Fabrication of Multilayer Assemblies Based on a Boronate-Terminated

Ovadia Abed, Meni Wanunu, Alexander Vaskevich, Rina Arad-Yellin, Abraham Shanzer, and Israel Rubinstein. Chemistry of Materials 2006 18 (5), 1247-1260...
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Langmuir 1998, 14, 2372-2377

Fabrication of Multilayer Assemblies Based on a Boronate-Terminated Self-Assembled Monolayer Shinji Kato and Chyongjin Pac* Kawamura Institute of Chemical Research, 631 Sakado, Sakura, Chiba 285-0078, Japan Received November 7, 1997. In Final Form: March 4, 1998 A new methodology for the fabrication of covalently bound multilayers has been developed on the basis of a self-assembled monolayer (SAM) prepared from 5-(2-methyl-1,3,2-dioxaborinan-5-yl)pentyltrichlorosilane (1). The SAM was prepared by immersion of an aluminized silicon wafer into a toluene solution of 1; its thickness was determined by ellipsometry to be 1.5 ( 0.3 nm, a value very similar to the molecular size of 1. The fabrication of a two-layer film was successfully achieved by hydrolysis of the boronate protecting group of the SAM with water/ethanol to the diol-terminated group followed by treatment of the hydrolyzed monolayer with a toluene solution of 1; the thickness (2.9 ( 0.3 nm) was nearly twice that of the starting monolayer. Similar treatment of the two-layer film with water/ethanol effected the selective hydrolysis of the boronate end group without significant loss of the second-layer unit, and subsequent reactions of the hydrolyzed two-layer film with 1 or octadecyltrichlorosilane smoothly occurred to give the corresponding three-layer assembly with thickness of 4.2 ( 0.4 or 5.4 ( 0.5 nm each.

Introduction Much attention has been currently directed to selfassembled monolayers (SAMs), typically represented by alkylsiloxane networks linked with oxide surfaces (e.g., glass and surface-oxidized metals)1 and alkanethiolate monolayers bound with the surface of gold, silver, or copper,1c,2 which are often regarded as a counterpart of Langmuir-Blodgett (LB) monolayers having no covalent bonding with the substrate surface.3 In general, SAMs have an important advantage over LB films in high chemical and physical stability due to the covalent bonding with substrate surfaces as well as in convenient processability in the film fabrication. On the other hand, a crucial drawback of SAMs compared with LB films can be found in limited structural variations of relevant molecules and, particularly, in difficulty in the construction of multilayers. In the case of LB films, a large number of functionalized multilayers have been constructed with structural versatility, because the LB method basically requires only solubilizability of relevant molecules in organic solvents but no covalent bonding with the substrate surface or in layer-to-layer lamination.3 Although many functional SAMs4 such as redox active SAMs5 were already reported, only a limited number of studies have been performed on * Author for correspondence. Tel: (+81)43-498-2111. Fax: (+81)43-498-2202. E-mail: [email protected]. (1) (a) Ulman, A. Adv. Mater. 1990, 2, 573. (b) Brzoska, J. B.; Ben Azouz, I.; Rondelez, F. Langmuir 1994, 10, 4367. (c) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) (a) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (b) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (3) (a) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (b) Tredgold, R. H. Order in Thin Organic Films; Cambridge University Press: New York, 1994. (4) For example, see: (a) Wollman, E. W.; Kang, D.; Frisbie, C. D.; Lorkovic, I. M.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 4395. (b) Motesharei, K.; Myles, D. C. J. Am. Chem. Soc. 1994, 116, 7413. (c) Lander, L. M.; Brittain, W. J.; DePalma, V. A.; Girolmo, S. R. Chem. Mater. 1995, 7, 1437. (5) For example, see: (a) Hickman, J. J.; Laibinis, P. E.; Auerbach, D. I.; Zou, C.; Gardner, T. J.; Whitesides, G. M.; Wrighton, M. S. Langmuir 1992, 8, 357. (b) Zhang, L.; Lu, T.; Gokel, G. W.; Kaifer, A. E. Langmuir 1993, 9, 786. (c) Yip, C. M.; Ward, M. D. Langmuir 1994, 10, 549.

