Bipolar properties of polythiophene derivatives with 1,3,5-triazine units

Electrochimica Acta 109 (2013) 395–402

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Bipolar properties of polythiophene derivatives with 1,3,5-triazine units Przemyslaw Ledwon a,∗ , Sandra Pluczyk a , Krzysztof R. Idzik c,d , Rainer Beckert c , Mieczyslaw Lapkowski a,b,∗,1 a

Silesian University of Technology, Faculty of Chemistry, Strzody 9, 44-100 Gliwice, Poland Centre of Polymer and Carbon Materials, Polish Academy of Sciences, Curie-Sklodowskiej 34, 41-819 Zabrze, Poland c Institute of Organic and Macromolecular Chemistry, Friedrich-Schiller University Jena, Humboldstrasse 10, 07743 Jena, Germany d Geoscience Centre of the University of Göttingen, Department of Applied Geology, Goldschmidtstr. 3, 37077 Göttingen, Germany b

a r t i c l e

i n f o

Article history: Received 21 May 2013 Received in revised form 18 July 2013 Accepted 24 July 2013 Available online 6 August 2013 Keywords: Bipolar Triazine Spectroelectrochemistry Polythiophenes ESR n-Doping

a b s t r a c t Bipolar properties of model polythiophene derivatives with 1,3,5-triazine units are investigated. The conjugation length is controlled by the meta substitution. In situ spectroelectrochemical UV–Vis–NIR and ESR measurement recorded during electrochemical reduction and oxidation are presented. Spectroelectrochemistry recorded during reduction of polymers in the solid state is especially interesting. Most literature data of spectroelectrochemistry of solid state polymers has been obtained during oxidation which is due to poor stability of radical anions in reduced polymers. Cathodic reduction leads to the formation of stable radical anions in a certain potential range. Exceeding this range leads to overreduction. A similar effect is observed during electrooxidation. Obtained results indicate that not only oligomers with 1,3,5-triazine units but also respective polymers are worth considering during studies on new bipolar polymer materials to broad range of optoelectronic and photovoltaic applications. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Organic -conjugated materials are an issue of interest in numerous studies owing to their inherent optoelectronic properties and their potential applications such as organic light-emitting diodes (OLEDs) [1,2], light-emitting electrochemical cells (LECs) [3], photovoltaic cells [4–6], organic thin film transistors (OFETs) [7,8] and sensors [9]. Two types of organic semiconductor can be distinguished low and high molecular weight. Depending on application both types of materials have certain advantages [10]. Increasing popularity of compounds with donor and acceptor groups in one molecule as a promising materials for optoelectronic and photovoltaic applications is observed [11]. For example in bulk heterojunction organic solar cells the donor–acceptor configuration considerably improves photoinduced charge separation in the excited state [12]. In order to improve electron transport properties, this type of materials is used in OLEDs [13] and OFETs [6].

∗ Corresponding authors at: Silesian University of Technology, Faculty of Chemistry, Strzody 9, 44-100 Gliwice, Poland. Tel.: +48 322371305; fax: +48 322371925. E-mail addresses: [email protected], [email protected] (P. Ledwon), [email protected] (M. Lapkowski). 1 The ISE member. 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.07.171

The most popular electron-accepting units are s-tetraazines [14], 1,3,5-triazines [15,16], naphthaleneimides [17], perylenediimides [18,19] and benzothiadiazoles [20,21]. 1,3,5-Triazine is a strong electron-accepting unit with good thermal stability and luminescence properties [22,23]. Radical anion can be effectively stabilized at this moiety and hence reversible reduction of this type of materials is observed [24]. It was found that triazine derivatives exhibit high electron affinities and reach LUMO values in the range of −2.7 to −3.1 eV [25]. 1,3,5Triazine is an important unit in low [26–28] and high molecular weight donor–acceptor conjugated molecules. The use of central 1,3,5-triazine core leads to the molecule with star-shaped architecture and C3-symmetry [29,30]. Molecules of this type were also tested as -conjugated columnar liquid crystals with bipolar charge carrier transport [31,32]. Basic electrochemical properties of different star-shaped 1,3,5triazines with thiophene derivative arms are described and show that electrochemical oxidation leads to the formation of a conjugated polymer [33]. The electrooxidation of star shaped monomers leads to the hyperbranched polymer with great electrochemical deviation from the linear analogues [34]. It was found that the combination of electron-rich thienyl group and the triazine-based system through the -system can be an efficient strategy to design new materials with strengthened third-order nonlinear optical

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S S

S

S

S S S

S

S

S

N

N N

S

S

N

N

N S

S S

S S S

6TPh

6TA 6T3PhA Fig. 1. Monomer structures.

