11. Nematic, columnar, and smectic mesomorphism exhibited by phasmidic molecules that self-assemble via hydrogen bonding

Acid-functionalized azo molecules, such as shown in Figure 1, self-assemble to form dimers (or, trimers) through hydrogen-bonding, resulting in phasmidic associations. We  explored equimolar binary mixtures with several simpler benzoic acids (e.g., 4-Octyloxy-benzoic acid) to gain insight of such associations by forming hetero-dimers. The pure compounds and their mixtures form the nematic phase with an underlying LC phase that can be either columnar or smectic, dependent on the composition of the mixtures. Results obtained under an in-situ magnetic field, reveal unique structural characteristics in x-ray diffraction patterns (Figure 2), which are unlike those of the conventional calamitic nematic phase and suggest the formation of non-linear dimers via hydrogen bonding. The mesomorphic properties of these new azo compounds are under investigation using the conventional characterization techniques. X-ray diffraction, electro-optical techniques, and micro-Raman spectroscopy are being used to determine the nature of molecular association with the goal of understanding the unusual x-ray diffraction pattern and switching properties of the nematic phase. Our results strongly suggest the formation of complex bent-core-like or tetramer mesogenic assemblies in pure system, rather than conventional coplanar linear dimers.

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Figure 1. Possible dimer formation of acid functionalized azo molecules via H-bonding.

Figure 2. XRD pattern of  4-Octyloxy-benzoic acid in the nematic phase. White double arrow indicate direction of magnetic field.

Presentations

1. Liquid Crystalline Behavior of the Dimeric Mesogens Formed by Intermolecular Hydrogen Bonding, S.-Y. Jeong, S.-W. Kang, V. Prasad, S. K. Varshney, and S. Kumar, Poster Presentation,23rd International Liquid Crystal Conference, Krakow, Poland, July 11–16 (2010).

2. Liquid Crystalline Behaviors of H-Bonded Dimer Formed from the Semi-Phasmidic Type Carboxylic Acid, S.-W. Kang, S.-W. Kang, S. Kumar, V. Prasad, and S. K. Varshney,APS March Meeting, Pittsburgh, PA, March 16-20 (2009)

3. Unconventional Dimerization in Mesogenic Semi-Phasmidic Type Carboxylic Acid, S.-W. Kang, S.-Y. Jeong, D. M. Agra-Kooijman, S. Kumar, V. Prasad, and S. K. Varshney, APS March Meeting, Pittsburgh, PA, March 16-20 (2009)

4. Unusual Supramolecular Associations of Phasmidic Type Carboxlic Acid: Formation of Bent-core Dimeric Mesogen and Its Mesophasic Behaviors S.-W. Kang, S.-Y. Jeong, S. Kumar, V. Prasad, S. K. Varshney, International Symposium on Biaxial Nematic Liquid Crystals, Kent, OH, October 27-28 (2008).

Project List: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11