Langmuir 1992,8, 3051-3056
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Structural Characterization of Langmuir-Blodgett Films of 2-Dodecyl-and 2-Pentadecyl-7,7,8,8-tetracyanoquinodimethane Shin-ichi Terashita, Kazumi Nakatsu, and Yukihiro Ozaki* Department of Chemistry, Kwansei Gakuin University, Uegahara, Nishinomiya 662, Japan
Tadashi Mochida and Toshinari Araki Frontier Technology Research Institute, Tokyo Gas Co., Ltd., Suehiro-cho, Tsurumi-Ku, Yokohama 230, Japan
Keiji Iriyama Department of Biochemistry, Institute of Medical Science, The Jikei University School of Medicine, Nishi-Shinbashi, Minato-ku, Tokyo 105, Japan Received April 29,1992. In Final Form: July 16,1992 Infraredtransmissionand reflection-absorption(RA)spectrahave been measuredfor Langmuir-Blodgett (LB) f i s of 2-dodecyl-7,7,8,8-tetracyanaquinodimethane(dodecyl-TCNQ)and 2-pentadecyl-7,7,8,8tetracyanaquinodimethane(pentadecyl-TCNQ)depositedon CaFz plates (fortransmissionmeasurements) and Au-evaporatedglass slides (forRA measurements),and the obtainedspectra have been compared with those of LB f i of 2-octadecyl-7,7,8,&tetracyanoquinodimethe(octadecyl-TCNQ)previouslyreported. The structure of the three kinds of TCNQ LB f i s shows dependenciesupon the length of the hydrocarbon chain and the substratesused. The shapes of the CH2 scissoringband suggest that the hydrocarbon chains in the dodecyl- and octadecyl-TCNQLB f i s are in an orthorhombic subcell packing while those in the pentadecyl-TCNQ LB films crystallize with a hexagonal or pseudohexagonal packing. Comparisons of the transmission and FtA spectra indicate that the hydrocarbonchains are tilted considerablywith respect to the surface normal in the three kinds of TCNQ LB films but the tilt angle of the chain is larger in the dodecyl-TCNQ LB f i than in the pentadecyl- and odadecyl-TCNQLB films. It haa been commonly observed that the TCNQ planes are nearly perpendicular to the substrate surface at least above the third monolayers deposited on the Au-evaporated glass slides but they are inclined appreciably in the first monolayers probably due to the interaction between the TCNQ chromophore and the substrate. Such dependency on the ordinal number of monolayers has not been observed for the films prepared on the CaFz plates.
Introduction A number of Langmuir-Blodgett (LB) films having tetracyanoquinodimethane(TCNQ) chromophores have recently been made to aim a t novel conducting materials.'-1° For example, Barraud et al.' reported the firat conducting LB film based on N-docosylpyridinium TCNQ while Kawabata et a L 4 p 6 developed LB films consisting of TCNQ and tetramethyltetrathiafulvalene (TMTTF) which show high conductivity (about 10-l S cm-') without iodine doping. The latter researchersfurther proposed conducting LB films based on TCNQ and a hydrophobicazobenzenegroupwhich showphotoewitching phenomenon via cis-trans isomeri~m.~*~
* To whom correspondence should be addressed.
(1) Ruaudel-Teixier, A.; Vandevyver, M.; Barraud, A. Mol. Cry&. Liq. Cryst. 1985,120,319. (2) Vandevyver, M.; Richard, J.; Barraud, A.; Ruaudel-Teixier, A.; Lequan,M.; Lequan, M. J. Chem. Phys. 1987,87,6754. (3) Nakamura, T.; Matsumoto, M.; Takei, F.; Tanaka, M.; Sekiguchi, T.; Manda, E.; Kawabata, T. Chem. Lett. 1986,709. (4) Kawabata, Y.; Nakamura, T.; Mataumoto, M.; Tanaka, M.; Sekiguchi, T.; Komizu, H.; Manda, E.; Saito, G.Synth. Met. 1987,19,663. (5) Mataumoto, M.; Nakamura, T.; Manda, E.;Kawabata, Y.; Ikegami, K.; Kuroda, S.; Sugi, M.; Saito, G. Thin Solid F i l m 1988,160,61. (6) Nakamura, T.; Takei, F.; Tanaka, M.; Mataumoto, T.; Sekiguchi, T.; Manda, E.; Kawabata, Y.; Saito, G. Chem. Lett. 1986, 323. (7) Richard, J.; Vandevyver,M.;Barraud,A.; Morand,J. P.; Lapouyade, Chem. R.; Delhaea, P.; Jacquinot, J. F.; Roulliary, M. J. Chem. SOC., Commun. 1988,754. (8) Tacbibana, H.; Komizu, H.; Nakamura,T.; Mataumoto,M.;Tanaka, M.; Manda, E.; Kawabata, Y.; Kato, T. Chem. Lett. 1989,841.
