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An Infrared Spectroscopy Study on Molecular Orientation and Structure in Mixed Langmuir-Blodgett Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and Deuterated Stearic Acid: Phase Separation and Freezing-in Effects of the Fatty Acid Domains Hai-Shui Wang and Yukihiro Ozaki* Department of Chemistry, School of Science, Kwansei Gakuin University, Uegahara, Nishinomiya 662-8501, Japan
Keiji Iriyama Institute of DNA Medicine, The Jikei University School of Medicine, Nishi-Shinbashi, Minato-ku, Tokyo 105-0003, Japan Received December 15, 1999. In Final Form: March 6, 2000 Mixed Langmuir-Blodgett (LB) films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C18TCNQ) and deuterated stearic acid (stearic acid-d35) were fabricated to investigate effects of incorporation of the fatty acid on molecular orientation and structure in LB films of C18TCNQ. A π-A isotherm of the mixed Langmuir film suggests that C18TCNQ and stearic acid-d35 are segregated in the film and that above 22 mNm-1 the C18TCNQ domains start to collapse or the C18TCNQ molecules are squeezed out from the fatty acid monolayer on the water subphase. Ultraviolet-visible and infrared transmission and reflection-absorption (RA) spectra were measured for one-, three-, and five-layer mixed LB films deposited at 18 and 35 mNm-1. A doublet peak caused by the CH2 scissoring mode of the alkyl chain of C18TCNQ suggests that the phase separation occurs in the mixed LB films. Infrared spectra of LB films of C18TCNQ alone give two bands at 2926 and 2918 cm-1 due to the CH2 antisymmetric stretching modes of the partially disordered and highly ordered parts of the alkyl chain of C18TCNQ, respectively. In the corresponding spectra of the mixed LB films, the band at 2926 cm-1 becomes very weak. Thus, the percentage of the partially disordered part is reduced markedly by mixing C18TCNQ with fatty acid. Molecular orientation of C18TCNQ is also changed significantly upon the formation of the mixed LB films. For example, the alkyl chain becomes more perpendicular, whereas the TCNQ plane becomes more parallel to the substrate surface. The reduction in the ratio of the partially disordered part of the alkyl chain and the change in the molecular orientation of C18TCNQ in the mixed LB films are probably due to freezing-in effects of the fatty acid domains which block the realignment of C18TCNQ during the film deposition. The infrared spectra of the mixed LB films deposited at 35 mNm-1 show that the molecular orientation and structure of C18TCNQ change greatly because of the collapse of the C18TCNQ layer at this pressure.
Introduction Mixed Langmuir-Blodgett (LB) films, which usually consist of functional dyes and dilutions, are of current interest for the following reasons:1-10 (1) It is possible to * To whom correspondence should be sent. Department of Chemistry, School of Science, Kwansei Gakuin University, Uegahara, Nishinomiya 662-8501, Japan. E-mail:
[email protected]. (1) Roberts, G. G. Langmuir-Boldgett Films; Plenum Press: New York, 1990. (2) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (3) Ruaudel-Teixier, A.; Vandevyver, M.; Roulliay, M.; Bourgoin, J. P.; Barraud, A.; Lequan, M.; Lequan, R. M. J. Phys. D: Appl. Phys. 1990, 23, 987. (4) Ahuja, R. C.; Matsumoto, M.; Mobius, D. J. Phys. Chem. 1992, 96, 1855. (5) Dringenberg, B. J.; Ahuja, R. C.; Mobius, D. Thin Solid Films 1994, 243, 569. (6) Song, X.; Miura, M.; Xu, X.; Taylor, K. K.; Majumder, S. A.; Hobbs, J. D.; Cesarano, J.; Shelnutt, J. A. Langmuir 1996, 12, 2019. (7) Zhang, Z.; Nakashima, K.; Verma, A. L.; Yoneyama, M.; Iriyama, K.; Ozaki, Y. Langmuir 1998, 14, 1177. (8) Iimura, K.; Ito, K.; Kato, T. Mol. Cryst. Liq. Cryst. 1998, 322, 117. (9) Parichha, T. K.; Tarapatra, G. B. J. Phys. Chem. Solids 1999, 60, 111. (10) Dutta, A. K.; Vanoppen, P.; Jeuris, K.; Grim, P. C. M.; Pevenage, D.; Salesse, C.; De Schryver, F. C. Langmuir 1999, 15, 607.
