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SAMs of Shape-Persistent Macrocycles: Structure and Binding on HOPG and Au(111) D. Borissov,† A. Ziegler,‡ S. Ho¨ger,*,‡,§ and W. Freyland*,† University of Karlsruhe, Institute of Physical Chemistry, Kaiserstrasse 12, 76128 Karlsruhe, Germany, and Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Received November 21, 2003. In Final Form: January 21, 2004 Self-assembled monolayers of shape-persistent macrocycles have been adsorbed on highly oriented pyrolytic graphite and Au(111) substrates. Their structure and binding characteristics have been investigated by scanning tunneling microscopy, X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy experiments. Special functionalization of the macrocycles for the first time has enabled their self-assembling and binding on a metal substrate and opened a new route for new functional nanostructures.
Self-assembly is of particular scientific and technological interest since it provides an attractive route to patterned nanostructures by the bottom-up approach. The pattern periodicities are related to the molecular weight of the organized objects that can be colloidal, macromolecular, molecular, and supramolecular.1-3 Assemblies of molecular and supramolecular building blocks have been intensively investigated regarding the type of interactions that govern self-assembly and the stability and structure of the assembled arrays.4-9 Among them, self-assembled monolayers lie at the forefront of current chemical and material research. Shape-persistent macrocycles such as 1 (Chart 1) are attractive objects for the preparation of self-assembled monolayers (SAMs).10-12 They are composed of a relatively rigid molecular backbone with flexible alkyl chains attached to the outside, and recently it has been shown that they form ordered monolayers at highly oriented pyrolytic graphite (HOPG) surfaces.13-18 In addition to most other molecules able to form SAMs, macrocycles such as 1 offer the possibility to incorporate functional groups * To whom correspondence should be addressed. E-mail:
[email protected] (W. Freyland) and
[email protected] (S. Ho¨ger). Fax: ++49 721 608 6662 (W. Freyland). † University of Karlsruhe. ‡ Max Planck Institute for Polymer Research. § Current address: Polymer Institute, University of Karlsruhe, Hertzstr. 16, 76187 Karlsruhe, Germany. (1) Lehn, J. M. Supramolecular Chemistry; VCH: Weinheim, 1995. (2) Whitesides, G. M.; Boncheva, M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4769. (3) Spatz, J. P.; Mo¨ssmer, S.; Hartmann, C.; Mo¨ller, M.; Herzog, T.; Krieger, M.; Boyen, H. G.; Ziemann, P.; Kabius, B. Langmuir 2000, 16, 407. (4) Yokoyanna, T.; Yokoyama, S.; Kamikado, T.; Okuno, Y.; Mashiko, S. Nature 2001, 413, 619. (5) Samori, P.; Rabe, J. P. J. Phys.: Condens. Matter 2002, 14, 9955. (6) Giancarlo, L. C.; Flynn, G. W. Annu. Rev. Phys. Chem. 1998, 49, 297. (7) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (8) Abdel-Mottaleb, M. M. S.; Gomar-Nadal, E.; De Feyter, S.; Zdanowska, M.; Veciana, J.; Rovira, C.; Amabilino, D. B.; De Schryver, F. C. Nano Lett. 2003, 3, 1375. (9) Rabe, J. P.; Buchholz, S.; Askadskaya, L. Synth. Met. 1993, 54, 339. (10) For recent reviews on shape-persistent macrocycles see, e.g.: Ho¨ger, S. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 2685. (11) Grave, C.; Schlu¨ter, A. D. Eur. J. Org. Chem. 2002, 3075. (12) Zhao, D.; Moore, J. S. Chem. Commun. 2003, 807. (13) Ho¨ger, S.; Bonrad, K.; Mourran, A.; Beginn, U.; Mo¨ller, M. J. Am. Chem. Soc. 2001, 123, 5651.
