Langmuir 1998, 14, 3921-3925
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Structural Study of Monolayers of Alkyl Side Chain Substituted Poly(ferrocenylsilane) Hyun Yim and Mark D. Foster* Maurice Morton Institute of Polymer Science, The University of Akron, Akron, Ohio 44325-3909
Dave Balaishis and Ian Manners Department of Chemistry, University of Toronto, 80 St. George Street, Toronto M5S 3H6, Ontario, Canada Received November 20, 1997. In Final Form: May 1, 1998 Monolayers of alkyl side chain substituted poly(ferrocenylsilane) (FCSIOCT) have been studied for the first time. The monolayer behavior is found to be strongly temperature dependent and also to change distinctly when the monolayer is oxidized by addition of FeCl3 to the polymer before spreading. The zero pressure area per repeat unit for the unoxidized polymers is ∼20 Å2 at 20 °C, which suggests that the ferrocene moiety must be excluded from the water surface and the silane repeat units with the aliphatic side chains alone occupy the surface area on the air-water interface. The FCSIOCT monolayer mixed with FeCl3 shows the liquid analogous state occurring at much larger areas, with a plateau region appearing in the isotherm at 44 mN/m. The zero pressure area in the liquid analogous state is ∼45 Å2, which is almost equal to the theoretical area required for both the silane and ferrocene moieties to be at the surface. For both monolayers, the area per repeat unit continuously decreases with time at constant pressure. Monolayers transferred onto substrates have been studied using X-ray reflectometry. The thickness of the unoxidized FCSIOCT monolayer is higher than that of the oxidized monolayer. When the oxidized monolayer relaxes on the water surface, the thickness increases and approaches the thickness of the pure FCSIOCT monolayer.
Polymers containing transition metals in the main chain are of considerable interest due to their potentially attractive electrical, electrochemical, and optical properties.1-5 Recently, the preparation of high-molecular-weight poly(ferrocenylsilane) for which the substituents on the silicon atom were methyl and octadecyl was reported by Manners and co-workers.6-8 The polymer was prepared with a monomodal molecular weight distribution and shows interesting physical and chemical properties. The Langmuir-Blodgett (LB) technique is an attractive means to fabricate a supramolecular assembly with a thickness controlled at the molecular level and welldefined molecular orientation.9 LB films of low-molecularweight amphiphiles containing the ferrocene moiety have been studied to elucidate the interfacial electron-transfer events between the ferrocene moiety and electrode surface.10-14 Also, LB films of polymers with ferrocene (1) Davies, S. J.; Johnson, B. F. G.; Khan, M. S.; Lewis, J. J. Chem. Soc., Chem. Commun. 1991, 187. (2) Fyfe, H. B.; Mlekuz, M.; Zargarian, D.; Taylor, N. J.; Marder, T. B. J. Chem. Soc., Chem. Commun. 1991, 188. (3) Brandt, P. F.; Rauchfuss, T. B. J. Am. Chem. Soc. 1992, 114, 1926. (4) Tenhaeff, S. C.; Tyler, D. R. J. Chem. Soc., Chem. Commun. 1989, 1459. (5) Manners, I. Angew. Chem., Int. Ed. Engl. 1996, 35, 1602. (6) Foucher, D. A.; Ziembinski, R.; Tang, B.-Z.; MacDonald, P. M.; Massey, J.; Jaeger, C. R.; Vancso, G. J.; Manners, I. Macromolecules 1993, 26, 2878. (7) Foucher, D. A.; Ziembinski, R.; Peterson, R.; Pudelski, J.; Edwards, M.; Ni, Y.; Massey, J.; Jaeger, C. R.; Vancso, G. J.; Manners, I. Macromolecules 1994, 27, 3992. (8) Pudelski, J. K.; Rulkens, R.; Foucher, D. A.; Lough, A. J.; MacDonald, P. M.; Manners, I. Macromolecules 1995, 28, 7301. (9) Ulman, A. An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (10) Facci, J. S.; Falcigno, P. A.; Gold, J. M. Langmuir 1986, 2, 732. (11) Widrig, C. A.; Miller, C. J.; Majda, M. J. Am. Chem. Soc. 1988, 110, 2009.
