Hydrogen-Bonding Networks of Dialkyl Disulfides Containing the Urea

Lateral interactions at functional monolayers. Shu-Han Hsu , David N. Reinhoudt , Jurriaan Huskens , Aldrik H. Velders. Journal of Materials Chemistry...
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Langmuir 2004, 20, 1674-1679

Hydrogen-Bonding Networks of Dialkyl Disulfides Containing the Urea Moiety in Self-Assembled Monolayers Jang Hoon Kim,† Hyeon Suk Shin,† Seung Bin Kim,*,† and Takeshi Hasegawa‡ Laboratory for Vibrational Spectroscopy, Department of Chemistry, Pohang University of Science & Technology, San 31, Hyojadong, Pohang 790-784, Republic of Korea, and Department of Applied Molecular Chemistry, College of Industrial Technology, Nihon University, 1-2-1 Izumi-cho, Narashino, Chiba 275-8575, Japan Received March 25, 2003. In Final Form: October 24, 2003 Self-assembled monolayers (SAM) of two newly synthesized dialkyl disulfides containing the urea moiety, 4-(3-octadecylureido)phenyl disulfide (ODPD) and 3-(octadecylureido)ethanedisulfide (ODED), were prepared and studied by infrared spectroscopy. Infrared reflection-absorption (IRRA) spectra of the SAMs showed almost no amide I band, whereas this band appeared strongly in diffuse reflectance infrared Fourier transform (DRIFT) spectra of the corresponding bulk systems. This suggests that the molecules in the SAMs form a hydrogen-bonding network in which the N-H and CdO bonds are parallel to the monolayer surface. Further evidence supporting the existence of hydrogen-bonding networks in the SAMs of ODPD and ODED was obtained from experiments in which octadecyl disulfide (ODDS) molecules, which act as barrier molecules, were added into the ODPD and ODED SAMs. Comparison of the IR spectra of SAMs with and without ODDS indicated that addition of ODDS degraded the hydrogen bonding between ODPD (or ODED) molecules. Furthermore, the orientations of the alkyl chains in the SAMs of ODPD and ODED were determined to probe the influence of the linker between the urea and sulfur moieties in the ODPD and ODED molecular structures (ODPD has a benzene ring as the linker and ODED has an ethylene linker). The analysis showed that the ODPD and ODED alkyl chains have tilt angles of 15.0° and 18.1° with respect to the surface normal, respectively. This result indicates that the molecular architecture of the SAMs containing the urea moiety is not affected by the type of linker; instead, hydrogen bonding and van der Waals interactions between the alkyl chains are the principal determinants of the SAM structure.

Introduction Self-assembled monolayers (SAMs) of molecules are spontaneously generated on metallic and semiconductor surfaces as a result of exothermic adsorption or the formation of covalent bonds between the adsorbing molecules and the surface.1-5 Studies of SAMs give insight into phenomena related to intermolecular, moleculesubstrate, and molecule-solvent interactions. For this reason, SAMs have attracted keen interest from researchers in various fields including surface chemistry, physical chemistry, and molecular spectroscopy.1,5 SAMs of alkanethiols on a gold substrate are known to be particularly interesting systems. Sulfur atoms have a strong chemical affinity (40-45 kcal/mol) for transition metal surfaces.2,3 The formation of SAMs of alkanethiols or dialkyl disulfides on gold is driven by this strong affinity along with the inactivity of gold surface against oxidization.4,5 SAMs are additionally stabilized by van der Waals interactions between the alkyl chains in the molecules.6 However, despite these stabilizing factors, SAMs of alkanethiols are easily removed under vacuum or hightemperature conditions. This problem has sparked numerous efforts to enhance the stability of alkanethiol SAMs by introducing functional groups such as diyne, sulfone, * Corresponding author: telephone +82-54-279-2106; fax +8254-279-3399; e-mail [email protected]. † Pohang University of Science & Technology. ‡ Nihon University. (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Dubois, L. H.; Nuzzo, R. G. Ann. Phys. Chem. 1992, 43, 437. (3) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (4) Nuzzo, R. G.; Allara, D. J. Am. Chem. Soc. 1983, 105, 4481. (5) Ulman, A. Ultrathin Organic Films; Academic: New York, 1991; p 237. (6) Clegg, R. S.; Hutchison, J. E. Langmuir 1996, 12, 5239.

