Towards highly efficient red thermally activated delayed fluorescence materials by the control of intra-molecular π-π stacking interactions.

Thermally activated delayed fluorescence (TADF) materials have attracted much attention as they can achieve 100% theoretical internal quantum efficiency without using expensive noble metals. However, efficient red TADF emitters are hard to realize according to the energy gap law. Here, three donor-acceptor-donor type TADF emitters with the same acceptor of o-phthalodinitrile (PN) but different donors (9, 9-dimethyl-9, 10-dihydroacridine (DMAC), phenoxazine (PXZ), and phenothiazine (PTZ) for DMAC-PN, PXZ-PN, and PTZ-PN, respectively) have been synthesized, and it is observed that the performance of the emitters can be improved by reducing the intra-molecular π-π stacking. DMAC-PN with reduced intra-molecular π-π stacking shows a photoluminescence quantum yield (PLQY) of 20.2% in degassed toluene solution, much higher than those of PXZ-PN, and PTZ-PN (0.8%, 0.2%, respectively). An organic light-emitting diode (OLED) employing DMAC-PN doped into 4,4'-bis(9H-carbazol-9-yl)biphenyl (CBP) as the emitting layer exhibits a maximum external quantum efficiency (EQE) of 10.2% with the emission peak at 564 nm. Moreover, when DMAC-PN is doped into a polar host, bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO), the OLED shows a large redshift of the emission maximum to 594 nm, while maintaining a peak EQE as high as 7.2%, indicating that efficient red TADF OLEDs can be fabricated by doping orange TADF emitters into hosts with proper polarity.