remove the dilution molecules after the fabrication of mixed LB films, by use of some suitable solvents. This greatly reduces the density of the films, and some properties of the films such as refractive index may be modified. (2) The film-forming ability of some functional dyes, which cannot form ideal and well-defined monolayers at the air-water interface or cannot be transferred onto plates very well by usual methods, may be improved greatly by mixing them with dilutions such as long-chain fatty acid. This provides a way to make LB films available for some useful functional dyes.9,10 (3) The structure and properties of the functional dyes in mixed LB films may be different from those in LB films of the dyes alone. One may be able to control molecular aggregation, orientation, and structure in the LB films and their physical properties by changing the ratio of each component in the mixed LB films. Despite the importance of the mixed LB films, their detailed structural studies have been limited.3-5,7 The purpose of this study is to investigate the molecular orientation and structure in mixed LB films consisting of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C18TCNQ) and deuterated stearic acid (stearic acid-d35) by use of
10.1021/la991643z CCC: $19.00 © 2000 American Chemical Society Published on Web 04/22/2000
Molecular Orientation of C18TCNQ and Stearic Acid-d35
π-A isotherm and ultraviolet-visible (UV-Vis) and infrared spectroscopy. We have been investigating the structure, morphology, and thermal and time-dependent behavior of LB films of 2-alkyl-7,7,8,8-tetracyanoquinodimethane (C12TCNQ, C15TCNQ, C18TCNQ) by means of UV-Vis and infrared spectroscopy and atomic force microscopy (AFM).11-20 As for the structure and morphology of the LB films of C18TCNQ, the following conclusions can be reached from our investigations:11-20 (1) The LB films of C18TCNQ consist of numerous platelike microcrystal domains in which the C18TCNQ molecules form a bilayer structure with the interdigitated alkyl chains. The domains in the first layer (4.3 nm) are thicker than those above the first layer (3.7 nm). C18TCNQ also assumes a bilayer or multilayer structure at the air-water interface.14,15 (2) The alkyl chain of C18TCNQ consists of highly ordered and partially disordered parts in the LB films.16 (3) Both the hydrocarbon chain and the TCNQ plane are tilted considerably with respect to the surface normal in the LB films of C18TCNQ.11,13 (4) The alkyl chains of C15TCNQ and C12TCNQ become more tilted with respect to the surface normal, and their TCNQ planes become more perpendicular with respect to the substrate surface during the time after the LB film deposition. Such time-dependent phenomena are not observed for the LB films of C18TCNQ.17-19 The present study has aimed at exploring the influence of incorporating stearic acid-d35 into the LB films of C18TCNQ on the molecular orientation and structure of C18TCNQ and investigating the possibility of the realignment of C18TCNQ during the film deposition. We have investigated the structure of a mixed Langmuir film of C18TCNQ and stearic acid-d35 at the water-air interface by measuring its π-A isotherm. Then, UV-Vis and infrared transmission and reflection-absorption (RA) spectra were