Langmuir 1991, 7, 160-165
160
Two-Dimensional Ordering of Viologen Polymers Fixed on Charged Surface of Bilayer Membranes: A Peculiar Odd-Even Effect on Redox Potential and Absorption Spectrum Masatsugu Shimomura,**tKoji Utsugi,? Jun Horikoshi,? Kenji Okuyama,? Osamu Hatozaki,i and Noboru Oyamat Department of Biotechnology and Department of Applied Chemistry, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan Received July 11, 1990. I n Final Form: October 8, 1990 Polyion complex films were prepared from viologen ionene polymers and anionic bilayer membranes. Electrochemical reduction and electrochromic behavior of the complex films were examined. A peculiar odd-even effect of alkyl chain connection among viologen groups was found in the absorption spectra, in the redox potential, and in the diffusion coefficients (DaPp) of the charge transport process. Blue shift in the absorption spectrum indicated dimer radical formation in the odd-numbered polymer. While in the even-numbered polymer, dimerization was suppressed. Cyclic voltammetry showed that the odd polymers were more easily reduced than the even polymers. The Dappvalue of the odd polymer was smaller than that of the even polymer. That suggests the dimer radical formed in the odd polymer acts as a trapping site of the hopping electron. The odd-even effect was assumed to be explained in the fixed conformation of the viologen polymers ordered on the regularly charged surface of the bilayer membranes.
Introduction Molecular organizates, such as Langmuir-Blodgett films or bilayer membranes, have attracted attention in the fields of "molecular devices"' and biomimetic chemistry.2 Many efforts have been made to find unique properties of the molecular organizates, which play an important role as organized media in chemical and/or physical processes. Synthetic bilayer membranes are two-dimensional molecular organizates and are useful media for ordering the guest molecules. Kunitake and co-workers found a unique molecular assemblage of the cyanine dyes3 and a large magnetic anisotropy of the metal chelates4 in the bilayer membranes. We have also reported a peculiar odd-even effect on absorption spectra of the reduced viologen polymers complexed with anionic bilayer membrane. The unique spectral properties are assumed to be ascribed to the fixed molecular orientation of the viologen polymers on the regularly oriented anion charges of the bilayer membranes5 Spectroscopy of the viologens has been intensively investigated in connection with their photo- and optoelectrochemical properties because of their utility in photoenergy converting systems as the electronhtransfer catalysts6 and for the electrochromic display devices.' The reduction properties of the viologens are well-known to be affected by the solvent polaritys and by the substituent t
Department of Biotechnology.
8 Department of Applied Chemistry.
(1)Papers presented at the Third International Conference on Lang muir-Blodgett Films are published in Thin Solid Films 1988, 159 and 160.
(2)For an example of the review, see Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. Engl. 1988,27, 113. (3)Nakashima, N.;Ando, R.; Fukushima, H.; Kunitake, T. J . Chem. Soc., Chem. Commun. 1982, 707. (4) Ishikawa, Y.; Kunitake, T. J. Am. Chem. SOC.1986, 108, 8300. (5)Shimomura, M.;Utsugi, K.; Okuyama, K. J . Chem. SOC.Chem. Commun. 1986, 1805. (6)For an exampleof the review, see Gratzel, M. Acc. Chem. Res. 1981, 14, 376. (7) Schoot, C. J.; Ponjee, J. J.; van Dan, H. T.; van Doorn, R. A.; Bolwijin, P. T. Appl. Phys. Lett. 1973, 23, 64. (8)Kawata, T.; Yamamoto, M.; Yamano, M.; Tajima, M.; Nakano, T. Jpn. J . Appl. Phys. 1975, 14, 725.
