Electrochemical synthesis of new conjugated polymers based on carbazole and furan units

Accepted Manuscript Electrochemical Synthesis of New Conjugated Polymers Based on Carbazole and Furan Units H. Esra Oğuztürk, Seha Tirkeş, Ahmet M. Önal PII: DOI: Reference:

S1572-6657(15)00221-0 http://dx.doi.org/10.1016/j.jelechem.2015.04.041 JEAC 2106

To appear in:

Journal of Electroanalytical Chemistry

Received Date: Revised Date: Accepted Date:

17 July 2014 26 March 2015 30 April 2015

Please cite this article as: H. Esra Oğuztürk, S. Tirkeş, A.M. Önal, Electrochemical Synthesis of New Conjugated Polymers Based on Carbazole and Furan Units, Journal of Electroanalytical Chemistry (2015), doi: http://dx.doi.org/ 10.1016/j.jelechem.2015.04.041

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Electrochemical Synthesis of New Conjugated Polymers Based on Carbazole and Furan Units

H. Esra Oğuztürka, Seha Tirkeşb, *, Ahmet M. Önala, *

a

Department of Chemistry, Middle East Technical University, 06800 Ankara, Turkey.

b

Atılım University, Chemical Engineering and Applied Chemistry, 06836 Ankara, Turkey

*Corresponding authors. (A.M. Önal) Tel.:+903122103188; Fax: +903122103200; E-mail: [email protected];

(S.Tirkeş)

Tel.:

+903125868390;

Fax:

+903125868091;

E-mail:

[email protected]

ABSTRACT

In this study, synthesis of four new monomers; 3,6-di(2-furyl)-9H-carbazole (M1), 3,6-di(2furyl)-9-ethyl-carbazole (M2), 2,7-di(2-furyl)-9-H-carbazole (M3), 2,7-di(2-furyl)-9-(tridecan-7yl)-9H-carbazole (M4), was achieved via Stille cross-coupling reaction. The monomers were electrochemically polymerized, via repetitive cycling in acetonitrile-tetrabutylammonium hexafluorophosphate electrolytic medium. Optical and electrochemical properties of the monomers and their corresponding polymers were investigated and it was found that optical properties show slight variations depending on the connectivity between the carbazole and furan moieties. However, all the monomers synthesized in this work exhibited an irreversible oxidation peak at around 1.0 V. Electrochemically obtained polymer films, on the other hand, exhibited quasi-reversible redox behavior due to doping/dedoping of the polymers which was accompanied by a reversible electrochromic behavior. Their band gap values (Eg) were elucidated utilizing spectroelectrochemical data and it was found that polymers obtained from

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2,7- substituted carbazole derivatives have slightly lower band gap values. Furthermore, scanning electron micrographs were used for morphological examinations.

Keywords: Carbazole, furan, electrochromic polymers, conjugated polymers.

1. Introduction

Conjugated polymers still are of great interest due to their advantages in various applications including solar cells [1,2] and light emitting diodes (LEDs) [3–5]. Furthermore, their electrical and optical properties might be easily tailored via functionalization of the monomer structure prior to polymerization. Among various conjugated polymers, polycarbazoles are of great importance due to their good electroactive and photochemical properties [6] which make polycarbazoles and their derivatives potential candidates for various applications including light emitting diodes [7,8], electroluminescent [9,10] and electrochromic displays [11]. Moreover, carbazole derivatives are widely used as charge-transporting materials. Although 3,6- or 2,7-linked polycarbazoles with different optoelectronic properties are known, due to higher reactivity of 3 and 6 positions, polymerization of carbazoles generally results in 3,6linked polycarbazoles [12,13]. Furthermore, direct electrochemical polymerization of carbazoles in aprotic solvents leads to oligomeric chains [14]. One of the most promising ways to obtain long polycarbazole chains via electrochemical polymerization is to introduce heteroaromatic substituents at 2,7- or 3,6- positions of the carbazoles as electron donor groups. These types of monomers, with proper donor side groups, are expected not only to have lower oxidation potentials but also may have lower band gap values. Thiophene and 3,4ethylenedioxythiophene (EDOT) are the two examples of heteroaromatics which were commonly used as donor group [15-17]. Reynolds and coworkers utilized pyrrole and EDOT

