Design Strategy for Efficient Solution-Processable Red Electrochromic

Sep 27, 2018 - ABSTRACT: In the quest for solution-processable red electro- chromic .... P1, P2, and P3 based on the long side-chain decorated buildin...
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Design Strategy for Efficient Solution-Processable Red Electrochromic Polymers Based on Unconventional 3,6Bis(dodecyloxy)thieno[3,2‑b]thiophene Building Blocks Yuyang Yin,† Weishuo Li,† Xianzhe Zeng,† Panpan Xu,† Imran Murtaza,‡ Yitong Guo,† Yumeng Liu,† Tingting Li,† Jupeng Cao,† Yaowu He,*,† and Hong Meng*,† †

School of Advanced Materials, Peking University Shenzhen Graduate School, Shenzhen 518055, China Department of Physics, International Islamic University, Islamabad 44000, Pakistan

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‡

S Supporting Information *

ABSTRACT: In the quest for solution-processable red electrochromic material with high performance, a series of 3,6bis(dodecyloxy)thieno[3,2-b]thiophene (DOTT)-based polymers have been designed by combining DOTT with different aromatic building blocks (bithiophene, benzene, and dimethoxybenzene) and prepared through Pd-catalyzed chemical copolymerization. Among them, the solution-processable polymer poly(2,5-dimethoxyphenyl-1,4-diyl-3,6-bis(dodecyloxy)thieno[3,2-b]thiophene) (P3) showed obvious yellow-green fluorescence in chloroform and demonstrated a rapid reversible switching (1.1 s/0.9 s for doping/dedoping process) between the red and bleached states. This brilliant red polymer exhibited moderate contrast (46% at 510 nm) and excellent cycling stability (in film: 78% for total amount and 63% for reduction after 12000 cycles; in device: 85.6% after 3000 cycles) which qualify it for further device fabrications. This study reveals that this polymer (P3) would be a promising high performance red electrochrome and could be a good candidate for flexible and large-scale organic electrochromic devices as well as for smart indicators or display applications.

1. INTRODUCTION In recent years, electrochromic polymers (ECPs) have been rapidly developed owing to their numerous superiorities over inorganic or organic small molecule materials, including their facile structural design, diverse coloration, high coloration efficiency, and great potential for fabrication of large-area or flexible devices.1,2 A variety of ingenious devices containing ECPs keep emerging for desirable applications, such as full color and portable displays,3,4 smart windows5 and glasses,6 camouflage technologies7 and data storage.8 However, it is quite challenging to develop satisfactory materials with desirable colors and favorable performance to fulfill the urgent demand for advanced and practical applications. Great efforts have been made to design and prepare soluble red, green, and blue (the three additive primary colors) ECPs with colored-totransmissive transformations. Among them, many outstanding blue or green materials have been well developed with desirable performances,9−12 while only a few soluble red materials have been reported to date, drawing our great attention to this field. Many efforts have been made by different research groups to explore red ECPs with saturated color and satisfactory performance (summarized as reference polymers A to I in Figure 1 and in Table S1 of the Supporting Information). Initially, thiophene-based ECPs with red neutral state were electrochemically prepared when researchers attempted to © XXXX American Chemical Society

optimize the polymerization methods and performance of the polymers.13−15 Afterward, Reynolds’ group started to develop red ECPs and prepared some processable polymers composed of 3,4-propylenedioxythiophene (ProDOT)16 or alkoxy-substituted thiophenes,17 but these polymers retained unavoidable hues of magenta or pink color. On the basis of these pioneering work, Xu’s group recently developed a series of processable decent red-to-transmissive copolymers by oxidative polymerization, which exhibited saturated neutral color, high contrast, and fast switching.18,19 Aiming to improve further the performance of ECPs, new building blocks such as benzotriazole,20 phenanthrocarbazole,21 and 4,9-dihydro-sindaceno[1,2-b:5,6-b]dithiophene (IDT)22 were introduced and copolymerized with conventional thiophene derivatives. Despite great efforts, it is still difficult to combine the desirable color with high performance, inspiring us to consider unconventional moieties to search novel solution-processable red electrochromic polymers with promising performance. Besides conventional monomer donors like thiophenes, 3,4ethylenedioxythiophenes (EDOTs), and ProDOTs, thieno[3,2-b]thiophene (TT) is another well-known donor unit. It has been widely exploited in the design of materials for active Received: June 7, 2018 Revised: September 11, 2018

