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Reactivity of Vinyl-Terminated Self-Assembled Monolayers on Gold: Olefin Cross-Metathesis Reactions Jungkyu K. Lee, Kyung-Bok Lee, Dong Jin Kim, and Insung S. Choi* Department of Chemistry and School of Molecular Science (BK21), Center for Molecular Design and Synthesis, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea Received May 17, 2003. In Final Form: August 16, 2003 Introduction of various organic functional groups onto organic thin films is a first step in their applications to sensors, catalysis, and nanotechnology in general. In this paper, we studied the reactivity of vinylterminated self-assembled monolayers (SAMs) of undec-10-ene-1-thiol on gold toward olefin cross-metathesis (CM). Vinyl groups on SAMs were successfully converted into R,β-unsaturated carbonyl groups by CM with acrylic acid, methyl acrylate, and acrylamide, and CM was confirmed by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and contact-angle measurement. Ruthenium-catalyzed olefin CM has offered a versatile strategy for functionalization in solution chemistry with mild reaction conditions and a high compatibility in functional groups. Our result shows that various useful functional groups can be introduced to SAMs on gold (and other solid surfaces) by olefin CM and suggests an alternative to the synthesis of desired molecules in solution.
Introduction Self-assembled monolayers (SAMs), particularly SAMs of alkanethiolates on Au(111) surfaces and of siloxanes on SiO2 surfaces, are one of the topics intensively studied in the past decade because of fundamental interest in interfacial reactions1 and many technological applications such as sensors, catalysis, and nanotechnology.2,3 Various organic functionalities have been introduced to SAMs by either separate solution synthesis of a desired molecule and subsequent formation of SAMs or direct chemical modification of the tail group of SAMs.1,4,5 The separate synthesis of the self-assembling molecules, in principle, gives an opportunity to introduce virtually any functional group but in practice requires cumbersome syntheses and shows a limited compatibility of functional groups (especially for siloxanes). Another approach, chemical transformation on the SAMs, has mainly focused on substitution reactions of activated carboxylic acid groups.6-8 Recently, Kwon and Mrksich used the Diels-Alder reaction of cyclopentadiene and benzoquinone to introduce various functional groups onto the surface.9 Among the coupling reactions between two molecular moieties applicable to the SAMs, olefin cross-metathesis (CM) shows several advantages, including mild reaction conditions and a great compatibility in functional groups.10 Vinyl-terminated SAMs have previously been studied in electrochemical and chemical oxidation to aldehyde and carboxylic acid groups,11 in-plane oligomerization by γ-ray exposure,12 * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (2) Kakkar, A. K. Chem. Rev. 2002, 102, 3579-3588. (3) Flink, S.; Veggel, F. C. J. M. V.; Reinhoudt, D. N. Adv. Mater. 2000, 12, 1315-1328. (4) Sullivan, T. P.; Huck, W. T. S. Eur. J. Org. Chem. 2003, 17-29. (5) Chechik, V.; Crooks, R. M.; Stirling, C. J. M. Adv. Mater. 2000, 12, 1161-1171. (6) Hutt, D. A.; Leggett, G. J. Langmuir 1997, 13, 2740-2748. (7) Yang, H. C.; Dermody, D. L.; Xu, C.; Ricco, A. J.; Crooks, R. M. Langmuir 1996, 12, 726-735. (8) Yan, L.; Marzolin, C.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 6704-6712. (9) Kwon, Y.; Mrksich, M. J. Am. Chem. Soc. 2002, 124, 806-812 and references therein. (10) Chatterjee, A. K.; Grubbs, R. H. Angew. Chem., Int. Ed. 2002, 41, 3171-3174 and references therein.
Scheme 1. Schematic Description of the Procedure.
