Open Access
Add to BibSonomyAbstract
The multi-color switching of electrofluorochromism was examined from the thin electrofluorochromic (EF) films of polyfluorene (PFO) and poly(propylenedioxythiophene–phenylene) (P(ProDOT-Ph)). The PFO and P(ProDOT-Ph) films showed vivid blue and yellow fluorescence, respectively, at neutral state but their emission was quenched upon application of oxidation potential leading to ion radical states. The fluorescence from the polymer films was reversibly switched to vivid color when the films were returned to their neutral states. The EF color switching ratio (rc1/c2) between the fluorescent state (c1) and dark (c2) state for the yellow color EF device with P(ProDOT-Ph) film was about four times higher than that of the PFO. Because the two polymer films have different colors and working potentials, a multi-color switching device was fabricated by coating the P(ProDOT-Ph) and PFO films onto working and counter electrodes, respectively. The multi-color EF device showed fluorescence switching from blue (B) to white (W) to yellow (Y), and vice versa, depending on the applied potential. The rc1/c2 for yellow (c1) and blue (c2) switching (Y/B) was larger (9.71) than those for Y/W and W/B. Moreover, the EF switching for Y/B in the multi-EF device was also very effective and showed the largest EF efficiency (EEF = 3.82 × 106) among the EF color switching.
© 2016 Optical Society of America
1. Introduction
Electrofluorochromic (EF) materials are fluorescent materials whose fluorescence can be modulated by the reversible electrochemical redox process. Because EF materials can convert electrical signals to visual signals, EF materials are receiving more and more interest in the opto-electronic area [1–12]. Since the tetrazine molecules showed reversible fluorescence switching visually in a device for the first time, several type of EF materials have been developed to show high EF switching in solution as well as in a thin film state. Furthermore, the electrically driven fluorescence modulation has been optimized in single fluorophores to explore their optical properties, such as wavelength, switching contrast, and stability. In addition, the EF switching has been observed not only in the visible but also in the NIR range. Thus, an EF device based on NIR emissive organic compounds [13] has shown EF switching in the NIR range [14].
Although the electrofluorochromic materials show reversible switching in their fluorescence intensity by applying a redox potential, multi-color switching of fluorescence could be challenging for wide applications, such as sensors and displays [15, 16]. To obtain multi-color switching, more than two states of reversible electro-conversion are required, and electrochemical conversions for each color should be precisely controlled in the same device. This should be possible in principle by using dyad-type chromophores and/or mixtures of fluorophores with different optical and redox properties. When an EF device is fabricated with a multi-component fluorophore mixture, it may provide not only the color switching for each fluorophore at their redox potential but also new colors by the combination of the colors of each of the fluorophores. Recently, our group reported multi-color switching from a mixture of naphthalimide-tetrazine dyad and (2-hydroxyethyl)-N-naphthalimide dissolved in liquid electrolyte medium [14]. Nonetheless, multi-color switching electrofluorochromism in a thin film state has only been rarely reported [17, 18]. To achieve this, electrochemically active fluorescent polymers are desirable because of their excellent stability against thermal and electrical stress, as well as the tunability of their color and electronic properties [19–21]. Moreover, these EF polymers can be solution processed to form thin films, which should be advantageous for the fabrication of EF devices.
In this paper, we report multi-color switching electrofluorochromism with high EF efficiency in a thin film state using yellow [fluorescent propylenedioxythiophene–phenylene; P(ProDOT-Ph)] and blue [poly(9,9-di-n-octylfluorenyl-2,7-diyl); PFO] emissive polymers. Since the fluorescence of each polymer film is quenched at different potentials, the emission color from the device could be tuned by varying the applied potentials. In addition, the EF device made using the PFO and P(ProDOT-Ph) films may show new colors owing to the combination of two colors when the polymer films are under neutral state.
