Multifunctional metallophosphors with anti-triplet–triplet annihilation properties for solution-processable electroluminescent devices

Abstract

With the goal to provide organometallic triplet emitters with good hole-injection/hole-transporting properties, highly amorphous character for simple solution-processed organic light-emitting diodes, and negligible triplet–triplet (T–T) annihilation, a series of new phosphorescent cyclometalated Ir^III and Pt^II complexes with triphenylamine-anchored fluorenylpyridine dendritic ligands were synthesized and characterized. The photophysical, thermal, electrochemical and electroluminescent properties of these molecules are reported. The incorporation of two sterically hindered electron-rich triphenylamino groups to the 9-position of the fluorene skeleton was found not only to afford triplet emitters in the glassy state with high T_g, but also to elevate the HOMO levels and confer the hole-injection ability to the phosphorescent center. These highly amorphous metal phosphors can serve as doped emitters in a small molecular host for spin-coated emission layer in suitable OLED structures to achieve good device performance with a maximum luminance of 29380 cd m⁻² at 23 V, a peak external quantum efficiency of 7.0%, a luminance efficiency of 21.4 cd A⁻¹ and a power efficiency of 2.9 lm W⁻¹. Both the electrophosphorescent device characterization as well as the theoretical simulation results show that these iridium electrophosphors show negligible T–T annihilation even at high operating current densities and moderately high doping levels. Our investigations indicate that attaching the triphenylamino moieties to the fluorene ring is an effective way to overcome the T–T annihilation caused by the strong interactions among the emitting molecules.