Effects of Ring Substitution on the Binding and Oxidation of

Apr 25, 1996 - Department of Chemistry, University of Illinois, 600 S. Mathews, ... Claire L Forryan , Nathan S Lawrence , Neil V Rees , Richard G Com...
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J. Phys. Chem. 1996, 100, 7204-7211

Effects of Ring Substitution on the Binding and Oxidation of Cyanophenols on Au(111) Electrodes Katherine M. Richard and Andrew A. Gewirth* Department of Chemistry, UniVersity of Illinois, 600 S. Mathews, Urbana, Illinois 61801 ReceiVed: December 13, 1995; In Final Form: February 2, 1996X

The behavior of 2-, 3-, and 4-cyanophenol adsorbed on Au(111) has been studied using electrochemistry, potential difference FTIR, and in situ scanning tunneling microscopy (STM). Cyclic voltammetry displays a decreased oxidation current for cyanophenols substituted at the 2 or 4 positions on the ring. Surface FTIR bands are observed for 4-cyanophenol, while the 2- and 3-substituted molecules display no bands in the ring-stretching region. STM shows that 4-cyanophenol forms an ordered structure similar to that seen for phenol, while 3-cyanophenol does not appear to adopt an ordered structure on the Au(111) surface. These different behaviors relate to the different packing abilities of the differently substituted cyanophenols.

I. Introduction There is currently widespread interest in the formation of ordered arrays of organic molecules on metal surfaces, especially concerning self-assembled monolayers (SAMs) of thiolates on gold surfaces.1 Understanding of the behavior of organic molecules on surfaces can be applied to a wide range of problems, from corrosion inhibition2 to oxidation of environmental contaminants.3 The extensive studies of SAMs include the impact of substrate on monolayer properties and structure4 and the impact of the constituent molecules on the formation and properties of the monolayer film.5 These studies have shown that well-ordered and oxide-free surfaces allow for the formation of reproducible monolayers, but that intermolecular interactions are important as well. Three of the apparent conditions for ordered monolayers are (1) a chemically stable surface, (2) a strong specific interaction between the molecule and substrate, and (3) dense packing between adsorbate molecules.6 Perturbation of ordered arrays with larger molecules is of interest for increasing the understanding of the factors that control assembly and order. Kaifer has performed several studies of larger molecules, including catenanes, functionalized to self-assemble on gold surfaces.7 There has also been extensive work completed on ordered systems in the UHV environment8 toward understanding the driving factors for molecules to order on surfaces. Coincident with the interest in conditions for ordered arrays is an interest in understanding how molecular structure affects reactivity. Recent studies have elucidated the impact of functionality on reactivity by comparing the oxidation of methanol, formic acid, and carbon monoxide on Pt-Ru alloys.9 Early work using scanning tunneling microscopy STM in situ focused on metal atoms,10 but recently this focus has been extended to organic molecules, as there is a better understanding about how to form ordered arrays of organic molecules. Organic molecules are difficult to investigate using X-ray techniques, due to the small masses of the constituent atoms. X-ray techniques are also unable to observe the interesting molecular transformations that organic molecules undergo. Ring-containing molecules have been adsorbed onto a variety of surfaces and imaged using scanning tunneling microscopy (STM). The structures of amino acids on graphite have been observed to orient flat both prior to and during the course of oxidation.11 X

Abstract published in AdVance ACS Abstracts, April 1, 1996.