2. Experimental part
2.1 Materials and preparation of EF devices
The blue fluorescent PFO was purchased from Sigma-Aldrich, and the yellow fluorescent P(ProDOT-Ph) was synthesized according to the reported method [22]. Bis(trifluoromethane)sulfonimide lithium (LiBTI) and propylene carbonate (PC) were purchased from Sigma-Aldrich and the solution of LiBTI dissolved in PC was used as an electrolyte layer. The EF device consisted of an electrolyte layer that was sandwiched between two indium tin oxide (ITO) electrodes. Onto the pre-cleaned ITO glass, EF polymers (5 mg/ml of P(ProDOT-Ph) or 1 mg/ml of PFO in chloroform) were spin-coated to form thin films, and then annealed at 100 °C for 30 min. A 120 μm thick polyimide tape was attached as a square-shape spacer on the ITO electrode. The EF device was prepared by assembling the two ITO electrodes and the electrolyte (0.1 M LiBTI in PC) was carefully injected within the spacer. Finally, the EF devices were completely sealed with an epoxy resin.
2.2 Measurements
Fluorescence spectra of polymer thin films and EF devices were obtained by fluorescence spectrometer (Model LS55, PerkinElmer, Waltham Massachusetts, USA). The electrochemical properties of the polymer thin films and EF devices were characterized using a universal potentiostat (Model CHI 624, CH Instruments Inc. Bee Cave, TX, USA) equipped with Iviumsoft, Ivium Tec. Eindhoven, Nederland. When recording the fluorescence under the application of an external voltage, the in situ fluorescence of the EF switching devices was characterized using a fluorescence spectrometer and a chronocoulometer through the potentiostat of the CHI 624B. Cyclic voltammetry (CV) was performed with the EF polymer-coated ITO glass, Ag/AgCl, and a stainless steel plate as the working, reference, and counter electrodes, respectively. The CVs were obtained after 5 min of nitrogen purging and then 1 min for stabilization. The photographic images were obtained with a digital camera (Canon 450d, sigma 30 mm f1.4). A UV lamp (365 nm; 60 μW cm–2, Spectronics ENF-260C/J) was used as the excitation source.
3. Results and discussion
The chemical structures and electrochemical conversion of EF polymers are presented in Fig. 1. Unlike low-molecular-weight fluorescent molecules, the polymers could be coated as a thin film on the ITO electrode by simple spin-coating method. PFO and P(ProDOT-Ph) are known as blue (λmaxfl = 428 nm) and yellow (λmaxfl = 545 nm) fluorescent polymers, respectively, in the thin film state.
Fig. 1 Chemical structures of (a) PFO and (b) P(ProDOT-Ph) and their electrochemical conversions. R = 2-ethylhexyl.
The structure of the EF devices and a schematic diagram for the characterization of EF switching are shown in Fig. 2(a) and 2(b), respectively. In the two-electrode system, the working electrode was coated by one of the EF polymers to prepare a mono-color EF device of PFO (Fig. 2(a), blue-EF) and P(ProDOT-Ph) (Fig. 2(a), yellow-EF). The electrolyte solution containing LiBTI was sandwiched between the working and counter electrode with a gap of 120 μm and the cell was sealed tightly. In the three-electrode system, a reference electrode (Ag wire) was inserted into the electrolyte layer, keeping the gap between the two ITO electrodes. The characterization for EF switching in Fig. 2(b) consists of CV, excitation light (365 nm), and a detector (fluorescence spectroscopy).
Fig. 2 (a) Structures of the mono- and multi-color electrofluorochromic devices containing working and counter electrodes, an electrolyte, and the polymer layers (P(ProDOT-Ph) and PFO). The polymers were spin-coated onto the working or counter electrodes. (b) Schematic diagram for the characterization of the electrofluorochromic switching with the light source (λexc = 365 nm), EF device, potentiostat, and fluorescence spectrometry.
For the multi-color EF device (Fig. 2(a), multi-EF), both electrodes were coated by one of the EF polymers. The electrolyte solution and space between the electrodes were the same as the mono-color EF device. The excitation wavelength for emission was chosen as 365 nm based on the absorption spectra of the polymers because 365 nm is in between the blue and yellow emission. In this way, the excitation system for the multi-color EF device could be simplified as it is possible to excite the two polymeric layers with a single source.
