Postformation Modification of SAMs: Using Click Chemistry to

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Postformation Modification of SAMs: Using Click Chemistry to Functionalize Organic Surfaces Rolf Chelmowski,† Daniel K€afer,† Stephan David K€oster,‡ Tim Klasen,† Tobias Winkler,§ Andreas Terfort,§ Nils Metzler-Nolte,‡ and Christof W€oll*,† †

Physikalische Chemie I and ‡Anorganische Chemie I, Ruhr-Universit€ at Bochum, 44780, Bochum, Germany, and §Anorganische Chemie, Philipps-Universit€ at Marburg, 35032 Marburg Received April 10, 2009. Revised Manuscript Received July 3, 2009

We have investigated a recently established strategy of modifying organic surfaces exposed by thiolate SAMs (selfassembled monolayers) deposited on Au substrates by employing so-called click chemistry. This term is used to denote a modified Huisgen 1,3-dipolar cycloaddition. We demonstrate the potential of this method by coupling ferrocene and azido acetic acid to alkyne/azide-terminated SAMs. After the surface reaction, the modified organic monolayers were analyzed using infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Under the conditions used in this study, only for the azide-terminated SAMs could successful grafting of the ferrocene be achieved whereas for the alkyne-terminated SAMs the spectroscopic studies reveal a rather low yield of the coupling reaction.

1. Introduction Among the many applications that self-assembled monolayers (SAMs) have found in recent years,1 the fabrication of welldefined organic surfaces with tailored properties is receiving an increasing amount of attention. Such model substrates are being used to study, for example, the metallization of organic films or the principles governing the biocompability of solid surfaces. The common approach to fabricating an organic surface terminated by a given functionality is to take an appropriate molecule and attach a thiol anchor group to the molecule. In many cases, it has been demonstrated that the additional insertion of a methylene or short alkyl spacer group between the SH anchor and the backbone with the functional group improves the properties of the SAM.2 For a number of applications, such as in combinatorial studies or in patterning approaches, it is desirable to modify the organic surface after the SAM has formed. In previous work, a number of different approaches have been studied with respect to such a postformation modification, for example, the attachment of isocyanates,3,4 the deposition of metal organic precursors,5 or deprotection reactions where a thiol-acetate unit was cleaved to yield an SH-terminated surface.5,6 Recently, click chemistry has been used in a large variety of coupling reactions in organic and inorganic chemistry and has been demonstrated to be highly selective.7,8 As an archetypal click reaction, the 1,3-dipolar *Corresponding author. E-mail: [email protected]. (1) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1169. (2) Cyganik, P.; Buck, M.; Strunskus, T.; Shaporenko, A.; Wilton-Ely, J. D. E. T.; Zharnikov, M.; W€oll, C. J. Am. Chem. Soc. 2006, 128, 13868–13878. (3) Dannenberger, O.; Weiss, K.; W€oll, C.; Buck, M. Phys. Chem. Chem. Phys. 2000, 2, 1509–1514. (4) Himmel, H.-J.; Weiss, K.; J€ager, B.; Dannenberger, O.; Grunze, M.; W€oll, C. Langmuir 1997, 13, 4943–4947. (5) Rajalingam, K.; Bashir, A.; Badin, M.; Schr€oder, F.; Hardman, N.; Strunskus, T.; Fischer, R. A.; W€oll, C. ChemPhysChem 2007, 8, 657–660. (6) Reiss, S.; Krumm, H.; Niklewski, A.; Staemmler, V.; W€oll, C. J. Chem. Phys. 2002, 116, 7704–7713. (7) Rostovtsev, V. V.; Green, L.G.; Fokin, V. V.; Sharpless, K. B. Angew. Chem. 2002, 114, 2708–2711. (8) Rodionov, V. O.; Fokin, V. V.; Finn, M. G. Angew. Chem. 2005, 117, 2250–2255. (9) Huisgen, R. 1,3-Dipolar Cycloaddition Chemistry; Padwa, A., Ed.; Wiley: New York, 1984; Vol. 1, pp 1-176.