2. Experimental 2.1. Materials The synthesis, structures and basic properties of monomers have been characterized in one of our previous papers [37]. They are triazines: 2,4,6-tris(2,2 -bithiophen-5-yl)-1,3,5-triazine (6TA), 2,4,6tris[p-(2,2 -bithiophen-5-yl)-phenyl]-1,3,5-triazine (6TtPhA) and benzene analogue 1,3,5-tris(2,2 -bithiophen-5-yl)-benzene (6TPh). Structures are shown in Fig. 1. 2.2. Measurements Electrosynthesis and studies on polymer films were performed on CH Instrument Electrochemical Analyzer model 600. Measurement was carried out in dichloromethane (CH2 Cl2 ; Sigma Aldrich ≥99.9%) or acetonitrile (CH3 CN; Sigma Aldrich ≥99.8%) containing 0.1 M tetrabutylammonium hexafluorophospate (Bu4 NPF6 ; Sigma Aldrich 98%) as a supporting electrolyte. The target polymer films were synthesized on the platinum wire or indium–tin-oxide (ITO) coated quartz electrode at a scan rate of 50 mV/s. An Ag pseudo-reference electrode was used and its exact potential was calibrated versus ferrocene/ferrocinium redox couple. Platinum wire served as a counter electrode. Appropriate vessels adapted for electrochemical cells were used. Spectral measurements were carried out using UV–vis Hewlett Packard spectrophotometer 8453 as well as JEOL JES-FA 200, X-band CW-EPR spectrometer, operating at 100 kHz field modulation. Concentration of paramagnetic species was estimated by double integration of the first-derivative ESR spectra. All electrochemical and spectroelectrochemical experiments were carried out in deaerated solutions and an additional argon cushion was maintained above the solutions.

3. Results and discussion 3.1. Electrochemical properties All investigated polymers were obtained by cyclic voltammetry. To elucidate the role of 1,3,5-triazine unit in the polymer backbone, electrochemical properties of this type of polymer were compared with its benzene analogue. All monomers polymerize at potentials related to the first oxidation peak. Fig. 2 presents the voltammogram recorded during electropolymerization of 6T3PhA. The first oxidation peak is located at approximately 0.65 V. In second scan at lower potentials a new peak appears, which increases with every subsequent scan. This indicates the formation of a conductive film on the electrode surface and the formation of products with higher conjugation, probably conjugated polymers. Similar results were obtained for electropolymerization of 6TA and 6TPh. Electrochemical properties of the studied polymers were investigated by cyclic voltammetry in a monomer free electrolyte solution. Measurements were made in broad range, in order to register reduction and oxidation peaks (Fig. 3). The value of onset oxidation potential is only slightly different from approximately 0.17 V for poly(6TPh) to approximately 0.26 V for 1,3,5-triazine derivatives. This is related to the oxidation of quaterthiophene segments. Larger differences are observed in the case of cathodic curves. The first reduction peak of poly(6TA) is located at approximately −1.5 V and the second at −1.95 V. After changing the potential sweep direction only one oxidation peak at −1.74 V is observed.

25 20 15

current / μA

properties [35]. The 1,3,5-triazine unit has also been incorporated into the polymer backbone in order to improve n-transport in OLEDs [36]. The objective of this study is to clarify the n- and p-doping processes taking place in conjugated polymers based on polythiophenes as well as to elucidate the role of 1,3,5-triazine units. Such donor–acceptor systems gain attention, however mainly as low molecular weight oligomers. The spectroelectrochemical properties of this type of conjugated polymers in the solid state have not been investigated in details so far. In this paper we characterize two polythiophene derivatives with 1,3,5-triazine core by electrochemical and spectroelectrochemical techniques. Moreover, we compare the electrochemistry of these polymers with benzene analogue.

