Unveiling the TADF Emitters with Apparent Negative Singlet-Triplet Gaps: Implications for Exciton Harvesting and OLED Performance

Intramolecular through-space charge transfer thermally activated delayed fluorescence (TSCT-TADF) has attracted much attention recently as it can achieve both small energy splitting and high emission efficiency. However, the relationship of excited states between TSCT and through-bond charge transfer (TBCT) remains a challenge in the TSCT-TADF molecules. Herein, three compounds DPS-m-bAc, DPS-p-bAc, and DPS-OAc that possess emissive TSCT and/or TBCT states are prepared. Interestingly, a so-called inverted energy gap is found for both DPS-m-bAc and DPS-p-bAc in toluene solution, which results from the different charge transfer states of ICThigh and ICTlow, as proved by the detailed transient photoluminescence and calculated results. Intense emission from blue to yellow associated with high photoluminescence quantum yields of 70-100% are measured in doped polymethyl(methacrylate) (PMMA) films. Notably, compound DPS-m-bAc achieves the highest reverse intersystem crossing rate constant (kRISC) of over 107 s-1 in a PMMA film, benefiting from close-lying TSCT and TBCT states. The solution-processed device with DPS-m-bAc displays a maximum external quantum efficiency of 21.7% and a relatively small efficiency roll-off (EQE of 20.2% @ 100 cd m-2). Overall, this work demonstrates how with judicious emitter engineering, a synergy between different charge transfer excited states, can be achieved, providing an avenue to achieve highly efficient solution-processed OLEDs. Three structurally related charge transfer-based compounds DPS-m-bAc, DPS-p-bAc, and DPS-OAc are prepared. Notably, compound DPS-m-bAc shows the fastest reverse intersystem crossing rate constant of the three of over 107 s-1 in a doped PMMA film as a result of the very small singlet-triplet gap. A maximum external quantum efficiency of 21.7% is achieved for the DPS-m-bAc-based solution-processed device.image