Langmuir 1994,10, 4599-4605
4599
Novel Orientation of Azobenzene Pendant Group in Hybrid Monolayers Composed of Polyimide Langmuir-Blodgett Film and Alkyl Polysiloxane Self-AssembledMonolayer Shiyoshi Yokoyama, Toshiki Yamada, Kotaro Kajikawa,? Masa-aki Kakimoto,* Yoshio Imai, Hideo Takezoe, and Atsuo Fukuda Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Meguro-ku, Tokyo 152, Japan Received August 9, 1993. In Final Form: September 13, 1994@ A new technique for preparing hybrid monolayers composed of azobenzene pendant type polyimide LB films and self-assembled alkyl polysiloxane monolayers is reported. The hybrid monolayers were prepared by octadecyltrichlorosilane (OTS) adsorption onto the azobenzene containing LB films. Contact angle measurements suggest a stable formation of the hybrid monolayers after the chemical adsorption of OTS molecules onto the LB films. The molecular orientation was determined by FTIR reflection-adsorption (RA)spectroscopy and second harmonic generation (SHG) technique. In the hybrid monolayers, the molecular orientation of the azobenzene units is sustained by the tightly packed structure of the selfassembled monolayers. The molecular tilt angles relative to the surface normal direction were estimated to be 16" and 39" for the alkyl chain and the azobenzene unit, respectively. Minimal conformational change was observed even after heating the films to 120 "C.
Introduction Artificial preparation techniques of highly organized monolayers are important to learn how to fabricate molecular level devices. Studies of molecular assemblies have shown that there is great potential for LangmuirBlodgett (LB) films and self-assembled monolayers to be used in optical and optoelectrical devices.l This is especially true of polymeric monolayers because of their thermal and chemical stability. We have developed a method for preparation of polyimide LB films via soluble polyamic acid alkylamine salt precursors (precursor methodh2J In our recent work, functional units were incorporated into the polymeric main chain backbone^.^,^ In these structures, the functional group, especially the azobenzene group, exhibited twodimensional order in the surface plane.5 However, we have not yet fabricated a pendant type polyimide LB film, in which the dipoles of the functional units are aligned in the surface normal direction. The electrical poling technique is a typical method used to induce molecular orientation parallel to a n applied dc electric field.'j-'l However, the resulting materials sometimes showed structural damage during the poling process and subsequent loss of orientation due to thermal relaxation. On the other hand, the preparation of LB films and self-assembled monolayers is interesting from the viewpoint of surface chemistry, because they permit finely organized and stable molecular structures to be made.l t Present address: Frontier Research Program, The Institute of Physical a n d Chemical Research (RIKEN), Hirosawa, Wako, Saitama 351-01, Japan. Abstract published in Advance A C S Abstracts, November 1, 1994. (1)Ulman, A. An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly;Academic Press: San Diego, CA, 1991. (2)Suzuki, M.; Kakimoto, M.; Konishi, T.; Imai, Y.; Iwamoto, M.; Hino, T. Chem. Lett. 1986, 395. (3)Kakimoto, M.; Suzuki, M.; Konoshi, T.; Imai, Y.; Iwamoto, M.; Hino, T.; Chem. Lett. 1986,823. (4)Nishikata, Y.; Fukui, S.; Kakimoto, M.; Imai, Y.; Nishiyama, K.; Fujihira, M. Mol. Cryst. Liq. Cryst. 1993,224,95. (5)Nishikata, Y.; Komatsu, IC;Kakimoto, M.; Imai, Y. Thin Solid Films 1992,210,29. @
Moreover, polymerizable self-assembled monolayers of alkyl polysiloxanes12-19exhibited the network orientation and excellent thermal stability.20p21Recent studies have been investigated to introduce some aromatic groups or chromophores into these monolayer system^.^^-^^ However, few attempts have been reported that two-dimensional structures composed of functional LB films and self-assembled monolayers were fabricated. In this study, we report the preparation of hybrid monolayers, which were composed of azobenzene pendant type polyimide LB films32 and self-assembled octadecylpolysiloxane(OPS)monolayers. We also determine the (6)Meredith, G. R.;Vandusen, J. G.; Williams, D. J. Macromolecules 1982,15,1385. (7)Ye, C.; Marke, T. J.;Yang, - J.;Wang, - G. K. Macromolecules 1987, 20, 2322. @)Ye, C.; Minami, N.; Markes, T. J.; Yang, J.; Wang, G. k. Macromolecules 1988,21,2899. (9)Hill, J. R.;Pantelis, P.; Abbasi, F.; Hodge, P. J.Appl. Phys. 1988, 64 2749 --I -.