A

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Figure 1. Representative red ECPs investigated in previous research works.14−22

layers in organic thin film transistors (OTFTs),23−25 organic photovoltaics (OPVs),26,27 and thermoelectric materials28−30 owing to the low band gaps of its polymers with electron-rich rigid conjugation structures.31 Recently, various TT-based ECPs have been investigated by our research group.32−36 The electrochromic devices based on these materials exhibited multiple colors, high optical contrast, short switching time, and stable cycling life indicating the potential of TT building blocks for design of novel electrochromic materials. Among all the TT-based electrochromic polymers we have investigated, 3,6bis(alkoxy)thieno[3,2-b]thiophenes (BOTTs) have been regarded as capable building blocks for comparable performances.37 Moreover, different side chains can be easily introduced at the 3,6-positions to effectively improve the processability of BOTT-based polymers.38,39 On the basis of these concepts, we have reported a BOTT-based donor− acceptor polymer poly(3,6-bis(hexyloxy)thieno[3,2-b]thiophene−benzo[c][1,2,5]thiadiazole) (PBOTT−BTD),40 which revealed favorable electrochromic performances and appeared to be a promising candidate applied in smart energy storage.41 Despite these, poly(3,6-bis(alkoxy)thieno[3,2-b]thiophenes) (PBOTTs) generally show more red-shifted spectra and narrower band gaps than neutral red polymers,37,39

inspiring us to introduce other cooperating moieties into the backbones of PBOTTs for red color tuning. Herein, we successfully synthesized three novel copolymers P1, P2, and P3 based on the long side-chain decorated building block 3,6-bis(dodecyloxy)thieno[3,2-b]thiophene (DOTT), which ensure them good solubility and performance, and a series of aromatic/heteroaromatic moieties 2,2′bithiophene, benzene, and 1,4-dimethoxybenzene have been introduced into the backbone for optical property modulation (Figure 2). First, 2,2′-bithiophene was introduced to broaden the band gap of the polymer (P1), but the color adjustment was insufficient. Therefore, we considered the benzene moiety which could significantly enlarge the band gap of the copolymer due to its higher aromaticity.42 However, the obtained orange-red tint of polymer P2 suggested the “overmagnified” band gap, inspiring us to further fine-tune the spectrum via the side-chain strategy. Considering that the electron donating groups (EDG) would lower the band gap when introduced into the polymer backbone,43 dimethoxysubstituted benzene was utilized in the design of P3 to achieve the desired optical characteristics. The spray-coated films of P3 were characterized showing fast switching between the saturated red and bleached states with a moderate optical B

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2.1. Optical Properties. The UV−vis and photoluminescence spectra of the polymers in solutions and in thin films were investigated to elucidate the relationship between their colors, optical absorbance/emission characteristics, and the molecular structures. Visually, P1 and P2 films are purple and yellow, respectively, similar to their chloroform solution, while P3 appears as red in film and green-yellow in chloroform solution. As seen from the UV−vis spectra (Figure 3a), in films, broad neutral absorption peaks of the complementary hues of their apparent colors are positioned in the visible region, and the absorption maxima are blue-shifted from P1 (538 nm, green region) to P3 (510 nm, cyan region) and to P2 (476 nm, blue region). Also, the absorption spectrum of P3 in film is structured, with shoulder peaks at 484 and 552 nm, owing to the strong aggregation effect in the film.26 In solutions, the absorption spectra of all polymers are structureless, with single narrow absorption peaks corresponding to π−π* transition, which are located in the shorter wavelength region. However, the maxima of absorption (λmax) follows a different order compared to that in thin films, i.e., P1 (514 nm) > P2 (464 nm) > P3 (444 nm). The significant bathochromic shift of the absorption peak for P3 (66 nm) could be attributed to the drastic π−π stacking effect.26 The optical energy gaps (Eg,opt) were determined from the onset absorptions in films (Table 1). As mentioned before, an insertion of additional aromatic rings into the backbone was chosen to adjust the energy band gap of DOTT-based polymers. When 2,2′-bithiophene was inserted into the polymer backbone (P1), the energy gap was only slightly enlarged to 1.84 eV compared to the dialkoxy-substituted TT homopolymers (Eg ∼ 1.60−1.69 eV37,39). In course of our experimental work on design of red electrochromic materials, we performed DFT B3LYP/6-31G(d) calculations for corresponding oligomers (the hexamers were found to be long enough to consider them as models for corresponding polymers, Table S4). The DFT calculations indicated that replacement of bithiophene moieties by thiophene building block should result in only small increase of the band gap (by

