Electrochemical synthesis and characterization of branched viologen derivatives

Electrochimica Acta 154 (2015) 361–369

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Electrochemical synthesis and characterization of branched viologen derivatives Nianxing Wang a,b , Anniina Kähkönen a , Pia Damlin a , Timo Ääritalo a , Jouko Kankare a , Carita Kvarnström a, * a Turku University Center for Materials and Surfaces, c/o Laboratory of Materials Chemistry and Chemical Analysis, University of Turku, Vatselankatu 2, FI20014 Turku, Finland b University of Turku Graduate School (UTUGS), FI-20014, Turku, Finland

A R T I C L E I N F O

A B S T R A C T

Article history: Received 17 November 2014 Received in revised form 12 December 2014 Accepted 12 December 2014 Available online 15 December 2014

In this work two three-branched cyanopyridine based monomers were synthesized and used as starting materials for the preparation of viologen derivatives with branched structures. The film synthesis was performed using reductive electropolymerization in aqueous solution. Both electrochemical and spectroelectrochemical characterization proved that the viologen films undergo a well-defined and reversible two step redox reaction, which is the typical performance for viologen materials. Furthermore, FTIR and SEM were utilized to characterize their structures and morphologies. SEM studies showed that the viologen films have a highly porous structure. Additionally, discharging experiments confirmed that the viologen films show different intercalation behavior as size of electrolyte anion had various effects on the redox potential. Finally, it has been proven that these two viologen derivative materials have the potential to be utilized as a supporting material with intrinsic redox activity and with excellent properties for hosting especially negatively charged particles, molecules and macromolecules. The promising properties of these materials make them good candidates in electronics and also in organic solar cells when loaded with large molecules such as fullerenes. In the organic rechargeable battery applications they can be used as anode-active materials as they exhibit high charging-discharging capacity at negative potentials. ã 2014 Elsevier Ltd. All rights reserved.

Keywords: viologen derivatives redox properties UV–Vis FTIR branched structure electropolymerization

1. Introduction The viologens, usually entitled as 1,10 -disubstituted-4,40 -bipyridinium salts, have been in the focus of increasing attention due to the unique properties they possess [1,2]. One of their key features is the three, one-electron redox states (V0, V+ and V2+) that exhibit rapid, reversible rates of electron transfer in the potential range of 0.4 V to 1.0 V vs Ag/AgCl. The three redox forms are shown in Scheme 1: the dication is the most stable of the three forms and usually colorless, however, the radial cation is strongly colored, and the neutral form can function as strong reductive agent [1,3]. The properties of the viologen unit can be tuned by functionalization by different groups resulting in changes in steric structures or redox potential of the formed polymer. Due to the unique redox property, the viologens are widely utilized in various applications,

* Corresponding author at: Vatselankatu 2, Turku, 20014, Finland. Tel.: +358 2 333 6729. E-mail address: carita.kvarnstrom@utu.fi (C. Kvarnström). http://dx.doi.org/10.1016/j.electacta.2014.12.075 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.

e.g. sensors, fuel cells, solar cells and electrochromic devices [4–8]. Additionally, during the past decade, viologens has attracted much attention as components in composites combined with carbon nanotubes [9], C60 [10], or with supra molecules having potential for being utilized in nano-devices [11,12]. Generally viologens are synthesized by chemical methods [13,14], however, they can also be synthesized through electrochemical pathways by reduction of cyanopyridine-based monomers, reaction mechanism shown in Scheme 2 [15–17]. Electrochemical synthesis has its own advantages in comparison to chemical methods, as efficiency and possibility to control film thickness of the produced materials. Additionally, electrochemical synthesis introduces fewer impurities to the polymer material and furthermore characterization can be made simultaneously during the synthesis and redox processes by applying spectroscopic or electrochemical techniques directly to the electrochemical cell. As mentioned above, viologens are due to their unique redox properties widely applied in various electronic devices. However, conventionally viologens are immobilized/attached as pendant

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polystyrene sulfonate (NaPSS) which was purchased from Aldrich was also prepared as the electrolyte with the concentration 0.02 g/ml. All solutions used in the experiments were prepared from deionized water and purged prior to use with N2. 2.2. Synthesis of monomers T1 and T2 2.2.1. 1,3,5-Tris(4-cyanopyridinium-1-ylmethyl)-benzene tribromide (T1) 1,3,5-tribromomethylenebenzene (5 g; 14 mmol) was dissolved in acetonitrile (100 ml). P-cyanopyridine (16 g; 150 mmol) was added and the solution was refluxed overnight. The formed precipitate was purified by dissolving it into methanol and then precipitating it with diethyl ether. 7.8 g of product was obtained with 83% yield. The synthesis was performed according to previously described procedure [15,16].

