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The effect of the linking topology on the electrochemical and spectroelectrochemical properties of carbazolyl substituted perylene bisimides
Electrochimica Acta 135 (2014) 487–494
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The effect of the linking topology on the electrochemical and spectroelectrochemical properties of carbazolyl substituted perylene bisimides Sandra Pluczyk a , Wojciech Kuznik b , Mieczyslaw Lapkowski a,c,∗,1 , Renji R. Reghu d , Juozas V. Grazulevicius d a
Silesian University of Technology, Faculty of Chemistry, Strzody 9, 44-100 Gliwice, Poland Institute of Electronics and Control Systems, Faculty of Electrical Engineering, Czestochowa University of Technology c Centre of Polymer and Carbon Materials, Polish Academy of Sciences, Curie-Sklodowskiej 34, 41-819 Zabrze, Poland d Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu pl. 19, LT-50254, Kaunas, Lithuania b
a r t i c l e
i n f o
Article history: Received 25 February 2014 Received in revised form 25 April 2014 Accepted 7 May 2014 Available online 21 May 2014 Keywords: Ambipolar Perylene diimide Carbazole Spectroelectrochemistry
a b s t r a c t We report on electrochemical and spectroelectrochemical characteristics of two isomeric, carbazolyl substituted perylene bisimides and their polymers. Investigations were carried out in order to determine the effect of the linking topology on their properties. Cyclic voltammetry (CV) measurements of the investigated monomers showed typical two-step reduction of the perylene diimide moiety. The electrochemical oxidation of the studied compounds gave the corresponding polymer, which was deposited on the electrode. The resulting films were also examined by cyclic voltammetry. Investigation indicated both: n-type and p-type doping of polymers. Analysis of redox process of the monomers and polymers was carried out using UV-Vis and EPR spectroscopy. Our studies point to differences in the kinetics of the reduction process, depending on the substitution of the carbazole moiety. Although both monomers easily undergo of electrochemical polymerization, an effect of the linking topology of carbazolyl substituted perylene bisimides on the polymerization process was evidently noticed. Monomer PE2 as well as PE3 led to stable polymers, but polymer based on PE3 displays the conjugation in a longer extend than that based on PE2, where carabazole is linked to perylene diimide at less favorable position 2. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction In the age of rapidly developing technology, there is a necessity to search for compounds with better properties, meeting the increasingly sophisticated requirements. Nowadays, one of the rapidly growing sectors with major potential is organic electronics [1,2], which involves organic materials in such devices as light emiting diodes (OLEDs)[3], light-emitting electrochemical cells (LECs) [4], photovoltaic cells [5–7], organic thin film field effect transistors (OFETs) [8,9] and sensors [10]. The main advantage of organic semiconductors over conventional conductive materials is their flexibility and wide range of optical, optoelectronic and photo- and electroluminescent properties [11].
∗ Corresponding author. 44-100 Gliwice, Strzody 9, Poland, Tel.: +48 322371509; fax: +48 322371509. E-mail address: [email protected] (M. Lapkowski). 1 1 the ISE member http://dx.doi.org/10.1016/j.electacta.2014.05.057 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
The previous investigation of organic, conjugated compounds which can be used as semiconductors in (opto)electronic systems are mainly related to the group of p-type semiconductors. Now, intense researches are focused on n-type semiconductors and ambipolar materials which are able to transport both kinds of charge carriers i.e. holes and an electrons. Despite of that fact, those types of compounds still lags behind p-type systems [12,13]. Among large number of conjugated materials, aryl imide derivatives appear to represent a promising group of organic compounds in the aspect of (opto)electronic applications mainly because of their good thermal and chemical stability. Additionally, due to their properties, the high value of electron affinity (strong electron withdrawing character), a low LUMO level and a good reversibility of the reduction process, aryl imide derivatives are the most important blocks for the preparation of n-doped materials [14–17]. There are known numerous perylene diimide derivatives with an excellent charge carrier transport properties, what makes them good candidates for the application in electronic and optoelectronic devices [18–28].
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On the other hand carbazole and its derivatives are widely used hole-transport (p-type semiconductor) materials, which also possess many other interesting properties. Carbazole-based compounds are known for their excellent photorefractive properties, intensive fluorescence and electroluminescence, high thermal and photochemical stability [29]. Carbazole derivatives are also able to form relatively stable radical cations. Because of that, compounds containing carbazole moieties are capable of electrochemical polymerization and formation of conductive layers during this process [30,31]. It is interesting to note that depending on the substitution of the carbazole ring, devices based on carbazole derivatives exhibit different properties. It was observed that, devices fabricated from 2,7-substiduted carbazole derivatives demonstrate better results than those prepared using 3,6-substituted derivatives [32]. Taking into account all the facts described above we studied the possibility of obtaining ambipolar polymers from perylene diimide derivatives directly substituted in 1,7 positions with carbazole groups via the electrochemical method and estimated the effect of the substitution pattern on the electrochemical and spectroelectrochemical properties of the obtained layers. It is worth of noting that such derivatives of perylene diimide and carbazole have not been yet extensive studied. Ambipolar polymers based on imide derivatives described in literature, were mainly obtained by chemical polymerization. There are no reports on electrochemical polymerization of such compounds. This fact motivated us to take this topic. 2. Experimental details 2.1. Materials The synthesis and basic properties of the investigated compounds were described in the earlier published paper [33]. Their chemical structures are shown in Fig. 1. All the measurements were carried out in supporting electrolyte which was 0,1 M solution of tetrabutylammonium hexafluorophospate (Sigma Aldrich 98%) in dichloromethane (Sigma Aldrich ≥99.9%). Electrodeposition and electrochemical characterization of the studied monomers was performed in 1 mM solution of a corresponding compound in the supporting electrolyte. Spectroelectrochemical investigations were made in 10−5 M solution of each monomer. The studies of