Figure 2. Illustration of design concept and the structures of copolymers P1−P3.

contrast and excellent capacity for long-term cycling. Thus, the strategy of fine-tuning the properties of DOTT-based copolymers by structural variations in incorporated aromatic building blocks led us to high-performance red electrochromic polymers.

2. RESULTS AND DISCUSSION The starting compounds and intermediates for synthesis of monomers M1−M5 were synthesized following the previously described procedures.40,44,45 Polymers P1−P3 were synthesized by Pd-catalyzed cross-coupling polymerization and purified by Soxhlet extraction (Scheme 1). The details of synthesis and structural characterizations are given in the Supporting Information, section S2. All synthesized polymers showed reasonably high and comparable molecular weights and polydispersity (Mn ∼ 7.1−13.8 kDa, PDI ∼ 1.9−2.2). Polymers P1 and P3 showed good solubility in common low/ medium-polar organic solvents (chloroform, dichloromethane, toluene), whereas the solubility of P2 was lower (difficult to dissolve in chlorobenzene or hot chloroform). We performed a series of investigations of optical, electrochemical, and electrochromic properties of these polymers and below we have discussed systematically on the results of these studies. Scheme 1. Synthesis of Polymers P1−P3

C

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Figure 3. (a) Normalized UV−vis absorption spectra of polymers P1−P3 in films (at applied bias of 0 V vs Ag wire) and in chloroform solutions (0.01 mg mL−1). A visible spectrum bar at the bottom of the graph is a reference for color analysis. The part on the right side of the gray dashed line indicates the red-light region. (b) Photoluminescence spectra of P3 in film on ITO/glass and in chloroform solution (0.01 mg mL−1). The excitation wavelengths are λexc = 480 and 365 nm for the film and the solution, respectively. Insets are photographs of the film and solution under UV light irradiation.

benzene (as in polymer P2), the higher degree of aromaticity of the latter and an increased dihedral angle (6.8° for P1 and 24° for P2 as calculated, Figure S14) between the DOTT and benzene units decrease the conjugation along the backbone, resulting in substantially higher optical energy gap of 2.26 eV, with a blue-shifted absorption.42 To slightly pull the spectrum back to the lower energy absorption, the EDG methoxy groups were added onto the benzene ring of polymer P3. The oxygen atoms participate in frontier orbitals perturbation to contribute through p−π conjugation and enhance the delocalization to narrow the band gap of the polymer to 2.06 eV. The suitable optical energy gap (∼2.0 eV) and maximum of absorption (510 nm) make P3 the most appropriate one among the three polymers because more cyan light (around 500 nm) and less

Table 1. Optical Properties of Synthesized Polymers P1−P3 λmaxa (nm) polymer

solution (CHCl3)

film

λonseta (nm)

Eg,optb (eV)

P1 P2 P3

514 469 444

538 476 484 sh,c 510, 552 sh

673 550 601

1.84 2.26 2.06

λmax is the absorption maximum; λonset is the onset absorption in film. Eg,opt is estimated from the equation Eg,opt = 1240/λonset (eV). csh: shoulder. a

b

ca. 0.1 eV, Table S4), so we declined the synthesis of corresponding polymer. When bithiophene was replaced with