Scheme 1. Three redox forms of viologens.

groups to the polymer [18,19], in which utilization of the electron transfer involved in the viologens is restricted and limits their application. In order to enhance the redox property and broaden the application of viologens, a series of branched viologen films have been synthesized in our group [20,21]. The viologen films were electrosynthesized from three-branched cyanopyridinebased monomers, with the goal of forming branched structures with charged cavities of desired sizes. Due to the special structure the viologen derivative is not simply working as a redox material, but also as a host material for immobilizing macromolecules. Furthermore, the highly branched viologen film has also a considerable number of viologen moieties per unit in order to obtain a rapid and stable electron transfer process. The promising properties of these materials make them good candidates in electronics and also in optoelectronics while the structure allows loading of pretty large molecules. In this work, the monomers (T1 and T2, sketched in Scheme 3) were synthesized as building block for electrochemical polymerization of branched viologen structures. Finally, the electrochemically synthesized viologen films (PolyT1, PolyT2) were characterized by electrochemical and different spectroscopic techniques (UV–Vis and FTIR) and the surface morphologies were studied using SEM. It was proven that both viologen materials show a reversible and rapid two-electron redox reaction in aqueous media. Depending on the size of anion a shift in the position of the redox peaks could be observed. 2. Experimental 2.1. Chemicals Potassium chloride (KCl) and sodium sulfate (Na2SO4) were purchased from Oy FF-Chemicals Ab, and both of them were prepared as electrolytes with the concentration of 0.1 M. Sodium

2.2.2. 1,3,5-Tris[(4-cyanopyridinium-1-ylmethyl)- phenyl] benzene tribromide (T2) 4-Methylacetophenone (6 ml, 45 mmol) was dissolved in acetonitrile (150 ml), radical activator benzoylperoxide was thereafter added and the mixture was refluxed until the starting material was consumed, (HPLC). After filtration, the filtrate was evaporated under reduced pressure and 3.2 g of the pure product, 4-(bromomethyl) acetophenone was obtained by crystallization from 1:10 ethyl acetate–petroleum ether. 4-(Bromomethyl) acetophenone (0.75 g; 3.52 mmol) was dissolved in ethanol (3.5 ml) and cooled to 0  C. Tetrachlorosilane (7.2 ml; 63 mmol) was added dropwise. HPLC shows several products and 1H-NMR confirms partial replacement of bromides with chlorides. Purification was done by filtration through a small silica layer using 1:1 hexane-DCM as eluent, m = 0.4 g. Terphenyl (2.9 g; 4.95 mmol) and p-cyanopyridine (5 g; 50 mmol) were mixed into a capped tube. The tube was then heated at 100  C for 3 days. 3.1 g of the pure product was obtained by dissolving the material into methanol and precipitated with diethyl ether. In Supplementary Information the synthesis scheme (Scheme S1) and 1H-NMR analysis results are shown for monomer T2. 2.3. Electrochemistry The cyclic voltammetric experiments were done in a three-electrode one-compartment cell connected to an Ivium Technologies potentiostat. The viologen films were synthesized electrochemically on a glassy carbon (GC) electrode with 1 mm Ø (purchased from Cypress Systems Inc.). The GC electrode was polished with diamond paste (1/4 to 1 mm, Struers A/s) and rinsed with deionized water before use. Ag/AgCl electrode was utilized as reference electrode obtained from eDAQ (2 mm Ø). A coiled Pt wire (1 mm Ø) was used as counter electrode. The electrochemical polymerization was carried out in aqueous solution according to the procedure reported for polyviologens [20]. Polymerization of both T1 and T2 were carried out in the concentrations of 5 mM in 0.1 M KCl solution, and the viologen films were synthesized either

Scheme 2. Coupling reaction of reduced cyanopyridine group.

