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Abstract
A conjugated polymer consisting of tri-EDOT and azine groups was synthesized and explored for a programmable dual electrochromic device. Electrochemical doping (oxidation) and de-doping (reduction) resulted in the two distinct redox pairs, depending on the applied potential range. In situ spectroelectrochemical analyses revealed that the stepwise redox process of two units was associated with dual electrochromic responses: tri-EDOT unit for purple electrochromism followed by azine unit for blue electrochromism. Dual electrochromism of purple and blue was attainable at below 1.0 V which could induce the color transition upon the potential application of around ± 1 V. Maximum coloration efficiency of 393 cm2/C with a response time of 1 s was obtained.
© 2017 Optical Society of America
1. Introduction
Electrochromic materials can undergo electrochemical oxidation/ reduction under an electric potential which is associated with an electronic band structure modification particularly locating in the visible region. As it exhibits a reversible color change, these materials have potential applications in optical modulation, displays, smart windows, and mirrors [1–4]. To be an ideal electrochromic material, it should have a large optical contrast in accordance with a small change in the applied electric field. Earlier studies of electrochromic device have started with inorganic material such as tungsten trioxide (WO3) and iridium dioxide (IrO2) [5, 6]. Later, organic materials such as violegen [7], phthalocyanine [8] and conjugated polymers [9] are receiving prior attention as electrochromic materials. Among these, conjugated polymers have been much of an interest recently which is attributed to their outstanding coloration efficiency, high switching speed, memory effect, possibilities to form multi-colored states under different electrical potentials and band gap tunability by structural modifications [10–17]. Yet, it is rare to find a programmable dual electrochromism in one molecular system.
Aromatic azines and polyazines have been widely studied for electronic, optoelectronic and photonic applications because of their high third-order non-linear susceptibility. In accordance with the perspectives attaining diverse electronic and optical properties, the utilization of conjugated copolymers containing azines or oligomeric azines has been demonstrated for channel waveguides, cathodic photocurrent generations, and light-emitting devices [18]. The presence of lone electron pair of the nitrogen atom can create a programmable unit to modify the electro-optical properties of pi-conjugated supramolecular system. However, neither their electrochromic properties nor the application for electrochromic devices has been provided yet.
Herein, we synthesized a poly(3,4-ethylenedioxythiophene) (PEDOT) derivative linked with azine unit (PEDAZ) and lateral alkyl spacers to conjugated main chain to improve solubility. PEDOT exhibits generally blue to transparent sky blue color transition upon oxidation [9, 12]. By bridging with an azine unit, the PEDAZ could show unprecedented dual electrochromism of ‘blue to transparent sky blue’ and ‘purple to transparent purple’ arising from the precise control of two different redox reactions of tri-EDOT and azine units. It implies that these two redox processes can be utilized to ‘program’ dual electrochromism at three different optical states, i.e, blue, purple, and transparent states, selecting one of the EC switching modes from a single EC material. This provides novel aspect of electrochromic devices based on the PEDOT based polyazine derivatives. Here we report redox properties, dual electrochromism, and electrochromic device (ECD) based on PEDAZ.
2. Experimental
2.1 Materials
Hydrazine monohydrate (1), 2,5-dibromo-3,4-ethylenedioxythiophene, palladium(II) acetate (Pd(OAc)2), N,N-dimethylacetamide (DMAc), potassium acetate, tetrabutylammonium hexafluorophosphate, chloroform, propylene carbonate were purchased from Aldrich Chemicals and used without further purification. Methanol, acetone, hexane and ethanol were purchased from Duksan Pure Chemicals. An ITO glass (Wooyang, Korea) was cleaned as reported before [2]. The azine 3a monomer was synthesized by a previously reported procedure [19].
2.2. Synthesis of PEDAZ
The polymer PEDAZ (6), was synthesized by direct polyarylation method using Pd(OAc)2 [19]. The di brominated EDOT (5) (150.0 mg, 0.496 mmol), azine 3a (320.0 mg, 0.496 mmol), Pd(OAc)2 (10.90 mg, 0.049 mmol), potassium acetate (146.0 mg, 1.486 mmol) were all transferred in a shlenk flask under nitrogen atmosphere. The anhydrous DMAc (5 mL) was then added and the whole reaction mixture was left to stir at 125 °C for 12 h. The resultant mixture was poured into 100 mL of methanol. The purple color precipitate was filtered and further purified by Soxhlet extraction using methanol, acetone and hexane to remove any remaining salts or oligomers to give the desired polymer PEDAZ as purple powder (300 mg, 77%); GPC (CHCl3, RI) Da, Mn = 11.857 × 103, Mw = 16.226 × 103; TGA%/°C; 90/313, 80/372; 1H NMR (400 MHz, CDCl3, δ): 4.33, 427 (8H, broad signal, 2 × (O-CH2CH2-O)2), 2.97 (4H, broad signal, 2 × (-CH2)), 1.51 (4H, broad signal, 2 × (-CH2)), 1.17 (32H, broad signal, 2 × (-CH2)8) and 0.80 (6H, broad signal, 2 × (CH3)).
