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Synthesis and electrochemical properties of mixed ionic and electronic modified polycarbazole
Electrochimica Acta 47 (2002) 2927 /2936 www.elsevier.com/locate/electacta
Synthesis and electrochemical properties of mixed ionic and electronic modified polycarbazole Franc¸ois Tran-Van, Thierry Henri, Claude Chevrot * Laboratoire sur les Polyme`res et les Mate´riaux Electroactifs, Equipe Re´activite´ aux Interfaces (EA 2528), Universite´ de Cergy Pontoise, 5 mail Gay Lussac, 95013 Cergy Pontoise Cedex, France Received 19 March 2002
Abstract A carbazole N-substituted by an oxyethylene group was polymerized using oxidative electro-polymerization. Due to the hydrophilic properties of the oligooxyethylene substituent, this monomer can be solubilized and electropolymerized in aqueous electrolyte solution. An acidic medium is particularly appropriate in order to decrease the oxidative potential and to obtain films mainly composed of short oligomers as revealed by GPC analysis. By comparison, the chemical polymerization of the corresponding 3,6 dibromo derivative leads to oligomers with well-defined structure and leads to the obtention of longer macromolecular chains. Electrochromic properties of these materials have been studied and a transition from colorless to deep green has been observed during the oxidation. More particularly interesting are the stability of such materials during polarization and the kinetics of diffusion of the counter ion in the film. In acidic medium 1.25 mol l 1 HClO4, 95% of the charge density was maintained after polarization for 7000 cycles which shows the good electrochemical stability of this material compared with other polycarbazoles. Diffusion coefficient have been evaluated in aqueous media about DClO4 / /10 11 cm2 s 1. Moreover, films are electroactive in oxyethylene based electrolytes. Then, they are probably well-compatible with hydrophilic poly(oxyethylene) solid electrolytes such that all-polymer devices may be prepared. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Mixed conducting polycarbazole; Electropolymerization; Dehalogenative polycondensation; Electrochromism
1. Introduction Although less studied than other families of conducting polymers such as polyaniline, polypyrrole, polythiophene or polyparaphenylene, polycarbazoles are of particular interest with regards to their photoconductivity [1,2], electrochromic properties [3 /6], for their application in electroluminescent devices [7 /13], such as biosensor [14 /16] and others applications[17 /19]. Two main routes are commonly used in order to synthesize poly (N-substituted carbazole)s: The first one is based on the activation of the carbon / halogen bond of a 3,6-dihalogenated monomer [20] in the presence of a zero valent nickel complex [21 /23]. This dehalogenative polycondensation can be carried out by a chemical [21] or an electrochemical reductive
* Corresponding author. Fax: /33-134-257-071 E-mail address: [email protected] (C. Chevrot).
route [22,23] and leads to well-defined polymers with 3,6 linkages strictly. The second and probably most commonly used route consists in the electrochemical [24 /34] or chemical [25 / 37] oxidation of carbazole derivatives in solution. In their work, Lacaze et al. electropolymerized N -ethyl and N -phenyl carbazole in acetonitrile [26]. The resulting films were composed of short oligomers [27]. Mengoli et al. showed that the electrooxidation of carbazole derivatives in hydroalcoholic acidic medium leads to films with good mechanical and well defined redox properties [28 /30]. Nevertheless, whatever the synthetic pathway, the electrochemical stability of polycarbazole films appears to be limited in comparison with the stability of some other conducting polymer films but it can be significantly improved by decreasing the polydispersity of the material [38]. This work deals with the electrochemical properties of oxyethylene N-substituted polycarbazoles (named PCzOE). Simonet et al. have previously studied the
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 1 7 1 - 8
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electrosynthesis of different oligoether substituted carbazoles in organic media [31]. These authors have successfully electropolymerized different monomers consisting in short oligoether chains disubstituted by carbazole moieties and observed that the resulting redox polymer was mainly made of dicarbazyl units linked to each other by polyether spacers. In this work, the electropolymerisation and chemical polymerization of the two monomers 1 and 2 was investigated.
graphy (SEC) in THF using polystyrene as standards on a Waters 510 HPLC pump and Waters 410 differential refactometer apparatus. Absorption spectra were measured with a Uvikon 922 Kontron spectrophotometer. Infra red spectra were recorded with a FTIR Equinox 55 Brucker spectrometer. NMR spectra were recorded on a Bruker avance DPX 250 apparatus in deuterated chloroform solution at 298 K. 2.2. Chemicals
Due to the hydrophilic properties of the oxyethylene substituent, we showed that monomer 1 can be solubilized and electropolymerized in aqueous electrolyte solution. The influence of the medium composition on the electropolymerization conditions and the macromolecular and electrochemical properties of the films is discussed in detail. In order to increase the molecular weight of the polymer and to compare properties of films we have also synthesized oxyethylene substituted polycarbazole by dehalogenative polycondensation of monomer 2. The stability and electrochromic properties of the films depending on the synthesis pathway, are reported. Moreover, the influence of the oxyethylene group on the diffusion coefficient of the counter ions in the film and on its electroactivity in hydrophilic oxyethylene based electrolytes is also presented.
2. Experimental 2.1. Instrumentation All electrochemical studies were performed using a PRT-20-2X potentiostat associated with IG6-N integrator (Radiometer). All curves were plotted on an X-Y recorder (DELFT BV BD94 Kipp and Zonen). Electrochemical experiments were run in a conventional three electrode cell. The working electrode was a platinum disc (1 cm diameter) or SnO2 coated glass (1 cm2 area). The counter electrode was a platinum wire. The reference electrode was a saturated calomel electrode (SCE) for all the electrochemical experiments with a salt bridge containing the supporting electrolyte. The macromolecular characteristics of the various samples were obtained by steric exclusion chromato-
Triphenylphosphine (Merck), NiBr2 /Bipy (Bipy / 2,2?-bipyridine) complex and the activated zinc powder were prepared as described previously [38]. N -N dimethylacetamide (Aldrich) was freshly distilled under reduced pressure over calcium hydride before use. Lithium perchlorate (Aldrich), ethyleneglycol (Aldrich) and perchloric acid (Carlo Erba) were used without further purification. Dodecylbenzene sulfonate (DBS) and sodium dodecylsulfate (SDS) (Aldrich) and polyoxyethylene-10lauryl-ether (Sigma) were used as received. 2.3. Synthesis of monomer 1 To 2.4 g (14.3 mmol) carbazole (Aldrich) dissolved in dimethylformamide were added 14.3 mmol sodium hydride at room temperature (r.t.). After 1 h stirring, 5 g (17.7 mmol) triethyleneglycolmonomethylether end terminated by a p-toluenesulfonate group was added and the resulting solution was stirred for 12 h at 50 8C under argon atmosphere. (end tosylated triethyleneglycolmonomethylether was obtained by reacting the commercially available hydroxylated compound with tosyl chloride in dichloromethane in the presence of pyridine). The mixture was hydrolyzed and the organic layer was separated and dried over MgSO4. After filtration and evaporation under vacuum, the crude product was purified by column chromatography on silica gel using ethyl acetate/cyclohexane 1/4 v/v as the eluent to obtain 2.5 g of yellow oil in 56% yield. 2.3.1. 1H NMR d (ppm) (CDCl3) (3.19, s, O /CH3); (3.31 /3.35, m, /(CH2 /CH2 /O) 2); (3.68, m, N/CH2 /CH2 /); (4.31, m; /N/CH2 /CH2); (7.11, d, H1,1?); (7.30, m, H2,2?,3,3?); (7.94, d, H4,4?). 2.3.2. 13C NMR d (ppm) (CDCl3) 141.0 (Ca,Ca?), 126.1 (C2,C2?), 123.3 (Cb,Cb?), 120.7 (C4,C4?), 119.4 (C3,C3?), 109.4 (C1,C1?) 69.7 /72.3 (CH2 /CH2 /O)2, 59.4 (CH3), 43.5 (CH2N).