derivatives in the side chain have been investigated by Aoki and Miyashita.15 In their study, it was seen that a copolymer of N-dodecylacrylamide (DDA) with ferrocenylmethyl acrylate (FcA) forms a stable condensed monolayer, while the copolymer of DDA with vinylferrocene (vFc) forms an unstable monolayer. They explained the difference in the monolayer properties for vFc and FcA on the water surface as resulting from two factors. One is the balance between the hydrophobic and hydrophilic interactions of the DDA copolymer formed, and the other is freedom in rotation of the ferrocene moiety around the polymer chain. In the present study, the structure of Langmuir monolayers and LB films from a polymer containing the ferrocene moiety in the main chain has been investigated for the first time. The behavior of the Langmuir monolayer under conditions in which the polymers are in one oxidation state and then under other conditions for which the oxidation state is changed has been studied using the surface pressure-area isotherm. LB monolayers transferred onto the solid substrate at different oxidation states have also been studied using X-ray reflectometry. Experimental Section Materials. The chemical structure of poly(ferrocenylmethyloctadecylsilane) (FCSIOCT) is shown in Figure 1. This polymer was prepared via thermal ring-opening polymerization of a silicon-bridged ferrocenophane pre(12) Charych, D, H,; Landau, E. M.; Majda, M. J. Am. Chem. Soc. 1991, 113, 3340. (13) Charych, D. H.; Majda, M. Thin Solid Films 1992, 210/211, 348. (14) Nakahara, H.; Katoh, T.; Sato, M.; Fukuda, K. Thin Solid Films 1988, 160, 153. (15) Aoki, A.; Miyashita, T. Macromolecules 1996, 29, 4662.
S0743-7463(97)01274-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/18/1998
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Figure 1. Chemical structure of the poly(ferrocenylmethyloctadecylsilane) (FCSIOCT) molecule.
cursor as described elsewhere.7 The polymer possessed a monomodal molecular weight distribution with Mw ) 2.1 × 106 and Mn ) 1.1 × 106 as indicated by gel permeation chromatography in tetrahydrofuran relative to polystyrene standards. Sample Preparation. Monolayers were prepared by spreading a dilute solution (below 0.1 mg/mL) in chloroform onto a Millipore-quality water subphase in a Nima 611 trough (300 mm × 200 mm). The monolayers were compressed at a rate of 40 cm2/min. When the concentration of solution was above 0.1 mg/mL, oily patches were observed on the water surface with the unaided eye. The size of these oily patches decreased as the solution concentration was decreased and could no longer be observed when the concentration was below 0.1 mg/mL. In a second kind of experiment, the polymer was mixed with an oxidizing agent being before spread. When the ferrocenylsilane polymer was mixed with FeCl3, the color changed from pale yellow to green. When the solution mixed with FeCl3 was spread onto the water surface, no oily patches were observed, even when the spreading concentration was above 0.1 mg/mL. Monolayers were deposited on 2-in.-diameter round silicon substrates (Semiconductor Processing Inc.). The substrates were cleaned by being treated first in chloroform for 10 min, then in a hot 7:3 solution of H2SO4-H2O2 for 1 h, and then rinsing in Millipore water. Warning: Piranha solution should be handled with extreme caution; in some circumstances (most probably when it has been mixed with significant quantities of an oxidizable organic material), it has detonated unexpectedly. The monolayers were transferred onto the substrates at a surface pressure of 25 mN/m with a dipping speed of 10 mm/min. X-ray Reflectometry. X-ray reflectometry measurements were performed on the reflectometer at the University of Akron using Cu KR radiation (λ ) 0.154 nm) selected with a pyrolytic graphite monochromator. The resolution of the wavelength (δλ/λ) was 0.015, and the angular divergence was ∼0.0002 rad. All measurements, typically lasting several hours, were performed in air at room temperature and all data sets were corrected for background. Results and Discussion Figure 2 shows the surface pressure-area isotherms of FCSIOCT with different temperatures. At 20 °C, the isotherm shows a liquid analogous behavior from about 0 to 20 mN/m surface pressure. A solid analogous behavior appears at higher surface pressure with a collapse pressure at ∼50 mN/m. The zero pressure area per repeat unit in this state is ∼20 Å2, in good agreement with what one would expect for closely packed aliphatic side chains alone.9 This area is not large enough to account for the presence of ferrocenylsilane moieties at the interface. Increasing temperature by a few degrees dramatically changes the shape of isotherm. A similar strong tem-
Figure 2. Pressure-area curves for a FCSIOCT monolayer at different temperatures.