2.3 Preparation and characterization of ECDs
All the electrochemical studies were performed using a universal potentiostat [model CHI 624B (CH Instruments, Inc.)]. Optical properties were obtained on a PerkinElmer Lambda750 UV/Vis/NIR Spectrophotometer. The film thickness was measured by an Alpha-Step IQ (Tencor Instruments). A conventional three-electrode system was used for cyclic voltammetry measurements consisting of the PEDAZ-coated ITO glass as the working electrode and a platinum wire as the counter electrode, and an Ag/AgCl reference electrode. Tetrabutylammonium hexafluorophosphate (Bu4NPF6, 0.1 M) in propylene carbonate was employed as a supporting electrolyte. Polymer solution (1wt% PEDAZ in CHCl3) was spin-coated on an ITO glass for 10 seconds at 1200 RPM (film thickness of 140 nm). Electrochromic properties were determined by an in situ spectroelectrochemical setup. The coloration efficiency and response time of the ECDs were determined at the absorption maxima under a square-wave switching potential using a chronocoulometry in a liquid electrolyte. The EC response time for coloration or bleaching was determined at a 90% absorption change under the given step potentials. Coloration efficiency (cm2/C) was determined by dividing the absorbance by the injected/ejected charge per unit area.
An electrochromic device (ECD) with a size of 1.0 x 1.0 cm2 and two electrodes was fabricated using PEDAZ-coated ITO glass as the working electrode and bare ITO glass as the counter electrode. A liquid electrolyte containing 0.1 M Bu4NPF6 in acetonitrile was isolated between the two electrodes by a Surlyn (Thermoplastic resins, DuPont) spacer (thickness of 100 μm) as well as the adhesive.
3. Results and discussion
3.1 Synthesis and optical properties of PEDAZ
PEDAZ (6) was synthesized from the (E, E) isomer 3a and 2, 2’-dibromo EDOT (5) (Fig. 1) via palladium catalyzed direct coupling in 77% yield, following the reported method [19]. The azine monomer (3a) was synthesized from 2-lauroyl EDOT (2) in the presence of hydrazine monohydrate (1) and the isomers were separated by column chromatography [19].
The optical and electrochemical properties of the PEDAZ were examined by UV-Visible absorption spectroscopy and cyclic voltammetry. The HOMO energy level of PEDAZ (5.1 eV) was much lower than that of the unsubstituted PEDOT [9], obviously due to the presence of electron withdrawing azine unit. The PEDAZ showed a deep purple color in its neutral state, which was matched to the optical band gap and absorption maxima (549 nm) in UV-Visible absorption spectrum. The band gap was determined as 1.91eV and the absorption maximum was observed at about 549 nm, which is slightly lower than PEDOT. The broad charge transfer band could be attributed to the electronic interaction between donor moiety (EDOT) and acceptor azine unit. In addition, the HOMO and LUMO energy level of PEDAZ was determined from the onset values of its oxidation and reduction potentials as −5.10 eV and −3.31 eV, respectively. PEDAZ exhibited lower HOMO and LUMO levels compared to those of PEDOT (−4.65 eV and −2.98 eV of HOMO and LUMO, respectively) [9]. The bandgap energy (Eg) of PEDAZ (1.87eV) was lower than that of polyazines with thiophene units (Eg of 1.92 eV) [18], as expected from the donating EDOT unit in PEDAZ.
3.2 Electrochemical properties
The cyclic voltammetry (CV) of PEDAZ thin film on ITO glass showed two quasi-reversible redox pair at 0.6 V and 1.0 V (vs Ag/AgCl) as shown in Fig. 2. The peak at 0.6 V was quite stable without any significant degradation when scanned within a potential range of 0 V ~0.8 V (dashed line, Fig. 2(a)). When scanned to a more positive potential up to 1.2 V, a new oxidation peak at 1.0 V was appeared. In a repeated scan under the potential range of 0~1.2 V, both redox peaks at 0.6 V and 1.0 V were shifted toward positive potential. The two peaks at 0.65 V and 1.13 V were reversible (solid line, Fig. 2(a)) but the initial redox pair at 0.6 V was not observed within 0~1.2 V. The initial first redox peak at 0.6 V was recovered only when scanned to a much negative potential up to −1 V (solid line, Fig. 2(b)) along with the broad second peak at around 1.0 V. Both redox pairs were slightly shifted to lower potential with higher reversibility compared to those of Fig. 2(a). Thus it seemed that the PEDAZ underwent two distinct redox reactions depending on the scanning potential range.
