Electrochromic materials based on novel polymers containing triphenylamine units and benzo[c][1,2,5]thiadiazole units

Synthetic Metals 259 (2020) 116235

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Electrochromic materials based on novel polymers containing triphenylamine units and benzo[c][1,2,5]thiadiazole units

T

Fanghong Maa, Fuhan Liua, Yanjun Houa,b,*, Haijun Niua,c, Cheng Wanga,c,* a

School of Chemistry and Materials Science, Heilongjiang University, Harbin, 150080, PR China Key Laboratory of Chemical Engineering Process and Technology for High-Efficiency Conversion, College of Heilongjiang Province, Heilongjiang University, Harbin, 150080, PR China c Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People's Republic ofChina, Heilongjiang University, Harbin, 150080, PR China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Electrochromism Triphenylamine Benzo[c][1,2,5]thiadiazole

Six novel electrochromic (EC) polymers containing triphenylamine units and benzo[c][1,2,5]thiadiazole (BTD) units, coded as TPABTA, TPABTD, TPABTD-a, TPABTD-b, TPABTD-c and TPABTD-d, were successfully synthesized through polymer condensation reaction and their electrochromic properties were characterized in detail. The films of these polymers exhibited reversible color changes, high optical transparency, good solubility in many organic solvents and good coloration efficiency. Among them, the TPABTD-b film showed a set of reasonable electrochromic features, including high coloration efficiency (270 cm2·C-1), appropriate transmittance change (23 % at 849 nm) and a relatively longer response time in bleaching process (tb = 3.7 s).

1. Introduction Electrochromism is a phenomenon in which a color is reversibly changed by the direction of an electric field when a voltage is applied. In recent years, electrochemical materials have received considerable attention and become remarkable synthetic materials since the first one was reported [1]. They have been widely used in the electrochromic fields such as organic light-emitting diodes (OLEDs) [2], antiglare rearview mirrors [3], sunglasses [4], smart electrochromic windows [5], and so on. Aromatic polyamides are popular kind of high-performance electrochromic materials as result of their high thermal stability [6], mechanical properties [7]and chemical resistance [8]. Moreover, most aromatic polyamide materials have exhibited huge potential in the field of gas separation [9–11] and memory characteristics [12–16]. However, high melting or glass transition temperatures and insolubility in most solvents make them hard to process, which restrict their application in many fields. It is necessary to modify the structures of aromatic polyamide polymers. Designing and synthesizing specific aromatic polyamides by introducing bulky and packing-disruptive groups into the polymer backbone [17–19] is one of the common approaches to improve the solubility and processability of aromatic polyamides without too much sacrificing their thermal stability. It has been demonstrated that aromatic polyamides containing three dimensional, propeller-shaped triphenylamine (TPA) unit had good morphological ⁎

stability [20], high thermal stability [21] and good solubility [22] in most organic solvents. TPA and its derivatives can be easily oxidized to form stable radical cations, and the oxidation process is always accompanied with a strong change of coloration. Liou, Hsiao and our groups have synthesized lots of aromatic polymers containing TPA unit, and they have been used widely in optoelectronic and electrochemical applications [23–25]. Hence, we designed and synthesized a series of new conjugated polymers containing TPA and benzo[c][1,2,5]thiadiazole groups, coded as TPABTA, TPABTD, TPABTD-a, TPABTD-b, TPABTD-c and TPABTD-d. The polymers were synthesized by direct polyamidation reaction and confirmed by 1H NMR and FT-IR techniques. The general properties of the polymers including thermal, solubility and SEM were investigated. The synthesized polymers showed good thermal stability due to their TPA groups. In addition. Electrochemical properties and spectroelectrochemical properities of the polymers were discussed herein as well. The polymers exhibited good electrochromic properties with high coloration efficiency. 2. Experimental 2.1. Materials and measurements Pyridine (Py), o-phenylenediamine, potassium hydroxide (90 %), mphenylenediamine, p-phenylenediamine, 1-fluoro-4-nitrobenzene (99

Corresponding authors at: School of Chemistry and Materials Science, Heilongjiang University, Harbin, 150080, PR China. E-mail addresses: [email protected] (Y. Hou), [email protected] (C. Wang).

https://doi.org/10.1016/j.synthmet.2019.116235 Received 20 July 2019; Received in revised form 26 October 2019; Accepted 11 November 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.