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Scheme 3. Structures of the cyanopyridine-based monomers. (a) 1,3,5-Tris(4-cyanopyridinium-1-ylmethyl)-benzene tribromide (T1). (b) 1,3,5-Tris[(4-cyanopyridinium-1ylmethyl)- phenyl] benzene tribromide (T2).

In the UV–vis experiments indium tin oxide (ITO) glass was used as a working electrode (8–12 ohms, Delta Technologies Limited). The ITO glass was cleaned with ultrasonication in acetone, ethanol and deionized water solutions for 15 minutes each. The measurements were made in a 1 cm path length quartz cuvette using a Pt-wire as counter-electrode and Ag/AgCl as reference electrode. The in situ UV–vis spectra were recorded between 300 and 1100 nm on a Hewlett Packard 8453 and Agilent Technology Cary 60 UV–vis spectrophotometer. The viologen films were deposited onto ITO-glass electrodes by potential cycling between 0 to 1.2 V using 50 mV/s as scan rate and 40/20 cycles. The UV–vis spectra from the redox response of the viologen films were measured in the potential range of 0 to 1.1 V at every 100 mV. The background spectrum was measured using a blank ITO glass in electrolyte solution.

The morphologies of viologen films (PolyT1 and PolyT2) were measured with scanning electron microscopy (SEM), model LEO 1530, LEO Electron Microscopy Ltd Germany. The viologen films were synthesized on ITO glass as described above in part 2.4. 3. Results and discussion 3.1. Electrochemical synthesis and characterization Fig. 1 shows the cyclic voltammograms recorded during electrochemical synthesis of PolyT1 in aqueous solution, using 20 cycles in the range of 0 to 1.2 V (vs. Ag/AgCl) and a scan rate of 50 mV/s. During the electrochemical reduction the monomers were initially reduced to their radical cation state at approx. 0.6 V after which the radicals are coupled and the cyano groups are removed from the pyridine leaving a viologen unit between two T1 monomers. Further increase of the negative potential involves the transformation of the formed viologen unit to its neutral form. In the reverse scan the viologen unit is reversible oxidized to the

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Fourier transform infrared (FTIR) spectra were measured by using a dry-air-purged Nexus 870 FTIR spectrometer (Nicolet) equipped with a MCT-A detector. The monomers were measured as KBr pellets but the viologen films were measured from the ITOglass directly by single reflection (Harrick SeagullTM Variable Angle Reflection Accessory). For each spectrum, 512 scans at a resolution of 4 cm1 were co-added. The monomers were dried and mixed with 200 mg KBr and pressed into pellets. The viologen films were synthesized on ITO glass as described above in part 2.4.

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by potentiodynamic cycling between 0 to 1.2 V during 20 or 10 consecutive cycles using 50 mV/s as scan rate or alternatively potentiostatically at 0.8 V for a specific time. All solutions were purged with nitrogen and experiments were performed under nitrogen. Prior to the measurement of the redox response in monomer free electrolyte solution the film was rinsed with deionized water and 0.1 M KCl solution to remove traces of monomer. In the discharging experiment, the films were synthesized in presence of KCl, Na2SO4 and NaPSS. These films were initially kept for 1 minute at the potential corresponding to the second reduction in different ion containing electrolytes. Thereafter, the potential was removed and the potential vs. time was recorded.

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dication state. Additionally partial oxidation of the coupled cyanopyridine groups might take place at 0.4 V depicted as the reversible step in Scheme 2. The monomer structure influences the reaction potential, and in an earlier work we studied the difference in redox properties of viologen films when using a linear monomer structure to that obtained upon copolymerization with a branched structure [20]. In the following scans, the reduction peak was successively shifted from 0.6 V to 0.7 V, simultaneously as another reduction peak was developed at approx. 0.42 V. The oxidation peak was also shifted from 0.45 V to 0.38 V and a new oxidation peak was formed at 0.68 V. The shift of the redox peaks can be explained by the formation of the dimer or oligomer leading to the deposition of the formed viologen films at the electrode surface, with increasing number of cycles the thickness of the film was increased. After electropolymerization a slightly yellow film was formed on the GC substrate that was insoluble in water and common organic solvents, which indicate that the formed viologen film possessed a highly cross-linked structure due to the branched monomer T1 and T2. After polymerization, the electrochemical response of PolyT1 was characterized by cyclic voltammetry. Fig. 2a shows the redox response of PolyT1 in monomer-free electrolyte from 0 to 1.2 V using scan rates ranging from 5, 10, 20, 50 to 100 mV/s. The first redox couple at approx. 0.39 and 0.32 V is from the

