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Voltammetric anion recognition by a highly cross-linked polyviologen film
www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 473 (1999) 145 – 155
Voltammetric anion recognition by a highly cross-linked polyviologen film Kaori Kamata, Takehiko Suzuki, Tadashi Kawai, Tomokazu Iyoda * Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan Uni6ersity, 1 -1 Minami-Ohsawa, Hachioji, Tokyo 192 -0397, Japan Received 19 October 1998; received in revised form 8 June 1999; accepted 6 July 1999
Abstract A highly cross-linked polyviologen film was prepared by reductive electropolymerization of a trifunctional monomer with three 4-cyanopyridinium moieties. The film was deposited on an ITO-deposited glass electrode at higher current efficiency than in the case of a linear polyviologen from a bifunctional monomer. The resulting polymer film shows a clear electrochromism characteristic of a viologen redox process. The film exhibits typical redox waves in a monomer-free electrolytic solution containing a smaller electrolyte anion than the anion used in electropolymerization. On the contrary, the film behaves as electrochemically inactive in a solution containing a larger electrolyte anion due to its reduced mobility through the cavity in the cross-linked polymer structure. This anion-controlled redox behavior suggested that the cavity size in the cross-linked polymer may be determined by the template effect of the electrolyte anion used in electropolymerization, called anion-imprinted electropolymerization. These features can be regarded as a new voltammetric anion recognition of the highly cross-linked polyviologen film. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Highly cross-linked polyviologen film; Anion-imprinted electropolymerization; Molecular imprint; Voltammetric anion recognition; Anion-controlled redox process
1. Introduction Viologens, a family of 1,1%-dialkyl-4,4%-bipyridine, have been well known for a long time in both electrochemistry and photochemistry [1], in particular for their reversible redox processes with conspicuous electrochromism and their strong electron acceptability as a mediator in electron transfer sequences [2 – 7].
Presented at the International Symposium on Electrochemistry of Ordered Interfaces, Sapporo, Japan, 11–12 September, 1998. * Corresponding author. Tel.: + 81-426-77-2833; fax: + 81-426-772821. E-mail address: [email protected] (T. Iyoda)
They have also played an active part as electron acceptors in charge transfer complexes in the solid state for electric and ionic conductors [8]. Very recently our group has begun to apply another feature of viologens to generate their stable ion-radicals photoelectrochemically to obtain a novel type of molecule-based magnetism including an optically tunable high spin system [9]. Therefore, the viologen has been regarded as and will be extended more widely to one of the promising molecular units for functional materials with electric, optical, and magnetic performance. For example, numerous studies [10–13] on functionalization of an electrode surface with viologen moieties for electrochromic displays and electrocatalysts have been reported along the lines mentioned above. The utilization of the viologen as a functional unit should include the preparative methods to exhibit its own functionality effectively, as follows: the Menshutkin reaction [1] of 4,4%-bipyridine with alkyl halides has been adopted in most of the viologen-based functional molecules and polymers [14]. In our previous
0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 9 9 ) 0 0 3 1 6 - 2
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work [15], a series of linear-type polyviologen (LPV) films on an electrode were prepared through an electrochemical coupling reaction of bifunctional 4-cyano-1pyridinium monomers (Scheme 1A and B). The film depositions on an electrode from their aqueous solutions were achieved by reduction of the solubilities of the resulting polymers due to their entangled structures or to the polyion complex formation with an anionic polymer such as potassium poly(vinylsulfate). In the present work, a highly cross-linked polyviologen film was prepared simply by electropolymerization of 1,3,5-tris(4%-cyanopyridinio-1%-methyl)-2,4,6-trimethylbenzene tribromide as a new branched monomer with three polymerizable groups (Scheme 1C). Reductive electropolymerization of the trifunctional monomer gave a completely insoluble polyviologen film on an electrode at a higher current efficiency than in the case of the bifunctional monomer, just because of its highly cross-linked structure. We also report that the highly cross-linked polyviologen (CPV) film exhibits a voltammetric anion recognition property in which the redox activity of the viologen moieties was controlled strongly by the size of the electrolyte anion. The present study will provide a new type of electrochemical sensing for anion size by the highly cross-linked redox-active polymer film, which was fabricated by anion-imprinted electropolymerization.
2. Experimental
2.1. General procedures Proton NMR spectra were recorded in D2O with a JEOL FX-270 spectrometer. All chemical shifts were reported in parts per million (d) down field from that of dioxane as the internal standard. The UV and visible spectra were recorded with a Shimadzu UV-3100s spectrophotometer. The FT-IR spectra were recorded in KBr pellets with a JASCO FT/IR-7000T spectrophotometer. Electrochemical polymerization and measurements were carried out in a three-electrode system using an HZ-3000 automatic polarization system (Hokuto Denko). An indium tin oxide (ITO) deposited glass (10 V of surface resistance, Evers) was used as a working electrode. A Pt plate or a bare ITO-deposited glass was used as a counter electrode. A reference electrode was a saturated calomel electrode (SCE) or an Ag AgCl wire. For spectroelectrochemistry, an ITO-deposited glass working electrode on which the highly crosslinked polyviologen (CPV) film was deposited, another bare ITO-coated glass counter electrode, and a 5 mm thick Teflon spacer were assembled into a sandwichtype electrolytic cell, where both ITO-deposited elec-
trodes were the optical windows for the spectrophotometric light beam. The SCE was connected electrochemically to the cell through a KCl-agar salt bridge. The cell was set in the sample holder of the spectrophotometer. Under potentiostatic conditions where an appropriate potential was applied to the CPV-coated working electrode with respect to SCE, the UV and visible spectra were recorded. The electrolytic solution was 0.1 M Na2SO4 aqueous solution. The electrode potential was set at intervals of 50 mV from 0.0 to − 1.0 V versus SCE. Every spectrum was taken 5 min after the electrode potential was applied.
2.2. Materials The trifunctional monomer having three polymerizable 4-cyanopyridinium moieties, 1,3,5-tris(4%-cyanopyrinio-1%-methyl)-2,4,6-trimethylbenzene tribromide (trifunctional monomer) for the highly cross-linked polyviologen (CPV) was prepared by refluxing CH3CN solution (10 ml) containing 1,3,5-tris(bromomethyl)2,4,6-trimethylbenzene (0.6 g, 1.5 mmol) and an excess amount of 4-cyanopyridine (1.9 g, 18 mmol) under N2 overnight. The yellow precipitate was collected and washed with CH3CN. Recrystallization with methanol gave 1.0 g of yellow powder (90% yield). Both 1HNMR and elementary analysis suggested the inclusion of methanol in the product with a 1:1 molar ratio. 1 H-NMR (D2O) d= 2.36 (–CH3, 9H, s), 3.79 (–CH3 of the included methanol, 3H, s), 6.26 (–CH2 –, 6H, s), 8.56 (b-H of pyridinium, 6H, d, J= 6.3 Hz), 9.09 (a-H of pyridinium, 6H, d, J= 6.3Hz). IR (KBr disk) n= 2244 (–CN), 2987, 2907 (C–H), 1505, 1409 (C–C of benzene ring), d= 1470 (C–H of methyl group), 871 (C–H of benzene ring), 506, 456 (C–C–C of benzene ring) cm − 1. Found: C, 49.89; H, 3.99; N, 11.09%. Calcd for C30H27Br3N6·CH3OH: C, 50.09; H, 4.20; N, 11.31%. The bifunctional monomer with two polymerizable 4-cyanopyridinium moieties, 1,4-bis(4%-cyanopyridinio1%-methyl)benzene dibromide (bifunctional monomer) for the linear-type polyviologen (LPV) was prepared from 4-cyanopyridine and 1,4-bis(bromomethyl)benzene in a similar manner to the case of the trifunctional monomer. The yellow precipitate was collected and recrystallized from acetone (78% yield). 1H-NMR (D2O) d= 6.02(–CH2 –, 4H, s), 7.65(H of phenylene), 8.53(b-H of pyridinium, 4H, d, J=6.8Hz), 9.23(a-H of pyridinium, 4H, d, J=6.8Hz). IR (KBr disk) n= 2241(–CN), 2987(C–H), 1505, 1409(C–C of benzene ring), d= 1471(C–H of methyl group), 872(C–H of benzene ring), 501, 464(C–C–C of benzene ring) cm − 1. Found: C, 50.71; H, 3.34; N, 11.70%. Calcd for C20H16Br2N4: C, 50.87; H, 3.42; N, 11.87%.
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Scheme 1. (A) The mechanism of the reductive coupling reaction of 4-cyanopyridinium to viologen. (B) Bifunctional and (C) trifunctional monomers and the repeated unit structures of their polymers. Bromide ion in the monomers is easily exchanged with an electrolyte anion X − in electropolymerization. LPV/X − and CPV/X − are a linear-type polyviologen and a highly cross-linked polyviologen obtained from the bifunctional and trifunctional monomers, respectively. A schematic network structure expected in CPV is also shown below. The counter anions are omitted for clarity.
