- Email: [email protected]
Fluorescence-color-tunable and transparent polyarylene ether nitrile films with high thermal stability and mechanical strength based on polymeric rare-earth complexes for roll-up displays
Materials Letters 91 (2013) 235–238
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Fluorescence-color-tunable and transparent polyarylene ether nitrile films with high thermal stability and mechanical strength based on polymeric rare-earth complexes for roll-up displays Hailong Tang, Zejun Pu, Junji Wei, Haoyu Guo, Xu Huang, Xiaobo Liu n Research Branch of Advanced Functional Materials, Institute of Microelectronics and Solid State Electronics, High Temperature Resistant Polymers and Composites Key Laboratory of Sichuan Province, University of Electronic Science and Technology of China, Chengdu 610054, PR China
a r t i c l e i n f o
abstract
Article history: Received 26 July 2012 Accepted 2 October 2012 Available online 10 October 2012
A novel series of transparent, flexible and fluorescent polymeric rare-earth complex films were successfully prepared by chemical coordination between carboxyl groups in polyarylene ether nitrile and rare-earth ions. The derived films showed high thermal stability with 5% weight loss temperature in the range of 402–420 1C, and also exhibited high mechanical strength ranging from 76 MPa to 83 MPa. Furthermore, all films demonstrated excellent macroscopic flexibility, since they can be easily curled in both natural and light-emitting states. Optical results indicated that the films possessed good optical transparency, and emitted intense fluorescence under ultraviolet excitation. More importantly, the fluorescence colors can be tuned between red and green by controlling the ratios of Eu(III)–Tb(III) ions in the films. & 2012 Elsevier B.V. All rights reserved.
Keywords: Fluorescence-color-tunable Polyarylene ether nitrile Rare-earth complex Mechanical properties Luminescence Thermal properties
1. Introduction Luminescent rare-earth (RE) complexes with organic ligands have many potential applications in light emitting diodes, fluorescent sensors, or laser materials due to their high fluorescence intensities [1–3]. However, organic low-molecular RE complexes have serious limitations in practical applications because of their poor mechanical properties and physicochemical stabilities. By contrast, polymeric high-molecular RE complexes not only possess unique fluorescence properties of the RE ions but also own good mechanical toughness, chemical stability and excellent processability [4–6]. Especially, the development of fluorescent films with robust flexibility, high transparency and excellent thermostability has become a hot research topic, for their good prospects of application in e-papers, roll-up displays and polymeric light-emitting devices [7]. As a new class of high-performance thermoplastic polymers, polyarylene ether nitriles possess high thermal stability, outstanding chemical inertia and excellent radiation resistance, and they have already been used as matrices in advanced composites in aerospace industries [8–11]. Recently, a series of new polyarylene ether nitriles containing pendant carboxyl groups were developed in our group, which possess a variety of favorable characteristics, such as high glass transition temperature (above
n
Corresponding author. Tel./fax: þ 86 28 83207326. E-mail address: [email protected] (X. Liu).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.10.010
180 1C), excellent mechanical strength and high transparency as well as good solubility in the common solvents and excellent film-forming property. Therefore, to develop high performance fluorescent films, polyarylene ether nitrile containing pendant carboxyl groups (PEN) was selected as the polymer matrix, and also as the macromolecular ligand to form PEN-RE complexes, by forming the chemical coordination bonds between RE ions and reactive carboxyl groups in PEN [12,13]. In this paper, we present the preparation and characterization of the PEN fluorescent films based on polymeric rare-earth complexes, and their thermal, mechanical, optical properties and flexibility were investigated in detail.
