Investigation of the Molecular Organization in Langmuir-Blodgett Films Using Polarized Infrared Spectra: Comparison of Two Methods H. Hui-Litwin, † L. Servant,* ,‡ M. J. Dignam, § and M. Moskovits Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, Ontario M5S 1A1, Canada Received June 24, 1997 X We compare two methods for obtaining structural information on Langmuir-Blodgett films from polarized infrared spectroscopy. For sufficiently uniform films, we have already shown (refs 11 and 12) that all polarized spectroscopic properties could be characterized by a single quantity that we call the “electrical surface susceptibility tensor”, γ ˜ . The imaginary parts of the susceptibility tensor could be readily obtained from reflectance measurements with the electric field parallel (Im(γ t)) and perpendicular (Im(γn)) to the plane of the film, independently of any specific properties of the film. This, in turn, could be related to the characteristics of individual molecules comprising the film, and their geometric disposition if the molecules are assumed to be interacting dipoles, which corrects, to a large extent, for the local field effects. We report the results of an infrared study performed on a dipalmitoylphosphatidylethanolamine monolayer. Attenuated total reflection spectra for both s and p polarization were recorded, and the spectra of Im(γ t) and Im(γ n), as well as those of the imaginary part of the refractive index (kt and kn), are presented. Finally, the information deduced from the Im(γx) and kx (x ) s, p) are compared, and the molecular orientation is discussed both in terms of Im(γ x) and kx. We show that by using this approach one obtains more truthworthy measurements of the disposition of oriented molecules at surfaces than those obtained from absorption coefficients. 1. Introduction Organic thin films offer an attractive method of design- ing molecular materials for various applications. The properties of such films arise not only from the specific characteristics of the molecules but also from their arrangement within the film. 1,2 Polarized infrared spec- troscopy has been widely used to obtain information on molecular organization. Since the infrared (IR) absorption depends on the relative orientation of the incident electric field and of the dipole transition moment belonging to the molecular vibration, it is possible to obtain information on the orientation of the molecules by using polarized radiation. In most studies incorporating this approach, the re- flectance spectra of a film lying on a substrate are recorded for the electric field parallel and perpendicular to the incident plane. The reflectance is defined as the ratio of the reflected intensity of the film-covered substrate to that of the bare substrate. The results are then discussed in terms of a three-phase stratified model, with an anisotropic film of thickness d, lying between the ambient and the substrate phase. 3-6 Here, the connection between the spectroscopic observables and the mean molecular orientation can be derived in principle, but it is not a simple one. The problem arises from the contribution of the radiation-induced dipole moment to the local electric field experienced by the oscillators. In general, the local electric field acting on a molecule is modified by the electric field radiated by all the surrounding induced dipoles. As a consequence, in an anisotropic medium, the effect of the local field is to alter the direction of the transition moments so that they do not correspond with those of an equivalent set of well-separated molecules. As pointed out by several authors, 1,2 the problem of taking into account local field effects in evaluating molecular parameters is not a simple task. 3-10 The Lorentz-Lorenz local field expressions are likely to be invalid for ordered Langmuir-Blodgett (LB) systems, where more detailed calculations are required. 7 With this in mind, we developed a new approach to evaluate the optical response of an anisotropic molecular layer, taking into account the local field effect, and directly connecting it to measurable spectroscopic quantities. 11-15 We show in particular, that the reflectance spectra of a thin uniaxial film lying on a substrate could be understood by considering the elements of a single tensor: the electric surface susceptibility tensor (γ ˜ ), which is related simply to the density of the induced dipoles per unit area. In addition, we show that the imaginary parts of the tangential and normal components of γ ˜ (Im(γ t ) and Im- (γ n )) could be readily obtained directly from the experi- mental data, without any assumptions regarding film † Now at Centre for Infection and Biomaterials Research, The Toronto Hospital, Bell Wing, Ground Fl., Rm. 631, 200 Elizabeth St., Toronto, ON M5G 2C4, Canada. ‡ Now at Laboratoire de Spectroscopie Mole ´ culaire et Cristalline, URA 124 CNRS, Universite ´ Bordeaux I, 351, Cours de La Libe ´ration, 33405 Talence Cedex, France. Author to whom correspondence should be sent. § Deceased. X Abstract published in Advance ACS Abstracts, December 1, 1997. (1) Roberts, G. Langmuir-Blodgett films; Plenum Press: New York, 1990. (2) Ulman, A. An Introduction to Ultrathin Organic films; Academic Press: London, 1991. (3) Buontempo, J. T.; Rice, S. A. J. Chem. Phys. 1993, 98, 5825. (4) Ahn, D. J.; Franses, E. I. J. Phys. Chem. 1992, 96, 9952. (5) Chollet, P. A. Thin Solid Films 1980, 68, 13. (6) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927. (7) Cnossen, G.; Drabe, K.; Wiersma, D. A. J. Chem. Phys. 1992, 97, 4512. (8) Brossard, Ch.; Kupfer, M.; Florsheimer, M.; Borer, T.; Gunter, P.; Tang, Q.; Zahir, S. Thin Solid Films 1992, 210/211, 198-201. (9) Shen, Y. R. Annu. Rev. Phys. Chem. 1989, 40, 327. (10) Munn, R. W.; Shabat, M. M. J. Chem. Phys. 1993, 99, 10059. (11) Servant, L.; Dignam, M. J. Thin Solid Films 1994, 242, 21. (12) Servant, L.; Dignam, M. J. Unpublished work. (13) Bardwell, J.; Dignam, M. J. Fourier Transform Polarimetry. In Fourier transform Infrared Characterization of Polymers; Ishida, H, Ed.; Plenum Publishing Corp.: New York, 1987. (14) Dignam, M. J. Fourier Transform Polarization Spectroscopy. Appl. Spectrosc. Rev. 1988, 24 (1&2), 99. (15) Dignam, M. J.; Moskovits, M.; Stobie, R. W. Trans. Faraday Soc. 1971, 67, 3306. 7211 Langmuir 1997, 13, 7211-7216 S0743-7463(97)00676-8 CCC: $14.00 © 1997 American Chemical Society