J . A m . Chem. SOC.1984, 106, 1991-1998 In light of the spectra discussed below for the shorter chain compounds it is likely that exchange interactions also play a significant role in the line broadening observed for the adducts of I11 and IV. The spectra of the six-coordinate adducts of I and I1 bear no resemblance to normal frozen-solution nitroxyl EPR spectra. The nitroxyl lines are severely broadened and spread over several hundred Gauss. For I-bipy the low-field and high-field turning points are a t 2600 and 3400 G, respectively (Figure 4A). The spectrum of I.(py), is slightly more spread out (turning points a t 2400 and 3600 G, respectively). Even for I1 ( n = 1) the low-field and high-field turning points in the spectrum of II.(py), are a t 2900 and 3600 G, respectively. The spectra of the six-coordinate adducts of I and I1 do not saturate a t the highest microwave powers available on our spectrometer. These observations indicate significant mixing of the nickel and nitroxyl wave functions.33 C P K molecular models indicate that the interspin distances are about 7 and 8 A respectively for the nitrogenous base adducts of I and 11. The observed splittings in the spectra are too large to be explained by purely dipolar interaction a t these distances. Thus it appears that a substantial exchange interaction must be invoked to explain the large shifts in the spectra. The observation of large splittings in the frozen-solution spectra is noteworthy in another respect. W e expect that when a nitroxyl interacts with a rapidly relaxing metal the appearance of the nitroxyl portion of the spin-coupled spectrum will be dependent on the rate of the metal relaxation compared with the splittings in the spectrum. When the metal relaxation rate is comparable to the splittings in the nitroxyl lines, a broadened spectrum is expected. When the metal relaxation rate is fast relative to the potential splittings of the nitroxyl lines, the splittings will be averaged out. In between these two limits, the widths of the nitroxyl lines should be a function of the metal relaxation rate (33) Throughout the text the observed EPR lines are referred to as nitroxyl lines which is the assignment of the transitions in the limit as J approaches 0.
1991
and of the spin-spin interaction. These arguments are analogous to those used in the analysis of temperature-dependent EPR and N M R spectra of dynamic systems. Since information is not available concerning the nickel relaxation rates in these compounds and since the transitions have not been assigned, it is not possible to determine the extent to which the line shapes and line positions reflect the partial collapse of larger splitting due to a nickel relaxation rate which is not sufficiently rapid to completely average out the spin-spin interaction. Further studies are needed to estimate the metal relaxation times, zero field splittings, and exchange interactions in these molecules. Conclusions The nickel-nitroxyl interaction in nitrogenous base adducts of I-V decreases rapidly as n increases. However, even in V the spin-spin interaction is detectable. In fluid solution the flexible hydrocarbon chains in the compounds with n = 2, 4, or 6 permit intramolecular nickel-nitroxyl interactions which are strongly dependent on the solution viscosity. The large splittings in the frozen solution spectra of the six-coordinate adducts of I and I1 indicate that exchange makes a significant contribution to the spin-spin interaction. Acknowledgment. This work was supported in part by NIH grant GM21156 and the donors of the Petroleum Research Foundation, administered by the American Chemical Society. We are grateful to Prof. Kosman (SUNY Buffalo) for providing us with unpublished details of the syntheses reported in ref 20 and to Prof. Hornback and Dr. More for helpful discussions concerning the syntheses. Registry No. I, 85361-86-0; 11, 85361-88-2; 111, 88945-48-6; IV, 88945-49-7; V, 88945-50-0; VI, 88932-76-7; VII, 88932-77-8; VIII, 31 146-15-3; IX, 88932-78-9; X, 88932-79-0; XI, 88932-80-3; XII, 88932-8 1-4; XIII, 88932-82-5; XIV, 88932-83-6; XV, 88932-84-7; XVI, 88932-85-8; XVII, 88932-86-9; XVIII, 88932-87-0; tempone, 2896-70-0; triethyl phosphonoacetate, 867-13-0; triethyl 4-phosphonocrotonate, 10236-14-3.
