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Preparation, Characterization, and Heck Reaction of Siloxane Films Derived from Carbosilane Dendrons with a Bromophenyl Group at the Focal Point and up to 27 SiCl3 Groups at the Periphery Maxence Deluge and Chengzhi Cai* Department of Chemistry & Center for Materials Chemistry, University of Houston, Houston, Texas 77204-5003 Received October 24, 2004. In Final Form: December 9, 2004 Multidentate organosiloxane thin films were prepared on SiO2/Si surfaces by solution phase deposition of carbosilane dendrons containing a bromophenyl group at the focal point and 3 (Br-G0), 9 (Br-G1), and 27 (Br-G2) SiCl3 groups at the periphery. The films were characterized by contact angle goniometry, ellipsometry, and X-ray photoelectron spectroscopy (XPS). The results indicated that about six Br-G0 molecules covered the same surface area as three Br-G1 molecules and one Br-G2 molecule. Hence, the density of the bromophenyl groups in the films could be defined by the size (generation) of the dendron adsorbates. We also demonstrated that the bromophenyl groups on the film surfaces could serve as a handle for attaching conjugated molecules via formation of C-C bonds. Thus, upon treatment of the films with 4-fluorostyrene under Heck reaction conditions, XPS analysis showed that about 90, 66, and 51% of the bromine atoms in the films prepared from Br-G0, Br-G1, and Br-G2 were consumed, and 94, 82, and 58% of the consumed bromine atoms were replaced by fluorostyryl groups. The remaining bromophenyl groups were probably not accessible to the reactants because of their unfavorable orientation. The overall yields for the surface Heck reaction were estimated to be 84, 54, and 30% for the films prepared from Br-G0, Br-G1, and Br-G2, respectively.
Introduction For many fundamental studies and applications of organic thin films, it is highly desirable to precisely control the spacing between functional groups on the film surface. For instance, for sensor applications, there exists an optimal spacing between the probe molecules on the surface for maximizing the bindings of target molecules. Recent studies have shown that a high density of probe molecules is not necessarily the best as it often hinders binding of the target molecules.1-4 To date, the average density of the functional groups on self-assembled monolayers (SAMs) is commonly adjusted by co-deposition of a mixture of inert and functional adsorbates.5,6 However, this method may not allow for a precise control of spacing between functional groups at the microscopic level, because inhomogeneous distributions of functional groups due to (microscopic) phase separation may be difficult to avoid, especially if the functional groups can strongly interact with each other via, for example, hydrogen bonding.6-8 In fact, STM images9,10 and force measurements11 have * To whom correspondence should be addressed. Tel.: 713-7432710. Fax: 713-743-2709. E-mail:
[email protected]. (1) Houseman, B. T.; Mrksich, M. Angew. Chem., Int. Ed. 1999, 38, 782. (2) Fryxell, G. E.; Rieke, P. C.; Wood, L. L.; Engelhard, M. H.; Williford, R. E.; Graff, G. L.; Campbell, A. A.; Wiacek, R. J.; Lee, L.; Halverson, A. Langmuir 1996, 12, 5064. (3) Spinke, J.; Liley, M.; Schmitt, F.-J.; Guder, H. J.; Angermaier, L.; Knoll, W. J. Chem. Phys. 1993, 99, 7012. (4) Spinke, J.; Liley, M.; Guder, H. J.; Angermaier, L.; Knoll, W. Langmuir 1993, 9, 1821. (5) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (6) Imabayashi, S.; Gon, N.; Sasaki, T.; Hobara, D.; Kakiuchi, T. Langmuir 1998, 14, 2348. (7) Heid, S.; Effenberger, F. Langmuir 1996, 12, 2118. (8) Heise, A.; Stamm, M.; Rauscher, M.; Duschner, H.; Menzel, H. Thins Solid Films 1998, 329, 199. (9) Stranick, S. J.; Parickh, A. N.; Tao, Y. T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636. (10) Kakiuchi, T.; Iida, M.; Gon, N.; Hobara, D.; Imabayashi, S.; Niki, K. Langmuir 2001, 17, 1599.
