Preparation, Characterization, and Heck Reaction of Multidentate

Langmuir , 2003, 19 (17), pp 6862–6868 ... Publication Date (Web): July 8, 2003 ... Multidentate thiolate films were prepared by solution phase depo...
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Langmuir 2003, 19, 6862-6868

Preparation, Characterization, and Heck Reaction of Multidentate Thiolate Films on Gold Surfaces Chi Ming Yam, Jaeeock Cho, and Chengzhi Cai* Department of Chemistry, and Center for Materials Chemistry, University of Houston, Houston, Texas 77204-5003 Received March 28, 2003. In Final Form: May 15, 2003 Multidentate thiolate films were prepared by solution phase deposition of dendritic oligothiols on gold surfaces. The dendritic molecules contain three (G0) and nine (G1) thiol groups at the periphery and a functional group (bromophenyl) at the corn. The films were characterized by contact-angle goniometry, ellipsometry, polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). Results of the XPS measurement indicate the presence of ∼20% of unbound S in the G0 films while ∼28% in the G1 films. Some of the unreacted thiol groups may be located at the upper part of the films and some of the bromophenyl groups at the lower part of the films. Three G0 molecules (containing three SH groups) cover about the same area on the substrate as one G1 molecule (containing nine SH groups), and both of the films exhibit similar thicknesses, indicating the flattening of the dendrons to maximize the bonding of SH groups to the gold surface. The G1 films exhibit higher stability than the G0 films, and both of them are much more stable than octadecanethiolate monolayers on gold in hot solvents, attributed to the formation of multiple S-Au bonds per dendron unit. The high stability of the films allows for the palladium-catalyzed Heck reaction on thiolate films. Treatment of both the G0 and G1 films with 4-fluorostyrene under Heck reaction conditions led to the disappearance of about 76 and 68% of the Br atoms among which about 60-70% were replaced by the 4-fluorostyryl groups, as shown by XPS. Prolonging the reaction time did not consume the remaining Br atoms, probably due to unfavorable orientation of the bromophenyl groups and/or the presence of adjacent thiol groups that may deactivate the catalyst in the film assembly.

Introduction Self-assembled monolayers (SAMs) on metal and semiconductor surfaces provide molecular platforms with a well-defined chemical functionality, allowing for the control of surface properties such as reactivity, specific affinity, wettability, and biocompatibility that are relevant to numerous applications.1-5 Such SAMs include organosiloxanes on SiO2/Si,6 alkanethiolates on gold,1,7 carboxylic acids8,9 and phosphonic acids10 on metallic oxides/ metal, and alkenes11 on hydrogen-terminated silicon surfaces. Among them, SAMs of alkanethiolates on gold are the most extensively studied, because not only are many of these SAMs highly ordered but also the atomically flat Au(111) surfaces are relatively easy to prepare and are stable under ambient conditions.12 The SAMs bearing active headgroups, such as carboxylic acid, amino, or hydroxy groups, can serve as platforms to covalently anchor a variety of functional moieties such as DNA, proteins, and carbohydrates for specific interaction with biomolecules and conjugated molecules for studies relevant * To whom correspondence should be addressed. Telephone: 713-743-2710. Fax: 713-743-2709. E-mail: [email protected]. (1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (2) Chidsey, C. E. D.; Murray, R. W. Science 1986, 231, 25. (3) Ulman, A. Adv. Mater. 1990, 2, 573. (4) Thomas, R. C.; Kim, T.; Crooks, R. M.; Houston, J. E.; Michalske, T. A. J. Am. Chem. Soc. 1995, 117, 3830. (5) Sullivan, T. P.; Huck, W. T. S. Eur. J. Org. Chem. 2003, 17. (6) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (7) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (8) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (9) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (10) Woodward, J. T.; Ulman, A.; Schwartz, D. K. Langmuir 1996, 12, 3636. (11) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3154. (12) Ulman, A. Chem. Rev. 1996, 96, 1533.

