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Novel Method for Preparing a Mixed-Stack Charge-Transfer Film of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and 3,3′,5,5′-Tetramethylbenzidine: The Use of a Mixed Langmuir-Blodgett Film of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and Stearic Acid as Starting Material for the Fabrication of the Charge-Transfer Film Hai-Shui Wang§ and Yukihiro Ozaki* Department of Chemistry, School of Science and Technology Kwansei Gakuin University, Gakuen, Sanda 669-1337, Uegahara, Japan Received July 8, 2002. In Final Form: September 18, 2002 The present paper reports a novel method for preparing a mixed-stack charge-transfer (CT) film having 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C18TCNQ) as an electron acceptor and the possibility of the control of molecular orientation in the CT film. A seven-layer mixed Langmuir-Blodgett (LB) film of C18TCNQ and stearic acid is deposited onto a CaF2 plate or a gold-evaporated glass slide at the surface pressure of 15 mN m-1. Dipping the mixed LB film into a petroleum ether solution of 3,3′,5,5′tetramethylbenzidine (TMB) results in the formation of a mixed-stack CT film of TMB‚C18TCNQ. Upon the formation of the CT film, the dilution molecules, namely stearic acid, are removed completely from the film. The appearance of a broad absorption band in the near-infrared region and the frequency shift of the CN stretching band (2208 cm-1) verify that the CT complex film of TMB‚C18TCNQ is formed by the TMB doping. IR spectra of the CT films do not show any bands assignable to stearic acid, suggesting that stearic acid molecules are removed from the thin films after the dipping. The band at 3496 cm-1 due to the antisymmetric stretching mode of the NH2 group does not appear in the IR transmission spectrum. Thus, it seems that the chromophore plane of TMB component is nearly perpendicular to the substrate surface with its long axis lying on the substrate in the CT films. The molecular structure and orientation in the CT films of TMB‚C18TCNQ prepared by the present method and those fabricated by dipping the LB films of C18TCNQ alone into the TMB solution have been compared. The comparison shows that the orientation of the chromophore planes of both donor and acceptor components and that of the alkyl chain of C18TCNQ in the CT films are controlled mainly by the doping process.
Introduction Thin films of the mixed-stack charge-transfer (CT) complex of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C18TCNQ) and 3,3′,5,5′-tetramethylbenzidine (TMB) have been prepared in our group by Langmuir-Blodgett (LB) and donor doping techniques.1-3 Namely, by dipping a LB film of C18TCNQ into a petroleum ether solution of TMB, we obtained the mixed-stack CT film (LB-doping film) of TMB‚C18TCNQ.1-3 The structure, morphology, and thermal behavior of the above CT films have been studied in considerable detail by use of infrared (IR) and ultravioletvisible-near-infrared (UV-Vis-NIR) spectroscopies and atomic force microscopy (AFM).1-4 The following conclusions could be reached from our previous investigations. (1) The chromophore planes of both TMB and C18TCNQ * Author to whom correspondence should be addressed. Tel: +81-79-565-8349. Fax: +81-79-565-9077. E-mail, ozaki@ kwansei.ac.jp. § Present address: Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun 130022, P. R. China. (1) Wang, Y.; Nichogi, K.; Iriyama, K.; Ozaki, Y. J. Phys. Chem. B 1996, 100, 17232. (2) Wang, Y.; Nichogi, K.; Iriyama, K.; Ozaki, Y. J. Phys. Chem. B 1996, 100, 17238. (3) Wang, Y.; Nichogi, K.; Iriyama, K.; Ozaki, Y. Mol. Cryst. Liq. Cryst. 1997, 294, 177. (4) Nichogi, K.; Taomoto, A.; Nambu, T.; Murakami, M. Thin Solid Films 1995, 254, 240.
components in the CT films are nearly perpendicular to the substrate surface with their long axes lying on the substrate.3 (2) AFM images of the CT films show that the films consist of numerous one-dimensional needlelike microcrystals and that a periodic structure exists inside these microcrystals.2 (3) Degree of charge transfer is estimated to be 0.4 from a downward shift of a CtN stretching band of the C18TCNQ component.1 Our previous studies have provided extensive insight into the molecular orientation and structure of the CT films.1-3,5 However, the influence of the orientation of C18TCNQ in a LB film before doping on the molecular orientation and structure in the CT films is still not clear. It is very important to obtain this sort of information in order to know how to control and modify the structure of the CT complex. The purposes of the present study are to propose a novel method for preparing mixed-stack CT films with C18TCNQ as an electron acceptor and to investigate the possibility of the control of molecular orientation in the CT films. We have employed a mixed LB film of C18TCNQ and stearic acid as starting material instead of a LB film of C18TCNQ alone. By incorporating stearic acid into a LB film of C18TCNQ, the molecular orientation and structure of C18TCNQ have (5) Wang, Y.; Nichogi, K.; Iriyama, K.; Ozaki, Y. J. Phys. Chem. B 1997, 101, 6367.
