Highly efficient TADF OLEDs with low efficiency roll-off based on novel acridine-carbazole hybrid donor-substituted pyrimidine derivatives

Dimethyl acridine (Ac) and carbazole (Cz) donors are fused to produce acridine-carbazole hybrid donors 12AcCz (8,8-dimethyl-5-phenyl-8,13-dihydro-5H-indolo[2,3-c]acridine) and 23AcCz (13,13-dimethyl-7-phenyl-7,13-dihydro-5H-indolo[3,2-b]acridine). By combining the TPPM (2,4,6-triphenylpyrimidine) acceptor with the Ac-Cz-fused donors of 12AcCz and 23AcCz, two emitters, 12AcCz-PM (13-(4-(2,6-diphenylpyrimidin-4-yl)phenyl)-8,8-dimethyl-5-phenyl-8,13-dihydro-5H-indolo[2,3-c]acridine) and 23AcCz-PM (5-(4-(2,6-diphenylpyrimidin-4-yl)phenyl)-13,13-dimethyl-7-phenyl-7,13-dihydro-5H-indolo[3,2-b]acridine), are designed and synthesized. The impact of the different fused topologies of the hybrid donors on the molecular configuration and photophysical and optoelectrical properties is systematically investigated. For 12AcCz-PM, the large steric hindrance between the donor and acceptor compels Ac of 12AcCz to be crooked, which endows it with a relatively planar quasi-axial conformation. No blatant TADF character is observed, but the 12AcCz-PM doped device still presents ultradeep blue emission with CIE_x,y of (0.15, 0.05) and a maximum external quantum efficiency (EQE_max) of 6.4%. Whereas, 23AcCz-PM shows a quasi-equatorial conformation and possesses notable TADF behavior. The 23AcCz-PM doped device reachs an EQE_max of 28.4% at low dopant concentration and 26.1% at high dopant concentration. Additionally, low efficiency roll-off (EQEs of 24.8% at 1000 cd m^-2 and 20.7% at 5000 cd m^-2) further confirms its superiority as a TADF emitter.