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2.2.2. Synthesis of TPABTD The synthesis of TPABTD was used as a case to illustrate the synthetic procedure of the five polymers. 1.0 g (1.3 mmol) of 4,4′-(benzo[c][1,2,5]thiadiazole-4,7-diyl)dibenzoic acid (M1), 0.8 g (1.5 mmol) DATPA, 4.0 mL of triphenyl phosphite (TPP), 2.0 mL of pyridine, 0.6 g of CaCl2 and 5 mL of NMP were placed in a 50 mL reaction flask. This mixture was heated at 125 ℃ for 5 h under nitrogen atmosphere. The resulting yellow viscous solution was poured into 300 mL of ice methanol to precipitate fiberlike solid. The pale yellow solid was obtained by reprecipitating the solid from NMP/methanol three times to remove the low molecular weight oligomers and residual monomers. The product was obtained through filtration and purified by a soxhlet extraction with methanol for 24 h and dried 24 h in vacuum. TPABTA, a rufous powder solid. 1H NMR (400 MHz, CDCl3, δ): 8.6 (s, 2 H), 8.14 (m, 6 H), 7.27–6.85 (m, 17 H). 13C NMR (101 MHz, CDCl3, δ) 157.39, 148.37, 143.45, 142.54, 132.42, 132.32, 129.87, 129.33, 129.09, 128.73, 128.61, 128.54, 128.04, 127.08, 123.19, 122.97, 122.30, 122.00, 116.39, 77.52. FT-IR (KBr, cm−1): 3364 (s, υN-H), 2771 (s υC=N) 1693 (vs, υC=C of benzenoid rings), 1585 (vs, υC=C of benzenoid rings), 1490 (vs, υC=C of benzenoid rings), 1309 (s, υC–N), 820 (s, υC-H). Anal. Calcd. for C38H25N5S: C, 78.19; H, 4.32; N, 12.00. Found: C, 78.05; H, 4.04; N, 11.78. TPABTD, a light yellow powder solid. 1H NMR (400 MHz, DMSO-d6, δ) 10.25 (s, 2 H), 8.25 (m, 6 H), 7.7 (m, 4 H) 7.32–6.83 (m, 13 H). 13C NMR (101 MHz, DMSO-d6 δ) 165.05, 159.29, 148.25, 143.60, 135.24, 131.07, 130.70, 130.02, 125.00, 122.90, 122.58, 122.38, 119.10. FT-IR (KBr, cm−1): 3301 (s, υN-H), 1750 (s, υC=O), 1649 (s, υC=C of benzenoid rings), 1579 (s, υC=C of benzenoid rings), 1496 (vs, υC=C of benzenoid rings), 1231 (s, υC–N), 769 (s, υC-H). Anal. Calcd. for C38H25N5O2S: C, 74.13; H, 4.09; N, 11.37. Found: C, 73.93; H, 3.82; N, 11.13. TPABTD-a, a yellow powder solid. 1H NMR (400 MHz, DMSO-d6, δ) 10.24 (s, 2 H), 8.050 (m, 8 H), 7.75 (m, 8 H), 7.03–7.20 (m, 23 H). 13C NMR (101 MHz, DMSO-d6 δ) 165.06, 159.29, 143.60, 135.23, 131.07, 130.76, 130.70, 130.03, 125.01, 122.90, 122.59, 122.38, 121.35, 121.20, 119.10, 115.88. FT-IR (KBr, cm−1): 3301 (s, υN-H), 1750 (s, υC=O), 1649 (s, υC=C of benzenoid rings), 1579 (s, υC=C of benzenoid rings), 1496 (vs, υC=C of benzenoid rings), 1231 (s, υC–N), 769 (s, υC-H). Anal. Calcd. for C64H41N9O4S2: C, 72.23; H, 3.88; N, 11.85. Found: C, 71.99; H, 3.59; N, 11.65. TPABTD-b, a yellow powder solid. 1H NMR (400 MHz, DMSO-d6, δ) 10.13 (s, 2 H), 10.17 (trace residual eCOOH), 8.10 (m, 8 H), 7.75 (m, 8 H), 7.03–7.23 (m, 23 H). 13C NMR (101 MHz, DMSO-d6 δ) 165.12, 164.86, 159.09, 143.40, 139.84, 135.05, 130.86, 130.63, 130.53, 130.02, 129.81, 127.86, 127.06, 126.12, 124.79, 122.70, 122.38, 122.20, 118.92, 118.88, 116.91, 115.78. FT-IR (KBr, cm−1): 3301 (s, υN-H), 1750 (s, υC=O), 1649 (s, υC=C of benzenoid rings), 1579 (s, υC=C of benzenoid rings), 1496 (vs, υC=C of benzenoid rings), 1231 (s, υC–N), 769 (s, υC-H). Anal. Calcd. for C64H41N9O4S2: C, 72.23; H, 3.88; N,

