Synthesis and electrochromic properties of benzonitriles with various chemical structures

Dyes and Pigments 171 (2019) 107783

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Synthesis and electrochromic properties of benzonitriles with various chemical structures

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Xin-cen Lin, Nan Li, Wei-jing Zhang, Zhen-jie Huang, Qian Tang, Chengbin Gong*, Xiang-kai Fu The Key Laboratory of Applied Chemistry of Chongqing Municipality, Chongqing Key Laboratory of Soft-Matter Material Chemistry and Function Manufacturing, College of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715, China

ARTICLE INFO

ABSTRACT

Keywords: Benzonitriles Electrochromic materials High coloration efficiency Low driving voltage Stable switching property

In this study, the electrochromic behavior of benzonitrile compounds were investigated. For this, a series of benzonitrile compounds with different chemical structures were synthesized and their electrochemical properties were investigated by cyclic voltammetry. Electrochromic behavior of the benzonitrile derivatives were also investigated by constructing sandwich type electrochromic devices and recording the changes in the UV–vis spectra as a function of applied potential. The compounds exhibited excellent electrochromic properties such as a high optical contrast, low driving voltage, good switching stability, high coloration efficiency, and a fast response time. All five compounds had different colors (orange, yellow-green, reddish-brown, green, blue) and driving voltages that were highly dependent on their chemical structures. The results indicate that benzonitriles are good electrochromic materials and should be of interest for applications such as electrochromic smart windows, information displays, and optical storage devices.

1. Introduction Color-changing materials that are responsive to specific stimuli such as temperature (thermochromic materials), light irradiation (photochromic materials), and electric fields (electrochromic materials, ECMs) have gained considerable research interest [1–5]. In particular, ECMs have attracted significant attention because they can produce reversible and long-lasting changes in their color and transparency through redox reactions at suitable applied voltages. They have the advantages of a broad working spectral range, low energy consumption, durability, and high controllability [6–9]. Since the first observation of electrochromism in dyes by Platt in 1961 [10], ECMs have generated significant research interest and gradually been widely used in such applications as electronic paper [11], electrochromic smart windows [12–14], information display [15,16], military security equipment [17], sunglasses [18], and anti-glare rearview mirrors [19]. The chemical properties of ECMs are mainly divided into inorganic and organic materials. Inorganic ECMs are mostly composed of transition metal oxides (such as W, Mo, V, Ti, and Ni) [20–22] or Prussian blue [23]. Compared with inorganic ECMs, organic ECMs have numerous advantages [8,24,25] such as synthetic flexibility, high contrast, rich color assortment, simple device preparation, and fast response time, resulting in them being the most widely applied type of electrochromic material. The active electrochromic moieties in organic ECMs generally contain *

triphenylamine [26–28], anthracene [29,30], thiophene [24,31], linear [32–36] and star-shaped [7,37–39] pyridinium salts, or benzoate esters [25,38,40]. For the current generation of ECMs, intense research work has been devoted to improve their electrochromic properties and widening their fields of application [5]; simultaneously, the development of new ECMs is another major field of interest. Benzonitriles contain a cyano group (-CN) that is conjugated with benzene. The triple bond between the carbon and nitrogen atoms in the cyano group results in it having a relatively high stability and being generally chemically unreactive. Because of their unique properties, nitrile compounds have been extensively used in perfumes, pharmaceuticals, pesticides, dyes, functional materials, and organic conductors [41–45]. However, to the best of our knowledge, they have rarely been used as ECMs. In this work, we designed and synthesized a series of benzonitriles 1–5 with different molecular structures (Scheme 1) and explored their application as ECMs. Their electrochemical and electrochromic properties were characterized by cyclic voltammetry and UV–Vis spectroscopy. The compounds exhibited good electrochromic properties including multiple colors, satisfactory optical contrast, conversion stability, high coloration efficiency, and fast response time. These unique features expand their suitability to applications such as smart windows, displays, sunglasses.

Corresponding author. E-mail address: [email protected] (C. Gong).

https://doi.org/10.1016/j.dyepig.2019.107783 Received 27 May 2019; Received in revised form 3 August 2019; Accepted 7 August 2019 Available online 08 August 2019 0143-7208/ © 2019 Elsevier Ltd. All rights reserved.

