Chemisorbed Monolayers of Corannulene Penta-Thioethers on Gold

Article pubs.acs.org/Langmuir

Chemisorbed Monolayers of Corannulene Penta-Thioethers on Gold Polina Angelova,*,† Ephrath Solel,‡ Galit Parvari,‡ Andrey Turchanin,† Mark Botoshansky,‡ Armin Gölzhaü ser,† and Ehud Keinan*,‡,§ †

Physics of Supramolecular Systems and Surfaces, University of Bielefeld, Germany Schulich Faculty of Chemistry, Technion - Israel Institute of Technology, Haifa 32000, Israel § Department of Molecular Biology and The Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, United States ‡

S Supporting Information *

ABSTRACT: Penta(tert-butylthio)corannulene and penta(4dimethylaminophenylthio)corannulene form highly stable monolayers on gold surfaces, as indicated by X-ray photoelectron spectroscopy (XPS). Formation of these homogeneous monolayers involves multivalent coordination of the five sulfur atoms to gold with the peripheral alkyl or aryl substituents pointing away from the surface. No dissociation of C−S bonds upon binding could be observed at room temperature. Yet, the XPS experiments reveal strong chemical bonding between the thioether groups and gold. Temperaturedependent XPS study shows that the thermal stability of the monolayers is higher than the typical stability of self-assembled monolayers (SAMs) of thiolates on gold.

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INTRODUCTION Formation of monolayers of pentagonal-shaped molecules is a challenging task, since the 5-fold rotational symmetry is incompatible with the translational order of a classical twodimensional (2D) crystal lattice. Pentagonal tiles cannot cover a planar surface without leaving open free spaces. Theoretical modeling and experiments with pentagonal tiles closely packed on a flat surface suggest two possible quasi-hexagonal crystalline patterns: either rotator phase or linear patterns of parallel aligned pentagons.1,2 Monolayers of corannulene3 derivatives are of particular interest because of their three-dimensional (3D) bowl shape and their affinity for metal ions. Indeed, as representatives of the curved π-conjugated carbon systems, their multisite coordination capacity to various alkaline4−6 and transition7−10 metals has been thoroughly studied. Thus, corannulene modified surfaces may provide interesting opportunities for patterned coordination of other molecules11 or various metal ions.12,13 Furthermore, such monolayers may allow for fabrication of metal selective and porous carbon nanomembranes.14−17 Recent reports on vapor deposited monolayers of corannulene and its derivatives on metal surfaces indicate that they organize in quasi-hexagonal patterns with their concave face pointing away from the surface and binding the metal via their π-system. Scanning tunneling microscopy (STM) showed18 that corannulene forms a closely packed parallel type linear pattern on Cu(110)2 and on Cu(111)19 surfaces. In contrast, pentachlorocorannulene adopts an antiparallel type linear pattern18 and pentamethylcorannulene can form a rotator phase,18,20 while penta-tert-butylcorannulene forms short-range domains of antiparallel type linear pattern on Cu(111).21 © 2013 American Chemical Society

Here we report on the formation of homogeneous monolayers of corannulene derivatives, 1,3,5,7,9-penta(tertbutylthio)corannulene, 1, and 1,3,5,7,9-penta(4-dimethylaminophenyl-thio)corannulene, 2 (Figure 1), on gold surfaces from solution. We also show by X-ray photoelectron spectroscopy data that both compounds strongly bind to gold via nondestructive chemisorption of the five thioether groups. Although dissociation of C−S bonds upon binding is not observed at room temperature, these compounds remain bound to the surface via S−Au bonds even at high temperatures (440 K), when cleavage of C−S bonds occurs with loss of the peripheral alkyl or aryl substituents. In contrast to the reported monolayers of corannulene, pentachloro- and pentamethylcorannulene, which bind to the metal at their convex face,20 the thioether groups in 1 and 2 lead to chemical adsorption of the molecules, thus presenting their concave face to the gold surface.