10.1021/la0206099 CCC: $22.00 © 2002 American Chemical Society Published on Web 11/21/2002
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Figure 2. UV-Vis-NIR absorption spectra of a seven-layer mixed-LB-doping film and a mixed-cast-doping film of TMB‚ C18TCNQ. Figure 1. Structures of (a) 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C18TCNQ), (b) 3,3′,5,5′-tetramethylbenzidine (TMB), (c) N,N,N′,N′-tetramethylbenzidine (N-TMB).
successfully been controlled and modified.6 For example, in contrast to a LB film of C18TCNQ alone, the alkyl chain becomes more perpendicular while the TCNQ plane becomes more parallel to the substrate surface in the mixed LB film of C18TCNQ and stearic acid.6 Therefore, it is possible to control the molecular orientation of C18TCNQ in the mixed LB films. If the doping of donor molecules to the mixed LB film is possible, we may be able to investigate the influence of the molecular orientation of C18TCNQ in the film before doping on the structure of the CT complex. The dilution molecules, such as stearic acid, have a function of aiding dye molecules such as C18TCNQ to form a high-quality LB film having the desired orientation. However, in the most cases, the physical properties of the functional dye, such as electrical conductivity, become poor in the mixed LB film because of the coexistence of the dilution molecules. The properties of the film of the functional dye can be improved by removing dilution molecules from the mixed LB film and adding acceptor/donor molecules to form CT complexes.7 Therefore, it is very important to select suitable dilution molecules and to remove them after the preparation of the mixed LB film. The present study aims at preparing the CT films of C18TCNQ and TMB from the mixed LB films of C18TCNQ and stearic acid by doping the donor molecules and removing the dilution molecules simultaneously. The molecular structure and orientation of the CT films are investigated by UV-Vis-NIR and IR spectroscopies. The fabrication of the CT films (mixed-LB-doping films) from the mixed LB films may provide a new way of controlling the molecular orientation in the CT films. Experimental Section Sample Preparation. C18TCNQ (Figure 1a) was purchased from Hayashibara Biochemical laboratory, Inc., Kankoh-Shikiso Institute (Japan), and used without further purification. The thin-layer chromatographic examinations revealed that the dye did not contain any other colored components. TMB (Figure 1b) and stearic acid were purchased from Sigma Co., and used without further purification. N,N,N′,N′-tetramethylbenzidine (N-TMB, Figure 1c) and petroleum ether were purchased from Wako Pure Chemical Industries, Ltd., and N-TMB was further purified by re-crystallization from ethanol. (6) Wang, H.; Ozaki, Y.; Iriyama, K. Langmuir, 2000, 16, 5142. (7) Roberts, G. G., Ed. Langmuir-Blodgett Films; Plenum Press: New York and London, 1990.