or amide into the alkanethiol molecule.6-9 Among the functional groups introduced into alkanethiols to increase monolayer stability, the amide group has been found to consistently enhance SAM stability due to its tendency to form strong intermolecular hydrogen bonds. Clegg et al.6,7 prepared a series of SAMs of alkanethiols containing amide groups on a gold substrate, and characterized the monolayers using Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), electrochemical measurements, and contact angle measurements. Their analytical results suggested that robust monolayers were formed when the number of carbons in the alkyl chain exceeded 15, although the ordering of the alkyl chains in these monolayers was disrupted by the amide moiety due to steric effect. The strength of the hydrogen-bonding network was evaluated by comparing the intensities and wavenumbers of the methylene stretching, amide I, and amide II bands in infrared reflection-absorption (IRRA) spectra of the SAMs with those of the infrared transmission spectra of the bulk alkanethiol sample. These comparisons revealed that SAMs of alkanethiols containing amide groups have strong hydrogen-bonding networks and are therefore more stable than the SAMs of alkanethiols that lack the amide group. They additionally studied the desorption profile of SAMs of octadecanethiol (ODT) and three amide-containing alkanethiols on heating the SAMs to 175 °C.8 They found that 50% of the ODT molecules were desorbed by this heating regime, whereas only 5% of the amide-containing alkanethiol molecules were desorbed. (7) Clegg, R. S.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 5319. (8) Clegg, R. S.; Hutchison, J. E. J. Am. Chem. Soc. 1998, 120, 2486. (9) Tam-Chang, S.; Biebuyck, H. A.; Whitesides, G. M.; Jeon, N.; Nuzzo, R. G. Langmuir 1995, 11, 4371.

10.1021/la030122f CCC: $27.50 © 2004 American Chemical Society Published on Web 01/29/2004

