Visualization of the Molecular Shape and the Orientational Distribution

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Visualization of the Molecular Shape and the Orientational Distribution in the Langmuir-Blodgett Film of a Bent-Shaped Achiral Molecule As Studied by Surface Second-Harmonic Generation Yoshitaka Kinoshita,† Byoungchoo Park,† Hideo Takezoe,*,† Teruki Niori,‡ and Junji Watanabe‡ Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-5882, Japan, and Department of Polymer Chemistry, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-5882, Japan Received May 7, 1998. In Final Form: August 10, 1998 We have studied the anisotropic molecular orientational distributions of two-dimensional bent-shaped molecular monolayers by optical surface second-harmonic generation (SHG). The molecular monolayer of 1,3-phenylene bis[4-(((4-dodecylphenyl)imino)methyl)benzoate] (12-PIMB) was prepared by the Langmuir-Blodgett (LB) technique. Oblique incident surface SHG measurements from the monolayer showed that the bent-shaped molecules preferably aligned toward the dipping direction. The analysis of the experimental results provided the nonlinear-optical tensor components of the 12-PIMB LB monolayer. Using these components and a simple superposition model, the molecular orientational distribution function was determined. We successfully showed that the distribution map provides a view of the molecular shape.

Introduction The understanding of the molecular orientational distribution in organic films is not only of fundamental interest but also of practical importance for designing organic devices such as electrooptic and photorefractive devices.1-5 Many kinds of organic molecules have been extensively studied for understanding their physical and chemical properties associated with the molecular orientations in the past few years.1-5 Most of them have a one-dimensional structure such as charge-transfer molecules (acceptor-π electron system-donor).1-6 Up to now, these one-dimensional molecules have widely been studied to obtain the highly functionalized organic films. Recently, the studies of two-dimensional charge-transfer molecules have been reported to achieve high optical nonlinearity.7-9 In these two-dimensional charge-transfer molecules, the molecules possess two equal donor-acceptor systems linked together such as bis-dipolar molecules. It has been successfully revealed that these molecules show a high * Corresponding author: Tel: +81-3-5734-2436. Fax: +81-35734-2876. E-mail: [email protected]. † Department of Organic and Polymeric Materials. ‡ Department of Polymer Chemistry. (1) Prasad, N. P.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley: New York, 1991. (2) Chemla, D. S.; Zyss, J. Nonlinear Optical Properties of Organic Molecules and Crystals; Academic: New York, 1987. (3) Shen, Y. R. The Principle of Nonlinear Optics; Wiley: New York, 1984. (4) Cheng, L. T.; Tam, W.; Stevenson, S. H.; Meredith, G. R.; Rikken, G.; Marder, S. R. J. Phys. Chem. 1991, 95, 10631. (5) Heinz, T. F.; Tom, H. W. K.; Shen, Y. R. Phys. Rev. A 1983, 28, 1883. (6) Rasing, Th.; Bevkovic, G.; Shen, Y. R., Grubb, S. G.; Kim, M. W. Chem. Phys. Lett. 1986, 130, 2. (7) Nalwa, H. S.; Watanabe, T.; Miyata, S. Adv. Mater. 1995, 7, 754. (8) Kelderman, E.; Derhaeg, L.; Heesink, G. J.; Verboom, W.; Engbersen, J. F.; Van Hulst, N. F.; Persoons, A.; Reinhoudt, D. Angew. Chem., Int. Ed. Engl. 1992, 31, 1075. (9) Deussen, H. J.; Hendrickx, E.; Boutton, C.; Krog, D.; Clays, K.; Bechgaard, K.; Persoons, A.; Bjornholm, T. J. Am. Chem. Soc. 1996, 118, 6841.