The electrochemical and fluorescent properties of PFO and P(ProDOT-Ph) were examined before preparing the multi-color EF device. Because the EF switching is based on the electrochemical conversion, the working potential for the switching of each component should be determined to optimize the multi-color condition in the multi EF device. The two polymers showed quasi-reversible redox processes (Fig. 3) in the cyclic voltammograms. The peak potential for oxidation in the three-electrode system (Ep,3ed), was observed at 0.97 V and 1.3 V for P(ProDOT-Ph) and PFO, respectively, and a reduction process to return to the neutral state was observed at 0.93 V and 1.2 V, respectively (Fig. 3(a)). These redox properties for the two polymers were well matched to the previous reports [10, 23]. In the two-electrode system, the redox peaks were shifted toward a larger value owing to the absence of a reference electrode. Nonetheless, the peak potential for oxidation in the two-electrode system (Ep,2ed) for PFO was higher than that for P(ProDOT-Ph), similar to the three-electrode system. The difference in the onset potential for oxidation in the two-electrode system (Eon,2ed) between the two polymer films was 0.6 V, which should be a large potential window for the control of EF switching in a multi-color device, in which both polymers are subjected to electrochemical reactions.
Fig. 3 (a) Cyclic voltammograms of PFO and P(ProDOT-Ph) coated on ITO electrode in PC containing 0.1 M LiBTI recorded at a scan rate of 20 mV s−1 with the polymer coated ITO glass, Ag/AgCl, and a stainless steel plate as the working, reference, and counter electrodes, respectively. (b) Cyclic voltammograms for the two-electrode system with two ITO electrodes of the same polymer films in (a). Eon and Ep represent the onset and peak potential for oxidation, respectively.
Upon application of oxidation potentials, the blue and yellow fluorescence from PFO and P(ProDOT-Ph), respectively, were quenched. Figure 4 shows the fluorescence spectral change for the mono-color switching EF device according to the applied potential in the two-electrode device. As the applied potential was increased, the fluorescence intensity was decreased and completely quenched to dark when an applied potential was reached to the peak potential for oxidation of polymers Ep (ox). The fluorescence quenching occurred reversibly without spectral shift for both polymers in both the mono- and multi-color EF devices.
Fig. 4 Fluorescence change of PFO (a) and P(ProDOT-Ph) (b) in the two-electrode system at different applied potentials. The inset photographic images were obtained under UV excitation at 0 V and 2.0 V for PFO and at 0 V and 1.5 V for P(ProDOT-Ph).
Interestingly, the EF color switching ratio (rc1/c2) between the fluorescent state (c1) and the dark state (c2) for the polymer EF device with (P(ProDOT-Ph) or PFO) film was larger than that of the small-molecule-based EF device, as shown in Table. 1. The peak intensity was used to calculate the value of rc1/c2 in the EF switching test in Fig. 7(a) for Y = yellow, B = blue, and W = white. Because the rc1/c2 is determined by dividing the fluorescence intensity at ON state by that at OFF state in the mono-color EF device, complete fluorescence quenching is essential to obtain a high rc1/c2 value. The rc1/c2 of PFO and P(ProDOT-Ph) was determined as 3.3 and 13.5, respectively, whereas that of the small molecules was less than 3 [13, 24, 25]. With the small molecules, the EF materials are dissolved in an electrolyte solution owing to the difficulty in the film fabrication with the small molecules. Thus, the electrochemical quenching for the small molecules in electrolyte layer may be incomplete and thus the EF device is not completely dark. On the other hand, the EF polymer films can possess intrinsically high rc1/c2 because of a very effective electrochemical reaction and little background fluorescence in a thin film. As shown in Fig. 4, the EF devices in the quenched state are dark, indicating the complete quenching of fluorescence in both polymer films by the oxidation reaction. This advantage of polymer films resulted in the high rc1/c2 (Table 1). It was noteworthy that the rc1/c2 value for the P(ProDOT-Ph) film was about four times higher than that of the PFO film, possibly owing to the lower oxidation potential of P(ProDOT-Ph) than the latter.
Table 1. The optical and electrochemical properties of multi-color electrofluorochromic device
In the multi-color EF device, fluorescence switching from blue (B) to white (W) to yellow (Y), and vice versa, was observed depending on the applied potential. As shown in Fig. 5(a), only yellow fluorescence was observed (Y-ON) at −2.0 V. At 0 V, white emission was observed (W-ON). The white emission faded away and yellow emission reappeared when a negative potential was applied (−2.0 V). On the other hand, the white emission changed to bluish color upon application of a positive potential. With the + 1.5 V potential, only blue emission was observed (B-ON) from the device.