In spite of the extensive studies of physical properties of TCNQ LB films, however, their structural characterization has fallen far behind. It is very important to investigate the structure-function relationship of the LB films to understand their physical properties at the molecular level and to design new films having more desirable properties. We, therefore, have undertaken structural studies of the LB films with TCNQ chromophores by use of Fourier transform infrared (FT-IR) spectrosc~py.~~ In our f i i t report of a aeries of the infrared studies on TCNQ LB films,the followingconclusionacould be reached for LB films of 2-octadecyl-7,7,8,8-teetracyanoquinodimethane (octadecyl-TCNQ; Figure 1):(i) the hydrocarbon chains of octadecyl-TCNQ in the LB films assume a uniasial orientation and are in an orthorhombic subcell packing; (ii) both the hydrocarbon chain and the molecular axis of TCNQ chromophore are tilted considerably with respect to the surface normal while the TCNQ plane is oriented along a direction approximately perpendicular to the substrate surface; (iii) the TCNQ plane in the l-monolayer assembly deposited on the Au-evaporated glass slide is inclined to some extent due (9) Tachibana, H.; Nakamura,T.; Mataumoto,M.; Kozumi, H.; Mauda, E.;Niino, H.; Yabe, A.; Kawabata, Y. J. Am. Chem. SOC.,1989,111,3080. (10) Vandevyver, M.; Ruaudel-Teixier, A.; Palacin, 5.;Bourgoin, J.P.; Barraud, A.; Bozio, R.; Meneghetti, M.; Pecile, C. Mol. Cryst. Liq. Cryst. 1990,187, 327. (11) Kubota, M.; Ozaki, Y.; Araki, T.; Ohki, 5.;Iriyama, K. Longmuir 1991, 7, 774.
0743-7463/92/2408-3051$03.00/00 1992 American Chemical Society
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3052 Langmuir, Vol. 8, No. 12, 1992
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Figure 1. Structure of 2-dodecyl- (n = 121,l-pentadecyl-(n = 15),and 2-octadecyl-7,7,8,8tetracyanoquinodimeth (n= 18) (dodecyl-,pentadecyl-, and octadecyl-TCNQ,respectively). In this paper the molecular axes of dodecyl-, pentadecyl-, and odadecyl-TCNQ are defined as shown in this figure.
i:b IC15
40\
e
lo
- 0
50 100 Molecrlar Area A/&&! Figure 2. Surface pressure-area curves of dodecyl- and pentadecyl-TCNQmonolayers on the surface of distilled water (pH 0
5.8).
to the interaction between the TCNQ chromophore and Au substrate. The purpose of the present study is to explore dependency of the structure of TCNQ LB films upon the length of the hydrocarbon chain substituted; structures of the LB films of 2-dodecyl- and 2-pentadecyl-TCNQ(hereafter, they are referred as dodecyl-TCNQand pentadecyl-TCNQ, respectively; Figure 1)have been inveetigated and compared with that of the LB films of octadecyl-TCNQ. This sort of study on dependency of the structure of LB film upon the length of the hydrocarbon chain is very rare except for LB films of simple fatty acids. The present study demonstrates that the length of the hydrocarbon chain is, indeed, a very important factor in determining the molecular orientation and structure of LB films. Experimental Section Dodecyl- and pentadecyl-TCNQwere generousgifts from Dr. S. Yasui (JapaneseResearch Institute for Photosensitizing Dyes Co., Ltd.) and used withoutfurther purircation. The thin-layer chromatographic examinationsrevealed that the dyes did not contain anyothercoloredcomponenta. AKyowa Kaimen Kagaku ModelHBM-AP Langmuir trough with a Wilhelmybalance was employed for the LB f i b fabrications. Several drops of a chloroformsolution (1X 10-8M)of dodecyl-or pentadecyl-TCNQ were placed onto an aqueous subphaeeof water doubly distilled from deionized water. After evaporation of the solvent, the monolayer was compressed at a constant rate of 30 cm2/minup to the surfacepressureof 10mN m-l. The FA isothermsshown in F w e2 revealthat the monolayerswere solid condensedf h at thispressure. The monolayersweretransferredby the vertical dipping method onto CaF2 platas (for transmission measurementa) or Au-evaporated glass slides (for RA measurementa).