group^.^ The colors of the reduced radicals and the redox potentials are also influenced by the molecular association of the viologen groups. In the classic paper of Kosower and Cotter,'O the color of methylviologen radicals ranged from blue to purple with increasing concentration in an aqueous solution. The reversible color change was ascribed to a monomer-dimer equilibrium of the radicals. As demonstrated by Bard, dimerization of methylviologen was facilitated in montmorillonite" but was depressed in an anionic micelle of sodium dodecyl sulfate.12 Owing to intramolecular association, polyviologen derivatives are favorable to form dimer radi~a1s.I~ The first reduction potential of the polyviologens often shows the positive shift compared with monomeric ~iologens.'~J5 Although the regulation of the aggregate structure could be useful to control photo- and optoelectrochemical properties, a systematic study of the relation among the aggregate structure, spectroscopy, and electrochemistry has not hitherto been carried out. In this paper, we find a peculiar odd-even effect on the redox potential, the absorption spectrum, and the diffusion coefficient of the viologen polymer 1 when the polymer is fixed on the charged surface of the anionic bilayer membrane 2. And we also find a linear relation between the redox potentials and absorption maxima. These findings are a fascinating example of controlled electrochemistry in organized media based on immobilized bilayer membranes. Experimental Section Materials. Poly(alkenylvio1ogendibromide) (1)was prepared by the Menschutkin reaction of equimolar amounts of 4,4'-bipyridine (5.0 g, 3.2 X mol) and the corresponding a,w-dibromoalkane in 100 mL of N,N-dimethylformamide (DMF) at (9) Hunig, S.; Schenk, W. Justus Liebigs. Ann. Chem. 1979, 1523. (10)Kosower, E.M.; Cotter, J. L. J . Am. Chem. Soc. 1964,86, 5524.
(11)White, J. R.; Bard, A. J. J. Electroanal. Chem. Interfacial Electrochem. 1986, 197, 233. (12)Kaifer, A. E.;Bard, A. J. J . Phys. Chem. 1985,89, 4876. (13)Furue, M.;Nozakura, S. Bull. Chem. SOC.Jpn. 1982,55, 513. (14) Deronzier, A.; Galland, B.; Vieira, M. Nouu. J . Chem. 1982,6,97. (15) Imabayashi, S.;Kitamura, N.; Tokuda, K.; Tazuke, S. Chem. Lett. 1987, 915.
0 1991 American Chemical Society
Viologen Polymers on Bilayer Membranes
- -
Table I. Characterization of Polyion Complex Films
R:fCHz.fn
X-
X-
Langmuir, Vol. 7, No. 4,1991 761
n=3.4.5.6.7.8.9.10
X=BF
1
w
9
Ci2Hz5-0-C-CH2 CIZH~~-O-$-~H -503 N d
ti
molar ratio of anion to cation
n
elemental analysis
Droton NMR
3 4 5 6 7 8 9 10
1.08
1.03 1.07 1.16 1.08 1.17 0.86 0.86 1.08
2
quarternization of the nitrogen atoms in the bipyridine ring. The average molecular weights of ionene polymers with and without the viologen group were reported to be 11 OO0l6 and 50 OOO,I7 respectively. The anionic amphiphile 2 was supplied from Sogo Pharmaceutical Co., Ltd., and was used without further purification. P r e p a r a t i o n of Polyion Complex. According to a slight modification of Kunitake's method,18 a polyion complex of the anionic bilayer membrane and the viologen polymer was prepared. An aqueous solution (15 mL) of the anionic amphiphile (157 mg, mol) and an aqueous solution (5 mL) of poly(pro2.8 X penylviologen dibromide) (50 mg, 1.4 X mol) were mixed. The precipitates were solved in chloroform and washed with water several times to remove water-soluble species. After evaporation of chloroform, the transparent and self-supporting complex films were obtained. Polyion complex films of the other polymers were prepared in the same manner. The molar composition of 1 and 2 in the polyion complexes was calculated not only from the sulfur and nitrogen content by elemental analysis but also from the ratio of aromatic and aliphatic protons of the 'H NMR spectrum. X-ray diffraction of the complex films was measured by RAD-C (Rigaku Corp. Cu Ka). Spectroscopic Measurement. Chemical reductions of the complex films cast on quartz slides (1 cm x 4 cm) were measured by a Shimazu UV-160 spectrophotometer. The quartz slide was immersed in an aqueous solution of sodium hydrosulfite (0.1 moledm-3, pH 6.2). In order t o obtain the electrochemical reduction, the complex film was cast on transparent SnO2 electrode (1.4 cm2area) from chloroform solution. Film thickness was measured