Monolayer Orientation of ω-Substituted Amide-Bridged Alkanethiols

J. Phys. Chem. C 2010, 114, 1253–1259

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Monolayer Orientation of ω-Substituted Amide-Bridged Alkanethiols on Gold Polina N. Angelova,† Karsten Hinrichs,*,‡ Irena L. Philipova,† Kalina V. Kostova,† and Dimiter T. Tsankov*,† Institute of Organic Chemistry, Bulgarian Academy of Sciences, Acad. G. BoncheV Str., Block 9, 1113 Sofia, Bulgaria and ISASsInstitute for Analytical Sciences, Department Berlin, Albert-Einstein-Str. 9, 12489 Berlin, Germany ReceiVed: October 15, 2009; ReVised Manuscript ReceiVed: NoVember 29, 2009

A new set of aryl-substituted amides of 16-mercaptohexadecanoic acid (R ) 4-OCH3; 3,5-di-OCH3) are synthesized using a simple and general procedure. Self-assembled monolayers of these molecules on Au(111) are studied by infrared and visible ellipsometry and contact angle measurements. Model calculations are employed to determine the molecular tilt angle. The SAM methylene chains exist in prevailing all-trans conformation and the tilt of the CCC plane is decreased by approximately 15° in comparison with the correspondent value for n-alkanethiols. Strong hydrogen bonds between the amide proton and the carbonyl oxygen are detected with CdO and N-H dipoles oriented parallel to the gold surface. The wetting of the outermost film surface reveals predominantly OCH3 groups exposed. 1. Introduction Self-assembled monolayers (SAM) of thiolates on metallic surfaces have been a subject of numerous studies in the past two decades because of their diverse application potential.1-3 Most frequently investigated and essentially well understood are the monolayers formed by n-alkanethiols with different chain lengths on gold. In attempts to create two- or three-dimensional networks with definite physical chemical properties, various functional groups and terminal substituents have been incorporated in the alkyl chains in order to engineer the outer film surface.4-17 The packing and stability of SAM formed by thiolates is governed by the strong gold-sulfur bonds (40-45 kcal/mol) and van der Waals interactions between the adjacent alkyl chains.2,3 Although they ensure enough steadiness at ambient conditions, some technological applications require enhanced stability against vacuum or thermal desorption. Additional stabilization of the monolayers can be achieved provided that intermolecular hydrogen-bonded networks between the alkyl chains can be created. Amide functionalities incorporated in alkanethiols with various chain lengths have been the most extensively studied objects because they are known to form strong intermolecular hydrogen bonds18-23 and are used to control the surface structure.24 Early studies of SAM-containing buried amide groups revealed that the hydrogen bonding and accompanying steric effects often disrupt the conformational ordering and packing efficiency within monolayers.19,20,24 Reflection-absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), and contact angle studies have disclosed that the SAM ordering is dependent on the length of the alkyl chain between the sulfur atom and the amide group.20,24-26 Clegg and Hutchinson reported on a series of secondary amide-containing SAM fabricated by alkanethiols, where the spacer between the sulfur atom and the amide group was kept constant, while the alkyl tail length was systematically * To whom correspondence should be addressed. Phone (+359 2 9606 150), Fax (+359 2 8700 225), E-mail: [email protected]; Phone (+49 30 6392 3541), Fax (+49 30 6392 3544), [email protected]. † Institute of Organic Chemistry, Bulgarian Academy of Sciences. ‡ ISASsInstitute for Analytical Sciences, Department Berlin.

varied.24 They found that the spacer is perfectly ordered in all cases, while the overlayers are only ordered when at least 15 carbon atoms are present in the aliphatic tail. In another study Boal and Rotello27 examined intra- and intermolecular bonding in monolayers with buried amide groups deposited on gold nanoparticles. In a series of thiolates with constant hydrocarbon chain length, but with variable distance between the head sulfur atom and the amide moiety, they detected intermolecular hydrogen bonding only in cases when the amide functionality was placed near the monolayer surface.27 This behavior was attributed to steric factors following from the radial orientation of the alkyl chains around the nanoparticles. Well-ordered SAMs were obtained by reaction of a long-chain OH-terminated alkanethiol template monolayer on gold with vapor-phase alkyl isocyanates.25 RAIRS data gave evidence for interchain hydrogen bonded carbamate network and conformationally well ordered underlayer. The overlayer becomes well oriented when the hydrocarbon chain exceeds five carbons.25 In another study, Kim et al.26 synthesized two novel dialkyl disulfides containing urea moiety and studied their monolayer orientation chemisorbed on gold substrates. Both disulfides contain short spacerssethylene and a benzene ring respectively, and equally long alkyl chains containing 18 carbons. The IR spectra demonstrated that the model disulfides form a strong hydrogen-bonded network between the urea moieties and well packed and ordered alkyl chains owing to reestablished van der Waals contacts.26 Amide stabilized SAMs containing spacers of different length and terminal oligo(ethylene glycol) group have been extensively studied both experimentally and theoretically by Liedberg et al.28-30 Ab initio optimized structure of hexagonal periodic array yielded different amide dihedral angles than those calculated for a single molecule and reduced by ∼0.6 Å H · · · O distance.30 In the present work, we report on newly synthesized arylsubstituted amides of 16-mercaptohexadecanoic acid (R ) 4-OCH3; 3,5-di-OCH3) self-assembled on Au(111). The spacer between the head sulfur atom and the amide group is extended in comparison to the formerly studied SAMs18-24,26,27 and contains 16 carbons while the tail terminus is comprised by voluminous substituents - 4-metoxy and 3,5-dimetoxy substi-

