Synthesis and characterization of novel graft copolymers of Poly(N-vinylcarbazole) and Poly(3-methylthiophene) for optoelectronic applications

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Synthetic Metals 160 (2010) 2306–2314

Contents lists available at ScienceDirect

Synthetic Metals

journa l homepage: www.e lsev ier .com/ locate /synmet

ynthesis and characterization of novel graft copolymers ofoly(N-vinylcarbazole) and Poly(3-methylthiophene) forptoelectronic applications

. Chemeka, J. Wéryb, M. Bouachrinec, M. Parisb, S. Lefrantb, K. Alimia,∗

Unité de recherche: Matériaux Nouveaux et Dispositifs Electroniques organiques, Faculté des Sciences de Monastir, Rue de l’environnement, 5000 Monastir, TunisiaInstitut des Matériaux Jean Rouxel, CNRS-UMR 6502, 2 Rue de la Houssinière, BP 32229, 44322 Nantes cedex 3, FranceLRMM, Faculté des Sciences et Techniques, B.P. 509, Boutalamine, Errachidia, Morocco

r t i c l e i n f o

rticle history:eceived 22 April 2010eceived in revised form 26 August 2010ccepted 1 September 2010

a b s t r a c t

In this paper, we report the synthesis and characterization of a new material based on Poly(N-vinylcarbazole) (PVK) and Poly(3-methylthiophene) (PMeT), which has been chemically prepared bycross-linking of PVK in the presence of 3-methylthiophene monomers in chloroform with anhydrousFeCl3. Thus, two samples were prepared, PVK–3MeT1 and PVK–3MeT2, obtained firstly in the fully doped

vailable online 8 October 2010

eywords:oly(N-vinylcarbazole) (PVK)oly(3-methylthiophene) (PMeT)pectroscopic methodshermal analysis

state and successfully dedoped by chemical treatment. They were characterized by various spectroscopicmethods as well as thermal analysis and conductivity measurements. Then, the formation of PMeT byoxidative coupling and its grafting into the skeleton of PVK is shown. Furthermore, the optical analysisshows that the new materials exhibit a blue-shift compared to that of the PMeT homo-polymer. Theobtained graft copolymer presents interesting optical and thermal properties compared to that of thehomo-polymers.

c conductivity

. Introduction

Recently, researchers have focused their attention in prepar-ng new blends and/or composites based on organic compounds,n order to obtain new inexpensive and promising materials forpplications in the field of organic optoelectronic devices, electro-hemical displays and sensors, such as OLEDs or PLEDS (organic orolymer light emitting diodes), and organic solar cells [1–4]. Fur-hermore, recent works have shown that the use of both electrononating materials (polyparaphenylene, Poly(3-alkylthiophene),arbazole based polymers, etc.) and electron acceptor materialsfullurenes, perylene derivatives, TiO2, etc.) in heterojunctionsan yield highly efficient photovoltaic conversion [5–9]. In fact,oly(3-alkylthiophene), which is an interesting conjugated poly-er regarding its optical, transport and electronic properties

10,11], has already been used to prepare new active layers forptoelectronic applications [12–14]. Moreover, the most promising

etero-junction was obtained by mixing Poly(3-hexylthiophene)PHT) as a donor polymer with phenyl C61-butyric acid methyl esterPCBM) as an acceptor compound [15] and the obtained materialsxhibit a conversion efficiency of ∼5%.

∗ Corresponding author. Tel.: +216 73 500 274; fax: +216 73 500 278.E-mail address: [email protected] (K. Alimi).

379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2010.09.001

© 2010 Elsevier B.V. All rights reserved.

Carbazole based polymers such as Poly(N-vinylcarbazole)(PVK) is currently used as a non-conjugated polymer, mainlyas a good hole transporting material in photovoltaic devices[16] or as a luminescent polymer emitting in the blue whenprepared in nanoparticles [17]. PVK has been mixed with inor-ganic or organic compounds to achieve emitting layers for newpromising optoelectronics devices [18,19]. In fact, early stud-ies show that the use of PVK, by modifying the luminescenceproperties of copolymers, strongly improve their lumines-cence efficiency [20]. Furthermore it had been shown that thenew composite based on Poly(N-vinylcarbazole) and Polythio-phene grows as a graft copolymer of PVK and Polythiophene[21–22].