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EXPERIMENTAL SECTION

General Methods. Commercially available starting materials and solvents were used without further purification, unless otherwise stated. All dry solvents were purchased (sure-seal) from Aldrich. 1,3,5,7,9-pentachlorocorannulene was synthesized as described earlier.22 1H NMR and 13C NMR spectra were recorded on a Bruker Ultrashield AV300 spectrometer, operating at 300 MHz (1H) or 75.44 MHz (13C) and AV500 operating at 500 MHz (1H) or 125.76 MHz (13C) using CDCl3 as a solvent. Chemical shifts are reported in ppm relative to internal standard, Me4Si (δ = 0.0). Mass-spectrometry analyses, MALDI-TOF, were carried out with a MALDI Micromass Received: November 18, 2012 Revised: January 17, 2013 Published: January 23, 2013 2217

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135.2, 135.0, 131.0, 124.7, 118.7, 113.3, 40.6. HRMS (ESI): m/z calculated for C60H56N5S5, 1006.3139 [M+H+]; found, 1006.3143. Preparation of Monolayers 1 and 2. Polycrystalline Au substrates (30 nm thermally evaporated Au on Ti-primed Si(100) wafers, grain size ∼ 50 nm, preferential (111) orientation)23 were purchased from Georg Albert PVD-Coatings. The Au substrates were cleaned in UV/ozone cleaner for 3 min and stored in absolute ethanol for 20 min before use. The substrates were immersed in 10−5 M solution of either 1 in dry DMF or 2 in dry CHCl3 at room temperature under air in the dark. After 24 h, the wafers were removed from solution, rinsed thoroughly with CHCl3, and blown dry with nitrogen. Characterization of the Monolayers. XPS measurements were performed under ultrahigh vacuum (UHV) conditions employing an Omicron Multiprobe spectrometer. Monochromatic Al Kα source (1486.7 eV, 250 W) and a hemispherical electron energy analyzer were used. The binding energies were referred to the Au4f7/2 signal at 84.0 eV, and the resolution of the spectra is 0.9 eV. An emission angle of photoelectrons of 13° was used. Shirley backgrounds and symmetrical Voigt functions with ratio of Gaussian−Lorentzian functions 70:30 were used for curve fitting. Thickness of the monolayers was evaluated from the exponential attenuation of the Au4f7/2 peak, employing the attenuation length, λ, of 36 Å.24,25 The calculation of the element ratios was based on the statistical model,26 assuming a homogeneous distribution of the atoms and applying the following values for λ and sensitivity factors, σ, for C1s, S2p, and N1s of 23.8, 27.7, and 21.2 Å and 1.0, 1.69, and 1.84, respectively. Bulk samples for the XPS analysis were prepared by drop casting of diluted solutions of the respective compounds. For the temperature dependent XPS measurements, the samples were in situ heated via resistive heating up to ∼440 K on a manipulator in the preparation chamber of the spectrometer. Temperature was increased in steps of 20 K with annealing time of 3−15 h.