The transfer ratio was found to be nearly unity (0.97 & 0.03) throughout the experimenta. All the LB f i i treated in this paper have the Y-typestructure. This was confiied by X-ray diffractionpatterns of 5- and 13-monolayerLB f i i of dodecyland pentadecyl-TCNQdeposited on the CaFzand Au-evaporated glass slides, obtained by a RIGAKU CN 2125 D2 powder diffractometerwith the use of Cu K a radiation. AU the infrared measurementa were performed by the instrumentation and methods described previous1y.l1J2 Results and Discussion Structureof Dodecyl-,Pentadecyl-,and OctadecylTCNQ in Solid States. Figure 3A shows infrared transmission spectra of dodecyl- (a),pentadecyl- (b), and octadecyl-TCNQ (c) in powdered states. Intense bands near 2920 and 2850 cm-l in the three spectra can be assigned to CHZantisymmetricand symmetric stretching modes of the hydrocarbon chains.ll Of note is that the bands near 2920 cm-l split into two bands as shown in Figure 3B. The doublet bands become singlets in bromoform solutions, indicating that the splittings do not arise from Fermi resonance. It is also very unlikely that the splittings are due to disorder in the hydrocarbon chains because the frequenciesof CH2 symmetricstretchingbands show that the chains are highly ordered.13 Therefore, we infer that the splittings are caused by crystal field splittings. Bands at 2222 cm-l are assignable to CN stretching modes of the four cyano groups and two bands at 1547and 1530 cm-' are attributed to C==Cstretching modes of the TCNQ chromophores. Doublets near 1465 cm-l are attributed to CHZscissoring modes of the hydrocarbon Parte. A band due to the CH2 scissoring mode of n-paraffins is a good marker for studying the lateral packing of the hydrocarbon chains.14-la As in the case of octadecylTCNQ,ll the splitting of the CH2 scissoring mode is observed for dodecyl-TCNQ, suggesting that its hydrocarbon chains crystallize with an orthorhombic subcell packing. In contrast, the CH2 scissoring band of pentadecyl-TCNQdoes not split into two bands, indicating that its hydrocrabon chains are in an hexagonal or pseudohexagonal packing. Molecular Orientation and Structure of LB Films of Dodecyl-TCNQ. Figure 4A exhibits infrared transmission spectra of (a) one-, (b) two-, and (c) threemonolayer dodecyl-TCNQ LB films deposited on both sides of CaFz substrates. Note that even a one-monolayer film gives a high-quality spectrum. The three spectra are very similar to each other except for band intensities which increase almost linearly with increasing the number of monolayers, n, suggesting that the structure of the LB films changes little as a function of n value and the interactionbetween the TCNQchromophore and substrate is very weak. As in the case of the solid state the band due to the CHZantisymmetric stretching mode splits into two bands; enlargements of the 3100-2700-~m-~ regions are shown in Figure 4B. This splitting is also probably due to the crystal field splitting. (12) Katayama, N.; Fukui, M.; Ozaki, Y.;Kuramoto, N.; Araki, T.; Iriyama, K. Langmuir 1991, 7, 2827. (13) Sapper, H.; Cameron, D. G.; Mantsch, H. H. Can. J. Chem. 1981, 59,2643. (14) Tasumi, M.; Shimanouchi, T.; Miyazawa, T. J. Mol. Spectroec. 1962,9,261. (16) Snyder, R. G.; Schachtschneider, J. H.Spectrochim. Acta 1968, 19,a. (16) Tasumi, M.; Shimanouchi, T. J. Chem. Phye. 1966,49, 1246. (17) Koyama, Y.;Yanagishita, M.; Toda, S.; Matauo, T. J. Colloid Interface Sci. 1977,61, 438. (18) Kimura, F.; Umemura, J.; Takenaka, T. Langnuir 1986, 2,96.
