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Electrochemical synthesis and characterization of copolyviologen films
Electrochimica Acta 90 (2013) 171–178
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical synthesis and characterization of copolyviologen films Nianxing Wang a,b , Pia Damlin a , Beatriz Meana Esteban a,1 , Timo Ääritalo a , Jouko Kankare a , Carita Kvarnström a,∗ a b
Turku University Center for Materials and Surfaces, c/o Laboratory of Materials Chemistry and Chemical Analysis, University of Turku, Vatselankatu 2, FI-20014 Turku, Finland Graduate School of Chemical Sensors and Microanalytical Systems (CHEMSEM), Finland
a r t i c l e
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Article history: Received 18 September 2012 Received in revised form 29 November 2012 Accepted 29 November 2012 Available online 7 December 2012 Keywords: Cyanopyridine monomers Polyviologens Spectroelectrochemistry Electrochemical synthesis
a b s t r a c t Two cyanopyridine-based monomers were synthesized and utilized for the cathodic electropolymerization of polyviologen (BPV) and copolyviologen (CoPV) films onto glassy carbon (GC) electrodes. One of the monomer material is branched (BCP) whereas the other one has a linear structure (LCP) the former bearing three the latter two polymerizable cyanopyridinium groups. The electrochemical synthesis of the polyviologen and copolyviologen films were made in aqueous solutions and the film properties were characterized by electrochemical and spectroscopic techniques. Atomic force microscopy was used in order to study the surface morphologies of the films. The BPV and CoPV films undergo a two-step reduction reaction and show redox properties typical for viologen derivatives in monomer free electrolyte solutions. In comparison to the polyviologen film, electrochemically synthesized from the branched monomer (BCP), the redox property of the copolymer was changed. The results from in situ UV–vis spectra show that the copolyviologen film has good stability and undergoes reversible electron transfer reactions successfully. The structure of the copolyviologen film was characterized by FTIR spectroscopy, confirming successful polymerization of both monomer materials. The copolymerization therefore opens up a direct electrochemical method to form viologen films with large cavities. This feature gives a potential to apply the partly conjugated copolyviologen film with redox properties as a conducting host material for immobilization of macromolecules. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Viologens, derivatives of diquaternized 4,4 -bipyridine [1], have been extensively studied in the last decades due to their reversible redox properties, electrochromic properties and good environmental stability. Viologens exhibit three different redox forms, all with conspicuous colors. The stable dication undergoes a stepwise two electron reduction to the radical monocation and thereafter further to its neutral form. The dication is usually colorless, while the radical cation is strongly colored. The color has been shown to be dependent on the viologen substituent [2] and this feature has been widely utilized in electrochromic displays [2,3]. The neutral form of the viologen is chemically reactive and a strong reducing agent [1]. Due to these special properties viologen-modified materials have been used in different applications such as mediators, electrochromic displays, bioelectronic devices and smart windows [4–9]. Furthermore, viologens and their derivatives are widely
∗ Corresponding author at: Vatselankatu 2, Turku 20014, Finland. Tel.: +358 2 333 6729; fax: +358 2 333 6700. E-mail addresses: carita.kvarnstrom@utu.fi, carkva@utu.fi (C. Kvarnström). 1 Present address: KWH Mirka Ltd., Jeppo 66850, Finland. 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.11.131
utilized in solar cell applications as accepter units together with different donor materials [8,10,11]. Different strategies for preparing insoluble viologenfunctionalized materials have been utilized for a wide range of applications [12]. Polyviologens can be synthesized for example by coupling of cyanopyridine derivatives either by chemical or electrochemical methods [1,13–15]. In comparison to the chemical reaction path, the electrochemical coupling offers a simple, clean and efficient way for synthesizing insoluble polyviologen films directly onto electrode substrates. Furthermore, the electrochemically synthesized viologen film can be easily studied and characterized in situ by different spectroelectrochemical techniques [16]. The mechanism for the electrochemical coupling of cyanopyridines is shown in Scheme 1. Firstly, the cationic cyanopyridinium derivative is reduced to its neutral form, after which it can undergo a dimerization reaction. As the result of the coupling reaction, followed by the splitting of the cyano groups, viologen in its dication form is obtained [1,13]. In the present work, copolyviologen films were synthesized by electrochemical reduction in aqueous solution from two cyanopyridine-based monomers: 1,3,5-Tris(4-cyanopyridinium-1ylmethyl)-2,4,6-trimethylbenzene tribromide (BCP) and 4,4 -bis(4cyanopyridinium-1yl)biphenyl bis-tosylate (LCP). The structures of
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Scheme 1. The mechanism of electrochemical reductive coupling of 4-cyanopyridinium to viologen.
BCP and LCP are shown in Scheme 2(a) and (b), respectively. In the polymerization process, the cyano moieties present in both monomers are reductively coupled, according to Scheme 1, and polyviologen units are formed and adsorbed onto the electrode surface as a film. There are reports on using the monomer material BCP for the electrochemical synthesis of polyviologen films and utilization of the material for electrochemical anion recognition [13,17]. In this work the electrochemical copolymerization concept is reported, aiming at preparing a redox and conjugated polymer in a one batch synthesis. The electrochemical polymerization constitutes a controllable way for synthesizing copolymers from monomers with slightly different reduction potentials. By tuning the monomer concentrations and the applied potential range used for polymerization the coupling sequence for the two monomer units can be tuned. Due to the unique structure, the monomer LCP has the potential to enlarge the cavity size in the copolyviologen film and in such a way to alter its properties. Different concentration ratios of the monomer materials were tested for electrochemical synthesis of the copolyviologen films. Finally, the polyviologen and copolyviologen films were characterized with cyclic voltammetry, UV–vis and FTIR spectroscopy and by the AFM technique. 2. Experimental 2.1. Chemicals Potassium chloride was purchased from Oy FF-Chemicals Ab and was used to prepare the electrolyte solution. All solutions used in the electrochemical experiments were prepared with deionized water, and purged with N2 before use. 2.2. Synthesis of monomers 1,3,5-Tris(4-cyanopyridinium-1-ylmethyl)-2,4,6-trimethylbenzene tribromide (BCP) was prepared according to previous reports [13,17]. The starting materials: 2,4,6-Tris(bromonethyl)mesitylene (98%, Sigma–Aldrich) and 4-cyanopyridine (98%,
Sigma–Aldrich) were dissolved in acetonitrile and refluxed for an hour. Recrystallization from water–methanol mixture gave the pure product. The unique synthesis of 4,4 -bis(4-cyanopyridinium-1yl) biphenyl bis-tosylate (LCP) was obtained by dissolving 1-(2,4dinitrophenyl)-4-cyanopyridinium tosylate (4 g, 9.0 mmol), which was synthesis by 2,4-dinitrophenol (MERCK) and p-toluenesulfonyl chloride (Sigma–Aldrich), in a 8:2 ethanol–water-solution. Benzidine (650 mg, 3.53 mmol, MERCK) in 8 ml of ethanol was added drop wise and the blackened mixture was refluxed for 40 h. The mixture was evaporated to dryness and dissolved into boiling ethanol and filtered while hot. The precipitate was recrystallized twice from a methanol–water mixture obtained by adding water to the boiling methanol mixture until the product dissolved. Filtration resulted in 1.63 g of product as 66% yield.
