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Electrosynthesis and characterizations of electrochromic and soluble polymer films based on N- substituted carbazole derivates
Synthetic Metals 260 (2020) 116253
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Electrosynthesis and characterizations of electrochromic and soluble polymer films based on N- substituted carbazole derivates
T
Hassan Elamin Elkhidra, Zeliha Ertekinb, Yasemin Arslan Udumc, Kadir Pekmezb,* a
Shendi University, Department of Chemistry, Shendi, 47711, Sudan Hacettepe University, Department of Chemistry, Ankara, 06800, Turkey c Technical Sciences Vocational School, Gazi University, Ankara, 06374, Turkey b
ARTICLE INFO
ABSTRACT
Keywords: Redox polymers Electropolymerization Polycarbazoles Electrochromic material
Carbazole derivatives are a class of nitrogen-containing aromatic heterocyclic compounds and they not only have various biological activities but also exhibit useful properties as organic materials due to their special structures. In this study, the compounds based on carbazole derivatives (N-positions were occupied by methanol, carboxylic acid, and cyanoethyl) were synthesized via a simple method. The electrochemistry and electropolymerization of these three monomers were investigated and compared with those of different N-substituted with groups attaching on the active sites of the carbazole units. The polymeric films were prepared on ITO/glass substrate by repetitive cyclic voltammetry (CV) scanning of the monomer solutions containing NaClO4-LiClO4 electrolyte dissolved in acetonitrile. The structures of the monomers and polymers were characterized by 1HNMR, 13C-NMR, and FTIR. The electro-generated polycarbazole films exhibited redox-activity and multichromic properties with increasing potential. The remarkable electrochromic behavior of the films was explained in detail with the help of spectroelectrochemical studies. Photoluminescence spectra and quantum yields were given of polycarbazole samples in solution. In addition, solubility, molecular weight, and conductivity properties of polymers containing N-substituted groups have been discussed further.
1. Introduction Carbazole gained attention as an important component of organic materials because it has excellent electrical and photoelectrical properties. Carbazole derivatives have also a great necessary in light emission diode (LEDs) devices [1–4] and non-linear optical materials with large photorefractive effect [5,6] because of their good hole transporting properties [7] and their charge transfer (CT) complexes [8]. The first report of electropolymerization of carbazole was carried out by Ambrose and Nelson in 1968 [9]. They have exhibited that carbazole could polymerize from 3, 6 and 9, positions and also noted that coupling could proceed through the 1 and 8 positions; however, these positions are sterically hindered due to rigid structure of carbazole. Carbazole based polymers also have been of greater interest because of their interesting thermal, electrical, and photophysical properties [10–13]. Therefore, numerous research focus on their properties and the performance of devices based on carbazole unit for a number of applications such as electrophotography and electroluminescence device hole transport layers, microcavity photoconduction, and as photovoltaic components [8,14,15]. The applications for electrochromic
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organic conjugated polymers, such as polycarbazole, polypyrrole, polyaniline, and polythiophene, have been found to be more promising as electrochromic materials [16,17]. Electrochromic polymers usually show fast switching times, allowing for rapid color changes from the materials upon application of appropriate potentials. In addition, they have excellent switching reproducibility, meaning that they can be switched between their various color states many times without any noticeable decline in performance [18]. Due to these properties, they are used widely in our daily life, such as smart windows, optical displays and e-paper [19,20]. Polycarbazole and its derivates can be built onto desired electrodes by electrochemically deposition technique to yield conductive thin films. The observed spectral changes of the polycarbazole film were fully reversible upon varying the applied potential; significant color changes from yellow-colored reduced polycarbazole films to green-colored oxidized polycarbazole films and easy to detect with the naked eye [21,22]. It was also reported that polycarbazole films are reduced form with low conductivity (> 10 −9 Scm1 ) whereas oxidized films have enhanced conductivity (> 10-3 Scm-1) [21]. Carbazole derivates can be polymerized either at 3- and 6- positions or 2- and 7- positions because these positions are more active than
Corresponding author. E-mail address: [email protected] (K. Pekmez).
https://doi.org/10.1016/j.synthmet.2019.116253 Received 24 July 2019; Received in revised form 5 November 2019; Accepted 26 November 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
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others. However, electrochemical polymerization of carbazole generally results in only poly(3,6-carbazole)s because the 3 and 6 positions are the most reactive sites in the structure. Also, poly(2,7-carbazole) derivatives have more extended conjugation lengths and lower energy band gaps than poly(3,6-carbazole) [23,24]. Electrochemical polymerization is one of the most useful methods for the preparation of conjugated polymers because electroactive polymer can be produced films without the need for purification. However, there is not intensive research on the electrosynthesis and electrochromic properties of redox-active conjugated polymers from soluble polymer films based on N- substituted carbazole derivates. According to this consideration, electropolymerization of the N- positions occupied by methanol (carbazol-9-yl-methanol), carboxylic acid (carbazol-9-yl- carboxylic acid) and cyanoethyl (carbazol-9-yl- cyanoethyl) was systematically carried out to obtain detailed information about using different substitution patterns and physical-chemical properties. In this study, the three different carbazole monomers are prepared successfully, and their electrochemistry and electropolymerization are investigated. On the other hand, solubility, molecular weight and conductivity properties of the polymers have been discussed in detail.
2.2.3. Carbazol-9-yl- cyanoethyl Carbazole (1.7 g, 0.01 mol) and acrylonitrile (12 g, 0.3 mol) were put into a 100-mL glass flask. Then, the flask was placed in an ice bath to cool for 15 min. Benzyltrimethylammonium hydroxide (40 % Triton B in MeOH) as catalyst was added dropwise to the reaction medium. The color of mixture was changed from white to yellow. The obtained paste was heated up to 65 °C, after 1 h the paste became to orange solution. Yellow precipitated was formed after cooling. Then, filtered and recrystallized from acetonitrile [27]. 1H NMR (400 MHz, DMSO) δ 8.17 (d, J =7.7 Hz, 1 H), 7.72 (d, J =8.2 Hz, 1 H), 7.48 (t, J =7.7 Hz, 1 H), 7.24 (t, J =7.4 Hz, 1 H). HRMS calcd. for C15H12N2 (m/z) 220.10, found 220.10 (+ mode). HRMS spectra of monomers were also shown in the supplementary data (Fig. S1). Fig. 1 shows the chemical structure of polycarbazole derivatives. 2.3. Electrochemical measurements and characterization Electrochemical and spectroelectrochemical studies were performed in a three-electrode cell configuration. ITO coated glass slide, platinum wire, and silver wire were used as the working, counter and pseudoreference electrodes, respectively. Pseudo-reference Ag wire electrodes used in electrochemical and spectroelectrochemical studies were adjusted versus the Fc/Fc+ redox couple (+0.3 V). The monomers were electrochemically deposited with cyclic voltammetry technique (CV) using Autolab PGSTAT100 potentiostat–galvanostat system. Lambda 75 UV–vis NIR spectrophotometer of Perkin Elmer was used to record the UV/Vis/NIR spectra with a scan rate of 2000 nm/min. The potentials were controlled by using a Solatron 1285 potentiostat/galvanostat. Color analysis and colorimetry measurements of the polymers were carried out via Konica Minalto CS-100 colorimeter. Polycarbazole samples were prepared in the same concentration of monomer solution (50 mM) using potentiodynamic electropolymerization method between 0.0 V and +1.6 V with 100 mV/s scan rate. The thickness of polymer films was tried to control by applying same cyclic number. Conductivity measurements were carried out using four-probe techniques. Our four- probe equipment contains four spring sensitive cylindrical platinum tips (d = 0.50 mm placed on linearly 5 mm PTFE head) distance between four tips are 0.50 mm. Film thickness of polymeric sample is measured using a digital micrometer. Thickness of polymeric film samples was selected about 100−150 μm for conductivity measurement. Polymer samples coated on platinum macro electrode were stripped and cut to 2 × 5 mm dimensions and placed under four probe tips. While conductivity measurement is carried out drive current from the two external tips of the four probe and measuring the voltage drop over the two internal tips. ENTEK conductivity measurement device was calculated automatically conductivity of sample using drive current, voltage drop and film thickness. Polycarbazole samples were prepared using same potentiodynamic electropolymerization method between 0.0 V and +1.6 V versus pseudo
2. Experimental 2.1. Materials Formaldehyde (%37, Merck), sodium hydroxide (Fluka), ethanol (Riedel-de Haën), n-hexane (BDH), carbazole (BDH), bromoacetic acid (Sigma-Aldrich), acrylonitrile (Merck), benzyltrimethyl ammonium hydroxide (Alfa Aesar) have been used. Acetonitrile (Merck, HPLC grade) and dimethyl sulfoxide (Riedel-de Haën) were of analytical grade and used as received. LiClO4 and NaLiClO4 as a supporting electrolyte were also obtained from Sigma Aldrich. All chemicals were employed without further purification. Transparent indium tin oxide (ITO) coated polyethylene terephthalate (60 Ω /sq) plates obtained from Sigma-Aldrich and glass plates (15 Ω/sq) obtained from Delta Technologies. 2.2. Synthesis 2.2.1. Carbazol-9-yl-methanol Tawney method was used to prepare Cz-OH. Carbazole (10 g, 0.06 mol) was added to successively with 25 mL of 37 % formaldehyde and 2 mL of 5 % sodium hydroxide. The carbazole had dissolved within 10 min and then, the crude product began to form. After 2.5 h, the solution was filtered at room temperature and dried under vacuum. Then, twice recrystallizations were made from n-hexane to obtain the product [25]. 1H NMR (400 MHz, DMSO) δ 8.17 (s, 1 H), 8.16 (d, J =7.5 Hz, 3 H), 7.69 (d, J =7.9 Hz, 3 H), 7.47 (d, J =7.5 Hz, 3 H), 7.24 (t, J =7.0 Hz, 3 H), 6.43 (s, 1 H), 5.79 (s, 2 H). HRMS calcd. for C13H11NO (m/z) 197.08, found 197.08 (+ mode). 2.2.2. Carbazol-9-yl- carboxylic acid Cz-COOH was synthesized using a procedure from the literatüre [26]. Carbazole (16.7 g, 0.1 mol) and sodium hydroxide (12 g, 0.3 mol) dissolved in DMSO (40 ml) and heated to 85 °C. Obtained dark brown solution was stirred for 30 min. Then, bromacetic acid (16.68 g, 0.12 mol) was added carefully. The solution was stirred overnight and then poured into 400 ml cold water. After filtration, the filtered solution separated from solid residue and pH value was adjusted to 3-4. Obtained white precipitate was seperated by second filtration, washed several times with water, and dried in air. 1H NMR (400 MHz, DMSO) δ 8.17 (d, J =7.7 Hz, 1 H), 7.58 (d, J =8.2 Hz, 1 H), 7.45 (d, J =7.9 Hz, 1 H), 7.23 (d, J =7.6 Hz, 1 H). HRMS calcd. for C14H11NO2 (m/z) 225.08, found 225.08 (+ mode).