with a Surfcom 550.4 (surface texture measuring instrument, Tokyo Seimitsu Co.) and was about 0.1 pm when 8 pL of 5 wt chloroform solution was spread on the electrode. T h e modified electrode was immersed in an aqueous solution (0.1 m ~ l - d m -KCl) ~ in a quartz cell and the spectroscopic measurement was carried out under and applied potential of -0.6 V vs SCE. In order to confirm the effect of complex formation, absorption spectra of the chemically reduced viologen polymers in an aqueous solution (1.0 x m ~ l - d m -without ~) bilayer membranes were also measured. Electrochemical Measurement. Cyclic voltammetry was carried in a nitrogen atmosphere in an aqueous 0.1 m ~ l . d m -KCl ~ solution. Five microliters of 2.5 wt % chloroform solution of the polyion complex was cast on a basal-plane pyrolytic graphite disk electrode (BPG, Union Carbide Co.; surface areaof electrode A = 0.19 cm2) prepared by O y a m a ' s m e t h ~ d .The ~ ~ film thickness was about 0.5 pm. A platinum ring and a saturated calomel electrode (SCE) were used as a counter and a reference electrode, respectively. Voltammograms were recorded on a potentiostat (NPGFZ-2501A, Nikko Keisoku) with a X-Y recorder (WXlOOO, Graphtec). '(
(16) Factor, A.; Heinsohn, G. E. J.Polym. Sci.,Polym. Lett. Ed. 1971,
9, 289.
(17) Rembaum, A.; Noguchi, H. Macromolecules 1972,5, 261. (18) Kunitake, T.; Tsuge, A.; Nakashima, N. Chem. Lett. 1984,1783. (19) Oyama, N.; Ohsaka, T.;Yamamoto,H.; Kaneko, M. J.Phys. Chem. 1986,90, 3850.
0.88
0.87 '0.95 0.97 0.78
Table 11. X-ray Diffraction of Cast Films
c
60 "C. Yellow precipitates were washed withethyl acetateseveral times and dried in vacuo (yield ca. 70%). Double doublet signals a t 8.5 and 9.2 ppm in lH NMR spectroscopy showed complete
0.83 0.82
spacing of reflection, nm
n 3
1
2
3
4
4 5 6 7 8 9 10
4.28 4.35 4.25 4.32 4.29 4.33 4.33 4.30
2.23 2.21 2.23 2.22 2.22 2.21 2.22
1.50 1.47 1.49 1.49 1.50 1.50 1.49
1.12 1.12 1.13 1.12 1.13
2
4.34
2.21
1.49
1.13
Potential-step chronocoulometry was employed to estimate the apparent diffusion coefficients (D,pp).20*21The surface concentration r of the electroactive site was calculated from the area of the cyclic voltammogram. The molar concentration C of the electroactive site was estimated from r and film thickness.22
Results Structure of Polyion Complex Films. The selfsupporting and optically transparent films were cast from chloroform. Presence of sulfur and nitrogen in elemental analysis of the cast films strongly suggested the complex formation of 1 and 2. Aromatic protons of the viologen groups and long-chain aliphatic protons observed in 'H NMR spectroscopy also suggested complex formation. Table I summarizes the molar ratio of anion to cation calculated from the elemental analyses and from the NMR spectra. Table I suggests that the anionic amphiphiles 2 take the place of bromide anions of the viologen polymers and one viologen unit has approximately two amphiphilic anions. The stoichiometric ion pair formation between a cationic amphiphile and an anionic polymer have been found in the Langmuir-Blodgett films prepared on an aqueous polymer s u b p h a ~ e . ~ ~ In a preliminary paper,5 we reported that the DebyeSherrer rings suggested as layered structure was found in the X-ray diffraction of the powdered samples of the complex films. The values of the long spacing in the previous experiments had a large experimental error, because the experimental setup was for the wide-angle X-ray diffraction and the diffraction intensity was not strong enough. In order to estimate the exact values, the middle-angle X-ray diffraction from the complex film cast on the glass side was measured in this experiment. The strong reflections from the cast films are summarized in Table 11. The number for each reflection corresponds to the higher order of diffraction from the long period, which is very consistent with the bimolecular length (20) Oyama, N.; Yamaguchi, S.; Nishiki, Y.; Tokuda, K.; Matsuda, H.; Anson, F. J . Electroanal. Chem. Interfacial Electrochem. 1982,139,371. (21) Ohsaka, T.; Okajima, T.; Oyama, N. J . Electroanal. Chem. Interfacial Electrochem. 1986, 215, 191. (22) Oyama, N.; Ohsaka, T.; Chiba, K.; Takahashi, K. Bull. Chem. SOC.Jpn. 1988, 61, 1095. (23) Shimomura, M.; Kunitake, T. Thin Solid Films 1985,132, 243.