2.3. Electrochemistry The cyclic voltammetric experiments were done in a threeelectrode one-compartment cell connected to an Autolab (PSTAT 10 and PGSTAT101) potentiostat. The polyviologen films were polymerized electrochemically on a glassy carbon (GC) electrode with 1 mm Ø (purchased from Cypress Systems Inc.). The GC electrode was polished with diamond paste (¼ to 1 m, Struers A/s) and rinsed with deionized water before use. Saturated calomel electrode (SCE) and Ag/AgCl electrode were utilized as reference electrodes, obtained from Radiometer Analytical and eDAQ, respectively. A coiled Pt wire (1 mm Ø) was used as a counter electrode. The electrochemical polymerization was carried out in aqueous solution. The polymerization of BCP was carried out in the concentration of 5 mM, whereas the co-polymerization was made in a solution of 2.5 mM BCP and 0.5 mM LCP. The polyviologen films were synthesized by potentiodynamic cycling between 0 and −1.2 V or −1.4 V during 20 consecutive cycles using 50 mV/s as scan rate. After polymerization the film was rinsed with deionized water and 0.1 M KCl solution after which the redox response
Scheme 2. Structures of cyanopyridinium-based viologen monomers (a) 1,3,5-Tris(4-cyanopyridinium-1-ylmethyl)-2,4,6-trimethylbenzene tribromide (BCP) and (b) 4,4 bis(4-cyanopyridinium-1yl)biphenyl bis-tosylate (LCP).
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was measured in monomer free electrolyte solution. A 0.1 M KCl solution was used as electrolyte in all electrochemical experiments. 2.4. In situ UV–vis NIR spectroscopy In the UV–vis experiments indium tin oxide (ITO) glass was used as a working electrode (8–12 , Delta Technologies Limited). The ITO glass was cleaned with ultrasonication in acetone, ethanol and deionized water solutions for 15 min each. The measurements were made in a 1 cm path length quartz cuvette using a Pt-wire as the counter-electrode and Ag/AgCl as the reference electrode. The in situ UV–vis NIR spectra were recorded between 300 and 1100 nm on a Hewlett Packard 8453 spectrophotometer. For the UV–vis spectroelectrochemical experiments the polyviologen/copolyviologen films were deposited onto ITO-glass electrodes by potential cycling between 0 and −1.2 V, using 20 cycles in the case of BPV and 40 cycles in the case of CoPV. With 50 mV/s was used as the scan rate. The UV–vis spectra from the redox response of the polyviologen/copolyviologen films were measured in the potential range of 0 to −1.1 V by changing the potential with 100 mV steps between the recordings. The background spectrum was measured using a blank ITO glass in electrolyte solution. 2.5. FTIR spectroscopy Fourier transform infrared (FTIR) spectra of polyviologen and copolyviologen films were measured by using a dry-air-purged Nexus 870 FTIR spectrometer (Nicolet) equipped with a DTGS detector in the transmission mode. For each spectrum, 256 scans at a resolution of 4 cm−1 were co-added. The polyviologen and copolyviologen films were synthesized on the ITO glass as described above, and the film was swelled in 0.1 M KCl. Samples of 2 mg of peeled films were oven dried and mixed with 200 mg KBr and pressed into pellets. 2.6. Atomic force microscopy (AFM) Samples for AFM were prepared by electrochemical synthesis of BPV and CoPV on silicon wafers. The BPV and CoPV were synthesized by cyclic voltammetry for 32 cycles from 0 to −1.2 V vs. Ag/AgCl. The AFM images were measured with a Veeco diCaliber AFM instrument (Veeco Instruments Inc.) under ambient conditions in the tapping mode using silicon tips with a resonance frequency of 320 kHz. The tip was 10–15 m in height and had the radius curvature of 8 nm. The polished silicon wafers were obtained from OKMETIC (phosphorus doped, 111) and cut to 10 by 10 mm chips. The Si-wafers were cleaned with ultrasonication in acetone, ethanol and deionized water solutions for 15 min each. 3. Results and discussion 3.1. Electroreductive polymerization of BCP Fig. 1 shows the cyclic voltammograms of the reductive polymerization of 5 mM of monomer BCP in 0.1 M KCl aqueous solution at a scan rate 50 mV/s on the GC-electrode. The first cycle can be seen in the insert in Fig. 1, in which a sharp reduction and oxidation peak can be observed at approx. −0.7 V and −0.5 V, respectively. The reduction peak is sharp due to the reduction of the 4-cyanopyridium moiety into its neutral form which involves a coupling of the moieties into a viologen unit. In the reverse scan, the viologen unit is oxidized to the dication form. During the following scans, the reduction peak becomes broad and is shifted successively from −0.7 V to −0.9 V. Additionally, another weak reduction peak is formed at approx. −0.55 V. The current for the oxidation peak is also radically decreased and shifted toward higher potentials and
Fig. 1. Cyclic voltammograms from the polymerization of the monomer BCP in 0.1 M KCl aqueous solution at a scan rate 50 mV/s on the GC-electrode. The inset graph shows the first cycle in the monomer solution.
