Cartesian-Structure Analysis in Cast Films by Advanced Infrared

Multiple-angle incidence resolution spectroscopy (MAIRS) has been improved to be an advanced algorithm so that the Cartesian structure in organic thin...
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Anal. Chem. 2004, 76, 3084-3090

Cartesian-Structure Analysis in Cast Films by Advanced Infrared Multiple-Angle Incidence Resolution Spectroscopy Takeshi Hasegawa,*,† Hiroyuki Kakuda,† and Norihiro Yamada‡

Department of Applied Molecular Chemistry, College of Industrial Technology, Nihon University, 1-2-1 Izumi-cho, Narashino, Chiba 275-8575, Japan, and Faculty of Education, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

Multiple-angle incidence resolution spectroscopy (MAIRS) has been improved to be an advanced algorithm so that the Cartesian structure in organic thin films can be analyzed. The infrared MAIRS technique was originally proposed as a totally new spectroscopic technique to reveal structural anisotropy in thin films on an infraredtransparent substrate, which yields both in-plane- (IP; X and Y) and out-of-plane (OP; Z)-mode spectra from an identical sample. Since this technique employs an analytical concept based on a signal decomposition of light intensity (not absorbance spectra), the algorithm intrinsically has high potential for further development. In the present study, the theoretically deduced matrix that correlates the light intensity to the angle of incidence has been modified to further decompose the IP-mode spectrum into X and Y components. As a result, anisotropic measurements of infrared spectra of thin film have become possible for the X, Y, and Z directions (Cartesian coordinate) simultaneously. With this advanced algorithm, the Cartesian structural changes in a cast film prepared on a germanium substrate have readily been analyzed, and a change from the biaxial to the uniaxial film structure with aging has spectroscopically been revealed. Multiple-angle incidence resolution spectroscopy (MAIRS) was proposed in previous papers1,2 as a conceptually new analytical technique to study structurally anisotropic thin materials by use of infrared absorption spectroscopy. MAIRS has some major unique benefits in that (1) the pure out-of-plane (OP)-mode absorption spectrum, which was conventionally observed on a limited basis on metallic surfaces, is now available on a nonmetallic surface; (2) the in-plane (IP)-mode spectrum and the OP-mode spectrum are simultaneously observed from an identical sample; (3) no polarizer is needed, and the spectra are free from the matter of polarization impurity; and (4) quantitative molecular orientation analysis is possible with no optical parameter. * To whom correspondence should be addressed. Fax: +81 47 474 2579. E-mail: [email protected]. † Nihon University. ‡ Chiba University. (1) Hasegawa, T. J. Phys. Chem. B 2002, 106, 4112. (2) Hasegawa, T.; Matsumoto, L.; Kitamura, S.; Amino, S.; Katada, S.; Nishijo, J. Anal. Chem. 2002, 74, 6049.

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The first benefit is of great importance to characterize nonmetallic surfaces, such as semiconductor devices. When a transition moment on a surface species is aligned perpendicular to the surface, the transmission spectroscopy (IP-mode active) does not vividly respond to a fine orientation change of the moment, and the OP-mode spectrum is required instead. With the conventional techniques, however, it has been impossible to observe the OP-mode spectrum on “nonmetallic” surfaces. For example, the issue of surface cleaning of germanium (Ge) and silicon (Si) substrates for LB film deposition has not been carefully discussed before, since transmission spectra of thin films prepared on a washed substrate by sonication in organic solvents are close to that measured on a fresh substrate.3 Nevertheless, infrared MAIRS clearly revealed that the OP-mode spectrum of the film on the recycled substrate was significantly different from that on a fresh substrate, although the IP-mode spectra are similar to each other. In this manner, this benefit sheds a new light on an unrecognized part of material characterization. The second benefit provides a new analytical point of view for analyses of surface adsorbates and thin films.4 It is well-known that the combination technique of the transmission and reflectionabsorption (RA) spectrometries5,6 are powerful for molecular orientation analysis. This combination technique requires, however, two different surfaces for the two different spectrometries. Since the film structure and properties are influenced by substrate surfaces, the matter of different substrates often becomes an intrinsic problem for the combination analysis. MAIRS, however, simultaneously provides IP- and OP-mode spectra from an identical sample, which correspond to the conventional transmission and RA spectra, respectively. MAIRS is now the only technique to overcome the matter of substrates, which would promisingly play an important role in surface analyses. The third benefit is useful for precise polarization analyses. A polarized ray generated through a polarizer contains impurity of polarization, and it makes the polarization analyses inaccurate. (3) Hasegawa, T.; Matsumoto, L.; Kitamura, S.; Amino, S.; Katada, S.; Nishijo, J. Can. J. Anal. Sci. Spectrosc. 2003, 48, 157. (The equation for the evaluation of molecular orientation in this paper is different from that in ref 1. The equation in ref 3 is the corrected equation.) (4) Hasegawa, T. Anal. Bioanal. Chem. 2003, 375, 18. (5) Mirabella, F. M. Modern Techniques in Appled Molecular Spectroscopy; WileyInterscience: New York, 1998; pp 11-124. (6) Umemura, J. In Handbook of Vibrational Spectroscopy; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley: Chichester, 2002; Vol. 2, pp 982-998. 10.1021/ac035496s CCC: $27.50