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2.4. Synthesis of monomer 2 The same procedure was used to synthesize the monomer 2 using 3,6 dibromo carbazole (Aldrich) as starting compound. Mixtures were stirred for 25 h at 50 8C. The crude product was purified by column chromatography on silica gel using chloroform then methanol as eluents to obtain a yellow oil in 30% yield. 2.4.1. 1H NMR d (ppm) (CDCl3) (3.27, s, O /CH3); (3.3 /3.4, m, /(CH2 /CH2 /O)2); (3.70, m, N /CH2 /CH2 /); (4.30; m; /N /CH2 /CH2); (7.19, d, H1,1?); (7.41, d, H2,2?); (7.95, s, H4,4?). 2.4.2. 13C NMR d (ppm) (CDCl3) 139.9 (Ca,Ca?), 129.4 (C2,C2?), 123.8 (Cb,Cb?), 123.4 (C4,C4?), 112.5 (C3,C3?) 111.2 (C1,C1?) 69.7 /72.3 (CH2 /CH2/O) 59.4 (CH3), 43.9 (CH2N). 2.5. Synthesis of poly(2) Activated zinc (0.450 g; 6.82 mmol) followed by 2.2 ml of DMA were mixed with NiBr2bipy (0.041 g; 1.1 mmol), triphenylphosphine (0.345 g; 1.32 mmol) and monomer 2 (1 g; 2.12 mmol) under an argon atmosphere. The color of the mixture became immediately red and remained unchanged after 3.5 h under stirring at r.t. The mixture was poured into 200 ml of sulfuric acid (5% weight). A grey solid precipitated and was filtered, washed three times with 50 ml distilled water, two times with 50 ml ether and three times with 50 ml methanol. After drying under vacuum, 0.58 g of a pale grey powder was obtained (yield: 87%). 2.5.1. 1H NMR d (ppm) (CDCl3) (3.2, s, O /CH3); (3.4, m, /(CH2 /CH2 /O)2); (3.85, m, N /CH2 /CH2 /); (4.45; m; /N /CH2 /CH2); (7.5, d, H1,1?); (7.8, d, H2,2?); (8.5, s, H4,4?).
3. Results and discussion 3.1. Electrochemical polymerization of monomer 1 The electropolymerization of carbazole derivatives in non-organic media has been generally studied in hydroalcoholic mixtures of perchloric acid and methanol which allows to solubilize poorly water soluble monomers [28 /30]. Thanks to the good hydrophilic properties of the oxyethylene chain, the monomer can be solubilized in water and electropolymerized without alcohol. The curve B of Fig. 1 shows the cyclic voltammograms of a 10 3 mol l 1 of monomer 1 in 0.1 mol l 1 LiClO4/H2O. On the anodic scan, we observe an irreversible oxidation peak at 0.96 V corresponding to the formation of the cation radical of carbazole which
Fig. 1. Cyclic voltammograms at V /50 mV s 1 of 10 3 M of monomer 1 (A) in 1.25 mol l 1 HClO4/H2O and (B) 0.1 mol l 1 LiClO4/H2O. Working electrode: Pt (F/1 cm).
entails, on the reverse scan, a reduction wave at 0.65 V. As it will be shown below, this cathodic peak corresponds to the reduction of a new species deposited as a thin film on the electrode surface. In order to decrease the oxidation potential of the monomer, which would improve the stability of the resulting polymer, we have studied the electroactivity of the monomer 1 in the presence of different anionic and neutral surfactants such as SDS, sodium DBS or polyoxyethylene-10-laurylether. Indeed, such micellar media are well known to decrease the oxidation potential of some monomers (benzene, ethylenedioxythiophene. . .) [29 /42]. Compared with the solution without surfactant we do not observe significant decrease of the oxidation peak. On the other hand, we have observed a large decrease of the oxidation potential of the monomer in acidic medium. Indeed, as shown in Fig. 1, curve A, the oxidation peak observed in HClO4 (1.25 mol l 1) is shifted to lower potential by 150 mV compared with the one obtained in neutral media. Moreover, it is noteworthy that any shift of potential has been observed in 1.25 mol l1 which indicate no influence of the ClO4 concentration on the oxidation potential of the monomer. This phenomenon, already observed for thiophene and benzene derivatives in strong acidic medium [43 /45], may be due to the formation of p-complexes between the aromatic ring and H which decreases the oxidation potential. Thus, an acidic aqueous medium would be the more appropriate for the electrochemical polymerization of monomer 1 since it decreases the applied potential necessary for the electropolymerization and increases the thermodynamic oxidation potential of H2O which lead to broaden the domain of electroactivity of the electrolyte in the oxidation process. Fig. 2 shows cyclic voltammo-
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Fig. 2. Electropolymerization of 10 3 M of monomer 1 in 1.25 mol l 1 HClO4/H2O. V /50 mV s 1. Working electrode: Pt (F /1 cm).
grams of a 10 3 M solution of monomer 1 in 1.25 mol l 1 HClO4/H2O. During successive scans, one welldefined redox systems quickly grow and correspond to the deposition of an electroactive film onto the electrode surface. The peak current of the redox system regularly increases during ten successive scans. The film was then washed with monomer-free electrolyte solution and its redox behavior was studied. Two oxidation peaks are observed at, respectively, 0.56 and 0.9 V by increasing the applied potential (Fig. 3). These anodic processes are associated with two cathodic waves occurring, respectively, at 0.54 and 0.89 V by reversing scans. Such a behavior might correspond to the formation of the radical cations of carbazolic units during the first oxidation step followed by their oxidation into dications through the second step as already described for poly (N -alkylcarbazole) [23]. Taking into account the Ipa/Ipc ratio, which is close to unity and the independance of the peak potential between 20 and 200 mV s 1, the first electronic transfer is fast and the redox process follows a quasi-reversible mechanism. On the other hand, the second redox process is not reversible according to the Ipa/Ipc ratio. Dications would be more reactive than cation radicals in this medium. The scan rate depen-
Fig. 3. Scan-rate dependance of the cyclic voltammetry of poly(1) in 1.25 mol l 1 HClO4/H2O. (a) V/20, (b) 50, (c) 100, (d) 200 mV s 1.
dance of the electroactive film peak current was investigated only on the first reversible system. Between 20 and 200 mV s 1, the peak current of the first system evolves linearly with the square root of the scan rate, which indicates diffusion limited redox process. Similar results were observed for films prepared by potentiostatic oxidation (0.8 /0.9 V). To our knowledge, it is the first time that films of poly(N-substituted carbazole) are electrodeposited in water without co-solvent. It is noteworthy that electropolymerisation in 0.1 mol l 1 LiClO4/H2O has also been carried out but in this case, the potential used to oxidize the monomer has to be increased (see Fig. 1). Here again, we can observe a linear change of peak current versus the square root of the scan rate. We have also tried to electropolymerize monomer 1 in micellar medium. In 0.01 mol l 1 sodium dodecyl sulfate (SDS) or DBS micellar media, the electropolymerization is totally inhibited. In a non ionic surfactant such as polyoxyethylene-10-lauryl-ether (0.03 mol l1) the growth of the film is stopped after about 50 potential scans. Then the electroactivity decreases quickly during subsequent scans.