functional multilayer assemblies based on SAMs, in particular, on multilayers formed by interlayer chemical bonding.6,7 This is certainly due to the limitation in preparation methodology associated with controlling film thickness and individual layer compositions of multilayer assemblies. In fact, a limited number of papers have appeared on the fabrication of multilayer assemblies based on layerto-layer chemical bonding, that is, covalent,8-10 hydrogen,11 ionic,7,12 or coordination13 bonding. Among them, covalent interlayer connections appear to be superior in the formation of highly stable, structurally well-defined, and neutral organic multilayers without participation of ionic and metallic species, though recent studies on the multilayer systems formed by interlayer hydrogen bonding have revealed their unexpected high stability.11 The formation of interlayer covalent bonds primarily requires (6) (a) Yitzchaik, S.; Marks, T. J. Acc. Chem. Res. 1996, 29, 197. (b) Lin, W.; Lin, W.; Wong, G. K.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 8034. (c) Roscoe, S. B.; Yitzchaik, S.; Kakkar, A. K.; Marks, T. J.; Xu, Z.; Zhang, T.; Lin, W.; Wong, G. K. Langmuir 1996, 12, 5338. (d) Lin, W.; Lee, T.-L.; Lyman, P. F.; Lee, J.; Bedzyk, M. J.; Marks, T. J. J. Am. Chem. Soc. 1997, 119, 2205. (7) (a) Katz, H. E.; Bent, S. F.; Wilson, W. L.; Schilling, M. L.; Ungashe, S. B. J. Am. Chem. Soc. 1994, 116, 6631. (b) Katz, H. E.; Wilson, W. L.; Scheller, G. J. Am. Chem. Soc. 1994, 116, 6636. (c) Byrd, H.; Suponeva, E. P.; Bocarsly, A. B.; Thompson, M. E. Nature 1996, 380, 610. (8) (a) Netzer, L.; Sagiv, J. J. Am. Chem. Soc. 1983, 105, 674. (b) Heid, S.; Effenberger, F.; Bierbaum, K.; Grunze, M. Langmuir 1996, 12, 2118. (9) (a) Tillman, N.; Ulman, A.; Penner, T. L. Langmuir 1989, 5, 101. (b) Pomerantz, M.; Segmuller, A.; Netzer, L.; Sagiv, J. Thin Solid Films 1985, 132, 153. (10) Collins, R. J.; Bae, I. T.; Scherson, D. A.; Sukenik, C. N. Langmuir 1996, 12, 5509. (11) (a) Maoz, R.; Yam, R.; Berkovic, G.; Sagiv, J. In Organic Thin Films and Surfaces: Directions for the Nineties; Ulman, A., Ed.; Academic: San Diego, CA, 1995; Vol. 20, p 41. (b) Maoz, R.; Sagiv, J.; Degenhardt, D.; Mohwald, H.; Quint, P. Supramol. Sci. 1995, 2, 9. (c) Maoz, R.; Matlis, S.; DiMasi, E.; Ocko, B. M.; Sagiv, J. Nature 1996, 384, 150. (12) (a) Lee, H.; Kepley, L. J.; Hong, H. G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618. (b) Cao, G.; Hong, H. G.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 420. (c) Thompson, M. E. Chem. Mater. 1994, 6, 1168. (d) Katz, H. E. Chem. Mater. 1994, 6, 2227. (13) (a) Evans, S. D.; Ulman, A.; Goppert-Berarducci, K. E.; Gerenser, L. J. J. Am. Chem. Soc. 1991, 113, 5866. (b) Bell, C. M.; Arendt, M. F.; Gomez, L.; Schmehl, R. H.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 8374. (c) Ansell, M. A.; Zeppenfeld, A. C.; Yoshimoto, K.; Cogan, E. B.; Page, C. J. Chem. Mater. 1996, 8, 591.

S0743-7463(97)01215-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/11/1998

Fabrication of Multilayer Assemblies

that the SAM surface be covered with functional groups (typically, the hydroxy groups) that can undergo smooth reactions with other bifunctional molecules to give an additional layer with a reactive surface. Repeated reactions of top-layer reactive surfaces with bifunctional molecules might thus give covalently linked multilayers. To fabricate uniform multilayer assemblies with welldefined structures, however, relevant reactions used for the covalent-linked layer lamination must be those which can control both the thickness and each layer composition with no random growth of chains. Therefore, reactant bifunctional molecules are required to undergo efficient reactions only with the film surface but no further reaction after the top-layer formation or self-condensation/polymerization reaction. A reasonable way is to employ particular bifunctional molecules in which one of the functional groups must be chemically protected or inert during the top-layer formation but can be readily deprotected or converted to a reactive site by in-situ chemical transformations on the two-dimensional film surface prior to the next layer lamination. In Sagiv’s pioneering work,8a they prepared the SAM of an ω-trichlorosilyl-1-alkene and then tried to convert the double-bond terminals to the hydroxy groups through hydroboration followed by oxidation. However, this procedure appears to be difficult in achieving the quantitative conversion to the hydroxylated surface on the twodimensional reaction field. An alternative approach was made by Ulman and co-workers to attempt the transformation of the terminal methoxycarbonyl functions to the hydroxylated SAM surface under rigorous conditions, i.e., lithium aluminum hydride reduction followed by hydrochloric acid treatment.9a Unfortunately, however, the hydroxylated SAM was significantly contaminated with inorganic residues.9a Another drawback of this method might be due to possible losses of SAM molecules that may occur under such reaction conditions. Recently, photolysis of a nitrate-bearing SAM was applied to create the hydroxylated surface, which was then subjected to the subsequent multilayer preparation.10 According to these precedents, an essential key for the construction of covalently bound multilayer assemblies is that an effective synthetic methodology must be developed for the quantitative creation of the hydroxylated SAM surface as well as for an efficient reaction of the surface with bifunctional molecules under mild conditions accompanied by no or least side reactions. We wish to report herein a novel methodology, in line of this perspective, for the creation of uniformly hydroxylated surface utilizing hydrolysis of the cyclic boronate linkage on SAM of 1 under mild conditions. The hydrolysis was successfully achieved in nearly quantitative yield, and the resulting hydroxylated surface allowed the sequential deposition of multilayer assemblies.

