Highly Efficient Luminescence from a Red Thermally Activated Delayed Fluorescence Emitter with Flexible Conformation of Ancillary Groups.

Robust scaffolds were typically applied in thermally activated delayed fluorescence (TADF) molecules to suppress the non-radiative decay, trigger the fast spin-flipping, and enhance the light out-coupling efficiency. Herein, we disclosed for the first time the positive effect of flexible conformation of ancillary groups on the photophysical properties of TADF emitter. The red TADF emitter Ph-TPA with flexible conformation demonstrated small excited-state structural distortion and low reorganization energy compared to the counterpart Mc-TPA with a rigid macrocycle. Consequently, Ph-TPA showed an excellent photoluminescent quantum yield (PLQY) of 92% and a state-of-the-art external quantum efficiency (EQE) of 30.6% at 630 nm. This work could deepen our understanding of structure-property relationships of organic luminophores and help us to rationalize the design of efficient TADF materials.