The Y-type mixed LB films of C18TCNQ and stearic acid were fabricated by use of a Kyowa Kaimen Kagaku model HBM-AP Langmuir trough with a Wilhelmy balance. C18TCNQ (2.5 × 10-4 M) and stearic acid (7.5 × 10-4 M) were dissolved in spectral grade chloroform (Dojin Chemical Co.) with the molar ratio of 1:3.6 The solution was spread onto a pure aqueous subphase that was prepared by the method reported previously.8 After evaporation of the solvent, a mixed Langmuir film was compressed up to the surface pressure of 15 mN m-1 at a constant rate of 20 cm2 min-1. The film was transferred by the vertical dipping method onto CaF2 plates (UV-Vis-NIR and IR transmission measurements) and onto gold-evaporated glass slides (IR RA measurements). For both the upstroke and downstroke process, the deposition rates were 10 mm min-1. After the deposition of the first layer, the substrate was kept in the air for about 1 h. Subsequent depositions were repeated with the dry time of about 20 min or more in the air. The transfer ratio was found to be nearly unity throughout the experiments. A detailed procedure for cleaning the CaF2 plates and gold-evaporated glass slides was described previously.8 Our previous study showed that the phase separation occurs in the mixed LB films of C18TCNQ and stearic acid.6 Donor doping of the mixed LB films was made by dipping the films into a petroleum ether solution of TMB (N-TMB) for 5 min or more to ensure its complete reaction.1,4 The yellow color of the mixed LB films changed into purple-red immediately after dipping them into the TMB solution. A CT film of TMB‚C18TCNQ (mixed-castdoping film) was prepared by dipping the cast film C18TCNQ and stearic acid into a TMB petroleum ether solution. Spectroscopy. IR spectra of the LB and cast films were measured at a 4 cm-1 resolution with a Nicolet Magna 760 FT-IR spectrometer equipped with a MCT detector. For the IR reflection-absorption (RA) measurements, a reflection attachment (Spectra-Tech, FT80 RAS) was employed at the incident angle of 80°. To yield spectra of high signal-to-noise ratio, 100 (cast film) or 1024 (LB film) interferograms were co-added. UV-VisNIR spectra of the LB and cast films were measured with a Shimadzu UV-visible 3101 PC spectrophotometer.
Results and Discussion UV-Vis-NIR Spectra. Figure 2 shows UV-Vis-NIR absorption spectra of a seven-layer mixed-LB-doping film of TMB‚C18TCNQ prepared from a seven-layer mixed LB film of C18TCNQ and stearic acid and a mixed-cast-doping film prepared from a mixed cast film of C18TCNQ and stearic acid. According to different stacking styles of donor and acceptor molecules, the CT complexes are classified into two types, namely, segregated-stack and mixed-stack forms.1,9,10 The mixed-stack CT complexes with TCNQ derivatives usually give a broad CT band in the NIR region (8) Myrzakozha, D. A.; Hasegawa, T.; Nishijo, J.; Imae, T.; Ozaki, Y. Langmuir 1999, 15, 6890. (9) Iwasa, Y.; Koda, T.; Koshihara, S.; Tokura, Y.; Iwasawa, N.; Saito, G. Phys. Rev. B 1989, 39, 10441.
Preparing a Mixed-Stack CT Film of TMB‚C18TCNQ
Figure 3. IR RA spectra of a seven-layer mixed LB film of C18TCNQ and stearic acid (a) and the corresponding mixedLB-doping film of TMB‚C18TCNQ (b).
while the segregated-stack ones have a broad CT band in the IR region.1,5,11,12 A broad feature centered about 1536 nm in Figure 2 shows that the CT complexes are formed in both films and that they are in the mixed-stack forms. The present results are in a good agreement with the results in our previous study,1 in which the mixed-stack CT film was formed by dipping the LB film of C18TCNQ alone into the TMB solution. IR Spectra. Figure 3 shows IR RA spectra of a sevenlayer mixed LB film of C18TCNQ and stearic acid (a) and the corresponding seven-layer mixed-LB-doping film of the CT complex (b), respectively. A comparison between the IR spectrum of the mixed LB film and that of the CT film (Figure 3a and 3b) reveals that some new bands, such as those at 3496 and 3402 cm-1, appear in the spectrum of the CT film. The bands at 3496 and 3402 cm-1 are assigned to the NH2 antisymmetric and symmetric stretching modes of the TMB component of the CT complex, respectively.3 The appearances of the two bands at 3496 and 3402 cm-1 reveal that the donor (TMB) has been introduced into the thin film by the dipping process. Bands due to the NH2 antisymmetric and symmetric stretching modes of neutral TMB appear at 3404 and 3325 cm-1, respectively.3,4 The large frequency shifts of the NH2 stretching bands between the TMB component in the CT complex and neutral TMB may be caused by the change in the force constant of the NH groups. In the X-H stretching vibrations, the force constant tends to increase as the electronegativity of X increases.13 In the present case, the TMB component is an electron donor, and thus, the TMB chromophore should have partial positive charge in the complex of TMB‚C18TCNQ. Therefore, the electronegativity of the N atoms increases in the CT complex, leading to the higher frequency shifts of both NH2 stretching bands. Another notable difference between Figure 3a and 3b is complete disappearance of bands due to stearic acid in Figure 3b. Figure 4 shows the enlargement of the spectra in Figure 3 in the 1800-1000 cm-1 region. Note that not only the CdO stretching band at 1703 cm-1 but also the band progression due to the CH2 wagging mode of stearic (10) Williams, J. M.; Ferraro, J. R.; Thorn, R. J.; Carson, K. D.; Geiser, U.; Wang, H. H.; Kini, A. M.; Whangbo, M. Organic Superconductors; Prentice Hall: Englewood Cliffs, NJ, 1992. (11) Meneghetti, M.; Girlando, A.; Pecile, C. J. Chem. Phys. 1985, 83, 3134. (12) Nakamura, T.; Yunome, G.; Azumi, R.; Tanaka, M.; Tachibana, H.; Matsumoto, M. J. Phys. Chem. 1996, 98, 1882. (13) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: Boston, 1990.