Experimental Section General. Proton (1H) and carbon (13C) NMR spectra were measured in CDCl3 on a JEOL GSX-400 instrument at 400 and 100 MHz, respectively, using tetramethylsilane as an internal reference. Infrared spectra were recorded on a JASCO diffraction grating infrared spectrophotometer. Capillary G C analyses were performed on a Shimadzu gas chromatograph GC-8A equipped with a capillary column (30 m, 0.25 mm i.d., GL Sciences Neutrabond-1). Mass spectra were acquired on a Shimadzu gas

Langmuir, Vol. 14, No. 9, 1998 2373 chromatograph-mass spectrometer system GCMS 9100-MK using chemical ionization (CI, isobutene). Materials. All the organic solvents (dichloromethane, THF, DMF, toluene, and ethanol) were anhydrous reagents purchased from Kanto Chemicals. Octadecyltrichlorosilane (2) was purchased from Shinetsu Silicon Chemicals and used as received. Triethylamine was used as received. 6,6-Bis(ethoxycarbonyl)-1-hexene (6). Diethyl malonate (13.9 g, 86.8 mmol) in DMF (15 mL) was added dropwise to a suspension of sodium hydride (oil free, 2.08 g, 86.8 mmol) in DMF (40 mL) at 0 °C, and the mixture was stirred for 15 min until most of the sodium hydride had been dissolved. To this mixture was added dropwise 5-bromo-1-pentene (6.50 g, 43.6 mmol) in DMF (25 mL) at 85 °C, and the resulting solution was further stirred for 6 h. After filtration, dichloromethane (200 mL) was added to the filtrate and washed with aqueous saturated NaHCO3 (150 mL) and then with aqueous saturated NaCl (150 mL × 3). The aqueous layers were combined and extracted with dichloromethane (200 mL). The combined organics were dried (MgSO4), filtered, and concentrated. Distillation of the residual oil gave 6 as clear colorless oil (8.17 g, 82%): bp 125-130 °C/12 mmHg; 1H NMR (400 MHz, CDCl3) δ 5.77 (ddt, 1H, CH2dCH), 4.95-5.05 (m, 2H, CH2dCH), 4.21 (q, 4H, CH2O), 3.33 (t, 1H, CHCO), 2.09 (dt, 2H, CH2dCHCH2), 1.90 (dt, 2H, CH2CHCO), 1.43 (tt, 2H, CH2dCHCH2CH2), 1.25 (t, 6H, CH3); 13C NMR (100 MHz, CDCl3) δ 169.46, 137.93, 114.90, 61.47, 61.26, 60.97, 51.89, 41.66, 33.77, 33.24, 31.75, 28.15, 26.53, 23.31, 14.05; IR (neat) 2980, 2930, 2860, 1730, 1635, 1442, 1368, 1332, 1300, 1268, 1240, 1218, 1175, 1150, 1098, 1032, 915, 860 cm-1. 7-Hydroxy-6-(hydroxymethyl)-1-heptene (7). To a suspension of lithium aluminum hydride (2.50 g, 65.8 mmol) in THF (120 mL) was added dropwise 6 (7.50 g, 32.9 mmol) in THF (30 mL) at 60 °C, and the mixture was refluxed for 3.5 h. After sequential quenching with water (3 mL), 10% aqueous NaOH (3 mL), and water (10 mL), the mixture was stirred for 15 min and then dried (MgSO4). Filtration followed by concentration in vacuo left an oil, which was then chromatographed on SiO2 (eluents, 7:3 hexane/ethyl acetate (v/v) f ethyl acetate) to afford a clear colorless oil (3.84 g, 81%): Rf 0.51 (SiO2, ethyl acetate); 1H NMR (400 MHz, CDCl3) δ 5.79 (ddt, 1H, CH2dCH), 4.94-5.03 (m, 2H, CH2dCH), 3.81 (dd, 2H, CH2O), 3.65 (dd, 2H, CH2O), 3.10 (br, s, 2H, OH), 2.05 (dt, 2H, CH2dCHCH2), 1.77 (ttt, 1H, CH2CHCH2), 1.43 (tt, 2H, CH2dCHCH2CH2), 1.26 (dt, 2H, CH2CH); 13C NMR (100 MHz, CDCl3) δ 138.51, 114.67, 66.16, 60.42, 41.82, 41.75, 33.88, 33.82, 27.14, 26.42; IR (neat) 3350, 2940, 2850, 1638, 1460, 1440, 1374, 1308, 1250, 1100, 1038, 1000, 916, 852 cm-1. 5-(2-Methyl-1,3,2-dioxaborinan-5-yl)-1-pentene (8). A solution of 7 (3.14 g, 21.8 mmol) and methaneboronic acid (1.30 g, 21.8 mmol) in toluene (80 mL) placed in a Dean-Stark flask was refluxed until a theoretical amount of water had been collected (2 h). Vacuum concentration followed by chromatography on SiO2 (eluents, 4:1 hexane/ethyl acetate (v/v)) gave 8 as clear colorless oil (3.12 g, 85%): Rf 0.61 (SiO2, 4:1 hexane/ethyl acetate (v/v)); 1H NMR (400 MHz, CDCl3) δ 5.77 (ddt, 1H, CH2dCH), 4.95-5.04 (m, 2H, CH2dCH), 3.98 (dd, 2H, CH2O), 3.59 (dd, 2H, CH2O), 2.06 (dt, 2H, CH2dCHCH2), 1.96 (ttt, 1H, CH2CHCH2), 1.43 (tt, 2H, CH2dCHCH2CH2), 1.22 (dt, 2H, CH2CH), 0.17 (s, 3H, BCH3); 13C NMR (100 MHz, CDCl3) δ 138.11, 114.95, 69.52, 66.30, 36.18, 33.71, 30.81, 27.62, 25.95; IR (neat) 3070, 2925, 2850, 1635, 1475, 1408, 1355, 1333, 1241, 1061, 1028, 995, 912, 742 cm-1. 5-(2-Methyl-1,3,2-dioxaborinan-5-yl)pentyltrichlorosilane (1). To a solution of 8 (2.52 g, 15.0 mmol) and hexachloroplatinic acid hexahydrate (5.3 mL of 4.18 mM THF solution, 2.2 × 10-5 mol) in THF (40 mL) was added dropwise trichlorosilane (14.4 g, 106 mmol) in THF (40 mL) at 60 °C, and the mixture was refluxed for 4 h. Vacuum concentration followed by distillation gave a clear colorless oil (3.52 g, 77%): bp 105107 °C/1.0 mmHg; 1H NMR (400 MHz, CDCl3) δ 3.98 (dd, 2H, CH2O), 3.59 (dd, 2H, CH2O), 1.96 (ttt, 1H, CH2CHCH2), 1.59 (t, 2H, CH2Si), 1.32-1.45 (m, 6H, CH2), 1.23 (dt, 2H, CH2CH), 0.17 (s, 3H, BCH3); 13C NMR (100 MHz, CDCl3) δ 66.33, 36.21, 31.87, 27.99, 26.21, 24.21, 22.11; IR (neat) 2935, 2860, 1479, 1408, 1337, 1242, 1100, 893, 834, 768, 746, 695, 657, 587, 565, 515, 472 cm-1;

