Enhancing Reverse Intersystem Crossing of Multiple Resonance Type Thermally Activated Delayed Fluorescence Emitter by Introducing Spatial Perturbation

For multiple resonance (MR) type thermally activated delayed fluorescence (TADF) emitters, electron cloud distributions of their pi-conjugated planes are crucial for determining their eventual performance. Currently, modulation attempts of MR-TADF emitters are mainly inside the pi-conjugated planes. Possible out-of-plane interactions may also significantly impact the photophysical properties, but the exploration is quite limited. Here, a novel concept of using out-of-plane (e.g., pi-pi and lone pair-pi) interactions to introduce spatial perturbation (SPPT) to improve TADF performance is proposed. Two newly developed MR-TADF emitters, namely, o-BNPO and BNPO, which both consist of a popular MR framework, DtBuCzB, and diphenylphosphine oxide (DPPO), are compared in depth. In particular, for o-BNPO, evident pi-pi interaction is observed between one side of the DtBuCzB pi-conjugated plane and a phenyl ring from DPPO, and lone pair-pi interaction with the oxygen atom from DPPO is noticed on the other side, resulting in significantly accelerated reverse intersystem crossing and better TADF without sacrificing narrowband emission features. Ultimately, in organic light-emitting diodes with sensitizer-free emitting layers, both emitters achieve similar narrowband emissions, while the o-BNPO-based device demonstrates a much higher external quantum efficiency of 36% and milder efficiency roll-off.