J. MATER. CHEM., 1991, 1(3), 457-460 Formation of Ordered Multilayers by Stepwise Oligomerisation Joseph Y. Jin and Robert A. W. Johnstone* Department of Chemistry, The University of Liverpool, P.O. Box 147, Liverpool L69 3BX, UK By stepwise oligomerisation, multilayer structures have been laid down on molecularly ordered substrates. The stepwise process utilises the reaction of alkenyl isocyanates with surface hydroxy groups. Keywords: Oligomerisation; Multilayer; Langmuir-Blodgett film; Alkenyl isocyanate Highly ordered monolayers of organic materials can be assembled through use of the Langmuir-Blodgett technique.' Subsequently, multilayers can be built up by repetitious deposition of such monolayers.' Although this is an excellent method for preparing molecularly highly ordered films of material, the substances used are constrained to be amphi- philic, and this requirement implies limitations on the types of film that can be assembled.Usually, the amphiphile contains a long hydrocarbon chain which has insulating properties and tends to isolate the individual layers from each other. A further problem with such multilayer structures lies in their relatively fragile nature due to the lack of chemical bonding between the substrate and the first layer, between any two layers and between molecules within the layers themselves. More recently, methods of laying down mono- and multi- layers of 'self-ordering' substances have been described. Alkenyltrichlorosilanes have been used to react with surface hydroxy groups on aluminium,2 glass2 and silicon3 to form a chemically bonded monomeric film showing considerable molecular ordering.The so-formed new upper surface contains vinyl groups which can be readily transformed into terminal hydroxys through treatment with B2H6 and basic H202.4 Also, hydroxy groups have been introduced through reduction of surface methyl ester functions with LiAlH4.3 Subsequently, self-ordered monolayers have been laid down on gold surfaces through reaction with alkanethiols,' although it is not clear whether there is any formal chemical bonding between the gold and the thiol groups. In developments of the work with trichlorosilanes, attempts have been made to assemble multilayers through stepwise iterative reactions.In particular, conversion of vinyl groups into hydroxy groups on the upper surface of a monolayer, was followed by further reaction with alkenyltrichlorosilane to lay down a second layer, etc6 It appears that the second and subsequent layers are not so highly ordered as the first layer, probably because of the need for each silicon atom to be tetrahedrally bonded to three oxygens, residing either on the surface or between silicon atoms. This requirement appears to lead to the formation of defects in the second and sub- sequent layers. Characterisation of the mono- and multi-layers has been in terms of the contact angle with infrared spec- tros~opy,~*~-'~ and X-ray diffraction.8 ellips~metry,~?~ In the present work, new methods for laying down layers on top of an ordered monolayer are being investigated.In many ways, the difficulty of bonding subsequent layers to a monolayer in a stepwise oligomerisation can be compared with the difficulties experienced in solid-phase peptide syn- thesis, where each reaction needs to proceed in as close to a 100% yield as is possible, and byproducts of reaction must be easily removable. '' However, unlike solid-phase peptide synthesis in which the growing peptide chains are kept as far apart as possible, stepwise reactions on an ordered solid surface need to proceed under conditions in which the reacting molecules become packed close together. This last requirement means that the kinetics of film formation will be controlled, especially in the terminal stages, by the degree of difficulty in access of the reactant molecules to the surface that is being covered and the need to remove byproducts from the reacting centres.Indeed, the distribution and separation of reactive sites across the upper layer may control the degree of orderli- ness achieved by oligomerisation. Too close a packing density of sites could make it extremely difficult to obtain complete coverage. Therefore, it would be useful to alter the density of reactive sites on the upper surface in a controlled manner and examine the degrees of order in subsequently formed layers. Work on this aspect of the oligomerisation method is underway. Another area which could affect order in the film lies in the choice of solvent, either through its entry into the layers or through its tendency