2374 Langmuir, Vol. 14, No. 9, 1998 MS (CI, isobutene) m/z 307 (35), 305 (100), 303 (M + 1, 99), 247 (12), 245 (33), 243 (35), 203 (2.8), 201 (2.2), 113 (0.6), 111 (2.5), 109 (2.6). Substrate Preparation. Silicon wafers (E&M Corp., pdoped, 0.37 mm thick) were cut into small strips (25 × 25 mm) and washed with deionized water and then with dichloromethane in an ultrasonicator bath. Aluminum (Rare Metal Co., 99.99%) was deposited in a thickness of ∼ 150 nm onto the cleaned silicon wafers at 6.7 × 10-3 Pa under monitoring with a quartz crystal thickness monitor. The aluminized silicon wafers were stored in a desiccator prior to SAM preparation (12-15 h). The thickness of the aluminum oxide layer (∼4.5 nm) formed on the aluminum surface during the storage was estimated by ellipsometry just before the SAM preparation. Preparation of SAMs. The aluminized silicon wafers were dipped in 3 mM anhydrous toluene solution of 1 or 2 in the presence of 3 equiv of triethylamine for 1 h at room temperature (20-23 °C) and then thoroughly washed with anhydrous toluene and with anhydrous ethanol. After heating at 120 °C for 1 h under reduced pressure (∼102 Pa), the SAM-deposited substrates were stored in a desiccator prior to follow-up procedures and surface characterization. Hydrolysis of SAMs. The SAM-deposited substrates of 1 were immersed into a 3:7 water/ethanol (v/v) mixture for a period of time ranging from 5 min to 3 h at room temperature (20-23 °C) and then thoroughly washed with anhydrous ethanol and dried at 50 °C for 30 min under reduced pressure (∼102 Pa). In some cases, this sequential procedure was repeated (two to five times). The hydrolyzed substrates were stored in a desiccator prior to follow-up procedures and surface characterization. Preparation of Multilayers. The preparation of two-layer films was performed by immersion of the hydrolyzed substrates into 3 mM anhydrous toluene solution of 1 or 2 in the presence of 3 equiv of triethylamine for a period of time ranging from 1 to 3 h at room temperature (20-23 °C). The two-layer substrates thus obtained were thoroughly washed with anhydrous toluene and with anhydrous ethanol and then heated at 120 °C for 1 h under reduced pressure (∼102 Pa). The two-layer films prepared with 1 were immersed into a 3:7 water/ethanol (v/v) mixture for 1 h at room temperature (20-23 °C) and then thoroughly washed with anhydrous ethanol; this hydrolysis treatment was repeated three times. The hydrolyzed two-layer substrates were dried at 50 °C for 30 min under reduced pressure (∼102 Pa). Similarly, three-layer films were fabricated by the procedure described for the preparation of the two-layer films. All the multilayerdeposited substrates were stored in a desiccator prior to followup procedures and surface characterization. Film Characterization. The fabricated monolayers and multilayers were analyzed by advancing contact angle measurements, ellipsometry, and X-ray photoelectron spectroscopy (XPS). Advancing contact angles were measured on sessile drops (2 µL) of doubly distilled pure water at room temperature (20-23 °C) under air by using a Kyowa Interface Science CA-X contact angle meter. All the observed values for each film were averaged for more than six measurements taken on different drops. For the determination of film thickness, ellipsometric measurements were performed for more than nine different spots of each film using a Photo Device Mary-102 ellipsometer equipped with a He-Ne laser (λ 632.8 nm); the incident angle of the laser light beam was 70.0°. All the observed values for each film were averaged, and the film thickness was calculated by assuming that the SAMs and multilayers are transparent materials with a common refractive index of 1.45. Elemental analyses of the films were carried out by XPS using a Shimadzu ESCA-850 X-ray photoelectron spectrometer (Mg KR X-ray source; 10-4-10-5 Pa) referenced to C1s at 284.6 eV. A survey spectrum (resolution 0.5 eV, one scan) and high-resolution spectra of the C1s, O1s, B1s, Si2p, and Al2p regions (resolution 0.1 eV, 5-30 scans) were taken for each sample.

