Highly effective organic light-emitting diodes containing thermally activated delayed fluorescence emitters with horizontal molecular orientation.

In this study, we report new thermally activated delayed fluorescence (TADF) emitters, AcPYM (10,10'-(pyrimidine-2,5-diylbis(4,1-phenylene))bis(9,9-dimethyl-9,10-dihydroacridine)) and PxPYM (10,10'-(pyrimidine-2,5-diylbis(4,1-phenylene))bis(10 H -phenoxazine)), by employing donor units at the 2,5-positions of the pyrimidine acceptor unit. The donor-acceptor-donor (D-A-D) units combined in the linear molecular structure of AcPYM or PxPYM enhanced the horizontally oriented alignment, and the horizontal transition dipole moments were realized by up to 87% in the host matrix. Organic light-emitting diodes (OLEDs) containing AcPYM and PxPYM emitters realized external quantum efficiencies ( η ext ) of 16.8% for blue and green emissions.