10.1021/jp909883b  2010 American Chemical Society Published on Web 12/22/2009

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Angelova et al.

SCHEME 1a

a

i: Zn/acetic acid, acetyl chloride; ii: H2N-R, EDCI, HOBT, 0.5 eq DMAP, 20, 24 h; iii: Sodium methoxide, methanol, 10% HCl.

tuted phenyl rings. The aim of this study is 2-fold: first, to explore the effect of the hydrogen-bonded network on the packing, orientational alignment and conformational ordering of the hydrocarbon chains and on the orientation of the end substituents within monolayers, and second, to estimate the influence of the steric hindrance imposed by the bulk substituents on the packing and the orientation of the hydrogen-bonded alkyl chains. The results are compared with data for an analogous CH3-terminated amide, which lacks voluminous end substituent. 2. Materials and Methods 2.1. Synthesis of the Model Compounds. The synthesis of the amides 4a-c is depicted in Scheme 1. The protection of the thiol group in 1 (Aldrich, 90%) was achieved with Zn dust (Fluka, purum) and acetyl chloride (Fluka, purum, g98%) in acetic acid (Fluka, 100%) with high yield.31 The coupling reaction of the acid 1 with 4-methoxyaniline (Fluka, purum, g98%) (a) and 3,5-dimethoxyaniline (Fluka, purum, g98%) (b) was carried out in methylene chloride (spectroscopy grade Merck) using N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDCI) (Fluka, purum, 98%) and 1-hydroxybenzotriazole (HOBT) (Fluka, purum, 98%) as activating reagents. The addition of 0.5 equiv 4-(dimethylamino)pyridine (DMAP) (Fluka, purum, 98%) increased the yields of the amides 3a and 3b to 77% and 85%, respectively. The 3c was synthesized in the same manner from methylamine hydrochloride (Fluka, purum, g98%) in 88% yield. Triethylamine (Fluka, puriss, for HPLC, g99.5%) was used to set free the methylamine. The thioacetates 3a-c were subsequently deprotected to the target compounds 4a-c under basic conditions in high yields (88-99%).31 The structure of all synthesized compounds was confirmed by IR, 1H-, and 13C NMR spectra. 2.2. Monolayer Preparation. Gold substrates (200 nm vacuum evaporated gold on microscope slides with 2 nm intermediate Ti layer) were purchased from Ssens bv (The Netherlands). Spectroscopy grade methylene chloride was obtained from Merck (Germany) and used as received. Glassware was cleaned with piranha solution (30% H2O2:98% H2SO4 ) 3:7) for 30 min at 90 °C. The gold substrates were first annealed for a few seconds on an acetylene burner (2300 °C, Contra AA 300, Analytik Jena, Germany) in order to remove possible organic contaminants. The annealing also smoothens the surface roughness and reveals large Au(111) terraces. The annealed substrates were cooled down in a stream of Ar and then immersed in a 10-3 M solution of the respective thiol in methylene chloride (Merck) at room temperature overnight. Before the ellipsometric measurements, the samples were rigorously cleaned in pure solvent by ultrasonic bath for 10 min, rinsed thoroughly with methylene chloride and blown dry with Ar. Such a procedure effectively removes the physisorbed molecules from the surface.

