A pyrimidine-5-carbonitrile acceptor combined with an ortho-linked donor for long lifetime through facilitated reverse intersystem crossing in thermally activated delayed fluorescence emitters

Abstract

Molecular design of thermally activated delayed fluorescence (TADF) emitters combining a 4,6-diphenylpyrimidine-5-carbonitrile (PyCN) acceptor and an ortho-linked donor was developed in order to extend the lifetimes of the TADF organic light-emitting diodes by shortening the delayed fluorescence lifetime and facilitating the reverse intersystem crossing by a TADF process. The ortho connection of the donor to the strong PyCN acceptor reduced the singlet-triplet energy splitting to 0.02 eV and the delayed fluorescence lifetime to 2.4 mu s compared to 0.18 eV and 12.8 mu s of the corresponding para-linked TADF emitter. The PyCN-derived emitter was even better than the well-known diphenyltriazine-derived emitter in terms of TADF parameters. Moreover, a high reverse intersystem crossing rate constant close to 1.0 x 10(6) s(-1) was noticed in the PyCN derived emitter. The high reverse intersystem crossing rate suppressed efficiency roll-off and extended device lifetime. Therefore, the design strategy merging the PyCN acceptor with an ortho-linked bicarbazole donor was successful in reducing efficiency roll-off and improving lifetime.