“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

Issues concerning excited state lifetime (τTADF) 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 τTADF 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 (kF), intersystem crossing (kISC), internal conversion (kIC) and reverse intersystem crossing (kRISC) are selectively altered, affording four representative TADF emitters. Based on lifetime and quantum yield measurements, kF, kISC, kIC and kRISC are calculated for four emitters and their interrelationship matches corrected time-dependent density functional theory simulation. Among them, even with a small kF, low photoluminescence quantum efficiency (Φ) and large kISC, molecules with a small singlet-triplet splitting energy (ΔEST) and lowest charge transfer triplet excited state (3CT) eventuate in shortening the τTADF. Herein, kRISC, which is inversely proportional to ΔEST, turns out to be the rate-limited factor in tuning the τTADF ("rate limited effect" of the RISC process). As revealed by flexible potential surface scanning, PyCN-ACR exhibited a moderate kF, reduced kIC and enlarged kRISC, resulting in a short τTADF 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.