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Zirconia-based luminescent organic-inorganic hybrid materials with ternary europium (III) complexes bonded
Optical Materials 55 (2016) 78–82
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Zirconia-based luminescent organic-inorganic hybrid materials with ternary europium (III) complexes bonded Jing Yang, Zhiqiang Li, Yang Xu, Yige Wang ⇑ School of Chemical Engineering and Technology, Hebei University of Technology, GuangRong Dao 8, Hongqiao District, Tianjin 300130, PR China
a r t i c l e
i n f o
Article history: Received 26 January 2016 Received in revised form 7 March 2016 Accepted 12 March 2016 Available online 18 March 2016 Keywords: Zirconia Europium (III) complex Hybrid materials Transparent film
a b s t r a c t In this work, a novel red-emitting organic-inorganic hybrid material with europium (III) lanthanide bdiketonate complexes linked to a zirconia was reported, which was realized by adduct formation with zirconia-tethered terpyridine moieties. Luminescence enhancement of the hybrid material has been observed compared with pure Eu(tta)32H2O. Transparent and strongly luminescent thin films based on PMMA were also prepared at room temperature, which are highly luminescent under UV-light irradiation and possess a promising prospect in the area of optics. Ó 2016 Published by Elsevier B.V.
1. Introduction Lanthanide complexes (LCs) are a great family of luminescent materials since they exhibit unique optical properties including large Stokes shifts, sharp emission profiles, and long-lived excited states [1–8]. However, their practical applications in optical devices are severely limited by their drawbacks such as low thermal and photochemical stability, poor mechanical and conduction properties, and the tendency to aggregate [9,10]. One acceptable and heavily investigated method to overcome these problems is to tethering LCs to the backbone of the sol-gel-derived silica networks via covalent bonds, which yields the so-called class II organic-inorganic hybrid materials [4,11–14]. The advantages of such kind of hybrid materials over the so-called class I hybrid ones in which LCs are physically doped in silica word are: (i) homogenous distribution of both components, (ii) no leaching of LCs from the hybrid materials, and (iii) higher loading of LCs in the final hybrid materials [15–17]. However, class II organic-inorganic hybrid materials were mainly developed for silicate systems so far, which might be due to the commercially available or easily synthesized sol-gel precursors that can sensitize Ln3+ luminescence [18–20]. The extension of this chemistry to metal oxides other than silica would allows interesting new options for the development of inorganic-organic hybrid materials, which can result in materials with interesting ⇑ Corresponding author. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.optmat.2016.03.018 0925-3467/Ó 2016 Published by Elsevier B.V.
properties and is a case study on the chemical possibilities to tether metal complexes to non-silicate metal oxides [21–23]. Nevertheless, LCs cannot be linked to metal centers by a hydrocarbon spacer as with silicon owing to the hydrolytic cleavage of metalcarbon bonds. Tethering LCs to titania has recently been realized by using iso-nicotinic acid as the complexing ligand, the carboxylic acid moieties of which can coordinate to titanium alkoxide to tailor the reactivity toward hydrolysis and condensation, while the heterocyclic groups can coordinate to lanthanide ions as well as sensitize the luminescence of them [22,23]. The aim of this paper is to extend the strategy to immobilize LCs to zirconia, because ZrO2 is well-known for its high transparency and mechanical/chemical durability [24–27]. Herein we report zirconia-based red-emitting organic-inorganic hybrid materials by linking europium (III) LC to zirconia using 40 paraphenyl-carboxyl-2,20 :60 ,200 -terpyridine (Carb-Terpy) a linker, the carboxylic acid group of which can coordinate to zirconium alkoxide to tailor the its reactivity of hydrolysis and condensation and the nitrogen atoms of which can coordinate to europium (III) LC. Furthermore, transparent and luminescent films were also prepared through casting THF solution of the luminescent materials with an appropriate amount of organic polymer like Polymethyl methacrylate (PMMA). 2. Results and discussion Carboxylic acid groups are commonly employed as the complexing agent for metal alkoxides to tailor their reactivity
