Thermal Behavior of Mixed-Stack Charge Transfer Films of 2

Thermal Behavior of Mixed-Stack Charge Transfer Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and 3,3',5,5'-Tetramethylbenzidine Prepared by t...
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J. Phys. Chem. 1996, 100, 17232-17237

Thermal Behavior of Mixed-Stack Charge Transfer Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and 3,3′,5,5′-Tetramethylbenzidine Prepared by the Langmuir-Blodgett Technique and Donor Doping. 1. Dependence of Thermal Behavior on the Number of Layers Studied by Ultraviolet-Visible-Near Infrared and Infrared Spectroscopies Yan Wang,† Katsuhiro Nichogi,‡ Keiji Iriyama,§ and Yukihiro Ozaki*,† Department of Chemistry, School of Science, Kwansei-Gakuin UniVersity, Uegahara, Nishinomiya 662, Japan, AdVanced Materials Research Laboratory, Matsushita Research Institute Tokyo, Inc., Higashimaita, Tama-ku, Kawasaki 214, Japan, and Institute of DNA Medicine, The Jikei UniVersity School of Medicine, Nishi-shinbashi, Minato-ku, Tokyo 105, Japan ReceiVed: March 12, 1996; In Final Form: June 26, 1996X

The dependence of thermal behaviors on the number of layers in Langmuir-Blodgett (LB) films of the mixedstack charge transfer (CT) complex of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (octadecyl-TCNQ) doped by 3,3′,5,5′-tetramethylbenzidine (TMB) has been investigated by using ultraviolet-visible-near infrared (UV-vis-NIR) and infrared (IR) spectroscopies. The appearance of a broad CT absorption band in the NIR region reveals that the mixed-stack CT films are formed by donor doping. The degree of the charge transfer determined by a shift of the CtN stretching band of the TCNQ chromophore verifies that the CT complex films are in a quasi-neutral state. Their temperature-dependent UV-vis-NIR and IR spectra show that donor molecules (TMB) dedope from the CT complexes in the LB films, resulting in the restoration of acceptor molecules (octadecyl-TCNQ) to their neutral states at their respective dedope temperatures. The dedope temperature of the CT complex films increases with the number of layers up to a seven-layer film. Both the UV-vis-NIR and IR spectra show that the one-layer CT complex film undergoes a progressive thermal process. In contrast, the seven- and 11-layer films are stable up to 120 °C and show rather abrupt changes near their dedope temperatures. Furthermore, pre-dedope phenomena are observed for the multilayer CT complex films but not for the one-layer CT film. This dependence of the thermal behaviors of the CT films on the number of layers may be attributed to the differences in the film thickness, the longitudinal interactions between the one-dimensional needle-like microcrystals of the CT complex, and the effects of the interaction between the first layer and a CaF2 substrate.

Introduction Recently, organic donor-acceptor charge transfer (CT) complexes have been extensively studied because they show a variety of physical properties, such as electrical conductivity, neutral-ionic phase transition phenomena, and nonlinear electrical property.1-20 These organic CT complexes are classified into two types according to their different stacking styles of donor and acceptor molecules, i.e. the so-called segregated-stack and mixed-stack forms.2,10 The mixed-stack CT complexes are composed of one-dimensional stacks of alternatively stacked donor (D) and acceptor (A) molecules. The CT complexes in the mixed stack form are further divided into quasi-neutral (N) and quasi-ionic (I) complexes by the magnitude of a partial electron transfer (F) from the D to A molecules. When F is less or greater than 0.5, a CT complex is in the quasi-neutral or quasi-ionic state, respectively.2,11,16 Langmuir-Blodgett (LB) films of the CT complexes have received keen interest because the LB technique may provide the desired control of the structure and function at the molecular level. However, so far, they have been investigated mainly for the segregated-stack forms, which exhibit a feature of highly * To whom correspondence should be addressed. † Kwansei-Gakuin University. ‡ Matsushita Research Institute Tokyo, Inc. § The Jikei University School of Medicine. X Abstract published in AdVance ACS Abstracts, October 1, 1996.

