Redox doping behaviour of poly(3,4-ethylenedithiothiophene) – The counterion effect

Optical Materials 33 (2011) 1405–1409

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Redox doping behaviour of poly(3,4-ethylenedithiothiophene) – The counterion effect Wojciech Domagala a,⇑, Dawid Palutkiewicz a, Diego Cortizo-Lacalle b, Alexander L. Kanibolotsky b, Peter J. Skabara b a b

Faculty of Chemistry, Silesian University of Technology, ul. M. Strzody 9, 44-100 Gliwice, Poland WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral street, Glasgow G1 1XL, United Kingdom

a r t i c l e

i n f o

Article history: Available online 22 March 2011 Keywords: PEDTT Redox doping Polaron Bipolaron Counterion PEDOT

a b s t r a c t Poly(3,4-ethylenedithiothiophene) – PEDTT, an alkylene sulphur derivative of PEDOT, presents itself as an interesting polymer with a number of disparate redox and chromic properties compared to its close analogue – PEDOT. In this study we present the results of an investigation into the electrochemical doping process of PEDTT, using four different electrolyte solutions, differing in anion content of the chosen salt. The results show that the anion identity plays a key role in the redox reactions accompanying these processes in what could be interpreted as anion ionochromism. In situ UV–Vis spectroelectrochemical experiments reveal an intriguing double electrochromic transition of PEDTT films during their oxidative doping, going from golden-yellow through green to pomegranate – a quality not so common within the family of electroactive conjugated polymers. The evolution of each UV–Vis spectrum over a potential range indicates that different redox states of the polymer are responsible for the chromatic changes. In the reduction half-cycle, the dedoping process of PEDTT appears to follow a path dissimilar to the p-doping one, featuring only one, direct electrochromic transition of the film’s colour, bypassing the green state, and a distinct two-step bleaching process of doping-induced charge carrier bands. The observed electrochemical and spectral phenomena have been accredited to the specific redox behaviour of doping-induced radical cation and cationic defect states interacting with the dithioalkylene sulphur atom. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Scientific and commercial success of the well-known poly(3,4ethylenedioxythiophene) PEDOT, has fuelled a drive to develop and investigate diverse new derivatives of this unusual polymer, in an attempt to discover materials featuring tailored molecular, electron and spectroscopic properties. This endeavour is indispensable for the maturation of the relatively young field of plastic electrochromics [1]. Various pathways have been undertaken to develop this fascinating field, involving the grafting of side chains [2], alteration of the alkylene bridge size [3], replacement of sulphur and oxygen with other chalcogenide atoms [4–6], alternating or statistical copolymerisation with other electron-abundant or deficient monomers, as well as combined implementation of two or more of these strategies together. In many cases, these strategies have yielded impressive results [1]. Application of one of these strategies involves the introduction of sulphur in place of oxygen atoms, affording 3,4-ethylenedithiothiophene – EDTT [4], whose properties turn out to be markedly different, if not contrasting to its closest relative of the 3,4-alkylenechalcogeno⇑ Corresponding author. E-mail address: [email protected] (W. Domagala). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.02.030

thiophene family – PEDOT [7]. Having a lower oxidation potential compared to EDOT, EDTT polymerises easily, albeit according to a different electrosynthetic mechanism [8]. The polymer PEDTT exhibits a considerably higher oxidation potential, attributed to a large torsion angle between the repeat units in the electroneutral polymer chain and considerably weaker chain ordering through sulphur – sulphur interactions, compared to sulphur – oxygen ones in PEDOT [9,10]. Substitution of sulphur for oxygen atoms increases the band gap of PEDTT to 2.2 eV [4] compared to 1.6 eV for PEDOT [11], resulting in a brown–yellow to dark green colour switch upon doping, as reported by Kanatzidis et al. The same group observed a surprisingly low concentration of defect and doping-induced spins, whose magnitude turned out to depend considerably on the doping agent employed. Comprehensive spectroelectrochemical studies by Cravino et al. [7] corroborated these findings, revealing a direct doping process of PEDTT, which yields bipolarons in a single oxidation step. Considering these distinct properties of PEDTT, in this study we decided to investigate its electrosynthesis and subsequent electrochemical doping using four different electrolyte salt solutions, differing in the chemical identity of the anions, in an effort to gain a better understanding of the redox processes taking place in this polymer upon its electrochemical doping.