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cycloaddition of azides and alkynes9,10 has been shown to be an effective way to produce connections between structures with a broad variety of functional groups. The introduction of copper catalysis for this reaction by Sharpless et al.7,11 has led to special molecules applicable in the fields of bioconjugation,12 organic synthesis,13 and combinatorial chemistry. Also, in the field of self-assembled monolayers click chemistry has been successfully employed in previous works.12,14-19 In principle, click chemistry can be used in two different ways to obtain functionalized SAMs: First, the organo thiols can be modified in solution by forming the click product and can then be adsorbed onto an Au substrate using the common protocol for SAM formation. Alternatively, the SAM can be formed first using the prefunctionalized organothiol with the click reaction carried out afterward. In the following text, these two methods will be referred to as click-in-solution (Figure 1A) and click-onSAM (Figure 1B,C). Modification of SAMs with biologically/electrochemically relevant molecules (oligonucleotides, ferrocene) has been shown by CV or XPS measurements using click chemistry on SAMs12,13,20-23 (10) Lummerstorfer, T.; Hoffmann, H. J. Phys. Chem. B 2004, 108, 3963–3966. (11) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem. 2001, 40, 2004– 2021. (12) Kleinert, M.; Winkler, T.; Terfort, A.; Lindhorst, T. K. Org. Biomol. Chem. 2008, 6, 2118–2132. (13) Devaraj, N. K.; Miller, G. P.; Ebina, W.; Kakaradov, B.; Collman, J. P.; Kool, E. T.; Chidsey, C. E. D. J. Am. Chem. Soc. 2005, 127, 8600–8601. (14) Metzler-Nolte, N. In Comprehensive Organometallic Chemistry III; Parkin, G., Ed.; Elsevier: New York, 2007; Vol. I, pp 883-920. (15) Metzler-Nolte, N. Chimia 2007, 61, 736–741. (16) Metzler-Nolte, N.; Salmain, M. In Ferrocenes; tpnicka, P., Ed.; Wiley-VCH: Weinheim, Germany, 2008; pp 499-639. (17) van Staveren, D. R.; Metzler-Nolte, N. Chem. Rev. 2004, 104, 5931–5985. (18) Chelmowski, R.; K€oster, S. D.; Prekelt, A.; Grunwald, C.; Winkler, T.; Metzler-Nolte, N.; Terfort, A.; W€oll, C. J. Am. Chem. Soc. 2008, 130, 14952–14953. (19) Lee, J. K.; Chi, Y. S.; Choi, I. S. Langmuir 2004, 20, 3844–3847. (20) Devaraj, N. K.; Dinolfo, P. H.; Chidsey, C. E. D.; Collman, J. P. J. Am. Chem. Soc. 2006, 128, 1794. (21) Collman, J. P.; Devaray, N. K.; Chidsey, C. E. D. Langmuir 2004, 20, 1051– 1053. (22) Collman, J. P.; Devaraj, N. K.; Eberspacher, T. P. A.; Chidsey, C. E. D. Langmuir 2006, 22, 2457. (23) Marrani, A. G.; Dalchiele, E. A.; Zanoni, R.; Decker, F.; Cattaruzza, F.; Bonifazi, D.; Prato, M. Electrochim. Acta 2008, 53, 3903.

Published on Web 08/27/2009

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Figure 1. Schematic view of SAM formation and modification via click reaction. (A) Reaction in solution with subsequent SAM formation (2 + 4 f 6). Below, the alternative path upon first forming the SAM and then using the click reaction is shown for (B) alkyne-terminated (2 + 4 f 6, 2 + 3 f 7) and (C) azide-terminated (1 + 5 f 8) SAMs.

and in solution.12,14-18 Also, the grafting of aromatic molecules, nucleotides, and carbohydrates on the organic surfaces as exposed by SAMs has been achieved19 (click on SAM). Very recently, carbohydrate-protein interactions were investigated using such an approach.12 For the click-on-SAM approach, one can still distinguish between two strategies: either the alkyne is immobilized (and the azide is deposited from solution, Figure 1B) or the azide is attached to the surface (with the alkyne in solution, Figure 1C, with the approach similar to that in refs 22 and 23). In the present work, we have investigated the suitability of alkyne- and azideterminated SAMs for click-on-SAM functionalization by using different spectroscopic techniques to characterize the reaction product. In a first set of experiments, alkyne-terminated SAMs of 11-thioacetyl-undecane acid-propargyl amid12 were prepared, and azides were clicked onto it (Figure 1B), whereas in the second approach SAMs from azide undecyl disulfide were prepared and alkynes were clicked onto it afterwards (Figure 1C).

