Molecular design strategy for orange-red thermally activated delayed fluorescence emitters via intramolecular energy transfer and their application in solution processable organic light-emitting diodes

Realizing effective orange-red/red emission is still far behind blue and green emission in thermally activated delayed fluorescence (TADF) materials. In this context, a novel strategy for achieving efficient orange-red/red TADF emitters is proposed via intramolecular energy transfer by encapsulating the red-emitting core with a blue emitting chromophore. Two novel compounds called TS-1 and TS-2 were prepared and characterized, in which two blue TADF emitting units are used as the end groups and a red TADF emitter as the molecular core (TA). Both compounds show clear TADF character with red emission at 655 nm in a neat film, suggesting an effective intramolecular energy transfer from the blue chromophores to the red one. Both TS-1 and TS-2 present higher photoluminescence quantum yields (70-90%) than the TA core (42%) due to the encapsulation and energy transfer. Solution-processable organic light-emitting diodes (OLEDs) exhibit a maximum external quantum efficiency of similar to 13% for compound TS-1, much higher than that of TA-based OLEDs (2.3%). This research is the first example for red TADF emitter via intramolecular energy transfer, which clearly provides an effective method for designing highly efficient red TADF molecules.