2. Experimental Polyarylene ether nitrile containing pendent carboxyl groups was synthesized from 2, 6-dichlorobenzonitrile with bisphenol A and phenolphthalin through nucleophilic aromatic substitution polymerization reaction, as described previously in detail [14]. In a typical procedure for preparing fluorescent films, a prescribed amount of EuCl3 6H2O and TbCl3 6H2O with different proportions were added in N, N-dimethylacetamide (DMAc) solvent in an ultrasonic water bath. Meanwhile, a certain amount of the carboxyl-functionalized PEN was dissolved in DMAc with heating and stirring. Then the rare-earth solution was slowly dropped into the PEN solution, and the mixture was refluxed at 165 1C for 2 h
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H. Tang et al. / Materials Letters 91 (2013) 235–238
accompanying with high-speed mechanical stirring. Finally, the resultant solution was cast onto a clean-horizontal glass plate and dried in an oven using a sequential mode of temperature program at 80 1C, 100 1C, 120 1C, 140 1C and 160 1C (2 h each) to evaporate solvent completely. The fluorescent films were obtained by shedding naturally after cooling to room temperature, with the thickness of 35–45 mm. In the experiment, through conversion by calculating, the mass fraction of EuCl3 in the films (marked as PEN-E0T8, PENE2T6, PEN-E4T4, PEN-E6T2, PEN-E8T0) was fixed at 0%, 2%, 4%, 6%, 8% (Eu(III) wt%: 0%, 1.18%, 2.35%, 3.53%, 4.71%), and the corresponding mass fraction of TbCl3 was fixed at 8%, 6%, 4%, 2%, 0% (Tb(III) wt%: 4.79%, 3.59%, 2.40%, 1.20%, 0%), respectively. Thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTG) were performed on a TA Q50 series analyzer system under a nitrogen atmosphere at a heating rate of 20 1C min 1. Mechanical properties were measured using a SANS CMT6104 series desktop electromechanical universal testing machine at room temperature, and the data was reported as the average value for five samples. Ultraviolet-visible (UV–vis) absorption and optical transmittance spectra were recorded by a Shimadzu 3100 UV–vis-near–IR spectrophotometer. Fluorescence emission spectra were measured on a Hitachi F-7000 FL spectrophotometer under UV irradiation of 254 nm.
potential applications in the field of flexible electronics such as e-papers and roll-up displays. Optical properties: The optical transmittance and UV–vis absorption spectra of the fluorescent films are shown in Fig. 2a and b respectively, and the inset of Fig. 2a displays the fluorescent film (36 mm in thickness) on a labeled paper. As we can see, the symbol on the paper can be clearly observed, and the optical transmittance spectra of the films further show a high transmittance of 83%–90% at the wavelength of 500–800 nm for films of 35–45 mm in thickness. Moreover, the fluorescent films show a strong and broad absorption band in the range of 200–325 nm, which is attributed to the p-pn electronic transitions of aromatic rings [15], and there is almost no absorption in the visible region. Fig. 3a–e depicts the fluorescence emission spectra of the PEN fluorescent films. As shown in Fig. 3a, the major emission peaks for the PEN-E0T8 film appear at 489 nm, 545 nm, 586 nm and 622 nm, corresponding to 5D4-7F6, 5D4-7F5, 5D4-7F4 and 5 D4-7F3 transitions of Tb(III) ions, respectively [13]. In addition, the emission peak at 545 nm is over four times higher than other
3. Results and discussion Thermal properties: The thermal stability of the PEN fluorescent films was characterized by TGA and DTG in nitrogen atmosphere, and evaluated by 5% weight loss temperature (T5%) and maximum decomposition rate temperature (Tmax). As summarized in Table 1, all the fluorescent films exhibited high thermal stability, with T5% in the range of 402–420 1C and Tmax over 514 1C. This is mainly due to the rigid conjugated aromatic structures of PEN matrix. In addition, the glass transition temperature of the films is high up to 192 1C, and it provides a good guarantee for their applications under high-temperature conditions. Mechanical properties and flexibility: The mechanical properties of the fluorescent films, such as tensile strength, breaking elongation and Young’s modulus, are also listed in Table 1. As can be seen, all the fluorescent films have a high tensile strength over 76 MPa, and the maximum is up to 83.9 MPa. Moreover, the fluorescent films show an elongation at break in the range of 3.9–4.5%, and a high Young’s modulus more than 2.6 GPa. This is mainly attributed to the high performance of PEN itself and its chemical coordination with the rare-earth ions, which brings the uniform dispersion and good compatibility. More importantly, all PEN fluorescent films demonstrate an excellent macroscopic flexibility. Fig. 1a–c shows the digital images of the fluorescent films in natural light and under UV light irradiation of 254 nm, respectively. As we can see, the fluorescent films can be easily bent and curled whether in the natural state or in the light-emitting state. This reveals that the films may have
Fig. 1. (a) Digital image of the PEN-E2T6 film to demonstrate the optical transparency and macroscopic flexibility. Photoluminescence images of (b) the PEN-E0T8 film (Green-light-emission) and (c) the PEN-E8T0 film (Red-lightemission) under UV light irradiation of 254 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Thermal and mechanical properties of the fluorescent films.