%), anhydrous calcium chloride (CaCl2), potassium carbonate (99 %), ethanol, triphenyl phosphite (TPP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), bromine, methanol, 4-fluorobenzonitrile Hydrobromic acid (40 %), aniline and potassium acetate (90 %) were bought from Tianjin Chemical Factory. Methylbenzene N-methyl2-pyrrolidinone (NMP), tetrabutylammonium perchlorate (TBAP), 4formylphenylboronic acid, hydrazinium hydrate (80 %) and dimethyl sulfoxide (DMSO) were purchased from Aladdin. Palladium (10 wt% on charcoal, noted as Pd/C), tetrakis (triphenylphosphine) palladium(0) and thionyl chloride (SOCl2) were purchased from Beijing Chemical Factory. 1H NMR and 13C NMR spectra were measured on a 400 MHz Bruker AC-400 NMR spectrometer using deuteriochloroform (CDCl3) and deuterated dimethyl sulfoxide (DMSO-d6). The Fourier transform infrared (FT-IR) spectra were measured by a Perkin-Elmer Spectrum 100 Model FT-IR spectrometer. The UV–vis spectra were recorded by a Shimadazu UV-2501spectrophotometer. The thermal stability of six kinds of polymers were recorded by thermogravimetric analysis (TGA) on PerkinElmer Pyris 6 TGA from 25 °C to 700 °C in air atmosphere with a rate of 10 °C min-1. Density-functional theory (DFT) was calculated on SGI Origin 350 server using Gaussian 03 software with the Becke’s three parameter gradient-corrected functional (B3LYP). Elemental analysis were performed by a Vario Miro elemental analyzer. Cyclic voltammetry (CV) were measured by a CHI660A electrochemical workstation in a three-electrode system. In the electrochemical measures, a Ag/ AgCl, KCl (sat.) electrode was used as a reference electrode, the ITO with polymer films (polymer film area is about 2.5 cm × 1 cm) was used as working electrode and a platinum wire was used as an auxiliary electrode, respectively. Electrochemical workstation and UV–vis spectra were used to test the spectroelectrochemical and kinetic properties. 2.2. Synthesis of the polymers DATPA were prepared according to the reported method [[26]]. A series of monomers were synthesized in our laboratory according to the reported method and shown in supporting information. Schemes 1 and 2 showed the synthetic routes of TPABTA, TPABTD, TPABTD-a, TPABTD-b, TPABTD-c and TPABTD-d. 2.2.1. Synthesis of TPABTA A mixture of 20 mL DMAc, 6 mL of toluene and 0.55 g (2 mmol) DATPA were placed in a 50 mL three necked flask with a magnetic stirred. 0.69 g (2 mmol) 4,4′-(benzo[c][1,2,5]thiadiazole-4,7-diyl)dibenzaldehyde (M2) was added into the above-mentioned solution slowly, and the reaction was heated at 140 ℃ for 2 days under nitrogen atmosphere. The obtained yellow polymer solution was cooled and added slowly into 100 mL of methanol, and the precipitate was collected by filtration.