a new oxidation peak is formed at approx. −0.8 V. The shift of the reduction peak toward more negative potentials can be explained by the dimer or oligomer formation leading to the deposition of the formed polyviologen chains at the electrode surface. Due to the structure of monomer BCP, the resulted polyviologen film was named as BPV (Branched PolyViologen). Fig. 2a shows the redox response of the film from monomer BCP in 0.1 M KCl cycled from 0 to −0.7 V to successively higher switching potentials reaching finally −1.3 V. The redox response from the formed polyviologen film is at a more negative potential in comparison to the cyanopyridine starting materials. Peaks from two redox reactions can be observed in the cyclic voltammograms; the first reduction and oxidation peaks (Er1 and Eo1) are at approx. −0.55 and −0.45 V, which is caused by the radical-cation formation of the viologen. The second redox couple (Er2 and Eo2) at approx. −0.9 and −0.8 V is from the conversion of the charged BPV to the neutral form. As already discussed, during electropolymerization of BCP, new reduction and oxidation peaks were formed in the cyclic voltammogram after continuous cycling. By comparing Figs. 1 and 2a, it can therefore be concluded that the cyanopyridine monomer BCP has been successfully polymerized on the electrode surface and that the obtained redox response shows typical polyviologen features. In order to know if the system is reversible according to the Randles–Sevcik equation [14], the redox response of the BPV was measured from 0 to −1.1 V in monomer-free electrolyte solution using 10, 20, 50 and 100 mV/s as scan rates. The response of BPV can be seen in Fig. 2b and the calculated result was shown in Fig. 2c. In Fig. 2c, it can be seen that the currents from all reduction/oxidation peaks are proportional to the square root of the scan rates indicating that the redox reaction of the polyviologen film is reversible and diffusion controlled, which suggests that the BPV film has an open, crosslinked structure. 3.2. Electroreductive polymerization of CoPV Due to its linear and conjugated structure the synthesized cyanopyridine monomer LCP has the potential to form new polyviologen structures (LPV), which would have good electrochemical properties [18]. During reductive polymerization of the monomer LCP a solid black product was formed at the electrode surface. The adsorbed material was very unstable and did not give a proper redox response during potential cycling. A series of experiments
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were repeated with various concentrations of the monomer material (2 mM, 1 mM, 0.5 mM), however, film formation was poor, presumably due to the high solubility of the intermediates. Based on the research with monomers of similar structure [19] and in order to enhance the redox performance and enlarge the cavities formed from the branched starting materials, the branched BCP and linear LCP were introduced into the system for the synthesis of copolyviologen materials. Fig. 3a shows the cyclic voltammograms of the copolymerization of 2.5 mM BCP and 0.5 mM LCP in 0.1 M KCl aqueous solution at a scan rate of 50 mV/s. CoPV films were synthesized at different concentration ratios of the monomer materials using otherwise identical conditions for polymerization. A comparison of the redox response in monomer free electrolyte solutions for these CoPV films is shown in Fig. 3b. Based on the redox response and charging capability, the CoPV which was synthesized at the concentration ratio of 2.5 mM BCP and 0.5 mM LCP gave the best results and therefore this was used onwards as the concentration ratio in the copolymerization experiments. The first cycles from the polymerization experiments of the CoPV, BPV and LPV are shown in Fig. 3c. In comparison to the first cycle of the synthesis of BPV the copolymer CoPV shows additional redox peaks at approx. −0.5 V and −0.35 V, nicely overlapping with the peaks obtained during the first polymerization cycle of BPV. The sharp reduction peak at approx. −0.5 V proves that the cyano group of the monomer LCP was successfully coupled, leading to the viologen unit. The oxidation peak at approx. −0.35 V in the reverse scan is from the viologen unit that is charged to its dication form. In the following scans the reduction peaks shift to more negative potentials and a new oxidation peak was formed at approx. −0.75 V. As described above, this indicates the formation of dimers or oligomers deposited onto the electrode surface. Fig. 4a shows the redox response of the copolyviologen film covered GC-electrode in the monomer free 0.1 M aqueous KCl solution. As described above, the redox couple at approx. −0.55 and −0.45 V originates from the charging of the dicationic viologen unit to the radical cation form and vice versa. The redox couple at approx. −0.9 and −0.8 V results from further reduction of the radical cation to its neutral form. The redox potentials are identical to the potentials obtained from the BPV film shown in Fig. 2a. However, it should be noticed that the currents of the second redox couple are much lower for the copolymer in comparison to the same redox response from the film of only BCP. In Fig. 4b, the redox response of the copolyviologen was measured at different scan rates and plotted vs. square root of the scan rate in Fig. 4c. The linear relationship between the current and the square root of the scan rate shows the diffusion dependence of the film charging. As described above, the redox reactions in the copolyviologen film are reversible, which indicates a film structure consisting of channels leading the counter ions in and out from the large cavities. 3.3. UV–vis spectra of polyviologen and copolyviologen films
Fig. 2. (a) Cyclic voltammograms of the BPV deposited GC-electrode in the monomer-free electrolyte from 0 to −0.7 V successively increasing the switching potential, finally reaching a value of −1.3 V using 50 mV/s as scan rate. (b) Cyclic voltammograms of the BPV from 0 to −1.1 V using different scan rates (10, 20, 50 and 100 mV/s). (c) The reduction/oxidation peak currents for the two redox pairs vs. the root of the scan rates.
The UV–vis spectra showing the –* transition of the two viologen precursors in aqueous solution (20 M) are shown in Fig. 5. BCP absorbs far below 300 nm, while LCP has a strong and broad absorbance band at approx. 350 nm. Fig. 6a shows the in situ UV–vis spectra of the BPV film covered ITO-glass under potentiostatic conditions recorded at every 100 mV from 0 to −1 V vs. Ag/AgCl electrode. The absorbance bands of the film at around 400 and 600 nm start to grow at potentials above 0.2 V. The rising of these bands can be explained by the formation of the radical cation in the viologen units. The potential correlates well to the potential seen in the cyclic voltammogram where the first reduction peak of the viologen starts to develop, shown in Fig. 2a. The absorbance of the band at 600 nm achieved
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Fig. 3. (a) Electropolymerization of the monomer BCP (2.5 mM) and LCP (0.5 mM) in 0.1 M KCl solution in the potential range 0 to −1.2 V with the scan rate 50 mV/s. (b) Cyclic voltammograms in the potential range 0 to −1.2 V of the various concentration-based CoPV films on the GC electrode in monomer free electrolyte solution with the scan rate 50 mV/s. The concentration ratios are given as concentration of BCP vs. LCP (c). The first cycles obtained from the electropolymerization of CoPV, BPV and LPV in the potential range 0 to −1.2 V.
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Fig. 4. (a) Cyclic voltammograms of the copolyviologen deposited GC-electrode in monomer free electrolyte in the potential range 0 to −0.7 V successively increasing the switching potential, finally reaching a value of −1.1 V using 50 mV/s as scan rate. (b) Cyclic voltammograms of the copolyviologen deposited GC-electrode in monomer free electrolyte solution with the scan rates of 10, 20, 50 and 100 mV/s. (c) The reduction/oxidation peak currents for the two redox pairs vs. the root of the scan rates.
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Fig. 5. The UV–vis spectra of the monomers BCP and LCP in aqueous solution.
Fig. 7. FTIR spectra of (a) the monomers BCP and LCP and (b) the BPV and CoPV films as KBr pellet.