Results and Discussion Design and Synthesis of Boronate-Terminated Alkyltrichlorosilane Compound. The strategy for the multilayer fabrication employed in the present study is shown in Scheme 1. The basic idea underlying this

Kato and Pac Scheme 1. Fabrication of Multilayer Assemblies by Protecting Methodology Based on Boronate Chemistry

strategy comes from the fact that the cyclic boronate linkage can be readily formed from and readily hydrolyzed to the 1,3-diol and the boronic acid under mild conditions.14 That is to say, the boronate groups 3 covering the surface of the SAM formed by silane compound 1 should be readily hydrolyzed to give the corresponding hydroxylated surface 4, which can in turn react with 1 to give a two-layer film with the interlayer siloxane bond. Consequently, this hydrolysis-deposition sequence might be applied to the lamination of multilayer assemblies bound by the interlayer siloxane bonds. Some interesting features can be found in this system as follows. (1) The hydrolysis of the boronate linkage for the creation of the hydroxylated surface can be achieved under mild conditions, thus leading to minimum or little damage of the film. This is certainly crucial in suppressing the generation of defects and pinholes in the multilayer assemblies. (2) Two hydroxylic reactive sites can be generated from one cyclic boronate group, so that the twodimensional molecular density may increase with the number of layers to result in effective coverage of the substrate surface. This appears to be related with the concept of dendrimer synthesis,15 being thus anticipated to lead to a possible formation of hyperbranched multilayer systems.16,17 On the basis of this strategy, the boronate-terminated alkyltrichlorosilane compound 1 was synthesized according to Scheme 2. The dihydroxyalkene 7 was obtained by the condensation of 5-bromo-1-pentene with diethyl malonate in the presence of sodium hydride, followed by lithium aluminum hydride reduction. The reaction of the diol function with methaneboronic acid gave the cyclic boronate linkage, which can work to protect the diol group in the subsequent hydrosilylation reaction and, more importantly, which can serve as the protected hydroxy function in the fabrication of hydroxylated films, as already described. The hydrosilylation of 8 using hexachloroplatinic acid hexahydrate as catalyst afforded the target trichlorosilyl compound 1 in a moderate net yield (77%). (14) (a) Lappert, M. F. Chem. Rev. 1956, 56, 959. (b) Torsell, K. Prog. Boron Chem. 1964, 1, 369. (15) (a) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A. Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (b) Frechet, J. M. J. Science 1994, 263, 1710. (c) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681. (16) For studies to prepare hyperbranched polymer thin films under the similar concept, see: (a) Zhou, Y.; Breuning, M. L.; Bergbreiter, D. E.; Crooks, R. M.; Wells, M. J. Am. Chem. Soc. 1996, 118, 3773. (b) Breuning, M. L.; Zhou, Y.; Aguilar, G.; Agee, R.; Bergbreiter, D. E.; Crooks, R. M. Langmuir 1997, 13, 770. (17) For adsorption of the presynthesized dendritic molecules to substrates, see: (a) Watanabe, S.; Regen, S. L. J. Am. Chem. Soc. 1994, 116, 8855. (b) Wells, M.; Crooks, R. M. J. Am. Chem. Soc. 1996, 118, 3988.

