Simultaneously Improving Radiative Decay and Reverse Intersystem Crossing in Space‐Confined Through‐Space Charge‐Transfer (TSCT) Emitter by Strong Intermolecular TSCT Enabled by a Planar Donor
Yongjun Song, Ke Zhang, Pingping Wang, Yuan Cao, Lei He
Abstract
Abstract Simultaneously improving radiative decay and reverse intersystem crossing (RISC) of through‐space charge‐transfer (TSCT) thermally‐activated delayed fluorescence (TADF) emitters has remained a challenge. Here, space‐confined TSCT‐TADF emitters are developed with a planar indolo[3,2,1‐ kl ]phenoxazine donor, a spiro‐carbon‐fixed benzophenone acceptor, and a fluorene (for 1 ) or xanthene (for 2 ) bridge. Close, cofacial donor/acceptor alignments and efficient intramolecular TSCT have been observed for both emitters. Intriguingly, emitter 1 exhibits an intermolecular donor/acceptor interaction as strong as the intramolecular one, which opens efficient intermolecular TSCT, improving the radiative decay rate ( k r,s ) to the same level as emitter 2 featuring a much stronger intramolecular donor/acceptor interaction. The intermolecular TSCT also largely enhances the RISC rate ( k RISC ). In the 20 wt.% doped films, emitters 1 and 2 show green TADF with high k r,s / k RISC at 1.1 × 10 7 s −1 /1.3 × 10 6 s −1 and 1.2 × 10 7 s −1 /7.7 × 10 5 s −1 , respectively. Organic light–emitting diodes (OLEDs) using the emitters show high external quantum efficiencies (EQEs) up to 27.5% and low‐efficiency roll‐offs. Hyperfluorescent OLEDs using the emitters as sensitizers afford narrowband blue‐green emission with high EQEs up to 30.6% and largely suppressed efficiency roll‐offs. The work reveals that strengthening intermolecular TSCT is a promising avenue to simultaneously improve k r,s, and k RISC of TSCT‐TADF emitters for high‐performance OLEDs.