Three-Step Redox in Polythiophenes: Evidence from Electrochemistry

Different oxidation levels, regarded as polaron, bipolaron, and metallic states, are usually found in conjugated heterocyclic polymers. We found that ...
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15202

J. Phys. Chem. 1996, 100, 15202-15206

Three-Step Redox in Polythiophenes: Evidence from Electrochemistry at an Ultramicroelectrode Xiwen Chen and Olle Ingana1 s* Laboratory of Applied Physics, Department of Physics (IFM), Linko¨ ping UniVersity, S-581 83 Linko¨ ping, Sweden ReceiVed: January 17, 1996; In Final Form: April 22, 1996X

Different oxidation levels, regarded as polaron, bipolaron, and metallic states, are usually found in conjugated heterocyclic polymers. We found that poly(3,4-ethylenedioxythiophene) (PEDOT) also has these different oxidation levels, using in situ UV-VIS-NIR spectroscopy. The transitions between the different oxidation levels were, however, never clearly observed in cyclic voltammetry (CV). Instead it usually shows a broad oxidation peak and two reduction peaks. The CV of PEDOT at macroelectrodes shows a pair of redox peaks separated by 0.9 V at low scan rates, indicating two irreversible electron transfer steps. Using an ultramicroelectrode, we found these two pairs of redox peaks in PEDOT, as well as for poly(3-methylthiophene) (PMeT). These two peaks cannot be explained by the existence of two forms of the materials, whether due to two different conjugation lengths, crystalline or noncrystalline phases, or created by conformational changes. We deduced that there should be a third redox peak. With fast scan cyclic voltammetry both PEDOT and PMeT on ultramicroelectrodes showed three reduction peaks and two oxidation peaks. We find three illresolved pairs of peaks in the CV of PEDOT in 1,1,1,3,3,3-hexafluoro-2-propanol (HFA) electrolytes, which was reported to be able to stabilize radical cations. Only at low temperatures can we find three reversible pairs of redox peaks in the ultramicroelectrode studies. We attribute the three peaks to three redox steps on conversion from the neutral state to polarons, from polarons to bipolarons, and finally from bipolaron to the metallic state, each step involving a one-electron transfer.

Introduction The nature of the doping-induced states in conjugated nondegenerate ground state polymers is still a matter of debate. As the highest level of doping attainable in conjugated polymers is of great importance for maximum charge storage in polymer electrodes, as are other excited states for the understanding of conjugated polymers in their applications as light emitting diodes and electrochemical sensors, this debate is of great importance. The charge in doped polythiophenes is understood to be stored in the form of polarons and bipolarons and at high doping levels also in a metallic state, but the nature of these states is not yet settled.1 Polythiophenes usually show two reduction peaks and at least one oxidation peak in their cyclic voltammograms (CV).2 The cause of this asymmetric CV has not been fully understood. Similar phenomena also exist in polypyrrole.3 Lukkari et al.4 observed two successive oxidation processes beneath the single oxidation peak in the CV for PMeT by cyclic spectrovoltammetry, but they explained the pre-peak as due to different conjugation lengths in the polymer. A large number of candidates for the different peaks have been suggested: different oxidation levels,2f different conjugation lengths,2g structural effects,5 structural relaxation and conformational changes,6 two different types of doping sites,3a,b swelling effects,2e chemical modification of the polymer,2i and resistivity changes.7 Recently, a reversible oxidation process regarded as π-dimers was found in oligothiophenes8 and also in polythiophenes.9 In some cases extreme environments, such as low-temperature SO2 electrolyte with a microelectrode array was used.10 In studying electrochemistry, ultramicroelectrodes have been used because of the advantages of rapid mass transport, small X

Abstract published in AdVance ACS Abstracts, August 1, 1996.

