Abstract
Abstract: 1. Os(II) blue emitters: A new series of Os(II) based complexes [Os(CO)2(py-tBu-tz)2] (1), [Os(py-tBu-tz)2(CO)2] (2) and [Os(py-CF3 -tz)2(CO)2] (3), in which bptz and fptz denote 3-t-butyl-(2-pyridyl) and 3-trifluoromethyl-5-(2-pyridyl) 1,2,4-triazolate, respectively, were synthesized in an aim to attain high efficiency room temperature blue phosphorescence. Although 1 and 2 are both geometric isomers, drastically different excited-state relaxation pathways were resolved. 1 exhibits strong phosphoresce (□p ~ 0.47) in CH3CN as well as in a single crystal, while 2 is nearly non-emissive. The associated relaxation dynamics have been investigated comprehensively from spectroscopy, dynamics and theoretical approaches. Our results lead to a conclusion that the trans effect imposed by the CO ligand plays a crucial role in fine-tuning the relative energy gap between the lowest 3□□* state and the metal centered dd state, which then triggers fast radiationless deactivation for the geometrical isomer 2. Fine adjustment is also achieved by functionalizing the ligands so that the electron withdrawing property of the CF3 group in 3 not only stabilizes the HOMO of the triazolate moiety, but also greatly enhances the □-acid nature of the ligand. Consequently, the □□* gap of ligand is increased and the d orbitals of the Os atom are stabilized by the enhanced metal-to-ligand back bonding, resulting in a highly efficient phosphorescence suited to application as a blue OLED emitter. 2. Os(II) red emitters: The OLED device using 6% of Os(py-CF3 -tz)2(PPh2Me)2 as the dopant emitter in CBP host and BPAPF as HTL shows an external quantum efficiency of 15.3% and luminous efficiency of 21.3 cd/A, power efficiency of 6.3 lm/W at 20 mA/cm2. An even higher external quantum efficiency of ~20 % was achieved at low current density of ~1 mA/cm2. 3. Ru(II) red emitters: The preparation of three charge neutral Ru(II) isoquinoline pyrazolate complexes, [Ru(ipy-tBu-pz)2(PPhMe2)2] (1) [Ru(ipy-tBu-pz)2(PPh2Me)2] (2) and [Ru(ipy-CF3 -pz)2(PPh2Me)2] (3) (ibpz: 3-tert-butyl-5-(1-isoquinolyl) pyrazolate, ifpz: 3-trimethylfluoro-5- (1-isoquinolyl) pyrazolate), is reported. An OLED device using 24 wt.% of 3 as a dopant emitter in 1,4-bis(N-carbazolyl) biphenyl host material and with 1,4-bis(1-naphylphenylamino) biphenyl as hole transport layer exhibits saturated red phosphorescence with an external quantum efficiency of 4.44%, luminous efficiency of 5.08 cd/A and power efficiency of 2.36 lm/W at the applied current density of 20 mA/cm2.