Fig. 6. The in situ UV–vis spectra of (a) polyviologen film (monomer BCP) and (b) copolyviologen film covered ITO-glass under potentiostatic conditions, spectra recorded at every 100 mV from 0 to −1 V (vs. Ag/AgCl electrode). The potential dependent intensities of the absorbance bands at 400 nm and 600 nm are shown as inserts in the top right of Fig. (a) and (b).
its highest value when the potential reached −0.6 mV. The intensity of this band starts thereafter to decrease with increasing negative potential, which is due to further reduction of the radical cation to its neutral form. The absorbance peak at 400 nm was increasing linearly from the start with increasing potential and the band intensity reached its maximum value at −1 V, shown in Fig. 6a. The continuous increase in absorbance throughout the whole reduction can be explained by that both the cationic form and the neutral form of viologens absorb close to 400 nm [12]. The neutral form absorbs more strongly than the cationic form resulting in the further increase of absorbance at higher negative potentials. The potential dependent intensities of the absorption bands at 400 nm and 600 nm are shown in the top right of Fig. 6a. A weakly increasing absorbance at 900 nm is regarded to be from an allowed transition within the viologen dimer unit, however not a charge-transfer transition according to Kosower and Cotter [15]. Fig. 6b shows the UV–vis spectra of the CoPV film covered ITOglass, and the intensities of the absorption peaks vs. potential are shown on the top right. The behavior of the absorption bands were almost the same as for BCP in Fig. 6a. The absorption bands at 400 nm and 600 nm were caused by the absorbance of the radical cation, and further increase of the band at 400 nm is due to the conversion into the neutral form of the viologen units. However, the continuous increase of the band at 400 nm is here interrupted
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Fig. 8. AFM images of (a) BPV and (b) CoPV add the size of the AFM pictures.
by a formation of a plateau in the intensity vs. potential plot shown as an insert in the top right of Fig. 6b. In this potential region there is a counter balance between the cation formation and the neutral form. In the copolyviologen these are due to incorporation of the LCP regions of longer effective conjugation, which probably stabilizes the cationic form thus slightly delaying the conversion to the neutral form. 3.4. FTIR characterization of the polyviologen and copolyviologen films In Fig. 7a, the FTIR spectra of the two monomers are shown. Since both of the monomers are cyanopyridine-based, the spectra are rather similar with a few exceptions. Both spectra have a strong band at approx. 580 and weaker at 2200 cm−1 , which are caused by the C N vibration of the aromatic nitriles. Strong bands can also be observed at 1500–1400 cm−1 , which are assigned to C C and C N stretch vibration [20]. The bands at 1250–1200 cm−1 are from the in-plane deformations of the pyridine groups. In the spectrum of the monomer BCP a series of weak bands unique for the monomer can be seen at 780–700 cm−1 from the 1,3,5-trisubstituted benzenes and at approx. 1480 cm−1 from the CH2 group. The monomer LCP has its own specific bands at approx. 1950 cm−1 from the 1, 4-disubstituted benzene and the bands at approx. 850 cm−1 from the biphenyl. The strong band at approx. 1020 cm−1 was caused by the counter anion tosylate (TOS− ). As described above in the polymerization process the cyano groups were reductively coupled to the viologen unit forming polyviologens. The FTIR spectra of the polymers were measured and are shown in Fig. 7b. The two polymer films have a similar spectrum, which indicates that the BCP is the major unit and works as the skeleton in the copolyviologen film. In the spectra, however there are weak bands at approx. 2300 and at 600 cm−1 , which indicates
that most cyano groups were reductively coupled into viologens. In the copolyviologen spectrum the bands around 1500 cm−1 and 1000 cm−1 are assigned to biphenyl structures in the monomer LCP indicating successful incorporation of both monomers. 3.5. AFM measurement of the BPV and CoPV The surface morphology of BPV and CoPV films was studied by AFM. The images are shown in Fig. 8. Film deposition from BCP is heterogeneous and the distribution of the film is irregular. The surface of CoPV was smoother in comparison to the BPV film, which means that by importing the LCP into CoPV the large cavity structure can be formed. The scratch seen in the AFM image 8b is probably from a crack while drying. An estimation of the film thickness of approx. 1.1 m was made from the crack. 4. Conclusion Two kinds of polyviologen structures were electrosynthesized. The three branched structure of the BCP precursor should result in a network of polyviologen chains with a specific mesh size. Introduction of the linear LCP precursor should enhance the length of the polymer chains and form a network with a bigger mesh size. It was shown that the viologen units in the copolymer film can successfully undergo two reversible reduction steps. Tuning of the mesh size in the network of polyviologens with both redox and conjugated conductance makes them interesting materials to utilize as redox-active host materials for immobilizing macromolecules of different size. Comparing the redox properties of both polyviologen and copolyviologen films, the redox potentials remained the same but the relative intensity of the current responses were more uniform in the copolyviologen in comparison to the plain BCP film.
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The UV–vis and FTIR spectra show the presence of both starting materials giving the evidence for a copolyviologen structure that could work as a functional material in electronics. Acknowledgment This work is funded by Academy of Finland and by Graduate School of Chemical Sensors and Microanalytical Systems (CHEMSEM, Finland). References [1] P.M.S. Monk, The viologens: physicochemical properties, synthesis, and applications of the salts of 4,4’-bipyridine, John Wiley & sons Ltd, West Sussex, England, 1998. [2] V. Jain, H. Yochum, H. Wang, R. Montazami, M.A.V. Hurtado, A. MendozaGalván, H.W. Gibson, J.R. Heflin, Solid-state electrochromic devices via ionic self-assembled multilayers (ISAM) of a polyviologen, Macromolecular Chemistry and Physics 209 (2008) 150. [3] D.M. DeLongchamp, M. Kastantin, P.T. Hammond, High-contrast electrochromism from layer-by-layer polymer films, Chemistry of Materials 15 (2003) 1575. [4] H.B. Yildiz, R. Tel-Vered, I. Willner, Solar Cells with Enhanced Photocurrent Efficiencies Using Oligoaniline-Crosslinked Au/CdS Nanoparticles Arrays on Electrodes, Advanced Functional Materials 18 (2008) 3497. [5] M. Riskin, R. Tel-Vered, I. Willner, The Imprint of Electropolymerized Polyphenol Films on Electrodes by Donor-Acceptor Interactions: Selective Electrochemical Sensing of N,N -dimethyl-4,4 -bipyridinium (Methyl Viologen), Advanced Functional Materials 17 (2007) 3858. [6] D. Cummins, G. Boschloo, M. Ryan, D. Corr, S.N. Rao, D. Fitzmaurice, Ultrafast Electrochromic Windows Based on Redox-Chromophore Modified Nanostructured Semiconducting and Conducting Films, Journal of Physical Chemistry B 104 (2000) 11449. [7] H.C. Ko, S. Kim, H. Lee, B. Moon, Multicolored Electrochromism of a Poly{1,4-bis[2-(3,4-ethylenedioxy)thienyl]benzene} Derivative Bearing Viologen Functional Groups, Advanced Functional Materials 15 (2005) 905.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical synthesis and characterization of copolyviologen films Nianxing Wang a,b , Pia Damlin a , Beatriz Meana Esteban a,1 , Timo Ääritalo a , Jouko Kankare a , Carita Kvarnström a,∗ a b
