Langmuir 1990,6, 709-712 tain data, for the reasons discussed above-are neglected, the regression coefficient is 0.995.) The interaction of a flexible chain polyelectrolyte with an oppositely charged rigid macroion was treated theoretically by Odijk.26 For a flexible polycation in the presence of a negatively charged rod, a critical ionic strength was predicted, below which the flexible polyion collapses onto the rod. The phase boundary of Figure 3 is consistent with this prediction. More quantitatively, there seems to be an inverse linear relationship between the critical polyion linear charge density which is proportional to /3 and the Debye length, which is proportional to I-”’. The influence of I on polyelectrolyte binding could indeed arise from a number of effects, including changes in micelle size and shapez7 and polyion chain extension. With regard to the former, it is worth noting
709
that the plots of Y, vs for PDMDAAC/TXlOO/SDS are linear through the regime of I and Y , where micelle dimensions change drastically; i.e., changes in micelle dimensions may play a secondary role. Despite these manifold possible factors, the observation of critical polyion binding at a well-defined polyelectrolyte degree of ionization is a significant finding, as is the empirical relationship between @, and Ill2,noted above. More detailed analyses will require measurements over a wide range of micelle compositions, using polymers of varying molecular weights and compositions.
Acknowledgment. Support from the National Science Foundation under Grant DMR-8507479 is acknowledged, as is assistance from Eli Lilly Corp. (27) Dubin, P. L.; Principi, J. M.; Smith, B. A.; Fallon, M. A. J. Colloid Interface Sci. 1989, 127, 558.
(26) Odijk, T. Macromolecules 1980,13, 1542.
Electron Transfer into Self-Assembling Monolayers on Gold Electrodes? K. A. Bunding Lee S.C. Johnson & Son, Inc., 1525 Howe St., M.S. 56, Racine, Wisconsin 53403 Received December 19, 1989. I n Final Form: January 10, 1990 Electron transfer through a nonconju ated hydrocarbon film was studied. 1-Methyl-1’-[ 10-(octadecylthio)decyl]-4,4’-bipyridiniumdibromife, 1-[7-(octadecylthio)heptyl]-l’-propyl-4,4’-bipyridinium dibrodibromide were synthesized and were mide, and l-hexyl-l’-[5-(octadecylthio)pentyl]-4,4’-bipyridinium separately self-assembled with octadecyl mercaptan onto Au electrodes. The sulfide attached to the gold, and the hydrocarbon chains oriented such that the resulting monolayer films had the electroactive group fixed a distance of 5, 7, or 10 CH, groups from the surface, which was about 7-12 A, depending on the angle of the chains to the surface. The films were characterized by cyclic voltammetry, FTIR, and ellipsometry. Electron transfer depended on the distance of the electroactive group from the surface and was facile in all cases.
Introduction Electron transfer is fundamental to most of chemistry, and questions relating to how fast, through what, and how far electrons will transfer provide the basis to understanding chemical phenomena. In an attempt to gain some insight to these questions, we devised a system whereby an electroactive species would be held a specific distance from an electrode in an otherwise passivating, well-ordered film. It has been shown previously that thiol molecules with long-chain alkyl groups would self-assemble on gold electrodes to form monolayers that were free of measurable pin holes, were substantial barriers to electron transfer, and were strongly resistant to ion penetration.’ Thus, by inserting an electroactive group into an alkyl chain with a sulfide group, electron transfer, or electron hopping, through a nonconjugated hydrocarbon film could be studied. Other studies of viologens in films on surfaces have been donezT5 This work is based on work regarding the use of thiols and mercaptans with bipyridinium groups presented at the Electrochemistry Gordon Conference, January 1987, by H. 0. Finklea and K. A. Bunding Lee. (I)Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. SOC. 1987,109, 3559-3568.