Chart 1
in their interior while keeping the molecular dimensions and thus the pattern periodicity unaffected.15-18 Functionalized shape-persistent macrocycles allow therefore not only the creation of nanopatterned surfaces but the formation of functionalized nanopatterned surfaces. Moreover, choosing the proper functionalization should tailor the binding properties toward a given substrate. In this communication, we present a detailed study of the arrangement and binding of SAMs of 1 and 2b on different substrates and show how it is affected by the effect of sulfur functionalization as well as the deposition procedure and experimental conditions (see the Supporting Information). (14) Fischer, M.; Lieser, G.; Rapp, A.; Schnell, I.; Mamdouh, W.; De Feyter, S.; De Schryver, F.-C.; Ho¨ger, S. J. Am. Chem. Soc. 2004, 126, 214. (15) Ho¨ger, S.; Meckenstock, A.-D. Chem.sEur. J. 1999, 5, 1686. (16) Fischer, M.; Ho¨ger, S. Eur. J. Org. Chem. 2003, 441. (17) Grave, C.; Lentz, D.; Scha¨fer, A.; Samori, P.; Rabe, J. P.; Franke, P.; Schlu¨ter, A. D. J. Am. Chem. Soc. 2003, 125, 6907. (18) Kro¨mer, J.; Rios Carreras, I.; Fuhrmann, G.; Musch, C.; Wunderlin, M.; Debaerdemaeker, T.; Mena-Osteritz, E.; Ba¨uerle, P. Angew. Chem., Int. Ed. 2000, 39, 3481.
10.1021/la030424h CCC: $27.50 © 2004 American Chemical Society Published on Web 03/05/2004
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Figure 1. STM images of self-assembly of the shape-persistent macrocycle 1 taken at the solution-HOPG interface with Vbias ) 1.7 V and It ) 1 nA; (a) 50 × 50 nm2; (b) 15 × 15 nm2. The STM images (and those in other figures) are not subjected to any manipulation or image processing.
Figure 1 shows scanning tunneling microscopy (STM) images of 1 spontaneously physisorbed as a monolayer on HOPG acquired with a tunneling current It ) 1 nA and a bias voltage Vbias ) 1.7 V. The image was taken during scanning at the solution-HOPG interface. A well-ordered array of the macrocycles can be clearly seen. The twodimensional unit cell contains one macrocycle (Z ) 1) and has lattice constants of A ) 3.8 ( 0.32 nm, B ) 6.1 ( 0.45 nm, and Γ ) 75 ( 2°. The drift-dependent effects on the lattice constants are considered as described by Staub at al.19 A well-defined arrangement of the molecules is achieved as long as the applied bias voltage is kept larger than 1.4 V. To understand this observation, the following aspects are of interest. In general, scanning tunneling microscopy maps the electron distribution at the interface and the tunneling current is determined by the transition probability and both the tip and substrate density of states. In the particular case of an adsorbed molecule, the tunneling process is enhanced if the highest occupied molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LUMO) matches with the Fermi level of the tip. It is reasonable to assume for molecule 1 that its HOMO and LUMO are separated by more than 1.4 eV. Therefore, when the bias voltage is in this range, a greater contrast is expected as seen in Figure 1.6 Furthermore, we have to consider the impact of the applied electric field between the tip and substrate on self-assembling itself during scanning. Taking into account that the interaction of molecule 1 with HOPG is rather weak and of van der Waals type, the effect of the electrostatic field on the selfassembly is not negligible. Another interesting problem concerns the lateral intermolecular interaction of the selfassembled molecules. Interdigitation of the peripheral alkyl chains obviously plays a role, which is indicated in the STM image taken at higher resolution (Figure 1b).9 Despite the ability of 1 to form well-organized monolayers on HOPG, the capability of the macrocycles to form organized monolayers on other substrates, such as gold, is an attractive goal for several applications (sensing, metal deposition, etc.).20 Attempts to form SAMs of 1 on gold substrates were not successful. This may have several reasons: for instance, the balance between the lateral (19) Staub, R.; Alliata, D. Rev. Sci. Instrum. 1995, 66, 2513. (20) For Calixarenes on Au(111) see: Pan, G.-B.; Liu, J.-M.; Zhang, H.-M.; Wan, L.-J.; Zheng, Q.-Y.; Bai, C.-L. Angew. Chem., Int. Ed. 2003, 42, 2747.
