The Synthesis And Optoelectronic Properties Of Nitrogen Heterocycle Host Materials And Red Iridium Complexes

From the working mechanism of phosphorescent organic light-emitting diodes(Ph OLEDs), we can realize that the host materials are crucial to devices performances. At present, the host materials of red and green Ph OLEDs have satisfied the commercial needs. However, development of highly efficient blue Ph OLEDs still remains a challenge as a higher triplet energy and carrier mobility are required for host materials. So, design and sythesis the high efficiency, stable and easy synthesis blue Ph OLEDs host materials is one of the urgent problems to solve. On the other hand, although Ph OLEDs have obtained excellent performances on small display devices, it is still needed to improve in solid lighting field. Recent resesrch demonstrates that using the long wavelengh red light in solid lighting devices can efficiently increase the luminance. So, development deep red emitting materials is another urgent problem to solve in OLED field. In order to solve these problems, this dissertation design and synthesis a series of azacarbazole(carboline) and aza-9,9’-spirobifluorene derivatives host materials and red iridium complexes emitting materials. The electrochemical, photophysical and optoelectronic properties of these materials were discussed in detail. The main content of every chapter is as follows:1. The Synthesis and Optoelectronic Properties of Azacarbazole(Carboline) DerivativesCarboline derivatives have been used as electron-withdrawing electron transport unit to derive many excellent bipolar host materials for blue Ph OLEDs. As a member of carboline, δ-carboline derivatives have rarely been reported as the host materials for Ph OLEDs. In this study, four δ-carboline derivatives, Cz BPDCb, BDCb BP, Cz35 Py DCb and Cz26 Py DCb, were designed and synthesized as host materials for Ph OLEDs. The electrochemical, photophysical and optoelectronic properties of these host materials were discussed in detail. It was demonstrated that the reduction potentials of Cz35 Py DCb and Cz26 Py DCb were lower than Cz BPDCb and BDCb BPCb due to the central pyridine moietiy, which was easy to be reduced. For the PL spectra, the small bandgap of pyridine unit compared with that of phenyl unit VIII induced the red shift of the PL emission peaks of Cz35 Py DCb and Cz26 Py DCb. Furthermore, it was illustrated that the four host materials have high triplet energy(high to 2.96 e V) and excellent transport properities of holes and electrons, which made them to be suitable as host materials for blue Ph OLEDs. The maximum current efficiency(CE), power efficiency(PE) and external quantum efficiency(EQE) of the blue Ph OLEDs based Cz35 Py DCb could be improved to 44.7 cd/A, 40.2 lm/W and 22.3% owing to its high charge mobilities and balanced bipolar transport properities.2. The Synthesis and Optoelectronic Properties of Aza-9,9’-spirobifluorene DerivativesHere, we designed and synthesized four aza-SBFs, α-aza-SBF, β-aza-SBF, γ-aza-SBF and δ-aza-SBF, with the nitrogen atom at different positions of azafluorene moiety. It was demonstrated that the four aza-SBFs have high triplet energy, and were suitable to be used as the host materials for blue Ph OLEDs. At the same time, it was noteworthy that α-aza-SBF and β-aza-SBF possessed almost completely separated the highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital(LUMO), while γ-aza-SBF and δ-aza-SBF didn’t. In addition, the aza-SBFs showed more excellent bipolar transport properities and thermal stabilities than SBF. The external quantum efficiency(EQE) of blue Ph OLEDs based on δ-aza-SBF could be improved to 9.5%, which was greater than the blue Ph OLEDs based on SBF(EQE, 6.6%).3. The Effect of Substitution Positions of Methyl Groups on Optoelectronic Properties of Arylquinoline Iridium ComplexesThe synthesis, electrochemical, photophysical properties and molecular simulation calculations of a series of methyl-substituted arylquinoline Ir(III) complexes were systematically studied. Compared to introducing methyl group to the 4’-position of the quinoline unit, the 6’-position can strongly stabilized the LUMO energy levels, resulting in lower reduction potentials and red-shifted absorbtion/emission spectra. On the other hand, introducing methyl group to the different positions of the HOMO dispersed phenyl unit, the absorbtion and emission spectra were different. As the 3-position of the phenyl unit was the para-position of central Ir atom, so introducing methyl group to the 3-position can efficiently increase the HOMO energy levels. Additionly, introducing methyl group to the 2-position of the phenyl unit leaded to apparent steric effect and a larger dihedral angle between the phenyl unit plan and quinoline unit plan, resulting in a longger absorbtion/emission spectra. Through the accommodation of methyl groups, we realized deep red emitting materials and the wavelengh can prolong to 646 nm. The devices performances based on 6’-substitued complexes and 4’-substitued were very close, which illustrated that introducing methyl groups to these two positions both can obtain excellent red emitting materials.