2. Experimental Section 2.1. Substrate Preparation. Polished Si/SiO2 wafers with a (100) surface termination were purchased from Prolog Semicor LTD, Ukraine. Prior to the experiments, they were rinsed with pure acetone and absolute ethanol (EtOH). After the substrates were dried in a stream of nitrogen, they were mounted in a Leybold Univac metal evaporator. First, a layer of 8 nm titanium was deposited to improve the adhesion of the subsequently deposited gold layer. Au (99.995%, 150 nm, Chempur) was then evaporated under high-vacuum conditions (10-7 mbar) at 300 K at a rate of 1.5 nm s-1 as monitored by a quartz microbalance. Before further processing, the Au/Si wafers were cut into pieces of 20 mm  30 mm. Langmuir 2009, 25(19), 11480–11485

Figure 2. Molecules used for click reactions: azide-terminated undecyl disulfide (1), 11-thioacetyl-undecane acid-propargyl amide (2), azido acetic acid (3), azido ferrocene (4), and ethynyl ferrocene (5).

2.2. Click Reaction and SAM Formation. Figure 2 shows all molecules used during this study. Syntheses of 2, 3, and 4 were carried out as described elsewhere (2,12 3,24 425), whereas 1 and 5 were commercially available (1 Asemblon, 5 ACROS). These molecules were used within four different reactions: the cycloaddition of 2 and 4 yielding N-((1H-1,2,3-triazol-4-yl)ferrocenyl)-11-mercaptoundecanamide 6 as click-in-solution and click-on-SAM, the cycloaddition of 2 and 3 (click-on-SAM only) yielding N-((1H-1,2,3-triazol-4-yl)acetyl)-11-mercaptoundecanamide 7, and finally the cycloaddition of 1 and 5 (click-on-SAM) (24) Dyke, J. M.; Groves, A. P.; Morris, A.; Ogden, J. S.; Dias, A. A.; Oliveira, A. M. S.; Costa, M. L.; Barros, M. T.; Cabral, M. H.; Moutinho, A. M. C. J. Am. Chem. Soc. 1997, 119, 6883–6887. (25) Shafir, A.; Power, M. P.; Whitener, G. D.; Arnold, J. Organometallics 2000, 19, 3978–3982.

