From these results it is possible to understand the competing effects imparted by peripheral bromine substitution on the emission energy and decay lifetimes of the DiKTa derivatives (Table and Figure ). ) when the emitter loading is too low. It appears, therefore, that the effect of peripheral heavy atoms on MR-TADF OLEDs -both directly on SOC through the HAE and indirectly through intermolecular interactions -can be both positive or negative depending on the exciton density.