Film code
PEN-E0T8 PEN-E2T6 PEN-E4T4 PEN-E6T2 PEN-E8T0
Thermal properties
Mechanical properties
T5% (1C)
Tmax (1C)
Tensile strength (MPa)
Breaking elongation (%)
Young’s modulus (GPa)
403 420 414 406 402
516 516 517 514 514
77.1 81.5 83.9 76.2 82.3
3.9 4.2 3.9 3.9 4.5
2.7 3.0 2.9 2.6 2.7
H. Tang et al. / Materials Letters 91 (2013) 235–238
237
Fig. 2. (a) Optical transmittance spectra (Inset: Transparent film on a labeled paper) and (b) UV–vis absorption spectra of the PEN fluorescent films.
Fig. 3. Fluorescence emission spectra of the PEN films: (a) PEN-E0T8, (b) PEN-E2T6, (c) PEN-E4T4, (d) PEN-E6T2, (e) PEN-E8T0 and (f) CIE 1931 x, y chromaticity diagram for the PEN films: (I) PEN-E0T8, (II) PEN-E2T6, (III) PEN-E4T4, (IV) PEN-E6T2, (V) PEN-E8T0.
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H. Tang et al. / Materials Letters 91 (2013) 235–238
peaks in intensity and plays a dominant role, so the PEN-E0T8 film displays a characteristic green-light-emission (Fig. 1b). Similarly, the major emission peaks for the PEN-E8T0 film are observed at 575 nm, 593 nm and 617 nm (Fig. 3e), which are assigned to the 5D0-7F0, 5 D0-7F1 and 5D0-7F2 transitions of Eu(III) ions, respectively [16]. The strongest emission peak at 617 nm plays a dominant role so that the PEN-E8T0 film emits characteristic red light under UV irradiation (Fig. 1c). As shown in Fig. 3b–d, the films of PEN-E2T6, PEN-E4T4 and PEN-E6T2 show both the characteristic emission peaks of Tb(III) and Eu(III) ions, and there has been no energy transfer phenomenon from the 5D4 level of Tb(III) ions to the 5D0 level of Eu(III) ions reported in the literature [17]. This may be attributed to the coordination of rare-earth ions with carboxyl groups, which makes a large separation distance between the rare-earth ions, and there will not form cluster structures. Furthermore, due to the different ratios of Eu(III) to Tb(III) ions in the films, their corresponding emission peaks have a different relative intensity. According to the RGB (red-green-blue) color model, all the intermediate colors between red and green can be obtained through controlling the intensity of red and green, such as yellow, orange, orange-red etc. Therefore, the fluorescence colors of the films can be tuned between red and green by controlling the ratios of Eu(III) to Tb(III) ions. In order to exhibit the trace of color variation, the CIE (Commission Internationale de L’Eclairage) color coordinates for the PEN fluorescent films are calculated based on their emission spectra [18], and the chromaticity diagram deduced as per CIE 1931 x, y coordinates is shown in Fig. 3f. Clearly, the CIE coordinates for the fluorescence of PEN-E0T8, PEN-E2T6, PEN-E4T4, PEN-E6T2 and PEN-E8T0 films indicate green, chartreuse, yellow, orange and red emissions, respectively.
4. Conclusion In summary, a series of PEN fluorescent films with good optical transparency and excellent macroscopic flexibility were prepared by coordination of PEN with rare-earth ions. The results showed that the films had a high thermal stability with T5% over 402 1C, and also possessed a high tensile strength exceeding 76 MPa and a high Young’s modulus greater than 2.6 GPa. Furthermore, the films could emit intense fluorescence under UV excitation.
Meanwhile, they can be easily curled whether in the natural state or in the light-emitting state. More importantly, the fluorescence colors can be tuned between red and green by controlling the ratios of Eu(III) to Tb(III) ions in the films. These characteristics are of great significance for their potential applications in the field of flexible electronics such as e-papers and roll-up displays.