S0022-3654(96)00767-8 CCC: $12.00

electrical conduction.12,18,19 The mixed-stack CT complexes have been studied in crystals which show neutral-ionic phase transition and/or nonlinear electrical features by applying pressure or changing temperature.2,4,5,9 The use of LB technique to prepare a thin film of mixed-stack CT complex was reported only once.20 Thermal behaviors of CT complex films have not been investigated yet for both the segregated- and mixed-stack forms. It is of great importance to explore them from the viewpoints of both basic research and practical applications. From the point of basic research, LB films of CT complexes may exhibit different thermal behaviors from those of crystals due to their low-dimensional characteristics. On the other hand, an understanding of their thermal behaviors is a prerequisite for the actual applications. Our group has been studying the structures of functional LB films of various dye molecules with an emphasis on their thermal behaviors.21-26 In our previous papers concerning LB films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (octadecyl-TCNQ), we reported studies on their order-disorder phase transition and annealing effects by atomic force microscopy (AFM) and ultraviolet-visible (UV-vis) and infrared (IR) spectroscopies.24,25 These studies elucidated that phase transition temperature increases with the number of layers and that the thermal behavior of the LB films depends upon the number of layers. Moreover, it was found that the annealing effects on domain structure in the LB films of octadecyl-TCNQ are clearly © 1996 American Chemical Society

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Figure 1. Structures of (a) 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (octadecyl-TCNQ) and (b) 3,3′,5,5′-tetramethylbenzidine (TMB).

different between the one-layer and multilayer films. These investigations on the LB films of octadecyl-TCNQ laid a solid foundation for the present and future research. The purpose of the present two papers (this and the following paper26) is to provide new insight into thermal behaviors of the LB films of the CT complex of octadecyl-TCNQ doped by TMB. The first paper is concerned with the dependence of the thermal processes on the number of layers in these LB films. Temperature-dependent structural changes in both the donor and acceptor molecules are discussed for the one-, three-, seven-, and 11-layer CT complex LB films on the basis of the UVvis-near infrared (NIR) and IR measurements at elevated temperatures. The second paper delineates morphological features of the CT complex films investigated by AFM. It also deals with the dependence of annealing effects on the number of layers for the films studied by AFM and spectroscopic techniques. Employing both microscopic and spectroscopic methods enables us to explore the thermal behavior extensively from various points of view. To the best of our knowledge, this sort of comprehensive study on the thermal behavior of the CT complex LB films has not yet been reported. Experimental Section Sample Preparation. Octadecyl-TCNQ (Figure 1a) was purchased from the Japanese Research Institute for Photosensitizing Dyes Co., Ltd., and used without further purification. A thin-layer chromatographic examination revealed that it did not contain any other colored components. Sample of TMB (Figure 1b) was obtained from the Tokyo Chemical Industry and purified by sublimation before use. The Y-type LB films of octadecyl-TCNQ were fabricated by using a Kyowa Kaimen Kagaku Model HBM-AP Langmuir trough with a Wihelmy balance. A detailed procedure for the fabrication of LB films of octadecyl-TCNQ was described in the preceding paper.24 The transfer ratio was found to be nearly unity (0.95 ( 0.05) throughout the experiments. Donor doping of the LB films of octadecyl-TCNQ was made by dipping the films into a petroleum ether solution of TMB for 5 min to ensure its complete charge conversion.20 The yellow color of the LB films of octadecyl-TCNQ changed into purple-red immediately after dipping them into the TMB solution. Spectroscopy. The instrumentations and sample-handling techniques employed for measuring the UV-vis-NIR and IR spectra were the same as those described before.24

Figure 2. UV-vis-NIR absorption spectra of LB films of a mixedstack CT complex of octadecyl-TCNQ doped by TMB measured at increments of 10 °C: (a) a one-layer film from 30 to 130 °C; (b) an 11-layer film from 30 to 150 °C.