(10) Eich, M.; Reck, B.; Yoon, D. Y.; Willson, C. G.; Bjorklund, G. C.
J.Appl. Phys. 1989,66,3241.
(11)Chen.M.:Yu.L.:Dalton.L. R.:. Shi.Y.: . , , , . Skier. W.Mucromolecules 1991,24,5421. (12)Sagiv, J. J. Am. Chem. SOC.1980,102,92. (13)Netzer, L.; Sagiv, J. J. Am. Chem. SOC.1983,105,674. (14)Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983,99,235. (15)Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983,100,67. (16)Maoz, R.;Sagiv, J. Thin Solid Films 1985,132, 135. (17)Maoz, R.;Sagiv, J. J. Colloid Interface Sci. 1984,100,465. (18)Gun, J.;Isovici, R.; Sagiv, J. J. Colloid Interface Sci. 1984,101, 201. (19)Gun, J.;Sagiv, J. J. Colloid Interface Sci. 1986,112,457. (20)Cohen, S.R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986,90, 3054. (21)Nuzzo, R.G.; Allara, D. L. J.Am. Chem. SOC.1983,105,4481. (22)Tillman, N.; Ulmann, A.; Elman, J. F. Langmuir 1990,6,1512. (23) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192. (24)Chen, S. H.;Frank, C. W. Lungmuir 1991, 7, 1719. (25)Gromez, M.; Li, J.; Kaifer, A. E. Langmuir 1991, 7, 797. (26)Steinberg, S.;Rubinstein, I. Langmuir 1992,8,1183. (27)Chen, K.; Herr, B. R.; Singeward, E. T.; Mirkin, C. A. Langmuir 1992,8,2585. (28)Kakkar, A.K.; Yitzchaik, S.; Roscoe, S. B.; Kubota, F.; Allan, D. S.;Marks, T. J.; Lin, W.; Wong, K. Langmuir 1993,9,388. (29)Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J.Am. Chem. SOC.1987, 109,2358. (30)Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J.Am. Chem. SOC.1988,110,6136. (31)Sum, W.; Kwan,V.; Atanasoka, L.; Miller, L. L. Lungmuir 1991, 7, 1419. I
Q743-7463/94l241Q-4599$Q4.5Q/Q 0 1994 American Chemical Society
Yokoyama et al.
4600 Langmuir, Vol. 10, No. 12, 1994 Scheme 1
la Nonpolar solvent N9N
m L
OTS
2a
J
Benzene I Acetic anhydride I Pyridine
or Heating
Sa
L
I
I l
3a Benzene I Acetic anhydride I Pyridine or Heating
6
0
J
4a molecular orientations of the hybrid monolayers by meansof FTIR reflection-absorption (RA) spect r o s ~ o p y ~and ~ second ~ ~ ~harmonic ~ ~ ~ , generation ~ ~ - ~ ~(SHG) measurement .37-40
Experimental Section Materials. Preparation of polyamic acid alkylamine salts la and lb in Schemes 1 and 2 has been reported in previouspapers.32,41 Octadecyltrichlorosilane (OTS) was purchased from
Langmuir, Vol. 10,No. 12, 1994 4601
Azobenzene Pendant Type Monolayers Scheme 2
0.03
L
I
J
1