reduction of the dicationic form to radical cation state, which are indicated as R1 and O1. Further reduction of the radical cation to the neutral state is seen as a second redox couple at approx. 0.82 and 0.68 V, which are indicated as R2 and O2. The influence of scan rate on the electrochemical response is shown in Fig. 2b. The oxidative currents grow with the scan rates (alternatively vs square root of the scan rates) although without showing any linearity. Neither the Randles–Sevcik or the ideal Nernstian behavior is fully valid for redox polymers. The ideal behavior is approached for relatively slow scan rates and for an adsorbed layer that shows no intermolecular interactions and fast electron transfer. However, the redox processes are quite complicated, and it may be that the diffusion control did not dominate the full range of used scan rates [22]. Furthermore, the peak-to-peak separation for the viologen film was DE1 = 70 mV and DE2 = 140 mV indicating electrochemically quasi reversible electron transfer within the viologen film. According to Bruinink et al. [24] the film deposition of viologen material takes place due to a radical salt formation. In cases when the potential scan is reversed directly after the first reduction the changes involved in the cathodic (Qc) and anodic (Qa) scans approach unity. Extending the scan over the second peak introduces aging products in the film that might cause crystallization which influences the electrochemical response. Additionally viologen dimerization takes place in the film as will be discussed later in the UV–vis section, interactions that influence the peak-to-peak separation as well. The slight asymmetry in the broadness of the first redox couple might be due to that the viologen was switched into its conductive phase from the non-conducting phase when it is reduced to the radical cation form. In the reverse scan, the film was brought from its conducting phase to its non-conducting phase. The electrochemical synthesis of PolyT2 in KCl solution was made by potential cycling between 0 and 1.2 V during 10 cycles using 50 mV/s as scan rate. The CVs from the electrochemical polymerization are shown in Fig. 3, and the insert on the top-left shows the first cycle during the polymerization reaction. The reduction and oxidation peak currents for T2 are at approx. 0.55 and 0.40 V respectively, and show a small shift in reduction potential compared to T1, indicating T2 being slightly easier to reduce. In the following scans, the reduction peak was shifted from 0.55 V to 0.8 V, meanwhile the oxidation peak was moving between 0.40 V to 0.3 V. The current potential behavior with increasing scans is explained by dimer and oligomer deposition onto the electrode surface during the polymerization of monomer

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T2. The larger shift in peak potential in comparison to polymerization of T1, together with the decrease in peak currents during cycling indicate the formation of a less conducting deposit. The electrochemical response of the PolyT2 was characterized in monomer free electrolyte by cyclic voltammetry, shown in Fig. 4a. The first redox couple is at 0.42 V and 0.30 V, and the second redox couple at approx. 0.82 V and 0.71 V. The presence of the two redox couples indicates a successful formation of a viologen film. The current vs scan rate plot of the PolyT2 film shows strong deviation from linearity at high scan rates, Fig. 4b. The peakto-peak separation for both redox couples was DE1 = DE2 = 120 mV. The first step is less reversible comparing to PolyT1, this might be due to lower redox activity in the formed structure. The UV–vis measurements (shown later) indicate a lower degree of viologen dimerization in polyT2 in comparison to polyT1. The lower degree of interactions in the film influences also the peak currents and DE. The value for the peak separation is similar to other polymers, which shows that the redox reactions are reversible and not diffusion controlled. The difference in internal current intensity of the redox couples in PolyT1 and PolyT2 is remarkable. Depending on the type of derivative the intensity of the first redox couple in the viologen unit has been found to vary a lot [24]. According to earlier reports [3] one reason could be that the viologen radical quickly reacts with traces of oxygen that might be dissolved in the electrolyte finally forming dicationic viologens and hydroxyl anions. In our work, however, the experimental conditions in