Results Ultraviolet-Visible-Near Infrared Spectroscopy. Figure 2a,b shows temperature-dependent UV-vis-NIR absorption spectra of one- and 11-layer LB films of the CT complex of octadecyl-TCNQ and TMB deposited on CaF2 plates, respectively. The spectra of the one- and 11-layer films were measured over a temperature range from 30 to 130 °C and from 30 to 150 °C at an increment of 10 °C, respectively. The noises appearing around 970, 1450, and 1940 nm are due to the second and first overtones and combination modes of OH vibrations of water, respectively. The spectra of both one- and 11-layer CT films measured at 30 °C show a broad absorption band at 1536 nm. This band is assigned to a CT excitation between octadecyl-TCNQ (a donor) and TMB (an acceptor) molecules,20 and thus it is proved that the CT complex films consisting of octadecyl-TCNQ and TMB have been formed by the TMB doping.20 A band at 538 nm is due to an intramolecular excitation of TCNQ molecules. The spectra of the CT films resemble that of a monoclinic TCNQ-TMB evaporated film, which is a mixed-stack type of complex.20 If the LB films of octadecyl-TCNQ doped by TMB were segregated-stack types, the absorbance of the CT excitation would be weak because the overlap of electronic orbitals between octadecyl-TCNQ and TMB is small.20 Other evidence for the formation of CT films is given by the IR spectra, as will be described below. Several spectral features of the CT complex films of octadecyl-TCNQ doped by TMB depend on the number of layers.

17234 J. Phys. Chem., Vol. 100, No. 43, 1996 It can be seen from Figure 2 that the CT excitation and intramolecular excitation bands decrease gradually in the spectra of the one-layer CT film, while those of the 11-layer CT film show only a small decrease below 140 °C and decrease largely from 140 to 150 °C. These thickness-dependent spectral changes may be explained by referring to the morphology of the CT films studied by AFM measurements.26 We discuss this point in the Discussion section. Another characteristic difference in the temperature-dependent spectral changes between the one- and 11-layer CT films is concerned with the dedope temperature of the CT complex films. It has turned out that the temperature strongly depends on the number of layers. The CT excitation and intramolecular excitation bands vanish at 130 and 150 °C for the one- and 11-layer CT complex films, respectively. The disappearance of the characteristic bands of CT complex films means that the complexes decompose. The temperature-dependent UV-visNIR spectra of the three- and seven-layer films show their dedope temperatures at 140 and 150 °C, respectively. These results reveal that the dedope temperature increases with the number of layers in the CT complex films of octadecyl-TCNQ doped by TMB. More accurate dedope temperatures of the CT complex films can be determined from the analyses of their IR spectra. Yet another notable feature of thermal behaviors depending on the number of layers is a pre-dedope process observed only for the multilayer CT complex films. A shoulder appears at 405 nm in the UV-vis-NIR spectra of the 11-layer CT complex film above 110 °C, which is much lower than the dedope temperature (Figure 2b). This band, which is attributable to the π-π* transition of the TCNQ chromophore of neutral octadecyl-TCNQ, does not appear in the spectra of the onelayer CT film below its dedope temperature (Figure 2a). With temperature this band becomes stronger and stronger in the spectra of the 11-layer film, indicating restoration of the neutral state of octadecyl-TCNQ from the electron-accepting state in the CT complex films. Finally, it becomes a dominant absorption band in the spectrum at 150 °C. The three- and seven-layer CT complex films show a pre-dedope process similar to that of the 11-layer CT film. In contrast, this prededope process is not observed for the one-layer CT complex film. This reflects a unique feature in the thermal behavior of the one-layer CT complex film. Infrared Spectroscopy. Figure 3a shows a temperature dependence of the IR spectrum of the 11-layer CT film. Band frequencies were reported previously and are in good agreement with those of the TCNQ-TMB evaporated films.20 It is wellknown that the frequency of a CtN stretching band depends upon the degree of charge transfer, i.e. formal charge (F); it is observed near 2184 and 2220 cm-1 for TCNQ-1 and TCNQ0 (the neutral species), respectively.20,27,30 In the present case it appears at 2208 cm-1 at 30 °C, revealing the formation of the CT complex film where partial charge transfer occurs. It is also noted that no band assignable to the neutral species is detected in the spectrum measured at 30 °C. This suggests that the CT formation proceeds completely in the film. The value of F for the CT films of octadecyl-TCNQ doped by TMB can be determined from the shift of the CtN stretching band.20,27,30 It was calculated to be 0.4, implying that the CT films are in a quasi-neutral state. (As a reference, neutral octadecyl-TCNQ and the completely charge-transferred octadecyl-TCNQ anion (K+-octadecyl-TCNQ-) powder were used. The peak positions of the CtN stretching band of octadecyl-TCNQ and octadecylTCNQ-1 are 2222 and 2184 cm-1, respectively.)

