9820
J. Phys. Chem. B 1998, 102, 9820-9824
Self-Assembled Monolayers on Gold of Thiols Incorporating Conjugated Terminal Groups† Scott Reese and Marye Anne Fox*,‡ Department of Chemistry and Biochemistry, UniVersity of Texas at Austin, Austin, Texas 78712 ReceiVed: May 5, 1998; In Final Form: October 8, 1998
Thiols 3, 6, and 14 incorporating ω-attached conjugated aromatic groups form self-assembled monolayers (SAMs) on polycrystalline gold. The thicknesses of the films as measured by optical ellipsometry correspond well to the expected thickness of monolayer films in which the components are oriented roughly perpendicular to the Au surface. Cyclic voltammetry measurements of K4Fe(CN)6 in aqueous KCl solution on a SAM modified gold electrode show that these monolayers are densely packed and attenuate electron transfer between the gold and the solution electrophore. Grazing angle reflectance-Fourier transform infrared spectroscopy (GAR-FTIR) shows that the conjugated arenes of 3, 6, and 14, as well as the C6 chains of thiols 6 and 14, are roughly perpendicular to the plane of the Au surface. Fluorescence spectroscopy indicates that the terminal arenes in SAMs of 6 and of 11 interact, causing red-shifted, structured emission in the SAM relative to that observed for the same species in dilute homogeneous solution.
Introduction Self-assembled monolayers (SAMs) of thiols on gold have received considerable attention in recent years.1-4 The high degree of control over the structure and composition of surfaces makes these systems an ideal environment for the study of interfacial phenomena in areas such as wetting,5-7 tribology,8 and electrochemistry.9 The synthetic accessibility of organic thiols also makes it possible to easily anchor complex molecules to a surface for the study of phenomena such as molecular recognition10 or catalysis.11 Because of their rich electronic and photonic properties, conjugated organic molecules are ideal candidates as probe components for the structure of SAMs that may find potential applications in optoelectronic devices.12 Several SAMs incorporating conjugated thiols have been reported.13-16 As the thiols incorporated into SAMs become more complex, a better understanding of the intermolecular interactions and their effect on the stability and morphology of these systems can be attained, permitting a delineation of the additional stabilizing forces operative in these functionalized monolayers beyond those encountered in simpler alkanethiols. We recently reported that SAMs of trans-4-(3-thiopropoxy)stilbene on gold blocked electron transfer between the electrode and a redox species in both aqueous and nonaqueous media more efficiently than did a SAM of dodecanethiol.17 We assigned the high passivating capacity of these SAMs to π-π interactions between neighboring conjugated subunits. Herein, we report the synthesis and characterization of thiols 3, 6, and 14 bearing terminal conjugated arenes that spontaneously self-assemble on polycrystalline gold to form monolayers. Electrochemical measurements of these SAMs indicate that they are densely packed and defect free. Furthermore, the chromophores in monolayers of 6 and 14 exhibit a high degree of intermolecular interaction, which is likely a driving force for the formation and stability of these monolayers. †
Dedicated to Allen J. Bard on the occasion of his 65th birthday. Current address: Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204. * Corresponding author. ‡
Experimental Section Spectra. Transmission FTIR spectra were acquired on a Nicolet 510P spectrometer. Grazing angle reflectance FTIR spectra were acquired on a Nicolet 550 FTIR instrument over 4096 scans at an incidence angle θ ) 80° and a resolution of 4 cm-1. Solution fluorescence spectra were acquired on a SLM Aminco SPF 500 fluorometer and surface fluorescence spectra on a SPEC Fluorolog 2 spectrometer with a 450 W Xe lamp arranged for front face excitation with emission detected at 11° from the incident light. Ellipsometric measurements were performed on a Gaertner L2W26D ellipsometer with a 70° angle of incidence and 633 nm wavelength. Measurements were performed in air, and film thicknesses were calculated with an estimated film refractive index of 1.45.1 Monolayer Formation. Gold surfaces for monolayer formation were prepared on a test grade Si wafer (Silicon Sense, Nashua, NH) by evaporation of Cr (200 Å), followed by Au (2000 Å). For electrochemical measurements, the Si wafer was masked prior to Cr and Au deposition to provide a 1 cm gold disk electrode. Monolayers of 3 and of 6 were deposited by allowing a freshly prepared gold wafer to soak for 8-24 h at room temperature in 10 mL of a 1 mM solution of 3 in ethanol. The deposition solutions had been degassed by bubbling with Ar for 5 min. After the gold surface had been added, the solutions
10.1021/jp9821174 CCC: $15.00 © 1998 American Chemical Society Published on Web 12/03/1998
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J. Phys. Chem. B, Vol. 102, No. 49, 1998 9821