Figure 3. Pressure-area curves for a FCSIOCT monolayer mixed with different amounts of FeCl3. The ratios of FeCl3 to FCSIOCT are shown in the inset. The area is shown normalized by the number of FCSIOCT repeat units.
perature dependence has also been observed for isotherms of octadecyl methacrylate monolayers.16 Upon increasing temperature, the monolayer becomes much more liquidlike and shows the appearance of a liquid analogous to solid analogous phase transition. The surface pressure-area isotherms of FCSIOCT mixed with different amounts of FeCl3 are shown in Figure 3. The FeCl3 oxidizes the polymer, which results in very strong change in the isotherm. However, the FeCl3 not only oxidizes the polymer, it also most likely provides FeCl4- ions in the subphase which associate with the ferrocene moieties. The area at which the liquid state occurs becomes larger with increasing amounts of FeCl3 until the ratio of repeat units of FCSIOCT to molecules of FeCl3 is 1:3 and then smaller with further addition of FeCl3. This indicates that there is a critical amount of FeCl3 needed for each repeat unit of FCSIOCT molecule to completely associate with it. It is known that FeCl3 will oxidize conjugated polymers with a stoichiometry of 2FeCl3 per cation produced.17 Nguyen et al.18 showed from UV-visible experiments that poly(ferrocenylsilanes) can be chemically fully oxidized by FeCl3 with a stoichiometry of 2 equiv of FeCl3/ferrocene and suggested the equation (16) Mu¨ller, G.; Riedel, C. Langmuir 1996, 12, 2556. (17) Fichou, D.; Horowitz, G.; Garnier, F. Synth. Met. 1990, 39, 125. (18) Nguyen, M. T.; Diaz, A. F.; Dement′ev, V. V.; Pannell, K. H. Chem. Mater. 1993, 5, 1389.
Alkyl-Substituted Poly(ferrocenylsilane) Monolayers
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for the stepwise oxidation of poly(ferrocenylsilanes) as
(FCSiR2FCSiR2) + 2FeCl3 f (FC+SiR2FCSiR2) + FeCl4- + FeCl2 (FC+SiR2FCSiR2) + 2FeCl3 f (FC+SiR2FC+SiR2) + FeCl4- + FeCl2 where FC and FC+ are ferrocene and ferrocenium centers. However, it was postulated3,19 that, due to the interactions between the iron atoms, initial oxidation for poly(ferrocenylsilanes) occurs at alternating sites and that subsequent oxidation of the remaining iron centers is energetically less favorable and occurs at a higher potential. Recently, new results supporting the idea that initially every other ferrocene unit is oxidized have been published.20 These come from electrochemical and X-ray diffraction studies of oligo(ferrocenyldimethylsilanes). In the rest of the experiments discussed in the present work, monolayers containing FeCl3 at a ratio of 1:3 are considered. The zero pressure area per repeat unit of the pure FCSIOCT molecule is ∼20 Å2. Thus, it is thought that the ferrocene moiety in the repeat unit occupies no area at the air-water interface because the expected zero pressure area per repeat unit of the FCSIOCT molecule is calculated to be ∼44 Å2. The ferrocene moiety must be excluded from the water surface, and the silane repeat units with the long aliphatic side chains alone occupy the surface area on the air-water interface. This is analogous to the behavior observed by Aoki and Miyashita for polymers with ferrocene derivative side chains.15 In their case, the ferrocene moieties were also excluded from the interface. However, the association of FeCl4- with the ferrocene moiety may increase the ionic nature of the monolayer, bringing the ferrocene moiety to the interface and causing the liquid analogous state to occur at large areas. This result is similar to that obtained for smallmolecule amphiphiles. It has been known that the addition of divalent ions to the water subphase, and the apparent formation of the acid salt, results in a decrease in ionic character in that case, whereas addition of salt with an univalent cation leads to the opposite effect.9 The ionic character of the ferrocene moiety may be enhanced by oxidation to the Fe3+ state and association with the FeCl4- ion. In our case, the zero pressure area in the liquid analogous state is around ∼45 Å2, which is almost equal to the theoretical area required for both the silane and ferrocene moieties to be at the surface. Further compression of the monolayer disrupts the association and forces the ferrocene moiety to be excluded from the interface. So it could be that the plateau region that appears at 44 mN/m surface pressure arises from the partial collapse of the layer due to rearrangement of the ferrocene moiety. Figure 4 shows the results of experiments performed to investigate the stability of the two types of monolayers at 25 mN/m. One curve is for pure FCSIOCT and the other for FCSIOCT mixed with FeCl3 at a ratio of 1:3. For both monolayers, the area per repeat unit decreases continuously with time and approaches 6-7 Å2 at 35 min, which indicates that the monolayer is collapsed and hence forms a multilayer. In this sense, the monolayers are not stable