According to the arguments given above, we have synthesized anew copolymer with Poly(3-methylthiophene) (PMeT) (Scheme 1)and Poly(N-vinylcarbazole) (PVK) (Scheme 2) due to their interest-ing properties. PMeT is obtained from 3-methylthiophene (3MeT)monomers by an in situ chemical oxidative way using FeCl3 as theoxidant in the presence of the PVK polymer. The obtained copoly-mer exhibits appropriate properties to be used as an active layer in

organic electronic devices.

First, we describe the route for preparing the new copoly-mer. Then, we report characterization measurements based onsolid-state Nuclear Magnetic Resonance (CP-MAS 13C NMR), IRabsorption, Raman scattering, UV–visible absorption, photolumi-

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M. Chemek et al. / Synthetic Meta

S

CH3

25 n

4 3

Scheme 1. Chemical structure of Poly(3-methylthiophene) (PMeT).

N2

3

7

6

1

45

8

4a5a

8a 1a

CH CH2

ne

2

3msa

aasd

tKwRasbnfCwt1

m6

p2m

TP

n

Scheme 2. Chemical structure of Poly(N-vinylcarbazole) (PVK).

escence spectroscopy, thermo-gravimetric analysis (TGA) andlectrical conductivity.

. Experimental part

Poly(9-vinylcarbazole) (PVK) powder (Mw ∼ 1.100.000),-methylthiophene (98%), ferrichloride (FeCl3), chloroform,ethanol, acetone and hydrazine used for the synthesis of the

tudied compounds were purchased from Aldrich chemistry, Merknd Fluka and were purified before use by usual procedures.

Microanalysis measurements were carried out with an EDAXpparatus, using a scanning electron microscope equipped withn EDX system (Jeol Jms-35C sonde tractor) and X-ray diffractiontudies on an INEL CPS 120 diffractometer equipped with a curvedetector.

Infrared spectra were obtained with a Brüker Vector 22 Fourierransform spectrophotometer. Samples were prepared in pellets ofBr mixed with the organic compound under study. Raman spectraere registered on a Fourier transform spectrophotometer BrükerFS 100, using a laser wavelength at 1064 nm. NMR spectra werecquired at room temperature (RT) using a Brüker Avance 500 MHzpectrometer operating at 125.7 MHz for 13C, using a 4-mm double-earing Brüker probehead. All rotors were spun under a dryitrogen flow. Spectra were referenced to tetramethylsilane (TMS)

or 13C (using adamantane as a secondary reference). {1H}–13CP-MAS (Cross-Polarization Magic-Angle-Spinning). NMR spectraere acquired using a ramp-amplitude sequence [23], a 2 ms con-

act time, a repetition time of 2 s and a 15 kHz MAS spinning rate.H decoupling during acquisition was achieved using the TPPMethod [24] with a radiofrequency (RF) field of approximately

0 kHz.Optical density measurements were carried out at room tem-

erature (RT) using a Cary 2300 spectrophotometer, in the range00–2200 nm. Continuous-wave (cw) photoluminescence (PL)easurements were collected on a Jobin-Yvon Fluorolog 3 spec-

able 1olymerisation yields of the synthesized PVK–3MeT1 and PVK–3MeT2.

PVK (g) 3MeT (ml) FeCl3 (g) Weight o(before de

PVK–3MeT1 0.4 2 2 1.1PVK–3MeT2 0.8 4 4 3

ls 160 (2010) 2306–2314 2307

trometer using a Xenon lamp (500 W) at room temperature. Wehave estimated the quantum efficiency with the method of de Melloet al. [25] in an integrating sphere of 3 in. in diameter. Signals wererecorded from the empty sphere, from the sphere with the sam-ple inside not directly illuminated (off-axis geometry), and fromthe sample inside the sphere directly in the exciting beam (on-axisgeometry).

Dynamic thermo-gravimetric analysis (TGA) was carried out ina Perkin-Elmer TGS-1 thermal balance with a Perkin-Elmer UV-1temperature program control. Samples were placed in a platinumsample holder and the thermal degradation measurements werecarried out between 300 and 973 K at a rate of 5 K/min under nitro-gen atmosphere.

