Fast Surface Modification by Microwave Assisted Click Reactions on

pubs.acs.org/Langmuir © 2009 American Chemical Society

Fast Surface Modification by Microwave Assisted Click Reactions on Silicon Substrates )

Claudia Haensch,† Tina Erdmenger,†,‡ Martin W. M. Fijten,†,‡ Stephanie Hoeppener,*,†,§ and Ulrich S. Schubert *,†,‡,§, †

)

Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology, Post Office Box 513, Eindhoven 5600 MB, The Netherlands, ‡Dutch Polymer Institute (DPI), Post Office Box 902, Eindhoven 5600 AX, The Netherlands, §Center for Nanoscience (CeNS), Ludwigs-Maximilians-University :: :: Munchen, Amalienstrasse 54, Munchen D-80799, Germany, and Laboratory of Organic and Macromolecular Chemistry, Friedrich-Schiller-University Jena, Humboldtstrasse 10, Jena D-07743, Germany Received February 9, 2009

Microwave irradiation has been used for the chemical modification of functional monolayers on silicon surfaces. The thermal and chemical stability of these layers was tested under microwave irradiation to investigate the possibility to use this alternative heating process for the surface functionalization of self-assembled monolayers. The quality and morphology of the monolayers before and after microwave irradiation was analyzed by surface-sensitive techniques, such as Fourier transform infrared (FTIR) spectroscopy, atomic force microscopy (AFM), and contact angle measurements. As a model reaction, the 1,3-dipolar cycloaddition of organic azides and terminal acetylenes was tested for the chemical modification of functional azide monolayers. Low and high molar mass compounds modified with an acetylene group were successfully clicked onto the surfaces as confirmed by FTIR spectroscopy and AFM investigations. It could be verified that the reaction can be performed in reaction times of 5 min, and a comparison to conventional heating mechanisms allowed us to conclude that the elevated reaction temperatures result in the fast reaction process.

Introduction The increasing use of microwaves (MWs) in organic and macromolecular synthesis has developed into an emerging field of basic and applied chemistry.1-10 The decrease of the reaction times from hours to minutes, the improvement of reaction yields, solvent-free reaction conditions, suppression of the formation of side products and the improvement of the reproducibility are major advantages of MW heating compared to conventional heating.4,11-13 The implementation of this technique into the field of surface functionalization holds promise for a simplified, fast, and straightforward modification of solid substrates. In particular, in the field of surface chemistry, the effective tuning of surface properties represents a key step toward the fabrication of functional materials that would benefit from faster processing times. A common reaction mechanism, which was found to be supported by MW irradiation, *To whom correspondence should be addressed. E-mail: s.hoeppener@tue. nl (S.H.); [email protected] (U.S.S.). (1) Gedye, R.; Smith, F.; Westaway, K.; Ali, H.; Baldisera, L.; Laberge, L.; Rousell, J. Tetrahedron Lett. 1986, 27, 279–282. (2) Giguere, R. J.; Bray, T. L.; Duncan, S. M.; Majetich, G. Tetraedron. Lett. 1986, 27, 4945–4948. (3) Abramovitch, R. A. Org. Prep. Proced. Int. 1991, 27, 4945–4958. :: (4) Lidstrom, P.; Tierney, J.; Wathey, B.; Westman, J. Tetrahedron 2001, 57, 9225–9283. (5) Caddick, S. Tetrahedron 1995, 51, 10403–10432. (6) In Microwaves in Organic Synthesis; Loupy, A., Ed.; Wiley-VCH: Weinheim, Germany, 2002. :: (7) In Microwave-Assisted Organic Synthesis; Lidstrom, P., Tierney, J. P., Eds.; Blackwell Publishing: Oxford, U.K., 2005. (8) Gabriel, C.; Gabriel, S.; Grant, E. H.; Halstead, B. S. J.; Mingos, D. M. P. Chem. Soc. Rev. 1998, 27, 213–223. (9) Wiesbrock, F.; Hoogenboom, R.; Schubert, U. S. Macromol. Rapid Commun. 2004, 25, 1739–1764. (10) Hoogenboom, R.; Schubert, U. S. Macromol. Rapid Commun. 2007, 28, 368–386. :: (11) Wathey, B.; Tierney, J.; Lidstrom, P.; Westman, J. Drug Discovery Today 2002, 7, 373–380. (12) Kappe, C. O. Angew. Chem., Int. Ed. 2004, 43, 6250–6284. (13) Zhang, X.; Hayward, D. O.; Mingos, D. M. P. Catal. Lett. 2003, 88, 33–38.