first hyperpolarizability β.9 Thus, it is clear that the twodimensional molecules can be an important candidate for various kinds of organic devices. However, the structural analysis in the film form of these molecules has scarcely been made. One of the recent topical molecules is achiral bentshaped molecules. These molecular systems have attracted much attention because of several novel properties: (1) They exhibit several smectic liquid crystal phases, in which the bent molecules are packed to form polar ordering.10-12 (2) Because of the characteristic packing, these systems exhibit ferroelectricity or antiferroelectricity.12-15 This is the first distinct example of (anti)ferroelectricity confirmed in liquid crystals consisting of achiral molecules.12,16 (3) Chirality is introduced because of the two-dimensional molecular shape and the particular packing.15,17-19 Moreover, helix formation is suggested in some phases.17-19 (4) Some of the bent-shaped molecular systems are highly second-harmonic generation (SHG) active, and the activity seriously depends on the small difference in the chemical structures.20 To further understand the above novel properties in these liquid (10) Matsunaga, Y.; Miyamoto, S. Mol. Cryst. Liq. Cryst. 1993, 237, 311. (11) Akutagawa, T.; Matsunaga, Y.; Yashuhara, K. Liq. Cryst. 1994, 17, 659. (12) Niori, T.; Sekine, T.; Watanabe, J.; Furukawa, T.; Takezoe, H. J. Mater. Chem. 1996, 6, 1231. (13) Sekine, T.; Takanishi, Y.; Niori, T.; Watanabe, J.; Takezoe, H. Jpn. J. Appl. Phys. 1997, 36, L1201. (14) Watanabe, J.; Niori, T.; Choi, S.-W.; Takanishi, Y.; Takezoe, H. Jpn. J. Appl. Phys. 1998, 37, L401. (15) Link, D. R.; Natale, G.; Shao, R.; Maclennan, J. E.; Clark, N. A.; Korblova, E.; Walba, D. M. Science 1997, 278, 1924. (16) Blinov, L. M. Liq. Cryst. 1998, 24, 143. (17) Sekine, T.; Niori, T.; Sone, M.; Watanabe, J.; Choi, S.-W.; Takanishi Y.; Takezoe, H. Jpn. J. Appl. Phys. 1997, 36, 6455. (18) Sekine, T.; Niori, T.; Watanabe, J.; Furukawa, T.; Choi, S.-W.; Takezoe, H. J. Mater. Chem. 1997, 7, 1307. (19) Heppke, G.; Moro. D. Science 1998, 279, 1872. (20) Choi, S. W.; Kinoshita, Y.; Park, B.; Takezoe, H.; Niori, T.; Watanabe, J. Jpn. J. Appl. Phys. 1998, 37, 3408.

S0743-7463(98)00542-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/19/1998

