Synthesis of a Photosensitive Thiocyanate-Functionalized

Chem. Mater. 2008, 20, 2009–2015

2009

Synthesis of a Photosensitive Thiocyanate-Functionalized Trialkoxysilane and Its Application in Patterned Surface Modifications Alexandra Lex,† Peter Pacher,‡ Oliver Werzer,‡ Anna Track,‡,§ Quan Shen,| Robert Schennach,‡ Georg Koller,§ Gregor Hlawacek,| Egbert Zojer,‡ Roland Resel,‡ Michael Ramsey,§ Christian Teichert,| Wolfgang Kern,†,⊥ and Gregor Trimmel*,† Institute for Chemistry and Technology of Organic Materials, Graz UniVersity of Technology, Stremayrgasse 16, 8010 Graz, Austria, Institute of Solid State Physics, Graz UniVersity of Technology, Petersgasse 16, 8010 Graz, Austria, Institute of Physics, UniVersity of Graz, UniVersitätsplatz 5, 8010 Graz, Austria, Institute of Physics, MontanuniVersität Leoben, Franz Josef Strasse 18, 8700 Leoben, Austria, and Institute of Chemistry of Polymeric Materials, MontanuniVersität Leoben, Franz Josef Strasse 18, 8700 Leoben, Austria ReceiVed September 26, 2007. ReVised Manuscript ReceiVed December 10, 2007

A bifunctional molecule, trimethoxy[4-(thiocyanatomethyl)phenyl]silane (Si-SCN), bearing both a photoreactive unit, the benzyl thiocyanate group, and an anchoring unit, the trimethoxysilyl group, was synthesized. Upon irradiation with UV light of 254 nm under inert atmosphere, the benzyl thiocyanate group undergoes an isomerization reaction to the benzyl isothiocyanate. Kinetic investigations of liquid films of Si-SCN by Fourier transform infrared (FTIR) spectroscopy show that the thiocyanate is almost quantitatively consumed during illumination, but only 25–30% of isothiocyanate is formed. From the subsequent reaction with propylamine from the vapor phase, the isothiocyanate groups react to the corresponding thiourea compound. Thin layers of Si-SCN were applied to modify oxidized silicon surfaces. X-ray reflectivity measurements revealed a layer thickness of 6 nm. The above-described photochemistry also proceeds in these very thin layers as determined by FTIR spectroscopy and X-ray photoelectron spectroscopy. Photopatterned surfaces were produced using a contact mask during illumination followed by postmodification with propylamine. The structures of the used photomask were clearly reproduced on the surface as revealed by friction force microscopy. Because of the versatility of this photochemistry, the new photosensitive silane Si-SCN is a promising molecule for applications in modern immobilization techniques and for the (structured) modification of inorganic surfaces.

Introduction Thin layers of bifunctional organosilanes, containing a chloro- or alkoxysilane moiety as anchoring group to surfaces and a second functionality, are of great interest for numerous applications and play an increasingly important role in nanotechnology, biotechnology, and molecular electronics.1–6 The range of organic functionalities spans from apolar to polar groups, from anionic to cationic groups, and includes, for example, also fluorescent dyes and electroactive moieties. While chlorosilyl and alkoxysilyl units bind to surface To whom correspondence should be addressed. Telephone: ++433168734958. Fax: ++43316-8738951. E-mail: [email protected]. † Institute for Chemistry and Technology of Organic Materials, Graz University of Technology. ‡ Institute of Solid State Physics, Graz University of Technology. § University of Graz. | Institute of Physics, Montanuniversität Leoben. ⊥ Institute of Chemistry of Polymeric Materials, Montanuniversität Leoben.

(1) Onclin, S.; Ravoo, B. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2005, 44, 6282 and references therein. (2) Ulman, A. Chem. ReV. 1996, 96, 1533. (3) Descalzo, A. B.; Martínez-Mánez, R.; Sancenón, F.; Hoffmann, K.; Rurack, K. Angew. Chem., Int. Ed. 2006, 45, 5924. (4) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (5) Senaratne, W.; Andruzzi, L.; Ober, C. K. Biomacromolecules 2005, 6, 2427. (6) Aswal, D. K.; Lenfant, S.; Guerin, D.; Yakhmi, J. V.; Villaume, D. Anal. Chim. Acta 2006, 568, 84.