© 2004 American Chemical Society Published on Web 05/04/2004

Chart 1. Chemical Structure of Leu3

We can generate highly pure polarized light by making the light pass through two or more polarizers in series. Nevertheless, the light intensity is largely lost in this optics. MAIRS requires no polarizer, in principle, and polarization spectra are available by a regression calculation process. This benefit was experimentally confirmed by measuring the Berreman effect7 in our previous work.8 To the best of our knowledge, MAIRS is the only method to employ the regression calculation to overcome the polarization matter. The fourth benefit may be the most outstanding of all the benefits of MAIRS. Orientation analysis needs optical parameters to model the dielectric function that elucidates the observed spectra, except for the parameter fitting method.8 A priori knowledge of optical parameters is a large limitation of spectral analysis. Of particular note is that the thickness of the film is an inevitable optical parameter,9,10 since the optical model is mostly constructed on a stratified-media model, in which optical coherency is required for Fresnel’s model. Therefore, molecular orientation analysis has been performed on a limited basis for optically coherent materials, such as Langmuir-Blodgett (LB) films.11 One of the characteristics of MAIRS, that no optical parameter is necessary for the molecular orientation analyses, means that optically incoherent films, such as cast films, can be analyzed. Along the fourth analytical concept, in the present study, MAIRS has first been employed to analyze the molecular orientation in cast films consisting of a newly synthesized tripeptide amphiphile molecule.12 In addition, the MAIRS technique has been developed so that biaxial orientation analysis would become possible. In the present study, however, the axes of the “biaxial” system are based on Cartesian coordinates. Therefore, strictly speaking, it is not appropriate to use the term “biaxial” defined in polar coordinates; instead, the term “Cartesian structure” of the cast film is used. The compound used for the cast film analysis is presented in Chart 1. The three-dimensional structure of this compound is difficult to understand simply by looking at the chemical structure, but fortunately, a figure of the accurate structure of its related compound is available in Bandekar’s review paper.13 According to the figure, the compound forms a stable (7) Berreman, D. W. Phys. Rev. 1963, 130, 2193. (8) Hasegawa, T.; Nishijo, J.; Umemura, J.; Theiss, W. J. Phys. Chem. B 2001, 105, 11178. (9) Hasegawa, T.; Takeda, S.; Kawaguchi, A.; Umemura, J. Langmuir 1995, 11, 1236. (10) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927. (11) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York, 1991. (12) Yamada, N.; Komatsu, T.; Yoshinaga, H.; Yoshizawa, K.; Edo, S.; Kunitake, M. Angew. Chem., Int. Ed. 2003, 42, 5496. (13) Bandekar, J. Biochim. Biophys. Acta 1992, 1120, 123.