0022-3654/96/20100-7204$12.00/0

Conversely, 2,2′-bipyridine has been shown to orient on end to graphite surfaces, with packing between rings of adjacent molecules as the driving force for ordering.12 The amino acids on gold and silver single crystals are observed to undergo changes in orientation and strength of binding as a function of potential.13 Modification of the Au(111) electrode with iodine allowed the observation of an ordered porphyrin array that was not observable on bare Au(111).14 Combining STM with potential-difference infrared spectroscopy (PDIR) has allowed for a greater understanding of the chemical, as well as structural, characteristics of surface species. The STM provides high-resolution structural information but chemical identification is only derived from shape, which can be ambiguous. IR provides information about surface vibrations and orientations but little information about lateral order. Carbon monoxide on Pt(111)15 and sulfate16 and thiols17 on Au(111) have been studied using this combination of techniques. We have previously reported the use of PDIR and STM to study the binding and initial stages of oxidation of phenol on Au(111).18 We are interested in further investigating the conditions which allow for or prevent the formation of ordered molecular arrays, as well as the impact of order on reactivity. In this study, we have substituted a CN group onto the ring as a spectral probe in hopes of gaining additional information about the tilt of the molecule with respect to the surface normal. We now report the effect of ring substitution on the binding and oxidation of 2-, 3-, and 4-cyanophenol studied by electrochemistry, surface Fourier-transform infrared spectroscopy (FTIR), and STM. We observed differences in behavior depending on the location of the cyanide on the ring. II. Experimental Section Cyclic voltammagrams were obtained using a Pine AFRDE-5 potentiostat in a two-compartment, glass electrochemical cell. A Ag/AgCl electrode in one compartment was connected via a capillary bridge to the other compartment of the cell serving as the reference electrode. The counter electrode was a gold wire. The working electrode was a Au(111) single crystal (Monocrystals, Cleveland, OH) characterized by Laue´ back-scattering performed at the Center for the Microanalysis of Materials at the University of Illinois. The crystal was placed in the cell in a hanging meniscus configuration. Solutions were prepared from 2-, 3-, and 4-cyanophenol (Aldrich), NaOH (EM Science), and purified water (Millipore-Q, Millipore Inc.). Solutions were © 1996 American Chemical Society

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deoxygenated prior to use and an Ar flow was maintained over the solution during potential cycling. All potentials in this paper are reported relative to NHE. Infrared spectra were collected on a Nicolet Magna-IR System 550 spectrophotometer, with optics modified for surface studies as described previously.18 The electrochemical sample cell was made of glass and the Ag/AgCl reference electrode was connected to the cell via a capillary bridge. The Au wire counter electrode was placed into the cell through a ground glass joint. A CaF2 trapezoidal prism (Wilmad Glass, NJ) was attached to the cell via a metal holder and sealed with a teflon-encapsulated O-ring. The Au(111) single-crystal working electrode was pressed against the prism with a plunger and maintained in tension against this surface with rubber bands. Spectra were collected with 8 cm-1 resolution. A total of 75 scans were taken at the background potential of -730 mV before the potential was stepped to the sample potential and an additional 75 scans were collected. This process was repeated until 750 scans were taken at each sample potential. Images were obtained with a Nanoscope II STM (Digital Instruments, Santa Barbara, CA) equipped with a bipotentiostat. STM tips were Pt/Ir and were coated with polyethylene to insulate from faradaic background. The substrate for imaging was again a Au(111) single crystal and the counter electrode was a gold wire. The reference electrode for STM was a silver wire which had a potential of -190 mV vs NHE in these solutions. III. Results A. Cyclic Voltammetry. Figure 1 shows the effect of 2-cyanophenol (Figure 1A), 3-cyanophenol (Figure 1B), and 4-cyanophenol (Figure 1C) on the cyclic voltammetry for Au(111) in a solution containing 0.1 M NaOH. In solutions without added cyanophenol (dotted line in all segments of Figure 1), the scan is essentially flat from the negative limit until the “OH” peak at +325 mV which has been ascribed to potentialinduced hydroxide adsorption.19 Surface-enhanced Raman spectroscopy has shown a distinction in surface-oxygen stretching frequencies of the surface oxide and the hydroxide adsorbed prior to oxide formation.20 The two other anodic peaks, A1 and A2, at +575 and +820 mV are due to oxide formation while the cathodic peak at +310 mV results from stripping of the oxide. The voltammagram in the presence of 1 × 10-4 M 2-cyanophenol (Figure 1A) is similar to that for the blank, with a few changes. The “OH” peak is decreased in size and shifted to slightly more positive potentials and a shoulder at +565 mV, labeled A2CN, appears on the side of peak A1. Peak A1 is also shifted 40 mV positive of its position in the blank solution. Scanning the potential positive just to the shoulder of this peak and no further results in the loss of the oxide stripping peak. On the basis of this behavior, we attribute the shoulder to the irreversible oxidation of 2-cyanophenol. A decrease in the size of the oxide stripping peak in the presence of 2-cyanophenol indicates that the oxidation products are remaining on the surface and interfering with normal electrode processes. The changes in the voltammetry due to the addition of 3-cyanophenol (Figure 1B) are more dramatic. The “OH” peak is shifted 50 mV negative relative to its position in solutions without added cyanophenols. The peak at +580 mV is greatly increased in size and occurs at a more negative potential than observed with cyanophenol-free solutions. Varying the positive scan limit of the voltammetry reveals that the initial portion of this peak is due to the oxidation of 3-cyanophenol, while the