to promote conformations that may not be consistent with a high degree of order.Thus, stepwise oligomerisation on a highly ordered surface preferably utilises fast, high-yielding reactions having either no or easily removable byproducts. One such reaction reported here involves the use of isocyanates, which react with alcohols, thiols and amines to give urethanes, thioure- thanes and ureas without byproducts RN=C=O+ R'XH-RNHCO-XR' (1) An added advantage of this reaction lies in the formation of a secondary amide bond, similar to that which exists in proteins, nylons and polyurethanes, and which is known to form strong intermolecular hydrogen bonds.I2 Where many such bonds occur, as in protein structures, considerable physi- cal strength is imparted to the structure.For multilayers formed through reaction of an isocyanate with a reactive substance [R'XH, eqn. (l)], this hydrogen bonding should help both to stabilise the structure and to encourage self- ordering within the layers. In the present work, such amide bonds have been formed by iterative (stepwise) reactions of alkenyl isocyanates with surface hydroxy groups. These step- wise reactions (oligomerisation) have given thin multilayers. Experimental 'H NMR spectra were obtained in CDCl,, standard TMS, on a Bruker AC 200 spectrometer, and IR spectra of liquids or solids (as Nujol mulls) were measured on a Perkin-Elmer 1720-X FTIR instrument.Water/Surface Contact-angle Measurements The method for measurement of static advancing contact angle has been well de~cribed.'~ Contact angles were deter- mined on sessile drops of doubly distilled water at room temperature (20-25 "C). The sessile drops were applied by first forming a 10-3cm3 drop at the tip of a syringe needle and allowing it to contact the surface, after which the drop size was increased to ca. 3 x cm3 and the needle with- drawn. Contact angles, measured within 1 min of applying the drop to the surface, were determined by measuring the tangent to the drop at its intersection with the surface, using a telescopic sight on a graduated (degrees) circular scale. All reported values are averages of at least three measurements.Although drop sizes were mostly ca. 3 x cm3, it was determined that contact angles were independent of drop size, within the range (1-20) x cm3. Ellipsometer Measurements An Applied Materials Laser Ellipsometer I1 was used, with an He-Ne laser (A=632.8 nm) as the light source. The incident angle was 70" and the quarter-wave compensator was set to -45". Each set of polariser and analyser angles was the result of at least three measurements taken at different locations on the sample. Film thicknesses were determined through calcu- lations based on previously derived equation^.'^ The refractive index of the base silicon oxide surface was calculated to be 3.902-0.152i. For calculation of film thickness of the organic layers, the refractive index was assumed to be 1.45.15 Deposition of Silane Monolayer This was carried out according to a reported procedure.2 A clean silicon wafer (2cm x 3 cm), for which the surface was believed16 to hold ca.5 x 1014 %-OH groups per cm2, was immersed in a solution of n-undec-10-enyltrichlorosilane (0.04 g, 0.21 mmol) in hexadecane (15 cm3) at 20-25 "C for 15 min. The wafer was removed from the solution, rinsed first in hexadecane at 20°C and then in light petroleum (b.p. 60-80 "C) several times before being dried off in a stream of nitrogen at room temperature. This coated water was used in the next process. Layer thickness was obtained by ellipso- metry, and water contact angle was measured. Conversion of the Upper Surface Vinyl Groups into Terminal Hydroxy Groups This type of reaction has been well doc~mented.'~The following method is closely similar except that the reaction was monitored with time to ensure complete conversion of vinyl to hydroxy.The silicon wafer, coated with an undecenylsiloxane monolayer was immersed in a solution of diborane in tetra- hydrofuran (1 mol dm -3, for 2 min under a dry argon atmos- phere at room temperature to allow tetrahydroborate addition to the terminal vinyl. The wafer 'was then dipped into an alkaline solution of H202 [l :l(v/v) of 30% H202 (m/v) and 0.1 mol dm-3 aq. NaOH] for 2 rnin at room temperature to convert the tetrahydroborate groups into hydroxy groups. The wafer was rinsed several times in ethanol and distilled water and then dried in a stream of nitrogen, ready for the next coupling stage.Layer thickness and water contact angle were measured at different times to determine optimum immersion time