St

SM

a

ARRAA

KTFC

1

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apdginc

0d

Synthetic Metals 161 (2011) 263–270

Contents lists available at ScienceDirect

Synthetic Metals

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

ynthesis and characterization of fluorene based �-conjugateder-copolymers

hyambo Chatterjee, Susanta Banerjee ∗, Pallab Banerjiaterials Science Centre, Indian Institute of Technology, Kharagpur 721302, West Bengal, India

r t i c l e i n f o

rticle history:eceived 23 August 2010eceived in revised form 6 October 2010ccepted 22 November 2010vailable online 8 January 2011

a b s t r a c t

Three conjugated ter-copolymers P1, P2 and P3 were prepared in which fluorene was a common unitand varying subunits were thiophene–pyridine, thiophene–anthracene and pyridine–anthracene, respec-tively. The number-average molecular weights (Mn) of the copolymers named as P1, P2 and P3 were foundto be 13,400, 14,200 and 15,800 g/mol, respectively. These copolymers were soluble in common organicsolvents such as chloroform, chlorobenzene, toluene, etc. and their tendency to form aggregates has been

eywords:er-copymersluorescence spectroscopyyclic voltammetry

observed by the atomic force microscopy (AFM). The copolymers were investigated for their thermal,optical, electrochemical and impedance properties. They showed high glass transition temperatures andimproved thermal and spectral properties. The copolymer P2 (thiophene/fluorene/anthracene) showedlower LUMO energy level and significant redox stability in respect to pyridine containing copolymersP1 (thiophene/fluorene/pyridine) and P3 (pyridine/fluorene/anthracene). The remarkable n-dope ability

ymerstud

and stability of these polimpedance spectroscopic

. Introduction

Aromatic �-conjugated polymers are of great interest inifferent applications due to their n- and p-doping ability and

ncrease in conductivity by chemical or electrochemical doping1–7]. It is known that poor stability (redox and thermal) of theonducting polymers is one of the main obstacles to their appli-ation in electrochemical devices [8,9]. Although relatively high-doping–undoping cycle stability has been achieved for manyolymers, the problem associated with these types of polymersas over oxidation that limits their usefulness [10–15]. It is

eported that n-doping �-conjugated polymers suffer from thensufficient stability of the resulting reduced forms of the polymer,

ore than that in case on p-doping polymers [15]. Therefore, theres an interest for preparation of stable n-doped polymers whichould be reversibly cycled in the electrochemical n-doping and-doping states [12,16–18].

There are reports that describe the presence of both donornd acceptor unit in the same polymer facilitating their n- and

-doping ability [19]. It is reported that poly(thiophene) can be n-oped [20,21]. Furthermore, introduction of electron withdrawingroup in thiophene ring along with electron withdrawing subunitn polymer backbone (e.g. oxadiazole, fluorenone) can increase the-doping ability and ensure high stability of the reduced form of theo-polymers [15]. It is also observed that incorporation of pyridinyl

∗ Corresponding author. Tel.: +91 3222 283972; fax: +91 3222 255303.E-mail address: susanta@matsc.iitkgp.ernet.in (S. Banerjee).

379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2010.11.032

s were discussed in the light of cyclic voltammetric and electrochemicalies.

© 2010 Elsevier B.V. All rights reserved.

moiety makes the copolymers n-dopable, more resistant to oxida-tion and easy to electron transportation [22–25].

The chromophoric and fluorophoric properties of anthracenehave long been of practical interest in polymer chemistry.Anthracene-based derivatives especially 9, 10-substituted havebeen widely used as organic optoelectronic materials due totheir high fluorescence quantum yields [26]. Electrochemicalimpedance spectroscopy (EIS) analysis reveals that the introduc-tion of anthracene moiety could suppress the charge recombinationarising from electrons donor unit [14].

It will be interesting to study whether the presence ofanthracene unit along with fluorene/thiophene or fluorene/pyridine makes the copolymers more stable at n-dope state ornot. Accordingly, in the present investigation we have preparedthree ter-copolymers namely P1 (thiophene/fluorene/pyridine),P2 (thiophene/fluorene/anthracene) and P3 (pyridine/fluorene/anthracene). The polymers have been well characterized by ther-mal, UV, IR and NMR spectroscopy, cyclic voltammetry andelectrochemical impedance spectroscopy (EIS).