2.3. Infrared Ellipsometry. The SAMs were examined by infrared spectroscopic ellipsometry. The measurements were performed with the IR ellipsometer, attached to a Vertex 70 FT IR spectrometer (Bruker) and described in detail elsewhere.32 All spectra were recorded in the 4000-800 cm-1 range with a resolution of 4 cm-1. The sample size (10 × 19 mm) restricted the incidence angles to 70°, because larger angles would have increased the error due to the opening angle in our setup.32 A special chamber purged with dry air maintained low humidity during the measurements. Data collection included set of intensity measurements taken with the analyzer fixed at 45° in relation to the plane of incidence and the front polarizer consecutively set at 0° and 90° and then at 45° and 135°. Good signal-to-noise ratios were achieved after averaging 24 cycles of measurements where 64 scans were coadded for each set. The whole measurement procedure was fully automated using Opus Macro software (Bruker). The optical response of the sample is quantified by means of the complex reflection coefficient F expressed in terms of the ellipsometric parameters tanΨ and ∆, according to the relation.

F)

rp ) tan Ψexp(i∆) rs

(1)

where rp and rs are the Fresnel coefficients for p- and s-polarization. The quantities tanΨ and ∆ are expressed by the following:

( ) |rp | |rs |

(2)

∆ ) δp - δs

(3)

tan Ψ )

where tanΨ is the amplitude ratio of the complex reflection coefficients and ∆ represents their phase difference. 2.4. Visible Ellipsometry. The visible ellipsometric measurements were performed by variable angle spectroscopic ellipsometer SE-801-E, SENTECH GmbH, Germany. The thicknesses were determined from a best-fit on three angles measurements at 60°, 65°, and 70° in the spectral range 400-700 nm. 2.5. Contact Angles. Advancing contact angles θa of sessile water drops deposited atop of the monolayer were determined by in-house built equipment. Diffuse rare illumination was used to obtain contrast images of drop profiles, which were captured by a CCD camera equipped with a long-distance objective, digitized by a frame-grabber and imported into a computer. Drawing a tangent at the three-phase contact point solidwater-air via standard Corel Draw program displayed the θa value. Such procedure yields contact angles at given points of the film surface. In order to obtain a macroscopic characteristic

ω-Substituted Amide-Bridged Alkanethiols on Gold

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of the dry monolayer surface several measurements were performed at different locations and the average θa value was taken. 2.6. Calculations. The procedure applied for calculation of SAM optical constants and thickness was based on the classical three phase model. The mathematical algorithm follows the model proposed by Azzam and Bashara,33 which accounts for the anisotropic properties of the film optical constants and therefore affords calculating the band intensities as a function of molecular orientation. The calculations were applied to simulate the ellipsometric tanΨ and ∆ spectra where the vibrational band shapes were approximated by independent Lorenzian oscillators with wavenumbers (ν˜ i0), parameters for the oscillator strengths (Fi) and full width at half-maximum fwhm (Γi) to yield the complex dielectric function εˆ ) ε′ + ιε′′ with:

ε′ ) ε∞ +

∑ (ν˜ 2 i

ε′′ )

i0

∑ (ν˜ 2 i

i0

2 Fi(ν˜ i0 - ν˜ 2)

- ν˜ 2)2 + (Γiν˜ )2 FiΓiν˜

- ν˜ ) + (Γiν˜ ) 2 2

2

(4) (5)

According to the uniaxial symmetry assumed for the SAMs studied, the components of the dielectric function ε are related by εx ) εy * εz. The tilt angle of the hydrocarbon chains for SAM formed by 4-methoxyphenyl amide 4a was evaluated following the method previously described.34 The film thickness and the high frequency refractive index were determined by visible ellipsometry. The initial approximate set for the CH2 oscillator strengths parameters was taken from model calculations carried out for reference molecules. Assuming predominant all trans conformation of the methylene chains and allowing for the mutual orthogonality of the transition dipole moments of both CH2 modes and the chain axis, the tilt angle of the chain can be evaluated from the following equation35

cos2 θs + cos2 θas + cos2 γchain ) 1

(6)

where θs and θas are the respective tilts of the symmetric and the antisymmetric CH2 stretching vibrations and γchain is the tilt of the hydrocarbon chain. θs and θas can be determined from the ratio of the respective oscillator strength parameters Fi according to the following equation35 2 Fix Mix 1 ) 2 ) Fiz Miz 2tg2θi

TABLE 1: Water Contact Angles for the SAMs of the Model Compounds 4a-c on Au(111) and Reference Data for Analogously Substituted n-Alkanethiols

(7)

