An Investigation On Physical Properties Of 1,3,4-Oxadiazole Derivatives At Ambient And High Pressure

In 1987, C.W. Tang used AlQ3 to prepare organic light-emitting diodes (LEDs), which stimulated organic light-emitting materials inventing. And the interests on polymeric light-emitting device increased, since J.H. Borroughes used PPV in LED preparing. Hence, with attractive characteristics including efficient light generation and colors tuning, organic and polymeric LEDs have gained considerable attentions for their potential applications in the luminescence and display technologies. Organic light-emitting diodes (OLEDs) show excellent properties in the aspects of highly efficient light emission and colors tuning, due to which it receives extraordinary recognitions. Because of high thermal stability, outstanding inoxidizability, tough mechanical behaviour, high efficient blue light emission and special electron transport ability, oxadiazole derivatives are one of the most optimal OLED materials, and deserve an in-depth study. An optimized LED requires efficient and balanced charge injection, however, most polymers are hole transport materials. Because of electron withdrawing effect of oxadiazole unit, the oxadiazole derivatives are electron transport materials. For better applications, it is necessary to particularly understand their solid-state structure, electronic structure, and other physical properties. Because organic materials have at least two states (ground state and lowest excited state), there photophysical properties are sensitive to the pressure. Multi-investigations of photophysical property under pressure will be helpful for understanding the courses and characteristics of electron transport of organic materials. In this thesis, three oxadiazole derivatives with different substitution of alkoxy groups, OXD-1 (no substitution of alkoxy group), OXD-2 (meta-substitution of alkoxy group) and OXD-3 (terminal substitution of alkoxy group) are investigated, because different substitutions lead to different electronic structures and electronic density distributions. Both OXD-1 and OXD-3 have rigid plane molecular structures. Because OXD-2 has meta-substitution of alkoxy group, it has a warped molecular structure. It is known that the crystal structures of oxadiazole derivatives show mainly monoclinic or orthorhombic symmetry and consist of stacks of molecules with one or two orientation within the stacks, but with a different arrangement of the stacks relative to each other. By using powder X-ray diffraction and computer simulation we have obtained 3-dimensional structures of these there oxadiazole derivatives, and analyzed their structural properties. OXD-1 belongs to P21/a group of monoclinic symmetry. Its lattice parameters are a=22.657(8)?, b=8.8430(7)?, c=9.9486(3) ?, β=112.45(7)°. OXD-2 belongs to P212121 of orthorhombic symmetry. Its lattice parameters are a=27.291(7)?,b=25.660(4)?,c=4.736(5)?. The symmetrical structure of OXD-3 is similar to that of OXD-1, having lattice parameters of a=17.328(2)?,b=6.915(6)?,c=13.923(8)?,β=114.03(3)°. In order to study optical properties of sample, we measure the photoluminescence (PL) and the UV-vis absorption spectra of oxadiazole derivatives. Except a small red shift and little higher emission intensity, OXD-1 has similar absorption peak with OXD-3. This is because they have similar molecular structures and electron disturbutions, but the substitution of alkoxy group that donates electrons in OXD-3 makes higher electron density in the molecular, which lead to a stronger absorption. Additionally, the absorption peaks of OXD-1 and OXD-3 have vibrational fine structures, which indicates their structures of excited state keep rigid plane structures. The dispersed-type UV adsorption spectrum of OXD-2 indicates that its basic and excited state have bad rigid plane. Because of the big conjugate πbonds and electron donating substitutions of alkoxy group, all the intensities of PL spectra are high,whether in solution or solidity. The maximum wavelengths of OXD-1, OXD-2, and OXD-3 are 378,425, and 400 nm in the solution, and are 410, 440, and 419nm in the solid-state. The PL wavelengths shift to red about 20 nm in solid-state, because of the smaller distances and the stronger π-interaction between molecules. In the thesis, photoluminescent spectra are also investigated under high pressure with diamond anvil cell (DAC), from which the pressure dependences of the wavelengths and the intensities of PL peaks are obtained. With pressure increase, the inter-molecule π-interactions become stronger and band gaps become smaller, so the PL wavelengths show red shifts. But because of the different electron densities, three oxadiazole derivatives have different velocity of red shift. As pressure increases, the band gap of molecular gradually approaches the energy of exciting beam (488.0 nm), which results in the increase of absorption efficiency and the raise of photoluminescent efficiency thereby. After the absorption efficiency reaches the maximum, further compression makes an increase of efficiency of internal conversion, and then leads to the decrease of absorption and photoluminescence. In addition, as the energy gap decreases to a given value, intra-molecular nonradiative transition increases, also leading to the decrease of luminescent efficiency. Therefore, the efficiency of luminescence decreases rapidly with pressure increase further. For OXD-1, OXD-2, and OXD-3, their maximum fluorescent peaks respectively take place at 13, 9.6, and 11.3 GPa, however, their photoluminescent wavelengths still show red shift with pressure. To investigate pressure-induced structure transitions and inter-and intra-molecules interaction changes, in situ high pressure X-ray diffraction experiments on oxadiazole derivatives were carried out using DAC. At 8 and 5 GPa, OXD-1 and OXD-2 turned to amorphous states. When the pressure is 5 GPa, OXD-3 occurred a structure phase transition, it changed from P21 to AN space group. And at 8 GPa, OXD-3 showed another phase transition -amorphous states. All changes under high pressure are reversible. With the experimental system of the metal electrode sputtered onto the surface of diamond anvil, we study the electrical character of oxadiazole derivatives under high pressure. And electrical field analysis,...