Figure 1. Cyclic voltammetry of Au(111) in solutions containing 0.1 M NaOH and (A) 2-cyanophenol, (B) 3-cyanophenol, and (C) 4-cyanophenol. Concentrations: s ) 1 × 10-4 M; ‚‚‚ ) 0 M.

latter part includes gold oxide formation. The peak is therefore labeled A3CN + A1. The oxide stripping peak is also reduced in size. Voltammetry in the presence of 4-cyanophenol (Figure 1C) is nearly identical to that observed for 2-cyanophenol. The “OH” peak is shifted slightly positive and 4-cyanophenol oxidation appears as a shoulder(A4CN) on peak A1. Figure 2 shows the double-layer region of the voltammagrams in Figure 1. The dotted line in all parts of Figure 2 is the voltammetry in the presence of 0.1 M NaOH without added cyanophenol. In the presence of 2-cyanophenol (Figure 2A), the current passed at the electrode begins to increase at -250 mV vs that passed in the blank solution. This increased current in the capacitive region of the voltammetry indicates that an adsorbate is becoming associated with the surface. The replacement of solvent molecules with adsorbates causes an increase in the amount of charge present on the surface and results in an increase of the capacitive current.21 The same type of behavior can be seen for 3-cyanophenol (Figure 2B), except that the

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Figure 2. Cyclic volatammetry of the double layer region of Au(111) in solutions containing 0.1 M NaOH and (A) 2-cyanophenol, (B) 3-cyanophenol, and (C) 4-cyanophenol. Concentrations: s ) 1 × 10-4 M. ‚‚‚ ) 0 M.

increased current begins at -325 mV. The greatly increased current at the positive end of this voltammetry is due to the early onset of the “OH” peak. 4-Cyanophenol (Figure 2C) displays similar behavior to 2-cyanophenol, with a very slight increase in the capacitive current beginning at -250 mV. B. Infrared Spectroscopy. We have previously reported the infrared spectra obtained in 0.1 M NaOH without added organic molecules at several potentials in the double-layer region.18 No features of interest are observed in the energy region 1500-1200 wavenumbers. Figure 3 shows the spectra obtained from Au(111) in solutions containing 0.1 M NaOH and 1 × 10-4 M 2-cyanophenol (Figure 3A), 3-cyanophenol (Figure 3B), or 4-cyanophenol (Figure 3C). All spectra are referenced to backgrounds taken at -730 mV, a potential at which capacitance measurements show nothing is specifically adsorbed to the electrode.22 Additional evidence for the lack of adsorbed material at the reference potential is the identical current in the negative end of the double layer voltammetry for solutions both with and without cyanophenols. Identical capacitive current to that observed in the blank solution indicates that the metal-solvent interactions have not yet been disrupted by adsorbates.21 Figure 3A shows the PDIR spectra for 2-cyanophenol, while Figure 3B shows the PDIR spectra for 3-cyanophenol. No bands are observed in the presence of 2- and 3-cyanophenol. In the case of 4-cyanophenol, however, two bands appear as the spectra are taken at more positive potentials. Band 1 is a