Chart 2. Schematic Illustrations of (Left Panel) Chemical Structure of Leu3, Corresponding to Chart 1, and (Right Panel) Molecular Interactions in Orthogonal Directions

parallel β-sheet structure via complimentary H-bond couplings between the N-H bond and one of the lone-pair electrons on CdO. The trans-zigzag structure that includes the sp2 configuration on nitrogen atoms is within the planer parallel β-sheet, which is in the a-c plane in the chart. On the other hand, the Y-shaped dimethylethane group in the leucine residue is oriented perpendicularly to the β-sheet, which is in the a-b plane. The structure of the compound is simplified in a model in Chart 2 (the left panel). The amide group is denoted by a short bar, and the dimethylethane group is denoted by the Y-shaped part. As illustrated in the right panel in Chart 2, the parallel β-sheet structure is formed by H-bonding in the a-c plane, whereas the Y-shaped groups are interdigitated to form a leucine fastener12,14 in the b-c plane. With the molecular interlocking system, the cast film of the compound has already been known to have unique mechanical strength and elasticity, although the molecules are not bonded covalently.12 The directions of the hydrogen bonding and the leucine fastener are designed to be mutually orthogonal to each other,12 and these orthogonal interactions are considered to influence the Cartesian structure in the cast film. The original MAIRS itself has great potential to reveal the anisotropic structure (14) Yamada, N.; Imai, T.; Koyama, E. Langmuir 2001, 17, 961.

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of incoherent thin films, but the advanced MAIRS technique has been found to be more useful, so we can discuss the structural changes with three spectra that correspond to X, Y, and Z directions in the film. MATERIAL AND METHODS Synthesis. Trileucine-containing amphiphile (N+-Leu3Glu(OC12)2; abbreviated as Leu3) was prepared in a manner similar to that described elsewhere.15 The chemical structure is presented in Chart 1. Didodecyl-L-glutamate was condensed with Boc-Leu3OH in the presence of diethylphosphoro-cyanidate at room temperature. After the protection group (Boc) was removed using HBr/acetic acid, the resulting compound was allowed to react with 11-bromoundecanoyl chloride in the presence of triethylamine with ice cooling. The alkyl bromide, thus obtained, was three times recrystallized from methanol, then quaternized by trimethylamine to give a colorless wax: mp 160.2-163.0 °C. Anal. Calcd. for C61H118N5O8Br‚H2O: C, 63.85; H, 10.54; N, 6.10. Found: C, 63.98; H, 10.65; N, 6.12. Infrared Spectroscopic Analysis. The FT-IR MAIRS measurements were performed by using a Harrick Scientific Co. (Ossining, NY) Brewster’s Angle Sample Holder (BXH-S1G). The angle of incidence was changed manually from 10° to 45° by 5° steps. As mentioned in earlier, no polarizer was used, and an unpolarized ray was irradiated onto the sample.1 The collection of infrared single-beam spectra was performed on a ThermoElectron Nicolet (Madison, WI) Magna 550 FT-IR spectrometer equipped with a mercury-cadmium telluride (MCT) detector with an aperture fully opened.2 The laser modulation frequencies for the interferogram collections were 60 kHz. The interferogram was accumulated 2000 times to improve the signal-to-noise ratio. The air in the sample room of the FT-IR was purged by dried air generated by an Airtech (Yokohama, Japan) AT-35H air dryer. The signal intensity of the residual water vapor does not have to do with the angle of incidence; therefore, it is not linearly correlated to the R matrix in MAIRS. As a result, most of the water signals were automatically discarded by the regression calculation. Regardless, water peaks still remained, particularly in the OP-mode spectra, because the OP mode spectra are very weak in intensity. The residual water peaks were readily corrected by subtraction of a water vapor spectrum. THEORY In the present study, the original MAIRS algorithm was modified to develop an advanced algorithm that enables us to analyze the Cartesian molecular structure in a thin film. MAIRS was originally developed on an assumption that the IP- and OPmode spectra are observed by use of an ordinary light and an imaginary light, respectively, that has an electric field vector parallel to the direction of light propagation.1 Therefore, the resolution matrix, R, has two columns for the distribution of electric-field components that correspond to the IP and OP modes. The IP mode comprises, however, mutually orthogonal X and Y components, and they individually appear in the R matrix. As mentioned in the original paper of the MAIRS technique,1 the contributions of electric fields to the IP and OP modes are schematically drawn in Figure 1. (15) Yamada, N.; Ariga, K.; Niato, M.; Matsubara, K.; Koyama, E. J. Am. Chem. Soc. 1998, 120, 12192.