Urea-Containing Dialkyl Disulfides in SAM

Tam-Chang et al.9 prepared SAMs of 4,4,4-trifluorobutanethiol and of an amide-containing alkanethiol and then placed them under ultrahigh vacuum (10-9 Torr) at room temperature. Under this condition, the amide-free alkanethiol SAM was desorbed, whereas almost no desorption of the amide-containing alkanethiol SAM was observed. They also found that SAMs of amide-containing alkanethiols were disordered when the linker between the sulfur and the amide group was a methylene group, whereas the monolayer became ordered when an ethylene linker was inserted and the number of carbon atoms in the alkyl chain was greater than 15.7 Thus, the stability of a SAM depends on various factors. The recent results from the Weiss group10,11 reported that binary mixed SAMs on crystalline gold were prepared from binary mixtures of adsorbates, alkanethiol (e.g., decanethiol), and amide-containing alkanethiol (3-mercapto-N-nonylpropionamide). The study, by use of scanning tunneling microscopy, showed that the introduction of a hydrogen-bonding functionality in the mixed SAMs causes the spontaneous phase separation on the nanometer scale and well-ordered SAMs through the hydrogenbonding and van der Waals interactions. Persson et al.,12 on the other hand, investigated the reaction of isocyanate molecules at the SAM surface to create urea-containing SAMs. Isocyanate is known to react with various nucleophiles such as amine, water, and alcohols. They found that ethylenediamine reacted with the isocyanate in these SAMs to form a urea moiety at the surface; however, little is known about the characteristics and stability of these urea-containing SAMs. In the present work, we investigate the hydrogen-bonding networks and molecular structures of urea-containing SAMs in detail. When alkanethiols containing an amide moiety are chemisorbed on gold, the character of the linker (methylene or ethylene) between the sulfur and the amide moiety can substantially influence the characteristics of the adsorbed layer.9 For isolated molecules, in the case of the ethylene linker, the trans conformation of the linker between the sulfur and amide is the most stable thermodynamically. However, this conformation is ill-suited to the formation of a hydrogen-bonding network structure among the molecules adsorbed on the gold substrate and may degrade the ordering of the alkyl chains on the surface. Collections of alkanethiols containing the ethylene linker can avoid this destabilization of the hydrogen-bonding network by changing the linker conformation from trans to gauche. In fact, the trans-gauche energy difference of the linker is only 0.8 kcal mol-1, which is greatly outweighed by the energy reduction of 3-5 kcal mol-1 resulting from the increased hydrogen bonding between molecules with linkers in the gauche conformation.7 In the present study, we synthesized two dialkyl disulfides containing the urea moiety and prepared SAMs of these compounds on a gold surface (Figure 1). The two types of dialkyl disulfide considered in this study contain linkers with different characteristics: one of them is capable of trans-gauche transformation (ethylene) and the other is not (benzene ring). A hydrogen-bonding network formed in the SAMs was investigated by infrared spectroscopy. In addition, orientational analysis of the (10) Smith, R. K.; Reed, S. M.; Lewis, P. A.; Monnel, J. D.; Clegg, R. S.; Kelly, K. F.; Bumm, L. A.; Hutchison, L. E.; Weiss, P. S. J. Phys. Chem. B 2001, 105, 1119. (11) Lewis, P.; Smith, R. K.; Kelly, K. F.; Bumm, L. A.; Reed, S. M.; Clegg, R. S.; Gunderson, J. D.; Hutchison, J. E.; Weiss, P. S. J. Phys. Chem. B 2001, 105, 10630. (12) Persson, H.; Caseri, W.; Suter, U. W. Langmuir 2001, 17, 3643.

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Figure 1. Models of dialkyl disulfides containing the urea moiety adsorbed on a gold surface: (a) ODPD and (b) ODED.

alkyl chains in the SAMs was performed to elucidate the molecular architecture of the SAMs. Experimental Section Synthesis of 4-(3-Octadecylureido)phenyl Disulfide (ODPD). 4-Aminophenyl disulfide (252 mg) (APDS, Aldrich, 98%) was dissolved in 2 mL of tetrahydrofuran (THF, Baker, 100.0%) and then 890 mg of octadecyl isocyanate (ODI, Aldrich, 98%) dissolved in 8 mL of THF was added dropwise to the solution. After 24 h, the THF was removed by evaporation. The precipitated solid was sonicated in 10 mL of acetone for 2 min, filtered, and washed with acetone. A white solid, 618 mg (72.3%; mp 188 °C), was obtained after drying in an oven at 70 °C. 1H NMR (DMSOd6): δ (ppm) 0.88 (t, 3H), 1.28 (m) 1.47(m, 2H), 3.11(q, 2H), 5.91 (s, 1H), 7.34 (d, 1H), 7.38 (d, 1H), 8.18 (s, 1H). 13C NMR (DMSOd6): δ (ppm) 12.8, 21.1, 25.7, 28.2, 29.0, 30.5, 118.0, 130.3, 140.6, 154.4. DRIFT IR (cm-1): 3322, 2956, 2921, 2848, 1624, 1580, 1559, 1469, 1396, 1235. Anal. calcd for C50H86N4O2S2: C, 71.53; H, 10.33; N, 6.676; S, 7.639. Found: C, 71.91; H, 10.14; N, 6.610; S, 7.908. Cystamine. Cystamine dihydrochloride (1000 mg) (Aldrich, 98%) was dissolved in 50 mL of methanol, and then 363 mg of NaOH was added. The methanol in the solution was then removed by evaporation, resulting in precipitation of NaCl. The NaCl was filtered off, and absolute ethanol was added to the filtrate to remove the remaining water through the formation of an azeotrope. A yellow liquid, 633 mg (93.5%), was obtained after removing the ethanol by evaporation. Transmission IR (ZnSe, cm-1): 3351, 3279, 2915, 2856, 1582, 1466. 3-(Octadecylureido)ethane Disulfide (ODED). Cystamine (245 mg) and ODI (1,000 mg) were poured into 60 mL of CH3CN and stirred for 6 h. A white solid, 723.7 mg (60.5%; mp 141 °C) formed as a precipitate and was collected by filtering. 1H NMR (DMSO-d6): δ (ppm) 0.88 (t, 3H), 1.28 (m), 1.41 (m, 2H), 2.81 (t, 2H), 3.01 (q, 2H), 3.31 (q, 2H), 5.62 (s, 1H), 5.77 (s, 1H). 13C NMR (DMSO-d6): δ (ppm) 12.8, 21.1, 25.7, 28.2, 29.3, 30.5, 157.4. DRIFT IR (cm-1): 3347, 2956, 2919, 2848, 1618, 1582, 1461, 1401, 1255. Anal. calcd for C42H86N4O2S2: C, 67.86; H, 11.66; N, 7.539; S, 8.626. Found: C, 67.96; H, 11.52; N, 7.441; S, 8.796. Octadecyl Disulfide (ODDS). Octadecanethiol (572 mg) was dissolved in 8 mL of 5% ethanolic NaOH solution, and then 508 mg of I2 was added. The mixture was stirred for 40 min at 50 °C. After cooling to room temperature, the mixture was filtered and the residue was washed with ethanol and then dried for 1 h under vacuum. A glossy white solid, 475 mg (83%; mp 60 °C), was obtained. 1H NMR (CDCl3): δ (ppm) 0.88 (t, 3H), 1.26 (m),