revealed the presence of discrete, nanometer scale molecular domains in mixed SAMs prepared by co-deposition of various pairs of alkanethiols such as CH3(CH2)15SH and CH3O2C(CH2)15SH that are non-hydrogen-bonding and have an identical alkyl chain lengths on gold surfaces.9 Recently, we and others have proposed a new strategy to control the spacing between surface functional groups, based on the use of multidentate adsorbates having only one functional group.12-18 An example of such adsorbates is the carbosilane dendrons with a bromophenyl group at the focal point and many thiol12 or allyl13 groups on the periphery of the dendrons. These adsorbates chemisorb on gold or hydrogen terminated silicon surfaces through formation of multiple Au-S or Si-C bonds. We have shown that the density of the bromophenyl groups in the resulting monolayers can indeed be defined by the size (generation) of the dendrons. In addition, the multidentate binding greatly improves the stability of the films against desorption. We envision that such large, focally functionalized dendritic adsorbate molecules are also useful building blocks for fabrication of molecular nanostructures at surfaces where the location of individual molecules is precisely controlled. The ability to precisely positioning single molecules at surfaces is a key step in the development of surface-based nanoscience and technology. Recently, nanolithography methods such as conductive AFM have been used to locally oxidize SAMs on silicon substrates to generate nanometric templates for selective (11) Brewer, N. J.; Legget, G. J. Langmuir 2004, 20, 4109. (12) Yam, C. M.; Cho, J.; Cai, C. Langmuir 2003, 19, 6862. (13) Yam, C. M.; Cho, J.; Cai, C. Langmuir 2004, 20, 1228. (14) Xiao, Z.; Cai, C.; Mayeux, A.; Milenkovic, A. Langmuir 2002, 18, 7728. (15) Yam, C. M.; Mayeux, A.; Milenkovic, A.; Cai, C. Langmuir 2002, 18, 10274. (16) Deng, X.; Mayeux, A.; Cai, C. J. Org. Chem. 2002, 67, 5279. (17) Deng, X.; Cai, C. Tetrahedron Lett. 2003, 44, 815. (18) Choi, Y.-S.; Yoon, C. W.; Lee, H. D.; Park, M.; Park, J. W. Chem. Commun. 2004, 1316.
10.1021/la0473815 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/03/2005
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Deluge and Cai Scheme 1. Preparation of the Dendron Br-G2
trichlorosilyl derivatives are highly moisture-sensitive and readily hydrolyzed into silanol derivatives that undergo cross-linking to form siloxane networks. We showed that molecularly flat monolayers could be prepared with these dendrons. While the substrates for alkanethiolate SAMs are limited to a few noble metals, organosiloxane films can be deposited on a wide variety of substrate surfaces including silicon, glass, and mica. In this study, we employed Br-G0, Br-G1, and Br-G2 as the dendron adsorbates that carry a focal functional group (bromophenyl), and silicon as the substrates that are technologically important and allow for convenient characterization of the films by ellipsometry and XPS. We show that the density of the bromophenyl groups in the films can be controlled by the size (generation) of the dendrons. We also demonstrate the attachment of conjugated molecules to the bromophenyl groups on the film surface through C-C bond formation using surface Heck reaction. Experimental Section
attachment of molecules such as alkyltrichlorosilanes19 and proteins.20 The feature size of the nanopatterns is currently in the range of 10-100 nm. It is expected that this size can be reduced to a few nanometers so that each patterned spot can accommodate no more than one “sticky” dendrons carrying only one functional group. In this way, individual functional groups can be precisely positioned on surfaces and serve as handles for attachment of other single molecules.1 Herein, we present the study of the films derived from the carbosilane dendrons Br-G0, Br-G1, and Br-G2 on silicon oxide surfaces. The Br-G0, Br-G1, and Br-G2 dendrons are composed of a bromophenyl group at the focal point and 3, 9 and 27 terminal SiCl3 groups at the periphery, respectively. In a previous study, we prepared analogue films by spin-coating of similar carbosilane dendrons on mica surfaces.14 These carbosilane dendrons contained up to 81 SiCl3 groups, but the focal point of the dendrons was not functionalized. It is well-known that (19) Liu, S.; Maoz, R.; Sagiv, J. Nano Lett. 2004, 4, 845. (20) Gu, J.; Yam, C. M.; Li, S.; Cai, C. J. Am. Chem. Soc. 2004, 126, 8098.
