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.