Turku University Center for Materials and Surfaces, c/o Laboratory of Materials Chemistry and Chemical Analysis, University of Turku, Vatselankatu 2, FI-20014 Turku, Finland Graduate School of Chemical Sensors and Microanalytical Systems (CHEMSEM), Finland
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Article history: Received 18 September 2012 Received in revised form 29 November 2012 Accepted 29 November 2012 Available online 7 December 2012 Keywords: Cyanopyridine monomers Polyviologens Spectroelectrochemistry Electrochemical synthesis
a b s t r a c t Two cyanopyridine-based monomers were synthesized and utilized for the cathodic electropolymerization of polyviologen (BPV) and copolyviologen (CoPV) films onto glassy carbon (GC) electrodes. One of the monomer material is branched (BCP) whereas the other one has a linear structure (LCP) the former bearing three the latter two polymerizable cyanopyridinium groups. The electrochemical synthesis of the polyviologen and copolyviologen films were made in aqueous solutions and the film properties were characterized by electrochemical and spectroscopic techniques. Atomic force microscopy was used in order to study the surface morphologies of the films. The BPV and CoPV films undergo a two-step reduction reaction and show redox properties typical for viologen derivatives in monomer free electrolyte solutions. In comparison to the polyviologen film, electrochemically synthesized from the branched monomer (BCP), the redox property of the copolymer was changed. The results from in situ UV–vis spectra show that the copolyviologen film has good stability and undergoes reversible electron transfer reactions successfully. The structure of the copolyviologen film was characterized by FTIR spectroscopy, confirming successful polymerization of both monomer materials. The copolymerization therefore opens up a direct electrochemical method to form viologen films with large cavities. This feature gives a potential to apply the partly conjugated copolyviologen film with redox properties as a conducting host material for immobilization of macromolecules. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Viologens, derivatives of diquaternized 4,4 -bipyridine [1], have been extensively studied in the last decades due to their reversible redox properties, electrochromic properties and good environmental stability. Viologens exhibit three different redox forms, all with conspicuous colors. The stable dication undergoes a stepwise two electron reduction to the radical monocation and thereafter further to its neutral form. The dication is usually colorless, while the radical cation is strongly colored. The color has been shown to be dependent on the viologen substituent [2] and this feature has been widely utilized in electrochromic displays [2,3]. The neutral form of the viologen is chemically reactive and a strong reducing agent [1]. Due to these special properties viologen-modified materials have been used in different applications such as mediators, electrochromic displays, bioelectronic devices and smart windows [4–9]. Furthermore, viologens and their derivatives are widely
∗ Corresponding author at: Vatselankatu 2, Turku 20014, Finland. Tel.: +358 2 333 6729; fax: +358 2 333 6700. E-mail addresses: carita.kvarnstrom@utu.fi, carkva@utu.fi (C. Kvarnström). 1 Present address: KWH Mirka Ltd., Jeppo 66850, Finland. 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.11.131
utilized in solar cell applications as accepter units together with different donor materials [8,10,11]. Different strategies for preparing insoluble viologenfunctionalized materials have been utilized for a wide range of applications [12]. Polyviologens can be synthesized for example by coupling of cyanopyridine derivatives either by chemical or electrochemical methods [1,13–15]. In comparison to the chemical reaction path, the electrochemical coupling offers a simple, clean and efficient way for synthesizing insoluble polyviologen films directly onto electrode substrates. Furthermore, the electrochemically synthesized viologen film can be easily studied and characterized in situ by different spectroelectrochemical techniques [16]. The mechanism for the electrochemical coupling of cyanopyridines is shown in Scheme 1. Firstly, the cationic cyanopyridinium derivative is reduced to its neutral form, after which it can undergo a dimerization reaction. As the result of the coupling reaction, followed by the splitting of the cyano groups, viologen in its dication form is obtained [1,13]. In the present work, copolyviologen films were synthesized by electrochemical reduction in aqueous solution from two cyanopyridine-based monomers: 1,3,5-Tris(4-cyanopyridinium-1ylmethyl)-2,4,6-trimethylbenzene tribromide (BCP) and 4,4 -bis(4cyanopyridinium-1yl)biphenyl bis-tosylate (LCP). The structures of
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Scheme 1. The mechanism of electrochemical reductive coupling of 4-cyanopyridinium to viologen.
BCP and LCP are shown in Scheme 2(a) and (b), respectively. In the polymerization process, the cyano moieties present in both monomers are reductively coupled, according to Scheme 1, and polyviologen units are formed and adsorbed onto the electrode surface as a film. There are reports on using the monomer material BCP for the electrochemical synthesis of polyviologen films and utilization of the material for electrochemical anion recognition [13,17]. In this work the electrochemical copolymerization concept is reported, aiming at preparing a redox and conjugated polymer in a one batch synthesis. The electrochemical polymerization constitutes a controllable way for synthesizing copolymers from monomers with slightly different reduction potentials. By tuning the monomer concentrations and the applied potential range used for polymerization the coupling sequence for the two monomer units can be tuned. Due to the unique structure, the monomer LCP has the potential to enlarge the cavity size in the copolyviologen film and in such a way to alter its properties. Different concentration ratios of the monomer materials were tested for electrochemical synthesis of the copolyviologen films. Finally, the polyviologen and copolyviologen films were characterized with cyclic voltammetry, UV–vis and FTIR spectroscopy and by the AFM technique. 2. Experimental 2.1. Chemicals Potassium chloride was purchased from Oy FF-Chemicals Ab and was used to prepare the electrolyte solution. All solutions used in the electrochemical experiments were prepared with deionized water, and purged with N2 before use. 2.2. Synthesis of monomers 1,3,5-Tris(4-cyanopyridinium-1-ylmethyl)-2,4,6-trimethylbenzene tribromide (BCP) was prepared according to previous reports [13,17]. The starting materials: 2,4,6-Tris(bromonethyl)mesitylene (98%, Sigma–Aldrich) and 4-cyanopyridine (98%,
Sigma–Aldrich) were dissolved in acetonitrile and refluxed for an hour. Recrystallization from water–methanol mixture gave the pure product. The unique synthesis of 4,4 -bis(4-cyanopyridinium-1yl) biphenyl bis-tosylate (LCP) was obtained by dissolving 1-(2,4dinitrophenyl)-4-cyanopyridinium tosylate (4 g, 9.0 mmol), which was synthesis by 2,4-dinitrophenol (MERCK) and p-toluenesulfonyl chloride (Sigma–Aldrich), in a 8:2 ethanol–water-solution. Benzidine (650 mg, 3.53 mmol, MERCK) in 8 ml of ethanol was added drop wise and the blackened mixture was refluxed for 40 h. The mixture was evaporated to dryness and dissolved into boiling ethanol and filtered while hot. The precipitate was recrystallized twice from a methanol–water mixture obtained by adding water to the boiling methanol mixture until the product dissolved. Filtration resulted in 1.63 g of product as 66% yield.