Wang et al.

Figure 3. (a) Temperature-dependent IR transmission spectra of an 11-layer CT complex film of octadecyl-TCNQ doped by TMB measured at various temperatures. (b) Enlargement of the CtN stretching band region measured in the temperature range 140-150 °C.

It can be seen from Figure 3a that marked spectral changes occur for all the bands between 140 and 150 °C. In this temperature range an N-H stretching band at 3402 cm-1 of TMB vanishes, showing that the donor molecules dedope from the CT complex film. To determine the dedope temperature more accurately, temperature-dependent spectral changes in the CtN stretching band region were measured from 140 to 150 °C at increments of 2 °C. The results are shown in Figure 3b. The figure clearly shows that the CtN stretching band shifts from 2207 to 2216 cm-1 in the temperature range 144-148 °C. At 146 °C, the CtN stretching feature consists of two bands at 2207 and 2216 cm-1 assignable to the CT and neutral states, respectively. The corresponding overlapped feature appears at 128, 134, and 144 °C for the one-, three-, and seven-layer CT complex films, respectively. On the basis of these results, the thermally induced dedope temperature is determined to be 128, 134, 144, and 146 °C for the one-, three-, seven-, and 11-layer CT complex films of octadecyl-TCNQ doped by TMB, respectively. The dedope temperature increases with the number of layers in the CT complex films. However, it increases little upon going from the seven- to 11-layer CT complex films. From 30 °C to the dedope temperature several notable changes are observed for bands due to the alkyl chain. Bands at 2922 and 2852 cm-1 are due to CH2 antisymmetric and symmetric stretching modes of the hydrocarbon chain of octadecyl-TCNQ in the CT state, respectively (Figure 3a). These bands appear at 2918 and 2848 cm-1 in LB films of octadecyl-TCNQ.22,24 It is of significance to monitor the shift

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Figure 4. Temperature dependence of the wavenumber of a CH2 symmetric stretching band of the hydrocarbon chain for the one-, three-, seven-, and 11-layer CT films of octadecyl-TCNQ doped by TMB.

of the CH2 symmetric stretching band because its frequency is sensitive to the conformation of the hydrocarbon chain31,32 (the CH2 antisymmetric stretching band splits into two in the IR spectra of the LB films of octadecyl-TCNQ,22 so that we do not use this band). The low-frequency (2848 cm-1) of the band is characteristic of a highly ordered (trans-zigzag) alkyl chain, while its upward shift is indicative of the increase in conformational disorder, i.e. gauche conformers in the chain.31,32 The upward shift of the CH2 symmetric stretching band in the spectra of the CT films indicates that the hydrocarbon chains are not as ordered as those in the LB films of octadecyl-TCNQ. A more detailed discussion about the structure of the hydrocarbon chain will be given in the succeeding paper by referring to AFM images of the CT films.26 In Figure 4, the wavenumber of the CH2 symmetric stretching band is plotted against temperature for the one-, three-, seven-, and 11-layer CT films. The plots clearly show that the hydrocarbon chain undergoes a melting step between 80 and 90 °C for all the films. The band shifts from 2852 to 2854 cm-1 in the above temperature range irrespective of the number of layers. Probably, the hydrocarbon chains become more disordered above 90 °C. The wavenumber and normalized intensity of the CtN stretching band are plotted as a function of temperature for the one-, three-, seven-, and 11-layer CT films in Figure 5a,b, respectively. It can be seen from Figure 5a that the frequencies of the CtN stretching band change little for all the films up to their respective dedope temperatures. This means that the CT complexes maintain their CT states if the temperature is kept below their dedope temperatures. The intensity changes (the experimental error is within (0.01) shown in Figure 5b reveal that the one-layer CT film undergoes a gradual change, while the seven- and 11-layer films experience a two-step process; that is, stepwise intensity decreases in the temperature ranges 80-90 and 140-150 °C. The second step may be brought about by the dedope of the CT complex films, while the first one might be concerned with the conformational changes in the hydrocarbon chains because their wavenumber shift takes place in the same temperature range. The thermal behavior of the three-layer CT film is something in between those of the one- and 11-layer CT films. An IR band at 3402 cm-1 is due to an N-H stretching mode of TMB and thus can be used as a probe for monitoring thermal behavior of the donor (TMB) in the CT films (Figure 3a). The normalized intensity of the N-H stretching band is plotted against temperature in Figure 6 for the one-, three-, seven-, and 11-layer CT complex films. The intensity decrease obviously depends upon the number of layers in the CT films. In the one-layer film it occurs gradually until a sudden decrease taking