hydroxy groups of glass and inorganic oxides, the second functionality, if desired, separated by an alkyl or aryl spacer from the anchoring group, determines the final properties of the surface. Because of the plenitude of available and described silanes, the polarity and chemical reactivity of the surface can be tailored over a wide range. These functional organosilanes are widely used in immobilization techniques, e.g., for the attachment of catalysts, (bio)molecules, nanoparticles, and analytes onto oxidic surfaces. For many applications, such as biochips and nanotechnology, twodimensional patterning of surface properties and site-selective immobilization is required. An elegant route for obtaining such features is the use of photolithographic techniques. Different patterning concepts using UV light have already been described. In most cases, they result in surface structures of hydrophobic and hydrophilic areas that can then be used for selective immobilization. An example for a patterning process is the light-induced oxidation of alkyl chains and phenyl alkyl chains resulting in aldehyde- and carboxylicacid-terminated layers (e.g., see refs 7–10). Alternatively, (7) Hong, L.; Sugimura, H.; Furukawa, T.; Takai, O. Langmuir 2003, 19, 1966. (8) Friedli, A. C.; Roberts, R. D.; Dulcey, C. S.; Hsu, A. R.; McElvany, S. W.; Calvert, J. M. Langmuir 2004, 20, 4295.

10.1021/cm702758n CCC: $40.75  2008 American Chemical Society Published on Web 02/19/2008

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Chem. Mater., Vol. 20, No. 5, 2008

benzyl compounds, such as chloromethylphenyl- and aminoethylaminomethylphenyl-terminated surfaces, have been oxidized to aromatic aldehydes and carboxylic acids.11–13 Using high-energy doses or short wavelength UV light, cleavage of the Si-C bond in aromatic silanes occurs, which leads to the removal of the whole organic layer. As a result, a Si-OH surface is created in the irradiated areas.11,14–20 As a consequence of the full degradation of the organic side chains, the residual SiO(OH) species are forming a new SiOx layer with controlled layer thickness as demonstrated for octadecyl-siloxane layers.21 A very convenient approach is the use of silanes already bearing a photoreactive moiety. This opens the way to alternative photoassisted surface modification schemes; e.g., siloxane layers bearing photoinitiators, such as azo compounds22–24 or dithiocarbamates,25,26 were used to initiate polymerizations only in the irradiated areas of the layer (“grafting from”). These methods result in polymer patterns covalently linked to the surface. Another interesting approach is the use of benzophenone-functionalized siloxane layers as photolinker of polymer chains27,28 and biomolecules29 to the surface. In this approach, radicals are formed on the benzophenone unit, which then reacts in a subsequent reaction with the molecules that are to be immobilized. In another approach, photoprotecting groups, such as photoreactive ortho-nitrobenzylesters, were used to mask amines. These groups can be cleaved by irradiation with 364 nm of light and give free amino groups at the surface.30 Del Campo et al. demonstrated that patterns of two different functionalities can be obtained using layers of silanes with different end groups: amino-, hydroxy-, or carboxylate-terminated silanes, masked by different light(9) Ye, T.; Wynn, D.; Dudek, R.; Borguet, E. Langmuir 2001, 17, 4497. (10) Howland, M. C.; Sapuri-Butti, A. R.; Dixit, S. S.; Dattelbaum, A. M.; Shreve, A. P.; Parikh, A. N. J. Am. Chem. Soc. 2005, 127, 6752. (11) Chen, M.-S.; Dulcey, C. S.; Chrisey, L. A.; Dressick, W. J. AdV. Funct. Mater. 2006, 16, 774. (12) Brandow, S. L.; Chen, M. S.; Aggarwal, R.; Dulcey, C. S.; Calvert, J. M.; Dressick, W. J. Langmuir 1999, 15, 5429. (13) Sun, S.; Montague, M.; Critchley, K.; Chen, M.-S.; Dressick, W. J.; Evans, S. D.; Leggett, G. J. Nano Lett. 2006, 6, 29. (14) Dulcey, C. S.; Georger, J. H.; Krauthamer, V.; Stenger, D. A.; Farer, T. L.; Calvert, J. M. Science 1991, 252, 551. (15) Dressick, W. J.; Dulcey, C. S.; Georger, J. H.; Calvert, J. M. Chem. Mater. 1993, 5, 148. (16) Dulcey, C. S.; Georger, J. H.; Chen, M. S.; McElvany, S. W.; OFerrall, C. E.; Benezra, V. I.; Calvert, J. M. Langmuir 1996, 12, 1638. (17) Masuda, Y.; Yamagichi, M.; Koumoto, K. Chem. Mater. 2007, 19, 1002. (18) Calvert, J. M.; Georger, J. H.; Peckerar, M. C.; Pehrsson, P. E.; Schnur, J. M.; Schoen, P. E. Thin Solid Films 1992, 210/211, 359. (19) Shirahata, N.; Sakka, Y.; Hozumi, A. Thin Solid Films 2006, 499, 293. (20) Rudolph, T.; Zimmer, K.; Betz, T. Mater. Sci. Eng., C 2006, 26, 1131. (21) Brunner, H.; Vallant, T.; Mayer, U.; Hoffmann, H. Langmuir 1996, 12, 4614. (22) Prucker, O.; Habicht, J.; Park, I.-J.; Rühe, J. Mater. Sci. Eng., C 1999, 8–9, 291. (23) Prucker, O.; Rühe, J. Langmuir 1998, 14, 6893. (24) Tsubokawa, N.; Satoh, M. J. Appl. Polym. Sci. 1997, 65, 2165. (25) Liang, L.; Feng, X.; Liu, J.; Rieke, P. C.; Fryxell, G. E. Macromolecules 1998, 31, 7845. (26) Liang, L.; Rieke, P. C.; Fryxell, G. E.; Liu, J.; Enehard, M. H.; Alford, K. L. J. Phys. Chem. B 2000, 104, 11667. (27) Prucker, O.; Naumann, C. A.; Rühe, J.; Knoll, W.; Frank, C. W. J. Am. Chem. Soc. 1999, 121, 8766. (28) Jeyaprakash, J. D.; Samuel, S.; Rühe, J. Langmuir 2004, 20, 10080. (29) Leshem, B.; Sarfati, G.; Novoa, A.; Breslav, I.; Marks, R. S. Luminescence 2004, 19, 69. (30) Jonas, U.; Del Campo, A.; Krüger, C.; Glasser, G.; Boos, D. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5034.

