Journal of Electroanalytical Chemistry 648 (2010) 184–189

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Journal of Electroanalytical Chemistry

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A ferrocene functionalized multichromic p and n dopable donor–acceptor–donortype conjugated polymer

S�erife Özdemir, Abidin Balan, Derya Baran, Özdemir Dogan, Levent Toppare *

Middle East Technical University, Department of Chemistry, 06531 Ankara, Turkey

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 April 2010Received in revised form 18 June 2010Accepted 19 July 2010Available online 27 July 2010

Keywords:FerroceneMultichromismDonor–acceptor–donorConjugated polymer

1572-6657/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.jelechem.2010.07.010

* Corresponding author. Tel.: +90 3122103251; faxE-mail address: toppare@metu.edu.tr (L. Toppare)

Ferrocene substituted donor–acceptor–donor type polymer; poly(5,8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2-(naphthalen-2-yl)-3-ferrocenyl-4a,8a-dihydroquinoxaline) (PDEFNQ) was syn-thesized and its electrochromic properties were investigated. Cyclic voltammetry and spectroelectro-chemistry studies for PDEFNQ showed that it is a multichromic green to transmissive polymer withhigh tendency to be both p and n doped. PDEFNQ is foreseen as a promising candidate for variousapplications.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Growing importance of conducting polymers [1] lies in the factthey can be exploited in a wide field of applications. Sensors [2],light emitting diodes [3], solar cells [4], field effect transistors[5], and electrochromic devices [6] are some of the importantindustrial applications of conducting polymers.

The reversible and visible change in transmittance or reflec-tance as a result of an applied voltage is called as electrochromism[7,8]. Conducting polymers can be used for electrochromic devicesdue to their fast switching times [9], high contrasts [10], process-ibility [6] and easy tuning of electronic and optical properties viasmall structural alterations [11].

The neutral state color of these conducting polymers is veryimportant for electrochromic devices. Most of the electrochromicpolymers in the literature reflect mainly blue and red colors intheir reduced state because they absorb only one dominant wave-length. The requirement for reflecting mainly green color is havingtwo simultaneous absorption bands in the red and blue regions ofthe visible spectrum. In addition these bands must be controlledwith the same applied potential. For completion of color space,neutrally green colored polymer should reveal also a transmissivestate. Recently, donor–acceptor–donor type polymers satisfy theserequirements [12].

Resonance which allows a stronger double bond characterbetween the donor and acceptor units can cause a significant

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: +90 3122103200..

decrease in band gap [13]. Band gap determines the conductivityand color of the neutral polymer. The logic behind the donor–acceptor approach is having a high HOMO of the donor and alow level of the LUMO moieties incorporated into the monomer[14]. The materials with low band gaps usually reveal a trans-missive oxidized state and colored reduced state as a conse-quence of lower energy transitions in the doped state. Inaddition, these types of materials usually have two absorptionmaxima [15].

In literature different type of conducting polymers containingmetal complexes on polymer backbone was synthesized. The ver-satility of approach to yield conductive thin films in which the me-tal is present in the polymer backbone is illustrated in the range ofcomplexes that were polymerized. Polythiophene-RuðbpyÞnþ3 hy-brids [16], ruthenium oligothienyl acetylide complexes [17], oli-gothienylferrocene complexes [18] were reported. Recently,attention on conducting polymers functionalized by ferrocene isaroused since these type of polymers show the redox propertiesof both groups [19]. As a result, these are useful in a range of appli-cations and studies such as sensors [20,21], electro active Lang-muir/Blodgett films [22], free-standing redox-active films [23].Conducting polymers containing ferrocene on main conjugationpath of polymer backbone were also synthesized [24,25]. Electro-chemical experiments revealed that the ferrocene clicked PEDOTfilms have fast electron transfer ability and the spectroelectro-chemical studies show that the conducting polymer can switchfrom opaque purple to red then blue [18].

We report here the synthesis and electrochemical properties ofa ferrocene containing donor–acceptor type green electrochromic

S�. Özdemir et al. / Journal of Electroanalytical Chemistry 648 (2010) 184–189 185

polymer 5,8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2-(naph-thalen-2-yl)-3-ferrocenyl-4a,8a-dihydroquinoxaline (DEFNQ) whichhas multi colored states and a highly transparent oxidized state.