Richard and Gewirth positive-going band at 1395 cm-1 that begins to appear in spectra taken at -200 mV, and grows in size as the sample potential is swept more positive. It has a full width at half maximum (FWHM) of 30 cm-1 and increases to three times its original area over the sampled potential range. Band 2 is a negative-going band at 1285 cm-1 that is apparent at potentials at and above -400 mV and increases in magnitude as the potential moves positive. This band has a FWHM of 25 cm-1. As with the other two cyanophenols, no bands are apparent in the C-N stretching region (∼2240 cm-1). In order to assign the bands in the PDIR spectra of 4-cyanophenol, we must consider both their energy and the surface selection rule. The surface selection rule states that only modes which transform the same as the surface normal under the operations of the point group of the system will exhibit IR intensity. In the Nujol spectrum23 of 4-cyanophenol there are several bands in this general energy region. First, the spectrum displays a relatively intense band in the energy region 1380-1400 cm-1, which is a composite of asymmetric ring modes and transforms as b1 + b2 under C2V symmetry. Additional bands in the this energy region include C-H in-plane bends at 1250-1307 cm-1 (b2 under C2V), and the aryl-O stretch at 1230-1282 cm-1 (a1 under C2V). Other bands in the Nujol spectrum are of appropriate symmetry (a1 under C2V) to be considered, such as the ringbreathing mode and C-H stretches, but are of inappropriate energy to fit well with our observed bands. In order for the solution symmetry of C2V for 4-cyanophenol to be maintained, the molecule would have to orient exactly normal to the surface. If this were the case, only bands which transform as a1 in solution would be allowed upon adsorption of 4-cyanophenol to the electrode surface. However, it would also create an unrealistic surface-O-C angle of 180°. A more realistic tetrahedral geometry around the O atom would result in a reduction of symmetry to Cs, under which the surface normal transforms as a′. The reduction in symmetry allows for some forbidden bands in solution to become allowed once the molecule adsorbs to the electrode surface. The C-H in-plane bending mode corresponding to ν3 is found in the energy region 1250-1307 cm-1 for para-substituted phenols, but has been found to be fairly weak and is rarely observed.26,27 It transforms as a′′ on the surface, which would make this band forbidden by the surface selection rule. The substituent-sensitive aryl-O band is found at 1230 cm-1 for unsubstituted phenol,24 but shifts to higher energy when the para position on the ring is substituted. Phenols with electron-withdrawing groups (like -CN) have this band at the highest energy of all phenols, placing it above 1280 cm-1 in energy.27 This band transforms as a′ on the surface under Cs symmetry. However, as the Au-O-C bond decreases from 180°, the projection of this mode on the surface normal would become smaller and the intensity of the band would decrease.25 On the basis of energy match and a similar FWHM, we assign band 1 to asymmetric CdC ring modes, specifically normal mode ν14 or ν19b (Wilson numbering). The asymmetric ring modes transform as b1 + b2 under C2V symmetry. Upon adsorbing to the surface these modes transform as a′ + a′′, and become allowed under the surface selection rule. The band for ν19b is stronger than that for ν19a when strong electronwithdrawing substituents are found on the ring.26 Band 1 falls into the energy region expected for this mode (1370-1470 cm-1). A study of various para-substituted phenols attributes the peak of this energy to ν14.27 The positive-going nature of band 1 indicates it is due to a band that becomes allowed upon

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Figure 3. PDIR spectra of Au(111) in solutions containing 0.1 M NaOH and 1 × 10-4 M added cyanophenol at several potentials within the double layer region of the voltammetry: (A) 2-cyanophenol, (B) 3-cyanophenol, and (C) 4-cyanophenol. The scale is the same for all three sets of spectra.

Figure 4. STM images of Au(111) in a solution containing 0.1 M NaOH and 1 × 10-4 M 3-cyanophenol: (A) 100 × 100 nm image of the (23 x x3) reconstruction at -60 mV; Itip ) 5 nA; Etip ) 30 mV. (B) 160 × 160 nm image of the surface at +190 mV, following gold oxide formation and stripping; Itip ) 5 nA; Etip ) 30 mV.

adsorption to the electrode surface. This fits with the symmetry characteristics of this band. The negative-going intensity for band 2 is due to the loss of an IR active species from solution. Band 2 is likely due to the loss of intensity for the aryl-O band as the 4-cyanophenol adsorbs to the electrode surface. The band is allowed in solution and remains allowed on the surface. However, intensity is lost due to the tilting of the aryl-O bond away from the surface normal upon adsorption. We can compare the PDIR spectra for 4-cyanophenol to those previously reported for unsubstituted phenol.18 The band 1 for 4-cyanophenol is narrower than the similar band in the phenol spectra. This could be due to the smaller number of contributing modes in the 4-cyanophenol. Both bands are shifted in energy from their frequencies in phenol-containing solutions. Band 1, due to ring modes, is shifted to slightly lower energy due to the change in substitution on the ring. Band 2, attributed to the aryl-O stretch, is shifted to higher energy due to the presence of the strongly electron-withdrawing CN group on the ring. C. Scanning Tunneling Microscopy. Figure 4 shows STM images of Au(111) obtained in a solution containing 0.1 M

