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Reactivity of Acetylenyl-Terminated Self-Assembled Monolayers on Gold: Triazole Formation Jungkyu K. Lee, Young Shik Chi, 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 January 29, 2004. In Final Form: March 22, 2004 We report the reactivity of acetylenyl-terminated self-assembled monolayers (SAMs) on gold toward “click” chemistry, Huisgen 1,3-dipolar addition, leading to the formation of triazoles. After the formation of acetylenyl-terminated SAMs, the triazole formation was performed on the SAMs and the reaction was confirmed by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, ellipsometry, and contact angle goniometry. “Click” chemistry has offered a versatile strategy for the functionalization in solution chemistry with mild reaction conditions and a high compatibility in functional groups, and our result shows that the reaction could be applied to acetylenyl-terminated SAMs for the introduction of useful functional groups to the surfaces.
Self-assembled monolayers (SAMs) of alkanethiolates on Au(111) surfaces have intensively been studied in various areas because of fundamental interest in interfacial reactions1 and many technological applications such as sensors, catalysis, microarrays, and molecular electronics.2-5 For a wider use of SAMs in the technologically important areas, it will be the first and important step to search for versatile organic reactions, which can be performed on SAMs, leading to the introduction of any required organic functionalities to the surface. Generally, organic functionalities have been introduced to SAMs by the separate solution synthesis of a desired molecule and subsequent formation of SAMs. Although the separate synthesis of self-assembling molecules, in principle, gives an opportunity to introduce virtually any functional group, in practice this approach requires cumbersome syntheses and shows a limited compatibility of functional groups.6 Another approach is a direct chemical transformation on the SAMs, exemplified by substitution reactions of activated carboxylic acid groups and Diels-Alder reactions.6-10 The direct transformation on the SAMs is an attractive alternative but is limited by the characteristics of the SAMs on gold: (1) the bond between gold and thiol is not thermally stable.11-15 The desorption of thiols occurs ∼ 60 °C, and, therefore, the reaction should be performed below
Scheme 1. Synthetic Procedure of the Acetylenyl-Terminated Dithiol 1a
* 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) Chechik, V.; Crooks, R. M.; Stirling, C. J. M. Adv. Mater. 2000, 12, 1161-1171. (5) Lahann, J.; Mitragotri, S.; Tran, T.-N.; Kaide, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371-374. (6) Sullivan, T. P.; Huck, W. T. S. Eur. J. Org. Chem. 2003, 17-29. (7) Kwon, Y.; Mrksich, M. J. Am. Chem. Soc. 2002, 124, 806-812 and references therein. (8) Hutt, D. A.; Leggett, G. J. Langmuir 1997, 13, 2740-2748. (9) Yang, H. C.; Dermody, D. L.; Xu, C.; Ricco, A. J.; Crooks, R. M. Langmuir 1996, 12, 726-735. (10) Yan, L.; Marzolin, C.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 6704-6712. (11) Kim, J.-B.; Breuning, M. L.; Baker, G. L. J. Am. Chem. Soc. 2000, 122, 7616-7617. (12) Shah, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Herdrick, J. L. Macromolecules 2000, 33, 597-605. (13) Camillone, N.; Chidsey, C. E. D.; Liu, G. Y.; Scoles, G. J. J. Chem. Phys. 1993, 98, 3503-3511. (14) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528-12536.
a (a) NaH, tetrahydrofuran (THF); (b) CH3COSH, ABCV, THF with UV irradiation; (c) NaH, THF; (d) MeOH-H2O (4:1), K2CO3; (e) BrCH2CtCH, NaH, THF.
that temperature. (2) As a result of the instability of the SAMs on gold, neither highly acidic nor highly basic conditions can be applied. (3) Because it is not easy to follow the reactions and calculate the yields of the reactions and practically not feasible to purify products at surfaces, it is desirable to perform highly yielding reactions especially for reactions at surfaces. With the aim of widening tools for introducing organic functionalities to SAMs directly, we have previously reported olefin cross-metathesis (CM) reactions of vinylterminated SAMs and R,β-unsaturated compounds.16 The (15) 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.