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Article yielding N-((1H-1,2,3-triazol-4-yl)ferrocenyl)-11-mercapto-undecanethiol 8. a. Click Reaction in Solution. For the click reaction in solution, 2 (0.074 mmol) and 4 (0.074 mmol) were incubated for 48 h in a mixture of CuSO4 pentahydrate (0.03 mmol) and sodium ascorbate (0.3 mmol) in a water/tert-BuOH mixture (1:1) yielding 6. Without further workup, the Au-covered Si/SiO2 wafers were immersed in this solution for 24 h to fabricate SAMs of 6. Afterwards, the samples were rinsed with pure ethanol and acetone and dried in a stream of nitrogen. b. Click Reaction on SAM. For the click-on-SAM reaction, the Au-covered Si/SiO2 wafers were immersed for 24 h in a solution of the corresponding thiol (1 mM) first, using 1 in ethanol (1 mM) for the azide-terminated and 2 for the alkynyl-terminated SAM. The azide/alkyne-terminated SAMs (1 and 2) were then incubated for 48 h in a mixture of CuSO4 pentahydrate (0.03 mmol) and sodium ascorbate (0.3 mmol) in 10 mL of a water/tert-BuOH mixture (1:1). The reaction solutions also contained the respective coupling partner 3, 4, or 5 (0.074 mmol). After removal from the reaction solution, the samples were rinsed with pure ethanol and acetone and dried in a stream of dry nitrogen. 2.3. Spectroscopic Investigations. The RAIRS and IR measurements were carried out using a dry-air-purged Bio-Rad Excalibur FTS-3000 FTIR spectrometer. IR spectra of SAMs on Au were recorded in grazing incidence reflection mode at a fixed angle of 82° relative to the surface normal using p-polarized light. The reflected light was detected by a liquid-nitrogen-cooled MCT (mercury-cadmium-telluride) narrow band detector with a resolution of 2 cm-1. For background compensation, all data were normalized by subtraction of a spectrum that was recorded for a perdeuterated docosanethiol SAM on Au acting as a reference sample that was protected against unintentional contamination by adsorption from the environment as compared to a clean gold surface. The IR spectra from bulk samples (KBr pellets) were recorded in transmission mode using a DTGS (deuterated triglycine sulfate) detector with the same resolution. Because product 6 was not isolated from solution, no such data was measured for this molecule. IR spectra of monolayers from the click reaction in solution were obtained using a Bruker Vertex 80v FTIR spectrometer. To aid the assignment of the vibrational bands, quantum chemical calculations employing density functional theory (DFT) were carried out. We used the Gaussian 98 package26 with the B3LYP hybrid functional and a 6-31+G(d,p) basis set. For free molecules 2, 4, and 6, normal-mode analyses were carried out after geometry optimization. The theoretical vibrational frequencies were scaled with an empirical factor of 0.963.27 The XPS data were recorded in normal emission mode in a multitechnique UHV instrument using Al KR radiation from a twin-anode X-ray source and a Leybold EA 200 electron energy analyzer. For quantitative analysis, a Shirley background has been removed from the spectra, and Voigt functions were used for fitting. The resulting peak areas were then used for stoichiometry analysis along with appropriate mean free path lengths28 and Scofield factors. (26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.11.1; Gaussian, Inc.: Pittsburgh, PA, 1998. (27) Rauhut, G.; Pulay, P. J. Phys. Chem. 1995, 99, 3093–3100. (28) NIST, Electron Inelastic-Mean-Free-Path Database V1.1; Gaithersburg, MD, 2000.

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Figure 3. IR spectra of ferrocene thiol SAM 6 (black curve) obtained by the click-in-solution synthesis route. For comparison, the educt spectrum of 2 (gray) is also shown.

The NEXAFS measurements were performed at the BESSY II synchrotron storage ring in Berlin, Germany at the HE-SGM dipole beamline using an end station described in detail elsewhere.6 All NEXAFS spectra were recorded using linearly polarized synchrotron light (polarization factor of P ≈ 85%) employing the partial electron yield (PEY) mode using a homemade electron detector based on a channel plate and a retarding field of -150 V. The energy resolution at the C K edge amounted to about 100 meV. The energy calibration of all NEXAFS spectra was carried out by recording simultaneously with each spectrum the photocurrent of a carbon-contaminated gold grid in the incident beam. This signal reveals a characteristic absorption peak at a photon energy of 284.9 eV that had been cross-calibrated by means of a graphite sample. For a scheme of the measuring geometry, see the inset in Figure 5b. The NEXAFS raw data have been normalized in a multistep procedure by considering the incident photon flux measured by the photocurrent on the grid and the background signal of the clean substrate. (For details, see ref 6.)