The parameters of the oscillator strengths Fj are related to the transition dipole moments Mj by Fj ≈ M2j and result from the simulations within the optical layer model. The components Fix associated with vibrations in the surface plane can be accounted for provided that the total transition dipole moment is known, because of the restrictions of the surface selection rule for metallic surfaces. 3. Results and Discussion 3.1. Wetting Properties. Contact angles measurements are insightful for both the hydrophilic character of the surface and the molecular structure of the monolayer. Table 1 lists the data of the water contact angle measurements of SAMs formed by compounds 4a-c on gold. For comparison, in the same table are given the contact angles of methyl- and methoxy-terminated

a

Data are taken from ref 4. b Data are taken from ref 37.

n-alkanethiols on gold,4 because they are indicative for SAM surface wetting when only these groups are exposed. The water contact angle for the methyl-terminated amide 4c (62°), is substantially lower than the corresponding well ordered n-alkanethiol (112°)4 and is very close to this measured for methyl-terminated long-chain ester (64°)37 (Table 1). The markedly increased hydrophilic character of these SAMs indicates presence of polar groups close to the interface that can be easily accessed by water molecules. Distinctly more hydrophobic is the monolayer formed by the compound 4a bearing a 4-methoxyphenyl end group. Contact angle of 75° is identical with the reference OCH3-terminated n-alkanethiol (74° on gold4) and likely reflects the fact that as the amide groups are buried deeper below the interface, their influence on the surface wetting properties became negligibly small. Although quite straightforward, this inference does not fully exhaust the role of buried functional groups. In a preceding paper35 we studied SAM comprised by an analogous ester compound - 4-methoxybenzyl-16-mercapto-hexadecanoat on gold. Even though the ester group was buried a little more below the interface than 4a, the contact angle for this monolayer (64°) indicates augmented wetting in which the participation of the carbonyl and/or ester group is indisputable.35 Seemingly, the presence of a flexible methylene linkage between the ester oxygen and the phenyl ring ensures enough orientational freedom for the bulky end group and thus plays a decisive role in the surface wetting properties. The contact angle for 3,5-dimethoxyphenyl compound 4b is 70° and is consistent with the result for 4a. The lower value can be ascribed to the presence of a second methoxy group, whose polar character contributes to increased hydrophilic properties of the SAM surface. 3.2. Visible and Infrared Ellipsometry. Visible and infrared ellipsometric spectra were simulated to evaluate the optical constants, thickness, and the molecular arrangement within the film. Each monomolecular layer is characterized by complex dielectric function and effective thickness which serve as input parameters and are adjusted in an iterative calculation process until obtaining the best fit between calculated and experimental spectra. Optical constants of the substrate, which also are parameters in the best-fit calculations, have been determined from independent measurement of an annealed gold substrate

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Figure 1. (Top) IR tanΨ ellipsometric spectrum of 4a monolayer on Au(111), measured at 70° angle of incidence and spectral resolution of 4 cm-1. (Bottom) IR absorption spectrum of the bulk compound in KBr. All IR absorption spectra were measured at sample concentration 1:300 mg KBr.

prior to SAM deposition. Nanometer thick monolayers are very sensitive to the surface morphology and ellipsometric measurements randomly performed over different blank and covered substrates might experience difficulties in the fitting procedure. Furthermore, spot-to-spot surface roughness on the same sample may vary and have to be accounted for upon sample positioning prior to the measurement. The evaluation of the SAM thickness was performed by visible ellipsometry fitting the three-phase model. Assuming inplane film isotropy and a value of 1.4 for the film high frequency refractive index n∞, the calculations ended with thickness of 2.27 nm, 2.20 and 2.10 nm for compounds 4a, 4b, and 4c, respectively. These calculations do not account for the surface roughness and possible interfacial layer between SAM and the substrate. More refined calculations using effective medium approach (EMA) performed for visible ellipsometric measurements of 1-octadecanethiol monolayers on gold estimated interfacial layer of 0.3÷0.4 nm.38 3.2.1. SAM Orientation Analysis. 3.2.1.1. Orientation of the Hydrocarbon Chains. The SAM orientational analysis is based on the IR ellipsometric tanΨ spectra. They are absolute measures and can be directly quantified with respect to complex optical constants, film thickness and molecular orientation. Quantitative simulations of the band shapes and intensities in tanΨ spectra, in conjunction with the surface selection rule for metallic substrates, are used to identify the tilt of the hydrocarbon chains and the orientation and the terminal substituents. The IR absorption in solid state and the tanΨ film spectra of 4a-c are displayed in Figures 1, 2, and 3, respectively. The assignments of the major vibrational bands are presented in Table 2, following literature available data.39-41 For comparison, in the table are also given the same band frequencies measured in diluted chloroform solution (10-3 M) where no intermolecular hydrogen bonds exist. The inspection of tanΨ spectra of 4a-c revealed that the molecules within the film are well packed, fully extended and exist in prevailing all-trans conformation. The antisymmetric stretching vibrations νas(CH2) for SAM 4a, 4b, and 4c (Figures 1, 2, and 3) are located at 2918, 2919, and 2917 cm-1, respectively.42 The wavenumbers of these vibrations are identical with the respective IR absorptions in solid state suggesting crystalline-like environment within monolayers. A small devia-

Angelova et al.