measured for the mixed LB films and were compared with those of the LB films of C18TCNQ alone. We have found that the incorporation of the long-chain fatty acid modifies the molecular orientation and structure of C18TCNQ in the mixed LB films even though the phase separation takes place and that the realignment of C18TCNQ, which probably occurs in the LB films of C18TCNQ alone during the film deposition, can be blocked. Experimental Section Sample Preparation. C18TCNQ was purchased from the Japanese Research Institute for Photosensitizing Dyes Co., Ltd., and used without further purification. The thin-layer chromatographic examinations revealed that the dye did not contain any other colored components. Stearic acid-d35 (98%) and stearic acid were obtained from Cambridge Isotope Laboratories and Sigma Co., respectively, and used without further purification. A Kyowa Kaimen Kagaku Model HBM-AP Langmuir trough with a Wilhelmy balance was used for the π-A isotherm (11) Kubota, M.; Ozaki, Y.; Araki, T.; Ohki, S.; Iriyama, K. Langmuir 1991, 7, 774. (12) Terashita, S.; Ozaki, Y.; Iriyama, K. J. Phys. Chem. 1993, 97, 10445. (13) Wang, Y.; Iriyama, K.; Ozaki, Y. Langmuir 1995, 11, 705. (14) Wang, Y.; Nichogi, K.; Terashita, S.; Iriyama, K.; Ozaki, Y. J. Phys. Chem. 1996, 100, 368. (15) Wang, Y.; Nichogi, K.; Iriyama, K.; Ozaki, Y. J. Phys. Chem. 1996, 100, 374. (16) Wang, H.; Ozaki, Y. Appl. Spectrosc., in press. (17) Morita, S.; Wang, H.; Wang, Y.; Iriyama, K.; Ozaki, Y. Mol. Cryst. Liq. Cryst. 1998, 322, 216. (18) Wang, H.; Morita, S.; Iriyama, K.; Ozaki, Y. Mol. Cryst. Liq. Cryst., in press. (19) Morita, S.; Iriyama, K.; Ozaki, Y. J. Phys. Chem. 2000, 104, 1183. (20) Morita, S.; Nichogi, K.; Ozaki, Y. J. Phys. Chem. submitted for publication.
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Figure 1. π-A isotherm of the mixed Langmuir film of C18TCNQ and stearic acid-d35 on a pure water surface at 20 °C. measurements and LB film preparation. C18TCNQ and stearic acid-d35 were dissolved in spectrograde chloroform (Dojin Chemical Co.) with the molar ratio of 1:3 (1.0 × 10-3 M in total). After they were mixed, the solution was spread onto an aqueous subphase at 20 °C. The distilled water used for the subphase, whose specific resistance was greater than 18.1 MΩcm, was prepared by the method reported previously.21 After evaporation of the solvent, the mixed Langmuir film was compressed at a constant rate of 20 cm2 min-1 up to the desired surface pressure. The Langmuir films were transferred by the vertical dipping method onto CaF2 plates (UV-Vis and infrared transmission measurements) and gold-evaporated glass slides (infrared RA measurements). The transfer ratio was about 1.00 ( 0.10. A detailed cleaning procedure for CaF2 plates and gold-evaporated glass slides was described previously.21 Spectroscopy. Infrared spectra of the LB films were measured at a 4 cm-1 resolution with a Nicolet Magna 760 FT-IR spectrometer equipped with a MCT detector. For the infrared RA measurements, a reflection attachment (Spectra-Tech, FT80 RAS) was used at the incident angle of 80°. To yield spectra of high signal-to-noise ratio, 512 interferograms were co-added. UV-Vis spectra of the LB films were measured with a Shimadzu UV-Vis 3101 PC spectrophotometer.
