Synthesis And Properties Of Perylenebisimide-Based Discotic Liquid Crystals

Recent year, organoelectronics has attracted much attention due to its several advantages, such as, light weight, easy modification low cost and so on. Discotic liquid crystals, characterized by the intermolecular coupling of π electrons on adjacent molecules along the columnar stack, have been recognized as an efficient structure to provide high charge carrier mobility. Their unique structures and electronic properties open completely different aspects of possible application in the field of molecular electronics.In this thesis, we focused on the synthesis and properties of perylenebisimide-based (PBI-based) discotic liquid crystals and its nanostructures were then assembled by solvent-exchange method. The main content includes three parts shown as follows:1. Synthesis and properties of PBI-based monomer discotic liquid crystals. A series of symmetric and asymmetric 1,6,7,12-tetrachloroperylene bisimides (PBICLs) modified by siloxane substituents at the imide nitrogen atom were synthesized. The effect of different siloxane substituents on their liquid crystal structure, thermotropic and lyotropic behaviors was discussed in detail. The structures of PBICLs were confirmed by 1H-NMR. Siloxane substitutions do not apparently affect the electronic properties of PBICLs as demonstrated by CV experiments. They display both thermotropic and lyotropic liquid crystalline behaviors. Small angle X-ray scattering (SAXS) indicates that PBICLs adopt hexagonal columnar packing in thermotropic liquid crystals. In addition, PBICLs exhibit good optical properties, good solubility and film-forming ability. Thus the oriented films of PBICLs liquid crystals could be easily fabricated by mechanical shear, which show anisotropic properties in UV-vis absorption spectra.2. Synthesis and properties of PBI-based polymers discotic liquid crystals.Homopolymer (P1 and P2) and alternating copolysiloxane with both electron acceptor perylenediimide and electron donor terthiophene derivative pendants (P1-alt-P2) were synthesized successfully. The structures of P1,P2 and P1-alt-P2 were confirmed by 1H-NMR and organic elemental analysis. P1, P2 and P1-alt-P2 showed excellent amorphous glass state stability and high thermal decomposition temperature by differential scanning calorrimety (DSC) and thermogravimetric analysis (TGA). Polarizing microscope (POM) and X-ray scattering (XRD) indicates that P1 and P2 adopt the lamellar mesophasein (DL) and the the columnar oblique mesophase (Colob) in lyotropic liquid crystal, respectively. P1-alt-P2 is solution crystallization. Compared with P1, the chromonic mesogens of P1-alt-P2 self-organize in solutions mainly as a result of intermolecular interactions of the aromatic rings.3. Highly conductive and stable polysiloxane-modified perylenebisimide nanosheets and nanowires by self-assembly and the following condensation.Firstly, PBI-based organosilane precursor PBI-Si(OC2H5)3 was synthesized. Its nanostructures were then assembled by solvent-exchange method. The nanostructures could be adjusted by assembling in different solvents. SEM image and TEM studies showed that these assemblies were nanowires in THF and nanosheets in DMF. The thickness of the nanosheets determined by AFM is about 4.6 ± 0.3 nm. The self-assembly mechanism of PBI-Si(OC2H5)3 could be confirmed by UV-vis spectra and XRD measurement. PBI molecules can assemble into the supramolecular nanostructures via strong π-stacking. This could be confirmed by Uv-vis spectra The morphological differences of PBI-Si(OC2H5)3 in THF and DMF suggest different packing modes, which is further supported by X-ray diffraction (XRD) measurements. The obtained nanostructures were finally fixed by acetic acid vapor treatment through polycondensation of the triethoxysilyl groups in the precursors. After polycondensation, the nanostructures can hardly be changed and become insoluble in common solvents. Moreover, the conductivity of the polycondensed nanowires and nanosheets are ca.35-40 times than that of polycondensed spin-coating PBI-Si(OC2H5)3 film by current sensing atomic force microscopy (CSAFM).