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(Konica Minolta, Inc., Japan) [38]. 2.2. Synthesis of compounds 1–5 Compounds 1–5 were synthesized by previously reported methods [46–49] with appropriate modifications. Detailed information on all the syntheses and characterization are listed in the supplemental information. 2.3. ECDs construction Sandwich type ECDs were constructed according to methods reported in the literature [7,8,40]. All the ECDs based on compounds 1–5 were assembled at room temperature in a glove box equipped with nitrogen gas. For this, glass coated with indium tin oxide (ITO) and a reflective metal surface (50 Ω/sq) (20 mm wide, 20 mm long, 0.035 mm thick; 50 cm2) were placed in parallel with each other and used as two electrodes. The electrolyte solution was prepared by dissolving compounds 1, 2, 4, or 5, (30 mmol L−1), ferrocene (45 mmol L−1), and tetrabutylammonium perchlorate (TBAP, 45 mmol L−1) in DMF. Here, ferrocene was used as a counter electrode material to stabilize the ECD [36,39,50]. Compound 3 was less soluble in DMF at room temperature, so N-methylpyrrolidone (NMP) was used as the solvent. For this electrolyte solution, compound 3, ferrocene, and TBAP in NMP at 20, 30, and 30 mmol L−1 were used, respectively. The resulting electrolyte solutions were then injected into the ECDs using a syringe and sealed with an epoxy adhesive.

Scheme 1. Chemical structures of compounds 1–5.

2. Experimental 2.1. Materials and instrument All chemical reagents used in this work were purchased from Aladdin Co., Shanghai, China, and used as received. All solvents were of analytical reagent grade and commercially available. Dimethyl formamide (DMF) was dried and purified before use. Cyclic voltammetry (CV) was carried out on a CHI 650B electrochemical workstation with a three-electrode system. The reference, working, and counter electrodes were Ag/AgCl, a platinum plate (0.02 cm2), and a platinum wire, respectively. UV–Vis spectra was obtained using a UV-4802 spectrophotometer (UNICO (Shanghai) Instruments Co. Ltd., China). The CIE (International Commission on Illumination) Lab color spaces for the electrochromic devices (ECDs) based on compounds 1–5 were measured on a color reader CR-10 plus

3. Results and discussion 3.1. Electrochemical properties The electrochemical behavior of compounds 1–5 was investigated by CV in DMF (1 mmol L−1) with purged nitrogen gas at room temperature using a three-electrode system and TBAP as the supporting

Fig. 1. Cyclic voltammograms of compounds 1 (A), 2 (B), 3 (C), 4 (D), and 5 (E) at a concentration of 1 mmol L−1 in DMF/TBAP (50 mmol L−1) vs. Ag/AgCl at room temperature and a scanning rate of 100 mV s−1.

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Fig. 2. UV–Vis/NIR spectroscopy for ECDs based on compounds 1 (A), 2 (B), 3 (C), 4 (D), and 5 (E) on ITO coated glass at different potentials. UV–Vis/NIR spectra of ECDs based on compounds 1–5 when on potential was applied (F).

redox reaction between the neutral state and anion-radical state, the anion-radical state and dianionic state, and the dianionic state and dianion-radical state (Scheme 2) [51–54], respectively. The irreversible cathodic peak at about −1.85 (2) V vs. Ag/AgCl could possibly be correlated to the reduction of the adsorbed materials. However, the star-shaped compound 5 with four cyano groups showed only one strong reversible redox couple at ca. −0.98 (1)/−0.87 (1′) V vs. Ag/ AgCl. This indicates that a one-electron reduction process occurred (Fig. S10 in Supporting Information) [51–54]. Compared with the benzene center, the –C]C- center is more favorable for electrochemical reaction, which is consistent with previous report [37,39]. Compounds 4 and 5 both showed good electrochemical reversibility and stability.