Figure 1. Synthesis of corannulene derivatives 1 and 2 and their proposed orientation on the gold surface. The three types of carbon atoms observed by XPS are color coded: C1 black, C2 blue, and C3 red. spectrometer using α-cyano-4-hydroxycinnamic acid as a matrix. Analytical thin layer chromatography (TLC) was performed on glass sheets precoated with silica gel (Merck, Kieselgel 60, F-254). Preparative thin layer chromatography (TLC) was performed on glass sheets precoated with silica gel (Merck, Kieselgel 60, F-254 0.5 mm). Column chromatography was performed with silica gel (Merck 60, 230−400 mesh) under pressure. Single crystal X-ray structure analysis was performed with a Nonius Kappa CCD diffractometer under stream of cold nitrogen. Data were collected using graphite monochromatized Mo Kα radiation. Nonius 2006 Collect was used for data collection and reduction. The structures were resolved by SHELXS-97 and refined using SHELXL-97. Non-hydrogen atoms were refined anisotropically and hydrogen atoms isotropically. Synthesis of 1,3,5,7,9-Penta(tert-butylthio)corannulene, 1. A 50 mL round-bottom flask was loaded with sodium 2-methyl-2propanethiolate (1.6 g, 14.2 mmol) in 30 mL of dry 1,3-dimethyl-2imidazolidinone (DMI) under argon. 1,3,5,7,9-Pentachlorocorannulene (0.3 g, 0.7 mmol) was added, and the mixture, which turned purple, was stirred at room temperature for 2 days. Toluene was added, and the organic phase was washed twice with water and once with brine. The organic phase was dried over Na2SO4, and the solvent was removed under reduced pressure. The crude residue was purified by column chromatography (silica gel, dichloromethane/hexane 2:8) to afford 0.11 g (22%) of 1, which was recrystallized from dichloromethane. 1H NMR (500 MHz, CDCl3): δ = 8.50 (s, 5H), 1.51 (s, 45H). 13C NMR (125.76 MHz, CDCl3): δ = 138.4, 135.6, 134.6, 132.6, 47.5, 31.3. MS (MALDI-TOF) m/z: 690.6 [M]. Tm= 307 °C (decomp.) 1,3,5,7,9-Penta(4-dimethylaminobenzenethio)corannulene, 2. A 10 mL round-bottom flask was loaded with sodium hydride (31.3 mg, 60% in mineral oil, 0.8 mmol) under argon. A solution of 4(dimethylamino)thiophenol (0.1 g, 0.6 mmol) in 4 mL of dry DMI was added, and the mixture was stirred at room temperature for 30 min. 1,3,5,7,9-Pentachlorocorannulene (18 mg, 0.043 mmol) was added and the mixture was stirred at room temperature for 2 days. Toluene was added, and the organic phase was washed twice with water and once with brine. The organic phase was dried over Na2SO4, and the solvent was removed under reduced pressure. The crude residue was purified by preparative TLC (first n-hexane and then hexane/dichloromethane 6:4) to afford 2 (26.7 mg, 62.3%). 1H NMR (300 MHz, CDCl3): δ = 7.68 (s, 5H), 7.31 (d, 10H), 6.68 (d, 10H), 2.99 (s, 30H). 13C NMR (75.44 MHz, CDCl3): δ = 150.5, 138.7,

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RESULTS AND DISCUSSION

XPS data for the bulk and monolayer samples of corannulenes 1 and 2 are presented in Figures 2 and 3 in (a) and (b), respectively. As can be seen, the S2p3/2,1/2 spectra of both monolayer samples are doublets with a branching ratio of 2:1 (spin−orbit coupling) and an energy difference between the

Figure 2. XP spectra of 1: (a) C1s and S2p spectra of a bulk sample; (b) C1s and S2p spectra of a monolayer sample on gold. 2218

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Figure 3. XP spectra of 2: (a) C1s, S2p, and N1s spectra of a bulk sample; (b) C1s, S2p, and N1s spectra of a monolayer sample on gold.