electrodeposited films were carried out in monomer-free solutions of the supporting electrolyte. 2.2. Instrumental characterization Electrochemical measurements were performed on Ecochemie AUTOLAB potentiostat-galvanostat model PGSTAT20. The obtained data were analyzed using the GPES program. Cyclic voltammetry (CV) was used for the electrochemical characterization. The typical three-electrode cell was applied. The platinum wire was used as working electrode, the platinum spiral was employed as an auxiliary electrode and the silver wire was used as a pseudo reference electrode which was calibrated versus ferrocene/ferrocinium redox couple. The solutions were purged with argon. Spectral measurements were carried out using UV-Vis Hewlett Packard spectrophotometer 8453 and JEOL JES-FA 200, X-band CW-EPR spectrometer operating at 100 kHz field modulation. Spectroelectrochemical investigations were made by connected spectrometers described above with OMNI potentiostat. UV-Vis measurements during reduction processes were carried out in a 2 mm quartz cell equipped with a platinum mesh as a working electrode, a platinum spiral as a auxiliary electrode and a silver wire as a pseudo reference electrode. UV-Vis measurements of oxidation processes which lead to the formation of the films on the surface of working electrode were performed with employed indium-tine-oxide (ITO) coated quartz electrode as working electrode. Investigations of redox process of polymer films by UV-Vis measurements were carried out with a ITO working electrode coated with films, which were obtained by electropolymerization. EPR measurements were carried out in a cylindrical cell equipped with set of electrode like in the case of electrochemical measurements. The spectra of the radical anions of investigated monomers were recorded during potentiostatic reduction. Investigations of polymer films by EPR measurements were carried out with the working electrode coated with films, which were obtained by electropolymerization. 3. Results and discussion. In this work the electrochemical and spectroelectrochemical characterization of the monomers and of their polymers obtained by electrochemical deposition on the surface of working electrodes was performed. The chemical structures of the investigated compounds give the opportunity to obtain ambipolar polymers capable of both n and p doping. The main purpose of this work was estimation of the influence of the linking topology of the carbazole units to the perylene core on the electrochemical and spectroelectrochemical properties of the compounds and of the electrochemically obtained polymers. 3.1. Electrochemical characterization
Fig. 1. Chemical structures of investigated monomers.
The basic electrochemical properties of the monomers were described in the former paper [33]. In this report we focus on the process of electropolymerization and on investigation of the obtained films. Both the investigated monomers undergo processes of electrochemical oxidation as well as reduction. The voltammograms of the monomers show that oxidation process starts at 0.71 V for PE2 and at 0.62 V for PE3. The potential of 0.71 corresponds to the value of ionization energy (IE) of 5.51 eV for PE2. The IE value of PE3 was found to be a little lower (5.42 eV). This observation indicates more extended conjugation in PE2 molecule than in PE3. Analysis of the shape of the monomer voltamograms revealed the wider peaks for PE3 monomer. In terms of the shape of cyclic voltammetry curves, the reduction processes for both compounds are very
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Fig. 2. Electropolymerization of PE2 (left) and PE3 (right) in 0.1 M Bu4 NPF6 dichloromethane solution.
Fig. 3. Cyclic voltammograms of polyPE2 (left) and polyPE3 (right) in 0.1 M Bu4 NPF6 dichloromethane solution.
similar. Two-stage reversible reductions typical for the derivatives of perylene diimide were observed [34,35]. For monomer PE2 the onset potential of the first step of reduction occurred at −1.03 V with the maximum at −1.15 V and −1.36 V. For PE3 the reduction onset occurred at-1.15 V with maxima at −1.26 V and −1.44 V. Higher oxidation potential and higher reduction potential of PE2 compared with those of PE3 makes the values of electrochemical band gap energy almost the same in both cases (1.74 eV for PE2 and 1.77 eV for PE3). Both monomer PE2 and monomer PE3 underwent electrochemical polymerization. After the first scan, when the monomers were oxidized, consecutive CV scans indicated the formation of electroactive layers on the surface of working electrodes (Fig. 2). During the second scan new peaks appeared at 0.51 V for PE2 and at 0.52 V for PE3 and grew continuously in the subsequent cycles. The position of the new peak maximum was stable (its potential did not change during the electropolymerization process) for PE2, whereas for PE3 the potential of the growing peak increased from 0.52 to 0.65 V in the subsequent scans. The peaks at the negative potentials also grew and merged into one wide peak. In the case of electropolymerization of PE2 increasing peaks at the positive potential range were clearly separated. It was not the case for electropolymerization of PE3. The reversible redox couples at the positive potential range can be distinguished in cyclic voltammetry curves of the polymeric films (Fig. 3). The oxidation onsets were observed at 0.18 V for polyPE2 and at 0.21 V for polyPE3. Thus the values of oxidation potentials are almost the same for the both polymeric layers. The values of ionization energy were also almost the same (4,98 eV for polyPE2 and 5.01 eV for polyPE3). The difference was observed in
the processes of reduction. The reduction onset of polyPE2 occured at −1.25 V, while the corresponding value for polyPE3 was found to be −1,37 V. In addition the shapes of the curves recorded during n-doping and n-dedoping were found to be different. Two-stage reduction merged into a single broad peak in the case of polyPE2. On the voltammograms of polyPE3, two peaks could be distinguished at the same conditions. The decrease in scan rate up to 5 mV s−1 allowed to identify two peaks in the anodic cycle of polyPE2 (Fig. 4). Thus, kinetics of reduction process varies depending on the pattern of substitution of the carbazole unit.
Fig. 4. Cyclic voltammogram of polyPE2 recorded at 5 mV s−1 potential sweep rate.
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Fig. 5. UV-Vis spectra recorded during reduction of 10−5 M solution of PE2 (left) and PE3 (right) in 0.1 M Bu4 NPF6 /dichloromethane. Table 1 Electrochemical and optical data.
