Preparation and Structural Characterization of Mixed-Stack Charge

Jul 18, 2000 - ... Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and N,N,N' ... P. Lozano, A. J. Fernández, J. J. Ruiz, L. Camacho, M. T. Mart...
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Preparation and Structural Characterization of Mixed-Stack Charge-Transfer Langmuir-Blodgett Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and N,N,N′,N′-Tetramethylbenzidine Hai-Shui Wang and Yukihiro Ozaki* Department of Chemistry, School of Science, Kwansei Gakuin University, Uegahara, Nishinomiya 662-8501, Japan Received April 4, 2000. In Final Form: May 30, 2000 One-, three-, and seven-layer Langmuir-Blodgett (LB) films of a mixed-stack charge-transfer (CT) complex of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C18TCNQ) and N,N,N′,N′-tetramethylbenzidine (N-TMB) were successfully deposited onto CaF2 plates and gold-evaporated glass slides at the surface pressure of 10 mN m-1. This is the first time that the mixed-stack CT films of N-TMB‚C18TCNQ have been prepared by the LB technique. Ultraviolet-visible-near-infrared (UV-vis-NIR) and infrared spectroscopies have been employed to investigate the formation, degree of charge transfer, molecular orientation, and structure of the CT complex in the LB films. The appearance of a broad absorption band with its center at about 1750 nm reveals that the CT complex in the LB films is in the mixed-stack form. The frequency of a CtN stretching band of C18TCNQ in the CT complex indicates that the degree of charge transfer (F) is about 0.24. A comparison of the infrared band intensities between the transmission and RA spectra suggests that the chromophore planes for both C18TCNQ and N-TMB are nearly parallel to the substrate surface in the CT LB films. This result is in a good agreement with the requirement for the formation of the mixed-stack complex in which the donor and acceptor components should be in a face-to-face contact with each other. The hydrocarbon chain of C18TCNQ is considerably tilted with respect to the substrate normal in the CT LB films. The CT film (LB-doping film) of N-TMB‚C18TCNQ was also prepared by LBand donor-doping techniques. It was found that N-TMB‚C18TCNQ molecules are partly dissolved in the petroleum ether solution during the doping procedure.

Introduction Charge-transfer (CT) complexes having tetracyanoquinodimethane (TCNQ) derivatives as electron acceptors are very attractive materials because they show novel and fascinating properties such as electrical conductivity, photoconductivity, nonlinear electrical conductivity, and neutral-ionic phase transition phenomena.1-12 A variety of structural factors and physical properties of the compounds, such as polarizability of the components, donor ionization potential, acceptor electron affinity, the size and shape of the donor and acceptor molecules, crystal structure, degree of charge transfer, donor-acceptor distance, and donor-acceptor relative orientation, are * To whom correspondence should be addressed. Fax: +81-79851-0914. Tel.: +81-798-54-6380. E-mail: [email protected]. (1) Torrance, J. B. Molecular Metals; Plenum Press: New York, 1979. (2) Torrance, J. B.; Vazquez, J. E.; Mayerle, J. J.; Lee, V. Y. Phys. Rev. Lett. 1981, 46, 253. (3) Torrance, J. B.; Girlando, A.; Mayerle, J. J.; Crowley, J. I.; Lee, V. Y.; Balail, P.; Laplaca, S. J. Phys. Rev. Lett. 1981, 47, 1747. (4) Jerome, D.; Schultz, H. J. Adv. Phys. 1982, 31, 299. (5) Jerome, D.; Schultz, H. J. Mol. Cryst. Liq. Cryst. 1985, 117, 121. (6) Ruaudel-Teixier, A.; Vandevyver, M.; Barraud, A. Mol. Cryst. Liq. Cryst. 1985, 120, 319. (7) Barraud, A.; Lesieur, P.; Ruaudel-Teixier, A.; Vandevyver, M. Thin Solid Films 1985, 134, 195. (8) Meneghetti, M.; Girlando, A.; Pecile, C. J. Chem Phys. 1985, 83, 3134. (9) Iwasa, Y.; Koda, T.;. Tokura, Y.; Kobayashi, A.; Iwasawa, N.; Saito, G. Phys. Rev. B 1990, 42, 2374. (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) Iwasa, Y.; Watanabe, N.; Koda, T.; Saito, G. Phys. Rev. B 1993, 47, 2920. (12) Nakamura, T. In Handbook of Organic Conductive Molecules and Polymers; Nalwa, H. S., Ed.; John Wiley & Sons: Chichester, 1997; Vol. 1, p 727.

