Chemisorbed Monolayers of Corannulene Penta-Thioethers on Gold

Chemisorbed Monolayers of Corannulene Penta-Thioethers on GoldPolina Angelova,*,† Ephrath Solel,‡ Galit Parvari,‡ Andrey Turchanin,† Mark Botoshansky,‡

Armin Golzhauser,† 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 NorthTorrey Pines Road, La Jolla, California 92037, United States

*S Supporting Information

ABSTRACT: Penta(tert-butylthio)corannulene and penta(4-dimethylaminophenylthio)corannulene form highly stablemonolayers on gold surfaces, as indicated by X-ray photo-electron spectroscopy (XPS). Formation of these homoge-neous monolayers involves multivalent coordination of the fivesulfur atoms to gold with the peripheral alkyl or arylsubstituents pointing away from the surface. No dissociation of C−S bonds upon binding could be observed at roomtemperature. Yet, the XPS experiments reveal strong chemical bonding between the thioether groups and gold. Temperature-dependent XPS study shows that the thermal stability of the monolayers is higher than the typical stability of self-assembledmonolayers (SAMs) of thiolates on gold.

■ INTRODUCTION

Formation of monolayers of pentagonal-shaped molecules is achallenging task, since the 5-fold rotational symmetry isincompatible with the translational order of a classical two-dimensional (2D) crystal lattice. Pentagonal tiles cannot cover aplanar surface without leaving open free spaces. Theoreticalmodeling and experiments with pentagonal tiles closely packedon a flat surface suggest two possible quasi-hexagonal crystallinepatterns: either rotator phase or linear patterns of parallelaligned pentagons.1,2

Monolayers of corannulene3 derivatives are of particularinterest because of their three-dimensional (3D) bowl shapeand their affinity for metal ions. Indeed, as representatives ofthe curved π-conjugated carbon systems, their multisitecoordination capacity to various alkaline4−6 and transition7−10

metals has been thoroughly studied. Thus, corannulenemodified surfaces may provide interesting opportunities forpatterned coordination of other molecules11 or various metalions.12,13 Furthermore, such monolayers may allow forfabrication of metal selective and porous carbon nano-membranes.14−17

Recent reports on vapor deposited monolayers ofcorannulene and its derivatives on metal surfaces indicate thatthey organize in quasi-hexagonal patterns with their concaveface pointing away from the surface and binding the metal viatheir π-system. Scanning tunneling microscopy (STM)showed18 that corannulene forms a closely packed paralleltype linear pattern on Cu(110)2 and on Cu(111)19 surfaces. Incontrast, pentachlorocorannulene adopts an antiparallel typelinear pattern18 and pentamethylcorannulene can form a rotatorphase,18,20 while penta-tert-butylcorannulene forms short-rangedomains of antiparallel type linear pattern on Cu(111).21

Here we report on the formation of homogeneousmonolayers of corannulene derivatives, 1,3,5,7,9-penta(tert-butylthio)corannulene, 1, and 1,3,5,7,9-penta(4-dimethylami-nophenyl-thio)corannulene, 2 (Figure 1), on gold surfaces fromsolution. We also show by X-ray photoelectron spectroscopydata that both compounds strongly bind to gold vianondestructive chemisorption of the five thioether groups.Although dissociation of C−S bonds upon binding is notobserved at room temperature, these compounds remain boundto the surface via S−Au bonds even at high temperatures (440K), when cleavage of C−S bonds occurs with loss of theperipheral alkyl or aryl substituents. In contrast to the reportedmonolayers of corannulene, pentachloro- and pentamethylcor-annulene, which bind to the metal at their convex face,20 thethioether groups in 1 and 2 lead to chemical adsorption of themolecules, thus presenting their concave face to the goldsurface.