3. Results 3.1. Click Reaction in Solution. The modification of the reactants by employing click chemistry in solution can be monitored conveniently with IR spectroscopy. Figure 3 shows RAIRS data recorded for a SAM made from 2 before (gray) and after (black) attaching 4 using the click reaction, yielding 6. The successful attachment of a ferrocene group and a triazole group is demonstrated by the presence of aromatic C-H stretching and out-of-plane modes (>3000 and ∼800 cm-1) and CdC stretching modes (1603 cm-1). Also, the absence of any vibrational bands originating from the alkyne or azide group (∼2100 and 2200 cm-1) as present in the educts indicates that neither 2 nor 4 is present on the surface.22 Note that the peak at 1733 cm-1 in the RAIRS data is caused by slight solvent contamination. The chemical differences between the initial alkyne thiol SAM and the click product SAM can be seen even more clearly using XPS; the corresponding data are displayed in Figure 4. In the C 1s region (Figure 4b), increases in the intensity and peak width (fwhm +0.2 eV) are seen for the peaks centered at 284.9 eV as a result of the additional cyclopentadienyl carbon atoms. In the N 1s region (Figure 4a), an increase in the peak area by the factor of 3.8 as a result of the additional nitrogen atoms in the triazole group is also found. However, the peak center also shifts from 399.2 to 400.1 eV along with broadening by +0.6 eV, which is explained by the different chemical environments of the amine nitrogen in the alkyne thiol and the three (partially aromatic) nitrogen atoms in the triazole group. The Fe 2p region (Figure 4c) provides the final proof of ferrocene attachment: whereas no Fe 2p signal is observed for the alkyne SAM, a distinct doublet at 708.1 and 721.0 eV (the doublet originates from the spin-orbit Langmuir 2009, 25(19), 11480–11485

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Figure 4. XP spectra of ferrocene thiol SAM 6 (gray curves) obtained by the click-in-solution synthesis route together with alkyne SAM 2 (black) showing the (a) N 1s, (b) C 1s, and (c) Fe 2p regions. (c) Ferrocene thiol SAM 6 obtained by the click-onSAM synthesis route has also been included for comparison.

splitting) can be detected for the SAM subjected to the click-insolution postformation modification. The Fe 2p3/2 peak position agrees well with literature data reported for ferrocene, where a peak at 707.95 eV (C 1s at 284.8 eV) had been found.29,30 Note, however, that this position is slightly lower than expected for a Fe atom in a +2 oxidation state but is still clearly above the binding energy of metallic (neutral) Fe. This slightly lower position can be explained by an effective charge of +0.7e at the Fe atom, as obtained from a Mulliken population analysis of ferrocene.30 A detailed quantitative analysis of the XP data of the click product yields a stoichiometry of C24N4.18Fe0.92, which compares well with the expected ratio of 6 (C24N4Fe1). Additional information on the ferrocene thiol SAM 6 can be obtained from an analysis of the NEXAFS data (Figure 5). The C 1s spectra show a broad π* resonance located at 284.95 eV characteristic of aromatic molecules, and a shoulder at 287.06 eV and at least two broad σ* resonances at 288.3 and 292.1 eV that can be assigned to the alkyl carbon atoms. A comparison with literature data recorded for gas-phase ferrocene31 exhibiting peaks at 285.71, 287.18, 292.2, and 297.7 eV suggests that this shoulder, which is not present in the C 1s NEXAFS of an alkyne thiol SAM (data not shown), can be assigned to corelevel excitations within the ferrocene group. In previous work, the first two of these peaks assigned to ferrocene have been (29) Barber, M.; Connor, J. A.; Derrick, L. M. R.; Hall, M. B.; Hillier, I. H. J. Chem. Soc., Faraday Trans. 1973, 69, 559–562. (30) Woodbridge, C. M.; Pugmire, D. L.; Johnson, R. C.; Boag, N. M.; Langell, M. A. J. Phys. Chem. B 2000, 104, 3085–3093. (31) Hitchcock, A. P.; Wen, A. T.; R€uhl, E. Chem. Phys. 1990, 147, 51–63. (32) Shaporenko, A.; R€ossler, K.; Lang, H.; Zharnikov, M. J. Phys. Chem. B 2006, 110, 24621–24628.

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Figure 5. NEXAFS spectra of ferrocene thiol SAM 6 obtained by the click-in-solution synthesis route for different angles of incoming synchrotron light, along with literature data of ferrocene in the gas phase31 for comparison. Curves at the C 1s edge are shown in plot a, and those at the Fe 2p edge are shown in plot b along with a scheme of the measuring geometry in the inset.

