Azaacene containing iridium(<sc>iii</sc>) phosphors: elaboration of the π-conjugation effect and application in highly efficient solution-processed near-infrared OLEDs

Azaacenes have attracted wide research interest due to their tremendous potential in organic electronics. However, near-infrared (NIR) light-emitting iridium(iii) phosphors bearing azaacene derivatives are rarely investigated. In this contribution, two solution-processable heteroleptic iridium(iii) complexes, namely DBPzIr and PPzIr, are rationally designed and synthesized, and they contain a rigid phenanthrene- or pyrene-fused diazaacene core and two peripheral groups of 4-tert-butyl-phenyl attached at the 12,13-positions in the core, respectively. The effects of the diazaacene core and appending groups on the optoelectronic properties of both complexes are systematically investigated. A dramatically red-shifted NIR emission peak at 789 nm with a photoluminescence quantum yield (PLQY) of 14% is observed in PPzIr compared with the 746 nm emission with a PLQY of 40% in DBPzIr. Taking advantage of their photophysical properties, the solution-processed device doped with DBPzIr achieves a maximum external quantum efficiency (EQEmax) of 8.00% with a radiance of 54 866 mW Sr-1 m(-2) at 716 nm and the device doped with PPzIr exhibits a significantly red-shifted emission at 772 nm with an EQE(max) of 3.53%. The achieved device performance is among the best values in the reported NIR-OLEDs based on iridium(iii) complexes via a solution process at the same color gamut. Our study indicates that the reasonable collocation of the rigid diazaacene chelating core and flexible peripheral groups in the iridium(iii) complex is of great significance in designing highly efficient NIR emitters.