10 5 0 -5 -10 -0.8

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potential ver. ferrocene / V Fig. 2. Cyclic voltammetry of 6T3PhA; potential sweep rate 50 mV/s; 1 mM monomer solution in 0.1 M Bu4 NPF6 in CH2 Cl2 .

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trapping caused by poor electric conductivity of conjugated polymers in the neutral state. A similar effect was observed in the case of the anodic curve after reduction. The second scan significantly differs from the first. A new sharp peak at 0.3 V appears. This is not observed during multiple scanning in the range from −0.5 V to 0.8 V. The cyclic voltammetry of poly(6T3PhA) shows the clear reduction peak (−2.2 V) at lower potential than poly(6TA). After changing the potential sweep direction the oxidation peak at approximately −1.93 V appears. It is related to the dedoping process. The second and subsequent scans almost overlap in the anodic as well as in the cathodic range. This indicates that the reduction and oxidation of poly(6T3PhA) are reversible. The reduction of poly(6TPh) begins at about −2.2 V and the value of the current increases sharply. After changing the potential sweep direction only a barely noticeable counterpeak is visible. From the second scan in every subsequent scan the current decreases. This indicates occurrence of irreversible reduction. The comparison of cyclic voltammetry of polymers with 1,3,5triazine units and polymer without these units clearly shows that 1,3,5-triazine moieties significantly improve the stability of radical cations in conjugated polymer backbone. To support this conclusion additional measurements, during multiple doping and dedoping of both poly(6TA) and poly(6T3PhA) were made (Fig. 4). After 100 scans in the restricted cathodic range, both polymers retained their electroactivity. The redevelopment of voltammetric curves is observed. 3.2. ESR spectroelectrochemistry

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potential (vs. Fc/Fc+) / V Fig. 3. Cyclic voltammetry of polymer films of: (a) poly(6TA); (b) poly(6T3PhA); (c) poly(6TPh). Polymer films at Pt electrode; potential sweep rate 100 mV/s; 0.1 M Bu4 NPF6 in CH3 CN.

The redox couple −1.95 V and −1.74 V is related to the doping and the dedoping of the polymer film. No counterpeak to the first reduction peak at −1.5 V is observed. Scanning only in the cathodic range results in the absence of this peak. This characteristic effect is well known in electrochemistry of conjugated polymers in the solid state. Such an effect is attributed to the charge

In order to investigate donor and acceptor character of poly(6TA) and poly(6T3PhA) in situ ESR measurement during electrochemical oxidation and reduction were performed. All spectra were recorded in potentiostatic conditions. The ESR spectra of poly(6TA) and poly(6T3PhA) show that in the potential range associated with the reduction (Fig. 5) or oxidation (Fig. 6) of polymers a single line appears. It is related to the delocalisation of the radical cations or radical anions along the polymer backbone. At higher potentials basic parameters of ESR lines such as line height and width are changing. In order to elucidate these relations ESR spectra were analyzed in details. The relative concentration of spins and linewidth of ESR signals recorded at different potentials are shown in Fig. 7. ESR spectra of poly(6TA) were recorded during cathodic doping and dedoping in the range from −1.4 V to −1.9 V. At the beginning the relative concentration of spins is low and stable. This is caused by spin-bearing species trapped in the film of electrochemically generated polymer. At the potential close to the polymer reduction onset (estimated from cyclic voltammetry) the relative concentration of spins begins to grow. It is related to the appearance at the ESR spectrum of a poorly visible line. With decreasing potential the relative concentration of spins increases up to −1.875 V. The plots with linewidth of ESR signals were made in order to investigate the interaction between paramagnetic species. The lower the linewidth the stronger exchange interactions take place between radicals. The linewidth of the ESR signal narrows at low doping levels up to the potential −1.75 V and then broadens. These results imply fast interchange between spinbearing radical anions at low doping levels. The slower interchange at higher doping levels can be caused by increasing interaction between different charge carriers and hence slower exchange taking place between radicals [38–41]. Inversion of the direction changes of potential steps at −1.9 V results in a slight increase of spin concentration and narrowing of the ESR signal. At the higher potential the hysteresis in plots of relative concentration of spin and linewidth of ESR signal is observed. These results compared

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Fig. 4. Cyclic voltammetry of polymer films during multiple doping and dedoping in cathodic range, 1st, 2nd and 100th scans of: (a) poly(6TA); (b) poly(6T3PhA). Polymer films at Pt electrode; potential sweep rate 100 mV/s; 0.1 M Bu4 NPF6 in CH3 CN.