and ring-opening metathesis polymerization.13,14 Herein, we report the reactivity of vinyl-terminated SAMs to CM and the formation of R,β-unsaturated carbonyl groups by CM. Results and Discussion Scheme 1 shows a general procedure for CM with vinylterminated SAMs. After formation of SAMs of undec-10ene-1-thiol on gold, R,β-unsaturated carbonyl compounds were screened for CM. To the reaction vial containing the SAM-coated gold substrate and CH2Cl2 (10 mL) were added ruthenium catalyst 1 (0.1 mmol) and a compound (2, 3, or 4; 2 mmol), and the mixture was heated at 50 °C for 4 h. We selected R,β-unsaturated carbonyl compounds as a model counterpart of CM reactions because the solution CM reactions between terminal olefins and R,βunsaturated compounds have been well-characterized15,16 and there are a variety of applications that require the introduction of carboxylic acid and its derivatives to SAMs.6-8 After the CM, the resulting gold substrate was (11) Maoz, R.; Cohen, S. R.; Sagiv, J. Adv. Mater. 1999, 11, 55-61 and references therein. (12) Peanasky, J. S.; McCarley, R. L. Langmuir 1998, 14, 113-123. (13) Juang, A.; Scherman, O. A.; Grubbs, R. H.; Lewis, N. S. Langmuir 2001, 17, 1321-1323. (14) Harada, Y.; Girolami, G. S.; Nuzzo, R. G. Langmuir 2003, 19, 5104-5114. (15) Choi, T.; Chatterjee, A. K.; Grubbs, R. H. Angew. Chem., Int. Ed. 2001, 40, 1277-1279. (16) Choi, T.; Lee, C. W.; Chatterjee, A. K.; Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 10417-10418.
10.1021/la034859g CCC: $25.00 © 2003 American Chemical Society Published on Web 09/03/2003
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Langmuir, Vol. 19, No. 20, 2003
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Figure 1. Polarized infrared external reflectance spectroscopy (PIERS) spectra and water contact angles of (a) SAMs of undec10-ene-1-thiol and SAMs after CM with (b) acrylic acid, (c) methyl acrylate, and (d) acrylamide. PIERS spectra were obtained in the single reflection mode using an argon-purged Digilab FT-IR spectrometer with a narrow band mercury-cadmium-telluride detector.
thoroughly washed with CH2Cl2, acetone, deionized water, and ethanol and dried under a stream of argon. The CM on the SAMs was characterized by Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and contact-angle measurement.17,18 IR spectra (Figure 1) show that CM was successfully applied to the vinyl-terminated SAMs. Initially, the IR spectrum of the vinyl-terminated SAMs of undec-10-ene-1-thiol showed peaks associated with the ω-olefin group:12 912 cm-1 (CsH out-of-plane deformation of dCH2 group), 993 cm-1 (CdC out-of-plane deformation), 1643 cm-1 (CdC stretch), 2983 cm-1 (dCH2 symmetric), 3007 cm-1 (β CsH stretch), and 3084 cm-1 (dCH2 antisymmetric; Figure 1a). After the CM, the peaks from the ω-olefin group disappeared and peaks from the CM product were observed. Figure 1b-d shows the IR spectra of the substrate coupled with acrylic acid, methyl acrylate, and acrylamide, respectively. The IR spectrum of the CM product with acrylic acid shows a complete disappearance of peaks from the ω-olefin group and appearance of CdO stretching peak from the CM product at 1723 cm-1. The CM with either methyl acrylate or acrylamide was not quantitative. We could still observe the characteristic peaks of the ω-olefin group (although with a low or negligible intensity) as well as the peaks from the CM adduct (1735 cm-1 for the CM with methyl acrylate and 1630 and 1685 cm-1 for the CM with acrylamide). The CM was also confirmed by contact-angle measurement and XPS. The water contact angle was changed from 103° (for vinyl-terminated SAMs) to 21° (after the CM with acrylic acid), 75° (after the CM with methyl acrylate), and 23° (after the CM with acrylamide). XPS spectra show the peaks from O(1s) and N(1s), which also support the successful CM on the SAMs (Figure 2). For example, the XPS spectrum of the CM product with acrylamide (Figure 2e) shows the peaks from nitrogen (at 402 eV) and oxygen (at 531 eV). The CM products with acrylic acid and methyl acrylate yielded the appearance of an oxygen peak at 531 eV in the XPS spectra. The C(1s) region of the XPS spectra further confirms the chemical transformation from vinyl(17) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (18) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569.