to potential optoelectronic and microelectronic applications.13-22 For this purpose, the studies of reactions on SAMs have recently attracted increasing interest.5,23 Interestingly, a high density of surface functional groups on closely packed SAMs may not facilitate, and may even hamper, the reactions especially for bulky reagents, due to the unfavorable steric interaction.24-26 Hence, controlling the spacing between the active headgroups in SAMs is highly desirable for certain surface reactions and interactions. The average surface density of functional groups can be adjusted by co-deposition of inert and active adsorbates to form mixed SAMs.27,28 However, the problem (13) Frey, B. L.; Jordan, C. E.; Kornguth, S.; Corn, R. M. Anal. Chem. 1995, 67, 4452. (14) Bierbaum, K.; Kinzler, M.; Woll, Ch.; Grunze, M.; Hahner, G.; Heid, S.; Effenberger, F. Langmuir 1995, 11, 512. (15) Fritz, M. C.; Hahner, G.; Spencer, N. D. Langmuir 1996, 12, 6074. (16) Jordan, C. E.; Frutos, A. G.; Thiel, A. J.; Corn, R. M. Anal. Chem. 1997, 69, 4939. (17) Wagner, P.; Nock, S.; Spudich, J. A.; Volkmuth, W. D.; Chu, S.; Cicero, R. L.; Wade, C. P.; Linford, M. R.; Chidsey, C. E. D. J. Struct. Biol. 1997, 119, 189. (18) Revell, D. J.; Knight, J. R.; Blyth, D. J.; Haines, A. H.; Russell, D. A. Langmuir 1998, 14, 4517. (19) Wadu-Mesthrige, K.; Xu, S.; Amro, N. A.; Liu, G.-Y. Langmuir 1999, 15, 8580. (20) Sieval, A. B.; Linke, R.; Heij, G.; Meijer, G.; Zuilhof, H.; Sudholter, E. J. R. Langmuir 2001, 17, 7554. (21) Kang, J. F.; Ulman, A.; Liao, S.; Jordan, R.; Yang, G.; Liu, G.-Y. Langmuir 2001, 17, 95. (22) Credo, G. M.; Boal, A. K.; Das, K.; Galow, T. H.; Rotello, V. M.; Feldheim, D. L.; Gorman, C. B. J. Am. Chem. Soc. 2002, 124, 9036. (23) Chechik, V.; Crooks, R. M.; Stirling, C. J. M. Adv. Mater. 2000, 12, 1161. (24) Houseman, B. T.; Mrksich, M. Angew. Chem., Int. Ed. 1999, 38, 782. (25) Li, M.; Lee, H. J.; Condon, A. E.; Corn, R. M. Langmuir 2002, 18, 805. (26) Wegner, G. J.; Lee, H. J.; Corn, R. M. Anal. Chem. 2002, 74, 5161.

10.1021/la0345385 CCC: $25.00 © 2003 American Chemical Society Published on Web 07/08/2003

