong chemical bonding. Differently, there are some new absorption peaks in the spectrum of Tb(acac)3/BN. The ~1615 cm-1 peak corresponds to the vibration of C-O bond in ligand acac; the ~531 cm-1 peak is due to the vibration of Tb-O bond,32 indicating the coordination of Tb3+ and acac ligands; the ~920 cm-1 peak corresponds to the vibration of the B-N-O bond, suggesting a strong interaction between the ligands and the BN host. According to the results of the above, it can be concluded that the organic complexes Tb(acac)3 ought to be introduced into the BN fibers successfully. The encapsulation of Tb(acac)3 into porous BN enables the hybrid material exhibits intense green emission. It is believed that the confinement of Tb(acac)3 molecules by the pores of BN could effectively decrease the aggregation of complexes and restrain the quenching effects, leading to the efficient luminescence. Considering that the introduce of second ligand into complexes could decrease the energy loss caused by ligand vibration, thus enhance the emission intensity of lanthanide ions, we further load of second ligand phen into Tb(acac)3/BN system. Through co-coordination of acac and phen with Tb3+, the Tb(acac)3phen/BN hybrid material has been obtained. The schematic diagrams of molecular structure of Tb(acac)3 and Tb(acac)3phen are shown in Figure 3e, respectively. The excitation and emission spectra of Tb(acac)3phen/BN were examined as shown in Figure 3a-d. We also include the spectra of pure BN, Tb3+/BN precursor, and Tb(acac)3/BN for comparison. As displayed in the excitation spectra (Figure 3a and 3b), Tb3+/BN 9



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precursor exhibits very weak excitation peaks, possibly due to the f-f electron transitions of Tb3+ ions. Differently, Tb(acac)3/BN displays a broad excitation band (~300 to 400 nm),which can be attributed to the π→π* electronic transitions in ligand acac.33, 34 After introducing of phen, the excitation peak of the hybrid material is red shifted and greatly enhanced, suggesting the successful co-coordination of acac and phen with Tb3+. Besides, the excitation peak at ~488 nm can be attributed to the f-f transition of Tb3+.35 The emission spectra monitored at 325 nm are shown in Figure 3c and 3d. The emission spectrum of Tb3+/BN shows similar UV emission from porous BN and very weak characteristic peaks of Tb3+. Compared with pure BN and Tb3+/BN, the Tb(acac)3/BN sample exhibits strong green emission, while the UV emission from BN almost disappears, revealing that efficient energy transfer from BN to acac and acac to Tb3+ ions occurred (will be discussed later). It is noteworthy that the emission intensities of Tb3+ have been remarkable increased after introducing of phen into the hybrid material, which indicates that phen also plays an energy donor role for the Tb3+ emission. The bright green emission of the two samples can be easily observed by eyes upon 365 nm UV excitation, as shown in Figure 3f. Then the fluorescence lifetime of Tb(acac)3/BN and Tb(acac)3phen/BN has also been studied, as shown in Figure 3g. The decay curves of Tb(acac)3/BN and Tb(acac)3phen/BN show monoexponentially luminescence decay with a lifetime of ~0.752 ms and 0.711 ms respectively, which is slightly longer than the lifetime of pure complex Tb(acac)3.nH2O (τ = 0.623 ms) and Tb(acac)3phen (τ = 0.596 ms).36 The 10


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result can be due to the suppression of nonradiative deactivation pathways of Tb complexes by their confinement in the pores of BN. After loading of phen ligands, the molecular structure of the composite becomes more rigid, thus the ligands will absorb energy at a faster rate and transfer it to the Tb3+, which leads to the increase of fluorescence intensity and decrease of fluorescence life. In addition, phen has greater rigidity and conjugation, so it can improve the luminous performance of the hybrid materials. Therefore the hybrid materials Tb(acac)3phen/BN exhibits stronger green luminescence than Tb(acac)3/BN. We also measured the quantum yield (QY) of the Tb(acac)3/BN sample is to be ~13.85%, while the QY of Tb(acac)3phen/BN increases to ~23.16%, indicating that the second ligand, phen, is involved in the energy transfer process.

