Liquid Crystalline Nano-Segregated Structures in Hydrogen-Bonded Complexes of Fluoroalkyl Substituted Benzoic Acids
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Liquid Crystalline Nano-Segregated Structures in Hydrogen-Bonded Complexes of Fluoroalkyl Substituted Benzoic Acids Etsushi Nishikawa, Jun Yamamoto and Hiroshi Yokoyama Japan Science and Technology Corporation, Yokoyama Nano-structured Liquid Crystal Project TRC, 5-9-9 Tokodai, Tsukuba, Ibaraki 300-2635, Japan ABSTRACT Several new benzoic acid derivatives having perfluorinated substituents were synthesized and their hydrogen-bonded complexes with 4,4’-dipyridyl were prepared. In these acid/base complexes the incompatibility between perfluoroalkyl moieties and hydrocarbon parts is large, which can lead to organize nano-segregation structures. We have found in one of such complexes, that has a long flexible fluorinated moiety, a thermotropic cubic phase with Ia3d symmetry formed by double gyroid of two interpenetrating jointed rod networks with an estimated cell parameter of 10.9 nm. Furthermore another complex with a branched long perfluoroalkyl terminal chain exhibits a first order smectic A to smectic C phase transition, which is confirmed with thermal analysis detecting a large enthalpy change of 5.3 kJxmol-1, X-ray scattering experiments revealing a tilt angle jump, and polarized optical microscopy observing a remarkable texture change at the phase transition temperature. INTRODUCTION Amphiphilic molecules composed of two different kinds of moieties with nature, e.g. polar /non-polar, rigid/flexible, bulky/planar and hydrophobic/hydrophilic etc. can self-organize a variety of supramolecular structures [1]. The fluorophilic/fluorophobic interaction, which exists among fluorinated moieties and hydrocarbon parts, has been also used as amphiphilic character to synthesize new materials, and the structures of organized liquid crystalline phases are investigated [3 – 5]. Kölbel et al., for instance, applied successfully this effect to form supramolecular liquid crystalline structures, e.g., rectangular columnar phases with c2mm or p2gg symmetry [6]. Recently inter-(and intra-)molecular hydrogen bonding is recognized as an effective interaction to organize self-assemblies forming liquid crystalline phases [7, 8]. One of the remarkable examples forming supramolecular liquid crystalline structures through hydrogen bonding is a homologue series of 4’-n-alkoxy-3’-nitrobiphenyl-4-carboxylic acids, which are well-known to show thermotropic cubic phases with e.g. the structure of double gyroid of two interpenetrating jointed rod networks with Ia3d symmetry [9, 10]. Therefore it is expected that combining such non-covalent interactions in one molecule can realize new materials showing intriguing supramolecular structures. In this paper we have reported compounds exhibiting hydrogen bonding and fluorophilic /fluorophobic interactions, investigating the role of non-covalent forces to form liquid crystalline phases with a strong tendency to nano-segregation. Such complexes can organize highly structured liquid crystalline phases and exhibit unique phase transition behavior [11].
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EXPERIMENTS Thermal analyses were performe
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