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Langmuir 1995,11, 1658-1665
Langmuir-Schaefer Films: Head Group Influence on Orientation of Substituted Styryl Bipyridines K. K. Balasubramanian,? V. Cammarata," and Q. Wu Department of chemistry, Auburn University, Auburn, Alabama 36849-5312 Received October 23, 1994. I n Final Form: January 26, 1995@ Nine new 4,4'-bis(4-substituted-styryl)-2,2'-bipyridineswere synthesizedand characterized. These new materials and 4,4'-bis(4-methylstyry1)-2,2'-bipyridine were applied to the subphase of a Langmuir trough. Eight of the ten compounds (R = CN, COOCH3, N(CH&, COOH, OCOCH3, NOz, OCH3, CH3) made stable Langmuir films. These films were transferred by horizontal lifting to a variety of solid surfaces with transfer ratios close to unity. The transferred films were analyzed by transmission UV-vis, transmission IR, grazing angle reflectance IR, and polarized attenuated total reflectance IR spectroscopies. Of the eight produce materials, two, 4,4'-bis(4-acetylstyry1)-2,2'-bipyridine and 4,4'-bis(4-methylstyryl)-2,2'-bipyridine, stable, highly anisotropic films. The other compounds form films with varying anisotropy from moderately anisotropic to totally isotropic. Thus, functional groups can have a profound influence on the ability of these materials t o form stable LS films and direct the net material orientation within the films. There is much current interest in the exploitation of anisotropicfilms for a variety of app1ications.lr2 Examples of applications in which anisotropy is necessary include nonlinear optic^,^ vectorial charge transport in photovolt a i c ~and , ~ piezoelectric and ferroelectric material^.^,^ A wide variety of molecules with the necessary properties have been produced, and their noncentrosymmetric arrangement onto a suitable substrate has been achieved in a number of ways.7 One of the most elegant methods is the engineering of molecular multilayers with the necessary macroscopic orientation using the LangmuirBlodgett (LB) and Langmuir-Schaefer (LS) deposition techniques. These techniques differ in only the geometry of the deposition. Typical LB molecules have a n active group, e.g., a chromophore or electrophore, and a long aliphatic chain, which provides organization within each layer.8 In order to expand the scope of LB and LS materials, it is of interest to study compounds which lack the organizing aliphatic taiLg LB and LS films from such compounds would, in principle, afford a much higher density of active groups in each layer. The organization
* Address correspondence to this author. +Authors a r e listed in alphabetical order. Abstract published in Advance A C S Abstracts, May 1, 1995. (1)Roberts, G. G. Ed. Langmuir-Blodgett Films, Plenum: NewYork, 1990. (2)Ulman, A. Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991. (3)Prasad, P.N.; Williams, D. J.;Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley: New York, 1991. Marder, S. R.; Sohn, J. E., Stuckey, G. D. Eds. Materials for Nonlinear Optics: Chemical Perspectiues;ACS Symposium Series 455;American Chemical Society: Washington, DC, 1991. (4)Fujihira, M.; Nishiyama, K.; Yamada, H., Thin Solid Films 1986, 132,77.Polymeropoulos, E. E.; Mobius, D.; Kuhn, H. Thin Solid Films 1980,68,173. Eichberger, R.; Willig, F.; Storck, W. Mol. Cryst. Liq. Cryst. 1989,175,19.Popovitz-Biro, R.; Hill, K.; Lahav, M.; Leiserowitz, L.;Sagiv, J.; Hsing, H.; Meredith, G. R.; Vanherzule, H. J.A m . Chem. SOC.1988,110,2672. (5)Novak, V. R.; Myagkov, I. V. Sou. Tech. Phys. Lett. 1986,11,159. Muller, P.; Gallet, F. J.Phys. (6)Eaton, D. E. Science 1991,253,281; Chem. 1991,95,3257. (7)Hann, R. A,;Bloor, D. Eds. Organic Materials forNonlinear Optics, Royal Society of Chemistry: London, 1981. (8)Schoondorp, M. A,; Schouren, B. J.; Hulshof, J. B. E.; Feringa, B. L. Langmuir 1992,8,1825. Sibouklis, J.; Cresswell, J. P.; Isalita, N.; Pearson, C.; Maddaford, P. J.;Ancelin, H.; Yanvood, J.; Goodwin, M. J.; Carr, N.; Feast, W. J.; Petty, M. C. J . Phys. D.: Appl. Phys. 1989, 22,1608.Ancelin, H.; Briody, G.; Lloyd, J . P.; Petty, M. C.; Ahmad, M. M.; Feast, W. J. Langmuir 1990,6, 172. Ulman, A,; Scarringe, R. P. Langmuir 1992,8,894.Ulman, A.Adu.Mater. 1991,3(61,298.Jones, R.; Tredgold, R. H.; Hodge, P. Thin Solid Films 1983,99,25.RuaudelTeixier, A.; Barraud, A.; Belbeoch, B.; Roulliay, M. Thin Solid Films 1983,99,33. @