were warmed to reflux under Ar for 5 min to ensure that the thiol was completely dissolved. The solutions were then allowed to cool and stand under Ar for 8-24 h at room temperature. The surfaces were rinsed sequentially with CH2Cl2, ethanol, and water just prior to any experimental manipulation. Monolayers of 14 were deposited in situ by treating 10 mL of a degassed 1 mM THF solution of 13 with one drop of concentrated NH4OH in the presence of a freshly prepared gold wafer. The mixtures were allowed to stand for 16 h under Ar. This method is effective for preparing SAMs of oxidatively unstable thiols.16 Electrochemistry. Cyclic voltammetry was performed using a Bioanalytical Systems BAS100/W electrochemical analyzer. Water used for electrochemical measurements was purified using a Millipore purification system and had a resistance of 4 Ω. An Ag/AgCl electrode was used as the reference electrode, and a Pt coil was used as the auxiliary electrode in a standard threeelectrode cell. Results and Discussion The distance from the thiol hydrogen to the para hydrogen of the ω-terminal arene rings of 3, 6, and 14 was determined by MM2 force field calculations6 to be 15.0, 21.1, and 27.7 Å, respectively. The thicknesses of the films of these thiols were found by optical ellipsometry to be 13 ( 2, 20 ( 2, and 27 ( 1 Å, respectively. The reported thicknesses are the average of 5-8 trials with the uncertainty given as the standard deviation. These measured thicknesses correspond well to the expected thickness of monolayer films in which the components are oriented roughly perpendicular to the Au surface. Electrochemistry. Cyclic voltammetry of electroactive species in a contacting aqueous solution is a valuable means of probing the integrity of a SAM on gold.1,18 Because electron transfer between a solution species and the electrode must occur either by tunneling through the monolayer or by approaching the electrode at a “pinhole” or defect in the monolayer, the extent of surface passivation to electron transfer is useful to detect defects in the monolayer.19,20 SAMs of n-alkanethiols with 10 or more carbons per chain form an impermeable barrier to electroactive species in aqueous electrolyte,1 but very few such studies have been done with monolayers comprised of conjugated thiols.14,15 In one such study, the passivating ability of SAMs of oligo(phenyl)thiols to cyclic voltammetry of contacting aqueous K4Fe(CN)6 was studied as a function of the number of phenyl units incorporated into the thiols. Though the SAMs became more passivating as the number of phenyl units increased, diffusion-limited Faradaic current was still observed at all of the SAMs in this study. Subsequent exposure of these SAMs to an alkanethiol resulted in an increase in the passivation. Similar behavior was observed at SAMs of rigid-rod arenethiols that contained a significant number of defect sites which were subsequently patched with alkanethiols.9 These experiments demonstrate that SAMs of these conjugated thiols are less passivating than SAMs of alkanethiols and that a primary mode of electrochemical communication between the electrode and the solution electrophore occurs at defect sites rather than by conduction through the monolayer. For cyclic voltammetry at an impermeable monolayer, Faradaic current is completely attenuated, and the only current response to the applied voltage is due to capacitive charging of the electrode. This capacitive current is proportional to the scan rate and inversely proportional to the thickness of the monolayer.1 Under ideal conditions, the capacitive current is
Figure 1. Cyclic voltammetry of 1.0 mM K4Fe(CN)6 in 0.1 M aqueous KCl at bare gold (top) and gold modified with SAMs of 3 (---), 6 (s), and 14 (‚‚‚). Potentials are measured vs Ag/AgCl reference elctrode at a sweep rate of 500 mV/s. A platinum coil was used as an auxiliary electrode.
independent of potential.21 At large overpotentials there may be a Faradaic contribution caused by electron tunneling, or permeation of the monolayer may contribute to the current response. Figure 1 shows a cyclic voltammogram of 1 mM K4Fe(CN)6 in 0.1 M aqueous KCl on a bare Au electrode (top) and an Au electrode modified with SAMs of 3, 6, and 14 (bottom). All three monolayers significantly attenuate the Faradaic response. At the SAM of 6, the current is independent of potential for the region scanned. At the SAM of 3 or of 14, however, there appears to be an increase of current at potentials higher than 300 mV. Thus, although these monolayers are passivating, they are more permeable than the SAMs of 6 at moderate overpotentials. Based on the cyclic voltammetric data, 6 forms the most impermeable SAM, followed by those of 14 and then by 3. Infrared Spectroscopy. Infrared spectroscopy is a powerful tool for investigating SAMs of thiols on gold.1,3 At the simplest level, reflectance spectroscopy can confirm the presence of molecules adsorbed on a surface. A comparison of the reflectance spectrum of the SAM with a transmission spectrum of the compound as a KBr pellet can confirm the identity of the molecules in the SAM, as well as orientation of specific groups relative to the surface normal.5,22 Infrared spectroscopy can also yield information about the intermolecular environment of SAMs on gold. The peak frequencies of the CH2 stretching modes of SAMs of alkanethiols are shifted to lower frequencies as the alkyl chain length increases because of lateral van der Waals interactions between neighboring chains.1 The frequencies of these modes in SAMs of short-chain n-alkanethiols (n ) 3-7) are close to the frequencies observed for bulk liquid samples of alkanethiols, whereas longer chain n-alkanethiols (n ) 21) exhibit frequencies close to those observed with bulk crystalline samples.
