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Issues concerning excited state lifetime ( τ  TADF ) tuning of thermally activated delayed fluorescence (TADF) materials are critical for organic light emitting diode (OLED) applications and other specific fields. For TADF-OLEDs, employing emitters with a short τ  TADF gives rise to suppressed singlet-triplet annihilation (STA) and triplet-triplet annihilation (TTA), leading to reduced efficiency roll-off at practical relevant brightness (100 and 1000 cd m -2 for display and illumination applications, respectively). Through molecular design, exciton dynamic process rate constants including fluorescence ( k  F ), intersystem crossing ( k  ISC ), internal conversion ( k  IC ) and reverse intersystem crossing ( k  RISC ) are selectively altered, affording four representative TADF emitters. Based on lifetime and quantum yield measurements, k  F , k  ISC , k  IC and k  RISC are calculated for four emitters and their interrelationship matches corrected time-dependent density functional theory simulation. Among them, even with a small k  F , low photoluminescence quantum efficiency ( Φ ) and large k  ISC , molecules with a small singlet-triplet splitting energy (Δ E  ST ) and lowest charge transfer triplet excited state ( 3 CT) eventuate in shortening the τ  TADF . Herein, k  RISC , which is inversely proportional to Δ E  ST , turns out to be the rate-limited factor in tuning the τ  TADF ("rate limited effect" of the RISC process). As revealed by flexible potential surface scanning, PyCN-ACR exhibited a moderate k  F , reduced k  IC and enlarged k  RISC , resulting in a short τ  TADF and a moderate Φ with orange-red emission. OLEDs containing PyCN-ACR as the emitting guest achieved orange-red TADF-OLEDs with an emission peak at 590 nm and the best external quantum efficiencies (EQEs) of 12.4%/9.9%/5.1% at practical luminances of 100/1000/10 000 cd m -2 .

chemical science

“Rate-limited effect” of reverse intersystem crossing process: the key for tuning thermally activated delayed fluorescence lifetime and efficiency roll-off of organic light emitting diodes