Acknowledgments The authors gratefully acknowledge support from the National Natural Science Foundation of China (No. 51173021) and ‘‘863‘‘ National Major Program of High Technology of China (2012AA03A212).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2012. 10.010.
References [1] Guerriero P, Tamburini S, Vigato PA. Coord Chem Rev 1995;139:17–243. [2] Campos RA, Kovalev IP, Guo Y, Wakili N, Skotheim T. J Appl Phys 1996;80:7144–50. [3] Gao XC, Cao H, Huang C, Li B, Umitani S. Appl Phys Lett 1998;72:2217–9. [4] Lee CI, Lim JS, Kim SH, Suh DH. Polymer 2006;47:5253–8. [5] Wang D, Zhang J, Lin Q, Fu L, Zhang H, Yang B. J Mater Chem 2003;13: 2279–84. [6] Yin W, Chen M, Lu T, Akashi M, Huang X. Eur Polym J 2006;42:1305–12. [7] Forrest S, Burrows P, Thompson M. IEEE Spectrum 2000;37:29–34. [8] Tang H, Huang X, Yang X, Yang J, Zhao R, Liu X. Mater Lett 2012;75:218–20. [9] Tang H, Ma Z, Zhong J, Yang J, Zhao R, Liu X. Colloids Surf A 2011;384:311–7. [10] Gao Y, Robertson GP, Guiver MD, Mikhailenko SD, Li X, Kaliaguine S. Macromolecules 2005;38:3237–45. [11] Saxena A, Rao VL, Ninan KN. Eur Polym J 2003;39:57–61. [12] Liu D, Wang Z. Polymer 2008;49:4960–7. [13] Liu D, Wang Z, Yu H, You J. Eur Polym J 2009;45:2260–8. [14] Tang H, Yang J, Zhong J, Zhao R, Liu X. Mater Lett 2011;65:1703–6. [15] Higuchi M, Imoda D, Hirao T. Macromolecules 1996;29:8277–9. [16] Li XF, Lau KT, An Y, Yin YS, Wong TT. Mater Lett 2008;62:4434–6. [17] Rosendo A, Flores M, Crdoba G, Rodrı´guez R, Arroyo R. Mater Lett 2003;57:2885–93. [18] Panigrahi S, Bera A, Basak D. J Colloid Interf Sci 2011;353:30–8.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Fluorescence-color-tunable and transparent polyarylene ether nitrile films with high thermal stability and mechanical strength based on polymeric rare-earth complexes for roll-up displays Hailong Tang, Zejun Pu, Junji Wei, Haoyu Guo, Xu Huang, Xiaobo Liu n Research Branch of Advanced Functional Materials, Institute of Microelectronics and Solid State Electronics, High Temperature Resistant Polymers and Composites Key Laboratory of Sichuan Province, University of Electronic Science and Technology of China, Chengdu 610054, PR China
a r t i c l e i n f o
abstract
Article history: Received 26 July 2012 Accepted 2 October 2012 Available online 10 October 2012
A novel series of transparent, flexible and fluorescent polymeric rare-earth complex films were successfully prepared by chemical coordination between carboxyl groups in polyarylene ether nitrile and rare-earth ions. The derived films showed high thermal stability with 5% weight loss temperature in the range of 402–420 1C, and also exhibited high mechanical strength ranging from 76 MPa to 83 MPa. Furthermore, all films demonstrated excellent macroscopic flexibility, since they can be easily curled in both natural and light-emitting states. Optical results indicated that the films possessed good optical transparency, and emitted intense fluorescence under ultraviolet excitation. More importantly, the fluorescence colors can be tuned between red and green by controlling the ratios of Eu(III)–Tb(III) ions in the films. & 2012 Elsevier B.V. All rights reserved.