10.1021/la049748b CCC: $27.50 © 2004 American Chemical Society Published on Web 04/07/2004
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Scheme 2. Schematic Description of the Procedure
olefin CM was successfully applied to the SAMs, but our study suggested that the strategy of olefin CM on the SAMs would not be generalized because of in-plane dimerization of vinyl-terminated thiols, the “biting-back” mode. Therefore, we surveyed reactions that could be applicable to the SAMs, and herein we report the reactivity of acetylenyl-terminated SAMs toward Huisgen 1,3dipolar addition (Sharpless “click” chemistry) and the formation of triazoles.17 Sharpless “click” chemistry has successfully been applied to various organic reactions in solution with a great regioselectivity.18 It does not require harsh reaction conditions and shows a great tolerance of functional groups. The SAM-forming acetylenyl-terminated dithiol 1 was synthesized from tri(ethylene glycol) (Scheme 1). Briefly, the substitution reaction with 11-bromo-1-undecene and subsequent addition of thioacetic acid yielded the compound 3. The acetyl group was migrated from thiol to the hydroxyl group by the treatment of sodium hydride (NaH), leading to the dithiol compound 4. Subsequent deprotection of the acetyl group and addition of propargyl bromide converted 4 to the compound 1 (see Supporting Information for the detailed description of the experimental procedures and spectroscopic data). The acetylenyl-terminated SAM was formed by immersing a freshly prepared, gold-coated (with a titanium adhesion layer of 50 Å and thermally evaporated gold layer of 1000 Å) silicon wafer in a 10 mM ethanolic solution of 1 for 12 h at room temperature. After the formation of the acetylenyl-terminated SAMs, the gold (16) Lee, J. K.; Lee, K.-B.; Kim, D. J.; Choi, I. S. Langmuir 2003, 19, 8141-8143. (17) While we were preparing for the manuscript, another approach to “click” chemistry on SAMs was reported. Collman et al. formed azidoterminated SAMs on gold and reacted the azido group at the surface with ethynyl ferrocene and propynone ferrocene to form triazoles. See: Collman, J. P.; Devaraj, N. K.; Chidsey, C. E. D. Langmuir 2004, 20, 1051-1053. (18) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004-2021 and references therein.
Figure 1. PIERS spectra of (a) SAMs of 1 and SAMs after triazole formation with (b) 4-azidobenzoic acid 6, (c) 3′-azido3′-deoxythymidine 7, and (d) 11-azido-3,6,9-trioxaundecan-1amine 8.
substrate was rinsed with ethanol several times and dried under a stream of argon. The formation of the SAMs was confirmed by polarized infrared external reflectance spectroscopy (PIERS), and the ellipsometric thickness was measured to be 21 Å. Scheme 2 shows a procedure for the triazole formation on the acetylenyl-terminated SAMs. To the reaction vial containing the SAM-coated gold substrate and H2O-EtOH (2:1, 15 mL) were added an azide compound (6, 7, or 8; 3 mmol, 0.2 M), copper(II) sulfate pentahydrate (1 mol % relative to the azide compound), and sodium ascorbate (5 mol % relative to the azide compound), and the mixture was stirred at room temperature for 12 h. After the reaction, the resulting gold substrate was thoroughly washed with deionized water and ethanol and dried under a stream of argon. The triazole formation on the SAMs was characterized by PIERS, X-ray photoelectron spectroscopy (XPS), ellipsometry, and contact angle goniometry.19 IR spectra (Figure 1) show that “click” chemistry was successfully applied to the acetylenyl-terminated SAMs. Initially, the IR spectrum of the acetylenyl-terminated SAMs of 1 showed a characteristic peak at 3322 cm-1 (tCsH
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Figure 2. Wide-scan XPS spectra of (a) SAMs of 1 and SAMs after triazole formation with (b) 4-azidobenzoic acid 6, (c) 3′-azido3′-deoxythymidine 7, and (d) 11-azido-3,6,9-trioxaundecan-1-amine 8. C(1s) region of the XPS spectra after triazole formation with (e) 6, (f) 7, and (g) 8.