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N,N%-Dibenzyl-4,4%-bipyridinium dibromide as a repeated unit of both CPV and LPV was synthesized by refluxing 15 ml CH3CN containing benzylbromide (4.4 g, 24 mmol) and 4,4%-bipyridine (1.0 g, 6.0 mmol) for 10 h. The yellow precipitate was collected and recrystallized from acetone (1.6 g, yield 77%). 1H-NMR(D2O) d = 5.97 (–CH2 –, 4H, s), 7.56 (H of phenyl, 10H, s), 8.56 (b-H of pyridinium, 4H, d, J = 6.9Hz), 9.19 (a-H of pyridinium, 4H, d, J = 6.9Hz). Found: C, 57.63; H, 4.71; N, 5.56%. Calcd for C24H22Br2N2: C, 57.85; H, 4.45; N, 5.62%. The reagents in the above synthetic procedures and other chemicals involving the electrolytes were used without any further purification.
Fig. 1. (a) The continuously scanned cyclic voltammogram of 0.02 M trifunctional monomer in a 0.1 M Na2SO4 aqueous solution at 50 mV s − 1 scan rate, using a 1.2 cm× 2.5 cm ITO-deposited glass electrode. The arrows show the increasing and the decreasing peaks during the scans. (b) The cyclic voltammogram of the CPV-deposited ITO electrode after 32 scans in a monomer-free 0.1 M Na2SO4 aqueous solution. The reference electrode was an SCE.
3. Results and discussion
3.1. Electropolymerization of the trifunctional monomer to highly cross-linked poly6iologen (CPV) film Fig. 1a shows a continuously scanned cyclic voltammogram of 0.02 M trifunctional monomer in a 0.1 M Na2SO4 aqueous solution at a scan rate of 50 mV s − 1, using a 1.2 cm× 2.5 cm ITO-deposited glass electrode. In the first scan from 0 V versus SCE, a pair of peaks consisting of a sharp reduction peak and an oxidation peak was observed at − 0.72 and − 0.48 V, respectively. The cathodic wave at − 0.72 V is assigned to the reduction of the 4-cyanopyridinium moiety to its neutral radical and the anodic one at −0.48 V corresponds to its reverse process. With repeating scans, these reduction and oxidation peaks decreased rapidly. Simultaneously new reduction peaks at − 0.56 and −0.94 V and an oxidation peak at − 0.80 V appeared and then increased. The oxidation peak at −0.48 V shifted slightly to − 0.44 V and broadened considerably to show a plateau. During the continuous scans, a homogeneous polymer film was deposited on the working electrode and showed a clear color change between light brown and light purple, depending on the applied potential. The cyclic voltammogram (Fig. 1b) of the film-coated electrode after 32 scans shows reversible two-step redox waves with half-wave potentials of − 0.52 and − 0.88 V in a monomer-free 0.1 M Na2SO4 aqueous solution. This voltammogram is in good agreement with that of N,N%-dibenzyl-4,4%-bipyridinium dibromide in CH3CN containing 0.1 M tetrabutylammonium perchlorate. This result suggests that the polymer film has a viologen structure resulting from the electropolymerization of the 4-cyanopyridinium moieties of the trifunctional monomer. Kosower and Cotter have reported that methylviologen was obtained by the reductive coupling reaction of 1-methyl-4-cyanopyridinium salts with alkaline aqueous sodium dithionite [16]. Similar coupling reactions of various pyridinium derivatives have been also investigated chemically and electrochemically [17–22]. Saika et al. also reported that the coupling reactions of 4cyanopyridinium salts were applied to the electrochemically successive coupling reaction of the bifunctional monomer with two 4-cyanopyridinium moieties, 1,vbis(4%-cyano-1%-pyridinio)alkane derivatives, to form linear-type polyviologen (LPV) films [15] (Scheme 1C). The feature of the voltammogram in Fig. 1a is almost identical to that of the bifunctional monomer, 1,4-bis (4%-cyanopyridinio-1%-methyl) benzene dibromide in a phosphate buffer solution in their study. Although similar reductive coupling polymerization of both the present trifunctional monomer and Saika’s bifunctional monomers should take place, the trifunctional monomer is expected to form a highly cross-linked
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Fig. 2. The FT-IR spectra of (1) the trifunctional monomer and (2) the CPV film and in KBr pellets, both of which are normalized with respect to the absorption band (a) at 2987 cm − 1. The arrows, a–d, are the significant absorption peaks mentioned in the text.
structure which may result in efficient deposition on an electrode. The following mechanism of the reductive coupling polymerization has been proposed [22]. 4Cyanopyridinium ion is reduced to the neutral 4cyanopyridinyl radical (process (a) in Scheme 1A), which is equilibrated to its dimer (process (b)). A part of the resulting neutral dimer, 4,4%-dicyano-4,4%bipyridine, loses two cyanide ions to form a viologen dication (process (c)). The overall reaction can be described as two-electron reduction and the elimination of two cyano groups from two 4-cyanopyridinium ions, giving the viologen skeleton. While a successive coupling reaction of the bifunctional monomer will give an LPV, efficient coupling of the trifunctional monomer could form a highly cross-linked structure, which is just our objective to demonstrate the anion-imprinted electropolymerization. At the electrode potential applied for the electropolymerization the resulting viologen moiety should be reduced to its cation radical (process (d)). A bulk electrolysis at −0.75 V of constant potential, corresponding to the first reduction peak of the 4cyanopyridinium moiety, also gave a polymer film on an ITO-coated glass electrode. A light purple film grew on the electrode, indicating that the resulting viologen moieties should be reduced to their cation radicals at this potential. After completing the electropolymerization the potential was set to + 0.02 V for a while, so that the film color turned light brown due to re-oxidation of the viologen cation radical back to the viologen dication. The polymer film was quite insoluble in any solvent, but became swollen or shrunk in a variety of solvents. Fig. 2 shows the FT-IR spectra of the resulting film and the trifunctional monomer in KBr pellets, both of which are normalized with respect to the absorption
band (arrow a in Fig. 2) at 2987 cm − 1 assigned to the C–H stretching vibration of the methyl groups. The absorption peak (b) at 2244 cm − 1, assigned to the C–N stretching vibration of the cyano group in the monomer, decreased down to B 7%, implying that most of the 4-cyanopyridinium moiety was consumed in the polymerization. The remaining 4-cyanopyridinium moiety resulted from incompleteness in the coupling reaction. A small amount of free CN − ((c) nCN − = 2076 cm − 1) resulting from the coupling reaction (process(c) in Scheme 1A) remained as a counter anion in the film, while an appreciable amount of sulfate ion was included, judging from the S–O vibration peak (d) at 618 cm − 1 and shoulder (e) at 1137 cm − 1. The remaining peaks of the film are not changed in comparison with those of the monomer. These results imply that the trifunctional monomer may be efficiently polymerized to form a highly cross-linked polyviologen (CPV) film and that both the bromide ion of the monomer and the released CN − should be exchanged by the electrolyte anion used, SO24 − , which is a counter anion of the CPV. The highly cross-linked structure (Scheme 1C) obtained plays an important role in the voltammetric anion recognition as will be discussed later. Compared with the bifunctional monomer for LPV, the electropolymerization of the trifunctional monomer gave an efficient deposition of the CPV film on an electrode. The current efficiency of the film deposition (CF) was evaluated by the ratio of the surface density (G) found to the ideal surface density (G0). CF= G/G0
(1)
Trifunctional and bifunctional monomers were electropolymerized at −0.75 V versus Ag AgCl in 0.1 M
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Na2SO4 aqueous solution to deposit CPV and LPV films on an ITO-coated glass electrode (1.2 cm× 2.5 cm). The G is derived from the charge passed in the cyclic voltammogram of the resulting film in monomerfree electrolytic solution1. G0 is the surface density assuming 100% of CF, i.e. that all the reduced 4cyanopyridinium moieties should be coupled completely and that the entire resulting viologen moiety should deposit on an electrode in its one-electron reduced state:
using the values m and n in order to extract only the charge passed in the reductive coupling of monomer2, where m is the fraction of the number of electrons consumed for the reductive coupling per repeated unit in the sum of the two contributions, so that m=2/3 for both the trifunctional and bifunctional monomers, and n is the number of the resulting viologen moiety per repeated unit, where n= 1.5 for the trifunctional monomer and n =1 for the bifunctional one, respectively. Finally the equation
G0 = (Q/F) (m/n)
G= (CF/F) (m/n) Q
(2)
where F is the Faraday constant. The charge passed in electropolymerization (Q) includes two contributions from the reductive coupling of 4-cyanopyridinium moieties (process (a)–(c) in Scheme 1A) and the one-electron reduction of the resulting viologen (process (d) in Scheme 1A). Then in equation (2), G0 was corrected by
(3)
is obtained. Fig. 3 shows that the relationship between G and Q is linear to 50 mC cm − 2 for CPV. The CF values were 5.7 and 0.6% for CPV and LPV films, respectively, which were determined from the slope of (CF/F) (m/n) in the linear region in Fig. 3. The larger value for CPV may be explained by its highly crosslinked structure (Scheme 1C).