Fig. 5 (a) The fluorescence changes in the multi-color EF device at different applied potentials ranging from 1.5 V (blue) to −2.0 V (yellow) with a 0.5 V decrease at each step in the two-electrode system. (b) The reversible color coordinates plot in chromaticity diagram corresponding for the multi-color EF device with different applied potentials.
The mechanism for the multi-color fluorescence switching is summarized in Fig. 6 based on the redox reaction of the polymers. As shown in Fig. 3(b), the oxidation reaction of PFO began at 1.8 V in the two-ITO electrode system. At −2.0 V, the PFO film at the counter (back) electrode is in an oxidized state. The blue contribution from the PFO disappeared at negative potentials. Thus, only yellow emission from the P(ProDOT-Ph) film on the working (front) electrode remained in the negative potential range (−2.0 V, Y-ON). Within this potential range, the electrochemical stability of P(ProDOT-Ph) was confirmed by the two-electrode CVs in Fig. 3(b). At 0 V, both P(ProDOT-Ph) and PFO films in the working and counter electrodes, respectively, are in their neutral state, thus, both blue and yellow emission are in the ON state. As expected from the color mixture of yellow and blue fluorescence, the emission was brighter than the mono-color EF device and the emission color was close to white (W-ON). When a positive potential ( + 1.5 V) was applied to the multi-color EF device, the P(ProDOT-Ph) undergoes an oxidation reaction at the working (front) electrode to turn the yellow emission off, while the PFO film at the counter electrode is in a neutral state (B-ON).
Fig. 6 The fluorescent emission mechanism of the multi-color EF device driven by the different applied potentials. The fluorescence images were captured with a visible camera in the two-electrode system.
As shown in Fig. 5(b), the color codes for the emission color change between blue and yellow were (0.22, 0.21) and (0.37, 0.43), respectively. As described above, the rc1/c2 of the polymeric EF device is higher than that of the small molecules. This advantage of polymer films resulted in the large color code changes in the multi-color EF device.
The step potentials for color switching among yellow (Y), blue (B), and white (W) were determined by CV. To optimize the stability and EF contrast in the EF device, we controlled the step potentials to match the color switching as −1.65 V/1.3 V, −1.65 V/0.5 V, and −0.5 V/1.3 V for Y/B, Y/W, and W/B switching, respectively. For the color switching for W/B and Y/W, an opposite over potential was applied (−0.5 V and + 0.5 V, respectively) to rapidly compensate the evolved positive or negative charge. Otherwise, the cyclability test of the multi-color EF device slowly decreased with the repeated switching cycle owing to the electrochemical decomposition of polymers. The unavoidable electrochemical oxidation of polymer occurred on the working electrode with the applied positive potentials. Therefore, we tried to minimize its decomposition with little loss of EF contrast ratio by applying low step potentials, as shown in Fig. 7(b). The rc1/c2 for yellow (c1) and blue (c2) switching (Y/B) was larger (9.71) than those for Y/W and W/B (Table 1).
Fig. 7 (a) Fluorescence switching responses of the multi-color EF device monitored at 545 nm under different applied potentials with a 10-s step duration time in the two-electrode system. (b) The cyclability test of the multi-color EF device monitored at 428 nm (black line) and 545 nm (red line) with a 10-s step duration time under switching potentials of Y/B. It was measured over 2000 s.
The EF switching was measured at 428 nm and 545 nm for those color switchings under the optimized step potential. As shown in Fig. 7(a), the Y/B switching intensity is the same as the sum of the W/B and Y/W contrast. Therefore, the EF switching for each color emission could be complemented by the emission of the other polymer. The electrochemical fluorescence switching efficiency (EEF, cm2/C) was determined from rc1/c2 and the consumed (injected/ejected) charge per unit area (C):
The rc1/c2 was determined from Fig. 4 and 5, and the consumed charge was from the chronocoulometric data for the EF switching. As described above, the rc1/c2 of P(ProDOT-Ph) was four times larger than that of PFO. Therefore, the EEF for yellow color switching was higher than that for the blue in the multi-color EF device. When monitored at a yellow color region (545 nm), the EEF for color switching was the highest for the multi-color EF device. The EF switching for Y/B in the multi-color EF device was very effective and showed the largest EF efficiency among the fluorescence color switching.
