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Preparation and Heck Reaction of Multidentate Carbosilane Films Derived from Focally Functionalized and Allyl-Terminated Dendrons on Hydrogen-Terminated Silicon(111) Surfaces Chi Ming Yam, Jaeeock Cho, and Chengzhi Cai* Department of Chemistry & Center for Materials Chemistry, University of Houston, Houston, Texas 77204-5003 Received September 9, 2003. In Final Form: November 24, 2003 Multidentate carbosilane films were prepared by thermally induced hydrosilylation of allyl-terminated carbosilane dendrons of generations 0, 1, and 2 (G0-G2) on hydrogen-terminated silicon(111) surfaces. The dendron molecules contain three (G0), nine (G1), and twenty-seven (G2) allyl groups at the periphery, and a bromophenyl functional group at the focal point. The dendron films were characterized by contactangle goniometry, ellipsometry, Fourier transform infrared spectroscopy in the attenuated total reflection mode, and X-ray photoelectron spectroscopy (XPS). Upon hydroboration of the remaining allyl groups in the films, the percentage of the introduced boron atoms in the films were measured by XPS. The results indicate the presence of roughly 20%, 27%, and 46% of unreacted allyl groups in the G0, G1, and G2 films, respectively. The mechanistic aspects of the chemisorption of these dendron molecules on H-Si(111) surfaces are discussed. XPS studies indicate that seven G0 molecules cover approximately the same area on the substrate as three G1 molecules and one G2 molecule. After treatment of the G0, G1, and G2 films with 4-fluorostyrene under the Heck reaction conditions, the XPS studies indicate that about 84%, 71%, and 55% of the Br atoms were consumed, yielding the replacement of ca. 58-70% of the reacted Br atoms by the fluorostyryl groups. The remaining bromophenyl groups were inactive toward the Heck reaction, probably due to their disfavorable position/orientation in the films.
Introduction Hydrosilylation of 1-alkenes on hydrogen-terminated silicon (H-Si) surfaces to form well-ordered and robust monolayers was first reported by Linford and Chidsey in 1993.1-4 Both flat H-Si(111)1-5 and H-Si(100)6 and H-terminated porous silicon3,7-9 have been used as the substrates. A number of unique features make this new class of thin films that directly couple organic materials and semiconductors promising for applications in sensors, microelectronics, microelectromechanical systems (MEMSs), and optoelectronics.10-15 These include the fact * To whom correspondence should be addressed. Phone: (713) 743-2710. Fax: (713) 743-2709. E-mail:
[email protected]. (1) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631. (2) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (3) Buriak, J. M. Chem. Rev. 2002, 102, 1271. (4) Sieval, A. B.; Linke, R.; Zuilhof, H.; Sudho¨lter, E. J. R. Adv. Mater. 2000, 12, 1457. (5) Terry, J.; Mo, R.; Wigren, C.; Cao, R. Y.; Mount, G.; Pianetta, P.; Linford, M. R.; Chidsey, C. E. D. Nucl. Instrum. Methods, B 1997, 133, 94. (6) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Linford, M. R.; van der Maas, J. H.; de Jeu, W. H.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 1998, 14, 1759. (7) Buriak, J. M. Adv. Mater. 1999, 11, 265. (8) Sailor, M. J.; Lee, E. J. Adv. Mater. 1997, 9, 783. (9) Bateman, J. E.; Eagling, R. D.; Worrall, D. R.; Horrocks, B. R.; Houlton, A. Angew. Chem., Int. Ed. 1998, 37, 2683. (10) Strother, T.; Cai, W.; Zhao, X. S.; Hamers, R. J.; Smith, L. M. J. Am. Chem. Soc. 2000, 122, 1205. (11) Stewart, M. P.; Buriak, J. M. Adv. Mater. 2000, 12, 859. (12) Song, J. H.; Sailor, M. J. Comments Inorg. Chem. 1999, 21, 69. (13) Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Nature 2000, 406, 48. (14) Wagner, P.; Nock, S.; Spudich, J. A.; Wayne, D. V.; Chu, S.; Cicero, R. L.; Wade, C. P.; Linford, M. R.; Chidsey, C. E. D. J. Struct. Biol. 1997, 119, 189. (15) Maboudian, R. Surf. Sci. Rep. 1998, 30, 209.