identified as C 1s f [Fe 3dxz,yz and π*Cp,e1g] (LUMO) and C 1s f [Fe 3dx2 - y2 and π*Cp,e2g] transitions.32,33 Using the experimental data recorded for three different angles of the incoming synchrotron light, quantitative analysis of the linear dichroism can be done using the relationship Ivec,σ g 3 = C(P cos2 θ(3 cos2 R - 1) + 1 - cos2 R).34 Considering only the σ* resonances, a molecular tilt angle of the hydrocarbon chain backbone versus the surface normal of around 36° ((8°) can be derived, which is very close to the value obtained for the pristine alkyne thiol SAM (31°, data not shown). The Fe 2p NEXAFS spectrum (Figure 5b) for ferrocene thiol SAM 6 shows a broad resonance at 710.3 eV with a shoulder at 708.8 eV and an even broader resonance at 723.0 eV. A comparison with the literature gas-phase data reported for ferrocene shows very good agreement, considering that the line width is much lower in gas-phase data. The two doublets in the literature spectrum at 708.9 and 711.5 eV and at 721.2 and 724.1 eV have been assigned to the Fe 2p3/2 f [Fe 3dxz,yz and π*Cp,e1g] and Fe 2p3/2 f [Fe 3dx2 - y2 and π*Cp,e2g] transitions and to the corresponding ones originating from Fe 2p1/2.33 In Figure 5b, only the spectrum recorded at the magic angle34 is shown. The spectra obtained at other angles of incoming light are virtually identical (i.e., no linear dichroism is present). Thus, the ferrocene functions at the outer surface or the film are either oriented randomly or exhibit a well-defined tilt angle of 54° (magic angle34). The low after-edge signal intensity before normalization to 1 (data not shown) is consistent with a rather low iron content in the sample, as expected from the stoichiometry of 6 (C24Fe1). 3.2. Click on SAM. The click reaction of alkyne 2 with azido ferrocene 4 was repeated using the click-on-SAM strategy with (33) Otero, E.; Wilks, R. G.; Regier, T.; Blyth, R. I. R.; Moewes, A.; Urquhart, S. G. J. Phys. Chem. A 2008, 112, 624–634. (34) St€ohr, J. NEXAFS Spectroscopy; Springer Series in Surface Science; SpringerVerlag: Berlin, 1992; Vol. 25.

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Chelmowski et al. Table 1. IR Band Assignment Based on the Calculation for Ferrocene Thiol SAM 8 via Click-on-SAM (Figure 6) wavenumber [cm-1] group

ν-CH (aromatic/in-plane) ν-Ν=Ν=Ν ν-CC (alkyne) δ-CH (aromatic/out-of-plane) δ-CH (aromatic/out-of-plane) δ-CH (aromatic/out-of-plane)

ethynyl ferrocene 5 3098

click product 8

calculation of 8

3102

3124

1104 1001 830

1011 979 803

2106 2102/3279 1105 1000 812

the SAM formed from molecule 2. However, the reaction did not proceed in high yield, as evidenced by the Fe 2p XP spectra (Figure 4c). The doublet peak area amounts to only 20% compared to the ferrocene thiol SAM obtained by the click-insolution method. Even if the alkyne SAMs were diluted with 50 or 90% mercapto undecanol (for the structure, see ref 35), no improvement could be achieved. Because we thought that the problems might result from the size of the ferrocene group in 4, we decided to use azido acetic acid 3 because of its smaller size. IR data (not shown) imply that indeed the desired click-on-SAM product N-((1H-1,2,3-triazol4-yl)acetyl)-11-mercaptoundecanamide 7 has formed then because bands indicative of alkyne groups (∼2200 cm-1) or azide groups (∼2100 cm-1) are absent. Thus, neither 2 nor 3 is present within the SAM. To further corroborate these conclusions, we inverted the functional groups in the system. Instead of the alkyne exposed at the organic surface of the SAM and the azides coming from solution (Figure 1B), we immobilized the azide group on the surface and kept the alkynes in solution (Figure 1C). Figure 6a shows IR data recorded for starting materials 1 and 5 and product N-((1H-1,2,3-triazol-4-yl)ferrocenyl)-11-mercapto-undecane thiol 8 for this inverse click-on-SAM reaction. A comparison of the experimental IR spectrum for ferrocene thiol SAM 8 (black) with the calculated vibrational bands (gray) reveals good agreement. Furthermore, the characteristic bands for azide groups (-NdNdN, 2106 cm-1) and alkyne groups (-CtC; 3279 and 2102 cm-1) are no longer present after the click reaction. Instead, aromatic C-H stretching and out-of-plane vibrations at 3100 and around 1000 cm-1 become visible in the IR spectrum recorded for the reaction product as expected for additional cyclopentadienyl rings. A detailed assignment of the major vibrational bands is given in Table 1. Note that the peaks around 1000 cm-1 show a (partially) reduced intensity in the product SAM compared to the ethynyl ferrocene KBr bulk spectrum. This is explained by the surface selection rule governing RAIRS measurements at metallic surfaces. This selection rule results from a metal surface screening all components of an electromagnetic field orientated parallel to the surface, which strongly reduces the excitation of vibrations with a transition dipole moment oriented parallel to the surface. In Figure 6b, the Fe 2p XP spectra of the three different synthesis routes are compared directly. As already stated above, the click-in-solution reaction (of 2 and 4, yielding 6; black curve) produces well-defined ferrocene thiol SAMs and the click-onSAM reaction of the same molecules (dashed black) only yields 20% of a SAM. The inverse click-on-SAM reaction of 1 and 5 yielding 8 (gray), in contrast, produces a ferrocene thiol SAM of nearly equal amount and quality as the solution approach presented above. (35) Chelmowski, R.; Prekelt, A.; Grunwald, C.; W€oll, C.; Case, A J. Phys. Chem. A 2007, 111, 12295–12303.

