High coloration efficiency and fast switching speed of poly(amic acid-imide)s containing triphenylamine in acidic electrolyte

Abstract

Two novel poly(amic acid-imide)s (PAA-IMs) were prepared from two electroactive diamines, 4,4′-diamino-4′′-methoxytriphenylamine and 4,4′-diamino-4′′-phenoxytriphenylamine, and pyromellitic dianhydride (PMDA) by conventional polycondensation, followed by partial imidization at 150 °C. The resulting PAA-IMs had significant levels of thermal stability associated with 2% weight-loss temperatures between 195 and 370 °C. Flexible PAA-IM films with light color showed excellent adhesion on the surface of an indium–tin oxide (ITO)-coated glass substrate. The electrochromic properties were examined by electrochemical and spectroelectrochemical methods. Cyclic voltammograms of the PAA-IM films cast onto ITO substrate exhibited a reversible oxidation at 1.26–1.35 V vs. Ag/AgCl in acid electrolyte consisting of acetonitrile and 4-toluene sulfonic acid (MBSA), revealed excellent stability of electrochromic characteristics with a color change from neutral yellowish pale form to the green or blue oxidized form at applied potentials ranging from −0.5 to 1.8 V. The anodically electrochromic PAA-IM films not only showed excellent reversible electrochromic stability with extremely high coloration efficiency (CE = 834 and 523 cm² C⁻¹) but also exhibited fast switching speed which required 1.7 to 3.8 s for color switching and 0.8 to 1.6 s for bleaching. After over 30 cyclic switches, the PAA-IMs films still exhibited stable electrochemical and electrochromic characteristics.

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Introduction

Electrochromic (EC) materials exhibit a reversible optical change in absorption or transmittance upon electrochemical oxidation or reduction and can be exploited for E-paper, optical switching devices, smart windows, and camouflage materials.^1–3 Triphenylamine (TPA) derivatives, including small molecules and polymers, are well-known as their photo- and electroactive properties have potential for optoelectronic applications, such as photoconductors, hole-transporters, light emitters, and memory devices.^4–7 Electron-rich triarylamines can be easily oxidized to form stable radical cations, and the oxidation process is always associated with a noticeable change of coloration.^8–10 In recent years, TPA-based EC polymers, for instance, polyimide, polyamide, epoxy resin and poly(azomethine), have been reported as a new and attractive family of electrochromic materials because of the high coloration efficiency, fast response time, and multicolored electrochromism of the TPA unit and high thermal stability of the polymer backbone.^11–15

The electrochromism of EC polymers related to the doping–dedoping process, the doping process modifies the polymer electronic structure, producing new electronic states in the band gap, causing color changes.^16,17 During electrochemical doping–dedoping, the polymers perform the oxidation–reduction of the polymer backbone resulting in dimensional changes.¹⁸ Generally, the substrate is an inorganic material (e.g., ITO) to deposit organic polymer films with significant differences exist between their natures. As a result, the polymers are deposited on substrates only through physical adsorption and this kind of adhesion is weak, which is resulting in peeling of polymer film from the substrate and breakdown of electrodes in the devices.¹⁹ In conducting polymer electrochromic electrodes, decay in performances is inevitable during repeated doping–dedoping processes and the poor adhesion of conducting polymer to the substrate is a key factor in the poor stability.²⁰ In the past few years, much work has been done on the improvement of adhesion of films to ITO substrate. One of the simple ways is to prepare copolymer containing carboxyl groups to improve adhesion, such as styrenesulfonate of polyaniline/acrylic copolymers.²¹ TiO₂ nanoparticle buffer layer imparted excellent adhesion between the P3MT polymer and the substrate, which prevented the electrode from delaminating during repeated stress. Increased adhesion promoted the long-term stability of the P3MT–TiO₂–ITO nanocomposite electrodes.²⁰

In this study, partially imidized poly(amic acid)s (PAAs) containing TPA unit were prepared by imidization at moderate temperature. The resulting poly (amic acid-imide)s (PAA-IMs) were expected to improve the adhesion between PAA-IMs and ITO substrate. Consequently, PAA-IMs exhibited excellent adhesion to substrate and prevented the film to peel off the substrate. However, the PAA-IMs in common supporting electrolyte tetrabutylammonium perchlorate (TBAP) as supporting electrolyte showed low and irregular optical contrasts, while the PAA-IMs in unexpected p-toluenesulfonic acid exhibited excellent electrochemical stability and high coloration efficiency and fast switching speed.