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Figure 3. Excitation (a) and emission spectra (c) of BN, Tb3+/BN, Tb(acac)3/BN and Tb(acac)3phen/BN samples, respectively. (b) and (d) is partial enlargement of (a) and (c). (e) The schematic diagrams of molecular structure of Tb(acac)3 and Tb(acac)3phen. (f) Photo images of Tb(acac)3/BN and Tb(acac)3phen/BN excited by UV light, showing bright green emission color. (g) Decay curves of Tb(acac)3/BN and Tb(acac)3phen/BN, fitted according to I(t)=A+B1*exp(-i/T1)+B2*exp(-i/T2).

In order to clarify the luminescence mechanism of the hybrid materials, we compare the three-dimensional excitation emission matrix (EEM) fluorescence spectroscopy of pure acac, pure BN, Tb3+/BN precursor, Tb(acac)3/BN and Tb(acac)3phen/BN, as shown in Figure 4. It clearly displays that pure BN shows a broad UV emission in 300-400 nm under deep UV excitation (~220 nm) (Figure 4a). After adsorption of Tb3+, the Tb3+/BN precursor displays both UV emission of BN and very weak characteristic line peaks of Tb3+ (~488 nm and 546 nm) (Figure 4b). The weak Tb emission indicates that the energy of excitation light cannot be absorbed by Tb ions efficiently. Differently, the hybrid Tb(acac)3/BN shows significant increased line emission of Tb3+ (Figure 4c), suggesting an efficient energy transfer from acac to Tb ions occurs through “antenna effect”.37, 38 As compared with Tb3+/BN, the greatly suppressed BN emission indicates the energy transfer from BN host to the acac ligands takes place. Moreover, the bright circle appears in Figure 4c indicates that the product can also be excited by ~450 nm light and emits a ~500 nm broad band, which matches the corresponding excitation spectrum in Figure 3b. It is believed that 12


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the newly generated broad band is probably due to the strong interactions between BN and organic ligands. When loading with phen, the green Tb emission has been remarkable increased, while the BN emission has disappeared (Figure 4d), also implying an efficient energy conversion existed in the Tb(acac)3phen/BN system. We also measured the excitation spectrum of the Tb(acac)3phen/BN and the absorption spectra of the acac and phen ligands, as shown in Figure S2 (Supporting Information). The excitation spectra of Tb(acac)3phen/BN overlap with the absorption spectra of acac and phen, which indicates that ligands acac and phen can effectively sensitize the central Tb3+ fluorescence emission.

Figure 4. EEM spectra of the acac, BN, Tb3+/BN, Tb(acac)3/BN and Tb(acac)3phen/BN are shown in (a), (b), (c) and (d). (e) The schematic illustration of the energy level transfer process in Tb(acac)3phen/BN system. ET=energy transfer, ISC=intersystem crossing.

Based on the above analysis, the energy transfer mechanism in the hybrid material 13



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can be described as follows: (1) The BN host absorbs the energy of UV light and transitions from the ground state to the excited state; (2) energy transfer takes place from BN to the ligand acac; (3) The chromophore group of the ligand acac absorbs energy and transitions from its ground state single state (S0) to the singlet excited state (S1). Then, the energy is transferred from acac to the singlet excited state of phen, and transferred to its triplet excited state (T1) through the intersystem crossing (ISC); (4) The energy is transferred from phen to the excited state of the Tb3+ ion; (5) Finally, the radiation from the excited state to the ground state transitions and emits the characteristic fluorescence of Tb3+. Compared with other kinds of BN-based hybrid materials, such as Tb(bpy)3/BN and Tb(phen)3/BN, the Tb(acac)3/BN sample displays superior green emission due to the more efficient energy transfer in the hybrid system. To ensure the efficient energy transfer in the hybrid system, the energy gap between the impurity energy levels in BN, the triplet state energy level (T1) of the ligand and the 5D4 energy level of Tb3+ should be proper. Compared with the T1 level of bpy (22900 cm-1) and phen (23000 cm-1), the impurity energy levels in BN, the T1 level of acac (25300 cm-1) and the 5D4 energy level of Tb3+ matches well, resulting in the efficient Tb3+ luminescence.39 When loading of second ligands phen, the energy absorbed by acac could be transferred to phen directly, resulting a positive effect on the Tb3+ luminescence. The thermal stability of Tb(acac)3/BN and Tb(acac)3phen/BN samples was studied by TG analysis in the presence of air. As shown in Figure 5a, The curve of Tb(acac)3/BN shows a weight loss of ~13% throughout the process. The first weight 14