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as heteroaromatic side groups in 3,6-substituded carbazoles. They reported that hybrid monomers exhibit lower oxidation potentials and their polymers can be readily obtained via repetitive cycling [15]. Heinze et al. reported that electrochemical polymerization of 3,6-bis(2thienyl)-N-ethylcarbazole in dichloromethane (DCM) [16] yields polymers exhibiting high conductivity and electrochromism. Furan on the other hand, is another five-membered heterocyclic unit which can be easily obtained from natural products [18]. However, its use as building block for the design and synthesis of new π-conjugated polymers has remained limited due to its lower stability especially in the oxidized state [19,20]. Woo et al. demonstrated that furan can be incorporated into conjugated polymer backbones and resulting polymers exhibited similar electrical and optical properties as thiophene counterparts [21]. A new donor-acceptor (D-A) type copolymer based on furan containing benzothiadiazole and benzodithiophene with good solubility and thermal stability was reported by Wang and coworkers [22]. More recently, we investigated electrochemical polymerization of a series of furan-fluorene and furanbenzochalcogeno-diazole-based hybrid monomers [23,24]. However, to the best of our knowledge, the only report utilizing furan as side group of carbazoles, is about the synthesis and polymerization of N-alkylated-2,7-di(2-furyl)carbazoles [25]. Keeping all this in mind, we have synthesized four new conjugated monomers to clarify the effect of linkage site of carbazole, with and without N-alkyl substitution, utilizing furan as the side group of hybrid monomers. carbazole

The (M2),

monomers,

3,6-di(2-furyl)-9-H-carbazole

2,7-di(2-furyl)-9-H-carbazole

(M3),

(M1),

3,6-di(2-furyl)-9-ethyl-

2,7-di(2-furyl)-9-(tridecan-7-yl)-9H-

carbazole (M4) were polymerized via potential cycling. Electrochemical and optical properties of the polymers (poly(3,6-di(2-furyl)-9-H-carbazole) (P1), poly(3,6-di(2-furyl)-9-ethyl-carbazole) (P2), poly(2,7-di(2-furyl)-9-H-carbazole) (P3), and poly(2,7-di(2-furyl)-9-(tridecan-7-yl)-9-Hcarbazole) (P4)) were investigated using cyclic voltammetry and in situ spectroelectrochemical technique, respectively. Photophysical and optical properties of the monomers were also investigated in terms of the connectivity between the carbazole and furan moieties.

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2. Experimental 2.1.General Information N-Bromosuccinimide (Fluka), CaH2 (Acros, 99%), Tetrakis(triphenylphosphine)palladium(0) (Aldrich, 99%), 2-(Tributylstannyl)furan (Aldrich, 97%), 2,7-dibromo-9-H-carbazole (Lumtec, >98%), 2,7-dibromo-9-(tridecan-7-yl)-9H-carbazole (Lumtec, >98%) were used as received. For electrochemical studies, tetrabutylammonium hexafluorophosphate (TBAH) (Fluka, ≥98 %), was used as a supporting electrolyte without further purification. Acetonitrile (ACN) (Merck) and dichloromethane (DCM) (Merck) were refluxed on CaH2 (Acros, ≥99 %) and then distilled. Platinum disc (0.02 cm2) and platinum wire electrodes were used as working electrode and counter electrode respectively. A Ag/AgCl electrode in 3M NaCl (aq) solution was used as a reference electrode. Polymer films were synthesized by both repetitive cycling and constant potential electrolysis. Repetitive cycling was used to obtain the polymer films for the investigation of optoelectronic properties of the polymers. On the other hand, to obtain polymer films in significant quantities for FTIR and SEM measurements, constant potential electrolysis was used. For the analysis of polymers, monomer-free electrolytic solution including ACN and 0.1 M TBAH was used and measurements were conducted at room temperature. In situ optical properties were investigated using an indium-tin oxide (ITO, Delta. Tech. 8-12 Ω, 0.7 cm x 5 cm) electrode in a UV cuvette. A platinum wire and silver wire were used as counter electrode and pseudo-reference electrode, respectively. The polymer films coated on ITO had been switched between the neutral and oxidized states several times in order to equilibrate its redox behavior in electrolytic solution prior to electrochemical and optical analyses. Electroanalytical measurements were performed using a Gamry PCI4/300 potentiostat-galvanostat and the electronic absorption spectra were monitored on a Hewlett– Packard 8453A diode array spectrometer. 1H and

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C NMR spectra were recorded on a

Bruker Spectrospin Avance DPX-400 Spectrometer and chemical shifts were given relative to

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tetramethylsilane as the internal standard. FT-IR spectra were recorded ex situ on a Bruker Vertex 70 Spectrophotometer with attenuated total reflectance (ATR).

