The locally twisted thiophene bridged phenanthroimidazole derivatives as dual-functional emitters for efficient non-doped electroluminescent devices

A series of locally twisted dual-functional materials namely PIPT, PITT and PIFT have been designed and synthesized by introducing different polyaromatic hydrocarbon groups to a phenanthroimidazole backbone through a thiophene bridge. In these molecules, the thiophene bridge and phenanthroimidazole platform are nearly coplanar and this endows these materials with relatively shallow HOMO levels (-5.35 to -5.21 eV). On the other hand, the bulky polyaromatic hydrocarbon units introduce non-planar twisty structures which reduce molecular aggregations. These three materials show color-tunable emission (emission peak from 468 to 532 nm in film) and high thermal stability (T_g > 160 °C). Simple trilayer devices using these three phenanthroimidazole derivatives as non-doped emitting layers exhibit low turn-on voltages (2.3-2.7 V) and high maximum efficiencies of 3.74, 6.15 and 6.89 cd/A for PIPT, PITT and PIFT, respectively. Above all, owing to their shallow HOMO levels for enabling efficient hole-injection, even simpler bilayer devices employing these materials as hole-transporting emitters show low turn-on voltages (2.6-2.8 V) and high efficiencies of 5.77 cd/A for PIPT, 6.03 cd/A for PITT and 6.04 cd/A for PIFT, respectively. These comparable performances with those of the trilayer configurations show the efficient hole-injection/transport ability of these three newly developed emitters.