Keywords: Fluorescence-color-tunable Polyarylene ether nitrile Rare-earth complex Mechanical properties Luminescence Thermal properties
1. Introduction Luminescent rare-earth (RE) complexes with organic ligands have many potential applications in light emitting diodes, fluorescent sensors, or laser materials due to their high fluorescence intensities [1–3]. However, organic low-molecular RE complexes have serious limitations in practical applications because of their poor mechanical properties and physicochemical stabilities. By contrast, polymeric high-molecular RE complexes not only possess unique fluorescence properties of the RE ions but also own good mechanical toughness, chemical stability and excellent processability [4–6]. Especially, the development of fluorescent films with robust flexibility, high transparency and excellent thermostability has become a hot research topic, for their good prospects of application in e-papers, roll-up displays and polymeric light-emitting devices [7]. As a new class of high-performance thermoplastic polymers, polyarylene ether nitriles possess high thermal stability, outstanding chemical inertia and excellent radiation resistance, and they have already been used as matrices in advanced composites in aerospace industries [8–11]. Recently, a series of new polyarylene ether nitriles containing pendant carboxyl groups were developed in our group, which possess a variety of favorable characteristics, such as high glass transition temperature (above
n
Corresponding author. Tel./fax: þ 86 28 83207326. E-mail address: [email protected] (X. Liu).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.10.010
180 1C), excellent mechanical strength and high transparency as well as good solubility in the common solvents and excellent film-forming property. Therefore, to develop high performance fluorescent films, polyarylene ether nitrile containing pendant carboxyl groups (PEN) was selected as the polymer matrix, and also as the macromolecular ligand to form PEN-RE complexes, by forming the chemical coordination bonds between RE ions and reactive carboxyl groups in PEN [12,13]. In this paper, we present the preparation and characterization of the PEN fluorescent films based on polymeric rare-earth complexes, and their thermal, mechanical, optical properties and flexibility were investigated in detail.
2. Experimental Polyarylene ether nitrile containing pendent carboxyl groups was synthesized from 2, 6-dichlorobenzonitrile with bisphenol A and phenolphthalin through nucleophilic aromatic substitution polymerization reaction, as described previously in detail [14]. In a typical procedure for preparing fluorescent films, a prescribed amount of EuCl3 6H2O and TbCl3 6H2O with different proportions were added in N, N-dimethylacetamide (DMAc) solvent in an ultrasonic water bath. Meanwhile, a certain amount of the carboxyl-functionalized PEN was dissolved in DMAc with heating and stirring. Then the rare-earth solution was slowly dropped into the PEN solution, and the mixture was refluxed at 165 1C for 2 h
236
H. Tang et al. / Materials Letters 91 (2013) 235–238
accompanying with high-speed mechanical stirring. Finally, the resultant solution was cast onto a clean-horizontal glass plate and dried in an oven using a sequential mode of temperature program at 80 1C, 100 1C, 120 1C, 140 1C and 160 1C (2 h each) to evaporate solvent completely. The fluorescent films were obtained by shedding naturally after cooling to room temperature, with the thickness of 35–45 mm. In the experiment, through conversion by calculating, the mass fraction of EuCl3 in the films (marked as PEN-E0T8, PENE2T6, PEN-E4T4, PEN-E6T2, PEN-E8T0) was fixed at 0%, 2%, 4%, 6%, 8% (Eu(III) wt%: 0%, 1.18%, 2.35%, 3.53%, 4.71%), and the corresponding mass fraction of TbCl3 was fixed at 8%, 6%, 4%, 2%, 0% (Tb(III) wt%: 4.79%, 3.59%, 2.40%, 1.20%, 0%), respectively. Thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTG) were performed on a TA Q50 series analyzer system under a nitrogen atmosphere at a heating rate of 20 1C min 1. Mechanical properties were measured using a SANS CMT6104 series desktop electromechanical universal testing machine at room temperature, and the data was reported as the average value for five samples. Ultraviolet-visible (UV–vis) absorption and optical transmittance spectra were recorded by a Shimadzu 3100 UV–vis-near–IR spectrophotometer. Fluorescence emission spectra were measured on a Hitachi F-7000 FL spectrophotometer under UV irradiation of 254 nm.
