Anisotropy in Langmuir Layers of a Bent-Core Liquid Crystal Ji Wang, ² Lu Zou, ² Antal Ja ´kli, ‡ Wolfgang Weissflog, § and Elizabeth K. Mann* ,² Department of Physics, and Liquid Crystal Institute, Kent State UniVersity, Kent, Ohio 44242-0001, and Institut fuer Physikalische Chemie, UniVersitaet Halle-Wittenberg, D-06109 Halle Germany ReceiVed NoVember 23, 2005. In Final Form: January 24, 2006 Langmuir layers of a symmetric bent-core molecule with hydrocarbon end chains and two chlorine atoms substituted on the central phenyl ring of the bent core were characterized by a combination of surface pressure isotherms, Brewster angle microscopy, and surface potential measurements. These layers were found to be optically anisotropic, in contrast to Langmuir layers of similar molecules with different substitutions on the core. After compression, the orientation of the optical axis was essentially uniform over the film. Upon decompression, the film broke into uniform islands or domains. Measuring domain reflectivity while changing the domain orientation allowed the determination of the tilt angle with respect to both domain features and the film normal, as well as the refractive index anisotropy. The tilt angle, near 90°, suggests that the bent-core molecules lie quite flat on the surface. Introduction Bent-core molecules have attracted more and more attention recently due to the rich variety of phases they exhibit. 1 Ferroelectric phases appear because the special structure of the bent-core molecules constrains the molecule packing and the mobility. Chiral liquid crystalline phases are obtained by the spontaneous chiral organization of the achiral bent-core mol- ecules. 2 A dozen different liquid phases 3 and five smectic phases 4 have been suggested. The long-sought biaxial nematic phase was recently claimed in molecules with this architecture. 5 The usefulness of bent-core molecules in scattering switching and in storage devices has been demonstrated. 6 It has also been suggested that the unique properties of these molecules can make them useful for electromechanical devices. 7 As molecular functional materials, bent-core molecules can be used in various kinds of organic devices. 8 Usually, organic functional materials have been processed in the form of films serving as active layers in devices; thus, studies of growth mechanism, molecular ordering, and the overall film morphology are of prime importance for device design. 7 Several groups have explored the molecular packing of such molecules in bulk both experimentally 9,10 and theoretically. 11 Dong et al., 10 using 13 C NMR, found the bending angle for different substitution compound of a series of bent-core molecules and a nonzero twist angle. Dewar et al. 11 found that a molecular model consisting of seven Lennard-Jones spheres gave better agreement with experimental phases than a less detailed model consisting of two hard-spherocylinders. They found that the phases exhibited by the bent-core molecules depend greatly on the bent- core angle, especially at a surface. Langmuir layers can give additional insight into the molecular packing within layers. A stable Langmuir layer, transferred to a solid interface, may form a natural alignment layer for bent- core liquid crystals. To our knowledge, four sets of articles consider Langmuir layers of such molecules. The first considers a single bent-core molecule with long hydrophobic side chains. 12 A very recent article, Blinov et al., 13 considers the dielectric, ferroelectric, and antiferroelectric properties of Langmuir- Blodgett films of similar bent core molecules. A third set of articles considers two different cores with very short hydrophobic side chains. 14 Previously, 15 we studied Langmuir layers of five bent core molecules varying both the core and the end-chains but maintaining molecular symmetry, with identical end-chains on either end of the core. The characterization includes systematic surface pressure isotherms, Brewster angle microscopy (BAM), 16 and surface potential measurements. We demonstrated that it is possible to make stable Langmuir layers of a variety of different bent-core molecules. Two were siloxane end-chain molecules. With these amphiphilic end-chains, the molecules lie quite flat on the surface, with both core and end-chains in direct contact with the air/water interface. The other three molecules were hydrocarbon end-chain molecules, with groups of different hydrophobicity substituted at the inner angle of the core. With these hydrophobic chains, the molecules form a complex multilayer structure; surface potential, surface pressure, and other * Corresponding author. E-mail: emann@kent.edu. Tel: 330-672-9750. Fax: 330-672-2959. ² Department of Physics, Kent State University. ‡ Liquid Crystal Institute, Kent State University. § Universitaet Halle-Wittenberg. (1) Pelzl, G.; Diele, S.; Weissflog, W. AdV. Mater 1999, 11, 707. (2) Link, D. R.; Natale, G.; Shao, R.; Maclennan, J. E.; Clark, N. A.; Korblova, E.; Walba, D. M. Science 1997, 278, 1924. (3) Lubensky, T. C.; Radzihovsky, L. Phys. ReV.E 2002, 66, 031704. (4) Brand, H. R.; Cladis, P. E.; Pleiner, H. Eur. Phys. J. B 1998, 6, 347. Roy, A.; Madhusudana, N. V.; Toledano, P.; Figueiredo Neto, A. M. Phys. ReV. Lett. 1999, 82, 1466. (5) Acharya, B.; Primak, A.; Kumar, S. Phys. ReV. Lett. 2004, 92, 145506. Madsen, L. A.; Dingemans, T. J.; Nakata, M.; Samulski, E. T. Phys. ReV. Lett. 2004, 92, 145505. 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