Scheme 1. Synthesis routes of TPABTD and TPABTA. 2

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Scheme 2. Synthesis routes of TPABTD a–d.

Fig. 1. FT-IR spectra (a) and TGA curves of six polymers (b).

11.85. Found: C, 72.01; H, 3.65; N, 11.59. TPABTD-c, a brown powder solid. 1H NMR (400 MHz, DMSO-d6, δ) 10.27 (s, 2 H), 8.03 (m, 6 H), 7.76 (m, 4 H), 7.044–7.25 (m, 22 H). 13C NMR (101 MHz, DMSO-d6 δ) 164.84, 148.04, 143.39, 135.02, 130.61, 130.50, 129.80, 124.787, 122.69, 122.37, 122.17, 118.88. FT-IR (KBr, cm−1): 3301 (s, υN-H), 1750 (s, υC=O), 1649 (s, υC=C of benzenoid rings), 1579 (s, υC=C of benzenoid rings), 1496 (vs, υC=C of benzenoid rings), 1231 (s, υC–N), 769 (s, υC-H). Anal. Calcd. for C64H41N9O4S2: C, 72.23; H, 3.88; N, 11.85. Found: C, 72.14; H, 3.76; N, 11.61. TPABTD-d, a brown powder solid. 1H NMR (400 MHz, DMSO-d6, δ) 10.24 (d, 2 H), 8.05 (m, 10 H), 7.73 (m, 10 H), 7.03–7.23 (m, 25 H). 13C NMR (101 MHz, DMSO-d6 δ) 165.15, 159.08, 153.31, 143.40, 135.02, 132.89, 130.88, 130.64, 130.58, 130.49, 130.04, 129.83, 127.08, 126.66, 124.81, 122.706, 122.16, 121.36, 120.82, 119.69, 118.91, 116.90. FT-IR (KBr, cm−1): 3301 (s, υN-H), 1750 (s, υC=O), 1649 (s, υC=C of benzenoid rings), 1579 (s, υC=C of benzenoid rings), 1496 (vs, υC=C of benzenoid rings), 1231 (s, υC–N), 769 (s, υC-H). Anal. Calcd. for C76H47N11O4S3: C, 71.63; H, 3.72; N, 12.09. Found: C, 71.38; H, 3.50; N, 11.86.

Table 1 Thermal properties of the polymers. Polymer code

T5%a

T10%a

T20%a

Char Yieldb (wt.%)

TPABTA TPABTD TPABTD-a TPABTD-b TPABTD-c TPABTD-d

295 241 206 218 192 148

310 376 327 318 253 248

461 555 484 509 415 397

11.6 36.7 27.5 34.7 31.4 28.7

a The decomposition temperatures (Td) at 5 %, 10 % and 20 % weight losses in air. b Measured at 700 °C in air.

Table 2 Solubility of TPABTA and TPABTDs in organic solvents. Polymer code

TPABTA TPABTD TPABTD-a TPABTD-b TPABTD-c TPABTD-d

Solubility in various solvents

a,b

DMF

DMAC

DMSO

THF

NMP

toluene

++ ++ ++ ++ ++ ++

++ ++ ++ ++ ++ ++

++ ++ ++ ++ ++ ++

+ + + + ---

++ ++ ++ ++ ++ ++

+++----

+ + -

2.3. Preparation of the polymer films 10 mg of the powder sample was dissolved into 3 mL of DMF, and the solution was cast on an ITO glass, followed by drying at 120 °C for 3 h in vacuum to achieve the final film. The remaining polymer films were prepared by a same method.

a Solvents: DMF: N,N-dimethylformamide; DMAc: N,N-dimethylacetamide; DMSO: dimethyl sulfoxide; THF: tetrahydrofuran; NMP: N-methyl-2-pyrrolidone. b The qualitative solubility of polymer was measured with 10 mg of specimen in 1 mL of solvent with stirring. ++, soluble under room temperature; +-, part of soluble under hot temperature; –, insoluble even when heated.