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Fig. 1. Cyclic voltammograms of 0.5 mM EDTT in 0.1 M solutions of: (a) Bu4NClO4, (b) Bu4NBF4, (c) Bu4NPF6, (d) Bu4NCF3SO3, in acetonitrile at a scan rate of 0.1 V s steady accumulation of conducting PEDTT layer at Pt electrode upon repetitive electrooxidation of EDTT. For clarity, only selected cycles are shown.

2. Experimental Electrochemical measurements were performed on an AUTOLAB potentiostat–galvanostat PGSTAT20 (EcoChemie, Netherlands) using a standard three electrode arrangement. Platinum wire with a geometric working area of 0.10 cm2 served as the working electrode for regular electrochemical experiments, whilst an ITO-covered quartz plate (Delta Technologies, USA) of 1 cm width was used to prepare PEDTT films for in situ UV–Vis-NIR spectroelectrochemical experiments. The two other electrodes were: Ag wire quasi-reference electrode, calibrated versus ferrocene/ferrocinium redox couple prior to each experiment, and platinum coil as an auxiliary electrode. Before each experiment the cell was deaerated with argon. 3,4-Ethylenedithiothiophene (EDTT), synthesised in our group using literature methods [12], was electropolymerised from 0.005 M solutions of the monomer in a 0.1 M environment of four different electrolyte salts: (Bu)4NClO4, (Bu)4NBF4, (Bu)4NPF6 and (Bu)4NCF3SO3 (Sigma–Aldrich, used as received) in CH3CN, by means of cyclic voltammetry with control of the maximum oxidation current in the early cycles [13].

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positioned perpendicular to the light beam, while the silver quasireference electrode was placed to the side and the platinum coil auxiliary electrode beneath the light path. UV–Vis-NIR spectra have been taken using a Cintra 5 spectrophotometer (GBC Scientific Equipment, Australia). PEDTT films have been investigated in 0.1 M acetonitrile solutions of the respective electrolyte salts, the same as those used for electropolymerisation. 3. Results and discussion Electrochemical oxidation of EDTT, in acetonitrile solutions in the presence of each of the four electrolyte salts, generates a thin, coloured, translucent layer at the surface of the working electrode. Upon repetitive cycling, a steady growth of current peaks at potentials below that of EDTT oxidation is observed, as shown in Fig. 1a– d, attributable to progressive accumulation of an insoluble, conducting PEDTT film at the surface of the electrode. The recorded voltammetric curves clearly show an influence of the supporting electrolyte composition on the electrochemical properties of the polymer films obtained. The nascent PEDTT layer displays two oxidation peaks whose potentials differ depending on the counterion – the first, smaller one between 0.13 V and 0.35 V, and the second, taller peak between 0.46 V and 0.59 V. The electrical processes represented by these peaks manifest themselves as colour changes of the polymer film, with the transition at the first peak going from golden-yellow to green, followed by re-colouration from green to pomegranate at potentials of the second peak. Mutual peak separation depends strongly on the counterion environment too, ranging from good resolution as in the case of PEDTT (BF4 ), to near coalescence observed for PEDTT (CF3 SO3 ). What is interesting to note is

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that, discounting the very first electropolymerisation cycles, the peak potential of the second, dominant peak is almost potential invariant, while the behaviour of the first peak is strongly dependent on the counterion environment with its peak potential appearing fixed for PEDTT (BF4 ) and PEDTT (CF3 SO3 ) (Fig. 1b and 1d), while clearly shifting towards the second peak with growing thickness in PEDTT (ClO4 ) and PEDTT (PF6 ) films (Fig. 1a and 1c). The two discernable, relatively narrow oxidation peaks of PEDTT at potentials relatively close to the oxidation potential of the monomer, are disparate features compared to its close relative PEDOT where a broad and unresolved oxidation wave, starting at ca. 0.7 V, is observed, spanning the whole potential range of the doped state of the polymer. The position of these peaks indicates a notably higher oxidation potential of PEDTT compared do PEDOT, a property attributed to the lower degree of polymerisation and a considerably less planar conformation of the PEDTT chain in its dedoped state, whilst their slim shape suggests a lower polydispersity index of PEDTT. The redox origins of these peaks are no less intriguing, for previous reports of Kanatzidis and co-workers [4] and Cravino et al. [7] suggest that the redox-generated charge car-