3. Experimental results and discussion

3.1. Synthesis procedure of the PVK–3MeT

The PVK–3MeT copolymer was prepared by a typical insitu chemical oxidative polymerisation of 3-methylthiophenemonomers in the presence of dissolved PVK in chloroform. First,Poly(9-vinylcarbazole) (PVK) was dissolved in approximately 50 mlof CHCl3, and then 3-methylthiophene monomers and FeCl3 dis-persed in 10 ml of chloroform were successively added to thedispersion under stirring. The colour of the solution changed fromcolourless to green dark then to black dark. The system was keptunder stirring at room temperature for 3 days. Then, an excess ofacetone (600–800 ml) was added to the solution for 12 h to removeresidual FeCl3. The solution was filtered and kept firstly under pres-sure for 30 min then under vacuum at 80 ◦C for 48 h. A dark blackpowder was obtained in the fully oxidative (doped) state.

For de-doping the sample, the collected powder was extractedin approximately 20 ml of ethanol for 24 h. Then, 5–8 ml of mono-hydrate hydrazine in water was added to the dispersion understirring for 12 h. The colour of the dispersion changed from darkblack to brownish, as a signal of successful de-doping of the pow-der. The sample was filtered under pressure then washed for severaltimes with acetone/methanol and chloroform and kept under vac-uum for 48 h at 80 ◦C. In such a procedure, two samples namedPVK–3MeT1 and PVK–3MeT2 were synthesized in the fully oxi-dized (doped) and de-doped state (Table 1), respectively. DopedPVK–3MeT1 contained approximately 1% of Fe and 2% of Cl, whilede-doped PVK–3MeT1 contained only approximately 0.2% of Fe and0.6% of Cl. It was shown that in doped poly-alkylthiophene withFeCl3 routes and dedoped using acetone and methanol, only lessthan 0.10% of Fe remained [26]. The polymerisation yields of de-doped PVK–3MeT1 and PVK–3Met2 were calculated to be equal to29% and 38%, respectively.

In a parallel way, 0.2 g of PVK was dissolved into 50 ml of CHCl3and 1.5 g of dispersed FeCl3 in 10 ml of CHCl3 was added to the solu-tion. The colour of the solution changed from colourless to greendark, suggesting the oxidation of PVK and its cross-linking. Thesystem was kept under stirring for 3 days, and then an excess of

acetone was added. The solution was filtered and about 0.26 g of agreen mass was collected and kept under vacuum for 1 h at 80 ◦C.For de-doping the sample, the powder was mixed with approxi-mately 20 ml of ethanol then 5 ml of hydrazine monohydrate inwater was added. The colour of the dispersion changed from green

f obtained sample-doping)

Weight of obtained sample(after de-doping)

Polymerisationyield (%)

0.7 291.85 38

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2308 M. Chemek et al. / Synthetic Metals 160 (2010) 2306–2314

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ig. 1. (a) FT-IR spectrum of PVK; (b) FT-IR spectrum of FeCl3 cross-linked PVKPVKC).

o clear brown, suggesting the de-doping of the sample. This sampleas labelled PVKC.

To facilitate the interpretation of our results, the PMeTomo-polymer was also synthesized by the usual oxidative poly-erisation with FeCl3: 2 ml of 3-methylthiophene was dispersed

nto 50 ml of CHCl3 and a saturated solution of FeCl3 was addedo the dispersion. The system was kept under stirring for 48 h. Theolymer was obtained in the fully doped state and the de-dopingas achieved by extracting the powder in methanol/acetone for

4 h and by treatment with hydrazine for 20 min. A red powderas collected and kept under vacuum for 48 h at 80 ◦C.