Results and Discussion Formation of C18TCNQ-Stearic Acid-d35 Mixed Monolayer at the Air-Water Interface. Figure 1 shows a π-A isotherm of mixed Langmuir film of C18TCNQ and stearic acid-d35 on the pure water subphase. Three characteristic features are observed for the π-A isotherm. First, a steeply rising region appears between 0 and 22 mNm-1. Second, the onset of surface pressure becomes gradual, between 22 and 32 mNm-1. The behavior of the π-A isotherm in this region is greatly different from that of stearic acid-d35 or C18TCNQ alone. It is very likely that some of the structural features of the mixed monolayer change in this region. Third, there is an abrupt rise of the slope for the π-A isotherm above 32 mNm-1. The shape of the π-A isotherm and the collapse pressure are useful guides to study the behavior of the mixed monolayer such as phase separation.1 The π-A isotherm of C18TCNQ-fatty acid mixed monolayer shown in Figure 1 indicates that the two components probably form immiscible domains at the air-water interface. In other words, they probably exist in the segregated domains (i.e., C18TCNQ domains and fatty acid domains). The final collapse pressure of the Langmuir film is about 47 mNm-1, which is close to the collapse pressure of fatty acid monolayer alone. Therefore, it seems that the fatty acid layer is hardly destroyed until 47 mNm-1. The extrapolating area per imaginary molecule, which consists of a quarter of a C18TCNQ molecule and three-quarters of a stearic acid-d35 molecule, is estimated to be 16.8 Å2 by extrapolating the second steeply rising part of the curve (21) Myrzakozha, D. A.; Hasegawa, T.; Nishijo, J.; Imae, T.; Ozaki, Y. Langmuir 1999, 15, 6890.
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Figure 2. UV-visible spectra of (a) a one-layer mixed LB film of C18TCNQ and stearic acid-d35 on a CaF2 plate and (b) a onelayer LB film of C18TCNQ alone on a CaF2 plate.
Figure 3. An infrared transmission spectrum of the threelayer mixed LB film of C18TCNQ and stearic acid-d35 deposited at 18 mNm-1 on a CaF2 plate.
to zero pressure. The area of 16.8 Å2 is much smaller than the area of 22 Å2, which is the extrapolating area per stearic acid molecule for the hypothetical state of an uncompressed close-packed layer.1 If the domains of fatty acid keep a monolayer structure and those of C18TCNQ keep a bilayer structure with the interdigitated alkyl chains on the water subphase above 32 mNm-1, the extrapolating area per imaginary molecule should not be smaller than the extrapolating area per stearic acid molecule because the size of one imaginary molecule is expected to be larger than that of the stearic acid molecule. Therefore, there is little doubt that one component, probably C18TCNQ, collapses at 32 mNm-1. It is very likely that the region between 22 and 32 mNm-1 corresponds to the collapse of the domains of C18TCNQ. In the study of the mixed monolayer of 5-(4-N-octadecylpyridyl)-10,15,20tri-p-tolylporphyrin and stearic acid-d35 at the air-water interface, it was found that the onset of surface pressure also becomes gradual when the functional dyes are being squeezed out from the dilution monolayer.7 Similarly, the C18TCNQ molecules may start to be squeezed out from the stearic acid-d35 monolayer at 22mNm-1 on the water subphase. Molecular Orientation and Structure of C18TCNQ in the Mixed LB Films. Figure 2 shows an UV-Vis spectrum of the one-layer mixed LB film of C18TCNQ and stearic acid-d35 deposited at the surface pressure of 18 mNm-1 (a), together with that of the one-layer LB film of C18TCNQ alone (b). The UV-Vis spectrum of the mixed LB film is very close to that of the LB film of C18TCNQ alone except the intensity13,20 and the shapes and patterns of UV-Vis spectra of the mixed LB films change little with the number of layers. We