at constant pressure. The Langmuir film’s change in time can be explained by considering two factors. One is amphiphilicity as suggested by Aoki and Miyashita.15 The polymer chain as a whole may need a certain degree of modest hydrophilicity before the layer at the water surface is stable at constant pressure. Whether or not so-called “hairy rod” polymers, which are more rigid, need some hydrophilic character in their backbones in order to form good monolayers has been debated. However, it appears the flexible FCSIOCT molecule does not have enough hydrophilic character in the main chain to be stabilized on the water surface. Abbott et al.21,22 insisted that the ferrocene moiety is more polar than an aliphatic chain but less polar than an ionic group and that the ionic character of the ferrocene moiety grows when going to the Fe3+ state by oxidation. The other factor determining the layer’s stability is the stiffness of the molecule. The FCSIOCT molecule is fairly flexible due to the free rotation of the cyclopentadienyl ligands in the ferrocene moiety. In fact, the iron atom in ferrocene has been likened to a “ball bearing”. So this flexibility of the ferrocenylsilane backbones contributes to the instability of the monolayer. Figure 5a presents semilogarithmic plots of X-ray reflectivity as a function of q ()4π(sinθ)/λ) for two monolayers. One monolayer does not contain an oxidizing agent and the other contains FeCl3 at a ratio of 1:3. The model curves shown in Figure 5a correspond to the scattering length density profiles shown in Figure 5b and the model parameters summarized in Table 1. To fit the experimental data, the monolayer was divided into two regions. One is the backbone region and the other the aliphatic side chain region. This separation into two regions is illustrated in Figure 6, which shows a schematic structure of the FCSIOCT molecule. In the illustration, the long aliphatic side chains are shown extending away from the backbone in fully extended conformations. When the FCSIOCT is placed on the surface of the water, one would expect the alkyl chains to move away from the water surface through rotation of the Si linkage or change in the conformation of the alkyl chain. The overall thickness of the FCSIOCT monolayer is higher than that of the FCSIOCT monolayer mixed with FeCl3 by ∼2 Å, which is consistent with the result that the
(19) Foucher, D. A.; Tang, B. Z.; Manners, I. J. Am. Chem. Soc. 1992, 114, 6246. (20) Rulkens, R.; Lough, A. J.; Lovelace, S. R.; Grant, C.; Geiger, W. E. J. Am. Chem. Soc. 1996, 118, 12683.
(21) Gallardo, B. S.; Hwa, M. J.; Abbott, N. L. Langmuir 1995, 11, 4209. (22) Gallardo, B. S.; Metcalfe, K. L.; Abbott, N. L. Langmuir 1996, 12, 4116.
Figure 4. Plot of change in area per repeat unit with the time for FCSIOCT monolayer (s) and FCSIOCT monolayer mixed with FeCl3 at a ratio of 1:3 (- - -).
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Figure 5. (a) XR curve for FCSIOCT monolayer (s) and FCSIOCT monolayer mixed with FeCl3 at a ratio of 1:3 (- - -) with model fits and (b) scattering length density profiles. Table 1. Fitting Parameters for FCSIOCT Monolayer and FCSIOCT Monolayer Mixed with FeCl3 at a Ratio of 1:3 scattering length density, b/V (×10-4/Å2) [(5 × 10-4/ Å2] FCSIOCT FCSIOCT / FeCl3 a
layer thickness, d (Å) [(0.5 Å]
rms interface roughness, σ (Å) [(0.5 Å]
side chaina
backboneb
side chain
backbone
air/side chain
side chain/backbone
0.0835 0.0985
0.1279 0.1429
13.1 13.8
8.2 5.8
3.2 3.2
2.5 2.5
Side chain, indicates the aliphatic side chain region. b Backbone, indicates the backbone region.
Figure 7. Schematic drawings of the FCSIOCT monolayers on the water surface: (a) oxidized monolayer; (b) pure FCSIOCT monolayer. The backbone is represented by an ellipsoidal cross section. In (a) all ferrocene moieties in each the backbone are in contact with the subphase, while in (b) the backbones are partially collapsed due to the rearrangement of the ferrocene moieties.
