| Since C.W.Tang fabricated the first organic light-emitting device (OLED) using tris(8-hydroxyquinoline)aluminum (Alq3) in 1987, plenty of novel organic electroluminescence materials (OELMs) have been synthesized to improve the performance of OLED. Among them, 8-hydroxyquinoline metal complex (Mqn) is widely used in all kinds of OLEDs as a kind of well-developed OELM, for its superior performance over other kinds of OELMs. Though, the performance and application of Mqn had been extensively investigated, there still exist problems to be solved, such as the effective and low-cost synthesis and purification for commercialization, and the well-established light-emitting mechanism of Mqn, especially the relationship between molecular spatial structure and material performance of Mqn. In this paper, Alq3, bis(8-hydroxyquioline)zinc (Znq2) and 8-hydroxyquioline lithium (Liq) were main research objects. In order to elucidate the relationship between molecular spatial structure and material performance of Mqn, the synthesis, performance modification and material performance of above Mqn molecules were studied experimentally and theoretically. The main research conclusions are as follows:1. The synthesis and purification of Alq3, Znq2 and Liq wereoptimized to satisfy the requirement for high yield and high purity of product. A novel vacuum evaporation equipment for Mqn purification was designed, which can increase purification efficiency and reduce product loss. This work may be helpful for preparing Mqn with low cost and high yield, and consequently for promoting the development of commercialization of Mqn.2. For the first time, blue-light-emittingδ-Alq3 was synthesized by vacuum heating and chemical purification. The analysis of molecular spatial structure and the characterization of material performance ofδ-Alq3 andα-Alq3 revealed that the molecular structure and molecule packing mode ofα-Alq3 can change when converted toδ-Alq3 by vacuum heating, resulting in strengthening intermolecularπ-πinteraction. In photoluminescence (PL) spectra, the maximum emission peak ofα-Alq3 blue-shifts relative toα-Alq3, andδ-Alq3 possesses blue fluorescence effect. In solid state, the fluorescence emission ofδ-Alq3 is attributed to the intermolecularπ-π* electron transition, while the fluorescence emission ofα-Alq3 is attributed to theπ→π* electron transition between the Highest Occupied Molecular Orbit (HOMO) located in phenol ring and the Lowest Unoccupied Molecular Orbit (LUMO) located in pyridine ring of 8-hydroxyquinoline. Becauseδ-Alq3 can convert toα-Alq3 again during the film preparation by vacuum evaporation, the electroluminescence (EL) spectra ofδ-Alq3 is nearly identical with that ofα-Alq3 whenδ-Alq3 is used as light emitting layer in OLED. Compared withα-Alq3, better film formability ofδ-Alq3 induces better electroluminescent performance.3. (Znq2)4 and Znq2 were synthesized by recrystallization and vacuum heating. The analysis of molecular spatial structure and the characterization of material performance of (Znq2)4 and Znq2 indicated that four Znq2 were connected by Zn-O-Zn bond bridges to form (Znq2)4, which can strengthen the rigidity of planar molecular structure of Znq2. Compared with Znq2, (Znq2)4 exhibits higher fluorescence quantum efficiency and better film formability. The intermolecularπ-πinteraction between adjacent (Znq2)4 molecules is stronger than that between adjacent Znq2 molecules, so the electron transporting performance of (Znq2)4 is better than Znq2. When used as light emitting layer in OLED, (Znq2)4 gives much better electroluminescent performance than Znq2. In (Znq2)4 molecule, the fluorescence emission is not only attributed to theπ→πelectron transition between HOMO and LUMO of hydroxyquinoline ring, but also attributed to theπ→πelectron transition between adjacent hydroxyquinoline rings. However, the fluorescence emission of Znq2 is attributed toπ→π* electron transition between HOMO and LUMO of hydroxyquinoline ring. Therefore, the EL spectra of (Znq2)4 is wider than that of Znq2.4. (Liq·Naq)2 was synthesized. The analysis of molecular spatial structure and the characterization of material performance of (Liq-Naq)2 and Liq showed that two Liq molecules and two Naq molecules were connected by Na-O-Na bond bridges to form (Liq·Naq)2. Compared with Liq, (Liq·Naq)2 exhibits stronger rigidity in planar molecular structure, larger steric hindrance and intermolecular distance, and much smaller molecular polarity, thus resulting in much longer fluorescence lifetime, much higher fluorescence quantum efficiency, wider energy bandgap and better film formability. When used as light-emitting layer in OLED, (Liq·Naq)2 shows lower formation probability of excited dimmer and exciplex formation than Liq, thus emits bluer light with higher current efficiency than Liq. When (Liq·Naq)2 ultrathin film is used as electron injection layer in OLED, it exhibits higher current density, higher luminance, lower turn-on voltage and higher current efficiency than Liq ultrathin film for the existence of sodium ions in (Liq·Naq)2 ultrathin film.5. The summarization the relationship between molecular spatial structure and material performance of Alq3, (Znq2) and Liq, lead the conclusion that the molecular spatial structure of Mqn affects its material performance in such aspects as the rigidity of planar molecular structure, intermolecular interaction, molecule stacking mode and intermolecular distance. On the base of this theory, the performance of different Mqn molecules can be modified at molecular level by changing their molecular spatial structure in response to different requirement for material performance. This would open a new route for the research on Mqn performance modification.
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