Multidentate Thiolate Films on Gold Surfaces

of (microscopic) phase segregation may be difficult to avoid, especially when the active headgroups can strongly interact with each other, e.g., via hydrogen bonding. Recently, we have proposed an alternative way to control the spacing between active headgroups in monolayers.29,30 This is based on the use of focally functionalized dendrons with many surface active groups at the periphery. Upon chemisorption on the surface, such dendrons are expected to flatten and spread out on the surface to maximize the bonding between the periphery and the surface,31-34 and the spacing between the adjacent functional groups at the focal point of the adsorbed dendrons is then determined by the size of the dendrons. In addition, the multidentate bindings between the dendron and the surface significantly increase the stability of the adsorbate. This is particularly important for thiolate SAMs where the S-Au bonds are relatively weak. Indeed, although alkanethiolate SAMs on gold are quite stable at elevated temperatures in ambient conditions, significant desorption in hot solvents has been reported.35 This drawback limits the scale of chemical derivatization and potential applications of the thiolate SAMs.36 Several types of multidentate thiolate monolayers on gold have been reported, including aminotrithiols,37,38 calix[4]resorcinarenetetrasulfides,39-42 cyclodextrin polythiols,43-45and thiol-terminated dendrimers.46,47 In particular, Crooks and co-workers showed that most of the thiol groups that are randomly distributed on the periphery of the fourth generation PAMAM dendrimer are covalently bonded to the gold surface due to the flexibility of the dendrimer allowing those thiol groups to reach the gold surface.46 Herein we reported the preparation, characterization, and Heck reaction of the films derived from dendrons containing three thiol groups (G0) and nine thiol groups (G1), both with a bromophenyl group at the focal point, on gold surfaces. We employed contact-angle goniometry, (27) Mrksich, M.; Grunwell, J. R.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 12009. (28) Rief, M.; Oesterhelt, F.; Heymann, B.; Gaub, H. E. Science 1997, 275, 1295. (29) Xiao, Z.; Cai, C.; Mayeux, A.; Milenkovic, A. Langmuir 2002, 18, 7728. (30) Yam, C. M.; Mayeux, A.; Milenkovic, A.; Cai, C. Langmuir 2002, 18, 10274. (31) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171. (32) Tsukruk, V. V. Adv. Mater. 1998, 10, 253. (33) Hierlemann, A.; Campbell, J. K.; Baker, L. A.; Crooks, R. M.; Ricco, A. J. J. Am. Chem. Soc. 1998, 120, 5323. (34) Tully, D. C.; Frechet, J. M. J. J. Chem. Soc., Chem. Commun. 2001, 1229. (35) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (36) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (37) Creager, S.; Collard, D. M.; Fox, M. A. Langmuir 1990, 6, 1617. (38) Fox, M. A.; Whitesell, J. K.; McKerrow, A. J. Langmuir 1998, 14, 816. (39) Adams, H.; Davis, F.; Stirling, C. J. M. J. Chem. Soc., Chem. Commun. 1994, 2527. (40) van Velzen, E. U. T.; Engbersen, J. F. J.; Delange, P. J.; Mahy, J. W. G.; Reinhoudt, D. N. J. Am. Chem. Soc. 1995, 117, 6853. (41) Friggeri, A.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Langmuir 1998, 14, 5457. (42) Faull, J. D.; Gupta, V. K. Langmuir 2002, 18, 6584. (43) Rojas, M. T.; Koniger, R.; Stoddart, J. F.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 336. (44) Nelles, G.; Weisser, M.; Back, R.; Wohlfart, P.; Wenz, G.; MittlerNeher, S. J. Am. Chem. Soc. 1996, 118, 5039. (45) Beulen, M. W. J.; Bugler, J.; Lammerink, B.; Geurts, F. A. J.; Biemond, E. M. E. F.; van Leerdam, K. G. C.; van Veggel, F. C. J. M.; Engbersen, J. F. J.; Reinhoudt, D. N. Langmuir 1998, 14, 6424. (46) Chechik, V.; Crooks, R. M. Langmuir 1999, 15, 6364. (47) Liebau, M.; Janssen, H. M.; Inoue, K.; Shinkai, S.; Huskens, J.; Sijbesma, R. P.; Meijer, E. W.; Reinhoudt, D. N. Langmuir 2002, 18, 674.

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ellipsometry, polarization-modulation infrared reflectionabsorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS) to characterize the dendron films. We show that the spacing between the functional group (-PhBr) can be controlled by the size (generation) of the dendron, and the thermal stability of such multidentate films is indeed greatly enhanced compared to octadecanethiolate SAMs on gold. We demonstrate that the -PhBr groups in the thiolate films can be used to anchor conjugated molecules onto the films under the Heck reaction. Although a variety of organic reactions, such as nucleophilic substitution, free radical halogenation, and oxidation/reduction, etc, have been performed on surfaces,5,23 and there are several reports relating to the complexation of palladium with bidentate sulfur ligands,48,49 it is the first palladium-catalyzed coupling reaction performed on the thiolate films.

