Electrochemistry Communications 11 (2009) 1972–1975

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Electrochemistry Communications

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Ferrocene clicked poly(3,4-ethylenedioxythiophene) conducting polymer:Characterization, electrochemical and electrochromic properties

Jingjing Xu, Yuan Tian, Ru Peng, Yuezhong Xian *, Qin Ran, Litong JinDepartment of Chemistry, East China Normal University, Shanghai 200062, China

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

Article history:Received 30 July 2009Received in revised form 16 August 2009Accepted 18 August 2009Available online 22 August 2009

Keywords:Click reactionFerroceneConducting polymerElectrochemistryElectrochromic property

1388-2481/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.elecom.2009.08.031

* Corresponding author.E-mail address: yzxian@chem.ecnu.edu.cn (Y. Xian

The ‘‘click” chemistry, Cu(I)-catalyzed azide–alkyne cycloaddition reaction, was applied to covalentlyfunctionalize the poly(3,4-ethylenedioxythiophene) (PEDOT) conducting polymer film with an excellentelectron transfer mediator (ferrocene). Scanning electron microscopy (SEM), Fourier transform infraredspectroscopy (FT-IR), and Raman spectroscopy were used to characterize the ferrocene-grafted PEDOTconducting polymer film, and it was proved that the grafting procedure via click reaction had a high effi-ciency. The ferrocene groups covalently grafted in the polymer films turned out to own a relatively fastelectron transfer rate and show multi-color states via adjusting applied potential.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

With the extensive applications of conductive polymers, moreand more efforts have been focused on the functionalization con-ducting polymers [1,2]. For example, poly(3-thiopheneaceticacid)(PTAA) film was covalently modified with a tripeptide, and it hadgreat affinity towards Cu2+ ions [3]. A carbon nanotube-conjugatedconducting polymer matrix was successfully prepared and itshowed electrical bistability and memory phenomenon [4]. Polya-nine-functionalized-MWCNTs containing noble metal (gold andsilver) nanoparticles were also prepared and the nanocompositewas successfully applied in gas sensing and catalysis [5].

Copper-catalyzed azide–alkyne cycloaddition, one of typicalclick reactions, has attracted wide attentions for its high reactionefficiency, mild reaction condition, and stability of the products[6,7]. Due to the excellent performance, click reaction has becomea suitable method for organic synthesis [8], surface modification[9], macromolecular architecture tailor and optimization [10]. Re-cently, click reaction is also applied as a versatile strategy for con-ducting polymer functionalization [11–13].

In this work, ferrocene functionalized poly(3,4-ethylenedi-oxythiophene) (PEDOT) conductive polymer was prepared viacopper-catalyzed azide–alkyne cycloaddition reaction (Scheme1). The resulting stable, conducting polymer films were charac-terized by scanning electron microscopy (SEM), Fourier trans-form infrared spectroscopy (FT-IR) and Raman spectroscopy. It

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indicates that click reaction is an efficient strategy for PEDOTmodification. The electrochemical experiments demonstratethat the ferrocene clicked PEDOT films have fast electron trans-fer ability. In addition, the spectroelectrochemical studies showthat the conducting polymer can switch from opaque purple tored, then blue with a contract of 20% in oxidized state andswitching time around 600 ms. It suggests that the ferroceneclicked PEDOT films have potential application in electrochro-mic devices.

2. Experiment

2.1. Reagent and apparatus

3,4-Dimethoxythiophene, ethynylferrocene and poly(sodium4-stryrene-sulfonate) (PSS) were purchased from Sigma–Aldrich.3,4-(1-Azidomethylethylene)dioxythiophene (N3-EDOT) was syn-thesized using previously established methods [13]. Other chem-icals were of analytical grade and used without furtherpurification.

The morphologies of the samples were observed by an S-4800cold field emission SEM (Hitach, Japan). FT-IR absorbance spectrawere conducted on 670 FT-IR Spectrometer (Nicolet). Raman spec-tra were obtained by a T64000 Raman spectrum instrument (JobinYvon, France). Electrochemical experiments were performed on aCHI-832 electrochemical analyzer (CHI, USA) with a three-elec-trode system. Spectroelectrochemical studies were performed atroom temperature on a Cary 50 UV–vis spectrophotometer (Varian,Australia).