Experimental section

Materials and measurements

4,4′-diamino-4′′-methoxytriphenylamine (mp: 150–152 °C) was synthesized according to the reported procedure.²² 4-Fluoronitrobenzene, hydrazine monohydrate (80%), palladium on charcoal (Pd/C), cesium fluoride (99%) and 4-phenoxyaniline (99%) were purchased from Sigma Aldrich. Pyromellitic dianhydride (PMDA, 98%), N,N-dimethylformamide (DMF, 99%), N,N-dimethylacetamide (DMAc, 99%) and acetonitrile (HPLC grade) came from TCI. ¹H nuclear magnetic resonance (NMR) spectra was obtained on a Bruker AC-400 MHz spectrometer in DMSO-d₆ or CDCl₃-d₆, using tetramethylsilane as an internal reference at room temperature. FT-IR spectra was recorded on a PerkinElmer Spectrum 100 Model FT-IR spectrometer. Thermogravimetric analysis (TGA) was performed on PerkinElmer Pyris 6 TGA at a heating rate of 10 °C min⁻¹. UV-vis spectra was measured with Shimadazu UV-3600 connected to a computer. CV measurements were recorded on CH Instruments 660A electrochemical analyzer at a scan rate of 50 mV s⁻¹.

The electrochemical properties of the PAA-IMs were investigated by cyclic voltammetry (CV) technique under nitrogen atmosphere. The cast films was conducted on an ITO-coated glass substrate as working electrode and saturated Ag/AgCl (218-type, saturated solution of KCl) as the reference electrode. The supporting electrolyte was dry acetonitrile (CH₃CN) containing 0.1 M of TBAP or MBSA (p-CH₃C₆H₄SO₃H).

Synthesis of monomers

Synthesis of 4,4′-dinitro-4′′-phenoxytriphenyla mine. Mixtures of 4-fluoronitrobenzene (2.85 g, 20 mmol), 4-phenoxyaniline (1.84 g, 10 mmol), cesium fluoride (3.80 g, 25 mmol) were dissolved in DMSO (25 mL) and were stirred at 120 °C for 24 h under nitrogen. After cooling to room temperature, the mixture was poured into 150 mL/150 mL methanol/water. The obtained yellow precipitate was filtered, washed with water and dried to obtain the crude production. The crude production was purified by recrystallization from DMF/methanol to obtain 3.8 g (89% in yield) of yellow crystals. Mp: 170 °C (by DSC). IR (KBr): 1579 cm⁻¹, 1305 cm⁻¹ (–NO₂ stretch). Anal. calcd (%) for C₂₄H₁₇N₃O₅ (427.42): C, 67.44; H, 3.98; N, 9.84. Found: C, 67.47; H, 3.84; N, 9.78.

Synthesis of 4,4′-diamino-4′′-phenoxytriphenylamine. A mixture of 4,4′-dinitro-4′′-phenoxytriphenylamine (4.27 g, 10 mmol), ethanol (100 mL), Pd/C (0.05 g, 10%) was refluxed at 80 °C under nitrogen. 4 mL hydrazine monohydrate was added slowly to the reaction mixture and reacted for 10 h. The resulting solution was filtered to remove Pd/C, and the filtrate was cooled to obtain precipitate. The product was collected by filtration and dried in vacuum at 70 °C to give 2.83 g (77% in yield) of light-gray needles. Mp: 185 °C (by DSC). IR (KBr): 3459, 3329 cm⁻¹ (–NH₂ stretch). ¹H NMR (DMSO-d₆, δ, ppm): 4.93 (–NH₂, 4H), 6.54 (g, 4H), 6.65 (d, 2H), 6.79 (e/f, 4H), 7.02 (a, 1H), 7.32 (b, 4H). Anal. calcd (%) for C₂₄H₂₁N₃O (367.45): C, 78.47; H, 5.72; N, 11.44. Found: C, 78.35; H, 5.66; N, 11.47.