would have to be intrinsic to the active group or one or more functional groups. Comparative studies of different functional groups can be particularly useful for systematic control of aggregate formation, molecular orientation, and electronic structure in LB and LS films. The information gathered may lead to molecular design of new LB and LS films having planned structures and properties. There have been few reports dealing with substituentdependent structural changes in LB films to date.1° The question is, what are the necessary molecular attributes that will lead to organized and transferable films? Also, if this can be accomplished, what exactly are the forces involved and how do they lead to organization? In this paper we will show that aggregation can lead to good anisotropy. A few materials lacking aliphatic chains have been shown to be oriented and transferred using LB and LS methods. These include oligomeric diimides and polyaromatic quinones by Miller and co-w~rkers,~Jbisepidithiotetracene by Wegmann et a1.,12 phenylenevinylene amphiphiles,13tetracyano-p-quinodimethaneder i v a t i v e ~ carotenoids,15 ,~~ and oligothiophenes.16 Our work in this direction has involved conjugated aromatic molecules like distyrylbipyridines with different substituents a t the styryl end (see Scheme 1). Here we report on LS films derived from distyrylbipyridines containing 10 different substituents. These substituents change both the hydrophobicity and the polarizability of the molecules. The substituents influence the net orientation of these kinds of molecules. Three polarized FTIR methods and transmission UV-visible spectroscopyhave (9)Cammarata, V.;Atanasoska, L.; Miller, L. L.; Kolaskie, C. J.; Stallman, B. J. Langmuir 1992,8, 876. (10)Lehmann, U. Thin SolidFilms 1988,160,257; Saito, K.; Ikegami, K.; Kurodan, S.; Tabe, Y.; Sugi, M. J. Appl. Phys. 1992, 71, 1401. Enomoto, S.;Ozaki, Y.; Kuramoto, N. Langmuir 1993,9,3219. (11)Kenny,P. W.;Miller,L.L.;Rak, S.F.;Jozefiak,T.H.;Christopfel, W. C.; Kim, J.-H.; Uphaus, R. A. J . A m . Chem. Soc. 1988,110,4445. (12)Wegmann, A.; Tieke, B.; Mayer, C. W.; Hilti, B. J.Chem. SOC., Chem. Commun. 1989,716. (13)Kunitake, T.; Watakabe, A. Thin Solid Films 1990,186,L21. (14)Barraud, A,; Lesieur, P.; Richard, J.; Ruaudel-Teixier, A.; Vandevyver, M. Thin Solid Films 1985,133,125.Bertho, F.;Talham, D.; Robert, A.; Batail, P.; Megtert, S.; Robin, P. Mol. Cryst. Lip. Cryst. 1988,156,339. Azuma, M.;Miura, A,; Naito, K. Langmuir 1991,7, 627. (15)Palacin, S.;Blanchard-Desce, M.; Lehn, J. M.; Barraud, A. Thin Solid Films 1989,178,387. (16)Nakahara, H.; Nakayama, J.; Hoshino, M.; Fukuda, K. Thin Solid Films 1988,160, 87.
0743-7463/95/2411-1658$09.00/00 1995 American Chemical Society
Langmuir, Vol. 11, No. 5, 1995 1659
Langmuir-Schaefer Films Scheme 1
sample compartment. ATR dichroicratios were correctedat each wavelength for polarizer efficiencyusing the followingequation: 18
R
R
cisoid
transoid
1 R=Br
6 R=CN
2 R=OH
7 R=OMe
3 R = COzH
8 R=N(Me)z
4 R=COzMe
9 R=N02
5 R=OzCMe
10R=Me
been used to elucidate the net orientation of transferred multilayer films.
Experimental Section Synthesis. All aldehydes (Aldrich) and 4,4'-dimethyl-2,2'bipyridine (Aldrich) were used as received. Acetic anhydride (Aldrich) was vacuum distilled. Anhydrous potassium acetate (Fischer) and anhydrous zinc chloride (Aldrich) were used as received. Diisopropyl amine (Aldrich)was distilled from KOH. n-Butyllithium in hexanes (Aldrich) was used as received. Trifluoroacetic acid (Aldrich)was used as received. Chloroform (Fisher) was passed through a column of adsorption grade alumina to remove acidic impurities. 13C and lH NMR spectra were obtained on a Bruker 250 multiprobe spectrometer. Elemental analyses were performed by Atlantic Microlabs. Mass spectrometry data were provided by the Auburn Mass Spectrometry Laboratory. The specific results for the synthesis and characterization are included in the supplementary material. Langmuir Trough. LS experiments were performed with a symmetric compression KSV3000 Langmuir-Blodgett trough. Surface pressures were measured with a flame-cleanedPt-plate Wilhelmy balance. Spreading solutions of distyrylbipyridines were typically 1.0 mg/mL in CHC13 or a 10% v:v TFAXHC13 mixture. The subphase was 18 MR Millipore filtered water. Trough temperature was fixed between 5 and 25 "C by a VWR model 1166 heating-cooling bath. Compression speeds were typically 5 mdmin. After the film was compressedto the desired pressure, 15 dydcm, the film was held at that pressure for 3060 min before transfer. Transfer was accomplished using the horizontal lifting technique.17 ATeflonvise was used to hold the substrate parallel to the monolayer film. Transfer was accomplished by lowering the substrate at 17 mdmin until the substrate just contacted the film. The motion of the substrate was then reversed until the substrate-subphase interface delaminated. Substrates. Au and Al films were thermally evaporated at a pressure of
