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Langmuir-Blodgett Film Assembly of a Novel TCNQ Amphiphile Substituted by a Steroid Skeleton Katsuyuki Naito,’ Akira Miura, and Makoto Azuma Advanced Research Laboratory, Research and Development Center, Toshiba Corp., 1 Komukai- Toshiba-cho, Saiwai- ku, Kawasaki 210, Japan Received August 17, 1990. In Final Form: January 23, 1991 Cholanyl-TCNQ (2,2/-( 2-(cholan-24-yl)-2,5-cyclohexadiene1,kdiylidene)bis(propanedinitri1e)) ( 1) was newly synthesized in order both to prove advantages of steroid skeletonsas hydrophobic tails for LangmuirBlodgett (LB) molecules and to obtain a uniform neutral TCNQ LB film. The film surfaces deposited onto hydrophobic substrates by the vertical dipping method were found to be smooth and homogeneous. The molecular orientationwas determined by X-ray diffractionand ultraviolet-visible and Fourier transform infrared polarized spectra. The molecular design rules for assembling uniform LB films are also discussed. Introduction The Langmuir-Blodgett (LB) technique has been recognized to construct organic thin films with controlled layer structures or superlattices, yielding potential applications to molecular devices.l Many efforts in fabricating high-quality LB films have been made, and many LB dye molecules have been synthesized to investigate their film-forming properties.2 Although the general rules for designing LB molecules have not been well established, with the exception of a rather qualitative one, hydrophobic long alkyl tails have most preferably been used to synthesize them. TCNQ (tetracyano-p-quinodimethane; 2,2’-(2,5-cyclohexadiene-l,4-diylidene)bis(propanedinitrile))is one of the most interesting functional dyes3 Thus several TCNQ derivatives with long alkyl chains have extensively been ~ t u d i e d .Inhomogeneous ~ film structures, however, have not yet been fully o ~ e r c o m e . ~Octadecyl-TCNQ (2) (1) (a) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987,3,932. (b) Ahmed, F. R.; Burrows, P. E.; Donovan, K. J.; Wilson, E. G. Synth. Met. 1988, 27, B593. (c) Girling, I. R.; Kolinsky, P. V.; Cade, N. A.; Earls, J. D.; Peterson, I. R. Opt. Commun. 1985,55,289. (d) Blinov, L. M.; Dubinin, N. V.; Mikhnev, L. V.; Yudin, S. G. Thin Solid Films 1984,120,161. (e) Roberts, G. G. Contemp. Phys. 1984,25, 109. (2) (a) Popovitz-Biro, R.; Hill, K.; Shavit, E.; Hung, D. J.; Lahav, M.; Leiserowitz, L.; Sagiv, J.; Hsiung, H.; Meredith, G. R.; Vanherzeele, H. J. Am. Chem. SOC.1990,222,2498. (b)Taylor, J. W.; Sloan,C. P.; Holden, D. A.; Kovacs, G. J.; Loutfy, R. 0. Can. J . Chem. 1989,67,2136,2142. (c) Grate, J. W.; Rose-Pehrsson, S.;Barger, W. R. Langmuir 1988,4,1393. (d) Durfee, W. S.; Storck, W.; Willig, F.; von Frieling, M. J. Am. Chem. SOC.1987, 109, 1297. (e) Penner, T. L.; Miibius, D. J. Am. Chem. SOC. 1982,204,7407. (f) Heeseman, J. J . Am. Chem. SOC.1980,202,2167. (g) Kuhn, H.; Miibius, D.; Baucher, H. Physical Methods of Chemistry;Weisberger, A., Rossiter, B., Eds.; Wiley: New York, 1972; Vol. 1, Part 3B. (3) (a) Carter, F. L., Ed. Molecular Electronic Devices; Marcel Dekker: New York, 1982. (b) Potember, R. s.;Poehler, T. 0. Appl. Phys. Lett. 1979,34,405. (c) Aviram, A.; Ratner, M. A. Chem.Phys. Lett. 1974, 29, 277. (4) (a) Matsumoto, M.; Nakamura, T.; Manda, E.; Kawabata, Y.; Ikegami, K.; Kuroda, S.; Sugi, M.; Saito, G. Thin Solid Films 1988, 160,61.
