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AIEE-active and Electrochromic Bifunctional Polymer and Device thereof Synchronously Achieving Electrochemical Fluorescence Switching and Electrochromic Switching Sai Mi, Jingchuan Wu, Jian Liu, Zhangping Xu, Xingming Wu, Gui Luo, Jianming Zheng, and Chunye Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09717 • Publication Date (Web): 19 Nov 2015 Downloaded from http://pubs.acs.org on December 1, 2015

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ACS Applied Materials & Interfaces

AIEE-active and Electrochromic Bifunctional Polymer and Device thereof Synchronously Achieving Electrochemical Fluorescence Switching and Electrochromic Switching Sai Mi, Jingchuan Wu, Jian Liu, Zhangping Xu, Xingming Wu, Gui Luo, Jianming Zheng and Chunye Xu* Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, P.R. China

KEYWORDS: polymer, fluorescence, electrochromic, aggregation-induced emission, thiophene, tetraphenylethene

ABSTRACT: A novel alternating polymer ProDOT-TPE with aggregation-enhanced fluorescent emission effect and electrochromic property based on thiophene and tetraphenylethene derivatives was designed, synthesized, and characterized. The polymer displays weak photoluminescence in tetrahydrofuran, but the corresponding film prepared by spray-coating exhibits yellow-green fluorescent light at 540 nm. The color of polymer film could be switched

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from bright yellow to navy blue by relatively low voltage. An electrochromic device (ECD) of the polymer was fabricated, being different with common ECDs, both of its color and fluorescent states could be switched by voltage synchronously, making the polymer a unique candidate for electrochemical fluorescence and electrochromic applications.

INTRODUCTION Organic luminophores have important applications in various areas, such as light-emitting diodes1-2, plastic lasers3-4, and fluorescent chemosensors and bioprobes5-6. The phenomenon that luminophore’s fluorescence decreases with increasing concentration in solution was found by Förster et al. about half a century ago, and then scientists realized that it was a general phenomenon

for

many

aromatic

compounds.

In

most

practical

applications,

this

aggregation-caused quenching (ACQ) effect has limited the scope of technological applications of many luminophoric molecules7, as the luminophoric molecules have to be used in the states of aggregation, such as a film or a rod state. Methods such as attaching block groups on aromatic rings, surfactant encapsulation and blending with other materials8 have been tried by many researchers to impede aggregate formation, but those were just passive ways to passivate the contradiction between fluorescence intensity and application in aggregation state, and ended up with limited success and consequent side effects such as weakened luminophore density and bad impact on stability9. Developing a system in which aggregation promotes rather than impedes the light-emitting processes of luminogens is crucial to solve above-mentioned problem. In 2001, Tang and co-workers found out that, hexaphenylsilole molecule with extensive conjugated structure was virtually non-luminescent when dissolved in good solvents, but became highly emissive when aggregated in poor solvents or fabricated into thin solid films. They coined


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the term ‘‘aggregation-induced emission’’ (AIE) for this novel phenomenon, because the non-luminescent silole solution were induced to emit by aggregate formation10-11. Being opposite to the ACQ effect, AIE effect has great academic and practical value. Organic luminophores with the AIE effect could play efficient roles in various fields without intentional work against aggregate formation or consequent side effects. Many organic fluorogens have been found to possess the AIE effect12-14, wherein, tetraphenylethene (TPE)15 is a typical molecule with simple molecular structure but a splendid AIE effect. Its propeller-like conformation, intersection angle between four phenyl rings and central alkene plane, and intermolecular hydrogen bonds, make TPE highly emissive in the aggregation state. Moreover, it is facile to modify or introduce to other molecular system to create new AIE molecules. Most of the AIE systems developed so far are small molecules. Troublesome techniques, such as vacuum sublimation and vapor deposition, have to be used to fabricate those molecules into solid films for practical applications. One method to solve this problem is to prepare polymers with AIE effect. AIE-active polymers with good solubility in conventional low-boiling point solvents are better choices, as they can be directly fabricated into large-area thin solid films and devices by simple methods such as spray-coating or spin-coating. Electrochromism (EC) is known as a reversible change of color or transmittance resulting from oxidation or reduction of materials by electrochemical means. In recent years, a large number of EC-based applications have emerged such as controllable light-reflective or light-transmissive devices for automatic anti-glazing mirrors16, sunglasses17, switchable displays18, and smart windows19. Transition-metal oxides (WO3, IrO2), inorganic coordination