Figure 4. (a) Cyclic voltammogram of P3 polymer, spray-coated on Pt electrode in 0.1 M TBAPF6/ACN at a scan rate of 100 mV s−1. The potentials were calibrated with ferrocene/ferrocenium redox couple, which showed the half-wave potential 0.77 V vs Ag wire pseudoreference electrode. (b) Linear dependence of the peak current density as a function of the scan rate. (c) Electrochemical stability of P3 film was tested in 0.1 M TBAPF6/propylene carbonate (PC) under square-wave voltammetry, switching the potentials between 0 and 1.0 V (vs Ag wire) in steps of 2 s. (d) Cyclic voltammograms of P3 film scanned between 0 and 1.3 V (vs Ag wire) at 100 mV s−1. Initial CV and CV after 6000 and 12800 cycles in SWV experiments are shown. D

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observations indicate that P3 film is reliable for redox processes and capable for long-term cycling stability. 2.3. Spectroelectrochemical Analysis. In situ spectroelectrochemical (SEC) measurements were performed to explore the optical−electrical relationship and evolution of the polymer’s electronic band structure. A neutral π−π* transition absorption peak in the visible region gets into the lower energy band in the NIR region due to formation of polarons and bipolarons with an increase of the applied potential, leaving a low-absorbing tail in the visible region. In total, P3 film is oxidized from the neutral colored state to pale blue when the applied potential increases from 0 to 1.3 V (vs Ag wire) (Figure 5). More specifically, a strong absorption

red light (Eg ≤ 2.0 eV) are absorbed, resulting in the highest transmission of the red light for the human eyes. The data of the absorption spectra measurements are summarized in Table 1. Interestingly, P3 polymer shows strong yellow-green fluorescence in chloroform solution and emits faint red light in thin films. Its photoluminescence spectra in chloroform solution and in films show the maxima at 521.5 nm (shoulder at 555 nm) and 617 nm (shoulder at 586 nm), respectively (Figure 3b). Similar to its absorption spectra, significant solidstate induced bathochromic shift (ca. 96 nm, between the strongest emission peaks) is observed for P3 emission, which makes it a potential component for smart devices such as chemical sensors. Thus, the polymer P3 with satisfactory red tint and distinctive optical properties was obtained, indicating that our strategy of precisely tuning the optical properties of DOTT-based polymers is efficient. This motivated us to investigate the electrochemical and spectroelectrochemical properties as well as electrochromic performance of P3 in more detail. 2.2. Electrochemical Properties. To study the electrochemical properties, cyclic voltammetry (CV) tests of P3 were performed (Figure 4a). The CV curve depicts quasi-reversible p-doping and dedoping processes in anodic region with distinct anodic peaks at 0.04, 0.17, and 0.4 V and cathodic peaks around 0.05 and 0.34 V (vs Fc/Fc+). Based on the onset of oxidation potential E[onset,ox vs Fc/Fc+] = −0.07 V and optical energy gap Eg,opt = 2.06 eV, HOMO and LUMO energy levels were estimated as −5.03 and −2.97 eV, respectively. The relationship between the current density and the scan rate in CV experiments was studied by varying the scan rates in the range of 50−300 mV s−1 (Figure S7a), showing reasonably good linear dependences of the current densities on both the scan rates (v) and their square roots (v1/2) (cf. Figure 4b and Figure S7b). Yet, compared to the plots of ip vs v1/2 (diffusioncontrolled redox process), better linear dependences (R2 > 0.997) were found for ip vs v, corresponding to non-diffusioncontrolled redox process and indicating that the electroactive film has been stably adhered onto the electrode surface.33,46 To test the long-term stability of the polymer films, squarewave voltammetry (SWV) experiments were performed by switching the doping/dedoping potentials for over 12000 times. The transported charges during the oxidation and reduction per unit area and the retained percentage of the total charge compared to its initial value as a function of the number of cycles are shown in Figure 4c. The amounts of charge were calculated by integrating the current density over the time at selected cycles (at the 2nd, 500th, and every 1000 until 12000). The charge per unit area at the oxidation potential (Qox) was about 1.2 mC cm−2 and kept relatively stable during cycling, while that at the reduction potential (Qred) underwent some losses from ca. −0.9 to −0.6 mC cm−2 before 4000 cycles. This indicates that the main factor which affects the stability is insufficient dedoping process induced by counterion trapping in the active layer.47 The initially observed reduction of the total charge (during the first 4000 cycles) then remains almost constant at the level of 80%. The charge recovered during the reduction decreases to 91% and 67% after 2000 and 4000 cycles, respectively, and then almost unchanged, showing 63% after 12000 cycles. Meanwhile, CV curves recorded before cycling and after 6000th and 12800th cycle show only slight peak shift and decent current retention (Figure 4d). These