in the reaction medium. Oligomerisa tion Steps In the first stage, the silicon wafer, supporting its silanised organic monolayer in a terminal hydroxy form, was immersed in a solution of n-dec-9-enyl isocyanate (0.04 g, 0.22 mmol) in hexadecane (15 cm3) for 40 rnin at 75 "C. After this, the wafer was washed in hexadecane at room temperature for 1 rnin and then rinsed several times in light petroleum (b.p. 60-80 "C) J. MATER. CHEM., 1991, VOL. 1 before being dried finally in a stream of nitrogen. Layer thickness and contact angle were measured.In the second stage, the new terminal vinyl groups were converted into terminal hydroxy groups by the same pro- cedure as described above for the first silanoxy layer, using diborane and alkaline hydrogen peroxide. After the final drying stage with nitrogen, layer thickness and contact angle were measured. The above two steps were repeated to add two further layers to give a total of four organic layers on top of the original silicon oxide surface layer of the silicon wafer. Preparation of n-Dec-9-enyl isocyanate (a) n-Undec-10-enoic acid (1 5.0 g, 0.08 mol) in dry ether (50 cm3) was refluxed with thionyl chloride (1 1.0 g, 0.092 mol) for 2 h. The solvent and excess of thionyl chloride were evaporated to give undec- 10-enoyl chloride which was used without further purification in the next stage. (b) The crude undec-10-enoyl chloride from stage (a) was added dropwise to concentrated aqueous NH, (d =0.88, 60 cm3) with stirring over 20 min. A solid separated immedi- ately and was filtered off to give undec-10-enamide (8.4 g, 56%), m.p.85 "C (from light petroleum, b.p. 60-80 "C; lit.'' m.p. 82 "C); 6, 1.3 (10 H, br s), 1.5 (2 H, m, H2C=CHCH,CH,-), 2.09 (2 H, 4, H2C=CHCHz--), 2.24, (2H, t, H2NCOCH2-), 4.95, 5.03 (2H, m, H2C=CH-), 5.55 (2 H, br d, NH2), 5.82 (1 H, m, H2C-CH-). (c) n-Undec-10-enamide (2.6 g) in dichloroethane (75 cm3) was reacted with oxalyl chloride (2.6 cm3) in dichloroethane (15 cm3) with stirring at 4 "C (i~e-water).'~The cooling bath was removed and stirring was continued for 1 h, after which the mixture was refluxed for 5 h.The solvent was removed to give a liquid, n-dec-9-enyl isocyanate (1.5 g, 58%), b.p. 68 "C at 0.1 mmHg.? Because of its reactivity to water, this isocyan- ate was characterised through its large IR band at 2243 cm-', which disappeared in moist air, being replaced by new bands at 3356 and 3192cm-' (-NH2); its conversion into the corresponding methyl carbamate was effected by reaction with methanol. Thus, the isocyanate (0.5 g) was dissolved in methanol (5 cm3) at room temperature with stirring for 10 min and the methanol was evaporated to give N-n-dec-9-enyl methyl carbamate, m.p. 101 "C (from MeOH); aH 1.30 (10 H, br s), 1.66 (2 H, m, H2C=CHCH2CH2-), 2.05 (9, H2C=CHCH2-), 2.75 (2 H, t, --CH,NH-), 3.79 (3 H, S, CH30-), 4.95, 5.05 (2 H, m, H2C=CH-), 5.80 (1 H, m, H2C=CH-), 7.85 (1 H, br s,-NH-).(Found: C, 67.3; H, 9.9; N, 5.7%. C13H23N02 requires: C, 67.6; H, 10.8; N, 6.6%). Preparation of n-Undec-lO-enyltrichlorosilane (a) Undec-10-enoic acid (58 g, 0.32 mol) was refluxed with LiA1H4 (12 g, 0.32 mol) in diethyl ether (400 cm3) for 18 h, after which excess of ethyl acetate was added cautiously to decompose any remaining hydride reagent. The mixture was washed with dil. H2S04 and water and dried (MgS04). Evaporation of the solvent and distillation of the residual oil gave n-undec-10-enol as a colourless liquid (40.2 g, 92%), b.p. 9 1-93 "C at 0.5 mmHg (lit.20 132- 133 "C at 15 mmHg).(b) To a solution of n-undec-10-enol (8.6g) in CC14 (100 cm3) was added dropwise tri-n-butylphosphine (30 cm3) over a period of 20min at room temperature. The solution became warm and was left to stir for a further 2 h. Evaporation of the solvent gave a residual oil which was distilled to give n-undec-10-enyl chloride as a colourless oil (58% yield), b.p. 71.5 "C at 0.3 mmHg (lit.2' 105-107 "C at 6 mmHg). t 1 mmHgx133.322 Pa. J. MATER. CHEM., 1991, VOL. 1 (c) A solution of n-undec-10-enyl chloride (3.14 g) in diethyl ether (20 cm3) was added to magnesium turnings (0.95 g) in diethyl ether (1 5 cm3) to which had been added a small crystal of 12.The mixture was stirred and heated under reflux for 18 h to give a solution of n-undec- 1 0-enylmagnesium chloride which was transferred to a dropping funnel, under N2; this Grignard reagent was added dropwise to a solution of SiC1, (6 g) in diethyl ether (30 cm3) with stirring and then the whole was heated under reflux for 18 h. The resulting mixture was filtered and the solvent was evaporated to yield an oil