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Figure 4. The enlargement of Figure 3 in the 1800-1000 cm-1 region.
Figure 5. IR transmission spectra of a seven-layer mixed LB film of C18TCNQ and stearic acid (a) and the corresponding mixed-LB-doping film of TMB‚C18TCNQ (b).
acid do not appear in the IR spectrum of the CT complex. These observations reveal that stearic acid molecules are removed from the mixed LB film on the gold-evaporated glass slide. Figure 5 shows IR transmission spectra of a seven-layer mixed LB film (a) and the corresponding seven-layer mixed-LB-doping film of the CT complex (b), respectively. Again, the band due to the CdO stretching mode disappears in the spectrum of the CT film. It is very likely that stearic acid molecules are removed completely also from the CaF2 plate. A CtN stretching band of the C18TCNQ component shifts to a lower frequency in IR spectra of the CT complexes with a TCNQ chromophore.1,3,5,14 This band appears at 2222 cm-1 in the spectra of the mixed LB films (Figures 3a and 5a), while it is observed at 2208 cm-1 in the spectra of the CT films (Figures 3b and 5b). This is additional evidence for the formation of the CT complex. We will discuss the degree of charge-transfer later on the basis of the shift of the CtN stretching band. In Figure 5b, bands at 2918 and 2850 cm-1 are due to the CH2 antisymmetric and symmetric stretching modes of the alkyl chain of the C18TCNQ component, respectively. It is well-known that the frequencies of the CH2 stretching bands are sensitive to the conformation of an alkyl chain.17,18 The frequencies of the CH2 stretching bands (14) Chappell, J. S.; Bloch, A. N.; Bryden, W. A.; Maxfield, M.; Poehler, T. O.; Cowan, D. O. J. Am. Chem. Soc. 1981, 103, 2442. (15) Roeges, Noe¨l P. G. A Guide to the Complete Interpretation of Infrared Spectra of Organic Structure; John Wiley & Sons: Chichester, 1994. (16) Iwasa, Y.; Koda, T.; Tokura, Y.; Kobayashi, A.; Iwasawa, N.; Saito, G. Phys. Rev. B 1990, 42, 2374.
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Figure 6. IR transmission spectra of a mixed cast film of C18 TCNQ and stearic acid (a) and the corresponding mixed-castdoping film of TMB‚C18TCNQ (b).
are significantly lower in the spectrum of Figure 5b than that of Figure 3b, suggesting that the alkyl chains in the film on the CaF2 are more ordered (trans zigzag) than those in the film on the Au-evaporated glass slide. One- and three-layer mixed-LB-doping films and a mixed-cast-doping film were also prepared by the same method. Figure 6 shows IR transmission spectra of a mixed cast film of C18TCNQ and stearic acid (a) and the corresponding mixed-cast-doping film of the CT complex (b), respectively. The IR spectra of the one- and threelayer mixed-LB-doping films and the mixed-cast-doping film are very close to those of the seven-layer mixed-LBdoping films in terms of both the frequencies and relative intensities. Our new method for preparing the CT films provides a novel way for fabricating various LB-doping CT films. For example, a LB-doping film of the CT complex of TMB‚ TCNQ (TCNQ itself without any alkyl tail) may be obtained by this method. A mixed LB film of TCNQ and stearic acid can be prepared by LB technique even though TCNQ itself cannot form a well-defined Langmuir film at the air-water interface. Thus, by a dipping process described above, one can obtain the LB-doping film. It was reported that the CT complex of TMB‚TCNQ shows a temperature-induced neutral-ionic transition and a stronger nonlinear electrical conduction than the CT complex of TMB‚C18TCNQ.4,16 Therefore, it is expected that the LB-doping film of the CT complex of TMB‚TCNQ may also show a variety of interesting physical properties. Degree of Charge Transfer. The degree of charge transfer (F) can be estimated from the frequency shift of the CtN stretching band.1,3,5,14 The frequencies of the Ct N stretching band of C18TCNQ and C18TCNQ-1 are 2222 and 2184 cm-1, respectively.1,5 In the present case it is observed at 2208 cm-1 for the CT film. Thus, the F value is calculated to be about 0.4. This value is the same as that for the CT film of TMB‚C18TCNQ prepared by dipping the LB film of C18TCNQ alone into the TMB solution.1 Molecular Orientation and Structure in the CT Films. IR spectroscopy is a powerful tool to explore molecular structure and orientation in thin films.7,19,20 The orientation of donor and acceptor components in the CT films of TMB‚C18TCNQ can be investigated by the (17) Takenaka, T.; Umemura, J. In Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: Amsterdam, 1991; Vol. 19, p 215. (18) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (19) Umemura, J.; Cameron, D. G.; Mantsch, H. H. Biochim. Biophys. Acta 1980, 602, 32. (20) Sapper, H.; Cameron, D. G.; Mantsch, H. H. Can. J. Chem. 1981, 59, 9, 2543.