Figure 5. Spectroelectrograms of P3 spray-coated on an ITO-coated glass slide electrode in 0.1 M TBAPF6/ACN at applied potentials from 0 to 1.3 V (vs Ag wire).

peak of π−π* transition at 510 nm observed at the applied potential of 0 V begins to decrease from 0.6 V. Simultaneously, a new peak around 710 nm and a broad band around 1800 nm start to grow owing to the appearance of doping energy levels. Growing the long-wavelength band in the visible region continues with saturation until the potential of 0.9 V (with some shift of its maximum to 738 nm). After that (0.9 → 1.3 V) its intensity is decreased, whereas the broad absorption in the NIR region continues to growth with some shift to ∼1500−1600 nm. This indicates the formation of two species in the SEC experiment (including formation and consumption of species with characteristic absorption at ∼730 nm) and can be attributed to the generation of polarons and bipolarons, as generally accepted for p-doping of many conjugated polymers including polythiophenes.48−50 Finally, at high potential of 1.3 V, the absorption in the NIR region becomes the strongest, with the tail into the visible region and a minor difference in the absorbance between 656 nm (a peak) and 466 nm (a valley). This makes the color of the film to be highly achromatic with a pale blue tint. In addition, the absorption in short-wavelength (π−π* transitions) region is blue-shifted owing to the deepened HOMO energy level alone with decreasing the concentration of the neutral states.51 These results demonstrate the transformation of electronic band structure, absorption, and polymer colors. 2.4. Electrochromic Performances. 2.4.1. Colorimetric Characteristics. On the basis of CIE 1976 L*a*b* color space [where L* represents lightness, a* represents the green (−a*) to red (+a*) variation, b* stands for blue (−b*) to yellow (+b*) variation, and the center of the a*b* plane means achromatic], the (a*, b*) values are located in a chromaticity diagram for quantitative comparison of the hue and chroma (Figure 6). The acceptable red region is defined by red outlines consisting of Munsell color points, where 7.5R and E

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Macromolecules *= Sab

* = ΔEab

a*2 + b*2 *2 Cab

+L

*2

× 100% =

* Cab *2 + L*2 Cab

(L 2* − L1*)2 + (a 2* − a1*)2 + (b2* − b1*)2

(1)

(2)

Here, the color saturation Sab * represents the proportion of chromatic color on a scale of 0−100% from gray to pure color. P3 film showed a good saturation of 72.7% in the neutral state and became unsaturated in the oxidized state (Sab * = 9.4%). The color difference ΔEab * between the neutral state of P3 and the target (L*a*b* = 51.43, 60.55, 30.32) was calculated. According to the just noticeable difference (JND) principle, the perceived color difference becomes harder to distinguish when the value gets lower.52 Polymer P3 showed a lower ΔEab * value of 15.3 compared to 25.7 and 74.4 for polymers D and F, respectively, confirming that its neutral red color is closer to the “true red” than other soluble polymers. All the above results demonstrate that we almost achieved a neutral red electrochromic polymer, although further efforts are demanded to adjust the color precisely and to improve the saturation of the colored neutral film and the transmittance of the doped film. 2.4.2. Switching Behaviors. The switching performance of P3 films was studied by chronoamperometry method, repeatedly switching the potentials between the neutral and fully oxidized states within the intervals of 5 s, and the transmittance was simultaneously measured at two wavelengths, i.e., at the maximum absorption of the neutral polymer (λmax = 510 nm) and at λ = 1500 nm. As seen from Figure 7a, P3 shows moderate optical contrasts of about 46% at 510 nm and 47% at 1500 nm. The response time was measured as the time to reach 95% transmittance change of the full response (Figure 7b), showing that polymer P3 rapidly switches to the bleaching state in 1.1 s and then turns back to the neutral state within 0.9 s. These switching times are shorter than those for most of the soluble polymers reported in the literature with red-to-transmissive transformations (the work conditions and results are shown in Table S3). The coloration efficiency (CE) can be calculated as the ratio of the optical density change to the charge injected/rejected per unit. The change in optical density is estimated by ΔOD = log(Tox/Tred), and Qd is the charge consumed during this process and is obtained by integrating the current density over the time. The corresponding current density vs time plots are shown in Figure S8. The CE of P3 film on ITO/glass were estimated to be 415 cm2 C−1 at 510 nm and 302 cm2 C−1 at 1500 nm.