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In conclusion HClO4 1.25 mol l1 HCLO4/H2O is the more convenient aqueous medium for the preparation of electroactive poly(1) films. An important parameter concerning conducting polymers is the doping level g, corresponding to the number of electron exchanged per aromatic ring. It can be easily determined electrochemically from the following equation:
g2=([Qi =Qf ]1) where Qi is the charge density required for the electrodeposition of the film and Qf is the faradic charge density recorded during the oxidation of the film. For the electrodeposited polycarbazole film, we obtained a g value of 0.5 both in LiClO4 0.1 mol l 1 and HClO4 1.25 mol l 1 in H2O. This value agrees with those determined (from elemental analysis) by Mengoli et al. [28 /30] for poly(N -ethylcarbazole). Electrochemical behavior of the film was also studied in LiClO4 /H2O in order to determine the influence of the electrolyte. Results are summarized in Table 1. Although the concentration of LiClO4 does not modify the redox properties of the material, one can clearly observe a large shift in the potential of the first redox system in strong acidic medium which stabilizes the oxidized film as already observed for poly(N-substituted carbazole) [28 /30] and other conducting polymers [43 / 45]. It must be noted that this modification of potential is reversible that is to say the redox potential return to its initial value when the film is dipped into the former electrolyte. Lacaze et al. suggest in the case of poly(para -phenylene) the formation of a p-complexe between the aromatic conjugated ring and the acid [45]. In any case, acidic medium lowers the redox potential of poly(1) films and will be an important parameter to increase the stability of the material during polarization (see Section 3.5). The surface morphology of the film deposited from potentiostatic way in HClO4 1.25 M has been checked by SEM (Fig. 4). We can see important surface roughness evens if, electrodeposited films are homogeneous in naked eyes. Table 1 Electrochemical characteristics of electropolymerized PCzOE film in different electrolytes PCz(POE)3
EPa1 (V)
DEp1 (mV)
Epa2 DEp2 (mV)
0.1 M LiClO4/H2O 1.25 M LiClO4/H2O 1.25 M HClO4/H2O
0.74 0.74 0.56
44 44 21
/ / 0.89
/ / 6
Fig. 4. Scanning electron micrographs of thin films of poly(1) electrosynthetized in 1.25 mol l 1 HClO4/H2O on ITO electrode.
3.2. Macromolecular analysis of polymer 1 It is now established that electrooxidation of carbazole derivatives in organic or inorganic media leads to short oligomers [26,27]. In order to determine the molecular weight of the electrosynthesized oligomers, we have performed potentiostatically macro-electrolyses on SnO2 electrodes of monomer in 0.1 mol l1 LiClO4/ H2O at 0.95 V and in 1.25 mol l1 HClO4/H2O at 0.8 V both electropolymerization potential values corresponding to the optimized values in such electrolytic solutions according to chronoamperometric studies. Thus, by electrochemical reduction at an applied potential of 0 V, the film solubilizes in acetonitrile, which is subsequently evaporated under vacuum. Products were characterized by SEC using THF as eluent and polystyrene standards in order to estimate the molecular weight. For example, Fig. 5 shows the chromatogram of the films obtained from electropolymerisation in acidic medium. Three well-defined short oligomers corresponding to dimer, tetramer and dodecamer (according to polystyrene standards) were identified. Although the proportion of each oligomer in the film depends on the synthesis
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substituent which increases with the number of oxyethylene units limit the accessibility of the reactive positions and prevents the polymerization. Indeed, for a substituent with seven oxyethylene units, the polymerization degree decreases to four and with eleven one, polymerization does not occur. 3.4. Electrochromic properties
Fig. 5. SEC of poly(1) electrosynthetized in 1.25 mol l 1 HClO4/H2O.
conditions (electrolyte, potential. . .), the main species are short oligomers in agreement with the literature [27]. 3.3. Chemical polymerization of the monomer 2 In order to increase the molecular weight of polycarbazoles N-substituted with oligooxyethylene group, we performed dehalogenative polycondensation of monomer 2 as previously described for other N-substituted carbazoles [31]. Due to the flexible oligooxyethylene group, the material is soluble in common organic solvents (THF, CHCl3, CH2Cl2,. . .) and its chemical and macromolecular structures can be easily determined. 1H NMR gave clear evidence that polycondensation has taken place. It leads to a shift of the three aromatic proton H1,1?, H2,2?, and H3,3? (8.5, 7.8 and 7.5 ppm, respectively) signals compared with the monomer, due to the coupling of the carbazole units and electronic delocalisation. IR spectra of dihalide monomer and polymer have also been compared (spectra not shown). For the monomer, the band at 1470 and 1434 cm 1 can be assigned to the antisymmetric and symmetric C /C stretching deformation. The broad band in the vicinity of 1100 cm 1 is due to the ether group (C/O stretching). The two bands at 863 and 797 cm1 are ascribed to the out-of-plane deformation vibration of trisubstituted benzene ring. Concerning the polymer, the presence of an extra band at 744 cm 1 corresponding to the end-chain disubstituted benzene ring may indicate that the end of macromolecular chains are not brominated. The SEC analysis indicates an average of about 20 carbazole units per polymer chain (Mn /6300 g mol 1) and a polydispersity of 2.5. Compared with electropolymerized carbazole derivative, the molecular weight of the chemical poly (N -oligoether)carbazole is increased and the chemical structure is better defined with strictly 3.6 linkages. We have also studied the influence of the length of the oxyethylene substituents on the polymerization degree of the substituted carbazoles. As already observed for electropolymerisation of substituted thiophene [46], steric hindrance of the
Optical absorption spectra corresponding to electropolymerized monomer 1 film on ITO at different applied potentials are shown on Fig. 6. In a reduced state (0 V), an absorption band is observed with a maximum at 310 nm which corresponds to the p /p* transition of carbazole derivatives. Due to CH2 /CH2 spacers between the oxygen atom and the polymer backbone we do not observe modifications of the optical spectra compared with poly(N -alkyl) carbazole. The electronic band gap Eg defined as the onset for the p/p* interband transition is close to 3.2 eV in agreement with the literature [3]. When a potential higher than 0.55 V is applied (for example the curve corresponding to an applied potential of 0.75 V), two absorption bands appear at 395 nm and around 800 nm which have been attributed in the case of poly(N -alkyl carbazoles) to the radical cation of carbazole diades [3]. The intensity of these bands increases with increasing potential, indicating a rise of the concentration of oxidized species in the film. This optical modification is clearly revealed as a change from colorless to deep green during the oxidation. Optical spectra of the chemically synthesized thin films are similar to the electropolymerized ones which is in agreement with an electronic delocalisation limited to two carbazole units. 3.5. Electrochemical behavior of the polycarbazole films For technological applications and particularly for electrochromic devices, the lifetime of conducting polymers under polarization is an important parameter. In the case of polycarbazole, the electrochemical stability during scan potentials is low and depends on the pH of the solution which leads to a complete deprotonation or degradation of the polymer at pH 3 [47,48]. The electrochemical behavior of electrosynthesized PCzOE films was investigated in different aqueous acidic media by successive scan potentials on the first redox system between 0 and 0.75 V at a scan rate of 0.05 V s 1 and compared with the behavior of the chemically synthesized ones. The electrochemical study was carried out in air without a deoxygenation of the solution in order to be close to standard use conditions. Fig. 7 shows the variation of the electrochemical activity (Qn / Q1) (where Qn and Q1 are the faradic charge density of the film during the n th and the first cycle, respectively) as a function of time for the two polymers. There are no
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Fig. 6. Optical absorption spectra of electropolymerized thin film on ITO electrode at different applied potentials vs. SCE.