which was distilled to give n-undec- 10-enyltrichlorosilane as a colourless oil (1.25 g, 26%), b.p.33 "C at 0.05 mmHg (lit.4 110 "C at 20mmHg); 8H 1.2 (14H, br), 1.48 (2 H, m, H2C=CHCH,CH,--), 1.95 (2 H, 9, H2C=CH-CH2), 4.84, 5.94 (2 H, m, H2C=CH-), 5.71 (H, m, H2C=CH-). Results and Discussion The experiments were carried out on pure silicon wafers, which had been left in contact with air for a sufficient period that an oxidised surface layer was produced, containing multiple hydroxy groups.I6 A monolayer was deposited on this silicon 'hydroxide' surface by immersing the silicon wafer in a solution of undecenyltrichlorosilane in hexadecane.6 Measurements of water contact angle and ellipsometry con- firmed that a monolayer having a hydrophobic surface had been deposited.By immersion of such silicon wafers in unde- cenyltrichlorosilane solution for periods of 2-15 min, it was determined that contact angle and film thickness reached limiting values of ca. 105' and 15 81, respectively, after 10 min (Table 1). The surface vinyl groups were converted into ter- minal hydroxy groups using B2H6 and alkaline H202. During this hydroxylation, measurements over a period of 5 min indicated that the water contact angle reached a limiting value of ca.46" after <2 min. This value is in accord with those found by other workers* and indicates the presence of hydroxy groups on the upper surface of the monolayer. The monolayer film on its silicon substrate was then immersed in a solution of n-dec-9-enyl isocyanate in hexa- decane at 75 "C for 40 min. Again, contact angle measurements indicated that a minimum time of 30 min was needed to reach a limiting value of ca. 82 "C, indicating a change in surface character from hydrophilic to hydrophobic as the hydroxy groups reacted with the isocyanate. Ellipsometry showed a new total film thickness of ca. 35 81, which indicates an added thickness of 1981 on top of the original monolayer.From molecular models, an extended (uncoiled) n-dec-9-enyl carba- mate chain would be expected to have a length of ca. 1681. Thus, allowing for errors in measurement, the added film thickness corresponds to a layer consisting of chains of n-dec- 9-enyl carbamate groups extending vertically from the original monolayer surface. These results are consistent with the formation of a close-packed second layer having terminal vinyl groups on its upper surface. As described above for the first layer, the vinyl groups of this second layer were converted into terminal hydroxy groups using B2H6 and alkaline H202. Table 1 Limiting values of water contact angle with time for reaction of n-dec-9-enyl isocyanate with surface hydroxy groups reaction time"/min contact angleb/" film thickness'/A 5 77 20.2 15 77 23.1 30 21.3 "Time of immersion of film in a solution of the isocyanate in hexadecane at 75 "C.bMean static advancing water contact angles. 'Estimated error 12 A. Table 2 Water contact angles and film thickness of multilayers ellipsometry measurements layernumber" contact angleb/" P/" Adlo total film thickness'/A 0 c25 42 10.7 - 1 105 39.8 10.8 15.7 1' 46 39.7 10.7 16.1 2 82 37.1 11.0 35.5 2' 53 37.0 11.2 37.0 3 78 35.2 11.1 49.6 3' 53 34.8 11.2 52.9 4 77 31.2 11.3 79.4 "The numerals indicate the number of each layer counted from the silicon oxide base with 0 =silicon dioxide surface alone.Thus, 1=first monolayer on the surface having vinyl groups and I' is the same layer except that the surface vinyl groups have been converted into terminal hydroxys. Layers 2,2' and so on (n, n') correspond to success- ive reactions with isocyanate, followed by conversion of vinyl to hydroxy, as described in the discussion section. Mean static advanc- ing contact angle, measured as described in the experimental section. 'P =polariser azimuth angle. A =analyser azimuth angle. See exper- imental section. Error in measurement of film thickness is estimated to be ca. +2 A, for each layer. For the purposes of this table, the silicon oxide surface is given a zero thickness, although it was actually 20A thick. Refractive index for the organic layers was taken to be 1.45.14 The water contact angle now changed to ca.52', consistent with the presence of hydroxy groups on the surface. These surface hydroxy groups were reacted with n-dec-9-enyl isocy- anate by repetition of the procedure described above. The water contact angle changed to 78" and the film thickness increased again by ca. 13 81. These stepwise reactions with the alkenyl isocyanate gave a thin film of four layers, the first being bonded to silicon through Si-0-Si bonds and the remainder by interlayer urethane (-NHC02-) linkages. Table 2 shows that the first layer gives a water contact angle (105") close to those reported for a highly ordered film having an upper hydrocarbon-like surface.2,6 As each of the layers, 2, 3,4, is prepared, the upper surface, which should also be hydrocarbon-like (terminal vinyl groups), gave water contact angles of 78-80', considerably less than that of the first layer.This result could be interpreted as being due to a film containing many defects, as demon- strated in earlier work with mixed thiols on a gold ~ubstrate.