Synthesis of PAA-IMs. The PAA-IMs were prepared from PMDA with an equimolar 4,4′-diamino-4′′-methoxytriphenylamine and 4,4′-diamino-4′′-phenoxytriphenylamine by conventional two-step method. A typical example for the preparation of PAA-IM-1 is given. 4-Methoxy-4′,4′′-diaminotriphenylamine (0.305 g, 1.0 mmol) was dissolved in 5 mL of DMF in a 25 mL three-necked flask and then dianhydride PMDA (0.218 g, 1.0 mmol) was added to the diamine solution. The solution was stirred at 5 °C for 24 h to yield a viscous PAA-1 solution. The resulting PAA-1 solution was poured slowly into an excess of methanol giving rise to a precipitate which was collected by filtration, washed thoroughly with methanol, and dried at 50 °C under vacuum. The inherent viscosity of the poly (amic acid) was 1.08 dL g⁻¹, measured in DMAc at a concentration of 0.5 dL g⁻¹ at 30 °C. IR (KBr): 2900–3300 cm⁻¹ (–COOH and NH₂), 1710 cm⁻¹ (C=O in COOH), 1660 cm⁻¹ (C=O in CONH), 1550 cm⁻¹ (C–NH). The PAA-2 with inherent viscosity of 0.92 was prepared by an analogous procedure.

For the thermal imidization method, the PAA-1 solution was cast on a glass slide, which was baked at 60 °C under vacuum to the remove the casting solvent. The dried PAA-1 film was converted to the PAA-IM-1 film by heating at 150 °C for 1 h under nitrogen. The IR spectrum of PAA-IM-1film exhibited characteristic imide absorption bands at 1780 cm⁻¹ (asymmetrical C=O stretch), and 1380 cm⁻¹ (C–N stretch). The PAA-IM-2 film was prepared by an analogous procedure.

Results and discussion

Synthesis of monomer and PAA-IMs

The monomer and PAA-IMs were synthesized by the synthetic route outlined in Scheme 1. 4-Phenoxy-4′,4′′-diaminophenylamine (mp = 185 °C) was prepared through the CsF mediated nucleophilic displacement reaction of 4-phenoxyaniline with 4-fluoronitrobenzene, followed by hydrazine Pd/C catalyzed reduction. The synthetic routines are outlined in Scheme 1.

Scheme 1 Synthesis routes of monomer and PAA-IMs.

Element analysis, IR, and ¹H spectroscopic techniques were used to identify the structures of intermediate compound (4,4′-dinitro-4′′-phenoxytriphenylamine) and 4,4′-diamino-4′′-phenoxytriphenylamine. The transformation from nitro to amino groups could be monitored by the changes in the IR spectra (Fig. 1). The nitro groups of the intermediate compound appeared two characteristic bands at around 1579 cm⁻¹ and 1305 cm⁻¹ (–NO₂ asymmetric and symmetric stretching). After reduction, the characteristic absorptions of the nitro group disappeared and the amino group showed the typical N–H stretching absorption pair in the region of 3329–3459 cm⁻¹. Fig. 2 illustrates the ¹H NMR spectrum of 4,4′-diamino-4′′-phenoxytriphenylamine. Assignments of each proton are also given in the figure, and agree well with the proposed molecular structure. The ¹H NMR spectra confirms that the nitro groups had been completely transformed to amino groups by the high field shift of the aromatic protons and the resonance signals at around 4.93 ppm corresponding to the amino protons.

Fig. 1 IR spectra of dinitro intermediate compound and diamine monomer.

Fig. 2 ¹H NMR spectrum of diamine monomer 2.