J. Yang et al. / Optical Materials 55 (2016) 78–82
and functionalize them [28]. In this paper, Carb-Terpy was utilized to modify zirconium alkoxide with the aim to link LCs to zirconia through adduct formation of tris(2-thenoyltrifluoroacetonato) lanthanide complexes with the nitrogen atoms of the terpyridine moieties in Carb-Terpy (Scheme 1). The reaction of Carb-Terpy with zirconium alkoxide can be clearly confirmed by FT-IR spectra (Fig. 1). The band at 1710 cm1 observed in the FT-IR spectrum of Carb-Terpy shown in Fig. 1a is attributed to the absorption of C@O for COOH group, which disappears after reaction with zirconium alkoxide and new bands at 1550 cm1 and 1410 cm1 (Fig. 1b), attributed to the antisymmetric stretching (vas) and symmetric stretching (vsym) vibration of ACOO group, respectively. The difference between carboxylate stretching frequencies (D = vas-vsym) is used to identify the bonding mode of the carboxylate ligand. The small frequency difference (D = 140 cm1) suggests that Carb-Terpy coordinate to the zirconium center in a bidentate mode [29]. The band at 480 cm1 observed in Fig. 1b is attributed to ZrAOAZr bond, indicating the formation of ZrAOAZr network, which results from the hydrolysis and condensation of the modified zirconium alkoxide [30]. Obvious changes in the FT-IR spectrum can be observed upon addition of Eu(tta)32H2O as shown in Fig. 1c. We can conclude the bands at 1609 cm1 and 1540 cm1 can be attributed to the C@O and C@C stretching vibrations of Eu(tta)32H2O by comparing Fig. 1c with Fig. 1d that shows the FI-IR spectrum of Eu(tta)32H2O. Theoretically, Eu(tta)32H2O can coordinate to the nitrogen atoms of terpyridine moieties in Carb-Terpy by replacing the coordinated water molecules as revealed by previously reported work [14,23]. However, this could not be observed from FT-IR spectra due to the great overlap of terpyridine absorption bands and those of Eu(tta)32H2O complexes. The resulting material (Eu(tta)3Terpy-carb-ZrO2) show bright red-emitting light when illuminated with a 365 nm UV light (as shown in Scheme 1). The formation of ternary europium (III)diketonate complex between Eu(tta)32H2O and the terpyridine moieties is further verified by the luminescence spectra shown in Fig. 2. The excitation spectrum of Eu(tta)32H2O is formed by a broad band in the range of 200–500 nm and weak sharp line at 465 nm (Fig. 2 left, line a). The former is attributed to the transitions of the tta ligand, whereas the weak sharper absorption line is assigned to the 7F0 ? 5D2 transition of Eu3+ ions, whose intensity is much weaker than that of the broad band, indicating that the ligand-excited states are the main intra-4f6 population path. Excitation into the tta ligand at 370 nm leads to the emission spectrum consisting of the characteristic sharp lines attributed to 7F0 ? 5DJ (J = 0–4) transition lines (Fig. 2 right, line b). The 5D0 decay curve of Eu3+ for Eu(tta)32H2O can be well fitted to a mono-
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exponential function and the lifetime is determined to be 0.25 ms (Fig. S1, a). The excitation spectrum of the resulting material is similar to that of Eu(tta)32H2O, the broad absorption band is due to tta ligand and the terpyridine moieties of Carb-Terpy (Fig. 2 left, line b). Less well-resolved emission lines at 580, 593, 613, 650 and 694 nm attributed to 5D0 ? 7FJ (J = 0–4), respectively, are observed for the resulting material (Fig. 2 right, line b), which can be attributed to the changes of the coordination sphere of Eu3+ due to the coordination of N atoms from Carb-Terpy [31]. This explanation can be supported by the behavior of the intensity ratio I(5D0 ? 7F2)/I(5D0 ? 