significative differences between the two materials in term of electrochemical stability. In 0.1 mol l 1 LiClO4/ H2O, 83% of the charge density was maintained after polarization for 2200 cycles and more than 55% after 6600 cycles. On the other hand, when the potential scans are carried out in 1.25 mol l 1 HClO4/H2O, the stability is exceptionally high particularly so far an aqueous electrolyte since there are no major variations of the electroactivity after 7000 cycles. By contrast, the charge density of unsubstituted polycarbazole in a similar electrolyte decreases quickly after 150 cycles [4] although for poly (N -alkyl carbazole) it decreases by about 25% after 500 cycles in organic electrolyte. It is noteworthy that in all cases, the electroactivity decreases drastically when the potentiodynamic scans are carried out on the second redox system of the film, which is not reversible. On the contrary, in 5 mol l 1 HClO4, the redox system of the film decreases quickly after several tens of cycles. We suggest that the drop in the electroactivity could be due to the degradation of ether chains in this medium, which may react with the oxidized carbazole species generated at the electrode.
Thus the oxyethylene substituent seems to be particularly adequate in order to stabilize oxidized species during the polarization in acidic electrolyte provided that the oxyethylene chain is not degraded. Another advantage of this substituent is its ionic conductivity, which could allow an increase in the kinetics of diffusion of the counter ion in the film during electrooxidation. Indeed, polyoxyethylene is well known to exhibit ionic conductivity when lithium salts are solubilized in the polymer [49 /51]. Ratner et al. have suggested a dynamic percolation model for description of ion transport in polymer electrolytes like PEO. This is a microscopic model that characterizes the ionic motion in terms of jumps between neighboring position [52 /55]. Thus, the presence of solvating oxyethylene units on the polycarbazole backbone leads to increase the ionic conductivity of polycarbazole. The mobility of anions in the modified polycarbazole film is increased which allows to improve the kinetics of insertion of anions in the film during polarization.
Fig. 7. Electrochemical activity of PCzOE films as a function of cycle number. Polarization limited on the first redox system.
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This parameter is particularly important since it is usually the limiting factor in the switching time of the electrochromic polymer devices [56,57]. As already observed, the proportionnality between peak current ip and square root of the scan rate v1/2 of the first redox process indicates that the kinetics of the electrochemical reaction is limited by ionic mass transport (see Section 3.2) which obeys Fick’s law. Potentiostatic step measurements were performed in order to investigate the type of diffusion. For example, Fig. 8 shows the chronoamperogram of a polymer 2 film in 0.1 mol l 1 LiClO4/H2O at an applied potential of 0.8 V vs SCE. The i/t1/2 prediction of the semi-infinite diffusionbased Cottrell equation: i nFAD1=2 C=(pt)1=2 (with D is the diffusion coefficient, C is the concentration of electroactive center [58,59], A is the geometrical area, n is the number of electrons involved in the process (n/1 for the first redox system))is observed at sufficiently short time and permits to determine the value of the diffusion coefficient. According to a uniform distribution of electroactive sites in the film of different thickness evaluated to 5/103 mol cm 3 as already obtained for other conducting polymers [58], we have obtained DClO4 / /10 11 cm2 s 1 which is of the same order of magnitude in 0.1 mol l 1 HClO4/H2O. An improvement in the ionic conductivity due to such a substituent has been shown to be an effective approach in order to decrease the electrochromic response time [60]. Moreover, when used in a PEO based solid electrolyte for electrochromic devices, oligoether substituents are well suited to improve the compatibility between elec-
Fig. 9. Electroactivity of thin film of a chemically synthesized PCzOE and Poly(N -hexyl) carbazole in ethyleneglycol 0.1 mol l 1 LiClO4. V /10 mV s 1. Working electrode: Pt (F/1 cm).
trode and electrolyte and to decrease the phase segregation, which could occur between hydrophilic electrolytes and hydrophobic electrochromic electrodes. In Fig. 9, we can clearly observe the difference of the electro-
Fig. 8. Chronoamperogram of chemically synthesized PCzOE film in 0.1 mol l 1 LiClO4/H2O. Eappl /0.8 V vs SCE. Working electrode: Pt (F/1 cm).
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chemical behavior of thin films of hydrophilic PCzOE and hydrophobic poly(N -hexylcarbazole) (PHCz) (synthesized as previously described [6]b/c) in ethyleneglycol/LiClO4 0.1 M electrolyte. The hydrophobic alkyl chains of PHCz prevent the counter ions to penetrate into the film, which limits the electroactivity of such PCz in this kind of hydrophilic electrolyte. In this case, intensity of the signal representative of the electroactivity is very low. On the other hand, PCzOE due to their oxyethylene units can be scanned from neutral to oxidized forms with a higher electrochemical signal. The electrochromic process is clearly visible during cycling which allows to consider the effective use of PCz(OE)3 in an oxyethylene based electrolyte.
4. Conclusion We have synthesized two N -oligoether carbazole monomers, which have been, respectively, electropolymerized and polymerized by polycondensation. Hydrophilic oligooxyethylene substituents, allowed solubility and electropolymerization of monomer 1 in aqueous electrolyte. An acidic medium is particularly appropriate to decrease the oxidation potential of the monomer which could lead to the formation of more stable materials. SEC analysis indicates that films are composed of short oligomers. By comparison, the chemical polymerization leads to longer macromolecular chains with well-defined structure. Spectroelectrochemical studies confirm the electrochromic properties of the films, which turn from colorless to deep green during the oxidation. More particularly interesting is the stability of such materials during polarization. In strong acidic medium (i.e 1.25 mol l 1 HClO4), 95% of the charge density was maintained after polarization during 7000 cycles. Due to its oligoether substituents, films are wellcompatible with oxyethylene based elecytrolytes or other hydrophilic electrolytes compared with N -alkyl substituted polycarbazoles and the ionic diffusion process could be improved. Experiments are in progress in the laboratory to study such polycarbazoles in an electrochromic device with a PEO based solid electrolyte.
