Stabilization of Blue Emitters with Thermally Activated Delayed Fluorescence by the Steric Effect: A Case Study by means of Magnetic Field Effects

The use of steric hindrance in blue thermally activated delayed fluorophores shows empirically the improvement of operational lifetime for organic light-emitting diodes (OLEDs). Nevertheless, the intrinsic mechanism of this strategy remains unclear and elusive. Here, the steric effects on rate coefficient and degradation probability of bimolecular quenching processes are decoupled by magnetic field effects and quantum-chemical calculation. The respective dependence of magnetoelectroluminescence on triplet density and charge-carrier density indicates that triplet-charge quenching (TCQ) is the dominant degradation mechanism over that of triplet-triplet annihilation, where steric hindrance effectively prevents the occurrence of Dexter-type TCQ and significantly reduces the TCQ rate coefficient. On the contrary, only a slight increase in degradation probability is generated by the steric effect from bulk inert groups. These findings will provide unambiguous guidance for the design of functional materials for OLEDs.