2.2. Monomer Synthesis

All monomers; 3,6-di(2-furyl)-9H-carbazole (M1); 3,6-di(2-furyl)-9-ethyl-carbazole (M2); 2,7di(2-furyl)-9-H-carbazole (M31) and 2,7-di(2-furyl)-9-(tridecan-7-yl)-9-H-carbazole (M4) were synthesized via Stille cross-coupling reaction of 2-(tributylstannyl)furan with corresponding 2,7- and 3,6-dibrominated carbazole derivatives in the presence of Pd as the catalyst. 2,7dibrominated carbazole derivatives were used as received (Lumtec). 3, 6-dibrominated carbazole derivatives were synthesized via bromination reaction from carbazole and N-ethylcarbazole (Fluka). Bromination reactions were performed with 1 equivalent of carbazole and N-ethyl-carbazole and 2 equivalents of N-Bromosuccinimide in DCM under N2 atmosphere for 6 hours at room temperature. Stille cross-coupling reactions were conducted with 1 equivalent of 2, 7 and 3, 6-dibrominated carbazole and its derivatives and 2 equivalents of 2-(tributylstannyl)furan in the presence of 0.1 equivalent of Pd catalyst in toluene at 90 °C under N2 atmosphere for 20-25 hours (Scheme1).

3,6-dibromo-9H carbazole: Beige Crystals, 1H NMR (400 MHz, CDCl3) δ 8.07 (s, 1H), 7.45 (dd, J = 8.6, 1.8 Hz, 2H), 7.25 (d, J = 8.6 Hz, 2H), 7.19 (s, 2H); 13C NMR (101 MHz, CDCl3) δ 129.71 (s), 123.67 (s), 112.66 (s). 3,6-dibromo-9-ethyl-carbazole: White Crystals; 1H NMR (400 MHz, CDCl3) δ 7.92 (d, J = 1.7 Hz, 2H), 7.38 (dd, J = 8.7, 1.8 Hz, 2H), 7.05 (d, J = 8.7 Hz, 2H), 4.07 (q, J = 7.2 Hz, 2H), 1.22 (t, J = 7.2 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 137.61 (s), 127.87 (s), 122.37 (s), 110.83 (s), 108.99 (s), 36.66 (s), 12.61 (s).

5

M1: Dirty white solid; yield: 75% ; 1H NMR (400 MHz, Acetone) δ 10.39 (s, 1H), 8.42 (d, J = 15.0 Hz, 2H), 7.68 (ddd, J = 8.5, 6.5, 1.7 Hz, 2H), 7.48 (d, J = 1.3 Hz, 2H), 6.69 (dd, J = 6.7, 3.4 Hz, 2H), 6.42 (dd, J = 3.3, 1.8 Hz, 2H); 13C NMR (101 MHz, Acetone) δ 156.03 (s), 142.36 (s), 140.91 (s), 124.32 (s), 123.75 (s), 123.30 (s), 116.56 (s), 112.46 (s), 111.94 (s) 104.24 (s); FTIR (ATR/ cm-1): 3400, 3150, 3115, 2850-2960, 1611, 1463, 1371, 1293, 1236, 1160, 1078, 1010, 932, 885, 723. M2: Yellow solid; yield: 72%; 1H NMR (400 MHz, CDCl 3) δ 7.71 – 7.57 (m, 2H), 7.40 (dd, J = 7.2, 1.8 Hz, 2H), 7.21 (t, J = 9.0 Hz, 2H), 6.53 (dd, J = 9.4, 3.3 Hz, 2H), 6.43 – 6.35 (m, 2H), 4.21 – 4.06 (m, 2H), 1.52 (qd, J = 15.0, 8.0 Hz, 3H);