but none with the viologen molecules bonded to the surface. The three following compounds were studied: l-methyl1’-[ 10-(octadecylthio)decyl] -4,4’-bipyridinium dibromide (abbreviated SClOVC), 1-[7-(octadecylthio)heptyl]l‘-propyl-4,4‘-bipyridinium dibromide (abbreviated SC7VC3), and l-hexyl-l’-[5-(octadecylthio)pentyl]-4,4’bipyridinium dibromide (abbreviated SC5VCS). (Nitrogensubstituted bipyridines are also called viologens.) When these organize by self-assembling on a Au electrode by attachment through the sulfur atom, the electroactive group is a fixed distance (of 5,7, or 10 CH, groups) from the electrode surface with the exact distance depending on the angle the hydrocarbon chains make with the surface.
Experimental Section The three compounds were synthesized by using a modification of a published synthesis of asymmetric dialkyl sulfides.6 (2) Garcia, 0. J.; Quintela, P. A.; Kaifer, A. E. Anal. Chem. 1989,61, 979-981. (3) Kaifer, A. J. Am. Chem. SOC. 1986, 108, 6837. (4) Lee, K. A. Bunding; Mowrey, R.; McLennan, G.; Finklea, H. 0. J. Electroanal. Chem. 1988,246, 217-224. (5) Lu, T.; Birke, R. L.; Lombardi, J. R. Langmuir 1986,2,305-309.
0743-7463/90/2406-0709$02.50/0 0 1990 American Chemical Society
710 Langmuir, Vol. 6, No. 3, 1990
Elemental analysis yielded the following. For SC5VC6: found (calcd): C, 62.07 (61.871%;H, 9.12 (9.07)%;S, 3.93 (4.24)%;N, 4.09 (3.70)%;Br, 21.33 (21.11)%. For SClOVC: C, 61.41 (61.87)%; H, 9.29 (9.07)%; S, 4.22 (4.24)%; N, 3.89 (3.70)%; Br, 21.18 (21.111%. For SC7VC3: C, 59.00 (61.43)%;H, 8.23 (8.97)%;S, 4.42 (4.321%; N, 4.05 (3.77)%;Br, 22.31 (21.50)%. Having the two chains on either side of the sulfide approximately equal in length has been shown to affect whether the film is oleophobic or ~elophilic.~ The Chem 3D Plus molecular modeling program developed by Cambridge Scientific Computing with MM2 energy minimization potential function developed by Norman Allinger at Indiana University was used to determine molecular distances for each of the alkyl chains of the sulfide molecules to assure they were approximately the same length. The saturated and unsaturated sides were 23.6 and 22.0 A, 23.8 and 19.5 A, and 23.7 and 20.6 A for SC5VC6, SC7VC3, and SClOVC, respectively. This approximate calculation indicates there are differences in chain length of about 2, 4, and 3 A, respectively, which may affect the oleophobicity of the self-assembled films. (The small differences in the saturated side length are a result of total molecule energy minimization.) The S to N distances were about 7.2, 9.0, and 12.9 A for SC5VC6, SC7VC3, and SClOVC, respectively. The substrates were 1-in.squares of float glass with 100 A of TiO, and 1000 A of gold on top prepared by Evaporated Metal Films (Ithica, NY). The counter electrode was a platinum wire and the reference electrode was a saturated calomel electrode (SCE); all potentials in this paper are reported against SCE. The electrodes were separated by glass frits and purged with argon gas, which was also used t o flow over the solution during the electrochemical cycling. The potentiostat was a BAS CV 27, and a Linseis LY 17100 XY recorder was used for data collection. The cleaning cycle consisted of alternating the potential between -1.86 and 0.65 V vs SCE for 10 s in a 0.5 mol/L solution of H,SO,, similar to the method previously publ i ~ h e d After . ~ being cleaned, the electrode was hydrophilic.The films were formed either by dipping in a solution of 5 mmol/L bipyridinium molecules for about 1 h7 followed by dipping in a 5 mmol/L solution of octadecyl mercaptan for a few minutes or by dipping in a mixed solution of 5 mmol/L bipyridinium and octadecyl mercaptan for 1 h. The solvent for the dipping solutions was 2:l ch1oroform:methanol (the molecules studied are insoluble in water). The films were rinsed with CHCl,. The electrolytefor theelectrochemicalanalysis was0.l mol/L NaNO, and 0.01 mol/L Na,HPO,. Infrared spectra were taken with a Nicolet 60SX equipped with a Harrick grazing angle reflectance attachment and polarizer. The angle used was 75'. The resolution was 2 cm-', and 1000 scans were taken with the sample ratioed to the spectrum of the cleaned substrate before film deposition. The spectrometer was purged with nitrogen. A Gaertner Model L116A/B ellipsometer was used. The wavelength was 632.8 nm, the angle of incident light with respect to the surface normal was 70°, the polarizer was 20°, and a refractive index of 1.5 was used for calculating the thickne~s.~