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complexes (Prussian blue), small organic molecules (viologens and phthalocyanines) and conjugated polymers are widely investigated as EC material candidates. Among them, conjugated electrochromic polymers exhibit unique advantages such as facile synthetic procedures20, tunable colors21, high coloration efficiencies22, fast switching capabilities18 and high optical contrast ratios23. Our group has been committing ourselves to the research and development of electrochromic field in the past years24-26. As far as we know, polymers that can achieve EC switching and fluorescence switching synchronously in film state are rarely reported before. In order to make this bi-function come true, the polymer should be AIE-active and EC-active simultaneously. In this article, a novel bifunctional polymer (ProDOT-TPE) with AIE effect and EC property is designed and synthesized. The polymer is an alternating polymer bearing TPE and 3,3-bis((2-ethylhexyloxy)methyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine, wherein, TPE is chosen to endow ProDOT-TPE with AIE effect. The thiophene derivative with long

alkoxy

3,3-bis((2-ethylhexyloxy)methyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine

chains, could

bring

ProDOT-TPE with EC property and good solubility in conventional low-boiling point solvents. The absorption, fluorescence, electrochemical and electrochromic properties of ProDOT-TPE were characterized in detail as presented below. An electrochromic device (ECD) of this polymer was fabricated, its electrochemical fluorescence switching and electrochromic switching can be achieved synchronously by voltage. EXPERIMENTAL SECTION Materials and Instrumentation. All chemicals used in this work were commercial products and used as received without further purification unless otherwise noted. Propylene carbonate


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(PC) was purchased from Sigma-Aldrich Chemical. Tetrahydrofuran (THF) was dried with lithium aluminium hydride in an argon atmosphere, and xylene was dried by refluxing with CaH2. Nuclear magnetic resonance (NMR) spectra were measured by Bruker Avance AV400. Infrared (IR) spectra were recorded on a Nicolet 8700 Fourier-transform infrared (FT-IR) spectrometer. Molecular weight and molecular weight distribution were determined by gel permeation chromatography (GPC) equipped with a Waters 1515 pump and a Waters 2414 differential refractive index detector, using HPLC-grade THF as mobile phase and polystyrene (PS) as standard. Thermogravimetric analyses (TGA) were performed by Q5000IR. Experiments were carried out on samples of approximately 4 to 6 mg in flowing nitrogen (flow rate = 20 cm3/min) heated at the rate of 20℃/min. Ultraviolet-visible-NIR (UV-vis-NIR) spectra were obtained by JASCO V-670 spectrophotometer. Photoluminescence (PL) spectra and solution fluorescent quantum yield were measured with JOBIN YVON Fluorolog-3-TAV fluorescence spectrophotometer. Particle size was measured with MALVERN Nano-ZS90. Cyclic voltammograms were collected using a CHI 660D electrochemical analyzer and a three-electrode cell, polymer film (0.7 cm × 2.5 cm) on indium tin oxide (ITO) as working electrode, a silver wire as pseudo-reference electrode, and a platinum wire as counter electrode in a solution of 0.1 M LiClO4/propylene carbonate (PC). Spectroelectrochemical analysis was carried out with electrolytic cells using a 1 cm UV-cuvette, polymer film (0.7 cm × 2.5 cm) on ITO glass as working electrode, a platinum wire as counter electrode, a silver wire as reference electrode, and a solution of 0.1 M LiClO4/PC as electrolyte. Polymer film was made by spraying chloroform solution of polymer onto ITO glass. Photographs of the polymer films and devices were taken with a Canon (IXUS 125 HS) digital camera.



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Polymer Synthesis. The synthetic route of ProDOT-TPE is shown in Scheme 1. 1,2-Bis (4-bromophenyl)-1,2-diphenylethene was synthesized according to a modified method reported in literature27. In a flame-dried three-neck flask under argon atmosphere, an ice-cooled (-5 °C) suspension of zinc powder (6.25 g, 95.6 mmol) in THF (120 ml) was prepared and titanium tetrachloride (10.00 ml, 91 mmol) was slowly added to the suspension. The resulting mixture was refluxed for 4 h. After cooling to room temperature, a solution of 4-bromobenzophenone (5.00 g, 19.15 mmol) in THF (25 ml) was slowly added to the mixture, and the mixture was then refluxed overnight. After cooling to room temperature, the mixture was diluted with saturated aqueous sodium hydrogen carbonate solution (until no bubble comes out) and diethyl ether and stirred for 2 h. Suction filtration was carried out to the mixture, and the filtrate was separated into an organic and an aqueous layers. The aqueous layer was extracted twice with diethyl ether, and the combined organic layer was dried with anhydrous Na2SO4 and the solvent was evaporated under reduced pressure. The resulting white crude product was purified by silica gel column chromatography using hexane as eluting solvent. White powder (8.0 g, 85% yield) was obtained, and then was further purified by multiple recrystallization with ethanol: toluene (2:1) and then hexane: ethyl acetate (5:1). The product was obtained as white crystal. 1H-NMR (300 MHz, CDCl3, ppm): δ 7.26 (4H, m), 7.15 (6H, m), 7.01 (4H, m), 6.90 (4H, m).