Materials. Dry THF and toluene were obtained by refluxing and distillation over sodium/benzophenone. The dendrons Br-G0 and Br-G112,13 were synthesized by successive Grignard reactions and hydrosilylation starting from dibromobenzene. Absolute ethanol, H2SO4, hydrogen peroxide (H2O2), trichlorosilane (HSiCl3), hydrogen hexachloroplatinate hydrate (H2PtCl6), tris(dibenzylideneacetone)-dipalladium(0), dioxane, 4-fluorostyrene, N-methyldicylohexylamine, and bis(tri-tert-butylphosphine)-palladium(0) (Pd(PtBu3)2) were all commercially available and used without further purification. Caution: HSiCl3 is highly toxic and hydroscopic and should be handled according to the guidance of MSDS. The glassware (Schlenck tubes and vials) were cleaned by immersion in piranha solution (H2SO4/H2O2 3:1) at 80 °C for 1 h, washed thoroughly with Millipore water, and oven-dried for at least 12 h. Synthesis of Br-G2 Dendrons. A mixture of Ally-G212 (150 mg, 0.70 mmol), HSiCl3 (1.0 mL, 8.3 mmol), and 0.5 M Speier’s catalyst solution (0.15 µL, 7 mmol H2PtCl6 in i-PrOH) in THF (1 mL) was stirred under N2 at room temperature for 24 h (Scheme 1). After the reaction was completed as indicated by the absence of the allyl signals in the 1H NMR spectrum, excess HSiCl3 and THF were removed in a vacuum into a liquid nitrogen trap. As a result of the high moisture sensitivity, elemental analysis and mass spectroscopy of the product were not performed. 1H NMR (300 MHz, CDCl3): δ 7.49 (d, 2H, J ) 8.1 Hz), 7.30 (d, 2H, J ) 8.1 Hz), 1.17-1.37 (m, 60H), 0.8-0.97 (m, 78H), 0.56-0.70 (m, 96H). Preparation of the Substrates. Single side polished silicon (111) wafers were used as substrates. They were cleaned by immersion in piranha solution (H2SO4/H2O2 3:1) at 80 °C for 1 h, washed thoroughly with Millipore water, and blow-dried under a stream of nitrogen. Preparation of Dendron Films. After completion of the hydrosilylation reactions, the crude Br-G0, Br-G1, and Br-G2
Siloxane Films Derived from Carbosilane Dendrons dendrons were dried in a vacuum at r.t. for at least 4 h, and used without further purification. The Br-G0, Br-G1, and Br-G2 dendrons were diluted with dry toluene to a concentration of 1 × 10-5, 1 × 10-6, and 5 × 10-7 M, respectively. The freshly prepared solutions were added to the Schlenk tubes containing the silicon substrate under N2. The Schlenk tubes were then immersed in an oil bath at 40 °C for 30 min. After cooling to room temperature, the solutions were carefully removed using a syringe, and the substrates were thoroughly rinsed sequentially with toluene, CH2Cl2, and ethanol and dried with a stream of N2. The films were finally annealed at 120 °C under N2 for 6 h. Surface Heck Reaction. Following the published procedures for Heck reaction in solution phase by Littke et al.,21 the prepared dendron films were placed in clean and dry Schlenk tubes equipped with a small magnetic bar. After repeating the evacuation and refilling with N2 several times, degassed dioxane (2 mL), 4-fluorostyrene (65 µL, 0.52 mmol) and dicyclohexylmethylamine (120 µL, 0.54 mmol) were added, followed by Pd2(dba)3 (2.25 mg, 0.00230 mmol) and Pd(PtBu)3)2 (2.5 mg, 0.0049 mmol). The Schlenk tubes were then placed in a 40 °C oil bath and gently stirred for 4 h (the stirring bar should not touch the substrate surface). The samples were taken out and thoroughly rinsed with diethyl ether, sonicated for 10 min in diethyl ether, rinsed with ethanol, and finally dried with a stream of pure N2. The samples were characterized by contact angle goniometry, 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 bromine photoelectron intensity remained the same, while there was no fluorine signal, indicating that fluorostyryl groups were not incorporated into the films in the absence of the Pd catalyst. Contact-Angle Goniometry. Water drops were dispersed onto the dendron film surfaces using Matrix Technologies microElectropipette 25. Advancing and receding contact angles were measured using 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 3 measurements were taken for each sample, and the mean values were reproducible within (1 Å. X-ray Photoelectron Spectroscopy (XPS). A PHI 5700 X-ray photoelectron spectrometer equipped with a monochromatic Mg KR X-ray source (hν ) 1486.7 eV) was employed. XPS measurements were performed at 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: C 1s (20 scans), F 1s (60 scans), Br 3d (60 scans), and Si 2p (8 scans). The binding energy scales were referenced to the Si 2p peak at 99.0 eV. XPS spectra were curvefitted, and the intensities measured as peak areas were calculated using Phi Multipak V5.0A from Physical Electronics.