Shimomura et al.
762 Langmuir, Vol. 7, No. 4, 1991 Table 111. AbsorDtion Maximum of Reduced Polymer
n
chemical reduction of film
,A,, nm electrochemical reduction of film"
chemical reduction of solution*
537 613 563 619 562 613 570 609
534 614 568 619 568 614 561 604
510 533 533 538 537 541 541 530
3 4 5 6 7 8
9 10
U
C
ra
n
i
0
m
n 4
02
Absorption maximum at stationary state of electrochemical reduction. A large excess of sodium hydrosulfite (10-3 mol.dm-3)
*
was used.
of 2. We expected elongation of the long spacing caused by polymer intercalation, but that was not found in this experiment. A similar result was reported by Kajiyama et al. for the thin layer films of fluorocarbon a m p h i p h i l e ~ . ~ ~ They showed that the long spacing of the Y-type LB film having poly(styrenesu1fonates) as counterions of the bimolecular layers was almost the same as that of the crystalline powder of the amphiphile without polymer. The X-ray experiment and the chemical analyses, however, strongly suggest that a well-oriented multibilayer structure was formed in the cast film of the polyion complex. Similar structural analysis of the polyion complex film was reported by Okahata.25 Spectroscopic Properties of the Complex Films. A strongly colored cation radical is well-known to be formed by a chemical or an electrochemical reduction of a viologen derivative. The absorption spectra of the reduced polymers in the aqueous solution without bilayer membranes showed two absorption peaks at around 540 and 900 nm. Spectral shape was similar to that of the dimer cation radical of methylviologen (MV2+)l0generated by the monomerdimer equilibrium expressed in eq 2. In the absence of
+
MV2+ e2MV"
-
MV"
(monomer radical)
(1)
(MV"),
(dimer radical)
(2)
the bilayer membranes, every viologen polymer used in this experiment showed intramolecular interaction between reduced viologen groups. As previously reported, a peculiar odd-even effect on the absorption spectra of the chemically reduced polymers was observed when the polymer was immobilized as a cast film complexed with the bilayer membranes.5 In the case of the odd polymer (the number of methylene groups, n, is odd), the spectral shape was similar to that of the aqueous solution, which indicated the strong interaction between the radical cations. While in the even polymer, an absorption spectrum similar to that of the monomer radical of methylviologen was observed. That meant the radicalradical interaction in the even polymer was suppressed. Spectral data are summarized in Table 111. A similar odd-even effect was found for the electrochromic property of the complex films cast on the SnOz electrodes. Figure l a shows the generation of blue radicals in the complex film of the even polymer ( n = 4) at the applied voltage of -0.6 V vs SCE. The electrochemical (24) Takahara, A.; Morotomi, N.; Hiraoka, S.; Higashi, N.; Kunitake, T.; Kajiyama, T. Maclomolecules 1989, 22, 617. (25) Okahata, Y.; En-na, G. J . Phys. Chem. 1988, 92, 4546.