S0022-3654(96)00177-3 CCC: $12.00

double-layer capacitance, and small ohmic losses.11 They can be employed to study fast homogeneous and heterogeneous electron transfer kinetics and electrode reaction mechanisms with fast scan cyclic voltammetry, also in highly resistive solvents.12 Low-temperature experiments were usually done to study electron transfer reactions with associated conformational changes,13 in which butyronitrile is used as solvent. We have combined fast scan cyclic voltammetry and low temperature to understand the anomalous CV in polythiophenes. We found that there are three steps in the redox transformation of poly(3,4-ethylenedioxythiophene) (PEDOT). This is also true for poly(3-methylthiophene) (PMeT). When the polymer changes from neutral to doped, the charge carriers are generated first as polarons, then bipolarons, and finally a phase transformation into the metallic state occurs. Each of these steps involves a one-electron transfer. Experimental Section The electrochemistry was conducted in a standard onecompartment, three-electrode cell with a Pt foil counter electrode and an Ag/AgCl reference electrode (unless otherwise specified). A Pt point ultramicroelectrode (10 µm), polished prior to use, was used as the working electrode. PEDOT was deposited in 0.05 M EDOT and 0.1 M LiClO4/acetonitrile (MeCN) either by applying 1.1 V vs. Ag/AgCl, consuming a charge of 1.1 µC in 1 s, or by CV to get a very thin film. Similarly, PMeT was obtained in 0.1 M 3-methylthiophene in 0.1 M LiClO4/MeCN by applying 1.35 V, consuming a charge of 2.2 µC in 1 s, or by scanning from 0.0 to 1.6 V to get a very thin film. In 1,1,1,3,3,3-hexafluoro-2-propanol (HFA) electrolyte, an Ag wire pseudoreference electrode was used instead of an Ag/AgCl electrode. We used aluminum foil for covering the cell instead © 1996 American Chemical Society

Three-Step Redox in Polythiophenes

J. Phys. Chem., Vol. 100, No. 37, 1996 15203

Figure 2. Cyclic voltammograms in 0.1 M LiClO4/acetonitrile solution for PEDOT-LiClO4(MeCN) deposited on a Pt wire electrode. The scan rate is (a) 100 mV s-1; (b) 10 mV s-1. Figure 1. In situ absorption spectroscopy of PEDOT during electrochemical doping and undoping in 0.1 M LiClO4/MeCN solution. The polymer is first undoped at -1.0 V and then doped with a series of increasing potentials. The PEDOT was electropolymerized on indiumtin oxide (ITO) at 1.1 V in 0.05 M EDOT/0.1 M LiClO4/MeCN solution, consuming a charge of 30 mC/cm2.

of a Faraday cage. A BAS-100A electroanalytical system was used for sample preparation, electrochemical doping, and cyclic voltammetry. The experiments at low-temperature cooling by a mixture of acetone and liquid nitrogen were carried out on Autolab and General Purpose Electrochemical System (GPES) for Windows from Eco Chemie, The Netherlands. No calibration was done for the reference electrode at low temperature. Optical measurements were performed on a Perkin-Elmer Lambda 9 UV/VIS/NIR spectrophotometer. In situ measurements during electrochemical doping were done in a quartz cell for electropolymerized PEDOT on indium-tin oxide (ITO) coated glass. Results and Discussion 1. In Situ UV-VIS-NIR Spectroscopy of PEDOT at Different Oxidation Levels. The spectroscopic evolution of electropolymerized PEDOT in situ at different potentials in 0.1 M LiClO4/MeCN is shown in Figure 1. With increasing potential, the bandgap absorption peak at 2.2 eV decreases and three new absorption peaks grow, at 1.3 eV, possibly another at 1.5 eV, and a peak around 0.75 eV, which cannot be clearly seen because of the strong absorption of the solvent. A spectrum taken without electrolyte demonstrates this peak (Figure 1S). However, these peaks, at 1.3 and 1.5 eV, at higher potentials join at 1.5 eV, while the one around 0.75 eV grows even stronger. At even higher potential, above 0.4 V, the peak at 1.5 eV disappears and only the peak at low energy remains. The doping appears to occur in three stages, very similar to the case of PMeT2e or polythiophene.1d,2e That is, when the subgap absorption at 1.3, 1.5, and 0.75 eV appears and the bandgap absorption decreases, the polymer is probably in the polaron state. The bandgap absorption disappears, and subgap absorptions of 1.3 and 1.5 eV merge into one absorption at around 1.5 eV, indicating the presence of bipolarons. Finally, the absorption peak at 1.5 eV disappears, and the polymer appears to be in a heavily doped, metallic state, as indicated by the Drude-like optical properties in the NIR range. These three steps are completed at around -0.3, 0.2, and 0.5 V vs Ag/AgCl, respectively. The isosbestic point at 1.75 eV implies the transformation between two states. 2. Electrochemistry of PEDOT and PMeT at Different Electrodes and Electrolytes. We have found14 that electrochemical doping causes the PEDOT film first to contract and