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is the widely used 1,3-dipolar cycloaddition of azides and acetylenes14-18 under copper(I) catalysis.19-22 Up to now, several examples are reported in the literature, where MW irradiation could be used to decrease the reaction times of the coupling of organic azides with terminal acetylenes in organic synthesis.23-35 (14) Huisgen, R. In 1,3-Dipolar Cycloaddition Chemistry; Padwa, A., Ed.; Wiley: New York, 1984. (15) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004–2021. (16) Binder, W. H.; Sachsenhofer, R. Macromol. Rapid Commun. 2007, 28, 15–54. (17) Binder, W. H.; Sachsenhofer, R. Macromol. Rapid Commun. 2008, 29, 952–998. (18) Meldal, M.; Tornoee, C. W. Chem. Rev. 2008, 108, 2952–3015. (19) Lutz, J.-F. Angew. Chem. Int. Ed. 2007, 46, 1018–1025. (20) Wu, P.; Fokin, V. V. Aldrichim. Acta 2007, 40, 7–17. (21) Appukkuttan, P.; van der Eycken, E. Eur. J. Org. Chem. 2008, 1133–1155. (22) Fournier, D.; Hoogenboom, R.; Schubert, U. S. Chem. Soc. Rev. 2007, 36, 1369–1380. (23) Appukkuttan, P.; Dehaen, W.; Fokin, V. V.; Van der Eycken, E. Org. Lett. 2004, 6, 4223–4225. (24) Katritzky, A. R.; Singh, S. K.; Meher, N. K.; Doskocz, J.; Suzuki, K.; Jiang, R.; Sommen, G. L.; Ciaramitaro, D. A.; Steel, P. J. ARKIVOC 2006, 43–62. (25) Ermolat’ev, D.; Dehaen, W.; Van der Eycken, E. QSAR Comb. Sci. 2004, 23, 915–918. (26) Guezguez, R.; Bougrin, K.; Akri, K. E.; Benhida, R. Tetrahedron Lett. 2006, 47, 4807–4811. (27) Bouillon, C.; Meyer, A.; Vidal, S.; Jochum, A.; Chevolot, Y.; Cloarec, J.-P.; Praly, J.-P.; Vasseur, J.-J.; Morvan, F. J. Org. Chem. 2006, 71, 4700–4702. (28) Bew, S. P.; Brimage, R. A.; L’Hermite, N.; Sharma, S. V. Org. Lett. 2007, 9, 3713–3716. (29) G
eci, I.; Filichev, V. V.; Pederson, E. B. Chem. Eur. J. 2007, 13, 6379–6386. (30) P
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alez, F. Org. Lett. 2003, 5, 1951–1954. (31) Yoon, K.; Goyal, P.; Weck, M. Org. Lett. 2007, 9, 2051–2054. (32) Khanetskyy, B.; Dallinger, D.; Kappe, C. O. J. Comb. Chem. 2004, 6, 884–892. (33) Joosten, J. A. F.; Tholen, N. T. H.; Maate, F. A. E.; Brouwer, A. J.; van Esse, G. W.; Rijkers, D. T. S.; Liskamp, R. M. J.; Pieters, R. J. Eur. J. Org. Chem. 2005, 3182–3185. (34) Rijkers, D. T. S.; van Esse, G. W.; Merkx, R.; Brouwer, A. J.; Jacobs, H. J. F.; Pieters, R. J.; Liskamp, R. M. J. Chem. Commun. 2005, 4581–4583. (35) Dijkgraaf, I.; Rijnders, A. Y.; Soede, A.; Dechesne, A. C.; van Esse, G. W.; Brouwer, A. J.; Corstens, F. H. M.; Boerman, O. C.; Rijkers, D. T. S.; Liskamp, R. M. J. Org. Biomol. Chem. 2007, 5, 935–944.