Results and Discussion In order to prove that the electrons can transfer from the electrode to an electroactive group separated from the electrode by a varied number of CH, groups, the appropriate orientation and organization first must be proven. Thus, molecular attachment to the surface (via the S atom), molecular organization with the hydrocarbon chains nearly perpendicular to the electrode surface, and lack of orientation with the electroactive group adjacent to the electrode must be proven. The orientation of the molecules must not be with the bipyridine group lying on the surface because there would be nothing unusual about the electron transfer in that case; it would not be through (6) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langrnuir 1988,4, 365-385. (7) Tillman, N.; Ulman, A.; Elman, J. F. Langrnuir 1989, 5 , 10201026. Tillman, N.; Ulman, A,; Schildkraut, J. S.; Penner, T. L. J . An. Chem. Soc. 1988, 110,6136-6144.
Letters
l
l
l .1'0
l
l
l l l " d 5 '
l
l
l
"
l '
l
6
l
UV vs. SCE
Figure 1. (A) Cyclic voltammogram of 1-methyl-1'-[lo-(octadecylthio)decyl]-4,4'-bipyridiniumdibromide deposited in a thick film by solvent evaporation; scan rate was 100 mV/s. (B) Cyclic voltammogram of a multilayer film of l-hexyl-l'-[5-(octadecylthio)pentyl]-4,4'-bipyridinium dibromide; scan rate was 200 mV/s.
the hydrocarbon spacer. This can be proven by considering the electrochemistry, ellipsometry, and spectrosCOPY. There are several measurable parameters that can be obtained from the cyclic voltammetry that indicate the structure of the films. The integrated area of the redox peaks indicates the charge transferred which relates to the coverage of electroactive species. This probably correlates to the coverage of molecules containing bipyridinium groups but is not necessarily the same since it is conceivable that the molecules are oriented such that electron transfer cannot occur. In order to show that the charge transferred was a good indication of whether the film was a monolayer or not, thick films were studied. Thick films were formed by dropping solutions of bipyridinium on the gold electrode and evaporating the solvent. These thick, visible films turned purple upon reduction. The color moved across the electrode in waves, and the color would also take several seconds to disappear when the potential was held at 0 V. An example of a cyclic voltammogram (CV) of a thick film is in Figure 1A. Thick films of the other molecules gave similar results. The coverage of electroactive species of 9 X lo-' mol/ cm2 was calculated by integrating the area under the first reduction peak in the cv in Figure 1A. This indicates a film much thicker than a monolayer and also indicates that even disorganization or film thickness did not prevent electron transfer. T h a t is, even without diffusion of electroactive species, electrons could be transferred to molecules that were not adjacent to the electrode. In Figure lB, the CV shows coverage of 6 X lo-'' mol/cm2 that is larger than the coverage expected of 2.2 X lo-'' mol/cm2 for a monolayer of bipyridinium molecules oriented with the chains perpendicular to the electrode if they had a cross section of 75 .&'/molecule. This coverage was higher than expected and implied a film thicker than a monolayer, the lack of distinct peaks and the redox peak separations indicated disorganization, and the relatively positive water reduction potential indicated lack of organized coverage and film defects. These again indicate that even disorganization or film thickness did not prevent electron transfer. The cyclic voltammograms from the monolayer films in Figure 2 had coverages of 6.3 X