7F1) and by the luminescence decay of the resulting material. The former changes from 16.18 for Eu(tta)32H2O to 13.10 for the resulting material. The lifetime of Eu3+ excited state for the resulting material is determined to be 0.56 ms (Fig. S1, b) from the decay curve that can be well fitted by means of mono-exponential function. The prolonged lifetime for the resulting material compared with that for Eu(tta)32H2O is attributed to the displacement of water molecules in the first coordination sphere of Eu3+ with the nitrogen atom of the terpyridine moieties of Carb-Terpy bond to zirconia matrices. The overall quantum yield (q) of the Eu(tta)3Terpy-carb-ZrO2 was determined with a calibrated sphere system and was found to be 37.54%, which exhibites a relatively low quantum yield compared to the similar silica-based luminescent hybrid materials (q = 97%) [14]. However, the q of Eu(tta)3Terpy-carb-ZrO2 is much higher than that of the titania-based luminescent materials that our group previously reported (q = 12.5%) [22]. Transparent and luminescent hybrid materials were also prepared by a simple casting procedure: casting THF solution of Eu (tta)3Terpy-carb-ZrO2 with an appropriate of PMMA polymer on glass slide and the petridish. The resultant film shows bright red light when illuminated with a UV light, demonstrating the presence of the luminescent material in the final polymer films (Fig. 3). Wave-guiding of the emitted light to the edge through total internal reflection is apparent. The obtained luminescent film is colorless and highly transparent as shown by the lack of visible absorption between 400 and 800 nm in the UV–Vis spectrum (Fig. 4). The absorption bands below 400 nm are attributed to the absorption of tta and terpyridine moieties. The top view image shows that the surface of the hybrid film is very uniform, flat, smooth, and free-defect (Fig. 5a). The cross-sectional image shows the transversal homogeneity and thickness uniformity of the hybrid film. The average thickness of the hybrid film measured from this image is 20 lm (Fig. 5b). The excitation and emission spectra of the luminescent films (Fig. 6) shows similar spectral features to Eu(tta)3Terpy-carb-
Scheme 1. Chemical structure of the organic ligands and the predicted ternary Eu3+ complexes as well as its digital photo taken under 365 nm UV light.
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Fig. 1. FT-IR spectra of (a) Carb-Terpy, (b) Terpy-carb-ZrO2, (c) Eu(tta)3Terpy-carb-ZrO2, (d) Eu(tta)32H2O.
Fig. 2. Excitation (left) monitored at 612 nm and emission (right) spectra excited at 380 nm of Eu(tta)32H2O (a) and of the Eu(tta)3Terpy-carb-ZrO2 (b).
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Fig. 4. UV–vis spectra of the luminescent film. Fig. 3. Digital photos of the luminescent films under daylight (right) and 365 nm UV-light (left).
ZrO2. The excitation spectrum is composed of a broad band ranging from 230 to 420 nm attributed to the absorption of organic ligands. Excitation at 343 nm leads to the same 7F0 ? 5DJ emission transitions as observed in Fig. 2 (right, line b) with the J = 2 transition as the most prominent feature. And no obvious shift in the wavelength of the emission bands can be observed. The decay curve (Fig. S1, c) of film is well produced by means of a singleexponential function, yielding a 5D0 lifetime value of 0.56 ms.
3. Conclusion The tethering europium (III) b-diketonate complexes to a zirconia has been realized by using 40 -paraphenyl-carboxyl-2,20 :60 ,200 -t erpyridine as a chemical additive which can modify the reactivity of the zirconia precursor and link LCs to zirconia via the coordination of the europium (III) b-diketonate complexes with the nitrogen atoms of the terpyridine moieties. Mixing the novel hybrid material with PMMA polymer leads to highly transparent and luminescent films, which show bright red luminescence under UV light and bear potential for improvement so that application
J. Yang et al. / Optical Materials 55 (2016) 78–82
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Fig. 5. (a) Top view and (b) cross-sectional SEM images of the luminescent film.
Fig. 6. Excitation (left) monitored at 612 nm and emission (right) spectra excited at 343 nm of the luminescent film.
as luminescence solar concentrator for solar cells might result in enhancement the photovoltaic conversion efficiency [32,33].
4. Experimental section 4.1. Material Poly(methyl methacrylate) (PMMA), 2-Thenoyltrifluoroacetone (tta), 2-acetylpyridine, Methyl 4-formylbenzoate and EuCl36H2O were purchased from Aldrich used as received. Zirconium npropoxide (abbreviated as ZNP) (70 wt%) in n-propanol was purchased from Alfa Aesar used as received. Eu(tta)32H2O and 40 -pa ra-phenylcarboxyl-2,20 :60 ,200 -terpyridine (Carb-Terpy) were synthesized by the reported methods [34,35].