Table 1 Electrochemical data for compounds 1–5 and optical data for ECDs based on compounds 1–5. Compounds

Epa (V vs. Ag/ AgCl)

Epc (V vs. Ag/ AgCl)

λonset (nm)

Eg (eV)

λmax (nm)

1 2 3 4

−1.20, −1.46, −0.46, −1.48, −1.97, −0.87

−1.30, −1.54, −0.56, −1.59, −2.14, −0.98

328 333 373 334

3.78 3.65 3.33 3.67

432 456 492 430

385

3.27

598

5

−1.95 −1.91 −0.97 −1.73, −2.17

−2.08 −2.01 −1.09 −1.85, −2.28

3.2. UV–Vis/NIR investigation

electrolyte (50 mmol L−1). The curve for the full CV scan range is shown in Fig. 1, and the anodic (Epa) and cathodic peak potentials (Epc) of compounds 1–5 are listed in Table 1. For the linear-shaped compounds 1–3 that contain two cyano groups, two distinct quasi-reversible redox couples were observed (Fig. 1A–C). The first redox couple 1/1′ (−1.30/−1.20, −1.55/−1.46, and −0.56/−0.46 V vs. Ag/AgCl for compounds 1, 2, and 3, respectively) represents the redox reaction between the neutral state and the electrochemically reduced anion-radical state. The second redox couple 2/2′ (−2.08/−1.95, −2.01/−1.91, and −1.09/−0.97 V vs. Ag/AgCl for compounds 1, 2, and 3, respectively) represents the redox reaction between the anion-radical and dianionic states. Scheme 2 shows their possible redox states and the electron accepting-donating process [51–54]. All three compounds showed good electrochemical reversibility and stability. Compared to compounds 1 and 2, compound 3 exhibited an anodic shift in the reversible reduction peak potential. This indicated that the azobenzene center was more beneficial to electrochemical process than the phenyl and biphenyl centers. The star-shaped compound 4 with three cyano groups showed three quasi-reversible redox couples at about −1.59/−1.48 (1′), −2.14 (3)/ −1.97 (3′), −2.28 (4)/−2.17 (4′) V vs. Ag/AgCl, which represents the

The spectral electrochemical properties of ECDs based on compounds 1–5 were investigated by UV–Vis spectroscopy using a conventional two-electrode configuration [38]. The UV–Vis absorption spectra of the ECDs were obtained under applied different DC voltages and are shown in Fig. 2. For the measurements, the ECDs were made using compounds 1–5 in DMF (3 in NMP), with ferrocene and TBAP serving as the counter redox material and the supporting electrolyte, respectively. For all the compounds, only one broad and weak absorption peak was observed when no voltage was applied at ca. 430 nm (458, 438, 460, 422, and 426 nm for 1–5, respectively) and could be attributed to the π-π* transition (Fig. 2F). As the applied voltage was increased to the onset value, the initial weak absorption peak disappeared and two to three new absorption peaks appeared. The onset bias and the absorption wavelengths are highly dependent on the chemical structure of the compound. The onset bias was −2.3, −2.1, −1.3, −2.2, and −1.8 V for compounds 1–5, respectively. As the conjugation degree increased, a red-shift of the absorption peak was observed, and a more positive onset bias was required. These onset biases were higher than those obtained via CV because of the overpotential caused by the ITO electrode [38]. Compounds 3 and 5 had

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Scheme 2. Possible redox states of compounds 1–5 in electrolyte solution.

lower onset biases, which was consistent with the CV results. For compounds 1 and 2, two new absorption peaks were observed (at ca. 340 and 432 nm for 1, Fig. 2A; and at ca. 456 and 756 nm for 2, Fig. 2B). As the applied voltage decreased, the absorption intensity increased and reached a saturation state at ca. −2.7 V. For compound 3, only one new absorption peak was observed at ca. 492 nm at an applied potential of −1.3 V, and two weaker and broader absorption peaks at ca. 588 and 650 nm appeared as the applied voltage was decreased to −1.5 V. As the applied voltage decreased, the absorption at 492 nm was strong and markedly increased, while the intensity of the other two broad absorption peaks increased slightly and reached saturation at −1.9 V (Fig. 2C). For compound 4, one new absorption peak at ca. 430 nm appeared at −2.2 V, and a second absorption peak at ca. 598 nm appeared as the applied voltage decreased to −2.2 V. Further decreasing the applied voltage caused the intensity of both peaks to significantly increase (Fig. 2D). For compound 5, a broad absorption peak at ca. 598 nm appeared at −1.8 V and a much weaker and a broader absorption peak at ca. 506 nm appeared at −2.1 V. The intensity of both the absorption peaks increased as the applied voltage was further decreased (Fig. 2E)" here. Fig. 3 shows the bleached and colored state of the ECDs based on compounds 1–5. The five compounds exhibited a similar bleached colorless state, while their colored states were all different (orange for 1, yellow-green for 2, reddish brown for 3, green for 4, and blue for 5, respectively) and highly dependent on the chemical structure. The CIE L*a*b coordinate values of the ECDs based on compounds 1–5 in the bleached and colored states are listed in Table 2. The optical band gap (Eg) of the electrochromic materials were calculated from their low energy absorption edges (λonset) according to