have assigned absorption bands at 365 and 405 nm of the LB films of C18TCNQ alone to the H-aggregation and monomer of the TCNQ chromophore, respectively.20 The UV-Vis spectrum of the mixed LB film shown in Figure 2a suggests that the H-aggregation form of the TCNQ chromophore (365 nm) is also the major component in the mixed LB films. In other words, the TCNQ chromophores, at least most of them, can interact with each other in the mixed films. This is consistent with the conclusion reached by the π-A isotherm measurement that the mixed Langmuir film consists of the segregated domains. Figure 3 shows an infrared transmission spectrum of the three-layer mixed LB film deposited at 18 mNm-1. Vibrational assignments of infrared spectra of C18TCNQ and stearic acid-d35 have been well established.11,22-25 Two intense bands at 2918 and 2849 cm-1 are assigned to CH2
antisymmetric and symmetric stretching modes of the hydrocarbon chain of C18TCNQ, respectively. The corresponding bands of stearic acid-d35 are observed at 2193 and 2088 cm-1. A band due to the CtN stretching mode of the four cyano groups should appear near 2220 cm-1, but in the present case this band is hidden by the CD3 stretching band of stearic acid-d35. Two bands at 1545 and 1531 cm-1 arise from CdC stretching modes of the TCNQ chromophore, and a doublet at 1471 and 1462 cm-1 is attributed to the CH2 scissoring mode of the hydrocarbon chain. For stearic acid-d35, a band due to the CD2 scissoring vibration appears at 1089 cm-1. Direct interactions between fatty acid and C18TCNQ in the mixed layers seem to be minimal because all the bands shift little between the infrared spectra of the mixed LB films and those of the LB films of C18TCNQ or stearic acid-d35 alone. The CH2 scissoring band of the hydrocarbon chain is sensitive to the intermolecular interaction and is often used to distinguish the lateral packing of the chains.26,27 As for C18TCNQ, our recent studies showed that the doublet characteristic of the CH2 scissoring mode is related to the structure of the interdigitated chains in the LB films of C18TCNQ alone.20 The mixed LB films also show the doublet at 1471 and 1462 cm-1. If most of the alkyl chains existed as independent chains (if each alkyl chain were surrounded by the deuterated alkyl chains) in the mixed LB films, the CH2 scissoring band should show a significant change,28 such as a frequency shift, a relative intensity change of the doublet features, etc. The similarity between the infrared spectra of mixed LB films and those of LB films of C18TCNQ alone, especially the similarity in the intensity pattern of the CH2 scissoring mode, implies that the structure of the interdigitated chains of C18TCNQ and the packing pattern of the alkyl chains of C18TCNQ change little upon forming the mixed LB films. It is well-known that the frequencies of CH2 antisymmetric and symmetric stretching bands are sensitive to the conformation of the hydrocarbon chains;29,30 low (22) Dote, J. L.; Mowery, R. L. J. Phys. Chem. 1988, 92, 1571. (23) Mowery, R. L.; Dote, J. L. Mikrochim. Acta 1988, 2, 69. (24) Cameron, D. G.; Martin, A.; Moffatt, D. J.; Mantsch, H. H. Biochemistry 1985, 34, 4355. (25) Koyama, Y.; Yanagishita, M.; Toda, S.; Matsuo, T. J. Colloid Interface Sci. 1977, 61(3), 438. (26) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1963, 19, 85. (27) Tasumi, M.; Shimanouchi, T. J. Chem. Phys. 1965, 43, 1245. (28) Shimomura, M.; Song, K.; Rabolt, J. F. Langmuir 1992, 8, 887. (29) Umemura, J.; Cameron, D. G.; Mantsch, H. H. Biochim. Biophys. Acta 1980, 602, 32.
Molecular Orientation of C18TCNQ and Stearic Acid-d35
Figure 4. The infrared transmission spectra in the 31002800 cm-1 region of (a) a three-layer mixed LB film of C18TCNQ and stearic acid-d35 and (b) a three-layer LB film of C18TCNQ alone.
