Enhancement of Reverse Intersystem Crossing in Charge‐Transfer Molecule through Internal Heavy Atom Effect

Abstract

Thermally activated delayed fluorescence (TADF) is beneficial for improving the efficiency of organic light-emitting diodes (OLEDs) by providing pathways to convert non-emissive triplet excitons into singlet excitons. To ensure TADF is efficient, it is critical to enhance the reverse intersystem crossing (RISC) rate. To this end, most approaches propose thus far have focused on reducing the energy difference between S 1 and T 1 states. The present study explores how incorporating the internal heavy atom (IHA) effect can impact the RISC and device performance. By introducing a series of halogen atoms to charge-transfer molecules, TADF molecules exhibiting RISC over 7 × 10 7 s −1 are realized. These molecules are then applied to OLEDs, and the effect of incorporating these moieties is investigated. The results show that efficiency roll-off is still significant even with RISC-enhanced TADF emitters. Spectroscopic and theoretical results indicate that a fast RISC may not be the sole factor important for reducing efficiency roll-off and that the spin-flip cycles considering both T 1 →S 1 and S 1 →T 1 should be carefully taken into account to derive a complete picture of the IHA effect on efficiency roll-off behavior.

Aggregation of gold(i) complexes: phosphorescence vs. singlet oxygen production

“…Nevertheless, for efficient production, the sensitizers should possess additional features such as high intersystem crossing (ISC) rates and long triplet lifetimes in order to increase the probability of energy transfer. In terms of molecular design, ISC may be favoured by enhancing the spinorbit coupling (SOC) with the introduction of heavy atoms [12][13][14] or heteroatoms 15,16 or by introducing donor/acceptor units with electron transfer steps. 17,18 In this way, organometallic chromophores containing heavy metals deserve their investigation in this field since they possess high SOC values and the tuneability of their photophysical properties through ligand modification can be easily performed.…”

Multiple resonance thermally activated delayed fluorescence enhanced by halogen atoms

“…The heavy-atom effect can be introduced not only by transition metals such as Ir or Pt but also by halogen atoms such as chlorine and bromine. 14–18…”

“…With the introduction of halogens into twisted TADF emitters, the performance of OLED devices improved in some cases. 16–18 However, a negative effect of halogens was observed when halogens were introduced into the TADF emitters with a large rate constant of reverse intersystem crossing ( k RISC ). 16 These molecules did not emit fluorescence from the excited state because the rate constants of intersystem crossing ( k ISC ) (∼10 8 ) and k RISC (∼10 7 ) were larger than the rate constant of radiative decay ( k s r ) (∼10 6 ).…”

“…16–18 However, a negative effect of halogens was observed when halogens were introduced into the TADF emitters with a large rate constant of reverse intersystem crossing ( k RISC ). 16 These molecules did not emit fluorescence from the excited state because the rate constants of intersystem crossing ( k ISC ) (∼10 8 ) and k RISC (∼10 7 ) were larger than the rate constant of radiative decay ( k s r ) (∼10 6 ). However, when halogens were introduced to twisted TADF molecules with a small k RISC , the PLQY, EQE, and roll-off were improved.…”

Highly Efficient Asymmetric Multiple Resonance Thermally Activated Delayed Fluorescence Emitter with EQE of 32.8 % and Extremely Low Efficiency Roll‐Off

“…Though the introduction of tert-butyl or steric hindrance groups on MR-TADF emitters helps to inhibit the notorious bimolecular interaction and prevent aggregation-caused emission quenching to some degree, the essential issue still cannot be well resolved due to the insubstantial improvement of the k RISC , which severely impede MR-TADF materials toward practical applications. [5d,e,h,i, 6d,e, 8a] According to the first-order perturbation theory, Fermi's Golden rule, the k RISC mainly depends on the singlet-triplet energy splitting (ΔE ST ) and spin-orbit coupling (SOC) between the lowest excited singlet (S 1 ) and the lowest excited triplet (T 1 ) states, as expressed in Equation ( 1): [9] k…”