13

C NMR (101 MHz, CDCl3) δ 154.06 (s),

140.20 (s), 138.66 (s), 122.15 (s), 121.58 (s), 121.38 (s), 114.97 (s), 110.61 (s), 107.73 (s), 102.15 (s), 36.62 (s), 25.75 (s); FTIR (ATR/ cm-1): 3145, 3112, 2961-2854, 1604, 1459, 1374, 1343, 1295, 1150, 1090, 1007, 940, 881, 790, 725. M3: Ash colored solid; yield: 79%; 1H NMR (400 MHz, Acetone) δ 10.36 (s, 1H), 8.00 (d, J = 8.2 Hz, 2H), 7.52 (d, J = 1.2 Hz, 2H), 7.46 (dd, J = 8.2, 1.4 Hz, 2H), 6.76 (d, J = 3.3 Hz, 2H), 6.45 (dd, J = 3.3, 1.8 Hz, 2H); 13C NMR (101 MHz, Acetone) δ 154.37 (s), 141.42 (s), 140.41 (s), 127.95 (s), 121.83 (s), 119.72 (s), 115.11 (s), 111.33 (s), 105.26 (s), 104.43 (s). FTIR (ATR/ cm-1): 3122, 2850-2970, 1601, 1458, 1374, 1291, 1225, 1146, 1090, 1013, 941, 881, 729. M4: Orange solid; yield: 88%; 1H NMR (400 MHz, CDCl3) ) δ 7.90 (t, J = 9.3 Hz, 2H), 7.72 (d, J = 62.5 Hz, 2H), 7.39 (d, J = 1.3 Hz, 2H), 6.60 (s, 2H), 6.38 (dd, J = 3.1, 1.7 Hz, 2H), 4.50 (tt, J = 10.1, 4.9 Hz, 1H), 2.23 (ddd, J = 19.8, 12.1, 7.2 Hz, 2H), 1.87 – 1.74 (m, 2H), 1.57 – 1.46 (m, 2H), 1.26 – 1.20 (m, 4H), 1.02 – 0.96 (m, 6H), 0.81 (t, J = 7.3 Hz, 4H), 0.63 (t, J = 7.0 Hz, 6H);

13

C NMR (101 MHz, CDCl3) δ 155.20 (s),143.00 (s), 141.84 (s), 120.60 (s), 120.35 (s),

115.59 (s), 111.89 (s), 106.71 (s), 104.87 (s), 104.09 (s), 56.65 (s), 33.93 (s), 31.65 (s), 29.20 (s), 26.95 (s), 22.68 (s), 14.10 (s); FTIR (ATR/ cm-1): 3150, 3113, 2958-2848, 1608, 1460, 1370, 1243, 1298, 1226, 1146, 1085, 1008, 944, 878, 793, 725. (Scheme1 hereabout)

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3. Results and discussions

3.1. Synthesis of monomers and their electrochemical and spectroscopic properties

In the present study four new conjugated hybrid monomers, M1-M4, containing furan as heterocyclic side groups were synthesized via Stille cross-coupling reactions. The monomers were characterized by 1H- and

13

C-NMR, UV-Vis and fluorescence spectroscopy. The

electronic absorption spectra of the monomers were recorded in ACN and the results are depicted in Figure 1. As seen in Figure 1a, M1 exhibits two main absorption bands at about 260 nm and 308 nm. The former absorption band has fine structure analogous to B band of benzene, on the other hand, the latter absorption band is due to π-π* transition. These bands are slightly red shifted (about 5 nm) in the case of M2 (Fig. 1b), due to ethyl substituent on the nitrogen atom. On the other hand, the electronic absorption spectra of M3 and M4 exhibit two absorption bands at about 270 nm and 350 nm with vibronic couplings. An inspection of Figure 1 reveals that characteristic absorption bands for M3 (Fig. 1c) and M4 (Fig. 1d) are red shifted (about 50 nm for π-π* transition band) as compared to M1 (Fig. 1a) and M2 (Fig. 1b) indicating that conjugation of p-orbitals of furan and carbazole units is stronger in 2,7substituted derivatives making M3 and M4 more planar as compared to M1 and M2. A similar observation was also reported for 2,7- and 3,6-linked carbazole-thiophene derivatives [26]. The fluorescence spectra of the monomers were recorded in ACN and the results are depicted in Fig. 1a-d. The fluorescence spectra of all the monomers exhibited strong luminescence maxima in the wavelength region 350–450 nm upon their excitation at 340 nm. One of the most striking differences in the fluorescence emission spectra between 3,6-linked carbazoles (M1 and M2) and 2,7-linked carbazoles (M3 and M4) is that former exhibit larger Stokes shift (around 100 nm) as compared to latter (around 10 nm) indicating conformational changes of the molecules upon excitation [27]. Moreover, the larger Stokes shift of M1 and M2 indicates 7

easier excited-state intramolecular charge transfer in M1 and M2 as compared to M3 and M4 [28,29].