potential applications in the field of flexible electronics such as e-papers and roll-up displays. Optical properties: The optical transmittance and UV–vis absorption spectra of the fluorescent films are shown in Fig. 2a and b respectively, and the inset of Fig. 2a displays the fluorescent film (36 mm in thickness) on a labeled paper. As we can see, the symbol on the paper can be clearly observed, and the optical transmittance spectra of the films further show a high transmittance of 83%–90% at the wavelength of 500–800 nm for films of 35–45 mm in thickness. Moreover, the fluorescent films show a strong and broad absorption band in the range of 200–325 nm, which is attributed to the p-pn electronic transitions of aromatic rings [15], and there is almost no absorption in the visible region. Fig. 3a–e depicts the fluorescence emission spectra of the PEN fluorescent films. As shown in Fig. 3a, the major emission peaks for the PEN-E0T8 film appear at 489 nm, 545 nm, 586 nm and 622 nm, corresponding to 5D4-7F6, 5D4-7F5, 5D4-7F4 and 5 D4-7F3 transitions of Tb(III) ions, respectively [13]. In addition, the emission peak at 545 nm is over four times higher than other
3. Results and discussion Thermal properties: The thermal stability of the PEN fluorescent films was characterized by TGA and DTG in nitrogen atmosphere, and evaluated by 5% weight loss temperature (T5%) and maximum decomposition rate temperature (Tmax). As summarized in Table 1, all the fluorescent films exhibited high thermal stability, with T5% in the range of 402–420 1C and Tmax over 514 1C. This is mainly due to the rigid conjugated aromatic structures of PEN matrix. In addition, the glass transition temperature of the films is high up to 192 1C, and it provides a good guarantee for their applications under high-temperature conditions. Mechanical properties and flexibility: The mechanical properties of the fluorescent films, such as tensile strength, breaking elongation and Young’s modulus, are also listed in Table 1. As can be seen, all the fluorescent films have a high tensile strength over 76 MPa, and the maximum is up to 83.9 MPa. Moreover, the fluorescent films show an elongation at break in the range of 3.9–4.5%, and a high Young’s modulus more than 2.6 GPa. This is mainly attributed to the high performance of PEN itself and its chemical coordination with the rare-earth ions, which brings the uniform dispersion and good compatibility. More importantly, all PEN fluorescent films demonstrate an excellent macroscopic flexibility. Fig. 1a–c shows the digital images of the fluorescent films in natural light and under UV light irradiation of 254 nm, respectively. As we can see, the fluorescent films can be easily bent and curled whether in the natural state or in the light-emitting state. This reveals that the films may have
Fig. 1. (a) Digital image of the PEN-E2T6 film to demonstrate the optical transparency and macroscopic flexibility. Photoluminescence images of (b) the PEN-E0T8 film (Green-light-emission) and (c) the PEN-E8T0 film (Red-lightemission) under UV light irradiation of 254 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Thermal and mechanical properties of the fluorescent films.
Film code
PEN-E0T8 PEN-E2T6 PEN-E4T4 PEN-E6T2 PEN-E8T0
Thermal properties
Mechanical properties
T5% (1C)
Tmax (1C)
Tensile strength (MPa)
Breaking elongation (%)
Young’s modulus (GPa)
403 420 414 406 402
516 516 517 514 514
77.1 81.5 83.9 76.2 82.3
3.9 4.2 3.9 3.9 4.5
2.7 3.0 2.9 2.6 2.7
H. Tang et al. / Materials Letters 91 (2013) 235–238
237
Fig. 2. (a) Optical transmittance spectra (Inset: Transparent film on a labeled paper) and (b) UV–vis absorption spectra of the PEN fluorescent films.
Fig. 3. Fluorescence emission spectra of the PEN films: (a) PEN-E0T8, (b) PEN-E2T6, (c) PEN-E4T4, (d) PEN-E6T2, (e) PEN-E8T0 and (f) CIE 1931 x, y chromaticity diagram for the PEN films: (I) PEN-E0T8, (II) PEN-E2T6, (III) PEN-E4T4, (IV) PEN-E6T2, (V) PEN-E8T0.