.2. Vibrational properties (IR and Raman spectroscopies)

FT-IR spectra of PVK and cross-linked PVK (PVKC) are shown inig. 1a and b, respectively. Both spectra show the bands character-stic of PVK: 715 cm−1 (ring deformation of substituted aromatictructure), 740 cm−1 (CH2 rocking vibration), 1150 cm−1 (C–H inlane deformation of aromatic ring), 1220 cm−1 (C–N stretchingf VK), 1315 cm−1 (C–H in plane deformation of vinylene group),405 cm−1 (CH2 deformation of vinylene group), 1452 cm−1 (ringibration of VK moiety), 1600 cm−1 (C–C + C C stretching in ben-ene ring), 1625 cm−1(C–C stretching) [27]. In addition to the bandsharacteristic of PVK, FT-IR spectra of PVKC exhibits a new oneocated at 790 cm−1 and attributed to the benzene ring vibration28], indicating the formation of dimeric carbazylium cation radi-als under oxidation by FeCl3 in solution. This is in agreement withhe infrared analysis of chemically or electrochemically doped PVK28,29].

Fig. 2a and b shows infrared spectra of de-doped PMeT and FeCl3oped PMeT. Un-doped PMeT (Fig. 2a) is characterized by the fol-

owing vibration bands: 823 cm−1 (C–H out-of-plane deformation)haracteristic of the 2,5-linked Poly(3-methylthiophene), 875 cm−1

−1 −1

C–S stretching), 1377 cm (CH3 vibration), 1440 and 1515 cmC C stretching vibration of thiophene ring), [30–32]. In the case ofeCl3 doped PMeT, the infrared spectrum (Fig. 2b) show the appear-nce of broad bands located at 700, 840, 970, 1000, 1120, 1300, 1380nd 1630 cm−1. The bands situated at 970, 1120 and 1300 cm−1 are

Fig. 2. (a) FT-IR spectrum of de-doped PMeT; (b) FT-IR spectrum of FeCl3 dopedPMeT.

related to the PMeT doping. This result is in good agreement withthat reported earlier [32]. It was shown that the position of thesebands does not depend on the type of doping or on the nature ofthe dopant anion [32].

Fig. 3a–d presents infrared spectra of PVKC, PVK–3MeT1 inthe doped and de-doped state and that of de-doped PVK–3MeT2,respectively. From infrared spectra of PVK–3MeT1 (Fig. 3c) and thatof PVK–3MeT2 (Fig. 3d), we observe firstly characteristic bandsof PVKC, which proves that PVK is partially cross-linked. How-ever, there are differences between the infrared spectra of thePVK–3MeT1, PVK–3MeT2 and that of PVKC. For PVK–3MeT1 andPVK–3MeT2 in the doped and de-doped state, band intensities at790 cm−1 are significantly weaker than those of PVKC (Fig. 3a).This indicates that the degree of cross-linking of PVK–3MeT islower than that of PVKC. Added to the main bands of PVKC, wedetect the characteristic vibrations of PMeT. This proves the for-mation of PMeT through the polymerisation of 3MeT monomers.In particular, infrared spectra show absorption bands at 830 cm−1

(C–H out-of-plane vibration in PMeT), a signature of 2,5-linkedpoly-methylthiophene [31], while no peaks are detected for the2–4 coupling methylthiophne ring observed at 730 and 840 cm−1

[33]. In fact, the polymerisation of 3-alkylthiophene throughFeCl3 induces only a 2–5 coupling. The polymerisation of 3-MeTmonomers proceeds through the formation of radical cation ini-tiated by chemical oxidation (see Scheme 3) and following themechanism reported by Niemi et al. [34].

By comparing FT-IR spectra of PVK–3MeT1 in the doped (oxi-dized) and de-doped state (Fig. 3b and c), we observe the effect

of the FeCl3 doping in the vibrational properties of our synthe-sized compounds. First, we notice the appearance of broad bands inthe infrared spectrum of the oxidized compound at 970, 1150 and1310 cm−1 essentially assigned to the vibration of charged polymerbackbone induced by doping. This is the signature of the forma-

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M. Chemek et al. / Synthetic Metals 160 (2010) 2306–2314 2309

FoP

tdfac

oP

Fig. 4. (a) Raman spectrum of de-doped PVK–3MeT2; (b) Raman spectrum of de-doped PVK–3MeT1; (c) Raman spectrum of FeCl3 doped PVK–3MeT2.

ig. 3. (a) FT-IR spectrum of FeCl3 cross-linked PVK (PVKC); (b) FT-IR spectrumf FeCl3 doped (oxidized) PVK–3MeT1; (c) FT-IR spectrum of de-doped (neutral)VK–3MeT1; (d) FT-IR spectrum of de-doped (neutral) PVK–3MeT2.

ion of free charge carriers into the PMeT backbone. We also note aecrease upon doping of the band intensity of the main vibrations ofunctional groups of PVKC into the compound situated at 720–740nd 1450–1485 cm−1. This is the signature of the oxidation of the

ross-linked PVK, as a consequence of the high level doping.

