Conformational isomerization imparts low concentration dependence to multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters

Electroluminescent performance of multiple resonance (MR) type thermally activated delayed fluorescence (TADF) emitters is highly dependent to the doping concentration due to the flat molecular structures, which is a major issue toward practical applications. Herein, an effective approach reducing the concentration dependence is proposed by introducing conformational isomerism for MR-TADF emitters. Two novel MR-TADF emitters were designed by decorating 10H-spiro[acridine-9,2 '-adamantane] (a-DMAc) as a dual-conformational moiety onto the MR-core BNCz with different connection methods, which could induce different conformational distributions. In particular, the conformational isomerism of BNCz-aDMAC leads to a lower fraction of QE forms compared to the QE-dominant BNCz-PDMAC in amorphous doped film states with identical doping weights, thus enabling effective suppression of concentration quenching with barely harming color purity. Therefore, BNCz-aDMAC-based organic light-emitting diodes (OLEDs) demonstrate consistent and high efficiency over a wide doping range, maintaining a maximum external quantum efficiency of 23.8 % and a full width at half maximum of 31 nm even at a high doping ratio of 30 wt%.