closely related to the physicochemical properties of the CT complexes.10,12-14 To design and construct the CT complexes with the desirable properties, it is very important to characterize their structure and further to understand the structure-function relationship. We have been investigating the structure of thin films of the mixed-stack CT complexes consisting of 2-octadecyl7,7,8,8- tetracyanoquinodimethane (C18TCNQ; Figure 1a) and donor molecules such as 5,10-dimethyl-5,10-dihydrophenazine ((Me)2P) and 3,3′,5,5′-tetramethylbenzidine (TMB) by use of ultraviolet-visible-near-infrared (UVvis-NIR) and infrared spectroscopies and atomic force microscopy (AFM).15-20 Thin CT films of TMB‚C18TCNQ and (Me)2P‚C18TCNQ were prepared by LangmuirBlodgett (LB) and donor-doping techniques.15,17 For example, dipping an LB film of C18TCNQ into a petroleum ether solution of TMB results in the formation of a CT LB-doping film of TMB‚C18TCNQ.15 Our studies have provided the following conclusions as to the molecular orientation and structure of the CT LB-doping films. (1) In the CT LB-doping films, the chromophore planes of (13) Andre, J. J.; Bieber, A.; Gautier, F. Ann. Phys. 1976, 1, 145. (14) Ahuja, R. C.; Matsumoto, M.; Mobius, D. J. Phys. Chem. 1992, 96, 1855. (15) Wang, Y.; Nichogi, K.; Iriyama, K.; Ozaki, Y. J. Phys. Chem. B 1996, 100, 17232. (16) Wang, Y.; Nichogi, K.; Iriyama, K.; Ozaki, Y. J. Phys. Chem. B 1996, 100, 17238. (17) Wang, Y.; Nichogi, K.; Iriyama, K.; Ozaki, Y. J. Phys. Chem. B 1997, 101, 6367. (18) Wang, Y.; Nichogi, K.; Iriyama, K.; Ozaki, Y. J. Phys. Chem. B 1997, 101, 6372. (19) Wang, Y.; Nichogi, K.; Iriyama, K.; Ozaki, Y. J. Phys. Chem. B 1997, 101, 6379. (20) Wang, Y.; Nichogi, K.; Iriyama, K.; Ozaki, Y. Mol. Cryst. Liq. Cryst. 1997, 294, 177.

10.1021/la000504a CCC: $19.00 © 2000 American Chemical Society Published on Web 07/18/2000

Mixed-Stack CT LB Films of C18TCNQ and N-TMB

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Figure 1. Structures of (a) 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C18TCNQ) and (b) N,N,N′,N′-tetramethylbenzidine (N-TMB).