Fig. 4a–c shows, respectively, Raman spectra at 1064 nmf neutralized PVK–3MeT2, neutralized PVK–3MeT1 and dopedVK–3MeT1. We can also note that Fig. 4c exhibits mainly an

S

CH3

n1 + 2 n1 FeCl3

Scheme 3. Mechanism of the formation of PMeT by Fe

Fig. 5. (a) Raman spectrum of neutral PMeT; (b) Raman spectrum of FeCl3 dopedP3MeT.

intense line at 1425 cm−1, whereas Fig. 4a and b shows mainly anintense Raman line at 1450 cm−1.

In Fig. 5, we present together Raman spectra of neutral (Fig. 5a)

and FeCl3 doped PMeT (Fig. 5b). It can be seen in the case ofFeCl3 doped PMeT (Fig. 5b) the appearance of an intense band at1425 cm−1 assigned to C C stretching deformation of the radicalcations, indicating the formation of a quinoid structure upon oxi-

S

CH3

n1

+ 2 n1 FeCl2 +2 n1 HCl

Cl3 polymerisation reported by Niemi et al. [34].

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2310 M. Chemek et al. / Synthetic Metals 160 (2010) 2306–2314

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formed in solution and in the solid state of un-doped PVK–3MeT2are shown in Figs. 8 and 9, respectively. The preparation of thePVK–3MeT2 solution starts by dissolving some amount of the sam-ple in chloroform. Then the solution is diluted several times and

Fig. 8. UV–vis spectrum of PVK–3MeT2 in chloroform solution (—). Decon-volution of UV–vis spectra of PVK–3MeT2 into components of Lorentzien

ig. 6. (a) NMR 13C CP-Mass spectra of PVK ( ); (b) 13C CP-Masspectrum of PVK–3MeT (—). The label s indicates CSA spinning sidebands. (For inter-retation of the references to colour in this figure legend, the reader is referred tohe web version of the article.)

ation [35,36]. In contrary, in the case of neutralized PMeT (Fig. 5a),n intense band at 1450 cm−1 assigned to the C� C� ring stretchingf the aromatic structure [35,36] is observed.

Thus, on one hand, the band located at 1425 cm−1 observedn the case of doped PVK–3MeT1 (Fig. 4c) is due to the for-

ation of a quinoid structure initiated by free charge carrierspolaron/bipolaron) upon doping. On the other hand, the bandocated at 1450 cm−1 in the case of neutralized PVK–3MeT1 orVK–3MeT2 (Fig. 4b and a) is the result of the vibration of theromatic structure showing their de-doping.

.3. CP-MAS 13C NMR spectroscopy and X-ray diffraction (XRD)

The 13C NMR spectrum of de-doped PVK–3MeT1 (Fig. 6b), con-rms the presence of both PVK and PMeT in the obtained structureince it displays two resonance regions. The first one (0–50 ppm)orresponds to the chemical shift of carbons of the aliphatic chainvinyl and methyl groups), while the second region located between00 and 150 ppm corresponds to aromatic carbons of carbazole andhiophene rings.

In the aliphatic region, the 13C NMR spectrum displays one res-nance band centred at about 16 ppm attributed to carbons of theethyl group (CH3) of PMeT [37], two resonance bands centred

t around 50 and 38 ppm corresponding, respectively, to methane–CH–) and methylene (–CH2–) carbons of the vinyl groups of PVK38].

In the aromatic region (100–150 ppm), the resonance bands cen-red at about 110, 120, 124.5 and 139 ppm are assigned to aromaticarbons of the carbazole rings of PVK. In fact, by comparison withP-MAS 13C NMR spectrum of PVK of this study (Fig. 6a) and thateported in the literature [39], the line centred at 110 ppm corre-pond to carbons C1 and C8 of carbazole rings, then that at 120 ppmo carbons C3, C4, C4a, C5 and C6 of carbazole rings, the line at24 ppm to carbon C2, C7 and C5a of carbazole rings and the line at39 ppm to C1a and C8a. In addition, in accordance with CP-MAS 13CMR of the PMeT homo-polymer [37], the lines at 128 and 134 ppm

re attributed to aromatic carbon on 3-methylthiophene ring.