13

C-NMR (100 MHz,

CDCl3): 142.93, 142.83, 142.39, 142.29, 140.31, 132.87, 131.21, 131.11, 130.91, 128.02, 127.82, 126.95, 126.83, 120.80, 120.66. ProDOT-TPE was synthesized according to a polymerization method reported by John R. Reynolds

group28,

and

concrete

(4-bromophenyl)-1,2-diphenylethene

steps (346

are mg,

as

follows: 0.71

1,2-bis mmol),

3,3-bis((2-ethylhexyloxy)methyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine (312 mg, 0.71


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mmol), K2CO3 (255.3 mg, 1.85 mmol), palladium acetate (6.7 mg, 0.03 mmol), pivalic acid (22 mg, 0.21 mmol) and 10 ml dry N-methyl pyrrolidone were added into dry two-neck flask in glove box. The mixture was heated up to 140 °C in oil bath for 3 hours under agitation situation. The solution was then precipitated into methanol (300 ml). The precipitate was filtered through a cellulose thimble and purified via Soxhlet extraction for 24 hours with methanol and hexane respectively. The polymer was extracted with chloroform, concentrated by evaporation, precipitated into 300 ml methanol again, and then was obtained as yellow solid (493 mg, 75% yield). 1H-NMR (300 MHz, CDCl3, ppm): δ 7.39~7.49 (4H, m), 6.89~7.08 (14H, m), 4.08 (4H, s), 3.48 (4H, s), 3.20 (4H, s), 1.54~1.59 (6H, m), 1.22~1.28 (12H, m), 0.81~0.88 (12H, m); 13

C-NMR (100 MHz, CDCl3): 146.53, 143.77, 142.34, 142.19, 140.63, 131.56, 131.51, 127.84,

127.62, 126.56, 125.38, 119.14, 74.31, 73.56, 70.00, 47.92, 39.52, 30.59, 29.14, 24.04, 22.93, 14.24, 11.17. Mn = 29700 g/mol, PDI = 2.38. Scheme 1. Synthesis route of ProDOT-TPE



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RESULT AND DISCUSSION The chemical structure of ProDOT-TPE was characterized by 1H NMR, 13C NMR and FT-IR, the results are shown in Supporting Information as Figure S1, S2, and S3 respectively. No thiophene characteristic peak can be found in the 1H NMR spectrum, and the integral ratio of H atoms of TPE group to methylene H atoms on seven membered ring conforms to the mole ratio of two monomers. In the

13

C NMR spectrum, the peak at 119.14 ppm, corresponding to 2, 5

positions of thiophene, is single and with no peak beside. The two positions have identical chemical shifts, which means 2, 5 positions of thiophene ring both connect TPE groups rather than anther thiophene rings. This proves that ProDOT-TPE is an alternating polymer. Additionally, GPC data also show that the polymer was well prepared. The thermal stability of ProDOT-TPE was characterized and the results are plotted in Figure S4. No phase transition peaks were observed by differential scanning calorimetry study up to 300 °C, indicating that it has good thermal stability. In addition, the polymer has good solubility in conventional low-boiling polar solvents such as tetrahydrofuran, dichloromethane, chloroform and toluene. The absorption spectra of ProDOT-TPE in THF solution and solid film states are shown in Figure 1. The polymer solution in THF exhibits yellow color and the maximum absorption (λmax) is 407 nm, corresponding to the π-π* transition of the polymer backbone. The polymer film shows luminous yellow color and its λmax is 417 nm. The red-shift and broadening of the absorption spectrum of polymer film could be ascribed to the aggregation of the polymer chains resulting in different energy levels distribution and enhanced inter-chain π-π* stacking interaction in the solid film.


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Figure 1. UV-vis spectra of ProDOT-TPE in THF solution and solid film state. The excitation and emission characters of ProDOT-TPE film spray-coated on quartz glass were characterized. As depicted in Figure 2, the maximum excitation wavelength is 372 nm, and the maximum emission wavelength is 540 nm.

Figure 2. Photoluminescence excitation and emission spectra of ProDOT-TPE film.