Fig. 1. The chemical structure of polycarbazole derivatives R: −OH (carbazol9-yl-methanol); −COOH (carbazol-9-yl- carboxylic acid) and −CH2CN (carbazol-9-yl- cyanoethyl). 2
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Ag reference electrode in monomer containing 0.1 M NaClO4-LiClO4/ ACN solution on Pt sheet macro working electrode. Obtained samples using same cycle number electropolymerization technique were used for FTIR-ATR (Diffuse Reflectance, Thermo model-NICOLET-IS 10 FTIR) characterization after washed several times with acetonitrile solution to remove electrolyte from polymer. All polymer films dried under vacuum at room temperature. Electropolymerized polycarbazole samples on Pt electrode were dissolved by immersion in dimethyl sulfoxide (DMSO) and this process was repeated several times to concentrate the solution. Purification of polycarbazole samples was carried out using reprecipitation method from obtained concentrated polycarbazole solution by addition of acetonitrile in their DMSO solution. Monomer and electrolyte were soluble in acetonitrile-DMSO mixture solution whereas polymer couldn’t soluble. By this way, reprecipitated polymeric samples were centrifuged and separated. Reprecipitated samples were used for NMR (Bruker 400 MHz AV NMR) spectroscopic characterization after filtered and dried under reduced pressure. Obtained all purified polycarbazole samples were redissolved in DMSO-d6 for 1H and 13C NMR methods in the supplementary data. Chemical shifts (δ, ppm) were determined with TMS as the internal reference. Polycarbazole samples were prepared using potentiodynamic electropolymerization method between 0.0 V and +1.6 V in 50 mM monomer containing 0.1 M NaClO4-LiClO4/ACN solution on Pt sheet macro working electrode for photoluminescence experiment. Obtained polycarbazole films were washed in acetonitrile solvent and dried nitrogen stream and then, polycarbazole samples on Pt electrode were dissolved by immersion in dimethyl sulfoxide (DMSO) solvent up to saturation (shown in Table 2). Polycarbazole solutions were diluted to 10−3 and 10-5 mol/L, respectively, using saturated solution. Fluorescence spectra were measured with a commercial spectrophotometer (Agilent Technologies, Cary Eclipse Fluorescence Spectrophotometer). 3. Results and discussion 3.1. Electrochemical polymerization and characterization studies Redox behavior of carbazol-9-yl-methanol (Cz-OH); carbazol-9-ylcarboxylic acid (Cz-COOH) and carbazol-9-yl- cyanoethyl (Cz-CN) was investigated by cyclic voltammetry technique in order the see the effect of substituent group in the monomer structure. Electropolymerization of the monomers was performed in an electrolyte solution of 0.1 M NaClO4-LiClO4 dissolved in acetonitrile a scan rate of 100 mV/s. The monomer concentrations were used as 1.10−2 M and electrode areas as 1 cm2 during the electrochemistry studies. The current density in the cyclic voltammogram was simply calculated by dividing the current to surface area. The irreversible oxidation peaks for Cz−OH, Cz−COOH, and Cz-CN were observed at +1.24 V, +1.50 V and +1.37 V at a bare ITO glass (Fig. 2). There is no reduction at the reverse cycle which also defined as an irreversible process. After the electrochemical polymerization of the monomers, reversible redox peaks of the polymers were obtained in blank solution (Fig. 2 inset graphs). The difference in monomer oxidation potentials can be due to the different electronic nature of substituent groups. The monomer oxidation peaks of the Cz−OH observed at lower potential as compared to Cz−COOH and CzCN. Moreover, the onset oxidation potential (Eonset) of Cz−OH shifts to lower potential than that of Cz−COOH and Cz-CN, which is consistent with the presence of the −OH group in the monomer structure, making it electron richer and easily oxidized [28]. With the increase of the cycles number, redox peaks were observed as reversible and at lower potentials due to the formation of the electroactive polymer film on the ITO/glass surface. Electrochemical behaviours of the polymeric films were carried out in monomer-free
Fig. 2. Cyclic voltammetry of (a) Cz−OH, (b) Cz−COOH, (c) Cz-CN at 100 mV/s in 0.1 M NaClO4/LiClO4/ACN solution. Each Inset graph shows CV of related polymeric films that were carried out in monomer-free same solution.
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reported that the peaks as values (aromatic range) at 8.16 (d, 2 H); 7.65 (d, 2 H); 7.55 (t, 2 H); 7.28 (s, 2 H), D-DMSO (ppm) [31]. Carbazoles could be easily functionalized at its 3,6- and 2,7- or N- positions and then covalently linked into polymeric systems [23]. In our study, the Npositions were occupied by methanol (carbazol-9-yl-methanol), carboxylic acid (carbazol-9-yl-carboxylic acid) and cyanoethyl (carbazol-9yl- cyanoethyl). For this reason, the poly N-substituted carbazole may be functionalized at 3,6- and 2,7- positions because these positions is more active than others. Poly(2,7-carbazole) derivatives have more extended conjugation lengths and lower energy bandgaps than poly (3,6-carbazole) [23]. 1 H NMR (DMSO-d6, 400 MHz) of Cz−OH at aromatic region δ(ppm):8.16(CH,d, J =7.5 Hz), 7.70(CH,d, J =7.9 Hz), 7.45 (CH,t, J =7.5 Hz), 7.25 (CH,t, J =7.0 Hz) and 6.45(OH,t), 5.8 (CH2,d) (Fig. S2) 1 H NMR (DMSO-d6, 400 MHz) of PCz−OH at aromatic region δ(ppm): 8.60, (CH,s), 8.31 (CH,d, J =7.8 Hz), 8.12 (CH,d), 7.90 (CH,d, J =8.5 Hz), 7.78 (CH,d), 7.70 ppm (CH,t), 7.40 ppm (CH,t), 7.25 ppm (CH,dd, J = 15.7, 7.8 Hz) (Fig. S4) 1 H NMR spectrum of monomer of Cz-OH (Fig. S2) was exhibit the doublet peak at 8.16 due to 1H at 4, 5 position, the doublet (CH, d) at 7.70 ppm due to 1H at 1,8 position, the triplet at 7.45 (CH, t) due to 1H at 2, 7 position, the other triplet peak at 7.25 ppm due to 1 H(CH, t), at 3,6 position [32]. In addition, the triplet peak which was appeared at 6.45 ppm (OH,t, disappeared after shaking with D2O) and the doublet at 5.8 due to aliphatic CH2 bounded to OH (CH2,d) (Fig. S2). As mentioned above, the polymerization of carbazoles mainly proceeds by CeC and CeN couplings but in our derivatives, C_N couplings don’t appear because the N atom closed by substituted. Proton peaks of PCz−OH are much broader than the corresponding peaks of Cz−OH due to the molar mass distribution of polymeric structure. In addition, chemical shift of the PCz−OH protons observed to much lower field because of the formation of extended conjugated delocalized structure [33,34]. If we suggested the polymerization coupled at 2, 7 position, 1H NMR of PCz−OH (Fig. S4) showed as the singlet peak at 8.60 (CH,s) due to 1H at 1, 8 position after electrochemical polymerization of monomers. We noticed that the new singlet peak appeared after polymerization when this peak is doublet in monomer, so this means, the polymerization occurred mainly in 2,7 position. The two doublet at 8.31, 8.12 ppm (CH,d) due to H in 4, 5 position and 3,6 position. The triplet peak at 7.70 ppm which may be due to terminal carbazole group. On the other hand, multiplet peak at 7.25 ppm also may be to positive charge on doped conducting (polaronic structure) polymer backbone. According to the formation or the coupling of polymer at 3,6 position, we obtained singlet at 8.6 ppm (CH,s) due to 1H, at 4,5 position. The two dublet peak at 8.12, 8.31 ppm(CH,d), due to 1H for 2, 7 position and 1,8 positions. If the polymer was attached to 1,5 position, the singlet peak should not be observed. If the polymerization coupled at 2,6 position we obtained four doublet for 3, 4, 7, 8 position and two different singlet for 1, 5 position (Fig. S2) 13 C NMR (DMSO-d6, 100 MHz) of Cz−OH at aromatic region δ(ppm): 139.68 (quaterner a), 125.90 (C2,7), 122.50 (quaterner b), 120,12 (C4,5), 119.31 (C3,6), 109.73 (C1,8). (Fig. S3). 13C NMR (DMSOd6, 100 MHz) of PCz-OH at aromatic region δ: 140.50, 139.31, 133.31, 126.22, 125.44, 123.67, 120.81, 122, 119.85, 118.61, 110.25, 110.18. (Figure S5). Equal six different carbon atoms (Ca, Cb, C1=C8, C2=C7, C3=C6, C4=C5) were observed according to literature at aromatic region of monomer Cz−OH 13C NMR spectra followed by twelve peaks of twelve different carbon atoms after polymerization [34]. In addition equal two different quaterner carbon atoms (Ca and Cb) were defined for monomer Cz−OH, six different quaterner carbon atoms due to the new substitution after polymerization according to APT (attached proton test) measurement. We also observed some other peaks with low intensity may be due to different coupling of electropolymerized PCz−OH and polaronic charge effect along to doped polymeric backbone.
