Fast reverse intersystem crossing over 107 s−1 via near-enantiomeric charge-transfer transitions

Abstract

Thermally activated delayed fluorescence (TADF) enables high-efficiency organic light-emitting diodes (OLEDs) through reverse intersystem crossing (RISC) of triplet excitons, yet existing systems struggle to achieve fast RISC rates (k_RISC >10⁷ s⁻¹), let alone balance a high radiative decay (k_r). Herein, we develop a dual-donor TADF emitter that bypasses the El-Sayed rule restriction by harnessing near-enantiomeric charge-transfer transitions between a high-lying triplet state and the lowest singlet state and that synergizes with heavy atoms to enhance spin-orbit coupling. This design yields a k_RISC of 1.54 × 10⁷ s⁻¹ alongside a k_r of 9.65 × 10⁶ s⁻¹, reducing the delayed fluorescence lifetime to 1.46 μs. The OLED exhibits a peak external quantum efficiency of 30.6% with minimal roll-off (30.4% at 5,000 cd m⁻²) and an operational lifetime exceeding 1,500 h at 1,000 cd m⁻² before the luminance decays to 95% of its initial value, which extends further to 3,876 h via sensitized narrowband emission. This work bypasses conventional localized triplet-intermediate-dependent RISC mechanisms and establishes a paradigm for designing high-efficiency, stable TADF materials.