NaOH and 1 × 10-4 M 3-cyanophenol. Figure 4A was taken at -60 mV, a potential where the double-layer voltammetry in Figure 2 suggests that 3-cyanophenol is already associated with the surface. The image shows the well-known (23 × x3) Au(111) reconstruction which is known to lift in the presence of strongly adsorbing species. We are unable to image the 3-cyanophenol on a molecular level at this potential. Upon sweeping to more positive potentials, the reconstruction is obscured by adsorbing material, but no order is apparent. Figure 4B shows the electrode surface at +190 mV following gold oxide formation and stripping. Pits and islands of monoatomic height can be observed, consistent with previous STM studies.28 The formation of pits is due to the Au-O rearrangements that take place during the formation and stripping of gold oxide. Figure 5 shows STM images of Au(111) in the presence of 0.1 M NaOH and 1 × 10-4 M 4-cyanophenol. Figure 5A shows (x3 x x3)R30° overlayer structure of phenol on Au(111) obtained in a solution containing 0.1 M NaClO4 with NaOH added to pH 9.3 + 1 × 10-4 M phenol (included for completeness). The reconstruction of Au(111) can be observed

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Figure 5. STM images of Au(111) in a solution containing: (A) 0.1 M NaClO4 (pH ) 9.3) and 1 × 10-4 M phenol, (B and D) 0.1 M NaOH and 1 × 10-4 M 4-cyanophenol. (A) 4.5 × 4.5 nm image of the (x3 × x3)R30° phenoxide overlayer structure; Itip ) 2 nA; Etip ) 28 mV. (B) 4.5 × 4.5 nm unfiltered image of the 4-cyanophenol overlayer at +90 mV; Itip ) 5 nA; Etip ) 30 mV. (C) The two-dimensional fast Fourier transform for B, showing two sets of spots rotated ∼27° with respect to each other. The spots due to the underlying gold are circled, while those due to the overlayer are indicated with arrows. (D) 4.5 × 4.5 nm filterd image of the 4-cyanophenol overlayer. Spacing is 0.49 ( 0.03 nm and lattice is rotated ∼27° with respect to the underlying gold lattice. This overlayer is close to the (x3 x x3)R30° structure.

at -60 mV in solutions containing 4-cyanophenol, but sweeping to more positive potentials results in the adsorption of an ordered 4-cyanophenol overlayer. Figure 5B shows an unfiltered image of the overlayer structure of 4-cyanophenol on Au(111) at +90 mV. The observed spacing is 0.49 ( 0.03 nm in a slightly distorted hexagonal lattice. A two-dimensional Fouriertransform of this image reveals two sets of spots rotated ∼27° with respect to each other (Figure 5C). One set of spots at 0.26 ( 0.02 nm is due to the underlying gold, while the other set at 0.43 ( 0.03 nm is due to the 4-cyanophenol overlayer. A filtered version of this image is shown in Figure 5D. We therefore describe the structure of 4-cyanophenol on Au(111) as a (x3 x x3)R30° overlayer. This structure has been seen previously with STM for unsubstituted phenol.18 IV. Discussion These results provide insight into some of the conditions required to order phenoxides on surfaces and also the correlation between these surface structures and reactivity. We compare the behavior of the cyanophenols to phenol to note the effect of ring substitution on the binding and reactivity of the molecules. For both phenol and 4-cyanophenol, we observe ordered structures on the electrode surface. 2-cyanophenol and 3-cyanophenol do not show any evidence of order upon adsorption to the electrode. Additionally, we have observed a

difference in the reactivity of the molecules as a function of substitution. Both 2- and 4-cyanophenol display inhibited oxidation, while phenol and 3-cyanophenol do not. The conditions for order and the conditions for reactivity are therefore not necessarily related. A. Ordered Molecular Association. We have previously reported18 evidence that phenol orients to the electrode surface through the deprotonated O atom. We expected similar behavior for the cyanophenols, as the Au-O interaction should be considerably stronger than any putative CN(π)-Au interaction. However, only the 4-cyanophenol displays active bands in the IR spectra. The bands we observe are due to the increase of intensity for asymmetric ring modes upon binding to the electrode surface and a loss of intensity in the aryl-O band due to tilting of the molecule away from the surface normal upon binding to the electrode. Infrared studies of the adsorption of benzonitrile onto Au(poly) from acidic solutions indicate that benzonitrile is oriented normal to the electrode, bound through the lone pair on the nitrogen.29 This is apparently not the case for 4-cyanophenol. Our observation of bands due to asymmetric ring modes contraindicates a normal orientation, as these bands would be forbidden by the surface selection rule in such a geometry. If the molecule were bound normal through the CN group, the aryl-O stretch would also lie along the surface normal and not