2. Experimental

2.1. Materials and methods

The compounds 9,9-dihexylfluorene-2,7-bis(trimethyleneboronate), 2,6-dibromopyridine, 2,5-dibromothiophene, 9,10-dibromoanthracene and tetrakis(triphenyl phosphine)palladium(0) [Pd(PPh3)4] were purchased from Aldrich Chemical Company

2 etic M

(wI

2

wlWeasssNmsbmwNrt2

64 S. Chatterjee et al. / Synth

USA) and used as received. Anhydrous Na2CO3 (E Merck, India)as further dried before use. Tetrahydrofuran (THF) (E Merck,

ndia) was freshly distilled over sodium before it was used.

.2. General measurement

Elemental carbon, hydrogen, and nitrogen of the compoundsere analysed by pyrolysis method using Euro EA elemental ana-

yzer. Gel permeation chromatography was performed using aaters 2414 instrument. Tetrahydrofuran (THF) was used as elu-

nt (flow rate, 0.5 mL/min), monodisperse polystyrene was useds standard and RI detector was used to record the signal. FT-IRpectra were recorded using a NEXUS 870 FT-IR (Thermo Nicolet)pectrophotometer at room temperature and humid free atmo-phere by making KBr pellets. 1H NMR and proton decoupled 13CMR were recorded on a Bruker 200 MHz and 500 MHz instru-ent. CDCl3 was used as solvent and TMS as reference. Chemical

hifts are reported in parts per million (ı) and the signals haveeen designated as follows: s (singlet), d (doublet), t (triplet) and

(multiplet). Glass transition temperature (Tg) of the polymers

ere analysed by differential scanning calorimetry (DSC) usingETZSCH DSC 200PC differential scanning calorimeter at a heating

ate of 20 ◦C/min under nitrogen atmosphere. Thermal decomposi-ion behavior of these polymers were measured on a NETZSCH TG09 F1 thermal analyzer at a heating rate of 10 ◦C/min under syn-

N

C6H13

Ar

Ar'Ar

N

S

+C6H13

BO

O

Ar BrBr

SC6H13 C6H13

Nn/2 n n/2

P1

n/2 C6H13

N

P3

n/2

Scheme 1. The synthes

etals 161 (2011) 263–270

thetic air. The absorption and fluorescence spectra of the polymerswere measured using Perkin Elmer UV-VIS-NIR, spectrophotome-ter and a Spex-Fluorolog-3 (model no.: FL3-11) spectrofluorimeter.The electrochemical cyclic voltammetry (CV) and electrochem-ical impedance spectroscopy (EIS) was conducted on a Gamryinstrument under inert atmosphere. A platinum (Pt) coated thinpolymer film was used as the working electrode and standardAg–AgCl electrode was used as a reference. While 0.1 (M) tetra-butylammonium perchlorate (nBu4NClO4) in acetonitrile was theelectrolyte, CV curves were calibrated using ferrocence as the stan-dard, whose oxidation potential was set at −4.4 eV with respectto zero vacuum level. The HOMO energy levels were obtainedfrom the equation HOMO = −(Eonset

ox − E(ferrocene)onset + 4.4) eV.The LUMO levels of polymer were obtained from the equationLUMO = −(Eonset

red − E(ferrocene)onset + 4.4) eV. The thickness of thecopolymer thin films above glass substrate and Pt was measured byalpha-step profilometer.

2.3. Synthesis and characterization of the copolymers

The copolymers were prepared by Pd (0) initiated cross couplingreaction of 9,9-dihexylfluorene-2,7-bis(trimethylene boronate)with different dibromo compounds in appropriate ratio adoptingSuzuki protocol [27]. The coupling reactions in THF in presence ofPd(PPh3)4 as a catalyst and sodium carbonate as a base under an

C6H13

Ar'

THF

Na2CO3

65 oC/ 3 days

[(PPh3)4]Pd

Ar' BrBr+C6H13

BO

O

n/2

nn/2S

C6H 13 C6H13

P2

n

n/2

C6H13

nn/2

is of copolymers.

tic Me

is

2

bp(0CFı((11218

2

29dPAF(HH111I17

2

2

S. Chatterjee et al. / Synthe

nert atmosphere resulted in a satisfactory yield about 62–74%. Theynthetic route is shown in Scheme 1.

