Blue Phosphorescent Iridium(III) Compounds with the 2′,6′-Diisopropoxy-2,3′-Bipyridine Ligand

초록

Cyclometallated iridium(III) complexes, especially those bearing the C, N-chelated bipyridine ligand, have attracted enormous attention for more than a decade due to their high efficiency in phosphorescent organic-light emitting diodes (PHOLEDs).(1) It is well known that the phosphorescent emission of iridium(III) complexes based on the C, N-chelating bipyridine ligand mostly originates from the triplet ligand-centered ((LC)-L-3) transition combined with a matal-to- ligand charge transfer transition (MLCT).(2) Therefore, a ligand characterized by a large triplet energy is needed to develop blue phosphorescent emitting materials. Recently, we reported the evidence that the triplet energy of the bipyridine ligand is greater than that of phenylpyridine (ppy).(3) In addition, we reported a difference in emission maxima greater than 50 nm and a significant efficiency change depending on the nature of the electron-donating substituent group in the bipyridine ligand. Therefore, by incorporating proper substituents into the bipyridine ligand, significant emission and efficiency changes can be expected. However, due to its high reactivity and low selectivity, the introduction of other functionalized substituents into the pyridine ring is very difficult and limited.(4) There is a recent increasing interest in the introduction of ancillary ligand into ligand frameworks because it can improve charge trapping and electron mobility in OLEDs as well as cause emission energy changes of dopant molecules.(5) From the perspective of an increasing demand for developing high efficiency and stable phosphorescent iridium(III), especially blue iridium(III) phosphor, the study of the effect on ancillary ligand should be continuous. Herein, we report the crystal structures of two blue phosphorescent Ir(III) compounds bearing bulky isopropoxy substituents of the bipyridine and ancillary ligands. Moreover, the photophysical and electrochemical properties of these compounds were systematically investigated for the purpose of determining the possibility of PHOLED application.

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