9822 J. Phys. Chem. B, Vol. 102, No. 49, 1998
Figure 2. Transmision FTIR spectra of 3 in KBr (top) and GARFTIR spectra of SAM of 3 (bottom).
Infrared spectroscopy can also yield information about the orientation of specific functional groups in the SAM with respect to the substrate surface.3,22 According to the selection rules for reflectance infrared spectroscopy of anisotropic films on a conducting surface, only those transition moments that are perpendicular to the plane of the substrate surface will exhibit absorbance.3 Therefore, the intensity of a peak for a given transition is proportional to the magnitude of its projection along the perpendicular axis. A comparison of this peak intensity relative to an isotropic bulk phase can yield information about the orientation of the molecules in the SAM relative to the surface normal.23,24 The peaks of the GAR-FTIR spectra of the films of 3 and 6 correspond well with those in transmission spectra of the bulk thiols in KBr (Figures 2 and 3). The thiol S-H stretch is not present in the bulk spectra, but these bands are characteristically weak so their absence is understandable.7 In all of the bulk spectra, the aryl C-H stretching bands are weak compared to those of the alkyl C-H stretching bands. The reason for this is not understood. The GAR-FTIR spectrum of 14 is compared to the transmission spectrum of the corresponding thioacetate 13 in Figure 4. The spectra of the film and of the bulk sample compare well with each other, and the expected disappearance of the thioacetate carbonyl stretch (1700 cm-1) as a result of the in situ deprotection is observed. Peak assignments for these spectra are given in Table 1. The aromatic C-H out-of-plane bending modes (823-883 cm-1) are present in all of the bulk IR spectra but are absent in the spectra of the thin films. This suggests that the aryl rings in the films are roughly perpendicular to the plane of the Au surface. The same is true for the alkyl CH out-of-plane bending modes (957-1080 cm-1). The alkyl C-H asymmetric stretching bands (2932-2937 cm-1) and symmetric stretching bands (2856-2861 cm-1) are moderately strong in all the bulk spectra but are weak in the GAR-FTIR spectrum of 3 and absent in
Reese and Fox
Figure 3. Transmision FTIR spectra of 6 in KBr (top) and grazing angle reflectance GAR-FTIR of SAM of 6 (bottom).
Figure 4. Transmision FTIR spectra of 13 in KBr (top) and GARFTIR of SAM of 14 (bottom).
the GAR-FTIR spectra of 6 and of 14. This indicates that the alkyl chains of the thiols in the films are roughly perpendicular to the Au surface. The alkyl chains in SAMs of simple alkanethiols tilt 2030° from the surface normal.1 Calculations suggest that, by tilting relative to the surface normal, van der Waals interactions between neighboring alkanethiols are maximized, and a densely
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J. Phys. Chem. B, Vol. 102, No. 49, 1998 9823
TABLE 1: Mode Assignmentsa and Frequencies (cm-1) for Transmission FTIR Spectra of Thiols 3 and 6 and of Thioacetate 13 and GAR-FTIR Spectra of SAMs of 3, 6, and 14 mode alkyl C-H asym str alkyl C-H sym str olefin CdC str aryl C-C ring str C-O-C str alkyl CH2 twist alkyl C-H out-of-plane bend aromatic C-H out-of-plane bend a
3 in KBr
SAM of 3
6 in KBr
SAM of 6
13 in KBr
SAM of 14
2932 2861 1597 1493 1247 1170 1080 1034
2932 2854 1597 1500 1247 1175 b
2937 2859 1605 1514 1250 1176 1030 967 957 829
b b 1603 1512 1253 1175 b
2936 2856 1601 1512 1254 1175 970
b b 1603 1512 1253 1176 b
883
b
b
823
b
b
Reference 7. Not present in the spectrum.
