Excited state engineering for efficient reverse intersystem crossing.

Reverse intersystem crossing (RISC) from the triplet to singlet excited state is an attractive route to harvesting electrically generated triplet excitons as light, leading to highly efficient organic light-emitting diodes (OLEDs). An ideal electroluminescence efficiency of 100% can be achieved using RISC, but device lifetime and suppression of efficiency roll-off still need further improvement. We establish molecular design rules to enhance not only the RISC rate constant but also operational stability under electrical excitation. We show that the introduction of a second type of electron-donating unit in an initially donor-acceptor system induces effective mixing between charge transfer and locally excited triplet states, resulting in acceleration of the RISC rate while maintaining high photoluminescence quantum yield. OLEDs using our designed sky-blue emitter achieved a nearly 100% exciton production efficiency and exhibited not only low efficiency roll-off but also a marked improvement in operational stability.