Over 260-fold enhancement of reverse intersystem crossing by a host–guest exciplex for a multiple resonance emitter toward efficient narrowband electroluminescence

Multiple resonance (MR) thermally activated delayed fluorescence (TADF) emitters hold great potential for fabricating high-efficiency narrowband organic light-emitting diodes (OLEDs) toward high-definition display applications. However, their slow reverse intersystem crossing (RISC) causes strong device efficiency roll-offs. Reported strategies to enhance the RISC rates (<i>k</i> _RISC) of MR-TADF emitters are based on chemical modification of the emitters, which complicates molecular design and synthesis and easily causes widened emission spectra. Here, by utilizing a delicately designed host-guest exciplex, the <i>k</i> _RISC of a MR-TADF emitter is significantly enhanced without sacrificing the narrow emission bandwidth. By closely aligning the energy levels of the host and MR-TADF guest, the host-guest ³exciplex state is efficiently formed, which serves as an intermediate triplet state to largely accelerate the RISC of the guest. By embedding a S/Se heavy atom into the host, the heavy atom is directly involved in the ³exciplex state, which markedly strengthens the spin-orbital coupling and boosts the RISC. With the above strategy, host materials for a typical MR-TADF emitter (DtBuCzB) are designed and synthesized. The formed host-guest exciplex significantly boosts the <i>k</i> _RISC of DtBuCzB by over 260-fold to 2.2 × 10⁶ s^-1, while the emission color and narrow emission bandwidth are both preserved. Narrowband OLEDs using the hosts and DtBuCzB guest show maximum external quantum efficiencies (EQEs) up to 28.7% and EQEs at 1000 cd m^-2 (EQE₁₀₀₀) up to 23.3%, with the EQE₁₀₀₀ values being the highest among non-sensitized narrowband OLEDs based on DtBuCzB reported so far.