Figure 2. XPS spectra of (a) a bare gold surface, (b) SAMs of undec-10-ene-1-thiol, and SAMs after CM with (c) acrylic acid, (d) methyl acrylate, and (e) acrylamide. XPS spectra were obtained by using a VG-Scientific ESCALAB 250 spectrometer with a monochromatized Al KR X-ray source.
terminated SAMs to the CM product: in addition to a largest peak at a binding energy of 284.6 eV, we observed additional peaks from sCOOH (at 288.5 eV for acrylic acid), sCOs and sOCH3 (at 286.7 and 288.9 eV for methyl acrylate), and sCONH2 (at 287.9 eV for acrylamide; Figure 3). The reactions were further characterized by analysis of stepwise CM reactions. The vinyl-terminated SAMs were mixed with 1 (0.1 mmol) in CH2Cl2 (10 mL) at room temperature for 4 h. The resulting substrate (substrate 5) was thoroughly washed with CH2Cl2, and a compound (2, 3, or 4; 2 mmol) was added to a freshly prepared CH2Cl2 solution containing the gold substrate. We did not observe any characteristic IR peaks of CM products from the resulting substrate. Of interest, the intensity of the bands characteristic of the ω-olefin group decreased and a new peak appeared at ∼965 cm-1 in IR spectrum, which was assigned as an internal double bond.12 The IR spectrum
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make sure that any physisorbed catalyst was removed. Even if 1 had been attached to some areas of the surface, the washing step would have released the attached catalyst into solution by the ring-closing metathesis between the attached catalyst and free vinyl groups present on the surface (i.e., “biting-back” mode). In summary, we report olefin CM between vinylterminated SAMs and R,β-unsaturated carbonyl compounds as one of our goals to introduce various functional groups onto surfaces. The introduction of carboxylic acid and its derivatives to SAMs previously required harsh oxidation conditions. CM could enable the introduction of carboxylic acid derivatives to SAMs under relatively mild conditions as well as other functional groups. CM has been shown to be a useful method for introducing various organic functional groups to substrates in solution, and we believe that this method has great potential as a useful tool for the development of tailored surfaces. Materials and Methods Figure 3. C(1s) region of the XPS spectra of the CM products with (a) acrylic acid, (b) methyl acrylate, and (c) acrylamide.
before the addition of a compound (2, 3, or 4), that is, the IR spectrum of substrate 5, showed the same peaks as the IR spectrum after the addition of a compound, which implies that no reaction occurred upon the addition of the compound. Therefore, it is likely that an attachment of the ruthenium catalyst 1 did not occur at the surface. The adjacent ω-olefin group (approximately 5-Å apart) might participate in the ring-closing metathesis, and the ruthenium catalyst was released from the surface. Grubbs and Coates reported the ring-closing metathesis of 1,2-polybutadiene and formation of poly(methylene-1,3cyclopentane),19 and other groups reported the “bitingback” mode in appropriately spaced vinyl groups.20,21 After the attachment of 1 to the gold substrate, we washed the substrate thoroughly with CH2Cl2 (20 times by using about 1.5 mL of CH2Cl2 for each washing step in a glovebox) to (19) Coates, G. W.; Grubbs, R. H. J. Am. Chem. Soc. 1996, 118, 229230. (20) Ivin, K. J.; Kenwright, A. M.; Khosravi, E. Chem. Commun. 1999, 1209-1210. (21) Barrett, A. G. M.; Cramp, S. M.; Roberts, R. S. Org. Lett. 1999, 1, 1083-1086.
Ru catalyst 1 was purchased from Strem, and all solvents and other reagents were purchased from Aldrich. Undec-10-ene-1thiol was synthesized by following a literature method.12 A representative procedure for olefin CM is as follows: The formation of SAMs was achieved by immersing a gold-coated (with a titanium adhesion layer of 50 Å and thermally evaporated gold layer of 1000 Å) silicon wafer in a 10 mM ethanol solution of undec-10-ene-1-thiol for 12 h at room temperature. After the formation of the vinyl-terminated SAMs, the gold substrates were rinsed with ethanol several times and then dried under a stream of argon. All the reaction preparations were performed under a nitrogen atmosphere in a glovebox. To the reaction vial containing the SAM-coated gold substrate and CH2Cl2 (10 mL) were added ruthenium catalyst 1 (0.1 mmol) and acrylic acid (137 µL, 2 mmol) in a glovebox. The CM reaction was carried out at 50 °C for 4 h. The resulting gold substrate was thoroughly washed with CH2Cl2, acetone, deionized water, and ethanol and dried under a stream of argon.
Acknowledgment. We are grateful for financial support of this work by National R&D Project for Nano Science and Technology. We thank Mr. Joon Sung Lee, Dr. Shinae Jun, and Ms. Sang Won Ko for experimental assistance and Dr. Won in Korea Basic Science Institute for XPS analysis. We are also grateful to Professor Sukbok Chang for a fruitful discussion. LA034859G