4. Conclusion
The electroactive and fluorescent PFO and P(ProDOT-Ph) were used to achieve multi-color fluorescence switching by coating them on a counter and working electrode, respectively. At −2.0 V, the PFO film at the counter (back) electrode is in an oxidized state. The blue contribution from the PFO disappeared at −2.0 V and only the yellow emission from the P(ProDOT-Ph) film on the working electrode remained in the negative potential range. At 0 V, both polymers were in their neutral state. Thus, their emissions were in the ON state and the color became close to white. When a positive potential ( + 1.5 V) was applied to the device, the PFO film at the counter electrode was in the neutral state while the P(ProDOT-Ph) undergo oxidation reaction with quenched fluorescence. Thus, the multi-color EF device showed B, Y, and W fluorescence depending on the applied potential. Moreover, the device showed large Y/B color switching along with high EEF (EEF = 3.82 × 106) among the fluorescence color switching.
Acknowledgments
This work was supported by the National Research Foundation (NRF) grant funded by the Korean government (Ministry of Science, ICT & Future Planning, MSIP) (2015M3D1A1069197 and 2015H1D3(a)1066519). This research was supported by a grant of the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), funded by the Ministry of Health & Welfare, Republic of Korea (grant number: HI15C0942). This work was supported (in part) by the Yonsei University Research Fund (Post Doc. Researcher Supporting Program) of 2015 (project no.: 2015-12-0003).
References and links
1. Y. Kim, E. Kim, G. Clavier, and P. Audebert, “New tetrazine-based fluoroelectrochromic window; modulation of the fluorescence through applied potential,” Chem. Commun. (Camb.) 34, 3612–3614 (2006). [CrossRef] [PubMed]
2. Y. Kim, Y. Kim, S. Kim, and E. Kim, “Electrochromic diffraction from nanopatterned poly(3-hexylthiophene),” ACS Nano 4(9), 5277–5284 (2010). [CrossRef] [PubMed]
3. S. Seo, Y. Kim, Q. Zhou, G. Clavier, P. Audebert, and E. Kim, “White electrofluorescence switching from electrochemically convertible yellow fluorescent dyad,” Adv. Funct. Mater. 22(17), 3556–3561 (2012). [CrossRef]
4. S. Seo, H. Shin, C. Park, H. Lim, and E. Kim, “Electrofluorescence switching of fluorescent polymer film,” Macromol. Res. 21(3), 284–289 (2013). [CrossRef]
5. G. Clavier and P. Audebert, “s-Tetrazines as building blocks for new functional molecules and molecular materials,” Chem. Rev. 110(6), 3299–3314 (2010). [CrossRef] [PubMed]
6. A. P. de Silva, H. Q. Gunaratne, T. Gunnlaugsson, A. J. Huxley, C. P. McCoy, J. T. Rademacher, and T. E. Rice, “Signaling recognition events with fluorescent sensors and switches,” Chem. Rev. 97(5), 1515–1566 (1997). [CrossRef] [PubMed]
7. G. Ding, H. Zhou, J. Xu, and X. Lu, “Electrofluorochromic detection of cyanide anions using a benzothiadiazole-containing conjugated copolymer,” Chem. Commun. (Camb.) 50(6), 655–657 (2014). [CrossRef] [PubMed]
8. M. Voicescu, D. Rother, F. Bardischewsky, C. G. Friedrich, and P. Hellwig, “A combined fluorescence spectroscopic and electrochemical approach for the study of thioredoxins,” Biochemistry 50(1), 17–24 (2011). [CrossRef] [PubMed]
9. J. H. Wu and G. S. Liou, “Fluorescence: High‐Performance Electrofluorochromic Devices Based on Electrochromism and Photoluminescence‐Active Novel Poly (4‐Cyanotriphenylamine),” Adv. Funct. Mater. 24(41), 6406 (2014). [CrossRef]