components of 1.2 eV. A binding energy of ∼162.0 eV was found for the S2p3/2 components. Note that a similar binding energy is typically observed for chemisorbed thiolate species on gold.27,28 The bulk samples of both corannulenes showed the S2p3/2 components at ∼163.5 eV, which is similar to physisorbed thiol species on gold.29 The observed shift in the binding energy clearly demonstrates the interaction between the thioether groups and the gold surface atoms in the monolayer formed. Moreover, the presence of only one sulfur species (one doublet) in the S2p spectra of the monolayers (cf. Figures 2b and 3b) unambiguously shows that all five thioether groups in the respective compounds undergo chemisorption on the gold substrate (Figure 1). The effective thicknesses of the monolayers, which were evaluated from the attenuation of the substrate Au4f7/2 signals, are ∼5 and ∼6 Å for compounds 1 and 2, respectively. These values are consistent with the expected thicknesses of the monolayers on the basis of the crystallographic structure of 1 (Supporting Information), which shows a distance of approximately 3.1 Å between the plane defined by the five sulfur atoms and the plane defined by the methyl groups of the tert-butyl substituents (Figure 4A). This distance, together with the known length of a typical Au−S bond (2.32 Å),30 matches the evaluated monolayer thickness. Comparison of the relative dimensions and symmetry of the pentathioethers 1 and 2 and the gold surface (Figure 4B) raises interesting questions about their binding mode. Based on the X-ray structure of 1 (Supporting Information), the average distance between two adjacent sulfur atoms is 5.692 Å, which is nearly twice the interatomic distance between two gold atoms, considering an unreconstructed Au(111) surface. These relative distances and the mismatch between the pentagonal symmetry of the sulfur atoms in 1, 2, and the hexagonal symmetry of the gold surface leads to the conclusion that at any given binding orientation only two sulfur atoms can undergo optimal binding while the binding efficiency of the other three sulfurs is compromised by less energetically favored adsorption sites. Since the XPS data indicate that there is only chemisorbed sulfur in each monolayer, we assume that the symmetrical restrictions are overcome by the rearrangement of substrate atoms or by a fluxional behavior of the adsorbed corannulenes. It is conceivable that the binding mode involves kinetic

Figure 4. Proposed binding geometry of 1 to the gold surface. The structure of 1 was adopted from its crystal structure. (A) Estimated thickness of the monolayer on the basis of the crystallographic data. (B) Overlay of the crystal structure over modeled Au(111) surface with matching scale. The hydrogen atoms and tert-butyl groups were omitted for clarity.

flexibility where the pentagonal molecule vibrates and rotates around its main axis of symmetry. Thus, on the average, each molecule would behave within the monolayer as a circular object rather than a pentagon. It seems likely that the fluxional behavior would result from symmetry considerations alone even without involving the sulfur binding. To obtain further details on the adsorption mode of corannulenes 1 and 2, we conducted a detailed analysis of their C1s, S2p, and N1s XP-signals (Table 1). The C1s signal for the monolayer of 1 consists of four components (Figure 2b). We assign the peak at 284.2 eV to the aromatic carbon (C1, black, Figure 1, Table 1) atoms in the corannulene core.31,32 We assign the peak at 284.8 eV to the methyl groups (C2, blue), and the peak at 285.6 eV to the carbon atoms bound to sulfur (C3, red).31 The low intensity peak at about 288 eV is attributed to final state effects.31 In comparison to the monolayer sample (Figure 2b), the binding energies of the C1s 2219

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the gold surface. Based on these findings, we propose an adsorption model for both compounds where the corannulene bowls present their concave face toward the gold surface (Figure 1). While thiols are known to bind noble metals via oxidative addition of the S−H bond to the metal surface, forming strong covalent bonds,35−39 the binding modes of thioethers are less well understood. Early studies suggested that thioethers adsorb on gold via dissociation of one of the C−S bonds and formation of thiolate species.40−42 However, current studies indicated that chemisorption of thioethers on metal surfaces is nondestructive,43−46 which is in agreement with our XPS data and quantitative analysis. Yet, the energy of adsorption of thioethers is usually assumed to be smaller than that of thiols.44,47 Our XPS spectra show that the S2p binding energy of the chemisorbed thioether species is identical to the S2p binding energy in alkanethiol SAMs. The binding energy correlates with the atomic charge at the sulfur and reveals similar energies of the formed molecule−substrate bonds in both cases. Since thioethers are considered to involve a sp3hybridized sulfur with two lone pairs30 and interact with gold surface atoms through one of these lone pairs,46 we assume that they form a dative covalent bond. Interestingly, this bond is of similar strength as the “regular” covalent bonds in thiolate monolayers. In order to further support the assumption that the studied pentathioethers have similar adsorption energy as thiolates, we investigated the thermal stability of the monolayers by temperature-dependent XPS measurements. Figure 5 shows the S2p and C1s XP spectra of the monolayer of 1, recorded at room temperature before and after thermal annealing in UHV.