Figure 2. IR tanΨ ellipsometric spectrum (top) of 4b SAM on Au(111). The angle of incidence was 70° and the spectral resolution 4 cm-1. (Bottom) IR absorption spectrum of the bulk compound in KBr.

Figure 3. IR tanΨ ellipsometric spectrum of 4c (top) and the IR spectrum of the bulk compound in KBr (bottom). The measurement conditions were the same as for 4a and 4b.

tion from the perfect all-trans conformation is only observed for 3,5-dimethoxyphenyl amide 4b presumably caused by the steric hindrance of the voluminous end substituents. Further comparison of tanΨ spectra with correspondent IR absorptions in polycrystalline (KBr) and liquid state (10-3M CHCl3 solutions) gives enough evidence for completely built hydrogen bonded network in the monolayers. For the sake of simplicity, the analysis will initially be restricted to SAM formed by 4-methoxyphenyl amide 4a. The dominant bands in the SAM spectrum are those at 1548 and 1252 cm-1 (Figure 1). The band at 1548 cm-1 can cogently be ascribed to the amide II band of a hydrogen-bonded trans conformer of a secondary amide.19-26 The amide II band mainly involves torsional modes of N-H and N-CdO moieties and this band in solution appears at 1513 cm-1. The commensurate amide III mode expected for a trans conformer appears at 1252 cm-1.24,25,40 The amide I band (predominantly CdO stretching) emerges as a very weak feature at 1647 cm-1. Similarly to KBr absorption, its peak position is shifted down by 32 cm-1 from that in solution as a consequence of participation in hydrogen bonds. Taking into account the surface selection rule, the substantially reduced intensity suggests that these bonds are aligned almost parallel to the surface. Another conclusive proof for the hydrogen-bonded chains in

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TABLE 2: Most Prominent Observed Bands for Compounds 4a-c Compound

TanΨ (film) KBr CHCl3 10-3M (cm-1) (cm-1) (cm-1)

4a 3001 2918 2848 1647

3299 3012 2918 2850 1647

2929 2856 1679

1608 1548

1610 1545

1601 1513

1518 1252

1517 1252 3297 3004 2916 2850 1652 1541

1523

4b 3003 2919 2844 1653 1558 1253 4c 2917 2848 1644 1569

a

3438

3438 2929 2855 1685 1530

1255 3316 2918 2850 1642 1565

3465 2929 2850 1664 1603

1472

1468

Assignmenta νNH str ν(CH)Ar 7b νas(CH2) νs(CH2) ν(CdO) Amide I ν(CC)Ar 8a NH, N-CdO Amide II ν(CC)Ar19a Amide III νNH str ν(CH)Ar 7b νas(CH2) νs(CH2) ν(CdO)AmideI NH, N-CdO Amide II Amide III νNH str νas(CH2) νs(CH2) ν(CdO)AmideI NH, N-CdO Amide II δ(CH2)

Assignments are made according refs 39-41.

the adsorbate is the location of the N-H stretching vibration. It appears at 3438 cm-1 in solution and upon establishing of a hydrogen-bonded network a broad and intense band downshifted by approximately 240 cm-1 emerges in KBr spectra. The corresponding band in the adsorbate spectrum is not apparent because of parallel alignment of these bonds with respect to the substrate. The examination of the adsorbate spectra of 4b,c also clearly demonstrates that the chains are hydrogen bonded. Amide I band for 4c is found as a weak feature at 1644 cm-1 and its location is fully consistent with the equivalent band in the hydrogen bonded polycrystalline state. The same band in 4b expected at about 1653 cm-1 in the monolayer is almost not discernible that clearly indicates close to parallel orientation of this bond to the metallic surface. By the same token, the corresponding N-H stretching modes, shifted down in KBr spectra, are not detected in the adsorbate spectra in Figure 2 and 3. Well pronounced are the amide II bands at 1569 cm-1 in 4c and at 1558 cm-1 in 4b. These vibrational modes give rise to transition dipole moments perpendicular to N-H bonds20,22,23 which suggests almost vertical arrangement of the hydrocarbon chains. All of these observations are in agreement with the existing literature data19-26,43 and support the conclusion that the chains in the monolayers formed by 4a-c are hydrogen bonded, despite the steric hindrance of bulky end substituents in 4a and 4b. The H-bonded network requires different organizational arrangement of the molecules on Au, than for monolayers composed of n-alkanethiols. The trans conformation of the secondary amide yields formation of linear hydrogen bonds with a chain tilt in the nearest neighbor direction and N-O bond distance of 2.8 Å.19 These steric constraints are not fully commensurate with the hexagonal overlayer (3 × 3)R30° generally accepted for bonding of thiols on Au(111)1-3 and some increase in S-S spacing, necessary to accommodate the hydrogen bonded chains would have reduced the chain tilt.19,22-24