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azide thiol 1

Figure 6. (a) IR spectra of ferrocene-terminated SAM 8 after the click-on-SAM reaction (lower black curve), along with a calculated spectrum for comparison (lower gray). The IR spectra of educts 1 (azido SAM, upper gray) and 5 (ethynyl ferrocene, upper black, KBr pellet) are also shown. (b) Fe 2p XP spectra for the ferrocene SAMs obtained by the click-in-solution (black) and click-on-SAM (dashed black, inverse gray) methods.

4. Discussion The spectroscopic investigations for the click-in-solution reaction clearly show that in general click chemistry works rather well for the fabrication of specifically functionalized organothiols by simply coupling an alkyne- (or azide-) functionalized S-containing anchor to the corresponding azide- (or alkyne-) functionalized target function. The absence of typical alkyne and azide bands and the appearance of aromatic vibrational modes in the IR spectra recorded for the SAMs that were made by solutionbased click chemistry documents that essentially quantitative reactions could be achieved. Whereas the changes in the C 1s and N 1s XP spectra are fully consistent with a high-yield click reaction, the most direct proof of successful attachment of the ferrocene unit to the alkyne- (or azide-) terminated thiol comes from the presence of an Fe 2p signal in the XP and NEXAFS spectra. The desired reaction proceeds very specifically, without any byproducts and under mild, simple conditions. Ferrocene as a reaction partner has the advantage of introducing electrochemical functionality for charge transport, for electrochemistry, or for further chemical modification and is a useful spectroscopic marker. Furthermore, its attachment to the organothiol does not substantially change the packing within the SAM made from the modified organothiol, as evidenced by the NEXAFS data. After clicking the ferrocene to the azide- (or alkyne-) terminated thiol, the molecular subunits within the corresponding SAM are still in a slightly tilted, mostly upright geometry (36°). No substantial dichroism for the ferrocene is observed. This observation would be consistent with the ferrocene unit rotating rather freely around the C-C (or C-N) bond connecting it to the SAM surface. Langmuir 2009, 25(19), 11480–11485