Published on Web 05/01/2009

DOI: 10.1021/la901140f

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Appukkuttan et al. were one of the first groups reporting the use of MW irradiation for the preparation of a series of 1,4-disubstituted 1,2,3-triazoles via a three-component reaction in a regioselective fashion.23 Other examples include the synthesis of a set of oligotriazoles using mono-, di-, tetra-, and hexaalkynes,24 the coupling of 2(1H)-pyrazinone with various saccharides to yield glycolpeptidomimetics,25 and the solvent-free synthesis of R- and β-20 -deoxy-1,2,3-triazolyl-nucleosides.26 The labeling of oligonucleotides with azide-functionalized galactosides was reported by Bouillon et al.27 Liskamp and Pieters et al. used the 1,3-dipolar cycloaddition for the synthesis of triazole-linked glycodendrimers,33 multivalent dendrimeric peptides,34 and 1,4,7,10-tetraazadodecane-N,N0 ,N00 ,N000 -tetraacetic acid (DOTA) conjugated multivalent cyclic-Arg-Gly-Asp (RGD) peptide dendrimers.35 All of these reactions were performed under MW irradiation and showed an acceleration of the reaction kinetics. However, these investigations were performed in the liquid state, and to the best of our knowledge, the transfer of these benefits of MW assisted reactions was not reported thus far for substrate supported chemical reactions. During the last few years, the self-assembly of silane molecules has attracted significant attention because of their possible use for the modification of technologically important substrates, such as silicon, glass, and others. In particular, functional molecules, which can be self-assembled on silicon surfaces, are an attractive concept to obtain tailor-made functional surfaces. The introduction of other chemical functionalities can be performed by chemical surface reactions. The 1,3-dipolar cycloaddition14,15 performed on surfaces has been introduced by Lummerstorfer et al. as click reaction sequences,36-40 performed on a functional self-assembled monolayer (SAM) under conventional reaction conditions.41 However, only conventional synthetic approaches have been reported thus far for the modification of surfaces. Here, we report on the use of MW irradiation as an alternative synthetic approach.

Experimental Methods Materials. Bicyclohexane (BCH), 11-bromoundecyltrichlorosilane, sodium azide, copper sulfate pentahydrate, sodium ascorbate, propargyl alcohol, ammonium citrate dibasics, toluene, N,N-dimethylformamide (DMF), and ethanol were purchased from various suppliers. Acetylene functionalized poly(2-ethyl-2oxazoline) was synthesized according to a literature procedure.42 BCH was distilled over sodium before use. The other reagents were used without further purification. Double-side polished p-type silicon wafers (100) were obtained from UniversityWafer. The substrates were treated on both sides for 30 min in an UV/ozone chamber before use. Instrumentation. The stability and functionalization sequences of the silicon substrates were performed in a CEM Discover43 MW system with a large reactor upgrade (up to 50 mL fill volume). In the single-mode MW system, the power was adjusted to 50 W. The temperature limit was set to 120 °C and was controlled with an IR sensor. The system was cooled with a compressed nitrogen flow. Fourier transform infrared (FTIR) measurements were performed on a Tensor 37 RT from Bruker with a grazing angle (36) Devaraj, N. K.; Collman, J. P. QSAR Comb. Sci. 2007, 26, 1253–1260. (37) Choi, I. S.; Chi, Y. S. Angew. Chem. Int. Ed. 2006, 45, 4894–4897.
Synthesis 2007, 11, 1589–1620. (38) Gil, M. V.; Ar
evalo, M. J.; L
opez, O. (39) Haensch, C.; Ott, C.; Hoeppener, S.; Schubert, U. S. Langmuir 2008, 24, 10222–10227. (40) Haensch, C.; Chiper, M.; Ulbricht, C.; Winter, A.; Hoeppener, S.; Schubert, U. S. Langmuir 2008, 24, 12981–12985. (41) Lummerstorfer, T.; Hoffmann, H. J. Phys. Chem. B 2004, 108, 3963–3966. (42) Fijten, M. W. M.; Haensch, C.; van Lankvelt, B. M.; Hoogenboom, R.; Schubert, U. S. Macromol. Chem. Phys. 2008, 209, 1887–1895. (43) www.cem.com (accessed on Sept 16, 2008).