that hydrosilylation can be induced photochemically,5,16 thus making it a suitable method for patterning monolayers on silicon surfaces.17,18 In addition, the growth conditions allow incorporation of a variety of functional groups.3,10,19,20 For example, very recently we demonstrated that oligo(ethylene glycol) derivatives can be incorporated in the films for resisting nonspecific adsorption of proteins.21,22 Finally, wide-scale chemical manipulation of these groups at the film surface is compatible with the stable adsorbate-substrate bonding (Si-C).1,2,23 For many thin film applications, it is desirable or necessary to control the density of functional groups in the films. The most common way to adjust this parameter is by codeposition with inert molecules, but sometimes phase separation can perturb the system.24-26 We have proposed a new strategy to improve the homogeneity of the functional groups at film surfaces by using focally functionalized dendrons with a periphery containing (16) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688. (17) Effenberger, F.; Gotz, G.; Bidlingmaier, B.; Wezstein, M. Angew. Chem., Int. Ed. 1998, 37, 2662. (18) Stewart, M. P.; Buriak, J. M. Angew. Chem., Int. Ed. 1998, 37, 3257. (19) Boukherroub, R.; Wayner, D. D. M. J. Am. Chem. Soc. 1999, 121, 11513. (20) Eagling, R. D.; Bateman, J. E.; Goodwin, N. J.; Henderson, W.; Horrocks, B. R.; Houlton, A. J. Chem. Soc., Dalton Trans. 1998, 1273. (21) Yam, C. M.; Xiao, Z.; Gu, J.; Boutet, S.; Cai, C. J. Am. Chem. Soc. 2003, 125, 7498. (22) Yam, C. M.; Lopez-Romero, J. M.; Gu, J.; Cai, C. Chem. Commun. submitted. (23) Sung, M. M.; Kluth, G. J.; Yauw, O. W.; Maboudian, R. Langmuir 1997, 12, 6164. (24) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (25) Imabayashi, S.; Gon, N.; Sasaki, T.; Hobara, D.; Kakiuchi, T. Langmuir 1998, 14, 2348. (26) Boukherroub, R.; Wayner, D. D. M. J. Am. Chem. Soc. 1999, 121, 11513.
10.1021/la035682t CCC: $27.50 © 2004 American Chemical Society Published on Web 01/13/2004
Preparation and Heck Reaction of Carbosilane Films
trichlorosilyl or thiol groups that can bind to mica, oxide, or gold surfaces.27-29 The density of the surface functional groups can then be controlled by the size of the dendron. Herein we report a new type of dendron films derived from carbosilane dendrons of generations 0-2 (G0-G2)
on hydrogen-terminated silicon(111) surfaces. The G0,29 G1,29 and G2 dendrons consist of three, nine, and twentyseven terminal allyl groups, respectively, at the periphery, and a bromophenyl group at the focal point. We demonstrated that the density of the bromophenyl groups in the films indeed correlated to the generation of the dendron, and these groups can undergo surface Heck reaction with 4-fluorostyrene. Experimental Section Materials. The syntheses of G0 and G1 were previously described.29 Mesitylene was freshly distilled under vacuum and degassed before use. Diethyl ether (Et2O) was dried by refluxing and distillation over sodium/benzophenone. Synthesis of the G2 Dendron. HSiCl3 (1.18 mL, 11.7 mmol) was added to a solution of G129 (764 mg, 1.00 mmol) in anhydrous Et2O (20 mL) under N2, followed by a solution of H2PtCl6 in 2-propanol (12.2 mL, 0.695 mmol). Caution: HSiCl3 is highly toxic and hydroscopic, and should be handled according to the guidance of the MSDS! The mixture was stirred for 16 h. Samples for 1H NMR analysis were taken from the reaction mixture, dried under high vacuum, and dissolved in CDCl3. When the reaction was completed as indicated by the disappearance of the allyl signals in the 1H NMR spectrum of the sample, excess HSiCl3 was removed in a vacuum through a liquid nitrogen trap. Et2O (20 mL) and allylmagnesium bromide (1 M in Et2O, 35 mL, 35.0 mmol) were slowly added to the residue at room temperature. The mixture was refluxed for 4 h, cooled to 0 °C, and hydrolyzed with 10% aqueous NH4Cl solution. The organic layer was separated, and the aqueous phase was extracted with Et2O (30 mL × 3). The organic phases were combined, washed with H2O, dried over MgSO4, and concentrated. The residue was purified by flash chromatograph (silica gel, 230-400 mesh, hexane) to yield G2 (1.09 g, 51%) as an oil. 1H NMR (300 MHz, CDCl3): δ 7.47 (d, 2H, J ) 7.8 Hz), 7.30 (d, 2H, J ) 7.8 Hz), 5.69-5.84 (m, 27H), 4.84-4.97 (m, 54H), 1.58 (d, 54H, J ) 9.3 Hz), 1.24-1.35 (m, 24H), 0.65 (t, 24H, J ) 11.1 Hz), 0.50-0.59 (m, 24H). 