both TMB and C18TCNQ are nearly perpendicular to the substrate surface with their long axes lying on the substrate.20 The hydrocarbon chain of C18TCNQ is nearly parallel to the substrate surface.20 (2) The chromophore planes of C18TCNQ and (Me)2P components in the CT LBdoping films, which were prepared by incorporating (Me)2P into the LB films of C18TCNQ in a vapor phase, are almost perpendicular to the substrate surface with their long molecular axes being nearly perpendicular to the substrate surface.17 The hydrocarbon chain of C18TCNQ is nearly parallel to the substrate surface.17 (3) The degree of charge transfer is about 0.4 for the LB-doping films of TMB‚ C18TCNQ and about 0.5 for the LB-doping films of (Me)2P‚ C18TCNQ.15,17 The above results show that the molecular orientation and structure of the CT complexes strongly depend on the doping procedure and/or the species of the donor molecules. The LB technique, which offers the unique advantage of experimental control for molecular organization parameters, enables us to control the molecular orientation and structure of the CT complexes to some extent.6,7,14,21 To the best of our knowledge, the preparation of the LB films of the mixed-stack CT complex of C18TCNQ and N,N,N′,N′- tetramethylbenzidine (N-TMB; Figure 1b) has not been reported yet. Therefore, the purpose of the present study is to prepare the mixed-stack CT films by use of the LB technique and to aim at the control of the molecular orientation and structure of the CT complex in the LB films. By selecting a suitable donor molecule (N-TMB) and fabricating thin mixed-stack CT films by the LB technique, it becomes possible to modify and control the molecular orientation and structure of the CT complexes. We have employed UV-vis-NIR and infrared spectroscopies to investigate the molecular orientation and structure of LB films of the CT complex. The preparation of the CT LB-doping film of N-TMB‚C18TCNQ, obtained by dipping an LB film of C18TCNQ into a petroleum ether solution of N-TMB, is also discussed. Experimental Section Sample Preparation. C18TCNQ 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. N-TMB was purchased from Wako Pure Chemical Industries, Ltd., and was purified by re-crystallization from ethanol. The Y-type LB films of the CT complex of N-TMB‚C18TCNQ were fabricated by use of a Kyowa Kaimen Kagaku Model HBMAP Langmuir trough with a Wilhelmy balance. C18TCNQ (5.0 × 10-4 M) and N-TMB (5.0 × 10-4 M) were dissolved in spectral(21) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991.

Figure 2. π-A isotherm of the Langmuir film of C18TCNQ and N-TMB on a pure water surface at 20 °C. grade chloroform (Dojin Chemical Co.) with the molar ratio of 1:1. The solution was spread onto a pure aqueous subphase, which was prepared by the method reported previously.22 After the evaporation of the solvent, the Langmuir film was compressed up to the surface pressure of 10 mN m-1 at a constant rate of 3.2 Å2/(molecule‚min-1). The relaxation time at 10 mN m-1 was about 10 min before the deposition and the area of the film at 10 mN m-1 deceased at a relaxation rate of 0.3%/min. The CT Langmuir film was transferred by the vertical dipping method onto CaF2 plates (UV-vis-NIR and infrared transmission measurements) and onto gold-evaporated glass slides (infrared reflectionabsorption (RA) measurements) at the pressure of 10 mN m-1. 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 2 h. Succeeding depositions were repeated with the dry time of about 15 min or more in the air and with the pause time of about 2 min under water. The transfer ratio was found to be nearly unity throughout the experiments. A detailed procedure for cleaning the CaF2 plates and the gold-evaporated glass slides was described previously.22 Spectroscopy. Infrared spectra of cast and LB films of the CT complex of N-TMB‚C18TCNQ were measured at a 4 cm-1 resolution with a Nicolet Magna 760 FT-IR spectrometer equipped with an MCT detector. For the infrared RA measurements, a reflection attachment (Spectra-Tech, FT80 RAS) was employed at the incident angle of 80°. To yield spectra of a high signalto-noise ratio, 512 or 1024 interferograms were co-added. UVvis-NIR spectra of the cast and LB films were measured with a Shimadzu UV-visible 3101 PC spectrophotometer.