PE2 polyPE2 PE3 polyPE3
max [nm]
Egopt [eV]
310; 428; 577 344, 416, 542 300; 466; 605 394, 550
1.85 1.68 1.79 1.69
IE [eV] 5.51 4,98 5.42 5.01
In both cases the oxidation potential of polymeric layers was lower than the potential of the corresponding monomers. The reduction potentials were also lower for the polymers. Despite of this, electropolymerization led to the formation of the systems with lower band gap energies than those of the starting monomers. This observation confirms that electrochemical oxidation leads to the formation of the systems with enhanced conjugation. 3.2. Spectroelectrochemical properties 3.2.1. UV-Vis spectroscopy The sets of UV-Vis spectra recorded during electrochemical reduction of the investigated compounds are shown in Fig. 5. In the neutral state UV-Vis spectra of both the monomers exhibit three peaks (Table 1). Absorbance of the two peaks located in the range of 400-700 nm in the spectra of PE3 is comparable while in the spectra of PE2 the peak at 577 nm is of the higher intensity than the second one located at 427 nm (Fig. 5). In order to carry out more thorough analysis of electronic transitions, UV-Vis spectra of the monomers were obtained by TDDFT calculations performed with B3LYP functional and 6-31G(d) basis set (Fig. 6). The simulation indicates that two peaks observed in the experimental spectra in the range of 400-700 nm in fact correspond to the three transitions. The difference in the relative intensities of the peaks in the UV-Vis spectra of the monomers is the result of the location of absorption band related to the transition from HOMO-2 to LUMO. In the case of PE2, HOMO-2 to LUMO transition enhances the peak of HOMO to LUMO and in the case of PE3 the peak of HOMO-4 to LUMO transition. The influence of the linking topology of carbazole and perylene bisimide moieties on the oxidation and reduction of the investigated monomers and polymers was also studied by spectroscopic methods.
LUMO [eV]
Egel [eV]
Egcalc 33 [eV]
−3.77 −3.55 −3.65 −3.43
1.74 1.43 1.77 1.58
2.41 2.33 -
During electrochemical reduction of the monomers, the absorption band related to the neutral monomer gradually lost its intensity and at the same time the new peaks appeared, for PE2 at 754, 841, 898 and 1028 nm and for PE3 at 758, 846, 905 and 1032 nm. It was possible to distinguish the isosbestic points for both sets of the spectra at 623 and 658 nm correspondingly (Fig. 5). It is clear that reduction takes place on the same unit of the molecules and the products of this process are the same for both compounds. The new peaks are apparently associated with the formation of radical anions during electrochemical reduction, what was confirmed by EPR spectroscopy (Fig. 10). When the absorption bands at 623 nm (for PE2) and at 658 nm (for PE3) achieved the maximum intensities, they began to lose their intensities and the other absorption bands (at 597-958 nm for PE2 and at 621-662 nm for PE3) gradually grew.
Fig. 6. Theoretical UV-Vis spectra of the investigated monomers.
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Fig. 7. UV-Vis spectra recorded during electropolymerization of PE2 (left) and PE3 (right) on ITO electrode.
In order to analyze the processes of electropolymerization, the sets of UV-Vis spectra were recorded during electrochemical oxidation (Fig. 7). New absorption bands formed during electropolymerization are located more or less at the same wavelengths, however the products of oxidation of PE3 exhibit higher absorbance of the UV-Vis radiation. The changes in the UV-Vis spectra can be attributed to the formation of radical cations (400-620 nm) and probably of bipolarons (800-1100 nm) in the polymers, which deposited on the ITO electrode during electrochemical oxidation [36]. The effect of the linking topology of carbazolyl substituted perylene bisimides on the electropolimerization process is apparent. In the case of PE2, where carbazole is linked to perylene bisimide at 2,2 positions, two isosbestic points located at 537 and 614 nm can be seen distinctly. In the case of PE3, where carbazole is linked to perylene bisimide at 3,3 position, the isosbestic points can be distinguished only in the early stages of polymerization. Comparing the UV-Vis spectra of monomers (Fig. 6 and 5- black curves), the redshift of HOMO energy can be noticed for PE3 compound. It demonstrates a better conjugation of carbazole units with perylene bisimide core in the case of a connection at position 3. Consequently, the P2 molecule of shorter conjugation is more reactive, thus quickly generates the final product - the polymer without intermediate products. In the case of PE3 monomer, the consecutive reaction of dimerization/polymerization is slower. Because of that, the reaction is accompanied by many intermediate products, such as dimers, trimmers etc. At the beginning of the process, when dimers are generated, the isosbestic points are distinguishable.
With the increasing concentration of trimers and other products of higher molecular weight, a broading of absorption peaks occurs and therefore a blurring of isosbestic point is observed. In order to investigate the processes which occur in the polymer films in the course of the redox process, the changes in UV-Vis spectra were traced. In the UV-Vis spectra of the neutral polyPE2 three maxima at 344, 416, 542 nm can be distinguished, whereas on the spectra of neutral polyPE3 the peaks at 394, 550 nm are observed. In the case of poly PE3 the absorption band with the maximum at 394 nm occurs. It corresponds to two absorption bands of polyPE2 at 344 and 416 nm (the black line in Figs. 8 and 9). Optical band gaps of the polymeric layers are lower than the band gaps of the corresponding monomers. This observation is in agreement with the electrochemical data. However in contrast to the electrochemical results the spectrally obtained value of the optical band gap of PE2 is higher than that of PE3. DFT calculations described in our previous paper revealed the same trend as the results of UV-Vis spectrometry [33]. The sets of the spectra recorded during electrochemical reduction of polymeric films deposited on ITO electrode are presented in Figure 8. For both the polymers the absorption band at ca. 550 nm lost its intensity and in the same time the new peaks at 657, 987 for polyPE2 and at 661, 872, 943 for polyPE3 appeared. The increase in the voltage applied caused the decrease of intensity of the peaks at 987 and 943 nm. With the decrease in intensity of these peaks the absorption bands at 337 nm for polyPE2 and at 381 nm for polyPE3 begin to grow. In addition the further increase of intensity of absorption bands at 657 nm for polyPE2 and at 661 nm for
Fig. 8. UV-Vis spectra recorded during electrochemical reduction of polyPE2 (left) and polyPE3 (right) deposited on ITO electrode.
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Fig. 9. UV-Vis spectra recorded during electrochemical oxidation of polyPE2 (left) and polyPE3 (right) deposited on the ITO electrode.