(Figure 1 hereabout)

The electrochemical properties of all monomers, synthesized in this work, were studied via cyclic voltammetry in an electrolyte solution containing 0.1 M TBAH dissolved in ACN. The voltammogram of M1 exhibited an irreversible peak at 1.0 V vs. Ag/AgCl during the first anodic scan (Fig. 2a). In the case of M2 and M3, the irreversible oxidation peaks appeared at 0.93 and 0.98 V in the same solvent-electrolyte couple (Fig. 2b and 2c), respectively. Oxidation peak for M4, on the other hand, appeared at slightly higher potential, 1.08 V, Fig. 2d. For the sake of comparison, cyclic voltammograms of furan and carbazole were also recorded in 0.1 M TBAH/ACN medium and results are shown in Fig. 2 a-d. An inspection of Fig. 2 reveals that onset oxidation potentials of N-alkyl substituted monomers (M2 and M4) are greater than those without N-substitution (M1 and M3), albeit alkyl chains are known to be an electron donating group. This might be due to steric repulsion between the substituents [25]. Furthermore, the oxidation potentials of the monomers synthesized in this work are lower than those of their smaller heterocyclic units (Eox= 1.60 V for furan and between 1.29 V to 1.49 V for carbazole units). This shift can be explained on the basis of increasing π-electron conjugation along the monomer molecules, which makes the loss of electron easier.

(Figure 2 hereabout)

3.2. Electrochemical polymerization of the monomers and investigation of their properties Electrochemical synthesis of the polymers was performed in ACN containing 0.1 M TBAH as supporting electrolyte using Pt-disc or ITO as working electrode. Electrochemically synthesized polymers were obtained with 25 cycles (200 mV/s) via repetitive cycling within the

8

range of 0.0-1.1 V for M1, M2, and M4, and 0.0-1.0 V for M3. As shown in Fig. 3, a new intensifying redox couple appeared around 0.8 V during successive scans. The increase in the current intensities of this new redox couple clearly indicated the formation of electroactive polymer film formation on the electrode surface. (Figure 3 hereabout)

After electrodeposition, the polymer film coated electrode was washed with ACN to remove any unreacted monomer and oligomeric species. The redox behavior of polymer films, P1-P4, was investigated by recording the cyclic voltammograms in monomer free electrolytic solution and the results are given in Fig. 4. The polymer films were found to exhibit one reversible redox couple representing the doping and de-doping of the polymer film at about 0.80 V. It is also observed that both anodic and cathodic peak currents increase linearly with increasing scan rate, indicating a non-diffusional redox process due to well adhered polymer film on the working electrode surface (See inset of Fig. 4 a-d). (Figure 4 hereabout)

The FTIR spectra of electrochemically obtained polymer films (P1-P4) and their corresponding monomers (M1-M4) were also recorded. FTIR spectra of the monomers exhibit the characteristics peaks due to furan and carbazole moieties. The peaks observed at 3150 and 3128 cm-1 due to aromatic C-H stretching, 1586 and 1487 cm-1 due to symmetric C=C ring stretching, 1171 cm-1 due to C-O-C stretching, 1050 and 985 cm-1 due to C-H in plane deformation, 915 and 865 cm-1 as a result of in plane and out of plane deformation of five membered ring [30] indicates the presence of furan moiety. The peaks due to carbazole moiety, on the other hand, are at 3400 cm-1 for N-H stretching, 2960-2850 cm-1 for aromatic ring stretching, at around 1600 cm-1 for C=C stretching, 1371, 1236 cm-1 for C-N stretching and 1293 cm-1 for C-C inter ring stretching. The in plane deformation of carbazole unit appears around 950-800 cm-1 and the peak at 723 cm-1 is due to 1,2- disubstituted benzene