Fig. 3. FTIR-ATR spectra of monomers; Cz−OH, Cz−COOH, Cz-CN and polymers; PCz−OH, PCz−COOH, PCz-CN.
solution of 0.1 M NaClO4/LiClO4 /ACN by separate cyclic voltammetric experiment and shown at inset graph in Fig. 2. Partially overlapped two successive anodic peaks were observed for carbazole derivatives versus pseudo reference electrode (Ag wire). As depicted in the insets of Fig. 2, on the anodic scans, polymer oxidations were observed at 1.04 V/ 1.20 V for PCz−OH, 1.12 V/1.48 V for PCz−COOH and 1.10 V/1.28 V for PCz-CN. On the reverse scans, reduction peaks evolved at 0.76 V/ 0.92 V for PCz−OH, 0.58 V/0.84 V for PCz−COOH and 0.73 V/ 0.90 V for PCz-CN. FTIR-ATR spectra of monomers and polymers were given in Fig. 3. The monomer of Cz−OH has characteristics peaks at 3403, 2913, 1614, 1599, 1448, 1330, 1228, 981, 742, 717 cm−1. The FTIR-ATR spectrum of Cz−OH indicated that the broad band at 3000−3500 cm−1 attributed to hydrogen bonded alcohol (OeH) band stretching, and 2913 cm−1 can be attributed to methyl (CH2) groups of Cz−OH, 1400−1600 cm−1 is due to aromatic ring of carbazole (Cz), and 1228 cm−1 (carbazole CeC deformation), at 745 cm−1 is evidence for the out of plane bending of aromatic C–H deformation [25]. The FT-IR spectrum of Cz−COOH has strong bands at 2910 cm−1(C–H), 1703 cm−1 (C]O), 1597 cm-1 (COO-), 1400−1600 cm−1 (aromatic ring of Cz) and broad IR absorbtion band at 3500−2500 cm−1 attributed to (OeH) band stretching with hydrogen bonded structure [29]. Observed FTIR peaks at 2944 cm−1 (C–H stretching), 2250 cm−1 (CΞN), 1191 cm−1 (CeN), 1325 cm−1 (CeC) 740 and 727 cm−1 (C–H aromatic) were obtained by monomer of Cz-CN [30]. According to FTIR-ATR spectra of polymer-coated Pt surfaces, some shifts and some new peaks appearances are observed when it is compared with the monomer spectra above. Also, all peaks are broader after electrochemical polymerization. At the same time, the band of Cz−OH at 1228 cm−1 related to CeC deformation is changed into one band of PCz−OH at 1263 cm−1 which indicates that a new CeC bond between two monomers has formed. The FT-IR spectra of PCz−OH has bands at 1602–1452 cm-1 (aromatic stretching of double bond C]C), 796 cm-1 (C–H deformation of out of plane of trisubstituted 1,2,4 carbazole cycle), 740 cm-1 (deformation out of plane-adjacent 4H 1,2,3,4 at the end of chains of disubstituted carbazole cycle), 1319 cm-1 (vibration of CeN bond of carbazole cycle) [25,29]. The FT-IR spectra of PCz−COOH film on Pt electrode have bands at 1703 cm−1 (C]O) and 1597 cm−1 attributed to COO- (carboxylate in neutral media) group in polymer and the band appears between 1050−1300 cm-1 is assigned to CeO group. The band of Cz-CN at 2240 cm-1 is attributed to nitrile group in and shifted to 2354 cm-1 in PCz-CN. The PCz-CN peaks at 1191 cm-1 and 1325 cm−1, as in Cz-CN, are attributed to CeN, CeC group respectively. According to literature, 1H NMR of carbazole monomer was 4
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Fig. 4. SEM image of polycarbazole films; a) PCz−OH, b) PCz−COOH, and c) PCz-CN. 1 H NMR (DMSO-d6, 400 MHz) of monomer Cz-CN at aromatic region δ(ppm):8.17(CH,d, J =7.7 Hz), 7.67(CH,d, J =8.2 Hz), 7.48 (CH,t, J =7.7 Hz), 7.24 (CH,t, J =7.4 Hz) (Fig. S10) 1 H NMR (DMSO-d6, 400 MHz) of PCz-CN at aromatic region δ(ppm): 8.65, (CH,s), 8.55 (CH,s), 8.35 (CH,d, J =7.7 Hz), 8.23 (CH,d, J =7.7 Hz), 8.10 (CH,d, J =8.7 Hz), 8.00 (CH,d), 7.90 (CH,dd, J =12 Hz, 6.4 Hz), 7.80 (CH,t, J =8.3 Hz), 7.55 (CH,t), 7.30 (CH,m), 7.17(CH,s), 7.05(CH,s) (Fig. S12). Similar 1H NMR results were observed that peaks much broader and shifted to down field for PCz-CN samples due to the polymerization [33,34]. However, 1H NMR spectra of PCz-CN showed that in this case the polymerization mainly may be occurred at 2,7 and 3,6 position (Fig. S12) because in this case two singlet peaks appeared at 8.65 and 8.55 ppm due to 1, 5 position. The four dublet at 8.35, 8.23, 8.10, 8.00 ppm due to 3,4,7,8 positions, also the triplet at 7.90, 7.80, 7.55 ppm and the other multiplet at 7.30 ppm may be due to positive charge on doped conducting polymer backbone or terminal group of polymer. The other singlet peaks were might be observed at 7.04 and 7.17 ppm due to oxidized quinoidal structure (polaronic doped) of PCzCN during oxidative electropolymerization process [35]. 13 C NMR (DMSO-d6, 100 MHz) of Cz-CN at aromatic region δ (ppm): 140.5 (quaterner a),126.43(C2,7), 122.80 (quaterner b), 120.92 (C4,5), 119.64 (C3,6), 119.20(CCN), 110.05 (C1,8). (Figure S11). 13C NMR (DMSO-d6, 100 MHz) of PCz-CN at aromatic region δ (ppm): 139.70, 138.51, 138.00, 131.61, 130.94, 130.57,124.72, 124.03, 123.63, 123.31, 123.19, 122.27, 121.94, 121.48, 121.17, 119.47, 119.22, 118.25, 118.18, 117.84, 117.88, 117.23, 116.85, 115.40, 109.11, 109, 108.77 (Fig. S13). Similarly, equal six different carbon atoms were observed according to literature at aromatic region at 13C NMR spectra of Cz-CN, several peaks of different carbon atoms due to the coupling of different positions after polymerization [34]. Three different quaterner carbon atoms (Ca, Cb and C of CN group) were defined for monomer Cz-OH but six different quaterner carbon atoms due to the new substitution after polymerization according to APT (attached proton test) measurement. Due to the polaronic charge effect and different coupling during electropolymerized some other peaks with low intensity also were observed of electropolymerized PCz-OH spectra. 1 H NMR (DMSO-d6, 400 MHz) of Cz-COOH at aromatic region: δ(ppm): 8.20(CH,d, J =7.7 Hz), 7.60(CH,d, J =8.2 Hz), 7.45 (CH,t, J =7.9 Hz), 7.25 (CH,t, J =7.6 Hz) (Fig. S6). 1 H NMR (DMSO-d6, 400 MHz) of PCz-COOH at aromatic region δ(ppm): 8.65 (CH,s)8.35 (CH,d, J =7.7 Hz), 8.20 (CH,d, J =7.7 Hz), 7.95(CH,d, J =8.6 Hz), 7.85(CH,d), 7.75(CH,d, J =8.2 Hz), 7.6(CH,d, J =8.2 Hz), 7.5(CH,t, J =7.9 Hz), 7.3(CH,t, J =7.2 Hz) 7.15(CH,s) 7.10(CH,s) 5.82(CH2-N-ring,t, J =19.1 Hz) (Fig. S8). 1 H NMR of PCz-COOH (Fig. S8) showed as the singlet peak at 8.65 (CH,s) due to 1H at 1, 8 position. We noticed that the new singlet peak
appeared after polymerization when this peak is doublet in monomer, so this means the polymerization was occurred mainly in may be in 2,7 position. In addition two weak singlet peak were also attributed to at 7.10 and 7.15 ppm due to oxidized quinoidal structure (polaronic doped) of PCz-COOH [35,36]. 13 C NMR (DMSO-d6, 100 MHz) of Cz-COOH at aromatic region δ (ppm): 139.35 (quaterner a), 124.63 (C2,7), 121.2 (quaterner b), 119.10 (C4,5), 118.04(C3,6), 108.24 (C1,8), and 169.15 ppm (C of COOH group) (Fig. S7). 13C NMR (DMSO-d6, 100 MHz) of PCz-COOH at aromatic region δ (ppm): 139.06, 138.09, 137.71, 131.66, 124.68, 124.26, 122.16, 121.86, 121.71, 119.33, 119.03, 118.00 (Fig. S9). Similarly, 13C NMR spectrum of PCz-COOH and APT results confirm the structure of the polymerization product as described in detail for other carbazole compounds above. Finally, after the electrochemical polymerization of all carbazole derivatives studied the broaden of all 1H NMR peaks and shifted to a lower field is one of the most important evidence of the formation of the polymer [33,34]. The three N-substituted carbazole derivatives studied can be polymerized mainly, from positions 2.7 or 3.6, but may also be partially coupled at 2.6 and 3.7 positions. Furthermore, the NMR spectra are greatly affected due to the polaronic charge centers generated by electrochemical oxidation. Scanning electron micrographs (SEM) were used to investigate the morphologies of the polymer films, which are related to spectroelectrochemical properties. Polycarbazole derivates on ITO electrode were grown potentiodynamically (by cyclovoltammetry) with scan rate of 50 mV/s from solution of containing NaClO4-LiClO4 electrolyte dissolved in acetonitrile. Fig. 4 shows the SEM images of PCz-OH, PCz-COOH and PCz-CN films. PCz-OH film shows some big clusters while PCz-COOH have a rough porous surface. On the other hand, PCz-CN film has non-uniform particles with few clusters. Formation of the PCz-CN film on ITO-PET electrode is confirmed by EDX spectrum in the supplementary data (Fig. S14). Before SEM- EDX analysis, all samples were coated by gold to obtain more clear images. Since the Indium Tin Oxide coated PET substrate is used as the electrode material during PCz-CN electropolimerization, the element of In is also clearly observed in EDX analysis. As known, perchlorate has been used as a counter anion during electropolimerization method. Therefore, chlorine peaks appear in the EDX analysis. Different morphology of polycarbazoles may be a benefit for the movement of the doping ion and improving the switching time. The average molecular weight of polycarbazole derivates was determined using gel permeation chromatography (GPC). Tetrahydrofuran (THF) was used as the solvent and polystyrene (PS) was used as a standard in the GPC analysis. The number-average molecular weight (Mw) of the prepared polycarbazole derivates were about 24724 Da, 130980 Da, 29520 Da for PCz-OH, PCz-COOH and PCz-CN respectively (given in detailed data in Table 1) and supplementary data (Fig. S15). Primary studies indicated that number-average
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Table 1 GPC analysis of polycarbazole derivates. Polymer Sample
Average molecular weight (Mw)
Number average molecular weight (Mn)
Dispersity (PDI)
PCz-OH PCz-COOH PCz-CN
24724 130980 29520
5676 98462 28745
4.35 1.33 1.03
Table 2 The solubility parameters of polycarbazole derivatives (mg/mL).
Table 3 Electrochemical and optical properties of monomers and polymers.