Table 1. Peak assignments and deconvolution parameters of the monolayer and bulk samples of corannulene 1 and 2 on gold samplea 1 MLb C1 C2 C3 shakeup satellite S2p3/2 1 bulk C1 C2 C3 shakeup satellite S2p3/2 thioether 2 ML C1 C2 shakeup satellite S2p3/2 N(Me)2 2 bulk C1 C2 shakeup satellite S2p3/2 thioether N(Me)2

binding energy (eV)

fwhm (eV)c

284.2 284.8 285.6 288.0 161.9

1.1 1.1 1.3 3.0 1.2

284.7 285.8 285.2 288.9 163.5

1.1 1.3 1.3 2.7 1.1

284.5 285.5 288.0 162.0 399.5

1.4 1.7 2.8 1.2 1.2

284.4 285.2 287.8 163.2 399.6

1.1 1.4 2.5 1.0 1.4

a Color coding of the carbons: C1, black; C2, red; C3, blue. bML: monolayer. cfwhm: full width at half-maximum.

signal for the bulk sample (Figure 2a) are shifted to higher values (see Table 1). This effect most probably arises from a poor electric coupling of the thicker organic film to the underlying gold substrate.34 The intensity ratio of C1:C2:C3, obtained from the spectrum, is 1:1:0.6 and stands in agreement with the carbon stoichiometry within 1. Also, an analysis of the elemental ratio between carbon and sulfur (C:S ≈ 8:1) corresponds well to the molecular composition (C40H50S5). Similar results were obtained for 2. In its monolayer phase, the C1s peak is fitted with three components (Figure 3b). The component at 284.5 eV is assigned to the aromatic carbon atoms of the corannulene core and the phenyl carbons remote from the heteroatoms (C1, black, Figure 1), the component at 285.5 eV is assigned to the carbon atoms, directly bound to S and N (C2, red), and the component at 288.0 eV is assigned to the final state effects. In agreement with the molecular stoichiometry, the intensity ratio of C1:C2 corresponds to 1.4:1. The elemental ratios (C/S ≈ 12/1, C/N ≈ 12/1 and N/ S ≈ 1/1), calculated by the XPS spectra, are consistent with the molecular formula of 2 (C60H25N5S5). The binding energy of the N1s peak is similar for both bulk and monolayer phases (Table 1). This similarity points at a lack of chemical interaction between the peripheral dimethylaminophenyl groups and the gold surface in the monolayer. The above-described XPS analysis of the two corannulene monolayers shows that the molecules (i) adsorb via chemical bonding of the five thioether groups to the gold substrate; (ii) preserve their stoichiometry, with no rupture of the C−S bonds during the chemisorption process; (iii) the side groups (tertbutyl in 1 and 4-dimethylaminophenyl in 2) do not interact with the surface, standing upward or inclined with respect to

Figure 5. XP spectra of a monolayer of 1 on Au at room temperature (top) and after annealing at different temperatures (the intensity scales can be quantitatively compared): (a) C1s signal; (b) S2p signal; (c) illustration of the proposed desorption sequence. 2220