Cyanophenols on Au(111) Electrodes

Figure 6. A space-filling model of a (x3 x x3)R30° overlayer of 4-cyanophenol on Au(111) created using Cerius2. The identities of the atoms are labeled in the figure.

suffer from a loss of intensity upon binding. We observe a loss of intensity for the aryl-O band, further evidence that 4-cyanophenol is oriented to the electrode surface through the O atom and is tilted away from the surface normal. Infrared studies of cyanide on Pt30 and Au31 surfaces show the CN group bound normal to the electrode surface, with a derivative-shaped band at ∼2100 cm-1 for both studies. The cyanide group on 4-cyanophenol in our proposed orientation would still have a component of the stretch along the surface normal, rendering it IR active. However, we do not observe a CN band for any of the cyanophenols. The fact that the CN stretch is not observed is not surprising when the changes in the CN region with changing spectral conditions are considered. The Nujol spectrum23 of 4-cyanophenol displays a very sharp peak at 2234 cm-1 which has a larger magnitude than any other bands. In the vapor spectrum of the molecule,32 the peak does not shift but decreases in size to about one-third the size of the aromatic ring stretches. This decrease in relative intensity may be due to a lack of hydrogen bonding and concerted dipoles which may be found in the solid state but are surely lacking in the isolated molecules of the vapor phase. Although the CN band should be allowed by surface selection, it is not observable in our experiment due to insufficient intensity to distinguish it from background noise. STM images show a (x3 x x3)R30° overlayer in solutions containing 4-cyanophenol. This structure is consistent with the IR data which indicates an end-on orientation of the molecule to the surface. A space-filling model of the (x3 x x3)R30° overlayer was created using Cerius2 on a Silicon Graphics computer, as shown in Figure 6. The model includes van der Waals, atomic, covalent, and metallic radii in the atomic attributes. The 4-cyanophenol molecules are packed very closely together, with the CN group extending away from the surface. The model yields a structure consistent with that observed with STM. 2- and 3-cyanophenol do not display any IR active bands in our experiment. The molecules could be orienting with the CN (and the ring) parallel to the surface, as has been observed for 4-methoxybenzyl cyanide on Au(111) when adsorbed from ethanol.33 In the flat orientation, no ring modes would be observed and the CN stretch would be absent as well. Alternatively, the molecules could form a disordered overlayer which would result in the magnitudes of the IR active bands to be one-third the height observed for 4-cyanophenol. The signal-

J. Phys. Chem., Vol. 100, No. 17, 1996 7209 to-noise ratio for our experiment is not sufficient to distinguish between these two possibilities. We are able to observe the reconstructed gold surface at potentials where the double-layer voltammetry in Figure 2 indicates the cyanophenols are already associated with the surface. Studies of the effect of adsorbates on the reconstruction of Au(111) have shown that the reconstruction lifts in the presence of strongly binding adsorbates, like Br or OH.34 The presence of weakly binding adsorbates, like pyridine, does not cause the reconstruction to lift.35 Capacitance studies using benzonitrile36 and phenol22 have found that the association between these molecules and the surface is weak and dominated by electrostatic interactions. Our observation of the reconstruction in the presence of the cyanophenols is likely due to the fact that the molecules bind only weakly and do not perturb the electronic structure of the surface sufficiently to lift the reconstruction. When the potential is swept positive into gold oxidation and back past the oxide stripping peak in the presence of 3-cyanophenol, behavior is observed that is similar to that observed in the blank solution.28 The electrode surface appears roughened due to the formation of pits and islands corresponding in height to a monoatomic layer of gold. These pits and islands form as a result of the Au-O place exchange reactions that occur during the formation and stripping of gold oxide. The voltammetry shows that oxidation of the 3-cyanophenol is occurring at the same potentials as the gold oxide formation, but the electrode is not completely passivated by the polymer formed. The amount of polymer formed during the brief excursion into oxidation is apparently not enough to interfere with imaging. We have observed that large amounts of organic material on electrode surfaces is deleterious to facile imaging.18 In order to understand why the 4-cyanophenol forms an ordered overlayer on the Au(111) surface and 2- and 3-cyanophenol do not, we must take into account both electronic and steric considerations. The CN group on a phenol is electronwithdrawing, deactivating, and meta-directing.39 Substitution in the 2 and 4 positions would have essentially the same effect on the pKa, with the substitution at the 3 position causing a slightly different pKa. If the reasons for order were purely electronic, we would expect the same ordering behavior from 2- and 4-cyanophenol. Instead, 4-cyanophenol is unique among the three molecules in that it forms an ordered overlayer. Surface-molecule interactions and ring stacking between the molecules preferentially create a (x3 x x3)R30° structure for phenoxides on Au(111). The model in Figure 6 shows how closely packed the 4-cyanophenol molecules must be to fit into the observed structure. We can consider the replacement of a proton with a CN group at other positions on the ring to discern the impact of the substitution on the (x3 x x3)R30° structure. There is no conformation that 2-cyanophenol can adopt that allows the (x3 x x3)R30° structure to form. Free rotation around the aryl-O axis would direct the CN group close to the surface or within the overlayer, creating steric repulsions with the surface or with another molecule, respectively. 3-cyanophenol could also rotate about this same axis, resulting in the CN pointing within the overlayer or above the overlayer. If the CN group is pointed within the layer, the 3-cyanophenol would have steric repulsions with other molecules. If the CN group is directed above the layer, the ordering behavior would be the same as for 4-cyanophenol. However, this would require one specific orientation of the 3-cyanophenol ring, while the