Redox Conduction in Single and Bilayer Films of Redox Polymer P. G . Pickup,$ W. Kutner,+C. R. Leidner, and Royce W. Murray* Contribution from Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 2751 4 . Received August 8, 1983
Abstract: Polymeric films made from Ru and Os polypyridine complexes and sandwiched between potential-controlled Pt and porous Au electrodes exhibit low resistance to current flow when the electrode potentials are such as to generate mixed-valent states in the polymers, and high resistances'when the films are composed of only one oxidation state. The observed current-potential curves with their limiting currents can be understood in terms of a concentration gradient (not potential gradient) driven flow of electrons with effective mobility characterized by an electron diffusion coefficient Del. Sandwich-electrode measurements of D,, for the M(III/II) mixed-valent state agree with those produced by other methods, and estimates from DCtof effective electron self-exchange rates in the polymers come close to those known for homogeneous solution self-exchange rates between [ M ( b ~ y ) ~ ]complexes. "+ For paly[O~(bpy),(vpy),]~+ films, the electron mobility varies in the proportion 1:3:25 for the three mixed-valent states Os(III/II), Os(II/I), and Os(I/O), respectively. The most highly conducting state can pass currents of 0.6 A/cm2.
Our' and other^'^,^ interest in electron transport in thin films of electroactive polymers stems from fundamental questions about electron mobility in these materials and from its connection to potential applications of the films to e l e ~ t r o c a t a l y s i s , photoe~'~~
scheme 1 6+t
II
polyOs(III/II)
I
Os(I1)-Os(III)
Permanent address: Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44.01-224 Warsaw, Poland. *Department of Chemistry, State University of New York at Buffalo.
0002-7863/84/1506-1991$01.50/0
II AuI
Os(III)-Os(II
1
lectrochemi~try,~ and chemical devices.Ih When studied by transient methods,'a.2bthe electron transport follows diffusion laws 0 1984 American Chemical Society
1992 J . Am. Chem. Soc., Vol. 106, No. 7, 1984
B
Pickup et al. contact to Au film
Au film
(I
\
c m s section
\ A uPtwiY film
Figure 1. Schematic sandwich electrode.
and can be characterized by a diffusion coefficient D,,.Electron transport is thought2e in most of these materials to occur by electron self-exchange between neighbor oxidized and reduced redox sites in the polymer, which is peculiar to the mixed-valent state of the polymer. Physical diffusion of the redox sites also effects electron transport, combining its rate with that of electron ~ e l f - e x c h a n g e , ’ ” ~especially ~ ~ j ~ ~ when the redox site is an ion electrostatically trapped3 in an ion-exchange polymer. When the redox site is affixed to the polymer framework, its physical diffusion is more restricted, but microscopically can still be important.” This report deals with polymers with affixed redox sites (redox polymer); we refer to the electron self-exchange-driven currents in these polymers as “redox conduction currents”. We recently describedkhhow to measure steady-state redox conduction Currents in ultrathin redox polymer films wetted with a solvent containing a supporting electrolyte. The polymer film is sandwiched between two metal electrodes, one of which is porous and contacted by the solvent. This paper describes “sandwich(1) (a) Daum, P.; Lenhard, J. R.; Rolison, D. R.; Murray, R. W. J . Am. Chem. SOC.1980, 102,4649. (b) Nowak, R. J.; Schultz, F. A,; Umana, M.; Lam, R.; Murray, R. W. Anal. Chem. 1980, 52, 315. (c) Daum, P.; Murray, R. W. J . Phys. Chem. 1981, 85, 389. (d) Denisevich, P.;Abruna, H. D.; Leidner, C. R.; Meyer, T. J.; Murray, R. W. Inorg. Chem. 1982, 21, 2153. (e) Ikeda, T.; Leidner, C. R.; Murray, R. W. J . Electroanal. Chem. 1982, 138, 343. (f) Burgmayer, P.; Murray, R. W. Ibid.1982, 135, 335. (g) Schmehl, R. H.; Murray, R. W. Ibid. 1983, 152, 97. (h) Pickup, P.; Murray, R. W. J . Am. Chem. SOC.1984, 105, 4510. (i) Denisevich, P.; Willman, K. W.; Murray, R. W. Ibid. 1981, 103, 4727. (j) Facci, J. S.; Schmehl, R. H.; Murray, R. W. Ibid. 1982, 104, 4959. (k) Nakahama, S.; Murray, R. W. J . Electroanal. Chem. 1983, 158, 303. (I) Kuo, K. N.; Murray, R. W. Ibid., 1982,131, 37. (m) Facci, J.; Murray, R. W. Ibid.,1982, 124,339. (n) Fa&, J.; Murray, R. W. J . Phys. Chem. 1981, 85, 2870. (2) (a) Peerce, P. J.; Bard, A. J. J . Electroanal. Chem. 1980, 114, 89. (b) Oyama, N.; Anson, F. C. J . Electrochem. SOC.1980, 127, 640. (c) Chambers, J. Q.J . Electroanal. Chem. 1981,130, 381. (d) Schroeder, A. H.; Kaufman, F. B.; Patel, V.; Engler, E. M. Ibid.,1980, 113, 193. ( e ) Kaufman, F. B.; Schroeder, A. H.; Engler, E. M.; Kramer, S. R.; Chambers, J. Q.J . Am. Chem. SOC.1980, 102,483. (f) Kaufman, F. B.; Engler, E. M. Ibid.,1979, 101, 547. (g) Thompson, W. A.; Schroeder, A. H.; Kaufman, F. B. J . Vac. Sci. Technol. 1981, 18, 243. (h) Shigehara, K.; Oyama, N.; Anson, F. C. J . Am. Chem. SOC.1981, 103,2552. (i) Anson, F. C. J . Phys. Chem. 1980,84, 3336. (j) Andrieux, C. P.; Saveant, J.-M. J . Electroanal. Chem. 1980, I l l , 377. (k) Laviron, E. Ibid. 1980, 112, 1 . (1) Laviron, E.; Roullier, L.; DeGrand, C. Ibid. 1980, 112, 1 1 . (m) Fukui, M.; DeGrand, C.; Miller, L. L. J . Am. Chem. Soc. 1982, 104, 28. (n) DeGrand, C.; Miller, L. L. J . Electroanal. Chem. 1982, 132, 163. (a) Albergy, W. J.; Boutelle, M. G.;Colby, P. J.; Hillman, A. R. Ibid. 1982, 133, 135. (p) Bookbinder, D. C.; Wrighton, M. S. J . Electrochem. SOC.,in press. (3) Anson, F. C.; Buttry, D. A. J . Am. Chem. SOC.1983, 105 658. (b) Oyama, N.; Anson, F. C. J . Electrochem. SOC.1980, 127, 247. (c) Buttry, D. A.; Anson, F. C. J . Am. Chem. SOC.1982, 104, 4824. (d) Oyama, N.; Yamaguchi, S.; Nishiki, Y.; Tokuda, K.; Matsuda, H.; Anson, F. C. J . Electroanal. Chem. 1982, 139, 371. ( e ) Anson, F. C . ;Saveant, J.-M.; Shigehara, K. J . Am. Chem. SOC.1984, 105, 1096. (f) Ohsaka, T.; Anson, F. C. J . Phys. Chem. 1983, 87, 640-647. (g) Rubinstein, I.; Bard, A. J. Am. Chem. SOC.1980, 102, 6641. (h) Rubinstein, I.; Bard, A. J. Anal. Chem. 1981, 53, 102. (i) Henning, T. P.; White, H. S.; Bard, A. J. J . Am. Chem. Soc. 1981, 103, 3937. Q) White, H. S.; Leddy, J. Bard, A. J. Ibid.1982, 104, 4811. (k) Martin, C. R.; Rubinstein, I.; Bard, A. J. Ibid. 1982, 104,4817. (1) Faulkner, L. R.; Majda, M. J . Electroanal. Chem. 1982, 137, 149. (m) Faulkner, L. R.; Shaw, B. R.; Haight, G. P., Jr. Ibid.1982, 140, 147. (4) (a) Andrieux, C. P.; Dumas-Bouchiat, J. M.; Saveant, J.-M. J . Electroanal. Chem. 1982, I31 1 . (b) Murray, R. W. Phil. Trans. R. SOC.London, Ser. A 1981, 302, 253. (5) Wrighton, M . S., ACS Symp. Ser. 1982, No. 192, 99.
b
ontact t o A u f i l m
Figure 2. Schematic representation of multi-mini-sandwich electrode assembly embedded in Teflon (a) and glass (b).
