Langmuir 1994,10,2235-2240
2235
A Vibrational Spectroscopic Study of the Structure of Electroactive Self-AssembledMonolayers of Viologen Derivatives Xiaoyan Tang, Thomas Schneider, and Daniel A. Buttry* Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071 -3838 Received October 15, 1993. In Final Form: February 18, 1994@ Surface-enhanced Raman spectroscopy (SERS)and infrared reflection absorption spectroscopy (IRRAS) are used to determine the orientation of the viologen redox group and the relative degree of ordering of the alkyl chain segments in self-assembled monolayers ofvarious alkylviologen derivatives. The monolayers were assembled and examined on rough Ag electrodes (SERS)and smooth, vapor-deposited Au electrodes (I=). Attachment to the Ag or Au surfaces which were studied occurs by virtue of strong interaction of a disulfidegroup on the molecule with the surface. In the present case, analysis ofthe relative intensities of the viologen A, N-R stretching mode at 1169 cm-l and two of the Ag ring modes at 1298 and 1637 cm-l reveals that the net orientation of the long axis of the viologen moiety is roughly perpendicular to the plane ofthe electrodesurface, in accordance with the surface coverages at saturation as determined from previous electrochemicaldata. The relative intensities of the methylene asymmetric (2890 cm-l) and symmetric (2850cm-l) stretches in the SERS spectra on rough Ag and the peak positions of the asymmetric and symmetric methylene C-H stretching modes in the IRRAS on smooth Au (at 2929 and 2857 cm-l, respectively) reveal a liquid-like environment for the alkyl chains. These results are consistent with the relative sizes of the viologen and alkyl groups and the expected packing arrangement based on these, as well as with previous electrochemicallyderived surface coverages for saturated self-assembledmonolayers of these compounds.
Introduction The spontaneous assembly of molecules at surfaces represents a simple and powerful method of constructing interfaces with well-defined structure and composition. Due to these characteristics, many groups are engaged in studies of self-assembled monolayers (SAM's).' A significant use of these assemblies is in understanding how interfacial structure influences chemistry and physics at the interface, with the long-range goal oflearning to control interfacial processes. Our group has also been using electroactive SAM's in an effort to understand how the interfacial environment affects interfacial redox processes. In particular, the influence ofion ~ a i r i n and 8 ~the position of the redox group within the monolayer structure4have been shown to significantly impact the electrochemistry (especially the formal potential) of the redox moiety within the SAM. The finding that subtle details of the S A M structure and positioning of the redox group within it can have such major influence on electron transfer to the pendent or embedded redox group has prompted us to more closely examine the structural details of these systems. Thus, as a complement to our previous work on the electrochemistry of viologen S A M systems, we present here a vibrational spectroscopic study of the structure of the SAM's formed from these viologen derivatives. Several other group^^-^ also have investigated monolayers of amphiphilic viologen derivatives which spontaneously adsorb at electrode surfaces either in the v2+ ~
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Abstract published in Advance ACS Abstracts, April 15,1994.
(1)Ulman, A. An Introduction to Ultrathin Organic Films: from Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (2)De Long, H. C.; Buttry, D. A. Lungmuir 1990,6,1319-22. (3)De Long, H. C.; Donohue, J. J.; Buttry,D. A. Langmuir 1991,7, 2196-202. (4) De Long, H. C.; Buttry, D. A. Langmuir 1992,8,2491-6. (5) Cotton, T. M.; Kim, J.-J.;Uphaus, R.A. Microchem. J . 1990,42, 44-71. (6)Gomez, M.; Li, J.; Kaifer, A. E. Langmuir 1991, 7, 1797-806. (7) Diaz, A.; Kaifer, A. E. J. Electruanal. C h m . 1988,249,333-8. ( 8 )Widrig, C. A.; Majda, M. Langmuir 1989,5,689-95. (9) Bae, I. T.; Huang, H.; Yeager, E. B.; Scherson, D. A. Langmuir 1991,1,1558-62.
state or in the one-electron-reduced V+ state. However, the SAM's formed from the derivatives described here differ in that they bear disulfide linkages capable of forming covalent anchors to the Ag electrode surfaces used in the study. Thus, these monolayers are expected to have less dynamic behavior than those without anchoring groups.