~.~ However, there is a consistency about the contact angles for layers 2, 3 and 4, which indicates that, if the result is due to defects, then the same degree of defect structure must be present in each layer except the first. Although this cannot be ruled out, it is also possible that the new urethane linkages, having strong dipoles and capable of very strong hydrogen bonding, may be able to make their presence felt at the surface.It has been demonstrated previously7 that any dipolar or other effect of ether linkages buried below a hydrocarbon surface become negligible beyond a depth of 5-10 81. The much stronger dipole of the urethane bond (amides have dipoles of ca. 3.7-4.1 D compared with ethers at ca. 1.1 D) may increase this depth effect markedly, and their hydrogen- bonding nature may lead to some water being entrained within the film. Further examination of this discrepancy, compared with earlier published data, is being carried out. Examination of the hysteresis of the contact angle should enable differentiation between a dipole effect and possible defects in the film.? An attempt was made to attach the alkenyl isocyanate ~~ ~ ~ t Constant hysteresis between the first and following layers could rule out an interpretation in terms of defects.460 directly to the silicon oxide surface, instead of onto a pre- viously deposited organic monolayer. Despite prolonged immersion of a silicon wafer (prepared as for deposition of alkenyltrichlorosilane, i.e. with surface hydroxy groups) in a solution of n-dec-9-enyl isocyanate, no organic film could be detected on the silicon by contact angle or ellipsometry after washing it with solvent and drying. It seems that, if the isocyanate had reacted with surface hydroxy groups, as seems likely, then the resulting carbamic acid derivatives probably decarboxylated readily to return the surface to a hydroxide state which desorbed the resulting amine; decarboxylation of free carbamic acids is well known.22 0 I I.I1-Si-OH + O=C=NR -+ -Si-O-CNHR I I I I -Si-OCONHR + H20 -+ -Si-OH + C02+ H2NR (2)I I Conclusion Thin multilayers can be assembled on molecularly ordered organic monolayers deposited on a silicon oxide surface by stepwise reactions (oligomerisation) using isocyanates to form urethane linkages. An advantage of this reaction is the absence of any byproducts, its high yield and ease of operation. Additionally, there is strong hydrogen bonding between adjacent urethane linkages, giving intralayer stabilisation. Differences between water contact angles for hydrocarbon surfaces and those reproted here may be due to this hydrogen- bonding character and/or the strong dipolar effects of urethane bonds.The authors thank SERC for a grant (Y.J.) and Dr. R. Greef for his kind help with calculating film thickness from our ellipsometer readings. J. MATER. CHEM., 1991, VOL. 1 References 1 See Memorial Edition, Thin Solid Films, 1980, 68, 1-133. 2 R. Maoz and J. Sagiv, J. Colloid Interface Sci., 1984, 100, 465; J. Gun, R. Iscovici and J. Sagiv, J. Colloid Interface Sci., 1984, 101, 201; J. Gun and J. Sagiv, J. Colloid Interface Sci., 1986, 112, 457. 3 S. R. Wasserman, Y. Tao and G. M. Whitesides, Langmuir, 1989, 5, 1074; N. Tillman, A. Ulman and T. L. Penner, Langmuir, 1989, 5, 101. 4 L. Netzer, R. Iscovici and J. Sagiv, Thin Solid Films, 1983, 99, 235.5 C. D. Bain, J. Eva11 and G. M. Whitesides, J. Am. Chem. SOC., 1989, 111, 7155. 6 L. Netzer, R. Iscovici and J. 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Vorst, Science, 1987, 237, 630; T. Zhuravlev, Langmuir, 1987, 3, 316. 17 H. C. Brown and B. C. Subba Rao, J. Am. Chem. SOC.,1959,81, 6428. 18 F. Becke and J. Gnad, Annalen, 1968, 713, 212. 19 For a comparable method see, Organic Syntheses, ed. E. C. Horning, Wiley, New York, 1955, vol. 3, p. 846. 20 R. Toubiana and J. Asselineau, Ann. Chem., 1962, 7, 593. 21 W. J. Gender and E. M. Behrmann and G. R. Thomas, J. Am. Chem. SOC., 1951,73, 1071. Paper 1/00237F; Received 17th January, 1991