.3.1. Copolymer P1

SC6H13 C6H13

Nn/2 n n/ P12

Monomer feed composition: 1 g of 9,9-dihexylfluorene-2,7-is(trimethylene boronate) (1.99 mmol), 0.235 g of 2,6-dibromo-yridine (0.99 mmol) and 0.24 g of 2,5-dibromothiophene0.99 mmol), Na2CO3 (2.11 g, 19.92 mmol), Pd(PPh3)4 (0.069 g,.06 mmol). Yield: 64% (yellow powder). Anal. Calcd. for29.5H34.5N0.5S0.5 (FW: 411.5): C, 86.02; H, 8.38; N, 1.70, S 3.88.ound: C, 85.92; H, 8.22, N, 1.63. 1H NMR (CDCl3, 200 MHz, ppm):8.20 (br, s, 0.5 H), 7.98 (d, J = 8 Hz, 1 H), 7.86–7.58 (m, 6 H), 7.42

br, s, 1 H), 2.09 (br, 4 H), 1.09 (m, 16 H), 0.76 (br, 6 H); 13C NMRCDCl3, 125 MHz, ppm): ı 152.27, 151.82, 151.60, 144.17, 142.22,40.31, 138.92, 126.28, 125.78, 124.68, 123.99, 121.52, 121.21,20.60, 120.18, 119.92, 119.07, 55.38, 40.45, 31.51, 29.71, 23.80,2.61, 14.03; IR (KBr) 3064, 3023, 2954, 2927, 2858, 2370, 2343,738, 1656, 1601, 1576, 1518, 1547, 1367, 1251, 1189, 1120, 1010,80, 832, 797 cm−1.

.3.2. Copolymer P2

n/2

nn/2S

C6H13 C6H13

P2Monomer feed composition: 1 g of 9,9-dihexylfluorene-

,7-bis(trimethylene boronate) (1.99 mmol), 0.334 g of,10-dibromoanthracene (0.99 mmol) and 0.240 g of 2,5-ibromothiophene (0.99 mmol), Na2CO3 (2.11 g, 19.92 mmol),d(PPh3)4 (0.069 g, 0.06 mmol). Yield: 67% (green fibrous solid).nal. Calcd. for C34H37S0.5 (FW: 461): C, 88.50; H, 8.02; S 3.47.ound: C, 88.12; H, 8.10.1H NMR (CDCl3, 200 MHz, ppm): ı 8.13d, J = 8.12 Hz, 2 H), 7.99 (d, J = 7.6 Hz, 2 H), 7.89–7.49 (m, 6), 7.45 (br, s, 1 H) 2.11 (br, 4 H), 1.29 (br, 16 H), 0.83 (br, 6).13C NMR (CDCl3, 125 MHz, ppm): ı 151.94, 151.27, 144.20,40.51, 140.31, 137.93, 137.75, 133.34, 132.19, 131.96, 130.18,28.85, 128.59, 127.73, 127.23, 126.27, 125.12, 124.72, 123.98,20.20, 119.89, 55.46, 40.56, 31.61, 29.72, 23.99, 22.57, 14.08.

R (KBr): 3064, 3030, 2927, 2858, 2364, 2336, 1608, 1560, 1518,470, 1367, 1285, 1244, 1196, 1113, 1017, 969, 880, 825, 804,63 cm−1.

