Intramolecular charge transfer assisted multi-resonance thermally activated delayed fluorescence emitters for...

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters have been actively employed in high-resolution solution-processed organic light emitting diodes (OLEDs) due to their excellent color purity. Nonetheless, they are always confronted with intrinsic slow spin flip of triplet excitons, impeding the electroluminescence properties, especially in non-sensitized OLEDs. Herein, we constructed intramolecular charge transfer (ICT) assisted MR-TADF emitters by grafting donor-acceptor-type moieties with a meta- or para-substitution as a pendant on an organoboron multi-resonance core. The newly designed MR-TADF emitters not only maintain short range charge transfer characteristics in emissive states without sacrificing color purity but the accelerated spin flips facilitated by the ICT process at a high-lying state are also confirmed by ultrafast spectroscopy and theoretical calculation, achieving over a 10-fold increase in the reverse intersystem crossing rate compared with unsubstituted counterpart emitters. In sensitizer-free solution-processed OLEDs, a cutting-edge external quantum efficiency of 27.8% can be achieved together with reduced efficiency roll-offs and an attractive full width at half maximum of 29 nm, representing a breakthrough in efficiency for solution-processed MR-TADF based narrowband OLEDs.