J. Phys. Chem. 1987, 91, 3932-3934
3932
in the fragment derives from the repulsive forces acting along the I-C axis, and in the present special case of ICN, this is the principal source. Although bending vibration energy is efficiently disposed to CN fragment rotation, as elementary calculations show, our comparison with the experimental data suggests that this source is not of central importance for molecules with a mass
distribution like the one we are studying. A similar analysis of BrCN dissociation shows similar agreement with experimental data.4 It should be realized that this simple treatment is useful only for molecules like ICN; it is not adquate when considering HCN. For that molecule, all three of the sources of fragment rotation must be considered.
Infrared Spectra and Dimer Structure of Reduced Vioiogen Compounds M. Ito,* H. Sasaki, and M. Takahashi Department of Chemistry, Faculty of Science and Technology, Keio University, Kohoku- ku, Hiyoshi, 223, Japan (Received: April 3, 1987) The self-dimer structure of electrochemically reduced methylviologen was confirmed by observing the remarkably strong totally symmetric modes of vibrations (a,) in the infrared spectra. The charge-transfer process in polymer-coated viologen on a Pt electrode was also studied by measuring the intensity growth of the ag bands.
Introduction The viologens were originally investigated as redox indicators in biological studies and, more recently, as electron mediators in biological systems.] On the basis of many available data2,3there are good grounds for expecting dimeric association of the cation radical, although no direct evidence for this structure has been reported. It is important to note that intermolecular interactions of viologen molecules or polymer viologens are closely related to the electron-transfer process. There have been a number of reportsks on the in situ Raman characterization of methylviologen (MV) generated at electrode surfaces. However, no infrared spectroscopic work has been reported on reduced MV. Girlando et aL9q10 have reported electron-molecular vibration interactions in many organic charge-transfer crystals. They showed that the totally symmetric Raman-active (a,) modes can couple with electronic wave functions, the coupling giving rise to vibronic absorptions in the IR. In this paper we report for the first time the infrared spectra of the three reversible oxidation states of some viologens and present results which show the vibronic activation of the totally symmetric (a,) modes of the viologen monocation based on the FergusonPerson vibronic model.iiJ2 The vibronic effects in various polymer viologens are also found and discussed.
Experimental Section A Perkin-Elmer 9 8 3 0 infrared spectrometer with a reflection attachment is used. The electrochemical celli3 was inserted in the sample beam. The broad absorption in Figure 1 around 1640 cm-’ is due to the absorption of a thin aqueous solution layer between the window and the electrode. A gradual cutoff due to the CaF2 appears below 1100 cm-’ in each spectrum. The spectra were recorded under potentiostatic conditions. (1) Michaelis, L.; Hill, E. S.J . Am. Chem. SOC.1933, 55, 1481. (2) Datta, M.; Jansson, R. E.; Freeman, J. J. Appl. Spectrosc. 1986, 40, 251. (3) Forster, M.; Girling, R. B.; Hester, R. E. J . Raman Spectrosc. 1982, 12, 36. (4) Regis, A.; Corset, J. J. Chim. Phys. 1981, 78, 687. (5) Lee,P. C.; Schmidt, K.; Gordon, S.; Meisel, D. Chem. Phys. Lett. 1981, 80, 242. (6) Lu Tianhong; Birke, R. L.; Lombardi, J. R. Langmuir 1986, 2, 305. (7) Ohsawa, M.; Nishijima, K.; Suetaka, W. Surf. Sci. 1981, 104, 281. (8) Benchenane, A,; Bernard, L.;Theophanides, T. J . Raman Spectrosc. 1974, 2, 543. (9) Girlando, A.; Bozio, R.; Pecile, C. Phys. Reu. B 1982, 26, 2306. (IO) Bozio, R.; Pecile, C. Solid State Commun. 1981, 37, 193. ( 1 1) Ferguson, E. E. J. Chim. Phys. 1964, 61, 257. (12) Friedrich, H. B.; Person, W. B. J. Chem. Phys. 1966, 44, 2161.