Experimental Section The silver surface-enhanced Raman spectroscopy (SERS) electrode was constructed from flattenedhigh-purity silverwire which was sealed in a Teflon plug with Torr Seal. The silver electrode surface was polished mechanically using increasingly smaller sizes of alumina powder (to 0.05 pm) and washed ultrasonically in deionized water. It was then roughened in 0.1 M NaCl by a double potential step using a conventional threeelectrode cell and a PAR Model 273 potentiostat. A double potential step sequence of -0.2 t o +0.2 or +0.25 V and back to -0.2 V versus SCE, with a dwell time of 2-4 s at $0.2 or +0.25 V,was usually used to roughen the surface. The dwell time was determined by the charge passed which was typically between 30 and 50 mC cm-2, a range which is generally agreed to give maximum SERS intensity. The dimensions of the exposed, rectangular electrode surface were 1 x 10 mm. The Raman cell which was used to obtain SERS spectra was constructed in the shape of a cylinder with an optically flat window at one end. A AglAgC1 electrode served as a reference electrode (to which all potentials are referenced), and a platinum wire was used as the counter electrode. The roughened silver electrode was used as the working electrode. The electrolyte in the Raman cell was deaerated by purgingwith argonbefore the experiments. Careful attention to exclusion of air is required for these experiments due to the extreme sensitivity of the reduced forms of viologens to oxygen. The optically smooth Au electrodes were prepared by vapor deposition in an Edwards 306A oil diffusion-pumped vacuum chamber. Float glass microscope slides were carefully cleaned with various acid treatments followed by rinsing with organic solvents and immediate placement into the vacuum chamber. A total of 10 nm of Cr was deposited as an adhesion layer after which 250 nm of Au was deposited. These substrates were cleaned with 50150 concentrated sulfuric acid and 30%HzOzjust prior to monolayer assembly. Note: This sulfuric acidhydrogen
0743-7463194l2410-2235$04.50/00 1994 American Chemical Society
Tang et al.
2236 Langmuir, Vol. 10,No. 7, 1994 peroxide solution reacts extremely vigorously with organics and should be used with extreme caution! The compounds are referred to in the following way: CH3( C H Z ) ~ V ~ + ( C H ~ ) ~ ~ S S ( C H ~ ) ~whereV+ ~ V ~ ~ ( Cdesignates H~)~CH~, redox group, the viologen (Le., N,WN'-dialky1-4,4'-bipyridinium) is referred to as (lOVlOS-)2, with the number referring to the number of carbons in the alkyl chain. Other compounds are referred to analogousljl. (lOVlOS-)2 was synthesized by the method described in ref 2. We note here that the synthetic route to these compounds described in ref 2 actually produces a disulfide, rather than a thiol, derivative. We will discuss this in a future paper along with a description of the differences in behavior of these two self-assembling systems. For the moment, we simply point out that these disulfidederivatives spontaneously adsorb to give surface coverages at saturation that are quite large, but not as large as those produced from the thiol derivative. Monolayers of all compounds were formed by immersing the roughened Ag electrode or the smooth Au surface into a dilute (typically 0.1 mM) aqueous electrolyte solution of the compound of interest for a time period ranging from several hours to days. In case the compound was not reasonably soluble in water, up to 50% ethanol was used to improve the solubility. Obtaining normal Raman spectra of these viologen derivatives required highly purified material due to the presence of traces of fluorescent impurities following the synthetic route which was employed. Purification was accomplished by repeated recrystallizations, typically at least five times. In-situ Raman spectra were obtained using a 90" excitation/ collection geometry. Spectra were taken with a SPEX 1700series single pass monochromator, a RCA C31034 PMT detector which was cooled by dry ice, and Ortec photon counting electronics. Raman spectra were collected and stored with an IBM personal computer interfaced to the monochromator. A 43068 interference filter (Edmund Scientific)and a holographic long pass 1005filter (Kaiser Electrooptics) were used t o remove plasma lines and to reduce the intensity of Rayleigh scattering, respectively. An fl1.8 collection lens was used to collect and focus the Raman scattered light through the monochromator. A Lexel Model 94 argonion laser provided 514.5-nmexcitation. Spectral resolution was typically 5 cm-l, and integration times were typically 1-10 s per point. Ex-situ infrared reflection absorption spectra (IRRAS) of monolayers on the Au electrodes were obtained with a Bomem MBlOO FTIR using a HgCdTe liquid nitrogen-cooled detector and a HanickVRAglancing angle attachment set at 80". Spectra of the viologen monolayer were ratioed against a perdeuterated hexadecanethiol monolayer (as background). This is a useful procedure that helps avoid spurious hydrocarbon contamination of the Au electrode used for the background, by virtue of the low surface energy, and consequently low level of surface contamination, ofthe perdeuterated S A M . Spectra were obtained at 4-cm-l resolution with 512 scans.