3

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Fig. 2. The SEM graph of films. (a) TPABTA film, (b) TPABTD-a film, (c) TPABTD film, (d) TPABTD-b film, (e) TPABTD-c film and (f) TPABTD-d film.

Fig. 3. Absorption spectra of six polymers in film (a) and in DMF solution (b).

stretching vibrations appeared around 2771 cm−1. The thermal stability of six polymers were evaluated in air atmosphere with a heating rate of 10 °C min−1 and shown in Fig. 1b, and the relevant data are summarized in Table 1. The 5 % weight loss temperatures in air of these conjugated polymers were around 190–300 ℃, and the char yields of the conjugated polymers at 700 ℃ were in the range of 11−36 wt.%. As shown in Fig. 1b, the TPABTDs have worse thermal stability than TPABTA, which may attribute to the negative effect of doping with different diamines in the polymeric main chain. TPABTA have good thermal stability, which is propitious to improve the morphological stability of the spin-coated films and increase the work time in device application.

Table 3 Optical properties of the polymers. Polymers

In solution

TPABTA TPABTD TPABTD-a TPABTD-b TPABTD-c TPABTD-d

294 335 330 328 333 325

a

a

λmax (nm)

As film λmax (nm) 416 435 429 429 428 434

The concentration measured in DMF is 10-5 mol·L-1.

3. Results and discussion

3.2. The solubilities and surface analysis by SEM of the polymers

3.1. Characterization

The solubilities of TPABTA and TPABTD a–d were tested in various solvents, and the results are summarized in Table 2. All of the polymers are well dissolved in polar aprotic solvents (such as NMP, DMAc and DMF). The high solubilities of these polymers are apparently due to the presence of the packing-disruptive TPA unit and electronic absorption properties of benzothiadiazole in the polymer backbone, which resulted in decreased intermolecular interactions. The morphologies and stacking modes of the polymers have important influence on their properties, such as electron transfer speed, stability, and so on. So the morphologies of film surfaces of the polymers were investigated by SEM and the results were shown in Fig. 2. TPABTD, TPABTD-a and TPABTD-

A series of monomers and six kinds of new polymers were synthesized. The structures of monomers were confirmed by 1H NMR (shown in Figs. S2, S4, S6). The structures of the polymers were confirmed via 1 H NMR, 13C NMR and FT-IR techniques. The 1H NMR and 13C NMR data were shown in Figs. S7–S18. Fig. 1 showed the FT-IR spectra (a) and TGA curves of six polymers (b). As shown in Fig. 1a, six polymers showed a similar FT-IR spectra and the NeH stretching vibrations appeared around 3301−3364 cm−1. The anti- and symmetrical stretching vibrations of C]O groups were around 1750 cm−1, the stretching vibration C–N groups were around 1231−1309 cm−1, the C]N 4

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Fig. 4. Cyclic voltammogram of TPABTA and TPABTD a–d in 0.1 M TBAP/MeCN solution at a scan rate of 50 mV·s-1.

TPABTA has good wetting property and film-forming capacity.

d exhibited unsmooth surface with many particles and some aggregates. TPABTD-b and TPABTD-c films displayed smoother surface, but many grains also can be observed. Compared with other polymers, TPABTA showed an excellent smooth surface on the morphology, which accelerates the charge transport. A smooth surface on ITO reveals that

3.3. Optical properties The optical properties of the polymers were investigated by UV–vis 5

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Table 4 Optoelectrochemical properties of the polymer films. Polymer code

EonsetOXEOX onset (V)

EHOMOa (eV)

ELUMOb (eV)

Egc (eV)

λonsetd (nm)

quantun EHOMO (eV)

quantun ELOMO (eV)