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the formation of the two polymers were identical which points to that the reason for the current intensity variations would origin from the structure of the viologen derivative or as discussed above from the electrolyte anions and their the probability of viologens radical salt formation. 3.2. Influence of the anion on the redox properties of the viologen films The influence of the anion on the reduction of the viologen unit has been thoroughly studied in water solutions [3]. It can be assumed that the same reactions take place to some extent also in the solid phase of a viologen film. The electrochemically made viologen films where therefore synthesized and cycled in different electrolytes within the potential range from 0 to 1.2 V in order to study the anion induced changes in the redox reactions of the polymer. Fig. 5a–e show the CVs of PolyT1 and PolyT2 polymerized in presence of KCl, Na2SO4 or sodium polystyrene sulphonate (NaPSS) and thereafter potentially cycled in each of the three different electrolytes. Fig. 5f–j shows the curves of decay in potential at open circuit displayed as potential vs time after that the films were charged at potentials corresponding to the neutral form of the polymer (approx. 0.8 V). Only films of PolyT1 made and cycled in KCl show a reversible two redox couple response of approximately equal current intensity from both redox couples. Fig. 5a shows that when the electrolyte is switched to Na2SO4 or NaPSS the current from the radical cation, 2PV+*, is damped meanwhile the current from the second reduction leading to the neutral form, PV , is dominating. There are several parameters causing this effect; the size, mobility and valence number of the anion (which counts in the beginning of the reduction), the interactions between the viologen unit and the anion and also the structure of the polymer material [3,23]. All the aforementioned parameters might influence the viologens radical salt formation which might lead to crystallization or recrystallization in the film upon cycling. These properties influences further the rate and presence of the dis- (2PV+* ! PV2+ + PV ) and comproportionation reactions (PV + PV2+ ! 2PV+*) taking place in the film. The highest charging/currents were always obtained for the films synthesized and cycled in presence of the same electrolyte. Fig. 5b shows the CV response of the film made in presence of Na2SO4. When cycling the film in a divalent anion containing (like SO42) electrolyte the radical formation takes place at slightly more negative potentials. The peak potentials for the radical formation in the different films when cycled in each of the three different electrolytes are listed in Table S1. From the Table and Fig. 5a–c one can notice that independently on which electrolyte was used for film preparation, the cycling in Na2SO4 always shows a more negative peak potential for the first redox reaction. In KCl and NaPSS the values of the peak potentials almost match indicating that it is more the valence and not the size of the anion that is important in the first one-electron transfer step (V to V+*). The NaPSS containing film showed a broad, potential shifted second reduction peak preceded by a pre-peak (Fig. 5c). The slight overpotential needed to obtain the neutral state is caused by the almost stationary counter ion, PSS. Other factors that might cause distortion of the voltammogram is the dimerization of the radical viologen cation [25], also referred to pi-dimerization [3] which can be seen in the UV–vis spectra as an absorbance at around 560 nm and second one around 900 nm [26,27], shown in Fig. 6a and b later on in this paper. In the E vs t curves of the PolyT1 films a very fast potential decay can initially be seen until reaching the potential region for the radical form at around 0.4 V (Fig. 5f and g). The stability of the radical cation varies with the type of anion and is at least stable with SO42 when films are made in presence of either Cl or SO42. In the case of films made in presence of

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Fig. 5. a–e show the CV:s of KCl, Na2SO4 or NaPSS prepared PolyT1 and PolyT2 in each of the three different electrolytesat a scan rate 50 mV/s,a) PolyT1(KCl), b) PolyT1 (Na2SO4), c) PolyT1(NaPSS), d)PolyT2(KCl), e) PolyT2(Na2SO4). f–j show the curves of decay in potential at open circuit displayed as potential vs time, f) PolyT1(KCl), g) PolyT1 (Na2SO4), h)PolyT1(NaPSS),i)PolyT2(KCl), j) PolyT2(Na2SO4).

