Electrochemical and electrochromic properties of two novel polymers containing carbazole and phenyl-methanone units

Journal of Electroanalytical Chemistry 689 (2013) 291–296

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Electrochemical and electrochromic properties of two novel polymers containing carbazole and phenyl-methanone units Bin Hu, Yujian Zhang, Xiaojing Lv, Mi Ouyang ⇑, Zhiyan Fu, Cheng Zhang ⇑ State Key Laboratory Breeding Base for Green Chemistry-Synthesis Technology, College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou, PR China

a r t i c l e

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Article history: Received 19 December 2011 Received in revised form 4 August 2012 Accepted 5 October 2012 Available online 26 October 2012 Keywords: Conducting polymer Electrochromism Electrochemical polymerization Spectroelectrochemistry

a b s t r a c t Two new electrochromic materials consisting of carbazole and phenyl-methanone units, 4-(9H-carbazol9-yl)-phenyl-methanone (CPM) and 4-(3,6-di(thiophen-2-yl)-9H-carbazol-9-yl)-phenyl-methanone (TCPM), were synthesized and characterized. Both the compounds show well-defined oxidation and reduction processes. Spectroelectrochemical analysis reveals that their polymers display reasonable optical contrast (25%, 41%) and fast switching time (3 s, 2 s). Cyclic voltammogram study confirms that the polymer with the existence of thiophene units is more stable in electrochemical environment. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Electrochromism is the phenomenon that some materials display the reversibly color change when a burst of charge is applied. Early studies about electrochromic materials were mainly focused on inorganic and organic small molecules. In recent years, electrochromic conducting polymers have received much attention due to several advantages such as their controllable band gap, fast switching speeds, high contrast ability and outstanding coloration efficiency [1–4]. For conjugated polymers, electrochromism is related to the doping-dedoping process, which leads to the alteration of band gap and causes color changes [5,6]. Therefore, control of band gap is an efficient way of gaining the multicolor-showing materials. Conjugated polymers base on carbazole unit have been widely used in the field of organic solar cells [7], organic field effect transistors [8], organic light emitting diodes [9] and organic electrochromic devices [10] because of their ease of formation of relatively stable radical, high charge carrier mobilities, high thermal and photochemical stabilities. However, carbazole is a material with a wide band gap. In order to decrease its band gap, many functional groups are introduced at its (3, 6) [11,12], (2, 7)position [13,14], and then covalently linked into polymeric systems in the main chain. Recently, Eyup Ozdemir group introduce 1,8-naphthalimide derivatives into the N-position, and the band ⇑ Corresponding authors. Tel./fax: +86 571 8832 0253. E-mail addresses: [email protected] (M. Ouyang), [email protected] (C. Zhang). 1572-6657/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2012.10.005

gap can also be decreased [15,16]. Phenyl-methanone is an n-type material and widely used in optical-electronic technology for its high electron affinity and excellent transport [17]. What is more, thiophene derivatives are also interesting EC materials due to their excellent stability, high conductivity, swift change of color with potential and low band gap [18,19]. It is expected that the carbazole derivatives containing phenyl-methanone and thiophene may possess some useful properties as an EC materials. In this article, two novel carbazole derivatives, 4-(9H-carbazol9-yl)-phenyl- methanone (CPM) and 4-(3,6-di(thiophen-2-yl)-9Hcarbazol-9-yl)-phenyl-methanone (TCPM), were synthesized. Electrochemical and electrochromic properties of their polymers were investigated using cyclic voltammetry (CV) and spectroelectrochemical test. The environment stability of the polymer films was also studied. The synthetic route was described in Scheme 1. 2. Experimental 2.1. Materials Carbazole (Aladdin-reagent, 96%). 3,6-Dibromocarbazole (Aladdin-reagent, 98%). Potassium tert-butoxide (Aladdin-reagent, 99%). 2-Thienylboronic acid (Energy Chemical, 98%). 4-Fluorophenylmethanone (Energy Chemical, 98%), Pd(PPh3)4 was purchased from Energy Chemical. Tetrabutyl ammonium perchlorate (TBAP, Acros Organics, 95%) was dried in vacuum at 80 °C for 24 h. Commercial HPLC grade acetonitrile (ACN, Shanghai Chemical Reagent Company) were used without further purification. All other regents were commercial products used as received.