2.3. Electrochemistry The cyclic voltammetric experiments were done in a threeelectrode one-compartment cell connected to an Autolab (PSTAT 10 and PGSTAT101) potentiostat. The polyviologen films were polymerized electrochemically on a glassy carbon (GC) electrode with 1 mm Ø (purchased from Cypress Systems Inc.). The GC electrode was polished with diamond paste (¼ to 1 m, Struers A/s) and rinsed with deionized water before use. Saturated calomel electrode (SCE) and Ag/AgCl electrode were utilized as reference electrodes, obtained from Radiometer Analytical and eDAQ, respectively. A coiled Pt wire (1 mm Ø) was used as a counter electrode. The electrochemical polymerization was carried out in aqueous solution. The polymerization of BCP was carried out in the concentration of 5 mM, whereas the co-polymerization was made in a solution of 2.5 mM BCP and 0.5 mM LCP. The polyviologen films were synthesized by potentiodynamic cycling between 0 and −1.2 V or −1.4 V during 20 consecutive cycles using 50 mV/s as scan rate. After polymerization the film was rinsed with deionized water and 0.1 M KCl solution after which the redox response
Scheme 2. Structures of cyanopyridinium-based viologen monomers (a) 1,3,5-Tris(4-cyanopyridinium-1-ylmethyl)-2,4,6-trimethylbenzene tribromide (BCP) and (b) 4,4 bis(4-cyanopyridinium-1yl)biphenyl bis-tosylate (LCP).
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was measured in monomer free electrolyte solution. A 0.1 M KCl solution was used as electrolyte in all electrochemical experiments. 2.4. In situ UV–vis NIR spectroscopy In the UV–vis experiments indium tin oxide (ITO) glass was used as a working electrode (8–12 , Delta Technologies Limited). The ITO glass was cleaned with ultrasonication in acetone, ethanol and deionized water solutions for 15 min each. The measurements were made in a 1 cm path length quartz cuvette using a Pt-wire as the counter-electrode and Ag/AgCl as the reference electrode. The in situ UV–vis NIR spectra were recorded between 300 and 1100 nm on a Hewlett Packard 8453 spectrophotometer. For the UV–vis spectroelectrochemical experiments the polyviologen/copolyviologen films were deposited onto ITO-glass electrodes by potential cycling between 0 and −1.2 V, using 20 cycles in the case of BPV and 40 cycles in the case of CoPV. With 50 mV/s was used as the scan rate. The UV–vis spectra from the redox response of the polyviologen/copolyviologen films were measured in the potential range of 0 to −1.1 V by changing the potential with 100 mV steps between the recordings. The background spectrum was measured using a blank ITO glass in electrolyte solution. 2.5. FTIR spectroscopy Fourier transform infrared (FTIR) spectra of polyviologen and copolyviologen films were measured by using a dry-air-purged Nexus 870 FTIR spectrometer (Nicolet) equipped with a DTGS detector in the transmission mode. For each spectrum, 256 scans at a resolution of 4 cm−1 were co-added. The polyviologen and copolyviologen films were synthesized on the ITO glass as described above, and the film was swelled in 0.1 M KCl. Samples of 2 mg of peeled films were oven dried and mixed with 200 mg KBr and pressed into pellets. 2.6. Atomic force microscopy (AFM) Samples for AFM were prepared by electrochemical synthesis of BPV and CoPV on silicon wafers. The BPV and CoPV were synthesized by cyclic voltammetry for 32 cycles from 0 to −1.2 V vs. Ag/AgCl. The AFM images were measured with a Veeco diCaliber AFM instrument (Veeco Instruments Inc.) under ambient conditions in the tapping mode using silicon tips with a resonance frequency of 320 kHz. The tip was 10–15 m in height and had the radius curvature of 8 nm. The polished silicon wafers were obtained from OKMETIC (phosphorus doped, 111) and cut to 10 by 10 mm chips. The Si-wafers were cleaned with ultrasonication in acetone, ethanol and deionized water solutions for 15 min each. 3. Results and discussion 3.1. Electroreductive polymerization of BCP Fig. 1 shows the cyclic voltammograms of the reductive polymerization of 5 mM of monomer BCP in 0.1 M KCl aqueous solution at a scan rate 50 mV/s on the GC-electrode. The first cycle can be seen in the insert in Fig. 1, in which a sharp reduction and oxidation peak can be observed at approx. −0.7 V and −0.5 V, respectively. The reduction peak is sharp due to the reduction of the 4-cyanopyridium moiety into its neutral form which involves a coupling of the moieties into a viologen unit. In the reverse scan, the viologen unit is oxidized to the dication form. During the following scans, the reduction peak becomes broad and is shifted successively from −0.7 V to −0.9 V. Additionally, another weak reduction peak is formed at approx. −0.55 V. The current for the oxidation peak is also radically decreased and shifted toward higher potentials and
Fig. 1. Cyclic voltammograms from the polymerization of the monomer BCP in 0.1 M KCl aqueous solution at a scan rate 50 mV/s on the GC-electrode. The inset graph shows the first cycle in the monomer solution.