Results
A. SAM and Model Compound IR and Raman Spectra. As discussed previously, these viologen systems self-assemble on Au electrodes to form stable monolayers which exhibit the well-defined electrochemistry of the viologen g r ~ u p . Similar ~ , ~ behavior is shown in Figure 1 for a S A M on a rough Ag electrode. The two, reversible, one-electron redox processes of the viologen redox group .areclearly observed. The potentials ofthe first and second redox processes (ca. -0.45 and -0.90 V, respectively) correlate well with those previously observed for SAM's of this same molecule on Au electrode^.^^^ The response of a typical S A M on a Ag electrode is stable for several hours, indicating nearly irreversible adsorption of these disulfide derivatives onto the Ag electrode. Figure 2A shows the normal Raman spectrum of a highly purified viologen derivative (N,W-dimethyl-4,4'-bipyridinium sulfate, MVS04),and Figure 2B shows the SERS spectrum of a (lOVlOS-)2 S A M on a rough Ag electrode in the 900-1700-cm-l region. A salient feature of these spectra is the difference in the integrated intensity ratios of the bands at 1169,1298, and 1637 cm-l for these two
I 6.00
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.ooo
3 -6.00
-12.0
1
-.m
-.600
Potential M
Figure 1. Voltammogram of a (1V12S-)2 S A M on rough Ag in 0.1 M NaN03 at a scan rate of 100 mV s-l, referenced versus Ag/AgCl, electrode area 0.1 cm2.
I
1169
I
1600
1200
1400 Wovenumber
/
1000
cm-'
Figure 2. Raman spectra in the ring mode region of (A, upper curve) N,"-dimethyl-4,4'-bipyridinium sulfate (MVSO4)and
(B, lower curve) a ( 10V10S-)2 SAM on a roughened Ag electrode at -0.1 V vs Ag/AgCl in 0.1 M phosphate buffer (pH 7.0).
samples. These bands correspond to the viologenk N-R stretching mode for the bond between the pyridinium nitrogen and the pendent alkyl chain (1169 cm-I for the S A M ,1193 cm-l for MVS04,where the shift is due to the difference in the reduced mass of the alkyl chain versus the methyl group) and Ag ring modes (1298 and 1637 cm-I for the S A M ,1292 and 1650 cm-l for MVS04). We return to a discussion of these assignments and relative intensities below. Figure 3A shows the C-H stretchingregion ofthe SERS spectrum of the (lOVlOS-)2 S A M on rough silver. This spectrum shows a strong band for the aromatic C-H stretching modes near 3080 cm-', as well as the methylene and methyl C-H stretching envelope between 2820 and 3000 cm-l. Figure 3B shows the same region for liquid octanethiol. The ratio (Zz89dZ2850) of the peak heights of the methylene asymmetric (2890 cm-l) and symmetric (2950 cm-l) C-H stretching modes, which has been used to reveal the degree of "order" in polymethylene systems,l0J' is seen to be quite similar for these two samples. This strongly suggests that the alkyl chains in these (10)Synder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta 1978, 34A,395-406.
Self-Assembled Monolayers of Viologen Derivatives
Langmuir, Vol. 10, No. 7, 1994 2237
J'-y
B
-.0002i
1700
Wovenumber / c m - l
Figure 3. Raman spectra in the C-H stretching region of (A, upper curve) a (lOVlOS-)2 S A M on a roughened Ag electrode at -0.1 V vs Ag/AgCl in 0.1 M phosphate buffer (pH 7.0) and (B,lower curve) liquid octanethiol.