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Scheme 1. Synthesis routes of compound CPM, TCPM and their polymers.

2.2. Characterizations A CHI 660C electrochemical analyzer (CH Instruments, China) was used to perform the electrochemical measurements. A Nicolet 6700 Fourier-transform infrared spectrometer (FTIR) (Thermo Fisher Nicolet, USA) were used to measure the infrared spectra. NMR spectra of the synthesized products were recorded on a Bruker AVANCE III instrument (Bruker, Switzerland) at 500 MHz (1H) and 125 MHz (13H). Mass spectrometry (MS) analysis was recorded on a GCT Premier spectrometer (Waters, USA) using the electron impact (EI) mass spectra technique. UV–vis spectra were recorded on a Varian Cary 100 UV–vis spectrophotometer (Varian, USA). The images of electrochromic films were taken by Nikon D90. 2.3. Synthesis of monomers 2.3.1. Synthesis of 4-(9H-carbazol-9-yl)-phenyl-methanone (CPM) Carbazole (1.6 g, 10 mmol) and Potassium tert-butoxide (1.1 g, 10 mmol) were dissolved in DMSO (150 mL) in a flask fitted with a magnetic stirrer and condenser. The mixture was heated at 70 °C for 10 min and 4-fluorophenyl-methanone (2.0 g, 10 mmol) was added with stirring for 12 h. The mixture was cooled to room temperature, poured into ice-water, filtered, and the crude residue was recrystallized from acetone. A white powder was obtained (2.4 g, 69% yield). 1H NMR (500 MHz, CDCl3) d: 8.18 (d, J = 7.5 Hz, 2H), 8.10 (d, J = 8.5 Hz, 2H), 7.93 (d, J = 6.5 Hz, 2H), 7.78–7.73 (d, J = 8.5 Hz, 2H), 7.66 (t, J = 7.5 Hz, 1H), 7.60–7.53 (d, J = 6.0 Hz, 4H), 7.47 (t, J = 9.0 Hz, 2H), 7.35 (t, J = 7.0 Hz, 2H). 13C NMR (126 MHz, CDCl3) d: 195.5, 141.6, 140.3, 137.4, 136.6, 132.6, 131.8, 130.2, 128.5, 126.6, 126.2, 123.6, 120.5, 120.6, 109.9. MS (EI): calculated for C25H17NO m/z: 347.4, found m/z: 347.6. 2.3.2. Synthesis of 4-(3,6-dibromo-9H-carbazol-9-yl)-phenylmethanone (DCPM) 3,6-Dibromocarbazole (1.6 g, 5 mmol) and Potassium tertbutoxide (0.6 g, 5 mmol) were dissolved in DMSO (150 mL) in a flask fitted with a magnetic stirrer and condenser. The mixture was heated at 70 °C for 10 min and 4-fluorophenyl-methanone (1.0 g, 5 mmol) was added with stirring for 12 h. The mixture was cooled to room temperature, poured into ice-water, filtered, and the crude residue was recrystallized from acetone. A white