electrode” measurements of D,,for the M(III/II) mixed-valent states of three related osmium and ruthenium complex polymer films, of two highly reduced mixed-valent oxidation states of one of the polymers, and of an overlaid film of two different polymers (a bilayer6). No previous electron mobility data exist for highly reduced, electrolyte, and solvent-swollen redox polymers, in part because of the poor stability of such states.’ Nor has the electron mobility in a series of mixed-valent states of the same redox polymer been compared, although results have been recently reported3a for redox ions electrostatically trapped in Nafion films. D,,measured as a function of mixed-valent state has use in assessing the relative importance of polymer lattice and counterion mobilities and intrinsic electron self-exchange rate in electron mobility. The sandwich electrode (Figure 1) consists of a ca. 1001000-nm polymer film (or bilayer of polymer films) reductively electropolymerized’d~8 onto a Pt disk electrode, with a porous gold film electrode (ca. 50-100 nm) vapor deposited over the polymer. The gold electrode porosity allows entry of solvent and supporting electrolyte ions into the polymer film. Charge-compensating counterions can then migrate in/out of the polyionic film as its oxidation state is changed, so that the polymer remains fully electroactive. Three of the sandwich electrodes described use films Pt/ of a single polymer, Le., Pt/p~ly[Os(bpy)~(vpy)~]~+/Au, poly [ R u ( b p ~ ) ~ ( v p y ) ~ l ~ +and / A uPt/poly , [Ru(vbpyM 2 + / A ~and , one has overlaid films of two different polymers (a bilayer6), Le., Pt/POlY [Ru(bPY)2(vPY)21Z+/PolY [Os(bpy)2(vpy)212+/Au,where bpy = 2,2’-bipyridine, vbpy = 4-methyl-4’-vinyl-2,2’-bipyridine, and vpy = 4-vinylpyridine. We cause one electrode to act as an electron sink and the other as an electron source, by applying potentials to them so as to oxidize and reduce, respectively, the polymer redox sites contacting the electrodes. Steady-state currents flow at both electrodes, since steady-state concentration gradients of oxidized and reduced sites are generated across the film. At applied electrode potentials which do not generate redox (6) Denisevich, P.; William, K. W.; Murray, R. W. J . Am. Chem. SOC. 1981, 103, 4727. (7) Kerr, J. B.; Miller, L. L.; Van De Mark, M. R. J . Am. Chem. SOC. 1980, 102, 3383. (8) (a) Abruna, H. D.; Denisevich, P.; Umana, M.; Meyer, T. J.; Murray, R. W. J . Am. Chem. SOC.1981, 103, 1 . (b) Calvert, J. M.; Schmehl, R. H.; Sullivan, B. P.; Facci, J. S.; Meyer, T. J.; Murray, R. W. Inorg. Chem. 1983, 22, 2151.
J . Am. Chem. Soc.. Vol. 106, No. 7 , 1984 1993
Redox Conduction i n Films of Redox Polymer site concentration gradients, t h e films are insulators in t h e sense t h a t there are no redox conduction currents. I t c a n be seen that electron mobility a n d t h e flow of redox conduction currents in these sandwich cells is determined by quite different considerations f r o m those in conductivity measurements on d r y films,9 w h e r e electron flow is driven by voltage, not concentration gradients.
1'
Experimental Section Potential Control. Of the several experimental ways'' to apply potentials to the two electrodes of the sandwich, the most versatile uses a four-electrode potentiostat (bipotentiostat) which independently controls their potentials relative to a reference electrode placed in the electrolyte/solvent part of the cell." The difference between the two potentials can be small or large; one can be fixed and the other scanned with time (both relative to the reference electrode), or both potentials scanned with a fixed difference between them. Most experiments here scan the Pt electrode potential (Ept)while the potential of the gold electrode (EAu) is kept constant. The current at each of the two working electrodes and therefore the current flowing through the polymer is measured as a function of the potentials of the two electrodes. Electrodes. A second-generation sandwich electrode was employed, based on a multi-mini-electrode assembly composed of the polished ends of six fine Pt wires (diameter 0.064 cm, A = 0.0032 cm2) embedded in a Teflon shroud (Figure 2a). This allowed five sandwich electrodes to be constructed simultaneously (the sixth electrode was not coated with polymer and served as a contact to the gold film), an important advantage considering that only ca. 50% of the sandwiches (compared to 10% in earlier experimentsIh) yielded any results (see below). An improved, glass-shrouded version of the electrode (Figure 2b) was used for the Os(II/I), Os(I/O), and variable-temperature parts of the study. The electrode was masked during Au deposition so as to form the indicated six polymer-coated electrode-connecting electrode pairs of Pt wire tips. [The glass shroud made it easier to maintain the Pt microdisk and surrounding shroud surface in the same plane during polishing. The polishing was done on a diamond (1 pm, Buehler) paste-coated paper card resting on a hard surface.] Syntheses of the electroactive monomers [ O s ( b ~ y ) ~ ( v p y ) ~[Ru]~+, (bpy)2(vpy)212+,and [Ru(vbpy),12+, and their