Langmuir, Vol. 6, No. 3, 1990 711
Letters
I '
. .,b
,
,
I
I '
I '
I
'
I '
I
d'6'
l
, "
I
J
'
b
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'
w V6. SCE
Figure 2. Cyclic voltammogram of a monolayer of (A) l-hexyl1'-[ 5-(octadecylthio)penty1]-4,4'-bipyridinium dibromide and octadecyl mercaptan. (B) l-[7-(octadecylthio)heptyl]-1'-propyl-4,4'-
bipyridinium dibromide and octadecyl mercaptan, and (C) 1methyl-1'- [ 10-(octadecylthio)decyl]-4,4'-bipyridinium dibromide and octadecyl mercaptan. Scan rates were 200-800 mV/s in 100 mV/s intervals, and phosphate buffer was the electrolyte. lo-", 1.4 X lo-", and 1.4 X 10-l' mol/cm2 for SC5VC6, SC7VC3, and SClOVC, respectively. This coverage of bipyridinium was well less than a monolayer (the rest of the monolayer was made up of electroinactive octadecyl mercaptan) and indicates there was no multilayer formation of bipyridinium-containing molecules as seen from the cyclic voltammograms in Figure 1. The coverage of the monolayer films, in particular, the ratio of electroactive species to C18SH, is governed by the surface binding affinity for the gold of the sulfide versus the mercaptan, which is orders of magnitude smaller than merc a p t a n ~ , ' as ~ ~ well as whether the films were made by sequential dipping or dipping into a mixed solution. These aspects were not studied for this paper. A good indication of a thorough coverge of the surface with an organic film is low charging current. This indicates that solution ions cannot easily penetrate the organic film and is characteristic of a compact organic layer that does not desorb in aqueous solution. If the charging current were not low, then this would imply there were defects in the film. The charging currents for the monolayer films illustrated in Figure 2 are all less than 1 pA/cm2, while that for the film in Figure l B , which is not well organized, is 20 pA/cm2. Charging currents for the Au electrode in buffer without an organized film are about 40 pA/cm2. (8) Strong, L.; Whitesides, G. M. Langmuir 1988, 4 , 546-558. (9) Bain, C.D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723-727.
Another characteristic of film conformation is water reduction. The more easily the water diffuses through the organic film, the more positive the water reduction. In some cases of film formation, the water reduction began a t about -0.8 V, a potential more positive of the second reduction couple of the bpy, and was so large as to obliterate or prevent the second reduction. Such films were considered disorganized with possible defects. Another parameter that indicates stability of the various species is redox potential and separation of the two one-electron-transfer peaks. The monolayer films that are well-organized and show water reduction more negative than -1 V have two one-electron redox waves at-0.48, 0.45, -0.96, and -0.95 V for SC5VC6; -0.54,-0.52, -0.98, and -0.95 V for SC7VC3; and -0.55, -0.52, -1.00, and 0.97 V for SClOVC, respectively, a t 200 mV/s scan rates (see Figure 2). The reproducibility of these potentials is about f 2 5 mV because they depend on the monolayer structure and organization, which was difficult to reproduce exactly. The redox potentials vary with changing number of CH, groups between the electroactive group and the electrode, but the electron transfer is facile within this range of spacing. The potential for the first electron transfer becomes more negative with increasing distance of the electroactive group from the surface, which would reflect the increased difficulty of electron transfer with distance. However, for the second electron reduction, the stability of the bpy+ becomes SC5VC6 > SClOVC > SC7VC3 based on the difference between the reduction potentials for