Results and Discussion Thin Film Preparation. Initially we prepared the Br-G0, Br-G1, and Br-G2 dendron films by spin-coating on silicon wafer surfaces using procedures similar to the one we developed for preparation of monolayers of analogous carbosilane dendrons on mica surfaces.14 However, the homogeneity of the films and the reproducibility of the procedure were not satisfactory for the present system. We then investigated the growth of the films by self-assembly in toluene solutions of the dendrons. A systematic study was carried out to optimize the deposition conditions. A series of films were prepared for concentrations of the dendrons ranging from 10-3 to 10-8 M, temperatures ranging from 25 °C to 80 °C, and deposition times ranging from 10 min to 2 h. Conditions leading to film thicknesses corresponding to monolayers were se(21) Littke, A. F.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 6989.
Langmuir, Vol. 21, No. 5, 2005 1919 Table 1. Ellipsometric Thickness (Te), Advancing/ Receding Water Contact Angle (θa/θr), and XPS Data for the Films Derived from Br-G0, Br-G1, and Br-G2 XPS binding energy (eV) film
Te (Å)
Br-G0 Br-G1 Br-G2
11 15 23
θa/θr (deg)
C 1s
Br 3d
Br/C
79/61 80/59 82/56
284.5 284.5 284.5
70.5 70.5 70.7
0.061 0.021 0.006
lected. The deposition conditions were then optimized to provide the lowest water contact angle hysteresis (difference between advancing contact angle and receding contact angle). A low contact angle hysteresis often corresponds to a high homogeneity in the films.22 Under the optimized deposition conditions described in the experimental part, we were able to reproducibly prepare homogeneous films of the dendrons. The ellipsometric thicknesses of these films and the corresponding water contact angles are presented in Table 1. Wettability. Ideally, the surfaces of the films derived from the dendrons should be composed mainly of -CH2CH2CH2Si- groups and a small fraction of bromophenyl groups. In fact, the contact angles of hexadecane for these films were all below 10°, as expected for surface dominantly composed of methylene groups. The reported advancing contact angles of water (θa) on SAMs with a methylene23,24 and phenyl25 headgroups are 100° and 80°, respectively. Therefore, θa of the ideal dendron monolayers should be in the upper range of 80-100°. In fact, films derived from similar carbosilane dendrons bound on silicon surfaces via Si-C bonds displayed θa of ∼90°.13 However, this was not the case for the present dendron films. θa of the films of Br-G0 to Br-G2 were in the range of 79-82°, similarly to the films prepared by spin-coating of analogous carbosilane dendrons (without bromine atom) on mica.15 The lowering of θa was probably due to the incomplete coverage of the underlying hydrophilic oxide substrate surface by the dendron monolayer. Previous study of similar dendron monolayers on mica has shown the existence of molecular scale pores or gaps probably resulting from the dendritic structure of the molecules.15 For chemically heterogeneous surfaces, the coverage of the monolayers can be estimated using the following modified Cassie’s law:26,27
(1 + cos θ)2 ) f1(1 + cos θ1)2 + f2(1 + cos θ2)2 (1) where f1 and f2 are the fractional areas occupied by components 1 and 2 and θ1 and θ2 are the contact angles of the pure surfaces of 1 and 2. Assuming that our dendron films are composed of only pure dendrons and bare Si/ SiO2, the surface coverage of dendron can be estimated from
fdendron )
(1 + cos θa)2 - (1 - cos θSi/SiO2)2 (1 + cos θmax)2 - (1 + cos θSi/SiO2)2
(2)
where fdendron is the fractional area occupied by the dendrons, θa is the measured advancing water contact (22) Fadeev, A. J.; McCarthy, T. J. Langmuir 1999, 15, 3759. (23) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 3665. (24) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (25) Sabatini, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (26) Israelachvili, J. N.; Gee, M. L. Langmuir 1989, 5, 288. (27) Drelich, J.; Miller, J. D. Langmuir 1993, 9, 619.