Figure 4. Simulated (dot line) and experimental (solid line) fragments of tanΨ spectrum of 4a on Au(111). The following optical constants were used for the calculations: n∝ )1.4, d ) 2.27 nm. The oscillator strengths parameters Fx and Fz were as follows: Fx ) 41450 cm-2 and Fz ) 2100 cm-2 for νs(CH2) at 2844 cm-1 and Fx ) 67200 cm-2 and Fz ) 7100 cm-2 for νas(CH2) at 2919 cm-1 .

Our model calculations performed for SAM formed by 4a (Figure 4) showed a tilt angle of 16°, which is lowered by 10-15° in comparison with n-alkanethiols with equal chain length,1-3 but is in a good agreement with data reported by others.20-26,28-30 The smaller tilt angle would have provided the necessary interchain distance that yields linear hydrogen bonds of proper length. Recent ab initio calculations of an amide bridged oligo(ethylene glycol) terminated thiol self-assembled on a Au(111) lattice have disclosed some details of the molecular arrangement within the film.30 Assuming hexagonal (3 × 3)R30° gold-sulfur lattice with fixed 5 Å spacing between sulfur atoms while all bonds, bond- and dihedral angles were left free to change, the authors confirmed a substantial decrease of the CCC plane tilt by ∼10-15° and commensurate reduction of the H · · · O distance to 1.9 Å.30 The optimized bond angles of γCdO and γN-H were calculated close to 90° which is consistent with the existing experimental data.30 3.2.1.2. Orientation of the End Methoxyphenyl Substituents. The measured contact angles for 4-methoxyphenyl- and 3,5dimethoxyphenyl-terminated amides are identical with the values reported for the reference n-alkanethiols4 suggesting that the surface wetting properties are entirely dominated by the end polar groups. The vastly increased hydrophilic character of methyl-terminated amide (62° vs 112° for the analogous alkanethiol) is attributable to the polar carbonyl oxygen obviously accessible for interaction with surface water molecules, albeit its involvement in hydrogen bonds. No augmented wetting expected on the same grounds was detectable for 4a and 4b. The reasons for this behavior could be either a specific orientation of the end methoxyphenyl substituents which shield the buried amide groups, or the impossibility of the carbonyl oxygen to interact with surface water molecules because of its involvement in more than one interchain hydrogen bond. The second possibility could be easily rejected, because it contradicts to the already established structural characteristics.19,23,28,30