electropolymerization to form films on electrodes are described elsewhere.'dB8 To prepare, for example, a Pt/p~ly[Os(bpy)~(vpy)~]~+/Au sandwich, the Pt disk electrode was coated with a film of the osmium polymer by repeated potential sweeps between -1.0 and -1.7 V vs. SSCE in a ca. 0.5 mM, thoroughly degassed, acetonitrile solution of the monomer complex. After the electrode was washed with acetone and air-dried, gold vapor deposition was performed using a Veeco V-300 depositor, resistively heating the gold to yield deposition rates on the polymer of 0.2 to 5 A/s at pressures of 2 to 5 X torr and gold films of average thickness 50-100 nm. The deposition rate was increased as deposition proceeded. The surface coverages of redox sites (l?T,Osor rT,Ru) were determined both before and after (with no significant changes in rTfor the electrodes used) gold deposition, by measuring the electrochemical charge under slow potential scan M(III/II) cyclic voltammograms. The values of r T are converted into film thickness using dry-weight polymer density, which corresponds to a redox site concentration in the p~ly[Os(bpy),(vpy)~]~+ polymer of of 1.6 X 1.5 X IO-, mol/cm3 and in the poly[R~(bpy)~(vpy)2]~+ mol/cm3. Electrochemical experiments were in 0.1 M Et4NCIO4/acetonitrile with conventional cells and equipment. A homemade bipotentiostat similar to that described in the Bard and Faulkner text" and a Model RDE3 bipotentiostat (Pine Instrument Co., Grove City, PA) were used. Electrochemical potentials are referenced to the NaCI-saturated S C E (SSCE). Electrode Conditioning. As before,Ih many sandwich electrodes initially exhibited an electrical short between the Pt electrode and the Au (9) (a) Cowan, D. 0.;Park, J.; Pittman, C. U.; Sasaki, Y.; Mukherjee, T.; Diamond, N. A. J . Am. Chem. SOC.1972, 94, 5110. (b) Pittman, C. U.; Sasaki, Y.; Mukherjee, T. Chem. Leu. 1975, 383. (c) Pladke, S. D.; Sathianandan, K.; Karekar, R. N. Thin Solid Films 1978, 51, L9. (10) In a three-electrode potentiostat, with working and auxiliary connections made to the two sandwich electrodes, the potential of only the working electrode is controlled. With a two-electrode potentiostat, the sandwichelectrode potentials are controlled only with respect to one another. Such experiments are described elsewhere.lh ( 1 1) An auxiliary electrode is also in the electrolyte solution, and a fourelectrode potentiostat is used in the manner of twin electrode thin layer electrochemistry.I2 (12) Anderson, L. 8.; Reilley, C. N. J . Electroanal. Chem. 1965, IO, 295. ( 1 3) Bard, A. J.; Faulkner, L. R. 'Electrochemical Methods"; Wiley: New York, 1980.
Eptvs SSCE
J'Pt
EA u
T
Q3)
(C)
ID+
'Pt
__
EAu
v
1.0
4 ,' I
I
I 'c
I
0.5
'0
E~~vs.SSCE
laU
(d)
1.0
0.5 EAUV5.SSCE
Figure 3. Four-electrode (bipotentiostat) cyclic voltammetry at 20 mV/s of 4.7 X lo-* mol/cm2 ( d = 280 nm) Pt/p~ly[Os(bpy)~(vpy)~]~+/Au sandwich electrode. Curves a-c: E , scanned at 20 mV/s and EA, = 0, +0.74, +0.9 V vs. SSCE, respectively. Curve D: E , = 0 V and E A u scanned at 20 mV/s. Electrode area = 0.0032 cm2, 0.1 M Et,NC104/ CH,CN. film, which could often be broken via oxidizing the shorting gold particles or filaments with a brief potentiostatting of the Pt electrode at +1.5 V, Le., "conditioning". Even so, many electrodes (individual surfaces on a multielectrode assembly) could not be used (failures). This depended strongly on the total surface coverage rTof polymer(s). In the case of the osmium polymer, for rT < 5 x mol/cm2, 90% were failures. For 5 X lo-* < rT< 1 X mol/cm2, only 30% were failures. For rT> 1 X lo-' mol/crri2, shorting was still a problem, but often when there was not a short there was poor electrical contact between the Au film and the polymer. Seventy-five percent of the glass-shrouded electrodes gave < rT< 1.5 X mol/cm2. redox conduction currents when 5 X or poly[RU(~bpy)~]~+ were used, When films of poly[R~(bpy)~(vpy),]~+ conditioning was not possible because the gold film is oxidized by the Ru(II1) states of these polymers. Fortunately, ca. 50% of these electrodes were not initially shorted. Results and Discussion Redox Conduction Currents for the M(III/II) Couple in Poly[WbPY)2(VPY)2I2+, POlY[RU(bPY),(vpY)2l+>and PolY[Ru(vbPY),12+ Films. To obtain a typical voltammogram for a Pt/poly[Os( b ~ y ) ~ ( v p y ) ~ ] ~ +sandwich /Au (Figure 3a), EA" was held a t 0 V, while Eptwas slowly scanned toward positive potentials past Eo' (+0.73 V) for t h e Os(II-+III) reaction of t h e polymer a n d back again. The currents a t each electrode a r e shown a s a function for half-maximum current of E , (vs. SSCE). N o t e that t h e is +0.74 V (very close t o Eo' for the polymer), a n d that a limiting current iL = 75 pA (23 mA/cm2) is observed a t sufficiently positive Ept. Except for a minor h y ~ t e r e s i s , 't~h e currents a t scan rates 120 m V / s a r e steady state a n d depend little on the scan direction of t h e Pt electrode. We symbolize this experiment with t h e notation shown in S c h e m e I. A t sufficiently positive potentials t h e P t electrode (14) The currents on the negative- and positive-going potential scans are not exactly the same.