the Planck equation (Eg = 1241/λonset) [7,39]. The λonset, Eg, and absorption maximum (λmax) of ECDs based on compounds 1–5 are listed in Table 1. 3.3. Electrochromic switching studies The optical contrast (ΔT%) is an important property of electrochromic materials [25,39,55]. It represents the difference in transmittance between the reduced and the oxidized states at a certain wavelength and is monitored as a function of time at the specified wavelength. The electrochromic switching studies of the ECDs were conducted under ambient conditions and the potential was swept repeatedly between the oxidized and reduced states with a residence time of 4 s at the characteristic absorption wavelength [25,36]. The ECDs based on compounds 1–5 were switched at ± 2.8, ± 2.6, ± 1.8, ± 2.6, and ± 2.0 V, respectively (Fig. 4). The maximum optical contrast was 41.2% at 432 nm for 1 (Figs. 4A), 68.9% at 456 nm for 2 (Figs. 4B), 66.1% at 492 nm for 3 (Figs. 4C), 64.5% at 430 nm for 4 (Figs. 4D), and 68.8% at 598 nm for 5 (Fig. 4E). Compounds 2–5 demonstrated a large optical contrast of > 60%. After 1000 cycles (4000 s), the change in optical contrast was 7.4% for 1, 6.3% for 2, 2.4% for 3, 16.2% for 4, and 22.2% for 5. This results show that the linear-shaped molecules (1–3) have a better switching stability than the star-shaped molecules (4 and 5). The switching stability of 1–3 increased in the order: 1 < 2 < 3. This indicated that the increase in the degree of conjugation of the linear molecules contributed to the improvement of the electrochromic properties. Additionally, compound 3 showed excellent stability and its ΔT% could be maintained at 57.9% even after cycling for 10,000 s. This may be attributed to the presence of the stable N]N

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Fig. 3. Photographs of the ECDs based on compounds 1–5 in their bleached and colored states.

are important parameters for ECMs. The ΔOD and CE values of compounds 1–5 were calculated using the following equations [40]:

Table 2 CIE L*a*b* coordinate values for the ECDs based on compounds 1–5. Compounds

L*a*b (Bleached state at 0.0 V)

L*a*b (Colored state at corresponding voltage)

1 2 3 4 5

48.2*-4.4*-11.2 48.9*-4.1*-10.5 47.4*-2.4*-10.7 48.8*-3.7*-11.4 48.6*-3.8*-10.5

45.2*-6.2*-4.8 43.8*-13.8*3.5 35.8*1.2*-10.5 41.2*-14.5*-11.7 33.1*-2.3*-21.1

ΔOD = log(Tb/Tc), CE (η) = ΔOD/Qd, where Tc and Tb are the light transmittance of the colored and the bleached states of the ECD at a specific wavelength, and Qd is the amount of injected charge per unit sample area in the redox step [36,38]. The CE values of compounds 1–5 were obtained by fitting the slope of linear plots concerning ΔOD versus Q (Figs. S11 and S12 in Supporting Information) [56], and the results are listed in Table 3. Compared with other organic ECMs such as linear [32] and star-shaped [7,37–39,57] pyridinium salts, organic molecules containing ester groups [25,40,58], triphenylamine [28,59], thiophene [31], dithienylpyrroles [60], and triazine based ambipolar compounds [61] (Table 4), the benzonitriles in this work (compounds 2–5) have a high ΔT% and CE, a low driving voltage, good switching stability, and a satisfactory response time. This endows them with considerable potential for applications such as smart windows, electronic paper, displays, and optical storage devices.