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Figure 1. Schematic diagram of electric fields across the film surface.

The s-polarization that has an electric field vector perpendicular to the plane of incidence (or the printed surface of the paper) and its electric field amplitude are normalized to be unity. The p-polarized light with an angle of incidence, θ, has the same electric field amplitude of unity, and the direction of the field is perpendicular to the light beam, which is in the plane of incidence (represented by the chain with arrow). This electric-field vector can be decomposed into the cos θ and sin θ components. The surface-perpendicular component of the p-polarized light (sin θ) goes across the sample surface obliquely, and it generates a virtual electric field that is parallel to the surface (sin θ tan θ),1 and the sin θ component is enlarged to be tan θ. Of note is that the amplitude of the sin θ component is enlarged, but it still contributes to the surface-perpendicular “direction”. Let us consider here a Cartesian coordinate represented by X, Y, and Z axes. In Figure 1, the Y and Z axes are drawn, and the X axis is directed perpendicular to the printed surface. In this Cartesian coordinate, the s-polarized component fully contributes to the X component, and the cos θ and sin θ tan θ components contribute to the Y component. In the same manner, the tan θ component contributes to the Z component. As a result, the conventional R matrix has been rearranged to be a three-column matrix, RXYZ.

4 RXYZ ) π

(

2

()

1 cos2 θj + sin2 θj tan2 θj tan2 θj • • • • • •

)

(1)

This new matrix yields three virtual single-beam spectra with the use of the following regression calculation. (S is a matrix consisting of collected single-beam spectra).

()

sX sY ) (RXYZTRXYZ)-1RXYZTS sZ

(2)

The superscript T denotes a transposed matrix ((RT)ij ) (R)ji). In this manner, the slight modification of the R matrix makes it possible to analyze the molecular Cartesian structure in thin films. It should be noted that the conventional experimental data, S, can be used as they are for the new analysis by only replacing R with RXYZ. RESULTS AND DISCUSSION Analysis of a Fresh Cast Film. The Leu3 gel dispersed in CCl4 was spread over the Ge substrate, and it was kept under the

Figure 2. Infrared MAIRS spectra of the fresh Leu3 cast film prepared on Ge.

saturated vapor of the solvent for ∼50 h, so that a cast film of Leu3 was gradually formed on the substrate. This cast film was subjected to the infrared MAIRS analysis. Since the cast film was formed on one side of the substrate, the other side was bare. When an infrared ray was irradiated onto the sample side, the infrared ray was a little scattered by the rough surface of the cast film, which was considered to make the MAIRS analysis inaccurate. Therefore, in the present study, an infrared ray was irradiated onto the bare side of the substrate, and the transmitted ray through the sample was collected. The infrared MAIRS spectra of the cast film presented in Figure 2 were measured immediately after the preparation of the film. The band locations of the N-H stretching vibration (ν(NH); 3284 cm-1) and amide I (1633 cm-1) modes suggest that these two groups are both hydrogen-bonded,14 and they are probably coupled with each other. The amide I band (mainly due to the CdO stretching vibration) at this wavenumber, which accompanies no band at ∼1690 cm-1, is characteristic of the parallel-chain β-sheet structure formed in the molecular aggregate.14-16 This ordered structure is supported by an apparent dichroic ratio of the ν(N-H) and amide I bands. MAIRS enables us to evaluate the molecular orientation angle (φ) by employing the following simple equation.3

φ ) tan-1(x2AIP/AOP)

(3)

