Synthesis and characterization of a series of conducting polymers based on indole and carbazole

A series of indole (In) and carbazole (Cz) derivative monomers have been synthesized, such as 4-[3-carbazolyl] indole (4In-3Cz), 5-[3-carbazolyl] indole (5In-3Cz), 6-[3-carbazolyl] indole (6In-3Cz), 7-[3-carbazolyl] indole (7In-3Cz). The comonomers synthesized by Stille coupling reaction were characterized by 1H-NMR and elemental analysis. Potentiodynamic method was used for electropolymerization of comonomers, Indole, Cz, and the mixture of In and Cz. Electrochemical activities of resulting P[4In-3Cz], P[5In-3Cz], P[6In-3Cz], P[7In-3Cz], polyindole (PIn), polycarbazole (PCz) and P[In-co-Cz] films were investigated comparatively by CV at different scan rates, electrochemical impedance spectroscopy (EIS) and spectroelectrochemical measurements. The ionization potentials, Ip, specific capacitance, Csp, and optical band gap, Eg, of copolymers were obtained from these measurements. In order to gain some preliminary information on the structure of the copolymers, DFT analysis was performed and dimers and tetramers were optimized. Results suggested that, in order to obtain an In-Cz copolymer with low oxidation potential and band gap, indole ring should be substituted through 5 position to the 3 position of Cz. If high specific capacitance value or high conductivity are desired, P[4In-3Cz] and P[6In-3Cz] are the best copolymers, respectively.

indoles have been prepared in one step by adding suitable organometallic reagents, followed by LiAlH 4 to a refluxing mixture of isatin in ether [43]. However, in this study, indole was substituted through the 2-position which limits the polymerization possibilities of indole ring.
In the literature, there are some studies on the synthesis of heteroaryl and allylindoles by Stille cross-coupling reactions [44] that offer advantages to the Suzuki reactions in view of the neutral conditions and the stability and accessibility of the tin derivatives.
Although there have been previous studies on the synthesis of indole and Cz comonomers with other monomers, studies on the synthesis of In-Cz comonomers are outside of our knowledge. In this study, 4In-3Cz, 5In-3Cz, 6In-3Cz, and 7In-3Cz were synthesized, polymerized, and comparatively characterized for the first time. Their physical properties were obtained by spectroscopic and electrochemical techniques. The results showed that P[4In-3Cz], P[5In-3Cz], P[6In-3Cz], P[7In-3Cz] are promising for applications as they have lower oxidation potentials and optical band gaps than homopolymers.

Measurements
NMR spectra were collected on a Brucker 250 MHz spectrometer and referenced to the residual proton solvent resonance. CHNS-932 LECO model device was used for elemental analysis.
ATR-FTIR measurements were performed with Perkin Elmer spectrum one spectrophotometer.
Keithley 2400 model multimeter connected to Lucas labs 302 model probe holder and SP4-180-TFS type probe was used to measure solid state electrical conductivity of polymers from pellets with a thickness of 0.8 mm. The following equation was used for calculation [45,46]: σ = V -1 I (ln 2/ π d n ) where σ = conductivity, V is the potential in volts, I is current in ampere, and d n is the thickness in cm which was measured by using a digital caliper.
PARSTAT 2263 model potentiostat/galvanostat controlled with Power Suite software via a computer was used for electrochemical polymerization and characterization. Measurements were performed in a three-electrode cell that contains a Pt button (area of 0.02 cm 2 ), Pt wire, and Ag wire as the working, axillary, and reference electrode, respectively. Calibration of pseudo-reference was performed externally by using a 5 mM ferrocene/ferrocenium solution that has a potential 0.35 V vs. Ag/AgCl. Spectroelectrochemical measurements were performed by Shimadzu 160A model UVvisible spectrophotometer in a quartz cell. Indium tin oxide (ITO) coated glass electrode with a size of 7 cm × 5 cm × 011 cm that has a resistivity R ≤12 Ω / cm obtained from Colorado Concept Coatings Company was used as working. Pt wire and Ag wire were counter and a reference electrode, respectively. Capacitive behavior of polymers was tested by electrochemical impedance spectroscopy (EIS) measurements that performed in the range of 10 × 10 3 Hz-10 × 10 -3 Hz with 10 mV amplitude at open circuit potential.
Density functional theory (DFT) method at 6-31g* level was used for optimization of comonomer and oligomer geometries. Gaussian software [47] was used for the calculations on the supercomputer of High-Performance Computing Center at ITU.