frequencies (2918 and 2848 cm-1) of the bands are characteristic of a highly ordered (trans-zigzag) hydrocarbon chain, whereas their high-frequency shifts (∼2926 and ∼2856 cm-1) are indicative of the increase in conformational disorder, that is, gauche conformers, in the hydrocarbon chain. The observation in Figure 3 indicates that the hydrocarbon chain of C18TCNQ is highly ordered in the mixed LB films. Of particular note in Figure 3 is the band shape of the CH2 antisymmetric stretching band of the mixed LB films, which is slightly different from that of the LB films of C18TCNQ alone.16,31 Figure 4 compares the infrared transmission spectrum in the 3100-2800 cm-1 region of the three-layer mixed LB film (a) with that of a threelayer LB film of C18TCNQ alone (b). The LB film of C18TCNQ alone yields a clear shoulder at 2926 cm-1, and this shoulder becomes very weak in the spectrum of the mixed LB film. Recently, by use of second derivative, Fourier self-deconvolution and two-dimensional correlation spectroscopy analysis, we attributed the bands at 2926 and 2918 cm-1 of the C18TCNQ LB film to disordered and highly ordered parts of the hydrocarbon chain, respectively.16 To investigate the structure of C18TCNQ in the mixed LB films in more detail, the second derivative of the spectrum in Figure 4a was calculated. The second derivative spectrum of the mixed LB film demonstrates the existence of a band at 2926 cm-1. Thus, there may still be some disordered parts in the alkyl chains; they decrease markedly in the mixed LB films. This result shows that the conformation of the alkyl chain of C18TCNQ is modified significantly, by incorporating fatty acid into the LB films. Vibrational modes with their transition moments perpendicular to the substrate surface are enhanced in a RA spectrum, whereas those with their transition moments parallel to the substrate surface appear strongly in a transmission spectrum.32,33 Thus, the ratio (R) of the absorbance between RA (AR) and transmission (AT) spectra should become smaller when the corresponding transition moment tends to become more parallel to the substrate surface. Now, we will compare the molecular orientation in the mixed LB films with that in the LB films of C18TCNQ alone by use of the R values. (30) Sapper, H.; Cameron, D. G.; Mantsch, H. H. Can. J. Chem. 1981, 59, 2543. (31) Terashita, S.; Nakatsu, K.; Ozaki, Y.; Mochida, T.; Araki, T.; Iriyama, K. Langmuir 1992, 8, 3051. (32) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (33) Chollet, P. A.; Messier, J.; Rosilio, C. J. Chem. Phys. 1976, 64, 1042.
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As we mentioned above, the CtN stretching band is overlapped with the CD3 stretching band of stearic acidd35. To observe the CtN stretching band, three-layer mixed LB films of C18TCNQ and stearic acid were prepared at 18 mNm-1 and their infrared spectra were measured. Figure 5A compares the infrared transmission (a) and RA (b) spectra of three-layer mixed LB films of C18TCNQ and stearic acid. Figure 5B shows their enlargement in the 2300-1000 cm-1 region. The intensity of the band at 2222 cm-1 due to the CtN stretching mode is comparable with the transmission and RA spectra. The R value of the band at 2222 cm-1 is 0.79 for the mixed LB films, whereas the R value is about 3.8 for the three-layer LB films of C18TCNQ alone.11 This result demonstrates that the TCNQ plane becomes more parallel to the substrate surface in the mixed LB films. Two bands at 1545 and 1531 cm-1, attributed to CdC stretching modes of the TCNQ chromophore, have their transition moments in different directions in the TCNQ plane.11,34 The R value of the band at 1531 cm-1 is 0.77 for the mixed LB films, whereas it is 1.25 for the LB films of C18TCNQ. This result supports the conclusion above that the TCNQ plane becomes more parallel to the substrate surface. As for the orientation of the alkyl chain of C18TCNQ, it is convenient to examine the R values of the CH2 stretching bands in the mixed LB films of stearic acid-d35 and C18TCNQ. Figure 6 shows infrared transmission (a) and RA (b) spectra in the 3100-2800 cm-1 region of the three-layer mixed LB films deposited at 18 mNm-1. The R values of the CH2 antisymmetric and symmetric stretching bands at 2918 and 2849 cm-1 are 0.56 and 0.80, respectively, for the mixed LB films, whereas these stretchng bands are 0.94 and 0.86 for the LB films of C18TCNQ. The alkyl chain therefore, becomes more perpendicular to the substrate surface in the mixed LB films. Freezing-in Effects by the Fatty Acid Domains. We compared the changes in the molecular orientation and structure of C18TCNQ in the mixed LB films with those of C12TCNQ and C15TCNQ in LB films during the time course. As described in the Introduction, the alkyl chains in the LB films of C15TCNQ and C12TCNQ become more tilted with respect to the surface normal and their TCNQ planes eventually become more perpendicular with respect to the substrate surface