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Studying SAM orientation in analogously long-chain benzyl esters on Ag(111), we ascertained alike wetting (∼90°) in a series of compounds notwithstanding the different polar character of the endmost substituents.44 This behavior was ascribed to a specific orientation of the phenyl rings that makes them exposed to the interface and responsible for the wetting. The same molecules showed much increased hydrophilic character when they were self-assembled on Au(111).35 This impressive behavior was attributed to enhanced conformational flexibility of the methylene linkage between the ester groups and the phenyl rings which allows them to change their orientation easily, when the SAMs are bound to energetically different surfaces.35 The end phenyl substituents in amides 4a and 4b do not possess such conformational flexibility because they are directly attached to the amide moiety and their orientation is a function of the chain conformation. The strong head gold-sulfur bonds and the terminal hydrogen bonds between amide groups make the chain conformation more rigid and fix the phenyl rings positions. Evidently, the surface wetting will be dominated by the polar character of the endmost substituents, such as methoxy groups. The emergence of some distinct characteristic modes in tanΨ spectra can serve as suitable markers for determination of the phenyl rings orientation. Bands, such as 8a and 19a (Table 2, Figure 1), for p-disubstituted benzene derivatives have transition dipole moments oriented along the both substituents.40,41 Both bands gain more intensity in tanΨ spectrum of 4a, as 19a becomes the third strongest band. The increased intensity is associated with larger projections of the correspondent dipole moments onto the surface normal as a consequence of the phenyl rings orientation. Particularly interesting is the intensity change of the band 7b located at 3001 cm-1 in tanΨ spectra40,41 (Figure 1). This vibration gives rise to a radial transition dipole moment whose vector is oblique to the line connecting the parasubstituents. Almost not discernible in absorption this band becomes as intense as the antisymmetric CH2 mode suggesting fixed oblique position of the phenyl rings. The same mode is also well pronounced in tanΨ spectrum of 4b. Presumably, the specific orientation of the end methoxyphenyl substituents, fixed by the rigid chain conformation is responsible for the SAM interface properties. 4. Conclusions Novel terminally substituted alkanethiols 4a-c containing amide moiety were synthesized and their self-assembled monolayers on Au(111) were examined by IR and visible ellipsometry and contact angle measurements. Introduction of an amide group created SAMs with extensive hydrogen-bonded networks having their CdO and N-H bonds parallel to the gold surface, as was established from comparison of IR absorptions in solid state with ellipsometric data. The hydrogen bonding substantially reduced the tilt angle but did not affect the conformational ordering of the alkyl chains. The ellipsometric spectra confirm that the hydrocarbon chains reside in crystalline-like environment with prevailing all-trans conformation and very low population of gauche defects. Apparently, the strong head gold-sulfur bonds and the amide hydrogen bonds at the end of the alkyl chains, supplemented by favorite van der Waals interactions, restrict the conformational freedom and provide enough steadiness of the whole periodic structure. This structure is rigid enough to sustain the steric hindrance caused by voluminous end groups such as 4-methoxyphenyl and 3,5dimethoxyphenyl groups. Such SAMs could effectively be used to control surface properties like wetting or lateral conductivity.

Angelova et al. Acknowledgment. The authors thank Prof. J. Petrov and Dr. T. Andreeva, Institute of Biophysics, Bulgarian Academy of Sciences for the contact angle measurements and some fruitful discussions. Thanks are due to Dr. C. Cobet and Prof. Dr. N. Esser, ISAS Berlin for making available the visible ellipsometer and for cooperation, and to Ms I. Fisher for technical assistance. Financial support by the Deutsche Forschungsgemeinschaft and the Bulgarian Academy of Sciences (contract 436 BUL 113/ 127), the Senatsverwaltung fu¨r Wissenschaft, Forschung und Kultur des Landes Berlin, the Bundesministerium fu¨r Bildung, Wissenschaft, and Forschung und Technologie is gratefully acknowledged. References and Notes (1) Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43, 437. (2) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (3) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (4) Bain, C. D.; Troughton, E. B.; Tao, Yu.-T.; Evall, J.; Whitesides, G.; Nuzzo, R. J. Am. Chem. Soc. 1989, 111, 321. (5) Buckel, F.; Effenberger, F.; Yan, Ch.; Go¨lzha¨user, A.; Grunze, M. AdV. Mater. 2000, 12, 901. (6) Sabatani, E.; Cohen, B. J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (7) Bandyopadhyay, K.; Patil, V.; Sastry, M.; Vijayamohanan, K. Langmuir 1998, 14, 3808. (8) Chang, S.-C.; Chao, I.; Tao, Y.-T. J. Am. Chem. Soc. 1994, 116, 6792. (9) Jin, Q.; Rodriguez, J. A.; Li, C. Z.; Darici, Y.; Tao, N. J. Surf. Sci. 1999, 425, 101. (10) Duan, L.; Garrett, S. J. J. Phys. Chem. B 2001, 105, 9812. (11) Ishida, T.; Mizutani, W.; Akiba, U.; Umemura, K.; Inoue, A.; Choi, N.; Fujihira, M.; Tokumotu, H. J. Phys. Chem. B 1999, 103, 1686. (12) Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L.; Wu, K.-C.; Chen, C.-H. Langmuir 1997, 13, 4018. (13) Jung, H. H.; Won, Y. D.; Shin, S.; Kim, K. Langmuir 1999, 15, 1147. (14) Gui, J. Y.; Stern, D. A.; Frank, D. G.; Lu, F.; Zapien, D. C.; Hubbard, A. T. Langmuir 1991, 7, 955. (15) Szafranski, C. A.; Tanner, W.; Laibinis, P. E.; Garrell, R. L. Langmuir 1998, 14, 3580. (16) Wacker, D.; Weiss, K.; Kazmaier, U.; Wo¨ll, Ch Langmuir 1997, 13, 6689. (17) Lee, S.; Puck, A.; Graupe, M.; Colorado, R.; Shon, Y.-S.; Lee, T. R.; Perry, S. S. Langmuir 2001, 17, 7364. (18) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 4610. (19) Tam-Chang, S.-W.; Biebuych, H. A.; Whitesides, G. M.; Jeon, N.; Nuzzo, R. G. Langmuir 1995, 11, 4371. (20) Clegg, R. S.; Hutchison, J. E. Langmuir 1996, 12, 5239. (21) Chechik, V.; Scho¨nherr, H.; Vansco, G. J.; Stirling, C. J. M. Langmuir 1998, 14, 3003. (22) Clegg, R. S.; Reed, S. M.; Hutchison, J. E. J. Am. Chem. Soc. 1998, 120, 2486. (23) Clegg, R. S.; Reed, S. M.; Smith, R. K.; Barron, B. L.; Rear, J. A.; Hutchison, J. E. Langmuir 1999, 15, 8876. (24) Clegg, R. S.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 5319. (25) Ferguson, M. K.; Low, E. R.; Morris, J. R. Langmuir 2004, 20, 3319. (26) Kim, J. H.; Shin, S. S.; Kim, S. B.; Hasegava, T. Langmuir 2004, 20, 1674. (27) Boal, A. K.; Rotello, V. M. Langmuir 2000, 16, 9527. ¨ stblom, M.; Svedhem, S.; Svenson, S. C. T.; (28) Valiokas, R.; O Liedberg, B. J. Phys Chem B 2001, 105, 5459. (29) Malysheva, L.; Onipko, A.; Valiokas, R.; Liedberg, B. J. Phys Chem. A 2005, 109, 7788. (30) Malysheva, L.; Onipko, A.; Liedberg, B. J. Phys Chem. A 2008, 112, 1683. (31) Svedhem, S.; Hollander, C.-A.; Shi, J.; Konradsson, P.; Liedberg, B.; Svensson, S. C. T. J. Org. Chem. 2001, 66, 4494. (32) Ro¨seler A., Korte E. H. Infrared Spectroscopic Ellipsometry. In Handbook of Vibrational Spectroscopy; Griffiths, P. R., Chalmers, J., Eds.; Wiley: Chichester, 2001; Chapter 2. (33) Azzam, R. M. A.; Bashara, N. M. In Ellipsometry and Polarized Light; North-Holland Publishing Co.: Amsterdam, New York, Oxford, 1977; Chapter 4. (34) Rosu, D. M.; Jones, J. C.; Hsu, J. W. P.; Kavanagh, K. L.; Tsankov, D.; Schade, U.; Esser, N.; Hinrichs, K. Langmuir 2009, 25, 919–923.