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[41] N. Sakmeche, S. Aeiyach, J.J. Aaron, M. Jouini, J.C. Lacroix, P.C. Lacaze, Langmuir 15 (1999) 2566. [42] A. Lima, P. Schottland, S. Sadki, C. Chevrot, Synth. Met. 93 (1998) 33. [43] V. Tsakova, S. Winkels, J.W. Schultze, Electrochim. Acta 46 (2000) 759. [44] E.A. Bazzaoui, S. Aeiyach, P.C. Lacaze, J. Electroanal. Chem. 364 (1994) 63. [45] P.C. Lacaze, S. Hara, P. Soubiran, S. Aeiyach, Synth. Met. 75 (1995) 111. [46] J. Roncali, Chem. Rev. 92 (1992) 711. [47] M.M. Verghese, T. Basu, B.D. Malhotra, Mater. Sci. Eng. C 3 (1995) 215. [48] M.M. Verghese, N.S. Sundaresan, T. Basu, B.D. Malhotra, J. Mat. Sci. Lett. 14 (1995) 401. [49] B. Fenton, J.M. Parker, P.V. Wright, Polymer 14 (1973) 589. [50] M. Armand, J.M. Chabagnot, M. Duclot, in: P. Vahista, J.N. Mundy, G.K. Shenoy (Eds.), Fast Ion Transport in Solids, NorthHolland, Amsterdam, 1979, p. 131.
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Synthesis and electrochemical properties of mixed ionic and electronic modified polycarbazole Franc¸ois Tran-Van, Thierry Henri, Claude Chevrot * Laboratoire sur les Polyme`res et les Mate´riaux Electroactifs, Equipe Re´activite´ aux Interfaces (EA 2528), Universite´ de Cergy Pontoise, 5 mail Gay Lussac, 95013 Cergy Pontoise Cedex, France Received 19 March 2002
Abstract A carbazole N-substituted by an oxyethylene group was polymerized using oxidative electro-polymerization. Due to the hydrophilic properties of the oligooxyethylene substituent, this monomer can be solubilized and electropolymerized in aqueous electrolyte solution. An acidic medium is particularly appropriate in order to decrease the oxidative potential and to obtain films mainly composed of short oligomers as revealed by GPC analysis. By comparison, the chemical polymerization of the corresponding 3,6 dibromo derivative leads to oligomers with well-defined structure and leads to the obtention of longer macromolecular chains. Electrochromic properties of these materials have been studied and a transition from colorless to deep green has been observed during the oxidation. More particularly interesting are the stability of such materials during polarization and the kinetics of diffusion of the counter ion in the film. In acidic medium 1.25 mol l 1 HClO4, 95% of the charge density was maintained after polarization for 7000 cycles which shows the good electrochemical stability of this material compared with other polycarbazoles. Diffusion coefficient have been evaluated in aqueous media about DClO4 / /10 11 cm2 s 1. Moreover, films are electroactive in oxyethylene based electrolytes. Then, they are probably well-compatible with hydrophilic poly(oxyethylene) solid electrolytes such that all-polymer devices may be prepared. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Mixed conducting polycarbazole; Electropolymerization; Dehalogenative polycondensation; Electrochromism
1. Introduction Although less studied than other families of conducting polymers such as polyaniline, polypyrrole, polythiophene or polyparaphenylene, polycarbazoles are of particular interest with regards to their photoconductivity [1,2], electrochromic properties [3 /6], for their application in electroluminescent devices [7 /13], such as biosensor [14 /16] and others applications[17 /19]. Two main routes are commonly used in order to synthesize poly (N-substituted carbazole)s: The first one is based on the activation of the carbon / halogen bond of a 3,6-dihalogenated monomer [20] in the presence of a zero valent nickel complex [21 /23]. This dehalogenative polycondensation can be carried out by a chemical [21] or an electrochemical reductive
* Corresponding author. Fax: /33-134-257-071 E-mail address: [email protected] (C. Chevrot).
route [22,23] and leads to well-defined polymers with 3,6 linkages strictly. The second and probably most commonly used route consists in the electrochemical [24 /34] or chemical [25 / 37] oxidation of carbazole derivatives in solution. In their work, Lacaze et al. electropolymerized N -ethyl and N -phenyl carbazole in acetonitrile [26]. The resulting films were composed of short oligomers [27]. Mengoli et al. showed that the electrooxidation of carbazole derivatives in hydroalcoholic acidic medium leads to films with good mechanical and well defined redox properties [28 /30]. Nevertheless, whatever the synthetic pathway, the electrochemical stability of polycarbazole films appears to be limited in comparison with the stability of some other conducting polymer films but it can be significantly improved by decreasing the polydispersity of the material [38]. This work deals with the electrochemical properties of oxyethylene N-substituted polycarbazoles (named PCzOE). Simonet et al. have previously studied the
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 1 7 1 - 8
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electrosynthesis of different oligoether substituted carbazoles in organic media [31]. These authors have successfully electropolymerized different monomers consisting in short oligoether chains disubstituted by carbazole moieties and observed that the resulting redox polymer was mainly made of dicarbazyl units linked to each other by polyether spacers. In this work, the electropolymerisation and chemical polymerization of the two monomers 1 and 2 was investigated.
graphy (SEC) in THF using polystyrene as standards on a Waters 510 HPLC pump and Waters 410 differential refactometer apparatus. Absorption spectra were measured with a Uvikon 922 Kontron spectrophotometer. Infra red spectra were recorded with a FTIR Equinox 55 Brucker spectrometer. NMR spectra were recorded on a Bruker avance DPX 250 apparatus in deuterated chloroform solution at 298 K. 2.2. Chemicals
Due to the hydrophilic properties of the oxyethylene substituent, we showed that monomer 1 can be solubilized and electropolymerized in aqueous electrolyte solution. The influence of the medium composition on the electropolymerization conditions and the macromolecular and electrochemical properties of the films is discussed in detail. In order to increase the molecular weight of the polymer and to compare properties of films we have also synthesized oxyethylene substituted polycarbazole by dehalogenative polycondensation of monomer 2. The stability and electrochromic properties of the films depending on the synthesis pathway, are reported. Moreover, the influence of the oxyethylene group on the diffusion coefficient of the counter ions in the film and on its electroactivity in hydrophilic oxyethylene based electrolytes is also presented.
2. Experimental 2.1. Instrumentation All electrochemical studies were performed using a PRT-20-2X potentiostat associated with IG6-N integrator (Radiometer). All curves were plotted on an X-Y recorder (DELFT BV BD94 Kipp and Zonen). Electrochemical experiments were run in a conventional three electrode cell. The working electrode was a platinum disc (1 cm diameter) or SnO2 coated glass (1 cm2 area). The counter electrode was a platinum wire. The reference electrode was a saturated calomel electrode (SCE) for all the electrochemical experiments with a salt bridge containing the supporting electrolyte. The macromolecular characteristics of the various samples were obtained by steric exclusion chromato-
Triphenylphosphine (Merck), NiBr2 /Bipy (Bipy / 2,2?-bipyridine) complex and the activated zinc powder were prepared as described previously [38]. N -N dimethylacetamide (Aldrich) was freshly distilled under reduced pressure over calcium hydride before use. Lithium perchlorate (Aldrich), ethyleneglycol (Aldrich) and perchloric acid (Carlo Erba) were used without further purification. Dodecylbenzene sulfonate (DBS) and sodium dodecylsulfate (SDS) (Aldrich) and polyoxyethylene-10lauryl-ether (Sigma) were used as received. 2.3. Synthesis of monomer 1 To 2.4 g (14.3 mmol) carbazole (Aldrich) dissolved in dimethylformamide were added 14.3 mmol sodium hydride at room temperature (r.t.). After 1 h stirring, 5 g (17.7 mmol) triethyleneglycolmonomethylether end terminated by a p-toluenesulfonate group was added and the resulting solution was stirred for 12 h at 50 8C under argon atmosphere. (end tosylated triethyleneglycolmonomethylether was obtained by reacting the commercially available hydroxylated compound with tosyl chloride in dichloromethane in the presence of pyridine). The mixture was hydrolyzed and the organic layer was separated and dried over MgSO4. After filtration and evaporation under vacuum, the crude product was purified by column chromatography on silica gel using ethyl acetate/cyclohexane 1/4 v/v as the eluent to obtain 2.5 g of yellow oil in 56% yield. 2.3.1. 1H NMR d (ppm) (CDCl3) (3.19, s, O /CH3); (3.31 /3.35, m, /(CH2 /CH2 /O) 2); (3.68, m, N/CH2 /CH2 /); (4.31, m; /N/CH2 /CH2); (7.11, d, H1,1?); (7.30, m, H2,2?,3,3?); (7.94, d, H4,4?). 2.3.2. 13C NMR d (ppm) (CDCl3) 141.0 (Ca,Ca?), 126.1 (C2,C2?), 123.3 (Cb,Cb?), 120.7 (C4,C4?), 119.4 (C3,C3?), 109.4 (C1,C1?) 69.7 /72.3 (CH2 /CH2 /O)2, 59.4 (CH3), 43.5 (CH2N).