Fig. 2 Cyclic voltammogram of PEDAZ film coated on ITO glass vs. Ag/AgCl at the scan rate of 50 mV/s. The first scan of each range is featured in red.
Within a narrow range (1 electron transfer), a redox pair appeared at 0.6 V, which could be attributed to the redox reaction of tri-EDOT unit in PEDAZ (Fig. 4). Once the potential went beyond 0.6 V, subsequent oxidation of azine group might occur which is associated with a redox peak at 1.0 V and presumably resulted in a more delocalized and stabilized form inducing the evolution of oxidation at a higher potential of 1.13 V. It indicates that the dedoping process after the first doping in the range from 0 V to 1.2 V could not retain the initial state of the PEDAZ. Upon scanning to a negative region (Fig. 2(b)), first redox pair were attainable (solid line). This is attributable to the higher energy barrier to undergo de-doping processes in both tri-EDOT and azine units in PEDAZ compared to PEDOT homopolymer. Additionally, it is well known that compounds with azine bonds are capable of rotation, protonation, and complexation [18]. Thus, it reflects plausible oxidation-induced conformational reorientation of conjugated polymer chain with azine moieties to a much stabilized form which would hinder the reduction of tri-EDOT moieties (Fig. 4).
3.3 Spectroelectrochemical properties
To carefully analyze the spectral changes according to the redox processes, we characterized PEDAZ film using an in situ spectroelectrochemical set-up. Transmittance spectra of PEDAZ film are shown in Fig. 3 at neutral state (at 0 V) and after doping/dedoping as a function of applied voltages in a three-electrode cell. Neutral state of PEDAZ showed an absorption maximum at 550 nm with purple color at 0 V. Upon applying 0.3 V, the transmittance at 550 nm was increased with bleaching of purple color, followed by the growth of absorption at around 800 nm, which was intensified upon application with a higher positive potential. Thus at 0.6 V, the transmittance of the 550 nm band was increased to 41% while that at 800 nm was decreased to 45%. The transmittance change was reversible within the potential range of 0 ~0.6 V. In a wide potential window (0 ~1.2 V), the transmittance was changed further to increase IR absorption (~1190 nm) in the expense of the 550 nm band at an applied potential larger than 0.9 V in the 1st oxidation process (solid lines, Fig. 3(a)). After the completion of oxidation at 1.2 V, the successive electrochemical reduction resulted in the decrease of transmittance at 800 nm (dashed line, Fig. 3(a)). Generally, electrochromic material is likely to retain its initial neutral state color upon dedoping process and thus, the spectral properties would also return to its initial state. However, reduction (dedoping) of the same film, which was once oxidized to 1.2 V, exhibited a deep blue color at 0.3 V, instead of the initial purple color. Furthermore the reduction at 0.3 V showed a red-shift of the band from 550 to 800 nm (dashed line, Fig. 3(a)), which shows quite different feature to that of the pristine neutral film (0 V and 0.3 V, solid line, Fig. 3(a)). The observed new absorption corresponds to the deep blue color from the dedoped polymer. Gradual oxidation to 1.2 V (Fig. 3(a), the second oxidation) induced the reoccurrence of transparent sky blue color without any trace of purple color.
Fig. 3 UV−vis−NIR spectral change upon series of applied potentials: (a) upon the first oxidation from 0 V to 0.3, 0.6, 0.9, and 1.2 V (solid line), followed by second oxidation from 0.3 to 1.2 V (dashed line), (b) reduction of the film of Fig. 3(a), oxidized fully at 1.2 V, from −0.3 to −1.2 V, (c) oxidation of the film at Fig. 3(b) from 0 V and 1.2 V.
As shown in Fig. 3(b) upon the subsequent reduction down to −1.2 V after the second oxidation, the PEDAZ film featured the recovery of the band at 550 nm, and the increase of transmittance at around 750 nm. Color transition from blue to deep purple at fully neutral state was also noticed at 0 ~0.6 V, just like the fresh PEDAZ film in Fig. 3(a) (solid line). The recovered neutral PEDAZ film by reduction down to −1.2 V underwent the third positive potential scanning (Fig. 3(c)). Reversible increase in transmittance at 550 nm with respect to the bleaching to transparent blue via transparent purple color was confirmed.