4.2. Preparation of Eu(tta)3Terpy-carb-ZrO2 A typical sample was prepared by adding 5 mmol of ZNP to 20 mL of EtOH under refluxing and stirring at 80 °C. 10 mmol of Carb-terpy dissolved in an appropriate amount of pyridine was added dropwise into the clear solution and a white precipitate appeared immediately, the stirring was continued for another 1 h. And then 10 mmol of Eu(tta)32H2O dissolved in absolute ethanol was added drop wise into the above solution, the stirring was continued for another 6 h to yield a colorless, translucent solution, which was recovered by evaporation and dried for 20 h under vacuum followed by fine grind, the resulting luminescent material (light-yellow powder) was named Eu(tta)3Terpy-carb-ZrO2.
4.3. Preparation of the luminescent polymer thin film 0.005 g Eu(tta)3Terpy-carb-ZrO2 was dispersed in 17 ml of THF, 4 g PMMA dispersed 12 ml of THF was then added into the clear solution, After stirring for 3 h, the resulting translucent solution was used to obtain the hybrid films by drop-casting onto 2.5 cm 2.5 cm quartz substrates, which were previously cleaned. The wet hybrid films were drying at room temperature for several hours. 4.4. Characterization The Fourier transform infrared spectrum (FT-IR) spectra were obtained on a Bruker Vector 22 spectrometer from 4000 to 400 cm1 at a resolution of 4 cm1 (16 scans collected). The UV– Visible transmittance spectrum was obtained on an Agilent Carry 100 UV–vis spectrometer, from 200 to 800 nm. The steady-state luminescence spectra and the lifetime measurements were measured on an Edinburgh Instruments FS920P spectrometer, with a 450 W xenon lamp as the steady state excitation source, a double excitation monochromator (1800 lines mm1), an emission monochromator (600 lines mm1) and a semiconductor cooled Hamamatsu RMP928 photo multiplier tube. All spectra were obtained at room temperature. SEM images were obtained from a Nova Nano SEM 450 atan acceleration voltage of 15 kV. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21271060 and 21502039), the Tianjin Natural Science Foundation (13JCYBJC18400), and Educational Committee of Hebei Province (LJRC021 and QN2015172).
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.optmat.2016.03. 018. References [1] L. Maggini, H. Traboulsi, K. Yoosaf, J. Mohanraj, J. Wouters, O. Pietraszkiewicz, M. Pietraszkiewicz, N. Armaroli, D. Bonifazi, Chem. Commun. 47 (2011) 1625. [2] S.V. Eliseeva, J.-C.G. Bunzli, Chem. Soc. Rev. 39 (2010) 189. [3] J. Zhang, W.-B. Yang, X.-Y. Wu, X.-F. Kuang, C.-Z. Lu, Dalton Trans. 44 (2015) 13586. [4] J. Feng, H. Zhang, Chem. Soc. Rev. 42 (2013) 387. [5] H. Li, W. Cheng, Y. Wang, B. Liu, W. Zhang, H. Zhang, Chem. – Eur. J. 16 (2010) 2125. [6] L.V. Meyer, F. Schonfeld, K. Muller-Buschbaum, Chem. Commun. 50 (2014) 8093. [7] X. Chen, P. Zhang, T. Wang, H. Li, Chem.– Eur. J. 20 (2014) 2551. [8] T.V. Usha Gangan, M.L.P. Reddy, Dalton Trans. 44 (2015) 15924. [9] H. Li, M. Li, Y. Wang, W. Zhang, Chem. – Eur. J. 20 (2014) 10392. [10] V. Divya, M.L.P. Reddy, J. Mater. Chem. C 1 (2013) 160. [11] L.D. Carlos, R.A.S. Ferreira, V.D. Bermudez, B. Julian-Lopez, P. Escribano, Chem. Soc. Rev. 40 (2011) 536. [12] M. Fernandes, V. de Zea Bermudez, R.A. Sá Ferreira, L.D. Carlos, A. Charas, J. Morgado, M.M. Silva, M. Smith, J. Chem. Mater. 19 (2007) 3892. [13] B. Yan, RSC Adv. 2 (2012) 9304. [14] K. Binnemans, Chem. Rev. 109 (2009) 4283.