600
403
800
1000
Wavelength ( n m )
Figure 1. Spectral change of the complex films due to the electrochemical reduction (a) of the even polymer ( n = 4) and (b) of the odd polymer ( n = 9). Reversible cycles coupled with oxidative decolorization were observed.
a
1 1
- 0.8
-04 0 E ( V vs.SCE)
1
- 0.8
-04
0
E ( V vs. SCE )
Figure 2. Cyclic voltammogram of the even polymer ( n = IO): (a) 1.0 x molsdm-3 in aqueous 0.1 m~l-dm-~ KCl solution, Pt wire used as a working electrode, scan rate = 50 mv-sec-l; (b) scan rate dependence of complex film cast on the BPG electrode, scan rate ranged from 5 to 10, 20, 50, 100, 200, and 500 m V d .
reaction proceeded to the stationary state of the reduction and oxidation equilibrium within several minutes. As shown in Table 111, the blue radical was also found in every even polymer. In contrast, the spectral change of the odd polymer is not as simple, Figure l b shows the typical spectral change of the odd polymer ( n = 9). At the beginning of the reduction, the absorption maximum was located at 610 nm. During the course of reduction, the absorption maximum shifted to 560 nm and a broad absorption at 900 nm appeared. The absorption peak at 400 nm also moved to 370 nm concomitantly with the spectral shift in the visible region. In the polymer of n = 3, dimer radicals were preferably formed. The absorption maximum at 534 nm appeared from the first stage of the reduction. Electrochemical reduction of the complex films also shows an odd-even effect as well as chemical reduction. Cyclic Voltammetry. A cyclic voltammogram with a sharp anodic peak suggesting the deposition of reduced species on the electrode surface was observed for an aqueous polymer solution without bilayer membranes (Figure 2a). Scan rate dependence of cyclic voltammograms obtained for the one-electron oxidaton-reduction
Viologen Polymers on Bilayer Membranes
Langmuir, Vol. 7, No. 4, 1991 763 I
I
nr 4
n=3
LO
120
i d
u -403
3 4 5 6 7 8 9 1 0
n Figure 3. Alkyl chain dependence of the formal redox potential of complex films (circles) and aqueous solutions (triangles). reaction of the complex film (n = 10) cast on the BPG electrode is shown in Figure 2b. The shape of the cyclic voltammogram of the cast film is quite different from that of the aqueous solution. Disappearance of the sharp anodic peak and the S-shaped voltammogram indicate that the charge transport through the cast film is diffusioncontrolled, as well observed in a liposomal solution of longchain viologen derivative^,^^,^^ in a polyion complex film of viologen polymer and anioic polymer,28 or in selforganized bilayer assemblies formed in microporous aluminum oxide electrode^.^^ A straight line given by the plots of the cathodic peak current against the square root of scan rate strongly suggests that the charge transport process within the film is apparently diffusion controlled.30 The formal redox potential ( E 4 for the first wave of V2+ to V'+ was calculated as an arithmetic average of the cathodic and the anodic peak potentials. As well as the absorption spectrum, the odd-even effect on the redox potential is also found in the cast films (Figure 3). It is noted that no odd-even effect has been found in an aqueous polymer solution without a bilayer membrane. Potential-Step Chronocoulometry. Figure 4 shows typical potential-step chronocoulometric charge-time responses of the complex films of the odd (n = 3) and even (n = 4) polymer on BPG electrodes. The potential was stepped from 0 to -0.8 V (vs SCE). The chronocoulometric Anson plots (Q vs t1/2)20for the reductive processes are also shown in Figure 4.