Here, AIP and AOP are the absorbance of an identical band appearing in the in-plane (IP)- and out-of-plane (OP)-mode spectra, respectively. Since the IP-mode spectra in the present paper are doubled (2 × IP), the evaluation of orientation angles using eq 3 is a very easy task. With the spectra in Figure 2, the ν(N-H) and amide I bands are calculated to have orientation angles of 65° and 64° from the surface normal, respectively. The good agreement of the two orientation angles supports that the N-H and CdO groups form hydrogen-bonding couplings, as suggested by the band location analysis. A similar agreement was previously reported in another amide-coupling system.17 In this manner, infrared MAIRS is useful to confirm hydrogen bond formation. Since unoriented (random) groups would yield an angle of 54.7°, the results suggest that the hydrogen bonding is a little oriented (16) Kunitake, T. Angew. Chem., Int. Ed. 1992, 31, 709. (17) Hasegawa, T.; Umemura, J.; Li, C.; Leblanc, R. M. J. Phys. Chem. B 2003, 107, 11996.

to the parallel direction of the film on average. The amide II band (1544 cm-1) does not exhibit clear dichroism, because the amide II mode is due to a mixed vibration of the N-H deformation and the C-N stretching vibrations,18 and the direction of its transition moment is not perfectly perpendicular to that of the amide I mode. Therefore, a discussion of this band does not appear in the present paper. On the other hand, it is interesting to take a look at the C-H stretching vibration region (2800-3100 cm-1), in which four major bands are found: the asymmetric CH3 stretching vibration (νa(CH3)); 2955 cm-1), the antisymmetric CH2 stretching vibration (νa(CH2)); 2924 cm-1), the symmetric CH3 stretching vibration (νs(CH3)); 2870 cm-1), and the symmetric CH2 stretching vibration (νs(CH2)); 2853 cm-1) bands. It is known that the νa(CH2) and νs(CH2) bands appear at ∼2917 and 2850 cm-1, respectively, for a methylene chain with the all-trans zigzag conformation, while the same modes appear at ∼2927 and 2855 cm-1 for the chain with the gauche conformation. The band locations in the MAIRS spectra suggest, therefore, that the hydrocarbon chains are in a mixed state of the trans and gauche conformers of the hydrocarbon chains. This is also supported by the fact that the two bands have relatively large full widths at half-maximum (fwhm). A fivemonolayer Langmuir-Blodgett film of cadmium stearate deposited on a germanium substrate, which is known to have a highly wellordered hydrocarbon chain with all-trans conformation, gives a fwhm of 8 cm-1 for the νs(CH2) band in its transmission spectrum.19 In the present spectra, the same band gives a fwhm of 17 cm-1, which is apparently larger than the well-ordered one. Regardless, the band location (2853 cm-1) is still lower than that for the perfectly disordered conformation (2855 cm-1). It can roughly be concluded, therefore, that the cast film comprises both ordered and disordered molecular species. Of further importance is the analytical result of the molecular orientation in the hydrocarbon chains. The orientation angles of the νa(CH2) and νs(CH2) modes are calculated to be 48° and 49°, respectively, with the use of eq 3. These values are quite small for the analytical results of a hydrocarbon chain, and the molecular image is needed. It would not be a good idea to calculate the molecular tilt angle of the hydrocarbon chain by use of the two orientation angles for the disordered chain, since the calculation (18) Miyazawa, T.; Shimanouchi, T.; Mizushima, S. J. Chem. Phys. 1958, 29, 611. (19) Hasegawa, T.; Amino, S.; Kitamura, S.; Matsumoto, L.; Katada, S.; Nishijo, J. Langmuir 2003, 19, 105.

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Figure 3. Schematic illustrations of β-sheets consisting of Leu3 for (a) the fresh cast film, and (b) the aged (dried) film that was kept in a desiccator for two weeks.

that employs “direction cosine” holds only when the tilt angle and the two orientation angles are mutually orthogonal to each other, which requires that the hydrocarbon chain is straight with alltrans zigzag conformation.9 Judging from the band location discussed above, an accurate analysis of tilt angle is impossible in the present case. It would still be interesting to perform the calculation to evaluate an averaged angle, however, to have a rough image of the molecules in the cast film. Direction cosines are formulated by the following equation:

cos2 R + cos2 β + cos2 γ ) 1

(4)