riers are predominantly spinless, with spin bearing polarons undergoing direct oxidation to bipolarons at the instant of oxidation of the polymer. In light of this interpretation, the first, minor redox peak could represent some sort of phase transition that the polymer undergoes at the very onset of its oxidation, but could well be a signature of the oxidation of residual polarons which could remain trapped in relatively thick films, such as those investigated in this study. Such surmise is further justified by the increasing ratio of the first and second oxidation peak currents, observed upon successive cycling, which supports the former peak’s relationship with PEDTT film thickness. Upon reduction, a markedly different behaviour is observed. Here a broad and almost featureless reduction wave develops, with two barely discernible reduction peaks located at ca. 0.2 to 0.4 V and 0.1 to 0.1 V (Fig. 1a–d). Such an extended redox signature points to a diffuse redox process taking place in the polymer, in this respect quite similar to PEDOT. Of the two distinguishable reduction peaks, again the first one has its peak potential almost fixed while the second one migrates along the potential axis. Upon traversing the reduction potential range, the colour of the polymer goes directly from pomegranate to golden-brown, unexpectedly missing out the intermediate green state and indicating a different redox state

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W. Domagala et al. / Optical Materials 33 (2011) 1405–1409

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Fig. 5. In situ UV–Vis-NIR spectra of PEDTT film during its electrochemical (a) doping, and (b) subsequent dedoping, conducted in 0.1 M Bu4NCF3SO3 solution in acetonitrile. The layout of results, analogous to Fig. 2 is adopted.

transition pathway of the polymer in the course of its dedoping process. In order to investigate the observed peculiar chromatic effects accompanying PEDTT doping, in situ UV–Vis-NIR spectroelectrochemical measurements have been carried out for films of all four PEDTT – counterion systems, the results of which are presented in Figs. 2–5. At first inspection, one can observe a non-trivial difference of the UV–Vis spectra collected during the doping and dedoping half-cycles of each PEDTT – counterion system. Considering PEDTT (ClO4 ) first (Fig. 2), an interesting feature of the very onset of the doping half-cycle is the bleaching of a broad band, centred at ca. 1.9 eV1, concurring with the evolution of the first oxidation wave on the cyclic voltammogram (CV) (see inset of Fig. 2a), and an observed first colour change of the film from golden-brown to green. Furthermore, this band vanishes once the first oxidation peak of PEDTT film develops fully. Such behaviour is rather peculiar as one would either expect quiescence or a first sign of polaron band development, but PEDTT (ClO4 ) is not unique in this respect, as this phenomenon is found for all four anions being considered. The energy span of this band and the magnitude of its attenuation,

appear to diminish in the following counterion order ClO4 > BF4 > PF6 > CF3 SO3 . Furthermore, a close inspection reveals that, as this band diminishes, absorption at the low energy edge of the UV– Vis-NIR spectrum increases with a discernable isosbestic point observed for PEDTT (PF6 ) and PEDTT (CF3 SO3 ) systems (Figs. 4 and 5, respectively). Upon further oxidation of the polymer film, two bands develop, one at 1.6 eV and the other well below 1.1 eV with only its high energy tail visible. This is accompanied by the simultaneous bleaching of the broad absorption wave at ca. 2.8 eV, manifested by the second chromatic transition of the film from green to pomegranate. As the doping level of the polymer increases, these two bands broaden and their maxima shift to higher energies, eventually coalescing at the highest doping levels. The Vis–NIR energy span of this band is different for each PEDTT – counterion system considered, but the position of its high energy limit once again increases in the order ClO4 > BF4 > PF6 > CF3 SO3 , correlating reasonably well with the base strength of these anions. Although the anions are weakly nucleophilic, this indicates that their coulombic interactions with the polymer borne charges appear to be influential enough to affect the course of the PEDTT doping process as well as the electronic properties of this polymer in its doped state. Considering the complex interplay of doping-induced spectral bands in the visible region of the electromagnetic spectrum, this