Q = 2n,FACD,pp'/2t1/2~-'~2
(3)
The linearity of the Anson plot is consistent with the cyclic voltammometry results where the charge transport process within the complex film follows Fickian diffusion law. Following eq 3, the apparent diffusion coefficient Dappis obtained from the slope of the plot, where ne denotes the number of electrons involved in the electron transfer reaction, F is the Faraday constant, A is the area of the electrode, and C is the molar concentration estimated from r and film thickness. The results are summarized in Table IV. The odd-even effect is also found for the diffusion coefficient. (26) Kaifer, A. E. J . Am. Chem. SOC.1986, 108,6837. (27) Lu, T.; Cotton, T. M.; Hurst, J.; Thompson, D. H. J . Electroanal. Chem. Interfacial Electrochem. 1988. 246. 337. (28) Ohsaka, T.; Oyama, N.; Sato, 'K.; Matsuda, H. J . Electrochem. SOC.1985, 132, 1871. (29) Miller, C. J.; Majda, M. J. J . Am. Chem. SOC.1986, 108, 3118. (30)Oyama, N.; Ohsaka, T.; Yamamoto, H.; Kaneko, M. J.Am. Chem. SOC.1986, 90, 3850.
tv*lmd/*
Figure 4. Typical potential-step chronocoulometric responses and Anson plots for the reduction of complex films: (a) n = 3. (b) n = 4.
Table IV. Apparent Diffusion Coefficients in Complex Films n
3 4
5 6 7
8 9 10
C , molecm-3 8.8X 7.0 x 10-4 9.3 x 2.1 x 5.6 x 2.3 x 5.0 x 1.6 x
10-4 10-4 10-4 10-4 10-4 10-4
Dapp,cm2.s-1 4.7 x 10-11 7.3 x 10-10 5.6 x lo-" 1.2 x 10-9 1.3 X 5.3 x 10-10 4.3 x 10-10 5.9 x 10-10
Discussion Spectroscopic Properties of Complex Films. Spectroscopic behavior of the viologens has been extensively investigated. The absorption spectrum of methylviologen is reported to be affected by concentration,'O temp e r a t ~ r eand , ~ ~microenvironment.l1J2 Solvent polarit? and substituent effect9 are not significant in this experiment, because the homologous series of the N-alkylated viologen are used in the same solvent. Intramolecular association of the viologen radicals is well-known in the polymeric viologen derivatives. Furue and Nozakura reported that photochemical reduction of poly(N-(vinylbenzy1)-N'-methylviologen) by 2-propanol gave a purple cation radical whose maximum was located at 530 nm.32 They verified the blue shift of the absorption spectrum due to the intramolecular association in the bis(viologen) derivatives 3 as a model compound.13 Deronzier et al.14 also observed the absorption spectra of the bis(viologen) radicals generated by the electrochemical reduction. In DMF solution, the absorption maximum was located a t 605 nm for n=2 and 535 nm for n=3, respectively. Spectral change from 605 nm to 545 nm during the process of reduction was found for n=4. The blue shift in the absorption spectra of the bis(vio1ogen) derivatives was attributed to the intramolecular association of two cation radicals. Spectral observation of the odd polymers in the cast film can be explained in terms of the intramolecular association. A qualitative estimation of the dimerization degree is able to be evaluated from the spectroscopic (31) Evans, A. G.; Dodson, N. K.; Rees, N. H. J . Chem. SOC.,Perkin Trans. 2 1976, 859. (32) Furue, M.; Yamanaka, S.; Phat, L.; Nozakura, S. J.Polym. Sci., Polym. Chem. Ed. 1981, 19, 2635.
Shimomura et al.