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intensities of the bands due to the two NH2 stretching modes and that due to the CtN stretching mode, respectively. The directions of the transition moments of the NH2 antisymmetric and symmetric stretching modes are perpendicular to each other and the direction of the transition moment of the NH2 antisymmetric stretching mode is perpendicular to the long axis of TMB chromophore.3 In the IR transmission spectrum of the mixedLB-doping CT film (Figure 5b), the band at 3496 cm-1 due to the NH2 antisymmetric stretching mode is almost missing. This suggests that the direction of the transition moment of the NH2 antisymmetric stretching mode is almost perpendicular to the substrate surface and also suggests that the long axis of TMB chromophore should be parallel to the substrate surface because of the perpendicular relation between the transition moment of the NH2 antisymmetric stretching and the long axis of TMB chromophore. Therefore, it seems that the chromophore plane of the TMB component is nearly perpendicular to the substrate surface with its long axis lying on the substrate (parallel to the substrate surface) in the mixed-LB-doping CT film as in the case of the LB-doping CT film prepared from a LB film of C18TCNQ alone.3-5 The NH2 symmetric stretching band shows a strong absorption in Figure 5b, giving additional evidence for the above conclusion. According to the requirement of the formation of mixedstack CT complexes, donor and acceptor molecules should be packed in a face-to-face pattern.4,5,10,16 Therefore, the chromophore plane of C18TCNQ component in the mixedLB-doping films should be nearly perpendicular to the substrate surface. The relative orientations of the donor and acceptor molecules can be investigated by the relative intensity ratio of the band at 2208 cm-1 due to the CtN stretching mode of C18TCNQ to the band at 3402 cm-1 due to the NH2 symmetric stretching mode of TMB. In the transmission spectrum of the mixed-LB-doping film (Figure 5b), the intensity ratio of the CtN stretching band to the NH2 symmetric stretching band is about 0.43, which is almost identical with the corresponding ratio of the LB-doping CT films (∼0.43).1-3 Therefore, it is very likely that the planes of the donor and acceptor molecules are both perpendicular to the substrate surface with their long axes lying in parallel with the substrate surface in the mixed-LB-doping film. The molecular orientation in the chromophore planes of the donor and acceptor is very close between the mixedLB-doping film and the LB-doping film. However, marked differences are observed in the orientation and structure of the alkyl chain of C18TCNQ between the CT films prepared by the two methods. The intensity ratios of either of the two CH2 stretching bands at 2918 and 2850 cm-1 to the band at 3402 cm-1 in the transmission spectra are much larger for the CT film prepared by the present method than for that fabricated by the previous method.1-3 In contrast, those in the RA spectra are larger for the CT film prepared by the previous method than that fabricated by the present method. These observations suggest that the alkyl chain is more perpendicular to the substrate surface in the present CT film than in the previous one. Thus, the orientation of the alkyl chain is controlled by the methods of preparing the films. The conformation of the alkyl chain of C18TCNQ also changes a little by the fabrication methods. It is wellknown that the frequencies of the CH2 stretching bands are sensitive to the conformation of an alkyl chain.19,20 It is noted that the frequencies of the two CH2 stretching bands in the transmission spectra are lower for the mixedLB-doping film prepared by the present method (2918
Preparing a Mixed-Stack CT Film of TMB‚C18TCNQ
