A Study On Liquid Crystalline Property Of Surfactant-Encapsulated Polyoxometalate Complexes

Polyoxometalates (POMs) are the polyoxoanion of the early transition elements, especially vanadium, molybdenum and tungsten. They are discrete and uniform cluster anions that assembled from small metal oxysalt fragments in water solution with a certain pH value. The variety of their composition and structures endowed them with many functional properties, such as catalyst, proton conductivity, nonlinear optics, medicine, photophysics and magnetism. However, the development of materials and devices based on POMs was restricted because of their higher lattice energies, lower stability and poor processibility. In is necessary that new methods should be explored to combine, position and orient the clusters, indicative of the physical and chemical properties of the POMs. Construction of well-ordered inorganic materials under the influence of organic molecular assemblies is an effective and facile strategy. Organic/inorganic complex can be formed by using cationic surfactants replaced the counterionscovered on the surface of POMs, it can be regarded as a novel supra-molecular soft material. Recent reports suggested that the different function of the assembly is a direct result of the formed structure. Therefore, it is significative to investigate the reversible transform of organizations in order to explore the molecular switch and data storage materials. Considering the flexible self-assemblies of liquid crystals (LCs), we prepared put the POMs into LC matrixs. Based on the anisotropic and organized nature of LCs, the properties of PMs can probably be modified and new functional materials can be constructed.Firstly, we have synthesized a series of surfactant-encapsulated terbium-constituted polyoxometalate complexes. Their LB and solvent-casting films have been prepared by LB technique and solvent-casting method, which are indicative of lamellar structures. But in the latter, DODA alkyl chains are partially interdigitated. Surfactant DODA-Br exhibited thermotropic liquid-crystalline behavior, but the corresponding complexes SECs are not LCs. Secondly, we designed and synthesized surfactant (L1) containing azobenzene group. The resulting complex SEC-4 can be processed into ordered casting film and LB films. In addition, the long axis of azobenzene unit of surfactant is perpendicular to the surface of the substrate. It is most important that SEC-4 exhibited thermotropic liquid-crystalline behaviors. The surfactant L1 and SEC-4 were characterized in detailed by differential scanning calorimetry (DSC), polarizing optical microscopy, temperature-dependent wide-angle X-ray diffraction (WXRD). The results suggest that SEC-4 can self-assemble into lamellar structure in its mesophases. Thirdly, we synthesized a series ofsurfactants containing biphenyl units, and the corresponding SECs exhibited typical LC behaviors. The results indicate that surfactants containing mesomorphous units absorbed on the surface of POMs can form hybrid LCs, it is a universal method. Thermal analysis showed the SECs can form Smectic A and Smectic C. Moreover, the alkyl chains of surfactants have significant effects on the LC behaviors of SECs. Finally, we trailed the phase behavior and the chain conformational dynamics and investigated the effects of charge number and shape of POMs by using temperature dependent FT-IR and XRD. The results suggested that the lower temperature phase transitions correspond to melting of the alkyl chains and the following disruption of the alkyl chain packing upon heating progress The clearing point transition does not have clear signature in the change of C-H stretching or wagging vibrations. That means in liquid-crystalline state, even after the alkyl chains have melted completely, the electrostatic interaction between cationic head and PM remains unbroken and free molecular motion of the surfactants is prevented by the rigidity of the ionic layers. Increasing surface charge density can enhance the inner layer interaction, which lead to increasing the phase transition temperature at clearing point. The fact suggests that the transition temperature at clearing peak can be controlled through adjusting the charge number of PMs.In summary, surfactants containing mesomorphous units encapsulated clusters can exhibit thermotropic liquid-crystalline behavior. The charge number and shape of POMs and the alkyl chains of surfactants have significant effects on the LC behaviors of SECs. The results provide new strategy to preparingorganic-inorganic hybrid liquid crystals. Further work will focus on both the choice for inorganic functional units and modification for surfactants in order to achieve the synergistic effects between the organizations and the functions of SECs.