Figure 3. Cyclic voltammograms in 0.1 M LiClO4/acetonitrile solution for PEDOT-LiClO4(MeCN) deposited on a Pt point microelectrode (10 µm): scan rate of (a) 500 mV s-1 on left axis; (b) 5 V s-1 on the right.

then to expand, at the doping potential around -0.5 V, independent of supporting electrolytes and counterions. To better understand this phenomenon, we began to study PEDOT’s electrochemistry carefully. Cyclic voltammograms of PEDOT, grown and characterized in LiClO4/MeCN, have two reduction peaks and a broad oxidation peak at normal scan rates, as shown in Figure 2a. However, we found that at the low scan rate of 10 mV s-1 (Figure 2b), the reduction peak around 0.1 V disappeared, and widely separated waves of oxidation at 0.2 V and reduction at -0.7 V are observed. In general, for a reversible electron transfer reaction, the peak separation is smaller at lower scan rates. The very large peak separation (around 0.9 V) therefore means that this is not one pair of redox peaks, because of the large deviation from the Nernst equation, but possibly two pairs of redox peaks in which intermediate unstable species cannot be seen at normal scan rates. We then chose an ultramicroelectrode to make use of fast scan CV. The CV of PEDOT-LiClO4/MeCN in monomer-free solution in Figure 3 clearly identified two pairs of possible redox peaks. One is around 0.0 V; the other is around -0.5 V (Figure 3a). The first oxidation product is relatively stable. With increasing scan rate (Figure 3b), an increase in the relative heights of the first to the second reduction peak, and also the first to the second oxidation peak, is observed. However, the second peaks become even weaker than the first peaks. We found that this observation also applied for potentiostatically prepared PMeT on a Pt ultramicroelectrode. Since PEDOT has no possibility of mislinking during polymerization nor the possibility of head-tail or head-head chain regioregular arrangement, we do not need to consider the possibility of a difference in chemical structure among the chains, as long as they form a conjugated chain. The two pairs of peaks may thus come from two forms of the material, such as two conjugation lengths, crystalline and amorphous phases,