■ EXPERIMENTAL SECTIONGeneral Methods. Commercially available starting materials and

solvents were used without further purification, unless otherwisestated. All dry solvents were purchased (sure-seal) from Aldrich.1,3,5,7,9-pentachlorocorannulene was synthesized as described ear-lier.22 1H NMR and 13C NMR spectra were recorded on a BrukerUltrashield AV300 spectrometer, operating at 300 MHz (1H) or 75.44MHz (13C) and AV500 operating at 500 MHz (1H) or 125.76 MHz(13C) using CDCl3 as a solvent. Chemical shifts are reported in ppmrelative to internal standard, Me4Si (δ = 0.0). Mass-spectrometryanalyses, MALDI-TOF, were carried out with a MALDI Micromass

Received: November 18, 2012Revised: January 17, 2013Published: January 23, 2013

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spectrometer using α-cyano-4-hydroxycinnamic acid as a matrix.Analytical thin layer chromatography (TLC) was performed on glasssheets precoated with silica gel (Merck, Kieselgel 60, F-254).Preparative thin layer chromatography (TLC) was performed onglass sheets precoated with silica gel (Merck, Kieselgel 60, F-254 0.5mm). Column chromatography was performed with silica gel (Merck60, 230−400 mesh) under pressure. Single crystal X-ray structureanalysis was performed with a Nonius Kappa CCD diffractometerunder stream of cold nitrogen. Data were collected using graphitemonochromatized Mo Kα radiation. Nonius 2006 Collect was used fordata collection and reduction. The structures were resolved bySHELXS-97 and refined using SHELXL-97. Non-hydrogen atomswere 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-2-propanethiolate (1.6 g, 14.2 mmol) in 30 mL of dry 1,3-dimethyl-2-imidazolidinone (DMI) under argon. 1,3,5,7,9-Pentachlorocorannu-lene (0.3 g, 0.7 mmol) was added, and the mixture, which turnedpurple, was stirred at room temperature for 2 days. Toluene wasadded, and the organic phase was washed twice with water and oncewith brine. The organic phase was dried over Na2SO4, and the solventwas removed under reduced pressure. The crude residue was purifiedby column chromatography (silica gel, dichloromethane/hexane 2:8)to afford 0.11 g (22%) of 1, which was recrystallized fromdichloromethane. 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 (31.3mg, 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 DMIwas added, and the mixture was stirred at room temperature for 30min. 1,3,5,7,9-Pentachlorocorannulene (18 mg, 0.043 mmol) wasadded and the mixture was stirred at room temperature for 2 days.Toluene was added, and the organic phase was washed twice withwater and once with brine. The organic phase was dried over Na2SO4,and the solvent was removed under reduced pressure. The cruderesidue was purified by preparative TLC (first n-hexane and thenhexane/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,

135.2, 135.0, 131.0, 124.7, 118.7, 113.3, 40.6. HRMS (ESI): m/zcalculated for C60H56N5S5, 1006.3139 [M+H+]; found, 1006.3143.

Preparation of Monolayers 1 and 2. Polycrystalline Ausubstrates (30 nm thermally evaporated Au on Ti-primed Si(100)wafers, grain size ∼ 50 nm, preferential (111) orientation)23 werepurchased from Georg Albert PVD-Coatings. The Au substrates werecleaned in UV/ozone cleaner for 3 min and stored in absolute ethanolfor 20 min before use. The substrates were immersed in 10−5 Msolution of either 1 in dry DMF or 2 in dry CHCl3 at roomtemperature under air in the dark. After 24 h, the wafers were removedfrom solution, rinsed thoroughly with CHCl3, and blown dry withnitrogen.

Characterization of the Monolayers. XPS measurements wereperformed under ultrahigh vacuum (UHV) conditions employing anOmicron Multiprobe spectrometer. Monochromatic Al Kα source(1486.7 eV, 250 W) and a hemispherical electron energy analyzer wereused. The binding energies were referred to the Au4f7/2 signal at 84.0eV, and the resolution of the spectra is 0.9 eV. An emission angle ofphotoelectrons of 13° was used. Shirley backgrounds and symmetricalVoigt functions with ratio of Gaussian−Lorentzian functions 70:30were used for curve fitting. Thickness of the monolayers was evaluatedfrom the exponential attenuation of the Au4f7/2 peak, employing theattenuation length, λ, of 36 Å.24,25 The calculation of the elementratios was based on the statistical model,26 assuming a homogeneousdistribution of the atoms and applying the following values for λ andsensitivity factors, σ, for C1s, S2p, and N1s of 23.8, 27.7, and 21.2 Åand 1.0, 1.69, and 1.84, respectively. Bulk samples for the XPS analysiswere prepared by drop casting of diluted solutions of the respectivecompounds. For the temperature dependent XPS measurements, thesamples were in situ heated via resistive heating up to ∼440 K on amanipulator in the preparation chamber of the spectrometer.Temperature was increased in steps of 20 K with annealing time of3−15 h.