PSS the trend is changing and here the radical form is stable in the case of cycling in Cl or SO42. The two-step plateau voltage near 0.4 and 0.8 V agree well with the redox potential of the films shown in Fig. 5. These results indicate that the charge diffusion for the second redox reaction of PolyT1 (PV to 2PV+*) was much faster than that of the first reaction. PolyT2 shows a very weak redox response from the radical form in all three electrolytes. Nevertheless upon open circuit measurements after charging, all the films finally relax towards a potential representing the state of the radical cation and forms a constant

potential plateau. The formation and presence of the radical cation can also clearly be seen in the UV–vis experiment as an absorbance response around 590 nm (Fig. 6a and b). There is always a strong intermolecular interaction between the viologen cation radicals which gives rise to the comproportionation. The type of anion does not seem to have the same influence on the redox response in PolyT2 as in PolyT1. It has also to be mentioned that in case of using NaPSS as electrolyte the film formation of polyT2 was poor and such films could not be included for the redox and discharging studies.

N. Wang et al. / Electrochimica Acta 154 (2015) 361–369

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The stable radical form of PolyT2 could be utilized in a process where a film needs to be reversibly conducting for some time. Similar systems containing bulky anions has been built from poly (ethylenedioxy thiophene), PEDOT-PSS, with the drawback that it stays in its conducting form and is not able to switch between non conducting-conducting states. In general conducting polymers undergo a conducting-non conducting redox switching but very few of them display a stable radical/polaron state. 3.3. UV–vis spectra of viologen films As described above, the viologen materials can undergo a two-step reductive process, which can also be followed by UV–vis spectroscopy. Fig. 6 shows the in-situ UV–vis spectra of PolyT1 and PolyT2 covered ITO-glass recorded at every 100 mV between 0.1 and 1.0 V vs. Ag/AgCl electrode. When successively higher negative potentials are applied to the PolyT1 film three absorbance bands can be observed at approx. 390, 590 and 920 nm shown in Fig. 6a. The bands were caused due to the formation of the radical cation in the viologen units. With increasing potential, the absorbance bands at 590 and 920 nm reach a maximum absorbance at 0.5 V, after that the intensity of these band starts to decrease with increasing potential during which the neutral state is formed. However, the absorbance of the band at 390 nm was steadily increasing with increasing potential with the exception in the potential region where the radical

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formation takes place, where the growth in absorbance of the 390 nm band is slowed down. This broad band contains information mainly from the neutral form of the viologen. The third transition in UV–vis at around 950 nm from PolyT1 can be assigned to the response from interacting closely spaced viologen units according to studies on viologen dimers or dimer-type interactions [28,29]. Dimerization should also give rise to a second absorbance response around 560 nm that is overlapped by the band around 600 nm from radical cations [27]. When comparing the absorbance of the radical formation for PolyT1 and PolyT2 a shift in the absorbance to longer wavelength can be observed in PolyT2 and hardly any absorbance around 900 nm is seen indicating lesser degree of dimerization. The shift in the radical cation band around 600 nm can be explained by a decrease of the part of the band representing the dimer and an increase in the slightly higher laying absorbance from the single viologen unit. The degree of dimerization is reported to increase with the hydrophobicity of the molecule or the substituent on the bipyridine ring [27]. In case of T1 and T2 it might also be the size of the molecule together with the amount of viologen units that might influence on the dimer formation. In situ UV–vis spectra were also recorded during the reverse scan for both polymers. In the discharging process the film shows the same absorbance behavior but in reverse direction, the viologen unit was re-oxidized from neutral state to its radical cation and finally to its dication state.

(b)

Monomer T2 PolyT2

2000

2300

Wavenumber (cm-1)

3000

2500

2000

2200

2100

2000

Wavenumbers (cm-1)

1500

Wavenumber (cm-1)

1000

500

3000

2500

2000

1500

1000

500

Wavenumber (cm-1)

Fig. 7. FTIR spectra of (a) monomer T1 and PolyT1 and (b) monomer T2 and PolyT2. The inset shows the enlargement of the wavenumber range 2000–2350 cm1 for the monomers.