with lone pair electrons in the structure, which is more conducive to the transfer of electrons during the electrochromic process. Compared with previously reported ester group-containing azobenzene derivatives [8,9,25], the switching stability of compound 3 was further improved. The response time is also an essential parameter for ECDs. It represents the time required for the electrochromic material to reach 95% of the complete absorbance change after switching between the oxidized and the reduced states [37]. As shown in Fig. 5 and Table 3, the compounds all had fast switching times and their optical response times for the coloring and bleaching processes were 1.00 and 0.5 s for compound 1, 0.99 and 0.9 s for compound 2, 1.66 and 1.16 s for compound 3, 2.05 and 1.00 s for compound 4, and 2.01 and 0.80 s for compound 5, respectively. Furthermore, the bleaching process took less time than the coloring process because of the inhibition of the discharge current flowing in the external circuit [25]. The coloring efficiency (CE) and the change in optical density (ΔOD)

4. Conclusions In this work, five benzonitriles with different chemical structures were synthesized and their electrochromic properties were characterized by a series of methods. It was found that these conjugated systems with cyano groups had excellent electrochromic behavior. Moreover,

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Fig. 4. Electrochromic switching responses for compounds 1–5 monitored at a monochromatic wavelength between −2.8 and + 2.8 V for 1 (A), −2.6 and + 2.6 V for 2 (B), −1.8 and + 1.8 V for 3 (C), −2.6 and + 2.6 V for 4 (D), and −2.0 and + 2.0 V for 5 (E).

Fig. 5. Optical switching studies of the ECDs based on compounds 1–5 monitored at square-wave potential of ± 2.8 V for 1 (A), ± 2.6 V for 2 (B), ± 1.8 V for 3 (C), ± 2.6 V for 4 (D), and ± 2.0 V for 5 (E).

ECDs based on compounds 1–5 exhibited different colors, good stability, high contrast, high coloration efficiency, and a fast switching time. The increase in conjugation was beneficial for the improvement of the electrochromic properties. Linear molecules showed better switching stability than the star-shaped molecules. Compounds 2 and 3 had improved electrochromic behavior compared with most of the reported

organic ECMs. The results demonstrate that these benzonitriles have potential for applications such as in electrochromic smart windows, information displays, and optical storage devices. Additionally, this work provides an alternate direction for the development of new organic ECMs.

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Table 3 Optical contrast, response times, and coloration efficiencies of the ECDs based on compounds 1–5. Compounds

1 2 3 4 5 a b

ΔT% (%)

41.2 68.9 66.1 64.5 68.8

Change in ΔT% after 1000 cycles (%)

Response time (s) Tc

Tb

7.4 6.3 2.4a/8.2a 16.2 22.2

1.68 0.99 1.66 2.05 2.01

0.65 0.90 1.16 1.00 0.80

CE (cm2 C−1) 593 2772 1435 2119 1765

After 1000 cycles. After 2500 cycles.

Table 4 Comparison of electrochromic properties of the benzonitriles with other organic ECMs. Compounds

ΔT% (%)

Driving voltage (V)

Tc

Tb

ΔOD

CE (cm2 C−1)

Ref. No

Benzonitriles Viologens Star-shaped bi-pyridinium salts Star-shaped tri-pyridinium salts Star-shaped bi-pyridinium salts containing ester Star-shaped tetra-pyridinium salts Azobenzene-4,4 ′-dicarboxylic acid dialkyl ester Star-shaped tri-esters Thiophene Triphenylamine Dithienylpyrroles Triazine based ambipolar compounds

66.1 82.9 72.64 65 50 75 62 69 48 71 32 79.79

1.3 0.9 1.5 1.5 1.5 1.2 1.5 1.7 1.8 – 1.0 0.4

1.66 – 0.97 3.95 1.93 0.47 0.82 1.7 2.4 1.1 5 –

1.16 – 0.96 1.85 0.97 0.93 0.78 0.92 1.7 0.8 4 –

– – 0.86 0.89 1.19 0.6 0.88 1.17 – 0.621 0.24 0.789

1435 315 929 958 1280 64.5 946 1259 214 294 125 365.83

This work 30 7 37,57 35 44 24 36 31 28,59 60 61

The “—” denotes the data were not reported.

Acknowledgements

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