LB Film of a Bent-Shaped Achiral Molecule

crystal systems, the study of the microscopic organization of these molecules is important. One of the powerful sample preparation techniques for this purpose is the Langmuir-Blodgett (LB) technique. As a technique to analyze the structure, SHG could be used. It is well-known that the optical SHG is a sensitive tool for studying the polar arrangement of the molecular layer.21-23 However, it was only possible to determine the average orientation of chromophores by assuming a very sharp δ-functional distribution in C∞v symmetry.5,21-42 Recently, we proposed a novel method to unambiguously determine the orientational distribution function (ODF) of molecules at interfaces with C∞v symmetry43,44 and applied this method to a spin-coated polyimide with SHGactive side chains. Application of this technique to the surfaces with in-plane anisotropy such as rubbed films for aligning liquid crystal molecules was also made using a modified maximum entropy method.45 This modified method is based on the idea of including an information of nonpolar axial ordering when using the maximum entropy method for analyzing ODF and proved to give better ODF than in the past.44,45 On the other hand, a simple model has been proposed to account for the bisdipolar character of the first molecular hyperpolarizability tensor.9 In this model, the components of the first hyperpolarizability can be expressed in terms of the hyperpolarizabilities of the one-dimensional monomeric units and the dihedral angle between two monomers. However, it is necessary to possess the dihedral angle beforehand to determine the components of the first hyperpolarizability. (21) Shen, Y. R. Annu. Rev. Phys. Chem. 1989, 40, 327. (22) Shen, Y. R. Nature 1989, 337, 519. (23) Jerome, B.; Shen, Y. R. Phys. Rev. E 1993, 48, 4556. (24) Barmentlo, M.; van Aerle, N. A. J. M.; Hollering, R. W. J.; Damen, J. P. M. J. Appl. Phys. 1992, 71, 4799. (25) Shen, Y. R. Liq. Cryst. 1989, 5, 635. (26) Rasing, Th.; Shen, Y. R. Phys. Rev. A 1985, 31, 537. (27) Rasing, Th.; Shen, Y. R.; Kim, M. W.; Grubb, S. Phys. Rev. Lett. 1985, 55, 2903. (28) Guyot-Sionnest, P.; Hsiung, H.; Shen, Y. R. Phys. Rev. Lett. 1986, 57, 2963. (29) Berkovic, G.; Rasing, Th.; Shen, Y. R. J. Opt. Soc. Am. B 1987, 4, 945. (30) Grubb, S. G.; Kim, M. W.; Rasing, Th.; Shen, Y. R. Langmuir 1988, 4, 452. (31) Mullin, C. S.; Guyot-Sionnest, P.; Shen, Y. R. Phys. Rev. A 1989, 39, 3745. (32) Shirota, K.; Kajikawa, K.; Takezoe, H.; Fukuda, A. Jpn. J. Appl. Phys. 1990, 29, 750. (33) Kajikawa, K.; Shirota, K.; Takezoe, H.; Fukuda, A. Jpn. J. Appl. Phys. 1990, 29, 913. (34) Kajikawa, K.; Takezoe, H.; Fukuda, A. Jpn. J. Appl. Phys. 1991, 30, 1050. (35) Barmentlo, M.; R. Hoekstra, F.; Willard, N. P.; Hollering, R. W. J. Phys. Rev. A 1991, 43, 5740. (36) Hsiung, H.; Beckerbauer, R. Chem. Phys. Lett. 1992, 193, 123. (37) Ashwell, G. J.; Hargreaves, R. C.; Baldwin, C. E.; Gahra, G. S.; Brown, C. R. Nature 1992, 357, 393. (38) Su, W. F. A.; Kurata, T.; Nobutoki, H.; Koezuka, H. Langmuir 1992, 8, 915. (39) Kajikawa, K.; Anzai, T.; Takezoe, H.; Fukuda, A.; Okada, S.; Matsuda, H.; Nakanishi, H.; Abe, T.; Ito, H. Chem. Phys. Lett. 1992, 192, 113. (40) Kajikawa, K.; Yamaguchi, T.; Anzai, T.; Takezoe, H.; Fukuda, A.; Okada, S.; Matsuda, H.; Nakanishi, H.; Abe, T.; Ito, H. Langmuir 1992, 8, 2764. (41) Kajikawa, K.; Anzai, T.; Takezoe, H.; Fukuda, A.; Okada, S.; Matsuda, H.; Nakanishi, H.; Abe, T.; Ito, H. Appl. Phys. Lett. 1993, 62, 2161. (42) Zhuang, X.; Wilk, D.; Marrucci, L.; Shen, Y. R. Phys. Rev. Lett. 1995, 75, 2144. (43) Yoo, J. G.; Hoshi, H.; Sakai, T.; Park, B.; Y.; Ishikawa, K.; Takezoe, H.; Lee, Y. S. J. Appl. Phys., in press. (44) Park, B.; Kinoshita, Y.; Sakai, T.; Yoo, J. G.; Hoshi, H.; Ishikawa, K.; Takezoe, H. Phys. Rev. E 1998, 57, 6717. (45) Yoo, J. G.; Park, B.; Sakai, T.; Kinoshita, Y.; Hoshi, H.; Ishikawa, K.; Takezoe, H. Jpn. J. Appl. Phys., 1998, 37, 4124.

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Figure 1. (a) Chemical structure of a 12-PIMB molecule and model geometry of the molecule. Both monomeric unit ξ’s are lying in the (1, 3) plane and make an angle of Θ. The 2-fold axis is chosen as the 3 axis of the reference frame of the molecule. (b) Experimental geometry for the surface SHG measurements from the 12-PIMB monolayer.