Fig. 10. EPR spectra of the radical anions generated during electrochemical reduction of investigated monomers.
polyPE3 is observed. These changes are more pronounced in the spectra of polyPE3, in which two isosbestic points at 566 and at 903 nm can be distinguished. These results are in agreement with electrochemical data, which also indicate the two-stage reduction. The results obtained by UV-Vis spectroelectrochemistry for the oxidation processes are presented in Figure 9. They are in agreement with the results of the CV measurements. In contrast to the reduction processes the peaks at 344 and 416 nm for polyPE2 and
at 394 nm for polyPE3 lose their intensity and at the same time new absorption bands appear. In the case of oxidation of polyPE2 as the result of formation of polarons in the polymer structure, broad band with the maximum at 1063 nm appears and the intensity of the narrow peak at 519 nm increases. As a result of the second stage of oxidation, the peaks at 684 and 480 nm emerge. The similar effects were observed for the oxidation of polyPE3, except the fact that during the oxidation process the absorption maxima are shifted
Fig. 11. EPR spectra recorded during electrochemical generation of the cation radical in the films of polyPE2 (left) and polyPE3 (right).
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Fig. 12. EPR spectra recorded during electrochemical generation of anion radical in polyPE2 (left) and polyPE3 (right) film.
to the lower wavelengths. In the first step the peaks at 1060 and 539 nm appear and they gradually shift to 989 and 519 respectively. In the second step the peak observed at 700 nm shifts to 620 nm and the peak observed at 507 nm shifts to 490 nm. The spectral changes observed during oxidation as well as during reduction are reversible in the investigated range of potentials (like in the cases of cyclic voltammetry measurements). 3.2.2. EPR spectroscopy The EPR spectra of radical anions generated during electrochemical reduction of the investigated compounds are shown in Figure 10. Reduction of both the monomers gives stable radicals. The single signal was observed in both cases which is the result of unification of the signals with very small hyperfine coupling coefficients. The peak-to-peak width (Bpp ) as well as the value of “g-factor” for both the radical anions is almost the same. In the case of PE2.− these values are equal to 0.380 mT and 2.0031 respectively, for PE2 they are 0,350 mT and 2.0036. Thus, it can be concluded that the structures of the paramagnetic centers and their environments are very similar in both cases. Subsequently, the changes in the EPR spectra of thin polymer films induced by the changes in electrode potential were investigated (Fig. 11 and Fig. 12). For both the polymers reduction, as well as oxidation of the films lead to the formation of stable radicals. The gradual increase of the EPR signal amplitude was observed with the increasing potential in the case of oxidation and with the decreasing potential in the case of reduction. The signal amplitude began to decrease after exceeding potential corresponding to the start of the second stage of the redox process, what is connected with the formation of dianions (for reduction) and bipolarons (for oxidation). After changing the direction of the electrode polarization, the initial increase and further slow decrease in the signal amplitude was observed, which confirms the reversibility of redox processes. 4. Conclusion We report on the effect of the linking topology of the electrophores on electrochemical and spectroelectrochemical properties of bay carbazolyl substituted perylene bisimide derivatives. Cyclic voltammetry measurements of the investigated monomers show typical two-step reduction of the perylene diimide moiety. The electrochemical oxidation of the studied compounds gives the corresponding polymers which deposit on the surface of a working electrode.
The resulting films were also examined by cyclic voltammetry. Investigation revealed formation of both n-type and p-type doping of polymers. Analysis of redox processes of monomers and polymers was carried out using UV-Vis and EPR spectroscopy. The results show a difference in the kinetics of the reduction of monomers as well as of the obtained polymer films, depending on the type of substitution of the carbazole moieties. Furthermore, the electropolymerization process of monomers does not proceed in the same way. The lower conjugation of PE2 molecules results in a rapid formation of polymer during electrochemical polymerization, whereas during electropolymerization of PE2 monomer the intermediate products are generated. However, in both cases electrochemical oxidation leads to stable polymers. The EPR measurements confirm formation of radical anions during reduction of studied monomers as well as polymers and radical cations during oxidation of polymers. Acknowledgements Calculations have been carried out in Wroclaw Centre for Networking and Supercomputing (http://www.wcss.wroc.pl, grant No. 135). Sandra Pluczyk and Wojciech Kuznik were scholars in SWIFT projectPOKL.08.02.01-24-005/10 which is co-financed by European Union within European Social Fund. This research was partially supported by FP7 REGPOT-2012-2013-1 REGPOT-2012-2013-1 ICT project CEOSeR (grant agreement No 316010). References [1] S.R. Forrest, M.E. Thompson, Introduction: Organic electronics and optoelectronics, Chem. Rev. 107 (2007) 923. [2] Y. Shirota, H. Kageyama, Charge carrier transporting molecular materials and their applications in devices, Chem. Rev. 107 (2007) 953. [3] D.Y. Kim, H.N. Cho, C.Y. Kim, Blue light emitting polymers, Prog. Polym. Sci. 25 (2000) 1089. [4] Q. Sun, Y. Li, Q. Pei, Polymer light-emitting electrochemical cells for highefficiency low-voltage electroluminescent devices, J. Disp. Tech. 3 (2007) 211. [5] A.W. Hains, Z. Liang, M.A. Woodhouse, B.A. Gregg, Molecular semiconductors in organic photovoltaic cells, Chem. Rev. 110 (2010) 6689. [6] J. Lia, A.C. Grimsdale, Carbazole-based polymers for organic photovoltaic devices, Chem. Soc. Rev. 39 (2010) 2399. [7] X. Zhan, D. Zhu, Conjugated polymers for high-efficiency organic photovoltaics, Polym. Chem. 1 (2010) 409. [8] C.D. Dimitrakopoulos, P.R.L. Malenfant, Organic thin film transistors for large area electronics, Adv. Mater. 14 (2002) 99. [9] M.E. Roberts, A.N. Sokolov, Z. Bao, Material and device considerations for organic thin-film transistor sensors, J. Mater. Chem. 19 (2009) 3351. [10] S. Cosnier, M. Holzinger, Electrosynthesized polymers for biosensing, Chem. Soc. Rev. 40 (2011) 2146.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The effect of the linking topology on the electrochemical and spectroelectrochemical properties of carbazolyl substituted perylene bisimides Sandra Pluczyk a , Wojciech Kuznik b , Mieczyslaw Lapkowski a,c,∗,1 , Renji R. Reghu d , Juozas V. Grazulevicius d a