Figure 6. In CIE 1976 L*a*b* color space, the colorimetry coordinates (a*, b*) of the neutral (solid star) and oxidized (hollow star) P3 films spray-coated on ITO-coated glass and representative polymer films in references (polymers B, D, F, H, and I; solid/hollow rectangles) are shown. The three red scattered lines frame the “red region”, and the 5R 5/14 point (framed by a yellow rectangular) were supposed as the target. The shown red and pale blue circles are cutouts from the photographs of P3 films.

2.5R lines are boundaries and the middle 5R line means “pure red”. The corresponding Munsell color patterns are displayed in Figure S9, and the relative definitions and details are described in the Supporting Information, section S4. Among them, the point with Chroma value of 14 on the 5R line (5R 5/14) is supposed to be the target, considering the chroma limit of reflective physical objects. For better comparison, coordinates of polymers B, D, F, H, and I mentioned in Figure 1 are shown in Figure 6 (the data for polymers A, C, E, and G are not available). Most of their neutral spots are settled out of the red region tending toward the red-purple area, among which only polymers D and F are near the red region for spray-casted films with colored-totransmissive switching. While polymers B and H are nearly on 5R line, they are not solution-processable polymers. For oxidation points, most of them are located near the original point, especially for polymer D as highly transmissive film. As for polymer P3 used in this research, its neutral point (L*a*b* = 55.01, 46.44, 35.04) is closer to the edge of the red region than that of the most of polymers in references, despite slightly leaning toward the orange hue, and its oxidized point (L*a*b* = 47.23, 0.68, 4.40) is close to the origin as a pale blue film, showing a decent color contrast with a reasonable difference between the neutral and oxidized points. To assess the parameters quantitatively, the saturation (Sab * ) and color difference from the red target (ΔEab * ) were calculated by eqs 1 and 2:3

Figure 7. (a) Transmittance changes of P3 film spray-coated on an ITO-coated glass slide at λmax (510 nm) and at 1500 nm. (b) Switching time of P3 film measured as the time to reach 95% of the full response in the transmittance change plot. F

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To figure out this influence, atomic force microscopy (AFM) (Figure 9) and scanning electron microscopy (SEM) (Figure S12) studies were performed on spray-coated P3 films on the ITO/glass substrate. The films showed a rough surface with randomly distributed pores of diverse size (ranging from dozens of nanometers to about 1 μm) and height. The film roughness Rq obtained from AFM image was 49.2 nm. Such morphology is favorable for electrochromic films and is beneficial for insertion and extraction of counterions and slowing down the damage of the film induced by redox process. Thus, the observed type of surface morphology can prove and explain the rapid switching ability and stable cycling capacity of P3 thin films.41,53

2.4.3. Optical Stability. To satisfy the demands of practical commercial applications, it is important to measure the stability of electrochromic devices. Therefore, we tested the device stability from the view of optical performance to cater for the visual perception. The device constituted a spray-coated P3/ITO/glass sheet as a working electrode and a blank ITO/ glass as a counter electrode, and CV behavior and absorption spectra were recorded before the stability tests (Supporting Information, section S5). In Figure 8, the results are similar to