Figure 5. Temperature dependence of (a, top) the wavenumber and (b, bottom) normalized intensity of a CtN stretching band of the TCNQ chromophore for the one-, three-, seven-, and 11-layer CT films of octadecyl-TCNQ doped by TMB.

Figure 6. Temperature dependence of the normalized intensity of an N-H stretching band of the TMB chromophore for the one-, three-, seven-, and 11-layer CT films of octadecyl-TCNQ doped by TMB.

place from 120 to 130 °C. On the contrary, the intensity decrease in the N-H band of the seven- and 11-layer films proceeds little up to 140 °C. The decreases in the intensities are attributable mainly to the sublimation of the TMB molecules from the CT films. Thus, the thermally induced dedope process of the donor molecules is in good agreement with that of the acceptor molecules (Figure 5a and b). Discussion Thermal Behavior of LB Films of the CT Complex of Octadecyl-TCNQ Doped by TMB. The UV-vis-NIR and IR studies described here reveal that the thermal behavior depends on the number of layers of the CT complex films. For example, the UV-vis-NIR and IR spectral features of the onelayer CT film exhibit gradual variations from 30 to 130 °C, while those of the multilayer films change little until the abrupt

17236 J. Phys. Chem., Vol. 100, No. 43, 1996 changes occurring near their respective dedope temperatures (Figures 2, 3a, 5b, 6). The second feature depending upon the number of layers is that the dedope temperature of the CT complex in the films increases with the number of layers. This can be clearly seen in Figure 5a, which plots the frequency of the CtN stretching band versus temperature. Our previous study on thermal behavior of LB films of octadecyl-TCNQ revealed that the band at 2216 cm-1 corresponds to a CtN stretching mode of the TCNQ chromophore in the melted state of the LB films.22,24 Therefore, the band shift from 2208 to 2216 cm-1 indicates that octadecyl-TCNQ molecules change from the state of the CT complex to the neutral melted state. The above-mentioned features, depending upon the number of layers, may be explained by referring to AFM images of the CT complex films shown in the following paper.26 According to the AFM images,26 the films consist of piles of onedimensional needle-like microcrystals. We think that it is the difference in the thickness of the CT films that dominates the dependence of thermal behavior upon the number of layers. Longitudinal interactions between the needle-like microcrystals become stronger with the increase in the thickness. The stronger longitudinal interactions prevent the needle-like microcrystals from becoming flatter, and thus the rearrangement of microcrystals consisting of donor-acceptor stacks cannot easily proceed. This is supported by the AFM images of the one-, three-, and 11-layer CT complex films after annealing from 110 °C.26 The thickness of the CT complex films is a key factor that determines the dedope temperature and temperaturedependent structural changes in the films, although the interactions between the first layer and substrate may also play an important role in the thermal process. If the number of layers reaches a certain number, seven layers in the present case, the dependence of the thermal behavior on the number of layers becomes less obvious. The third thickness-dependent feature is that the thermally induced pre-dedope process is detected for the multilayer CT complex films but not for the one-layer CT film (Figure 2). As shown in Figure 2, the shoulder peak appears at 405 nm in the spectra of the 11-layer CT film above 110 °C, and its intensity becomes stronger with temperature. At 150 °C, which is above the dedope temperature of the 11-layer CT film (146 °C), this peak is dominant in the spectra while other characteristic bands of the CT films disappear. The IR spectrum of the 11-layer CT film measured at 150 °C suggests that it consists only of neutral octadecyl-TCNQ molecules. Therefore, there is no doubt that the band at 405 nm is due to the π-π* transition of TCNQ chromophores. The thermally induced dedope process of the CT films can also be monitored by the N-H stretching band of the TMB molecules (Figure 6). The intensity of this band provides complementary information to that about the thermal behaviors of acceptor molecules. Comparison of Figure 6 with Figure 5b reveals that the donor and acceptor molecules undergo concerted structural changes near the dedope temperatures. Order-Disorder Transition of Hydrocarbon Chains of Octadecyl-TCNQ Molecules in the CT Films. The orderdisorder transition of the hydrocarbon chains in the one-, three-, seven-, and 11-layer CT complex films of octadecyl-TCNQ doped by TMB can be studied by the frequency and intensity of the CH2 symmetric stretching band. As shown in Figure 4, the hydrocarbon chains in the CT films undergo a sudden melting process from 80 to 90 °C irrespective of the number of layers. Ulman33 summarized that kinks and jogs are distributed along hydrocarbon chains and a strong increase in rotation about