2. Experimental

2.1. Materials and equipments

All chemicals were purchased from Aldrich except anhydrousTHF and chloroform, which were purchased from Merck. 1H NMRand 13C NMR spectra were recorded at 25 �C in CDCl3 at 400 MHzand 101 MHz respectively, with Me4Si as the internal standard. AVoltalab potentiostat was used for all electrochemical studies.Electropolymerization was performed in a three-electrode cell con-sisting of an Indium Tin Oxide doped glass slide (ITO) as the work-ing electrode, platinum wire as the counter electrode, and Ag wireas the reference electrode. Tetrabutylammonium tetrafluoroborate(Bu4NBF4) in ACN/DCM (5:95, v:v) was used as the electrolyticmedium. 1H and 13C NMR spectra were recorded in CDCl3 on Bru-ker Spectrospin Avance DPX-400 Spectrometer. Chemical shiftswere given in ppm downfield from tetramethylsilane. Varian Cary5000 UV–Vis spectrophotometer was used to perform the spectro-electrochemical studies of the polymer. Colorimetry measure-ments were done via Minolta CS-100 Spectrophotometer.

2.2. Synthesis

2-Hydroxy-1-(naphthalen-2-yl)-2-ferrocenylethanone [26],1-ferrocenyl-2-naphthylethanedione [26], 4,7-dibromo-2,1,3-ben-zothiadiazole [27], 3,6-dibromo-1,2-phenylenediamine [28], 5,8-dibromo-2-(naphthalen-2-yl)-3-ferrocenyl-4a,8a-dihydroquinox-aline [29] and tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)stannane [30] were synthesized according to the previouslyreported methods.

H

O

Fe H

O

+ KCNFe O

O

Fe

NNSS

O OO O

NS

N HBr

Br2

NS

N

BrBr

NaBH4

1 2 3

5 6

DEFNQ

Scheme 1. Synthetic route

2.2.1. Synthesis of 5,8-dibromo-2-(naphthalen-2-yl)-3-ferrocenyl-4a,8a-dihydroquinoxaline

A solution of 3,6-dibromo-1,2-phenylenediamine (168.6 mg,0.64 mmol) and 1-ferrocenyl-2-naphthylethanedione (231 mg,0.63 mmol) in EtOH (40.6 mL) was refluxed overnight with a cata-lytic amount of p-toluene sulfonic acid (PTSA). The mixture wascooled to 0 �C. The precipitate was isolated by filtration andwashed with EtOH several times to afford the desired compoundin 89% yield (140 mg, 0.279 mmol). 1H NMR (CDCl3) (r) 8.15 (s,1H), 7.87–7.76 (m, J = 14.4 and 7.1 Hz, 5H), 7.63 (dd, J = 8.5 and1.6 Hz, 1H), 7.51–7.45 (m, 2H), 4.60–4.57 (m, 2H), 4.28–4.25 (m,2H), 3.93 (s, 5H). 13C NMR (CDCl3) (r) 156.05, 153.75, 139.70,138.24, 135.95, 133.66, 133.00, 132.94, 131.96, 129.58, 128.71,127.81, 127.76, 127.07, 127.04, 126.49, 123.72, 123.14, 71.72,70.52, 70.23.

2.2.2. Synthesis of 5,8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2-(naphthalen-2-yl)-3-ferrocenyl-4a,8a-dihydroquinoxaline

5,8-Dibromo-2-(naphthalen-2-yl)-3-ferrocenyl-4a,8a-dihydro-quinoxaline (50 mg, 0.0836 mmol) and tributyl (2,3-dihydrothie-no[3,4-b][1,4]dioxin-5-yl)stannane (478.8 mg, 0.753 mmol) weredissolved in dry THF (25 mL). The solution was purged with argonfor 30 min. and PdCl2(PPh3)2 was added under argon atmosphere.The mixture was stirred at 100 �C under argon atmosphere for15 h, cooled and concentrated on the rotary evaporator. The residuewas subjected to column chromatography (silica gel, CHCl3:hexane,2:1) to afford a red solid in 88.0% yield (53 mg, 0.074 mmol). 1HNMR (CDCl3) (r) 8.63–8.49 (m, 2H), 8.23 (s, 1H), 7.89–7.83 (m,3H), 7.75 (dd, J = 8.5 and 1.6 Hz, 1H), 7.52–7.44 (m, 2H), 6.61 (s,1H), 6.42 (s, 1H), 4.76–4.70 (m, 2H), 4.40–4.35 (m, 2H), 4.34–4.30(m, 2H), 4.30–4.26 (m, 2H), 4.25 (dd, J = 4.9 and 2.9 Hz, 2H), 4.24–4.19 (m, 2H), 3.91 (s, 5H). 13C NMR (CDCl3) (r) 151.99, 150.10,141.49, 141.32, 140.32, 140.23, 137.48, 136.90, 135.95, 133.39,133.10, 129.63, 128.90, 128.66, 128.01, 127.81, 127.71, 127.67,127.56, 127.01, 126.67, 126.26, 113.47, 103.10, 102.78, 82.32,77.21, 71.87, 70.06, 70.04, 65.01, 64.96, 64.39, 64.35, 53.40, 29.69.

