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 nm), whereas (dfpmpy)2Ir(prz) shows red phosphorescence (λmax=543 nm) 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 cd/A, respectively. Maximum luminance values were approximately 10,000 cd/m2 for the both compounds.