a new oxidation peak is formed at approx. −0.8 V. The shift of the reduction peak toward more negative potentials can be explained by the dimer or oligomer formation leading to the deposition of the formed polyviologen chains at the electrode surface. Due to the structure of monomer BCP, the resulted polyviologen film was named as BPV (Branched PolyViologen). Fig. 2a shows the redox response of the film from monomer BCP in 0.1 M KCl cycled from 0 to −0.7 V to successively higher switching potentials reaching finally −1.3 V. The redox response from the formed polyviologen film is at a more negative potential in comparison to the cyanopyridine starting materials. Peaks from two redox reactions can be observed in the cyclic voltammograms; the first reduction and oxidation peaks (Er1 and Eo1) are at approx. −0.55 and −0.45 V, which is caused by the radical-cation formation of the viologen. The second redox couple (Er2 and Eo2) at approx. −0.9 and −0.8 V is from the conversion of the charged BPV to the neutral form. As already discussed, during electropolymerization of BCP, new reduction and oxidation peaks were formed in the cyclic voltammogram after continuous cycling. By comparing Figs. 1 and 2a, it can therefore be concluded that the cyanopyridine monomer BCP has been successfully polymerized on the electrode surface and that the obtained redox response shows typical polyviologen features. In order to know if the system is reversible according to the Randles–Sevcik equation [14], the redox response of the BPV was measured from 0 to −1.1 V in monomer-free electrolyte solution using 10, 20, 50 and 100 mV/s as scan rates. The response of BPV can be seen in Fig. 2b and the calculated result was shown in Fig. 2c. In Fig. 2c, it can be seen that the currents from all reduction/oxidation peaks are proportional to the square root of the scan rates indicating that the redox reaction of the polyviologen film is reversible and diffusion controlled, which suggests that the BPV film has an open, crosslinked structure. 3.2. Electroreductive polymerization of CoPV Due to its linear and conjugated structure the synthesized cyanopyridine monomer LCP has the potential to form new polyviologen structures (LPV), which would have good electrochemical properties [18]. During reductive polymerization of the monomer LCP a solid black product was formed at the electrode surface. The adsorbed material was very unstable and did not give a proper redox response during potential cycling. A series of experiments
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were repeated with various concentrations of the monomer material (2 mM, 1 mM, 0.5 mM), however, film formation was poor, presumably due to the high solubility of the intermediates. Based on the research with monomers of similar structure [19] and in order to enhance the redox performance and enlarge the cavities formed from the branched starting materials, the branched BCP and linear LCP were introduced into the system for the synthesis of copolyviologen materials. Fig. 3a shows the cyclic voltammograms of the copolymerization of 2.5 mM BCP and 0.5 mM LCP in 0.1 M KCl aqueous solution at a scan rate of 50 mV/s. CoPV films were synthesized at different concentration ratios of the monomer materials using otherwise identical conditions for polymerization. A comparison of the redox response in monomer free electrolyte solutions for these CoPV films is shown in Fig. 3b. Based on the redox response and charging capability, the CoPV which was synthesized at the concentration ratio of 2.5 mM BCP and 0.5 mM LCP gave the best results and therefore this was used onwards as the concentration ratio in the copolymerization experiments. The first cycles from the polymerization experiments of the CoPV, BPV and LPV are shown in Fig. 3c. In comparison to the first cycle of the synthesis of BPV the copolymer CoPV shows additional redox peaks at approx. −0.5 V and −0.35 V, nicely overlapping with the peaks obtained during the first polymerization cycle of BPV. The sharp reduction peak at approx. −0.5 V proves that the cyano group of the monomer LCP was successfully coupled, leading to the viologen unit. The oxidation peak at approx. −0.35 V in the reverse scan is from the viologen unit that is charged to its dication form. In the following scans the reduction peaks shift to more negative potentials and a new oxidation peak was formed at approx. −0.75 V. As described above, this indicates the formation of dimers or oligomers deposited onto the electrode surface. Fig. 4a shows the redox response of the copolyviologen film covered GC-electrode in the monomer free 0.1 M aqueous KCl solution. As described above, the redox couple at approx. −0.55 and −0.45 V originates from the charging of the dicationic viologen unit to the radical cation form and vice versa. The redox couple at approx. −0.9 and −0.8 V results from further reduction of the radical cation to its neutral form. The redox potentials are identical to the potentials obtained from the BPV film shown in Fig. 2a. However, it should be noticed that the currents of the second redox couple are much lower for the copolymer in comparison to the same redox response from the film of only BCP. In Fig. 4b, the redox response of the copolyviologen was measured at different scan rates and plotted vs. square root of the scan rate in Fig. 4c. The linear relationship between the current and the square root of the scan rate shows the diffusion dependence of the film charging. As described above, the redox reactions in the copolyviologen film are reversible, which indicates a film structure consisting of channels leading the counter ions in and out from the large cavities. 3.3. UV–vis spectra of polyviologen and copolyviologen films
Fig. 2. (a) Cyclic voltammograms of the BPV deposited GC-electrode in the monomer-free electrolyte from 0 to −0.7 V successively increasing the switching potential, finally reaching a value of −1.3 V using 50 mV/s as scan rate. (b) Cyclic voltammograms of the BPV from 0 to −1.1 V using different scan rates (10, 20, 50 and 100 mV/s). (c) The reduction/oxidation peak currents for the two redox pairs vs. the root of the scan rates.
The UV–vis spectra showing the –* transition of the two viologen precursors in aqueous solution (20 M) are shown in Fig. 5. BCP absorbs far below 300 nm, while LCP has a strong and broad absorbance band at approx. 350 nm. Fig. 6a shows the in situ UV–vis spectra of the BPV film covered ITO-glass under potentiostatic conditions recorded at every 100 mV from 0 to −1 V vs. Ag/AgCl electrode. The absorbance bands of the film at around 400 and 600 nm start to grow at potentials above 0.2 V. The rising of these bands can be explained by the formation of the radical cation in the viologen units. The potential correlates well to the potential seen in the cyclic voltammogram where the first reduction peak of the viologen starts to develop, shown in Fig. 2a. The absorbance of the band at 600 nm achieved
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Fig. 3. (a) Electropolymerization of the monomer BCP (2.5 mM) and LCP (0.5 mM) in 0.1 M KCl solution in the potential range 0 to −1.2 V with the scan rate 50 mV/s. (b) Cyclic voltammograms in the potential range 0 to −1.2 V of the various concentration-based CoPV films on the GC electrode in monomer free electrolyte solution with the scan rate 50 mV/s. The concentration ratios are given as concentration of BCP vs. LCP (c). The first cycles obtained from the electropolymerization of CoPV, BPV and LPV in the potential range 0 to −1.2 V.
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Fig. 4. (a) Cyclic voltammograms of the copolyviologen deposited GC-electrode in monomer free electrolyte in the potential range 0 to −0.7 V successively increasing the switching potential, finally reaching a value of −1.1 V using 50 mV/s as scan rate. (b) Cyclic voltammograms of the copolyviologen deposited GC-electrode in monomer free electrolyte solution with the scan rates of 10, 20, 50 and 100 mV/s. (c) The reduction/oxidation peak currents for the two redox pairs vs. the root of the scan rates.
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Fig. 5. The UV–vis spectra of the monomers BCP and LCP in aqueous solution.
Fig. 7. FTIR spectra of (a) the monomers BCP and LCP and (b) the BPV and CoPV films as KBr pellet.