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1.68 (m, 2H), 2.69 (m, 2H). 13C NMR (CDCl3): δ (ppm) 14.5, 23.1, 28.9, 29.9, 32.3, 39.6. DRIFT IR (cm-1): 2954, 2919, 2851, 1471. Formation of SAMs. The gold substrate was purchased from Lance Goddard Associates (Foster City, CA). ODPD (8.39 mg) and the gold substrate were dipped in 20 mL of 1-butanol that was deoxygenized by bubbling with nitrogen gas, and the solution including the gold substrate was maintained at 100 °C for 24 h. To rinse the gold substrate, it was placed in 20 mL of 1-butanol at 100 °C for 5 min, sonicated for 2 min, and dried with N2 gas. This process was repeated several times. In a similar manner, ODED (1.49 mg) and the gold substrate were placed in 20 mL of chloroform, and the solution including the gold substrate was maintained at 50 °C for 24 h. To rinse the gold substrate, it was placed in chloroform at 50 °C for 5 min, sonicated for 2 min, and dried with N2 gas. This process was repeated several times. IRRA Spectra. IRRA spectra of the SAMs were measured at a spectral resolution of 4 cm-1 with a Bruker (Karlsruhe, Germany) IFS 66v/s FT-IR spectrometer equipped with a liquid nitrogen-cooled MCT detector. A Bruker A513 reflection attachment was used for the IRRA measurements, and a p-polarized infrared ray was used at an angle of incidence of 82°. The p-polarized infrared ray was generated by a SPECAC (Orpington, U.K.) wire-grid infrared polarizer. To ensure a high signal-tonoise ratio, 1024 interferograms were co-added for each measurement. Both sample and source compartments were evacuated. Study of the Barrier-Molecule Effect. To break the intermolecular hydrogen bonds among the monolayer molecules, ODDS was mixed with the molecules of interest. In this sense, the ODDS molecules work as “barrier” molecules. ODDS was chosen instead of the simpler octadecanethiol because thiols tend to be adsorbed onto a gold surface at a much faster rate than disulfides. For example, Bain et al.13 reported that undecanethiol adsorbs on a gold surface 75 times faster than diundecyl disulfide. In the present study, SAMs comprising a mixture of ODPD and ODDS were prepared under the same conditions that were used to prepare the SAM of ODPD alone. Molecular ratios of ODPD to ODDS of 20:1, 15:1, 10:1, 2:1, and 1:5 were used, with a constant total molar concentration of 2.5 mM in 1-butanol. Another series of SAMs comprising mixtures of ODED and ODDS was prepared at the same mixing ratios with a constant total molar concentration of 0.5 mM in chloroform. The ODED/ODDS SAMs were prepared under the same conditions that were used to prepare the ODED SAM. IRRA spectra of these SAMs were recorded and analyzed. Measurement of SAM Thickness by XPS. The X-ray photoelectron spectrometer (XPS) used in this study was an Escalab 250 manufactured by Thermo VG Scientific (West Sussex, U.K.), with Al KR as the X-ray source. Photoelectron intensity was measured at a pressure of 7.2 × 10-11 kPa and at angles of 20.01°, 41.66°, 63.31°, and 84.96° between the gold substrate and the analyzer. The variation of Au(4f7/2) intensity with monolayer thickness can be represented as follows:14