Results and Discussion Surface Pressure-Area (π-A) Isotherm. The π-A isotherm of N-TMB‚C18TCNQ is shown in Figure 2. The behavior of the π-A isotherm of the Langmuir film of N-TMB‚C18TCNQ is quite different from that of C 18TCNQ alone.23 For example, the compressibility in the pressure region of 0-10 mN/m is much lower in the Langmuir film of N-TMB‚C18TCNQ than in the corresponding film of C18TCNQ alone.23 Namely, the surface pressure increases much faster in the Langmuir film of N-TMB‚C18TCNQ than in that of C18TCNQ as the surface area is reduced at the same compression rate. The area per N-TMB‚ C18TCNQ complex is about 64 Å2 at the pressure of 10 mN/m. The area of 64 Å2 is much larger than the area of about 23 Å2, which is the area per C18TCNQ molecule for the Langmuir film of C18TCNQ alone at the pressure of 10 mN/m.23 When the Langmuir film of N-TMB‚C18TCNQ was overcompressed and the collapsed layer became thick (22) Myrzakozha, D. A.; Hasegawa, T.; Nishijo, J.; Imae, T.; Ozaki, Y. Langmuir 1999, 15, 6890. (23) Nichogi, K.; Nambu, T.; Miyamoto, A.; Murakami, M. Jpn. J. Appl. Phys. 1995, 34, 4956.

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Figure 4. Infrared transmission spectrum of a seven-layer LB film of the charge transfer complex of N-TMB‚C18TCNQ.

Figure 3. UV-vis-NIR absorption spectra of (a) a seven-layer LB film and (b) a cast film of the charge-transfer complex of N-TMB‚C18TCNQ.

enough, it was found that the collapsed layer shows a dark-red color at the air-water interface. Therefore, it is suggested that C18TCNQ and N-TMB are in the CT complex form at the air-water interface. Existence Form of C18TCNQ and N-TMB in the LB and Cast Films. Figure 3 shows a UV-vis-NIR absorption spectrum of a seven-layer LB film of N-TMB‚ C18TCNQ deposited at 10 mN m-1 (a), together with the spectrum of a cast film of N-TMB‚C18TCNQ (b). The cast film was prepared by dropping several drops of the solution of the mixture of C18TCNQ and N-TMB onto a CaF2 plate. The solution of C18TCNQ and N-TMB shows a yellow color. After the evaporation of the solvent, the cast film on the CaF2 plate shows a dark-red color, giving a sign of the formation of the CT complex. CT complexes are classified into two types, namely, segregated-stack and mixed-stack forms, according to their different stacking styles of donor and acceptor components.2,15,24 The mixed-stack CT complexes with TCNQ derivatives usually give a broad CT band in the NIR region while the segregated-stack ones have a broad CT band in the IR region.8,15,17,25 A broad band with its center at about 1750 nm in Figure 3 indicates that C18TCNQ and N-TMB form the mixed-stack CT complex. Figure 4 shows an infrared transmission spectrum of a seven-layer LB film of the CT complex. A band at 2213 cm-1 is due to a CtN stretching mode of the C18TCNQ component in the CT complex. The CtN stretching band appears at 2228 cm-1 in an infrared spectrum of neutral TCNQ and it shows a low-frequency shift by 5-45 cm-1 in the spectra of the CT complexes consisting of TCNQ.26,27 Therefore, this band can be used as a marker for the formation of the CT complex. In an infrared spectrum of neutral C18TCNQ, the CtN stretching band appears at 2222 cm-1.15 Therefore, the appearance of the CtN stretching band at 2213 cm-1 in Figure 4 confirms the formation of the CT complex. Composition Ratio of C18TCNQ:N-TMB in the CT Complex. It was reported that the composition ratio of (24) Iwasa, Y.; Koda, T.; Koshihara, S.; Tokura, Y.; Iwasawa, N.; Saito, G. Phys. Rev. B 1989, 39, 10441. (25) Nakamura, T.; Yunome, G.; Azumi, R.; Tanaka, M.; Tachibana, H.; Matsumoto, M. J. Phys. Chem. 1996, 98, 1882. (26) Chappell, J. S.; Bloch, A. N.; Bryden, W. A.; Maxfield, M.; Poehler, T. O.; Cowan, D. O. J. Am. Chem. Soc. 1981, 103, 2442. (27) Robles-Martinez, J. G.; Salmeron-Valverde, A.; Alonso, E.; Soriano, C. Inorg. Chim. Acta 1991, 179, 149.