PAA-IMs were prepared in conventional two-step method by the polycondensation reaction of 4-methoxy-4′,4′′-diaminophenylamine and 4-phenoxy-4′,4′′-diaminophenylamine with PMDA to form PAAs, followed by thermal imidization. The PAA precursors have inherent viscosity of 0.92 ∼ 1.08 dL g⁻¹. The PAA solutions were cast on a glass slide and dried to form solid films. The thermal conversions to PAA-IMs were carried out by successive heating the PAA films at 130, 150, and 200 °C for 1 h, respectively. Because of the imidization at moderate temperature, the partially imdized polymer is composed of amic acid and imide, called PAA-IM. The structures of PAA-IMs were confirmed by IR spectroscopy (Fig. 3). PAA-IM-1 and PAA-IM-2 showed the characteristic absorbance bands of PAA.

Fig. 3 FTIR spectra of PAA-IMs in different temperatures.

Imidization degree

Fig. 3 shows the FTIR spectra of PAA-IMs in different temperatures. The imidization of PAA-1 was carried out by heating of the sample at 100, 150 and 200 °C for 0.5 h and then at 300 °C for 1 h to ensure a complete imidization. The PAA-IMs are confirmed by the occurrence of the C=O stretch peak at 1780 cm⁻¹ and the C=C stretching of the p-substituted benzene backbone peak at 1500 cm⁻¹, and the typical C–N stretch peak at 1380 cm⁻¹. The spectra exhibit the typical absorption bands of PAA-1 and PI after imidization at 130, 150 and 200 °C, respectively.

Imidization degree (ID) of PAA-IM sample was determined according to a reported method,²³ in which the band of 1380 cm⁻¹ (stretching vibration of C–N) was selected for quantifying ID, and the aromatic band at 1500 cm⁻¹ (C=C stretching of the p-substituted benzene backbone) was selected as the internal standard. ID was calculated using the following equation:

ID% = (A₁₃₈₀/A₁₅₀₀)_T/(A₁₃₈₀/A₁₅₀₀)_{T=300 °C} (1)

where A is the normalized peak height of absorption band, T is the cured temperature of the samples, T = 300 °C is taken as the temperature of completely imidized PI. All the experimental IDs are calculated according to the eqn (1) and summarized in Table 1, in which it can be seen that IDs increase with the increasing of imidization temperatures.

Table 1 Imidization degree and thermal stability of PAA-IM-1 and PAA-IM-2

PAA-IMs		Imidization degree (%)	Temperature/°C (weight-loss 2%)
PAA-IM-1	130 °C	61	262
	150 °C	69	370
	200 °C	73	433
PAA-IM-2	130 °C	51	196
	150 °C	60	250
	200 °C	71	402

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Thermal stability of PAA-IMs

The thermal properties of PAA-IMs were investigated by TGA in nitrogen. Typical TGA curves for PAA-IM-1 and PAA-IM-2 imidized at 150 °C are reproduced in Fig. 4, and the thermal behavior data of PAA-IMs for different imidization temperatures are summarized in Table 1. The initiate decomposition temperatures of PAA-IM-1 and PAA-IM-2 were recorded at 255 °C and 195 °C respectively. The temperature section of 2% weight loss was observed at 255–370 °C for PAA-IM-1 and 195–250 °C for PAA-IM-2. In this range, the weight loss was associated with the elimination of water generated by imidization. Comparison of the thermal behaviors of PAA-IMs showed that the thermal stability of PAA-IM-1 was better than PAA-IM-2, which was attributed to the higher ID of the former. In consideration of thermal stability and film color, 150 °C was set as the imidization temperature for PAA-IMs throughout electrochemical and spectroelectrochemical studies.

Fig. 4 TGA curves of PAA-IM-1 and PAA-IM-2 with a heating rate of 10 °C min⁻¹ in nitrogen.