Despite the synthesis of novel molecules such as of bentshaped and the development of the experimental technique and analysis, the quantitative structural analysis on the well-aligned macroscopic systems of the two-dimensional molecules has not been presented yet. In this respect it is important and useful in controlling molecular alignment to clarify the surface structure of the two-dimensional system. In this paper, we report on the orientational anisotropy in a Langmuir-Blodgett (LB) film of twodimensional bent-shaped molecules by using the second harmonic generation (SHG) measurement. From the SHG intensity profiles, we determined the optical symmetries and the nonlinear optical (NLO) susceptibility of the bentshaped molecular monolayers. The associated orientational distribution function of the bent-shaped molecular monolayers was also determined quantitatively, visualizing the molecular shape and giving the dihedral angle of the two mesogenic units. Experimental Procedure The bent-shaped molecule used in this study was 1,3-phenylene bis[4-(((4-dodecylphenyl)imino)methyl)benzoate] (12-PIMB), whose chemical structure is shown in Figure 1a. The monolayer film of 12-PIMB molecules was deposited onto a glass substrate by using the conventional LB technique (vertical deposition). A 1 mM solution of 12-PIMB molecules dissolved in chloroform was prepared as a spreading solution. The measurement of the π-A isotherm and the deposition of the monolayer were carried out by using a commercial LB trough (Nippon Laser & Electronics Laboratory, NL-LB 240-MWC). SHG experiments were performed using the output beam (λ ) 600 nm) from a pulsed optical parametric oscillator (OPO, BMI AP-061) laser, producing pulses of 12 ns duration at a repetition rate of 10 Hz with a pulse energy of 8 mJ. The focused beam with a diameter of approximately 1 mm was directed onto the glass side of the sample at an incidence angle, θ0, of 45° after selecting the polarization of the incident beam. The transmitted SHG output (300 nm) from the LB film was selected by passing through a monochromator for both p and s polarizations after blocking the transmitted fundamental beam. The SHG output was detected by using a photomultiplier and boxcar system. While the sample film was rotated around the surface normal, the SHG

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Figure 2. π-A isotherm of the bent-shaped 12-PIMB molecules on the water surface. signals from the LB sample were measured in four different polarization combinations; p-p, p-s, s-p, and s-s, where the former and the latter symbols stand for the polarization directions of the polarizer and analyzer, respectively. The SHG wavelength (300 nm) is close to an absorption peak (350 nm) of 12-PIMB and therefore in all cases the SHG background of the substrates is negligible compared with the signal from the LB film. By continuously monitoring the SHG from the monolayer as time elapsed, we confirmed that neither 600 nm laserinduced damage nor molecular desorption occurred in the LB sample. All the measurements were performed at a temperature of 25 °C. Figure 1b shows the surface SHG experimental geometry, where the angle φ denotes the azimuthal angle for sample rotation with respect to the surface normal in the sample coordinates (x, y, z), where x represents the dipping direction, y is perpendicular to x axis in the film surface plane, and z is the direction normal to the film surface.

Experimental Results Figure 2 shows the π-A isotherm of the bent-shaped molecules on the water surface. As shown in the figure, the surface pressure rose steeply from 5 mN/m, and the collapse pressure was as high as 40 mN/m. The extrapolated molecular area is about 50 Å2. The total molecular length estimated from the smectic layer spacing is about 43 Å.17 Even the layer spacing extrapolated to zero chain length is 25 Å.17 Hence the molecular area 50 Å2 seems to be too small. The film may have a multilayered structure. However, the overall π-A isotherm shows that the stable quasi-monolayer of the 12-PIMB is formed at the air-water interface on compression. At the surface pressure of π ) 20 mN/m, a closely packed LB monolayer was transferred onto the glass substrate by the vertical dipping method. The surface specific SHG measurements were performed to clarify the surface structure of the 12-PIMB LB film. The anisotropic surface SHG response with respect to the sample rotation φ about the substrate normal directly reflects the symmetry of the monolayer. Here φ ) 0° when the dipping direction (x) is in the incidence plane. Figure 3 shows the polar plots of the SHG intensities from the 12-PIMB monolayer for the four different polarization combinations. The radial length represents the SHG intensity. If the 12-PIMB monolayer on the glass substrate exhibits C∞v symmetry, the spolarized SHG signal from the monolayer is forbidden and the p-polarized signal should be isotropic. However, the observed SHG signals from the 12-PIMB monolayer

Figure 3. SHG intensity from the monolayer of 12-PIMB on a glass substrate as a function of rotation angle φ. Open circles are experimental data, and the best theoretical fit is also shown.