Fig. 6. The in situ UV–vis spectra of (a) polyviologen film (monomer BCP) and (b) copolyviologen film covered ITO-glass under potentiostatic conditions, spectra recorded at every 100 mV from 0 to −1 V (vs. Ag/AgCl electrode). The potential dependent intensities of the absorbance bands at 400 nm and 600 nm are shown as inserts in the top right of Fig. (a) and (b).
its highest value when the potential reached −0.6 mV. The intensity of this band starts thereafter to decrease with increasing negative potential, which is due to further reduction of the radical cation to its neutral form. The absorbance peak at 400 nm was increasing linearly from the start with increasing potential and the band intensity reached its maximum value at −1 V, shown in Fig. 6a. The continuous increase in absorbance throughout the whole reduction can be explained by that both the cationic form and the neutral form of viologens absorb close to 400 nm [12]. The neutral form absorbs more strongly than the cationic form resulting in the further increase of absorbance at higher negative potentials. The potential dependent intensities of the absorption bands at 400 nm and 600 nm are shown in the top right of Fig. 6a. A weakly increasing absorbance at 900 nm is regarded to be from an allowed transition within the viologen dimer unit, however not a charge-transfer transition according to Kosower and Cotter [15]. Fig. 6b shows the UV–vis spectra of the CoPV film covered ITOglass, and the intensities of the absorption peaks vs. potential are shown on the top right. The behavior of the absorption bands were almost the same as for BCP in Fig. 6a. The absorption bands at 400 nm and 600 nm were caused by the absorbance of the radical cation, and further increase of the band at 400 nm is due to the conversion into the neutral form of the viologen units. However, the continuous increase of the band at 400 nm is here interrupted
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Fig. 8. AFM images of (a) BPV and (b) CoPV add the size of the AFM pictures.
by a formation of a plateau in the intensity vs. potential plot shown as an insert in the top right of Fig. 6b. In this potential region there is a counter balance between the cation formation and the neutral form. In the copolyviologen these are due to incorporation of the LCP regions of longer effective conjugation, which probably stabilizes the cationic form thus slightly delaying the conversion to the neutral form. 3.4. FTIR characterization of the polyviologen and copolyviologen films In Fig. 7a, the FTIR spectra of the two monomers are shown. Since both of the monomers are cyanopyridine-based, the spectra are rather similar with a few exceptions. Both spectra have a strong band at approx. 580 and weaker at 2200 cm−1 , which are caused by the C N vibration of the aromatic nitriles. Strong bands can also be observed at 1500–1400 cm−1 , which are assigned to C C and C N stretch vibration [20]. The bands at 1250–1200 cm−1 are from the in-plane deformations of the pyridine groups. In the spectrum of the monomer BCP a series of weak bands unique for the monomer can be seen at 780–700 cm−1 from the 1,3,5-trisubstituted benzenes and at approx. 1480 cm−1 from the CH2 group. The monomer LCP has its own specific bands at approx. 1950 cm−1 from the 1, 4-disubstituted benzene and the bands at approx. 850 cm−1 from the biphenyl. The strong band at approx. 1020 cm−1 was caused by the counter anion tosylate (TOS− ). As described above in the polymerization process the cyano groups were reductively coupled to the viologen unit forming polyviologens. The FTIR spectra of the polymers were measured and are shown in Fig. 7b. The two polymer films have a similar spectrum, which indicates that the BCP is the major unit and works as the skeleton in the copolyviologen film. In the spectra, however there are weak bands at approx. 2300 and at 600 cm−1 , which indicates
that most cyano groups were reductively coupled into viologens. In the copolyviologen spectrum the bands around 1500 cm−1 and 1000 cm−1 are assigned to biphenyl structures in the monomer LCP indicating successful incorporation of both monomers. 3.5. AFM measurement of the BPV and CoPV The surface morphology of BPV and CoPV films was studied by AFM. The images are shown in Fig. 8. Film deposition from BCP is heterogeneous and the distribution of the film is irregular. The surface of CoPV was smoother in comparison to the BPV film, which means that by importing the LCP into CoPV the large cavity structure can be formed. The scratch seen in the AFM image 8b is probably from a crack while drying. An estimation of the film thickness of approx. 1.1 m was made from the crack. 4. Conclusion Two kinds of polyviologen structures were electrosynthesized. The three branched structure of the BCP precursor should result in a network of polyviologen chains with a specific mesh size. Introduction of the linear LCP precursor should enhance the length of the polymer chains and form a network with a bigger mesh size. It was shown that the viologen units in the copolymer film can successfully undergo two reversible reduction steps. Tuning of the mesh size in the network of polyviologens with both redox and conjugated conductance makes them interesting materials to utilize as redox-active host materials for immobilizing macromolecules of different size. Comparing the redox properties of both polyviologen and copolyviologen films, the redox potentials remained the same but the relative intensity of the current responses were more uniform in the copolyviologen in comparison to the plain BCP film.
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The UV–vis and FTIR spectra show the presence of both starting materials giving the evidence for a copolyviologen structure that could work as a functional material in electronics. Acknowledgment This work is funded by Academy of Finland and by Graduate School of Chemical Sensors and Microanalytical Systems (CHEMSEM, Finland). References [1] P.M.S. Monk, The viologens: physicochemical properties, synthesis, and applications of the salts of 4,4’-bipyridine, John Wiley & sons Ltd, West Sussex, England, 1998. [2] V. Jain, H. Yochum, H. Wang, R. Montazami, M.A.V. Hurtado, A. MendozaGalván, H.W. Gibson, J.R. Heflin, Solid-state electrochromic devices via ionic self-assembled multilayers (ISAM) of a polyviologen, Macromolecular Chemistry and Physics 209 (2008) 150. [3] D.M. DeLongchamp, M. Kastantin, P.T. Hammond, High-contrast electrochromism from layer-by-layer polymer films, Chemistry of Materials 15 (2003) 1575. [4] H.B. Yildiz, R. Tel-Vered, I. Willner, Solar Cells with Enhanced Photocurrent Efficiencies Using Oligoaniline-Crosslinked Au/CdS Nanoparticles Arrays on Electrodes, Advanced Functional Materials 18 (2008) 3497. [5] M. Riskin, R. Tel-Vered, I. Willner, The Imprint of Electropolymerized Polyphenol Films on Electrodes by Donor-Acceptor Interactions: Selective Electrochemical Sensing of N,N -dimethyl-4,4 -bipyridinium (Methyl Viologen), Advanced Functional Materials 17 (2007) 3858. [6] D. Cummins, G. Boschloo, M. Ryan, D. Corr, S.N. Rao, D. Fitzmaurice, Ultrafast Electrochromic Windows Based on Redox-Chromophore Modified Nanostructured Semiconducting and Conducting Films, Journal of Physical Chemistry B 104 (2000) 11449. [7] H.C. Ko, S. Kim, H. Lee, B. Moon, Multicolored Electrochromism of a Poly{1,4-bis[2-(3,4-ethylenedioxy)thienyl]benzene} Derivative Bearing Viologen Functional Groups, Advanced Functional Materials 15 (2005) 905.
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