1 For interpretation of colour in Figs. 2 and 5, the reader is referred to the web version of this article.

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phenomenon can be considered a manifestation of anion ionochromism of PEDTT. Moving on to the dedoping half-cycle for all the PEDTT – counterion systems studied, an asynchronous attenuation of the NIR doping-induced bands is observed upon a decrease of the doping level of the polymer. In the initial stage of the reduction of doped PEDTT, strong attenuation of the lowest energy band takes place first, exposing a stable, second doping-induced band at ca. 1.4 eV. Once again the intensity ratio of the lowest energy band to this one depends on the doping counterion, with the highest ratio being observed for ClO4 (Fig. 2b), and the lowest for CF3 SO3 (Fig. 5b), suggesting either differences in the abundance of species absorbing at these energies, or dissimilarities of their interactions with counterion molecules. Upon further dedoping, this second band diminishes as well, triggering growth of a broad and featureless UV–Vis absorption band spanning the energy range of 2.3 eV and upwards, together with the evolution of a new band at ca. 1.9–2.1 eV, reminiscent of the band observed in the dedoped polymer. These changes, taking place at potentials of a broad CV reduction peak of the polymer, manifest themselves through a direct colour change of its film from pomegranate to golden-brown, missing out the intermediate green state observed during doping. Being a general observation for all the counterions employed, this curious manifestation of contrasting doping and dedoping mechanisms presents PEDTT as a ‘doping history’ sensitive material. Gathering together the results presented, the following interpretation of the observed doping behaviour of PEDTT can be put forward. At the first redox step of an electro-dedoped polymer, proceeding at potentials of the first anodic CV redox peak (Fig. 2), oxidation of residual polarons to bipolarons takes place leading to bleaching of the VIS shoulder band at ca. 1.9 eV with simultaneous evolution of the band at 1.1 eV, ascribable to nascent bipolarons. Considering the contrasting results of earlier in situ ESR spectroelectrochemical studies carried out on PEDTT (PF6 ) [7], indicating a relatively low abundance of polarons throughout a broad doping level range, in line with first reports of unusually low concentration of paramagnetic centres in PEDTT (ClO4 ) samples [4], this disparate inference can be supported by the fact that notably thicker films have been investigated in this study compared to the ones in the reports cited above. Next, at potentials of the second anodic redox peak, electroneutral PEDTT chains whose twisted conformation accounts for their elevated oxidation threshold [5,9], undergo direct oxidation to bipolarons. The resulting doping charge, accumulated in PEDTT, can be quite high compared to other polythiophenes as reported by Zhan and co-workers [14], a phenomenon ascribed to effective localisation of positive charge on moderately electronegative dithioethylene sulphur atoms. In light of this hypothesis, the observed doping-induced band at 1.6 eV could be attributed to electron transitions involving these localised states. Furthermore, the observed counterion dependent tailing of this band into the VIS energy range can originate from differences in strengths of coulombic interactions of each of the four counterions employed, correlating the binding (localisation) energy of sulphur borne cations with increasing anion base strength. Considering the dedoping process, a rapid depletion of an absorption band at 1.1 eV signifies the reduction of bipolarons delocalised on the polythiophene main chain in the first place, leading to the generation of an insulating layer of dedoped, electroneutral PEDTT chains effectively fencing off the remaining bulk of doped polymer film from the electrode metal. Consequently, as a result of such inferior electrical contact, the reduction process of the film spreads out on the potential scale with reduction peaks moving to considerably lower cathodic potentials. This in turn facilitates a stepwise, rather than direct reduction process