[15] H.R. Li, J. Lin, H.J. Zhang, H.C. Li, L.S. Fu, Q.G. Meng, Chem. Commun. (2001) 1212. [16] Q.M. Wang, B. Yan, Cryst. Growth Des. 5 (2005) 497. [17] A.-C. Franville, D. Zambon, R. Mahiou, Y. Troin, Chem. Mater. 12 (2000) 428. [18] J. Graffion, X. Cattoën, M. Wong Chi Man, V.R. Fernandes, P.S. André, R.A.S. Ferreira, L.D. Carlos, Chem. Mater. 23 (2011) 4773. [19] P.P. Lima, F.A. Almeida Paz, R.A.S. Ferreira, V. de Zea Bermudez, L.D. Carlos, Chem. Mater. 21 (2009) 5099. [20] P. Zhang, Y. Wang, H. Liu, Y. Chen, J. Mater. Chem. 21 (2011) 18462. [21] U. Schubert, J. Mater. Chem. 15 (2005) 3701. [22] P. Liu, H. Li, Y. Wang, B. Liu, W. Zhang, Y. Wang, W. Yan, H. Zhang, U. Schubert, J. Mater. Chem. 18 (2008) 735. [23] H. Li, P. Liu, Y. Wang, L. Zhang, J. Yu, H. Zhang, B. Liu, U. Schubert, J. Phys. Chem. C 113 (2009) 3945. [24] C.C. Cheng, S.Y. Yang, D. Lee, Sensors 14 (2014) 19493. [25] R. Di Maggio, L. Fambri, A. Guerriero, Chem. Mater. 10 (1998) 1777. [26] Y. Hu, S. Zhou, L. Wu, Polymer 50 (2009) 3609. [27] N. Nakayama, T. Hayashi, Compos. A Appl. Sci. Manuf. 2007 (1996) 38. [28] U. Schubert, Acc. Chem. Res. 40 (2007) 730. [29] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, John Wiley & Sons, New York, 1963. [30] H. Hayashi, H. Suzuki, S. Kaneko, J. Sol-Gel. Sci. Technol. 12 (1998) 87. [31] P. Li, D. Wang, D. Liang, L. Zhang, S. Zhang, Y. Wang, Mater. Res. Bull. 55 (2014) 216. [32] V.T. Freitas, L. Fu, A.M. Cojocariu, X. Cattoën, J.R. Bartlett, R. Le Parc, J.-L. Bantignies, M. Wong Chi Man, P.S. André, R.A.S. Ferreira, L.D. Carlos, ACS Appl. Mater. Interfaces 7 (2015) 8770. [33] L. Wilson, B. Richards, Appl. Opt. 48 (2009) 212. [34] J.-C. Terrillon, S. Akamatsu, Vision Interface, vol. 99, Citeseer, 1999, p. 1821. [35] D.G.-H.J. Xiao-Yuan, Chin. J. Inorg. Chem. (2004) 7.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Zirconia-based luminescent organic-inorganic hybrid materials with ternary europium (III) complexes bonded Jing Yang, Zhiqiang Li, Yang Xu, Yige Wang ⇑ School of Chemical Engineering and Technology, Hebei University of Technology, GuangRong Dao 8, Hongqiao District, Tianjin 300130, PR China
a r t i c l e
i n f o
Article history: Received 26 January 2016 Received in revised form 7 March 2016 Accepted 12 March 2016 Available online 18 March 2016 Keywords: Zirconia Europium (III) complex Hybrid materials Transparent film
a b s t r a c t In this work, a novel red-emitting organic-inorganic hybrid material with europium (III) lanthanide bdiketonate complexes linked to a zirconia was reported, which was realized by adduct formation with zirconia-tethered terpyridine moieties. Luminescence enhancement of the hybrid material has been observed compared with pure Eu(tta)32H2O. Transparent and strongly luminescent thin films based on PMMA were also prepared at room temperature, which are highly luminescent under UV-light irradiation and possess a promising prospect in the area of optics. Ó 2016 Published by Elsevier B.V.