Polymers
DMSO
NMP
THF
DMAC
PCz-OH(n) PCz-COOH(n) PCz-CN(n)
50 75 80
2 .0 4.0 2.4
2.0 3.0 2.0
2.8 3.6 4.0
E ox, monomer (V) λmax (nm) E ox, polymer (V) Ered, polymer (V) HOMO (eV) LUMOa (eV) Egop (eV) λmax (nm) Optical Contrasts (%) Switching Times (s)
molecular weight of the polymers was in the range of 2000–5000 Da [37,38]. Relatively high molecular weight polycarbazole derivates were obtained by potentiodynamic electropolymerization method. Polycarbazole was found to be insoluble in almost all inorganic solvents and some of the organic solvents, but the nitrogen atom in the carbazole unit can be easily substituted with a wide variety of functional groups to help polymer solubility [34,35]. The high molecular weight polycarbazoles, PCz-OH, PCz-COOH, and PCz-CN have good solubilities in common organic solvents such as Dimethyl sulfoxide (DMSO), N-Methyl-2-pyrrolidone (NMP), Tetrahydrofuran (THF), and N,N-dimethylacetamide (DMAC) as seen in Table 2. Polycarbazole (PCz) can be electrochemically deposited to yield conducting thin films on desired electrodes and electrical conductivity
Cz-OH
Cz-COOH
Cz-CN
1.24 230/290/325 PCz-OH 1.04/1.20 0.76/0.92 −5.50 eV −2.30 eV 3.14 eV 318/395/875 % 26 2.1 s
1.50 235/290/325 PCz-COOH 1.12/1.48 0.58/0.84 −5.35 eV −1.75 eV 3.61 eV 305/393/845 %17 2.2 s
1.37 231/290/327 PCz-CN 1.10/1.28 0.73/0.90 −5.51 eV −2.11 eV 3.42 eV 303/405/770 %10 4.2 s
of PCz at room temperature is quite low (between 10−4 and 5 × 10-7 Scm-1) [39–41]. The changing of conductivity has been attributed to preparation techniques of the conductive polymers such as chemical or electrochemical methods. Their conductivities are also affected by the degree of moisture content, the morphology and texture of the polymer, its chain length the mode of the deposition and temperature and the
Fig. 5. Spectroelectrochemical behavior of (a) PCz−OH (b) PCz−COOH and (c) PCz-CN under different potentials in 0.1 M NaClO4/LiClO4/ACN solution (d) Comparison of λmax of π−π* transitions for reduced (neutral state) PCz−OH, PCz−COOH and PCz-CN.
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Table 4 The colors and color coordinates of conducting polymers, PCz−OH, PCz−COOH and PCz-CN in accordance with CIE standards. PCz-OH
PCz-COOH
PCz-CN
0.0 V
0.9 V
1.6 V
0.0 V
1.1 V
1.6 V
0.0 V
1.1 V
1.6 V
L:59.100 a:0.438 b:18.920
L:45.710 a:-1.718 b:22.910
L:29.620 a:-11.870 b:15.550
L:57.390 a:0.453 b:7.703
L:52.690 a:-5.116 b:13.030
L:44.990 a:-10.230 b:1.922
L:60.970 a:0.188 b:16.800
L:57.750 a:-3.804 b:15.630
L:46.450 a:-11.690 b:7.398
degree of crystallization [42]. However, the dry conductivity of the carbazole derivatives was measured using four probe conductivity technique as 7.6 × 10−3 S/cm, 4.3 × 10−2 S/cm and 1.62 × 10−4 S/ cm for PCz-OH, PCz-COOH, and PCz-CN respectively. The conductivity of the films was determined at room temperature (25 °C).
PCz−OH, PCz−COOH and PCz-CN respectively. A smaller band-gap value for −OH containing polymer was calculated than that of –CN and −COOH containing polymer, which is compatible with the electrochemical results (Table 3). The UV–vis-NIR absorption data of the monomers and their polymers are summarized in Table 3 together with the corresponding data for the structurally related monomers and polymers. Electrochromic switching studies for the polymers were performed by double potential step chronoamperometry technique (Fig. 6). The optical contrast (ΔT %) can be defined as the transmittance difference between the redox states of the polymer and monitored as a function of time at their absorption maximum (λmax). The optical contrast of PCz−OH, PCz−COOH, and PCz-CN were found to be 26 %, 17 % and 10 % at 875 nm, 845 nm and 770 nm, respectively. Switching time is the necessary time for 95 % of the full optical switch and to determine the switching time by stepping potential repeatedly between the neutral and oxidized states. The switching time of PCz−OH from the reduced to the oxidized state was found to be 2.1 s at 875 nm (Table 3). The color change of the polymers electrodeposited on ITO-coated glass electrodes observed during electrochemical redox processes. To make the measurement of the color in objective and quantitative practice, redox switching of conjugated polymers were further examined through colorimetry using the Commission Internationale de l’Eclairage system (CIE). In CIE system, luminance or brightness, hue and saturation, symbolized with L, a, and b, respectively, are determined to qualify color. Table 4 shows the change of the CIE spectral color coordinates for the polymers. Photoluminescence spectra of polycarbazole samples were shown in Fig. 7. The excitation wavelengths were 272 nm for PCz−OH, 302 nm PCz−COOH and PCz-CN. The emission maxima recorded were 327 nm for PCz−OH, 412 nm for PCz−COOH and PCz-CN. Photoluminescence quantum yields were calculated using a common method with anthracene standard solution (Φ = 28 %) [43–48]. We obtained percentage of quantum yield of polycarbazole derivatives; PCz−OH, PCz−COOH, PCz-CN are 2.5, 2.7 and 2.7 respectively. While photoluminescent materials was not the main issue of this study, the preliminary results given here suggest that the polymers described may be useful for applications such as luminescent sensors.
3.2. In- situ Spectroelectrochemical Studies The spectroelectrochemistry experiments give not only information about the intergap states that appear upon doping but also possible to detect the electronic structure of the conducting polymer such as optical band gap. The UV–vis-NIR absorption spectra of the polymer films in solid state on an ITO electrode are illustrated in Fig. 5. In the reduced form of the polymers, there is no strong absorption peak in the visible region and the polymer films almost transparent. Therefore, an absorption peak belongs to the π-π* was observed at UV region at about 300 nm. The polymer films show the absorption maximum of 305 nm for PCz−COOH and 303 nm for PCz-CN. On the other hand, −OH substituted PCz−OH exhibits longer absorption maxima at 318 nm (Fig. 5d). Stepwise oxidation of the films show newly generated bands due to the formation of charge carriers. Characteristic electronic spectra of the carbazole derivatives are shown in Fig.5. When positive potentials are applied, the spectra exhibit intense bands corresponding to the presence of a polaronic charge carrier. Further oxidation leads to the generation of bipolaronic bands on the polymer chains at longer wavelengths. The formation of these new bands lead to different colorations for conducting polymer films. In-situ spectroelectrochemical studies revealed that PCz−OH, PCz−COOH and PCz-CN films exhibited a multichromic behaviour under various applied potentials (Table 4). The different structures of the polymers due to the differences in substituent groups affect not only the monomer oxidation and the polymer redox potential but also the maximum absorption wavelengths of the corresponding transition. It is known that electron donating group into the polymer backbone has the tendency to increase its HOMO energy level, resulting in a decrease in polymer’s band gap [28]. The optic band gaps of the polymers, defined as the onset of the ππ* transitions in other words from their onset absorption edge of the neutral state spectrum, were determined as 3.1 eV, 3.6 eV and 3.4 eV for
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Fig. 7. Photoluminescence; excitation (line) and emission (dotted line) spectra of polycarbazole in 10−5 mol/L DMSO solution.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests. 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. 116253. References [1] S. Maruyama, X.T. Tao, H. Hokari, T. Noh, Y. Zhang, T. Wada, H. Sasabe, H. Suzuki, T. Watanabe, S. Miyata, Electroluminescent applications of a cyclic carbazole oligomer, J. Mater. Chem. 9 (1999) 893–898, https://doi.org/10.1039/A809313J. [2] N. Ikeda, T. Miyasaka, A solid-state dye-sensitized photovoltaic cell with a poly(Nvinyl- carbazole) hole transporter mediated by an alkali iodide, Chem. Commun. (Camb.) (2005) 1886–1888, https://doi.org/10.1039/b416461j. [3] J.X. Yang, X.T. Tao, C.X. Yuan, Y.X. Yan, L. Wang, Z. Liu, Y. Ren, M.H. Jiang, A facile synthesis and properties of multicarbazole molecules containing multiple vinylene bridges, J. Am. Chem. Soc. 127 (2005) 3278–3279, https://doi.org/10. 1021/ja043510s. [4] Ö. İpsiz, H.Y. Yenilmez, K. Kaya, A. Koca, Z.A. Bayır, Carbazole-substituted metallophthalocyanines: synthesis, electrochemical, and spectroelectrochemical properties, Synth. Met. 217 (2016) 94–101, https://doi.org/10.1016/j.synthmet.2016.03.023. [5] Q. Wang, L. Wang, H. Saadeh, L.P. Yu, A new family of amorphous molecular materials showing large photorefractive effect, Chem. Commun. (Camb.) (1999) 1689–1690, https://doi.org/10.1039/A904767K. [6] P. Cheben, F. Del Monte, D.J. Worsfolds, D.J. Carlsson, C.P. Grover, J.D. Mackenzie, Nature 408 (2000) 64–67, https://doi.org/10.1038/35040513. [7] H. Hoegl, On photoelectric effects in polymers and their sensitization by dopants, J. Phys. Chem. 69 (1965) 755–766, https://doi.org/10.1021/j100887a008. [8] G. Krucaite, S. Grigalevicius, A review on low-molar-mass carbazole- based derivatives for organic light emitting diodes, Synth. Met. (2019) 90–108, https://doi. org/10.1016/j.synthmet.2018.11.017. [9] J.F. Ambrose, R.F. Nelson, Anodic Oxidation Pathways of Carbazoles I. Carbazole and N‐Substituted Derivatives, J. Electrochem. Soc. 115 (11) (1968) 1159–1164, https://doi.org/10.1149/1.2410929. [10] Y. Morishima, Photophysics in amphiphilic polyelectrolyte systems, Prog. Polym. Sci. 15 (1990) 949–997, https://doi.org/10.1016/0079-6700(90)90026-W. [11] M. Nowakowska, B. White, J.E. Guillet, Studies of the antenna effect in polymer molecules. 13. Preparation and photophysical studies of poly (sodium styrenesulfonate-co-2-vinylnaphthalene), Macromolecules 22 (1989) 3903–3908, https://doi.org/10.1021/ma00200a016. [12] C.L. McCormick, C.E. Hoyle, M.D. Clark, Water-Soluble Copolymers. 35. Photophysical and Rheological Studies of the Copolymer of Methacrylic Acid with 2- (1 -Naphthylacetyl) ethyl Acrylate, Macromolecules 23 (1990) 3124–3129, https://doi.org/10.1021/ma00214a015.
Fig. 6. Electrochromic switching responses: optical absorbance change monitored at (a) 875 nm for PCz−OH, (b) 845 nm for PCz−COOH and (c) 770 nm for PCz-CN.
4. Conclusions In this study, we have prepared three carbazole derivatives carrying different N-positions occupied by methanol, carboxylic acid, and cyanoethyl groups. The structures of the monomers and polymers were confirmed by 1H-NMR, 13C-NMR and FTIR spectroscopic techniques. Polycarbazol derivates show different electrochemical and optical properties due to the existence of the different substituent groups. PCz−OH, PCz−COOH, and PCz-CN are soluble in common organic solvents such as DMSO, NMP, THF, DMAC and their high molecular weight allow the preparation of flexible films onto various substrates. These properties indicate that carbazole-based materials will certainly be explored in future years for many optoelectronic applications. What is more, light-harvesting devices and molecular electronics are the most likely fields of research that will take advantage of the great potential of carbazole-based materials.