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Deluge and Cai Table 2. Ellipsometric Thickness (Te), Water Contact Angle (θa/θr), Percentage of Unreacted Br Atoms, F/C and F/Bra Ratios, Percentage of Reacted Br Atoms Replaced by Fluorostyryl Groups, and Overall Yield of the Heck Reaction dendron films after Heck reaction Te (Å) θa/θr (deg) % of unreacted Br F/C F/BrTheoreticala F/BrExperimental % of reacted Br atoms replaced by fluorostyryl groups (F/Brreacted) overall reaction % yield
FS-G0
FS-G1
15 79/60 10.3 0.039 8.71 8.17 93.8
17 80/60 34.1 0.013 1.93 1.57 81.7
24 82/59 48.6 0.0031 1.05 0.61 58.1
FS-G2
84.1
53.8
29.8
a
F/BrTheoretical based on ratio of Br supposedly reacted over Br left on the film.
Figure 1. XPS of Br 3d for films derived from dendrons Br-G0, Br-G1, and Br-G2 on SiO2/Si.
angles (Table 1), θSi/SiO2 (20°) is the contact angle for a freshly cleaned Si/SiO2 wafer, and θmax (102°) is the advancing water contact angle obtained with thick films of the dendrons. The thick films were prepared following the procedure described in the Experimental Section, except that a high concentration (10-3 M) of the dendron solutions were used, leading to a film thickness of at least 100 nm. We found that the contact angles of such films derived from different generation dendrons were about the same (102°). Using eq 2, the coverage for the films derived from Br-G0, Br-G1, and Br-G2 were estimated to be 75%, 76%, and 79%, respectively. XPS. XPS data of the films are listed in Table 1. Two intense Si 2p peaks corresponding to Si atoms in the dendron and bulk silicon substrate, and the SiO2 layer, appeared at 99.0 and 103.1 eV, respectively. The C 1s signal corresponding to the methylene and phenylene carbons of the dendron appeared as a strong peak centered at 284.5 eV. The weak Br 3d peak that appeared at 70.5 eV (Figure 1) is consistent with the presence of bromophenyl groups in the film.28 Derived from the molecular formulas, the theoretical Br/C ratios of Br-G0, Br-G1, and Br-G2 are 1/15 (0.066), 1/42 (0.024), and 1/123 (0.00813), respectively. After correction with sensitivity factor,29 the Br/C ratios of Br-G0, Br-G1, and Br-G2 measured by XPS were 0.061, 0.021, and 0.006, in good agreement with the theoretical values. The bromine peak intensity decreased for higher generation of dendrons as shown in Figure 1. By comparing the integrations of the Br 3d signals, the relative packing density of the dendron films can be estimated. Thus, the ratio Br-G0/Br-G1/ Br-G2 of Br 3d intensities was 6.3:3:1, suggesting that approximately six molecules of Br-G0 (with three SiCl3 groups) covered about the same area as three molecules (28) Briggs, D.; Seah, M. P. Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy; John Wiley & Sons: New York, 1984. (29) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129.