1. Introduction Lanthanide complexes (LCs) are a great family of luminescent materials since they exhibit unique optical properties including large Stokes shifts, sharp emission profiles, and long-lived excited states [1–8]. However, their practical applications in optical devices are severely limited by their drawbacks such as low thermal and photochemical stability, poor mechanical and conduction properties, and the tendency to aggregate [9,10]. One acceptable and heavily investigated method to overcome these problems is to tethering LCs to the backbone of the sol-gel-derived silica networks via covalent bonds, which yields the so-called class II organic-inorganic hybrid materials [4,11–14]. The advantages of such kind of hybrid materials over the so-called class I hybrid ones in which LCs are physically doped in silica word are: (i) homogenous distribution of both components, (ii) no leaching of LCs from the hybrid materials, and (iii) higher loading of LCs in the final hybrid materials [15–17]. However, class II organic-inorganic hybrid materials were mainly developed for silicate systems so far, which might be due to the commercially available or easily synthesized sol-gel precursors that can sensitize Ln3+ luminescence [18–20]. The extension of this chemistry to metal oxides other than silica would allows interesting new options for the development of inorganic-organic hybrid materials, which can result in materials with interesting ⇑ Corresponding author. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.optmat.2016.03.018 0925-3467/Ó 2016 Published by Elsevier B.V.
properties and is a case study on the chemical possibilities to tether metal complexes to non-silicate metal oxides [21–23]. Nevertheless, LCs cannot be linked to metal centers by a hydrocarbon spacer as with silicon owing to the hydrolytic cleavage of metalcarbon bonds. Tethering LCs to titania has recently been realized by using iso-nicotinic acid as the complexing ligand, the carboxylic acid moieties of which can coordinate to titanium alkoxide to tailor the reactivity toward hydrolysis and condensation, while the heterocyclic groups can coordinate to lanthanide ions as well as sensitize the luminescence of them [22,23]. The aim of this paper is to extend the strategy to immobilize LCs to zirconia, because ZrO2 is well-known for its high transparency and mechanical/chemical durability [24–27]. Herein we report zirconia-based red-emitting organic-inorganic hybrid materials by linking europium (III) LC to zirconia using 40 paraphenyl-carboxyl-2,20 :60 ,200 -terpyridine (Carb-Terpy) a linker, the carboxylic acid group of which can coordinate to zirconium alkoxide to tailor the its reactivity of hydrolysis and condensation and the nitrogen atoms of which can coordinate to europium (III) LC. Furthermore, transparent and luminescent films were also prepared through casting THF solution of the luminescent materials with an appropriate amount of organic polymer like Polymethyl methacrylate (PMMA). 2. Results and discussion Carboxylic acid groups are commonly employed as the complexing agent for metal alkoxides to tailor their reactivity
J. Yang et al. / Optical Materials 55 (2016) 78–82
and functionalize them [28]. In this paper, Carb-Terpy was utilized to modify zirconium alkoxide with the aim to link LCs to zirconia through adduct formation of tris(2-thenoyltrifluoroacetonato) lanthanide complexes with the nitrogen atoms of the terpyridine moieties in Carb-Terpy (Scheme 1). The reaction of Carb-Terpy with zirconium alkoxide can be clearly confirmed by FT-IR spectra (Fig. 1). The band at 1710 cm1 observed in the FT-IR spectrum of Carb-Terpy shown in Fig. 1a is attributed to the absorption of C@O for COOH group, which disappears after reaction with zirconium alkoxide and new bands at 1550 cm1 and 1410 cm1 (Fig. 1b), attributed to the antisymmetric stretching (vas) and symmetric stretching (vsym) vibration of ACOO group, respectively. The difference between carboxylate stretching frequencies (D = vas-vsym) is used to identify the bonding mode of the carboxylate ligand. The small frequency difference (D = 140 cm1) suggests that Carb-Terpy coordinate to the zirconium center in a bidentate mode [29]. The band at 480 cm1 observed in Fig. 1b is attributed to ZrAOAZr bond, indicating the formation of ZrAOAZr network, which results from the hydrolysis and condensation of the modified zirconium alkoxide [30]. Obvious changes in the FT-IR spectrum can be observed upon addition of Eu(tta)32H2O as shown in Fig. 1c. We can conclude the bands at 1609 cm1 and 1540 cm1 can be attributed to the C@O and C@C stretching vibrations of Eu(tta)32H2O by comparing Fig. 1c with Fig. 1d that shows the FI-IR spectrum of Eu(tta)32H2O. Theoretically, Eu(tta)32H2O