Highly Luminescent Tetradentate Platinum Complexes:Design,Synthesis,Photophysics,And Electroluminescence Application

Organic light-emitting diodes(OLEDs) have attracted great interest in both the scientific and industrial communities due to their advantages such as high-efficiency, light weight, wide viewing angle and low-cost. OLEDs technology has been widely used in flat panel displays and solid state lighting. The emitting layer materials play an important role in the efficiency and stability of the device. There is a surge of interest to develop phosphorescent Ir(III) and Pt(II) complexes which can realize a theoretical internal quantunm efficeiency of 100% since their electroluminescence can be generated from both singlet and triplet excited states. Platinum(II) complexes bearing rigid tetradentate chelating ligands have emerged as an important class of phosphorescent materials because their rigid structure can reduce the probability of nonemissive decay through suppressing the vibration and rotation around metal ions, thus leading to high luminescence efficiency. Meanwhile the chelated Pt(II) complexes showed very high thermal stability. However, the lack of efficient and stable emitting layer materials limits OLEDs further application. In this thesis, our work mainly focuses on the design and synthesis of tetradentated platinum complexes as novel dopants for phosphorescent OLEDs, emission tunability via the incorporation of heteroatoms is also investigated.In the first work, robust phosphorescent Pt(II) complexes(Pt1, Pt2, Pt3) supported by benzothiazole, benzoxazole, and benzimidazole groups were designed and synthesized. A bulky group 2-ethylhexyl was introduced into Pt3 to increase solubility. The effect of heteroatoms on photoluminescence and electroluminescence properties of this series of Pt(II) complexes was studied systematically. There is a red-shift in the absorption spectrum, photoluminescence spectra and electroluminescence spectra of Pt1 relative to those of Pt2 and Pt3, which could be ascribed to low electro-negativity and high polarizability of the S atom in Pt1, so we can tune the emission by changing the hetero atomswhile the backbones of these complexes are essentially the same.OLEDs based on these Pt complexes exhibited excellent device performance. For Pt1 and Pt2, the maximum device efficiencies(73.5 cd A-1, 66.8 lm W-1 and 19.8% for Pt1, and 62.0 cd A-1, 56.2 lm W-1 and 19.0% for Pt2) were obtained at the same doping level(5 wt%), but as the doping ratios were increased from 1 wt% to 15 wt%, a new emission band with the wavelength> 600 nm appeared gradually, suggestingthe excimer-based emission. Particularly, the introduction of the 2-ethylhexyl group into Pt3 discouraging the strong intermolecular interaction, not only increased solubility, but also suppressed excimerbased emission.For Pt3, the best device performance data was achieved at the doping level of 10 wt% with peak ηc of 75.0 cd A-1, ηp of 70.1 lm W-1 and ηEQE of 21.4%. Pt3 exhibited low efficiency roll-off at a brightness of 1000 cd m-2 with external quantum efficiency ηEQE of 20.3%(5.1% roll-off).In the second work, our work focused on the synthesis and characterization of blue emitter based on platinum complexes Pt4 and Pt5. In Pt4 and Pt5, oxygen atom was introduced to link two bidentate units together to realize the blue emission.The effect of hetero-atom on the emission properties has also been investigated. The trimethylbenzene group was introduced into thiazole unit to suppresse the intermolecular interaction. Pt4 shows alarge band gap with the emission centered at 440 nm. The Pt4 shows high thermal stability with Td up to 441 oC, which allows the purification by sublimation. The HOMO energy levels of these complexes were determined by UPS, which provides the guideline for choice of the host materials.The maximum emission of device is 456 nm with CIEx, y=(0.21, 0.31).This work demonstates emission tenabilityby changing the hetero atom and how to improve the efficiency and stability by introducing bukly groups, which may pave the way for the design of novel luminescent materials.