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H. Tang et al. / Materials Letters 91 (2013) 235–238
peaks in intensity and plays a dominant role, so the PEN-E0T8 film displays a characteristic green-light-emission (Fig. 1b). Similarly, the major emission peaks for the PEN-E8T0 film are observed at 575 nm, 593 nm and 617 nm (Fig. 3e), which are assigned to the 5D0-7F0, 5 D0-7F1 and 5D0-7F2 transitions of Eu(III) ions, respectively [16]. The strongest emission peak at 617 nm plays a dominant role so that the PEN-E8T0 film emits characteristic red light under UV irradiation (Fig. 1c). As shown in Fig. 3b–d, the films of PEN-E2T6, PEN-E4T4 and PEN-E6T2 show both the characteristic emission peaks of Tb(III) and Eu(III) ions, and there has been no energy transfer phenomenon from the 5D4 level of Tb(III) ions to the 5D0 level of Eu(III) ions reported in the literature [17]. This may be attributed to the coordination of rare-earth ions with carboxyl groups, which makes a large separation distance between the rare-earth ions, and there will not form cluster structures. Furthermore, due to the different ratios of Eu(III) to Tb(III) ions in the films, their corresponding emission peaks have a different relative intensity. According to the RGB (red-green-blue) color model, all the intermediate colors between red and green can be obtained through controlling the intensity of red and green, such as yellow, orange, orange-red etc. Therefore, the fluorescence colors of the films can be tuned between red and green by controlling the ratios of Eu(III) to Tb(III) ions. In order to exhibit the trace of color variation, the CIE (Commission Internationale de L’Eclairage) color coordinates for the PEN fluorescent films are calculated based on their emission spectra [18], and the chromaticity diagram deduced as per CIE 1931 x, y coordinates is shown in Fig. 3f. Clearly, the CIE coordinates for the fluorescence of PEN-E0T8, PEN-E2T6, PEN-E4T4, PEN-E6T2 and PEN-E8T0 films indicate green, chartreuse, yellow, orange and red emissions, respectively.
4. Conclusion In summary, a series of PEN fluorescent films with good optical transparency and excellent macroscopic flexibility were prepared by coordination of PEN with rare-earth ions. The results showed that the films had a high thermal stability with T5% over 402 1C, and also possessed a high tensile strength exceeding 76 MPa and a high Young’s modulus greater than 2.6 GPa. Furthermore, the films could emit intense fluorescence under UV excitation.
Meanwhile, they can be easily curled whether in the natural state or in the light-emitting state. More importantly, the fluorescence colors can be tuned between red and green by controlling the ratios of Eu(III) to Tb(III) ions in the films. These characteristics are of great significance for their potential applications in the field of flexible electronics such as e-papers and roll-up displays.
Acknowledgments The authors gratefully acknowledge support from the National Natural Science Foundation of China (No. 51173021) and ‘‘863‘‘ National Major Program of High Technology of China (2012AA03A212).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2012. 10.010.
References [1] Guerriero P, Tamburini S, Vigato PA. Coord Chem Rev 1995;139:17–243. [2] Campos RA, Kovalev IP, Guo Y, Wakili N, Skotheim T. J Appl Phys 1996;80:7144–50. [3] Gao XC, Cao H, Huang C, Li B, Umitani S. Appl Phys Lett 1998;72:2217–9. [4] Lee CI, Lim JS, Kim SH, Suh DH. Polymer 2006;47:5253–8. [5] Wang D, Zhang J, Lin Q, Fu L, Zhang H, Yang B. J Mater Chem 2003;13: 2279–84. [6] Yin W, Chen M, Lu T, Akashi M, Huang X. Eur Polym J 2006;42:1305–12. [7] Forrest S, Burrows P, Thompson M. IEEE Spectrum 2000;37:29–34. [8] Tang H, Huang X, Yang X, Yang J, Zhao R, Liu X. Mater Lett 2012;75:218–20. [9] Tang H, Ma Z, Zhong J, Yang J, Zhao R, Liu X. Colloids Surf A 2011;384:311–7. [10] Gao Y, Robertson GP, Guiver MD, Mikhailenko SD, Li X, Kaliaguine S. Macromolecules 2005;38:3237–45. [11] Saxena A, Rao VL, Ninan KN. Eur Polym J 2003;39:57–61. [12] Liu D, Wang Z. Polymer 2008;49:4960–7. [13] Liu D, Wang Z, Yu H, You J. Eur Polym J 2009;45:2260–8. [14] Tang H, Yang J, Zhong J, Zhao R, Liu X. Mater Lett 2011;65:1703–6. [15] Higuchi M, Imoda D, Hirao T. Macromolecules 1996;29:8277–9. [16] Li XF, Lau KT, An Y, Yin YS, Wong TT. Mater Lett 2008;62:4434–6. [17] Rosendo A, Flores M, Crdoba G, Rodrı´guez R, Arroyo R. Mater Lett 2003;57:2885–93. [18] Panigrahi S, Bera A, Basak D. J Colloid Interf Sci 2011;353:30–8.