XRD patterns of PMeT homo-polymer prepared by FeCl3 cou-ling reaction and that of PVK–3MeT1 show weak intensityeflection in the small angle region (∼1–2◦), indicating that theMeT formed into the copolymer is regio-irregular and therefore

Fig. 7. TEM micrograph of PVK–3MeT1.

mostly amorphous. In fact, this is different to the oxidative poly-merisation with FeCl3 which gives Poly(3-alkylthiophene) with adegree of regio-regularity of about 70–80% [40] with a defect cou-pling of ranging from 15% to 25%.

In Fig. 7, we present a TEM micrograph of the PVK–3MeT1in which we can clearly see the presence of mainly amorphousdomains, together with small crystalline regions. This agrees withX-ray data which suggest rather short coherence lengths.

3.4. Optical absorption and photoluminescence measurements

UV–visible absorption and fluorescence emission spectra per-

profile ( ) PL spectra in chloroform solution of PVK–3MeT2[( ) excitation at 380 nm, ( ) excitation at470 nm]. PL spectra of PVK in chloroform solution (——) (excitation 380 nm). PLspectra of PVKC in chloroform (—) (excitation 380 nm). (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web versionof the article.)

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M. Chemek et al. / Synthetic Metals 160 (2010) 2306–2314 2311

Fig. 9. UV–vis spectrum of PVK–3MeT2 in condensed state (—) (Solution dis-posed in silica). deconvolution of UV–vis spectra of PVK–3MeT2 into componentsof Lorentzien profiles ( ). PL of PVK–3MeT2 (condensed state)[4r

toiscCss

tawcd

cal absorption spectra of doped PVK–3MeT1 (Fig. 10) for the short

( ) excitation at 380 nm, ( ) excitation at70 nm]. (For interpretation of the references to colour in this figure legend, theeader is referred to the web version of the article.)

reated in ultrasonic bath at (RT) for 1 h. Despite the poor solubilityf the sample in chloroform, spectra are obtained in dilute solutionn order to avoid the possibility of concentration quenching. Allolutions were degassed prior measurements. Films on silica wereast from chloroform solution by simple disposition onto silica andHCl3 was slowly evaporated. Otherwise, UV–visible absorptionpectra of doped PVK–3MeT1, shown in Fig. 10, were obtained inolid-state using KBr pressed pellets [41].

On reacting PVK with anhydrous FeCl3 in chloroform solution,he colour of the solution changed from colourless to dark green

nd a green dark mass was obtained. We believe that this featureas due to the cross-linking of PVK. In fact, as reported earlier, the

arbazole units on the PVK backbone or N-alkylcarbazole derivativeimerised via the 3 or 6 positions by oxidative cross-linking when

N

CH CH2

n

lCeF2+2 3

Scheme 4. Mechanism of chemical

Fig. 10. UV–vis spectrum of doped PVK–3MeT1in a KBr pressed pellet.

they were submitted to an electrochemical or chemical oxidationinvolving cation-radical species of a dark green colour solution[42,43]. The oxidation of Poly(N-vinylcarbazole) with FeCl3 in solu-tion produces cross-linked PVK which contain dimeric carbazyluimcation radicals.

In the light of these observations, we tentatively describe inScheme 4 the mechanism of the chemical reaction of PVK withFeCl3.

The deconvolution of the optical absorption spectra of de-dopedPVK–3MeT2 in the solid state and in solution into components ofLorentzien profile shows that the sample absorbs at 410, 330, 270and 240 nm in solution (Fig. 8) and at 425, 330, 270 and 240 nmin the solid state (Fig. 9). Moreover, the deconvolution of the opti-

wavelength (200–650 nm) show that PVK–3MeT1 absorbs in theUV part of the electromagnetic spectrum with absorption bandscentred at 240, 310 and 360 nm and in the visible part with anabsorption band centred at around 440 nm.