Figure 5. Solution fluorescence spectrum of 5 in CH2Cl2 (- - -) and reflectance spectrum of SAM of 6 (s).
Figure 6. Solution fluorescence spectrum of 13 in CH2Cl2 (- - -) and reflectance spectrum of SAM of 14 (s).
packed, crystalline environment is obtained.25 Even though the alkyl groups in the SAMs of 3, 6, and 14 do not tilt and thus obtain the maximum van der Waals stabilization, these SAMs are still very densely packed and defect free, as indicated by the observed passivation of the electrochemical response of a contacting electroactive solution. This suggests that the packing is driven by the aryl groups. Strong attractive interactions between π-systems contribute to order and stability of Langmuir-Blodgett (LB) films incorporating extended conjugated moieties such as transstilbenes,13,26-28 trans-azobenzenes,27,29 and oligo(phenylenevinylene).30,31 These interactions also influence how these molecules pack into crystals.32 The stabilizing energy due to noncovalent interactions between extended conjugated moieties is stronger than simple hydrophobic interactions.13 In the case of SAMs of 6 and of 14, these interactions result in a very stable monolayer, even though the alkyl portion of the component thiols do not adopt the optimum orientation for van der Waals stabilization. Fluorescence. Fluorescence spectroscopy can provide valuable information about the intermolecular interactions of molecules in molecular crystals33 and monolayers.34 Figure 5
shows fluorescence spectra of 5 in CH2Cl2 and 6 as a SAM on gold. The solution spectrum consists of a broad emission with a maximum at 370 nm, whereas the monolayer emission maximum is red-shifted by 40 nm and exhibits more fine structure. This emission is very similar to that reported for crystalline trans-stilbene35 but is red-shifted by 2500 cm-1. The emission spectra of 13 in solution and 14 as a SAM show similar behavior (Figure 6). The solution spectrum consists of two broad, overlapping bands at 390 and 420 nm, whereas the maximum of the SAM emission is at 490 nm and the spectra are more structured. The observed spectral shift in the emission of monolayers of 6 and of 14 relative to solution spectra is very similar to what was observed for LB films incorporating stilbene13,26,28 and oligo(phenylenevinylene)30,31 and is attributed to intermolecular interaction among the chromophores on the surface. The highly structured shape of these emissions suggests that the interaction is not of an excimer,36 but rather a consequence of intermolecular exciton interactions37 between the H-aggregated arenes.29-33 Excitonic interactions between neighboring chromophores in the array cause a splitting of the excited state into exciton bands (Figure 7).30,37 In the case of a “card-packed”
9824 J. Phys. Chem. B, Vol. 102, No. 49, 1998
Reese and Fox References and Notes
Figure 7. Energy level splitting due to exciton interactions among H-aggregates in a “card-pack” array.
array, such as with stilbene and oligo(phenylenevinylene), only absorbance to the highest exciton level is allowed, causing a blue shift in the absorbance spetrum.30,37 Fluorescence occurs from the low-energy band and is therefore red-shifted.33 The highly structured shape of the emission suggests that the array is crystalline in nature. Summary Thiols 3, 6, and 14 incorporating ω-attached conjugated aromatic groups form SAMs on polycrystalline gold. The thicknesses of the films as measured by optical ellipsometry correspond well to the expected thickness of monolayer films in which the components are oriented roughly perpendicular to the Au surface. The films are densely packed and are passivating to electron transfer between the Au surface and K4Fe(CN)6 in a contacting aqueous KCl solution. GAR-FTIR spectra show that the conjugated arenes as well as the C6 chains of thiols 6 and 14 are roughly perpendicular to the plane of the Au surface. Even though a perpendicular orientation of the C6 chains does not allow for maximum van der Waals stabilization between neighboring alkyl groups, a densely packed, crystalline monolayer is still obtained. This is due to strong attractive interactions between the neighboring π-systems of the component thiols. Strong π-π interaction is also evident from the emission spectra of SAMs of 6 and 14 which are red-shifted as a result of exciton interactions between the chromophores in the highly crystalline environment of the film. This study clearly demonstrates that SAMs of 6 and 14 exhibit stabilizing forces beyond those encountered in simpler alkanethiols. As the thiols incorporated into SAMs become more complex, a better understanding of the intermolecular interactions and their effect on the stability and morphology of the films will be an important aspect of molecularly designed architectures. Model systems such as the ones presented here may play an important role in the elucidation of these interactions. Acknowledgment. This work was supported by the U. S. Department of Energy and by the Robert A. Welch Foundation. Supporting Information Available: Synthesis and characterization of compounds 3, 6, and 14.
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