Synthesis And Properties Of Phosphorescent And Thermally Activated Delayed Fluorescence (TADF) Host Materials

Electrophosphoresecnt and thermally activated delayed fluoresecence (TADF) materials can fully harvest both singlet and triplet excitons in Organic Linght-emitting Diodes (OLEDs) to achieve the internal quantum efficiency of unity. Because of their excellent efficiency merits, they have always been the the hotspots in OLED. In order to avoid the aggregation quenching and exciton annihilation of triplet excitons and fulfill the efficient utilization of electric excitons, light-emitting layers featuring host-guest system should be applied in both phosphorescent and TADF devices.The ideal host materials should have high triplet energy, suitable HOMO/LUMO energy levels, certain charge transporting abilities and fine spectral overlap with luminophers. Bipolar host materials possessing both hole and electron transpoting abilities could not only meet the above requirements, but also banlance the hole and electron fluxes which is superior to unipolar hosts and proved benefical to expand the charge recombination zone and thus improve the luminous efficiency and alleviate the efficiency roll-off. So in this thesis, by adopting the electron-withdrawing pyridine or triazole groups and varying the species of electron-donating units and their molar ratio to acceptor ones, we developed a series of phosphoresecent and TADF host materials. Details are as follows:(1) Small and solution-processible bipolar host materials. Four novel hosts, namely G1CzPy, G1CzTz, G2CzTz and G2CzPy, were developed by combining 1 generation or 2 generaton oligo-carbazole dendrimers with electron-withdrawing units, pyridine or triazole, in the linkage form of D-?-A. The separation of HOMO and LUMO orbitals confirmed their bipolar transporting abilities. Moderate extent of ? conjugation in these molecules not only facilitated the charge transporting, but also guaranteed high triplet energy of these molecules. The dendritic structure and t-Butyl groups at the surface gave these molecules wonderful solution-processible properties.(2) Bicarbazole based blue host materials with high triplet energy. Two blue hosts, namely CzCzPy and CzCzTz, were synthesized by connecting the bicarbazole with pyridine or triazole units through meta-substituton strategy. Low temperature spectra indicated that they had very high triplet energy. The maximum external quantum efficiencies of phosphorescent and TADF devices hosted by CzCzPy were 27.30% and 10.03%, respectively.(3) Phenothiazine modified green host materials. Two host materials, namely CzPTZPy and CzPTZTz, were reported by adding phenothiazine modified carbazole to the electron-withdrawing pyridine or triazole moieties by meta-substituton strategy. Their optical bandgaps were decreased compared with the reference molecule by introducing electron-withdrawing groups. By virtue of rigid and highly distorted molecular conformation, they presented moderate ET values.(4) Blue host materials from the linkage in the 3 position of carbazole. Two host materials, namely 3-DCzPy and 3-DCzTz, were synthesized by connecting the N-phenylcarbazole units with the twice number of pyridine or triazole units through meta-substituton strategy. Theoretical simulations showed that there were some overlaps between their HOMO and LUMO contours. Introducing the electron-withdrawing groups to some extent lowered their HOMO/LUMO energy levels.3-DCzPy hosted blue phosphorescent device achieved a maximum current efficiency of 33.10 cd A-1.