Triazatruxene: A Rigid Central Donor Unit for a D–A₃ Thermally Activated Delayed Fluorescence Material Exhibiting Sub‐Microsecond Reverse Intersystem Crossing and Unity Quantum Yield via Multiple Singlet–Triplet State Pairs

Abstract

By inverting the common structural motif of thermally activated delayed fluorescence materials to a rigid donor core and multiple peripheral acceptors, reverse intersystem crossing (rISC) rates are demonstrated in an organic material that enables utilization of triplet excited states at faster rates than Ir‐based phosphorescent materials. A combination of the inverted structure and multiple donor–acceptor interactions yields up to 30 vibronically coupled singlet and triplet states within 0.2 eV that are involved in rISC. This gives a significant enhancement to the rISC rate, leading to delayed fluorescence decay times as low as 103.9 ns. This new material also has an emission quantum yield ≈1 and a very small singlet–triplet gap. This work shows that it is possible to achieve both high photoluminescence quantum yield and fast rISC in the same molecule. Green organic light‐emitting diode devices with external quantum efficiency >30% are demonstrated at 76 cd m−2.

Recent Progress of Singlet‐Exciton‐Harvesting Fluorescent Organic Light‐Emitting Diodes by Energy Transfer Processes

“…After commercialization of phosphorescent OLEDs, a new technology utilizing an up‐conversion process of triplet excitons, thermally activated delayed fluorescence (TADF), evolved and also proved excellent as a high‐EQE approach . The device performance of TADF OLEDs slowly caught up with that of phosphorescent OLEDs, although the device lifetime of TADF OLEDs was not as long as that of phosphorescent OLEDs …”

Balancing charge-transfer strength and triplet states for deep-blue thermally activated delayed fluorescence with an unconventional electron rich dibenzothiophene acceptor

“…Unfortunately, no clear rules currently exist to guide molecular design of high efficiency TADF emitters with sufficiently fast radiative decay to avoid the build-up of triplet states and non-radiative channels. 35 A multitude of various colour TADF emitters have been reported to date. [36][37][38][39][40][41][42] The majority are composed of well-decoupled D and A units, which create excited states with strong CT character.…”

The influence of molecular geometry on the efficiency of thermally activated delayed fluorescence

“…15 OLEDs fabricated with TADF emitters have shown impressive performances, sometimes with external efficiencies (EQE) above 30%. [16][17][18] However, the performance of TADF-based OLEDs is often weaker in the red and blue regions, 19 and is in general affected by poor stability showing significant device efficiency roll-off at high current densities. The causes of poor stability are not always clear, but triplet-triplet annihilation and tripletpolaron processes are often responsible for the luminescence quenching.…”