ω-Substituted Amide-Bridged Alkanethiols on Gold (35) Angelova, P.; Hinrichs, K.; Kostova, K.; Tsankov, D. J. Phys. Chem. C 2008, 112, 17683–17687. (36) Handbook of Infrared Spectroscopy of Ultrathin Films; Tolstoy V. P., Chernyshova I. V., Skryshevsky V. A., Eds.; Wiley & Sons, Inc.: New York, 2003. (37) Engquist, I.; Lestelius, M.; Liedberg, B. Langmuir 1997, 13, 4003–4012. (38) Prato, M.; Moroni, R.; Bisio, F.; Rolandi, R.; Mattera, L.; Cavalleri, O.; Canepa, M. J. Phys. Chem.C 2008, 112, 3899. (39) Roeges, N. P. G. A Guide to the Complete Interpretation of Infrared Spectra of Organic Molecules, Wiley: Chichester, 1994. (40) Varsa´nyi, G. Vibrational Spectra of Benzene DeriVatiVes; Akade´miai Kiado´: Budapest, 1969; Chapter 3. (41) Varsa´nyi, G. Assignments for Vibrational Spectra of SeVen Hundred Benzene DeriVatiVes; John Wiley & Sons: New York, 1974.

J. Phys. Chem. C, Vol. 114, No. 2, 2010 1259 (42) Apart from the wavenumber shift owing to the transition from ordered to disordered state, one has to be aware that the peak positions in the experimental ellipsometric spectra of oriented films may slightly differ from the true resonance frequencies due to pure optical effect.34 The wavenumber shift for SAMs on metallic surfaces is about 1-2 cm-1 only and therefore hardly discernible, while for monolayers on semiconductors it could be more pronounced34. (43) Uvdal, K.; Ekeroth, J.; Konradsson, P.; Liedberg, B. J. Colloid Interface Sci. 2003, 260, 361. (44) Angelova, P.; Hinrichs, K.; Esser, N.; Kostova, K.; Tsankov, D. Vibr. Spectrosc. 2007, 45, 55.

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