9

ring [31,32]. This peak at 723 cm-1 disappeared after coupling reaction and new peaks due to 1,2,4-trisubstituted benzene rings at about 840 and 960 cm-1 were observed indicating C-C bond formation between furan and carbazole units. The peak due to N-H stretching appears at 3400 cm-1 for M1 and M3. However, in the FTIR spectra of M2 and M4, N-H stretching peak is not present because of the alkyl chains attached to nitrogen atom. A comparison of FTIR spectra of monomers with their corresponding polymers indicates the disappearance of the peak at around 740 cm-1 (α-C-H stretching of furan) due to the polymerization via α-α’ coupling. In addition, at 840 cm-1 new peak appears indicating the presence of PF6- dopant ion. Electro-optical properties of the polymer films deposited on ITO electrode via potential cycling were investigated by monitoring the changes in the electronic absorption spectra as a function of applied potential in a monomer free electrolyte solution and the results are depicted in Fig. 5. The electronic absorption spectra of neutral forms of the films exhibit an absorption band at around 370 nm for P1 and P2, and around 420 nm for P3 and P4 due to π–π* transition. During oxidation, these bands lose intensity which is accompanied by the formation of new intensifying band at about 550-600 nm. A new broad band at about 800 nm starts to evolve upon further oxidation of P1 and P2. On the other hand this newly evolving band appears beyond 800 nm for P3 and P4. Appearance of these new bands indicates the formation of charge carriers. All spectra recorded during potential cycling between 0.0 V and 1.1 V pass through a clear isosbestic points at 435 nm, 438 nm, 480 nm, and 495 nm for P1, P2, P3, and P4, respectively, indicating that polymer films were being interconverted between neutral and oxidized states. The changes in the electronic absorption spectra of polymer films are also accompanied by a color change, green to blue for P1 and P2, and yellow to green for P3 and P4 as they are exhibited in the inset of Fig. 5.

(Figure 5 hereabout)

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Optical band gap (Eg) values of the polymer films in their neutral states were calculated from the onset of the low energy end of π-π* transitions and were found to be 2.67 eV, 2.65 eV, 2.42 eV, and 2.45 eV for P1, P2, P3, and P4, respectively. HOMO/LUMO energy levels of the polymers were elucidated utilizing their ionization potentials and electron affinities obtained from experimental data. The onset of oxidation potentials of P1 (0.56 V), P2 (0.53 V), P3 (0.53 V), and P4 (0.71 V) were used as Eox in the following empirical equation [33]

Ip = (Eox + 4.8) eV

where the energy level of Fc/Fc+ was taken as 4.8 eV under vacuum [34]. Electron affinities were estimated by subtracting the band gap energy from Ip and the results are tabulated in Table 1. As seen from table, HOMO levels of the polymers are almost the same. This is not unexpected because for all polymers donor groups are the same. However, their LUMO levels differ depending on the 2,7- and 3,6-substitution of carbazole derivatives. In the case of 2, 7linked carbazole derivatives, P3 and P4, the LUMO levels are lowered, thus they have slightly lower Eg values as compared to 3,6-substituted carbazole derivatives which is in accordance with the theoretical calculations of Doskocz et al. [35] indicating 2,7-diheteroaromatic carbazoles have lower band gaps as compared to 3,6-diheteroaromatic carbazoles because of linear conjugation of the 2,7- positions [36].

Due to its importance in electrochromic applications, switching times and optical contrast of the polymer films on ITO were also investigated under square wave input of 0.0 and +1.1 V in 10 s intervals by monitoring the visible transmittance and the kinetic responses of the film, and the results are given in Table 1. Coloration efficiency values are also included in the table. As seen from table among four polymers synthesized in this work, P1 exhibits the highest coloration efficiency and the lowest switching time. In order to investigate the effect of substitution site on the conductivity, conductivities of polymer films were determined using four-probe technique and the results are given in Table 11

1. Since the attempts to prepare free standing films of P3 were unsuccessful, measuring its conductivity was not possible. The conductivities of P1, P2, and P4 were found to be 6.25x10-4 S cm-1, 2.38x10-5 S cm-1 and 1.54x10-2 S cm-1, respectively. The higher conductivity of P4 can be explained in terms of higher conjugation length and stronger π-stacking in accordance with the red shift observed in the electronic absorption spectrum of P4 as compared to P1 and P2. (Table 1 hereabout)