The averaged molecular tilt angle, γ, can be calculated by putting the two orientation angles into R and β, and the calculated tilt angle is calculated to be 70° from the normal. This result suggests that most of the Leu3 molecules are lying on the Ge substrate with the nearly parallel orientation. The schematic image of the β-sheet aggregates is presented in Figure 3(a). Analysis of an Aged Cast Film. The cast film of Leu3 was kept in a vacuum desiccator for two weeks, and the aged cast film was analyzed by the same technique. In this aging process, it was expected that residual solvent (CCl4) would be removed enough under a water-free condition. Figure 4 presents the infrared MAIRS spectra of the aged cast film. There is no band shift in comparison to Figure 2, which means that the hydrogenbonding between the N-H and the CdO groups is maintained. It is impressive, however, that the dichroic ratio found in Figure 2 is largely depressed in Figure 4. For example, the band intensity ratio of the 2 × IP and OP-mode spectra for the ν(N-H) is found to be 4.6 in Figure 2, whereas the same ratio is found to be 1.9 in Figure 4. This depressed ratio gives rise to the orientation angle of 54° from the surface normal by use of eq 3, which is apparently smaller than the previous result (65°), and the angle of 54° corresponds to the unoriented condition. The orientation angle of the perfectly unoriented condition can be calculated by use of the equation of the direction cosine (eq 4). Under the perfectly random condition (R ) β ) γ), cos2 R becomes 1/3, which yields R ) β ) γ ) 54.7°, which is known to be “magic angle”, particularly popular in NMR spectroscopy. In this manner, it is concluded that the N-H bond is randomly oriented in the aged cast film. In response to the N-H bond analysis, the amide I band was also analyzed. As a result, the ν(CdO) mode gave the same orientation angle of 54°. As previously discussed with the band location, the hydrogen-bonding coupling was maintained after the aging. It is expected, therefore, that the N-H and CdO groups would yield the same orientation angles, which has been confirmed. 3088

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As for the C-H stretching vibration region, both orientation angles of the νa(CH2) and νs(CH2) modes are calculated to be 44° for the dried cast film. This value itself seems not significantly different from those for Figure 2 (48° and 49°), but it is a critical value. If the hydrocarbon chain is perfectly aligned parallel to the surface (γ ) 90°), the averaged R and β should be 45° (eq 4). Therefore, the experimental result of 44° strongly suggests that the hydrocarbon chains lie roughly on the surface (γ ≈ 90°), and methylene groups are unoriented (R ) β), which is also supported by the wavenumber locations. The tilt angle of the hydrocarbon chain (γ ≈ 90°) is merely an averaged angle, and it does not mean that the molecules are “oriented”. It should be noted that the aged cast film exhibits more disordered molecular architecture. The schematic molecular architecture of the dried cast film is illustrated in Figure 3b. In this fashion, MAIRS analyses of the two cast films have suggested that the removal of solvent (or aging) makes the cast film disordered, probably due to the molecular aggregation between the β-sheets, although the hydrogen bondings are kept in a good condition. Nonetheless, it is still unclear how the rearrangement of molecules goes in the film. Then the advanced MAIRS analysis was employed so that the Cartesian structural information would be revealed. Analysis by the Advanced Infrared MAIRS Technique. Figure 5 presents the “advanced infrared MAIRS” spectra of the Leu3 cast film from the CCl4 dispersion, which corresponds to Figure 2. Since the R matrix is now modified to resolve the singlebeam spectra into X, Y, and Z components, three spectra are presented. As found in Figure 2, large dichroic ratios between the 2 × X and Z spectra are found for the ν(N-H) and amide I bands. It is striking in this advanced MAIRS analysis that the 2 × Y spectrum does not have the same intensity as the 2 × X spectrum, although they are both attributed to the IP-mode spectrum. This result suggests that the molecular orientation of the freshly cast film is not uniaxial, but biaxial in molecular orientation. In other words, the cast film has in-plane orientation, in addition to the IP and OP anisotropy. In our study, the X axis is aligned to the short axis of the Ge substrate, and the Y axis is aligned to the long axis. When the cast film was prepared, the monomer gel was spread in the direction of the long axis. Unlike the preparation of LB films, it is difficult to evaluate the “flow orientation”20 in the present study, since no external surface pressure is applied to the molecular aggregation. We can consider, however, that the spreading process of the gel is considered to cause the in-plane orientation. The advanced MAIRS analysis of the “aged” film is interesting. The XYZ spectra are presented in Figure 6. It should be noted that the X and Y spectra have become very similar to each other for the aged film. These analytical results strongly suggest that the cast film has become close to the uniaxial molecular architecture. As discussed above for Figure 4, the aged cast film lost molecular order, and each chemical group exhibited unoriented structure, which suggested the lying molecular arrangement. Figure 6 agrees with the discussion, and it further suggests that the in-plane orientation has also been lost during the aging process. Cartesian structural changes have never been reported for cast films thus far, and it is emphasized that the analysis has (20) Ikegami, K.; Mingotaud, C.; Delhae´s, P. Phys. Rev. E 1997, 56, 1987.