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For the click-on-SAM reaction, however, the situation is quite different. As evidenced by IR and XPS data, the direct approach of linking the azido-terminated ferrocene to the alkyne SAM on the Au surface does not proceed in good yield. This surprising result, which is apparently inconsistent with the successful click reaction in solution result (see above), can be rationalized by considering the fact that in this case one of the reaction partners is immobilized at an organic surface.1 Then the azido group has to approach the immobilized alkyne group in a certain molecular orientation from the solution for the reaction to proceed. It is thus not unreasonable that the reactivity is strongly reduced with regard to the solution method. The presence of such steric constraints has been reported before for chemical reactions with one reaction partner confined to a surface. (See ref 5 and references cited therein.) In the present case, the importance of such steric aspects is nicely corroborated by the much higher reactivity seen when using the same chemistry to attach a smaller molecule, azido acetic acid, to the alkyneterminated SAM. In the latter case, the IR data show successful coupling to the alkyne-terminated organic surface exposed by the SAM. The effect of steric hindrance is well known from solution chemistry but normally comes into play for larger systems only. On the SAM surface, large molecules will block several (other) reaction sites and exhibit small reactivities because the incoming orientation may be unfavorable. This is also the likely reason that in previous work large (bio-) molecules have been grafted to Au substrates almost exclusively via the direct attachment of one (or more) thiol groups and subsequent dilution in a matrix of unreactive thiols. A successful direct linking of such larger molecules by coupling chemistry to the organic surface of a SAM has to our knowledge not yet been reported. Such steric constraints can also explain why for the rather stiff azido-terminated ferrocene with a height of 3.3 A˚ only small reactivities are observed upon linking these molecules to the organic surface exposed by a corresponding SAM, whereas the smaller (and more flexible) azido acetic acid can be linked successfully without any problems. These observations demonstrate that in principle click chemistry on a SAM surface is highly desirable because it allows the stepwise processing of organic surfaces exposed by SAMs under very mild conditions, opening up possibilities to construct larger systems. The method, however, suffers from a substantial reduction in reactivity when using sterically demanding and unflexible reaction partners. However, this study has also shown that these problems can be overcome to some extent by exchanging the headgroup of the

Langmuir 2009, 25(19), 11480–11485

Article

SAM (alkyne f azide) and the functional group of the ferrocene derivative in solution (azide f alkyne), thus employing the inverse click reaction. At first, this is surprising because previous work showed that the presence of the alkyne in the solution together with the copper-based catalyst led to precipitation of insoluble copper acetylides instead of the desired coupling to the azide-terminated SAM surface.11 A plausible explanation for this observation might be the different solvent employed (acetonitrile vs water/tert-butanol in this study). More systematic studies to be carried out in the future will be needed to clarify the reason for these differences. However, this observation also demonstrates that the click-on-surface method has some intrinsic flexibility that can be utilized to optimize reaction yields of the grafting reaction.

5. Conclusions We have investigated the fabrication of ferrocene-terminated organic surfaces using two different approaches based on click chemistry (i.e., the 1,3-dipolar cycloaddition of azides to alkynes). First, the conventional approach employing solution chemistry was investigated and was found to have high yields; the correspondingly modified organothiolates led to SAMs with a similar packing to that of the unmodified organothiolates. Second, the click-on-SAM approach was investigated, where the reaction was carried out with one of the partners (azide or alkyne) being exposed on the organic surface. We have demonstrated that in this case complications occur from steric effects and the reactivity of an alkyne-terminated SAM with regard to an azide-functionalized ferrocene was found to be very low. At the same time, the coupling of a smaller azide proceeded without major problems. However, when the reaction system was inverted, with the azido groups being exposed on the organic surface, the respective ferrocene alkyne could be attached in a straightforward fashion. Click chemistry is therefore a promising tool for a postformation synthesis of alkyne- (or azide-) terminated SAM surfaces. Such a modification of organic surfaces has recently attracted substantial interest in many fields, including their use as substrates for the liquid-phase epitaxy of metal-organic frameworks (MOFs).36,37 The results also demonstrate, however, that for the coupling of larger molecules some optimization of reaction conditions may be necessary. Acknowledgment. This work was supported by the EU under FP6 (contract SURMOF, NMP4-CT-2006-032109). (36) Shekhah, O.; Wang, H.; Strunskus, T.; Cyganik, P.; Zacher, D.; Fischer, R. A.; Woll, C. Langmuir 2007, 23, 7440. (37) Shekhah, O.; Wang, H.; Paradinas, M.; Ocal, C.; Schuepbach, B.; Terfort, A.; Zacher, D.; Fischer, R. A.; Woell, C. Nat. Mater. 2009, 8, 481.

DOI: 10.1021/la9012813

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