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2.4. Synthesis of monomer 2 The same procedure was used to synthesize the monomer 2 using 3,6 dibromo carbazole (Aldrich) as starting compound. Mixtures were stirred for 25 h at 50 8C. The crude product was purified by column chromatography on silica gel using chloroform then methanol as eluents to obtain a yellow oil in 30% yield. 2.4.1. 1H NMR d (ppm) (CDCl3) (3.27, s, O /CH3); (3.3 /3.4, m, /(CH2 /CH2 /O)2); (3.70, m, N /CH2 /CH2 /); (4.30; m; /N /CH2 /CH2); (7.19, d, H1,1?); (7.41, d, H2,2?); (7.95, s, H4,4?). 2.4.2. 13C NMR d (ppm) (CDCl3) 139.9 (Ca,Ca?), 129.4 (C2,C2?), 123.8 (Cb,Cb?), 123.4 (C4,C4?), 112.5 (C3,C3?) 111.2 (C1,C1?) 69.7 /72.3 (CH2 /CH2/O) 59.4 (CH3), 43.9 (CH2N). 2.5. Synthesis of poly(2) Activated zinc (0.450 g; 6.82 mmol) followed by 2.2 ml of DMA were mixed with NiBr2bipy (0.041 g; 1.1 mmol), triphenylphosphine (0.345 g; 1.32 mmol) and monomer 2 (1 g; 2.12 mmol) under an argon atmosphere. The color of the mixture became immediately red and remained unchanged after 3.5 h under stirring at r.t. The mixture was poured into 200 ml of sulfuric acid (5% weight). A grey solid precipitated and was filtered, washed three times with 50 ml distilled water, two times with 50 ml ether and three times with 50 ml methanol. After drying under vacuum, 0.58 g of a pale grey powder was obtained (yield: 87%). 2.5.1. 1H NMR d (ppm) (CDCl3) (3.2, s, O /CH3); (3.4, m, /(CH2 /CH2 /O)2); (3.85, m, N /CH2 /CH2 /); (4.45; m; /N /CH2 /CH2); (7.5, d, H1,1?); (7.8, d, H2,2?); (8.5, s, H4,4?).
3. Results and discussion 3.1. Electrochemical polymerization of monomer 1 The electropolymerization of carbazole derivatives in non-organic media has been generally studied in hydroalcoholic mixtures of perchloric acid and methanol which allows to solubilize poorly water soluble monomers [28 /30]. Thanks to the good hydrophilic properties of the oxyethylene chain, the monomer can be solubilized in water and electropolymerized without alcohol. The curve B of Fig. 1 shows the cyclic voltammograms of a 10 3 mol l 1 of monomer 1 in 0.1 mol l 1 LiClO4/H2O. On the anodic scan, we observe an irreversible oxidation peak at 0.96 V corresponding to the formation of the cation radical of carbazole which
Fig. 1. Cyclic voltammograms at V /50 mV s 1 of 10 3 M of monomer 1 (A) in 1.25 mol l 1 HClO4/H2O and (B) 0.1 mol l 1 LiClO4/H2O. Working electrode: Pt (F/1 cm).
entails, on the reverse scan, a reduction wave at 0.65 V. As it will be shown below, this cathodic peak corresponds to the reduction of a new species deposited as a thin film on the electrode surface. In order to decrease the oxidation potential of the monomer, which would improve the stability of the resulting polymer, we have studied the electroactivity of the monomer 1 in the presence of different anionic and neutral surfactants such as SDS, sodium DBS or polyoxyethylene-10-laurylether. Indeed, such micellar media are well known to decrease the oxidation potential of some monomers (benzene, ethylenedioxythiophene. . .) [29 /42]. Compared with the solution without surfactant we do not observe significant decrease of the oxidation peak. On the other hand, we have observed a large decrease of the oxidation potential of the monomer in acidic medium. Indeed, as shown in Fig. 1, curve A, the oxidation peak observed in HClO4 (1.25 mol l 1) is shifted to lower potential by 150 mV compared with the one obtained in neutral media. Moreover, it is noteworthy that any shift of potential has been observed in 1.25 mol l1 which indicate no influence of the ClO4 concentration on the oxidation potential of the monomer. This phenomenon, already observed for thiophene and benzene derivatives in strong acidic medium [43 /45], may be due to the formation of p-complexes between the aromatic ring and H which decreases the oxidation potential. Thus, an acidic aqueous medium would be the more appropriate for the electrochemical polymerization of monomer 1 since it decreases the applied potential necessary for the electropolymerization and increases the thermodynamic oxidation potential of H2O which lead to broaden the domain of electroactivity of the electrolyte in the oxidation process. Fig. 2 shows cyclic voltammo-
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Fig. 2. Electropolymerization of 10 3 M of monomer 1 in 1.25 mol l 1 HClO4/H2O. V /50 mV s 1. Working electrode: Pt (F /1 cm).
grams of a 10 3 M solution of monomer 1 in 1.25 mol l 1 HClO4/H2O. During successive scans, one welldefined redox systems quickly grow and correspond to the deposition of an electroactive film onto the electrode surface. The peak current of the redox system regularly increases during ten successive scans. The film was then washed with monomer-free electrolyte solution and its redox behavior was studied. Two oxidation peaks are observed at, respectively, 0.56 and 0.9 V by increasing the applied potential (Fig. 3). These anodic processes are associated with two cathodic waves occurring, respectively, at 0.54 and 0.89 V by reversing scans. Such a behavior might correspond to the formation of the radical cations of carbazolic units during the first oxidation step followed by their oxidation into dications through the second step as already described for poly (N -alkylcarbazole) [23]. Taking into account the Ipa/Ipc ratio, which is close to unity and the independance of the peak potential between 20 and 200 mV s 1, the first electronic transfer is fast and the redox process follows a quasi-reversible mechanism. On the other hand, the second redox process is not reversible according to the Ipa/Ipc ratio. Dications would be more reactive than cation radicals in this medium. The scan rate depen-
Fig. 3. Scan-rate dependance of the cyclic voltammetry of poly(1) in 1.25 mol l 1 HClO4/H2O. (a) V/20, (b) 50, (c) 100, (d) 200 mV s 1.
dance of the electroactive film peak current was investigated only on the first reversible system. Between 20 and 200 mV s 1, the peak current of the first system evolves linearly with the square root of the scan rate, which indicates diffusion limited redox process. Similar results were observed for films prepared by potentiostatic oxidation (0.8 /0.9 V). To our knowledge, it is the first time that films of poly(N-substituted carbazole) are electrodeposited in water without co-solvent. It is noteworthy that electropolymerisation in 0.1 mol l 1 LiClO4/H2O has also been carried out but in this case, the potential used to oxidize the monomer has to be increased (see Fig. 1). Here again, we can observe a linear change of peak current versus the square root of the scan rate. We have also tried to electropolymerize monomer 1 in micellar medium. In 0.01 mol l 1 sodium dodecyl sulfate (SDS) or DBS micellar media, the electropolymerization is totally inhibited. In a non ionic surfactant such as polyoxyethylene-10-lauryl-ether (0.03 mol l1) the growth of the film is stopped after about 50 potential scans. Then the electroactivity decreases quickly during subsequent scans.