.3.3. Copolymer P3

n/2 nC6H13 C6H13

N

n/2P3Monomer feed ratio: 1 g of 9,9-dihexylfluorene-

,7-bis(trimethylene boronate) (1.99 mmol), 0.334 g of

tals 161 (2011) 263–270 265

9,10-dibromoanthracene (0.99 mmol) and 0.235 g of 2,6-dibromopyridine (0.99 mmol), Na2CO3 (2.11 g, 19.92 mmol),Pd(PPh3)4 (0.069 g, 0.06 mmol). The yield was 62% (green fibroussolid). Anal. Calcd. for C34.5H37.5N0.5 (FW: 459.2): C, 90.23; H,8.23; N 3.47. Found: C, 89.23; H, 7.84; N, 2.93. 1H NMR (CDCl3,200 MHz, ppm): ı 8.26 (d, J = 7.8 Hz, 2 H), 8.07–7.29 (m, 15 H),2.05 (br, 4 H), 1.13 (br, 16 H), 0.78 (br, 6 H). 13C NMR (CDCl3,125 MHz, ppm): ı 157.25, 151.95, 151.62, 151.27, 142.08, 141.78,140.60, 140.25, 138.75, 138.09, 137.72, 133.02, 132.18, 131.96,130.18, 128.58, 127.73, 127.15, 126.18, 125.11, 121.51, 120.27,120.08, 118.58, 55.45, 40.66, 31.64, 29.79, 24.07, 22.58, 14.06. IR(KBr): 3071, 3023, 2948, 2927, 2851, 2370, 2329, 1745, 1690, 1649,1586, 1518, 1443, 1374, 1244, 1209, 1168, 1113, 1017, 886, 797,742, 694, 667 cm−1.

3. Result and discussion

The molecular structures of the copolymers were confirmed by1H NMR, FT-IR, and UV–vis spectroscopy. The elemental analysesof the copolymers were in good agreement with the experimen-tal and calculated values from the polymer repeat unit structure.The molecular weights of the polymers were measured by GPC andthe values indicated the formation of high molar masses under thechosen reaction conditions that are typically obtainable for thesetypes of polymers [25]. The number average molecular weightswere about 13,400–15,800 g/mol with a polydispersity index of1.4–1.5 against linear polystyrene as the standard and with THF aseluent. All the copolymers were highly soluble in common organicsolvents such as THF, CHCl3, CH2Cl2, 1,2-dichlorobenzene, tolueneetc. at room temperature.

For all the copolymers similar characteristic chemical shiftswere observed in 1H NMR spectra. The chemical shifts of thepyridinyl protons were manifested at ı 8.26 and 7.97 ppm while thethiophene protons appeared at 7.42 ppm. The remaining resonanceat ı 2.05–2.16 and 1.29–0.80 ppm can be correlated to the hexylpendant chains on fluorene unit. From 13C NMR spectroscopy, thecarbon corresponding to >C N appeared at ı 151 ppm. FT-IR spec-tra of the polymers exhibited characteristics of following stretchingvibration at 3023–3071 cm−1 (aromatic � C–H and ϕ C–H stretch-ing), 2851–2961 cm−1 (aliphatic C–H stretching), 1440–1500 cm−1

(ring stretching vibration of thiophene), 1367–1374 cm−1 (–CH3deform) and the presence of stretching vibration at 825 cm−1 (C–Hout of plane vibration of thiophene) confirm the presence of thio-phene. The vibrational band at 1518 cm−1 in pyridine containingcopolymers were due to the stretching vibration of the >C N group,which is characteristics of the pyridine unit, and also 886, 1374,1586 cm−1 stretching vibrations supported the presence of pyri-dine unit after polymerization.

3.1. Thermal properties

The thermal stability of the polymers was investigated by TGAunder synthetic air at 10 ◦C/min heating rate and the phase transi-tion behavior was investigated by DSC at a heating rate of 20 ◦C/minunder nitrogen. The polymers showed good thermal stability in airas expected from the aromatic ring containing polymers. The poly-mers showed 10% weight losses at 352, 360 and 365 ◦C for P1, P2 andP3, respectively as provided in Table 1. The 10% weight loss temper-ature of these polymers was above 350 ◦C indicating good thermalstability of these classes of polymers [12,28]. When the degrada-

tion temperature of synthesized copolymers were compared withthe reported copolymers like thiophene–fluorene (Td, 326 ◦C), itwas observed that presence of another aromatic unit causes higherdegradation temperature, indicating higher stability of the copoly-mers [29]. A differential scanning calorimetry (DSC) curve show

266 S. Chatterjee et al. / Synthetic Metals 161 (2011) 263–270

Table 1Molecular weight and thermal analysis data of P1–P3.