Synthesis
In-Cz comonomers were synthesized by Stille coupling reactions between haloindoles and 3-trimethyl tin Cz which was synthesized by the reaction of Cz and Sn (CH 3 ) 3 Cl.
i) Synthesis of 3-(trimethyl stannyl)carbazole Firstly, a 100-mL Schlenk flask equipped with a stirring bar and N 2 inlet/outlet was dried and charged with ~80 mL of THF, 2.00 g (12 mmol) of Cz, and 1.39 g (12 mmol) of TEMED. After cooling the flask to −78 °C, 2.5 M (12 mmol) t-BuLi in hexanes was added and stirred at this temperature for 1 h. Then, 2.39 g (12 mmol) of Sn (CH 3 ) 3 Cl in 15 mL of THF was added dropwise. The mixture was stirred overnight at room temperature. After evaporation of solvent under vacuum, 100 mL of CHCl 3 was added to the mixture. The extraction of product from solution was performed by CHCl 3 and washing with water. Organic phase was collected dried by MgSO 4 . The solvent was evaporated under vacuum and a dark yellow product was obtained. The product was characterized with 1 H NMR and FT-IR measurements.
At the end of the reaction, mixture was poured into water and extracted with CH 2 Cl 2 . The process was repeated three times and the organic phases were combined and dried with K 2 CO 3 . After evaporation of some solvent, purification of the product was carried out by flash chromatography that contains silica gel as a stationary phase and CHCl 3 /hexane/THF; 1:1:1, v/v mixture as eluent.
Synthetic route and the structures of synthesized comonomers are shown in Schemes 1 and 2, respectively. Comonomers were fully characterized by 1 H-NMR and elemental analyses and results are given below. Observation of all numbered protons (inset) of 4In-3Cz comonomer, as an example, given in 1 H-NMR spectrum, supported the synthesis of comonomers ( Figure 1).
The yields and the elemental analysis of the comonomers are given below:  Elemental analysis results of comonomers agreed with theoretical ones; C: 85.71 %, H: 4.29 %, N: 10.00 %. These results suggested that the comonomer structure contains one Cz unit for one indole unit.

Electropolymerization
The electropolymerization was performed in 0.1 M NaClO 4 containing ACN on Pt by potentiodynamic method in the potential range of -0.2 V (vs. SCE) up to 1.8 V at 20 mV s -1 . Different cycle numbers were applied during electropolymerization to obtain polymer films at different thicknesses and optimum thicknesses were determined according to the redox behaviors.
For comparison, random copolymer of indole and Cz was also prepared starting from their mixtures under the same conditions and called P[In-co-Cz]. The exact concentrations of pristine monomers (Cz, In, and In-Cz comonomer) were 1.0 × 10 -3 M and the total concentration in the mixture of In and Cz was also 1.0 × 10 -3 M by using 0.5 × 10 -3 M In and 0.5 × 10 -3 M Cz.
All polymeric films were washed with ACN before characterization.

