Sky-blue phosphorescence from bis- and tris-cyclometalated iridium(III) complexes bearing carbazole-based dendrons: fabrication of non-doped multilayer organic light-emitting diodes by solution processing

Novel phosphorescent bis- and tris-cyclometalated iridium(III) complexes bearing carbazole-appended dendrons were successfully synthesized, and fabrication of non-doped multilayer OLEDs by solution processing was demonstrated by using these complexes as emitting materials. The complexes exhibited sky-blue photoluminescence (PL) at 460-464 nm with excellent PL quantum yields of ca. 0.8, and their neat films also emitted sky-blue photoluminescence: the dendrons effectively suppressed aggregation between the luminescent cores, although referential core complexes, bearing no dendrons, exhibited aggregate-based green PL even when the core complexes were doped into a model host material of the dendron in the same core-dendron molar ratios as the present dendritic bis- and tris-cyclometalated iridium(III) complexes. As hydrophobic tert-butyl groups are placed on the periphery, the dendritic complexes exhibit good solubility in cyclohexane and poor solubility in 2-propanol. Such special properties allow for fabrication of solution-processed non-doped multilayer OLEDs consisting of a stack of the hole-transporting layer (HTL)/emitting layer (the dendritic complex)/electron-transporting layer (ETL). The devices showed sky-blue electroluminescence with excellent electron-hole charge balance factors (gamma) of ca. 0.9, which were much higher than those of simple non-doped devices without HTL and ETL (gamma; ca. 0.1).