Langmuir, Vol. 8, No. 12,1992 3053
LB Films of TCNQ 3
If 3200 2800 2400 2ooo
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Figure 3. (A) Infrared transmission spectra of dodecyl- (a), pentadecyl- (b), and octadecyl-TCNQ(c) in powdered solid states. (B) Enlargements of the 3100-2700-~m-~regions of Figure 3A. The frequency (2850 cm-l) of the CH2 symmetric stretching band and the splitting of the scissoring band (1471and 1462cm-') suggest that the hydrocarbon chains in the dodecyl-TCNQ LB films assume trans zigzag conformation and are in the orthorhombicsubcell packing. Probably, the structure and subcell packing of the hydrocarbon chain of dodecyl-TCNQ in the LB films are similar to those in the solid state. Infrared reflection-absorption (RA) spectra of (a) one-, (b) two-, and (c) three-monolayer LB f i b s of dodecylTCNQ deposited on the Au-evaporated glass slides are presented in Figure 5. It is noted that in contrast to the transmission spectra shown in Figure 4A the spectral pattern depends clearly on the number of monolayers; for example, the interesting ratio of bands at 2926 and 2222 cm-' changes markedly with the number of monolayers. This observation strongly suggests that the molecular orientation in the dodecyl-TCNQ LB films alters with the ordinal number of monolayer at least up to the third monolayer. In general, a comparison of infrared transmission and RA spectra of LB filmsenables one to discussthe molecular orientation in However, since the RA spectra of one-, two-, and three-monolayer LB films of dodecylTCNQ show clear dependency on the number of monolayers, it is difficult to investigate the molecular orientation ~~~
~~
(19)Greenler, R. G . J. Chem. Phys. 1966,44, 310. (20) Chollet, P.-A.; Meseier, J.; Roeio, C. J. Chem. Phys. 1976,64, 1042. (21) Chollet, P.-A. Thin Solid Films 1978,52, 343. (22) Umemura,J.; Kamnta, T.;Kawai, T.; Takenaka,T.J. Phys. Chem. 1990,94,62.
in the dodecyl-TCNQ LB films by comparing the spectra in Figure 4A and those in Figure 5. We, therefore, compared infrared transmission and RA spectra of 13monolayer LB films of dodecyl-TCNQ. Parta a and b of Figure 6 show them, respectively. The intensities of bands in the RA spectrum of the 13-monolayerLB film are about 3 times stronger than those of corresponding bands in the RA spectrum of the three-monolayer LB film (Figure 5c), indicating that above the third monolayer the structure of monolayers changes little even in the LB films deposited on the Au-evaporated glass slides. Of particular note in the comparison to the 13-monolayer film spectra is that the intensity of a band at 2222 cm-' due to the CN stretching mode and those of bands at 1547 and 1531 cm-' arising from the C 4 stretching modes of TCNQ chromophore are much stronger in the RA spectrum than in the transmission spectrum. According to the surface selection rule in infrared RA spectro~copy,~'~~ vibrational modes with their transition momenta perpendicular to the substrate surface are enhanced in a RA spectrum. Therefore, the above observations for the TCNQ chromophore bands suggest that the TCNQ plane is oriented nearly perpendicular to the substrate surface in the LB films of dodecyl-TCNQ because the above three modes are all in-plane vibrational modes. The two C 4 stretching modes corresponding to the 1547- and 1531-cm-' bands have transition momenta in different directions; one (the mode corresponding to the 1547-cm-' band) is parallel with the molecular axis of the TCNQ group and the other (the mode corresponding to the 1531-cm-' band) is perpendicular to it (the molecular
Tercurhita
3054 Langmuir, Vol. 8, No.12,1992 B
A
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Figure 4. (A) Infrared transmission spectraof one- (a),two- (b),and three-monolayer(c)LB filmsof dodecyl-TCNQ. (B)Enlargements of the 3100-2700-~m-~ regions of Figure 4A.