,002
c C
; 9
.oo 1
3000
2900
2800
Wovenumber / cm-'
Figure 4. Infrared spectrum in the C-H stretching region of a (lOVlOS-)2 S A M on a smooth Au surface. monolayers are in a liquid-like environment. We return t o a discussion of this ratio below. Several SERS spectra were obtained in the 400-600cm-l region in a effort to determine whether or not the disuflide S-S bond had been cleaved during adsorption. In no case was there any evidence for the S-S stretch (which would occur near 500 cm-l) in these spectra. This strongly suggests that the S-S bond does cleave upon adsorption, and in that regard is in agreement with expectations baesd on a fairly large body of previous w0rk.l Figure 4 shows an ex-situ IRRAS glancing angle spectrum in the C-H stretching region of a (lOVlOS-)2 SAM on a smooth Au surface. The peak intensities of the methylene C-H stretching modes are consistent with formation of a single monolayer of material in these SAM'S.^^-^^ Previous researchers have used the peak (11)Gaber, B.P.; Peticolas, W. L. Biochim. Biophys.Actu 1977,465, 260-74. (12)Porter, M.D.;Bright, T. B.;Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. SOC.1987,109,3559-68. (13)Walczak, M.M.;Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung,C.;Porter, M. D. Lungmuir 1991, 7 , 2687-93. (14)Popenone, D. D.;Deinhammer, R. S.; Porter, M. D. Langmuir 1992,8,2521-30.
, 1500
1600 Wavenumber
2800
3000
, /
y 1400
cm-'
Figure 5. Infrared spectra in the 1000-1800-cm-1region of (1, upper curve) the dibromide salt of (lOVlOS-)2 in KBr and (2, lower curve) a (10VlOS-)Z SaM on smooth Au. positions of the asymmetric and symmetric methylene C-H stretches to reveal the degree of ordering of the alkyl chains in thiol-anchored SAM's of alkanethiols12 and ferrocene-terminated, electroactive SAM's.13J4 In the present case, these peak positions (asymmetric at 2929 cm-l and symmetric at 2857 cm-') are consistent with a liquid-likeenvironment for the alkyl chains, in agreement with the SERS results. Figure 5 shows the normal FTIR spectrum of a bulk sample of the dibromide salt of (lOVlOS-)2 (curve 1)and the IRRAS glancing angle spectrum of a (lOVlOS-)2 S A M in the 1000-1800-~m-~ region. This spectrum is of the same S A M as that shown in Figure 4 above. Note that while the ring mode at 1637 cm-' is the most pronounced peak in this region of the bulk spectrum of the dibromide salt of (lOVlOS-)2, this band is somewhat weaker in the S A M ,compared to the 1462-cm-' band (and also to the C-H stretches in the 2800-3000-cm-' region). We return to this point below. B. Characterization of the Rough Ag Surfaces. Given the observations of liquid-like alkane chains in the viologen SAM's described above, we undertook a set of experiments designed to characterize the extent to which these SERS-active, rough Ag surfaces could support formation of reasonably well-ordered SAM's. The question is whether the liquid-like nature of the viologen SAM's is due to the surface roughness of the Ag SERS substrates or the intrinsic ability of these molecules to pack efficiently. Assembly of SAM's of n-alkane thiols was choosen as a good test of this tendency, since these compounds are known to have a tendency to form well-ordered monolayers whose degree of order is easily discernible from their spectroscopic response.'J2 Figure 6 shows the SERS spectrum of a SAM formed from octanethiol on a Ag electrode roughened in the same way as those used for the viologen SAM studies described above. Comparison of the ratio (1289d12850) of the peak heights of the methylene asymmetric (2890 cm-9 and symmetric (2850 cm-l) C-H stretching modes for the S A M (IzSsdI2860 = 1.2) with that of liquid octanethiol in Figure 3b above (IzsgdI28,o = 0.85) indicates substantially more order in the octanethiol SAM than in the liquid. Another measure of the degree of order within a SAM can be obtained by examination of the Raman spectral region which contains the C-S stretching modes. Two distinct bands are seen for the C-S stretches which correspond to gauche (635 cm-') and trans (705 cm-l) conformations around the two carbon atoms adjacent to
Tang et al.
2238 Langmuir, Vol. 10,No. 7,1994
3000
2900
2800
Wovenumber / cm-'
Figure 6. SERS spectrum in the C-H stretchingregion of an octanethiol S A M on a roughened Ag surface.