Voltametric measurements
Electrochemical oxidation onset potentials (E ox ) from CVs were obtained at the position where the current starts to increase. Figure 2A shows the first anodic scan of CVs for In, Cz, In + Cz mixture, and 4In-3Cz and their E ox values were determined from this figure as 1.14, 1.24, 1.16, and 1.06 V, respectively. The mixture of In and Cz has a current value that is between what In and Cz individually have, as expected. A solution containing 1.75 × 10 -4 M 4In-3Cz showed lower E ox than the others and the current value was the highest. This result suggested that the 4In-3Cz behaved differently than the mixture of In and Cz, which supported the formation of comonomer. When the polarization curves of comonomers were compared, it can be seen that the E ox values were close to each other and current intensities at maximum peak potentials were shifted and 6In-3Cz has the lowest value ( Figure 2B).
Ionization potential (I p ) was calculated as suggested in the literature [48]: (1) E ox and calculated I p values of monomers are summarized in Table 1.
The results suggested that it is possible to obtain new comonomers by cross-coupling reactions of Cz and In with lower oxidation potential than the value of starting monomers.
In, Cz, and In + Cz were polymerized potentiodynamically, and electroactivities of the polymers were tested by CV performed at variety of scan rates and results are illustrated in Figure 3. Oxidation started at 1.14 V, 1.24 V, and 1.16 in the first scan of In, Cz, and In + Cz, respectively, which were attributed to formation of cation radicals by oxidation of the monomers (Figures 3a-3c). In the first cycle, when potential up to 1.4 V is applied for In + Cz similarly with the polymerization of In and Cz, a faster current increase was observed after the peak potential of around 1.1 V. For this reason, the applied potential has been reduced to prevent overoxidation of the copolymer film formed on the surface, which has lower redox activity (Figure 3c). In the following cycles, a new peak appeared at lower potentials, 0.62 V (E P1 ), 0.87 V (E P1 ) and 0.52 V (E P1 ), 0.93 V (E P2 ) for In, Cz, and In + Cz respectively. The peak current increased upon successive cycling and it was an indication of the deposition of polymer.
The redox activity of the polymers was tested by CV in ACN containing 0.1 M NaClO 4 at different scan rates. Comparison of the CVs of resulted polymers namely, PIn, PCz, and P[In-co-Cz] is given in Figure 3d. P[In-co-Cz] has a peak potential in between PIn and PCz, as expected.
During electropolymerization of In-Cz comonomers, CVs were obtained and are given in Figure 4. The anodic potential limit of polymerization in all cases was selected as 1.75 V, since 4In-3Cz, comonomer does not polymerize at lower potentials. In order to optimize the polymerization conditions, the same potential range was selected for all monomers. Increase in current intensity was higher in the case of 4In-3Cz and 6In-3Cz than 5In-3Cz and 7In-3Cz and this result suggested that thicker film formation occurred in the case of 4In-3Cz and 6In-3Cz comonomers.  The redox behaviors of resulting polymeric films obtained by applying different cycle numbers were compared ( Figure  5). For example, for the P[7In-3Cz], the optimum number of cycles was accepted as 4, since the redox behavior of the film obtained by applying 4 cycles was better than the film obtained in 2 and 8 cycles.
Polymerizations were also carried out in different potential ranges for 5In-3Cz, 6In-3Cz, 7In-3Cz. It was found that the redox behavior of polymer depends on the anodic potential limit and at lower potential limit; more electrochemically reversible polymeric films were obtained as expected [49]. This comparison was given for P[7In-3Cz] as an example ( Figure 6). As it can be seen the polymer film obtained at the potential limit of 1.75 V has quasireversible redox behavior due to some degradation in the polymer chain.
Scan rate dependence of P[In-Cz]'s is given in Figure 7.
De-doping peak of P[6In-3Cz] was not significant as much as the peaks of P[4In-3Cz], P[5In-3Cz] and P[7In-3Cz]. Generally, CV of conducting polymers demonstrates "irreversible" anodic and cathodic peaks and there are several explanations for such behaviors [50]. Free rotation of the molecule during the transition from benzonoid to quinoid structure might be hindered by deposited oligomers on the electrode as a solid matrix and the typical voltammogram is the superposition of different redox waves, i.e. a mixture of oligomers, and polymers of different chain lengths and possibly different cross-linking. Role of coupling position seems to affect the redox behavior of polymer and superposition of different redox waves became more pronounced at lower scan rates for P[5In-3Cz] and P[7In-3Cz].
The comparison of CVs of all In-Cz polymer films is given in Figure 8a. As it can be seen, although the peak potentials of polymers were close to each other, the highest peak current was observed for P[7In-3Cz]. This result indicated that the coupling position affected the electroactivity of P[In-Cz] films and P[7In-3Cz] seems to be the most electroactive one.
Scan rate dependence on the anodic and cathodic peak currents (I pa and I pc ) of P[4In-3Cz], P[5In-3Cz], P[6In-3Cz] and P[7In-3Cz] was investigated comparatively with PIn and PCz obtained under the same experimental condition. It was found that I pa and I pc scale linearly with scan rates not with square root of scan rates and this result indicate an electroactive thin film behavior instead of diffusion control one as suggested in literature [49] (Figure 8b). The order of the current intensities was found as follows: P[7In-3Cz] > PCz > PIn > P[4In-3Cz] > P[6In-3Cz] > P[5In-3Cz].