Figure 6. Schematic representation of the three-dimensional structure of FCSIOCT molecule. The molecule is divided into two regions, backbone and aliphatic side chain region.
area taken up by one repeat unit is increased dramatically with oxidation. One may consider first the backbone region. Of importance here is the observation that the thickness of the backbone region of the FCSIOCT monolayer is 2.4 Å larger than that of the oxidized monolayer, while the scattering length density of the backbone region is lower than that for the oxidized monolayer. The higher scattering density of the backbone region of the oxidized monolayer indicates that it has FeCl4- anions associated with the ferrocene moieties. The thickness of the backbone region of the oxidized monolayer, which is used to fit the XR, data is 5.8 Å. This value is reasonable considering that the d spacing of dihydro-substituted poly(ferrocenylsilane) in bulk is known to be 5.87 Å from X-ray diffraction study.8 The thickness increase of the backbone region of the pure FCSIOCT monolayer may be ascribed to the location of some of the ferrocene moieties away
from the substrate surface as shown in the cartoon of Figure 7. This sketch is not meant to suggest the presence of multilayers on the surface of the water, but rather the location of some ferrocene moieties in one chain at the surface, while other moieties in the same chain are not at the surface. This idea is consistent with the result that the zero pressure area of the pure FCSIOCT monolayer is much smaller than that required for both the silane and ferrocene moieties to be at the interface. Turning attention to the side-chain region, one notes that the electron scattering density of the aliphatic side-chain region of the pure FCSIOCT is lower than that of the oxidized layer although the thickness is almost same. This result seems at first to be contrary to that obtained from the surface pressure-area isotherms which indicates that the alkyl side chains are much more closely packed and uniformly oriented for the pure FCSIOCT monolayer. This discrepancy may be explained, however, by the possibility that FeCl4- ions are also included in the aliphatic sidechain region in the oxidized layer. Variable incident angle evanescent wave induced X-ray fluorescence measurements are being pursued to clarify where in the transferred monolayer the ions reside. To investigate what happens to the structure as the monolayer was relaxing on the water surface, monolayers were transferred at different times after compression and
Alkyl-Substituted Poly(ferrocenylsilane) Monolayers
Figure 8. Comparison of the XR data for transferred FCSIOCT monolayers which were allowed to relax on the water surface for different times before transfer.
studied with XR. It is important to note that for smallchain amphiphiles it has been demonstrated that the transfer process can sometimes alter the layer structure.23 So what one sees on the solid substrate is not necessarily directly related to what is on the water subphase. However, it is thought that helpful information about the monolayer structure may be obtained nonetheless. The first monolayer was compressed to a target pressure of 25 mN/m and transferred immediately. The second and third monolayers were compressed to the same target pressure, allowed to relax for 5 and 15 min, respectively, and then transferred. Figures 8 and 9 show the X-ray data for the pure FCSIOCT monolayer and the FCSIOCT monolayer mixed with FeCl3, respectively. The reflectivity curves of the two samples of pure FCSIOCT monolayers are very similar. The appearance of the data suggests that the layer transferred after 5 min of relaxation has almost the same thickness but is slightly more dense. Interesting results are seen in Figure 9. The monolayer of FCSIOCT mixed with FeCl3 changes more distinctly than does the monolayer of pure FCSIOCT, and the reflectivity curve obtained after 15 min of relaxation is similar to that obtained for the pure FCSIOCT monolayer. This is a reasonable result considering that in the stabilization (23) Riegler, H.; Spratte, K. Thin Solid Films 1992, 210/211, 9.
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Figure 9. Comparison of the XR data for monolayers of FCSIOCT mixed with FeCl3 at a ratio of 1:3 which were allowed to relax on the water surface for different times before transfer.
experiment the area per repeat unit of the oxidized FCSIOCT monolayer decreased to 17 Å2 after 15-min relaxation. Conclusions Monolayers of alkyl side chain substituted poly(ferrocenylsilane) have been studied for the first time. The pressure-area isotherm is strongly dependent on temperature. The zero pressure area per repeat unit obtained at 20 °C is ∼20 Å2, which is lower than that expected for both the silane and ferrocene moieties to be at the airwater interface. An increase in area per repeat unit with the addition of FeCl3 indicates that the ferrocene moiety is brought to the interface by the association of the ferrocene moiety with the FeCl4- anion. For both monolayers, the area per repeat unit continuously decreases with time at constant pressure. The monolayer structure and its change during relaxation on the water surface have been investigated indirectly by X-ray reflectometry using transferred monolayers. The thickness of the poly(ferrocenylsilane) monolayer is higher than that of the oxidized monolayer. When the oxidized monolayer relaxes on the water surface, its structure becomes similar to that of the pure poly(ferrocenylsilane) monolayer. LA971274B