TCNQ:N-TMB in the CT complex is 1:1.28 In the present case, the composition ratio of C18TCNQ:N-TMB may be determined by studying the relation between the intensity changes of the two CtN stretching bands (2222 and 2213 cm-1) and the molar ratio changes of C18TCNQ:N-TMB. In Figure 4, it is noted that a band does not appear at 2222 cm-1, a frequency corresponding to the CtN stretching mode of neutral C18TCNQ. This result suggests that all the C18TCNQ molecules are involved in the CT states. A similar result was obtained for a CT cast film of C18TCNQ and N-TMB. It must be kept in mind here that the CT LB and cast films were prepared from the 1:1 mixture of C18TCNQ and N-TMB. A cast film was also fabricated from a 1:0.9 mixture of C18TCNQ and N-TMB on a CaF2 plate and its infrared transmission spectrum was measured. In this case, the band at 2222 cm-1 still appears, indicating that C18TCNQ is the excess species when the molar ratio of C18TCNQ:N-TMB is 1:0.9. Therefore, the composition ratio of C18TCNQ:N-TMB in the CT complex seems to be 1:1 because the molar ratio of organic donor-acceptor complexes is usually a ratio of integral numbers.29 Degree of Charge Transfer. It is well-known that the frequency of the CtN stretching band is related to the degree of charge transfer (F), and the value of F for the CT complexes consisting of C18TCNQ can be derived from the degree of the shift of the CtN stretching band.15,17,26,27 As references, the peak at 2222 cm-1 of neutral C18TCNQ and that at 2184 cm-1 of C18TCNQ-1 can be employed.15,17 The CtN stretching band of N-TMB‚C18TCNQ in the CT LB film appears at 2213 cm-1 in Figure 4, indicating that the value of F is about 0.24. The mixed-stack CT complexes can be divided into two types, namely, quasi-neutral complexes (F < 0.5) and quasi-ionic complexes (F > 0.5).2,9,15 The value of 0.24 indicates that the CT complex investigated is in a quasineutral state. The type of CT complex of N-TMB‚C18TCNQ is similar to that of the CT complex of N-TMB‚TCNQ.28 Band Assignments. In Figure 4, bands at 2922 and 2852 cm-1 are assigned to CH2 antisymmetric and symmetric stretching modes of the alkyl chain of C18TCNQ, respectively. Bands at 1606 and 1508 cm-1 probably arise from aromatic rings of the N-TMB component because the quadrant and semicircle stretching modes of substituted benzene usually give infrared bands in the above region.30 A band at 1357 cm-1 can be assigned to the C-N stretching mode (arylamine, Ph-N) of N-TMB.30 It is noteworthy that the above three vibrational modes at 1606, (28) Amano, T.; Kuroda, H.; Akamatu, H. Bull. Chem. Soc. Jpn. 1969, 42, 671. (29) Eastman, J. W.; Androes, G. M.; Calvin, M. J. Chem. Phys. 1962, 36, 1197.

Mixed-Stack CT LB Films of C18TCNQ and N-TMB

Figure 5. Infrared reflection-absorption spectra of three-layer LB films of the charge-transfer complex of N-TMB‚C18TCNQ deposited (a) on a gold-evaporated glass slide precoated with a two-layer LB film of stearic acid and (b) on a gold-evaporated glass slide directly.