3.2. Spectroelectrochemistry of CPV film
Fig. 3. The relationship between the surface density of the electrochemically active viologen moiety (G) and the charge passed in electropolymerization (Q). The surface density was obtained from the charge passed in the cyclic voltammogram of the polyviologen film in a monomer-free aqueous solution. The current efficiency was derived from the slope by using equation (3) in the text. The bifunctional and trifunctional monomers were electropolymerized at − 0.75 V versus Ag AgCl in 0.1 M Na2SO4 aqueous solution and deposited on an ITO-coated glass electrode (1.2 cm × 2.5 cm). 1 When the weight of the resulting thick films (Q \ 100 mC cm − 2) was adopted to estimate the current efficiency (CF) for the film deposition, the CF values were 49% for CPV and 1% for LPV, respectively. The elementary analysis indicated only two water molecules in a repeated unit of the CPV/SO24 − dried in vacuo at room temperature for 10 min. The inconsistent results between the coulometric and the gravimetric determinations might be interpreted by additional inclusion of electrolyte Na2SO4 and its low conductivity. The authors think throughout this study that an appreciable fraction of the counter anion would be confined in the highly cross-linked structure and therefore become electrochemically inactive when the cyclic voltammetry was carried out in a thick CPV film, relating to the structural consideration on the present voltammetric anion size recognition. So, the current efficiency derived from the gravimetric determination seemed larger than that from coulometry in this study. Based on the referee’s suggestion, a coulometric method for a relatively thin film was adopted. A gravimetric study by EQCM is in progress.
Fig. 4a shows the UV–vis spectra of the CPV film on an ITO-deposited electrode under potentiostatic conditions at every 50 mV and the spectrum at + 0.02 V was subtracted from all the spectra taken at 0.0 to −1.0 V versus SCE so as to avoid the contribution from partially reduced viologen moieties. In the potential region less negative than − 0.35 V, a new small band appeared around 480 nm, but no redox wave was observed in the cyclic voltammetry (Fig. 1b), where the assignment of this unknown band is under investigation. In the potential range from − 0.50 to −0.75 V a broad absorption band appeared over the 500–700 nm region and an intense absorption band around 400 nm, assigned to viologen cation radical. The first inflection points in the absorbance titration curves at 400–600 nm are observed around − 0.60 V (Fig. 4b), close to the first reduction potential −0.52 V obtained from the cyclic voltammogram (Fig. 1b). Very different from the wellknown behavior of the concentrated solution of viologen cation radical, it is noticeable that the spectrum shows little indication of the dimer formation of the resulting viologen cation radicals, probably because the individual viologen moieties are fixed tightly in the highly cross-linked structure. In a more negative potential region, this broad band in the visible region van2
The fraction m indicates the ratio of the number of electrons consumed for the complete coupling of 4-cyanopyridinium moieties to the total number of electrons passed in electropolymerization. In the case of one molecule of trifunctional monomer, three electrons are required for the coupling, and 1.5 electrons are consumed for the one-electron reduction of the resulting viologen moieties. So the fraction m should be given by a ratio of 3 to (3 + 1.5), i.e., 2/3. The repeated unit produces 1.5 equivalents of the viologen moiety (n= 1.5). In the case of the bifunctional monomer, m= 2/3 and n=1.
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Fig. 4. (a) The UV – vis spectra of the CPV film on an ITO-deposited electrode under potentiostatic conditions. All the spectra were subtracted by the spectrum at + 0.02 V versus SCE. The solid lines are the spectra taken at 50 mV intervals from 0 to −0.75 V (thick solid line) and the broken lines are those from − 0.8 to − 1.0 V (thick broken line) with arrows indicating the absorbance changes. (b) The absorbances at 400 nm ( ) and 600 nm () as a function of the applied potential.
ished, while both the 400 nm intense band and the 480 nm shoulder band remained. The titration curve (Fig. 4b) shows a second inflection point around − 0.85 V, in good agreement with −0.88 V of the second reduction potential. The spectra in the region still more negative than −0.95 V are assigned to neutral quinoid-like viologen as a two-electron reduced species.
3.3. Voltammetric anion recognition of CPV film The redox behavior of the CPV-coated electrode was dependent greatly on a variety of electrolyte anions used in monomer-free aqueous electrolytic solutions. The CPV film with Cl − as counter anion, CPV/Cl − , on an ITO-coated glass electrode (0.4cm× 2.5cm) was prepared beforehand by electropolymerization in 0.1 M NaCl aqueous solution in 50 mC cm − 2 of electropolymerization. The CPV/Cl − film shows two reversible redox waves with half-wave potentials of − 0.57 and − 0.91 V versus Ag AgCl in a monomer-free 0.1 M NaCl aqueous solution (Fig. 5a). No redox wave appeared in the cyclic voltammogram of the same CPV/Cl − -deposited ITO electrode in a monomer-free 0.1 M sodium tetraphenylborate aqueous solution (Fig. 5b). When the electrolytic solution was replaced again by the original monomer-free
NaCl solution, the original two-step redox waves appeared in cyclic voltammetry (Fig. 5c). The CPV/Cl − film, which was electrochemically active in the NaCl solution, became completely inactive in the sodium tetraphenylborate solution. Again in the NaCl solution, the voltammetric response was almost restored. This striking contrast in a series of cyclic voltammograms illustrated that the CPV film should recognize electrochemically the anion size of the electrolyte used in the viologen redox process with high selectivity. Similar voltammetric anion recognition was demonstrated with Cl − and Br − as electrolyte anions. The diameter of Br − (1.96 A, ) is only 8.3% larger than that of Cl − (1.81 A, ) [23]3, which means a 27% difference in excluded volume. Fig. 6 shows typical cyclic voltam3 Several sets of radii of ions [23] have been proposed such as crystallographic radii (1.81, 1.96, and 2.16 A, for Cl − , Br − , and I − by Goldschmidt, 1.81, 1.95, and 2.20 by Pauling, 1.64, 1.81, and 2.05 by Gourary and Adrian, and 1.70, 1.87, and 2.12 by Ladd), hydrodynamic radii of solvated ions (1.21, 1.18, and 1.19 A, for Cl − , Br − , and I − ), and hydrated ion radii (3.32, 3.30, and 3.31 by Nightingale). Considering the procedures to derive these values, actual ionic radii in the CPV film would be different from them because of much difference in ionic strength and water structure in the film. Therefore, the crystallographic radii on the Goldschmidt scale were adopted here [23] as reliable ones.
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mograms (50 mV s − 1 scan rate) of the CPV/Cl − -deposited ITO electrodes (0.4 cm×2.5 cm) in both monomer-free 0.1 M KCl (solid line) and KBr (broken line) solutions. The CPV/Cl − films were prepared by electropolymerization of the trifunctional monomer (0.02 M) in 0.1 M KCl aqueous solution at − 0.75 V versus SCE for 10, 20, 30, 40, 50, 100, 200, and 300 mC cm − 2 of the charge passed. The voltammetric response of the CPV/Cl − film was depressed in KBr solution, compared with that in KCl solution. The reduction of the responses in KBr solution became more significant as the charge passed on electropolymerization increased, i.e. the film become thick. Then, the reduction reached a saturation up to 100 mC cm − 2 of charge passed on electropolymerization. This voltammetric feature indicated that the mass transport of the counter anion accompanied by the redox process of the viologen moiety can be controlled by the highly cross-linked structure in the CPV/Cl − , and then that the anion size recognition might be evaluated simply by a current ratio in both solutions, as will be described later. Another feature was that the voltammetric response even
Fig. 6. A series of cyclic voltammograms (50 mV s − 1 scan rate) of the CPV/Cl − films on ITO-coated glass electrodes (0.4 cm ×2.5 cm) with different surface density in both monomer-free KCl (solid line) and KBr (broken line) solutions. The surface density was controlled by the amount of the charge passed in electropolymerization of 0.02 M trifunctional monomer aqueous solution containing 0.1 M KCl at −0.75 V versus SCE. The CPV/Cl − films were prepared by electropolymerization with (a) 10; (b) 20; (c) 40; (d) 50; (e) 100; and (f) 200 mC cm − 2.
