Structurally Nontraditional Bipolar Hosts for RGB Phosphorescent OLEDs: Boosted by a “Butterfly‐Shaped” Medium‐Ring Acceptor

The emitting layer based on a host-guest system plays a crucial role in organic light-emitting diodes (OLEDs). While emitters have witnessed rapid progress in structural diversity, hosts still rely heavily on traditional structures and are underdeveloped. Herein a "medium-ring" strategy has been put forward to design structurally nontraditional host molecules, which are not only rotatable enough to suppress close π-π stacking, thus reducing exciton annihilation, but also rigid enough to prevent excessive conformational flipping, thus inhibiting non-radiative decay. Accordingly, a brand-new type of bipolar hosts with a twisted "butterfly-shaped heptagonal acceptor (EtBP), which features an electron-deficient benzophenone fragment with a flexible ethylidene bridge, has been developed. With satisfactory morphological stability and well-balanced hole- and electron-transporting properties, the EtBP-based bipolar hosts enable high-performance RGB phosphorescent OLEDs with small efficiency roll-off, which are superior to those of acyclic benzophenone-based devices.