Janus-type dendritic organoiridium (III) complex bearing holeand electron-transporting moieties: Synthesis, luminescence properties, and OLED applications

Abstract

A novel tris-cyclometalated iridium(III) complex bearing carbazole-based hole-transporting and triarylphosphine oxidebased electron-transporting dendrons was synthesized through a convergent synthetic route, and its facial and meridional isomers were isolated by conventional silica gel chromatography. The facial isomer exhibited sky-blue photoluminescence (PL) from the phosphorescent core even in the neat film state with a relatively high PL quantum yield (ΦPL) of 0.57 because the dendrons efficiently suppress aggregate formation between the cores. The meridional isomer was less emissive (ΦPL; 0.029 in deaerated dichloromethane) but photoisomerized to the facial isomer by UV light irradiation (365 nm) in spite of large steric hindrance between the bulky dendrons. A non-doped organic light-emitting diode (OLED) employing the facial isomer exhibited sky-blue electroluminescence with a higher maximum external quantum efficiency (EQEmax) of 1.6% than that of a comparable device employing a reference complex bearing only hole-transporting dendrons (EQEmax; 0.5%). As lipophilic and apolar tert-butyl groups are placed on the molecular surface, the present dendritic complex is soluble in cyclohexane and insoluble in ethanol. Utilizing this characteristic solubility, non-doped multilayer OLEDs consisting of a stack of hole-transporting layer/emitting layer (the dendritic complex)/electron-transporting layer were fabricated by solution processing. One of the multilayer devices exhibited an improved EQEmax of 5.2%.