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of doped PEDTT, involving polarons as the intermediate species. What is interesting at this point is that the growth of the polaron attributed band at ca. 1.9–2.1 eV appears closely tied associated with the depletion of the band at 1.4 eV, suggesting that the radical cations are in fact harboured by the alkylene bridge sulphur atoms in the form of stable thioether radical cations [15]. This could help explain the observed counterion influence on both the intensity (hence population size) and the energies of their related spectral bands, as well as the observed anion ionochromic effect on the doping process of PEDTT, being in principle a manifestation of subtle differences in the interactions between the sulphur borne charge carriers and their charge balancing counterions. 4. Conclusions Unusual colouration changes of poly(3,4-ethylenedithiothiophene) – PEDTT taking place upon electrochemical doping of this polymer have been investigated using in situ UV–Vis-NIR spectroelectrochemistry. Upon doping, a double chromatic transition of the polymer film, switching between golden-brown, green and pomegranate, has been correlated with a two-step redox doping act involving, firstly, the oxidation of residual polarons, followed by the direct oxidation of electroneutral PEDTT chains to bipolarons. This sequence yields two groups of positively charged sites, one delocalised on the polythiophene backbone and the other localised at the thioethylene sulphur atoms, featuring pronounced interactions with charge balancing counterions. Subsequent reduction of the doped polymer film restores the golden-brown film colour directly in a broad and diffused reduction process, whereby the population of delocalised bipolarons decreases first, followed by a gradual transformation of the sulphur cationic defects to thioalkylene radical cations, which, on account of their stability and low cathodic potential, become confined in an electro-dedoped polymer film. The observed doping mechanism, contrasting with that found in PEDTT’s close relative PEDOT, offers a fresh look at the mutual interactions of doping-induced charged defects and counterion pairs, demonstrating yet another example of the profound impact of side-chain architecture, on the electrochromic properties of 3,4-chalcogenoalkylene heterocyclic conjugated polymer families. References [1] P.M. Beaujuge, J.R. Reynolds, Chem. Rev. 110 (2010) 268–320. [2] B.L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J.R. Reynolds, Adv. Mater. 12 (2000) 481–494. [3] L. Groenendaal, G. Zotti, P.-H. Aubert, S.M. Waybright, J.R. Reynolds, Adv. Mater. 15 (2003) 855–879. [4] C.G. Wang, J.L. Schindler, C.R. Kannewurf, M.G. Kanatzidis, Chem. Mater. 7 (1995) 58–68. [5] H. Pang, P.J. Skabara, S. Gordeyev, J.J.W. McDouall, S.J. Coles, M.B. Hursthouse, Chem. Mater. 19 (2007) 301–307. [6] E. Aqad, M.V. Lakshmikantham, M.P. Cava, Org. Lett. 3 (2001) 4283–4285. [7] A. Cravino, H. Neugebauer, A. Petr, P.J. Skabara, H.J. Spencer, J.J.W. McDouall, L. Dunsch, N.S. Sariciftci, J. Phys. Chem. B 110 (2006) 2662–2667. [8] H. Randriamahazaka, G. Sini, F. Tran Van, J. Phys. Chem. C 111 (2007) 4553– 4560. [9] M. Turbiez, P. Frère, M. Allain, N. Gallego-Planas, J. Roncali, Macromolecules 38 (2005) 6806–6812. [10] H.J. Spencer, P.J. Skabara, M. Giles, I. McCulloch, S.J. Coles, M.B. Hursthouse, J. Mater. Chem. 15 (2005) 4783–4792. [11] G. Sonmez, H.B. Sonmez, C.K.F. Shen, F. Wudl, Adv. Mater. 16 (2004) 1905– 1908. [12] F. Goldoni, B.M.W. Langeveld-Voss, E.W. Meijer, Synth. Commun. 28 (1998) 2237–2244. [13] A. Zykwinska, W. Domagala, B. Pilawa, M. Lapkowski, Electrochim. Acta 50 (2005) 1625–1633. [14] J. Tang, Z.-P. Song, N. Shan, L.-Z. Zhan, J.-Y. Zhang, H. Zhan, Y.-H. Zhou, C.-M. Zhan, J. Power Sources 185 (2008) 1434–1438. [15] W.K. Musker, Acc. Chem. Res. 13 (1980) 200–206.