exhibit substantial anisotropy. This reveals that the 12PIMB monolayer shows a C1v symmetric arrangement on the glass substrate. Particularly, the different responses of SHG for φ ) 0° and φ ) 90° directly reflect the preferential alignment of 12-PIMB molecules along the dipping (x) direction. Discussion Since the monolayer film takes C1v symmetry, the surface NLO susceptibility tensor χ(2) can be described in terms of six independent components:23,44 χ1 ) χzzz, χ2 ) χxxx, χ3 ) χzyy ) χyzy ) χyyz, χ4 ) χzxx ) χxzx ) χxxz, χ5 ) χzxz ) χzzx ) χxzz, and χ6 ) χxyy ) χyxy ) χyyx. Here Kleinmann’s symmetry is assumed for simplicity, though the SH wavelength is in the absorption region. After the components of χ(2) are properly transformed, the effective NLO coefficients can be deduced theoretically for the inputoutput polarization combinations with the appropriate local field factors.15,17 With these theoretical expressions of the effective NLO coefficients, one can obtain all six independent components of χ(2) for the monolayers by fitting the experimental SHG data. In the practical fitting, we assumed the refractive indices, nω ) 1.5 and n2ω ) 1.6 and a thickness of 1 nm. From the best theoretical fit to the SHG data of the 12-PIMB monolayer, we determined the relative values of the χ(2) elements for the 12-PIMB monolayer: χ1:χ2:χ3:χ4:χ5:χ6 ) 1.00:-0.29:0.87:1.88:-0.01:0.02. The fitted results are also shown in Figure 3. As shown in the figure, it is clear that the fitted results are in good agreement with the experimental results. It is noted that the components of χ2, χ5, and χ6 are smaller than the others; i.e., the symmetry of this monolayer is close to C2v. A model has been developed to express the first hyperpolarizability β of the bis-dipolar molecules in terms of the hyperpolarizabilities of the one-dimensional monomeric units and the dihedral angle Θ between the two monomers.9 For the linked rodlike monomeric unit, it is assumed that the first hyperpolarizability is dominated by a single element βξξξ, where ξ is in the direction of the charge transfer in the monomeric unit, as shown in Figure 1a. According to this model, the β components of the twodimensional bis-dipolar molecule can be analyzed by the

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superposition of the monomeric contribution. Since the molecule in this model has C2v symmetry, there are only four nonzero β components

β333 ) 2 cos3(Θ/2)βξξξ β311 ) β131 ) β113 ) 2 cos(Θ/2) sin2(Θ/2)βξξξ

(1)

where (1, 2, 3) refers to the reference frame of the bisdipolar molecule, as shown in Figure 1a. Thus, if the angle Θ/2 is determined, then one can calculate the ratio of the two independent tensor components according to β311/β333 ) (tan(Θ/2))2. In this paper, we extend this model to the macroscopic nonlinear susceptibility tensor χ(2) without explicitly introducing the dihedral angle Θ. Namely, χ(2) of the bis-dipolar monolayer can be written in terms of the monomeric hyperpolarizability βξξξ by

ˆ )(jˆ‚ξˆ )(kˆ ‚ξˆ )〉βξξξ χ(2) ijk ) 2Ns〈(ıˆ‚ξ

(2)

where Ns is the surface density of the bis-dipolar molecules, (i, j, k) refers to the sample coordinates (x, y, z), and the brackets denote the orientational average weighted by a distribution function f. Thus, χ(2) of the monolayer of the bent-shaped molecular system was expressed as a sum of molecular hyperpolarizability βξξξ of the one-dimensional monomeric unit free from the separate determination of Θ. Since the monolayer film takes C1v symmetry, as shown in the above section, the χ(2) tensor can be described in terms of six independent components:

χ1 ) 2Ns〈cos3(θm)〉βξξξ χ2 ) 2Ns〈sin3(θm)cos3(φm)〉βξξξ χ3 ) 2Ns〈(cos(θm) - cos3(θm))(1 - cos2(φm))〉βξξξ (3) χ4 ) 2Ns〈(cos(θm) - cos3(θm)) cos2(φm)〉βξξξ χ5 ) 2Ns〈(sin(θm) - sin3(θm)) cos(φm)〉βξξξ χ6 ) 2Ns〈sin3(θm)(cos(φm) - cos3(φm))〉βξξξ where θm is the polar angle between the monomeric axis ξ and the z axis in sample coordinates. φm is the azimuthal angle between ξ and the x axis in sample coordinates. We can then estimate the orientational distribution f(θm, φm) of the monomer units with respect to the sample coordinates (x, y, z) by using the modified maximum entropy method with the determined NLO coefficients.44,45 The obtained orientational distribution function f(θm, φm) of the monomeric units for the 12-PIMB monolayer is shown in Figure 4a. The directional density of βξξξ in the (x, y, z) coordinate system is also shown in Figure 4b, where the length between the origin (0, 0, 0) and each point (x, y, z) on the surface is proportional to the number of βξξξ directing toward (x, y, z). As shown in the figures, the azimuthal distribution of monomeric units for 12PIMB monolayer exhibits an anisotropy; i.e., the monomeric units align preferentially along the dipping (x) direction. The two major molecular tilt angle distributions are found at (θm ) 64.9°, φm ) 0°) with a half-width ∆θ of 26.4° and (θm ) 63.0°, φm ) 180°) with a ∆θ of 27.0°. It is noted that the polar angles of the 12-PIMB molecules