Silesian University of Technology, Faculty of Chemistry, Strzody 9, 44-100 Gliwice, Poland Institute of Electronics and Control Systems, Faculty of Electrical Engineering, Czestochowa University of Technology c Centre of Polymer and Carbon Materials, Polish Academy of Sciences, Curie-Sklodowskiej 34, 41-819 Zabrze, Poland d Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu pl. 19, LT-50254, Kaunas, Lithuania b
a r t i c l e
i n f o
Article history: Received 25 February 2014 Received in revised form 25 April 2014 Accepted 7 May 2014 Available online 21 May 2014 Keywords: Ambipolar Perylene diimide Carbazole Spectroelectrochemistry
a b s t r a c t We report on electrochemical and spectroelectrochemical characteristics of two isomeric, carbazolyl substituted perylene bisimides and their polymers. Investigations were carried out in order to determine the effect of the linking topology on their properties. Cyclic voltammetry (CV) measurements of the investigated monomers showed typical two-step reduction of the perylene diimide moiety. The electrochemical oxidation of the studied compounds gave the corresponding polymer, which was deposited on the electrode. The resulting films were also examined by cyclic voltammetry. Investigation indicated both: n-type and p-type doping of polymers. Analysis of redox process of the monomers and polymers was carried out using UV-Vis and EPR spectroscopy. Our studies point to differences in the kinetics of the reduction process, depending on the substitution of the carbazole moiety. Although both monomers easily undergo of electrochemical polymerization, an effect of the linking topology of carbazolyl substituted perylene bisimides on the polymerization process was evidently noticed. Monomer PE2 as well as PE3 led to stable polymers, but polymer based on PE3 displays the conjugation in a longer extend than that based on PE2, where carabazole is linked to perylene diimide at less favorable position 2. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction In the age of rapidly developing technology, there is a necessity to search for compounds with better properties, meeting the increasingly sophisticated requirements. Nowadays, one of the rapidly growing sectors with major potential is organic electronics [1,2], which involves organic materials in such devices as light emiting diodes (OLEDs)[3], light-emitting electrochemical cells (LECs) [4], photovoltaic cells [5–7], organic thin film field effect transistors (OFETs) [8,9] and sensors [10]. The main advantage of organic semiconductors over conventional conductive materials is their flexibility and wide range of optical, optoelectronic and photo- and electroluminescent properties [11].
∗ Corresponding author. 44-100 Gliwice, Strzody 9, Poland, Tel.: +48 322371509; fax: +48 322371509. E-mail address: [email protected] (M. Lapkowski). 1 1 the ISE member http://dx.doi.org/10.1016/j.electacta.2014.05.057 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
The previous investigation of organic, conjugated compounds which can be used as semiconductors in (opto)electronic systems are mainly related to the group of p-type semiconductors. Now, intense researches are focused on n-type semiconductors and ambipolar materials which are able to transport both kinds of charge carriers i.e. holes and an electrons. Despite of that fact, those types of compounds still lags behind p-type systems [12,13]. Among large number of conjugated materials, aryl imide derivatives appear to represent a promising group of organic compounds in the aspect of (opto)electronic applications mainly because of their good thermal and chemical stability. Additionally, due to their properties, the high value of electron affinity (strong electron withdrawing character), a low LUMO level and a good reversibility of the reduction process, aryl imide derivatives are the most important blocks for the preparation of n-doped materials [14–17]. There are known numerous perylene diimide derivatives with an excellent charge carrier transport properties, what makes them good candidates for the application in electronic and optoelectronic devices [18–28].
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On the other hand carbazole and its derivatives are widely used hole-transport (p-type semiconductor) materials, which also possess many other interesting properties. Carbazole-based compounds are known for their excellent photorefractive properties, intensive fluorescence and electroluminescence, high thermal and photochemical stability [29]. Carbazole derivatives are also able to form relatively stable radical cations. Because of that, compounds containing carbazole moieties are capable of electrochemical polymerization and formation of conductive layers during this process [30,31]. It is interesting to note that depending on the substitution of the carbazole ring, devices based on carbazole derivatives exhibit different properties. It was observed that, devices fabricated from 2,7-substiduted carbazole derivatives demonstrate better results than those prepared using 3,6-substituted derivatives [32]. Taking into account all the facts described above we studied the possibility of obtaining ambipolar polymers from perylene diimide derivatives directly substituted in 1,7 positions with carbazole groups via the electrochemical method and estimated the effect of the substitution pattern on the electrochemical and spectroelectrochemical properties of the obtained layers. It is worth of noting that such derivatives of perylene diimide and carbazole have not been yet extensive studied. Ambipolar polymers based on imide derivatives described in literature, were mainly obtained by chemical polymerization. There are no reports on electrochemical polymerization of such compounds. This fact motivated us to take this topic. 2. Experimental details 2.1. Materials The synthesis and basic properties of the investigated compounds were described in the earlier published paper [33]. Their chemical structures are shown in Fig. 1. All the measurements were carried out in supporting electrolyte which was 0,1 M solution of tetrabutylammonium hexafluorophospate (Sigma Aldrich 98%) in dichloromethane (Sigma Aldrich ≥99.9%). Electrodeposition and electrochemical characterization of the studied monomers was performed in 1 mM solution of a corresponding compound in the supporting electrolyte. Spectroelectrochemical investigations were made in 10−5 M solution of each monomer. The studies of
