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

Abstract

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 (ΔE_ST) and enhanced spin-orbit coupling matrix element (<S|Ĥ_SOC|T>) synergistically contribute to fast k_RISC. The k_RISCs increases from 1.23 × 10⁴ s⁻¹ (BN-TP-FH), 4.05 × 10⁴ s⁻¹(BN-TP-TH) to 6.32 × 10⁴ s⁻¹ (BN-TP-FL) and 14.8 × 10⁴ 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.