stretching;20 Figure 1a). After the triazole formation with 4-azidobenzoic acid 6, the peak from the acetylene group disappeared and characteristic peaks from the formed triazole were observed (Figure 1b): 1723 cm-1 (CdO stretching) and 1607 cm-1 (benzene ring).15,21,22 The widescan XPS spectrum (Figure 2b) shows a peak from N(1s) (402 eV), which also supports the successful triazole formation on the SAMs, and the absence of Cu peaks in the XPS spectrum indicates that the washing step removed the copper catalyst from the substrate. The C(1s) region of the XPS spectra (Figure 2e) further confirms the chemical transformation from acetylenyl-terminated to triazole-functionalized SAMs: in addition to peaks at binding energies of 284.6 eV (CsC) and 286.0 eV (CsO, (19) PIERS spectra were recorded on a Thermo Nicolet Nexus Fourier transform infrared spectrometer in a SAGATM mode. An ellipsometer (Gaertner L116s) equipped with a He-Ne laser (632.8 nm) was used to determine the thickness of the films. Contact angles were determined using a Phoenix 300 apparatus (SEO, Ltd., Korea). The XPS study was performed with a VG-Scientific ESCA-LAB 250 spectrometer with a monochromatized Al KR X-ray source. (20) Yam, C. M.; Tong, S. S. Y.; Kakkar, A. K. Langmuir 1998, 14, 6941-6947 and references therein. (21) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. (22) Arnold, R.; Azzam, W.; Terfort, A.; Wo¨ll, C. Langmuir 2002, 18, 3980-3992.
from the ethylene glycol group), we observed an additional peak from sCOO (at 288.9 eV).15,16 The ellipsometric thickness increased from 21 to 25 Å after the triazole formation and the water contact angle was changed from 100 to 40°. After the confirmation of the successful formation of the triazole group from the acetylenyl-terminated SAMs, we investigated the reactivity and functionality tolerance of the triazole formation on the SAMs. We chose 3′-azido3′-deoxythymidine 7 and 11-azido-3,6,9-trioxaundecan1-amine 8. The compound 7 contains secondary azide and a biologically important nucleoside moiety, and the compound 8 contains primary amine at the other end with the ethylene glycol linker, which could be utilized for subsequent functionalizations. The IR and XPS spectra after the reactions are shown in Figures 1 and 2, respectively. After the reaction with 7, we observed a Cd O stretching peak at 1700 cm-1 in the IR spectrum (Figure 1c) and an N(1s) (402 eV) peak in the XPS spectrum (Figure 2c). The C(1s) region of the XPS spectrum (Figure 2f) shows an additional peak from sCON (at 287.9 eV).15,16 The thickness increased by 7 Å, and the water contact angle was changed to 70°. All the data imply that triazole was formed with secondary azide on the SAMs, although we could not confirm how well the reaction proceeded with
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the bulky and secondary azide 7 because of the relatively weak IR intensity of the acetylene group. In the reaction with 8, we did not observe any appearance of characteristic peaks in the IR spectrum except the disappearance of the peak from the acetylene group. The XPS spectra confirm the transformation: the N(1s) peak appeared after the reaction (Figure 2d). In summary, we report “click” chemistry, Huisgen 1,3dipolar addition, between acetylenyl-terminated SAMs and azide compounds as one of our goals to introduce various functional groups onto surfaces. Three azide compound having different functional groups were successfully coupled with the acetylene group at the surface. “Click” chemistry has been shown to be a useful method for introducing various organic functional groups to substrates in solution under mild conditions with a great
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functionality tolerance, and we believe that the triazole formation by “click” chemistry on acetylenyl-terminated SAMs could be used as a versatile tool for tailoring surface functionalities. 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 and Ms. Sangwon Ko for experimental assistance and Dr. Won in Korea Basic Science Institute for XPS analysis. Supporting Information Available: Detailed experimental procedure for the synthesis of compound 1. This material is available free of charge via the Internet at http://pubs.acs.org. LA049748B