15204 J. Phys. Chem., Vol. 100, No. 37, 1996 or any other forms in which one of two species can independently be responsible for one of the two redox pairs. These two redox peaks are not reversible, however, indicating the close relation between the two species. Depending on the relative peak heights at different scan rates, there must be one stable form in the doped or the undoped state. If there are two conjugation lengths and if their oxidation potentials have a difference larger than 0.5 V, there must be two lengths with different bandgaps. If these two lengths come from different polymer chains, they have to transform between each other by polymerization/depolymerization. This will require additional electron transfer. Since we obtained the undoped state with only one characteristic bandgap, and also three other states, none of which indicate a multiplicity of phases, we can surely exclude the possibility of two conjugation lengths. As for the order of the polymer, it was reported2a that electropolymerized PEDOT does not show any crystallinity. The chemically polymerized PEDOT has similar electrochemical properties on a macroelectrode. This material is partially crystalline, but we cannot say that crystallinity is responsible for one of the redox peaks. If the crystalline phase should be responsible for this, we would have found that the crystalline and amorphous phase will transform with each other. R. D. McCullough et al.15 claimed that self-oriented, highly ordered crystalline head-to-tail polythiophenes also have two reversible oxidation potentials, but these polymers show no amorphous phase. Other regioregular polyalkylthiophenes have also shown two reversible peaks.16 This also indicates that crystallinity is not responsible for one of the redox peaks. Another possibility is that of conformational change.6,17 Depending on the relative peak heights at different scan rates, if one conformation is stable in the doped state, then another conformation will be stable in the undoped state. Both forms will transform with each other, in the doped or undoped state. Even if these two forms have same optical absorption at room temperature, we should have found a corresponding change of optical absorption at high temperature for doped or undoped PEDOT (not shown here), related to a transition between these two forms. We find, however, that PEDOT is quite thermally stable. A last cause of the different redox peaks has been suggested in the form of the varying resistance of the polymer film during doping/undoping. As recently reported,18 the electrochemistry of PEDOT can also be performed without a supporting current collector. This is mainly due to unusually high conductivity, and thus also hole mobility, of PEDOT in the undoped state. In addition, the peak separation of the anodic and cathodic waves is almost independent of the scan rates, also indicating that the IR drop due to resistance can be excluded. For PEDOT on ultramicroelectrodes, we do not need to consider the small resistance modulation. If there are two redox processes only, with unstable intermediates, we would have found the first oxidation or reduction peak to increase in amplitude but not surpass the second reduction or oxidation peak at high scan rates. This is not the case. So we expect that there is a third process behind this behavior. Actually, there may be more than two electron transfers for the transition of a conjugated polymer from the neutral to the metallic state. We can assume that the first oxidation step converts the polymer from the neutral to the polaron state, and the second oxidation step from the polaron through the bipolaron to the metallic state. Since this second step involves more electrons than the first, we therefore cannot see the same peak heights in these two steps even at higher scan rates, up to the limit of our equipment. Theoretically, it

Chen and Ingana¨s

Figure 4. Cyclic voltammograms between -1.0 and 1.4 V of 0.05 M EDOT in 0.1 M LiClO4/MeCN solution on a Pt point microelectrode (10 µm). The scan rate is 40 V s-1.

should be possible to detect all electron transfer reactions by the corresponding CV peaks. We thus tried to make a very thin polymer film on the ultramicroelectrode by fast scan CV, and we obtained three reduction peaks and two oxidation peaks. We found a third reduction peak around 0.5 V and another two at 0 and -0.6 V in the CV during PEDOT growth (Figure 4) and its characterization (Figure 2S) in monomer-free solution. These data are in good agreement with our results from optical spectroscopy. For PMeT (Figure 3S) we also found three reduction peaks at 0.15, 0.45, and 0.8 V vs Ag/AgCl and two oxidation peaks at 0.3 and 0.6 V in which the three peaks have almost the same peak heights. Since Lukkari et al.4 have found two oxidation processes very close to each other in one broad single oxidation peak, where we also have found only one broad peak but expected two, we suggest that there are also two oxidation processes in the second broad peak in our case. The reason for only one broad oxidation peak, not the two expected, may be due to the unstable intermediate or to the very close potentials which may defy the appearance of individual oxidation process. Since the three reduction peaks are almost of the same height, these three oxidation processes may possibly involve a one-electron transfer in each step. It was reported that HFA can stabilize the radical cation used in EPR (electron paramagnetic resonance) and in anodic electrochemistry.19 We sought to use it for characterizing PEDOT with an ultramicroelectrode. The result shows an illresolved CV on a Pt point ultramicroelectrode, possibly with three pairs of redox peaks. The peak positions have some shifts compared with the earlier results. This is not as clear as we desired; although HFA can stabilize radical cations, in our case it may not be able to stabilize a special cation with three positive charges. Surprisingly EDOT cannot grow continuously in HFA electrolyte by electropolymerization. This also means that HFA changes the stability of some growing radical cations, hindering the polymerization. We thus attempted a low-temperature electrochemical study, expecting the intermediate to be more stable in this case. CV of PEDOT on Pt wire in Figure 5 at -78 °C shows three reduction peaks and two oxidation peaks. Only on Pt ultramicroelectrode at low temperature can we find three reversible pairs of redox peaks, as shown in Figure 6. Very recently Sato et al.20 have obtained a result similar to ours in hexylsexithiophene and determined the presence of a trication with 50% charge (one charge per two monomers). There are also three quasireversible redox waves found in some oligothiophenes.21 It was reported that in PMeT22 a doping level as high as 52% was reached. The high doping levels in polythiophenes may imply the possible existence of trications in six thiophene rings.