Au ) (Au)0 exp(-d/λ sin γ)

(1)

where Au and Au0 are the intensities of Au(4f7/2) with and without the monolayer, respectively, d is the monolayer thickness, λ is the inelastic mean free path of the photoelectrons, and γ is the angle between the sample and the analyzer. By measuring intensities at various angles, the monolayer thickness can be determined from the slope of a plot of ln (Au/Au0) against (sin γ)-1. By this method, the thicknesses of the ODPD, ODED, and ODDS SAMs were found to be 31, 30, and 24 Å, respectively. Determination of Molecular Orientation. The molecular orientation in the SAMs was determined by analyzing the IRRA spectra using a calculation procedure developed for the analysis of the electric-field distribution on a dielectric surface. Here we present only a brief outline of this procedure, which is described in detail elsewhere.15 When a material of interest consists of stratified layers, an infrared ray striking that material at an (13) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723. (14) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 1670.

angle of θ1 with respect to the layer plane is transmitted and reflected at each of the interfaces between the layers. As for the IRRA measurements of single-monolayer films directly deposited on a metal surface, a three-layer model can be used to represent this layer structure: air, SAM layer, and the metal layer. In this three-layer system, the electric fields accompanying the transmitted and reflected rays obey the following equations:

M ˜ film )

[

] [

cos β˜ film m ˜ 11 m ˜ 12 (-i/q˜ film) sin β˜ film ≡ m ˜ 21 m ˜ 22 -iq˜ film sin β˜ film cos β˜ film

]

(2) q˜ film )

ξ˜ film n˜ film,on˜ film,e

(3)

where ξ˜ film ) (n˜ film,e2 - nair2sin2 θair)1/2 and

β˜ film )

n˜ film,o 2π hfilm ξ˜ λ n˜ film,e film

(4)

where n˜ , h, and λ represent the complex refractive index, the film thickness, and the wavelength of the infrared ray, respectively. In these equations, the subscripts (film or air) specify the layer position in the three-layer system, and o and e represent directions parallel and perpendicular to the film, respectively. Thus, this theory takes into account the optical anisotropy based on a uniaxial distribution in the reflection calculation. The reflection coefficient for p-polarization is calculated as follows:15

r˜ p )

(m ˜ 11 + m ˜ 11qgold)q1 - (m ˜ 21 + m ˜ 22qgold) (m ˜ 11 + m ˜ 11qgold)q1 + (m ˜ 21 + m ˜ 22qgold)

(5)

From this coefficient, the reflectivity, R, is calculated as

R ) |r˜ p|2

(6)

Thus, within this formalism the expected intensities of the background and the sample are determined individually, so that absorbance would finally be evaluated. The calculation procedure was coded by the last author (T.H.) in MathWorks (Natick, MA) MATLAB Release 12.1.