after the film deposition.17-19 In the present case of the mixed LB films, the orientations of the alkyl chain and of the TCNQ plane of C18TCNQ change in reverse directions; namely, the alkyl chain becomes more perpendicular, whereas the TCNQ plane becomes more parallel to the substrate surface. The modification of the orientation of C18TCNQ in the mixed LB films implies that C18TCNQ undergoes an inverse timedependent realignment. However, the inverse time course is very unlikely. To investigate the influence of the fatty acid on the structure of CnTCNQ in the mixed LB films, the timedependent infrared spectral changes were studied for a one-layer mixed LB film of C15TCNQ and stearic acid-d35. The time-dependent realignment of C15TCNQ was blocked in the mixed one-layer LB film after the film deposition. Therefore, the domains of the fatty acid probably have a freezing-in effect on the molecular orientation and structure of C15TCNQ molecules in the mixed LB film. A similar freezing-in effect probably exists in the mixed LB films of C18TCNQ and fatty acid. Two possibilities exist for the changes in the molecular orientation and structure of C18TCNQ in the mixed LB (34) Takenaka, T. Spectrochim. Acta, Part A 1971, 27A, 1735.
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Figure 5. (A) A comparison of infrared transmission (a) and RA (b) spectra of three-layer mixed LB films of C18TCNQ and stearic acid. (B) The enlargement in the 2300-1000 cm-1 region of the spectra in panel A.
Figure 6. A comparison of infrared transmission (a) and RA (b) spectra in the 3100-2800 cm-1 region of three-layer mixed LB films of C18TCNQ and stearic acid-d35.
Figure 7. Infrared transmission spectra in the region of 30002000 cm-1 of one-layer mixed LB films of C18TCNQ and stearic acid-d35 deposited at 18 and 35 mNm-1.
films: one is the structural modification of C18TCNQ occurring at the water-air interface, and the other is the structural modification of C18TCNQ induced mainly by the freezing-in effect of fatty acid domains. If the molecular orientation and structure of C18TCNQ in the mixed Langmuir layer were different from those in the Langmuir layer of C18TCNQ alone, it would be very likely that the orientation and structure of C18TCNQ in the mixed LB films are different from those in the LB films of C18TCNQ alone. However, in the present case, the molecular orientation and structure of C18TCNQ may be similar between the Langmuir film of C18TCNQ alone and the mixed Langmuir film because of the phase separation. In that case, the freezing-in effect can account for the structural modification of C18TCNQ in the mixed LB films. Because of the freezing-in effect, the molecular orientation and structure of C18TCNQ in the mixed LB films change little, at least not so much, during and after the film deposition. As for the LB films of C18TCNQ alone, the realignment process should occur during the film deposition, because the alkyl chain becomes more tilted with respect to the surface normal and the TCNQ plane becomes more perpendicular with respect to the substrate surface. The ratio of the partially disordered part of the alkyl chain in the LB films of C15TCNQ alone increases during the time course,18 whereas it decreases in the mixed LB films. Therefore, the changes of the molecular orientation and structure of C18TCNQ in the mixed LB films are all in the opposite direction. These results strongly suggest
that the freezing-in effect probably plays a key role in the structural modification of C18TCNQ. Dependence of the Molecular Structure and Orientation of C18TCNQ in the Mixed LB Films on the Deposition Pressure and the Number of Layers. Figure 7 shows infrared transmission spectra of one-layer mixed LB films of C18TCNQ and stearic acid-d35 deposited at 18 and 35 mNm-1, respectively. The intensities of the CH2 antisymmetric and symmetric stretching bands change with the alteration in the chain packing density and the orientation of the alkyl chain. The packing densities of both C18TCNQ and stearic acid-d35 at 35 mNm-1 are larger than those at 18 mNm-1; therefore, the band intensities should increase upon going from 18 to 35 mNm-1 unless the orientation of the alkyl chain changes significantly in a reverse direction. Inspection of Figure 7 shows that the absorbances of two CD2 stretching bands at 35 mNm-1 are greater than those at 18 mNm-1, probably partly resulting from the higher packing density and the orientation changes. In the LB films of fatty acid alone, the alkyl chain usually becomes more perpendicular to the substrate surface at higher pressure,35 and this kind of orientation change makes the two CD2 stretching bands become stronger at 35 mNm-1. As for the alkyl chain of C18TCNQ, the surface pressure-dependent spectral changes are quite different from those of stearic acid-d35. The intensities of the two CH2 stretching bands at 35 mNm-1 (35) Umemura, J.; Takeda, S.; Hasegawa, T.; Takenaka, T. J. Mol. Struct. 1993, 297, 57.