3. CONCLUSION In conclusion, a series of electrochromic copolymers P1−P3 containing unconventional 3,6-bis(dodecyloxy)thieno[3,2-b]thiophene (DOTT) building block combined with different aromatic moieties (bithiophene, benzene, and dimethoxybenzene) were chemically synthesized. The molecular engineering strategy toward optical absorption and color adjustment of novel electrochromic polymers was disclosed. Among them, polymer P3 (DOTT−dimethoxybenzene) was found to be the best red electrochromic polymer which showed excellent electrochromic performance such as satisfactory red color in the neutral state, fast switching time, moderate optical contrasts, and outstanding cycling stability, making it a promising material for further device fabrications. Moreover, the strong fluorescence in solution and significant bathochromic shifts from solution to the solid state for both absorption and fluorescence spectra endow it with the potential for various applications. Consequently, this solution-processable polymer completes the color palette and provides more possibilities for exploring multifunctional applications such as smart warning signs, displays, indicators, and so on. Further studies on color tuning for purer red tints and better performance are in progress in our group.

Figure 8. In situ measurements of square-wave potential switching between −2 and 2.3 V (within 5 s interval) and transmittance detection at 510 nm to monitor the transmittance and contrast changes of P3 device during the long-term cycling. The data were picked out at every 100 cycles, and the percentages of the contrast retention versus the initial value are shown.

the electrochemical stability studies described in section 2.2, in which the transmittance at bleaching state is steady after the fluctuation stage before 1000 cycles, while a trend to higher transmittance of the coloring state is observed which mainly narrows the contrast. During the test, the transmittance and contrast underwent a very gentle change, indicating a mild and gradual process of activity loss. The contrast nearly keeps around 100% of the initial value during the first 1000 cycles, shows only 5.5% loss after 2000 cycles, and finally retains 85.6% after 3000 cycles, which is an excellent cycle life for monoactive-layer device operated in ambient conditions. The observed decreases might be due to many factors, including counterion trapping,47 film structure changes and/or exfoliation of the active material, or sealing problem and applying too wide a switching potential window. From the above results, one can conclude that our polymer is suitable for further device fabrications and applications. 2.5. Morphology. Normally, the surface morphology of the polymer films affects their redox behaviors in some ways.

4. EXPERIMENTAL SECTION 4.1. Chemicals and Instruments. All chemicals were purchased from Aldrich. Tetrahydrofuran (THF) and acetonitrile (ACN) were purified and dried by an organic solvent purification system. N,NDimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and propylene carbonate (PC) were dried over molecular sieves. 1H NMR spectra were recorded using Bruker Avance NMR spectrometers, with chloroform-d (CDCl3) as the solvent and trimethylsilane (TMS) as the internal standard. Average molecular weights and polydispersity indices (PDIs) were measured by gel permeation chromatography (GPC; Agilent PL-GPC-220). Elemental analyses were performed on