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setup. Spectra were recorded at 4 cm-1 resolution using 500 scans and a clean silicon substrate as a reference. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a VG Escalab MKII spectrometer, equipped with a dual Al/Mg KR X-ray source and a hemispherical analyzer with a five channel-tron detector. Spectra were obtained using a magnesium anode (Mg KR = 1253.6 eV) operating at 480 W and a constant pass energy of 20 eV, with a background pressure of 2  10-9 mbar. Spectra were referenced to the Si(2p) peak at 103.3 eV of the native SiO2 layer on the substrate.44 Contact angle measurements were performed on an OCA30 optical contact angle measuring instrument from Dataphysics. Advancing contact angles were determined by placing a drop of water on the surface, and measurements were performed within 30 s after application of the water drop. The obtained contact angles were averages from four to six measurements on different points on the surface. The investigations on the functionalized surfaces were carried out on a Digital Instruments Nanoscope IIIa Multimode scanning force microscope. Commercially available SFM tips (μ-Mash, NSC36/AIBS) were used for the measurements of the surfaces. Images were acquired on the basis of a 512  512 pixel image. Monolayer Preparation. Bromine terminated monolayers were prepared by immersing the wafer in a solution of 11bromoundecyltrichlorosilane (5 μL) in BCH (5 mL) for 5 min, followed by ultrasonicating in toluene for several minutes. The substrates were finally dried in a stream of air. Substitution with Sodium Azide. Bromine terminated silicon substrates were immersed in a saturated solution of sodium azide in DMF for 24 h at room temperature. The substrates were then sonicated in DMF, water, and ethanol for 3 min each and dried in a stream of air. For a full characterization, see ref 45. Stability Tests in the MW. Azide terminated substrates were placed in a reaction vessel on top of a glass support to avoid contact to the glass wall of the vial. A mixture of 1:1 ethanol/water (25 mL) was added to the substrate. The power of the CEM MW was adjusted to 50 W; the temperature limit was adjusted to 120 °C; and the reaction time was varied between 5 and 45 min.

1,3-Dipolar Cycloaddition Reactions with Propargyl Alcohol. Cycloaddition reactions were carried out by immersing the azide functionalized substrate in a solution of propargyl alcohol (50 mg, 0.892 mmol) dissolved in a mixture of 1:1 ethanol/water (25 mL). CuSO4  5H2O (11.14 mg, 0.045 mmol, 5 mol %) and sodium ascorbate (17.67 mg, 0.089 mmol, 10 mol %), each dissolved in 1 mL of water, were added as catalysts. Subsequently, the substrates were sonicated in ethanol and water for 3 min each and dried in a stream of air. Removal of the Catalyst. The 1,2,3-triazole terminated silicon substrates were stirred for 24 h at room temperature in a 1 M solution of ammonium citrate dibasics in water. Afterward, the substrate was sonicated for several minutes in water and finally dried in a stream of air.

1,3-Dipolar Cycloaddition Reactions with an AcetyleneFunctionalized Poly(2-ethyl-2-oxazoline). Cycloaddition reactions were carried out by immersing the azide functionalized substrate in a solution of P(EtOx)21 (10 mg, 4.55  10-3 mmol) dissolved in a mixture of 1:1 ethanol/water (25 mL). A stock solution of CuSO4  5H2O (3.41 mg, 0.014 mmol, 0.3 equiv) and sodium ascorbate (4.50 mg, 0.023 mmol, 0.5 equiv), each dissolved in 10 mL of water, was prepared. A total of 1 mL of the CuSO4  5 H2O solution and 1 mL of the sodium ascorbate solution were added to the reaction mixture as the catalytic system. Subsequently, the substrates were sonicated in ethanol and water for 3 min each and dried in a stream of air. (44) Cardinaud, C.; Rhounna, A.; Turban, G.; Grolleau, B. Rev. Phys. Appl. 1989, 24, 309–322. (45) Haensch, C.; Hoeppener, S.; Schubert, U. S. Nanotechnology 2008, 19, 035703/1–035703/7.

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Figure 1. (a) Experimental setup of the reaction vessel with the azide terminated silicon substrate on a glass support under MW irradiation and (b) photograph of the CEM Discover43 MW system.