Polymer Mn (g/mol) Mw (g/mol) Mw/Mn Tg (◦C) Td (◦C)

P1 13,400 19,400 1.45 110 352P2 14,200 21,000 1.48 135 360

Ms

apCt(tdvvttpiPgd

3

(it

50 100 150 200-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

P1 P2 P3

Hea

t flo

w (W

/g)

P3 15,800 24,300 1.53 120 365

w = weight average molecular weight with respect to mono-disperse polystyrenetandard. Td = 10% weight loss temperature in air.

clear glass transition and indicate the amorphous nature of theolymers (Fig. 1). The incorporation of different aromatic unit at the-2, 7 position in fluorene, changes the glass transition tempera-ure of the polymers from 110 ◦C (P1), to 135 ◦C (P2) and 120 ◦CP3) (Table 1). The copolymers showed a higher glass transitionemperature (Tg) and was comparable to the homopolymer 9,9-ihexylfluorene which show a Tg of about 103 ◦C [29,30]. The Tg

alue of the copolymers were also higher than reported Tg (83 ◦C)alue of fluorene–thiophene copolymer [31]. So the incorpora-ion of different aromatic rings in the polymer backbone changeshe rigidity and planarity of the molecule and that affect thermalroperty of the co-polymers [31]. Copolymers (P2, P3) contain-

ng anthracene unit show higher glass transition temperature than1. This is due to the bulkiness of the rigid anthracene unit. Highlass transition temperature of a polymer is essentially required forevice longevity [32].

.2. Film morphology

The AFM images of the copolymer film cast from THF solution2 mg/mL) on platinum substrate are shown in Fig. 2. The AFMmage shown in Fig. 2 was obtained from the filtered polymer solu-ion. It has been observed that polymer film morphology changes

Fig. 2. AFM images of a

Temperature (°C)

Fig. 1. Differential scanning calorimetry (DSC) of copolymers.

with thickness due to formation of aggregation. The smooth sur-face is a very important factor for impedance analysis to decreaseinternal resistance which limits electron transfer at polymer/metalinterface [21]. The very first layer deposited on the solid substratetends to be more compact whereas the outer layer becomes rougheras the thickness increases. At a lower concentration (filtered dilute

polymer solution), AFM images show a rather smooth surface thanhigher concentration. In higher concentration aggregation affectsthe electrical properties. Therefore, the film casts from the filtereddilute solution which showed very small number of aggregateswere used for impedance analysis.

smooth solid film.

S. Chatterjee et al. / Synthetic Me

350 400 450 500 550 600 6500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1In

tens

ity (a

.u.)

P1 P2 P3

F

3

qrrldmsbmflTbtstfitiatc

Wavelength (nm)

ig. 3. Photoluminescence spectra of the copolymers in THF solution (10−5 M).

.3. Optical absorption and fluorescence behavior

Transparent and uniform films of polymers were prepared onuartz plates by spin casting filtered dilute polymer solution atoom temperature. The optical spectra of the copolymers wereecorded to understand the region of electromagnetic spectrum ofight in which polymer absorbs or emits. The measurements wereone at a concentration of 1 × 10−5 (M) in THF solution of the poly-er solutions. The well defined vibronic structures in the emission

pectra indicate that the polymers have a rigid and well definedackbone structure (Fig. 3). The optical properties of the copoly-ers P1, P2, and P3 are donor–acceptor type. Both thiophene and

uorene act as donors and pyridine acts as an acceptor unit [33].he films of P1 and P2 emit intensive green light while P3 emitlue light by excitation of UV light. A comparison of the absorp-ion and fluorescence maxima of the polymers (P1, P2 and P3) inolid and liquid state revealed that P1 and P2 copolymer show theendency of red shifting whereas a large blue shift was observedor P3. This red shifting is due to the presence of electron donat-ng thiophene unit in P1 and for the overlap of absorption band of

hiophene and anthracene in P2. Here anthracene unit participatesn conjugation rather than interrupting the polymer conjugationnd hence more red shift is observed in P2 [34]. The lone pair elec-rons of the sulfur atom in thiophene units increase the electronloud density in the polymer main chain, thereby increasing its abil-

0.0-0.5-1.0-1.5-2.0 0.5 1.0 1.5 2.0 2.5-1.0

-0.5

0.0

0.5

1.0

1.5

curr

ent (

mic

ro A

)

E / V

1st cycle(P1) 25 cycle(P1) 40 cycle(P1) 50 cycle (P1)