Electrochemical properties of PAA-IMs

The CV diagrams of PAA-IMs were shown in Fig. 5. The PAA-IM-1 (Fig. 5(A)) and PAA-IM-2 (Fig. 5(B)) in TBAP/CH₃CN electrolyte exhibited the oxidation potentials of 1.01 V and 1.10 V, respectively, corresponding to TPA oxidation. During the procedure of electrochemical oxidation, the color of PAA-IM films changed from light yellow to blue for PAA-IM-1 and from light yellow to green for PAA-IM-2. As expected, PAA-IM-1 exhibited a lower oxidation potential of 1.01 V than PAA-IM-2 of 1.10 V because methoxy substituents is a more electron-donating group than phenoxy substituent group on phenyl ring of the TPA unit. Fig. 5(C) and (D) showed the CV diagrams of PAA-IM-1 and PAA-IM-2 in MBSA/CH₃CN electrolyte. These exhibited the oxidation potentials of 1.26 V and 1.35 V, respectively. Compared to common TBAP supporting electrolyte for triarylamine derivatives, oxidation potentials of PAA-IM-1 and PAA-IM-2 in MBSA are higher, respectively. It was attributed to the formation of hydrogen bonding between MBSA and PAA-IMs. The possible redox behavior of PAA-IM in MBSA is proposed in Scheme 2. The hydrogen bonding changes the electronic structure of triarylamine to higher oxidation potentials of PAA-IMs.

Fig. 5 (A) and (B) Cyclic voltammograms of PAA-IMs films at 50 mV s⁻¹ in 0.1 mol L⁻¹ TBAP/CH₃CN electrolyte; (C) and (D) cyclic voltammograms of PAA-IMs films at 50 mV s⁻¹ in 0.1 mol L⁻¹ MBSA/CH₃CN electrolyte.

Scheme 2 Postulated redox behavior of PAA-IMs in MBSA/CH₃CN electrolyte.

Because of the stability of the films and their excellent adhesion on the surface of ITO substrate, both PAA-IM-1 and PAA-IM-2 exhibited excellent reversibility of electrochromic characteristics during fifty repeated cyclic scans, which should be attributed to the formation hydrogen bonding between carboxyl groups of PAA-IMs and hydrogen groups on ITO surface. For comparison, the PI-1 film prepared based on PAA-1 by imidization at 300 °C began to peel off the ITO substrate after a few cycles.

Electrochromism of the PAA-IM films was monitored by a UV-vis spectrometer at different applied potentials. The electrode preparations and solution conditions were identical to those used in cyclic voltammetry. The absorption spectral changes of PAA-IMs are shown in Fig. 6. When the applied potentials increased positively from 0 to 1.35 V, the peak of characteristic absorbance at 318 nm of neutral form PAA-IM-1 decreased gradually while new band grew up at 385 nm, and the color of the film changed to blue as shown in Fig. 6(A). The spectral changes were clearly due to the formation of the cationic states of PAA-IM-1. Similar spectroelectrochemistry result for the PAA-IM-2 was included in Fig. 6(B). Onset of the optical absorption wavelength (λ_onset), optical band gap (E_g), HOMO and LUMO energy levels of the PAA-IMs were clearly exhibited in Table 2.

Fig. 6 Electrochromic behavior of PAA-IM-1 (A) and PAA-IM-2 (B) in CH₃CN with 0.1 M MBSA as the supporting electrolyte.

Table 2 Electrochemical properties of the PAA-IMs

Code	λonset/nm	EHOMO/eV	ELUMO/eV	Eg/eV
PAA-IM-1	385	−5.507	−2.2862	3.2208
PAA-IM-2	393	−5.6803	−2.5251	3.1552

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Spectroelectrochemical and electrochromic properties

Optical switching studies were investigated to manifest the outstanding electrochromic characteristics of these obtained novel anodically electrochromic materials. The switching time can be defined as the time that required for reach 90% of the full change in absorbance after switching potential because it is difficult to perceive any further color change with naked eye beyond this point.^24,25 For optical switching studies, PAA-IM films were cast on ITO-coated glass in the same manner as described above, and for each film the potential was stepped between its neutral and oxidized state. Although the films were switched, the absorbance at the given wavelength was monitored as a function of time with UV-vis spectroscopy. Switching data of the PAA-IM films are shown in Fig. 7. As depicted in Fig. 7(A), PAA-IM-1 film revealed a switching time of 1.7 s between −0.5 and 1.3 V for color change procedure at 385 nm and 0.8 s for bleaching. The magnification of the switching peak of (A) demonstrated the procedure of optical switching on the right side. PAA-IM-2 film was found to be a switching time of 3.8 s between −0.3 and 1.4 V for color change procedure at 393 nm and 1.6 s for bleaching (Fig. 7(B)).