Fabrication of Multilayer Assemblies

Langmuir, Vol. 14, No. 9, 1998 2375

Scheme 2. Synthetic Route for Boronate-Terminated Alkyltrichlorosilane Compound 1

Table 1. Advancing Contact Angles, Film Thickness, and Elemental Ratio for SAMs and Multilayer Assemblies Fabricated on Aluminized Silicon Wafers no. of surface immersion layers groupa time/hb 1 2

3

a

3 4f 5 3 4f 5 5 5 3 4f 5

1 1 3 1 2 3 3 3

contact angle θa/degc

thickness/ nmd

72 ( 1 34 ( 1 110 ( 2 69 ( 1 33 ( 2 94 ( 2 103 ( 4 109 ( 3 69 ( 2 33 ( 3 110 ( 2

1.5 ( 0.3 1.3 ( 0.2 2.4 ( 0.2 2.9 ( 0.3 2.6 ( 0.3 3.5 ( 0.3 3.8 ( 0.2 4.0 ( 0.3 4.2 ( 0.4 4.1 ( 0.5 5.4 ( 0.5

elemental ratioe C1s Si2p 0.47 0.44 0.55 0.57 0.54

0.051 0.039 0.048 0.072 0.076

0.72 0.69 0.66 0.83

0.072 0.11 0.13 0.10

b

See Scheme 1. For layer formation in toluene solution of 1 or 2 (3 mM) at room temperature in the presence of triethylamine (9 mM). c Averaged values of advancing contact angles for water at gsix measurement points. d Averaged values obtained by ellipsometric measurements at gnine different points; the thickness was calculated by assuming that the SAMs and the multilayers are transparent materials with a common refractive index of 1.45.e Area ratios of the XPS signals againset the sum of C1s, Si2p, and Al2p taken at takeoff angle of 90°. f Obtained by three cycles of repeated immersion of 3 into fresh water/ethanol for 1 h at room temperature.

Preparation of Boronate-Terminated SAM. The preparation of SAM using 1 or 2 was carried out essentially according to reported procedures.18 Immersion of an aluminized silicon wafer into an anhydrous toluene solution of 1 (3 mM) for 1 h in the presence of 3 equiv of triethylamine gave a SAM with thickness of 1.5 ( 0.3 nm, which is exactly that expected from the molecular length of 1. Table 1 summarizes advancing contact angles for water on the SAM surface, ellipsometric thickness, and elemental ratios determined by XPS analysis. The water contact angles on the SAM surface were 72 ( 1°, which seems to be reasonable for the relatively hydrophilic and polar nature of the methylboronate surface.3a It is therefore indicated that the SAM of 1 was successfully formed on the substrate. While the SAM of 1 was made up by heating at 120 °C for 1 h to facilitate the siloxane network formation, it was confirmed that the cyclic boronate linkage is thermally stable enough to be left unchanged in the heating process.19 With regard to hydrolytic reactivity of the boronate surface 3, moreover, it is of crucial significance to note that the water contact angle was gradually decreased with time during the measurement, from 72° immediately after the application of water to 60° after 5 min. Another important observation is that a film obtained by immersing the substrate into a toluene solution of 1 (3 mM) in the absence of triethylamine showed ellipso(18) (a) Cohen, S. R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986, 90, 3054. (b) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 110, 6136. (c) Tillman, N.; Ulman, A.; Elman, J. F. Langmuir 1990, 6, 1512.