Results and Discussion Formation of Hydrogen-Bonding Networks in ODPD and ODED SAMs. If molecules adsorbed on a surface form a hydrogen-bonding network, the intensities of their vibrational modes parallel to the surface will largely be suppressed because of the surface selection rule of IRRA spectroscopy. For the SAMs considered in the present work, the intensity of the stretching mode of the carbonyl group in the IRRA spectra of the SAMs should provide key information on the formation of hydrogen bonding having the N-H and CdO bonds parallel to the surface in the monolayer. Figure 2 shows the DRIFT spectrum of bulk ODPD along with the IRRA spectrum of the SAM of ODPD, and Figure 3 presents the corresponding ODED spectra. Comparison of the DRIFT and IRRAS spectra reveals that the N-H stretching (3322 and 3346 cm-1 for ODPD and ODED, respectively) and CdO stretching (amide I; 1624 and 1621 cm-1 for ODPD and ODED, respectively) modes appear in the DRIFT spectra but not in the IRRA spectra. On the other hand, the intensities of the amide II bands in the IRRA spectra (1555 and 1582 cm-1 for ODPD and ODED, respectively) are stronger than in the corresponding DRIFT spectra (1559 and 1581 cm-1 for OPDP and ODED, respectively). (15) Hasegawa, T.; Takeda, S.; Kawaguchi, A.; Umemura, J. Langmuir 1995, 11, 1236.

Urea-Containing Dialkyl Disulfides in SAM

Langmuir, Vol. 20, No. 5, 2004 1677 Table 1. Band Assignments of ODPD and ODED in the DRIFT and IRRA Spectraa ODPD (cm-1) assignments

DRIFT

ν(NH) νas(CH3) νas(CH2) νs(CH2) amide I ν(CdC) amide II amide III

3322 2956 2921 2848 1624 1580/1589 1559 1235

a

ODED (cm-1)

IRRAS

DRIFT

IRRAS

2965 2921 2854

3346 2956 2919 2849 1621

2965 2920 2851

1581 1256

1582 1256

1589 1555 1237

Band assignments are based on refs 6-9.

Figure 2. (a) DRIFT spectrum of bulk ODPD and (b) IRRA spectrum of the SAM of ODPD.

Figure 3. (a) DRIFT spectrum of bulk ODED and (b) IRRA spectra of the SAM of ODED.

This suggests that the N-H stretching and amide I modes are parallel to the surface, whereas the amide II mode is nearly perpendicular to the surface. Given that the amide II band is known to have a transition moment that is almost normal to the N-H bond,6-8 these results are reasonable. Thus, the spectral data strongly suggest that both the N-H and CdO bonds are oriented parallel to the surface. Moreover, this additionally suggests that the N-H and CdO groups readily form hydrogen-bonding networks in ODPD and ODED SAMs as shown in Figure 1. Details of the band assignments are listed in Table 1. Hydrogen-Bonding Networks of SAMs Containing Barrier Molecules. When ODDS as a barrier molecule is mixed with ODPD (or ODED) to form a SAM on a gold surface, the ODDS molecules are expected to degrade the hydrogen bonding between ODPD (or ODED) molecules. Smith and co-workers10,11 performed a similar experiment, and they reported that 3-mercapto-N-nonylpropionamide and n-decanethiol exhibit phase-separated domains in a mixed SAM. Therefore, the ODDS molecules are expected to form self-aggregations rather than to be incorporated into ODPD (or ODED) molecules. Figure 4 presents IRRA spectra of ODPD/ODDS mixed SAMs at various mixing ratios. The corresponding spectra from the ODED/ODDS mixed SAMs are shown in Figure

Figure 4. IRRA spectra of the SAM of a mixture of ODPD and ODDS in (a) the amide stretching region and (b) the CH2 stretching region. The ratios of ODPD to ODDS are 1:0, 20:1, 15:1, 10:1, 2:1, and 1:5.