(b) Sotnikov, P. S.; Berzina, T. S.; Troitsky, V. I.; Valter, R. E.; Karlivan, G. A.; Neiland, 0. Ya. Thin Solid Films, 1989,279,267. (c) Barraud, A.; Fliirsheimer, M.; Miihwald, H.; Richard, J.; Ruaudel-Teixier, A.; Vandevyver, M. J. Colloid Interface Sci. 1988,221,491. (d) Tachibana, H.; Nakamura, T.; Matsumoto, M.; Komizu, H.; Manda, E.; Niino, H.; Yabe, A.; Kawabata, Y. J . Am. Chem. SOC.1989,211,3080. (e) Metzger, R. M.; Panetta, C. A. J. Chim. Phys. 1988,85,1125. (f) Metzger, R. M.; Schumaker, R. R.; Cava, M. P.; Laidlaw, R. K.; Panetta, C. A.; Torres, E. Langmuir 1988,4, 298. (g) Geddes, N. J.; Sambles, J. R.; Jarvis, D. J.; Parker, W. G. Appl. Phys. Lett. 1990,56, 1916. ( 5 ) (a) Matsumoto, M.; Uyeda, N. Bull. Inst. Chem. Res., Kyoto Uniu. 1989, 66, 554. (b) Vandevyver, J. R.; Lesieur, P.; Ruaudel-Teixier, A., Barraud, A. J . Chem. Phys. 1987,86,2428. (c) Komizu, H.; Matsumoto, M.; Nakamura, T.; Tanaka, M.; Manda, E.; Kawabata, Y.; Honda, K. 54th Spring Meeting of Chem. SOC.Jpn., Preprints. 1987, 564 (in Jananese).
F (dynlcm)
-i 10
’
Me
0
” 30
I
40
L
50
,A
(i2/molecule)
Figure 1. Surfacepressure (F)-area (A)curveof cholanyl-TCNQ (1) monomolecular film on a pure water surface at 18 “C. The toluene solution of 1 (100 pL, 0.5 mM) was poured onto a water surface. The spread film was compressed at the rate of 1.5 A2/ molecule-min from the initial area of 115 A2/molecule.
(commercially available), for example, has been reported to form neither a monomolecular film nor homogeneous two-dimensional molecular arrays5* These inhomogeneities are probably due to their improper molecular structures as LB materials. In a previous paper: we have investigated the relationships between molecular structures and monomolecular film properties. We have proposed that steroid skeletons are superior both to long monoalkyl chains and to long dialkyl chains in forming homogeneous LB films of bulky dyes. To our knowledge, however, dye molecules possessing a hydrophobic steroid group have never been used as LB molecules except cholestanyl-TTF, which does not form a monomolecular film on a water surface.4f This is probably due to the absence of a hydrophilic group. TCNQ, however, possesses four hydrophilic cyano groups and can be expected to be a hydrophilic moiety in LB molecules. The purpose of this study is to extend the applicability of the molecular design rule to functional dyes, in (6) Naito, K.; Iwakiri, T.; Miura, A.; Azuma, M. Langmuir 1990, 6, 1309.
0743-7463/91/2407-0627$02.50/00 1991 American Chemical Society
628 Langmuir, Vol. 7, No. 4,1991
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Figure 2. (a) Differential interference optical micrograph of the cholanyl-TCNQ (1) bilayer deposited onto a methylated Si substrate by the vertical dipping method. On the upper region, no film exists. (b) Micrograph of the octadecyl-TCNQ (2) monolayer deposited by the horizontal dipping method. The scale bars indicate 50 pm length.
particular, to verify the advantages of steroid hydrophobic groups that enable us to form neutral TCNQ LB films with the improved qualities.
Experimental Section A steroid-TCNQ, cholanyl-TCNQ (2,2'-( 2-(cholan-24-~1)-2,5-
cyclohexadiene-1,4-diylidene)bis(propanedinitri1e)) (l),was synthesized from 50-cholanic acid (Sigma) and p-dimethoxybenzene according t o the synthetic routes of TCNQ and 2.' T h e total yield was 0.5 96. Characterization data for 1 are as follows: IR (KBr, cm-l) 3066 w,.2932 s, 2862 s, 2218 m (CN), 1552 m, 1533 m, 1468 m, 1448 m, 1375 m, 894 m, 815 m, 738 vw; lH NMR 6 (CDC13, ppm) 7.69,7.58,7.47 d, 7.35,7.26 (TCNQ-H, 3.OH), 2.97 t (TCNQ-CHz-, J = 7 Hz, 1.9 H), 2.0-0.65 (cholane skeleton, 39 H). Anal. Calcd for CxHMN4: C, 81.16; H, 8.32; N, 10.52. Found: C, 81.01; H, 8.27; N, 10.23. TLC Rf (silica gel, CHCl3) 0.27 (single spot). The method of measuring surface viscosity was described in the previous paper.6 T h e LB apparatus and the treatment of the substrate were also reported elsewhere.8
Results The spread molecules of 1 on a water surface were uniformly compressed and then closely packed a t 12.5dyn/ cm and a t the occupied area of 43 f 2 A2/molecule,giving a stable film. The surface pressure (F)-area ( A ) curve of 1 is shown in Figure 1. The surface viscosity of the monolayer of 1 was 0.15 f 0.05 g/s at 10dyn/cm at 18"C and small enough to reduce the surface pressure drop699 and the pressure distribution,lO that cause the inhomogeneities of the LB film." (7) (a) Acker, D. S.; Hertler, W. R. J . Am. Chem. SOC.1962,84,3370. (b) Kawabata, Y.; Nakamura, T.; Matsumoto, M.; Tanaka, M.; Sekiguchi, T.; Komizu, H.; Manda, E.; Saito, G. Synth. Met. 1987,19,663; and
Synth. Met 1987,22,92. (8) Naito, K. J. Colloid Interface Sci. 1989, 131, 218. (9) Nakayama, T.;Egusa, S.;Gemma, N.; Miura, A.; Azuma, M. Thin Solid Films 1989, 178, 137. (10) Egusa, S.; Nakayama, T.;Gemma, N.; Miura, A.; Azuma, M. Thin Solid Films 1989, 178, 165.