Lex et al. Scheme 1. Photoisomerization of Benzyl Thiocyanates

sensitive protecting groups, which can be selectively cleaved by choosing the appropriate wavelength.31 In the present contribution, we introduce the benzyl thiocyanate group as a photosensitive group for organomodified silanes. Upon irradiation, the thiocyanate (-SCN) undergoes a photoisomerization to the corresponding isothiocyanate (-NCS), which is accompanied by a drastical change in the physical and chemical reactivity. The isomerization proceeds via a radical mechanism, as shown in Scheme 1. The advantage of the benzyl thiocyanate group is the fact that this unit can be easily introduced by substitution of benzyl halides with rhodanide ions, as applied in the synthesis of photosensitve monomers and polymers.32–34 Over the last few years, we have shown that polymers bearing this photosensitive group have potential as photoresists and as active layers for optical applications (e.g., DFB laser gratings35–37) and for the immobilization of amines,38 biomolecules,39 and gold nanoparticles.40 However, the thickness of the applied polymer films is in the range from 100 nm to several micrometers, so that the applicability for surface modifications of inorganic materials is limited. In this work, we present the synthesis of a new bifunctional molecule bearing both the photoreactive benzyl thiocyanate unit and the trimethoxysilyl group as an anchoring unit. This molecule provides the possibility to transfer the thiocyanate photochemistry to thin layers on inorganic surfaces. We investigate the photoreactivity of this compound in substance as a liquid layer and its potential in surface modification and immobilization techniques. The photoreaction as well as postmodification reactions with gaseous amines are monitored by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The layer thickness is determined by X-ray reflectivity (XRR), and the photopatterned surfaces are analyzed by friction force microscopy (FFM). (31) Del Campo, A.; Boos, D.; Spiess, H. W.; Jonas, U. Angew. Chem., Int. Ed. 2005, 44, 4707. (32) Kern, W.; Hummel, K. Eur. Polym. J. 1995, 31, 437. (33) Lex, A.; Trimmel, G.; Kern, W.; Stelzer, F. J. Mol. Catal. A: Chem. 2006, 254, 174. (34) Schöfberger, W.; Zaami, N.; Mahler, K. A.; Langer, G.; Jakopic, G.; Pogantsch, A.; Kern, W.; Stelzer, F. Macromol. Chem. Phys. 2003, 204, 779. (35) Kavc, T.; Langer, G.; Kern, W.; Kranzelbinder, G.; Toussaere, E.; Turnbull, G. A.; Samuel, I. D. W.; Iskra, K. F.; Neger, T.; Pogantsch, A. Chem. Mater. 2002, 14, 4178. (36) Kranzelbinder, G.; Toussaere, E.; Zyss, J.; Kavc, T.; Langer, G.; Kern, W. Appl. Phys. Lett. 2003, 82, 2203. (37) Weinberger, M. R.; Langer, G.; Pogantsch, A.; Haase, A.; Zojer, E.; Kern, W. AdV. Mater. 2004, 16, 130. (38) Langer, G.; Kavc, T.; Kern, W.; Kranzelbinder, G.; Toussaere, E. Macromol. Chem. Phys. 2001, 202, 3459. (39) Preininger, C.; Sauer, U.; Kern, W.; Dayteg, J. Anal. Chem. 2004, 76, 6130. (40) Weinberger, M. R.; Rentenberger, S.; Kern, W. Monatsh. Chem. 2007, 138, 309.