electrodeposited films were carried out in monomer-free solutions of the supporting electrolyte. 2.2. Instrumental characterization Electrochemical measurements were performed on Ecochemie AUTOLAB potentiostat-galvanostat model PGSTAT20. The obtained data were analyzed using the GPES program. Cyclic voltammetry (CV) was used for the electrochemical characterization. The typical three-electrode cell was applied. The platinum wire was used as working electrode, the platinum spiral was employed as an auxiliary electrode and the silver wire was used as a pseudo reference electrode which was calibrated versus ferrocene/ferrocinium redox couple. The solutions were purged with argon. Spectral measurements were carried out using UV-Vis Hewlett Packard spectrophotometer 8453 and JEOL JES-FA 200, X-band CW-EPR spectrometer operating at 100 kHz field modulation. Spectroelectrochemical investigations were made by connected spectrometers described above with OMNI potentiostat. UV-Vis measurements during reduction processes were carried out in a 2 mm quartz cell equipped with a platinum mesh as a working electrode, a platinum spiral as a auxiliary electrode and a silver wire as a pseudo reference electrode. UV-Vis measurements of oxidation processes which lead to the formation of the films on the surface of working electrode were performed with employed indium-tine-oxide (ITO) coated quartz electrode as working electrode. Investigations of redox process of polymer films by UV-Vis measurements were carried out with a ITO working electrode coated with films, which were obtained by electropolymerization. EPR measurements were carried out in a cylindrical cell equipped with set of electrode like in the case of electrochemical measurements. The spectra of the radical anions of investigated monomers were recorded during potentiostatic reduction. Investigations of polymer films by EPR measurements were carried out with the working electrode coated with films, which were obtained by electropolymerization. 3. Results and discussion. In this work the electrochemical and spectroelectrochemical characterization of the monomers and of their polymers obtained by electrochemical deposition on the surface of working electrodes was performed. The chemical structures of the investigated compounds give the opportunity to obtain ambipolar polymers capable of both n and p doping. The main purpose of this work was estimation of the influence of the linking topology of the carbazole units to the perylene core on the electrochemical and spectroelectrochemical properties of the compounds and of the electrochemically obtained polymers. 3.1. Electrochemical characterization
Fig. 1. Chemical structures of investigated monomers.
The basic electrochemical properties of the monomers were described in the former paper [33]. In this report we focus on the process of electropolymerization and on investigation of the obtained films. Both the investigated monomers undergo processes of electrochemical oxidation as well as reduction. The voltammograms of the monomers show that oxidation process starts at 0.71 V for PE2 and at 0.62 V for PE3. The potential of 0.71 corresponds to the value of ionization energy (IE) of 5.51 eV for PE2. The IE value of PE3 was found to be a little lower (5.42 eV). This observation indicates more extended conjugation in PE2 molecule than in PE3. Analysis of the shape of the monomer voltamograms revealed the wider peaks for PE3 monomer. In terms of the shape of cyclic voltammetry curves, the reduction processes for both compounds are very
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Fig. 2. Electropolymerization of PE2 (left) and PE3 (right) in 0.1 M Bu4 NPF6 dichloromethane solution.
Fig. 3. Cyclic voltammograms of polyPE2 (left) and polyPE3 (right) in 0.1 M Bu4 NPF6 dichloromethane solution.
similar. Two-stage reversible reductions typical for the derivatives of perylene diimide were observed [34,35]. For monomer PE2 the onset potential of the first step of reduction occurred at −1.03 V with the maximum at −1.15 V and −1.36 V. For PE3 the reduction onset occurred at-1.15 V with maxima at −1.26 V and −1.44 V. Higher oxidation potential and higher reduction potential of PE2 compared with those of PE3 makes the values of electrochemical band gap energy almost the same in both cases (1.74 eV for PE2 and 1.77 eV for PE3). Both monomer PE2 and monomer PE3 underwent electrochemical polymerization. After the first scan, when the monomers were oxidized, consecutive CV scans indicated the formation of electroactive layers on the surface of working electrodes (Fig. 2). During the second scan new peaks appeared at 0.51 V for PE2 and at 0.52 V for PE3 and grew continuously in the subsequent cycles. The position of the new peak maximum was stable (its potential did not change during the electropolymerization process) for PE2, whereas for PE3 the potential of the growing peak increased from 0.52 to 0.65 V in the subsequent scans. The peaks at the negative potentials also grew and merged into one wide peak. In the case of electropolymerization of PE2 increasing peaks at the positive potential range were clearly separated. It was not the case for electropolymerization of PE3. The reversible redox couples at the positive potential range can be distinguished in cyclic voltammetry curves of the polymeric films (Fig. 3). The oxidation onsets were observed at 0.18 V for polyPE2 and at 0.21 V for polyPE3. Thus the values of oxidation potentials are almost the same for the both polymeric layers. The values of ionization energy were also almost the same (4,98 eV for polyPE2 and 5.01 eV for polyPE3). The difference was observed in
the processes of reduction. The reduction onset of polyPE2 occured at −1.25 V, while the corresponding value for polyPE3 was found to be −1,37 V. In addition the shapes of the curves recorded during n-doping and n-dedoping were found to be different. Two-stage reduction merged into a single broad peak in the case of polyPE2. On the voltammograms of polyPE3, two peaks could be distinguished at the same conditions. The decrease in scan rate up to 5 mV s−1 allowed to identify two peaks in the anodic cycle of polyPE2 (Fig. 4). Thus, kinetics of reduction process varies depending on the pattern of substitution of the carbazole unit.
Fig. 4. Cyclic voltammogram of polyPE2 recorded at 5 mV s−1 potential sweep rate.
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Fig. 5. UV-Vis spectra recorded during reduction of 10−5 M solution of PE2 (left) and PE3 (right) in 0.1 M Bu4 NPF6 /dichloromethane. Table 1 Electrochemical and optical data.