A
654
800
600
Wovenumber / c m - '
Figure 7. Raman spectra in the C-S stretchingregion of (A) an octanethiol SAM on a roughened Ag surface and (B)liquid octanethiol. the S atom. This region has been used by Joo and cow o r k e r ~and ~ ~Pemberton and co-workers16 to estimate the relative amounts of gauche and trans conformers in SAM's. They showed that highly ordered SAM's and solid alkane thiols show large proportions of the trans conformer compared to the gauche conformer, in accordance with expectations. Figure 7A shows the SERS spectrum of an octanethiol S A M on a rough Ag surface, and Figure 7B shows a normal Raman spectrum of liquid octanethiol. The liquid spectrum clearly shows both trans (ca. 734 cm-l) and gauche (ca. 654 cm-l) conformers to be present in the octanethiol liquid sample. On the other hand, the SERS spectrum of the octanethiol S A M shows a very strong band at 701 cm-' and only a very small band at ca. 632 cm-l. These bands are attributed to the trans and gauche conformers, respectively. The large energy shifts of these bands have been suggested to be due to strong interaction with the Ag surface atoms.16 The predominance of the trans band strongly suggests that it is possible to form reasonably (15)Joo,T.H.; Kim, R;Kim, M. S. J.Phys. Chem. 1986,90,5816-9. (16)Bryant, M. A.;Pemberton, J. E. J.Am.Chem. SOC.1991,113, 3629-37.
well-ordered SAM's on these rough Ag surfaces. The relative intensities of the trans and gauche bands (ca. 20:l) suggest a defect density of 1-5%. Another test of the extent to which formation of reasonably well-ordered SAM's can form on these rough Ag surfaces is based on relative permeability of the SAM toward solution-phase redox couples. To address this question, the currents for methylviologen reduction at a rough, bare Ag surface and at a rough, SAM-covered Ag surface were examined. Avery crude measure of the defect density in the monolayer can be obtained from the ratio of the plateau current a t the (microelectrode-like) S A M surface17to the peak current at a rough, bare Ag surface. This ratio is ca. 0.01, consistent with a SAM which contains some defects (pinholes, etc.), but is not so lacking in structural integrity as to allow unrestricted access of solution-phase species to the underlying Ag electrode.
Discussion The results presented above clearly show that these viologen disulfide derivatives spontaneously adsorb onto Ag and Au to give a self-assembled monolayer with a fairly high surface coverage of viologen groups. The lack of an observable S-S stretch provides evidence for cleavage of the S-S bond upon adsorption. The essentially irreversible nature of the adsorption and the high saturation surface coverage measured electrochemically are signatures of strong interaction between the S atoms and the metal surface atoms. We turn now to a discussion of the relative orientation of the viologen groups and the degree of ordering within the alkyl chain regions of these monolayers. The vibrational spectroscopic properties of viologens and their reduction products, especially the symmetrically substituted N,"-dimethylviologen (MV+), have been frequently studied. The first paper concerning the Raman spectra of methylviologen was published by Benchenane et al. in 1974.lS Since then many papers on the Raman and infrared spectra of viologens, mainly methylviologen, have been published. Among them the works of Cotton and c ~ - w o r k e r s , ~ Hester J ~ - ~ ~ and ~ o - w o r k e r s , ~and ~-~~ Poizat and c o - ~ o r k e r s are ~ ~ very - ~ ~ representative. The first normal coordinate analysis of the in-plane vibrational modes o f W +and its reduction product, the cation radical MV+, was made in 1982.26This was then extended to the three redox forms of methylviologen in 1988.21 These normal mode calculations combined with all the experi(17)Sabatani, E.;Rubinstein, I. J. Phys. Chem. 1987,91,6663-9. (18)Benchenane, A,; Bernard, L.; Theophanides, T. J. Raman Spectrosc. 1974,2,543-53. (19)Kim, Jae-Ho; Cotton, T. M.; Uphaus, R. A. J.Phys. Chem. 1989, 93,3713-20. (20)Feng, Q.;Yue, W.; Cotton, T. M. J.Phys. Chem. 1990,94,208291. (21)Ghoshal, S.;Lu,T.; Feng, Q.; Cotton, T. M. Spectrochim.Acta 1988,44,651-60. (22)Lu,T.; Cotton, T. M.; Hurst, J. K.; Thompson, D. H. P. J.Phys. Chem. 1988,92,6978-85. (23)Lu,T.;Cotton, T. M. J. Phys. Chem. 1987,91,5978-85. (24)Feng, Q.;Cotton, T. M. J. Phys. Chem. 1986,90,983-7. (25)Forster, M.; Girling, R. B.; Hester, R. E. J. Raman Spectrosc. 1982,12,36-48. (26)Hester, R. E.;Suzuki, S. J. Phys. Chem. 1982,86, 4626-30. (27)Metcalfe, K.;Hester, R. E. J.Chem. SOC., Chem. Commun. 1983, 133-5. (28)Poizat, 0.;Giannotti, C.; Sourisseau, C. J. Chem. Soc.,Perkin Trans. 2,1987,829-34. (29)Poizat, 0.;Sourisseau, C.; Corset, J. J.Mol. Struct.1986,143, 203-6. (30)Poizat, 0.; Sourisseau,C.; Giannotti, C. Chem. Phys. Lett. 1986, 122,129-32. (31)Poizat, 0.;Sourisseau, C.; Mathey, Y. J. Chem. SOC., Faraday Trans. 1 1984,80,3257-74. (32)Regis,A,; Corset, J. J.Chim. Phys. Phys.-Chim. Biol. 1981,78, 687-93.