Fig. 4. Cyclic voltammogram recorded during electrochemical doping of P1 and P3 po

tals 161 (2011) 263–270 267

ity to act as an electron donor and decreasing its band gap. Thus,thiophene containing co-polymers, P1 and P2 was associated withhigher co-planarity, lower band gap (Table 2), and red shifting of itsabsorption spectrum. In case of pyridine containing copolymer P3,strong electron withdrawing effect of pyridine is more dominant.Apart from the electron withdrawing effect of pyridine, the bulkyanthracene unit in P3 causes reduction of the effective conjugationlength. This causes the decrease in electron density in the back-bone of polymer main chain of P3. P1 copolymer consisting of boththiophene and pyridine units but still shows red shifted becauseelectron donating power is more dominated as not only thiophenebut also fluorene too act as electron donor. So, presence of thio-phene changes the absorption and emission band towards the redregion and in case of incorporation of pyridine unit in P3 (causeslesser extent of electron delocalization through meta linkages) itbecomes blue shifted as expected [15]. The quantum efficienciesof each of the polymers in THF solution was determined by usingdilute quinine sulfate as a standard (1 × 10−5 M solution in 0.10 MH2SO4). The PL efficiency of quinine sulfate in 0.10 M H2SO4 solu-tion was taken to be 0.55 at 239 nm excitation [35]. Using thesevalues, the quantum yields of P1, P2 and P3 were calculated to0.79, 0.60 and 0.88, respectively. The lower quantum yield in P2 isdue to electronic interaction of anthracene leading to an undesiredbroadening of emission spectrum and decrease in fluorescenceyield [36].

3.4. Electrochemical behavior

The electrochemical behavior of the copolymers was studiedto understand the electrochemical stability in n-doped states. Theelectrochemical properties of the polymers in the solid films wereinvestigated by cyclic voltammetry in degassed acetonitrile withnBu4NClO4 0.1 (M) as the electrolyte. Figs. 4 and 5 show the cyclicvoltammogram of freshly prepared copolymer films within theelectrode potential range of n and p doping states at a scan rateof 50 mV s−1. There is no obvious decrease in peak current from2nd to 50th cycle, which indicates better long-term stability ofthe copolymers. HOMO and LUMO energy level were calculatedfrom onset oxidation and onset reduction potential which is givenin Table 2. The HOMO energy levels of the copolymers were veryclose to the data reported for the polyfluorene homopolymer butLUMO energy levels differ from the LUMO value of polyfluorene

homopolymer. This indicates that the oxidation processes of thesecopolymers originated from the fluorene segments but reductionprocess has the contribution of another two sub units which causeslowering of LUMO level and easy n doping ability [37,38]. Thereported LUMO energy level of polyfluorene, thiophene/fluorene

-3 -2 -1 0 1 2 3-3

-2

-1

0

1

2

3

4

curr

ent (

mic

ro A

)

E / V

1st cycle (P3) 25 cycle (P3) 40 cycle (P3) 50 cycle (P3)

lymer film electrode in 0.1 (M) nBu4NClO4 solution at a scan rate of 50 mV s−1.

268 S. Chatterjee et al. / Synthetic Metals 161 (2011) 263–270

Table 2Absorption, emission maxima and redox potential for P1–P3 copolymer.

Polymer In solutiona In solid filmb Oxidation Reduction Band gap

�abs (nm) �PL (nm) ϕPL �abs (nm) �PL (nm) Eg (eV)c Eonset (V) EHOMO (eV) Eonset (V) ELUMO (eV) Eg (eV)d

P1 407 463 0.79 405 503 2.55 0.74 5.14 0.97 3.43 1.71P2 404 464 0.60 403 503 2.57 1.07 5.47 0.50 3.90 1.57P3 341 383 0.88 352, 401 492 2.84 1.64 6.04 0.69 3.71 2.33

a Measured with THF solution at a concentration of 10−5 M.b

a[dtcwLd−n

ttaiorcpwatdTaadt

Fm

ers occurred during oxidation or reduction and charge trapping

100 (±10) nm thickness.c Band gap estimated from the wavelength of optical absorption of solid film.d Band gap estimated from the onset oxidation and reduction potential

nd pyridine/fluorene are 2.12–2.6, 2.86 and 2.88, respectively39,40]. Apparently, the incorporation of subunits (thiophene, pyri-ine) into the polymer backbone (fluorene) was effective to reducehe LUMO energy and thus the ease of n-doping ability [32]. Thealculated LUMO energy level of the ter-copolymers P1, P2 and P3ere P1LUMO = 3.43 eV, P2LUMO = 3.90 eV, P3LUMO = 3.71 eV (Table 2).

