Synthesis And Optoelectronic Properties Of Phosphorescent Iridium Complexes And Wide Band Gap Polymer Hosts

Phosphorescent iridium complexes have drawn much attention since they were used in EL devices by Forrest et al. in 1999. Iridium complexes show high PL efficiency and their emission colors can be tuned by the structure of the ligands. Highly efficient blue, green, red and white devices have been reported based on iridium complexes, but the stability of phosphorescent devices can not satisfy the request of practice yet,and highly efficient deep blue emissive iridium complexes are still scarce. Deeper researches on the materials are needed to solve these problems. The effect of ancillary ligands on luminescent properties of iridium complexes needs further understanding on the mechanism. This kind of research can help to understand the luminescence process of the complexes deeply, and provide ideas for materials development. Because of the long life time of phosphorescent materials, the intermolecular interaction between excited molecules would strongly quench the luminescence. So it is necessary to disperse the phosphorescent materials in the hosts. Polymer hosts have the advantage of allowing inexpensive solution processing technologies, but wide band gap polymer hosts are scarce, which limits the development of blue and white PLEDs. Starting from the above questions, in this thesis, we designed a series of iridium complexes to research the effect of ancillary ligands on luminescent properties of iridium complexes, and understand the basic luminescence process; developed stable and high luminescent efficiency iridium complexes and wide band gap polymers, and researched their photophysical, electrochemical and EL properties.It was considered that the emission colors of iridium complexes were determined by the nature of cyclometalating ligands, while the ancillary ligands operated insignificant control. In Chapter 2, we chose high energy phenylpyrazole (ppz) as the cyclometalating ligand and introduced different ancillary ligands to obtain four iridium complexes. These complexes show either no emission or medium intensity emission at room temperature. But at low temperature (77 K), all complexes emit strongly and the emission colors change from deep blue (422 nm) to orange red (587 nm). The results from electrochemistry and DFT data demonstrate ancillary ligands affect the electronic structure of the complexes mainly on LUMO, while with little effect on HOMO. When the LUMO level of ancillary ligand is lower than the cyclometalating ligand, the LUMO of the complex would be determined by the ancillary ligand. The study by TD-DFT demonstrates that the emission nature of four complexes is MLCT essentially. When ancillary ligand has lower LUMO level than the cyclometalating ligand, it would take part in the MLCT process that contributes the lowest excited state of the complex. The MLCT excited state with the lowest energy would determine the luminescent properties of the complex.To improve the thermal stability of iridium complexes and decrease the emission quenching induced by molecular interaction, in Chapter 3, we designed and synthesized three complexes using spirobifluorene-pyridine (SBFP) as the cyclometalating ligand. Owing to the introduction of spirobifluorene part, the complexes show good thermal stability with weight loss temperature higher than 400°C. All three complexes show yellow emission, and the PL efficiencies of Ir(SBFP)3 and Ir(SBFP)2(acac) in solution are 0.30 and 0.28, respectively. We emphatically researched the EL properties of Ir(SBFP)2(acac). The complex showes anti-aggregation property in EL devices. The device performances change little when the doping concentration lies between 12 wt% and 25 wt%. The optimized device shows pure yellow emission, and the luminous and external quantum efficiency of the device exceed 50 cd/A and 15%, respectively. At the luminance of 1000 cd/m2, the efficiency retains 90% of the maximum value. By breaking the connection of the two benzenes in spirobifluorene-pyridine to modify the structure of Ir(SBFP)2(acac), we obtained the complex Ir(DPFP)2(acac), which maintains highly efficient pure yellow emission with PL efficiency of 0.26 in solution. The evaporation temperature of Ir(DPFP)2(acac) (270°C) is 130°C lower that that of Ir(SBFP)2(acac) (400°C), this demonstrates the free vibration of the two benzenes in ligands could improve the sublimation property of the material as the rigidity of the molecule decreased, which is beneficial for fabricating EL devices with good stability. By simple molecular design, the evaporation temperature of the material is much decreased.Theδ-Si interrupted main chain can confine the effective conjugation length, and increase the band gap of the polymer. In Chapter 4, byδ-Si interrupted main chain, we designed and synthesized two wide band gap polymers P36HCTPSi and P27HCTPSi, with derivatives of tetraphenylsilicane and carbazole connected by meta- and para-linkage, respectively. P36HCTPSi and P27HCTPSi show deep blue emission in solution with PL efficiency of 0.19 and 0.67, and the emission peaks are 392 and 410 nm, respectively. The blue emission of P27HCTPSi is not stable, and its triplet energy is relatively low (2.44 eV), which limit its application as a host. P36HCTPSi shows good thermal stability (Tg = 217°C), good film formation, wide HOMO-LUMO gap (HOMO and LUMO levels are -5.5 and -2.3 eV, respectively), stable blue emission (422 nm) in device and high triplet energy (2.67 eV), which can be used as a wide band gap host. The blue, green and yellow devices with P36HCTPSi doped iridium complexes as the light emitting layer show luminous efficiency up to 3.4,27.6 and 61.7 cd/A, respectively. The white device with red, green and blue iridium complexes doped shows luminous efficiency up to 8.6 cd/A, while the efficiency of yellow and blue complexes doped white device up to 25.1 cd/A. The device performances demonstrate P36HCTPSi is a good universal host material.In a blending system of polymer host and complex guest, there are problems of phosphor aggregation, phase separation and incomplete energy transfer. Combing the host and guest in one material can solve these problems. In Chapter 5, we added blue iridium complex FIrpic onto the side chain of P36HCTPSi by Suzuki coupling reaction. The content of FIrpic in the polymers could be controlled by feed ratio of the monomers. We synthesized two polymers PCzSiIr2.5 and PCzSiIr5, with the FIrpic content of 2.5 and 5 mol%, respectively. The FIrpic content calculated from 1H NMR spectra is similar to the monomer ratio. The polymers show good film formation, the phase separation is depressed in films compared with the blending system and more efficient energy transfer between the host and guest is realized. Using grafting FIrpic as the"internal standard", EL spectra of PLEDs with PCzSiIr5 doped by a red iridium complex as the light emitting layer were compared before and after annealing, and we found that the doped material is not as stable as FIrpic in film. PLEDs based on the polymers show blue phosphorescence from FIrpic with a maximum luminous efficiency of 2.3 cd/A. The efficiency roll-off at high current densities is suppressed in the devices.