Hanging heavy atom-containing chains onto a multiple resonance framework: Influence on the TADF properties and device performances

Multiple resonance (MR)-based thermally activated delayed fluorescence (TADF) emitters are promising candidates for ultra-high definition (UHD) organic light-emitting diodes (OLEDs). However, the device performances of MR-TADF emitters are usually encumbered with their slow reverse intersystem crossing (RISC) rates (kRISCs). Recently, applying heavy atom effect to MR emitters has been proved to be a plausible solution for enhancing their kRISC and improving TADF properties. Nevertheless, to date, heavy atoms have all been directly fused in MR frameworks, leading to limitations in molecular design and additional structure relaxation from heavy atoms. Herein, we incorporate heavy atom effect via facilely hanging heavy atom-containing chains onto a MR framework and thus synthesize two MR emitters (sulfur ether group for TCzBN-S and sulfone group for TCzBN-SO). An enhanced kRISC of 1.40 × 105 s−1 is reached for TCzBN-S due to sufficient natural transition orbital (NTO) contributions from the heavy atom, which is absent for TCzBN-SO, with a relatively slow kRISC of 5.6 × 104 s−1. Consequently, OLEDs employing TCzBN-S as emitter achieve a maximum external quantum efficiency (EQE) of 30.0% and an alleviated efficiency roll-off of 44% at 1000 cd m−2. In contrast, OLEDs based on TCzBN-SO achieve a maximum EQE of 33.0% but a more severe efficiency roll-off1000 of 65%. Moreover, solution-process OLEDs based on TCzBN-S and TCzBN-SO are also fabricated and achieved high EQEs of 23.3% and 25.5%, respectively, demonstrating that peripheral flexible and hanging heavy atom chains are also beneficial to solution processing. Our work not only extends heavy atom involving modes in MR emitters, which are important for further developing MR derivatives by using the heavy atom effect, but also confirms that such MR derivatives with hanging chains are applicable to both vacuum-evaporation and solution-process OLEDs.