ower LUMO level makes the ter-copolymers easily n-dopable. Theifference of the reduction potential of P1 (−1.0, −2.25 V), P2 (−0.7,2.80 V) and P3 (−1.0, −2.20 V) indicates that P2 is more easily-doped than P1 and P3.

It is observed from cyclic voltammogram (Figs. 4 and 5) thathe start of n doping of P2 copolymer is shifted to more posi-ive (about −0.5 V) value than other two copolymers P1 (−0.97 V)nd P3 (−0.67 V). The electrode shows increased n-dope stabil-ty up to −2.8 V when it was cycled up to fifty times in casef P2 (Fig. 5). Throughout this cycling in anodic and cathodicange, cyclic voltammogram represented the broad anodic andathodic peaks. As the densities of peak current in n-doped and-doped state were both proportional to the scan rates, so thereere reversible redox behavior of the polymers in a particular

pplied voltage [41]. These films can be cycled repeatedly betweenhe conducting and insulating (neutral) states without significantecomposition, indicating the high stability of the copolymers.he thiophene/fluorene/anthracene (P2) copolymer first n-dopedt −0.7 V then reach up to −2.5 V to −2.8 V. So the copolymer P2llow to attain highest n doping capacity and also lower charge

ensity and more effective delocalization of �-electron density inhe conjugated system.

ig. 5. Cyclic voltammogram recorded during electrochemical n doping of P2 poly-er film electrode in 0.1 (M) nBu4NClO4 solution at a scan rate of 50 mV s−1.

The band gap of the copolymers was determined both fromthe UV–vis absorption spectra and from cyclic voltammetry mea-surements from the HOMO and LUMO energy values. The HOMOand LUMO energy values were shown in Table 2. A differencewas observed between these measurements, higher band gap wasobserved from the UV–vis absorption measurements. Similar resulthas also been reported previously [37,40].

3.5. Electrochemical impedance spectroscopy (EIS)

We have investigated the impedance spectroscopy of the freshlyprepared copolymer films in neutral state and in the range of n-and p-doping process (after first n-doping process) in a knownconcentration of tetrabutylammonium perchlorate (0.1 M) solutionunder different applied potential. This investigation was under-taken to understand charge trapping phenomena in oxidized andreduced states without any particular physical model. The twosemi circle; small one at high frequency and a large one at lowfrequency induced by the processes at the metal/polymer andpolymer/solution interfaces are not always noticeable (Figs. 6–9).Moreover these semicircles are often depressed [42].

The low frequency impedance at onset doping was observed byapplying voltage and after first n-doping it transformed to ratherlarge low frequency semicircle (n-dope state of P1 copolymer,Fig. 6). This could be explained that the trapping of charge carri-

phenomena can be removed after electrochemical doping pro-cess. This trapping of negatively charge depend upon redox unitor subunits present in the polymer [40]. As the potential increases(low frequency region) in the anodic direction, the impedance plot

0 70006000500040003000200010000

500

1000

1500

2000

2500

3000

3500

4000

4500

-1.09 V-1.20 V-1.30 V

imag

inar

y im

peda

nce

(Ohm

)

real impedance (Ohm)

Fig. 6. Impedance plots from measurements on P1 copolymer coated electrodes atneutral and n doping state in nBu4NClO4 0.1 (M) as the electrolyte.