Figure 4. 3-dimensional plots of the orientational distribution function f(θm, φm) of the 12-PIMB molecules: (a) the distribution as functions of θm and φm; (b) an alternative polar illustration of f(θm, φm), where the radial length between each point at the surface and the origin represents the amount of the distribution.

in the parallel (φ ) 0°) direction and in the antiparallel (φ ) 180°) direction are nearly the same. The small difference may be caused by the dipping action. The distribution density (51.4%) and shape of the distribution function in the +x direction are nearly the same as those in -x direction. The dihedral angle between the two directions of the largest density is determined to be 127°, which agrees with the angle determined by NMR, 122°.46 Thus, it is safely concluded that the one monomeric unit of the 12-PIMB molecule aligns toward the +x direction and the other unit aligns toward the -x direction. Moreover, we successfully showed that the plot of ODF and the directional density of β provide a means to visualize the shape of the molecule. Let us estimate the in-plane anisotropy of the 12-PIMB monolayer. A surface in-plane order parameter of monomeric units, 〈cos 2φm〉, is introduced for the estimation. Here, values of 〈cos 2φm〉 between 0 and 1 describe a degree (46) Grande, S. Abstract published in Workshop of Banana-Shaped Liquid Crystals: Chirality by Achiral Molecules; Berlin, 1997; p 5.

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of in-plane ordering between completely isotropic and completely ordered. For the 12-PIMB film, 〈cos 2φm〉 induced by dipping was 0.33. Another order parameter for the in-plane polar anisotropy is 〈cos φm〉. 〈cos φm〉 directly represents the parallel/antiparallel asymmetry of the orientation for monomeric units. For an antiparallel distribution, 〈cos φm〉 is zero, and for completely parallel ordering 〈cos φm〉 is 1. For the 12-PIMB LB film, the value of 〈cos φm〉 was 0.03. Therefore, the molecular orientation of 12-PIMB with respect to the dipping direction is nearly symmetric. Two surface-order parameters are also introduced to estimate the polar angle distributions of the 12-PIMB monolayer: 〈cos θm〉 and 〈cos2 θm〉. 〈cos θm〉 and 〈cos2 θm〉 resemble the polar- and the axial-order parameters with respect to the surface normal direction for the monolayer, respectively. For the 12-PIMB LB film, the obtained value of 〈cos θm〉 was 0.42 and that of 〈cos2 θm〉 was 0.19. Thus the 12-PIMB molecules are highly polar ordered. Finally, we want to comment on the hyperpolarizability β333 of the 12-PIMB molecules. With the assumption that the dihedral angle Θ is the same as the angle between the two major molecular tilt angles in f(θm, φm), Θ ) 127°, we could deduce that β311/β333 for the bent-shaped molecules is 4.18. We expect that this ratio can be used for analyzing the nonlinear optical response of the 12-PIMB molecular systems, such as hyper-Rayleigh scattering or electricfield-induced second-harmonic generation.

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Conclusions In summary, we studied the molecular orientation in the monolayer LB films of two-dimensional bent-shaped 12-PIMB molecules by using the surface SHG measurements. From the SHG results, it was found that the symmetry group of the LB monolayers is C1v, it is very close to C2v. The relative values of the second-order NLO coefficients of the LB monolayers were determined selfconsistently from the SHG intensity profiles. The associated orientational distribution function of the LB monolayer was also determined quantitatively by using the modified maximum entropy method. The result clearly visualizes the molecular shape and the orientational distribution. From the obtained f(θm, φm), we could also estimate the degrees of asymmetry from the in-plane and the polar orientational order parameters for the monolayer system. Moreover, the ratio of the molecular hyperpolarizability tensor components was deduced from the measurement. It was demonstrated that one can make the well-aligned LB film of two-dimensional bent-shaped molecules. Both the experimental technique and the analysis can be easily generalized to the study of various organic systems of two-dimensional bis-dipolar molecules. Acknowledgment. B. Park is supported by the Korean Science & Engineering Foundation (KOSEF). LA980542I