3200
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Figure 6. Infrared transmission (a) and RA (b) spectra of 13monolayer LB films of dodecyl-TCNQ. 3200
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WA-air’)
Figure 5. Infrared RA spectra of one- (a),two- (b), and threemonolayer (c) LB films of dodecyl-TCNQ.
axis of the TCNQ group is defined in Figure l).23 The relative intensity of the 1547-and 1531-cm-I bands changes between the transmission and RA spectra but both have comparable intensities in the two spectra. This result (23) Takenaka, T.SpeCtrOChiM. Acta, Part A 1971, 27A, 1735.
implies that the molecular axis is neither parallel nor perpendicular to the surface, being in an intermediate direction. The intensity of the band due to the CH2 symmetric stretching mode is much stronger in the transmission spectrum than in the RA spectrum, whereas that of the band arising from the CH2 antisymmetricstretching mode does not showa significantchangebetween the two spectra Figure 7 shows the orientations of a hydrocarbon chain and the directions of transition momenta of CH2 antisymmetric and symmetric stretching modes. If the hy-
LB Films of TCNQ
symmatrlc slrelchlng
Langmuir, Vol. 8, No. 12,1992 3056
s n l l s y m m ~ t r l cslrmtshlnp
Figure 7. Orientationsofa hydrocarbonchain and the directions of transient momenta of CHZantisymmetric and symmetric stretching modes. The hydrogen atoms are not shown in (a-c)
3200 2800
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and (a'+).
drocarbon chain is oriented nearly perpendicular to the substrate surface like Figure 7a or 7a', both the bands due to CH2 antisymmetric and symmetric stretching modes should be very weak in a RA spectrum because their transition momenta are parallel with the substrate surface. In contrast, if it is oriented approximately parallel with the surface as shown in Figure 7b and 7b', only the CH2 symmetric (for the case of Figure 7b) or antisymmetric (for the case of Figure 7b') band is strong in a RA spectrum because the transition momenta of CHZantisymmetric and symmetric stretching modes are nearly parallel and perpendicular to the surface, respectively, in the case of Figure 7b and those are nearly perpendicular and parallel to the surface, respectively, in the case of Figure 7b'. In the cases of Figure 7c and 7c', where the long chain is neither parallel nor perpendicular to the surface, being in an intermediate direction,both bands should have medium intensities. The consideration in Figure 7 leads us to conclude that the hydrocarbon chain of dodecyl-TCNQ is tilted largely with ita molecular plane nearly parallel to the surface in the LB film as shown in Figure 6b'. Now, we can discuss the molecular orientation in the first monolayer of LB films of dodecyl-TCNQ. The RA spectrum of the 1-monolayer LB film is clearly different from that of the 13-monolayerLB film in two pointa (Figure 5a and Figure 6b). One is that the relative intensity of the band at 2222 cm-' is weaker in the former than in the latter. The other difference ie the intensity ratio of the two bands at 1547 and 1529 cm-I. These differences suggest that the TCNQ plane and molecular axis of the TCNQ chromophore are more tilted in the first monolayer. Probably, there is a significant interaction between the TCNQ chromophore and the Au substrate, and it changes the orientations of the TCNQ plane and ita molecular axis. However, the interaction should be weak because the frequencies of bands due to the TCNQ part do not show appreciable changes upon the LB film fabrications. Molecular Orientationand Structureof LB Films of Pentadecyl-TCNQ. Figure 8 shows infrared transmission (a) and RA (b) spectra of 13-monolayerLB films of pentadecyl-TCNQ. The transmission spectrum is very close to that of 13-monolayer LB film of dodecyl-TCNQ (Figure 6a). There are, however, two important differences between the spectra in Figures 8a and 6a. One is the band shape of CHz scissoringmode; the scissoringband appears
Figure 8. Infrared transmission (a) and RA (b) sp&tra of 13monolayer LB fiis of pentadecyl-TCNQ.
I
. ' * 3200 2800
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.
.
'
.