764 Langmuir, Vol. 7, No. 4, 1991 L ]
I measurement. As shown in Figure 1, the hyposchromic shifts from 610 to 560 nm and from 400 to 370 nm were found in the cast film of the odd polymer. Thus the relative absorbance of 560-610 nm (A560/&10) and 370-400 nm (A370IA400) should be a parameter of the dimer radical formation. A good correlation of A560/A610 and A370/A400 for various viologen derivatives shown in Figure 5 strongly suggests that most of the radical cations in the even polymer are standing as the monomeric radical, because the spectral parameters are very similar to those of the typical monomeric radical of ethyl~iologen'~ or bis(vio1ogen) derivative ( n = 2).14 Why is the dimerization of the viologen groups in the even polymer so suppressed in the cast film? We proposed one probable structural model of the complex films in a previous r e p ~ r t . Judging ~ from the X-ray structural experiments and the chemical analyses (EA and NMR), the positive-charged viologen polymer is intercalated into the anionic interface of the each bimolecular layer in the cast film. The spectroscopic characteristics of the complex films should be related to the molecular orientation of the viologen polymers fixed on the two-dimensional organized media. If a gauch-(trans),-3-gauch conformation of the alkyl chain between two viologen groups is fixed a t the charged interface, the intramolecular association of the neighboring viologen groups within the linear polymer is probable in the odd polymer but is suppressed in the even polymer. Electrochemical Properties of Complex Films. Because of the highly conducting redox properties, electrochemical characteristics of the poly(vio1ogens) have been extensively examined. Due to the intramolecular interaction, redox behavior of the poly(vio1ogens) is different from that of methylviologen. Even in the bis(viologen) derivatives (V2+-V2+),the most simple poly(viologens),two reduction processes should be considered
(4) Since the waves of each reduction-oxidation teps were not able to be discriminated in the normal cyclic voltammogram, Deronzier and co-workers estimated the formal redox potentials, Elb and Ezb, of the bis(vio1ogen) derivatives, 3, from the peak separation value of the cathodic and anodic peaks of the cyclic v01tammograms.l~ In the case of n = 3, whose dimer radical is favorably formed, the second reduction step (formation of I11 in eq 4) is easier than the first step. In contrast, the second step is slightly harder for n = 2, whose spectral behavior is similar to methylviologen. Wilkins and Tsukahara showed that the overall potential of eq 4 calculated as a numerical average of the cathodic and the anodic peak potentials in the cyclic voltammogram was identical with the average value of Elb and Ezb. The overall potential of n = 3 was shown to be more positive than those of n = 2 and 4.33 As shown in Figure 3, the overall potentials of the odd polymers are more positive than those of the even polymers. The results of the cyclic voltammetry of the complex films are also explained by the fixed molecular orientation proposed in the previous report. Owing to the intramolecular association the formerly formed cation radical (33) Atherton, S. J.; Tsukahara, K.; Wilkins, R. G. J. Am. Chem. SOC.
1986, 208,3380.
1
-u
0
2
1
F i g u r e 5. Linear relation of A560/A610 vs A3,0/Am. The number for the close circle corresponds to alkyl chain length n. The values of the open circles are the typical monomer radicals calculated from the literature of ethylviologenl5 and bis(vio1ogen) derivative 3 (n = 2).14
h
> u N
w
- 0.4
1 1
1 ~
,
2.0
2.1
2.2
23
E T ( ~ v ) F i g u r e 6. Linear relation between redox potential and transition energy of the viologen polymer complex with bilayer membrane.
affects the subsequent reduction process of the neighboring viologen group in the odd polymers. On the contrary, the intramolecular interaction is suppressed in the even polymer; the second reduction process should be almost independent of the presence of the formerly reduced species Since the viologen polymer is tightly fixed on the charged surface of the bimolecular layer, the physical diffusion of the polymer might be impossible in the complex film. A self-exchange mechanism of electron transferN is probable. As shown in Table IV, the odd-even effect is also found in the charge transport process. The Dappvalues of the odd polymers are smaller than those of the even polymers. In the process of the electron hopping along the polymer chain, the dimer radical formed in the odd polymer should act as a trapping site of the hopping electrons. It is noted that no odd-even effects have been found for Dappof the polyion complex of polymer 1 and poly(styrenesu1fonic acid) prepared on IT0 electrode^.^^ Inverse Correlation between Redox Potential and Transition Energy. A peculiar odd-even effect is found for the redox potential (Figure 3) as well as for absorption spectra (Table 111)of the reduced polymers. A good linear relation between the transition energy calculated from the spectral data of Table I11 and the redox potential is shown in Figure 6. An inverse correlation means that the odd polymer, which has higher energy in the electronic transition, is more easily reduced than the even polymer.
.
(34) Ohsaka, T.; Yamamoto, H.; Kaneko, M.; Yamada, A.; Nakamura, M.; Nakamura, S.; Oyama, N. Bull. Chem. SOC.Jpn. 1984,57, 1844. (35)Ohsaka, T.;Nakanishi, M.; Hatozaki, 0.;Oyama, N. Electrochim. Acta 1990, 35, 63.