powder was obtained (1.8 g, 72% yield). 1H NMR (500 MHz, CDCl3) d: 8.22 (d, J = 2.0 Hz, 2H), 8.09 (d, J = 8.0 Hz, 2H), 7.91 (d, J = 7.0 Hz, 2H), 7.67 (d, J = 7.5 Hz, 3H), 7.56 (t, J = 9.0 Hz, 4H), 7.38 (d, J = 8.5 Hz, 2H). 13C NMR (126 MHz, CDCl3) d: 195.6, 140.8, 139.5, 137.3, 136.8, 132.7, 131.9, 130.2, 129.7, 128.5, 126.8, 124.4, 123.4, 113.7, 111.8. MS (EI): calculated for C25H15Br2NO m/z: 505.2, found m/z: 505.3. 2.3.3. Synthesis of 4-(3,6-di(thiophen-2-yl)-9H-carbazol-9-yl)-phenylmethanone (TCPM) DCPM (1.1 g, 2 mmol) and 2-thienylboronic acid (0.6 g, 5 mmol), Pd (PPh3)4 (0.2 mmol), aqueous Na2CO3 (2.0 M, 5 mL), THF (20 mL) and toluene (30 mL) were mixed in a flask. The mixture was degassed and then refluxed for 48 h under a nitrogen atmosphere. After being cooled, a lot of water was added and the mixture was extracted with DCM. The organic phase was dried with MgSO4. After the solvent was evaporated, the product was processed by column chromatography on silica gel with petroleum ether- acetic ether to give a yellow power (0.6 g, 58% yield). 1H NMR (500 MHz, CDCl3) d: 8.37 (d, J = 9.0 Hz, 2H), 7.94 (m, 2H), 7.74 (d, J = 11.0 Hz, 4H), 7.68 (t, J = 12.0 Hz, 2H), 7.60 – 7.49 (m, 5H), 7.40 (m, 2H), 7.32 (d, J = 10 Hz, 2H), 7.16 (d, J = 9.0 Hz, 2H). 13 C NMR (126 MHz, CDCl3) d: 195.3, 140.5, 139.3, 137.2, 136.8, 132.7, 131.9, 130.2, 129.6, 129.3, 126.5, 124.4, 123.4, 113.7, 116.8. MS (EI): calculated for C33H21NOS2 m/z: 511.6, found m/z: 511.8. 2.4. Electrochemistry The electro-syntheses and measurements were performed in a conventional three-electrode cell with an ITO-coated glass (CSG holding Co. LTD, Rs 6 10 X h1. the active area: 1.0 cm  2.0 cm) as working electrode which was sequencely washed with ethanol, acetone and deionized water under ultrasonic before use, a platinum sheet and a double-junction Ag/AgCl electrode (silver wire coated with AgCl in saturated KCl solution, 0.1 M TBAP in ACN solution as the second junction) were applied as the counter electrode and the reference electrode, respectively. The concentration of CPM and TCPM used for polymerization were all 0.002 M. All the electrochemistry experiments were carried out at 25 °C under N2 atmosphere.

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3. Results and discussion 3.1. Electrochemical polymerization The cyclic voltammogram (CV) curves of CPM and TCPM in TBAP/ ACN solution containing 0.002 M monomers at a potential scan rate of 100 mV s1 are shown in Fig. 1. Both the CV curves present well-defined oxidation and reduction peak. The oxidation current increases with the increasing scanning cycle, and the observable polymer films can be found on the surface of ITO/glass electrode for all the processes of polymerization. For CPM, the first scan gives evidence of the monomer oxidation (Fig. 1a). After the second scan, a reversible redox system appears due to the doping and dedoping process of its polymer film under different applied potential [20,21], and the corresponding oxidation and reduction peaks locate at +1.10 and +0.90 V, respectively. After thiophene units are introduced, the CV curves of TCPM (Fig. 1b) display a broad oxidation wave and slightly shift to lower potential due to the fact that aromatic conjugation becomes more obvious in the polymer backbone [12]. During the film growth on the electrode, two obvious reduction peaks can be seen at +0.76 and +1.08 V, which is attributed to the formation of polaronic and bipolaronic forms [22,23]. The results are consonant with the spectroelectrochemistry properties of the polymer.

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0.002 M monomers, then both the films were dedoped at 0 V in monomer free solution for 5 min and washed with clean ACN to remove the residual supporting electrolyte and the monomers, and their CV curves were investigated in monomer-free solution. Fig. 2 shows the CV curves of the PCPM and PTCPM film at different scanning rates between 50 and 250 mV s1. For PCPM, the oxidation and reduction peaks locate at about +1.15 and +1.05 V (Fig. 2a). While the CV of PTCPM present the oxidation and reduction at +1.16 and 0.56 V (Fig. 2b), respectively. Both the peak current densities are directly proportional to the scan rates (Fig. 2c), indicating that the well-adhered polymer films are formed on the ITO/glass electrode surface and the electrochemical processes of the polymer films are reversible and not diffusion limited [24].

3.2. Electrochemistry of polymer films The polymer films of PCPM and PTCPM on the ITO/glass electrode were prepared potentiostatically (polymerization charge: 0.06 C) at 1.38 V and 1.25 V in 0.1 M TBAP/ACN solution containing

Fig. 1. Cyclic voltammogram curves of (a) 0.002 M CPM, (b) 0.002 M TCPM in 0.1 M TBAP/ACN solution at a scan rate of 100 mV s1.

