Sky-blue phosphorescent iridium(III) complexes with two substituted 2-phenylpyridine derivatives and one picolinic acid for organic light-emitting diodes

Two sky-blue phosphorescent heteroleptic cyclometalated iridium(III) complexes, bis(2-(2′,4′-difluoro-3′-trifluoromethylphenyl)-4-methylpyridinato-N,C2′)iridium(picolinate) [(dfpmpyCF3)2Ir(pic)] (1) and bis(2-(2′,4′-difluoro-3′-hexylcarbazolephenyl)-4-methylpyridinato-N,C2′)iridium(picolinate) [(dfpmpyh-Cz)2Ir(pic)] (2), were synthesized and characterized. Energy transfer from the carbazol moiety to the iridium core in compound 2 resulted in higher quantum efficiency than with compound 1. The electronegative effect of the –CF3 group on the phenyl ring in the phenylpyridine main ligand resulted in a hypsochromic shift of the emitting spectra in compound 1. In contrast, the sky-blue emission without a red shift in compound 2 indicated a negligible inductive effect of the long chain hexylcarbazole unit on the electronic state of the phenyl ring. The single crystal X-ray structural analysis showed that compound 1 adopts a distorted octahedral geometry around the iridium metal exhibiting cis-C,C′ and trans-N,N′ chelate disposition. An electroluminescent devices using the iridium complexes as a dopant in the emitting layer has been fabricated. The best device performance based on compound 1 demonstrated a maximum luminance (L), external quantum efficiency (EQE), luminance efficiency (LE) and power efficiency (PE) of 5750 cd/m2, 7.53%, 16.86 cd/A and 5.88 lm/W, respectively with Commission Internationale de LʹEclairage (CIE) coordinates of (0.17,0.40).