New heteroleptic cyclometalated iridium(Ⅲ) complexes containing 2-(2’,4’-difluorophenyl)-4-methylpyridine for organic light-emitting diode applications

Two phosphorescent iridium(Ⅲ) complexes (dfpmpy)₂Ir(ppc) and (dfpmpy)₂Ir(prz) [dfpmpy=2-(2’,4’- difluorophenyl)-4-methylpyridine, ppc=pipecolinate, prz=2-pyrazine carboxylate] were synthesized from the reaction of the chloro-bridged dimeric complex [(dfpmpy)₂Ir(μ-Cl)]₂ and the ancillary ligand. Their structures and photoluminescence properties were investigated and device performances for application in organic light-emitting diodes (OLEDs) were studied. The complexes adopt a distorted, octahedral geometry around the iridium metal, exhibiting cis C–C and trans N–N arrangements. The photoluminescent (PL) properties reveal that (dfpmpy)₂Ir(ppc) emits in the blue-green region (λmax=497 ㎚), whereas (dfpmpy)₂Ir(prz) shows red phosphorescence (λmax=543 ㎚) in the film state (5% wt. doped in PMMA). The (dfpmpy)₂Ir(ppc)-and (dfpmpy)₂Ir(prz)-based OLEDs exhibited sky-blue and greenish-yellow electroluminescence with similar current–voltage characteristics, repectively. Maximum current efficiency of (dfpmpy)₂Ir(ppc) and (dfpmpy)₂Ir(prz) were 4.4 and 7.4 ㏅/A, respectively. Maximum luminance values were approximately 10,000 ㏅/㎡ for the both compounds.