5. When the ratio of ODPD to ODDS is less than or equal to 10, bands appear at 1685 (amide I), 1645 (amide I), and 1532 cm-1 (amide II) that are not observed at higher ratios. This suggests that the presence of the ODDS molecules degrades the hydrogen-bonding networks in the SAMs. An interesting aspect of these results is the observation of two bands for the amide I mode, which suggests that there are at least two different types of hydrogen bonding. The addition of ODDS also influences the ordering of the alkyl chains. In the IRRA spectra of ODPD/ODDS mixed SAMs (Figure 4), the CH2 antisymmetric stretching mode appears at 2921 cm-1 for a molecular ratio of 20:1

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than the corresponding bands in the ODED spectrum, respectively. These shifts to lower wavenumber are considered to be due to a conjugation effect associated with the phenyl ring. Comparison of the spectral features associated with the alkyl chains in the mixed SAMs provides additional information on the effect of the phenyl ring in ODPD on the hydrogen-bonding network. In the spectra of the ODPD/ODDS mixed SAMs (Figure 4), the CH2 antisymmetric stretching mode is located at 2921-2923 cm-1, which is 2 or 3 cm-1 higher than that of the ODED/ODDS mixed SAMs (Figure 5). This suggests that the alkyl chains of ODPD are more loosely packed than those of ODED, which may be due to the weaker hydrogen bonding of the urea moiety in ODPD. In the SAM of ODED, the compactness and flexibility of the ethylene linker is expected to lead to a structure with strong hydrogen bonds and well-packed alkyl chains.7,9 In the SAM of ODPD, on the other hand, the interplane interactions between the phenyl groups are expected to disturb the hydrogen bonding and ordering of the alkyl chains. Orientation Analysis of Alkyl Chains in the SAMs. To determine the molecular orientation in SAMs, optical parameters are required for the thin film and the substrate. Previously, the absorption coefficient (k) of the νs(CH2) mode at 2850 cm-1 for stearic acid (16 methylene groups) has been shown to be 0.2.16 In the present study, all the absorption coefficients of the bands were evaluated by measuring KBr-pellet transmission spectra. The absorbance of a band is proportional to the absorptivity (R), which is formulated according to the Beer-Lambert law:

A ) pR

(7)

where p is a proportionality coefficient. It should be noted that the absorptivity is related to the wavenumber (ν˜ ) by

R ) 4πkν˜ Figure 5. IRRA spectra of the SAM of a mixture of ODED and ODDS in (a) the amide stretching region and (b) the CH2 stretching region. The ratios of ODED to ODDS are 1:0, 20:1, 15:1, 10:1, 2:1, and 1:5.

(ODPD:ODDS) and shifts to 2925 cm-1 when the ratio is 1:5. This indicates that the order of the alkyl chains is degraded in the SAM with a high molecular ratio of ODDS. The barrier effects of ODDS in the SAMs of ODED and ODPD were compared. In the IRRA spectra of the ODED/ ODDS mixed SAMs (Figure 5), the amide I band is not observed even at the ratio of 2:1; it is only at a ratio of 1:5 that a broad amide I band appears. New amide II band is not observed except for the shift and broadness of the band at 1582 cm-1, suggesting that the barrier effect is much weaker in the ODED/ODDS SAMs than in the ODPD/ODDS SAMs. In regard to the molecular orientation, the appearance of the amide I band in the spectra of ODPD or ODED SAMs containing sufficient ODDS represents indirect proof that the N-H and CdO bonds of the hydrogenbonding networks in the SAMs without ODDS were oriented parallel to the surface. Effect of the Linker between the Sulfur and Urea Moieties. The linker part of the ODPD molecule is a phenyl group, whereas that of ODED is an ethylene moiety. This structural difference manifests in the infrared spectra of these molecules, both in the bulk and in SAMs. (Figures 2 and 3). In the DRIFT spectra, for example, the N-H stretching mode of ODPD appears 24 cm-1 lower than that of ODED (see Table 1). In a similar way, the amide II and III bands of ODPD appear 22 and 21 cm-1 lower