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First derivative ESR spectra [a.u.]

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Magnetic field - Bo [mT]

Fig. 5. Selected ESR spectra recorded during n-doping and dedoping (from bottom to top) of: (a) poly(6TA); (b) poly(6T3PhA). Polymer films at Pt electrode in 0.1 M Bu4 NPF6 in CH3 CN.

with charge trapping revealed in voltammetric curves of poly(6TA) imply that doping in the cathodic range is much faster than dedoping. In the potential range from −0 V to 0.25 V the relative concentration of spins in poly(6TA) is low. At the potential close to the polymer oxidation onset (estimated from cyclic voltammetry) the relative concentration of spins begins to grow. Spin concentration is increased up to 0.7 V indicating the formation of spin-bearing polarons. At higher potentials bipolarons start to dominate. Up to the potential 0.55 V the narrowing of ESR line is observed. After exceeding this potential the line is slightly broader. Similar characteristic features present ESR plots of poly(6T3PhA). The relative concentration of spins increases in the range from 0.25 V to 0.65 V. This plot is well correlated with the cyclic voltammogram. The increase of relative concentration of paramagnetic species is related to the first oxidation wave. The first wave is fully reversible. Additional cyclic voltammetry and ESR experiments demonstrate that after exceeding 0.65 V doping is only partially reversible. It suggests

the formation of stable species at low oxidation level and less stable species at high oxidation level. The g-factor brings information about the environment of radicals. Herein g-factor values scaled from Mn lines are collected in Table 1. Values of the g-factor of both polymers in their oxidized form are almost the same. This indicates the distribution of the radical cation on the quaterthiophene segment. A clear difference between the g-factor of radical cations and radical anions implies that the distribution of radicals is different in the oxidized and reduced state. Strong shift of g-factor indicates the distribution of radical anion mainly on 1,3,5-triazine units. The experimental Table 1 g-Factor of paramagnetic species recorded during n- and p-doping.

n-Doping p-Doping

Poly(6TA)

Poly(6T3PhA)

2.00413 2.00266

2.00337 2.00264

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Magnetic field - Bo [mT]

Fig. 6. Selected ESR spectra recorded during p-doping and dedoping (from bottom to top) of: (a) poly(6TA); (b) poly(6T3PhA). Polymer films at Pt electrode in 0.1 M Bu4 NPF6 in CH3 CN.

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Fig. 7. Concentration of paramagnetic centres (black) and linewidth of ESR lines (red) as a function of applied potential recorded during n-doping and dedoping (top); p-doping and dedoping (bottom) of: (a) poly(6TA); (b) poly(6T3PhA). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

P. Ledwon et al. / Electrochimica Acta 109 (2013) 395–402

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Fig. 8. Selected UV–Vis–NIR spectra recorded during reduction (n-doping) of: (a) poly(6TA); (b) poly(6T3PhA). Polymer films at ITO electrode in 0.1 M Bu4 NPF6 in CH3 CN.

results are in accordance with theoretical calculations of spin density of 1,3,5-triazine molecules [34]. 3.3. UV–Vis–NIR spectroelectrochemistry

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UV–Vis–NIR spectra of polymer films deposited on an ITO electrode were recorded in situ during electrochemical oxidation and reduction. All spectra were recorded in potentiostatic conditions after reaching the equilibrium. This procedure eliminates charge trapping in the electrodeposited polymer film. Polymer films were electrochemically deposited on ITO electrode. UV–Vis–NIR spectroelectrochemistry can bring information about charge carriers formed during doping processes and clarify doping processes. UV–Vis–NIR spectra recorded during reduction of poly(6TA) and poly(6T3PhA) are presented in Fig. 8. Neutral poly(6TA) has two absorption maxima of –* transition at approximately 380 nm and 500 nm with a long arm extending to the wavelength exceeding 600 nm. At the potential of −1.6 V the absorbance of the neutral polymer begins to decrease. Simultaneously a new, broad absorption band appears with the maximum at near-infrared. This is simultaneous with the increase in measured spin concentration in the sample. This is attributed to the formation of polarons (radical anions). The application of more negative potential results in further diminishing of the peak at 500 nm, shifting of higher energy peak to 365 nm, while the isosbestic point located at 571 nm. The clear isosbestic point up to −1.8 V and reversibility of these changes indicate good stability of poly(6TA) in this condition (see supporting materials). After exceeding −1.85 V the disappearance of peak at 365 nm is observed. Simultaneously a small peak below