Fig. 5. The cyclic voltammograms (50 mV s − 1 scan rate) of the CPV/Cl − -deposited ITO electrode (a) in a monomer-free 0.1 M NaCl aqueous solution, (b) in a monomer-free 0.1 M sodium tetraphenylborate aqueous solution, and (c) again in the monomer-free 0.1 M NaCl aqueous solution. These three voltammograms were taken sequentially on the same CPV/Cl − -deposited electrode. The CPV/ Cl − -deposited electrode was prepared beforehand by electropolymerization of the trifunctional monomer (0.02 M) in 0.1 M NaCl aqueous solution by 50 mC cm − 2. The reference electrode was an Ag AgCl electrode.
in KCl solution became reduced and distorted in a \ 100 mC cm − 2 region. Mass transport of the same anion as that imprinted on the highly cross-linked structure could be limited in the case of such a thick film. Here, great emphasis should be placed on the following mechanistic consideration on these anion-controlled redox processes of the CPV/Cl − films. Basically, any redox processes accompanying mass transport of the counter ion maintain the electroneutrality, as Scheme 2 shows. When the CPV film having a counter anion X − , defined as CPV/X − , is reduced in aqueous solution
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Scheme 2. Schematic mass transport of counter anions on the redox cycle of the viologen moiety (V) in the CPV/X − film. The counter anion X − in CPV/X − was released stepwise when the electrode potential was swept negatively (V2 + V + V0). The electrolyte anion Y − was incorporated stepwise as a new counter anion when the two-electron reduced viologen V0 was re-oxidized by the positive sweep of electrode potential (V0 V + V2 + ).
Scheme 3. The highly cross-linked structures with different cavity sizes which can be fabricated by the template effect of the electrolyte anions used in the electropolymerization, called anion-imprinted electropolymerization here. The CPV film shows voltammetric anion recognition through the anion-imprinted cavity.
containing an electrolyte anion Y − , the X − is released stepwise from the film into the electrolytic solution. That is to say, the viologen cation radical has one X − and the two-electron reduced neutral species has no counter anion. When the latter neutral CPV film is re-oxidized stepwise to the cation radical and then to the original dication state, Y − should be incorporated stepwise into the film as a new counter anion because the concentration of Y − is much higher than that of the released X − in the electrolytic solution. In the redox processes of the viologen moieties the counter anion should transport through the cavities of the cross-linked structure in the CPV films. The cavity size should be crucial for the anion transport accompanied by these redox processes. The viologen resulting from
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electropolymerization of the trifunctional monomer took counter anions from an excess amount of the electrolyte in the bulk solution. Since the highly crosslinked structure in the CPV film is fabricated by the template effect of the electrolyte anion used on the electropolymerization, the cavity size in the network structure would depend on the size of the electrolyte anion used, which could be regarded as a new type of molecular imprint [24–27], i.e. anion-imprinted electropolymerization. Therefore, the anion transport in the CPV film would be limited by the cavity size and then determine the electrochemical activity on the redox processes of the CPV film in a monomer-free electrolytic solution, as Scheme 3 illustrates. When Y − is equal to or smaller than X − in size, Y − can transport easily through the cavities in the CPV/X − film on the redox process. The CPV film behaves as electrochemically active in this case. On the contrary, the transport of larger Y − than X − in size should be considerably depressed, and then the CPV film becomes inactive electrochemically. Surprisingly, the cavity of the crosslinked structure recognized only an 8% difference in diameter or a 27% difference in excluded volume between Cl − and Br − as the voltammetric response. A significant question arises here about the dependence of the CPV film thickness on the voltammetric response, which was evaluated qualitatively as the ratio of the second cathodic currents in the cyclic voltammograms in the KCl and KBr solutions, as is illustrated in Fig. 6d. The amount of redox active sites, called the surface density of the viologen moieties in the film, can be adjusted accurately by the charge passed on electropolymerization. Therefore, on the basis of the linear relationship between the CPV film thickness and the voltammetric response of the viologen moiety, the film thickness may be discussed by using the surface density, which corresponds to the charge passed on the electropolymerization. Fig. 7 shows the peak current ratios IBr/ICl as a function of the charge passed on electropolymerization. A 100% ratio means that the CPV film should show the same redox activity in KBr solution as in the case of KCl solution, and would be achieved in an infinitely thin CPV film. These cathodic current ratios decrease suddenly with the increasing surface density of the film and are saturated down to about 40% in the CPV/Cl − films prepared by more than 40 mC cm − 2 of electropolymerization. Another significant feature is that the cathodic current ratio IBr/ICl was almost constant in a wide range of scan rate from 1 to 200 mV s − 1. This evidence indicated that the CPV/Cl − films lost a maximum of 60% of their redox activity in the KBr solution. That is to say, the CPV/Cl − films can distinguish a small difference in the anion size of Cl − and Br − (Scheme 3). In addition, these results suggest that the cavity size in the CPV film can be determined by the anion-imprinted electropolymerization
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Fig. 7. The ratios (IBr/ICl) of the second cathodic peak currents of the cyclic voltammograms (50 mV s − 1 scan rate) in monomer-free 0.1 M KBr and KCl solutions as a function of the amount of the charge passed (Q) in the preparation of the CPV/Cl − -coated ITO electrodes. The CPV/Cl − films were prepared by electropolymerization with (a) 10; (b) 20; (c) 40; (d) 50; (e) 100; and (f) 200 mC cm − 2.
and that the resulting CPV film shows clearly voltammetric anion recognition. The increasing surface density in the region B40 mC cm − 2 causes the selectivity of the voltammetric anion recognition to be elevated. In the 100 mC cm − 2 region, the present estimation of the anion-controlled redox process becomes invalid because of distorted voltammograms with considerable irreversibility of the redox processes even in the KCl solution. So far we have described the electrochemical anion recognition in size through the redox properties in an aqueous solution containing an electrolyte anion larger than the cavity formed in the CPV film. Hereafter, three kinds of polyviologen films with different cavity sizes, CPV/Cl − , CPV/Br − , and CPV/I − , were prepared by electropolymerization of the trifunctional monomer (0.02 M) in 0.1 M KCl, KBr, and KI aqueous solutions, respectively. The diameters of Cl − , Br − , and I − are 1.81, 1.96, and 2.19 A, [23], and their excluded volumes are 3.10, 3.94, and 5.50 A, 3, respectively. Fig. 8a shows the cyclic voltammograms of the CPV/Cl − films in 50 mC cm − 2 of electropolymerization in three kinds of electrolytic solutions containing KCl, KBr, and KI, respectively. The CPV films become electrochemically inactive in the order of KCl, KBr, and KI solutions. As the anion size in the electrolytic solution increases, the mass transport of the anion in the cavity of the CPV film tends to be limited and then the redox current decreases. Fig. 8b shows the cyclic voltammograms of the CPV/Br − films (50 mC cm − 2) in the three solutions. While the CPV/Br − becomes almost inactive in the KI solution, the CPV/Br − film exhibits a similar redox response in the KCl solution to that in the KBr solution. This evidence suggests that Cl − , which is smaller than the cavity in the CPV/Br − film, may transport with almost the same mobility in the viologen redox process as Br − . While the CPV/Br −
Fig. 8. The cyclic voltammograms of (a) CPV/Cl − films, (b) CPV/ Br − films, and (c) CPV/I − films in monomer-free 0.1 M KCl (solid line), KBr (dotted line), and KI (chain line) aqueous solutions. Every CPV/X − film was prepared by electropolymerization of 0.02 M trifunctional monomer aqueous solution containing 0.1 M KX (X = Cl, Br, I) with 50 mC cm − 2
film exhibits an appreciable voltammetric response for Br − and Cl − , but not for the larger I − as a counter anion, almost similar electrochemical activities were realized in the voltammograms of the CPV/I − films in the three solutions as Fig. 8c shows. Both Br − and Cl − move easily through the larger cavity in the CPV/I − film in the viologen redox processes. These results of voltammetric anion recognition are consistent with the mechanistic considerations mentioned in Scheme 3 and support strongly the supposition that the cavity size in the CPV film should be controlled by the template effect of the electrolyte anion used in electropolymerization.
Acknowledgements This work was partially supported by Grants-in-Aid for Scientific Research on Priority Area of Electrochemistry of Ordered Interfaces (No.282/09237256) and New Polymers and Their Nano Organized Systems (No. 277/08246254) from the Ministry of Education, Science, Sports, and Culture, Japan. One of the authors (T.I.) acknowledges the financial support of the CASIO Science Promotion Foundation and the Tokuyama Science Promotion Foundation.