can coordinate to the nitrogen atoms of terpyridine moieties in Carb-Terpy by replacing the coordinated water molecules as revealed by previously reported work [14,23]. However, this could not be observed from FT-IR spectra due to the great overlap of terpyridine absorption bands and those of Eu(tta)32H2O complexes. The resulting material (Eu(tta)3Terpy-carb-ZrO2) show bright red-emitting light when illuminated with a 365 nm UV light (as shown in Scheme 1). The formation of ternary europium (III)diketonate complex between Eu(tta)32H2O and the terpyridine moieties is further verified by the luminescence spectra shown in Fig. 2. The excitation spectrum of Eu(tta)32H2O is formed by a broad band in the range of 200–500 nm and weak sharp line at 465 nm (Fig. 2 left, line a). The former is attributed to the transitions of the tta ligand, whereas the weak sharper absorption line is assigned to the 7F0 ? 5D2 transition of Eu3+ ions, whose intensity is much weaker than that of the broad band, indicating that the ligand-excited states are the main intra-4f6 population path. Excitation into the tta ligand at 370 nm leads to the emission spectrum consisting of the characteristic sharp lines attributed to 7F0 ? 5DJ (J = 0–4) transition lines (Fig. 2 right, line b). The 5D0 decay curve of Eu3+ for Eu(tta)32H2O can be well fitted to a mono-
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exponential function and the lifetime is determined to be 0.25 ms (Fig. S1, a). The excitation spectrum of the resulting material is similar to that of Eu(tta)32H2O, the broad absorption band is due to tta ligand and the terpyridine moieties of Carb-Terpy (Fig. 2 left, line b). Less well-resolved emission lines at 580, 593, 613, 650 and 694 nm attributed to 5D0 ? 7FJ (J = 0–4), respectively, are observed for the resulting material (Fig. 2 right, line b), which can be attributed to the changes of the coordination sphere of Eu3+ due to the coordination of N atoms from Carb-Terpy [31]. This explanation can be supported by the behavior of the intensity ratio I(5D0 ? 7F2)/I(5D0 ? 7F1) and by the luminescence decay of the resulting material. The former changes from 16.18 for Eu(tta)32H2O to 13.10 for the resulting material. The lifetime of Eu3+ excited state for the resulting material is determined to be 0.56 ms (Fig. S1, b) from the decay curve that can be well fitted by means of mono-exponential function. The prolonged lifetime for the resulting material compared with that for Eu(tta)32H2O is attributed to the displacement of water molecules in the first coordination sphere of Eu3+ with the nitrogen atom of the terpyridine moieties of Carb-Terpy bond to zirconia matrices. The overall quantum yield (q) of the Eu(tta)3Terpy-carb-ZrO2 was determined with a calibrated sphere system and was found to be 37.54%, which exhibites a relatively low quantum yield compared to the similar silica-based luminescent hybrid materials (q = 97%) [14]. However, the q of Eu(tta)3Terpy-carb-ZrO2 is much higher than that of the titania-based luminescent materials that our group previously reported (q = 12.5%) [22]. Transparent and luminescent hybrid materials were also prepared by a simple casting procedure: casting THF solution of Eu (tta)3Terpy-carb-ZrO2 with an appropriate of PMMA polymer on glass slide and the petridish. The resultant film shows bright red light when illuminated with a UV light, demonstrating the presence of the luminescent material in the final polymer films (Fig. 3). Wave-guiding of the emitted light to the edge through total internal reflection is apparent. The obtained luminescent film is colorless and highly transparent as shown by the lack of visible absorption between 400 and 800 nm in the UV–Vis spectrum (Fig. 4). The absorption bands below 400 nm are attributed to the absorption of tta and terpyridine moieties. The top view image shows that the surface of the hybrid film is very uniform, flat, smooth, and free-defect (Fig. 5a). The cross-sectional image shows the transversal homogeneity and thickness uniformity of the hybrid film. The average thickness of the hybrid film measured from this image is 20 lm (Fig. 5b). The excitation and emission spectra of the luminescent films (Fig. 6) shows similar spectral features to Eu(tta)3Terpy-carb-
Scheme 1. Chemical structure of the organic ligands and the predicted ternary Eu3+ complexes as well as its digital photo taken under 365 nm UV light.
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Fig. 1. FT-IR spectra of (a) Carb-Terpy, (b) Terpy-carb-ZrO2, (c) Eu(tta)3Terpy-carb-ZrO2, (d) Eu(tta)32H2O.
Fig. 2. Excitation (left) monitored at 612 nm and emission (right) spectra excited at 380 nm of Eu(tta)32H2O (a) and of the Eu(tta)3Terpy-carb-ZrO2 (b).