Three-Step Redox in Polythiophenes

Figure 5. Cyclic voltammograms in 0.5 M Bu4NClO4/BuCN at -78 °C for PEDOT on a Pt wire at a scan rate of 100 mV s-1. The polymerization is carried out in a 0.05 M EDOT/0.05 M Et4NClO4/ MeCN solution by applying a potential of 1.1 V.

Figure 6. Cyclic voltammograms in 0.5 M Bu4NClO4/BuCN at -78 °C for PEDOT on a Pt point microelectrode (10 µm) at a scan rate of 20 V s-1. The polymerization is carried out in a 0.05 M EDOT/Et4NClO4/MeCN solution by scanning from 0.0 to 1.5 V at a scan rate of 10 V s-1.

This concept of trications in a conducting polymer is not understood within the current polaron/bipolaron model. The very high doping levels discussed here have been observed, however, by other methods. In a study of Cs vapor deposition on oligomers of phenylene, doping levels of one charge per nine carbons were observed.23 Also, at high doping levelssmore than one charge for eight carbonsspolyalkylthiophene can reach into the metallic state.24 Multistep redox processes in oligomers have been observed.10 The charge injection mechanism depends sensitively upon the structure of the oligomer. In these cases the multistep redox process is attributed to the weak conjugation between individual mers along the oligomer chain, due to torsion caused by the significant steric hindrance. The phase transition from the insulating to the metallic state, which is not yet understood, may therefore be the formation of a high doping level, washing out all the local structure of the polaron/bipolaron states. The structural conditions for the the stability of intermediate states, however, are unclear at present. We are not aware of the relation between π-dimers and the bipolaron in this discussion. In summary, we have first found a three step redox process, the evidence coming from fast scan and low-temperature electrochemistry with a conducting polymer deposited on a Pt ultramicroelectrode. This sequence of redox steps is probably a general property of conducting polymers. The polaron/ bipolaron concept that comes from semiconductors should be modified when applied to the conducting polymers. The exact physics of these states need further theoretical explanation. References and Notes (1) (a) Heeger, A. J.; Kivelson, S.; Schrieffer, J. R.; Su, W.-P. ReV. Mod. Phys. 1988, 60, 781. (b) Roncali, J. Chem. ReV. 1992, 92, 711. (c) Moraes, F.; Chen, J.; Chung, T.-C.; Heeger, A. J. Synth. Met. 1985, 11,

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Chen and Ingana¨s (22) (a) Tourillon, G.; Garnier, F. J. Polym. Sci., Polym. Phys. Ed. 1984, 22, 33. (b) Bach, C. M.; Reynolds, J. R. J. Phys. Chem. 1994, 98, 13636. (23) Ramsey, M. G.; Schatzmayr, M.; Stafstro¨m, S.; Netzer, F. P. Europhys. Lett. 1994, 28, 85. (24) Dyreklev, P.; Ingana¨s, O. Submitted. (25) We thank Bayer Inc. for providing the EDOT monomer and acknowledge financial support from the Swedish Research Council for Engineering Sciences (TFR).

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