the first and second reductions. However, the difference here is 30 mV, which is almost within experimental error. T h e oxidations are ordered as expected, with the neutral SClOVC being most difficult to oxidize and neutral SC5VC6 being the easiest. The solubility of viologens in a nonionic micellar phase increases in the order dication < radical < fully reduced form of the vio1ogen.l' Thus, increased order and stability would be expected for the reduced films. It must be shown that the molecules do not diffuse to the electrode but are attached to it. Surface confinement of reactants (lack of diffusion) is indicated by the small separation of the oxidation and reduction peak potentials of a redox couple." For the monolayer films illustrated in Figure 2, the peak separation is 10-30 mV a t 200 mV/s, which is much smaller than the 59 mV expected for a one-electron reduction in the case of diffusion. Peak potential separations greater than 59 mV can indicate slow electron transfer but can also be caused by film resistance effects." The separation of the redox peaks is also a function of reversibility, which is affected by the electron-transfer kinetics between substrate and film, intrdilm electron transfer, chemical factors, counterion diffusion, and uncompensated solution resistance.'"12 Thus, the separation of the redox peaks as a function of scan rate also may be due to one or more of these factors. Sometimes, the monolayer films formed from these compounds show a definite dependence of the peak separation of the first reduction couple with scan rate, with the peak separation going from 30 mV at 200 mV/s to 110 mV a t 800 mV/s, as in Figure 2C for SClOVC, while the peak separation of the second redox couple hardly varies with scan rate. This is not peculiar to the SClOVC compound and was seen for some monolayer films of the (10) Hoshino, K.; Sasaki, H.; Suga, K.; Saji, T. Bull. Chem. Soc. Jpn. 1987,60, 1521. (11)Peerce, P. J.; Bard, A. J. J. Electroanal. Chem. 1980, 114, 89115. (12)Tsoq Y.; Liu, H.; Bard, A. J. J. Electrochen. Soc. 1988, 135, 1669-1675. ~~~
712 Langmuir, Vol. 6 , No. 3, 1990 other compounds as well. There was a smaller increase in the peak separation for the CV of the SC5VC6 film shown in Figure 2A which was 30 mV a t 200 mV/s and 70 mV a t 800 mV/s. The cyclic voltammograms show variability for different film preparations which was most likely associated with variations in coverage and degree of organization. Other indications of surface confinement are proportionality of peak current with scan rate instead of the square root of the scan rate and symmetrical shape of the peaks." The monolayer films show both of these characteristics for scan rates of 200-800 mV/s. The full width a t half-maximum (fwhm) of the reduction peaks can be indicative of surface confinement. The theoretical fwhm for a surface-confined species is 90.6 mV a t 25 "C for one-electron-transfer processes but more frequently is 150-540 mV. The fwhm can be increased due to repulsive interactions and differences in spatial distributions of redox centers" as well as aging effects, reorientation, or phase transition processes.13 The monolayer films show fwhm of 100 and 110 mV, 120 and 110 mV, and 110 and 130 mV for the cathodic and anodic peaks of the first redox couple of SC5VC6, SC7VC3, and SClOVC, respectively, and thus indicate surface confinement. The films are fairly sturdy in that the charge transferred does not diminish much with repeated cycling, and the films can be left immersed in electrolyte overnight with little loss of signal. This also indicates the sulfides are bonded to the surface and are organized in an organic film. In other cases of organized monolayers on gold, which were not covalently bound, faradaic processes did destroy the integrity of the mon01ayer.l~ Ellipsometry indicated average film thickness of 24, 21, and 20 i 5 A for SC5VC6, SC7VC3, and SClOVC, respectively. These data support the proposed film organization as monolayers with some small (0-30") molecular chain tilt. There is no interdigitation, which would have yielded greater thicknesses. FTIR spectra of the thin films are shown in Figure 3. These spectra showed no clear bands in the 1600-1300cm-' region either due to low intensity or water vapor interference. The spectra do have the relative intensities and band positions in the CH stretching regions that are typical of monolayers organized with the alkyl chains oriented nearly perpendicular to the electrode surface.