New boomerang-shaped host materials for phosphorescent organic light-emitting diodes

Abstract

Inspite of being a good triplet sensitizer, benzophenones are not useful as host materials for phosphorescent organic light-emitting diodes (PhOLEDs), primarily due to their low thermal stability. In this work, we have addressed this shortcoming by judicial tethering of benzoyl units onto different positions of a rigid Tröger’s base (TB) scaffold to develop two new benzophenone tethered TBs, namely TB-Bz1, and TB-Bz2 as thermally stable host materials for applications in PhOLEDs. The TB-based host molecules were readily synthesized in good yields from the corresponding amines, and were fully characterized using various spectroscopic techniques. These TBs displayed high thermal decomposition temperatures above 320 °C, imparted by the rigid, non-planar, boomerang-shaped scaffold of the Tröger’s base. It was further observed that the tethering position of the benzoyl units can significantly affect the photophysical properties of the TBs due to the twisting of the benzoyl unit with respect to the TB-core. This assumption was further confirmed by DFT simulations. Taking advantage of the high triplet energy of TBs (>2.8 eV), PhOLED devices were fabricated exploring TBs as the host material and Ir(ppy)3 as the dopant. The maximum external quantum efficiency of 6.1 %, and 6.9 % and maximum luminance of greater than 5000 cd/m2 were obtained using TBs as the host for the proof-of-concept PhOLED devices, where Ir(ppy)3 was employed as the dopant.