Scheme 1. The copper-catalyzed azide–alkyne cycloaddition reaction between N3-PEDOT and ethynylferrocene.

J. Xu et al. / Electrochemistry Communications 11 (2009) 1972–1975 1973

2.2. Electrochemical polymerization of the N3-PEDOT

The electrochemical polymerization of the conducting polymer(N3-PEDOT) was carried out at room temperature in N3-EDOT/poly(sodium 4-stryrene-sulfonate) (PSS) solution by cyclic scan-ning between 0 and 1.3 V with scan rate of 100 mV/s. In this work,PSS was used as a cosolvent and dopant during the process of elec-trochemical polymerization.

2.3. Preparation of ferrocene-grafted PEDOT conducting polymer

Ferrocene-grafted PEDOT conducting polymer was prepared viaclick reaction, which is similar to the work of Baeuerle and cowork-ers [13]. In briefly, the N3-PEDOT modified electrode was im-mersed in 1.0 mM of ethynylferrocene in 2:1 water/ethanolsolutions. Then, 1.0 mol% copper (II) sulfate pentahydrate and5.0 mol% sodium ascorbate were added as catalysts at room tem-perature. After incubation for 24 h, the electrodes were washedwith ethanol and twice distilled water to remove excess adsorbateand then dried under nitrogen.

2.4. Electrochemical and electrochromic properties

Electrochemical and electrochromic properties were carried outin a 0.1 M LiClO4 aqueous solution at room temperature with ferro-cene-grafted PEDOT modified indium tin oxide (ITO) electrode asthe working electrode.

3. Result and discussion

3.1. Characterization of the N3-PEDOT and ferrocene ‘‘clicked” PEDOTfilm

The electrochemical polymerization of N3-EDOT was similar tothat of EDOT (data not shown). The anodic current density of N3-EDOT starts to grow rapidly around 0.95 V, which is higher thanthat of the EDOT (0.76 V) [14]. It may ascribe to the influence ofazido, an electron-attracting group.

The morphologies of conducting films are characterized by SEM.The N3-PEDOT films grow regularly and uniformly and present a

Fig. 1. SEM images of N3-PEDOT films (a), ferrocene clicked PEDOT fi

compact, dense and homogeneous structure (Fig. 1a). Fig. 1b showsthe ferrocene ‘‘clicked” PEDOT films, and a rough and porous struc-ture is observed. According to the EDS data (Fig. 1c), the composi-tion of the conductive polymer films can be defined. The two Speaks and the O peak indicate that the ‘‘clicked” PEDOT conductingfilms remain their initial composition. The appearance of Fe peakafter click reaction demonstrates the successful proceeding of the1,3-dipolar cycloaddition reaction. The atom ratio of Fe/S (2.12/2.41) is close to 1/1, which suggests that the click reaction has car-ried out with a high yield.

Fig. 2A shows the FT-IR spectra of the conducting films. Thepeak at 2104 cm�1 (Fig. 2A(a)) attributes to the azido group inthe N3-PEDOT films [15]. After click reaction, the peak at2104 cm�1 disappears completely (Fig. 2A(b)), which further indi-cates the high efficiency of the Cu(I)-catalyzed azide–alkyne cyclo-addition. The success of the covalent click process is also confirmedby Raman spectra (Fig. 2B). As shown in Fig. 2B, the broad peakfrom 1230 to 1268 cm�1 ascribe to the C–N asymmetric stretching[16] and the N–N asymmetric stretching [17]. The peak at1285 cm�1 assigns to the symmetric N@N stretching [18]. Thepeaks around 1380 cm�1 attributes to the asymmetric stretchingof C–N bond [19] and asymmetric N@N stretching [18]. The peaksat 990, 1347 and 1533 cm�1 characterize the formation of triazolering, indicating that click reaction between azide and alkyne hasbeen successfully conducted [20]. The strong peak at 1432 cm�1

assigns to the symmetric stretching of C@C bond [21]. Becausethe C@C stretching widely exists in initial PEDOT films and ferro-cene, the peaks around 1432 cm�1 is strong and sharp. The peaksat 1106 and 1503 cm�1 are due to the deformation of C–O–C [20]and asymmetric C–C stretching [22], respectively. The weak bandsaround 700–730, 875 and 1054 cm�1 attribute to C–H deformation[23], S–C asymmetric stretching [18] and ring vibration [24],respectively.