Self-Assembled Monolayers of Viologen Derivatives X
f
R
7
L
JU Y+J J I.
R
Figure 8. Molecular axes of a viologen derivative used in this work.
Langmuir, Vol. 10, No. 7, 1994 2239 cm-l ring mode by virtue of the surface selection rules under conditions in which the long axis of the molecule is along the surface normal.6933-36This argument has generally been consistent with experimental results from other sources (e.g., electrochemical studies) and appears to be valid. One manner of examining the enhancement due to orientation of the viologen group is to compare the ratio of the integrated intensities of the 1169- and 1298-cm-l bands (i.e.,z116$11298) for an isotopic (i.e., unoriented) bulk sample and for a SAM. In the present case these ratios are 0.5 for theMVS04sample and 1.3 for the SAM. Thus, the integrated intensity ratio for the SAM is increased by a factor of 2.6 relative to that for the bulk MVS04 sample. This implies a high degree of Orientation of the long axis of the viologen group along the surface normal in these
SAM'S.
The IRRAS data given in Figure 5 also provide mental data available (including isotopic substitutions) convincing evidence of the degree of orientation of the have made it possible to reach a general agreement on viologen group in the SAM. These data show that, while assignment of most bands of the three redox forms of the 1637-cm-l band is the most intense band in the 1000methylviologen. These assignments are used as the basis 1800-cm-l region of the FTIR spectrum of the bulk sample, of the discussion below. it is relatively weaker in the S A M IRRAS spectrum. This The ready availability of strong Raman bands which band has been assigned as due to a BzUring mode by both have polarizability changes perpendicular to each other Cotton and co-workers21and Hester and co-workers.26This allows use of the surface selection ru1e33-36to estimate assignment defines the dipole moment change as along the orientation of the viologen ring system with respect the y-axis for the coordinate system shown in Figure 8. to the surface normal. Briefly, because of the relative Thus, the observation of a weak absorption for this mode enhancement (via surface plasmon excitation at the rough suggests that the y-axisof the molecule is roughly paallel surface) of the component of the optical field which is to the Au surface. This could be achieved with either perpendicular to the surface over that which is parallel flat-on or perpendicular orientation of the ring system (or t o the surface, those modes which produce polarizability any angle between). However, this result precludes edgechanges perpendicular to the surface are enhanced relative on orientations. to those which produce polarizability changes parallel to Taken together, the SERS and IRRAS data suggest that the surface. In favorable cases, it is possible to use this the viologen group is roughly perpendicular to the electrode effect to quantitativelycalculate molecular orientation^.^' surface. This is in accordance with expectations based on In the present case, the fact that these bands contain the saturation coverage of this molecule on smooth Au contributions from more than one mode (see below) surfaces, which has been previously found to be (3.5-3.9) precludes a detailed, quantitative orientational analysis. x mol cm-2.2,4 Other studies on similar selfHowever, it is possible to make qualitative statements assembling viologens with long alkyl chains which are about relative orientation, as has been done in the past not functionalized with groups capable of anchoring the in analyses of this type on monolayer and multilayer molecule to the s ~ r f a c e , as ~ -well ~ as Langmuir-Blodgett deposits of viologens at e l e c t r o d e ~ . ~ J ~ - ~ (LB) monolayers of long-chain viologen d e r i v a t i ~ e s , 5 ~ ~ ~ ~ ~ ~ For purposes of discussion, it is useful to impose a have reported similar coverages ((2-4) x mol cm-9 coordinate axis on the viologen molecule. Figure 8 shows With the exception of one LB monolayer study5 which the structure of a generalized viologen derivative, along reported a relatively low surface coverage of 2 x 10-lomol with an arbitrarily chosen axis system. The symmetry is cm-2, most studies have reported saturation surface taken to be Da,implying a coplanar arrangement