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[13] T. Nagamura, K. Sakai, T. Ogawa, J. Chem. Soc., Chem. Commun. (1988) 1035. [14] A. Factor, G.E. Heinsohn, J. Polym. Sci, Part B 9 (1971) 289. [15] T. Saika, T. Iyoda, T. Shimidzu, Bull. Chem. Soc. Jpn. 66 (1993) 2054. [16] E.M. Kosower, J.L. Cotter, J. Am. Chem. Soc. 86 (1964) 5524. [17] W.M. Schwarz, E.M. Kosower, I. Shain, J. Am. Chem. Soc. 83 (1961) 3164. [18] I. Carelli, M.E. Cardinali, J. Electroanal. Chem. 124 (1981) 147. [19] A. Webber, E.K. Eisner, J. Osteryoung, J. Hermolin, J. Electrochem. Soc. 129 (1982) 2725. [20] A. Webber, J. Osteryoung, J. Electrochem. Soc. 129 (1982) 2731. [21] K. Ageishi, T. Endo, M. Okawara, J. Polym. Sic, Polym. Chem. Ed. 21 (1983) 293. [22] N. Kitamura, Y. Nambu, T. Endo, J. Polym. Sic, Polym. Chem. Ed. 26 (1988) 993. [23] B.E. Conway, Ionic Hydration in Chemistry and Biophysics, Chapter 4, Elsevier, New York, 1981. [24] G. Wulff, A. Sarhan, Angew. Chem. 84 (1972) 364. [25] F. Flam, Science 263 (1994) 1221. [26] K.J. Shea, Trends Polym. Sci. 2 (1994) 166. [27] G. Wulff, Angew. Chem. Int. Ed. Engl. 34 (1995) 1812.
.
Voltammetric anion recognition by a highly cross-linked polyviologen film Kaori Kamata, Takehiko Suzuki, Tadashi Kawai, Tomokazu Iyoda * Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan Uni6ersity, 1 -1 Minami-Ohsawa, Hachioji, Tokyo 192 -0397, Japan Received 19 October 1998; received in revised form 8 June 1999; accepted 6 July 1999
Abstract A highly cross-linked polyviologen film was prepared by reductive electropolymerization of a trifunctional monomer with three 4-cyanopyridinium moieties. The film was deposited on an ITO-deposited glass electrode at higher current efficiency than in the case of a linear polyviologen from a bifunctional monomer. The resulting polymer film shows a clear electrochromism characteristic of a viologen redox process. The film exhibits typical redox waves in a monomer-free electrolytic solution containing a smaller electrolyte anion than the anion used in electropolymerization. On the contrary, the film behaves as electrochemically inactive in a solution containing a larger electrolyte anion due to its reduced mobility through the cavity in the cross-linked polymer structure. This anion-controlled redox behavior suggested that the cavity size in the cross-linked polymer may be determined by the template effect of the electrolyte anion used in electropolymerization, called anion-imprinted electropolymerization. These features can be regarded as a new voltammetric anion recognition of the highly cross-linked polyviologen film. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Highly cross-linked polyviologen film; Anion-imprinted electropolymerization; Molecular imprint; Voltammetric anion recognition; Anion-controlled redox process
1. Introduction Viologens, a family of 1,1%-dialkyl-4,4%-bipyridine, have been well known for a long time in both electrochemistry and photochemistry [1], in particular for their reversible redox processes with conspicuous electrochromism and their strong electron acceptability as a mediator in electron transfer sequences [2 – 7].
Presented at the International Symposium on Electrochemistry of Ordered Interfaces, Sapporo, Japan, 11–12 September, 1998. * Corresponding author. Tel.: + 81-426-77-2833; fax: + 81-426-772821. E-mail address: [email protected] (T. Iyoda)
They have also played an active part as electron acceptors in charge transfer complexes in the solid state for electric and ionic conductors [8]. Very recently our group has begun to apply another feature of viologens to generate their stable ion-radicals photoelectrochemically to obtain a novel type of molecule-based magnetism including an optically tunable high spin system [9]. Therefore, the viologen has been regarded as and will be extended more widely to one of the promising molecular units for functional materials with electric, optical, and magnetic performance. For example, numerous studies [10–13] on functionalization of an electrode surface with viologen moieties for electrochromic displays and electrocatalysts have been reported along the lines mentioned above. The utilization of the viologen as a functional unit should include the preparative methods to exhibit its own functionality effectively, as follows: the Menshutkin reaction [1] of 4,4%-bipyridine with alkyl halides has been adopted in most of the viologen-based functional molecules and polymers [14]. In our previous
0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 9 9 ) 0 0 3 1 6 - 2
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work [15], a series of linear-type polyviologen (LPV) films on an electrode were prepared through an electrochemical coupling reaction of bifunctional 4-cyano-1pyridinium monomers (Scheme 1A and B). The film depositions on an electrode from their aqueous solutions were achieved by reduction of the solubilities of the resulting polymers due to their entangled structures or to the polyion complex formation with an anionic polymer such as potassium poly(vinylsulfate). In the present work, a highly cross-linked polyviologen film was prepared simply by electropolymerization of 1,3,5-tris(4%-cyanopyridinio-1%-methyl)-2,4,6-trimethylbenzene tribromide as a new branched monomer with three polymerizable groups (Scheme 1C). Reductive electropolymerization of the trifunctional monomer gave a completely insoluble polyviologen film on an electrode at a higher current efficiency than in the case of the bifunctional monomer, just because of its highly cross-linked structure. We also report that the highly cross-linked polyviologen (CPV) film exhibits a voltammetric anion recognition property in which the redox activity of the viologen moieties was controlled strongly by the size of the electrolyte anion. The present study will provide a new type of electrochemical sensing for anion size by the highly cross-linked redox-active polymer film, which was fabricated by anion-imprinted electropolymerization.
2. Experimental
2.1. General procedures Proton NMR spectra were recorded in D2O with a JEOL FX-270 spectrometer. All chemical shifts were reported in parts per million (d) down field from that of dioxane as the internal standard. The UV and visible spectra were recorded with a Shimadzu UV-3100s spectrophotometer. The FT-IR spectra were recorded in KBr pellets with a JASCO FT/IR-7000T spectrophotometer. Electrochemical polymerization and measurements were carried out in a three-electrode system using an HZ-3000 automatic polarization system (Hokuto Denko). An indium tin oxide (ITO) deposited glass (10 V of surface resistance, Evers) was used as a working electrode. A Pt plate or a bare ITO-deposited glass was used as a counter electrode. A reference electrode was a saturated calomel electrode (SCE) or an Ag AgCl wire. For spectroelectrochemistry, an ITO-deposited glass working electrode on which the highly crosslinked polyviologen (CPV) film was deposited, another bare ITO-coated glass counter electrode, and a 5 mm thick Teflon spacer were assembled into a sandwichtype electrolytic cell, where both ITO-deposited elec-
trodes were the optical windows for the spectrophotometric light beam. The SCE was connected electrochemically to the cell through a KCl-agar salt bridge. The cell was set in the sample holder of the spectrophotometer. Under potentiostatic conditions where an appropriate potential was applied to the CPV-coated working electrode with respect to SCE, the UV and visible spectra were recorded. The electrolytic solution was 0.1 M Na2SO4 aqueous solution. The electrode potential was set at intervals of 50 mV from 0.0 to − 1.0 V versus SCE. Every spectrum was taken 5 min after the electrode potential was applied.
2.2. Materials The trifunctional monomer having three polymerizable 4-cyanopyridinium moieties, 1,3,5-tris(4%-cyanopyrinio-1%-methyl)-2,4,6-trimethylbenzene tribromide (trifunctional monomer) for the highly cross-linked polyviologen (CPV) was prepared by refluxing CH3CN solution (10 ml) containing 1,3,5-tris(bromomethyl)2,4,6-trimethylbenzene (0.6 g, 1.5 mmol) and an excess amount of 4-cyanopyridine (1.9 g, 18 mmol) under N2 overnight. The yellow precipitate was collected and washed with CH3CN. Recrystallization with methanol gave 1.0 g of yellow powder (90% yield). Both 1HNMR and elementary analysis suggested the inclusion of methanol in the product with a 1:1 molar ratio. 1 H-NMR (D2O) d= 2.36 (–CH3, 9H, s), 3.79 (–CH3 of the included methanol, 3H, s), 6.26 (–CH2 –, 6H, s), 8.56 (b-H of pyridinium, 6H, d, J= 6.3 Hz), 9.09 (a-H of pyridinium, 6H, d, J= 6.3Hz). IR (KBr disk) n= 2244 (–CN), 2987, 2907 (C–H), 1505, 1409 (C–C of benzene ring), d= 1470 (C–H of methyl group), 871 (C–H of benzene ring), 506, 456 (C–C–C of benzene ring) cm − 1. Found: C, 49.89; H, 3.99; N, 11.09%. Calcd for C30H27Br3N6·CH3OH: C, 50.09; H, 4.20; N, 11.31%. The bifunctional monomer with two polymerizable 4-cyanopyridinium moieties, 1,4-bis(4%-cyanopyridinio1%-methyl)benzene dibromide (bifunctional monomer) for the linear-type polyviologen (LPV) was prepared from 4-cyanopyridine and 1,4-bis(bromomethyl)benzene in a similar manner to the case of the trifunctional monomer. The yellow precipitate was collected and recrystallized from acetone (78% yield). 1H-NMR (D2O) d= 6.02(–CH2 –, 4H, s), 7.65(H of phenylene), 8.53(b-H of pyridinium, 4H, d, J=6.8Hz), 9.23(a-H of pyridinium, 4H, d, J=6.8Hz). IR (KBr disk) n= 2241(–CN), 2987(C–H), 1505, 1409(C–C of benzene ring), d= 1471(C–H of methyl group), 872(C–H of benzene ring), 501, 464(C–C–C of benzene ring) cm − 1. Found: C, 50.71; H, 3.34; N, 11.70%. Calcd for C20H16Br2N4: C, 50.87; H, 3.42; N, 11.87%.