300 nm appears. Irreversible changes after exceeding −1.85 V indicate overreduction of the conjugated system. The UV–Vis–NIR spectrum of poly(6T3PhA) in neutral state contain overlapping bands with peaks located at approximately 480 nm and 396 nm. The trend of poly(6T3PhA) spectra recorded during reduction is similar to poly(6TA). First change is recorded at slightly lower potential. At −1.95 V new bands appear. Isosbestic point at 552 nm is stable up to −2.05 V. After exceeding this potential a sharp drop of the peak at 480 nm is observed and the decrease of the peak at 396 nm. These results indicate that 1,3,5-triazine is a good electron acceptor unit in the polythiophene chain. Control of applied potential as well as the use of anhydrous and deoxygenated environment are crucial parameters which determine the possibility to use 1,3,5-triazine in conjugated polymers as a unit which stabilizes the radical anion. Oxidation of poly(6TA) results in the decrease of the peak located at approximately 500 nm originating from the absorbance of the neutral polymer (Fig. 9). Simultaneously, at least two, red-shifted absorption bands appear as a manifestation of the absorbance of the doped polymer. The first is located at approximately 730 nm. The second broad band is located in the near-infrared at approximately 1010 nm and extends beyond the measured range. After exceeding 0.7 V, an isosbestic point, located at 548 nm, slightly shifts and the peak at 642 nm increases. These results correlate well with ESR measurements. These changes at the beginning of oxidation are attributed to the absorption of radical cations (polarons). Further oxidation leads to the formation of other spin-less species probably dications (bipolarons).

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0.55 V 0.6 V 0.65 V 0.7 V 0.75 V 0.8 V 0.85 V

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Fig. 9. Selected UV–Vis–NIR spectra recorded during oxidation (p-doping) of: (a) poly(6TA); (b) poly(6T3PhA). Polymer films at ITO electrode in 0.1 M Bu4 NPF6 in CH3 CN.

P. Ledwon et al. / Electrochimica Acta 109 (2013) 395–402

Changes between spectra recorded at subsequent potentials during oxidation of poly(6T3PhA) are more visible than in the case of spectroelectrochemistry of poly(6TA). At higher potentials (more than 0.6 V) the peak at 480 nm completely disappears. This can indicate that all chromophores responsible for this peak undergo oxidation after exceeding 0.6 V.

[12] [13]

[14]

4. Conclusions [15]

In situ spectroelectrochemical UV–Vis–NIR and ESR were made in order to explain the doping and dedoping processes taking place during electroreduction and electrooxidation of bipolar solid state conjugated polymers. These results imply some similarities as well as differences between doping processes in the cathodic and anodic range. Polymers are stable in a certain potential range for both doping types. The interaction between charge carriers changes during redox processes, which is manifested as narrowing of ESR lines at low doping levels and then broadening at higher doping levels. Clear differences in the g-factor of cation and anion radicals indicate their different distribution. The charge trapping recorded by cyclic voltammetry and hysteresis in ESR plots implies that doping in the cathodic range is much faster than dedoping. Electrochemical and spectroelectrochemical measurements confirm that 1,3,5-triazine units serve well as anion stabilizing moieties in the polythiophene matrix. Acknowledgements This work was supported by a research grant no. 2011/01/N/ST4/02251 from National Science Centre, Poland. P. Ledwon and S. Pluczyk are scholars in SWIFT project POKL.08.02.0124-005/10 which is co-financed by European Union within European Social Fund. This work was also partially realized within the European Union Project (SNIB, MTKD-CT-2005-029554).

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Appendix A. Supplementary data [26]

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2013.07.171.

[27]

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