Studies On Supramolecular Liquid Crystals Of Fully Rigid,Multi-armed Molecules

Due to coupling hierarchical structures of supramolecular self-assembly and unique properties of liquid crystal, supramolecular liquid crystals open a new approach to develop superstructures and multiple functionalities in liquid crystalline materials, and thus have gained increasing attention in recent years. Up to now, a great number of molecular building blocks with diverse geometrical shapes such as rod, fan, disc, cone and other exotic shapes, have been synthesized to construct supramolecular liquid crystals via various intermolecular forces. The supramolecular liquid crystals with well-defined nanostructures and unique properties also show great potential for application in electron, ion, or molecular transporting, catalytic, optical, sensory, and bio-active materials. However, the studies on supramolecular liquid crystals were intensively carried on in thermotropic situation, few work was conducted to program lyotropic liquid crystal by using fully rigid, multi-armed molecules. Rigid molecules with multiple arms may not only contribute to strong intermolecular forces and robust superstructures, but also benefit the control of liquid crystalline structures and properties through tuning various structural parameters of molecular building blocks, such as geometrical shape, symmetry, ion group, and chemical constitution.According to the background mentioned above, we synthesized a series of fully rigid, multi-armed molecules with ion groups at their periphery, then investigated their supramolecular liquid crystal in water via diverse methods used for structureal and performance determination. The main work and conclusions of this dissertation are described as follows:(1) We designed and prepared nine rigid multi-armed molecules with different geometrical shapes and symmetries, which were designated as P73, P72, P71, P83, P52C, P62C, P72C, P64and P104, respectively, according to the length of hydrophobic arm and distribution of ion at molecular periphery.(2) The supramolecular liquid crystalline behaviors of C3symmetric and unsymmetric molecules (P73, P72, P71) in water were investigated by various methods, such as POM, XRD, CD and IR. It was found that fully rigid three-armed molecules can self-organize into supramolecular columnar liquid-crystalline phases with triplehelical rosette nanotubes as the columns, which are robust and stable over a wide concentration range in water. The helical nanotubes are hardly affected by ionic defects at the molecular periphery, and this would no doubt benefit further functionalization of their liquid crystals. The formed hexagonal phases exhibit two distinctive properties. First, the supramolecular helical cylinders can be aligned macroscopically by simple shear, which leads to anisotropic ionic conductivities in the hexagonal phases. Second, the ordered liquid-crystalline structures of organized achiral molecules showed macroscopic chirality, which indicates the occurrence of spontaneous chiral symmetry breaking in these systems. These interesting findings open the possibility to prepare low-dimensional ion-conductive materials and some other functional liquid-crystalline materials with singlehanded helices based on only achiral molecules.(3) To understand lyotropic behaviors of bent-core molecule P52C in water, counterion species, concentration of sample, ion substitution ratio were taken into account to influence the supramolecular assembly, and were studied by diverse methods (XRD, POM, and TEM). It was found that,1) Bent-core molecule P52C can self-organize into smetic phase in water, increasing molecular concentration will lead to decrease of the lamellar long period, because the lyotropic lamellas already possess the molecular array feature in solid state, drying process only gives rise to decrease of interlayer spacing without changing intermolecular organization.2) Upon replacing hydrogen ion in P52C with alkaline ion (Li, Na, K, Cs), a phase transition from semtic to hexagonal array takes place, and the hexagonal liquid crystalline phase is formed by supramolecular helical columns with a diameter of about8nm.3) The concentration of alkaline ion determines the transition process, taking potassium ion for example,0.4mol ratio of K+/P52C is already able to induce the formation of hexagonal phase with long range order, while mol ratio above2.0will destroy the ordered hexagonal array. These interesting features suggest potential application of P52C molecules for ion detection.(4) The lyotropic systems of P62C, P72C, P83, P64, and P104were also studied. Through taking all sample systems we studied into consideration, we discussed the structural factors of rigid multi-armed molecules for the formation of lyotropic liquid crystal. Some qualitative conclusions have been drawn as follows:1) liquid crystalline structures are mainly determined by geometrical shapes of rigid molecules;2) counter ions perhaps play an important role in tuning liquid crystalline phase;3) longer hydrophobic arms of rigid molecules would make supramolecular assembly even more robust and stable provided that superstructures in liquid crystal are preserved. Our work on fully rigid, multi-armed molecules demonstrates a new research area of supramolecular liquid crystal. Based on these new-type mesogens, lyotropic liquid-crystalline systems with unique nanostructures and properties can be built up and potentially be used for optical, electrical device, ion transport, biosensor, etc.