Experimental Section Materials. All solvents and chemicals required for the synthesis of G0 and G1 were purchased from commercial suppliers of high quality and were used without purification. Et2O was dried and distilled over sodium/benzophenone. G0 and G1 were synthesized via hydrosilylation and allylation.50,51 Caution: SiCl4 and HSiCl3 are highly toxic and hydroscopic and should be handled according to the guidance of the MSDS! Bromophenyltrichlorosilane (2). A solution of 1,4-dibromobenzene (1, 40 g, 0.17 mol; Scheme 1) in 80 mL of Et2O was slowly added to a mixture of Mg (4.2 g, 0.17 mol) in Et2O (10 mL). The reaction mixture was stirred for 8 h and slowly added to a mixture of SiCl4 (27 mL, 0.24 mol) in Et2O (40 mL) within 2 h. The mixture was refluxed for 16 h, and unreacted SiCl4 and Et2O were removed in vacuo. Et2O (100 mL) was added, and the mixture was stirred for 0.5 h. The remaining solid was filtered off under N2. The filtrate was concentrated and fractionally distilled through a Vigreux column in vacuo to provide 2 as an oil (9.53 g, 19%). 1H NMR (300 MHz, CDCl3): δ 7.67 (s). 13C NMR (75 MHz, CDCl3): δ 134.43, 131.85, 130.22, 128.19. Bromophenyltriallylsilane (G0-(allyl)3, 3). To a solution of 2 (2.95 g, 10.2 mmol) in Et2O (20 mL), allylmagnesium bromide (1 M in Et2O, 40 mL, 40 mmol) was added slowly at room temperature under N2. The mixture was refluxed for 16 h, cooled to 0 °C, and hydrolyzed with 10% H2SO4. The organic layer was separated, and the aqueous layer was extracted with Et2O (50 mL × 3). The organic layers were combined and washed with water, dried over MgSO4, and concentrated. The residue was purified by flash chromatography (silica gel, 230-400 mesh, n-hexane) to give 3 as an oil (2.86 g, 92%). 1H NMR (300 MHz, CDCl3): δ 7.52 (d, 2H, J ) 7.8 Hz), 7.37 (d, 2H, J ) 7.8 Hz), 5.70-5.84 (m, 3H), 4.89-4.96 (m, 6H), 1.86 (d, 6H, J ) 8.1 Hz). 13C NMR (75 MHz, CDCl ): δ 135.67, 133.87, 133.27, 130.82, 3 124.16, 114.52, 19.25. Anal. Calcd: C, 58.63; H, 6.23. Found: C, 58.98; H, 6.02. (48) Huck, W. T. S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Angew. Chem., Int. Ed. Engl. 1996, 35, 1213. (49) Huck, W. T. S.; Snellink-Ruel, B. H. M.; Lichtenbelt, J. W. T.; van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Chem. Soc., Chem. Commun. 1997, 9. (50) Motesharei, K.; Myles, D. C. J. Am. Chem. Soc. 1997, 119, 6674. (51) Kim, M.-K.; Jeon, Y.-M.; Jeon, W. S.; Kim, H.-J.; Hong, S. G.; Park, C. G.; Kim, K. J. Chem. Soc., Chem. Commun. 2001, 667.

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Yam et al. Scheme 1. Synthesis of G0 and G1

Bromophenyltris(3-thioacetylpropyl)silane (G0-(SAc)3, 4). A solution of 3 (307 mg, 1.00 mmol) in toluene (10 mL) was treated with thioacetic acid (457 mg, 6.00 mmol) and azobis(isobutyronitrile) (AIBN; 15 mg, 0.090 mmol). The solution was then refluxed for 40 min. The reaction was quenched by addition of aqueous NaHCO3 (1 M, 15 mL), and the mixture was extracted with ethyl acetate. The organic layer was washed with aqueous NaHCO3 (1 M, 15 mL × 3) and brine (15 mL) and dried over MgSO4. Flash chromatography (silica gel, 230-400 mesh, 10% ethyl acetate/n-hexane) gave 4 as an oil (430.8 mg, 80%). 1H NMR (300 MHz, CDCl3): δ 7.50 (d, 2H, J ) 8.1 Hz), 7.30 (d, 2H, J ) 8.1 Hz), 2.87 (t, 6H, J ) 7.2 Hz), 2.34 (s, 9H), 1.49-1.59 (m, 6H), 0.83-0.88 (m, 6H). 13C NMR (75 MHz, CDCl3): δ 195.77, 136.64, 135.58, 131.24, 124.24, 32.52, 30.76, 24.15, 11.69. Bromophenyltris(3-mercaptopropyl)silane (G0-(SH)3, G0). To a solution of 4 (340 mg, 0.66 mmol) in dry tetrahydrofuran (THF; 20 mL) was slowly added NaBH4 (75 mg, 2.0 mmol). After being stirred for 1 h, the reaction mixture was quenched with 15% aqueous NaOH solution and filtered. The organic layer was dried over MgSO4. Flash chromatography (silica gel, 230-400 mesh, n-hexane) gave G0 as an oil (249 mg, 92%). 1H NMR (300 MHz, CDCl3): δ 7.51 (d, 2 H, J ) 8.4 Hz), 7.35 (d, 2H, J ) 8.4 Hz), 2.54 (m, 6H), 1.55-1.68 (m, 6H), 1.35 (t, 3H, J ) 8.1 Hz), 0.87-0.99 (m, 6H). 13C NMR (75 MHz, CDCl3): δ 134.46, 133.72, 130.09, 123.06, 27.17, 27.09, 10.19. FT-IR: 2557 cm-1 (SH). Anal. Calcd: C, 43.99; H, 6.15. Found: C, 44.53; H, 6.04. G1-(allyl)9 (5). A mixture of 3 (3.28 g, 10.67 mmol), HSiCl3 (5.6 mL, 42 mmol), and H2PtCl6/i-PrOH (1.22 M, 0.070 mmol) in THF (40 mL) was stirred at 40 °C for 16 h. When the reaction was complete as indicated by the absence of the allyl signals in the 1H NMR spectrum, excess HSiCl3 was removed in vacuo into a liquid nitrogen trap. Allylmagnesium bromide (0.75 M in Et2O, 60 mL, 40 mmol) was added slowly at room temperature. The reaction mixture was refluxed for 8 h, cooled to 0 °C, and hydrolyzed with 10% H2SO4. The organic layer was separated, and the aqueous phase was extracted with Et2O (50 mL × 3). The organic layers were combined, washed with water, and dried over MgSO4. Flash chromatography (silica gel, 230-400 mesh, n-hexane) gave 5 as an oil (6.68 g, 82%). 1H NMR (300 MHz, CDCl3): δ 7.49 (d, 2H, J ) 7.8 Hz), 7.30 (d, 2H, J ) 7.8 Hz), 5.67-5.79 (m, 9H), 4.83-4.88 (m, 18H), 1.56 (d, 18H, J ) 8.7 Hz), 1.33-1.36 (m, 6H), 0.82 (m, 6H), 0.65 (m, 6H). 13C NMR (75 MHz, CDCl3): δ 136.32, 135.51, 134.23, 130.82, 123.59, 113.52, 19.63, 18.06, 17.22, 16.45. G1-(SAc)9 (6). A solution of 5 (312 mg, 0.409 mmol), thioacetic acid (561 mg, 7.38 mmol), and AIBN (15 mg, 0.091 mmol) in toluene (10 mL) was refluxed for 2 h. The reaction was quenched by addition of aqueous NaHCO3 (1 M, 15 mL). The mixture was diluted with ethyl acetate, washed with aqueous NaHCO3 (1 M, 15 mL × 3) and brine (15 mL), and dried over MgSO4. Flash chromatography (silica gel, 230-400 mesh, 20% ethyl acetate/