PE2 polyPE2 PE3 polyPE3
max [nm]
Egopt [eV]
310; 428; 577 344, 416, 542 300; 466; 605 394, 550
1.85 1.68 1.79 1.69
IE [eV] 5.51 4,98 5.42 5.01
In both cases the oxidation potential of polymeric layers was lower than the potential of the corresponding monomers. The reduction potentials were also lower for the polymers. Despite of this, electropolymerization led to the formation of the systems with lower band gap energies than those of the starting monomers. This observation confirms that electrochemical oxidation leads to the formation of the systems with enhanced conjugation. 3.2. Spectroelectrochemical properties 3.2.1. UV-Vis spectroscopy The sets of UV-Vis spectra recorded during electrochemical reduction of the investigated compounds are shown in Fig. 5. In the neutral state UV-Vis spectra of both the monomers exhibit three peaks (Table 1). Absorbance of the two peaks located in the range of 400-700 nm in the spectra of PE3 is comparable while in the spectra of PE2 the peak at 577 nm is of the higher intensity than the second one located at 427 nm (Fig. 5). In order to carry out more thorough analysis of electronic transitions, UV-Vis spectra of the monomers were obtained by TDDFT calculations performed with B3LYP functional and 6-31G(d) basis set (Fig. 6). The simulation indicates that two peaks observed in the experimental spectra in the range of 400-700 nm in fact correspond to the three transitions. The difference in the relative intensities of the peaks in the UV-Vis spectra of the monomers is the result of the location of absorption band related to the transition from HOMO-2 to LUMO. In the case of PE2, HOMO-2 to LUMO transition enhances the peak of HOMO to LUMO and in the case of PE3 the peak of HOMO-4 to LUMO transition. The influence of the linking topology of carbazole and perylene bisimide moieties on the oxidation and reduction of the investigated monomers and polymers was also studied by spectroscopic methods.
LUMO [eV]
Egel [eV]
Egcalc 33 [eV]
−3.77 −3.55 −3.65 −3.43
1.74 1.43 1.77 1.58
2.41 2.33 -
During electrochemical reduction of the monomers, the absorption band related to the neutral monomer gradually lost its intensity and at the same time the new peaks appeared, for PE2 at 754, 841, 898 and 1028 nm and for PE3 at 758, 846, 905 and 1032 nm. It was possible to distinguish the isosbestic points for both sets of the spectra at 623 and 658 nm correspondingly (Fig. 5). It is clear that reduction takes place on the same unit of the molecules and the products of this process are the same for both compounds. The new peaks are apparently associated with the formation of radical anions during electrochemical reduction, what was confirmed by EPR spectroscopy (Fig. 10). When the absorption bands at 623 nm (for PE2) and at 658 nm (for PE3) achieved the maximum intensities, they began to lose their intensities and the other absorption bands (at 597-958 nm for PE2 and at 621-662 nm for PE3) gradually grew.
Fig. 6. Theoretical UV-Vis spectra of the investigated monomers.
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Fig. 7. UV-Vis spectra recorded during electropolymerization of PE2 (left) and PE3 (right) on ITO electrode.
In order to analyze the processes of electropolymerization, the sets of UV-Vis spectra were recorded during electrochemical oxidation (Fig. 7). New absorption bands formed during electropolymerization are located more or less at the same wavelengths, however the products of oxidation of PE3 exhibit higher absorbance of the UV-Vis radiation. The changes in the UV-Vis spectra can be attributed to the formation of radical cations (400-620 nm) and probably of bipolarons (800-1100 nm) in the polymers, which deposited on the ITO electrode during electrochemical oxidation [36]. The effect of the linking topology of carbazolyl substituted perylene bisimides on the electropolimerization process is apparent. In the case of PE2, where carbazole is linked to perylene bisimide at 2,2 positions, two isosbestic points located at 537 and 614 nm can be seen distinctly. In the case of PE3, where carbazole is linked to perylene bisimide at 3,3 position, the isosbestic points can be distinguished only in the early stages of polymerization. Comparing the UV-Vis spectra of monomers (Fig. 6 and 5- black curves), the redshift of HOMO energy can be noticed for PE3 compound. It demonstrates a better conjugation of carbazole units with perylene bisimide core in the case of a connection at position 3. Consequently, the P2 molecule of shorter conjugation is more reactive, thus quickly generates the final product - the polymer without intermediate products. In the case of PE3 monomer, the consecutive reaction of dimerization/polymerization is slower. Because of that, the reaction is accompanied by many intermediate products, such as dimers, trimmers etc. At the beginning of the process, when dimers are generated, the isosbestic points are distinguishable.
With the increasing concentration of trimers and other products of higher molecular weight, a broading of absorption peaks occurs and therefore a blurring of isosbestic point is observed. In order to investigate the processes which occur in the polymer films in the course of the redox process, the changes in UV-Vis spectra were traced. In the UV-Vis spectra of the neutral polyPE2 three maxima at 344, 416, 542 nm can be distinguished, whereas on the spectra of neutral polyPE3 the peaks at 394, 550 nm are observed. In the case of poly PE3 the absorption band with the maximum at 394 nm occurs. It corresponds to two absorption bands of polyPE2 at 344 and 416 nm (the black line in Figs. 8 and 9). Optical band gaps of the polymeric layers are lower than the band gaps of the corresponding monomers. This observation is in agreement with the electrochemical data. However in contrast to the electrochemical results the spectrally obtained value of the optical band gap of PE2 is higher than that of PE3. DFT calculations described in our previous paper revealed the same trend as the results of UV-Vis spectrometry [33]. The sets of the spectra recorded during electrochemical reduction of polymeric films deposited on ITO electrode are presented in Figure 8. For both the polymers the absorption band at ca. 550 nm lost its intensity and in the same time the new peaks at 657, 987 for polyPE2 and at 661, 872, 943 for polyPE3 appeared. The increase in the voltage applied caused the decrease of intensity of the peaks at 987 and 943 nm. With the decrease in intensity of these peaks the absorption bands at 337 nm for polyPE2 and at 381 nm for polyPE3 begin to grow. In addition the further increase of intensity of absorption bands at 657 nm for polyPE2 and at 661 nm for
Fig. 8. UV-Vis spectra recorded during electrochemical reduction of polyPE2 (left) and polyPE3 (right) deposited on ITO electrode.
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Fig. 9. UV-Vis spectra recorded during electrochemical oxidation of polyPE2 (left) and polyPE3 (right) deposited on the ITO electrode.