13C NMR (75 MHz, CDCl3): δ 135.65, 135.23, 134.46, 130.98, 123.68, 112.94, 19.78, 18.35, 18.18, 17.81, 17.56, 16.70, 16.63. Anal. Calcd: C, 69.19; H, 9.96. Found: C, 69.23; H, 10.07. Substrate Preparation. Single-sided polished silicon(111) wafers were cleaned and etched similar to the reported procedures:30,31 (i) washing with H2O2/NH4OH/H2O (1:1:4 in volume) for 10 min at 80 °C, followed by rinsing with abundant H2O; (ii) repeating step i; (iii) etching with buffer-HF (Transene) for 2 min, followed by rinsing with abundant H2O; (iv) repeating steps (27) Xiao, Z.; Cai, C.; Mayeux, A.; Milenkovic, A. Langmuir 2002, 18, 7728. (28) Yam, C. M.; Mayeux, A.; Milenkovic, A.; Cai, C. Langmuir 2002, 18, 10274. (29) Yam, C. M.; Cho, J.; Cai, C. Langmuir 2003, 19, 6862. (30) Hines, M. A. Int. Rev. Phys. Chem. 2001, 20, 645. (31) Wade, C. P.; Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1679.
Langmuir, Vol. 20, No. 4, 2004 1229 i and iii; (v) etching with 40% NH4F (General Chemical) under N2 for 15 min, followed by immersing in H2O for 10 s, and immediately drying with a stream of N2. The prepared hydrogenterminated silicon(111) substrates were immediately used for the film deposition. Dendron Film Preparation. Solutions of dendrons G0, G1, and G2 of 1 mM concentration in mesitylene32 were prepared in Schlenk tubes that were precleaned by piranha solution (H2SO4/H2O2, 3:1 in volume). Caution: Piranha solution reacts violently with organic substances, and care should be taken while handling this mixture! The mesitylene solutions were repeatedly degassed and refilled with N2 three times. The clean hydrogenterminated silicon substrates were immersed in the solutions, and gently refluxed for 2 h under N2. After the solutions were cooled to room temperature, the substrates were taken out, rinsed sequentially with petroleum ether, ethanol, and dichloromethane, and finally dried with a stream of N2. Surface Hydroboration. For hydroboration of the alkene groups remaining in the dendron films,33 the samples were placed in dry Schlenk tubes, and the tubes were evacuated and refilled with N2. Each sample surface was covered with 1 M BH3 in tetrahydrofuran (THF) for 15 min. The samples were then washed sequentially with THF, 1 M aqueous HCl, Millipore water (resistivity 18.2 MΩ), and dichloromethane, and finally dried with a stream of N2. The samples were characterized by contactangle goniometry, ellipsometry, and X-ray photoelectron spectroscopy (XPS). Upon hydroboration, the water contact angles of the films were slightly lowered by 2-4°, and the ellipsometric thicknesses remained nearly the same ((1 Å). Surface Heck Reaction. Following the procedures of Heck reaction in the solution phase reported by Littke et al.,34 the dendron films were placed in dry Schlenk tubes. After the evacuation was repeated and the Schlenk tubes were refilled with N2 several times, a degassed solution of 4-fluorostyrene (0.1 mL, 0.8 mmol) and dicyclohexylmethylamine (0.2 mL, 0.9 mmol) in dioxane (1 mL) was added, followed by tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3; 3.6 mg, 0.0039 mmol) and bis(tri-tert-butylphosphine)palladium(0) (Pd(P(t-Bu)3)2; 4.2 mg, 0.0082 mmol). The Schlenk tubes were then placed in a 40 °C oil bath for 2 h. The samples were taken out, rinsed with THF and ethanol, and dried with a stream of N2. The samples were immediately characterized by contact-angle, ellipsometry, and XPS measurements. A control experiment repeating the above procedures but in the absence of Pd catalysts was performed. XPS of the films showed that the Br 3d photoelectron intensity remained unchanged, and no F 1s signal was observed, indicating that no reaction occurred in the