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Electrosynthesis and characterizations of electrochromic and soluble polymer films based on N- substituted carbazole derivates
T
Hassan Elamin Elkhidra, Zeliha Ertekinb, Yasemin Arslan Udumc, Kadir Pekmezb,* a
Shendi University, Department of Chemistry, Shendi, 47711, Sudan Hacettepe University, Department of Chemistry, Ankara, 06800, Turkey c Technical Sciences Vocational School, Gazi University, Ankara, 06374, Turkey b
ARTICLE INFO
ABSTRACT
Keywords: Redox polymers Electropolymerization Polycarbazoles Electrochromic material
Carbazole derivatives are a class of nitrogen-containing aromatic heterocyclic compounds and they not only have various biological activities but also exhibit useful properties as organic materials due to their special structures. In this study, the compounds based on carbazole derivatives (N-positions were occupied by methanol, carboxylic acid, and cyanoethyl) were synthesized via a simple method. The electrochemistry and electropolymerization of these three monomers were investigated and compared with those of different N-substituted with groups attaching on the active sites of the carbazole units. The polymeric films were prepared on ITO/glass substrate by repetitive cyclic voltammetry (CV) scanning of the monomer solutions containing NaClO4-LiClO4 electrolyte dissolved in acetonitrile. The structures of the monomers and polymers were characterized by 1HNMR, 13C-NMR, and FTIR. The electro-generated polycarbazole films exhibited redox-activity and multichromic properties with increasing potential. The remarkable electrochromic behavior of the films was explained in detail with the help of spectroelectrochemical studies. Photoluminescence spectra and quantum yields were given of polycarbazole samples in solution. In addition, solubility, molecular weight, and conductivity properties of polymers containing N-substituted groups have been discussed further.
1. Introduction Carbazole gained attention as an important component of organic materials because it has excellent electrical and photoelectrical properties. Carbazole derivatives have also a great necessary in light emission diode (LEDs) devices [1–4] and non-linear optical materials with large photorefractive effect [5,6] because of their good hole transporting properties [7] and their charge transfer (CT) complexes [8]. The first report of electropolymerization of carbazole was carried out by Ambrose and Nelson in 1968 [9]. They have exhibited that carbazole could polymerize from 3, 6 and 9, positions and also noted that coupling could proceed through the 1 and 8 positions; however, these positions are sterically hindered due to rigid structure of carbazole. Carbazole based polymers also have been of greater interest because of their interesting thermal, electrical, and photophysical properties [10–13]. Therefore, numerous research focus on their properties and the performance of devices based on carbazole unit for a number of applications such as electrophotography and electroluminescence device hole transport layers, microcavity photoconduction, and as photovoltaic components [8,14,15]. The applications for electrochromic
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organic conjugated polymers, such as polycarbazole, polypyrrole, polyaniline, and polythiophene, have been found to be more promising as electrochromic materials [16,17]. Electrochromic polymers usually show fast switching times, allowing for rapid color changes from the materials upon application of appropriate potentials. In addition, they have excellent switching reproducibility, meaning that they can be switched between their various color states many times without any noticeable decline in performance [18]. Due to these properties, they are used widely in our daily life, such as smart windows, optical displays and e-paper [19,20]. Polycarbazole and its derivates can be built onto desired electrodes by electrochemically deposition technique to yield conductive thin films. The observed spectral changes of the polycarbazole film were fully reversible upon varying the applied potential; significant color changes from yellow-colored reduced polycarbazole films to green-colored oxidized polycarbazole films and easy to detect with the naked eye [21,22]. It was also reported that polycarbazole films are reduced form with low conductivity (> 10 −9 Scm1 ) whereas oxidized films have enhanced conductivity (> 10-3 Scm-1) [21]. Carbazole derivates can be polymerized either at 3- and 6- positions or 2- and 7- positions because these positions are more active than
Corresponding author. E-mail address: [email protected] (K. Pekmez).
https://doi.org/10.1016/j.synthmet.2019.116253 Received 24 July 2019; Received in revised form 5 November 2019; Accepted 26 November 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
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others. However, electrochemical polymerization of carbazole generally results in only poly(3,6-carbazole)s because the 3 and 6 positions are the most reactive sites in the structure. Also, poly(2,7-carbazole) derivatives have more extended conjugation lengths and lower energy band gaps than poly(3,6-carbazole) [23,24]. Electrochemical polymerization is one of the most useful methods for the preparation of conjugated polymers because electroactive polymer can be produced films without the need for purification. However, there is not intensive research on the electrosynthesis and electrochromic properties of redox-active conjugated polymers from soluble polymer films based on N- substituted carbazole derivates. According to this consideration, electropolymerization of the N- positions occupied by methanol (carbazol-9-yl-methanol), carboxylic acid (carbazol-9-yl- carboxylic acid) and cyanoethyl (carbazol-9-yl- cyanoethyl) was systematically carried out to obtain detailed information about using different substitution patterns and physical-chemical properties. In this study, the three different carbazole monomers are prepared successfully, and their electrochemistry and electropolymerization are investigated. On the other hand, solubility, molecular weight and conductivity properties of the polymers have been discussed in detail.
2.2.3. Carbazol-9-yl- cyanoethyl Carbazole (1.7 g, 0.01 mol) and acrylonitrile (12 g, 0.3 mol) were put into a 100-mL glass flask. Then, the flask was placed in an ice bath to cool for 15 min. Benzyltrimethylammonium hydroxide (40 % Triton B in MeOH) as catalyst was added dropwise to the reaction medium. The color of mixture was changed from white to yellow. The obtained paste was heated up to 65 °C, after 1 h the paste became to orange solution. Yellow precipitated was formed after cooling. Then, filtered and recrystallized from acetonitrile [27]. 1H NMR (400 MHz, DMSO) δ 8.17 (d, J =7.7 Hz, 1 H), 7.72 (d, J =8.2 Hz, 1 H), 7.48 (t, J =7.7 Hz, 1 H), 7.24 (t, J =7.4 Hz, 1 H). HRMS calcd. for C15H12N2 (m/z) 220.10, found 220.10 (+ mode). HRMS spectra of monomers were also shown in the supplementary data (Fig. S1). Fig. 1 shows the chemical structure of polycarbazole derivatives. 2.3. Electrochemical measurements and characterization Electrochemical and spectroelectrochemical studies were performed in a three-electrode cell configuration. ITO coated glass slide, platinum wire, and silver wire were used as the working, counter and pseudoreference electrodes, respectively. Pseudo-reference Ag wire electrodes used in electrochemical and spectroelectrochemical studies were adjusted versus the Fc/Fc+ redox couple (+0.3 V). The monomers were electrochemically deposited with cyclic voltammetry technique (CV) using Autolab PGSTAT100 potentiostat–galvanostat system. Lambda 75 UV–vis NIR spectrophotometer of Perkin Elmer was used to record the UV/Vis/NIR spectra with a scan rate of 2000 nm/min. The potentials were controlled by using a Solatron 1285 potentiostat/galvanostat. Color analysis and colorimetry measurements of the polymers were carried out via Konica Minalto CS-100 colorimeter. Polycarbazole samples were prepared in the same concentration of monomer solution (50 mM) using potentiodynamic electropolymerization method between 0.0 V and +1.6 V with 100 mV/s scan rate. The thickness of polymer films was tried to control by applying same cyclic number. Conductivity measurements were carried out using four-probe techniques. Our four- probe equipment contains four spring sensitive cylindrical platinum tips (d = 0.50 mm placed on linearly 5 mm PTFE head) distance between four tips are 0.50 mm. Film thickness of polymeric sample is measured using a digital micrometer. Thickness of polymeric film samples was selected about 100−150 μm for conductivity measurement. Polymer samples coated on platinum macro electrode were stripped and cut to 2 × 5 mm dimensions and placed under four probe tips. While conductivity measurement is carried out drive current from the two external tips of the four probe and measuring the voltage drop over the two internal tips. ENTEK conductivity measurement device was calculated automatically conductivity of sample using drive current, voltage drop and film thickness. Polycarbazole samples were prepared using same potentiodynamic electropolymerization method between 0.0 V and +1.6 V versus pseudo
2. Experimental 2.1. Materials Formaldehyde (%37, Merck), sodium hydroxide (Fluka), ethanol (Riedel-de Haën), n-hexane (BDH), carbazole (BDH), bromoacetic acid (Sigma-Aldrich), acrylonitrile (Merck), benzyltrimethyl ammonium hydroxide (Alfa Aesar) have been used. Acetonitrile (Merck, HPLC grade) and dimethyl sulfoxide (Riedel-de Haën) were of analytical grade and used as received. LiClO4 and NaLiClO4 as a supporting electrolyte were also obtained from Sigma Aldrich. All chemicals were employed without further purification. Transparent indium tin oxide (ITO) coated polyethylene terephthalate (60 Ω /sq) plates obtained from Sigma-Aldrich and glass plates (15 Ω/sq) obtained from Delta Technologies. 2.2. Synthesis 2.2.1. Carbazol-9-yl-methanol Tawney method was used to prepare Cz-OH. Carbazole (10 g, 0.06 mol) was added to successively with 25 mL of 37 % formaldehyde and 2 mL of 5 % sodium hydroxide. The carbazole had dissolved within 10 min and then, the crude product began to form. After 2.5 h, the solution was filtered at room temperature and dried under vacuum. Then, twice recrystallizations were made from n-hexane to obtain the product [25]. 1H NMR (400 MHz, DMSO) δ 8.17 (s, 1 H), 8.16 (d, J =7.5 Hz, 3 H), 7.69 (d, J =7.9 Hz, 3 H), 7.47 (d, J =7.5 Hz, 3 H), 7.24 (t, J =7.0 Hz, 3 H), 6.43 (s, 1 H), 5.79 (s, 2 H). HRMS calcd. for C13H11NO (m/z) 197.08, found 197.08 (+ mode). 2.2.2. Carbazol-9-yl- carboxylic acid Cz-COOH was synthesized using a procedure from the literatüre [26]. Carbazole (16.7 g, 0.1 mol) and sodium hydroxide (12 g, 0.3 mol) dissolved in DMSO (40 ml) and heated to 85 °C. Obtained dark brown solution was stirred for 30 min. Then, bromacetic acid (16.68 g, 0.12 mol) was added carefully. The solution was stirred overnight and then poured into 400 ml cold water. After filtration, the filtered solution separated from solid residue and pH value was adjusted to 3-4. Obtained white precipitate was seperated by second filtration, washed several times with water, and dried in air. 1H NMR (400 MHz, DMSO) δ 8.17 (d, J =7.7 Hz, 1 H), 7.58 (d, J =8.2 Hz, 1 H), 7.45 (d, J =7.9 Hz, 1 H), 7.23 (d, J =7.6 Hz, 1 H). HRMS calcd. for C14H11NO2 (m/z) 225.08, found 225.08 (+ mode).