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The spectrum changes above 350 K, exhibiting broadening of the S2p peak due to the formation of a second doublet with a binding energy of 161.3 eV (S2p3/2). We assign the new doublet to the formation of gold sulfide species,47,48 which indicates cleavage of both C−S bonds and desorption of various molecular fragments. In the temperature range of 350−440 K, the relative intensity of the atomic sulfur is ∼35% of the overall sulfur intensity (Figure 5b). The intensity ratio of S2p(T)/ S2p(RT) was nearly constant over a broad temperature range with only a slight decrease of ∼10% upon annealing at 440 K. In contrast, the carbon intensity decreased significantly over the entire temperature range, down to 50% at 440 K, indicating that the C−S bonds are more thermally labile than the S−Au bonds. The temperature dependence of the C1s peak reveals some mechanistic details of the decomposition process of monolayer 1. The intensity ratio of the C1:C2 components increased from 1:1 at room temperature to 2:1 after annealing at 420 K, indicating preferential loss of the tert-butyl groups rather than the corannulene core (Figure 5c). Even after annealing at 440 K where partial desorption of sulfur atoms occurred, most of the corannulene moieties remained chemically adsorbed on the surface via S−Au bonds (S2p3/2, 162.0 eV). The monolayer of 2 exhibited similar behavior as observed by its S2p, C1s, and N1s XP spectra (Figure 6) recorded at

Figure 7. Plot of the temperature dependence of the XPS intensities of the C1s and S2p signals of monolayers 1 and 2 as a function of temperature.

studied corannulene monolayers was much higher than the literature data for thiolate SAMs.33,49,50 We attribute this enhanced thermal stability to the multiple binding of each molecule to the gold surface. An additional interaction between the corannulene π-system and the substrate could also contribute to their stronger binding.

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CONCLUSIONS Both penta-thioether corannulene derivatives, 1 and 2, form highly stable monolayers on gold substrates. Their nondestructive, multivalent coordination mode is verified by XPS. Formation of these homogeneous monolayers involves binding of all five sulfur atoms to the gold, while the peripheral groups on sulfur are oriented away from the surface. The binding energies of the thioether groups and the thermal stability of both monolayers reveal strong chemical interaction between the thioether groups and the gold substrate, comparable to the bonding between thiolates and gold surfaces. In contrast to previously reported monolayers of pentasubstituted corannulenes, which bind the metal at their convex face,18,21 our compounds present their concave face to the gold surface, opening new coordination sites and opportunities for surface chemistry reactions. Further studies into the structure and applications of these unique monolayers are currently underway in our laboratories.

Figure 6. XP spectra of a monolayer of 2 on Au at room temperature (top) and after annealing at different temperatures (the intensity scales can be quantitatively compared): (a) C1s signal; (b) S2p signal; (c) N1s signal; (d) illustration of the proposed desorption sequence.

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various temperatures. The temperature-induced changes in the S2p spectra are detected above 370 K, and the C1:C2 intensity ratio increased from 1.4:1 to 4:1 (Figure 6a) together with a complete loss of nitrogen-related species (Figure 6c, d). The binding energy of the C1s peak at 440 K (284.3 eV), characteristic of aromatic carbon species, and the gradual decrease of C:S ratio from 12:1 at room temperature to 5:1 were consistent with the formation of surface-bound pentamercaptocorannulene. Figure 7 plots the above-described changes of the S2p and C1s XP signal intensities. The temperature-dependent XPS data demonstrate the comparable or even higher thermal stability of our adsorbed monolayers on gold in comparison with typical oligophenylthiol monolayers (Tdes ∼ 390 K)33 and alkanethiol monolayers (desorption temperature, Tdes ∼ 350 K).40 Also, after annealing at 440 K, the percentage of chemisorbed sulfur species in the

ASSOCIATED CONTENT

S Supporting Information *

Figure S1: Single crystal structure of 1. ORTEP representation of the molecular structure (left) and structure of the unit cell (right). Table S1: Crystallographic data of 1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(P.A.) Telephone: +49-521-106-5351. E-mail: polina@physik. uni-bielefeld.de. (A.G.) Telephone: +49-521-106-6995. E-mail: [email protected]. (E.K.) Telephone: 9724-829-39-13. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 2221

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ACKNOWLEDGMENTS The authors thank the German Bundesministerium für Bildung und Forschung (BMBF) and the Deutsche Forschungsgemeinschaft (SFB 613) for financial support. E.K. is the incumbent of the Benno Gitter & Ilana Ben-Ami Chair of Biotechnology, Technion.

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