■ RESULTS AND DISCUSSION

XPS data for the bulk and monolayer samples of corannulenes1 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 bothmonolayer samples are doublets with a branching ratio of 2:1(spin−orbit coupling) and an energy difference between the

Figure 1. Synthesis of corannulene derivatives 1 and 2 and theirproposed orientation on the gold surface. The three types of carbonatoms observed by XPS are color coded: C1 black, C2 blue, and C3red.

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.

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components of 1.2 eV. A binding energy of ∼162.0 eV wasfound for the S2p3/2 components. Note that a similar bindingenergy is typically observed for chemisorbed thiolate species ongold.27,28 The bulk samples of both corannulenes showed theS2p3/2 components at ∼163.5 eV, which is similar tophysisorbed thiol species on gold.29 The observed shift in thebinding energy clearly demonstrates the interaction betweenthe thioether groups and the gold surface atoms in themonolayer formed. Moreover, the presence of only one sulfurspecies (one doublet) in the S2p spectra of the monolayers (cf.Figures 2b and 3b) unambiguously shows that all five thioethergroups in the respective compounds undergo chemisorption onthe 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. Thesevalues are consistent with the expected thicknesses of themonolayers on the basis of the crystallographic structure of 1(Supporting Information), which shows a distance ofapproximately 3.1 Å between the plane defined by the fivesulfur atoms and the plane defined by the methyl groups of thetert-butyl substituents (Figure 4A). This distance, together withthe known length of a typical Au−S bond (2.32 Å),30 matchesthe evaluated monolayer thickness.Comparison of the relative dimensions and symmetry of the

pentathioethers 1 and 2 and the gold surface (Figure 4B) raisesinteresting questions about their binding mode. Based on theX-ray structure of 1 (Supporting Information), the averagedistance between two adjacent sulfur atoms is 5.692 Å, which isnearly twice the interatomic distance between two gold atoms,considering an unreconstructed Au(111) surface. These relativedistances and the mismatch between the pentagonal symmetryof the sulfur atoms in 1, 2, and the hexagonal symmetry of thegold surface leads to the conclusion that at any given bindingorientation only two sulfur atoms can undergo optimal bindingwhile the binding efficiency of the other three sulfurs iscompromised by less energetically favored adsorption sites.Since the XPS data indicate that there is only chemisorbedsulfur in each monolayer, we assume that the symmetricalrestrictions are overcome by the rearrangement of substrateatoms or by a fluxional behavior of the adsorbed corannulenes.It is conceivable that the binding mode involves kinetic

flexibility where the pentagonal molecule vibrates and rotatesaround its main axis of symmetry. Thus, on the average, eachmolecule would behave within the monolayer as a circularobject rather than a pentagon. It seems likely that the fluxionalbehavior would result from symmetry considerations aloneeven without involving the sulfur binding.To obtain further details on the adsorption mode of

corannulenes 1 and 2, we conducted a detailed analysis oftheir C1s, S2p, and N1s XP-signals (Table 1). The C1s signalfor the monolayer of 1 consists of four components (Figure2b). We assign the peak at 284.2 eV to the aromatic carbon(C1, black, Figure 1, Table 1) atoms in the corannulenecore.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 atomsbound to sulfur (C3, red).31 The low intensity peak at about288 eV is attributed to final state effects.31 In comparison to themonolayer sample (Figure 2b), the binding energies of the C1s

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.