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In conclusion HClO4 1.25 mol l1 HCLO4/H2O is the more convenient aqueous medium for the preparation of electroactive poly(1) films. An important parameter concerning conducting polymers is the doping level g, corresponding to the number of electron exchanged per aromatic ring. It can be easily determined electrochemically from the following equation:
g2=([Qi =Qf ]1) where Qi is the charge density required for the electrodeposition of the film and Qf is the faradic charge density recorded during the oxidation of the film. For the electrodeposited polycarbazole film, we obtained a g value of 0.5 both in LiClO4 0.1 mol l 1 and HClO4 1.25 mol l 1 in H2O. This value agrees with those determined (from elemental analysis) by Mengoli et al. [28 /30] for poly(N -ethylcarbazole). Electrochemical behavior of the film was also studied in LiClO4 /H2O in order to determine the influence of the electrolyte. Results are summarized in Table 1. Although the concentration of LiClO4 does not modify the redox properties of the material, one can clearly observe a large shift in the potential of the first redox system in strong acidic medium which stabilizes the oxidized film as already observed for poly(N-substituted carbazole) [28 /30] and other conducting polymers [43 / 45]. It must be noted that this modification of potential is reversible that is to say the redox potential return to its initial value when the film is dipped into the former electrolyte. Lacaze et al. suggest in the case of poly(para -phenylene) the formation of a p-complexe between the aromatic conjugated ring and the acid [45]. In any case, acidic medium lowers the redox potential of poly(1) films and will be an important parameter to increase the stability of the material during polarization (see Section 3.5). The surface morphology of the film deposited from potentiostatic way in HClO4 1.25 M has been checked by SEM (Fig. 4). We can see important surface roughness evens if, electrodeposited films are homogeneous in naked eyes. Table 1 Electrochemical characteristics of electropolymerized PCzOE film in different electrolytes PCz(POE)3
EPa1 (V)
DEp1 (mV)
Epa2 DEp2 (mV)
0.1 M LiClO4/H2O 1.25 M LiClO4/H2O 1.25 M HClO4/H2O
0.74 0.74 0.56
44 44 21
/ / 0.89
/ / 6
Fig. 4. Scanning electron micrographs of thin films of poly(1) electrosynthetized in 1.25 mol l 1 HClO4/H2O on ITO electrode.
3.2. Macromolecular analysis of polymer 1 It is now established that electrooxidation of carbazole derivatives in organic or inorganic media leads to short oligomers [26,27]. In order to determine the molecular weight of the electrosynthesized oligomers, we have performed potentiostatically macro-electrolyses on SnO2 electrodes of monomer in 0.1 mol l1 LiClO4/ H2O at 0.95 V and in 1.25 mol l1 HClO4/H2O at 0.8 V both electropolymerization potential values corresponding to the optimized values in such electrolytic solutions according to chronoamperometric studies. Thus, by electrochemical reduction at an applied potential of 0 V, the film solubilizes in acetonitrile, which is subsequently evaporated under vacuum. Products were characterized by SEC using THF as eluent and polystyrene standards in order to estimate the molecular weight. For example, Fig. 5 shows the chromatogram of the films obtained from electropolymerisation in acidic medium. Three well-defined short oligomers corresponding to dimer, tetramer and dodecamer (according to polystyrene standards) were identified. Although the proportion of each oligomer in the film depends on the synthesis
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substituent which increases with the number of oxyethylene units limit the accessibility of the reactive positions and prevents the polymerization. Indeed, for a substituent with seven oxyethylene units, the polymerization degree decreases to four and with eleven one, polymerization does not occur. 3.4. Electrochromic properties
Fig. 5. SEC of poly(1) electrosynthetized in 1.25 mol l 1 HClO4/H2O.
conditions (electrolyte, potential. . .), the main species are short oligomers in agreement with the literature [27]. 3.3. Chemical polymerization of the monomer 2 In order to increase the molecular weight of polycarbazoles N-substituted with oligooxyethylene group, we performed dehalogenative polycondensation of monomer 2 as previously described for other N-substituted carbazoles [31]. Due to the flexible oligooxyethylene group, the material is soluble in common organic solvents (THF, CHCl3, CH2Cl2,. . .) and its chemical and macromolecular structures can be easily determined. 1H NMR gave clear evidence that polycondensation has taken place. It leads to a shift of the three aromatic proton H1,1?, H2,2?, and H3,3? (8.5, 7.8 and 7.5 ppm, respectively) signals compared with the monomer, due to the coupling of the carbazole units and electronic delocalisation. IR spectra of dihalide monomer and polymer have also been compared (spectra not shown). For the monomer, the band at 1470 and 1434 cm 1 can be assigned to the antisymmetric and symmetric C /C stretching deformation. The broad band in the vicinity of 1100 cm 1 is due to the ether group (C/O stretching). The two bands at 863 and 797 cm1 are ascribed to the out-of-plane deformation vibration of trisubstituted benzene ring. Concerning the polymer, the presence of an extra band at 744 cm 1 corresponding to the end-chain disubstituted benzene ring may indicate that the end of macromolecular chains are not brominated. The SEC analysis indicates an average of about 20 carbazole units per polymer chain (Mn /6300 g mol 1) and a polydispersity of 2.5. Compared with electropolymerized carbazole derivative, the molecular weight of the chemical poly (N -oligoether)carbazole is increased and the chemical structure is better defined with strictly 3.6 linkages. We have also studied the influence of the length of the oxyethylene substituents on the polymerization degree of the substituted carbazoles. As already observed for electropolymerisation of substituted thiophene [46], steric hindrance of the
Optical absorption spectra corresponding to electropolymerized monomer 1 film on ITO at different applied potentials are shown on Fig. 6. In a reduced state (0 V), an absorption band is observed with a maximum at 310 nm which corresponds to the p /p* transition of carbazole derivatives. Due to CH2 /CH2 spacers between the oxygen atom and the polymer backbone we do not observe modifications of the optical spectra compared with poly(N -alkyl) carbazole. The electronic band gap Eg defined as the onset for the p/p* interband transition is close to 3.2 eV in agreement with the literature [3]. When a potential higher than 0.55 V is applied (for example the curve corresponding to an applied potential of 0.75 V), two absorption bands appear at 395 nm and around 800 nm which have been attributed in the case of poly(N -alkyl carbazoles) to the radical cation of carbazole diades [3]. The intensity of these bands increases with increasing potential, indicating a rise of the concentration of oxidized species in the film. This optical modification is clearly revealed as a change from colorless to deep green during the oxidation. Optical spectra of the chemically synthesized thin films are similar to the electropolymerized ones which is in agreement with an electronic delocalisation limited to two carbazole units. 3.5. Electrochemical behavior of the polycarbazole films For technological applications and particularly for electrochromic devices, the lifetime of conducting polymers under polarization is an important parameter. In the case of polycarbazole, the electrochemical stability during scan potentials is low and depends on the pH of the solution which leads to a complete deprotonation or degradation of the polymer at pH 3 [47,48]. The electrochemical behavior of electrosynthesized PCzOE films was investigated in different aqueous acidic media by successive scan potentials on the first redox system between 0 and 0.75 V at a scan rate of 0.05 V s 1 and compared with the behavior of the chemically synthesized ones. The electrochemical study was carried out in air without a deoxygenation of the solution in order to be close to standard use conditions. Fig. 7 shows the variation of the electrochemical activity (Qn / Q1) (where Qn and Q1 are the faradic charge density of the film during the n th and the first cycle, respectively) as a function of time for the two polymers. There are no
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Fig. 6. Optical absorption spectra of electropolymerized thin film on ITO electrode at different applied potentials vs. SCE.