Equantun (eV) g

TPABTD TPABTD-a TPABTD-b TPABTD-c TPABTD-d TPABTA

0.72 0.80 0.68 0.70 0.71 0.24

−5.15 −5.23 −5.11 −5.13 −5.14 −4.67

−2.09 −2.15 −1.91 −1.98 −1.95 −1.37

3.06 3.08 3.20 3.15 3.19 3.30

405 403 390 393 389 370

−4.49 −5.25 −5.09 −5.12 −5.25 −4.68

−2.72 −2.64 −2.69 −2.69 −2.80 −2.53

1.77 2.61 2.40 2.43 2.45 2.15

Equntum: theoritical calculation of the TPABTDs. a EHOMO = -e (EOX onset vs Ag/AgCl + 4.43) eV. b ELUMO = EHOMO + Eg. c Eg = 1240/λonset. d UV–vis absorption onset of the polymers in DMF.

Fig. 5. Pictorial representations of the electron density in the frontier molecular orbitals of TPABTA and TPABTD a–d.

Fig. 6. Spectroelectrochemistry of the polymer films (a) TPABTA and (b) TPABTD applying different voltages in 0.1 M TBAP/CH3CN solution.

3.4. Electrochemical properties

absorption spectra at room temperature. Fig. 3 showed the UV–vis absorption spectra of TPABTA and TPABTD a–d in the film states and in DMF solution (10−5 mol·L−1), and the results are summarized in Table 3. As shown in Fig. 3a, the absorption spectra in the film state of these polymers show obvious red shift of ca. 100–110 nm compared with the corresponding spectra in dilute solution (Fig. 3b), which may attributed to the tighter packing of the polymer backbones in the state of thin films and the strong intermolecular interactions. As shown in Fig. 3b, every polymer demonstrates a broad and strong absorption with two absorption peaks at 294 nm and approximately 330 nm. The absorption peaks at 294 nm and around 330 nm of the polymers originate from the centered π-π* transition of TPA.

The electrochemical properties of the polymers were tested by cyclic voltammetry. An ITO conductive glass with polymer films, platinum wire and saturated calomel electrode were used as the working electrodes, counter and reference electrodes, respectively. Fig. 4 showed the cyclic voltammetry (CV) diagrams of TPABTA and TPABTD a–d. There was no obvious oxidation peak in the beginning few cycles for TPABTDa and TPABTD-b, as shown in the CV diagrams, while the electrolyte can gradually diffuse into the polymers films after hundreds of scans and the oxidation peaks became clear. Besides, an enhancement of EC reversibility and stability for the hybrid system could also be observed over 500 cycle scans. Other polymers have similar properties. The CV of 6

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Fig. 7. Dynamic changes of the current and transmittance (a, c, e) and switching time (b, d, f) for TPABTA, TPABTD-b and TPABTD-c (in 0.2 mol·L-1 TBAP/CH3CN) via applying a potential step.

TPABTD-a, TPABTD-b, TPABTD-c, TPABTD-d are 0.72 V, 0.80 V, 0.68 V, 0.69 V and 0.70 V, respectively. The TPABTD-b, TPABTD-c, TPABTD-d showed a lower onset oxidation potentials by doping different types of diamine. Among them, TPABTD-b showed the lowest onset oxidation potential 0.68 V, which may be caused by the planar and conjugated polymer chain.

TPABTD and TPABTD a–d exhibited one oxidation peak at 1.00 V, 1.30 V, 1.17 V, 1.08 V, 1.27 V, respectively. The CV of TPABTA exhibited two oxidation peaks and the oxidation peak at 0.70 V was ascribed to the radical cation (TPA +·) formation according to previous report [27]. The oxidation peak of TPABTA at 1.14 V, may be attributed to the oxidation of benzo[c][1,2,5]thiadiazole (BTD) unit. The initial oxidation potentials (EOX onset ) of the polymers were calculated by the cyclic voltammograms. The onset oxidation potentials of TPABTD, 7

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3.6. Spectroelectrochemical properities

Table 5 Electrochromic properties of the polyamide films. Polymer code

λmaxa(nm)