Benzyl-thiocyanate-Functionalized Silane

Experimental Section General Procedures. All chemicals were purchased from commercial sources and used without further purification. All experiments were carried out under argon atmosphere using Schlenk techniques. As substrates, p-type-doped (B) single-side-polished silicon wafers with native silicon oxide (resistivity 9–18 Ω cm) from Infineon Technologies Austria AG were used. Hazard Warnings. For the preparative work, hazardous chemicals and solvents are used (ammonium thiocyanate, methanol, propylamine, 2,2,2-trifluoroethylamine, and piranha solution). In addition, piranha solution is explosive, and its preparation is highly exothermic (up to 120 °C). Therefore, reactions must be carried out in a fume hood, and protective clothes and goggles must be used! UV irradiation causes severe eye and skin burns. Precautions (UV protective goggles and gloves) must be taken! Synthesis of Trimethoxy[4-(thiocyanatomethyl)phenyl]silane (Si-SCN). To 7.62 g (100 mmol) dried ammonium thiocyanate dissolved in 50 mL of dried methanol, 20 mL (91 mmol) of (4(chloromethyl)phenyl)trimethoxysilane was added under an inert atmosphere. The reaction flask was shaded with aluminum foil and stirred under reflux until full conversion was achieved, which was monitored by FTIR spectroscopy. The obtained white precipitate was filtered off under an inert atmosphere, and the solvent was removed in vacuum. The residue was redissolved in 30 mL of n-heptane, and the remaining white precipitate was filtered off again. n-Heptane was evaporated under vacuum to give a yellow oil as product. 1H NMR (500 MHz, 20 °C, CDCl3) δ: 7.68 (d, 2H, Ph-H2,4), 7.39 (d, 2H, H3,5), 4.16 (s, 2H, -CH2-), 3.63 (s, 9H, -OCH3). 13C{1H} NMR (125 MHz, 20 °C, CDCl3) δ: 136.8 (1C, Ph1), 135.7 (2C, Ph3,5), 130.7 (1C, Ph4), 128.6 (2C, Ph2,6), 111.9 (1C, -SCN), 51.0 (3C, -OCH3), 38.3 (1C, -CH2-). FTIR (film on Si wafer): 2976–2892 (m, νCH), 2155 (m, νSCN), 1603 (w), 1476 (w), 1443 (w), 1392 (m), 1294 (w), 1246 (w), 1166 (m), 1124–1079 (s, νSiO), 1022 (w), 962 (m), 785 (m), 715 (m), 653 (w), 525 (m), 491 (m) cm-1. Sample Preparation. For the preparation of thin layers, silicon wafers were pretreated with a 30 s oxygen plasma-cleaning step and subsequently dipped into 18 MΩ cm Milli-Q water. Afterward, the samples were dried with a stream of CO2 gas. All glassware was first cleaned with piranha solution (solution of 3:1 concentrated H2SO4 and 35% H2O2), washed repeatedly with deionized water, sonicated in Hellmanex solution (2% in deionized water) for 20 min, thoroughly rinsed with deionized water, subsequently rinsed with 18 MΩ cm Milli-Q water, and dried in an oven at 80 °C overnight. The clean substrates were placed in a 0.1 vol % solution of Si-SCN in HPLC-grade toluene at room temperature under ambient conditions for 3-4 days. Afterward, they were transferred to cleaned glasses with fresh toluene, sonicated for 2 min, rinsed with toluene, dried with CO2, and annealed for 30 min at 100 °C in vacuum. Liquid layers were prepared by placing a drop of Si-SCN between two CaF2 substrates. For postmodification reactions, one substrate was removed, so that the layer could react with gaseous amines. FTIR Spectra of Liquid Layers. Liquid layers were measured with a Perkin-Elmer Spectrum One instrument (spectral range between 4000 and 450 cm-1). All FTIR spectra of the samples were recorded in transmission mode. FTIR Spectra of Thin Layers. FTIR spectra of the thin layers were recorded with a Bruker IFS 66v/S IR spectrometer equipped with a mercury-cadmium-telluride (MCT) detector (cooled with liquid nitrogen) at an angle of 74° between the IR beam and the sample normal at a pressure of