H

Fe O

OMnO2

Fe

BrBr

NN

S

OO

SnBu3

PdCl2(PPh3)2

BrBr

NH2H2N

4

7

9 8

of monomer DEFNQ.

Fig. 2. Multiple scan voltammogram for polymerization of DEFNQ in DCM/ACN/TBAPF6 solution.

186 S�. Özdemir et al. / Journal of Electroanalytical Chemistry 648 (2010) 184–189

HRMS (EI) for C40H26FeS2N2O4 calculated 721.0919, found721.0918.

3. Results and discussion

3.1. Synthesis

The synthetic route to the monomer is shown in Scheme 1. Thereagent 2-hydroxy-1-(naphthalen-2-yl)-2-ferrocenylethanone wassynthesized by benzoin condensation of ferrocene carboxaldehydeand b-naphthaldehyde [26] and oxidized by MnO2 to obtain 1-ferr-ocenyl-2-naphthylethanedione as a red solid [26]. The reagent2,1,3-benzothiadiazole was brominated with a HBr/Br2 mixturein high yields [27] and reduced with NaBH4 to afford 3,6-dibro-mo-1,2-phenylenediamine as described previously [28]. Thepurified compound was condensed with 1-ferrocenyl-2-naphthy-lethanedione in ethanol to afford 5,8-dibromo-2-(naphthalen-2-yl)-3-ferrocenyl-4a,8a-dihydroquinoxaline in high yields [29].Stannylation of EDOT was achieved in two steps according topreviously reported methods [30]. The Stille coupling reaction ofdibromoquinoxaline with tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)stannane was performed in dry THF withPd(PPh3)2Cl2 as the catalyst to give the desired compound DEFNQ.

3.2. Cyclic voltammetry

Reversible redox couple of ferrocene was recorded in 0.1 MTBAPF6/ACN mixture on ITO. Characteristic oxidation and reduc-tion peaks of Fc (ferrocene) were observed at 0.78 V and 0.17 Vrespectively, versus Ag wire pseudo reference electrode. The cyclicvoltammogram (CV) of acceptor unit containing Fc group was alsorecorded in order to observe the effect of acceptor unit on redoxbehavior of Fc. The oxidation potential of Fc was shifted to aslightly higher potential due to electron deficient quinoxaline unit.Fig. 1 summarizes the CV of Fc, the acceptor unit and the first runof the electropolymerization of DEFNQ in ACN/DCM (5:95, v:v)medium with TBAPF6 electrolyte. During stepwise oxidation be-tween �0.3 V and +1.2 V, first peak (Fc/Fc+) was observed at0.72 V and the monomer peak was appeared at 1.1 V (Fig. 1).

After the monomer oxidation, a reduction appears at 0.63 V as aresult of deposition of polymer chains on ITO. This reduction leadsto an insulator electrode surface hence; ferrocene reduction peakcannot be seen. The repetitive cycles of electropolymerization ofthe monomer on ITO was performed in a 0.1 M TBAPF6 and

Fig. 1. Single scan voltammograms of (a) Fc , acceptor unit in ACN and (b) thsolvent–electrolyte couple.

0.01 M DEFNQ solution at a scan rate of 100 mV/s in ACN/DCM(5:95, v:v) solvent mixture (Fig. 2). An oxidation peak at 0.82 Vand its reverse cathodic peak at 0.45 V corresponding to PDEFNQappeared with increasing number of cycles (Fig. 2).

The polymer also revealed n-doping property where a revers-ible redox couple at �1.7 V and �1.35 V, versus the Ag wire pseudoreference electrode, was observed for the n-type doping and ded-oping processes, respectively (Fig. 3). The anodic and cathodicpeaks of PDEFNQ were proportional to each other even at differentscan rates, which is an indication of a well adhered polymer filmand a non diffusion controlled charge transfer process [31].