Results and Discussion The functionalization of industrially important surfaces plays an important role in the field of nanotechnology. Monolayers are formed by an adsorption process of surfactant molecules onto solid substrates.46 Hydroxyl terminated surfaces, such as silicon and glass, can be coated with functional trichloro-, triethoxy-, and trimethoxysilanes. The use of such SAMs holds limitations with respect to their thermal and chemical stability toward MW assisted processing. This aspect was investigated because MW irradiation can effect the ordering of the monolayer, as well as the integrity of the end groups. Stability Tests on Azide Terminated Silicon Substrates under MW Irradiation. To investigate the applicability of MW irradiation to perform surface reactions, the stability of selfassembled monolayers on silicon was analyzed. Therefore, functional azide terminated monolayers were used to investigate the ordering of the monolayer, as well as the chemical stability of the -N3 functionalities. These functionalities can be subsequently coupled in 1,3-dipolar cycloadditions with acetylene compounds to yield 1,2,3-triazole systems, and their stability is therefore of special interest. The experimental setup and the used MW system are shown in Figure 1. The stability of the functional monolayers was investigated in a CEM Discover43 MW system with a large reactor upgrade (up to 50 mL fill volume). In the single-mode MW system, the temperature limit was set to 120 °C at a constant power supply of 50 W. The irradiation times were varied from 15 to 45 min. The maximum temperatures that could be generated during the reaction process, as measured by the IR sensor, were found to be 90 °C for 15 min, 95 °C for 30 min, and 100 °C for 45 min, respectively. The characterization of the substrates was performed by FTIR spectroscopy, contact angle measurements, and atomic force microscopy (AFM) investigations. The stability of the monolayers was investigated by analysis of the FTIR spectra of the initial azide terminated monolayers, which were compared to spectra of the monolayers recorded after MW irradiation. Thereby, the signal positions as well as signal intensities of the -CH2 and -N3 vibrations were analyzed (Figure 2). The positions of the -CH2 vibrations provide important information about the ordering of the monolayer, whereas the intensity of the absorption peak for the -N3 group (46) Ulman, A. Chem. Rev. 1996, 96, 1533–1554.

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Figure 2. FTIR spectra of the silicon substrates before and after MW irradiation using different irradiation times.

is a measure for the integrity of the functional end group. After reaction times of 15 and 30 min, the peak positions of the alkyl chains remained at 2927 and 2855 cm-1 and did not shift with respect to the peak position of a non-irradiated monolayer (Figure 2). The peak position of the strong absorption peak for the azide moiety also remained at 2099 cm-1. Furthermore, it could be observed that the signal intensities of the analyzed vibrations are preserved after 15 and 30 min of MW irradiation, respectively. This is regarded as an indication that the monolayer is not degraded or influenced by the applied MW irradiation. This was furthermore confirmed by analysis of the water contact angle measured on the wafers. Figure 3 depicts the dependence of the contact angle upon the irradiation time. The initial azide monolayer exhibits a water contact angle of 83°. The contact angle of the SAM after 15 min of irradiation time remains at 82° and 81° after 30 min of irradiation time. A further increase of the irradiation time to 45 min results in a significant decrease of the FTIR signal intensity of the azide functionality, even though the peak position is still located at 2099 cm-1 (Figure 2). The -CH2 vibrations of the alkyl chains are not influenced by the increased reaction time. The positions remained at 2928 and 2856 cm-1, and the signal intensity stayed constant. These results identify the chemical stability of the functional SAM as a limiting parameter for the application of MW irradiation for surface reactions, because the azide moieties are partially destroyed under DOI: 10.1021/la901140f

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Haensch et al. Scheme 1. Schematic Representation of the Modification Sequence of an Azide Terminated Monolayer: (a) Cycloaddition with Propargyl Alcohol and (b) Cycloaddition with an Acetylene Functionalized Poly (2-ethyl-2-oxazoline)

Figure 3. Contact angle versus the irradiation time in the CEM MW.