Fig. 10. EPR spectra of the radical anions generated during electrochemical reduction of investigated monomers.
polyPE3 is observed. These changes are more pronounced in the spectra of polyPE3, in which two isosbestic points at 566 and at 903 nm can be distinguished. These results are in agreement with electrochemical data, which also indicate the two-stage reduction. The results obtained by UV-Vis spectroelectrochemistry for the oxidation processes are presented in Figure 9. They are in agreement with the results of the CV measurements. In contrast to the reduction processes the peaks at 344 and 416 nm for polyPE2 and
at 394 nm for polyPE3 lose their intensity and at the same time new absorption bands appear. In the case of oxidation of polyPE2 as the result of formation of polarons in the polymer structure, broad band with the maximum at 1063 nm appears and the intensity of the narrow peak at 519 nm increases. As a result of the second stage of oxidation, the peaks at 684 and 480 nm emerge. The similar effects were observed for the oxidation of polyPE3, except the fact that during the oxidation process the absorption maxima are shifted
Fig. 11. EPR spectra recorded during electrochemical generation of the cation radical in the films of polyPE2 (left) and polyPE3 (right).
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Fig. 12. EPR spectra recorded during electrochemical generation of anion radical in polyPE2 (left) and polyPE3 (right) film.
to the lower wavelengths. In the first step the peaks at 1060 and 539 nm appear and they gradually shift to 989 and 519 respectively. In the second step the peak observed at 700 nm shifts to 620 nm and the peak observed at 507 nm shifts to 490 nm. The spectral changes observed during oxidation as well as during reduction are reversible in the investigated range of potentials (like in the cases of cyclic voltammetry measurements). 3.2.2. EPR spectroscopy The EPR spectra of radical anions generated during electrochemical reduction of the investigated compounds are shown in Figure 10. Reduction of both the monomers gives stable radicals. The single signal was observed in both cases which is the result of unification of the signals with very small hyperfine coupling coefficients. The peak-to-peak width (Bpp ) as well as the value of “g-factor” for both the radical anions is almost the same. In the case of PE2.− these values are equal to 0.380 mT and 2.0031 respectively, for PE2 they are 0,350 mT and 2.0036. Thus, it can be concluded that the structures of the paramagnetic centers and their environments are very similar in both cases. Subsequently, the changes in the EPR spectra of thin polymer films induced by the changes in electrode potential were investigated (Fig. 11 and Fig. 12). For both the polymers reduction, as well as oxidation of the films lead to the formation of stable radicals. The gradual increase of the EPR signal amplitude was observed with the increasing potential in the case of oxidation and with the decreasing potential in the case of reduction. The signal amplitude began to decrease after exceeding potential corresponding to the start of the second stage of the redox process, what is connected with the formation of dianions (for reduction) and bipolarons (for oxidation). After changing the direction of the electrode polarization, the initial increase and further slow decrease in the signal amplitude was observed, which confirms the reversibility of redox processes. 4. Conclusion We report on the effect of the linking topology of the electrophores on electrochemical and spectroelectrochemical properties of bay carbazolyl substituted perylene bisimide derivatives. Cyclic voltammetry measurements of the investigated monomers show typical two-step reduction of the perylene diimide moiety. The electrochemical oxidation of the studied compounds gives the corresponding polymers which deposit on the surface of a working electrode.
The resulting films were also examined by cyclic voltammetry. Investigation revealed formation of both n-type and p-type doping of polymers. Analysis of redox processes of monomers and polymers was carried out using UV-Vis and EPR spectroscopy. The results show a difference in the kinetics of the reduction of monomers as well as of the obtained polymer films, depending on the type of substitution of the carbazole moieties. Furthermore, the electropolymerization process of monomers does not proceed in the same way. The lower conjugation of PE2 molecules results in a rapid formation of polymer during electrochemical polymerization, whereas during electropolymerization of PE2 monomer the intermediate products are generated. However, in both cases electrochemical oxidation leads to stable polymers. The EPR measurements confirm formation of radical anions during reduction of studied monomers as well as polymers and radical cations during oxidation of polymers. Acknowledgements Calculations have been carried out in Wroclaw Centre for Networking and Supercomputing (http://www.wcss.wroc.pl, grant No. 135). Sandra Pluczyk and Wojciech Kuznik were scholars in SWIFT projectPOKL.08.02.01-24-005/10 which is co-financed by European Union within European Social Fund. This research was partially supported by FP7 REGPOT-2012-2013-1 REGPOT-2012-2013-1 ICT project CEOSeR (grant agreement No 316010). References [1] S.R. Forrest, M.E. Thompson, Introduction: Organic electronics and optoelectronics, Chem. Rev. 107 (2007) 923. [2] Y. Shirota, H. Kageyama, Charge carrier transporting molecular materials and their applications in devices, Chem. Rev. 107 (2007) 953. [3] D.Y. Kim, H.N. Cho, C.Y. Kim, Blue light emitting polymers, Prog. Polym. Sci. 25 (2000) 1089. [4] Q. Sun, Y. Li, Q. Pei, Polymer light-emitting electrochemical cells for highefficiency low-voltage electroluminescent devices, J. Disp. Tech. 3 (2007) 211. [5] A.W. Hains, Z. Liang, M.A. Woodhouse, B.A. Gregg, Molecular semiconductors in organic photovoltaic cells, Chem. Rev. 110 (2010) 6689. [6] J. Lia, A.C. Grimsdale, Carbazole-based polymers for organic photovoltaic devices, Chem. Soc. Rev. 39 (2010) 2399. [7] X. Zhan, D. Zhu, Conjugated polymers for high-efficiency organic photovoltaics, Polym. Chem. 1 (2010) 409. [8] C.D. Dimitrakopoulos, P.R.L. Malenfant, Organic thin film transistors for large area electronics, Adv. Mater. 14 (2002) 99. [9] M.E. Roberts, A.N. Sokolov, Z. Bao, Material and device considerations for organic thin-film transistor sensors, J. Mater. Chem. 19 (2009) 3351. [10] S. Cosnier, M. Holzinger, Electrosynthesized polymers for biosensing, Chem. Soc. Rev. 40 (2011) 2146.
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