Synthesis And Study Of Lamellar-smectic Phase Polyoxometalate Liquid Crystal Materials

Nano-hybrid liquid crystal is a kind of functional material combining the physical and chemical properties of nanoparticles with the ordered character of liquid crystal.Nano-hybrid liquid crystals have optical,electrical and magnetic properties due to their combination with nanoparticles,which provides a possibility for the development of multi-functional liquid crystal materials.In addition,the ordered assembly structure of liquid crystals provides the possibility for us to dynamically regulate the functional properties of nanoparticles through external stimuli.Polyoxometalates(also known as polyacids)are not only characterized by diverse chemical compositions,rich topological structures and excellent chemical and physical properties,but also a class of functional nanoscale inorganic clusters,and have developed into excellent supramolecular building elements gradually.The multi-functional organic-inorganic hybrid liquid crystal materials can be obtained by modifying the organic components containing mesogenic units to the periphery of the polyoxometalates by electrostatically encapsulating or covalently bonding.However,most of the mesophase of polyoxometalate liquid crystal materials are mainly concentrated on simple structures such as the smectic or the columnar phase et al.,and the polyoxometalate liquid crystal materials with novel assembly structure are rarely reported.The bonding mode of mesogenic units and inorganic nanoparticles has an important effect on the assembly structure of the composite.In general,the terminal attachment of the mesogenic units often leads to the assembly of the complex into the smectic phase or columnar phase,while the lateral attachment of the branched mesogenic units tends to produce more novel assembly structures(e.g.,nematic phase).Based on above,a novel lamellar-smectic polyoxometalate liquid crystal material was obtained by electrostatically encapsulating europium-containing polyoxometalate Na₉[Eu W₁₀O₃₆]with quaternary ammonium salt amphiphilic molecular A6 containing carboxylic acid group.(1)We have designed and synthesized a quaternary ammonium amphiphilic molecule A6 containing carboxylic acid group,and electrostatically encapsulated the polyoxometalate Na₉[Eu W₁₀O₃₆]containing europium to obtain complex A6Eu.We used ¹H NMR,TGA,IR,elemental analysis to characterize the complex structure in detail.The results confirmed that the dimerization of carboxylic acid groups through intermolecular hydrogen bond interaction resulted in the formation of branched bolaform in organic components.The branched structure makes the mesogenic units encapsulated in the periphery of the polyoxometalate in a lateral attachment under the synergistic drive of multiple supramolecules such as electrostatic interaction,hydrogen bond interaction and microphase separation interaction,which provides the possibility for the composite to assemble into a novel liquid crystal phase.(2)Temperature-dependent infrared spectroscopy confirmed that the hydrogen bond dimer between carboxylic acid groups is still maintained in the liquid crystal temperature range.The thermotropic liquid crystal behaviors of the complexes were confirmed by polarized light microscopy,differential scanning calorimetry and variable temperature X-ray diffraction.The lateral attachment mode of the mesogenic units drives the composite to assemble into a novel quasi-2D lamellar-smectic phase.In the lamellar-smectic liquid crystal structure,the mesogen is organized parallel to the layer planes.Variable temperature emission spectra shows that the fluorescence properties of polyoxometalates are maintained in the complex and the luminescence properties of the complex can be regulated by temperature.Hydrogen bond dimer formed by amphiphilic molecules containing carboxylic acid groups results in the branched bolaform structure of the mesogen.This structure further promotes the lateral attachment between the mesogen group and the polyoxometalate.The lateral attachment between the mesogenic units and the polyoxometalates has an important effect on the assembly structure of the composite,which provides an effective strategy for the further development of multi-functional hybrid liquid crystal materials with novel assembly structure.