1994 J . Am. Chem. SOC.,Vol. 106, No. 7, 1984
Pickup et al.
Table I. Electron Diffusion Coefficients Expressed as D c t l i 2 C for ~ Os(lII/II) and Ru(III/II) Mixed-Valent Polymers, by Different Methods D c t " ' C ~ X l o * , mol/cm' s"' polymer
sandwich electrode'
poly [Os(bpy), (VPY),1 '+
8.5, 7.3, 10.7, 13.2 9.0, 11.0, 16.0, 11.4
poly[Os(bpy),~-cinn),]
'+
chronoamperometry
10.6e
av 10.9 i 2 . 0 ~ 5.1, 3.4, 3.2, 3.4, 7.2f 3.8. 3.2
4.3g 2.1, 2.4h av 2.2
poly[Ru(bpy),(vpy),
I,+
mediationC
i
3.3,4.8,4.8, 3.2' av 4.0 i 0.8 6.9, 5.0,4.4,4.@'
0.2
av 5.1 4.7, 5.9,3.5, 3.1
i
,
0.6
3.2'
av 4.3 i 1.0 a Using sandwich electrodes as in I'igure 2a.
Potential step chronoamperometry. M(II1) states in the film mediate oxidation of a solution complex or of a second polymer film in a bilayer electrode at a rate limited by D,t. Measurements on electrodes with I'T= 1.8, 2.6, 4.7, 6.4, 6.5, 7.3, 7.4, 8.0 X IO-' mol/cm2. in order given. e Reference 21. Referencc Ih. Refcrencc I j . Refercncc i d . Rcfcrcncc I C . Leidncr, C. R.; Denisevich, P.;Willman, K . ; Murray. R. W. J . Electroanul. Chern., in presc. K u o , I 3 : >25 in the poly[Osan apparent electron self-exchange rate kzp, for comparison to the barrier-controlled self-exchange rate k::'"' known19 for [Os( b ~ y ) ~ ( v p y ) ~polymer ]"+ film. ( b ~ y ) ~ ] ~in+ homogeneous /~+ solution. Converting D,, to E,Pp is Homogeneous kti'"' values do not, except as estimations,22exist straightforward if one assumes that the collision rate via polymer for comparison to the apparent Os(II/I) and Os(I/O) self-exchange lattice mobility (process ii) is controlling, using the Smoluchowski rates. Heterogeneous electron-transfer rate constants for [OsequationZofor a collision rate-controlled reaction ( b ~ y ) ~ ] "a't Hg electrodes in D M F solvent have been reported,23 however, and their relative values surprisingly fall in a different k,",PP= 4 X 10-3aN(4rD,,) (7) order from that observed in the polymer, namely, 1 : 0.2 : 0.3 for the Os(III/II), Os(II/I), and Os(I/O) electrode reactions. The where for the Os polymer the collision radius, r, is taken as 0.7 reported rationalez3 for the Os(II/I) rate being slower than that nm, and Do, and Dredare taken as equal since the electron and for Os(III/II) was that the Os(III/II) reaction involves loss or hole motions are correlated. Note that assuming rate control by gain of an electron in a primarily metal-centered orbital, whereas process ii is equivalent to stating that kgl >> k:,Pp. the Os(II/I) reaction involves a change in electronic configuration Converting D,, to kt,PP is less obvious for control by process i by placing the added electron in a ligand centered orbital. If the (Le., k,P' k:,PP), but the following line of reasoning suggests that same argument were applied to the polymer kg' data, one would eq 7 allows a conceptually valid approximation of k:XpP even in this conclude that such a change in electronic configuration does not case. Consider that, in a barrier-controlled homogeneous solution reaction, the reactant diffusion rate is relatively fast, and so many occur in making poly[Os(bPY)2(vPY)21I + and Poly [~(bPY.)2(VPY)zl0 states, Le., that in producing these states the electrons enter orbitals collisions occur which are not fruitful in producing electron that are more metal centered than would be the case