368

N. Wang et al. / Electrochimica Acta 154 (2015) 361–369

Table 1 Assignments to the IR bands of PolyT1 and PolyT2 films [31,32].

n cm1 PolyT1

n cm1

755, 818, 850

818, 851 781, 825 1215 1452 1510, 1550,

1,3,5-tri-substituted benzene Para-di-substituted benzene N—CHm stretch CH2 scissor vibration C¼N vibration

1683, 1567 1165, 3029 2229 2950

C¼C vibration CHring vibration CRN group C—H stretching vibration

1210 1456 1506, 1519, 1560,1643 1690, 1560 1168, 3031 2235 2930

Assigned to

PolyT2

3.4. FTIR characterization of the viologen films The FTIR technique was utilized to characterize the structures of both monomers T1, T2 and their formed viologen films, and the obtained spectra are shown in Fig. 7. Absorbances observed at different wavenumbers in the spectra together with assignments according to what have been reported in literature for similar vibrational bands are listed in Table 1. When comparing the spectra of monomers T1 and T2 to the ones obtained for the viologen films, the peaks at 2244 (T1) and 2238 cm1 (T2), assigned to the vibration of the —CN group, have substantially diminished. This together with the vibrations from the C¼N stretching vibrations (for bipyridinium) from the viologen groups, observed at 1643 cm1 [30,31] in the spectra of PolyT1 and PolyT2, provides evidence for successful coupling of the viologen from cyanopyridine. However, it should be noticed that there are still weak absorbances in the range 2200–2260 cm1 from the CRN, indicating the presence of some uncoupled cyanopyridine groups in the films. It has also to be pointed out that for PolyT1 and PolyT2 the spectra in Fig. 7

have been multiplied by a factor of 5 in order to increase the intensity of the observed IR peaks. The peaks are broadened and peak positions are shifted downwards in the polymer spectrum in comparison to those observed for the monomer materials, in particular for T1. Furthermore, it can be noticed that the absorbance peaks at around 1690 and 1560 cm1 from the C¼C vibrations are more pronounced for PolyT1. Vibrations due to 1,3,5-tri-substituted benzene can be observed at 850, 818 and 755 cm1 and show for PolyT2 overlapping with the vibrations at 825 and 781 cm1 from the di-substituted benzene rings. As shown in Scheme 3 the monomer materials resembles very much in structure why the obtained IR spectra for the films mainly differ in internal intensity. The CHring vibrations can be observed for PolyT1 and PolyT2 at 1165 and at 3030 cm1. Furthermore, in the high wavenumber region PolyT1 and PolyT2 shows C—H stretching vibrations from the aromatics (3000–3100 cm1) and from alkanes (3000– 2850 cm1). The peak at 1210 cm1 was assumed to be due to the N—CHm stretching. Especially for PolyT2 with an asymmetric group aside the viologen units it is appearing as a strong peak. 3.5. Scanning electron microscope The morphology of the viologen derivative films were studied by SEM. As discussed earlier and from the monomer structures shown in Scheme 3 the synthesized viologen films are expected to form network like structures. In Fig. 8a and b, a top view from PolyT1 and PolyT2 are shown. PolyT1 resulted in a porous film that homogeneously covered the substrate, while again polymerized T2 resulted in less homogenous film morphology. In Fig. 8c and d, the films were detached from the electrode surface in order to estimate the thickness of the films. From the SEM pictures an approximate thickness of 570 nm for PolyT1 and 640 nm for PolyT2 could be obtained.

Fig. 8. SEM images of PolyT1 (a), (c) and PolyT2 (b), (d).

N. Wang et al. / Electrochimica Acta 154 (2015) 361–369

4. Conclusions In this work, two three-branched cyanopyridine monomers (T1 and T2) were synthesized. Furthermore, these monomers were successfully electrosynthesized forming two branched viologen films which were characterized using electrochemical and spectroscopic techniques. The electrochemical characterization of the formed films showed that both PolyT1 and PolyT2 undergo a two-step reversible redox reaction. According to UV–vis results the stability of the radical form for PolyT2 was enhanced probably due to its lesser tendency to form dimers. Furthermore, due to the different structures of the monomers, the two polymers will end up having different cavity sizes showing potential for the viologen films to be utilized as host material. Acknowledgements Financial support from the Academy of Finland is gratefully acknowledged.

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

Appendix A. Supplementary data [19]

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2014.12.075.

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