Figure 4. Infrared MAIRS spectra of the dried Leu3 cast film prepared on Ge.

Figure 5. Advanced infrared MAIRS spectra of the fresh Leu3 cast film prepared on Ge, which corresponds to Figure 2.

Figure 6. Advanced infrared MAIRS spectra of the dried Leu3 cast film prepared on Ge, which corresponds to Figure 3.

easily been performed by use of the advanced infrared MAIRS technique, as done by the conventional MAIRS technique. The advanced MAIRS provides other interesting molecular information for the ester CdO stretching vibration band at ∼1740 cm-1. This band is split into two components at 1755 and 1737 cm-1. Leu3 (Chart 1) has two ester CdO groups in the molecule, and they can be unequally oriented. Therefore, the two CdO groups can have different molecular interactions with neighbor molecules, which would result in two different band locations. In the conventional analyses of these bands, the relative band intensities have always been unchanged. The advanced MAIRS has revealed, however, that they exhibit different band intensity ratios, depending on a sample condition. Figure 7 presents magnified spectra of the bands for the X and Z components found in Figure 5. It is apparent that the two bands have different intensity ratios in the X and Z components.

Figure 7. Magnified advanced infrared MAIRS spectra of the X and Z components for the ester CdO stretching vibration band after Figure 5.

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The orientation angles of these modes have been calculated after curve fittings to be 38° and 46°, which is an 8° difference. In this manner, the advanced MAIRS enables us to discuss overlapped bands with the aid of the molecular orientation difference. On the other hand, the same analysis has also been performed for Figure 6. The same modes gave orientation angles of 38° and 43°. The difference between the two angles (5°) is smaller than that for Figure 5. If the Leu3 molecules had a totally disordered orientation, the two angles would be close to each other, irrespective of their intrinsic orientation in the fresh cast film. Therefore, the closer angles of the two CdO groups support that the aged cast film has lost molecular orientation, and the hydrocarbon chains are roughly lying on the surface. CONCLUSION The conventional and advanced infrared MAIRS analyses of cast films of Leu3 on Ge prepared from a CCl4 dispersion gel suggest that the fresh cast film had relatively ordered architecture, which exhibited biaxially anisotropic spectra, whereas the aged cast film lost biaxial structure, and disordered orientations are

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found for the major bands. The intermolecular hydrogen bondings were maintained even after the ordered architecture was lost. The discussion of the Cartesian structure with the use of the advanced infrared MAIRS technique has been found to be powerful to characterize cast thin films. ACKNOWLEDGMENT This work was financially supported by a Grant-in-Aid for Scientific Research for Priority Areas “Dynamic Control of Strongly Correlated Soft Materials” (No. 413/14045267 (T.H.) and 413/ 14045208 (N.Y.)), Exploratory Research (No. 15659011 (T.H.)), and a Grant-in-Aid for Scientific Research (No. 14550831 (N.Y.)) from the Ministry of Education, Science, Sports, Culture, and Technology, Japan.

Received for review December 17, 2003. Accepted March 16, 2004. AC035496S