Design, Synthesis And Self-assembly Of Coil-Rod-Coil Oligomers

Recent study on design, self-assembling behavior of rod-coil block oligomers is important research interest in supramolecular materials and polymer science realms. The research results showed that the driving forces of rod-coil molecules forming supramolecular building units with high ordering wereπ-πinteraction of rigid units, even the microphase separation of flexible segments and rod groups in rod-coil oligomers. Compared to the traditional flexible coil-coil and rigid rod-coil block polymers, rod-coil block oligomers exhibited different phase behaviors, self-assembling characters and microstructures. And correlative investigations showed that the rod-coil block oligomers could come into being several nanostructures, such as mushroom-like nature, smectic phase, cubic phase and hexagonal columnar phase, and so on. In the realm, many recent researches focused on the synthesis of end-on rod-coil oligomers, formation mechanism of supramolecular order structure, and functional study of the materials. Study on the flexible coil lateral-substituted rigid oligomers had proved that the molecules could self-assemble into columnar liquid crystal phase.In order to fabricate more efficient and high performance device, new materials with high electron-transporting capability are demanded. Typical electron-transporting materials usually contain aπ-electron deficient heterocyclic moiety, such as oxadiazoles. The oxadiazole moieties had been demonstrated to possess high potential for electron transporting. This gives us a strong motivation for the synthesis and structural investigation of Coil-Rod-Coil oxadiazole derivatives, aimed for good electron transport.In this thesis, a series of novel Coil-Rod-Coil functional oligomers, dihydrazide compounds with different alkoxy chain-length and chain-number (C-Tn, n= 7, 8, 10, 12, 16), imide compounds (BI-C-m, m= 8S, T12), 1,3,4-oxadiazole derivatives (D-n, n= 8S, 10, 12, 16; D-T1), D-8S derivatives (F-8S-R, R= C12, PEG) were prepared. The chemical structures of the compounds were confirmed by FT-IR, 1H NMR spectroscopy and elemental analysis. We focused on the self-assembling behaviors of hydrazide derivatives, photophysical properties, electrochemical properties, crystal structure, electronic structures and energy levels of oxadiazole derivatives. And the results are summarized as following:1. Self-assembly of Coil-Rod-Coil hydrazide compoundsBased on the SEM observations, X-ray diffraction, FT-IR and 1H NMR spectroscopy methods, we studied the self-assembling behaviors of the hydrazide compounds C-Tn and BI-C-m in solution. Investigating the minimum concentrations of gelling, aggregation morphology and structure of gels, and gelation driving forces, the results showed, (1) intermolecular H-bonds origining from C=O and N-H existed in hydrazide compounds C-Tn and BI-C-m, respectively, and it could freeze organic solvent forming gel. (2) The change of H-bonds donor-accepter numbers of hydrazide compounds affected gelling ability remarkably, it indicated that intermolecular H-bonds was the main driving force for the formation of self-assembled gel. Furthermore, the longer the end-on attached alkyl chains, the stronger the gelation ability. (3) The structure of C-T7 xerogel was formed by bundles of the fibres with average diameter of 100 nm, and the molecules stacking into hexagonal columnar phase.2. Photophysical properties of Coil-Rod-Coil oxadiazole derivatives By using the UV-vis absorption and fluorescence spectroscopy methods, we examined the photophysical properties of oxadiazole derivatives with an alternating phenylene-oxadiazole conjugated structure. The results showed, (1) introduction of end-on attached alkyl chain with electron-donating increases the electron density of the conjugated segment of D-n, which reduces the energy ofπ-π* transitions and thus led to the emission maximum wavelength red-shifted; (2) the emission maximum wavelength of D-8S blue-shifted showed remarkablely concentration-dependent fluorescence in non-polar solvents, and introduction of polar solvents in the system also resulted in the emission maximum wavelength blue-shifting because of the intermolecular aggregation weakened; (3) introduction of laterally attached two carboxyls with electron-accepting to the central phenyl, D-n showed significantly difference in fluorescent properties, thereinto, the maximum emission red-shift was about 70 nm, compared with that of OXD3-7 without lateral substitutents.3. Thermal properties of Coil-Rod-Coil oxadiazole derivativesBased on the TGA method, we studied thermal properties of the oxadiazole derivatives. The results showed that oxadiazole compounds, D-n and D-T1, took on high melting points and thermal stability, and their decomposed temperature were above 270℃; because of introduction laterally attached flexible chains to the central phenyl, the melting points of F-8S-R lowered remarkably about 240℃, compared to that of D-8S, and the longer the alkoxy groups, the lower the melting points.4. Crystal structure of Coil-Rod-Coil oxadiazole derivativeBy using powder X-ray diffraction and computer simulation (Material Studio, MS) methods, we had obtained 3-dimensional structure of D-T1, and analyzed its structural property. D-T1 belongs to P21 space group of monoclinic symmetry. Its lattice parameters are a= 2.3106(4) nm, b= 0. 9677(1) nm, c= 0.6447(1) nm,β= 96.81(0)°. 5. Energy levels of Coil-Rod-Coil oxadiazole derivativesThe effect of the changes of side chain (chain type and substituents bonding model) substituents on the energy level structure of Coil-Rod-Coil oxadiazole derivatives had been studied by cyclic voltammetry (CV), UV-vis absorption spectroscopy and quantum chemical calculation (Calculations of the electronic structure of D-8S and D-T1 were performed by the Dmol3 package of MS Modeling 3.0 through the geometry optimization using GGA method with PW91 function). It is indicated that the frontier molecular orbits of the oxadiazole compounds were perturbed, and the distribution of energy level orbits was changed because of improvement of the electron-transport ability. It had been investigated in the effect of the position changes of the symmetrical alkoxy substituents on the energy level structure of this series of oxadiazole derivatives. With the type and position changes of the symmetrical alkoxy substituents, from terminal phenyl to central phenyl, the HOMOs, LUMOs and Egopt were affected by the electron-donating abilities. It is shown that HOMO energy levels changes from -7.24 eV to -6.90 eV, and LUMO energy levels are in the range of -3.62 eV to -3.58 eV, exhibiting reversible redox behaviors, which means the oxadizole derivatives should behave good electron-transport property.