in solutions transfer. In this case, k:,PP calculated from eq 7 (using the true of the monomers of these complexes. That this indeed may be reactant diffusion coefficient) would clearly be too large. Consider, the case is given precedent in a recent report by DeArmond et on the other hand, knowing in the solution (somehow) the rate al.24that precipitation of doubly reduced Ru and Fe polypyridine of reactant diffusion which would lead only to electron-transcomplexes from acetonitrile solution is accompanied in the prefer-producing collisions; this (smaller) diffusion coefficient used cipitate by electronic isomerization from a ligand localized conin eq 7 would yield a k:$P more closely approximating the barfiguration to a more metal- than ligand-centered one. This rarier-controlled k::'"'. Such a measure of solely fruitful collisions tionale for the observed polymer kg' ordering is at this point clearly is not easily made (as a diffusion coefficient) in homogeneous speculative, and should bear further scrutiny. solution, but in the electron transport problem in fixed site redox Redox Conduction Currents across a Bilayer, Pt/poly[Rupolymers, this is, in fact, the diffusion coefficient that is measured. That is, Da measures inherently only the electron-transfer fruitful (~PY)~(~PY)zI~+/P~~Y[O~(~PY)~(~PY )21z+/Au. When a redox polymer film is separated from an electrode by a different redox collision rate; excess, non-electron-transferringcollisions do not polymer, then oxidation or reduction of the outer polymer by the produce a current flow. electrode must be mediated by redox conduction through the inner Using D,,= 5 X cm2/s in eq 7, we then calculate k:,PP = polymer.6 For example, a p ~ l y [ O s ( b p y ) ~ ( v p y2+) ~film ] (Eo'= 1 x 107 M-Ls-1 for Os(III/II). This value is close to the ho0.73 V) separated from the electrode by a poly[Ru(bpy),(vpy),12+ mogeneous rate," 2.2 X lo7 M-' s-I, Le., k:$P k::"'. From the film (Eo'= 1.22 V) is not oxidized a t an electrode potential of 0.73 V, because a t this potential the ruthenium polymer is in its (18) (a) Ikeda, T.; Schmehl, R.; Denisevich, P.; Willman, K.; Murray, R.
-
-
-
-
-
~~~
~~
W. J . Am. Chem. SOC.1982, 104, 2683. (b) Facci, J. S. University of North Carolina, unpublished results, 1982. (c) P and D,,,, represent the partition and diffusion coefficients of C1- in the polymer, respectively. (19) Chan, M. S.; Wahl, A. C. J . Phys. Chem. 178, 82, 2543. (20) Kahlweit, M. In 'Physical Chemistry An Advanced Treatise", Eyring, H., Ed., Academic Press: New York, 1975; Vol. VII, pp 507, 677.
(21) Leidner, C. R., Murray, R. W. J . Am. Chem. Soc. 1984, 106, 1606. (22) Sutin, N.; Creutz, C. Pure Appl. Chem. 1980, 2717. (23) Saji, T.; Aoyagui, S . J . Electroanal. Chem. 1975, 63, 31. (24) Morris, D. E.; Hanck, K. W.; DeArmond, M. K. J . Am. Chem. SOC. 1983, 105, 3032.
1998
J. A m . Chem. SOC.1984, 106, 1998-2001
I
I
OV /lpt
f t
Eptvs SSCE
EAy s SSC E Figure 8. Four-electrode cyclic voltammetry of a bilayer sandwich electrode Pt/poly[Ru(bpy)z(vpy),12~/poly[Os(bpy)z(vpy)z]2C/A~with rRu= 1.0 X and rOs= 9.4 X mol/cm2. Curve A: EAu= 0 V vs. SSCE and E , scanned at 20 mV/s, S = 5 and 250 PA for positive and negative E,, respectively. Curve B: E , = 0 V vs. SSCE and EAu scanned at 20 mV/s, S = 5.0 uA for ia.. and 0.25 uA for io,, .. 0.1 M Et4NCIO,/CH,CN.
