Supramolecular Liquid Crystals Of Polyoxometalate Complexes

Polyoxometalates (PMs) are a kind of monodisperse and uniform molecular clusters of the early transition metal oxides, especially for the elements of vanadium, molybdenum and tungsten. The variety of their composition and structures endowed them with many functional properties, such as catalysis, proton conductivity, nonlinear optics, medicine, proton acidity, photophysics and magnetism. However, the development of materials and devices based on PMs was restricted because of their high lattice energies, low pH stability and poor processibility, and great efforts have tried to improve the functionality performance of PMs with the assistance of other matrices. Organic liquid crystal (LC) system is considered as the suitable candidate for combination, location and orientation the clusters, representing the physical and chemical properties of the PMs, due to its organizable properties and good processibility. In this thesis, we have prepared organic/inorganic hybrid supramolecular liquid crystal material, through the supramolecular interaction on multi-component and multi-forms, based on the surfactant encapsulated polyoxometalate complexes (SEP). In the hybrid materials, the synergy of polyoxometalates and organic liquid crystal molecules realized the mutual adjustment of the intrinsic properties of polyoxometalates and the liquid crystal structures of the complexes.Firstly, we encapsulated Keggin-type heteropolyanions, H₃PW₁₂O₄₀ (HPW), Na₃PW₁₂O₄₀ (NaPW), H₄SiW₁₂O₄₀ (HSiW) and K₄SiW₁₂O₄₀ (KSiW), by a kind of azobenzene-containing surfactant, through the replacement of counterions. The resulting surfactant-encapsulated PM complexes were characterized in detail by UV-vis, Raman, NMR spectra and elemental analysis. The measurement results indicate that some azobenzene groups in the complex L/HPW, L/NaPW and L/HSiW were protonated during the encapsulation due to polyoxometalates'intrinsic strong acidity. In the protonated complex, the ammonium head group of the surfactant and polyoxometalate combine through the electrostatic interaction, and the bromonium ion derived from ion replacement binds to the protonated azobenzene group as a counterion. The thermotropic liquid crystal properties of these complexes were investigated by differential scanning calorimetry, polarized optical microscopy and variable-temperature X-ray diffraction. The protonated complex HL/HSiW reveals SmA and SmC phases, while the corresponding non-protonated complex L/KSiW exhibits only SmB phase. The protonated complexes HL/HPW and HL/NaPW self-organize into SmB phase, similar to that of non-protonated L/HPW. The competitive balance between the phase separation and the volume minimization of surfactants is supposed to play an important role and could be employed to explain the self-organized LC structures of these protonated and non-protonated complexes. Both the number of surfactants on the surface of PMs and the protonation to the azobenzene groups can make the LC phases of SEPs become diversiform, which may exhibit potential applications in developing chemistry stimulation-response hybrid LC materials.Following, we constructed multi-component hybrid LC materials through both intermolecular hydrogen bonding and ionic interaction. In order to achieve this motivation, we designed and synthesized a surfactant with a benzoic acid group at one tail end, and encapsulated the Keggin-type heteropolyanions using this surfactant, obtaining the supramolecular complex which could be served as the donor of the hydrogen bonding. If there is not a mesogen in the hybrids, they will not exhibit LC properties during heating and cooling cycles. So, we chose four pyridine derivatives containing conjugated groups, and have then prepared the hybrids through intermolecular hydrogen bonding between the supermolecular complex and pyridine derivatives. The results imply that the isotropic phase transition temperature of the hybrids increase with increasing alky chain length, number of the pyridine derivatives. And the introduction of stilbene unit to the pyridine derivatives also benefits for the higher isotropic phase transition temperature. We also study the effect of the charge number of polyoxometalate. The organic/inorganic hybrid liquid crystals constructed by multi-forms supramolecular interaction is convenient for us to adjust the liquid crystal phases, through the introduction of the different organic and inorganic components into the liquid crystal system.As the benzoic acid dimer is not a real conjugated group, it should not quench the luminescence of the inorganic PM core while it directs the formation of liquid crystal phases. Based on this point, we encapsulated three Eu-containing PMs with the surfactant containing benzoic acid group, and obtained a serious of surfactant-encapsulated PM complexes with intrinsic luminescence. The carboxyls bearing in the complexes were confirmed existing in the dimer state through intermolecular hydrogen bonding, which leads to stable and reversible thermotropic liquid crystal properties of these complexes. These complexes displayed intrinsic luminescence both in the amorphous powder states and in their mesophases. The photophysical properties showed the dependence on the existing states of samples, and the quantum yields of the complexes in the liquid crystal structures are higher than the corresponding amorphous powders. Specifically, Eu3+ in the complexes shows higher asymmetry in the liquid crystal structure due to its anisotropy than the corresponding amorphous powder, and thus, we have obtained the complexes with more pure red emission in their liquid crystal phases. The present investigation provides an example for developing hydrogen-bonding-induced polyoxometalate-containing hybrid liquid crystal materials with intrinsic luminescence.In summary, we have fabricated functional supramolecular LCs, by using the liquid crystal surfactants to drive and link PMs into the organic LC system. The intrinsic properties of polyoxometalates are well retained in the hybrid liquid crystals, and show an important effect on the liquid crystal phases. And, the liquid crystal phases could also adjust the property of polyoxometalates.