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

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signal for the bulk sample (Figure 2a) are shifted to highervalues (see Table 1). This effect most probably arises from apoor electric coupling of the thicker organic film to theunderlying gold substrate.34 The intensity ratio of C1:C2:C3,obtained from the spectrum, is 1:1:0.6 and stands in agreementwith the carbon stoichiometry within 1. Also, an analysis of theelemental 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). Thecomponent at 284.5 eV is assigned to the aromatic carbonatoms of the corannulene core and the phenyl carbons remotefrom the heteroatoms (C1, black, Figure 1), the component at285.5 eV is assigned to the carbon atoms, directly bound to Sand N (C2, red), and the component at 288.0 eV is assigned tothe final state effects. In agreement with the molecularstoichiometry, the intensity ratio of C1:C2 corresponds to1.4:1. The elemental ratios (C/S ≈ 12/1, C/N ≈ 12/1 and N/S ≈ 1/1), calculated by the XPS spectra, are consistent with themolecular formula of 2 (C60H25N5S5). The binding energy ofthe N1s peak is similar for both bulk and monolayer phases(Table 1). This similarity points at a lack of chemicalinteraction between the peripheral dimethylaminophenylgroups and the gold surface in the monolayer.The above-described XPS analysis of the two corannulene

monolayers shows that the molecules (i) adsorb via chemicalbonding of the five thioether groups to the gold substrate; (ii)preserve their stoichiometry, with no rupture of the C−S bondsduring the chemisorption process; (iii) the side groups (tert-butyl in 1 and 4-dimethylaminophenyl in 2) do not interactwith the surface, standing upward or inclined with respect to

the gold surface. Based on these findings, we propose anadsorption model for both compounds where the corannulenebowls 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 strongcovalent bonds,35−39 the binding modes of thioethers are lesswell understood. Early studies suggested that thioethers adsorbon gold via dissociation of one of the C−S bonds andformation of thiolate species.40−42 However, current studiesindicated that chemisorption of thioethers on metal surfaces isnondestructive,43−46 which is in agreement with our XPS dataand quantitative analysis. Yet, the energy of adsorption ofthioethers is usually assumed to be smaller than that ofthiols.44,47 Our XPS spectra show that the S2p binding energyof the chemisorbed thioether species is identical to the S2pbinding energy in alkanethiol SAMs. The binding energycorrelates with the atomic charge at the sulfur and revealssimilar energies of the formed molecule−substrate bonds inboth cases. Since thioethers are considered to involve a sp3-hybridized sulfur with two lone pairs30 and interact with goldsurface atoms through one of these lone pairs,46 we assume thatthey form a dative covalent bond. Interestingly, this bond is ofsimilar strength as the “regular” covalent bonds in thiolatemonolayers.In order to further support the assumption that the studied

pentathioethers have similar adsorption energy as thiolates, weinvestigated the thermal stability of the monolayers bytemperature-dependent XPS measurements. Figure 5 showsthe S2p and C1s XP spectra of the monolayer of 1, recorded atroom temperature before and after thermal annealing in UHV.

Table 1. Peak assignments and deconvolution parameters ofthe monolayer and bulk samples of corannulene 1 and 2 ongold

samplea binding energy (eV) fwhm (eV)c

1 MLb

C1 284.2 1.1C2 284.8 1.1C3 285.6 1.3shakeup satellite 288.0 3.0S2p3/2 161.9 1.21 bulkC1 284.7 1.1C2 285.8 1.3C3 285.2 1.3shakeup satellite 288.9 2.7S2p3/2 thioether 163.5 1.12 MLC1 284.5 1.4C2 285.5 1.7shakeup satellite 288.0 2.8S2p3/2 162.0 1.2N(Me)2 399.5 1.22 bulkC1 284.4 1.1C2 285.2 1.4shakeup satellite 287.8 2.5S2p3/2 thioether 163.2 1.0N(Me)2 399.6 1.4