Viologen Polymers on Bilayer Membranes EL2
t
;“F
Ev2’
EL o
ELo monomer
dimer (aggregate)
Figure 7. Tentative energy level diagrams of monomer radical and dimer radicals.
Such a linear relation, which is found for the first time in this study, may be explained by the tentative energy level proposed by Kosower and Cotterlo (Figure 7). As the result of dimerization an energy-level of the radical electron (ELI) is split into two levels (ELI’ and ELI”). Two courses of the electronic transition from the split ELI’S to the excited state ( E L 3 are assigned to the two absorption bands in the visible (ET’, ca. 550 nm) and in the near-infrared regions (ET”, ca. 900 nm). The energy gap between the ground state (ELo) and ELI, assumed to be corresponding to the energy level of formal redox potential, E1p and Elp‘, should be lower in the odd polymer than in the even polymer. It seems reasonable that the odd polymer, whose radical needs more energy for electronic transition in the visible region, is more easily reduced than the even polymer. Another significant finding is also shown in Figure 6. We succeeded in regulating the redox potentials of the simple viologen polymers over the range of 150 mV. We believe that our results are the first example of the controlled electrochemistry of the viologen polymers. That was realized by taking advantage of the ordering nature of bilayer membranes, because we could not find any oddeven effect either on electrochemistry or on spectroscopy in aqueous solutions without bilayer membranes. Regulated Orientations of Guest Molecules on the Surface of Bilayer Membranes. Well-organized molecular assemblies, such as Langmuir-Blodgett (LB) films and bilayer membranes, are suitable matrices for ordered
Langmuir, Vol. 7, No. 4 , 1991 765 arrangement of guest molecules. Nakashima and Kunitake found that the J aggregate of an anionic cyanine dye was formed on the charged surface of the cationic bilayer membrane.3 The J aggregates of water-soluble cyanide dyes were also found a t the interface of the m o n ~ l a y e r ~ ~ , ~ ’ and the Langmuir-Blodgett films.3s The regularly charged surface of two-dimensional molecular assemblies may be considered as a specific adsorption site of the guest molecules. Water-soluble polymers are also the guest of the charged matrices. Electrostatic interaction between the monolayer of dioctadecyldimethylammonium bromide and potassium poly(viny1 sulfate) was observed as a drastic change of the surface pressure-molecular area isotherm.39 A stoichiometric ion pairing of the cationic amphiphile and the anionic polymer in the deposited LB film23suggests that the random-coil conformation of the polymer chain should not be allowed. The peculiar odd-even effect of the complex film is assumed to be ascribable to the restricted conformations of the alkyl chain between two viologen groups fixed on the regularly oriented charged surface of the bilayer assembly.
Conclusion By taking advantage of well organized bilayer membranes, we succeeded in connecting the electrochemistry and the spectroscopy of the viologen polymers with the molecular orientation. The linear relation between the redox potential and the transition energy will become an important index for the energy level calculation of the molecular orbitals. We believe that bilayer membranes are powerful tools not only for the molecular ordering but also for a new approach of electrochemistry. Detailed kinetic and thermodynamic experiments of the electron transfer process are in preparation for p u b l i ~ a t i o n . ~ ~ (36) Lehmann, U. Thin Solid Films 1988, 160, 257. (37) Kirstein, S.;Shimomura,M.; Mohwald, H. Chem.Phys. Lett. 1989, 154, 303. (38) Era, M.; Hayashi, S.; Tsutsui, T.; Saito, S.; Shimomura, M.; Nakashima, N.; Kunitake, T. Chem. Lett. 1986, 53. (39) Shimomura,M.;Fujii, K.; Karg, P.; Frey, W.;Sackmann,E.; Meller, P.; Ringsdorf, H. Jpn. J . Appl. Phys. 1988,27, L1761. (40) Oyama, N.; Hatozaki, 0.;Utsugi, K.; Shimomura, M. Manuscript
in preparation.