~
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2400 2000 1600 1200 WAVENUMBER(cm-1) Figure 9. Infrared RA spectrum of a one-monolayerLB f i i of pentadecyl-TCNQ.
as a singlet in Figure 8a while it is seen as the doublet in Figure 6a. Therefore, it seems that the lateral packing of the hydrocarbon chains is different between the dodecyland pentadecyl LB films. The other difference is the intensity ratio of two bands due to CHZantisymmetric and symmetricmodes. This difference indicates that the molecular orientation of the hydrocarbon chain is significantly different between the dodecyl- and pentadecylTCNQ LB films as discussed later. A comparison of parts a and b of Figure 8 shows that bands at 2222, 1549, and 1531 cm-' due to the in-plane modes of TCNQ chromophore are again much stronger in the RA spectrum than in the transmission spectrum. It is, therefore, considered that the TCNQ plane is nearly perpendicular to the substrate surface. The intensityratio of two bands at 2918 and 2850 cm-I due to the CHz antisymmetricand symmetric stretching modes showsthat the hydrocarbon chain is tilted like Figure 7c and 7c', but the tilt angle from the surface normal may be smaller in the pentadecyl-TCNQLB f i i s than in the dodecyl-TCNQ LB films. As in the case of the dodecyl-TCNQLB films,RA spectra of the pentadecyl-TCNQLB films on the Au-evaporated glass slides show a dependency upon the number of monolayers. The RA spectrum of a one-monolayer LB film of pentadecyl-TCNQ is shown in Figure 9. The relative intensities of bands due to the TCNQ in-plane modes are much weaker in the 1-monolayerfilm spectrum than in the 13-monolayer film spectrum (Figure 8b),
l
3056 hngmuir, Vol. 8, No. 12,1992 indicating that the TCNQ plane is inclined appreciably in the first monolayer. Comparison among the Structure of LB Films of Dodecyl-, Pentadecyl-, and Octadecyl-TCNQ. The structure of LB films of dodecyl-, pentadecyl-, and octadecyl-TCNQ shows significant differences as well as similarities." The most marked difference between the LB films of pentadecyl-TCNQand those of dodecyl- and octadecyl-TCNQ is the subcell packing; the hydrocarbon chains in the LB fiis of dodecyl- and octadecyl-TCNQ" crystallize with the orthorhombic subcell packing while those in the LB films of pentadecyl-TCNQ are in the hexagonal or pseudohexagonal subcell packing. At the moment, it is rather difficult to discuss the cause of difference in the subcell packing, but the difference in an even or odd number of the length of hydrocarbon chain might be an important factor. There is one more important difference among the three kinds of TCNQ LB films. The comparisons of the infrared and RA spectra show that the hydrocarbon chains are tilted considerably with respect to the surface normal for all the TCNQ LB films investigated. However, it seems that the tilt angle of the chain is larger in the dodecylTCNQ films than in the pentadecyl-TCNQ films as discussed above, and judging from the intensity ratio of
Terashita the two bands due to the CH2 antisymmetric and symmetric stretching modes, the corresponding angle in the octadecyl-TCNQ LB fiis is more or lees similar to that of the pentadecyl-TCNQ LB films.ll These findings indicate that the length of the hydrocarbon chain is a very important factor in determiningthe molecular orientation and structure of TCNQ LB films. The three kinds of TCNQ LB f i i exhibit similar dependencyon the substrates.l' The structure of the LB films deposited on the CaF2 plates alters little with the number of monolayersand there seems to be no significant interaction between the TCNQ plane and the substrate. In contrast, strong evidence for the interaction between the TCNQ plane and the substrate has been obtained commonly for the three kinds of TCNQLB fiisdeposited on the Au-evaporated glass slides.
Acknowledgment. We thank Mr. H. Yoshioka (Kwansei Gakuin University) for his assistance and expertise in measuring X-ray diffraction patterns. We are also grateful to Dr. S. Yasui (Japanese Research Institute for Photosensitizing Dyes Co., Ltd.) for his generous donation of dodecyl- and pentadecyl-TCNQ. Registry No. dodecyl-TCNQ, 105314-21-4; pentadecyl-TCNQ, 105344-15-8;CaF2, 7789-75-5.