Δ%T

tbb (s)

tcb (s)

ΔODc

Qd d (mC·cm-2)

CEe (cm2·C-1)

TPABTA TPABTD TPABTD-a TPABTD-b TPABTD-c TPABTD-d

835 848 837 849 848 845

14 5 11 23 13 14

3.2 4.3 4.9 3.7 3.6 3.9

4.4 4.5 4.0 4.2 3.8 4.3

0.19 0.20 0.48 0.36 0.13 0.21

1.35 5.70 5.90 1.33 2.70 4.50

141 35 81 270 99 47

Spectroelectrochemistry studies were performed by electrochemical workstation and UV–vis spectra. During the test, a three-electrode configuration was used and the polymer films drop-coated onto ITO conduvtive glass were immerged in a 0.1 M TBAP/acetonitrile electrolyte solution (Fig. 6). The TPABTA (Fig. 6a) film exhibited a color change from reddish brown (neutral form) to blue (oxidized state) upon applying voltage 1.6 V. The neutral absorptions gradually decreased in intensity, and two new peaks appeared at 497 nm and 898 nm, respectively, which can be attributed to the production of a monocation radical of the atomnitrogen in TPA unit. The TPABTD film exhibited a color change from yellow (neutral form) to green (oxidized state) upon applying voltage, where the neutral absorptions gradually decreased in intensity, and two new peaks appeared at 613 nm and 848 nm, respectively. The polymer films of TPABTD a–d have similar electrochemical characteristics with TPABTD and shown in Fig. S19.

a

Maximum absorption wavelength of TPABTD and TPABTDs. Time for 90 % of the full-transmittance change. c Optical density (ΔOD) = log [Tbleached/Tcolored]. Tbleached is the maximum transmittance in the neutral states and Tcolored is the maximum transmittance in the oxidized states. d Qd is an ejection charge that tested by experiments. e Coloration efficiency (CE) = ΔOD/Qd. b

3.5. Quantum chemistry calculation

3.7. Electrochromic switching

Density functional theory calculations were calculated on the molecular structures using Gaussian 03 software with the Becke’s three parameter gradient-corrected functional (B3LYP). The LUMO and HOMO energy levels are estimated according to the equations:

Electrochromic switching studies for the polymer films were performed to monitor the percent transmittance changes (Δ%T) as a function of time at their absorption maximum and to determine the response time by stepping potential repeatedly between their neutral and oxidized states. While the films were switched, the absorbance at selected wavelengths was monitored as a function of time with UV–vis spectrometer. Switching data for TPABTA, TPABTD-b and TPABTD-c were studied by applying potentials between 0 V and 1.6 V with a switching interval of 10 s, as shown in Fig. 7 (others were shown in Fig. S20). The switching time was calculated at 90 % of the full switch. The coloring time of TPABTA is 4.4 s, and the bleaching time is 3.2 s at 898 nm (as shown in Fig. 7b). The coloring time of TPABTD-b is 4.2 s, and the bleaching time is 3.7 s at 849 nm (as shown in Fig. 7d). The coloring time of TPABTD-c is 3.8 s, and the bleaching time is 3.6 s at 845 nm (as shown in Fig. 7f). Comparing with the polymer TPA-PMPI (color switching for 9.2 s and bleaching for 2.2 s at 424 nm, color switching for 4.1 s and bleaching for 1.4 s at 740 nm) in the literature [28], TPABTD-c exhibits shorter coloring response time. By comparing with the polymer BCT-1 (color switching for 3.89 s and bleaching for 3.71 s at 778 nm) in the paper of our group [29], TPABTD-c exhibits better bleaching response time and the coloring response time by doping o-phenylenediamine. This phenomenon may attribute to high internal resistance and relatively difficult ion permeation. The optical contrast measured as T% of these polymers between neutral and oxidized state was found to be 14 % for TPABTA, 23 % for TPABTD-b and 13 % for TPABTD-c, respectively. Table 5 shows the optical contrasts of

ELUMO = EHOMO + Eg

(1)