n-hexane) gave 6 as an oil (369 mg, 61%). 1H NMR (300 MHz, CDCl3): δ 7.50 (d, 2H, J ) 8.1 Hz), 7.29 (d, 2H, J ) 8.1 Hz), 2.82 (t, 18H, J ) 7.2 Hz), 2.31 (s, 27H), 1.41-1.52 (m, 18H), 1.23-1.27 (m, 6H), 0.77-0.83 (m, 6H), 0.50-0.59 (m, 24H). 13C NMR (75 MHz, CDCl3): 195.64, 136.34, 135.70, 131.04, 123.72, 32.33, 30.74, 24.33, 18.37, 17.40, 17.11, 11.93. G1-(SH)9 (G1). To a solution of 6 (152 mg, 0.105 mmol) in dry THF (20 mL) was slowly added NaBH4 (38 mg, 0.99 mmol). After being stirred for 2 h, the reaction mixture was quenched with a 15% aqueous NaOH solution. After filtration, the organic layer was dried over MgSO4. Flash chromatography (silica gel, 230400 mesh, 10% ethyl acetate/n-hexane) gave G1 as an oil (108 mg, 91%). 1H NMR (300 MHz, CDCl3): δ 7.52 (d, 2H, J ) 8.1 Hz), 7.33 (d, 2H, J ) 8.1 Hz), 2.47-2.55 (m, 18H), 1.49-1.60 (m, 18H), 1.29-1.38 (m, 15H), 0.82-0.88 (m, 6H), 0.56-0.62 (m, 24H). 13C NMR (75 MHz, CDCl3): δ 136.39, 135.73, 131.07, 123.78, 28.52, 28.80, 18.45, 17.37, 17.20, 11.58. FT-IR: 2557 cm-1 (SH). Anal. Calcd: C, 47.10; H, 8.00. Found: C, 47.95; H, 8.10. Substrate Preparation. Substrates were prepared by deposition of 100 Å of chromium, followed by 1000 Å of gold onto silicon wafers at 10-6 mbar. The gold-coated substrates were rinsed with absolute ethanol, followed by drying with a stream of pure N2 before deposition of the dendron films. Preparation of Dendron Films. Solutions of freshly synthesized G0 and G1 in degassed anhydrous THF of 1 mM concentration were prepared in glass vials, which were precleaned with Piranha solution (H2SO4/H2O2 3:1) at 80 °C. Caution: Piranha reacts violently with organic compounds, and care should be taken while handling this mixture. The clean gold-coated substrates were immersed in the dendron solutions for 24 h in N2, followed by thoroughly rinsing with THF and ethanol and finally drying with a stream of pure N2. Thermal Stability Tests. The dendron films on gold surfaces were heated in a degassed solution of hexadecane at 80 °C for 1 h and toluene at 110 °C for 2 h, respectively, followed by rinsing with THF and ethanol and drying with a stream of pure N2. The samples were then immediately subjected to contact-angle and ellipsometry measurements. Surface Heck Reaction. Following the published procedures for the Heck reaction in solution phase,52 the prepared dendron films on gold substrates were placed in oven-dried Schlenk tubes that were cleaned with Piranha solution at 80 °C. After repeating the evacuation and refilling with N2 several times, degassed dioxane (1 mL), 4-fluorostyrene (0.1 mL, 0.8 mmol), and dicyclohexylmethylamine (0.2 mL, 0.9 mmol) were added, followed by Pd2(dba)3 (3.6 mg, 0.0039 mmol) and 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 and rinsed thoroughly (52) Littke, A. F.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 6989.