Wang et al. the chain axis may bring about the disorder process in the temperature range 80-90 °C. Although his conclusion was drawn from the analyses of simple amphiphilic compounds such as fatty acids and their salts, it may be applicable to the thermal behavior of the CT complex LB films of TCNQ and TMB. For both the CT complex films and the LB films of octadecylTCNQ,22,24 it was found that the alkyl chains experience marked conformational changes in the temperature range 80-90 °C. The thermal behavior of alkyl chains plays an essential role in the thermal stability of functional LB films. Chromophores are, in general, more stable than the alkyl chains, so that the first step of the thermally induced changes usually happens on the alkyl parts. This first disturbance on the alkyl chains becomes a trigger for further changes in molecular orientation and structure in the LB films. Conclusions This paper has reported a detailed study on the thermal behavior of mixed-stack CT films of octadecyl-TCNQ and TMB prepared by the LB technique and donor doping. The following conclusions can be reached from the temperature-dependent UV-vis-NIR and IR spectral changes for the one-, three-, seven-, and 11-layer CT complex films. Thermally induced structural changes in the mixed-stack CT complex films strongly depend upon the number of layers. First, the dedope temperature of the CT complex films increases with the number of layers. Second, the progressive thermal process of the one-layer CT complex film changes into a sudden variation one as the number of layers increases. Third, the prededope phenomenon occurs in the multilayer CT complex films. The dependence of the thermal behavior on the number of layers becomes less obvious if the number of layers is more than seven. There may be both external and internal factors that determine the thermal behavior of the CT complex films of octadecylTCNQ doped by TMB. The major external factor is concerned with the existence of longitudinal interactions between the onedimensional needle-like microcrystals. Another one is the interaction between the first layer and the substrate. There are two internal factors determined by the stacking pattern of donor and acceptor molecules, as well as a thermally-induced disorder disturbance of alkyl chains in the temperature range 80-90 °C. References and Notes (1) Torrance, J. B. Ann. N. Y. Acad. Sci. 1978, 313, 210; Acc. Chem. Res. 1979, 12, 79. (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) Hubbard, J.; Torrance, J. B. Phys. ReV. Lett. 1981, 47, 1750. (5) Tokura, Y.; Koda, T.; Mitani, T.; Saito, G. Solid State Commun. 1982, 43, 757. (6) Jerome, D.; Schultz, H. J. AdV. Phys. 1982, 31, 299; Mol. Cryst. Liq. Cryst. 1985, 117, 121. (7) Miller, J. S., Ed. Extended Linear Chain Compounds; Plenum Press: New York, 1982; Vols. 1 and 2. (8) Ferraro, J. R.; Williams, J. M. Introduction to Synthetic Electrical Conductors; Academic Press: Orlando, FL, 1987; pp 1-337. (9) Tokura, Y.; Okamoto, H.; Koda, T.; Mitani, T.; Saito, G. Phys. ReV. B 1988, 38, 2215. (10) Iwasa, Y.; Koda, T.; Koshihara, S.; Tokura, Y.; Iwasawa, N.; Saito, G. Phys. ReV. B 1989, 39, 10441. (11) Iwasa, Y.; Koda, T.; Tokura, Y.; Kobayashi, A.; Iwasawa, N.; Saito, G. Phys. ReV. B 1990, 42, 2374. (12) Okamoto, H.; Mitani, T.; Tokura, Y.; Koshihara, S.; Komatsu, T.; Iwasa, Y.; Koda, T. Phys. ReV. B 1991, 43, 8224. (13) 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. (14) Iwasa, Y.; Watanabe, N.; Koda, T.; Saito, G. Phys. ReV. B 1993, 47, 2920.