Fig. 1. The chemical structure of polycarbazole derivatives R: −OH (carbazol9-yl-methanol); −COOH (carbazol-9-yl- carboxylic acid) and −CH2CN (carbazol-9-yl- cyanoethyl). 2
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Ag reference electrode in monomer containing 0.1 M NaClO4-LiClO4/ ACN solution on Pt sheet macro working electrode. Obtained samples using same cycle number electropolymerization technique were used for FTIR-ATR (Diffuse Reflectance, Thermo model-NICOLET-IS 10 FTIR) characterization after washed several times with acetonitrile solution to remove electrolyte from polymer. All polymer films dried under vacuum at room temperature. Electropolymerized polycarbazole samples on Pt electrode were dissolved by immersion in dimethyl sulfoxide (DMSO) and this process was repeated several times to concentrate the solution. Purification of polycarbazole samples was carried out using reprecipitation method from obtained concentrated polycarbazole solution by addition of acetonitrile in their DMSO solution. Monomer and electrolyte were soluble in acetonitrile-DMSO mixture solution whereas polymer couldn’t soluble. By this way, reprecipitated polymeric samples were centrifuged and separated. Reprecipitated samples were used for NMR (Bruker 400 MHz AV NMR) spectroscopic characterization after filtered and dried under reduced pressure. Obtained all purified polycarbazole samples were redissolved in DMSO-d6 for 1H and 13C NMR methods in the supplementary data. Chemical shifts (δ, ppm) were determined with TMS as the internal reference. Polycarbazole samples were prepared using potentiodynamic electropolymerization method between 0.0 V and +1.6 V in 50 mM monomer containing 0.1 M NaClO4-LiClO4/ACN solution on Pt sheet macro working electrode for photoluminescence experiment. Obtained polycarbazole films were washed in acetonitrile solvent and dried nitrogen stream and then, polycarbazole samples on Pt electrode were dissolved by immersion in dimethyl sulfoxide (DMSO) solvent up to saturation (shown in Table 2). Polycarbazole solutions were diluted to 10−3 and 10-5 mol/L, respectively, using saturated solution. Fluorescence spectra were measured with a commercial spectrophotometer (Agilent Technologies, Cary Eclipse Fluorescence Spectrophotometer). 3. Results and discussion 3.1. Electrochemical polymerization and characterization studies Redox behavior of carbazol-9-yl-methanol (Cz-OH); carbazol-9-ylcarboxylic acid (Cz-COOH) and carbazol-9-yl- cyanoethyl (Cz-CN) was investigated by cyclic voltammetry technique in order the see the effect of substituent group in the monomer structure. Electropolymerization of the monomers was performed in an electrolyte solution of 0.1 M NaClO4-LiClO4 dissolved in acetonitrile a scan rate of 100 mV/s. The monomer concentrations were used as 1.10−2 M and electrode areas as 1 cm2 during the electrochemistry studies. The current density in the cyclic voltammogram was simply calculated by dividing the current to surface area. The irreversible oxidation peaks for Cz−OH, Cz−COOH, and Cz-CN were observed at +1.24 V, +1.50 V and +1.37 V at a bare ITO glass (Fig. 2). There is no reduction at the reverse cycle which also defined as an irreversible process. After the electrochemical polymerization of the monomers, reversible redox peaks of the polymers were obtained in blank solution (Fig. 2 inset graphs). The difference in monomer oxidation potentials can be due to the different electronic nature of substituent groups. The monomer oxidation peaks of the Cz−OH observed at lower potential as compared to Cz−COOH and CzCN. Moreover, the onset oxidation potential (Eonset) of Cz−OH shifts to lower potential than that of Cz−COOH and Cz-CN, which is consistent with the presence of the −OH group in the monomer structure, making it electron richer and easily oxidized [28]. With the increase of the cycles number, redox peaks were observed as reversible and at lower potentials due to the formation of the electroactive polymer film on the ITO/glass surface. Electrochemical behaviours of the polymeric films were carried out in monomer-free
Fig. 2. Cyclic voltammetry of (a) Cz−OH, (b) Cz−COOH, (c) Cz-CN at 100 mV/s in 0.1 M NaClO4/LiClO4/ACN solution. Each Inset graph shows CV of related polymeric films that were carried out in monomer-free same solution.
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reported that the peaks as values (aromatic range) at 8.16 (d, 2 H); 7.65 (d, 2 H); 7.55 (t, 2 H); 7.28 (s, 2 H), D-DMSO (ppm) [31]. Carbazoles could be easily functionalized at its 3,6- and 2,7- or N- positions and then covalently linked into polymeric systems [23]. In our study, the Npositions were occupied by methanol (carbazol-9-yl-methanol), carboxylic acid (carbazol-9-yl-carboxylic acid) and cyanoethyl (carbazol-9yl- cyanoethyl). For this reason, the poly N-substituted carbazole may be functionalized at 3,6- and 2,7- positions because these positions is more active than others. Poly(2,7-carbazole) derivatives have more extended conjugation lengths and lower energy bandgaps than poly (3,6-carbazole) [23]. 1 H NMR (DMSO-d6, 400 MHz) of Cz−OH at aromatic region δ(ppm):8.16(CH,d, J =7.5 Hz), 7.70(CH,d, J =7.9 Hz), 7.45 (CH,t, J =7.5 Hz), 7.25 (CH,t, J =7.0 Hz) and 6.45(OH,t), 5.8 (CH2,d) (Fig. S2) 1 H NMR (DMSO-d6, 400 MHz) of PCz−OH at aromatic region δ(ppm): 8.60, (CH,s), 8.31 (CH,d, J =7.8 Hz), 8.12 (CH,d), 7.90 (CH,d, J =8.5 Hz), 7.78 (CH,d), 7.70 ppm (CH,t), 7.40 ppm (CH,t), 7.25 ppm (CH,dd, J = 15.7, 7.8 Hz) (Fig. S4) 1 H NMR spectrum of monomer of Cz-OH (Fig. S2) was exhibit the doublet peak at 8.16 due to 1H at 4, 5 position, the doublet (CH, d) at 7.70 ppm due to 1H at 1,8 position, the triplet at 7.45 (CH, t) due to 1H at 2, 7 position, the other triplet peak at 7.25 ppm due to 1 H(CH, t), at 3,6 position [32]. In addition, the triplet peak which was appeared at 6.45 ppm (OH,t, disappeared after shaking with D2O) and the doublet at 5.8 due to aliphatic CH2 bounded to OH (CH2,d) (Fig. S2). As mentioned above, the polymerization of carbazoles mainly proceeds by CeC and CeN couplings but in our derivatives, C_N couplings don’t appear because the N atom closed by substituted. Proton peaks of PCz−OH are much broader than the corresponding peaks of Cz−OH due to the molar mass distribution of polymeric structure. In addition, chemical shift of the PCz−OH protons observed to much lower field because of the formation of extended conjugated delocalized structure [33,34]. If we suggested the polymerization coupled at 2, 7 position, 1H NMR of PCz−OH (Fig. S4) showed as the singlet peak at 8.60 (CH,s) due to 1H at 1, 8 position after electrochemical polymerization of monomers. We noticed that the new singlet peak appeared after polymerization when this peak is doublet in monomer, so this means, the polymerization occurred mainly in 2,7 position. The two doublet at 8.31, 8.12 ppm (CH,d) due to H in 4, 5 position and 3,6 position. The triplet peak at 7.70 ppm which may be due to terminal carbazole group. On the other hand, multiplet peak at 7.25 ppm also may be to positive charge on doped conducting (polaronic structure) polymer backbone. According to the formation or the coupling of polymer at 3,6 position, we obtained singlet at 8.6 ppm (CH,s) due to 1H, at 4,5 position. The two dublet peak at 8.12, 8.31 ppm(CH,d), due to 1H for 2, 7 position and 1,8 positions. If the polymer was attached to 1,5 position, the singlet peak should not be observed. If the polymerization coupled at 2,6 position we obtained four doublet for 3, 4, 7, 8 position and two different singlet for 1, 5 position (Fig. S2) 13 C NMR (DMSO-d6, 100 MHz) of Cz−OH at aromatic region δ(ppm): 139.68 (quaterner a), 125.90 (C2,7), 122.50 (quaterner b), 120,12 (C4,5), 119.31 (C3,6), 109.73 (C1,8). (Fig. S3). 13C NMR (DMSOd6, 100 MHz) of PCz-OH at aromatic region δ: 140.50, 139.31, 133.31, 126.22, 125.44, 123.67, 120.81, 122, 119.85, 118.61, 110.25, 110.18. (Figure S5). Equal six different carbon atoms (Ca, Cb, C1=C8, C2=C7, C3=C6, C4=C5) were observed according to literature at aromatic region of monomer Cz−OH 13C NMR spectra followed by twelve peaks of twelve different carbon atoms after polymerization [34]. In addition equal two different quaterner carbon atoms (Ca and Cb) were defined for monomer Cz−OH, six different quaterner carbon atoms due to the new substitution after polymerization according to APT (attached proton test) measurement. We also observed some other peaks with low intensity may be due to different coupling of electropolymerized PCz−OH and polaronic charge effect along to doped polymeric backbone.
Fig. 3. FTIR-ATR spectra of monomers; Cz−OH, Cz−COOH, Cz-CN and polymers; PCz−OH, PCz−COOH, PCz-CN.