To investigate the morphologies of the polymers, scanning electron microscope was used. Electrochemically synthesized films were peeled from ITO glass with the help of diethyl ether after being washed with ACN. It was observed, that the surface of the polymers were porous with small granules (Fig. 6). This surface structure provides easy movement of counter-ions into and out of the polymer film during doping and de-doping processes [37, 38]. (Figure 6 hereabout) 4. Conclusions In this study, furan and carbazole based four new monomers, M1, M2, M3, and M4, and their corresponding polymers, P1, P2, P3, and P4, were synthesized. The electrochemical and optical properties of monomers (M1 and M3) and their polymers (P1 and P3) were investigated in terms of linkage site of carbazole. Comparison of electronic and photophysical properties of 2,7- and 3,6-linked carbazole-furan derivatives indicated that the 2,7-linked monomers (M3 and M4) have higher extend of conjugation as compared to 3,6-linked monomers (M1 and M2) due to more planar structure of 2,7-linked monomers. The larger Stoke’s shift observed in the fluorescence spectra of 3,6-linked monomers as compared to 2,7-linked monomers indicated creation of conformational changes in M1 and M2. The optical band gap values of P3 and P4 (about 2.42 eV) were found to be lower than those of P1 and P2 (about 2.65 eV) due to extended conjugation and higher planarity of 2,7-linked polymers. Electrochemically obtained polymer films have p-type doping property which is accompanied

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with an electrochromic response from green to blue for P1 and P2 and yellow to green for P3 and P4 upon moving from neutral state to the oxidized state. This electrochromic behavior makes them potential materials for electrochromic devices. Work in this line is currently underway in our laboratories. Acknowledgement The authors express their gratitude to the Middle East Technical University Research Fund for financial support.

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Figure Captions Fig. 1. Electronic absorption spectra and fluorescence spectra of monomers in ACN solution a) M1, b) M2, c) M3, and d) M4.

Fig. 2. Cyclic voltammograms of monomers and their constituents. a) M1, b) M2, c) M3, and d) M4 recorded in 0.1 M TBAH/ACN electrolytic solution on Pt electrode vs. Ag/AgCl.

Fig. 3. Electropolymerization of monomers at 200 mV/s in ACN and 0.1 M TBAH as electrolyte a) M1, b) M2, c) M3, and d) M4 on Pt electrode vs. Ag/AgCl.

Fig. 4. Cyclic voltammograms of polymers at different scan rates changing between 20- 200 mV/s in ACN and 0.1 M TBAH as electrolyte a) P1, b) P2, c) P3, and d) P4 on Pt electrode vs. Ag/AgCl (Inset: Relationship of anodic and cathodic current peaks as a function of scan rate between neutral and oxidized states).

Figure 5. Optical absorption spectra of the polymer films on ITO electrode in 0.1 M TBAH/DCM at a potential range between 0.0 and 1.1 V. a) P1, b) P2, c) P3, and d) P4. Inset: Color of the polymer films on ITO electrodes at their neutral and oxidized states.

Figure 6. SEM micrographs of P1, P2, and P4. Magnification: a) x2500 and b) x15000.

Table Captions Table 1. Optical and electrochemical properties of the polymers. Scheme Captions Scheme 1. Synthetic route for monomer synthesis. 19

20

21

22

23

24

25

26

Polymer

Eonset [V]

HOMO [eV]

LUMO [eV]

Eg [eV]

CE [cm2C-1]

Switching time [s]

Conductivity [Scm-1]

P1

0.56

5.36

2.69

2.67

190

1.2

6.25x10 -4

P2

0.53

5.33

2.68

2.65

144

2.4

2.38x10 -5

P3

0.50

5.30

2.88

2.42

177

1.4

-

P4

0.71

5.51

3.06

2.45

154

1.4

1.54x10 -2

27

28

Highlights

•

Four furan- and carbazole-based monomers and their polymers were synthesized.

•

Optical and electrochemical properties were investigated.

•

All polymer films show p-type doping and exhibit electrochromic properties.

29