of the coverages in the range of (3-4) x mol cm-2. This two rings, consistent with previous vibrational spectroran e yields an area per molecule in the range of ca. 41scopic s t ~ d i e s ~and J ~ X-ray - ~ ~ crystal s t r ~ c t u r e s . From ~ ~ - ~ ~ 55 2/molecule,which is consistent with the area expected the body of work described above, there is general for a perpendicular o r i e n t a t i ~ n . ~Thus, * ~ - ~these findings agreement that the band at 1169 cm-l is mainly derived all support a structure for saturated monolayers ofviologen from the 4 N-R stretch between the ring nitrogen and species bearing long alkyl chains in which the long axis the first methylene group in the chain. Thus, the of the ring system is roughly perpendicular to the electrode predominant component of the polarizability change for surface. this mode is along thex-axis ofthe coordinate system (the The SERS and IRRAS spectra also offer information on long axis of the molecule). The 1298-and 1637-cm-' bands the relative degree of order within the alkyl chain regions are 4 ring modes which are mainly derived from C-C of the viologen SAM's. In particular, the ratio (z289dh50) and C-N stretches for atoms within the ring system. It of the peak heights of the methylene asymmetric (2890 has been previously argued that, since the N-R stretch cm-l) and symmetric (2850 cm-') C-H stretching modes has its polarizability change directly along the x-axis, it which has been used to reveal the degree of order in should be more enhanced than either the 1298- or 1637polymethylene systems has proven useful in the present case. Snyder and co-workers1°have discussed the physical (33)Moskovits, M.;Suh, J. S. J . Phys. Chem. 1964,88,5526-30. meaning of this peak height ratio in considerable detail, (34)Moskovits, M. Rev. Mod. Phys. 1985,57,783-826. and make several important points about its use. First, (35)Creighton, J. A. S u ~ Sci. . 1983,124,209-19. (36)Creighton, J. A. Sur$ Sci. 1985,158,211-21. this ratio responds to both the environment (i.e., lateral (37)Jiang, Y.;Carron,K. To be submitted for publication toJ. Chem. packing) and conformational order. Second, since both Phys . (38)Russell,J.H.;Wallwork,S.C.ActaCrystaZlogr. 1972,B28,1527-
R
33. (39)Rout, C. K.;Murray-Rust, P. J . Chem. Soc. A lS69,1520-5.
(40)Lee, C.-W.; Bard, A. J . Chem. Phys. Lett. 1990,170,57-60. (41)Lee,C.-W.;Bard, A. J. J.Ekctroanul. Chem. 1988,239,441-6.
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Si43of ap-oxyphenyl sulfone derivative (i.e., CH~(CHZ)IIthe breadth and height of these peaks are variable with S O Z ( C ~ H ~ ) O ( C H Z ) ~ ~They S ~ Cused ~ ~ ) . a wide variety of these environmental and conformational effects, an acmethods to characterize these SAM's, including contact ceptable procedure is to use raw intensities at the positions angle measurements, FTIR, ellipsometry, surface potenof the peak centers (as opposed to using the heights or tial measurements, X P S , and SERS. The conclusions of integrated intensities of deconvoluted peaks). The previthese two complementary studies were that these molous studies using this approach have given reasonable ecules form relatively high quality SAM's which generally results, even under conditions where the peaks are exhibit spectroscopicand wetting behavior similar to that relatively obscured by virtue of the large number of Raman bands in this region of the spectrum.loJ1 With these of SAM's formed from the simpler n-alkyanethiols on Au. precautions in mind, we note that in the present case the In particular, the FTIR data were consistent with an increasingly more ordered environment for the alkyl similar values of this ratio and the very similar appearances of the C-H stretching envelope for the liquid chains as the length of the alkyl chain above the buried octanethiol and viologen S A M would seem to imply that polar group was increased. However, the data also the alkyl chains are highly disordered and in a liquid-like revealed that the SAM's with buried polar groups are not, environment. The IRRAS results also demonstrate a in general, as highly ordered as the n-alkanethiol SAM's. liquid-like environment for the alkyl chains, based on the Their findings are consistent with the conclusions of the positions of the symmetric and asymmetric methylene