0022-3654/87/2091-3932$01 .50/0
All the reagents were used as received (Wako Chemical Co.). The redox polymer viologens, (poly(p-xylylviolgen dibromides) @-PXVBr2), and a mixed polymer complex composed from p PXVBr2 and potassium poly(styrenesu1fonate) (PXV-(PSS)2) were used. The syntheses of polymer viologens were prepared as described by the 1 i t e r a t ~ r e . IThe ~ ~ ~PXV-(PSS)2 ~ film on Pt was prepared by a dipcoating method.14 The film of the polymer layer was allowed to completely dry, and the thickness was roughly estimated from the amount of polymer used (0.5-1.0 pm). A saturated calomel electrode (SCE) was used as a reference electrode. Argon gas was bubbled through the solutions in order to prevent reactions of the reduced films with oxygen. The cyclic voltammetry of MVC1, was examined in an aqueous phosphate buffered solution (pH 7.2).
Results and Discussion Methyluiologen. Cyclic voltammetry a t a Pt electrode in deaerated 0.1 M MV in 0.1 M phosphorous acid buffered aqueous solution scanned between 0 and -1.4 V showed the first (-1.0 V) and the second (-1.3 V) electron reduction peaks and the corresponding oxidation peaks (-0.3, -1 .O V). Therefore, we chose two reduction potentials, -1.0 and -1.4 V, at which the spectrum of the two reduced species are measured. Figure 1 shows the reflection absorption spectra (IRRAS) of electrochemically reduced methylviologen in 50 mM MV2+ aqueous solution. The spectrum at 0 V where no reduction occurs is also included in the figure. The bands at 1605, 1511, 1340, 1201, and 1184 cm-I appeared with large intensities. All of those bands begin to glow at -0.4 V and reach maximum intensities at -1.0 V and decrease in intensities at more negative potentials, while the frequencies of those bands remain unchanged throughout the potential regions. The intensity changes against the potentials are rapid and mostly reversible. Monomer cation radical species cannot exist stably in an aqueous solution phase, although an adsorbed monomeric species on the electrodes from dilute solution (less than 2 mM of MV2+) has been reported. Since a highly concentrated solution (0.1 M MV2+)is used in the present work, the precipitated species in the solution may be the “dimer species” as indicated by both the surface-enhanced Raman scattering and the resonance Raman However, there have been so far no detailed structural data for the dimer species. ESRI6 or UV-visible s p e c t r o ~ c o p y ~ ~ ~ ~ ” (13) Kitamura, F.; Takahashi, M.; Ito, M. Chem. Phys. Lett. 1986, 123, 273. (14) Factor, A.; Heinshon, G. E. J . Polym. Sci., Part B 1971, 9, 289. (1 5) Wiley, R. H.; Smith, N. R.; Ketterer, C. C. J. Am. Chem. SOC.1954, 76, 720. (16) Ivanov, V. F.; Grishnia, A. D.; Shapiro, B. I. Izu. Akad. Nauk SSSR, Ser. Khim. 1976. 1383.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 15, 1987 3933 \
MV-CIZ
1
1700
1500
1300
'J
PXV-( PSS);!
1197
I100
WAVENUMBER ( cm-' ) Figure 1. IR spectra for 0.1 M MVCI, in an aqueous solution a t (a) 0, (b) -1.0, and (c) -1.4 V.
1700
1
1500
'(C) 1
1
.
1300
l
1100
WAVENUMBER ( cm -I ) Figure 3. IR spectra for a PXV-(PSS)2 film in an aqueous solution a t (a) 0, (b) -0.8, and (c) -1.4 V.