of Br-G1 (with nine SiCl3 groups) and one molecule of Br-G2 (with twenty-seven SiCl3 groups). These results demonstrate that the density of functional groups in the films is indeed defined by the size of the dendron. Modification of the Films by Heck Coupling Reactions. With the dendron monolayers in hand, we went on to demonstrate that the bromophenyl groups in these films can be used as a handle for attaching other molecules through formation of C-C bonds via Pdcatalyzed coupling reactions such as Heck reaction.30 To date, many types of organic reactions have been carried out on SAMs,31-33 but few palladium-catalyzed coupling reactions on surfaces have been reported.12,13 These reactions allow for the direct attachment of conjugated molecules. To validate the feasibility of Heck reaction on the dendron film surfaces, 4-fluorostyrene was employed as the reactant which allows us to monitor the reaction by measuring the F 1s photoelectron intensity. The experimental conditions reported by Fu’s group21 for highly efficient Heck reaction in solution were adapted to our biphasic system. The reaction was carried out at room temperature for 4 h; longer reaction time did not change the intensity of both the F 1s and Br 3d signals. After the reaction, the films were characterized by contact angles, ellipsometry and XPS. As shown in Table 2, the water contact angles (θa/θr) of the films remained nearly unchanged upon completion of the reaction. This result is understandable because the replacement of the bromophenyl groups with fluorostyryl groups, as minor components on the film surface, may not induce a significant change in the hydrophobicity of the film. Compared to the thicknesses of the dendron films, the ellipsometry measurement of the films after Heck reaction showed an increase of film thicknesses by 4, 2, and 1 Å for the films derived from Br-G0 (FS-G0), Br-G1 (FS-G1), and Br-G2 (FS-G2) (Table 1 and Table 2). These ellipsometric thicknesses and contact angles are consistent with the results obtained for similar films made on gold with thiol-terminated dendrons12 or made on H-terminated silicon (111) surfaces with allyl-terminated dendrons.13 The attachment of the fluorostyryl groups on the films upon Heck reaction was supported by the appearance of the XPS F 1s peak at 687 eV (Figure 2). By comparing the Br 3d peak intensities for the films before and after Heck (30) Heck, R. H. Acc. Chem. Res. 1979, 12, 146. (31) Tillman, N.; Ulman, A.; Penner, T. L. Langmuir 1989, 5, 101. (32) Balachander, N.; Sukenik, C. N. Langmuir 1990, 6, 1621. (33) Lee, Y. W.; Reed-Mundell, J.; Sukenik, C. N.; Zull, J. E. Langmuir 1993, 9, 3009.
Siloxane Films Derived from Carbosilane Dendrons
Langmuir, Vol. 21, No. 5, 2005 1921 Scheme 2. Proposed Mechanism for Surface Heck Reaction between the Bromophenyl Group of the Immobilized Dendron and 4-Fluorostyrene (Gn is dendron of generation n, n ) 1, 2, or 3)
Figure 2. X-ray photoelectron spectra of F 1s for films derived from dendrons Br-G0, Br-G1, and Br-G2 on Si/SiO2 after 4 h of Heck reaction.
coupling, we estimate that about 10, 34 and 49% of the bromine atoms present in the Br-G0, Br-G1, and Br-G2 films did not react (Table 2), even after prolonging the reactions. The failure of these bromophenyl groups to undergo the sterically demanding Heck reaction might be due to their unfavorable orientation or location in the films. Applying the commonly accepted mechanism for Heck reaction21 to our system (Scheme 2), the covalent attachment of the fluorostyryl to the dendron films proceeds via oxidative addition of the bromophenyl group present in 1 to the Pd(0) catalyst to form the intermediate 2, followed by the insertion of fluorostyrene leading to the bulky intermediate 3 that undergoes reductive elimination to give the product 4. Ideally, upon hydrolysis and adsorption, the dendrons Br-G0, Br-G1, and Br-G2 should orient the hydrophilic silanol groups on the periphery to bind to the substrate surface, thus exposing the focal functional group (Br-Ph) on the film surface (Figure 3a). However, the entropy factor favors the orientation of some of the peripheral groups on the film surface, leading to the tilting of the bromophenyl
groups (Figure 3b) and even the formation of aggregates via vertical polymerization (Figure 3c).34-36 It is conceivable that it is extremely difficult for the bromophenyl groups adopting the orientations shown in Figure 3b,c to form the bulky organo-palladium complex intermediate 3 (Scheme 2). The number of these unfavorable orientations likely increases for higher generation dendrons having more peripheral SiCl3 groups, which is consistent with the experimental results. As shown by the ratios of fluorine atoms over bromine atoms (Table 2), not all of the reacted bromine atoms in the films were converted into fluorostyryl groups. On the basis of these ratios, approximately 94, 82, and 58% of the reacted bromine atoms were replaced by fluorostyryl groups in the Br-G0, Br-G1, and Br-G2 films, respectively. Hence, the overall yields for the Heck coupling reaction on the dendron films were 84, 54 and 30% respectively for Br-G0, Br-G1, and Br-G2 (Table 2). Again, steric effect of the Heck reaction may be the reason for the increasing difficulty of the desired transformation for the higher generation dendrons. Specifically, even when the steric hindrance would not prevent the formation of the bulky intermediate 2, the insertion of the fluorostyryl group to form the bulkier intermediate 3, and consequently the desired product 4, would be inhibited. Instead, phenyl groups could then be formed as byproducts via dehalo-
Figure 3. Illustration of possible molecular configurations on monolayers derived from Br-G0 (a and b) and Br-G2 (c) on hydroxylterminated silicon (111).