Fig. 7 (a) Current consumption and (b) electrochromic switching response of the cast films of PAA-IM (A) 1 and (B) 2 on the ITO-glass slide (active area 1 cm²) between 0 and 1.1, 1.2 V, respectively, in a 0.1 mol L⁻¹ MBSA/MeCN, 10 s per cycle; (C) PAA-IM-1 in a 0.1 mol L⁻¹ TBAP/CH₃CN).

It is obvious that the switching response of PAA-IM-1 was faster than PAA-IM-2. After 50 cyclic scans between neutral and oxidized state, the PAA-IM films still exhibited excellent electrochemical and electrochromic stability. However, it can be seen in Fig. 7(C), after continuous scans between 0 and 1.0 V at 404 nm in dry acetonitrile (CH₃CN) containing 0.1 M of tetrabutylammonium perchlorate (TBAP) as the supporting electrolyte, the PAA-IM-1 film on an ITO-coated glass exhibited low and irregular optical contrasts. The less stability might be attributable to an opaque product formation between carboxyl groups on PAA-IM-1 and TBAP. The electrochromic coloration efficiency (CE) was also an important characteristic to measure the power requirements for the electrochromic materials.²⁶ It has the definition of the change in the optical density (δ_OD) for the charge consumed per unit electrode area (Q). Derived from the Beer–Lambertlaw, it is given by the equation below:^27,28

δ_OD = lg(T_b/T_c) and η = δ_OD/Q (2)

T_b and T_c are the transmittances before and after coloration, respectively. δ_OD is the change of the optical density, which is proportional to the amount of created color centers. Q (mC cm⁻²) is the amount of injected/ejected charge per unit sample area. On the basis of the eqn (2), the CE of PAA-IM-1 was calculated to be 834 cm² C⁻¹ (385 nm) and PAA-IM-2 was measured at 523 cm² C⁻¹ (393 nm). The parameters of PAA-IMs were presented in Table 3.

Table 3 Optical and electrochemical data collected for coloration efficiency measurements of polymers

Polymer^a	λ (nm)^b	δOD^c	Q (mC cm^−2)^d	η (cm^2 C^−1)^e
a Switching between 0 and 1.20 for PAA-IMs (V vs. Ag/AgCl).b Given wavelength where the data was determined.c Optical density change at the given wavelength.d Ejected charge is determined from the in situ experiments.e Coloration efficiency is derived from the equation η = δOD/Q.
PAA-IM-1	385	0.2132	0.178	834
PAA-IM-2	393	0.5822	0.3049	523

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Conclusions

Two novel PAA-IMs were prepared by the polycondensation reactions of 4,4′-diamino-4′′-methoxytriphenylamine and 4,4′-diamino-4′′-phenoxytriphenylamine with PMDA, followed by partial imidization at moderate temperatures. Because of the presence of carboxyl groups, the PAA-IM films have excellent adhesion to the ITO substrate during electrochemical redox procedure. In acid electrolyte consisting of acetonitrile and MBSA (p-CH₃C₆H₄SO₃H), the PAA-IM films showed excellent reversible electrochromic stability, very high coloration efficiency and short switching time. Thus, the PAA-IMs could be well candidates as anodic electrochromic materials.

Acknowledgements

The authors are grateful to the support of the National Science Foundation of China (Grant no. 21372067, 51373049), Doctoral Fund of Ministry of Education of China (20132301110001 and 20132301120004).

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