7210 J. Phys. Chem., Vol. 100, No. 17, 1996 4-cyanophenol ring has two orientations which allow order. Hence, the 3-cyanophenol may not order because of the increased entropic barrier to ordering. When steric constraints keep the 2- or 3-cyanophenol molecule from forming this preferred structure, there is inherently less order in the overlayer. Similar results have been seen when the alkyl chains of thiols are replaced by cholesterol groups. The resultant monolayers are less ordered due to the more sterically bulky cholesterol subunits.37 Additionally, the increased rigidity of ring-containing molecules causes the formation of the monolayers to be kinetically slower than those formed from alkyl mercaptans.5 The lack of bands in the IR and the lack of order in the STM can be attributed to steric constraints preventing the binding of the 2- and 3-cyanophenol in its preferred structure. The inability of the molecule to adopt its preferred structure requires it to orient to the surface in another wayseither flat or disordered, as mentioned previously. B. Reactivity. 3-Cyanophenol displays voltammetry nearly identical to that previously reported18 for unsubstituted phenol in 0.1 M NaOH. The shifting of the “OH” peak to more negative potentials is indicative of a stabilization of the surface hydroxide with the addition of 3-cyanophenol. The oxidation of 3-cyanophenol does not appear to be hampered by the addition of the cyanide moiety to the ring in the meta position. In the case of 2- and 4-cyanophenol, the “OH” peak is shifted to more positive potentials, indicating a destabilization of the surface hydroxide relative to the blank. Additionally, the oxidation current for both 2- and 4-cyanophenol is significantly decreased. In order to understand this reduction in oxidation current, we must consider both the steric and electronic effects of the addition of the CN group to the molecule. From the perspective of steric considerations, it is important to note that phenol polymerizes through the ortho and para positions on the ring.38 The blockage of one of these positions (2, 4, and 6) with a cyanide group would therefore interfere with the polymerization of the molecule. Substitution in the 3 position leaves all polymerization sites open and therefore would not inhibit the oxidative coupling of the molecule. Electronically, substituents have little impact on the acidity of phenols and therefore the reactivity of the O atom. A notable exception to this is the presence of strongly electronwithdrawing groups, such as CN and NO2, in the ortho or para positions on the ring.39 Electron-withdrawing groups in these positions stabilize the phenoxide ion by bearing a part of the negative charge. Electron-withdrawing groups in the meta position stabilize the phenoxide to lesser extent. The decreased amount of oxidation current for the 2- and 4-cyanophenol is likely due to a combination of steric and electronic effects, which block polymerization sites and stabilize the individual phenoxide ions. The shift of the “OH” peak relative to the blank may also be explained by the electronic effects of substituting electronwithdrawing groups into the 2 or 4 positions on the ring. The unsubstituted phenol and 3-cyanophenol both display the “OH” peak shifted to more negative potentials than observed in solutions without added organics. This may be due to a coadsorption phenomenon involving hydrogen bonding between the organic molecule and solution hydroxide. The presence of the CN in the 2 or 4 position serves to remove electron density from the O atom and may result in a decrease in hydrogen bonding. The smaller amount of hydroxide associated with the adsorbing molecules causes a positive shift in the “OH” peak.

