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. [ABSTRACT FROM AUTHOR]