Acceleration of Reverse Intersystem Crossing using Different Types of Charge Transfer States

There is a need to boost the rate constant of reverse intersystem crossing (k_RISC ) in thermally activated delayed fluorescence (TADF) materials for applications to organic light-emitting diodes. Recently, energy level matching of the locally excited state (LE) and charge transfer state (CT) has been reported to enhance k_RISC . In this study, we conceptually demonstrate that k_RISC can be improved even between CT states without LE states, through the use of different types of CT states. On the basis of this concept, we design a new compound, named DMAC-bPmT, where two phenyl groups of a well-known TADF material DMAC-TRZ are substituted by pyrimidine groups. Theoretical calculations indicated that the energy levels of the different CT states of DMAC-bPmT are very close and enhanced spin orbit coupling may be expected between them. As predicted, DMAC-bPmT experimentally exhibited a k_RISC three times as high as that of DMAC-TRZ.