intermolecular and the adsorbate-Au(111) interactions might not be optimal; the lack of needed functionalization which could bind 1 to Au(111); the roughness of the Au(111) surface in comparison with the large flat HOPG surface. Alkanethiols easily form a stable adlayer on Au(111) yielding (x3 × x3)R30° superstructures.21-24 Recently, even alkyl sulfides have been shown to produce steady self-assembled monolayers on Au(111).25 Therefore, macrocycles 2b were prepared by carbodiimide coupling of 2a and 2-(ethylmercapto)-ethylamine hydrochloride yielding the ethylmercapto-ethylamide containing compounds 2b. These structures now contain intraannular functional groups (amides) and additional thioether groups for the binding at the Au substrate. The effect of the sulfur functionalization on its adsorption properties is remarkable which is clearly seen in the STM images showing an ordered assembly of the 2b molecules adsorbed on a singlecrystal Au(111) (Figure 2a). The images were taken at the air-SAM interface after preparation of the SAM by adsorption from solution at 70 °C. Although the arrangement is not as perfect as in the case of molecule 1 on HOPG, a uniform average superstructure in a wide range all over the scan area is clearly visible (see e.g. Figure 2a). To make sure that the observed periodicity is not an imaging artifact, we have rotated the scanning area by different angles. The molecules are organized into rows densely packed which is indicative of vigorous intermolecular interactions. The impact of the roughness of the substrate underneath on the assembly is clearly exhibited in several places (right top of Figure 2a) where the order is perturbed. The width of the molecular rows is approximately 5 ( 0.4 nm, indicating that here also interdigitation of the alkyl periphery to a highly dense packed adlayer occurs. To investigate how the packing of 2b is influenced at the vacuum interface, ultrahigh vacuum scanning tunneling microscopy (UHV-STM) experiments were carried out. Figure 2b displays an image revealing the molecular resolution topography of a SAM of 2b on Au(111) under UHV conditions showing that the (21) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (22) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (23) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (24) Camillone, N.; Chidsey, C. E. D., III; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (25) Beulen, M. W. J.; Kastenberg, M. I.; Van Veggel, F. C. J. M.; Reinhoudt, D. N. Langmuir 1998, 14, 7463.
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Figure 2. STM images of SAMs of molecule 2b adsorbed on Au(111). (a) Image recorded at the air-substrate interface (200 × 200 nm2); the z-profile taken along the black line exhibits a row width of 5 ( 0.4 nm. (b) The same SAMs but studied with higher magnification at the vacuum-substrate interface (40 × 40 nm2).
molecular arrangement does not change significantly. From the z-profile measurements, we conclude that the molecule 2b is lying flat which is characteristic for planar molecules with a π system parallel to the surface.26 In the case of 2, we could not resolve the empty space in the macrocycle which presumably is due to the functionalization of the ring with the amides and thiolether groups. To understand the binding of 2b on the Au(111) substrate, X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) measurements have been carried out after UHV-STM characterization. As expected, the C 1s, Au 4f, and O 1s core level peaks are clearly resolved. The C 1s peak seems to be shifted to higher binding energies (BEs) by 0.3 eV. However, this lies within the error limits of the spectrometer. The pronounced Au 4f7/2 “bulk” peak (83.84 eV) is found to be in good agreement with literature values (83.95 eV).27 In Figure 3a are shown the N 1s (399.5 eV) and S 2p (163.4 eV) emissions. The peaks exhibit a low intensity because of the small photoionization cross-section of these atoms at the respective excitation energy. Nevertheless, the N/S intensity ratio is consistent with the chemical composition of the 2b molecules. The N 1s peak at 399.5 eV corresponds to an oxidation state of N which is the same for similar nitrogen organic compounds.28 The interaction of sulfur with the Au surface can be elucidated by examining the BEs of S 2p. It is known that elemental sulfur (S8) adsorbed on a metal has an S 2p3/2 BE of 164.2 eV.29 Furthermore, sulfur in unbound alkylthiols and disulfides exhibits a positive shift (BE > 163 eV).27 Although in our spectra (see the inset of Figure 3a) we were not able to resolve the S 2p doublet, the peak located at 163.4 eV suggests that some amount of sulfur (26) Bohringer, M.; Morgenstern, K.; Schneider, W.-D.; Berndt, R.; Mauri, F.; De Vita, A.; Car, R. Phys. Rev. Lett. 1999, 83, 324. (27) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (28) Wagner, C. D.; et al. Handbook of X-ray Photoelectron Spectroscopy; Muilenburg, G. E., Ed.; Perkin-Elmer Corp.: Eden Prairie, MN, 1979. (29) Riga, R.; Verbist, J. J. J. Chem. Soc., Perkin Trans. 2 1983, 10, 1545.