absence of Pd catalysts. Contact-Angle Goniometry. Water or hexadecane drops were dispersed onto the dendron film surfaces using a microElectrapette 25 (Matrix Technologies). Advancing and receding contact angles were measured using a goniometer (Rame-Hart, model 100). The pipet tip was kept in contact with the drop during the measurements. At least four drops of probe liquid were measured for each sample, and the mean values were reproducible within (1°. Ellipsometry. An ellipsometer (Rudolph Research, Auto EL III), operated with a 632.8 nm He-Ne laser at an incident angle of 70°, was employed for thickness measurements. A refractive index of 1.45 was assumed for all dendron films. At least four measurements were taken for each sample, and the mean values were reproducible within (1 Å. Fourier Transform Infrared Spectroscopy in the Attenuated Total Reflection Mode (FTIR-ATR). The dendron films were deposited on the (111) surfaces of the single-sided polished silicon wafers (1.0 × 5.0 cm2). A silicon ATR crystal with a 45° angle of incidence was sandwiched between the reflective surfaces of two silicon wafers before and after deposition of the dendron films. The former was used for background correction. All spectra were run for 2000 scans at a resolution of 4 cm-1, using a Nicolet MAGNA-IR 860 Fourier transform spectrometer. (32) Sieval, A. B.; Vleeming, V.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 1999, 15, 8288. (33) Buriak, J. M. J. Chem. Soc., Chem. Commun. 1999, 1051. (34) Littke, A. F.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 6989.
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Figure 1. Schematic representation of possible orientations of a G0 dendron molecule on H-Si(111). Table 1. Advancing/Receding Water Contact Angle (θa/θr), Experimental Ellipsometric Thickness (Te), and XPS Data (C 1s and Br 3d) for the Dendron Films Derived from G0, G1, and G2 on H-Si(111) binding energy (eV) film
θa/θr (deg)
Te (Å)
C 1s
Br 3d
G0 G1 G2
88/78 90/78 90/78
12 16 20
284.5 284.5 284.6
70.2 70.2 70.3
XPS. A PHI 5700 X-ray photoelectron spectrometer, equipped with a monochromatic Al KR X-ray source (hν ) 1486.7 eV) at a takeoff angle (TOA) of 45° from the film surface, was employed for XPS measurements. High-resolution XPS spectra were obtained by applying a window pass energy of 23.5 eV and the following numbers of scans: Si 2p, 5 scans; C 1s, 10 scans; B 1s, 30 scans; F 1s, 40 scans; Br 3d, 40 scans. The binding energy scales were referenced to the Si 2p peak at 99.0 eV. XPS spectra were curve fitted, and the intensities measured as peak areas were calculated using Phi Multipak V5.0A from Physical Electronics.
Results and Discussion Contact-Angle Goniometry and Ellipsometry. As shown in Table 1, the advancing contact angles of water (θa(H2O)) for the G0-G2 films were in the range of 8890°, which are slightly higher than those reported for phenyl-terminated self-assembled monolayers (SAMs) such as aryl thiol on gold and phenylpropanol on Si/SiO2 (80-85°),35,36 but lower than those for polyethylene films (∼100°).37,38 Considering the surface structures illustrated in Figure 1, the probe liquid (H2O) interacts with a mixture of -PhBr, -CH2Si-, -CH2-, and -CHdCH2 groups. The advancing contact angles of the surfaces terminated with these groups are in the range of 80-100°. Therefore, the dendron surfaces are expected to display a similar range of θa, with the exact value depending on the relative percentage and density of these surface groups, which in turn is determined by the orientation (e.g., a-c in Figure 1) and density of the dendron molecules on the surfaces. Hexadecane (HD) almost completely wetted the dendron film surfaces (θa < 5°), further suggesting the interaction of the probe liquid with the CH2 groups.39 The water contact-angle hysteresis (10-12°) was comparable to those of alkyl monolayers on silicon prepared from 1-alkenes on hydrogen-terminated silicon (∼10°),2,32 but larger than those of trichlorosilane monolayers on silicon and alkanethiolate monolayers on gold (