N

CH CH2

n

+2 FeCl2 + 2 HCl

2

+

N

CH CH2

n 2

+.

FeCl4

-+ FeCl2

2FeCl3

cross-linking of PVK by FeCl3.

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2312 M. Chemek et al. / Synthetic Metals 160 (2010) 2306–2314

F(

PPiebpcst

itah3aota[bciol

tTefihtmpq(

b[i(3mn

ig. 11. UV–vis spectrum of: (a) PVK; (b) neutral PVKC; (c) oxidized PVK by FeCl3doped PVKC).

Based on optical absorption spectra of oxidized and neutralizedVKC, it is clear that the optical absorption spectra of de-dopedVK–3MeT2 and that of doped PVK–3MeT1 show firstly character-stic absorption of neutral PVKC and that of doped PVKC, putting invidence the formation of bicarbazole units under oxidation of PVKy FeCl3. Moreover, the absorption detected in the visible regionresumably originates from the � to �* transition of PMeT in theopolymer. As it is known, PMeT absorbs at 500 nm in the solidtate [44], whereas, a blue shift of about 50–70 nm is observed inhe case of PMeT grafted with PVK.

The effect of the oxidation of the synthesized copolymer is seenn the optical absorption spectrum of doped PVK–3MeT1, charac-erized by polaronic/bipolaronic bands located at 820 (1.5 eV) andt wavelengths � > 1500 nm. Meanwhile, the FeCl3 oxidized PVKomo-polymer (oxidized PVKC) exhibits an absorption at around60–380 nm in the optical absorption spectra (Fig. 11c), while nobsorption band is observed for � > 400 nm. The same results arebtained for iodine or bromine doped PVK [45,46]. Furthermore,he FeCl3 doped Poly(3-alkylthiophene) induces the appearance ofwide band in the optical absorption spectra at around 800 nm

47]. In the case of FeCl3 doped PVK–3MeT1, a wide absorptionands at 820 and for � > 1500 nm are observed resulting from theoexistence of polaronic and bipolaronic charge carriers, prov-ng the formation of new backbone resulting from the graftingf Poly(3-methylthophene) (PMeT) into the skeleton of the cross-inked PVK.

Luminescence properties of de-doped PVK–3MeT2 were inves-igated in chloroform and in solid cast films at room temperature.he fluorescence spectrum of the compound displays a maximummission at 570 nm in solution (Fig. 8) and at 628 nm in solid castlm (Fig. 9) with excitations at 380 and 470 nm, respectively. Filmsave their emission and absorption maxima red-shifted comparedo the corresponding solutions. This can be explained by confor-

ational changes which increase the degree of conjugation in theolymer backbone of the condensed state [48]. The luminescenceuantum yield of the PVK–3MeT2 cast film is measured to be 0.1313%) with an excitation at 470 nm.

It was reported that PVK, carbazole derivatives and bicar-azole systems, exhibit interesting emission properties in the blue49–51], while the PMeT emission is in the red [52]. Normal-

zed photoluminescence spectra of PVK homo-polymer in solutionFig. 8) display a maximum emission at 450 nm when excited at80 nm. However, PVKC exhibits an emission in the blue with aaximum at 480 nm when excited at 380 nm (Fig. 8). Furthermore,

o emission in the blue is detected in the normalized photo-

Fig. 12. PL spectra of de-doped PVK–3MeT1 ( ). PL spectra ofPMeT synthesized by FeCl3 oxidation (this study) (—). (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web versionof the article.)

luminescence spectra of PVK–3MeT2 when excited at 380 nm.The changes observed in photoluminescence measurement arerelated to the incorporation of PMeT chain into the skeletonof PVK.

