Heteroatom location‐ and orientation mode‐dependent reverse intersystem crossing rate in multiple resonance emitters

Constructing multiple resonance thermally activated delayed fluorescence (MR‐TADF) emitters with fast reverse intersystem crossing rate constant ( k RISC ) and reducing electroluminescence efficiency roll‐off is an essential challenge for practical applications. Herein, two pairs of isomers, O‐containing BN‐TP‐FH/BN‐TP‐FL and S‐containing BN‐TP‐TH/BN‐TP‐TL, are constructed by leveraging different heteroatom anchoring positions to systematically investigate the relationship between heteroatom location and orientation mode in MR emitters and k RISC . All emitters exhibit yellow‐green emission (537–546 nm) with narrow full‐widths at half‐maximum of ≤38 nm in toluene solution. By manipulating the heteroatom location and orientation, the reduced singlet–triplet energy splitting (Δ E ST ) and enhanced spin‐orbit coupling matrix element (<S| Ĥ SOC |T>) synergistically contribute to fast k RISC . The k RISC s increases from 1.23 × 10 4 s −1 (BN‐TP‐FH), 4.05 × 10 4 s −1 (BN‐TP‐TH) to 6.32 × 10 4 s −1 (BN‐TP‐FL) and 14.8 × 10 4 s −1 (BN‐TP‐TL). As a result, BN‐TP‐TL‐based single‐host device demonstrates the best electroluminescence performances, achieving a maximum external quantum efficiency of 31.1% and effectively suppressing the efficiency roll‐off.