FT-IR results
The FT-IR spectra of PIn and PCz, P[In-co-Cz]) and P[4In-3Cz] are given in Figure 9. The characteristic vibrations of Cz and indole rings are very similar. Therefore, it is difficult to interpret the coupling position. Two rings give similar vibration at very close wave numbers and when they coupled each other these peaks are split. -C -H out of plane deformation peaks were observed at 739 and 794 cm -1 for PIn, and at 727, 746, and 800 cm -1 for PCz and split into three peaks in the range of The peak at 1400 cm -1 , which belongs to the aromatic -C -H vibration, was observed in PCz, but not in PIn, and shifted to 1393 cm -1 in the case of P[4In-3Cz]. This result indicates that Cz is included in the structure. Similarly, the presence of the peak at 927 cm -1 for PIn but the absence for PCz, observed at 927 cm -1 for P[4In-3Cz] showed the inclusion of indole to the P[4In-3Cz] structure. An absorption band at 1100 cm -1 which is attributed to ClO 4 -, showed doping of the polymers with this anion (Figure 9). The peak at 740 cm -1 corresponding to the out of plane deformation of aromatic -C -H of Cz and indole, was observed with higher intensity for P[In-co-Cz] by splitting in two (745 and 720 cm -1 ). This indicated less coupling possibility through the benzene ring during the random copolymerization of In and Cz.

Optical properties
Spectroelectrochemical methods were also employed to study optical properties of polymers by applying anodic potentials which results polaron and bipolaron formations. P[4In-3Cz], P[5In-3Cz], P[6In-3Cz] and P[7In-3Cz] films were electrochemically polymerized on ITO for spectroelectrochemical analysis and subsequently placed in a quartz cuvette with Pt counter and Ag reference electrodes. For reduction and to obtain their neutral states, -1.0 V was applied to the polymers. UV-visible spectrum of P[4In-3Cz] obtained by applying different potential is shown in Figure 10. The peak at 350 nm was observed it was attributed to π-π* transition. The new peak at 870 nm was started to form by application of anodic potential at 0.8 V and this might be due to polaron and/or bipolaron formation. As the anodic potential was increased (up to 1.1 V and further), absorption maximum shifted to 700 nm. This might occur by oxidation of polymer chains that results better and less ordered polymer structures as suggested for other PTh derivatives [51][52][53]. Another explanation might be formation of the segments with different conjugation length [54] and these two approaches are very close to each other.
E g values of polymers were estimated from UV-visible spectrophotometric measurements, by extrapolation of the low energy edge to the baseline of the spectra and results are given in Table 2. While P[4In-3Cz] and P[6In-3Cz] have lower E g than PIn and PCz, P[5In-3Cz] has similar value with Pin, and P[7In-3Cz] has the highest E g . It is found that E g could be decreased by obtaining polymers from In-Cz comonomer that synthesized by coupling of In and Cz instead of random copolymerization.
E ox of P[In-Cz]s were calculated from the equation (1) above and showed similar trends with comonomers. Results are given in Table 2. It can be seen that E ox of P[In-Cz]s was slightly different from the values of PIn and PCz. E ox and E g values of conjugated polymers are expected to decrease with the extension of π-conjugation length. When the E ox of polymers and monomers are compared (Table 2), it can be seen that E ox significantly decreases in the case of polymers. Variation in molecular architecture of resulting polymer has different effects on ionization potential and electron transition between the HOMO and LUMO levels; therefore, changes in E ox values were different from variation of E g values.