Those unprecedented spectral changes depending on the applied potentials are consistent to the results of cyclic voltammogram. Obviously, the first oxidation might be originated from the tri-EDOT unit of the polymer chain (6b, Fig. 4(a)) accompanying purple electrochromism. At higher doping, azine group also would start to oxidize, which would create new resonance structures and finally rearrange as depicted in (6c, Fig. 4(a)). Similar kind of system was reported by Hünig et.al in their series of study over violegen/cyanine hybrid electrochromics [20]. After a complete oxidation at 1.2 V, the polymer chain was attained a highly conjugated resonance structure (6d, Fig. 4(a)). At this form, polymer exhibited absorption at the longest wavelength which produces a transparent blue color. Due to the enhanced stability of the 6d, subsequent reduction to 0 V could not afford 6a but 6f. This corresponds to the red-shift of the band from 550 to 800 nm as noticed in Fig. 3(a). 6a could be recovered upon further reduction to −1 V as described in Fig. 4(b). Consequently, the structural rearrangement of polymer backbone and the recovery of fully dedoped state of polymer require much larger oxidation and reduction potentials, respectively.
Fig. 4 A schematic representation of doping induced rearrangement of PEDAZ. (a). At the potential range 0 V to 0.6 V: a redox reaction of tri-EDOT unit (benzoid-quninoid) was responsible for the purple electrochromism. At the potential range 0.6V to 1.2V: further oxidation produces more charged and stabilized species (6d) consequently resulting in transparent blue electrochromism. (b). At the potential range 1.2 V to 0 V: a redox reaction was responsible for the blue electrochromism of the rearranged structure (6d). At the potential 0 V to −1.0 V: further reduction recovered 6a invoking purple electrochromism
Based on the intrinsic EC properties of PEDAZ film, two kinds of electrochromism were programmable for purple and blue as featured in Fig. 5. Specifically, purple switching could be achieved by applying 0.6 V and 0 V for bleaching (oxidation) and coloration of tri-EDOT unit, respectively. It could be also programmed for blue electrochromism by applying potential over 1 V to oxidize azine unit, followed by EC switching between 0.9 V and 0 V. The ‘reset’ to purple EC mode can be achieved by applying −1 V resulting in neutral PEDAZ film.
Fig. 5 Dual electrochromism observed at below 1.0 V, and the transition between purple and blue electrochromic switching by inducing fully dedoped (−1 V, reduction) or highly doped (1.2 V, oxidation) states.
3.4 Electrochromic switching properties
The EC device was fabricated to elucidate the response behavior of PEDAZ film. EC switching of the device between bleached and colored state was carried out by alternating voltage application between 0.6 V and 0 V inducing oxidation (doping) and reduction (de-doping), respectively. In terms of the blue switching, EC device was firstly applied with 1.2 V for 30 seconds to attain the resonance structure 6d, and then subsequently applied the alternating potentials of 0.9 V and 0 V as described in Fig. 5. The devices exhibited reversible color switching and the resultant optical response was measured at 550 nm and 750 nm where the maximum absorption contrast was observed as shown in Fig. 6. At both wavelengths, decent reversibility was confirmed on repetitive switching even with the shortening the interval from 10 s to 1 s.
Fig. 6 Optical responses of electrochromic device based on PEDAZ film containing liquid electrolyte at 0 V and 0.6 V with a switching interval of 10 s, 5 s, 3 s, and 1 s per step monitored at 550 nm (a and c) and 750 nm (b and d): (a and b) switching between deep purple and transparent purple, (c and d) switching between deep blue and transparent blue (applied 1.2 V for 30 s to induce blue switching mode).
Electrochromic properties are summarized in Table 1. The optical response time estimated at 90% of the full switching showed similar response time of around 3 s for coloration and within 5 s for bleaching. In terms of coloration efficiency, maximum value of 393 cm2/C (Fig. 6(a), monitored at 550 nm) was achieved with purple electrochromism. In contrast, blue EC switching showed much lower efficiencies at both wavelengths, and this huge difference might originate from the stabilized resonance form of PEDAZ in blue EC process (6d in Fig. 4). High absorption (low %T) with subtle switching at bipolaronic region (Fig. 6(d) monitored at 750 nm) strongly suggests that the stabilized and rearranged blue EC state led to the limited de-doping process of PEDAZ. As described above, distinguished redox behaviors of tri-EDOT and azine units resulted in the unique EC device performing dual electrochromism of purple and blue. The reversible transition between two electrochromism -purple and blue- was also successfully attainable.
Table 1. Electrochromic properties of PEDAZ film
4. Summary
A conjugated polymer based on tri-EDOT and azine (PEDAZ) was synthesized and explored for a programmable electrochromic switching device. Redox behaviors of two units were confirmed exhibiting higher oxidation potential for azine unit than that of tri-EDOT unit accompanying much stabilized resonance structure in the doped state. Stepwise reduction could induce the recovery to neutral form of tri-EDOT unit followed by azine unit. Reversible dual electrochromism of purple and blue was attainable at below 1.0 V which could switch the color upon the potential application of around ± 1 V. Maximum coloration efficiency of 393 cm2/C with a response time of 1 s was obtained.
Funding
Global Research Laboratory (GRL) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2016K1A1A2912753). JSPS KAKENHI Grant Number (JP16K17886).
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