Thermal Behavior of Mixed-Stack Charge Transfer Films (15) Meneghetti, M.; Girlando, A.; Pecile, C. J. Chem. Phys. 1985, 83, 3134. (16) McConnell, H. M.; Hoffman, B. M.; Metzger, R. M. Proc. Natl. Acad. Sci. 1965, 53, 46. (17) Nakamura, T.; Kojima, K.; Matsumoto, M.; Tachibana, H.; Tanaka, M.; Manda, E.; Kawabata, Y. Chem. Lett. 1989, 367. (18) Ruaudel-Teixier, A.; Vandevyver, M.; Barraud, A. Mol. Cryst. Liq. Cryst. 1985, 120, 319. (19) Barraud, A.; Lesieur, P.; Ruaudel-Teixier, A.; Vandevyver, M. Thin Solid Films 1985, 134, 195. (20) Nichogi, K.; Taomoto, A.; Nambu, T.; Murakami, M. Thin Solid Films 1995, 254, 240. (21) Katayama, N.; Enomono, S.; Ozaki, Y.; Kuramoto, N. J. Phys. Chem. 1993, 97, 6880. (22) Terashita, S.; Ozaki, Y.; Iriyama, K. J. Phys. Chem. 1993, 97, 10445. (23) Wang, Y.; Ozaki, Y.; Iriyama, K. Langmuir 1995, 11, 705. (24) Wang, Y.; Nichogi, K.; Terashita, S.; Iriyama, K; Ozaki, Y. J. Phys. Chem. 1996, 100, 368. (25) Wang, Y.; Nichogi, K.; Iriyama, K.; Ozaki, Y. J. Phys. Chem. 1996, 100. 374.

J. Phys. Chem., Vol. 100, No. 43, 1996 17237 (26) Wang, Y.; Nichogi, K.; Iriyama, K.; Ozaki, Y. J. Phys. Chem. 1996, 100, 17238. (27) Chappell, J. S.; Bloch, A. N.; Bryden, W. A.; Maxfield, M. Poehler, T. O.; Cowan, D. O. J. Am. Chem. Soc. 1981, 103, 2442. (28) Matsuzaki, S.; Kuwata, R.; Toyoda, K. Solid State Commun. 1980, 33, 403. (29) Bandrank, A. D.; Traong, K. D.; Carlone, C. Can. J. Chem. 1982, 60, 588. (30) Terashita, S.; Nakatsu, K.; Ozaki, Y.; Takagi, S. J. Phys. Chem. 1995, 99, 3618. (31) Umemura, J.; Cameron, D. G.; Mantsch, H. H. Biochim. Biophys. Acta 1980, 602, 32. (32) Sapper, H.; Cameron, D. G.; Mantsch, H. H. Can. J. Chem. 1981, 59, 2543. (33) Ulman, A. An Introduction to Ultrathin Organic Films, From Langmuir to Self-Assembly; Academic Press, Inc.: San Diego, CA, 1991. Part Two.

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