* Though the relative peak height indicates these films have slightly more disorder in the alkyl chains than is typical of a pure octadecyl mercaptan film, considerable order still is evident, especially considering the size of the bipyridinium group and the bromide ions. The calculated S to N distances were about 7.2,9.0, and 12.9 A for SC5VC6, SC7VC3. and SClOVC, respectively, which, when corrected for up to an approximate angle of 30' that the hydrocarbon chain may make with the surface and the electrode to sulfur distance, puts the bipyridinium group about 7.2, 8.8, and 12.2 A from the surface, which gives the approximate variation of electron-transfer distances. Since the electroactive molecules are fixed to the electrode and oriented with their alkyl chains nearly perpendicular t o the surface, and since there is no mediator present, electron transfer through the u bonds or longrange electron tunneling must be taking place. The distance the electroactive group is spaced by the varied num--__ (13) Lu, T.; Cotton, T. M.; Hurst, J. K.: Thompson, D. H. P. J . Electroanal. Chem. 1988,246,337-347. (14) Finklea, H. 0.; Robinson, L. R.; Blackburn, A,: Richter, B. Langrnrrir 1986. 2. 239-244
Letters
-
m
A
0
11
1
UlC
0
WAVENUMBER
B
.r
n
0 1
il
1
h
C
m 0
91
1
iLI
z:
00
E? Lo D
-
mm a0 0
03-:00
3600 2400 2600 WAVENUMBER
2jOO
Figure 3. FTIR spectra of (A) l-hexyl-l'-[5-(octadecylthio)pentyl]-4,4'-bipyridinium dibromide and octadecyl mercaptan, (B) 1- [ 7 -(octadecylthio)hepty1]-l'-propyl-4,4'-bippidiniumdibro-
mide and octadecyl mercaptan, and (C) 1-methyl-1'-[lo-(octadecylthio)decyl]-4,4'-bipyridiniumdibromide and octadecyl mercaptan. ber of CH, groups from the electrode then gives a distance range of electron transfer, or electron hopping of 7-12 A, with the limit still to be found for these systems. This implies that electrons may be able to hop from inside a biological membrane to the outside but certainly to an electroactive species embedded in the membrane. This also implies that such an electroactive material can be embedded in the membrane despite steric and charge restraints and still undergo electron transfer. Related studies include electron transfer "through space" in biological molecules,'5 through membranes,16 and into polymer-modified electrodes,'* and electron transfer "through'! insulating u bond^.'^^^^ The latter was described as longrange electron tunneling based on the LUMO of the spacer and the negligible probability of finding the electron in the spacer region." I t has also been reported that electron transfer across an insulating barrier as wide as 10 A could occur in about 100 ps, with dependence on exothermicity and solvent polarity." With the novel system presented in this paper, this type of long-range electron tunneling can be easily studied, and the effect of distance and environment can be determined.
Acknowledgment. We would like t o thank Gregory Russel for his help with the molecular distance calculations. (15) Gray, H. B. Chem. SOC. Reu. 1986,15,17-30 and references therein. (16) Baumgartner, E.; Fuhrhop, J.-H. Angew. Chem., Int. Ed. Engl. 1980,19, 550-551. Tundo, P.; Kurihara, K.; Kippenberger, D. J.; Politi, M.; Fendler, J. H. Angew. Chem., Int. Ed. Engl. 1982, 1, 81, 82. (17) Calcaterra, L. T.; Closs, G . L.; Miller, J. R. J . Am. Chem. SOC. 1983, 105,670-671. (18) Miller, J. R.; Calcaterra. L. T.: Closs, G . L. J . Am. Chem. Soc. 1984, 106, 3047-3049.