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Scheme 1. (A) The mechanism of the reductive coupling reaction of 4-cyanopyridinium to viologen. (B) Bifunctional and (C) trifunctional monomers and the repeated unit structures of their polymers. Bromide ion in the monomers is easily exchanged with an electrolyte anion X − in electropolymerization. LPV/X − and CPV/X − are a linear-type polyviologen and a highly cross-linked polyviologen obtained from the bifunctional and trifunctional monomers, respectively. A schematic network structure expected in CPV is also shown below. The counter anions are omitted for clarity.
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N,N%-Dibenzyl-4,4%-bipyridinium dibromide as a repeated unit of both CPV and LPV was synthesized by refluxing 15 ml CH3CN containing benzylbromide (4.4 g, 24 mmol) and 4,4%-bipyridine (1.0 g, 6.0 mmol) for 10 h. The yellow precipitate was collected and recrystallized from acetone (1.6 g, yield 77%). 1H-NMR(D2O) d = 5.97 (–CH2 –, 4H, s), 7.56 (H of phenyl, 10H, s), 8.56 (b-H of pyridinium, 4H, d, J = 6.9Hz), 9.19 (a-H of pyridinium, 4H, d, J = 6.9Hz). Found: C, 57.63; H, 4.71; N, 5.56%. Calcd for C24H22Br2N2: C, 57.85; H, 4.45; N, 5.62%. The reagents in the above synthetic procedures and other chemicals involving the electrolytes were used without any further purification.
Fig. 1. (a) The continuously scanned cyclic voltammogram of 0.02 M trifunctional monomer in a 0.1 M Na2SO4 aqueous solution at 50 mV s − 1 scan rate, using a 1.2 cm× 2.5 cm ITO-deposited glass electrode. The arrows show the increasing and the decreasing peaks during the scans. (b) The cyclic voltammogram of the CPV-deposited ITO electrode after 32 scans in a monomer-free 0.1 M Na2SO4 aqueous solution. The reference electrode was an SCE.
3. Results and discussion
3.1. Electropolymerization of the trifunctional monomer to highly cross-linked poly6iologen (CPV) film Fig. 1a shows a continuously scanned cyclic voltammogram of 0.02 M trifunctional monomer in a 0.1 M Na2SO4 aqueous solution at a scan rate of 50 mV s − 1, using a 1.2 cm× 2.5 cm ITO-deposited glass electrode. In the first scan from 0 V versus SCE, a pair of peaks consisting of a sharp reduction peak and an oxidation peak was observed at − 0.72 and − 0.48 V, respectively. The cathodic wave at − 0.72 V is assigned to the reduction of the 4-cyanopyridinium moiety to its neutral radical and the anodic one at −0.48 V corresponds to its reverse process. With repeating scans, these reduction and oxidation peaks decreased rapidly. Simultaneously new reduction peaks at − 0.56 and −0.94 V and an oxidation peak at − 0.80 V appeared and then increased. The oxidation peak at −0.48 V shifted slightly to − 0.44 V and broadened considerably to show a plateau. During the continuous scans, a homogeneous polymer film was deposited on the working electrode and showed a clear color change between light brown and light purple, depending on the applied potential. The cyclic voltammogram (Fig. 1b) of the film-coated electrode after 32 scans shows reversible two-step redox waves with half-wave potentials of − 0.52 and − 0.88 V in a monomer-free 0.1 M Na2SO4 aqueous solution. This voltammogram is in good agreement with that of N,N%-dibenzyl-4,4%-bipyridinium dibromide in CH3CN containing 0.1 M tetrabutylammonium perchlorate. This result suggests that the polymer film has a viologen structure resulting from the electropolymerization of the 4-cyanopyridinium moieties of the trifunctional monomer. Kosower and Cotter have reported that methylviologen was obtained by the reductive coupling reaction of 1-methyl-4-cyanopyridinium salts with alkaline aqueous sodium dithionite [16]. Similar coupling reactions of various pyridinium derivatives have been also investigated chemically and electrochemically [17–22]. Saika et al. also reported that the coupling reactions of 4cyanopyridinium salts were applied to the electrochemically successive coupling reaction of the bifunctional monomer with two 4-cyanopyridinium moieties, 1,vbis(4%-cyano-1%-pyridinio)alkane derivatives, to form linear-type polyviologen (LPV) films [15] (Scheme 1C). The feature of the voltammogram in Fig. 1a is almost identical to that of the bifunctional monomer, 1,4-bis (4%-cyanopyridinio-1%-methyl) benzene dibromide in a phosphate buffer solution in their study. Although similar reductive coupling polymerization of both the present trifunctional monomer and Saika’s bifunctional monomers should take place, the trifunctional monomer is expected to form a highly cross-linked
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Fig. 2. The FT-IR spectra of (1) the trifunctional monomer and (2) the CPV film and in KBr pellets, both of which are normalized with respect to the absorption band (a) at 2987 cm − 1. The arrows, a–d, are the significant absorption peaks mentioned in the text.
structure which may result in efficient deposition on an electrode. The following mechanism of the reductive coupling polymerization has been proposed [22]. 4Cyanopyridinium ion is reduced to the neutral 4cyanopyridinyl radical (process (a) in Scheme 1A), which is equilibrated to its dimer (process (b)). A part of the resulting neutral dimer, 4,4%-dicyano-4,4%bipyridine, loses two cyanide ions to form a viologen dication (process (c)). The overall reaction can be described as two-electron reduction and the elimination of two cyano groups from two 4-cyanopyridinium ions, giving the viologen skeleton. While a successive coupling reaction of the bifunctional monomer will give an LPV, efficient coupling of the trifunctional monomer could form a highly cross-linked structure, which is just our objective to demonstrate the anion-imprinted electropolymerization. At the electrode potential applied for the electropolymerization the resulting viologen moiety should be reduced to its cation radical (process (d)). A bulk electrolysis at −0.75 V of constant potential, corresponding to the first reduction peak of the 4cyanopyridinium moiety, also gave a polymer film on an ITO-coated glass electrode. A light purple film grew on the electrode, indicating that the resulting viologen moieties should be reduced to their cation radicals at this potential. After completing the electropolymerization the potential was set to + 0.02 V for a while, so that the film color turned light brown due to re-oxidation of the viologen cation radical back to the viologen dication. The polymer film was quite insoluble in any solvent, but became swollen or shrunk in a variety of solvents. Fig. 2 shows the FT-IR spectra of the resulting film and the trifunctional monomer in KBr pellets, both of which are normalized with respect to the absorption
band (arrow a in Fig. 2) at 2987 cm − 1 assigned to the C–H stretching vibration of the methyl groups. The absorption peak (b) at 2244 cm − 1, assigned to the C–N stretching vibration of the cyano group in the monomer, decreased down to B 7%, implying that most of the 4-cyanopyridinium moiety was consumed in the polymerization. The remaining 4-cyanopyridinium moiety resulted from incompleteness in the coupling reaction. A small amount of free CN − ((c) nCN − = 2076 cm − 1) resulting from the coupling reaction (process(c) in Scheme 1A) remained as a counter anion in the film, while an appreciable amount of sulfate ion was included, judging from the S–O vibration peak (d) at 618 cm − 1 and shoulder (e) at 1137 cm − 1. The remaining peaks of the film are not changed in comparison with those of the monomer. These results imply that the trifunctional monomer may be efficiently polymerized to form a highly cross-linked polyviologen (CPV) film and that both the bromide ion of the monomer and the released CN − should be exchanged by the electrolyte anion used, SO24 − , which is a counter anion of the CPV. The highly cross-linked structure (Scheme 1C) obtained plays an important role in the voltammetric anion recognition as will be discussed later. Compared with the bifunctional monomer for LPV, the electropolymerization of the trifunctional monomer gave an efficient deposition of the CPV film on an electrode. The current efficiency of the film deposition (CF) was evaluated by the ratio of the surface density (G) found to the ideal surface density (G0). CF= G/G0
(1)
Trifunctional and bifunctional monomers were electropolymerized at −0.75 V versus Ag AgCl in 0.1 M
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Na2SO4 aqueous solution to deposit CPV and LPV films on an ITO-coated glass electrode (1.2 cm× 2.5 cm). The G is derived from the charge passed in the cyclic voltammogram of the resulting film in monomerfree electrolytic solution1. G0 is the surface density assuming 100% of CF, i.e. that all the reduced 4cyanopyridinium moieties should be coupled completely and that the entire resulting viologen moiety should deposit on an electrode in its one-electron reduced state:
using the values m and n in order to extract only the charge passed in the reductive coupling of monomer2, where m is the fraction of the number of electrons consumed for the reductive coupling per repeated unit in the sum of the two contributions, so that m=2/3 for both the trifunctional and bifunctional monomers, and n is the number of the resulting viologen moiety per repeated unit, where n= 1.5 for the trifunctional monomer and n =1 for the bifunctional one, respectively. Finally the equation
G0 = (Q/F) (m/n)
G= (CF/F) (m/n) Q
(2)
where F is the Faraday constant. The charge passed in electropolymerization (Q) includes two contributions from the reductive coupling of 4-cyanopyridinium moieties (process (a)–(c) in Scheme 1A) and the one-electron reduction of the resulting viologen (process (d) in Scheme 1A). Then in equation (2), G0 was corrected by
(3)
is obtained. Fig. 3 shows that the relationship between G and Q is linear to 50 mC cm − 2 for CPV. The CF values were 5.7 and 0.6% for CPV and LPV films, respectively, which were determined from the slope of (CF/F) (m/n) in the linear region in Fig. 3. The larger value for CPV may be explained by its highly crosslinked structure (Scheme 1C).