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

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

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The spectrum changes above 350 K, exhibiting broadening ofthe S2p peak due to the formation of a second doublet with abinding energy of 161.3 eV (S2p3/2). We assign the newdoublet to the formation of gold sulfide species,47,48 whichindicates cleavage of both C−S bonds and desorption of variousmolecular fragments. In the temperature range of 350−440 K,the relative intensity of the atomic sulfur is ∼35% of the overallsulfur intensity (Figure 5b). The intensity ratio of S2p(T)/S2p(RT) was nearly constant over a broad temperature rangewith only a slight decrease of ∼10% upon annealing at 440 K.In contrast, the carbon intensity decreased significantly over theentire temperature range, down to 50% at 440 K, indicatingthat the C−S bonds are more thermally labile than the S−Aubonds. The temperature dependence of the C1s peak revealssome mechanistic details of the decomposition process ofmonolayer 1. The intensity ratio of the C1:C2 componentsincreased from 1:1 at room temperature to 2:1 after annealingat 420 K, indicating preferential loss of the tert-butyl groupsrather than the corannulene core (Figure 5c). Even afterannealing at 440 K where partial desorption of sulfur atomsoccurred, most of the corannulene moieties remained chemi-cally adsorbed on the surface via S−Au bonds (S2p3/2, 162.0eV).The monolayer of 2 exhibited similar behavior as observed

by its S2p, C1s, and N1s XP spectra (Figure 6) recorded at

various temperatures. The temperature-induced changes in theS2p spectra are detected above 370 K, and the C1:C2 intensityratio increased from 1.4:1 to 4:1 (Figure 6a) together with acomplete loss of nitrogen-related species (Figure 6c, d). Thebinding energy of the C1s peak at 440 K (284.3 eV),characteristic of aromatic carbon species, and the gradualdecrease of C:S ratio from 12:1 at room temperature to 5:1were consistent with the formation of surface-bound pentam-ercaptocorannulene. Figure 7 plots the above-describedchanges of the S2p and C1s XP signal intensities.The temperature-dependent XPS data demonstrate the

comparable or even higher thermal stability of our adsorbedmonolayers on gold in comparison with typical oligophenylth-iol monolayers (Tdes ∼ 390 K)33 and alkanethiol monolayers(desorption temperature, Tdes ∼ 350 K).40 Also, after annealingat 440 K, the percentage of chemisorbed sulfur species in the

studied corannulene monolayers was much higher than theliterature data for thiolate SAMs.33,49,50 We attribute thisenhanced thermal stability to the multiple binding of eachmolecule to the gold surface. An additional interaction betweenthe corannulene π-system and the substrate could alsocontribute to their stronger binding.

■ CONCLUSIONSBoth penta-thioether corannulene derivatives, 1 and 2, formhighly stable monolayers on gold substrates. Their non-destructive, multivalent coordination mode is verified by XPS.Formation of these homogeneous monolayers involves bindingof all five sulfur atoms to the gold, while the peripheral groupson sulfur are oriented away from the surface. The bindingenergies of the thioether groups and the thermal stability ofboth monolayers reveal strong chemical interaction betweenthe thioether groups and the gold substrate, comparable to thebonding between thiolates and gold surfaces. In contrast topreviously reported monolayers of pentasubstituted corannu-lenes, which bind the metal at their convex face,18,21 ourcompounds present their concave face to the gold surface,opening new coordination sites and opportunities for surfacechemistry reactions. Further studies into the structure andapplications of these unique monolayers are currently underwayin our laboratories.

■ ASSOCIATED CONTENT*S Supporting InformationFigure S1: Single crystal structure of 1. ORTEP representationof the molecular structure (left) and structure of the unit cell(right). Table S1: Crystallographic data of 1. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*(P.A.) Telephone: +49-521-106-5351. E-mail: [email protected]. (A.G.) Telephone: +49-521-106-6995. E-mail:[email protected]. (E.K.) Telephone: 972-4-829-39-13. E-mail: [email protected] authors declare no competing financial interest.

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

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

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■ ACKNOWLEDGMENTSThe authors thank the German Bundesministerium fur Bildungund Forschung (BMBF) and the Deutsche Forschungsgemein-schaft (SFB 613) for financial support. E.K. is the incumbent ofthe Benno Gitter & Ilana Ben-Ami Chair of Biotechnology,Technion.

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