Figure 9. AFM images of P3 films spray-coated on ITO/glass. G

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an Elementar vario EL cube elemental analyzer (Germany). Electrochemical measurements were performed using a CHI620E electrochemical workstation. UV−vis−NIR electron absorption spectra were recorded using a PerkinElmer Lambda 750 spectrophotometer combined to a tandem setup with the electrochemical workstation for SEC experiments. Photoluminescence spectra were recorded using a PerkinElmer LS55 fluorescence spectrometer. Film surface morphology was studied using atomic force microscopy (AFM) Bruker MultiMode 8 and scanning electron microscope (SEM) Carl Zeiss ZEISS SUPRA 55. 4.2. Film Formation. For films preparation, all polymers were dissolved in chloroform at concentrations of 1 mg/mL. Electrochromic polymer films were prepared by spray-coating, after filtering the solutions through 0.45 μm pore size PTFE membrane syringe filters to prevent nozzle clogging. The ITO-coated glass slides (2.6 × 0.6 cm2) and Pt button electrodes (A = 0.0314 cm2) were used as substrates. 4.3. Electrochemistry and Spectroelectrochemistry. The electrochemical properties of these polymers were tested by cyclic voltammetry in a three-electrode cell system containing electrolyte solution of 0.1 M TBAPF6/ACN, Pt and Ag wires as counter and pseudoreference electrodes, respectively, and a polymer-coated Pt disk as working electrode. The reference electrode was calibrated versus the ferrocene/ferrocenium redox couple (Fc/Fc+) (E1/2,Fc/Fc+ = 0.77 V). The energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were estimated by the equations40 EHOMO = −(E[onset,ox vs Fc/Fc+] + 5.1) (eV) and ELUMO = (EHOMO + Eg,opt) (eV), where E[onset,ox vs Fc/Fc+] represents the onset potential of the first oxidation peak and Eg,opt was estimated from the onset of longest wavelength absorption. Before the electrochemical stability tests, the P3 film was coated on a Pt electrode, covered by a drop of Nafion solution (5% Nafion in ethanol), and dried naturally to protect the active film; the electrolyte solution was purged with N2 for 20 min. The measurements were done in 0.1 M TBAPF6/PC under a N2 atmosphere. Spectroelectrochemical experiments (SEC) for in situ absorption and switching studies were performed on spray-coated polymer films on ITO glass slides under same conditions in a quartz cuvette. The optical absorption spectra of the films at different applied potentials and their transmittance at given wavelengths were recorded on a UV− vis−NIR spectrophotometer combined with an electrochemical workstation in chronoamperometry and repeating chronoamperometry modes. 4.4. Colorimetric Characteristics. The color of electrochromic films was measured by a colorimeter (CHN Spec, CS-820) with a D65 light source at 10° observer angle and a shutter with a minimum hole (0.3 cm × 0.8 cm) under the Specular Component Included (SCI) mode. The colorimetric values in color space including CIELAB, XYZ, and Yxy were obtained synchronously. 4.5. Device Fabrication. The electrochromic device was fabricated with the following structure: ITO-coated glass/ECP film/ gel electrolyte/ITO-coated glass. The gel electrolyte consisting of 30 g of PC, 30 g of poly(ethylene glycol acrylate) (Mn = 700), 6 g of lithium trifluoromethanesulfonate, and 0.105 g of 2-dimethoxy-2phenylacetophenone (DMPAP) (Mn = 256.3) was made following the reported method.54 A spray-coated P3/ITO/glass sheet was used as working electrode, where 3M double-side tape was stuck on for presealing and solid electrolyte was dropped on. Another ITO/glass sheet as a counter electrode was capped onto the ECP-coated sheet, and the cell was exposed by UV light to solidify the electrolyte. The device was finally sealed by applying the sealant (AG-7103122, Shenzhen Yingtai Technology Co., Ltd.) on the edges. The area of the active material in the device was 30 mm × 30 mm. All device assembly processes were performed in air. The device performances were measured in a two-electrode system in air.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01213. Summary of previous research work on red electrochromic polymers, the details of synthesis and characterization of novel compounds M1−M5, P1−P3, the scan rate dependency tests, illustrations of colorimetric analyses, comparison of test conditions of switching time, device characterizations, SEM images, and DFT calculations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(H.M.) E-mail: [email protected]. *(Y.H.) E-mail: [email protected]. ORCID

Yaowu He: 0000-0003-2887-735X Hong Meng: 0000-0001-5877-359X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Igor F. Perepichka (Bangor University) for helpful discussions and suggestions. This work was financially supported by Shenzhen Science and Technology Research Grant (JCYJ20160331095335232 and JCYJ20160510144254604), National Natural Science Foundation of China (51761145101, 51603003 and 51873002), the Shenzhen Engineering Laboratory(Shenzhen development and reform commission [2016]1592), the Shenzhen Peacock Plan (KQTD2014062714543296), Shenzhen Science and Technology research grant (JCYJ20170818085725139 and JCYJ20170818090312652), the National Basic Research Program of China (973 Program, No. 2015CB856500), and NSFC (21704040).

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REFERENCES

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