Tailored Energy Landscape to Foster Reverse Intersystem Crossing and Radiative Rates in TADF Emitters

Effective triplet exciton utilization is essential for high electroluminescence efficiency and device durability in the development of OLEDs. Thermally activated delayed fluorescence (TADF) has proven to be a powerful strategy for this purpose. However, balancing a low singlet-triplet energy offset (ΔEST) with a high oscillator strength (f) remains a significant challenge, as these two factors often conflict due to their different dependencies on orbital overlap integrals. In this study, we developed a TADF emitter with a minimal ΔEST, where both short-range and long-range charge transfer transitions contributed to a high f and an enhanced krISC. A reliable strategy for fostering electronic coupling between the1CT and3LE or hybridized triplet states in newly synthesized TADF emitters featuring a strongly twisted D-A structure is reported here. Furthermore, a suitable dielectric solvation technique is employed to effectively modulate the3CT energy, revealing a clear correlation between the photoluminescence quantum yield and the dielectric constant of the matrix. © 2024 American Chemical Society.