3.2. Electrochemical and electrochromic properties of the ferrocene-grafted PEDOT conducting polymer films

According to Fig. 3a and b, it is very clearly aware that the clickreaction cannot perform without the catalyst. After the click reac-tion, a couple of redox peak has been observed (Fig. 3c), whichindicates that ferrocene group is successfully grafted onto PEDOTfilm. The formal potential (E�0 (=(Epa + Epc)/2) of conducting film is332 mV, which is considerably more positive than that of free fer-rocene (E�0 = 120 mV) [25]. It is due to the conjugation effects of thetriazole ring and the conducting polymer chains, which reduces theelectron density of the ferrocene and makes the oxidation of theferrocene unit more difficult than that of unsubstituted metallo-cenes. In addition, the well-defined voltammetric response witha peak-to-peak separation of 82 mV suggests a relatively fast andquasi-reversible electron transfer.

lms (b), and the EDS image of ferrocene clicked PEDOT films (c).

Fig. 2. (A) FT-IR spectra of N3-PEDOT films (a) and the ferrocene clicked PEDOTfilms (b), and (B) Raman spectrum of the ferrocene clicked PEDOT films.

Fig. 3. Cyclic voltammograms of the conducting polymer films in PBS buffer: N3-PEDOT (a), N3-PEDOT film in 1.0 mM of ethynylferrocene solutions for 24 h without(b), and with Cu(I) catalyst (c).

Fig. 4. Spectroelectrochemical spectra of the ferrocene-grafted PEDOT films in0.1 M LiClO4 with potential of �0.4 (a), �0.3 (b), �0.2 (c), �0.1 (d), 0 (e), 0.1 (f), 0.2(g), 0.3 (h), 0.4 (i), 0.5 (j), and 0.6 (k) V. Inset: multi-color states of ferrocene-graftedPEDOT films at different potentials (V): from left to right: �0.4 (dark purple), �0.2(purple), 0 (maroon), 0.2 (brown), 0.5 (grey green). (For interpretation of thereferences to colours in this figure legend, the reader is referred to the web versionof this paper.)

1974 J. Xu et al. / Electrochemistry Communications 11 (2009) 1972–1975

The effect of scan rates on the electrochemical response of fer-rocene-grafted PEDOT film is also studied. Ipc and Ipa are linearlyproportional to the scan rate from 10 to 300 mVs�1, which suggeststhat the reaction on the modified system is a surface-controlled

process [26]. According to the theory of Laviron [27], the electrontransfer rates ks of ferrocene clicked on PEDOT is calculated to be1.15 s�1, which is very close to that of ferrocene ‘‘clicked” toSWCNs (1.2 s�1) [28]. In addition, the surface coverage of the ferro-cene on the conductive polymer films is calculated to be1.45 � 10�9 mol cm�2 using the Laviron’s equation [27].

The spectroelectrochemical features of ferrocene clicked PEDOTwere investigated in 0.1 M LiClO4 aqueous solution (Fig. 4). Whilevarying the potential from �0.4 to 0.6 V, the absorption peak at580 nm decreases and the absorption peak around 830 nm in-creases concomitantly. In this work, the conducting polymer canswitch from opaque purple to red, then blue, indicating that ferro-cence clicked PEDOT films can display multi-color states viaadjusting applied potential.

The electrochromic switching were also conducted by steppingpotential repeatedly between reduced (�0.4 V) and oxidized states(+0.6 V). The ferrocene-grafted PEDOT conducting polymer filmsreveals exceptional optical contrasts of 20% at 580 nm and 12% at830 nm with switching time about 0.6 s (calculated from 95% ofits full contrast). However, the N3-PEDOT conducting film displaysa contrast of 11% at its kmax (540 nm) and 4% at 800 nm, and theaverage switching time turns out to be 1.2 s. The kinetic studiesclearly shows that ferrocene clicked PEDOT film has better electro-chromic properties than that of N3-PEDOT.