10. C. Xia and R. C. Advincula, “Decreased Aggregation Phenomena in Polyfluorenes by Introducing Carbazole Copolymer Units,” Macromolecules 34(17), 5854–5859 (2001). [CrossRef]
11. J. Yoo, T. Kwon, B. D. Sarwade, Y. Kim, and E. Kim, “Multistate fluorescence switching of s-triazine-bridged p-phenylene vinylene polymers,” Appl. Phys. Lett. 91(24), 241107 (2007). [CrossRef]
12. C. Yun, J. You, J. Kim, J. Huh, and E. Kim, “Photochromic fluorescence switching from diarylethenes and its applications,” J. Photochem. Photobiol. Photochem. Rev. 10(3), 111–129 (2009). [CrossRef]
13. S. Seo, S. Pascal, C. Park, K. Shin, X. Yang, O. Maury, B. D. Sarwade, C. Andraud, and E. Kim, “NIR electrochemical fluorescence switching from polymethine dyes,” Chem. Sci. (Camb.) 5(4), 1538–1544 (2014). [CrossRef]
14. S. Seo, C. Allain, J. Na, S. Kim, X. Yang, C. Park, J. Malinge, P. Audebert, and E. Kim, “Electrofluorescence switching of tetrazine-modified TiO2 nanoparticles,” Nanoscale 5(16), 7321–7327 (2013). [CrossRef] [PubMed]
15. T. Bhuvana, B. Kim, X. Yang, H. Shin, and E. Kim, “Reversible Full-Color Generation with Patterned Yellow Electrochromic Polymers,” Angew. Chem. Int. Ed. Engl. 52(4), 1180–1184 (2013). [CrossRef] [PubMed]
16. S. Seo, C. Park, X. Yang, J. You, Y. Kim, and E. Kim, “Reversible multi-color electrofluorescence switching,” in SPIE OPTO, (International Society for Optics and Photonics, 2012), 82580N.
17. Y. C. Kung and S. H. Hsiao, “Pyrenylamine‐functionalized aromatic polyamides as efficient blue‐emitters and multicolored electrochromic materials,” J. Polym. Sci. A Polym. Chem. 49(16), 3475–3490 (2011). [CrossRef]
18. C.-P. Kuo, C.-L. Chang, C.-W. Hu, C.-N. Chuang, K.-C. Ho, and M. K. Leung, “Tunable electrofluorochromic device from electrochemically controlled complementary fluorescent conjugated polymer films,” ACS Appl. Mater. Interfaces 6(20), 17402–17409 (2014). [CrossRef] [PubMed]
19. M. R. Craig, M. M. de Kok, J. W. Hofstraat, A. P. H. J. Schenning, and E. W. Meijer, “Improving color purity and stability in a blue emitting polyfluorene by monomer purification,” J. Mater. Chem. 13(12), 2861–2862 (2003). [CrossRef]
20. A. P. Kulkarni, Y. Zhu, and S. A. Jenekhe, “Quinoxaline-containing polyfluorenes: synthesis, photophysics, and stable blue electroluminescence,” Macromolecules 38(5), 1553–1563 (2005). [CrossRef]
21. J.-I. Lee, G. Klaerner, and R. D. Miller, “Oxidative stability and its effect on the photoluminescence of poly (fluorene) derivatives: end group effects,” Chem. Mater. 11(4), 1083–1088 (1999). [CrossRef]
22. T. Bhuvana, B. Kim, X. Yang, H. Shin, and E. Kim, “Reversible Full-Color Generation with Patterned Yellow Electrochromic Polymers,” Angew. Chem. Int. Ed. Engl. 52(4), 1180–1184 (2013). [CrossRef] [PubMed]
23. T. Bhuvana, B. Kim, X. Yang, H. Shin, and E. Kim, “Electroactive subwavelength gratings (ESWGs) from conjugated polymers for color and intensity modulation,” Nanoscale 4(12), 3679–3686 (2012). [CrossRef] [PubMed]
24. O. Galangau, I. Fabre-Francke, S. Munteanu, C. Dumas-Verdes, G. Clavier, R. Méallet-Renault, R. Pansu, F. Hartl, and F. Miomandre, “Electrochromic and electrofluorochromic properties of a new boron dipyrromethene–ferrocene conjugate,” Electrochim. Acta 87, 809–815 (2013). [CrossRef]
25. C. Quinton, V. Alain-Rizzo, C. Dumas-Verdes, F. Miomandre, G. Clavier, and P. Audebert, “Redox-controlled fluorescence modulation (electrofluorochromism) in triphenylamine derivatives,” RSC Advances 4(65), 34332–34342 (2014). [CrossRef]