solution of 0.1 M NaClO4/LiClO4 /ACN by separate cyclic voltammetric experiment and shown at inset graph in Fig. 2. Partially overlapped two successive anodic peaks were observed for carbazole derivatives versus pseudo reference electrode (Ag wire). As depicted in the insets of Fig. 2, on the anodic scans, polymer oxidations were observed at 1.04 V/ 1.20 V for PCz−OH, 1.12 V/1.48 V for PCz−COOH and 1.10 V/1.28 V for PCz-CN. On the reverse scans, reduction peaks evolved at 0.76 V/ 0.92 V for PCz−OH, 0.58 V/0.84 V for PCz−COOH and 0.73 V/ 0.90 V for PCz-CN. FTIR-ATR spectra of monomers and polymers were given in Fig. 3. The monomer of Cz−OH has characteristics peaks at 3403, 2913, 1614, 1599, 1448, 1330, 1228, 981, 742, 717 cm−1. The FTIR-ATR spectrum of Cz−OH indicated that the broad band at 3000−3500 cm−1 attributed to hydrogen bonded alcohol (OeH) band stretching, and 2913 cm−1 can be attributed to methyl (CH2) groups of Cz−OH, 1400−1600 cm−1 is due to aromatic ring of carbazole (Cz), and 1228 cm−1 (carbazole CeC deformation), at 745 cm−1 is evidence for the out of plane bending of aromatic C–H deformation [25]. The FT-IR spectrum of Cz−COOH has strong bands at 2910 cm−1(C–H), 1703 cm−1 (C]O), 1597 cm-1 (COO-), 1400−1600 cm−1 (aromatic ring of Cz) and broad IR absorbtion band at 3500−2500 cm−1 attributed to (OeH) band stretching with hydrogen bonded structure [29]. Observed FTIR peaks at 2944 cm−1 (C–H stretching), 2250 cm−1 (CΞN), 1191 cm−1 (CeN), 1325 cm−1 (CeC) 740 and 727 cm−1 (C–H aromatic) were obtained by monomer of Cz-CN [30]. According to FTIR-ATR spectra of polymer-coated Pt surfaces, some shifts and some new peaks appearances are observed when it is compared with the monomer spectra above. Also, all peaks are broader after electrochemical polymerization. At the same time, the band of Cz−OH at 1228 cm−1 related to CeC deformation is changed into one band of PCz−OH at 1263 cm−1 which indicates that a new CeC bond between two monomers has formed. The FT-IR spectra of PCz−OH has bands at 1602–1452 cm-1 (aromatic stretching of double bond C]C), 796 cm-1 (C–H deformation of out of plane of trisubstituted 1,2,4 carbazole cycle), 740 cm-1 (deformation out of plane-adjacent 4H 1,2,3,4 at the end of chains of disubstituted carbazole cycle), 1319 cm-1 (vibration of CeN bond of carbazole cycle) [25,29]. The FT-IR spectra of PCz−COOH film on Pt electrode have bands at 1703 cm−1 (C]O) and 1597 cm−1 attributed to COO- (carboxylate in neutral media) group in polymer and the band appears between 1050−1300 cm-1 is assigned to CeO group. The band of Cz-CN at 2240 cm-1 is attributed to nitrile group in and shifted to 2354 cm-1 in PCz-CN. The PCz-CN peaks at 1191 cm-1 and 1325 cm−1, as in Cz-CN, are attributed to CeN, CeC group respectively. According to literature, 1H NMR of carbazole monomer was 4
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Fig. 4. SEM image of polycarbazole films; a) PCz−OH, b) PCz−COOH, and c) PCz-CN. 1 H NMR (DMSO-d6, 400 MHz) of monomer Cz-CN at aromatic region δ(ppm):8.17(CH,d, J =7.7 Hz), 7.67(CH,d, J =8.2 Hz), 7.48 (CH,t, J =7.7 Hz), 7.24 (CH,t, J =7.4 Hz) (Fig. S10) 1 H NMR (DMSO-d6, 400 MHz) of PCz-CN at aromatic region δ(ppm): 8.65, (CH,s), 8.55 (CH,s), 8.35 (CH,d, J =7.7 Hz), 8.23 (CH,d, J =7.7 Hz), 8.10 (CH,d, J =8.7 Hz), 8.00 (CH,d), 7.90 (CH,dd, J =12 Hz, 6.4 Hz), 7.80 (CH,t, J =8.3 Hz), 7.55 (CH,t), 7.30 (CH,m), 7.17(CH,s), 7.05(CH,s) (Fig. S12). Similar 1H NMR results were observed that peaks much broader and shifted to down field for PCz-CN samples due to the polymerization [33,34]. However, 1H NMR spectra of PCz-CN showed that in this case the polymerization mainly may be occurred at 2,7 and 3,6 position (Fig. S12) because in this case two singlet peaks appeared at 8.65 and 8.55 ppm due to 1, 5 position. The four dublet at 8.35, 8.23, 8.10, 8.00 ppm due to 3,4,7,8 positions, also the triplet at 7.90, 7.80, 7.55 ppm and the other multiplet at 7.30 ppm may be due to positive charge on doped conducting polymer backbone or terminal group of polymer. The other singlet peaks were might be observed at 7.04 and 7.17 ppm due to oxidized quinoidal structure (polaronic doped) of PCzCN during oxidative electropolymerization process [35]. 13 C NMR (DMSO-d6, 100 MHz) of Cz-CN at aromatic region δ (ppm): 140.5 (quaterner a),126.43(C2,7), 122.80 (quaterner b), 120.92 (C4,5), 119.64 (C3,6), 119.20(CCN), 110.05 (C1,8). (Figure S11). 13C NMR (DMSO-d6, 100 MHz) of PCz-CN at aromatic region δ (ppm): 139.70, 138.51, 138.00, 131.61, 130.94, 130.57,124.72, 124.03, 123.63, 123.31, 123.19, 122.27, 121.94, 121.48, 121.17, 119.47, 119.22, 118.25, 118.18, 117.84, 117.88, 117.23, 116.85, 115.40, 109.11, 109, 108.77 (Fig. S13). Similarly, equal six different carbon atoms were observed according to literature at aromatic region at 13C NMR spectra of Cz-CN, several peaks of different carbon atoms due to the coupling of different positions after polymerization [34]. Three different quaterner carbon atoms (Ca, Cb and C of CN group) were defined for monomer Cz-OH but six different quaterner carbon atoms due to the new substitution after polymerization according to APT (attached proton test) measurement. Due to the polaronic charge effect and different coupling during electropolymerized some other peaks with low intensity also were observed of electropolymerized PCz-OH spectra. 1 H NMR (DMSO-d6, 400 MHz) of Cz-COOH at aromatic region: δ(ppm): 8.20(CH,d, J =7.7 Hz), 7.60(CH,d, J =8.2 Hz), 7.45 (CH,t, J =7.9 Hz), 7.25 (CH,t, J =7.6 Hz) (Fig. S6). 1 H NMR (DMSO-d6, 400 MHz) of PCz-COOH at aromatic region δ(ppm): 8.65 (CH,s)8.35 (CH,d, J =7.7 Hz), 8.20 (CH,d, J =7.7 Hz), 7.95(CH,d, J =8.6 Hz), 7.85(CH,d), 7.75(CH,d, J =8.2 Hz), 7.6(CH,d, J =8.2 Hz), 7.5(CH,t, J =7.9 Hz), 7.3(CH,t, J =7.2 Hz) 7.15(CH,s) 7.10(CH,s) 5.82(CH2-N-ring,t, J =19.1 Hz) (Fig. S8). 1 H NMR of PCz-COOH (Fig. S8) showed as the singlet peak at 8.65 (CH,s) due to 1H at 1, 8 position. We noticed that the new singlet peak
appeared after polymerization when this peak is doublet in monomer, so this means the polymerization was occurred mainly in may be in 2,7 position. In addition two weak singlet peak were also attributed to at 7.10 and 7.15 ppm due to oxidized quinoidal structure (polaronic doped) of PCz-COOH [35,36]. 13 C NMR (DMSO-d6, 100 MHz) of Cz-COOH at aromatic region δ (ppm): 139.35 (quaterner a), 124.63 (C2,7), 121.2 (quaterner b), 119.10 (C4,5), 118.04(C3,6), 108.24 (C1,8), and 169.15 ppm (C of COOH group) (Fig. S7). 13C NMR (DMSO-d6, 100 MHz) of PCz-COOH at aromatic region δ (ppm): 139.06, 138.09, 137.71, 131.66, 124.68, 124.26, 122.16, 121.86, 121.71, 119.33, 119.03, 118.00 (Fig. S9). Similarly, 13C NMR spectrum of PCz-COOH and APT results confirm the structure of the polymerization product as described in detail for other carbazole compounds above. Finally, after the electrochemical polymerization of all carbazole derivatives studied the broaden of all 1H NMR peaks and shifted to a lower field is one of the most important evidence of the formation of the polymer [33,34]. The three N-substituted carbazole derivatives studied can be polymerized mainly, from positions 2.7 or 3.6, but may also be partially coupled at 2.6 and 3.7 positions. Furthermore, the NMR spectra are greatly affected due to the polaronic charge centers generated by electrochemical oxidation. Scanning electron micrographs (SEM) were used to investigate the morphologies of the polymer films, which are related to spectroelectrochemical properties. Polycarbazole derivates on ITO electrode were grown potentiodynamically (by cyclovoltammetry) with scan rate of 50 mV/s from solution of containing NaClO4-LiClO4 electrolyte dissolved in acetonitrile. Fig. 4 shows the SEM images of PCz-OH, PCz-COOH and PCz-CN films. PCz-OH film shows some big clusters while PCz-COOH have a rough porous surface. On the other hand, PCz-CN film has non-uniform particles with few clusters. Formation of the PCz-CN film on ITO-PET electrode is confirmed by EDX spectrum in the supplementary data (Fig. S14). Before SEM- EDX analysis, all samples were coated by gold to obtain more clear images. Since the Indium Tin Oxide coated PET substrate is used as the electrode material during PCz-CN electropolimerization, the element of In is also clearly observed in EDX analysis. As known, perchlorate has been used as a counter anion during electropolimerization method. Therefore, chlorine peaks appear in the EDX analysis. Different morphology of polycarbazoles may be a benefit for the movement of the doping ion and improving the switching time. The average molecular weight of polycarbazole derivates was determined using gel permeation chromatography (GPC). Tetrahydrofuran (THF) was used as the solvent and polystyrene (PS) was used as a standard in the GPC analysis. The number-average molecular weight (Mw) of the prepared polycarbazole derivates were about 24724 Da, 130980 Da, 29520 Da for PCz-OH, PCz-COOH and PCz-CN respectively (given in detailed data in Table 1) and supplementary data (Fig. S15). Primary studies indicated that number-average
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Table 1 GPC analysis of polycarbazole derivates. Polymer Sample
Average molecular weight (Mw)
Number average molecular weight (Mn)
Dispersity (PDI)
PCz-OH PCz-COOH PCz-CN
24724 130980 29520
5676 98462 28745
4.35 1.33 1.03
Table 2 The solubility parameters of polycarbazole derivatives (mg/mL).
Table 3 Electrochemical and optical properties of monomers and polymers.