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To verify the AIE effect of ProDOT-TPE, we tested its fluorescence emission property in THF/water mixtures of different water fractions, and the result is shown in Figure 3a. Compared with TPE micromolecule, which is non-emissive when dissolved in good solvents caused by the rotations of its phenyl rotors29, weak photoluminescence can be detected from dilute THF solution of ProDOT-TPE. The difference between the micromolecule and macromolecule may be due to the partial restriction to TPE luminogen group exerted by polymer skeleton. Addition of a little amount of water (10% by volume) into the THF solution causes the polymer to aggregate, owning to the immiscibility of the hydrophobic luminogen with the hydrophilic medium. The aggregate formation activates the RIR process and boosts the light emission, the photographs can be intuitively observed in Figure 3b. The fluorescent quantum yields of the solutions in Figure 3b were measured with integrating sphere, and detailed results are given out in Table 1. The quantum yields exhibit an obvious increasing tendency as water fractions enlarge. In order to directly verify the existence of aggregate state, the Z-average diameter of polymer in solutions are measured. The Z-average diameter of polymer solution obviously increases after water is added. As the water fraction raises, the value exhibits an increasing trend as well. Detailed results are listed in Table 1 along with the quantum yield data. The enlargement of polymer’s Z-average diameter strongly confirms the existence of aggregate state in solutions.


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Figure 3. (a) PL spectra of ProDOT-TPE in THF/water mixtures with different water content, polymer concentration is 0.15 mg/ml, excitation wavelength used is 372 nm. (b) Photographs of polymer in THF/water mixtures with different water content under 365 nm UV light, polymer concentration is 0.1 mg/ml; water fractions (vol %) are 0%, 10%, 20%, 40%, 50%, 70%, 80% and 90% successively from left to right. Table 1. Fluorescent quantum yields and Z-average diameters of the solutions with different water fractions Water fraction Quantum yield Z-Diameter(nm)

0% 0.09 4.9

10% 0.15 99

20% 0.31 109

40% 0.35 114

50% 0.41 146

70% 0.44 165

80% 0.46 174

90% 0.46 179



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The electrochemical properties of ProDOT-TPE were investigated by cyclic voltammetry (CV). The resulting CV curve can be seen in Figure 4, in which the oxidation voltage of polymer is 1.40 V, and two reduction peaks appear at 0.86 V and -0.76 V respectively. The film started to be reduced at 0.86V, and became yellow simultaneously, and then it was totally reduced at -0.76 V, giving out a bright yellow color again.

Figure 4. Cyclic voltammogram of ProDOT-TPE in 0.1 M LiClO4/PC at a scan rate of 100 mV/s. Insert: photographs of ProDOT-TPE film in oxidized state and reduced state respectively. Color changes and detailed optical properties could be observed when different potentials were applied to the films. The spectrum behavior of ProDOT-TPE film under different voltages was investigated with a UV–vis–NIR spectrophotometer. The resulting spectroelectrochemical spectra of ProDOT-TPE are plotted in Figure 5, wherein as the potential enlarged, the transmittance from 400 nm to 500 nm increased, while the transmittance in near-infrared and infrared regions decreased. This phenomenon indicates that the polymer exists in a non-planar (less conjugated) form in the neutral state, and a low concentration of localized portions of the


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polymer chain become oxidized to a more planar (highly conjugated) form at the expense of the π-π* transition in the oxidized state30. The largest transmittance change in visible region occurs at 608 nm, and the value is 40%.

Figure 5. Spectroelectrochemistry of ProDOT-TPE film on ITO-coated glass in 0.1 M LiClO4/PC at applied potentials of 0.0 V, 0.9 V, 1.0 V, 1.1 V, 1.2 V, 1.3 V, 1.4 V ( by the direction of the arrow). Response time is defined as the time needed by EC film to achieve 95% of its largest transmittance change. A double-step chronoamperometry technique and a UV-vis-NIR spectrophotometer were used to measure the response time. As shown in Figure S5, the response time for oxidation is 1.5 s, and the response time for reduction is 4.0 s. An electrochromic device (ECD) (3.45 cm × 3.45 cm) of the polymer was manufactured, its structural schematic diagram is shown as Figure S6. Polymer film was conveniently prepared by spray-coating on ITO, as it has good solubility in chloroform. The film was used as working electrode, V2O5 film was used as counter electrode due to its outstanding properties of lithium



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ion intercalation/deintercalation and large charge capacity31, and 0.1 M LiClO4/PC solution was injected into the device and used as electrolyte. The EC and fluorescence behavior of this ECD were tested. The CV curve of ECD is shown in Figure S7. The ECD rapidly became blue from yellow neutral state at 2.5 V oxidation voltage, and started to be reduced at 1.65 V, meanwhile it slowly switched back to yellow again. The photographs of ECD in neutral and oxidized states are shown in Figure 6a and Figure 6b respectively, and corresponding UV-vis spectra are given in Figure 6c. The largest transmittance change of the ECD is 36% at 614 nm.