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Fig. 4. UV–vis spectra of the luminescent film. Fig. 3. Digital photos of the luminescent films under daylight (right) and 365 nm UV-light (left).
ZrO2. The excitation spectrum is composed of a broad band ranging from 230 to 420 nm attributed to the absorption of organic ligands. Excitation at 343 nm leads to the same 7F0 ? 5DJ emission transitions as observed in Fig. 2 (right, line b) with the J = 2 transition as the most prominent feature. And no obvious shift in the wavelength of the emission bands can be observed. The decay curve (Fig. S1, c) of film is well produced by means of a singleexponential function, yielding a 5D0 lifetime value of 0.56 ms.
3. Conclusion The tethering europium (III) b-diketonate complexes to a zirconia has been realized by using 40 -paraphenyl-carboxyl-2,20 :60 ,200 -t erpyridine as a chemical additive which can modify the reactivity of the zirconia precursor and link LCs to zirconia via the coordination of the europium (III) b-diketonate complexes with the nitrogen atoms of the terpyridine moieties. Mixing the novel hybrid material with PMMA polymer leads to highly transparent and luminescent films, which show bright red luminescence under UV light and bear potential for improvement so that application
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Fig. 5. (a) Top view and (b) cross-sectional SEM images of the luminescent film.
Fig. 6. Excitation (left) monitored at 612 nm and emission (right) spectra excited at 343 nm of the luminescent film.
as luminescence solar concentrator for solar cells might result in enhancement the photovoltaic conversion efficiency [32,33].
4. Experimental section 4.1. Material Poly(methyl methacrylate) (PMMA), 2-Thenoyltrifluoroacetone (tta), 2-acetylpyridine, Methyl 4-formylbenzoate and EuCl36H2O were purchased from Aldrich used as received. Zirconium npropoxide (abbreviated as ZNP) (70 wt%) in n-propanol was purchased from Alfa Aesar used as received. Eu(tta)32H2O and 40 -pa ra-phenylcarboxyl-2,20 :60 ,200 -terpyridine (Carb-Terpy) were synthesized by the reported methods [34,35].
4.2. Preparation of Eu(tta)3Terpy-carb-ZrO2 A typical sample was prepared by adding 5 mmol of ZNP to 20 mL of EtOH under refluxing and stirring at 80 °C. 10 mmol of Carb-terpy dissolved in an appropriate amount of pyridine was added dropwise into the clear solution and a white precipitate appeared immediately, the stirring was continued for another 1 h. And then 10 mmol of Eu(tta)32H2O dissolved in absolute ethanol was added drop wise into the above solution, the stirring was continued for another 6 h to yield a colorless, translucent solution, which was recovered by evaporation and dried for 20 h under vacuum followed by fine grind, the resulting luminescent material (light-yellow powder) was named Eu(tta)3Terpy-carb-ZrO2.
4.3. Preparation of the luminescent polymer thin film 0.005 g Eu(tta)3Terpy-carb-ZrO2 was dispersed in 17 ml of THF, 4 g PMMA dispersed 12 ml of THF was then added into the clear solution, After stirring for 3 h, the resulting translucent solution was used to obtain the hybrid films by drop-casting onto 2.5 cm 2.5 cm quartz substrates, which were previously cleaned. The wet hybrid films were drying at room temperature for several hours. 4.4. Characterization The Fourier transform infrared spectrum (FT-IR) spectra were obtained on a Bruker Vector 22 spectrometer from 4000 to 400 cm1 at a resolution of 4 cm1 (16 scans collected). The UV– Visible transmittance spectrum was obtained on an Agilent Carry 100 UV–vis spectrometer, from 200 to 800 nm. The steady-state luminescence spectra and the lifetime measurements were measured on an Edinburgh Instruments FS920P spectrometer, with a 450 W xenon lamp as the steady state excitation source, a double excitation monochromator (1800 lines mm1), an emission monochromator (600 lines mm1) and a semiconductor cooled Hamamatsu RMP928 photo multiplier tube. All spectra were obtained at room temperature. SEM images were obtained from a Nova Nano SEM 450 atan acceleration voltage of 15 kV. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21271060 and 21502039), the Tianjin Natural Science Foundation (13JCYBJC18400), and Educational Committee of Hebei Province (LJRC021 and QN2015172).
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