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genation, a side reaction of the Heck coupling.37 This possible side reaction would explain the discrepancy between the estimated number of reacted bromine atoms and the number of fluorostyryl groups introduced in the films. The overall yields for the surface Heck coupling reaction on the films were estimated to be 84, 54, and 30% for the Br-G0, Br-G1, and Br-G2 films. Interestingly, these yields (84-30%) were similar to the yields of the same reactions performed on monolayers derived from the analogous carbosilane dendrons of the same generation and with a bromophenyl focal group but different peripheral groups including ethenyl (59-31%), and thiol (53-41%) on the hydrogen-terminated silicon and gold surfaces, respectively. This result suggests that the percentages of the bromophenyl groups with an unfavorable orientation (tilting) in these films were similar. The tilting of the bromophenyl groups probably results from the flexibility of the branches of these carbosilane dendrons. Conclusion Multidentate siloxane thin films on silicon surfaces bearing silanol groups could be reproducibly grown from solutions of Br-G0, Br-G1, and Br-G2, containing a bromophenyl group at the focal point and 3, 9, and 27 SiCl3 groups at the periphery, respectively. The results showed that the surface coverage for all the monolayer films were about 75-80%. XPS data indicate that about six Br-G0 molecules (with three SiCl3 groups at the periphery) cover about the same surface area as three (34) Brandriss, S.; Margel, S. Langmuir 1993, 9, 1232. (35) Trau, M.; Murray, B. S.; Grant, K.; Griezer, F. J. Colloid Interface Sci. 1992, 148, 182. (36) Fadeev, A. Y.; McCarthy, T. J. Langmuir 2000, 16, 7268. (37) Djakovitch, L.; Wagner, M.; Hartung, C. G.; Beller, M.; Koehler, K. J. Mol. Catal. A: Chem. 2004, 219, 121.
Deluge and Cai
Br-G1 molecules bearing nine SiCl3 groups and one Br-G2 molecule bearing 27 SiCl3 groups. These films were derivatized with 4-fluorostyrene using palladium-catalyzed Heck coupling reaction. XPS study of the resulting films indicated that about 90, 66, and 51% of the bromine atoms in the films of Br-G0, Br-G1, and Br-G2 were consumed, and 94, 82 and 58% of these consumed bromine atoms were replaced by fluorostyryl groups. The overall yields for the surface Heck coupling reaction on the films were estimated to be 84, 54 and 30% for the Br-G0, Br-G1, and Br-G2 films. Some bromophenyl groups are probably not accessible for the reaction because of their unfavorable location and/or orientation in the films, mainly because of the flexibility of the branches of the carbosilane dendrons. As suggested by a reviewer, the accessibility of the focal functional group could be improved by using a long spacer such as a conjugated oligomer to connect the functional group to the focal point of the dendron. To increase the rigidity of the molecular framework, we recently designed and synthesized tripod-shaped oligophenylenes with a bromophenyl group at the focal point of the tripod and many surface active groups at the base of each tripod leg. Preliminary results have shown that the monolayers derived from these adsorbates exhibited a nearly quantitative yield for similar palladium-catalyzed coupling reactions. The results will be reported in due course. Acknowledgment. This work was supported by the Robert A. Welch Foundation, the Petroleum Research Fund (type G) administrated by the American Chemical Society, and the GEAR Program at the University of Houston. LA0473815