Fig. 2. Cyclic voltammogram curves of (a) PCPM film and (b) PTCPM film in monomer free solution of 0.1 M TBAP/ACN at different scan rates. (c) Scan rate dependence of the anodic and cathodic peak current densities graph.

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3.3. FTIR spectra of monomers and polymers Fig. 3 presents the FTIR spectra of CPM, TCPM, PCPM and PTCPM. According to FTIR spectra of two monomers, the C@O vibration can be observed at 1652 and 1599 cm1. The peak at 1275 cm1 is ascribed to the stretching of CAN. The bands at 1516, 1448 and 753 cm1 are due to the stretching of phenylene rings [25]. Comparing with the spectrum of CPM, the spectrum of TCPM shows some new bands at 1464 and 805 cm1, which should be assigned to the C–H and C–S vibration [26]. After polymerization, these bands become very weak. Compared to the spectrum of respective monomer, the spectrum of PCPM exhibits new peak at about 801 cm1 and the band at 851 cm1 almost disappears, indicating the polymer occurs at 3,6-position of CPM [27,28]. The band at 700 cm1 of the spectrum of PTCPM becomes narrow due to the fact that the out of plane C–H bending vibrations of mono-substituted thiophene rings at 699 cm1 disappears, implying that the polymerization occurs at the a-position of thiophene rings [29]. What is more, the special peak at 1087 cm1 is ascribed  to the existence of ClO4 [30]. The differences of spectrums confirm the occurrence of polymerization. 3.4. Scanning electron microscopy (SEM) of polymer films The polymer films of PCPM and PTCPM prepared by constant potential electrolysis (polymerization charge: 0.06 C) were dedoped at negative potential in monomer-free solution and then washed with clean ACN for several times. The surface morphology of polymer film was investigated by SEM. As shown in Fig. 4a, PCPM film displays fibrous surface and the thickness is almost 694 nm. While a cohesive structure with clusters of granules can be observed for PTCPM film, which is more favorable for the movement of doping anions into and out of the polymer film which may benefit for the improvement of the switching time. And the thickness of PTCPM film can be considered as 784 nm (Fig. 4b). 3.5. Spectroelectrochemical behavior of PCPM and PTCPM Spectroelectrochemistry is a useful way to study the changes in the absorption spectra of conducting polymer films under different applied potentials [31]. The spectrochemical and electrochromic properties of the resultant films are studied in 0.1 M TBAP/ACN solution without the monomer. Fig. 5a shows the spectrochemical property of PCPM film by various applying potential ranging from +1.0 to +1.5 V. At the neutral state, the peak at 380 nm is attributed to the p–p transtions of the polycarbazole backbones. With the applied potential increasing, a new absorption wave is formed at 950 nm, which is related to the evolution of polaron. For PTCPM

Fig. 4. SEM images of PCPM (a) and PTCPM (b) deposited on ITO/glass electrode, the inset graph is the cross-sectional images of respective polymer film.

(Fig. 5b), the maximum absorption value of the p–p transition of polymer is 415 nm for the neutral state. The amplitude of the p–p transition band at 415 nm decreased as the applied potential increased from +0.6 to +1.5 V, and charge carrier bands are formed. The isosbestic point of PTCPM is located at 476 nm. Two new absorption waves at around 580 nm and >900 nm are assigned to the evolution of the polaron and bipolaron bands, respectively [32,33]. In addition, both the polymer films possess distinct color changes, which are shown in Fig. 6. PCPM film has a dual-color electrochromism, which shows light yellow at the neutral state due to the fact that it only has an absorption around 380 nm. As the new electronic transitions progressively intensified at 950 nm upon the applied potential increasing, brown-yellow is observed due to the absorption at 380 nm and 800 nm. While PTCPM film exhibits yellow color as a result of an absorption at 415 nm at the neutral state. Since these transitions lie in the visible region in its intermediate doping levels, yellow color of the polymer film changes into brown-black. Moreover, further oxidation of PTCPM results in blue-purple color due to increased absorption at 590 nm together with a diminished neutral state absorption. 3.6. Electrochromic switching of PCPM and PTCPM

Fig. 3. FTIR spectra of CPM, TCPM, PCPM and PTCPM films.