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loss stage (~2%) is from room temperature to ~50°C, which is related to the residual gas desorption of the sample. Then, the curve shows a weight loss peak at 358°C due to the decomposition of Tb(acac)3. (The decomposition temperature of pure Tb(acac)3 is about 118°C). The TG curve of Tb(acac)3phen/BN displays a total weight loss of ~17%. The decomposition temperature of Tb(acac)3phen/BN (~260°C) was higher than that of pure Tb(acac)3phen complex (~150°C).34 Compared with other hybrid materials, such as Tb(ACAC-SBA-15)3phen with a higher weight loss of ~23%,34 the relatively lower weigh loss indicates that porous BN can be used as a better substrate for the protection of organic molecules.40, 41 We consider that the spatial limitation of porous BN to terbium (III) complex is the main factor to improve its thermal stability.

Figure 5. TG-DTA curves of (a) Tb(acac)3/BN and (b) Tb(acac)3phen/BN. (c) Kinetics curves of Tb(acac)3/BN, Tb(acac)3phen/BN and Tb(acac)3.

To further demonstrate the photo stability of the hybrid luminescent materials, kinetics curves of Tb(acac)3/BN and Tb(acac)3phen/BN samples were measured. The maximum luminescence intensities of 5D4→7F5 transitions in different samples were monitored when the samples were continuously exposed to UV light irradiation for two hours. As shown in Figure 5c, the emission intensities of Tb(acac)3/BN and

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Tb(acac)3phen/BN are greatly enhanced compared with pure Tb(acac)3. Besides, the intensity of Tb(acac)3/BN gradually decreases and tends to be stable under the long-time UV irradiation. After loading of phen ligands, a relatively faster intensity decrease of Tb(acac)3phen/BN can be observed in the initial irradiation period (0~20 min), which may be attributed to the photo degradation of residual ligands exist in the sample. Similarly, the emission intensity tends to be stable after 2 h’s UV irradiation, also indicating the stabilization of the hybrid Tb(acac)3phen/BN sample. The spatial skeleton of BN provides a stable chemical microenvironment for the lanthanide complex, which is the key factor for stabilization effect on the complexes.

Conclusions In summary, we have synthesized a novel kind of BN-based hybrid material with tunable luminescent properties through controllable encapsulation of devise Tb complexes into porous BN host. A unique synergistic effect between the BN host and Tb complexes has been realized, resulting in the strong interaction and efficient energy transfer in the BN-ligands-Tb3+ system. Especially, due to the efficiently BN-acac-phen-Tb3+ energy transfer process, significantly enhanced green emission and high quantum yield has been obtained for the Tb(acac)3phen/BN product. Moreover, the confinement of Tb complexes within porous BN leads to the resultant Tb(acac)3/BN and Tb(acac)3phen/BN samples showing improved thermal and photo stabilities. These organic-inorganic hybrid luminescence materials are expected to find applications in lighting devices and biomedical analysis. 16


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Supporting Information Figure S1 shows the pore size distribution and pore volume of porous BN. Figure S2 shows the excitation spectrum of Tb(acac)3phen/BN samples and UV-vis absorption spectrum of ligands acac and phen.

Acknowledgements This work was supported by the National Natural Science Foundation of China (51572068, 51402086, 51372066, 51772075), the Natural Science Foundation of Hebei Province (E2016202122, B2015202346), the Tianjin Research Program of Application Foundation and Advanced Technology (14JCYBJC42200), the Hundred Talents Program of Hebei Province (E2014100011), and the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT: IRT13060).

Notes and references (1) Yan, B.; Zhou, B. Two Photoactive Lanthanide (Eu3+, Tb3+) Hybrid Materials of Modified β-Diketone Bridge Directly Covalently Bonded Mesoporous Host (MCM-41). J. Photochem. Photobiol. A 2008, 195, 314-322. (2) Moran, C. E.; Hale, G. D.; Halas, J, N. Synthesis and Characterization of Lanthanide-Doped Silica Microspheres. Langmuir 2008, 17, 8376-8379. (3) Li, Y. J.; Wang, L.; Yan, B. Photoactive Lanthanide Hybrids Covalently Bonded to Functionalized Periodic Mesoporous Organosilica (PMO) by Calix[4]Arene Derivative. J. Mater. Chem. 2011, 21, 1130-1138. 17



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