(8)

where k is the absorption coefficient. Taking into account the number of methylene groups (n) in ODPD and ODED (n ) 17 and 19, respectively), the absorbance of the νs(CH2) mode at 2850 cm-1 for ODPD and ODED is represented by (p′ ≡ 4πp)

A2850 ) p′k2850ν˜ 2850 ) 2850p′

(0.2n 16 )

(9)

In this formulation, the linker group in ODED consisting of two methylene groups cannot be discriminated from the long hydrocarbon chain. Thus, the analytical molecular orientation angle of ODED corresponds to the average tilt angle of the methylene groups. For a band at ν˜ cm-1 in the KBr spectra of ODPD and ODED, in general, the following equation holds:

Aν˜ ) p′kν˜ ν˜

(10)

Thus, by use of eqs 9 and 10, the absorption coefficient, kν˜ , at ν˜ can be calculated from the KBr spectra:

kν˜ )

Aν˜ 35.625n A2850 ν˜

(11)

Since Aν˜ and A2850 can be approximately determined from measurements of the band intensities in the KBr spectra, the absorption coefficient is easily calculated. (16) Popenoe, D. D.; Stole, S. M.; Porter, M. D. Appl. Spectrosc. 1992, 46, 79.

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With the calculated optical parameters, the chain orientation in the ODPD and ODED SAMs was determined from the IRRA spectra by use of the calculation procedure described in the Experimental Section. The axes of the alkyl chains in the ODPD and ODED SAMs exhibit average tilt angles of 15.0 and 18.1° from the surface normal, respectively, by use of the following relationship that is called direction cosine.

cos2 R + cos2 β + cos2 γ ) 1

(12)

Here R and β represent the orientation angles for the two transition moments, and γ corresponds to the tilt angle. The error range in the calculated angles is (3%. Although these angles are smaller than the tilt angle of ca. 30° reported previously for SAMs of n-alkanethiols with no amide group on Au, they are consistent with the results of Tam-Chang et al.,9 who showed using a spacefilling model that the chain tilt of the CH3(CH2)11NHCOCH2SH SAM on Au cannot exceed ∼18°. It should be noted that the difference of chain-tilt angle between ODPD with benzene ring as a linker and ODED with ethylene linker is only 3°. This suggests that ethylene linker does not significantly influence the tilt angle of the entire molecule in the SAMs and that the molecular architecture of these SAMs is mostly driven by the hydrogen-bonding network and the van der Waals interaction between the alkyl chains.

Conclusion Two dialkyl disulfides containing the urea moiety, ODED and ODPD, were successfully synthesized. Introduction of the urea moiety into the dialkyl disulfide molecule created SAMs with hydrogen-bonding networks having the N-H and CdO bonds parallel to the gold surface, which was characterized from comparisons of IRRA spectra of ODPD and ODED SAMs and DRIFT spectra of bulk ODPD and ODED. Comparisons of the CH2 antisymmetric stretching vibrational modes in the IRRA spectra of ODPD/ODDS and ODED/ODDS mixed SAMs suggested the SAM of pure ODED is characterized by stronger hydrogen bonds and better packing of alkyl chains than the SAM of pure ODPD. The higher stability of the ODED SAM was attributed to the compactness and flexibility of the ethylene linker in ODED compared to the phenyl linker in ODPD. Quantitative analysis of the orientations of the alkyl chains in ODPD and ODED SAMs revealed they have chain-tilt angles of 15.0° and 18.1°, respectively, indicating that the linker character has little effect on the tilt angle of the entire molecule in the SAMs. Thus, the present results collectively suggest that the molecular architecture of urea-containing SAMs is principally driven by hydrogen bonding and van der Waals interactions between the alkyl chains. Acknowledgment. This work was supported by the Ministry of Education (BK 21 Project). LA030122F