The monomolecular film was successfully transferred by the vertical dipping method (dipping speed, 5 mm/ min) a t 12.5dyn/cm onto a hydrophobic substrate,yielding a Y-type multilayer film. The transfer ratios were almost unity. The morphologies and structures of the built-up films were then characterized. Figures 2 and 3 show the surface morphologies of 1 and 2 films, observed respectively with a differential interference optical microscope and with a field-emission type scanning electron microscope.ll The film surfaces of 1 were smooth and homogeneous, while those of 2 were not, as reported by Matsumoto et al.5aIn Figure 3, the light and dark patches for 2 can be seen, which indicate the bare surface of the substrate and the surface covered with the partly piled-up film. An alternating heterostructure (S(STTS)&),consisting of a cadmium stearate bilayer (SS)and a bilayer of 1 (TT), was fabricated and the distance between the cadmium layers (STTS) was determined to be 86.5 A by the lowangle X-ray diffraction method. The thickness of the stearate monolayer film (S) was known to be 25 A;therefore the monolayer thickness of 1 (T) was determined to be 18.3 f 0.5 A. The UV-visible and Fourier transform infrared polarized spectral2 indicated that both the cholane skeleton (longitudinal axis) and the TCNQ molecular plane were almost perpendicular to the film surface. The absorption intensities a t about 400 nm, assigned to the transition dipole parallel to the long axis of the TCNQ moiety (bl"), were measured a t three different incident angles of Oo, 25", and 50". The absorbance ratio of s-polarized to p-polarized light was 1.0 a t Oo, 1.2 a t 2 5 O , and 1.6 a t 50°, respectively. The average declining angle of the long axis was estimated to be 90°, and the dispersion was 20" by the assumption of a normal distribution. The IR absorptions (11) Nakayama, T.; N. Miura, A.; Azuma, M. Thin Solid Films 1989, 178,477. (12) Vandevyver, M.; Barraud, A. J . Mol. Electron. 1988,4, 207.
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Figure 3. (a) Scanning electron micrographof the cholanyl-TCNQ (1) bilayer deposited onto a methylated Si substrate by the vertical dipping method. (b) Micrograph of the octadecyl-TCNQ (2) bilayer deposited by the horizontal dipping method. The scanning electron microscope images were taken without metal coating at the accelerating voltage of 1.5 kV. The scale bars indicate 200 nm length.
at 830 and 1380 cm-l were assigned to the CH deformation of the TCNQ moiety and to the symmetric CH deformation of the methyl groups of the cholane skeleton. The corresponding transition dipole moments were perpendicular to the TCNQ plane and almost perpendicular to the cholane rings, respectively. At an incident angle of 45", the absorbance ratio of s-polarized to p-polarized light was 2.0 a t 830 cm-l and 1.4 a t 1380 cm-l, respectively. Hence, the dipole moments were almost parallel to the film surface. The proposed molecular orientation of 1 is given in the inset of Figure 1.
Discussion The results in the previous paper,6 based on the molecular models of fatty acids possessing various hydrophobic tails, revealed t h a t two molecular factors of geometric size and cohesive interaction profoundly affect the monolayer properties. TCNQ moieties have strong cohesion and are apt to form three-dimensional aggregations on a water surface. For construction of a stable monolayer, the geometric requirement S h / & k 1, where s h means a cross-section of a hydrophobic tail and s d means that of a dye moiety,
should be satisfied to prevent the dye aggregations. The cross-sections of the cholane skeleton6 and of the edge-on oriented TCNQ moiety4e in Figure 1 are almost the same and are estimated to be 43 A2. The bulky cholan skeleton is therefore suitable for the requirement. Furthermore, the fusion enthalpy of 1 in solid powder (6.85 kcal/mol, mp 188 "C) was smaller than that of 2 (12.2 kcal/mol, mp 125 "C). The cohesion of the cholan skeleton itself was also reported to be much smaller than those of long alkyl chains.6 We consider that this weak cohesion generates the low surface viscosity, resulting in the formation of the uniform LB films.6*&11 We have reported herein the first demonstration of steroid-dye LB films with two-dimensional uniformity, cholanyl-TCNQ (1) being an example. This may break new ground for TCNQ applications. Furthermore, the findings are applicable to the design of LB molecules containing bulky functional moieties.
Acknowledgment. The authors thank Dr. Nobuhiro Gemma, Dr. Syun Egusa, and Mr. Toshio Nakayama (Toshiba Corp. R&D Center) for the helpful discussion on this study. Thanks are also due to Sogo Pharmaceutical Co., Ltd. (Japan),for the preparation of the compound 1.