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Table 1. Advancing/Receding Water Contact Angle (θa/θr), Experimental Ellipsometric Thickness (Te), and XPS Data (C 1s, S 2p, Si 2p, and Br 3d) for G0 and G1 Films on Gold G0 θa/θr (deg) Te (Å) XPS binding energy (eV) C 1s S 2p Si 2p Br 3d

82/60 10 284.5 162.0, 163.2 99.6 70.3

G1 82/61 13 284.4 161.9, 163.2 99.7 70.2

with THF and ethanol, and dried with a stream of pure N2. The samples were immediately characterized by contact-angle, ellipsometry and XPS measurements. A control experiment repeating the above procedure but in the absence of Pd catalysts was performed. XPS of the film showed that the Br 3d photoelectron intensity remained the same, while there was no F 1s signal, indicating that there was no reaction in the absence of Pd catalysts. Contact-Angle Goniometry. Water or hexadecane drops were dispersed onto the dendron film surfaces using Matrix Technologies micro-Electrapette 25. Advancing and receding contact angles were measured using a Rame-Hart model 100 goniometer. The pipet tip should be kept in contact with the drop during the measurements. At least 4 drops of probe liquids were measured for each sample, and the mean values were reproducible within (1°. Ellipsometry. A Rudolph Research Auto EL III ellipsometer, operated with a 632.8 nm He-Ne laser at an incident angle of 70°, was employed for thickness measurement. 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. Polarization-Modulation Infrared Reflection-Absorption Spectroscopy. A Nicolet MAGNA-IR 860 Fourier transform spectrometer, equipped with a liquid nitrogen-cooled mercurycadmium-telluride (MCT) detector and a Hinds Instruments PEM-90 photoelastic modulator (37 Hz), was employed to obtain IR spectra. All spectra were run for 256 scans at a resolution of 4 cm-1 using p-polarized light incident at 80°. X-ray Photoelectron Spectroscopy. A PHI 5700 X-ray photoelectron spectrometer equipped with a monochromatic Al KR X-ray source (hν ) 1486.7 eV) was employed. XPS measurements were performed at a takeoff angle (TOA) of 45° from the film surfaces. High-resolution XPS spectra were obtained by applying a window pass energy of 23.5 eV and the following numbers of scans: Au 4f, 2 scans; C 1s, 10 scans; F 1s, 40 scans; S 2p, 40 scans; Si 2p, 40 scans; Br 3d, 40 scans. The binding energy scales were referenced to the Au 4f7/2 peak at 84.0 eV. XPS spectra were curve-fitted and the intensities measured as peak areas were calculated using Phi Multipak V5.0A from Physical Electronics.

Figure 1. Schematic representation of possible orientations of the G0 dendron on gold.

Figure 2. PM-IRRA spectra in the region 2700-3100 cm-1 for G0 and G1 films on gold.

The G0 and G1 dendron films were characterized by contact-angle goniometry, ellipsometry, PM-IRRAS, and XPS. The data are presented in Table 1. Contact-Angle Goniometry. Contact-angle goniometry has been extensively employed to characterize surface wettability of organic thin films.53 As shown in Table 1, the advancing contact angles of water for both the G0 and G1 thin films were 82°, which is similar to the phenyl-54 or bromoalkyl-terminated55 SAMs on gold (8085°) but lower than the methyl- (∼110°) or methyleneterminated35,55 (∼100°) SAMs. The lower water contact

angle of the films may be attributed to the exposure of a small portion of thiol (see below) and bromophenyl headgroups in addition to methylene chains on the surface (Figure 1). Hexadecane (HD) almost completely wets the film surface (