Molecular Orientation of C18TCNQ and Stearic Acid-d35
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C18TCNQ species, except for the peak intensities, which increase with the number of LB layers. Therefore, the molecular orientation and structure of C18TCNQ in the one- and five-layer mixed LB films may be similar to those in the three-layer LB film discussed above. Conclusions
Figure 8. Infrared transmission spectra of one-, three-, and five-layer mixed LB films of C18TCNQ and stearic acid-d35 deposited at 18 mNm-1.
are almost as equivalent as those at 18 mNm-1. The packing density of C18TCNQ should increase to an extent similar to that of stearic acid-d35; therefore, the intensities of the CH2 stretching bands are expected to increase. Because this is not the case, it is likely that the orientation of the alkyl chain of C18TCNQ changes greatly. In other words, the observation in Figure 7 suggests that the alkyl chain becomes more tilted at 35 mNm-1. Only two cases can induce those changes in the molecular orientation of the alkyl chain: one is the collapse of C18TCNQ layer at the air-water interface, and the other is the squeezing out of the C18TCNQ layer from the fatty acid layer. This conclusion about the collapse of C18TCNQ at 35 mNm-1 agrees with the result of the π-A isotherm. Figure 8 compares the infrared transmission spectra of one-, three-, and five-layer mixed LB films deposited at 18 mNm-1. There is no perceptible spectral change for the
This study has provided new insight into the molecular orientation and structure in the mixed LB films of C18TCNQ and fatty acid. (1) The π-A isotherm of the mixed Langmuir film at the air-water interface suggests that above 22 mNm-1 the C18TCNQ domains start to collapse or the C18TCNQ molecules are squeezed out from the fatty acid monolayer. The infrared transmission spectrum of the mixed LB films deposited at 35 mNm-1 shows that the relative intensity of the CH2 stretching bands of C18TCNQ decreases; this result also supports the conclusion regarding the collapse of C18TCNQ. (2) The π-A isotherm and UV-Vis spectrum of the mixed LB films suggest that the films consist of the segregated domains of C18TCNQ and stearic acid-d35. The splitting of the CH2 scissoring mode in the infrared spectra also supports the conclusion that the phase separation takes place in the mixed LB films. (3) By incorporating fatty acid into the LB films of C18TCNQ, the percentage of the disordered part of the hydrocarbon chain is markedly reduced. (4) In comparison with the orientation in the LB films of C18TCNQ alone, the alkyl chain becomes more perpendicular whereas the TCNQ plane becomes more parallel to the substrate surface in the mixed LB films. (5) Both the conformation and orientation changes of C18TCNQ are probably caused by the freezing-in effect of the domains of fatty acid. It is very likely that the realignment of C18TCNQ molecules in the mixed LB films is blocked by the effect during the film deposition. LA991643Z