film/film junction, with results shown in Figure 8. For E,, = 0 V vs. SSCE, the redox conduction waves have a similar appearance to those obtained for a sandwich using a film of a single polymer. The redox conductions due to the M(II/I) and M(II/O) mixed-valent states (Schemes I1 and 111) occur with similar magnitude (iL = 700 FA) as observed for osmium, and a t the same potentials as for nonsandwich films of osmium or ruthenium polymer alone, since Eo’for the M(II/I) and M(I/O) reactions are approximately the same for both polymers. The redox conduction due to the M(III/II) reaction occurs with similar i , (23 FA) to that with just one polymer, and it is centered on Eo’ = 1.22 V vs. SSCE for the ruthenium polymer. Redox conduction is not observed a t Ept near the osmium polymer EO’. The latter is the expected result6 since the bilayer cannot conduct electrons until both polymer films are mixed valent, which requires that Eptbe close to Eo‘ for the ruthenium polymer. A quantitative treatment of i L for the bilayer is given elsewhere.25 The corollary result is obtained if Eptis kept constant a t 0 V vs. S S C E and E,, is varied. As shown in Figure 8B, a nonsteady-state wave for oxidation and reduction of the osmium part of the film is observed a t the Au electrode, centered a t Eo’ for the osmium polymer. No significant current is seen a t the Pt electrode, which has a potential sufficient to re-reduce the Os(II1) generated by the Au electrode. This is, of course, because the inner ruthenium polymer film remains as Ru(I1) and is an insulator in this non-mixed-valent state. Acknowledgment. This research was supported in part by the Office of Naval Research and by grants from the National Science Foundation and from the Chevron Research Company.
..I
Ru(I1) state, not a mixed-valent state and so is an insulator. At potentials near 1.22 V the ruthenium polymer conducts and so the osmium polymer is oxidized. W e have prepared a bilayer electrode sandwich of these two polymers to illustrate the rectifying behavior of the polymer
Registry No. Poly[Ru(bpy)z(vpy)z]2+, 88670-65-9; p~ly[Ru(vbpy),]~+, 81206-05-5;p~iy[Os(bpy),(~py),]~+, 88728-97-6.
(25) Pickup, P. G.; Leidner, C. R.; Denisevich, P.; Murray, R. W., J . Electroana/. Chem., in press.
Water-Soluble a,o-Bis(paraquat) Amphiphiles Form Monolayer Membrane Vesicles, Micelles, and Crystals by Stepwise Anion Exchange or Photochemical Reduction Jurgen-Hinrich Fuhrhop,* Detlev Fritsch, Bernd Tesche, and Hardi Schmiady Contribution from the Institut f u r Organische Chemie der Freien Uniuersitat Berlin, 1000 Berlin 33, West Germany, and Fritz-Haber-Institut der Max-Planck-Gesellschaft, Abt. Elektronenmikroskopie. 1000 Berlin 33, West Germany. Received July 13, 1983
Abstract: 1,l”-[( 1,8-Dioxo-1,8-octanediyl)bis(oxy-ll,l-undecanediyl)] bis[ l’-methyl-4,4’-bipyridinium] tetrachloride (3a), 1,l”-[( 1,12-dioxo-1,12-dodecanediyl)bis(oxy-ll,l-undecanediyl)]bis[ l’-methyl-4,4’-bipyridinium] tetrachloride (3b), the corresponding dibromide diiodide (30, l,l”-[(1,18-dioxo-l,18-octadecanediyl)bis(oxy-ll,l-undecanediyl)]bis[1’-methyland trichloride monoperchlorate (3e) have been 4,4’-bipyridinium] tetrachloride (3c), the corresponding tetraperchlorate (M), prepared. The solubility and the aggregation behavior of these compounds largely depend on the nature of the inorganic anions. The tetrachlorides 3a-c are readily soluble in water. Titration of 3c with 1 equiv of sodium perchlorate or irradiation of 3c with visible light in the presence of ferrocyanide lead to vesiculation. Vesicles were also obtained by dissolving the dibromide diiodide 3f in water. A turbid emulsion of the tetraperchlorate 3d clears on heating to 70 O C . After cooling to rmm temperature vesicles were formed; after 24 h at room temperature crystals of a thickness 5100 8, were observed in electron micrographs. This corresponds to a crystalline monolayer. The electron micrographs also show micelles of diameters 100 A.
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Our general investigation of monolayer vesicle membranes has shown that a,w-bifunctional amphiphiles produce unsymmetric vesicles if the size of both head groups is different.’ Curvature is produced by orientation of the large head groups to the outside Address correspondence to this author at the Freien Universitat.
and the small head groups to the inside. An early report by our group that symmetric a,o-bis(bipyridinium) amphiphile 1 gives vesicles2 was partly in error. Vesicle formation rather depended (1) Fuhrhop, J.-H.; Mathieu, J . J . Chem. SOC.,Chem. Commun. 1983, 144. See also: Okahata, Y . ;Kunitake, T.J . Am. Chem. SOC.1979, 101, 5231.
0002-7863/84/1506-1998$01.50/00 1984 American Chemical Society