Figure 1. Plots of B1s ratio and advancing contact angle for water (θa) versus the period of time for immersion of SAM of 1 in water/ethanol. The parameter B1s ratio denotes relative areas of the XPS B1s signals before and after the hydrolysis treatment.

metric thickness of 34 ( 2 nm, which roughly corresponds to a chain length of over 20 molecules of 1. Probably, hydrochloric acid generated during the SAM formation could catalyze the hydrolysis of the boronate linkage to cause unreclaimed and uncontrollable condensation/ polymerization reactions on the surface. It should be however emphasized that the cyclic boronate linkage of 1 is stable enough under the conditions for the SAM preparation; the use of both the anhydrous solvent and the base can safely avoid possible hydrolysis of the boronate group by hydrochloric acid and moisture. Hydrolysis of Boronate Terminal Groups on SAMs. As already mentioned, it is required that hydrolysis of the boronate linkages in the SAM of 1 must occur completely and selectively under mild conditions to create the uniform hydroxylated surface 4 with no significant destruction of the SAM. In this regard, it is suggestive that the water contact angle on the SAM of 1 was decreased with time during the measurement, an observation that the selective hydrolysis might be able to be achieved with water under neutral conditions at room temperature. To confirm this possibility, we investigated details of what occurred by immersion of the SAM-deposited substrate into a 3:7 water/ethanol (v/v) mixed solvent at room temperature. Figure 1 shows plots of relative areas of XPS B1s signals before and after the hydrolysis treatment (B1s ratio) and advancing contact angles for water against the immersion period, and Figure 2 displays typical XPS spectra for the B1s region. The B1s ratio value was decreased with immersion time, certainly indicating that the hydrolytic loss of the boronate groups proceeds by contact with water/ ethanol. It is important to note that the B1s ratio value levels off at g 1 h to reach a constant value (0.42-0.45) with no further change. Similarly, the water contact angle exhibited a decrease with immersion time to reach a constant value (51-52°) at g 1 h. These results imply that the extent of hydrolysis increases with immersion time but settles down at a certain level, presumably due to an equilibrium between bond breaking and forming of the boronate linkage. To achieve the complete hydrolysis of the boronate surface 3, we repeated several times the hydrolysis treatments of the SAM of 1 with fresh water/ethanol solvent for 1 h at room temperature. Plots of B1s ratio values and advancing contact angles for water against (19) For stability of the ester-linkage formed by the boronic acidterminated SAMs with some alcohols or phenols on gold, see: Carey, R. I.; Folkers, J. P.; Whitesides, G. M. Langmuir 1994, 10, 2228.

2376 Langmuir, Vol. 14, No. 9, 1998

Figure 2. XPS spectra of the SAM of 1 in the B1s region (a) before and (b) after immersion in water/ethanol for 1 h and (c) after three-cycle repetitions of immersion in water/ethanol for 1 h.

Figure 3. Plots of B1s ratio and advancing contact angle for water (θa) versus the number of repetitions for immersion of SAM of 1 in water/ethanol. The parameter B1s ratio denotes relative areas of the XPS B1s signals before and after the hydrolysis treatment.

the number of repeated treatment are shown in Figure 3, clearly demonstrating that considerable decreases of both the B1s ratio value and the contact angle were attained. It should be noted that the water contact angle reached a minimum value of 33 ( 1°, which is nearly corresponding to a uniformly hydrophilic surface.9a,10 Therefore, it appears that hydrolysis of the boronate linkage is mostly completed by at least three cycles of repeated immersion of the substrate into the fresh solvent. Likewise, no peak was recognized in the B1s region of the XPS spectrum after three cycles of repeated immersion (Figure 2c). It should be however noted that unavoidable noise in the XPS signals contributed to the B1s ratio since the absolute intensity in the B1s region is quite low, thereby the B1s ratio value still showed 0.12 even after three cycles of repeated immersion. The repetitive treatment of the SAM of 1 with fresh aqueous solvent was thus proved to be an effective method to obtain the uniformly hydroxylated surface. Ellipsometric thickness of the hydrolyzed SAM displayed a reasonable value, 1.3 ( 0.2 nm. The hydrolyzed SAM was allowed to dry under reduced pressure (∼102 Pa) after the sequential hydrolysis treatment. In the literature,9a,20 it was pointed out that such (20) (a) Ulman, A.; Evans, S. D.; Shnidman, Y.; Sharma, R.; Eilers, J. E. Adv. Colloid Interface Sci. 1992, 39, 175. (b) Hautman, J.; Bareman, J. P.; Mar, W.; Klein, M. L. J. Chem. Soc., Faraday Trans. 1991, 87, 2031.