Figure 3. XPS and UPS spectra of SAMs of molecule 2b on Au(111). (a) XPS spectra of the SAMs and the chemical shift of the S 2p level relative to unbound S (inset). (b) Comparison of the UPS spectra of the bare Au(111) crystal and SAM-covered Au(111) (dotted line); the inset shows the respective Fermi level calibrations.
of the 2b SAM is not bound to Au(111). Nevertheless, the shoulder at 162.1 eV clearly seen in the spectra is in good agreement with what has been generally found for alkylthiols bound to Au.30-34 The observed difference in BEs with respect to the unbound sulfur is explained as a chemical shift due to the formation of a sulfur-gold
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bond and by screening of the S 2p core hole by the substrate electrons. This indicates that some “thiolate-like” sulfur is formed on the Au surface causing an enhanced adhesion strength of the 2b monolayer on Au. It is quite unlikely that the S-C bond cleavage takes place upon the adsorption. Moreover, the chemical state of the sulfur is not explicitly known which is not surprising bearing in mind that even in the case of alkylthiols adsorbed on Au the S-H bond dissociation has been questioned recently.35 The work function of Au(111) modified by the 2b SAMs has been determined by ultraviolet photoelectron spectroscopy simultaneously with the XPS spectra. Figure 3b presents the spectra of the 2b SAM on Au(111) and the bare gold substrate prepared by sputtering the SAMAu(111) sample (dashed line) for 15 min with an Ar-ion gun. In the case of the sputtered Au substrate, the spectra in the range of 2-7 eV show the characteristic Au 5d signal. Indeed, the shoulder at 2.5 eV and the peaks at 4.75 and 6.5 eV match with the main peaks of Au 5d. By comparing both spectra, a difference of 0.65 eV in the work function of Au(111) functionalized with 2b molecules was found. For the evaluation of the work function, we took the difference of the total spectral width between the sample with the 2b SAM and the sputtered bare Au(111) (30) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (31) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (32) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 4058. (33) Himmel, H.-J.; Wo¨ll, C.; Gerlach, R.; Polanski, G.; Rubahn, H.G. Langmuir 1997, 13, 602. (34) Zhong, C.-J.; Brush, R. C.; Anderegg, J.; Porter, M. D. Langmuir 1999, 15, 518. (35) Gro¨nbeck, H.; Curioni, A.; Andreoni, W. J. Am. Chem. Soc. 2000, 122, 3843.
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substrate.36 The inset of the picture represents the calibration of the Fermi levels of both spectra. The change in the work function following the adsorption of molecules onto a clean surface is a well-established fact, and the reduced work function can be associated with a partial filling of antibonding orbitals of the adsorbate. This is also supported by theoretical calculations which claim that the S atom in the SAM systems bears a charge of about -0.4e.37 In summary, we have shown that shape-persistent macrocycles can be functionalized in the interior with thioether groups so that they are able to form SAMs on Au(111) substrates. The stability of these monolayers allows a detailed investigation of the binding by XPS and UPS measurements which yields binding characteristics similar to those of alkanethiols with a shift of the Au(111) work function by 0.65 eV. The special functionalization of 2b shows a potential for new functional nanostructures. Acknowledgment. We thank the CFN at the University of Karlsruhe, the DFG and the Zentrum fu¨r Multifunktionelle Werkstoffe und Miniaturisierte Funktionseinheiten (BMBF 03N 6500) as well as the Fonds der Chemischen Industrie for financial support of this work. Supporting Information Available: The synthesis of 1 and 2; STM, XPS, and UPS experimental details; preparations of the substrates and SAMs. This material is available free of charge via the Internet at http://pubs.acs.org. LA030424H (36) Christmann, K. Introduction to Surface Physical Chemistry; Springer: New York, 1991. (37) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (38) Fischer, M.; Ho¨ger, S. Tetrahedron 2003, 59, 9441.
