Narrowband TADF emitters with high utilization of triplet excitons: theoretical insights and molecular design

Narrowband emitters with thermally activated delayed fluorescence (TADF) features, known as multi-resonant TADF (MR-TADF) emitters, are drawing increasing research interest owing to their properties of high efficiency and excellent color purity. However, MR-TADF-based devices often face serious efficiency roll-off at high luminance intensity, which could be attributed to undesired triplet-triplet annihilation (TTA) caused by the structural planarity and relatively small reverse intersystem crossing rate constants (k_risc) of MR-TADF emitters. Herein, combining a sp³-C inserted strategy to suppress harmful bimolecular interactions and chalcogens to improve the k_risc, a series of asymmetric narrowband emitters, namely, DMAC-O, DMAC-S, and DMAC-Se, have been theoretically designed to break the slow rate-limiting step of k_risc of experimental BN-DMAC. For comparison, both O and Se atoms were doped into the MR skeleton to substitute two sp³-inserted units, yielding BN-O-Se. The combination of TD-DFT and the wavefunction-based STEOM-DLPNO-CCSD approach exhibits that those asymmetric molecules are promising for simultaneously exhibiting narrow emission spectral full-width at half-maximums (FWHMs) and high luminous efficiencies. The contributions of chalcogens to hole distributions result in red-shifted fluorescent peaks, and the asymmetric strategy also helps with twisted molecular configuration, which is beneficial for suppressing unfavorable TTA. Furthermore, the incorporation of chalcogens is sufficient to promote the intersystem crossing and reverse intersystem crossing channels of asymmetric emitters. More importantly, the doped heavy Se atom results in a significantly increased k_risc of 2.32 × 10⁶ s^-1 for DMAC-Se, which is more than 200 times larger than 1.09 × 10⁴ s^-1 of pristine BN-DMAC. These results suggest that the combination of the heavy Se atom and an sp³-inserted unit is a feasible strategy for achieving poor planarity and significantly enhancing k_risc, which will help in harvesting triplet excitons, thereby inhibiting efficiency roll-off in corresponding narrowband devices.