present and previous studies regarding the structure of C-H stretches. Taken in total, these results seem to the viologen SAM's, namely, that the ring system is clearly indicate a lack of conformational ordering of the reasonably well-oriented, and predominantly perpendicular to the surface a t saturation coverage. A caveat is that alkyl chains in these SAM's. the slightly larger size of the viologen ring system The lack of a highly ordered alkyl chain region in these (including its associated counterions) compared to the viologen SAM's seems reasonable given the fact that the polar ring systems studied by Ulman and co-workers projected area of the viologen group of these molecules precludes efficient packing of the alkyl chains and (ca. 41-55 A21molecule,depending on whether or not the adsorption of the all-trans conformation. counterions are included) is about twice that of the alkyl chains (ca. 18-20 &/molecule). The differencein projected Conclusions area of these moieties is readily apparent in the difference The picture which emerges from the present work is in the saturation coverage for the viologen SAM (ca. 4 x one in which the SAM's formed at saturation coverage mol cm+ 2, and for a n n-alkane thiol S A M (ca. 8 x from these viologen derivatives place the long axis of the mol cm-242). The fact that the larger viologen viologen ring systems roughly perpendicular to the determines the packing density (because of the size electrode surface, and have alkyl chain regions which are disparity) is likely to be a significant impediment to liquid-like. This abilityto impose orientation on the buried efficientpacking of the alkyl chains. This failure to achieve group implies that it should be possible for these and other closest-packingof the alkyl chains is undoubtedly the cause systems with embedded groups to control (and also of the spectral signatures for highly disordered alkyl chains systematically vary) a variety of structural parameters of which were observed. The results described for the C-S the S A M ,including the distance of the redox group from stretching region of octanethiol SAM's on rough Ag make the electrode surface or the monolayerlsolution interface, it clear that the surface roughness of the Ag SERS and the degree of exposure of the redox group to the substrates is not the cause of the poor degree of order of solution-phase environment. Control of this sort allows the alkyl chains in the viologen SAM's. for the fabrication of experimental systems designed to The data reported here for these viologen derivative probe the distance dependence ofvarious phenomena, such SAM's and those previously reported by others for the as electron transfer rates45)46 and the interfacial electric alkylviologen SAM's and LB monolayers can also be compared to two studies by Ulman and c o - w ~ r k e r sof~ ~ , ~ field ~ t r e n g t h . ~It' might also allow one to manipulate the optical properties of monolayer or multilayer films by monolayers formed from n-alkyl chains with polar aromanipulating the distance between the chromophores and matic groups embedded within the chain and having either the underlying surface and between the chromophore a trichlorosilane or thiol group for S A M formation on Si layers, a degree of control which may prove essential to or Au surfaces, respectively. Specifically, they studied any potential nonlinear optical applicationsbased on SAM assembly on Au4 of molecules with either ap-oxyphenyl technology. sulfone or p-oxyphenyl sulfide group embedded within the alkyl chain (Le., CH~(CHZ)~-~SO~(CGHG)O(CHZ)~SH Acknowledgment. We are grateful to the Office of or CH3(CH2)m-1S(CsHs)O(CH2)nSH,with the following m, Naval Research for full support of this work. We n combinations, 12,4; 8,8; 4,12; 11,5),and assembly on acknowledge Professor Carron for helpful discussions and a preprint. (42)Widrig, C. A.;Chung, C.; Porter, M. D. J . Electroanal. Chem. 1991, 310, 335-59. (45)Chidsey, C. E.D. Science 1991,251, 919-22. (43) Tillman, N.; Ulman, A,; Elman, J. F. Langmuir 1990,6, 1512-
8. _.
(44)Evans, S .D.; Urankar, E.; Ulman, A,; Ferris, N. J.Am. Chem. SOC.1991, 113, 4121-31.
(46) Finklea, H.0.;Hanshew, D. D. J.Am. Chem. SOC.1992,114, 3173-81. (47)Pope, J. M.;Tan, Z.; Kimbrell, S.; Buttry, D. A. J . Am. Chem. SOC.1992,114, 10085-6.