has been the basis for most of the evidence for radical cation dimerization or association. Khaskin et al. suggest a quinhydrone structure for the dimer, and such a dimer does not give ESR signals.I8 Although there has been no available IR data for feduced viologens, some of those bands, 1605, 1511, 1340, 1201, and 1184 cm-', cannot be explained by IR-active modes of vibration. The positions of all the major bands in the spectrum at -1 .O V are in good agreement with those reported by other workers2 for the monovalent cation dimer (MV'+), formed in the aqueous solution at higher concentration. The bands apparently belong to the totally symmetric modes of vibrations. In comparing the present data with those of organic conductor films, we rely on the theoretical predictions of Rice et al.;19320the dimer model is predicted in the electron-vibration (e-v) coupling approximation, and ag modes will appear in the IR spectra as Qu modes, Le. the out-of-phase vibration of the two paired molecules. Thus, the pronounced bands, 1605, 1511, 1340, and 1201 cm-I, are assigned to the ag modes of the methylviologen molecule, which are allowed by e-v coupling. When the potential is swept to the more cathodic region than -1.0 V, the above-mentioned major bands start to decrease in intensity. The bands at -1.4 V probably correspond to the electrochemically generated MVo species. The species is slightly soluble in an aqueous solution. Between intermediate potential regions (-0.8 to -1.4 V) many bands that originated from both (MV*+), and MVo species appeared with various intensities, but the frequencies of those bands remained unchanged. This means two kinds of species exist on the electrode. The intensities for
1700
1500
1300
1100
WAVENUMBER ( cm-') Figure 2. IR spectra for 0.1 M PXVBr, in an aqueous solution at (a) 0, (b) -0.8, and (c) -1.4 V.
(17) Stargardt, J. F.; Hawkridge, F. M. Anal. Chim. Acta 1983, 146, 1. (18) Melnikov, N. N.; Novikov, E. G.; Khaskin, B. I. Chemistry and Biological Activify of Bipyridyls and Their Deriuatives; Gosimdat: Moscow, 1975; p 35. (19) Rice, M. J. Solid Stare Commun. 1979, 31, 93. (20) Rice, M. J.; Yartsev, V.M.; Jacobsen, C. S.Phys. Reu. B 1980, 21, 3437.
3934
J . Phys. Chem. 1987, 91, 3934-3936
TABLE I: Frequencies of the ag Bands for Reduced Vialogens (cm-')
(MV'+),O
PXVb
PXV-PSS'
(MV'+),d
1605 1511 1340 1184
1589 1503 1330 1169
1597 1505 1334 1172
1604 1512 1340 1188
a MVCI,. bPXVBr2. 'PXV-PSS. dResonanceRaman data for the (MV'+), dimer.2
those species do not change for the p or s-polarized infrared beam, indicating that no oriented film formed on the surface. When acetonitrile was used as a solvent instead of water, neither the precipitation of the reduced viologen nor the appearance of the characteristic bands assignable to the self-dimer (MV'+), species occurred at -1.0 V. Therefore, the dimer association of two viologen skeletons would not occur in the acetonitrile solution. PXVand PXV-PSS. Polymer viologens with a charge similar to PXV are soluble in water. The cyclic voltammogram of the PXVBr, in an aqueous solution shows two reduction peaks at -0.5 and -1.2 V as is similar with MV2+. Figure 2 shows the IRRAS spectra of PXV reduced at -0.8 or -1.4 V from an aqueous solution as well as the reference spectrum at 0 V. As in the case of (MV'+), the reduced species of PXV precipitated on the electrode. The spectral features are very similar to those of (MV"), although small frequency shifts are observed. Therefore, even for PXV, dimer association through bipyridyl skeletones occurs at these potentials. The bands at 1169 (PXV) and 1172 cm-I (PXV(PSS),) correspond to the band at 1201 cm-I of (MV'+), and are found to be substituent sensitive. Thus, Freeman et al. assigned the band to the N-CH3 stretching vibration. The polymer complex (PXV-(PSS)2)21 which was coated on (21)
Akahoshi, H.; Toshima, S.; Itaya, K. J . Phys. Chem. 1981, 85, 818.