4. Conclusions

In this work, ferrocene was successfully grafted to the PEDOTconducting polymer films via click reaction with a high yield. Theferrocene in the PEDOT films displays a relative fast electron trans-fer rate and the resulted conducting films have potential applica-tion in electrochromic devices.

Acknowledgements

This work was supported by the National Natural Science Foun-dation of China (No. 20875031), Shanghai Rising-Star Program(09QH1400800) and Shanghai Municipal Education Commission(No. 08ZZ25).

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References

[1] A.B. Sanghvi, K.P.H. Miller, A.M. Belcher, C.E. Schmidt, Nature Mater. 4 (2005)496.

[2] D. Wei, C. Kvarnstrom, T. Lindfors, A. Ivaska, Electrochem. Commun. 9 (2007)206.

[3] M. Lin, M.S. Cho, W.S. Choe, Y. Lee, Biosens. Bioelectron. 25 (2009) 28.[4] B. Pradhan, S.K. Batabyal, A.J. Pal, J. Phys. Chem. B 110 (2006) 8274.[5] K.R. Reddya, B.C. Sina, K.S. Ryua, J.C. Kimb, H. Chung, Y. Lee, Synth. Met. 159

(2009) 595.[6] V.V. Rostovtsev, L.G. Green, V.V. Fokin, K.B. Sharpless, Angew. Chem. Int. Ed. 41

(2002) 2596.[7] S. Ciampi, T. Böcking, K.A. Kilian, J.B. Harper, J.J. Gooding, Langmuir 24 (2008)

5888.[8] Q. Zhang, J.M. Takacs, Org. Lett. 10 (2008) 545.[9] P. Mansky, Y. Liu, E. Huang, T.P. Russell, C.J. Hawker, Science 275 (1997) 1458.

[10] D.Y. Ryu, K. Shin, E. Drockenmuller, C.J. Hawker, T.P. Russell, Science 308(2005) 236.

[11] A. Jatsch, A. Kopyshev, E. Mena-Osteritz, P. Bauerle, Org. Lett. 10 (2008) 961.[12] M. Ak, B. Gacal, B. Kiskan, Y. Yagci, L. Toppare, Polymer 49 (2008) 2202.[13] H.B. Bu, G. Goetz, E. Reinold, A. Vogt, S. Schmid, R. Blanco, J.L. Segura, P.

Baeuerle, Chem. Commun. 11 (2008) 1320.

[14] C. Li, T. Imae, Macromolecules 37 (2004) 2411.[15] B.A. Laurent, S.M. Grayson, J. Am. Chem. Soc. 128 (2006) 4238.[16] R.J. Davis, J.E. Pemberton, J. Phys. Chem. C 112 (2008) 4364.[17] L. Kabalan, S.F. Matar, Chem. Phys. 359 (2009) 14.[18] A.A. Jbarah, A. Ihle, K. Banert, R. Holze, J. Raman Spectrosc. 37 (2006) 123.[19] N.G. Tognalli, A. Fainstein, J. Phys. Chem. C 112 (2008) 3741.[20] B.K. Yoo, S.W. Joo, J. Coll. Interf. Sci. 311 (2007) 491.[21] E. Tamburria, S. Orlanducci, F. Toschi, M.L. Terranova, D. Passeri, Synth. Met.

159 (2009) 406.[22] J. Casado, R.P. Ortiz, M.C.R. Delgado, V. Hernndez, J.T.L. Navarrete, J.M.

Raimundo, P. Blanchard, M. Allain, J. Roncali, J. Phys. Chem. B 109 (2005)16616.

[23] F. Nastasea, D. Mihaiescub, C. Nastasea, C. Mireac, I. Burzob, I. Stamatin,Composites: Part A 36 (2005) 503.

[24] J. Zhang, M.Z. Wu, T.S. Pu, Y.Z. Zhang, R.P. Jin, Z.S. Tong, D.Z. Zhu, D.X. Cao, F.Y.Zhu, J.Q. Cao, Thin Solid Films 307 (1997) 14.

[25] B. Jin, F. Tao, P. Liu, J. Electroanal. Chem. 624 (2008) 179.[26] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and

Applications, Wiley, New York, 2001.[27] E. Laviron, J. Electroanal. Chem. 101 (1979) 19.[28] E.C. Landis, R.J. Hamers, Chem. Mater. 21 (2009) 724.