EIS measurements
To explore the capacitive behavior of polymers, EIS measurements were performed and Nyquist diagrams of PIn, PCz, P[In-co-Cz], and P[4In-3Cz] films are given in Figure 11. Specific capacitance values (C sp ) were calculated from the slope of imaginary component of the impedance, -Z im versus 1/frequency (f) plot, by using the equation below as suggested in the literature [55] and the results are summarized in Table 2.
Capacitive behaviors of polymers depend on the structure, and the highest C sp value was obtained in the case of P[4In-3Cz]. This structure seems to be the most favorable one for charge storage applications. The C sp values of the copolymers needed to be improved by changing reaction conditions for application.
Experimental EIS results were analyzed by fitting an equivalent circuit model. Different electrical models were tried and with the one that has the lowest chi-squared value, an excellent agreement between experimental and simulated data was obtained as shown in Figure 12. The values of the circuit elements obtained as a result of the simulation are summarized in the Table 3. The value of R1 in the circuit belongs to the solution resistance of the cell, constant phase elements (CPEs) and R 2 and R 3 belong to the nonideal capacitances and resistances of the polymer film and electrode, respectively, and W to the Warburg element related to mass transfer.
The impedance of a capacitor is obtained by the following equation and the CPE value is used in case of nonideal capacitors account for surface roughness, nonuniform current distribution etc. and the exponent n is less than one: where Y 0 is the capacitance, C, and n is an exponent equaling 1 for ideal capacitor The equation for Warburg impedance and can be written as follows: where ω = radial frequency (s −1 ) D o = diffusion coefficient of the oxidant (cm 2 /s) D R = diffusion coefficient of the reductant (cm 2 /s) A = surface area of the electrode (cm 2 ) n = number of electrons transferred Table 3. Impedance parameters of P[4In-3Cz] film obtained from R(Q(R(Q(RW)))) type equivalent circuit model.

C = bulk concentration of the diffusing species (moles/cm 3 )
Total capacitance, C spEIS , was calculated and is given in Table 3. When experimentally determined and simulated capacitance values were compared, they were found to be in good agreement.