Photoluminescence properties of neutralized PVK–3MeT1 andthat of the homo-polymer PMeT were investigated in powder. Thepowder was collected between two silica plates and excited at450 nm for the PVK–3MeT1 and at 550 nm for PMeT. Photolumi-nescence spectra of the two compounds are presented in Fig. 12.The fluorescence emission maximum of PVK–3MeT1 is at 573 nm,whereas that of PMeT is at 688 nm. The emission of PVK–3MeT1is blue shifted around 110 nm by comparison with that of PMeThomo-polymer. In addition, we observe an enhancement in theintensity when we use PVK in the synthesis. However, we detecta blue shift of the characteristic �–�* optical absorption of PMeTin the PVK–3MeT2 around 70–50 nm. The blue shift of the �–�*transition in the optical properties of PVK–3MeT1 or PVK–3MeT2,in comparing with that of homo-polymer PMeT is the result of thegrafting of the PMeT main chain into the formed bicarbazole unit.In this context, In this context, Siove and co-workers [51–53] havealso synthesized soluble copolymer of bis(N-alkylcarbazole) and 3-octhylthiophene by chemical oxidation of the co-monomers withFeCl3 in CHCl3 and suggest that the copolymer consist of an alter-nating pattern of Poly(3-octhylthiophene) (P3OT) and an (ethyl) or(octhyl)-carbazole dimer (ET-CZ)2 or (Oc-CZ)2. So we believe thatour prepared sample cannot be regarded as a simple mixture of twohomo-polymers, but instead as a graft copolymer, by alternatingpattern of Poly(3-methylthiophene) (PMeT) and the dicarbazolylunits, resulting of the cross-linking of carbazole pendant group inPVK. Thus, we tentatively propose in Scheme 5 the possible mech-anism reaction of the formation of the graft copolymer:

3.5. Thermal properties and conductivity measurements

Thermal properties of the graft copolymer were investigated bythermo-gravimetric analysis (TGA) under nitrogen atmosphere andresults are shown in Fig. 13.

PMeT remains stable until 280 ◦C with a weight loss between300 and 600 ◦C. However, it is known that PVK is more stable than

◦

PMeT, since its decomposition starts at around 380–400 C with acomplete decomposition at 500 ◦C [54].

The PVK–3MeT1 remain stable until 400–420 ◦C, indicating thatthe graft copolymer has a better thermal stability than PMeT homo-polymer. This may be ascribed to the incorporation of PMeT chains

Page 8

M. Chemek et al. / Synthetic Metals 160 (2010) 2306–2314 2313

S

CH3

n1

N

CH CH 2

n 2

+2 FeCl3

+2 FeCl3

S

CH3

n1

FeCl4

-+.

+ FeCl2

N

CH CH2

n 2

FeCl4

-+ FeCl2

+

+

N

CH CH2

n 2

S

CH3

n1

.

rmati

aa

pu

F(

Scheme 5. Possible mechanism of the fo

nd its grafting into the skeleton of PVK to form new homogenous

nd compact structure.

Added to the thermal analysis, the electrical conductivity waserformed. Measurements were carried out at room temperaturesing four-point technique in the pressed pellet sample. Dc conduc-

ig. 13. TGA micrograph of: (a) neutral PMeT; (b) TGA micrograph of de-dopedneutral) PVK–MeT1.

+2 FeCl2 +2 HCl

on of graft copolymer of PVK and PMeT.

tivity of de-doped PVK–3MeT1 and PVK–3MeT2 show a significantdecrease of conductivity relative to PMeT but much enhancedconductivity relative to PVK. In fact, the conductivity of PVK homo-polymer has been reported to be in the range of 10−12–10−16 S cm−1

[55]. However, polythiophene derivatives had low and variableconductivities in the neutral state at room temperature from10−9 to 10−5 S cm−1 [56,57]. PVK–3MeT1 presents a conductivityevaluated around 1.42 × 10−10 S cm−1 in the de-doped state and0.21 S cm−1 on the doped state. PVK–3MeT2 presents conductivityin the de-doped (neutralized) state around 52.6 × 10−10 S cm−1.

4. Conclusion

In this paper, we describe the synthesis of a new organic graftcopolymer based on PVK and PMeT with two different relativemasses. The formation of PMeT and its grafting into the skele-ton of PVK to form a new homogenous and compact material isconfirmed. In the doped state, the compound exhibits a high con-ductivity. These polymeric hybrids exhibit modified properties ascompared to their pristine components and then can constituteexcellent candidates for applications in organic electronics.

Acknowledgements

This work has been supported by the Tunisian-French cooper-ative action CMCU/07G1309. Authors are grateful to Dr. EtienneJanod for the conductivity measurements.

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