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Figure 6. (a) Photograph of ECD in neutral state. (b) Photograph of ECD in oxidized state. (c) UV-vis spectra of ECD in neutral state and oxidized state.



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The ECD remains strong fluorescence of film in its yellow neutral state, and most importantly, its fluorescence can be switched on/off by voltage along with its color. The photograph of ECDs in UV light (365 nm) is shown in Figure 7a. Corresponding PL spectra in neutral and oxidized states are given in Figure 7b, in which the PL spectrum of ECD in oxidized state is nearly a straight line overlapping zero axis. This means that the yellow fluorescent light totally disappeared when the device was oxidized and became blue synchronously. The possible explanation to this oxidative fluorescence quenching process of ProDOT-TPE is as follows: when an electrical potential (2.5 V) was applied, oxidation selectively occurred on TPE moieties and gave rise to the corresponding radical cations. Since the radical cation is known to have a strong absorption to the blue-light and UV light, it acts as an effective fluorescence quencher so that the photoluminescence of ProDOT-TPE is efficiently quenched. When a reverse sweep was applied, the radical cations were annihilated and the fluorescence at neutral state was recovered. The synchronous fluorescence switching and electrochromic switching of the ECD can be repeatedly driven for tens of time. Similar bifunctional polymers with better switching stability are being designed and constructed in our group, and they are hopefully to be reported in the near future.


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Figure 7. (a) Photographs of ECDs in neutral state (right) and oxidized state (left) in 365nm UV light. (b) PL curves of ECD in neutral and oxidized states. CONCLUSION A

novel

alternating

polymer

bearing

3,3-bis((2-ethylhexyloxy)methyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine

TPE

and

was

designed,

synthesized, and characterized. It possesses both aggregation-enhanced fluorescent emission



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effect and electrochromic property. The dilute polymer solution in THF only has feeble fluorescence, yet the polymer film that was fabricated by spray-coating gives out strong yellow-green fluorescence of 540 nm. Furthermore, the color of polymer film can be switched from bright yellow to blue by voltage within ±1.5V, and the largest transmittance change is 40% at 608 nm. An ECD of the polymer was manufactured, its fluorescence can be switched on/off by voltage within ±2.5V synchronously with its EC behavior. The ECD exhibits bright yellow color and fluorescence in neutral state, however, in oxidized state, it becomes navy blue and the fluorescence disappears. This unique property makes the polymer a special candidate for electrochemical fluorescence and electrochromic applications. ASSOCIATED CONTENT Supporting Information. 1H NMR spectra of ProDOT-TPE, 13C NMR spectra of ProDOT-TPE, FT-IR spectra of ProDOT-TPE, TGA curve of ProDOT-TPE at a heating rate of 20 °C/min under nitrogen, Response time of ProDOT-TPE film on ITO-coated glass in 0.1 M LiClO4/PC measured by double-step chronoamperometry method, Structural schematic diagram of ECD, and CV curve of ECD. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ACKNOWLEDGMENT We gratefully acknowledge financial supports of this work by the National Natural Science Foundation of China (21273207, 21274138 and 21474096).


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ABBREVIATIONS EC, electrochromic; ECD, electrochromic device; ACQ, aggregation-caused quenching; AIE, aggregation-induced emission; AIEE, aggregation-induced emission enhancement; RIR, restriction of intramolecular rotation; TPE, tetraphenylethene; PL, photoluminescence; PC, propylene carbonate. REFERENCES 1. Tang, C. W.; VanSlyke, S. A., Organic Electroluminescent Diodes. Appl. Phys. Lett. 1987, 51, 913-915. 2. Hu, R.; Lam, J. W. Y.; Liu, J.; Sung, H. H. Y.; Williams, I. D.; Yue, Z.; Wong, K. S.; Yuen, M. M. F.; Tang, B. Z., Hyperbranched Conjugated Poly(tetraphenylethene): Synthesis, Aggregation-Induced Emission, Fluorescent Photopatterning, Optical Limiting and Explosive Detection. Polym. Chem. 2012, 3, 1481. 3.

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AIEE-Active and Electrochromic Bifunctional ... - ACS Publications

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