3.2. Spectroelectrochemistry of CPV film
Fig. 3. The relationship between the surface density of the electrochemically active viologen moiety (G) and the charge passed in electropolymerization (Q). The surface density was obtained from the charge passed in the cyclic voltammogram of the polyviologen film in a monomer-free aqueous solution. The current efficiency was derived from the slope by using equation (3) in the text. The bifunctional and trifunctional monomers were electropolymerized at − 0.75 V versus Ag AgCl in 0.1 M Na2SO4 aqueous solution and deposited on an ITO-coated glass electrode (1.2 cm × 2.5 cm). 1 When the weight of the resulting thick films (Q \ 100 mC cm − 2) was adopted to estimate the current efficiency (CF) for the film deposition, the CF values were 49% for CPV and 1% for LPV, respectively. The elementary analysis indicated only two water molecules in a repeated unit of the CPV/SO24 − dried in vacuo at room temperature for 10 min. The inconsistent results between the coulometric and the gravimetric determinations might be interpreted by additional inclusion of electrolyte Na2SO4 and its low conductivity. The authors think throughout this study that an appreciable fraction of the counter anion would be confined in the highly cross-linked structure and therefore become electrochemically inactive when the cyclic voltammetry was carried out in a thick CPV film, relating to the structural consideration on the present voltammetric anion size recognition. So, the current efficiency derived from the gravimetric determination seemed larger than that from coulometry in this study. Based on the referee’s suggestion, a coulometric method for a relatively thin film was adopted. A gravimetric study by EQCM is in progress.
Fig. 4a shows the UV–vis spectra of the CPV film on an ITO-deposited electrode under potentiostatic conditions at every 50 mV and the spectrum at + 0.02 V was subtracted from all the spectra taken at 0.0 to −1.0 V versus SCE so as to avoid the contribution from partially reduced viologen moieties. In the potential region less negative than − 0.35 V, a new small band appeared around 480 nm, but no redox wave was observed in the cyclic voltammetry (Fig. 1b), where the assignment of this unknown band is under investigation. In the potential range from − 0.50 to −0.75 V a broad absorption band appeared over the 500–700 nm region and an intense absorption band around 400 nm, assigned to viologen cation radical. The first inflection points in the absorbance titration curves at 400–600 nm are observed around − 0.60 V (Fig. 4b), close to the first reduction potential −0.52 V obtained from the cyclic voltammogram (Fig. 1b). Very different from the wellknown behavior of the concentrated solution of viologen cation radical, it is noticeable that the spectrum shows little indication of the dimer formation of the resulting viologen cation radicals, probably because the individual viologen moieties are fixed tightly in the highly cross-linked structure. In a more negative potential region, this broad band in the visible region van2
The fraction m indicates the ratio of the number of electrons consumed for the complete coupling of 4-cyanopyridinium moieties to the total number of electrons passed in electropolymerization. In the case of one molecule of trifunctional monomer, three electrons are required for the coupling, and 1.5 electrons are consumed for the one-electron reduction of the resulting viologen moieties. So the fraction m should be given by a ratio of 3 to (3 + 1.5), i.e., 2/3. The repeated unit produces 1.5 equivalents of the viologen moiety (n= 1.5). In the case of the bifunctional monomer, m= 2/3 and n=1.
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Fig. 4. (a) The UV – vis spectra of the CPV film on an ITO-deposited electrode under potentiostatic conditions. All the spectra were subtracted by the spectrum at + 0.02 V versus SCE. The solid lines are the spectra taken at 50 mV intervals from 0 to −0.75 V (thick solid line) and the broken lines are those from − 0.8 to − 1.0 V (thick broken line) with arrows indicating the absorbance changes. (b) The absorbances at 400 nm ( ) and 600 nm () as a function of the applied potential.
ished, while both the 400 nm intense band and the 480 nm shoulder band remained. The titration curve (Fig. 4b) shows a second inflection point around − 0.85 V, in good agreement with −0.88 V of the second reduction potential. The spectra in the region still more negative than −0.95 V are assigned to neutral quinoid-like viologen as a two-electron reduced species.
3.3. Voltammetric anion recognition of CPV film The redox behavior of the CPV-coated electrode was dependent greatly on a variety of electrolyte anions used in monomer-free aqueous electrolytic solutions. The CPV film with Cl − as counter anion, CPV/Cl − , on an ITO-coated glass electrode (0.4cm× 2.5cm) was prepared beforehand by electropolymerization in 0.1 M NaCl aqueous solution in 50 mC cm − 2 of electropolymerization. The CPV/Cl − film shows two reversible redox waves with half-wave potentials of − 0.57 and − 0.91 V versus Ag AgCl in a monomer-free 0.1 M NaCl aqueous solution (Fig. 5a). No redox wave appeared in the cyclic voltammogram of the same CPV/Cl − -deposited ITO electrode in a monomer-free 0.1 M sodium tetraphenylborate aqueous solution (Fig. 5b). When the electrolytic solution was replaced again by the original monomer-free
NaCl solution, the original two-step redox waves appeared in cyclic voltammetry (Fig. 5c). The CPV/Cl − film, which was electrochemically active in the NaCl solution, became completely inactive in the sodium tetraphenylborate solution. Again in the NaCl solution, the voltammetric response was almost restored. This striking contrast in a series of cyclic voltammograms illustrated that the CPV film should recognize electrochemically the anion size of the electrolyte used in the viologen redox process with high selectivity. Similar voltammetric anion recognition was demonstrated with Cl − and Br − as electrolyte anions. The diameter of Br − (1.96 A, ) is only 8.3% larger than that of Cl − (1.81 A, ) [23]3, which means a 27% difference in excluded volume. Fig. 6 shows typical cyclic voltam3 Several sets of radii of ions [23] have been proposed such as crystallographic radii (1.81, 1.96, and 2.16 A, for Cl − , Br − , and I − by Goldschmidt, 1.81, 1.95, and 2.20 by Pauling, 1.64, 1.81, and 2.05 by Gourary and Adrian, and 1.70, 1.87, and 2.12 by Ladd), hydrodynamic radii of solvated ions (1.21, 1.18, and 1.19 A, for Cl − , Br − , and I − ), and hydrated ion radii (3.32, 3.30, and 3.31 by Nightingale). Considering the procedures to derive these values, actual ionic radii in the CPV film would be different from them because of much difference in ionic strength and water structure in the film. Therefore, the crystallographic radii on the Goldschmidt scale were adopted here [23] as reliable ones.
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mograms (50 mV s − 1 scan rate) of the CPV/Cl − -deposited ITO electrodes (0.4 cm×2.5 cm) in both monomer-free 0.1 M KCl (solid line) and KBr (broken line) solutions. The CPV/Cl − films were prepared by electropolymerization of the trifunctional monomer (0.02 M) in 0.1 M KCl aqueous solution at − 0.75 V versus SCE for 10, 20, 30, 40, 50, 100, 200, and 300 mC cm − 2 of the charge passed. The voltammetric response of the CPV/Cl − film was depressed in KBr solution, compared with that in KCl solution. The reduction of the responses in KBr solution became more significant as the charge passed on electropolymerization increased, i.e. the film become thick. Then, the reduction reached a saturation up to 100 mC cm − 2 of charge passed on electropolymerization. This voltammetric feature indicated that the mass transport of the counter anion accompanied by the redox process of the viologen moiety can be controlled by the highly cross-linked structure in the CPV/Cl − , and then that the anion size recognition might be evaluated simply by a current ratio in both solutions, as will be described later. Another feature was that the voltammetric response even
Fig. 6. A series of cyclic voltammograms (50 mV s − 1 scan rate) of the CPV/Cl − films on ITO-coated glass electrodes (0.4 cm ×2.5 cm) with different surface density in both monomer-free KCl (solid line) and KBr (broken line) solutions. The surface density was controlled by the amount of the charge passed in electropolymerization of 0.02 M trifunctional monomer aqueous solution containing 0.1 M KCl at −0.75 V versus SCE. The CPV/Cl − films were prepared by electropolymerization with (a) 10; (b) 20; (c) 40; (d) 50; (e) 100; and (f) 200 mC cm − 2.