cm-1

1508, and 1357 have their transition moments in the ring plane of the N-TMB component. Molecular Orientation and Structure. Molecular orientation in an LB film can be investigated by comparing the intensities of infrared bands in a reflection-absorption (RA) spectrum with those in a transmission spectrum.21,31-33 If the molecular orientation in a film is affected by the difference in substrates, a direct comparison between the RA and transmission spectra may be inappropriate. In the present case, however, such a problem does not exist because the molecular orientation in the CT LB film has little dependence on the substrates as described below. To investigate substrate dependence of the molecular orientation and structure, we precoated a two-layer LB film of stearic acid (SA) under the deposition pressure of 30 mN m-1 on a gold-evaporated glass slide (Au-SA plate). Figure 5 shows infrared RA spectra of three-layer LB films of the CT complex deposited on the Au-SA plate (a) and directly on a gold-evaporated glass slide (b), respectively. Of note is that the two spectra in Figure 5 are close to each other, except for bands due to stearic acid. For example, a band at 2213 cm-1 due to the CtN stretching band has almost the same intensity. Other bands due to the CT complex such as bands at 1606 and 1232 cm-1 also show little/minor changes between Figure 5a and Figure 5b. These observations suggest that the molecular orientation and structure in the CT LB films have little dependence on the substrate. Similar results were obtained for the CaF2 and CaF2-SA plates. According to the surface selection rule,31-33 vibrational modes with their transition moments perpendicular to the substrate surface are strong in an RA spectrum. In Figure 6, an infrared transmission spectrum of the three-layer LB film of the CT complex on a CaF2 plate (a) is compared with an RA spectrum of the same layer LB film on a gold-evaporated glass slide (b). It is noted that the intensities of the bands at 1606, 1508, and 1357 cm-1 are much stronger in the transmission spectrum than in the RA spectrum. As we described above, the three bands arise from the N-TMB component and have their transition moments in the ring plane. Therefore, the comparison (30) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: Boston, 1991. (31) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (32) Chollet, P. A.; Messier, J.; Rosilio, C. J. Chem. Phys. 1976, 64, 1042. (33) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62.

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Figure 6. Infrared (a) transmission and (b) reflectionabsorption spectra of three-layer LB films of the charge-transfer complex of N-TMB‚C18TCNQ.

between the infrared transmission and RA spectra indicates that the ring plane of N-TMB is nearly parallel to the substrate surface in the LB films. The CtN stretching band (C18TCNQ has four CtN groups and their transition moments are in the ring plane of C18TCNQ) at 2213 cm-1 shows a similar intensity pattern to the bands at 1606, 1508, and 1357 cm-1, suggesting that the ring plane of the C18TCNQ component is also almost parallel to the substrate surface. Therefore, it may be concluded that the ring planes of C18TCNQ and N-TMB are nearly parallel to each other. This conclusion is in a good agreement with the requirement of the formation of the mixed-stack CT complex. In the mixed-stack CT complex, the donor and acceptor should be packed in a face-to-face pattern.9,17,28 Amano et al.28 reported that the donor and acceptor components are in a face-to-face contact in the CT complex of N-TMB‚TCNQ. Bands at 2922 and 2852 cm-1 are assigned to the CH2 antisymmetric and symmetric stretching modes of the hydrocarbon chain of C18TCNQ, respectively. It is noted that the intensities of these two bands are comparable between the transmission and RA spectra. This suggests that the hydrocarbon chain is tilted considerably with respect to the substrate surface. The frequencies of the CH2 stretching bands are sensitive to the conformation of a hydrocarbon chain; low frequencies (∼2918 and ∼2848 cm-1) of the bands are characteristic of a highly ordered (trans-zigzag) alkyl chain, while their upward shifts are indicative of the increase in conformation disorder, i.e., gauche conformers, in the chain.34,35 The observation that the two CH2 stretching bands appear at 2922 and 2852 cm-1 in Figure 6 suggests that the hydrocarbon chain of C18TCNQ contains several gauche conforms. The chromophore planes of the CT complex in the LB films are parallel to the substrate surface, and thus they occupy a relative large surface area with respect to the hydrocarbon chain. Therefore, it is expected that the hydrocarbon chain tilts and/or folds in order to fill the space. In fact, the conclusion of the tilt of the chain has been reached above by making the comparison between the transmission and RA spectra. The disordered conformation of the chain is probably related to the tilt of the chain and to the low packing density of the chain in the CT LB films. The molecular orientation and structure of the CT complex in the one-layer LB film is similar to those in the (34) Umemura, J.; Cameron, D. G.; Mantsch, H. H. Biochim. Biophys. Acta 1980, 602, 32. (35) Sapper, H.; Cameron, D. G.; Mantsch, H. H. Can. J. Chem. 1981, 59, 2543.