the Pt electrode is stable and totally insoluble in water. The structure of the polymer complex is highly cross-linked by the formation of a complex between the polymer cation (PXV2+) and the polymer anion (PSS-). This cross-linking strongly reduces the solubilities of both the oxidized and the reduced polymer films. The similar intensity increase of the ag bands which are found in the reduced PXV or (MV'+), species is also seen in the IRRAS spectra of the PXV-(PSS), reduced film (Figure 3). The frequencies of the bands are listed in Table I together with those of reduced MV and PXV. The ag bands of reduced (MV"), studied by the resonance Raman work are included in the last column. Reversible intensity changes are also observed by the potential sweep in the range of 0 to -1 .O V. The intensity change of the ag bands of the reduced PXV-(PSS), associated with the potential change (0 -0.8 V) is very slow; it takes 30 min to reach intensity saturation, which is in marked contrast to the appearance of the instantaneous intensity changes of the ag bands for reduced (MV"),. The totally symmetric modes of the reduced MV (the outof-phase modes), inducing electron oscillations in the dimer from one molecule to the other, are coupled to the C T excitation. The C T interactions through bipyridyl skeletons are important to understand the electron-transfer mechanisms of the viologen redox center in the film. Electron transfer in the PXV-(PSS)z film during electroreduction propagates along the surface normal from the surface layer to the topmost layer successively in such a way that the two bipyridyl skeletons overlap to form the self-dimer as is seen in one-dimensional organic C T complexes. The large PXV-(PSS)2 polymer molecule coated on the electrode swells and can move for diffusion in an aqueous solution. It takes a much longer time to determine the dimer configuration, compared with the instantaneous reversible changes of (MV"),. Thus, we conclude that the rate-determining step for the slow intensity gain of the ag bands during the electroreduction to be the slow diffusion of the large polymer chains.
-
Effect of Added Water on Voltammetry in Near-Critical Carbon Dioxide Mark E. Pbilips, Mark R. Deakin, Milos V. Novotny, and R. Mark Wightman* Department of Chemistry, Indiana University, Bloomington, Indiana 47405 (Received: April 14, 1987)
The use of microvoltammetric electrodes in near-critical carbon dioxide has been investigated. The working electrode is a platinum disk of 5 pm radius and the test compound employed is ferrocene. Voltammetry is not possible without the addition of water to the electrochemical cell. At temperatures and pressures above the critical point for pure C02,a well-defined voltammetric wave for ferrocene is obtained in the presence of 0.64 M water. Added water also enables the dissolution of tetrahexylammonium hexafluorophosphatein this medium. The diffusion coefficient obtained from voltammogramsof ferrocene recorded in these fluids is similar to that reported for other compounds in supercritical carbon dioxide., However, in the presence of a high concentration (0.05 M) of the added salt, the value of the diffusion coefficient is lowered. These results demonstrate that the addition of water to near-critical carbon dioxide results in a fluid which has much greater solvating power than the pure supercritical fluid.
Supercritical fluids are becoming of increasing importance in a number of areas of chemistry.' Unusual solvating capabilities are among the main reasons for this interest. In particular, supercritical carbon dioxide has seen increased use. It has been shown to be a useful mobile phase for capillary chromatography.2 The change in solvation properties which occur due to increasing density can be used in a similar way as temperature changes are used in gas chromatography. The addition of various organic solvents to supercritical CO,has been shown to modify the solvating power of this fluid.3 Supercritical conditions for COz are *To whom correspondence should be addressed.
0022-3654/87/2091-3934$01.50/0
readily obtained at moderate temperatures ( T , = 3 1.3 "C) and pressures (P,= 72.9 atm).4 In this Letter we describe the use of voltammetry in near-critical COz, in the presence of certain modifiers, to assess the solvating power of the resulting fluid. Voltammetry has been used for similar purposes in supercritical HzO and NH3.5-7 However, the (1) Lamb, D. M.; Barbara, T. M.; Jonas, J. J . Phys. Chem. 1986, 90, 42 10-42 15. (2) Novotny, M.; Springston, S . R.; Peaden, P. A.; Fjeldsted, J. C . ; Lee, M. L. Anal. Chem. 1981, 53, 407A-414A. (3) Levy, J. M.; Ritchey, W. M. J. Chromatogr. Sci. 1986, 24, 242-248. (4) Gouw, T.; Jentoft, R. J . Chromatogr. 1972, 68, 303-323.
0 1987 American Chemical Society