Conductivity measurements
Electrodeposition leads to a doped and conductive polymer, where the perchlorate ion is the dopant. The solid-state conductivities of the polymer films are given in Table 2.
The conductivity values of the polymers agree with literature and have the expected values in the range for PIn and PCz [1,56,62,63]. The highest conductivity was obtained in the case of P[6In-3Cz]. This could be due to higher conjugation possibility and change in growth behavior of P[6In-3Cz] that affects the inclusion of dopant ion. 4.6. Possible dimers and tetramers structures of comonomers E ox , E g , conductivity, C sp values of copolymers suggested that different structures were obtained by polymerization of comonomers and the mixture of two monomers. Although only random copolymers can be obtained from the mixtures, it is possible to obtain alternated structures by using newly synthesized comonomers which have improved properties. Theoretical and experimental methods were used together in order to have an idea about the relationship between the application-oriented properties of polymers such as E ox , E g , and C sp .
The possibility of polymerization from 2 and 3 positions of indole [57][58][59][60][61] and 3 position of Cz [62-64] allows the formation of many dimers for each comonomer as given in Scheme 3 due to asymmetric structure of comonomers. Dimers 7 and 8 are less likely to occur during the polymerization since they coupled through the 2 position of Cz.
As it can be seen, by oxidation of 4In-3Cz comonomer cation radicals can be formed which was coupled in the following step and formed 2,3-indole-indole dimer, called D[2,3(In-In)] 4In-3Cz . The numbers and abbreviation of the name of dimers that may occur from 4In-3Cz cation radicals are summarized in Table 4.
In the propagation step, after oxidation, these dimers can couple with each other and formed tetramers, for instance, D[2,3(In-In)] 4In-3Cz can couple through 3,3' position of Cz and give the tetramer named as T(3,3'(Cz-Cz)] 4In-3Cz (Scheme 4).   Similarly, other possible dimer structures of 4In-3Cz might be obtained by coupling of Cz cation radicals that formed at the 3 position of Cz rings and called D[3,3'(Cz-Cz)] 4In-3Cz . Further oxidation of this dimer might result a tetramer by coupling of indole rings through their 2 positions which resulted a tetramer named T[2,2'(In-In)] 4In-3Cz . Coupling positions have significant role in the properties of resulting copolymers. DFT allowed to resolve the problems concerning the monomer linkage as suggested in the literature for polymerization of 1,8-diaminocarbazole [65]. It would be better to carry out similar calculations to clarify the most probable structures.
Preliminary calculation was carried out for T(3,3'(Cz-Cz)] 4In-3Cz and T[2,2'(In-In)] 4In-3Cz and optimized geometries of them are given in Schemes 5 and 6. As can be seen, the geometry of the tetramer changes significantly as the coupling position changes, and since the electronic properties of polymers are formed by the contribution of all oligomers, it is difficult to make a precise prediction about the mechanism and structure-property relationship without further theoretical investigation Scheme 5. Optimized geometry of T(3,3'(Cz-Cz)] 4In-3Cz tetramer formed from D[2,3(In-In)] 4In-3Cz dimer. Scheme 6. Possible optimized geometry of T[2,2'(In-In)] 4In-3Cz tetramer that formed from D[3,3'(Cz-Cz)] 4In-3Cz dimer.

Conclusion
The Stille cross-coupling reaction was applied to obtain various carbazolylindoles and their chemical structures were fully characterized by common techniques.
Although random copolymers can be obtained by polymerization of In and Cz mixture, alternated copolymers have been obtained by using newly synthesized In-Cz comonomers.
In + Cz mixture and In-Cz comonomers were electropolymerized and the important properties of their polymers for applications such as redox potential, conductivity, optical band gap and specific capacitance were investigated comparatively.
The changes of optical and electrical properties of In-Cz copolymers synthesized by different indole derivatives showed the role of coupling position of indole ring with Cz. For example, the higher conductivities were obtained in the case of P[6In-3Cz]. Conductivity depends on the conjugation length and geometries of structure. Therefore, it can be said that P[6In-3Cz] has the most planar geometries and/or the highest conjugation length. The evaluation of the relationship between the chemical structure of copolymers and their specific properties were supported by theoretical calculations.
To obtain an In-Cz copolymer with low oxidation potential and band gap, one must substitute indole ring through 5 to the 3 position of Cz. indicate that it has a more planar conjugation keeping the HOMO-LUMO energy level at the lowest. The specific capacitance values of copolymers were lower than that of homopolymer. P[4In-3Cz] has the highest C sp value among the series and this can be explained by the combination of oligomers formed from the 4In-3Cz comonomer that highly porous structure. In other words, P[4In-3Cz] possessed the most suitable structure for capacitive applications, while P[5In-3Cz] and P[6In-3Cz] were the most favorable candidates for electronic applications.