and 2850 cm-1, Figure 5b) than in the corresponding film prepared by the previous method (2922 and 2852 cm-1),1-3 suggesting that the alkyl chain is more ordered in the CT film prepared by the present method. Relation between the Molecular Orientation of C18TCNQ before Doping and the Structure of the CT Complex. The orientation of C18TCNQ component in the LB-doping CT film of TMB‚C18TCNQ is quite different from that in the LB film of C18TCNQ alone before doping.5 For example, both the C18TCNQ and TMB chromophore planes in the CT films are almost perpendicular to the substrate surface with their long axes lying on the substrate surface, while the long axis of the chromophore plane of C18TCNQ in the LB film before doping is neither parallel nor perpendicular to the substrate surface.5 Therefore, the orientation of C18TCNQ changes significantly during donor doping. The change of the orientation probably does not depend on the molecular orientation of C18TCNQ before doping but depends on the formation process of the CT complex, e.g., the growth process of microcrystals. The molecular orientation and structure of C18TCNQ in the mixed LB film of C18TCNQ and stearic acid are quite different from those in the LB film of C18TCNQ alone.6 For example, in contrast to the LB film of C18TCNQ alone, the alkyl chain becomes more perpendicular and the TCNQ plane becomes more parallel to the substrate surface in the mixed LB film of C18TCNQ and stearic acid.6 However, the orientations of the chromophore planes of both TMB and C18TCNQ are very similar to each other between the mixed-LB-doping CT films and the LB-doping CT films. In other words, the molecular orientation of C18TCNQ before doping has little influence on the final orientation of the planes of C18TCNQ and TMB components in the CT films, although it affects significantly the orientation and structure of the alkyl chain of C18TCNQ. Therefore, it is very likely that the orientation of the chromophore planes in the CT films is controlled by the doping process (the formation process of the CT complex). One may be able to modify the molecular orientation and structure of the CT films by adjusting the concentration of TMB in the solution, the temperature of the solution, and the nature of the substrate. The doping reaction takes place in a liquid phase, so that the solvent molecules may have some effects on the final orientation of C18TCNQ and TMB components in the CT films. The Doping of N-TMB. In our previous paper,21 we reported the fabrication of LB films of the CT complex of N-TMB‚C18TCNQ by the LB technique. In the present study, we have found that one can prepare the CT film by dipping a mixed LB film of C18TCNQ and stearic acid into a petroleum ether solution of N-TMB. Figure 7 shows a UV-Vis-NIR absorption spectrum of a seven-layer mixed(21) Wang, H.; Ozaki, Y. Langmuir 2000, 16, 7070.
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Figure 7. UV-Vis-NIR spectrum of a seven-layer mixedLB-doping film of N-TMB‚C18TCNQ.
LB-doping film of N-TMB‚C18TCNQ. A broad band appears in the NIR region, indicating that the CT complex of N-TMB‚C18TCNQ is formed in the thin film. An IR spectrum of the above CT film demonstrates that stearic acid molecules are removed from the thin film upon the formation of the CT film. Conclusions The present study has shown that by dipping a mixed LB film of C18TCNQ and stearic acid into a petroleum ether solution of TMB, the mixed-LB-doping film can be obtained. This procedure provides a new method to prepare CT films. For example, in the case of an evaporated film of TCNQ, the doping of TMB hardly proceeds, probably because of no space permission for the entering of donor molecules.4 In the case of the mixed films of functional dye and stearic acid, we may make the desired space inside the films by removing the stearic acid molecules. Therefore, the donor (acceptor) molecules could diffuse into the whole films to form the CT films. In the mixed-LB-doping CT films, the chromophore planes of C18TCNQ and TMB components are nearly perpendicular to the substrate surface with the long axes lying in parallel with the substrate surface in the CT films. These results are similar to those of the CT films obtained by dipping the LB film of C18TCNQ alone into TMB solution. Therefore, the final orientation of the chromophore planes of donor and acceptor components is controlled mainly by the doping process. The orientation and structure of the alkyl chain of C18TCNQ are different between the CT films prepared by the two kinds of fabrication methods. For example, the alkyl chain is more perpendicular to the substrate surface in the mixed-LBdoping CT film than in the LB-doping CT film prepared previously. LA0206099