significative differences between the two materials in term of electrochemical stability. In 0.1 mol l 1 LiClO4/ H2O, 83% of the charge density was maintained after polarization for 2200 cycles and more than 55% after 6600 cycles. On the other hand, when the potential scans are carried out in 1.25 mol l 1 HClO4/H2O, the stability is exceptionally high particularly so far an aqueous electrolyte since there are no major variations of the electroactivity after 7000 cycles. By contrast, the charge density of unsubstituted polycarbazole in a similar electrolyte decreases quickly after 150 cycles [4] although for poly (N -alkyl carbazole) it decreases by about 25% after 500 cycles in organic electrolyte. It is noteworthy that in all cases, the electroactivity decreases drastically when the potentiodynamic scans are carried out on the second redox system of the film, which is not reversible. On the contrary, in 5 mol l 1 HClO4, the redox system of the film decreases quickly after several tens of cycles. We suggest that the drop in the electroactivity could be due to the degradation of ether chains in this medium, which may react with the oxidized carbazole species generated at the electrode.
Thus the oxyethylene substituent seems to be particularly adequate in order to stabilize oxidized species during the polarization in acidic electrolyte provided that the oxyethylene chain is not degraded. Another advantage of this substituent is its ionic conductivity, which could allow an increase in the kinetics of diffusion of the counter ion in the film during electrooxidation. Indeed, polyoxyethylene is well known to exhibit ionic conductivity when lithium salts are solubilized in the polymer [49 /51]. Ratner et al. have suggested a dynamic percolation model for description of ion transport in polymer electrolytes like PEO. This is a microscopic model that characterizes the ionic motion in terms of jumps between neighboring position [52 /55]. Thus, the presence of solvating oxyethylene units on the polycarbazole backbone leads to increase the ionic conductivity of polycarbazole. The mobility of anions in the modified polycarbazole film is increased which allows to improve the kinetics of insertion of anions in the film during polarization.
Fig. 7. Electrochemical activity of PCzOE films as a function of cycle number. Polarization limited on the first redox system.
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This parameter is particularly important since it is usually the limiting factor in the switching time of the electrochromic polymer devices [56,57]. As already observed, the proportionnality between peak current ip and square root of the scan rate v1/2 of the first redox process indicates that the kinetics of the electrochemical reaction is limited by ionic mass transport (see Section 3.2) which obeys Fick’s law. Potentiostatic step measurements were performed in order to investigate the type of diffusion. For example, Fig. 8 shows the chronoamperogram of a polymer 2 film in 0.1 mol l 1 LiClO4/H2O at an applied potential of 0.8 V vs SCE. The i/t1/2 prediction of the semi-infinite diffusionbased Cottrell equation: i nFAD1=2 C=(pt)1=2 (with D is the diffusion coefficient, C is the concentration of electroactive center [58,59], A is the geometrical area, n is the number of electrons involved in the process (n/1 for the first redox system))is observed at sufficiently short time and permits to determine the value of the diffusion coefficient. According to a uniform distribution of electroactive sites in the film of different thickness evaluated to 5/103 mol cm 3 as already obtained for other conducting polymers [58], we have obtained DClO4 / /10 11 cm2 s 1 which is of the same order of magnitude in 0.1 mol l 1 HClO4/H2O. An improvement in the ionic conductivity due to such a substituent has been shown to be an effective approach in order to decrease the electrochromic response time [60]. Moreover, when used in a PEO based solid electrolyte for electrochromic devices, oligoether substituents are well suited to improve the compatibility between elec-
Fig. 9. Electroactivity of thin film of a chemically synthesized PCzOE and Poly(N -hexyl) carbazole in ethyleneglycol 0.1 mol l 1 LiClO4. V /10 mV s 1. Working electrode: Pt (F/1 cm).
trode and electrolyte and to decrease the phase segregation, which could occur between hydrophilic electrolytes and hydrophobic electrochromic electrodes. In Fig. 9, we can clearly observe the difference of the electro-
Fig. 8. Chronoamperogram of chemically synthesized PCzOE film in 0.1 mol l 1 LiClO4/H2O. Eappl /0.8 V vs SCE. Working electrode: Pt (F/1 cm).
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chemical behavior of thin films of hydrophilic PCzOE and hydrophobic poly(N -hexylcarbazole) (PHCz) (synthesized as previously described [6]b/c) in ethyleneglycol/LiClO4 0.1 M electrolyte. The hydrophobic alkyl chains of PHCz prevent the counter ions to penetrate into the film, which limits the electroactivity of such PCz in this kind of hydrophilic electrolyte. In this case, intensity of the signal representative of the electroactivity is very low. On the other hand, PCzOE due to their oxyethylene units can be scanned from neutral to oxidized forms with a higher electrochemical signal. The electrochromic process is clearly visible during cycling which allows to consider the effective use of PCz(OE)3 in an oxyethylene based electrolyte.
4. Conclusion We have synthesized two N -oligoether carbazole monomers, which have been, respectively, electropolymerized and polymerized by polycondensation. Hydrophilic oligooxyethylene substituents, allowed solubility and electropolymerization of monomer 1 in aqueous electrolyte. An acidic medium is particularly appropriate to decrease the oxidation potential of the monomer which could lead to the formation of more stable materials. SEC analysis indicates that films are composed of short oligomers. By comparison, the chemical polymerization leads to longer macromolecular chains with well-defined structure. Spectroelectrochemical studies confirm the electrochromic properties of the films, which turn from colorless to deep green during the oxidation. More particularly interesting is the stability of such materials during polarization. In strong acidic medium (i.e 1.25 mol l 1 HClO4), 95% of the charge density was maintained after polarization during 7000 cycles. Due to its oligoether substituents, films are wellcompatible with oxyethylene based elecytrolytes or other hydrophilic electrolytes compared with N -alkyl substituted polycarbazoles and the ionic diffusion process could be improved. Experiments are in progress in the laboratory to study such polycarbazoles in an electrochromic device with a PEO based solid electrolyte.
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