Richard and Gewirth V. Conclusions We have observed differences in the binding and oxidation of cyanophenols depending on the ring position of their substitution. 2-Cyanophenol is hindered both in ordered binding and in electrooxidation. 3-Cyanophenol oxidizes in manner similar to unsubstituted phenol, but does not form an ordered overlayer upon binding to the Au(111) surface. 4-Cyanophenol forms an ordered overlayer on Au(111), confirmed with STM and FTIR, that is similar to that seen for phenol. The oxidation of 4-cyanophenol is inhibited, however, by the effects of cyanide substitution in the para position on the ring. These results show that assembly of molecules can be controlled by steric interactions between molecules. Additionally, the combination of STM and IR has again served as a powerful tool for both providing both chemical identification and structural information about surface adlayers. Acknowledgment. We thank Brian Niece for performing Laue´ measurements which were obtained at the Center for the Microanalysis of Materials, University of Illinois, which is supported by the Department of Energy under Contract DEFG02-91ER45349. A.A.G. acknowledges a Presidential Young Investigator Award (CHE-90-57953) and an A. P. Sloan Research Fellowship. This work was funded by the NSF (DMR-89-20538) through the Materials Research Laboratory at the University of Illinois. References and Notes (1) Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43, 437 and references therein. (2) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022. (3) Kummert, R.; Stumm, W. J. J. Colloid Interface Sci. 1980, 75, 373. (4) (a) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. T.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (b) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Bucholz, S.; Whitesides, G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813. (5) Tour, J. M.; Jones, L.; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. M.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529. (6) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (7) (a) Lu, T.; Zhang, L.; Gokel, G. W.; Kaifer, A. E. J. Am .Chem. Soc. 1993, 115, 2542. (b) Zhang, L.; Lu, T.; Gokel, G. W.; Kaifer, A. E. Langmuir 1993, 9, 786. (c) Rojas, M. T.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 5883. (8) Hubbard, A. T. Chem. ReV. 1988, 88, 633. (9) (a) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. Electrochim. Acta 1994, 39, 1825. (b) Markovic, N.; Gasteiger, H. A.; Ross, P. N.; Jiang, X.; Villegas, I.; Weaver, M. J. Electrochim. Acta 1995, 40, 91. (10) Staikov, G.; Lorenz, W. J. in Nanoscale Probes of the Solid/Liquid Interface, Gewirth, A. A.; Siegenthaler, H., Eds., Kluwer Academic: Dordrecht, 1995; chapter 11 and the references therein. (11) (a) Tao, N. J.; Shi, Z. Surf. Sci. 1994, 321, L149. (b) Tao, N. J.; Shi, Z. J. Phys. Chem. 1994, 98, 7422. (12) Cunha, F.; Tao, N. J. Phys. ReV. Lett. 1995, 75, 2376. (13) (a) Ho¨lzle, M. H.; Wandlowski, T.; Kolb, D. M. Surf. Sci. 1995, 335, 281. (b) Ho¨lzle, M. H.; Krznaric, D.; Kolb, D. M. J. Electroanal. Chem. 1995, 386, 235. (14) Kunitake, M.; Batina, N.; Itaya, K. Langmuir 1995, 11, 2337. (15) (a) Yau, S.-L.; Gao, X.; Chang, S.-C.; Schardt, B. C.; Weaver, M. J. J. Am. Chem. Soc. 1991, 113, 6049. (b) Vitus, C. M.; Chang, S.-C.; Schardt, B. C.; Weaver, M. J. J. Phys. Chem. 1994, 95, 7559. (16) Edens, G. J.; Gao, X.; Weaver, M. J. J. Electroanal. Chem. 1994, 375, 357. (17) Alves, C. A.; Porter, M. D. Langmuir 1993, 9, 3507. (18) Richard, K. M.; Gewirth, A. A. J. Phys. Chem. 1995, 99, 12288. (19) Hamelin, A.; Sottomayer, M. J.; Silva, F.; Chang, S.-C.; Weaver, M. J. J. Electroanal. Chem. 1990, 295, 291. (20) Desilvestro, J.; Weaver, M. J. J. Electroanal. Chem. 1986, 209, 377.

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