Rigid Polyimides with Thermally Activated Delayed Fluorescence for Polymer Light‐Emitting Diodes with High External Quantum Efficiency up to 21 %.

A series of rigid nonconjugated polyimide (PI)‐based thermally activated delayed fluorescence (TADF) polymers were reported for the first time, based on a "TADF‐Linker‐Host" strategy. Among of which, the TADF unit contains a typical TADF luminous core structure, the "Host" unit exhibits effective conjugation length that endows polyimide with high triplet energy, and the "Linker" unit has an aliphatic ring structure to improve solubility and inhibits intramolecular charge transfer effect. All the TADF polymers exhibit high thermal stability (Tg>308.7 °C) and refractive index (1.76–1.79). Remarkably, highly‐efficient polymer light‐emitting diodes (PLEDs) based on the polymers are successfully realized, leading to a maximal external quantum efficiency of 21.0 % along with low efficiency roll‐off. Such outstanding efficiency is amongst the state‐of‐the‐art performance of nonconjugated PLEDs, confirming the effectiveness of structural design strategy, providing helpful and valuable guidance on the development of highly‐efficient fluorescent polymer materials and PLEDs. [ABSTRACT FROM AUTHOR]