| In recent years, organic light emitting devices (OLEDs) have played a key role as a new type of technology in information display and solid-state lighting fields. However, the search for efficient deep blue phosphorescent materials is an important challenge and has become the bottleneck of high efficiency white OLEDs.In this paper, in order to obtain high-efficiency blue phosphorescent Ir(III) complexes,2-(2-pyridyl)Phenylimidazole (Hpybi),2-(5-(trifluoromethyl)-1,2,3-triazole)pyridine (Htfmptz),2-(5-(-(trifluoromethyl)phenyl)-1,2,3-triazole) pyridine (Htfmpptz) and2-(5-(4-fluorophenyl)-1,2,3-triazole) pyridine (Hfpptz) were introduced as auxiliary ligand in low-temperature phosphorescent material of tris(phenylpyrazole)iridium (Ir(ppz)3) and green-phosphorescent tris(2-phenylpyridine)iridium (Ir(ppy)3). Their molecular structures were defined by X-ray single crystal diffractions. The photophysical properties were investigated by UV-Vis absorption, excitation, emission spectra and cyclic voltammetry. The phosphorescent organic electroluminescent devices (PhOLEDs) were fabricated by using Ir (Ⅲ) complexes doped in CBP as emitting layer.As we all known that Ir(ppy)3is the best green electrophosphorescent materials up to now, which had excellent electron injection and transport characteristics. By introducing N^N auxiliary ligands, four (ppy)2Ir(N^N) complexes of (ppy)2Ir(pybi),(ppy)2Ir(tfmptz),(ppy)2Ir(fpptz) and (ppy)2Ir(tfmpptz) were prepared. The results indicated that blue-shifted emission spectra were obtained relative to Ir(ppy)3. The single-crystal structures of (ppy)2Ir(tfmpptz),(ppy)2Ir(tfmptz) are triclinic system, space group P-1. The titled Ir(III) complexes have green-blue emission in the CH2C12solution, especially (ppy)2Ir(tfinpptz) with the emission peak of483nm. Compared with Ir(ppy)3,(ppy)2Ir(tfmpptz) has a34nm blue shift. The emission maximum peaks of (ppy)2Ir(tfmpptz),(ppy)2Ir(fpptz),(ppy)2Ir(tfmptz) and (ppy)2Ir(pybi) appear at485,487,483and495nm, respectively. To character the electroluminescent properties of these complexes, the PhOLEDs with stuctures of ITO/NPB (30nm)/CBP:(ppy)2Ir(N^N)(6%complexes,30nm)/BAlq (10nm)/Alq3(30nm)/LiF (1nm)/Al (100nm) were prepared. The order of maximum current efficiency is (ppy)2Ir(pybi)>(ppy)2Ir(tfmpptz)>(ppy)2Ir(fpptz)>(ppy)2Ir(tfmptz). The roll-off phenomenon was not obvious. The true-blue fac-Ir(ppz)3was reported with an emission peak wavelength of λmax=414nm at77K, while is nonemissive at room temperature due to the small activation energy to a non-emissive dd excited state and thus is lack of practical value. In this paper, the room-temperature electrophosphorescent (ppz)2Ir(N^N) complexes of (ppz)2Ir(pybi),(ppz)2Ir (tfrnptz),(ppz)2Ir(fpptz) and (ppz)2Ir(tfmpptz) were prepared by using N, N-aromatic heterocyclic (N^N) as ancillary ligands and ppz acting as blue chromophore. The single-crystal structures of (ppz)2Ir(tfmpptz),(ppz)2Ir(tfmptz) are monoclinic system, space group P2(1)/n. The blue light emissions were observed in CH2C12solution of these complexes, thereinto blue color purity of (ppz)2Ir(tfmptz) is the best with peak wavelength ofλmax=474nm. The emission peak wavelength increases in the following order:(ppz)2Ir(pybi)(525nm)>(ppz)2Ir(fpptz)(495nm)>(ppz)2Ir(tfmpptz)(485nm)>(ppz)2Ir(tfmptz)(474nm). The HOMO/LUMO levels of complexes (ppz)2Ir(tfmpptz),(ppz)2Ir(fpptz),(ppz)2Ir(tfmptz) and (ppz)2Ir(pybi) are6.01/3.22,5.95/3.26and6.59/3.26, and5.92/3.32eV, respectively. The PhOLEDs were fabricated by doping Ir (III) complexes in4,4’-N,N’-dicarbazole-biphenyl (CBP) as lighte-emitting layers with the architecture of ITO/NPB (30nm)/CBP:(ppz)2Ir(N^N)(6%complexes,30nm)/BAlq (10nm)/Alq3(30nm)/LiF (1nm)/Al (100nm). The emission maximum of electroluminscent (EL) spectra appeared at480nm based on (ppz)2Ir(tfmpptz) with maximum current efficiency (ηc) of6.0cd/A. The ηc reach to5.7cd/A at the current density of100mA/cm2. The roll-off phenomenon of current efficiency is very weak. The maximum brightness of PhOLED is20033cd/m2.On the basis of experiments, the relationship between the molecular structure and photophysical performance was discussed. The results indicated that the larger steric hindrance of ancillary ligands can efficiently suppress the roll-off of efficiency at high current density. |
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