Abstract
Complexes based on iridium and platinum have taken the privilege of the second-generation phosphorescence OLED materials, which can theoretically utilize both singlet and triplet excitons to achieve unity internal quantum efficiency (IQE). This is due to their stability and color tunability achieved through the strategic design of typical ligands. However, there are still some problems that require further investigation. Stable and efficient blue phosphor materials run through the OLED research process; the other is that the energy gap law has always hindered the NIR phosphor material, and it is difficult to obtain typical NIR emission materials beyond 900 nm. The thesis focuses on three more specific objectives: 1. Explore more stable multi-dentate iridium complexes for OLED application and enrich the coordination strategies; 2. Try to achieve highly efficient, small roll-off deep blue phosphors and use them as sensitizers for MR-TADF materials to achieve deep blue hyperfluorescence; 3. Delve deeper into NIR emissions beyond 900 nm without utilizing the intended stacking strategy.In the second chapter, three multi-dentate coordinated Iridium complexes Ir(κ4-L1)(thd), were designed and synthesized, composing of a linked 1-(pyridin-2-yl)ethylbenzene and one pyrazolyl pyridine unit, and are capable of adopting either tridentate or tetradentate coordination modes. Meanwhile, we figured out the synthesis mechanism of stepwise forming [Ir(κ4-Ln)(μ-Cl)]2 from respective intermediate [Ir(κ3-LnH)Cl(μ-Cl)]2 bearing the tridentate coordination chelate κ3-LnH (1a, n = 1; 1b, n = 2; 1c, n = 3). Concurrently, methylation of 2c in the presence of nBu4NCl afforded tridentate Ir(κ3-L3HMe)Cl3 (4) and, next, can be converted to tetradentate Ir(κ4-L3Me)Cl2 (5) by further cyclometallation and HCl elimination. The Ir(III) complexes 3a, 4, and 5 were unambiguously identified using spectroscopic methods and X-ray crystallographic analyses on Ir(III) derivatives 3a, 4, and 5. Their photophysical and electrochemical properties were also investigated and compared with results from theoretical studies. With an EQE of about 8% for 3b and 5, this demonstrates their potential for OLED applications.
In the third chapter, three typical sterically shielding deep-blue N-heterocyclic carbene (NHC) Iridium emitters with short radiative exciton lifetime were developed, namely ƒ-ct7a, ƒ-ct7b and ƒ-ct7c, 1) bulk structure with phenyl and 4-tBuphenyl substituted pyrimidine imidazole to protect the N-heterocyclic carbene (NHC) Ir(III) complexes from intermolecular interactions 2) short excited lifetime can efficiently reduce the concentration of the triplet excitons at high current density, thus relieving the severe roll-off in the OLED device. The emission peaks are at 452 - 461 nm, with a lifetime range of 0.66-0.97 μs and PLQY to 80% in degassed toluene. Radiative decay rate as high as 10^6 s-1. Phosphorescence OLED was fabricated, giving maximum external quantum efficiency (EQE) of 26.6%, CIEy 0.14, with an extremely low roll-off of 23.3% (EQE) at 1000 cd m-2. Furthermore, Ir phosphors were utilized to sensitize a deep blue multiple-resonance TADF (MR-TADF) emitter (t-DABNA); the corresponding device simultaneously realizes a maximum external quantum efficiency of 36.1%, CIEy = 0.09, full width at half maximum (FWHM) of 26.5 nm with a relatively low roll-off of 13.3% (EQE) at 1000 cd m-2. To our knowledge, this is the first time to achieve such a high EQE with CIEy 0.09.
In the fourth chapter, four binuclear Pt complexes using rigid carboline as the bridge with the NIR emission peak beyond 900 nm were prepared, namely L-CF3-Pm5, R-CF3-Pm5, L-tBu-Pm5 and R-tBu-Pm5. Without intended stacking to utilize the 3MMLCT, simple strategies have been applied in designing new double-decker Pt complexes with simple synthesis procedures. single-molecule emitting beyond 1000 nm was successfully achieved for the first time. In particular, L-CF3-Pm5 gives the emission to 1024 nm. Besides integrated characterizations, NIR OLED fabrication devices have been thoroughly investigated. This research can provide guidance for developing typical double-decker Pt complexes in the NIR region. The result indicated the potential use in biomedical sensors, night vision, security, and photodynamic therapy applications.
| Date of Award | 7 Sept 2023 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Yun CHI (Supervisor) & Fu-Rong CHEN (Co-supervisor) |