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Figure 7. Infrared transmission spectra of (a) a one-layer LBdoping film and (b) a one-layer LB film of the charge-transfer complex of N-TMB‚C18TCNQ.

three-layer LB film discussed above. For example, the chromophore planes for both C18TCNQ and N-TMB components are almost parallel to the substrate surface. Comparison between Two Kinds of Thin CT Films. It is of interest to make a comparison between a one-layer CT LB film of N-TMB‚C18TCNQ and its one-layer CT LBdoping film that was obtained by LB and donor-doping techniques. The donor-doping was made by dipping a onelayer LB film of C18TCNQ into a petroleum ether solution of N-TMB for about 5 min according to the method described previously.15 Figure 7 compares infrared transmission spectra of the one-layer CT LB film and the onelayer CT LB-doping film. The CtN stretching band of the LB-doping film is located also at 2213 cm-1, indicating the formation of the CT complex by the doping. Of particular note is that the intensities of the two CH2 stretching bands at 2922 and 2852 cm-1 and of the CtN stretching band at 2213 cm-1 are much stronger in the LB film than in the LB-doping film. The chromophore plane of C18TCNQ is nearly parallel to the substrate surface in the CT LB film of N-TMB‚ C18TCNQ while that is tilted considerably with respect to the substrate surface in an LB film of C18TCNQ alone.17,36 Therefore, it is very likely that a C18TCNQ molecule in the LB film of C18TCNQ alone occupies a smaller area on the substrate surface than a N-TMB‚C18TCNQ complex consisting of one C18TCNQ component in the CT LB film. Thus, the number of C18TCNQ per unit area should be (36) Kubota, M.; Ozaki, Y.; Araki, T.; Ohki, S.; Iriyama, K. Langmuir 1991, 7, 774.

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larger in the one-layer LB film of C18TCNQ alone than in the one-layer CT LB film of N-TMB‚TCNQ. If almost all the C18TCNQ molecules lie on the plate after a doping procedure, the number of N-TMB‚C18TCNQ per unit area should be larger in the CT LB-doping film than in the CT LB film. However, infrared band intensities in Figure 7 are much weaker in the CT LB-doping film than in the CT LB film. There are two possibilities about the intensity changes of the infrared bands between the CT LB-doping film and the CT LB film. One is a change in the molecular orientation of the CT complex, and the other is that N-TMB‚C18TCNQ molecules are partly dissolved in the petroleum ether solution during the doping procedure. The dissolution of N-TMB‚C18TCNQ into the petroleum ether solution can be verified easily by an experiment. After the CT film of N-TMB‚C18TCNQ is rinsed with the solvent of petroleum ether (∼5 mL), almost N-TMB‚ C18TCNQ molecules are removed from the CaF2 plate. The problem of the dissolution of the CT complex has been reported for a CT LB-doping film of C18TCNQ and N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD).37 Conclusions LB films of the CT complex of N-TMB‚C18TCNQ have been successfully deposited onto CaF2 plates and onto gold-evaporated glass slides at the surface pressure of 10 mN m-1 by use of a vertical dipping method. This is the first time that the mixed-stack CT LB films of N-TMB‚ C18TCNQ have been prepared by the LB technique. A broad band in the NIR region indicates that the CT complex molecules in the LB films are in the mixed-stack style. The degree of charge transfer has been found to be 0.24 from the frequency shift of the CtN stretching band. A comparison of the infrared band intensities between the transmission and RA spectra suggests that the chromophore planes for both C18TCNQ and N-TMB are nearly parallel to the substrate surface in the LB films. Thus, the donor and acceptor components are in a faceto-face contact with each other. This result is in good agreement with the requirement for the formation of the mixed-stack complex. The hydrocarbon chain of the C18TCNQ component is considerably tilted with respect to the substrate surface and the tilt of the chain probably results in significant disorder in the alkyl chain. LA000504A (37) Nichogi, K.; Taomoto, A.; Nambu, T.; Murakami, M. Thin Solid Films 1995, 254, 240.