Multi-π-Stacked Through-Space Charge-Transfer Emitters with Accelerated Radiative Decay for High-Efficiency Organic Light-Emitting Diodes with Suppressed Efficiency Roll-Offs

Through-space charge-transfer (TSCT) thermally-activated delayed fluorescence (TADF) emitters have been extensively explored for organic light-emitting diodes (OLEDs), but their low radiative decay rates (kr,s) have remained a formidable obstacle to enhancement of luminescent efficiency (& Fcy;PL) and suppression of efficiency roll-off of the OLED. Here, multi-pi-stacked TSCT-TADF emitters with accelerated radiative decay are developed with a 3D [fixed acceptor]/donor/[fixed acceptor] structure formed on two individual fluorene bridges. Single-crystal structures and theoretical calculations reveal cis or trans-configurations of the two acceptors around the central donor and double TSCT channels in the emitter. In doped films, the emitters show green-blue TADF with high kr,s up to 1.1 x 107 s-1 and near-unity & Fcy;PL, which are notably improved compared to the single-pi-stacked analog (kr,s = 2.1 x 106 s-1, & Fcy;PL = 0.89). OLEDs based on the emitters show high external quantum efficiencies (EQEs) up to 26.2% and the EQEs at 1000 cd m-2 (EQE1000) remain up to 23.4%, which are superior over the device based on the single-pi-stacked analog (EQEmax/EQE1000 = 20%/15.6%). A narrowband blue-green OLED with one emitter as a sensitizer shows high EQEmax/EQE1000 at 37.6%/27.7%. The work demonstrates the great potential of multi-pi-stacked structure in enhancing the kr,s of TSCT-TADF emitters for high-efficiency OLEDs with low efficiency roll-offs.