the applied conditions. The loss of N2 is responsible for the degradation of the azide group, because it is known that they are unstable under elevated thermal conditions and tend to decompose.47-49 The resulting nitrene can subsequently react to amide, amine, imine, or oxime species.48 Chemical degradation was moreover confirmed by the decrease of the contact angle to 66° (Figure 3) because of the fact that the formed reaction products increase the hydrophilicity of the surfaces. Further characterization includes the investigation of the morphology of the monolayers after MW irradiation. Figure 4 shows tapping mode height AFM images of the irradiated azide terminated monolayers. In accordance with the FTIR measurements, the AFM investigations revealed smooth surfaces for shorter irradiation times, with an average surface roughness of 0.19 nm [root mean square (rms) on a 2.5 μm scan] after 15 min (Figure 4a) and 0.14 nm after 30 min (Figure 4b) of irradiation time, respectively. Only minor changes in the film morphology could be observed for 30 min of irradiation time. For longer irradiation times of 45 min (Figure 4c), a significant change in the surface roughness could be observed, resulting in a surface roughness of 0.87 nm (rms on a 2.5 μm scan) and larger grains formed. These investigations confirmed the thermal and chemical stability of the SAM for reaction times of 15 and 30 min, which resulted in no change or destruction of the monolayer. MW Assisted Functionalization by 1,3-Dipolar Cycloaddition. Further investigations targeted the use of the azide terminated SAMs to induce additional surface functionalization using the 1,3-dipolar cycloaddition, where an azide compound reacts with a terminal acetylene reactant; this reaction is used here as a test reaction, whereby other reaction pathways might also be implemented. Scheme 1 displays the reaction sequences of the performed surface reactions. As acetylene compounds, propargyl alcohol and an acetylene functionalized poly(2-ethyl-2-oxazoline)42 were chosen to demonstrate that both low molar mass compounds as well as polymers can be successfully clicked onto azide terminated silicon surfaces using MW assisted conditions. Different reaction conditions were investigated to identify the optimal conditions for the two different acetylene compounds, whereby the tested reaction conditions were limited to those that have been identified to result in stable -N3 terminated SAMs. (47) Farber, M.; Harris, S. P.; Srivastava, R. D. Combust. Flame 1984, 55, 203– 211. (48) Oyumi, Y.; Brill, T. B. Combust. Flame 1986, 65, 127–135. (49) Tang, C.-J.; Lee, Y. J.; Litzinger, T. A. Combust. Flame 1999, 117, 244–256.

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Table 1 summarizes the reaction conditions of the coupling reactions between the azide terminated silicon substrates and propargyl alcohol as well as an acetylene functionalized poly(2ethyl-2-oxazoline) with an average number of 21 units, respectively. MW Assisted Clicking of Propargyl Alcohol onto Azide Terminated Silicon Substrates. The first acetylene compound that was tested here was propargyl alcohol. The reaction time was varied between 5 and 25 min to ensure the chemical integrity of the azide terminated monolayer (sample numbers 1-4), as catalytic system CuSO4 and the reducing agent sodium ascorbate were added. To investigate the conversion of the azide moieties, FTIR spectroscopy was performed (Figure 5). The symmetric and asymmetric vibrations for the -CH2 groups remained at 2856 and 2928 cm-1 for sample number 4, indicating, in agreement with the previous investigations, the stability of the monolayers. A reaction time of 25 min resulted in the quantitative conversion of the azides, which could be confirmed by the complete disappearance of the strong absorption peak at 2098 cm-1. The reaction time was further decreased from 25 to 15, 10, and 5 min, respectively. The complete disappearance of the absorption peak of the -N3 and the constant intensities of the methylene vibrations indicate a complete reaction even for the shortest irradiation time of 5 min. After the chemical modification in the MW, sample number 1 was analyzed by AFM. After additional cleaning steps with ammonium citrate, a clean surface could be obtained. The corresponding AFM and XPS investigations are provided in the Supporting Information. MW Assisted Clicking of an Acetylene Functionalized Poly(2-ethyl-2-oxazoline) onto Azide Terminated Silicon Substrates. For the second set of experiments, an acetylene functionalized poly(2-ethyl-2-oxazoline)42 was used, which represents a more bulky molecule in contrast to the previously used propargyl alcohol. The reaction times were varied between 15 and 25 min (sample numbers 5-7). To investigate the conversion of the cycloaddition under MW irradiation, also here FTIR spectroscopy was used as a main characterization tool. The FTIR spectra of the modified substrates are shown in Figure 6. A reaction time of 15 min (number 5) resulted in a non-quantitative conversion of the azide groups to triazole functionalities. This was concluded Langmuir 2009, 25(14), 8019–8024