The ability of a polymer to present a rapid color change is necessary for electrochromic applications. Double potential step chronoamperometry technique is common method to investigate such a behavior between its neutral and doped states [34]. Fig. 7 dis-

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Fig. 7. Electrochromic switching response for PCPM film monitored at 940 nm in 0.1 M TBAP/ACN solution between 0 and +1.5 V with a residence time of 5 s.

the optical contrast(DT%) of PTCPM film at 415 and 1100 nm to be 20% and 41% and both the switching time are found to be 2 s. The coloration efficiency (CE) is also an important characteristic for the electrochromic materials. CE can be calculated by using CE ¼ DOD=Q d and DOD ¼ logðT colored =T bleached Þ, where Qd is the injected/ejected charge between neutral and oxidized states, Tcolored and Tbleached are the transmittance in the oxidized and neutral states, respectively. Using this equation, CE of PCPM is measured as 110.48 cm2 C1 (at 940 nm) at full doped state which is a little lower than the other carbazole derivatives [35] and CE of PTCPM at 1100 nm is 186.18 cm2 C1 which can be considered to be a reasonable coloration efficiency. Fig. 5. Spectroelectrochemical spectra of (a) PCPM film and (b) PTCPM film on ITO/ glass under different potentials in 0.1 M TBAP/ACN solution.

3.7. Stability of PCPM and PTCPM The stability of polymer towards multiple redox processes is another important parameter for its applications. For this reason, both the polymer are tested by Cyclic Voltammetry under the applied potential between 0 and +1.2 V with 100 mV s1 in blank solution and the results are illustrated as Fig. 9. After 300 cycles, an obvious decrease can be observed for the PCPM (Fig. 9a). Comparing with the PCPM, the PTCPM retains better stability (Fig. 9b). The introduction of thiophene units can lead to highly conjugated structure which can be reason for better stability [14,36]. The re-

Fig. 6. Multichromic behaviors of PCPM film and PTCPM film at different potentials.

plays the electrochromic switching property of PCPM film at 940 nm upon the applied potential between 0 and +1.5 V with regular interval of 5 s. The contrast is measured as the difference between T% in the reduced and oxidized forms. Owing to light yellow color at the neutral state, the contrast is as high as 80%; brown-yellow color at the oxidized state reduces the contrast to 55%. The optical contrast(DT%) is calculated as 25% and the switching time required to reach 95% of the ultimate T is found to be 3 s. The electrochromic switching properties of PTCPM film at 415 and 1100 nm upon the applied potential between 0 and +1.5 V with regular interval of 6 s are also investigated. As shown in Fig. 8, the change between yellow color and blue-purple make

Fig. 8. Electrochromic switching response for PTCPM film monitored at 415 nm and 1100 nm in 0.1 M TBAP/ACN solution between 0 and +1.5 V with a residence time of 6 s.

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Acknowledgements The authors gratefully thank the support of National Basic Research Program of China (2010CB635108, 2011CBA00700), National Natural Science Foundation of China (51203138, 51273179), Major Science and Technology, Special and Priority Themes of Zhejiang Province, China (2009C14004). References

Fig. 9. Cyclic voltammogram of (a) PCPM film and (b) PTCPM film on ITO/glass electrode as a function of repeated scans 100 mV s1 in 0.1 M TBAP/ACN.

sults indicate that TCPM can be considered as a promising candidate material for EC devices.

4. Conclusion Two novel electrochromic materials, 4-(9H-carbazol-9-yl)-phenyl-methanone (CPM) and 4-(3,6-di(thiophen-2-yl)-9H-carbazol9-yl)-phenyl-methanone (TCPM), were synthesized by simple chemical route. Both the materials show well-defined CV curves during the process of electropolymerization. Spectroelectrochemical reveals that PCBM film exhibits two colors in different states and holds the optical contrast of 25%. Comparing with PCBM film, PTCPM film displays different colors with the optical contrast reaching 41%. In addition, the PTCPM film possesses better stability than PCBM, which makes PTCPM can be promising candidates for electrochromic devices.

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