3.3. Spectroelectrochemistry

To probe the optical behaviors of PDEFNQ upon doping, spectralchanges were investigated by UV–Vis–NIR spectrophotometer in amonomer free, 0.1 M TBAPF6, ACN solution with gradually increas-ing potentials between �0.3 V and 1.2 V. Since PDEFNQ is a donor–acceptor type polymer and this type of materials usually show twodistinct absorption maxima, peaks at 450 nm and 760 nm were ob-served (Fig. 4). The second peak at longer wavelength corresponds

e first run of electropolymerization of DEFNQ in a DCM/ACN (5:95, v:v)/TBAPF6

Fig. 3. Single scan voltammogram of PDEFNQ upon both p and n-type doping.

Fig. 6. Percent transmittance changes as a function of time at 730 nm and 1750 nmfor PDEFNQ.

Fig. 5. Electronic absorption spectra for PDEFNQ upon n doping between �0.5 Vand �2.1 V with 0.2 V potential intervals.

Fig. 4. Electronic absorption spectra for PDEFNQ upon p doping between �0.3 Vand 1.2 V with 0.1 V potential intervals in a monomer free solution.

S�. Özdemir et al. / Journal of Electroanalytical Chemistry 648 (2010) 184–189 187

to the lowest energy p–p� transition of the neutral polymer. Theband gap calculated from the onset of this transition was 1.3 eV.

Two absorption peaks in the red and blue regions of the visiblespectrum resulted in a neutral state green polymer, PDEFNQ,

which is a rarely seen property among conjugated polymers.These peaks were decreased simultaneously upon stepwise oxida-tion and the polymer revealed a highly transmissive state. As aresult of charge carrier formation upon oxidation, the absorptionin the visible region reached a minimum value and new absorp-tion band evolved at 1000 nm. Bipolaron formation was trackedat 1750 nm in NIR region. Up to date almost all neutral stategreen polymers were accompanied by only a transmissive oxi-dized state when oxidized [12]. Introduction of ferrocene unitsinto the polymer backbone makes PDEFNQ a promising multi-chromic neutral state green to transmissive electrochromic poly-mer during p-type doping. PDEFNQ film is saturated green (Y:17.2, x: 0.30, y: 0.39) in its neutral state, the color changes to ol-ive (Y: 35.83, x: 0.39, y: 0.45) when partially oxidized and finallyhighly transparent (Y: 57.2, x: 0.31, y: 0.36) in its fully oxidizedstate.

Although conducting polymers have tendency to exhibit both p-and n-doping ability, only a few of them show this notable prop-erty [32]. The n-type polymers can be used in the fabrication ofmany different types of polymer based electronic devices such asLEDs [33] and ambipolar field effect transistors [5]. True n-typedoping process can be proved by a reversible redox couple at neg-ative potentials and spectroscopic changes. We achieved thereduction of a Fc containing conjugated polymer, PDEFNQ byinserting Fc units to a material with donor–acceptor–donor units[12a,12b].

Despite the common belief that the spectra for n-doping cannotbe recorded under ambient conditions [34], charge carrier forma-tion upon reduction of PDEFNQ was observed in open air medium

Fig. 7. Relative luminance (Y%) values for PDEFNQ upon applied potential.

188 S�. Özdemir et al. / Journal of Electroanalytical Chemistry 648 (2010) 184–189

(Fig. 5). Polymer revealed purple color when totally reduced (Y:24.7, x: 0.34, y: 0.33).

3.4. Kinetic studies

Kinetic measurements were done in a monomer free, ACN/TBAPF6 solvent–electrolyte system. The polymer film was switchedbetween its reduced and oxidized states with 5 s time intervals.Percent transmittance changes in the visible and NIR regions andthe switching times (the time required for one full switch) weredetermined. In the visible region PDEFNQ revealed 33% optical con-trast with a switching time of 0.8 s. The percent transmittancechange and the switching time in the NIR region for the polymerfilm were measured as 40% and 1.6 s, respectively (Fig. 6).

Relative luminance studies which measure the film lightness ordarkness (the amount of transmitted light through the polymerfilm) were performed on electrochemically polymerized PDEFNQfilms (Fig. 7). The values ranged from 17% i.e. absorbed state, to55% when the polymer film is transmissive.

4. Conclusion

PDEFNQ, a ferrocene functionalized, both p and n dopable do-nor–acceptor–donor type conjugated polymer was synthesizedand characterized. PDEFNQ is a neutrally green polymer but ithas multicolored states with a transmissive oxidized state. Accord-ing to the electrochemical and spectroscopic results PDEFNQ is aninteresting candidate for optoelectronic applications.

Acknowledgments

Authors thank TUBA and METU grants.

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