Fig. 5. The cyclic voltammograms (50 mV s − 1 scan rate) of the CPV/Cl − -deposited ITO electrode (a) in a monomer-free 0.1 M NaCl aqueous solution, (b) in a monomer-free 0.1 M sodium tetraphenylborate aqueous solution, and (c) again in the monomer-free 0.1 M NaCl aqueous solution. These three voltammograms were taken sequentially on the same CPV/Cl − -deposited electrode. The CPV/ Cl − -deposited electrode was prepared beforehand by electropolymerization of the trifunctional monomer (0.02 M) in 0.1 M NaCl aqueous solution by 50 mC cm − 2. The reference electrode was an Ag AgCl electrode.
in KCl solution became reduced and distorted in a \ 100 mC cm − 2 region. Mass transport of the same anion as that imprinted on the highly cross-linked structure could be limited in the case of such a thick film. Here, great emphasis should be placed on the following mechanistic consideration on these anion-controlled redox processes of the CPV/Cl − films. Basically, any redox processes accompanying mass transport of the counter ion maintain the electroneutrality, as Scheme 2 shows. When the CPV film having a counter anion X − , defined as CPV/X − , is reduced in aqueous solution
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Scheme 2. Schematic mass transport of counter anions on the redox cycle of the viologen moiety (V) in the CPV/X − film. The counter anion X − in CPV/X − was released stepwise when the electrode potential was swept negatively (V2 + V + V0). The electrolyte anion Y − was incorporated stepwise as a new counter anion when the two-electron reduced viologen V0 was re-oxidized by the positive sweep of electrode potential (V0 V + V2 + ).
Scheme 3. The highly cross-linked structures with different cavity sizes which can be fabricated by the template effect of the electrolyte anions used in the electropolymerization, called anion-imprinted electropolymerization here. The CPV film shows voltammetric anion recognition through the anion-imprinted cavity.
containing an electrolyte anion Y − , the X − is released stepwise from the film into the electrolytic solution. That is to say, the viologen cation radical has one X − and the two-electron reduced neutral species has no counter anion. When the latter neutral CPV film is re-oxidized stepwise to the cation radical and then to the original dication state, Y − should be incorporated stepwise into the film as a new counter anion because the concentration of Y − is much higher than that of the released X − in the electrolytic solution. In the redox processes of the viologen moieties the counter anion should transport through the cavities of the cross-linked structure in the CPV films. The cavity size should be crucial for the anion transport accompanied by these redox processes. The viologen resulting from
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electropolymerization of the trifunctional monomer took counter anions from an excess amount of the electrolyte in the bulk solution. Since the highly crosslinked structure in the CPV film is fabricated by the template effect of the electrolyte anion used on the electropolymerization, the cavity size in the network structure would depend on the size of the electrolyte anion used, which could be regarded as a new type of molecular imprint [24–27], i.e. anion-imprinted electropolymerization. Therefore, the anion transport in the CPV film would be limited by the cavity size and then determine the electrochemical activity on the redox processes of the CPV film in a monomer-free electrolytic solution, as Scheme 3 illustrates. When Y − is equal to or smaller than X − in size, Y − can transport easily through the cavities in the CPV/X − film on the redox process. The CPV film behaves as electrochemically active in this case. On the contrary, the transport of larger Y − than X − in size should be considerably depressed, and then the CPV film becomes inactive electrochemically. Surprisingly, the cavity of the crosslinked structure recognized only an 8% difference in diameter or a 27% difference in excluded volume between Cl − and Br − as the voltammetric response. A significant question arises here about the dependence of the CPV film thickness on the voltammetric response, which was evaluated qualitatively as the ratio of the second cathodic currents in the cyclic voltammograms in the KCl and KBr solutions, as is illustrated in Fig. 6d. The amount of redox active sites, called the surface density of the viologen moieties in the film, can be adjusted accurately by the charge passed on electropolymerization. Therefore, on the basis of the linear relationship between the CPV film thickness and the voltammetric response of the viologen moiety, the film thickness may be discussed by using the surface density, which corresponds to the charge passed on the electropolymerization. Fig. 7 shows the peak current ratios IBr/ICl as a function of the charge passed on electropolymerization. A 100% ratio means that the CPV film should show the same redox activity in KBr solution as in the case of KCl solution, and would be achieved in an infinitely thin CPV film. These cathodic current ratios decrease suddenly with the increasing surface density of the film and are saturated down to about 40% in the CPV/Cl − films prepared by more than 40 mC cm − 2 of electropolymerization. Another significant feature is that the cathodic current ratio IBr/ICl was almost constant in a wide range of scan rate from 1 to 200 mV s − 1. This evidence indicated that the CPV/Cl − films lost a maximum of 60% of their redox activity in the KBr solution. That is to say, the CPV/Cl − films can distinguish a small difference in the anion size of Cl − and Br − (Scheme 3). In addition, these results suggest that the cavity size in the CPV film can be determined by the anion-imprinted electropolymerization
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Fig. 7. The ratios (IBr/ICl) of the second cathodic peak currents of the cyclic voltammograms (50 mV s − 1 scan rate) in monomer-free 0.1 M KBr and KCl solutions as a function of the amount of the charge passed (Q) in the preparation of the CPV/Cl − -coated ITO electrodes. The CPV/Cl − films were prepared by electropolymerization with (a) 10; (b) 20; (c) 40; (d) 50; (e) 100; and (f) 200 mC cm − 2.
and that the resulting CPV film shows clearly voltammetric anion recognition. The increasing surface density in the region B40 mC cm − 2 causes the selectivity of the voltammetric anion recognition to be elevated. In the 100 mC cm − 2 region, the present estimation of the anion-controlled redox process becomes invalid because of distorted voltammograms with considerable irreversibility of the redox processes even in the KCl solution. So far we have described the electrochemical anion recognition in size through the redox properties in an aqueous solution containing an electrolyte anion larger than the cavity formed in the CPV film. Hereafter, three kinds of polyviologen films with different cavity sizes, CPV/Cl − , CPV/Br − , and CPV/I − , were prepared by electropolymerization of the trifunctional monomer (0.02 M) in 0.1 M KCl, KBr, and KI aqueous solutions, respectively. The diameters of Cl − , Br − , and I − are 1.81, 1.96, and 2.19 A, [23], and their excluded volumes are 3.10, 3.94, and 5.50 A, 3, respectively. Fig. 8a shows the cyclic voltammograms of the CPV/Cl − films in 50 mC cm − 2 of electropolymerization in three kinds of electrolytic solutions containing KCl, KBr, and KI, respectively. The CPV films become electrochemically inactive in the order of KCl, KBr, and KI solutions. As the anion size in the electrolytic solution increases, the mass transport of the anion in the cavity of the CPV film tends to be limited and then the redox current decreases. Fig. 8b shows the cyclic voltammograms of the CPV/Br − films (50 mC cm − 2) in the three solutions. While the CPV/Br − becomes almost inactive in the KI solution, the CPV/Br − film exhibits a similar redox response in the KCl solution to that in the KBr solution. This evidence suggests that Cl − , which is smaller than the cavity in the CPV/Br − film, may transport with almost the same mobility in the viologen redox process as Br − . While the CPV/Br −
Fig. 8. The cyclic voltammograms of (a) CPV/Cl − films, (b) CPV/ Br − films, and (c) CPV/I − films in monomer-free 0.1 M KCl (solid line), KBr (dotted line), and KI (chain line) aqueous solutions. Every CPV/X − film was prepared by electropolymerization of 0.02 M trifunctional monomer aqueous solution containing 0.1 M KX (X = Cl, Br, I) with 50 mC cm − 2
film exhibits an appreciable voltammetric response for Br − and Cl − , but not for the larger I − as a counter anion, almost similar electrochemical activities were realized in the voltammograms of the CPV/I − films in the three solutions as Fig. 8c shows. Both Br − and Cl − move easily through the larger cavity in the CPV/I − film in the viologen redox processes. These results of voltammetric anion recognition are consistent with the mechanistic considerations mentioned in Scheme 3 and support strongly the supposition that the cavity size in the CPV film should be controlled by the template effect of the electrolyte anion used in electropolymerization.
Acknowledgements This work was partially supported by Grants-in-Aid for Scientific Research on Priority Area of Electrochemistry of Ordered Interfaces (No.282/09237256) and New Polymers and Their Nano Organized Systems (No. 277/08246254) from the Ministry of Education, Science, Sports, and Culture, Japan. One of the authors (T.I.) acknowledges the financial support of the CASIO Science Promotion Foundation and the Tokuyama Science Promotion Foundation.
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