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Figure 4. Tapping mode AFM height images of the modified SAMs: (a) 15, (b) 30, and (c) 45 min of irradiation time. Table 1. Reaction Conditions for the MW Assisted Click Reactions (Temperature Limit of 120 °C and Power at 50 W) number

acetylene compound

reaction time (min)

maximum measured reaction temperature (°C)

mol % of CuSO4  5H2O

mol % of sodium ascorbate

1 2 3 4 5 6 7

propargyl alcohol propargyl alcohol propargyl alcohol propargyl alcohol functionalized poly(2-oxazoline) functionalized poly(2-oxazoline) functionalized poly(2-oxazoline)

5 10 15 25 15 20 25

75 115 115 120 118 120 115

5 5 5 5 30 30 30

10 10 10 10 50 50 50

Figure 5. FTIR spectra of the sample numbers 1-4.

from the remaining absorption peak at 2098 cm-1 for the azide moieties. Increase of the reaction time to 20 min (number 6) led to a further decrease in the absorption peak of the azide groups. However, still a small absorption peak was visible at 2100 cm-1. Sample number 7 showed a complete disappearance for the azide groups, indicating a quantitative conversion to triazole functionalities after 25 min of MW irradiation. The required increase of the reaction time of the acetylene functionalized poly(oxazoline) compared to the propargyl alcohol might be attributed to the relatively bulky character of the polymer. Figure 6 also depicts the symmetric and asymmetric -CH2 vibrations of the alkyl chains of the modified surfaces, which demonstrate also in their case the integrity of the functional monolayers. An additional peak at 2981 cm-1 appeared, which can be assigned to the -CH3 groups of the poly(2-ethyl-2-oxazoline). This can be regarded as a further Langmuir 2009, 25(14), 8019–8024

indication for the successful coupling of the acetylene functionalized polymer to the azide terminated silicon surface. Comparison to Reactions under Conventional Heating. The reported fast modification times, which are much shorter than the reaction times that are usually reported in the literature for the 1,3-dipolar cycloaddition performed on SAMs, motivated the comparison of MW assisted reactions to reactions performed under conventional heating conditions. Therefore, an experiment was conducted under conventional reaction conditions in a pressurized polymerization tube. Propargyl alcohol was used for the click reaction onto -N3 substrates at a reaction temperature of 75 °C and a reaction time of 5 min (comparable reaction conditions as for sample number 1). FTIR spectroscopy showed a full conversion of the azide groups, as indicated by the complete disappearance of the -N3 absorption peak. This finding indicated DOI: 10.1021/la901140f

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Figure 6. FTIR spectra of the clicked monolayers with acetylene functionalized poly(2-oxazoline) after (a) 15, (b) 20, and (c) 25 min reaction times.

that the short reaction times observed in the MW reaction are a result of the increased temperature; moreover, no non-thermal MW effect was observed. However, the performance of such reactions has to be carried out with dedicated equipment, such as pressure bombs or autoclaves, to ensure a safe handling of the reaction. The use of MW systems represents therefore an important practical alternative, because these systems are equipped with temperature and pressure control.

Conclusion A practical approach for the use of MW irradiation was established to modify solid substrates by means of the 1,3-dipolar

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cycloaddition reaction. Stability tests of azide terminated substrates were performed to investigate the influence of the MW irradiation on the surface morphology, the order of the alkyl chain, and the contact angle. FTIR spectroscopy could provide valuable information about the yield of the reactions. AFM confirmed that no degradation of the surfaces was caused by the MW irradiation for irradiation times less than 30 min. Low and high molar mass probe molecules with acetylene functionalities were successfully coupled to azide terminated silicon surfaces via a copper catalyzed Huisgen reaction. As a result, MW assisted click chemistry was identified as a potentially powerful tool for the functionalization of substrates. In particular, the evaluated temperatures could be identified as an important parameter for the shorter reaction times, compared to conventional heating/reaction conditions. In addition, the investigations of the stability of the SAMs under MW irradiation show the possibility to apply MW irradiation also for other chemical modification reactions; this aspect represents an important future perspective for the effective functionalization of surfaces with tailor-made functional groups. Acknowledgment. The authors thank the Nederlandse Wetenschappelijke Organisatie (NWO, VICI grant awarded to U. S. Schubert). Part of this work was performed within the Projects 502 and 543 funded by the Dutch Polymer Institute (DPI). Supporting Information Available: AFM and XPS investigations on the removal of the Cu catalyst. This material is available free of charge via the Internet at http://pubs. acs.org.

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