Polymers
DMSO
NMP
THF
DMAC
PCz-OH(n) PCz-COOH(n) PCz-CN(n)
50 75 80
2 .0 4.0 2.4
2.0 3.0 2.0
2.8 3.6 4.0
E ox, monomer (V) λmax (nm) E ox, polymer (V) Ered, polymer (V) HOMO (eV) LUMOa (eV) Egop (eV) λmax (nm) Optical Contrasts (%) Switching Times (s)
molecular weight of the polymers was in the range of 2000–5000 Da [37,38]. Relatively high molecular weight polycarbazole derivates were obtained by potentiodynamic electropolymerization method. Polycarbazole was found to be insoluble in almost all inorganic solvents and some of the organic solvents, but the nitrogen atom in the carbazole unit can be easily substituted with a wide variety of functional groups to help polymer solubility [34,35]. The high molecular weight polycarbazoles, PCz-OH, PCz-COOH, and PCz-CN have good solubilities in common organic solvents such as Dimethyl sulfoxide (DMSO), N-Methyl-2-pyrrolidone (NMP), Tetrahydrofuran (THF), and N,N-dimethylacetamide (DMAC) as seen in Table 2. Polycarbazole (PCz) can be electrochemically deposited to yield conducting thin films on desired electrodes and electrical conductivity
Cz-OH
Cz-COOH
Cz-CN
1.24 230/290/325 PCz-OH 1.04/1.20 0.76/0.92 −5.50 eV −2.30 eV 3.14 eV 318/395/875 % 26 2.1 s
1.50 235/290/325 PCz-COOH 1.12/1.48 0.58/0.84 −5.35 eV −1.75 eV 3.61 eV 305/393/845 %17 2.2 s
1.37 231/290/327 PCz-CN 1.10/1.28 0.73/0.90 −5.51 eV −2.11 eV 3.42 eV 303/405/770 %10 4.2 s
of PCz at room temperature is quite low (between 10−4 and 5 × 10-7 Scm-1) [39–41]. The changing of conductivity has been attributed to preparation techniques of the conductive polymers such as chemical or electrochemical methods. Their conductivities are also affected by the degree of moisture content, the morphology and texture of the polymer, its chain length the mode of the deposition and temperature and the
Fig. 5. Spectroelectrochemical behavior of (a) PCz−OH (b) PCz−COOH and (c) PCz-CN under different potentials in 0.1 M NaClO4/LiClO4/ACN solution (d) Comparison of λmax of π−π* transitions for reduced (neutral state) PCz−OH, PCz−COOH and PCz-CN.
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Table 4 The colors and color coordinates of conducting polymers, PCz−OH, PCz−COOH and PCz-CN in accordance with CIE standards. PCz-OH
PCz-COOH
PCz-CN
0.0 V
0.9 V
1.6 V
0.0 V
1.1 V
1.6 V
0.0 V
1.1 V
1.6 V
L:59.100 a:0.438 b:18.920
L:45.710 a:-1.718 b:22.910
L:29.620 a:-11.870 b:15.550
L:57.390 a:0.453 b:7.703
L:52.690 a:-5.116 b:13.030
L:44.990 a:-10.230 b:1.922
L:60.970 a:0.188 b:16.800
L:57.750 a:-3.804 b:15.630
L:46.450 a:-11.690 b:7.398
degree of crystallization [42]. However, the dry conductivity of the carbazole derivatives was measured using four probe conductivity technique as 7.6 × 10−3 S/cm, 4.3 × 10−2 S/cm and 1.62 × 10−4 S/ cm for PCz-OH, PCz-COOH, and PCz-CN respectively. The conductivity of the films was determined at room temperature (25 °C).
PCz−OH, PCz−COOH and PCz-CN respectively. A smaller band-gap value for −OH containing polymer was calculated than that of –CN and −COOH containing polymer, which is compatible with the electrochemical results (Table 3). The UV–vis-NIR absorption data of the monomers and their polymers are summarized in Table 3 together with the corresponding data for the structurally related monomers and polymers. Electrochromic switching studies for the polymers were performed by double potential step chronoamperometry technique (Fig. 6). The optical contrast (ΔT %) can be defined as the transmittance difference between the redox states of the polymer and monitored as a function of time at their absorption maximum (λmax). The optical contrast of PCz−OH, PCz−COOH, and PCz-CN were found to be 26 %, 17 % and 10 % at 875 nm, 845 nm and 770 nm, respectively. Switching time is the necessary time for 95 % of the full optical switch and to determine the switching time by stepping potential repeatedly between the neutral and oxidized states. The switching time of PCz−OH from the reduced to the oxidized state was found to be 2.1 s at 875 nm (Table 3). The color change of the polymers electrodeposited on ITO-coated glass electrodes observed during electrochemical redox processes. To make the measurement of the color in objective and quantitative practice, redox switching of conjugated polymers were further examined through colorimetry using the Commission Internationale de l’Eclairage system (CIE). In CIE system, luminance or brightness, hue and saturation, symbolized with L, a, and b, respectively, are determined to qualify color. Table 4 shows the change of the CIE spectral color coordinates for the polymers. Photoluminescence spectra of polycarbazole samples were shown in Fig. 7. The excitation wavelengths were 272 nm for PCz−OH, 302 nm PCz−COOH and PCz-CN. The emission maxima recorded were 327 nm for PCz−OH, 412 nm for PCz−COOH and PCz-CN. Photoluminescence quantum yields were calculated using a common method with anthracene standard solution (Φ = 28 %) [43–48]. We obtained percentage of quantum yield of polycarbazole derivatives; PCz−OH, PCz−COOH, PCz-CN are 2.5, 2.7 and 2.7 respectively. While photoluminescent materials was not the main issue of this study, the preliminary results given here suggest that the polymers described may be useful for applications such as luminescent sensors.
3.2. In- situ Spectroelectrochemical Studies The spectroelectrochemistry experiments give not only information about the intergap states that appear upon doping but also possible to detect the electronic structure of the conducting polymer such as optical band gap. The UV–vis-NIR absorption spectra of the polymer films in solid state on an ITO electrode are illustrated in Fig. 5. In the reduced form of the polymers, there is no strong absorption peak in the visible region and the polymer films almost transparent. Therefore, an absorption peak belongs to the π-π* was observed at UV region at about 300 nm. The polymer films show the absorption maximum of 305 nm for PCz−COOH and 303 nm for PCz-CN. On the other hand, −OH substituted PCz−OH exhibits longer absorption maxima at 318 nm (Fig. 5d). Stepwise oxidation of the films show newly generated bands due to the formation of charge carriers. Characteristic electronic spectra of the carbazole derivatives are shown in Fig.5. When positive potentials are applied, the spectra exhibit intense bands corresponding to the presence of a polaronic charge carrier. Further oxidation leads to the generation of bipolaronic bands on the polymer chains at longer wavelengths. The formation of these new bands lead to different colorations for conducting polymer films. In-situ spectroelectrochemical studies revealed that PCz−OH, PCz−COOH and PCz-CN films exhibited a multichromic behaviour under various applied potentials (Table 4). The different structures of the polymers due to the differences in substituent groups affect not only the monomer oxidation and the polymer redox potential but also the maximum absorption wavelengths of the corresponding transition. It is known that electron donating group into the polymer backbone has the tendency to increase its HOMO energy level, resulting in a decrease in polymer’s band gap [28]. The optic band gaps of the polymers, defined as the onset of the ππ* transitions in other words from their onset absorption edge of the neutral state spectrum, were determined as 3.1 eV, 3.6 eV and 3.4 eV for
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Fig. 7. Photoluminescence; excitation (line) and emission (dotted line) spectra of polycarbazole in 10−5 mol/L DMSO solution.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests. 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. 116253. References [1] S. Maruyama, X.T. Tao, H. Hokari, T. Noh, Y. Zhang, T. Wada, H. Sasabe, H. Suzuki, T. Watanabe, S. Miyata, Electroluminescent applications of a cyclic carbazole oligomer, J. Mater. Chem. 9 (1999) 893–898, https://doi.org/10.1039/A809313J. [2] N. Ikeda, T. Miyasaka, A solid-state dye-sensitized photovoltaic cell with a poly(Nvinyl- carbazole) hole transporter mediated by an alkali iodide, Chem. Commun. (Camb.) (2005) 1886–1888, https://doi.org/10.1039/b416461j. [3] J.X. Yang, X.T. Tao, C.X. Yuan, Y.X. Yan, L. Wang, Z. Liu, Y. Ren, M.H. Jiang, A facile synthesis and properties of multicarbazole molecules containing multiple vinylene bridges, J. Am. Chem. Soc. 127 (2005) 3278–3279, https://doi.org/10. 1021/ja043510s. [4] Ö. İpsiz, H.Y. Yenilmez, K. Kaya, A. Koca, Z.A. Bayır, Carbazole-substituted metallophthalocyanines: synthesis, electrochemical, and spectroelectrochemical properties, Synth. Met. 217 (2016) 94–101, https://doi.org/10.1016/j.synthmet.2016.03.023. [5] Q. Wang, L. Wang, H. Saadeh, L.P. Yu, A new family of amorphous molecular materials showing large photorefractive effect, Chem. Commun. (Camb.) (1999) 1689–1690, https://doi.org/10.1039/A904767K. [6] P. Cheben, F. Del Monte, D.J. Worsfolds, D.J. Carlsson, C.P. Grover, J.D. Mackenzie, Nature 408 (2000) 64–67, https://doi.org/10.1038/35040513. [7] H. Hoegl, On photoelectric effects in polymers and their sensitization by dopants, J. Phys. Chem. 69 (1965) 755–766, https://doi.org/10.1021/j100887a008. [8] G. Krucaite, S. Grigalevicius, A review on low-molar-mass carbazole- based derivatives for organic light emitting diodes, Synth. Met. (2019) 90–108, https://doi. org/10.1016/j.synthmet.2018.11.017. [9] J.F. Ambrose, R.F. Nelson, Anodic Oxidation Pathways of Carbazoles I. Carbazole and N‐Substituted Derivatives, J. Electrochem. Soc. 115 (11) (1968) 1159–1164, https://doi.org/10.1149/1.2410929. [10] Y. Morishima, Photophysics in amphiphilic polyelectrolyte systems, Prog. Polym. Sci. 15 (1990) 949–997, https://doi.org/10.1016/0079-6700(90)90026-W. [11] M. Nowakowska, B. White, J.E. Guillet, Studies of the antenna effect in polymer molecules. 13. Preparation and photophysical studies of poly (sodium styrenesulfonate-co-2-vinylnaphthalene), Macromolecules 22 (1989) 3903–3908, https://doi.org/10.1021/ma00200a016. [12] C.L. McCormick, C.E. Hoyle, M.D. Clark, Water-Soluble Copolymers. 35. Photophysical and Rheological Studies of the Copolymer of Methacrylic Acid with 2- (1 -Naphthylacetyl) ethyl Acrylate, Macromolecules 23 (1990) 3124–3129, https://doi.org/10.1021/ma00214a015.
Fig. 6. Electrochromic switching responses: optical absorbance change monitored at (a) 875 nm for PCz−OH, (b) 845 nm for PCz−COOH and (c) 770 nm for PCz-CN.
4. Conclusions In this study, we have prepared three carbazole derivatives carrying different N-positions occupied by methanol, carboxylic acid, and cyanoethyl groups. The structures of the monomers and polymers were confirmed by 1H-NMR, 13C-NMR and FTIR spectroscopic techniques. Polycarbazol derivates show different electrochemical and optical properties due to the existence of the different substituent groups. PCz−OH, PCz−COOH, and PCz-CN are soluble in common organic solvents such as DMSO, NMP, THF, DMAC and their high molecular weight allow the preparation of flexible films onto various substrates. These properties indicate that carbazole-based materials will certainly be explored in future years for many optoelectronic applications. What is more, light-harvesting devices and molecular electronics are the most likely fields of research that will take advantage of the great potential of carbazole-based materials.
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