Study Of Ionic Liquid Crystals Materials Based On Non-mesogenic Cations And Polyoxometalate Anions

Polyoxometalates （POMs）, a class of discrete, negative charge early transition metaloxide clusters, possess a wealth of topologies and physical and chemical properties. Theanionic character of POMs cluster allows its incorporation into materials possessing cationicgroups to yield various inorganic organic hybrid materials. Among varieties of routes to thepreparation of these hybrid materials, supramolecular self assembly as a controllabletechnique at the nanosized level has been successfully introduced to organize POMs withsurfactants into ordered assemblies. The properties of the inorganic and organic speciesshould be maintained in the complexes, and eventually new desired properties could also beobtained by adjusting the composition and ratio of cations and polyoxoanions.We now chose tetra n alkylammonium cation and Keggin tpye polyoxoanions(PW₁₂O₄₀3, GeW₁₂O₄₀4, SiW₁₂O₄₀4，BW₁₂O₄₀5), Dawson tpye polyoxoanion （P2W18O626-）and Lindqvisn tpye polyoxoanion （W6O192-） to prepare three series surfactant/POMs hybridsby straightforward ion exchange reaction, respectively. Composition and structurally ofcomplexes are characterized by elemental analyses, IR, TGA and single crystal X-raydiffractions. And the thermotropic liquid crystal （LCs） properties of these complexes havebeen studied by differential scanning calorimetry （DSC）, optical polarizing microscopy （PM）,variable-temperature IR and variable-temperature XRD.1. Three new POMs based ionic complexes without mesogen units have been obtainedby straightforward ion exchange reaction.[(C₈H₁₇)₄N]₃[PW₁₂O₄₀]（TPW）,[(C₈H₁₇)₄N]₄[GeW₁₂O₄₀]（TGeW）,[(C₈H₁₇)₄N]₅[BW₁₂O₄₀]（TBW）。Single crystal of TPW was only obtained, and the packing in the crystal structure of TPW canbe described as a pseudo lamellar system along the b axis. The LCs properties of TGeW andTBW were identified by using DSC, PM, variable-temperature IR and variable-temperatureXRD. Inspired by the single crystal results of TPW and the XRD data of TGeW and TBW,we presumed that the superstructures of TGeW and TBW show a tendency to layeredaggregation. In such system, the POMs cluster is a polarizable rigid group that is suitable as apotential mesogenic core. The TOA cation is a hydrophobic flexible group and increasesconformational freedom. Despite the fact that neither the POMs nor tetra n octylammoniumcations are mesogens, the microsegregation of incompatible units, the aggregation ofcompatible units, and the minimization of volume can give rise to ionic self assemblyformation ordered supramolecular structure. In the light of the low phase transition temperature and anisotropic properties, TGeW and TBW can also be regarded as the orderedionic liquids.2. A series of composition analogous polyoxometalate based ionic complexes aresynthesized.[（C₄H₉）₄N]₄[SiW₁₂O₄₀]（4-SiW）,[(C₆H₁₃)₄N]₄[SiW₁₂O₄₀]（6-SiW）,[(C₈H₁₇)₄N]₄[SiW₁₂O₄₀]（8-SiW）,[(C₁₀H₂₁)₄N]₄[SiW₁₂O₄₀]（10-SiW）.The study focuses on the effects of length of cationic alkyl chains on the self-assemblybehavior and thermal properties. Through FTIR analysis, the presence of highly disorder ofalkyl chains in the complexes. In addition, the disordered conformer of sample s exhibits adecreasing tendency with chain length increasing. Characterized by DSC and PM,8SiW and10SiW are manifested tending to thermodynamically stable mesophases. The results of thevariable-temperature XRD study show that length of cationic alkyl chains affects theself assembly behavior and thermal properties of ionic complexes. Shorter alkyl chaincomplexes tend to form cubic supramolecular aggregates and exhibit high thermalstabilization, whereas longer alkyl chains are found to be more conductive to mesophases andtend to form lamellar aggregates.3. Two new POMs based ionic complexes have been prepared.[(C₈H₁₇)₄N]₆[P₂W₁₈O₆₂]（8PW）,[(C₈H₁₇)₄N]₂[W₆O₁₉]（8WO）.Combining with [(C₈H₁₇)₄N]₄[SiW₁₂O₄₀]（8SiW） in the above chapter, we emphaticallystudied the effect of anionic charge density and the size of anion on thermal properties ofionic complexes. TGA analysis shows that the weight loss of8-WO occurs at310oC, and itsthermal stability was higher than8-PW and8-SiW by about100oC. After investigation,8-PWand8-SiW exhibit liquid crystalline properties, and8-WO is a typical ionic liquid. The causeis mainly related to the charge density and the size of the anion. The higher the charge densityof anion is, the stronger the interaction force between ions will be and the more orderly thecomplex structure will be. Conversely, if the lower the charge density anion is, the weaker theinteraction between the ions will be and the more disordered the structure will be, the easier toform liquid crystalline phase. At the same time, the smaller the anion size is, the more easilythe ions will make free movement. Although the interaction between ions is the strongest in8-WO, but because the size of anion is the smallest, ions easy to move,8-WO exhibitedthermal behavior is from crystalline directly into liquid phase.