Kato and Pac

Figure 4. A linear plot of ellipsometric thickness versus the number of layers for the hydrolyzed monolayer and multilayers with the surface group 4.

drying treatment of hydroxy group-bearing SAMs might induce molecular reorientation to hide the hydroxy groups inside the films. In those cases, therefore, the water contact angle should be higher than the anticipated value. In the present hydrolyzed SAM, however, the water contact angle was still 33 ( 1° even after drying for 30 min under reduced pressure, demonstrating that the film surface is unchanged by the drying treatment. This is probably due to high hydroxy density given from the diol functions and/or to difficulty in the reorientation of the relatively short pentyl spacer of 1. Preparation of Multilayer Assemblies. Deposition of the second layer of 1 or 2 onto the hydrolyzed SAM was successfully performed by the procedure employed for the SAM preparation, though the prolonged immersion (3 h) was required (see the two-layer film data of surface group 5 in Table 1). The two-layer films with the surface groups of 3 and 5 displayed, respectively, 2.9 ( 0.3 and 4.0 ( 0.3 nm for ellipsometric thickness and 69 ( 3° and 109 ( 3° for water contact angle. Area ratios of C1s and Si2p against Al2p in the XPS signals of these films are significantly higher than those of the monolayer systems, in accord with the formation of two-layer films. In contrast, only minor changes of the water contact angle and the film thickness occurred after similar treatment of the unhydrolyzed intact SAM of 1 with anhydrous toluene solution of 1 and 2 (data not shown), clearly indicating that creation of the hydroxylated surface 4 by hydrolysis is necessary for effective deposition of the second layers. It should be noted that the two-layer film is substantially stable under the hydrolysis treatment for the surface boronate group (vide infra), indicating the involvement of the interlayer covalent siloxane bonds. Moreover, it has been already established that such siloxane bond formation readily occurs on hydroxylated surfaces upon treatment with alkyltrichlorosilanes.1,3,8-11 Hydrolysis of the top-layer boronate group 3 in the twolayer film can be performed by the treatment essentially identical with that employed for the monolayer system, that is, three cycles of repeated immersion of the substrate into a 3:7 water/ethanol (v/v) mixture for 1 h at room temperature. The resulting water contact angle and ellipsometric thickness of the hydrolyzed two-layer film showed 33 ( 2° and 2.6 ( 0.3 nm, respectively. Similarly, the procedures employed for the two-layer films can be applied to the fabrication of the third layer on the hydrolyzed two-layer film using 1 or 2 as well as to the subsequent hydrolysis of the boronate surface 3 of the corresponding three-layer film. Table 1 shows that the water contact angles of the resulting three-layer films are reasonable, that is, 69 ( 2°, 33 ( 3°, and 110 ( 2° for

Fabrication of Multilayer Assemblies

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films were estimated by subtracting the thickness of the repeating units from the observed thickness; the values obtained for the two- and three-layer films are commonly 1.6 nm for 3 and 2.7-2.8 nm for 5, values close to the length of each component unit. Thus, it can be concluded that the successive iteration of the deposition-hydrolysis sequence of the layer of 1 effectively allows the fabrication of the multilayer assemblies bound by the interlayer siloxane bonds.

Figure 5. A schematic illustration for the plausible structure of the multilayer assemblies. For clarity, small number of the methylenes out of the actual layer components are indicated.

each film with the surface groups of 3, 4, and 5, and that the XPS elemental ratios of both C and Si in these films are higher than those of the two-layer films. The ellipsometric thickness of the hydrolyzed three-layer film showed 4.1 ( 0.5 nm. Figure 4 shows a linear plot of the film thickness versus the number of layers for the monolayer and multilayer films with the hydrolyzed surface 4, the slope of which gives the average thickness per layer of 1.4 ( 0.1 nm, a value in good accordance with the size of the repeating unit in the multilayer assemblies. Moreover, the net thickness of the top layers with the surface groups of 3 and 5 for the two- and three-layer

Conclusions The methodology developed in the present investigation has been proved to be effective to the fabrication of covalently bound multilayer assemblies utilizing the chemical characters of the cyclic boronate linkage, in particular, the stability during the layer lamination but a high susceptibility to hydrolysis under mild conditions. With regard to the latter, it is of particular significance to note that the surface boronate groups of multilayers can be completely hydrolyzed on the two-dimensional reaction field to give the uniformly hydroxylated surface without significant loss of top-layer molecular units. An attractive possibility in the present method can be found in the hyperbranched structure of the multilayer assemblies (Figure 5), which should give rise to an increase of molecular density with the number of layers favoring the effective coverage of the substrate surface. Moreover, it can be predicted that surface-functionalized multilayers might be readily fabricated by the use of designed silane compounds in the top-layer lamination. LA971215Y