S. Chatterjee et al. / Synthetic Metals 161 (2011) 263–270 269

0 500 1000 1500 2000 25000

200

400

600

800

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40 50 60 70 80 90 120110100468

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real impedance (Ohm)40 60 80 100 120 140

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-2.7 V

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Fig. 7. Impedance plots from measurements on P2 copolymer coated ele

Figs. 6 and 9) begin to show a decrease in the resistance value andoints become scattered. This indicates there is some relaxationrocess due to oxidation and reduction in p-dope and n-dope states43]. Conductivity relaxation of the phase with oxidized repeat unitenerates peak at lower frequency and that at higher frequency dueo the relaxation of the phase with reduced repeat unit. So it cane concluded that the high frequency semicircle with low valuef resistivity is due to the conductivity relaxation of the reducedepeat units and the low frequency semicircle having high value ofesistivity is because of the relaxation of the oxidized repeat units44–46].

Figs. 6 and 7 show a family of impedance plots at their reduction

otential and Figs. 8 and 9 show a family of impedance plots at theirxidation potential during the course of n- and p-doping in an ace-onitrile based nBu4NClO4 electrolyte. Sometimes two semicirclesn p-dope state of P1 copolymer are to some extent overlap withach other (Fig. 8), or at other times one or one half semi-circle

10000800060004000200000

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1.01 V 1.32 V 1.60 V

18000160001400012000

ig. 8. Impedance plots from measurements on P1 copolymer coated electrodes ateutral and p doping state in nBu4NClO4 0.1 (M) as the electrolyte.

e (Ohm)

es at neutral and n doping state in nBu4NClO4 0.1 (M) as the electrolyte.

is observed (EIS of p-dope P2 copolymer, Fig. 9). These semi-circle plots of electrode/polymer/electrolyte system show that thecurrent flow requires the transport of electron across the metalpolymer interface and ions through polymer electrolyte interface[47–51]. Thus the impedance study of the polymers leads to boththe nature of ion transport process of polymer/electrode interfaceand the oxidation/reduction property of the polymer. Additionallyit is well known that the hole current is dominated by space-chargeeffects whereas the electron current is reduced strongly by traps.So, the nature of the interface plays an important role in the chargetransport. Even the polymer/electrode interface can be a rectifyingone limiting the current flow. In disordered solids broad distribu-tion in transit times of the individual carriers leads to dispersive

current traces. In such a system, the dispersion of transit times canbe represented by a chain of hopping events. Thus the effective bar-rier height between the polymer/electrode will further increase. Ina random network of localized states as is found in case of an amor-phous material, the Coulomb interaction between ionized dopant

0 14000120001000080006000400020000

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1.5 V3.5 V

Fig. 9. Impedance plots from measurements on P2 copolymer coated electrodes atneutral and p doping state in nBu4NClO4 0.1 (M) as the electrolyte.

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70 S. Chatterjee et al. / Synth

toms and the resulting localized charge carriers leads to changesn the electronic density-of-states distribution. This will lead to aand tail (LUMO), a characteristic of an amorphous material with-doping state.

. Conclusions

In summary, three ter-copolymers of fluorene were synthesizedy the Suzuki coupling reaction and characterized. The copolymershowed good solubility, reasonably high thermal stability in airnd high glass transition temperature. Absorption and fluorescencepectra revealed that the optical properties of the polymers dependn the presence of sub units (thiophene, pyridine and anthracene).olymers exhibited strong photoluminescence emission in greennd blue region. All the copolymers have the contribution ofetal/polymer and polymer/film interfaces on electrochemical

mpedance behavior. Conductivity relaxation of oxidized repeatnit generates peak at lower frequency and that at higher frequencyue to the relaxation of the phases with reduced repeat unit. Allhe polymers are easily n dopable as they have very low LUMOnergy level (P1LUMO = 3.43 eV, P2LUMO = 3.90 eV, P3LUMO = 3.71 eV).mong these polymers P2 (thiophene/fluorene/anthracene) showsery low onset reduction potential (−0.5 V), highest n dop-ng ability (P2LUMO = 3.90 eV) and stability (upto −2.8 V). Tohe best of our knowledge the n-dope stability value isven lower than any other earlier reported copolymers basedn fluorene/thiophene (−2.31 V), fluorene/oxadiazole (−2.1 V),uorene/phenylene (−2.56 V), fluorene/pyridine (−2.13 V). Thisuggests the use of these ter-copolymers for electrochemical devicepplication.

cknowledgement

The authors thank to the CSIR (Grant No. 01(2110)/07/EMR-II)f India for financial support.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.synthmet.2010.11.032.

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