EHOMO = − e (EOX onset vs Ag/ AgCl + 4.43) eV

(2)

In order to obtain the accurate redox potential, assuming that the HOMO energy level for the ferrocene/ferrocenium (Fc/Fc+) standard is 4.80 eV with respect to the zero vacuum level. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the investigated polymers were estimated from the EOX onset values, which are summarized in Table 4. Fig. 5 showed the electron cloud distribution in HOMO and LUMO state. The electrons of the highest occupied molecular orbital (HOMO) states were localized at the TPA group and the electrons of the lowest unoccupied molecular orbital (LUMO) states were localized at the BTD group. Among these oligomers, the experimental trend of Eg tend to E(TPABTA) > E(TPABTDHowever, in b) > E(TPABTD-d) > E(TPABTD-c) > E(TPABTD-a) > E(TPABTD). theoretical data, E(TPABTD-a) > E(TPABTD-d) > E(TPABTD-c) > E(TPABTDb) > E(TPABTA) > E(TPABTD). The slight deviation may be resulted from the influence of solvent and electrolyte. In addition, theoretical data came from the structures of the oligomers rather than polymer chains, and the adjacent benzene rings also play important roles in the result.

Fig. 8. Proposed simplified redox process and the resonance form of TPABTD and TPABTD-b. 8

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the polymer films. The coloration efficiency (CE: η) can be calculated using the related equations given below: ΔOD = lgTb/Tc

(3)

η=ΔOD/Q

(4)

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The coloration efficiencies of TPABTD-b, TPABTA and TPABTD-c are 270 cm2·C-1, 141 cm2·C-1 and 99.1 cm2·C-1, respectively (Table 5). By comparing with polymers BCT - 1 (CE = 266.8 cm2·C-1) and TPAPMPI (CE = 127 cm2·C-1 at 424 nm, CE = 172 cm2·C-1 at 740 nm), TPABTD-b features a highest CE that up to 270 cm2·C-1 at 849 nm due to its high transmittance at oxidized states as shown in Fig. 7(c). This means that TPABTD-b can change color easily. According to these results, the mechanisms of oxidation reactions for TPABTD and TPABTDb are proposed in Fig. 8. The mechanisms of other poymers are in the same way. 4. Conclusions In conclusion, a series of new conjugated polymers bearing TPA units and BTD groups have been successfully synthesized via direct polycondensation reactions. The structures of the polymers were confirmed by 1H NMR and FT-IR. The resulting polymers exhibited good solubility in many organic solvents due to the bulky triarylamine units and BTD groups. The cyclic voltammograms of all the new polymer films displayed stable color changes from electro-oxidation to reversible redox processes. Furthermore, these conjugated polymer films also displayed unique electrochromic feature, from their colourless neutral states to colours in related oxidation forms. In a comparison with BCT 1, TPABTD-c exhibits better bleaching response time and coloring response time by doping o-phenylenediamine. The TPABTD-b film features a better reasonable optical contrast ΔTmax = 23 % at 849 nm, a better coloration efficiency of of 270 cm2·C-1 during bleaching process than the polymers BCT - 1 and TPA-PMPI. Based on these results, these novel polymers may be have great potential in optoelectronics applications as electrochromic materials. Declaration of Competing Interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Acknowledgements This work was financially supported by the National Science Foundation of china [grant numbers 51773053, 51973051] and the Natural Science Foundation of Heilongjiang Province of china [grant number B2017010]. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.synthmet.2019. 116235. References [1] JungTsu Wu, Y.Z. Fan, G.S. Liou, Synthesis, characterization and electrochromic properties of novel redox triarylamine-based aromatic polyethers with methoxy protecting groups, Polym. Chem. 10 (2019) 345–350. [2] X.Y. Du, J.W. Zhao, W. Liu, K. Wang, S.L. Yuan, C.J. Zheng, H. Lin, S.L. Tao, X.H. Zhang, Bromine-substituted triphenylamine derivatives with improved holemobility for highly efficient green phosphorescent OLEDs with a low operating

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